Dehydration of acetic acid by pervaporation using filled IPN membranes

Dehydration of acetic acid by pervaporation using filled IPN membranes

Separation and Purification Technology 81 (2011) 295–306 Contents lists available at SciVerse ScienceDirect Separation and Purification Technology jou...

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Separation and Purification Technology 81 (2011) 295–306

Contents lists available at SciVerse ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Dehydration of acetic acid by pervaporation using filled IPN membranes S.B. Kuila a, S.K. Ray b,⇑ a b

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

a r t i c l e

i n f o

Article history: Received 29 May 2011 Received in revised form 20 July 2011 Accepted 21 July 2011 Available online 28 July 2011 Keywords: Pervaporation Acetic acid Crosslink copolymer Partial permeability Diffusion coefficient

a b s t r a c t Polyvinyl alcohol (PVOH) was chemically modified by crosslink copolymerization of acrylic acid (AA) and acrylamide (AM) in aqueous solution of PVOH and finally crosslinking the copolymer of AA and AM designated as PAAAM with N,N0 -methylenebisacrylamide (NMBA) and PVOH with glutaraldehyde to produce a full interpenetrating network (FIPN) membrane. Accordingly, a membrane containing PVOH:PAAAM of 1:0.5 designated as FIPN500 was synthesized. Filled FIPN membranes were synthesized by in situ incorporation of highly hydrophilic aluminosilicate filler during copolymerization of the monomers in PVOH matrix to produce three filled membranes designated as FIPN502, FIPN505, and FIPN510 containing 2, 5 and 10 mass%, respectively (of total polymer) of the filler. PVOH membrane cosslinked with 2 mass% glutaraldehyde, PAAAM copolymer modified PVOH membrane i.e. FIPN500 and the three filled FIPN membranes were used for sorption and pervaporative dehydration of acetic acid. The filled IPN membranes were found to show higher flux and water selectivity than the unfilled membranes. Among the three filled membranes, FIPN510 was found to show the highest flux (6.612 kg m2 h1 lm) and water selectivity (325.53) at 0.953 mass% water in feed. Interaction parameters, partial permeability, intrinsic membrane selectivity and concentration average diffusion coefficients for all the membranes were also evaluated. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Acetic acid is one of the top 20 organic intermediates in chemical industries. It is used for manufacturing many important chemicals like acetic anhydride, phthalic anhydride, vinyl acetate, terephthalic acid, etc. It is also used for production of various cellulose plastics, vinyl plastics, hot melt adhesive and textile finishes [1–4]. Mixture of acetic acid–water is encountered as by product in the production of acetic acid itself, vinyl acetate, catalytic esterification of alcohols, etc. Hence, separation of acetic acid–water mixture is very important. However, due to small difference in volatility of water and acetic acid, separation of this mixture is an energy expensive process. Further, due to the closeness of boiling point of water and acetic acid a large number of trays and a high reflux ratio are necessary for separation of acetic acid–water mixture by distillation. Aqueous acetic acid containing more than 80 mass% acid also corrodes the distillation plates. Thus, in recent times an energy saving alternative separation candidate like pervaporation is being tried for dehydration of highly concentrated acetic acid. From the available literature it is observed that for dehydration of acetic acid by pervaporation membranes based on polyvinyl alcohols [1–4], natural polymer like sodium alginate ⇑ Corresponding author. Tel.: +91 033 23508386; fax: +91 033351 9755. E-mail address: [email protected] (S.K. Ray). 1383-5866/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.07.033

[5], nafion, polycarbonate, copolymer, polymer blend, [2], doped and undoped polyaniline (PANI) (6) and inorganic polymer [7,8] have been used. However, most of these membranes show either high flux with very low water selectivity or high selectivity with very low flux. In the present work AM and AA monomers were copolymerized in the matrix of PVOH followed by crosslinking of the copolymer with NMBA and crosslinking of PVOH with glutaraldehyde to produce a full interpenetrating (FIPN) polymer. In this case the resulting FIPN is expected to be more hydrophilic than conventional glutaraldehyde crosslinked PVOH because of the presence of copolymer in its matrix. The FIPN containing 1:50 mass ratio of PVOH:copolymer showed optimum performance of flux and selectivity for pervaporative dehydration of ethylene glycol [9]. Thus, in the present work FIPN500 membrane was chosen and further filled in situ during copolymerization with 2, 5, and 10 mass% hydrophilic aluminosilicate filler to produce three different filled membranes i.e., FIPN502, FIPN505, and FIPN510, respectively. In one of our previous works aluminosilicate fillers synthesized in the laboratory was found to increase water selectivity of filled PVOH membrane manifold for dehydration of dioxane [10]. These filled membranes were used for sorption and pervaporative dehydration of acetic acid (HAC) over the concentration range of 80–100 mass% of acid in water. For comparison, glutaraldehyde crosslinked PVOH and copolymer modified FIPN500 (both unfilled) membranes were also studied for this separation.

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Nomenclature

u v

Abbreviations PVOH polyvinyl alcohol AA acrylic acid AM acrylamide NBA N0 ,N-methylene bis acrylamide FIPN full interpenetrating network PAAAM copolymer of AA and AM PSI pervaporation separation index DC diffusion coefficient

a x y f Pi ps pf pp J l

Symbols

a

Intrinsic membrane selectivity () Enrichment factor () activity coefficient () solubility parameter (MPa0.5)

b

c d

1.1. Theory 1.1.1. Thermodynamic interaction parameter In pervaporation process polymeric membrane and the binary liquid mixtures form a ternary system. Thermodynamic interaction parameters in a ternary system of a polymer and two solvents were first determined by Aminabhavi and Munk [11] by measuring preferential adsorption of one solvent using ultracentrifuge as a differential refractometer with the help of Flory-Huggins free energy of mixing. Mulder and Smolder [12] obtained the interaction parameters between two solvents for pervaporation system using the following Eq. (1).

vij ¼

1 xi v j

" xi ln

  xi

vi

þ xj ln

  xj

vj

þ

DGE RT

# ð1Þ

Excess free energy, DGE can be expressed as

DGE ¼ RTðxi ln ci þ xj ln cj Þ

ð2Þ

Here vi and vj are volume fraction of i and j in liquid feed mixture of i and j. The activity coefficient of solvent may be calculated from its molar concentration using van Laar or Willson equation [13]. In the present system acetic acid is polar in nature and miscible in all proportion with water. Thus, activity coefficient of water (component i) and acetic acid (component j) of different feed mixtures were obtained with good degree of accuracy using Wilson equation [13]. The Wilson parameter for water and acetic acid was obtained from its vapor–liquid equilibrium data [13] as 0.23965 and 1.67589, respectively. The interaction parameter of component i or j with membrane polymer i.e., vip or vjp may be obtained using Flory-Huggins theory from solubility parameter of polymer and solvent using Eq. (3) [14].

