Halogenated fire suppressants: A symposium hosted by the Southwest Research Institute, San Antonio, Tex., April 23-24, 1975 : proceedings (ACS symposium series ; 16)

Halogenated Fire Suppressants Richard G. G a n n , EDITOR Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.f...

Author: Richard G. Gann

82 downloads 591 Views 10MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

Halogenated Fire Suppressants

Richard G. G a n n , EDITOR

Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.fw001

Naval Research Laboratory

A symposium hosted by the Southwest Research Institute, San Antonio, Texas, April 23-24, 1975.

ACS

AMERICAN WASHINGTON,

SYMPOSIUM

CHEMICAL D.

SERIES

SOCIETY C.

1975

16


Library of Congress

Data

Halogenated fire suppressants. (ACS symposium series; 16

ISSN 0097-6156)

Includes bibliographical references and index. 1. Fire extinguishing agents—Congresses. 2. Organohalogen compounds—Congresses. I. Gann, Richard G., 1944II. Series: American Chemical Society. A C S symposium series; 16. TP266.H34 ISBN 0-8412-0297-4

628.9'254 75-25638 ACSMC8 16 1-453 (1975)

Copyright © 1975 American Chemical Society All Rights Reserved PRINTED IN THE UNITED STATES OF AMERICA


ACS Symposium Series Robert F. Gould, Series Editor


FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide

a medium for publishing symposia quickly in book form. The format of the SERIES parallels that of its predecessor, ADVANCES IN CHEMISTRY SERIES, except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. As a further means of saving time, the papers are not edited or reviewed except by the symposium chairman, who becomes editor of the book. Papers published in the ACS SYMPOSIUM SERIES are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.

PREFACE "In many areas of the fire problem, proposed solutions rest on limited experience, shaky assumptions, and guesswork."

Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.pr001

(i)

"More research is needed on extinguishing agents . . . , to improve the effectiveness of existing agents and to inves-

tigate the chemical and physical mechanisms of new agents." (I) A ny of us who have been near a roaring fire or who have seen gutted buildings or who have known burn victims are aware of the need for rapid, effective fire suppression. In many cases, especially for large buildings, the use of fire trucks or sprinkler systems is unavoidable. Despite the foreknowledge that the water damage could match or exceed the direct fire damage, water is still the most deliverable, often the only available suppressant for a fully developed conflagration. However, in certain situations, particularly indoors, something less massive is desired. Perhaps people need to remain in the area to secure valuables, perhaps expensive equipment is present, or perhaps the fire is too small to warrant calling the fire department. Whatever the specific reason, the need exists for a conveniently local, non-destructive and physiologically tolerable fire suppressant. An early alternative candidate to water was carbon tetrachloride, CCI4. Although it is effective for flame quenching, we now know that CCU is too toxic to use indoors. However, the idea of halogenated agents was sufficiently attractive that copious testing was performed on a wide variety of these compounds, commonly referred to as halons (2). Many were found effective; many others posed patently unacceptable health hazards. The three major halon agents in commercial use today evolved from this previous work. These are C F B r (halon 1301), C F C l B r (halon 1211), and C F B r (halon 2402). In 1972, in response to controversy concerning the safety and effectiveness of these compounds, the National Academy of Science's Committee on Fire Research and the National Research Councils Committee on Toxicology held a symposium to appraise the toxicology and usage of at least the three aforementioned principal halons (3). They concluded 3

2

4

2

vii

2


that the halons displayed variable degrees of effectiveness in fire suppression and that some potential physiological hazard exists in the agents themselves as well as in their fire-generated breakdown products. Although thoroughly covering the two principal goals, the 1972 appraisal, by its definition, stopped short of considering what else was needed to better the existing situation. There are several complementary approaches to improving our knowledge in this direction. The one chosen for this symposium was to look at the basic processes occurring in inhibited flames and to deduce mechanistic elements for suppressing fires by halons. Fortified with this knowledge, we can perhaps better discuss their usage conditions, screen suggested compounds, and even provide guidelines for developing better agents. Thus, the authors of these papers, representing a wide range of disciplines, assembled to examine each others work in flame inhibition, to look for mutual assistance, and to ascertain how far along we are in understanding the processes involved. The papers review prior classical and contemporary bulk flame inhibition studies, describe various studies of chemically and dynamically different flame-halon systems, review the current state of the pertinent kinetics, and attempt to gather, model, and interpret the previously presented data. Much of the oral discussion has been transcribed and included, as has the general discussion which took place at the conclusion of the symposium. Hopefully, this collaborative effort will isolate and highlight the significant information we have already evolved and point out research recommendations for each other and for new workers in the field.

Acknowledgments In the process of organizing this symposium I have received help from several sources, and these deserve a large measure of credit for the success and the very existence of the conference. My thanks go to E. I. du Pont de Nemours & Co., Inc., and the National Bureau of Standards, especially to Charles Ford and Andrej Macek, respectively, forfinancialassistance. The officers and program chairmen of the Central States and Western States Section and the national office of the Combustion Institute helped to publicize and coordinate the symposium. In particular, I acknowledge the efforts of the Southwest Research Institute, especially David Black, William McLain, and W . D . Weatherford for invaluable assistance with the local arrangements and with the techniques of running a meeting. I am also grateful to the Naval Research Laboratory for allowing me the time to prepare both the conference and this volume and to Homer W. Carhart for his active encouragement throughout. In addition, I thank Bunny Hampton for her able help in preparing these proceedings for publication. viii

Literature Cited 1. "America Burning: The Report of the National Commission on Fire Prevention and Control," U. S. Government Printing Office, Washington, D. C. 20402 (1973). 2. Halon is an abbreviation devised by the Army Corps of Engineers to designate halogenated hydrocarbon. If the compound under discussion has the formula C F Cl Br I , it is designated as halon abcde. Terminal zeros are dropped, allowing halon "names" to consist of fewer than five digits. 3. "An Appraisal of Halogenated Fire Extinguishing Agents," National Academy of Sciences, Washington, D. C., 1972. a

b

c

d

e

RICHARD G. G A N N


Naval Research Laboratory Washington, D . C. 20375

June 1975

ix

1 An Overview of Halon 1301 Systems CHARLES L. FORD

Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch001

E. I. du Pont de Nemours & Co., Wilmington, Del. 19898

A symposium to focus upon the mechanism by which halogenated extinguishants operate i s commendable, and I am honored to be a participant. This paper does not speak directly to the mechanism of halogenated agents, or even of Halon 1301, but instead speaks around it. In the thirty-odd years since Halon 1301 was f i r s t synthesized, the Du Pont Company has assumed a major role in developing practical application data on this agent for the fire protection industry. Although virtually none of this effort has been aimed directly at mechanisms, there are nevertheless many relationships which have been observed and which must be explained by a valid mechanism. Further, perhaps an awareness of these relationships in light of the papers which follow will help to form the nucleus of a viable mechanism. These relationships include differences i n extinguishing requirements between agents, between fuels, between methods of application, between conditions of application, and other variables which exist in a fire situation. This paper will therefore be of a historical nature, examining 28 series of tests conducted in the past. Because most of the work with which Du Pont has been associated has dealt with Halon 1301, most of the studies included here also deal with Halon 1301. However, I have also included studies on Halon 1211 and a few points of comparison with other agents. As the t i t l e implies, the discussion will be heavy on systems applications, but again not solely so. Three studies on portable extinguishers are reported. A l l i n all, I believe this paper represents one of the most comprehensive compilations of prior fire test work on the "modern" halogenated agents which has been published to date. I offer it with the hope that it will indeed contribute to the formulation of an acceptable mechanism for these agents. History. Halogenated compounds have been employed as fire suppressants throughout the 20th Century, although until recently their use was rather limited. It was recognized in the late 1800's that carbon tetrachloride was an effective fire 1


2

HALOGENATED FIRE SUPPRESSANTS

extinguishant. In the early 1900's, when the electrolytic process for the production of caustic soda frem brine provided large quantities of cheap chlorine, hand pump extinguishers containing carbon tetrachloride were introduced cxxrmercially (1). These extinguishers became very popular and continued i n service up until the mid-1960's when their listings were discontinued by the testing laboratories. In the 1920's, carbon tetrachloride was applied experimentally i n small aircraft systems to extinguish engine fires. These systems were later adopted i n Europe, but were rejected by the U. S. Army Air Corps in favor of carbon dioxide systems in 1931. In the late 1920's, methyl brcmide was considered as a fire extinguishant and was found to be more effective than carbon tetrachloride, but also was considerably more toxic. Its use i n the United States was rejected, largely based on its high toxicity, but in England about 1938, i t became the standard agent for aircraft extinguishing systems. Seme of these systems remain in use today. Methyl bromide was also widely used by the Germans in Vforld War II, both for aircraft and for marine fire protection systems. A number of casualties in the German Navy were attributed to methyl brcmide poisoning (2). Beginning in the late 1930's, the Germans developed a third fire extinguishing agent, côrobrcmomethane, somewhat less toxic than methyl brcmide but s t i l l a highly effective extinguishing agent. It quickly replaced methyl bromide i n both the Luftwaffe and the Kriegsmarine. CMorobromcmethane was evaluated by the U. S. C i v i l Aeronautics Administration in 1939 to 1941, and by the C-O-Two Company i n 1948. Chlorobranramethane was later adopted by the U. S. Air Force for on-board fire extinguishing systems for aircraft power plants, for use i n portable extinguishers aboard aircraft, and for airport ramp patrol vehicles(3). Many of these units are s t i l l in service. About 1948, the U. S. Army initiated a research program to develop an extinguishing agent which had the high effectiveness of chlorcteonrarethane or methyl brcmide but without the attendant high toxicity of these agents. Approximately 60 candidate agents were collected and evaluated both for extinguishing effectiveness and toxicity. Most of these compounds were halogenated hydrocarbons. Because the names of these compounds are rather long, the Army devised a numbering system known as the "halon" (contraction of halogenated hydrocarbon) nomenclature system. This system identifies the empirical formula of the compound with a five-digit number which represents, i n order, the number of carbon, fluorine, chlorine, bromine, and iodine atoms in the molecule. Terminal zeroes are dropped so that the resulting number may have only two, three, or four digits. In the remainder of the paper, halogenated extinguishants are identified by their Halon numbers. As a result of the Army studies, four agents were selected for further evaluations. These were: Halon 1301, bromotri-


1.

FORD

Halon 1301 Systems

3

fluoromethane, Halon 1211, bromochlorodifluoromethane, Halon 1202, dihranodifluorcmethane, Halon 2402, dibraiotetrafluoro-ethane. These further evaluations resulted eventually in the selection of Halon 1301 by the Arnry for a snail 2-3/4 pound portable extinguisher for battle tanks and electronic vans, and by the Federal Aviation Administration for protecting aircraft engines in oonmercial transport aircraft. Halon 1211 was selected by the British for use i n both military and oonmercial aircraft. Halon 1202 was selected by the U. S. Air Force i n military aircraft protection. Recently, Halon 2402 has seen some limited use i n southern and eastern Europe. On both continents, halogenated agent systems were applied to protect *the engines of many types of military land and water craft such as battle tanks, personnel carriers, trucks, hydrofoils, and hovercraft. Gradually, the systems were expanded to cover other areas of these vehicles. In the United States, the use of Halon 1301 i n oonmercial total flooding systems began in the early 1960's. Many of these early systems utilized carbon dioxide hardware and technology. In 1965, a project was begun with Factory Mutual Research Corporation to define the requirements for Halon 1301 i n total flooding systems. In 1966, the National Fire Protection Association organized a technical oonmittee on halogenated fire extinguishing systems to develop standards for systems using Halon 1301 and other halogenated agents. A tentative Standard NFPA 12A-T was adopted i n 1968 and made a permanent Standard NFPA No. 12A in 1970 (£). At the present time, there are perhaps 10,000 Halon 1301 systems i n service in the world. Typical applications include computer rooms, magnetic tape storage vaults, electronic control rocms, storage areas for art work, books, and stamps, aerosol f i l l i n g rooms, nachinery spaces i n ships, cargo areas in large transport aircraft, processing and storage areas for paints, solvents, and other flartmable liquids, instrument trailers, and compressor stations. In Europe, development of Halon 1211 closely paralled that of Halon 1301 i n the States. However, there has been somewhat greater emphasis on the use of Halon 1211 in portable fire extinguishers than i n the U. S. Halon 1211 total flooding systems, using suitable safeguards such as time delays, are enployed for similar applications as previously mentioned for Halon 1301. NFPA Standard No. 12B on Halon 1211 systems was adopted by NFPA tentatively in 1971, and permanently in 1972 (5). Some work has been initiated i n Europetowardsapplying Halon 1211 to local application systems. Halon 1211 portables have recently been introduced into the United States and currently several models from several manufacturers have been listed or approved by the testing laboratories. Applications for halogenated fire suppressants can be categorized as follows:

4

HALOGENATED

FIRE SUPPRESSANTS


Portable extinguishers, in which a supply of agent i s directed onto the burning surface by a person; Total flooding systems, in which a supply of agent i s arranged to discharge into an enclosure and f i l l i t uniformly with a concentration of agent sufficient to extinguish the fire. Subclassifications of total flooding systems include: Flame extinguishment, in which the agent i s applied to an existing fire, usually a diffusion flame; Inerting, in which a suppressing concentration of agent is developed within an enclosure to prevent ignition by an ignition source; Explosion suppression, in which agent i s rapidly injected into a developing flame front to suppress i t before i t has reached damaging proportions; Local application systems, in which a supply of agent i s arranged to discharge directly onto a burning surface. Special systems designed to protect specific hazards such as aircraft engines, race cars, restaurant hood and ducts, etc. Conceivably, a l l the halogenated agents could be used in any of the above applications, and most of them have been evaluated in several of them. We may, therefore, utilize these past evaluations to provide comparisons between agents, between method of application, and between various fuels to perhaps shed seme light on mechanisms of these agents. The primary objective of this paper i s to examine seme of these relationships. We shall begin by examining the evaluations on portable extinguishers. Portable Extinguisher Evaluations Some factors which are known to influence the performance of portable extinguishers are: 1. Extinguishing agent 2. Fuel and fuel tenperature 3. Fuel configuration 4. Discharge rate 5. Discharge pattern 6. Nozzle motion Fuel configuration includes the shape of the fire pan, the amount of freeboard, and obstructions which are present. The discharge pattern i s largely governed by the type of extinguishing agent and the design of the discharge nozzle. Nozzle motion i s iitparted by the operator and can be quite influential on the performance of the extinguisher. Three evaluation programs for portable extinguishers are reviewed here: U. S. Army Corps of Engineers - 1954 Ansul Company Portable Evaluations - 1955

1.

FORD

5

Halon 1301 Systems


Du Pont Portable Extinguisher Design - 1958-1973 U. S. Army Corps of Engineers(2). The project undertaken by the U. S. Army Corps of Engineers in 1947 had as its primary goal the development of a snail portable fire extinguisher. Following an i n i t i a l screening of agents by Purdue Research Foundation using an explosion burette technique (2) / the Corps of Engineers conducted a series of practical extinguishment tests in which 12 halogenated agents were applied to two standardized fires. One fire was a flammable liquid contained in a 2 f t diameter steel tub f i l l e d with water to within 10-1/2 i n of the rim. Two quarts of gasoline were poured on top of the water and a 20 sec preburn was allowed before the fire was attacked. A second fire was ocmposed of 8 pounds of cotton waste spread over a 2 x 4 f t area and wetted with two quarts of gasoline. A 10 sec preburn time was alleged before attacking the fire. A 2-3/2 pound carbon dioxide extinguisher was used to apply the agents. Tests were run with the extinguishers pressurized with nitrogen to both 800 psi and to 400 psi total pressure. For each agent/ fire/pressure level combination, 10 tests were made and the results averaged. From the average weight of each agent required to extinguish each fire, a weight effectiveness value was calculated based on Halon 1001 equal to 100. Results of these tests are shown i n Table I for Halons 1201, 1301, 2402, and for carbon dioxide. TABLE I PORTABLE EXTINGUISHER EVALUATIONS Army Corps of Engineers R&D Laboratories 1951

Agent Halon Halon Halon Halon C02

1001 1202 1301 2402

24 i n Tub Fire Weight Weight to EffectiveExtinguish ness oz % 8.0 6.6 7.6 10.8 9.1

100 120 105 74 88

2x4 f t Cotton Waste Fire Weight Weight to EffectiveExtinguish ness oz % 20 20 15.6 19.8 32

100 100 126 N.E. N.E.

Of the three halogenated candidates, Halon 1202 was most effective on the tub fire and Halon 1301 most effective on the cotton waste fire. Halon 1211 was not evaluated i n this series of tests, having been eliminated as a result of the Purdue study (see below). Based upon this test program and upon toxicology studies conducted by the Army Chemical Center which shewed Halon 1301 to be the least toxic of these candidates, Halon 1301 was



100

200

300

400

500

600

700

800

Agent Raft,g/sec.

Figure 2. Performance curves for Halons 1301,1211,1202, and 2402. Ansul Co. portable evaluations—1955. Nozzle AX-11, P — 550 psi.

1.

FORD

Halon 1301 Systems

7


selected by the Anny as i t s sole halogenated fire extinguishing agent. Ansul Company Portable Evaluations (6). In 1955, the Ansul Ocmpany and Du Pont jointly conducted a second series of agent evaluations aimed at portable extinguishers. For this series of tests, the human operator factor was eliminated by utilizing fixed nozzles. A sketch of the test arrangement i s shown i n Figure 1. Pour fan spray nozzles were installed along the inner periphery of a circular tank of approximately 4 f t diameter. The nozzles were connected through a manifold to the agent supply cylinder. The tank was f i l l e d with water and a 2 in layer of unleaded gasoline to leave a 12 in freeboard in the tank. Eight halogenated agents and a sodium bicarbonate-base dry chemical agent were evaluated i n these tests. The evaluation was conducted i n a manner described by Guise (7). Each agent was applied to the fire at various discharge rates, observing i n each case the quantity of agent required for extinguishment. For each agent, several different discharge nozzles were used, and the nozzle giving optimum performance was selected. The data for each agent was then plotted to shew quantity of agent for extinguishment vs discharge rate. Figure 2 shows curves obtained in this manner for the four primary halogenated agents, Halons 1301, 1211, 1202, and 2402. The minimum i n each curve represents the minimum quantity of agent Qm required for extinguishant and the optimum discharge rate Bq. These values are tabulated in Table II for the eight halogenated agents and for the sodium-based dry chemical. From these tests also, Halon 1301 was shown to be more effective than the other halogenated candidates and somewhat more effective than sodium bicarbonate-base dry chanical. Also, compounds containing bromine were a l l more effective than any of the agents containing no bromine. This data was utilized to reinforce the Army's previous selection of Halon 1301. IflftTJ? TT ANSUL PORTABLE EVALUATIONS 1955 Qm g/sec

Agent Halon Halon Halon Halon Halon Halon Halon Halon

1301 1202 2402 1011 1211 113 (R-ll) 104 122

509 670 836 884 952 988 1064 1103

125 183 300 232 417 350 458 472

8

HALOGENATED

Sodium Bicarbonate Dry Chemical

FIRE SUPPRESSANTS

813

218

Acoonpanying the main development thrust of halogenated agents on portable extinguishers in these early years, a number of tests were made by the Du Pont Company to determine the requirements of Halon 1301 i n portable extinguishers having standardized Underwriters Laboratories ratings. The UL rating system i s based on a square fire pan 12 in deep containing 4 in of water and 2 in of normal heptane given a 1 minute preburn time. The number of the rating refers to the area in square feet of fuel surface which can be extinguished by an untrained operator. At the testing laboratories, the extinguisher must be shewn capable of extinguishing a fire 2.5 times greater i n surface area than the numerical UL size.


1

Du Pont Portable Extinguisher Design (8,9, W,ll,12,13). Du Pont between the years 1958 and 1973 has determined Halon 1301 requirements for portable extinguishers ranging from size IB (2.5 ft2) to 40B (100 ft2). These requiranents are summarized in Table III. TABLE III DU PONT DATA - PORTABLE EXTINGUISHERS

UL Fire Size

Surface Area ft2

Minimum Discharge Rate g/sec

IB 2B 4B (5B) 6B 20B 30B 40B

2.5 5 10 (12.5) 15 50 75 100

25 50 100 (125) 150 589 760 (1000)

Minimum Quantity for 10 sec Discharge lb 0.61 1.25 2.5 (3.2) 3.75 14.5 18.5 (25)

For each rate shown, an optimum nozzle design i s required. This design represents a balance between agent penetration and nozzle coverage of the fire area. In the case of Halon 1301, i t must also exhibit a minimum amount of nozzle "drool" (liquid agent which i s separated from the discharging stream within the nozzle and which drips from the end of the nozzle, thus representing loss of agent). The data show that over a 40-fold range of fuel surface area, a constant discharge rate per unit surface area of 10 g/sec/ft2 (based on actual fuel area) i s required. Subsequent testing by others has shown that Halons 1211 and

1.

FORD

Halon 1301 Systems

9

2402, when used i n optimized equipment, are much more effective than the foregoing analyses indicate. In addition, these agents have some limited effectiveness on standard Class A wood crib fires which Halon 1301 does not possess. Therefore, most of the current activity i n halogenated agents for portable extinguishers is being conducted with these two agents, rather than with Halon 1301. Their lower volatility also gives them the capability to be projected for a considerable distance to the fire. Their discharge streams are also more visible, and hence more easily directed than that of Halon 1301.


Total Flooding Systems The basic components of total flooding systems are shown in Figure 3, Schematic Diagram of Total Flooding Fire Suppression Systems. A supply of agent contained under pressure (or pressurized at the time of discharge) i s connected by piping to one or more discharge nozzles located within the enclosure. Upon detection of a fire or a hazardous condition, a control valve on the agent supply container releases agent through the piping to one or more discharge nozzles located within the enclosure. The nozzles disperse the agent in such a manner as to form a uniform extinguishing (or inerting) concentration throughout the enclosure. Total flooding systems may be of two types: flame extinguishment and inerting (defined on page 4). For the purpose of this discussion, we will also define a third type of total flooding suppression system, i.e., for Class A fires. Halogenated agents are particularly attractive for use i n total flooding systems because of their cleanliness (lack of particulate residue), low toxicity, lack of vision obscuration, compactness and cost. Halon 1301, having the lowest toxicity of the halogenated agents currently i n use, has found its greatest application in this area. Most Halon 1301 systems are for flame extinguishment or for Class A protection, although a few have been installed for inerting. Flame Extinguishment Evaluations. Two test methods have been employed to determine the requirements of halogenated agents to achieve flame extinguishment in total flooding systems. The first i s a "dynamic" test in which a flame, usually a diffusion flame, i s surrounded by a flowing mixture of air and agent. Combustion and decomposition gases are not recirculated into the fire zone. "Static" test methods are also employed, particularly at large scale, wherein pure agent i s dispersed into an enclosure containing the fire. This method i s often desirable for demonstration since i t closely approximates the conditions under which the system operates i n practice. A number of investigations have been conducted using both test methods. In this discussion the results of six investigations of dynamic tests and four of static tests are examined:


10


Test

Discharge

Enclosure

Nozzle

A

Fire Detector

Control Valve

Agent Supply Container

~~

i t -

Figure 3. Schematic of totalfloodingfiresuppression system

5cm - Glass chimney

r 18 cm

L

- G l a s s bead packing

3 cm

Air & Agent

t Fuel & Agent

Figure 4. Creitz* diffusionflameextinction apparatus (1961)


1.

FORD

11

Halon 1301 Systems

Dynamic Tests Creitz - 1961 ICI Hemispherical Burner - 1970 ICI Cup Burner - 1973, 1974 Factory Mutual Research Corporation Cup Burner - 1973 Factory Mutual Research Corporation Vapor Burner - 1974 Cardox Intermediate-Scale - 1974 Static Tests Factory Mutual Research Corporation QmallScale - 1967 Factory Mutual Research Corporation Intermediate and Large Scale - 1967 to 1968 ICI Large-Scale - 1970 Ansul Large-Scale - 1972 Dynamic Tests. Creitz (14). In 1961, Creitz studied the effects on diffusion flames of four gaseous hydrocarbon fuels plus carbon monoxide and hydrogen, of adding Halons 1001 and 1301 and nitrogen to the air side and to the fuel side of the flame. His apparatus, shown i n Figure 4, consisted of a 5.4 mm i.d. tube upon which a diffusion flame of the fuel was supported. The flame was surrounded by a 5 cm i.d. glass chimney through which a mixture of air plus extinguishing agent flowed upwards past the flame. He observed the minimum concentration of agent i n either the air or i n the fuel to produce extinguishment of the flame. In some cases, his point of extinguishment was considered to be the point at which the flame lifted off the 5.4 mm tube. His results are shown i n Table IV. TABLE IV CREITZ FLAME EXl'lNUTION 1961 Flame supported on 5.4 mm i.d. glass tube Surrounded by 5 cm i.d. glass chimney Air velocity past flame = 16 an/sec Fuel, agent, and air at room temperature Agent Concentration i n M r or Fuel

N2 H2

CH4 C2H6 C3H8

94.1 83.1 85.6 83.7

Added to Air Halon Halon 1001 1301 11.7 2.5 4.0 3.1

17.7 1.5 3.0 2.7

N£ 52.4 51.0 57.3 58.3

Added to Fuel Halon Halon 1301 1001 52.6 22.9 35.1 37.6

58.1 28.1 36.6 34.0

12


C4H10 CD


83.7 90

2.8 7.2

2.4 0.8

56.8 42.8

37.9

40.0 19.9

The results illustrate several points. First, extinction concentrations of both Halon 1001 and Halon 1301, in terms of additional concentrations added to the system, are considerably lower than for nitrogen. The nitrogen values are somewhat deceptive. For example, to increase the concentration of nitrogen i n air from 80% to 85.6%, requires the addition of 39% additional nitrogen by volume. This compares to 4% additional Halon 1011 or 3% Halon 1301 to air to extinguish ethane. Second, when added to the air side, Halon 1301 i s more effective than Halon 1011 on the four hydrocarbon fuels and on carbon monoxide. Halon 1011 i s more effective than Halon 1301, however, on the hydrogen fire. When added to fuel, the relationship appears to be reversed. Here Halon 1011 i s generally more effective than Halon 1301. For the hydrogen fire, nitrogen appears to be the most effective extinguishant when added to the fuel. Third, the concentration of extinguishants in the fuel are considerably higher than for the agents added to the air side. Again, however, this i s somewhat deceptive. Considering the relative ratios of fuel to air, a smaller quantity of agent would be required when added to the fuel than when added to the air. This would be especially true when air-to-fuel ratios greatly exceed stoichiometric. A fourth observation i s that the lower concentration of Halon 1301 required to extinguish carbon monoxide i s considerably lower than for the aUcanes; with Halon 1011, a considerably greater concentration i s required for CO than for the hydrocarbon fuels. ICI Hemispherical Burner (15). Imperial Chemical Industries reported i n 1970 a series of flame extinction tests with Halon 1211 on 18 liquid and gaseous fuels which were supported on a 28.5 mm diameter porous metal hemisphere. The apparatus i s illustrated i n Figure 5. Fuels were fed through jacketed tubing onto the hemisphere until the lower surface of the hemisphere was wetted with fuel. The fuel was ignited. For gaseous fuels, the fuel rate was adjusted to give the flame approximately 6 to 9 in long before addition of Halon 1211. Once the flame was established, Halon 1211 was gradually added to the air stream until extinction was achieved. The concentration of Halon 1211 at the extinguishment point was calculated from the respective air and Halon 1211 addition rates from calibrated rotameters. For these tests, the air flow rate was held constant at 45 1/roin, which produced a velocity past the burner of approximately 13 can/sec. Extinction concentrations obtained are shown i n Table V. The data shew that for the alkanes butane through hexane, the Halon 1211 requirement for extinction i s relatively constant at 3.5 to 3.7% by volume. Avtur (aviation turbine fuel) and

FORD

Halon 1301 Systems

TABLE V ICI HEMISPHERICAL BURNER 1970 1 1/8 in dia Sintered Stainless Steel Hemisphere 96 nm i.d. glass chimney Air velocity past burner = 13.4 cm/sec Fuel at room temperature


Halon 1211 Concentration at Extinction % by Vol. Methane Ethane Propane n-Butane i-Butane n-Hexane Cyelohexane n-Heptane Avtur Kerosene Ethylene Methanol Ethanol Acetone Carbon Disulfide Hydrogen*

3.91 5.02 4.37 3.7 3.5 3.49 3.45 3.45 2.50 2.60 5.87 7.70 4.50 3.54 2.40 20.5

*Air velocity past hemisphere = 7.6 cm/sec Fuel

Water-cooled jacket

Glass chimney 9 6 mm ID.

30 cm

Sintered stainless-steel hemisphere 28.5 mm dia. Packed bed ('A"glass balls) Air & Agent

Figure 5. ICI hemispherical burner (1970)

14


ICI COP BURNER (1974) 28 itm o.d. Glass Cup in 85 nm i.d. Glass Chimney Air Velocity Past Cup: 13 cm/sec


Fuel Methane Ethane Propane n-Butane i-Butane n-Pentane n-Hexane n-Heptane 2,2,5 Trimethyl Hexane Avtag Avgas Avtur Avcat Gasoline (98 octane) Diesel Oil Transformer Oil Cyclohexane Benzene Toluene Ethyl Benzene Mixed Xylenes Ethylene Propylene 1-Butene 1,3-Butadiene Decalin Methanol Ethanol n-Propanol i-Propanol n-Butanol i-Butanol sec-Butanol Amyl Alcohol n-Hexanol Benzyl Alcohol

Agent Cone, i n Air at Extinction, % Fuel at 20°C Fuel at t°C Halon Halon Halon Halon 1301 1211 t°C 1301 1211 1.53 3.35 3.5 3.0 3.0 3.7 3.7 3.8

1.33 2.85 3.1 2.7 2.5 3.3 3.3 3.5

60 90

4.0 4.4

3.3 4.0 3.5 3.7 3.5

3.0 3.8 3.3 3.4

80 170

3.8 4.0

3.6

3.9

3.5

45 150

3.8 3.4

3.4

3.9 2.9 2.2 3.1 2.5 5.3 4.1 3.8 4.4 2.9 8.2 4.5 4.3 3.8 4.4 4.3 3.8 4.2 4.5 2.9

3.5 2.9 1.9 3.0 2.3 5.1 3.9 3.7 4.5 2.6 7.3 3.9 3.8 3.4 3.9 4.0 3.4 3.7 3.9 3.1

200 75 70 100 130 100

2.8 4.0 2.7 2.7 3.7 2.5

170

4.0

55 75 90 75 100 100 95 130 150 190

8.5 4.5 4.3 3.9 4.7 4.6 4.0 4.2 4.7 4.3

4.3

2.4

8.0 4.3 3.7

1.

FORD

Halon 1301 Systems


Ethylene Glycol

Acetone Acetyl Acetone Methyl Ethyl Ketone Ethyl Acetate Ethyl Aoetoaoetate Methyl Acetate Diethyl Ether Petroleum Ether Ethylene Gylcol Chlorobenzene Acetonitrile Acrylonitrile Eîchlorohydrin Nitramethane NN-Dimethyl Poiiramide Carbon Disulfide Isopropyl Nitrate Hydrogen

15

3.0

2.6

3.5 3.7

100 125 150 165 180 50 130

3.1 3.5 4.0 5.0 5.2 3.7 4.0

3.8 4.1 3.9 3.3

3.6 2.9

70

3.3

3.6 3.3 4.4 3.7 3.0 0.9 3.0 4.7 5.5 4.9

3.2 2.7 3.9 3.3 2.6 0.9 2.6 4.7 5.3 4.1

170 50

3.9 3.4

180 120 75

5.2 1.4 30

100 85

5.5 8.0

3.6 1.6 7.5 15.0

3.1 2.6

140

3.8

95

7.7

16



kerosene flames required somewhat less agent for extinction. Ethylene, methanol, and hydrogen a l l required considerably higher concentrations. Because seme back-diffusion of agent decomposition products onto the surface of the hemisphere occurred, and because the burner proved difficult to clean, the use of this apparatus was discontinued in favor of the ICI cup burner described next. ICI Cup Burner (16). The hemispherical burner described above was modified by replacing the hemisphere with a streamlined 28.5 mm o.d. glass cup, as shown in Figure 6. The inner rim of the cup was ground to a sharp 45° edge so that the liquid level of fuel could be maintained at the very outside edge of the cup. In 1973, a series of tests were reported i n which 58 gaseous and liquid fuels ware examined with Halon 1211. In 1974, the cup was further modified to include a heating coil within the structure of the cup itself. For gaseous fuels the cup was replaced with an 8 nm o.d. glass tube having the same length as the cup. Air rates of 10 1/min to 50 1/min were examined, with a rate of 40 1/min becoming standard. Air velocity past the cup was approximately 13 cm/sec. For gaseous fuels, the linear velocity of the gas stream was matched to that of the air plus extinguishing agent. Many liquid fuels were examined both at roan temperature (20°C) and at elevated temperatures near the boiling point of each fuel. Several fuels were evaluated with Halon 1301 as well as with Halon 1211. Results from these tests are shown in Table VI. Generally speaking, Halon 1211 requirements determined i n these tests are somewhat greater than for Halon 1301. For gaseous fuels with both agents, the aUcanes of ethane through butane shew relatively constant requirements of 3 to 3-1/2% Halon 1211 and 2-1/2 to 3% Halon 1301. Olefins showed higher requirements, as did most oxygenated fuels. The highest requirement was observed with hydrogen, requiring 15% Halon 1211 for extinction. Some fuels showed a higher requirement for both agents when heated than when the fuel was at room temperature; for others the difference only marginal. This suggests that seme types of fuels exhibit seme temperature sensitivity i n their extinction requirements, while others do not. FMBC Cup Burner (17). Factory Mutual Research Corporation in 1973 attempted to reproduce the ICI cup burner apparatus with a series of measurements on 10 liquid fuels with Halons 1211 and 1301. A diagram of their apparatus i s shown i n Figure 7. An internal heater was placed i n the cup so that the fuels could be heated. The range of volumetric a i r flews examined was similar to that examined by ICI, but due to a larger diameter chimney, the velocity of air past the cup was only about 60% of that obtained in the ICI apparatus at an equivalent volumetric rate. The results are shown i n Table VII.

1.

FORD

Halon 1301 Systems h-85mnH A= 50.6

Glass chimney

53 cm

28 mm


- Streamlined fuel cup

23 cm

• A " glass bead packing

II J

Fuel •

'««

Air & Agent

Figure 6. ICI cup burner (1973,1974)

•-105 mm

40 cm

-Glass chimney

28 mm - Streamlined fuel cup

22 cm

Glass bead packing 8 cm

-

TI

FT & Agent

Fuel

Figure 7. FMRC cup burner (1973)

Aluminum inlet manifold

18


TABLE VII EMRC COP BURNER 1973


Fuel

Peak Agent Concentration in Air at Extinction, % Fuel at Room Temp. (24°C) Fuel at Elevated Temp. Halon Halon Halon Halor 1211 1301 1211 t,°C 1301

Methanol Ethanol Acetone Diethyl Ether Ethylene Glycol Nitocmethane n-Heptane Kerosene Navy Distillate

7.85 4.16 3.62 4.17

7.06 3.66 3.29 3.67

4.74 3.55

4.11 3.29

Xylene

1.96

2.08

75

3.43

150 85

5.45

4.66 7.40

60

3.63

3.48

90 250 100

3.75 2.91 2.29

3.38 2.93

The FMRC roomtemperaturedata shows the same relationships as observed by ICI, but the extinction concentrations seem to be uniformly lower than ICI's by approximately 0.5% (absolute). In this series, ethanol exhibited a lower extinction concentration with Halon 1211 at elevated tanperature than at room temperature. With Navy distillate, increasing the fuel temperature from 90°C to 250°C reduced the extinction concentrations of both Halon 1211 and Halon 1301. For nitrcmethane, a substantially higher concentration of Halon 1301 was required at 85°C than at room temperature. FMBC Vapor Burner (18). Following the series of tests with the liquid cup burner reported above, Factory Mutual Research Corporation modified the apparatus to permit liquid fuels to be flash-vaporized prior to being fed to the burner. Fuels were fed into the flash vaporizer from a syringe driven by an Orion injector. The vaporized fuel then passed up the 38.5 mm burner tube and was ignited at the top. Air containing extinguishing agent was fed into the bottom of the 105 mm i.d. glass chimney surrounding the burner. The apparatus is shown diagranmatically in Figure 8. For each fuel/agent combination, the concentration to achieve extinction was plotted against the ratio of the air plus agent velocity to the fuel vapor velocity. The peak concentration obtained from this plot was then reported for each fuel. This data i s shown in Table VIII below.

1.

FORD

Halon 1301 Systems

19

— 105 mm


Glass chimney

61cm

38.5 mm

3cm 2

TO

ir & Agent

Heated block "

Syringe & injector

Figure 8. FMRC vapor burner (1974)

20

HALOGENATED FERE SUPPRESSANTS

TABLE VIII EMRC VAPOR BURNER 1974


Peak Agent Concentration at Extinction, % tpor Tenp. Approx. 3-4°C Above Boiling Point) Halon "1301 Halon 1211 Pentane Hexane Heptane Kerosene Xylene Methanol Ethanol Acetone Diethyl Ether Nitronethane

4.3 4.4 4.0 2.7 2.5 9.1 4.6 3.9 4.4 12.3

4.6 5.1 5.2 2.6 8.6 5.2 4.8 5.1 10.1

This data generally shows a higher extinction requirement for Halon 1211 than for Halon 1301. Exceptions are methanol and nitromethane. Many of these fuels shew higher extinguishing concentrations i n the vapor burner than were observed in the cup burner. In general, however, the same relationships between fuels was observed in this series as in the previous ones. The extinguishing concentrations for both methanol and for nitromethane are substantially higher than was obtained frem cup burner data at either room temperature or at elevated temperatures. Cardox 21.5 Cubic Foot Cabinet (19). A series of intermediate scale dynamic tests were undertaken at Cardox R&D Laboratories in 1974. Thirty-five liquid and ten gaseous fuels were evaluated with Halon 1301, and ten liquids and two gases with Halon 1211. The liquid fuels were contained in a 5 i n diameter x 3/8 in deep steel pan placed in a 2 f t x 2 f t x 5.4 f t high cabinet. An air flow of 50 ft3/inin was established upwards through the cabinet. The a i r velocity past the fuel pan was 6.5 cm/sec. Gaseous fuels were ignited in a 1-1/4 in diameter Fisher laboratory burner. A schematic diagram of the apparatus is shown in Figure 9. In each test, the fuel was allowed to burn until an equilibrium tenperature was established. Then the concentration of extinguishant in air was slowly increased until extinction was obtained. Data for 18 fuels i s presented in Table IX. In almost every case, a slightly lower extinguishing concentration was required for Halon 1301 than for Halon 1211. In the case of xylene, the two agents displayed equal effectiveness. For alkanes, alcohols, ketones, and ethers, the data shows good agreement with that from the laboratory cup burner with the fuels

1.

FORD

21

Halon 1301 Systems

TABLE IX CARDOX INIERMEDIATE-SCAIE TESTS (1974) 3

21.5 f t Cabinet 50 ft3/min Air Flew Rate


Fuel Methane Ethane Propane n-Butane i-Pentane neo-Pentane n-Heptane Petroleum Naphtha JP-4 JP-5 Diesel Fuel Navy Distillate Kerosene Stoddard Solvent Transformer O i l Cyclopentane Cyclohexane Benzene Toluene Xylene (mixed) Acetylene Ethylene Propylene Methanol Ethanol i-Propanol Dimethyl Ether Diethyl Ether Acetone Methyl Ethyl Ketone Ethyl Acetate Amyl Acetate Vinyl Acetate Methyl Methacrylate Ethylene Oxide Dioxane Acrylic Acid Ethylene Glycol i-Propyl Amine Acetonitrile Acrylonitrile

Agent Concentration at Extinction, % Halon 1301 Halon 1211 1.3 2.8 2.9 2.3 3.0 2.9 3.8 3.4 3.3 3.5 3.8 3.3 3.7 3.5 3.5 3.3 3.5 3.4 2.7 2.9 8.8 4.6 3.9 7.3 3.9 3.3 4.6 3.7 3.2 3.7 3.0 3.8 4.3 5.6 7.9 6.0 4.4 3.9 2.6 2.8 4.6

1.5 3.2

4.0

4.1 3.4 2.9

4.7 4.2 4.2 3.7 4.0

4.5

22


Tetrahydrofuran Carbon Disulfide Hydrogen

4.2 2.8 (>10)* 18


*With wire screen obstruction i n fire pan.

Exhaust

Fire Chamber

Diffuser

y-F\re pan

\

Stand

5.2 cu.ft.

iiniin1111 in it titi ii mi i mil 1111111 it

5.2 cu.ft.

in ti|

•Gas

Sample

AirExtinguishing Agent

Figure 9. Schematic of Cardox apparatus

1.

FORD

Halon 1301 Systems

23


at room temperature. For nitrcmethane, reasonably good agreement is obtained with the laboratory cup burner with the fuel at elevated tatperature. For Halon 1301, the extinction concentrations range from 1.3% for methane up to about 18% for hydrogen. Most of the alkanes require concentrations between 2.3 and 3.8% by volume. Oxygenated fuels and unsaturated hydrocarbon fuels require higher concentrations of Halon 1301 for extinction. Static Flame Extinguishment Tests FMBC Snail-Scale Ttests(20). In 1967, Factory Mutual Research Corporation conducted a series of tests using a 180 ft3 glove box as a test enclosure. Fuels examined were CDA-19 (ooranercial denatured alcohol) and n-heptane using both Halon 1301 and carbon dioxide as extinguishing agents. The enclosure was equipped with recirculating fans as well as an exhaust fan which could maintain approximately 75 ft^/min air flow through the cabinet. Four sizes of fire pans were utilized: 1. 2-1/8 i n diameter x 1-1/8 i n high copper 2. 3-1/8 i n diameter x 1-1/2 in high copper 3. 6 x 6 x 2 i n high steel 4. 12 x 12 x 2 i n high steel Agent concentrations of both Halon 1301 and carbon dioxide were monitored using a Ranarex specific gravity meter. A Bailey oxygen meter recorded oxygen concentrations before and during extinction. In one series of tests, the extinguishing agent vas injected directly to the cabinet with the ventilating fan turned off. In a second series of tests, the ventilating fan remained i n operation and the extinguishing agent was slowly added to the inlet air stream until extinction of the fire was obtained. The results from this series of tests are shown i n Table X. A schematic diagram of the apparatus employed i s shown in Figure 10. TABLE X FMRCTOTALFLOODING TESTS 1967 180 ft3 Glovebox Fire Pan Sizes: 2-1/8 i n diameter 3-1/8 i n diameter 12 i n x 12 in x 2 i n Min. Halon 1301 Concentration for Extinction, Direct Injection Added to Vent. Inlet 1301 002 1301 C02~ CDA-19 n-Heptane

3.6

23

3.6 3.3

23 19



1.

FORD

25

Halon 1301 Systems


With the exception of the 12 in x 12 in fire pan, the extinguishing concentration was found to be dependent upon the agent employed and the fuel. Essentially no difference was observed between the direct injection tests and the agent slowly added to the air inlet. A somewhat lower concentration was observed for both Halon 1301 and carbon dioxide on n-heptane than on CDA-19. FMBC Intermediate and Larcre Scale Tests (21). To demonstrate the validity of the determinations made i n the glovebox tests above, Factory Mutual Research Corporation undertook in 1968 a series of intermediate- and large-scale tests with Halon 1301 on t**D fuels, CDA-19 and n-heptane. The tests were conducted i n three stages. First, using a 1,000 f t ^ enclosure (10 x 10 x 10 ft) and twD snail fire pans, 2-1/8 i n diameter and 3-1/8 in diameter copper pan, the fuel was ignited and the agent slowly bled into the enclosure over a 6 to 18 roin time period. At extinction, the agent flew was shut off and the agent concentration i n the enclosure was determined by calculation from the net weight of agent discharged. In a second series of tests i n an 893 ft3 enclosure using the same two fire pans plus a 12 x 12 i n steel pan, the extinguishing agent was discharged rapidly into the enclosure within a 1-min period. The minimum weight of agent to extinguish the fire was determined and the agent concentration calculated from this weight. Finally, two tests were conducted i n a 9,282 cu f t enclosure using a 38 i n square fire pan (10 ft2 surface), again injecting the agent into the enclosure rapidly. In the large tests, only n-heptane was used as fuel. Results from this series i s shown in Table XI below. TABLE XI FFMC INTERMEDIATE AND LARGE SCALE FLAME EXTINGUISHMENT TESTS 1968 Minimum Halon 1301 Concentration in Enclosure at Extinction, % Enclosure Size, ft3 Fire Size

Agent Discharge Time: CDA-19 n-heptane

1000 2-1/8 in dia 3-1/8 in dia 6-18 2.35 2.50

roin

893 9242 2-1/8 in dia 38 i n x 38 i n 3-1/8 i n dia 12 i n x 12 i n 11.8

2.0*

30

Extinguished

10

Extinguished

*Threshold extinguishing concentration

American Chemical Society Library 1155 16th St. N. W. Washington, D. C. 20036

48


TABLE XXVII SAFETY FIRST PRODUCTS CORPORATION Class A Fire Tests

Test No.


1 2 3 4 5

6

7

8 9

10

11

Fuel Wbod crib 24 pes. 2x2x18, 4 rows Wbod crib, UL Std. 1A

Halon Preburn 1301 Soaking Enclosure Time Cone. Time ft3 min:sec % Min

Results

336

9:00

3.8

5

336

10:00

3.8

5

3 lbs excel336 sior on floor 3 lbs shredded 336 paper on floor 5 lbs shredded 336 paper i n 2 f t dia wire basket 5 lbs shredded 336 paper i n 2 f t dia wire basket 3 sheets 336 fiberglass sheet 1/16 i n x 16 i n x 16 in 7 lbs PVC 336 insulated wire in loose mass 2 lbs PVC 2000 wire i n 27 ft3 subenclosure 1/8 i n thick 336 polyurethane sheets piled on floor 7 f t x 3 f t x 1.5 f t high 4 cardboard 336 boxes of punch cards 40 lbs total

0:50

3.8

2:30

0:15

4

0:30

Extgsh'd, cold to touch Extgsh'd, cold to touch Extgsh'd, no reflash Extgsh'd

0:15

3.8

5:45

Extgsh'd

0:35

3.8

5:00

Reignited when door opened

1:30

3.8

0:20

Extgsh'd

1:40

3.5

0:30

Extgsh'd

0:20

3.8

5:00

Extgsh'd

0:38

3.8

0:30

Extgsh'd

20:00

6.5

10:00

Slight wisps of smoke when rocm opened

1.

FORD

49

12

Glazed fox fur with lining

336

0:30

3.8

5:00

13

Same as No. 12 21 lbs. corrugated cardboard 4 pes. (12 lbs) wood flake board 15 pes (63 oz) wood fiberboard 3 lbs cotton lint 1 pail each crumpled paper and carbon paper

336

0:20

7.6

5:00

Fur out, Lining still smoldering slightly Extgsh'd

336

2:40

7.6

30:00

Reignited

336

4:00

20

10:00

Reignited

336

2:00

20

10:00

Reignited

336

0:10

3.8

1:05

Extgsh'd

336

0:15

3.8

5:00

Extgsh'd

14 15 Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch001

Halon 1301 Systems

16 17 18


50


The test enclosure i s the same as reported previously i n the FMRC flammable liquid fires. Of most interest were tests with charcoal and wood blocks. In the charcoal fire tests, 21 oz of oven dried hardwood charcoal in 1 i n to 1-1/2 in cubes were arranged in a wire basket measuring 6-1/2 in dia by 4-1/2 in deep. The charcoal was ignited by exposure to a gas-fired radiant heater for 4 min. The basket was placed on a scale i n the glovebox and allowed to burn an additional 11 min. A predetermined quantity of Halon 1301 was discharged into the glovebox and allowed to remain until the basket temperatures and weight loss readings indicated the fire had been extinguished. The glovebox was then opened to the attnosphere and ventilation restored to determine whether reignition would occur. Approximately 15 determinations were made, with Halon concentrations ranging from 1.1% to 25%. Results of these tests are shown i n Figure 15. Halon 1301 concentrations as low as 1-1/2 to 2% were effective in extinguishing these fires completely, provided minimum soaking times of 30-40 minutes were maintained. To keep the soaking time below 10 min, a Halon 1301 concentration of about 13% was required. The wood block fires consisted of two oven-dried 2 x 4 f i r blocks 1 f t long. The two blocks were mounted vertically with the 4-in faces parallel and separated by approximately 3/8 in. The blocks were ignited with a match and allowed to preburn 15 min before injecting agent. Halon 1301 concentrations were varied from 10.2 to 13.2%. Soaking times between 3 and 60 min were required. The curve represented by these four tests i s also shewn i n Figure 15. Approximately 12% Halon 1301 i s required for this fuel to keep the soaking time below 10 min. 1

Underwriters' Laboratories(34). Underwriters Laboratories, Inc., compiled a set of fire test data accumulated from test programs for equipment listings. Their ccmpilation included four types of Class A fires: standard 1-A wood cribs, shredded paper, excelsior, and roofing brands. Results from these tests are shown i n Table XXVI. A l l fires except the shredded paper fires were extinguished with Halon 1301 concentrations of 6% or less maintained for 10-min soaking periods. With the shredded paper fires, Halon 1301 concentrations below 18% gave erratic results. With concentrations above 18%, extinguishment was achieved within the 10-min soaking time. In the shredded paper fires, either 5 or 10 lb of shredded paper was contained in an open wire basket elevated approximately 1 f t above floor level and ignited at the bottom center. Fenwal, Inc., Polystyrene and Polyethylene Tests(35). As a part of an industry effort to examine fires which might typically occur i n computer rooms, Fenwal, Inc., determined the minimum extinguishing concentration of Halon 1301 for polystyrene computer tape reels and tape cases and for polyethylene sealing


1.

FORD

Halon 1301 Systems

70 -

-Parallel wood blocks

Charcoal : 21 oz. total I to I/*" 1

cubes in 6^2"dia. x 41/2 deep

wire basket - 15 min. preburn time Wood block : 2 fir 2 x 4 x 1 2 " long blocks mounted vertically 8 parallel with 4" faces separated by 3/8* space

-Charcoal

_J_ 10

_L_ 20

30

40

50

60

Halon 1301 Concentration, % by vol.

Figure 15. Soaking time vs. Halon 1301 concentration for charcoal and wood block test fires. FMRC (1967). 180-ft enclosure. 3


52

bands. The test enclosure measured 12 x 16 x 8 f t high and contained a volume of 1,536 f t . The test fire consisted of six empty polystyrene reels, four empty polystyrene tape cases, and six empty polystyrene reels sealed with polyethylene bands placed in each of the two lower shelves of a six shelf storage rack. The rack of cxanponents was ignited frcm 40 ml of n-heptane in a steel pan measuring about 8 x 16 x 1 in deep placed under the bottom row of ocmponents. Ten tests were performed i n which Halon 1301 ocncentrations ranging frcm 2 to 6% by volume were applied using Fenwal commercial Halon 1301 ocmponents. In a l l tests in which the Halon 1301 concentration was 3% or greater, the fires were completely extinguished within a 10-min soaking time. Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch001

3

Ansul Cellulosic Tests (35). As a part of the same industry computer fire test programlnentioned above, Ansul Company examined the extinguishment characteristics of cellulosic materials ccmmonly used in computer rooms. Fuels used were a 5 lb stack of computer print-out paper, and 5 lbs (total) of paper punch tape, computer print-out paper, and data tabulating cards arranged randomly in an open wire basket. In tests containing the stacked print-out paper, one test was run in which the fire was ignited by placing an electrical heater wound around a single data. tabulating card i n the center of the stack. The ignitor was energized and the stack allowed to burn until a 0.5 lb weight loss had been observed. At that point, a Halon 1301 concentration of 5.1% was established in the 14.75 x 14.75 x 8 f t high (1,729 f t ) enclosure. Following a 10-min soaking time, the fire was observed to be completely extinguished. In four tests performed on random arrangements of paper tape, computer print-out paper, and data tabulating cards, concentrations of 5.1% were found to extinguish flaming combustion but permitted the smoldering fire to continue throughout a 10-min soaking time. In two tests with concentrations of 11.8 and 21.0% Halon 1301, the fire was completely extinguished at the end of a 30-min soaking period. 3

Du Rant FVC Tests (35). As a final portion of the industry computer fire test program, Du Bont determined the threshold extinguishing concentration for Halon 1301 on 5 lbs of polyvinyl chloride tubing contained in an open wire basket. The tests were run i n an enclosure measuring 12.5 x 15.5 x 8.75 f t high (1,695 ft3). The fire was ignited from 25 ml of methanol in a 5 x 8 x 1 in deep aluminum pan placed directly on the floor beneath the fuel basket. Approximate burning time of the methanol was 1 min. Five tests were performed i n which Halon 1301 concentrations were varied from 2.6% to 5% by volume. It was found that a 2.6% Halon 1301 concentration was sufficient to extinguish the polyvinyl chloride fire ocnpletely with a 1-min soaking period.

1.

FORD

Halon 1301 Systems

53

Safety First Class A Tests (36). Cholin in 1972 reported results from 24 tests conducted with Halon 1301 in 336 f t and 2,000 f t test buildings. Eighteen of these tests were conducted to determine the effects of Halon 1301 on various types of Class A fuels. The Halon 1301 was discharged into the test enclosures using ccitmercial Safety First systems components. The results of this test series are shown in Table XXVII. Of the fuels examined, the following materials were completely extinguished with concentrations of 3.5 to 4% Halon 1301: 1. Small wood crib 2. UL size 1-A wood crib 3. Three pounds excelsior on floor 4. Three pounds shredded paper on floor 5. Three sheets fiberglass 6. PVC cable insulation 7. Polyurethane sheets 8. Cotton l i n t 9. Crumpled paper and carbon paper i n solid wall pails The following materials were not extinguished with low Halon 1301 concentrations for short soaking times: 1. Shredded paper in wire basket 2. Corrugated cardboard 3. Wood flake board 4. Wbod fiberboard The cardboard boxes of punches cards and the glazed fox fur with lining showed marginal results. Although neither fuel configuration was completely extinguished within the test conditions, the burning rate had been reduced to an extremely low level. 3


3

3

Williamson 21.5 f t Cabinet Tests (37). Using the Cardox test cabinet described above, Williamson i n 1972 determined the Halon 1301 extinguishing requirements for eight fuels under flow conditions. The fuel arrangement consisted of five 1 f t square panels mounted parallel and vertically with a 3/8-in space between each panel. The array was ignited frcm a small pan containing 40 ml of alcohol placed under the array which provided a burning time of about 1 min. Halon 1301 was added to the ventilating air inlet stream to establish the desired concentration within the cabinet. After a 10-min exposure period to the agent, the door to the cabinet was opened and the sample observed for continuing combustion. Results frcm the tests are given in Table XXVI. Corrugated cardboard, low^-density ceiling t i l e , plywood, and masonite required concentrations between 20 and 30% for extinguishment. Four plastic materials investigated (fiberglass polyester resin, polyether foam, cellulose acetate, and polyvinyl chloride trim) required only 3.3 to 3.5% for extinguishment. These materials would not be considered to produce deepseated fires.

54



Conclusion From the foregoing analyses of individual test series, several "facts" may be extracted which hopefully will provide clues for a mechanism: 1. Halogenated agents containing bromine or iodine in the molecule are more effective extinguishants than those cxmtaining only chlorine or fluorine. Chlorine imparts a greater degree of effectiveness than fluorine. Gonpounds containing only fluorine do not show significantly greater effectiveness than inert gases. Fluorine atoms tend to impart stability to the compound. 2. One atom of an active halogen (CI, Br, or I) in a compound gives a narked increase in extinguishing effectiveness over a similar compound which contains none of that halogen. Adding a second atom of the same halogen produces only marginal additional increase in effectiveness. 3. Required concentrations of a given halogenated agent to extinguish established fires of liquid or gaseous fuels depend primarily upon the fuel. Oxygenated or unsaturated hydrocarbon fuels (such as methanol, dimethyl ether, ethylene oxide, ethylene, and acetylene) require higher agent concentrations for extinguishment than do aliphatic hydrocarbon fuels. Hydrogen requires very high concentration of halogenated agents for extir Tuishment. 4. Halogenated agents added to the fuel side of a diffusion flame require higher concentrations for extinguishment than when added to the air side of the flame. In terms of the rate of agent injection for the total system, addition to the fuel side i s probably more efficient than addition to the air side. 5. Halon 1301 concentrations to extinguish carbon monoxide diffusion flames are significantly lower than for most other fuels. With Halon 1001, carbon monoxide requires a higher concentration for extinguishment than do other fuels. 6. The extent to which minimum concentrations of halogenated agents to extinguish diffusion flames depend upon fuel temperature varies with type of fuel. Some fuels, such as alkanes, show l i t t l e effect withtemperature,while others, notably oxygenated fuels, show large effects.


1.

FORD

Halon 1301 Systems

7. Dynamic tests for determining flame extinguishment requirements of halogenated agents for a given fuel give somewhat higher threshold concentrations than do static tests. This i s probably due to oxygen depletion or recirculation of combustion and decomposition products into the flame, which accompany static determination. For this reason, a dynamic procedure i s preferred because of its conservative result. 8. Concentrations of halogenated agents to inert a given fuel/air mixture are significantly greater than to extinguish diffusion flames of that fuel. 9. Inerting requirements for halogenated agents vary with fuel type and temperature and conditions (ignition energy, container geometry, and amount of flame propagation permitted) of the system. These variables appear to operate as follows: a. The effect of fuel type i s similar to that for extinguishment of diffusion flames. Paraffins require lower concentrations than olefins or oxygenated fuels. Hydrogen requires about the highest concentrations of any fuel. b. Increasing temperature produces increased flanmability peak concentrations of agent. There i s a suggestion that the extent of the dependence varies with fuel type. c. Increasing ignition energy increases the inerting requirement for a given agent/fuel system. Hie extent of this effect depends upon fuel type. Oxygenated fuels, such as dimethyl ether, exhibit greater effects than paraffinic hydrocarbons. d. Increasing the anount of flame propagation permitted i n the system permits lower agent concentrations for inerting. This effect i s greater when a high (>100 J) ignition energy is used. e. At high ignition energies, snaller scale equipment produces higher inerting requirements. At lower ignition energies, l i t t l e dependence on scale i s observed. (This assumes that the scale i s not so small to

55


56

produce wall-quenching effects.) Successful extinguishment of Class A (solid oaribustihle) fuels with halogenated agents in total flooding systems depends upon the fuel and i t s configuration, preburn time, agent concentration, and soaking time. Thermoplastics, paper products i n stacks or in solid wall waste containers, and wood arrangements which do not contain closelyspaced vertical surfaces, are completely extinguished with Halon 1301 concentrations of 5 to 6% and soaking times of 10 minutes or less. Wbod or wood products oontaiiiing closely-spaced vertical surfaces, corrugated cardboard, shredded paper in open wire baskets, and some types of thermosetting plastics, tend to develop deep-seated fires which require much higher agent concentrations and long soaking times for complete extinguishment. Application of low concentrations of Halon 1301 will "control" deep-seated fires during the soaking period. The questions which these observations pose can serve as a check l i s t to test proposed mechanisms. These relationships must be explained i f a mechanism i s to have practical significance to the fire protection engineer. While a valid mechanism will aid research for improved extinguishants, i t must also act as a guide for application of existing agents to the variety of fuels, hazards, and conditions found i n industry. The level of understanding necessary to formulate a workable mechanism cannot be programmed as an experiment, but must result from i n s i s t and inspiration. With this conference, we are taking the f i r s t step towards this understanding.


10.

Literature Cited 1.

Wharry, David and Ronald Hirst, Fire Technology: Chemistry and Combustion, Institution of Fire Engineers, Leicester, England (1974). 2. The Halogenated Extinguishing Agents, NFPA Q-48-8, National Fire Protection Association, Boston, Massahcusetts (1954). 3. Strasiak, Raymond R., The Development History of Bromo-chloromethane (CB), WADC Technical Report 53-279, Wright Air Development Center, Ohio, January 1954. 4. Standard on Halogenated Fire Extinguishing Agent Systems -Halon 1301. NFPA No. 12A - 1973. National Fire Protection Association, Boston, Massachusetts (1973). 5. Standard on Halogenated Fire Extinguishing Agent Systems -Halon 1211. NFPA No. 12B - 1973. National Fire Protection Association, Boston, Massachusetts (1973).

1.

FORD

6. 7. 8.


9.

10. 11.

12.

13. 14.

15. 16.

17.

18.

19. 20.

Halon 1301 Systems

The Ansul Company, Research Department Project No. 147, Technical Report No. A-11, January 30, 1956. Guise, A. B., "Extinguishants and Extinguishers", presented at 61st Annual Meeting, National Fire Protection Association, Los Angeles, May 1957. Gorski, R. A., "Experimental 4 and 6B Extinguishers Containing 'Frean' FE 1301 Fire Extinguishing Agent", Technical Report KSS-2720A, E. I. du Pont de Nemours & Co., "Frean" Products Laboratory, November 9, 1960. Gorski, R. A., "Experimental 6B Fire Extinguisher Containing 'Freon' FE 1301 Fire Extinguishing Agent", Technical Report KSS-3466A, E. I. du Pont de Nemours & Co., "Freon" Products Laboratory, November 17, 1961. Stewart, R. L., "Design of F.P.L. 2:B Fire Extinguishing Head", Technical Report KSS-6269, E. I. du Pont de Nemours & Co., "Freon" Products Laboratory, May 27, 1968. Mader, G. N., "Design Parameters for an FE-1301 Portable Fire Extinguisher with a 1-B Rating", Technical Report KSS-7289, E. I. du Pont de Nemours & Co., "Freon" Products Laboratory, October 25, 1971. Ovens, R. J., "Modification of a 50 lb. Carbon Dioxide Fire Extinguisher for Use with FE 1301 Fire Extinguishant", Technical Report KSS-7343, E. I. du Pont de Nemours & Co., "Freon" Products Laboratory, February 2, 1972. Ford, Charles L., Unpublished Data, E. I. du Pont de Nemours & Co., Germay Park, Wilmington, Delaware, June 27-28, 1973. Creitz, E. C., "Inhibition of Diffusion Flames by Methyl Bromide and Trifluoromethyl Bromide Applied the Fuel and Oxygen Sides of the Reaction Zone", J. Res. of Nat'l Bur. of Stds., 65, No. 4, 1961. Fletcher, N., "Halon 1211 Systems: Determination of Extinction Concentrations in the Laboratory", PN-67-39-2, ICI Mond Division, Winnington Laboratory, February 4, 1970. Booth, K., B. J. Melia and R. Hirst, "Critical Concentration Measurements for Flame Extinguishment of Diffusion Flames Using a Laboratory 'Cup Burner' Apparatus", ICI Mond Division, Winnington Laboratory, August 31, 1973. Bajpai, S. N., "An Investigation of the Extinction of Diffusion Flames by Halons", Ser. No. 22391.2, Factory Mutual Research Corporation, Norwood, Massachusetts, November 1973. Bajpai, S. N., "Extinction of Vapor Fed Diffusion Flames by Halons 1301 and 1211 - Part I", Ser. No. 22430, Factory Mutual Research Corporation, Norwood, Massachusetts, November 1974. Ford, Charles L., "Intermediate-Scale Flame Extinguishment Tests with Halon 1301 and Halon 1211", E. I. du Pont de Nemours & Co., Wilmington, Delaware, May 8, 1974. Fitzgerald, P. M. and M. J. Miller, "Evaluation of the Fire Extinguishing Characteristics of 'Freon' FE 1301 on

58

21.

22.


23. 24.

25.

26.

27. 28.

29. 30. 31. 32.

33.

34.


Flammable Liquid Fires", Ser. No. 16234.1, Factory Mutual Research Corporation, Norwood, Massachusetts, February 21, 1967. Miller, M. J., "Determination of Design Criteria and Performance Testing of Prototype Fire Extinguishing Systems Using 'Freon' FE 1301", Ser. No. 16234.1, Factory Mutual Research Corporation, Norwood, Massachusetts, October 1968. Fletcher, N., "Halon 1211 Systems: Threshold Concentration for Extinguishment of Various Fuels", PN-67-39-2, ICI Mond Division, Winnington Laboratory, February 3, 1970. Wickham, Robert T., "Final Report on the Halon 1301 Threshold Fire Extinguishing Program", Wickham Associates, Marinette, Wisconsin, September 10, 1972. "Final Report on Fire Extinguishing Agents for the Period September 1, 1947, to June 30, 1950", Contract No. W44-099eng-507, Purdue Research Foundation, Lafayette, Indiana, July 1950. Floria, Joseph A., "Du Pont FE 1301 Fire Extinguishant Fuel Inerting Concentrations by Mason Jar Technique", Technical Report KSS-7149, E. I. du Pont de Nemours & Co., "Freon" Products Laboratory, March 17, 1971. Owens, R. J., "Determination of Inerting Concentrations of FE 1301 Fire Extinguishant i n Intermediate Scale Equipment", Technical Report KSS-7310, E. I. du Pont de Nemours & Co., "Freon" Products Laboratory, November 10, 1971. Floria, Joseph A., "Du Pont FE 1301 Fire Extinguishant Fuel Inerting Concentrations - Summary Report", E. I. du Pont de Nemours & Co., "Freon" Products Laboratory, March 20, 1972. Lewis, D. J., "Provisional Draft: Comparison of Fuel-Air-BCF Flammability Limits in 2" and 4" Diameter Vertical Tube Apparatus - Part II", ICI Mond Division, Winnington Laboratory, September 13, 1973. Bajpai, S. N. and J . P. Wagner, "Inerting Characteristics of Halogenated Hydrocarbons (Halons)", I & EC Product Research & Development, 14, No. 1, March 1975. Bajpai, S. N., "Inerting Characteristics of Halons 1301 and 1211", Ser. No. 22391.3, Factory Mutual Research Corporation, Norwood, Massachusetts, August 1974. Ford, Charles L., Unpublished Data, E. I. du Bant de Nemours & Co., Germay Park, Wilmington, Delaware, June 1973 - June 1974. Miller, M. J. and R. L. Pote, "Fire Tests of Two Remote Area Fire Suppression System Concepts", Ser. No. 15974.1, Factory Mutual Research Corporation, Norwood, Massachusetts, November 29, 1965. Miller, M. J., "Extinguishment of Charcoal, Wood, and Paper Fires by Total Flooding with 'Freon' FE 1301/Air and Carbon Dioxide/Air Mixtures", Ser. No. 16234.1, Factory Mutual Research Corporation, Norwood, Massachusetts, July 28, 1967. Private Communication, W. A. Gawin (Underwriters' Labora--

1.

FORD

Halon 1301 Systems

59


tories, Inc.) to C. L. Ford (Du Font), August 17, 1971. 35. Ford, Charles L., "Halon 1301 Computer Fire Test Program Interim Report", E. I. du Pont de Nemours & Co., Germay Park, Wilmington, Delaware, January 10, 1972. 36. Cholin, Roger R., "How Deep i s Deep? (Use of Halon 1301 on Deep-Seated Fires)", Fire Journal 66, No. 2, January 1972. 37. Williamson, H. V., "Halon 1301 - Minimum Concentrations for Extinguishing Deep-Seated Fires", Fire Technology, 8, No. 4, November 1972.

60



DISCUSSION W. D. WEATHERFORD, JR: I t i s of i n t e r e s t r e l a t i v e to mechanism clues to point out that the noneffectiveness of normally-liquid halon 2402 on cotton waste f i r e s (observed by the Army Corps of Engineers) agrees with our observations with normally-liquid halon 1011 i n d i e s e l f u e l . I t has been shown by the Army B a l l i s t i c Research Laboratory that 5 l i q u i d volume percent halon 1011 prevents burning of the bulk l i q u i d d i e s e l f u e l , but data obtained at Southwest Research I n s t i t u t e demonstrates that t h i s concentration of i n h i b i t o r does not retard the development of mist f i r e or wick-supported combustion. In f a c t , halon concentrations i n excess of 25 l i q u i d volume percent do not prevent wick-supported combustion. An apparentl y obvious explanation, based on enhanced surface area f o r evaporation (as i n a kerosene latern) and consequent s e l e c t i o n vaporization of the i n h i b i t o r does not completely explain these observations. On the other hand, d i l u t i o n of the normally nonflammable mixture with a i r i n a d i f f u s i o n flame (after t o t a l vaporization) could cause the mixture to pass through the flammable zone (as could be i l l u s t r a t e d on a diagram of i n h i b i t o r vapor concentration vs f u e l concentration). Coupling of both phenomena could possibly explain observations. J . DEHN: (In reply to comment of D. Weatherford that wicking f i r e s with binary l i q u i d mixtures of f u e l and halons i n a i r p e r s i s t at halon concentrations s u f f i c i e n t f o r e x t i n c t i o n i n the absence of a wick.) A wick evaporates l i q u i d s i n a binary mixture e s s e n t i a l l y i n proportion to t h e i r concentrations i n the l i q u i d phase, independent of t h e i r v o l a t i l i t i e s . In the case of surface evaporation without a wick, the more v o l a t i l e component w i l l be present i n the vapor phase at a higher concentration than i t i s present i n the l i q u i d phase, thus, by choosing a l i q u i d halon which i s more v o l a t i l e than a l i q u i d f u e l a smaller l i q u i d volume percentage i s required than might be supposed from a glance at vapor phase peak i n e r t percentages as determined i n explosion burettes, provided no wick i s present. (cf. a paper to be published i n Combustion and Flame by J . Dehn on the

1.

FORD

Halon 1301 Systems

s u b j e c t o f Flammable L i m i t s o v e r L i q u i d

61 Surfaces.)


E. T. McHALE: How do the l a r g e - s c a l e "Fenwal s i l o " r e s u l t s f o r peak p e r c e n t a g e compare w i t h s m a l l e r s c a l e d a t a from the b u r e t t e method? C. L. FORD: The Halon 1301 v a l u e f o r propane i n the Fenwal s i l o t e s t s was 6.5%. T h i s compares t o about 5.25% (vented bottom) and 8.0% (vented top) from the F a c t o r y Mutual l a b o r a t o r y s t u d y . For ethylene, the Fenwal v a l u e o f 9% compares t o F a c t o r y Mutual v a l u e s of 8.3% and 10.5%, r e s p e c t i v e l y . The l a b o r a t o r y r e s u l t s w i t h t h e v e n t e d - t o p a p p a r a t u s g i v e somewhat h i g h e r i n e r t i n g c o n c e n t r a t i o n s t h a t were o b s e r v e d a t large scale. R. G. GANN: In the e a r l y A n s u l and Army t e s t s which you r e p o r t e d , h a l o n 2402 appears some 30% l e s s e f f e c t i v e than h a l o n 1301 a t pan f i r e s u p p r e s s i o n . T h i s i s c o n t r a r y t o flame speed r e d u c t i o n measurements, such as t h o s e o f M i l l e r , Evans and S k i n n e r , (Comb, and Flame, (1963) , 1_, 137) which showed t h a t h a l o n 2402 i s f a r more e f f e c t i v e . As w i t h the poor showing o f h a l o n 1211 i n t h e s e t e s t s , can the r e s u l t s be a t t r i b u t e d t o the experiment d e s i g n and agent d e l i v e r y technique? C. L. FORD: Yes, I t h i n k so. Optimization of d i s charge n o z z l e d e s i g n i s v e r y i m p o r t a n t i n the d e s i g n of a p o r t a b l e e x t i n g u i s h e r . In the e a r l y t e s t s , t h e agents were e v a l u a t e d i n p o r t a b l e e x t i n g u i s h e r s which were d e s i g n e d f o r another type o f agent, namely CO-. I t h i n k t h a t i f the n o z z l e s had been o p t i m i z e d , Halons 1211 and 2402 would have showed s u p e r i o r performance then, as they a r e d o i n g now. T e s t s by Montedison i n I t a l y show Halon 2402 t o be v e r y effective i n portable extinguishers. R. W. BILGER: How does the f i r e go out? Does i t blow o f f o r g r a d u a l l y d e c r e a s e the i n t e n s i t y o f f i r e ? P l e a s e comment on b o t h r e a l f i r e s and the dynamic laboratory tests. C. L. FORD: The e f f e c t s a r e somewhat d i f f e r e n t i n a t o t a l f l o o d i n g room a p p l i c a t i o n than i n a l a b o r a t o r y cup b u r n e r . In the room s i t u a t i o n , f o r l i q u i d f u e l s , s u b - e x t i n g u i s h i n g c o n c e n t r a t i o n s have l i t t l e e f f e c t on the apparent b u r n i n g r a t e o f the f i r e . The flame h e i g h t i s about the same as w i t h no i n h i b i t o r p r e s e n t . Then, as s l i g h t l y more i n h i b i t o r i s added, the f i r e

62



s u d d e n l y goes o u t . A p e r s o n s e e i n g t h i s f o r t h e f i r s t time i s u s u a l l y a s t o n i s h e d . In t h e l a b o r a t o r y cup b u r n e r , as t h e e x t i n g u i s i n g c o n c e n t r a t i o n i s approached, t h e flame l i f t s o f f t h e cup and may burn q u i t e s t a b l y s e v e r a l c e n t i m e t e r s above the f u e l s u r f a c e . I n c r e a s i n g t h e agent concent r a t i o n by o n l y a few t e n t h s o f a p e r c e n t w i l l d r i v e t h e flame f u r t h e r up t h e tube, u n t i l i s e v e n t u a l l y goes o u t . C. HUGGETT: The d a t a shown i n d i c a t e t h a t Halon 1301 i s much more e f f e c t i v e than m e t h y l bromide i n ext i n g u i s h i n g a c a r b o n monoxide f l a m e . I t i s well known t h a t t h e combustion o f carbon monoxide i s a c c e l e r a t e d by t r a c e s o f hydrogen, presumably t h r o u g h p a r t i c i p a t i o n of hydrogen-containing r a d i c a l s i n a c h a i n r e a c t i o n p r o c e s s . The commonly a c c e p t e d mechanism f o r t h e a c t i o n o f t h e h a l o n e x t i n g u i s h i n g agent i n v o l v e s r e a c t i o n w i t h t h e s e same r a d i c a l s . The c o n c e n t r a t i o n o f hydrogen w i l l be v e r y low i n t h e c a r b o n monoxide flame, a r i s i n g from water vapor and other trace impurities. Only a s m a l l amount o f e x t i n g u i s h i n g agent i s needed t o i n t e r a c t w i t h t h e small concentration of hydrogen-containing r a d i c a l s present. The methyl bromide w i l l i n t r o d u c e l a r g e q u a n t i t i e s o f hydrogen i n t o t h e flame, i n c r e a s i n g the c o n c e n t r a t i o n o f hydrogen-containing r a d i c a l s and g r e a t l y i n c r e a s i n g t h e i n h i b i t o r r e q u i r e m e n t . T h i s s u g g e s t s t h a t the s t u d y o f c a r b o n monoxide flames c o n t a i n i n g s m a l l , c o n t r o l l e d amounts o f hydrogen might p r o v i d e a v e r y u s e f u l t o o l f o r t h e s t u d y o f t h e mechanisms o f h a l o n e x t i n g u i s h m e n t processes. A. S. GORDON: There i s a s u r p r i s i n g l y l a r g e e f f e c t o f h a l o n 1301 on t h e e x t i n g u i s h m e n t o f c h a r c o a l . Has the e f f e c t o f N been compared under t h e same experimental conditions? 2

C. L. FORD: N i t r o g e n was n o t examined, b u t c a r b o n d i o x i d e was. To g i v e comparable e f f e c t s , t h e c o n c e n t r a t i o n s o f C 0 were about 6 t o 7 times h i g h e r t h a n Halon 1301. F o r a 10-minute s o a k i n g time, about 65% by volume o f C 0 would be r e q u i r e d . 2

2

R. G. GANN: L e t me p r o v i d e some d e t a i l s on r e c e n t work performed a t NRL. F i e l d i n g , Woods and Johnson ( J . F i r e and Flam. (1975) , 6>, 37) f l o w e d premixed 10% h a l o n 1301 i n a i r t h r o u g h a h e a t e d c h a r c o a l bed. The s m o l d e r i n g combustion was n o t s u p p r e s s e d and t h e

1.

FORD

Halon 1301 Systems

63

CO/CO2 r a t i o i n the e f f l u e n t gas was much higher than that observed when normal a i r was flowed through the bed. They i n t e r p r e t e d t h e i r r e s u l t as i n d i c a t i n g l i t t l e e f f e c t of CF~Br on the surface generation of CO and s i g n i f i c a n t e f f e c t on the gas phase oxidation of CO to C0 -


2

C. L. FORD: The Factory Mutual work was performed i n a quiescent atmosphere, with no forced a i r flow through the bed. Some work which Cardox has performed i n a flow cabinet suggests that higher concentrations are required when a flow i s present. From t e s t s with other f u e l s , concentrations of Halon 1301 perhaps as high as 30% might be required f o r extinguishment under flow conditions.

