Packed Bed Columns: For absorption, desorption, rectification and direct heat transfer

Preface Separation processes in gas (vapour) - liquid systems, like absorption, desorption and rectification, are estimated to account for 40%-70% of both capital and operating costs in process industry [1]. A significant part of the costs are connected with the packed bed columns used for these processes. The employment of these apparatuses also for direct heat transfer between gas and liquid, including utilization of waste heat from flue gases, enlarges their importance. Packed bed columns are the best type of apparatuses, from thermodynamical point of view, for carrying out of mass and heat transfer processes between gas and liquid phase. It is because of all types of highly effective apparatuses, they operate as near as possible to the conditions of countercurrent flow, i.e., at maximum driving force for given initial and end concentrations of the two phases and a given ratio between their flow rates. The history of packed bed columns has begun with absorption and desorption processes. The first apparatus of this type was proposed by Gay-Lussac in 1827 who at this time was a consulting chemist of Saint Gobain Company [2]. The column, called by his name, was designed to absorb NOX after the nitrosyl method for production of sulphuric acid. 10 years later, filled up with coke, it was implement in industry. After 1980 a gradual displacement of the tray-type contactors by packet columns began in the distillation plants, and until 1990 it was considered the greatest novelty in the area of distillation [42]. Because of high efficiency, low pressure drop, and high admissible flow rates, packed bed columns are now largely used in chemical and food industry, environmental protection and also for carrying out of some processes in thermal power stations such as water purification, flue gas heat utilization and SO 2 removal. This book is intended for chemical and heat engineers working in these industries and also especially for chemical engineers and scientists working on development of new technologies which include absorption, desorption, distillation, and direct heat transfer processes in gas-liquid systems. It is especially proper for designers of such types of apparatuses.

N. Kolev

Vll

Acknowledgements My thanks to the Alexander von Humboldt foundation for supporting my scientific carrier and especially for the financial support during the writing of this book. Many thanks to Dr. Daniela Dzhonova-Atanasova for her assistance, language correction and discussions. I am grateful to my colleagues Prof. Reinhard Billet from the Ruhr-University of Bochum and Prof. Johann Stichlmair from the Technical University of Munich for the cooperation in the area of packed bed columns. The results of these works are used in this book. My thanks also to my former PhD students Prof. Rumen Daraktchiev, Assoc. Professors Krum Semkov and Svetoslav Nakov, Dr. Dimitar Kolev and Dr. Elena Razkazova-Velkova, to Dr. Daniela Dzhonova-Atanasova, and to my PhD student Borislav Kralev for the long and useful cooperation in the development of the theory and practice of packed bed columns. Many of the results of this cooperation have been used in writing the book. I would like to thank Dr. Michael Schultes from Raschig Ltd and Dr. Lothar Spiegel from Sulzer Chemtech Ltd for the technical information about the packings and column internals of these companies which was very useful. I am also grateful to Dr. Dimitar Kolev, Dr. Elena Razkazova-Velkova and Borislav Kralev for helping me with this book.

IX

About the author Prof. Kolev is leading the laboratory of "Heat and mass transfer processes in gas-liquid systems" at the Institute of Chemical Engineering of the Bulgarian Academy of Sciences. He was born in 1933. In 1957 he graduated from the University of Chemical Technology and Metallurgy in Sofia. For two years he worked as shift engineer in a plant for ammonia production. From 1959 to 1962 he was an assistant professor at the Department of Chemical Engineering of the same University. His Ph.D. thesis (1967) is about the influence of surface-active agents on hydrodynamics and mass-transfer in packed bed columns. His D.Sc. thesis (1980) is "About some basic problems of chemical engineering in creation of highly effective packed bed columns". In 1970 he became an associate professor. Since 1982 he has been a full professor. From 1967 to 1969 he specialized at Prof. Brauer's institute at the Technical University-Berlin, funded by the Foundation Alexander von Humboldt. He has published about 128 papers and has 51 patented inventions, most of them in the area of packed bed columns. On the basis of his patented inventions about uniform liquid phase distribution over the whole cross-section of the apparatus, and by using his own mathematical model, he succeeded to introduce in industry new own packings and new processes without any pilot plant investigations for a given system. His team has developed 18 types of installations for different processes, such as absorption, desorption, rectification and direct heating. The whole number of his apparatuses, operating in chemical industry, food industry, power stations, and environment protection, is over 750.

Table of Contents Chapter 1. Basic Information Chapter 2. Investigation of the Main Performance Characteristics of Packed Bed Columns Chapter 3. Industrial Packings Chapter 4. Marangoni Effect and its Influence on the Mass Transfer in Packings Chapter 5. Mass Transfer in Packed Bed Columns Accompanied by Chemical Reaction Chapter 6. Fouling on Packings Chapter 7. Column Internals Chapter 8. Distribution of the Liquid and Gas Phase over the Cross-Section of a Packed Bed Column Chapter 9. Examples

Chapter 1

Basic information 1.1. Basic information about packed bed columns Up to now more than 5000 papers and patents connected with packed bed columns have been published. Some of the best books related to these apparatuses are given in the references [3-10], 1.1.1. Short description of a packed bed column A principle scheme of a packed bed column is presented in Fig. 1 [4]. The apparatus consists of the body 1, in a cylindrical or parallelepiped form, in which on the support grid 3 the packing 2 is installed.

4

1

y

/\

/%

/\ 2

5

/A \

3

i i i i i i r

3

Fig. 1. Principle scheme of a packed bed column

A distributor 4 for the liquid phase is mounted over the packing. The gas phase enters the apparatus trough pipeline 5, it flows trough the packing 2,

and exits the column through pipeline 6. In the packing the gas is treated with liquid, mainly in a eountercurrent flow. The last enters trough pipe 7 in the distributor 4 distributes over the packing and leaves the column through pipe 8. The distillation (rectification) columns are more complicated. A principle construction of these apparatuses is given in Fig, 1 of Chapter 7. /. 1.2. Some terms largely used in the field of packed bed columns A packing has three important geometrical characteristics: size dp, specific surface area a, and void fraction (free volume) e. The specific surface area of the packing is its area related to 1 m3 of the packing volume. The packing void fraction is the volume of the free space of the packing related also to 1 m of its volume. Obviously, the value of the packing specific area is equal to the value of the sum of the perimeters of the channels formed in the packing related to 1 m2 cross-section, and that value of e is equal to the cross-section of these channels related also to 1 m2 of the cross-section of the packing. Another important value is the equivalent (hydraulic) diameter of the packing dh. There are two different possibilities to define this value. The first of them comes from the consideration of the packing structure in terms of the so called inner problem, i.e. when the packing is considered as a system of connected channels with different dimensions. The equivalent diameter by this model is equal to the average hydraulic diameter of these channels which is equal to 4 times the hydraulic radius, or: dh = 4efa.

(1)

E is the packing void fraction in m3/m3, and a -the specific packing area in m2/m3. The second model for defining the equivalent diameter is the so called outside problem. By this model the equivalent diameter is determined as a diameter of a spherical packing element ensuring the same values of specific surface and void fraction as those of the real packing. To distinguish the two equivalent diameters, the second one is marked later as Da and is usually called arithmetical. Simple calculations show that it is determined by the equation

Da=6(l-e)/a

(2)

The great disadvantage of the second definition is that the hypothetical spheres with the arithmetical diameter are quite smaller than the real packing

elements and are not touching each other. To make the equations with the arithmetical diameter useful, in the presented equations the arithmetical £

diameter is multiplied additionally by

. Thus, an equation with equivalent 1-e and with arithmetical diameter gives practically the same results by compensating the difference between them with a constant.

D

1-e

a

1-e

a

The effective surface area of the packing ae, through which the mass transfer process takes place, is the wetted area of the packing aw and also the surface of the liquid drops and jets trickling through the free space of the packing. In case of small packing elements, especially at very high liquid loading, it is possible that the area between the gas and the liquid phase is smaller than the wetted area of the packing, i.e. it is possible that

The dimension of ae and a** is m2/m3. The liquid superficial velocity L in m3/(m2s) is the liquid flow rate for the whole column divided by its whole cross-section. The liquid holdup Hf, is the volume of the liquid in the packing related to the whole packing volume. Its value is important for strength calculation of the support grid of the column, and especially for determination of the residence time for the liquid phase, a value very important for thermo-unstable liquids. The total liquid holdup Hi, consists of two components: static and dynamic liquid holdups. The static holdup Hs is mis part of the holdup which remains in the packing because of the capillary forces when the irrigation is stopped. It depends on the wettability of the packing, the surface tension and density of the liquid and increases with the packing specific surface. The dynamic holdup Ha is the difference between total and static holdup of the packing. It is the liquid in the packing held by the resistance forces. In case of large industrial packings, and they are more important, the value of the static holdup is to be neglected. The dynamic holdup increases with the liquid superficial velocity and packing surface, and decreases with increasing of the liquid density.

The gas velocity, or to be precise, the superficial gas velocity wo, in m/s, is the average value of the gas velocity defined as volumetric gas flow rate related to 1 m2 of the whole cross-section of the column. The real gas velocity w in the dry packing is calculated with the real cross-section for the gas flow equal to e, that is

(4)

Because the cross-section of the irrigated packing is smaller than that of the dry one with the holdup, the real gas velocity in this case is calculated by:

(5)

1-B-H k

An important hydrodynamic parameter of the packed bed column is the packing pressure drop AP equal to the difference between the pressures at inlet and outlet of the packing. To characterize the packing, the pressure drop related to a unit of packing height is used. The concentrations of the phases are given in two principle ways, in kg/m3 (kmol/m3) and kg/kg (mol/mol). In the first, the concentrations are denoted by CQ (for the gas phase) and by Q (for the liquid). In the second, they are denoted by Fand Xalso for the gas and for the liquid phase. The rate of mass transfer is given by the equation: WA = KG .FACG = Kt .FACt,

(6)

or WA = KY .FAY = Kx .FAX,

(7)

where WA is the quantity of substance transferred through the interface per unit of time in kg/(m2s) or kmol/(m2s); ACG in kg/m3 (or kmol/m3) and AY in mol/mol - driving force of the mass transfer process calculated as a difference between the concentration of the transferred component in the bulk of the gas phase and the equilibrium

concentration of the same component corresponding to its concentration in the bulk of the liquid; ACL in kg/m3 (or kmol/m3) and AX in mol/mol - driving force of the mass transfer process calculated as a difference between the concentration of the transferred component in the bulk of the liquid phase and the equilibrium concentration of the same component corresponding to its concentration in the bulk of the gas; .KG and Ky- overall mass transfer coefficients, when calculating the driving force with the concentration in the gas phase in m/s and mol/(m2s) respectively; Ki and Kjr overall mass transfer coefficients, when calculating the driving force with the concentration in the liquid phase in m/s and in mol/(m2s) respectively; F- interfacial mass transfer area, m29 calculated by the equation; (8)

Vp is the volume of the packing in m3. The methods for calculating the overall mass transfer coefficients and the driving forces are considered below. The products KGa = KG.ae,

(9)

KYa = KY.ae,

(10)

respectively KLa = KLja9,

(11)

Kxa = Kx.ae,

(12)

are called volumetric mass transfer coefficients for the gas (vapour), and for the liquid phase. 1.1,3. Hydrodynamic regimes of packed bed columns The liquid phase influences the hydrodynamics of the gas phase in two different ways. The first of them is by the liquid holdup, according to Eq. (5) it leads to increasing of the gas velocity. The second one is because the pressure

drop is connected directly not with the gas velocity, but with its relative value equal to the gas velocity w» only in case of dry packing, In case of irrigated packing, it is the algebraic sum of the gas velocity and the liquid velocity at the contact surface with the gas phase. That is why there is a difference in the influence of the liquid phase on the gas hydrodynamics for co-current and countercurrent flow. Because usually the gas velocity is many times greater than the liquid one, in many cases the second effect can be neglected. In Fig. 2 in double logarithmic coordinates, three typical lines for the packing pressure drop AP versus the gas velocity Wo are presented.

logw. Fig. 2, The influence of the gas velocity on the pressure drop of the packing in double logarithmic coordinates: 1- not irrigated packing; 2- irrigated packing- co- current flow; 3- irrigated packingcountereurrent flow.

