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polysulfone composite membrane
j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 116 (1996) 67-76
Pervaporation with chitosan membranes. I. Separation of water from ethylene glycol by a chitosan/polysulfone composite membrane Xianshe Feng, Robert Y.M. Huang * Department of Chemical Engineering, Uni~,ersity of Waterloo, Waterloo, ON, Canada N2L 3G1 Received 5 September 1995; revised 19 December 1995; accepted 3 January 1996
Abstract A chitosan/polysulfone composite membrane was prepared. The preparation procedure involved dissolution of chitosan in dilute aqueous acetic acid to form chitosan salt, coating of the chitosan salt solution on a porous polysulfone substrate, and regeneration of chitosan by alkaline treatment. The membrane was tested for selective removal of water from aqueous ethylene glycol solutions by pervaporation. The effects of operating parameters, including feed concentration, temperature, and downstream pressure, on the separation performance of the membrane were investigated. At 35°C and 60 Pa downstream pressure, a permeation flux of 0.3 k g / m 2 h and permeate water concentration higher than 92 wt% was achieved at a feed water content of 10 wt%. This study demonstrated the potential of membrane pervaporation as an alternative to conventional distillation for the given separation. It was also shown that conditioning of the membrane in the pervaporation system at the maximum operating temperature led to a quasi-permanent change in membrane permselectivity.
Keywords: Pervaporation; Ethylene glycol/water separation; Chitosan membrane; Composite membrane
1. I n t r o d u c t i o n Pervaporation is an attractive alternative to conventional processes for the separation of liquid mixtures. This technique depends on the fact that certain membranes permit selective permeation of different species in a mixture, producing a permeate stream enriched in the preferentially-permeating species. It
materials from which the membrane is formed and an appropriate procedure to manufacture a thin membrane. Almost all industrially important pervaporation membranes are in composite forms where a thin
is the selective permeability of the membrane that forms the basis of separation. A successful pervaporation membrane lies primarily in selecting proper
" a c t i v e " membrane layer is deposited on a porous support to achieve high membrane productivity yet to retain the necessary mechanical strength. Separation processes to recover and purify products account for over 40% of overall energy consumption in the chemical processing industry, and distillation is used to make about 95% of all the
* Corresponding author. Fax: (519)746-4979; E-mail: [email protected],
separations [1]. Membrane pervaporation is believed to be especially valuable for accomplishing separations that are not cost-effective by distillation since
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved Pll S 0 3 7 6 - 7 3 8 8 ( 9 6 ) 0 0 0 2 2 - 1
68
X. Feng, R.Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
the separation efficiency of pervaporation is not limited by the relative volatility of the species to be separated. The thermodynamic efficiency of distillation is inherently low whereas that for membrane processes can be very high [2,3]. Currently, dehydration of alcohols, in particular ethanol and isopropanol, by pervaporation is the best developed application of pervaporation due to commercial interest of augmenting distillation systems [4,5]. Extensive studies have been carried out to investigate pervaporation for the separation of various azeotropic and close-boiling mixtures by using a number of different membranes. However, not as much effort has been directed at pervaporation application for dehydrating non-azeotropic systems such as ethylene glycol/water systems, Ethylene glycol is an important chemical widely used for non-volatile antifreeze and coolant as well as an intermediate in the manufacture of polyesters [6]. At present the main commercial route of ethylene glycol production is the direct oxidation of ethylene to ethylene oxide followed by hydrolysis of ethylene oxide [7]. The large excess of water from the hydrolysis is removed by multi-stage evaporation, and the glycol-enriched product from the reboiler is then dehydrated in another distillation (drying) unit. Although ethylene glycol and water do not form an azeotrope over the entire composition range, the separation of water from ethylene glycol by distillation has proved to be costly because high pressure steam is required for the reboiler due to the high boiling point of ethylene glycol (198°C). In fact, ethylene glycol/water separation by distillation is ranked the eighth most energy intensive distillation operations in the chemical process industry [8]. Pervaporation is expected to provide an energy efficient alternative. In pervaporation only that fraction
of feed stream that permeates the membrane undergoes liquid to vapor phase change. Moreover, pervapotation can be operated at lower temperatures than distillation, making it possible to utilize waste-heat as the heat of vaporization needed for pervaporation. Pervaporation is often used in conjunction with distillation and as such the enthalpy of the heated stream from a distillation unit can be directly applied as the heat input for the pervaporation unit. A case study showed that substantial reductions in utility costs for ethylene glycol/water separation could be achieved by a hybrid process of integrating distillation with pervaporation [9]. In the present study the separation of ethylene glycol/water mixtures by pervaporation through a chitosan membrane was attempted. Chitosan is the N-deacetylated product of chitin, a natural polymer that can be extracted from outer shells of crustaceans. Chitosan is a linear polymer composing primarily of glucosamine (see Scheme 1). In this study chitosan was chosen as the membrane material because of its favorable permselectivity and solvent stability as well as good film forming properties [10]. In order to increase the permeation rate a composite membrane comprising a thin chitosan barrier layer and a porous polysulfone support was prepared, and the pervaporation characteristics of the membrane for the given separation was investigated.
2. Experimental 2.1. Materials
Chitosan flakes (Flonac N) were supplied by Kyowa Technos Co., Ltd., Japan. Polysulfone powder (Udel P-3500) was supplied by Amoco Performance
~H20H
,
Chitosan
C.HzOH
Chitosan/acid salt
Scheme 1.
X. Feng, R.Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
CHITOSAN
Dissolutionin dilute aceticacid (formationofchitosan/acidsalt) I Coating chitosan salt solution
on polysulfonesubstrate
Alkaline treatment
(regeneration
of chitosan)
I washin]/rinsin9 ]
CHITOSAN/POLYSULFONE COMPOSITEMEMBRANE Fig. 1. Membranepreparation sequence,
Products, OH, USA. All organic solvents used in the experiments were of reagent grade; they were obtained from BDH Chemicals Co. except for methyl Cellosolve (ethylene glycol monomethyl ether), which was obtained from Fisher Scientific Co. Water was deionized and distilled before use. A non-woven fabric was used as the backing material for the membrane.
69
The polysulfone membrane was coated with chitosan. Since chitosan was insoluble in water, the coating solution was prepared by dissolving 0.53 wt% chitosan in a 0.79 wt% aqueous acetic acid solution. After being filtered to remove any undissolved residual solid, the chitosan solution was coated onto the polysulfone substrate membrane. The casting procedure is as follows. First, the substrate membrane was mounted in a straight-sided wide-mouth bottle that contained the chitosan coating solution with the aid of a closure. The top surface of the substrate membrane was brought into contact with the coating solution by leaving the bottle upside down for 30 min. After the residual solution on the membrane surface was drained, the membrane was dried at 70°C for 2 h in an oven with forced air circulation. Then the coated membrane was treated with an alkaline solution containing 2.6 wt% sodium hydroxide in methanol, followed by further alkaline treatment with a 4.3 wt% aqueous sodium hydroxide solution. Finally, the membrane was washed and rinsed repeatedly with water, and then air-dried. The total thickness of the dry composite membrane was found to be 240 /xm. Fig. 2 shows an scanning electron micrograph of the membrane. Dense homogeneous chitosan films were used to measure the swelling degree of chitosan in the permeating species. These fihns were prepared by pour-
2.2. Mernbraneprepara tion
~z.