2 The Relevance of Fundamental Studies of Flame Inhibition to the Development of Standards for the Halogenated Extinguishing Agent Systems MYRON J. MILLER


Factory Mutual Research Corp., Norwood, Mass. 02062

The principal responsibility of National Fire Protection Association (NFPA) committees i s to provide unambiguous guidelines for the design and evaluation of safe, effective systems. Some of the factors that must be considered i n the design of Halon Systems are shown as a Fault Tree i n Figure 1. The behavior of the agent i n preventing or suppressing f i r e and explosion and the effect of the agent on personnel who might be exposed have received a large amount of attention over the past twenty years. About two years ago inconsistencies were noted i n extinguishing and inerting data which had been accumulated since the NFPA Halogenated Fire Extinguishing Agent Systems Committee was established i n 1966. Of particular concern was the lack of a systemati c relationship between f i r e extinguishment and inerting data and increasing discrepancy between the results obtained by different investigators and by different experimental methods. It would be satisfying if the data available could be tested against a theoretical ideal. However, f i r e (being the varied event that it i s ) has so far frustrated efforts to develop an all-encompas vances i n the past ten years toward an understanding of f i r e , and it i s my purpose here to present some of the data developed by members of the NFPA Halon Committee i n a way that w i l l highlight our conclusions to date and perhaps stimulate further investigation responsive to the questions that remain. Perhaps the best way to convey the information is to organize it into segments based upon our principal concerns regarding the Precision, Accuracy, Predictability and Applicability of the data. Since it i s my contention that flame extinguishment, inerting and explosion suppression concentration requirements should all be related i n some l o g i c a l way, I w i l l avoid breaking the discussion into these categories. We have also made an attempt to publish all experimental work conducted by the Factory Mutual Research Corporation. These and other sources are referenced, and I w i l l avoid discussing the details of these investigations. (1-39) 64

PROBABILITY ESTIMATE WRONG

FIRE BEHAVIOR NOT PREDICTED

EXCESSIVE LEAKAGE

CALCULATION WRONG

ASSUMPTIONS WRONG

LAB DATA WRONG

FLOW CALCULATION WRONG

INSUFFICENT DESIGN QUANTITY

PROPERTIES NOT PREDICTED

GEOMETRY NOT PREDICTED

ACTUATOR STRESS NOT PREDICTED

INSUFFICIENT DEGIGN DELIVERY RATE

MATERIALS NOT PREDICTED

POOR COMMUNICATIONS

INADEQUATE INSTRUCTIONS

IMPROPER TEST DESIGN

IMPROPER DESIGN RESPONSE

EFFECT BY AGENT NOT PREDICTED

EFFECT ON SYSTEM NOT PREDICTED

HUMAN FAILURE

CONDUIT FAILURE

STORAGE CONTAINER FAILURE

ACTUATOR FAILURE

DETECTOR FAILURE

ELECTRICAL FAILURE -|

IMPROPER TEST OR STRESS

DAMAGE DURING TRANSPORT

IMPROPER INSTALLATION

SABOTAGE

MAINTENANCE AT TIME OF FIRE

POOR COMMUNICATIONS

INADEQUATE SECURITY

INADEQUATE SUPERVISION

SYSTEM DISARMED

FUNCTIONAL FAILURE

Figure 1. System effectiveness considerations

IMPROPER LOCATION

IMPROPER CHOICE OF MATERIALS

NO PREVENTIVE MAINTENANCE PROGRAM

DESIGN RELIABILITY WRONG

DETECTOR TYPE/LOCATION WRONG

AGENT NOT SUITED TO FIRE

LOCATION NOT PREDICTED

HARDWARE FAILURE

AGENT FAILURE

DESIGN BASIS FIRE - NOT APPROPRIATE

DESIGN FAILURE

SYSTEM FAILURE


__

INCAPACITATED

INADEQUATE TRAINING

66


(§)


(jj)

g)

@

LEGEND Fuel reservoir ft burner level control device Teflon joint 3mm id. capillary Teflon stop cock Glass-metal bellows joint Epoxy seal Swageiok fitting Teflon coated aluminum burner support block 105mm i.d. pyrex glass chimney Teflon coated aluminum sleeve Neoprene 0-rings Teflon coated perforated circulor plate Nichrome wire heating coil Cup burner Matheson flowmeters 8cm deep bed packed with glass beads Graduated level indicator arm

Figure 2. Experimental setup showing "cup burner*

2. MILLER

Development of Standards

67

Mr. C.L. Ford of DuPont has discussed a number of these in the preceding paper.


Precision In f i r e research ve are often confronted with the need to produce results which are approximately correct rather than precisely wrong from the standpoint of the ultimate application. It i s more appropriate from an applicability standpoint to conduct full-scale simulations and apply a safety factor to the highest value obtained. Precise, reproducible experimental data must be available, however, to test theoretical models and form a basis for establishing confidence i n the predictability of the results. Since individual investigators are quite l i k e l y to reproduce their own experimental data, we have initiated an interlaboratory comparison of essentially equivalent experiments. Figure 2 shows the general arrangement of a laboratory "cup burner" experiment from which data were obtained. Figure 3 shows the relationship between results obtained using Halons 1211 and 1301 i n which the fuel was heated to maintain bulk fuel temperature at the boiling point. A 1:1 equivalence l i n e i s shown for reference. Constructing an envelope and comparing possible answers at a common value of 5% indicates a potential error of up to 20% without considering whether the test accurately represents a r e a l i s t i c application condition. The points outside the envelope represent a research question, why? It would be quite useful to users of data i f individual Investigators would provide r e a l i s t i c assessments of the precision of their own experiments when only one laboratory i s involved. Accuracy To some extent predictability i s a measure of the accuracy of the experiment. While predictions based on fundamentals w i l l be covered i n the next section some comparisons of agents and experimental systems w i l l be covered here. A comparison of Halons 1211 and 1301 i n various flame extinguishment and inerting (explosion prevention) experiments i s shown i n Figure 4. The vapor burner for which data are presented is shown schematically i n Figure 5. The inerting data were obtained for both agents using an open top, closed bottom, 2-in. diameter tube with ignition at the bottom. (14) Comparison of flame extinguishment and inerting data i n general shows a systematic relationship as shown i n Figures 8 (ICI) and 9 (Du Pont). The Du Pont "Mason Jar" Technique i s described i n reference (23). To date there i s l i t t l e published data on the use of Halons for explosion suppression, and the data available show l i t t l e systematic relation to inerting or extinguishment data (Figure 10). A comparison of inerting data obtained i n open top versus



•

ICI Cup Burner

O

Vapor Burner - Nov. 74 FMRC

•

Cup Burner - Nov. 73 FMRC

A

Inerting - Aug.73 FMRC

10

12

(1301)

Figure 4. Relationship between Halons 1211 and 1301

.2*

(1)

I n f u s i o n Pump

(2)

Gear D r i v e

O)

Syringe

(4)

G C Type

(5)

Vaporizer

Inlet

(6) C a r t r i d g e (7)

1QI 5)

Halon

lOj

A k r @ ©

Heater

Power

Supply

(8) V a p o r

Burner

(9)

Rotameters

(10)

M i x i n g Tee

(ID

Pyrex

(12)

Sleeve

(13)

0-ring

(14)

Chimney

G l a s s Reads

(15)

Perforated

(16)

Support

(17)

Tripod

Figure 5. Experimental setup showing "vapor burner'

Disk

Block


2.

MILLER


(1301)

Figure 6. Analysis of ICI cup burner data for Halon 1211 and Halon 1301 with fuel at room temperature

(1301)

Figure 7. Analysis of ICI cup burner data with fuel at boiling point

69



(1211) Extinguishment

Figure 8. Relation between extinguishment and inerting (ICI data)

Ethylene Methanol

®

O

Wmethyiether

/

•NX

/ /

© Dlethylether

® Methyl Amine

/

/

/

/ X

/

10

3

10

20

30

(1301) Extinguishment

Figure 9. Analysis of Dupont "Mason jar* data for flame extinguishment and inerting with Halon 1301

2.

MILLER


71

open bottom tubes shows a reasonably tight correlation (Figure 11). Added experimental work i s presently being conducted. Data comparing a porous glass burner (Figure 12) show a better correlation (Figure 13) with cup burner data for liquids than for gases, which give higher results due to greater preheating of the fuel. Correlation of these laboratory experiments with the results of larger scale tests w i l l be covered i n the section on Applicability.


Predictability Maximum confidence i n design data would require that a l l values be entirely predictable and that these predictions be verified by an experiment that adequately represents a l l aspects of a predictable event. A complete model based on the material properties and the chemistry and physics of the situation i s s t i l l somewhat beyond us; however, Tewarson (22) has made significant strides i n developing a theoretical model of the radiation augmented oxygen index type experiment shown i n Figure 14. Data for the extinguishment of plastics using Halon 1301 (with oxygen mole fraction held constant) are shown i n Figure 15 and compared with oxygen index values for the same materials. This and other papers show that energy balance has a significant effect on results. Additional evidence of significant thermal effects on some materi a l s i s shown In Figure 16 which shows the effect of temperature on the extinguishing concentrations for ethylene glycol obtained by ICI using their heated cup burner. A similar effect on i n erting concentrations was obtained by Lewis of ICI for n-Hexane and Halon 1211, as shown i n Figure 17. Since we have mentioned the oxygen index type of experiment i t i s noted that a systematic relationship seems to exist between Halon extinguishing and inerting requirements and the oxygen Index values for the same materials as shown i n Figure 18. Creitz (Figure 19) (30) has also suggested that the amount of inhibitor required for extinguishment i s related to the amount of oxygen above that required for combustion (the oxygen index). This also suggests that there i s a relationship between the Halons and the inert gases. If the inerting values for hydrogen, ethylene, propane-butane, and methane are plotted as a function of the heat capacity, a general trend i s shown (Figure 20). Larsen (29) develops a thermal explanation of inhibition even further by c a l culating the total heat capacity of the system. A general trend of agent requirements i s shown with fuel-air system parameters such as the lean limit equivalence ratio (Figure 21) and adiabatic flame temperature (Figure 22) reported by Hertzberg (24). Bajpai (14) In his paper also proposes a correlation based on flame temperature (Figure 23). Consideration of other materials using values for flame and autoignition temperatures reported by Woinsky (Figure 24) (25) throws some doubt on this approach. His i n t r i n s i c safety "Group Classification Temperature" also seems to


O Supression X No supression Numbers indicate fuel concentration.

22 Q

*


6.25

°0

2

4 (1211)

6 Inerting

8

10

12

Figure 10. Relation between inerting and explosion suppression

O- Halon 1211

Open Bottom

Figure 11. Comparison between Factory Mutual Research Corp. inerting experiments using two different techniques

MILLER



2.

73

Figure 12.—ICI porous burner H (15,20.5) t

6 8 ICI Cup Burner of Ambient (1211)

10

12

Figure 13. Comparison of ICI cup and porous burners



Figure 14. Experimental apparatus IS I

)i IS

1

1

1

1

1

1

1

.16

.17

.18

.19

Oxygen

1 .20

Index

Figure 15. Comparison of oxygen index and Halon concentration (at constant 0.21 oxygen) (Data from Tewarson, 22)


75


2. MILLER

Figure 17. Inerting concentration for n-hexane vs. temperature

Tamperotur*

*c

76


30

i

i

1

25

t

-

20

-

I


Inerting ®C0

x

5 .-Extinguishing 0 0

O

oco

t

1 10 !5 Oxygen Index(Ref. 31,38)

5

— r ° 20

Figure 18. Correlation of Halon 1301 concentrations for inerting and extinguishment with the oxygen index of the fuel

0.31 -0.4

1 -0.3

1

1

1

1

1

1

-0.2

-0.1

0.0

0.1

0.2

0.3

1 0.4 LOG

1

1

1

0.5

0.6

0.7

1 0.8

1 0.9

1

1

1.0

1.1

1 1.2

L_ 1.3

(CF Br) 3

Figure 19. Relationship between oxygen concentration above the minimum required for combustion and the amount of CF Br required for extinction of a 2.4-cm high diffusion flame [O/] = [0 ] (total)—[d] (required for combustion) (data from Creitz, s

2

30)

2.

MILLER


77


1001

1—i—i

i i i

1

Lean Limit Equivalence Ratio

Figure 21. Relation of agent requirements to combustion properties of fuel

ol 0

I 500

I 1000

I 1500

I 2000

I 2500

Tadiabatic °K at Lean Limit

Figure 22. Relation of agent requirements to flame properties (24)

1

1

r

HALOGENATED

FIRE

SUPPRESSANTS

36 p 34-

O Halon 1301 1

32-

A

Halon 1211 J Onerting)

A

Halon 1211 J (Extinguishment)

F

M

R

C

30 282624-

• |

22-

• |20^


I

1850

2000

1950 Flame Temperature,*C

Figure 23. Correlation of inerting and extinguishment data with flame temperature (14)

T > Flame Temperature \ | >Autoignitk>n Temperature F

-

O (CH ) 0 2

O

-

OH

2

O

^

- ^ o u 0

1

1

1

1

2

CH3OH

«

»

»

» —1

«

Qo

0

i

i

-

""""

^

* — » — 1 — • — • — • — 1 — • — • —

3000 T

C ^ O

3500

F - A | f > < * e f 25) T

F

Figure 24. Relationship between combustion properties of fuel and Halon 1301 inerting concentrations


2. MILLER


79

be of l i t t l e use i n correlating Inerting data (Figure 25) and the reciprocal nature of flammability and extinguishability i s questionable. If the analysis i s limited to simple paraffinic hydrocarbons, data can be correlated using fuel boiling point (Figure 26), l i m i t flame temperature (Figure 27), etc. However, when more complex fuels are added, simple correlations, such as with the number of carbon atoms, produce trends only within natural groups as shown i n Figure 28. Use of "B" number or other parameter based on heat of combustion, boiling point or other thermal properties does l i t t l e to improve the situation since they ignore chemical kinetic effects. Such combustion system parameters as flame velocity which contain both thermal and kinetic effects provide another possible basis for comparison (Figure 29) and directly relatable effects such as quenching distance provide a basis for correlating data on a wide variety of fuels as shown i n Figure 30. Other combustion system parameters which include both thermal and kinetic effects, e.g., minimum ignition energy and ignition delay should provide useful correlations. Forman Williams (26) has proposed the use of the Damkohler Number for correlating f i r e extinguishment data. This has yet to be demonstrated, and this or any other approach needs to be tempered by consideration of i t s practical as well as theoretical u t i l i t y and the ease with which numerical values can be obtained. I have belaboured this point principally because there i s a need for improved understanding to provide added confidence that unexpected effects w i l l not n u l l i f y our design assumptions and create catastrophic consequences. Applicability The ultimate test of a l l laboratory data i s i t s pertinence to the f i r e event which actually occurs. In this respect, even f u l l scale f i r e extinguishment tests represent only one set of conditions, although i t i s hoped that these conditions are representative of most, i f not a l l , f i r e events that might occur. For instance, a test program oriented toward computer rooms might i n clude f i r e s i n various configurations of plastics and cellulosic materials but not consider a spray f i r e caused by a leaking hydraulic system for tape drive units. Often these are considered "improbable" events without considering the actual numerical probability. Most of the laboratory extinguishment studies of flammable liquids have been oriented toward applications i n which a twodimensional fuel surface i s assumed. In the absence of unusual heating effects there i s generally a rather good correlation between laboratory, intermediate and larger scale experiments as shown i n Figures 32-35. By varying experimental apparatus and technique some materials, such as nitromethane, ethylene glycol and isopropyl nitrate have been shown to be thermally sensitive i n that modest i n -


601

i

1

1

i

1

1

r


50h

Figure 26. Relationship of agent requirements to boiling point of fuel


2. MILLER


14

81

T

1

1

-

1

i

»

Paraffin Hydrocarbons

—

Orefins Acetylsnss

#—

* — - A —-

12

\A

y o

Cyccic Hydrocarbons Alcohols

0

Acids

•

Esttr

— • —

—

A

*/

Ketones

—

Oxides Aminos

6 9

_

\ v

*v

\ \

N

_

Aromatics

Chloride Ether

\

4 i \ \

2

0 0,

1

1

1

1

i

1

1

Number of C - Atoms

Figure 28. Relation of Halon concentration to number of carbon atoms in fuel (36)


82

H A L O G E N A T E D F I R E SUPPRESSANTS

Figure 29. Halon concentration vs.flamevelocity

Figure 30. Halon concentration vs. quenching distance (22)

MILLER


j


Holon 1301 Supply 4x 150-lb. Cylinders - Vapor Phase Removal

Cardox Gas Analyzer Halon 1211 Supply Liquid-Phase Removal

Figure 31. Schematic of Cardox 21.5-f$ cabinet

1

O A

1

A

1301 1211

O

3] mole r a t i o s o f 6 and 2, r e s p e c t i v e l y . Under c o n d i t i o n s o f comp l e t e r e a c t i o n , the observed r a t i o o f 5 would r e q u i r e r e a c t i o n (1) to be three times more e f f e c t i v e than r e a c t i o n (la) i n t r a n s p o r t i n g antimony t o the vapor phase. A check on these mass-balance cons i d e r a t i o n s i s p r o v i d e d by the areas under the SbOCl and SbCl3 pressure-temperature curves o f F i g u r e 3 . These curves i n d i c a t e an o v e r a l l r a t i o o f SbCl3/SbOCl ^2.0 which compares reasonably with the mass balance p r e d i c t i o n o f 3.0, p a r t i c u l a r l y as the r e a c t i o n s are not completely t o the r i g h t . In order t o express the pressure-temperature data, contained i n the curves o f F i g u r e 3 , i n terms t h a t may r e a d i l y be t r a n s f e r r e d to o t h e r experimental or even " r e a l - f i r e " c o n d i t i o n s i t i s necessary t o d e f i n e apparent e q u i l i b r i u m constants f o r the v a r i ous p o s s i b l e r e a c t i o n s . Thus, f o r r e a c t i o n (1) we have: 2

(PSbCl ) (PH 0) 3

3

2

K, = 1

(PHC1)

6

a[Sb 0 ] 2

3

where Ρ represents s p e c i e s p a r t i a l pressure and a[Sb2Û3] i s the thermodynamic a c t i v i t y o f condensed [Sb203] which, by d e f i n i t i o n , i s u n i t y f o r the pure [Sb2Û3] phase. For the present, we c o n s i d e r K^ as an "apparent" e q u i l i b r i u m constant s i n c e we have y e t t o e s t a b l i s h whether r e a c t i o n (1) i s a t e q u i l i b r i u m . A l s o , [Sb2Û3] i s assumed f o r the moment t o be p r e s e n t a t u n i t a c t i v i t y . Values of K^ as a f u n c t i o n o f temperature are g i v e n i n F i g u r e 4. S i m i l a r l y ,


124

H A L O G E N A T E D FIRE SUPPRESSANTS

Figure 3. Partial pressure of volatile species resulting from the thermal interaction (heating rate 2°C/min) of an ethylene chlorinated polymer (40% CI, Borden Chemical) with an equal weight of [Sb O li. Thermogravimetric analysis curve of Touval (12) is also shown for comparison. Curve for Shfie is based on absolute pressure data given by Jungermann and Plieth (6). 2

TEMPERATURE

c

e

s

4.

Antimony-Halogen System

HASTiE AND MC BEE

125

values o f K3 f o r the homogeneous r e a c t i o n : SbCl

3

+ H0 2

= SbOCl + 2HC1

(3)

are a l s o i n d i c a t e d i n the F i g u r e 4. Data f o r K\ have not been p l o t t e d as t h i s r e a c t i o n i s merely another form of r e a c t i o n s (1) and (3). Note t h a t Κ χ i s p a r t i c u l a r l y s e n s i t i v e to the temperature and the absence o f a s i n g l e monotonie dependence of Κ χ on the r e c i p r o c a l temperature i s i n d i c a t i v e of the presence o f a d d i t i o n a l r e a c t i o n s , other than r e a c t i o n (1), i n v o l v i n g the vapor s p e c i e s S b C l , HCl and H 0. a


3

2

Reaction of HCl with [SbÔ^]. In order to determine the nature o f the a d d i t i o n a l r e a c t i o n s i n f e r r e d above, i t i s necessary to c o n t r o l the t o t a l pressure o f HCl e x t e r n a l l y r a t h e r than by the i n t e r n a l r e l e a s e mechanism provided by the p y r o l y s i s of the c h l o r i n a t e d polymer. F i g u r e 5 i n d i c a t e s the nature o f the Κ χ tempera ture dependence'for t o t a l HCl pressures i n the r e g i o n o f 10~ -10" atm over an i n i t i a l l y pure [Sb 03] s u b s t r a t e . Note t h a t Κχ reaches a maximum value i n the r e g i o n of 420°C which i s a s i m i l a r r e s u l t to t h a t obtained f o r the c h l o r i n a t e d polymer -[Sb 03] system. Again, more than one process f o r the production o f SbCl3 i s evident from the slope change of the l o g Κ χ versus T"" curve. I t i s noteworthy that the slope o f t h i s curve between temperatures o f about 420 and 520°C y i e l d s a r e a c t i o n enthalpy of -20 kcal/mol, i n agreement w i t h a value based on the l i t e r a t u r e heats o f formation f o r the v a r i o u s r e a c t i o n components (see Appendix ). F i g u r e 5 a l s o i n d i c a t e s the dependence of Κ3 on temperature f o r the same system. The molecularity#or s t o i c h i o m e t r i c dependences, of the r e a c t i o n ( s ) l e a d i n g t o the formation of SbCl3 and SbOCl can be i n f e r r e d from the dependence of the products p a r t i a l pressure on the r e a c t a n t p r e s s u r e f o r isothermal c o n d i t i o n s . For i n s t a n c e , i f r e a c t i o n (1) i s predominant i n forming SbCl3 and Κχ i s pressure independent {i.e. the system i s a t e q u i l i b r i u m ) then the pressure of SbCl3 should vary as the 6/5 power of the HCl p a r t i a l p r e s s u r e . T h i s r e s u l t assumes that H 0 i s formed p r i m a r i l y by r e a c t i o n (1) and i s not e x t e n s i v e l y i n v o l v e d i n secondary processes such as r e a c t i o n (3). As i s shown i n F i g u r e 6 the pressure o f SbCl3 shows the expected 6/5 power dependence with HCl pressure thus i n d i c a t i n g r e a c t i o n (1) t o be the main overall r e a c t i o n f o r these p a r t i c u l a r conditions. The r e a c t i o n s (1) and (3) may not be i n a s t a t e o f general thermodynamic e q u i l i b r i u m . A c o n d i t i o n o f g e n e r a l thermodynamic e q u i l i b r i u m r e q u i r e s Κ t o be pressure independent f o r constant Τ and t h a t the v a r i a t i o n o f Κ with Τ y i e l d a second law r e a c t i o n enthalpy i n agreement with the corresponding t h i r d law value. In p r a c t i c e , we f i n d t h a t Κχ decreases w i t h i n c r e a s i n g Ρ(HCl) where Τ < 420°C. The value of K3 i n c r e a s e s w i t h i n c r e a s i n g Ρ(HCl) where Τ SbOCl + 2HC1

5[SbOCl] -> S b C l

3

4.

2

H 0 + SbCl

3

3.

2

2

[T=693] 97

15.8[Τ=693]-45.8

32

4.5[T=640]^16

8.2[T=743]

8.2[T=663]

8.7[T=538]^25

18.7[T=693] 40

-14[T=700]

+ H0

2

2b. 2HC1 + 3 [ S b 0 ] -> 2 [SbOCl ' S b ^ ]

3

2

-17[T=600]

2

3

+ H0

2

-11 [T=693] -20

2a. 2HC1 + 2 [ S b 0 ] -> [2Sb0Cl - S b ^ ]

2

+ 3H 0 -26[T=500]

3

2HC1 + [ S b 0 ] -> 2 [SbOCl] + H 0

3

2.

2

6HC1 + [ S b 0 ] -> 2 S b C l

Obs.

1.

Reaction τ

63

-17

-23

-20

m/

23 [T=693]

44

41

103

33

(β/

168Î/

*

[T=298]

* 35^

41[T=693]

19[T=693]

42[T=590]

27[T=693]

m

τ

64

16

m/

36 [T=590]

44[T=693]

AS L i t . Obs. L i t .

76&30[T=526] 25

0[T=693]

-20[T=693]

Obs.

ΔΗ

Summary o f main r e a c t i o n s i n the antimony oxide-halogen s u b s t r a t e systems.

Table I


4.

HASTTE AND MC BEE

Footnotes

131


f o r Table I

1. A l l l i t e r a t u r e data are based on the thermodynamic parameters given i n the Appendix. m. From the observed Δ Η f o r r e a c t i o n s 4(a) and (b) we can show t h a t f o r these data to be s e l f c o n s i s t e n t , AH [SbOCl-Sb 0 ] = -356 kcal/mol; t h i s i s w i t h i n the u n c e r t a i n t y f o r the reported l i t e r ature value (see the Appendix). n. Based on an estimated S 98[SbOCl] = 37±5 cal/deg mol. p. By analogy with above r e a c t i o n 2. q. From S [ S b O C l ] = 71.6±0.8 cal/deg mol (cale.) and S s [ S b O C l ] = 37 cal/deg mol ( e s t . ) . r. Based on AS f o r r e a c t i o n s 2 and 2a and the l i t e r a t u r e S g8 f o r S b C l , HCl, [ S b 0 ] , and H 0. s. Based on As f o r r e a c t i o n 2a. t. Using AH [SbOCl] = -28.7 kcal/mol (see Appendix), u. Our P(Sbi 0e) data, determined using the t o t a l v a p o r i z a t i o n method o f c a l i b r a t i o n f o r the k - f a c t o r , were found to be i n s a t i s f a c t o r y agreement with the vapor pressure data g i v e n by Jungermann and P l i e t h (6) f o r senarmonite. Τ

f

2

3

2

29

2 9 8

2


3

2

3

2

f

t

Mass Transport o f Retardant

to Flame Front

C o n t r o l l i n g F a c t o r s . Under a c t u a l f i r e c o n d i t i o n s the r e lease and t r a n s p o r t o f a flame i n h i b i t i n g s p e c i e s , o r some s u i t able molecular precursor, from a decomposing polymer substrate to the flame w i l l be determined p r i m a r i l y by: - the p a r t i a l vapor pressure f o r the i n h i b i t o r a t the polymer s u r f a c e , and - the formation o f a d i f f u s i v e b a r r i e r or boundary l a y e r a t the surface-gas i n t e r f a c e which l i m i t s the t r a n s p o r t o f r e t a r d a n t to the flame. These flame i n h i b i t i o n process-determining-factors w i l l i n t u r n be dependent upon the temperature, the r a t e o f bulk gas flow across the s u r f a c e , the e f f e c t i v e thickness o f the flame f r o n t , and the thermodynamic a c t i v i t y o f r e t a r d a n t i n the polymer s u b s t r a t e . Consider f i r s t the p o s s i b l e l i m i t a t i o n o f S b C l and HCl t r a n s p o r t from the s u b s t r a t e to the flame due to boundary l a y e r forma t i o n between the v a p o r i z i n g s u r f a c e and the bulk gas, as shown s c h e m a t i c a l l y i n F i g u r e 10. The e f f e c t o f such a b a r r i e r would be to reduce the f l u x o f r e t a r d a n t reaching the bulk gas and hence the flame f r o n t . From F i c k * s law o f mass d i f f u s i o n (15) and the accepted concept of a boundary l a y e r we have: 3

J ( S b C l ) = (DM/6RT)[P(surface)-P ( b u l k ) ] , 3

132

Σ.

4

0

HALOGENATED

ο Ε

Γ

8

20h

SbCI,

(SbOCI)

FERE

SUPPRESSANTS

(4) obs

(2 SbOCl ·


(SbOCl* Sb,0.)

TEMPERATURE °C

Figure 9. Reaction free energy dia gram showing the main reaction products at various temperatures, based on data in Table I. Condensed phases indicated by parentheses.

VERTICAL DISTANCE FROM SUBSTRATE SURFACE

Figure 10. Idealized fire system including a boundary layer between substrate and flame

4.


HASTiE AND MC BEE

133

2 where J(SbCl3) i s the f l u x o f SbCl3 (units g/cm - s e c ) , D i s the d i f f u s i o n c o e f f i c i e n t o f S b C l i n the atmospheric gas (mainly N ) * i n u n i t s o f cm sec" , δ i s the e f f e c t i v e t h i c k n e s s o f the boundary l a y e r (cm) Ρ(surface) = P(SbCl3) a t the s u r f a c e , and Ρ (bulk) = P(SbCl3) i n the bulk gas (units atm). In the absence o f such a d i f f u s i o n l i m i t e d boundary the f l u x would be g i v e n by the Langmuir expression f o r f r e e molecular v a p o r i z a tion: J = P/[2.557x10 ( T / M ) ' ] g / c m -sec. 3

2

2

D i f f u s i o n C o e f f i c i e n t s f o r S b C l and HCl. The d i f f u s i o n c o e f f i c i e n t f o r a s p e c i e s i n a gas mixture can be estimated from the e x p r e s s i o n :


?

= 2.62xl0" [T (Mi+M )/2MiM 3 3

D

3

2

1

2

Ρ σ 12

2

1/2

c m

2

s e c

-l

2

as d e r i v e d from hard sphere ( i . e . , a d i a b a t i c e l a s t i c c o l l i s i o n s ) k i n e t i c theory, e.g., see H i r s c h f e l d e r e t a l . [p. 544 (16)]. The average c o l l i s i o n diameter 2 between u n l i k e s p e c i e s (e.g. N and SbClg) i s estimated from the e x p r e s s i o n : 2

( σ

°12 "

ΐ

+

0

2

) / 2

A

'

°

Values o f the molecular diameters

and

are taken a s :

a(SbCl~) i>7.OA ο

J

a(HCl) = 2.54 A, and σ ( Ν ) = 2.10 A, 2

using molecular parameters g i v e n by Sutton (17.) · 350 °C, D(SbCl

3

Therefore, a t

2 -1 i n N ) = 0.28 cm sec , 2

and D(HC1

2 -1 i n N ) = 1.34 cm sec . 2

Boundary Layer Thickness δ. An upper l i m i t to the value o f δ i s g i v e n by the flame s t a n d - o f f d i s t a n c e , which i s t y p i c a l l y ob served t o be o f the order o f 0.1 cm (21). A l t e r n a t i v e l y , i f we assume t h a t the common gas dynamic case o f laminar flow over a f l a t p l a t e a p p l i e s here, then δ can be estimated from the exprès6 = 1.5 L H - ^ V S R

1

7

2

'

134


where L i s the l e n g t h o f t r a v e l by the gas over the p l a t e , Ν Ν

= v/Dp

(Schmidt number),

= VLp/v

(Reynolds number), and

R


where V i s the bulk gas v e l o c i t y ν i s the c o e f f i c i e n t o f v i s c o s i t y f o r the gas, and ρ i s the gas d e n s i t y (p = PM/RT), e.g., see Graham and Davis (18) and Turkdogan e t a l . (19). To a good approximation both ν and ρ can be taken as the e s t a b l i s h e d values f o r a i r i . e . , -4 ν = 3.18x10 gm/cm-sec and -4 Q ρ = 5.6x10 gm/cm* a t 350

°C

[Handbook o f Chemistry and Physics (20)]. The bulk gas v e l o c i t y V i s an a d j u s t a b l e q u a n t i t y . For the p r e s e n t purpose we assume a value o f : V = 10 cm sec , which i s s i m i l a r t o the experimental values o f Hirano e t a l . (21) f o r a burning cardboard system. T h i s choice o f V i s not p a r t i c u l a r l y c r i t i c a l t o the arguments which f o l l o w . The e f f e c t i v e sample l e n g t h L i s a l s o an a d j u s t a b l e parameter. In more c l a s s i c a l boundary l a y e r problems, L i s d e f i n e d as the sample l e n g t h and may be e x p e r i m e n t a l l y measured. However, f o r a propagating f i r e system some g e o m e t r i c a l assumptions are r e q u i r e d i n determining L. In the absence o f d i r e c t evidence, the f o l l o w ing arguments are used to estimate l i k e l y v a l u e s f o r L. Since the phenomenon o f flame spread allows f o r a c o n t i n u a l replenishment o f f u e l and heat across the s u r f a c e , the c o n v e c t i v e movement o f gas i s maintained a t the flame f r o n t and a small e f f e c t i v e value f o r L can t h e r e f o r e be expected. Now a f i r e can be considered as an aggregation o f i n d i v i d u a l flame elements. Each flame element w i l l c o n s i s t o f a pre-flame, r e a c t i o n zone and burnt gas r e g i o n , as suggested i n F i g u r e 10. For purposes o f flame propagation and i n h i b i t i o n , the r e g i o n near and a t the r e a c t i o n zone w i l l be o f primary s i g n i f i c a n c e . T h i s i s the r e g i o n where the c o n c e n t r a t i o n o f propagating r a d i c a l s , such as H, i s i n a p p r e c i a b l e excess o f the thermodynamic e q u i l i b r i u m v a l u e . I t i s known t h a t f o r e i t h e r d i f f u s i o n o r premixed f u e l - r i c h CH^-air flames the t h i c k n e s s o f t h i s r e g i o n v a r i e s between about 0.1-0.4 cm. Likewise, the con c e n t r a t i o n o f suspected i n h i b i t o r s p e c i e s such as HCl, HBr o r SbO i s predominant i n t h i s r e g i o n , as i n d i c a t e d i n F i g u r e 10 (22). Thus as the flame f r o n t moves across the combustible s u r f a c e the passage o f an i n h i b i t o r , such as SbCl , through the boundary l a y e r

4.

HASTiE

AND M C BEE


135

i s l i k e l y t o be most s i g n i f i c a n t over a d i s t a n c e o f about 0.4 cm, c o i n c i d i n g with the passage o f the pre-flame and r e a c t i o n zone r e g i o n s . Given the non-ideal geometry o f r e a l f i r e s , t h i s d i s t a n c e c o u l d vary and an estimated f a c t o r o f three u n c e r t a i n t y i n L should be allowed f o r . From the above c o n s i d e r a t i o n s , a v a l u e o f the boundary l a y e r t h i c k n e s s δ i s c a l c u l a t e d as: δ = 0.24

cm f o r V = 10 cm/sec .


This d i s t a n c e i s o f a s i m i l a r order o f magnitude as the flame s t a n d - o f f d i s t a n c e which tends to support our i n t e r p r e t a t i o n o f L f o r flame-spread a p p l i c a t i o n s .

F l u x P r e d i c t i o n s . With these estimated data f o r D and δ, e i t h e r the mass t r a n s p o r t f l u x or the p a r t i a l pressure d i f f e r e n t i a l across the boundary l a y e r can be c a l c u l a t e d from the f l u x r e l a t i o n s h i p g i v e n above. The p a r t i a l pressure d i f f e r e n t i a l f o r an i n h i b i t o r s p e c i e s , such as SbCl3 , can be estimated Λ pKLOKL provided there are no k i n e t i c l i m i t a t i o n s t o the movement o f SbCl3 and HCl through the polymer s u b s t r a t e to the v a p o r i z i n g s u r f a c e . T h i s i n v o l v e s a determination o f the value o f P(SbCl3) a t the s u r f a c e from the r e a c t i o n f r e e energies g i v e n i n Table I. The f l u x o f r e t a r d a n t e n t e r i n g a flame f r o n t may be d e r i v e d as f o l l o w s . Consider a polymer s u b s t r a t e c o n t a i n i n g a halogen source equiva l e n t to 6 mole % HCl, together with 2 mole % [Sb2Û3]. We a l s o assume t h a t the p a r t i a l pressure o f Η 0 i n the atmosphere i s f i x e d by the a i r humidity which i s t y p i c a l l y e q u i v a l e n t to 3xl0~ atm. From the p r e v i o u s l y determined e q u i l i b r i u m constants f o r r e a c t i o n (1), the amount o f SbCl3 p r e s e n t a t the s u r f a c e o f the s u b s t r a t e can be c a l c u l a t e d . F i g u r e 11 shows the general r e l a t i o n s h i p between P(SbCl3> and the i n i t i a l unreacted Ρ(HCl) f o r v a r i o u s t y p i c a l p y r o l y s i s temperatures. Thus f o r the assumed c o n d i t i o n o f Ρ(HCl) t o t a l = 6 x l 0 " atm we have P(SbCl3) = 1.3xl0" atm and Ρ(HCl) = 2.1xl0" atm a t 350 °C, and 65% o f the HCl i s converted to SbCl3The e f f e c t o f an order o f magnitude i n c r e a s e i n P(H20) on P(SbCl3) may a l s o be noted i n F i g u r e 11. From the previous determinations o f K3, the p r o d u c t i o n o f SbOCl can be shown t o be n e g l i g i b l e f o r these c o n d i t i o n s . 2

2

2

2

2

These p a r t i a l pressures are assumed to represent the halogen r e t a r d a n t s p e c i e s c o n c e n t r a t i o n s a t the s u b s t r a t e s u r f a c e . The corresponding c o n c e n t r a t i o n s i n the bulk gas can be expected to be s i g n i f i c a n t l y reduced by the p r e d i c t e d presence o f a mass t r a n s p o r t - l i m i t i n g boundary l a y e r . An upper e f f e c t i v e l i m i t f o r P(SbCl3) i n the bulk gas can be estimated from the amount o f SbCl3 r e q u i r e d i n the flame f o r s i g n i f i c a n t flame i n h i b i t i o n to occur. From our observations o f burning v e l o c i t y and H atom c o n c e n t r a t i o n r e d u c t i o n s i n the presence o f SbCl3, t h i s pressure i s known t o



2

H 0=3 χ 10 atm 2

I

ι

0

2

ι

ι

ι

ι

4 6 8 10 TOTAL HC1 PRESSURE χ 10 ATM

ι

1

12

2

Figure 11. Dependence of SbCl partial pressure on total HCl pressure and temperature for conditions of H 0 par tial pressure equal to 3 X 10~ or 3 X 10~ atm, based on the K, values in Figure 4. Broken line indicates maximum pos sible SbCh pressure, i.e., complete conversion of HCl to SbCl . Note that SbOCl is a negligible species for these conditions. s

2

2

3

3

4.

HASTiE

AND


M C BEE

137

f a l l i n the range o f 10 -10 atm (see the f o l l o w i n g d i s c u s s i o n ) . In p r a c t i c e , t h i s i s an upper l i m i t s i n c e the flame removes SbCl3 from the bulk gas (see F i g . 10). We can t h e r e f o r e reasonably assume the p a r t i a l pressure o f SbCl3 i n the bulk gas t o be small compared with the corresponding pressure a t the s u r f a c e . From these estimates i t f o l l o w s t h a t the f l u x o f SbCl3 i s J(SbCl ) 3

-5 2 -7 2 = 6.8 χ 10 g/cm sec (=3.0x10 mole/cm s e c ) .

T h i s value may be compared with the maximum Langmuir f l u x no boundary l a y e r ) o f 0.35 g/cm sec. Thus,

(i.e.,

2


J/J(max) ^2χ1θ"

4

and a s i g n i f i c a n t r e d u c t i o n i n f l u x over the maximum allowed by f r e e molecular v a p o r i z a t i o n appears l i k e l y under a c t u a l f i r e con ditions . S i m i l a r c o n s i d e r a t i o n s t o those given above f o r sbCl3 i n d i c a t e the HCl f l u x t o the flame f r o n t t o be: J(HCl) = 0.9χ1θ"

4

2

g/cm sec a t 350 °C.

At 450 °C the f l u x e s a r e c a l c u l a t e d a s : J(SbCl ) 3

-4 2 = 0.9x10 g/cm sec,

and -4 2 J(HCl) = 0.4x10 g/cm sec. Thus the amount o f SbCl3 r e l a t i v e t o HCl i n c r e a s e s with i n c r e a s i n g p y r o l y s i s temperature. T h i s e f f e c t i s l a r g e l y a r e s u l t o f the absence o f s t a b l e antimony o x y c h l o r i d e s o l i d s a t elevated tempera tures ( i . e . , >420 °C). P r e d i c t e d Flame I n h i b i t o r Species Concentrations. From a knowledge o f the f l u x o f i n h i b i t o r e n t e r i n g a flame f r o n t we can d e r i v e i n h i b i t o r species concentrations f o r a w e l l d e f i n e d f i r e system, such as t h a t represented i n F i g u r e 10. F o r example, con s i d e r a h y p o t h e t i c a l polyethylene - t e r e p h t h a l a t e s u b s t r a t e c o n t a i n i n g [Sb2Û3] together with a halogenated hydrocarbon e q u i v a l e n t t o 6 mole % HCl. The predominant gaseous f u e l component i n t h i s system i s acetaldehyde, CH3CHO. A s t o i c h i o m e t r i c combust i o n r e a c t i o n between CH3CHO and a i r , i . e . : CH CH0 + 5/20 3

2

= 2C0

2

+ 2^0

138

HALOGENATED

FIRE SUPPRESSANTS

consumes 0.078 mole f r a c t i o n CH CH0, with r e s p e c t to the f u e l - a i r mixture. The N component i n the bulk gas mixture amounts to 0.725 mole f r a c t i o n . T h i s amount can a l s o be considered as an upper l i m i t to the N c o n c e n t r a t i o n a t the substrate surface. Based on experimental observations f o r analogous systems such as polymer candle burning, e.g., see Fenimore and Martin (23), we may c o n s i d e r 0.4 N mole f r a c t i o n as a l i k e l y lower l i m i t surface concentration. I f we assume 0 to be o f n e g l i g i b l e c o n c e n t r a t i o n a t the surface then the f u e l component c o n c e n t r a t i o n should t h e r e f o r e be i n the r e g i o n o f 0.27-0.6 mole f r a c t i o n . Consider now the e f f e c t o f the d i f f u s i o n boundary l a y e r on the f l u x o f CH3CHO e n t e r i n g the flame f r o n t . At a temperature o f 350 °C we c a l c u l a t e : 3

2

2

2


2

D(CH CH0) i n N 3

I f Ρ(CH CH0) = 0.26

2

= 0.82

cm

2

1

s e c " [ u s i n g σ(CH CH0) 2.9 3

A].

atm a t the surface then

3

-4 2 J(CH CH0) = 7.6x10 g/cm sec. 3

Comparison of t h i s f l u x with t h a t f o r S b C l ratio of: J(SbCl )/J(CH CH0) = 1.68xl0" 3

3

g i v e s a molar f l u x

2

3

Now a t the flame r e a c t i o n zone, under the c h a r a c t e r i s t i c s t o i c h i o m e t r i c r e a c t i o n c o n d i t i o n o f a d i f f u s i o n flame, 0.078 mole f r a c t i o n o f CH3CHO i s r e q u i r e d . Hence, from the above f l u x r a t i o , the amount o f SbCl3 w i t h i n the flame i s : SbCl

3

i n flame = 1.2x10

3

mole f r a c t i o n .

From the observed burning v e l o c i t y ( 2 ) — a n d Η atom c o n c e n t r a t i o n — reductions (see F i g . 12 g i v e n l a t e r ) i n premixed flames t h i s amount of SbCl3 i s more than s u f f i c i e n t to s t r o n g l y i n h i b i t a hydrocarbon — or even H — f u e l e d flame. S i m i l a r arguments t o those above i n d i c a t e : 2

-2 HCl i n flame = 1.0x10 mole f r a c t i o n . An a l t e r n a t i v e method o f c o n v e r t i n g the SbCl3 f l u x to a flame c o n c e n t r a t i o n i s to c o n s i d e r the f l u x o f a i r e n t e r i n g the flame. For example, consider a volume element determined by the boundary l a y e r t h i c k n e s s 6(= 0.24 cm), the e f f e c t i v e r e a c t i o n zone t h i c k ness L(= 0.4 cm) and a u n i t width (see F i g . 10). Let the flow o f a i r across the surface i n t o t h i s volume element be V(= 10 cm/sec). Then the molar a i r f l u x w i l l be given by: J(air) =

V/V

4.

HASTiE

139


AND M C BEE

J (air) = 2.1x10 3 where V (=22.4x10 From b e f o r e :

°C,

3 cm

m

mole/cm sec a t 350

a t N.T.P.) i s the molar volume o f a i r .

-7 2 J ( S b C l ) = 3.0x10 mole/cm sec, 3

hence the c o n c e n t r a t i o n o f SbCl^ i n a flame under these assumed c o n d i t i o n s o f a i r f l u x i s determined as:


SbClg i n flame = 1.3xl0~

3

mole f r a c t i o n ,

which i s s i m i l a r t o the p r e v i o u s estimate. We may r e c a l l t h a t macroscopic f l a m m a b i l i t y t e s t s f o r sub s t r a t e s c o n t a i n i n g a s i m i l a r chloride/antimony oxide content t o t h a t assumed here i n d i c a t e an e f f e c t i v e degree o f flame retardancy, e.g. see the F i g u r e 8 g i v e n by P i t t s (13). Note a l s o from P i t t s t h a t f o r s u b s t r a t e mole r a t i o s o f Sb:Cl > 1:3 very l i t t l e g a i n i n flame retardancy i s achieved. Our data a r e i n accord with t h i s o b s e r v a t i o n owing to the f i n i t e values o f Κ ensuring the presence o f excess s u b s t r a t e [SbÔ^], even f o r s t o i c h i o m e t r i c r e a c t i o n con ditions. Fate o f SbClg i n the Pre-Flame

Region

Once SbCl3 e n t e r s a flame i t encounters a f a r d i f f e r e n t chemical environment to t h a t a t the s u b s t r a t e s u r f a c e . In p a r t i c u l a r , the d i f f u s i o n o f H 0 as a combustion product i n t o the p r e flame region, and the i n c r e a s e d temperature, w i l l tend to favor the conversion o f SbCl3 t o SbOCl v i a r e a c t i o n (3). For i n s t a n c e , a t some p o i n t i n the pre-flame r e g i o n we can expect the f o l l o w i n g c o n d i t i o n s to occur: 2

Τ = 1200

Κ

Ρ(Η 0) = 10

atm

2

P ( S b C l ) = 5x1θ" 3

5

atm

-3 and

Ρ(HCl) = 10

atm.

From the thermodynamic data g i v e n i n the Appendix, the f r e e energy change f o r r e a c t i o n (3) a t 1200 Κ i s 19.7 kcal/mol. Hence, we c a l c u l a t e : Ρ(SbOCl) = 1 . 3 x l 0 "

3

atm.

and SbOCl becomes the predominant antimony c o n t a i n i n g s p e c i e s a t

140


the expense o f SbCl.3 f o r these c o n d i t i o n s . From premixed flame s t u d i e s ( 3 ) , we know t h a t mass d i f f u s i o n from the r e a c t i o n zone provides a s i g n i f i c a n t c o n c e n t r a t i o n o f H-atoms i n the pre-flame r e g i o n (e.g. ^ 1 0 - 1 0 atm). The exothermic bi-molecular r e a c t i o n : 3


SbOCl + H

HCl + SbO

h

(F.l)

can t h e r e f o r e be expected to occur r e a d i l y . As r e a c t i o n (3) i s c o n s i d e r a b l y endothermic i t w i l l most l i k e l y be slow as compared with r e a c t i o n F . l . I t t h e r e f o r e f o l l o w s t h a t the steady-state c o n c e n t r a t i o n o f SbOCl would be small and t h i s accounts f o r i t s non-observation i n our mass spectrometric flame sampling s t u d i e s (22). On the other hand, both HCl and SbO were observed as major products o f SbCl3 decomposition i n the pre-flame r e g i o n ( 2 2 ) . Thus SbOCl i s most l i k e l y a s i g n i f i c a n t intermediate species (but o f low steady-state concentration) i n the pre-flame conversion of SbCl3 to the flame i n h i b i t i n g moieties of SbO and HCl. These arguments do not preclude the presence o f a d d i t i o n a l r e a c t i o n s such as: SbCl

3

+ H + HCl +

which were suggested e a r l i e r

SbCl , 2

(22).

A c t i o n of SbCl3 and HCl i n Flame I n h i b i t i o n Thus f a r we have considered the chemistry o f SbCl3 from the p o i n t o f i t s i n i t i a l s u b s t r a t e production to i t s conversion to other species i n the pre-flame r e g i o n . These secondary species o f SbO and, to a l e s s e r extent, HCl are considered to be d i r e c t l y r e s p o n s i b l e f o r flame i n h i b i t i o n . As has been argued elsewhere ( 1 , 2 2 ) , we c o n s i d e r t h a t these species are c a t a l y t i c a l l y i n v o l v e d i n a c c e l e r a t i n g the recombination of flame r a d i c a l s and p a r t i c u l a r l y H atoms. For i n s t a n c e , H + HCl = H

2

+ CI

C l + RH + HCl + R CI + H 0

2

-* HCl +

0

2

C l + H + M + HCl + M where RH i s a f u e l species and M a t h i r d body such as N , 2

SbO

+ H -> SbOH

SbOH + H -> SbO

+

H2

and

4.

HASTiE

AND


M C BEE

141

These processes are l i k e l y to be more r a p i d than the normal r a d i c a l recombinations o f : H + OH + M + H 0

+ M

2

H + H + M->H +M.


2

Using the Li/Na technique f o r monitoring H atom concentrations we have demonstrated t h a t the presence o f antimony does indeed cause a r e d u c t i o n i n the steady-state flame H atom c o n c e n t r a t i o n as shown, f o r example, i n F i g u r e 12. Further evidence f o r SbO as a s i g n i f i c a n t flame species was a l s o obtained by the o b s e r v a t i o n o f known emission s p e c t r a f o r SbO (System A a t 506.2 nm)-as shown i n the F i g u r e 12. I t i s i n t e r e s t i n g to note t h a t a s i m i l a r degree o f e f f e c t i v e n e s s i n reducing H atom concentrations i s shown by HCl a t 30 times the c o n c e n t r a t i o n o f SbCl3T h i s r e s u l t i s s i m i l a r to t h a t found by burning v e l o c i t y measurements (2) . From the f l u x arguments given above an HCl/SbCl3 flame c o n c e n t r a t i o n r a t i o o f about 8 was i n d i c a t e d . Hence, f o r these c o n d i t i o n s , the HCl cont r i b u t i o n to flame i n h i b i t i o n would be n e g l i g i b l e as compared with t h a t provided by the antimony component o f SbCl3Further flame measurements are i n progress to e s t a b l i s h the molecular d e t a i l s o f the apparent c a t a l y t i c r o l e o f antimony species i n e f f e c t i n g r a d i c a l recombination. Conclusions Our s t u d i e s i n d i c a t e t h a t the e f f e c t i v e n e s s o f antimony-halogen flame r e t a r d a n t systems depends on the f o l l o w i n g p r i n c i p a l factors : a. The degree of conversion of the s u b s t r a t e halogen component to SbCl3 and SbOCl s p e c i e s , b. The f l u x o f these antimony h a l i d e species from the subs t r a t e s u r f a c e through a d i f f u s i o n boundary l a y e r to the flame front, c. The conversion i n the flame o f the species SbCl3 and SbOCl to SbO and HCl, d. The i n t e r a c t i o n o f SbO and, to a l e s s e r extent, HCl with H atoms with a r e s u l t a n t l o s s o f chain branching and reduced burning v e l o c i t y . We have shown t h a t the f i r s t f a c t o r (a) i s s t r o n g l y i n f l u e n c e d by the formation o f intermediate s o l i d o x y c h l o r i d e phases. The production o f the v o l a t i l e species SbOCl can a l s o be s i g n i f i c a n t i n the presence o f H 0 vapor. As o x y c h l o r i d e s are g e n e r a l l y more v o l a t i l e species than the corresponding halides,we are c o n s i d e r i n g t h e i r p o s s i b l e r o l e as r e t a r d a n t species f o r other metal oxide systems such as [Sn0 ], [ A l 0 3 ] , [ T i 0 ] and [ C r 0 3 ] . Here-to-fore, the l e s s than f a v o r a b l e thermodynamics f o r the conversion of HCl (or HBr) to v o l a t i l e metal c h l o r i d e s such as TiCli+ or CrCl3 has l i m i t e d c o n s i d e r a t i o n o f such systems as [Sb 03] s u b s t i t u t e s . 2

2

2

2

2

2



Figure 12. Concentration profiles for H atoms in the H /O /N flame 3/1/3.7 (1650 K) showing catalytic effects of 3.2 χ 10* mole fraction SbCl and 9.6 χ JO" mole fraction HCl on the loss of Η atoms. Additives were introduced with the premixed combustion mix ture nebulized aqueous droplets. 2

g

s

s

4

4.


H A S T i E AND MC BEE

143

With regard t o r o l e o f SbCl3 o r SbOCl i n flame i n h i b i t i o n , i t appears t h a t the halogen moiety i s o f secondary importance? i t s primary f u n c t i o n being t o t r a n s p o r t the metal component t o the flame r e a c t i o n zone.


Appendices:

B a s i c Data

Estimated Thermodynamic P r o p e r t i e s f o r the Species SbOCl. The s p e c i e s SbOCl has not been p r e v i o u s l y i d e n t i f i e d and hence no thermodynamic data e x i s t . In view o f i t s s i g n i f i c a n c e t o t h e present work we have made the f o l l o w i n g e s t i m a t i o n s f o r the s t a b i l i t y and entropy p r o p e r t i e s o f SbOCl. These estimates a r e b e l i e v e d t o be s u f f i c i e n t l y r e l i a b l e t o a l l o w c a l c u l a t i o n s o f i t s probable c o n c e n t r a t i o n i n a " r e a l f i r e " environment. The heat o f formation AHf i s estimated as f o l l o w s . Consider the a t o m i z a t i o n energy f o r the process: CI - Sb =

0

+ CI + Sb +

0,

as ΔΗ atoms = D(Sb-Cl) + D(Sb=0) . We assume D(Sb-Cl) t o be the average bond energy i n SbCl ? namely, 7 4 . 8 kcal/mol. A l s o D(Sb=0) i s taken as the bond d i s s o c i a t i o n energy f o r t h e f r e e diatomic s p e c i e s SbO. L i t e r a t u r e v a l u e s o f D(SbO) vary from about 9 0 t o 1 0 5 kcal/mol. From the known p e r i o d i c trends f o r the s i m i l a r (157±3),

s p e c i e s o f GeO

AsO

(114±3),

SeO

(100±15),

SnO

(126±2)

and TeO ( 9 2 . 5 ± 2 ) i t appears t h a t D(SbO) ^ 1 0 5 kcal/mol i s the most appropriate v a l u e . T h i s r e s u l t i s supported by a s i m i l a r value o f 1 0 4 r e p o r t e d by Labsham as c i t e d by the review o f Brewer and Rosen b l a t t ( 2 4 ) . Hence, the a t o m i z a t i o n energy f o r SbOCl i s taken as 1 7 9 . 8 ± 1 0 kcal/mol. This i s equivalent to ΔΗ 298(SbOCl) = - 2 8 . 7 kcal/mol £

which compares reasonably with the estimated value o f - 2 5 . 5 k c a l / mole g i v e n by Wagman e t a l ( 2 6 ) . The entropy o f SbOCl can be s a t i s f a c t o r i l y determined from estimated geometrical and v i b r a t i o n a l parameters u s i n g the w e l l known treatment o f s t a t i s t i c a l thermodynamics ( 2 6 ) . Based on t h e known f r e q u e n c i e s o f v i b r a t i o n f o r S b C l [see Nakamoto ( 2 7 ) ] and SbO [see Brewer and Rosenblatt (24)] we estimate f o r SbOCl f r e 3

quencies o f 7 1 0 ± 3 0

1

cm" ,

360±20

1

cm"

and

200±50

1

cm" .

An

0 =

Sb-Çl

bond angle o f 1 2 0 ° i s assumed and i n t e r n u c l e a r d i s t a n c e s o f 2 . 3 2 A and 1 . 8 5 A f o r Sb-Cl and Sb = 0 , r e s p e c t i v e l y , a r e used. I t f o l l o w s from these estimates and the s t a t i s t i c a l treatment t h a t S__ (SbOCl) = Q

71.6±0.8

cal/deg mol.

L i t e r a t u r e Thermodynamic Data. The l i t e r a t u r e thermodynamic data f o r the other s p e c i e s o f i n t e r e s t may be obtained from the compilations o f Wagman e t a l ( 2 5 ) and the JANAF Tables ( 2 8 ) ,

144


with s e v e r a l exceptions. P e r t i n e n t data are summarized i n Table A. I t should be noted t h a t Δ Η and A s can be determined to a good approximation from these Δ Η - 2 9 8 and S298 data s i n c e , f o r the r e a c t i o n s o f i n t e r e s t , the neat content and entropy changes are almost independent o f temperature. τ

T

TABLE A Summary o f B a s i c Thermodynamic Data

Species

AH

S °

f


kcal/mol

[SbOCl] [2Sb0Cl-Sb 0 ] 1

3

c

a

l

2

/

9

8

d

e

Comments g

-344.3

52.8

- 89.4

37±5

m

o

1

Senarmonite, i . e . , c u b i c form. S

°298

o

u

r

e

s

t

i

m

a

t

e

-

-346.9, -354.5*

HCl

- 22.06

H0

- 57.8

2

SbCl-

44.64 45.1

- 75.0

80.7

- 28.7 or - 25.5

71.6

or

82.1

Larger S ° 2 9 8 value i s from Wilmshurst (29).

•5

SbOCl

Larger Δ Η ^ value i s our estimate.

0.8

Mass S p e c t r a l Fragmentation Data. In order to u t i l i z e mass s p e c t r a l i o n i n t e n s i t i e s f o r species p a r t i a l pressure déterminat i o n s i t i s necessary to e s t a b l i s h the mass s p e c t r a l fragmentat i o n p a t t e r n f o r each s p e c i e s o f i n t e r e s t . These are given as f o l l o w s f o r 90 eV i o n i z i n g e l e c t r o n energy: SbCls SbOCl Sbi+Ofi

1

SbCl

+ 3

:

• Sb0Cl :

• Sb 0 4

6

(22.6), S b C l

+

(18.9), S b 0

+

(9.2), S b 0 3

+

(60.5), S b C l

(19.9), +

5

The r e s u l t s f o r S b C l and SbÔ^ l i t e r a t u r e data, i . e . : 3

+ 2

SbCl

(2.1), S b 0 3

(7.2), S b

(20.2), S b

4

+ 4

+

+

+

(17.2), S b 0

(9.7) (41.0) +

(71.5)

compare f a v o r a b l y with a v a i l a b l e

4.

AND MC BEE

HASTiE

SbCl

3


[ P r e i s s (30)] SbCl (23.2), S b C l +

3

+ 2

(58.5), S b C l

Sbi*06 [Boerboom e t a l ( 3[1) ] SbO (68), SbÔe"* (13.6), S b O i * +

145

1

3

+

+

(7.8), S b

+

(10.5)

(6.8).

I t i s p e r t i n e n t t o note t h a t , c o n t r a r y t o the observations o f Boerboom e t a l (31) and Kazanas e t a l (32), the r e l a t i v e abundances f o r the mass s p e c t r a l ions i n the Sbi*06 system were temperature independent, as i s t o be expected i f SbÔô i s the s o l e neutral precursor.


Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12.

13.

14. 15. 16.

H a s t i e , J . W., J . Res. N.B.S. (1973) 77A, 733. Lask, G. and Wagner, H. G., E i g h t h Symposium ( I n t e r n a t i o n a l ) on Combustion (1962) W i l l i a m s and W i l k i n s Co., p. 432. H a s t i e , J. W., Comb. Flame (1973) 21, 187. H a s t i e , J . W., I n t . J. Mass Spec. Ion Phys. (1975) 16, 89. Grimley, R. Τ., Mass Spectrometry in "The C h a r a c t e r i z a t i o n o f High Temperature Vapors", 195, Ed. J. L. Margrave, John Wiley and Sons, Inc., New York (1967). Jungermann, E. and Plieth, Κ., Z. Physik. Chem. (1967) 53,1. Drowart, J . and G o l d f i n g e r , P., Angew Chem. I n t . Ed. (1967) 6, 581. Gaydon, A. G., "Spectroscopy o f Flames", Chapman and H a l l , London (1957). Bulewicz, Ε. M. and Sugden, Τ. Μ., Trans. Faraday Soc. (1956) 52, 1475. Bulewicz, Ε. Μ., James C. G., and Sugden, Τ. Μ., Proc. Roy. Soc. (1956) A255, 89. Mavrodineanu, R. and Boiteux, H., "Flame Spectroscopy", John Wiley and Sons, New York (1965). Touval, I . , "A comparison o f antimony oxide and s t a n n i c oxide hydrate as flame r e t a r d a n t s y n e r g i s t " (1973) Polymer Conf. S e r i e s , U n i v e r s i t y D e t r o i t , May 22, 1973; see a l s o J . F i r e and Flammability (1972) 3, 130. Pitts, J . J., "Inorganic flame r e t a r d a n t s and their mode o f a c t i o n " p. 133 in "Flame Retardance o f Polymeric M a t e r i a l s " , V o l . I , Eds. W. C. K u r y l a and A. J. Papa, Marcel Dekker, I n c . , New York (1973). Behrens, R. G. and Rosenblatt, G. M., J. Chem. Thermodynamics, (1973) 5, 173. J o s t , W., " D i f f u s i o n in S o l i d s , L i q u i d s , Gases", Academic Press, New York, (1960). H i r s c h f e l d e r , J . O., C u r t i s s , C. F., and B i r d , R. Β., "Molecu lar Theory o f Gases and L i q u i d s " , John Wiley and Sons, New York (1954).


146


17. Sutton, L. Ε., "Tables o f Interatomic D i s t a n c e s and C o n f i g u r a t i o n in Molecules and Ions", Spec. Publ. No. 18, Chemical S o c i e t y , Londong (1965). 18. Graham, H. C. and Davis, H. H., J. Amer. Ceram. Soc. (1971) 54, 88. 19. Turkdogan, E. T., Grieveson, P. and Darken, L. S., J. Phys. Chem. (1963) 67, 1647. 20. "Handbook o f Chemistry and P h y s i c s " , 55th E d i t i o n , CRC Press, New York (1974). 21. Hirano, T., N o r e i k i s , S. E. and Waterman, Τ. Ε., Comb. Flame (1974), 22, 353; see a l s o I n t e r i m T e c h n i c a l Report No. 2, P r o j e c t J1139 "Measured v e l o c i t y and temperature p r o f i l e s o f flames spreading over a t h i n combustible solid", I I T Research I n s t i t u t e , Chicago, Illinois, May (1973). 22. H a s t i e , J . W., Comb. Flame (1973) 21, 49. 23. Fenimore, C. P. and M a r t i n , F. J . , "The Mechanisms o f Pyroly sis, Oxidation, and Burning o f Organic M a t e r i a l s " , Nat. Bur. Stand. (U.S.) Spec. Publ. 357 (June 1972). 24. Brewer, L. and Rosenblatt, G. Μ., "Advances in High Tempera t u r e Chemistry", (L. E y r i n g , Ed.) 2, 1 Academic Press, New York (1969). 25. Wagman, D. D., Evans, W. Η., Parker, V. B., Halow, I . , B a i l e y , S. M. and Schumm, P. Η., "Selected Values o f Chemical Thermo dynamic P r o p e r t i e s " , N.B.S. Tech. Note 270-3 (U. S. Govt. P r i n t i n g O f f i c e , Washington, D. C.) (1968). 26. Herzberg, G., " I n f r a r e d and Raman Spectra o f Polyatomic Molecules", Van Nostrand, New York (1945). 27. Nakamoto, Κ., " I n f r a r e d Spectra o f Inorganic and C o o r d i n a t i o n Compounds", John Wiley & Sons, Inc., New York (1963). 28. JANAF T a b l e s , J o i n t Army Navy Air Force Thermochemical T a b l e s , 2nd Ed. NSRDS-NBS 37, U. S. Govt. P r i n t i n g O f f i c e , Washington, D. C. (1971). 29. Wilmshurst, J . K., J. Mol. Spec. (1960) 5, 343. 30. P r e i s s , Η., Z. Anorg. Allg. Chem. (1972) 389, 280. 31. Boerboom, A. J. H., Reyn. H. W., Vugts, H. F. and Kistemaker, J . , "Thermochemistry o f antimony and antimony t r i o x i d e " p. 945 in Advances in Mass Spectrometry 3, Ed. W. L. Mead (1966). 32. Kanzanas, E. K., C h i z h i k o v , D. Μ., Tsvetkov, Yu.V. and O l ' S h e v s k i i , M. V., Russ. J . Phys. Chem. (1973) 47, 871.

4.

HASTiE

AND M C BEE


147


DISCUSSION A. W. BENBOW: Perhaps t h e major d i f f i c u l t y i n h e r e n t i n t h e study o f t h e mechanism o f t h e s y n e r g i s t i c flame-retardant a c t i o n o f metal oxides ( p a r t i c u l a r l y antimony o x i d e ) w i t h h a l o g e n - c o n t a i n i n g compounds i s t h a t i t i s a m u l t i s t a g e p r o c e s s . Dr. H a s t i e ' s work and r e c e n t work c a r r i e d o u t a t t h e C i t y U n i v e r s i t y , London, have c o n c e n t r a t e d on d i f f e r e n t a s p e c t s o f t h e i n h i b i t i o n p r o c e s s and t h e r e s u l t s t e n d t o be complementary. F o l l o w i n g o u r e x p e r i m e n t a l work i n which l a r g e l y u n s u c c e s s f u l attempts were made t o i n c o r p o r a t e a wide range o f m e t a l h a l i d e s i n t o v a r i o u s polymers, i t became o b v i o u s t h a t an i m p o r t a n t s t a g e i n t h e m e t a l o x i d e / h a l o g e n compound i n t e r a c t i o n was t h e r e a c t i o n between them i n t h e condensed phase t o produce v o l a t i l e s p e c i e s such as t h o s e mentioned by Dr. H a s t i e . As an example o f o u r s t u d i e s , a h a l o g e n a t e d compound ( a c t u a l l y I . C . I . C e r e c l o r 70, a c h l o r i n a t e d wax w i t h 70% c h l o r i n e c o n t e n t ) and antimony o x i d e were h e a t e d i n a i r i n a s e n s i t i v e thermobalance, and t h e k i n e t i c s o f t h e v o l a t i l i z a t i o n r e a c t i o n were m o n i t o r e d . V a r i a t i o n o f t h e r a t i o by w e i g h t o f antimony t o h a l o g e n heated t o g e t h e r showed t h a t t h e predominant v o l a t i l i z e d s p e c i e s was t h e trihalide. T h i s was c o n f i r m e d by c h e m i c a l a n a l y s i s . Other h a l o g e n compounds, which v o l a t i l i z e d i n p r e f e r e n c e t o decomposing i n t h e condensed phase, r e q u i r e d h i g h e r than t h e o r e t i c a l r a t i o s o f h a l o g e n : antimony but t h e v o l a t i l i z e d s p e c i e s was t h e t r i h a l i d e i n each case. I was v e r y i n t e r e s t e d t o hear Dr. H a s t i e s t r e s s i n g t h e importance o f antimony o x y h a l i d e s (e.g., SbOCl) i n t h e f l a m e . Such e n t i t i e s have p r e v i o u s l y been c o n s i d e r e d i m p o r t a n t i n t h e condensed phase r e a c t i o n s between antimony o x i d e and h a l o g e n compounds. I n f a c t t h e endothermic breakdown o f antimony o x y c h l o r i d e s i n t h e condensed phase was f r e q u e n t l y p o s t u l a t e d as an i m p o r t a n t p a r t o f t h e i r mode o f a c t i o n . However, o u r p r e s e n t work has shown t h a t t h e r e a c t i o n between antimony o x i d e and h a l o g e n a t e d compounds i s i n f a c t m o d e r a t e l y e x o t h e r m i c . Thus t h e i n v o l v e m e n t o f o x y h a l i d e s i n t h e condensed phase seems u n i m p o r t a n t . When attempts a r e made t o use o t h e r m e t a l o x i d e s

148

HALOGENATED

F I R E SUPPRESSANTS


as s u b s t i t u t e s f o r antimony o x i d e i n r e a l f l a m e retardant compositions the r e s u l t s are g e n e r a l l y very poor. The o n l y e x c e p t i o n s a r e some o f t h e h y d r a t e d m e t a l o x i d e s (aluminum, s t a n n i c , z i n c ) which a r e now b e i n g brought on t o t h e market, and i t seems t h a t g e n e r a l l y t h e s e m e t a l o x i d e s have a d i f f e r e n t mode o f a c t i o n from t h a t o f antimony o x i d e . J . W. HASTIE: I agree w i t h t h e g e n e r a l comments o f Benbow w i t h t h e f o l l o w i n g e x c e p t i o n . Our r e s u l t s do show t h a t o x y c h l o r i d e s a r e p r e s e n t as condensed phase i n t e r m e d i a t e s . T h i s does n o t p r e c l u d e t h e p o s s i b i l i t y o f an o v e r a l l e x o t h e r m i c r e a c t i o n (observed by Benbow and C u l l i s ) between antimony o x i d e and t h e h a l o g e n s o u r c e s i n c e t h e condensed o x y c h l o r i d e f o r m i n g r e a c t i o n s a r e exothermic as shown i n T a b l e I o f o u r paper.

5 Effect of CF Br on Counterflow Combustion of Liquid Fuel with Diluted Oxygen 3

K. SESHADRI and F. A. WILLIAMS


Department of Applied Mechanics and Engineering Sciences, University of California, San Diego, La Jolla, Cal. 92037

T o o b t a i n basic i n f o r m a t i o n on m e c h a n i s m s of fire s u p p r e s s i o n , well c o n t r o l l e d e x p e r i m e n t s o n f l a m e inhibition and extinction are n e e d e d . M o s t such laboratory t e s t s c o n c e r n p r e m i x e d f l a m e s and p r o v i d e information relating to the e f f e c t s of s u p p r e s sants o n b u r n i n g velocity, flammability limits and p r e m i x e d r e a c t i o n - z o n e s t r u c t u r e (1, 2). Y e t fires g e n e r a l l y a r e thought to be d i f f u s i o n flames. A f e w b a s i c s t u d i e s (3-8) h a v e b e e n f o c u s e d on the s t r u c t u r e and e x t i n c t i o n of diffusion flames s u b j e c ted to i n h i b i t i n g a g e n t s . All of t h e s e i n v o l v e gaseous f u e l s instead of the m o r e c o m m o n l y e n c o u n t e r e d liquid fuels. R e c e n t l y a n e x p e r i m e n t has b e e n d e s i g n e d (9) f o r t e s t i n g the e f f e c t s of s u p p r e s s a n t s o n diffusion f l a m e s of liquid f u e l s u n d e r carefully controlled c o n d i t i o n s . The p r e s e n t w o r k p r o v i d e s f u r t h e r q u a n t i t a t i v e d a t a h e l p i n g to v a l i d a t e the t e c h n i q u e and e x p l o i t s the m e t h o d b y a p p l y i n g it to a variety of d i f f e r e n t liquid fuels. C o u n t e r f l o w and c o f l o w i n g c o n f i g u r a t i o n s a r e e m p l o y e d i n the two principal c o m p e t i n g a p p r o a c h e s f o r s t u d y i n g diffusion f l a m e s . The former c o n t a i n s a s t a g n a t i o n p o i n t t o w a r d w h i c h f u e l a n d oxidizer flow from o p p o s i t e directions. F o r the l a t t e r , a n e x a m p l e of which is p r o v i d e d by fuel issuing from a tube i n t o a q u i e s c e n t o x i d i z i n g a t m o s p h e r e , the f u e l and oxidizer s t r e a m s f l o w in a p p r o x i m a t e l y the s a m e direction, and a s t a g n a t i o n p o i n t need not be p r e s e n t , a l t h o u g h o f t e n t h e r e i s a s t a g n a t i o n l i n e i n the gas n e a r the lip of the wall at w h i c h the two s t r e a m s first m e e t . B o t h c o f l o w i n g (3, 4 , 8) and c o u n t e r f l o w (5, 6, 7) c o n f i g u r a t i o n s w e r e u s e d in earlier s t u d i e s of e f f e c t s of inhibitors o n g a s e o u s f u e l s . In c o f l o w i n g s y s t e m s r e s i d e n c e t i m e s and o b s e r v a t i o n v o l u m e s typically are larger, c o n v e c t i o n t o w a r d the f l a m e zone is at a m i n i m u m t h r o u g h o u t m o s t of the field, but c o n c e r n 149


150


c a n a r i s e o v e r p o s s i b l e p r e m i x i n g n e a r the c o o l w a l l w h e r e f u e l and o x i d i z e r g a s e s f i r s t c o m e i n t o c o n t a c t . In c o u n t e r f l o w s y s t e m s c l o s e r c o n f i n e m e n t u s u a l l y i s needed to a s s u r e s t a b i l i t y of the f l o w and r e s u l t s i n s m a l l e r o b s e r v a t i o n v o l u m e s and s h o r t e r r e s i d e n c e t i m e s , c o n v e c t i o n n o r m a l to the r e a c t i o n z o n e a l w a y s is present and affects concentration and t e m p e r a t u r e p r o f i l e s , but the p o s s i b i l i t y of p r e m i x i n g o t h e r than that r e s u l t i n g d i r e c t l y f r o m d i f f u s i o n of r e a c t a n t s t h r o u g h the f l a m e i s e l i m i n a t e d c o m p l e t e l y . T h a t both t y p e s of c o n f i g u r a t i o n s a r e u s e f u l i s d e m o n s t r a t e d by the f a c t that c o n c l u s i o n s d r a w n f r o m c a r e f u l w o r k w i t h b o t h s y s t e m s tend to a g r e e . The p r e s e n t s t u d i e s e m p l o y a counterflow configuration i n which a flat l a m i n a r diffusion flame i s e s t a b l i s h e d i n a s t a g n a t i o n - p o i n t b o u n d a r y l a y e r by d i r e c t i n g the o x i d i z i n g gas s t r e a m d o w n w a r d onto the s u r f a c e of a b u r n i n g liquid fuel. In e x p e r i m e n t s w i t h l i q u i d f u e l s the s u p p r e s s a n t i s added to the o x i d i z i n g g a s . A l t h o u g h t h e r e i s g r e a t e r f r e e d o m i n w o r k i n g w i t h g a s e o u s f u e l s , i n that the s u p p r e s s a n t a l s o c a n be added to the f u e l , r e a l - w o r l d s i t u a t i o n s g e n e r a l l y a r e a p p r o x i m a t e d b e t t e r b y a d d i t i o n to the o x i d i z e r . D e t a i l s of the a p p a r a t u s e m p l o y e d i n the p r e s e n t e x p e r i m e n t s m a y be o b t a i n e d f r o m F i g u r e 1; a d e s c r i p t i o n i s g i v e n b y K e n t a n d W i l l i a m s (9). The p r e s s u r e i s at m o s p h e r i c , and the gas c o n s i s t s of m i x t u r e s of θ £ , N2 and C F 3 B r . The t h i n f l a m e s i t s a f e w m i l l i m e t e r s above the l i q u i d s u r f a c e , w e l l o n the o x i d i z e r s i d e of the s t a g n a t i o n p o i n t f o r a l l stoichiometries achievable with undiluted liquid fuels. Thus, c o n v e c t i v e v e l o c i t i e s i n the f l a m e z o n e a r e d i r e c t e d t o w a r d the f u e l i n a l l t e s t s . The p r e s e n t p a p e r r e p o r t s o n m e a s u r e m e n t s of t e m p e r a t u r e s w i t h i n the l i q u i d f u e l , of g a s - p h a s e t e m p e r a t u r e p r o f i l e s , of c o n c e n t r a t i o n p r o f i l e s f o r m a j o r s t a b l e s p e c i e s , and of e x t i n c t i o n c o n d i t i o n s o b s e r v e d f o r m e t h a n o l , b e n z e n e , i s o - p r o p y l ether, b u t y l - v i n y l ether, methyl methacrylate, polym e t h y l m e t h a c r y l a t e , i s o - o c t a n e , k e r o s e n e and v a r i o u s n o r m a l a l k a n e s . The l a t t e r r e s u l t s p r o v i d e o v e r a l l k i n e t i c i n f o r m a t i o n of a q u a n t i t a t i v e c h a r a c t e r i n the a b s e n c e of Ο Γ β Β τ but as yet enable o n l y q u a l i t a t i v e d e d u c t i o n s to be d r a w n c o n c e r n i n g k i n e t i c s i n the p r e s e n c e of the c h e m i c a l i n h i b i t o r . L i t e r a t u r e for Gaseous Fuels A l l of the e x i s t i n g e v i d e n c e s u g g e s t s that the g a s - p h a s e c o m b u s t i o n p r o c e s s e s a r e s i m i l a r f o r gaseous a n d l i q u i d f u e l s . T h e r e f o r e e a r l i e r r e s u l t s o n c h e m i c a l i n h i b i t i o n of d i f f u s i o n f l a m e s f o r g a s e o u s f u e l s a r e r e l e v a n t to the p r e s e n t i n v e s t i g a t i o n .

SESHADRi A N D W I L L I A M S

Counterflow Combustion

Metered i

V«

—

Baffle


Τ

Oxidizer and

Inert Gas Entry

Scale

Figure 1. Schematic of apparatus

10 mm — '

1


152

H A L O G E N A T E D FERE SUPPRESSANTS

Coflowing Streams. In the early works of Simmons and Wolfhard (3) and Creitz (4) results may be found for gaseous hydrocarbons as well as for carbon monoxide and hydrogen, with the inhibitors CH3 Br, C F 3 B r and Br£. Flame extinction occurred when the added inhibitor concentration i n the fuel or air stream was high enough to lift the flame off the burner r i m . It was found that the suppressants acted more efficiently as extinguishers when added to the a i r side of the flame rather than to the fuel side; this effect was explored i n detail by Creitz (4) for C H 3 B r and C F 3 Br. The result is understandable f r o m the fact that the reaction zone i n diffusion flames seeks out a position where the fuel-oxidizer ratio i s approximately stoichiometr i c . Since there is more oxygen than fuel i n a stoichiometric mixture, there is more suppressant i n the flame zone when it is added to the oxidizer. Simmons and Wolfhard (3) made spectroscopic examinations of the flames and found that the flame structure was considerably altered by addition of the halide inhibitors. Halons increased carbon luminosity on the fuel side of the flame. In the case of methyl bromide added to air, an extra reaction zone was formed on the a i r side, i n which premixed combustion of the inhibitor with a i r occurred when the mixture encountered sufficiently high temperatures. These results and the observation that smaller volumes of halide s than of nitrogen needed to be added to produce extinguishment (3, 4) led to the belief that the effect of the halide inhibitors is chemical i n nature and not merely thermal quenching. More recently, Creitz (8) made composition and temperature measurements i n a coflowing propane-air diffusion flame inhibited with C F 3 B r . The measurements showed that carbon, hydrogen and carbon monoxide were the only fuel species on the air side of the yellow zone, indicated that the oxygen concentration vanished before reaching the yellow zone, and demonstrated that the concentration of C F 3 Br, which was added to the air, dropped to zero well away from the blue flame reaction zone. It was concluded that decomposition of C F 3 B r occurred by chemical means rather than thermally, since the measured temperature when C F 3 B r disappeared was only about 1080°K. It was inferred that inhibiting effects were due to the decomposition products rather than the intact molecule. Counterflow Streams. Friedman and Levy (5) used opposed-jet methane-air diffusion flames to test the effects of alkali metal vapor and organic halide inhibitors. Extinction


5.

