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polyvinyl pyrrolidone blend membranes
ARTICLE IN PRESS
Vacuum 82 (2008) 579–587 www.elsevier.com/locate/vacuum
Sorption, diffusion, and pervaporation characteristics of dimethylformamide/water mixtures using sodium alginate/polyvinyl pyrrolidone blend membranes Ebru Kondolot Solaka, Gu¨lsen Asmanb, Pınar C - amurluc, Oya S- anlıb, a
Gazi U¨niversitesi, Atatu¨rk Meslek Yu¨ksekokulu, Kimya Bo¨lu¨mu¨, Teknikokullar, 06500 Ankara, Turkey b Gazi U¨niversitesi, Fen Edebiyat Faku¨ltesi, Kimya Bo¨lu¨mu¨, Teknikokullar, 06500 Ankara, Turkey c Akdeniz U¨niversitesi, Fen Edebiyat Faku¨ltesi, Kimya Bo¨lu¨mu¨, 07058 Antalya, Turkey Received 9 April 2007; received in revised form 16 August 2007; accepted 20 August 2007
Abstract Separation of aqueous dimethylformamide (DMF) solutions in the concentration range of 0–100 wt% were studied using sodium alginate (NaAlg)/polyvinyl pyrrolidone (PVP) blend membranes. The NaAlg was blended in different ratios with PVP. Prepared membranes were crosslinked with CaCl2 for testing in pervaporation (PV) separation of DMF/water mixtures. Effects of feed composition (0–100 wt%), operating temperature (30–50 1C), and membrane thickness were investigated. Best results were obtained at the conditions of 75/25 NaAlg/PVP blend ratio (w/w), 40 1C temperature, 20 wt% DMF concentration, and 70 mm membrane thickness. Blending of PVP with NaAlg increased permeation flux whereas it decreased the separation factor. NaAlg/PVP membranes gave separation factors of 5.5–27 for permeation flux of 0.96–1.81 kg/m2h depending on the operating temperature and the feed mixture composition. Arrhenius plot of permeation flux data versus reciprocal of temperature exhibited linear trends. Permeation activation energy of DMF and water in the PV was calculated as 6.76 and 1.88 kcal/mol, respectively, using an Arrhenius type relationship. Sorption–diffusion properties of the NaAlg/PVP membranes were also investigated at the operating temperature and the feed composition. r 2007 Elsevier Ltd. All rights reserved. Keywords: Pervaporation; Sorption; Diffusion; Dimethylformamide; Polyvinyl pyrrolidone.
1. Introduction Pervaporation (PV) is a useful alternative method for solvent extraction. This process is most often used for the dehydration of organic solvents and the separation of binary organic mixtures [1–4]. In PV, liquid mixtures are separated using membrane and vacuum pump. The liquid mixture is placed in contact with one side of membrane and permeate is removed as vapor from the other side. The membrane acts as a selective barrier between the two phases. One of the most widely used polymers in PV is sodium alginate (NaAlg). But this polymer is not necessarily suitable for the dehydration process; because of its water solubility Corresponding author.
E-mail address: [email protected] (O. S- anlı). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.08.012
and excess hydrophilicity as a membrane material. Usually, when a highly permeable polymer material is preferred, this membrane material should be modified to have the suitable combination of permeation flux and separation factor. To improve the properties of the desired polymer is possible with different form of polymer blends. Dimethylformamide (DMF) is an important solvent; it is primarily used as a solvent in the production of polyurethane products and acrylic fibers. It is also used in the pharmaceutical and petrochemical industries, in the formulation of pesticides and in the manufacture of synthetic leathers, fibers, films, and surface coating [5,6]. In PV separation studies, many research groups have focused on the membrane development for the separation of DMF–water mixtures at different feed concentrations [7–11]. Shah et al. [7] prepared hydrophilic zeolite NaA
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membranes for the PV of alcohol water and DMF–water mixtures. They have reported that the water flux for the DMF–water system decreases rapidly with an increase in the feed DMF concentration. Aminabhavi and Naik [8] grafted poly(vinyl alcohol) (PVA) with acrylamide for the separation of water/DMF mixtures. It was found that these membranes are more selective to water than DMF. Separation factors increased with grafting, but permeation flux did not considerably change with grafting. Kurkuri and Aminabhavi [9] grafted PVA with polyacrylonitrile and used it in the separation of DMF and water mixtures in a range of 10–90 wt% in the feed at 25, 35, and 45 1C. They have reported that by the increase in the grafting percentage of the membrane, flux decreased whereas selectivity increased slightly over that of pure polyvinyl pyrrolidone (PVP) membrane. Devi et al. [10] studied PV separation of DMF/water mixtures through PVA/poly(acrylic acid) blend membranes. They reported that membrane selectivity is improved with decreasing feed water concentration. Das et al. [11] crosslinked polyurethane urea–polymethylmethacrylate (PMMA) interpenetrating network (IPN) membrane for the PV separation of DMF–water. They have reported that with an increase in PMMA content of the membrane DMF flux and separation factor values are increased due to the presence of more polar groups of PMMA. The highest DMF flux was obtained as 0.231 kg/m2h at a temperature of 60 1C for 80 wt% DMF in the feed. In recent years alginic acid based membranes and their modified forms are also used in PV dehydration of DMF solutions [12,13]. Kondolot Solak and S- anlı [12] studied separation characteristics of DMF/water mixtures through alginate membranes by PV and vapour permeation with temperature difference methods. They found that increase in the operating temperature in PV and vapour permeation method increased the permeation flux whereas it decreased the separation factor. However permeation flux decreased whereas separation factor increased as the DMF content of the feed increased in all of the methods. The highest permeation flux (1.2 kg m2 h1) was observed in PV method. Kondolot Solak and S- anlı [13] also found that increase in the operating temperature increased the permeation flux whereas the separation factor decreased on using NaAlg-g-n-vinyl-2-pyrrolidone membranes with PV method. In this study, NaAlg/PVP blend membranes were prepared in different ratios and crosslinked with calcium chloride. These membranes were used in the PV separation of DMF/water mixtures and the separation performance of the membranes as a function of temperature, feed composition, and blend ratios was studied.
2.2. Blend membrane preparation NaAlg and PVP were dissolved in water. Blend solutions were prepared by mixing these solutions in different ratios (w/w), stirred, and then casted onto rimmed round glass dishes. Solvent was evaporated at 60 1C to form the membrane. The dried membrane was crosslinked with calcium chloride (0.1 M) for 24 h. The thickness of the membranes thus prepared was 70 (710) mm. Membranes prepared in this research were used at least 10 times without any deformation during the PV process. 2.3. Swelling degree measurements The dried membranes were immersed in different concentrations of DMF water mixtures at 40 1C for 48 h. The membranes were wiped with cleansing tissue to remove the solvent mixture. These samples were dried at 60 1C until a constant weight. The swelling degree (SD) was calculated as SD ¼
ðM S M D Þ 100, MD
where MS is the mass of the swollen membrane in the feed solution and MD the mass of the dried membrane. 2.4. Pervaporation experiments Separation of DMF/water mixtures by using PV method were carried out over the full range of compositions (0–100 wt%) at temperatures varying from 30 to 50 1C by using NaAlg membranes. The membrane surface area was 16.5 cm2 and pressure was kept at 0.6 mbar with a vacuum pump (Edwards). The feed mixture was circulated between PV cell and feed tank at constant temperature and permeate was collected in liquid nitrogen traps [2,14,15] (Fig. 1). Composition of the permeate that was collected after steady state conditions were attained was analyzed with Atago DD-5 type digital refractometer. The membrane performance was expressed by separation factor a, permeation rate J, and separation index (SI).