~.~
The s e q u e n c e for p r e p a r a t i o n of the chitosan/polysulfone composite membrane is summarized in Fig. 1. The porous polysulfone substrate membrane was prepared via the phase inversion process from a homogeneous casting solution composing of 12 wt% polysulfone, 11 wt% methyl Cellosolve, and 77 wt% N,N-dimethyl formamide. The casting solution was cast onto the backing material held on a glass plate, and then the cast film was gelled in ice-cold water ( ~ 2°C) for 2 h. The polysulfone membrane so obtained was washed with water before being dried in air at ambient temperature. Such a porous membrane exhibited a pure water permeation rate of 218 k g / m 2 h at 22°C under a transmembrane pressure difference of 100 psi (690 kPa).
!~:~';
~i~
i
[
Fig. 2. An SEM picture of a chitosan/polysulfone membrane.
70
X. Feng, R.Y.M. Huang/ Journal of Membrane Science 116 (1996) 67-76
ing the chitosan solution onto a rimmed poly(vinyl chloride) plate, followed by the evaporation and alkaline treatment procedures, as described above, It should be noted that chitosan in its free amine form is insoluble in water at neutral pH. However, in some acidic solutions (e.g. acetic acid), the free amino groups ( - N H 2) become protonated to form water-soluble chitosan-acid salts. Alkaline treatment of the latter converts the cationic amine groups (-NH~-) into - N H 2 form, thereby accomplishing regeneration of chitosan to the free amine form (see Scheme 1). 2.3. Pervaporation Pervaporation experiments were conducted in a stirred batch cell as described before [11]. The effective area of the membrane in the permeation cell was 8.55 cm 2. The temperature was controlled by circulating water from a thermostatic bath (VWR Scientific 1167) to a water jacket that surrounded the feed compartment. The feed solution was agitated by a magnetic stirrer located 3 mm above the membrane surface. The vacuum applied to the downstream side of the membrane was controlled by a vacuum regulator and measured by a Pirani gauge, while the upstream pressure was kept at atmospheric pressure since hydrodynamic pressure can hardly affect pervaporation transport. The permeate was condensed and collected in a cold trap immersed in liquid nitrogen, and the permeation rate was determined from the amount of permeate sample collected for a given period. The permeate composition was analyzed either by a Hewlett Packard gas chromatograph (model 5890, series II) equipped with a flame ionization detector or by an Atago high-precision refractometer (type 3T), depending on permeate concentration and the amount of permeate sample. For each pervaporation run, the permeate removed by membrane was kept below 0.5% of initial feed load and the permeate sample was returned to the feed compartment after analysis for composition to maintain a constant concentration of feed. The study was concerned with steady state of pervaporation. When the permeation rate and permeate concentration became constant, the steady state of permeation was assumed. The time required to reach the steady state varied between 0.8 to 2.5 h,
depending on such factors as feed concentration and operating conditions. The membrane performance was characterized in terms of permeation flux and permeate concentration, two quantities obtainable directly from pervaporation experiments.
3. Results and discussion 3.1. Effect of feed agitation For majority of pervaporation runs, the stirrer speed was set at 200 rpm. It was found that the speed of stirrer in feed compartment, in the range 150-240 rpm, did not affect total flux and permeate composition appreciably, indicating that concentration polarization at the feed side of the membrane was negligible in the range of stirrer speeds investigated. Hence, the pervaporation characteristics reported below is the separation performance intrinsic to the membrane, free from complication due to boundary layer effect. 3.2. Effect of feed concentration Fig. 3 shows the total permeation flux and permeate concentration as a function of feed concentration. In the entire composition range, the concentration of water in permeate is higher than that in feed, suggesting that water is preferentially permeable through the membrane. Chitosan is a highly hydrophilic polymer. The permselectivity of chitosan membrane to water is believed to be due to stronger affinity of chitosan to water molecules and smaller molecular size of water as compared with ethylene glycol. Fig. 3a also shows the data of equilibrium vapor composition versus liquid composition for the binary water-ethylene glycol mixtures at 101.3 kPa [12]. It appears that at a given feed liquid concentration, the selectivity of pervaporation separation using the chitosan membrane is higher than the selectivity of distillation separation, though the difference in the selectivities of the two processes becomes less significant as the water concentration in feed increases. At water content of 5 and 10 wt% in feed liquid, the equilibrium vapor phase concentration is 49.4 and 74.1 wt% water, respectively, which values are considerably lower than the corresponding single-pass
X. Feng, R. EM. Huang / Journal of Membrane Science 116 (1996) 67-76
permeate concentration of 85.5 and 92.4 wt% water, Since pervaporation also involves a phase change of permeated species from liquid to vapor phase, it is obvious that pervaporation with the chitosan merebrane works best for low feed water concentrations. In subsequent investigations of operating temperature and downstream pressure effects, the feed concentration has been fixed at a 10 wt% water concentration. The concentration dependence of total permeation flux, shown in Fig. 3b, seems to follow an exponential fashion. The increase in the total permeation flux with feed water concentration can be explained in the following manner. Owing to the high hydrophilicity of chitosan material, the chitosan merebrane swells more significantly in a feed solution with higher water content, thereby making it easier for permeating species to penetrate the membrane. Fig. 4 shows the swelling data of chitosan in ethylene g l y c o l / w a t e r solutions; these data were ob-
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Mass fraction of HzO in feed Fig. 3. Effect of feed concentration on (a) permeate concentration, and (b) permeation flux. Operating temperature, 35°C; downstream pressure, 60 Pa. Dotted line in Fig. 3a represents the vapor-liquid equilibrium (VLE) data at 101.3 kPa.
