Permeation in filled membranes: Role of solute-filter interactions

SCIENCE ELSEVIER

Journal of Membrane Science 134 (1997) 225-233

Permeation in filled membranes: Role of solute-filter interactions S h i v K u m a r , J.N. S h a h , S.B. S a w a n t , J.B. J o s h i , V.G. P a n g a r k a r * Department of Chemical Technology, University of Bombay, Matunga, Mumbai 400 019, India Received 16 May 1996; received in revised form 29 April 1997; accepted 1 May 1997

Abstract An experimental study of permeation in membranes containing adsorptive fillers is reported. The aim of the study was to investigate the effect of solute adsorption on the permeation flux. Pervaporation of pure components in the homologous series of lower (C1-C3) alcohols and lower carboxylic acids was used for the experimental studies which involved measurement of sorption and permeation data. The results indicate that the permeation flux decreases with increasing equilibrium sorption capacity of the filler for iso-propanol and the carboxylic acids which are strongly adsorbed by the fillers. Using the theoretical model proposed by Cussler et al. [1] as the basis, a correlation for the permeation flux of these solutes is obtained. This correlation incorporates the effect of solute adsorption on the permeation flux. For ethanol and methanol which are weakly adsorbed implying that desorption is easy, the permeation flux increases with the incorporation of the fillers.

Keywords: Filled membranes; Adsorption fillers; Permeation

1. Introduction The most common reason for adding fillers, especially high aspect ratio fillers, to polymers is to improve physical properties such as increased modulus (stiffness) or reduced creep. Besides this major use, fillers have been added to polymers for a variety of other purposes. They include improved thermal stability, high voltage resistance, electrical conductivity, radiation shielding and optical and aesthetic effects. In addition, filled polymeric membranes show permeabilities much lower than the conventional unfilled membranes, and hence can serve as barriers for oxygen, water and other solutes. *Corresponding author. Fax: 91-22-4145614; e-mail: [email protected]. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00119- 1

The use of polymeric membranes as barriers constitutes an interesting area of polymer research which has revolutionised the packaging industry. Besides providing a container to prevent scattering and bulk phase mixing of components, modern packages control the exchange of components between the package contents and the external environment. For instance, protection from attack by oxygen is the most common requirement in the case of food packaging. The oxygen barrier properties of the plastic food container is frequently a major consideration influencing its suitability for a specific application [2]. Several attempts have been made in the literature to model this system of fillers dispersed in a polymer film in order to predict the relative reduction in flux in a filled membrane. One of the earliest models proposed in this regard was by Prager [3]. The major assumption

226

S. Kumar et al./Journal of Membrane Science 134 (1997) 225-233

in this model was random orientation of fillers of various shapes. The results showed that flakes would give a greater improvement of barrier performance over cylinders and spheres. Barter [4] modelled the system as a uniform dispersion of lattices of rectangular parallelopipeds. Using this model, Murthy [5] studied the intrinsic characteristics of talc-filled polyethylene and Nylon-6 membranes. The most significant work in this area has been by Cussler et al. [ 1]. They modelled the system as that of regular arrays and developed the following mathematical equations to predict the variation of flux with the filler loading, 6: J o / J n = 1 + 0~2q52/(1 - ~b)

for c~/a << 1

Jo/Jn = 1 + c~b for ~ / a >> 1

(1) (2)

Eq. (1) is applicable to the case where the size of the filler particle is very large as compared to the slit size, while Eq. (2) is applicable to the reverse case. In the above equations, as q~increases, the flux Jn decreases and thus Jo/Jn increases. The above equations are applicable to the case where the filler is non-sorbing, and the reduction in flux is attributed to the geometric hindrance offered by the filler. Zeolites have been used in membrane separation processes, especially pervaporation, for continuous separation of organics from aqueous streams. The addition of an adsorptive filler results in an appreciable increase in the permeation selectivity of the preferentially sorbed component of a binary system. Te Hennepe et al. [6] achieved this by incorporating silicalite - a hydrophobic zeolite, for the separation of alcohols from water, and obtained improved selectivity with respect to alcohols. In a subsequent publication, te Hennepe et al. [7] developed a resistance-inseries model, which explained the permeability of a silicalite-filled silicone rubber membrane in terms of the permeabilities of the membrane constituents. Jia et al. [8] used silicalite-filled silicone rubber membranes for gas permeation experiments and showed that silicalite-filled poly(dimethyl siloxane) membranes possess higher oxygen selectivities than unfilled membranes for an oxygen-nitrogen mixture. Netke et al. [9] carried out a systematic study of permeation characteristics of silicalite-filled silicone rubber membranes for acetic acid-water system and found a considerable improvement in the sorption selectivity for acetic acid, but a decrease in flux with

