- Email: [email protected]
Pervaporation dehydration of isopropanol with chitosan membranes
j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 124 (1997) 53-62
Pervaporation dehydration of isopropanol with chitosan membranes M. Ghazali M. Nawawi, Robert Y.M. Huang * Department of Chemical Engineering, Unit'ersity of Waterloo, Waterloo, Ontario, Canada N2L 3G 1
Received 3 June 1996; revised 15 July 1996: accepted 15 July 1996
Abstract Homogeneous and composite chitosan based membranes were prepared by the solution casting technique. The membranes were investigated for the pervaporation dehydration of isopropanol-water systems. The effects of feed concentration and temperature on the separation performance of the membranes were studied. In terms of the pervaporation separation index (PSI), the composite membrane was more productive than the homogeneous membrane for pervaporation of feed with high isopropanol content. It was observed that permeation increased and the separation factor decreased with the temperature. Modification of the homogeneous chitosan membrane by chemical crosslinking with hexamethylene diisocyanate improved the permselectivity but reduced the permeation rate of the membrane. Keywords: Dehydration; Pervaporation; Chitosan membrane; Composite membrane; Crosslinking; Hexamethylene diisocyanate (HMDI): lsopropanol-water separation
1. Introduction Pervaporation, a hybrid between a liquid- and a gas-separation process, is a very effective and economical technique for the separation of water from organic solvents and solvent mixtures. One of the potential applications of pervaporation is in the removal of trace water contaminants from bulk organics. A specific example of alcohol dehydration includes pervaporation dehydration of isopropanol to allow in-plant recycle as a cleaning agent in compact disks and semi-conductor chips manufacturing. Chitosan, a natural, non-toxic, biodegradable
" Corresponding author. Fax: +1-519-7464979; [email protected]
e-maih ry-
polymer, has been studied as a membrane [1-4] because of its good film forming properties, highly hydrophilic, and good chemical resistant properties. Chitosan is a high molecular weight polymer, which is a linear polyamine whose amino groups are readily available for chemical reactions and salt formation with acids. Although chitosan is insoluble in water, chitosan films swell significantly, and thus are soft and elastic in aqueous solutions. In this study, homogeneous and composite chitosan membranes were developed for the pervaporation dehydration of isopropanol-water systems. The objective is to develop water permselective chitosan membranes having high selectivity and good permeation rate, and long-term durability. Chemical reaction crosslinking with HMDI was investigated as a method to improve the stability of the membranes.
0376-7388/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PI1 S 0 3 7 6 - 7 3 8 8 ( 9 6 ) 0 0 2 1 6 - 5
54
M.G.M. Nawawi, R.EM. Huang / Journal of Membrane Science 124 (1997) 53-62
2. Experimental
2.1. Materials Chitosan flakes of Flonac-N grade were obtained from Kyowa Technos, Japan. Reagent grade acetic acid was purchased from Canlab, Canada. Polysulfone powder (Udel P-3500) was supplied by Amoco Performance Product, USA. All chemical including 1,6-hexamethylene diisocyanate obtained from Sigma, USA, methyl Cellosolve (ethylene glycol monomethyl ether) from Fisher Scientific and isopropanol from Commercial Alcohols, Canada were of reagent grade. Water was deionized and distilled before use. A non-woven polyester fabric was used as the backing material for the membrane.
2.2. Membranes preparation The preparation of both the homogeneous and composite membranes involved dissolution of the polymer in a solvent to form the casting solution, casting of the polymer solution onto a plate to form a membrane film, treatment in sodium hydroxide solution to regenerate chitosan, washing with water to remove trace of alkaline solution, and drying in air to evaporate the solvent. The porous polysulfone substrate was prepared via phase inversion process from a casting solution containing 12 wt% polysulfone, 11 wt% methyl Cellosolve, and 77 wt% N,N-dimethyl formamide. The casting solution was cast onto a non-woven polyester fabric held on a glass plate with the aid of a Gardner knife. The cast film was immediately immersed into a gel bath consisting of 50 wt% dimethyl formamide in deionized water at room temperature for 10 rain. The resulting porous membrane was washed with deionized water for 24 h and dried in air at room temperature. The substrate membrane exhibited a pure water permeation rate of 235 k g / m 2 h at a cross-membrane pressure difference of 100 psi. The composite membrane was prepared by coating a 0.5 wt% chitosan solution onto the porous polysulfone membrane with the aid of a dropper and a glass rod adjusted to the appropriate thickness. The chemical crosslinking of the membranes with HMDI was done via the solution technique. The pre-dried membranes were immersed in an aqueous
solution consisting of 1 wt% HMDI and 50 wt% acetone for 6, 12, 18, and 24 h followed by heat treatment at 150°C in an oven for 1 h. The role of acetone is to control the swelling balance of the chitosan membrane in the aqueous solution.
