Separation of acidwater mixtures by pervaporation

Research Article Received: 30 August 2011

Revised: 24 September 2011

Accepted: 24 September 2011

Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jctb.2752

Separation of acid–water mixtures by pervaporation using nanoparticle filled mixed matrix copolymer membranes Himadri Sekhar Samanta, Samit Kumar Ray,∗ Paramita Das and Nayan Ranjan Singha Abstract BACKGROUND: Low energy and less expensive membrane based separation of acetic acid-water mixtures would be a better alternative to conventional separation processes. However, suitable acid resistant membranes are still lacking. Thus, the objective of the present study was to develop mixed matrix membrane (MMM) which would allow high flux and water selectivity over a wide range of feed concentrations of acid in water. RESULTS: Three MMMs, namely PANBA0.5, PANBA1.5 and PANBA3 were made by emulsion copolymerization of acrylonitrile (AN) and butyl acrylate (BA) with 5.5 : 1 comonomer ratio and in situ incorporation of 0.5, 1.5 and 3 wt%, sodium montmorilonite (Na-MMT) nanofillers, respectively. For a feed concentration of 99.5 wt% of acid in water the membranes show good permeation flux (2.61, 3.19, 3.97 kg m−2 h−1 µm−1 , for PANBA0.5, PANBA1.5 and PANBA3 membrane, respectively) and very high separation factors for water (1473, 1370, 1292 for PANBA0.5, PANBA1.5 and PANBA3 membrane, respectively) at 30 ◦ C. Similarly for a dilute acid–water solution, i.e. for 71.6 wt% acid the membrane showed a very high thickness normalize flux (8.67, 9.44, 11.56 kg m−2 h−1 µm−1 , for PANBA0.5, PANBA1.5 and PANBA3 membrane, respectively) and good water selectivity (101.7, 95.3, 79 for PANBA0.5, PANBA1.5 and PANBA3 membrane, respectively) at the same feed temperature. The permeation ratio, permeability, diffusion coefficient and activation energy for permeation of the membranes were also estimated. CONCLUSION: Unlike most of the reported membranes, the present MMMs allowed high flux and selectivity over a wide range of feed concentrations. These membranes may also be effective for separating other similar organic-water mixtures. c 2012 Society of Chemical Industry Keywords: pervaporation; acetic acid; nanoparticle; mixed matrix membranes; partial permeability; diffusion coefficient

NOTATION α β γ δ φ χ a X Y x y f Pi p ps pf pp ppt J l

Intrinsic membrane selectivity (−) Enrichment factor (−) activity coefficient (−) solubility parameter (MPa0.5 ) volume fraction (−) interaction parameter (−) activity (−) Feed concentration (wt%) Permeate concentration (wt%) mole fraction, feed (−) mole fraction, permeate (−) fugacity (cm of water) permeability of component i (barrer) pressure (cm Hg) saturated vapor pressure (cm Hg) feed side partial pressure (cm Hg) permeate side partial pressure (cm Hg) permeate side total pressure (cm Hg) Flux (kg m−2 h−1 ) membrane thickness (µm)

J Chem Technol Biotechnol (2012)

INTRODUCTION Acetic acid is one of the top 20 organic intermediates used in chemical industries.1 It is produced mainly by oxidation of petrochemical feedstocks.2 Acetic acid is also produced from renewable sources such as corn or biomass by fermentation.2 Mixtures of acetic acid–water are encountered as byproducts in the production of acetic acid itself, vinyl acetate, catalytic esterification of alcohols, etc. Hence, separation of acetic acid–water mixtures is very important. However, separation of acetic acid from water by normal binary distillation is difficult due to the low relative volatility, especially at acetic acid concentrations in excess of 90 wt% acid. It also forms a pinch on the vapor–liquid equilibrium curve. Thus, more energy intensive and expensive azeotropic distillation is required for separation of acetic acid–water mixtures.1 Further, separation or recovery of acetic acid from fermentation

∗

Correspondence to: Samit Kumar Ray, Department of Polymer Science and Technology, University of Calcutta, 92, A.P.C. Road, Kolkata-700009, India. E-mail: [email protected] Department of Polymer Science and Technology, University of Calcutta, 92, A.P.C. Road, Kolkata-700009, India

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c 2012 Society of Chemical Industry

www.soci.org broth should be carried out at low temperatures so that the bacteria used for the fermentation process are not killed. Thus, in recent times a low temperature and low energy membrane based separation process such as pervaporation has been tried by many researchers1,3 – 19 for separation of acetic acid–water mixtures. Hydrophilic membranes are used for pervaporative separation of water from binary mixtures with acids. Hydrophilic pervaporation (PV) membranes are mainly prepared from polymers having solubility in water. The main drawbacks of most of the present hydrophilic membranes used for pervaporative dehydration of acid are extensive swelling in high feed concentrations of water or collapsing in high acid feed concentrations because of the corrosive nature of acetic acid. Since most hydrophilic membranes used for this separation process are made by crosslinking watersoluble polymers (e.g. crosslinked PVOH), these membranes show extensive swelling when the water content of feed solution is high. Another way of making hydrophilic membranes is using water-insoluble polymer having functional groups with affinity for water (e.g. acrylonitrile copolymers). These polymers show comparatively less swelling in feed containing high amounts of water but most of these polymers collapse when corrosive organic like acetic acid content (as in our present work) in feed is very high (greater 99%). Further, these membranes also yield low flux at high organic feed concentrations. Considering the limitations of these membranes, the objective of our present work was to make a membrane that would exhibit high flux and separation factor for water at both low and high feed concentrations of water. The water selectivity of hydrophilic membranes may be improved by reducing swelling of the membranes. Swelling of the membranes may be reduced by crosslinking,11,20 blending with less water swelling polymer,3 copolymerization of two functional monomers,1,9,13 allowing polymerization of one hydrophilic monomer in the matrix of a polymer to produce IPN type polymer21 and using composite membranes.10,12,14,16,19,22 These modifications improve water selectivity of the resulting membranes to some extent but flux is not increased because of the dense network structure formed by most of these modifications. This dense network reduces the number of available hydrophilic channels required for water permeation resulting in lower flux.19 Mixed matrix membranes (MMM) are used8,23,24 to increase both flux and water selectivity even at higher feed water concentrations. Hydrophilic MMMs are prepared by incorporating hydrophilic adsorbent in the matrix of hydrophilic polymeric membranes. The hydrophilic adsorbents improve the membrane permeability and selectivity by increased sorption and a higher rate of diffusion. Further, it also improves mechanical properties of the membranes. Among the various adsorbents zeolite,7,10,25 metal oxides,4 sodium montmorillonite (Na-MMT),24,26,27 etc., have been tried for making MMM. Incorporation of nanosize fillers as adsorbent in dense membrane improves its performance (normalized flux and selectivity) in both gas separation and pervaporation for several types of feed mixtures.24,26 – 28 Most porous fillers reported up to now have particle sizes in the micron range (∼50–60 micron).8,26 As a consequence, the minimum membrane thickness of the filled MMM was inherently higher than that of most unfilled membranes and the absolute fluxes remained low. This apart, MMMs made by incorporating hydrophilic fillers of micron size show low water selectivity at higher filler loading because of defects in the polymer–filler interphase due to poor polymer–filler compatibility. In contrast, nanosize filler may contribute to improved flux and selectivity of a MMM even at a much lower concentration than fillers of micron size. However, nanosize filler is also very difficult to mix with a polymer

