Pervaporation behaviour of chlorinated hydrocarbons through organophilic membranes

Desalination, 91 (1993) 177-186 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

177

Pervaporation behaviour of chlorinated hydrocarbons through organophilic membranes C. Dotremont, S. Goethaert and C. Vandecasteele Departmentof Chemical Engineering, KatholiekeUniversiteitLeuven, De Croylaan46, B-3001 Leuven (Belgium) (Received November 12,1992; in revised form February 10,1993) SUMMARY

The pervaporation behaviour of chlorinated hydrocarbons (Cl-HC’s) was studied in detail. The permeability of different Cl-HC’s through a hydrophobic membrane was considered in relation to their molecular structure and some of their physicochemical properties. The hydrocarbons selected for this study differed in: the number and position of chlorine atoms, the chain length, the degree of branching and the presence of a double bond. The membranes were composite membranes: a dense top layer consisting of PDMS (polydimethylsiloxane), filled with hydrophobic zeolite on a layer consisting of porous PAN (polyacrylonitrile) on polyester fabric. SYMBOLS

J; pi D; S; I

-

ci

-

Pi

-

s 1,2 -

molar flux of component i (mol/m2s) permeability of component i (m2/s) diffusion coefficient of component i (m2/s) solubility coefficient of component i (-) membrane thickness (m) molar concentration of component i (mol/m3) partial pressure of component i (Pa) solution retentate and permeate side of the membrane respectively

Keywords: membrane separation, pervaporation, chlorinated hydrocarbons, organophilic membrane, permeability OOll-9164/93 /$06.00 01993

Elsevia

Science Publishers B.V. All rights reserved.

178 INTRODUCTION

Chlorinated hydrocarbons (Cl-HC’s) arc undoubtedly among the main toxic components in waste waters and constitute a serious threat to ground and surface water. They originate from industrial processes. The traditional ways of removing halogenated organic compounds include intensive aeration, ozonisation and adsorption on activated carbon. However, these techniques are either inadequate and do not yield the required low levels of pollutant, or are too expensive for waste water treatment [ 11. Pervaporation is a membrane process which can be used for the direct removal, e.g., of Cl-HC’s from waste water. This can be achieved by using very selective hydrophobic membranes. Since the energy demand in the process is proportional to the amount of the component to be evaporated, pervaporation is only suitable to remove small quantities of contaminants from a bulk liquid [24]. Pervaporation performance can be expressed by two parameters: flux and selectivity. According to the solution-diffusion model of Lee [5], the flux of component i through a homogeneous flat membrane under steady isothermal conditions can be described as

(1) Pi = S;D;

(2)

From the literature it is already known that the pervaporation performance of different mixtures under identical conditions can vary quite remarkably [6-g]. The purpose of this study was to get more information about the permeability of different Cl-HC’s through organophilic membranes and to relate the obtained performance to their structure and some of their properties such as molecular geometry, polarity and molar volume. With the obtained results, predictions could be made about the permeability of an organic component and it may even be possible to improve the membrane properties. EXPERIMENT&

Membranes

All experiments were carried out with flat, composite membranes consisting of a dense top layer (ca. 30 pm) of polydimethylsiloxane (PDMS), filled with hydrophobic zeolite (silicalite, 60% filling degree). The PDMS is fixed on a porous polyacrylonitrile (PAN) layer on polyester fabric.

179

Apparatus All experiments were carried out in the laboratory test cell (Lab Test Cell Unit from GFT4.e Carbone, Neunkirchen-Heinitz, Germany). The feed is warmed up in a pressure-tight stainless steel reservoir with an electrical heater. The recirculation pump extracts liquid from the reservoir and feeds it to the membrane module which contains a flat sheet membrane with a diameter of 6”, from which it returns to the reservoir. The permeate is collected in a glass condenser placed in a dewar filled with liquid nitrogen. The permeate collector is connected to a vacuum pump, the total permeate pressure being kept constant by a vacuum controller. Concentration polarization was studied extensively for this type of test cell by varying the Reynolds number between 100 and 700. For Reynolds numbers below 380, a decrease of the flux could be observed. In this study, however, all experiments were carried out at the same feed flow rate, corresponding with a Reynolds number of 380 so that any influence of concentration polarization could be ignored. RESULTS

