coupling phenomena in the removal of chlorinated hydrocarbons by means of pervaporation

Journal of Membrane Science, 78 (1993) 135-145 Elsevier Science Publishers B.V., Amsterdam

135

Coupling phenomena in the removal of chlorinated hydrocarbons by means of pervaporation

S. Goethaert, C. Dotremont, M. Kuijpers, M. Michiels and C. Vandecasteele* Department of Chemical Engineering, Katholieke Universiteit Leuven, De Croylaan 46, B-3001 Leuven (Belgium)

(Received July 20,1992; accepted in revised form November 11,1992)

Abstract

In some pervaporation systems, coupling effects may play an important role. This occurs e.g. in the pervaporation of a multicomponent mixture of chlorinated hydrocarbons (Cl-HCs) in water through organophilic membranes. Several mixtures of Cl-HCs were pervaporated through organophilic PDMS membranes. The membranes were composite membranes: a dense top layer consisting of PDMS (poly- dimethylsiloxane), filled with hydrophobic zeolite on a layer consisting of porous PAN (polyacryloni- trile) on polyester fabric. The pervaporation experiments were carried out with ternary mixtures of water, chloroform and a second Cl-HC, as well as with acetone and isopropanol. The concentration of chloroform was kept constant during all experiments. The concentration of the other organic component was varied. In all experiments a decrease of the flux of the organic components was noticed for the ternary mixtures with respect to the binary mixtures. To investigate whether the coupling occured due to the zeolite, additional pervaporation experiments were performed with PDMS membranes without zeolite, with binary mixtures as well as with ternary mixtures of all components. With these membranes, almost no coupling phenomena could be observed.

Keywords: membrane separation; pervaporation; aqueous mixture of chlorinated hydrocarbons; COU-

pling; organophilic membrane

1. Introduction

Chlorinated hydrocarbons (Cl-HC) are widely used in industrial processes. They are in general very toxic and thus constitute a great threat for the quality of ground- and surface water. Normally these contaminants are re- moved by means of one of the following techniques: aeration, ozonisation, adsorption on activated carbon, steam stripping. These techniques all have in general at least one major disadvantage: aeration moves the problem to-

*To whom correspondence should be addressed.

wards air pollution, so that an post-treatment, such as adsorption on activated carbon, is nec- essary; ozonisation can form new products which are more harmful than the original ones; adsorption on activated carbon is an expensive technique and should therefore only be used with very small quantities of contaminants in the wastewater. The major cost of this technique is the regeneration of the activated carbon.

Pervaporation can in principle be used to re- move Cl-HC from water in an economically interesting way [ 11. Adsorption on activated carbon seems to be most interesting for the removal

0376-7388/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

136

of very low concentrations of Cl-HCs from wastewater ( < 10 g/m”); whereas for higher concentrations pervaporation is one of the least expensive techniques, since in these cases the cost for regeneration and replacement of the activated carbon filter becomes too high [ 21. The main advantages of pervaporation are the flexibility in design, the low energy cost and the possibility of solvent recycling. As the energy demand is proportional to the amount of the component to be evaporated, pervaporation is most suitable for the removal of small quantities of contaminants from a bulk liquid [ 31.

Usually, the wastewaters considered consist of a mixture of Cl-HCs and other organic components. For this reason it is important to take into account the influence of other components on the fluxes of the Cl-HCs (coupling).

Coupling phenomena [ 41 are difficult to de- scribe or predict quantitatively, or even to mea- sure quantitatively. Coupling phenomena may occur in the sorption of the molecules in the membrane as well as in the kinetic aspects of the diffusion through the membrane [4]. In- direct information about flow coupling is thus obtained when thermodynamic interactions (or preferential sorption) are considered in rela- tion to selective transport. Preferential sorption has been studied for many systems and it has been shown that in many different polymeric materials and with many different liquid mixtures the component that is sorbed prefer-

S. Goethaert et al/J. Membrane Sci. 78 (1993) 135-145

entially also permeates preferentially [ 41. However, as will be shown, preferential sorption is not the only parameter in the description of coupling phenomena. Flow coupling [ 41 may indeed also affect the pervaporation results.

