ZSM-5 filled polyurethaneurea membranes for pervaporation separation isopropyl acetate from aqueous solution

Separation and Purification Technology 85 (2012) 8–16

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ZSM-5 filled polyurethaneurea membranes for pervaporation separation isopropyl acetate from aqueous solution Chunfang Zhang, Le Yang, Yunxiang Bai, Jin Gu, Yuping Sun ⇑ School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Jiangsu, China

a r t i c l e

i n f o

Article history: Received 19 April 2011 Received in revised form 7 July 2011 Accepted 8 July 2011 Available online 9 December 2011 Keywords: Polyurethaneurea membrane ZSM-5 Pervaporation Isopropyl acetate Aroma compounds

a b s t r a c t ZSM-5 filled hydroxyl terminated polybutadiene (HTPB)-based polyurethaneurea (PU) membranes, HTPB-PU/ZSM-5, were prepared by a two-step polymerization process, which were used as membrane material to recover aroma, isopropyl acetate (IPAC), from aqueous solution by pervaporation (PV). The membranes demonstrated high IPAC permselectivity and with the increase of ZSM-5 loading, the separation factor increased initially and then decreased, while the total flux demonstrated the similar variation until the ZSM-5 loading was 40 wt.%, at which it reached the lowest value. After that, it began to increase again. The HTPB-PU/ZSM-5 membranes containing 20 wt.% ZSM-5, HTPB-PU/ZSM-5-20, showed the highest separation factor. On the other hand, the separation factor and total flux of HTPB-PU/ZSM-5-20 membrane increased with the increase of feed concentration and temperature. The best separation factor and total flux reached 288.72 and 53.21 g m2 h1, respectively, at 60 °C when the feed concentration of IPAC is 0.39 wt.%. Ó 2011 Published by Elsevier B.V.

1. Introduction Volatile organic compounds (VOC) of foods and beverages, generally called aroma compounds, usually ester, alcohol, aldehyde and so on, contribute significantly to the flavor. Analysis of different kinds of fruits indicates more than 6000 compounds are constituents of their aroma. For instance, juices of passion fruit and orange have about 200 compounds responsible for their aroma [1]. But most of these VOCs are lost during the processing of drinks (beverages, juices) and foods by conventional process (evaporation, distillation, air stripping, etc.) because aroma compounds are heat sensitive. Isopropyl acetate (IPAC) is such a typical aroma compound giving rise to a fruit odor which can be found in grape and is widely used in processing foods, beverages, wines and tobacco [2]. Pervaporation (PV) is a promising separation technique and is becoming recognized as an energy efficient alternative to distillation and other separating methods for liquid mixtures, especially in cases that the traditional separation techniques are not efficient, such as separating of azeotropic mixtures, isomeric components and close-boiling point systems [3]. Nowadays, a major hurdle limits the commercialization of PV process for recovery of aroma from aqueous solution, namely, a lack of proper membrane materials with high flux and separation factor for recovery of so many kinds of

⇑ Corresponding author. Tel./fax: +86 510 85917763. E-mail address: [email protected] (Y. Sun). 1383-5866/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.seppur.2011.07.008