2.3. Pervaporation The pervaporation dehydration experiments were carried out using the apparatus which has been described elsewhere [5]. The downstream pressure was kept below 3 mmHg and the experiments were conducted at different temperatures, 30, 40, 50, and 60°C in the entire range of isopropanol feed composition. The analysis of the permeate composition was done by using a Hewlett Packed 5890 Series II gas chromatography equipped with a 6' X 0.125'-column packed with Porapak T 80/100 mesh. The study was concerned with the steady state pervaporation dehydration of the isopropanol-water system. The membrane performance was characterized in terms of permeation flux and permeate concentration, two quantities which are obtainable directly from the pervaporation experiments. The separation factor, which was derived from the feed and permeate concentration, was defined as =
yj
(1)
where x and y are the weight fraction of feed and permeate respectively. In this study, the subscripts i and j refer to water and isopropanol, respectively.
3. Results and discussion
3.1. Pervaporation performance of the membranes The individual permeation fluxes of the permeating species were calculated from the total permeation rate and the permeate composition. Fig. la and b show the permeation fluxes as a function of isopropanol concentration for the uncrosslinked homogeneous and composite chitosan membranes, respectively. In the entire composition range, water permeates predominantly and the total flux is almost the
M.G.M. Nawawi, R. KM. Huang / Journal of Membrane Science 124 (1997)53-62
same as the water flux suggesting that water is preferentially permeable through both the homogeneous and composite membranes. Since chitosan is relatively hydrophilic in a sense that it has a polar amino group attached to a repeat unit of the main chain, it is understandable that chitosan membrane shows high water permselectivity. Stronger affinity
55
of chitosan to water and the tact that molecular size of water is relatively smaller than that of isopropanol would make the chitosan membrane more selective to water. Both the homogeneous and composite membranes exhibit similar concentration dependence of permeation fluxes. The total and water permeation fluxes
4.000
(a) 3200-
%
+
Total Flux
-o-
Water Flux
24.00-
o=
1800G.
800 -
0
.A
0
2'0
J_
4.'0 8'0 Weight fraction of i-PrOH in feed, %
8'0
.A
!C@
5000
(b)
+
Total flux
~ 4000 -
"-Sm3000-
2000-
1000-
0 20
40
~0
80
100
Weight fraction of i-PrOH in feed, % Fig. I. Effect of teed concentration on the individual permeation flux fi)r (a) homogeneous chitosan membrane: (b) composite chitosan/polysulfone membrane. Operating temperature, 30°C.
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 124 (1997) 53-62
56
decrease with isopropanol concentration in the feed solution. Owing to the high hydrophilicity of the chitosan material, the chitosan membranes swell more significantly in the solution with high water content. As the water concentration in feed increases, the amorphous regions of the membrane are more swollen, and the polymer chains become more flexible and increase the space available for diffusion, thus decreasing the energy required for diffusive transport through the membrane. As a result, the water and, in this case, the total permeation fluxes through the highly hydrophilic chitosan membranes increase with an increase in water concentration in the feed solution. Interestingly, for both the homogeneous and composite membranes, the isopropanol permeation flux did not increase monotonically with the increase in isopropanol in feed as shown in Fig. 2. In case of the homogeneous membrane, an increase in isopropanol concentration increased the isopropanol flux when the isopropanol concentration is less than about 25 wt%, whereas after this point a reverse trend was observed. In the case of the composite membrane, an increase in isopropanol concentration increased the
isopropanol flux when the isopropanol concentration is less than about 40 wt%, whereas after this point the flux started to decrease with the isopropanol concentration. The existence of the maximum flux suggests that the permeation flux of isopropanol was greatly affected by the presence of water in the feed. The maxima in the isopropanol fluxes in Figs. 1 and 2 can be attributed to two effects: decrease of swelling at increased isopropanol feed concentration; decrease of water flux at increased isopropanol concentration. The latter effect reduces the friction between water molecules and isopropanol molecules in the membrane and so the 'drift' transport of isopropanol is reduced. The coupled transport of isopropanol and water through the membrane can be modelled with the Maxwell-Stefan theory. The influence of coupled diffusion of the permeating components according to the Maxwell-Stefan theory can be introduced to the widely accepted solution-diffusion model to describe the transport processes of mixtures through dense membranes in pervaporation [6]. Fig. 3a and b show the concentration dependence of separation factor for both the homogeneous and
800 --I--
Homogeneous CS
600-
<,_
m .2.
-5 400-
o_ 200-
0 o
20
~o
50
80
Ioo
Weight fraction of i-PrOH in feed, %
Fig. 2. Effect of feed concentration on the isopropanol permeation flux. Operating temperature, 30°C.
M.G.M. Nawawi, R, EM. Huang / Journal of Membrane Science 124 (1997) 53-62
composite membrane, respectively. As the feed isopropanol concentration increases from 70 to 95 wt% the decrease of isopropanol flux is more significant than that of water, resulting in an exponential increase in the separation factor. The increase in the isopropanol concentration restricts the degree of
57
swelling of the membranes, thus reducing the diffusivity of the permeating species. Since the molecular size of isopropanol is smaller than those of water, the permeation flux of the former is more affected than that of the latter. At 30°C, the separation factor tot the homogeneous and composite membranes for 90
5000
(a)
• i;2°°
~--000-
,a00
t
30002
= 2000-
2
&
looo-
20
40 60 Weight fraclion of i-PrOH in feed. %
20
100
5000
~C00
(b) 4000-
~
O
L-~C0
{ 3000-
/~F ~c°
•.~
-~ 7~
~ooo-
~o 2
? 1000-
0
~
0
t 20
~ 200
0
~ ~ i ' 40 60 Weight fraction of i-#rOH in feed, %
~ 80
-',, 43 I oo
Fig. 3. Effect of feed concentration on the separation factor and total permeation flux for (a) homogeneous chitosan membrane: (b) composite chitosan/polysulfone membrane. Operating temperature, 30°C.
