Pervaporation dehydration of fusel oil with sulfated polyelectrolyte complex hollow fiber membrane

Journal of the Taiwan Institute of Chemical Engineers 95 (2019) 627–634

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Pervaporation dehydration of fusel oil with sulfated polyelectrolyte complex hollow fiber membrane Pei-Yao Zheng b, Wen-Hai Zhang a, Kai-Fan Chen b, Nai-Xin Wang a, Quan-Fu An a,b,∗ a

Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China b MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 2 July 2018 Revised 11 September 2018 Accepted 19 September 2018 Available online 15 October 2018 Keywords: Fusel oil Pervaporation Sulfated polyelectrolyte complex Hollow fiber membrane

a b s t r a c t Recycling and utilizing fusel oil, the by-product of ethanol fermentation, is of significant importance for both environmental and economic perspective. In this study, cross-linked sulfated polyelectrolyte complex hollow fiber membranes were applied to pervaporative remove the water in fusel oil. The membranes were fabricated from chitosan and dextran sulfate sodium via the “complexation–sulfation” method followed by chemical cross-linking and surface coating. The membrane preparation condition and pervaporation operation condition were optimized to achieve satisfactory alcohol dehydration performance. Where a high flux of 1354 gm−2 h−1 and a permeate water content of 99.4 wt% in dehydrating 10 wt% ethanol/water mixture at 60 °C was achieved. Moreover, when applied in actual fusel oil dehydration, the organics content in feed can be increased from 90 to 99 wt% with high efficiency and excellent water selectivity (water content in permeate always above 98.5 wt%). This research offered an effective approach to remove water in multi-component fusel oil. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction The ethanol fermentation yeast, especially when sugar or amylum is used as raw materials, also produces a small amount of other organic by-products. After separation by distillation, these organics are enriched and removed from the bottom of the rectification column [1,2]. The oil-like multi-component solution containing water, ethanol, iso-propanol, iso-pentanol, iso-butanol is known as fusel oil. Typically, the fusel oil will take 0.3–0.7 v% of the total ethanol yield. Since vehicle ethanol–gasoline is now being placed in great expectations [3], the issue of fusel oil disposal can no longer be neglected. On the other hand, some of the components in fusel oil are considered as highly-valued chemicals, for example, the main component iso-pentanol, is crucial solvents for perfume and liquid crystal industry [4,5]. Therefore, separating and utilizing fusel oil not only make it less polluting but also of significant commercial value [6]. Water forms azeotropes with those C2 –C5 alcohols in fusel oil, making it difficult to separate and purify into individual component. Pervaporation, an emerging mem-

∗ Corresponding author at: Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China. E-mail address: [email protected] (Q.-F. An).

brane technology not limited by vapor–liquid equilibrium, is just well suit in this case [7,8]. An ideal membrane is expected to process both high permeate flux and selectivity [9]. The most direct and effective way to achieve this is designing appropriate membrane material and structure [10,11]. In recent years, the pervaporation performance of alcohol dehydration is significantly improved by applying a newly developed material, namely, polyelectrolyte complex (PEC) [12,13]. Formed by the electrostatic complexation of oppositely charged polyelectrolytes, PECs are endowed with high hydrophilicity, unique dual free volume characteristic and are well-suited for organic dehydration [14,15]. Dispersible solid PECs proposed by our group facilitated the membrane preparation process and thereby made it possible to translate from lab-scale to industrialscale. The early generation of solution processable PEC membranes is prepared by the so-called acid protection–deprotection method with weak polyanions containing carboxyl groups [16]. The water selectivity in iso-propanol dehydration of such membrane could be close to infinite, that is, the permeate iso-propanol was below the lower limit of detection [16]. But the separation performance in ethanol dehydration was less satisfactory, because ethanol molecule is smaller and its interaction with water is stronger [17,18]. Previous work had proved that PEC membranes were highly selective for dehydrating the main components (permeate water content > 99 wt%) of fusel oil, except for ethanol