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HS Samanta et al.

because of its tendency to agglomeration. In the present work, MMM was made by in situ incorporation of nanosize (∼30–90 nm) filler during copolymerization of hydrophilic but water-insoluble monomers. Among the various nanosize fillers, the recent development of small ‘nanosized’ Na-MMT leads to the possibility for the preparation of thin, defect-free, filled polymer layers. Na+MMT is also a cation exchanger with the properties of good intercalation in polymer matrix. This smectite clay with particle size in the nanometer range can form layered polymer nanocomposite with improved physical and mechanical properties. Thus, in the present work nanosize Na+MMT clay was incorporated in situ in copolymers of two water-insoluble monomers, i.e. AN and BA to form mixed matrix copolymer membranes. These membranes were used for separation of acetic acid–water mixtures over the concentration range 72–99.5 wt% acid in water.

THEORY Sorption Total sorption (S) of solvent or binary mixtures of two solvents may be obtained from the following equation: S=

Mf − Mi Mi

(1)

Here Mf is mass of wet membrane after reaching sorption equilibrium and Mi is initial mass of dry membrane before it is immersed in solvent or binary mixtures of two solvents. Similarly, sorption selectivity is obtained from the following equation: Ym(Water) Y (2) αS = m(HAC) Xf (Water) Xf (HAC) where Ym1 and Xf 1 denote membrane phase and feed concentration (wt %) of component 1, respectively. Relative sorption of two components i.e. water and acetic acid in a membrane, depends on the interaction parameter between these two components as well as on the interaction parameter between these components and the membrane polymer. Interaction parameter Mutual interaction between the permeating components (water and acetic acid) in feed liquid mixtures Interaction of component i and j in feed mixture is assumed to be the same as it will be in the membrane after sorption equilibrium is reached.29 The interaction of component i with component j (i.e. water and acetic acid) may be obtained using the following equation: χij =

xj GE 1 xi + xj ln + xi ln xi vj vi vj RT

(3)

Excess free energy, GE can be expressed as GE = RT(xi ln γi + xj ln γj )

(3a)

Here vi and vj are volume fraction of i and j in a liquid feed mixture of i and j.



Separation of acid–water mixtures using nanoparticle/copolymer membranes In the present system acetic acid is polar in nature and miscible in all proportions with water. Thus, activity coefficient of water and acetic acid of different feed mixtures may be obtained using a two-parameter Wilson equation, i.e. Equation (4) and (5) for water and acetic acid, respectively:

ji ij − xi + ij xj xj + ji xi ji ij lnγj = −ln(xj + ji xi ) − xi − xi + ij xj xj + ji xi lnγi = −ln(xi + ij xj ) + xj

(4) (5)

The Wilson parameter ij and ij for water and acetic acid was obtained from its vapor–liquid equilibrium data30 as 0.23965 and 1.67589, respectively. Interaction parameter between membrane polymer and water or acetic acid The interaction parameter χip between component i (say water) and membrane polymer p or χjp between component j (acetic acid) and membrane polymer may be obtained using the following equation based on Flory–Huggins theory:31 lna1 = ln(1 − ϕp ) + ϕp + χ1p ϕp2

x1 and γ1 are mole fraction and activity coefficient of component 1. The pure component i or j may be assumed ideal. Accordingly, ai = aj = 1. Hence, Equation (6) will reduce to −ln(1 − ϕp ) − ϕp ϕp 2

where Q is the mass of permeates collected in time interval t, A is the effective membrane area. However, thickness has a significant influence on flux. Thus, to eliminate this effect of thickness, thickness normalized flux (Jn , kg m−2 h−1 µm−1 ) was calculated by multiplying flux by the thickness (l) of the membrane using: (11) Jn = l × Jtotal Separation factor The permeation selectivity (αpv ) of water expressed as separation factor for water was calculated from a similar type of equation to sorption selectivity, i.e.

αPV

1 ρp 1+ S ρi

(8)

The density of filled copolymer membrane ρp was obtained by dividing a specified mass of the membrane by its volume. Volumes of the membranes were obtained by a method reported elsewhere.32 Sorption selectivity, αs may be obtained in terms of the interaction parameter32 from the following equation: Vi ϕi − 1 ln( ) − χij (ϕj − ϕi ) lnαs = Vj vj Vi − χij (vi − vj ) − ϕp χip − χjp Vj

PSI = Jtotal × αPV

Permeation Total flux Total flux (Jtotal , kg m−2 h−1 ) of each membrane was determined by weighing the permeate mass containing both water and acid


(13)

Enrichment factor Pervaporation selectivity is also described by another parameter β, which is the ratio of permeate concentrations of component i and j, i.e. Ywater β= (14) YHAC Permeability and permeance Based on a solution diffusion model permeation of component i through a dense pervaporation membrane may be described in terms of its vapor pressure difference (driving force) on the feed and permeate side by the following equation: Ji =

where V1 is molar volume, ϕ1 is volume fraction in sorbed membrane and v1 is volume fraction in bulk feed of component 1.

(12)

Pervaporation separation index (PSI) The performance of a pervaporation membrane is evaluated in terms of pervaporation separation index (PSI), which is the product of total flux and separation factor, i.e.

(9)

Ywater Y = HAC Xwater XHAC

where Y1 and X1 denote permeate and feed concentration of component 1, respectively.