The first of them is for L=0, the second two - for irrigated packing with constant value of liquid superficial velocity. Lines 2 and 3 are respectively for co-current and countercurrent flow. In a wide range of w0, between points A and B, (Fig. 2), the lines for dry and irrigated packing are parallel. The main influence of the liquid phase in this region is trough covering of a part of the free cross-section of the packing with liquid. The effect of friction between gas and liquid on the liquid holdup is to be neglected in comparison to the gravity force. That is why the liquid holdup in this regime is practically constant, independent of the gas phase velocity. It means that the real cross-section for the gas phase between points A and B is

constant at constant liquid superficial velocity and the lines for dry and irrigated packing in double logarithmic coordinates are parallel. The lines for co-current and countercurrent flow coincide. Under point A the gas velocity is very low and the influence of the liquid velocity on the relative gas velocity cannot be neglected. In case of countercurrent flow the effect of the liquid superficial velocity leads to significant increasing of the relative gas velocity and, that is why, to increasing of the ratio of the pressure drop of irrigated packing to the pressure drop of dry packing. The increasing of the liquid superficial velocity leads to increasing of this ratio. The opposite effect is observed at a co-current flow. Over point B the influence of the friction force between gas and liquid in case of countercurrent flow leads to increasing of the liquid holdup, i.e. to additional increasing of the real gas velocity and of the pressure drop. That is why after point B, called loading point, the slope of the line for irrigated packing is greater than that for dry one. Just on the contrary, in case of cocurrent flow, the friction between gas and liquid acts in the gravity direction, i.e. in direction of decreasing of the holdup. That is why the slope of the line for irrigated packing in this case is lower than that for dry one. The increasing of the liquid holdup with the gas velocity for countercurrent flow, and the increasing of the gas velocity with increasing of the liquid holdup lead to additional increasing of the slope of the lines for the pressure drop. For countercurrent flow in point C, called flooding point, this leads to transition of the disperse phase into disperse medium and vice versa. In case of co-current flow it leads to additional decreasing of the line slope in comparison to that for dry packing. As already mentioned, the greatest advantage of the packed bed column is the possibility for carrying out the processes in a countercurrent flow. The flow rates in case of a co-current flow, as expected and seen from Fig. 2, are not limited by loading and flooding points. But this regime is not proper in case of equilibrium because of the strong reduction of the driving force. However, in industry there are a lot of processes for which the equilibrium is not of importance. Just for these processes the co-current packed bed columns are very proper. The maximal gas and liquid superficial velocity for these apparatuses are limited only by economical considerations, namely by the strong increasing of the pressure drop when increasing the gas velocity. Besides the countercurrent and co-current flow regimes, the packed bed column can operate in a crossflow. A principle scheme of such a column [41] is presented in Fig. 3.

«

liquid flow




The scale multipliers being constants are taken before the differentiation operator as follows: ^

^ . a. at m

(96)

X

From Eqs, (95) and (96) it is obtained

a. arCl=-J-^

=L

(97)

By replacing in the similarity indicator (C») the scale multipliers with the respective ratios of physical values, we obtain: ft

(98)

mw where the index (1) is for the similar system. The comparison between Eqs. (88) and (98) shows that the obtained ratio is the Newton number, 1,3.2. Dimensional analysis As already mentioned the similarity theory gives the possibility to obtain equations for calculation of a given class of phenomena based on differential equations. Unfortunately, in industrial practice there are many cases, especially the more complicated, for which it is impossible or very difficult to write these equations. The theory of dimensional analysis helps to solve the problem in these cases. For this analysis it is necessary to know only what

33

parameters influence the value to be calculated. For example we know that the partial heat transfer coefficient or is a function of the following parameters: (99) The dimensional analysis is a very powerful method not only to find the form of the necessary equation, but also to give information about the limits of its validity. The basic theorem of the dimensional analysis is the n theorem of Buckingham which states that the general dependence between » values at m basic dimension units can be presented as a function of (M-»I) dimensionless ratios of these values, and in case of similarity, of (n-m) criteria of similarity. If for example a given phenomenon can be described as a function of five values

f(a,fl,r,T,fi). and if these values are expressed using e.g. 3 basic units, £-for size, T- for time, and M- for mass, based on the n theorem the number N of the dimensionless ratios is N=m-n-=5-3=2.

I.e., the necessary equation can be presented in the form of a function of two terms.

A simpler example of the application of dimensional analysis can be given in case of 4 values with 3 basic units.

f(a,p,Y.t),

(100)

or a = f1(j3,y,T),

(101)

34

when the described value is a. According to the JT theorem the number of the dimensionless ratios is N=3-2=L If all values are expressed in one system of measurements, for example the international system of units (SI), the dimensionless form of a can be presented as yy.T') ,

(102)

where x, y and z are the powers at the respective values. Later it is shown how to determine them. Let the dimensions of the values in Eq. (102) be respectively: [aJ = [LaTbMcJ

(103)

= [LdTeMf]

(104)

= [LsThMiJ

(105)

[Tj = [LaT"Mrj

(106)

From Eqs. (101) to (106) it follows [IfTbMc] = [L

(257)

72

It is easy to see that the Bodenstein numbers can be obtained by multiplying the corresponding Peclet numbers by H/dp. In fact the piston flow model, as well as the diffusion model, gives too ideal picture of the structure of the flows. They take into account neither the diffusion boundary layer nor the real movement of the phases. These models are especially far from the real situation in the apparatus in respect to the liquid phase which moves not like a piston flow but in the form of film, drops and jets which are not only separate in space but have also different and continuously changing velocities. Nevertheless, not only the diffusion model but in some cases also its simpler variant, the piston flow model, gives often very good description of the mass transfer processes in industrial apparatuses. This can be explained with the comparatively weak influence of the real structure of the flows on the mass transfer. On the other side using in the model such experimentally obtained values as mass transfer coefficient, effective surface, and Peclet number, it is possible to take into account the important for the mass transfer rate characteristics of the flows structures. In Chapter 8 the cases when it is possible to use the simpler piston flow model, and when it is necessary to use the diffusion model are considered and specified. Theoretically it is possible to write the differential equations for the mass transfer in case of a nonuniform distribution of the phases over the column cross-section. As boundary conditions in this case also the initial distribution over the column cross-section for both of the phases and the conditions on the column wall should be given. Of course, the equations for calculating the Bodenstein or Peclet numbers in radial direction have to be also known. The determination of the necessary additional conditions in comparison to the simpler model, Eqs. (250) to (257), is very complicated and together with the difficulties to solve the system make the whole model unusable. Nevertheless, in literature there are some investigations on calculating the Peclet number in radial direction. 1.6. Principle types of equations for calculation of the performance characteristics of the packing The similarity theory and the dimensional analysis give the possibility to obtain equations for calculation of the performance characteristics of the packings in dimensionless form. Each equation has two important characteristics, the type of the function and the experimental constants. The complete equations, with their experimental constants are presented in Chapter 3 together with the description of the respective packings. Besides the dimensionless equations valid for different dimensions and often for different types of packings, there are a lot of equations with experimental constants valid not only for one packing type but also only for one

73

packing size. Such equations are also presented in Chapter 3. Usually their precision is higher than that of the equations of wider validity. Their disadvantage is that, especially in case of random packings, such very important values as specific surface area and void fraction depend not only on the packing construction and dimensions but also on the dumping of the packing in the column. That is why their constants, at which they have their higher precision, are obtained not only for a given size of the elements, but also for a given specific surface area and void fraction. It is well known, for example, that the pressure drop of random packings varied from experiment to experiment about 10% only because of the refilling of the packing. Without refilling the difference in parallel experiments, at least under the loading point, is not more than 1%. Since there is information general for the most of the dimensionless equations for one performance characteristic of different packing types, it is useful to gather this information in one place which is done in this part of the book. 1.6.1. Pressure drop 1.6.1.1. Pressure drop of dry packing The most of the equations for calculation of the pressure drop of irrigated packings need knowledge for the pressure drop of dry one. The main form of the equation is:

W = f(ReG,eJi.JJ,

(258)

where y/ = —-—^—r- is the dimensionless pressure drop, often called Euler H.pG.w0 number (Eu); ReG = W°" A"^° - Reynolds number; pG - gas density in kg/m3; juG - dynamic gas viscosity in Pa.s; e - free column cross-section, equal to the free volume of the packing; //.../„- dimensionless geometrical parameters of the packing. In some cases to Eq. (258) as a dimensionless geometrical parameter, the so called way factor (Fi) offered by Kast [28] can be added. This factor is

74

experimentally determined to take into account the different way for the gas phase due to different form of the packings. As geometrical factors for some structured packings also the maximal and the minimal free cross-section can be also used. 1.6.1.2. Pressure drop of irrigated packing The irrigated liquid affects the pressure drop as follows: 1. By occupying a part of the free cross-section of the packing and thus increasing the gas velocity; 2. By smoothing the rough surface of the packing or its edges; 3. Due to the velocity of its free surface, it increases the relative gas velocity essential for the pressure drop. It is practically impossible to take into account in the form of the equation all the above mentioned phenomena. That is why the equations take directly into account only the first one. The rest of them influence through the proper determined experimental constants. Using a simple model of irrigated parallel tubes Zhavoronkov, Aerov and Umnik [29] offered the equation:

AP=

**" . .

(259)

(1-Af where A is a dimensionless value related to the liquid holdup. Theoretically, when having one vertical pipe with surface folly wetted by a film of equal thickness, the value of A is equal to the liquid holdup. A can be presented formally [30] as a sum of two values: A = A0+M,

(260)

where Ag is the value of A under the loading point and AA - the increasing of A over the loading point. The types of equations for describing Ao and AA [30,31] are: Ao = f(ReL,FyL,Ed,e,ll...JJt

(261)

M = f(-^-,^^,^1Eo,sJ,.Jn),

(262)

gdh

g

L

75

where ReL =

4L

is the Reynolds number for the liquid phase;

a.vL FrL = — : — Froude number for the liquid phase; g Ed=—*z— -EStvos number;

a a

pL - liquid phase density in kg/m3; vh - liquid phase kinematic viscosity in m2/s; In equation (262) the viscosities of both phases are omitted because at developed turbulence the effect of the viscous forces is negligible. In case of a fully wetted packing, like some packings with vertical walls, Ed can also be omitted. 1.6.2. Liquid holdup As already mentioned the value of A is closely connected with the liquid holdup. The last can also be divided, some formally, into two parts; Hd=Hd0+AHd,

(263)

where Hat is the holdup under the loading point and AHd - its increase over this point because of the forces between the gas and the liquid phase. The equations for these two parts are similar to the equations for A and Ad, namely: Hd0 = f(ReL,FrL.E6,e,lr.JJ, — f(

1

~,——iEd,e,l,....L).

