HEC) composite membranes

HEC) composite membranes

Journal of Membrane Science 195 (2002) 143–151 Pervaporation dehydration of ethanol–water mixtures with chitosan/hydroxyethylcellulose (CS/HEC) compo...

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Journal of Membrane Science 195 (2002) 143–151

Pervaporation dehydration of ethanol–water mixtures with chitosan/hydroxyethylcellulose (CS/HEC) composite membranes I. Effect of operating conditions R. Jiraratananon a,∗ , A. Chanachai a , R.Y.M. Huang b , D. Uttapap a a

Department of Chemical Engineering, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand b Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 Received 1 March 2001; received in revised form 28 May 2001; accepted 18 June 2001

Abstract Composite hydrophilic pervaporation membranes were prepared from chitosan blended with hydroxyethylcellulose using cellulose acetate as a porous support. The membranes were tested for dehydration performance of ethanol–water mixtures of ethanol concentrations 70–95 wt.% in the laminar flow region, at temperatures 50–70◦ C and at permeate pressures of 3–30 mmHg. The composite membrane showed an improved dehydration performance compared with dense CS/HEC membrane developed earlier. The effects of operating conditions also revealed that pervaporation of low water content feed carried out at high feed flow rate and at low temperature and permeate pressure was an advantage. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Composite membrane; Ethanol–water mixture; Dehydration; Pervaporation separation index; Separation factor

1. Introduction Dehydration of aqueous ethanol solutions by pervaporation has been commercialized since 1982. The expansion of this technology, however, is still limited compared with other membrane processes, partly due to the low selectivity and permeability of hydrophilic pervaporation membranes. Hydrophilic membranes such as polyvinyl alcohol (PVA) and chitosan (CS) are required for dehydration purpose. A number of techniques such as copolymerization, blending [1,2], and crosslinking [3] have been shown to be effective for improving pervaporation performance of the membranes. The preparation of com∗ Corresponding author. E-mail address: [email protected] (R. Jiraratananon).

posite membranes, which possess thin and dense skin on the top of porous support, can also significantly improve pervaporation performance [4]. The PVA/CS blended membranes crosslinked with multicomponent acid were successfully used for dehydration of alcohol–water mixtures [1]. Chanachai et al. [5] reported that dense CS/HEC blended membrane (blend ratio 3/1) crosslinked with urea–formaldehyde–sulfuric acid mixture showed promising results on pervaporation dehydration of ethanol–water mixtures. Performance of pervaporation is dependent not only upon the membranes but also upon the operating parameters such as feed concentration, temperature, permeate pressure and feed flow rate. A number of researchers reported the effect of these parameters on various pervaporation systems [6–10]. Feed

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 5 6 3 - 4

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concentration refers to the concentration of the more permeable (usually minor) component in the solution. A change of feed concentration directly affects sorption at the liquid/membrane interface, i.e., the concentration of the components in the membrane tends to increase with the feed concentration. Since diffusion in the membrane is concentration dependent the permeation flux, generally, increases with feed concentration. Mass transfer in the liquid feed side may be limited by the extent of concentration polarization. In principle, an increase of feed flow rate should reduce concentration polarization and increase flux due to a reduction of transport resistance in liquid boundary layer. Flux and selectivity in pervaporation are also affected by feed temperature and flux, generally, follows an Arrhenius law:   Ep,i Ji = Ai exp − (1) RT where, Ji is the permeation flux of i (g/m2 s), Ai the pre-exponential parameter (g/m2 s), R the gas constant, T the absolute temperature (K) and Ep,i is the activation energy for permeation (kJ/mol) which depends on both activation energy for diffusion and heat of solution. In most cases, a small decrease of separation factor with increasing feed temperature was found [8]. However, an increase of separation factor with feed temperature was also reported [11,12]. Permeate pressure is also an important parameter since the high vacuum is costly. The maximum driving force can be obtained at zero pressure. The effect of permeate pressure change on flux was described mathematically and confirmed experimentally by many investigators [13,14]. For separation factor, its variation with permeate pressure relies on the relative volatility of the permeating components. This research is divided into two parts, which is a continuation of our previous work [5]. In this first paper, we investigate the performance of the blended CS/HEC-CA composite membrane on dehydration of ethanol–water mixtures. In the second paper to follow the analysis of mass transfer based on the resistance-in-series model is presented. The objectives of this paper are to develop the composite membrane consisted of CS/HEC top layer on cellulose acetate

(CA) porous support and to test its dehydration performance as affected by the operating conditions. The results of this work are expected to offer useful criteria for selection of appropriate operating conditions for each pervaporation system and ensure that the system be operated at its highest performance and is economically viable.

