Synthesis of full interpenetrating network membranes

Chemical Engineering and Processing 50 (2011) 391–403

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Synthesis of full interpenetrating network membranes of poly(acrylic acid-co-acrylamide) in the matrix of polyvinyl alcohol for dehydration of ethylene glycol by pervaporation S.B. Kuila a , S.K. Ray b,∗ , Paramita Das b , N.R. Singha b a b

Department of Chemical Engineering, Haldia Institute of Technology, India Department of Polymer Science and Technology, University of Calcutta, India

a r t i c l e

i n f o

Article history: Received 8 July 2010 Received in revised form 12 February 2011 Accepted 15 February 2011 Available online 4 March 2011 Keywords: Pervaporation Ethylene glycol Crosslink copolymer IPN Permeability Diffusion coefficient

a b s t r a c t Polyvinyl alcohol (PVOH) has been chemically modified by crosslink copolymerization of acrylic acid (AA) and acrylamide (AM) in aqueous solution of PVOH and finally crosslinking the copolymer with methylene bis acrylamide (MBA) and PVOH with glutaraldehyde to produce a full interpenetrating network (FIPN) membrane. Accordingly, three such fully crosslinked IPNs i.e. FIPN25, FIPN50 and FIPN75 have been synthesized with different mass ratio of PVOH:copolymer i.e. 1:0.25 (FIPN25), 1:0.50 (FIPN50) and 1:0.75 (FIPN75). These full IPN membranes were used for pervaporative dehydration of ethylene glycol (EG). All of these IPN membranes were characterized with various conventional methods like FTIR, mechanical properties, DTA and SEM. The performances of the membranes were evaluated in terms of sorption and pervaporative dehydration of EG. The IPN membranes were found to show preferential sorption and diffusion for water. Flux and water selectivity of these membranes were found to increase with increasing amount of copolymer in PVOH matrix. However, among the three membranes, FIPN75 were found to show the highest flux but lower selectivity for water while FIPN50 membrane showed optimum performance in terms of both flux and selectivity. Diffusion coefficient and plasticization interaction of water and EG through all the IPN membranes were determined using modified solution-diffusion model. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Ethylene glycol (EG) is an important chemical intermediate widely used as a nonvolatile antifreeze and coolant agent and also as an intermediate for producing different polymers like polyethylene glycol and polyester. EG is industrially produced by hydrolysis of ethylene oxide [1]. The equilibrium composition of EG in the product stream is around 15 mass% which is concentrated by distillation. However, this distillation is very expensive as high-pressure steam is required for the reboiler [1] due to the high boiling point of EG (198 ◦ C). In fact, EG–water separation by distillation is ranked as the eighth most energy intensive distillation operation in the chemical process industry [2]. As an alternative and more economic route, EG could be concentrated to 70–80 mass% by distillation followed by further concentration by pervaporation using a hydrophilic membrane [3]. Thus, in recent times various hydrophilic membranes are being tried for pervaporative dehydration of EG though it does not form any azeotrope with water over

∗ Corresponding author. Tel.: +91 033 23508386; fax: +91 033 351 9755. E-mail address: [email protected] (S.K. Ray). 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.02.011

the entire concentration range. Hydrophilic membranes based on chitosan [3–6], polyvinyl alcohol [7–10], sulfonated polyetherketone (SPEK) [11], polysulfone [12], thin film composite membrane [13], inorganic membranes [14,15] and inorganic–organic hybrid membrane [16] have been tried for dehydration of EG. Dehydration of EG is different from dehydration of other alcohols or acids because of distinct physicochemical properties of EG such as strong polarity, high viscosity, high boiling point and low relative volatility. The interaction between water and EG being strong, coupling effect will be very significant for this kind of separation system. This apart, water and EG will strongly swell the membranes. In our previous works PVOH was chemically modified by copolymerizing acrylic acid with HEMA in the matrix of PVOH with different mass ratio of PVOH:copolymer. These membranes were found to give encouraging results when used for dehydration of IPA [17] and removal of methanol from its mixtures with MTBE [18]. Extensive swelling of PVOH membranes and thus loss in water selectivity may be reduced by copolymerizing acrylic acid and acrylamide in the matrix of PVOH followed by crosslinking with MBA (for the copolymer) and glutaraldehyde (for PVOH matrix) to form full interpenetrating network (IPN) membranes. Thus, in the present work three full IPN, i.e. FIPN25, FIPN50 and FIPN75 have

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S.B. Kuila et al. / Chemical Engineering and Processing 50 (2011) 391–403

been synthesized with mass ratio of PVOH:poly(acrylic acid-coacrylamide) [polyAAAM] of 1:0.25, 1:0.50 and 1:0.75, respectively. In each of these polymers, the molar ratio of acrylic acid (AA) to acrylamide (AM) was kept constant at 10:1. Polyacrylic acid is hydrophilic and soluble in water but cannot form a suitable membrane. Homopolymer of acrylamide i.e. polyAM with high glass transition temperature is a brittle polymer which cannot produce a suitable pervaporation membrane. The copolymer of AA and AM (polyAAAM)in the matrix of PVOH would result in a highly water selective membrane. The membranes cast from these polymers i.e. FIPN25, FIPN50 and FIPN75 have been used for pervaporative dehydration of EG–water mixture over the concentration range of 0–20 mass% EG in water. 2. Theory

Taking logarithm on both side

ln JW +

D0w m L

= ln

D 0w m L

+ (Cmw )

(9)

Diffusion coefficient of water at infinite dilution (D0w ) and plasticization coefficient of water () may be obtained using the above equation. Further, according to solution-diffusion theory pervaporation selectivity (˛PV ) is a product of diffusion selectivity (˛D ) and sorption selectivity (˛S ) i.e. ˛PV = ˛D · ˛S