v1p ¼

V 1 ðdp  d1 Þ RT

volume fraction () interaction parameter () activity () mole fraction, feed () mole fraction, permeate () fugacity (cm of water) permeability of component i (barrer) saturated vapor pressure (cm Hg) feed side partial pressure (cm Hg) permeate side partial pressure (cm Hg) flux (kg m2 h1) membrane thickness (ml)

ð3Þ

Here v1p is interaction parameter between component 1 and polymer while V1 and d1 are volume fraction and solubility parameter, respectively of component 1. The values of solubility parameter of solvent like water or acetic acid and a pure polymer may be obtained from literature. For blend polymers it may be obtained by multiplying mass fraction of polymers with its solubility parameter and adding these values for all of the constituent polymers of the blend [15]. However, it is difficult to correctly calculate solubility parameter for a synthesized new IPN type polymer filled with inorganic filler as in the present system. Calculation based on molar attraction constants (E) of the different functional groups of the

polymer will not be accurate since exact structure and composition of the ter IPN polymer in filled network form is not known [16]. Thus, for the present system interaction parameter between solvent and polymer was obtained using Eq. (4) [17].

ln a1 ¼ lnð1  /p Þ þ /p þ v1p /2p

ð4Þ

Here a1 is activity of component 1. The pure component i or j may be assumed as ideal. Accordingly, ai or aj will be unity. Thus, the above equation reduces to

v1p ¼

 lnð1  /p Þ  /p /2p

ð5Þ

Volume fraction, up of polymer membrane may be obtained from density of polymer, solvent and total mass of solvent swollen polymer [9]. 1.1.2. Permeability and permeance Based on solution diffusion model mass flux of component i through a dense pervaporation membrane may be described in terms of its vapor pressure difference (driving force) on feed and permeate side by the following Eq. (6).

Ji ¼

Pi ðp  pp Þ l f

ð6Þ

Here ‘‘l’’ is membrane thickness, pf and pp are feed and permeate side vapor pressure of this i component. Pi is intrinsic membrane permeability and Pi/l is membrane permeance [18,19]. Vapor pressure of component i on feed side may be written as

pf ¼ xi ci ps

ð7Þ

Here xi and ci are mole fraction and activity coefficient of component i on feed side and ps is saturated vapor pressure of component i. On permeate side pressure is very low and hence it may be assumed to behave like ideal gas. Thus, partial vapor pressure of component i on permeate side (pp) may be written as [20].

pp ¼ yi P y

ð8Þ

Here Py is total permeate pressure and yi is mole fraction of i component on permeate side. Now Eq. (6) may be written as

Ji ¼

Pi ðxi ci ps  yi Py Þ l

ð9Þ

Further, fugacity of component i is defined as

fi ¼ xi ci ps

ð10Þ

S.B. Kuila, S.K. Ray / Separation and Purification Technology 81 (2011) 295–306

Ji l ðfi  yi Pp Þ

297

2. Experimental

65 °C for 3 h using ammonium persulfate and sodium metabisulfide (each, 0.5 mass% of the total monomer mass) as redox pair of initiators. In all of these polymerizations 10:1 molar ratio of AM and AA was used and filler was mixed in situ during polymerization. The reactor was fitted with a stirrer, a thermometer pocket and a condenser. At first 5 mass% PVOH solutions was made in deionised water in a 250 ml glass beaker by gradual addition of required amount of PVOH to boiling water in several intervals with constant stirring to obtain a viscous clear PVOH solution. The clear and homogeneous PVOH solution was then mixed with aqueous dispersion of required amounts of fine sodium alumino silicate (inorganic filler) and stirred with magnetic stirrer for 8 h to get filler incorporated stable polymer dispersion. Required amounts of acrylamide and acrylic acid were then added to this aqueous dispersion in the three neck flask placed on a constant temperature bath. Temperature was raised to 65 °C and aqueous solution of initiators was added to the reactor. After 2 h of polymerization, NMBA was added and polymerization was continued for another 15 min. After completion of polymerization, the reaction mixture was cooled to ambient temperature and mixed with 25% aqueous solution of glutaraldehyde, 10% solution of H2SO4 (to catalyze the reaction), 50% aqueous methanol (to quench the reaction) and 10% solution of acetic acid (pH controller) [10]. The polymerization mixture was then immediately cast on a clean and smooth glass plate to avoid gelling before casting with an applicator. 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 FIPN polymer was maintained at 50 lm. The thickness was measured by Test Method ASTM D 374 using a standard dead mass thickness gauge (Baker, Type J17). The composition of the membrane polymers are shown in Table 1.

2.1. Materials

2.4. Membrane characterization

High purity analytical grade acetic acid used for this study was purchased from E. Marck, Mumbai. The monomers and crosslinker i.e., acrylic acid (AA), acrylamide (AM), N0 ,N-methylene bis acrylamide (NMBA) and glutaraldehyde all 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 mass 1,25,000 and hydrolysis of 98–99% was obtained from S.d. fine chemicals, Mumbai and used as obtained. Sodium aluminate and sodium silicate used for making the hydrophilic sodium aluminosilicate filler was laboratory reagent grade and obtained from the same company. Deionized water, having a conductivity of 20 lS/cm, was produced in the laboratory itself from a RO module using polyamide reverse osmosis (RO) membrane. This water was used for making PVOH solution and also for making feed acid–water mixtures to be used for sorption and permeation studies.

The characterizations of the unfilled FIPN membranes are reported elsewhere [9]. The distribution of fillers in the filled FIPN membranes was characterized with mechanical properties and scanning electron microscopy (SEM). The experimental procedure for mechanical properties and (SEM) was similar to those reported in detail for unfilled IPN membranes [9].

Hence; Pi ¼

ð11Þ

Here saturated vapor pressure of component i and j i.e. water and acetic acid may be calculated using Antoine equation [13]. Activity coefficient of water and acetic acid of different feed mixtures may be determined using two parameter Wilson equation. 1.1.3. Intrinsic membrane selectivity The intrinsic membrane selectivity, amem may be calculated by dividing permeability of water (Pi) with permeability of acetic acid (Pj) i.e.,

amem ¼

Pi Pj

ð12Þ

1.1.4. Diffusion coefficient The concentration average diffusion coefficients of component i or j through the various used membranes may be obtained from the following Eq. (13) [21].

D1 ¼

J 1diff  l m1  qp

ð13Þ

Here m1 is membrane phase concentration of component 1 obtained from sorption experiment and J1diff is diffusive flux of component 1 which is related to mass flux J1 by the following Eq. (14).