3
2
6
2. Experimental 1
2.1. Materials NaAlg was provided from Sigma (medium viscosity). DMF, calcium chloride, and PVP were provided by Merck and used as supplied.
(1)
4
10
7
11
5 9
8
Fig. 1. Schematic diagram of the pervaporation apparatus: (1) vacuum pump, (2–4, 6) permeation traps; (5) McLeod manometer; (7) vent; (8) permeation cell; (9) constant temperature water bath; (10) peristaltic pump; and (11) temperature indicator.
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The separation factor (asep.W/DMF) was defined as asep:W=DMF ¼
PW =PDMF , F W =F DMF
(2)
where PW, PDMF, and FW, FDMF are the mass fractions (wt%) of water and DMF components in the permeate and feed, respectively. The permeation flux (rate) J was calculated as follows: J¼
W , At
(3)
where W is the mass of permeate (kg), A the membrane surface area (m2), and t the experiment time (h). The SI was calculated as SI ¼ Ja,
(4)
where J and a are the total permeation flux and separation factor, respectively. The diffusion selectivity can be calculated from the separation factor and the sorption selectivity as follows [15]: Diffusion selectivity adif:W=DMF ¼
asep:W=DMF . asorp:W=DMF
(5)
2.5. Sorption measurements The membrane was immersed into the DMF/water mixture for 48 h at 40 1C. To remove excess solvent the membrane was blotted between tissue paper. This membrane was placed into the empty PV cell, the sorbed mixture was collected in the traps. The composition of permeate was determined by Atago DD-5 type digital refractometer. The separation factor for sorption was calculated as Sorption selectivity ðasorp:W=DMF Þ ¼
Y W =Y DMF , X W =X DMF
(6)
where XDMF, XW and YDMF, YW are the mass fractions of DMF and water in the DMF solution (feed) and membrane, respectively.
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Table 1 Effect of the separation factor and permeation flux with different ratios of NaAlg/PVP NaAlg/PVP ratio (w/w)
Separation factor a
Permeation flux J (kg/m2h)
100/0 95/5 90/10 85/15 80/20 75/25
30.31 29.59 28.16 28.78 27.65 27.67
0.672 0.692 0.709 0.722 0.758 0.790
[C3H7NO]: 20 wt%, membrane thickness: 70 mm, pressure: 0.6 mbar, operating temperature: 40 1C.
3.2. Characterization of membranes Crosslinked membrane was scanned with Mattson 1000 Fourier transfer infrared spectroscopy (FTIR, Fig. 2). In the FTIR spectrum of NaAlg/PVP and NaAlg, the peak at 3000–3500 cm1 area indicates the presence of the stretching vibration of –OH band. In the FTIR spectrum of NaAlg/PVP these stretching vibrations appear as a wider band than in the spectrum of NaAlg. This peak appears at 3445 cm1 in the spectrum of PVP. The peak at 1625 cm1 in the spectrum of NaAlg is due to the stretching band of C ¼ O. The spectrum of PVP appears as a strong absorption band at 1640 cm1, due to the presence of the C ¼ C–N group. In the FTIR spectrum of NaAlg/PVP, these bands are seen together. The spectrums of PVP, NaAlg, and NaAlg/PVP appear as stretching bands of C–H group at 2964, 2946, and 2954 cm1, respectively. The morphology of the NaAlg and NaAlg/PVP membranes was observed using scanning electron microscopy (SEM, JEOL JSM-6400, Figs. 3(a,b)). It is seen from the SEM results that the NaAlg membrane surface (Fig. 3(a)) has a smoother appearance than the blend membrane. The thermal analysis was performed with differential scanning calorimeter (DSC, General V4.1C Dupont 2000) and the results are illustrated in Fig. 4. The NaAlg polymer used in this study showed a Tg of 66 1C, however this value for NaAlg/PVP has been found to be 67 1C. The increase in Tg could also be the evidence of the blending reaction between PVP and NaAlg in the membrane.
3. Results and discussion 3.3. Effect of the membrane thickness 3.1. Effect of the different blend ratios The effect of blend ratio was studied with 20 wt% DMF/water mixture in ratios of 100/0, 95/5, 90/10, 85/15, 80/20, 75/25 (NaAlg/PVP, w/w) at 40 1C. The results are presented in Table 1. As is seen from the table the permeation flux decreases, whereas the separation factor increases with the blend ratio. Blend ratio of 75/25 (NaAlg/PVP, w/w) was preferred in the rest of the study due to the acceptable flux and separation factor.
The influence of membrane thickness (l) was studied with 20 wt% DMF mixture using the NaAlg membranes of the thickness ranging from 30 to 90 mm at 40 1C and the results are presented in Fig. 5. As is seen from the figure the permeation flux decreases with membrane thickness whereas the separation factor increases. Membranes of 70 mm thickness were preferred in the rest of the study due to their acceptable flux and separation factor. Fig. 6 shows a linear relationship between the total permeation flux and the reciprocal of the membrane
ARTICLE IN PRESS E. Kondolot Solak et al. / Vacuum 82 (2008) 579–587
582 100
PVP
2964 NaAlg
Transmittance (%)
90
1640
3445 2964 NaAlg/PVP 80
1625 2954
70 4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers (cm )
Fig. 2. IR spectra of NaAlg/PVP, PVP and NaAlg membranes.
Fig. 3. (a) Scanning electron microscopic picture of NaAlg membrane and (b) scanning electron microscopic picture of NaAlg/PVP membrane.
thickness as is predicted by the solution-diffusion model (Fickian behavior). Different results concerning the effect of membrane thickness were reported in the literature [14,16,17]. Is-ıklan and S- anlı [14] studied the separation characteristics of acetic acid–water mixtures by PV using PVA membranes modified with malic acid. They have reported that as the membrane thickness increases the permeation flux decreases whereas separation factor stays almost constant below membrane thicknesses of 70 mm and then increases sharply between 70 and 100 mm membrane thickness. Alghezawi et al. [16] studied the effect of membrane thickness on the separation factor and permeation flux for 20 wt% acetic acid solutions
at 30 1C. They have reported that as the membrane thickness increases permeation flux decreases whereas separation factor increases. Koops et al. [17] investigated the PV 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 mm. 3.4. Effect of the feed composition By using the NaAlg/PVP (75/25, w/w) membrane, the PV separation of DMF/water mixtures was carried out
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583
0.5
0.0
67 °C
-0.5
218.14 °C
Heat Flow (w/g)
NaAlg/PVP
-1.0
64 °C
206.08 °C
PVP
-1.5 66 °C
NaAlg
230.08 °C
-2.0
2954 -2.5 25
75
125
175
225
Temperature (°C)
275 General V4.1C Dupont 2000
Fig. 4. Differential scanning calorimeter curves of NaAlg/PVP and NaAlg membranes.