(10)

To obtain diffusion selectivity of EG at infinite dilution (D0eg ) and also plasticization coefficient of water on EG i.e. ϕ, a brief mathematical derivation is required. Ratio of permeation rate of water to EG may be written as

According to modified solution diffusion model [11] diffusion coefficient of a binary ethylene glycol (EG)–water mixtures through a dense pervaporation membrane is

JW =− JEG

D = D0 exp(Cmw + ϕCme )

Integrating over membrane thickness and ignoring concentration of water and EG on downstream side because of its low values

(1)

Here Cmw and Cme are local concentration of water and EG in the membrane and and ϕ are the plasticization coefficients of water and EG, respectively. D0 is diffusion coefficient at infinite dilution. Because of high water selectivity of the membranes the concentration of EG in the membrane is very low and may be ignored. Hence, diffusion coefficient of water may be written as Dw = D0w exp(Cmw )

D dC m w mw 1 − Cmw

(3)

dl

For low Cmw , the above equation reduces to, JW = −m Dw

dCmw dl

(4)

Substituting Eq. (2) in Eq. (4), permeation rate of water is JW = −m D0w exp(Cmw )

dCmw dl

(5)

JEG = −m D0e exp(ϕCmw )

dCme dl

(6)

In this case membrane phase concentration of EG being very small, its plasticization effect on its own diffusion coefficient is ignored. Integrating Eq. (5) over the membrane thickness (L) results in total permeation rate of water through the membrane

Cwf

JW dl = −m D0w 0

exp(Cmw )dCmw

(7)

Cwi

Ignoring the very low concentration of the permeating component on the downstream side (because of very low pressure on this side), Eq. (7) reduces to JW =

D0w Cmw exp{( − ϕ)Cmw } − 1 D0eg ( − ϕ)Cme

(12)

Or, expanding exponential of ( − ϕ)Cmw , we have JW D0w Cmw =− JEG D0eg Cme

1+

( − ϕ)C

mw

2!

( − ϕ)C 2 mw +

3!

+ ···

D0w m [exp(Cmw ) − 1] L

(13) Considering definition of permeation selectivity and sorption selectivity, Eq. (13) is equivalent to ˛PV

D0w = ˛S D0eg

1+

(−ϕ)C

mw

( − ϕ)C 2 mw +

2!

3!

+ ···

(14)

Comparing Eqs. (10) and (14) diffusion selectivity, ˛D may be expressed as D0w ˛D = D0eg

1+

( − ϕ)C

mw

2!

( − ϕ)C 2 mw +

3!

+ ···

(15)

The diffusion coefficient of EG at infinite dilution i.e. D0eg and plasticization coefficient of EG i.e. ϕ may be obtained using Eqs. (10) and (15). 3. Experimental

Similarly, permeation rate of EG is

L

(11)

(2)

Cmw may be obtained for any feed (bulk) concentration of water (Cfw ) using regressed equation based on curve fitting of variation of Cmw with Cfw by best fit method in sorption experiment. Using Fick’s first law permeation rate of water through the membrane may be written as JW = −

JW =− JEG

D0w exp(Cmw )(dCmw /dl) D0eg exp(ϕCmw )(dCme /dl)

(8)

3.1. Materials High purity analytical grade ethylene glycol (EG) used for this study was purchased from E. Marck, Mumbai. The monomers and crosslinker i.e. acrylic acid (AA), acrylamide (AM), methylene bis acrylamide (MBA) and glutaraldehyde all synthesis grade were procured from S.d. fine chemicals, Mumbai and used as obtained. Ammonium persulfate and sodium metabisulfide was used as redox initiator pair for the copolymerization reaction. Polyvinyl alcohol (PVOH) of number average molecular mass 125,000 and hydrolysis of 98–99% was obtained from S.d. fine chemicals, Mumbai and used as obtained. 3.2. Synthesis, crosslinking and casting of full IPN (FIPN) membrane The three IPNs i.e. FIPN25, FIPN50 and FIPN75 were synthesized by solution polymerization in a three-necked reactor at 65 ◦ C for


393

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 this synthesis 10:1 molar ratio of acrylic acid and acrylamide was used. The reactor was fitted with a stirrer, a thermometer pocket and a condenser. At first 5 mass% PVOH solution 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. Required amounts of acrylic acid and acrylamide were then added to 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, MBA 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 H2 SO4 (to catalyze the reaction), 50% aqueous methanol (to quench the reaction) and 10% solution of acetic acid (pH controller) [19]. Thus, for 2 mass% crosslinking of PVOH and its IPNs the aqueous polymer solution was mixed with 0.4 ml glutaraldehyde, 0.2 ml sulfuric acid, 0.6 ml acetic acid and 0.4 ml methanol. 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 ␮m. The thickness was measured by Test Method ASTM D 374 using a standard dead mass thickness gauge (Baker, Type J17). 3.3. Membrane characterization 3.3.1. Fourier transform infrared spectroscopy (FT-IR) The FT-IR spectra of the three FIPN membranes were recorded on a Jasco (FT/IR-460 plus, Jasco Corporation, Japan) FT-IR spectroscope using a thin film (10 ␮m) of the polymer. The FTIR of the FIPN membranes i.e. FIPN25, FIPN50 and FIPN75 are shown in Fig. 1a–c, respectively. 3.3.2. Mechanical strength The tensile strength (TS) and elongation at break (EAB) of the polymer film was determined by an Instron-Tensile tester (Instron 4301, Instron Limited, England). The experiment was performed according to ASTM D 882-97. In this work, length of the specimens was 250 mm, the thickness of the specimens was around 0.1 mm and the thickness was uniform to within 5% of the thickness between the grips. The width of the specimens was 20 mm and edges were parallel to within 5% of the width over the length of the specimen between the grips. The mechanical properties of the membranes i.e. TS and EAB were also evaluated after 1 week immersion of the test samples in pure water and pure ethylene glycol to study stability of the membranes during pervaporation experiments. The TS and EAB of the membranes are given in Table 1. 3.3.3. Scanning electron microscope (SEM) The three IPN membranes were coated with gold (Au). The morphology of the membranes was observed using SEM (scanning electron microscope, Model No. S3400N, VP SEM, Type-II, made by Hitachi, Japan) with the accelerating voltage set to 3 kV. The SEM of the three IPN membranes i.e. FIPN25, FIPN50 and FIPN75 are shown in Fig. 2a–c, respectively. 3.3.4. Thermal analysis Differential thermal analysis (DTA) of the polymer samples were carried out in a Mettler instrument in nitrogen atmosphere at the