Pervaporation experiments were carried out with different binary mixtures (Cl-HC/water). The retentate and permeate were analysed with gaschromatography at frequent intervals. From these data and flux measurements, the permeability (P*) of Cl-HC’s can be calculated, using Eqn. (1) (P* = Pi/Z). The obtai ned permeabilities (P*), the dipole moment (p) and the molar volume (M-V.) are listed in Table I. All experiments were carried out under the same conditions: feed temperature = 5o”C, permeate pressure = 10 mbar. EfSect of a double bond The effect of a double bond is clearly illustrated by considering the

permeability of trichloroethene (C~HCIJ) and 1,1,2-trichlorocthane (1,1,2CzHjClj). The former shows a considerably higher permeability than the latter. Comparing the permeability of 1,l -dichloroethane ( 1,l -C$I&) with 1,1-dichloroethene (l,l-C2H2C12) and the permeability of 1,1,2,2tetrachloroethene (1 ,1,2,2-C2H2C14)with tetrachloroethene (CzC14) leads to the same conclusion: the Cl-HC with double bond shows the highest permeabiltity.

180

TABLE I Dipole moment, molar volume and measured permeability through a zeolite filled PDMSmembrane for different Cl-HC’s

P* (10-3m/h)

Component

P(D)

CHzClz

1.60

64.43

20

CHC13

1.01

80.66

67

CC14

0

97.15

29

l,l-C*l!I&l*

2.06

84.72

18

1,l -CzH2C12

1.34

86.8

37

l,l,l-C2H$&

1.78

100.28

37

1,l ,2-C2H3Clj

1.25

93.0

43

1,i ,2,2-cz~2c4

1.29

105.8

38

C2HC13

0.77

90.3

148

C2C4

0

102.8

50

l-CjHyCl

2.05

91.7

86

2-C3H7C1

2.17

91.84

25

l-&H&l

2.05

105.2

59

i-C,H&l

2.0

105
36

t-C4H&1

2.13

110.7

0

M.V. (cm3/mol)

Effect of the chain length The permeability of 1-chloropropane (l-CsH,Cl) is considerably higher than the permeability of 1-chlorobutane (l-C4H&l). The same trend, although not always very pronounced, can be observed when the permeability of dichloromethane (CH2C12)is compared with the one of 1, ldichloroethane ( l , l-C&C12) and the permeability of chloroform (CHCl3) with the one of 1 ,1,1-trichloroethane ( 1,l,l-C2H3Cls). It is apparent that a longer chain length results in a lower permeability. The molecules considered differ each time by one CHz-group.

181

Position of the chlorine atom

Although 1-chlorofiopane (lGH7Cl) and 2chloropropane (2-C3H$l) are very similar molecules, they differ quite considerably in permeability. However, the influence of the position of the chlorine atoms has to be considered in each individual case. The difference in permeability between l,l,l-trichloroethane (l,l,l-C~H3Cl3) and l,l,Ztrichloroethane (l,l,2-C2H3C13) is much smaller, the latter showing a somewhat higher permeability. Degree of branching The permeability of 1-chlorobutane ( 1-C4H&l),

1-chloro-Zmethylpropane (i-CaH&l) and 2-chloro-Zmethylpropane (t-CdH&l) were mutually compared: the higher the degree of branching, the lower the resulting fluxes. Additional experiments with 2-chloro-Zmethyl-butane (t-CsHiiCl) confirmed this statement, since this highly branched organic component showed hardly any permeability. Number of chlorine atoms

To see the influence of the number of chlorine atoms, pervaporation experiments were carried out with dichloromethane (CH2C12), chloroform (CHCl3) and tetrachloromethane (CCb). Chloroform showed the highest permeability, followed successively by tetrachloromethane and dichloromethane. The same observation was made when the permeabilities of 1, I-dichloroethene (l,l-C2H2Q), trichloroethene (CzHC13) and tetrachloroethene (C&b) are compared: the trichloro-HC has the highest flux and the dichloro-HC the lowest flux. DISCUSSION

In order to explain these phenomena, the transport mechanism through the zeolite-filled membrane must be sufficiently well understood. According to the model of te Hennepe [lo] the transport of the organic molecules takes place mainly through the hydrophobic zeolite pores. During the transport through the active layer, the organic molecules are adsorbed and desorbed by subsequent zeolite particles. Consequently, the organics can travel along a straight line. Water molecules, on the other hand, are far too polar to sorb in the zeolite, and have to travel around the zeolite particles.

