Chemical Engineering and Processing 44 (2005) 51–58
Separation of acetic acid–water mixtures through acrylonitrile grafted poly(vinyl alcohol) membranes by pervaporation a,∗ , L. Aras b , G. Asman a N. Alghezawi a , O. Sanlı ¸ a
Gazi Universitesi Fen-Edebiyat Fakültesi Kimya Bölümü 06500 Teknik Okullar Ankara, Turkey b Orta Doˇ gu Teknik Universitesi Fen-Edebiyat Fakültesi Kimya Bölümü, Ankara, Turkey Received 19 May 2003; received in revised form 29 March 2004; accepted 30 March 2004 Available online 15 June 2004
Abstract The pervaporation separation of acetic acid–water mixtures was performed over a range of 10–90 wt.% acetic acid in the feed at temperatures ranging 25–50 ◦ C using acrylonitrile (AN) grafted poly(vinyl alcohol) (PVA) membranes. The permeation and separation characteristics of PVA-g-AN membranes were studied as a function of membrane thickness, feed composition, operating temperature and pressure. When the downstream pressure increased permeation rate increased with decreasing separation factor and at high acetic acid concentrations PVA-g-AN membranes shows grater tendency for the separation of acetic acid–water mixtures. Depending on the membrane thickness, feed composition and temperature PVA-g-AN membranes gave separation factors 2.3–14 and permeation rates 0.18–1.17 kg/m2 h. It was also determined that PVA-g-AN membranes were found to have lower permeation rate and grater separation factors than PVA membranes. © 2004 Elsevier B.V. All rights reserved. Keywords: Membranes; Selectivity; Separations; Transport processes; Pervaporation; Acetic acid–water mixtures; Poly(vinyl alcohol) membranes; Graft copolymerization
1. Introduction Pervaporation is a membrane separation process where the liquid feed mixture is in contact with the membrane in the upstream under atmospheric pressure and permeate is removed from the downstream as vapor by vacuum or a swept inert gas. Most of the research efforts of the pervaporation have concentrated on the separation of alcohol–water system [1–20] but the separation of acetic acid–water mixtures has received relatively little attention [21–34]. Acetic acid is an important basic chemical in the industry ranking among the top 20 organic intermediates. Because of the small differences in the volatility’s of water and acetic acid in dilute aqueous solutions, azeotropic distillation is used instead of normal binary distillation so that the process is an energy intensive process. From this point of view, the pervaporation separation of acetic acid–water mixtures can be one of the alternate processes for saving energy. ∗ Corresponding author. Tel.: +90-312-2122900; fax: +90-312-2122279. E-mail address:
[email protected] (O. Sanlı). ¸
0255-2701/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2004.03.007
PVA is a possible candidate for the separation of aqueous mixtures [5,7,12–14,21–25,35] because of its good chemical stability, film forming ability and high hydrophilicity, except for its poor stability in aqueous solution. Therefore PVA must be insolubilized by crosslinking, grafting or other modification reactions to create a stable membrane with good mechanical properties and selective permeability to water. Huang and Yeom [23,24] studied the separation of ethanol–water and acetic acid–water mixtures using amic acid as the crosslinking agent. Yeom and Lee [36] prepared PVA membranes by crosslinking with gluteraldehyde for the pervaporation of acetic acid–water mixtures. Aminabhavi and Naik [37] prepared poly(vinyl alcohol)-g-poly(acrylamide) copolymeric membranes to separate acetic acid–water mixtures. Acrylonitrile based membranes were also used in acetic acid–water separation. Lee and Oh [11] copolymerized 4-vinylpyridine with acrylonitrile in order to prepare a membrane for the dehydration of water–acetic acid mixture by pervaporation. Yoshikava et al. [25] reported that membranes prepared from poly (acrylic acid-co-acrylonitrile)
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were effective in the selective separation of water from acetic acid–water mixtures. In the present study we have aimed to separate acetic acid–water mixtures using AN grafted PVA membranes and studied the permeation characteristics of PVA-g-AN membranes as a function of membrane thickness, temperature, feed composition and pressure.
