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
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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 membrane residual solution on the membrane surfaces. area and the operation time, respectively. Then, the swollen membranes were weighted by a mass balance (accuracy ± 0.0001 g). The degree of swelling was defined as
푊 −푊 푆퐷 (%) = 푆 퐷 × 100% (1) 푊퐷
Wherein, WS (g) and WD (g) were the mass of the swollen membrane and the mass of the dried membrane, respectively. The data of swelling degree were collected from four replicate experiments. Figure 1. The schematic diagram of the lab scale pervaporation unit 2.4. Pervaporation performance The composition of permeated product was The separation performance of prepared specified by measuring the refractive indices membranes was investigated for ethanol/water with a digital differential refractometer mixture of 80/20 wt%, using a lab scale (Reichert, AMETEK GmbH, Germany) with the pervaporation unit. The schematic diagram of aid of a calibration curve for ethanol/water the pervaporation setup was illustrated in Figure mixture prepared using known quantities of the 1. A module membrane designed with the two components. The refractometry membrane area of 19 cm2 with a channel height of 2 mm. The feed solution was circulated from
Trang 100 TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 19, SOÁ K7- 2016 measurement was carried out at 25 oC. The Figure 2. For the non-modified membrane, the separation factor (α) was defined as broad peak at 3000-3600 cm-1 attributed to the -1 푦 /푦 hydroxyl group and the peak at 1578 cm was 훼 = 푊 퐸푡푂퐻 (3) 푥푊/푥퐸푡푂퐻 characteristic of acetate group of PVA. For the crosslinked membranes, the intensity of the Where, xW, xEtOH, yW, yEtOH were the weight broad peak at 3000-3600 cm-1 was reduced and fractions of water and ethanol in the feed and the peak at 1578 cm-1 was disappeared. In the permeate side, respectively. The data of spectra of the membrane crosslinked by three permeation flux and separation factor reported dicarboxylic acids, a new small peak at 1725 in this work were based on the average of four cm-1 attributed to the ester group was observed experimental runs. as compared to the non-crosslinked membrane The pervaporation separation index (PSI) [7]. Whereas, the PVA membrane crosslinked of the membranes was calculated by the with GA exhibited the stretch of 1050 cm-1 and following equation 1140 cm-1 for acetal and ether linkages [4-6]. 푃푆퐼 = 퐽 ×∝ (4) The changes in the FTIR spectra implied that the chemical structure of the PVA membrane was 3. RESULTS AND DISCUSSION altered by the crosslinking linkages compared to 3.1. Physicochemical properties of the the plain PVA membrane. crosslinked PVA membranes Figure 3 showed the contact angle and Four crosslinkers including GA, fumaric swelling degree of the prepared membranes. The acid, maleic acid and malic acid with different contact angle, indicating the hydrophilicity of chemical structures were investigated for the crosslinked membrane surfaces was between modifying PVA composite membrane. The 50o and 57o. It indicated that the hydrophilicity FTIR spectra of virgin PVA and crosslinked of crosslinked membranes was lower than that PVA composite membranes were shown in of non-crosslinked membrane.