(i) an increasing loading of the zeolite, and (ii) increasing hydrophobicity of the zeolite. A dual mode model was developed to explain permeation in filled membranes containing such adsorptive fillers. The present work, a continuation of the work reported by Netke et al. [9], is an attempt to quantify the effect of adsorption on the rate of permeation in a filled membrane.

2. Experimental The method of pervaporation of liquids was employed for studying permeation in filled membranes. In order to facilitate experimentation, it was decided to employ pure components for the pervaporation experiments. The feed liquids used were pure acetic, propionic and butyric acids from the homologous series of alkanoic acids and methanol, ethanol and iso-propanol from the homologous series of aikanols. It was felt that the adsorption phenomenon on the filler may be the key factor influencing permeation in filled membranes. To study its role, sorption and permeation studies were carried out at different temperatures and filler loadings for each filled membranesolute system.

2.1. Membrane preparation Silicone rubber (RTV 2/VP 7660) was kindly supplied by Wacker Chemie, Germany. Silicone rubber and cross linker were mixed in 9 : 1 proportion and then spread on a perspex sheet with a bar coater and cured overnight. Silicone rubber was chosen as the membrane polymer since it wetted the fillers used in this work. Wetting of the filler by the polymer is important since otherwise, air gaps are created at the filler-polymer interface. These air gaps distort the performance and true barrier properties/permeation data cannot be measured. The resulting film was annealed at 80°C for 8 h. Silicalite samples (SA-5 and S-115) were kindly supplied by Universal Oil Products, USA. The properties of the silicalites are given in Table 1. Spherical quartz particles of diameter 14 lam were procured locally. For casting filled membranes, the filler was first homogeneously dispersed in the silicone fluid with the help of a turbine-type impeller. The cross linker was then added and mixed

227

S. Kumar et al./Journal of Membrane Science 134 (1997) 225-233

Table 1 Properties of silicalites used

Approximate Si/A12 Pore diameter, A Pore volume, cc/gm

S-115

SA-5

140 6 15

450 6 15

sorption studies. In view of the variation in membrane thickness and the dependence of flux on the same, the fluxes reported here are normalised to a membrane thickness of 100 ktm.

3. Results and discussion

3.1. Sorption studies thoroughly. The resulting mixture was then degassed under vacuum, spread on a perspex sheet with a bar coater and cured overnight. The resulting film was then annealed at 80°C for 8 h. Membranes with 10, 20, 30 and 40 wt% filler loading were prepared. The membranes had a thickness ranging from 40 to 150 ~tm.

2.2. Sorption studies Films of known weight were immersed in pure acetic, propionic and butyric acid and methanol, ethanol and iso-propanol. These films were allowed to equilibrate for 48 h at a given constant temperature. These films were removed and weighed after the superfluous liquid was wiped with tissue paper. The increase in weight is due to the amount of solute taken up by the membrane. Sorption studies were performed at 30, 40 and 50°C for acetic acid, methanol, ethanol and iso-propanol; 40, 50 and 60°C for propionic acid and 50, 60 and 70°C for butyric acid with each filled membrane.