2. Experimental 2.1. Membrane preparation The materials and procedures for preparation of the top dense layer of the composite membrane were previously described [5]. The CS/HEC blend ratio of 3/1 which possessed highest pervaporation separation index (PSI) was selected to prepare the casting solution which was cast on the porous CA support with the aid of a Garder casting knife. The porous support was prepared by the phase inversion method [15]. A homogeneous casting solution consisted of 17.5% CA, 35% dimethylformamide, and 47.5% acetone was cast on a glass plate. After evaporation of solvent for 0, 2, or 5 min, the cast film was gelled in water at 25◦ C for 30 min, then rinsed with deionized water. The CA membrane obtained was hydrolyzed with 0.1N NaOH at room temperature for different periods of time, washed repeatedly with water, then dried in an oven at room temperature. Water flux of CA supports was measured at room temperature and a transmembrane pressure of 5170 mmHg. After casting of the dense skin layer on the CA porous support, the resulted composite membrane was dried and crosslinked by immersing the membrane in a mixture of 25 wt.% urea–2.2 wt.% formaldehyde–2.5 wt.% sulfuric acid containing 50 wt.% ethanol at room temperature for 2 h. The total and the skin thicknesses of the membrane measured by scanning electron microscope model JSM-840 of JEOL Co. were 110 and 4 ␮m, respectively. 2.2. Pervaporation experiments The detailed experimental set up for pervaporation was the same as described previously [5]. For each experiment, the operating parameter was varied one at a time and covered the following ranges — feed

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concentration: 70, 80, 90 and 95 wt.% ethanol; feed temperature: 50, 60 and 70◦ C; permeate pressure: 3, 10, 20 and 30 mmHg. Feed rates were varied in the range of 74–542 ml/min in which the average velocities across the membrane surface were 0.012–0.09 m/s. The corresponding Reynolds number were 16–190 which are in the laminar flow region. At the end of each experiment, which lasted for 2 h, retentate and permeate samples were collected for analysis of fluxes and compositions. Total flux was calculated directly from the permeate weight. The retentate and permeate compositions were analyzed either by a Hewlett-Packard gas chromatography (model 5890, series II) with a flame ionization detector or by an Atago high-precision refractometer (type 3T), depending on the concentration. Knowing of the composition allows for the ethanol and water fluxes to be calculated. The separation factor (α) and pervaporation separation index (PSI) are calculated as follows: separation factor =

yw /ya xw /xa

PSI = total flux × (separation factor − 1) where y w and y a are the weight fractions of water and ethanol in the permeate and x w and x a are the weight fractions of water and ethanol in the feed, respectively.

3. Results 3.1. Effect of CA porous support preparation on the performance of CS/HEC-CA composite membranes Table 1 shows that clean water fluxes of CA porous supports, which fell in the range of ultrafiltration membranes, decreased with solvent evaporation time but increased with hydrolysis time. The composite membranes with different preparation conditions of CA porous supports gave the pervaporation performance as shown in Fig. 1. By phase inversion method the skin layer is generally formed during an evaporation of solvent and its thickness increases, but the porosity decreases with time [16]. Therefore, flux decreased but separation factor increased with increasing evaporation time. When CA supports were hydrolyzed,

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Table 1 Water flux of CA ultrafiltration substrates under a transmembrane pressure of 5170 mmHg Evaporation time (min)

Hydrolysis time (min)

Water flux (kg/m2 h)

0

0 2 10 30 60

198.85 222.19 264.97 270.08 271.56

2

0 2 10 30 60

129.65 190.34 250.12 260.82 265.52

5

0 2 10 30 60

56.56 90.53 107.55 130.15 150.48

it can be explained that large hydrophobic acetate groups were replaced by smaller hydrophilic hydroxyl groups. Such replacement increased hydrophilicity and void volumes [17] of the substrates and enhanced pervaporation flux of composite membranes (Fig. 1). An increase in hydrophilicity enhanced the affinity of the CA substrates to water vapor which related to higher substrate selectivity. On the other hand, an increase in void of a substrate increased diffusion of both water and ethanol vapor which reduced the selectivity. The results indicated that the hydrolysis time of 10 min gave the optimal balance between hydrophilicity and void of the substrates and separation factor was highest. The composite membrane CS/HEC of blend ratio 3/1 on CA porous support prepared by using 2 min evaporation time and 10 min hydrolysis time was then selected for pervaporation experiments. 3.2. Effect of operating parameters on the performance of CS/HEC-CA composite membranes 3.2.1. Effect of water content in feed From Fig. 2, an increase of water content in feed resulted in a sharp increase of water, ethanol and total fluxes but separation factor and PSI were decreased as also reported by other researchers [3,18]. When the