(7b)

Volume fraction, ϕp of polymer membrane may be obtained using the following equation: ϕp =

(kg) collected after a definite interval of time and dividing it by the time interval (h) and membrane area (m2 ), i.e. using the following equation: Q Jtotal = (10) A t

(6)

Here, a1 is activity of component 1 which is related to concentration by (7a) a1 = x1 γ1

χ1p =

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Pi (p − pp ) l f

(15)

Here, l is membrane thickness, pf and pp are feed and permeate side vapor pressure for this i component. Pi is intrinsic membrane permeability and Pi /l is membrane permeance. Vapor pressure of component i on the feed side may be written as (16) pf = xi γi ps Here xi and γi are mole fraction and activity coefficient of component i on feed side and ps is saturated vapor pressure of component i.



www.soci.org On the permeate side, pressure is very low and hence it may be assumed to behave like an ideal gas. Thus, partial vapor pressure of component i on permeate side (pp ) may be written as pp = yi ppt

(17)

Here, ppt is total permeate pressure and yi is mole fraction of i component on permeate side. Now Equation (15) may be written as Pi Ji = (xi γi ps − yi ppt ) (18) l 8 Further, fugacity of component i on feed side is defined as fi = xi γi ps

HS Samanta et al.

Permeation ratio Permeation ratio gives a quantitative idea about the effect of one component on the permeation rate of the other component. Huang and Lin35 have defined this permeation ratio, θ, 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 as described by Equations (25) and (26): θi = −

Ji expt at ai . J0 i expt at ai

J0 i (at ai ) = J0 (pure i)×ai

(25) (26)

(19)

Hence, Ji l (20) (fi − yi ppt ) Saturated vapor pressure of component i and j, i.e. water and acetic acid, were calculated using the Antoine equation. Activity coefficient of water and acetic acid of different feed mixtures were obtained using a two-parameter Wilson equation, i.e. Equation (4) and Equation (5) for water and acetic acid, respectively. Pi =

Intrinsic membrane selectivity The intrinsic membrane selectivity, αmem was calculated by dividing permeability of water by permeability of acetic acid (Pj ), i.e. Pi αmem = (21) Pj Diffusion coefficient In pervaporation, the diffusion coefficient of the permeating species depends on its local concentration. Hence, the assumption of a constant diffusion coefficient would be erroneous. The diffusion coefficient of component i through the membrane is obtained using the following equation based on Fick’s law of diffusion: dCi Ji = −Di (22) dx where Di is diffusion coefficient of component i and dCi is concentration gradient across the membrane. Naidu et al.33 described a more accurate way of calculating diffusion coefficient where instead of mass%, solvent volume in feed and permeate were considered for computing diffusion coefficient. In the present work concentration average diffusion coefficients through the various membranes tested were obtained from the following equation:34 Ji diff × l Di = (22a) mi × ρp Here mi is membrane phase concentration of component i obtained from the sorption experiment Jidiff is diffusive flux of component i which is related to mass flux of component i by the following equation:34 Jidiff =

mi [1 + r−1 ] Ji ln[1 − mi (1 + r−1 )]−1

(23)

Similarly for component j Jjdiff =

mj [1 + r] ln[1 − mj (1 + r)]−1

Jj

(24)

Here r is the ratio of flux of component i to component j, i.e. r = Ji /Jj .


EXPERIMENTAL Materials High purity analytical grade acetic acid used for this study was purchased from M/s. E. Merck (India) Ltd, Mumbai. The monomers used for membrane synthesis, i.e. laboratory reagent grade acrylonitrile (AN) and butyl acrylate (BA), emulsifier sodium lauryl sulfate and initiator potassium peroxodisulfate were also obtained from the same company. The monomers were freed from their inhibitor by washing with 5% sodium hydroxide solution in water. Sodium lauryl sulfate and potassium peroxodisulfate were used as obtained without any further purification. Nanosize Na+MMT clay was procured from M/S Aldrich, Mumbai and used after drying in a hot oven at 110 ◦ C for 2 h. The specification and physical properties of the nanofiller are given in Table 1. Deionized water, having a conductivity of 20 µS cm−1 , was produced in the laboratory from a RO module using polyamide reverse osmosis (RO) membrane. This water was used for emulsion polymerization and also for making feed acid–water mixtures to be used for sorption and permeation studies. Synthesis of polymer The copolymerizations of acrylonitrile (AN) and butyl acrylate (BA) with 5.5 : 1 comonomer ratios (AN: BA = 5.5 : 1) was carried out by emulsion polymerization in a three-necked reactor at 70 ◦ C for about 4 h. The reactor was fitted with a stirrer, a thermometer pocket and a condenser. Water was used as the dispersion medium. Sodium lauryl sulfate and potassium peroxodisulfate was used as the emulsifier and initiator, respectively. The required amount of Na+MMT filler was mixed in the reactor during polymerization. After polymerization the filler-incorporated polymer was precipitated using common salt and washed repeatedly with distilled water, toluene and ethyl acetate to

Table 1.

Physical properties of Na-montmorillonite used in the study

Color

Off white

Grade Particle size Structure

Polymer (98% montmorillonite) 30–90 nm M+ y(Al2−y Mgy )(Si4 ) O10 (OH)2 nH2 O 120 300–500 2.6 1 nm 9–10

CEC: (meq/100 g) Aspect ratio Specific gravity Mineral’s thickness: pH (5% dispersion):



Separation of acid–water mixtures using nanoparticle/copolymer membranes

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H

(a) CH2

CH2

C CN X

CH y C O O (CH2)3 CH3

(b) 99

Transmittance [%]

98 97 96 95 94

3500

3000

2500

2000

1500

1000

544.75

1020.39

1169.64

1453.09

1724.11

2312.25

2927.68

3323.17

3737.52

93

500

Wavenumber cm-1 Figure 1. (a) Chemical structure of PANBA copolymer; (b) FTIR of unfilled PANBA copolymer.

remove unreacted monomer and emulsifier. The purified filled copolymer was then dried at 70 ◦ C for 4 h in a vacuum dryer. The structure of the copolymer is given in Fig. 1(a). Membrane casting The membranes were prepared by casting a dimethyl formamide (DMF) solution of the copolymers on a clean smooth glass plate with an applicator and dried at 60 ◦ C for 2 h. Subsequently, the membrane was annealed at 80 ◦ C for an additional 6 h. The membrane thickness was maintained at ∼50 micron. Membrane characterization The copolymer PANBA (unfilled) was characterized with FTIR for functional groups while distribution of fillers in the polymer for filled membranes was characterized using scanning electron microscopy (SEM). The membranes were also characterized for their mechanical properties. Sorption and permeation study Sorption of mixtures of water and acetic acid were carried out at different temperatures for the three copolymer-filled membranes.


All of these filled membranes were also used for permeation experiments by pervaporation using a batch stirred cell1 for varied concentrations of acid–water mixtures at different feed temperatures. Each experiment 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 3.0%. From these experiments total sorption, sorption selectivity, flux, separation factor and permeation ratio were determined.