J 1diff ¼

m1 ½1 þ r 1  ln½1  m1 ð1 þ r 1 Þ1

J1

ð14Þ

Here r is ratio of flux of components i and j i.e., r = Ji/Jj.

2.2. Preparation of sodium aluminosilicate Sodium aluminosilicate filler was synthesized in the laboratory from sodium and aluminum silicate by an ion exchange method. The spray dried powder of this sodium aluminosilicate, also called synthetic zeolite was then characterized in terms of alumina–silica molar ratio, surface area and cation exchange capacity as reported elsewhere [10].

2.5. Sorption experiment Two unfilled and three filled membrane samples of known mass were immersed in different known concentrations of acetic acid– water mixtures and were allowed to equilibrate for 96 h at four different experimental temperatures i.e., 30, 40, 50 and 60 °C. Each

Table 1 Properties of the polymer membranes. Name of the polymer membrane

Composition

PVOH

Polyvinyl alcohol crosslinked with 2 mass% glutaraldehyde Full interpenetrating network of copolymer of acrylamide-co-hydroxyethyl methacrylate of 5:1 molar ratio in PVOH matrix with 0.5:1 mass ratio of copolymer to polymer. PVOH matrix crosslinked with 2 mass% glutaraldehyde. Copolymer crosslinked with 0.5 mass% MBA FIPN500 is incorporated with 2 mass% alumino silicate filler FIPN500 is incorporated with 5 mass% alumino silicate filler FIPN500 is incorporated with 10 mass% alumino silicate filler

FIPN500

FIPN502

2.3. Synthesis, crosslinking and casting of filled FIPN membrane

FIPN505

The filled FIPNs i.e., FIPN502, FIPN505, and FIPN510 were synthesized by solution polymerization in a three-necked reactor at

FIPN510

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sample was weighed periodically until no mass change was observed. These membranes were taken out from the solutions and weighed after the superfluous liquid was wiped out with tissue paper. The increment in mass is equal to the total mass of water and acetic acid sorbed by the membrane. After measuring the total mass of the sorped membranes from the above experiment, these thick samples were taken in a 250 ml conical flask kept in a constant temperature bath and connected to a cold trap and vacuum pump in series [9]. The cold trap was immersed in liquid nitrogen flask. The sorped sample was heated under vacuum and the vapor coming out of the thick sorped membranes were freezed and collected in the cold trap immersed in liquid nitrogen. The amount of water sorped by the membranes was obtained by analyzing the composition of the liquefied vapor from the cold trap by a digital refractometer (model. Abbemat-HP, made by Anton Paar, Austria) at 25 °C temperatures for all the samples. From the total sorption mass and corresponding water content (mass) of the membrane, sorption selectivity of the membrane for water was calculated. 2.6. Permeation experiment All of the five membrane samples were also used for permeation experiments by pervaporation using a batch stirred cell [9] for varied concentrations of acid–water mixtures at four different feed temperatures. Each of the sorption and pervaporation experiments was repeated three times and the results were averaged to minimize error. The results for pervaporation separation of acetic acid/water mixtures were reproducible and the errors inherent in the pervaporation measurements were less than 3%. From the sorption and pervaporation experiments total sorption, sorption selectivity, flux, separation factor and permeation ratio were determined [9]. The performance of the pervaporation membranes were also evaluated in terms of pervaporation separation index (PSI) and enrichment factor (b) using Eqs. (15) and (16), respectively [22].

PSI ¼ J total  aPV y b ¼ water yHAC

ð15Þ ð16Þ

3. Results and discussion 3.1. Membrane synthesis Earlier, the FIPN membranes containing varied amounts of the copolymer PAAAM and PVOH were used for dehydration of ethylene glycol [9] and FIPN500 containing PVOH: copolymer of 1:0.5 was found to show optimum performance in terms of flux and water selectivity. Thus, in the present work this FIPN500 membrane was further filled with 2, 5 and 10 mass% aluminosilicate filler to produce FIPN502, FIPN505 and FIPN5010 membranes, respectively.

3.2. Membrane characterization 3.2.1. Membrane characterization by mechanical properties PVOH membrane shows a good balance of tensile strength (T.S) and elongation at break (E.A.B) and hence considered as a good membrane forming polymer. From Table 2 it is observed that the T.S and EAB of PVOH membrane is higher than FIPN membranes. PVOH membrane shows high T.S because of its stiffness due to crystallinity and it also shows high elongation because of its low Tg. The incorporation of high Tg PAAAM copolymer in PVOH decreases its crystalline structure. Further, crosslinking of both PVOH and PAAAM reduces chain flexibility of the FIPN membranes. Thus, both T.S and E.A.B of FIPN membranes are lower than T.S and E.A.B of PVOH membrane. From the values given in Table 2 it is also observed that the filled membranes i.e., FIPN502, FIPN505, and FIPN510 membranes show slightly higher T.S but lower E.A.B in comparison to unfilled FIPN500 membrane. Incorporation of inorganic filler in the matrix of FIPN membrane increases its stiffness but reduces chain mobility. Hence, though the filled membranes show higher TS, EAB of the membranes are reduced after immersion in water and acetic acid for one week. Since the membrane is water selective, the water swollen membrane shows somewhat lower TS than acid swollen membrane while both of these membranes show higher EAB than the dry (not immersed in solvent) membranes. However, the TS and EAB of the solvent swollen membranes are still high enough for pervaporative applications. The mechanical properties of the membranes in different feed mixtures as used for pervaporation experiments will be intermediate in values in between the mechanical properties in pure water and pure acid.

3.2.2. Membrane characterization by SEM SEM studies of the unfilled FIPN500 and the three filled FIPN membranes i.e., FIPN502, FIPN505, and FIPN5010 are shown in Fig. 1a–d, respectively. SEM of a pure polymer like PVOH always gives a dense feature. IPN are different from a blend in that due to interpenetration of the constituent polymers the extent of compatibility is very high in an IPN. Thus, much higher magnification is required (higher than those used for conventional blend) for getting morphology of an IPN through SEM. Hence, SEM of the FIPN500 membrane was carried out at 14 KV in 5 lm scale to get morphology of the constituent polymers. In IPN the size and shape of the polymer II domains (i.e., the copolymer PAAAM) are controlled by the cross-link density of polymer I (PVOH) and the relative proportions of the two polymers [9]. The SEM of the filled IPN are shown at lower magnification (50 lm) to understand the distribution of fillers physically dispersed in the IPN matrix. The distribution of filler in the polymer matrix is understood from SEM of the filled polymer. From Fig. 1b–d it is clear that with increasing filler loading distribution becomes more uniform and Fig. 2c i.e., SEM of FIPN505 gives a well distributed morphology. However, as the FIPN505 is loaded with more filler, filler-polymer