60
1
40
0.5
20
α
J (kg /m2h)
1.5
0
0 30
40
50
60 l (μm)
70
80
90
Fig. 5. Effect of membrane thickness on the permeation rate and the separation factor.
over a range of 0–100 wt% DMF in the feed solution at 40 1C. The results obtained are presented in Fig. 7. As seen from Fig. 7 the separation factor increased while permeation flux decreased at high DMF concentrations. This phenomenon can be explained through the examination of the individual component permeation flux (Fig. 8) and the swelling measurements presented in Fig. 9. Looking at the permeation flux of each individual component in Fig. 8, the water permeation flux is too large compared with DMF permeation flux curve. As the water concentration in the feed increases, the amorphous regions of the membrane becomes more swollen; hence the flexibility of polymer chains increases
and the energy required for diffusive transport through the membrane decreases, resulting in increase in the swelling of the membrane (Fig. 9). In the literature similar results were reported concerning the effect of the feed composition on the PV performance [2,18]. Asman and S- anlı [2] investigated PV separation of acetic acid–water mixtures through PVA membranes modified with poly(acrylic acid). They observed that the separation factor decreased at low acetic acid concentrations whereas the permeation flux increased. Yeom and Lee [18] studied PV separation of water–acetic acid mixtures through PVA membranes crosslinked with glutaraldehyde. They found that permeation flux decreased while
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increasing the acetic acid content, whereas separation factor increased. Fig. 10 shows the relationship between the sorption selectivity and the diffusion selectivity for NaAlg/PVP membrane at different DMF concentrations. The sorption selectivity and the diffusion selectivity increased with increasing DMF concentration in the feed solution. It was found that the sorption selectivity was the dominant factor for the separation of DMF/water mixtures. Chen et al. [19] investigated the sorption–diffusion selectivity as a function of PV properties for the polysulfone membrane in the PV separation of water/ ethanol mixture and reported that sorption selectivity increased and diffusion selectivity decreased with increasing ethanol composition in the feed. Toti et al. [20] prepared Na-Alg and guar gum-grafted polyacrylamide membranes in different ratios and used in the PV separation of acetic acid–water mixtures. They also
J (kg/m2h)
1.5
1
0.5 1.0
1.5
2.0
2.5
3.0
3.5
1/lx102 (μm)-1
Fig. 6. Permeation rate as a function of the reciprocal of the membrane thickness.
2
30
1.5
1
α
J (kg/m2h)
20
10 0.5
0
0 0
20
40
60
80
90
100
C3H7NO in feed (wt %)
Fig. 7. Effect of the feed composition in PV. The permeation conditions; membrane thickness: 70 mm, operating temperature: 40 1C, pressure: 0.6 mbar.
Permeation rate of water (kg/m2h)
0.4 1.5 0.3 1.0 0.2 0.5 0.1
0 0
20
40
60
80
90
Permeation rate of DMF (kg/m2h)
2.0
0.5
0.0 100
C3H7NO in feed (wt %)
Fig. 8. Variation of the permeation rate of water and dimethylformamide with the feed composition.
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The temperature dependence of permeation flux can be expressed by an Arrhenius type relationship:
80
J p ¼ Ap expðE p =RT Þ,
60 Swelling degree
585
40
20
0 0
20
40
60
80
100
C3H7NO in feed (wt %)
Fig. 9. Change in the swelling degree with the feed composition.
where Ap is the pre-exponential factor and Ep the apparent activation energy of permeation. Fig. 12 shows the Arrhenius plot of individual permeation flux which is used to calculate the permeation activation energy. The permeation activation energy of water and DMF are calculated to be 1.88 and 6.76 kcal/mol, respectively. The solubility of DMF in the membrane was smaller than in water, leading to larger heat of solution (DH) and DMF having larger permeating molecule size can have higher diffusion activation energy (Ed) than the case of water. Thus Ep of DMF is calculated to be larger than that of water for 20 wt% DMF (Eq. (8)). Ep ¼ DH þ Ed:
60
α
40
20
0 0
20
40
60
80
100
C3H7NO in feed (wt %)
Fig. 10. Relationship between the selectivities at different feed compositions. Separation selectivity (~), sorption selectivity (’), and diffusion selectivity: (m).
found that sorption selectivity increased with an increase in the amount of acetic acid in the feed mixture. 3.5. Effect of the operating temperature Fig. 11 presents the effect of the operating temperature on the permeation flux and the separation factor of the NaAlg/PVP membrane for a 20 wt% DMF solution. As expected, when the temperature was increased, the permeation flux increased, but the separation factor decreased. These results can be explained with the freevolume theory. According to this theory, the thermal motion of polymer chains in the amorphous regions randomly produces free volume. As the temperature increases the frequency and the amplitude of chain jumping increase, so the free volume becomes larger, both DMF and water molecules pass through the membrane, resulting in increased total permeation flux and decreased separation factor.
(7)
(8)
The relationship between the sorption selectivity and the diffusion selectivity for NaAlg/PVP membrane at various permeation temperatures by PV is shown in Fig. 13. The sorption selectivity and the diffusion selectivity of NaAlg/ PVP membrane decreased with an increase in the permeation temperature. It is well known that the decrease of the sorption selectivity and the diffusion selectivity was due to the increase of membrane swelling with increasing feed temperature. Inui et al. [15] have investigated the relationship among the separation factor, the sorption selectivity, and the diffusion selectivity for a benzene/cyclohexane (Bz/Chx) mixture of 10 wt% benzene through the nematic and smectic side-chain liquid-crystalline polymer (n- and s-LCP) membrane in the liquid-crystalline state at various permeation temperatures by PV. They have reported that the sorption selectivity of the n-LCP and s-LCP membranes was lowered with an increase in the permeation temperature. On the other hand, though the diffusion selectivity of the n-LCP membrane was considerably lowered with an increase in the permeation temperature that of the s-LCP membrane was almost constant. In order to estimate the performance of the blend membranes, SI values were calculated by Eq. (6) and are presented in Fig. 14. As is seen from the figure the SI’s of 75/25 blend membrane are higher than that of other membranes, especially at low DMF concentrations. Results of the studies reported in the literature on the PV performance of the NaAlg membranes are listed in Table 2 for comparison purposes. As is seen from the table permeation rates of NaAlg/PVP membranes are higher than those of NaAlg membrane without sacrifying the selectivity so much. Also NaAlg /PVP membranes have high selectivity compared to NaAlg-g-NVP membranes. As a result it can be said that NaAlg/PVP blend membranes have good separation performance and are more suitable than the NaAlg-g-NVP membranes since the preparation method is cheap and easy compared to NaAlg-g-NVP membranes.