organic compounds. Many kinds of hydrophobic membrane materials, such as polydimethylsiloxane (PDMS) [4–6], poly(dimethylsiloxane)-poly(methyl hydrogen siloxane) (PDMS/PMHS) [7], polyoctylmethylsiloxane (POMS) [8,9], poly(vinylidene-fluorideco-hexafluoropropene (P(VDF-co-HFP)) [10], polyether block amide (PEBA) [11–13], PU [14,15], ethylene-propylene-diene monomer (EPDM) [1], perfluoro-alkylsilane modified Al2O3 [16] and so on, have shown promise in recovery of aroma from water by PV. However, the tradeoff between permeability and selectivity is one of the biggest problems faced by pure polymer membranes, which greatly limits their further application in the chemical industries. In the past two decades, mixed matrix membranes started to emerge as an alternative approach in pervaporative membrane technology for separation of organic compounds from aqueous solutions. The PV performances as well as heat-resistance were proved to be enhanced by incorporating inorganic fillers into polymer membranes. These inorganic fillers include zeolite [17–26], carbon black [27], carbon molecular sieve [28] and so on. Among them, zeolite with high Si/Al ratio (ZSM-5) and silicalite (Al free zeolite) is intensely investigated to remove different organic compounds from water due to its high hydrophobicity, surface area and void volume as well as uniform pore size distribution. For example, Kittur and co-workers [17] prepared ZSM-5/PDMS blend membranes for recovery isopropanol from water. Their results showed that both permeation flux and selectivity increased simultaneously with increasing zeolite content in the membrane matrix. The highest separation selectivity reached 80.84 and flux of 67.8 g/m2 h at 30 °C for 5 wt.% of isopropanol in the feed when

C. Zhang et al. / Separation and Purification Technology 85 (2012) 8–16

zeolite loading was 30 wt.%. Vane et al. [18] prepared ZSM-5 filled PDMS membranes and showed that the separation factor of ethanol to water increased monotonously from 8.7 to 43.1 when ZSM-5 loading increased from 0 to 65 wt.%, and the ethanol flux also increased from about 40–300 g/m2 h. Lu et al. [22] found that the incorporation of silicalite into PDMS enhanced both the total flux and acetic acid/water separation factor which were from 57 to 150 g/m2 h and from 1.35 to 2.75, respectively with silicalite content of 49.9 wt.%. Nevertheless, inevitably non-selective voids are generated near the interface of polymer and inorganic particles due to their incompatibility. Attempts were made to enhance the compatibility between the fillers and polymers by introducing silane coupling agent [29], but chemical modifying the fillers will partially block their pores. So it is probably more reasonable and convenient to enhance physical affinity between polymer segments and the inorganic fillers by selecting or synthesizing suitable polymer materials. Hydroxyl terminated polybutadiene (HTPB) based polyurethaneurea is an alternate copolymer material, which could provide hydrophobic and flexible soft segments (polyol) for facilitating organic diffusion and rigid hard segments (isocyanate and chain extender, viz. diol, diamine) for mechanical strength. So inorganic fillers should have good affinity to PU segments and are prone to be dispersed well in HTPB-PU membrane because the polymer segments are amphiphilic. On the other hand, we once found that pure HTPB-PU membranes can successfully separate aroma, ethyl acetate, from water [14,15]. In the present study, to enhance the PV performance of HTPBPU membrane, ZSM-5-filled HTPB based PU membranes, HTPBPU/ZSM-5, were prepared and their PV performance with different ZSM-5 loading, feed concentration and temperature for recovering IPAC from aqueous solution were investigated. Furthermore, the PV performances with increasing of ZSM-5 loading were discussed from the viewpoint of the chemical and physical structure of HTPBPU/ZSM-5 membranes. 2. Experimental 2.1. Materials HTPB (hydroxyl value = 1.14 mmol KOH g1) obtained from Qiluyixi Chemical Co. Ltd. (Shandong, China) was dealt with vacuum drying before used. Isophorone diisocyanate (IPDI) was purchased from Mingda Macromolecule Science and Technology Co. Ltd. (Jiangsu, China). 1,2-diaminocyclohexane was bought from Jiachen Chemical Co. Ltd. (Shanghai, China) and used as a chain-extender. Tetrahydrofuran (THF) was purchased from National Pharmaceutical Group Chemical Reagent Co. Ltd. and used as a solvent, which was dried by molecular sieves and distilled under nitrogen. Dibutyltin dilaurate (DBTDL, catalyst) was purchased from Hangzhou Chemical Agent Co. Ltd.; ZSM-5 (Si/Al was 360) was bought from Tianjin Nankai Catalyst Co. Ltd. (Tianjin, China). Isopropyl acetate (IPAC) was purchased from National Pharmaceutical Group Chemical Reagent Co. Ltd. 2.2. Membrane preparation The synthesis of HTPB-PU was carried out as described in our previous work [15]. Prepolyurethane was prepared by a reaction between HTPB and IPDI at a mole ratio 2:1 of NCO:OH in dry THF with 0.05 wt.% DBTDL as a catalyst at 30 °C for 60 min. Polyurethaneurea was prepared by adding the chain-extender 1,2-diaminocyclohexane (OH:NH2 = 1:1) into the prepolyurethane with mechanical stirring for 15 min. A known mount of ZSM-5 was added into the polyurethaneurea solution. The polyurethaneurea solution with ZSM-5 loading of 0, 5, 10, 20, 30, 40, 50 and

