Conversion of Polymers Wastes & Energetics

U. Hansen and A. Rinschede

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Waste Disposal Logistics a Prerequisite for Effective Recycling

U. Hansen and A. Rinschede Fraunhofer-Institut für Materialfluß und Logistik, Department of Waste Disposal Logistics, Joseph-von-Fraunhofer Straße 2-4, D-44227 Dortmund, Germany

Legislation has reacted to increasing environmental problems by changing basic regulations especially regarding waste disposal. The recycling of waste will become a standard practice. It is therefore necessary to develop new concepts and technologies which allow for recycling at a high level. Consequently, the use of logistics is an important part of the solution to present problems. The following publication discusses a new software for optimization of internal waste disposal. Also, examples of recycling of plastics and cars are given and approaches towards the development of logistics for suitable redistribution and dismantling are analyzed.

Today, recycling is of growing importance and of great interest to the public. The recycling effort is endless in itself and a basic necessity because our civilization is threatened by growing amounts of waste. The Waste Law stipulates a higher degree of waste avoidance and mandatory recycling. This regulation requires large-scale recycling of industrial products. It also offers the possibility of maintaining responsible care of product content within a production-consumption-disposal cycle. The large product range and high number of component materials call for complex but inexpensive recycling processes. Figure 1 shows present recycling options. It must be understood that not every level of recycling is attainable or feasible for all aggregates, components, or materials; for example, the thermal transformation of duroplastic and elastomers is not possible.

8

Waste disposal logistics

The aim of all efforts should be recycling at a high level. Here lies great development potential for innovative technologies such as recycling of components. The basic aim is to find a scope of use similar to the original product. Fiber reinforced plastics, characterized by high stiffness and low weight, can be used as an example. Instead of milling for use as filler, areas of application should be found where the characteristics of these components can be used directly.

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This procedure is similar to the present material recycling principles where the so-called “cascade of re-utilization” aims for an optimal utilization of existing characteristics in a secondary use. In an economy reorganized into a cyclical system, components and materials would be re-entered into the economic cycle as secondary raw materials, secondary semi-finished products or secondary components. In our highly industrialized society, based on the division of labor, the great variety of material flows cannot be directed to their destinations without thorough planning and coordination. Without planned development, large material quantities would grow to intolerable dimensions because of under-utilization of residues. Conurbational traffic has already reached its limit. Therefore, material and information flows have to be optimized to realize recycling and transport effectively. This is a logistical task. The great importance of logistics in enterprises and economy has been realized only in the last few years. Consequently, its acceptance, and above all, the efficiency of its technical components has grown. This can be seen from an example of manufacturing industry. Here, besides complex production cells, modern logistic components such as fully automated multilevel warehouses, automatically guided transporting systems, automated loading, and transfer points with order-picking and warehouse robots are used. Logistics is divided into four main fields: • procurement logistics • production logistics • distribution logistics • waste disposal logistics. In waste disposal logistics, collecting and recycling are added to the traditional logistic functions such as transport, trans-shipment, and storage. Production and services now use logistics to reduce costs and rationalize operations. In waste disposal, however, the quantity of product-specific residues and waste shows that the logistics is still unknown to many enterprises. This is a disadvantage; only if the quality and the quantity of waste and residues are known, suitable measures can be undertaken to avoid waste and decisions made as to which material can be re-utilized. In order to intensify internal and external recycling, the use of logistics is absolutely necessary. The problems mentioned earlier explain the need for waste disposal logistics.

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Waste disposal logistics

Another important task of waste disposal logistics is to expand internal and external organization systems, which, at present, are mostly concerned with procurement, production, distribution, and waste disposal (Figure 2). This can be achieved, for example, by extending existing material management systems to include waste disposal systems, which either already exists or which

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must be developed. This leads to change in material and information flow because recycling has to be performed to a greater extent in an increasing number of fields. The establishment of a modified business structure is a frequent result (Figure 3). For the effective use of waste disposal logistics, the separation of micro and macro logistics has proven successful (Figure 4): • •

In macro-logistics, elements and systems which connect plants widely separated by geographical distribution (collection, redistribution) are studied. Micro-logistics, on the other hand, combines these elements and systems within a relatively close area, for example within a production or dismantling plant.

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Waste disposal logistics

Below, we give a short description of the software tool for optimization of internal waste disposal and recycling (ELOS) which has been developed by the Fraunhofer - Institut für Materialfluß und Logistik (IML): The basis of the optimization is the collection of data on all internal waste flow systems, which are

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stored in the form of current-state-database. The necessary data can be collected by using existing EDP material or from interviews using machine-readable questionnaires. The actual situation can be graphically presented in the form of a company layout with qualitative and quantitative information concerning existing mate-

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Waste disposal logistics

rial flows and can be statistically evaluated with the help of a related analyzing system. Thus, the cost assessment and analysis of weak points can be performed (Figure 5). A second important part of the system is a database which contains technical information on containers, transportation, warehousing, collecting strategies, methods of waste disposal, recycling potential, and cost assessment. After entering planned procedures, such as the potential for utilizing recycled materials, minimization of waste disposal costs or high disposal security, in a second database further design variants can be generated by interactive planning steps with the help of databases, which are evaluated in respect to their targets. Finally, a choice of evaluated, partially optimized alternatives is available. In the following discussion, two examples of micro and macro-logistics are given followed by some suggestions. The examples were chosen for the following reasons: • Part of the Packing Ordinance has already come into force. Comparable decrees for other products (electronic scrap, used cars) are under discussion in draft form. • Plastics are often used in complex products. If the products are not returned and recycled this has a negative effect on the possible future use of plastics (catchword “recycling quota”). EXAMPLE 1: RETURN LOGISTICS - REDISTRIBUTION (MACRO-LOGISTICS)

Before the planned regulations obliging a producer to take back and recycle certain products come into force, suitable general return strategies have to be developed. The direct technical design of the respective logistics depends on the products and the recycling plans. From economical and ecological points of view, the first requirement should be to use the supply vehicles not only as distribution vehicles but also as waste disposal redistribution vehicles. If this should prove unrealistic, a new concept should be developed. In both cases the concept has to comprise several plans: • a long term (strategic) • a medium term (tactical) • a short term (operational) level

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Strategic planning involves investments of working funds as well as development of concepts for a suitable network of collection points. Here, the collection areas must be defined and the question answered “should centralized or decentralized sites be established (catchword ”combined planning of sites and area")?"

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Waste disposal logistics

FhG has developed a software tool called “Displan”, which can be used for strategic planning in order to find suitable places to concentrate the supply. It should also be possible, with the help of the same tool, to plan the collection points (Figures 6 and 7).