Fig. 1. FTIR of the FIPN membranes.

scanning rate of 10 ◦ C/min in the temperature range of 60–600 ◦ C. The DTA of the polymer samples are shown in Fig. 3. 3.4. Swelling study 3.4.1. Sorption experiment Membranes of known mass were immersed in different known concentrations of aqueous EG solutions and were allowed to equilibrate for 96 h at 30 ◦ C. Each 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 EG sorped by the membrane. After measuring the total mass of the sorped membranes from the above

394


Table 1 Mechanical properties of the membranes. Polymer membrane

Tensile strength (MPa) of unused membrane

Tensile strength (MPa) of membrane after 1 week immersion in pure water

Tensile strength (MPa) of membrane after 1 week immersion in pure ethylene glycol

Elongation at break (%) of unused membrane (EAB)

Elongation at break (%) of membrane after 1 week immersion in pure water (EAB)

Elongation at break (%) of membrane after 1 week immersion in pure ethylene glycol (EAB)

PVOH FIPN25 FIPN50 FIPN75

42 35.2 29.5 21.6

38.5 29 24.3 17.5

39 34.5 27 21

218 122 105 75.5

282 188 167 121

245 183 156 118

Fig. 3. DTA of the FIPN and PVOH membranes.

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 (Fig. 4). The cold trap was immersed in liquid nitrogen flask. The sorped sample was heated under vacuum and the vapour 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 were obtained by analyzing the composition of the liquefied vapour from the cold trap by an Abbe type Refractometer (Model No. AR600, MISCO, USA) 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

Fig. 2. SEM of the FIPN membranes.

Fig. 4. Experimental set up for sorption selectivity.


calculated from the following equation ˛S =

ym(Water) /ym(EG) Xf(Water) /Xf(EG)

(16)

Here Ymi and Xfi denote membrane phase and feed concentration of ‘i’ component 3.5. Permeation study 3.5.1. Pervaporation experiment Pervaporation experiments were carried out in a batch stirred cell [21] with adjustable downstream pressure that was maintained at 6 ± 1 mbar using a double stage vacuum pump (Basynth, India). The feed compartment of the pervaporation cell was equipped with a stirrer to ensure adequate mixing of the liquid feed so as to eliminate any concentration or temperature gradient. Effective membrane area (A) in contact with the feed mixture was 19.6 cm2 and the feed compartment volume was 150.0 cm3 . The EG–water mixtures in contact with the membrane were allowed to equilibrate for around 3 h for the first experiment and 1 h for the subsequent experiments with different feed compositions. The PV experiment was performed at a constant temperature by circulating constant temperature water around the jacket of the PV cell. When the steady state was reached the permeate was collected in traps immersed in liquid nitrogen. The results for pervaporation separation of EG/water mixtures were reproducible, and the errors inherent in the pervaporation measurements were less than 1.0%. The mass of permeate was determined by a digital electronic balance. The pervaporation performance of the membranes was evaluated by permeation rate (J) using Eq. (17). J=

Q At

(17)

where Q is the mass of permeates collected in time interval t, A is the effective membrane area. However, pervaporation flux is inversely proportional to thickness of the membrane and in the present study, thick (50) membrane was used. Thus, to eliminate the thickness effect for comparison of data with those reported in literatures thickness normalized flux (Jn , kg m−2 h−1 ␮m) was calculated using the following equation. Jn = J × L

(18)

Here J is flux in kg m−2 h−1 and L is membrane thickness in ␮m The water content of the permeate was determined in a similar way like sorption selectivity by an digital Refractometer at 30 ◦ C temperatures for all the samples. The permeation selectivity (˛PV ) of water expressed as separation factor for water was calculated from a similar type of equation as sorption selectivity i.e. Eq. (19) as given below ˛PV =

ywater /yethylene glycol xwater /xethylene glycol

(19)

4. Results and discussion 4.1. Membrane synthesis The objective of chemically modifying the PVOH matrix with poly(acrylic acid-co-Acrylamide) [polyAAAM] was two fold. By incorporation of poly(acrylic acid-co-acrylamide) in PVOH matrix, the crystallinity of PVOH would be reduced to give enhanced water permeability. Further, the highly hydrophilic polyAAAM, a water soluble polymer would give added hydrophilicity to the modified PVOH membrane. Earlier, crosslink copolymer of acrylamide and HEMA was used for separation of MTBE–methanol [20] and dehydration of corrosive organic like DMF [21]. However, polyacrylic acid being a high Tg polymer, the copolymer polyAH would