https://doi.org/10.1016/j.jtice.2018.09.025 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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(permeate water content ∼ 94 wt%) [19]. Given that the percentage of ethanol is not negligible, it is necessary to design materials with better ethanol dehydration performance. The structure of PECs could be tailored by employing strong polyanions with sulfate groups, and further chemical modified to introduce free sulfate groups. It has been established that ion pairs based on strong polyelectrolytes displays higher binding strength and the resultant membranes are more stable in structure [20–22]. Moreover, the high hydrophilicity of free sulfate groups can greatly increase the permeate flux and water selectivity of the pervaporation membranes according to the theoretical solution–diffusion model as well as experimental result [23,24]. For example, An et al. fabricated sulfate poly(sodium vinylsulfonate)/chitosan membranes containing sulfonate ionic cross-linking and free sulfate groups, the optimal membrane showed a ultra-high flux of 1980 gm−2 h−1 and permeate water content above 99.5 wt% in dehydrating 10 wt% water/ethanol mixture at 70 °C [24]. The above analysis hint sulfated polyelectrolyte complex (SPEC) being a promising material for fusel oil dehydration [25]. SPEC is dispersible in water, which facilitates the membrane fabrication process, but at the same time, leading to the instability in aqueous solution. The water content of fusel oil differs from source to source, and sometimes could be very high. To meet the demand of practical applications, the water resistance of SPEC should be enhanced by chemical crosslinking. Herein, the “complexation–sulfation” method was adopted to synthesis SPEC using dextran sulfate sodium and chitosan as starting material, and sulfur trioxide trimethylamine complex as sulfation agent. Crosslinked SPEC hollow fiber membranes were prepared by surface coating method at the presence of silane coupling agent. The influences of SPEC composition, operating conditions and the composition of the hollow fiber membrane module on its pervaporation separation performance were discussed in detail. The membrane was further applied to the fusel oil dehydration.

Fig. 1. Schematic diagram of the preparation process of SPECM.

This was because part of amino groups in CS chains remained uncharged state at pH = 6.10. The DSS/CS PEC precipitates were then collected and washed by deionized water and stored for later use.

2.3. Fabrication of sulfated polyelectrolyte complexes (SPECs) SPECs were fabricated through an N-sulfation process of above mentioned DSS/CS PECs. In detail, 1.0 g PEC powder was dispersed in 100 ml Na2 CO3 aqueous solution followed by the addition of sulfation agent (CH3 )3 N. SO3 . The reaction was carried out at 60 °C and nitrogen atmosphere for 6 h, the pH value was controlled at ca. 8.0 during the entire sulfation process. When the dispersion turned to a uniform and transparent solution, the crude product was precipitated in excess acetone, then re-dissolved in deionized water and further purified by the dialysis to remove any inorganic salts and unreacted components. Finally, solid SPEC was obtained by lyophilization. SPECs with different degree of sulfation were prepared by changing the mole ratio of (CH3 )3 N. SO3 to –NH2 in CS (X) and were designated as SPEC-X. The preparation procedure of SPECs as well as hollow fiber morphology were illustrated in Fig. 1.

2. Experimental 2.1. Materials

2.4. Fabrication of sulfated polyelectrolyte complex membranes (SPECMs)

Polyvinylidene fluoride ultrafiltration support membrane (PVDF-UF, outside diameter 1.5 mm) was kindly provided by R&D Center for Membrane Technology of Chung Yuan University. Chitosan (CS, 20 0–40 0 mPa s, degree of deacetylation ≥ 95%) and γ -glycidyloxypropyltrimethoxysilane (GPTMS) were acquired from Sigma-Aldrich Co. Ltd. Dextran sulfate sodium (DSS), sulfur trioxide trimethylamine complex ((CH3 )3 N·SO3 ), sodium carbonate (Na2 CO3 ), sulfuric acid (H2 SO4 ), hydrochloric acid (HCl), ethanol, acetone, iso-propanol, iso-butanol, iso-pentanol and decyl acetate were obtained from Sinopharm chemical reagent Co., Ltd., Shanghai, China. All the reagents were used as received without any further purification. Deionized water (resistance, 18.2 MΩ cm) was used in all experiments.

SPEC was dispersed in water and the homogeneous SPEC aqueous dispersion (0.8 wt%) was prepared. Then cross-linker GPTMS was then added into the SPEC dispersion followed by continual stirring for 4 h. Then the PVDF hollow fiber membrane was immersed horizontally in the solution and each fiber was coated twice (60 s + 60 s). After uniformly coated, the membranes were dried at 40 °C for 12 h and then annealed at 80 °C for another 5 h. In this article, the resulting membranes were designed as SPECM, SPECM/GPTMS-5, SPECM/GPTMS-10, SPECM/GPTMS-15, SPECM/GPTMS-20 and PECM/GPTMS-25, where the mass fractions of GPTMS relative to SPEC were 0, 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt%, respectively.