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60 wt.% (the ratio of ZSM-5 weight to HTPB weight) was cast onto a clean Teflon plate by knife-coating. The cast film of HTPB-PU/ ZSM-5 about 300 lm thick was dried at room temperature for 2 h followed by a 10 h thermal curing at 80 °C. The obtained membranes were designated as HTPB-PU, HTPB-PU/ZSM-5-5, HTPB-PU/ ZSM-5-10, HTPB-PU/ZSM-5-20, HTPB-PU/ZSM-5-30, HTPB-PU/ ZSM-5-40, HTPB-PU/ZSM-5-50, HTPB-PU/ZSM-5-60, respectively. 2.3. FT-IR measurement The chemical structures of ZSM-5, HTPB-PU, HTPB-PU/ZSM-520 and HTPB-PU/ZSM-5-60 were characterized by a FTLA2000 type Fourier transform infrared (FT-IR) spectrometer. The samples for FT-IR measurement were obtained by spreading a thin film of their solutions in THF on a potassium bromide flake and evaporated the solvent under vacuum at room temperature. 2.4. Scanning electron microscopy Scanning electron micrographs of ZSM-5, HTPB-PU, HTPB-PU/ ZSM-5-20 and HTPB-PU/ZSM-5-60 were performed on a Hitachi S4800 scanning electron microscope (SEM) instrument. All the samples were coated with a thin layer of gold to prevent charging. 2.5. TGA measurement Thermal stability of the ZSM-5, HTPB-PU, HTPB-PU/ZSM-5-20 and HTPB-PU/ZSM-5-60 were examined with a METTLER 1/ 1100SF Thermogravimetric analyzer (TGA). The temperature profile was from 30 to 800 °C with a heating rate of 10 °C/min and a nitrogen flow of 50 mL/min. 2.6. Mechanical properties studies Stretching testing of HTPB-PU, HTPB-PU/ZSM-5-20, HTPB-PU/ ZSM-5-40 and HTPB-PU/ZSM-5-60 were performed at room temperature using an electronic universal testing machine (Shenzhen, China) with a crosshead speed of 30 mm/min. The width and length of the sample was 10 and 50 mm. The membranes were evaluated by two parameters as shown in the Eqs. (1) and (2):

r¼ E¼

F bd

r DL=L

ð1Þ ð2Þ

where r is the tensile stress, and F is the maximum load, b and d represent the width and thickness of the samples, respectively. E represent the young’s modulus, DL and L are the extension and the original length. 2.7. Static contact angle measurement Static contact angles for water of HTPB-PU/ZSM-5 membranes were measured by sessile drop method [30] using a Contact Angle Meter (OCA 20, Dataphysics Instruments GmbH Germany) at 25 °C and about 65% relative humidity. The volume of the water drop used was always 2 lL. All reported values were the average of at least eight measurements taken at different locations of the film surface and had a typical mean error of ±1. 2.8. Degree of swelling measurement The dried HTPB-PU/ZSM-5 membranes were immersed in IPAC/ water solution at 30 °C. At regular intervals, the swollen membranes were wiped out carefully with filter paper to remove

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superficial liquid and weighted quickly. The degree of swelling (DS) was calculated using the following Eq. (3):