58
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 124 (1997) 53-62
wt% isopropanol feed concentration were 491 and 348, respectively. When the isopropanol concentration was increased to 95 wt%, the separation factor for the homogeneous and composite membranes increased to 1096 and 807, respectively. The composite membrane comprising of a thin chitosan barrier and a porous polysulfone support was prepared to increase the permeation flux of water with a minimal sacrifice of separation factor. In order to measure the performance of the composite membrane as compared to the homogeneous one, both factors have to be taken into consideration. Since there is a trade-off, between the permeation flux ( J ) and separation factor (o~), that is when one factor increases, the other decreases, the third parameter, the pervaporation separation index (PSI), which is defined as the product of J and ( a - 1), has been widely accepted to evaluate the overall membrane pervaporation performance [7]. The results are shown in Fig. 4. It was found that in terms of the PSI, the composite membrane was more productive for the feed with high isopropanol content especially when
the isopropanol feed concentration exceeds 70 wt%, whereas for the feed with low isopropanol content the reverse trend was observed. This implies that if the PSI were the determining factor, composite chitosan would be more effective for the pervaporation dehydration of aqueous isopropanol near its azeotropic concentration, while homogeneous chitosan membrane would be useful for the separation of dilute aqueous isopropanol solutions. Note that separation of azeotropic and close-boiling mixtures by pervaporation through a number of different membranes including dehydration of ethanol and isopropanol is the best developed application of pervaporation due to commercial interest in combining pervaporation and conventional hybrid systems [8,9]. When polymer membranes are crosslinked, in general, the permselectivity is improved but the permeation rate is decreased because the degree of swelling of the membrane is lowered. This is also the case when the chitosan membranes were reacted with HMDI as shown in Fig. 5a and b. As the polymer was crosslinked, the polymer chains in the
1000000
100000 Z~ ff
10000
loooI
100
2b
~'o Weight fra~on
3
HomogeneousCS
•
Comoosr{eCS/PSf
6'o
8'0
~oo
of i-PrOH in feea, %
Fig. 4. Pervaporation separation index (PSI) of homogeneous and composite as a function of feed concentration. Operating temperature, 30°C.
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 124 (1997) 53-62
crosslinked membrane become more immobile and the available free volume for sorption and diffusion was substantially reduced. Since the membranes were crosslinked by the solution technique and the reaction was carried out by diffusion of HMDI through the membrane network, the degree of crosslinking was presumed to increase with the duration of reaction period. As the reaction time was increased, more
59
crosslinked structures were formed, the membrane become more rigid and less permeable to permeating species, thus the swelling degree decreased accordingly. At 30°C, the separation factor increased up to 1964, while the total permeation flux decreased down to 82 g / m 2 h when a 24-h-crosslinked homogeneous chitosan membrane was used for the pervaporation of 95 wt% isopropanol feed solution.
2400
(a) ---I-
2000-
90 w~ % i-PrCH
--o,-- 95 v4 % i-PrOH
1600-
.--~ 1200-
U~
80040o -~
1'2 Cross-linking reac:ion period, hr 210
1'8
1 II
2'4
(b)
2 18o] 15o ff _u
£ -4 "5
12o J
90-
50-
p-
J Z
30
90 wl' % i-PrOH 95 wt % i-PrOH
0 h
1L8
2'4
Cross-linking reaction period, hr Fig. 5. Effect of crosslinking reaction time on the separation factor and permeation flux fl~r the homogeneous chitosan membrane. Operating temperature, 30°C.
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 12,i (1997) 53-62
60
The effect of temperature on the pervaporation performance was investigated on the homogeneous chitosan membrane by carrying out the experiments at different feed temperature; 30, 40, 50, and 60°C. Fig. 6 illustrates the effect of temperature on the individual permeation flux for the pervaporation dehydration of 90 wt% isopropanol aqueous solution using homogeneous chitosan membrane, while Fig. 7 illustrates the typical separation factor with respect to temperature. It was observed that the permeation flux increases progressively with the increase in temperature. As the temperature increases, the thermal motion of polymer chains is intensified, creating more free volume in the polymer matrix, through which the permeating molecules of water and isopropanol can diffuse. As a result, the transport of both the permeating species is enhanced, leading to an increase in the total permeation flux and a decrease in the separation factor. The dependence of the permeation flux on tem-
perature can be related by an Arrhenius-type expression of the form J = aexp( -
E/RT)
(2)
where A is a constant, E the apparent activation energy for permeation, T the absolute temperature, and R the gas constant. The apparent activation energy for transport of each component, water and isopropanol, could be calculated from the slope of the Arrhenius plot of the flux as shown in Fig. 5. The apparent activation energy thus calculated for water and isopropanol are 20.59 and 35.49 k J / k g mol, respectively. The activation energy for transport of water is lower than that of isopropanol, indicating that water molecules requires less energy than isopropanol molecules do to facilitate permeation through the membrane. This difference in the activation energy for water and isopropanol allows us to speculate that isopropanol permeates by a random molecular diffusion within the amorphous matrix and water permeates by a selective transport through
1000
100"
g
~-
10,
--.il-- Isopropanot flux --o.- Water flux 1
2.9
3
311
312
313
3.4
1000/-[', 1/K Fig. 6. Effect of temperature on the individual permeation flux for the homogeneous chitosan membrane. Feed isopropanol concentration, 90 wt%.