2.2. Preparation of PEC

2.5. Membrane characterizations

DSS/CS was synthesized according to our previous works [24]. In detail, DSS and CS aqueous solutions were mixed dropwise, the pH value of each solution was adjusted to 6.10. The mixed solution turned turbid immediately as the –SO3 − and NH3 + groups formed ionic bonds and produced water-insoluble PEC (DSS/CS). After a certain amount of DSS solution was added, coacervation and precipitation occurred due to the destruction of electrical double layer of DSS/CS aggregates. Specifically, the molar ratio of added DSS and CS were close to 0.46 in our experiment, indicating the degree of complexation (DC) of as-prepared DSS/CS PEC being around 0.46.

Chemical composition and structure of SPECs were determined with X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCALAB250Xi, USA) and fourier transform infrared spectroscopy (FTIR, Nicolet 470, Waltham, MA). The cross-sectional and surface morphologies of SPECMs were observed by field emission scanning electron microscope (FESEM, Hitachi S4800, Japan). The AFM results have been removed form the manuscript according to the reviewers comments. Water contact angle (WCA, Data physics instruments GmbH, Germany) measurement was carried out at ambient temperature to evaluate surface hydrophilicity of SPECMs.

P.-Y. Zheng et al. / Journal of the Taiwan Institute of Chemical Engineers 95 (2019) 627–634 Table 1 Compositions of PEC and their SPEC determined by XPS. S (mol%)

N (mol%)

S:N

DS

CS/DSS SPEC-1 SPEC-2 SPEC-3 SPEC-4

3.61 4.52 4.87 5.52 5.67

7.81 6.18 7.76 6.54 6.38

0.462 0.628 0.731 0.844 0.89

– 16.6 26.9 38.1 42.8

2.6. Equilibrium swelling measurement Free-standing SPEC flat-sheet membranes were fabricated by casting SPEC dispersion on a Teflon plate. The membranes were then dried at 40 °C for 12 h and further dried at 80 °C for 5 h (i.e. the same drying process with hollow fiber membranes). The dried membrane (pristine weight marked as M0 ) was immersed into 10 wt% ethanol/water mixtures at 50 °C for at least 48 h. Each swelled membrane was weighed several times until the weight was unchanged (final weight marked as M∞ ). The equilibrium swelling degree (ESD) was calculated through the following formula:

SO3- C-O-C

Free NHSO3-

CS/DSS SPEC-1

Transmlttance

Sample

NH2

629

SPEC-3 SPECM-3

2000

1600

1200

800

400

Wavenumber (cm-1) Fig. 2. ATR–FTIR spectra of CS/DSS, SPEC-1, SPEC-3 and SPECM-3.

ESD = (M∞ − M0 )/M0 × 100% The solubility test was carried out by immersing dried freestanding membranes in 25 wt% and 50 wt% ethanol/water solution at 50 °C for 48 h. The membranes were then taken out and dried in oven to remove all the absorbed solvent before weighed again. The solubility was estimated by the weight loss after immersing. 2.7. Pervaporation experiments In pervaporation experiments, two fibers were sealed in each module unless specially mentioned. The feed solution was circulated through the shell side of the membrane module and a vacuum was applied on the tube side (downstream) using a vacuum pump. The downstream pressure was fixed at ca. 180 Pa. The permeate was completely collected by a liquid nitrogen cold trap (77 K) and was then analyzed by a gas chromatograph (GC1690A, Hangzhou Ke Xiao Chemical Instrument Co., Ltd., China). The pervaporation performances of SPECMs were evaluated by flux, permeance (P/l) and water content in permeate. The flux was defined as: J = g/(A × t) where g is the permeate weight; t is the operation time; A is the membrane area. The outer diameter of the hollow fiber is 1.5 mm and the length is 100–250 mm. J The permeance was defined as: Pl = sat where l is the memxi γi pi