2. Experimental

All experiments were carried out in the laboratory test cell (Lab Test Cel Unit from GFT- Le Carbone, Neunkirchen-Heinitz, Germany) shown in Fig. 1. The feed is warmed up in a pressure-tight stainless steel reservoir with an electric heater. The recirculation pump ex- tracts liquid from the reservoir and feeds it to the membrane module which contains flat sheet membranes with a diameter of 6”) from which it returns to the reservoir. The permeate is col- lected in a glass condenser placed in a dewar filled with liquid nitrogen. The permeate col- lector is connected to a vacuum pump, the total pressure being kept constant by a vacuum con- troller. All experiments were performed under the same conditions: feed temperature = 50’ C, permeate pressure = 15 mbar.

The coupling phenomena were studied with ternary mixtures of Cl-HCs (mainly water, chloroform and a second Cl-HC ). To get a clear view on the interferences occurring with the pervaporation of multicomponent mixtures, results for the pervaporation of the third com-

I ‘U

retentete

c membrane

feed I module

tank 1 I

/ pWlVtOd@

electtkal heater

Fig. 1. Experimental set up.

S. Goethaert et al./J. Membrane Sci. 78 (1993) 135-145 137

cont. in retentate [ppml 400,

cont. in permeate IwtKl 140

100 - 3-

so -

0 0

1 20

1 40

time [mid

-a 1 0

60 t 80

- chloroform In rot. + component 2 In rd.

+ chloroform In perm. -a- component 2 In perm.

Fig. 2. Concentration of chloroform and component 2 vs. time in retentate and permeate.

ponent should be obtained for constant chloroform (CHCl,) concentration. We selected a chloroform concentration of 250 ppm (0.003774 mole% ) . Since it was not possible to recycle the permeate in order to keep the feed concentration constant, the concentration of all components both in the retentate and in the permeate was measured as a function of time. One can then easily determine at what time the concentration of chloroform reaches 250 ppm and de- duce the corresponding concentrations of the other components (Fig. 2). All components were also investigated as binary mixtures (water-Cl-HC).

In the experiments two types of organophilic composite membranes were used: zeolite-filled membranes consisting of a dense PDMS top layer (ca. 30 pm) filled with hydrophobic zeolite (silicalite, 60% filling degree) on a support layer composed of porous PAN on polyester fabric, and non-filled membranes with a uni- form PDMS top layer (20 pm). The membranes were supplied by GFT-Le Carbone ( Neunkirchen-Heinitz, Germany).

2.1. Pervaporation of a water-chloroform- trichloroethene mixture

Zeolite-filled membrane According to Fig. 3 the flux of chloroform de-

creases with increasing concentrations of trichloroethene ( C2HC13). The decrease for a water-chloroform-trichloroethene mixture is extremely rapid at low concentrations of trichloroethene. The flux reaches an asymptotic value of about 35% of the binary flux of chloroform (i.e. concentration of C2HC13 = 0). The flux of trichloroethene is also lower than the corresponding binary flux. The pervaporation performance of both components is thus clearly reduced by the presence of the other Cl-HC and both components present a lower alpha-selectivity in the ternary mixtures with respect to the binary mixtures. In the area where the chloroform flux reaches an asymptotic value, the mean selectivity of chloroform is about 450 (binary mixture: lOOO), and that of trichloroethene is 600 (binary mixture: 1200). The water flux remains almost constant for all concentra-

138 S. Goethaert et al./J. Membrane Sci. 78 (1993) 135-145

flux Cl-HC [mole/m2.h1 0.7

x 0.6 -

ss ,_e ____

_<*-

_- ,’ *’

0 10 20 30 40 60 60 70

mole% C2HCl3 in retentate (*E+04)

- CHCl3 ternary * C2HCl3 ternary +-- C2HCl3 binary

zeollto-flllod mombruw

Fig. 3. Flux of Cl-HC vs. mole% trichloroethene in retentate for a zeolite-filled membrane.

tions of trichloroethene (6.5 moles/m’-hr ) . alpha selectivity:

Xi ( > l -Xi permeate

feed

where Xi is weight fraction of component i.

Non-filled membrane Figure 4 shows the fluxes for the pervapora-

tion of the same system through a non-filled membrane. A similar but much smaller decline of the chloroform flux can be noticed. The flux of trichloroethene is hardly influenced by the presence of chloroform. The water flux remains almost constant (20.5 moles/m2-hr). The selectivity of the membrane for the Cl-HC is lower than with the filled membrane due to the increase in water flux.