(264) (265)

1.6,3. Effective staface area The effective surface area ae is the surface of the interface between the gas and the liquid phases per 1 m3 of the packing volume. It is a sum of the effective wetted surface of the packing and the surface of the jets and drops

76

trickling in the free volume. According to the dimensional analysis, under the loading point it can be presented as follows:

L,Rel,Ed,e0tl1.JJ

,

(266)

a where Bg is the angle of wettability of the packing material, called also contact angle. The influence of three important physical parameters on the effective area of the packing are discussed below. They are the wettability of the packing material expressed through the angle of wettability, the liquid surface tension, and the liquid viscosity. 1.6.3.1, Influence of the contact angle ofwettability The forces responsible for the holding of the liquid phase on the surface of the packing are the intermolecular adhesion forces. Their effect can be measured through the angle ofwettability presented in Fig. 12.

solid Fig. 12. Angle ofwettability

Upon contact of three different phases, solid, liquid and gas (or vapour), on the phase boundary line, three different boundary surface tensions act. They are: 1. Boundary surface tension between the liquid and the solid phase aLS, 2. Boundary surface tension between the liquid and the gas phase c ^ , 3. Boundary surface tension between the solid and the gas phase crSG. These three boundary surface tensions, the values of which are dependent on the chemical properties of the phases, the temperature, and the pressure, are in equilibrium according to the equation of Young [49]. °~m = ^LS +cm

B

e®w

(267)

77

The angle of wettability varied from zero (fully wetted) to 90° (not wetted material). The difficult measuring of this angle, dependent on the packing material and its preliminary treating, and also on the gas and liquid phase properties, is the reason why, in many equations in literature for calculating the effective packing area, this angle is not taken into account. The other reason is that this angle is partially related to the liquid surface tension, Eq. (267). As already mentioned the wettability is connected with the intermolecular forces. A simple rule of chemistry says that the similar dissolves in a similar. The rule can be expanded for wettability. I.e., if the packing material is a not polar one, it is better wetted by not polar liquids, and vice versa. For example, good results are achieved in improving the wettability of a plastic packing by treating with oxidants. Moreover, it is well known that after some time of operation the wettability of a plastic packing improves. The reason is the chemical reaction on its surface. The investigation of Kolev [47], carried out with a very wettable ceramic packing and PVC Raschig and Pall rings which have been under the influence of atmospheric air for a long time, has achieved a good coincidence with the data for all packings without using the angle of wettability in the equations. All 16 different packings (Raschig rings, Pall rings, Intalox saddles and shorten rings) with sizes from 15 to 50 mm, made of ceramic and PVC, obtained by 4 authors, have good agreement with the obtained correlation which does not include the angle of wettability. Only the data of Danckwerts and Rizvi [48], obtained for polypropylene newly produced Intalox saddles not preliminary treated, show quite smaller effective area than the predicted one. The reason is that the difference in the angle of wettability is not taken into account. Besides the contact angle, defined by the equation of Young which is not depending on the surface geometry, there is another contact angle which is depending on it and which can be measured in the same way. This angle is responsible for the wettability of the surface important for the effective area of the packing. The existing of this second angle of wettability is connected with the following. Let us assume that the surface is covered with small sharp channels, Fig. 13, with width and distance between them quite less than the dimension of the liquid drop. When the channels are crossed, the liquid phase, fallen on the surface, spreads in the channels thus wetting the whole surface. As it is easy to see from Fig. 13 (a), the real value of the angle of wettability, in relation to the horizontal plane, is less than zero, i.e., the material is fully wetted. There are different ways to obtain small crossing channel structure on the packing surface,

78

for example treating with sand or using of sintered porous materials. Especially proper is the sintered PVC [51-53].

(b)

Fig. 13. Filling up with liquid of the channels on a solid surface; (a) single channel; (b) many channels.

In details the problem of wettability of materials with rough (textured) surfaces is investigated by Palzer et al. [50]. 1.6.3.2. Influence of the swfaee tension The liquid surface tension influences the effective area of the packing through its effect on the contact angle, Eq. (267), and also directly on the surface of drops and jets trickling in the free volume of the packing. It is clear that the reduction of the surface tension leads to increasing of the effective packing area. 1.6.3.3. Influence of the viscosity The forces which are responsible for the holding of the liquid phase on the surface of the packing are the intermolecular adhesion forces. That is why there is no physical reason to expect any direct effect of the liquid viscosity, at least under the loading point. Nevertheless, there are a great number of equations in which the effective surface is presented as a function only of ReL, assuming that the effect of viscosity is equal to that of the liquid superficial velocity. Moreover, according to this type of equations the increasing of the viscosity leads to reduction of the effective surface. By simple physical considerations, it is to be expected that the increasing of the dynamic holdup can lead to some increasing of the wetted and also of the effective surface area. Because the two values are not proportional and the liquid holdup depends weakly on the viscosity, the expected effect is very small being of the experimental error magnitude for this type of investigations. In case of small packing elements, there is a reason to expect also some reduction of the effective surface area with increasing of the viscosity. It is because of some reduction of the ratio of wetted packing area to

79

the gas-liquid interfacial area with reduction of the packing size and at high liquid superficial velocity. As already mentioned, there are a lot of experimental equations according to which the effective surface is presented as a function only of Rei. But there is not any investigation with changing of the liquid viscosity, where this is experimentally proved. It is noteworthy that these wrong equations are a result of an incorrect use of dimensional analysis. 1.6.4, Partial mass transfer coefficients The similarity theory and the dimensional analysis give the following types of equations for the partial mass transfer coefficients; for gas phase controlled processes ShG=f(ReG,ScGllJJ,

(268)

for liquid phase controlled processes

ScJ^lJ,

where: ShG =

(269)

kGd * - Sherwood number for the gas phase; DG

kd ShL = —-— Sherwood number for the liquid phase; «.d3 GaL =^—z—the number of Galilei; v

i

ScG — —— - Schmidt number for the gas phase; ScL = —^- - Schmidt number for the liquid phase; Li

vL - the gas kinematic viscosity; d- the diameter of the packing elements or other characteristic geometrical value of the packing in m.

80

1.6.5. Peclet numbers As a geometrical characteristic in Bodenstein numbers, used by the diffusion model (Eqs.(250) to (255)), the height of the packing is employed. Because the axial mixing is a hydrodynamic process, the axial mixing coefficients are functions of the geometrical parameters of the packing, important for the hydrodynamics, like its specific surface area, but not its height. That is why for the dimensionless equations taking into account the axial mixing coefficients, the Bodenstein number is not proper. Instead of it, similar number called Peclet number is used. It can be obtained for the gas and for the liquid phases as follows:

Per =Bor

= aH

PeT =Bo, L aH

=

^ r , e(l-Hd).a.Dc r• Hd.a.DL

(270)

(271)

The dimensional analysis gives the following types of equations for determination of the Peclet numbers: for the gas phase 2

Per =f(Re,,Rer,Gar,——,—,£,l,..L),

(272)

for the liquid phase f

.

(273)

The equation for the gas phase is especially complicated because it should reflect the influence not only of the gas hydrodynamics, but also of the liquid phase. Because of the small effect of the axial mixing in the gas phase on the mass transfer in the packing and the great experimentel difficulties in this area, the number of investigations in M s field is insufficient. The problem is discussed in details in Chapter 3.

81

To obtain equations for calculating of industrial apparatuses using the similarity theory and the dimensional analysis theory we should have reliable experimental data for any given value. The methods for investigation of the packings to obtain the necessary data are considered in Chapter 2. 1.7. About the possibility of purely theoretical calculation of the performance characteristics of a packed column The development of the computational methods and the computers put the question whether the time of a calculation without any experiment of the performance characteristics of the packed bed column is already coming, or whether it is possible to come. Some circumstances in this area are discussed below. The first step of any model of this type is to make a mathematical model of the packing form. For random packings this problem can be solved only as a statistical task. It is impossible to know for sure whether this statistical description of the form of the packing reflects the real hydrodynamic and mass transfer situation in the column, especially having in mind the great capital investments for an industrial column. In case of some types of structured packings, especially these with smooth walls, the mathematical description of the packing form is easier. But in all cases the industrial packings operate at gas velocity for which the gas flow is turbulent. To be more precise, they operate in an area where the influence of Reo is not to be neglected, and it cannot be described theoretically without any experimentally obtained constants. The prediction of the hydrodynamics of the liquid phase is some easier when the wetted area of the packing is known, i.e. again in case of smooth packings with vertical walls. The determination of the surface of the drops and jets, trickling in the packing, without any experiment is also a problem with no solution, at least until now. At the same time the similarity theory and the dimensional analysis, as already mentioned, give the possibility, by using experimental data, to obtain dimensionless equations which are solutions of the differential equations of the corresponding processes. Nevertheless, in the literature there are purely theoretical solutions [51] of the problems. In all eases the obtained results should be validated by comparison with independent experimental data. Nomenclature Latin A- dimensionless number in Eq (259) related to the liquid holdup;

82

Ar value of A under the loading point; a- specific packing area, m2/m3; ae- effective (interface) area, m2/m3; att =

coefficient of temperature conductivity, m2/s;

c.p a,- dimensionless parameter, constant of similarity or scale multiplier; a*,- wetted packing area m2/m3; aT - similarity constant for time; BoG = BoL =

"w U

e(l-Hd).DG

; —Bodenstein

number for the gas phase;

r - Bodenstein number for the liquid phase;

Hd.DL C- concentration, kg/m3 or kmol/m3; CG - concentration of the gas phase, kg/m3 or kmol/m3; CGiH- concentration of the gas phase, kg/nm3inert gas or kmol/nm3inert gas; CL - concentration of the liquid phase, kg/m3 or kmol/m3; CLb - concentration of the absorbed component in the flow bulk, kmol/m3; CG - equilibrium concentration of the gas phase, kg/m3 or kmol/m3; CL - equilibrium concentration of the liquid phase, kg/m3 or kmol/m3; Cr indicator of similarity; Or constant given in Table 1; Co*- equilibrium concentration of the transferred component in the gas, for example kg/m3; CL- equilibrium concentration of the transferred component in the liquid phase for example in kg/m3; Cr coefficient of radiation of grey material, W/m2; 0.5

Cs =

—-—JJ - capacity factor to account for the effect of vapour and

liquid density, m/s', cL - liquid heat capacity, J/(kg.K); cp - gas heat capacity at constant pressure, J/(kg.K); D- diffusivity, m2/s; DAB- diffusivity in system A-B m2/s; DQ- diffusivity for the gas phase, m2/s;

83

Di- diffusivity for the liquid phase, m2/s; DG - axial mixing coefficient for the gas phase, m2/s; DL - axial mixing coefficient for the liquid phase in m2/s; Da = 6(1 — e) / a - arithmetical hydraulic diameter of the packing, m; d- diameter of the packing elements or other characteristic geometrical value of the packing, m; dt - bubble diameter, m; de = 4 e/a- equivalent (hydraulic) packing diameter, m; dp- packing size, m; dt temperature gradient, K/m; dn -Edtvos number; Ed=z£iJL a a F - mass transfer area in m2; FrL = — : — Froude number for the liquid phase; g FG = WQ^PG - vapour (gas) capacity factor, without taking into account the liquid density, kg^m'^s" 1 ; F"w-waJ—

vapour (gas) capacity factor, taking into account the

liquid density, m/s; / = se~s0 - function depending on the distribution of the contact time; G - mass of the transferred substance, kg/m2s or mol /m 2 s; GaL = — z — Galilei number for the liquid phase;

n

Gj- mol flow of liquid phase, kmol/(m2.s); Gr- mol flow of gas phase, kmol/(m2.s); Gg — mass flow rates of the gas phase, kg/s; Gm - mol flow rates of the gas phase, kmol/s; Gm - mol velocities of the gas phase, kmol. m"2. s"1; Gk - mass flow of the dry gas - kg/(m2.s); g- gravity constant, m/s ; H- packing height, m;

84

HTUX =

—— height of one mass transfer unite, calculated for the liquid

phase, m; HTUY = — - — height of one mass transfer unite, m; KrMe Hj - dynamic liquid holdup, m3/m3; Hat - liquid holdup under the loading point, m3/m3; Hk - liquid holdup of the packing, m /m3; Hs - static liquid holdup of the packing, mVm3; He- Henry constant, Pa; HETP - height equivalent to one theoretical stage (theoretical plate); HETPx - height equivalent to one theoretical stage {theoretical plate) for the liquid phase; HETPY - height equivalent to one theoretical stage (theoretical plate) for the gas phase; Hh - liquid holdup, m3/m3; Hs - static liquid holdup m3/m3; h- packing height, current coordinate, m; a - index for the absorbed component; K- overall heat transfer coefficient, W/(m2 °C); KG- overall mass transfer coefficient for the gas phase, m/s; Ky overall mass transfer coefficient for the gas phase, kg/(s,m2) or kmol/(s.m2); KGa = KG.ae - volumetric mass transfer coefficient for the gas phase, 1/s; Ki~ overall mass transfer coefficients for the liquid phase, m/s; Kx- overall mass transfer coefficients for the liquid phase, kg/(s.m2) or kmol/(s.m2); KLa = KL.ae - volumetric mass transfer coefficient for the liquid phase, 1/s; Kg- coefficient of radiation of an absolutely black body; her partial mass transfer coefficient, for the gas phase, m/s; kL- partial mass transfer coefficient for liquid phase, m/s; L- liquid, superficial velocity in m3/(m2s); £/, L2, L3 Lm - geometrical sizes, m; Lg - flow rates of the liquid phase in kg/s; Zga, - mol flow rates of the liquid phase in kmol/s; Lm - mol velocities of the liquid phase, kmol m"2. s"1; la - ratio between the molar flow rates of the liquid and of the gas phase; //, I2,13, 4-geometrical sizes, m; ML -mol mass of the liquid phase;

85

MOG and Mai- average mol mass of the gas and of the liquid phase in kg/kmol. m = He/P - equilibrium constant; mc- equilibrium coefficient given with the equation me= CG I C £ ; N- number of kmols in 1 m3 liquid and number of absorbed components; NA- mass flux of substance A, transferred for unit of time trough unit of interface in kg/(m2s) or kmol/(m2s); Ndm - average mol flux of the component A, kmol/m .s; Ne- criterion of Newton; X ~X NTfJm = — — - overall number of mass transfer units defined with the concentration in the liquid phase; - overall number of mass transfer units for the gas phase; NTP- number of theoretical stages (theoretical plates); Y —Y NTUfe = — — - number of mass transfer units for one theoretical stage for the gas phase; 'xc= — 2 T^7—~ number of mass transfer units for one theoretical stage for the liquid phase; P- pressure, Pa, bar: PA -equilibrium pressure of the pure component A, Pa, bar; Pg -equilibrium pressure of the pure component B, Pa, bar; p A - equilibrium partial pressure of absorbed component A, Pa, bar; p B - equilibrium partial pressure of absorbed component B, Pa, bar, PeG — BoG

= aH

PeL = BoL

=

p -Peclet number for the gas phase; e(l-Hd).a.DG r - Peclet number for the liquid phase;

aH Hd.a.DL Q- transferred heat, J, or heat flux, J/mzs; q - the heat of absorption in J/kmol; R- universal gas constant; ReG = °" h G - Reynolds number for the gas phase;

86

4£ Re, =

Reynolds number for the liquid phase; a.vL r - evaporation heat- J/kg. ScG = —— - Schmidt number for the gas phase; DG ScL = —— - Sehmidt number for the liquid phase; ShG =

kG dh

- Sherwood number for the gas phase;

A?

