Separation of toluene–methanol mixtures by pervaporation using crosslink IPN membranes

Chemical Engineering and Processing 48 (2009) 1560–1565

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Separation of toluene–methanol mixtures by pervaporation using crosslink IPN membranes N.R. Singha a , S.B. Kuila b , Paramita Das a , S.K. Ray a,∗ a b

Department of Polymer Science and Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India Department of Chemical Engineering, Haldia Institute of Technology, West Bengal, India

a r t i c l e

i n f o

Article history: Received 7 April 2009 Received in revised form 8 September 2009 Accepted 9 September 2009 Available online 17 September 2009 Keywords: Pervaporation Toluene Methanol Crosslink copolymer Full IPN

a b s t r a c t Polyvinyl alcohol (PVOH) has been chemically modified by crosslink copolymerization of acrylic acid (AA) and hydroxyethylmethacrylate (HEMA) in aqueous solution of PVOH and finally crosslinking PVOH to produce a full interpenetrating network (IPN) membrane termed as PVAH. Accordingly, three such full crosslink IPNs membranes, i.e. PVAHI, PVAHII and PVAHIII containing varied weight ratio of PVOH and copolymer have been synthesized and used for pervaporative separation of methanol from its mixtures with toluene. For comparison, a conventional PVOH membrane crosslinked with glutaraldehyde has also been used for the same pervaporation study. The flux and selectivity of these IPN membranes were found to be much higher than the conventional glutaraldehyde crosslinked PVOH membrane. Among the three membranes, PVAHII with 50 wt% polyAH incorporation showed optimum performance in terms of flux and methanol selectivity. © 2009 Published by Elsevier B.V.

1. Introduction In recent times organic–organic mixtures like aromatic– aliphatic mixtures, mixtures of oxygenated hydrocarbons, etc. are being tried to separate by an energy intensive second generation membrane process like pervaporation (PV). PV is carried out at low temperature and the membrane can be reused with minimum environmental emission of the treated chemicals. Thus, in terms of energy and material consumption as well as environmental emission PV is an ideal example of process intensification. The major field of pervaporative separation is dehydration of organic using a hydrophilic membrane or for the removal of organics from water using an organophilic membrane. However, for the separation of organic–organic mixtures neither of these hydrophilic and organophilic membranes can be used. In this case a suitable membrane selection is based on closeness of solubility parameter value of the desired permeate with the membrane material [1]. In recent times pervaporative separation of mixtures of alcoholoxygenated hydrocarbon have been reported by many. Cellulose acetate (CA) membrane filled with metal oxide particles of Al2 O3 and ZnO were found to show higher methanol flux and selectivity in comparison to unfilled CA membrane for methanol–MTBE mixtures [2]. CA blended with 15 wt% polyvinyl pyroliddone showed a high separation factor of 411 and total flux of 430 g m−2 h−1 for

∗ Corresponding author. Tel.: +91 033 2350 8386; fax: +91 033 2351 9755. E-mail address: [email protected] (S.K. Ray). 0255-2701/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.cep.2009.09.002

a methanol–MTBE feed mixtures composed of 20 wt% methanol at 313 K [3]. Wang et al. [4] tried poly(vinyl alcohol) (PVA)poly(acrylic acid) blend membrane crosslinked with varied doses of glutaraldehyde for separation of methanol from its mixtures with dimethyl carbonate. For these membranes methanol concentration in permeate side was always higher than 98.6 wt% for separating feed mixtures of 40–70 wt% methanol at 50–70 ◦ C. Novel hybrid membranes were prepared by incorporating H-␤zeolite or Rh-loaded H-␤-zeolite (Rh/H-␤-zeolite) into polyvinyl chloride (PVC) for the pervaporative separation of benzene and cyclohexane [5]. The pervaporation performance for membranes filled with Rh/H-␤-zeolite was much better than that for pure PVC membrane and membranes filled with H-␤-zeolite. Zhao et al. [6] tried PDMS rubber–polyetherimide blend membrane for separation of model gasolines, i.e. heptane–thiphene mixtures with sulfur enrichment in the permeate. Membrane made from polyvinyl acetate [7] has been found to permeate preferentially benzene from benzene–isooctane mixtures. Mixed matrix membrane based on chitosan and silicalite [8] preferentially permeated toluene from its mixtures with methanol. From the above discussion it appears that in most of the organic–organic separations glassy membranes have been used. Rubbery membranes give high flux but poor selectivity because of its amorphous structures and hence presence of appreciable amount of free volume in the bulk of the elastomeric chains. Efficiently vulcanized natural rubber and SBR membrane has been used [9,10] for separation of toluene–methanol mixtures with preferential toluene permeation through these rubbery membranes.