2.3. Permeation studies Pervaporation experiments were carried out in a batch stirred cell with adjustable downstream pressure. Details of the experimental setup are available [10]. The membrane was equilibrated with the feed solution for 1 h before starting the experiment. Longer equilibration time was not necessary as there was no significant change in the results after 30 min equilibration. Vacuum was applied and permeation was continued for 1 h, after which the permeate sample was collected in a liquid nitrogen trap. All the experiments were of 1 h duration and were carried out at constant permeate pressure of 1 mmHg absolute. Permeation studies were carried out for each solute-filled membrane system at the temperatures mentioned in

From the sorption experiments, the weight of solute sorbed per unit dry weight of the membrane was determined. The adsorption capacity, Ks (moles of solute sorbed/dry wt filler), was then calculated for each case, which gave a measure of the adsorbability of the solute on the filler. Ks is a lumped parameter, which accounts for the effects of change in temperature, volume fraction of the filler and the nature of the filler and solute.

3.1.1. Effect of filler loading Figs. 1 and 2 show the typical plots of the adsorption capacity, Ks, with the volume fraction of the filler, 4, for SA-5 filled membrane at different temperatures. The marginal increase in Ks may be due to the

0.09

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H

~ 0.07

~

0.06

~ 0.05

~ o

0.04

0.03

0.02

ii,iltll,lj,l,l~jJ,l~Fllll~lllll~lllllll, ~ 0.03 0.05 0.08 0.10 0.13 0.15 0.18 0.20 0.28 0.25 VOLUME FRACTION OF FILLER IN THE M]~MBRANE

Fig. 1. Variation of adsorption capacity with volume fraction of the filler for sorption of acetic acid. []: SA-5, 30°C; A: SA-5, 40°C; ~: SA-5, 50°C; ~-: S-115, 30°C; +: S-115, 40°C; x: S-115, 50°C; *: quartz, 30°C; "k: quartz, 40°C; ~c': quartz, 50°C.

228

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S. Kumar et al./Journal of Membrane Science 134 (1997) 225-233

0.0450

0.085

0.0425

0.080 E-; 0.075

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~ 0,070

8

0.0375

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m

~

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Z o

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0.040 IIIIIIIII]IIIIIIIII~FIIIIIIIIIILI~I Ill Jll~ 0.05 0.08 0.10 0.13 0.15 0.18 0.20 0.28 0.25 VOLUME FRACTION OF FILLER IN THE MEMBRANE

0.0~

Fig. 2. Variation of adsorption capacity with volume fraction of the filler for sorption of methanol. I-1: SA-5, 30°C; A: SA-5, 40°C; ¢: SA-5, 50°C; ~,: S-115, 30°C; +: S-115, 40°C; x: S-115, 50°C; *: quartz, 30°C; ~lr: quartz, 40°C; .~: quartz, 50°C.

incomplete wetting of the filler during membrane preparation. Incomplete wetting of the filler by the polymer phase may leave air gaps at the filler-polymer boundary in which the solute may accumulate. As the filler loading increases, these air gaps around the filler particles also probably increase, resulting in marginally higher values of Ks. It is evident that, for a particular solute and filler, with an increase in the volume fraction of the filler, the weight of solute sorbed per unit dry weight of the composite membrane increases, thus confirming the fact that the inclusion of an organophilic adsorbent in the membrane matrix results in an increase in the sorption capacity of the membrane.

3.1.2. Effect of feed temperature As expected, for all the combinations of solutefiller systems at various filler loadings, a decrease in Ks was observed with an increase in the temperature. This variation could be correlated with Arrhenius type of equations.

3.1.3. Effect of nature of filler Figs. 1 and 2 show typical plots of Ks against ~bfor a given solute at different temperatures. At any given

0.035

iiitl lllllll ii i i i r l l t l l l l l r l l l F J l r l l l l l l l l l 0 [ 0.03 0.05 0.08 0. i0 0.13 0.15 0.18 0,20 0.23 .25 VOLUME FRACTION OF FILLER IN THE MEMBRANE

Fig. 3. Variation of adsorption capacity with volume fraction of the filler for sorption in SA-5 filled membrane for alkanoic acid. [-]: acetic acid, 30°C; A: acetic acid SA-5, 40°C; (>: acetic acid, 50°C; ~ : propionic acid, 40°C; +: propionic acid, 50°C; ×: propionic acid, 60°C; *: butyric acid, 50°C; ~ : butyric acid, 60°C; 4 : butyric acid, 70°C.

filler loading, it is seen that SA-5 affords the greatest sorption, followed by S-115; and lastly quartz. SA-5 being the most organophilic, affords the maximum sorption capacity.