182

Properties of the Cl-HC’s that will enhance the sorption in the top layer (in the zeolite) can be summarized as follows: the molecules must be apolar and must have a structure that allows them to penetrate in the zeolite without any steric hindrance. Permeability, however, is not only the result of sorption, but is also dependent on the diffusion velocity of the organic component through the membrane; the smaller the Cl-HC’s, the faster their transport through the membrane and the larger their resulting diffusion coefficient. The favourable effect of a double bond on the permeability can be explained by the reduced polarity of these molecules with respect to analogous components without a double bond (see Table I). In addition, the double bond gives these molecules a plane geometry which favours transport through the small zeolite pores without steric hindrance. With increasing chain length, the size of the molecules increases, of course, which results in a slower diffusion and by consequence, a smaller permeability of the Cl-HC’s in the pervaporation process. This effect is especially pronounced for linear molecules with a given chain length (e.g. l-C3H7Cl and l-CdH&l). From a structural point of view, the mentioned molecules are identical. Moreover, they show the same polarity (Table I). The large difference in permeability can only be explained by their different chain length. For smaller molecules (Ci and Cz), on the other hand, the diffusion velocity is not critical, so that other parameters, for instance, polarity and molecular geometry, determine the permeability. Although 2-CsH7Cl and l-C3H7Cl present nearly the same p and M.V. (Table I), their difference in permeability is quite remarkable. The position of the chlorine-atom must play a prominent role. The molecular structure of l-C3H7Cl can be considered as a polar chlorine atom and an apolar tail (&chain). The apolarity of the 2-C3H7Cl-tail is destroyed by the addition of a chlorine atom on the second carbon atom. Besides, l-C3H7Cl has a linear structure, while 2-C3H7Cl can be seen as an iso-geometry, its linearity being interrupted by the voluminous chlorine atom. For 1,l , l-C2HQ and 1,1,2-C2H&% other assumptions should be made, since these molecules differ substantially in M.V. and ~1(Table I). Both parameters favour the permeability Of 1, 1,2-C2H3Cl3. By comparing the permeability of l-C+H&l, i-&H&l and t-C4H9Cl, the influence of the degree of branching can be studied. There is hardly any difference in polarity between these Cl-HC’s. The difference in permeability between these organic molecules can only be explained by their molecular structure. The molecule with the highest degree of branching

183

(t-C,+H&l) showed hardly any permeability. Preferential sorption of organic molecules occurs in the pores of the hydrophobic zeolite (silicalite) which have a diameter of about 0.55 nm. One can easily understand that the size of the molecules determines whether or not penetration in the pores occurs. Totally branched molecules like t-CdH&l and t-CsHirCl are rejected from the silicalite pores due to their large size. To prove this statement, an additional pervaporation experiment (feed temperature 5o”C, feed concentration 2940 ppm, permeate pressure 10 mbar) was carried out with the mixture t-&H&l/water. An unfilled composite membrane was used: PDMS without zeolite as top layer on a layer of porous PAN on polyester fabric. The feed concentration in function of the pervaporation time for the filled and unfilled membrane is shown in Fig. 1. With the retentate concentration t-C4HQCI (90

80 -

80 -

40 -

20

t OI 0

I

60

100

*

I

I

I

200 160 250 pervaporation time (mid

without zeolite

q

I

300

350

with zeolite

Fig. 1. Retentate concentration in 96 of the initial feed concentration in function of the pervaporation time with a filled and an unfilled membrane.

filled membrane, the observed removal of t-&H&l was marginal; with the unfilled membrane, a 20% removal was reached after less than 5 hr of pervaporation time. The obtained mean partial fluxes are given in Table II. The partial fluxes oft-&H&l are rather small for the unfilled as well as for the filled membrane. However, the former presented the highest butylchloride flux. The water flux, as could be expected for this type of membrane, was more than twice as high as for the filled membrane. This experiment undoubtedly proves that t-&H&l is rejected from the pores of the zeolite. Instead of improving the sorption, the zeolite has become an obstacle for the butylchloride molecules. The organic and water molecules have to pass

184

TABLE II Mean fluxes by the pervaporation of the mixture t-C.+H&l/waterwith a filled and an unfilled membrane

unfilled membrane filled membrane

flux t-&H&l (g/m’h)

flux Hz0 (g/m2h)