brane thickness; xi , γ i and psat is the mole fraction of component i. i The composition of model fusel oil is, 10 wt% water, 51 wt% isopentanol, 10 wt% ethanol, 15 wt% iso-butanol, 13 wt% iso-propanol and 1 wt% decyl acetate. While fusel oil contains different portion of water was prepared by turning the water content at a fixed organic ratio (ethanol: pentanol: butanol: propanol: decyl acetate = 10 : 51 : 15 : 13 : 1). 3. Results and discussion The degree of sulfation (DS) is defined as the portion of sulfated amino groups (i.e. –NHSO3 − ) in total amino groups (i.e. –NH2 and –NHSO3 − ), it can be calculated by the molar ration of sulfur to nitrogen in SPEC minus that in unreacted CS/DSS (i.e. degree of complexation). In order to investigate the degree of sulfation of each SPECs, the elemental contents of SPEC samples were characterized by XPS. The results are shown in Table. 1. It can be seen that degree of sulfation increases from 16.6% to 42.8% as the molar ratio of

(CH3 )3 N. SO3 to chitosan monomer in the polyelectrolyte complex increases from 1 to 4, indicating the successful introduction of sulfate groups (–SO3 − ) in polyelectrolyte complexes. The higher the degree of sulfation, the more sulfonic acid groups are introduced, which means the amount of sulfate groups can be well controlled by adjusting the dosage of (CH3 )3 N. SO3 . In addition, the degree of sulfation of SPEC-3 and SPEC-4 are similar (ca. 40%), indicating that the degree of sulfation is relatively complete at these dosages. The chemical structures of solid CS/DSS, SPEC as well as SPECM membranes were explored by ATR–FTIR. The characterization results are shown in Fig. 2. The peak at 1230 cm−1 corresponds to the peak of the bending vibration of –SO3 − group [26]. The peak at 1540 cm−1 is the characteristic bending vibration of – NH2 groups on CS [27]. Compared with the spectra of CS/DSS, the absorption peak of –NH2 on the SPEC infrared spectrum is obviously weakened, and the absorption peak of –SO3 − group (1230 and 625 cm−1 ) is obviously enhanced by contrast. This result, in accordance with the result of XPS, indicates that the –NH2 group of CS/DSS was successfully N-sulfated and sulfate groups were introduced. In addition, the ether bond (Si–O–C) absorption peak appeared at 1070 cm−1 in the infrared spectrum of SPECM-3 after the membrane was chemically cross-linked by GPTMS. These newly formed ether linkages were formed by the crosslinking of the hydroxyl groups on SPEC with the silanol groups on GPTMS, indicating the successful chemical cross-linking of SPECM-3. Fig. 3 shows the surface (a), (b) and cross-section (c), (d) FESEM morphologies of SPECM-4. It can be seen that the surface of SPECM-4 is in smooth and non-porous structure. When refer to the cross-section, it can be found that the membrane is uniform in thickness of about 5 μm. This indicates that a dense, uniform and defect-free active separation layer is formed on the surface of the outer surface of PVDF support. In addition, the degree of sulfation seems to have no obvious effect on the membrane surface morphology. In order to understand the hydrophilicity of the prepared membranes more specifically, the contact angles of SPECM with different degrees of sulfation were characterized in the study. The results are shown in Fig. 4. All the membranes are highly hydrophilic with continuous decreasing water contact angles lower than 40°. The equilibrium contact angles of SPECM-1, SPECM-2, SPECM-3 and SPECM-4 are approximately 26°, 23°, 16° and 13°, respectively. The

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a 30

ESD(%)

25 20 15 10 5 0 Fig. 3. Surface (a, × 20 k and b, × 40 k) and cross-section (c, × 50 and d, × 1 k) FESEM morphologies of SPECM-4.

0

5

10

15

20

25

GPTMS content (wt%)

b 30

SPECM-1 SPECM-2 SPECM-3 SPECM-4

40

30

25

Weight loss (%)

Water Contact Angle (o)

50

20 15 10 5

20

0 10

0

20

40

60

80

100 120 140 160

Time (s)

0

5

10

15

20

25

GPTMS content (wt%) Fig. 5. (a) The equilibrium swelling degree of SPECM in 10 wt% water/ethanol mixture and (b) the solubility of SPECM in 50 wt% water/ethanol mixture.