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Tactical planning concerns the combining of orders with operation days as well as tour plans for the individual collection trucks. The operational level includes planning of daily collection tours. By densifying planning data on the operational level input, data for tactical planning are generated. Individual planning should be carried out with the help of a PC where a great range of optimization modules are available. Tour information is taken from street database which are available with high resolution. To implement the plans, data collection trucks should be equipped with a motor fleet information system. Besides the development of such a concept, technical aspects have to be given first consideration. Suitable vehicles are needed for collection of used articles. Requirements depend on size and type of products. An example for this is the recycling of used cars: Besides its topicality (draft of an objective by the Federal Government), this subject is especially interesting because of the great quantity of plastics. On average, 10.4% (about 120 kg) of the weight of a car is made up of different plastics (H. Steinert: Recycling must be given early consideration in the design of a car, FAZ 13/11/1989). At present, the following collection techniques for used cars are possible: 1. Collection via a mobile scrap baling press 2. Collection via retrieving trucks and mounted grapple crane 3. Collection via haulaway trucks 4. Other systems. Given the composition of used cars, variants 1 and 2 would not work. Even cars involved in collisions should not be further damaged during collection. Therefore, collection trucks should be designed in such a way that the outer contour of used cars can be maintained so that parts of the car body can still be re-utilized. Generally, haulaway trucks should be equipped with collecting basins to contain leaking fuels and lubricants. Suitable loading aids must to be developed to allow for continuous loading. If the outer contour of a used car is even partly maintained it can be loaded by a crane and be stacked in the collection truck. Here, loading techniques must be developed carefully so that the car bodies are not dented during loading. Otherwise, window breakage would lead to the materials in the interior being unusable also.

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Waste disposal logistics

As for plastics it should be pointed out that they have a great volume at low weight (EPS is an example). A suitable collection technique must be developed for these materials. Here, the combination of mobile swing-hammer crashers or shrinking devices is possible to improve the loading capacity of the trucks. The collection method must meet quality and purity requirements. EXAMPLE 2: DISMANTLING (MICRO-LOGISTICS)

Before aggregates, components and materials can be recycled, the collected products must be dismantled. At first sight, it seems to be favorable to use production strategies and techniques. A detailed analysis shows, however, that the change from one complex product to a variety of ordered components, raw material, and waste results in definitely changed parameters and problems (Figure 8). Examples for this are: • • • •

increased cycle times due to a variety of types and condition of products high set-up time due to necessary variety of tools changed order conditions possible use of destruction techniques

This field also offers a significant potential for development. The internal information flow connects the necessary work steps. Among others, it should allow for time control. For this purpose, all data on the products which are to be recycled, should be collected before delivery. Important data can be entered into the system with minimum effort by using a scanner-readable questionnaire. Changes or damage to delivered used products should also be considered. At the end of these procedures, all relevant data on delivered products and the whole stock are stored in a central database. In the next step, the necessary recycling processes or the method of retrieval of components or elements must be determined. Depending on production structure and throughput, different informational concepts have to be studied to determine their suitability. For a centrally organized system, all necessary information is stored in a database. Information on each work step is available before the next starts. It is well-understood that such a system requires a widely spread and well functioning communication network. Apart from a high data-processing rate and storage capacity, transmission times have to be considered as well. The advantage of

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this concept is that after determination of the current data, products need only be identified during each work step. The assignment of information to the processed subject is an alternative to central information management. Here, the item is accompanied by all necessary data in machine-readable form (e.g., in the form of programmable data carriers). This allows for a direct control of an automated transport technique. The central production control system is informed only about the part movement. With the help of adequate loading aids, it is relatively easy to automate transport techniques. Requirements for dismantling are, however, more demanding. Prerequisites for automation are either a far-reaching standardization of processes or the use of highly flexible techniques. Both cases differ widely in the size of investment needed, therefore it is important to evaluate the feasible alternatives in detail.

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Waste disposal logistics

This report has shown the role of waste disposal logistics in effective recycling. In the same way as procurement, production, and distribution must be optimized, the various processes in material and information flows must also be optimized and coordinated during recycling and disposal. Here, it has to be considered that in a market economy components and raw materials will only be recycled when the process is as smooth as possible, i.e., at minimum cost. It is the task of waste disposal logistics to make the necessary concepts and technologies available.

F. Pilati et al.

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Powder Coatings from Recycled PET

F. Pilati, M. Toselli, and S. Torricelli Dipartimento di Chimica Applicata e Scienza dei Materiali, Università di Bologna, Italy

C. Stramigioli Dipartimento di Ingegneria Chimica e di Processo, Università di Bologna, Italy

M. Dinelli Inver Spa, Bologna, Italy

INTRODUCTION

The human activities in their different forms are responsible for the pollution problems that have become more and more critical in the last few years. Some of more common actions that have been undertaken to reduce pollution are the recycling of post-consumer materials and elimination of toxic solvents from products such as coatings and adhesives, or from chemical processes in general. The study reported in this paper proposes a contribution to the solution of the pollution problems by suggesting that it is possible to recycle poly(ethylene terephthalate), PET, derived from post-consumer beverage bottles, for the preparation of solvent-free coatings. Research work in the field of polymer recycling is more frequently conducted in the last few years, and different types of recycling have been explored. In several cases, recycled polymers can be considered as a source of molecular fragments which can be used to build again new polymeric materials. A process like this, commonly named as tertiary or chemical recycling, can be used to pro-

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Powder coatings from recycled PET

duce monomers or mixtures of oligomers which can be employed, alone or with other monomers, to produce virgin polymers. The main requirement for the success of this type of recycling is the chemical feasibility and a convenient economic balance. The chemical feasibility of the process depends mainly on the nature of polymer to be recycled. Also, the polymer must be collected in a relatively pure form and must undergo a very extensive demolition of the polymer chains by ‘clean’ chemical reactions, that is by reactions leading to a limited number of chemical compounds at a high yield. On the other hand, the real possibility of recycling is based on the reliability of the process and, particularly, on the economic convenience; the polymeric materials produced by this process should have lower prices than those obtained from monomers. In general, the main prerequisites to meet this last goal are the possibility of a large-scale collection of a high quality waste and an economic chemical process. All the above requirements are commonly met for PET that can easily be transformed (by exchange reactions such as methanolysis or glycolysis) into a mixture of products from which monomers or oligomers can be recovered by a limited number of simple unit operations. Work done recently has shown that, recycled PET, mainly recovered from beverage bottles or films, can be used to produce dimethyl terephthalate, DMT, 1-4 by methanolysis, or bis(hydroxyethyl) terephthalate, BHET, by glycolysis with ethylene glycol,5 or polyols by glycolysis with various glycols.1-4,6-9 DMT and BHET can be re-used to prepare colorless PET of grade suitable for food-contact applications, PET oligomers, mainly for the preparation of PET for non-food products, while polyols can be used as intermediates for new polymeric materi1,4 1,6,7,10,11 However the als such as polyurethanes and unsaturated polyesters. chemical recycling of PET is not limited to the above products, but, in principle, it can be extended to the production of every type of resin which contains terephthalate and/or ethylene glycol units. In other words, PET may be seen as a source of molecular fragments which can be used, alone or together with other monomers, as ‘bricks’ in processes for the rebuilding of new resins. Polymers that contain terephthalate units are employed for instance in coatings and adhesives other than in unsaturated polyesters and in a large number of thermoplastic copolyesters.

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This paper investigates the fundamental aspects (feasibility, properties and economics) of a process which uses PET wastes for the preparation of powder coatings (see Figure 1). The recycling of PET by this process will help in solving of some problems with solid plastic waste and some pollution problems arising from the use of solvents in coatings. A number of resins can be used to prepare powder coatings, among these, for demonstrating the feasibility of the process, we choose as a reference, a polyester resin based on terephthalate (T), neopentyl glycol (NPG) and trimellitic anhydride (TMA) units, similar to some commercial resins. The polyester resins (either obtained from PET or from monomers) were then mixed with an epoxy resin, TiO2 and a catalyst, applied to standard metal sheets and cured in a oven. The properties of the coatings obtained by the resins prepared from PET were then compared with those of the coatings obtained from the reference resin and, finally, an economic balance of the process was established.