SESHADRi AND WILLIAMS


153

o c c u r e d i n the c e n t e r of the f l a m e s h e e t u p o n a p p l i c a t i o n of the i n h i b i t o r , as i s u s u a l f o r o p p o s e d - j e t f l a m e s . I n o u r e x p e r i m e n t , o n the c o n t r a r y e x t i n c t i o n o c c u r s s u d d e n l y a n d c o m p l e t e l y o v e r the e n t i r e s u r f a c e of the l i q u i d p o o l ; the d i f f e r e n c e m a y be a t t r i buted to the f a c t that a f t e r b u r n i n g i s q u e n c h e d b y a w a t e r s p r a y i n o u r a p p a r a t u s , w h i l e l o n g e r r e s i d e n c e t i m e s at e l e v a t e d t e m p e r a t u r e s a r e a v a i l a b l e at the s i d e s i n t y p i c a l o p p o s e d - j e t t e s t s . F r i e d m a n and L e v y ( 5 ) g i v e c u r v e s of i n h i b i t o r v o l u m e c o n c e n t r a t i o n s at v a r y i n g f l o w r a t e s , s h o w i n g that o n t h i s b a s i s , w h e n a d d e d to the f u e l s t r e a m C F 3 B r and CH3 B r a r e m o r e e f f e c t i v e t h a n CH^Ct and C C t ^ and c o n s i d e r a b l y m o r e effective than I b i r i c u and G a y d o n (6) m a d e s p e c t r o g r a p h ! c s t u d i e s a n d t e m p e r a t u r e m e a s u r e m e n t s of i n h i b i t e d h y d r o c a r b o n and h y d r o g e n f l a m e s . T h e i r r e s u l t s s h o w that the h a l i d e i n h i b i t o r s g e n e r a l l y d e c r e a s e the e m i s s i o n of O H and H C O a n d i n c r e a s e the e m i s s i o n of C a r b o n l u m i n o s i t y i n c r e a s e s and temperature decreases with increasing inhibitor concentration. M i l n e , G r e e n a n d B e n s o n (7) m e a s u r e d the e f f e c t i v e n e s s of v a r i ous h a l i d e i n h i b i t o r s and p o w d e r s o n h y d r o c a r b o n f u e l s . Their c u r v e s of gas v e l o c i t y v e r s u s m o l e f r a c t i o n of i n h i b i t o r r e q u i r e d to e x t i n g u i s h the f l a m e , s h o w that C F 3 B r i s m o r e e f f e c t i v e t h a n CH3 B r a n d c o n f i r m that the i n h i b i t o r s a r e m o r e e f f e c t i v e w h e n added to the a i r s i d e . The E x p e r i m e n t a n d i t s I n s t r u m e n t a t i o n T h e a p p a r a t u s e m p l o y e d i n the p r e s e n t t e s t s d i f f e r e d f r o m that d e s c r i b e d p r e v i o u s l y (9) i n o n l y one r e s p e c t . The f u e l f e e d s y s t e m w a s r e p l a c e d b y a n a r r a n g e m e n t s i m i l a r to the B a j p a i f e e d d e v i c e (10 , 11). T h i s s i m p l i f i e d the o p e r a t i o n , e l i m i n a t i n g the n e e d to r u n the p e r i s t a l t i c p u m p and h e a t e x c h a n g e r d u r i n g the t e s t s ; the p u m p w a s r u n o n l y i n i t i a l l y to f i l l the B a j p a i l e v e l ing r e s e r v o i r . T h i s m o d i f i c a t i o n c a u s e d no o b s e r v a b l e d i f f e r ences i n e x p e r i m e n t a l r e s u l t s . Temperature Measurement. Liquid temperatures were m e a s u r e d w i t h c h r o m e l - a l u m e l t h e r m o c o u p l e s and gas t e m p e r a t u r e s w i t h c o a t e d P t - P t 10% R h t h e r m o c o u p l e s (12). Wire d i a m e t e r s of 0. 076 m m w e r e e m p l o y e d f o r the t h e r m o c o u p l e s u s e d i n the p r e s e n t t e s t s . W i t h i n the l i q u i d , t h i s p r o v i d e d e x c e l l e n t s p a t i a l r e s o l u t i o n , and m e a s u r e m e n t s w i t h d i f f e r e n t l e a d c o n f i g u r a t i o n s d e m o n s t r a t e d that c o n d u c t i v e l o s s e s w e r e n e g l i g i b l e . W i t h i n the g a s , c o n d u c t i v e l o s s e s w e r e m a d e n e g l i g i b l e b y a l i g n i n g the l e a d s p a r a l l e l to the f l a m e sheet so that t h e y e x tended a l o n g a n i s o t h e r m f o r a l e n g t h of a p p r o x i m a t e l y 5 m m .

154

H A L O G E N A T E D FIRE

SUPPRESSANTS


A l t h o u g h s l i g h t m i s a l i g n m e n t has no e f f e c t , e r r o r s up to 200°C c a n o c c u r i f the l e a d s a r e o r i e n t e d n o r m a l to the f l a m e s h e e t . The n o n c a t a l y t i c c o a t i n g (12) c a n be i m p o r t a n t i n the d i f f u sion flame; a bare thermocouple indicated a flame temperature about 200°C h i g h e r t h a n a s i m i l a r c o a t e d t h e r m o c o u p l e . A c o r r e c t i o n f o r r a d i a n t l o s s of e n e r g y a l s o i s i m p o r t a n t i n the f l a m e . The t w o - t h e r m o c o u p l e m e t h o d w a s e m p l o y e d , w i t h a l i n e a r e x t r a p o l a t i o n to z e r o w i r e d i a m e t e r , f o r c a l i b r a t i n g t h e r m o c o u p l e s of the s i z e e m p l o y e d . R a d i a t i o n c o r r e c t i o n s d i d not e x c e e d 150°C. E s t i m a t e d a c c u r a c i e s of f i n a l c o r r e c t e d t e m p e r a t u r e s a r e ± 50°C i n the f l a m e . C o m p o s i t i o n M e a s u r e m e n t . C o n c e n t r a t i o n s of g a s e o u s s p e c i e s w e r e m e a s u r e d by gas c h r o m a t o g r a p h i c a n a l y s i s of samples w i t h d r a w n through fine quartz p r o b e s . Data w e r e obtained for 0 , N , H 0 , C 0 , C O , H , C H , C H , C H , C H £ , C3H6, C F ^ B r a n d the f u e l . The c h r o m a t o g r a p h c o n t a i n e d two 6 . 4 m m d i a m e t e r c o l u m n s , e a c h 305 c m l o n g , one a P o r a p a k Q and the o t h e r a m o l e c u l a r s i e v e 5 A . A v a l v e a l l o w e d the m o l e c u l a r s i e v e to be e i t h e r i s o l a t e d o r c o n n e c t e d i n s e r i e s d o w n s t r e a m f r o m the P o r a p a k c o l u m n . The o v e n w a s o p e r a t e d at a c o n s t a n t t e m p e r a t u r e of 80°C, and the c a r r i e r gas w a s h e l i u m f l o w i n g at a r a t e of 80 m l / m i n . A t h e r m a l c o n d u c t i v i t y c e l l s e r v e d as the d e t e c t o r . C a l i b r a t i o n of the i n s t r u m e n t w a s p e r f o r m e d b y m i x i n g g a s es w i t h n i t r o g e n i n a 1 l i t e r p r e s s u r e c y l i n d e r . The c o m p o s i t i o n s of the m i x t u r e s w e r e d e t e r m i n e d f r o m p a r t i a l p r e s s u r e s w h i c h w e r e r e a d o n a m e r c u r y m a n o m e t e r . A f t e r a l l o w i n g the g a s e s t i m e to m i x t h o r o u g h l y i n the c y l i n d e r , the m i x t u r e w a s i n t r o d u c e d v i a the s a m p l i n g v a l v e to the 3 m l s a m p l i n g l o o p of the gas c h r o m a t o g r a p h . T h e s a m p l i n g l o o p , as w e l l as the s a m p l i n g and c o l u m n s w i t c h i n g v a l v e s , a r e l o c a t e d i n the g a s c h r o m a t o g r a p h o v e n and a r e at o v e n t e m p e r a t u r e to p r e v e n t c o n d e n s a t i o n of v a p o r s i n the s a m p l e . The a r e a s of the c h r o m a t o g r a m s w e r e a p p r o x i m a t e d b y the p r o d u c t of the p e a k h e i g h t and w i d t h at h a l f h e i g h t . A s c h e m a t i c f l o w d i a g r a m of the s a m p l i n g s y s t e m i s s h o w n i n F i g u r e 2. S a m p l e s a r e w i t h d r a w n f r o m the f l a m e w i t h a q u a r t z m i c r o p r o b e of the type d e s c r i b e d b y F r i s t r o m and W e s t e n b e r g (13). The p r o b e has a s o n i c o r i f i c e w i t h a t h r o a t d i a m e t e r of about 60 m i c r o n s . A f l e x i b l e t e f l o n tube c o n n e c t s the p r o b e to a s t a i n l e s s - s t e e l tube of l a r g e r d i a m e t e r , w h i c h t h e n l e a d s to the o v e n - m o u n t e d s a m p l i n g v a l v e and the s a m p l i n g l o o p of the gas c h r o m a t o g r a p h . The c o m p l e t e s a m p l i n g l i n e , e x c e p t 2

2

2

2

2

2

4

2

2

2

4



(no.


0>_>

0D>

α. ο en σ Ε ο

.So

sf

Ν

^1.

ω σ

I

4

S s'J* Θ

3

Is

CO Û .

-

-

156


SUPPRESSANTS

f o r the p r o b e i t s e l f , i s w r a p p e d i n h e a t i n g tape to a v o i d c o n d e n s a t i o n of the s a m p l e o n the w a l l s . If c o n d e n s a t i o n o c c u r s i n the p r o b e , i t i s e a s i l y s e e n and m a y be d i s p e l l e d b y t e m p o r a r i l y h e a t i n g the p r o b e e x t e r n a l l y w i t h a b u n s e n f l a m e . D o w n s t r e a m of the gas c h r o m a t o g r a p h , the s a m p l i n g l i n e l e a d s to the s u c t i o n pump. S a m p l e s w e r e w i t h d r a w n f r o m the f l a m e at a l i n e p r e s s u r e of 63. 5 c m H g b e l o w a t m o s p h e r i c . A s a m p l i n g t i m e of about 4 m i n u t e s i s r e q u i r e d w i t h a 60 m i c r o n p r o b e . D u r i n g t h i s t i m e , the s a m p l e f l u s h e s t h r o u g h the e n t i r e f l o w l i n e , i n c l u d i n g the s a m p l i n g l o o p of the gas c h r o m a t o g r a p h . T h i s p r o c e d u r e l e a d s to good r e p e a t a b i l i t y f o r H 0 m e a s u r e m e n t . W a t e r tends to a d s o r b o n the w a l l s of the s a m p l i n g l i n e , and the s y s t e m m u s t be " c o n d i t i o n e d " i n t h i s w a y to o b t a i n r e l i a b l e r e s u l t s . A f t e r the s a m p l i n g s y s t e m h a s b e e n c o m p l e t e l y f l u s h e d , the s u c t i o n v a l v e , d o w n s t r e a m of the s a m p l i n g l o o p , i s c l o s e d and the l i n e i s q u i c k l y b r o u g h t to a t m o s p h e r i c p r e s s u r e b y o p e n i n g the a i r v a l v e s h o w n i n F i g u r e 2. The tube v o l u m e ahead of the s a m p l i n g v a l v e i s l a r g e enough to p r e v e n t a i r f r o m r e a c h i n g the s a m p l i n g l o o p w h e n p r e s s u r i z a t i o n t a k e s p l a c e . W h e n the s y s t e m h a s r e a c h e d a t m o s p h e r i c p r e s s u r e , the s a m p l i n g v a l v e i s s w i t c h e d to s w e e p the s a m p l e onto the c o l u m n s . T o a n a l y z e the s a m p l e , the c o l u m n s i n i t i a l l y a r e a d j u s t e d so that the m o l e c u l a r s i e v e i s i n s e r i e s w i t h the P o r a p a k Q . D u r i n g the f i r s t s i x t y s e c o n d s , the s p e c i e s H , 0 , N , C O , and C H ^ e l u t e f r o m the P o r a p a k Q a n d a r e s w e p t onto the m o l e c u l a r s i e v e . A t 60 s e c o n d s a f t e r the s a m p l e i s i n t r o d u c e d , the m o l e c u l a r s i e v e i s i s o l a t e d . The s p e c i e s C 0 , C H , C H , C H £ , H 0 and C ^ H ^ elute f r o m the P o r a p a k Q c o l u m n i n that o r d e r a n d go to the d e t e c t o r . T h e n at about s e v e n m i n u t e s a f t e r the s a m p l e w a s i n t r o d u c e d , the m o l e c u l a r s i e v e c o l u m n i s a g a i n s w i t c h e d on l i n e , and the s p e c i e s H , 0 , N , C H ^ , a n d C O a r e e l u t e d i n that o r d e r . A t about f o u r t e e n m i n u t e s a f t e r the s a m p l e i s i n t r o d u c e d , the m o l e c u l a r s i e v e c o l u m n i s s w i t c h e d out and the P o r a p a k Q c o l u m n i s r e v e r s e f l u s h e d . T h i s c a u s e s the h e a v y c o m ponent, the f u e l , to be e l u t e d , w h i c h c o m p l e t e s the a n a l y s i s . A c c u r a c i e s of c o m p o s i t i o n m e a s u r e m e n t s a r e e s t i m a t e d to be better than ±10%.


2

2

2

2

2

2

2

2

2

2

4

2

)

2

2

T h e r m a l E n v i r o n m e n t of the L i q u i d M u c h w o r k has s h o w n that the t h e r m a l e n v i r o n m e n t of the l i q u i d i s i m p o r t a n t i n e x t i n g u i s h m e n t of b u r n i n g p o o l s (14, 15). S i n c e the o b j e c t i v e of the p r e s e n t study i s to e x t r a c t i n f o r m a t i o n c o n c e r n i n g g a s - p h a s e k i n e t i c s , c o n s i d e r a t i o n m u s t be g i v e n to


5.



157

p o s s i b l e t h e r m a l i n f l u e n c e s f r o m the c o n d e n s e d p h a s e . A n a l y s i s of the data i s b a s e d o n a p a r a l l e l t h e o r e t i c a l e f f o r t (16) i n w h i c h the i n f l u e n c e of h e a t i n g and v a p o r i z a t i o n of the f u e l i s t a k e n i n t o a c c o u n t . H o w e v e r , the t h e o r y i g n o r e s l a t e r a l l i q u i d m o t i o n and h e a t l o s s f r o m the l i q u i d to i t s s u r r o u n d i n g s . T o t e s t the a c c u r a c y of t h e s e a p p r o x i m a t i o n s , t e m p e r a t u r e m e a s u r e m e n t s w i t h i n the l i q u i d w e r e m a d e f o r r e p r e s e n t a t i v e f u e l s . H o r i z o n t a l p r o f i l e s of t e m p e r a t u r e s at v a r i o u s depths w i t h i n the l i q u i d a r e s h o w n i n F i g u r e 3 f o r m e t h y l m e t h a c r y l a t e . Corr e s p o n d i n g v e r t i c a l t e m p e r a t u r e p r o f i l e s a l o n g the c e n t e r l i n e and at the w a l l of the cup a r e s h o w n i n F i g u r e 4 . It i s s e e n that the m a x i m u m l i q u i d t e m p e r a t u r e i s a c h i e v e d at the top n e a r the r i m of the c u p and the m i n i m u m o n the b o t t o m at the c e n t e r . T h e t e m p e r a t u r e s a r e c o n s i s t e n t w i t h the f a c t that the c u p i s c o o l e d w i t h w a t e r f r o m the b o t t o m . The t h e r m a l c o n d u c t i v i t y of the m e t a l w a l l s e x c e e d s that of the l i q u i d , t h e r e b y a c c o u n t i n g f o r the s h a l l o w e r g r a d i e n t a l o n g the w a l l . T h i s c a u s e s h e a t to be t r a n s f e r r e d f r o m the l i q u i d to the s i d e w a l l n e a r the top of the c u p and f r o m the s i d e w a l l to the l i q u i d n e a r the b o t t o m ( F i g u r e 3). O b s e r v a t i o n s of the m o t i o n of d u s t o n the s u r f a c e of the l i q u i d i n d i c a t e a r a d i a l l y o u t w a r d f l o w at v e l o c i t i e s of a f e w c m / s , s m a l l c o m p a r e d w i t h gas v e l o c i t i e s . T h i s o u t w a r d f l o w and the t e m p e r a t u r e p r o f i l e s s u g g e s t the e x i s t e n c e of a w e a k t o r o i d a l v o r tex i n the l i q u i d . The v o r t e x m a y be d r i v e n b y s h e a r at the g a s l i q u i d i n t e r f a c e due to the gas f l o w , b y f l o w f r o m the n e a r l y c e n t r a l l y l o c a t e d f u e l i n l e t i n the b o t t o m of the c u p (a v e r y s m a l l effect) o r by b u o y a n c y f o r c e s due to the n o n u n i f o r m t e m p e r a t u r e of the l i q u i d . W i t h r e s p e c t to the l a s t e f f e c t , the v e r t i c a l s t r a t i f i c a t i o n a l w a y s i s s t a b l e , but the l o w e r t e m p e r a t u r e a l o n g the u p p e r w a l l , i n c o m p a r i s o n w i t h the tempe r a t u r e s of the n e a r b y l i q u i d , c a n gene r a t e h o r i z o n t a l p r e s s u r e g r a d i e n t s that tend to p r o d u c e m o t i o n i n the o b s e r v e d d i r e c t i o n . T h e t e m p e r a t u r e j u s t b e l o w the l i q u i d - g a s i n t e r f a c e i s s e e n i n F i g u r e 3 to p e a k n e a r the s i d e w a l l . C o n t i n u e d h e a t i n g of t r a n s v e r s e l y c o n v e c t e d l i q u i d m a y c o n t r i b u t e to t h i s , but the p r i n c i p a l c a u s e i s b e l i e v e d to be that due to i n c r e a s e d gas f l o w v e l o c i t i e s p r o d u c e d p a r t i a l l y b y e x h a u s t s u c t i o n , the f l a m e i s s l i g h t l y c l o s e r to the l i q u i d n e a r the p e r i p h e r y . T h i s i n c r e a s e s the l o c a l h e a t t r a n s f e r f r o m the gas to the l i q u i d . The i n t e r p r e t a t i o n i s s t r e n g t h e n e d by F i g u r e 5, w h i c h s h o w s that m o v i n g the f l a m e c l o s e r to the l i q u i d s u r f a c e at the s t a g n a t i o n p o i n t b y i n c r e a s i n g the m a s s f r a c t i o n of o x y g e n i n the gas s t r e a m , has the e f f e c t of r e d u c i n g the d i f f e r e n c e b e t w e e n n e a r - s u r f a c e l i q u i d t e m p e r a t u r e s at the c e n t e r and o n the p e r i p h e r y .


Ί

1

1

J

J

1

1

Γ

H0RIZ0NTAL TEMPERATURE PROFILES AT VARIOUS DEPTHS FUEL METHYL METHACRYLATE :

N IN 0 2

2

Y 2= 0.183 , U = 0.53 m/sec 0

70-

,

ΤΤ

JÊ

ί

15 16

kl


"TV -42all dimensions in mm

2QL

12

16

20

28

24

32

30

HORIZONTAL DISTANCE (mm)

Figure 3. Horizontal profiles of liquid temperature at various depths for methyl methacrylate burning in an 0 -N mixture having Y 2 = 0.183 with U = 0.53 m/s 2

1

1

1

1

0

2

1

1

r

VERTICAL TEMPERATURE PROFILES METHYL METHACRYLATE N IN 0 2

ο FUEL SIDE OF THE RIM

2

Υθ2= 0.183

{

β 21 mm FROM RIM

U = 0.53 m/sec HEPTANE

I 20

N

2

Y

02

IN 0

2

= 0.185

J • 15 mm FROM RIM

U =0.305 m/s

30

40

50

90

60

TEMPERATURE (°C)

Figure 4. Vertical profiles of liquid temperature for methyl metha crylate and heptane burning in O -N mixtures g

2



88 FUEL METHYL METHACRYLATE N IN 0 :

2

2

U =0.53 m/s

80-

1.5 mm

;j

τ 1 * 70


I mm

16mm |«_2i mm-

-42 mm-

60 0.17

0.18

0.19

0.20

0.21

OXYGEN MASS FRACTION IN STREAM

Figure 5. Dependence of liquid temperatures on oxygen content of gas stream at U = 0.53 m/s for 0 -N mixtures 2

2

Figure 6. New extinction curves for various liquid fuels


160


SUPPRESSANTS

The m e a s u r e d t e m p e r a t u r e g r a d i e n t s w e r e u s e d to e s t i m a t e the h e a t l o s s f r o m the l i q u i d to i t s s u r r o u n d i n g s . V a l u e s l e s s than 0. 5 c a l / s w e r e o b t a i n e d ; t h i s i s l e s s t h a n 5 % of the h e a t f l o w r e q u i r e d to v a p o r i z e the f u e l . E r r o r s of t h i s m a g n i t u d e , e v e n i f s y s t e m a t i c a l l y dependent o n c o n d i t i o n s of gas f l o w , a r e too s m a l l to i n f l u e n c e c a l c u l a t e d k i n e t i c p a r a m e t e r s s i g n i f i c a n t l y . Theref o r e i t w a s c o n c l u d e d that v a r i a t i o n s i n the t h e r m a l e n v i r o n m e n t of the l i q u i d a f f e c t d e d u c e d g a s - p h a s e k i n e t i c q u a n t i t i e s n e g l i g i b l y i n the p r e s e n t e x p e r i m e n t s . A n additional o b s e r v a t i o n was made to strengthen this conc l u s i o n a n d to i n d i c a t e f u r t h e r that the l i q u i d m o t i o n p e r se w a s u n i m p o r t a n t . E x t i n c t i o n c u r v e s f o r heptane i n O2 - N2 m i x t u r e s w e r e g e n e r a t e d f o r c o n d i t i o n s u n d e r w h i c h the c u p w a s f i l l e d w i t h s t e e l w o o l and s c r e e n i n g to a h e i g h t 1 m m b e l o w the l i q u i d - g a s s u r f a c e . W i t h i n the a c c u r a c y of r o t a m e t e r m e a s u r e ment, extinction conditions w e r e i d e n t i c a l w i t h those obtained i n the a b s e n c e of t h i s f l o w - i n h i b i t i n g , c o n d u c t i o n - e n h a n c i n g m o d i fication. R e s u l t s f o r L i q u i d F u e l s i n M i x t u r e s of N i t r o g e n w i t h O x y g e n S i n c e the e f f e c t of n i t r o g e n a d d i t i o n i s k n o w n to be p u r e l y t h e r m a l , a n u n d e r s t a n d i n g of the i n f l u e n c e of n i t r o g e n m u s t be d e v e l o p e d b e f o r e a c o m p a r a b l y t h o r o u g h u n d e r s t a n d i n g of e f f e c t s of c h e m i c a l i n h i b i t o r s c a n be o b t a i n e d . E x p e r i m e n t s w i t h n i t r o g e n a d d i t i o n h e l p t o v a l i d a t e the a p p r o a c h . A n u m b e r of r e s u l t s w i t h n i t r o g e n , beyond those g i v e n e a r l i e r (9), h a v e r e c e n t l y b e e n o b t a i n e d and a r e r e p o r t e d h e r e . N i t r o g e n Q u e n c h i n g . The s i m p l e s t m e a s u r e m e n t s to o b t a i n a r e the v e l o c i t y of the o x i d i z i n g s t r e a m at e x t i n c t i o n as a f u n c t i o n of the c o m p o s i t i o n of the s t r e a m . N e w data of t h i s t y p e , g e n e r a t e d f o r v a r i o u s f u e l s , a r e g i v e n i n F i g u r e 6. T h i s d a t a , as w e l l as d a t a o b t a i n e d e a r l i e r f o r o t h e r f u e l s , a r e s u m m a r i z e d i n F i g u r e 7. The m e a s u r e m e n t s a r e h i g h l y r e p r o d u c i b l e o n t h e s e g r a p h s ; s y s t e m a t i c d i f f e r e n c e s i n the t h i r d d i g i t f o r the e x t i n g u i s h m e n t v a l u e of Y02 at a g i v e n gas f l o w r a t e w e r e o b s e r v e d f o r d i f f e r e n t s a m p l e s of m e t h y l m e t h a c r y l a t e and of h e p t a n e . The o x y g e n m a s s f r a c t i o n w a s s e l e c t e d as the v e r t i c a l c o o r d i n a t e b e c a u s e i n the e a r l i e r w o r k (9) a c o r r e l a t i o n w a s o b t a i n e d o n t h i s b a s i s f o r e x t i n g u i s h m e n t w i t h the d i f f e r e n t i n e r t d i l u e n t s N2 a n d CO2 . It i s c l e a r f r o m F i g u r e s 6 and 7 that data f o r d i f f e r e n t f u e l s do not c o r r e l a t e o n s u c h a g r a p h . M o r e o v e r , s y s t e m a t i c v a r i a t i o n s w i t h e i t h e r s t o i c h i o m e t r y o r e x p e c t e d r e a c t i v i t y a r e not e v i d e n t . S i m i l a r g r a p h s w e r e p r e p a r e d u s i n g m o l e f r a c t i o n s and a l s o


5.

SESHADRi

AND WILLIAMS


161

u s i n g s t o i c h i o m et r i c a l l y a d j u s t e d q u a n t i t i e s ( m e a s u r i n g m o l a r and m a s s c o m p o s i t i o n s i n the f l a m e z o n e ) , but no b e t t e r c o r r e lations were obtained. S i n c e the e f f e c t s of n i t r o g e n a r e t h e r m a l , i t i s m o r e l o g i c a l to s e e k a c o r r e l a t i o n on the b a s i s of the a d i a b a t i c f l a m e t e m p e r a t u r e . E q u i l i b r i u m d i s s o c i a t i o n i n f l u e n c e s the a d i a b a t i c f l a m e t e m p e r a t u r e a p p r e c i a b l y o n l y at the h i g h e r c a l c u l a t e d t e m p e r a t u r e s (above 2 0 0 0 ° K ) , but r e g a r d l e s s of w h e t h e r d i s s o c i a t i o n i s t a k e n i n t o a c c o u n t , the dependence of the a d i a b a t i c f l a m e t e m p e r a t u r e o n the e x t i n c t i o n v e l o c i t y i s f o u n d to be q u a l i t a t i v e l y s i m i l a r to the d e p e n d e n c e of o x y g e n m a s s f r a c t i o n s h o w n i n F i g u r e s 6 and 7; the c o r r e l a t i o n i s no b e t t e r . The d i f f i c u l t y i s that the e x t i n c t i o n v e l o c i t y a l o n e i s not a s u i t a b l e p a r a m e t e r f o r c h a r a c t e r i z i n g d i f f e r e n c e s i n the s t r u c t u r e of the s t a g n a t i o n point boundary l a y e r for different fuels. T h e t h e o r y (16) o n w h i c h a n a l y s i s of the e x t i n c t i o n e x p e r i m e n t s i s b a s e d h a s b e e n s u m m a r i z e d e a r l i e r (9). The f l a m e sheet a p p r o x i m a t i o n i s e m p l o y e d i n l o w e s t o r d e r to d e s c r i b e the s t a g n a t i o n - p o i n t f l o w f i e l d . The e f f e c t of f i n i t e - r a t e c h e m i s t r y i s t h e n i n c o r p o r a t e d a s a s i n g u l a r p e r t u r b a t i o n . The c o m p l e x c h e m i c a l k i n e t i c s i n the f l a m e a r e a p p r o x i m a t e d as a o n e - s t e p A r r h e n i u s p r o c e s s , of f i r s t o r d e r w i t h r e s p e c t to both f u e l and o x i d i z e r . The j u s t i f i c a t i o n for this a p p r o x i m a t i o n , g r o s s f r o m the v i e w p o i n t of u n d e r l y i n g c h e m i s t r y , i s that e x t i n c t i o n t e s t s c a n p r o v i d e at m o s t two p a r a m e t e r s , a n o v e r a l l a c t i v a t i o n e n e r gy and a n o v e r a l l p r e - e x p o n e n t i a l r a t e f a c t o r . C l e a r l y a f o r m i d able task w i l l r e m a i n i n r e l a t i n g these o v e r a l l rate p a r a m e t e r s to c h e m i c a l m e c h a n i s m s . H o w e v e r , the g r o s s d e s c r i p t i o n i t s e l f c a n be u s e f u l , e v e n i n the a b s e n c e of f u r t h e r f u n d a m e n t a l e l u c i d a t i o n . The r a t e p a r a m e t e r s o b t a i n e d w i l l be s p e c i f i e d to d i f f u s i o n f l a m e s and a s s u c h s h o u l d be a p p l i c a b l e b e t t e r t h a n a v a i l a b l e o v e r a l l p r e m i x e d r a t e p a r a m e t e r s to d i f f u s i o n f l a m e s i n other configurations. T h e t h e o r y (16) p r e d i c t s that U F M p / T (see N o m e n c l a t u r e ) at e x t i n c t i o n s e r v e s to e x t r a c t the o v e r a l l c h e m i c a l k i n e t i c s , b e i n g p r o p o r t i o n a l to the p r e - e x p o n e n t i a l f a c t o r ( c m ^ / m o l e sec) t i m e s the A r r h e n i u s f a c t o r . T h e f a c t o r F a c c o u n t s f o r the i n f l u e n c e s of the s t a g n a t i o n - p o i n t f l o w f i e l d w i t h v a p o r i z a t i o n . It i s c a l c u l a t e d f r o m c o m p u t e r s o l u t i o n s f o r the f l o w i n the f l a m e sheet a p p r o x i m a t i o n , and i t depends on p h y s i c o - c h e m i c a l p r o p e r t i e s of the f u e l . The d e f i n i t i o n of F e m p l o y e d h e r e i s that of K r i s h n a m u r t h y and W i l l i a m s (_16) and d i f f e r s s l i g h t l y f r o m that of K e n t and W i l l i a m s (9). F u e l s h a v i n g the s a m e p r e - e x p o n e n t i a l f a c t o r a n d a c t i v a t i o n e n e r g y w i l l f o l l o w the s a m e e x t i n c t i o n


162


c u r v e i n a p l o t of a d i a b a t i c f l a m e t e m p e r a t u r e as a f u n c t i o n of U F M p / T ^ . S i n c e a c t u a l f l a m e t e m p e r a t u r e s at e x t i n c t i o n a r e so l o w that e q u i l i b r i u m d i s s o c i a t i o n i s n e g l i g i b l e , f r o m the v i e w p o i n t of o v e r a l l e n e r g e t i c s i t i s p r o p e r to e x c l u d e d i s s o c i a t i o n w h e n c a l c u l a t i n g the a d i a b a t i c f l a m e t e m p e r a t u r e f o r use i n c o n n e c t i o n w i t h the t h e o r y . T h e r e s u l t i n g p l o t i s s h o w n i n F i g u r e 8. F r o m t h i s g r a p h , i t f i n a l l y b e c o m e s p o s s i b l e to i d e n t i f y t r e n d s in chemical reactivity. C o n t r a r y to F i g u r e s 6 and 7, i t i s s e e n that w i t h the e x c e p t i o n of the l i n e f o r i s o p r o p y l e t h e r , a l l l i n e s i n F i g u r e 8 h a v e a p p r o x i m a t e l y the s a m e s l o p e . T h i s t u r n s out to i m p l y r o u g h l y e q u a l o v e r a l l a c t i v a t i o n e n e r g i e s f o r t h e s e f u e l s . The h i g h e r c u r v e s i m p l y e x t i n g u i s h m e n t at h i g h e r a d i a b a t i c f l a m e t e m p e r a t u r e s and t h e r e f o r e c o r r e s p o n d to l e s s r e a c t i v e f u e l s . It i s s e e n that l i n e s f o r the n o r m a l a l k a n e s and a l s o f o r benzene n e a r l y c o i n c i d e , s u g g e s t i n g that t h e s e s p e c i e s have a p p r o x i m a t e l y e q u a l r e a c t i v i t i e s . K e r o s e n e and p a r t i c u l a r l y i s o - o c t a n e e x t i n g u i s h at h i g h e r f l a m e t e m p e r a t u r e s , i m p l y i n g that t h e s e f u e l s a r e s o m e w h a t l e s s r e a c t i v e . The o x y g e n a t e d f u e l s a r e s e e n to be a p p r e c i a b l y m o r e r e a c t i v e as m i g h t be e x p e c t e d . The v a r i a t i o n i n o v e r a l l a c t i v a t i o n e n e r g y i s m u c h g r e a t e r f o r the o x y g e n a t e d f u e l s , and b e c a u s e of t h i s the d i f f e r e n c e s i n t h e i r r e a c t i v i t i e s a r e l e s s t h a n F i g u r e 8 s u g g e s t s ; the cube of the a c t i v a t i o n e n e r g y b e l o n g s i n the n u m e r a t o r i n s i d e the l o g a r i t h m i f r e a c t i v i t i e s of f u e l s w i t h d i f f e r e n t a c t i v a t i o n e n e r g i e s a r e to be c o m p a r e d (16). T h i s c o r r e c t i o n f o r a c t i v a t i o n e n e r g y shows that butyl v i n y l ether i s l e s s reactive and i s o p r o p y l ether m o r e r e a c t i v e t h a n F i g u r e 8 w o u l d i n d i c a t e . The d i f f e r e n c e b e t w e e n the o v e r a l l r e a c t i o n r a t e s at a n y g i v e n t e m p e r a t u r e f o r the l e a s t and m o s t r e a c t i v e of a l l of the f u e l s i s not q u i t e one o r d e r of magnitude. It i s t e m p t i n g to s e e k c o r r e l a t i o n s w i t h the c a r b o n to h y d r o gen r a t i o C / H of the f u e l . If t h i s i s done b y p l o t t i n g the a d i a b a t i c f l a m e t e m p e r a t u r e at e x t i n c t i o n as a f u n c t i o n of C / H f o r a f i x e d v a l u e of U , t h e n a v e r y r o u g h c o r r e l a t i o n i s o b t a i n e d , i n w h i c h the e x t i n c t i o n f l a m e t e m p e r a t u r e i n c r e a s e s w i t h C / H . If the b e t t e r a p p r o a c h of k e e p i n g U F M p / T ^ ( i n s t e a d of U) f i x e d i s e m p l o y e d , t h e n t h e r e i s no c o r r e l a t i o n f o r the c o m p l e t e set of f u e l s , a l t h o u g h c o r r e l a t i o n s of s u b c l a s s e s ( e . g . , o x y genated f u e l s and n o r m a l a l k a n e s ) a p p e a r , i n w h i c h the e x t i n c tion flame temperature decreases with increasing C / H . F r o m the f u n d a m e n t a l v i e w p o i n t , C / H c o r r e l a t i o n s a r e to be e x p e c ted o n l y f o r s u b c l a s s e s , i f at a l l . A s y e t , too f e w f u e l s h a v e b e e n t e s t e d to a s c e r t a i n w h e t h e r s u c h c o r r e l a t i o n s a c t u a l l y e x i s t .




VELOCITY (m/s)

Figure 7. Summary of extinction curves for all liquid fuels tested

2400i

1

1

Γ

Figure 8. Dependence of adiabatic flame temperature on UFM /T for various liquid fuels F

2

164


Table I


Fuel

T h e r m a l P r o p e r t i e s of F u e l s (JL7, jy$,19, 20)

H e a t of Formation (gas) at 25°C kcal/mole

Latent Heat at N o r m a l Boiling Point

Boiling Point

cal/gm

Benzene

19.82

94

Iso-propyl Ether

68.40 (Bond Energy Calc. )

Butyl Vinyl Ether

33.28 (Bond Energy Calc. )

Heptane

44.89

75

Methyl Methacry late

81.46

85

L i q u i d Heat C a p a c i t y at Boiling Point cal/gm°C

80.1

0.456

80 (Same as D i propyl Ether)

68

0.55 (Same as D i propyl Ether)

74 (Same as D i propyl Ether)

93

0.58 (Same as D i propyl Ether)

98.4

0.602

101

0.46

R a t e c o n s t a n t s a r e d e r i v e d f r o m A r r h e n i u s p l o t s of the type s h o w n i n F i g u r e 9. F r o m the t h e o r e t i c a l d e v e l o p m e n t (16) i t i s k n o w n that c a l c u l a t e d r a t h e r t h a n m e a s u r e d f l a m e t e m p e r a t u r e s a r e to be u s e d i n the c o m p u t a t i o n s . The m e a s u r e d f l a m e t e m p e r a t u r e s , s h o w n i n F i g u r e 9, o b t a i n e d u n d e r c o n d i t i o n s n e a r e x t i n c t i o n a r e 4 5 0 ° C to 500°C b e l o w the a d i a b a t i c v a l u e s , i n a g r e e m e n t w i t h e a r l i e r o b s e r v a t i o n s f o r heptane (9); the d i f f e r e n c e i s due m a i n l y to i n c o m p l e t e r e a c t i o n . T h e p h y s i c a l p r o p e r t i e s of f u e l s l i s t e d i n T a b l e I w e r e e m p l o y e d i n c a l c u l a t i n g the r a t e c o n s t a n t s ; w h e n p r o p e r t i e s w e r e not a v a i l a b l e i n t a b l e s , those of s i m i l a r c o m p o u n d s w e r e u s e d , or bond-energy calculations were made. D e r i v e d r a t e c o n s t a n t s a r e l i s t e d i n T a b l e Π. T h e m a i n s o u r c e of e r r o r i n the c a l c u l a t i o n of the a c t i v a t i o n e n e r g y i s the

5.



T a b l e II

benzene


isopropyl ether butyl-vinyl ether

O v e r a l l Rate Constants

Chemical Formula

Fuel

C

Activation Energy (kcal/mole)

6 6 H

[(CH ) CH] 0 3

2

2

C

5 8°2 H

polym ethyl methacrylate methanol

CH OH 3

heptane

C

decane hexadecane iso-octane kerosene

C

7 16 H

C

10 22

C

16 34

C

8 18

1 ( )

H

H

H

H

2 ( )

(approx.)

Pre- Exponential Factor ( c m ^ / m o l e sec)

36

6.0

X io14

50

1.2

X io17

5.0

X

io

1 4

37

2.4

X

io

1 5

42

7.6

X

io

1 5

40

4.6

X

io

1 5

38

2.2

X

io

1 5

37

1.6

X

io

1 5

35

8.2

X

io

1 4

35

4.5

X io14

35

5.4

X

CH^CKÔCH: C

methyl methacrylate

165

io

1 4

d e t e r m i n a t i o n of the v a l u e of F . T h i s e r r o r w a s e s t i m a t e d not to e x c e e d ± 2 k c a l / m o l e . O t h e r s o u r c e s of e r r o r , s u c h as u n c e r t a i n t i e s i n the data l i s t e d i n T a b l e I, a n d e x p e r i m e n t a l i n a c c u r a c i e s , w e r e e s t i m a t e d to p r o d u c e u n c e r t a i n t i e s not e x c e e d i n g ± 1 kcal/mole. T h e m o s t s t r i k i n g a s p e c t of T a b l e II i s the s i m i l a r i t y i n o v e r a l l r a t e s of the f u e l s t e s t e d . C e r t a i n s y s t e m a t i c t r e n d s m a y be s e e n , s u c h as a t e n d e n c y f o r the o v e r a l l a c t i v a t i o n e n e r g y to

166



d e c r e a s e w i t h i n c r e a s i n g c h a i n l e n g t h f o r the a l k a n e s . M a n y of the d i f f e r e n c e s l i e w i t h i n e x p e r i m e n t a l e r r o r . H o w e v e r , the o v e r a l l a c t i v a t i o n energy f o r i s o p r o p y l ether quite d e f i n i t e l y i s d i f f e r e n t f r o m those of the other f u e l s . F l a m e S t r u c t u r e . It h a s b e e n i n d i c a t e d (9) that f o r a l k a n e s the d i f f u s i o n f l a m e s i n the p r e s e n t a p p a r a t u s a r e p a l e b l u e , t h e r e b e i n g i n s u f f i c i e n t r e s i d e n c e t i m e on the f u e l s i d e f o r f o r m a t i o n of a l u m i n o u s c a r b o n - c o n t a i n i n g z o n e . F o r the f u e l s t e s t e d h e r e , the o n l y e x c e p t i o n to t h i s r e s u l t o c c u r r e d w i t h b e n z e n e , w h i c h e x h i b i t e d a b r o a d o r a n g e z o n e b e n e a t h the b l u e f l a m e . W i t h i n c r e a s i n g d i l u t i o n the o r a n g e z o n e d i m i n i s h e d i n s i z e and d i s a p p e a r e d e n t i r e l y j u s t b e f o r e e x t i n c t i o n . R a p i d b u i l d u p of c a r b o n d e p o s i t s at the r i m of the cup l i m i t e d t e s t i n g t i m e s w i t h b e n z e n e . A d d i t i o n a l c a l c u l a t i o n s w e r e m a d e f o r heptane u s i n g t e m p e r a t u r e and c o m p o s i t i o n p r o f i l e s p r e v i o u s l y r e p o r t e d (9). The m e a s u r e d c o m p o s i t i o n p r o f i l e s w e r e e m p l o y e d to c a l c u l a t e a l o c a l a d i a b a t i c t e m p e r a t u r e . The r e s u l t i n g t e m p e r a t u r e p r o f i l e , s h o w n i n F i g u r e 10, a g r e e s w e l l w i t h the m e a s u r e d t e m p e r a t u r e p r o f i l e . The e n t i r e d i s c r e p a n c y c a n be a t t r i b u t e d to the u n c e r t a i n t y i n the a b s o l u t e v a l u e of the v e r t i c a l p o s i t i o n ; the z e r o p o i n t s f o r t e m p e r a t u r e and c o n c e n t r a t i o n m a y d i f f e r b y a f r a c t i o n of a m i l l i m e t e r . T h e a g r e e m e n t i m p l i e s the a b s e n c e of a n a p p r e c i a b l e d i f f e r e n c e b e t w e e n the c o e f f i c i e n t of d i f f u s i o n f o r h e a t and the a v e r a g e c o e f f i c i e n t of d i f f u s i o n f o r c h e m i c a l s p e c i e s . It a l s o s u p p o r t s the v a l i d i t y of the t e m p e r a t u r e and c o m p o s i t i o n m e a surements. If r e s i d e n c e t i m e w e r e s u f f i c i e n t l y l a r g e , c o m p l e t e e q u i l i b r i u m w o u l d be e x p e c t e d to e x i s t at e a c h p o i n t i n the d i f f u s i o n f l a m e . The f a c t that m e a s u r e d f l a m e t e m p e r a t u r e s l i e a p p r e c i a b l y b e l o w e q u i l i b r i u m a d i a b a t i c v a l u e s ( F i g u r e 9) i m p l i e s that c o m p l e t e e q u i l i b r i u m does not e x i s t . N e v e r t h e l e s s , i t i s of i n t e r e s t to t e s t w h e t h e r e q u i l i b r i u m m a y o c c u r f o r c e r t a i n r e a c t i o n s . The t e m p e r a t u r e and c o m p o s i t i o n p r o f i l e s f o r the heptane f l a m e (9) w e r e e m p l o y e d f o r t h i s p u r p o s e . The e q u i l i b r i u m c o n stant K p w a s e v a l u a t e d s e p a r a t e l y f r o m the m e a s u r e d t e m p e r a t u r e a n d f r o m the m e a s u r e d c o n c e n t r a t i o n s f o r the o v e r a l l r e actions H + | θ * H 0 , C O + | 0 ^ C 0 and C 0 + H ^ C O + H 0 . T h r o u g h o u t the f l a m e , the K p e v a l u a t e d f r o m t e m p e r a t u r e w a s m u c h l a r g e r t h a n that e v a l u a t e d f r o m c o n c e n t r a t i o n s f o r the f i r s t two r e a c t i o n s . The K p ' s f o r the t h i r d r e a c t i o n a r e s h o w n i n F i g u r e 11. The a g r e e m e n t i s r e a s o n a b l y good i n the c e n t r a l r e g i o n w h e r e the c o m p o s i t i o n m e a s u r e m e n t s a r e m o s t a c c u r a t e . T h e a g r e e m e n t m i g h t be q u i t e g e n e r a l l y v a l i d i n 2

2

2

2

2

2

2

2




-I4.6 4.4

_i_

L

4.8

5.2

5.6

6.4

6.0

7.2

6.8

T " ' x l 0 (°K) 4

Figure 9. Arrhenius plot for adiabatic and measured flame tem peratures of isopropyl ether Ί

1

Γ

FUEL HEPTANE :

INERT N 1600-

Y

02

2

= 0.185

U =0.305 m/s

Measured Temp. Profile Adiabatic Flame Temp,

(

From Measured Concen tration of Species

3

2

1

0

1

2

DISTANCE FROM LUMINOUS ZONE (mm)

3

Figure 10. Measured temperature profile vs. that calculated from measured local composition on the basis of adiabaticity for heptane

168


hydrocarbon-air diffusion flames. A p r o f i l e of the C / H r a t i o w a s c a l c u l a t e d f r o m the m e a s u r e d c o n c e n t r a t i o n p r o f i l e s (9) f o r the heptane f l a m e . The r e s u l t , s h o w n i n F i g u r e 12, i n d i c a t e s that the r a t i o tends to p e a k i n the c e n t e r of the f l a m e . A r e s u l t of t h i s type i s a c c e p t a b l e i f i t i s a s s u m e d that H i s p r o d u c e d n e a r the c e n t e r of the f l a m e and d i f f u s e s p r e f e r e n t i a l l y i n b o t h d i r e c t i o n s b e c a u s e of i t s h i g h e r d i f f u s i o n c o e f f i c i e n t . If the a l t e r n a t i v e v i e w i s t a k e n that the variation arises f r o m inaccuracies i n composition measurem e n t s , t h e n c o r r e c t e d p r o f i l e s , s h o w n i n F i g u r e 12, c a n be g e n e r a t e d by a s s u m i n g a c o n s t a n t C / H r a t i o , the c o r r e c t i o n s b e i n g m a d e i n the s p e c i e s w h o s e m e a s u r e m e n t s w e r e d e e m e d l e a s t a c c u r a t e . The c o r r e c t i o n g r e a t l y i n c r e a s e s w a t e r c o n c e n t r a t i o n i n the c e n t e r of the f l a m e ; the t o t a l h y d r o g e n e l e m e n t concentrat i o n t h e r e m u s t be i n c r e a s e d by about a f a c t o r of t w o . S i n c e i t i s u n l i k e l y that the e x p e r i m e n t a l e r r o r c a n be so l a r g e , the r e s u l t s tend to s u p p o r t the o c c u r r e n c e of p r e f e r e n t i a l H d i f f u s i o n .


2

2

E x t i n g u i s h m e n t of P o l y m e t h y l M e t h a c r y l a t e T h e r e h a s b e e n c o n t i n u i n g i n t e r e s t (21, 22) i n the g a s - p h a s e c h e m i c a l k i n e t i c s f o r c o m b u s t i o n of p o l y m e t h y l m e t h a c r y l a t e , w h i c h s e r v e s as a m o d e l s o l i d f u e l . E x t i n c t i o n m e a s u r e m e n t s have b e e n m a d e i n o x y g e n - e n r i c h e d a i r by u s e of a c o m b u s t i o n t u n n e l (22). S i n c e t h e r e i s e v i d e n c e that the p o l y m e r u n z i p s to the m o n o m e r i n g a s i f i c a t i o n , the g a s - p h a s e k i n e t i c s m a y be e x p e c t e d to be the s a m e a s f o r m e t h y l m e t h a c r y l a t e . One of the r e a s o n s f o r i n c l u d i n g m e t h y l m e t h a c r y l a t e i n the p r e s e n t e x p e r i m e n t s w a s to t e s t t h i s h y p o t h e s i s . It w a s f o u n d to be p o s s i b l e to t e s t the p o l y m e r i t s e l f i n the p r e s e n t a p p a r a t u s . C y l i n d e r s 5 c m i n d i a m e t e r w e r e cut i n t o s l i c e s 2 c m t h i c k , and e a c h s l i c e w a s p l a c e d o n top of the c u p a n d i g n i t e d . The o u t l e t of the d u c t f o r d e l i v e r i n g the o x i d i z i n g gas m i x t u r e w a s p o s i t i o n e d 1 c m above the b u r n i n g p o l y m e r s u r f a c e , and e x t i n c t i o n c o n d i t i o n s w e r e m e a s u r e d i n t h e s a m e m a n n e r as f o r l i q u i d f u e l s . The f l a m e w a s s i m i l a r i n a p p e a r a n c e to t h o s e of the l i q u i d f u e l s , e x c e p t that a r o u n d the s i d e s of the p o l y m e r a y e l l o w zone e x i s t e d . S i n c e the o x i d i z i n g gas i s m o r e d i l u t e t h a n a i r , the new e x t i n c t i o n r e s u l t s c o m p l e m e n t t h o s e o b t a i n e d e a r l i e r (22). E x t i n c t i o n c u r v e s f o r the p o l y m e r and i t s m o n o m e r a r e g i v e n i n F i g u r e 13. The p o l y m e r d a t a at the h i g h e r f l o w v e l o c i t i e s a r e p o o r l y r e p r o d u c i b l e b e c a u s e i n the a l t e r e d e x p e r i m e n t a l c o n f i g u r a t i o n the f l a m e i s l e s s s t a b l e . The p o i n t s s h o w n c o r r e s p o n d to the m i n i m u m o x y g e n content f o r e x t i n c t i o n ; o f t e n

5.


Counterflow Combustion 1 FUEL

:

169 ι

HEPTANE

Y 2 = 0.185

7-

0

U =0.305 m/s INERT N

2

REACTION C0 #

2

+H ^C0+H 0 2

2

From Measured Concen trations


ο From Temp. Profile

5 3

ο

Figure 11. Equilibrium con stant for: C0 + H ^± CO + H 0, calculated separately from measured temperatures and measured compositions, for heptane 2

2

2

U =

0.305m/s

3

2

1

0

1

DISTANCE FROM LUMINOUS ZONE(mm)

DISTANCE FROM LUMINOUS FLAME ZONE (mm)


170


r a n d o m d i s t u r b a n c e s c a u s e the f l a m e to e x t i n g u i s h at c o n s i d e r a b l y h i g h e r o x y g e n c o n c e n t r a t i o n s , at the h i g h e s t v e l o c i t i e s . A c t i v a t i o n e n e r g i e s d e d u c e d f r o m the n i t r o g e n - q u e n c h data p l o t t e d i n F i g u r e 13 f o r the p o l y m e r v a r y w i t h o x y g e n m a s s f r a c t i o n , r a n g i n g f r o m 4 2 k c a l / m o l e at the l o w e r v e l o c i t i e s to 55 k c a l / m o l e at the h i g h e r velocities. S i n c e the h i g h - v e l o c i t y r e s u l t s a r e i n a c c u r a t e , the l o w - v e l o c i t y a c t i v a t i o n e n e r g y has b e e n adopted as m o s t r e l i a b l e and i s r e p o r t e d i n T a b l e Π. The r a t e c o n s t a n t s s h o w n i n T a b l e II f o r the p o l y m e r and the m o n o m e r d i f f e r s o m e w h a t , but the a c c u r a c y of the e x p e r i m e n t i s not quite s u f f i c i e n t to e s t a b l i s h w h e t h e r the d i f f e r e n c e i s r e a l . The p o l y m e r c o m b u s t i o n m i g h t i n v o l v e m o n o m e r c o m b u s t i o n i n the gas p h a s e . It m a y be noted that a c t i v a t i o n e n e r g i e s r e p o r t e d e a r l i e r f o r e n r i c h e d a t m o s p h e r e s (21, 22) w e r e i n the v i c i n i t y of 2 0 k c a l / m o l e . In the c a l c u l a t i o n s that p r o d u c e d t h i s v a l u e , d i s s o c i a t i o n w a s n e g l e c t e d e v e n though, c o n t r a r y to the p r e s e n t e x p e r i m e n t s , a c t u a l f l a m e t e m p e r a t u r e s at e x t i n c t i o n w e r e s u f f i c i e n t l y h i g h f o r e q u i l i b r i u m d i s s o c i a t i o n to be i m p o r t a n t e n e r g e t i c a l l y . T h i s p r o c e d u r e w a s f o l l o w e d b e c a u s e t h e r e e x i s t s no t h e o r y f o r e x t r a c ting accurate o v e r a l l activation energies f r o m experiments i n which dissociation occurs. However, approximate corrections c a n be m a d e i n an ad hoc m a n n e r by i n c l u d i n g d i s s o c i a t i o n i n the c a l c u l a t i o n of the a d i a b a t i c f l a m e t e m p e r a t u r e and by c o r r e c t i n g the o v e r a l l h e a t r e l e a s e f o r d i s s o c i a t i o n i n d e r i v i n g v a l u e s of F . A p p l i c a t i o n of t h e s e c o r r e c t i o n s to the data o b t a i n e d i n e n r i c h e d atmospheres produces activation energies ranging f r o m 42 k c a l / m o l e i n a i r to 4 8 k c a l / m o l e at Y Q = 0. 53 . A l t h o u g h these c o r r e c t e d results are i n reasonable agreement w i t h results ob t a i n e d f r o m the new data r e p o r t e d h e r e i n , i t s h o u l d be e m p h a s i z e d that the c o r r e c t i o n p r o c e d u r e s a r e open to q u e s t i o n . T h e r e i s need f o r f u r t h e r t h e o r e t i c a l w o r k d i r e c t e d t o w a r d a c c o u n t i n g f o r e f f e c t s of d i s s o c i a t i o n . N e v e r t h e l e s s , i t m a y be s i g n i f i c a n t that a l l of the c a l c u l a t i o n s f o r the p o l y m e r s e e m to show a tendency f o r the e f f e c t i v e o v e r a l l a c t i v a t i o n e n e r g y to i n c r e a s e w i t h i n c r e a s i n g o x y g e n content. 2

Results for Liquid Fuels i n C F ^ B r - A i r Mixtures Q u e n c h i n g by C F ^ B r . V a r i o u s l i q u i d f u e l s b u r n i n g i n a i r w e r e e x t i n g u i s h e d by a d d i n g C F 3 B r to the a i r s t r e a m . The c o n v e n t i o n a l p l o t of the v o l u m e p e r c e n t of C F 3 B r i n the o x i d i z i n g g a s , r e q u i r e d f o r e x t i n g u i s h m e n t , as a f u n c t i o n of the v e l o c i t y of the o x i d i z i n g g a s , i s s h o w n i n F i g u r e 14. A t the l o w e r


SESHADRI A N D W I L L I A M S

0

23|

,

,

1

1

1

1

1

Γ

1 0.2

1 0.4

1 0.6

I 0.8

I 1.0

I 1.2

I 1.4

1 1.6


0.22-

0.161 0

1 1.8

VELOCITY (m/s)

Figure 13. Extinction curves for poly (methyl methacrylate) and for its monomer, burning in air with added N and CF Br 2

9|

1

1

o«

1 0.2

1 0.4

1

s

Γ

1 I I 0.6 0.8 1.0 VELOCITY (m/s)

I 1.2

1 1.4

1 1.6

Figure 14. Vol % CF Br in air required to quench various fuels s


172


v e l o c i t i e s , the c o n c e n t r a t i o n s needed f o r e x t i n g u i s h m e n t a r e q u i t e s i m i l a r to those r e p o r t e d by m a n y o t h e r i n v e s t i g a t o r s , f o r m e t h a n o l and h e p t a n e . D a t a f o r i s o p r o p y l e t h e r and f o r m e t h y l m e t h a c r y l a t e h a v e not b e e n r e p o r t e d i n the p a s t ; h e r e they a r e found to n e a r l y c o i n c i d e w i t h the d a t a f o r h e p t a n e . The r e s u l t s for i s o p r o p y l ether a r e quite c o m p a r a b l e w i t h e a r l i e r r e s u l t s for diethyl ether. The C F 3 B r v o l u m e p e r c e n t a g e r e q u i r e d f o r e x t i n g u i s h m e n t of b e n z e n e w a s found to be l e s s t h a n the l o w e s t v a l u e (2. 9%) r e p o r t e d e a r l i e r , a l t h o u g h the g e n e r a l t r e n d t o w a r d c o m p a r a t i v e l y e a s y e x t i n g u i s h m e n t of benzene i s v e r i f i e d . It i s u n d e r s t a n d a b l e that l o w e r i n h i b i t o r c o n c e n t r a t i o n s a r e needed at h i g h e r a i r v e l o c i t i e s b e c a u s e as the v e l o c i t y i n c r e a s e s the r e s i dence t i m e i n the r e a c t i o n zone d e c r e a s e s . It has b e e n e m p h a s i z e d (9) that q u e n c h i n g e f f i c i e n c i e s o n a m a s s b a s i s c a n d i f f e r f r o m those on a v o l u m e b a s i s . M a s s b a s e d e x t i n c t i o n c u r v e s , s i m i l a r to F i g u r e 6, a r e s h o w n i n F i g u r e 15 f o r e x t i n g u i s h m e n t w i t h C F 3 B r . A l t h o u g h r e l a t i v e r a n k i n g s of v a r i o u s e x t i n g u i s h a n t s d i f f e r o n m a s s and v o l u m e b a s e s (9), i t i s s e e n that the dependence o n the type of f u e l i s unchanged. F o r n i t r o g e n q u e n c h i n g , p l o t s l i k e F i g u r e 15 w e r e t u r n e d into plots l i k e F i g u r e 9 f o r extracting o v e r a l l kinetic data. This has not y e t b e e n a t t e m p t e d w i t h C F 3 B r b e c a u s e of u n c e r t a i n t y i n the p r o p e r t h e o r e t i c a l a d i a b a t i c f l a m e t e m p e r a t u r e to adopt. If C F 3 B r i s t r e a t e d as an i n e r t , t h e n l o w f l a m e t e m p e r a t u r e s a r e o b t a i n e d , but i f e q u i l i b r i u m f o r m a t i o n of s p e c i e s s u c h a s H F and H B r i s i n c l u d e d , t h e n the f l a m e t e m p e r a t u r e s a r e h i g h . E x p l o r a t o r y c a l c u l a t i o n s of the s e n s i t i v i t y of d e r i v e d o v e r a l l k i n e t i c p a r a m e t e r s to d i f f e r e n t e q u i l i b r i u m a s s u m p t i o n s w o u l d be of i n t e r e s t . D i f f e r e n c e s i n i n h i b i t o r e f f e c t i v e n e s s w i t h a l k a n e s and m e t h a n o l , f o r e x a m p l e , often a r e a t t r i b u t e d to d i f f e r e n c e s i n s t o i c h i o m e t r y . S i n c e the f l a m e s i t s at the s t o i c h i o m e t r i c p o s i t i o n , p r o p o r t i o n a l l y l e s s i n h i b i t o r p e n e t r a t e s to the f l a m e f o r m e t h a n o l . T o see i f t h i s e f f e c t c a n e x p l a i n o b s e r v e d d i f f e r e n c e s i n e f f e c t i v e n e s s f o r d i f f e r e n t f u e l s , a n e x t i n c t i o n p l o t u s i n g the s t o i c h i o m e t r i c a l l y a d j u s t e d m o l e p e r c e n t a g e w a s p r e p a r e d ( F i g u r e 16). The a d j u s t e d p e r c e n t a g e i s d e f i n e d as the m o l a r r a t i o of i n h i b i t o r to o x y g e n i n the gas s t r e a m , d i v i d e d b y one p l u s t h i s q u a n t i t y p l u s the s t o i c h i o m e t r i c m o l a r r a t i o of f u e l to o x i d i z e r . S i n c e the m e t h a n o l c u r v e l i e s c l o s e r to that f o r heptane i n F i g u r e 16 than i n F i g u r e 15, i t a p p e a r s that t h e r e does e x i s t a n e f f e c t of the a c c e s s i b i l i t y of the r e a c t i o n z o n e . H o w e v e r , i t i s c l e a r that there exist a d d i t i o n a l f u e l - s p e c i f i c i n f l u e n c e s . In p a r t i c u l a r ,

SESHADRI AND WILLIAMS


τ

Counterflow

ComhuStlOYl

Γ

VELOCITY (m/s)

Figure 15. Mass-based extinction curves of CF Br in air for various fuels s

π

1 1 I EXTINCTION CF Br IN AIR

Γ

3

© ο ο Δ •

0.2

0.4

0.6

0.8

METHYL ALCOHOL METHYL METHACRYLATE HEPTANE ISOPROPYL ETHER BENZENE

1.0

1.4

1.6

1.8

VELOCITY (m/s)

Figure 16. Stoichiometrically adjusted mole % CF Br required to quench various fuels burning in air s

174


r e l a t i v e p o s i t i o n s of f u e l s r e m a i n the s a m e i n a l l p l o t s y e t c o n structed. E f f e c t of V a r y i n g R e l a t i v e P r o p o r t i o n s of , 0 and C F ^ B r . A g r a p h f o r heptane e x t i n c t i o n has b e e n g i v e n p r e v i o u s l y (9) s h o w i n g that a l t h o u g h C F 3 B r i s l e s s e f f e c t i v e i n p u r e o x y g e n t h a n i n a i r , c h e m i c a l i n h i b i t i o n r e m a i n s a n i m p o r t a n t a s p e c t of i t s m o d e of o p e r a t i o n . F i g u r e 17 shows that the s a m e p h e n o m e non o c c u r s f o r b e n z e n e . It a p p e a r s that c u r v e s of the shape s h o w n i n F i g u r e 17 w i l l be r e p r e s e n t a t i v e of a l l h y d r o c a r b o n f u e l s . I n 0 - Ο Ε β Β τ m i x t u r e s , e x t i n c t i o n o c c u r s at v e r y h i g h flame temperatures. 2


2

Flame Structure. A d d i t i o n of C F ^ B r to the a i r s i d e of the f l a m e p r o d u c e s a n o r a n g e c a r b o n zone on the f u e l s i d e . It a l s o changes the c o l o r of the f l a m e f r o m b l u e to b l u e - g r e e n . T h e s e o b s e r v a t i o n s tend to a g r e e w i t h the r e s u l t s r e p o r t e d b y S i m m o n s and W o l f h a r d (3) and by I b i r i c u and G a y d o n (6) f o r g a s eous f u e l s , the g r e e n b e i n g a t t r i b u t a b l e to e n h a n c e d C emis s i o n . C l e a r l y , the i n f l u e n c e of the i n h i b i t o r p e n e t r a t e s w e l l i n t o the f u e l s i d e of the f l a m e . The a u g m e n t a t i o n i n c a r b o n p r o d u c t i o n w a s so g r e a t that c o a t i n g of p r o b e s by soot p r e v e n t e d a c c u rate temperature or composition measurements f r o m being made on the f u e l s i d e of the c e n t e r of the b l u e - g r e e n f l a m e . C o m p o s i t i o n p r o f i l e s o n the a i r s i d e of a heptane f l a m e w i t h added C F 3 B r h a v e b e e n r e p o r t e d e a r l i e r (9). A t e m p e r a t u r e p r o f i l e h a s now been o b t a i n e d f o r the s a m e c o n d i t i o n s , and the c o m p l e t e r e s u l t s a r e s h o w n i n F i g u r e 18. T h e s e r e s u l t s s h o u l d be c o m p a r e d w i t h c o r r e s p o n d i n g r e s u l t s of C r e i t z (8); t h e r e a r e both s i m i l a r i t i e s and d i f f e r e n c e s , the d i f f e r e n c e s p r o b a b l y b e i n g a t t r i b u t a b l e to l o n g e r r e s i d e n c e t i m e s i n the f l a m e s t u d i e d by Creitz. The o n l y f u e l s p e c i e s o t h e r t h a n c a r b o n that C r e i t z found o n the a i r s i d e of the y e l l o w z o n e w e r e C O and H . In F i g u r e 18, some C H a n d C H ^ a l s o a r e p r e s e n t , but i n l o w e r c o n c e n t r a t i o n s t h a n C O and H , a n d a l s o i n s l i g h t l y l o w e r c o n c e n t r a t i o n s t h a n found (9) i n the c o r r e s p o n d i n g u n i n h i b i t e d f l a m e . It m a y be a s s u m e d that the l o n g e r r e s i d e n c e t i m e s i n the f l a m e of C r e i t z allowed time for C H and CH4 to be c o n v e r t e d to c a r b o n , H and C O b e f o r e e m e r g i n g f r o m the y e l l o w z o n e and that a d d i t i o n of C F 3 B r e n h a n c e s f u e l p y r o l y s i s . C o n t r a r y to the r e s u l t s of C r e i t z , t h e o x y g e n c o n c e n t r a t i o n i s a p p r e c i a b l e at the edge of the y e l l o w zone i n F i g u r e 18; i t c e r t a i n l y cannot be s t a t e d that f u e l p y r o l y s i s o c c u r s w i t h o u t 2

2

2

2

2

2

2

2

5.



175

1

.28

1

Ί

2000

BENZENE (U=0.32m/s)

rjeoo

-1600


- 1400 £

Temperature (Benzene)

•

Temperature(Heptane)

-

1200

.08

-1000

.04

-800

Figure 17. Effects of varying CF Br-N ratio on oxygen content and tem perature at extinction for benzene s

•

2

0.2

0.8

0.6

0.4 l i l

CF Br/ " CF Br % (

,

3

+

1.0

)

3

s Ι s* 3

I 1

FUEL

!

N-HEPTANE

--I600

Y -.203 °2 n

14

--I400 U » 3 0 2 m/s

χ» I --1200

* I 10 Λ

1

-1000 £

-800 eu

w

-600

]

2x

Figure 18. Compo sition and tempera ture profiles on the air side of a heptane flame with added CF Br 3

4

3

DISTANCE

-400

-200

2 FROM

I LUMINOUS

Ο FLAME

Τ ZONE (mm)

2


176


SUPPRESSANTS

o x y g e n i n the p r e s e n t e x p e r i m e n t . The t e n d e n c y t o w a r d a h i g h e r o x y g e n c o n c e n t r a t i o n i n the f l a m e a g a i n m a y be a t t r i b u t e d at l e a s t p a r t i a l l y to the s h o r t e r r e s i d e n c e t i m e . H o w e v e r , a n a d d i t i o n a l e f f e c t o b s e r v e d i n the p r e s e n t p r o g r a m i s a t e n d e n c y f o r the o x y g e n c o n c e n t r a t i o n i n the f l a m e to be h i g h e r w i t h C F 3 B r a d d i t i o n t h a n i n the c o r r e s p o n d i n g f l a m e w i t h o u t CT^BT (9). I n the c e n t e r of t h e b l u e z o n e of o u r u n i n h i b i t e d f l a m e s , the o x y g e n m o l e f r a c t i o n w a s found i n r e p e a t e d t e s t s to r a n g e f r o m 1. 5% to 4%, w h i l e two m e a s u r e m e n t s of i n h i b i t e d f l a m e s gave v a l u e s of 6% and 9%. A l t h o u g h t h e r e i s u n c e r t a i n t y i n t h e s e v a l u e s , m a i n l y b e c a u s e of u n c e r t a i n t y i n m e a s u r e m e n t of a b s o lute p o s i t i o n s , the f l a m e - z o n e o x y g e n contents w i t h C F 3 B r d e f i n i t e l y tend to be two to t h r e e t i m e s as l a r g e as i n the a b s e n c e of the i n h i b i t o r . C h e m i c a l m e c h a n i s m s p r o p o s e d f o r the a c t i o n of the i n h i b i t o r w i l l h a v e to t r y to e x p l a i n t h i s o b s e r v a t i o n . F i e l d i n g , W o o d s and J o h n s o n (23) m a d e c o m p o s i t i o n m e a s u r e m e n t s i n the e f f l u e n t g a s e s of a f u r n a c e i n w h i c h c a r b o n w a s burned i n an a i r s t r e a m w i t h added C F 3 B r . They demonstrated that the i n h i b i t o r d e c r e a s e d the C 0 c o n c e n t r a t i o n and i n c r e a s ed the C O c o n c e n t r a t i o n by about a f a c t o r of t w o . N o s u c h changes w e r e found i n the p r e s e n t s t u d y ; a d d i t i o n of C F 3 B r h a d v e r y l i t t l e e f f e c t o n the C O o r C 0 p r o f i l e s . The d i f f e r e n c e s u g g e s t s that e v e n i n the s o o t y , i n h i b i t e d , d i f f u s i o n f l a m e , c a r b o n c o m b u s t i o n i s not the m a j o r s o u r c e of C O . F i n a l l y , the p r o f i l e of C F ^ B r i t s e l f s h o u l d be c o n s i d e r e d . U n l i k e the i n e r t N , the i n h i b i t o r d r o p s v e r y n e a r l y to z e r o c o n c e n t r a t i o n at the c e n t e r of the f l a m e , as i f i t w e r e a n a i r - s i d e r e a c t a n t . I n f a c t , the a p p a r e n t s i n k i n t o w h i c h C F ^ B r d i f f u s e s s e e m s to be s l i g h t l y o n the a i r s i d e of the 0 s i n k , the f l a m e center. T h i s c e r t a i n l y i m p l i e s p r o d u c t i o n of d i s s o c i a t i o n o r r e a c t i o n p r o d u c t s of C F 3 B r a n d , i n a g r e e m e n t w i t h the c o n c l u s i o n of C r e i t z (8), i m p l i e s that the i n t a c t C F 3 B r m o l e c u l e i s not r e s p o n s i b l e f o r the c h e m i c a l i n h i b i t i o n e f f e c t . T h a t not C F 3 B r but r a t h e r i t s p r o d u c t s c a u s e the e n h a n c e d f u e l p y r o l y s i s and c a r b o n p r o d u c t i o n , i s c o n s i s t e n t w i t h the o b s e r v a t i o n of S i m m o n s a n d W o l f h a r d (3) that a d d i t i o n of B r p r o d u c e d a p p r o x i m a t e l y the s a m e i n t e n s i f i c a t i o n of the c a r b o n z o n e as a d d i t i o n of C ^ B r . B r o m i n e a t o m s o r H B r a r e a m o n g the c a n d i d a t e s f o r the p y r o l y s i s - a u g m e n t i n g a g e n t s , but a p p a r e n t l y no k i n e t i c m e c h a n i s m f o r the p r o c e s s h a s b e e n p r o p o s e d . E n h a n c e m e n t of o x y g e n l e a k a g e , f o l l o w e d b y o x y g e n - c a t a l y z e d p y r o l y s i s , i s another p o s s i b i l i t y which is consistent with our observations. A l t h o u g h C F 3 B r disappears m o r e r a p i d l y than 0 , i t i s 2

2

2

2

2

2


5.



177

not c o n s u m e d n e a r l y as e a r l y as i n the f l a m e of C r e i t z (8). F r o m the t e m p e r a t u r e p r o f i l e i n F i g u r e 18 i t i s s e e n that at the C r e i t z d i s a p p e a r a n c e t e m p e r a t u r e of 1080°K, m o r e t h a n h a l f of the o r i g i n a l C F 3 B r r e m a i n s i n the c o u n t e r f l o w f l a m e . It i s u n l i k e l y that t h i s d i f f e r e n c e i s e f f e c t e d b y u s e of heptane r a t h e r t h a n p r o p a n e a s the f u e l . I n s t e a d i t m a y be a t t r i b u t a b l e to d i f f e r e n c e s b e t w e e n d i f f u s i v i t i e s f o r h e a t and f o r C F 3 B r , w i t h the i n f l u e n c e of the r e l a t i v e i m m o b i l i t y of the l a r g e r m o l e c u l e m a g n i f i e d at the l o n g e r r e s i d e n c e t i m e i n the c o f l o w i n g s y s t e m . The m e c h a n i s m b y w h i c h C F 3 B r i s a t t a c k e d r e m a i n u n c l e a r . T h e r m a l d e c o m p o s i t i o n s e e m s u n l i k e l y at t h e s e t e m p e r a t u r e s . The attack by oxygen atoms, p r o p o s e d by F r a n k i e w i c z , W i l l i a m s and G a n n (24), i s a n a t t r a c t i v e p o s s i b i l i t y , e s p e c i a l l y s i n c e the i n h i b i t o r tends to be c o n s u m e d o n the a i r s i d e of the f l a m e . O n the o t h e r h a n d , i n F i g u r e 18 the r e l a t i v e p o s i t i o n s of H and C F ^ B r p r o f i l e s a r e s o m e w h a t r e m i n i s c e n t of a d i f f u s i o n f l a m e i n w h i c h t h e s e two s p e c i e s r e a c t r a p i d l y . U n f o r t u n a t e l y , the H2 p r o f i l e i s not a c c u r a t e enough to w a r r a n t drawing conclusions. However, since H diffuses easily ( r e c a l l C / H p r o f i l e i n F i g u r e 12), and s i n c e H a t o m s d i f f u s e e v e n m o r e q u i c k l y t h a n H , the p o p u l a r i d e a that H a t o m s a t t a c k C F 3 B r cannot be e x c l u d e d o n the b a s i s of o u r d a t a , and p r o b a b l y not e v e n on the b a s i s of the data of C r e i t z . T h e r e i s a g r e a t d e a l of c h e m i s t r y o c c u r r i n g w i t h i n the c o n c e n t r a t i o n p r o f i l e s s h o w n i n F i g u r e 18. 2

2

2

Concluding Remarks T h e r e i s n e e d f o r a c o n s i d e r a b l e a m o u n t of a d d i t i o n a l w o r k b e f o r e the m e t h o d s e m p l o y e d h e r e i n c a n a c h i e v e c l a r i f i c a t i o n of r e a c t i o n r a t e s and m e c h a n i s m s f o r c h e m i c a l l y i n h i b i t e d d i f f u s i o n f l a m e s , c o m p a r a b l e to the c l a r i f i c a t i o n that h a s b e e n o b t a i n e d i n the a b s e n c e of c h e m i c a l i n h i b i t i o n . The a p p r o a c h to determining o v e r a l reaction rates, discussed i n connection with n i t r o g e n q u e n c h i n g , s h o u l d be e x p l o r e d b o t h t h e o r e t i c a l l y a n d e x p e r i m e n t a l l y f o r f l a m e s to w h i c h c h e m i c a l s u p p r e s s a n t s a r e a p p l i e d . In a d d i t i o n , m e a s u r e m e n t s of c o n c e n t r a t i o n and t e m p e r a t u r e p r o f i l e s , of the t y p e d i s c u s s e d i n the p r e c e d i n g s e c t i o n , s h o u l d be r e p e a t e d to t e s t r e p r o d u c i b i l i t i e s and s h o u l d be e x t e n d e d to o t h e r c o m p o s i t i o n s and v e l o c i t i e s of the o x i d i z i n g gas and to o t h e r f u e l s , so that s y s t e m a t i c t r e n d s c a n be i d e n t i f i e d . K n o w l e d g e of s u c h t r e n d s m a y be quite h e l p f u l i n s h e d d i n g l i g h t o n the i n h i b i t i o n m e c h a n i s m s of c h e m i c a l f i r e s u p p r e s s a n t s .

178


Acknowledgement The c o n t r i b u t i o n s of J . H . K e n t and of L . K r i s h n a m u r t h y to these studies a r e g r a t e f u l l y acknowledged. The w o r k was s u p p o r t e d p a r t i a l l y b y the N a v a l R e s e a r c h L a b o r a t o r y , U . S . N a v y a n d p a r t i a l l y b y the N a t i o n a l S c i e n c e F o u n d a t i o n t h r o u g h the p r o j e c t o n R e s e a r c h A p p l i e d t o N a t i o n a l N e e d s , G r a n t N o . GI-36528X.


Nomenclature F Mp Τ U YQ2

Stagnation-point flow parameter M o l e c u l a r w e i g h t of f u e l Theoretical adiabatic flame temperature G a s v e l o c i t y i n the a p p r o a c h s t r e a m M a s s f r a c t i o n of o x y g e n i n the a p p r o a c h s t r e a m

Literature 1. 2.

3.

4.

5.

6. Diffusion 7.

Cited

F r i s t r o m , R. Μ . , "Combustion Suppression", Fire Research A b s t r a c t s and R e v i e w s (1967), 9, 1 2 5 - 1 5 2 . M c H a l e , E. T., " S u r v e y of V a p o r P h a s e C h e m i c a l A g e n t s f o r C o m b u s t i o n S u p p r e s s i o n " , Fire R e s e a r c h Abstracts and R e v i e w s (1969), 11, 9 0 - 1 0 4 . S i m m o n s , R . F. and W o l f h a r d , H. G., " T h e I n f l u e n c e of M e t h y l Bromide o n Flames. P a r t 2 Diffusion Flames", T r a n s a c t i o n s of the F a r a d a y S o c i e t y (1956), 52, 53-59. C r e i t z , E. C., " I n h i b i t i o n of Diffusion F l a m e s b y M e t h y l B r o m i d e and Trifluoromethyl B r o m i d e Applied to the Fuel and O x y g e n S i d e s of the R e a c t i o n Z o n e " , J o u r n a l of Research of t h e N a t i o n a l B u r e a u of S t a n d a r d s - Α . Phy sics and Chemistry (1961), 6 5 A , 3 8 9 - 3 9 6 . F r i e d m a n , R . a n d Levy, J. Β., " I n h i b i t i o n of O p p o s e d - j e t Methane-air D i f f u s i o n Flames, T h e E f f e c t s of Alkali M e t a l V a p o r s and O r g a n i c Halides", C o m b u s t i o n a n d F l a m e (1963), 7, 1 9 5 - 2 0 1 . I b i r i c u , M. M. and G a y d o n , A. G., " S p e c t r o s c o p i c S t u d i e s of the E f f e c t of I n h i b i t o r s o n C o u n t e r f l o w Flames", C o m b u s t i o n and Flame (1964), 8, 51-62. M i l n e , Τ . Α., G r e e n , C . L. a n d B e n s o n , D . Κ., " T h e U s e of the C o u n t e r f l o w Diffusion Flame in S t u d i e s of I n h i b i t i o n E f f e c t i v e n e s s of G a s e o u s and P o w d e r e d A g e n t s " , C o m b u s t i o n and Flame (1970), 15, 2 5 5 - 2 6 3 .

5.

SESHADRi

A N D WILLIAMS


C r e i t z , E. C., " G a s C h r o m a t o g r a p h i c D e t e r m i n a t i o n of C o m p o s i t i o n Profiles of Stable S p e c i e s A r o u n d a P r o p a n e Diffusion Flame", J o u r n a l of C h r o m a t o g r a p h i c S c i e n c e (1972), 10, 1 6 8 - 1 7 3 . 9. K e n t , J. H. and Williams, F. Α., "Extinction of Lami n a r D i f f u s i o n F l a m e s f o r Liquid Fuels", F i f t e e n t h S y m p o s i u m (International) on C o m b u s t i o n , The Combus tion I n s t i t u t e , P i t t s b u r g h (1975), 3 1 5 - 3 2 5 . 10. B a j p a i , S. Ν . , "An I n v e s t i g a t i o n of the Extinction of Diffusion F l a m e s b y Halons", F a c t o r y M u t u a l Research Corp., R e p t . N o . 2 2 3 9 1 . 2 , N o v . 1973. 11. K u b i n , R . F. and G o r d o n , A. S., " I n h i b i t i o n of M e t h a n o l Pool Fires", p r e s e n t e d at Clay Preston B u t l e r M e e t i n g o n E x p e r i m e n t a l M e t h o d s in Fire R e s e a r c h , S t a n f o r d Research I n s t i t u t e , M a y 1974. 12. Kent, J. Η., "A Noncatalytic Coating for Platinum -Rhodium Thermocouples", Combustion and Flame (1970), 14, 2 7 9 - 2 8 1 . 13. F r i s t r o m , R . M. a n d W e s t e n b e r g , A. A., " F l a m e Structure", McGraw-Hill, New York (1965), 197. 14. R a s h b a s h , D . J., "Extinction of Fires b y W a t e r S p r a y s " , Fire R e s e a r c h Abstracts and R e v i e w s (1962), 4, 2 8 - 5 3 . 15. R o b e r t s , A. F., "Extinction P h e n o m e n a in Liquids", Fifteenth S y m p o s i u m (International) on C o m b u s t i o n , The C o m b u s t i o n I n s t i t u t e , P i t t s b u r g h (1975). 16. K r i s h n a m u r t h y , L. and Williams, F. A., " A s y m p t o t i c T h e o r y of Diffusion-Flame Extinction in the S t a g n a t i o n - P o i n t B o u n d a r y Layer", u n p u b l i s h e d . 17. Gallant, R. W., Physical Properties of H y d r o c a r b o n s , V o l u m e 1, G u l f Publishing C o m p a n y , H o u s t o n (1968). 18. Gallant, R. W . , Physical Properties of H y d r o c a r b o n s , V o l u m e 2, G u l f Publishing C o m p a n y , H o u s t o n (1970). 19. R o s s i n i , F. D . et. al., S e l e c t e d V a l u e s of Properties of H y d r o c a r b o n s , N B S Circular C 4 6 1 , N a t i o n a l B u r e a u of S t a n d a r d s , W a s h i n g t o n (1947). 20. S t u l l , D . R . et. al., JANAF T h e r m o chemical T a b l e s , U . S . D e p a r t m e n t of C o m m e r c e , N a t i o n a l B u r e a u of S t a n d a r d s , W a s h i n g t o n (1971). 21. K r i s h n a m u r t h y , L. and Williams, F. Α., "Laminar C o m b u s t i o n of P o l y m e t h y l m e t h a c r y l a t e in O /N Mixtures", Fourteenth S y m p o s i u m (International) on C o m b u s t i o n , The C o m b u s t i o n I n s t i t u t e , P i t t s b u r g h (1973), 1151-1164. Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch005

8.