71
a.0 z.~ "~, 6~z.0 ;z E 1.5 .E = 1.0 w 0.5
°'°0.0
q
r
i
~
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Fig. 4. Swelling ratio of chitosan in ethylene glycol/water solu-
tions at 35°C.
tained using the conventional procedure of sorption intake measurements [13,14]. Clearly, the degree of swelling of chitosan increases with an increase in water concentration. It is generally accepted that membrane swelling by a permeating species increases its diffusivity through the membrane. In a swollen polymer, the mobility of the polymer segments is promoted, and the space available for diffusion is increased. When the concentration dependence of diffusivity follows an exponential relationship, which is one of the most commonly used phenomenological expressions, the above observed exponential increase in permeation flux with an increase in water concentration will result. The pervaporation of ethanol/water mixtures through the phosphorylated chitosan membranes also exhibits a similar behavior [15]. The individual permeation fluxes of the permeating species were calculated from total permeation rate and permeate composition. Figs. 5 and 6 show the permeation fluxes of water and ethylene glycol, respectively. It can be seen that water permeation rate increases as the water content in feed increases. However, the water permeation rate was lower than i d e a l o v e r a large c o n c e n t r a t i o n r a n g e , a s represented by the dotted line in Fig. 5. In an ideal permeation, there are no permeant-permeant and permeantmembrane interactions, and the permeation flux of a component is proportional to its concentration in the feed. The negative deviation of water permeation
X. Feng, R.Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
72
z0
15
. . . 9 " . /
~: ~to "~ 5
."" ~
ID~
et al. [16] reported similar results for ethanol/water pervaporation through a modified chitosan membrane. Also shown in Fig. 6 is that the permeation of ethylene glycol deviated from ideal case positively at a feed glycol content up to ~ 30 wt%, above which the deviation became negative. This behavior seems to account for two aspects: one is the remarkable membrane swelling by water at low concentrations of glycol, which facilitated the permeation, the other is the tendency of ethylene glycol to form interchain hydrogen bonding at high concentrations of glycol, thus retarding its permeation through the membrane.
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o
OG
0.0
I
i
0.4 0.6 0.8 Mass f r a c t i o n of H~O in f e e d
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3.3. Effect of temperature
Fig. 5. The partial permeation flux of water as a function of water content in feed. Dotted line represents the ideal permeation flux that would be obtained if there were no permeant-permeant and permeant-membrane interactions. Operating conditions were the same as those given in Fig. 3.
from ideal indicates that the permeation of water is retarded by the presence of ethylene glycol. Interestingly, the glycol flux did not increase monotonically with increasing concentration of ethylene glycol in feed, as shown in Fig. 6. The figure shows that an increase in feed glycol concentration increased the glycol flux when then glycol concentration is less than 10 wt% or higher than 50 wt%, whereas in the intermediate range a reverse trend was observed. Lee 0.t0
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.-" .//
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,
i , ~ , , , i 0.2 0.4 0.6 0.8 Mass fraction of EG in feed
, 1.0
Fig. 6. The partial permeation flux of ethylene glycol as a function of ethylene glycol content in feed. Dotted line represents the ideal permeation flux. Operating conditions were the same as those given in Fig. 3.
The effect of temperature on pervaporation performance was tested by carrying out the experiments different temperatures. The m e a s u r e m e n t s w e r e conducted first under the conditions of systematically increasing the temperature from 22 to 80°C, and then the membrane was maintained at 80°C for 5 days to be "conditioned" before another series of measurements in the sequence of systematically decreasing the temperature. Figs. 7 and 8 illustrate the permeation flux and permeate concentration, respectively, with respect to temperature at a feed concentration of 10 wt% water. Empirically, the temperature dependence of permeation flux often follows an exponential fashion, as shown by the semi-logarithmic plots depicted in Fig. 7, though the curves are inflected at ~ 57°C. While the permeation flux increased with an increase in temperature, the permeation selectivity decreased gradually, as reflected by the gradual change in permeate concentration (Fig. 8). According to the classical Eyring theory of diffusion, the thermal tootion of polymer chains generates penetrant-scale transient gaps in the polymer matrix, through which the permeating molecules can diffuse. As the temperature increases, the thermal motion is intensified, and the transient diffusive gaps become larger. Consequently, the transport of bulkier ethylene glycol molecules, along with water, is enhanced, leading to an increase in permeation flux and a less-significant decrease in permeation selectivity. at
By comparing t h e d a t a (using symbols • and O) in Figs. 7 and 8, it can be seen that there is a shift in both permeation flux and permeate concentration.
X. Feng, R. EM. Huang / Journal of Membrane Science 116 (1996) 67-76
lo
~
1
0.1 ~ Measurement Sequence: ~) Temp. increasing Temp. decreasin9
[] Random change in temp.
~ °%.'8
2.9
3.0
~ ~ ~ 3.2 3.3 1000/T, K-' 3.1
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3.5
Fig. 7. Effect of operating temperature on permeation flux. Feed concentration, 10 wt% water; downstream pressure, 60 Pa. Measurement sequence: (1) temperature increasing from 22 to 80°C (O), (2) membrane maintained at 80°C for 5 days (membrane "conditioning"), (3) temperature decreasing from 80 to 22°C (O), and (4) random change of temperature in the same range
(a). However, continued measurements under the conditions of randomly changing temperature within the same temperature range exhibited no additional shift in the membrane performance, as shown by the consistency of the data (represented by symbols O
73
and D) in the figures. Steady state was maintained when taking the performance data, at least within the time frame of measurements. An explanation of the aforementioned behavior may be that the membrane morphology was changed during the course of membrane "conditioning". However, an examination of the membrane cross section by a scanning electron microscope showed no visible change in the membrane structure after conditioning, suggesting that the morphological change is due to subtle perturbations of the polymer matrix that constitute the membrane. It seems that the morphological change is quasi-permanent since the experimental data in the second and the third series of experiments agree while prolonged tests showed no definite trend in the change of membrane performance. It is not yet clear at present time whether the inflection in Fig. 7 is caused by the secondary transition of the membrane material. The glassy transition temperature of a dry chitosan material is over 150°C [17], which value is expected to be lowered substantially in the case of a wet chitosan membrane because of the strong plasticization effect of water in the feed. This is supported by the experimental observation that the dense homogeneous chitosan films are soft and elastic in water but become quite rigid when completely dried. The flux vs. temperature plots reported by Lee et al. [16,18] for waterethanol pervaporation through modified chitosan membranes also displayed the tendency of inflection, although such a tendency was not elaborated explic-
ty "~
Based on the data (represented by symbols O and
D) in Figs. 7 and 8, the permeation fluxes of .~ 0.9
% O :~
o.8
Measurement Seguence:
• Temp. increasing o Temp. decreasing [] Random change in temp.
z~ °72o
30L 4o~ 5o~ 6o~ vo~ 8o~ 9or loo Ternperature.°C
Fig. 8. Effect of operating temperature on permeate concentration. Operating conditions and meaning of the symbols are the same as those given in Fig. 6.
individual species after membrane "conditioning" are depicted in Fig. 9, which again shows an inflection at 57°C. The temperature dependence of permeation flux is due to the joint effects of temperature on the driving force for permeation and the membrane permeability. The chemical potential gradient or the partial pressure difference across the membrahe is normally considered as the driving force for the pervaporative mass transport [19]. The activation energy for permeation can be calculated from an Arrhenius-type expression of the form
(J/Ap)
i = aiexp( -Ei/RT
)
(1)
X. Feng, R.Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
74
101
I0 o Water [] Ethylene glycol
,~ ~
0 Wafer 1:3 Ethylene glycol
o.1
0.00
.