DSð%Þ ¼

mt  m0  100 m0

ð3Þ

where m0 and mt are the weights of dry and swollen membranes, respectively. 2.9. Determination of diffusion coefficient The diffusion coefficient was estimated to describe the diffusion behavior of the system. The diffusion coefficient of component i was calculated from Eq. (4) as previously reported by Ma [31] and evolved from the Fick’s law of Eq. (5) when assuming that the concentration profile along the diffusion length x is linear:

Di ¼

Ji d Ci

J i ¼ Di

ð4Þ dC i dx

ð5Þ

where Di represents concentration-averaged diffusion coefficient of component i (m2/s), Ji is the flux of component i, Ci is the concentration (kg/m3), d is the membrane thickness and x is the diffusion length (m). 2.10. PV experiments The PV experiment apparatus used in this study was shown in Fig. 1. The membrane was installed in the cell and the effective area was 35.24 cm2. The feed solution was continuously circulated from a feed tank to the upstream side of the membrane by a pump. The downstream pressure was kept at about 0.3 kPa and the permeate was collected in a cold trap. The compositions of the feed and the permeate were measured using GC900 gas chromatography equipped with a Thermal Conductivity Detector (TCD). The permeation flux (J) and the separation factor (a) for all membranes were calculated according to the Eqs. (6) and (7):

J¼

Q At

asep;IPAC=water ¼

ð6Þ Y IPAC =Y water X IPAC =X water

ð7Þ

Fig. 2. FT-IR spectra of (a) ZSM-5, (b) HTPB-PU, (c) HTPB-PU/ZSM-5-20 and (d) HTPB-PU/ZSM-5-60 membranes.

where Q is the weight of permeate collected in time t, and A is the effective membrane area, X and Y represent the mass fractions of the organic in the feed and permeate, respectively. 3. Results and discussion 3.1. Membrane characterization 3.1.1. FT-IR analysis Fig. 2 shows the FT-IR spectra of ZSM-5, HTPB-PU, HTPB-PU/ ZSM-5-20 and HTPB-PU/ZSM-5-60. In Fig. 2a a characteristic band appeared around 3400 cm1 which is corresponding to the stretching vibrations of O–H and the band appeared at about 1100 cm1 assigning to Si–O. The peak at around 800 cm1 represented the stretching vibrations of Al–O, which was weaker than that of Si–O due to the ratio of Si/Al was 360. In Fig. 2b–d, the band of N@C@O stretching disappeared near 2270 cm1, which indicated the completion of the reaction of chain extending. On the other hand, the peaks of 1730 cm1 (C@O) and 1531 cm1 (C–N) were observed, which confirmed the formation of the urethane group. Compared with the spectra of HTPB-PU, HTPB-PU/ZSM-5-20 and HTPB-PU/ZSM-5-60, no new absorption peak could be observed except Si–O peak, suggesting that ZSM-5 particles were physically blended within the polymer matrix. 3.1.2. SEM analysis Fig. 3 showed the scanning electron micrographs of ZSM-5 particles as well as the cross-section of HTPB-PU, HTPB-PU/ZSM-5-20 and HTPB-PU/ZSM-5-60 membranes. As shown in Fig. 3a, the particle size of ZSM-5 was about 3–5 lm. From Fig. 3b–f, no appreciable pore could be observed, indicating that defect-free dense membrane was synthesized. It could be seen from Fig. 3c–f that ZSM-5 particles were uniformly dispersed in the membrane matrix.

Fig. 1. Schematic diagram of the experimental equipment: (1) digital constant temperature bath, (2) feed tank, (3) recycle pump, (4) membrane cell, (5) membrane, (6) cold trap, (7) permeate collection tube, (8) vacuometer, (9) desiccator, (10) vacuum pump.