2.2. Pervaporation of a water-chloroform- tetrachloromethane mixture

Zeolite-filled membrane Similar phenomena can be observed for the

pervaporation of a water-chloroform-tetrachloromethane mixture (Fig. 5) through a zeolite-tilled membrane. The selectivity for chloroform diminishes (ternary mixture: 500) with respect to the binary pervaporation experiment (binary mixture: 1000). The mean selectivity of tetrachloromethane (Ccl, ) is 700 (binary: 1000). The influence of chloroform on the flux or permeability of tetrachloromethane is rather small.

Non-filled membrane The decline of the flux of chloroform is

smaller than in the pervaporation through the zeolite-filled membrane (Fig. 6). The flux of tetrachloromethane is practically equal to the binary flux.

S. Goethaert et al./J. Membrane Sci. 78 (1993) 135-145

flux Cl-HC [mole/m2.hl 0.7

0.3 -

0.6 -

0.4 -

1 I

J

10 20 30 40 60 60

mole% C2HCl3 in retentate (*E+04)

0 C2HCl3 ternary -X- C2HCl3 binary

non-llllod mombruw

Fig. 4. Flux of Cl-HC vs. mole% trichloroethene in retentate for a non-filled membrane.

flux Cl-HC [mole/m2.h1

10 20 30

mole% CC14 in retentate (*E+04)

40

* CHCD ternary + CC14 ternary -X- CC14 binary

139

zoollto-flllod membrane

Fig. 5. Flux of Cl-HC vs. mole% tetrachloromethane in retentate for a zeolite-filled membrane.

2.3. Pervaporation of a water-chloroform- isoproparwl mixture

Zeolite-filled membrane Isopropanol (C,H,OH) is a component with

a very low permeability through zeolite-filled

membranes. However the effect on the flux of chloroform is comparable in magnitude with the influence of the previously mentioned Cl-HC (Fig. 7) and the same asymptotic character of the decrease is noticed (note two different scales for the fluxes).

140 S. Goethaert et al.fJ. Membrane Sci. 78 (1993) 135-145

flux Cl-HC [mole/m2.hl

0 6 10 16 20

pole% CC14 in retentate (+E+04)

- WC13 ternary + CC14 ternary -*- CC14 binary

non-fIlled mombrano

Fig. 6. Flux of Cl-HC vs mole% tetrachloromethane in retentate for a non-filled membrane.

flux CHC13 [mole/m2*hl

- 0.016

* _-

_** - 0.01

- 0.006

0

mopB”x C3H70H ;?retentate $?l4) 200

. CHCI3 lomary f C3H70H ternary *. C3HlOH binary

zeollto-flllod mombrano

Fig. 7. Flux of chloroform and isopropanol vs. mole% isopropanol in retentate for a zeolite-filled membrane.

The selectivity of isopropanol is very small ( < 10) with respect to chloroform (500). Cou- pling effects also have a negative influence on the flux of isopropanol. The binary flux of chlo-

roform in this and in the next experiment is smaller than in the previous experiments. The reason is that a new set of membranes was used which was produced on an industrial scale


whereas the first set was produced in the laboratory.

Non-filled membrane Figure 8 shows that the effect of isopropanol

on the flux of chloroform is similar to the influence of the previously discussed Cl-HC. The flux of chloroform reaches asymptotically a minimum value. Coupling effects have no ef- feet on the flux of isopropanol.

flux CHCl3 Imole/mS*hl flux C3H70H Imole/mPhJ 0.3 0.06

0 60 100 160 200

mole% C3H70H in retentate (*E*04)

. CHCIS twnuy + C3WOH ternary -* C3H70H binary

non-filled mwnbruw

Fig. 8. Flux of chloroform and isopropanol vs. mole% isopropanol in retentate for a non-filled membrane.

flux CHC13 lmole/m2*hl

0.1

i 0.06

mo6PB% C3H60 il%tentate (+E?4)

flux C3H60 lmole/m2*hl 0.08

m CHC13 torn& i- CBHBO tornair -*- C3HBO blnalr

-oIlto-flllod membrane

Fig. 9. Flux of chloroform and acetone vs. mole% acetone in retentate for a zeolite-filled membrane.

14z S. Goethaert et al. jJ. Membrane Sci. 78 (1993) 135-145

flux CHCl3 Imole/mP*hl 0.3.

0.2 -

flux C3H60 Imole/m2*hl o ,2

-0.1

* .’