395

not be an effective PV membrane. Thus, in our previous works [17,18] synthesis of the copolymer polyAH was carried out in the matrix of PVOH followed by crosslinking with glutaraldehyde. In a similar way, in this work, polyAAAM has been synthesized in presence of PVOH solution in water. In this case two different kinds of crosslinkers have been used i.e. methylene bis acrylamide for crosslinking the polyAAAM copolymer and glutaraldehyde to crosslink the PVOH matrix. As both of these polymers (copolymer and PVOH) are crosslinked, a full IPN or FIPN is produced. Accordingly, three FIPNs i.e. FIPN25, FIPN50 and FIPN75 were produced with PVOH to copolymer ratio of 1:25, 1:50 and 1:75. By this chemical modification of PVOH, hydrophilicity of PVOH was increased and with reduced crystallinity the resulting FIPN membranes yielded increased flux. The crosslinking of polyAAAM was fixed at 2 mass%(MBA) of acryl amide and acrylic acid while crosslinking of PVOH matrix was fixed at 2 mass% (glutaraldehyde) of PVOH. Crosslinking% was not further increased as it would give decreased permeability. In fact, increase in crosslinking density gives higher selectivity with reduced flux. Thus, for most of the crosslinkable polymeric membranes, crosslink density is varied to obtain a value which would give optimum flux and selectivity. However, for a crystalline polymer like PVOH, its crystallinity acts like crosslinking and even uncrosslink PVOH can be used for pervaporative dehydration of an organic with more than 99 mass% concentration in water. 2 mass% crosslinking produces a membrane which remains intact even after 48 h of dipping in pure water. 4.2. Membrane characterization 4.2.1. FTIR study FTIR spectra of FIPN25, FIPN50 and FIPN75 membranes are shown in Fig. 1a–c, respectively. The peaks observed between 3580 and 3050 cm−1 corresponds to hydrogen bonded (bridged) O–H of acrylamide/MBA units [22]. The strong band appearing at around 1626.8–1637.9 cm−1 in all the FIPN membranes corresponds to carboxyl (COOH) peak due to acrylic acid present in the FIPN polymers. This peak has been shifted with more intensity with increasing amount of acrylic acid from FIPN25 (1626.8 cm−1 ) to FIPN50 (1629.0 cm−1 ) and FIPN75 (1637.9 cm−1 ). Similarly, O–H bending vibrations at around 1115 cm−1 corresponds to polyvinyl alcohol (PVOH) [12]. The bands at around 3436 cm−1 and 3754 cm−1 corresponds to crosslinked and uncrosslinked hydroxyl group of FIPN polymer. These apart, the strong band at around 600 cm−1 and 2920 cm−1 are due to (–CH2 –) methylene and C–H alkane group of the IPN membranes [21,23]. 4.2.2. Mechanical strength PVOH membrane shows a good balance of tensile strength (TS) and elongation at break (EAB) and hence considered as a good membrane forming polymer. From Table 1 it is observed that the TS and EAB of FIPN membrane is found to decrease with increasing amount of poly(acrylic acid-co-acrylamide) from FIPN25 to FIPN75. PVOH membrane shows high TS because of its stiffness due to crystallinity and it also shows high elongation because of its low Tg . The increasing amount of high Tg copolymer in PVOH decreases its crystalline structure as well as crosslinking of both PVOH and polyAAAM reduces chain flexibility of the FIPN membranes. Thus, with increasing amount of polyAAAM both TS and EAB of the polymer decreases. From the values given in Table 1 it is also observed that the membranes show lower TS and higher EAB after immersion in water and ethylene glycol for 1 week. Since the membrane is water selective, the water swollen membrane show somewhat lower TS than EG 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


4.2.4. Membrane characterization by thermal analysis Thermal behavior of all the used membranes in terms of temperature differential against sample temperature i.e. DTA is shown in Fig. 3. From these figures it is observed that glass transition temperature of the membranes increases in the following order: PVOH (112 ◦ C) > FIPN25 (97.5 ◦ C) < FIPN50 (119 ◦ C) ◦

< FIPN75 (129 C) Uncrosslinked PVOH shows glass transition temperature of around 85 ◦ C. Crosslinking of PVOH by glutaraldehyde restricts segmental motion of its chain with increase in Tg to 112 ◦ C. Incorporation of the small amount of copolymer decreases crystalline symmetry of PVOH matrix with increased free volume and hence, decrease in Tg due to easier segmental motion of polymer chain. As the amount of copolymer increases from FIPN25 to FIPN75 Tg increases due to more restriction in movement of the polymer chain. Further, both polyacrylic acid and polyacrylamide being high Tg polymer, stiffness of the polymer chain increases (as also evidenced by its mechanical properties given under Section 3.3.2) due to presence of large amount of copolymer of these two homopolymer in FIPN50 and FIPN75 with increase in Tg . 4.3. Swelling studies 4.3.1. Effect of feed concentration on sorption isotherms Fig. 5a shows the variation of total sorption of EG and water by PVOH and the three chemically modified PVOH membranes i.e. FIPN25, FIPN50 and FIPN75 membranes with feed concentration of water at 30 ◦ C. Similar kind of relationship was also observed at the other two temperatures of sorption experiments i.e. at 40 and 50 ◦ C. From this figure it is found that total sorption increases almost linearly in the following order:

120

a

1.2

100

1 80 0.8 60 0.6 40 0.4

Sorption selectivity for water (-)

Total sorption (g/g of dry membrane)

4.2.3. Membrane characterization by SEM SEM studies of the three FIPN membranes are shown in Fig. 2a–c, respectively. SEM of a pure polymer like PVOH always gives a dense feature. SEM is usually carried out for a polymer blend to evaluate the extent of compatibility in terms of morphology of the blend. The poorer the compatibility, the coarser is the morphology. IPN are different from a blend in that due to interpenetration of the constituent polymers the extent of compatibility is very high in a 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 membranes was carried out at 14 kV in 5 ␮m scale to get morphology of the constituent polymers. In IPN the size and shape of the polymer II domains (i.e. the copolymer polyAAAM) are controlled by the cross-link density of polymer I (PVOH) and the relative proportions of the two polymers [24]. Close examination of Fig. 2a–c suggests that with increasing amount of polymer-II domain i.e. polyAAAM the morphology becomes coarser from FIPN25 to FIPN75. The microphase separation of PVOH and interpenetrating copolymer is maximum in FIPN75 resulting in needle like morphology [25,26].