3.1.3. TGA analysis The TGA plots of ZSM-5, HTPB-PU, HTPB-PU/ZSM-5-20 and HTPB-PU/ZSM-5-60 membranes were shown in Fig. 4. In Fig. 4a, the ZSM-5 presented a total weight loss about 3.2% from 30 to 800 °C, mainly due to the loss of water molecules attached to the ZSM-5, indicating that the ZSM-5 was stable at high temperature.

C. Zhang et al. / Separation and Purification Technology 85 (2012) 8–16

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Fig. 3. SEM images of ZSM-5 particles and the cross-section of PU membranes, (a) ZSM-5 particles, (b) and (e) HTPB-PU, (c) HTPB-PU/ZSM-5-20, (d) and (f) HTPB-PU/ZSM-560.

Fig. 4. TGA plots, (a) ZSM-5 particles, (b) HTPB-PU, HTPB-PU/ZSM-5-20 and HTPB-PU/ZSM-5-60 membranes.

It could be seen from Fig. 4b that the HTPB-PU/ZSM-5 membranes were stable below 200 °C, and above 200 °C there are mainly two sharp decompositions in the TGA curve, as can be ascribed to the decomposition of the polyurethane segment and the polybutadiene segment respectively. Consequently, the total weight loss of

the HTPB-PU, HTPB-PU/ZSM-5-20, HTPB-PU/ZSM-5-60 membrane decreased and the decomposition temperature increased with the increasing loading of ZSM-5, indicating that the incorporation of ZSM-5 improved the thermal stability of HTPB-PU/ZSM-5-20 membranes.

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Fig. 5. Tensile strength and Young’s modulus of HTPB-PU/ZSM-5 membranes with different ZSM-5 loading. Fig. 7. Effect of ZSM-5 loading on the contact angle of HTPB-PU/ZSM-5 membranes.

3.1.4. Mechanical properties In order to study the effect of ZSM-5 on mechanical properties, the tensile strength and Young’s modulus of HTPB-PU, HTPB-PU/ ZSM-5-20, HTPB-PU/ZSM-5-40 and HTPB-PU/ZSM-5-60 were measured as shown in Fig. 5. From Fig. 5, the tensile strength demonstrated downtrend with increasing ZSM-5 loading, which indicating that the incorporation of ZSM-5 weakened the mobility of the PU chains and consequently the mechanical strength of HTPB-PU/ZSM-5 membranes decreased. As shown in Fig. 5, it could be seen that with the increase of ZSM-5 loading, the Young’s modulus of the HTPB-PU/ZSM-5 membranes increased correspondingly due to the increased rigidity of HTPB-PU/ZSM-5 chains. 3.2. Effect of ZSM-5 loading on membrane swelling The equilibrium DS values of HTPB-PU/ZSM-5 membranes in pure water, pure IPAC and IPAC/water mixtures with different IPAC concentration at 30 °C were shown in Fig. 6. It could be seen that the DS values of HTPB-PU/ZSM-5ZSM5 membranes decreased in pure IPAC and IPAC aqueous solutions, while nearly unchanged in pure water, with increasing ZSM-5 loading. It may be explained that the incorporation of ZSM-5 decreased the mobility of the HTPB-PU chain and blocked the free movement of IPAC and water molecular into HTPB-PU/ZSM-5 membranes. From Fig. 6, it could

Fig. 6. Effect of ZSM-5 loading and feed concentration on the equilibrium DS of HTPB-PU/ZSM-5 membranes at 30 °C.