RESULTS AND DISCUSSION Membrane synthesis AN and BA monomers were copolymerized by emulsion polymerization with 5.5 : 1 comonomer ratio to produce poly (AN-co-BA) copolymer, designated here as PANBA copolymer. The structure of PANBA is given in Fig. 1(a). Free radical polymerization may be carried out by bulk, solution, suspension and emulsion polymerization. Since both BA and AN monomer are water-insoluble monomer, solution polymerization in water was not possible. Solution polymerization in highly polar solvent like DMF, DMSO or



www.soci.org NMP, in which both AN and BA are soluble, often leads to chain transfer of these polar solvents to the propagating polymer radicals at an early stage of polymerization.36 Due to the high heat of polymerization of vinyl monomer, control of reaction in bulk polymerization is difficult. Further, in bulk polymerization viscosity builds up rapidly leading to low molecular weight polymer, so that membrane of required mechanical integrity cannot be prepared. In contrast, emulsion polymerization may be carried out easily in water and it leads to high molecular weight polymer with low polydispersity, and is an ideal choice for making membrane of good mechanical strength and consistent permeation behavior. Tian et al.37 reported synthesis of PANBA by emulsion polymerization to use the polymer as an electrode membrane for lithium-ion batteries. Pervaporation membranes were reported to be made from copolymer of acrylonitrile with acrylic acid, vinyl acetate, styrene and methyl methacrylate.38 All of these acrylonitrile copolymers were synthesized by emulsion polymerization. Thus, in the present work PANBA were synthesized by emulsion polymerization to obtain membrane with improved properties. The nanofiller was mixed in situ during copolymerization of AN and BA to produce a new kind of nanofiller-impregnated mixed matrix polymeric membrane. Nanofiller is difficult to mix with any polymer because of its tendency to agglomerate even at very low concentration. Thus, instead of physical mixing of the filler with polymer (which is usually done when making filled membranes) nanofiller was mixed in situ by adding it to the monomer mixtures during polymerization to ensure better mixing. Copolymer composition The approximate copolymer composition of the resulting copolymer i.e. PANBA may be obtained from molar composition of the comonomer and their mutual reactivity ratios using the following equation: d[MAN ] rAN m2 AN + mAN mBA (27) = d[MBA ] rBA m2 BA + mAN mBA where rAN , the reactivity ratio of AN is 0.403, rBA , the reactivity ratio of BA is 0.825. Hence, for 5.5 : 1 comonomer ratio, i.e. for mAN : mBA of 5.5 : 1 as used for the present study, approximate copolymer composition, i.e. d[MAN ]/d[MBA ] will be 2.79 : 1. In fact, PANBA copolymer is a combination of glassy (polyacrylonitrile, the homopolymer of AN is a hard glassy polymer) and rubbery (polybutylacrylate, the homopolymer of BA is a soft rubbery polymer) polymer. Several copolymers with varied comonomer ratios were synthesized. However, the copolymer with 5.5 : 1 comonomer ratio and calculated copolymer ratio of ∼2.79 : 1 (AN : BA moiety in the copolymer) gave membrane suitable for (a)

CU-2551 10.0kV x1.50k SE

HS Samanta et al.

pervaporation application. Thus, in the present study copolymers with 5.5 : 1 comonomer ratio were used for making filled membranes. Membrane characterization Membrane characterization by FTIR FTIR detects the functional groups present in a polymer. The FTIR of unfilled PANBA membrane is shown in Fig. 1(b). The nitrile (CN-) functional group of AN is observed at around 2312 cm−1 while carbonyl of ester of BA moiety is shifted to 1724 cm−1 . The stretching bands appearing at 1453 and 1020 cm−1 correspond to methylene and butyl groups of the copolymer.37 The stretching band at around 1635 cm−1 due to nonsaturation of monomer is absent in the FTIR of the copolymer, signifying conversion of the double bond of AN or BA monomer to C–C single bond in the resulting PANBA copolymer.37 Membrane characterization by SEM SEM studies of the three filled PANBA copolymer membranes i.e. PANBA0.5, PANBA1.5 and PANBA3 are shown in Fig. 2(a), (b) and (c), respectively. The distribution of nanosize filler in the matrix of the copolymer is clearly observed in the filled membranes. From Fig. 2(a), (b) and (c) it is clear that with increasing % of filler the distribution becomes poor and non uniform with coarser morphology. Membrane characterization by mechanical properties A pervaporation membrane should have a good balance of tensile strength (TS) and elongation at break (EAB). In the present copolymer PANBA, the AN moiety provides strength since polyacrylonitrile, the homopolymer of AN monomer, is a glassy polymer with high TS because of its high glass transition temperature (Tg ∼ 88 ◦ C). On the other hand poly butyl acrylate, the homopolymer of the other comonomer BA is soft and elastomeric in nature with very low Tg (−49 ◦ C) and high EAB. Thus, copolymers of AN and BA were synthesized with varied comonomer ratios. However, copolymer with 5.5 : 1 comonomer ratio of AN: BA was found to give polymer with appropriate strength (TS) and flexibility (EAB). The mechanical strength of this copolymer was further improved by incorporation of nanosize Na-MMT filler, as is evident from the values of TS and EAB of the filled polymer given in Table 2. From this table it is observed that with increase in filler loading TS of the filled membrane increases while EAB decreases. Incorporation of nanofiller in the matrix of the PANBA copolymer membrane

(b)

30.0um

CU-2547 10.0kV x1.50k SE

(c)

30.0um

CU-2549 10.0kV x1.50k SE

30.0um

Figure 2. SEM of the membranes: (a) PANBA 0.5; (b) PANBA1.5; (c) PANBA 3.0.




Separation of acid–water mixtures using nanoparticle/copolymer membranes increases its stiffness but reduces chain mobility. To study the solvent resistance of the membranes, TS and EAB of the membranes were also recorded after immersion in pure acetic acid and water for 1 week. From Table 2 it is observed that the solvent-swollen membranes show somewhat lower TS but higher EAB in comparison with dry (not immersed in water or acid) membranes. Further, the change in TS and EAB of waterswollen membranes are observed to be much higher than those of acid-swollen membranes, which signifies water selectivity of the hydrophilic filled copolymer membranes. However, the TS and EAB of the solvent-swollen membranes are still high enough for pervaporative applications. The mechanical properties of membranes made from PANBA copolymer to be used for different feed mixtures of acetic acid–water is expected to be intermediate between the mechanical properties of the dry (and unused) membranes and wet membranes swollen in pure water and pure acetic acid. Swelling studies Effect of feed concentration on sorption and sorption selectivity Figure 3(a) shows the variation of total sorption of acetic acid and water mixtures by the three filled membranes. A similar kind of relationship was observed at the other three temperatures of the sorption experiments, i.e. at 40, 50 and 60 ◦ C. From this figure it is found that initially with increase in feed concentration of water, total sorption increases steadily for all the membranes and above around 10 wt% water in feed it reaches a plateau. Thereafter sorption increases further at a much lower rate. This

Table 2.

