Preparation of poly(vinyl alcohol)-sodium alginate hollow-fiber composite membranes and pervaporation dehydration characterization of aqueous alcohol mixtures

Preparation of poly(vinyl alcohol)-sodium alginate hollow-fiber composite membranes and pervaporation dehydration characterization of aqueous alcohol mixtures

Desalination 193 (2006) 202–210 Preparation of poly(vinyl alcohol)-sodium alginate hollow-fiber composite membranes and pervaporation dehydration cha...

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Desalination 193 (2006) 202–210

Preparation of poly(vinyl alcohol)-sodium alginate hollow-fiber composite membranes and pervaporation dehydration characterization of aqueous alcohol mixtures Y.Q. Dong, L. Zhang, J.N. Shen, M.Y. Song, H.L. Chen* College of Materials Science and Chemical Engineering, Zhejiang University, Hangzhou, 310027, PR China Tel. +86 (571) 8795-2121; email: [email protected] Received 31 March 2005; accepted 3 August 2005

Abstract A hollow-fiber composite membrane, poly(vinyl alcohol) (PVA)-sodium alginate (SA) blend, supported by a polysulfone (PS) hollow-fiber ultrafiltration membrane, was prepared for pervaporation dehydration from isoproanol, n-butanol, tert-butanol and ethanol aqueous solutions. The compatibility of PVA–SA was characterized by FTIR and SEM; the mechanism of the cross-linking reaction is discussed. The results of pervaporation showed that high selectivity and promising permeability were obtained at a 4:1 ratio of the composition of the blended membrane crosslinked with 1.5 wt% maleic acid for 8 h. With alcohol concentration at 90 wt% in the feed at 45EC, the separation factors and permeation fluxes were 1727, 414 g/m2.h; 606, 585 g/m2.h; 725, 370 g/m2.h; and 384, 384 g/m2.h for four alcohol aqueous solutions through the blended membranes. Keywords: PVA–SA blend; Hollow-fiber composite membrane; Pervaporation dehydration; Alcohol

1. Introduction Over the past decades, pervaporation was considered as one of the most effective separation units and energy-saving technologies for azeotropic and close boiling point mixtures. Now, *Corresponding author.

there are over 100 pervaporation plants in the world, most of which use frame-and-plate membrane modules for pervaporation dehydration of alcohols. However, there are some disadvantages to the plate-and-frame module, such as its large size and low-packet density. Recently, compact hollow-fiber modules are considered to be effective, and the development of such membrane

Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.

doi:10.1016/j.desal.2005.08.023

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modules for pervaporation depends on desirable membrane and anti-leakage encapsulation technology. Only a few previous studies [1,2] have reported on the hollow-fiber module for pervaporation dehydration. Poly(viny1 alcohol) (PVA) is one of the important membrane pervaporation materials for dehydration from organic solvents for its chemical stability, membrane-forming ability and heatresistant properties. But PVA membrane shows lower permselectivity and higher permeability due to good hydrophilicity [3]. Sodium alginate (SA), which is one of the polysaccharides extracted from seaweed, has shown excellent water solubility [4], but the mechanical weakness of SA membranes has been a drawback as a pervaporation membrane material. Yeom [5] solved the relaxation of SA membrane polymeric chains by blending PVA with SA. Aminabhavi [6] prepared SA–PVA blended membranes by physical mixing in different ratios and investigated the pervaporation dehydration characteristics. Joncceon [7] blended SA with PVA to increase the water permeability. All these PVA and SA blended membranes are flat. PVA–SA hollowfiber composite membranes for organic dehydration by pervaporation have not been reported till now. In the work, crosslinked hollow-fiber composite membranes were prepared by coating PVA– SA blended solutions on a PS hollow-fiber membrane. The pervaporation dehydration characteristics of the membranes for four alcohol aqueous solutions — isopropanol, n-butanol, tertbutanol and ethanol — were investigated. Batch operations of pervaporation dehydration from the alcohols were also carried out. 2. Experimental 2.1. Materials PVA (95% hydrolyzed with a 1790 degree of polymerization), SA (viscosity above 0.02 Pa.s in