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Fig. 1. Effect of evaporation time and hydrolysis time of CA porous supports on pervaporation performances of CS/HEC:3/1-CA composite membranes at 60◦ C, 90 wt.% ethanol solution: (a) flux; (b) separation factor; (c) PSI.

water content in feed was increased both water and ethanol sorption increased [5]. Consequently, water flux and ethanol flux increased due to the increase of driving force for the transport of water and ethanol. The CS/HEC blended membrane showed a strong affinity to water, which led to an extensive swelling of the membrane [5]. The same thing was expected for composite membrane in which the free volume and swelling increased significantly with water content in the feed. Ethanol is normally difficult to diffuse into the non-swollen membrane due to its large size while it can diffuse easily through swollen membrane and in an amorphous (hydrophobic) region of the membrane.

Therefore, sorption selectivity toward water decreased with water concentration in feed. Diffusion of ethanol through a swollen membrane may also increase due to a coupling effect. A larger increase of ethanol sorption and diffusion in the swollen membrane resulted in larger increase of ethanol flux than water flux and caused a reduction of the separation factor. The PSI curve (Fig. 2), which showed a higher PSI value at lower water concentration indicated that pervaporation performance of lower water content feed was an advantage. It is known that pervaporation is especially attractive when the concentration of the component to be removed is relatively low.

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Fig. 2. Effect of water content in feed on (a) flux and (b) separation factor and PSI in pervaporation of ethanol solution at 60◦ C, feed flow rate 542 ml/min and permeate pressure 3 mmHg with CS/HEC:3/1-CA composite membrane.

3.2.2. Effect of feed flow rate Water, ethanol, and total fluxes, all increased, presumably, linearly with feed flow rate as shown in Fig. 3a due to a decrease of concentration polarization. In general, the concentration of water, a more permeable component, on the membrane surface is lower than that in the bulk phase. The opposite is true for ethanol which is a less permeable component. A reduction of concentration polarization means that water concentration near the membrane surface was close to the water concentration in the bulk. An increase of water concentration on the membrane surface with feed rate enhanced water as well as ethanol sorption in the membrane such that water and ethanol fluxes increased (due to an increase of the driving force to transport across the membrane and an increase of membrane swelling) and the separation factor was reduced. An increase of PSI with feed flow rate (Fig. 3b) indicated that the operation of pervaporation system at high flow rate was an advantage. 3.2.3. Effect of feed temperature The influences of feed temperature on pervaporation performances are given in Fig. 4. As expected, when

temperature was increased, the total permeation flux increased, but the separation factor decreased. Similar trends were also reported [8,12]. Temperature affected the transport of components in the liquid feed and in the membrane. Both mass transfer coefficient of components in the liquid and sorption of components into the membrane increase with feed temperature. In addition, the polymer chains were more flexible at higher temperatures and caused larger available free volume of polymer matrix for diffusion. By all given reasons, flux increased and separation factor decreased with temperature. The experimental data were fitted with the Arrhenius-type relationship (Eq. (1)). Fig. 5 shows that linearity existed for the relationship between fluxes and the reciprocal of the absolute temperature at different feed concentrations. The apparent activation energy of permeation of water (EP,w ) and ethanol (EP,e ) in the membrane were taken from the slope of the Arrhenius plot, and the results are shown in Fig. 6. The EP,e was always higher than EP,w over the compositions studied, indicating that water preferentially permeated by this membrane. This is due to the affinity between water and membrane which is stronger

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Fig. 3. Effect of feed flow rate on (a) flux and (b) separation factor and PSI in pervaporation of 90 wt.% ethanol solution at 60◦ C and permeate pressure 3 mmHg with CS/HEC:3/1-CA composite membrane.

Fig. 4. Effect of feed temperature on (a) flux and (b) separation factor and PSI in pervaporation of 90 wt.% ethanol solution, feed flow rate 542 ml/min and permeate pressure 3 mmHg with CS/HEC:3/1-CA composite membrane.

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Fig. 5. Effect of feed temperature on (a) water flux and (b) ethanol flux at different feed compositions, feed flow rate 542 ml/min and permeate pressure 3 mmHg with CS/HEC:3/1-CA composite membrane.

than that between ethanol and membrane since water can better form H-bonds with the membrane and its dipole moment is also higher than that of ethanol. Lower EP,w also suggests that water permeation was less sensitive to temperature change in the membrane or ethanol permeability increased to a greater extent with temperature than that of water, leading to a decrease in the separation factor as shown before in Fig. 4.