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Sciem'c 124 (1997) 53-62
61
1200 --'11-- 90 wt % i-PrOH .--o--
95 w't % i-PrOH
900
s0
,*4
.,..~ .,J
600
r~
300
0 20
4'0
6'0
80
Temperature, C Fig. 7. Effect of temperature on the separation factor for the homogeneous chitosan membrane.
hydrophilic moiety in the polymer. The difference in the activation energy for each component may arise from several material factors such as the molecular size and the affinity between permeates and membrane. As a consequence, water molecules are preferentially absorbed and permeates more easily as compared with isopropanol due to the smaller molecular size as well as the higher affinity for the membrane. Note, however, that the activation energy for transport is only an empirical parameter that shows the relationship between the permeation flux and temperature. Since flux in a membrane is determined by both the solubility and the diffusion of the permeants, the activation value should depend on both the activation energy for diffusion through the membrane and the heat of sorption [10].
4. Conclusion Water-isopropanol mixtures can be effectively separated by pervaporation through homogeneous and composite chitosan membranes. Being hy-
drophilic, the membranes were highly water selective in the entire concentration range. Chemical modification of the chitosan membranes via crosslinking improved their separation performance. The separation factor increased and total permeation flux decreased with the relative increase in the crosslinking time. The separation performance of the membrane was affected by both operating parameters, teed concentration and temperature. In terms of the PSI, composite chitosan/polysulfone membrane exhibited an advantage over the homogeneous membrane when the isopropanol concentration in the feed solutions was more than 70 wt%.
Acknowledgements The generous supply of chitosan material by Kyowa Technos Co., Japan, the financial support by the Environmental Science and Technology Alliance Canada (ESTAC) and the Natural Science and Engineering Research Council of Canada (NSERC) for funding this research program, and the financial
62
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 124 (1997) 53-62
support by the Malaysian Government in the form of a graduate fellowship, are gratefully acknowledged. [5]
References [6] [1] A. Mochizuki, Y. Sato, H. Ogawara and S. Yamashita, Pervaporation separation of water-ethanol mixtures through polysaccharide membranes. I. The effects of salts on the permselectivity of cellulose membrane in pervaporation, J. Appl. Polym. Sci., 37 (1989) 3357. [2] A. Mochizuki, Y. Sato, H. Ogawara and S. Yamashita, Pervaporation separation of water-ethanol mixtures through polysaccharide membranes. IL The permselectivity of chitosan membrane, J. Appl. Polym. Sci., 37 (1989) 3375. [3] T. Uragami and K. Takigawa, Permeation and separation characteristics of ethanol-water mixtures through chitosan derivative membranes by pervaporution and evapomeation, Polymer, 31 (1990) 668. [4] A. Mochizuki, Y. Sato, H. Ogawara and S. Yamashita, Pervaporation separation of water-ethanol mixtures through
[7]
[8] [9]
[10]
polysaccharide membranes. IV. The relationships between the permselectivity of alginic acid structure and its solid state structure, J. Appl, Polym. Sci., 40 (1990) 385. R.Y.M. Huang and Y. Wei, Pervaporation with latex membranes. II. Pervaporation properties and aging effect, J. Membrane Sci., 87 (1994) 257. A. Heintz and W. Stephan, A generalized solution-diffusion model of the pervaporation process through composite membranes. Part II. Concentration polarization, coupled diffusion and the influence of the porous support layer, J. Membrane Sci., 89 (1994) 153. R.Y.M. Huang and X. Feng, Dehydration of isopropanol by pervaporation using aromatic polyetherimide membrane, Sep. Sci. Tech., 28 (1993) 2035. H.L. Fleming, Membrane pervaporation: Separation of organic/aqueous mixtures, Sep. Sci. Tech., 25 (1990) 1239. R. Rautenbach, S. Klatt and J. Vier, State of the art of pervaporation - - 10 years of industrial PV, in: Proc. Sixth Int. Conf. on Pervaporation Processes in Chem. Ind., R. Bakish Material Corp., Englewood, NJ, 1992, pp. 2-15. P.M. Bungay, Synthetic Membrane: Science, Engineering and Application, D. Reidel, Dordrecht, 1986.