2. Experimental 2.1. Materials PVA (Merck) with molecular weight of 72,000 and degree of saponification of 98.5–99.2%, was used as supplied, AN (Merck) was purified by reduced pressure distillation. Ceric ammonium nitrate (CAN) (Merck) was used after vacuum drying. Acetic acid (Merck), dimethylformamide (DMF) and dimethylsulfoxide (DMSO) (Merck) were used without further purification. Other reagents used in the study were also all Merck products. 2.2. Synthesis of PVA-g-AN The reaction was carried out in a three-necked flask equipped with stirrer, condenser and nitrogen gas inlet. Five grams of PVA was dissolved in 95 mL of distilled water at 80 ◦ C. Fifteen grams of AN was added to the solution. Stirring was continued for 1 h at room temperature under N2 atmosphere. Ten mililiter portion of 1.0 × 10−2 M HNO3 containing 10 mL, 1.0 × 10−2 M of CAN was added to the reaction vessel and polymerization was carried out for a period of 3 h at 30 ◦ C. The reaction was quenched with 10 mL of aqueous ferrous salt solution (FeSO4 ) (5.0 × 10−2 M) and pH of graft copolymer latexes were adjusted to 1.5 with 1.0 M HCl solution, then dried in air at room temperature. The product was extracted with water at 80 ◦ C then with DMF at 55 ◦ C for 2 days to remove the unreacted PVA and polyacrylonitrile (PAN) homopolymer. After the extraction, graft copolymer was dried in vacuum oven at 60 ◦ C [5,38]. The proposed grafting mechanism between PVA and AN is given in Fig. 1.
Fig. 1. Proposed reaction mechanism between PVA and AN for the synthesis of PVA-g-AN copolymer.
downstream side was kept at 23 Pa by a vacuum pump (Vacuu Brand RD 15, GMBH Co., Germany). Pervaporation experiments were conducted at constant temperatures ranging 25–50 ◦ C using different feed compositions. Upon reaching steady state flow conditions, the permeate vapor was collected in liquid nitrogen traps. The composition of permeates was deduced refractometrically by digital differential refractometer (Atago DD-5, Atago Co. Ltd., Japan). 2.5. Representation of the results The permeation and separation characteristics of PVA-g-AN membranes toward acetic acid–water mixtures were expressed as permeation rate (flux) (Q), separation factor (α) and pervaporation separation index (PSI). The permeation rate, Q was determined by using the equation m Q= (1) At where m, A and t represent the weight of the permeate (kg),
2.3. Preparation of membranes 1.2% (w/v) solution of PVA-g-AN in DMSO were prepared. The solutions of 20–30 mL were poured on to petri-dishes (9.2 cm in diameter) and the solvent was evaporated at 80 ◦ C to form the membrane (15 m–40 m). 2.4. Pervaporation experiment The apparatus used in this study were illustrated in Fig. 2. The pervaporation cell was a two compartment cell with a 150 mL upper compartment, 75 mL of lower compartment. Effective membrane area was 12.5 cm2 . The pressure at the
Fig. 2. Schematic diagram of the pervaporation apparatus: (1) vacuum pump; (2–4, 6) permeation traps; (5) Mc Leod manometer; (7) vent; (8) permeation cell; (9) constant temperature water bath.
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effective membrane area (m2 ) and operating time (h) respectively. Separation factor was calculated using: α=
YH2 O /YCH3 COOH XH2 O /XCH3 COOH
(2)
where XH2 O , XCH3 COOH , YH2 O and YCH3 COOH denote the weight fractions of water and acetic acid in the feed and in the permeate, respectively. Pervaporation separation index (PSI), which is a measure of the separation ability of a membrane was defined by Huang and Yeom [7] and expressed as the product of separation factor and permeation rate. PSI = Qα
(3)
3. Results and discussion 3.1. PVA-g-AN copolymer PVA-g-AN copolymer was synthesized by using PVA and AN in a weight ratio of (PVA/AN) (1/2.5), (1/3.0) and (1/5.0) in the polymerization solution; compositions of the copolymer were determined by elemental analysis (Table 1). FTIR spectrum of the copolymer was taken (Fig. 3). The stretching vibrations of –OH and –CH appears at 3340 cm−1 and 2947–2910 cm−1 , respectively, and stretching vibration of –CN appears at 2240–2225 cm−1 were taken as the evidence of grafting.