2

2

180 22.


SUPPRESSANTS

K r i s h n a m u r t h y , L., "Diffusion-Flame Extinction in the S t a g n a t i o n - P o i n t B o u n d a r y Layer of PMMA in O /N Mixtures", C o m b u s t i o n S c i e n c e and T e c h n o l o g y , to appear. F i e l d i n g , G. Η . , W o o d s , F. J. and J o h n s o n , J . Ε., "Halon 1301: M e c h a n i s m of Failure to Extinguish Deep-Seated Fires", J o u r n a l of Fire and Flammability (1975), 6, 3 7 - 4 3 . F r a n k i e w i c z , T . C., Williams, F. W . a n d G a n n , R . G., " R a t e C o n s t a n t for O ( P ) + CF Br from 800° to 1 2 0 0 ° K " , J o u r n a l of C h e m i c a l Physics (1974), 6 1 , 402-406. 2

23.

24.


3

3

2

5.

SESHADRi

AND WILLIAMS


181


DISCUSSION A. S. GORDON: We have found t h a t h y d r o g e n - a i r d i f f u s i o n flames p a r t i a l l y i n h i b i t e d by CF~Br a r e v e r y luminous. A t l e a s t p a r t o f t h e l u m i n o s i t y must be from c a r b o n p a r t i c l e s , s i n c e we see b l a c k p a r t i c l e s c i r c u l a t o r y i n s i d e t h e flame cone. We have a l s o noted t h a t HBr added t o a hydrogen flame c a u s e s l u m i n o s i t y , so t h a t p a r t o f the l u m i n o s i t y from t h e CFgBr flame a r i s e s from t h e HBr. F. A. WILLIAMS: That i s v e r y i n t e r e s t i n g . I do n o t u n d e r s t a n d t h e l u m i n o s i t y i n the c a s e o f HBr and hydrogen, but I can b e l i e v e t h a t c a r b o n would be formed by a d d i t i o n o f CF~Br t o the hydrogen f l a m e . In our e x p e r i m e n t s , a l l o f which i n v o l v e d c a r b o n c o n t a i n i n g f u e l s , I f e e l s t r o n g l y t h a t the g r e a t l y enhanced l u m i n o s i t y i s a t t r i b u t a b l e m a i n l y t o carbon t h a t o r i g i n a t e d i n the f u e l i t s e l f . The s u p p r e s s a n t enhances f u e l p y r o l y s i s . T h i s agrees with observat i o n s o f Simmons and W o l f h a r d who found t h a t a d d i t i o n o f B r t o h y d r o c a r b o n - a i r d i f f u s i o n flames i n c r e a s e d carbon p r o d u c t i o n . 2

L. D. SAVAGE: In premixed flame s h e e t s o f methane, we see no f o r m a t i o n o f c a r b o n a c i o u s m a t t e r when CF^Br i s added, however t h e s p e c t r a l changes from the bromine compounds g i v e a r e s u l t i n g c o l o r s i m i l a r t o t h a t from c a r b o n p a r t i c u l a t e e m i s s i o n . F. A. WILLIAMS: In o u r d i f f u s i o n flames t h e r e i s d e f i n i t e e v i d e n c e f o r enhanced p r o d u c t i o n o f carbonaceous m a t e r i a l by a d d i t i o n o f CF-Br. For example, w i t h benzene, our f u e l t h a t e x h i b i t e d t h e g r e a t e s t p r o p e n s i t y f o r s o o t p r o d u c t i o n , i n the absence o f CF~Br, we c o u l d burn f o r t e n minutes b e f o r e carbonaceous d e p o s i t s t h a t b u i l t up on t h e s i d e o f the cup produced f l o w n o n u n i f o r m i t i e s , but when CF-Br was added we were l u c k y t o have two minutes r u n n i n g time b e f o r e s o o t d e p o s i t i o n d i s r u p t e d the f l o w . The magnitude o f t h i s e f f e c t appears t o be much t o o g r e a t t o be a t t r i b u t e d o n l y t o t h e c a r b o n c o n t a i n e d i n t h e CF^Br. In l e a n premixed flames c a r b o n f o r m a t i o n s h o u l d n o t o c c u r even i n t h e p r e s e n c e o f CF~Br, b u t i n s u f f i c i e n t l y r i c h f l a m e s s o o t i n g might be

182

H A L O G E N A T E D FERE SUPPRESSANTS

i n f l u e n c e d by t h e a d d i t i v e i f i t c a n p a r t i c i p a t e d i r e c t l y i n t h e p y r o l y s i s mechanism. One o f t h e p o s s i b l e explanations f o r our observations i s that the i n h i b i t o r enhances oxygen l e a k a g e t h r o u g h t h e d i f f u s i o n f l a m e , which i n t u r n promotes oxygencatalyzed p y r o l y s i s ; i f t h i s i s the operative mechanism, t h e n i n premixed f l a m e s CF^Br need n o t increase sooting a t a l l .


J . DEHN: Have you used HBr, and how do HBr, CF^Br and N2 compare i n e f f i c i e n c y ? F. A. WILLIAMS: We have n o t used HBr a t a l l . On a molar b a s i s , CF~Br i s more e f f i c i e n t t h a n N^. The same i s u s u a l l y t r u e on a mass b a s i s , b u t t o a l e s s e r e x t e n t . We have emphasized (9) t h a t r e l a t i v e e f f i c i e n c i e s on mass molar bases c a n d i f f e r and t h a t e f f i c i e n c i e s o f CF~Br a r e lower i n p u r e oxygen t h a n i n a i r because o f e x o t h e r m i c i t y . R. G. GANN: I n y o u r g r a p h o f c o n c e n t r a t i o n p r o f i l e s , you show no f l u o r i n a t e d o r b r o m i n a t e d s p e c i e s , a l though we might e x p e c t them s i n c e t h e CF-Br i s v i r t u a l l y t o t a l l y consumed. COF , HBr and HF a r e d i f f i c u l t t o sample, b u t CH^Br has been o b s e r v e d t o appear j u s t a s CF-Br i n d e c l i n i n g . Would you comment on t h e non-appearance o f h a l o g e n a t e d s p e c i e s o t h e r t h a n t h e p a r e n t CF^Br. 2

F. A. WILLIAMS : Our system i s d e s i g n e d t o measure o n l y major s t a b l e s p e c i e s . I t s s e n s i t i v i t y as employed i s such t h a t s p e c i e s p r e s e n t i n c o n c e n t r a t i o n s l e s s t h a n a p p r o x i m a t e l y one h a l f mole p e r c e n t would n o t be d e t e c t e d . Although exhaustive searches f o r a d d i t i o n a l s p e c i e s were n o t p e r f o r m e d , no uni d e n t i f i e d peaks were a p p a r e n t i n t h e chromatograms. I t h i n k t h a t o n l y HBr and HF, w h i c h c o n d e n s a t i o n problems p r e v e n t e d us from measuring, would be p r e s e n t i n h i g h enough c o n c e n t r a t i o n s t o be seen a t o u r l e v e l of s e n s i t i v i t y .

6 Halomethane and Nitrogen Quenching of Hydrogen and Hydrocarbon Diffusion and Premixed Flames R. F. KUBIN, R. H . KNIPE, and ALVIN S. GORDON


Naval Weapons Center, China Lake, Cal. 93555

It has been shown in previous work (1-4) that flame inhibitors may function by chemical as well as physical inhibition. Often the efficiency of inhibitors has been determined by the amounts of inhibitor required to produce a given percentage reduction in the flame speed of a premixed flame (5,6) or by the observed flame speed reduction for a given constant inhibitor percentage (1). However, it should be noted that the ability to reduce the flame speed may not bear any relationship to quenching efficiency. For example, low concentrations of hydrocarbonssubstantiallyreduce the speed of a hydrogen flame. Also, it has been reported that inhibitors are more effective in rich rather than lean premixed hydrogen flames (1). The ambiguities introduced by such methods are eliminated by considering the amount of inhibitor required to quench. For diffusion flames few inhibitors have been studied. Quenching concentrations have been reported for CF Br, CHBr and N for several hydrocarbons, hydrogen and carbon monoxide flames (7). In more recent studies the opposed jet diffusion flame has been employed to quantify inhibitor effectiveness (8,9,10). In this work we have compared premixed and diffusion flame results for various inhibitors in a bunsen flame configuration to try to gain insight into the combustion and inhibition process. This paper contrasts the effect of CF Br with those of CF and N on both premixed and diffusion flames. Data are also presented on CF Br , HBr, and He inhibition. 3

3

2

3

2

4

2

2

Experimental The burner used for most of the work was a 10.6 mm ID quartz burner approximately 50 cm long. The remainder of the apparatus consisted of separate air and fuel systems. The air and fuel systems each contained an appropriate number of gas metering devices and a gas sampler for stream analysis. In the premixed flame configuration the metered air and fuel streams were led to a mixing tee before going to the burner. The 183


184


burner was surrounded with an approximately 15 cm diameter fine mesh screen to minimize drafts. For the diffusion flame configuration the fuel was metered to the burner and the a i r metered to a 30 mm ID quartz chimney surrounding the burner, subsequently termed the standard con figuration. The chimney was sealed at the bottom and extended approximately 10 cm above the top of the burner to minimize secondary a i r effects. Additionally, to test the effects of burn er and chimney size, a 18.9 mm ID quartz burner, subsequently termed the large burner, and a 65.2 mm ID quartz chimney, sub sequently termed the large chimney, were substituted i n some of the experiments. The metering devices were Fischer Porter T r i Flat Precision Rotameters calibrated against a wet test meter. The reproduc i b i l i t y of gas calibrations was better than 2% for those rotam eters with a nominal diameter greater than 1/16-inch. For the 1/16-inch rotometer the reproducibility was 3-5% which was depend ent on the flow rate, the slower the flow, the poorer the repro d u c i b i l i t y . Because of the c r i t i c a l nature of the gas mixing to the experiment, a i r , fuel, and inhibitor gas mixtures were sampled and analyzed by mass spectrometry and gas chromatography. These analyses showed that to better than the reproducibility of the experiments (5-8%), the rotometers were stable analytical devices. Because of i t s corrosiveness and solubility i n water, a calculated calibration was used for HBr flows. Observation of the quench points was visual except for those hydrogen flames where the inhibitor imparted no color to the flame. For these essentially invisible flames a simple biased PbS IR detector with maximum sensitivity at 2.5μ was used. The d.c. signal intensity was monitored by an oscilloscope using a d i f f e r e n t i a l amplifier. At the quench point, the signal dropped sharply to essentially zero. For some diffusion flames, the flame did not actually quench but would rise from a few mm to 1 cm or more off the burner and stabilize. The point of l i f t off was reproducible and was taken as the quench point since i t was f e l t that i f the chimney did not extend beyond the burner the flame would have blown off. This definition of quench was also employed by Creitz (7). The quench points for both premixed and diffusion flames were observed to be dependent on the gas flow rates. In order to correlate the results for differing flow rates, we have chosen to use the volumetric flow rate divided by the cross-sectional area of the flow channel to parameterize the flow. For the pre mixed flames and the fuel stream of the diffusion flames, the flow channel i s the burner tube. For the oxidizer stream of the diffusion flames, the flow channel i s the annular region between the burner tube and the chimney. This flow parameter i s designat ed by v. For f u l l y developed laminar flow where the velocity distribu tion can be scaled by the channel dimensions, the velocity


6.

KUBIN E T

AL.

Halomethane and Nitrogen Quenching

185

distribution corresponding to a given ν i s independent of the nature of the gas. We recognize that i n the c r i t i c a l region near the burner port the large temperature and concentration gradients w i l l influence the velocity distribution i n a manner which cannot be described by the parameter. Air was dried and compressed into cylinders and the oxygen used was aviators grade. The hydrogen was industrial grade, methane u l t r a high purity, butane industrial grade, acetylene technical grade and carbon monoxide Matheson purity grade. The reagent gases were used as available and not further purified except for CO and C^^. The acetylene was further purified by scrubbing with concentrated sulfuric acid and drying over calcium sulfate; the acetone being removed quantitatively and the loss of acetylene small. The carbon monoxide was run through the same system since i t removed the iron pentacarbonyl contaminant always present. Mass spectrometrie analysis showed no acetone or other contaminant present i n the C H * pentacarbonyl i n the CO after treatment. For the inhibitors, CF^Br was approximately 98% pure, the remainder showing up as unidentified peak on the gas chromatograph, the CF^Br,^ and CF^ were both found to be mass spectrometrically pure ana N was technical grade. A l l experiments were performed at ambient pressure, approxi mately 700 torr. anc

2

n o

i r o n

2

2

Results Premixed Flames. A summary of our results of quenching of some premixed flames i s shown i n Figure 1. To minimize the effect of oxygen i n the a i r external to the flame, the flames were made up to be slightly on the lean side of stoichiometric. A l l fuela i r mixes were such that the adiabatic flame temperature was kept constant at 2214+14°K. It i s seen that a l l the hydrocarbons and CO exhibit a characteristic flow dependence for quenching by halomethanes ; faster flows require less inhibitor to quench the flame. In contrast, hydrogen flames require more CF^Br and 2 2 ^ increasing flow. When nitrogen i s the quencher, hydrogen shows l i t t l e flow dependence. In the case of CO, at the flow rates used i n this work, the pure gas would not stabi l i z e and bum i n a i r u n t i l 7% hydrogen was added to the fuel. CF Br tends to be less than twice as effective as CF^Br per mole under our test conditions. For methane and butane at the lowest flows, quenching tends to correlate with the number of bromine atoms but the relationship becomes poorer at the higher flow rates. This relationship i s i n qualitative agreement with the work of Rosser, et a l , (6), based on flame speed reduction measurements for near stoichiometric methane-air flames. The effect of fuel/air ratio on the amount of CF^Br required to quench premixed butane or hydrogen flames at a constant ν of 75 cm/sec i s illustrated i n Figure 2. Over the range from slightly fuel rich to lean mixtures, the correlation between the quencher/ C F

B r

w i t

2

2


10


ADDED Ν

C

CF Br / 2

2

0.01

=

CH

•

CO + 7% H

4

9

H

2 2

rj^

CK

0.008

A

0

CFnBro^ ^^

GF,Br

0.006 — 0.004

-

0.002

-

30

60

90

150

120

180

210

v~ CM/SEC

Figure 1. Quencher/fuel ratios at quench for slightly lean premixed fames as a function of flow parameter v. (Added N is nitrogen in excess of that present in the air premixed in the fuel stream.) s


6.

KUBIN E T A L .

Ο


4

8

12

16

20

24

28

32

36

187

40

AIR/FUEL Figure 2. Effect of air/fuel ratio on CF Br requirement for quenching pre mixed butane and hydrogen flames with y = 75 cm/sec s

ADDED N / C H 2

4

1 0

Figure 3. CF Br/butane ratio at quench vs. added N / butane in rich flames (v = 75 cm/sec) :{

2


188

fuel ratio at quench and the a i r / f u e l ratio i s approximately linear. For rich butane/air mixtures, the quencher/fuel ratio shows l i t t l e dependence on the a i r / f u e l ratio. In this a i r / f u e l regime flames with various (added Έ^) /butane ratios were studied. The effect of added nitrogen on the CF^Br/butane ratio at quench i s illustrated in Figure 3 where an approximately linear relationship may be noted. Diffusion Flames. In studying diffusion flames, we have investigated the effects of quenchers i n both the fuel stream and i n the oxidizer stream. As a consequence, there are two flow parameters whichjare under experimental control:__ that of the oxidizer stream ν and that of the fuel stream ^. The quali tative effect of independent variation of these parameters on the amount of quencher required to quench the flame i s illustrated i n Figures 4 and 5 for butane and i n Table I for methane. The figures suggest that the effect of having different flow parameters for the two streams i s not a great deal larger than the reproducibility of the quench points (^8%) over our experimental range. As a convenience i n presenting the results, much of the data were taken where the two flow parameters were equal at the quench point within their experimental uncertainty. This entailed an iterative technique where i n i t i a l fuel and oxidizer flows were established and quencher added to one of the flows u n t i l the flame quenched. Based on the disparity of the two flows, new i n i t i a l flows were established and a new quench point was determined. This was continued u n t i l the disparity of the flows was negligible. The quench point was found to be insensitive to the disparity i n the flow parameters near equality. The effect of nitrogen concentration i n the oxidizer on the amount of fuel stream nitrogen required for quench i s illustrated in Figure 6. The approximately linear relationships exhibited suggest that there i s an equivalence between nitrogen i n the oxi dizer stream and i n the fuel stream such that at quench


χ

(N /fuel) 2

fuel stream

+ q (N /0 ) f

2

2

o x i d i z e r

s

t

r

e

a

m

(1)

where q^ and D^ are parameters dependent on the particular fuel. The data i n Figure 6 show that when atmospheric a i r i s used as the oxidizer, the nitrogen/fuel ratio necessary to quench i s approximately the same for hydrogen and butane flames. Figure 7 shows much the same effect for other quenchers considered i n this study. In each case, the relative quencher requirement for the hydrogen and butane flames depends on the flow parameter v. The trend of the data with ν i s similar to that observed for the premixed flames (Figure 1) as are the relative magnitudes of the quencher/fuel ratios at quench. In Figure 8, both the fuel stream and the a i r plus quencher stream have the same flow parameter v. Since nitrogen i s always present i n the a i r stream, the effect of additional nitrogen

6.

KUBiN E T A L .


189

1.0 r 0.8 0.6 0.4

BO

CO


0.2

J

l__J1 3 V

1

1—ι

L

Γ

10

'OX

FUEL

Figure 4. Effect of oxidizer stream flow parameter v on CF,Br addition to fuel stream to quench a butane diffusion flame ( v = 3.6) M

/M)

0.030 r 0.025

ec

0.020

CQ 0.015

CO

Ο

0.010 0.005

J 5 7

I

I

6

L

tJ

8

FUEL

7

9

10

0X

Figure 5. Effect of fuel streamflowparameter v on CF,Br addition to oxidizer stream to quench a butane diffusionflame(v = 8.8) fMt

ox

H

2

2

2

36.5

CO +1% H

0.058

0.056

0.056

0.16

0.16

0.21

0.22

Fuel

2

2

13

1.9

2.2

2.9

Fuel

2

0.017

0.037

0.038

0.038

0.085

0.088

0.092

0.020

0.016

0.013

0.016

0.019

Air

1.01

1.02

0.98

0.36

0.26

0.19

0.21

0.26

0.36

Air

OXIDIZER STREAM ADDITION CF^Br Added N £

3

2

3

1.6

1.5

1.5

8.8

8.6

8.0

20

19

22

21

18

13

10

10

11

2

CF Br/Air Added N /Air

CF Br/EueL Added N /Fuel

Q x

and V are the values of the flow parameters for the fuel and a i r streams determined prior to Fuel addition of the quencher.

16.3

16.3

36.5

2

12.2

8.2

27.3

18.2

0.75

15.2

6.3

20.4

0.76

10.2

4.2

8.4

0.74

37.7

0.22

0.24

13.8

28.2

2 CO +2% H

CO +2%

CO +2%

2

2

2

C H

H

2 2 C H

C

H

4

CH

10.3

4

0.39

CH

4.9

6.9

4

CH

4

0.29

0.30

0.30

9.8

14.7

0.33

19.6

10.3

9.8

0.34

Fuel

3.4

4

CH

6.9

4.9

*Ox

3

13.8

4

CH

3-4

Fuel

a

CH. 4 CH

4

CH

Fuel

V

FUEL STREAM ADDITION CF Br CF Br Added N

TABLE I. QUENCHER REQUIREMENTS FOR SOME DIFFUSION FLAMES.


6.

KUBIN ET A L .


191

18 FUEL

v

16 •

Ο

14

12

C/3

S

10

g

8

Ο

-4

r

™

Δ

C

H

4 10

H

2

CH

4

C H 2

2

ox

V

FUEL/(1 + N / F U E L ) 2

8.7 CM/SEC

2.2 CM/SEC

6.6 CM/SEC

19.6 CM/SEC

8.2 CM/SEC

6.9 CM/SEC

8.5 CM/SEC

4.2 CM/SEC

Ζ

Psi

6

Ο


"psi 2

4 2 10

14

18

22

26

30

38

N / F U E L IN FUEL STREAM 2

Figure 6. Quench point relationship between N in oxidizer stream and N in fuel stream for diffusion flames of several fuels. (v and Vfuei are flow parameters for oxidizer stream and fuel stream.) s

g

ox

2 PTS —

N /H 2

CF./H,

2

N /BUTANE 2

CF /BUTANE 4

HBr/H -f'

2

^

CF Br/H 3

2

^*-CF Br/BUTANE 3

HBr/BUTANE

0

f-O

2

•

Figure 7. Quencher/fuel ra tios for hydrogen and butane diffusion flames v s . flow parameter. Flow parameter for the fuel plus quencher stream is equal to that of the air stream at quench. (Re producibility of data shown by the many points defining trend for nitrogen quenching of hydrogen.)

«'L

=

Δ

Ζ • 0.04 -

0

0

6

> 0.02 J- Q

0.01

H

) STANDARD

2

BUTANE

BURNER

CONFIGURATION

BUTANE LARGE CHIMNEY AND LARGE BURNER BUTANE LARGE CHIMNEY AND SMALL BURNER H

2

BUTANE

CF Br AS 3

QUENCHER

WITH Η Br AS QUENCHER STANDARD BURNER CONFIGURATION

X

X

10

15

20

-

X

X

25

30

35

a

5.5 5.4 5.9

10.6 10.3 10.6

11.3 12.3

1.9

1.8

10

1 0

C H

(15)

(10)

Both the fuel stream and the oxidizer stream have the same flow parameter V·

4

4

C H

1.9

8.8

4

C H

2

(5)

6.0

4.2

1.8

H (15)

2

1 0

3.8 6.2

4.4

1.7

H (10)

3.4

4.2

9.6

6.6

2

2.3

(15)

H (5)

4

FUEL STREAM

5.3

13.2

6.8

1 0

C H

(10)

2.5

6.5

14.5

11.2

2.2

1 0

4

C H

5.2

13.8

21.1

2.6

4

C H

2

(5)

4.1

11.4

2.6

H (15)

1 0

4.3

11.4 10.6

4.5

12.4

2

2

0

CF,/CF Br 4 3

2.7

3

H (10)

2

N /CF Br

2

N /HBr

11.9

4

OXIDIZER STREAM 9.7

2

N /CF

2.6

a

H (5)

Fuel ( V )

TABLE II. RELATIVE QUENCHER CONCENTRATION REQUIREMENTS AT QUENCH.


6.

Halomeihane and Nitrogen Quenching

KUBiN E T A L .

193

rather than total nitrogen i s compared with the addition of the other quenchers. This i s consistent with the striking equivalence of the incremental addition of nitrogen and CF-Br i n a rich butane flame exhibited i n Figure 3. The flow dependence exhibited by these data i s somewhat different from that shown i n Figure 7 for the same systems. The significance of the difference i s open to question since i n general with the same ν the velocity distribu tion i s different i n a tube and an annular channel. _In spite of this deficiency, i t i s evident that, independent of v, that Ν , CF^, CF«Br and HBr show approximately the same relative efficiency for both fuels independent of whether they are added to the fuel stream or to the oxidizer stream (see Table II). It i s particu l a r l y significant that the relative efficiencies of N and CF^Br are similar i n magnitude to that exhibited by the average slope of the data for the premixed flame plotted i n Figure 3 (^14). Based on the approximately linear correlations shown i n Figure 6, i t i s of interest to compare the quencher requirements for fuel stream addition and for a i r stream addition. The apparent flow dependence i s different for the two cases, and this comparison reflects that dependence (see Figure 9). It i s significant that within the uncertainty associated with the flow dependence, the air stream to fuel stream comparison correlates well with the a i r / fuel ratios for stoichiometric reaction to 1^0 and C0«. This correlation i s also illustrated i n Figure 10, where the range of the (quencher/fuel)//(quencher/air) values for hydrogen and butane as well as the data from Table I, where the flows were not as closely similar, are plotted as a function of the stoichiometric O^/fuel ratio. Simmons and Wolfhard (11) considered the possibility of this type of stoichiometric correlation for the quenching of a number of different hydrocarbon diffusion flames and the hydrogen d i f fusion flame by CH^Br. In these cases, they showed that there was no evidence of such a correlation. It must be recognized however that CH^Br i s i t s e l f a fuel (7). At the level of addition required to quench (quencher/fuel ranged from 0.9 for CH^ to 3 for H«) the additional oxygen requirement would significantly alter the stoichiometry. Thus, at quench any such correlation i s strongly dependent on the extent to which the CH^Br i s participat ing as a fuel molecule. Further evidence of the correlation with stoichiometric 0«/ fuel ratio i s shown i n Figure 7, where the lines were constructed such that the value of q i n Eq. (1) was the stoichiometric 0^/ fuel ratio and D was the mean value of (N /fuel) + q^C^/Og) for the measured points. The values of D thus obtained for H~, C«H«, CH and C,H were 8.7 + 1, 23 + 1, 10.5 + 0.7, and 30 + 1 where the quoted uncertainties represent the extremes of the range of the individual values. Apparently similar conclusions were drawn by Wolfhard but no data have been published (11). On the basis of the above correlations we are led to general ize Eq. (1) and express the quench condition by


2

f

f

2

f

4

1()

194

HALOGENATED

OH — —

•

2

>

BUTANE i

·

H

2

\

FIRE

SUPPRESSANTS

STANDARD BURNER CONFIG WITH Η Br AS QUENCHER

-

Ο BUTANES STANDARD BURNER CONFIG •ADDED N (H ) 2

s?-0- -0--0-

2

CF (H ) 4

2

ADDED N ( C H 2

4

1 Q

)

— — CF Br(H ) 3

2

HBr(H )


2

0.1 ' CF (C H )

0.08

4

0.06

Δ

4

1 0

BUTANE WITH C F B r AS QUENCHER

-

3

0.04

LARGE CHIMNEY AND LARGE BURNER

Figure 8. Quencher/air ratios for hydrogen and butane diffusion flames vs. flow parameter. Flow parameter for fuel stream is equal to that of the air plus quencher stream.

HBr(C H )

0.02

4

10

CF Br(C H ) 3

J

1.01 10

15

4

1Q

I I L 20

25

STOICHIOMETRIC

•I

J

30

35

^

ρ

T

BUTANE

•

QUENCHER

ο

CF

ο

CF Br

Δ

Η Br

ADDED

N

2

4

3

Ο -

Δ

Ο

STOICHIOMETRIC

AIR FUEL

10

15 V CM/SEC

20

25

30

Figure 9. Ratio of quencher/ fuel ratios from data infigure8 to the quencher/air ratios from data in Figure 9 vs.flowparam eter ν


KUBIN ET AL.

/ STOICHIOMETRIC

1.0

-

0.8

-

AIR ( —

(C H ) 4

1 0

L

(C H ) 2

QUENCHER KEY

2

HBr

{ù JT}


ADDED N

0

ft]

C F

B

3 r

CF>,

[ (CO + 2% H ) 2

I I I MIN 0.1

0.2

0.4

L

0.6 0.81.0

ι 11 nu

2.0

4.0

6.0 8.0 10.0

STOICHIOMETRIC ( 0 / F U E L ) 2

Figure 10. Range of values of quencher/fuel/quencher/air from diffusion flame data of Figures 8 and 9 and Table II vs. stoichio metric Ο /fuel ratio. (Stoichiometric air/fuel ratio is plotted on the same scale.) 2

ADDED N /AIR 2

0.0S

0.10

.7 " I I .6 .5 yj Ζ

2.4 CD

0.15 1

0.20

0.25

I

0.30

1

1

0.020

^

1

•\ •

•

1

èâ .3

• \

CO

A-*-

A

là

".2

0.0151

0.010 «

I 0.005

.1

0

1

2 3 4 5 ADDED N / B U T A N E

6

7

2

Figure 11. Effect of nitrogen addition on CF Br requirement for quenching butane diffusion flames (V/uei = v = 10 cm/sec) s

oa?

196

HALOGENATED

£(Q/F) + (N /F) + q ( N / 0 ) + q ( Q / 0 ) F

2

F

f

2

2

0

f£

2

Q

FIRE

* D

SUPPRESSANTS

(2)

f

In this latter equation, (Q/F) i s the quencher to fuel ratio for quencher Q added to the fuel stream, (N /F) i s the nitrogen to fuel ratio for nitrogen added to the fuel stream, (Q/° ô 3 to oxygen ratio for quencher Q added to the oxidizer stream, ( N / 0 ) i s the nitrogen to oxygen ratio for nitrogen added to the oxidizer stream, and ε i s the effectiveness parameter for Q r e l a tive to nitrogen where a l l quantities are evaluated at quench. This relationship has been investigated i n detail for nitro gen and CF^Br quenching of butane diffusion flames. In Figure 11, the relative scaling for fuel stream and a i r stream addition of the quenchers_was determined from the empirical result taken from Figure 9 for v=10 cm/sec rather than the stoichiometric equiva lence ratio. As a consequence, the points corresponding to single component addition both to the fuel stream and to the a i r stream are plotted as a common point. The fact that the remainder of the points where both quenching agents were simultaneously added tend to scatter about a common curve suggests that the scaling param eters are only slightly affected by the level of addition. On the_other hand, Figure 9 shows that there i s a significant effect of ν as well as the particular quenching agent on the value of q-. The curve drawn through the data i n Figure 11 i l l u s t r a t e s the large decrease i n relative effectiveness of CF^Br as a quenching agent as the amount of nitrogen i s decreased. A less complete but similar study of the addition of nitrogen and CF^Br to the hydrogen diffusion flame i s illustrated i n Figure 12 where the relative scaling of the fuel stream and a i r stream data was again based on the empirical values from Figure 9. The gross behavior of the data i s similar to that for butane cited above. However, the consistent difference between the results for fuel stream and a i r stream addition of the quenching agents sug gests that not only i s there a difference i n the scaling param eters q- dependent on the quencher but also that the dependence of the efficiency parameter ε on nitrogen level i s somewhat different for fuel side and oxidizer side addition. These effects are, how ever, small compared to the gross change i n the magnitude of these parameters over the total range of the data. As an extreme example of the reduction of efficiency of CFJSr at low nitrogen levels, we have quenched the hydrogen-oxygen d i f fusion flame both by fuel stream and oxidizer stream addition where no nitrogen was present. These limiting values were CF^Br/H =1.8 and CF-Br/0 =2.7 where the flow parameters of both streams was kept at v=10 cm/sec. By way of comparison, the corresponding values when a i r i s the oxidizer are CF Br/H =0.77 and CF Br/0 =0.95 at v=10 cm/sec. The complete range of CFJJr efficiencies relative to nitrogen i s shown i n Figure 13 where the relative scaling of the fuel stream and oxidizer stream data i s such that the points without nitrogen addition are forced to coincide. Relative to the bromine atom effect, i t i s of interest that 2

F

i

s

t

h

e

u e n c l i e l

2


2

2

Q

2

3

2

3

2

6.

KUBIN E T A L .


197

ADDED N/H 1.0 2.0 3.0 4.0 5.0 6.0 2

2

oc


ο

0.5 1.0 1.5 2.0 2.5 3.0 ADDED N/AIR 2

Figure 12. Effect of nitrogen addition on the CF Br requirement for quenching hydrogen diffusion flames ( v = ν fuel = 10 cm/sec) 3

oaî

198


HBr has approximately the same quenching a b i l i t y per mole as CFJBr (see Figures 7 and 8 ) . The data for fuel stream quenching of methane quoted i n Table I show that, per mole, CF Br PP " mately 1.5 times as effective as CFJBr. The data for the CO flames quoted i n Table I are of some interest relative to the effect of hydrogen i n stabilizing the CO flame. It i s evident from the nitrogen quench data i n Figure 7 that CO i s behaving as a fuel i n the C0-H flame, since the mix has about ten times as much CO as the nitrogen necessary to quench a hydrogen diffusion flame. However, the experimental concentrations of CF^Br necessary to quench the CO-H^ diffusion flame are roughly proportional to the hydrogen content of the fuel, and are consistent with the hypothesis that CF«Br i s reacting with those flame species which are common to the HL-O^ and the hydrogen stabilized C0-0 flames. i s

2

a

r o x i

2


2

2

Discussion A phenomonological description of the conditions for the s t a b i l i t y of a burner flame was given by Lewis and von Elbe (13). According to this description, the s t a b i l i t y i s controlled by the relationship between the gas flow speed and the flame propagation speed adjacent to the surface of the burner. Due to the quenching action of the surface on the radical species responsible for flame propagation, the propagation speed increases rapidly with distance normal to the burner surface from zero to a value characteristic of the gas mixture. In contrast, the gas flow speed which i s also zero at the surface increases more gradually. A stable flame results when the two speeds become equal at some value less than the homogeneous flame propagation speed. As the gas flow to the burner i s increased a stable flame becomes unstable. The c r i t i c a l gas flow occurs when the slopes of the two speeds are equal at the point of intersection and with larger gas flows the flame blows o f f . This description has been generalized i n terms of the transport properties of the gas and reaction rates to a broadly applicable theory of flames through the use of the Damkohler number (14). Lewis and von Elbe demonstrated the consistency of their description for laminar natural gas (principally methane)-air flames. Wohl, est _al, investigated butane-air flames from the same point of view (15). For their experimental conditions employing large flow rates, the latter group observed that blow off was replaced by l i f t off where, after leaving the immediate v i c i n i t y of the burner surface, the flame was subsequently stabilized at a greater distance from the surface by turbulent effects. In both of the above investigations, the flame increased i n s t a b i l i t y as the fuel/air ratios were increased to the rich side of stoichiometric. This was interpreted as due to the modification of the gas composition i n the c r i t i c a l region at the base of the flame by admixture of a i r from the surrounding atmosphere. For butane, i t was shown that the blow o f f / l i f t off limit as a function of percentage butane


6.

KUBiN E T A L .


199

increased monotonically to a limit characteristic of the diffusion flame. This i s qualitatively similar to our observations for the quenching of butane flames with CF^Br as plotted i n Figure 2. Rather than inducing blow off by increasing the gas flow rate, we induce blow off by reducing the flame speed i n that c r i t i c a l region adjacent to the burner l i p as a consequence of adding quench ing agents. In the diffusion flame studies reported here, the flow parameters are at least an order of magnitude smaller than those required for l i f t off i n the absence of the quenching agents. For the premixed flame studies this i s not the case. In fact, the data of Wohl, e£ a l , (15) suggests that the upper limit of flow parameter for the butane data plotted i n Figure 1 i s close to the blow off limit. With the exception of the data for hydrogen flames, the trends of the data with flow parameter are i n qualitative accord with the simple s t a b i l i t y criterion proposed by Lewis and von Elbe (13). It must be recognized that the velocity distribution adjacent to the burner surface cannot be described i n terms of a single variable, e.g., the flow parameter, and the same i s true for the gas compo s i t i o n which controls the flame propagation velocity. In fact the locus of f i n a l flame attachment to the burner surface at quench may i n general be different from that of the uninhibited flame. This was very evident i n the case of quenching butane flames by fuel side addition of CF-Br. The uninhibited flame was attached near the a i r side edge or the burner l i p . Near quench, attachment was ^1 mm below the l i p on the a i r side of the burner tube. We do not believe that the anomolous dependence of hydrogen flame quench ing on flow parameter negates the basic description of flame sta b i l i t y , but rather reflects the details of the velocity and concen tration distributions over the burner surface for high mobility fuel under flow conditions which are far from uninhibited blow off or l i f t o f f . The p o s s i b i l i t y that the anomalous flow dependence of the hydrogen quench points might be associated with the high thermal conductivity of hydrogen was considered. I f this were true, i t should be possible to alter the flow dependence of the butane quench points by using helium as the quenching agent. Some helium quench points were determined for butane diffusion flames for flow parameters i n the range 5 to 15 cm/sec. The results showed that helium was very nearly equivalent to nitrogen as a quenching agent and the dependence of the quench point on ν was essentially the same as that exhibited by the nitrogen quench points i n Figures 7 and 8. On this basis, i t i s concluded that thermal conductivity of the gas does not play an important role i n determining the flow dependence of the quench point. Those data i n Figures 7 and 8 which show the effect of the large chimney and large burner on the CF^Br quenching of the butane diffusion flame suggest that the flow dependence of the quench points i s largely controlled by the detailed f l u i d dynamics i n the neighborhood of the burner l i p . In the case of the large chimney


200


and l a r g e burner where the apparent flow dependence Is much l e s s than f o r the standard c o n f i g u r a t i o n , i t would be a n t i c i p a t e d that f o r f u l l y developed laminar flow, the flow v e l o c i t y would change more slowly w i t h the d i s t a n c e from the burner s u r f a c e than would be the case f o r the standard c o n f i g u r a t i o n . C o n s i d e r i n g the comp l e x i t y of a d e t a i l e d d e s c r i p t i o n of the c o n d i t i o n s at the base of the flame, the o v e r a l l c o n s i s t e n c y of the r e s u l t s f o r d i f f e r e n t f u e l s and d i f f e r e n t quenching c o n d i t i o n s w i t h the c o r r e l a t i o n given by Eq. (1) and (2) i s a l l the more remarkable. The l e f t - h a n d s i d e of Eq. (1) i s the t o t a l moles of n i t r o g e n per mole of f u e l that would be obtained as a r e s u l t of simply mixing a p p r o p r i a t e a l i q u o t s from the f u e l and o x i d i z e r streams to o b t a i n a s t o i c h i o m e t r i c f u e l / oxygen mix. The l e f t - h a n d s i d e of Eq. (2) i s the n i t r o g e n equival e n t of the t o t a l quencher that would be present i n a s i m i l a r l y constructed s t o i c h i o m e t r i c mix. While i t i s obvious that these equations are not q u a n t i t a t i v e f o r the r e p r e s e n t a t i o n of the data that have been presented, they provide a u s e f u l s e m i - q u a n t i t a t i v e t o o l f o r d i s c u s s i n g the e f f e c t s of the quenchers. For example, i t i s r e a d i l y apparent t h a t , r e l a t i v e to hydrogen, butane d i f f u s i o n flames r e q u i r e much l e s s o x i d i z e r stream quencher as a consequence of i t s g r e a t e r oxygen demand. I t i s a l s o apparent that the appare n t l y t r i v i a l o x i d i z e r stream a d d i t i o n of CF^Br (^2% i n the flow r e g i o n of t h i s data) i s the order of one molecule of CF^Br f o r every two molecules of butane, when considered r e l a t i v e to the f u e l molecules which enter i n t o the r e a c t i o n . In order to d i s c u s s the chemistry of burner flame quenching, i t would be d e s i r a b l e to know the composition of the gas i n that c r i t i c a l volume at the base of the flame which determines the s t a b i l i t y of the flame r e l a t i v e to blow o f f . On the s t r e n g t h of the c o r r e l a t i o n s demonstrated i n t h i s paper, we propose that f o r q u a l i t a t i v e c o n s i d e r a t i o n s t h i s composition can be represented by a s t o i c h i o m e t r i c mix c o n s t r u c t e d by mixing a p p r o p r i a t e a l i q u o t s of the o x i d i z e r and f u e l streams. In the case o f d i f f u s i o n flames, we have probed the dark r e gion of a methane flame, between the flame and the port and f i n d that there are a l l mixes of f u e l and a i r present. Since mixtures i n the neighborhood of s t o i c h i o m e t r i c have the greatest flame v e l o c i t y , i t i s t h i s mix which w i l l c o n t r o l the flame attachment ( a l l other mixes would be i n the blow o f f regime). I n t e r e s t i n g l y , t h i s concept can account f o r the great r e s i s t a n c e of d i f f u s i o n flames to blow o f f , r e l a t i v e to s t o i c h i o m e t r i c premixed f u e l - a i r flames. In the c r i t i c a l r e g i o n of flame attachment, the d i f f u s i o n flame s t a b i l i t y i s c o n t r o l l e d by a premixed s t o i c h i o m e t r i c flame, whereas the s t o i c h i o m e t r i c premixed flame i s weakened by the admixture of o u t s i d e a i r i n t o t h i s r e g i o n . The quenching of a burner flame by n i t r o g e n i s due to p h y s i c a l i n t e r a c t i o n s i n the sense that n i t r o g e n i s not c h e m i c a l l y r e a c t i n g w i t h any c o n s t i t u e n t of the flame. I f we consider the base of the flame, there i s a bulk flow of gas i n t o the flame zone. With a i r as the o x i d i z e r , f o r a l l of the flames we have considered, n i t r o -


6.

KUBIN ET AL.


201

gen i s the p r i n c i p l e c o n s t i t u e n t of the gas that i s flowing i n t o the flame zone. By i n c r e a s i n g the n i t r o g e n i n t h i s flow over i t s normal a i r v a l u e r e l a t i v e to oxygen e i t h e r by a d d i t i o n from the f u e l s i d e or from the a i r s i d e i n a d i f f u s i o n flame, we are i n e f f e c t decreasing the f u e l and oxygen flow per u n i t volume of t o t a l flow and thus decreasing the heat r e l e a s e a v a i l a b l e per mole of n i t r o g e n i n that flow. This i m p l i e s an o v e r a l l r e d u c t i o n i n the temperature of the flame. In a d d i t i o n , the reduced c o n c e n t r a t i o n of f u e l and o x i d i z e r w i l l r e s u l t i n the r e d u c t i o n of the r a t e s of r e a c t i o n and hence, the r a t e at which heat i s r e l e a s e d . Therefore, the net r e s u l t of n i t r o g e n a d d i t i o n i s to reduce the flame tempera t u r e , the temperature gradient i n t o the unburned gas, and the r e a c t i o n r a t e i n the unburned gas. Taken together these e f f e c t s r e s u l t i n a r e d u c t i o n i n the flame propagation speed. Thus, r e l a t i v e to blow o f f , f o r a s u f f i c i e n t a d d i t i o n of n i t r o g e n to the supply gases, the flame propagation speed i s reduced t o the p o i n t where i t i s no longer s u f f i c i e n t to keep up w i t h the flow of n i t r o gen and r e a c t a n t s i n t o the flame zone. In a quiescent fuel-oxygen mix, the a d d i t i o n of n i t r o g e n would e v e n t u a l l y g i v e the c r i t i c a l s i t u a t i o n where the heat l o s s from the unburned mix would be g r e a t e r than the heat r e l e a s e r a t e from the chemical r e a c t i o n . While quenchers such as CF^Br, C F B r and HBr w i l l p a r t i c i p a t e i n a p h y s i c a l way much the same as n i t r o g e n , the f a c t that they are e f f e c t i v e at an order of magnitude lower c o n c e n t r a t i o n than n i t r o g e n suggests that t h e i r primary i n t e r a c t i o n i s chemical. Since CF~Br r e a c t s to form other molecules i n the flame, i t i s reasonable to i n q u i r e as to whether the heat of r e a c t i o n i s endothermic enough to s u b s t a n t i a l l y reduce the flame temperature. A c t u a l l y , to form CO, HF and HBr, the r e a c t i o n i s somewhat exothermic. Flame temperature measurements have shown that CF^Br a d d i t i o n a c t u a l l y increases the flame temperature (10). Thus, i t must be concluded that the chemical i n t e r a c t i o n must be such that i t r e s u l t s i n a s u b s t a n t i a l r e d u c t i o n i n the r a t e s of those r e a c t i o n s which are i n v o l v e d i n the u n i n h i b i t e d flame. The case of quenching by CF^, i s somewhat ambiguous, i t s heat c a p a c i t y i s 2 or 3 times that of n i t r o g e n depending on temperature; however, i f i t were to r e a c t to CO and HF c o n s i d e r a b l e heat would be produced. We have observed that the hydrogen flame which i s normally non-luminous becomes h i g h l y luminous upon a d d i t i o n of amounts of CF^Br which are s m a l l compared to the amounts necessary to quench the flame. The l u m i n o s i t y , i n p a r t , i s due to carbon, s i n c e carbon p a r t i c l e s can be seen i n s i d e the flame cone. In cont r a s t there i s no s i g n i f i c a n t v i s i b l e l u m i n o s i t y induced i n the hydrogen flame by CF, even w i t h the r e l a t i v e l y l a r g e amounts r e q u i r e d to quench. The i n i t i a l chemical process would be f l u o r i n e atom a b s t r a c t i o n i n the case of CF, and bromine atom a b s t r a c t i o n i n the case of CF^Br to y i e l d CF^ where the CF^ r a d i c a l i s the source of the carbon which i s r e s p o n s i b l e f o r at l e a s t p a r t of the flame l u m i n o s i t y . We, t h e r e f o r e , conclude that there i s l i t t l e i f any chemical a t t a c k on the CF. molecule i n the hydrogen flame. This 2

2



202

i s i n accord with the much greater bond strength for F relative to Br (125 kcal vs 68 kcal). On this basis, we conclude the CF, i s primarily a physical quencher and that i t s greater effect relative to nitrogen i s a consequence of i t s greater heat capacity. There i s a body of evidence which suggests that i n the flame zone of a hydrocarbon-air flame, the predominant chemistry i s that of the hydrogen-air flame. The hydrogen present i n the reaction zone of the hydrocarbon-air flame i s produced by the pyrolysis of the hydrocarbon i n the lower temperature regions adjacent to the flame. Thus, i f one understands the quenching chemistry of the hydrogen-air flame, one can extend this understanding to hydrocarbon-air flames i n general. Fristrom and Sawyer (16) have analyzed chemical quenching relative to hydrogen-oxygen flame chemi s t r y . From an investigation of the influence of HBr on the p a r t i a l equilibrium concentrations of the various radicals, they conclude the quenching action must be kinetic i n nature. They also conclude that the quenching action must occur i n the low temperature region of the flame volume, rather than i n the high temperature region. An extension of these considerations has proven f r u i t f u l i n defining the nature of the kinetic interaction. The following reactions are postulated to be the important steps i n developing the radicals which lead to flame propagation: H

+ 0

2

OH

+ 0

(3)

0

+ H

2

-* OH

+ H

(4)

2

+ H0 + H

(5)

OH + H

2

Without the interference of other processes, once i n i t i a t e d , these reactions would proceed to a p a r t i a l equilibrium limit at a temperature determined by the i n i t i a l temperature and composition. At any point i n this process, d[H 0] 9

d[H]

d[0]

where this last relation would continue to hold even i f the process OH + OH

H0 + 0 2

and i t s reverse were to play a role. Therefore, i f the i n i t i a l radical concentrations necessary to start the reaction system (3), (4) and (5) were neglected, at p a r t i a l equilibrium [H 0] - [H] + [0]. We have computed the p a r t i a l equilibrium concentration constrained by this condition using JANAF thermochemical data (17). Starting with a stoichiometric H /0 mixture at 298°K, p a r t i a l equilibrium would be achieved at a temperature of M>50°K with ^2/3 of the hydrogen present as water and the remainder essentially present as hydrogen atoms. In the presence of diluent gases the temperature would 2

2

2

6.

KUBIN ET AL.

203


be lower. Thus, other processes must play an important role to obtain the high temperature and water yields characteristic of the hydrogen a i r flames. These processes are the termolecular radical recombination reactions, the most important of which are: H + H

+M


H + OH + M

-*H

+ M

2

(7)

H0 + M

(8)

2

H + 0

+M

-* OH + M

0 + 0

+M

->0

(9)

+ M

2

(10)

At 600°K, the relative p a r t i a l equilibrium concentrations were com puted to be [H]

[0] [H,] [0.] " °·"> ΤτΓΤΓΤ * 0.01, ηΓΚΤ 0.05, TwnT [Η 0] " ' [Η 0] ' [Η 0] ^· [Η 0] ν β

w

2

2

where [0Η]/[0] *

w x >

ν β

2

0.26

2

0.02.

It i s clear that hydrogen atoms are the dominant radicals. With these concentrations, we compare the relative probability of a hydrogen atom reacting by reaction (3) or the reverse of reactions (2) and (5) with that for reacting v i a reactions (7), (8) or (9), using rate constants tabulated by Kondratiev (18). The result i s a factor of ^10 i n favor of reacting v i a the termolecular re actions. Therefore, long before p a r t i a l equilibrium i s achieved, the termolecular reactions w i l l enter into the reaction scheme i n an important way. The effect of the termolecular reactions i s twofold: 1) relation (6) becomes d[H 0] d[H] d[0] 2

—

dt

>

~dt~

+

~dt~

thus allowing an increased y i e l d of water from the overall reaction system, and 2) the enthalpy that goes into radical production v i a reactions (3), (4) and (5) i s returned to the system as heat through the recombination of the radicals, thus allowing high temperature to be achieved. While the relative activation energies of reaction (3), (4) and (5) suggest that hydrogen atoms w i l l tend to build up in the system relative to 0 and OH, this w i l l probably not be as extreme as suggested by the p a r t i a l equilibrium results quoted above as a consequence of the termolecular reactions. Thus relative to the propagation of the flame into unburned gases which are supplied to the reaction zone both the bimolecular reactions and the termolecular reactions play important roles: the bimolecular reaction through the production of radicals and water and the termolecular reactions through the production of heat which in turn leads to an acceleration of the bimolecular reactions lead-

204

HALOGENATED

FIRE SUPPRESSANTS

i n g to HO. I f bromine i s introduced i n t o the system e i t h e r as bromine molecules, HBr or as p a r t of an organic compound, four a d d i t i o n a l r e a c t i o n s need be considered* H + Br + M -* HBr + M

(11)

H + HBr -* H

(12)

2

+ Br

Br + Br + M + B r


H + Br«

2

+ M

HBr + Br

(13) (14)

The net r e s u l t of these r e a c t i o n s i s to i n c r e a s e the competition f o r hydrogen atoms by processes other than r e a c t i o n (3) with an accompanying r e d u c t i o n i n the r a t e of r a d i c a l production by the bimolecular processes (3), (4) and (5). With s u f f i c i e n t a d d i t i o n of bromine to the system, the burning speed decreases because the flame propagation depends on the r e a c t i o n r a t e of the mix and t h i s i s decreased by a d d i t i o n of bromine. The flame e v e n t u a l l y blows o f f as described p r e v i o u s l y . For a quiescent flammable mix, the e f f e c t of bromine on the spread of flame i s s i m i l a r . Eventually with bromine a d d i t i o n , the r e a c t i o n r a t e i s i n h i b i t e d to the p o i n t where heat leak out of the r e a c t i o n volume i s greater than the r a t e of heat generation and the flame does not propagate. To the extent that the above i s a v a l i d d e s c r i p t i o n of the quenching e f f e c t r e s u l t i n g from the a d d i t i o n of bromine to the system, i t i m p l i e s that quenching depends on the number of bromine atoms introduced, independent of the bromine compound (bromine, organic bromides, or hydrogen bromide). Semiquantitative evidence that t h i s i s the case a l s o has been reported by Rosser, et al (6). The increased e f f e c t i v e n e s s of bromine a d d i t i o n with increased n i t r o g e n can a l s o be understood i n terms of the temperature reduct i o n a s s o c i a t e d w i t h n i t r o g e n d i l u t i o n . The r a t e of r e a c t i o n (3) decreases e x p o n e n t i a l l y with the r e d u c t i o n i n temperature thus i n c r e a s i n g the e f f e c t i v e n e s s of r e a c t i o n s (11) t h r u (14) r e l a t i v e to r e a c t i o n (3) s i n c e the former r e a c t i o n s have a lower energy of a c t i v a t i o n than the l a t t e r . C r e i t z has p r e v i o u s l y proposed that the r o l e of bromine as a quencher was p r i n c i p a l l y to recombine oxygen atoms (19). Based on the a n a l y s i s presented above, we f e e l that the hydrogen atom recombination r e a c t i o n s are most probably dominant i n the c r i t i c a l r e g i o n of the flame a s s o c i a t e d with i t s propagation i n t o the unburned gases. On the b a s i s of the above mechanism, i t i s now p o s s i b l e to i d e n t i f y those unique p r o p e r t i e s which d i s t i n g u i s h bromine c o n t a i n i n g compounds as good quenching agents. Where bromine i s i n t r o duced i n t o the flame as a hydrocarbon or fluorocarbon s u b s t i t u e n t *For an organic bromide, the i n i t i a l r e a c t i o n s are: H + RBr ·* R + HBr and p y r o l y s i s , RBr ·* R + Br.

6.

KUBIN ET AL.

205


or as B r the strongest bond that can be made w i t h the species present i n the flame r e g i o n i s the HBr bond. Thus, t o the extent that bond strengths a r e r e f l e c t e d i n the r a t e s of the a b s t r a c t i o n and p y r o l y s i s processes by which Br i s introduced i n the system, the k i n e t i c d r i v e i n the flame r e g i o n i s t o form HBr. The HBr bond i s i n t u r n weaker than the other p o s s i b l e H atom bonds i n the system. As a consequence the k i n e t i c d r i v e i s t o l i b e r a t e the bromine from HBr. T h i s leads t o the sequence of r e a c t i o n s (11) t h r u (14) where the bromine atoms are r e c y c l e d w i t h an attendant i n c r e a s e i n the r a t e of hydrogen atom recombination i n c o m p e t i t i o n w i t h r a d i c a l p r o d u c t i o n r e a c t i o n s (3) t h r u ( 5 ) . 2 >


Summary and C o n c l u s i o n I t has been shown that r e l a t i v e t o the f u e l , the quencher r e quirements are s i m i l a r f o r d i f f u s i o n flames and premixed flames. For those quenchers and f u e l s which we have s t u d i e d , the r e l a t i v e e f f e c t i v e n e s s of the quenchers i s approximately the same f o r a l l of the f u e l s where a i r i s the o x i d i z e r . I n p a r t i c u l a r CF^Br i s about 10 times as e f f e c t i v e as n i t r o g e n on a per mole b a s i s , and twice as e f f e c t i v e on a weight b a s i s . Because of the l a r g e cost d i f f e r e n c e between these agents, l i q u i d n i t r o g e n may i n f a c t be the quencher of choice i n s p e c i a l i z e d a p p l i c a t i o n s . In our s t u d i e s , a comparison of the quencher requirements f o r f u e l stream i n t r o d u c t i o n and o x i d i z e r stream i n t r o d u c t i o n i n t o d i f f u s i o n flames, has shown that they are q u a l i t a t i v e l y r e l a t e d t o the s t o i c h i o m e t r y of the u n i n h i b i t e d flame r e a c t i o n . This suggests that f o r burner flames quenching i s dominated by a r e d u c t i o n of the a b i l i t y of the flame t o propagate i n t o an e s s e n t i a l l y s t o i c h i o m e t r i c mixture of f u e l and o x i d i z e r which i s being c o n v e c t i v e l y s u p p l i e d to the base of the flame. From t h i s p o i n t of view i t i s seen t h a t the c o n c e n t r a t i o n of bromine c o n t a i n i n g species r e q u i r e d f o r quench i s of the same order of magnitude as the other r e a c t i v e species i n the flame r e g i o n . I t a l s o f o l l o w s that premixed flames are e a s i e r to quench than d i f f u s i o n flames, because of the d i l u t i o n of the mixture at the base of the flame by entrainment of gas from the surrounding environment. The f l o w dependence of the quencher requirement f o r most flames s t u d i e d was q u a l i t a t i v e l y c o n s i s t e n t w i t h a blow o f f d e s c r i p t i o n of quenching. Hydrogen flames, both premixed and d i f f u s i o n flames, e x h i b i t e d an anomalous f l o w dependence. Experiments w i t h h e l i u m designed t o d e l i n e a t e the cause of the anomaly e s t a b l i s h e d that the anomaly was not due t o thermal c o n d u c t i v i t y . We concluded that the probable source was r e l a t e d t o the d e t a i l e d f l u i d dynamics i n that r e g i o n at the base of the flame which i s a s s o c i a t e d w i t h l a r g e gradients o f temperature, gas v e l o c i t y , and c o m p o s i t i o n . Of the quenchers s t u d i e d , the l a r g e amounts of N and CF^ r e q u i r e d f o r quench are c o n s i s t e n t w i t h the c h a r a c t e r i z a t i o n of these agents as p h y s i c a l quenchers. The bromine compounds, CF^Br, C F B r and HBr behave as chemical quenchers where t h e i r r e l a t i v e e f f e c t i v e 2

2

2


206


ness is determined primarily by the number of bromine atoms they carry into the flame. For CF^Br quenching of butane and hydrogen diffusion flames, it was clearly demonstrated that the effectiveness of CF^Br as a quencher relative to nitrogen decreased markedly as the nitrogen content of the flame system was reduced. In the case of hydrogen diffusion flames in the absence of nitrogen, the level of CF^Br addition required for quench was sufficiently large that an appreciable dilution would result. This suggests that for some fires, the application of nitrogen and CF«Br concurrently could greatly enhance the effectiveness of CF^Br, and minimize the toxic and corrosive products. An overall consistent picture was obtained from a consideration of the dominant chemical processes which relate to the ability of the reaction to propagate into the unburned gas. On this basis, it was inferred that the dominant radical species are hydrogen atoms which are produced, concurrently with water, by a bimolecular sequence of reactions where the slow step is H + 0^ -*· OH + 0. The reaction heat is liberated primarily by a set of termolecular recombination reaction, principally H + H + M-*H +M, which competes with the hydrogen atom generating reactions. Both of these sets of processes are essential for flame propagation. The effect of physical quenchers is to dilute the reaction species to the point where the rates of reaction and, as a consequence, the temperature is reduced to the point where propagation cannot continue. The chemical quenchers introduce bromine into the system which catalytically recombine the hydrogen atoms via a sequence of reactions. With sufficient bromine addition, the recombination reactions become dominant over the radical, and water producing bimolecular reactions. As a consequence the hydrogen atom concentration is reduced to a point where the rate of the reaction and as a consequence the temperature is reduced so that flame propagation ceases. 2

Acknowledgements The authors are indebted to Messers. J . H. Johnson and J . Calderwood who obtained many of the quench points which we report, to Dr. W. H. Thielbahr for illuminating discussions of the fluid dynamics at the base of the flame, and to Professors F. A. Williams and R. C. Corlett for valuable comments and suggestions on many aspects of this work.

Literature Cited 1.

Miller, D. R., R. L. Evers, and G. B. Skinner; Combust. Flame 7 137 (1963). 2. Fristrom, R. M.; Fire Research Abstr. Re . 9 125 (1967). 3. Wilson, W. E . , Jr., J . T. O'Donovan, and R. M. Fristrom; Twelfth Symposium (International) on Combustion, p. 929, 1968.


6.

KUBiN E T A L .


207

4. Simmons, R. F. and H. G. Wolfhard; Trans. Faraday Soc. 51 1211 (1955). 5. Lask, G. and H. Gg. Wagner; Eighth Symposium (International) on Combustion, p. 432, Williams and Wilkins, Baltimore, 1962. 6. Rosser, W. Α., H. Wise, and J. Miller; Seventh Symposium (International) on Combustion, p. 175, Buttersworth, London (1959). 7. Creitz, E. C.; J. Res. Nat. Bur. Stand. 65A 389 (1961). 8. Friedman, R. and J. Β. Levy; Combust. Flame 7 195 (1963). 9. Ibiricu, M. M. and A. G. Gaydon; Combust. Flame 8 51 (1964). 10. Kent, J. H. and F. A. Williams; Fifteenth Symposium (Inter national) on Combustion, p. 315, Combustion Institute, 1975. 11. Simmons, R. F. and H. G. Wolfhard; Trans. Faraday Soc. 52 53 (1956). 12. Simmons, R. F. and H. G. Wolfhard; Combust. Flame 1 155 (1957). 13. Lewis, B. and G. von Elbe; J . Chem. Phys. 11 75 (1942). 14. Williams, F. A.; Combustion Theory; p. 131; Addison-Wesley Co., 1965. 15. Wohl, Κ., Ν. M. Kapp, and C. Gazley; Sixth Symposium (Inter national) on Combustion, p. 3-21; The Williams and Wilkins Co., Baltimore, 1949. 16. Fristrom, R. and R. Sawyer, Flame Inhibition Chemistry, Thirty-Seventh AGARD Symposium, The Hague, 12 May 1971. 17. Stull, D. R. and H. Prophet, Project Directors; JANAF Thermo -chemical Tables, 2nd Ed., U.S. Dept. of Commerce, NSRDS-NBS -37, June 1971. 18. Kondratiev, V. N.; Rate Constants of Gas Phase Reactions; (in English Translation) U.S. Dept. of Commerce COM-72-10014, 1972. 19. Creitz, E. C.; J. Res. Nat. Bur. Stand. 74A 521 (1970).

7 The Effect of Halogens on the Blowout Characteristics of an Opposed Jet Stabilized Flame R. W. SCHEFER, N. J. BROWN, and R. F. SAWYER


Department of Mechanical Engineering, University of California, Berkeley, Cal. 94720

Flame inhibitors are broadly classified as being physical or chemical. Physical inhibitors (e.g., N ) are believed to act as diluents which lower flame temperatures. This results in slower reaction rates which in turn reduce flame propagation velocity. In contrast, chemical inhibitors (e.g., HBr) directly participate in the chemistry of the system and are believed to reduce reaction rates via a reduction in reactive radical concentrations. There were two principal reasons for the present study of flame inhibition. First, the detailed mechanism of flame inhibition is not well understood. Even for those reactions which are thought to be important, rate constants are not known accurately over the temperature range of interest in practical combustion systems. Second, few quantitative measures of inhibitor effectiveness exist and relatively few measurements have been made on different inhibitors in identical combustion environments. This investigation can be divided into two parts. In the first part, experimental measurements of the extinction characteristics of inhibited propane-air mixtures as a function of inhibitor type and inhibitor concentration were made. The inhibitors investigated were CH Br, HBr, HCl, N , and Ar. Measurements were done in an opposed reacting jet (ORJ). This system is characterized by intense turbulent mixing so that stability characteristics are strongly influenced by chemical kinetics as opposed to mixing phenomena. Part two is a computational study of inhibition in a well stirred reactor. The well stirred reactor represents the limiting case of a turbulent flow system in which mixing is assumed to occur instantaneously, making the system chemically limited. Inhibitor effectiveness based on an inhibition parameter proposed by Fristrom and Sawyer (1) is evaluated for both the opposed reacting jet and the well stirred reactor and the results are compared. 2

3

2

Work supported by NSF-RANN under Grant Number GI-43. 208


7.

SCHEFER

ET AL.

Opposed Jet Stabilized Flame

209

Experimental Apparatus and Procedure Figure 1 shows the approximate flowfield in an opposed reacting jet. The opposed reacting jet used in the present study consists of a main premixed stream of fuel and air and a smaller stream of premixed air and inhibitor which is injected at a high velocity along the centerline and in a direction opposite to the main stream. The result is the formation of a stagnation region and zone of strongly recirculating combustion gases. A stable flamefront forms at the upstream end of the recirculation zone which spreads outward into the main flow. A schematic diagram of the experimental apparatus used is shown in Figure 2. Air was supplied by the house air system. Main stream air was metered upstream of the combustor using a standard 1.52 cm diameter orifice. The pressure was reduced to approximately atmospheric prior to the combustor inlet. Relative humidity of the reactants was measured at each data point and was found to remain relatively constant at 20%. Propane used was Matheson commercial grade 99.9% pure with impurities consisting mainly of iso-Butane (0.35%) and n-Butane (0.05%). The propane was metered through a calibrated rotameter and then mixed with the main stream air in a venturi section. The flow then passes down a 1.0 meter long straightening section to assure a fully developed velocity profile and uniform composition. Measurements were made of the radial velocity profile at the test section inlet using a stagnation pressure probe in conjunction with a static pressure tap in the wall. The velocity profile indicated turbu lent flow existed under the entire range of experimental condi tions. Composition measurements using a quartz sampling probe and a mass spectrometer for analysis of 0 , N and C H were made. The mixture was ignited by a spark plug located just upstream of the combustor test section. Jet air and inhibitor were metered separately through rota meters and mixed in a tee before passing into the jet injector. The HC1 and CHBr had a minimum purity of 99.5%, with trace amounts of hydrocarbons and water. The HBr had a minimum purity of 99.8% with a maximum of 0.2% HC1. Inhibitor concentration in the jet stream was varied up to a maximum of 5% by volume of the total jet flow. No inhibitor was added to the main stream. The inhibitor was added in such a way as to replace air and not 0 or N separately. The combustor test section consists of a 58 mm I.D. χ 356 mm O.D. high temperature vycor liner which was wrapped in asbestos lagging, making the combustor more nearly adiabatic. Temperatures were measured at several locations inside the lagging to determine lagging effectiveness. The resulting temperature profiles indi cated a heat loss of less than 0.3% of the total energy release due to combustion. The stainless steel jet injector extended 6.35 cm into the test section. It had a 6.35 mm O.D. and a 2

2

3

8

3

2

2

210



Figure 1. Opposed reacting jet flowfield

y Warn ι fcrt / czz AIR

•VENTURI MIXER

/ /

HEATER

STRAIGHTENING SECTION

PROPANE

INHIBITOR"

Figure 2. Experimental apparatus

COMBUSTOR /C( / JET

7.


SCHEFER ET AL.

211


3.97 mm I.D. except at the exit where the inner diameter converged to 1.60 mm. Water cooling of the jet walls allowed operation at high temperatures. For all of the work presented here the total jet mass flow rate (air + inhibitor) was maintained constant at .25 gm/sec, corresponding to a jet exit velocity of-96.0 m/sec. To obtain extinction points for a given inhibitor concentration either the fuel flow could be decreased while maintaining a constant main stream air flow, or the air flow could be increased with a constant fuel flow. The former method of decreasing the fuel flow was found to provide the best reproducibility and was used in all cases. Typical main stream mass flow rates were in the range of 18.6 gm/sec to 32.2 gm/sec. Computational Study of Inhibition Introduction. Computational studies of inhibition were conducted for the inhibitors Ar, N, HC1 and HBr. The case of inhibition by CHBr was not investigated since thermal data for CHBr was unavailable. A well stirred reactor model was used for this part of the study because it most closely simulates the experimental studies. Although the ultimate goal of companion experimental and computational studies is quantitative agreement, qualitative agreement between the two is useful and often contributes to a general understanding. Here, there were two difficulties which made comparison at best qualitative: 1) possible uncertainties in the validity of the assumptions used to describe the fluid mechanics of the experimental system and 2) uncertainty in the kinetic data used in the computational studies. Hence the computational studies were directed toward assessing the sensitivity of various reaction steps in the proposed mechanism using the experimental results as guidelines. Rate coefficients required for inhibition modeling have not been measured in the appropriate temperature range so that kinetic data can only be inferred from extrapolation of low or high temperature results; hence it is impossible to absolutely quantify error bounds on the rate coefficients. To assess the effects of various reactions in the assumed inhibition mechanism, the rate coefficients were varied by factors assumed to be extreme bounds on their uncertainties. The remainder of this section contains a description of the well-stirred reactor formalism and a discussion of the mechanisms and rate data used. 2

3

3

Well-stirred reactor. The idealized well stirred reactor is a constant volume steady flow reactor in which mixing is assumed to occur instantaneously between cold incoming reactants and the reacting gases existing in the reactor. Thus the composition within the reactor is homogeneous and the process is kinetically

212


3) Equation of State: p =

*r

(7)

m o is the inverse of the mixture molecular weight and ρ is the density. Given the pressure ρ and rate of heat loss from the system Q, equations (1) and (2) form a.set of 1+1 equations with 1+1 unknowns , 1=1,...,I and either m/V or T. A computer program developed by Pratt and Bowman (3) and based on a Newton-Raphson iteration technique was used to solve the above set of equations. Kinetic Mechanism. The chemistry of the present system can be divided into three parts; the oxidation of propane to the intermediate combustion products H and CO, a mechanism for H and CO combustion, and a mechanism which accounts for the effect of chemical inhibitor addition. Due to the complexity of hydrocarbon oxidation (except per haps for methane), little is known in the way of a detailed reaction mechanism. Thus, past studies have almost exclusively relied on some type of a global or quasi-global mechanism to describe hydrocarbon combustion. Two approximations for the mechanism have been widely used. The first (mechanism 1) and simplest assumes infinitely fast oxidation of the hydrocarbon to H and CO (4). The second (mechanism 2), perhaps a somewhat better approach, is the use of a finite rate global expression for the oxidation of the hydrocarbon based on overall experimental observations in a particular system. Edelman and Fortune (5) derived such an expression for the oxidation of a parrafin Hydro carbon based on shock tube ignition delay times. For propane, the overall reaction can be written


m

2

2

z

3 8 I °2 * and the global rate is given by C

H

3 C 0 + 4 H

+

1

R-

2

(C H ) / (0 )T exp(- I^jJuoles cm sec 3

p

2

8

2

(8)

Caution should be excerised whenever a global reaction rate derived for one combustion configuration is used for another. Such an approximation entirely neglects any reactions between hydrocarbon intermediates. In two such different systems as a shock tube, where low radical concentrations are present during the initial stages of hydrocarbon oxidation, and a strongly backmixed system such as a well stirred reactor where high radical concentrations are present throughout the combustion process, it is probable that both the important reaction paths and the rate

7.

213


SCHEFER ET AL.

controlled. The governing equations, using the notation of Jones and Prothero (2), are as follows: 1) Conservation of Energy: I * I Σ o.h. - Σ o h. = Q/m i=l i=l 11

*

(1)

4

σ· and σ· are the concentrations of the ith species (mole/gm) at the reactor inlet and exit respectively, hj is the species enthalpy, Q is the rate of heat loss from the system and m is the total mass flow rate through the reactor. 2) Conservation of Species: Ί


11

Ί

m

*

J

γ (σ σ.) = Σ (αί-α^.) (Rj-R.j), 1 - 1,2...I. (2) J1 αΚ· and aljj are the stoichiometric coefficients for the ith species in the jth reaction as defined by the general reaction for the species S-μ Γ

—

I I Σ α!.S. + 5 . M = Σ o! S. + â.M, j = 1,2...J. (3) •1 ij ι j · ι ij ι j 4

=

Ί

=

Here âj denotes the stoichiometric coefficient of the third body M and J is the total number of reactions considered in the system. Rj and R-j, the forward and backward reaction rates of the jth reaction, are -I R

j

=

k

j

( p c T

m

) a j 1

?

( p c T 1

1

, a i J

*

j

=

1

- --* 2

J

(

4

)

and - I „ R-,J· k~.J (ρσm ) j j_i Π (pa.) ij, ι j « 1,2,...,J; =

a

a

(5)

k. is an Arrhenius rate coefficient of the form Β. N. k. = 10Texp(-E./RT). J

J

J

J

Reverse rate coefficients were assumed to be related to the forward rate constants through the equilibrium constant Κ k. j c

214


will be different. Bearing in mind the above limitations, it was decided as a comparative measure to use both an infinitely fast rate and the global reaction rate given by equation (8) for propane oxidation. The reaction mechanisms and rate constants for H and CO combustion are relatively well known. These are shown in Table I. The mechanism for H consists of four propagation reactions and four recombination/dissociation reactions. In addition, three reactions involving H0 (reactions 9-11) were found to be important. Three reactions (12-14) were used to describe CO kinetics. Fristrom and Sawyer (1) proposed a model for flame inhibition based largely on the experimental results of Wilson, et al. (11) The major assumption of their inhibition model was that the sTiiïffling reaction 2

2


2

H + HX + H + X is in competition with the branching reaction 2

H + 0 - OH + 0 . 2

For the case of H + 0 flames, inhibition would then occur through the replacement of the chain initiator (H atom) with an inhibitor atom (Br or Cl). A second radical-molecule shuffling reaction 2

2

HX + X ^ X + H 2

was included since it is competitive with the abstraction reaction. Although dissociation reactions are likely to be slow in the temperature range 1000°Κ to 2000°K, it was felt the reverse recombination reactions might be important. Thus HX + M^H + X + M and X + M**X + X + M 2

were included to assess their realtive importance in the inhibition mechanism. The kinetic data associated with the four halogen reactions is shown in Table II. The data are believed to represent the best available data for these reactions and their selection was based on a survey of halogen kinetics by Brown (19). Experimental Results The first step in determining the effect of inhibitor addition on the stability characteristics of the 0RJ was to obtain extinction curves. As discussed previously, extinction

7.

SCHEFER

215


ET AL.

Table I C-H-O-N ChemicalKinetic Reaction Mechanism Reaction 6

14>342

6

13

833

6

10

255

6

10 exp(-5140/RT)

2. H + 0 - OH + 0

10 exp(-16780/RT)

3. 0 + H0 - OH + OH

10 ' exp(-18350/RT)

4. 0 + H + OH + H

10- T'exp(-8900/RT) 17.806-1

2

2

2

2


13,342

1. OH + H - H + H0

2

5. H + H + M-H + M

10

T

2

18

139

_1

7. 0 + Η + Μ-0Η + Μ

10 ' T exp(338/RT) 15.86

8. H + OH + M - H0 + M

10 - exp(500/RT)

9. H + H0 - OH + OH

10 exp(-1890/RT)

10. Η + 0 + Μ + H0 + M

10 exp(1000/RT)

6. 0 + 0 + M^0 + M 2

10

16

2

2

2

2

788

6

13

6

740

10 exp(-1080/RT)

13. CO + 0 + M - C0 + M

10-exp(-2500/RT)

14. co + 0 - CO + o 15. Ν + NO * N + 0

10 ' exp(-54130/RT) 13.204

16. Ν + 0 - 0 + NO

10- T'exp(-6256/RT)

2

2

2

2

11,748

9

14 00

9

13

9

279

10

2

9 806

2

3

n

_1

8

15,176

12. CO + OH - C0 + H

2

7 6

10 * exp(-57790/RT)

2

7

14,398

11. H + 0 * H + H0 2

6

10 10

*units - k: (cmmole"^)"^sec where η is reaction order; R: cal mole'^K"; T: °K . 1

216


Table II


HBr - HC1 Reaction Mechanism Reaction 17. H + CI 2

18. H + Cl

2

13

532

HC1 + H

10 ' exp(-4480/RT)

HC1 + CI

10 exp(-1800/RT)

14,57

13

623

19. HC1 + M

H + Cl + M 10 * exp(-81000/RT)

20. Cl + M

Cl + Cl + M 10-exp(-48494/RT)

21. H + Br

HBr + H

10'exp(-19700/RT)

22. H + Br

HBr + Br

10 * exp(-2200/RT)

23. HBr + M

H + Br + M 10'T"exp(-88000/RT)

2

2

2

24. Br + M 2

14 15

14 43

16

14

16

833

11,33

2

0,5

Br + Br + M 10 T exp(-31300/RT)

*units . - k: cnrmole-1sec-1; 1

13

13 94

21 78

±?

12

R: cal mole'^K"; T: °K

17 18

HUNTER ET AL.


8.

CO-H -O -Ar g

g

217

Diffusion Flame

points for a given inhibitor concentration were determined by maintaining a constant flow velocity and decreasing the fuel flow until stable combustion could not be maintained and extinction occurred. These measurements yielded extinction curves which are plots of main stream velocity at blowout as a function of main stream equivalence ratio. A curve was obtained for each concentration of a particular inhibitor. Typical extinction curves for an ORJ system are parabolic in shape with the maximum occurring at an equivalence ratio of approximately unity. Figure 3 shows extinction curves for the various inhibitors. Fuel supply limitations made it possible to obtain only the lean portion of these curves. The region to the right of the curves represents stable combustion, and the region to the left represents a region in which stable combustion cannot be maintained. The effect of inhibitor addition is to shift the extinction curve to higher equivalence ratios (relative to the curve for an air jet). This corresponds to a reduction in stability limits for a given main stream velocity. Table III shows the reduction in the lean stability limit obtained for the inhibited mixtures of Figure 3. This reduction is indicated by the percent increase in equivalence ratio for the inhibited mixture relative to the uninhibited case. -

TabTëTn

Inhibitor

Mole Percent in Jet Stream

Percent Increase in Main Stream Equivalence Ratio

N

2

3

0.45

HC1

3

1.40

CHgBr

1.5

6.80

HBr

1.5

6.80

CHgBr

3

8.30

HBr

3

9.10

N is least effective and HC1 is slightly more effective while CHBr and HBr are the most effective. Figure 3 gives a qualitative idea of the effect of inhibitor addition on the opposed reacting jet, however, it is important to quantify these results and have a measure of inhibitor effectiveness. An inhibition parameter which is independent of the experimental apparatus has been proposed by Fristrom and Sawyer (χ) (see Appendix 1) as a quantitative means of evaluating the effectiveness of inhibitors. It is defined as 2

3

218

HALOGENATED

t°2> δν "TTT

FIRE SUPPRESSANTS

0)

7"


where δν — is the fractional change in propagation velocity per unit inhibitor normalized to the oxygen consumption. If attention is confined to the effect of added inhibitor on the amount of additional fuel necessary to sustain a stable flame at a constant extinction velocity, then the ratio of inhibition parameters for two different inhibitors A and Β becomes ."HE 'B

(10a)

w

since

_ δν 00b) A " Τ" Β for constant extinction velocity. In a premixed lean system where nearly all the fuel is consumed the 0 consumption approxi mately equals the initial fuel concentration, and equation (10a) can be approximated as: (fuel) W (Π) TfùeïT Β B (Β) To use the relationship expressed by equation (11) for the ORJ, the concentration terms appearing in equation ( Π ) must be related to conditions existing in the ORJ. Past investigations of the ORJ have indicated that flame stabilization is determined by conditions in a small critical zone located just behind the nose of the flame in the stagnation region (20). Turbulent mixing in this region is sufficiently intense that the zone acts as a homogeneous reactor. Thus the concentrations of fuel and inhibitor to be used in equation (11) are those that enter this zone. Based on sodium tracer studies, Schaeffer (20) concluded the total mass flow into the critical zone could be divided into three parts: the primary and jet stream contributions, and the products of combustion entrained by the jet stream. No fuel exists in the jet stream, and under fuel lean conditions very little unburned fuel should exist in the entrained products of combustion. Thus the only fuel entering the critical zone is due to the main stream contribution. Bellamy (21) derived an empirical correlation for the ratio of main stream to jet stream δν V

2

A

V

7.


SCHEFER ET AL.