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sorption behavior is close to the type-IV isotherm of Rogers.39 In this case initially the solutes are absorbed at specific sites of the membranes due to chemical interaction (hydrogen bonding, etc.). Since the membranes are hydrophilic, initially water is sorped more than acid because of its increased interaction with membranes. Thus, initially sorption selectivity of water for all the membranes is high as seen in the same figure. With increase in sorption these specific sites of the membranes are exhausted and preferential water sorption by these sites decreases. Thus, sorption selectivity for water decreases and total sorption reaches plateau values. Beyond this plateau value the increase in sorption is due to solute–solute bonding of type-IV sorption. From this figure it is also observed that for the same feed concentration total sorption increases with increasing filler loading from PANBA0.5 to PANBA3 while sorption selectivity for water decreases in the same order. However, over the entire feed concentration all the membranes show very high total sorption and sorption selectivity for water. From Fig. 3(b) it is clearly observed that for the same feed concentration partial water sorption is much higher than partial acid sorption signifying water selectivity of the membranes. Above 1.5 wt% filler loading agglomeration takes place as also observed in SEM of PANBA3 filled membrane containing 3 wt% filler. Thus, free volume increases in PANBA3 membrane due to increase in defects in the interphase of polymer and filler in the membrane resulting in increased sorption and lower selectivity in comparison with PANBA0.5 and PANBA1.5 membranes, which show comparable sorption and sorption selectivity.

Mechanical properties of the membranes

Tensile Tensile strength Tensile strength (MPa) Elongation Elongation at break Elongation at break (%) strength (MPa) (MPa) of membrane of membrane after at break (%) of (%) of membrane after of membrane after one Polymer of unused after one week one week immersion unused one week immersion in week immersion in pure membrane membrane immersion in pure water in acetic acid membrane (EAB) pure water(EAB) ethylene glycol (EAB) 5.25

4.73

4.81

8.86 16.54 41.53

7.55 12.35 37.56

8.24 15.56 39.17

0.8 0.7

400

0.6 0.5

PANBA 0.5 SORP PANBA 1.5 SORP PANBA 3 SORP PANBA 0.5 SEL PANBA 1.5 SEL PANBA 3 SEL

0.4 0.3 0.2

300

200

100

0.1 0 0

5

10

15

20

25

Feed concentration of water (wt%)

0 30

17.51

14.23

12.34 9.25 8.67

9.21 6.82 5.13

(c) 2.5 0.8

0.8

0.7

0.7

0.6

0.6

0.5

0.5

0.4

0.4 PANBA 0.5 water PANBA 1.5 water PANBA 3 water PANBA 0.5 acid PANBA 1.5 acid PANBA 3 acid

0.3 0.2

0.3 0.2

0.1 0

0.1

0

5

10

15

20

25

0 30

Feed conc. of water (wt%)

Membrane phase conc. of acid (g/g of membrane)

500

Sorption selectivity for water (-)

0.9

Total sorption (%)

8.53 4.582 2.697

(b) 600

1

Membrane phase conc. of water (g/g of membrane)

(a)

12.25

ln(Sorption selectivity), Calculated

Unfilled PANBA PANBA0.5 PANBA1.5 PANBA3

2

1.5

1

PANBA 0.5 PANBA 1.5 PANBA 3

0.5

0 2

2.5

3

3.5

4

4.5

5

5.5

6

ln(Sorption selectivity), Experimental

Figure 3. (a) Variation of total sorption and sorption selectivity with feed water concentration: PANBA 0.5SORP; PANBA 1.5SORP; PANBA 3.0SORP; PANBA 0.5SEL; ♦ PANBA 1.5SEL; PANBA 3.0SEL. (b) Variation of membrane phase concentration of water and acid with feed conc. of water: PANBA 0.5water; PANBA 1.5water; PANBA 3.0water; PANBA 0.5acid; ♦ PANBA 1.5acid; PANBA 3.0acid (c) Parity plot for sorption selectivity: PANBA 0.5; PANBA 1.5; PANBA 3.0.




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100 90 80 Permeate conc. of water (wt%)

Interaction parameter The interaction parameter of (1) membrane–acetic acid and (2) membrane–water is given in Table 3 while the interaction parameter between water and acetic acid is given in Table 4. The higher the values of interaction parameter, the less the interaction. From the values given in Table 3 it is observed that the interaction parameter of water–membrane is less than that of the acid–membrane. Accordingly, water–membrane interaction will be much more than acid–membrane interaction, which is also reflected in total sorption and sorption selectivity, i.e. total sorption increases with increase in feed concentration of water, and over the entire concentration range used for sorption experiments the filled membranes show more water sorption and sorption selectivity for water. Thus, the calculated interaction parameter values explain well the experimental sorption and sorption selectivity values. Similar values of interaction parameters were also reported elsewhere.8 The calculated sorption selectivity based on Equation 9 is shown in Fig. 3(c) in terms of parity plot of calculated sorption selectivity versus experimental sorption selectivity. It is clear from this figure that experimental sorption selectivity matches well with experimental values.

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Table 3. Density of the polymer and interaction parameter between membrane polymer and pure solvents Name of the polymer membrane PANBA0.5 PANBA1.5 PANBA3

Density of the polymer (g cm−3 )

Interaction parameter between water and membrane (χip )

Interaction parameter between acid and membrane (χjp )

1.321 1.375 1.410

0.692 0.673 0.659

1.037 1.013 0.997

Figure 4. Variation of permeate concentration of water with feed concentration of water at 30 ◦ C: ANBA 0.5; ANBA 1.5; ANBA 3.0.