Journal of Membrane Science 124 (1997) 53-62
Pervaporation dehydration of isopropanol with chitosan membranes M. Ghazali M. Nawawi, Robert Y.M. Huang * Department of Chemical Engineering, Unit'ersity of Waterloo, Waterloo, Ontario, Canada N2L 3G 1
Received 3 June 1996; revised 15 July 1996: accepted 15 July 1996
Abstract Homogeneous and composite chitosan based membranes were prepared by the solution casting technique. The membranes were investigated for the pervaporation dehydration of isopropanol-water systems. The effects of feed concentration and temperature on the separation performance of the membranes were studied. In terms of the pervaporation separation index (PSI), the composite membrane was more productive than the homogeneous membrane for pervaporation of feed with high isopropanol content. It was observed that permeation increased and the separation factor decreased with the temperature. Modification of the homogeneous chitosan membrane by chemical crosslinking with hexamethylene diisocyanate improved the permselectivity but reduced the permeation rate of the membrane. Keywords: Dehydration; Pervaporation; Chitosan membrane; Composite membrane; Crosslinking; Hexamethylene diisocyanate (HMDI): lsopropanol-water separation
1. Introduction Pervaporation, a hybrid between a liquid- and a gas-separation process, is a very effective and economical technique for the separation of water from organic solvents and solvent mixtures. One of the potential applications of pervaporation is in the removal of trace water contaminants from bulk organics. A specific example of alcohol dehydration includes pervaporation dehydration of isopropanol to allow in-plant recycle as a cleaning agent in compact disks and semi-conductor chips manufacturing. Chitosan, a natural, non-toxic, biodegradable
" Corresponding author. Fax: +1-519-7464979; [email protected]
e-maih ry-
polymer, has been studied as a membrane [1-4] because of its good film forming properties, highly hydrophilic, and good chemical resistant properties. Chitosan is a high molecular weight polymer, which is a linear polyamine whose amino groups are readily available for chemical reactions and salt formation with acids. Although chitosan is insoluble in water, chitosan films swell significantly, and thus are soft and elastic in aqueous solutions. In this study, homogeneous and composite chitosan membranes were developed for the pervaporation dehydration of isopropanol-water systems. The objective is to develop water permselective chitosan membranes having high selectivity and good permeation rate, and long-term durability. Chemical reaction crosslinking with HMDI was investigated as a method to improve the stability of the membranes.
0376-7388/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PI1 S 0 3 7 6 - 7 3 8 8 ( 9 6 ) 0 0 2 1 6 - 5
54
M.G.M. Nawawi, R.EM. Huang / Journal of Membrane Science 124 (1997) 53-62
2. Experimental
2.1. Materials Chitosan flakes of Flonac-N grade were obtained from Kyowa Technos, Japan. Reagent grade acetic acid was purchased from Canlab, Canada. Polysulfone powder (Udel P-3500) was supplied by Amoco Performance Product, USA. All chemical including 1,6-hexamethylene diisocyanate obtained from Sigma, USA, methyl Cellosolve (ethylene glycol monomethyl ether) from Fisher Scientific and isopropanol from Commercial Alcohols, Canada were of reagent grade. Water was deionized and distilled before use. A non-woven polyester fabric was used as the backing material for the membrane.
2.2. Membranes preparation The preparation of both the homogeneous and composite membranes involved dissolution of the polymer in a solvent to form the casting solution, casting of the polymer solution onto a plate to form a membrane film, treatment in sodium hydroxide solution to regenerate chitosan, washing with water to remove trace of alkaline solution, and drying in air to evaporate the solvent. The porous polysulfone substrate was prepared via phase inversion process from a casting solution containing 12 wt% polysulfone, 11 wt% methyl Cellosolve, and 77 wt% N,N-dimethyl formamide. The casting solution was cast onto a non-woven polyester fabric held on a glass plate with the aid of a Gardner knife. The cast film was immediately immersed into a gel bath consisting of 50 wt% dimethyl formamide in deionized water at room temperature for 10 rain. The resulting porous membrane was washed with deionized water for 24 h and dried in air at room temperature. The substrate membrane exhibited a pure water permeation rate of 235 k g / m 2 h at a cross-membrane pressure difference of 100 psi. The composite membrane was prepared by coating a 0.5 wt% chitosan solution onto the porous polysulfone membrane with the aid of a dropper and a glass rod adjusted to the appropriate thickness. The chemical crosslinking of the membranes with HMDI was done via the solution technique. The pre-dried membranes were immersed in an aqueous
solution consisting of 1 wt% HMDI and 50 wt% acetone for 6, 12, 18, and 24 h followed by heat treatment at 150°C in an oven for 1 h. The role of acetone is to control the swelling balance of the chitosan membrane in the aqueous solution.