3.1.4. Effect of solute~carbon chain length Figs. 3 and 4 show data for adsorption capacity, Ks, for the acids and alcohols, respectively. With increasing carbon number the molecular weight and size increases. As a result, although the total weight of the sorbed solute increases with increase in carbon number, the number of moles adsorbed decreases and a lower Ks value is obtained. Amongst the alcohols Ks values for ethanol and iso-propanol are not significantly different.

3.2. Permeation studies Since permeation studies with different solutes were performed at different temperatures, a uniform basis for comparison of the fluxes was felt necessary. This was achieved by calculating Jo/Jn, that is, the ratio of the flux for the unfilled membrane to that for a given filled membrane, at each temperature. This

S. Kumar et al./Journal of Membrane Science 134 (1997) 225-233 0.0700

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'

I

'

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I

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I

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E

,

,

I

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0.050 0.100 0.150 0.200 0.250 VOLUME FI~CT]ON OF FILLER IN TH]E MP-JI~R&NE

Fig. 4. Variation of adsorption capacity with volume fraction of the filler for sorption in SA-5 filled membrane for alkanols. I-7: isopropanol, 30°C; A: iso-propanol, 40°C; ¢: iso-propanol, 50°C; ~ : ethanol, 30°C; +: ethanol, 40°C; ×: ethanol, 50°C; *: methanol, 30°C; "A': methanol, 40°C; -~: methanol, 50°C.

Fig. 5. Variation of the flux ratio, Jo/Jn, with volume fraction of the filler for permeation through SA-5 f'dled membranes at 50°C. I-q: butyric acid; A: propionic acid; ¢: acetic acid; "~: isopropanol; +: ethanol; x: methanol.

definition used by Cussler et al. [1] eliminates the effect of varying vapour pressure of the solutes and gives a measure of the reduction in flux due to the presence of the filler, at a given temperature. An increase in Jo/Ja reflects decrease in Jn, the flux in the presence of the filler.

For 00 << 1, that is, for weakly sorbed solutes such as methanol and ethanol,

P = kM[DMM + (DzM + DZZ + Dz)kzC* + DMZ]

3.2.1. Effect of filler loading

P = kMDMM

From Fig. 5 two distinct types of behaviour are observed. For all acids and iso-propanol, Jo/Jn increases or the flux decreases in the presence of the filler below that when the filler was absent. On the other hand, for methanol and ethanol, incorporation of filler increases the flux. This behaviour can be explained on the basis of the dual mode model for filled membranes of Netke et al. [9]. According to this model the permeability through the filled membrane is given by

Thus the flux for the filled membrane is more than that for the unfilled membrane. For/90 ,-~ l, that is, for strongly sorbed solutes such as iso-propanol, acetic acid, etc.

P

=

kM/DMM + [(DzM + Dzz + Oz)kzC* - DMZ] × (1 - 00) - 2DMz T 0 0 0 ln(l -- 0o) }

(3)

(4) This permeability is greater than that for filled membranes which is given by

P = kMDMM

(6)

This permeability is the same as that for the unfilled membrane. But

DMM ~ (1 -- ¢)

(7)

Thus DMM decreases with an increase in ¢ and the flux decreases with an increase in the filler loading. Sorption of the solute on the filler is of the Langmuir type and obeys the following equation: C*kzC M Cz

The above equation has the following asymptotes:

(5)

-

1 +kzCM

(8)