Fig. 6. Activation energy for permeation of water and ethanol obtained from the Arrhenius plot (Fig. 5).

Because an increase of temperature can reduce the amount of water–water clusters, which are high for high water content feed, and thus can significantly enhance the permeation of water. In other words, for high water content feed, permeation flux depends largely on temperature. This caused an increase in EP,w with water content in feed (Fig. 6). Similar results were also reported for the system of water–ethanol mixture and CS membrane [19] and PVA membrane [20]. Since ethanol transports through the hydrophobic amorphous network of the membrane, ethanol flux depends strongly on the polymer chain segment mobility. Because an increase in temperature can increase polymer mobility, permeation flux of ethanol strongly increased with feed temperature and was more strongly dependent on temperature than water as described previously. At low water content in the feed, the polymer chain is only slightly plasticized by water so an increase of temperature can rapidly increase polymer chain mobility which benefits the permeation of ethanol. With high water content in feed, the plasticization effect of water also enhanced ethanol flux. However, below

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Fig. 7. Effect of permeate pressure on (a) flux and (b) separation factor and PSI in pervaporation of 90 wt.% ethanol solution at 60◦ C, and feed flow rate 542 ml/min with CS/HEC:3/1-CA composite membrane.

20 wt.% water content, a plasticization effect of water may still be small and thermodynamic interaction between permeants dominated the transport of ethanol. Therefore, the ethanol flux strongly depended on temperature when water concentration was increased, so E P,e increased. At high water content in the feed (higher than 20 wt.%), the increase of plasticization effect of water can significantly enhance the free volume and diffusion of ethanol in amorphous network. The effect of temperature then become weaker and E P,e decreased as shown in Fig. 6.

3.2.4. Effect of permeate pressure From Fig. 7, both flux and separation factor decreased with increasing permeate pressure since there is a reduction of driving force for transport of components. Similar results were reported by a number of researches [9,12,18,21]. However, the decrease of water flux was larger than that of ethanol. Since the saturated vapor pressure of water is lower than that of ethanol, an increase of permeate pressure slowed down evaporation of water and caused a decrease of separation factor.

Table 2 Pervaporation dehydration performance of various membranes Membrane

EtOH concentration (wt.%)

Temperature (◦ C)

Total flux (g/m2 h)

Separation factor

PSI (g/m2 h)

CS-PES composite [21] CS/PVA-Psf composite [1] CS-PAN composite [3] CS/HEC dense [5]

95 94 95 90 95

80 – 75 60 60

350 150 600 112 45

500 3000 5000 10491 33231

174650 449850 2999400 1178278 1516411

CS/HEC-CA composite (this work)

90 95

60 60

424 220

5469 16606

2332188 3665381

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4. Conclusions Composite hydrophilic pervaporation membrane was prepared from CS/HEC on a porous CA support. The dehydration performance of the membrane was superior to that of dense CS/HEC previously reported. The comparison shown in Table 2, along with those reported in the literature, indicated the satisfactory and better performance of the membrane developed. The results of this work also show that an increase of temperature, feed flow rate, and feed concentration enhanced flux but reduced separation factor. Operating the pervaporation system at low permeate pressure can increase both flux and separation factor. However, a trade-off between flux and separation factor has to be considered carefully together with the economic feasibility of the system. Acknowledgements The authors would like to gratefully acknowledge the research funds supported by the National Science and Technology Development Agency of Thailand (NSTDA), and by the Natural Sciences and Engineering Research Council of Canada (NSERC). References [1] L.G. Wu, C.L. Zhu, M. Liu, Study of a new pervaporation membrane. Part 1. Preparation and characteristics of the new membrane, J. Membr. Sci. 90 (1994) 199–205. [2] H.C. Park, R.M. Meertens, M.H.V. Mulders, C.A. Smolders, Pervaporation of alcohol–toluene mixtures through polymer blend membranes of poly(acrylic acid) and poly(vinyl alcohol), J. Membr. Sci. 90 (1994) 265–274. [3] K. Watanabe, S. Kyo, Pervaporation performance of hollow-fiber chitosan–polyacrylonitrile composite membrane in dehydration of ethanol, J. Chem. Eng. Jpn. 25 (1992) 17– 21. [4] X. Feng, R.Y.M. Huang, Liquid separation by membrane pervaporation: a review, Ind. Eng. Chem. Res. 36 (1997) 1048–1066. [5] A. Chanachai, R. Jiraratananon, D. Uttapap, G.Y. Moon, W.A. Anderson, R.Y.M. Huang, Pervaporation with chitosan/hydroxyethylcellulose (CS/HEC) blended membranes, J. Membr. Sci. 166 (2000) 271–280.

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