also be seen that the DS values of HTPB-PU/ZSM-5 membranes increased with the increase of IPAC concentration, suggesting that the HTPB-PU/ZSM-5 membranes had preferential selective adsorption for IPAC. 3.3. Contact angle Fig. 7 shows effect of ZSM-5 loading on the contact angle for water of HTPB-PU/ZSM-5 membranes at 30 °C. As can be seen from Fig. 7, the contact angle for water of HTPB-PU/ZSM-5 membranes increased with increasing ZSM-5 loading. This result suggests that the hydrophobicity of HTPB-PU/ZSM-5 membranes were enhanced with increasing ZSM-5 loading. The more hydrophobic membrane surface favors absorption of IPAC while repelling water from its surface. In other words, the introduction of ZSM-5 could enhance the affinity of HTPB-PU/ZSM-5 membranes to IPAC. 3.4. Effect of ZSM-5 loading on PV performance of HTPB-PU/ZSM-5 membranes Fig. 8 shows the variation of separation factor (a) and permeation flux (b) as a function of ZSM-5 loading in the HTPB-PU/ ZSM-5 membranes with about 0.37 wt.% of IPAC in feed at 30 °C. As shown in Fig. 8a, a considerable increase in separation factor achieved when the ZSM-5 loading was 20 wt.%, and the separation factor of all HTPB-PU/ZSM-5 membranes was higher than that of pure HTPB-PU membrane. The incorporation of ZSM-5 improved the hydrophobic property of membrane and selective adsorption for IPAC as can be confirmed by the contact angle measurements as shown in Fig. 7. But when ZSM-5 loading was higher than 20 wt.%, the increasing defect between polyurethane segments and ZSM-5 allowed IPAC and water permeated simultaneously, resulting in the decrease of the separation factor. From Fig. 8b, water flux decreased initially and then increased with the increase of ZSM-5 loading, and when the ZSM-5 loading was 40 wt.%, the water flux reached the lowest value and began to increase again. While the IPAC flux increased initially until ZSM-5 loading was 20 wt.% and began to drop down slowly and increased a little when the ZSM-5 content was more than 40 wt.%. These phenomena can be ascribed to the multiple effects of the incorporation of ZSM-5 particles into HTPB-PU membrane, i.e. higher hydrophobic, rigidity of HTPB-PU segments and more defects owing to the aggregation of ZSM-5 particles. When ZSM-5 loading was larger than 20 wt.%, the incorporation of ZSM-5 brought about two contradict effect

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Fig. 8. Effect of ZSM-5 loading on PV performance of HTPB-PU/ZSM-5 membranes: (a) separation factor, asep,IPAC/water, (b) flux (IPAC concentration in feed, 0.37 wt.%, feed temperature, 30 °C).

Table 1 Diffusion coefficient of IPAC and water through HTPB-PU/ZSM-5 membranes with different ZSM-5 loading. ZSM-5 loading (wt.%)

DIPAC (1011 m2 s1) Dwater (1012 m2 s1) CIPAC (gm L1) d (lm)

0

10

20

30

40

50

60

5.19 1.85 0.0036 311

11.64 1.67 0.0035 307

12.25 1.59 0.0039 286

9.41 1.36 0.0035 296

5.03 1.21 0.0039 281

6.72 1.68 0.0036 331

6.74 1.78 0.0036 318

to the membranes, i.e. one, the rigidity of HTPB-PU segments which is harmful for both the permeation of IPAC and water, and the other, defects resulted from the aggregation of ZSM-5 particles made IPAC and water flux increase. For water, at a lower ZSM-5 loading, the declined flux was enhanced owing to the inclined hydrophobic surface of HTPB-PU/ZSM-5 membranes that further forbidding the permeation of water molecular. While, for IPAC, when ZSM-5 loading was lower than 20 wt.%, the increase in IPAC flux can be attributed to the pore size effect of ZSM-5 and a close contact between the polymer and ZSM-5 particles. But when the ZSM-5 loading was higher than 20 wt.%, the rigidity of polymer

chains caused by incorporating ZSM-5 particles blocked the diffusion of IPAC, so the separation factor reaches the highest value at a ZSM-5 loading of 20 wt.%. When the ZSM-5 loading exceeded 40 wt.%, more interfacial defects generated, which could provide the opportunity for IPAC to pass through a HTPB-PU/ZSM-5 membrane. The diffusion coefficients of IPAC/water component were calculated through Eq. (4) and the calculated Di values at 30 °C were shown in Table 1. The tendency of diffusion coefficient for IPAC and water with the increase of ZSM-5 is similar to their fluxes. As a whole, for all the HTPB-PU/ZSM-5 membranes, the diffusion

Fig. 9. Effect of feed concentration on PV performance of HTPB-PU/ZSM-5-20 membrane at 30 °C: (a) separation factor, (b) permeation flux.