219

contribution to the critical zone for an ORJ based on the fact that the equivalence ratio in the critical zone at the peak in extinction curves is unity. This has been verified by spectroscopic measurements (22). The resulting equation is 02)

where Mm and Mj are the rates of momentum transport (gm cm /sec) at the main stream inlet and jet stream exit respectively. In the present model it was assumed that all of the jet stream enters the critical zone. This assumption has been used quite successfully in the past to model the extinction characteristics of an opposed reacting jet (20), (21)· Bellamy further assumed that the contribution of the entrainêa combustion products was of the same order as the jet stream. Since the entrained products contain very little unburned fuel (fuel lean system) and no inhibitor, the net effect of the entrained gases is to act as a diluent in the critical zone. The amount of jet entrained gases depends primarily on the jet velocity, which is maintained constant in the present experiment. Thus the dilution effect would be the same for all extinction conditions. Since interest is in the relative concentrations existing in the critical zone at extinction, uncertainties in the entrained products contribution to the critical zone should have a negligible effect on the relative inhibition parameters calculated from equation (11). Here assuming that the jet stream and entrained combustion products contributions are equal, propane and inhibitor concentrations in the critical zone were calculated. The results are shown in Figure 4 where normalized propane mole fraction in the critical zone at extinction is plotted against inhibitor mole fraction for a main stream velocity of 7.6 m/s. Propane mole fraction is normalized by the fuel mole fraction entering the critical zone at extinction for an all air jet. Figure 4 shows that HBr and CHBr are the most effective inhibitors, with the effectiveness of CHBr falling slightly below that of HBr at mole fractions greater than 0.3%. This decrease in effectiveness is attributed to the added hydrocarbon fuel associated with CHBr. Next in decreasing order of effectiveness is HC1, followed by the two physical inhibitors N and Ar. The lower specific heat for Ar (Cp/\/Cp|^ * 0.7) would account for the difference between N and Ar effectiveness. To quantify the results shown in Figure 4, equation (11) was used to calculate relative inhibition parameters using N as a reference case. This was accomplished by computing the slopes of the curves in Figure 4 relative to the slope of the N curve. Only the linear part of the curve (mole fraction less than 0.003 was used for CHBr. A standard linear least squares fitting


2

3

3

3

2

r

2

2

2

2

3

220


10 -

3%HCL

.5% C H B T Δ 3


J . 5 % ΗΒΓ

0.65

0.70

0.75

<J

Figure 3. Effect of inhibi tor addition on opposed reacting jet stability limits. Inlet temperature = 300°K; jet velocity = 96 m/s.

0.80

MAIN STREAM EQUIVALENCE RATIO, 0

o/ / ο

°

-

1.075 / ^ C H

3

Br

-

V

1.050 -

-

1.025

^^ï^^xr""—

A 1.000 0

- f f t j , —Q—-n -ΖΓ~Δ

"

4

—ft—•

ft

\ ""—" x

A r

1

1

1

1

1

0.002

0.004

0.006

0.008

0.010

INHIBITOR

MOLE

FRACTION IN

CRITICAL

~~

-

ZONE

Figure 4. Effect of inhibitor addition on normalized fuel mole fraction in critical zone at extinction. Main stream velocity = 7.6 m/s; inlet temperature = 300°K; jet velocity = 96 m/s.

7.

SCHEFER ET AL.

221


technique was used to compute the slopes, and the standard deviations were used to place error limits on the relative inhibition parameters. The results are shown in Table IV where Φν(max) and Φγ^'η) indicate the maximum and minimum values, respectively, or the relative inhibition parameter. These values were computed using the average slope and adding or subtracting the standard deviation. Note that the value shown in parenthesis for CHBr is the experimentally determined relative inhibition parameter reported in (]_). 3


Inhibitor

Table IV *v(max)

Φ

*v(m1n)

N

2

1.00

1.00

1.00

Ar

0.55

0.37

0.22

HCl

2.90

2.44

2.08

HBr

15.0

12.95

11.3

CHBr

15.0

12.95

11.3

3

Computational Results A useful comparison between the ORJ and the well stirred reactor can be obtained through curves similar to those of Figure 4. The slopes of these curves were used to determine relative inhibitor effectiveness. The governing differential equations for the well stirred reactor were solved wijfh tempera ture as the dependent variable while mass flow rate (m/V) was independently increased until extinction was reached. A series of curves were obtained for fuel mole fraction at extinction as a function of (m/V) for various inhibitor concentrations. The experimental fuel mole fraction at extinction fçr an all air jet was used as a reference point. An equivalent (m/V) was found for the well stirred reactor which would give extinction at this fuel mole fraction. Fuel mole fraction at extinction was then found as a function of mole percentage inhibitor added for the equivalent (m/V). Figure 5 shows the results for the physical inhibitors Ar and N. The computed curves fall below the experimental curves. Two curves were computed for N, one using mechanism (1) and the other using mechanism (2) for propane oxidation. The curve using the Edelman-Fortune global mechanism (mechanism 2) are somewhat closer to the experimental value; however the differences between 2

2

222


Ζ

2ί Ο

1.075

< lâJ ^

1.050

2


Ο iy Ν

1.025

< oc .000 EXPERIMENTAL A r X

t

^ A f - C g H ^ MECHANISM (2).

0.006

INHIBITOR

0.008

0.010

MOLE FRACTION IN CRITICAL ZONE

Figure 5. Comparison of experimental values of normalized fuel mole fraction in critical zone at extinction and theoretical values predicted for a well stirred reactor for physical inhibitors N and Ar. Main stream velocity = 7.6 m/s; inlet temperature = 300° K; jet velocity = 96 m/s. Mechanism (1) = infinite propane oxidation rate; mechanism (2) = finite oxidation rate (5). t

1.075

CjH,

MECHANISM (I)

oc Ζ

I

C H

1.050

9

t

MECHANISM (2)

"-EXPERIMENTAL

1.025

1.000 0.008

0.002

HCl

0.010

MOLE FRACTION IN CRITICAL ZONE

Figure 6. Comparison of experimental values of normalized fuel mole fraction in critical zone at extinction and theoretical values predicted for a well stirred reactor for HCl addition. Main stream velocity — 7.6 m/s; inlet temperature = 300°K; jet velocity = 96 m/s. Mechanism (1) = infinite propane oxidation rate; mechanism (2) = finite propane oxidation rate (5).


7.

SCHEFER

ET AL.


223

the two computed curves isn't significant enough to draw any conclusions about the suitability of one mechanism over the other. The relative inhibition parameter for Ar (using mechanism 2) was computed as 0.21. This falls slightly outside the range of experimental values where the lower limit was v(min) - 0.22. Figure 6 shows the computed curves for HCl inhibition using the rate coefficients of Table II and mechanisms (1) and (2) for propane oxidation. The agreement between the computed and experimental curves is poor, although when relative slopes are compared the agreement is somewhat improved. The lack of agreement between theory and experiment will be discussed in the next section. In an effort to determine the relative importance of the four chlorine-hydrogen reactions on predicted inhibitor effectiveness, the well stirred reactor calculations were repeated for several variations of the chlorine-hydrogen kinetic data using mechanism (2) for propane-oxidation. The rate coefficients were varied by factors assumed to represent extreme bounds on their values. The results are shown in Figure 7. The lowest curve is the case where the dissociation-recombination reactions were eliminated from the mechanism. Extinction limits (reflected by changes in slopes) were found to be most sensitive to variation in the rate coefficients for the reaction H + Cl + M -> HCl + M. Well stirred reactor calculations were made for the case of HBr inhibition using the rate coefficients of Table II and mechanisms (1) and (2) for propane oxidation. Figure 8 shows that the computed results are significantly lower than the experimental values. Figure 9 shows the effects of rate coefficient variation on the computed curves. As with HCl, inhibitor effectiveness was found to be most sensitive to the H + X + M -> HX + M recombination rate coefficient. Increasing the rate coefficients by an order of magnitude resulted in some improvement but the slope was still lower than the experimental value. Discussion The large discrepancy between computed and experimental results can most likely be attributed to the kinetic data used in the calculations rather than errors in the interpretation of the experimental results resulting from uncertainties in the model of the fluid mechanics of the experimental system. The ORJ was assumed to have a homogeneous reaction zone that could be approximated as a well stirred reactor. It was assumed that inhibitor and fuel concentrations in the zone could be computed from jet and mainstream inlet conditions. These approximations have been used previously (20,21) to model the extinction characteristics of an ORJ. An additional argument against the large discrepancy being attributed to the assumed fluid mechanics is that relative

HALOGENATED FIRE SUPPRESSANTS " !

NORMAUZED


FUEL MOLE

FRACTION

I

" -

1

Τ""— '•

1

-

1.078

-

1.080

-

^H+CI+M χ 1

/

EXPERIMENTAL ~

1.025

/

m k

onvr

s££^"^

1.000
c o

2

- H 0

+ I0

H

+ Η

- H + OH

2

2

2

- OH + Ο

CO + ! o

OH + H

2

2

- HF + OCF

CO + OH - C 0

Η + 0

2

- Ο + OCF

- HF + C F

- HBr + C F

Η + OCF - HF + CO

Η + OCF

CF

H + CF

3

H + CF Br

Reactions 3

2

-60.15

-66.42

-14.8

+2.09

+15.67

-21.07

-106.08

- 9.81

-47.26

-52.88

-22.27

Δ H a t 2000°K

TABLE I I . THERMOCHEMISTRY AND MECHANISMS OF H , CO and CFgBr at 2000°K



8.

HUNTER

ET

AL.

CO-HÔÂr

Diffusion Flame

245

d i f f u s e b l u e l u m i n o s i t y , a s s o c i a t e d w i t h CO o x i d a t i o n , f o u n d i n CO f l a m e s and i n h y d r o c a r b o n f l a m e s w h i c h i n v o l v e CO as an i n t e r m e d i a t e . Due t o the low p r e s s u r e t h i s l u m i n o s i t y e x t e n d e d o v e r a l m o s t a two c e n t i m e t e r r e g i o n ( F i g u r e s 1 and 2 ) . The a d d i t i o n o f CF3Br t o the f u e l j e t i n t r o d u c e d an u n e x p e c t e d second l u m i n o s i t y a s s o c i a t e d w i t h the d i s a p p e a r a n c e o f CF3Br ( F i g u r e s 5A and SB). I t was a b r i g h t t h i n cap w h i c h showed a most i n t e r e s t i n g behavior. I t s p o s i t i o n moved up the f u e l j e t a l m o s t l i n e a r l y w i t h i n c r e a s i n g CF3Br c o n c e n t r a t i o n u n t i l r e a c h i n g a p o s i t i o n w i t h i n a m i l l i m e t e r o f the CO b l u e l u m i n o s i t y , F i g u r e 5A, where i t s t a y e d f i x e d f o r a s h o r t c o n c e n t r a t i o n range d i s a p p e a r i n g q u i t e s h a r p l y a t an i n l e t f r a c t i o n o f 0.035. When the a d d i t i v e was i n c r e a s e d t o 0.045, a r e d h a l o appeared o u t s i d e the b l u e CO m a n t l e . The d a r k cone i n s i d e the b l u e m a n t l e appeared u n a f f e c t e d by CF3Br a d d i t i o n . I n a n o t h e r e x p e r i m e n t , the f l o w r a t e was h e l d c o n s t a n t b u t the p r e s s u r e was v a r i e d by c h a n g i n g the pumping s p e e d ; then the p o s i t i o n of the cap remained f i x e d . A t d e c r e a s i n g p r e s s u r e the CF3Br l u m i n o s i t y became f a i n t e r and more d i f f u s e , and d i s a p p e a r e d a t a p r e s s u r e o f 75 t o r r . A d d i n g s m a l l amounts o f CF3Br t o the o x i d i z e r f l o w gave no v i s i b l e e f f e c t u n t i l a f l u x f r a c t i o n o f 0.007 was r e a c h e d . A t t h i s p o i n t a r e d h a l o appeared around the b l u e CO m a n t l e s i m i l a r t o t h a t o b s e r v e d when e x c e s s CF3Br was added on the f u e l s i d e . I t s h o u l d be n o t e d t h a t t h i s c o n c e n t r a t i o n i s s i m i l a r t o the o v e r a l l c o n c e n t r a t i o n when i n j e c t e d from the f u e l s i d e i f i t i s assumed t h a t the f u e l r e a c t s s t o i c h i o m e t r i c a l l y w i t h the o x i d i z e r . A p o s s i b l e e x p l a n a t i o n of t h i s b e h a v i o r can be made by assuming t h a t the flame i s an a x i a l l y symmetric f u e l j e t e n t e r i n g an o x i d i z i n g atmosphere c o n t a i n i n g the CO-O2-H2 r e a c t i o n zone w h i c h we t e n t a t i v e l y i d e n t i f i e d w i t h the b l u e luminous m a n t l e . T h i s m a n t l e i s a t h i g h t e m p e r a t u r e (- 2000K) and a c t s as a s o u r c e o f H atoms w h i c h a t t a c k the 0 Ρ Β Γ . Because of the e x c e s s o f H2 and CO, OH and 0 cannot p e n e t r a t e the j e t , but h e a t can be c o n d u c t e d from the s i d e s and oxygen, w a t e r , and Η atoms can d i f f u s e i n w a r d . C o n v e r s e l y , CO and hydrogen d i f f u s e l a t e r a l l y outward. I f one assumes the t i p r e p r e s e n t s the p o i n t where the f l u x o f Η atoms i s s u f f i c i e n t t o d e s t r o y a l l o f the CF^Br, the h e i g h t of t h i s p o i n t should vary l i n e a r l y w i t h i n l e t f l u x of additive. T h i s s h o u l d c o n t i n u e u n t i l the CF3Br cap r e a c h e s the t i p o f the dark r e g i o n i n s i d e the CO m a n t l e . W i t h f u r t h e r CF3Br, the l u m i n o s i t y can be no l o n g e r seen presumably because i t d i f f u s e s i n t o the mantle. 3

246


The b a s i c a s s u m p t i o n i s t h a t CF3Br r e a c t s p r i n c i p a l l y w i t h H atoms i n a r e g i o n where r a d i c a l s a r e not generated. They t h e r e f o r e must d i f f u s e l a t e r a l l y from the CO-O2-H2 r e a c t i o n r e g i o n where H atoms are assumed t o be g e n e r a t e d u n i f o r m l y and i n d e p e n d e n t l y o f CF3Br. We i d e n t i f y t h i s zone w i t h the o u t e r b l u e luminous m a n t l e . I t i s o b s e r v e d t h a t the geometry i s approximately c y l i n d r i c a l . We assume t h a t the t i p o f the luminous r e g i o n r e p r e s e n t s the p o i n t where the i n l e t f l u x o f C F B r i s b a l a n c e d by the t o t a l f l u x o f H atoms i n t o the c y l i n d r i c a l f u e l j e t ( f o u r b e i n g r e q u i r e d f o r completion (Table I I ) ) : 3

(1) 4 1 r r V ( C F 3 B r ) = 2πrhV (H) (298/T ) = 2 π h D ( H ) ( 2 9 8 / T ) 2


0

o

H

£

f

H

f

f

S i n c e d e t a i l e d a x i a l c o n c e n t r a t i o n and t e m p e r a t u r e p r o f i l e s a r e n o t a v a i l a b l e we a p p r o x i m a t e the a x i a l f l u x o f H atoms w i t h the term D ( H ) f / r . This i s a common o r d e r o f magnitude a p p r o x i m a t i o n . S o l v i n g (1) f o r h we g e t : (2)

2

h =

1

1

2r V D ' (H) ' (F /298)(CF Br) 0

H

f

f

3

o

This p r e d i c t s that h should vary l i n e a r l y with i n l e t c o n c e n t r a t i o n o f CF*Br. I t s h o u l d be n o t e d t h a t i f the p r e s s u r e i s changed w h i l e k e e p i n g the i n l e t mass f l o w c o n s t a n t , the o n l y two p r e s s u r e dependent f a c t o r s i n the c o e f f i c i e n t o f ( C F j B r ) a r e V and Djj. They b o t h are p r o p o r t i o n a l t o i n v e r s e p r e s s u r e , and s i n c e t h e y appear as a r a t i o , the c o e f f i c i e n t i s p r e s s u r e i n d e p e n d e n t ( n e g l e c t i n g the s m a l l change i n ( H ) ) . Hence, the p o s i t i o n o f the luminous cap s h o u l d be p r e s s u r e i n d e p e n d e n t . T h i s i s i n agreement w i t h the o b s e r v a t i o n s ( F i g u r e s 5A and 5B). The model a l l o w s one t o e s t i m a t e the c o n c e n t r a t i o n o f H atoms from the o b s e r v e d h e i g h t o f the CF3Br cap a t a known i n l e t mole f r a c t i o n . By r e a r r a n g i n g Eq. ( 2 ) , we have Q

£

(3)

(H)

£

=

2

1

1

2r V D " h" (CF Br) (T /298). 0

H

3

0

£

U s i n g an a p p r o x i m a t e d i f f u s i o n c o e f f i c i e n t f o r H atoms under flame c o n d i t i o n s , D = 282 cm s e c " , and the f a c t t h a t h = 1.1 cm when ( C F 3 B r ) = 0.022 MF, the v a l u e o f (H)f = O.OI5 i s o b t a i n e d . T h i s i s f o u r times the c a l c u l a t e d a d i a b a t i c e q u i l i b r i u m v a l u e o f 0.0034 MF. The t r u e v a l u e i s e x p e c t e d t o be somewhat l a r g e r t h a n the e q u i l i b r i u m v a l u e but o f the same o r d e r o f magnitude. The somewhat s i m p l e r J o s t model ( r « 2Dt where t 2

0

1

8.

HUNTER ET

CO-H O -Ar

AL.

r

Diffusion Flame

z

247

= H / V Q ) g i v e s an e s t i m a t e o f the h e i g h t H o f the dark zone i n s i d e the CO m a n t l e . Here we assume t h a t the CO must d i f f u s e out o f the j e t t o r e a c t . T h i s d i f f e r s from our model f o r CF3Br, w h i c h i s presumed t o be a t t a c k e d i n p l a c e by the more r a p i d l y d i f f u s i n g H atoms· The o b s e r v e d a x i a l c o n c e n t r a t i o n p r o f i l e s o f CO and CF3Br a r e c o n s i s t e n t w i t h t h e s e a s s u m p t i o n s . U s i n g an a p p r o x i m a t e d i f f u s i o n c o e f f i c i e n t f o r CO, D = 7.6 cm sec' a t flame c o n d i t i o n s , and the v a l u e s r = 0.22 cm, V = 250 cm s e c " , and Tf = 2450°K ( c a l c u l a t e d ) , the h e i g h t o f the dark zone i s e s t i m a t e d t o be 0.8 cm. T h i s v a l u e i s comparable w i t h the o b s e r v e d v a l u e o f 1.8 cm. The n o t a t i o n used i n t h e s e e q u a t i o n s may be summarized as f o l l o w s : 2

1

1


Q

V D r h Τ () ο f t Η

-

f l o w v e l o c i t y (cm see"*) diffusion coefficient r a d i u s o f the f u e l j e t (cm) h e i g h t o f the luminous cap (cm) flame t e m p e r a t u r e (°K) mole f r a c t i o n a subscript indicating i n l e t conditions a s u b s c r i p t i n d i c a t i n g flame c o n d i t i o n s time ( s e c ) h e i g h t o f dark zone (cm).

In the c a l c u l a t i o n s , the d i f f u s i o n c o e f f i c i e n t s a t flame c o n d i t i o n s were e s t i m a t e d from v a l u e s a t room t e m p e r a t u r e and p r e s s u r e by the f o r m u l a (9) (4)

D - D

1

2 9 8

1

P " (T /298) f

6 7

£

where Ρ (atm) i s the flame p r e s s u r e . V.

Summary

We have r e p o r t e d the appearance and b e h a v i o r o f a low p r e s s u r e c a r b o n monoxide-oxygen d i f f u s i o n flame w i t h and w i t h o u t added CF3Br. Measurements were made of a x i a l t e m p e r a t u r e and c o m p o s i t i o n and the s p a t i a l b e h a v i o r o f the v a r i o u s l u m i n o s i t i e s . Based on the o b s e r v e d c o m p o s i t i o n p r o f i l e s a mechanism has been p r o p o s e d f o r the d e s t r u c t i o n o f CF3Br i n t h i s system. The Burke-Schumann-Jost model f o r d i f f u s i o n f l a m e s has been a p p l i e d i n a m o d i f i e d form t o e x p l a i n the b e h a v i o r of the luminous cap a s s o c i a t e d w i t h CF3Br d e s t r u c t i o n . To f u r t h e r j u s t i f y t h i s s i m p l e i n t e r p r e t a t i o n o f the system a c a l c u l a t i o n was made o f the h e i g h t o f the dark cone o f the f l a m e and i t s p r e s s u r e dependence. The


248


reaction region. An i n t e r e s t i n g experiment would be the rigorous exclusion of hydrogen containing species from each stream separately. Since this requires control below parts per m i l l i o n i t appears to be too d i f f i c u l t to be worthwhile with the available less than perfect apparatus. The p r o f i l e s of CF3Br and i t s products at the two pressures (50 and 100 t o r r ) were q u a l i t a t i v e l y s i m i l a r , but the reaction thicknesses scaled with inverse pressure. This i s common behavior i n flames controlled by bimolecular reactions (9J. Since the i n i t i a l concentrations of CF3Br d i f f e r e d , the r e l a t i v e positions of the reaction zones also d i f f e r e d . The species which were found from the reaction of CF3Br i n this flame included HBr, HF, B r and COF2. Our sampling techniques do not allow d i r e c t detection of atoms and r a d i c a l s , but we are f a i r l y confident that the major part of the analyzed molecular bromine was sampled i n the form of atomic bromine. This species i s almost surely formed as atoms by the reaction of hydrogen atoms with HBr (Table II) and thermodynamic calculations indicate that i n the temperature regime where this species i s detected the maximum value attained at equilibrium would be less than 1% of the observed bromine (Table I ) . Therefore, the Br2 curve is interpreted as representing atomic bromine. Both COF2 and HBr were intermediates i n this flame as might be expected from thermodynamic considerations (Table I) . There was no evidence of perfluoro compounds such as CF4 and C2F5, nor did C2F4 appear. The f i r s t product to appear was HBr followed by bromine atoms. The CF3 must be r a p i d l y attacked since no evidence of i t s recombination products was found. Attack was probably by hydrogen atoms forming HF and CF2. The l a t t e r r a d i c a l reacts rapidly with molecular oxygen under flame conditions (15) to form COF . Under our conditions COF i s r a p i d l y attacked presumably by H atoms with the ultimate formation of HF and CO (Table II) . A mechanism consistent with these observed facts i s presented i n Table I I . No evidence of BrF or F2 was found mass spectrometrically. This probably r e f l e c t s a r e l a t i v e l y low F atom concentration and even lower molecular f l u o r i n e concentration i n the sample. This would be expected from the s t a b i l i t y of HF (Table I) and the fact that there appears to be no obvious k i n e t i c mechanism for maintaining a high nonequilibrium concentration of F atoms. 2

2

2

Luminosity. Two luminosities are associated with this flame ; the f i r s t was the expected t y p i c a l

8.

HUNTER

ET AL.

CO-H -0 -Ar 2

2

Diffusion Flame

249

results were i n reasonable agreement with the experiment. A f i n a l piece of corroborating evidence was obtained by estimating the maximum concentration of H atoms i n the flame from the p o s i t i o n a l behavior of the CF-Br luminous cap. A reasonable value was obtained.


Acknowledgements This work was supported by the National Science Foundation RANN (Research Applied to National Needs) Program, under grant GI-440088. The authors thank Luigi P e r i n i for performing the computer c a l c u l a t i o n s . One of use (RMF) would l i k e to thank the NATO S c i e n t i f i c Community for supporting exchange of i n f o r mation between the Applied Physics Laboratory and Laboratorie de Physico-Chemie de l a Combustion, Université^ de Louvain-la-Neuve, Belgium.

250


Literature Cited 1. 2.

Libouton, C . , Dormal, Μ., and Van Tiggelen, P.J., "15th Symposium on Combustion," The Combustion Institute, Pittsburgh, Pa. (in press 1975). Van Tiggelen, P.J. and Fristrom, R.M., (Compara tive Inhibition Studies in Detonation and Deflagration in the CO-O System) submitted to the Second European Symposium on Combustion (scheduled September 1975). Minkoff, G . J . and Tipper, C . F . H . , "The Chemistry of Combustion Reactions," Butterworths, London (1963). Fristrom, R., "Survey of Progress in Chemistry," Vol. 3, ed. A . F . Scott, Academic Press (1966). Jost, W., "Explosion and Combustion Processes in Gases," (in German 1938), translated by H.O. Croft, McGraw-Hill Book Co., New York (1946). Fristrom, R., "Katalyse" (in German), ed. K. Haufe, W. de Gruyter and Co., Berlin (in press 1975). Lewis, B. and von Elbe, G . , "Combustion, Flames and Explosions," Academic Press, New York, 2nd ed. (1961). Potter, A . E . , Heimel, Α . , and Butler, J., in "8th Symposium on Combustion," The Combustion Insti tute, Pittsburgh, P a . , pg. 1027 (1962). Fristrom, R. and Westenberg, A . A . , "Flame Structure," McGraw-Hill, New York (1965). Lask, G. and Wagner, H . G . , in "8th Symposium on Combustion," Williams and Wilkins, Baltimore, pg. 432 (1962). Bonne, U . , Jost, W. and Wagner, H . G . , Fire Research Abstracts and Reviews, 4, (6) (1962). Burke, S. and Schumann, T.E.W., in "1st Symposium on Combustion," reprinted Combustion Institute, Pittsburgh, Pa., pg. 1 (1964). Hoyermann, K . H . , Priv. Comm., Thesis, Gottingen, W. Germany, paper in preparation for submission to Combustion and Flame (1975). Hirschfelder, J.O., J. Chem. Phys. 26, 274 (1957). Matula, R., "Combustion Kinetics of Reentry Vehicle Ablation Materials," Air Force Office of Scientific Research Report No. DUCK 731, September 1973. 2

3.


4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

8.

HUNTER

ET

CO-H -O Ar

AL.

£

r

Diffusion Flame

251

DISCUSSION R. G. GANN: I n your sampling system w i t h C O F and water c o - f l o w i n g t o t h e mass s p e c t r o m e t e r , wouldn't you e x p e c t some s e r i o u s h y d r o l y s i s o f t h e C O F ? 2


2

L. W. HUNTER: We do n o t e x p e c t t h e h y d r o l y s i s t o be i m p o r t a n t s i n c e our p r e s s u r e s were low, t h e r e s i d e n c e time between flame and s p e c t r o m e t e r was short, and the H 0 was o n l y s l i g h t l y absorbed. 2

C. HUGGETT: What was t h e c o n c e n t r a t i o n o f H flame r e l a t i v e t o t h e CF^Br c o n c e n t r a t i o n ?

2

i n your

L. W. HUNTER: The f u e l c o n t a i n e d e i g h t mole p e r c e n t hydrogen and i n a d d i t i o n t h e r e was some hydrogen p r e s e n t i n t h e o x i d i z e r stream. The t o t a l hydrogen a v a i l a b l e was thus i n s t o i c h i o m e t r i c e x c e s s o f CF Br. 3

C. HUGGETT: T h i s flame w i l l have a r e l a t i v e low low H 0 c o n t e n t . T h i s may a c c o u n t f o r t h e o b s e r v a n c e of C 0 F i n t h i s c a s e . The proposed mechanism s u g g e s t s t h a t CF-Br p r o v i d e s a v e r y e f f e c t i v e s i n k f o r hydrogen atoms, consuming about 4 H atoms p e r CF-Br m o l e c u l e . I t would be i n t e r e s t i n g t o o b s e r v e t h e B e h a v i o r o f the flame i n t h e r e g i o n where t h e hydrogen c o n c e n t r a t i o n i s reduced t o the r e g i o n of equivalence w i t h the CF^Br c o n c e n t r a t i o n . T h i s mechanism a l s o s u g g e s t s t h a t CF-Br s h o u l d be as e f f e c t i v e o r somewhat more e f f e c t i v e t h a n C F B r , on a molar b a s i s . Why i s t h i s n o t t h e case? Perhaps i t would be found t o be t r u e i n flames c o n t a i n i n g low c o n c e n t r a t i o n s o f hydrogen. 2

2

2

2

L. W. HUNTER: In comparing t h e e f f e c t i v e n e s s o f CF^Br and C F B r as i n h i b i t o r s i t i s u s u a l l y assumed that chemical i n h i b i t i o n i s a r e s u l t of reduction of r a d i c a l c o n c e n t r a t i o n i n t h e p r i m a r y r e a c t i o n zone. One can v i s u a l i z e two g e n e r a l mechanisms f o r t h i s : (1) s c a v e n g i n g o f r a d i c a l s , i . e . , the r e a c t i o n o f r a d i c a l s t o produce s t a b l e m o l e c u l e s o r u n r e a c t i v e r a d i c a l s ; (2) c a t a l y t i c r e c o m b i n a t i o n o f r a d i c a l s . The f i r s t i s a b i m o l e c u l a r p r o c e s s and t h e r e i s a s t o i c h i o m e t r i c r e l a t i o n between r a d i c a l c o n c e n t r a t i o n 2

2

252


and r a d i c a l c o m p l e t i o n . I n t h e second p r o c e s s t h e l i m i t i n g slow s t e p i s a t e r m o l e c u l a r r e c o m b i n a t i o n , but the processes can r e c u r w i t h a long c h a i n l e n g t h . F o r example, S c a v e n g i n g : H + CF^Br Recombination:

H + Br+M

HBr + CF^ HBr + M*

(slow)

HBr + OH(H,0) H 0 + Br ( f a s t , w i t h r e g e n e r a t i o n o f Br)


2

In comparing t h e two m o l e c u l e s , i f one assumes o u r mechanism i s c o r r e c t , b o t h m o l e c u l e s have t h e same s c a v e n g i n g p o t e n t i a l , i . e . , f o u r H atoms p e r m o l e c u l e . I n s o f a r as r e c o m b i n a t i o n i s concerned, C F B r produces two Br atoms and s h o u l d show t w i c e t h e r e c o m b i n a t i o n c a p a c i t y o f CF-Br. By comparing t h e e f f e c t i v e n e s s o f t h e s e two o r s i m i l a r compounds i t might be p o s s i b l e t o e s t a b l i s h t h e r e l a t i v e c o n t r i b u t i o n s o f t h e two mechanisms. P a r e n t h e t i c a l l y , s i n c e s c a v e n g i n g i s b i m o l e c u l a r and w h i l e t h e slow s t e p i n c a t a l y z e d r e c o m b i n a t i o n i s t e r m o l e c u l a r , t h e two mechanisms might be d i s t i n g u i s h ed by s t u d y i n g t h e p r e s s u r e dependence o f i n h i b i t i o n . 2

2

J . BIORDI: I would s i m p l y l i k e t o s t a t e t h a t y o u r proposed mechanism f o r t h e d i s a p p e a r a n c e o f CF^Br and subsequent f a t e o f t h e CF- and CF^ s p e c i e s i s e n t i r e l y c o n s i s t e n t w i t h our o b s e r v a t i o n s i n s t o i c h i o m e t r i c premixed methane flames c o n t a i n i n g CF-Br, and, i n f a c t , we used a s i m i l a r mechanism t o a c c o u n t f o r t h e s e o b s e r v a t i o n s . R e c e n t l y , we have detected the C F r a d i c a l i n these flames, although t h e CF~ r a d i c a l remains u n d e t e c t a b l e . I t seems t o me t h a t t h e r e a c t i o n CF +H+HF+CF , b e i n g a t r u e s i n k f o r H atoms, must p r o v i d e some i n h i b i t i o n i n t h i s system. What s o r t o f b e h a v i o r would you have e x p e c t e d o f your flame system i f i t were " i n h i b i t e d " i n t h e u s u a l sense? 2

3

2

L. W. HUNTER: We agree t h a t t h e r e a c t i o n you quote s h o u l d p r o v i d e some i n h i b i t i o n . Our thoughts on t h i s m a t t e r a r e p r e s e n t e d i n t h e r e s p o n s e t o Dr. Huggett's q u e s t i o n . We would e x p e c t t h a t i n o u r system an i n h i b i t e d flame would show an i n c r e a s e i n t h e s i z e o f t h e mantle and a drop i n t h e peak flame temperature. The c o n c e n t r a t i o n p r o f i l e s o f t h e major s t a b l e s p e c i e s s h o u l d a l s o show some change. We d i d n ' t o b s e r v e t h e s e


HUNTER ET AL.

CO-H -O -Ar t

t

Diffusion Flame

HALOGENATED

254

FIRE SUPPRESSANTS

changes because o f o u r low p r e s s u r e and because H (a c a t a l y s t o f CO r e a c t i o n s ) was p r e s e n t i n e x c e s s o v e r t h e CF^Br. 2


F. A. WILLIAMS: Your model f o r t h e i n n e r flame seems t o i n v o l v e a more o r l e s s c o n t i n u o u s d i f f u s i o n p r o c e s s which might l e a d t o a g r a d u a l i n c r e a s e i n l u m i n o s i t y i n s t e a d o f t h e a p p a r e n t l y f a i r l y s h a r p cap t h a t you o b s e r v e d . Would you l i k e t o comment on t h i s ? L. W. HUNTER: T h i s i s an i n t e r e s t i n g q u e s t i o n , t o which we do n o t have a complete answer. Our model may be t o o n a i v e t o a l l o w d e t a i l e d s t r u c t u r a l c a l c u l a t i o n s s i n c e o u r o r i g i n a l aim was t o e x p l a i n t h e p e c u l i a r behavior of the t i p . I d e a l l y i f the j e t v e l o c i t y p r o f i l e i s u n i f o r m and t h e r e a c t i o n o f H atoms w i t h CF-Br and i t s fragments i s r a p i d we s h o u l d e x p e c t a s h a r p c o n i c a l luminous zone r e p r e s e n t i n g t h e i n t e r f a c e between H and CF-Br w i t h i n a l a r g e r c o n i c a l dark zone r e p r e s e n t i n g t h e o n s e t o f CO r e a c t i o n (see figure). We o b s e r v e i n s t e a d a c r e s c e n t shaped i n n e r luminosity i n s i d e a s l i g h t l y tapered c y l i n d r i c a l dark r e g i o n w i t h h e m i s p h e r i c a l c a p . (Refer t o o u r f i r s t figure). We f e e l t h a t t h e d e v i a t i o n s from t h e e x p e c t e d i d e a l c o n i c a l geometry i n t h e two c a s e s a r e due t o a nonuniform v e l o c i t y p r o f i l e a c r o s s t h e f u e l j e t and t h a t o u r f a i l u r e t o o b s e r v e a c o n t i n u o u s i n n e r luminous s u r f a c e e x t e n d i n g t o t h e l i p o f t h e f u e l n o z z l e i s t h a t when t h e CF-Br and CO l u m i n o s i t i e s l i e c l o s e t o one a n o t h e r , t h e C r - B r l u m i n o s i t y i s o b s c u r e d . Note t h a t when t h e i n n e r luminous c a p approaches t h e t i p o f t h e d a r k zone i t s t a y s s t a t i o n a r y about two m i l l i m e t e r s below t h e t i p o f t h e dark zone o v e r a s h o r t c o m p o s i t i o n range and t h e n disappears a t a r a l a t i v e l y reproducable concentration r a t h e r t h a n t r a v e l i n g smoothly i n t o t h e b l u e CO l u m i n o s i t y as c o n c e n t r a t i o n o f CF^Br i s i n c r e a s e d . YEN-HSIUNG KIANG: I n t h i s paper, o n l y one low c o n c e n t r a t i o n o f H i n t h e f u e l i s used. Because o f t h i s low H c o n c e n t r a t i o n , a l l o f t h e bromine appears as B r i n t h e p r o d u c t g a s . We have done some t e s t work on t h e b u r n i n g o f b r o m i n a t e d h y d r o c a r b o n s i n a c o m m e r c i a l b u r n e r , t h e d a t a show t h a t when t h e [ H ] / [ B r ] r a t i o i n c r e a s e s , [ B r ] i n t h e p r o d u c t gas d e c r e a s e s and [HBr] i n c r e a s e s . I think the possible reactions are 2

2

2

2

2

2

Br

0

+ H + HBr + Br

8.

HUNTER

ET

CO-H.-OÂr

AL.

l/2Br

2

+ H0 2

HBr

Diffusion Flame +

255

OH

due t o h i g h e r [ H ] . F u r t h e r works on v a r y i n g t h e H~ c o n c e n t r a t i o n i n t h e f e e d and d e t e r m i n i n g c o r r e s p o n d i n g [ B r ] / [ H B r ] r a t i o s i n t h e p r o d u c t gas w i l l be q u i t e interesting. 2

2


L. W. HUNTER: i n t h e atomic correspond t o we agree w i t h i n flames a r e

I n t h i s system we b e l i e v e bromine o c c u r s form. The e q u a t i o n s you p r e s e n t o v e r a l l s t o i c h i o m e t r y and i n t h i s sense your comments. The elementary r e a c t i o n s n o t between whole m o l e c u l e s .

9 The Effect CF Br on Radical Concentration Profiles in Methane Flames 3

JOAN C. BIORDI, CHARLES P. LAZZARA, and JOHN F. PAPP


Pittsburgh Mining and Safety Research Center, Bureau of Mines, U. S. Department of the Interior, Pittsburgh, Pa. 15213

That complicated reaction systems involving chain reactions may be extremely sensitive to small amounts of substances that interfere with the chain propagation is well known in chemistry (1). Flames are examples of such reaction systems and the recognition that certain molecules retard flame propagation out of proportion to their thermal or mechanical influence leads to the supposition that their effect is a matter of chemical reactivity. The idea of "chemical extinguishment" becomes very attractive when it is appreciated, as pointed out by Friedman and Levy (2), that chemical reactivity varies by orders of magnitude and so offers the promise of extinguishing agents significantly more effective than those now in use. Aside from screening tests, the development and efficient use of such agents requires an understanding of the mechanisms by which uninhibited flames propagate and of the mechanisms by which chemical agents impede that propagation. The survey of extinguishment mechanisms by Friedman and Levy (2) has been followed by periodic reviews (3-8) which, in their entirety, cover physical as well as chemical extinguishment, heterogeneous and homogeneous systems of fuel, oxidant, and extinguishing agent, and a variety of measurements of agent effectiveness. We wish to narrow this discussion primarily to volatile halogenated inhibitors in premixed flames studied principally by the technique of flame microstructure, and, further, to scrutinize the available data for its implications on the effects of these molecules on the primary chain propagating radical species in the flame. The papers that, in our view, are most pertinent to this discussion are summarized in Table I, and they have been divided into those providing indirect and direct evidence regarding atom and radical concentrations in the presence of inhibitor. It may be seen that a wide variety of conditions have been employed in these studies. This fact precludes the formulation of a generalized mechanism whose consequences can be predicted and directly tested. 256


9. B i O R D i

ET AL.

257

Methane Flames

Rosser, Wise and Miller (9) measured the effect of various halogenated inhibitors on CH^-air flame speeds and observed Br containing inhibitors to be more effective in rich than in lean mixtures. Their effectiveness (in a 10% CH^-air flame) was proportional to the number of Br atoms in the inhibitor molecule. They also verified that small quantities of inhibitors could significantly reduce flame speed without affecting the final flame temperature. More precise measurements of methane-air-CH^Br burning velocities gave similar results (10) and were considered to support the mechanism suggested in (9). In that mechanism the critical species were the halogen atom and the halogen acid, the former considered to arise from the thermal decomposition of the inhibitor and the latter from hydrogen atom abstraction (from the fuel or any other hydrogen containing species) by the halogen atom. The acid in turn may react with any available radical species, H, 0, OH, CH, to regenerate the halogen atom. In effect, it is a matter of substituting a halogen atom for the usual chain propagating radicals with the result that reactions usually associated in a critical way with flame propagation, H+0->0H+0, 0+CH-»HC0...-£0, and C0+0H-+C0+H are reduced in rate, and that reduction is observed macroscopically as a reduction of burning velocity. This interference was considered to occur when the temperature was high. The halogen atoms may initiate a competing chain, but they cannot serve the function of H, 0, or OH in the reactions cited. Although recognizing the possibility, the authors did not think a reduction in the rate of diffusion of H, 0 and OH was a critical feature of combustion inhibition. One of the first structure studies of inhibited flames (11) supported this mechanism to the extent that the concentration of CO was found to be reduced in a lean, atmospheric CH.-Op-Ar flame containing 0.36% HBr, relative to the clean flame. It was suggested that formaldehyde should also be markedly affected by HBr. Such observations were later made in structure studies of lean (12) and stoichiometric (13) low pressure methane flames, the latter with CFBr inhibitor. The most concentrated effort in determining the structure of inhibited flames was that of Wilson and co-workers (12, 14), who reported the structures of two clean and four inhibited (CH3Br, HBr, HCl, Cl ) flames, all lean CH-0 maintained at 0.05 atm. Common to all the inhibited flames was an early appearance of new products related to the inhibitor, e.g. CHjX when the inhibitor was HX. The maximum H concentration was greater in flames containing HX than in clean flames. The data could be analyzed for individual net chemical reaction rates, , and in each inhibited flame the profile of K ^ was sharper, narrower, and shifted to a higher-temperature part of the flame. Profiles for OH and H were calculated from the 3

2

3

2

)

2

3

2

4

2

2

CH

rich range

2

2

2

2

2

2

2

2

2

2

H-N0-N H-0-N

2

2

2

2

range rich rich

H-0-N H-N0-Ar H-0-N

4

2

3

stoich.

CH-0-Ar

2 3

4

4

2

4

4

HBr, CHjBr, C H HBr, HI, HC1

3

3

CHBr(1-3), CH(1-3) CHBr(5) HBr (0.027,0.1)

2

HBr(0.36) CHBr(1.71),HBr(1.88), HC1(2.69), Cl(1.71) CFBr (0.3)

lean lean

CH-0-Ar CH-0

4

2

range

CH-air

Variety of Br, I, CI contg. organics («f-

-> H Br, b e f o r e i t becomes b a l a n c e d , w i l l a c t t o reduce t h e H atom concentration. I f , as we suggest, HF i s produced by a r e a c t i o n such as (9) , CF~ + H -> HF + C F , i t s v e r y e a r l y appearance i n t h e i n h i b i t e d flame a l s o i m p l i e s H atom s c a v e n g i n g . I n a d d i t i o n , removal o f methyl r a d i c a l s i s o c c u r r i n g t o produce C H C F , presumably v i a a r e c o m b i n a t i o n o f methyl and t r i f f u o r o m e t h y l radicals. A l l o f t h e s e r e a c t i o n s w i l l cause i n h i b i t i o n o f t h e flame's u s u a l p r o p a g a t i o n r e a c t i o n s . Now i f HBr can be r e g e n e r a t e d , o r , i n f a c t , i f any r e a c t i o n o c c u r s which causes r e a c t i o n (1) t o p r o c e e d i n t h e f o r w a r d d i r e c t i o n , then the i n h i b i t i n g a c t i o n becomes c a t a l y t i c . A t p r e s e n t , t h e view seems t o be t h a t H + Br + M + HBr + M i s t h e most l i k e l y r e a c t i o n t h a t can r e g e n e r a t e HBr. We suppose (without t h e b e n e f i t o f a measured r a t e c o n s t a n t f o r t h i s r e a c t i o n , though) t h a t t h i s r e a c t i o n i s r e l a t i v e l y unimportant i n our low p r e s s u r e f l a m e . However, there i s s t i l l the p o s s i b i l i t y t h a t r e a c t i o n (3), Br + H 0 j HBr + Oy c o u l d be s i g n i f i c a n t even a t these pressures. 2

2


2

2

2

10 Inhibition of the Hydrogen-Oxygen Reaction by CF Br and CF BrCF Br 3

2

2

GORDON B. SKINNER


Department of Chemistry, Wright State University, Dayton, Ohio 45431

About ten years ago we published results of shock-tube experiments in which ignition delays of H - O - Ar mixtures containing either CF Br or CF BrCF Br, were measured (1 , 2). Substantial increases in the ignition delay times were found for the gas mixtures containing halocarbons. These studies also included some product analyses of partially reacted H -O -Ar-halocarbon mixtures, and product analyses of shock heated Ar-halocarbon mixtures (with no H or O present). An attempt was made to interpret the ignition delay data using a set of eleven elementary reactions for hydrogen-oxygen itself, five additional reactions for the CF Br experiments, and four additional reactions for the CFBrCFBr experiments. We were able to adjust the rate constants to give quite good agreement of the calculations with experiment, but it was recognized that rate constants for many of the reactions in the hydrogen-oxygen scheme were uncertain, so that the rate constants deduced for the inhibition reactions might well be substantially in error. This suspicion was strengthened by the fact that some of the deduced rate constants were substantially larger than would be expected from general chemical considerations. Now that much more accurate date are available for the hydrogen-oxygen reaction, it appeared to be worthwhile to carry out a new kinetic analysis of our data. 2

3

2

2

2

2

2

2

2

3

2

2

The Hydrogen-Oxygen-Argon System The 13-reaction scheme we have used for hydrogen oxidation is shown in Table I. This was developed during a study of methane oxidation (3,4) and while it does not contain all reactions of possible importance for hydrogen oxidation, it includes those most important for stoichiometric and richer mixtures between 900 and 1600 K. While the evidence concerning the initiation step is not entirely conclusive yet, the date of Jachimowski and Houghton (5) strongly suggest that our reaction 1, rather than 295


296 Table I

Reactions and Rate Constants Assumed for the 2 " °2 * i t h Argon Diluent H

R e a c t

Reaction

w

Afforward) A(forward) A(reverse)

a

Ε (forward) (forward")^ Ε(reverse) u

Ref.

1.36E+ +1313 1.36E 4.48E + 11

48.15 48.15 29.85

2. OH+H = OH+H

+ 13 2.20E + 8.40E + 13

5.15 20.10

(6)

3. H+0 = OH+0

2.20E ++ 14 1.30E + 13

16.80 0.00

(6)

4. 0+H = OH+H

5.90E ++ 13 2.50E + 13

11.20 9.05

(6)

5. 0+H 0 = 20H

+ 13 6.80E + 6.80E + 12

18.35 1.13

(6)

+ 15 2.60E + 4.90E + 16

-2.30 117.20

(6),c

7. H+0H+H 0 = Η 0+Η 0

+ 16 4.35E + 8.00E + 17

-2.30 117.20

(6),c

8. H+H+Ar = H +Ar

2.40E ++ 14 1.00E + 15

-2.30 102.20

(6),c

+ 15 4.00E + 1.60E + 16

-2.30 102.20

(6),c

10. 0+0+Ar = 0 +Ar

+ 13 1.28E + 1.28E + 15

-2.30 121.90

(7),c

11. H+0 +Ar = H0 +Ar

+ 15 1.68E + 1.20E ++ 15

45.90 -1.00

(6),d

12. H0 +H = Η 0 +Η

+ 12 1.10E + 2.55E + 12

18.70 3.75

(6),e

13. H 0 +Ar = 20H+Ar

+ 17 1.20E + 9.00E + 14

45.50 -5.07

H

+ 0

=

2 2

2

0

H

2


o n

2

2

2

6. H+OH+Ar = H 0+Ar 2

2

2

2

2

9. H+H+H 0 = Η + Η 0 2

2

2

2

?

2

2

?

2

2

2

2

1

1

(5)

(6)

2

(a) Units: mole' cc sec for bimolecular reactions and mole c c sec for termolecular reactions. A l l reactions are assumed to be elementary and to maintain the same order under a l l conditions considered. "E" means "times 10 to the". 1

2


10.

Hydrogen-Oxygen Reaction

SKINNER

297

(b)

Units:

(c)

Equation i n t a b l e matches references r a t e constant at 1200K. Ε o f -2.30 K c a l . corresponds to a 1/T dependence o f the r a t e constant i n t h i s temperature range.

(d)

Equation i n t a b l e gives r a t e constants 0.8 times those o f reference.

(e)

Equation i n t a b l e gives r a t e constants 1.5 times those o f reference.

Table I I

Kcal.

Comparison o f Observed and C a l c u l a t e d I g n i t i o n Delay Times f o r H - 0 - Ar Mixtures ?

ι υ Α

H

2

*

0

l c m p

0

2

*

?

Microseconds

r

Κ

atm

Obs

Calc.

Reference

8

2

1000

5

2630

3400

(1)

8

2

1050

5

410

325

(1)

8

2

1100

5

135

144

(1)

4

2

1200

1

230

230

(8)

4

2

1600

1

52

43

(8)


298

Reactions Considered t o be Involved i n I n h i b i t i o n

Table I I I

A(forward) A(reverse)


Reaction

a

Ε(forward) Ε(reverse)

b

Ref.

14.

CFgBr+H = CF^+HBr

2.50E + 13 4.55E 11

3.40 24.00

15.

CF +H = CF H

5.00E + 13 2.80E + 15

4.00 110.30

16.

CF +H = CF H+H

3.50E + 11 5.20E 12

9.20 11.20

(£)

17.

HBr+H = Br+H

1.00E 14 2.70E + 15

2.90 19.70

(£)

18.

Br +Ar = 2Br+Ar

1.18E + 13 8.70E + 12

32.40 -11.40

19.

H+Br

= HBr+Br

1.40E + 14 8.00E + 13

0.90 41.70

20.

C F B r = CF +Br

1.68E + 13 3.83E + 11

66.30 0.50

21.

CF Br+Br = C F + B r

6.00E 13 1.87E + 12

23.00 1.00

22.

C F

2.10E 17 1.60E + 13

92.00 0.00

(15)

23,

HBr+Ar = H+Br+Ar

1.00E + 15 7.20E + 14

83.80 3.70

(16,17?

24.

C,F-Br-+H = C«F-Br+HBr 2 4 2 2 4

5.00E 13 4.15E + 10

3.00 20.80

25.

C F Br

2.00E + 13 1.46E + 09

68.60 1.00

1.00E + 14 4.00E 11

20.00 4.00

26.

3

3

3

2

3

n

2

2

3

3

3

2

2

C

3

6

- 2CF

4

2 4 F

B r

2

2

2

3

= C^^Br+Br

+ B r

=

C

2 4 F

B r + B r

2

27.

C^F.Br+H = C F,+HBr 2 4 2 4

2.50E + 13 1.80E + 11

2.00 74.40

28.

C^F.Br = C F . + B r 2 4 2 4

1.00E + 13 7.10E + 09

50.00 36.70

0

0

(10)

c

(9)

10.


SKINNER

Table I I I

teont'd.)

Reaction

A(forward) a A(reverse)

Ε(forward) Ε(reverse)

Ref.

29.

C F Br Br = C^+Br,,

(a)

U n i t s : S e c f o r u n i m o l e c u l a r r e a c t i o n s , m o l e cc sec for b i m o l e c u l a r r e a c t i o n s and m o l e c c sec-1 f o r termolecular r e a c t i o n s . A l l r e a c t i o n s are assumed to be elementary and to m a i n t a i n the same order under a l l c o n d i t i o n s c o n s i d e r e d . " E " means "times 10 to t h e " .

2

4

+

5.00E + 13 1.72E + 12

1

8.00 46.10 1

2


299

-1

2

(b)

Units:

Kcal.

(c)

In the c a l c u l a t i o n s these r a t e constants were m u l t i p l i e d by 1.2 to allow f o r the f a c t that most o f the reactants present would be more e f f i c i e n t energy t r a n s f e r agents than A r .

300



H

2

+ 0

2

= H0 + H 2

may* be the most important one. Most o f the other r a t e c o n s t a n t s , as i n d i c a t e d i n the t a b l e , have been taken from the review by B a u l c h , D r y s d a l e , Home and Lloyd ( 6 ) , but a few m o d i f i c a t i o n s w i t h i n t h e i r recommended e r r o r l i m i t s have been made to give b e t t e r agreement o f c a l c u l a t e d i g n i t i o n delays w i t h experimental v a l u e s . Table I I shows the extent o f agreement o f i g n i t i o n delay times c a l c u l a t e d by t h i s scheme w i t h l i t e r a t u r e d a t a . In our experimental work the end o f the i g n i t i o n p e r i o d was d e f i n e d as the moment when the i n t e n s i t y o f OH emission a t 309-310 nm reached i t s maximum. I t has been shown f o r both hydrogen and methane o x i d a t i o n t h a t t h i s maximum OH emission occurs at n e a r l y the same p o i n t i n the r e a c t i o n as the maximum OH c o n c e n t r a t i o n (which can be detected by a b s o r p t i o n i n the same region) and the maximum r a t e o f temperature r i s e . In our c a l c u l a t i o n s we have d e f i n e d the end o f the i g n i t i o n p e r i o d as the p o i n t where the OH c o n c e n t r a t i o n reaches a maximum v a l u e , except f o r the l a s t two values reported i n Table I I , where both experimental and c a l c u l a t e d i g n i t i o n times correspond t o the p o i n t where the c o n c e n t r a t i o n of OH reaches 10" mole/cc. Experimental temperatures were based on measurements o f i n c i d e n t shock speeds. Pressures behind the r e f l e c t e d shock waves were a l s o measured. These were i n good agreement ( w i t h i n a few percent) o f the c a l c u l a t e d values r i g h t behind the r e f l e c ted shock waves, but u s u a l l y the pressure rose s e v e r a l percent d u r i n g the i g n i t i o n p e r i o d , apparently due to a s l i g h t pressure g r a d i e n t behind the i n c i d e n t shock wave due t o a s m a l l amount o f attenuation. (Since the pressure r i s e occurred behind shocks i n pure argon, i t could not be due to chemical r e a c t i o n s . ) Temperature changes due to these pressure changes were c a l c u l a t e d assuming i s e n t r o p i c c o n d i t i o n s , and the r e p o r t e d temperatures are a r i t h m e t i c averages o f those c a l c u l a t e d d u r i n g the i g n i t i o n p e r i o d s . No allowance f o r heats o f r e a c t i o n was made i n these calculations. I n h i b i t i o n by CF^Br The r e a c t i o n s considered p o t e n t i a l l y important f o r i n h i b i t i o n o f the H - 0 r e a c t i o n are shown i n Table I I I . Of the ten r e a c t i o n s considered f o r CF^Br (14-23), f i v e (16-19 and 22) could be s p e c i f i e d w i t h i n f a c t o r s o f l e s s than two from l i t e r a t u r e d a t a . These f i v e s e t s o f r a t e constants were not v a r i e d during the a n a l y s i s . Forward and reverse r a t e constants o f these and a l l other r e a c t i o n s i n v o l v e d i n CF^Br i n h i b i t i o n could be checked f o r thermodynamic consistency by means o f the JANAF t a b l e s (11). These t a b l e s a l s o were used to c a l c u l a t e reverse r a t e constants f o r those r e a c t i o n s where forward r a t e constants had t o be estimated. 2

2

10.

SKINNER


301

For Reaction 15 a forward r a t e constant i d e n t i c a l to one we developed f o r the r e a c t i o n


CH. + H = CH. 3 4 was assigned. The methane r a t e parameters were a r r i v e d at from an RRKM c a l c u l a t i o n t h a t r e c o n c i l e s as w e l l as p o s s i b l e the data o f H a r t i g , Troe and Wagner (12) and Skinner and Ruehrwein (13). The Arrhenius parameters are a p p l i c a b l e i n a d i l u t e argon mixture o f 5 atm. pressure i n our temperature range, a r e g i o n i n which the r a t e constants as w r i t t e n f o r second- and f i r s t - o r d e r r e a c t i o n s would show a dependence on the argon c o n c e n t r a t i o n . A c t u a l l y , t h i s r e a c t i o n was not very important u n t i l the end o f the i g n i t i o n p e r i o d , and hence d i d not have a s t r o n g i n f l u e n c e on the r e s u l t s o f the c a l c u l a t i o n s . Rate constants f o r Reaction 20 have been r e p o r t e d by Sehon and Szwarc (14) and d i s c u s s e d by Benson and OVNeal (15). The l a t t e r authors recommend an Arrhenuis A o f 5 x 1 0 * secf*" and Arrhenius Ε o f 66.3 K c a l . We have used the somewhat lower A value o f Table I I I s i n c e i t leads to b e t t e r agreement w i t h our experimental p y r o l y s i s r e s u l t s . This r e a c t i o n was not i n c l u d e d i n our o r i g i n a l scheme s i n c e i t seemed l i k e l y from the observed r e s u l t t h a t CF^Br disappeared, at a given temperature, much f a s t e r i n the presence than the absence o f the and 0^, t h a t d u r i n g the i n h i b i t i o n Reaction 14 removes C F . B r much f a s t e r than Reaction 20 does. While t h i s c o n c l u s i o n s t i l l appears c o r r e c t , we have r e a l i z e d t h a t Reaction 20, coupled w i t h Reactions 16 and 17 ( r e v e r s e ) , can be an e f f e c t i v e i n i t i a t i n g r e a c t i o n even i f i t s r a t e i s not very l a r g e , s i n c e the r a t e o f Reaction 1 i s not very l a r g e e i t h e r . Rate constants f o r Reaction 21 are e s s e n t i a l l y those r e ported f o r 3

CH Br + Br = C H + B r 3

3

2

by Trotman-Dickenson and M i l n e (9). The s i m i l a r i t y i n r a t e constant would be expected from both the general s i m i l a r i t y o f the r e a c t i o n s and the c l o s e s i m i l a r i t y o f the thermochemistry. Reaction 23, and e s p e c i a l l y i t s r e v e r s e , the combination o f Η and Br t o form HBr, was i n c l u d e d mainly because o f i t s pos s i b l e e f f e c t on C F ^ r C F - B r i n h i b i t i o n , i n which i t was expected t h a t r e l a t i v e l y l a r g e concentrations o f Br atoms would be p r o duced, and i t had o n l y a s m a l l e f f e c t on CF-Br i n h i b i t i o n . Rate constants were obtained from data o f Westberg and Greene(16) and Cohen, G i e d t and Jacobs (17) who s t u d i e d HBr d i s s o c i a t i o n i n the 1500-2500K range. The two s t u d i e s d i f f e r e d by about a f a c t o r o f two i n r a t e c o n s t a n t s , and an e x t r a p o l a t i o n from 1500K down to our 900-1300K temperature range was needed, so an uncer t a i n t y o f a f a c t o r o f 3 i n these r a t e constants c o u l d be p r e s e n t . As mentioned above, t h i s u n c e r t a i n t y i s not very s i g n i f i c a n t f o r CF-Br i n h i b i t i o n because the Br atom c o n c e n t r a t i o n remains low.

302


In r e a c t i o n 14, which we c o n s i d e r to be the c h i e f i n h i b i t i n g one, a low Arrhenius a c t i v a t i o n energy (no more than 5 Kcal) and a h i g h Arrhenius A (above 2 x l 0 m o l e " c c s e c ) would be expected by comparison w i t h s i m i l a r r e a c t i o n s such as 19. W i t h i n these l i m i t a t i o n s , r a t e constants f o r t h i s r e a c t i o n were considered v a r i a b l e parameters, w h i l e r a t e constants f o r the other r e a c t i o n s (15, 20, 21 and 23) were considered v a r i able w i t h i n l i m i t e d ranges o f experimental u n c e r t a i n t y . C a l c u l a t i o n s on the p y r o l y s i s o f CF_Br were made f i r s t . From the product analyses o f our o r i g i n a l s t u d y , i t seemed t h a t the p r i n c i p a l course o f the r e a c t i o n i s 13

2CF Br = C F Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch010

3

2

6

+ Br

1

1

2

where, at e q u i l i b r i u m , a c o n s i d e r a b l e amount o f the bromine would be present as Br atoms. The e q u i l i b r i u m constant f o r the o v e r a l l r e a c t i o n changes s l o w l y , from 0.76 at HOOK t o 0.62 a t 1300K, so t h a t ( a l l o w i n g f o r formation o f Br atoms) 50 t o 60% o f the CF^Br would be decomposed at e q u i l i b r i u m . This c a l c u l a t i o n i n d i c a t e s t h a t e q u i l i b r i u m i s being approached i n our higher-temperature r u n s , but t h a t a t lower temperatures j u s t above HOOK, e q u i l i b r i u m was not being approached and the observed conversions were determined by the k i n e t i c s . In our experimental s t u d y , f i n a l concentrations o f both CF^Br and C F . were measured f o r 1% and 4% mixtures o f CF^Br i n argon, at 5 atmospheres t o t a l p r e s s u r e , heated f o r times o f about 14 m i l l i s e c o n d s . At low c o n v e r s i o n s , i t i s a p p r o p r i a t e to g i v e g r e a t e r weight t o the C F . a n a l y s e s , s i n c e a t 5% conv e r s i o n a 2% e r r o r i n CF^Br would correspond t o a 40% e r r o r i n C F . From smooth curves o f the d a t a , weighted as i n d i c a t e d , i t appears t h a t 6.0% o f the C F . B r i n the 1% mixture would be converted to C F i n 14 m i l l i s e c o n d s and 1120K, w h i l e 4.3% o f the 4% mixture would be converted. These, t h e n , are the " e x p e r i m e n t a l " date t o which c a l c u l a t e d conversions should be compared. P y r o l y s i s o f CF^Br i n v o l v e s o n l y a small subset o f the r e a c t i o n s we have assumed f o r i n h i b i t i o n . These a r e : 2

2

2

6

2

6

CF^Br = C F + Br

(Reaction 20)

3

CF^Br + Br = C F + B r 3

2

(Reaction 21)

C F . = 2CF_

(Reaction 22)

+ A r = 2Br + A r

(Reaction 18)

0

Br

2

Of c o u r s e , Reaction 22 w i l l proceed mainly i n the reverse d i r e c t i o n d u r i n g the p y r o l y s i s . With the r a t e constants o f Table I I I , i t was c a l c u l a t e d t h a t 5.6% o f the CF,_Br i n t h e 1% m i x t u r e , and 5.8% i n the 4% m i x t u r e , would be converted t o C F ^ . This seems to be good agreement w i t h the experimental 2


10.


SKINNER

303

results. Some features o f the p y r o l y s i s process brought out by the c a l c u l a t i o n s may be n o t e d . Most o f the Br atoms produced by Reaction 20 reached f u r t h e r by Reaction 21 r a t h e r than by the reverse o f 18. As a r e s u l t , Reaction 18 proceeded mainly i n the forward d i r e c t i o n , producing Br atoms, and i n the end s e v e r a l times as much CF Br disappeared v i a Reaction 21 than v i a 20. The reverse r a t e o f Reaction 22 was always at l e a s t 100 times as great as the forward r a t e , but f o r a l l the other r e a c t i o n s the forward and reverse r a t e s had come w i t h i n a f a c t o r o f 2 o f one another a f t e r 14 m i l l i s e c o n d s o f r e a c t i o n . At 14 m i l l i s e c o n d s , the Br« concentrations i n the 1% and 4% mixtures were 2.1 and 4.8 times the Br c o n c e n t r a t i o n s , r e s p e c t i v e l y . For i n h i b i t i o n o f the H -CL r e a c t i o n by CF^Br, experimental data to be matched by the c a l c u l a t i o n s were the delay times shown i n F i g u r e 1 and the analyses o f p a r t i a l l y reacted m i x t u r e s . S p e c i f i c a l l y , we found t h a t h e a t i n g an i n h i b i t e d gas mixture at 1000K f o r 14 m i l l i s e c o n d s r e s u l t e d i n conversion o f 17% o f the CF^Br to CF H . With the r a t e constants o f Table I I I , good agree ments o f c a l c u l a t e d i g n i t i o n times and CF_H y i e l d was o b t a i n e d , as shown i n F i g u r e 2. The r a t e e x p r e s s i o n f o r Reaction 14 ap pears t o be i n a reasonable range, w i t h both A and Ε o f the r i g h t magnitude. I t should be p o i n t e d o u t , however, t h a t the c a l c u l a t i o n s are i n s e n s i t i v e to s m a l l changes i n A and Ε t h a t r e s u l t i n unchanged r a t e constants i n the middle o f our temperature range. For example, an i n c r e a s e o f Ε by 1.6 K c a l . ( t o 5.0 Kcal) and a corresponding i n c r e a s e i n A o f a f a c t o r o f 2 r e s u l t i n essen t i a l l y unchanged i g n i t i o n delay times and a very s m a l l drop i n the c a l c u l a t e d conversion o f CF_Br to C F J i . Changes i n the ab s o l u t e magnitude o f the r a t e constant a r e , o f course, more impor tant. Doubling o f the r a t e constant f o r Reaction 14 increased the c a l c u l a t e d i g n i t i o n delay from 3.0 to 5.0 m i l l i s e c o n d s at 1200K, w h i l e decreasing the c a l c u l a t e d conversion o f CF Br to CF Η at 1000K from 11% to 6%. B i o r d i . L a z z a r a and Papp (18) have r e p o r t e d an Arrhenius A o f 1 . 2 2 x 1 ο * molé^cc sec" and an Ε of 8,430 c a l . f o r Reaction 14, At 1150K t h e i r r a t e constant i s 0.55 times t h a t o f Table I I I , which might be considered reasonably good agreement. C o n s i d e r i n g the u n c e r t a i n t i e s i n the many other r e a c t i o n s i n v o l v e d i n the combustion p r o c e s s , as w e l l as i n the experimental d a t a , an u n c e r t a i n t y o f about a f a c t o r o f 3 i n , may be e s t i m a t e d , i n the temperature range we have been c o n s i d e r i n g . I t i s o f i n t e r e s t to note the r a t e s , and consequently the r o l e s and r e l a t i v e importance, o f the v a r i o u s r e a c t i o n s i n the i n h i b i t i o n process. Table IV shows r a t e s ( i n mole c c s e c ) f o r a c a l c u l a t i o n f o r a mixture o f 8% H , 2% 0 , and 0.8% CF^Br at 5 a t m . , i n i t i a l temperature 1200K, at 1.5 m i l l i s e c o n d s , which i s h a l f - w a y through the i g n i t i o n p e r i o d . At t h i s t i m e , about 96% o f the o r i g i n a l Η , 97% o f the 0 , and 69% o f the CF^Br remain, w h i l e the temperature has r i s e n by 20 degrees. Those r e a c t i o n s 1

1

1

2

2

2

1


304 HALOGENATED FIRE SUPPRESSANTS

10.

SKINNER


305

not l i s t e d c o n t r i b u t e t o o n l y a minor extent at t h i s stage o f the p r o c e s s . I t i s c l e a r t h a t Reaction 20 i s a much more impor t a n t chain i n i t i a t o r than Reaction 1. Even s o , over 10 times as much CFgBr disappears by Reaction 14 as by Reaction 20. Much more H atom i s produced and consumed by Reactions 14, 16 and 17 than by the hydrogen-oxygen combustion r e a c t i o n s , w i t h the r e s u l t t h a t the C F . B r disappears much more r a p i d l y than the H and 0 · Once the CF^Br i s gone, the s l i g h t i n h i b i t i n g e f f e c t o f Br« and HBr are incapable o f c o n t r o l l i n g the growth o f Η (and otner a c t i v e s p e c i e s ) so t h e i r concentrations grow r a p i d l y and i g n i t i o n occurs. 2


2

Figure 2. Observed (O) and calculated ( , X) ignition delays and product yields for mixture of 8% H , 2% O and 0.8% CF Br at 5 atm 2

gj

s

306


Table IV Rates o f Elementary Reactions at 1.4 M i l l i s e c o n d s f o r Heating o f M i x t u r e o f 8% H , 2%02, 0.8% CF Br at 5 Atm. and an I n i t i a l Temperature o f 1200K 2

Rate, Mole Forward


Reaction 1.

H +0

2.

= 20H

cc sec Reverse

1.22E-07

1.77E-17

0H+H = H 0+H

3.13E-05

3.44E-06

3.

H+e> = OH+0

1.25E-05

2.18E-10

4.

0+H = OH+H

1.25E-05

1.08E-10

2

2

2

2

2

2

11.

H+0 +Ar = H0 +Ar

7.23E-07

4.81E-06

12.

H0 +H

3.10E-06

2.60E-09

13.

H 0 + A r = 20H+Ar

3.05E-06

3.08E-12

14.

CF^Br+H = CF +HBr

1.02E-04

3.09E-09

15.

CF +H = CF H

6.32E-07

3.82E-12

16.

CF +H = CF H+H

6.85E-05

5.00E-07

2

2

2

2

2

- H 0 +H 2

2

2

3

3

3

3

2

3

17.

HBr+H = Br+H

19.

H+Br

2.21E-04

2.26E-04

= HBr+Br

6.08E-07

2.40E-10

20.

C F B r = CF +Br

6.05E-06

2.53E-07

21.

CF Br+Br = C F + B r

9.19E-07

1.47E-07

1.36E-07

1.99E-05

22.

2

3

3

3

C

2 6 " F

2

3

2 C F

3

2

10.


SKINNER

307

I n h i b i t i o n by CF^BrCF^Br Our k i n e t i c a n a l y s i s f o r t h i s i n h i b i t i o n process has been l e s s s a t i s f a c t o r y than t h a t f o r CF^Br, l a r g e l y because o f the absence o f thermodynamic o r k i n e t i c data f o r C F B r , an expected intermediate i n the r e a c t i o n scheme. Even f o r C F B r , thermo dynamic data are incomplete. The N a t i o n a l Bureau o f Standards t a b l e s (19) i n c l u d e an estimate o f the heat o f formation o f C F B r at 298 Κ (AHf°=-186.5 K c a l ) . We lowered t h i s value to - i 8 9 Kcal to make i t more compatible w i t h JANAF data f o r r e l a t e d compounds, which tend to be a l i t t l e lower than the NBS ones where comparisons can be made. The heat o f formation of CJF^Bx was estimated by assuming t h a t the d i f f e r e n c e between i t s neat o f formation and t h a t o f C F . B r i s the same as the d i f f e r e n c e between t h a t o f CF_ and CF-Br (53 K c a l ) s i n c e i n both cases a Br atom i s being removed. This leads to -146 Kcal f o r AHf° f o r C F B r at 298 K. Entropies o f C J Br and CJj^Br at 298 Κ were e s t i m a t e d , r e s p e c t i v e l y , at 88 and 82 c a l deg* mole" , by compar i s o n w i t h data on a v a r i e t y o f r e l a t e d compounds i n the NBS tables. Once these estimates were made, e q u i l i b r i u m constants f o r each o f the i n h i b i t i n g r e a c t i o n s was obtained by c a l c u l a t i n g AS° and ΔΗ° f o r each r e a c t i o n at 298K, and assuming these q u a n t i t i e s are constant w i t h temperature. This approximation was t e s t e d on some o f the r e a c t i o n s i n v o l v i n g CF^Br f o r which complete data are a v a i l a b l e , and appears to be reasonably accurate f o r com pounds o f t h i s t y p e . Reactions 24-29 o f Table I I I , along w i t h 17, 18, 19, and 23, were considered to be o f p o t e n t i a l importance f o r i n h i b i t i o n by C F B r « This mechanism i s a c l o s e p a r a l l e l t o the CF^Br scheme, w i t h Reaction 24 corresponding to 14, 25 t o 20, and 26 to 21, w h i l e 27-29 are CLF.Br analogues o f 24-26. As w i t h CF^Br, i t seemed d e s i r a b l e to use the p y r o l y s i s data i f p o s s i b l e . Steps i n v o l v e d i n p y r o l y s i s would be expected to be 2

4

2

4

2


2

2

2

2

4

1

2

4

2

C

2 4 2 F

C F Br 2

4

B r

2

=

C

2 4 F

B r

(Reaction

B r

+ Br = C ^ B r + B r

C F Br = C F 2

+

4

2

2Br + Ar = B r

2

(Reaction

2

+ Br

4

+ Ar

25)

(Reaction

26)

28)

(Reaction 18, reverse)

w i t h the o v e r a l l r e a c t i o n being C F Br 2

4

2

= C F 2

4

+

Br

2

under our experimental c o n d i t i o n s , although a few percent o f the bromine would be expected t o end up as Br atoms.

308


E q u i l i b r i u m c a l c u l a t i o n s i n d i c a t e that approximately 17% o f the C F B r should be decomposed at 1000K, and 50% at 1200K, which are temperatures near the ends o f the experimental range. Since experimental conversions at these temperatures were 15% and 50% ( f o r p y r o l y s i s o f 2% C F . B r i n argon at 5 atm t o t a l pressure and a r e a c t i o n time of 15 m i l l i s e c o n d s ) i t seems c l e a r that e q u i l i b r i u m was being approached i n a l l o f our experiments, so that q u a n t i t a t i v e k i n e t i c data cannot be deduced from them. From Figure 3 i t may be seen t h a t C F . B r p y r o l y s i s i s a good deal more r a p i d than that o f CF„Br. f t does not seem l i k e l y t h a t Reaction 25 w i l l be f a s t e r than Reaction 20, s i n c e our e s t i mation o f i t s a c t i v a t i o n energy i s about 2 K c a l . h i g h e r than that o f Reaction 20, and the Arrhenius A should not be much d i f f e r e n t . The probable cause i s a l i n e a r r e a c t i o n chain i n v o l v i n g Reactions 26 and 28, which p a r a l l e l the r e a c t i o n s 2

4

2

2

2


2

C H 2

H

+

C

5

2 6 H

= C H 2

=

C

H

•H

4

2 5

2

+

H

2

i n ethane decomposition. This sequence leads to s e v e r a l molecules o f C F B r being decomposed f o r each one t h a t decomposes v i a Reaction 25. Because o f the lack o f k i n e t i c d a t a , r a t e constants f o r Reaction 25 were set by comparison w i t h 20. The s l i g h t l y h i g h e r A v a l u e r e f l e c t s the dual r e a c t i o n path a v a i l a b l e f o r 25, p a r t i a l l y compensated by the expected s m a l l e r moment o f i n e r t i a change i n the formation of an a c t i v a t e d complex f o r C F B r decom p o s i t i o n , as compared t o C F - B r . S i m i l a r l y , r a t e constants f o r Reaction 26 were set by comparison w i t h those o f 21. For Reaction 29, which proved t o be unimportant s i n c e concentrations o f both C F . B r and Br were low i n i n h i b i t i o n experiments, A was set at h a l t that f o r Reaction 26 because o f the s i n g l e r e a c t i o n p a t h , w h i l e Ε was lowered s i n c e Reaction 29 i s exothermic. The r a t e constant f o r Reaction 22 cannot be d e r i v e d , even approximately, from any o f our d a t a , and i t i s d i f f i c u l t t o e s t i mate from general c o n s i d e r a t i o n s . One appealing p o s s i b i l i t y i s t h a t the r e a c t i o n may be so f a s t t h a t a l l of the C^F^Br produced by r e a c t i o n s 24-26 w i l l immediately decompose t o ^ F ^ and B r . This w i l l e l i m i n a t e c o n s i d e r a t i o n o f Reactions 27-29, a great s i m p l i f i c a t i o n . Under these c o n d i t i o n s , Reactions 24 to 26 would be w r i t t e n as 2

4

2

2

4

2

2

C F Br 2

4

2

• Η= C F 2

C F Br 2

C F Br 2

4

2

4

2

= C F 2

• Br = C F 2

+ HBr + Br

4

4

+ 2Br

4

+ Br

2

(Reaction 24A) (Reaction 25A)

+ Br

(Reaction 26A)

Since these r e a c t i o n s are no longer elementary, i t was assumed

10.

SKINNER


309

that reverse r a t e constants would be 0 (a reasonable assumption, s i n c e i n c a l c u l a t i o n s w i t h the f u l l set of r e a c t i o n s the reverse r a t e s f o r 24-26 were much s m a l l e r than the forward r a t e s ) . It was found that w i t h forward r a t e constants f o r Reactions 25A and 26A as given i n Table I I I f o r Reactions 25 and 26, c a l c u l a t e d i g n i t i o n delays were low f o r a l l reasonable r a t e constants f o r Reaction 24A. For an Arrhenius A o f 4.0 X I0 and an a c t i v a t i o n energy o f 3.0 Kcal (which seems an upper l i m i t f o r the r a t e con s t a n t ) c a l c u l a t e d i g n i t i o n d e l a y s , as i n d i c a t e d i n F i g u r e 4, are low by a f a c t o r o f 2 to 3. For what seem the "most reasonable" k i n e t i c parameters f o r Reaction 24A, the A and Ε shown i n Table I I I , c a l c u l a t e d delay times are low by a f a c t o r o f 4 to 5. Com p a r i s o n w i t h the 11% o f 4 2 * m i l l i s e c o n d s at 935K showed t h a t w i t h the upper l i m i t c a l c u l a t i o n 4% o f the C^F^Br^ was l o s t , w h i l e w i t h the "most reasonable" c a l c u l a t i o n si o f the C F B r was l o s t . An important reason f o r the low i g n i t i o n delay times i n the above c a l c u l a t i o n s i s the r a p i d p r o d u c t i o n o f Br atoms by decom p o s i t i o n o f C F ^ B r , and the subsequent r e a c t i o n o f these Br atoms w i t h H v i a Reaction 17 (reverse) to produce Η atoms. A f i n i t e r a t e o f d i s s o c i a t i o n o f CLF.Br w i l l slow t h i s p r o c e s s , thereby lengthening the i g n i t i o n delay time. The approach used was to assign an Arrhenius A o f 1 0 s e c t o Reaction 28 ( t h i s being j u s t h a l f the A v a l u e f o r Reaction 25) and to t r y i n c r e a s i n g values o f the Arrhenius Ε u n t i l a n o t i c e a b l e e f f e c t on the i g n i t i o n delay times was observed. I t was found t h a t Ε had t o be increased to 45 or 50 Kcal to produce an a p p r e c i a b l e d i f f e r e n c e i n the c a l c u l a t e d delay t i m e s . The value o f 50 K c a l . leads to good agreement between c a l c u l a t e d and observed i g n i t i o n delays when combined w i t h the "most reasonable" A v a l u e of 5 Χ 1 0 for Reaction 24, and these f i g u r e s have been l i s t e d i n Table I I I . Good agreement w i t h observed i g n i t i o n delays can a l s o be obtained however, by u s i n g l a r g e r A values f o r Reaction 24, up to 4 X 1 0 , w i t h somewhat s m a l l e r Ε values f o r Reaction 28, so our c a l c u l a t i o n s do not d e f i n e a unique set o f r a t e constants f o r the i n h i b i t i o n reactions. The slower r a t e s o f decomposition o f C F B r r e s u l t i n sub s t a n t i a l l y s m a l l e r conversions o f C F B r to C F . than were found f o r " i n s t a n t a n e o u s " decomposition - l e s s than 6.1% decomposition at 935K f o r the r a t e equations that gave i g n i t i o n delays i n agreement w i t h experiment. This poor agreement at 935K suggests that slow decomposition o f C F . B r may not be the r i g h t approach to i n t e r p r e t i n g our r e s u l t s . An a c t i v a t i o n energy o f 50 K c a l . does seem very h i g h f o r Reaction 28. The r e a c t i o n i s endother mic by o n l y about 15 K c a l , so t h a t an a c t i v a t i o n energy o f about 20 Kcal would seem more reasonable. One a l t e r n a t i v e approach would be t o consider p o s s i b l e changes i n c e r t a i n o f the other r e a c t i o n r a t e c o n s t a n t s . For example, a r e d u c t i o n f o r Reaction 17 would reduce the r a t e o f conversion o f Br atoms to H , w h i l e increases f o r Reactions 18 and 23 would l e a d to r e d u c t i o n s i n Br and Η atom c o n c e n t r a t i o n s . lk


c

2

4

2

F

B r

l

o

s

t

n

1 5

2

2

2

1 3

1

1 3

l l +

2

2

2

4

2

4

2

310



τ

1

1

1

1

Γ

Figure 3. Percent of halocarbons disappearing in pyrolysis experiments with total pressure 5 atm, heating time averaging 15 msec, with argon as diluent. Mixtures are: 4% CF Br, A; 1% CF Br, O; 2% C F Br , X. s

s

i

i

i

Figure 4. Ignition delays for mixture of 8% H , 2% O and 0.4% C F Br at 5 atm: observed, O; calculated from rate constants of Table TV, ; Calculated using rate constants erf Table IV for Reactions 24, 25, and 26, and instantaneous decomposition of CFfir,--g

ty

2

4

s

10.