ShL = —-— Sherwood number for the liquid phase; s- rate of renewal of the interface, i.e, the part of the surface renewed, s"1; T- absolute temperature in K; t- temperature, °C; tG - temperature of the gas, °C; tt - temperature of the liquid phase, °C; Uh - bubble velocity in equation (148), m/§; UL- liquid velocity in equation (149 ), m/s; V- volume and gas volume in m3; Vp- packing volume in m3; w- real gas velocity (flow velocity), m/s; wh=Ti Ek

(17)

120

Bo,,

A cr^; A T Ek

H

AT

(a)

(b)

Fig. 15. Values determined from the impulse answer of the whole system (a) and of the measurement cell only (b).

The signal from the delta function at section 1, Fig. 15 is independent of the measurement cell, but cannot be measured without it. After Bishoff [85] the change of the dispersion and of the residence time because of the measurement cell can be described by the equations:

• +

BoIk

(18)

4 exp(-BO

Lk

exp(Bou )-5]\

and

=

tJl—J—[l-exp(-Boa)i. [ Bon J

(19)

The values of 4 and Bon are obtained by the measurements with the measurement cell using equations (17) and (18). The values of tSk, AtEk

cr^,

and A<j2tk for the measurement cell, shown in Fig. 14, are presented in Fig. 16.

121

\ \

\ \ \ -X-\\ * \\ A\ s1

"b t of s

\

i

N*

JtP

4

«8K)'J


and this increasing is not only limited by the loading and flooding points but also leads to strong increasing of the pressure drop, namely of the energy loss. That is why for every technological process and type and dimension of the packing, the gas and liquid velocities optimal from economical point of view are firmly determined. Therefore, the only possibility for significant intensification of the processes in the packed bed column is the changing of the oldest, not so effective packings, with new more effective constructions. Thus the problem for intensification of the packed bed columns is usually transformed to a problem of creating of new more effective packings and their proper selection for every concrete technological process. The models, discussed in Chapter 1, of the elementary act of the mass transfer processes are able to predict the mass transfer coefficient only for a limited circle of hydrodynamic regimes and types of interface, which are usually far away from the conditions of the industrial packed bed columns. Nevertheless, they allow tracing the ways for creation of new, more effective packings. According to the models of Higbie [1], Danckwerts [2], and Kischinevsky [3], the increasing of the mass transfer coefficient is connected with the reduction of the average contact time on the interface. This time decreases with increasing of the flow velocity and with reduction of the length of the liquid film, respectively the length of the packing element on which the film is flowing down. The model of the diffusion boundary layer [4-7] also leads to the same conclusion. According to this model, both the increasing of the flow rate and the

151

reduction of the length of the film lead to reduction of the diffusion layer and in this way to increasing of the mass transfer coefficient. Consequently from the point of view of the mass transfer theory, the intensification of the packed bed column can be achieved in the following two ways: 1. Creating packings which allow operation at high gas and liquid superficial velocities at small hydraulic diameter of the packing; 2. Creating packings which promote frequent breaks of the liquid film, and coalescence and break of the drops and jets in the packing free volume. From economical point of view the packings must have also low pressure drop, low price, corrosion stability, and so on. Usually the packings are compared on the base of their pressure drop per one mass transfer unit as a function of FG:

NTUG

= f(FG)-

(2)

A more useful comparison is presented by Elenkov et al. [144], given principally in Fig. 1. Kca at the abscissa of the figure is reverse proportional to the packing volume of the column. The ordinate

HTUG is the pressure drop per one mass transfer

unit. For a given process it expresses the pressure drop independent of the height of the packing, but dependent on its type. It is easy to see that of the two lines for different packings in these coordinates, the line No. 1, Fig. 1, corresponds to the packing with higher pressure drop at a given volumetric mass transfer coefficient, and lower mass transfer coefficient at a given pressure drop. Le. the packing of line No. 2 is characterized by lower operating cost for covering the pressure drop at given capital investments, and by lower capital investments at a given operating cost. Of course, such a conclusion is true assuming that the price of 1 m3 of the column is practically the same for the compared packings. A very important possibility for intensification of the column is to use the best from economical point of view dimensions of the packing elements. It is easy to see, that the reduction of the element dimensions leads to respective increasing of the specific surface area, which leads to increasing of the effective surface of the packing. At the same time it leads to reduction of the hydraulic diameter and in this way to increasing of the gas-side controlled mass transfer,

152

and to reduction of the gas and liquid velocities at the loading and flooding points. The reduction of the packing element dimensions leads also to increasing of the pressure drop. That is why the selection of the dimensions of the packing elements can be made only through an optimization procedure, taking into account not only the process type and the concentrations of the flows, but also the loading of the apparatus.

Fig 1. Principle comparison of different packings in coordinates

NTUQ

as a function of

H

The possibility to increase the efficiency of the packing by applying the results of the mass transfer theory was proved by us experimentally in 1969 [8, 9] using ceramic 25 mm Rasehig rings whit a height equal to their diameter and also rings with smaller height of the element. The ratio of the height to the diameter varied from 0.43 to 1. The results showed [8] that the volumetric liquid-side controlled mass transfer coefficient in case of shorter elements was up to 45% higher. In case of gas-side controlled process, the increasing was up to 35%. It must be mentioned also that the reduction of the height of the elements leads not only to increasing of the mass transfer rate but also to reduction of the pressure drop, the value of which can be reduced up to 20%.

153

The effect is connected with reduction of the real way of the gas phase through the packing and therefore with reduction of the real gas velocity. It could be interesting to mention that at the same time when the above described experiments were carried out in Bulgaria, Dr. Reichelt [10] in Germany, who is the author of a good book [11] in the area of packed bed columns, came to the same idea and obtained the same results for both mass transfer coefficient and pressure drop. Since he obtained lower pressure drop for shorter packing elements, contrary to his expectation, he was not quite sure in his experimental data and delayed their publication [9]. His idea was that the pressure drop increased because of increasing of the number of changes of the flow direction per 1 m of the packing without taking into account that the real way of the gas flow is shorter for the shorter packings. The results of Kolev [8, 9] are confirmed by Ellis [340] in case of rectification and also by Rao [190] and by James Jobling et al. cited by Ellis [340] as a private communication. 3.2. Types of packings Since 1827 when Gay-Lussac has offered the first packed bed column filled with coke [12] till now, hundreds of packings of different types have been proposed and patented. At the end of 19 century the first of the ring form packings were implemented as arranged packing in sulphuric acid manufacture. Some later, in 1914, Raschig patented the well known Raschig ring [13] which was one of the most widespread packings to about the middle of 20th century, used either as random or arranged. It is characterised by equal height and diameter of the packing element. As already mentioned, a packing with a smaller height is more effective. For a long time Berl [21, 96, p. 372] and Intalox [15] saddles (Fig. 2) were also largely used. In 1953 the Pall rings (Fig. 3) were patented [16]. The existing now packings can be divided into two big groups: random and structured packings. A group of packings operating in the inversion regime over the flooding point and moving together with the liquid phase can be consider as an additional third group. These packings have very high pressure drop and are proper, first of all, for cases when the usual mass transfer processes of absorption, desorption, or rectification are accompanied with crystallization on the packing walls or with purification from dust. These types of packings are not considered in this book.

154

3.2.1, Random packings 3.2.1.1. Description of the random packings The random packings consist of elements with a given form dumped in the column over its supporting grid. By form they can be divided into rings, saddles and packings with other form. Another possibility is specification into packings with continuous surface and packings with surface divided into connected lamellas with small heights. The oldest type of elements, for example Raschig rings, Berl saddles, Intalox saddles and others do not answer the requirements following from the theory of the mass transfer processes. The first packing construction that partially corresponds to these requirements is the Pall ring [16]. In 1970 Eckert, comparing different packings largely used at this time, wrote [293] that it exceeded all other packings (Intalox and Berl saddles, and Raschig rings too). Now we can be sure that its advantage is connected just with the fact that its constraction fulfils the requirements of mass transfer theory better than that of any other packing at this time. For the last thirty years of the last century practically there is no patented random packing not corresponding more or less to these requirements. Another requirement to the lamellas of the random packing elements is that their width must not be greater than about 5 mm, at least for liquids with properties similar to these of the water. This requirement follows from the results of the experiments of Kolev and Razkazova-Velkova [242] who measured the leakage of liquid flowing on a vertical well wetted plate. It was found that at the bottom edge of the plate a long "drop" with a height of about 5 mm, in case of irrigation with water, is formed. It is able to lead away the liquid in horizontal direction without leakage. I. e., if the width of the lamellas is not greater than 5 mm, they can be completely wetted with moving liquid at extremely low liquid superficial velocity. As shown later this phenomenon can be used for creation of special structured packings able to operate at practically fully wetted surface at extremely low liquid superficial velocity. At the same time this phenomenon gives the possibility to create more effective random packmgs too. The packing elements are produced of ceramic metal and plastic. The metal usually used is stainless steel. A mechanical or chemical treatment of the plastic elements leads to increasing of their wettability. A small group of packings are made of net which is wetted significantly better than the net material itself. In some special cases of corrosive systems, carbon packings are used. Some of the most popular and widely used packing elements are presented in Figs. 2 to 5. The first two of mem, figures 2 and 3, are taken from

155

the book by Reichelt [11] published in 1974. The most of them are not in use any more. Main Forms of Cylindrie Packings

0 Pall Ring Rings with connecting strips

Mesh Ring (with and without a strip)

}—

Spiral

Pertb-Ring Spiral Rings

• Spring Spiral Expanded Metal Ring Ring with a corrugated ^surface

Fig. 2. Some ring form packing elements [II].

Main Form

Bert Saddle

Me Machon Saddle

Super Saddle

Intalox (Novalox) Saddle

Fig. 3. Some saddle form packing elements [11].

156

As packings which correspond very well to the requirements of the mass transfer theory, the following packings can be specified; Hiflow Ring, Ralu-Flow, IMTP, Nutter ring, and especially Raschig Super Ring. They are presented in Figs. 4, 5, 6, 7 and 8. Metal Packings

Mini Ring

Top Pak

Hiflow Ring

VSP Ring

Plastic Packings

Nor Pac Ring

Hackette

Hiflow Ring

SuperTorus Saddle

RBIU Ring

Pall Ring

ENVIPAC

Ceramic Packings

Hitlow Ring

Pall Ring

Intalox Saddle

Fig. 4. Typical examples of random packings taken from Billet [177],

157

Fig, 5. Geometry of metal Nutter ring [214].