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1400 1200 1000

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Effect of feed concentration on separation Figure 4 shows the variation of wt% of water in the permeate against wt% of water in the feed for acetic acid–water mixtures with the three filled membranes, i.e. PANBA0.5, PANBA1.5 and PANBA3 at 30 ◦ C. Similar trends were also obtained at the three other PV temperatures. It appears from these McCabe–Thiele type xy diagrams that the filled membranes show measurable separation of water–acetic acid mixtures over the concentration range used without any pervaporative azeotrope. The vapor–liquid equilibrium (VLE) data30 are shown on the same figure. It is evident from the figure that over the concentration range used, all the mem-

Total flux (kg/m2 hr µm)

PERVAPORATION(PV) STUDIES

400 2 200 0 0

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Figure 5. Variation of total flux and selectivity with feed concentration of water at 30 ◦ C: PANBA 0.5 FLUX; PANBA 1.5 FLUX; PANBA 3.0 FLUX; PANBA 0.5 SEL; ♦ PANBA 1.5 SEL; PANBA 3.0 SEL. Table 4. Interaction parameter between toluene and methanol at feed concentration used Feed concentration of acid in water (wt%) 0.462 2.31 4.638 6.971 9.31 18.76 28.37


Interaction between toluene and methanol (χij ) 0.614 0.701 0.487 0.909 0.983 1.080 1.184

branes show much higher concentration of water in permeate than its VLE value. 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. 5 for all the membranes. In Fig. 6 thickness normalized partial flux of water and acetic acid is plotted against feed concentration of water. From Fig. 5 it is observed that total flux increases while selectivity decreases with feed water concentration for all the membranes. From Fig. 6 it is observed that over the entire concentration range




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Figure 6. Variation of partial flux with feed concentration of WATER at 30 ◦ C: PANBA 0.5 WATER; PANBA 1.5 WATER; PANBA 3.0 WATER; PANBA 0.5 ACID; PANBA 1.5 ACID; PANBA 3.0 ACID.

all the membranes show much higher water flux than acid flux, indicating water selectivity of the filled membranes. It is also observed from Fig. 6 that acid flux remains marginally constant over the entire concentration range. From both Fig. 5 and Fig. 6 it is observed that total flux or water flux increases linearly at the lower feed concentration range, i.e. up to around 7.5 wt% water in feed and thereafter the rate of increase of total or partial water flux is very high indicating plasticization of the membranes above this feed concentration. With increase in feed concentration the water selective membranes becomes plasticized with increased flux but reduced selectivity, as observed in Fig. 5. Effect of filler loading on flux and selectivity For the same feed concentration, total flux (Fig. 5) or partial flux (Fig. 6) of water is observed to increase with increase in filler concentration from PANBA0.5 to PANBA3, while water selectivity changes in the reverse order, i.e. for any feed concentration it decreases from PANBA0.5 to PANBA3. The hydrophilic nanofiller increases preferential water–membrane interaction. Further, similar to total sorption or sorption selectivity of water, at higher filler loading the free volume of the membrane increases due to increased defects in the filler–polymer interphase with increased flux and reduced selectivity. However, over the concentration range used the filled membrane showed high flux and selectivity while acid flux remains constant. From Fig. 5 it is also observed that for the separation of around 30 wt% acetic acids from its mixture with water PANBA1.5 membrane will yield a high flux of around 10 kg m−2 h−1 µm−1 and a very good selectivity of around 100. In a similar way for dehydration of 99.5 wt% acetic acid, PANBA3 membrane will yield a good flux of around 4 kg m−2 h−1 µm−1 and a very high water selectivity of 1356. Thus, the filled membranes used in this study will be suitable for separation of both low and high concentrations of acetic acid from water. Effect of feed concentration on PSI and enrichment factor (EF) Figure 7 shows the variation of PSI and EF for water with feed concentration of water. In general, flux and selectivity have an opposite relationship with respect to feed concentration.


0

5

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15

20

25

0 30


Figure 7. Variation of PSI and EF with feed concentration of water: PANBA 0.5 PSI; PANBA 1.5 PSI; PANBA 3.0 PSI; PANBA 0.5 EF; ♦ PANBA 1.5 EF; PANBA 3.0 EF.

Pervaporation separation index or PSI relates both permeation flux and selectivity of the desired component in one equation (Equation (13)) and hence, the optimum performance of a membrane can be evaluated in terms of its PSI. For all the filled 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. 5). Thus, PSI, a product of flux and selectivity decreases with feed concentration. EF (Equation (14)) is also shown for all the membranes in the same figure. From the figure it is observed that all the membranes show high values of EF (EF1) signifying water selectivity of the membranes. It is also observed that EF increases with feed concentration, i.e. in spite of plasticization of the membranes permeate water concentration increases with increase in feed concentration. Effect of feed concentration on intrinsic membrane properties Intrinsic membrane properties, i.e. partial permeability of water and acetic acid and the ratio of these two permeabilities (intrinsic membrane selectivity), were obtained from partial flux, thickness of membrane and vapor pressure differential on feed and permeate side using Equation (20) and (21), respectively. The variation of partial permeability of water and acetic acid with feed concentration is shown in Fig. 8(a) while Fig. 8(b) describes the variation of membrane selectivity with feed concentration. From Fig. 8(a) it is observed that all the membranes show much higher water permeability than acid permeability, signifying water selectivity of the filled membranes. However, in contrast to flux, partial permeability is found to decrease almost exponentially up to around 4 wt% feed water concentration and thereafter the change in partial permeability with feed concentration is marginal. In this case decrease of permeability with feed concentration may be due to increase of fugacity (f) of water with feed concentration as shown in Fig. 9. With increase in feed concentration though partial flux (numerator of Equation (20)) of both water and acid increases (Fig. 6), fugacity (fugacity, f given in denominator of Equation (20)) increases at a much higher rate for water and overall partial permeability for water decreases drastically. It is also observed from Fig. 8(a) that change of acid permeability with feed concentration is small. This is because of marginal change of



www.soci.org 0.4

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Partial permeability of water (Barrer) x 10-10

(a) 180

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Figure 9. Variation of fugacity with feed concentration of water: water fugacity; acid fugacity.

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PANBA 3

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PANBA 0.5 PRWATER

200

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0 0

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Figure 8. (a) Variation of partial permeability with feed concentration of water: PANBA 0.5 water; PANBA 1.5 water; PANBA 3.0 water; PANBA 0.5 acid; ♦ PANBA 1.5 acid; PANBA 3.0 acid. (b) Variation of membrane selectivity with feed concentration of water: PANBA 0.5; PANBA 1.5; PANBA 3.0.