SKINNER

311

These changes would i n c r e a s e i g n i t i o n delay t i m e s , but at the same time reduce the a l r e a d y low amount o f C F . B r decomposed i n the c a l c u l a t i o n at 935K. Another approach would ue t o c o n s i d e r a d d i t i o n a l r e a c t i o n s l e a d i n g to formation o f C0F (which was observed i n somewhat l a r g e r r e l a t i v e concentrations i n C 2 F B r i n h i b i t i o n than i n CF^Br i n h i b i t i o n ) and o f HF and CO, which were not measured but are expected r e a c t i o n p r o d u c t s . Perhaps these r e a c t i o n s would r e s u l t i n slower i g n i t i o n . U n f o r t u n a t e l y , knowledge o f even which r e a c t i o n s o c c u r , not to mention t h e i r r a t e c o n s t a n t s , i s s c a n t , and i t seems o f d o u b t f u l v a l u e t o modi f y an already somewhat questionable r e a c t i o n scheme by a d d i t i o n o f such r e a c t i o n s . 2

4

2


Conclusions The k i n e t i c a n a l y s i s f o r CF^Br i n h i b i t i o n reproduces the experimental data u s i n g e i t h e r l i t e r a t u r e values o f the r a t e c o n s t a n t s , o r " r e a s o n a b l e " values where l i t e r a t u r e data are unav a i l a b l e o r u n c e r t a i n . While the r e a c t i o n scheme i s incomplete, i n the sense t h a t i t does not account f o r the appearance o f C0F and other p r o d u c t s , such as CO and HF, which would be expected to appear i n l a t e r stages o f the r e a c t i o n , the f a c t that o n l y a small f r a c t i o n o f CF.Br i s converted to C0F up t o the p o i n t where 50 to 60% o f tne CF.Br has decomposed s t r o n g l y suggests t h a t these l a t e r processes have a s m a l l e f f e c t on the delay t i m e s . Hence, i f they were i n c l u d e d , the r a t e constant f o r Reaction 14 would not have t o be changed much (probably l e s s than a f a c t o r o f two) to accommodate f o r t h e i r e f f e c t . I t seems p o s s i b l e t o s t a t e t h a t the r e a c t i o n scheme f o r CF.Br i n h i b i t i o n i s b a s i c a l l y c o r r e c t as f a r as i t goes, and o n l y cnanges i n d e t a i l would be necessary as a d d i t i o n a l r e a c t i o n s to account f o r other p r o d u c t s , or updated values o f some o f the r a t e c o n s t a n t s , are i n c o r p o r a t e d into i t . Much l e s s confidence can be p l a c e d i n the proposed schemes f o r C « F B r i n h i b i t i o n , s i n c e none o f the a l t e r n a t i v e s proposed i s able to reproduce a l l o f the data w i t h " r e a s o n a b l e " r a t e constants. I t seems t h a t one has a choice o f an unreasonably h i g h r a t e constant f o r Reaction 24, o r an unreasonably low one f o r Reaction 28. From general k i n e t i c comparisons (such as the decomp o s i t i o n o f C H to C H and H) the most l i k e l y e x p e c t a t i o n i s that C ^ ^ B r decomposition w i l l be f a s t , w i t h an a c t i v a t i o n energy o f no more than 20 o r 25 K c a l , so t h a t the approximation o f i n stantaneous decomposition i s r e a l i s t i c , and t h a t some a d d i t i o n a l r e a c t i o n s which we have not i n c l u d e d w i l l r e s u l t i n f u r t h e r i n h i b i t i o n . The importance o f l a t e r r e a c t i o n s i n C F B r i n h i b i t i o n i s suggested from the f a c t t h a t C F B r disappears somewhat f a s t e r than CF.Br at low temperatures, yet i g n i t i o n delays at h i g h e r temperatures are somewhat longer f o r C F . B r . While at the moment i t i s not c l e a r what these a d d i t i o n a l r e a c t i o n s should be, a k i n e t i c a n a l y s i s along these l i n e s made i n the f u t u r e when more data 2

2

4

2

2

5

2

4

2

2

4

2

2

2

4

2

312


are a v a i l a b l e might w e l l be s u c c e s s f u l . Literature Cited

1. 2. 3. 4. 5. Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch010

6.

Skinner, G.B. and Ringrose, G.H., J. Chem. Phys. (1965), 42, 2190. Skinner, G.B. and Ringrose, G.H., J. Chem. Phys. (1965), 43, 4129. Skinner, G.B., Lifshitz, Α., Scheller, K. and Burcat, Α., J. Chem. Phys. (1972), 56, 3853. Skinner, G.B., J. Chem. Phys. (1973), 58, 412. Jachimowski, C.J. and Houghton, W.M., Combustion and Flame (1971), 17, 25. Baulch, D.L., Drysdale, D.D., Home, D.G. and Lloyd, A.C., "Evaluated Kinetic Data for High Temperature Reactions. Vol. 1, Homogeneous Gas Phase Reactions of the H + O System", CRC Press, Cleveland, Ohio (1972). Browne, W.G., White, D.R. and Smookler, G.R., Symp. Combus tion (1969), 12, 559. Schott, G.L. and Kinsey, J.L., J. Chem. Phys. (1958), 29, 1177. Trotman-Dickenson, A.F. and Milne, G.S., "Tables of Bimolec ular Gas Reactions, NSRDS - NBS 9", U.S. Government Printing Office, Washington, D.C. (1967). Warshay, M., J. Chem. Phys. (1971), 54, 4060. S t u l l , D.R. and Prophet, H . , "JANAF Thermochemical Tables, Second Edition, NSRDS-NBS 37", U.S. Government Printing Office, Washington, D.C. (1971). Hartig, R., Troe, J. and Wagner, H.G., Symp. Combustion (1971), 13, 145. Skinner, G.B. and Ruehrwein, R.A., J. Phys. Chem. (1959), 63, 1736. Sehon, A.H. and Szwarc, Μ., Proc. Roy. Soc. (London)(1951), A209, 110. Benson, S.W. and O'Neal, H.E., "Kinetic Data on Gas Phase Unimolecular Reactions, NSRDS-NBS 21", U.S. Government Printing Office, Washington, D.C. (1970). Westberg, K. and Greene, E.F., J. Chem. Phys. (1972), 56, 2713. Cohen, N . , Giedt, R.R. and Jacobs, T.A., Int. J. Chem. Kinet. (1973), 5, 425. Biordi, J.C., Lazzara, C.P. and Papp, J.F., "Abstracts of Papers, Fifteenth Symposium (International) on Combustion", The Combustion Institute, Pittsburgh, Pa. (1974), 171. Wagman, D.D., Evans, W.H., Parker, V.B., Halow, I., Bailey, S.M. and Schumm, R.H., "Selected Values of Chemical Thermo dynamic Properties, NBS Technical Note 270-3", U.S. Govern ment Printing Office, Washington, D.C. (1968). 2

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

2

10.


SKINNER

313

DISCUSSION A. S. GORDON:


and H

2

+ 0

Comparison o f

•+ H 0

2

2

+ H

E 2

* °2 ~*

2

0

H

(D (la)

would seem t o f a v o r t h e l a t t e r because, w h i l e i t s endothermicity i s greater, the 4 center r e a c t i o n y i e l d i n g OH would have a c o n s i d e r a b l e lower (approx. χ 10 ) p r e - e x p o n t i a l f a c t o r . G. B. SKINNER: H

2

+ 0

I do n o t f e e l v e r y s t r o n g l y t h a t 2

= 20H

(1)

i s t h e i n i t i a t i n g r e a c t i o n i n hydrogen o x i d a t i o n . Jachimowski and Houghton (Combustion and Flame (1971), 17, 25) have p o i n t e d o u t t h a t R e a c t i o n 1 i s ex p e r i m e n t a l l y i n d i s t i n g u i s h a b l e from H

2

+ 0

2

H0

2

+ H

(la)

so t h a t t h e c h o i c e between them does n o t a f f e c t c a l c u l a t i o n s o f i g n i t i o n d e l a y s o r measurable p r o d u c t distributions. They appear t o f a v o r R e a c t i o n 1 because t h e i r c a l c u l a t e d a c t i v a t i o n energy f o r t h e i n i t i a t i n g r e a c t i o n was 48.1 K c a l , w h i c h i s l e s s than the e n d o t h e r m i c i t y (57.7 K c a l ) o f R e a c t i o n 1A, and I agree t h a t t h e i r argument has some m e r i t . I suspect t h a t b o t h r e a c t i o n s o c c u r a t comparable r a t e s , and t h a t t h e r a t e c o n s t a n t s c a l c u l a t e d by Jachimowski and Houghton a r e t h e sum o f t h e two. S c a t t e r o f t h e d a t a i s s u f f i c i e n t t o obscure curvature o f the Arrhenius p l o t o v e r t h e temperature range s t u d i e d . A. S. GORDON : A t flame temperatures, one would have t o have v e r y h i g h p r e s s u r e s b e f o r e CF^Br CF^ + B r would be c o n s i d e r e d as an u n i m o l e c u l a r p r o c e s s i n i t s p r e s s u r e independent regime. G. B. SKINNER: I do n o t t h i n k t h a t CF~Br decomposi t i o n i s a l o n g way from i t s f i r s t - o r d e r l i m i t under our c o n d i t i o n s . I have n o t made an RRKM c a l c u l a t i o n f o r CFgBr, b u t have made a comparison t o an e a r l i e r c a l c u l a t i o n (G. B. S k i n n e r and B. S. R a b i n o v i t c h ,


314


J . Phys. Chem. (1972), 76, 2418) u s i n g Golden, S o l l y , and Benson's method ( J . Phys. Chem. (1971), 75, 1333). The v i b r a t i o n a l s p e c i f i c heat, t h e e f f e c t i v e p r e s s u r e and t h e a c t i v a t i o n energy f o r t h e r e a c t i o n a r e r e quired. From t h e JANAF t a b l e s , C .,/R f o r CF~Br a t 1300K i s 8.5; t h e e f f e c t i v e pressXèe, assuming a c o l l i s i o n e f f i c i e n c y o f 10% f o r argon, i s 0.5 atm., and t h e a c t i v a t i o n energy i s 66 K c a l . Our e a r l i e r c a l c u l a t i o n , f o r a m o l e c u l e w i t h C .,/R o f 8.6 and a c t i v a t i o n energy o f 61 K c a l , was Xlso made a t 1300K and a t 0.5 atm., so t h e p a r a l l e l i s c l o s e . The c o n c l u s i o n i s t h a t k/k«> > 0.5 under t h e s e c o n d i t i o n s . I t c o u l d a l s o be p o i n t e d o u t t h a t , t o t h e e x t e n t t h a t o u r r a t e c o n s t a n t s f o r CF~Br d e c o m p o s i t i o n a r e based on o u r p y r o l y s i s r e s u l t s , t h e f a c t t h a t b o t h p y r o l y s i s and i n h i b i t i o n were c a r r i e d o u t a t 5 atmospheres t o t a l p r e s s u r e w i l l l e a d t o a c a n c e l l a t i o n of errors. That i s , t h e r a t e c o n s t a n t s we used may be c o n s i d e r e d " e f f e c t i v e " f i r s t - o r d e r r a t e c o n s t a n t s f o r use a t 5 atmospheres t o t a l p r e s s u r e i n a gas m i x t u r e c o n s i s t i n g most o f argon. K. L. WRAY: A t times as l o n g as 20 ms behind t h e r e f l e c t e d shock wave, I would e x p e c t t h a t boundary l a y e r e f f e c t s would have s i g n i f i c a n t l y changed t h e f r e e stream gas temperature from t h a t g i v e n by t h e s i m p l e shock e q u a t i o n s . Have you c o r r e c t e d f o r this effect? I f n o t , perhaps t h i s c o u l d be r e s p o n s i b l e f o r t h e d i f f i c u l t y you a r e h a v i n g i n g e t t i n g reasonable a c t i v a t i o n energies. G. B. SKINNER: No c o r r e c t i o n s f o r c o o l i n g o f t h e gas sample by c o n t a c t w i t h t h e w a l l s were made. A few y e a r s ago ( J . Phys. Chem. (1971), 75, 1) we made c a l c u l a t i o n s on t h e e f f e c t s o f w a l l c o o l i n g i n a 1 1/2-inch d i a m e t e r shock tube f o r h e a t i n g times up t o 5 m i l l i s e c o n d s . F o r a t y p i c a l r e a c t i o n w i t h an a c t i v a t i o n energy o f 50 K c a l , t h e e f f e c t o f w a l l c o o l i n g was t o reduce t h e amount o f r e a c t i o n (and hence t h e c a l c u l a t e d r a t e c o n s t a n t where c o n v e r s i o n i s s m a l l ) by a f a c t o r o f 0.8 compared t o no w a l l cooling. S i n c e t h e h a l o c a r b o n work was done i n a 3 - i n c h tube, and s i n c e t h e w a l l c o o l i n g e f f e c t i n c r e a s e s w i t h time a t l e s s than t h e f i r s t power o f the t i m e , I would e x p e c t about a f a c t o r o f 0.7 i n amount o f r e a c t i o n . W a l l c o o l i n g w i l l have a s u b s t a n t i a l (perhaps l a r g e ) e f f e c t on a c t i v a t i o n e n e r g i e s i f c o n v e r s i o n s a t h i g h temperatures a r e l a r g e , b u t n o t i f they a r e s m a l l ( i . e . , a k i n e t i c s t u d y w i t h c o n v e r s i o n s r a n g i n g from 0.5 t o 1 0 % ) .

10.


SKINNER

315

R. FRISTROM: Have you c o n s i d e r e d r e a c t i o n s C F and C F 0 , e.g., 2

involving

2

CF

+ H

3

HF + C F

2

or CF

2

+ 0

CF 0 + 0

2

?

2

I s r e a c t i o n 15, H + CF~ + C F H , r e a s o n a b l e ? (See Hunter, F r i s t r o m , G r u n r e l d e r , t h i s symposium.) 3

G. B. SKINNER: R e a c t i o n s i n v o l v i n g C F and C F 0 were not i n c l u d e d , undoubtedly some such r e a c t i o n s must o c c u r , s i n c e C F 0 was observed as a r e a c t i o n p r o d u c t , but they were n o t i n c l u d e d p a r t l y because C F 0 appear ed toward t h e end o f t h e i n d u c t i o n p e r i o d , so t h a t i t probably d i d not a f f e c t the length of the i n d u c t i o n p e r i o d v e r y much, and p a r t l y because k i n e t i c d a t a on such r e a c t i o n s a r e hard t o f i n d . As I suggest i n the paper, such r e a c t i o n s may be more important i n C F - B r ^ i n h i b i t i o n than w i t h CF^Br. I agree t h a t i t would be a good i d e a t o d e v e l o p a more complete model t h a t would i n c l u d e such r e a c t i o n s and account f o r a l l observed p r o d u c t s . The r e a c t i o n 2

2

2


2

2

H + CF

3

= CF H 3

i s " r e a s o n a b l e " i n t h e sense t h a t i t c o u l d o c c u r , w i t h a c o l l i s i o n w i t h (probably) an argon atom t o de-energize the CF H. I n our c a l c u l a t i o n s t h i s r e a c t i o n was r e l a t i v e l y unimportant because i n a H ~ r i c h m i x t u r e most o f t h e CF~ w i l l r e a c t w i t h H r a t h e r than w i t h H d u r i n g t n e g r e a t e r p a r t o f t h e induction period. 3

2

2

J . BIORDI: Our e x p e r i e n c e i n low p r e s s u r e methane flames c o n t a i n i n g CF~Br suggests t h a t the r e a c t i o n CF +H-+CF H i s n o t v e r y important r e l a t i v e t o p H->HF+CF2· The reasons a r e t h e s e : C F H i s n o t d e t e c t a b l e i n a s t o i c h i o m e t r i c flame ( & Λ H atom dominated) c o n t a i n i n g 0.3% C F B r , b u t HF i s formed r a p i d l y and e a r l y i n t h i s flame. The C F r a d i c a l i s a l s o unobservable, a l t h o u g h o t h e r r a d i c a l s p e c i e s w i t h mole f r a c t i o n o f the o r d e r 10~ - 10~ can be d e t e c t e d . The C F r a d i c a l i s a l s o unobservable i n flames c o n t a i n i n g 1% C F B r , a l t h o u g h i n t h i s case we do see s m a l l amounts o f CF-H, s u g g e s t i n g t h a t the a b s t r a c t i o n r e a c t i o n s do o c c u r a t h i g h enough C F c o n c e n t r a t i o n . F i n a l l y , and most s i g n i f i c a n t l y , we do observe the C F r a d i c a l i n s u b s t a n t i a l amounts i n both t h e 0.3% and t h e 1% CF~Br c o n t a i n i n g flame. 3

C

3

+

3

3

3

3

3

3

2

316


G. B. SKINNER: I would agree t h a t C F + H = CF-H would be unimportant i n l o w - p r e s s u r e flames because of the much s m a l l e r c o l l i s i o n number compared t o our shock-tube e x p e r i m e n t s . Low-pressure flames c o n t a i n r e l a t i v e l y more H atoms than our shock tube r e a c t i o n s , because o f slow c o m b i n a t i o n i n the hot zone o f the flame, and ease o f back d i f f u s i o n o f H atoms. I think t h a t C F + H = C F H + H i s the most i m p o r t a n t r e a c t i o n o f C F d u r i n g most o f the i n d u c t i o n p e r i o d i n the shock tube e x p e r i m e n t s , s i n c e most o f our C F B r goes t o CH H. Toward the end o f the i n d u c t i o n p e r i o d , the r e a c t i o n C F + H = C F + HF p r o b a b l y o c c u r s , l e a d i n g t o COF r as Dr. F r i s t r o m s u g g e s t s . 3

3

2

3

3

3

3

3

2


2

R. C. GANN: There i s s t i l l some u n c e r t a i n t y i n the l i t e r a t u r e c o n c e r n i n g some o f the r a t e c o n s t a n t s which, o f n e c e s s i t y , you assumed i n your model. For i n s t a n c e , your v a l u e s f o r r e a c t i o n s 17 and 19 a r e c l o s e t o , but not the same a s , the v a l u e s i n Nancy Brown's r e v i e w ( t h i s symposium). As you mention, your r a t e c o n s t a n t e x p r e s s i o n f o r r e a c t i o n 14 i s comparable i n magnitude w i t h , but s t i l l d i f f e r e n t from, the r e s u l t s o f B i o r d i , L a z z a r a and Papp. Have you performed a p a r a m e t r i c study t o put e r r o r b a r s on k ^ and determine t o which o f t h e o t h e r reaction rates k i s particularly sensitive? 4

1 4

G. B. SKINNER: H o n e s t l y , I d i d not make note o f Nancy Brown's v a l u e s f o r R e a c t i o n s 17 and 19. For R e a c t i o n 17 I used the v a l u e recommended by F e t t i s and Knox and r e p o r t e d by Trotman-Dickenson (page 22 of R e f e r e n c e 9 o f t h e paper) f o r the r e v e r s e r e a c t i o n , and c a l c u l a t e d an A r r h e n i u s e q u a t i o n f o r the f o r w a r d r e a c t i o n from the JANAF t a b l e s , u s i n g l o g K values at 900 and 1300K. For R e a c t i o n 19 I chose the v a l u e f o r the r e v e r s e r e a c t i o n recommended by Benson and Buss and r e p o r t e d by Trotman-Dickenson (page 24) and a g a i n o b t a i n e d the forward r a t e c o n s t a n t from the JANAF t a b l e s . In f a c t , f o r a l l o f the i n h i b i t i n g r e a c t i o n s f o r C F B r i n h i b i t i o n , forward and r e v e r s e r e a c t i o n s have been r e l a t e d by e q u a t i o n s o f the A r r h e n i u s t y p e v a l i d f o r the 900-1300K range. I f a l l the o t h e r r a t e c o n s t a n t s were known e x a c t l y , t h e r a t e c o n s t a n t f o r R e a c t i o n 14 c o u l d p r o b a b l y be g i v e n t o a f a c t o r o f 2, but c o n s i d e r i n g o t h e r u n c e r t a i n t i e s t h e f a c t o r i s more l i k e l y 3. The v a l u e of k j - i s most s e n s i t i v e t o changes i n R e a c t i o n 16, than t o R e a c t i o n s 17 and 20. The a n a l y s i s i s not s e n s i t i v e t o s m a l l changes (1-2 K c a l ) i n a c t i v a t i o n energy p r o v i d e d compensating changes a r e made i n A f

3

10. to

SKINNER


317

keep the r a t e c o n s t a n t the same a t 1150K.


Ε. T. McHALE: D i d you p e r f o r m any p a r a m e t r i c - t y p e a n a l y s i s f o r t h e CF-Br c a s e t o determine whether any of t h e seven assumed i n h i b i t i o n r e a c t i o n s a r e r e l a t i v e l y unimportant? F o r example, how w e l l can you reproduce your i g n i t i o n d e l a y times i f o n l y the r e a c t i o n CF^Br + H CF^ + HBr i s c o n s i d e r e d ? G. B. SKINNER: We d i d n o t v a r y the r a t e c o n s t a n t s o f a l l the r e a c t i o n s s y s t e m a t i c a l l y t o o b s e r v e the e f f e c t on c a l c u l a t e d r e s u l t s . However, by l o o k i n g a t the r a t e s o f a l l the r e a c t i o n s d u r i n g the c a l c u l a t i o n s i t was p o s s i b l e t o t e l l which were c o n t r i b u t i n g most t o the o v e r a l l p r o g r e s s o f t h e r e a c t i o n . From T a b l e IV o f t h e paper i t can be seen t h a t a t the half-way p o i n t i n t h i s c a l c u l a t i o n , R e a c t i o n 16, CF

3

+ H

2

= CF H + H 3

i s the o n l y one p r o c e e d i n g a t a r a t e comparable t o t h a t o f R e a c t i o n 14 CF Br + H = CF 3

3

+

HBr.

R e a c t i o n 20, a l t h o u g h slow i n comparison w i t h the o t h e r s , i s i m p o r t a n t i n the e a r l y s t a g e s as an initiator. A l l o f the r e a c t i o n s i n the scheme were i n c l u d e d because t h e y were expected t o o c c u r , b u t i t i s t r u e t h a t some, such as 15, 18, 19 and 21 have l i t t l e e f f e c t on the r e s u l t s . One c a l c u l a t i o n , made t o s p e c i f i c a l l y answer your l a s t q u e s t i o n , was made w i t h a l l i n h i b i t i o n r e a c t i o n r a t e constants s e t t o 0 except f o r Reaction 14. T h i s r e s u l t e d i n a d e l a y time o f >10 m i l l i seconds, a t 1200K, compared t o the e x p e r i m e n t a l v a l u e o f about 3 m i l l i s e c o n d s . P r o b a b l y the most i m p o r t a n t r e a s o n f o r the change was t h a t , i n t h i s c a l c u l a t i o n , C F r a d i c a l s "could not" r e a c t with H by r e a c t i o n 16. 3

2

11 Initial Reactions in Flame Inhibition by Halogenated Hydrocarbons RICHARD G. GANN


Chemical Dynamics Branch, Chemistry Division, Naval Research Laboratory, Washington, D. C. 20375

Over the past three decades, many halogenated molecules have been investigated as fire suppressants. Some tests have been carried out on full-scale fires while others have been performed under controlled laboratory conditions. Generally, the effectiveness of the suppressant has been characterized by changes in one or more bulk properties, such as reduction of flame velocity, narrowing of flammability limits, final flame temperature, or simply how much was needed to quench a "standard" f i r e . A l l of these allow for the fact that chemical activity of the suppressant is involved, but do not directly examine it. There are two major considerations in designing laboratory flame studies: fluid mechanics and chemistry. Because of the inherent great d i f f i c u l t i e s in analyzing combustion systems, we generally simplify the situation by working as nearly as possible in one of two extreme regions. In the f i r s t , one assumes "fast kinetics," and thus allows the fluid dynamics to determine the flame propagation. Alternatively, one creates a well-stirred reactor in which the gas mixing is assumed complete and the chemistry determines the system. In the former situation, one must be assured that the chemistry is complete on a time frame which is fast relative to gas motion. In the latter, a more precise knowledge of the rate data is needed to understand the time evolution of the various species that are observed. In all situations a knowledge of the pertinent kinetics is necessary. Several reviewers in the past decade discussed classical and contemporary studies of flame inhibition chemistry (1, 2, 3, 4, 5). Their analyses of kinetic results was brief for the simple reason that, in general, the reactions involving the halons had not 318

11.

GANN

Initial Reactions in Inhibition

been s t u d i e d . In t h e p a s t few y e a r s , i n t e r e s t i n t h e s e p r o c e s s e s has i n c r e a s e d , and k i n e t i c d e t e r m i n a t i o n s a r e b e g i n n i n g t o appear i n t h e l i t e r a t u r e . While t h e i r number i s s t i l l few, the need f o r t h e r a t e d a t a i s as g r e a t as e v e r . T h i s Symposium p r o v i d e s a good o p p o r t u n i t y t o l o o k a t what has been done and t o p o i n t a f i n g e r a t f u r t h e r needs. In g e n e r a l , the b u l k o f t h e d a t a a p p l i e s t o CF~Br c h e m i s t r y and t h i s w i l l be r e f l e c t e d i n t h i s paper. Where p o s s i b l e , e x t e n s i o n s t o two o t h e r h a l o n s which a r e used as f i r e s u p p r e s s a n t s , C F C l B r and C F , B r , w i l l be made. An attempt t o span t h e f u l l flame h i s t o r y o f the i n h i b i t o r m o l e c u l e s and t h e i r o f f s p r i n g would be f r u i t l e s s , i f o n l y because t h e f u l l sequence o f r e a c t i o n s i s b a r e l y b e i n g guessed a t p r e s e n t . Rather, t h i s d i s c u s s i o n w i l l r e v i e w the e a r l i e s t s t e p s i n the scheme. Good k i n e t i c d a t a here i s c r i t i c a l , f o r t h e s e r e a c t i o n s may w e l l be d e t e r m i n i n g s t e p s f o r suppressant e f f e c t i v e n e s s . S i n c e the a v a i l a b l e d a t a i s s t i l l s p a r s e , my approach t o each r e a c t i o n w i l l be t o d i s c u s s t h e r e l e v a n c e o f the r e a c t i o n , r e v i e w t h e e x p e r i m e n t a l d a t a where p o s s i b l e , d i s c u s s t h e k i n e t i c s by a n a l o g y i f n e c e s s a r y , and guess an approximate value f o r the r a t e constant i f desperate. The o v e r a l l d i s c u s s i o n can be c o n s i d e r e d i n two s e c t i o n s . The f i r s t d e a l s w i t h t h e r e a c t i o n s which i n t r o d u c e t h e halon i n t o the i n h i b i t i o n process. The second d e a l s w i t h some avenues f o r t h e p r i m a r y p r o d u c t s from t h e s e reactions. Throughout the paper, I have c o n v e r t e d a melange o f u n i t s t o the f o l l o w i n g : rate constants i n cm, m o l e c u l e , seconds, a c t i v a t i o n e n e r g i e s i n k c a l / m o l e , and temperatures i n K e l v i n . Thermodynamic d a t a has been t a k e n from t h e JANAF Thermochemical T a b l e s (6) and Benson's monograph on thermochemical k i n e t i c s (7). 2


319

Primary

2

2

Reactions

Thermal Decomposition. The f i r s t q u e s t i o n i n h a l o n flame i n h i b i t i o n i s whether the agent t h e r m a l l y decomposes. I f so, h a l o g e n - c o n t a i n i n g s p e c i e s w i l l be g e n e r a t e d u n i m o l e c u l a r l y as w e l l as by b i m o l e c u l a r r e a t i o n s w i t h flame s p e c i e s . There i s j u s t one avowed e x p e r i m e n t a l d e t e r m i n a t i o n o f the homogeneous r a t e constant f o r CF Br * CF 3

3

+ Br

i n the l i t e r a t u r e . Sehon and Szwarc (8) value of rxcF^-Br) e q u a l t o 64.5 k c a l / m o l e

(1) determined from

a

320


1020-1090K u s i n g t h e t o l u e n e c a r r i e r t e c h n i q u e and assumed E = D ( C F - - B r ) . They a l s o assumed an A r r h e n i u s 13 -1 A f a c t o r o f 2 χ 10 sec based on t h e i r s t u d i e s o f CH^Br p y r o l y s i s . Benson and O'Neal (BO) (9) f e e l t h a t v a l u e s f o r both A and E a r e t o o low. Based on an RRK c a l c u l a t i o n t h e y p r e f e r k, = 5 χ l o e " ' ^ . Darwent's (D) c o m p i l a t i o n (10) recommends a C F - B r bond d i s s o c i a t i o n energy o f 68 k c a l / m o l e a t 298K and the JANAF (6) thermochemical d a t a (J) produces ΔΗ = 69.5 k c a l / m o l e , b o t h o f which imply a l a r g e r A f a c t o r i s n e c e s s a r y t o o v e r l a p the Sehon and Szwarc d a t a . T a b l e I shows v a l u e s f o r k, c a l c u l a t e d from the f i r s t two r e f e r e n c e s as a f u n c t i o n o f temperature. o

a

1 3

6 6

3 0 0

R T


3

TABLE I Rate C o n s t a n t s Τ

(Κ) 900 1020 1090 1200 1300 1400

f o r Thermal Decomposition

SS 0.0043 0.30 2.3 36 290 1700

BO 0.0040 0.31 2.5 42 360 2230

D 0.0037 0.32 2.8 49 440 2900

o f CF-Br J 0.0033 0.33 3.0 57 534 3670

The two l a t t e r columns a r e o b t a i n e d by a r b i t r a r i l y s e l e c t i n g A f a c t o r s such t h a t t h e Darwent- and JANAFd e r i v e d r a t e s would i n t e r s e c t t h e e x p e r i m e n t a l d a t a a t 983K, t h e temperature a t which the Benson-O'Neal and Sehon-Szwarc f o r m u l a s i n t e r s e c t . In t h e narrow temperature range 1020-1090K, t h e f o u r f o r m u l a t i o n s a r e q u i t e c l o s e t o each o t h e r . However, upon e x t r a p o l a t i o n t o h i g h e r temperatures, s t i l l w i t h i n the flame regime, some d i v e r g e n c e o c c u r s . L o o k i n g a t t h e d a t a from a n o t h e r v i e w p o i n t p r o v i d e s a b e t t e r y a r d s t i c k f o r a s s e s s i n g the s i g n i f i cance o f t h i s s p r e a d . L e t us c o n s i d e r the time frame i n which d e c o m p o s i t i o n might be e x p e c t e d . As an example, T a b l e I I l i s t s t h e temperatures a t which 1% d e c o m p o s i t i o n o c c u r s f o r t h e f o u r models a t v a r i o u s heating times.

11.

TABLE Temperature

ss 864 921 985 1059 1145 1246 1514

10 1 -3


ioJ 1

0

- 6

10

II

R e q u i r e d f o r 1% Thermal o f CF-Br

t(sec)

1 0

321


GANN

6

BO

868 923 985 1058 1141 1239 1494

Τ (Κ) D

870 924 985 1056 1136 1230 1474

Decomposition

J 873 926 985 1055 1133 1225 1460

C e r t a i n l y t h e l o c a l temperature fluctuations i n real f i r e s f a r exceed even t h e l a r g e s t d i f f e r e n c e among t h e models shown h e r e , and t h i s might l e a d t o t h e c o n c l u s i o n t h a t k, i s s u f f i c i e n t l y well-known. However, we must keep i n mind t h e f a c t t h a t t h e a b s o l u t e magnitude o f a l l f o u r f o r m u l a t i o n s i s based on an assumed A f a c t o r , and t h e r e l a t i v e magnitudes d e r i v e from an a r b i t r a r y i n t e r s e c t i o n temperature. Over t h e wide temperature range r e l e v a n t t o f l a m e s , d i f f e r e n t A r r h e n i u s parameters w i l l render g r e a t l y varying e s t i m a t e s o f t h e degree o f t h e r m a l d e c o m p o s i t i o n . A t t h e l e a s t , an a b s o l u t e r a t e c o n s t a n t d e t e r m i n a t i o n i s needed. Sehon and Szwarc's e x p e r i m e n t s were c a r r i e d o u t i n t h e p r e s e n c e o f a l a r g e amount o f t o l u e n e , an e f f i c i e n t Br s c a v e n g e r . S k i n n e r and R i n g r o s e (11) (SR) have p y r o l y z e d 4% CF-Br - 96% Ar m i x t u r e s a t 5 atm i n a shock tube from 1112-1982K. A t t h e i r lower v a l u e s o f temperature and f r a c t i o n d i s s o c i a t e d , c a l c u l a t e d v a l u e s o f k, a r e c l o s e t o SS o r BO. However, s e v e r e d i f f e r e n c e s o c c u r a t h i g h e r decomposi t i o n r e s u l t i n g i n an a p p a r e n t Ε ^ 20kcal/mole. We (12) have performed a few e x p e r i m e n t s w i t h a 13% CF3Br-87% N m i x t u r e a t 26 t o r r i n our f l o w r e a c t o r (13) and found %5% d e c o m p o s i t i o n a t 1020K and ^8.5% a t 1070K w i t h a r e s i d e n c e time o f ^0.8 s e c . SS found 12% and 52% d e c o m p o s i t i o n a t s i m i l a r temperatures and ^1/2 t h e r e s i d e n c e t i m e . In b o t h our and t h e SR s t u d i e s , t h e r e i s no e f f i c i e n t bromine scavenger; and some CF^Br r e g e n e r a t i o n , perhaps by 2

CF

0

+ Br

CF Br 0

(2)

322


or

CF

+ Br -*CF Br + Br,

3

2

(3)

3

may

be o c c u r r i n g . To a f f e c t t h e a p p a r e n t r a t e c o n s t a n t f o r r e a c t i o n 1, r e a c t i o n 2 o r 3 must compete w i t h 2 CF +C F


3

2

(4)

6

T h i s b i m o l e c u l a r recombination proceeds a t c l o s e t o gas k i n e t i c f r e q u e n c y a t t h e s e e x p e r i m e n t a l tempera t u r e s (1A, 15). A l s o , a t these temperatures, the bromine atom c o n c e n t r a t i o n i s but a s m a l l f r a c t i o n o f t h e m o l e c u l a r bromine c o n c e n t r a t i o n (14) and t h e t e r m o l e c u l a r bromine atom r e c o m b i n a t i o n r a t e i s slow r e l a t i v e t o r e a c t i o n 4 (16). Thus [CF 1 i s much lower t h a n [ B r ] , which i s i n t u r n lower t h a n [ B r ] . (This i s , of course, not n e c e s s a r i l y v a l i d i n a c t u a l flames). Amphlett and W h i t t l e (17) have found 3

2

1 2

7

R

3

k = 3.8 χ - ° 0 / T c m / m o l e c u l e - s from 451 t o 600K, which e x t r a p o l a t e s t o 3 χ 10~ cm / m o l e c u l e - s a t 100OK. Thus, when s i g n i f i c a n t d e c o m p o s i t i o n o f C F B r has o c c u r r e d , t h e r a t e o f r e a c t i o n 3 i s a l m o s t c e r t a i n l y c o m p e t i t i v e w i t h t h a t o f 4. T h i s not o n l y p e r t u r b s t h e e x p e r i m e n t a l system but can l e a d t o t h e re-formation of parent CF Br. There a r e no r e p o r t e d d e t a i l e d s t u d i e s f o r C^F^Br and C F C l B r t h e r m a l d e c o m p o s i t i o n . However, t h r e e l e s s f o r m a l s t u d i e s bear m e n t i o n i n g . S k i n n e r and R i n g r o s e (11) f i n d f a r more d e c o m p o s i t i o n o f C F B r than C F B r a t r o u g h l y e q u i v a l e n t h e a t i n g t i m e s and temperatures. Engibous and T o r k e l s o n (18) have r e p o r t ed s t u d i e s i n which a m i x t u r e o f 2.5% h a l o n 1301, 2402 o r 1211 i n anhydrous a i r was passed t h r o u g h a l o n g , t h i n s t a i n l e s s s t e e l tube which i n t u r n l a y i n s i d e a f u r n a c e . In a q u e s t f o r a s e l e c t i v e a n a l y t i c a l t e c h n i q u e f o r h y d r o c a r b o n s , Stone and W i l l i a m s (19) passed e x t r e m e l y d i l u t e (^100 ppm) h a l o n s i n an 0 -He c a r r i e r t h r o u g h a q u a r t z tube f i l l e d w i t h q u a r t z beads which a l s o l a y i n s i d e a f u r n a c e . The c o n t a c t time i n t h e former s t u d y i s g i v e n as 2 s e c ; i n t h e l a t t e r , i t was t h e same magnitude. Both s t u d i e s were a t 1 atm. N e i t h e r c o r r e c t e d f o r a dependence o f l i n e a r f l o w v e l o c i t y and thus r e s i d e n c e time on tem p e r a t u r e . Each s t u d y produced p l o t s o f % decomposi t i o n v s . temperature. A q u a l i t a t i v e p l o t of the data o f Stone and W i l l i a m s appears i n F i g u r e 1. The Engibous and T o r k e l s o n d a t a i s s i m i l a r . In both s t u d i e s , t h e s t e e p s l o p e f o r C F B r C l o c c u r r e d somewhat p a r a l l e l t o and about 100K lower than t h a t f o r CF-Br. 1 0

e

3

3

2

2

3

2

2

4

2

11.

GANN


A l s o i n both cases, the C F B r decomposition begins a t a s t i l l lower temperature, w i t h the s t u d i e s d i s a g r e e i n g a t h i g h e r temperatures and d i s s o c i a t i o n fractions. The e x t r a c t i o n o f q u a n t i t a t i v e k i n e t i c d a t a i s n o t m e r i t e d due t o t h e p r e s e n c e o f oxygen, i n s u f f i c i e n t temperature c o n t r o l and i n t h e Engibous and T o r k e l s o n work, t h e l a c k o f an e f f i c i e n t scavenger t o a v o i d the a f o r e m e n t i o n e d r e c o m b i n a t i o n . What i s a p p a r e n t i s t h a t t h e q u a r t z o r s t a i n l e s s s t e e l w a l l s s i g n i f i c a n t l y enhance t h e t h e r m a l decom p o s i t i o n o f t h e s e h a l o n s . One can o n l y s u r m i s e t h e e f f e c t w h i c h would be r e a l i z e d due t o t h e p r e s e n c e of t h e s o o t p a r t i c l e s p r e v a l e n t i n r e a l f i r e s . One f u r t h e r o b s e r v a t i o n . In a flame, t h e h a l o n m o l e c u l e s a r e b e i n g b o t h t h e r m a l l y and c h e m i c a l l y decomposed. The r e l a t i v e importance o f t h e s e two c o m p e t i t i v e p r o c e s s e s w i l l v a r y w i t h t h e flame conditions. In a r e a l f i r e , t h e h a l o n e n t e r s from the a i r s i d e , and the c h e m i c a l s p e c i e s and c o n c e n t r a t i o n s i t e n c o u n t e r s f i r s t a r e l i k e l y t o be t h o s e g e n e r a t e d l a t e i n the combustion sequence. By c o n t r a s t , i n a l a b o r a t o r y study where t h e h a l o n i s premixed w i t h f u e l and o x i d a n t , the e a r l i e s t c h e m i c a l a t t a c k on t h e h a l o n w i l l come from e a r l y flame components, w i t h the i n t a c t m o l e c u l e p r o b a b l y n o t s u r v i v i n g t o meet t h o s e c o n d i t i o n s mentioned e a r l i e r . Thus, even i f t h e t h e r m a l c o n d i t i o n s were t h e same i n t h e two c a s e s , d i f f e r e n t d e c o m p o s i t i o n p r o f i l e s s h o u l d be o b s e r v e d . The c o n c l u s i o n t o be drawn i s t h a t t h e homogeneous decomposition data i s not s u f f i c i e n t to l a b e l the r a t e s f o r t h e s e p r o c e s s e s as e s t a b l i s h e d . Furthermore, t h e r e i s a l s o a need f o r d e t a i l e d s t u d i e s o f s u r f a c e e f f e c t s on the r a t e s o f h a l o n d e c o m p o s i t i o n . 2


323

4

Radical Reactions. H + CF Br-*HBr + C F 3

3

ΔΗ = -18

kcal/mole

(5)

T h i s a t t a c k on t h e i n h i b i t o r m o l e c u l e has a l r e a d y been shown t o be t h e p r i m a r y r e a c t i v e c h a n n e l i n premixed s t o i c h i o m e t r i c , low p r e s s u r e , methaneoxygen flames (20). In t h o s e s i t u a t i o n s , i t i s a l s o t h e f i r s t s t e p i n which t h e h a l o n d e p l e t e s a c t i v e s p e c i e s w i t h i n the flame. Even though as much a t t e n t i o n has been p a i d t o r e a c t i o n 5 as t o any o t h e r r e a c t i o n d i s c u s s e d here, i t s t i l l e x e m p l i f i e s the u n c e r t a i n s t a t e of the k i n e t i c data. In 1965, S k i n n e r and R i n g r o s e (SR) c a r r i e d out shock tube s t u d i e s o f H - 0 - A r - C F B r m i x t u r e s a t 5 atm (11). o

o

o


324

Based on t h e i r n u m e r i c a l m o d e l l i n g o f the 9

1

7

4

5

R

results,

T

3

they e x t r a c t e d k = 7.2 χ " # °/ m /molecules from 920-1280K. In 1974, B i o r d i , L a z z a r a and Papp 5

1

0

e

1 0

C

9 4 6 0 / R T

3

(20) r e p o r t e d k = 3.7 χ 1 0 ~ e ~ cm /molecule-s from 700-1550K. T h e i r v a l u e i s based on d e r i v a t i v e s o f c h e m i c a l p r o f i l e s o f CH - Ο - A r - C F ~ Br flame a t 0.04 atms. In t h i s symposium, S k i n n e r (ST has re-examined the e a r l i e r SR r e s u l t s i n the l i g h t o f more r e c e n t k i n e t i c d a t a on competing r e a c t i o n s , and a more complete r e a c t i o n scheme, o b t a i n i n g an a c t i v a t i o n energy o f 3.4 k c a l / m o l e and an A f a c t o r o f 2.5 χ 10 cm / m o l e c u l e - s . Using a f a s t flow r e a c t o r , F r a n k i e w i c z , W i l l i a m s - a n d Gann o b t a i n e d a lower l i m i t for k o f ^1 χ 10 ^cnT/molecule-s from 750-1200K (21). T h i s was based on t h e o b s e r v e d e f f e c t o f t r a c e water i n an oxygen d i s c h a r g e on the 0 + CF^Br r e a c t i o n and i s o n l y used t o e s t a b l i s h the magnitude o f k^. A l l o f t h e s e r e s u l t s a r e p l o t t e d o v e r the a p p r o p r i a t e temperature ranges i n F i g u r e 2. I t i s apparent t h a t f u r t h e r i n f o r m a t i o n i s needed on t h i s i m p o r t a n t reaction. The analogous r e a c t i o n 5


4

2

5

H + CH Br-*HBr + C F 3

(6)

3

i s thermodynamica1ly and s t r u c t u r a l l y s i m i l a r t o r e a c t i o n 5 and s h o u l d go t h r o u g h a s i m i l a r t r a n s i t i o n state. Thus i t would n o t be s u r p r i s i n g i f the r a t e c o n s t a n t were s i m i l a r . S e i d e l (S') (22) has s t u d i e d r e a c t i o n 6 u s i n g c r o s s e d m o l e c u l a r beams. Jones, Macknight and Teng (23), i n t h e i r r e c e n t r e v i e w o f hydrogen atom r e a c t i o n s , have used S e i d e l s a c t i v a t i o n energy c r o s s s e c t i o n v a l u e s t o d e r i v e 1

1 0

6

k = 5.9 χ 1 0 ~ e " " ' Tnrush and Tuck (DTT) £

9 0 0 / R T

3

cm /molecule-s. (24) have s t u d i e d

D + CH Br+DBr + CH 3

Davies,

(7)

3

using a d i s c h a r g e - f l o w apparatus i:L

w i t h ESR

4f 3 0 0 / R T

an

d e t e c t i o n , C F B r + HBr 2

4

2

2

(8)

4

Elsewhere i n t h i s symposium, S k i n n e r has r e - e v a l u a t e d t h i s d a t a , and cannot o b t a i n a unique f o r m u l a t i o n f o r kg due t o a l a c k o f knowledge about t h e u n i m o l e c u l a r decay, C F Br-^C F 2

4

2

4

+ Br.

0 + CF Br+OBr + C F Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch011

3

(9)

3

ΔΗ = +13 k c a l / m o l e

(10)

In 1974, we r e p o r t e d what i s s t i l l t h e o n l y d i r e c t measurement o f t h i s r e a c t i o n r a t e (13). Using a d i s c h a r g e - f l o w a p p a r a t u s w i t h microprobe sampling and gas c h r o m a t o g r a p h i c d e t e c t i o n o f C F B r , we 3

i : L

1 3

5 0 0 / R T

3

determined k = 1.5 χ 1 0 ~ e ~ ' cm molecule-s from 800-120uk. Computer m o d e l l i n g p r e d i c t i o n s o f p o s s i b l e e f f e c t s from secondary r e a c t i o n s were t e s t e d against widely-varied experimental c o n d i t i o n s , with the c o n c l u s i o n t h a t these r e a c t i o n s d i d not a f f e c t our v a l u e s f o r k , . The f a c t t h a t t h e e x p e r i m e n t a l a c t i v a t i o n energy e q u a l s t h e known e n d o t h e r m i c i t y o f r e a c t i o n 10 a l s o l e n d s c o n f i d e n c e t o t h e d a t a . The r e s u l t s a r e shown i n F i g u r e 3. Comparison w i t h s t u d i e s o f Ο + methyl h a l i d e s i s not a p p l i c a b l e s i n c e t h e a b s t r a c t i o n o f a hydrogen atom i s exothermic w i t h a low Ε and e a s i l y p r e v a i l s a t lower temperatures (25). A c a l c u l a t i o n i n v e s t i g a t i n g the competition between r e a c t i o n 10 and l n

Q

a

Ο + H -*H + OH 2

(11)

was performed (13), u s i n g t h e Leeds d a t a f o r r e a c t i o n 11 (26) and C r e i t z ' propane d i f f u s i o n flame d a t a (27). The c a l c u l a t i o n shows t h a t r e a c t i o n 10 i s more e f f i c i e n t than r e a c t i o n 11 a t removing 0 atoms from t h e flame a t 1000K. R e a c t i o n 11 i s f o l l o w e d by: Η + 0 -*OH + Ο 2

(12)

i n a p r i n c i p a l c h a i n b r a n c h i n g sequence i n t h e h o t region o f flames. Removal o f any one r a d i c a l , H, 0, o r OH, o f n e c e s s i t y r e d u c e s t h e c o n c e n t r a t i o n s o f t h e o t h e r two and t h u s t h e i r r e a c t i o n r a t e s w i t h t h e

11.

GANN


1250


LOxlO"

13

1110

327

TEMPERATURE (K) 1000 909 833

769

I.OxlO" hl 4

I.OxlO'

15

10.0 M.O Τ ( Κ Γ ' Χ I0

13.0

4

Figure 3. Arrhenius plot for the reaction: Ο + CF Br -» OBr + CF . Data from Ref. 13. s

S

328


halon. C o m p e t i t i o n w i t h t h e r e v e r s e o f r e a c t i o n 12 might a l s o be i m p o r t a n t , depending on t h e l o c a l OH concentration.

Br + C F B r - * B r 3

+ CF

2

3

ΔΗ = + 23 k c a l / m o l e

(13)

Consideration of this reaction i s included to c o v e r t h e p o s s i b i l i t y t h a t atomic bromine, once formed by a t t a c k on t h e h a l o n , might i n c r e a s e t h e y i e l d o f i n h i b i t i n g s p e c i e s , s i n c e r e a c t i o n 13 would perhaps be f o l l o w e d by H + Br +HBr + Br.

(14)


2

The h i g h e n d o t h e r m i c i t y f o r r e a c t i o n 13, even i f combined w i t h a g a s k i n e t i c A f a c t o r , makes k , t o o s m a l l t o compete w i t h e i t h e r Ο o r H a t t a c k . No ex p e r i m e n t a l d e t e r m i n a t i o n s o f k,~ o r i t s a n a l o g s f o r t h e o t h e r h a l o n s have been r e p o r t e d . 3

C F B r + OH+HOBr + C F 3

3

ΔΗ = + 14 k c a l / m o l e

(15)

T h i s i s t h e t h i r d o f t h e r e a c t i o n s i n which C F B r c o u l d i n t e r a c t d i r e c t l y w i t h t h e 0, OH, H c h a i n b r a n c h i n g sequence, r e a c t i o n s 11 and 12. Once a g a i n , t h e r e i s no d i r e c t k i n e t i c d a t a f o r r e a c t i o n 15. The analogous r e a c t i o n o f CH-Br w i t h OH l e a d s t o H r a t h e r than B r a b s t r a c t i o n . S i n c e ΔΗ i s about t h e same a s f o r r e a c t i o n 10, and t h e t r a n s i t i o n s t a t e s must be s i m i l a r , i t i s l i k e l y t h a t k ^ w i l l approximate k ^ (2)· An a l t e r n a t i v e r e a c t i o n p a t h : 3

5

C F B r + OH+HF + C 0 F 3

2

Q

+ B r ΔΗ = -66 k c a l / m o l e (16)

i s h i g h l y exothermic, but t h e concerted process r e q u i r e s t h e f o r m a t i o n o f a s t r a i n e d c o l l i s i o n complex w i t h 5 bond r u p t u r e - f o r m a t i o n s , and i s thus n o t l i k e l y .

CH

3

+ CF Br+CH Br + C F 3

3

3

ΔΗ = -1 k c a l / m o l e

(17)

T h i s r e a c t i o n a p p a r e n t l y t r a d e s one i n h i b i t o r molecule f o r another o f s i m i l a r e f f e c t i v e n e s s . How e v e r , CELBr i s s u b j e c t t o exothermic a t t a c k a t t h e hydrogen p o s i t i o n s . Furthermore, C H B r appears e a r l y as C F B r d i s a p p e a r s i n low p r e s s u r e C H - 0 - A r - C F B r flames (20) and i n 1 atm CH-CHO-air-CF^Br c o o l flames (28). Tomkinson and P r i t c h a r d (TP) (29) have 3

3

4

2

3

11.


GANN

measured A r r h e n i u s parameters

329

in a static

system U

1

2

#

5

0

0

/

from 363-518K, d e r i v i n g k,- = 3.3 χ 1 0 " e " cm m o l e c u l e - s . B i o r d i , L a z z a r a and Papp (BLP) i n

R

T

1 1

t h e i r low p r e s s u r e flames f i n d k , = 1.6 χ Ι Ο * — 5 200/RT e ' ' from 800-1600K. The two c u r v e s a r e shown i n F i g u r e 4. The dashed p o r t i o n s o f t h e l i n e s i n d i c a t e e x t r a p o l a t i o n f o r comparison a t common t e m p e r a t u r e s . Tomkinson and P r i t c h a r d c o n c l u d e t h a t D(CF~Br) D(CF--Br) — 4 k c a l / m o l e , which i s a t odds w i t h t h e g e n e r a l l y a c c e p t e d thermochemistry which f i n d s t h e two bonds e q u i v a l e n t . B i o r d i , L a z z a r a and Papp base t h e i r r e s u l t s on f i r s t and second d e r i v a t i v e s o f s p e c i e s p r o f i l e s , and i t i s n o t y e t c e r t a i n t h a t t h e sampling o f f l a m e s i s a t a r e l i a b l e enough l e v e l t o s u p p o r t r i g o r o u s k i n e t i c d e t e r m i n a t i o n . A t any r a t e , t h e disagreement between t h e two r e p o r t e d e x p r e s s i o n s f o r k,~ i s s i g n i f i c a n t and i n d i c a t e s a need f o r further investigation.


7

H0

2

+ CF Br-*HOOBr + C F

3

(18)

ROO

+ CF Br-HROOBr + C F

3

(19)

3

3

RO + CF Br+R0Br + C F 3

3

(20)

These r e a c t i o n s where R i s a h y d r o c a r b o n r a d i c a l , a r e grouped t o g e t h e r m e r e l y f o r c o n v e n i e n c e . They r e p r e s e n t a t t a c k by oxygenated s p e c i e s which appear e a r l y i n t h e flame sequence (30), (31), (32). Both a r e c e n t s u r v e y (33) and a c u r r e n t l i t e r a t u r e s e a r c h r e v e a l no d a t a on r e a c t i o n 18 o r any analogous system, nor was any found on r e a c t i o n s 19 o r 20. Other I n t a c t Halon M o l e c u l e E f f e c t s . In a d d i t i o n t o i t s c h e m i c a l i n t e r a c t i o n w i t h flame s p e c i e s , C F B r e x h i b i t s a p h y s i c a l presence. The p r i m a r y e f f e c t i n v o l v e s t r a n s l a t i o n a l c o o l i n g o f the h o t flame gases, o r on a b u l k s c a l e , b a t h e f f e c t s which reduce t h e flame temperature t o a l e v e l t o o low f o r flame propagation. T h i s i s r e l a t e d t o the a d d i t i o n a l heat c a p a c i t y o f t h e added h a l o n . L a r s e n , l a t e r i n t h i s Symposium, d i s c u s s e s t h i s i n d e t a i l , and i t w i l l n o t be pursued f u r t h e r h e r e . R e l a t i v e t o KU, 0 , o r t h e i n e r t gases, t h e h a l o n s , w i t h t h e i r i n c r e a s e d number o f i n t e r n a l ( v i b r a t i o n a l ) degrees o f freedom, s h o u l d quench e x c i t e d molecules with higher e f f i c i e n c y . I t has been 3

2



Figure 4. Arrhenius plots for the reaction: CH + CF Br -» CH Br + CF . BLP: Ref. 20; TP: Ref. 29. S

S

s

s

11.


GANN

331

known f o r some time t h a t r a d i c a l s can be g e n e r a t e d e i t h e r e l e c t r o n i c a l l y e x c i t e d and/or w i t h nonBoltzmann r o t a t i o n a l and v i b r a t i o n a l d i s t r i b u t i o n s . I f t h e s e s p e c i e s have s u f f i c i e n t l y l o n g l i f e t i m e s and p a r t i c i p a t e i n t h e combustion p r o p a g a t i o n , t h e n p h y s i c a l quenching by newly i n t r o d u c e d , " c o l d e r " h a l o n s w i l l c o n t r i b u t e t o flame i n h i b i t i o n . No measurements have been r e p o r t e d on t h e s e e f f e c t s , but s i n c e i n t e r n a l energy can s i g n i f i c a n t l y i n c r e a s e r e a c t i o n r a t e s , some i n v e s t i g a t i o n o f h a l o n quenching rates i s necessary. The h a l o n s s h o u l d a l s o be more e f f i c i e n t t h a n 0- o r N i n promoting three-body r e a c t i o n s . This e f f e c t i s w e l l demonstrated by t h e work o f Walkauskas and Kaufman (34). They measured s i g n i f i c a n t i n c r e a s e s i n the r a t e of


2

H + H + M+H

2

+ M

(21)

i n v a r y i n g M from N t o CH- o r S F . Whether such enhancement, o r t h e e q u i v a l e n t s t a b i l i z a t i o n o f e x c i t e d c o l l i s i o n complexes, i s i m p o r t a n t can o n l y be r e s o l v e d by d a t a which i s n o t now a v a i l a b l e . 2

Secondary

g

Fragment R e a c t i o n s

CF^ R a d i c a l s . R e s u l t s such as t h o s e o f R o s s e r , Wise and M i l l e r (35) i n d i c a t e t h a t CF^Br i s about 50% more e f f i c i e n t t h a n CH-Br i n r e d u c i n g flame speed. T h i s p r o b a b l y i m p l i e s some a c t i v e p a r t i c i p a t i o n i n the i n h i b i t i o n p r o c e s s by the CF- r a d i c a l . Biordi, L a z z a r a and Papp (20) were unable t o d e t e c t CF- i n t h e i r system, i m p l y i n g a r a p i d c o n v e r s i o n t o o t h e r products. Thus a l o o k a t t h e k i n e t i c d a t a p u b l i s h e d on i t s r e a c t i v i t y w i t h flame p r o p a g a t i n g s p e c i e s i s worthwhile. No f u r t h e r comment w i l l be made on t h e C F B r o r C F C 1 r a d i c a l s , which would be produced by analogous r e a c t i o n s w i t h t h e o t h e r two h a l o n s , s i n c e no d a t a e x i s t s on any o f t h e i r r e a c t i o n s . 2

4

2

Ο + CF -K)CF 3

2

+ F ΔΗ = -81 k c a l / m o l e

(22)

I n t h e i r r e v i e w o f oxygen atom r e a c t i o n s (36), Huie and Herron c o n c l u d e t h a t t h i s i s p r o b a b l y a major c h a n n e l f o r the 0-CF- r e a c t i o n . B i o r d i , Lazzara and Papp, i n s e e k i n g t o e x p l a i n the r i s e o f [CF 0] i n t h e i r f l a m e s , c o n c l u d e t h a t r e a c t i o n 22 i s not f a s t enough t o be t h e p r i n c i p a l e x p l a n a t i o n (20). There i s no r e p o r t e d d i r e c t s t u d y o f k , t h e assumption 2

9 9

332


b e i n g made t h a t the r a t e c o n s t a n t i s l a r g e , comparable t o t h a t f o r t h e analogous m e t h y l r a d i c a l r e a c t i o n . We have, however, performed a few experiments (37) on Ο + CF I+CF 3

+ 01

3

(23)

which i s f o l l o w e d by r e a c t i o n 22. In our f a s t f l o w r e a c t o r (13), p r e h e a t e d CF~I was i n t r o d u c e d t o a 3% 0 i n He m i x t u r e a t 3 t o r r and 850K, w i t h o r w i t h o u t t n e microwave d i s c h a r g e on. The p r i n c i p a l s p e c i e s , Ο-* C F I , C F , 0 C F were d e t e c t e d by gas cnromatography. With t h e microwave u n i t o f f , no C F I t h e r m a l d e c o m p o s i t i o n ( i n d i c a t e d by appearance o f C 0 F o r C F ) was o b s e r v e d i n t h e a p p r o x i m a t e l y 25 ms r e s i d e n c e time i n the h e a t e d s e c t i o n o f t h e flow r e a c t o r . With 0 atoms f l o w i n g , a l l t h e C F I was consumed and b o t h 0 C F and CJFV appeared, i n t h e rough r a t i o o f 3 : 1. The b i m o l e c u l a r r e c o m b i n a t i o n o f C F r a d i c a l s ( r e a c t i o n 4) p r o c e e d s a t c l o s e t o gas k i n e t i c f r e q u e n c y a t t h e s e temperatures (14, 1 5 ) . S o l u t i o n o f the r e a c t i o n scheme i n d i c a t e s t h a t f o r [ 0 C F ] t o be comparable t o [ C F g ] , k must a l s o be o f gas k i n e t i c magnitude. O b v i o u s l y , a more c l e a r cut determination i s desirable. 2

3

2

6

2

3


2

2

3

2

3

2

2

°2

+

C

F

3^

O

C

F

+

0

F

2 2

ΔΗ « +1 k c a l / m o l e

2

(24)

T h i s r e a c t i o n i s a l s o a p o s s i b l e source f o r the formation of 0 C F i n i n h i b i t e d flames. Again, no d i r e c t r a t e d e t e r m i n a t i o n e x i s t s . However, i n our same s e r i e s o f experiments d e s c r i b e d i n t h e p r e v i o u s p a r a g r a p h , t h e m i x t u r e was exposed t o 1000K w i t h t h e microwave u n i t o f f . T o t a l t h e r m a l decompo s i t i o n of the C F I occurred, with approximately e q u a l amounts o f 0 C F and C F formed. S i n c e [ 0 ] >> [ C F ] , i t i s p r o b a b l e t h a t iLJ i s a t l e a s t two o r d e r s o f magnitude lower t h a n k . We i n t e n d t o l o o k more c l o s e l y a t t h i s i n the f u t u r e . 2

3

2

2

g

2

3

2 2

H + CF +HF + C F 3

2

Η + CF ^CF H 3

ΔΗ = -106

3

OH + CF -K)CF 3

ΔΗ = -45

2

+ HF

kcal/mole kcal/mole

ΔΗ = -3 k c a l / m o l e

(25) (26) (27)

I n each o f t h e s e t h r e e e x o t h e r m i c r e a c t i o n s , a p o t e n t i a l c h a i n c a r r i e r i s bound i n t o a c h e m i c a l l y quite stable species. B i o r d i , L a z z a r a and Papp (38)

11.

GANN


333

a t t r i b u t e t h e l a c k o f a CF- peak and t h e appearance CF i n t h e i r f l a m e s t o r e a c t i o n 25. S k i n n e r (39)

of

0

assigns a value of k = 8 χ 10- ecm /molecules, by a n a l o g y w i t h h i s H + CH~ r e c o m b i n a t i o n r e s u l t s , t o e x p l a i n t h e o b s e r v a t i o n o f p r o d u c t CF-H i n h i s e a r l i e r schok tube s t u d i e s (11). However, w h i l e t h e s e r e a c t i o n s c o u l d w e l l be i m p o r t a n t i n flame i n h i b i t i o n , as y e t t h e r e has been no k i n e t i c s t u d y o f them. 1 1

4 0 0 0 / R T

3


2 6

OBr R a d i c a l s . OBr i s formed as a p r o d u c t o f r e a c t i o n 10 and perhaps by o t h e r r e a c t i o n s , and i t s p r o g r e s s t o r e l e a s i n g Br atoms i n t o t h e i n h i b i t i o n scheme i s o f i n t e r e s t . U n t i l very recently, there had been no r e p o r t o f any k i n e t i c s t u d i e s o f OBr reactions. However, work by C l y n e and Watson w i l l soon appear. F o r r e a c t i o n 28 0 + OBr-MD + Br

ΔΗ = -63

2

kcal/mole

(28)

-11 3 t h e s e a u t h o r s quote L = 8 χ 10 cm / m o l e c u l e - s a t 30OK (40), and i t i s r e a s o n a b l e t o e x p e c t t h a t the r e a c t i o n proceeds a t n e a r l y u n i t c o l l i s i o n e f f i c i e n c y a t flame t e m p e r a t u r e s . A l t h o u g h no one has y e t r e p o r t e d on g

H + OBr-*OH + Br

ΔΗ = -46

kcal/mole

(29)

H + OBr-Κ) + HBr

ΔΗ = -31

kcal/mole

(30)

C l y n e and Watson have b r i e f l y l o o k e d a t t h e analogous OC1 r e a c t i o n and found t h a t the c h a n n e l which l e a d s t o OH f o r m a t i o n predominates, p r o b a b l y a t a gas k i n e t i c r a t e (41). F i n a l l y , t h e y have s t u d i e d t h e r e a c t i o n 2 OBr-K)

2

+ 2 Br

and found k = 6.4 χ 1 0 " This r e a c t i o n i s probably flames s i n c e t h e c h e m i c a l w i l l be f a r t o o s h o r t f o r b u i l d up. 3 1

ΔΗ = -7 k c a l / m o l e 12

(31)

3

c m / m o l e c u l e - s a t 300K (40). unimportant i n i n h i b i t e d l i f e t i m e o f t h e OBr r a d i c a l an a p p r e c i a b l e d e n s i t y t o

HOBr. A l i t e r a t u r e s e a r c h r e v e a l s no d a t a on the r e a c t i o n s o r f o r m a t i o n o f t h i s m o l e c u l e . However, i t has now been o b s e r v e d i n 1 atm, premixed, a c e t a l d e h y d e - a i r - CF-Br c o o l f l a m e s (42), perhaps due t o r e a c t i o n 15 o r

334


Table


Reactions

i n CF^Br Flame

(1)

CF Br

(5)

H +

CF Br

->

HBr + C F

3

(10)

0 +

CF Br

->

OBr + C F

3

(13)

Br +

CF Br

•>

Br

3

(15)

OH +

CF Br

->·

HOBr

(17)

CH

3

+

CF Br

CH Br + C F

3

(18)

H0

2

+

CF Br

HOOBr + C F

3

(19)

ROO +

CF Br

ROOBr + C F

3

(20)

RO +

(22)

0 + CF

3

°2

3

(24)

4

3

+

CF

III

3

3

3

3

3

3

3

+

3

+

-»»

OCF

2

+ F

->

OCF

2

+ OF

(25)

H + CF

3

HF + C F

(26)

H + CF

3

C

(27)

OH + C F

(3)

Br 2 + C F

(28)

0 + OBr

(29)

H + OBr

(30)

H + OBr

(31)

20Br

3

3

C F

ROBr

->

3

C F

+ CF

2

3

3

CF Br

+ Br

->

->

F

3

C F

3

2

H

OCF

2

+ HF

Br

+ CF Br

°2

+

3

Br

OH + B r 0 + HBr °2

+

2Br

Inhibition

11.

GANN


OH + Br +HOBr + Br

335 ΔΗ = -9 k c a l / m o l e

0

(37)

Summary K i n e t i c data f o r the r e a c t i o n s l i s t e d i n Table I I I have been surveyed and d i s c u s s e d . These r e a c t i o n s a r e t h o s e which must be c o n s i d e r e d i n t h e e a r l y s t a g e s o f flame i n h i b i t i o n by CF~Br, h a l o n 1301. The e q u i v a l e n t s o f r e a c t i o n 1 f o r C F C l B r and 2 4 2 and t h a t o f r e a c t i o n 5 f o r C F B r have a l s o been included. I n a l l t o o many c a s e s , no r e s u l t s have been reported. D e s p i t e t h e use o f measurement t e c h n i q u e s i n which t h e c h e m i s t r y i s o f t e n s i m p l i f i e d r e l a t i v e t o a c t u a l f l a m e s , f o r v i r t u a l l y none o f t h e r e a c t i o n s i s the r a t e data s u f f i c i e n t l y e s t a b l i s h e d t o support accurate modelling of i n h i b i t e d flames. Certainly t h i s s i t u a t i o n bears c o r r e c t i o n . R e l i a b l e measure ments f o r r e a c t i o n s 1, 3, 5, 10, 15, 17, 22, 24, 25, and 27 a r e a minimum r e q u i r e m e n t . Consideration of r e a c t i o n s 18-20 becomes i m p o r t a n t i n f i r e - i n e r t i n g situations. Furthermore, i n o r d e r t o e x p l a i n on a chemical b a s i s the observed v a r i a t i o n i n e f f i c i e n c y of the d i f f e r e n t halons i n a given f i r e suppression p r o c e d u r e , some k i n e t i c d a t a on t h e s e r e a c t i o n s i s s o r e l y needed f o r a t l e a s t h a l o n s 2402 and 1211. While t h i s may appear t o be a burdensome demand on the rank o f k i n e t i c i s t s , we s h o u l d bear i n mind t h e urgency and s u b l i m i t y o f t h e end r e s u l t . C

F

B r

2


2

4

2

Acknowledgements The a u t h o r i s p l e a s e d t o thank t h e f o l l o w i n g f o r d i s c u s s i o n o f r e c e n t r e s u l t s or the r e c e i p t o f p r e - p u b l i c a t i o n copies of papers: Dr. G. B. S k i n n e r , Dr. R. E. Huie, Dr. J . T. Herron, Dr. J . C. B i o r d i , Dr. R. T. Watson, Dr. F. W. W i l l i a m s , Mr. J . P. Stone, and Dr. J . R. Wyatt. H e l p f u l d i s c u s s i o n s w i t h Dr. D. J . Bogan and Dr. M. C. L i n a r e a l s o a p p r e c i a t e d . The f i n a n c i a l s u p p o r t o f t h e N a v a l A i r Systems Command i s g r a t e f u l l y acknowledged.

Literature Cited 1. 2. 3.

F r i s t r o m , R. Μ., F i r e Res. Abst. and Rev., (1967) 9, 125. McHale, E . T . , F i r e Res. Abst. and Rev., (1969) 11 90. C r e i t z , E . C . , J . R e s . N a t ' 1 . Bur. S t d s . , (1970) 74A, 521.

336


4.

Fristrom, R. M. and Sawyer, R. F., 37th AGARD Symposium, Conference Proceedings, No. 84 on Aircraft Fuels, Lubricants and Fire Safety, AGARD -CP-84-71, Section 12, North Atlantic Treaty Organization (1971). Baratov, A. N. in Ryabov, I. V . , Baratov, A. N. and Petnor, I. I . , eds., "Problems in Combustion and Extinguishment," Amerind Publishing Co., New Delhi, 1974; available as NTIS TT-71-58001. S t u l l , D. R. and Prophet, Η., eds., "JANAF Thermochemical Tables 2nd Edition," NSRDS - NBS 37, U. S. Government Printing Office, Washington, D. C. 20402 (1971). Benson, S. W., "Thermochemical Kinetics," J. Wiley and Sons, New York, 1968. Sehon, A. H. and Szwarc, Μ., Proc. Roy. Soc., (London) (1951) A209, 110. Benson, S. W. and O'Neal, Η. Ε., "Kinetic Data on Gas Phase Unimolecular Reactions," NSRDS -NBS 21, U. S. Government Printing Office, Washington, D. C. 20402 (1970). Darwent, B. deB., "Bond Dissociation Energies in Simple Molecules," NSRDS - NBS 31 (1970). Skinner, G. B. and Ringrose, G. H . , J. Chem. Phys. (1965) 43, 4129. Frankiewicz, T. C., Gann, R. G. and Williams, F. W., Code 6180, Naval Research Laboratory, Washington, D. C. 20375, unpublished results. Frankiewicz, T. C., Williams, F. W. and Gann, R. G., J. Chem. Phys., (1974) 61, 402. Kondratiev, V. Ν . , "Rate Constants of Gas Phase Reactions," National Technical Information Service, Springfield, Va. 22151, 1972. Hiatt, R. and Benson, S. W., Int. J. Chem. K i n . , (1972) 4, 479. Brown, N. J., This Symposium. Amphlett, J. C. and Whittle, Ε . , Trans. Faraday Soc., (1966) 62, 1962. Engibous, D. L . and Torkelson, T. R., AD239021, National Technical Information Service, Spring f i e l d , V a . , 1960. Stone, J. P. and Williams, F. W., Code 6180, Naval Research Laboratory, Washington, D. C. 20375, unpublished results. Biordi, J. C., Lazzara, C. P. and Papp, J. F., "15th Symposium (International) on Combustion," The Combustion Institute, Pittsburgh, Pa., 1975. Gann, R. G . , Frankiewicz, T. C. and Williams, F. W., J. Chem. Phys., (1974) 61, 3488. Seidel, W., Z. Phys. Chem. (Frankfurt am Main)

5.


6.

7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

11. gann 23. 24. 25. 26.


27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.


337

(1969) 65, 95. Jones, W. E., MacKnight, S. D. and Teng, L., Chem. Rev., (1973) 73, 407. Davies, P. Β . , Thrush, B. A. and Tuck, A. F., Trans. Faraday Soc., (1970) 66, 886. Herron, J. T. and Huie, R. E., J. Phys. and Chem. Ref. Data., (1974) 2, 467. Baulch, D. L., Drysdale, D. D . , Home, D. G. and Lloyd, A. C., "Evaluated Kinetic Data for High Temperature Reactions," Volume 1, Chemical Rubber Co., Cleveland, Ohio, 1972. Creitz, E. C., J. Chromatographic Science, (1972) 10, 168. Wyatt, J. R., DeCorpo, J. J., McDowell, M. V. and Saalfeld, F. E., Int. J. Mass Spec, and Ion Phys., (1975) 16, 33. Tomkinson, D. M. and Pritchard, H. O., J. Phys. Chem., (1966) 70, 1579. Hastie, J. W., Chem. Phys. L e t t . , (1974) 26, 338. Fish. Α . , Angew. Chem. (International Edition) (1968) 7, 45. Baker, R. R. and Yorke, D. A., J. Chem. E d . , (1972) 49, 351. Lloyd, A. C., Int. J. Chem. K i n . , (1974) 6, 169. Walkauskas, L. P. and Kaufman, F. "15th Symposium (International) on Combustion," The Combustion Institute, Pittsburgh, P a . , 1975. Rosser, W. A., Wise, H. and Miller, J., "7th Symposium (International) on Combustion," p. 175, Butterworths, London, 1959. Huie, R. Ε., Herron, J. T . , "Progress in Reaction Kinetics," in press. Frankiewicz, T. C., Gann, R. G. and Williams, F. W., Code 6180, Naval Research Laboratory, Washington, D. C. 20375, unpublished results. Biordi, J. C., U. S. Bureau of Mines, 4800 Forbes Ave., Pittsburgh, Pa. 15213, private Communication. Skinner, G. B . , this Symposium. Clyne, M. A. A. and Watson, R. T . , J. Chem. Soc., in press. Watson, R. T . , Dept. of Chem., University of Maryland, College Park, Md. 20742, private communication. Wyatt, J. R., Code 6110, Naval Research Laboratory, Washington, D. C. 20375, private communication.


338


DISCUSSION J . C. BIORDI: Our e x p e r i e n c e a l s o r a i s e s some q u e s t i o n s about t h e temperature dependence f o r t h e t h e r m a l d e c o m p o s i t i o n o f CF~Br. A l t h o u g h t h e r m a l d e c o m p o s i t i o n , u s i n g Benson & O'Neal's v a l u e , i s n e g l i g i b l y i m p o r t a n t i n a CF-Br c o n t a i n i n g methane flame a t 1300°K (where the d i s a p p e a r a n c e r a t e f o r CF-Br i s a t i t s maximum), i t would dominate above ^1500°K. T h i s i n t u r n , would imply a n e g a t i v e a c t i v a t i o n energy f o r t h e r e a c t i o n H+CF-Br+HBr+CFabove about 1400°K, c l e a r l y u n t e n a b l e . I t may be, though, t h a t a t the number d e n s i t y i n t h i s flame (P=32 t o r r ) , t h i s r e a c t i o n may be nowhere near i t s high pressure l i m i t . S t u d i e s d e f i n i n g the p r e s s u r e dependence o f the t h e r m a l d e c o m p o s i t i o n o f CF^Br would be h e l p f u l . A. S. GORDON: I n d e a l i n g w i t h t h e r e a c t i o n CF^+O-* CF 0+F as an e x o t h e r m i c p r o c e s s , one has t o c o n s i d e r t h e p r e s s u r e regime, because f o r CF^+O+M+CF-O+M, f o l l o w e d by CF-O+M+CF0+F+M, the l a t t e r r e a c t i o n i s now c o n t r o l l i n g f o r t h e f o r m a t i o n o f C F 0 and i s endothermic. 2

2

2

R. G. GANN: T h i s i s t r u e r e g a r d i n g t h e thermodynamics. However, i f t h e r a t e o f the b i m o l e c u l a r d i s p l a c e m e n t i s e x t r e m e l y f a s t , as our r e s u l t s i n d i c a t e , then t h e t e r m o l e c u l a r a d d i t i o n r e a c t i o n w i l l be h a r d p r e s s e d t o compete, g i v e n t h e gas d e n s i t i e s a t flame temperatures and a t p r e s s u r e s o f the o r d e r o f 1 atm or l e s s . J . C. BIORDI: The d e t e r m i n a t i o n o f r a t e c o n s t a n t s i n f l a m e s , as i n any o t h e r c o m p l i c a t e d r e a c t i n g system, depends upon knowledge o f the mechanisms i n v o l v e d and u s u a l l y , a l s o upon "known" r a t e c o n s t a n t s f o r some o f t h e r e a c t i o n s . The r e a c t i o n CH^+CF-Br-* CH^Br+CFg t h a t you mentioned i s an e x c e l l e n t c a s e in point. What we can determine from our d a t a i s t h e n e t r e a c t i o n r a t e f o r CH~Br i n the flame, i . e . , t h e sum o f the r a t e s a l l the r e a c t i o n s o c c u r r i n g t h a t form CH~Br minus the sum o f the r a t e s o f a l l t h e r e a c t i o n s o c c u r r i n g t h a t d e s t r o y CH^Br. Somet i m e s one i s l u c k y and f i n d s t h a t perhaps two o r

11.