2.9
3.0
3.1
3.E
3.3
3.4
3.5
2.9
3.0
IO00/T, K -~
3.1
3.2
3.3
3.4
3.5
tOOO/T, K -i
Fig. 9. Partial fluxes of water and ethylene glycol after membrane "conditioning" vs. reciprocal temperature. Feed concentration, 10 wt% water; downstream pressure, 60 Pa.
Fig. 10. Arrhenius plots showing permeation flux normalized for partial pressure difference across membrane versus reciprocal temperature. The conditions were the same as for Fig. 9.
where ( J / Z i p ) is the permeation flux normalized for transmembrane partial pressure difference, A is a constant, E is the activation energy for permeation, T is the absolute temperature, and subscript i denotes the permeating species. Here the driving force-normalized flux is used to evaluate activation energy for permeation because the variation of flux with temperature due to the effect of temperature on the driving force should be excluded, although many researchers [20] simply used In J vs. 1 / T plots to calculate the activation energy. The Arrhenius plots for water and ethylene glycol permeation through the chitosan/polysulfone membrane are shown in Fig. 10. The corresponding activation energy data are listed in Table 1. The activation energy is only an empirical parameter that relates temperature dependence of membrane permeability. If pervaporation transport follows the solution-diffusion mechanism, then the permeability coefficient is given as a prod-
endothermic or exothermic. It may be mentioned that when the polymer undergoes a glass transition, both the magnitude and the sign of sorption enthalpy can change [21]. The results in Table 1 seem to indicate that the sorption enthalpy is more significant than the diffusion activation energy at temperatures below 57°C; at higher temperatures, the reverse situation holds.
Table 1
uct of diffusion coefficient and solubility coefficient.
Activation energy for pervaporation after membrane conditioning a
Consequently, the activation energy for permeation is a combination of enthalpy of permeant sorption in the membrane and the activation energy of permeant diffusion through the membrane. While the activation energy of diffusion is normally positive, the enthalpy of sorption can be either positive or negative, depending on whether the sorption process is
Species
E (kJ/mol)
Temperature t (°C)
Water
8.9 49.8 -28.5
< 57 > 57 <57
3.4. Effect of downstream pressure The permeate pressure influenced membrane performance considerably, as illustrated in Fig. 11. Since the driving force for pervaporation transport is provided by lowering the downstream pressure, it is natural that permeation rate decreases with increases
Ethylene glycol
-
31.4
range,
> 57
a Feed concentration, 10 wt% water; downstreampressure, 60 Pa.
X. Feng, R. Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
100 98
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09
96 .E o 94 o~ 92 "~ 90 , 2.4 ~" 42.0 ~ 1.6
~
,
r
,
~ ,
~
,
~
,
~
75
permeate water concentration higher than 92 wt% was achieved for a feed with water content of 10 wt%. These results demonstrate the potential of pervaporation as an alternative to conventional distillation for separation of w a t e r - e t h y l e n e glycol mixtures. C o n d i t i o n i n g the m e m b r a n e at the m a x i m u m operating temperature led to a change in m e m b r a n e performance, and such a change was found to be quasi-permanent. Because the m e m b r a n e was not prepared under optimized conditions, c o n t i n u e d investigations are needed to improve m e m b r a n e performance.
1.2
o.8
Acknowledgements
~0.4 °'°0.'0
The chitosan material used in the study was gen-
i
I
0.2 0.4 0.6 0.8 1.0 1.2 Downstream pressure, kPa Fig. 11. Effect of permeate pressure on permeation flux and permeate concentration. Feed concentration, 10 wt% water; operating temperature, 75°C.
erously provided by K y o w a Technos Co., Ltd., Japan. The financial support by the E n v i r o n m e n t a l Science and T e c h n o l o g y Alliance C a n a d a and the Natural Sciences and E n g i n e e r i n g Research Council of C a n a d a is gratefully acknowledged.
in permeate pressure. Fig. 11 shows that at a feed water content of 10 wt%, an increase in d o w n s t r e a m
References
pressure from 60 to 200 Pa led to a reduction in p e r m e a t i o n flux from 2.05 to 0.64 k g / m 2 h and an increase in water concentration in permeate from 92.7 to 95.9 wt%. The reduction in permeation rate due to increase in d o w n s t r e a m pressure is more significant than the gains in permeate water concentration. Therefore, the permeate pressure should be m a i n t a i n e d as low as e c o n o m i c a l l y feasible to accomplish an efficient separation. However, maintaining a high v a c u u m in pervaporation has the effect of increasing operating costs.
4. C o n c l u s i o n s A c h i t o s a n / p o l y s u l f o n e composite m e m b r a n e was prepared and tested for the separation of water from ethylene glycol. It was shown that the m e m b r a n e was selective to water permeation over the entire concentration range, and the separation performance of the m e m b r a n e was affected by changes in feed concentration, temperature, and permeate pressure, At 35°C a permeation flux of 0.3 k g / m 2 h and
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[18]
[19] [20] [21]
water-ethanol through modified chitosan membranes. IV. Phosphorylated chitosan membranes, J. Membrane Sci., 64 (1991) 145. Y.M. Lee, E.M. Shin and S.T. Noh, Pervaporation separation of water-ethanol through modified chitosan membranes. II. Carboxymethyl-, carboxyethyl-, cyanoethyl-, and amidoxime-chitosan membranes, Angew. Makromol. Chem., 192 (1991) 169. M. Kakizaki, H. Yamamoto, T. Ohe and T. Hideshima, Molecular motions and dielectric relaxations in chitin, chitosan and related polymers, in G. Skjak-Braek, T. Anthonsen and P. Sandford (Eds.), Chitin and Chitosan, Elsevier, London, 1989, pp. 511-517. Y.M. Lee, E.M. Shin and K.S. Yang, Pervaporation separation of water-ethanol through modified chitosan membranes. 1. Chitosan-acetic acid and -metal ion complex membranes, Polymer(Korea), 5 (1991)182. J.G. Wijmans and R.W. Baker, The solution-diffusion model: a review, J. Membrane Sci., 107 (1995) 1. R.Y.M. Huang, Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, The Netherlands, 1991. V. Stannett, Simple gases, in J. Crank and G.S. Park (Eds.), Diffusion in Polymers, Academic Press, New York, 1968.