16.10

441.20

7.20

181.29

around the silica&. Since the filling degree of the zeolite filled membrane is 60% there is only 40% PDMS left for both butylchloride and water to pass. The result is a lower t-&H&l and water flux with respect to the partial fluxes obtained with the unfilled membrane. When the number of chlorine atoms increases from two to four, an optimum in permeability is achieved for the trichloro-HC’s [7]. Nevertheless, the polarity decreases radically when the number of chlorine atoms increases (Table I). One would expect consequently a high sorption capacity for CC4 and C2C4 (p = 0) and thus a high permeability. To know which component sorbs best, sorption experiments from the liquid phase were performed with five binary mixtures. The top layer of the zeolite-filled membrane was immersed 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-HC’s could be calculated (Fig. 2). Trichloroethene and tetrachloroethene sorb best, dichloromethane and chloroform show an intermediate sorption behaviour, whereas tetrachloromethane exhibits the lowest sorption. Due to their plane structure and low polarity, the high sorption capacity for trichloroethene and tetrachloroethene was expected and is in full agreement with their high permeability. The sorption of CC4, however, is outstandingly low and can only be explained by its molecular structure and size. Tetrachloromethane has an analogous structure as t-butylchloride, although it is somewhat smaller. It can thus be assumed that, like t-butylchloride, tetrachloromethane has also to pass around the zeolite instead of penetrating into the pores, which results in a lower flux. Since CC4 is smaller than t-butylchloride, its exclusion from the pores is not as complete.

185 sorbed (mol/g) (lE-4) 30 25 -

0.002

’

CHZCM

0.004

0.006 0.008 0.01 concentration (mol/l) +

CHCIO

*

CZHCl3

0

0.012

C2Cl4

0.014

x

0.018

cc14

Fig. 2. Sorption-isotherms of Cl-HC’s in the top layer of a zeolite filled membrane.

CONCLUSION

Permeability of an organic component is the result of its sorption and diffusion in the membrane. All factors (exterior and interior) that improve the sorption and/or the diffusion of the HC will enhance its permeability. Small, linear (unbranched) and apolar molecules present a high permeability through a zeolite filled PDMS membrane. Trichloro-HC’s show a higher performance than dichloro-HC’s. The highest permeabilities are obtained for Cl-HC’s with a double bond. The smallest permeabilities are established for t-C4H&l and t-C5HiiCl. Both molecules are liable to steric hindrance and due to their size they are excluded from the zeolite pores. One should better use in this case an unfilled membrane or a membrane filled with a zeolite with a pore size larger than 0.55 nm. ACKNOWLEDGEMENTS

Grateful acknowledgement is made to dr. Brtischke (Deutsche Cat-bone) and ing. Mangelschots (Le Carbone Belgium) for providing the membranes, and for useful discussions. We would like to thank S. Schoeters and S. Opdenakker for their contribution to this work. This study has been performed partly within the framework of the Glaams Impulsprogramma Milieutechnologie” of the “Gemunschapsminister van Leefmilieu, Naturrbehoud in Landinrichting.”

186 REFERENCES 1

I. Blume and C.A. Smolders, Membrane Processes in Environmental Technology, Seminar Application of Membrane Processes in Environmental Problems, Maastricht, 1991.

2

H. Nijhuis, Removal of trace organics from water by pervaporation, Ph.D. thesis, University of Twente, Enschede, The Netherlands, 1990.

3

M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, 1991.

4

C. Dotremont, S. Goethaert and C. Vandecasteele, Verwijdering van gechloreerde koolwaterstoffen uit afvalwater door pervaporatie. Het Ingenieursblad, 7/8 (1992) 27-33.

5

C.H. Lee, Theory of reverse osmosis and other membrane permeation operations, J. Appl. Pol. SC., 19 (1975) 83-95.

6

K.W. Boddeker, G. Bengtson and H. Pingel, Pervaporation of isomerlc butanols, J. Membr. Sci., 54 (1990) 1-12.

7

I. Blume, P.J.F. Schwering, M.H.V. Mulder and C.A. Smolders, Vapour sorption and permeation properties of poly(dimethylsiloxane) films, J. Membr. Sci., 61 (1991) 85-97.

8

K.W. B&ldeker and G. Bengtson, Pervaporation of low volatility aromatics from water, J. Membr. Sci., 53 (1990) 143-158.

9

J.M. Watson and PA. Payne, A study of organic compound pervaporation through silicone rubber, J. Membr. Sci., 49 (1990) 171-205.

10

J. te Hennepe, Zeolite filled polymeric membranes, Ph.D. Thesis, Chapter 3, University of Twente, Enschede, The Netherlands, 1988.