1.4

PVOHsorption FIPN25sorption FIPN50sorption FIPN75sorption PVOHselectivity FIPN25selectivity FIPN50selectivity FIPN75selectivity

20

0.2

0

0 0

2

4

6

8

10

12

14

Feed conc. of water (wt%), C fw 100

b

90

Membrane phase water (wt%), C mw

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 ethylene glycol.

80 70 60 50 40 PVOH

30

FIPN25 FIPN50

20

FIPN75

10 0 0

2

4

6

8

10

12

14

Feed conc of water (wt%), C fw 1.8

c Total sorption (g/g of dry membrane)

396

1.6 1.4 1.2 PVOH

1

FIPN25 FIPN50 FIPN75

0.8 0.6 0.4 0.2 0

FIPN75 > FIPN50 > FIPN25 > PVOH.

25

30

35

40

45

50

55

60

65

0

The increase in total sorption with increase in feed concentration of water may be attributed to increased hydrophilicity of the chemically modified PVOH membranes. As the mass% of the copolymer PAAAM increases from FIPN25 to FIPN75, total

Feed Temperature C Fig. 5. (a) Sorption isotherm and sorption selectivity of the membranes at 30 ◦ C. (b) Variation of membrane phase water with feed cone of water at 30 ◦ C. (c) Variation of total sorption with feed temperature.

S.B. Kuila et al. / Chemical Engineering and Processing 50 (2011) 391–403 Table 2 Values of ‘a’, ‘b’ and r2 . Polymer membrane

A

b

r2


41.36 50.21 53.15 42.42

0.29 0.24 0.22 0.29

0.9262 9423 0.9269 0.96

sorption increases due to increased hydrophilicity in the membranes. The FIPN membranes also show much higher sorption than chemically unmodified but glutaraldehyde crosslinked PVOH membrane at higher feed concentration of water because of increased water–membrane interaction through extensive hydrogen bonding between both of the permeants and the membranes. It is also evident from the figures that these sorption isotherms closely resemble Rogers Type-III sorption [27]. 4.3.2. Feed concentration and sorption selectivity From Fig. 5a it is also observed that sorption selectivity of water for all the membranes decreases almost exponentially with increase in feed concentration of water. It is also observed that for all the membranes with increase in feed concentration of water, sorption selectivity decreases in the following order: FIPN50 > FIPN25 > FIPN75 > PVOH

the used concentration range, all the used membranes show high water concentration in the permeate. Among the used membranes FIPN50 shows marginally higher water wt% in the permeate. Incorporation of hydrophilic crosslink copolymer polyAAAM in PVOH matrix increases water affinity of the membranes. However, it also decreases crystallinity of PVOH matrix as its intramolecular hydrogen bonding is reduced. The slightly lower water concentration in the permeate for FIPN75 may be due to increased void space in the membrane. 4.4.2. Effect of feed concentration on flux and permeation selectivity The effect of feed concentration of water on normalized partial flux of water and EG and permeation selectivity for water at 30 ◦ C are shown in Fig. 6b and c, respectively. Similar kind of relationship was also observed at the other two temperatures of PV experiments i.e. at 40 and 50 ◦ C. From Fig. 6b it is observed that water partial flux increases linearly for all the membranes at lower feed concentrations of water. At higher feed concentration the feed vs. 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 at higher feed concentration of water. For the same feed concentration partial water flux is also observed to increase in the following order. FIPN75 > FIPN50 > FIPN25 PVOH

In comparison to total sorption the different trend in sorption selectivity is due to incorporation of more hydrophilic copolymer in the PVOH matrix which gives added hydrophilicity. Thus, with incorporation of increasing amount of copolymer water selectivity increases from PVOH to FIPN50. However, incorporation of the copolymer also reduces crystalline symmetry of the PVOH matrix and the loss in crystallinity is maximum in FIPN75. The lower sorption selectivity of FIPN75 in comparison to FIPN25 or FIPN50 may be due to this loss in symmetry of PVOH matrix resulting in highest sorption but lower sorption selectivity. In Fig. 5b membrane phase concentration of water (Cmw ) is plotted against feed concentration of water (Cfw ) to obtain the following regressed equation b Cmw = a · Cfw

397

(20)

The values of ‘a’ and ‘b’ for all the membranes are obtained from this Fig. 5b by power regression of the trend lines. The values of ‘a’ and ‘b’ along with regression coefficients (r2 ) for the four membranes are given in Table 2. The membrane phase concentration of water for any feed water concentration may be obtained using Eq. (20). 4.3.3. Effect of feed temperature on total sorption Fig. 5c shows the effect of temperature on total sorption for the four different membranes i.e. PVOH, FIPN25, FIPN50, FIPN75 for 1.032 mass% of water in water–EG mixtures. From this figure it is observed that with increase in temperature total sorption decreases almost exponentially from PVOH to FIPN75 signifying negative enthalpy of sorption. 4.4. Pervaporation (PV) studies 4.4.1. Effect of feed concentration on dehydration Fig. 6a shows the variation of mass% of water in the permeate against mass% of water in the feed for dehydration of EG with crosslinked PVOH and the three FIPN membranes at 30 ◦ C. Similar kind of relationships were also observe at the two other PV temperatures i.e. at 40 and 50 ◦ C. It appears from these McCabe-Thiele type xy diagrams that all the membranes show measurable dehydration characteristics over the used concentration range without any pervaporative azeotrope. It is also observed from the figure that over