Figure 2. FTIR spectra of non-crosslinked and Figure 3. Hydrophilicity and swelling degree of crosslinked PVA membranes prepared membranes
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(a) (b)
(c) (d)
Figure 4. SEM surface and cross-section images of non-crosslinked (a,b) and crosslinked (c,d) membranes
Additionally, the hydrophilicity of the structures of the crosslinking agents. Malic acid prepared membrane surface was different exhibited the highest steric hindrance due to the depended on the chemical structure of hydroxyl group in its molecule. Both fumaric crosslinking agents. For crosslinked membranes, acid and maleic acid had a carbon double bond the GA-PVA membrane showed the lowest on their molecules, but fumaric acid had the hydrophilicity, while the malic acid-PVA trans isomerism in comparison with the cis membrane had the highest hydrophilicity due to structure of maleic acid. Hence, the steric the additional hydroxyl group on acid malic hindrance effect of fumaric acid was lower than molecules [2,3,8]. The maleic and fumaric acid- that of maleic acid [2,3]. Although the GA PVA membranes were less hydrophilic owing to showed the lowest steric hindrance, the GA- the double bond in in their molecules. From PVA membrane exhibited slightly more Figure 3, the swelling degree of the prepared swelling degree compared to PVA membrane membranes was observed to follow the trend of crosslinked with carboxylic acids. It was due to plain PVA > GA-PVA > malic acid-PVA > the lower crosslinking temperature of GA-PVA maleic acid-PVA> fumaric acid-PVA. It would compared to dicarboxylic acid-PVA membrane. come from steric effect of the various chemical Previous studies reported that the crosslinking in
Trang 102 TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 19, SOÁ K7- 2016 lower temperature might induce the protonation separation factor showed the opposite trend. The of function groups of GA, which lessened the plain PVA membrane was easily swelled in etherification and acetalization reactions [4-6,8]. aqueous solution during pervaporation test due to the high affinity of the hydroxyl group with Figure 4. presented the SEM images of the water and hence, the permeation flux was high surface and cross-section structure of un- but the selectivity was insignificant. The results crosslinked and crosslinked PVA membranes. showed that the permeation flux of the There was no observed difference of surface and crosslinked membranes was lower than that of cross-section morphology between the virgin the plain PVA membrane. The crosslinking in and the modified membranes. The derived the PVA layer resulted in the rigid structure. membranes had a composite structure, involving The rigid structure of the polymer chains had a dense and uniform PVA film on the top of the less mobility and hence, the solubility and PAN porous layer, and the non-woven fabric diffusive ability of the water and ethanol substrate at the bottom of the membrane. It was molecules were hindered. Accordingly, the observed that macro voids or defects were not crosslinked membranes obtained a lower found on and in the selective PVA film. permeation flux but higher selectivity. For PVA Moreover, the interface between the PVA film membrane crosslinked by dicarboxylic acids, the and PAN support was difficult to discriminate permeation flux and separation factor varied for the crosslinked membrane, implying the with the chemical structure of the crosslinker. good bond formed between the PAN support The membrane crosslinked by malic acid layer and PVA selective film. The thickness of showed the highest flux but the lowest the derived membranes was approximately 10 separation factor in comparison with that by m via maintaining the gap of the casting knife fumaric and maleic acid due to the higher steric at 70 m. hindrance of hydroxyl functionality of malic 3.2. Separation performance of the acid. Despite having the same carbon double crosslinked PVA membranes bond, fumaric acid owned trans isomerism with
Figure 5 presented the permeation flux and smaller steric hindrance than cis isomerism structure of maleic acid. Therefore, the fumaric separation factor of prepared membranes, which acid crosslinked membrane exhibited the lowest was evaluated for the pervaporation dehydration flux but the highest separation factor. For of 80 wt% ethanol solution at 60 oC. The crosslinking PVA membrane with GA at permeation flux of the derived membrane was observed to decrease in the order of plain PVA ambient temperature, the separation factor was observed to significantly enhance while the flux > GA-PVA > Malic acid-PVA > maleic acid- PVA > fumaric acid-PVA. Meanwhile, the was declined partially as compared to the plain PVA membrane.