Table 2 Mechanical properties of the membranes. Polymer membrane

Tensile strength (MPa) of unused membrane

Tensile strength (MPa) of membrane after one week immersion in pure water

Tensile strength (MPa) of membrane after one week immersion in acetic acid

Elongation at break (%) of unused membrane(E.A.B)

Elongation at break (%) of membrane after one week immersion in pure water(E.A.B)

Elongation at break (%) of membrane after one week immersion in pure acetic acid (E.A.B)

PVOH FIPN500 FIPN502 FIPN505 FIPN510

42 29.5 30.4 31.2 33.5

38.5 24.3 28.7 29.5 30.2

39 27 29 30.3 31.5

218 105 101 98.5 88.6

282 167 171 113 105

245 156 155 103 98.6

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Fig. 1. SEM of the membranes. (a) FIPN500, (b) FIPN502, (c) FIPN505, (d) FIPN510.

0.12

50

2

FIPN502filler FIPN505filler

PVOHsorp FIPN500sorp

1.8

45

FIPN505sorp FIPN510sorp PVOHsel

1.4

35

FIPN500sel FIPN502sel

1.2

30

FIPN505sel FIPN510sel

1

25

0.8

20

0.6

15

0.4

10

0.2

5

0

0

0

5

10

15

20

FIPN510filler ONLY FILLER

0.1

40

25

Feed conc. of water (mass%)

Total sorption (mass%)

1.6

Sorption selectivity (-)

Total sorption (g/g dry membrane)

FIPN502sorp

0.08

0.06

0.04

0.02

0

0

5

10

15

20

25

Feed conc. of water (mass%)

Fig. 2a. Variation of total sorption and sorption selectivity with feed conc. of water at 30 °C.

Fig. 2b. Variation of total sorption with feed concentration for filler in membrane at 30 °C.

compatibility becomes poorer and the morphology becomes coarser as seen in SEM of FIPN510 containing 10 mass% filler in Fig. 1d.

its matrix resulting in increased free volume and higher sorption of FIPN500 than PVOH. As this FIPN500 is further filled with hydrophilic aluminosilicate filler total sorption increases with increasing filler loading from FIPN502 to FIPN510. From Fig. 2a it is also observed that at very low concentration of water total sorption of water increases almost linearly with feed concentration which may be described by Henry’s law. However, there is a tendency of saturation at higher feed concentration resembling type-II sorption isotherm of Rogers [9].

3.3. Swelling studies 3.3.1. Effect of feed concentration on total sorption Fig. 2a shows the variation of total sorption of acetic acid and water by PVOH, FIPN500 and the three filed IPN i.e., FIPN502, FIPN505, and FIPN510 membranes. Similar kind of relationship was also observed at the other three temperatures of sorption experiments i.e., at 40, 50, and 60 °C. From this figure it is found that total sorption increases in the following order: FIPN510 > FIPN505 > FIPN502 > FIPN500 > PVOH. PVOH shows the lowest sorption because of its semi crystalline nature. The crystallinity of PVOH is reduced by copolymerization in

3.3.2. Effect of feed concentration on sorption selectivity Sorption selectivity for water for all the five membranes are also shown in the same Fig. 2a Like total sorption, sorption selectivity for water also follows the same trend i.e., it also increases from PVOH to FIPN510.

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1.2

50 45

1

40

Total sorption

35

PVOHsorp

0.8

FIPN500sorp

30

FIPN502sorp FIPN505sorp FIPN510sorp

0.6

25

PVOHsel FIPN500sel FIPN502sel

20

Table 3 Density of the polymer and interaction parameter between membrane polymer and pure solvents. Name of the polymer membrane

Density of the polymer (g/cm3)

Interaction between water and membrane vip

Interaction between acetic acid and membrane vjp

PVOH FIPN500 FIPN502 FIPN505 FIPN510

1.7782 1.8132 1.8543 1.8953 1.9432

0.543528 0.502062 0.501681 0.501373 0.501167

1.846457 1.820393 1.724629 1.308794 1.240833

FIPN505sel

0.4

FIPN510sel

15 10

0.2

5

0

0 25

30

35

40

45

50

55

60

65

Feed temperature,0C Fig. 2c. Variation of total sorption and sorption selectivity for water with feed temperature. Feed conc. 0.953 mass% water.

Ratio of water to acid conc in membrane (-)

Feed concentration used for sorption and permeation (mass% water in feed)

Interaction between water and acetic acid vij

0.95 2.09 5.73 9.57 19.23

1.15 1.09 0.88 0.84 0.70

3.3.4. Effect of temperature on total sorption and sorption selectivity From Fig. 2c it is observed that total sorption of all the membranes increases almost linearly with temperature. Chemical modification of PVOH by copolymerization as well as incorporation of filler reduce the crystallinity and increases free volume resulting in higher sorption of FIPN500 or the other three filled membranes. At higher temperature chain mobility of the polymer membranes increases with increased sorption. Similar result was reported by Xiao et al for isopropyl alcohol–water system with PVOH membrane crosslinked with trimesoyl chloride [23]. Sorption selectivity of the membranes was found to decrease with temperature as shown in the same figure.

3

2.5

2

1.5

Table 4 Interaction parameter between water and acetic acid at used feed concentration.

PVOH FIPN500 FIPN502 FIPN505 FIPN510

1

PVOHexpt FIPN500expt FIPN502expt FIPN505expt

0.5

FIPN510expt

0

0

5

10

15

20

25

Feed concentration of water (mass%) Fig. 2d. Effect of feed concentration of water on experimental and calculated membrane phase concentration at 30 °C.

Sorption selectivity of the membranes is also found to decrease almost exponentially with feed concentration of water. 3.3.3. Effect of filler on sorption The contribution of the aluminosilicate fillers to total sorption may be obtained by a simple material balance.

St ¼ Sm þ Sf

ð17Þ

Assuming sorption of water by the polymer part of filled membranes remaining unaffected by the presence of filler, Sf, the amount of sorption by only filler in the membranes may be obtained by subtracting total sorption (Sm) of unfilled IPN membrane i.e., FIPN500 from total sorption of filled membrane (St). The total sorption of water and acetic acid by the aluminosilicate filler and this filler incorporated in the three filled membranes are shown in Fig. 2b. From this figure it is evident that sorption by the filler incorporated in membranes are higher than sorption by the same amount of filler in absence of any membrane.