Permeation ratio (-)

Membrane selectivity (-)

500

PANBA 1.5 PRWATER

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PANBA 3 PRWATER PANBA 0.5 PRACID PANBA 1.5 PRACID

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PANBA 3 PRACID

20

10

0

acid flux (Fig. 6) or acid fugacity (Fig. 9) with feed concentration of water. Similar results were reported8,20 elsewhere for acetic acid–water mixtures and also other organic–water mixtures. From Fig. 8(b) it is observed that membrane selectivity also decreases with feed concentration. Effect of feed concentration on permeation ratio From Fig. 10 it is observed that at very low feed concentration of water, permeation ratio (PR) of both water and acid is very high (PR1) for all the membranes, which may be due to the positive coupling effect of acid/water on partial flux of water/acid. Thus, at around 0.5 wt% water in feed PR of water for PANBA0.5 membrane is observed to be as high as 25, which further increases to a value of 31 for PANBA1.5 and 39 for PANBA3 membrane. However, for the same feed concentration PR of acid for PANBA0.5 is around 50 which reduce to a value of 41 for PANBA1.5 and 34 for PANBA3 membrane. With increase in feed concentration of water, PR of both water and acid decreases and at around 30 wt% water in


0

5


25

30

Figure 10. Variation of permeation ratio of water and acid with feed concentration of water: PANBA 0.5 PRWATER; PANBA 1.5 PRWATER; PANBA 3.0 PRWATER; PANBA 0.5 PRACID; ♦ PANBA 1.5 PRACID; PANBA 3.0 PRACID.

feed, PR of water is found to be close to 1 while PR of acid becomes less than 1 for all three membranes. At very low concentrations of water, acid–water interaction is more than water–membrane interaction because of very high acid concentration in the feed. Thus, PR of water is very high for all the membranes due to the strong coupling effect between acid and water flux. At this low feed water concentration due to increased acid–water interaction, the PR of acid is also very high. As the water concentration in feed is increased, PR of water decreases drastically for all water selective membranes and become close to unity, i.e. coupling effect of acid on water becomes negligible at high feed water concentration




1

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4 PANBA 0.5 water

3.5

PANBA 3 water

0.8

PANBA 0.5 acid

Diffusion coefficient (m2/s) x 109

Ratio of diffusive to permeation flux (-)

PANBA 1.5 water

PANBA 1.5 acid PANBA 3 acid

0.6

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PANBA 1.5 water PANBA 3 water PANBA 0.5 acid

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PANBA 1.5 acid PANBA 3 acid

0 0

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Figure 11. Variation of ratio of diffusive to permeation flux with feed concentration of water: PANBA 0.5 water; PANBA 1.5 water; PANBA 3.0 water; PANBA 0.5 acid; ♦ PANBA 1.5 acid; PANBA 3.0 acid.

Figure 12. Variation of diffusion coefficient with feed concentration: PANBA 0.5 water; PANBA 1.5 water; PANBA 3.0 water; PANBA 0.5 acid; ♦ PANBA 1.5 acid; PANBA 3.0 acid;.

because of much higher water–membrane interaction (through hydrogen bonding) than acid–water interaction. This increased water–membrane interaction also reduces the PR of acid. From Fig. 10 it is observed that for the same feed water concentration, PR for water increases from PANBA0.5 to PANBA3 while the PR of acid shows the reverse order, i.e. it decreases from PANBA0.5 to PANBA3. With increase in concentration of hydrophilic nanofiller the filled membranes becomes increasingly hydrophilic from PANBA0.5 to PANBA3. Thus, the PR for water increases in the same order while the PR of acid shows the opposite trend. This result is in good agreement with the variation of partial water and acid flux with filler loading (Fig. 6).

overall permeation flux. Thus, the ratio of diffusive to permeation flux decreases from PANBA0.5 to PANBA3. The concentration average diffusion coefficients (DC) of water and acetic acid were calculated using Equation (22). The variation of DC of water and acid with feed concentration of water is shown in Fig. 12. A similar order of DC for benzene and cyclohexane was reported by Peng et al.40 through substituted silane-filled PVOH membrane. Schaetzel et al.41 also reported a similar order of DC for ethanol and water through PVOH–polyacrylic acid blend membranes. From this figure it is observed that over the entire feed concentration range, the DC of water is much higher than that of acid. This may be ascribed to lower kinetic diameter of water (0.296 nm) than acetic acid (0.436 nm).32 Thus, for the same concentration gradient water moves faster than acid through the membranes. It is also observed from the figure that at very low water concentration, the DC of water through the membranes is of the order of 10−9 m2 s−1 while the DC of acid is of the order of 10−10 m2 s−1 . However, as water concentration increases in the feed, the hydrophilic membrane becomes plasticized with increase in DC of acid. Thus, at higher water feed concentration, the DC of acid increases and the difference in DC between water and acid also decreases.

Determination of concentration average diffusion coefficient of water and acetic acid Diffusive flux of water and acetic acid was calculated using Equation (23) and (24), respectively. The variation of the ratio of diffusive flux to permeation flux with feed concentration at 30 ◦ C is shown in Fig. 11 for all the filled membranes. From this figure it is observed that at very low feed water concentration all the membranes show very high ratio of diffusive to permeation flux (0.6–0.9), which decreases almost exponentially with increase in feed concentration of water, signifying plasticization of the membranes. It is observed from Fig. 11 that over the entire concentration range used in this study, around 60–90% of the permeation flux of water is due to its diffusive flux. For acid, only 15–60% of its permeation flux is due to diffusion. This indicates that the overall permeation of water is dominated by diffusion while for acid, diffusive flux becomes significant at very low feed concentration of water. From this figure it is also observed that for the same feed concentration of water, the ratio of diffusive to permeation flux decreases from PANBA0.5 to PANBA3. As the membrane becomes more hydrophilic by incorporation of hydrophilic nanofiller from PAN0.5 to PANBA3, sorption becomes more significant with decreasing contribution of diffusive flux to


Effect of temperature on flux, selectivity and permeability With increasing temperature, flux increases almost linearly for all the membranes while selectivity decreases at higher temperature in the same order, as shown for all the membranes in Fig. 13 with 28.37 wt% feed water concentration. Frequency and amplitude of segmental motion of the polymeric chains increases at higher temperature with increased free volume. This increased free 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) cause increased flux at higher temperature for all the membranes. From Fig. 14 it is observed



6

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PANBA 3 water sep PANBA 0.5 water sep

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Figure 13. Variation of total flux and selectivity with feed temperature. Feed concentration 28.37 wt% water in acid: PANBA 0.5 FLUX; PANBA 1.5 FLUX; PANBA 3.0 FLUX; PANBA 0.5 SEL; ♦ PANBA 1.5 SEL; PANBA 3.0 SEL.

Figure 15. Variation of partial permeability and separation selectivity of membranes with feed temperature: PANBA 0.5 water per; PANBA 1.5 water per; PANBA 3.0 water per; PANBA 0.5 acid per; PANBA 1.5 acid per; PANBA 3.0 acid per; PANBA 0.5 water sep; ♦ PANBA 1.5 water sep; PANBA 3.0 water sep.