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1 wt% aqueous solutions at 20EC) was purchased from the Shanghai Chemical Reagent Supply Station. The PS hollow-fiber ultrafiltration membrane was supplied by the Development Center of Water Treatment Technology, Hangzhou. Maleic acid, ethanol, isopropanol, n-butanol and tertbutanol were obtained from Shanghai Chemical Reagent. All products were used directly without further purification. 2.2. Membrane preparation and intrinsic viscosity measurement PVA and SA solutions with a concentration of 2 wt% were prepared by dissolving PVA and SA in distilled water for 2 h at 90EC. The casting solution was prepared by uniformly mixing together the two solutions with different ratios. The hollow-fiber composite membranes were prepared by coating the blended solution onto the PS hollow-fiber membrane, and then they were dried at room temperature in a dust-free atmosphere. Then the dry membranes were immersed in a reaction solution containing maleic acid, sulfuric acid and ethanol for some time at ambient temperature. Then they were removed from the reaction solution, and washed several times with pure ethanol to eliminate any possible residual crosslinking agent and dried at room temperature. Solutions of PVA–SA blends in water or water and alcohol mixture solvents were prepared for 24 h, and then filtered through G2 sintered glass filters. Viscosity was measured with an Ubbelohde dilution viscometer at 25EC. An extrapolation procedure from data obtained for five concentrations of solutions was used to evaluate intrinsic viscosity. 2.3. FTIR and SEM measurements An FTIR spectrometer (SX) was used to characterize the PVA, SA and PVA–SA blended membrane and the crosslinked PVA–SA membrane with maleic acid. SEM (XL30-ESEM,

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Phillips) was used to study the cross-section morphology of the composite membranes and to measure the thickness of the membrane at 15 kV. 2.4. Module construction and pervaporation experiment The hollow-fiber modules were prepared by sealing the bottom of the hollow-fiber composite membranes in a polyester or glass pipe with epoxy resin. In the pervaporation experiments, the effective membrane area was 18.54 cm2, and the feed is into the module shell. The permeate was condensed downstream by liquid nitrogen. A vacuum pump maintained the downstream press at 100±30 Pa. The flux was determined by Eq. (1). (1) where J, W, A and t represent the flux (g m!2h!1), weight of the permeate (g), effective membrane area (m2), and operation time (h), respectively. The separation factor, α, is calculated from Eq. (2): (2) where XW, XA, YW and YA are the weight fractions of water and alcohol in the feed and permeate, respectively.

Fig. 1. FTIR spectrum of PVA–SA blended membranes. (a) PVA, (b) PVA–SA (4:1), (c) PVA–SA (3:2), (d) PVA–SA (2:3), (e) PVA–SA (2:3), (f) SA.

-COO peaks of blend membranes are shifted to a higher wave number, which illustrates the good interaction between PVA and SA. 3.2. Scanning electromicrograph of blended membranes SEM photographs of the PVA–SA blended composite membrane are shown in Fig. 2. It is obvious that the surface of the blended membrane is flat, and has no pores nor cracks. The inner structure of the membrane is homogeneous with no pores nor obvious phenomena of microphase separation. This illustrates the good compatibility between PVA and SA. A PVA–SA blended thin layer is properly cast on the surface of the PS hollow-fiber membrane (Fig. 2b), and the thickness of thin layers is about 1.8 µm.

3. Results and discussion

3.3. Mechanism of PVA–SA crosslinking reaction

3.1. FTIR spectra analysis of blended membranes

Fig. 3 gives the ideal crosslinking reaction mechanism of PVA–SA membranes with maleic acid. In the crosslinking reaction, the -OH groups of PVA and SA react with the group of

FTIR spectra of PVA, SA and their blended membranes are shown in Fig. 1. For the SA membrane, characteristic peaks of -OH are at 3386 cm!1, and asymmetrical and symmetrical stretching vibration peaks of -COO are at 1604 and 1411 cm!1, respectively [8]. From Fig. 2, the

maleic acid to form the

group.