3.3.5. Thermodynamic interaction parameter The interaction parameter of water–acetic acid, membrane polymer–water and membrane polymer–acetic acid as calculated using the above Eqs. (1), (2), and (5) are given in Tables 3 and 4 for all of the unfilled and filled membranes used for this study. The interaction parameter between solutes i and j i.e., vij or between solute and polymer i.e., vip or vjp bears an inverse relationship with extent of interaction. The higher the extent of interaction between solute i and j, the lower will be the values of vij. Similarly, higher the extent of sorption or swelling of the polymer membrane by the solute i or j molecules, the lower will be the values of vip or vjp. From the values of vip or vjp for all of the used polymer membranes as given in Table 3, it is observed that for any polymer membrane vjp i.e., interaction parameter between polymer and acetic acid (component j) is much higher than the same between water and membrane (vip) signifying more polymer–water interaction than polymer–acetic acid interaction. Further, both of these interaction parameters decrease from PVOH to FIPN510 signifying more interaction of the filled polymer membranes with water and acetic acid. With incorporation of hydrophilic filler the copolymer modified PVOH membranes show higher interaction with the penetrant molecules which is also in good agreement with the result of total sorption or sorption selectivity as shown in Fig. 2a. Similar range of interaction parameter values were reported for toluene– methanol binary mixtures with polyimide–polyacrylonitrile blend membranes [17] and alcohol–water binary mixtures with alginate–cellulose blend [15] and mixed matrix membranes [24]. The variation of the ratio of membrane phase concentration of water to acid with feed concentration of water is shown in Fig. 2d for both experimental and theoretical values. The theoretical

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concentration, FIPN500 membrane yields higher mass% of water in permeates. As the FIPN500 is filled with hydrophilic aluminosilicate filler, hydrophilicity of the filled membranes increases and overall for any feed concentration, permeate concentration of water shows the following trend: FIPN510 > FIPN505 > FIPN502 > FIPN500 > PVOH.

100

Permeate conc. of water (wt%)

90 80 70 60 PVOH

50

FIPN500 FIPN502

40

FIPN505 FIPN510

30

tie VLE

20 10 0 10

0

20

30

40

50

60

70

80

90

100

Feed conc. of water (wt%) Fig. 3a. Variation of permeate conc of water with its feed conc. at 30 °C.

400 14 350

10 250

PVOHflux FIPN500flux

8

FIPN502flux FIPN505flux

200

FIPN510flux PVOHsel

6

150

FIPN500sel FIPN502sel

Water selectivity (-)

300

-2

-1

Total Flux (kgm hr µm)

12

FIPN505sel

4

100

FIPN510sel

2

50

0

0 0

5 10 15 20 Feed concentration of water (mass%)

25

Fig. 3b. Variation of normalized total flux and selectivity with feed concentration of water at 30 °C.

membrane phase water concentration was determined from interaction parameters using Flory Huggins theory [21]. It is evident from this figure that experimental values are in close agreement with theoretical values.

3.4.2. Effect of feed concentration on flux and permeation selectivity The effect of feed concentration of water on thickness normalized total flux and water selectivity is shown in Fig. 3b for all the membranes. From Fig. 3b it is observed that for any feed concentration permeation flux and selectivity shows similar trend like sorption i.e., it increases from PVOH to FIPN510. The lowest flux of PVOH may be ascribed to its high degree of crystallinity due to intra and intermolecular hydrogen bonding. Incorporation of PAAAM copolymer in the matrix of PVOH increases its hydrophilicity and decreases its crystallinity. Thus, copolymer modified FIPN500 membrane shows higher flux than PVOH membrane. The hydrophilicity of FIPN500 membrane is further increased by incorporating hydrophilic aluminosilicate filler in its matrix. Thus, with increasing % of this filler flux increases from FIPN502 to FIPN510. Water selectivity is also found to decrease almost exponentially with feed concentration for all the membranes. It is also observed that at lower feed concentration rate of increase in permeation flux is very high but above around 5 mass% feed concentration there is a tendency for saturation. Above this concentration the fall in water selectivity is also rapid signifying plasticization of the membranes. Variation of normalized partial flux of water and acid with feed water concentration for all the membranes are shown in Fig. 3c. From Fig. 3c it is observed that water partial flux increases linearly at lower feed water concentration for all the membranes. At higher feed concentration the feed versus partial flux plot for water is no longer linear (the slopes are not constant and overall the plots show a polynomial trend with degree of freedom 2) signifying plasticization of the membranes. From Fig. 3c it is also observed that for the same feed concentration water flux is much higher than acid partial flux signifying high water selectivity of the membranes. In fact, acid flux remains marginally constant over the concentration range used for this study for all the membranes. 3.4.3. Effect of feed concentration on pervaporation separation index (PSI) and enrichment factor (b) Fig. 4 shows variation of PSI and enrichment factor for water with feed concentration of water. In general, flux and selectivity bears an opposite relationship with respect to feed concentration as also seen for the used membranes. PSI relates both permeation flux and

3.4. Pervaporation(PV) studies 12

Normalised partial Flux (kgm-2hr-1µm)

3.4.1. Effect of feed concentration on dehydration Fig. 3a shows the variation of mass% of water in the permeate against mass% of water in the feed for dehydration of acetic acid with the two unfilled and three filled membranes at 30 °C. Similar kind of relationships was also observed at the three other PV temperatures i.e., at 40, 50, and 60 °C. It appears from these McCabeThiele type xy diagrams that all of the five membranes show measurable dehydration characteristics over the used concentration range without any pervaporative azeotrope. The vapor–liquid equilibrium (VLE) data [13] is also shown in the same figure. It is evident from the figure that over the used concentration range, all of the used membranes show much higher water concentration in the permeate than its VLE values. Incorporation of hydrophilic crosslink copolymer PAAAM in PVOH matrix increases water affinity of the membranes. However, it also decreases crystallinity of PVOH matrix as its intramolecular hydrogen bonding is reduced. Thus, in comparison to PVOH, for the same feed water

PVOHWATER

10

FIPN500WATER FIPN502WATER

8

FIPN505WATER FIPN510WATER

6

PVOHACID FIPN500ACID

4

FIPN502ACID FIPN505ACID

2

FIPN510ACID

0 0

5

10

15

20

25

Feed conc of water (mass%) Fig. 3c. Variation of normalized partial flux with feed concentration of water at 30 °C.