230

S. Kumar et al./Journal of Membrane Science 134 (1997) 225-233

In Eq. (8) the value ofkz determines the occurrence of the plateau in the sorbed concentration (Cz) with respect to the concentration in the membrane, Cm (or solute activity in the membrane). For the solute-filler system with kz >> 1, the isotherm rises sharply approaching C* at very low values of Cm. At the permeate side the value of Cm is low as decided by the low pressure maintained on the permeate side. But because of the large values of kz, Cz is still higher and desorption of the solute phase sorbed in the filler (zeolite) is not effective. This fraction of the solute sorbed in the filler cannot contribute effectively towards the flux which is consequently reduced. Boom [11] has given the above explanation for methanolhexane system. In the present case the same reasoning may be applicable. For instance, according to the data reported by Lin and Ma [ 12] for the sorption of ethanol and iso-propanol vapours in silicalite, the kz values are 12.7 and 56.5 1/mol, respectively, and the plateau of the sorption curve develops at approx. 15 and 6 mmHg, respectively. The pressure at which the plateau value of the sorption isotherm develops decreases with increase in kz, whereas kz increases with decrease in polarity. Thus according to the above logic, the residual concentration of iso-propanol and carboxylic acids would be much higher than that of methanol and ethanol at the operating permeate pressure of 1 mmHg absolute. In other words, there would be retardation in desorption of iso-propanol and carboxylic acid, resulting in reduction in flux for filled membranes. Fig. 5 also indicates that for all acids and isopropanol Jo/Jn varies linearly with q~ and its variation with ~b can be described by Eq. (2). This will be discussed in detail later.

1.90 1.80 1.70 1.60 ~'o1.50

1.40 1.30 1.20 1.10 1,00

0.90 0.00

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Illlllllllllllllllllll 0.08 0.10 0.13 0.15

0.18

Illlllllll I 0.20 0.23 0,25

VOLUME FRACTION OF FILLER IN THE IdEMBP.ANE

Fig. 6. Variationof the flux ratio, Jo/Jn, with volume fraction of the filler for permeationof acetic acid. Vq: SA-5, 30°C; A: SA-5, 40°C; 0: SA-5, 50°C; ~-: S-115, 30°C; +: S-115, 40°C; x: S-115, 50°C; *: quartz, 30°C; ~-: quartz, 40°C; ~: quartz, 50°C. 1.1000

1.0000 i 0.9000 0.8000 0.7000 o ~ 0.6000 ~ 0.5000 0.4000 0.0000

3.2.2. Effect of temperature From Figs. 6 and 7 it is clear that at higher temperatures, the effect of change in flux is comparatively higher in filled membranes as compared to the same in the case of the unfilled membrane. This results in a lower value of Jo/Jn or lesser change in flux at a given ~b at lower temperatures. Further, a filled membrane which contains a filler with high sorption capacity for the solute (e.g. SA-5 in Fig. 6) yields higher values of Jo/J~. However, the effect of temperature on Jo/J, is not very clear at lower values of ~b.

0.2000 ,111,,,,l,,,t 11,t11,,lll,t,il,ll,lt11,11,,11,,1 0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250 VOLUME FRACTION OF FILLER IN THE I / l ~ u t n ~

Fig. 7. Variation of the flux ratio, Jo/Jn, with volume fraction of the filler for permeation of methanol. I-1: SA-5, 30°C; A: SA-5, 40°C; O: SA-5, 50°C; ~ : S-115, 30°C; +: S-115, 40°C; x: S-115, 50°C; *: quartz, 30°C; ~tC,:quartz, 40°C; ~ : quartz, 50°C.

3.2.3. Effect of nature of filler The effect of the filler type can be explained through its adsorption capacity Ks. Figs. 8 and 9 show typical plots of Jo/Jn against Ks for a particular solute. There

S. Kumar et al./Journal of Membrane Science 134 (1997) 225-233

231

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ii i i i i i i i i i r 0.070 0.080 0.000 ADSORPTIONEQUILIBRIUMCONSTANT,Ks

1.00 iiil[llll[llll[lllllllltlllllll~tllllll]llll]lll 0.035 0.040 0.045 0.0fi0 0.055 0.060 0.065 0.070 0.075 0.080 0.065 ADSORPTION R Q ~ R I U M CONSTANT, Ks

Fig. 8. Variation of the flux ratio, Jo/Jn, with adsorption capacity for permeation of acetic acid. IS]: SA-5, 30°C, A: SA-5, 40°C; ~: SA-5, 50°C; ~ : S-115, 30°C; +: S-115, 40°C; ×: S-115, 50°C; *: quartz, 30°C; .~: quartz, 40°C; -~: quartz, 50°C.