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C. Zhang et al. / Separation and Purification Technology 85 (2012) 8–16

Fig. 10. Effect of operating temperature on PV performance of HTPB-PU/ZSM-5-20 membrane: (a) separation factor, asep,IPAC/water, (b) permeation flux (IPAC concentration in feed, 0.39 wt.%).

coefficients of IPAC were about 10 times larger than those of water, indicating HTPB-PU/ZSM-5 membranes were good for the transportation of IPAC than for water owing to their hydrophobic properties though the water molecular is rather smaller. 3.5. Effect of feed concentration on PV performance of HTPB-PU/ZSM5-20 membrane Effect of feed concentration on PV performance of HTPB-PU/ ZSM-5-20 membrane was studied at 30 °C. As shown in Fig. 9, the PV flux and separation factor increased with the increase of feed concentration. The relationship among the permeation flux, liquid and vapor phase concentrations follows the Eq. (8) for recovery of VOC from aqueous solution [32]:

J i ¼ K i q½ðC i ÞL  ðC i ÞV 

ð8Þ L

V

where Ki (m/s), q (mol/m3), (Ci) (dimensionless) and (Ci) (dimensionless) are the overall mass transfer rate coefficient, total molar density of feed, bulk liquid phase concentration (mole fraction), and bulk vapor phase concentration (mole fraction, reported as an equivalent liquid phase mole fraction), respectively, for component i. The increase of feed concentration is nearly equal to the increase of the driving force (Ci)L because (Ci)V is usually small and can be neglected. Therefore, the permeation flux increased usually with an increase of feed concentration. In this study water flux decreased slightly and IPAC flux increased from 1.98 to 23.94 g/m2 h when the feed concentration increased from 0.16 to 0.85 wt.% as can be seen from Fig. 9b. This is because the driving force of IPAC was enhanced and that of water nearly depressed or unchanged with increasing feed concentration. Uragami et al. [33] proved that organic molecules mainly permeated through the hydrophobic phase of a polymer membrane containing micro-phase structure, while water molecules mainly permeated through hydrophilic matrix. For HTPB-PU membranes, the hydrophobic and prevalent polybutadiene segments formed the main matrix of the whole membrane and the low content of polar hard segments was dispersed evenly in the polybutadiene matrix. The soft segments of polybutadiene, which have the strong affinity to IPAC, provide a path for the permeation of IPAC. But the diffusion of water became very difficult because the low content of hard and polar segments of urethane could not form a continuous phase in the HTPB-PU/ZSM-5 membrane. In addition, in the studied feed concentration range the mild DS of HTPB-PU/ZSM-5-20 membrane, from 2.3% to 5.5% seen from Fig. 6, will also be resistant to water permeation.

3.6. Effect of operating temperature on PV performance of HTPB-PU/ ZSM-5-20 membrane Fig. 10 shows the variation of the separation factor asep,IPAC/water and flux of HTPB-PU/ZSM-5-20 membrane with feed temperature. From Fig. 10, it can be seen that both asep,IPAC/water and total flux increased with increasing the feed temperature. Commonly, the separation factor decreases with increasing feed temperature in PV process. This anomalous phenomenon was reported by Yeom et al. [34] when PDMS membrane was used to separate chloromethane from chloromethane/water mixtures. They considered that it is attributed to the decreasing of water flux, due to water cluster formation with increasing the feed temperature. Also, Wu et al. [35] discovered the anomalous phenomena when PDMS membrane was used to separating p-cresol from p-cresol/water mixtures. They explained it with the fact that the activation energy of p-cresol was a little larger than that of water from 50 to 80 °C, which resulted in the increase of selectivity with increasing feed temperature. The larger permeation activation energy of organic implies that the organic permeation flux is more sensitive to the increase of temperature compared to that of water permeation flux. We [36] also found the anomalous phenomena when using EVA

Fig. 11. Arrhenius plots of HTPB-PU/ZSM-5-20 membrane.