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3.5

9 PVOHWATER

2000 3

FIPN502WATER

2 1000

1.5 1

FIPN500P SI FIPN502P SI FIPN505P SI FIPN510P SI PVOHEF FIPN500EF

FIPN505WATER

7 Permeation ratio (-)

PSI (kg/m2hr)

1500

Enrichment factor(-)

PVOHPSI

2.5

FIPN500WATER

8

FIPN510WATER PVOHacid

6

FIPN500acid FIPN502acid

5

FIPN505acid FIPN510acid

4 3

FIPN502EF

500

2 0.5

1 0 5

10

15

20

0

25

0

Feed concentration of water (mass%)

10

15

20

Fig. 5. Variation of permeation ratio with feed concentration at 30 °C.

1800

300

Partial permeability of water (Barrer)

1600 250

1400 1200

200

1000

3.4.5. Effect of feed concentration on intrinsic membrane properties Fig. 6a describes the variation of partial permeability of water and acetic acid with feed water concentration at 30 °C for all the five membranes. Partial permeability of the membranes was calculated from activity, partial flux, and vapor pressure difference on feed and permeate side of the membranes using Eq. (11). Intrinsic membrane properties like permeability, permeance or membrane

150

PVOHwater

800

FIPN500water FIPN502water

600

FIPN505water

100

FIPN510water

400

PVOHacid

50

FIPN500acid FIPN502acid

200

FIPN505acid FIPN510acid

0 0

5

10

15

20

0 25

Feed concentration of water (mass%) Fig. 6a. Variation of partial permeability with feed concentration of water at 30 °C.

25

16

Fugacity of water Fugacity of acid

14

PVOHsel

20 Fugacity (cm water)

3.4.4. Effect of feed concentration on permeation ratio From Fig. 5 it is observed that at very low feed concentration of water the permeation ratio of water is far above unity for all the membranes signifying positive coupling effect of acid on partial flux of water. At this high acid feed concentration, acid–water interaction is more than water–membrane interaction because of very low water concentration in feed. As the water concentration in feed increases, permeation ratio of water decreases drastically for all the water selective membranes and become close to unity i.e., coupling effect of acid on water becomes negligible because of much higher water–membrane interaction (through hydrogen bonding) than acid–water interaction. Similarly, acid permeation ratio increases with increase in water feed concentration because of increasing water–acid interaction and above an water concentration of around 5 mass%, acid permeation ratio exceed unity signifying positive coupling affect of water on acid permeation. It is also observed from the figure that for the same feed concentration of water permeation ratio of water increases from PVOH to FIPN510 because of increasing hydrophilicity in the same order. In a similar way acid permeation ratio follows an opposite trend i.e., it decreases from PVOH to FIPN510.

25

Feed concentration of water (mass%)

Fig. 4. Variation of pervaporation separation index (PSI) and enrichment factor (EF) with feed concentration of water at 30 °C.

selectivity of the desired component in one equation (Eq. (15)) and hence, the optimum performance of a membrane can be evaluated in terms of its PSI. For all of the used membranes, PSI is found to decrease with feed concentration. The rate of decrease of selectivity is much higher than rate of increase of flux with feed concentration (Fig. 3b). Thus, PSI, which is a product of flux and selectivity decreases with feed concentration. Enrichment factor (b, Eq. (16)) is also shown for all the membranes in the same figure. From the figure it is observed that for any feed concentration except PVOH (for the first feed concentration showing b < 1) all of the membranes show b > 1 signifying water selectivity of the membranes. However, with increasing feed concentration b decreases due to plasticization though the change of b with concentration is marginal above around 10 mass% feed water concentration.

5

Partial permeability of acetic acid (Barrer)

0

12

FIPN500sel FIPN502sel

10

FIPN505sel

15

FIPN510sel

8

10

6 4

5

Membrane selectivity (-)

0

2 0

0

0

5

10 15 20 Feed concentration of water (mass%)

25

Fig. 6b. Variation of intrinsic membrane selectivity and fugacity of water and acid with feed concentration at 30 °C.

selectivity (ratio of permeability of component i to j) decouples the driving force of vapor pressure differential from operating parameters like feed concentration or temperature. Accordingly, flux or selectivity showing considerable variation with feed concentration or temperature may remain practically constant or

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may show a different trend with feed concentration or temperature in terms of driving force normalized permeability or membrane selectivity. In tune with this argument as given by Baker et al. [18] and Guo et al. [20], in the present work partial permeability (Fig. 6a) is also observed to show a different trend from partial flux (Fig. 3c) for all the membranes. In this case, unlike flux, partial permeability is observed to decrease almost exponentially with feed concentration for all the membranes. Similar results were obtained with one of our previous works with rubber membranes [25] as shown by Baker et al. [18]. The opposite trend of water permeability (Fig. 6a) with respect to water flux (Fig. 3c) as a function of feed concentration may be ascribed to change of fugacity with feed concentration as shown in Fig. 6b for water and acid. Comparing Fig. 3c and Fig. 6a, it is evident that the rate of change of water fugacity (Eq. (10) and denominator of Eq. (11) defining permeability) with feed water concentration is much higher than change of partial flux (numerator of Eq. (11)) with concentration. As a result, water permeability decreases with increase in feed concentration because of higher rate of increase of fugacity of water with feed concentration. The change of fugacity of acid with feed concentration of water is marginal and hence acid permeability show a different trends i.e., for filled membranes like partial acid flux, partial acid permeability also increases with feed concentration. A close look on Fig. 6a reveals that FIPN510, FIPN505, and FIPN502 filled membranes show very high water permeability ranging from 1550 barrer (FIPN510) to around 750 barrer (FIPN502) and very low acid permeability (100–200 barrer) at low feed water concentration which is usually practiced in industrial pervaporation applications. As the feed water concentration is increased, the water permeability decreases by 60–70% and reaches a plateau above 10 mass% feed water concentration. On the other hand, acid permeability increases by around 100% at this concentration. This change in membrane permeability may be ascribed to plasticization of the water selective membranes at higher feed concentration. Chemical modification by copolymerization in the matrix of PVOH not only reduces its crystallinity but also increases its hydrophilicity by incorporating hydrophilic carboxylic (from AA monomer) and amide groups (from AM monomer) of copolymer. Incorporation of hydrophilic filler further improves its hydrophilicity. Thus, at 0.95 mass% water in feed crosslink PVOH shows water permeability as low as 187 barrer while at the same feed concentration FIPN510 shows a water permeability

1.2

FIPN500water

Ratio of diffusive to permeation flux (-)