ARTICLE IN PRESS E. Kondolot Solak et al. / Vacuum 82 (2008) 579–587
586 1
25
0.9
0.8 15
α
J (kg/m2h)
20
0.7 10 0.6
0.5
5 30
35
40
45
50
Temperature (°C)
Fig. 11. Effect of the operating temperature in PV. The permeation conditions; [C3H7NO]: 20 wt%, membrane thickness: 70 mm, pressure: 0.6 mbar.
0
0
-0.1
-1
-0.2 -0.3 -3 -0.4
lnJ (H2O)
lnJ (DMF)
-2
-4 -0.5 -5
-0.6 -0.7
-6 3.3
3.25
3.19
3.14
3.1
1/Tx103(K-1)
Fig. 12. Arrhenius plot of ln(dimethylformamide permeation rate) (~) and ln(water permeation rate) (’) with 1/T for NaAlg/PVP membrane in PV.
20 25 Separation index (kg/m2h)
α
15
10
5
20
15
10
5
0 30
35
40
45
50
°
Temperature ( C) Fig. 13. Relationship between the selectivities at various permeation temperatures. Separation selectivity (~), sorption selectivity (’), and diffusion selectivity: (m).
0
20
40
60
80
100
C3H4NO in feed (wt%)
Fig. 14. Change of separation index in PV for blend membranes. Blend ratio (NaAlg/PVP), 75/25 (K), 80/20 (’), 85/15 ( ), 90/10 (m), 95/15 (), and 100/0 (~).
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Table 2 Comparison of the performance of the membranes for separation of dimethylformamide–water mixtures Membrane
Feed composition % (w/w)
Temperature (1C)
Permeation flux J (kg/m2h)
Separation factor a
Separation method
Ref.
NaAlg NaAlg/PVP NaAlg-g-NVP
0–100 0–100 0–100
30–50 30–50 40
0.264–1.417 0.960–1.810 0.963–2.046
13.3–31.3 5.5–27 5.6–15.4
PV PV PV
[12] Present study [13]
4. Conclusion In this study the NaAlg/PVP membranes were prepared and their PV performance was studied for DMF–water mixtures. It was found that an increase in the operating temperature increased the permeation flux whereas it decreased the separation factor. As the membrane thickness increased permeation flux decreased whereas separation factor increased. Sorption and diffusion selectivity slightly increased with increasing DMF concentration in the feed. Permeation flux decreased whereas separation factor increased as the DMF content of the feed increased. Blending of PVP with the NaAlg membranes increased the permeation flux compared to the NaAlg membranes and increased the separation factor compared to the NaAlg-gNVP membranes. The highest separation factor of 27 and permeation flux of 1.81 kg/m2h were obtained in this study. Acknowledgment We are grateful to Gazi and Akdeniz Universities Research Fund for the support of this study. References [1] Ray SK, Sawant SB, Pangarkar VG. Development of methanol selective membranes for separation of methanol–methyl tertiary butyl ether mixtures by pervaporation. J Appl Polym Sci 1999;74:2645–59. [2] Asman G, S- anlı O. Characteristics of permeation and separation for acetic acid–water mixtures through poly(vinyl alcohol) membranes modified with poly(acrylic Acid). Sep Sci Technol 2003;38:1963–80. [3] Ray SK, Sawant SB, Joshi JB, Pangarkar VG. Methanol selective membranes for separation of methanol–ethylene glycol mixtures by pervaporation. J Membr Sci 1999;154:1–13. [4] Johnson T, Thomas S. Pervaporation of acetone–chlorinated hydrocarbon mixtures through polymer blend membranes of natural rubber and epoxidized natural rubber. J Appl Polym Sci 1999;71: 2365–79. [5] Howard PH, editor. Solvents 2. In: Handbook of environmental fate and exposure data for organic chemicals, vol. IV. Chelsea, MI: Lewis Publisher Inc.; 1993. [6] Gescher A. Metabolism of N,N-dimethylformamide: key to the understanding of its toxicity. Chem Res Toxicol 1993;6(3):245–51.
[7] Shah D, Kissick K, Ghorpade A, Hannah R, Bhattacharyya D. Pervaporation of alcohol–water and dimethylformamide–water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results. J Membr Sci 2000;179:185–205. [8] Aminabhavi TM, Naik HG. Pervaporation separation of water/ dimethylformamide mixtures using poly(vinyl alcohol)-g-polyarylamide copolymeric membranes. J Appl Polym Sci 2002;83:273–82. [9] Kurkuri MD, Aminabhavi TM. Polyacrylonitrile-g-poly(vinyl alcohol) membranes for the pervaporation separation of dimethyl formamide and water mixtures. J Appl Polym Sci 2004;91:4091–7. [10] Devi DA, Smitha B, Sridhar S, Aminabhavi TM. Pervaporation separation of dimethylformamide/water mixtures through poly(vinyl alcohol)/poly(acrylic acid) blend membranes. Sep Purif Technol 2006;51:104–11. [11] Das S, Banthia AK, Adhikari B. Pervaporation separation of DMF from water using a crosslinked polyurethane urea-PMMA IPN membrane. Desalination 2006;197:106–16. [12] Kondolot Solak E, S- anlı O. Separation characteristics of dimethylformamide/water mixtures through alginate membranes by pervaporation, vapor permeation and vapor permeation with temperature difference methods. Sep Sci Technol 2006;41:627–46. [13] Kondolot Solak E, S- anlı O. Separation characteristics of dimethylformamide/water mixtures using sodium alginate-g-N–vinyl–2-pyrrolidone membranes by pervaporation method. Chem Eng Process, in press. [14] Is-ıklan N, S- anlı O. Separation characteristics of acetic acid–water mixtures by pervaporation using poly(vinyl alcohol) membranes modified with malic acid. Chem Eng Process 2005;44:1019–27. [15] Inui K, Okazaki K, Miyata T, Uragami T. Effect of mesogenic groups on characteristics of permeation and separation for benzene/ cyclohexane mixtures of side-chain liquid-crystalline polymer membranes. J Membr Sci 1998;143:93–104. [16] Alghezawi N, S- anlı O, Aras L, Asman G. Separation of acetic acid–water mixtures through acrylonitrile grafted poly(vinyl alcohol) membranes by pervaporation. Chem Eng Process 2005;44:51–8. [17] Koops GH, Nolten JAM, Mulder MHV. Selectivity as a function of membrane thickness: gas separation and pervaporation. J Appl Polym Sci 1994;53:1639–51. [18] Yeom CK, Lee KH. Pervaporation separation of water–acetic acid mixtures through poly(vinyl alcohol) membranes crosslinked with glutaraldehyde. J Membr Sci 1996;109:257–65. [19] Chen SH, Liou RM, Hsu CS, Chang DJ, Yu KC, Chang CY. Pervaporation separation water/ethanol mixture through lithiated polysulfone membrane. J Membr Sci 2001;193:59–67. [20] Toti US, Kariduraganavar MY, Soppimath KS, Aminabhavi TM. Sorption, diffusion, and pervaporation separation of water–acetic acid mixtures through the blend membranes of sodium alginate and guar gum-grafted-polyacrylamide. J Appl Polym Sci 2002;83:259–72.