2.3. Pervaporation The pervaporation dehydration experiments were carried out using the apparatus which has been described elsewhere [5]. The downstream pressure was kept below 3 mmHg and the experiments were conducted at different temperatures, 30, 40, 50, and 60°C in the entire range of isopropanol feed composition. The analysis of the permeate composition was done by using a Hewlett Packed 5890 Series II gas chromatography equipped with a 6' X 0.125'-column packed with Porapak T 80/100 mesh. The study was concerned with the steady state pervaporation dehydration of the isopropanol-water system. The membrane performance was characterized in terms of permeation flux and permeate concentration, two quantities which are obtainable directly from the pervaporation experiments. The separation factor, which was derived from the feed and permeate concentration, was defined as =
yj
(1)
where x and y are the weight fraction of feed and permeate respectively. In this study, the subscripts i and j refer to water and isopropanol, respectively.
3. Results and discussion
3.1. Pervaporation performance of the membranes The individual permeation fluxes of the permeating species were calculated from the total permeation rate and the permeate composition. Fig. la and b show the permeation fluxes as a function of isopropanol concentration for the uncrosslinked homogeneous and composite chitosan membranes, respectively. In the entire composition range, water permeates predominantly and the total flux is almost the
M.G.M. Nawawi, R. KM. Huang / Journal of Membrane Science 124 (1997)53-62
same as the water flux suggesting that water is preferentially permeable through both the homogeneous and composite membranes. Since chitosan is relatively hydrophilic in a sense that it has a polar amino group attached to a repeat unit of the main chain, it is understandable that chitosan membrane shows high water permselectivity. Stronger affinity
55
of chitosan to water and the tact that molecular size of water is relatively smaller than that of isopropanol would make the chitosan membrane more selective to water. Both the homogeneous and composite membranes exhibit similar concentration dependence of permeation fluxes. The total and water permeation fluxes
4.000
(a) 3200-
%
+
Total Flux
-o-
Water Flux
24.00-
o=
1800G.
800 -
0
.A
0
2'0
J_
4.'0 8'0 Weight fraction of i-PrOH in feed, %
8'0
.A
!C@
5000
(b)
+
Total flux
~ 4000 -
"-Sm3000-
2000-
1000-
0 20
40
~0
80
100
Weight fraction of i-PrOH in feed, % Fig. I. Effect of teed concentration on the individual permeation flux fi)r (a) homogeneous chitosan membrane: (b) composite chitosan/polysulfone membrane. Operating temperature, 30°C.
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 124 (1997) 53-62
56
decrease with isopropanol concentration in the feed solution. Owing to the high hydrophilicity of the chitosan material, the chitosan membranes swell more significantly in the solution with high water content. As the water concentration in feed increases, the amorphous regions of the membrane are more swollen, and the polymer chains become more flexible and increase the space available for diffusion, thus decreasing the energy required for diffusive transport through the membrane. As a result, the water and, in this case, the total permeation fluxes through the highly hydrophilic chitosan membranes increase with an increase in water concentration in the feed solution. Interestingly, for both the homogeneous and composite membranes, the isopropanol permeation flux did not increase monotonically with the increase in isopropanol in feed as shown in Fig. 2. In case of the homogeneous membrane, an increase in isopropanol concentration increased the isopropanol flux when the isopropanol concentration is less than about 25 wt%, whereas after this point a reverse trend was observed. In the case of the composite membrane, an increase in isopropanol concentration increased the
isopropanol flux when the isopropanol concentration is less than about 40 wt%, whereas after this point the flux started to decrease with the isopropanol concentration. The existence of the maximum flux suggests that the permeation flux of isopropanol was greatly affected by the presence of water in the feed. The maxima in the isopropanol fluxes in Figs. 1 and 2 can be attributed to two effects: decrease of swelling at increased isopropanol feed concentration; decrease of water flux at increased isopropanol concentration. The latter effect reduces the friction between water molecules and isopropanol molecules in the membrane and so the 'drift' transport of isopropanol is reduced. The coupled transport of isopropanol and water through the membrane can be modelled with the Maxwell-Stefan theory. The influence of coupled diffusion of the permeating components according to the Maxwell-Stefan theory can be introduced to the widely accepted solution-diffusion model to describe the transport processes of mixtures through dense membranes in pervaporation [6]. Fig. 3a and b show the concentration dependence of separation factor for both the homogeneous and
800 --I--
Homogeneous CS
600-
<,_
m .2.
-5 400-
o_ 200-
0 o
20
~o
50
80
Ioo
Weight fraction of i-PrOH in feed, %
Fig. 2. Effect of feed concentration on the isopropanol permeation flux. Operating temperature, 30°C.
M.G.M. Nawawi, R, EM. Huang / Journal of Membrane Science 124 (1997) 53-62
composite membrane, respectively. As the feed isopropanol concentration increases from 70 to 95 wt% the decrease of isopropanol flux is more significant than that of water, resulting in an exponential increase in the separation factor. The increase in the isopropanol concentration restricts the degree of
57
swelling of the membranes, thus reducing the diffusivity of the permeating species. Since the molecular size of isopropanol is smaller than those of water, the permeation flux of the former is more affected than that of the latter. At 30°C, the separation factor tot the homogeneous and composite membranes for 90
5000
(a)
• i;2°°
~--000-
,a00
t
30002
= 2000-
2
&
looo-
20
40 60 Weight fraclion of i-PrOH in feed. %
20
100
5000
~C00
(b) 4000-
~
O
L-~C0
{ 3000-
/~F ~c°
•.~
-~ 7~
~ooo-
~o 2
? 1000-
0
~
0
t 20
~ 200
0
~ ~ i ' 40 60 Weight fraction of i-#rOH in feed, %
~ 80
-',, 43 I oo
Fig. 3. Effect of feed concentration on the separation factor and total permeation flux for (a) homogeneous chitosan membrane: (b) composite chitosan/polysulfone membrane. Operating temperature, 30°C.