Fig. 6. Metal Rasehig Supper Ring.

Fig. 7. Intalox metal tower packing (IMTP).

Fig. 8. Plastic Ralu-Flow.

The advantages of the random packings are easy production using highly effective technology and easy dumping in the column. Their great disadvantages are poor distribution of the phases over the cross-section of the apparatus in comparison to the structured packings and often higher pressure drop. The geometrical characteristics of some of these packmgs are presented in Table 1.

158 Table 1. Geometrical characteristics of some random packings after Billet [177] Packing

Pall ring

Pall ring

Material

Metal

Plastic

Size dp

N

a

E

mm

3

1/m

m /m

3

m'/m 1

50

6242

112.6

0.951

0.763

38

15772

149.6

0.952

1.003

35

19517

139.4

0.965

0.967

25

53900

223.5

0J54

0.957

15

229225

368.4

0.933

0.999

50

6765

111.1

0.919

0.698

35

17000

151.1

0.906

0.927

25

52300

225.0

0.877

0.865

2

Pall ring

Ceramic

50

6215

116.5

0.783

0.662

Muring

Plastic

50

5770

95.2

0.938

0.468

SOhydr.

7720

95.2

0.939

0.439

50

5000

92.3

0.977

0,421

25

40790

202.9

0.962

0.689

90

1340

69.7

0.968

0.276

50

6815

117.1

0.925

0.327

SOhydr.

6090

118.4

0.925

0.311

25

46100

194.5

0.918

0.741

75

1904

54,1

0.868

0.435

50

5120

89.7

0.809

0.538

35

16840

108.3

0.833

0.621

20,4 webs

110741

261.2

0.779

0.628

20,6 webs

110688

265.8

0.776

6050

82.0

0.942

0.414

82.0

0.942

0.414

Hiflow ring

Hiflow ring

Hiflow ring

Metal

Plastic

Ceramic

Hiflow ring Super

Plastic

50

Hiflow saddle

Plastic

50

NORPAC ring

Plastic

50

7330

86.8

0.947

0.350

35

17450

141.8

0.944

0,371

25, type A

52356

211.0

0.951

25, type B

50000

202.0

0.953

25, type C

47619

192.0

0.922

0.397

159 Table 1. Geometrical characteristics of some random packings after Billet [177] Packing

NORPAC ring

Bialecki ring

Material

Plastic

Metal

Size dp

J¥

cp

e

a 3

2

3

3

3

mm

1/m

m /m

25,10 webs

48920

197.9

0.920

0.383

22

69274

249.0

0.913

0.397

15

167729

309.8

0.920

0.365

50

6278

121.0

0.966

0.719

m /m

35

18200

155.0

0.967

1.011

25

41353

210.0

0.956

0.891

Bialecki ring

Plastic

50

3900

100

0,956

Rasehig ring

Ceramic

SO

5990

95.0

0.830

38

13275

118.0

0,680

25

47700

190.0

0.680

15

189091

312.0

0.690

13

378000

370.0

0.640

10

672000

440,.

0.650

8

1261000

550.0

0.650

6

3022936

771.9

0.620

1.329

Rasehig ring

Metal

15

260778

378.4

0.917

Rasehig ring

Carbon

25

50599

202.2

0.720

13

378000

370.0

0.640

50, No 2

7841

104.6

0.980

25,Nol

33434

199.0

0.975

38

60744

174.9

0,974

25

158467

232.5

0.971

16

553950

340.0

0.951

13

560811

356.8

0.955

80, No 3

2000

60.0

0.955

0.358

60, No 2

6800

98.

0.961

0.338

32, No 1

53000

138.9

0.936

0.549

29039

135.3

0.921

307.9

0.894

VSPring

Glich, CMR ring

Envi Pack ring

Metal

Metal

Plastic

Dinpac ring

Plastic

45, No 1

Raflux Rings

Plastic

15

0.595

160 Table 1. Geometrical characteristics of some random packings after Billet [177] Packing

Intalox saddle

Material

Plastic

Size dp

N

a 3

cp

e 3

mm

1/m

rtfW

nrVm

50

9422

133.0

0.900

35

25867

169.0

0.910

114.6

0.761

0.758

0.747

Intalox saddle

Ceramic

50

Malax saddle; grid

Plastic

50

12429

108.0

0.950

Telerete

Plastic

25

37037

190.0

0.930

0.538

Hackete

Plastic

45

12000

139.5

0.928

0.399

Sphere

Glass

25

66664

134.5

0.430

13

561877

282.2

0.400

38

24928

164.0

0.700

25

80080

260.0

0.680

13

691505

546.0

0.650

106.6

0.956

Berl saddle

Top-pak

Ceramic

Alum.

50

0.604

Abbrev.: hydr.=hydrofllized

dp is the nominal size of the packing, N- number of packing elements in 1 m35 aspecifie surface area in m2/m3, and e -void fraction in m3/m3. The data in Table 1 are taken from 4 tables given by Billet [177]. Small differences in some of the values are observed in them, but since they are in the range of the differences from experiment repetition, including re-dumping of the packing, they are not discussed later. By simple stereometrieal examination of geometrically similar packings, it is easy to obtain the following expressions, independent of their type: -const.,

(3)

dnM = const.,

(4)

N.d3p - const.,

(5)

e

161

where the corresponding constants depend on the form of the packing, and dp is the nominal packing size, respectively its diameter. Among the contemporary packings, the Raschig Supper-Ring of metal and plastic, the metal packing IMTP, and the plastic Ralu-Flow, presented in Figs. 6 to 8, investigated at the author's laboratory, are especially effective. The geometrical characteristics of these packings measured by us are presented in Table 2. It is well known that these characteristics depend on the column diameter and are not perfectly reproducible. That is why in Table 3 the corresponding data for metal Raschig Super-Ring after the prospectus of Raschig Company [17] at ratio of the column diameter dc, to the packing size dp equal to 20 are also presented.

Table 2. Name

Material

freeVol

Nom. diam.

m /m

%

mm

Surface area 2

3

Raschig Super Ring No.0.5

Metal

236.2

96

20

Raschig Super-Ring No. 0.7

Metal

175.9

97

25

Raschig Super-Ring No. 1

Metal

155.5

98

30

Raschig Super Ring No. 1.5

Metal

105J

98

38

Raschig Super Ring No. 2

Metal

100.6

98

50

Raschig Super Ring No. 3

Metal

74.9

98

70

IMTP 25

Metal

242.8

97

25

IMTP 40

Metal

171.6

97

40

IMTP 50

Metal

107.1

98

50

IMTP 70

Metal

69.1

98

70

Ralu-Flow No. 1 PP

Plastic

177

95

25

Ralu-Flow No.2PP

Plastic

98.4

95

50

Raschig Super Ring No. 0.6

Plastic

206.3

96

25

Raschig Super Ring No. 2

Plastic

117.2

96

50

162 Table 3. Technical dala for various Raschig Super-Rings

Material

Name

Surface area nrAir

Free Vol.

Nominal diameter

mm

Raschig Super-Ring Nr. 0.3

metal

315

96

15

Raschig Super-Rmg Nr. 0.5

metal

250

97

20

Raschig Super-Ring Nr. 0.7

metal

ISO

98

25

Raschig Super-Ring Nr, 1

metal

150

98

30

Raschig Super-Ring Nr. 1.5

metal

120

98

38

Raschig Super>Ring Nr. 2

metal

100

98

50

Raschig Super-Ring Nr. 3

metal

SO

98

70

Raschig Super-Ring Nr. 2

plastic

100

96

50

In Fig. 9 a compensation for the "decrease in volume" for dumped packings is presented after the same prospectus [17]. 12 rt- = diameter of the vessel • = diameter or nominal sisle

8 IB |

-8

Q

/ /

8

1'

/

is e S 5

/

i

/ /

I1

/

£ o ^

20

30

40

50

60 70 80 DiamatBr ratio tijdp

90

100

Fig. 9. Compensation for the "decrease in volume" for dumped packings after [17].

110

120

163

3,2.1.2. Performance characteristics of random packings As already mentioned in Chapter 2, the investigations on the performance characteristics are carried out in two types of installations, one for obtaining of experimental data necessary for determination of constants in the equations for calculation of the columns, and another for investigation of rectification to obtain first of all the values of HTUand HETP. The latter can be used for design of industrial rectification columns, and also for comparison of the efficiency of different packings. Of course in the second type of installations, data for holdup, pressure drop, and loading and flooding point can be obtained. That is why for easier comparison, it is expedient to consider separately the performance characteristics of the packings obtained in every type of experimental installation. 3.2.1.2.1. Performance characteristics of random packings obtained in cold experimental installations 3.2.1.2.1.1. Pressure drop 3.2,1.2.1.1.1. Experimental data Investigations on the pressure drop of random packings are carried out by Mach [21], Zhavoronkov [20, 22], Kafarov and Bliachman [23], Glaser [24], Schrader [25], Brauer [26], Teutsch [27], Leva [28, 29], Kast [30], Reichelt [31], Kolev [B]s Billet [177] and others. Data for the pressure drop of some highly effective packings versus the gas capacity factor are presented in Fig. 10 and Figs, 1A to 9A in the appendix of this chapter. The investigations are carried out [18] in a column with a 470 mm diameter with system air- water. The geometrical characteristics of the packings are presented in Table 2. Data for some performance characteristics of other highly effective random packings are also presented there. From the figures it is seen that the increase of the packing size leads to a significant reduction of its pressure drop. In Fig, 11, taken from Schultes [321], the history of development of the random packings after the time of Gay-Lussac is presented schematically. Nevertheless that the classification of packing generations is offered by Schultes [321], the author of the Raschig Super-Ring marked by him as a forth generation packing, this classification corresponds very well not only to the time of development but also to the real advantages of the best variants of M s packing. This can be seen later from the comparison of this packing with other highly effective ones.

164 2000

Lh=10, m3/(m2h) • •

Lh=20, m3/(mah) Lh=40, m3/(mzh) Lh=B0, m3/(m2h) L=120. ma/m2h

10 0.3 0.4

0.6 0.8 1

Fig, 10, Pressure drop of metal Rasohig Super Rings No. 3 4>=7Q mm versus the gas capacity factor FQ.

165 Raschig Super-Ring

Third Generation (Late 1970's- 199O's} CMR Ring Nutter Ring IMTP Ring Fleximax

Second Generation (Late 1950' s- Early 1970'si Pall Ring Intalox Saddle

First Generation {1895 - 1950) Raschig Ring

Beri-Saddle

Fig. 11. Scheme of the history of development of the random packings after the time of GayLussac [321].

3.2.1.2.1.1.2. Equation for determination of the presswe drop 3.2.1.2.1.1.2.1. Presstire drop of dry packings The pressure drop of a dry packing can be calculated using the equation:

(6) dk

2

where AP0 is the pressure drop in Pa;

166

£ - coefficient which characterizes the type of the packing; w = —— - real gas velocity in m/s; s w0 - gas velocity determined for the column cross-section in m/s; e - bed void fraction (free packing volume, equal to its free cross-section); pa- gas density in kg/m3; 4e dh = hydraulic diameter of the packing in m; a a- packing specific surface area, m2/m3. Eq. (6) can be written also as follows:

APB = ffl™ " " 8e3

.

(7)

The coefficient f takes into account the pressure drops both from friction of the gas on the packing surface and from changing of its direction and velocity of moving trough the packing. Ergun [104] proposed the equation

VE=-Z-

+ *-75,

(8)

where the dimensionless pressure drop (friction factor ysE ) is defined as

' •

H I - .

*p

'

(9>

or:

H

WE G

° e3Da

The Reynolds number Re0 is given by the expression:

(10)

167

The arithmetical diameter of the packing Da is the diameter of a spherical packing with the same value of e and a as the real packing in the column. It is easy to see that it is calculated by the equation: Da=6(l-B)/a.

(12)

For determination of £ in Eq. (7), Aerov [103] recommended equation like that of Ergun [104]. ^ + 2.34, ReG

(13)

where S

*o=-

s

•

HQ - dynamic gas viscosity in Pa.s. For determination of the pressure drop of ring random dry packings, Zhavoronkov et al. [20] presented the equations: at Rea40

= —j}

1—•

The average deviation of this equation is 17%.