Vacuum 82 (2008) 579–587 www.elsevier.com/locate/vacuum
Sorption, diffusion, and pervaporation characteristics of dimethylformamide/water mixtures using sodium alginate/polyvinyl pyrrolidone blend membranes Ebru Kondolot Solaka, Gu¨lsen Asmanb, Pınar C - amurluc, Oya S- anlıb, a
Gazi U¨niversitesi, Atatu¨rk Meslek Yu¨ksekokulu, Kimya Bo¨lu¨mu¨, Teknikokullar, 06500 Ankara, Turkey b Gazi U¨niversitesi, Fen Edebiyat Faku¨ltesi, Kimya Bo¨lu¨mu¨, Teknikokullar, 06500 Ankara, Turkey c Akdeniz U¨niversitesi, Fen Edebiyat Faku¨ltesi, Kimya Bo¨lu¨mu¨, 07058 Antalya, Turkey Received 9 April 2007; received in revised form 16 August 2007; accepted 20 August 2007
Abstract Separation of aqueous dimethylformamide (DMF) solutions in the concentration range of 0–100 wt% were studied using sodium alginate (NaAlg)/polyvinyl pyrrolidone (PVP) blend membranes. The NaAlg was blended in different ratios with PVP. Prepared membranes were crosslinked with CaCl2 for testing in pervaporation (PV) separation of DMF/water mixtures. Effects of feed composition (0–100 wt%), operating temperature (30–50 1C), and membrane thickness were investigated. Best results were obtained at the conditions of 75/25 NaAlg/PVP blend ratio (w/w), 40 1C temperature, 20 wt% DMF concentration, and 70 mm membrane thickness. Blending of PVP with NaAlg increased permeation flux whereas it decreased the separation factor. NaAlg/PVP membranes gave separation factors of 5.5–27 for permeation flux of 0.96–1.81 kg/m2h depending on the operating temperature and the feed mixture composition. Arrhenius plot of permeation flux data versus reciprocal of temperature exhibited linear trends. Permeation activation energy of DMF and water in the PV was calculated as 6.76 and 1.88 kcal/mol, respectively, using an Arrhenius type relationship. Sorption–diffusion properties of the NaAlg/PVP membranes were also investigated at the operating temperature and the feed composition. r 2007 Elsevier Ltd. All rights reserved. Keywords: Pervaporation; Sorption; Diffusion; Dimethylformamide; Polyvinyl pyrrolidone.
1. Introduction Pervaporation (PV) is a useful alternative method for solvent extraction. This process is most often used for the dehydration of organic solvents and the separation of binary organic mixtures [1–4]. In PV, liquid mixtures are separated using membrane and vacuum pump. The liquid mixture is placed in contact with one side of membrane and permeate is removed as vapor from the other side. The membrane acts as a selective barrier between the two phases. One of the most widely used polymers in PV is sodium alginate (NaAlg). But this polymer is not necessarily suitable for the dehydration process; because of its water solubility Corresponding author.
E-mail address: [email protected] (O. S- anlı). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.08.012
and excess hydrophilicity as a membrane material. Usually, when a highly permeable polymer material is preferred, this membrane material should be modified to have the suitable combination of permeation flux and separation factor. To improve the properties of the desired polymer is possible with different form of polymer blends. Dimethylformamide (DMF) is an important solvent; it is primarily used as a solvent in the production of polyurethane products and acrylic fibers. It is also used in the pharmaceutical and petrochemical industries, in the formulation of pesticides and in the manufacture of synthetic leathers, fibers, films, and surface coating [5,6]. In PV separation studies, many research groups have focused on the membrane development for the separation of DMF–water mixtures at different feed concentrations [7–11]. Shah et al. [7] prepared hydrophilic zeolite NaA
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membranes for the PV of alcohol water and DMF–water mixtures. They have reported that the water flux for the DMF–water system decreases rapidly with an increase in the feed DMF concentration. Aminabhavi and Naik [8] grafted poly(vinyl alcohol) (PVA) with acrylamide for the separation of water/DMF mixtures. It was found that these membranes are more selective to water than DMF. Separation factors increased with grafting, but permeation flux did not considerably change with grafting. Kurkuri and Aminabhavi [9] grafted PVA with polyacrylonitrile and used it in the separation of DMF and water mixtures in a range of 10–90 wt% in the feed at 25, 35, and 45 1C. They have reported that by the increase in the grafting percentage of the membrane, flux decreased whereas selectivity increased slightly over that of pure polyvinyl pyrrolidone (PVP) membrane. Devi et al. [10] studied PV separation of DMF/water mixtures through PVA/poly(acrylic acid) blend membranes. They reported that membrane selectivity is improved with decreasing feed water concentration. Das et al. [11] crosslinked polyurethane urea–polymethylmethacrylate (PMMA) interpenetrating network (IPN) membrane for the PV separation of DMF–water. They have reported that with an increase in PMMA content of the membrane DMF flux and separation factor values are increased due to the presence of more polar groups of PMMA. The highest DMF flux was obtained as 0.231 kg/m2h at a temperature of 60 1C for 80 wt% DMF in the feed. In recent years alginic acid based membranes and their modified forms are also used in PV dehydration of DMF solutions [12,13]. Kondolot Solak and S- anlı [12] studied separation characteristics of DMF/water mixtures through alginate membranes by PV and vapour permeation with temperature difference methods. They found that increase in the operating temperature in PV and vapour permeation method increased the permeation flux whereas it decreased the separation factor. However permeation flux decreased whereas separation factor increased as the DMF content of the feed increased in all of the methods. The highest permeation flux (1.2 kg m2 h1) was observed in PV method. Kondolot Solak and S- anlı [13] also found that increase in the operating temperature increased the permeation flux whereas the separation factor decreased on using NaAlg-g-n-vinyl-2-pyrrolidone membranes with PV method. In this study, NaAlg/PVP blend membranes were prepared in different ratios and crosslinked with calcium chloride. These membranes were used in the PV separation of DMF/water mixtures and the separation performance of the membranes as a function of temperature, feed composition, and blend ratios was studied.
2.2. Blend membrane preparation NaAlg and PVP were dissolved in water. Blend solutions were prepared by mixing these solutions in different ratios (w/w), stirred, and then casted onto rimmed round glass dishes. Solvent was evaporated at 60 1C to form the membrane. The dried membrane was crosslinked with calcium chloride (0.1 M) for 24 h. The thickness of the membranes thus prepared was 70 (710) mm. Membranes prepared in this research were used at least 10 times without any deformation during the PV process. 2.3. Swelling degree measurements The dried membranes were immersed in different concentrations of DMF water mixtures at 40 1C for 48 h. The membranes were wiped with cleansing tissue to remove the solvent mixture. These samples were dried at 60 1C until a constant weight. The swelling degree (SD) was calculated as SD ¼
ðM S M D Þ 100, MD
where MS is the mass of the swollen membrane in the feed solution and MD the mass of the dried membrane. 2.4. Pervaporation experiments Separation of DMF/water mixtures by using PV method were carried out over the full range of compositions (0–100 wt%) at temperatures varying from 30 to 50 1C by using NaAlg membranes. The membrane surface area was 16.5 cm2 and pressure was kept at 0.6 mbar with a vacuum pump (Edwards). The feed mixture was circulated between PV cell and feed tank at constant temperature and permeate was collected in liquid nitrogen traps [2,14,15] (Fig. 1). Composition of the permeate that was collected after steady state conditions were attained was analyzed with Atago DD-5 type digital refractometer. The membrane performance was expressed by separation factor a, permeation rate J, and separation index (SI).