. ..I. ,’ - 0.08

- 0.06

I ‘0 0

mr%% C3H60 itkntate (*E1+6ooq) 200

- CHC13 tomary + C3tte.O ternary -* C3H30 bhty

non-tlllod mombrano

Fig. 10. Flux of chloroform and acetone vs. mole% acetone in retentate for a non-filled membrane.

2.4. Pervaporation of a water-chloroform- acetone mixture

Zeolite-filled membrane The permeability of acetone (C,H,O)

through the zeolite-filled membrane is comparable with the results for isopropanol. How- ever, one can see very clearly in Fig. 9 that the flux of acetone in the pervaporation experiment with the ternary mixture is higher than for the binary mixture. The effect of chloroform on acetone is positive, although not very large. The decrease of the chloroform flux is also more gradual.

Non-filled membrane The same gradual decrease of the chloroform

flux can be observed in Fig. 10. The flux of acetone is again higher when the ternary mixture of water, chloroform and acetone is pervaporated.

3. Discussion

The function of the zeolite in the dense top

layer of the membrane is to build up a barrier for the permeation of the water molecules; whereas the organic molecules sorb preferentially in the zeolite [ 51. This effect can be noticed when the partial fluxes in the pervaporation of binary mixtures (water + Cl-HC ) with zeolite-filled and non-filled membranes (Fig. 11) are compared. When the non-filled membrane is used the water flux doubles with respect to the water flux obtained with the filled membrane. The flux of thrichloroethene is influenced positively by the presence of the zeolite, which indicates the preferential sorption of the organic component into the zeolite. Pref- erential sorption of alcohols in the zeolite of zeolite-filled membranes was also observed by Bartels-Casper et al. [ 61.

When the partial fluxes in the pervaporation of ternary mixtures of water, chloroform and another Cl-HC with zeolite-filled and non-filled membranes are compared, it is noticed that the zeolite has a smaller influence. In Figs. 3-6 the flux of the Cl-HC (except for chloroform) in the ternary mixtures is almost the same for both types of membranes. Likewise the asymptotic


flux C2HCl3 lmole/m2.h1 flux water [mole/m2.hl 1 ,25

P

0.8 ‘t water _

“20

0 10 20 30 40 50 80

mole% C2HC13 in retentate (*E+O4)

+ Mod mombrano

-A- Illlod mombrano

* non-fIlled membrane

-s- non-llllod mombrano

Fig. 11. Flux of trichloroethene and water vs. mole% tricbloroethene for a non-filled membrane and a zeolite-filled membrane.

value of the chloroform flux, and thus the over- all permeability, is almost identical for both types of membranes. The only remaining function of the zeolite is to reduce the water flux.

A remarkable observation can be made when a mixture of water, chloroform and a third component such as isopropanol or acetone is pervaporated. Although the permeability for isopropanol and acetone is very low, the flux of chloroform decreases with increasing concentration of these two components. Comparing the flux of isopropanol and acetone through zeolite-filled and non-filled membranes, one can see that the flux of acetone is higher in the experiment with the ternary mixture than in the one with the binary mixture; the opposite effect appears when isopropanol is pervaporated.

An explanation of the coupling effects can be found in the observation of the coupling between the fluxes of the different components. Coupling phenomena can be observed on two levels: (1) sorption and (2) diffusion.

The organic component which is sorbed preferentially hampers the sorption of the other components. If the difference in sorption behaviour is large, some components can be excluded from the zeolite so that the transport of these molecules takes place through the PDMS film. To know which component sorbs best, binary and ternary sorption data would be useful.

Sorption experiments from the liquid phase were performed with five binary mixtures. The toplayer of the zeolite-filled membrane was im- mersed in a mixture of water and a Cl-HC. After 24 hr (when equilibrium was reached) the difference in concentration between the sample and a blank (without membrane) was measured. From these data, sorption isotherms of the five Cl-HCs could be calculated (Fig. 12). Trichloroethene sorbs at a higher level, dich- loromethane and chloroform show an inter- mediate sorption behaviour, whereas tetrachloromethane exhibits the lowest sorption. Analogous experimental work was done by

144 S. Goethuert et al./J. Membrane Sci. 78 (1993) 135-145

30 norbed (mol/g) dE-4)

25 -

J

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 concentration (mol/l)

- CH2Cl2 + click3 * C2HCl3 0 CPCI4 x cc14

Fig. 12. Sorption isotherms for five Cl-HCs in the top layer of a zeolite-filled PDMS membrane.