The lowest flux of PVOH may be ascribed to its high degree of crystallinity due to intra and intermolecular hydrogen bonding. Incorporation of polyAAAM copolymer in the matrix of PVOH increases its hydrophilicity and decreases its crystallinity. Thus, with increase in amount of the copolymer polyAAAM, flux increases in the above order. From Fig. 6b it is also observed that for the same feed concentration water flux is much higher than EG partial flux signifying high water selectivity of the membranes. In fact, EG flux remains marginally constant over the concentration range used for this study for all the membranes. In Fig. 6b water selectivity of the membranes is observed to decrease almost exponentially with its feed concentration. It is also observed from this figure that for the same feed concentration water selectivity shows the following trend FIPN50 > FIPN25 > FIPN75 > PVOH Incorporation of hydrophilic polyAAAM copolymer in PVOH matrix increases its hydrophilicity and hence water selectivity of the membranes increases from FIPN25 to FIPN50 with increasing amount of the copolymer. However, the amount of copolymer in FIPN75 is very high (75 mass% of PVOH) and it causes decrease in water selectivity because of increased void space. 4.4.3. Membrane permeability and membrane selectivity Flux and selectivity of the membrane can also be expressed in terms of concentration dependent permeability and membrane selectivity [28]. Based on solution-diffusion theory the flux of component ‘i’ through the pervaporation membrane may be expressed as [28,29] ji =

Pig (ai0 xi psat L i

− ppi )

(21)

Here Pig is gas phase membrane permeability, L is thickness of membrane, ai0 is activity coefficient of component ‘i’ in feed liquid at infinite dilution, xi is mole fraction of component ‘i’ in feed liquid, psat is vapour pressure of pure component ‘i’ in feed and ppi is partial pressure of component ‘i’ on permeate side. Eq. (21) may be written as ji =

Pig (Hi xi L

− ppi )

(22)


a

6

5

PVOHwater

100

FIPN25water FIPN50water FIPN75water PVOHEG

-1

90 80

FIPN25EG

2

Permeate conc. of water (wt%)

b Normalised Partial Flux (kg m hr )μm

398

70 60

PVOH FIPN25

50

FIPN50 FIPN75 VLE

40 30 20

FIPN50EG

4

FIPN75EG

3

2

1

10 0

0 20

40

60

80

0

100

2

4

Feed conc. of water (wt%)

3500

Water Selectivity (-)

3000

2500

2000

PVOH FIPN25 FIPN50

1500

FIPN75

1000

500

0 0

2

4

6

8

10

12

14

Partial parmeability of water (kgmolem-1s-1) x 10 6

d c

2.5

8

10

12

0.01

PVOHwater FIPN25water FIPN50water FIPN75water PVOHEG FIPN25EG FIPN50EG FIPN75EG

2

0.008

1.5

0.006

1

0.004

0.5

0.002

0

14

0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Feed conc. of water (mole fraction)

Feed conc. of water(wt%)

e

6

Feed conc. of water (wt%)

Partial parmeability of ethylene glycol (kgmolem-1s1) x 10 6

0

16000

PVOH FIPN25 FIPN50

Membrane selectivity (-)

14000

FIPN75

12000 10000 8000 6000 4000 2000 0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Feed conc of water (mole fraction) ◦

Fig. 6. (a) Variation of permeate cone with its feed cone, at 30 C. (b) Variation of partial fluxes of water and EG with its feed concentration at 30 ◦ C. (c) Variation of water selectivity of the membranes with Feed concentration at 30 ◦ C. (d) Variation of partial permeability with feed cone, of water. (e) Variation of membrane selectivity with feed concentration of water.


Here Hi is Henry’s coefficient such that Hi = ai0 psat i

(23)

a

ji =

xi −

ppi Hi

(25)

As the pervaporation experiments were carried out at very low permeate pressure, the term ppi /Hi may be ignored. Hence, Eq. (25) reduces to ji =

Pic x L i

0.4

0.5

0.6

0.7

0.8

0.9

PVOH

(24)

Hence, Eq. (22) reduces to Pic L

-4

-4.5

FIPN25 FIPN50 FIPN75

ln (Flux of water)

Pig =

Pic Hi

membrane phase concentration of water (g/cm3) 0.3

Further, gas phase membrane permeability, Pig is related to concentration based permeability Pic by the following Eq. (24) [28]

399

-5

-5.5

(26)

-6

(27)