Figure 1. Scheme of the use of recycled PET for the production of powder coatings.

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Powder coatings from recycled PET

THE CHEMISTRY OF THE PROCESS

The overall chemical process, leading to the formation of a coating, consists of three principal steps, in the first step polyester resin with hydroxyl terminal groups is prepared, then it is reacted with TMA in order to transform hydroxyl terminal groups into carboxyl groups, and finally this latter resin is mixed with an epoxy resin with which it reacts during the curing stage. The quality of the final coating is expected to depend on the characteristics of the polyester resin, and mainly on its chemical composition, determined by molecular weight distribution, acid value (usually expressed as mg of KOH/g of resin), and functionality. In particular, it is important that the polyester resin has a suitable acid value and carboxylic functionality after the reaction with the TMA because the curing reaction proceeds mainly via reaction of carboxyl with epoxy groups. The purpose of this work was to demonstrate that it is possible to transform recycled PET into a polyester resin with chemical structure, molecular weight distribution, acid value and functionality similar to that of resins (R1 and R2) prepared from monomers and taken as references. The composition of the polyester reference resins was chosen from those typically used for powder coatings. Resin R1 was obtained by reacting first DMT with an excess of NPG, in the presence of Ti(OBu)4 as catalyst, and then with TMA (reactions 1 and 2) (similar polyester resin is marketed for powder coatings by Eastman Chemicals). Resin R2 was obtained in a similar way using a mixture of glycols, NPG and EG, instead of NPG alone (see reactions 1 and 2). Table 1: Characteristic of recycled PET from Tecoplast Size

5 mm

Moisture

0.2 wt%

Elemental analysis Fe

12 ppm

Cl

0.02 wt%

Al

10 ppm

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Recycled PET, in the form of flakes, was purchased from Tecoplast (Italy), and was obtained from the collection of PET beverage bottles clear and light-blue colored; its main characteristics are reported in Table 1. The reactions for preparation of the resins both from monomers and PET, described below, were carried out in a stainless steel reactor (1.8 L of capacity) mounting a paddle agitator, usually driven at 30 rpm, and equipped with facilities for distillation of volatile by-products, for charging reactants and with a valve for the recovering of the products from the bottom of the reactor. As far as

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Powder coatings from recycled PET

possible the reactions were carried out under similar conditions, but we did not try to optimize them with respect to neither molecular weight nor acid value. The amount of reactants used for the preparation of the resins are reported in Table 2. Table 2: Amount of reactants* DMT

EG

PET

NPG

TMA

NPG/PET

g

g

g

g

g

mol/mol

R1

582

-

-

374

109

-

R2

582

38

-

312

125

-

S1

-

-

576

312

109

1

S2

-

-

422

458

92

2

S3

-

-

400

434

87

2

S4

-

-

300

489

66

3

Sample

*all reactions were carried out using Ti(OBu)4 as catalyst (0.1 wt% with respect to DMT or PET)

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REACTION OF PET WITH NPG

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The first goal of the process was to prepare polyester resins from recycled PET with characteristics similar to reference resins obtained from monomers. The more convenient process for this purpose is to cleave back the PET chains to oligomers by glycolysis with NPG (reaction 3), followed by selective removal of ethylene glycol (EG) to decrease the EG/NPG ratio in the reaction mixture. The glycolysis of PET did not present any particular problem, but the presence of a catalyst is needed to reduce the reaction time; in our experiments the rate of glycolysis was increased by adding Ti(OBu)4 as the catalyst (0.1 wt% with respect to the initial amount of PET). Unfortunately the difference in volatility between EG and NPG is not very large and part of NPG distilled off the reactor along with EG, making the complete elimination of EG moieties from the final resin very difficult. Of course, the residual amount of EG moieties in the chains of the final resin can be decreased by increasing the relative amount of NPG added for glycolysis. For this reason we performed several reactions with various NPG /PET ratio; data are reported in Table 2. The first stage, of glycolysis (reaction 3), was carried out at 220-230oC in the closed reactor (there is a slight increase of the pressure up to about 1.5 bar) o for 60 minutes. Then the reaction was continued at 230-240 C with the reactor opened allowing the distillation of a mixture of EG and NPG which was collected in a condenser. When the rate of distillation became very low the pressure in the reactor was decreased (down to about 2 m<W1%-2>bar during 20 minutes) and maintained for 5 minutes. The reactor was then filled with nitrogen and the o temperature was allowed to decrease to 160 C (before the addition of TMA). The chemical composition of the products at the end of this step was deduced from 1H-NMR spectroscopy. As it appears, there is a strong decrease of the EG/NPG molar ratio in the product when the NPG/PET ratio increases; a polyester resin with a residual content of EG moieties as low as 3 mol% was obtained when the initial NPG/PET molar ratio was 4 (based on the monomeric unit of PET). The mol% of EG in the resin, after the first stage of glycolysis, is also reported in Figure 2 against the starting NPG/PET molar ratio.

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Powder coatings from recycled PET

REACTION OF THE HYDROXYL-TERMINATED POLYESTER WITH TRIMELLITIC ANHYDRIDE

The mixtures of oligomers, obtained from monomers, in the case of the reference resin, and from recycled PET, after the glycolysis step, were reacted with TMA under the same reaction conditions. TMA was added when the temperature in o o the reactor was decreased to 160 C, and then allowed to react at 180 C for 110 min (reactions 2 and 4 for resins from monomers and recycled PET, respectively). The final resins were then collected, ground, and analyzed for their

F. Pilati et al.

Figure 2. Resin composition vs the initial (NPG/repeating units of PET) molar ratio in the reactor.

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Powder coatings from recycled PET 1

chemical composition (by FTIR and H-NMR), acid value (by titration), molecular weight (by GPC), and glass transition temperature (by DSC). While the reference resins were white powders, the products obtained from recycled PET were discolored appearing as green-gray powders. However the difference in the chemical structures was limited to the presence of residual EG in the resins derived from PET as measured by spectroscopic techniques. It will be shown below that the color of the polyester resins will not affect significantly the final color of the coating. From 1H-NMR, it was possible to calculate the ratios of the EG/NPG units in the resins. The results obtained, for the reference resin and for samples derived from PET, are reported in Table 3. As expected the EG/NPG ratio is the same at the end of the first step of glycolysis, decreasing from samples S1 to S4. The NPG to terephthalate units (T) ranges from 1.18 to 0.82. Table 3: Composition and characteristic of polyester resins a

a

b

c

f

e

Acid value

mol/mol

mol/mol

mg KOH/mg

R1

-

1.18

62

1520

1.7

58

R2

0.17

1.00

98

1510

2.6

63

S1

0.43

0.82

66

1850

2.2

67

S2

0.20

1.00

58

1800

1.9

63

S3

0.18

0.95

70

1810

2.3

65

S4

0.09

1.10

84

1830

2.7

69

Sample

Mn

d

NPG/T

EG/NPG

Tg o

C

a - from 1H-NMR, b - from titration, c - from GPC using PS standards for calibration, d - average carboxylic functionality, i.e., number avarage of carboxyl groups per molecule, e - from DSC