Journal of Membrane Science 116 (1996) 67-76
Pervaporation with chitosan membranes. I. Separation of water from ethylene glycol by a chitosan/polysulfone composite membrane Xianshe Feng, Robert Y.M. Huang * Department of Chemical Engineering, Uni~,ersity of Waterloo, Waterloo, ON, Canada N2L 3G1 Received 5 September 1995; revised 19 December 1995; accepted 3 January 1996
Abstract A chitosan/polysulfone composite membrane was prepared. The preparation procedure involved dissolution of chitosan in dilute aqueous acetic acid to form chitosan salt, coating of the chitosan salt solution on a porous polysulfone substrate, and regeneration of chitosan by alkaline treatment. The membrane was tested for selective removal of water from aqueous ethylene glycol solutions by pervaporation. The effects of operating parameters, including feed concentration, temperature, and downstream pressure, on the separation performance of the membrane were investigated. At 35°C and 60 Pa downstream pressure, a permeation flux of 0.3 k g / m 2 h and permeate water concentration higher than 92 wt% was achieved at a feed water content of 10 wt%. This study demonstrated the potential of membrane pervaporation as an alternative to conventional distillation for the given separation. It was also shown that conditioning of the membrane in the pervaporation system at the maximum operating temperature led to a quasi-permanent change in membrane permselectivity.
Keywords: Pervaporation; Ethylene glycol/water separation; Chitosan membrane; Composite membrane
1. I n t r o d u c t i o n Pervaporation is an attractive alternative to conventional processes for the separation of liquid mixtures. This technique depends on the fact that certain membranes permit selective permeation of different species in a mixture, producing a permeate stream enriched in the preferentially-permeating species. It
materials from which the membrane is formed and an appropriate procedure to manufacture a thin membrane. Almost all industrially important pervaporation membranes are in composite forms where a thin
is the selective permeability of the membrane that forms the basis of separation. A successful pervaporation membrane lies primarily in selecting proper
" a c t i v e " membrane layer is deposited on a porous support to achieve high membrane productivity yet to retain the necessary mechanical strength. Separation processes to recover and purify products account for over 40% of overall energy consumption in the chemical processing industry, and distillation is used to make about 95% of all the
* Corresponding author. Fax: (519)746-4979; E-mail: [email protected],
separations [1]. Membrane pervaporation is believed to be especially valuable for accomplishing separations that are not cost-effective by distillation since
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved Pll S 0 3 7 6 - 7 3 8 8 ( 9 6 ) 0 0 0 2 2 - 1
68
X. Feng, R.Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
the separation efficiency of pervaporation is not limited by the relative volatility of the species to be separated. The thermodynamic efficiency of distillation is inherently low whereas that for membrane processes can be very high [2,3]. Currently, dehydration of alcohols, in particular ethanol and isopropanol, by pervaporation is the best developed application of pervaporation due to commercial interest of augmenting distillation systems [4,5]. Extensive studies have been carried out to investigate pervaporation for the separation of various azeotropic and close-boiling mixtures by using a number of different membranes. However, not as much effort has been directed at pervaporation application for dehydrating non-azeotropic systems such as ethylene glycol/water systems, Ethylene glycol is an important chemical widely used for non-volatile antifreeze and coolant as well as an intermediate in the manufacture of polyesters [6]. At present the main commercial route of ethylene glycol production is the direct oxidation of ethylene to ethylene oxide followed by hydrolysis of ethylene oxide [7]. The large excess of water from the hydrolysis is removed by multi-stage evaporation, and the glycol-enriched product from the reboiler is then dehydrated in another distillation (drying) unit. Although ethylene glycol and water do not form an azeotrope over the entire composition range, the separation of water from ethylene glycol by distillation has proved to be costly because high pressure steam is required for the reboiler due to the high boiling point of ethylene glycol (198°C). In fact, ethylene glycol/water separation by distillation is ranked the eighth most energy intensive distillation operations in the chemical process industry [8]. Pervaporation is expected to provide an energy efficient alternative. In pervaporation only that fraction
of feed stream that permeates the membrane undergoes liquid to vapor phase change. Moreover, pervapotation can be operated at lower temperatures than distillation, making it possible to utilize waste-heat as the heat of vaporization needed for pervaporation. Pervaporation is often used in conjunction with distillation and as such the enthalpy of the heated stream from a distillation unit can be directly applied as the heat input for the pervaporation unit. A case study showed that substantial reductions in utility costs for ethylene glycol/water separation could be achieved by a hybrid process of integrating distillation with pervaporation [9]. In the present study the separation of ethylene glycol/water mixtures by pervaporation through a chitosan membrane was attempted. Chitosan is the N-deacetylated product of chitin, a natural polymer that can be extracted from outer shells of crustaceans. Chitosan is a linear polymer composing primarily of glucosamine (see Scheme 1). In this study chitosan was chosen as the membrane material because of its favorable permselectivity and solvent stability as well as good film forming properties [10]. In order to increase the permeation rate a composite membrane comprising a thin chitosan barrier layer and a porous polysulfone support was prepared, and the pervaporation characteristics of the membrane for the given separation was investigated.
2. Experimental 2.1. Materials
Chitosan flakes (Flonac N) were supplied by Kyowa Technos Co., Ltd., Japan. Polysulfone powder (Udel P-3500) was supplied by Amoco Performance
~H20H
,
Chitosan
C.HzOH
Chitosan/acid salt
Scheme 1.
X. Feng, R.Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
CHITOSAN
Dissolutionin dilute aceticacid (formationofchitosan/acidsalt) I Coating chitosan salt solution
on polysulfonesubstrate
Alkaline treatment
(regeneration
of chitosan)
I washin]/rinsin9 ]
CHITOSAN/POLYSULFONE COMPOSITEMEMBRANE Fig. 1. Membranepreparation sequence,
Products, OH, USA. All organic solvents used in the experiments were of reagent grade; they were obtained from BDH Chemicals Co. except for methyl Cellosolve (ethylene glycol monomethyl ether), which was obtained from Fisher Scientific Co. Water was deionized and distilled before use. A non-woven fabric was used as the backing material for the membrane.
69
The polysulfone membrane was coated with chitosan. Since chitosan was insoluble in water, the coating solution was prepared by dissolving 0.53 wt% chitosan in a 0.79 wt% aqueous acetic acid solution. After being filtered to remove any undissolved residual solid, the chitosan solution was coated onto the polysulfone substrate membrane. The casting procedure is as follows. First, the substrate membrane was mounted in a straight-sided wide-mouth bottle that contained the chitosan coating solution with the aid of a closure. The top surface of the substrate membrane was brought into contact with the coating solution by leaving the bottle upside down for 30 min. After the residual solution on the membrane surface was drained, the membrane was dried at 70°C for 2 h in an oven with forced air circulation. Then the coated membrane was treated with an alkaline solution containing 2.6 wt% sodium hydroxide in methanol, followed by further alkaline treatment with a 4.3 wt% aqueous sodium hydroxide solution. Finally, the membrane was washed and rinsed repeatedly with water, and then air-dried. The total thickness of the dry composite membrane was found to be 240 /xm. Fig. 2 shows an scanning electron micrograph of the membrane. Dense homogeneous chitosan films were used to measure the swelling degree of chitosan in the permeating species. These fihns were prepared by pour-
2.2. Mernbraneprepara tion
~z.