Table 1 Elemental analysis of PVA-g-AN Membrane
PVA/AN (w/w)
C (%)
H (%)
N (%)
PVA-1 PVA-2 PVA-3
1/2.5 1/3.0 1/5.0
59.71 60.47 60.77
6.56 7.13 7.34
13.62 13.80 16.29
53
Table 2 Effect of membrane thickness on permeation rate and separation factor Thickness (m)
Q (kg/m2 h)
α
15 25 35 40
Low mechanical strength 1.09 0.66 0.11
2.30 3.00 5.50
20 wt.% CH3 COOH; P = 23 Pa, T = 30 ◦ C.
3.2. Effect of membrane thickness Membranes with varying thickness (15–40 m) were prepared from the copolymer by casting method. The permeation rate and separation factor as a function of membrane thickness was studied for 20 wt.% acetic acid solutions at 30 ◦ C and the results were presented in Table 2. As reflected from the table as the membrane thickness increases permeation rate decreases whereas separation factor increases as expected from the Fick’s first law [39]. Different results concerning the effect of membrane thickness on pervaporation performance was reported in the literature [39,40,42]. Koops et al. [39] investigated the pervaporation selectivity as a function of membrane thickness for the polysulfone, poly(vinyl chloride) and poly(acrylonitrile) membranes in the dehydration of acetic acid and reported that selectivity decreases with decreasing membrane thickness, below a limiting value of about 15 m. Aptel et al. [40] observed a reduction of separation factor as a function of decreasing membrane thickness for grafted polytetrafluoroethylene films using a water/dioxane mixture. They have shown in a different study [41] that transport rate is inversely proportional to thicknes in the pervaporation separation of water through poly(tetra fluoroethylene)–poly(4-vinylpyridine) membranes. Brun et al. [42] studied the influence of membrane thickness on the separation factor using nitrile rubber membranes for 60/40 mixture of butadiene and isobutene. They have concluded that the separation factor was constant above 100 m membrane thicknesses. In this present study membranes of 35 m thicknesses were preferred in the rest of the study due to their acceptable flux and separation factor.
Table 3 Comparison of PVA and PVA-g-AN membranes T (◦ C)
25 45 50 Fig. 3. FTIR spectrum of PVA-g-AN.
α
Q (kg/m2 h) PVA
PVA-g-AN
PVA
PVA-g-AN
0.95 4.47 5.70
0.18 1.10 1.17
1.76 1.23 1.05
2.58 3.03 3.03
20 wt.% CH3 COOH; P = 23 Pa, t = 1.5 h.
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14. 0
0.4
12. 0 10. 0
0.
8.0 0.2
α
Q (kg/m 2 h)
hydrogen bonding between acetic acid and –CN groups of AN; so it becomes difficult for acetic acid molecules to diffuse through the membrane and separation factor toward water increases. Decrease in permeation rate can be explained by the dense structure of the grafted membrane. In Fig. 4 permeation rate of grafted membranes at different grafting ratio with respect to the feed composition was given. As it seen from the graph although permeation rate decreases with the amount of AN separation factors were almost the same.
16. 0
0.5
6.0 4.0
0.1
2.0
0
3.4. Effect of temperature on pervaporation
0.0 0
10
30
50
70
90
100
The effect of temperature on the separation performance of PVA-g-AN (PVA-2) membranes was studied in the temperature range of 25–50 ◦ C and the results were given in Table 4. As it is reflected from the table, as the temperature increases permeation rate increases, however, separation factor stays constant above 30 ◦ C. Aminabhavi and Naik [37] obtained similar results in the pervaporation of acetic acid–water mixtures by using PVA membranes. They reported that for 20 wt.% acetic acid solutions separation factor stayed constant at 35 and 45 ◦ C although permeation rate increased with temperature. The permeation at various operating temperatures is thought to be governed by three factors [7,23].