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However, for selective permeation of methanol from its mixtures with toluene, hydrophilic membrane is to be used as methanol is similar to water in polarity [11]. Thus, in the present work chemically modified polyvinyl alcohol membranes, i.e. semi-IPN membranes of poly (acrylic acid-co-hydroxy ethyl methacrylate) in the matrix of polyvinyl alcohol termed as PVAHI, PVAHII and PVAHIII containing 25, 50 and 75 wt% of the copolymer in PVOH have been used for separation of methanol from its mixtures with toluene over the concentration range of 0.5–20 wt% methanol. Earlier, these membranes have been found to show high water selectivity for dehydration of isopropanol [12].

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The above equation reduces to J=

DCmf ı

(2)

If sorption by the membrane bears a linear relationship with feed concentration, Cf , feed side membrane phase concentration, Cmf , may be expressed in terms of feed concentration Cf by sorption coefficient S as Cmf = SCf

(3)

Combining Eqs. (2) and (3) 2. Experimental

J=

2.1. Materials

(DS)Cf ı

(4)

or,

High purity analytical grade toluene and methanol used for this study were purchased from E. Marck, Mumbai. The monomers, i.e. acrylic acid and hydroxyethylmethacrylate (HEMA), both synthesis grade were procured from S.d. fine chemicals, Mumbai and used as obtained. Ammonium persulfate and sodium metabisulfide were used as redox initiator pair for the copolymerization reaction. Polyvinyl alcohol (PVOH) of number average molecular weight 1,25,000 and hydrolysis of 98–99% was obtained from S.d. fine chemicals, Mumbai and used as obtained.

J=

PCf ı

(5)

Here, P is permeability. At lower feed concentration, solute flux J increases linearly with its concentration in feed and thus, permeability, P will be constant and can be found from the slope of the J vs. Cf plot. As permeability is independent of feed concentration it can be suitably used for comparison of membrane performance for different feed concentration [15].

The three PVAH, i.e. PVAHI, PVAHII and PVAHIII with 10:1 molar ratio of acrylic acid and HEMA in all of these IPNs were synthesized by solution polymerization in a three-necked reactor at 65 ◦ C for 3 h using ammonium persulfate and sodium metabisulfide (each, 0.5 wt% of the total monomer weight) as redox pair of initiators [12]. The resulting IPNS were crosslinked with glutaraldehyde. Membrane was prepared by casting this aqueous solution of the PVAH with an applicator on a clean and smooth glass plate. It was kept overnight at room temperature and then dried at 60 ◦ C for 2 h under vacuum. Subsequently, the membrane was annealed at 80 ◦ C for an additional 6 h under vacuum. The membrane thickness for the PVAH polymer was maintained at ∼50 ␮m. The thickness was measured by Test Method ASTM D 374 using a standard dead weight thickness gauge (Baker, Type J17).

2.4.2. Pervaporation experiment Pervaporation experiments were carried out in a batch stirred cell [12] with adjustable downstream pressure that was maintained at 1 mm Hg by liquid (mercury) column method using a manometer. The feed compartment of the pervaporation cell was equipped with a stirrer to ensure adequate mixing of the liquid feed so as to eliminate any concentration or temperature gradient. Effective membrane area (A) in contact with the feed mixture was 19.6 cm2 and the feed compartment volume was 150.0 cm3 . The organic–organic mixtures in contact with the membrane was allowed to equilibrate for around 3 h for the first experiment and 2 h for the subsequent experiments with different feed compositions. When the steady state was reached the permeate was collected in traps immersed in liquid nitrogen. Permeation flux (J) was calculated by dividing the amount of total permeate (W) by the time (t) of experiment and area of the membrane from Eq. (6).