Fig. 4. The water contact angle of SPECM-1, SPECM-2, SPECM-3 and SPECM-4.

trend of contact angle change is quite clear that higher degree of sulfation leads to lower contact angle. This is because charged – SO3 − groups are highly hydrophilic [21,28], as the degree of sulfation increases, more –SO3 − groups are introduced, and the hydrophilicity of the membrane surface is continuously increased. A hydrophilic membrane surface is conducive to absorb more water molecules and repel alcohol molecules, which could be beneficial to pervaporation dehydration process. Pervaporation has a love–hate relationship with membrane swelling. Higher degree of swelling favors the sorption behavior and plasticizing effect, which may lead to a higher flux. On the other hand, excess swelling in feed solution will reduce the membrane selectivity due to the enlarged free-volume [29]. In extreme cases, the membrane will even dissolve in feed solution, leading to irremediable damage. To this end, the equilibrium swelling degree of SPECM with different cross-linking agent loadings were investigated in 10 wt% water/ethanol solution and the dissolution rate measured by higher water content (25 wt% and 50 wt%) solution. The results are presented in Fig. 5. It can be seen from Fig. 5a that the uncross-linked membrane SPECM shows a high degree of swelling (ca. 24 wt%) because the structure of PEC contains a large amount of hydrophilic groups –OH and charged group. In addition, sulfation reaction further increases the hydrophilicity by introducing –SO3 − groups, which has been revealed by water contact

angle measurement, resulting in a greater degree of swelling. After chemical cross-linked by GPTMS, the equilibrium swelling degrees are somehow lowered. In detail, the equilibrium swelling degrees SPECM/GPTMS-5, -10, -15, -20 and -25 are 15.0 wt%, 13.1 wt%, 12.0 wt%, 10.5 wt% and 10.1 wt%, respectively. This is because the cross-linking agent GPTMS builds Si–O–C bonds between the –OH groups of each polyelectrolyte chain, making the inter-molecular spacing more compact and there by the anti-swelling property is greatly enhanced. That being said, the final degree of swelling of SPECM/GPTMS-25 is still above 10 wt%, thus the pervaporation flux would not decrease too much. An opposite phenomenon was observed when the membranes were immersed in 50 wt% water/ethanol solution (Fig. 5b). That is, the weight of membrane decreases because of the partial dissolution of SPEC nanoparticles. After cross-linking, this phenomenon is significantly suppressed, and the cross-linked membrane has a much better water resistance. The weight loss of SPECM, SPECM/GPTMS-5, -10, -15, -20 and -25 are 22.0 wt%, 10.2 wt%, 9.1 wt%, 6.3 wt%, 5.5 wt% and 5.2 wt%, respectively. No obvious weight loss was observed in 25 wt% water/ethanol mixture (thus the figure is not shown here), which means all the membranes were basically stable at water content lower than 25 wt%. As for the separation performance, the effect of the degree of sulfation on the pervaporation behavior of SPECM was investigated (Fig. 6). The fluxes of SPECM-1, SPECM-2, SPECM-3 and

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1400

98

1300

96

1200

94

1100

92

1000

b

90

SPECM-1 SPECM-2 SPECM-3 SPECM-4

1500

100

1400

98

1300

96

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92

1000

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20

90

25

GPTMS content (wt%) 8000

6000 water ethanol

6000

Permeance (GPU)

Permeance (GPU)

10

b

8000

4000

5

Water content in permeate(wt%)

100

Flux (gm-2h-1)

a

1500

Water content in permeate (wt%)

Flux (gm-2h-1)

a

631

2000 10 8 6

2000 10 8

4

6

2

4

0

2

SPECM-1 SPECM-2 SPECM-3 SPECM-4 Fig. 6. The pervaporation performances of SPECM-1, SPECM-2, SPECM-3 and SPECM-4 in 90 wt% ethanol/water mixtures at 60 °C. The GPTMS content here was fixed at 5 wt%.

SPECM-4 are 1222 gm−2 h−1 , 1298 gm−2 h−1 , 1332 gm−2 h−1 and 1354 gm−2 h−1 . And the permeate water contents corresponded to 99.0 wt%, 99.2 wt%, 99.3 wt% and 99.4 wt%, respectively. It is obvious that the separation performance is in proportion to the degree of sulfation. Among which, SPECM-4 has the best pervaporation performance in both flux and water selectivity. This is because the polyelectrolyte complex has an increased amount of –SO3 − after sulfation, making the membrane more hydrophilic. When refer to the permeance of water and ethanol, the water permeance increases from 6239 to 6938 GPU while ethanol permeance is always lower than 6 GPU. Similar results were also obtained elsewhere [25]. Therefore, the following study was carried out under the conditions of a polyelectrolyte complex sulfating agent and chitosan monomer ratio of 4 (SPEC-4). The effect of GPTMS content on the pervaporation performance of SPECM-4 was investigated with a feed composition of 90 wt% ethanol/water and the feed temperature was 60 °C. The results are presented in Fig. 7. The permeate water contents of cross-linked SPECM-4 are between 98.5 wt% and 99.4 wt%. As the content of GPTMS increases, the pervaporation selectivity of the hybrid membrane decreases slightly. This can be explained by the fact that glycidyloxypropyl group in GPTMS is less hydrophilic than –SO3 − group in SPEC. The flux of SPECM-4 decreases from 1354 gm−2 h−1 to 1256 gm−2 h−1 when GPTMS content increases from 5 wt% to 25 wt%. This is because GPTMS mainly undergoes a cross-linking reaction and the self-polymerization proceeds at a slower rate