FlPN502water FIPN505water

1

FIPN510water PVOHacid FIPN500acid

0.8

3.4.6. Determination of concentration average diffusion coefficient of water and acetic acid Diffusive flux of water and acetic acid was calculated using Eq. (14). The variation of ratio of diffusive flux to permeation flux with feed concentration is shown Fig. 7a at 30 °C for all the five membranes. From Fig. 7a it is observed that for PVOH membrane more than 95% of total permeation flux of water is due to diffusion while for acid it is around 71%. As this PVOH membrane is chemically modified with copolymer, in FIPN500, diffusive flux reduces to 90% for water and 65% for acid. When this chemically modified FIPN500 membrane is further physically modified by incorporating filler, in FIPN505 or FIPN510, diffusive flux of water further reduces to around 80% while diffusive flux of acid becomes as low as 30% of its permeation flux. As the membrane becomes more hydrophilic by chemical or physical modification, sorption becomes more significant with decreasing contribution of diffusive flux to overall permeation flux. From Fig. 7a it is observed that with increasing feed concentration of water the ratio of diffusive to permeation flux of water decreases almost exponentially for all the membranes signifying plasticization of the membranes. Above around 5 mass% feed water concentration, there is a marginal increase of the flux ratio of acid. In fact, as the membranes become plasticized, it also allows diffusion of more acid with increased diffusive flux. It is observed from Fig. 7a that over the entire concentration range used in this study, 87–95% of permeation flux of water is due to its diffusive flux. For acid, 30–70% of its permeation flux is due to diffusion.

Concentration average Diffusion coefficient (cm2/s) x 108

PVOHwater

of 1532 barrer. However, at higher feed concentration plasticization of the membrane reduces water permeability of the membranes. The intrinsic membrane selectivity is also reduced at higher feed concentration as also shown in Fig. 6b for all the membranes. The crystallinity of PVOH is not only reduced by crosslinking, copolymerization or filler incorporation [13,14] but also by increased feed concentration of water. Hodge et al. [26] studied reduction of crystallinity of PVOH by water absorption and found that when the absorbed water reaches 60–62%, the otherwise semi crystalline PVOH polymer becomes amorphous with increased free volume. In a similar way, the chemically and physically modified PVOH membranes in the present study looses its crystallinity at high feed water concentration and hence with increased free volume show lower water permeability or intrinsic membrane selectivity.

FlPN502acid FIPN505acid FIPN510acid

0.6

0.4

0.2

0

4 PVOHwater FIPN500water

3.5

FlPN502water FIPN505water

3

FIPN510water PVOHacid FIPN0acid

2.5

FlPN2acid FIPN5acid FIPN10acid

2 1.5 1 0.5 0

0

5

10

15

20

25

Feed concentration of water (mass%) Fig. 7a. Variation of ratio of diffusive to permeation flux with feed concentration at 30 °C.

0

5

10 15 20 Feed concentration of water (mass%)

25

Fig. 7b. Variation of concentration average diffusion coefficient with feed concentration of water at 30 °C.

S.B. Kuila, S.K. Ray / Separation and Purification Technology 81 (2011) 295–306

It indicates that the overall permeation of water for any feed concentration is dominated by diffusion while for acid diffusive flux becomes significant at higher feed water concentration. The concentration average diffusion coefficient of water and acetic acid as calculated using Eq. (13) is shown in Fig. 7b. The diffusion coefficient (DC) of water is observed to be in the order of 108 cm2/s while DC of acetic acid is observed to be in the order of 1010 cm2/s. Similar order of DC for benzene and cyclohexane was reported by Peng et al. [27] through substituted silane filled PVOH membrane. Schaetzel et al. [28] also reported similar order of DC for ethanol and water through PVOH-polyacrylic acid blend membranes. It is observed from Fig. 7b that for any feed concentration DC of water is much higher than DC of acid. This may be ascribed to lower kinetic diameter of water (0.296 nm) than acetic acid (0.436 nm) [29] for which for the same concentration gradient water moves faster than acid through the membranes. 3.4.7. Effect of temperature on flux, selectivity and permeability With increase in temperature flux increases exponentially for all the membranes while selectivity decreases at higher temperature in the same order as shown for all the membranes in Fig. 8 with 0.953 mass% feed water concentration. At higher temperature frequency and amplitude of segmental motion of the polymeric chains increases with increased free volume. This increased free

250

500

200

350 300

150

250 200

100

150 100

50

PVOHflux FIPN500flux FIPN502flux FIPN505flux FIPN510flux PVOHsel FIPN500sel FIPN502sel FIPN505sel FIPN510sel

50 0

Table 5 Activation energy for permeation of water and acetic acid Name of the polymer membrane

Ew/R, water

R2

Activation energy for permeation of water, Ew, kJ/deg mole

Ea/R, acid

R2

Activation energy for permeation of acid, Ea, kjoule/deg mole

PVOH FIPN500 FIPN502 FIPN505 FIPN510

3558 3658.5 2828.3 2826.8 2219.5

0.9951 0.9755 0.8948 0.8488 0.8725

29.58 30.41 23.51 23.50 18.45

3865.9 4159.7 3654.4 3628.7 3796.9

0.9518 0.9542 0.8916 0.8719 0.917

32.14 34.58 30.38 30.16 31.56

0 25

35

45

55

65

75

85

Feed temperature,0C

Fig. 8. Variation of total flux and permeation selectivity for water with feed temperature. Feed conc. 0.953 mass% water in feed.

4000

16

3500

14 PVOHWATER

12

2500

10 8

2000 1500

6

1000

4

300

600

250

500

200

400

PVOHflux

FIPN500WATER FIPN502WATER FIPN505WATER FIPN510WATER PVOHACID FIPN500ACID FIPN502ACID FIPN505ACID FIPN510ACID PVOHsel FIPN500sel FIPN502sel

2

3000

Membrane selectivity (-)

Partial Permeability (Barrar)

3.4.8. Apparent activation energy for permeation Apparent activation energy for permeation (EP) can be obtained from the slope of the Arrhenius type linear plot of logarithmic of partial molar flux against inverse of absolute temperature (1/T). The apparent activation energy for permeation of water (Ew) and acetic acid (Ea) for 0.953 mass% of water in feed are given in Table 5. The values of slope and regression coefficient, r2 for all of these linear trend lines are also shown in Table 5. From this table it is observed that activation energy for water is much lower than acetic acid for all the membranes. Permeation of smaller water molecules through these highly hydrophilic membranes are much easier than acid resulting in lower activation energy. The values of r2 for all the membranes are also found to be close to unity signifying good fit of the linear trend lines.

Total Flux (g/m hr)

Total Flux (gm -2hr)

400

Permeation selectivity for water (-)

450

volume facilitates transport of permeants. Further, vapor pressure of the permeating molecules also increases at higher temperature. Both of these effects i.e., increased free volume and driving force (vapor pressure difference) causes increased flux at higher temperature for all the membranes. Partial permeability of water and acid as shown in Fig. 9 are also observed to increase with temperature while change of intrinsic membrane selectivity with feed temperature is not very significant for filled membranes as shown in the same figure. It is worth mentioning that rate of increase of water flux or permeability with temperature is higher than the rate of increase of acid flux or acid permeability. Thus, at higher temperature, though acid flux or permeability increases, overall the decrease of water selectivity with temperature (Fig. 8) or membrane selectivity with temperature (Fig. 9) is not as high as decrease of selectivity with feed concentration (Fig. 3b). Thus, these membranes may also be suitably used at higher temperature with increased flux at the cost of a slight decrease in selectivity.