2.3. Membrane characterization

J=

2.2. Synthesis and casting of membranes

The resulting membranes were characterized with FTIR, mechanical properties, DSC, TGA, XRD [12] and used for pervaporative studies. 2.4. Permeation study 2.4.1. Theory Based on solution–diffusion model in absence of concentration polarization, flux of a solute through pervaporation membrane is given by [13–15]: J=

D(Cmf − Cp ) ı

(1)

Here Cmf is membrane phase feed side concentration and Cp is permeate concentration of the solute, D is diffusion coefficient and ı is membrane thickness. Since vapor phase concentration of the solute on the permeate side, i.e. Cp ≈ 0

W At

(6)

The PV experiment was performed at a constant temperature by circulating constant temperature water around the jacket of the PV cell. The methanol content of the permeate was determined by an Abbe type digital Refractometer (model no. AR600, MISCO, USA) at 25 ◦ C temperatures for all the samples. The permeation selectivity (˛pv ) of methanol was calculated from Eq. (7) as given below. ˛PV =

ymethanol /ytoluene xmethanol /xtoluene

(7)

Here yi and xi are weight fraction of component i (methanol) in permeate and feed, respectively. The performance of the membrane was also evaluated in terms of permeation separation index (PSI) and enrichment factor (ˇ) as obtained from the following Eqs. (8) and (9), respectively. PSI = Jmethanol (˛PV − 1) ˇ=

ymethanol xmethanol

(8) (9)

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Table 1 Variation of performance of the membranes (membrane thickness 40 ␮m) with feed concentration of methanol at 30 ◦ C. (a) Total flux (g/m2 h): Feed conc. of methanol wt%

PVOH

PVAHI

PVAHII

PVAHIII

1.8307 4.5887 9.217 13.886 18.596

13.903 14.059 18.664 24.189 33.65

18.321 21.433 27.603 41.235 54.965

18.99 25.158 31.618 47.654 72.012

19.021 26.36 35.331 57.564 81.023

(b) Permeate concentration (wt%): Feed conc. of methanol wt%

PVOH

PVAHI

PVAHII

PVAHIII

1.8307 4.5887 9.217 13.886 18.596

88 88.5 78 73 71.5

89.15 89.03 79.52 74.75 73.9

92.06 91.31 85.76 82.18 78.25

91.2 90.34 82 78.32 75.3

(c) Methanol selectivity (−): Feed conc. of methanol wt%

PVOH

PVAHI

PVAHII

PVAHIII

1.8307 4.5887 9.217 13.886 18.596

393 160 34 16.7 10.98

440.60 168.74 38.24 18.35 12.39

621.74 218.47 59.31 28.59 15.748

555.73 194.45 44.87 22.40 13.34

3. Results and discussion 3.1. Pervaporation (PV) studies 3.1.1. Effect of feed concentration on methanol separation The PV experiments were carried out at five different feed concentration of methanol in toluene, i.e. 1.83, 4.58, 9.21, 13.88 and 18.59 wt% methanol in toluene. The PV performance of PVOH and the three IPN membranes, i.e. PVAHI, PVAHII and PVAHIII are shown in Table 1 in terms of total flux, permeate concentration of

Fig. 1. Variation of permeate concentration of methanol with its feed concentration at 30 ◦ C; 䊉 PVOH;  PVAHI;  PVAHII;  PVAHIII.

