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Pervaporation Dehydration of Ethanol-Water Mixture Using Crosslinked Poly(Vinyl Alcohol) Membranes

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Pervaporation Dehydration of Ethanol-Water Mixture Using Crosslinked Poly(Vinyl Alcohol) Membranes TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 19, SOÁ K7- 2016 Pervaporation dehydration of ethanol-water mixture using crosslinked poly(vinyl alcohol) membranes Tran Le Hai Vuu Ngoc Duy Minh Hoang Minh Quan Nguyen Thi Nguyen Mai Thanh Phong* Ho Chi Minh City University of Technology, VNU-HCM (Manuscript Received on October 19th, 2016, Manuscript Revised November 28th, 2016) ABSTRACT Crosslinked poly(vinyl alcohol) (PVA) membrane was changed via crosslinking composite membranes were synthesized by reaction. The physicochemical properties casting selective crosslinked PVA films on the (hydrophilicity and swelling degree) and polyacrylonitrile (PAN) porous substrates. The separation performance of the prepared PVA films were prepared by in-situ crosslinking membranes were affected by the chemical technique using four different crosslinking structures of the crosslinking agents. agents, such as glutaraldehyde, fumaric acid, Furthermore, there was a trade-off between maleic acid and malic acid. The separation permeation flux and selectivity of the resulting performance in terms of permeation flux and membranes. When the flux increased, the separation factor of prepared membranes were separation factor decreased. The results of this evaluated for pervaporation dehydration of study contributed to enrich the data of the ethanol/water mixture of 80/20 wt% at 60 oC. crosslinking reaction of PVA membranes, and The prepared membranes were also expected to help researcher in suitable choosing characterized by FTIR, SEM, swelling and crosslinking agent for producing pervaporation sessile drop contact angle measurements. It was PVA membrane for dehydration of ethanol found that the chemical structure of the PVA solutions. Keywords: Poly (vinyl alcohol), crosslinking, membrane, pervaporation, dehydration. 1. INTRODUCTION energy consumption, benign operating conditions, no emission to the environment, no Nowadays, pervaporation has been gaining involvement of additional species into the feed much attention as an effective membrane stream and simplicity of the process [1,3]. technique for separating heat sensitive; close Polymer based pervaporation membranes were boiling and azeotropic mixtures due to its low Trang 97 SCIENCE & TECHNOLOGY DEVELOPMENT, Vol 19, No.K7- 2016 the most widely used for dehydration of 6], but the PVA crosslinked with dicarboxylic ethanol/water mixtures because of their acids need higher temperature (>100 oC) inexpensive fabrication and operation costs. The [2,3,7,8]. However, to our knowledge, the effect commercialized pervaporation membrane of different chemical structures of crosslinking employed in ethanol/water separation is agent on the physicochemical properties and PERVAP membrane, which is formed by Sulzer separation performance of modified membrane (Germany). It is a composite membrane with a was not concerned sufficiently. In this study, we PVA active layer coated on a polyacrylonitrile developed the composite membranes by casting (PAN) porous support membrane. The main a crosslinked PVA layer on a PAN porous advantage of the composite membrane is the substrate. The PAN porous could suppress the higher permeation flux, stability and good swelling of the PVA selective film at the selectivity as compared to traditional one-layer interface and thus retain the dense skin. membrane [2,3]. Accordingly, the high permeation flux and good selectivity as well as the durability of the So far, poly(vinyl alcohol) (PVA) has been pervaporation membranes could be achieved [1- attracting for development of pervaporation 3,8]. In-situ crosslinking technique was membranes for ethanol dehydration [1-8]. It is employed to prepared crosslinked PVA selective due to the inherent hydrophilicity, excellent layer using four different crosslinking agents thermal, mechanical, chemical stability and such as glutaraldehyde (GA), fumaric acid, good film-forming ability of PVA. However, maleic acid and malic acid in the presence of because of the high hydrophilicity, the PVA HCl as catalyst. The fumaric and maleic acids films is not stable in aqueous solutions, leading had the same number of carbon atoms and also to low separation performance of the derived the carbon double bond. The difference of the membranes. Therefore, PVA membranes should two acids was that the fumaric acid had the trans be modified to offer long-term durability in a configuration while the maleic acid owned the pervaporation process. Some techniques usually cis configuration. The trans structure of fumaric used for modifying PVA membrane are heating, acid was expected to much more pack the grafting, crosslinking, irradiating and blending