1000

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Feed temperature (°C)

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Figure 14. Variation of partial flux with feed temperature. Feed concentration 28.37 wt% water in acid: PANBA 0.5 WATER; PANBA 1.5 WATER; PANBA 3.0 WATER; PANBA 0.5 ACID; ♦ PANBA 1.5 ACID; PANBA 3.0 ACID.

that both partial flux of water and acid increases with temperature. However, the rate of increase of water flux with temperature is higher than the rate of increase of acid flux with temperature. Thus, at higher temperature, although acid flux increases, overall the decrease of water selectivity with temperature is not as high as the decrease of selectivity with feed concentration (Fig. 5). Partial permeability of water and acid are also observed to decrease with temperature, as shown in Fig. 15. However, there is little change of permeability above 45 ◦ C. This trend is the combined effect of change of fugacity and partial flux with temperature. From the same figure it is also observed that for the filled copolymer membranes the change of intrinsic membrane selectivity with feed temperature is not very significant. Thus, these filled membranes


Figure 16. Variation of PSI and enrichment factor with feed temperature: PANBA 0.5 PSI; PANBA 1.5 PSI; PANBA 3.0 PSI; PANBA 0.5 EF; ♦ PANBA 1.5 EF; PANBA 3.0 EF.

may also be used at higher feed temperature with increased flux at the cost of a slight decrease in permeation or intrinsic membrane selectivity. This is also evident from Fig. 16 where PSI, the product of flux and selectivity and enrichment factor, is observed to change little above 45 ◦ C. Apparent activation energy for permeation Apparent activation energy for permeation (EP ) at a specific feed concentration may be obtained from the slope of the Arrhenius type linear plot of logarithmic partial molar flux (Q) against inverse of absolute temperature (1/T). The variation of apparent activation energy for permeation of water and acetic acid with feed concentration of water is shown in Fig. 17. From this figure it is observed that for any feed concentration, EP of water is much lower than EP of acid. It is also observed that




Activation energy for permeation, Ep, kj/degmolF

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membranes, the activation energy needed for permeation further decreases.

PANBA 0.5 water PANBA 1.5 water PANBA 3 water

50

PANBA 0.5 acid PANBA 1.5 acid

COMPARISON WITH REPORTED DATA

PANBA 3 acid

The pervaporative separation of water–acetic acid mixtures by various reported membranes along with the present membranes are shown in Table 5. It is observed that most of the membranes report dehydration of acetic acid, i.e. flux and separation factor for acetic acid–water mixtures at acid concentration in the range of 80–90 mass% acid in water. In contrast, few membranes are reported which show high flux and selectivity at both high and low concentration of acid in water (less than 75% or above 99% acid). This may be due to the very corrosive as well as polar nature of acetic acid. Above 90 wt% acid concentration most of the membranes collapse and at high feed water concentration (above 20% water in feed) water selectivity becomes very low with most of the hydrophilic membranes. In the present work the nanoparticlefilled mixed matrix copolymer membranes maintained high flux and selectivity for both high and low feed water concentration as observed in Table 5. Thus the present membranes would be very suitable for pervaporative separation of acetic acid–water or any similar organic–water mixtures over a wide range of feed concentration.

40

30

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0 5

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Figure 17. Variation of activation energy for permeation with feed concentration of water: PANBA 0.5 water; PANBA 1.5 water; PANBA 3.0 water; PANBA 0.5 acid; ♦ PANBA 1.5 acid; PANBA 3.0 acid.

at very low feed water concentration EP of both water and acid is high, which decreases with increase in feed water concentration and above around 5 wt% water in feed the change of EP with feed concentration is not appreciable. Because of easier permeation of water molecules through the hydrophilic filled membrane, EP of water is much lower than EP of acid at any concentration. With increase in feed concentration of water due to plasticization of the

Table 5.

CONCLUSIONS Hydrophilic membranes were synthesized by free radical emulsion copolymerization of AN and BA monomer with 5.5 : 1 comonomer ratio. This hydrophilic copolymer was further filled with 0.5, 1.5 and 3 wt% nanosize bentonite filler to produce three

Comparison of performances of membranes reported for pervaporative separation of acetic acid-water mixtures

Membrane PVOH-PAM graft copolymer Na-Alg silicotungstic acid hybrid membrane FIPN500 FIPN502 FIPN505 FIPN510 ZSM-5 zeolite PVOH-GLU (Pervap 2201,Sulzer) PVOH-g-AN membrane PVOH TEOSPVOH-Na-Alg blend Na-Alg-HAD Charged Nafion Mordenite PANBA0.5 PANBA1.5 PANBA3 PANBA0.5 PANBA1.5 PANBA3

Feed water concentration (wt%) 70 10 0.953

Temperature (◦ C)

Normalized Flux, (kg m−2 h−1 µm−1 )

Water selectivity (−)

25 30

11.3 9.7 2.924 4.242 5.188 6.612 0.62 12 1 kg/ m2 h 1.75 3.32 0.717 0.262 32.2 0.614 2.6065 3.1945 3.978 8.6725 9.4385 11.556

1.7 22,491 110.49 168.72 204.18 325.533 8 20 450 14 2423 40.3 161 243 299 1473.23 1370.65 1355.43 101.67 95.32 79.02

30

50 10 50 10 10 10 15 10 50 0.46

70 25 60 30 30 30 70 25 80 30

28.37

30

Reference [9] [19] [8]

[10] [11] [12] [27,13] [28,14] [29,15] [30,16] [31,17] [32,18] Present work

Present work

PVOH-polyvinyl alcohol, PAM-polyacryamide, FIPN500-full IPN of 5 : 1 copolymer in PVOH, FIPN505-same IPN but with 5% aluminosilicate filler, etc., 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




www.soci.org mixed matrix membranes designated PANBA0.5, PANBA1.5 and PANBA3. These membranes were characterized and used for pervaporative separation of acetic acid–water mixtures over the range 28.3 to 0.5 wt% water in feed. The filled membranes showed high flux and very high water selectivity over the entire feed concentration range. PANBA3 membrane was observed to show the highest flux while water selectivity of PANBA1.5 or PANBA3 was comparable and slightly lower than the selectivity of PANBA0.5 membranes. The filled membranes were also found to show higher flux at higher feed temperature with little change of selectivity. The effect of operating parameters such as feed concentration and feed temperature on sorption and permeation were studied. Intrinsic membrane properties such as permeability and membrane selectivity were compared with flux and selectivity under different operating conditions for all the membranes. The diffusion coefficients for water and acid through the membranes were also evaluated. The filled membranes used in the present work were found to give very high flux and water selectivity at both low and high acid concentrations in feed. Thus, these mixed matrix membranes would be very effective for pervaporative separation of acid–water or any similar organic–water mixtures over a wide range of feed concentration and temperatures.

ACKNOWLEDGEMENTS The authors are grateful to Center for Research in Nano Science and Nano Technology, University of Calcutta for sponsoring the work.

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