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(a)

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(b)

Fig. 2. SEM photographs of the PVA–SA (4:1). (a) Surface; (b) Cross-section.

Fig .3. Crosslinking reaction mechanisms of PVA and SA with maleic acid.

Fig. 4 is the FTIR spectra of the PVA–SA membrane and crosslinked PVA–SA membrane with maleic acid. The spectrum of the crosslinked PVA–SA membrane shows an absorbance band

at 1725 cm!1, which is the characteristic band for the group of crosslinked PVA–SA by maleic acid.

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excessive swelling of membranes. In principle, a polymer material with higher crosslinking density has lower membrane mobility and a more compact network structure, resulting in less flux and less liquid solubility. In this study meleic acid was used as the cross linker for the PVA–SA blended membrane, and pervaporation dehydration of the crosslinked PVA–SA membrane was carried out for aqueous solutions with 90 wt% alcohols. It is shown in Fig. 6 that the permeation flux decreases and separation factor increases with increasing maleic acid content. Fig. 4. FTIR spectra of PVA–SA (4:1) membrane (1) and crosslinked PVA–SA membrane with maleic acid (2).

3.4. Effect of PVA–SA blended ratio on pervaporation performance The effect of PVA–SA blended ratios on pervaporation characteristics is shown in Fig. 5. Separation factors decrease and permeation fluxes increase with an increasing PVA content in the blended membranes. As PVA content ranged from 0 wt% to 80 wt%, separation factors decreased slowly and permeation flux greatly increased. However, when the PVA content varied from 80 wt% to 100wt%, separation factors decreased drastically, but permeation fluxes still increased greatly. This may be the reason why PVA belongs to the flexible polymers, and is easily oriented in feed, leading to molecule chain expansion and increasing of free volume. However, SA is a rigid chain polymer whose molecule movement ability is small and is hard to disorient. The SA molecule is filled in the PVA molecule chain for blended membranes with lower SA content, which limits the disorientation of membrane in the feed. 3.5. Effect of crosslinking agent on pervaporation performance Crosslinking is an efficient strategy to control

3.6. Pervaporation performance of alcohol aqueous solutions From the above results, the PVA–SA blended membrane with a 4:1 ratio, crosslinked in 1.5 wt% maleic acid for 8 h, was used for dehydration from alcohol aqueous solutions. Figs. 7 and 8 give the pervaporation characteristics of the PVA–SA blended membrane for four alcohol aqueous solutions. For all alcohol aqueous solutions, the permeation flux complies with typical increases with increased water content in the feed; the selectivity increases initially — but then decreases — with an increase of alcohol concentration in the feed. The membranes show higher selectivity for isopropanol, n-butanol and tertbutanol aqueous solutions than for the ethanol aqueous solution. This can be explained by the interaction between solution and membrane. The solubility parameter is an intrinsic physicochemical property of a substance, which can be used to explain the structure–activity relationship. Hansen has defined the parameter ijR as Eq. (3) based on solubility parameters, which can be used to evaluate the interaction between the polymer and solute, or two solutes. The smaller the value of ijR, the stronger the interaction between two substances. Table 1 lists the parameters of ijR between membrane or water and alcohols, and the solubility parameters of n-

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Fig. 5. Variation of pervaporation characteristics of PVA–SA blended membranes for aqueous alcohols mixture with different PVA contents (90 wt% alcohol concentration in feed, at 45EC).

Fig. 6. Variation of pervaporation performance of crosslinked PVA–SA membranes with maleic acid content in reaction solution (feed composition =90 wt% alcohol, at 45EC). Table 1 Value of ijR between membrane or water and alcohols and their mole volumes ij

Water

Ethanol

Isopropanol

n-butanol

ter-butanol

Membrane (Cal1/2/cm3/2) Water (Cal1/2/cm3/2) Vi (cm3/mol)

9.2677

3.1241 11.7358 58.5

4.4967 13.0526 76.8

5.668 14.0314 91.8

5.6187 14.1379 91.3

R

butanol and tert-butanol are obtained from Ref. [9], their mole volumes (Vi) are calculated by the group contribution method [10].