The acid value is a parameter that is critical to control because it depends primarily on the amount of TMA relative to the molecular weight of the hydroxyl-terminated resins, which in turn can change significantly in a relatively short time of reaction. This leads to a relatively high dispersion of the acid values found for both the reference and the PET-derived resins. Except for resin R2, they are smaller than those of the commercial resins, however, if the acid

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value would be a very critical parameter for the coating properties, it would not be a problem to obtain higher values (and the same values for resins from recycled PET or monomers) by optimizing reaction conditions. Similar comments and conclusions can be extended to the molecular-weight results derived from GPC curves (some of which are reported in Figure 3). The shape of the curves are similar for the resins R1 and R2, obtained from monomers, and those obtained from recycled PET (S1-S4); these latter show very similar molecular weights, independently from the starting ratio NPG/PET, that are slightly higher than those found for the reference resins. As said before for the acid value, it would not be a serious problem to reduce or increase the molecular weight of the resins if necessary.

Figure 3. Typical GPC curves for reference and PET-derived resins; molecular weight calibration is based on polystyrene standards.

From the acid values and the number-average molecular weight, it is possible to calculate the average carboxylic functionality of the polyester resins expressed as an average number of carboxyl groups per chain; the relative data are reported in Table 3. They are relatively low and spread, but this short-comings depend on the reaction conditions, which were not optimized, rather than on the use of recycled PET. o The values of Tg, obtained from DSC curves recorded at 10 C/min, are similar (see Table 3) for both reference and PET-derived resins.

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Powder coatings from recycled PET

CURING AND PROPERTIES OF THE POWDER COATINGS CURING

The polyester resins, prepared as described above, were compounded with the other ingredients of the coating in a twin-screw extruder; all powder coatings were formulated with the same composition as reported in Table 4. The blends were then micronized in a dry mill micronizer under the same conditions. Then the powder coatings were applied on standard UNI panels using an electrostatic o spray gun and cured in an oven at 180 C for 20 minutes. Dry film thicknesses of 170-180 µm were measured. Table 4: Coating composition Components

wt%

Polyester resin

35.2

Epoxy resina

35.2

Flow control agent Degassing agent TiO2d

c

b

1.6 1.0 27.0

a - Araldite GT 7004 from Ciba-Geigy, b - Modaflow from Monsanto, c- Benzoin from BASF, d - Tioxide TR/92 from Tioxide Europe

COATING PROPERTIES

After curing, the coatings obtained from both the reference resins and from PET did not show any readily visible difference. To check the differences in the coatings obtained from monomers and from recycled PET, the most important properties of the coatings (color, gloss, adhesion, and mechanical properties) were evaluated according to ASTM methods for panels after conditioning for 24 h at room temperature. The method employed and the results are listed in Tables 5 and 6 respectively. Based on the properties reported in Table 6 the coatings obtained from monomers and from recycled PET can be considered equivalent. In particular, the color of coating derived from the recycled PET does not show significant difference with that of coatings obtained from monomers.

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Table 5: Tests performed for coatings Test Specular gloss at 60

Method o

ASTM D523 o

Color δE (CIELAB, 10 , D65, specularity included)

ASTM D2244

Buchholtz hardness

DIN 53153

Direct impact

ASTM D2794

Conical mandrel

ASTM D522

Flow

visual

Cross-cut adhesion test

ASTM D3395

Table 6: Results of tests listed in Table 5 Test

R1

R2

S1

S2

S3

S4

Gloss (%)

88

87

80

80

82

90

Color

0

-

0.39

0.37

-

-

Hardness (Buchholz units)

100

100

100

100

110

110

Impact (kg cm)

10

20

20

10

10

10

severe cracking

few cracks

few cracks

severe cracking

no cracks

few cracks

Flow

good

good

good

good

good

good

Adhesion (%)

100

100

100

100

100

100

Mandrel

The differences in molecular weight, molecular weight distribution, acid value, and carboxylic functionality of various resins may be responsible for a slight difference in gloss, impact resistance, and flexibility, however it does not seem that these characteristics of the resins have a strong influence on the properties.

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Powder coatings from recycled PET

ECONOMICS CAPITAL COST

As it is well-known, the economical feasibility of production strongly depends on the size of plant. In the following analysis, a plant able to treat about 1100 t/year of recycled PET corresponding to the production of coating powders of a medium factory is taken into account. The plant consists of a batch reactor with a jacket for heating and cooling, a cooling conveyer belt with a flaker at the end, a tank to collect glycol mixture distilled during the reaction, and a pump. The plant is considered to operate 230 d/year with a daily production of two batches and to be an addition to the existing plant for the production of powder coatings. The installed cost, as sum of the FOB cost, transportation, cost of foundation, erection, and connection to service facilities, was estimated as 0.882 M$ (late 1993). All prices and costs are pertinent to the Italian market. A rate of change 1$=1700 It£ was used. MATERIAL BALANCE

A NPG/EG molar ratio equal to 3 was considered in this situation; as previously stated, 91% of EG units are substituted by NPG units. A summary of the results of the material balance is shown in Table 7. Table 7: Summary of material balance Reactants (g)

Products (g)

PET

1000

-

NPG

1625

1132

TMA

220

-

EG

-

294

Resin

-

1419

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Table 8: Raw material unitary cost Material

Unitary cost ($/kg)

PET

0.53

NPG

1.09

EG

0.39

TMA

2.85

It is easy to note that a glycol mixture is obtained consisting of the replaced EG and excess of NPG. In the following we assume that this mixture can be used, without further purification, in other production in the factory, we consider only the NPG effectively consumed, and we take advantage of a negative cost for the EG obtained. OPERATING COSTS

The unit costs of raw materials used for the economic analysis are reported in Table 8. The incidence of raw materials on the resin cost is: (0.53x1 + 1.09x0.493 + 2.85x0.220 - 0.39x0.294)/1.419 = 1.11 $/kg Other operating costs are quoted in the following: Methane Electrical energy Water Nitrogen Total Utilities Labor (3 units) R&D and quality control Maintenance

Thousand $/year 22.6 26.3 1.0 8.9 58.8 97.1

(5% of the installed cost)

44.1

(11% of the installed cost)

97.0

Depreciation

144.1

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Powder coatings from recycled PET

Table 9: Total production cost 1000$/ year Raw materials Utilities Other operating costs

$/kg

1732.6

1.110

58.8

0.038

382.3

0.245

The results are summarized in Table 9. The total production cost compares favorably with the purchase price of a low-priced polyester resin equal to 1.56 $/kg. Obviously, the results obtained are dependent on the assumed values for all pertinent costs. In particular, the cost of the recycled PET varies considerably depending on the market situation. From a sensitivity analysis performed in correspondence of variations of PET cost, we obtained a total production cost for the resin of 1.319 $/kg and 1.473 $/kg for PET-cost variations of + \20%, respectively. CONCLUSIONS