-6.5

Combining Eqs. (5) and (26), Jn xi

Concentration based permeability coefficient of component ‘i’ (water) and ‘j’ (ethylene glycol) i.e. Pic and Pjc for each feed concentration (in mole fraction) were calculated and plotted against feed concentration in Fig. 6d. From Fig. 6d it is observed that the permeability of water is much higher than the permeability of EG signifying high water selectivity of the membranes. It is also observed that permeability of water decreases almost exponentially with feed concentration while permeability of ethylene glycol increases with feed concentration of water though the overall increment in permeability of EG over the entire range of feed concentration is insignificant. These trends of Fig. 6d is in contrast to the trend observed in Fig. 6b where partial flux of both water and ethylene glycol increases with increasing feed concentration. This opposite trends of flux and permeability was also reported by Baker et al. [28] for several binary mixtures. This result indicates that the permeability of the IPN membranes are strongly concentration dependent. In Fig. 6e membrane selectivity which is the ratio of permeability of water to the same of ethylene glycol (Pic /Pjc ) is plotted against feed concentration of water. From Fig. 6e it is observed that the membranes show high water selectivity at low feed concentration and it decreases exponentially with concentration. This trend is similar to those observed in Fig. 6c where permeation selectivity based on permeate and feed composition (Eq. (19)) is plotted against feed concentration. 4.4.4. Determination of diffusion coefficient of water and EG 4.4.4.1. Diffusion coefficient of water. In Fig. 7a logarithmic permeation rate of water (ignoring the D0 m /L term on left hand side of Eq. (9)) is plotted against its membrane phase concentration. Membrane phase concentration for all the pervaporation feed concentration was obtained from Eq. (20). From slope and intercept of this linear trend lines (Fig. 7a) the values of diffusion coefficient of water at infinite dilution (D0w ) and its plasticization coefficient () are obtained as given in Table 3. 4.4.4.2. Diffusion coefficient of EG. The values of diffusion selectivity, ˛D at different membrane phase concentration of water, Cmw may be obtained from pervaporation selectivity and sorption selectivity using Eq. (10). Second order polynomial regression of the curves obtained by plotting ˛D against Cmw (Fig. 7b) results in a quadratic regressed equation. Comparing this equation with Eq. (15) gives values of plasticization coefficient of EG i.e. ϕ and its diffusion coefficient at infinite dilution (D0w ) as given in Table 3. The higher diffusion coefficient of water than EG for all the membranes may be attributed to bigger kinetic diameter of EG than water. The plasticization coefficient of EG i.e. ϕ is also much higher than that

-7

b

45

40 PVOH FIPN25

35

Diffusion selectivity (-)

Pic =

FIPN50 FIPN75

30

25

20

15

10

5

0 0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

3

Membrane phase conc. of water (g/cm ) Fig. 7. (a) Variation of logarithm of water flux with membrane phase concentration at 30 ◦ C. (b) Variation of diffusion selectivity with membrane phase concentration of water at 30 ◦ C.

of water signifying increasing rate of diffusion coefficient of EG at higher feed concentration of water. 4.4.5. Effect of temperature on flux and selectivity 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. 8a and b for partial flux and selectivity, respectively, with 2.9 mass% feed water concentration. Apparent activation energy for permeation (EP ) can be obtained from the slope of the Arrhenius type linear plot of logarithmic of partial flux (Q) against inverse of absolute temperature (1/T) as shown in Fig. 9a for water and in Fig. 9b for EG for 2.9 wt%

400


Table 3 Diffusion coefficient of water and EG at infinite dilution and its plasticization coefficients. Polymer membrane

Plasticization coefficient for water,

D0w × 1010 cm2 /s

Plasticization coefficient for EG (ϕ)

D0e × 1012 cm2 /s


1.7782 3.233 3.119 3.055

1.06 1.59 1.85 1.97

5.264 6.728 6.327 6.362

1.04 1.59 1.89 2.11

a

0.1

6

PVOHwater FIPN25water FIPN50water FIPN75water PVOHEG FIPN25EG FIPN50EG FIPN75EG

12

Normalised partial Permeability of water

Normalised partial Flux (kgm-2hr-1) μm

5 PVOHwater

10

FIPN25water FIPN50water FIPN75water PVOHEG

8

FIPN25EG FIPN50EG FIPN75EG

6

4

4

0.09

0.08

0.07

0.06

0.05

3

0.04 2 0.03

0.02 1

2

Normalised partial Permeability of ethylene glycol

c

0.01

0 20

0 30

40

50

60

70

80

0 25

35

0

55

65

75

85

0

Feed temperature ( C)

b

45

Feed temperature ( C)

d

4000

3000 PVOH FIPN25 FIPN50 FIPN75

PVOH FIPN25

3500

2500

FIPN50 FIPN75

Membrane selectivity (-)

Water selectivity(-)

3000 2500 2000 1500

2000

1500

1000

1000 500 500 0 25

35

45

55

Feed temp,0C

65

75

85

0 25

35

45

55

65

75

85

0

Feed tempearure ( C)

Fig. 8. (a) Variation of partial flux with feed temperature for 2.9 wt% water in feed. (b) Variation of water selectivity of the membranes with feed temperature for 2.90 wt% water in feed. (c) Variation of partial permeability with feed temperature for 2.9 wt% feed concentration. (d) Variation of membrane selectivity with feed temperature for 2.9 wt% water in feed.


401

Table 4 Apparent activation energy for permeation of the membranes. Polymer membrane

Apparent activation energy for water (kJ)

Regression coefficient for Arrhenius plot for water (r2 )

Apparent activation energy for EG

Regression coefficient for Arrhenius plot for EG (r2 )


15.51 30.24 30.74 24.43

0.988 0.992 0.990 0.942

74.10 89.26 85.02 83.03

0.991 0.980 0.963 0.999

feed concentration of water. The apparent activation energy for permeation of water and EG for 2.9 mass% of water in feed are given in Table 4. From this table it is observed that activation energy for water is much lower than EG for all the membranes. Perme-

a

4.4.6. Effect of temperature on permeability and membrane selectivity The variation of permeability of the membranes with temperature is shown in Fig. 8c for 2.9 wt% feed concentration. Similar kind of trend lines were obtained at other feed concentrations of the pervaporation experiments. The variation of membrane selectivity with feed temperature for the same feed concentration is shown in Fig. 8d. From these figures it is observed that similar to flux, permeability also increases with temperature while like permeation selectivity, membrane selectivity decreases with temperature. In fact, the vapour pressure of the permeants are embedded in concentration based permeability and with increasing temperature as vapour pressure of the permeant increases, permeability also increases almost exponentially [28].