CH3 COOH (wt %) Fig. 4. Effect of PVA:AN ratio on the permeation rate and separation factor (T = 30 ◦ C): (䊊) α (PVA-2); (䊉) Q (PVA-2); (䊐) α (PVA-1); (䊏) Q (PVA-1). Table 4 Effect of temperature on permeation rate and separation factor T (◦ C)
Q (kg/m2 h)
α
25 30 40 50
0.18 0.66 1.07 1.17
2.60 3.00 3.00 3.00
20 wt.% CH3 COOH; P = 23 Pa, t = 1.5 h.
1. Change in the free volume of the polymer membrane swollen by pure components, which affects mainly permeation rates. 2. Change in the free volume due to the plasticizing effect. 3. Change in the interaction between permeants in the membrane.
3.3. Effect of AN grafting on the performance of the PVA membranes In order to understand the effect of grafting on the performance of PVA membranes, PVA-g-AN (PVA-2) membranes were compared with the heat treated PVA membranes that were prepared by casting method from 7% (w/v) PVA solution (Table 3). As it is reflected from the table that the presence of AN decreases the permeation rate while increasing the separation factor. Increase in the separation factor is caused from the
According to the free volume theory as the temperature increase the frequency and amplitude of chain jumping (i.e. thermal agitation) increase and the resulting free volumes become larger. Increase in temperature also decreases the interaction between acetic acid and water molecules so it will be easy for both acetic acid and water molecules to 16 .0
0. 5
14 .0 12 .0 10 .0
0. 3
8. 0 0. 2
α
Q (kg/m 2 h)
0. 4
6. 0 4. 0
0. 1
2. 0 0. 0
0. 0 0
10
30
50
70
90
100
CH 3 COOH (wt %)
Fig. 5. Change of total permeation rate and separation factor with feed composition: (䊊) Q (30 ◦ C); (䊉) Q (40 ◦ C); (䊐) α (30 ◦ C); (䊏) α (40 ◦ C).
N. Alghezawi et al. / Chemical Engineering and Processing 44 (2005) 51–58
55 5.0
0.4
4.0
2
0.2 2.0 0.1
2
3.0
Q(CH3COOH) (kg/m h)x10
Q (H2O) (kg/m h)
2
0.3
1.0
0
0.0 10
30
50
70
90
CH3COOH (wt %)
Fig. 6. Effect of feed composition on permeation rate of water and acetic acid: (䊊) Q (H2 O) (30 ◦ C); (䊉) Q (H2 O) (40 ◦ C); (䊐) Q (CH3 COOH) (30 ◦ C); (䊏) Q (CH3 COOH) (40 ◦ C).
diffuse through the free volumes resulted by the increase in temperature. This situation leads to constant separation factors.
crease steadily with the increase in acetic acid content of feed. Similar results were obtained in the pervaporation separation of acetic acid–water mixtures using blended polyacrylic acid–nylon 6 membranes [22], poly(4-vinylpyridine-coacrylonitrile) membranes [11] and in the permeation and separation of aqueous alcohol solutions through PVA-ANHEMA latex membranes [5]. However, in the studies of Huang and Yeom [23,24] using chemically crosslinked PVA membranes, maximum permeation rates were reported at low acetic acid concentrations (10 wt.%). Looking at the permeation rate of individual components (Fig. 6), the permeation rate of water component decreases with the decrease in the water content of the feed solution
3.5. Effect of feed composition At 30 and 40 ◦ C the effect of feed concentration, in the concentration range of 10–90 wt.% of acetic acid solutions, on the permeation rate and the separation factor was investigated and the results were given in Fig. 5. From the figure it is clear that when acetic acid concentration in the feed solution increases the permeation rate decreases, whereas the separation factor increases. It can also be seen that the separation factors obtained at low temperatures are higher than those obtained at high operating temperatures and they in1.0
0.8
PSI
0.6
0.4
0.2
0.0 0
10
20
30
40
50
60
70
80
CH3COOH (wt%)
Fig. 7. Change of pervaporation separation index with feed composition (T = 30 ◦ C).