As an overall conclusion, we can state that it is possible to prepare powder coatings using recycled PET by a chemical process which is feasible and, pro<W1%-2>bably, economically convenient. More detailed conclusions about feasibility and economics of the proposed process can be summarized in the following comments. As regard to feasibility, our results demonstrate that it is possible to use recycled PET for the preparation of polyester resins which are suitable for curing with epoxy resins and which give coatings with properties similar to those obtained from the reference resins prepared from monomers. The major difference between resins prepared from monomers and recycled PET is a residual content of EG moieties in the resins derived from recycled PET which depend by the NPG/PET starting ratio. None of the properties examined seem to be changed significantly by the presence of residual EG up to 30 wt%. Some other minor differences between the coatings prepared from recycled PET and those obtained from monomers, such as acid value and molecular weight, seem to be irrelevant for the properties of the coatings, and also these

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parameters can be quite easily modified by a better control of the reaction conditions during the preparation of polyester resin. Thus, the differences in the acid value, molecular weight, and functionality of the resins prepared in our laboratories (either from recycled PET or from monomers) with respect to those of equivalent commercial resins are acceptable considering that we did not try to optimize the reaction conditions (temperature, time, and catalyst of the reaction). Regarding the economics of the process, we have to emphasize that it depends mainly on the price paid for recycled PET which is largely dependent on the collection system developed (which can vary in different countries), and on the market demands. Therefore, the economic convenience of such a process can change from country to country and fluctuate with the cost of the raw materials and of recycled PET. In any case, the process can be carried out in chemical plants which are very similar to those commonly used for the preparation of polyester resins from monomers, and therefore it should not create a problem to design a plant suitable for an easy change of the feed from recycled PET to monomers following the fluctuation of the prices of the raw materials. 1.

REFERENCES

J. Milgrom in Plastics Recycling, Ed. R. J. Ehring, Hanser Publ., Munich, 1989, p. 59. 2. W. De Winter in Review on PET-film Recycling in Recycling of Plastic Materials, Ed. F. P. La Mantia, ChemTec Publishing, Toronto, 1993. 3. Chem. Eng. News, 67, 7 (1989). 4. W. De Winter, A. Marien, W. Heirbaut, and J. Verheijen, Makromol. Chem., Makromol. Symp., 57, 253 (1992). 5. Unpublished results. 6. U. R. Vaida and V. M. Nadkami, Ind. Eng. Chem. Res., 26, 194 (1987). 7. U. R. Vaida and V. M. Nadkami, J. Appl. Polym. Sci., 34, 235 (1987). 8. D. Gintis, Makromol. Chem., Makromol. Symp., 57, 185 (1992). 9. S. Baliga and W. T. Wong, J. Polym. Sci. Part-A, Polym. Chem., 27, 2071 (1989). 10. S. N. Tong, D. S. Chen, C. C. Chen, and L. Z. Chung, Polymer, 24, 469 (1983). 11. K. S. Rebeiz, D. W. Flower, and D. R. Paul, Polym. -Plast. Technol. Eng., 30, 809 (1991).

M. Teller

31

Possible Applications of Pyrolysis Technology in Treatment of Hazardous Wastes and Valuable Materials Recovery

Matthias Teller BC Berlin-Consult GmbH, Am Karlsbad 11, D-10785 Berlin, Germany

Pyrolysis technology has reached by now a state of development that opens more favorable possibilities for waste disposal than some of the traditional techniques. Special complex waste materials can now be recycled or disposed of properly by pyrolytic treatment. Composite materials such as circuit-board waste and complete circuit-boards, mixed plastics, flameproof plastics and shredder residues from car scrapping can be converted into useful materials and environmentally neutral residues by a combination of pyrolysis, gas scrubbing, processing of residues, and incineration. In this process, metals are separated without being oxidized, oils recovered in the gas purification stage, and they can be recycled into the raw material pool with standard methods of petrochemistry. The remaining clean pyrolysis gas can be used directly in the process, and under certain conditions, depending on the type of the input material, permits autothermal operation. A suitable design of the process stages permits the concentration of pollutants and leads to very small mass flow resulting in low disposal costs.

STATE OF DEVELOPMENT IN PYROLYSIS TECHNOLOGY

Since the beginning of the seventies, engineers in industry, and universities are working on the technical development of pyrolysis processes. So far, and with a few exceptions, this technology is not used on a large-scale for treatment of waste and residues. However, this approach undergoes fundamental changes at present. The deciding factors are two development tendencies: On one hand, the number of wastes and residues containing valuable materials which are not

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Pyrolysis technology in treatment of hazardous wastes

suitable for the traditional thermal disposal process - the incineration - is steadily increasing, and dumping of such materials is more and more given up. On the other hand, basic legal requirements to be met by the emission standards for thermal disposal or utilization are increasingly tightened up, a fact which affects the profitability of incineration plants to a considerable extent. Solutions to this problem are offered by a combination of pyrolysis and incineration, as described in the following. In the past, pyrolysis (i. e., the thermal decomposition of a material under the exclusion of air oxygen) was mainly tested in shaft furnaces, autoclaves, chamber pyrolyses, rotary kilns, and fluidized-bed reactors. Meanwhile, the indirectly heated rotary reactor is considered to be a mature solution for a large-scale technical level. In the field of waste management, pyrolysis was used so far for treating household refuse, hazardous waste, special residues such as tires, sewage sludge, residual oils, oil muds, shredder residues, plastics, and circuit-board 1,2,3 scrap, and for a pyrolytic treatment of contaminated soils. Figure 1 shows a selection of pyrolysis plants for waste treatment. As can be seen, plants with a capacity of 5 to 6 tons per hour are state-of-art.

M. Teller

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From a technology view point, the processes employed can be divided into three groups of a coupling of pyrolysis and final incineration which is called “graduated incineration” (Figure 2). The first process type directly feeds the pyrolysis gas to the incineration and burns it (plants by Babcock and BC). In the second process type, the pyrolysis gas is cracked in a gas converter, afterwards, cleaned and is then available as lean gas (plant by KWU). In the third process type, the cracked gas is cleaned in a multi-step gas scrubbing before being used for thermal purposes (plants by BC, Kaminsky, Noell, MVU, Veba Oel). The clean pyrolysis gases obtained from the third process type can be used totally or partly for running the pyrolysis. The graduated incineration offers three essential process advantages as compared with the sole incineration. These are: • • •

Resources can be separated and isolated out of the process gently. Noxious materials can be retained and concentrated with a comparably low expenditure for process engineering. The inhomogeneous waste material flows are homogenized to a large extent and can then be burn as pyrolysis gas or pyrolysis coke under very favorable process conditions. The result is a reduction of the flue gases to 30 %.

34

Pyrolysis technology in treatment of hazardous wastes

PROCESS BALANCE

The process advantages of the graduated incineration become clear when a more detailed balance of the process is evaluated. The pyrolysis provides two flows of products (Figure 3): • •

First, the volatile phase, a dust-laden pyrolysis gas, containing constituents of acid gas, ammonia, water, volatile heavy metals, nitrogen, sulphur, and carbon compounds as well as H2, CO, CO2, and a multitude of organics Second, a solid pyrolysis residue is obtained which contains the remaining carbon generated in the thermal cracking and all non-volatile components.