(15)

168

The equation presented by Kast [30] takes into account that plotted in double logarithmic coordinates the relation of the pressure drop is not exactly a straight line. The equation is as follows: Ws

=Kw.(64/ReG+2.6/Re^),

(16)

where Kw is the so called way factor depending on the packing type and taking into account the ratio of the average way of the gas phase in the packing to its height. In Table 4 the values of this constant for different types of packings are presented. For practical calculation of the pressure drop Billet [177] proposed the equation

„

=

-system air-water -system air-machine oil with viscosity 0.0108 Pa.s and density 889 kg/m3 -system air- machine oil with viscosity 0.0373 Pa.s and density 900 kg/m3

In Fig, 13 a comparison between the experimental data for the packings presented in Table 7 and these calculated by equations (259) and (260), Chapter 1, and Eqs. (25) and (26) for the same conditions is presented. It is worth noting that the equations (259) and (260) from Chapter 1, and (25) and (26) described well also the data of Reichelt [35] obtained for short ceramic rings filled not only in a column but also in vertical tubes. The geometrical data for the corresponding packings are presented in Table 8.

180

15 \ 10 S 8 7

\

\\ \ \ \

\U

8

+20%

^30%

A

0

\\ 10

w

3

0

-30%\

V

V

a

°

s

6

A

2

"IF

4 1

04

0.6

0.8

1

Fig. 12, Comparison between the experimental dHte for the packings given in Table 7 and the calculated line obtained using Eq. (259), Chapter 1, and Eq. (25).

0.2

0.4

0.6 0.8 1

Fig. 13. Comparison between the experimental data and those calculated by Eq. (259) and (260) from Chapter 1 and equations (25) and (26) for the pressure drop over the loading point of the packings given in Table 7.

To calculate the pressure drop of the irrigated packings under the loading point, Bemer and Kalis [245] proposed the equation:

AP

(27)

APn where % is an experimentally obtained construction factor based on the mean diameter of the packing channels. Its value is as follows [245]: for Raschig rings - % = 0.435; for Pall rings - % = 0.485.

181

The value of the constant^ is determined experimentally. It is as follows [245]: for Raschig rings - $ = 0.6; for Pall rings- ^ = 0 J . The comparison of the calculated data and the experimental ones shows a significant disagreement in some cases. For example, for 12.5 mm Raschig rings and system dichloroethanol-benzene, the difference is up to 2.5 times. For the most of the experiments the difference is less than 30%. Simplifying Eq. (27) and taking into account the influence of the liquid superficial velocity on H4 according to Eqs. (79) and (80), Billet [177] proposed the equation

Table 8. Geometrical characteristics of the packings investigated by Reichelt and used for testiog of equations (25) and (26) Sizes mm

Diameter mm

Free volume 3

3

m /m

Specific surface area

Material

Symbols

itftta3 26.08x25.1x0.5

15.4x15.1x0.5

11.85x12.05x0.3

8.5x8.05x0.3

15.95x1410x2.65

9.73x10.06x1.685

41.5

0.9®

127

m§

0.962

155

41.5

0.933

277

79J

0.024

314

41.5

0.944

377

79.8

0.941

404

139.0

0.939

416

41.5

0.921

494

79.8

0.913

543

139.0

0.913

543

41.5

0.746

210

79.8

0.698

250

41.5

0.6S5

437

79.8

0.652

471

139.0

0.655

475

Steel

O •

Steel

X

Steel

A • CD

Copper

•

Porcelain

Q »

e

O Porcelain

+ ©

182

where the factor Cw expresses the hydraulic status of a wetted packing

Cw can be found experimentally either directly by pressure drop tests or by evaluation of Co by means of the following expression: y/j

on the basis of holdup measurements and by determination of the values of ^ and x -vo i § the kinematic viscosity of reference liquid (water), m2/s. For calculating the pressure drop of irrigated packings, Billet [177] offered the following equation:

(31)

where the resistance factor of the irrigated packing is given by the equation

(32)

where Cp is a constant called "packing factor for pressure drop" depending on the type and dimensions of the packing. Its values for different packings are presented in Table 1. The wetting factor W, in the equation is given by the expressions:

183

for regimes under the loading point

W = expl —^-1 ;

(33)

{200J for regimes over the loading point

»T= —*•

expl-^-}.

(34)

The Reynolds numbers for the liquid and for the gas phase are defined as follows:

= ^

and Rem =6^^

a.ftL

ajuG

]

,

(35)

l + 4/(dca)

where dc is the column diameter. For the liquid holdup over the loading point H& Billet [177] proposed the equation

X-\

•

06)

where the index UF" is for the flooding point. The value H^F can be calculated by the equation

HdF =0.3741 e \ ^ \

.

(37)

From equations (31) to (35) it is easy to see that the effect of the column diameter is to be neglected for big columns. The above presented equations for determination of the pressure drop of irrigated packings are suggested only for a liquid which does not form froth. The investigations [74, 75] show that the presence of froth can increase many

184

times the pressure drop even under the loading point. The reason is that the formed froth occupies a significant part of the column free cross-section, which leads to significant increasing of the gas velocity and therefore of the pressure drop. The investigation shows the following: 1. The froth is formed because of the transformation of a part of the kinetic energy of the liquid flow in energy of foam connected with its larger surface. That is why the generation of froth increases with increasing of the liquid velocity, respectively of the liquid superficial velocity. But the increase of the liquid velocity leads also to destroying of the already existing froth in the packing. 2. The foam stability, or the froth life, depends not only on the composition of the liquid, but also on the hydrodynamie regime in the apparatus. It is found, for example, that when we have two foam forming liquids and two methods of measurement of their foam stability, if the stability for the first liquid according to one of these methods is higher, according to the other it can be measured as lower. 3. The increasing of the gas velocity also acts in direction of forming and destroying of the foam, 4. A mechanism and a mathematical model for calculating the pressure drop of foam forming liquids is presented and experimentally proved. 5. It is predicted and experimentally proved that in a packing with long vertical walls on which the liquid is fed with proper low velocity almost parallel to the packing surface, froth is not formed. Nevertheless that it is possible to build a packed bed column able to operate in presence of from forming substances, it is better to use anti-foaming agents to prevent foaming than to construct the column so as to operate with foam forming liquids. For calculation of the pressure drop under and over the loading point of irrigated packings Billet and Schultes [316] offered the following equation:

4F

(64 13300

1.8

)(S-Htf(HtT

185

FrL =

L2a

is the Froude number, and Ha is the total holdup under the loading g point. Ht is the total holdup for the corresponding regime. The value of Cpg is given in Table 6. 3.2.1.2.1.2. Loading, flooding and maximum efficient capacity In the area of the critical points in loading and flooding, a great number "W

of investigations are carried out [22, 36-39]. Usually, they use the ratio —-, or 0 in the critical points as a function of the ratio of the friction to the L.pL gravity forces, A dimensionless number equal to the ratio of the packing pressure drop to the weight of the liquid per m2 of the packing cross-section is offered by Zhavoronkov [22]. The following equation is proposed:

_

AP

gpLH

where FrrG =

W jp

2

PG G

w2 gdk

pL aw 4ge>'

In his method for determination of the loading and flooding, Zhavoronkov used the value of y/ for dry packings (Eq. (15)) and the equation:

(40)

In [22] Zhavoronkov presented the equation (40) in graphical form. Later [4] he proposed the equation: T = B,(1-B2A?

,

(41)

186

where A is the dimensionless parameter calculated by Eq. (22). The values of the constants Bi and B^ depending on the type and dimensions of the packing and also on A are presented in Table 9. Eq. (41) in comparison to Eq. (40) has the advantage that on the righthand side there are no terms including the gas velocity, which makes it easier to determine w§. So calculating Fby Eq. (41) using Eq. (39), we can easy obtain wg at the loading or flooding point. For determination of the flooding point Sherwood et al. [41] have offered the equation:

(42) Pi

The equation is used by Labo et al. [42] who offered a diagram based on it, presented in Fig. 14. It is noteworthy that the differences between the experimental points and the curve in the figure in some cases are more than 300%.

Table 9, The values of A, B, and B2 in Eq. (41) for different packings A

B,

Bt

Loading point

0-07

0.046

1.0

Flooding point

0-0.2

0.168

1.83

Flooding point

0.2-1

0.081

1.22

Loading point

0.2 - 0.7

0.0243

1.165

Flooding point

0.3 - 0.7

0.118

1.22

Loading point

0.3 - 0.55

0.081

1.44

Flooding point

0.55 - 0.8

0.067

0.96

Ceramic Raschig rinp (3Q mm)

Steel Raschig rings (25 - 50 mm)

187 187

Fig. 14. Correlation of the data for flooding. 1-system water-air; 2- system water - H2; 3- system water - CO2; 4- system glycerol - air; 5system butter acid - air; 6- system methanol - air; 7- system turbine oil - air; 8- system transformer oil B100 - air; 9- system oil B100 - air; 10- system oil IOC - air; 11- system oil No. 1 - air; 12- system oil No. 1 - COj; 13- system oil No. 1 - H2; 14- system oil No. 2 - air; 15- system oil No.3 - air.

The curve in Fig. 14 can be described by the equation: y = 0J922x3 + 0.2Q41X + 0.5584 = 0 ,

where y^J^BLfA

and ,«2+ J± J&).

Similar equation is published in [71];

(43)

188

y'+0.2422x2 -L9461x + 4.6457 = 0 ,

(44)

(L2a \ where y'=lg\ —jMi Eq. (44) gives the possibility to obtain the gas velocity at the flooding point:

(45)

KH-*'

where igFj = -4.12881$ ^tul

2

OJ

1-8.622

Another possibility to determine the loading point is connected with its definition and with the employment of equations (259) and (260) from Chapter 1 and (25) and (27). By definition the loading point is the point where the line of the irrigated packing pressure drop as a function of the gas velocity, in double logarithmic coordinates, changes its slope and is no more parallel to the respective line for dry packing. In other words, it is the point in which the influence of AA on A cannot be neglected any more, that is the point in which the ratio

(46)

is significantly greater than 1. Investigations carried out with different packings show that the value of fa can be taken equal to 1.03 for all of them. As it has been already mentioned the presence of foam building substrates (surfactants) in the liquid phase leads to increasing of the pressure drop which causes also reduction of the gas and liquid superficial velocities at the loading and flooding points [74, 75]. Using foam destroying agents the harmful effect of such substances can be eliminated. For calculation of the gas velocity at the loading point Billet [177] proposed the equations:

189

(47) Pa

and J/3

f)

a

i/e

(48)

1/6

where

g -o.es

(49)

U Obviously, the last three equations are not dimensionless. The values of Cs for different random packings are presented in Table 10. For calculation of the gas velocity in the flooding point Billet [177] proposed the equation

(50)

J/2

where

g 0,2

1-0,39

(51)