methanol, and methanol selectivity at 30 ◦ C. Fig. 1 shows the variation of wt% of methanol in the permeate against wt% of methanol in the feed for all of the used five feed concentrations at 30 ◦ C. Similar kind of relationships were also observed at the two other PV temperatures, i.e. at 40 and 50 ◦ C. It appears from Table 1 and these McCabe-Thiele type xy diagrams that all the membranes show measurable separation characteristics for methanol over the used concentration range without any pervaporative azeotrope. It is also observed from the figure and Table 1 that over the used concentration range, all the four membranes show high methanol concentration in the permeate. Among these four membranes, methanol concentration in the permeate increases in the following order: PVAHII > PVAHIII > PVAHI > PVOH. It is interesting to note that all the three IPN membranes show higher methanol selectivity than the conventional PVOH membrane crosslinked with glutaraldehyde. Incorporation of hydrophilic crosslink copolymer polyAH in PVOH matrix increases methanol affinity of the membranes. However, it also decreases crystallinity of PVOH matrix as intramolecular and intermolecular hydrogen bonding in PVOH matrix is reduced by this IPN formation [12]. The slightly lower methanol concentration in the permeate for PVAHIII in comparison to PVAHII may be due to increased void space in the membrane. 3.1.2. Effect of feed concentration on flux and permeation selectivity The effect of feed concentration of methanol on methanol and toluene partial flux and permeation selectivity for methanol is shown in Fig. 2a and b, respectively, for PVOH and the three IPN membranes at 30 ◦ C. The partial methanol and toluene flux was calculated from permeate concentration of methanol and total flux. Similar kind of relationship was also observed at the other two temperatures of PV experiments, i.e. at 40 and 50 ◦ C. From Fig. 2a it is observed that at lower range of feed concentration, flux increases linearly but above around 10 wt% feed concentration due to plasticization and swelling of the membrane, flux increment is drastic and overall flux increases almost exponentially for all the membranes in the following order: PVAHIII > PVAHII > PVAHI > PVOH. The increasing order of flux from PVAHI to PVAHIII may be ascribed to decreasing extent of crystallinity from PVAHI to PVAHIII. Incorporation of the copolymer polyAH, not only increases affinity

N.R. Singha et al. / Chemical Engineering and Processing 48 (2009) 1560–1565

Fig. 2. (a) Variation of methanol and toluene flux with feed concentration of methanol at 30 ◦ C;  PVOHmethanol;  PVAHImethanol; 䊉 PVAHIImethanol;  PVAHIIImethanol;  PVOHtoluene;  PVAHItoluene; 䊉 PVAHIItoluene;  PVAHIIItoluene; (b) variation of methanol selectivity with its feed concentration at 30 ◦ C;  PVOH;  PVAHI; 䊉 PVAHII;  PVAHIII.

for methanol but also void space in these IPN membranes because of loss in crystallinity. Thus, flux increases from PVAHI to PVAHIII. The much lower methanol flux of PVOH in comparison to the IPN membranes may be due to its high degree of crystallinity. From Fig. 2a it is also observed that for the same feed concentration, methanol flux is much higher than toluene flux at all the used concentrations signifying methanol selectivity of the membranes. Further, rate of increase in methanol flux with increasing feed concentration is much higher than toluene flux. It is also observed from this figure that up to 10 wt% feed concentration of methanol in feed, the toluene partial flux is very low and the difference in the extent of toluene flux among the membranes are marginal. Above this (∼10 wt% methanol in feed) feed concentration, the methanol selective membranes becomes plasticized with increase in toluene flux in the same order with an exponential trend. The permeabilTable 2 Methanol and toluene permeability (cm3 cm/cm2 h) of the membranes. Name of the membrane

Pmethanol

Ptoluene

PVOH PVAHI PVAHII PVAHIII

1.645 3.725 6.31 7.63

1.758 2.594 2.075 3.27

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Fig. 3. (a) Variation of PSI of methanol with its feed concentration at 30 ◦ C;  PVOH;  PVAHI; 䊉 PVAHII;  PVAHIII; (b) variation of enrichment factor of methanol with its feed concentration at 30 ◦ C;  PVOH;  PVAHI; 䊉 PVAHII;  PVAHIII.