~.~
The s e q u e n c e for p r e p a r a t i o n of the chitosan/polysulfone composite membrane is summarized in Fig. 1. The porous polysulfone substrate membrane was prepared via the phase inversion process from a homogeneous casting solution composing of 12 wt% polysulfone, 11 wt% methyl Cellosolve, and 77 wt% N,N-dimethyl formamide. The casting solution was cast onto the backing material held on a glass plate, and then the cast film was gelled in ice-cold water ( ~ 2°C) for 2 h. The polysulfone membrane so obtained was washed with water before being dried in air at ambient temperature. Such a porous membrane exhibited a pure water permeation rate of 218 k g / m 2 h at 22°C under a transmembrane pressure difference of 100 psi (690 kPa).
!~:~';
~i~
i
[
Fig. 2. An SEM picture of a chitosan/polysulfone membrane.
70
X. Feng, R.Y.M. Huang/ Journal of Membrane Science 116 (1996) 67-76
ing the chitosan solution onto a rimmed poly(vinyl chloride) plate, followed by the evaporation and alkaline treatment procedures, as described above, It should be noted that chitosan in its free amine form is insoluble in water at neutral pH. However, in some acidic solutions (e.g. acetic acid), the free amino groups ( - N H 2) become protonated to form water-soluble chitosan-acid salts. Alkaline treatment of the latter converts the cationic amine groups (-NH~-) into - N H 2 form, thereby accomplishing regeneration of chitosan to the free amine form (see Scheme 1). 2.3. Pervaporation Pervaporation experiments were conducted in a stirred batch cell as described before [11]. The effective area of the membrane in the permeation cell was 8.55 cm 2. The temperature was controlled by circulating water from a thermostatic bath (VWR Scientific 1167) to a water jacket that surrounded the feed compartment. The feed solution was agitated by a magnetic stirrer located 3 mm above the membrane surface. The vacuum applied to the downstream side of the membrane was controlled by a vacuum regulator and measured by a Pirani gauge, while the upstream pressure was kept at atmospheric pressure since hydrodynamic pressure can hardly affect pervaporation transport. The permeate was condensed and collected in a cold trap immersed in liquid nitrogen, and the permeation rate was determined from the amount of permeate sample collected for a given period. The permeate composition was analyzed either by a Hewlett Packard gas chromatograph (model 5890, series II) equipped with a flame ionization detector or by an Atago high-precision refractometer (type 3T), depending on permeate concentration and the amount of permeate sample. For each pervaporation run, the permeate removed by membrane was kept below 0.5% of initial feed load and the permeate sample was returned to the feed compartment after analysis for composition to maintain a constant concentration of feed. The study was concerned with steady state of pervaporation. When the permeation rate and permeate concentration became constant, the steady state of permeation was assumed. The time required to reach the steady state varied between 0.8 to 2.5 h,
depending on such factors as feed concentration and operating conditions. The membrane performance was characterized in terms of permeation flux and permeate concentration, two quantities obtainable directly from pervaporation experiments.
3. Results and discussion 3.1. Effect of feed agitation For majority of pervaporation runs, the stirrer speed was set at 200 rpm. It was found that the speed of stirrer in feed compartment, in the range 150-240 rpm, did not affect total flux and permeate composition appreciably, indicating that concentration polarization at the feed side of the membrane was negligible in the range of stirrer speeds investigated. Hence, the pervaporation characteristics reported below is the separation performance intrinsic to the membrane, free from complication due to boundary layer effect. 3.2. Effect of feed concentration Fig. 3 shows the total permeation flux and permeate concentration as a function of feed concentration. In the entire composition range, the concentration of water in permeate is higher than that in feed, suggesting that water is preferentially permeable through the membrane. Chitosan is a highly hydrophilic polymer. The permselectivity of chitosan membrane to water is believed to be due to stronger affinity of chitosan to water molecules and smaller molecular size of water as compared with ethylene glycol. Fig. 3a also shows the data of equilibrium vapor composition versus liquid composition for the binary water-ethylene glycol mixtures at 101.3 kPa [12]. It appears that at a given feed liquid concentration, the selectivity of pervaporation separation using the chitosan membrane is higher than the selectivity of distillation separation, though the difference in the selectivities of the two processes becomes less significant as the water concentration in feed increases. At water content of 5 and 10 wt% in feed liquid, the equilibrium vapor phase concentration is 49.4 and 74.1 wt% water, respectively, which values are considerably lower than the corresponding single-pass
X. Feng, R. EM. Huang / Journal of Membrane Science 116 (1996) 67-76
permeate concentration of 85.5 and 92.4 wt% water, Since pervaporation also involves a phase change of permeated species from liquid to vapor phase, it is obvious that pervaporation with the chitosan merebrane works best for low feed water concentrations. In subsequent investigations of operating temperature and downstream pressure effects, the feed concentration has been fixed at a 10 wt% water concentration. The concentration dependence of total permeation flux, shown in Fig. 3b, seems to follow an exponential fashion. The increase in the total permeation flux with feed water concentration can be explained in the following manner. Owing to the high hydrophilicity of chitosan material, the chitosan merebrane swells more significantly in a feed solution with higher water content, thereby making it easier for permeating species to penetrate the membrane. Fig. 4 shows the swelling data of chitosan in ethylene g l y c o l / w a t e r solutions; these data were ob-
®
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o
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~
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o o.84 o.8o 10o
(a) t ,~ ~
,
~
,
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,
~
,
~-
ea
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'
0.41
r 0.61
,
0.81
~ 1.0
Mass fraction of HzO in feed Fig. 3. Effect of feed concentration on (a) permeate concentration, and (b) permeation flux. Operating temperature, 35°C; downstream pressure, 60 Pa. Dotted line in Fig. 3a represents the vapor-liquid equilibrium (VLE) data at 101.3 kPa.
71
a.0 z.~ "~, 6~z.0 ;z E 1.5 .E = 1.0 w 0.5
°'°0.0
q
r
i
~
i
i
i
' 02 0.4 0.0 0.8 Mass fraction of H20 in feed
i
1.o
Fig. 4. Swelling ratio of chitosan in ethylene glycol/water solu-
tions at 35°C.