Fig. tO. Variation of the flux ratio, Jo/J,, with adsorption capacity for permeation through SA-5 filled membrane for carboxylic acids at 50°C. ~: acetic acid; +: propionic acid; *: butyric acid.

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0.023 0.025 0.025 0.030 0.033 0.035 0.038 0.040 0.043 0.045

VOLUME FRACTIONOF FILLER IN THE MEMBRANE Fig. 9. Variation of the flux ratio, Jo/Jn, with adsorption capacity for permeation of methanol, n : SA-5, 30°C; A: SA-5, 40°C; ~: SA-5, 50°C; ~ : S-115, 30°C; +: S-115, 40°C; x: S-115, 50°C; *: quartz, 30°C; "k': quartz, 40°C; @: quartz, 50°C.

is a clear dependence of J0/Jn on Ks indicating that Ks is an important factor. For ethyl alcohol and methyl alcohol the trend is reversed for reasons discussed in Section 3.2.1.

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Fig. 11. Variation of the flux ratio, Jo/Jn with adsorption capacity for permeation through SA-5 filled membrane for alkanols at 50°C. ~: iso-propanol; ×: ethanol; +: methanol.

3.2.4. Effect of solute/carbon chain length Figs. 10 and 11 show typical plots of Jo/Jn against Ks for various solutes but a given filled membrane. From these figures as well as from Figs. 8 and 9 it is clear that there is a definite relationship between Jo/Jn and Ks.

232

S. Kumar et al./Journal of Membrane Science 134 (1997) 225-233

4. Application of the theory of permeation in filled membranes to the case of adsorptive fillers As discussed earlier, all acids and iso-propanolfiller systems used in this work exhibit a linear dependence of Jo/Jn on ~b (Fig. 5). Thus it is clear that the data for alkanoic acids and iso-propanol obey Eq. (2) given by Cussler et al. [1]. The data were also tested for Eq. (1) but evidently Eq. (I) was not applicable. Eq. (2) is valid for o~/a >> 1. In the present work the maximum filler loading used was 24 vol%. At these low loadings of low aspect ratio filler particles, the gaps between the filler particles are likely to be much larger than the filler particle size. Thus, permeation occurs through the gaps and the basis of Eq. (2) is satisfied. It is interesting to note that although all the acids and iso-propanol-filter systems obey Eq. (2), they cannot be collectively described by a single equation of the type of Eq. (2). Thus, the model of Cussler et al. [1] for c@r >> 1 is valid for individual solutes. It may be possible to obtain a unique single equation of the type given by Eq. (2) by making some minor modifications. It has been shown earlier that Jo/Jn shows good correlation with Ks. Thus, it was felt that Ks may be included in Eq. (2) to obtain a single unique equation.

Correlationfor flux in membranes containing adsorptive fillers 4.1.

From the above discussion, it is clear that the sorption capacity of the filler for the solute represented by Ks is an important parameter. A detailed statistical analysis of the 138 data points generated for all acids and iso-propanol revealed that both ~b and Ks are significant parameters through the product ~bKs. In order to obtain a correlation for explaining permeation in filled membranes, the sorption and permeation data were subjected to linear regression. The range of ~b values was 0.050-0.24 (volume fraction) and the range of Ks values used was 0.00038-0.0013 (moles of solute/wt of dry filler). The following equation was obtained:

Jo/Jn = 1 +

ff(bKs1"42

(9)

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o

°

1.4 1.2

1

.

1

1.2

1.4

.

.

.

1.6

1.8

2

Jo/Jn PREDICTED Fig. 12. Parity plot of the experimental values Of Jo/Jn against the predicted values. O: acetic acid; [=]: propionic acid; A: butyric acid; +: iso-propanol.

The value of ~ obtained from regression analysis was 3.605 × 10 4, A correlation coefficient of 0.931 was obtained and the standard error of estimate was 0.0743. An error analysis was made which showed no systematic deviation. The maximum deviation was 11.46%. The experimental values of Jo/Jn have been plotted against the predicted values, in the parity plot shown in Fig. 12. The fit of Eq. (9) with the data is satisfactory and hence Eq. (9) may be considered as a satisfactory correlation to predict the decrease in permeation flux in filled membranes containing adsorptive fillers.