ity of both methanol and toluene were calculated based on Eq. (5) from the slop of linear plot (regression coefficient was > 0.9) of the first three feed concentrations of methanol against corresponding partial flux and these permeability values are given in Table 2. In Fig. 2b methanol selectivity of all the membranes are plotted against feed concentration of methanol. From this figure it is observed that the used membranes show the following trend of selectivity: PVAHII > PVAHIII > PVAHI > PVOH. This trend is different from the trend of flux data in that in this case PVAHII shows higher selectivity than PVAHIII. Chemical modification of PVOH by incorporating polyAH copolymer in its matrix increases methanol selectivity from PVOH to PVAHII as this copolymer absorbs more of methanol than toluene. However, incorporation of the copolymer also reduces crystallinity and increases free void in the IPN membranes. In PVAHIII, the amount of copolymer is high (75 wt% of PVOH) enough to cause increase in much of its void space and hence reduced methanol selectivity. 3.1.3. Effect of feed concentration on permeation separation index (PSI) and enrichment factor Fig. 3a shows variation of PSI of methanol with its feed concentration at 30 ◦ C. PSI is found to be maximum at the lowest used feed concentration, i.e. around 2.5 wt% methanol signifying optimum flux and selectivity at lower feed concentration of methanol.

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N.R. Singha et al. / Chemical Engineering and Processing 48 (2009) 1560–1565 Table 3 Activation energy for permeation (Eact ) of methanol and toluene for 4.6 wt% feed concentration of methanol. Name of the membrane

Activation energy for methanol (kJ/mol/K)

Activation energy for toluene (kJ/mol/K)

PVOH PVAHI PVAHII PVAHIII

0.540 0.425 0.576 0.253

0.368 2.018 2.586 3.363

from the following equation ln Q = ln A −

Fig. 4. (a) Variation of methanol and toluene flux with feed temperature for 4.6 wt% methanol in feed;  PVOHmethanol;  PVAHImethanol; 䊉 PVAHIImethanol;  PVAHIIImethanol;  PVOHtoluene;  PVAHItoluene; 䊉 PVAHIItoluene;  PVAHIIItoluene; (b) variation of methanol selectivity with feed temperature for 4.6 wt% methanol in feed;  PVOH;  PVAHI; 䊉 PVAHII;  PVAHIII.

E  P

RT

(10)

Here ‘A’ is a pre-exponential factor and ‘R’ is universal gas constant. Thus, activation energy for permeation of toluene and methanol for 4.38 wt% feed concentration of methanol was calculated as shown in Table 3. From this table, it is observed that activation energy for permeation of methanol is much lower than that of toluene. The much smaller kinetic diameter of methanol (methanol −0.38 nm [11], toluene −0.61 nm [16]) makes its permeation much easier through these methanol selective membranes with lower activation energy.

PSI decreases exponentially with increase in feed concentration. Among the four membranes, PVAHII shows maximum PSI. Enrichment factor for all the membranes are shown in Fig. 3b which also shows a maximum at the lowest feed concentration. Enrichment factor is also found to decrease from PVAHIII to PVAHI. 3.1.4. Effect of temperature on flux and selectivity With increase in temperature both methanol and toluene flux increases while methanol selectivity decreases at higher temperature in the same order for all the used membranes as shown for 4.38 wt% feed concentration of methanol in Fig. 4a and b, respectively. At higher temperature flux increases due to increased rate of diffusion. The increased rate of diffusion at higher temperature is caused by increased thermal motion of the polymer chains at higher temperatures. Activation energy for permeation (EP ) of both methanol and toluene was obtained from the slope of the Arrhenius type linear plot of logarithmic of partial flux (Q) against inverse of absolute temperature (1/T), as shown in Fig. 5

Fig. 5. Arrhenius plot for methanol and toluene flux for 4.6 wt% methanol in feed;  PVOH;  PVAHI; 䊉 PVAHII;  PVAHIII.

Fig. 6. (a) Variation of permeation ratio of methanol with its feed concentration at 30 ◦ C;  PVOH;  PVAHI; 䊉 PVAHII;  PVAHIII; (b) variation of permeation ratio of toluene with its feed concentration at 30 ◦ C.  PVOH;  PVAHI; 䊉 PVAHII;  PVAHIII.