GANN


339

o r even one r e a c t i o n dominates the b e h a v i o r o f a species. In t h i s c a s e , however, we were r e q u i r e d t o c o n s i d e r f o r f o r m a t i o n a l s o the r e a c t i o n C H + B r + CH-Br+Br; and t h e a b s t r a c t i o n r e a c t i o n s f o r d i s appearance, CH^Br+H(OH)+HBr(HOBr)+CH , c o u l d never be n e g l e c t e d . Thus, the quoted r a t e c o n s t a n t depends upon t h e r a t e c o n s t a n t s f o r t h o s e o t h e r r e a c t i o n s and i t i s not a l l easy t o a s s i g n an u n c e r t a i n t y t o a r a t e c o n s t a n t so d e t e r m i n e d . I n r e c e n t y e a r s k i n e t i c i s t s have been d e a l i n g q u a n t i t a t i v e l y w i t h more and more c o m p l i c a t e d r e a c t i n g systems because they have l e a r n e d t o use the computer t o t e s t t h e i r s p e c u l a t i o n s on mechanisms. Good p r o g r e s s i s b e i n g made on m o d e l l i n g o f flame systems and e v e n t u a l l y i t w i l l be p o s s i b l e t o use t h e s e t e c h n i q u e s t o make more comprehensive a n a l y s i s o f flame p r o f i l e and r a t e d a t a than i s now practical. 3

2


3

A. W. BENBOW: T h i s comment i s i n s u p p o r t o f your statement t h a t R00* i n t e r a c t i o n s w i t h flame i n h i b i t o r s need t o be s t u d i e d . Dr. A. W. Bastow, w o r k i n g i n Professor C u l l i s combustion r e s e a r c h group a t the C i t y U n i v e r s i t y , London, has performed some v e r y d e t a i l e d work on the c h e m i c a l mechanism whereby hydrogen bromide a f f e c t s c o o l flame p r o p a g a t i o n i n npentane - oxygen m i x t u r e s i n the r e g i o n o f 250-350°C. I n the absence of t h e h a l o g e n compound, an i m p o r t a n t r e a c t i o n i s t h e i n t r a m o l e c u l a r rearrangement o f p e n t y l p e r o x y r a d i c a l s as a r e s u l t s o f t h e t r a n s f e r o f hydrogen from a p o i n t i n t h e C (-chain t o t h e o u t e r oxygen o f the peroxy group, e.g., 1

00· I CH CHCH CH CH 3

2

2

3

00· I CH CHCH CHCH 3

2

3

T h i s l e a d s t o the f o r m a t i o n o f C O - h e t e r o x y c l i c compounds and f i s s i o n carbony1 compounds and a l k e n e s as the p r i n c i p a l c o o l flame p r o d u c t s . In t h e p r e s e n c e o f even s m a l l amounts o f HBr Κ 2 mole%), t h e r e i s a s t r i k i n g change i n the p r o d u c t s o f the c o o l flame r e a c t i o n , which now c o n s i s t p r e dominantly o f C ketones. I t can be shown t h a t t h i s d r a m a t i c d i f f e r e n c e can be a s c r i b e d t o t h e replacement o f the i n t r a m o l e c u l a r hydrogen a b s t r a c t i o n p r o c e s s by an i n t e r m o l e c u l a r r e a c t i o n i n v o l v i n g t r a n s f e r o f hydrogen from t h e HBr d i r e c t l y t o t h e p e n t y l p e r o x y r a d i c a l t o form a pentylmonohydroperoxide: 5

5


00· I CH CHCH CH CH


3

2

2

3

OOH I + H B r -> C H C H C H C H C H 3

2

2

3

+ Br-

12 Halogen Kinetics Pertinent to Flame Inhibition: A Review N. J. BROWN


Department of Mechanical Engineering, University of California, Berkeley, Cal. 94720

The objectives of this paper have been somewhat tempered by what may be termed the total immersion approach to halogen kineti c s . There is a vast amount of literature on this subject, and realization of this and the complexity of the f i e l d has forced the author to rethink and moderate the original objectives. The objective, simply stated, i s to review some of the chemistry believed to play an important role in gas phase halogen flame inhibition and not to review the immense f i e l d of flame inhibition chemistry. Chemical inhibitors are believed to alter the composition profiles of radical species in flames. The detailed mechanism i s not understood because of the lack of radical species concentration profile measurements and pertinent kinetic rate data. Radical species represent a small fraction of total flame composition and their profiles change dramatically in the thin reactive zones of flames. Because of the inherent d i f f i c u l t y in their measurement, radical concentrations are often inferred by measurements of stable species and conservation equations and/or by invoking partial equilibrium approximations. The present state-of-the art is such that the reactions of the hydrogen-halogen chains are believed to play an important role in flame inhibition. The major emphasis w i l l be placed on the kinetics of the inhibitors HCl and HBr. These species were selected because 1) they represent the simplest type of inhibitor, 2) their chemistry has been the object of extensive study, and 3) they play an integral part in the inhibition by more complex chlorine and bromine containing compounds. Reactions with oxygen containing species which may also be important in the inhibition mechanism w i l l not be discussed. The reasons for emphasizing HCl and HBr are that they 1) have more practical importance, 2) are less expensive than iodine compounds, and 3) are more effective inhibitors than fluorine compounds. Creitz (1), Fristrom and Sawyer (2) and Hastie (3) have written recent review articles on flame Inhibition. Fristrom and Sawyer develop a model in their paper which is based on the 341


342

experiments of Wilson et a K (4,5). The major premise of their model 1s that the Inhibitor molecule reacts by abstracting H atom In competition with the chain branching reaction H + 0 ** OH + 0. The choice of reactions considered in this review which was strongly Influenced by their model, are those of the hydrogenhalogen chain: 2

H + HX - H

+ X

(1)

H + X ^HX + X

(2)

H + X + M + HX + M

(3)

X + X + M + Xg + M

(4)

2


2

Reaction (1) is thought to be competitive with the chain branching reaction and reaction (2) i s Included since i t 1s competitive with (1). Although the reverse reactions (3) and (4) are slow at flame temperatures, and are not important reaction steps in the halogen inhibition mechanism, we believe that the forward recombination reactions may play an important r o l e . High temperature dissociation kinetics w i l l be discussed since the data is frequently extrapolated to lower temperatures and combined with equilibrium constants to obtain recombination rate coefficients. Part two of the paper is a review of the chlorine kinetics and part three discusses the bromine kinetics. All activation energies are given in cal/mole and a l l temperatures are In kelvins. Second order rate coefficients appear in cm mole" sec" and the third order recombination rate coefficients are in units of cm mole" sec" . 3

6

2

l

1

x

Chlorine Reactions Introduction. Reactions (1) through (4) for chlorine w i l l be discussed in this section. Early workers 1n the f i e l d studied what may best be described as competitive chlorinatlon reactions, and prior to the last decade, chlorine reactions were studied more extensively than any other halogen. An early review by Fettis and Knox (6) and later reviews by Benson et al_. (7) and White (8) provide good coverage of the early work. Recent work on the kinetics of C10 reactions of atmospheric Interest has been reviewed by Watson (9). Here, emphasis w i l l be placed on more recent work. x

H + HC1 H + CI. This reaction and i t s reverse are not only important in flame i n h i b i t i o n , but the reverse rate is used as a primary standard in the measurements of rate coefficients of hydrocarbon chlorinatlon. Four review articles discuss the early studies of this reaction 1) Fettis and Knox, (6) 2) Benson 2


12.

Halogen Kinetics

BROWN

343

et a l . (7), 3) White (8)and 4) Watson (9). The analysis and correction of early results with modern thermo data by Benson et a i . 1s excellent. The kinetics of this reaction are s t i l l quite controversial and the rate coefficients for the forward and reverse reactions have been the object of recent theoretical and experimental studies. The f i r s t measurement of the reverse rate was that of Rodenbush and Klingelhoeffer (10) who studied the reaction at 273 and 298K. The product HC1 concentration was measured as a function of time; however k decreased consistently with time during the course of a run. The authors suggest the effect might be due to a general activation of the glass walls of the reaction vessel to the recombination of CI atoms. Their reported value of k is most l i k e l y too low and should not be weighted too heavily due to the large amount of scatter in the data. Steiner and Rideal (11) measured the forward rate between 900 and 1070K and determined the H concentration from the equilibrium H * 2H assuming the normal ortho to para hydrogen ratio of 3:1 obtained In their experiment. Benson et a l . point out that the equilibrium o-p H ratio did not exist in the SteinerRideal experiments and have recomputed the rate coefficient. They also report Steiner and Rideal used an equilibrium constant for KH+2H that is 55 percent higher than the JANAF (12) value, and conclude that the Steiner-Rideal rate coefficienfshould be 20 percent higher. Benson et a l . further suggest that this corrected rate coefficient shouTï be decreased by 50 percent, since oxygen did not diffuse through the walls of the reaction vessel. Ashmore and Chanmugam (23) studied the reverse reaction at 523K by determining the rate coefficient relative to that of the reaction, NO + N0C1. They obtained the l a t t e r coefficient by extrapolating the 298-328K results of Burns and Dainton (14) to 523K. The method of Ashmore and Chanmugam was based on approximations, and the result of Burns and Dainton which they used was dependent upon the evaluation of other rate c o e f f i cients; hence their results could easily be in error. Benson et a l . recorrected a numerical error in the Ashmore-Chanmugam results and used more modern thermo data for the C l + NO equilibrium constant to obtain k = 4.79 χ 10 cm mole" sec" at 523K. Clyne and Stedman (15) studied the forward reaction 1n a glass flow system by monitoring the Η atom concentration Indi rectly by measuring the emission of ΗΝ0 formed via Η + NO + ΗΝ0 (the NO was added to the stream at a fixed point after the Η + HC1 reaction zone). They attempted to make the reverse reaction and the reaction Η + C l negligible by careful choice of experi mental parameters; their Η decay plots, however, exhibit some curvature at long reaction times which could Indicate competition from other reactions. There is also slight curvature in their Arrhenius plots which they f i t to a straight U n e . Their results appear in Table I. 2

2

f

2

11

2

3

1

l

15

Flow tube, monitored Η atom via HNO emission Flow tube, monitored monitorec H atom decay with ESR

Reference Measurement

Westenberg and de Haas 16

Clyne and Stedman

Investigators

2

195-497

195-373

Temperature (K)

HC1 + H - H + CI

Table I


2.3 χ 10exp(-3500/RT)

13

3.5 χ lO^TêxpÔO/RT)

k(cm /mole sec)

12.

Halogen Kinetics

BROWN

345

Benson et a l . (7) studied the reverse reaction between 476 and 610K using I l T as a thermal source for CI atoms and measuring C l , I 2 , and ICI spectrophotometrically. A mechanism for the H + CI2 + 12 system was postulated and an expression for the rate coefficient for the CI + H reaction was derived. They combined their work with the corrected early results and obtained an Arrhenius expression for the temperature range 273 to 1071K; their results are given 1n Table I I . Westenberg and de Haas (16) studied both forward and reverse reactions in a flow system using ESR detection. Their convention for forward and reverse is opposite ours and the use of "forward and reverse" w i l l apply to the way the reaction is written here, I.e. H + HC1. The forward rate was studied under pseudo f i r s t order conditions in HC1 between 195 and 497K by monitoring the H atom concentration via ESR. The H atoms were formed from discharged H . In measuring this rate, competition from the reverse reaction was probably not important since kn+HCl kH +Cl reaction was studied in excess HC1; however competition rrom the fast C l + H reaction would have been a problem i f adequate production of C l had occured via wall recombination. The authors admit this possibility in stating that some C l was present and that the slope of the H decay plots increased s l i g h t l y with increasing H concentration. The effect 1s not due to increased H but to increased i n i t i a l H (resulting from increased H ) which reacted to give more CI which, in turn, could produce more C l . In the evaluation of the rate coefficient, they retained only the data associated with smallest possible i n i t i a l H concentrations. The results are summarized in Table I . Measurement of the reverse reaction was accomplished by monitoring the concentration of the ground state ( P3/2) chlorine atom as a function of time with ESR. The reaction was studied under pseudo f i r s t order conditions with a large concentration of H and chlorine atom was produced by discharging small amounts of C l . The competitive reactions were the reactions H + HC1 and H + C l ; however, attempt to minimize these reactions was made by keeping the H concentration as large as possible. The i n i t i a l slopes of the CI decay plots had good f i r s t order lineari t y but showed curvature at longer residence times Indicating possible competition from other reactions. The Arrhenius parameters are given in Table I I . Westenberg and de Haas calculated the ratio of the measured forward and reverse rate coefficients and the computed ratio was 2 or 3 times larger than the thermodynamic equilibrium constant over the experimental temperature range. This result has been the subject of several current papers. The f i r s t question to raise is "does this experiment i l l u s t r a t e a violation of microscopic reversibility" and our opinion is no. Westenberg and de Haas explained their results (in the original paper) as being due to the greater probability of obtaining rotationally excited 2

2


2

2

a

>

n

d

t

h

e

2

2

2

2

2

2

2

2

2

z

2

2

2

2

20

Clyne Evaluation 17

Davis et al

2

3

2

2

2

3

298

II - above excluding Benson et al. evaluation

195-496

I using Clyne and Stedman 195-610 and Westenberg and de Haas values for reverse reaction and calculating k from equi librium constant, combined with Davis et al. results and Benson et_ iT. evaluation.

2

Flow tube, measured Cl( P /2) with resonance fluorescence.

2

2

2

3

13

3.4 χ 10exp(-4480/RT)

13

2.23 χ 10exp(-4260/RT)

8.4 χ 10

13

13

Table II H + CI -> HC1 + H k(cm/mole sec) Inve st i ga tors Reference Measurement Temperature (K) ICI as source of CI 476-610 4.8 χ 10exp(-5260/RT) Benson et al. Measured ICI, I and H evaluation of early respectrophotometrically suits implies expression assumed mechanism for the valid for 476-1071 K. H + CI + I system. 1.2 χ 10exp(-4300/RT) Flow tube, pseudo first 251-456 Westenberg and 16 de Haas order, measured Cl( P /2) with ESR.


12.

Halogen Kinetics

BROWN

347

product HC1 in the reverse direction than excited product H in the forward direction. Clyne and Walker (T7)suggest that the reverse rate might have been systematically too low and that the forward rate was correct. They attribute this to CI wall recombination to Cl and subsequent reaction via H + Cl -* HC1 + CI; hence, the production of CI via the competitive reaction results in a smaller CI decay plot slope. This explanation is reasonable and merits further consideration. Snider (18) gives another plausible explanation for the Westenberg and~cTe Haas results, that is, that the CI (*P ) more reactive than CI ( P ) which Westenberg and de Haas meas ured, and this effect could produce a significantly non-Boltzmann distribution of CI among its electronic states during the forward reaction. Snider supports his point of view with a three step mechanism: 2

2

2

1 S

1/2

2


3/2

2

Cl( P ) + H ^ HC1 + H 3/2

(1)

2

2

Cl( P ) + H ^ HC1 + H 1/2

(2)

2

2

2

Cl( P ) + H 1/2

Cl( P ) + H (3)

2

3/2

2

If k > ki and the quenching reaction (3) is not fast enough to restore equilibrium, then Snider's arguments could be correct, however Cadman and Polanyi (19) have shown this not to obtain for the H + HI reaction. Davis et al_. (20) have studied the H + CI reaction at room temperature monitoring the CI atom concentration using a resonance fluorescence technique capable of detecting CI ( P / and P / ) and they report that the ( P ) state made negligible contribution. Their results agree with the Westenberg and de Haas results within 10 per cent at 298 K. In a further attempt to resolve the controversy, Galante and Gislason (21) argue that the two CI electronic states are in equilibrium. They reject the proposed two reaction channel mech anism and argue that the measured H + HC1 rate is too large rather than the H + CI rate being too small. They assume the recombin ation H + Cl + M + HC1 + M interferes and consumes an additional H atom, and as a consequence the H + HC1 rate is large by a factor of two. Our conclusion with regard to the apparent violation of k/k = Κ is that the measured rate for the CI + H reaction is too low. Whether this can be attributed to Cl production and the competitive reaction Cl + Η or to the two channel reaction mech anism via the ( P ) and ( P ) still remains an unanswered question. The role of the atomic ( Ρ / ) electronic state in 2

2

2

3

2

z

t

2

I/2

2

f

2

2

2

2

1/2

2

3/2

2

χ

2

2




BROWN

Halogen Kinetics

350


halogen kinetics is a question often raised by many investigators. Experiments which 1) determine the rate of quenching ( Ρ J adorns at various temperatures, 2) investigate the production of ( P-i/p) atoms formed by discharging halogen molecules, and 31 measure the reactivity of the ( P-i/o) state related to the ( P3/0) are needed. Careful considerations of these problems by theoret icians would be extremely valuable. More studies of both forward and reverse reactions should be made to investigate whether microscopic reversibility is indeed maintained for systems reacting far from equilibrium. Figure 1 is an Arrhenius plot for the H + HC1 reaction; the agreement between investigators is good; however, the Westenbergde Haas results are preferable in our opinion, since their meas urement was more direct. There are no measurements in the 500 to 2000 Κ range and the extrapolated results disagree by a factor of 3 at 2000 K. Figure 2 shows the results of studies of the Hp + CI reac tion. The Westenberg-de Haas results are low. In our opinion, Benson et al. placed too much weight on the low temperature re sults of RôHenbush and Klingelhoefer. The Clyne I and Clyne II entries are the values recommended by Clyne and Walker. Clyne I, which is the one we favor, is based on the results of Clyne and Stedman and Westenberg and de Haas for the reverse reaction and the data of Davis et al_. Clyne II is the same as Clyne I with the addition of the results of the Benson et al_. evaluation. Cl + H ^ HC1 + CI


Ί

2

There has been relatively little work done on the alleged fast reaction, Cl + H + H + CI. Klein and Wolfsberg (22) measured the rate relative to the H + HC1 reaction using tritium labeled HC1. The temperature interval investigated was 273 335 K, and species concentrations were measured with mass spectrometry. The results reported as: 2

2

n

^ = (0.143 ± 0.033)exp(-1540 ± 130/RT) C1 + H K

2

might be subject to error because of reliance on previously determined rate data. Their paper does not contain enough primary data to verify the above expression with newer kinetic data. The rate coefficient for H + Cl was measured directly between 294 and 565 Κ by Dodonov et af. (23) with a technique that is best described as molecular beam sampling of a diffusion cloud in a flow. They studied the reaction under pseudo first order conditions, maintaining a nearly constant concentration of Η atoms produced from H with a stable discharge. The reactor was a capillary tube containing a large amount of He and a small amount of Η atom. Mixtures of Cl and He were introduced at a fixed 2

2

0

12.

BROWN

Halogen Kinetics

351

point and the Cl was measured as a function of distance along the reactor axis. The diffusion coefficient of Cl in He, required for analysis of their results, was measured in their apparatus. Their first order and Arrhenius plots are of good linearity, and the reported rate coefficient is 2

2

k

+ H= 3

7 x

1

Cl+H ' 10êxp(-1800/RT) cm^mole^'sec'. Jacobs et al. (24) measured the same ratio of rates as Klein and Wolfsberg. Tiïe reactions were studied in a shock tube between 3500 and 5200 Κ by measuring HC1 concentrations by infra red emission. They indicate that a factor of 2 is the probable error in their experimental results. The ratio Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch012

2

= .2 exp(-1800/RT) is in reasonable agreement with the value reported by Klein and Wolfsberg. The Klein and Wolfsberg ratio and the ky ri coefficient reported by Westenberg and de Haas (16) can be used to compute kçj ^ and Figure 3 in an Arrhenius plot of this calculated rat? coefficient and the Dodonov result. Considering the errors quoted by Klein and Wolfsberg (as a standard deviation) and by assuming the Westenberg and de Haas rate as correct for H + HC1, the agreement is surprisingly good. + H

+

H + Cl + Ar HC1 + Ar Four shock tube studies of HC1 dissociation with Ar as third body spanning the temperature regime 2900 - 5400 Κ have been con ducted. The experimental results are summarized in Table III. Note all the activation energies are less (by several RT) than the HC1 bond dissociation energy (102,170 cal/mole), and that they differ considerably. The agreement between the various rate coefficients is reasonably good in the common temperature regime. Of the three experiments employing spectroscopic detection techniques, the absorption measurements of Seery and Bowman (25) are preferable since absorption techniques are generally more reliable. The Breshears and Bird (26) measurement, if their approximations are correct, has the distinct advantage of not assuming anything about the distribution of molecular states. One difficulty in determining the rate coefficient for HC1 dissociation at high temperatures is correctly assessing the con tributions of competitive reactions of the H + Cl chain. The kinetic data of competitive chain reactions fs frequently measured at low temperatures and extrapolation of this data to the high temperatures of shock tube experiments may give rate coefficients 2

2

352


H A L O G E N A T E D FIRE SUPPRESSANTS

II 1 0.5

I 1.0

I 2.0

I 3l0

I 4.0

I0y[î(

I 5.0

1

1

6.0

K)]

Figure 3. Arrhenius plot of H + Cl -» HCl + CI rate coefficients; dotted lines indicate extrapolation f

28

27

25

26

29

Fishburn

Jacobs et al.

Seery and Bowman

Breshears and Bird

Giedt and Jacobs

Investigators Reference

3

3000 -> 4000

1600 2800

shock tube, monitored radiative recombination of CI atoms

1 3

shock tube, measured 2900 + 4000 HC1 concentration using vacuum ultraviolet absorption spectroscopy

shock tube, laser schliern technique to monitor post shock density gradient

1 2

1/2

1 2

6.6 χ 10exp(-70,000/RT)

4.78 χ 10exp(-82,700/RT)

1 3

4.2 χ 10exp(-81,000/RT)

6.6 χ 10exp(-70,000/RT)

n

1.92 χ 10T exp(-69,700/RT)

k(cm/mole«sec)

shock tube, ir emission 2800 4600 of HC1 invoking trans parent gas approximation

shock tube, monitored 3300 - 5400 reaction; progress by measuring IR emission from (1-0) band of HCL

Table III HC1 + Ar + H + Cl + Ar Measurement Temperature (K)


354


which are unreliable. Systematically varying the rate coeffi cients over the probable error limits is a common method of assessing the effects of uncertainties in the kinetic data and the contributions of competing reactions. The paper of Jacobs et al. (27) gives a good discussion of this approach. Jacobs et al. found that the activation energy of dissociation reaction was sensitive to the Cl + HC1 Cl + H rate coefficient; however, the rate coefficient of this reaction has only been measured at low temperatures. Increasing k , was found to increase the HC1 dissociation activation energy- Fishburn (28) also noted a dependence of the activation energy on the reaction rates for Cl + HC1 and H + HC1 reactions. The only measurement of the HC1 dissociation rate at temperatures important to combustion was that of Giedt and Jacobs (29). The extended the temperature range to 1600 Κ monitoring the radiative recombination of CI atom. They report few experi mental details and their measurements raise some doubt since they assumed the intensity of emission varied as [CI] where η = 2; η was subsequently shown by Clyne and Stedman (30) to vary with wavelength. In addition their experimental and calculated reaction profiles did not show good agreement. Their results are shown in Table III. Figure 4 is an Arrhenius plot of the experimental results. In the common temperature range the agreement is within a factor of 2. Jacobs et a]_. (27) estimate that the accuracy of their rate coefficient is about a factor of 2. Figure 5 is an Arrhenius plot of the HC1 dissociation rate coefficients extrapolated to 1300 K. The variation between the rate coefficients at the lower temperature is more than a factor of 10 and the extrapolation introduces errors that are impossible to ascertain. Little study has been directed toward assessing the relative efficiences of various third bodies in HC1 dissociation. Giedt and Jacobs reported no difference in HC1 and Ar while Breshears and Bird found HC1 10 times more effective than Ar, and that the temperature dependence of M = HC1 departs from the Arrhenius form, with the apparent activation energy increasing at lower temper atures. If one can argue by analogy, using HBr as an example, the Giedt and Jacobs result is incorrect. More work in this area is warranted. The relatively large difference between bond dissociation energies and activation energies is not an anomaly of the HC1 dissociation but a general characteristic of high temperature dissociation reactions. There have been many explanations for the phenomena, 1) impurities, 2) improper consideration of com petitive reaction, 3) nonequilibrium distribution of vibrational states, and 4) nonequilibrium distribution of electronic states. Most probably each of the four explanations has some validity in specific experimental environments but the author favors reason (3). Johnston and Birks (31) have written an excellent theoretical paper in which model calculations are presented which 2


f

H C 1

12.

355

Halogen Kinetics

BROWN

TEMPERATURE Κ 4000

3000

I

1

10

HCI + Ar -

H + Cl + Ar

9 FISHBURN JACOBS ET AL

y'

\ ο

ε

m β

BRESHEARS AND BIRD

%^χΧ

ε

log k ι


"SEERY AND BOWMAN

u

NX

\ \

7

1

ZO

1

2.5

3.0

1 35

4

Ι0 /[Τ( K)] Figure 4. Arrhenius plot of HCI + Ar-+H -\-ClAr indicate extrapolation

rate coefficients; dotted Unes



12.

BROWN

Halogen Kinetics

357

support reason (3). The paper gives a discussion of activation energy concept. There are no reported measured values of rate coefficients for the reaction, H + Cl + M, and the reaction should be studied in a temperature regime where both forward and reverse reactions could be investigated. Judging from the poor agreement among the extrapolated high temperature dissociation results, calculation of the recombination rate coefficient from the equilibrium con stant and the dissociation rate coefficient between 1000 and 2000 Κ would be unwise.


Cl f Cl + M

Cl

2

+ H

This recombination reaction and the reverse dissociation reaction have been critically reviewed by Lloyd (32). The recommended value for the forward rate coefficient is based largely on the results of Clyne and Stedman (33) who studied the reaction with Ar as third body. The recommended rate coefficient for the temperatures 200 and 500 Κ is 1 4

6

2

1

k = 2.19 ± 1.6 χ 10exp(+1800 ± 500/RT)cmmole"sec". Ay%

CI2 was found 4.5 times more efficient than Ar as a third body between 200 and 500 K; however, the ratio k-, /k« is probably temperature dependent. 2 The "negative" activation energy for recombination arises, in part, from the redissociation of highly excited product Cl an effect that increases with temperature. The product molecule formed via recombination is usually formed in a highly excited vibrational state and it can be thought of as being closer to reactants than products. The recommended value of the dissociation rate coefficient with Ar as third body in the temperature range 1500 - 3000 Κ is 2>

k

s

1

Cl2+Ar 8.7+2.0χ1ΰ%φ(-48,500 ± 2000/RT) oAiole^sec" .

This value was based primarily on the results of Jacobs and Giedt (34). The activation energy is approximately 8500 cal/mole less than the bond dissociation energy. Jacobs et al. (35) in a later study reported that CI was 10 times more efficient than Ar in CI dissociation. The high efficiencies of atoms as third bodies indicates that control of their concentrations in dissocia tion studies is extremely important. Radicals in flames are also likely to be efficient third bodies and the use of data applicable for M Ar or Ν might introduce serious errors in modeling calculations. Dissociation reactions are slow at temperatures < 2000K and probably are not an important reaction step in halogen flame inhibition. Dissociation rate coefficients are, however, fre quently used with equilibrium constants to compute recombination s

358


rate coefficients. Since recombination may play an Important role 1n Inhibitions 1t 1s Important to compare the direct recombina tion rate coefficient with that computed from extrapolated d i s sociation data. The two rate coefficients for the chlorine recombination reaction are shown in Figure 6. The agreement (indeed a misnomer) 1s very poor and indicative that more study of recombination and dissociation reactions in a common tempera ture interval is required. The relationship Κ = k f / k s t i l l has not been confirmed experimentally for reactions measured in shock tubes, and this and the fact that activation energies may well be temperature dependent are further reasons to merit additional study, r


HBr Reactions Introduction. The HBr reactions (1) and (2) have not been studied as extensively as the HCI reactions. There are two reviews of the literature associated with HBr kinetics, an early review by Fettis and Knox (6) and a later review by White (8). Hydrogen-bromine reactions are a classic example of a free radi cal chain mechanism. It is d i f f i c u l t to experimentally study an Isolated reaction of the chain, free from competitive reactions, and the d i f f i c u l t y of assessing the effects of competitive reac tions has been largely responsible for the errors in interpreta tion of early data. Η + HBr ^ H? + Br. This reaction has been studied simul taneously with the competitive hydrogen bromine chain reactions. There are many errors in the literature due to incorrect evalua tion of the rate coefficient ratio kH+Br /kH+HBr Inaccurate a

n

d

2

values of the equilibrium constant Κ » [ B r ] V [ B r ] . The following discussion w i l l pertain to studies of the chain mechanism and w i l l not always be chronological. Effort w i l l be made to point out rather than correct errors in early data. The H + B r reac tion w i l l be discussed here, since most of the data associated with i t are extracted from studies of the chain mechanism. Bodenstein et al_. (36,37,38) studied H + Br reactions between 498 and 577K. Known amounts of H + Br were mixed 1n a bulb, allowed to react, and the HBr and Br concentrations were measured as a function of time using wet chemical techniques; no rate coefficients were reported. The data were interpreted later by Christiansen (39), Herzfeld (40), and Polanyi (4]J " pendently showed that the system reacted via a radical chain mechanism. The Br atom concentration was assumed to be governed by the equilibrium 2

w

Br

2

h

^ 2Br.

With this interpretation, the now accepted rate law i s :

o

i n d e


BROWN

Halogen Kinetics


360

d

LHBr] -

2

m

2

k

where Κ = [Br] /[Br] and m - kH+Br / H+HBr The original data of Bodensteln et a i . and the rate law have been used by other Investigators to determine the rate c o e f f i cients kH +Br and k ; these "derived" values have frequently been incorrect because m was not properly evaluated and Κ was in error. Bodensteln and Jung (38) performed photochemical experiments in the temperature interval 303 to 575K and reported m • 8.61 at low temperatures and 8.4 at high temperatures. Sullivan (42) has recently shown that the atomic chain mechanism does not obtain in room temperature photochemical experiments. He reports that Bodenstein and Jung did not measure the ratio but Instead meas ured the recombination rate coefficient for the reaction 2


2

H + H B r

2Br + HBr

B r + HBr. 2

If this interpretation is correct, results computed with the Bodenstein-Jung ratio are incorrect. Bach et a i . (43) studied reactions of D and H with Br between 549 and 612K~By measuring the Br concentration spectrophotometrically. Pease (44) gives a good summary of these results in his text; however m i s not evaluated correctly and the value of the equilibrium constant is not the currently accepted value. Steiner (45) studied the reaction between 820 and 980K using a technique identical to that of Steiner and Rideal ( ϋ ) discussed earlier. The errors in this study are 1) an incorrect assumption regarding the ortho-para hydrogen equilibrium which makes their rate coefficient about 5 percent too high and 2) use of an equi librium constant for the reaction H 2H which i s 55 percent higher than the JANAF value. The latter error makes their rate coefficient 20 percent too low. Fass (46) studied the relative rates of the reactions H + Br and H + HBr between 300 and 523K by photodissociation of HBr at 1850 A and 2480 A . The H atoms produced were translationa l l y hot, and the experiment was contrived to measure both rate coefficient r a t i o s , one associated with hot H atom reactions, the other with the thermalized H atoms. Reaction progress was monitored by using absorption spectroscopy to measure HBr and B r concentrations as a function of time. An Arrhenius type expression for the rate coefficient r a t i o , m, was reported as ^ 2

2

2

in *

H

Br

+ 2 = 6.8

exp(800/RT).


12.

Halogen Kinetics

BROWN

361

The ratio 1s 26 at room temperature, 15 at 500K and 8.9 at 1500K. Fass Indicates that this ratio i s valid between 300 and 1400K since his results were combined with the high temperature results of other Investigators (47,48,49). The Arrhenius expression for the ratio f i t s the low temperature data better than the high temperature data. Fass used He to thermalize the H atoms and he assumed 15 percent remained unthermalized. We believe the Fass results are correct i f the assumption of 85 percent thermalizatlon is accu rate. Levy (47) studied H + Br reactions in a flow system between 600 and 1470K. The mixture was thermally quenched after reacting for a specified time interval, then separated by frac tional condensation, and the concentrations of HBr and Br were measured as gas volumes. Incomplete quenching and poor separa tion are two potential sources of error in these data. Levy assumed m = 8.4 in his analysis which, i f the Fass results are correct, underestimates the ratio by approximately a factor of 3 at 600K. Leyy's Arrhenius plots appear to be reasonably linear; however the scatter in the high temperature data is large. The Arrhenius parameters are given in Table IV and Figure 7 i s an Arrhenius plot of the rate coefficient. The next study was that of Britton and Cole (48) who studled the chain reactions in a shock tube between 1300 and 1700K. Reaction progress was monitored by measuring Br concentration spectrophotometrlcally. The Br recombination rate coefficient, which was required for data analysis, was computed from dissocia tion data, and the error 1n this coefficient affects their results. They reported that their results were most l i k e l y uncertain by 25 percent. Their Arrhenius plot was of reasonable l i n e a r i t y , and their rate coefficient 1s given 1n Table IV. The ratio m was measured in their experiment and reported as m = 8.3; however the constant value of m is d i f f i c u l t to under stand since their tabulated results indicated temperature depend ence. Vidal (50) studied the rate of formation of HBr from v a r i ous mixtures of H and Br in the temperature range 508-566K. Reaction progress was followed by measuring Br concentrations spectrophotometrlcally. After quenching the reaction, the HBr concentration was determined by an acid-base t i t r a t i o n and the Br , by an iodometric t i t r a t i o n . A modern value of Κ = [ B r ] / [ Κ [ B r ] / [ B r ] was used in data analysis and the ratio m was determined experimentally and agreed favorably with the Fass results. The results of earlier workers were re-interpreted with the new m and Κ values and agreement between Vidal s results and those of Bodensteln et al_-» ÊÏ. * ! · > Britton and Cole was very good. The Levy data rather than the Levy Arrhenius expression were also compared with the results of others. The agreement at high temperatures between the Levy data and the extrapolated Vidal results was reasonable. Vidal's 2

s

2

2

1

B a c h

a

n

d

3

1/2

14

2

1 4

1.35 χ 10exp(-18400/RT) 508-566 Flow tube, measured Br2 concentration by absorp expression valid tion spectroscopy, final for 500-1700. HBr and Br concentrations by titration.

1.79 χ 10exp(-19200/RT)

50

1300-1700

Vidal

2

Shock tube, measured Br concentration by absorp tion spectroscopy

48

2

12

Measurement Temperature (K) k(cm/mole*sec) Reference 600-1470 2.04 χ 10 T exp(-17280/RT) 47 flow tube, separated gases by fractional condensation and measured HBr and Br pressure

Britton and Cole

Levy

Iηves ti ga tors

2

Br + H - HBr + Η

Table IV


12.

BROWN

Halogen Kinetics

363

Temperature ( I2i

Λ

1000 I

1

Br+ H

\ Vv^Britton


K) 500

and

2

-•"HBr + Η

Cole

V \ 8

7

-

6

-

\

\\

\ Vidal

\

1.0

2.0 3

I0 /[T(

\ \

ι

I1

_J

1 3.0

K)]

Figure 7. Arrhenius plot of Br + H* -> HBr + H rate co efficients; dotted lines indicate extrapolation

364


expression for the rate coefficient is given in Table IV. Con sidering the agreement with the higher temperature results, the Vidal Arrhenius expression is assumed valid for the temperature interval 500 to 1700 K. Figure 7 shows a comparison of the Levy, Britton and Cole and Vidal rate coefficients. Takacs and Glass (51) report the only direct measurement of the rate coefficient of the H + HBr reaction. The reaction was studied at 295 Κ in a flow system by monitoring the Br( P /2) and H(S!/) via EPR spectroscopy; no Br( Pi/ ) was detected. The rate coefficient at 295 Κ was: 2

2

3

2

2

2


k

1

= 2.05 χ lO^cm^Wle^sec".

H+HBf

If the rate coefficient reported by Vidal is used to compute the kH+HBr coefficient via the equilibrium constant, the value at 295 Κ is 5 χ lO^cn^mole^sec" which is 2.4 times larger than the Takacs and Glass value. 1

Br + Η •» HBr + Br. No direct measurements of this rate co efficient have been reported. If one assumes the Fass ratio m is correct between 300 and 1400 Κ and that the Vidal value of ky XD„ is correct in the same temperature interval, then the rate coefficient for the Br + H reaction can be computed using these results and the equilibrium constant: 2

2

432

Κ=

= 10"'exp(16697/RT).

(2)

K

H+Br 2

Assuming k

= 1 , 3 5x

H+Br 2

14

3

1

1

10exp(-18400/RT)cmmole"sec"

the value: k

13

3

1

1

H+HBr - 5.0 χ 10exp(-1703/RT)cmmole"sec"

obtains. When k

H+HBr

is used with the Fass ratio,

k

H+Br - = 6.8 exp(800/RT) H+HBr the rate coefficient is: H+Br TOêxpf-gOS/RTjcmôle^sec". 9

τ

k

= 3 , 4x

2

1


12.

BROWN

Halogen Kinetics

365

HBr + M ^ H + Br + M. The HBr dissociation has been studied by three investigators in shock tubes between 1500 and 4200 K, and a brief discription of the work appears in the following paragraphs. The first determination of the rate coefficient was by Giedt et al_. (52) who studied the reaction in Ar between 2100 and Ï2Û0 K. The initial amount of HBr was varied between 2 and 10 percent and HBr concentrations were measured by monitoring both the HBr, IR emission and UV absorption. The calibration plots exhibited scatter; however, no difference between the results of the emission and absorption measurements was reported. Their first order plots are not of good linearity, indicative of competitive reactions and/or experimental error. Competitive reactions of the H + Br chain were accounted for when shock profiles were computed; however the profiles were most sensitive to the dissociation reaction and less sensitive to the competitive reactions. HBr was reported 15 times more efficient than Ar. The rate coefficient which gave the best fit to their profiles is given in Table V. Westberg and Green (53) studied HBr dissociation between 1500-2700 Κ in a shock tube by measuring the emission from Br recombination to Br^ ^). Concentrations were corrected for boundary layer growth; however the calibration measurements showed a great deal of scatter. They list their probable experi mental errors as 1) errors in determining shock velocity, 2) uncertainty in determining location of the shock front, and 3) errors due to boundary layer effects, and conclude that the value of the rate coefficient is not seriously jeopardized by these errors. The competitive reactions of the HBr chain were accounted for and it was noted that no realistic variations of the rate coefficients of the competitive reactions produced the rate coefficient reported earlier by Giedt et al_. HBr and Ar were found to be equally efficient as third~Fodies in contrast to the earlier results of Giedt et al_. The rate coefficient which was estimated to be accurate within a factor of three is given in Table V. The most recent study of HBr dissociation is due to Cohen et a]_. (54) who studied the reaction in pure HBr in the tempera ture interval 1450-2300 Κ monitoring the radiative recombination of Br atom. They presented a review of the literature of the HBr chain reactions and were particularly conscientious about assessing the effects of these competitive reactions. Their calibration measurements showed large scatter. The earlier (52) value of activation energy, 50,000 cal/mole, was not valid in the low temperature range and profile matching was only possi ble if the HBr/Ar efficiency was reduced from the former value of 15 to 3.75. In examining their computed and experimental profiles, one would conclude that no single rate coefficient and choice of rate coefficients for competitive reactions gave an 2

2

1

Shock tube study monitored HBr concentration with both IR emission and UV absorption

Shock tube study, monitored Br radiative recombination emission

Shock tube study monitored Br radiative recombination emission

52

53

54

Giedt et al.

Westberg and Green

Cohen et al,

Measurement

Reference

Investigators

1.2 χ 10T"exp(-88,000/RT)

22

6.0 χ 10Vexp(-88,000/RT)

1500-2700 1450-2300

2

2

2

1.5 χ 10exp(-50,000/RT)

1 2

3

k(cm/mole*sec)

2100-4200

Temperature (K)

HBr + Ar -* H + Br + Ar

Table V


12.

BROWN

Halogen Kinetics

367

equally good fit over the entire temperature interval. The rate coefficient given in Table V is recommended by Cohen et aK for the temperature interval 1500-3800 K. The efficiency of HBr/Ar for the HBr dissociation was approximated at 4:1 and the HBr/Ar efficiency for Br dissociation was approximated as 1:1 while Westberg and Green gave the former ratio the value 1:1 and the latter 3:1. The results of the three measurements are shown in Figure 8. The agreement is satisfactory and is a bit misleading since there was difficulty in fitting the Cohen et aK results satisfactorily over a large temperature interval ancTmany assumptions were made regarding relative efficiencies and competitive rates. The role of Br atom efficiency was not explored. Our strong conclusion regarding HBr dissociation is that more study is required. The competitive reactions, H + Br and Br + HBr, should also be studied at temperatures above 1000 Κ using modern kinetic tech niques since these competitive reactions play an important role in unraveling the dissociation data. The reverse recombination reaction has not been studied and measurement of this rate in a common temperature range where dissociation studies could be made would contribute to our understanding of these complicated kinetics.


2

Br + Br + M Br + M. This reaction and its reverse have been reviewed by Cohen et a]_. (54). The dissociation reaction has been studied in shock tubes with Ar as the third body by this investigation; reaction progress was monitored by measurement of Br concentration via absorption spectroscopy. The results are summarized in Table VI which shows that all the reported activa tion energies are less than the Br bond dissociation energy, 45,500 cal/mole. The dissociation reaction was studied between 1200 and 2225 Κ by Palmer and Hornig (55) who observed that Br was more efficient than Ar, and that the relative efficiency was tempera ture dependent. The next study was by Britton et ajL (56, 57, 58); however, the results of their last study, tiïat of Johnson and Britton (58) are considered preferable by the authors. This rate coefficient (58) which is given in Table VI is about 40 percent less than tRe earlier results (56, 57) with the differbeing attributed to the faulty photomultiplier power supply used in the earlier studies. The Johnson-Britton paper gives a good comparison of the use of reflected and incident shocks in determining dissociation rates. The rate coefficient given here is from an incident shock study which they demonstrated to be the superior technique for Br dissociation studies in the temperature interval 1500 to 1900 K. No attempt was made to correct for the Br efficiency relative to Ar in their 95 percent Ar and five percent Br mixtures. 2

2

2

2

2

2

2

368


Κ


TEMPERATURE

,1

I

I

I

1

I

1

\—i

2

3

4

5

6

7

8

4

Ι 0 / [ Τ ( K)]

Figure 8. Arrhenius plot of HBr + Ar-*H + Br + Ar rate coefficients; dotted lines indicate extrapolation

55

58

59

Palmer-Hornig

Johnson-Britton

Warshay

Inve s ti ga tors Reference

3

1200-2000

Shock tube, measured Br concentration by absorption spectroscopy 2

1500-1900

2

Shock tube, monitored Br concentration by absorption spectroscopy

2

1Ί

1/2

1/2

2.14 χ 10T exp(-31300/RT)

n

5.12 χ 10 exp(-37700/RT)

13

k(cm/mole*sec) Measurement Tempera ture(K) 1200-2225 2.51 χ 10 Τ βχρ(-30690^Τ) Shock tube, monitored Br concentration by absorption spectroscopy

2

Br + Ar 2Br + Ar

Table VI


370


Warshay has reported a carefully detailed shock tube study of Br dissociation between 1200 and 2000 Κ in 1:99 percent mix ture of the rare gases He, Ne, Ar, Kr and Xe. Boundary layer corrections were made which increased the rate coefficients with the largest corrections found at low temperatures. Mixtures 3:97, Br to Xe were studied to ascertain the Br/Xe relative efficiency. The Br/Xe relative efficiency was temperature dependent with the values 3.8 and 1.5 at the temperature 1250 and 1900 K, respec tively. On this basis, a correction to the 1:99 percent mixtures for Br efficiency was neglected. The relative efficiencies of the rare gases were temperature dependent and at 1500 Κ the following order obtained: Kr > Xe > Ar > Ne > He. An explana tion for the large Kr efficiency is that it is the rare gas whose mass nearly equals the Br atom mass and hence the maximum kinetic energy is transferred in Kr, Br collisions. The Warshay results are summarized in Table VI and plotted in Figure 4. A fourth study of Br dissociation has been made by Boyd et al. (60). The reaction was studied in shock tubes between TÏÏ0TTandT985 Κ monitoring reaction progress via the two body emission of Br atoms. The results of this investigation agree with the high temperature absorption results of Warshay but the rate coefficient determined by emission is about 1/3 that deter mined by absorption at 1300 K. To add fuel to the fire, the recombination rate coefficient determined via the "emission" dissociation rate coefficient agrees with the recombination rate coefficient determined by flash photolysis by Ip and Burns (61). In the Boyd et al. paper, comparison was made with earlier results of Warsfiay {62) and not the later work (59). Iρ and Burns point out that beyond 1300 K tïïë extrapolated flash photolysis results diverge from the shock tube emission results. Boyd et al_. used k /k^ /\ = 2 in their data reduction; however in a later paper (63) they indicate that the low temperature relative efficiency is approximately 5. This would reduce the emission recombination and dissociation rate coefficient and also the agreement between the photolysis experiments and shock experiments. Studying reactions by measuring emission intensities of recombining atoms is still, in the author's opinion, inferior to absorption measurement techniques. With specific reference to Br dissociation, the ( P / ) Br atoms recombine to the Br ( n + ) state though this state does not correlate with ground state atoms. The amount of quenching of the ( n + ) state by atoms or molecules is unclear as is the amount of recombination that occurs via the ( Πi ) state. Boyd et al. believe they measured emission from the Jl -*'Eg transition, and, if this is so, are the various molecular elec tronic states equilibrated? The relative concentration of the atomic ( Pi/ ) state has not been determined in shock experiments and its role in the radiative process is unknown. As a counter argument one could state that the absorption rate measurements give results which are too high, since absorption from low lying 2

2

2

2


2

2

2

dB r

r

2

2

3

2

0

3

2

U

3

0

u

3

u

t

2

2

+

u

12.

BROWN

Halogen Kinetics

371

Temperature

( Κ)

2000

1250 Br

2

+ Ar — 2 Br +Ar


10.0 Palmer- Hornlg

9.0 Johnson-Britton

8.0 Warshay

Boyd'' et al

7.0

5.0

6.0

7.0 4

Ι0 /[Τ(

8.0

9.0

Κ)]

Figure 9. Arrhenius phi of Br + Ar-» H + Br + Ar rate coefficients; dotted lines indicate extrapolation

372


states is measured and these states could be underpopulated due to relaxation into higher states. Further studies are required in our opinion before the discrepancy i s proven real and is de termined not to be an experimental a r t i f a c t . The recombination rate of Br atoms in He, Ne, A r , Kr, N and 0 was determined in the flash photolysis experiment of Ip and Burns (6]J. The reaction was studied between room temperature and 1275 Κ by measuring the absorption of B r at 443 mu. Flash photolysis reactions are exothermic; hence the equilibrium concentration of Br atoms increases during the course of an experiment because of the temperature r i s e . Correction for this effect became important above 1000 K. The recombination rate coefficient 2

2


2

log k

A

f

= 15.381 + 0.016 - (2.287 + 0.125)log(T/300) 2

1

+ (1.154 + 0.194)log (T/300) cn^mole^sec- . 15

6

2

1

The value for Ar at toom temperature is 2.4 χ 10 cm mole" sec" . Clyne and Woon-Fat studied the recombination reaction at room temperature in a discharge-flow system monitoring reaction progress by absorption spectroscopy of B r at 415 nm. Their room temperature rate coefficient for Br + Br + Ar -*· Br + Ar i s : 2

2

k

l5

f

A

r

6

2

= 3.4 χ 10 cm mole" sec"

l f

which is 1.4 times greater than the Ip and Burns results. Acknowledgement The assistance of Mr. Raymond Yu for his help in the literature search is gratefully noted.

Literature Cited 1. Creitz, E.C., NBS J. of Res. (1970), 74A, 521. 2. Fristrom, R.M., and Sawyer, R.F., "Flame Inhibition Chem.", AGARD Conference Proceedings No. 84 on Aircraft Fuels, Lubricants, and Fire Safety, AGARD-CP-84-71 (1971), Section 12, North Atlantic Treaty Organization. 3. Hastie, J.W., NBS J. of Res. (1973), 77A, 733. 4. Wilson, W.E., Jr.,"10th Symposium (International) on Combustion," 47, Combustion Institute, Pittsburgh, Pa., (1965). 5. Wilson, W.E., J r . , O'Donovan, J.T., and Fristrom, R.M., "12th Symposium (International) on Combustion," 929, Combus tion Institute, Pittsburgh, Pa., (1969)


12. brown

Halogen Kinetics

373

6. Fettis, G.C., and Knox, J.H., "Progress in Reaction Kinetics," (Ed. G. Porter) 2, 1, Pergamon Press, London, (1964) 7. Benson, S.W., Cruickshank, F.R., and Shaw, R., Int. J. Chem. Kinet. (1969), 1, 29. 8. White, J.H., "Comprehensive Chem. Kinetics," (Eds., C.H. Bamford and C.F.H. Tipper) 6, 201, Elsevier, Amsterdam (1972). 9. Watson, R.T., Nat. Bur. Stand., NBSIR 74-516 (1974). 10. Rodenbush, W.H., and Klingelhoefer, W.C., J. Am. Chem. Soc. (1933), 55, 130. 11. Steiner, H. and Rideal, E.K., Proc. Roy. Soc. (1939), A173, 503. 12. JANAF Thermochemical Tables, NSRDS-NBS 37 (1971). 13. Ashmore, P.G. and Chanmugam, J., Trans. Faraday Soc. (1953), 49, 254. 14. Burns, W.G. and Dainton, F.S., Trans. Faraday Soc. (1952), 48, 52. 15. Clyne, M.A.A., and Stedman, D.H., Trans. Faraday Soc. (1966), 62, 2164. 16. Westenberg, A.A., and deHaas, N., J. Chem. Phys. (1968), 48, 4405. 17. Clyne, M.A.A. and Walker, R.F., JCS Faraday Trans. I (1973), 69, 1547. 18. Snider, N.S., J. Chem. Phys. (1970), 53, 4116. 19. Cadman, P. and Polanyi, J.C., J. Phys. Chem. (1968), 72, 3715. 20. Davis, D.D.,, Braun, W., and Bass, A.M., Int. J. Chem. Kinet. (1970), 3, 223. 21. Galante, J.J. and Gislason, E.A., Chem. Phys. Letters (1973) 18, 231 22. Klein, F.S. and Wolfsberg, M., J. Chem. Phys. (1961), 34, 1494. 23. Dodonov, A.F., Lavrovskaya, G.K. and Morozov, I.I., Kinetika: Kataliz (1970), 11, 821. 24. Jacobs, T.A., Giedt, R.R., and Cohen, N., J. Chem. Phys. (1968), 49, 1271. 25. Seery, D.J. and Bowman, C.T., J. Chem. Phys. (1968), 48, 4314. 26. Breshears, W.D. and Bird, P.E., J. Chem. Phys. (1972), 56, 5347. 27. Jacobs, T.A., Cohen, N. and Giedt, R.R., J. Chem. Phys. (1967), 46, 1958. 28. Fishburn, E.S., J. Chem. Phys. (1966), 45, 4053. 29. Giedt, R.R. and Jacobs, T.A., J. Chem. Phys. (1971), 55, 4144. 30. Clyne, M.A.A. and Stedman, D.H., Trans. Faraday Soc. (1968), 64, 1816. 31. Johnston, Η., and Birks, J., Accounts Chem. Res. (1972), 5, 327. 32. Lloyd, A.C., Int. J. Chem. Kinet. (1971), 3, 39.

374 33. 34. 35. 36. 37. 38.


39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62.


Clyne, M.A.A. and Stedman, D.H., Trans. Faraday Soc., (1968), 64, 2698. Jacobs, T.A. and Giedt, R.R., J. Chem. Phys., (1963), 39, 749. Jacobs, T.A., Giedt, R.R. and Cohen, N., J. Chem. Phys., (1968), 49, 1271. Bodenstein, M. and Lind, S.C., Z. Phys. Chem., (1906), 57, 168. Bodenstein, M. and Lutkemeyer, H., Z. Phys. Chem., (1924), 114, 208. Bodenstein, M. and Jung, Ζ., Z. Phys. Chem. (1920), 121, 127. Christiansen, J.Α., Kgl. Danske Videnskab Selskab, Mat. Fys. Medd., (1919), 1, 14. Herzfeld, Κ., Ann. Phys., (1919), 59, 635. Polanyi, M., Z. Elektrochem, (1920), 26, 50. Sullivan, J.H., J. Chem. Phys., (1968), 49, 1155. Bach, F . , Bonhoeffer, F.K. and Moelwyn-Hughes, E.A., Z. Phys. Chem., (1934), B27, 71. Pease, R.N., "Equilibrium Kinetics of Gas Reactions", 121, Princeton University Press, 1942. Steiner, H., Proc. Roy. Soc., (1939), A173, 531. Fass, R.A., J . Phys. Chem., (1970), 74, 984. Levy, Α., J. Phys. Chem., (1958), 62, 570. Britton, D. and Davidson, N., J. Chem. Phys., (1956), 25, 810. Cooley, S.D. and Anderson, R.C., Ind. Eng. Chem., (1952), 44, 1402. Vidal, C., J. Chim. Phys., (1971), 68, 1360. Takacs, G.A. and Glass, G.P., J . Phys. Chem., (1973), 77, 1060. Giedt, R.R., Cohen, N. and Jacobs, T.A., J. Chem. Phys., (1969), 50, 5374. Westberg, K. and Greene, E.F., J. Chem. Phys., (1972), 56, 2713. Cohen, Ν., Giedt, R.R. and Jacobs, T.A., Int. J. Chem. Kinet., (1973), 5, 425. Palmer, H.B. and Hornig, D.F., J. Chem. Phys., (1957), 26, 98. Britton, D., and Davison, N., J. Chem. Phys., (1956), 25, 810. Britton, D., J. Phys. Chem., (1960), 64, 742. Johnson, C.D. and Britton, D., J. Chem. Phys., (1963), 38 1455. Warshay, M., J . Chem. Phys., (1971), 54, 4060. Boyd, R.K., Burns, G., Lawrence, T.R. and Lipiatt, J.H., J. Chem. Phys., (1968), 49, 3804. Ip, J.K.K. and Burns, G., J . Chem. Phys., (1969), 51, 3414. Warshay, M., Proc Intern. Shock Tube Symp. 5th, White Oak, Md., 1965, (1966), 631.

12.

63. 64.

BROWN

Halogen Kinetics

375

Boyd, R.K., Brown, J.D., Burns. G. and Lippiatt, J.H., J. Chem. Phys., (1968), 49, 3822. Clyne, M.A.A. and Woon-Fat, A.R., Dis. Faraday Soc., (1972), 53, 412.


This Work was supported by NSF-RANN under Grant Number GI-43.

DISCUSSION R. M. FRISTROM: With r e s p e c t t o t h e problem o f e x t r a p o l a t i n g low temperature k i n e t i c d a t a t o h i g h temperature combustion systems, t h e r e i s one r e c e n t h o p e f u l development. The r e a c t i o n s o f hydrogen atoms w i t h s e v e r a l m e t h y l h a l i d e s (CH-F, CH C1 and CH^Br) have been s t u d i e d by a new t e c h n i q u e c a l l e d t h e " p o i n t s o u r c e method" which uses f l a m e s as h i g h temperature r a d i c a l s o u r c e s (L. H a r t , C. G r u n f e l d e r , and R. F r i s t r o m , "The P o i n t Source Technique U s i n g Upstream Sampling," Combustion and Flame, (1974), 23, 109). The m o l e c u l e t o be s t u d i e d i s i n j e c t e d a t a p o i n t and t h e d i s a p p e a r a n c e i s f o l l o w e d as a f u n c t i o n o f d i s t a n c e from the s o u r c e . T h i s i s an e x t e n s i o n o f an o l d method t o t h e flame temperature regime. These same r e a c t i o n s have a l s o been s t u d i e d by t h e c o n v e n t i o n a l d i s c h a r g e tube - ESR Methods o v e r a wide temperature range (A. A. Westenberg and N. deHaas, "Rages o f H + CH-X R e a c t i o n s , " J . Chem. Phys., (1975), 62, 3321) and t h e r e i s an o v e r l a p o f s e v e r a l hundred degrees i n t h e r e g i o n around 1000K. Here t h e a b s o l u t e r a t e s agree between t h e two methods w i t h i n a f a c t o r o f 2-3; most o f the o b s e r v e d d i s c r e p a n c y i s p r o b a b l y due t o t h e c r u d e method used f o r d e t e r m i n a t i o n o f H atom c o n c e n t r a t i o n s i n t h e p o i n t s o u r c e method s i n c e i f r e l a t i v e r a t e s between the h a l i d e s a r e compared t h e two methods a g r e e w i t h i n 15%. Thus, a t l e a s t i n t h i s one c a s e , i t appears t h a t flame k i n e t i c s c a n r e l i a b l y i n f e r r e d from lower temperature c o n v e n t i o n a l s t u d i e s . 3

13 Halogenated Fire Extinguishants: Flame Suppression by a Physical Mechanism? ERIC R. LARSEN


Halogens Research Laboratory, The Dow Chemical Co., Midland, Mich. 48640

The mechanism by which halogenated agents act as flame suppressants has been the subject of intensive research for about twenty five years. In the early 1950's Fryburg (1) proposed that the outstanding effectiveness of these agents may be due to a mechanism that involved the chemistry of the combustion processes rather than one or another of the mechanisms which were thought to be applicable to the inert gases. A chemically based mechanism was felt to be attractive since i t might explain the fact that certain halohydrocarbons, especially those containing bromine, were from five to eight times as effective as carbon dioxide and nitrogen. Fryburg's proposal led directly to very extensive studies to elucidate the chemistry of flames into which halogenated materials were introduced. These studies, in turn, led to the development of the radical trap mechanism for flame suppression that is in vogue today. That the halogenated agents enter into the chemistry of the various combustion processes has been adequately demonstrated by studies too numerous to into here. That this chemical reactivity is involved in the primary mechanism by which the halogenated agents act as flame suppressants i s , however, open to question. Several years ago it was reported by the author that an investigation of the flammability of halohydro-carbons showed that these compounds were nonflammable in air only when they contained more than about 70 wt.% halogen (2,3). The type of halogen present did not seem to be important. In other words, when the halogen concentration was expressed on a weight basis, rather than on the commonly employed molar basis, the various halogens were essentially equivalent. This was later shown to also be true for mixtures of halogenated 376

13.

LARSEN

Flame Suppression Mechanism

377

agents and h y d r o c a r b o n based f u e l s when the h a l o g e n c o n t e n t o f the m i x t u r e o f agent and f u e l was d e t e r mined a t the " f l a m m a b i l i t y peak" (A) . The h a l o g e n c o n t e n t o f peak c o m p o s i t i o n f o r some s i x t y agents i n h e p t a n e - a i r systems were found t o obey the r e l a t i o n ship:


1.

[ [ A ]

+

3 [ p ]

. 100 = 69.8

±

3.5%

where [H] i s the weight o f h a l o g e n i n t h e m i x t u r e o f agent [A] and f u e l [F] t h a t i s p r e s e n t a t the f l a m m a b i l i t y peak when a l l c o n c e n t r a t i o n s a r e e x p r e s s e d i n mg/l. T h i s r e l a t i o n s h i p was found t o h o l d r e g a r d l e s s o f the t y p e o f h a l o g e n p r e s e n t i n the agent, and showed t h a t the halogens were e f f e c t i v e flame supp r e s s a n t s i n d i r e c t p r o p o r t i o n t o t h e i r atomic w e i g h t s , i . e . F:Cl:Br:I::1.0:1.9:4.2:6.7. These r e s u l t s a r e c l e a r l y i n c o n s i s t e n t w i t h the p o s t u l a t e d c h e m i c a l mechanisms o f flame s u p p r e s s i o n . P r i o r t o d i s c u s s i n g the p r i m a r y mechanism by which h a l o g e n a t e d compounds a c t as flame s u p p r e s s a n t s i t i s n e c e s s a r y t o re-examine the r o l e o f the i n e r t gases, s i n c e we must e s t a b l i s h the b a s e l i n e b e h a v i o r o f t h e s e agents b e f o r e we can hope t o s t u d y the v a r i a n t b e h a v i o r o f t h e h a l o g e n a t e d a g e n t s . The i n e r t gas a g e n t s , e.g. He, A r , N2, C02# CF4, e t c . , a r e g e n e r a l l y conceded t o a c t by a p h y s i c a l mechanism s i n c e t h e y do not e n t e r i n the c h e m i s t r y o f the v a r i o u s combustion p r o c e s s e s t o any s i g n i f i c a n t e x t e n t . F i g u r e 1 shows the i n f l u e n c e o f t h e i n e r t gases upon the f l a m m a b i l i t y l i m i t s o f p e n t a n e - a i r mixt u r e s ( 5 ) . The p o i n t a t which the upper and lower l i m i t c u r v e s meet i s commonly c a l l e d the f l a m m a b i l i t y peak and r e p r e s e n t s the minimum agent c o n c e n t r a t i o n needed t o p r e v e n t flame p r o p a g a t i o n i n any and a l l f u e l - o x i d a n t m i x t u r e s . While the minimum agent conc e n t r a t i o n , o r peak v a l u e s , o f N2 and CO2 a r e r e l a t i v e l y h i g h - 42 v o l % and 29 v o l %, r e s p e c t i v e l y the h a l o g e n a t e d agents g e n e r a l l y show peak v a l u e s o f l e s s than 10 v o l %. A l t h o u g h the most e f f i c i e n t h a l o genated agent (CF2Br2, 4.4 v o l % i n heptane) and n i t r o g e n d i f f e r i n peak c o n c e n t r a t i o n s by about a f a c t o r o f 10, t h e d i f f e r e n c e between CF2Br2 and C3F8 one o f the most e f f e c t i v e i n e r t gases - i s o n l y about a f a c t o r o f two. T h i s r e l a t i v e l y narrow s p r e a d i n d i f f e r e n c e s h a r d l y seems t o j u s t i f y a c o m p l e t e l y new mechanism. A c l o s e i n s p e c t i o n o f the i n e r t gas agents shows t h a t when the c o n c e n t r a t i o n s o f pentane, oxygen, and i n e r t gas a r e r e p l o t t e d on a heat c a p a c i t y b a s i s , o r



,1

0

I 10

I I 20 30 Added Inert, (Vol %)

I 40

I 50

Bureau of Mines Bulletin

Figure 1. Limits of flammability of various n-pentane-inert gas-air mixtures at 25°C and atmospheric pressure (5)

13.

LARSEN


Figure 2. Limits of flammobility of various n-pentane-inert gas-'air" mixtures. Heat capacity basis.

κ IF/

InertV?

• 38.7% 0 • 31.3% 02 a 21.0% 0 2


2

Figure 3. Limits of fiammability of various n-pentane—inert gas-air mixtures at 25°C and atmos pheric pressure. N from air deleted. Heat capacity basis. 8


380


f o r t h a t m a t t e r on a weight b a s i s , the d i f f e r e n c e s between t h e v a r i o u s i n e r t gases d i s a p p e a r s (6). F i g u r e 2 shows such a p l o t u s i n g t r i a n g u l a r c o o r d i n a t e s . The heat c a p a c i t y c o n t r i b u t i o n o f the i n e r t gases i s shown here as the sum o f the i n d i v i d u a l c o n t r i b u t i o n s o f the i n e r t gases i . e . N2 and C F A # p r e s e n t i n t h e system. The upper l i m i t (I) and lower l i m i t ( J ) o f pentane i n oxygen a r e shown a l o n g the base o f the diagram. The upper l i m i t (G) and lower l i m i t (H) i n a i r a r e shown a l o n g the l i n e AD. P o i n t F, the p o i n t a t which t h e l i m i t c u r v e s converge, r e p r e s e n t s t h e t h e o r e t i c a l f l a m m a b i l i t y peak, and p o i n t Ρ the ex p e r i m e n t a l l y d e t e r m i n e d f l a m m a b i l i t y peak. The d i f f e r e n c e s between t h e two i s p r o b a b l y due t o a v a r i e t y o f f a c t o r s , c h i e f o f which appear t o be t h e s t r e n g t h o f the i g n i t i o n s o u r c e and the p r a c t i c a l d i f f i c u l t y o f making up a c c u r a t e gas m i x t u r e s when t h e d i f f e r ences between the upper and lower l i m i t become v e r y small. T h a t the e x p e r i m e n t a l peak l a y s on t h e f u e l r i c h s i d e o f the s t o i c h i o m e t r i c m i x t u r e l i n e , i . e . C s t C , i s i n k e e p i n g w i t h the f i n d i n g t h a t the most r e a d i l y i g n i t a b l e mixtures are s l i g h t l y f u e l r i c h . The e x p e r i m e n t a l p o i n t s shown i n F i g u r e 2 a r e d e r i v e d from a s t u d y , c a r r i e d o u t by Moran and B e r t s c h y (1), on the e f f e c t o f C F on the flamma b i l i t y l i m i t s o f p e n t a n e - " a i r " systems. The " a i r " employed h e r e c o n t a i n e d e i t h e r 21.0%, 31.3%, o r 38.7% oxygen. The s o l i d l i n e r e p r e s e n t s the flammable l i m i t c u r v e s f o r t h e system p e n t a n e - o x y g e n - n i t r o g e n . I t i s o b v i o u s from t h i s diagram t h a t t h e system pentane-oxygen-CF s h o u l d e x h i b i t e s s e n t i a l l y the same l i m i t diagram as p e n t a n e - o x y g e n - n i t r o g e n . The r e g i o n bound by the t r i a n g l e ADC i s shown i n g r e a t e r d e t a i l i n F i g u r e 3. In t h i s f i g u r e the con t r i b u t i o n t o the heat c a p a c i t y due t o the n i t r o g e n c o n t e n t o f the a i r employed as the o x i d a n t was d e l e t e d , and t h e d a t a r e n o r m a l i z e d . I t i s o b v i o u s from t h e s e f i g u r e s t h a t a l l o f t h e i n e r t gases show e s s e n t i a l l y the same flammable l i m i t s , and t h a t t h e i r heat capac i t y c o n t r i b u t i o n s are a d d i t i v e . There a r e s e v e r a l p o i n t s i n F i g u r e 2 t h a t a r e i m p o r t a n t t o the a n a l y s i s o f f l a m m a b i l i t y peak (P) d a t a ; t h e s e a r e shown as K and K j p . K i s the c o m p o s i t i o n o f t h a t i n e r t gas-oxygen m i x t u r e which w i l l g i v e a peak (P) m i x t u r e upon the a d d i t i o n o f f u e l . S i m i l a r l y , Kjp i s the c o m p o s i t i o n o f i n e r t gas which upon a d d i t i o n o f oxygen w i l l y i e l d a peak m i x t u r e . These c o m p o s i t i o n s a r e r e p r e s e n t e d by t h e e q u a t i o n s : [I]Cp 4

4

I 0

I 0

T

13.

LARSEN

381

Flame Suppression Mechanism [IlCPj

3

*

K

IF

[I]C

P l

+ [Fuel]Cp

F

*

1

0

0

where [ I ] C p j , [02]CPQ and [FuelJCpp r e p r e s e n t the i n d i v i d u a l heat capacity c o n t r i b u t i o n s of the i n e r t g a s e s , o x y g e n , and f u e l , r e s p e c t i v e l y . The r a t i o o f the heat c a p a c i t y c o n t r i b u t i o n s of a l l of the i n e r t g a s e s , Σ [ I ] C p j , a t t h e p e a k (P) t o t h e t o t a l h e a t c a p a c i t y o f t h e s y s t e m c a n be r e p r e s e n t e d b y t h e equation:


2

w h e r e Σ [ C ] C p i s t h e sum o f t h e i n d i v i d u a l h e a t c a p a c i t y c o n t r i b u t i o n s o f a l l components o f the system (3,6) . T a b l e I shows t h e f l a m m a b i l i t y peak c o m p o s i t i o n s f o r v a r i o u s methane-oxygen-agent and p e n t a n e - o x y g e n agent systems. Those systems marked w i t h an a s t e r i s k c o n t a i n o n l y a s i n g l e i n e r t gas w h i l e t h e o t h e r s y s t e m s c o n t a i n N2 a l o n g w i t h t h e i n d i c a t e d a g e n t . This d a t a shows q u i t e c l e a r l y t h e a d d i t i v i t y o f t h e h e a t c a p a c i t y c o n t r i b u t i o n s o f the i n e r t gases t o the peak c o m p o s i t i o n , and c o n f i r m s the f a c t t h a t a l l o f the i n e r t g a s e s behave as f l a m e s u p p r e s s a n t s by v i r t u e of t h e i r c o n t r i b u t i n g to the t o t a l heat c a p a c i t y of the system w i t h o u t c o n t r i b u t i n g to the heat producing capacity of the system. T h e d a t a f o r A r a n d He a l s o suggests that n e i t h e r d i l u t i o n of the f u e l or oxygen, nor the thermal c o n d u c t i v i t y of the agent are p a r t i c u l a r l y important to the flame suppression process. T h e mean v a l u e o f K (86.9 ± 1.2) f o r t h e i n e r t g a s e s i s p a r t i c u l a r l y germane t o t h e mechanism by w h i c h t h e h a l o g e n a t e d a g e n t s a c t as f l a m e s u p p r e s s a n t s . R e c e n t l y , C r e i t z on the b a s i s o f an e x t e n s i v e s t u d y of t h e f l a m e i n h i b i t i n g a b i l i t y o f CF3Br d e t e r m i n e d t h e c o n c e n t r a t i o n of CFoBr r e q u i r e d t o prevent flame p r o p a g a t i o n i n v a r i o u s f u e l - o x y g e n m i x t u r e s (8). The " l i m i t i n g c o n c e n t r a t i o n of oxygen-agent atmosphere" d e t e r m i n e d b y C r e i t z f o r t h e f u e l s CH4, C2H5, C3H8F a n d C4H10 a r e s h o w n i n T a b l e I I a l o n g w i t h t h e e q u i v a l e n t v a l u e s f o r v a r i o u s other oxygen-agent atmo spheres . When CF3Br i s c o m p a r e d t o t h e i n e r t g a s e s , i . e . A r , H e , a n d N2, o n a v o l u m e % b a s i s i t w o u l d appear t h a t the d a t a might s u p p o r t a c h e m i c a l mech anism of flame suppression. Such s u p p o r t i s l e s s c l e a r w h e n CF3Br i s c o m p a r e d w i t h CF4. When t h e v a r i o u s agents are compared on a heat c a p a c i t y b a s i s i t would appear t h a t such support i s e n t i r e l y absent. c

I

0


382

Table I F l a m m a b i l i t y Peak C o m p o s i t i o n s

Fuel


CH

C

H

4

5 12

Agent

Fuel Vol%

Peak Agent Vol%

(7,11,12)

KCp

K

IO

K

T F

6.3

81.5

80.3

86.9

91.3

7.5

22.3

77.8

84.7

89.9

He

5.8

36.3

78.8

85.5

91.0

He*

5.0

85.0

79.0

85.8

92.6

Ar

5.0

48.4

88.0

92.2

Ar*

4.0

88.2

81.7 83.2

88.8

93.9

N

2.1

86.2

80.8

88.0

90.9 88.7

w

2 co

2

2 co

2

2.8

28.0

77.5

CF

4

2.5

26.0

79.7

86.1 86.5

3.0

15.7

79.7

87.1

91.0 90.4

2.8

10.6

80.6

87.1

91.5

S F

C

6 F

3 8

Mean V a l u e s • S o l e Agent

KCp = 80 .2 ± 1.7 K K

I 0

TTTI

= 86 .9 ± 1.2 = 91.0 ± 1.5

E x p e r i m e n t a l C o n d i t i o n s : Fuel-C^-No- Agent M i x t u r e s w i t h Flame P r o p a g a t i o n V e r t i c a l l y t h r o u g h Tube

13.

LARSEN


383

Table I I L i m i t i n g C o n c e n t r a t i o n s o f Oxygen-Agent


Fuel

Agent

o

2

Vol%

Atmospheres

Agent Vol%

02 C %

Agent Cp%

P

CH

4

Ar

8.2

91.8

11.2

88.8

CH

4

He

10.5

85.9

14.2

85.8

CH

4

N

2

13.0

87.0

13.0

87.0

2

11.9

88.1

12.0

88.0

12.0

88.0

12.0

88.0

25.0

75.0

13.7

86.3

34.0

66.0

17.9

82.1 83.4

C

3 8

H

N

C

5 12

H

N

C

H

5 12 CH * 4

2 CF

4

CF Br 3

CF Br

32.0

68.0

16.6

H

CF Br

33.3

66.7

17.5

82.5

H

CF Br

33.8

65.3

18.0

82.0

W

3

C

3 8*

C

4 10*

3

2

• D i f f u s i o n Flame - E . C. C r e i t z , F i r e Tech., 8_ 131 (1972) (8)


384

The v a l u e which appears i n t h e l a s t column (Agent Cp) o f T a b l e I I i s the v a l u e f o r K discussed previously. The d i f f e r e n c e between t h e v a l u e o f K J Q f o r the i n e r t gases ( 8 6 . 9 ± 1 . 2 ) and f o r CF-Br ( 8 2 . 5 ± 0 . 6 ) i s s m a l l enough t o be a t t r i b u t a b l e t o the f a c t t h a t the i n e r t gas d a t a i s d e r i v e d from premixed combustion tube d e t e r m i n a t i o n s which the d a t a f o r CF3Br i s d e r i v e d from a s t u d y on the agent c o n c e n t r a t i o n s r e q u i r e d t o extinguish small d i f f u s i o n flames. T a b l e I I I shows t h e f l a m m a b i l i t y peak c o m p o s i t i o n s f o r a number o f h a l o g e n a t e d agents t h a t were s t u d i e d by the Purdue Research F o u n d a t i o n (9). These agents a r e r e p r e s e n t a t i v e o f a t o t a l o f 4 6 agents s t u d i e d w i t h r e s p e c t t o t h e i r a b i l i t y t o s u p p r e s s flame p r o p a g a t i o n i n h e p t a n e - a i r m i x t u r e s . T h i s s t u d y by Purdue has been f r e q u e n t l y employed t o s u p p o r t the premise t h a t the h a l o g e n a t e d agent a c t as flame s u p p r e s s a n t s by a c h e m i c a l , r a t h e r t h a n p h y s i c a l , mechanism (1) . The v a r i e t y o f agents shown a r e r e p r e s e n t a t i v e o f the d i f f e r e n t h a l o g e n s , s i n g l e l y and i n c o m b i n a t i o n . The v a l i d i t y o f t h i s premise i s q u e s t i o n a b l e when the agents a r e compared on a heat c a p a c i t y b a s i s r a t h e r than on t h e u s u a l volume b a s i s . When compared on a h e a t c a p a c i t y b a s i s no one t y p e o f h a l o g e n s t a n d s o u t as c l e a r l y s u p e r i o r t o any o t h e r . I t i s a x i o m a t i c o f the r a d i c a l t r a p mechanism t h a t bromine c o n t a i n i n g a g e n t s , which e n t e r i n t o the combustion c h e m i s t r y , s h o u l d be s u p e r i o r t o the f l u o r i n e c o n t a i n i n g compounds and CO2 which behave as i n e r t g a s e s . The mean v a l u e o f K f o r t h e s e systems i s 8 4 . 0 ± 2 . 9 which l a y s midway between the v a l u e s o f K J O found f o r the i n e r t gas systems ( 8 6 . 9 ± 1 . 2 , T a b l e I) and f o r C F B r ( 8 2 . 5 ± 0 . 6 , T a b l e I I ) d e t e r mined from C r e i t z ' s d a t a . The v a l u e o f Kjp found f o r t h e s e h a l o g e n a t e d agents compares c l o s e l y w i t h t h a t found f o r the i n e r t g a s e s ; 8 7 . 9 ± 2 . 5 and 9 1 . 0 ± 1 . 5 , respectively. In a more r e c e n t study by B a j p a i and Wagner ( 1 0 ) t h e e f f e c t o f t h e agents CF3Br and C F 2 C l B r upon the f l a m m a b i l i t y l i m i t s o f s e v e r a l h y d r o c a r b o n f u e l s were determined i n a m o d i f i e d combustion t u b e . The f l a m m a b i l i t y peak v a l u e s d e r i v e d from t h i s s t u d y a r e shown i n T a b l e IV, a l o n g w i t h the c o r r e s p o n d i n g v a l u e s o f KCp, KJO# and K . T h i s study tends t o bear o u t the o v e r a l l f i n d i n g s c i t e d above, and a g a i n f a i l s t o s u p p o r t any c l e a r s u p e r i o r i t y o f the h a l o g e n a t e d agents o v e r the i n e r t g a s e s . The v a l u e o f Kjp h e r e c o r r e s p o n d s even more c l o s e l y w i t h t h a t found f o r t h e i n e r t gases t h a n does t h e Purdue study (9). The v a l u e o f K j which i n t h i s c a s e i s the sum o f the h e a t


I

I

3

I F

0

0

0

13.

LARSEN

385

Flame Suppression Mechanism Table Flammability

III

Peak

Compositions

Experimental Conditions: n-Heptane-Air Mixtures with Flame P r o p a g a t i o n V e r t i c a l l y t h r o u g h Tube, Spark Ignition


Purdue Res. Found, and Dept. o f Chem.: Sum. R e p ' t S e p t . 1 , 1947 t o A u g . 3 1 , 1 9 4 8 , P u r d u e U n i v . (Contract W - 4 4 - 0 0 9 e n g - 5 0 7 w i t h A r m y E n g . R&D L a b s ) (9)

Agent

Agent Vol%

Peak

Heptane Vol%

KCp

CF Br 2

2

4.2

4.4

67.2

81.2

79.5

CH Br

2

5.2

2.1

73.5

80.9

89.0

C H I

5.6

3.2

70.5

81.4

84.7

CH Br

6.1

2.1

73.4

80.7

88.8

CH I

6.1

2.1

73.1

80.6

88.8

CF Br

6.1

3.0

71.8

81.8

85.4

C H Br

6.2

2.3

73.5

81.6

88.4

11.2

3.2

74.6

84.6

86.4

12.3

3.3

74.0

84.4

85.4

13.4

3.0

77.8

86.6

88.6

14.9

3.9

73.8

85.2

84.7

17.5

3.6

74.6

85.9

85.2

17.8

2.2

77.3

84.8

89.7

18.1

2.3

85.3

90.5

89.7

2

2

5

3

3

3

2

5

cci

4

CC1F C

3

2 6 F

2 2 CHC1 C F

C 1

3

CHF

3

c-C F 4

8

CF

4

26.0

1.8

82.1

87.7

92.7

co

2

26.0

1.8

78.8

85.5

91.1

Mean

Values

KCp = 7 5 . 1 ± 4 . 4 IO K „ IF K

T

=

8 4 . 0 ± 2 .9

= 8 7 . 9 ± 2 .5


386

Table Flammability

Fuel


CH

C

Agent

4

n-C H 4

1 Q

AgenP t eak F u e l Vol% Vol%

(10)

KCp

KTF

6.8

9.2

74

82

88

CF ClBr 2

4.3

9.9

72

81

87

CF Br

8.0

4.3

75

83

89

CF ClBr 2

7.0

5.2

73

82

87

CF Br

7.2

3.0

75

82

90

CF ClBr 2

6.3

2.8

75

82

90

CF Br

7.2

4.0

73

82

87

CF ClBr

6.3

2.9

75

82

90

3

H

Peak C o m p o s i t i o n s

CF Br 3

3 8

IV

3

3

2

Mean V a l u e s

KCp = 7 4 . 0

± 1.2

K

IO

=

82.0

± 0.5

K

IF

=

88.5

± 1.4


13.

LARSEN


387

c a p a c i t y c o n t r i b u t i o n s o f the agent and t h e n i t r o g e n from the a i r employed as the o x i d a n t a g r e e s e x a c t l y w i t h t h e v a l u e f o r CF3Br d e r i v e d from C r e i t z ' s d a t a ( 8 ) . On t h e b a s i s o f t h e d a t a d e v e l o p e d h e r e and from the p r e v i o u s f i n d i n g t h a t t h e h a l o g e n s a r e e f f e c t i v e as flame s u p p r e s s a n t s i n d i r e c t p r o p o r t i o n t o t h e i r atomic weight i t i s f e l t t h a t the h a l o g e n a t e d agents probably p l a y a d u e l r o l e i n the s u p p r e s s i o n of flames. T h e i r primary r o l e i s s i m i l a r t o t h a t of the i n e r t gases, t h a t i s , t h e y a c t as h e a t s i n k s w i t h o u t cont r i b u t i n g s i g n i f i c a n t l y t o the h e a t p r o d u c i n g p o t e n t i a l o f the system. In t h i s r e s p e c t i t i s a n t i c i p a t e d t h a t they a c t i n t h e r e g i o n ahead o f the moving flame f r o n t r a t h e r than w i t h i n the flame p e r s e . In a d d i t i o n t o t h i s p r i m a r y mechanism t h e h a l o genated a g e n t s , w i t h a few e x c e p t i o n s , a l s o appear t o a c t as f u e l s once they a r e h e a t e d t o a h i g h enough temperature. T h i s a b i l i t y t o a c t as a f u e l i s c o n s i s t e n t w i t h the r e l a t i o n s h i p shown i n e q u a t i o n 1 and i t i s p r o b a b l y i n t h i s r o l e t h a t t h e s e agents p a r t i c i p a t e i n t h e c h e m i s t r y o f f l a m e s . In t h i s r o l e t h e h a l o genated agents would be flame s u p p r e s s a n t s o n l y w i t h i n the c o n t e x t t h a t the a d d i t i o n o f e x c e s s f u e l t o a f l a m mable m i x t u r e s produces a non-flammable m i x t u r e . In o r d e r t o make any k i n d o f a s t r o n g c a s e f o r a more complex mechanism o f flame i n h i b i t i o n t h a n i s found f o r the i n e r t gases i t would appear t o be nece s s a r y t o show t h a t t h e agent produces a d e f i n i t e e f f e c t upon t h e o v e r a l l flammable e n v e l o p e . T h i s w i l l be d i f f i c u l t t o a c c o m p l i s h u s i n g a i r as the o x i d a n t as has been done w i t h t h e g r e a t b u l k o f t h e work c a r r i e d o u t t o d a t e , and i t i s s t r o n g l y recommended t h a t f u t u r e s t u d i e s on t h e mechanism o f flame i n h i b i t i o n be c a r r i e d o u t u s i n g a i r t h a t i s s i g n i f i c a n t l y e n r i c h e d w i t h oxygen, eg. >50% O2. Experimental

Data

The d a t a r e l i e d upon i n t h i s paper were d e v e l o p e d by a number o f d i f f e r e n t groups, i n c l u d i n g Coward and coworkers (11,12), Z a b e t a k i s ( 5 ) , Moran and B e r t s c h y (7) 9 C r e i t z (S), B a j p a i and Wagner (10) and o t h e r s ( 9 ) . The " f l a m m a b i l i t y peak" v a l u e s d e r i v e d from t h e s e r e f e r e n c e s are g i v e n i n the v a r i o u s t a b l e s . The r e a d e r i s r e f e r r e d t o t h e o r i g i n a l a r t i c l e s f o r the complete data. Only t h o s e s t u d i e s were e v a l u a t e d i n which a number o f i n e r t gases on h a l o g e n a t e d agents were e v a l u a t e d f o r t h e i r a b i l i t y t o p r e v e n t flame p r o p a g a t i o n by d e t e r m i n i n g the e f f e c t o f the added i n e r t


388


gases upon t h e l i m i t s o f f l a m m a b i l i t y o f v a r i o u s f u e l oxidant mixtures. I n most c a s e s t h e l i m i t d a t a was o b t a i n e d by upward flame p r o p a g a t i o n i n an a p p a r a t u s s i m i l a r t o t h a t d e s c r i b e d by t h e Bureau o f Mines (12), the one e x c e p t i o n b e i n g C r e i t z * s study (£) which em ployed a d i f f u s i o n flame. I n t h e combustion tube s t u d i e s t h e tubes employed were s u f f i c i e n t l y l a r g e so t h a t i n c r e a s i n g t h e d i a m e t e r o f t h e tube would have l i t t l e e f f e c t upon t h e v a l u e s o b t a i n e d . F o r t h e method employed t o c o n v e r t t h e c o m p o s i t i o n o f t h e systems from t h e normal volume p e r c e n t b a s i s t o t h e heat c a p a c i t y b a s i s employed here t h e r e a d e r i s referred t o reference ( 6 ) .