3
2
6
2. Experimental 1
2.1. Materials NaAlg was provided from Sigma (medium viscosity). DMF, calcium chloride, and PVP were provided by Merck and used as supplied.
(1)
4
10
7
11
5 9
8
Fig. 1. Schematic diagram of the pervaporation apparatus: (1) vacuum pump, (2–4, 6) permeation traps; (5) McLeod manometer; (7) vent; (8) permeation cell; (9) constant temperature water bath; (10) peristaltic pump; and (11) temperature indicator.
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The separation factor (asep.W/DMF) was defined as asep:W=DMF ¼
PW =PDMF , F W =F DMF
(2)
where PW, PDMF, and FW, FDMF are the mass fractions (wt%) of water and DMF components in the permeate and feed, respectively. The permeation flux (rate) J was calculated as follows: J¼
W , At
(3)
where W is the mass of permeate (kg), A the membrane surface area (m2), and t the experiment time (h). The SI was calculated as SI ¼ Ja,
(4)
where J and a are the total permeation flux and separation factor, respectively. The diffusion selectivity can be calculated from the separation factor and the sorption selectivity as follows [15]: Diffusion selectivity adif:W=DMF ¼
asep:W=DMF . asorp:W=DMF
(5)
2.5. Sorption measurements The membrane was immersed into the DMF/water mixture for 48 h at 40 1C. To remove excess solvent the membrane was blotted between tissue paper. This membrane was placed into the empty PV cell, the sorbed mixture was collected in the traps. The composition of permeate was determined by Atago DD-5 type digital refractometer. The separation factor for sorption was calculated as Sorption selectivity ðasorp:W=DMF Þ ¼
Y W =Y DMF , X W =X DMF
(6)
where XDMF, XW and YDMF, YW are the mass fractions of DMF and water in the DMF solution (feed) and membrane, respectively.
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Table 1 Effect of the separation factor and permeation flux with different ratios of NaAlg/PVP NaAlg/PVP ratio (w/w)
Separation factor a
Permeation flux J (kg/m2h)
100/0 95/5 90/10 85/15 80/20 75/25
30.31 29.59 28.16 28.78 27.65 27.67
0.672 0.692 0.709 0.722 0.758 0.790
[C3H7NO]: 20 wt%, membrane thickness: 70 mm, pressure: 0.6 mbar, operating temperature: 40 1C.
3.2. Characterization of membranes Crosslinked membrane was scanned with Mattson 1000 Fourier transfer infrared spectroscopy (FTIR, Fig. 2). In the FTIR spectrum of NaAlg/PVP and NaAlg, the peak at 3000–3500 cm1 area indicates the presence of the stretching vibration of –OH band. In the FTIR spectrum of NaAlg/PVP these stretching vibrations appear as a wider band than in the spectrum of NaAlg. This peak appears at 3445 cm1 in the spectrum of PVP. The peak at 1625 cm1 in the spectrum of NaAlg is due to the stretching band of C ¼ O. The spectrum of PVP appears as a strong absorption band at 1640 cm1, due to the presence of the C ¼ C–N group. In the FTIR spectrum of NaAlg/PVP, these bands are seen together. The spectrums of PVP, NaAlg, and NaAlg/PVP appear as stretching bands of C–H group at 2964, 2946, and 2954 cm1, respectively. The morphology of the NaAlg and NaAlg/PVP membranes was observed using scanning electron microscopy (SEM, JEOL JSM-6400, Figs. 3(a,b)). It is seen from the SEM results that the NaAlg membrane surface (Fig. 3(a)) has a smoother appearance than the blend membrane. The thermal analysis was performed with differential scanning calorimeter (DSC, General V4.1C Dupont 2000) and the results are illustrated in Fig. 4. The NaAlg polymer used in this study showed a Tg of 66 1C, however this value for NaAlg/PVP has been found to be 67 1C. The increase in Tg could also be the evidence of the blending reaction between PVP and NaAlg in the membrane.
3. Results and discussion 3.3. Effect of the membrane thickness 3.1. Effect of the different blend ratios The effect of blend ratio was studied with 20 wt% DMF/water mixture in ratios of 100/0, 95/5, 90/10, 85/15, 80/20, 75/25 (NaAlg/PVP, w/w) at 40 1C. The results are presented in Table 1. As is seen from the table the permeation flux decreases, whereas the separation factor increases with the blend ratio. Blend ratio of 75/25 (NaAlg/PVP, w/w) was preferred in the rest of the study due to the acceptable flux and separation factor.
The influence of membrane thickness (l) was studied with 20 wt% DMF mixture using the NaAlg membranes of the thickness ranging from 30 to 90 mm at 40 1C and the results are presented in Fig. 5. As is seen from the figure the permeation flux decreases with membrane thickness whereas the separation factor increases. Membranes of 70 mm thickness were preferred in the rest of the study due to their acceptable flux and separation factor. Fig. 6 shows a linear relationship between the total permeation flux and the reciprocal of the membrane
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582 100
PVP
2964 NaAlg
Transmittance (%)
90
1640
3445 2964 NaAlg/PVP 80
1625 2954
70 4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers (cm )
Fig. 2. IR spectra of NaAlg/PVP, PVP and NaAlg membranes.
Fig. 3. (a) Scanning electron microscopic picture of NaAlg membrane and (b) scanning electron microscopic picture of NaAlg/PVP membrane.
thickness as is predicted by the solution-diffusion model (Fickian behavior). Different results concerning the effect of membrane thickness were reported in the literature [14,16,17]. Is-ıklan and S- anlı [14] studied the separation characteristics of acetic acid–water mixtures by PV using PVA membranes modified with malic acid. They have reported that as the membrane thickness increases the permeation flux decreases whereas separation factor stays almost constant below membrane thicknesses of 70 mm and then increases sharply between 70 and 100 mm membrane thickness. Alghezawi et al. [16] studied the effect of membrane thickness on the separation factor and permeation flux for 20 wt% acetic acid solutions
at 30 1C. They have reported that as the membrane thickness increases permeation flux decreases whereas separation factor increases. Koops et al. [17] investigated the PV 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 mm. 3.4. Effect of the feed composition By using the NaAlg/PVP (75/25, w/w) membrane, the PV separation of DMF/water mixtures was carried out
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583
0.5
0.0
67 °C
-0.5
218.14 °C
Heat Flow (w/g)
NaAlg/PVP
-1.0
64 °C
206.08 °C
PVP
-1.5 66 °C
NaAlg
230.08 °C
-2.0
2954 -2.5 25
75
125
175
225
Temperature (°C)
275 General V4.1C Dupont 2000
Fig. 4. Differential scanning calorimeter curves of NaAlg/PVP and NaAlg membranes.