The composition of these two product flows is substantially influenced by the way in which the process is conducted. In general, a dehydrohalogenation and, linked to it, a decomposition of the halogen-organic compounds take place (Figure 4). It can be seen that most of the remaining halogen-organics such as dioxines and furanes enter into the volatile phase and that the remaining solid pyrolysis residue is burdened with noxious materials to such an extent that it is harmless

M. Teller

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for further handling and utilization. Halogen-organic compounds, still remaining in the volatile phase, do not cause any difficulty. Such compounds are not detectable in the sewage of pyrolysis gas scrubbings because of their good oil solubility and their hydrophobic behavior. They are retained quantitatively in 6 the condensate oils and can be separated from them by well-known processes. Depending on the kind of input material, the residue is more or less contaminated with heavy metals. Because of the activated coke-like structure of the carbon proportions in the residue, however, these heavy metals are bound in such a good manner that elution tests according to DEVS4 do not lead to significant leachings as compared with the limits suggested in the “Technical Directions Waste” (TA Abfall).4 Depending on the input material, metals and inert materials can be separated from the solid residue and be re-used. If there is a sufficient poorness of noxious materials, the remaining pyrolysis coke can be used as a source of substitution energy, for instance, in cement production or also, in combination with a traditional combustible, in a coal-fired power stations.

36

Pyrolysis technology in treatment of hazardous wastes

Pyrolysis tests with plastics have shown1,5 that the volatile phase, i.e., the pyrolysis gases, can reach up to 75 mass percents of the input material. Another 45 mass percents can be condensed as liquid phase by a pyrolysis gas purification in the form of condensation-washing steps. This phase is first of all a mixture of light petrol and coal tar. If the process is conducted properly, aromatic compounds, with the main components such as benzene, toluol, styrene, and naphthalene, constitute up to 95 mass percents. Higher condensing aromatic compounds are mainly anthracene, phenanthrene, pyrene, and chrysene. Thus, the purification of low-temperature carbonization gas provides a raw material much sought after by the petrochemical industry. 5 The quantities of water built by the reaction are between 0.1 and 5 %. Part of the water, obtained from washing the condensate, must be taken from the process to subtract the salts produced. This sewage is of course also loaded with organic materials such as toluol, dimethylacetamide, and diethylacetamide. Tests carried out at Berlin Technical University by order of BC Berlin-Consult GmbH have proven that a biological-adsorptive purification method can decompose the COD of such pyrolysis sewage up to a residual concentration of 300 mg/l of COD. With the same process, nitrogen compounds can be reduced to residual ammonia concentrations of < 10 mg/l.

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After the condensation washing, a clean gas is obtained, free of noxious materials and precursors of noxious materials such as chlorine, sulphur, ammonia, volatile heavy metals, dioxines, and furanes. This clean gas, built up from usual components of burnable gas, which can reach up to 60 mass percents of the input 3 material and thermal values of up to 45 MJ/m , can be directly used for running the pyrolysis. Thus, depending on the type of input material, an autothermal pyrolysis operation becomes possible, and, if occasion arises, part of the clean gas flow can be used for other purposes. These balance values make clear that the pyrolysis technology, including a washing of the pyrolysis gases, permits the splitting up of very heterogeneous input materials, as for instance compounded materials, electronic scrap or shredder residues from car recycling, into the product flows of metals, inert materials, pyrolysis coke, oil, and clean gas. As the following examples will show, the technological expenditures remain far behind that of a traditional thermal disposal.

38

Pyrolysis technology in treatment of hazardous wastes

EXAMPLE: PYROLYSIS OF CIRCUIT-BOARDS

From 1985 to 1987 BC Berlin-Consult GmbH erected a recovery and disposal plant for circuit-boards in a manufacturing plant for a basic circuit-board material in Bernau near Berlin. In this process combination of pyrolysis and incineration (Figure 5), the pyrolysis gas obtained is directly fed to the incineration. It results from the mass balance (Figure 6) that up to 13 mass percents of the input material are recovered in the form of copper. 50 kg per ton of input material are obtained as residue from the flue-gas purification and have to be dumped. Taking the toxicity equivalents of the analogous chlorinated compounds as a basis, the results of measurements, carried out in the clean gas, showed 0.146 ng/m3 for the sum of the brominated dioxines and furanes. This value was reached when a basic circuit-board material was used, the resins of which contained up to 6 mass percents of bromine in the form of flame-retarding additions made of pentabromine-diphenylether. The corresponding coke loads amount to 0.011 ng/g.

M. Teller

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If the gas is washed between the pyrolysis and the final incineration processes, there is of course no emission of halogenized hydrocarbon on the side of the clean gas. EXAMPLE: PYROLYSIS OF ELECTRONIC SCRAP

A corresponding combination of processes was used in tests to carbonize electronic scrap at low temperatures (Figure 7). In the first test phase, the pyrolysis was followed by a gas quench and washing stage by means of a packed column. Four different charges of electronic scrap, obtained from Siemens, IBM, Edelhoff, and two collectors of electronic scrap in Dresden and Hamburg, were carbonized at low temperatures. Pyrolysis took place at temperatures between o 575 and 645 C, with an average residence time of the materials in this temperature range of 30 minutes. As expected, the tests showed that the pyrolytic treatment of circuit-boards can be applied without any problem to the carbonization of electronic scrap. The following results were obtained as far as the balance of the overall process and the contents of noxious materials are concerned.

40

Pyrolysis technology in treatment of hazardous wastes

A rough balance of the pyrolysis products of the different charges shows that the portion of the remaining solid residue is a subject to strong fluctuations, depending on the input material, and that it can amount up to 55 to 84 mass percents of the input. The solid residue can be mechanically separated into the fractions of metals, glass fibre, inert material, and a fine portion. By the kind of input, the quantities of the individual fractions (Figure 8) can also vary considerably. Processing of metals has to be done in metal-separating plants. In the first step, iron parts and granules of solidified melt (alloys from copper, aluminum, lead, and zinc) can easily be separated. As far as the fraction of glass fibre is concerned, it is checked at present in what quantities and under which conditions they can be integrated into the melting process for recycled colored glass. The fractions of inert materials which, on an average, amount to approximately 8 mass percents only (Figure 9), have to be disposed of in a household-refuse dump. It remains a fine portion with approximately 15 mass percents of the input. Noxious materials which were not converted into the volatile phase, i.e., into the pyrolysis gas, during the pyrolysis process, are bound to the fine portion. An analysis of these fractions suggests to use them separately for thermal purposes. In the sense of a well-rounded disposal concept, it is rec-

M. Teller

41

ommended to design this thermal utilization in such a way that the incineration residues are obtained in the form of vitrified slag (Figure 10). The remainders are the constituents of the volatile phase, the condensate oils from washing the pyrolysis gases, and the clean gas. Present results of condensate oil analyses give no concern regarding their subsequent use. The clean gas is free of any noxious material and can be used for running the pyrolysis. EXAMPLE: PYROLYSIS OF SHREDDER RESIDUES

The process depicted here is also suited for the disposal of shredder residues. So far, this remaining material, of which approximately 400,000 tons are obtained in the Federal Republic of Germany every year from car-crushing plants, is mainly dumped. Due to the content of different noxious materials such as PCB or mineral oils, the acceptance of this disposal option is steadily decreasing, and Federal laws are being prepared which will extremely aggravate a dumping of shredder residues in future.