Blume et al. for PDMS membranes without filler [7].

From this point of view the results of the water-chloroform-trichloroethene experiment can be explained. Since trichloroethene is sorbed preferentially, chloroform is excluded from the zeolite. For this reason chloroform diffuses mainly through the PDMS, not through the zeolite, which results in a smaller permeation rate. Since the zeolite contains mainly trichloroethene, one would expect that the flux of trichloroethene in the binary and ternary mixture is almost equal. However, the flux of trichloroethene has clearly decreased in the ternary system so that apparently not only sorption effects occur. The diffusion of trichloroethene through the zeolite may be ob- structed by small amounts of chloroform in the zeolite. Comparing the results of the water- chloroform-trichloroethene and the water- chloroform-tetrachloromethane mixtures, the ternary fluxes of all components are practically equal. Although trichloroethene sorbs better, and tetrachloromethane sorbs worse than chloroform, no difference between the fluxes can be

noticed. This again indicates that not only sorption effects occur.

Flow coupling can also be observed for the pervaporation of water, chloroform and a non- chlorinated organic component. Although the sorption of isopropanol and acetone is much smaller than that of chloroform, the influence on the chloroform flux is comparable with the results of the Cl-HC.

Comparing the decline of the flux of chloroform in the experiments with the two types of membranes, the influence on the flux is deli- nitely smaller with non-filled membranes.

The decline of the flux in ternary mixtures could also be explained as a consequence of the lower concentration in the zeolites. If the num- ber of vacancies in the zeolite is limited, the sorption of both components will be lower than in the pervaporation experiments with binary mixtures, unless all components except one are excluded from the zeolite.

The water flux is almost constant for all experiments with one type of membrane, and is thus not affected by the presence nor the concentration of the HC. The transport of water

S. Goethaert et al./J. Membrane Sci. 78 (1993) 135-145

takes place through the PDMS, since the zeolite is hydrophobic.

Since coupling more usually refers to in- creases of flux due to the presence of a third component, one might be reluctant for using the term “coupling” in this case. Here, in most cases inhibitions to transport phenomena occur due to the presence of a second organic component.

4. Conclusion

The transport phenomena in the pervaporation of multicomponent mixtures are much more complicated than in the case of binary mixtures. The permeation behaviour of each component is influenced by the other components. Coupling phenomena occur on two levels: preferential sorption and flow-coupling. By adding zeolite to the membrane the description of the process becomes even more difficult. Whereas transport takes place partly through the PDMS film and partly through the zeolite, coupling phenomena occur in both areas, albeit mainly in the zeolite.

6. Acknowledgements

Grateful acknowledgement is made to dr. Briischke (Deutsche Carbon) and ing. Man-

145

gelschots (Le Carbone Belgium) for providing the membranes, and for useful discussions. This study has been performed partly within the framework of a project supported by the Ge- meenschapsminister van Leefmilieu, Natuur- behoud en Landinrichting in accordance with the Vlaams Impulsprogramma Milieu- technologie.

References

I. Blume and C.A. Smolders, Membrane processes in environmental technology, Seminar application of Membrane Processes In Environmental Problems, Maastricht, 1991. H. Nijhuis, Removal of trace organics from water by pervaporation, Ph.D Thesis, University of Twente, Enschede, The Netherlands, 1990, Chapter 6. C. Dotremont, S. Goethaert and C. Vandecasteele, Ver- wijdering van gechloreerde koolwaterstoffen uit afwal- water door pervaporatie, Het Ingenieursblad, 7/8 (1992) 27-33. M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1991. H.J.C. te Hennepe, Zeolite filled polymeric membranes, Ph.D Thesis University of Twente, Enschede, The Netherlands, 1988, Chapter 3. C. Bartels-Caspers, E. Tusel-Langer and R.N. Lichten- thaler, Sorption isotherms of alcohols in zeolite-filled silicone rubber and in PVA-composite membranes, J. Membrane Sci., 70 (1992) 75-83. I. Blume, P.J.F. Schwering, M.H.V. Mulder and C.A. Smolders, Vapour sorption and permeation properties of poly(dimethylsiloxane) films, J. Membrane Sci., 61 (1991) 85-97.

coupling phenomena in the removal of chlorinated hydrocarbons by means of pervaporation

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