60
1
40
0.5
20
α
J (kg /m2h)
1.5
0
0 30
40
50
60 l (μm)
70
80
90
Fig. 5. Effect of membrane thickness on the permeation rate and the separation factor.
over a range of 0–100 wt% DMF in the feed solution at 40 1C. The results obtained are presented in Fig. 7. As seen from Fig. 7 the separation factor increased while permeation flux decreased at high DMF concentrations. This phenomenon can be explained through the examination of the individual component permeation flux (Fig. 8) and the swelling measurements presented in Fig. 9. Looking at the permeation flux of each individual component in Fig. 8, the water permeation flux is too large compared with DMF permeation flux curve. As the water concentration in the feed increases, the amorphous regions of the membrane becomes more swollen; hence the flexibility of polymer chains increases
and the energy required for diffusive transport through the membrane decreases, resulting in increase in the swelling of the membrane (Fig. 9). In the literature similar results were reported concerning the effect of the feed composition on the PV performance [2,18]. Asman and S- anlı [2] investigated PV separation of acetic acid–water mixtures through PVA membranes modified with poly(acrylic acid). They observed that the separation factor decreased at low acetic acid concentrations whereas the permeation flux increased. Yeom and Lee [18] studied PV separation of water–acetic acid mixtures through PVA membranes crosslinked with glutaraldehyde. They found that permeation flux decreased while
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increasing the acetic acid content, whereas separation factor increased. Fig. 10 shows the relationship between the sorption selectivity and the diffusion selectivity for NaAlg/PVP membrane at different DMF concentrations. The sorption selectivity and the diffusion selectivity increased with increasing DMF concentration in the feed solution. It was found that the sorption selectivity was the dominant factor for the separation of DMF/water mixtures. Chen et al. [19] investigated the sorption–diffusion selectivity as a function of PV properties for the polysulfone membrane in the PV separation of water/ ethanol mixture and reported that sorption selectivity increased and diffusion selectivity decreased with increasing ethanol composition in the feed. Toti et al. [20] prepared Na-Alg and guar gum-grafted polyacrylamide membranes in different ratios and used in the PV separation of acetic acid–water mixtures. They also
J (kg/m2h)
1.5
1
0.5 1.0
1.5
2.0
2.5
3.0
3.5
1/lx102 (μm)-1
Fig. 6. Permeation rate as a function of the reciprocal of the membrane thickness.
2
30
1.5
1
α
J (kg/m2h)
20
10 0.5
0
0 0
20
40
60
80
90
100
C3H7NO in feed (wt %)
Fig. 7. Effect of the feed composition in PV. The permeation conditions; membrane thickness: 70 mm, operating temperature: 40 1C, pressure: 0.6 mbar.
Permeation rate of water (kg/m2h)
0.4 1.5 0.3 1.0 0.2 0.5 0.1
0 0
20
40
60
80
90
Permeation rate of DMF (kg/m2h)
2.0
0.5
0.0 100
C3H7NO in feed (wt %)
Fig. 8. Variation of the permeation rate of water and dimethylformamide with the feed composition.
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The temperature dependence of permeation flux can be expressed by an Arrhenius type relationship:
80
J p ¼ Ap expðE p =RT Þ,
60 Swelling degree
585
40
20
0 0
20
40
60
80
100
C3H7NO in feed (wt %)
Fig. 9. Change in the swelling degree with the feed composition.
where Ap is the pre-exponential factor and Ep the apparent activation energy of permeation. Fig. 12 shows the Arrhenius plot of individual permeation flux which is used to calculate the permeation activation energy. The permeation activation energy of water and DMF are calculated to be 1.88 and 6.76 kcal/mol, respectively. The solubility of DMF in the membrane was smaller than in water, leading to larger heat of solution (DH) and DMF having larger permeating molecule size can have higher diffusion activation energy (Ed) than the case of water. Thus Ep of DMF is calculated to be larger than that of water for 20 wt% DMF (Eq. (8)). Ep ¼ DH þ Ed:
60
α
40
20
0 0
20
40
60
80
100
C3H7NO in feed (wt %)
Fig. 10. Relationship between the selectivities at different feed compositions. Separation selectivity (~), sorption selectivity (’), and diffusion selectivity: (m).
found that sorption selectivity increased with an increase in the amount of acetic acid in the feed mixture. 3.5. Effect of the operating temperature Fig. 11 presents the effect of the operating temperature on the permeation flux and the separation factor of the NaAlg/PVP membrane for a 20 wt% DMF solution. As expected, when the temperature was increased, the permeation flux increased, but the separation factor decreased. These results can be explained with the freevolume theory. According to this theory, the thermal motion of polymer chains in the amorphous regions randomly produces free volume. As the temperature increases the frequency and the amplitude of chain jumping increase, so the free volume becomes larger, both DMF and water molecules pass through the membrane, resulting in increased total permeation flux and decreased separation factor.
(7)
(8)
The relationship between the sorption selectivity and the diffusion selectivity for NaAlg/PVP membrane at various permeation temperatures by PV is shown in Fig. 13. The sorption selectivity and the diffusion selectivity of NaAlg/ PVP membrane decreased with an increase in the permeation temperature. It is well known that the decrease of the sorption selectivity and the diffusion selectivity was due to the increase of membrane swelling with increasing feed temperature. Inui et al. [15] have investigated the relationship among the separation factor, the sorption selectivity, and the diffusion selectivity for a benzene/cyclohexane (Bz/Chx) mixture of 10 wt% benzene through the nematic and smectic side-chain liquid-crystalline polymer (n- and s-LCP) membrane in the liquid-crystalline state at various permeation temperatures by PV. They have reported that the sorption selectivity of the n-LCP and s-LCP membranes was lowered with an increase in the permeation temperature. On the other hand, though the diffusion selectivity of the n-LCP membrane was considerably lowered with an increase in the permeation temperature that of the s-LCP membrane was almost constant. In order to estimate the performance of the blend membranes, SI values were calculated by Eq. (6) and are presented in Fig. 14. As is seen from the figure the SI’s of 75/25 blend membrane are higher than that of other membranes, especially at low DMF concentrations. Results of the studies reported in the literature on the PV performance of the NaAlg membranes are listed in Table 2 for comparison purposes. As is seen from the table permeation rates of NaAlg/PVP membranes are higher than those of NaAlg membrane without sacrifying the selectivity so much. Also NaAlg /PVP membranes have high selectivity compared to NaAlg-g-NVP membranes. As a result it can be said that NaAlg/PVP blend membranes have good separation performance and are more suitable than the NaAlg-g-NVP membranes since the preparation method is cheap and easy compared to NaAlg-g-NVP membranes.