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Figure 5. Permeation flux and separation factor of Figure 6. Relationship between swelling degree and prepared membranes flux of prepared membranes
Figure 7. Relationship between swelling degree and Figure 8. Pervaporation separation factor of non- separation factor of prepared membranes crosslinked and crosslinked PVA membranes
From Figure 6 and Figure 7, it was found capability of the pervaporation membrane. The that the permeation flux and separation factor PSI of the prepared membranes were showed in were closely related to the swelling degree of Figure 8. Among the prepared membranes, GA the derived membrane. The higher swelling crosslinked membrane exhibited the highest PSI. degree gave the lower separation factor but the In comparison, GA was a good crosslinker for higher flux. Both flux and selectivity were the modifying membrane due to good flux and good essential parameters for the pervaporation selectivity, while the crosslinking condition was process. It was observed that there was a trade- not required high temperature. off between the permeation flux and selectivity 4. CONCLUSIONS of the membrane. When the flux increased, the separation factor decreased. Therefore, the term The PVA membrane crosslinked with of pervaporation separation index (PSI) was various crosslinking agents was successfully employed for measuring the separation prepared by in-situ crosslinking technique. The
Trang 104 TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 19, SOÁ K7- 2016 results showed that crosslinking PVA membrane temperature was found to achieve higher improved the separation performance of the separation factor but lower flux as compared to resulting membrane. The chemical structure of that with GA. The GA exhibited as a proper the crosslinkers affected the physicochemical crosslinker for modifying PVA membrane due properties and separation performance of the to good flux and good selectivity, while the crosslinked membrane. Particularly, the swelling crosslinking condition was not required high degree exhibited a significant relationship with temperature. the separation performance of the prepared Acknowledgment: The authors gratefully membrane. Moreover, elevating the crosslinking acknowledge the financial support provided by temperature was observed to improve the the Ministry of Industry and Trade of the selectivity, but decrease the permeation flux of Socialist Republic of Vietnam under Grant the resulting membrane. Crosslinking PVA 11/HĐ-ĐT.11.14/NLSH. membrane with dicarboxylic acids at high
Làm khan hỗn hợp ethanol-nước bằng phương pháp thẩm thấu bốc hơi sử dụng màng poly(vinyl alcohol) nối mạng Trần Lê Hải Vưu Ngọc Duy Minh Hoàng Minh Quân Nguyễn Thị Nguyên Mai Thanh Phong*
Trường Đại Học Bách Khoa, ĐHQG-HCM
TÓM TẮT
Trong bài báo này, chúng tôi đã tạo ra nhau bao gồm glutaraldehyde, axít fumaríc, axít màng lọc compozít trên nền poly(vinyl alcohyol) malêíc, axít malíc. Hiệu quả phân tách của (PVA) bằng cách phủ một lớp màng chọn lọc màng tạo thành qua các thông số thông lượng PVA được nối mạng lên lớp đế xốp nước thẩm thấu, hệ số phân tách được đánh giá polyacrylonitrile (PAN). Phương pháp nối mạng bằng quá trình tách nước hỗn hợp ethanol-nước in-situ được sử dụng để tạo ra màng mỏng PVA với tỷ lệ nồng độ 80/20 %kl ở 60 oC. Các nối mạng với bốn tác nhân khâu mạng khác phương pháp đo phổ hồng ngoại FTIR, SEM, đo
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độ trương nở và đo góc tiếp xúc của màng với thành. Khi thông lượng thẩm thấu qua màng nước cất được sử dụng để đánh giá các đặc tính tăng thì độ chọn lọc của màng giảm. Kết quả của màng lọc tạo thành. Kết quả cho thấy các của thí nghiệm góp phẩn làm giàu dữ liệu về tính chất lý hóa (tính ưa nước và độ trương nở) phản ứng nối mạng của màng PVA, nhằm giúp và hiệu quả phân tách của màng chịu ảnh các nhà khoa học chọn lựa phù hợp các tác hưởng bởi cấu trúc hóa học của các tác nhân nhân nối mạng để biến tính màng thẩm thấu bốc nối mạng. Ngoài ra, có sự cân bằng giữa thông hơi trên nền PVA, ứng dụng để tách nước dung lượng thẩm thấu và độ chọn lọc của màng tạo dịch ethanol. Từ khóa: poly (vinyl alcohol); nối mạng; màng lọc; thẩm thấu - bốc hơi; tách nước
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