tained using the conventional procedure of sorption intake measurements [13,14]. Clearly, the degree of swelling of chitosan increases with an increase in water concentration. It is generally accepted that membrane swelling by a permeating species increases its diffusivity through the membrane. In a swollen polymer, the mobility of the polymer segments is promoted, and the space available for diffusion is increased. When the concentration dependence of diffusivity follows an exponential relationship, which is one of the most commonly used phenomenological expressions, the above observed exponential increase in permeation flux with an increase in water concentration will result. The pervaporation of ethanol/water mixtures through the phosphorylated chitosan membranes also exhibits a similar behavior [15]. The individual permeation fluxes of the permeating species were calculated from total permeation rate and permeate composition. Figs. 5 and 6 show the permeation fluxes of water and ethylene glycol, respectively. It can be seen that water permeation rate increases as the water content in feed increases. However, the water permeation rate was lower than i d e a l o v e r a large c o n c e n t r a t i o n r a n g e , a s represented by the dotted line in Fig. 5. In an ideal permeation, there are no permeant-permeant and permeantmembrane interactions, and the permeation flux of a component is proportional to its concentration in the feed. The negative deviation of water permeation
X. Feng, R.Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
72
z0
15
. . . 9 " . /
~: ~to "~ 5
."" ~
ID~
et al. [16] reported similar results for ethanol/water pervaporation through a modified chitosan membrane. Also shown in Fig. 6 is that the permeation of ethylene glycol deviated from ideal case positively at a feed glycol content up to ~ 30 wt%, above which the deviation became negative. This behavior seems to account for two aspects: one is the remarkable membrane swelling by water at low concentrations of glycol, which facilitated the permeation, the other is the tendency of ethylene glycol to form interchain hydrogen bonding at high concentrations of glycol, thus retarding its permeation through the membrane.
"
o .1"
o
OG
0.0
I
i
0.4 0.6 0.8 Mass f r a c t i o n of H~O in f e e d
1.0
0.2
3.3. Effect of temperature
Fig. 5. The partial permeation flux of water as a function of water content in feed. Dotted line represents the ideal permeation flux that would be obtained if there were no permeant-permeant and permeant-membrane interactions. Operating conditions were the same as those given in Fig. 3.
from ideal indicates that the permeation of water is retarded by the presence of ethylene glycol. Interestingly, the glycol flux did not increase monotonically with increasing concentration of ethylene glycol in feed, as shown in Fig. 6. The figure shows that an increase in feed glycol concentration increased the glycol flux when then glycol concentration is less than 10 wt% or higher than 50 wt%, whereas in the intermediate range a reverse trend was observed. Lee 0.t0
1 /
~o.o8
I
~0 w" a~ 0.06
.-" .//
M
• 0.04 ~ o~ ~
"
/
/
o x ~
~o.o2
-"
..... ,"
. 0.o0 0.0
,
i , ~ , , , i 0.2 0.4 0.6 0.8 Mass fraction of EG in feed
, 1.0
Fig. 6. The partial permeation flux of ethylene glycol as a function of ethylene glycol content in feed. Dotted line represents the ideal permeation flux. Operating conditions were the same as those given in Fig. 3.
The effect of temperature on pervaporation performance was tested by carrying out the experiments different temperatures. The m e a s u r e m e n t s w e r e conducted first under the conditions of systematically increasing the temperature from 22 to 80°C, and then the membrane was maintained at 80°C for 5 days to be "conditioned" before another series of measurements in the sequence of systematically decreasing the temperature. Figs. 7 and 8 illustrate the permeation flux and permeate concentration, respectively, with respect to temperature at a feed concentration of 10 wt% water. Empirically, the temperature dependence of permeation flux often follows an exponential fashion, as shown by the semi-logarithmic plots depicted in Fig. 7, though the curves are inflected at ~ 57°C. While the permeation flux increased with an increase in temperature, the permeation selectivity decreased gradually, as reflected by the gradual change in permeate concentration (Fig. 8). According to the classical Eyring theory of diffusion, the thermal tootion of polymer chains generates penetrant-scale transient gaps in the polymer matrix, through which the permeating molecules can diffuse. As the temperature increases, the thermal motion is intensified, and the transient diffusive gaps become larger. Consequently, the transport of bulkier ethylene glycol molecules, along with water, is enhanced, leading to an increase in permeation flux and a less-significant decrease in permeation selectivity. at
By comparing t h e d a t a (using symbols • and O) in Figs. 7 and 8, it can be seen that there is a shift in both permeation flux and permeate concentration.
X. Feng, R. EM. Huang / Journal of Membrane Science 116 (1996) 67-76
lo
~
1
0.1 ~ Measurement Sequence: ~) Temp. increasing Temp. decreasin9
[] Random change in temp.
~ °%.'8
2.9
3.0
~ ~ ~ 3.2 3.3 1000/T, K-' 3.1
~ 3.4
3.5
Fig. 7. Effect of operating temperature on permeation flux. Feed concentration, 10 wt% water; downstream pressure, 60 Pa. Measurement sequence: (1) temperature increasing from 22 to 80°C (O), (2) membrane maintained at 80°C for 5 days (membrane "conditioning"), (3) temperature decreasing from 80 to 22°C (O), and (4) random change of temperature in the same range
(a). However, continued measurements under the conditions of randomly changing temperature within the same temperature range exhibited no additional shift in the membrane performance, as shown by the consistency of the data (represented by symbols O
73
and D) in the figures. Steady state was maintained when taking the performance data, at least within the time frame of measurements. An explanation of the aforementioned behavior may be that the membrane morphology was changed during the course of membrane "conditioning". However, an examination of the membrane cross section by a scanning electron microscope showed no visible change in the membrane structure after conditioning, suggesting that the morphological change is due to subtle perturbations of the polymer matrix that constitute the membrane. It seems that the morphological change is quasi-permanent since the experimental data in the second and the third series of experiments agree while prolonged tests showed no definite trend in the change of membrane performance. It is not yet clear at present time whether the inflection in Fig. 7 is caused by the secondary transition of the membrane material. The glassy transition temperature of a dry chitosan material is over 150°C [17], which value is expected to be lowered substantially in the case of a wet chitosan membrane because of the strong plasticization effect of water in the feed. This is supported by the experimental observation that the dense homogeneous chitosan films are soft and elastic in water but become quite rigid when completely dried. The flux vs. temperature plots reported by Lee et al. [16,18] for waterethanol pervaporation through modified chitosan membranes also displayed the tendency of inflection, although such a tendency was not elaborated explic-
ty "~
Based on the data (represented by symbols O and
D) in Figs. 7 and 8, the permeation fluxes of .~ 0.9
% O :~
o.8
Measurement Seguence:
• Temp. increasing o Temp. decreasing [] Random change in temp.
z~ °72o
30L 4o~ 5o~ 6o~ vo~ 8o~ 9or loo Ternperature.°C
Fig. 8. Effect of operating temperature on permeate concentration. Operating conditions and meaning of the symbols are the same as those given in Fig. 6.
individual species after membrane "conditioning" are depicted in Fig. 9, which again shows an inflection at 57°C. The temperature dependence of permeation flux is due to the joint effects of temperature on the driving force for permeation and the membrane permeability. The chemical potential gradient or the partial pressure difference across the membrahe is normally considered as the driving force for the pervaporative mass transport [19]. The activation energy for permeation can be calculated from an Arrhenius-type expression of the form
(J/Ap)
i = aiexp( -Ei/RT
)
(1)
X. Feng, R.Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
74
101
I0 o Water [] Ethylene glycol
,~ ~
0 Wafer 1:3 Ethylene glycol
o.1
0.00
.