190 Table 10, The values of the constant Q, C, C r a CL and Crfor different packings [177] Dumped packings

Material

Pall ring

Size, mm

Ch

cs

CFl

Q

Cy

50

58.70

2.725

1.580

1.192

0.410 0.341

38 Metal

Pall ring

Plastic

2.597

1.76

1.227

35

43.12

2.629

1.679

1.012

25

31.74

2.627

2.083

1.440

0.336

15

20.17

2.550

2.081

50

67.95

2.816

1.757

1.239

0.368

35

49.18

2.654

1.742

0.856

0.310

25

22.23

2,696

2.064

0.905

0.446

1.227

0.415

1.520

0.303

1.481

0.341

1.168

0.408

Pall ring

Ceramic

50

70.29

2.846

1.913

Pall ring grid

Plastic

50

70.29

2.846

1.913

Raul ring

Plastic

50

2J43

1J12

50, hydr Raul ring Raul ring

Metal Plactic

50

81.71

2.702

1.626

25

42.93

2.918

2.177

2,853

1.597

2.894

1.871

90 50

73.03

50, hydr Raul ring

Hiflow ring; Super NOR PAK ring

Ceramic

Plastic Plastic

0.402 1.487

0.345

1.553

0.369

1.577

0.309

1.377

0.379

25

2.842

1.989

75

2.903

1.565

50

2.819

1.694

35

2.664

1.667

2.875

2.410

1.744

0.547

20,4wb

58.05

20,6wb

45.53 2.886

1.702

1.219

0.342

50

143.96

2.959

1.786

1.080

0.322

35

57.62

3.179

2.242

0.756

0.425

3.419

2.656

0.862

3.277

2.472

0J83

2.990

2.156

0.888

50

25,typA 25, typ B 25, typC

41.01

0.366

191 Table 10. The values of the constant C* C, Cm, CL and C>for different packings [177] Dumped packings

Bialecki ring

Raschig ring

Material

Metal

Size, mm

c,

Ca

CL

Cv

25,10 wb

2.865

2.083

0.976

0.410

22

2.893

2.173 0.302

15

21.7

2.911

2.406

50

44.04

2.916

1J96

1.721

35

44.92

2.753

1.885

1.412

25

32.23

2.521

1.885

1.461

2.558

1.540

1.798

2.482

1.574

1.416

0.210

1.536

0.230

1.361

0.412

1.276

0.401

1.367

0.265

1.303

0.272

Plastic

50

Metal

15

Ceramic

50

10.60

38 25

54.04

15

29.61

2.454

1.899

13 10

Carbon

28.16

8

1.210

6

1.130

25

1.379

0.471

1.419 13 VSPring Glich, CMR ring

Envi Pac ring

Dinpac ring

Metal Metal

Plastic

Plastic

50, No 2

94.17

2.106

1.689

1.222

0.328

32, No 1

33.90

2.755

1.970

1.376

0.387

38

121.62

2.697

1.841

25

13.97

2.703

1J96

16

54.4

13

11.04

80, No.3

152.60

2.846

1.522

1.603

0.257

60, No.2

111.50

2.987

1.864

1.522

0.296

32, No.l

279.21

2.944

2.012

1.517

2.929

1.991

45, No.l

192 Table 10. The values of the constant Ch Cm CFh CL and C r &r different packings [177] Dumped packings

Material

Size, mm

C,

Intalox saddle

Plastic

50

2.382

1.548

35

2.317

1.600

2.675

1.657

2.913

2.132

Intalox saddle; grid

Plastic

50

Tellercte

Plastic

25

Sphere

Glass

25

32.21

0.899 1.335

13 Haekette

Plastic

45

Berl saddle

Ceramic

38

59.91

1.364 L568

0.244

25

53.25

1.246

0.2387

13

37.07

1.416

0.232

Abbrev.: hydr.= hydrophilized

(theoretical liquid holdup at the flooding point) is given by the equation:

(52)

8 It is easy to see that the last three equations are not dimensionless either. The values of CFI for different random packings are presented in Table 10. For calculating the maximum efficient capacity factor which determines the value of the gas velocity at maximal mass transfer efficiency of the column for the packing IMTP, Rukovena and Koshy [291] presented figure IS. Here the value of Co for packings with different dimensions is presented as a function of the flow parameter

193

In details the problem with the maximum efficient capacity factor CS€ (called also maximum entrainment parameter) is considered in Chapter 1, Fig. 11, and equation (249). 0.3

*15 IMTP-i 0.2 •—

^_

—

1

mm —

B

=

=

_

»25 IMTP-r

« Pi w0 where w^ is the value of w§ at the flooding point, m/s; The equation is valid for

-3all ring 25

B 8 10'

2

,rrfi/(m2s)

Fig. 84. The volumetric mass transfer coefficient for CO2 desorption as a function of the liquid superficial velocity (see Table 31 and 33).

A very useful comparison of the efficiency of different packings is this based on the pressure drop per mass transfer unit (AP/ NTUG). It gives the

309

possibility to compare easy the necessary pressure drop for a given process, as a measure of a significant part of the operating costs, independent of the packing efficiency. In Fig. 87 [156] this ratio is plotted versus the F-factor,

s

4 ,

S

Is

-

So

ft

s '/ 2

/

4

r J-Hiflo»riii 8 50 3-HHlnwin

S_f

ft

•ack

5-Pall rira SO B-P»ll rini 15 + - 7-Turbo-Pack P0-5I Q - S-Turbd-Pec* P3.75-5B 4

B

B

10'

Z

A

L.103,m3/(m2.s) Fig. 85. Plots of pressure drop per mass transfer unit for COj desorption versus the liquid superficial velocity at wfl =0.8 m/s (see Table 31 and 33).

Fig, 86. Plots of pressure drop per mass transfer unit for COa desorption versus the volumetric mass transfer coefficient at a gasliquid ratio W(/I=300 m3/m3 (see Tables 31 and 33).

In Fig. 88 the same ratio is plotted versus koa [156] as a measure of the packing efficiency. It is easy to see that fea is approximately reverse proportional to the capital investments. It means that in such coordinates the lines for the better packings from economical point of view, taking into account both capital investment and operating cost, lie in the lower part of the figure. The comparison shows that in Fig. 87 the pressure drop of Turbo-Pack PO-58 is the lowest. Practically this packing is a packing with smooth walls, without turbulizers. At Fig. 88 the packing P3.75 has the lowest pressure drop. That is, the presence of turbulizers leads not only to increasing of the mass transfer coefficient, but also to reduction of the pressure drop per mass transfer unit. The 50 mm Pall Ring has the highest pressure drop. The reduction of the dimension of this packing leads to reduction of its pressure drop in these coordinates.

310

10*

15

tU-2

ff r 1, i/P

1-fcuroforni 3-Ralu-Pnek 3-Hif lwring 50 4-Hif Inuring 25 S-Impulae-Pack B-Pall ring 50 7-Pall ring 8-Intalo« SQ S-Sul«a10-Hellapak ~ H-Turbo-Pack m-58

arm

1

If J

4-Hiflo 5-Iapijl -ing

50

1 II

1-Pall ring e-NU

S

V //

l-Eurtrf *ring 50

' // l-Hiflarfring

1Q 1 B 6 5

o - lZ-Turbo-Pack P3 75-3B

Iff —

\

25

'+~ll-Turbo-PBck PD-58 o-12-Turbu-Pack PI75-58

5 6

9

10

kea, Fig. 87. Pressure drop per mass transfer unit for the packings compared versus FQ factor (see Table 34).

Fig. 88. The pressure drop per mass transfer unit for the reference packings (Table 34) plotted versus the volumetric mass transfer coefficient

3.2.2.3. Structwedpackings of expanded metal 3.2.2.3.1. Description of the structured packings of expanded metal The expanded metal sheets, Fig, 89, are very proper for constructing packings. As already mentioned the reduction of the length of the packing elements leads to increasing of the mass transfer coefficient. As it is easy seen from Fig. 89, the expanded metal sheets consist of many lamellas with small width. That is, from this point of view this material is very proper for production of highly effective packings. Another advantage of this material is that it is produced easy by highly effective technology without waste material from making orifices in it. All these advantages have oriented the investigators to apply it for a packing design. Up to now the following main constructions are presented in the literature: the packing of Stage [178], Spraypack [182, 192, 193], the packing of Kolar [179-181], and the Holpack packing of Kolev and Daraktschiev [183-189, 194].

311

The packing of Stage consists of vertical plates of expanded metal bent so that to build channels with square cross-sections between them. The long axes of the expanded metal orifices are oriented vertically. A great advantage of this packing, in addition to the main advantages of the expanded metal, is their very low pressure drop due to the vertical channels. That is why the packing is proper for processes which need extremely low pressure drop, such as for example vacuum rectification processes. The great disadvantage of the packing is that the slope of the constituent lamellas is oriented more to one side, which leads to one side spreading of the liquid to the edge of the square channels. This together with the additional capillary forces acting on the edges leads to strong maldistribution of the liquid after a given height. That is why after some packing height, the liquid should be collected and distributed again over the packing surface. The scheme of the packing Spraypaek is presented in Fig. 90. The packing consists of horizontal sheets of expanded metal or nets connected with vertical bars and bent so that to build V-formed equal perforated "channels".

sheet length

Fig. 89. Expanded metal sheet.

312

Fig. 90. Scheme of the Spraypaek packing.

The investigated packing have pitch/? equal to 125 - 300 mm and step h equal to 50 - 150 mm. The dimension of the orifices is 3.2 mm. The results show that at very low gas velocity, the liquid flows as a film, and gas bubbles exist only in some points. The increasing of the gas velocity leads to strewing the liquid in drops. At additional increasing of the gas velocity to the loading point, the liquid is collected in the bottom part of the channels and the pressure drop strongly increases up to the flooding point. At additional increasing of the gas velocity in the regime of strewing, the pressure drop remains constant. After the flooding point the liquid goes out together with the gas phase. The advantage of the packing is that it operates at high gas velocity- 2.5 to 3.5 tn/s, at not very high pressure drop (500- 750 Pa/m). The packing of Kolar [179-181] is presented in Fig. 91. It consists of vertical plates of expanded metal assembled in a package. The long axes of the orifices in the expanded metal of the Kolar packing are oriented vertically. The loading and the flooding in the packings with parallel vertical sheets begin at their top or bottom part. To increase the permissible gas and liquid velocities of the packing, the inlet and the outlet of the gas phase in (out) the packing take place not in vertical direction, which is the case for all other packings, but in horizontal. This gives the possibility to reach gas and liquid superficial velocities in this packing quite greater than in all other packings, for example gas velocity up to 6 m/s at high liquid superficial velocity. The visual investigations show that at liquid superficial velocity over 0.01 m3/(m2.s), the entire surface of the packing is wetted. The greatest advantage of the packing is the possibility to operate at extremely high gas and liquid superficial velocities. Its greatest disadvantage, mentioned already for the packing of Stage [17i], is that the slope of the constituent lamellas is oriented

313

more to one side which leads to one side spreading of the liquid, i.e. to maldistribution. The idea to construct the packing Holpack comes from the need to design a packing with a surface covering the most of the column cross-section, to prevent great axial mixing in the liquid phase, and having at the same time great fee cross-section for the gas phase to reduce the pressure drop. The other requirement to this packing is to turbulize additionally the liquid phase tough collision with the solid surface of the packing lamellas. At the same time the packing effective surfece area is quite greater than the specific one because of the area of drops and jets trickling between the horizontal sheets. To provide for these conditions, the packing consists of horizontal sheets of expanded metal mounted at a distance between them. There are two possible arrangements of the sheets, opposite and cross. A principle vertical cross-section in case of opposite arrangement of the sheets, towards the long axis of their orifices (Fig. 89) is shown in Fig. 92.

Fig. 91. The packing of Kolar 1- gas inlet pipe; 2- gas outlet pipe; 3- liquid Met pipe; 4- liquid outlet pipe; 5- liquid distributor; 6- packing.

314

The liquid from a given sheet dripstothe lower one and turbulizes after coming into collision with it. This leads to strong increasing of the liquid-side controlled mass transfer coefficient. An additional possibility to increase the liquid-side mass transfer is shown schematically in Fig. 93.

§\\\\\\\\\\\\\\\V

P/f/////Y///////A Fig. 92. Principle scheme of opposite arrangement of the horizontal sheets of expanded metal.

Fig. 93. Scheme of obtaining fresh interface upon turning the liquid film in the Holpack,

315

The collision of the liquid with the sheets, together with the free volume between them, prevent the building of sediments in the packing, and make it very proper for operating with liquids containing solid phase. The inclination of the constituent lamellas of the expanded metal sheet leads to spreading of the liquid phase in one direction, i, e. to non-uniform distribution over the cross-section of the apparatus. To reduce this effect every next sheet is oriented so that the inclination of each two neighbour sheets is opposite. Another possible arrangement of the sheets is when the inclination of every second sheet is opposite and the long axes of the packing orifices for the neighbour sheets are oriented at 90° (cross arrangement). The geometrical characteristics of the investigated packings [183-189] are presented in Table 35. To prevent the flowing of the liquid phase to the column wall, which leads to strong reduction of the driving force of the mass transfer process, respectively of the packing efficiency, in all investigations of these packings special deflecting rings are mounted on the column wall [196]. The principle of these rings is described in Chapter 8. 3.2,2.3.2. Performance characteristics of the structured packings of expanded metal and their comparison with other types of packings 3.2.2.3.2.1. Pressure drop and loading point. Experimental data and equations Some experimental data for the pressure drop of Holpaek are presented in Fig. 94. The pressure drop of the dry packing can be calculating using the equation [184]:

(212)

where w, =

2,AP0.Ej

7—— — 1 and s, is the free cross-section area of the sheets, H.wopGN, which here is not equal to the free volume of the packing. Ni is the number of expanded metal sheets per 1 m packing, 1/m. The comparison of the experimental data and the values calculated by equation (212) is presented in Fig. 95. The deviations of 96% of the experimental data do not exceed + 15%.