(3)

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Y.Q. Dong et al. / Desalination 193 (2006) 202–210 Table 2 Intrinsic viscosity of PVA–SA in different solvents Solvent Water Water–ethanol Water–isopropanol Water–n-butanol Water–tert-butanol

Fig. 7. Effect of feed composition on the permeation flux of PVA–SA hollow-fiber composite membranes at 45EC.

[η] 143.45 103.47 160.27 97.67 118.14

of the membrane is changed. The intrinsic viscosity is a basic property of polymer dilute solutions, and it describes the effective hydrodynamic volume of an isolated polymer molecule in solutions [Eq. (4)], and characterizes the interaction between polymer species. It is also known that solvent is the most important factor to change solution properties of a polymer; therefore, we can evaluate the change of membranes in different solvents by investigating the intrinsic viscosity of PVA–SA in solvents of water and alcohol. (4)

Fig. 8. Effect of feed composition on the separation factor of PVA–SA hollow-fiber composite membranes at 45EC.

From Table 1 it was found that the value of ijR between ethanol and membrane is smaller than those between other alcohols and membranes, and the interaction of ethanol with the membrane is the strongest. In addition, the ijR between ethanol and water is also lowest in all alcohols. Thus, the separation selectivity of blended membranes for an ethanol–water mixture is lower than that for other alcohol–water mixtures. Generally, there is a plastification of polymer membranes in butanol, and then the configuration

where [η] is intrinsic viscosity; Φ is a universal constant, is the mean-square end-to-end distance of the polymer, and M is the molecular weight of the polymer. Table 2 gives the intrinsic viscosity of PVA– SA with a 4:1 blended ratio in different solvents; the alcohol volume concentrations in mixture solvents are 10%. The [η] of PVA–SA in different solvents is different. The [η] of PVA–SA in water/n-butanol is the smallest, and the meansquare end-to-end distance of the polymer chain is smallest in water/n-butanol; that is to say, the n-butanol induces the polymer coil to shrink. The result of intrinsic viscosity is in agreement with the pervaporation characteristics based on Table 2.

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(a)

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(b)

Fig. 9. Batch dehydration of alcohols. (a) isopropanol; (b) tert-butanol.

3.7. Batch dehydration experiment of alcohol For estimating the process of preparing the PVA–SA blended membrane, the crosslinked blended membrane was used for the batch dehydration experiments from isopropanol and tertbutanol aqueous mixtures. The membrane area was 0.082 m2. Results are shown in Fig. 9. The permeation fluxes of isopropanol and tert-butanol decreased with a longer operation time. The decrease of permeation flux was much greater for the isopropanol content — from 83 wt% to 94 wt%, then above 94 wt%. However, the water content in the permeate decreased greatly, and these phenomena validated the change law between alcohol content in the feed and separation factor. The variation of flux and separation factor shows that the crosslinked PVA–SA blended membrane can be used in industry. 4. Conclusions PVA and SA are compatible polymers, and they can be used to prepare blended membranes by solution casting. FTIR spectra show that maleic acid can crosslink PVA–SA blended membranes very well. The optimal process of preparing membranes is as follows: 80 wt% PVA and 20 wt% SA are blended, and the casting solution

of the PVA–SA blend with a concentration of 2 wt% is obtained by dissolving the blend in water; then the blend solution is cast onto the PS hollow-fiber membrane, and the composite membrane is crosslinked with 1.5 wt% maleic acid and 0.05 wt% H2SO4 in ethanol solvent for 8 h. For isoproanol, n-butanol, tert-butanol and ethanol aqueous solutions, as the alcohol concentration is 90 wt% at 45EC, higher separation factors and permeation fluxes of crosslinked PVA–SA blended membranes are obtained: 1727, 414 g/m2.h; 606, 585 g/m2.h; 725, 370 g/m2.h and 384, 384 g/m2.h, respectively. In addition, the blended membranes can potentially be used in industry. Acknowledgements This work was supported by the National Fundamental Research Foundation of China (2003CB615706) and the High Technology Research and Development Program of China (No. 2003AA328020).

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