150

300

100

200

50

100

FIPN505sel

500

2

0

0

FIPN510sel

FIPN500flux

Water Selectivity (-)

304

FIPN502flux FIPN505flux FIPN510flux PVOHsel FIPN500sel FIPN502sel FIPN505sel

25

35

45

55

Feed temperature,

65

75

85

0C

Fig. 9. Variation of partial permeability and membrane selectivity with feed temperature. Feed conc. 0.953 mass% water.

0 0

0.01

0.02

0.03

0.04

0 0.05

FIPN510sel

Membrane thickness (micron) Fig. 10. Variation of total flux and water selectivity with membrane thickness. Feed conc. 0.953 mass% water.

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Feed water concentration (wt%)

Temperature °C

Normalized Flux, (kg/m2 h) ml

Water selectivity ()

Reference

PVOH-GLU (Pervap 2201, Sulzer PVOH-g-AN membrane PVOH TEOSPVOH-Na-Alg blend Na-Alg-HAD Na-Alg silicotungstic acid hybrid membrane Charged Nafion Asym. Polycarbonate Mordenite ZSM-5 zeolite FIPN500 FIPN502 FIPN505 FIPN510 FIPN500 FIPN502 FIPN505 FIPN510

10 50 10 10 10 15 10

25 60 30 30 30 70 30

12 1 kg/m2 h 1.75 3.32 0.717 0.262 9.7

20 450 14 2423 40.3 161 22,491

[1] [2] [3] [4] [5] [6] [7]

10 70 50 50 9.58

25 25 80 70 30

30

243 1.7 299 8 12.16 15.84 18.27 20.43 110.49 168.72 204.18 325.533

[8] [9] [10] [11] Present work

0.953

32.2 11.3 0.614 0.62 4.917 9.234 11.0 11.7 2.924 4.242 5.188 6.612

Present work

PVOH, polyvinyl alcohol; GLU-PVOH, glutaraldehyde crosslinked PVOH; PVOH-g-AN, acrylonitrile grafted PVOH; PVOH-TEOS, tetraethyl orthosilicate croslinked PVOH; PVOH-Na-Alg blend, sodium alginate-PVOH blend; Na-Alg-HAD, sodium alginate crosslinked with hexane diamine.

3.4.9. Effect of membrane thickness on flux and selectivity For any homogeneous membrane used for pervaporative studies thickness of the membrane maintains a linear relationship with flux i.e., flux decreases linearly with increase in thickness of the membrane. Thus, for comparison of performance of different membranes with varied thickness, the effect of thickness is normalized by multiplying flux with thickness. Accordingly, in the present study thickness normalized flux has been presented. However, in the present study the homogeneity of the filled membranes depends on both distributive and dispersive mixing of filler in the polymer matrix [27]. From Fig. 10 it is observed that flux through the filled membranes i.e., FIPN502, FIPN505, and FIPN510 increases linearly (with regression coefficient, r2 > 0.9) with inverse of membrane thickness. It is also interesting to note that as the filler loading increases from 2 mass% (in FIPN502) to 10 mass% (in FIPN510), regression coefficient of the linear trend lines increasingly deviates from unity signifying less uniform distribution of fillers in the membrane matrix [27]. Water selectivity of the membranes is also found to increase with increase in membrane thickness for all the membranes. In a similar way, here also linearity of the trendlines decreases from PVOH to FIPN510 as shown in Fig. 10 where also r2 deviates increasingly in the same order from unity.

3.4.10. Comparison with reported data The pervaporative dehydration of acetic acid by various reported membranes along with the present membranes are shown in Table 6. From Table 6 it is observed that most of the membranes report pervaporation data i.e., flux and separation factor for acetic acid–water mixtures at acid concentration of 90 mass%. Some of the reported membranes showed these data even at lower concentration of acid. This may be due to very corrosive nature of acetic acid. Above 90 mass% acid concentration most of the membranes collapse. The present membranes appeared to be very stable even above 90 mass% concentration of acid. It is also worth mentioning that pervaporative dehydration of any organic is relevant at very high concentration of the organic. The present membranes showed good flux and high water selectivity for highly concentrated acid and thus would be very suitable for pervaporative dehydration of corrosive organics.

4. Conclusion PVOH membrane was chemically modified by allowing copolymerization of acrylic acid and acrylamide with 10:1 comonomer ratio in the matrix of PVOH with PVOH:copolymer of 1:0.5 followed by crosslinking of copolymer with N,N0 -methylene bis acrylamide and crosslinking of PVOH with glutaraldehyde. This chemically modified full inter penetrating network (FIPN) type polymer designated as FIPN500 was further physically modified by incorporating 2, 5, and 10 mass% aluminosilicate filler to produce three filled FIPN membranes designated as FIPN502, FIPN505, and FIPN510. All the filled membranes show high flux and water selectivity. Among the used membranes FIPN510 were found to show the highest flux and selectivity. The effect of operation parameters like feed concentration and feed temperature on sorption and permeation were studied. The intrinsic membrane properties like permeability and membrane selectivity were compared with flux and selectivity at different operating conditions for all the membranes. Diffusion coefficient for water and acid through the membranes were also evaluated. These highly hydrophilic but acid resistant filled IPN membranes may also be used at higher temperature with increased flux at the cost of slight decrease in water selectivity. As these membranes tolerate highly concentrated corrosive acetic acid, these may also be used for dehydration of other organics. Acknowledgement The authors are grateful to DST-SERC for sponsoring the works. References [1] S.P. Kusumocahyo, K. Sano, M. Sudoh, M. Kensaka, Water permselectivity in the pervaporation of acetic acid–water mixture using crosslinked poly(vinyl alcohol) membranes, Sep. Purif. Technol. 18 (2000) 141–150. [2] T.M. Aminabhavi, U.S. Toti, Pervaporation separation of water–acetic acid mixtures using polymeric membranes, Des. Monomers Polym. 6 (3) (2003) 211–236. [3] D. Van Baelen, B. Van der Bruggen, K. Van den Dungen, J. Degreve, C. Vandecasteele, Pervaporation of water–alcohol mixtures and acetic acid–water mixtures, Chem. Eng. Sci. 60 (2005) 1583–1590.

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