42

Pyrolysis technology in treatment of hazardous wastes

M. Teller

43

Experience made so far with the carbonization of shredder residues shows that, as long as the suitable process components are chosen, this material can be processed resulting in fractions of inert materials, metals, and vitrified slag (Figure 11). Relative to the waste input, the mass of the remainders obtained from the pyrolysis is between 40 and 55 mass percents. Analysis of the washout of solid residues provides values which meet even the most restrictive effluent 7 conditions in the Federal Republic of Germany. The solid residues of the pyrolysis can be separated into the fractions of metals, inert materials, and carbonated fine portion (Figure 12). Via the process already depicted, the volatile low-temperature carbonization products are decomposed into condensation products and clean gas. The clean gas is used to heat the pyrolysis process, whereas the oils, after a possible decontamination, are used to vitrify the fine portion rich in carbon. 1. 2.

3. 4. 5. 6. 7. 8.

REFERENCES

G. P. Bracker, G. Collin, and E. Michel, Pyrolytische Rohstoff-Rückgewinnung aus unterschiedlichen Sonderabfällen in einem Drehtrommelreaktor. Versuchsergebnisse im halbtechnischen Maßstab, Chem.-Ing.-Tech., 53, 10 (1981). W. Bischofsberger and R. Born in Verfahrens- und umwelttechnische Analyse neuer thermischer Prozesse in der Abfallwirtschaft, Phase 1: Pyrolyse, Berichte aus Wassergütewirtschaft und Gesundheitsingenieurwesen, TU München, 1989. H. Piechura in Erprobung und Optimierung der SalzgitterPyrolyse-Anlage zur thermischen Zersetzung von Sonderabfall mit Energie- und Rohstoffrück gewinnung, Schlußbericht BMFT, Förderkennzeichen 01 VQ 8519, 1989. C. G. B. Frischkorn in Bericht zu Deponieverhalten des Pyrolysereststoffes aus der PKA- Müllpyrolyse und seine Nutzung für die Entsorgung organisch belasteter wässriger Emissionen, Sonderdruck KFA, Julich, 1991. W. Kaminsky, Wertstoffrückgewinnung aus Altgummi durch Pyrolyse, in Elastomere und Umwelt, VDI- Verlag, Düsseldorf, 1991. E. Bilger, Enthalogenierung halogenkohlenwasserstoffhaltiger organischer Flüssigkeiten, Chem.-Ing.-Tech., 62, 4 (1990). E. Pruckner in Verschwelung von Shredderabfällen, KWU Umwelttechnik. R. Martin in Thermische Behandlung der Shredderleichtfraktion, Bayer AG, Leverkusen, 1991.

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Continuous Determination of Volatile Organic Breakdown Products of Propellants in Water

Günther Hambitzer and Martin Joos Fraunhofer Institut für Chemische Technologie (ICT), 76327 Pfinztal 1, Joseph von Fraunhofer Straße 7, Germany

Volatile products such as aniline, toluene, and alkylamines are often formed by the decomposition of propellants. These substances can be continuously detected in aqueous solutions by a new mass spectrometric setup. The essential part of this setup is a flow cell coupled with a mass spectrometric unit. A porous, hydrophobic membrane fixes the phase boundary of liquid/gas and serves as a gas inlet for the vacuum. A model explains the pathway of volatile substances dissolved in water. It describes the diffusion in the flow cell, the transfer of volatiles through the phase boundary, and the membrane into the vacuum system. The sensitivity determination of volatile, organic compounds is generally within the range of 1 µg/L to 1 mg/L. Due to the short reaction times, this method enables continuous control of the process and waste water.

INTRODUCTION

The environment of former East Germany has been subjected to considerable environmental pollution as a result of emissions from propellants over many years. On account of their high toxicity, there is a need to determine these substances and their breakdown products in contaminated waters. The disposal of propellants produces process waters in which the breakdown products have to be monitored.1

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Continuous determination of volatile breakdown products

Figure 1. Schematic presentation of the mass spectrometric apparatus and membrane inlet system.

Many propellants, especially nitroglycerin, contain Centralite 1, which, apart from stabilizing properties, has also gelatinizing (plasticizing) properties. Centralite 1 is a symmetric diethylphenylurea (C17H2O2N2). Its breakdown mostly produces volatile alkylamines and aniline derivatives. In the breakdown of TNT (trinitrotoluene), volatile toluene and partially volatile nitrotoluenes are produced. The rapid analysis and the continuous determination of these volatile breakdown products is needed and can be accomplished by a recently developed mass spectrometric apparatus. EXPERIMENTAL SETUP MASS SPECTROMETRIC APPARATUS WITH MEMBRANE INLET SYSTEM

The core of the setup consists of a flow cell with a porous membrane which allows only gases but no liquids to pass through. A mass spectrometric apparatus is coupled with this flow cell. The porous membrane plays the role of the liquid/gas

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phase boundary and also the inlet system to the mass spectrometer. The mass spectrometric apparatus ( Figure 1) can be broken down into two parts, i.e., the mass spectrometer itself as a recording unit and the vacuum apparatus. The vacuum apparatus with its connection to the membrane inlet system (1) possesses two differentially pumped volumes. In the first step, the main part of the gas flowing through the membrane is pumped by a membrane pump (4). A small part of the gas flows via a pressure converter (2), equipped with an adjustable -5 shutter, into the high vacuum (approx. 10 bar) necessary for operating the mass spectrometer. The high vacuum is generated by a turbomolecular pump (5) operated by a preconnected membrane pump (6). The measurement head of the mass spectrometer is arranged so that the molecular stream from the shutter opening is aimed directly at the ion source. This has an effect on reduction of desorption and adsorption processes on the inner wall and thus increase of sensitivity of measurement. FLOW CELL WITH CONTINUOUS DETECTION OF VOLATILE SUBSTANCES IN LIQUIDS

For continuous recording, a flow cell (Figure 2) is flanged onto the mass spectrometric apparatus. A liquid pump conveys a solution through the cell. Gaseous substances are transferred from the liquid into the vacuum through the membrane responsible for the liquid/gas phase boundary. A layer of steel frits (φ = 8 mm) (2) supports the membrane (3), preventing its rupture due to the prevailing difference in pressure. The liquid flows at pressure approaching atmospheric pressure through the cell, whereas on the vacuum side the pressure is 2.2 mbar.

Figure 2. Schematic diagram of flow cell with liquid pump.