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C. Zhang et al. / Separation and Purification Technology 85 (2012) 8–16 Table 2 Comparison of PV results of HTPB-PU/ZSM-5-20 membrane for IPAC/water mixture with literatures. Ref.

Membrane

aIPAC/Water

Normalized IPAC flux (kg lm/m2 h)

C (wt.%)

m (lm)

T (°C)

[9] [16] This work

POMS Perfluoro-alkylsilane modified Al2O3 HTPB-PU/ZSM-5-20

900 90 99

0.07 0.68 2.23

0.04 0.3 0.39

4.8 4 286

20 40 40

m: Membrane thickness; T: feed temperation; C: feed concentration; POMS: poly(octyl)methylsiloxane.

membrane for the separation of ethylene acetate from water mixtures. For the results in this study, in Fig. 10b, IPAC flux increased from 6.0 to 26.8 g/m2 h, i.e. more than four times, but water flux increased from 19.9 to 26.4 g/m2 h, i.e. less two times, at the same temperature range studied. This could result in the increase of asep,IPAC/water with increasing feed temperature according to equation asep,IPAC/water = JIPAC/CJwater in mathematics, where C is a constant and equals to the ratio of FIPAC to Fwater. Additionally, from Fig. 11, the permeation activation energy of IPAC (Ea(IPAC) = 59.95 kJ/mol) was higher than that of water (Ea(Water) = 9.68 kJ/ mol) for the HTPB-PU/ZSM-5-20 membrane. It is similar to the case reported by Wu et al. [35]. 3.7. PV performance for separating IPAC/water mixtures with literatures Table 2 compares the PV performance for separating IPAC/water mixtures with literatures. The HTPB-PU/ZSM-5-20 membrane in this study exhibited comparatively higher IPAC normalized flux (defined as the IPAC flux multiplied by the membrane thickness) though separation factor is not outstanding enough compared to other membrane materials. 4. Conclusions Defect free ZSM-5 filled polyurethaneurea membranes, HTPBPU/ZSM-5, were prepared for the separation of IPAC from its aqueous solutions by pervaporation. The chemical structure, morphology and thermal stability of these filled membranes were characterized. It could be seen that ZSM-5 dispersed uniformly in the membrane. With the incorporation of ZSM-5 in the membrane, the thermal stability of the membrane increased, while the swelling degree decreased. The separation factor increased first and then decreased with the increase of the ZSM-5 loading. The diffusion coefficient of IPAC was much larger than that of water for the HTPB-PU/ZSM-5 membranes, indicating that the membranes were highly IPAC permselective. The HTPB-PU/ZSM-5 membranes containing 20 wt.% ZSM-5, HTPB-PU/ZSM-5-20, showed the highest separation factor at 30 °C. With the increase of the operating temperature and feed concentration, both the permeation flux and separation factor increased. The best PV performance of the HTPB-PU/ ZSM-5 membranes containing 20 wt.% ZSM-5, separation factor and total flux were 288.72 and 53.21 g m2 h1, respectively with feed concentration of 0.39 wt.% IPAC at 60 °C. Acknowledgements The authors acknowledge financial supports for this work from Education Ministry of China (JD09011), the Funds for Science and Technology Innovation of Jiangsu Province (BC2010017) and the Fundamental Research Funds for the Central Universities (JUSRP21113). References [1] C.C. Pereira, J.R.M. Rufino, A.C. Habert, R. Nobrega, L.M.C. Cabral, C.P. Borges, Aroma compounds recovery of tropical fruit juice by PV: membrane material selection and process evaluation, J. Food Eng. 66 (2005) 77–87.

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