N.R. Singha et al. / Chemical Engineering and Processing 48 (2009) 1560–1565

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Table 4 Comparison of performance of various membranes reported for pervaporative separation of methanol–toluene mixtures. Polymer (Ref.)

Methanol in the feed (wt%)

Temperature of experiment (◦ C)

Normalized flux (kg ␮m/m2 h)

Selectivity

PVA-PAA (17) Composite membrane of N-acetylated chitosan supported on polyetherimide [18] PPY-PTS [19] PPY-PF [20] PVAHII (this work)

20 10

60 30

Good 4.84

148 607

1–95 1–80 1.8–18.6 wt%

57.5 –

0.05–10 0.05–10 0.759–2.88

5–60 10–600 16–622

3.1.5. Effect of feed concentration on permeation ratio Permeation ratio gives a quantitative idea about the effect of one component on the permeation rate of the other component. Huang and Lin [17] has defined this permeation ratio, , as a measure of the deviation of the actual permeation rate, Jexpt , from the ideal rate, J0 , to explain interactions between membrane polymer and permeants. Thus, i = −

Ji

expt at x conc. Ji0expt at x conc.

0 0 = J(pure × xi Ji(at x conc.) i)

(11)

(12)

where i denote ‘ith’ component in the binary mixture, x is the weight fraction in feed mixture, superscript 0, denotes ideal permeation. From Fig. 6a it is observed that at very low concentration of methanol, i.e. at very high concentration of toluene (around 99 wt% or more) in feed, the permeation ratio of methanol is far above unity for all the membranes signifying positive coupling effect of toluene on methanol flux. In this case, toluene–methanol interaction is more than methanol–membrane interaction. As the methanol % in feed increases, permeation ratio of methanol decreases drastically for all the methanol selective membranes and become close to unity, i.e. the coupling effect of toluene on methanol flux becomes negligible because of much higher methanol–membrane interaction (through hydrogen bonding) than toluene–methanol interaction at higher feed concentration of methanol. From the figure it is also observed that for the same feed concentration, permeation ratio increases from PVOH to PVAHIII. The highest permeation ratio of PVOH is due to its minimum methanol selectivity. As methanol selectivity of the membranes increases from PVOH to PVAHIII, the extent of methanol–membrane interaction increases in the same order resulting in the above order of permeation ratio of methanol. Fig. 6b shows the variation of permeation ratio of toluene with feed concentration which shows the opposite trend among the used four membranes.

4. Conclusion Three crosslinked IPN membranes termed here as PVAHI, PVAHII and PVAHIII, earlier used for dehydration of IPA [12] were used for the separation of toluene–methanol mixtures. These membranes showed measurable flux and methanol selectivity. The flux and selectivity of these IPN membranes were found to be much higher than conventional polyvinyl alcohol membrane (PVOH) crosslinked with glutaraldehyde. Among the three membranes, PVAHII with 50 wt% polyAH incorporation showed optimum performance in terms of flux and selectivity. Table 4 shows a comparison of the relative performances of various membranes reported for the separation of methanol from its mixtures with toluene. From this table it is observed, that these PVAH membranes show significant improvement in flux and selectivity when compared to various membranes reported for this separation.