58
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 124 (1997) 53-62
wt% isopropanol feed concentration were 491 and 348, respectively. When the isopropanol concentration was increased to 95 wt%, the separation factor for the homogeneous and composite membranes increased to 1096 and 807, respectively. The composite membrane comprising of a thin chitosan barrier and a porous polysulfone support was prepared to increase the permeation flux of water with a minimal sacrifice of separation factor. In order to measure the performance of the composite membrane as compared to the homogeneous one, both factors have to be taken into consideration. Since there is a trade-off, between the permeation flux ( J ) and separation factor (o~), that is when one factor increases, the other decreases, the third parameter, the pervaporation separation index (PSI), which is defined as the product of J and ( a - 1), has been widely accepted to evaluate the overall membrane pervaporation performance [7]. The results are shown in Fig. 4. It was found that in terms of the PSI, the composite membrane was more productive for the feed with high isopropanol content especially when
the isopropanol feed concentration exceeds 70 wt%, whereas for the feed with low isopropanol content the reverse trend was observed. This implies that if the PSI were the determining factor, composite chitosan would be more effective for the pervaporation dehydration of aqueous isopropanol near its azeotropic concentration, while homogeneous chitosan membrane would be useful for the separation of dilute aqueous isopropanol solutions. Note that separation of azeotropic and close-boiling mixtures by pervaporation through a number of different membranes including dehydration of ethanol and isopropanol is the best developed application of pervaporation due to commercial interest in combining pervaporation and conventional hybrid systems [8,9]. When polymer membranes are crosslinked, in general, the permselectivity is improved but the permeation rate is decreased because the degree of swelling of the membrane is lowered. This is also the case when the chitosan membranes were reacted with HMDI as shown in Fig. 5a and b. As the polymer was crosslinked, the polymer chains in the
1000000
100000 Z~ ff
10000
loooI
100
2b
~'o Weight fra~on
3
HomogeneousCS
•
Comoosr{eCS/PSf
6'o
8'0
~oo
of i-PrOH in feea, %
Fig. 4. Pervaporation separation index (PSI) of homogeneous and composite as a function of feed concentration. Operating temperature, 30°C.
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 124 (1997) 53-62
crosslinked membrane become more immobile and the available free volume for sorption and diffusion was substantially reduced. Since the membranes were crosslinked by the solution technique and the reaction was carried out by diffusion of HMDI through the membrane network, the degree of crosslinking was presumed to increase with the duration of reaction period. As the reaction time was increased, more
59
crosslinked structures were formed, the membrane become more rigid and less permeable to permeating species, thus the swelling degree decreased accordingly. At 30°C, the separation factor increased up to 1964, while the total permeation flux decreased down to 82 g / m 2 h when a 24-h-crosslinked homogeneous chitosan membrane was used for the pervaporation of 95 wt% isopropanol feed solution.
2400
(a) ---I-
2000-
90 w~ % i-PrCH
--o,-- 95 v4 % i-PrOH
1600-
.--~ 1200-
U~
80040o -~
1'2 Cross-linking reac:ion period, hr 210
1'8
1 II
2'4
(b)
2 18o] 15o ff _u
£ -4 "5
12o J
90-
50-
p-
J Z
30
90 wl' % i-PrOH 95 wt % i-PrOH
0 h
1L8
2'4
Cross-linking reaction period, hr Fig. 5. Effect of crosslinking reaction time on the separation factor and permeation flux fl~r the homogeneous chitosan membrane. Operating temperature, 30°C.
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 12,i (1997) 53-62
60
The effect of temperature on the pervaporation performance was investigated on the homogeneous chitosan membrane by carrying out the experiments at different feed temperature; 30, 40, 50, and 60°C. Fig. 6 illustrates the effect of temperature on the individual permeation flux for the pervaporation dehydration of 90 wt% isopropanol aqueous solution using homogeneous chitosan membrane, while Fig. 7 illustrates the typical separation factor with respect to temperature. It was observed that the permeation flux increases progressively with the increase in temperature. As the temperature increases, the thermal motion of polymer chains is intensified, creating more free volume in the polymer matrix, through which the permeating molecules of water and isopropanol can diffuse. As a result, the transport of both the permeating species is enhanced, leading to an increase in the total permeation flux and a decrease in the separation factor. The dependence of the permeation flux on tem-
perature can be related by an Arrhenius-type expression of the form J = aexp( -
E/RT)
(2)
where A is a constant, E the apparent activation energy for permeation, T the absolute temperature, and R the gas constant. The apparent activation energy for transport of each component, water and isopropanol, could be calculated from the slope of the Arrhenius plot of the flux as shown in Fig. 5. The apparent activation energy thus calculated for water and isopropanol are 20.59 and 35.49 k J / k g mol, respectively. The activation energy for transport of water is lower than that of isopropanol, indicating that water molecules requires less energy than isopropanol molecules do to facilitate permeation through the membrane. This difference in the activation energy for water and isopropanol allows us to speculate that isopropanol permeates by a random molecular diffusion within the amorphous matrix and water permeates by a selective transport through
1000
100"
g
~-
10,
--.il-- Isopropanot flux --o.- Water flux 1
2.9
3
311
312
313
3.4
1000/-[', 1/K Fig. 6. Effect of temperature on the individual permeation flux for the homogeneous chitosan membrane. Feed isopropanol concentration, 90 wt%.