5. Conclusion An attempt has been made to quantify the effect of solute adsorption on the rate of permeation in membranes containing adsorptive fillers. It has been found that increasing the filler volume fraction results in a decrease in the flux through the filled membrane. Cussler et al.'s [1] equation for a/a << I has been modified to include the effect of solute adsorption. This modification results in a satisfactory correlation for predicting solute flux in such filled membranes.

S. Kumar et al./Journal of Membrane Science 134 (1997) 225-233

233

6. Nomenclature

References

C C*

[1] E.L. Cussler, S.E. Hughes, W.J. Ward and R. Aris, Barrier membranes, J. Membr. Sci., 38 (1988) 161. [2] T.C. Bissot, Performance of high-barrier resins with platelet type fillers: Effects of platelet orientation, in: Barrier Polymers and Structures, J.J. Koros (Ed.), American Chemical Society, Washington, DC, 1990, Chapter 5. [3] S. Prager, Diffusion in homogeneous media, J. Chem. Phys., 33 (1960) 122. [4] R.M. Barrer, Diffusion in Polymers, J. Crank and G.S. Park (Eds.) Academic Press, New York, 1968, p. 165. [5] N.S. Murthy, Structure and properties of talc-filled polyethylene and Nylon-6 films, J. Appl. Polym. Sci,, 31 (1986) 2569. [6] H.J.C. te Hennepe, D. Bargeman, M.H.V. Mulder and C.A. Smolders, Zeolite filled silicon rubber membranes. Part 1. Membrane preparation and pervaporation results, J. Membr. Sci., 37 (1987) 39. [7] H.J.C. te Hennepe, D. Bargeman, M.H.V. Mulder and C.A. Smolders, Exclusion and tortuosity effects for alcohol/water separation by zeolite filled PDMS membranes, Sep. Sci. Technol, 26(4) (1991) 585. [8] M. Jia, K.V. Peinemann and R.D. Behling, Molecular sieving effect of the zeolite filled silicone membranes in gas permeation, J. Membr. Sci., 57 (1991) 297. [9] S.A. Netke, S.B. Sawant, J.B. Joshi and V.G. Pangarkar, Sorption and permeation of acetic acid through zeolite filled membranes, J. Membr. Sci., 107 (1995) 23. [10] S.A. Netke, S.B. Sawant, J.B. Joshi and V.G. Pangarkar, Sorption and permeation of aqueous picolines through elastomeric membranes, J. Membr. Sci., 91 (1994) 163. [11] J. Boom, Transport through zeolite filled polymeric membranes, Ph.D. Thesis, University of Twente., 1994. [12] Y.S. Lin and Y.H. Ma, A comparative study of adsorption and diffusion of vapor alcohols and alcohols from aqueous solutions in silicalite, in: P.A. Jacobs and R.A. van Santen (Eds.), Zeolites: Facts, Figures, Future, Vol. 49b, Elsevier Science, Amsterdam, 1989.

D J kM kz Ks P

C o n c e n t r a t i o n (tool/l) S a t u r a t i o n c o n c e n t r a t i o n in the L a n g m u i r isotherm equation (mol/1) Diffusivity (m2/s) Flux ( k g / s m z) Constant for solute sorption (kg s o l u t e / k g dry membrane) C o n s t a n t in L a n g m u i r i s o t h e r m e q u a t i o n (1/mol) Adsorption capacity of the filler (moles of solute s o r b e d / g r a m s o f dry filler) Permeability (mZ/s)(kg s o l u t e / k g dry m e m brane)

6.1. Greek letters OL O"

Filler aspect ratio ( - ) Pore aspect ratio ( - ) Volume fraction of the filler in the m e m b r a n e

(-) 00

Fraction of zeolite site occupied at feed side conditions ( - ) Constant

6.2, Subscripts

M n 0 S Z

M e m b r a n e phase G i v e n filler loading Unfilled membrane Sorption Zeolite phase

Acknowledgements The work reported was supported by a grant under the I n d o - U S research p r o g r a m ( D S T / I N T / U S H R F / 336/92).