water ethanol

4000

0

5

10

15

20

25

GPTMS content (wt%) Fig. 7. Impacts of GPTMS content on the pervaporation performances of SPECM-4 in 90 wt% ethanol/water mixtures at 60 °C.

under preparation conditions [30]. And an increase in the amount of the cross-linking agent decreases the degree of swelling of the membrane, resulting in a denser membrane. So permeability of the pervaporation membrane is reduced. In summary, 5 wt% of GPTMS is enough to obtain a robust membrane. To optimize the separation performance, we also looked into the effects of membrane module, including aspect ratio of fiber and packing density (Fig. S1) as well as feed concentration (Fig. S2). And hence the optimal membrane composition and module design have been determined. That is, PEC prepared from CS and DSS at pH = 6.10, where the degree of complexation is around 46%. SPEC prepared from sulfation reaction of as-prepared CS/DSS at the molar ratio of sulfate reagent to CS monomer equals 4 (degree of sulfation = 43%). The hollow fiber membranes were in-situ crosslinked with 5 wt% GPTMS and each module is sealed with two fibers (10 cm in length and 1.5 mm in outer diameter) to reconcile the maximal average flux and space-efficiency. In the following study, the above mentioned membrane and module was applied to the dehydrating of actual fusel oil. SPECM4 shows excellent performance and the results are shown in Fig. 8. When the temperature increased from 30 °C to 60 °C, the flux increases from 637 gm−2 h−1 to 1493 gm−2 h−1 , and the permeate water content remaines above 99.3 wt%. The increase of temperature enhances the driving force as well as the diffusion rate of the components in the membrane. Moreover, the increase of

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Feed temperature (oC)

4000

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0

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Water content in fusel oil(wt%)

Fig. 8. Impacts of feed temperature (10 wt% water) and water content (60 °C) on pervaporation performances of SPECM-4 in fusel oils.

temperature does not affect the separation performance due to the existence of stable cross-linked network structure in the membrane [31,32]. The effect of water content in fusel oil on the pervaporation properties of SPECM-4 was also explored. It can be seen that the flux of SPECM-4 reduces from 1493 gm−2 h−1 to 206 gm−2 h−1 and water permeance from 4760 to 2020 GPU, when the water content in fusel oil is reduced from 10 wt% to 2 wt%. As the water content in feed decreases, the concentration of water molecules decreases, resulting in the reduction of the adsorption of water molecules. In addition, the degree of swelling of the membrane decreases. The water content in permeate also lowers with the water content in feed. The water content in permeate of SPECM-4 decreases from 99.5 wt% to 98.5 wt% when the water content in fusel oil reduces from 10 wt% to 2 wt%, but still quite satisfactory. Most polymeric membranes, such as commercial PVA membrane, suffer from too low permeate water content at low feed composition [31]. Given that the water content gradually decreases during the dehydration process, our cross-linked SPECM is particularly advantageous for real-time fusel oil dehydration. Finally, SPECM-4 was conducted in a lab-scale simulation of continuous in-situ removal of water form fusel oil solution (Fig. 9). The feed solution contained 10 0 0 g fusel oils with above mentioned composition and was kept at 50 °C without adding extra water or organics. The area of hollow fiber membranes in this con-