316

317

318

L=0 L= 0.0018 m/s 3- L= 0.0084 m/s 4- L= 0.01 m/s 5-L= 0.017 m/s

Fig. 94. Pressure drop of Holpack Nr. 1, Table 35, with hydraulic diameter 6.5 mm.

319

•

3.0 2.5

i [-145%

« 2.D

1.0

8 102

6

8 103

Fig. 95. Comparison of the experimental date, for dry Holpaek, presented in Table 35, with the values calculated after Eq. (212).

The experiments [194] carried out with expanded metal sheets, covered with plastic and without covering, show that in both cases the pressure drop depends only on the geometrical parameters of the sheets, according to Eq. (212). For calculating the value of A in Eq, (259) from Chapter 1, under the loading point the following equation is proposed [184]: -o.i

JJ66

(213)

where ReL and FrL are defined as follows:

and

g.dh.ef ' The average square error of Eq. (213) is ± 5%. Comparison between the experimental data and the value of AP/ AP0 calculated by means of Eq. (259) from Chapter 1, and Eq, (213) is presented in Fig. 96. The dependence between the gas and liquid velocity at the loading point of the Holpack packings given in Table 35 is presented in Fig. 97. From the figure it is seen that the data for packings with equal hydraulic diameters lie on one and the same line. It must be mentioned also that the packings with cross arrangement of the expanded metal sheets permit about

320

12% higher gas velocity in the loading point than at opposite arrangement. The difference between the gas velocity for the packings with equal values of e ; and dk, but different distances between the sheets is not more than + 5%. Using the experimental date for a system air-water, the following equation for calculating the gas and liquid velocity in the loading point is proposed [184]: (214)

where rrG =—:

.

The values ofFr^, Fro and Ej in the experiments used for obtaining the constants of Eq. (214) are changed as follows: FTL from 12.8 to 55.2, Fr® from 2.35xlO"5 to 7xlO"3, e,from 0.1 to 0.8. The Galilei number, with negligible influence, is changed from 2.7x106 to 8.8x108. The values of ^-

and of

-^- are not changed in the experiments. Pa The comparison between the calculated values using Eq. (214) and the experimental ones is shown in Fig. 97. The errors are less than 15%. A special eharaeteristie of Holpack is that at reduction of the gas velocity after the flooding, a hysteresis is observed [307] (see Fig. 10 in Chapter 2). The expanded metal elements in these investigations are specially fixed to the column wall. 3.2.2.3.2.2. Dynamic holdttp of Holpack packing Some experimental data for the dynamic holdup [189] of different Holpack packings for a system air-water under the loading point are presented in Fig. 98. For calculating the dynamic holdup of these packings (under the loading point), the following equation is proposed [189]: Hd = 0.47.Gaf05Fr°J2,

(215)

321

2 1.9 1.8 A ~l

\,l

1.6

15

v. \\\ \

1

\

\

\ Jv \ \ \v >\ \, \ . \ \ \ \+10%

1 *\

\i

,-\

V

13

-10 %\

%

\

\.N

»\\ \* }I \

\\j i *>

\ l\*\

1.1

1

0.85

0.90

\ \, \ \

0.95

1

1-A Fig. 96. Comparison between the ejqjerimental data and the values of AP/ means of Eq. (259) from Chapter 1, and Eq. (213) versus 1-A (see Table 35).

APg calculated by

322

8

10

12

14

16

18

LKP.m/s Fig. 97. The dependence between the gas and liquid velocity at the loading point of the Holpack packings presented in Table 35. The curves are calculated using Eq. (214). The symbols are presented in Table 35.

where: GaL =

g

and Frt =

In obtaining the experimental constants of this equation, not only date for different construction sizes (Table 35), but also for different liquid viscosities (from lxlO' 3 to 1.3xlO"2Pa.s.) and different liquid densities (between 1000 end 1230 kg/m3) are used. In Fig. 99 a comparison between the experimental data and the calculated line is presented. 3.2,2.3.2.3. Effective surface area ofHolpack packing Some experimental data [187,195,196] for the effective surface area of the Holpack packings, given in Table 35, are represented in Fig. 100. The data are obtained using the method of Danckwerts for absorption of CO2 in NaOH solution. For comparison some data for Intalox saddles and Pall rings are also presented. The results in Fig. 100 show that the slopes of the lines for Holpack are quite smaller than those for the random packings. In Fig. 101 the same data for the effective surface area divided by the specific surface area of the packing are presented.

323

4

6

8 1Q-2

L, m/s Fig. 98. Holdup of different constructions of Holpack, Table 35, for a system air-water, as a function of the liquid superficial velocity.

6

8 ia- s

4

es

Fig. 99. Comparison of the experimental date for the dynamic holdup and the line calculated using Eq. (215), see Table 35.

324

200

4

6

8 10"2

L,m/s Fig. 100. Dependence of the effective surface area of Holpack (Table 35) on the liquid superficial velocity. For comparison date, of Raschig rings, 25x25x3 mm (1), Intalox saddles 50 mm (2) and Pall rings 50 mm (3) are also presented.

The data show that the ratio of the two areas reaches up to 3. That is, the surface area of the drops and jets trickling in the free volume of the packing is up to two times larger than the area of the packing itself. For calculating the effective surface area of the Holpack packings the following equation is proposed [187]: 0J8 / , j

L .a

\ 0.066

(216)

a The constant K depends on the type of arrangement of the expanded metal sheets. In case of countercurrent arrangement it is equal to 0.82. At cross arrangement K=QS5. As already noted, the exponents in equation (216) are obtained using the data from absorption of CO2 in NaOH solutions. The influence of CT is additionally proved by evaporation of ethanol solution in air stream according to the method described in 2.1.2.5.9. 93% of the experimental points show differences from the equation less than ± 15%.The comparison of the experimental data and the line after Eq. (216) is presented in Fig. 102.

325

4

6

8 10"2

L ,m/s Fig. 101. Dependence of the ratio of the effective surface area to the specific surface area of the packing Holpack (Table 35) on the liquid superficial velocity. For comparison data of Raschig rings, 25x25x3 mm (1), Intalox saddles 50 mm (2) and Pall rings 50 mm (3) are also presented.

!L OJ

1.0 0.8 0.6

+15%

j^^a—f-trrr •1

as

to

' -

0.4 0.2

10" 5

104

FrL=L2a/g

10" 3

10" 2

Fig. 102. Comparison of the experimental data for the effective surfmee area of the packings given in Table 35 and the line calculated using Eq. (216).

3.2.2.3.2.4. Mass transfer coefficients of Holpack packing 3.2.2.3.2.4.1, Liquid-film controlled mass transfer of Holpack packing. The investigations of the liquid-film controlled mass transfer coefficient are carried out with the packing presented in Table 35 [188], The results under the loading point are given in Fig. 103. For comparison on the same figure data for Intalox saddles and Pall rings are also presented.

326

i

i

to-2

L,m/s Fig. 103. Dependence between the liquid-film controlled volumetric mass transfer coefficient for Holpack packings and the liquid superficial velocity. The symbols are presented in Table 35. 1Pall rings 50 mm; 2- Intalox saddles 50 mm

The comparison shows that the slopes of the lines for Holpack are quite smaller than those for the random packings, which corresponds with the respective slopes for the effective surface area. The comparison of the data for Holpack shows also that the packing of expanded metal sheets with narrower lamellas has lower liquid film controlled mass tansfer coefficient. This fact seems to contradict with the principle that with reduction of the length of the liquid film, the mass transfer coefficient increases. In case of Holpack the main reason for the increasing of the liquid phase controlled mass transfer coefficient is the turbulization of the liquid phase because of collision with the solid surface of the packing lamelks. The reduction of the lamella width leads to reduction of the strength of that collision, and respectively to reduction of the liquid film mass transfer coefficient. A similar effect on the gas-film controlled mass transfer is not to be expected. An equation for calculating of fe of this packing is proposed in [188]:

= 0.0Q113.Retm.S4\Ga™m(sl

\6.1

The dimensionless numbers in this equation are defined as follows:

(217)

327

/?/ is the thickness of the expanded metal sheets, m; The geometrical parameters are explained in Fig. 89. The effective surface area is calculated using Eq. (216). The comparison between the calculated and the experimental data is presented in Fig. 104. The hysteresis observed for the pressure drop of Holpack packing (Fig. 10 of Chapter 2) leads to hysteresis of the liquid-side controlled volumetric mass transfer coefficient [307], In Fig. 11, Chapter 2, the value of Kia at liquid superficial velocity equal to 0.0057 m3/(m2s) in the flooding regime versus the gas velocity wg is presented.

I J8

10-' 8 6 4 +20%

V

4

2

10-2 6

8

10z

S

103

Re. Fig. 104. Comparison of the experimental data for the liquid-film controlled mass transfer coefficient of the packings presented in Table 35 with the line calculated using equation (217).

3.2.2,3.2.4,2. Gas-film controlled mass transfer of Holpack packing. The investigations of the gas-film controlled mass transfer of Holpack are carried out by Daraktschiev, Kolev and Tschapkanova [185] for the packings presented in Table 35. As a model process the absorption of NH3 is used. The values of fe for the packing Nr 2 obtained for two different liquid superficial velocities, 0.0039 and 0.017 m3/(m2s), are presented in Fig. 105. They show that the influence of £ is to be neglected.

328

10-1

9

8

1

6

•- 4

Y

0.2

0-L =0. D039 ,m/s • -L =0. D17 ,m/s

0.4

0.6 0.8 1

wo,m/s

Fig. 105. Dependence between the gas-film controlled mass transfer coefficient of the packing Nr 2 on the gas velocity at two different values of the liquid superficial velocity.

Some of the obtained data [185] for the gas-film controlled mass transfer coefficients are presented in Fig. 106. The results show that practically the distance between the expanded metal sheets influences the gas-side controlled mass transfer only through the effective surface area. It means that the conditions for the mass transfer in the orifices of the expanded metal and in the area between the sheets (see Table 35) are practically the same in regard to the gas-film controlled mass transfer. Nevertheless, the reduction of the hydraulic diameter of the packing leads to increasing of the mass transfer coefficient, like for all other types of packings. For calculation of fcthe following equation is proposed [185]: JUS

(218)

where She and Rec are determined with the hydraulic diameter of the orifices of the expanded metal. The comparison of the experimental data with the calculated line is presented in Fig. 107.

329

3.2.2.3.3. Comparison of the packing Hoipack with other highly effective types ofpackings The comparison of the pressure drop of different packings of expanded metal [169] is presented in Fig. 108. The geometrical characteristics of the compared packings are given in Table 36. Here the Kolar packings are marked with K, Holpack- with H and Spraypack- with S. From the figure it is seen that with comparable specific surface areas and at the same gas velocity, the pressure drop of the Kolar packing is about 20 times lower than that of Holpack and about 100 times lower than of Spraypack. The comparison of the liquid-side mass transfer coefficients of Holpack and of the packing of Kolar at equal liquid superficial velocities shows that they are significantly higher for the Holpack. The maximal values of these coefficients are approximately the same. The comparison of the gas-film controlled mass transfer coefficients shows also that the values for the Holpack at the same gas velocity are quite higher. The comparison of the maximal values for both types ofpackings shows that they are practically the same.

10 3




-n.

OB

12

ft*

r 24

w

/I

2fl

12

16

05

Gas capacity factor F& (m/s)(kg/i7fi)

Fig. 117, Performance data of Ralu Pak 250YC sheet metal structured packing (Fig. 111) at three different pressure levels (total reflux).

342

A shorteut method for estimation of corrugated structured packings is presented by Carrilo et al. [275]. The date for the liquid holdup of Montz sheet metal structured packing [177] versus the liquid superficial velocity are presented in Fig. 125, for Mellapak 250 Y and for Gempak 230 A2T at Fig. 126 and 127 respectively. 4 3 : 2 : 1 : 0

D——c

120 100 80

.-6 g ^I

60fin-

Jin—£>•——O-jL3

A

_ • — * >."

—

"

\c

11

Ger npakA2T 66.1^mbar d G =