polymer chain and give better selectivity for the [2]. Among them, crosslinking is more efficient modified membrane. The malic acid had an because the physicochemical properties and additional hydroxyl group which was separation performance of the prepared hypothesized to provide more hydrophilic membranes are easily controlled by varying the property for the crosslinked membrane, whereas crosslinking agents, crosslinker concentration GA with the reduced oxygen content was looked and reaction conditions. PVA membrane could forward to produce the tightest crosslinked crosslink by dicarboxylic acids, dialdehydes, membrane. The effects of crosslinker’s chemical and alkoxysilanes [2,3]. Many crosslinkers such structure on the physicochemical properties (i.e., as fumaric acid, maleic acid and glutaraldehyde chemical structure, hydrophilicity and swelling are investigated for crosslinking PVA degree) of the crosslinked membranes were membrane. Previous studies showed that characterized using FTIR, SEM, swelling and glutaraldehyde reacted with PVA at ambient sessile drop contact angle measurements. The temperature to produce crosslinking linkages [2- Trang 98 TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 19, SOÁ K7- 2016 separation performance of the prepared malic acid (>98%, Sigma, USA). The 2M HCl membrane in terms of flux and separation factor solution as catalyst was subsequently added was evaluated for pervaporation dehydration of under vigorous stirring for 3h to produce PVA the ethanol/water mixture of 80/20 (wt%) at 60 casting solution. Then the resulting solution was oC. degassed before casting. The composite membranes were prepared by casting PVA 2. EXPERIMENTAL aqueous solution on the PAN support 2.1. Membrane preparation membranes (PAN200, Ultura, USA) by using a casting knife with a constant gap of 70 µm. The PVA (Mw 146-186 kDa, 99%, Sigma) solutions with the concentration of 10 wt% were resulting membranes were dried at room prepared by dissolving PVA in deionized (DI) temperature for 48h. After that, the prepared water at 90 oC under agitation for 6 h. Next, membranes were crosslinked at elevated o PVA solution was cooled to room temperature temperature in 2 h at 120 C for the three and the crosslinkers with the concentration of 5 dicarboxylic acids [2,3,7], while the ambient o wt% per weight of PVA was added under temperature (30 C) was fixed for GA [2-6]. continuous stirring for 24 h. Four crosslinkers Finally, the derived membranes were immersed used in this study were glutaraldehyde (GA, in 80wt% ethanol solution for 3 h and dried at 25% in water), fumaric acid, maleic acid and room temperature for 24 h before pervaporation tests. Table 1. The characteristics of original PAN200 support substrate Membrane Recommended operating limits Material Total thickness1 Nominal MWCO2 pH Pressure Temperature (µm) (Da) range range (bar) range (oC) PAN 165 20,000 2-10 1-10 20-80 1Including the non-woven support of thickness approximately 100 µm 2Based on above 70% rejection of PEG Table 2. Chemical formula and structure of investigated crosslinking agents Crosslinker Chemical formula Chemical structure Mw (g/mol) Glutaraldehyde C5H8O2 OHC(CH2)3CHO 100.12 Fumaric acid C4H4O4 Trans - HO2CCH=CHCO2H 116.07 Maleic acid C4H4O4 Cis - HO2CCH=CHCO2H 116.07 Malic acid C4H6O5 HO2CCH2CH(OH)CO2H 134.09 Trang 99 SCIENCE & TECHNOLOGY DEVELOPMENT, Vol 19, No.K7- 2016 2.2. Membrane characterization the feed tank through the membrane cell with a flow rate of 90 L/h by a pump. The temperature The surface and thickness of the PVA of the feed solution was maintained at 60 ± 1 oC selective layers was determined from SEM by a laboratory recirculating heater. The images by using Scanning Electron Microscopy pressure of the feed side was at atmosphere (S4800, FE-SEM) and digital micrometer pressure and the pressure on the permeate side (accuracy ± 1 µm, Series 293, Mitutoyo). The was maintained lower than 1 mbar by using a chemical structure of the membranes was vacuum pump (Robinair, USA). The separation characterized using ATR-FTIR spectroscopy. process was conducted for 2 h, and the permeate The hydrophilicity of the membrane surfaces vapor was condensed in a cold trap by a was investigated using sessile drop water laboratory chiller and heat exchanger using pure contact angle measurement. ethanol liquid (-15 oC ÷ -20 oC). The collected 2.3. Swelling experiments permeate was weighed using a mass balance The pieces of dried membranes with the (accuracy ± 0.0001 g) for determining the dimension of 3×3 cm were immersed in both permeation flux. The permeation flux (J) was water and ethanol/water solution of 80/20 wt% determined using the following equation o at 30 C for 48h to reach an equilibrium 퐽 = 푄/(퐴푡) (2) swelling. The swollen membranes were wiped Where, Q (g), A (m2) and t (h) represented carefully using tissue paper for removing the weight of permeate, effective
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