Literature Cited 1. 2. 3. 4. 4 5. 6. 7. 8. 9.

10. 11. 12.

Fryburg, G . , NACA Tech. Note 2102, p. 27 (1950). Larsen, E. R., in Handbook of Experimental Phar macology, New Series, Volume XXX, (M. B. Chenoweth, ed.), Springer-Verlag, New York, 1972, p. 28. Larsen, E. R., Abs. 166th National ACS Meeting, Chicago, I l l i n o i s , Aug. 26-31 (1973), Paper No. INDE 054. Larsen, E. R., JFF/Fire Retardant Chemistry, 1 (1974). Zabetakis, M. G . , "Flammability Characteristics of Combustible Gases and Vapors," Bull. 672, Bur. Mines, 1965. Larsen, E. R., JFF/Fire Retardant Chemistry, 2 5 (1975). Moran, Jr., Η. Ε., and Bertschy, A. W., "Flam mability Limits for Mixtures of Hydrocarbon Fuels, A i r , and Halogen Compounds", NRL Report 4121, (1953). Creitz, E. C., Fire Tech., 8 131 (1972). Purdue Research Foundation and Department of Chemistry: Fire Extinguishing Agents, Summary Report Sept. 1, 1947 to Aug. 31, 1948, Purdue University (Contract W-44-009 eng. 507 with Army Eng. Res. and Dev. Labs.). Bajpai, S. N. and Wagner, J. P . , I&EC Product R&D, 14 54 (1975). Coward, H. F . and Hartwell, F . J., J. Chem. Soc., 1926, 1522. Coward, H. F . and Jones, G. W. : "Limits of Flam mability of Gases and Vapors", Bull. 503, Bur. Mines, 1952.

13.

LARSEN


389

DISCUSSION


D. CHAMBERLAIN: D i d you i n c l u d e bromine bromide i n y o u r c o r r e l a t i o n ?

and

hydrogen

E. R. LARSEN: Data on t h e e f f e c t o f HBr was not a v a i l a b l e f o r t h e f u e l s which were d i s c u s s e d i n t h i s paper. Data t h a t i s a v a i l a b l e f o r t h e system propanea i r - h y d r o g e n bromide i n d i c a t e s t h a t HBr i s n o t s i g n i f i c a n t l y d i f f e r e n t i n i t s behavior. R. ALTMAN: Don't you need t o use the H - # 98 instead of C (298) t o demonstrate t h e v e r a c i t y o f your argument, and i s n ' t t h e Τ t o use the flame temperature o r some f r a c t i o n t h e r e o f i n s t e a d o f 298K? 2

T

d

a

t

a

E. R. LARSEN: As I p o i n t e d o u t i n r e f e r e n c e 6, t h e method used here which employs C (298) a l l o w s a good a p p r o x i m a t i o n o f the f l a m m a b i l i t p b e h a v i o r o f t h e s e systems. In a more r i g o r o u s t r e a t m e n t t h e s p e c i f i c heat o f each component s h o u l d be i n t e g r a t e d o v e r t h e temperature range between t h e temperature o f the c o l d gas and the l i m i t flame t e m p e r a t u r e . T h i s i s more i m p o r t a n t w i t h r i c h m i x t u r e s than w i t h l e a n m i x t u r e s s i n c e , w h i l e t h e l i g h t gases show a s l i g h t l i n e a r i n c r e a s e i n C w i t h i n c r e a s i n g temperature, t h e h e a v i e r more complex f u e l s and a g e n t s , e.g., Cj-H, , C 0 , CH^, CF^, e t c . show a l a r g e n o n - l i n e a r i n c r e a s e i n C o v e r t h e range o f 300-1000°K. This factor i s n o t p a r t i c u l a r l y i m p o r t a n t i n systems c o n t a i n i n g l a r g e amounts o f l i g h t components such as N and 0 , i . e . , s t u d i e s employing a i r as the o x i d a n t , and, c o n s e q u e n t l y , was f o r the most p a r t i g n o r e d i n e v a l u a t i n g the "peak" d a t a . I n some c a s e s , e s p e c i a l l y t h o s e i n v o l v i n g e x t i n c t i o n , o r flame v e l o c i t y r e d u c t i o n , i n pre-mixed gas b u r n e r s t h e use o f C (700°K) seems t o g i v e b e t t e r r e s u l t s . There a r e ? however, n o t enough examples o f such systems where a l a r g e v a r i e t y o f agents were employed t o determine whether t h i s approach o f f e r s any d i s t i n c t advantage. 2

2

2

2

A. D. LEVINE: Your c o r r e l a t i o n s based on h e a t c a p a c i t y a r e v a l i d i n a p r a c t i c a l sense f o r p r e d i c t i n g flammability l i m i t s . W i l l t h i s c o r r e l a t i o n work when agents a r e added t o a h o t flame where c h e m i c a l k i n e t i c s

390 may


be

important?


E. R. LARSEN: The c o r r e l a t i o n appears t o h o l d i n t h o s e c a s e s where t h e agent i s i n j e c t e d i n t o e i t h e r the f u e l s i d e o r t h e a i r s i d e o f a d i f f u s i o n f l a m e , or f o r t h a t m a t t e r i n t o a b u r n e r which i s f u e l e d w i t h a premixed f u e l - a i r m i x t u r e . Some adjustment o b v i o u s l y has t o be made s i n c e the b u r n e r i t s e l f may cont r i b u t e t o the heat l o s s e s i n c u r r e d by t h e system. A. S. GORDON : I f we c o n s i d e r h e a t c a p a c i t y on a molar b a s i s , t h e n CF. and CF-Br s h o u l d have t h e same h e a t c a p a c i t y / m o l e t o a good f i r s t a p p r o x i m a t i o n . Now i f we add the same quencher/fue1 r a t i o t o a g i v e n f u e l / o x y g e n mix, t h e two systems s h o u l d have t h e same h e a t r e l e a s e p o t e n t i a l and t o t a l h e a t c a p a c i t y . Yet a t a C F - B r / f u e l r a t i o where t h e flame i s e x t i n g u i s h e d , f u l l r e p l a c e m e n t o f CF~Br by CF. does n o t e x t i n g u i s h the f l a m e . E. R. LARSEN: I agree t h a t CF. and CF-Br have e s s e n t i a l l y the same h e a t c a p a c i t y . I do n o t , however, have d a t a t h a t e i t h e r s u p p o r t s o r r e f u t e s your second statement and I am n o t s u r e t h a t a s e t o f c i r c u m s t a n c e s such as you p o s t u l a t e i s i n f a c t p o s s i b l e . CF. i s e s s e n t i a l l y a t r u e i n e r t gas which c o n t r i b u t e s n o t h i n g t o t h e energy p r o d u c i n g a b i l i t y o f t h e system and c a u s e s m i n i m a l , i f any, change i n t h e f u e l - o x y g e n stoichiometry. CF-Br, on t h e o t h e r hand, does e n t e r the combustion p r o c e s s and cause a marked change i n the f u e l - o x y g e n s t o i c h i o m e t r y . CF~Br, i n f a c t , becomes p a r t o f the f u e l c o n t e n t o f t h e system. W i t h i n t h i s c o n t e x t CF-Br would a l s o a c t as a flame s u p p r e s s a n t i n t h e same manner t h a t t h e a d d i t i o n o f e x c e s s f u e l t o a flammable m i x t u r e can r e s u l t i n the p r o d u c t i o n o f a non-flammable m i x t u r e . Cons e q u e n t l y , i t would n o t s u r p r i s e me t o f i n d t h a t f u l l r e p l a c e m e n t o f CF~Br by CF. d i d n o t e x t i n g u i s h t h e flame. F. A. WILLIAMS : In f i g u r e 17 o f my paper i s shown the dependence o f measured flame temperature a t e x t i n c t i o n on t h e n i t r o g e n - CF-Br r a t i o f o r a heptane d i f f u s i o n flame b u r n i n g i n a m i x t u r e o f oxygen, n i t r o g e n and CF-Br. The temperature a t e x t i n c t i o n i n CF-Br i s more tnan 500°C above t h a t i n n i t r o g e n . T h i s t e l l s me t h a t thhe c o o l i n g caused by CF^Br a t e x t i n c t i o n i s c o n s i d e r a b l y l e s s than t h a t caused by nitrogen. The c h e m i c a l i n h i b i t i o n , i . e . , s l o w i n g o f t h e o v e r a l l r e a c t i o n , by t h e s u p p r e s s a n t a t any

13.

LARSEN


391

g i v e n flame temperature, t h e r e f o r e must be i m p o r t a n t . In a paper t o be p u b l i s h e d i n t h e F i f t e e n t h I n t e r n a t i o n a l Symposium on Combustion, Kent and I a m p l i f y this point further. Our c o n c l u s i o n i s t h a t your c o r r e l a t i o n can work w e l l because o f two competing effects that cancel. E. R. LARSEN: That the temperature a t e x t i n c t i o n i n CF~Br i s more than 500°C above t h a t i n n i t r o g e n comes as no g r e a t s u r p r i s e t o me, i n f a c t i t i s what I would have e x p e c t e d i f my t h e o r y i s c o r r e c t . As I have p o i n t e d o u t N s e r v e s s t r i c t l y as a h e a t s i n k , but CF^Br can p l a y a d u e l r o l e , i . e . , heat s i n k and fuel. In o r d e r t o e v a l u a t e you comments i t i s n e c e s s a r y t o know the c o n c e n t r a t i o n s o f a l l components o f b o t h systems. However, i n view o f the f a c t t h a t t h e i g n i t i o n energy o f a C F ~ B r - 0 - h e p t a n e m i x t u r e near the f l a m m a b i l i t y peak i s s e v e r a l o r d e r s o f magnitude h i g h e r than i s t h e i g n i t i o n energy o f a c o r r e s p o n d i n g N ~ 0 - h e p t a n e m i x t u r e I would assume t h a t a much h o t t e r flame must o c c u r i n o r d e r t o o b t a i n a s e l f - p r o p a g a t i n g flame.


2

2

2

2

W. D. WEATHERFORD, JR.: Southwest Research I n s t i t u t e has o b t a i n e d d a t a which c o u l d be c o n s i d e r e d t o s u p p o r t a p h y s i c a l i n h i b i t i o n mechanism under c e r t a i n conditions. I w i l l mention i t a t t h i s time t o f u r t h e r i l l u s t r a t e the c o m p l e x i t i e s o f h a l o g e n compound flame i n h i b i t i o n mechanisms. We have c o n f i r m e d t h a t 5 l i q u i d volume p e r c e n t h a l o n 1011 (The h a l o n 1011 c o m p o s i t i o n was 85% (wt) 1011, 10% 1020, and 5% 1002.) i n d i e s e l engine i s s a t i s f i e d w i t h such a f u e l . In f a c t , t h e t o t a l i g n i t i o n d e l a y i n a s i n g l e c y l i n d e r r e s e a r c h d i e s e l engine i s not i n f l u e n c e d by t h e p r e s e n c e o f t h i s c o n c e n t r a t i o n o f h a l o n 1011 i n the f u e l . I n a d d i t i o n , minimum a u t o i g n i t i o n temp e r a t u r e measurements y i e l d AIT v a l u e s o f about 480°F f o r h a l o n 1011 c o o n c e n t r a t i o n s i n d i e s e l f u e l from 0 t o 30 l i q u i d volume p e r c e n t , whereas the v a l u e i s about 920°F f o r t h e f u e l - f r e e h a l o n 1011. The o b s e r v e d m i s t f l a m m a b i l i t y , b o t h a t ambient c o n d i t i o n s and i n a d i e s e l e n g i n e , p r o b a b l y stems from t h e vapor f l a m m a b i l i t y c h a r a c t e r i s t i c s shown i n the f o l l o w i n g i l l u s t r a t i o n . These d a t a were measured by Southwest Research I n s t i t u t e Army F u e l s and L u b r i c a n t s L a b o r a t o r y (Wimer, W. W., e t a l , " I g n i t i o n and F l a m m a b i l i t y P r o p e r t i e s o f F i r e - S a f e F u e l s , " NTIS, AD 784281, 1974). The r e s u l t s c l e a r l y i n d i c a t e t h a t 5 l i q u i d volume p e r c e n t t e c h grade h a l o n 1011 can be flammable under c e r t a i n c o n d i t i o n s

392


r


12

MOLE % BROMOCHLOROMETHANE *

Flammability characteristics of diesel fuel bromochloromethane * mixtures in air at 150 dfc 10°C and 1 atm

13.

Fhme Suppression Mechanism

LARSEN

and nonflammable under

393

others.

J . DEHN: The a u t h o r g i v e s as h i s most s u c c e s s f u l a n a l y s i s o f Purdue U n i v e r s i t y r e s u l t s the approximate constancy o f the r a t i o (heat c a p a c i t y of i n e r t s ) / ( t o t a l heat capacity) (Egn. 4 ) . For n-heptane+air systems he f i n d s t h a t when t h e n i t r o g e n + h a l o n i n e r t s c o n t r i b u t e 75% t o t h e h e a t c a p a c i t y o f t h e m i x t u r e t h e n no f l a m e c a n p r o p a g a t e . I submit that f o r the c a s e s he has t r e a t e d t h i s r a t i o i s b a s i c a l l y (heat capacity N )/(heat capacity a i r ) with observed v a r i a t i o n s i n t r o d u c e d by t h e o t h e r components o f t h e system and e x p e r i m e n t a l e r r o r s i n the d a t a . For example, the s p e c i f i c heats (cal/mole-°K) o f n i t r o g e n and oxygen (and so o f a i r ) a r e e q u a l t o 7 . 0 , w h i l e those o f heptane and CF~Br a r e 39.0 and 16.5 r e s p e c t i v e l y a t the i n i t i a l temperature o f t h e gas mixture. F o r p e a k v a l u e s o f 6 . 1 % C F ~ B r , 3% n - h e p t a n e a n d t h e r e s t a i r ( a u t h o r ' s T a b l e I I I ; we o b t a i n


2

.061(16.5) + .73(7.0) .03(39) + .91(7.0) + .061(16.5)

. X

X

Ω

U

η

=

U

1 Δ

*

A c t u a l l y t h e P u r d u e r e p o r t (pp 65 a n d 146) g i v e s 2 . 5 % f u e l i n s t e a d o f 3% w h i c h r a i s e s t h e a b o v e s l i g h t l y t o 73%. I f we n e g l e c t t h e h a l o n a n d t h e f u e l we o b t a i n * ' .91(7.0) 7

3

(

7

Q

)

χ 100 = 80%,

t h e a p p r o x i m a t i o n we a r e u s i n g f o r t h e p e r c e n t a g e o f nitrogen i n a i r . S i m i l a r calculations f o r the other c o m p o u n d s c i t e d r e v e a l t h e same b a s i c r e a s o n f o r t h e c l u s t e r a r o u n d 75%. I f o n l y weight f r a c t i o n s are used w i t h o u t heat capacities 0.3(100) If

.061(149) + .73(28) + .73(28) _ .18(32)

we n e g l e c t

'Mill)

+ .061(149)

X

1

Q

Q

±

υ

υ

=

?

? ,

1 /

e

%

"

t h e h a l o n a n d f u e l we h a v e

+ .18(32)

x

1

0

0

»

7

8

%

'

e

t

C

'

The a u t h o r o b s e r v e s a n o t h e r a p p r o x i m a t e l y c o n s t a n t weight r a t i o (agent)/(agent+fuel) and c l a i m s t h a t somehow t h i s d e m o n s t r a t e s a p u r e l y p h y s i c a l m e c h a n i s m . Why t h i s s h o u l d b e s o i s n o t c l e a r . A t any r a t e , I submit that t h i s r a t i o i s b a s i c a l l y u n i t y (halon


394


w e i g h t d i v i d e d by i t s e l f ) w i t h d e v i a t i o n s i n t r o d u c e d m a i n l y by t h e weight o f the f u e l and w i t h o t h e r v a r i a t i o n s due t o t h e f u e l p e r c e n t a g e found a t a peak and e x p e r i m e n t a l u n c e r t a i n t i e s . F o r example, a l l r a t i o s i n v o l v i n g n-pentane c e n t e r around 0.89 w h i l e t h o s e i n v o l v i n g t h e h e a v i e r η-heptane c e n t e r around t h e v a l u e 0.83. The a u t h o r a l s o c l a i m s t h a t h a l o n s o r hydrocarbon+halon m i x t u r e s become non-flammable i n a i r when h a l o g e n atoms a c c o u n t f o r more t h a n about 70% o f the fuel+halon weight. The examples shown i n t h e a u t h o r ' s r e f e r e n c e 4 do n o t c o n v i n c e one t h a t t h i s f i g u r e i s a p r a c t i c a l guide f o r avoiding f i r e s s i n c e v a l u e s as low as 63% and as h i g h as 80% appear t h e r e . Moreover, C 0 and N which c o n t a i n no h a l o g e n atoms f i t w e i g h t f r a c t i o n schemes elsewhere i n the paper why s h o u l d n ' t they a l s o f i t here? As f o r pure h a l o n s i n a i r b e i n g non-flammable when h a l o g e n atoms make up a t l e a s t 70% o f t h e i r w e i g h t , we might r e f e r t o t h e Z a b e t a k i s r e p o r t c i t e d by t h e a u t h o r (Bureau o f Mines B u l l 627, pp 102-5). Here d a t a i s g i v e n t o show t h a t CH~Br (84% Br by w e i g h t ) , CH^Cl^ (84% CI by weight) and C H C 1 (81% CI by weight) can be flammable i n a i r a t one atmosphere p r e s s u r e o r below, g i v e n a p r o p e r i g n i t i o n s o u r c e and c o n t a i n e r s i z e . S i m i l a r m a t t e r s a r e d i s c u s s e d i n Bur. Mines B u l l . 503, pp 101 f f . 70% by w e i g h t h a l o n i s a poor p r a c t i c a l guide. We may.ask why t h a t f r a c t i o n o f t h e t o t a l m i x t u r e h e a t c a p a c i t y c o n t r i b u t e d by t h e i n e r t s s h o u l d have any p a r t i c u l a r s i g n i f i c a n c e . I t would seem more reasonable t o look f o r a c r i t i c a l heat c a p a c i t y v a l u e of the t o t a l mixture i f heat c a p a c i t y i s indeed the dominant f a c t o r . In p u r e l y t h e r m a l t h e o r i e s o f flame p r o p a g a t i o n i n pre-mixed gases a t l e a s t two p h y s i c a l f a c t o r s a r e u s u a l l y t a k e n t o be i m p o r t a n t . These a r e the s p e c i f i c h e a t , c , and the t h e r m a l conduc t i v i t y , λ. Simple t h e o r i e s l e a d t o a flame v e l o c i t y p r o p o r t i o n a l t o some power o f the r a t i o λ/c , s i n c e a flame s h o u l d p r o p a g a t e w e l l when i t i s e a l y t o t r a n s m i t h e a t and r a i s e t h e temperature o f t h e unburned gas (at l e a s t f o r t y p i c a l h y d r o c a r b o n + a i r m i x t u r e s i n t w o - i n c h t u b e s ) . A good f i r e s u p p r e s s a n t s h o u l d d e c r e a s e λ and i n c r e a s e c , l e a d i n g t o a d e c r e a s e i n flame v e l o c i t y and e v e n t u a l e x t i n c t i o n . The T a b l e below g i v e s the average c and λ v a l u e s f o r a s t o i c h i o m e t r i c n-heptane+air fiixture and f o r Purdue peak p e r c e n t a g e m i x t u r e s w i t h v a r i o u s compounds. The N peak m i x t u r e however was t a k e n from t h e Z a b e t a k i s r e p o r t a l r e a d y c i t e d (p. 36) s i n c e t h e 2

2

2

2

3

13.

395


LARSEN

Purdue work d i d n o t i n c l u d e Ν

λ n-heptane+a i r +4.2% CF Br~ + 5 . 2 % CH^Br^ + 6 . 1 % CFÎBr + 7 . 6 % CH^ClBr + 17.5% CHC1-* + 9 . 3 % HBr +25.5% HCI +29.5% C 0 + 33% H 0 + 42% N 9


9

9

2

NOTE:

56.5 54.9 54.5 54.0 53.4 49.6 52.4 50.3 51.0 50.7 56.7

7.6 8.1 8.1 8.6 8.1 9.1 7.56 7.47 8.1 8.2 7.3

6.78 6.73 6.28 6.59 5.45 6.93 6.73 6.30 6.18 7.77

λ u n i t s : 10 cal/cm-sec-deg Cp u n i t s : c a l / m o l e - d e g

With t h r e e e x c e p t i o n s ( N , HBr and HCI) c i s i n c r e a s e d compared t o t h e r u e l + a i r r e f e r e n c e , w h i l e a l l e x c e p t N lower λ i n t h i s sample. The f i r s t t h i n g we note i s t h e f a c t t h a t t h e r e i s no c r i t i c a l v a l u e o f c o r λ o r o f t h e r a t i o λ/c which c a n be c o r r e l a t e d w i t h t h e phenomenon o f a peak p e r c e n t a g e . I f we s i m p l y l o o k a t averages w i t h o u t p a y i n g a t t e n t i o n t o t h e type o f d e v i a t i o n s we might d e c e i v e o u r s e l v e s i n t o t h i n k i n g t h a t we have found c r i t i c a l v a l u e s and p r o c e e d t o argue t h a t no c h e m i c a l e x p l a n a t i o n i s needed. T h i s s h o u l d be a v o i d e d . The brominated methane examples g i v e n have r e l a t i v e l y s m a l l peak percentages i n s p i t e o f the r e l a t i v e l y high λ values of t h e m i x t u r e s and t h e c o n t r i b u t i o n s which some o f t h e s e compounds might be e x p e c t e d t o make t o t h e h e a t of combustion. T h i s s u g g e s t s t h a t f o r such compounds where d i l u t i o n i s much l e s s i m p o r t a n t t h a n f o r o t h e r compounds i n t h e T a b l e f a c t o r s b e s i d e s λ and c a r e at work. HBr and HCI a r e p a r t i c u l a r l y i n t e r e s t i n g s i n c e t h e y a c t u a l l y lower t h e average heat c a p a c i t y of t h e m i x t u r e y e t a r e more e f f e c t i v e on a volume basis. Even on a weight b a s i s HCI i s more e f f e c t i v e than C 0 . The f a c t t h a t N i n c r e a s e s t h e λ/c ratio p a r t i a l l y e x p l a i n s t h e jump between peak v a l u e s o f H 0 and Ν · We p a r t i c u l a r l y note t h a t N i s h e a v i e r than H 0 y e t more i s r e q u i r e d . I f we i n s e r t a water c u r v e on F i g u r e I o f t h e paper under d i s c u s s i o n , we see t h a t a s i m p l e c o r r e l a t i o n o f e f f e c t i v e n e s s w i t h weight breaks down i m m e d i a t e l y . F i n a l l y we note t h a t CHCl^ i s n o t e s p e c i a l l y e f f e c t i v e i n s p i t e o f i t s 2

2

p

2

2

2

2

2

2


396


f a v o r a b l e λ/c r a t i o i n the m i x t u r e . A l l of t h i s l e a v e s us w i t n o u t a c o r r e l a t i o n between e f f e c t i v e n e s s i n t h i s p a r t i c u l a r experiment and t h e s e two t h e r m a l parameters. We can e i t h e r e l a b o r a t e our p h y s i c a l e x p l a n a t i o n by i n c l u d i n g o t h e r f a c t o r s o r can t u r n to a p a r t i a l chemical e x p l a n a t i o n ( i f indeed i t i s p r o f i t a b l e t o make such d i s t i n c t i o n s ) . While I d i s p u t e t h e a u t h o r ' s c l a i m o f showing t h e r e i s o n l y one p h y s i c a l mechanism a t work i n h a l o n flame s u p p r e s s i o n , I do not n e c e s s a r i l y deny h i s assertion. He may be r i g h t i n some c a s e s i n s p i t e of t h e f a c t t h a t he has not p r o v e d h i s c a s e h e r e . My p e r s o n a l p r e j u d i c e i s toward the view t h a t some c o m b i n a t i o n o f p h y s i c a l and c h e m i c a l mechanisms i s p r o b a b l y a t work i n most i n h i b i t i o n o r promotion phenomena, w i t h one o r the o t h e r dominant perhaps i n p a r t i c u l a r c a s e s , a l t h o u g h no one has y e t e s t a b l i s h e d t h e c o n d i t i o n s which must h o l d . While s t r o n g i n d i c a t i o n s o f a t l e a s t p a r t i a l c h e m i c a l mechanisms have been p r e s e n t e d a t t h i s symposium and elsewhere, t h e r e i s s t i l l a l i n g e r i n g doubt s i n c e l i t t l e attempt i s u s u a l l y made t o e v a l u a t e p o s s i b l e p h y s i c a l f a c t o r s w h i c h must a l s o be a t work. S i m i l a r l y the p r e s e n t paper and my comments make no attempt t o e v a l u a t e t h e c h e m i s t r y o f e x p l o s i o n b u r e t t e work. I t might be w o r t h w h i l e t o employ c h e m i c a l a n a l y s i s w i t h i s o t o p i c l a b e l i n g i n such pre-mixed gas f l a m e p r o p a g a t i o n experiments as has been done i n s t a t i c o r f l o w r e a c t o r systems which p r o p a g a t i o n d i s t a n c e i s not a f a c t o r . The c h e m i s t r y o f combustion i n h i b i t i o n i s a d i f f i c u l t f i e l d which has opened up o n l y i n t h e p a s t few y e a r s . K i n e t i c i s t s a r e t o be c o n g r a t u l a t e d f o r the f a c t u a l l i g h t t h e y have a l r e a d y shed on o l d theories. While the g u l f between our p r e s e n t c h e m i c a l knowledge and the g o a l o f improved f i r e s u p p r e s s i o n i s s t i l l q u i t e wide, the p r a c t i c a l importance o f t h e g o a l i s c e r t a i n l y worth the e f f o r t . E. R. LARSEN: Dr. Dehn's c r i t i q u e o f my paper i s q u i t e l e n g t h y and d e t a i l e d and r e q u i r e s a s i m i l a r i l y d e t a i l e d answer. I t s h o u l d be n o t e d t h a t Dr. Dehn's c r i t i q u e i s s t r i c t l y a p p l i c a b l e t o a paper ( r e f . 2) t h a t I gave s e v e r a l y e a r s ago. In t h a t e a r l i e r paper I had d e a l t i n l e n g t h upon the use o f mass f r a c t i o n as a means o f e x p r e s s i n g l i m i t d a t a , w i t h o n l y a c u r s o r y r e f e r e n c e t o the f a c t t h a t e x p r e s s i n g t h e c o m p o s i t i o n on a h e a t c a p a c i t y b a s i s appeared t o be superior. In the p r e s e n t paper the o p p o s i t e t a c k was t a k e n . In s p i t e o f t h i s change Dr. Dehn, a f t e r


13.

LARSEN


397

h e a r i n g my paper, d e c i d e d t o s t a n d by h i s c r i t i q u e . Dr. Dehn's p o i n t s a r e w e l l t a k e n and were c o n s i d e r e d d u r i n g the a n a l y s i s o f the f l a m m a b i l i t y l i m i t data t h a t i s a v a i l a b l e i n the l i t e r a t u r e . U n f o r t u n a t e l y , I can n o t agree w i t h h i s r e a s o n i n g , nor w i t h h i s c o n c l u s i o n s . F l a m m a b i l i t y l i m i t , and peak, d a t a w h i c h appears i n t h e l i t e r a t u r e has been determined i n a v a r i e t y o f systems and w i t h a v a r i e t y of i g n i t i o n sources. As I p r e v i o u s l y s t r e s s e d t h e Purdue s t u d y was c a r r i e d o u t twenty, o r so, y e a r s ago and employed a spark i g n i t i o n s o u r c e o f unknown s t r e n g t h . From our e x p e r i e n c e spark s o u r c e s employed d u r i n g t h a t p e r i o d were n o t i n t e n s e enough t o i g n i t e a l l flammable mixtures. I t i s p r o b a b l e t h a t the l i m i t s o f i g n i t a b i l i t y were b e i n g measured i n the Purdue study; r a t h e r than the l i m i t s o f f l a m m a b i l i t y . T h i s would r e s u l t i n an e x p e r i m e n t a l f l a m m a b i l i t y peak t h a t would be somewhat lower than i t s h o u l d be. Such an e f f e c t i s a p p a r e n t when a comparison i s made o f t h e Purdue peak v a l u e f o r CF~Br (6.1 v o l %) and the CF~Br peak (7.2 v o l %) c i t e d i n T a b l e IV. For t h i s reason I have s h i e d away from l a y i n g undue emphasis upon t h e peak v a l u e o f any s i n g l e agent. In s p i t e o f t h i s problem the Purdue study i s i n t e r n a l l y s e l f - c o n s i s t e n t and i s l a r g e enough (46 agents) so t h a t t h e t r e n d s drawn from i t s h o u l d be v a l i d even though t h e i n d i v i s u a l peak v a l u e s may be u n r e l i a b l e . It i s partly f o r t h i s r e a s o n t h a t the d i f f e r e n c e s between t h e v a l u e s o f KC f o r the i n e r t gases and f o r t h e h a l o n s do n o t o v e r l y d i s t u r b me. The r e a s o n i n g employed by Dr. Dehn i n c r i t i z i n g my a n a l y s i s o f t h e peak d a t a i s o b v i o u s l y i n v a l i d ; b o t h when the c o m p o s i t i o n s a r e e x p r e s s e d on a mass f r a c t i o n b a s i s o r on a h e a t c a p a c i t y b a s i s . To deny the v a l i d i t y o f a d i r e c t c o n v e r s i o n o f v o l % d a t a t o t h e s e o t h e r u n i t s , a s i m p l y m a t h e m a t i c a l manipulation» i s t o deny the v a l i d i t y o f the volume % d a t a i t s e l f . Dr. Dehn submits t h a t f o r the c a s e s i n which a i r i s employed as the o x i d a n t , as i n the Purdue s t u d y , KC i s " b a s i c a l l y (heat c a p a c i t y N ) / ( h e a t c a p a c i t y a i r ? w i t h o b s e r v e d v a r i a t i o n s i n t r o d u c e d by o t h e r components o f t h e system and e x p e r i m e n t a l e r r o r s i n the d a t a . " Dr. Dehn's r a t i o (heat c a p a c i t y N ) / ( h e a t c a p a c i t y a i r ) i s l i t e r a l l y p o i n t D i n f i g u r e 2. If the "observed v a r i a t i o n i n t r o d u c e d by the o t h e r components o f t h e system and the e x p e r i m e n t a l e r r o r i n the d a t a " a r e r e a l l y t h i s l a r g e then I submit t h a t a l l o f our s t u d i e s a r e w o r t h l e s s , s i n c e the same argument can be employed when volume % d a t a i s employed. 2

2


398


Dr. Dehn's comments must a l s o be viewed as i n v a l i d s i n c e t h e y a r e a p p l i c a b l e t o a l l p o i n t s bound by t h e t r i a n g l e ADC i n f i g u r e 2, and a p p l y e q u a l l y w e l l t o the i n e r t gases and t o the h a l o n s , i n c l u d i n g p o i n t s l a y i n g near, b u t not on, s i d e AC, and c e r t a i n l y t o a l l p o i n t s l a y i n g a l o n g l i n e AD. The w e i g h t r a t i o ( a g e n t ) / ( a g e n t + f u e l ) was e x p r e s s e d i n the c u r r e n t paper on a h e a t c a p a c i t y b a s i s r a t h e r than on a mass f r a c t i o n b a s i s as was done i n t h e paper r e f e r r e d t o by Dr. Dehn. The importance o f t h i s r a t i o i s a g a i n o b v i o u s i n f i g u r e 2, s i n c e t h i s r a t i o s i m p l y r e p r e s e n t s any c o m p o s i t i o n c o n s i s t i n g s o l e l y o f agent and f u e l : t h o s e composi t i o n s l a y i n g a l o n g s i d e AC. As i s a p p a r e n t i n f i g u r e 2 any m i x t u r e o f i n e r t gas and f u e l w h i c h has a c o m p o s i t i o n l a y i n g between p o i n t s A and Κ will, upon the a d d i t i o n o f oxygen, e v e n t u a l l y y i e l d a flammable m i x t u r e , i . e . , pass t h r o u g h the flammable envelope ( I P J ) . One would a n t i c i p a t e t h a t i f t h e h a l o n s were d i s p r o p o r t i o n a t e l y more e f f e c t i v e t h a n the i n e r t gases t h e y would show v a l u e s f o r t h i s r a t i o w h i c h a r e much s m a l l e r than i s found f o r the i n e r t gases. I t might a l s o be e x p e c t e d t h a t t h i s r a t i o would be much s m a l l e r w i t h the bromine c o n t a i n i n g h a l o n s than f o r the c h l o r i n e or f l u o r i n e c o n t a i n i n g halons. That t h i s i s not the c a s e i s shown by t h e v a l u e s o f K , i . e . , the v a l u e o f t h i s r a t i o f o r peak c o m p o s i t i o n s , shown i n T a b l e I I I . The c o n s t a n c y o f t h i s r a t i o i s s t r o n g e v i d e n c e i n s u p p o r t o f an argument t h a t t h e p r i m a r y mechanism o f flame s u p p r e s s i o n i s t h e same f o r b o t h the i n e r t gases and the h a l o n s , and, c o n v e r s e l y , i s d i f f i c u l t t o r a t i o n a l i z e away w i t h a c h e m i c a l mechanism. The o b s e r v a t i o n t h a t the h a l o n s and h a l o n h y d r o c a r b o n m i x t u r e s g e n e r a l l y become non-flammable i n a i r when t h e h a l o g e n atoms a c c o u n t f o r more t h a n 70% o f the f u e l = h a l o n w e i g h t i s d e a l t w i t h i n d e t a i l i n r e f e r e n c e 4. As p o i n t e d o u t i n r e f e r e n c e 4 the f l a m m a b i l i t y peak c o m p o s i t i o n i s most i n t e r e s t i n g from a m e c h a n i s t i c s t a n d p o i n t s i n c e i t r e p r e s e n t s t h a t c o m p o s i t i o n o c c u r r i n g a t t h e p o i n t o f convergence of the upper and lower l i m i t c u r v e s . The peak com p o s i t i o n r e p r e s e n t s t h e minimum amount o f agent r e q u i r e d t o i n e r t any and a l l f u e l - o x i d a n t m i x t u r e s . T h e r e f o r e , the c o m p o s i t i o n o f t h e system a t t h i s p o i n t i s unique w i t h r e s p e c t t o the agent and f u e l i n each a g e n t - f u e l - o x i d a n t system, and c o m b i n a t i o n s o f f u e l and agents t h a t o c c u r a t the peak can be t r e a t e d as single entities. The h a l o g e n c o n t e n t o f the m i x t u r e I F

13.

LARSEN


399

i s f i x e d , and may be c a l c u l a t e d , i n much t h e same way t h a t t h e h a l o g e n c o n t a n t o f a f i n i t e compound i s fixed. The h a l o g e n c o n t e n t f o r some 60 peak c o m p o s i t i o n s were c a l c u l a t e d and t h e mean h a l o g e n c o n t e n t o f t h e m i x t u r e s was found t o be 69.8 ± 3.5 wt %, i . e . ,


wt

% Halogen i n _ wt Halogen χ 100 the mixture wt Agent + wt f u e l

t o determine whether o r n o t any one s p e c i e s o f h a l o g e n was d i s p r o p o r t i o n a t e l y more e f f e c t i v e than any o t h e r , as demanded by t h e r a d i c a l t r a p t h e o r y o f flame s u p p r e s s i o n , t h e peak d a t a f o r t h e 46 compounds i n c l u d e d i n t h e Purdue s t u d y were broken down by h a l o g e n t y p e , and t h e mean v a l u e s o f κ f o r each group determined. One group (9 agents) c o n t a i n e d o n l y f l u o r i n e (κ=68.0), and a f o u r t h and f i n a l group (5 agents) c o n t a i n e d i o d i n e o r i o d i n e and f l u o r i n e (κ=73). On t h e b a s i s o f t h i s a n a l y s i s , which i s somewhat more complete i n r e f . 4, no e v i d e n c e was found t o s u p p o r t t h e c o n t e n t i o n t h a t any one h a l o g e n was more e f f e c t i v e t h a n any o t h e r , and t h e r e l a t i v e e f f e c t i v e n e s s o f t h e h a l o g e n i s shown t o be d i r e c t l y p r o p o r t i o n a l t o t h e i r atomic weights, i . e . , F : C l : Br : 1 = 1 . 0 : 1.9 : 4.2 : 6.7. The f a c t t h a t one bromine atom i s 4.2 t i m e s as e f f e c t i v e as a f l u o r i n e atom on a molar b a s i s i s not s u p p o r t i v e o f t h e con t e n t i o n t h a t t h e y f u n c t i o n by d i f f e r e n t mechanisms. The f a c t t h a t t h o s e h a l o n s and h a l o n - h y d r o c a r b o n systems s t u d i e d showed a mean v a l u e o f 70 wt % h a l o g e n does not mean t h a t t h e r e a r e no e x c e p t i o n s : t h r e e b e i n g c i t e d by Dr. Dehn. I t s h o u l d be p o i n t e d o u t t h a t w h i l e CH~Br i s flammable i n a i r even though i t c o n t a i n s 84 wt % Br, t h e Purdue s t u d y shows a peak p e r c e n t a g e f o r t h e heptane-CH-Br system than c o n t a i n s 73 wt % bromine. I would be r e m i s s here i f I d i d not t u r n Dr. Dehn's q u e s t i o n about and ask how the c h e m i c a l mechanism e x p l a i n s t h e f l a m m a b i l i t y o f CH^Br i n a i r i n view o f i t s h i g h molar c o n c e n t r a t i o n o f bromine. I do agree w i t h Dr. Dehn t h a t employing 70 wt % h a l o g e n as a p r a c t i c a l g u i d e i s poor p o l i c y , e x c e p t f o r s a y i n g i f t h e compound, o r m i x t u r e o f f u e l and h a l o n , c o n t a i n s l e s s t h a n 70 wt % h a l o g e n i t s h o u l d be r i g o r o u s l y t e s t e d b e f o r e assuming t h a t i s non flammable. I n g e n e r a l , any f l a m m a b i l i t y l i m i t d a t a o b t a i n e d u s i n g an unknown s p a r k s o u r c e i s p o t e n t i a l l y a booby-trap ( r e f . 2).

400


Dr. Dehn asks why the f r a c t i o n o f the t o t a l m i x t u r e heat c a p a c i t y c o n t r i b u t e d by t h e i n e r t s s h o u l d have any p a r t i c u l a r s i g n i f i c a n c e . I do n o t have a good answer f o r t h i s q u e s t i o n a t t h i s time o t h e r than i t seems t o work. I f we examine the peak d a t a f o r the system p e n t a n e - a i r - a g e n t we f i n d t h a t t h e agent c o n c e n t r a t i o n d e c r e a s e s as t h e t o t a l h e a t c a p a c i t y o f the m i x t u r e i n c r e a s e s , but the r a t i o o f the h e a t c a p a c i t y c o n t r i b u t i o n o f the agent and the t o t a l heat c a p a c i t y o f the m i x t u r e remains r e a s o n a b l y c o n s t a n t . The N i n the a i r i s p r o p e r l y p a r t o f the agent system and i t s heat c a p a c i t y c o n t r i b u t i o n i s i n c l u d e d i n Σ[I]Cp . 2


T

Agent ( v o l %) Pentane

N (42.2) C0 (28.0) CF~ (20.6) SF* (15.7 CJ* (10.6) CF " ( 4 1 . 2 ) * C F j (46.8)** 9

9

Σ[Ι]0

Ρ ι

6.00 6.29 7.25 8.12 8.62 8.97 8.67

* Oxygen e n r i c h e d " a i r " - 31.3% ** Oxygen e n r i c h e d " a i r " - 38.7%

E[C]Cp

KC

c

7.42 8.11 9.10 10.19 10.69 11.45 11.02 0 0

p

80.8 77.5 79.5 79.7 80.6 78.7 78.3

2 2

On t h e b a s i s o f the b e h a v i o r o f t h e i n e r t gases I see l i t t l e r e a s o n t o e x p e c t t h a t a c r i t i c a l v a l u e o f Cp w i l l be found, u n l e s s t h i s r a t i o i s t h a t v a l u e . I am aware o f the f a c t t h a t i n p u r e l y t h e r m a l t h e o r i e s o f flame p r o p a g a t i o n i n pre-mixed gases a t l e a s t two p h y s i c a l f a c t o r s a r e u s u a l l y t a k e n t o be important: t h e s p e c i f i c heat (Cp) and t h e t h e r m a l c o n d u c t i v i t y (λ). As f a r as I can a s c e r t a i n λ was o r i g i n a l l y proposed as b e i n g i m p o r t a n t by Coward and H a r t w e l l i n an attempt t o j u s t i f y the f a c t t h a t h e l i u m appeared t o be more e f f e c t i v e than t h e y thought i t s h o u l d be i n view o f i t s low h e a t capacity: e s p e c i a l l y when compared t o argon which has the same h e a t c a p a c i t y but a much lower λ v a l u e . They v e r y h i g h t h e r m a l c o n d u c t i v i t y o f h e l i u m then was employed t o r a t i o n a l i z e t h e e f f i c i e n c y o f h e l i u m . In a p r e v i o u s paper ( r e f . 6 ) , I n o t e d t h a t n e i t h e r He nor Ar were s i g n i f i c a n t l y d i f f e r e n t than N , H 0, o r C 0 , when compared t o a pure heat c a p a c i t y b a s i s . I f the p r e s e n t t h e r m a l t h e o r i e s r e q u i r e t h a t λ p l a y an i m p o r t a n t r o l e i n flame p r o p a g a t i o n then I s u g g e s t 2

2

2

13.


LARSEN

401

t h a t t h e s e t h e o r i e s might r e q u i r e r e t h i n k i n g . I f Dr. Dehn had extended h i s t a b l e t o i n c l u d e more i n e r t gases he would have o b s e r v e d t h a t Cp f o r "peak" c o m p o s i t i o n s can extend from a t l e a s t 5.3 t o about 13.5 λ(10~ ) can extend from about 42 t o 295, and h i s " c o n s t a n t " r a t i o λ/Cp from about 5.5 t o 55. I agree w i t h Dr. Dehn t h a t l o o k i n g a t a v e r a g e s , e s p e c i a l l y w i t h a l i m i t e d d a t a base such as employed by Dr. Dehn, can cause one t o be d e c i e v e d i n t o t h i n k i n g t h a t we have found c r i t i c a l v a l u e s and p r o c e e d t o argue t h a t no c h e m i c a l e x p l a n a t i o n i s needed. C o n v e r s e l y , one can be d e c e i v e d by o b s e r v i n g t h a t because t h e h a l o g e n a t e d agents undergo t h e r m a l c r a c k i n g and e n t e r i n t o the flame c h e m i s t r y t h i s c h e m i s t r y must be i n h i b i t o r y i n n a t u r e . Proposed mechanisms o f flame s u p p r e s s i o n i n which the h a l o g e n behaves as a r a d i c a l t r a p g i v e r i s e t o a dichotomy which i s n o t g e n e r a l l y d e a l t w i t h . On t h e one hand, the h a l o g e n s and t h e i r compounds a r e w e l l known as e f f e c t i v e flame s u p p r e s s a n t s w i t h bromine o c c u p y i n g a prominent p o s i t i o n w i t h r e s p e c t to e f f i c i e n c y . On the o t h e r hand, i t i s a l s o known t h a t bromine i s e s p e c i a l l y e f f e c t i v e as an e f f e c t i v e o x i d a t i o n , and hence combustion, promoter. I s i t not p o s s i b l e t h a t t h e f r e e r a d i c a l c h e m i s t r y t h a t i s r e p u t e d t o r e s u l t i n flame i n h i b i t i o n might be t h e c h e m i s t r y o f flame promotion, o r the i n i t i a t i o n o f the combustion a t a temperature lower than i t might o t h e r w i s e o c c u r i n a g i v e n system. I t s h o u l d be p o i n t e d o u t here t h a t t h e d a t a f o r KCp i n T a b l e I I I does have a b i a s b u i l t i n t o i t , and t h a t t h e s i m p l e use o f a v e r a g e s can be m i s l e a d i n g . As mentioned i n my o r a l p r e s e n t a t i o n , b u t o v e r l o o k e d i n the w r i t t e n manuscript, I ran a r e g r e s s i o n a n a l y s i s o f t h i s d a t a and found i t t o f i t t h e e q u a t i o n :


f

EtUCpj

= 1.0

E[C]Cp

c

-

2.0

which tends t o a c c o u n t f o r t h e f a c t t h a t t h e v a l u e o f KCp tends t o i n c r e a s e as we descend t h e column. T h i s b i a s i s a s s o c i a t e d w i t h the f a c t t h a t each a g e n t - f u e l - o x i d a n t system has i t s own unique f l a m m a b i l i t y diagram. I f we l o o k a t the diagrams f o r a s e r i e s o f a g e n t s , such as was done i n f i g u r e 3, we f i n d t h a t t h e y form a f a m i l y o f diagrams h a v i n g a common base ( l i n e AB). F o r each d i f f e r e n t agent the apex C o f t h e diagram w i l l be e v a l u a t e d above the p l a n e o f t h e page, so t h a t our c o l l e c t i o n o f diagrams open somewhat l i k e the l e a v e s o f a book.

402


The diagram which o c c u p i e s t h e page i s t h a t o f N and t h e p o i n t s f o r t h e o t h e r agents a r e shown as p r o j e c t i o n s upon t h e N diagram. I submit t h a t t h e burden o f p r o o f f o r t h e s u p p o r t o f a new and n o v e l mechanism l i e s upon t h o s e who propose i t . I do not deny t h a t t h e h a l o n s e n t e r i n t o t h e flame c h e m i s t r y and i n most c a s e s appear t o a c t as f u e l s . T h e i r b e h a v i o r i n premixed gas systems, such as employed i n combustion t u b e s , i s e s s e n t i a l l y i n d i s t i n g u i s h a b l e from t h a t o f t h e i n e r t gases. I t i s f o r t h e r e a s o n s s e t down i n t h i s paper and i n p r e v i o u s papers t h a t I m a i n t a i n t h a t t h e p r i m a r y r o l e o f t h e h a l o n s i n flame s u p p r e s s i o n i s as h e a t s i n k s , and t h a t t h e y have a common mechanism w i t h the i n e r t g a s e s . As a f u r t h e r note i n p r o o f , I would l i k e t o say t h a t j u s t p r i o r t o t h i s meeting I r e q u e s t e d Dr. R. V. P e t r e l l a o f our l a b o r a t o r y t o determine whether o r not t h e c o m p o s i t i o n 48.9 v o l % CF-Br, 7.2 v o l % 5 12' ·*·8 v o l % 0 , t h e whole a t one atmosphere p r e s s u r e , was flammable. Indeed, i t p r o v e d t o be. On a weight f r a c t i o n b a s i s the r a t i o 2 #


2

C

H

a

n

d

4

2

[CF^Br] χ 100 [C H I+[o ]+[CF Br] 5

1 2

2

79.2

3

i s r e a s o n a b l y c l o s e t o t h e r a t i o found i n the Purdue s t u d y f o r t h e system H e p t a n e - A i r - C F - B r , which was c r i t i c i z e d by Dr. Dehn above, i . e . , [CF^Br]+[N ] x 100 = [C H ]+fo ]+[N^]+[CF Br] 0

7

16

2

3

77.1

14 The Role of Ions and Electrons in Flame Inhibition by Halogenated Hydrocarbons: Two Views EDWARD T. McHALE Atlantic Research Corp., Alexandria, Va. 22314 ALEXANDER M A N D L


Avco Everett Research Laboratory, Inc., Everett, Mass. 02149

(Editor's note: The question of whether charged species, known to exist in flames, are important in flame inhibition is a continuing matter of controversy. Dr. Edward T. McHale addressed the subject at the Symposium. The entire text of his presentation is not given here because essentially the same material, co -authored with Dr. D. Spence, appears elsewhere [Combustion and Flame (1975), 24, 211]. The following brief summary contains the essential points of the paper. Following it is a reply to Dr. McHale's paper by Dr. Alex Mandl.) Ε. T. McHALE: Experimental measurements of the kinetic cross-sections were made as a function of temperature for seven alkyl halides (CH Br , CCl , CH3-Br, CF Br, CHCl, CH I, CFC1 ) and SF for the election dissociative attachment reactions 2

3

3

3

2

4

3

6

-

RX + e ->R +

-

X

(a)

The results of these measurements were compared with "peak percentage" values for the same compounds. Since peak percentages represent a measure of i n hibiting effectiveness, one would expect to find some correlation between these and the kinetic rate constants i f ions or electrons had a significant role in flame suppression. The complete lack of correla tion that was found represents substantial evidence that charged species are not important in inhibition processes involving halogenated hydrocarbons. Further evidence for the absence of ionic reactions in inhibition was also presented in the form of calculations of the relative rates of reaction (a) and reaction (b):

403

404


RX

4-

H ·* R + HX

(b)

In a sample c a l c u l a t i o n f o r a s t o i c h i o m e t r i c methanea i r flame w i t h CF~Br as t h e RX s p e c i e s , t h e r a t i o (b)/(a) was shown t o be


Rate R e a c t i o n (b) Rate R e a c t i o n (a)

=

3

χ

ô

2

The c a l c u l a t i o n was based a l m o s t e n t i r e l y on ex p e r i m e n t a l l y measured v a l u e s o f t h e n e c e s s a r y parameters r e p o r t e d i n t h e l i t e r a t u r e . T h i s l a r g e c a l c u l a t e d value of the r a t i o of n e u t r a l to i o n i c i n h i b i t i o n r e a c t i o n r a t e s r e i n f o r c e s the previous c o n c l u s i o n t h a t combustion s u p p r e s s i o n i s not a r e s u l t o f charged s p e c i e s i n flames i n h i b i t e d w i t h a l k y l h a l i d e compounds. A. MANDL: Dr. McHale has commented on some work he has r e c e n t l y completed w i t h Dr. Spence from which he has c o n c l u d e d t h a t i o n s a r e not i m p o r t a n t i n flame i n h i b i t i o n . Whereas under the assumptions and c o n d i t i o n s o f h i s mechanism one c o u l d c o n c l u d e t h a t i o n s do n o t p l a y an i m p o r t a n t r o l e , I t h i n k t h a t a g e n e r a l statement i m p l y i n g t h a t i o n c h e m i s t r y i s unimportant i n flame i n h i b i t i o n cannot be made. In p a r t i c u l a r , i f one s t a r t s w i t h t h e same s e t o f e q u a t i o n s t h a t Dr. McHale has p r e s e n t e d , RX + e" Χ" + Η

X" + R

(1)

HX + e"

(2)

RX + Η •+ HX + R

(3)

where RX = i n h i b i t o r and X = h a l o g e n , and compares r a t e s (2) and (3) i n s t e a d o f (1) and ( 3 ) , one has a d i r e c t comparison between t h e e f f e c t o f i o n s and i n h i b i t o r s on t h e f r e e r a d i c a l , atomic hydrogen. Thus Rate 2 Rate 3

=

k~[X~][H] — k [RX][H] 3

and s i n c e a t flame temperature, t y p i c a l r a t e s f o r r e a c t i o n s (2) and (3) might be k ~ 2 χ 10 cm / 2

m o l e c u l e - s and L

- 3 χ 10

cm3/molecule-s.

(4)

14.

MCHALE

at

g- e

Role of Ions and Electrons

AND MANDL

2

Rate 3

* io ULi

(5)

3

x

u

405

[RX]

Typical i n h i b i t i o n concentrations reported a t t h i s meeting i n d i c a t e t h a t [RX] - Ις mole f r a c t i o n . Thus, f o r systems i n which [X ] > 10" i s a h i g h e q u i l i b r i u m d e n s i t y , i t s h o u l d be p o i n t e d o u t t h a t there a r e n o n e q u i l i b r i u m paths f o r i o n f o r m a t i o n . F o r example, my own shock tube work (1) on t h e cesium h a l i d e s has shown t h a t t h e y c o l l i s i o n a l l y d i s s o c i a t e d almost completely i n t o the n o n e q u i l i b r i u m i o n i c branch. R. S. B e r r y and h i s coworkers (2) have shown t h a t i n g e n e r a l many o f t h e a l k a l i h a l i d e s d i s s o c i a t e p r e f e r e n t i a l l y i n t o the nonequi1ibrium i o n i c branch. T h i s i s due t o t h e f a c t t h a t t h e i o n i c and n e u t r a l p o t e n t i a l c u r v e s c r o s s a l a r g e i n t e r n u c l e a r s e p a r a t i o n f o r a l a r g e number o f t h e s e s a l t s . I n c a s e s f o r which a s a l t such a s KC£ i s i n j e c t e d i n t o a flame i t s d i s s o c i a t i o n w i l l be m a i n l y i n t o Κ and C £ ~ . I f a p e r c e n t o f KCil i s added t o a flame i t i s n o t _ u n r e a s o p a b l e t o assume t h a t t h i s w i l l r e s u l t i n [X~] > 10" . T h i s would i m p l y t h a t t h e i n h i b i t i o n i n t h i s c a s e would be t h r o u g h an i o n i c mechanism. J. C. B i o r d i e t a l have shown a t t h i s meeting t h a t n o n e q u i 1 i b r ium c h e m i s t r y i s q u i t e i m p o r t a n t i n flames. A d d i t i o n a l l y , l a r g e nonequi1ibrium e l e c t r o n d e n s i t i e s have been o b s e r v e d (3) i n f l a m e s ; and i n e x p e r i m e n t s (4) i n which a l k a l i m e t a l s a l t s were added t o f l a m e s , l a r g e i n c r e a s e s i n flame c u r r e n t were o b s e r v e d f o r many o f t h e e f f e c t i v e flame i n h i b i t o r s . I t i s t h e r e f o r e not unreasonable t o assume t h a t i o n c h e m i s t r y c a n p l a y an i m p o r t a n t r o l e i n t h e e x p l a n a t i o n o f how some quenchers work.


10

+

Literature Cited 1. 2. 3.

4.

Mandl, Α., J. Chem. Phys., (1971), 55, 2918. Ewing, J. J., Milstein, R. and Berry, R. S., J. Chem. Phys., (1971), 54, 1752. Fenimore, C. P., "Int. Encyclopedia of Phys. Chem. and Chem. Phys., " Topic 19, Vol. 5, "Chemistry of Premixed Flames," McMillan, New York, (1964). Birchall, J. D., Combustion and Flame, (1970),

15 Theoretical Investigation of Inhibition Phenomena in Halogenated Flames SERGE GALANT and J. P. APPLETON


Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Mass. 02139

Literature reviews on chemical inhibition (1),(2),(3) reveal that a great deal of experimental work has been carried out in an attempt to gain a better understanding of inhibition phenomena in flames. Since strong experimental evidence (2),(3),(4) supports the view that a basic understanding of inhibition phenomena can be obtained by studying inhibition effects in premixed laminar flames, researchers have studied inhibition effects in such systems. Yet, two somewhat different approaches have prevailed. One crude but efficient way of screening potential inhibiting agents has been to measure the decrease in nominal laminar flame speed as a function of the inhibitor concentration. (5),(6),(7), (8),(9),(10). Another related type of experiment has been concerned with the modifications of the ignition limits of a given mixture when inhibiting agents are introduced. (3),(6),(9). On the basis of such experimental evidence, a plethora of speculative kinetic models have been proposed (for exhaustive listings, see References (1),(2),(3)). A feature common to almost all these proposals is their lack of conclusive agreements when it comes to making "ab initio" predictions of the reduction of the laminar flame speed. On the other hand, it can be argued that a better insight into inhibition can be gained by looking at flames on a molecular level (2). In particular, spectroscopic measurements (11),(12), (13),(14) have yielded a wealth of information by providing actual nonequilibrium concentration and temperature profiles. Most important, speculations have been quantified into models which identified some of the critical kinetic steps. However, the uncertainty in those measurements and the lack of detailed knowledge about the various nonlinear couplings between transport phenomena and chemical reactions which occur in flames, have impaired any conclusive description of the actual inhibiting effects. Our theoretical approach is aimed at reconciling these two experimental approaches: using the rate coefficients and diffusion parameters as obtained from "molecular experiments" (11), 406


15.

Inhibition Theory

GALANT AND APPLETON

407

(12),(15),(16),(17), we intend to use a new numerical technique to solve the general one dimensional unsteady flame equations and, i n turn, to predict the local and global behavior of i n h i bited flames, v i z . temperature and concentration profiles but also, flame velocities and ignition limits. Such an approach i s not new per se. Day et_ a l . (6) solved approximations to the flame equations to study inhibition of rich hydrogen-air flames. In doing so, they were able to pinpoint the fundamental kinetic steps needed to account for experimental observations. Yet a proper adjustment of unknown reaction rates was required to match to the experimentally observed decrease of the laminar flame speed. In the present investigation, a computing procedure which had been successfully applied to the ozone-oxygen flame (18) has been improved and generalized to be used for an automatic computer code which describes both ignition and steady propagation of deflagrations i n planar and spherical coordinates. The calculations reported here have been obtained with a mathematical model which includes a l l effects that are believed to be important, i.e., molecular diffusion, heat conduction and thermal diffusion, together with a general reaction kinetic scheme and f u l l y r e a l i s t i c thermodynamic data. A short overview of the equations and of the computational method to solve them i s given in the f i r s t section of this paper. As noted earlier, a f i r s t logical approach i s to model i n h i bition phenomena i n flames for which the kinetic, transport and thermodynamic data are f a i r l y well established. The archetype i s evidently the hydrogen-oxygen (or hydrogen-air) flame which is abundantly documented i n the literature. In the second section we consider inhibition effects for a low pressure rich H2-O2 flame (19) supported on a porous plug burner. In so doing, we hope to demonstrate the capability of the numerical method to describe actual experiments. These calculations are the f i r s t i n a series of simulations dealing with inhibition phenomena i n hydrogen-oxygen a i r flames. Further work on ignition limits and flame velocity computations w i l l be reported later. Theory The Mathematical Model The equations which describe the unsteady one-dimensional propagation of a premixed flame i n a frame of reference fixed with respect to the moving flame front are (20) mass continuity: species continuity: where

+ r ^ n

p^ j

(r ^ pv) = 0

pv

=

r"^3 (r^pG,)

W

j


408

m

η = Σ

η

energy equation:

equation of s t a t e :

ρ = P/(RTn)

o v e r a l l mass c o n s e r v a t i o n :

m Σ w.m. 3=1


3

= 0

3

m

Σ j=l m

Σ J-l

n m 3

m.G. J

= 1

3

- 0

3

Here t i s the time, r i s the coordinate along the dimension of i n t e r e s t , ν i s the v e l o c i t y component i n the r d i r e c t i o n , ρ i s the l o c a l d e n s i t y of the f l o w i n g gas, n^ i s the c o n c e n t r a t i o n of the j - t h species i n moles per gram of mixture, mj i t s molecular weight,C$+ i t s molar heat c a p a c i t y a t constant pressure, Hj i t s molar enthalpy, Dj i t s molecular d i f f u s i o n c o e f f i c i e n t , D.T i t s thermal d i f f u s i o n c o e f f i c i e n t and WJ i s the net mass production (or d e s t r u c t i o n ) r a t e of the j - t h s p e c i e s per u n i t volume. I t has been t a c i t l y assumed t h a t s p e c i e s d i f f u s e according to F i c k ' s law, that heat t r a n s f e r by conduction w i t h i n the system obeys F o u r i e r ' s law, λ being the heat conduction c o e f f i c i e n t , and that the mixture c o n s i s t s of i d e a l gases a t constant pressure P. F i n a l l y , by s e t t i n g γ = 0, 1 or 2 the equations can be obtained i n terms of c a r t e s i a n , p o l a r or s p h e r i c a l coordinates respectively. Together w i t h the set of equations (1) there must be a s e t of adequate i n i t i a l and boundary c o n d i t i o n s which complete the d e s c r i p t i o n of the system. Two types of i n i t i a l c o n d i t i o n s have been d e a l t with i n the past. For steady s t a t e flame s t r u c t u r e c a l c u l a t i o n s , guessed Sshaped p r o f i l e s are assumed to d e s c r i b e the v a r i a t i o n s of the dependent v a r i a b l e s ( c o n c e n t r a t i o n , temperature) through the flame r e g i o n i n i t i a l l y , I f enough energy i s s t o r e d i n these i n i t i a l p r o f i l e s , a flame w i l l propagate and the p r o f i l e s u l t i mately assume t h e i r steady-state shapes. P h y s i c a l l y , t h i s means that steady s t a t e flame propagation does not depend upon the way i n which the system was i g n i t e d . Yet, such a r b i t r a r y i n i t i a l c o n d i t i o n s have no p h y s i c a l b a s i s . In order to consider the i g n i t i o n phase, i t would be p r e f e r a b l e to use i n i t i a l c o n d i t i o n s


15.

Inhibition Theory

GALANT AND APPLETON

409

which were compatible w i t h a more p h y s i c a l s i t u a t i o n such as e x posing " p o c k e t s " of combustion products of d i f f e r e n t dimensions to a l a r g e body of combustible mixture ( 1 8 ) . More g e n e r a l l y , i n i t i a l c o n d i t i o n s which a r e square (discontinuous) waves i n concentrations and/or temperature might be thought of as r e p r e s e n t i n g the e a r l y phases of i g n i t i o n ( e x t i n c t i o n ) phenomena: i n deed, they a r e approximate models of the l a r g e c o n c e n t r a t i o n and temperature g r a d i e n t s which i n i t i a l l y do e x i s t i n , s a y , spark o r l a s e r i g n i t e d systems. Boundary c o n d i t i o n s f o r steady flame propagation have been d i s c u s s e d a t l e n g t h . The problem i s to s p e c i f y i n t e r n a l l y c o n s i s t e n t boundary c o n d i t i o n s f o r an a c t u a l n o n - a d i a b a t i c m u l t i - d i m e n s i o n a l flame. For the c a l c u l a t i o n s presented i n the second p a r t , we use the concept of a porous p l u g burner (20),(21) which i s v a l i d o n l y f o r p l a n a r c o o r d i n a t e s , i . e . , Y=0. A t the flameholder a temperature g r a d i e n t i s assumed t o e x i s t which tends t o s t a b i l i z e the p o s i t i o n of the flame. Be cause of d i f f u s i o n of the product s p e c i e s from the flame a l l the way back t o the f l a m e h o l d e r , the chemical composition of the gas at the flameholder i s not s p e c i f i e d . However, s i n c e the mass f l u x of the j - t h s p e c i e s must be continuous a t the c o l d boundary (r=0), we o b t a i n

(n

j P

V - PGÎ^Q = p° u°

η.

0

where the s u p e r s c r i p t denotes c o n d i t i o n s i n the mixing chamber, and r = 0 a t the c o l d boundary. Using the d e f i n i t i o n of G . i n (1) we get 0

3

Τ

> j - Ψ- I V ë>- ' νν ' {u

+

ν (

} r = 0 c >0

A l s o , the temperature a t the flameholder i s supposed t o remain constant w i t h t i m e . F i n a l l y , a t the hot boundary ( r - * » ) , i t i s assumed that the s p e c i e s d i f f u s i o n f l u x e s and heat d i f f u s i o n f l u x a r e z e r o . Therefore, the s e t of boundary c o n d i t i o n s r e a d s : Τ

r

=

0

:

ί ΐ

-

ψ

i f -

+

^

Τ (0,t) = Τ ο r + ~ ^ = 0 ,

g}

+

P

O

U

O

(

n

j

0 _

n

j

)

.

o,

t>0

, t > 0 g

=0, t >

0

Before s e a r c h i n g f o r s o l u t i o n s of s e t ( 1 ) , f u r t h e r i n f o r m a t i o n i s needed about the f u n c t i o n a l form of the d i f f u s i o n c o e f f i c i e n t s , heat c a p a c i t i e s , e n t h a l p i e s , and chemical p r o d u c t i o n terms. These m o d e l l i n g questions are considered i n the Appendix


410


Computational Method. It i s only recently that numerical techniques to describe one-dimensional premixed laminar flames have become available. Spalding (21) and Dixon-Lewis (22) dev eloped similar relaxation methods where the unsteady flow equations are solved for arbitrary i n i t i a l conditions. Although computation times seem quite reasonable, t r i a l and error must be used to determine an economical stable step size, because the equations are linearized locally i n time. Alternatively, the steady state equations and their corresponding eigenvalue—the adiabatic flame velocity—may be solved via shooting methods quasi-linearization or f i n i t e difference techniques (23), (24). The latter method can become very time consuming, whereas, the former two present d i f f i c u l t i n s t a b i l i t y problems i n both the backward and forward integration because of the sensitivity of the numerical solutions to the guessed boundary conditions (19). From this brief glance at already existing methods to solve the flame equations, i t can be recognized that the choice of a numerical technique i s governed by trade-offs between computation time and problems of s t a b i l i t y , convergence and accuracy. Since one of our objectives was to develop a generalized computer pro gram to provide the user with a fast and reliable technique which could analyze a wide range of physical situations, i t was our belief that none of the methods discussed above f u l f i l l e d a l l the requirements for a general user package. The technique presented now i s the so-called "method of l i n e s " which was f i r s t applied to one dimensional flame propagation by Bledjian (18). Basically, instead of approximating the p a r t i a l derivatives with respect to a l l the independent variables by f i n i t e different expressions, one can convert the original set of parabolic par t i a l d i f f e r e n t i a l equations (1) to a system of ordinary d i f f e r ential equations by discretizing only the space variable, the time being l e f t continuous. In our case, for a system of m reacting species, the system of m parabolic nonlinear differen t i a l equations, m-1 species continuity equations and 1 energy equation , i s transformed into a set of m Ν nonlinear d i f f e r t i a l equations where Ν i s the number of grid points in the spatial direction: the i n i t i a l conditions to solve this system are provided by the i n i t i a l conditions applied to the original partial d i f f e r e n t i a l equations. Basic numerical advantages characterize this method • once the problem i s reduced to an i n i t i a l value problem, very efficient numerical methods of integration are available, with a computation cost proportional to the dimension of the system (mN) • unsteady problems can be dealt with i n a general fashion in particular, the modelling of ignition and/or extinction pheno mena becomes feasible. • flames i n spherical or c y l i n d r i c a l coordinates can be treated with no major inherent d i f f i c u l t y . The immediate


15.

GALANT AND APPLETON

411

Inhibition Theory

shortcomings of the technique stem from the particular nature of the resulting set of d i f f e r e n t i a l equations. • Since rapid changes within the flame occur over a very limited spatial domain, a large number of discretized points Ν i s needed to accurately describe those changes. In case of an insufficient number of grid points, strong i n s t a b i l i t i e s which most often yield negative concentrations appear readily. • On the other hand, i t has been shown with the help of simple linear parabolic equations (25) that, as the number of grid points Ν i s increased, the "stiffness" of the d i f f e r e n t i a l equations becomes very c r i t i c a l , i . e . , smaller and smaller time step sizes are needed to obtain stable and accurate results. • Moreover, since the original equations are already " s t i f f " due to the presence of nonlinear chemical source terms, this immediately suggests the use of an implicit " s t i f f " method of integration such as Gear's code (26). But then the cost of computation i s no longer proportional to the dimension of the system, m Ν: i t varies as (mN) or even (mN) because the Jacobian matrix of the nonlinear system must be inverted when a loss of accuracy i s detected. Most of the numerical problems inherent to the solution of the flame equations i n their original form were overcome by transforming both the dependent and independent variable. The use of the von Mises transformation (21) together with a mapping (27) bringing the i n f i n i t e or semi-infinite domains ]-°°, + °°[, [0,°°] into [0,1] allowed for a non-uniform grid size i n the r direction and improved the s t a b i l i t y characteristics of the re sulting set of equations. The stiffness of the equations was further reduced by using the new dependent variables 2

n. = log n. 3 3 f

3

i=l,...m

• log Τ

which also eliminates the problem of dealing with negative con centrations. The discretization of the spatial derivatives was accomplished by using a non-central five-point difference scheme (28) . Such a scheme allows for a sensible reduction i n the num ber of spatial grid points needed to follow the flame history within a given accuracy. Finally, a slightly modified version of the variable step size variable order integration method devised by Krogh (29) was implemented: i t leads to stable, ac curate and relatively inexpensive calculations for the set of equations considered above. Numerical Simulations Motivations. The flame which was chosen i s a low pressure, rich, H2-O2 flame, for which temperature and concentration measurements exist (19). Although such a flame i s not a very


412


good approximation to the burning of inhibited polymers i n an atmospheric environment, i t was selected to i ) demonstrate the a b i l i t y of the computational scheme to reliably describe an actual burner type experiment, i i ) pinpoint eventual improvements to be made i n the model l i n g (kinetics, transport, properties,...)* i i i ) obtain a general description of inhibition phenomena when the chosen flame i s seeded with halogenated compounds (HBr, Br. 2 •··), iv) check the influence of thermal diffusion which had been considered as a secondary effect but which i s claimed to be im portant under some experimental conditions (14). Within this context, no attempt was made to c r i t i c a l l y evaluate the parameters which are needed to describe both unin hibited and inhibited flames. Rather, the results shown here should be considered as a check for the v a l i d i t y of our flame model and a f i r s t step toward a basic understanding of i n h i b i tion phenomena i n gaseous mixtures. Results Ife-(>2 flame. The present predictions have been obtained using a set of rate constants l i s t e d i n Table I (from there on, referred to as Flame 1 : 75% H , 25% 0 ) . 2

2

TABLE I Reaction Mechanism and Reaction Rate Data (a) Reaction Mechanism and Forward Rate Constants Reaction Rates are expressed as Κ = AT exp (-E/RT) with k i n cm mole" sec" (cm mole"" sec" for three body reactions), Τ i n Κ and Ε i n kilocalories C

3

1

1

6

2

1

Flame 1 Reaction No. Reaction 1 0H+H = Η + H 0 2 H+0 = 0H+ 0 3 H +0 = Η + OH 4 0 +H20 = 0H+ OH 5 H+OH-m = E43+M 6 H+H +M = H + M 7 H+0 +M = 0H+ M 8 H2+O2 = 0H+ OH 9 H+0 2 4M = HO2+M 10 Η +H0 = OH+OH 11 HO 2 +0 = OH+O2 12 OH +H0 = O2+H2O 13 Η +H0 = H2+O2 2

2

2

2

2

2

2

2

A ~ 10 ,9 χ 74x 10 1 3 74x 1 0 75x 1 0 0 χ 10 1 5 2.04X10 4.6 xlO 15 2.1 xlO Ik 1.5 x l O 13 7. xlO 1. xlO 1 3 1. xlO 13 6.3 xlO 12 1 3

1 3

1 3

16

1 5

C

0. 0. 0. 0. 0. -0.31 -1.75 0. 0. 0. 0. 0. 0.

ε 5.T55 16.815 9.459 18.016 -1.49 0.0 -0.777 58.133 -0.993 0. 0. 0. 0.

Ref.

w (30) (30) (30) (31) (31) (31) (30) (31) (30) (30) (30) (30)

15.

GALANT

AND APPLETON

413

Inhibition Theory

(b) Third Body Efficiencies. For Reactions (6) and (7) a l l third bodies are taken to be equally e f f i c i e n t . For reactions (5) and (9), the following relative efficiencies are used, a l l others being set equal to 1.

Reaction No. 5 9

H0

H

5. 16.25

1. 2.5

2

References (30),(31) (30),(31)

The species considered were U2,02, H2O, Η, 0, OH, HO2. H 0 was set aside because of i t s numerically proven minor importance, and the lack of thermodynamic data above 1500°K. The computation of the transport coefficients i s detailed i n Appendix A. The pre dicted and experimental values of temperature are compared i n Figure l a whereas i n Figure lb the heat release rate along the burner i s plotted. The agreement i s f a i r at low temperature: the deviations from experimental values might be due to modelling i n accuracies when computing the thermal conductivity and the ther mal diffusion coefficients. The heat loss by conduction to the burner i s equal to 0.062 cal.cm s"" , whereas, Eberius et a l . found i t equal to 0.15 aal.cm"" s " (19). At higher tempera ture the agreement i s very poor; this can be explained by experi mental heat losses along the burner which tend to flatten the temperature p r o f i l e . However, since this i s a low pressure flame, the recombination of radicals i n the hotter regions i s extremely slow: hence, the high temperature profiles which are displayed have not reached their steady state values (19). In Fig. 2, the predicted and measured concentrations profiles for H2,02,H 0, Η and OH are compared, together with the theoreti cal values displayed i n Ref. 19 for a model without thermal diffusion effects. The agreement i s improved and emphasis should be put on the influence of thermal diffusion over most of the reaction zone. (See Figure 4a—dotted l i n e s — t o compare the relative magnitudes of the various Η atom fluxes.) The importance of those effects i s illustrated by the f a i r l y good agreement between experimental and predicted values for the Η and OH concentration at low temperature. This also demonstrates that the contribution from thermal diffusion effects cannot be correctly estimated on the basis of steady-state flux calculations alone, as i t has been done before (22). In fact, the preflame region i s characterized by strongly nonlinear feedback effects which are hardly predictable without an adequate numerical treat ment. Finally, i t should also be mentioned that polarity effects of the water vapor molecule, which have not been considered i n those calculations, can account for the discrepancy i n the concentra tion profiles over the cold part of the flame. They may change,


2

-2

2

2

1

1

2


1300 FLAME 0

2

1100 Lu

cc

£ 900

FLAME

< cc

1


£ 700 500 3οα

Ό

HEIGHT

A B O V E B U R N E R , CM

Figure la. Temperature distribution above the burner Δ Experimental points taken from Ref. 19 Solution of the flame equations taken from Ref. 19 Solution of the flame equations for Flames 1 and 2, including thermal diffusion

Lu

Halogenated fire suppressants: A symposium hosted by the Southwest Research Institute, San Antonio, Tex., April 23-24, 1975 : proceedings (ACS symposium series ; 16)

Carbohydrate Antigens (Acs Symposium Series)

Isotopes and chemical principles: A symposium (ACS symposium series ; 11)

Environmental Catalysis (Acs Symposium Series)

Fermentation Biotechnology (Acs Symposium Series)

Polyethers (ACS symposium series ; 6)

Hemicelluloses: Science and Technology (Acs Symposium Series)

Light-Activated Pesticides (ACS Symposium Series 339)

Ultraclean Transportation Fuels (Acs Symposium Series)

Chemical Sensors and Microinstrumentation (Acs Symposium Series)

Food and Packaging Interactions (Acs Symposium Series)

Dietary Supplements (ACS Symposium Series 987)

Light-Activated Pesticides (ACS Symposium Series 339)

Zeolite Synthesis (ACS Symposium Series 398)

Smart Coatings II (ACS Symposium Series 1002)

Drug Metabolism Concepts (ACS symposium series ; 44)

Clinical Chemistry (ACS symposium series ; 36)

Metal-DNA Chemistry (Acs Symposium Series)

Chiral Separations by Liquid Chromatography (Acs Symposium Series)

Separation and Purification by Crystallization (ACS Symposium Series 667)

Patras Logic Symposium: Proceedings

L.E.J.Brouwer Centenary Symposium Proceedings

Nuclear Energy and the Environment (ACS Symposium Series)

Constructivity in Computer Science: Summer Symposium, San Antonio, TX, June 19-22, 1991. Proceedings

Symposium

symposium

Symposium

Symposium

Symposium

Molybdenum Enzymes, Cofactors, and Model Systems (Acs Symposium Series)

Introduction to Microlithography: Theory, Materials, and Processing (Acs Symposium Series)

Halogenated fire suppressants: A symposium hosted by the Southwest Research Institute, San Antonio, Tex., April 23-24, 1975 : proceedings (ACS symposium series ; 16)

Carbohydrate Antigens (Acs Symposium Series)

Isotopes and chemical principles: A symposium (ACS symposium series ; 11)

Environmental Catalysis (Acs Symposium Series)

Fermentation Biotechnology (Acs Symposium Series)

Polyethers (ACS symposium series ; 6)

Hemicelluloses: Science and Technology (Acs Symposium Series)

Light-Activated Pesticides (ACS Symposium Series 339)

Ultraclean Transportation Fuels (Acs Symposium Series)

Chemical Sensors and Microinstrumentation (Acs Symposium Series)

Food and Packaging Interactions (Acs Symposium Series)

Dietary Supplements (ACS Symposium Series 987)

Light-Activated Pesticides (ACS Symposium Series 339)

Zeolite Synthesis (ACS Symposium Series 398)

Smart Coatings II (ACS Symposium Series 1002)

Drug Metabolism Concepts (ACS symposium series ; 44)

Clinical Chemistry (ACS symposium series ; 36)

Metal-DNA Chemistry (Acs Symposium Series)

Chiral Separations by Liquid Chromatography (Acs Symposium Series)

Separation and Purification by Crystallization (ACS Symposium Series 667)

Patras Logic Symposium: Proceedings

L.E.J.Brouwer Centenary Symposium Proceedings

Nuclear Energy and the Environment (ACS Symposium Series)

Constructivity in Computer Science: Summer Symposium, San Antonio, TX, June 19-22, 1991. Proceedings

Symposium

symposium

Symposium

Symposium

Symposium

Molybdenum Enzymes, Cofactors, and Model Systems (Acs Symposium Series)

Introduction to Microlithography: Theory, Materials, and Processing (Acs Symposium Series)

Recommend Documents