90
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Table 5 Comparison of the performance of the membranes based on pva for the pervaporation separation of acetic acid–water mixtures Polymer
Temperature (◦ C)
Mass % of acetic acid in the feed
Permeation rate (kg/m2 h)
Separation factor (α)
Ref. no.
PVA-g-PAAm PVA/PAA (Blend membranes) PVA/PHC PVA/PVP PVA PVA PVA-g-AAm (48%) PVA-g-AAm (93%) PVA-g-AN (52%) (PVA-2)
35 25 25 25 35 45 35 35 30
80 90 90 90 80 80 80 80 90
0.056 0.300 0.140 0.800 0.056 0.124 0.086 0.094 0.090
3.90 6.60 7.90 2.40 6.71 6.39 5.38 3.81 14.60
[45] [46] [46] [46] [37] [37] [37] [37] This study
PAAm, poly(acryl amide); PAA, poly(acrylic acid); PHC, poly(hydroxycarboxylic acid); PVP, poly(N-vinyl-2-pyrolidone); PVA, poly(vinyl alcohol); Aam, acryl amide; AN, acrylonitrile.
and the permeation rate of water is higher than that of acetic acid regardless of feed composition. These phenomena can be explained in terms of plasticizing effect of water. As the water concentration in the feed is increased, the amorphous regions of the membrane becomes more swollen; hence the flexibility of polymer chains increases the energy required for diffusive transport through the membrane decreases. This could be the reason why low separation factors were obtained at low acetic acid concentrations. Additionally in pervaporation separation follows the solution-diffusion mechanism. Therefore the molecular size of the permeating molecules becomes very important to characterize the permeation behavior [43]. It is known that acetic acid has larger molecular size (0.40 nm) than water molecules (0.28 nm). As the amount of acetic acid increases in the feed mixture it becomes difficult for acetic acid molecules to diffuse through the less swollen membrane, so separation factor increases at high acid concentrations. Pervaporation separation index values were calculated using Eq. (3) and presented in Fig. 7. As it is reflected from
the figure that PSI values of the membranes increases with the acetic acid concentration which shows that the performance of PVA-g-AN (PVA-2) membranes are good at high acetic acid concentrations For comparison purposes several results regarding the separation of acetic acid–water mixtures obtained by various authors with different type of membranes listed in Table 5. One can see that PVA-g-AN membranes have high selectivity although they have almost the same permeation rate depending on the feed composition and temperature than the membranes studied up in the table. 3.6. Effect of pressure on pervaporation Effect of the down stream pressure on the permeation rate and the separation factor was shown in Fig. 8 for 20 wt.% acetic acid solution at 30 ◦ C. Permeation rate increases with the pressure which complies well with theoretical prediction. However separation factor decreases as the down stream
0.6
4.0
3.0
2.0
α
2
Q (kg/m h)
0.4
0.2 1.0
0.0
0.0 2
4
6
8
-3
P (Pa x 10 )
Fig. 8. Effect of down stream pressure on the total permeation rate and separation factor (20 wt.% CH3 COOH; T = 30 ◦ C): (䊉) Q (䊏) α.
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pressure increases. Similar results were also reported in the literature [44].
4. Conclusions Acrylonitrile grafted poly(vinyl alcohol) membranes were found to be capable of permeating water in preference to acetic acid from aqueous acetic acid mixtures. The permeation rate of water increases whereas permeation rate of acetic acid decreases as the water content of the feed increases. It was also determined that increase in temperature increased the permeation rate without affecting the separation factor much. As the downstream pressure increased permeation rate increased whereas separation factor decreased and from the PSI values of the membranes it could be said that especially at high acetic acid concentrations membranes behaved more separable.
Acknowledgements Three of grateful to Gazi University Research Fund for the support of this study.
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