tinuous operation is 56.5 cm2 . The content of feed organics gradually as time goes on, since the water molecules are selectively removed. The flux of SPECM-4 decreased gradually because the content of water in feed declined with time. And the water content in permeate decreases slightly after 20 h, however, even when the feed water content is as low as 1 wt%, the permeate still maintains a high water concentration of 98.5 wt%, which further indicated the structures were stable and highly selective towards water. In general, the final efficiency of the fusel oils separation and utilization depends strongly on the water content in the fusel oils mixtures. The lower the initial water content, the more thoroughly fusel oils can be fractionated. After a continuous operation of 36 h, the organics content in feed can be increased to 99.0 wt%. The throughput capacity is calculated to be 4.9 kgm−2 h−1 , much higher than the reported commercial membrane PERVAP-1001 [5]. A comparison of pervaporation dehydration performance of recently reported membranes for various alcohols and fusel oil are presented in Table 2 [5,8,19,33-38]. It is seen that the vast majority of membranes documented in literature normally show inferior selectivity (usually permeate water content < 99 wt%) in ethanol dehydration than other higher alcohols, only a handful of membranes exhibit high selectivity at the expense of flux. Moreover, the investigation about fusel oil dehydration via pervaporation requires more attention. Under these circumstances, SPECM/GPTMS

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633

Table 2 Comparison of pervaporation dehydration performance of recently reported membranes for various alcohols and fusel oil. Membrane

Feed organic

Temperature(°C)

Flux(gm−2 h−1 )

Water in permeate(wt%)

Separation factora

Ref.

CS/PSS TETA–TMC

90 wt%ethanol 90 wt%ethanol 70 wt% iso-propanol 85 wt%ethanol 85 wt%iso-propanol 85 wt%iso-butanol 90 wt%ethanol 90 wt%ethanol 90 wt%ethanol 85 wt%iso-propanol fusel oil10 wt% water fusel oil10 wt% water 90 wt%ethanol 90 wt%isopentanol fusel oil10 wt% water 90 wt%ethanol fusel oil10 wt% water

70 25 25 60 60 60 25 60 60 600 70 60 60 60 60 60 60

495 1475 1521 ∼320 ∼160 ∼150 458 325 ∼100 ∼220 ∼300 ∼4680 972 2325 1323 1354 1493

99.0 99.2 99.1 ∼81 ∼99.7 ∼99.9 ∼99.6 98.4 ∼92 ∼98.6 ∼99.9 ∼96 93.9 99.5 98.9 99.4 99.5

891 1116 257 ∼51 ∼1128 ∼5600 2471 564 ∼100 ∼400 ∼90 0 0 ∼216 139 1791 809 1491 1791

[33] [34]

Polyimide/UiO-6

CAU-11(W)@CS ZIF-90/PVA Pervap® 2201 PERVAP-1001 PVA–PAA PECM/SiO2

SPECM/GPTMS a

[8] [36] [37] [5] [38] [19]

This work

Separation factor for fusel oil dehydration is water to total organics.

4. Conclusions

100

Cross-linked sulfated polyelectrolyte complex was fabricated by the ionic complexation of chitosan and dextran sulfate sodium and further sulfated to introduce free sulfate groups. The hollow fiber membranes were prepared by dip-coating method with the presence of cross-linking agent GPTMS. The membrane exhibited outstanding alcohol dehydration performance. In detail, SPECM/GPTMS-5 showed a high flux of 1354 gm−2 h−1 and a permeate water content of 99.4 wt% in dehydrating 10 wt% ethanol/water mixture at 60 °C. When applied in actual fusel oil dehydration, the organics can be enriched from 90 to 99 wt% in 36 h (initial 10 0 0 g fusel oil, membrane area 56.5 cm2 , 50 °C). Compared with other fusel oil dehydration technology, pervaporation with SPECM/GPTMS-5 is of high efficiency and application potential.

98 96 94 92 90 0

10

20

30

40

Operation time (h)

1500

Acknowledgment

100 90

1000

80 70 60

500

50 0

10 20 30 Operation time (h)

Water content in permeate (wt%)

Organics content in feed (wt%)

a

b

Flux (gm-2h-1)

[35]

40 40

Fig. 9. Batch pervaporation dehydration of fusel oils for SPECM-4 with operation time at 50 °C. (a) organics content in feed, (b) flux and water content in permeate.

fabricated in this work are more precious. All in all, SPECM4/GPTMS-5 has high efficiency in both steady-state and continuous pervaporation dehydration of fusel oil, indicating its potential application in pervaporation dehydration of organic compounds.

This research was financially supported by National Natural Science Foundation of China (No. 21376206) and The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD20170305).

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