3 PVOH FIPN25 FIPN50

2.5

FIPN75

ln(Flux)

2

1.5

1

0.5

0 0.0028

0.0029

0.003

0.0031

0.0032

0.0033

0.0034

-0.5

b

1 (1/T)K-1

0 0.0028

0.0029

0.003

0.0031

0.0032

0.0033

0.0034

-1 PVOH

ln(Flux)

FIPN25

-2

ation of smaller water molecules through these highly hydrophilic membranes are much easier than EG resulting in lower activation energy.

FIPN50 FIPN75

-3

4.4.7. Effect of feed concentration on permeation ratio Permeation ratio gives a quantitative idea about the effect of one component on the permeation rate of the other component. Huang and Lin [30] have defined this permeation ratio, , as a measure of the deviation of the actual permeation rate, Jexpt , from the ideal rate, J0 , to explain interactions between membrane polymer and permeants. From Fig. 10a it is observed that at very low concentration of water i.e. at very high concentration of EG (around 99 mass% or more) in feed, the permeation factor of water is far above unity for all the membranes signifying positive coupling effect of EG on partial flux of water. In this case EG–water interaction is more than water–membrane interaction. As the water% in feed increases to around 5 wt%, permeation ratio of water decreases drastically for all the water selective membranes and become close to unity i.e. coupling effect of EG on water becomes negligible because of much higher water–membrane interaction (through hydrogen bonding) than EG–water interaction. Fig. 10b shows the effect of water concentration in feed on permeation ratio of EG. It is observed that at very low water concentration in feed permeation factor of EG is much higher that that of water as seen in Fig. 10b. However, with increase in feed concentration of water permeation factor of EG decreases at a much higher rate and approaches unity. In the highly hydrophilic membranes, above 5 mass% feed concentration of water the EG flux is hardly influenced by water.

-4

5. Comparison of present work with reported data -5

-6 Fig. 9. (a) Arrhenius plot for water for 2.90 mass% water in feed. (b) Arrhenius plot for ethylene glycol for 2.90 mass% water in feed.

The performance of the FIPN membranes synthesized in the present work is compared with other reported membranes in Table 5 for pervaporative dehydration of EG. The water selectivity of the present membranes are found to be much higher than most of the reported membranes under similar operating conditions while normalized flux is comparable to many of the reported membranes.

402


a

b

10

PVOH

PVOH

FIPN25

4.5

FIPN50

8

Permeation Factor of Ethylene glycol (-)

Permeation Factor of water(-)

9

5

FIPN75

7 6 5 4 3 2 1

FIPN25 FIPN50

4

FIPN75

3.5 3 2.5 2 1.5 1 0.5 0

0 0

20

40

60

80

100

0

20

40

60

80

100



Fig. 10. (a) Variation of permeation factor of water with its feed concentration at 30 ◦ C. (b) Variation of permeation factor of ethylene glycol with feed concentration of water at 30 ◦ C.

Table 5 Comparison of performances of membranes reported for pervaporative dehydration of ethylene glycol. Membrane

Feed water concentration (wt%)

Temperature(◦ C)

Normalized flux (kg/m2 h) ␮m

Surface crosslinked chitosan Chitosan–poly(acrylic acid) polyelectrolyte Cross-linked chitosan Chitosan/polysulfone blend membrane PVA/PES blend Surface crosslinked PVA PVA/MPTMS/Silica PAA/PVA/IPN SPEEK PDMAEMA/PSF Polyacrylic–polyethyleneamine polyelectrolyte Chitosan coated zeolite filled cellulose membrane FIPN25 FIPN50 FIPN75 FIPN25 FIPN50 FIPN75

10 20 12.32 10 17.5 20 20 10 10 6 3 10

75 70 30 35 80 70 70 30 32 30 40 30

2.9

30

2.9

75

0.560 0.216 2.250 30.0 0.845 3.165 3.015 17.0 0.750 33.3 8.086 0.311 1.505 1.870 2.755 8.928 10.630 12.255

6. Conclusion The matrix of polyvinyl alcohol (PVOH) membrane was chemically modified by copolymerizing acrylic acid with acrylamide (polyAAAM) in aqueous solution of PVOH with three different mass ratio of PVOH:polyAAAM followed by crosslinking with methylene bis acrylamide to form three full interpenetrating network (FIPN) membranes i.e. FIPN25, FIPN50 and FIPN75 containing 25 mass%, 50 mass% and 75 mass% copolymer, respectively in the FIPN membranes. The FIPN membranes were characterized with, FTIR, SEM, DTA and mechanical properties. With incorporation of polyAAAM, the FIPN membranes becomes stiff with lower elongation at break and less tensile strength. All the membranes show measurable flux and very high separation factor for water. The concentration dependent permeability and membrane selectivity were also calculated. Diffusion selectivity at infinite dilution and plasticization coefficients of the membranes were determined using modified solution-diffusion model. Intrinsic permeability of all the mem-

Water selectivity (−) 141 105 148 104 231 933 311 500 1800 600 340 50 2512 2655 1859 121 148 101

Reference

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [16] Present work

Present work

branes were also determined. Among the three membranes, FIPN50 with 50 mass% polyAAAM incorporation shows optimum performance in terms of flux and selectivity. FIPN75 membrane shows highest flux but lower water selectivity. These highly water selective membranes could be effectively used for dehydration of many organics. Acknowledgement The authors are grateful to Department of Science and Technology (DST-SERC), Govt. of India (SR/S3/CE/056/2009) for sponsoring this work. Appendix A. Nomenclature The authors are grateful to Department of Science and Technology (DST-SERC), Govt. of India (SR/S3/CE/056/2009) for sponsoring this work.


A C D Ea H J L P t W X Y ai0 psat i x

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