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586 1
25
0.9
0.8 15
α
J (kg/m2h)
20
0.7 10 0.6
0.5
5 30
35
40
45
50
Temperature (°C)
Fig. 11. Effect of the operating temperature in PV. The permeation conditions; [C3H7NO]: 20 wt%, membrane thickness: 70 mm, pressure: 0.6 mbar.
0
0
-0.1
-1
-0.2 -0.3 -3 -0.4
lnJ (H2O)
lnJ (DMF)
-2
-4 -0.5 -5
-0.6 -0.7
-6 3.3
3.25
3.19
3.14
3.1
1/Tx103(K-1)
Fig. 12. Arrhenius plot of ln(dimethylformamide permeation rate) (~) and ln(water permeation rate) (’) with 1/T for NaAlg/PVP membrane in PV.
20 25 Separation index (kg/m2h)
α
15
10
5
20
15
10
5
0 30
35
40
45
50
°
Temperature ( C) Fig. 13. Relationship between the selectivities at various permeation temperatures. Separation selectivity (~), sorption selectivity (’), and diffusion selectivity: (m).
0
20
40
60
80
100
C3H4NO in feed (wt%)
Fig. 14. Change of separation index in PV for blend membranes. Blend ratio (NaAlg/PVP), 75/25 (K), 80/20 (’), 85/15 ( ), 90/10 (m), 95/15 (), and 100/0 (~).
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Table 2 Comparison of the performance of the membranes for separation of dimethylformamide–water mixtures Membrane
Feed composition % (w/w)
Temperature (1C)
Permeation flux J (kg/m2h)
Separation factor a
Separation method
Ref.
NaAlg NaAlg/PVP NaAlg-g-NVP
0–100 0–100 0–100
30–50 30–50 40
0.264–1.417 0.960–1.810 0.963–2.046
13.3–31.3 5.5–27 5.6–15.4
PV PV PV
[12] Present study [13]
4. Conclusion In this study the NaAlg/PVP membranes were prepared and their PV performance was studied for DMF–water mixtures. It was found that an increase in the operating temperature increased the permeation flux whereas it decreased the separation factor. As the membrane thickness increased permeation flux decreased whereas separation factor increased. Sorption and diffusion selectivity slightly increased with increasing DMF concentration in the feed. Permeation flux decreased whereas separation factor increased as the DMF content of the feed increased. Blending of PVP with the NaAlg membranes increased the permeation flux compared to the NaAlg membranes and increased the separation factor compared to the NaAlg-gNVP membranes. The highest separation factor of 27 and permeation flux of 1.81 kg/m2h were obtained in this study. Acknowledgment We are grateful to Gazi and Akdeniz Universities Research Fund for the support of this study. References [1] Ray SK, Sawant SB, Pangarkar VG. Development of methanol selective membranes for separation of methanol–methyl tertiary butyl ether mixtures by pervaporation. J Appl Polym Sci 1999;74:2645–59. [2] Asman G, S- anlı O. Characteristics of permeation and separation for acetic acid–water mixtures through poly(vinyl alcohol) membranes modified with poly(acrylic Acid). Sep Sci Technol 2003;38:1963–80. [3] Ray SK, Sawant SB, Joshi JB, Pangarkar VG. Methanol selective membranes for separation of methanol–ethylene glycol mixtures by pervaporation. J Membr Sci 1999;154:1–13. [4] Johnson T, Thomas S. Pervaporation of acetone–chlorinated hydrocarbon mixtures through polymer blend membranes of natural rubber and epoxidized natural rubber. J Appl Polym Sci 1999;71: 2365–79. [5] Howard PH, editor. Solvents 2. In: Handbook of environmental fate and exposure data for organic chemicals, vol. IV. Chelsea, MI: Lewis Publisher Inc.; 1993. [6] Gescher A. Metabolism of N,N-dimethylformamide: key to the understanding of its toxicity. Chem Res Toxicol 1993;6(3):245–51.
[7] Shah D, Kissick K, Ghorpade A, Hannah R, Bhattacharyya D. Pervaporation of alcohol–water and dimethylformamide–water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results. J Membr Sci 2000;179:185–205. [8] Aminabhavi TM, Naik HG. Pervaporation separation of water/ dimethylformamide mixtures using poly(vinyl alcohol)-g-polyarylamide copolymeric membranes. J Appl Polym Sci 2002;83:273–82. [9] Kurkuri MD, Aminabhavi TM. Polyacrylonitrile-g-poly(vinyl alcohol) membranes for the pervaporation separation of dimethyl formamide and water mixtures. J Appl Polym Sci 2004;91:4091–7. [10] Devi DA, Smitha B, Sridhar S, Aminabhavi TM. Pervaporation separation of dimethylformamide/water mixtures through poly(vinyl alcohol)/poly(acrylic acid) blend membranes. Sep Purif Technol 2006;51:104–11. [11] Das S, Banthia AK, Adhikari B. Pervaporation separation of DMF from water using a crosslinked polyurethane urea-PMMA IPN membrane. Desalination 2006;197:106–16. [12] Kondolot Solak E, S- anlı O. Separation characteristics of dimethylformamide/water mixtures through alginate membranes by pervaporation, vapor permeation and vapor permeation with temperature difference methods. Sep Sci Technol 2006;41:627–46. [13] Kondolot Solak E, S- anlı O. Separation characteristics of dimethylformamide/water mixtures using sodium alginate-g-N–vinyl–2-pyrrolidone membranes by pervaporation method. Chem Eng Process, in press. [14] Is-ıklan N, S- anlı O. Separation characteristics of acetic acid–water mixtures by pervaporation using poly(vinyl alcohol) membranes modified with malic acid. Chem Eng Process 2005;44:1019–27. [15] Inui K, Okazaki K, Miyata T, Uragami T. Effect of mesogenic groups on characteristics of permeation and separation for benzene/ cyclohexane mixtures of side-chain liquid-crystalline polymer membranes. J Membr Sci 1998;143:93–104. [16] Alghezawi N, S- anlı O, Aras L, Asman G. Separation of acetic acid–water mixtures through acrylonitrile grafted poly(vinyl alcohol) membranes by pervaporation. Chem Eng Process 2005;44:51–8. [17] Koops GH, Nolten JAM, Mulder MHV. Selectivity as a function of membrane thickness: gas separation and pervaporation. J Appl Polym Sci 1994;53:1639–51. [18] Yeom CK, Lee KH. Pervaporation separation of water–acetic acid mixtures through poly(vinyl alcohol) membranes crosslinked with glutaraldehyde. J Membr Sci 1996;109:257–65. [19] Chen SH, Liou RM, Hsu CS, Chang DJ, Yu KC, Chang CY. Pervaporation separation water/ethanol mixture through lithiated polysulfone membrane. J Membr Sci 2001;193:59–67. [20] Toti US, Kariduraganavar MY, Soppimath KS, Aminabhavi TM. Sorption, diffusion, and pervaporation separation of water–acetic acid mixtures through the blend membranes of sodium alginate and guar gum-grafted-polyacrylamide. J Appl Polym Sci 2002;83:259–72.