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Continuous determination of volatile breakdown products

A teflon-coated Viton ring (4) tightly seals the system preventing any gas from interfering. The membrane is made of Tefzel, a copolymer of Polyethylene and Teflon. The type used has a mean pore radius of 0.05 µm, the thickness of the membrane of 60 µm, and porosity 60 %. EVAPORATION PROCESSES AT THE POROUS MEMBRANE AND MEASUREMENT OF SENSITIVITY

In the following, a model concept is developed which describes the transport of volatile substances, dissolved in water, from the flow cell through the porous membrane, into the vacuum system. CHANNEL FLOW

In the thin layer flow cell (height 0.2 mm), the hydrodynamics are similar to a 2 channel flow. The channel flow in electrochemical cells has been subject to intensive investigation over the last years. The conversion of electrochemical active substances dissolved in water on a channel electrode is comparable with the passage of volatile substances dissolved in water through the phase liquid/gas boundary. The volatile substances are transferred via the membrane into the vacuum system, whereby the concentration drops at the phase boundary. Subsequent transport then takes place from the interior of the solution. In the case of channel flow, a defined diffusion layer is formed. Apart from the substance-specific diffusion constant, the particle flow density is provided through the difference of the concentrations in the liquid interior (bulk) and at the phase boundary. The density of the diffusion layer depends on the third root of the flow rate:2 jLi = Kz D2/3 v1/3 (cBi - cPhi) jLi Kz D2/3 v1/3 cBi cPhi

particle flow density of the volatile species [mol/(min cm2)] cell constant (dependent on cell geometry) diffusion coefficient of the volatile species i [cm2s-1] flow rate of the moving solution [ml/min] bulk concentration of the species i in the solution [mol/L] concentration of the species i at the phase boundary [mol/L]

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FLOW RESISTANCE OF THE MEMBRANE

The evaporating water flows through the membrane into a fine vacuum of approximately two millibars. If the water vapor produced at the phase boundary is completely pumped off, approximately eight milliliters of water per hour and square centimeter of the membrane surface ought to evaporate, according to cal3 culation of the surface-related evaporation rate. In actual fact, however, only 2 th 0.25 ml/(h cm ) evaporates, i.e., less than 1/30 of the calculated quantity. This results in the following model: Directly at the liquid/gas phase boundary, a thin vapor layer is present of saturation pressure of water vapor (approx. 23 mbar at room temperature). Accordingly, a pressure gradient in a direction of a fine vacuum is present in the pores of the membrane. The membrane thus acts as a flow resistance for the water vapor flowing through its pores. On account of the small pore size, there exists a molecular flow of transported gas in the membrane, so that the gas particles are more frequently in contact with walls than other particles of gas. The flow is laminar in the fine vacuum of approx. 2 mbar behind the membrane. PARTIAL PRESSURES OF COMPONENTS IN THE VAPOR LAYER

The concentration of the volatile species at the phase boundary determines their partial pressure in the evaporation area. Raoult’s Law applies to the vapor pressure found above liquid mixture at high concentrations of the components: pA = xA PA pA xA PA

partial pressure of the component A [mbar] mol fraction of the component A in the solution saturation evaporation pressure of component A [mbar]

Accordingly, the partial pressure of the component A is proportional to its molar fraction in the solution and its vapor saturation pressure. Raoult’s Law only applies to ideal solution, i.e., the components of mixture must have chemical similarity and be present at sufficiently high concentration. The usual concentration of components of propellant breakdown are in the region of milligrams to micrograms per liter (relatively low concentration). In this case, Henry’s Law applies:

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Continuous determination of volatile breakdown products

pB = xB KH,B pB partial pressure of the component B [mbar] mol fraction of component B in the solution xB KH,B Henry’s constant of the component B [mbar].

Although the vapor pressure of the dissolved substance, at low concentration, is also proportional to the mol fraction but the proportionality constant is not. However, the vapor pressure of the pure substance is proportional to a substance-specific constant KH (with a dimension of the pressure). Henry’s Law describes a thermodynamic state of equilibrium and is only valid for closed systems in which a vapor medium (area) saturated with the components establishes itself. As described, there exists a practically saturated vapor layer of water (in vapor form) directly at the liquid/gas phase boundary. The flow resistance for the molecular flow in the membrane is, for the evaporated substances, such as gaseous aniline, comparable with that of water vapor. It increases with the root of the molar mass. In practice, the saturated vapor layer, almost equal to partial pressures, must then be formed for the volatile components as corresponds to Henry’s Law. The particle flow density of the volatile substances, from the solution in the vapor layer, occurs because of the low difference of the partial pressure, pH, in accordance with Henry’s Law, and the partial pressure pd, in the vapor layer. jdi = Kdi (pHi - pdi) jdi Kdi pHi pdi

particle flow density of the volatile species i from the phase boundary in the vapor phase [mol/(min cm2)] proportionality constant partial pressure of species i according to Henry Law [mbar] real partial pressure of species i [mbar]. FLOW IN THE MEMBRANE AND IN THE FINE VACUUM

The flow resistance of the membrane produces the difference between the partial pressures of the vapor layer, almost saturated with the components, and the partial pressures in the fine vacuum of the vacuum apparatus. The pump suction output of the vacuum pump (membrane pump) determines the total pressure in the fine vacuum. Thus, the vacuum pump also determines the degree of saturation of the vapor layer and in accordance with the partial pressures of the

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components in the vapor layer via the particle stream of the vapor layer through the membrane into a fine vacuum. The pressure gradient in the membrane determines the particle stream density of the volatile species: jvi = KPi(pdi - pvi) jvi KPi pdi pvi

particle flow density of the volatile species in the fine vacuum [mol/(min cm2)l guideline value of the membrane for the species i partial pressure of the volatile species in the almost saturated vapor layer [mbar] partial pressure of the volatile species in the fine vacuum [mbar].

Via a pressure converter, with an adjustable shutter/opening, a small part of the gas flows into the vacuum, necessary for the mass spectrometer, of ap-5 proximately 10 mbar. The mass spectrometer measures the partial pressure in high vacuum, proportional to the partial pressures in fine vacuum. Figure 3 shows the evaporation process of volatile substances dissolved in water through an idealized pore in the membrane.

Figure 3. Schematic diagram of the evaporation of volatile substances dissolved in water through a membrane pore.

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Continuous determination of volatile breakdown products

The liquid/gas phase boundary is formed in the membrane pores. According to Henry’s Law, a specific partial pressure of this substance is reached in the practically saturated vapor layer at the phase boundary corresponding to the concentration of the volatile species. STATIONARY CONDITIONS

In the stationary conditions, all particle flow densities are equal. The following equation applies: jLi = jdi = jvi Consequently, in the stationary conditions, the mass signal is also constant. In external conditions, such as flow rate and temperature of the solution, constantly maintained, the mass signal is directly proportional to the concentration of the volatile species in the solution. The height of the mass signal and/or 4 the limit of evidence is then provided by Henry’s Constant. APPLICATIONS

With the apparatus, aqueous solutions of standard hydrocarbons are measured. Table 1 shows the measured mass signals (always 10 µl/L). With the example of chloroform, trichloroethylene, and methylene chloride, it was possible to demonstrate that the apparatus gives linear readings.5 The limits for other substances were estimated under assumption of linearity and at the region of a few µg/L. With a detection limit of 1 mg/L, aniline shows a considerably lower measurable sensitivity. The diffusion coefficients of the substances listed only differ to a slight extent. With the exception of aniline, all carbohydrates listed possess similar Henry’s constants and thus similar limits of detection. Aniline is markedly adsorbed on the stainless steel wall in the high vacuum system. To obtain short response times and constant mass signals, the high vacuum recipient must be heated.

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Table 1: Mass signals of aqueous solutions of hydrocarbons (10 µl/l) and estimated detection limits

Concentration

Signal

Detection limit

mg/l

10-12 Å

µg/l

83

14.82

9.1

1

Trichloroethylene

95

14.70

14.0

1

Methylene chloride

49

13.36

18.0

1

Methyleneisobutylketone (2-methyl-1-pentanone-4)

43

8.01

7.3

1

Diethylether

31

7.14

2.1

4

Ethyl acetate

43

9.01

8.5

1

Toluene

91

8.72

13.0