2.9
3.0
3.1
3.E
3.3
3.4
3.5
2.9
3.0
IO00/T, K -~
3.1
3.2
3.3
3.4
3.5
tOOO/T, K -i
Fig. 9. Partial fluxes of water and ethylene glycol after membrane "conditioning" vs. reciprocal temperature. Feed concentration, 10 wt% water; downstream pressure, 60 Pa.
Fig. 10. Arrhenius plots showing permeation flux normalized for partial pressure difference across membrane versus reciprocal temperature. The conditions were the same as for Fig. 9.
where ( J / Z i p ) is the permeation flux normalized for transmembrane partial pressure difference, A is a constant, E is the activation energy for permeation, T is the absolute temperature, and subscript i denotes the permeating species. Here the driving force-normalized flux is used to evaluate activation energy for permeation because the variation of flux with temperature due to the effect of temperature on the driving force should be excluded, although many researchers [20] simply used In J vs. 1 / T plots to calculate the activation energy. The Arrhenius plots for water and ethylene glycol permeation through the chitosan/polysulfone membrane are shown in Fig. 10. The corresponding activation energy data are listed in Table 1. The activation energy is only an empirical parameter that relates temperature dependence of membrane permeability. If pervaporation transport follows the solution-diffusion mechanism, then the permeability coefficient is given as a prod-
endothermic or exothermic. It may be mentioned that when the polymer undergoes a glass transition, both the magnitude and the sign of sorption enthalpy can change [21]. The results in Table 1 seem to indicate that the sorption enthalpy is more significant than the diffusion activation energy at temperatures below 57°C; at higher temperatures, the reverse situation holds.
Table 1
uct of diffusion coefficient and solubility coefficient.
Activation energy for pervaporation after membrane conditioning a
Consequently, the activation energy for permeation is a combination of enthalpy of permeant sorption in the membrane and the activation energy of permeant diffusion through the membrane. While the activation energy of diffusion is normally positive, the enthalpy of sorption can be either positive or negative, depending on whether the sorption process is
Species
E (kJ/mol)
Temperature t (°C)
Water
8.9 49.8 -28.5
< 57 > 57 <57
3.4. Effect of downstream pressure The permeate pressure influenced membrane performance considerably, as illustrated in Fig. 11. Since the driving force for pervaporation transport is provided by lowering the downstream pressure, it is natural that permeation rate decreases with increases
Ethylene glycol
-
31.4
range,
> 57
a Feed concentration, 10 wt% water; downstreampressure, 60 Pa.
X. Feng, R. Y.M. Huang / Journal of Membrane Science 116 (1996) 67-76
100 98
~
09
96 .E o 94 o~ 92 "~ 90 , 2.4 ~" 42.0 ~ 1.6
~
,
r
,
~ ,
~
,
~
,
~
75
permeate water concentration higher than 92 wt% was achieved for a feed with water content of 10 wt%. These results demonstrate the potential of pervaporation as an alternative to conventional distillation for separation of w a t e r - e t h y l e n e glycol mixtures. C o n d i t i o n i n g the m e m b r a n e at the m a x i m u m operating temperature led to a change in m e m b r a n e performance, and such a change was found to be quasi-permanent. Because the m e m b r a n e was not prepared under optimized conditions, c o n t i n u e d investigations are needed to improve m e m b r a n e performance.
1.2
o.8
Acknowledgements
~0.4 °'°0.'0
The chitosan material used in the study was gen-
i
I
0.2 0.4 0.6 0.8 1.0 1.2 Downstream pressure, kPa Fig. 11. Effect of permeate pressure on permeation flux and permeate concentration. Feed concentration, 10 wt% water; operating temperature, 75°C.
erously provided by K y o w a Technos Co., Ltd., Japan. The financial support by the E n v i r o n m e n t a l Science and T e c h n o l o g y Alliance C a n a d a and the Natural Sciences and E n g i n e e r i n g Research Council of C a n a d a is gratefully acknowledged.
in permeate pressure. Fig. 11 shows that at a feed water content of 10 wt%, an increase in d o w n s t r e a m
References
pressure from 60 to 200 Pa led to a reduction in p e r m e a t i o n flux from 2.05 to 0.64 k g / m 2 h and an increase in water concentration in permeate from 92.7 to 95.9 wt%. The reduction in permeation rate due to increase in d o w n s t r e a m pressure is more significant than the gains in permeate water concentration. Therefore, the permeate pressure should be m a i n t a i n e d as low as e c o n o m i c a l l y feasible to accomplish an efficient separation. However, maintaining a high v a c u u m in pervaporation has the effect of increasing operating costs.
4. C o n c l u s i o n s A c h i t o s a n / p o l y s u l f o n e composite m e m b r a n e was prepared and tested for the separation of water from ethylene glycol. It was shown that the m e m b r a n e was selective to water permeation over the entire concentration range, and the separation performance of the m e m b r a n e was affected by changes in feed concentration, temperature, and permeate pressure, At 35°C a permeation flux of 0.3 k g / m 2 h and
[1] J.L. Humphery, A.F. Seibert and R.A. Koort, Separation Technologies - Advances and Priorities, US DOE Final Report, Contract no. DE-AC07-90IDI2920, US Department of Energy, Washington, DC, 1991. [2] J.L. Humphery and A.F. Seibert, Separation Technologies: An opportunity for energy savings, Chem. Eng. Prog., March (1992) 32-41. [31 M. Mulder, Energy requirements in membrane separation processes, in J.G. Crespo and K.W. BSddeker (Eds.), Membrane Processes in Separation and Purification, Kluwer Academic, Dordrecht, The Netherlands, 1994, pp. 445-475. [41 H.L. Fleming, Membrane pervaporation: separation of organic/aqueous mixtures, Sep. Sci. Technol., 25 (1990) 1239. [5] R. Rantenbach, S. Klatt and J. Vier, State of the art of pervaporation - 10 years of industrial PV, in R. Bakish (Ed.), Proc. Sixth Int. Conf. Pervaporation Processes Chem. Ind., Bakish Material Corp., Englewood, NJ, 1992, pp. 2-15. [6] B.F. Greek, Ethylene-glycol supplies will be sufficient to meet rising demand, Chem. Eng. News, 69(33) (1991) 11. [7] M.W. Forkner, J.H. Robson, and W.M. Snellings, Glycol, in J.I. Kroschwitz and M. Howe-Grant (Eds.), Kirk-Othmer Encyclopedia of Chemical Technology, 4th edn., Vol. 12, Wiley, New York, 1994, pp. 695-714. [81 J.L. Bravo, J.R. Fair, J.L Humphery, C.L. Martin, A.F. Seibert and S. Joshi, Fluid Mixture Separation Technologies for Cost Reduction and Process Improvement, NoTes Data Corp., Park Ridge, NJ, 1986.
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