Acknowledgement The first author of this manuscript is grateful to CSIR, India for providing fellowship. References [1] S.K. Ray, S.B. Sawant, J.B. Joshi, V.G. Pangarkar, Development of new synthetic membranes for separation of benzene-cyclohexane mixture by pervaporation—a solubility parameter approach, Ind. Eng. Chem. Res. 36 (1997) 5265–5276. [2] Y. Wang, L. Yang, G. Luo, Y. Dai, Preparation of cellulose acetate membrane filled with metal oxide particles for the pervaporation separation of methanol/methyl tert-butyl ether mixtures, Chem. Eng. J. 146 (1) (2009) 6–10. [3] H. Wu, X. Fang, X. Zhang, Z. Jiang, B.L. Xia, Cellulose acetate–poly(N-vinyl-2pyrrolidone) blend membrane for pervaporation separation of methanol/MTBE mixtures, Sep. Purif. Technol. 64 (2) (2008) 183–191. [4] L. Wang, J. Li, Y. Lin, C. Chen, Crosslinked poly(vinyl alcohol) membranes for separation of dimethyl carbonate/methanol mixtures by pervaporation, Chem. Eng. J. 146 (1) (2009) 71–78. [5] X. Zhang, L. Oian, H. Wang, W. Zhong, O. Du, Pervaporation of benzene/cyclohexane mixtures through rhodium-loaded ␤-zeolite-filled polyvinyl chloride hybrid membranes, Sep. Purif. Technol. 63 (2) (2008) 434–443. [6] C. Zhao, J. Li, R. Oi, J. Chen, Z. Lu, Pervaporation separation of n-heptane/sulfur species mixtures with polydimethylsiloxane membranes, Sep. Purif. Technol. 63 (1) (2008) 220–225. [7] M.K. Mandal, P.K. Bhattacharya, Poly(vinyl acetal) membrane for pervaporation of benzene–isooctane solution, Sep. Purif. Technol. 61 (3) (2008) 332–340. [8] M.B. Patil, T.M. Aminabhavi, Pervaporation separation of toluene/alcohol mixtures using silicalite zeolite embedded chitosan mixed matrix membranes, Sep. Purif. Technol. 62 (1) (2008) 128–136. [9] S. Ray, S.K. Ray, Separation of organic mixtures by pervaporation using crosslinked rubber membranes, J. Membr. Sci. 270 (2006) 132–145. [10] S. Ray, S.K. Ray, Separation of organic mixtures by pervaporation using crosslinked and filled rubber membrane, J. Membr. Sci. 285 (2006) 108– 119. [11] J.E. ten Elshof, C.R. Abadal, J. Sekuli, S. Roy Chowdhury, D.H.A. Blank, Transport mechanisms of water and organic solvents through microporous silica in the pervaporation of binary liquids, Micropor. Mesopor. Mater. 65 (2003) 197–208. [12] N.R. Singha, S. Kar, S. Ray, S.K. Ray, Separation of isopropyl alcohol–water mixtures by pervaporation using crosslink IPN membranes, Chem. Eng. Process. 48 (2009) 1020–1029. [13] J.M. Neto, M.N. Pinho, Mass transfer modelling for solvent dehydration by pervaporation, Sep. Purif. Technol. 18 (2000) 151–161. [14] J.G. Wijmans, R.W. Baker, The solution–diffusion model: a review, J. Membr. Sci. 107 (1995) 1–21. [15] D.M. Kanani, P. Bhaurao, P. Nikhade, P. Balakrishnan, G. Singh, V.G. Pangarkar, Recovery of valuable tea aroma components by pervaporation, Ind. Eng. Chem. Res. 42 (2003) 6924–6932. ´ M. Hassan, Z. Niaki, S. Vasenkov, Towards Observation of Single[16] A. Iliyas, M. Eic, File Diffusion Using TZLC, diffusion-fundamentals, Open-Access J Basic Princ, Diffus. Theory Exp. Appl. (2003). [17] R.Y.M. Huang, V.J.C. Lin, Separation of liquid mixtures by using polymer membranes. I. Permeation of binary organic liquid mixtures through polyethylene, J. Appl. Polym. Sci. 12 (1968) 2615–2631. [18] H.C. Park, R.M. Meertens, M.H.V. Mulder, C.A. Smolders, Pervaporation of alcohol–toluene mixtures through polymer blend membranes of poly(acrylic acid) and poly(vinyl alcohol), J. Membr. Sci. 90 (1994) 265–274. [19] R.Y.M. Huang, G.Y. Moon, R. Pal, N-acetylated chitosan membranes for the pervaporation separation of alcohol/toluene mixtures, J. Membr. Sci. 176 (2000) 223–231. [20] M. Zhou, M. Persin, J. Sarrazin, Methanol removal from organic mixtures by pervaporation using polypyrrole membranes, J. Membr. Sci. 117 (1996) 303–309.