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Sciem'c 124 (1997) 53-62
61
1200 --'11-- 90 wt % i-PrOH .--o--
95 w't % i-PrOH
900
s0
,*4
.,..~ .,J
600
r~
300
0 20
4'0
6'0
80
Temperature, C Fig. 7. Effect of temperature on the separation factor for the homogeneous chitosan membrane.
hydrophilic moiety in the polymer. The difference in the activation energy for each component may arise from several material factors such as the molecular size and the affinity between permeates and membrane. As a consequence, water molecules are preferentially absorbed and permeates more easily as compared with isopropanol due to the smaller molecular size as well as the higher affinity for the membrane. Note, however, that the activation energy for transport is only an empirical parameter that shows the relationship between the permeation flux and temperature. Since flux in a membrane is determined by both the solubility and the diffusion of the permeants, the activation value should depend on both the activation energy for diffusion through the membrane and the heat of sorption [10].
4. Conclusion Water-isopropanol mixtures can be effectively separated by pervaporation through homogeneous and composite chitosan membranes. Being hy-
drophilic, the membranes were highly water selective in the entire concentration range. Chemical modification of the chitosan membranes via crosslinking improved their separation performance. The separation factor increased and total permeation flux decreased with the relative increase in the crosslinking time. The separation performance of the membrane was affected by both operating parameters, teed concentration and temperature. In terms of the PSI, composite chitosan/polysulfone membrane exhibited an advantage over the homogeneous membrane when the isopropanol concentration in the feed solutions was more than 70 wt%.
Acknowledgements The generous supply of chitosan material by Kyowa Technos Co., Japan, the financial support by the Environmental Science and Technology Alliance Canada (ESTAC) and the Natural Science and Engineering Research Council of Canada (NSERC) for funding this research program, and the financial
62
M.G.M. Nawawi, R.Y.M. Huang / Journal of Membrane Science 124 (1997) 53-62
support by the Malaysian Government in the form of a graduate fellowship, are gratefully acknowledged. [5]
References [6] [1] A. Mochizuki, Y. Sato, H. Ogawara and S. Yamashita, Pervaporation separation of water-ethanol mixtures through polysaccharide membranes. I. The effects of salts on the permselectivity of cellulose membrane in pervaporation, J. Appl. Polym. Sci., 37 (1989) 3357. [2] A. Mochizuki, Y. Sato, H. Ogawara and S. Yamashita, Pervaporation separation of water-ethanol mixtures through polysaccharide membranes. IL The permselectivity of chitosan membrane, J. Appl. Polym. Sci., 37 (1989) 3375. [3] T. Uragami and K. Takigawa, Permeation and separation characteristics of ethanol-water mixtures through chitosan derivative membranes by pervaporution and evapomeation, Polymer, 31 (1990) 668. [4] A. Mochizuki, Y. Sato, H. Ogawara and S. Yamashita, Pervaporation separation of water-ethanol mixtures through
[7]
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[10]
polysaccharide membranes. IV. The relationships between the permselectivity of alginic acid structure and its solid state structure, J. Appl, Polym. Sci., 40 (1990) 385. R.Y.M. Huang and Y. Wei, Pervaporation with latex membranes. II. Pervaporation properties and aging effect, J. Membrane Sci., 87 (1994) 257. A. Heintz and W. Stephan, A generalized solution-diffusion model of the pervaporation process through composite membranes. Part II. Concentration polarization, coupled diffusion and the influence of the porous support layer, J. Membrane Sci., 89 (1994) 153. R.Y.M. Huang and X. Feng, Dehydration of isopropanol by pervaporation using aromatic polyetherimide membrane, Sep. Sci. Tech., 28 (1993) 2035. H.L. Fleming, Membrane pervaporation: Separation of organic/aqueous mixtures, Sep. Sci. Tech., 25 (1990) 1239. R. Rautenbach, S. Klatt and J. Vier, State of the art of pervaporation - - 10 years of industrial PV, in: Proc. Sixth Int. Conf. on Pervaporation Processes in Chem. Ind., R. Bakish Material Corp., Englewood, NJ, 1992, pp. 2-15. P.M. Bungay, Synthetic Membrane: Science, Engineering and Application, D. Reidel, Dordrecht, 1986.