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Pervaporation dehydration of ethylene glycol by NaA zeolite membranes
chemical engineering research and design 9 0 ( 2 0 1 2 ) 1372–1380
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Pervaporation dehydration of ethylene glycol by NaA zeolite membranes Congli Yu, Chao Zhong, Yanmei Liu, Xuehong Gu ∗ , Gang Yang, Weihong Xing, Nanping Xu State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, PR China
a b s t r a c t Home-made NaA zeolite membranes were used for pervaporation dehydration of ethylene glycol (EG)/water mixtures. Hydrothermal stability of the membranes in pervaporation was investigated for industrial application purpose. The membranes exhibited good stability for water content of less than 20 wt.% at 100 ◦ C. The reduction of operating temperature was effective to improve membrane stability for operating at high feed water content (e.g. 30 wt.%). The influence of feed water content and operating temperature on dehydration of EG was extensively investigated. A permeation flux of 4.03 kg m−2 h−1 with separation factor of >5000 was achieved at 120 ◦ C for the separation of the solution with 20 wt.% water content. A pilot-scale pervaporation facility with membrane area of 3 m2 was built up for dehydration of EG with the water content of 20 wt.%, which showed technical feasibility for industrial application. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Ethylene glycol; NaA zeolite membranes; Pervaporation; Dehydration
1.
Introduction
Ethylene glycol (EG) is one of the major chemicals, which can be used as precursors to polyester, non-volatile antifreeze, plasticizer, etc. The conventional synthesis route via hydrolysis of ethylene oxide uses excess water to improve EG yield, which requires an extra dehydration process to acquire pure product. Multistage evaporation and distillation are commonly employed for the dehydration, which are highly energy-consuming due to high boiling point of EG. In the past decades, pervaporation technique based on membrane separation has gained increasing attention in many chemical processes. Since only a small amount of permeate has to be vaporized, the separation process can save more energy compared with conventional separation techniques, such as distillation and adsorption (Lipnizki et al., 1999; Hinchliffe and Porter, 2000; van Hoof et al., 2004; Naidu and Malik, 2011). The separation efficiency of pervaporation mainly depends on sorption equilibrium and mobility of components through membrane channels, which is almost independent of vapor–liquid equilibrium associated with feed mixture. For EG
∗
dehydration, pervaporation has been considered as a promising technique with water content of less than 30 wt.% (Jehle et al., 1995). Polymeric membranes have been investigated for dehydration of EG by several groups. Feng and Huang (1996) first demonstrated pervaporation dehydration of EG by using chitosan membranes. A permeation flux of 0.3 kg m−2 h−1 and the water content of >92 wt.% in permeate were achieved at 35 ◦ C for the feed of 90 wt.% EG. Since then, various polymeric membranes were employed for dehydration of EG, such as surface crosslinked PVA membranes (Guo et al., 2007), poly (N,N-dimethylaminoethyl methacrylate)/polysulfone composite membranes (Du et al., 2008), chitosan-poly (vinyl alcohol) blend membranes (Hyder and Chen, 2009), chitosan coated zeolite filled regenerated cellulose membranes (Dogan and Hilmioglu, 2010), etc. Although continuous progress has been made on water selectivity for polymeric membranes, the achieved water fluxes were still modest for practical utilization. The low water flux is related with the inherent property of polymeric membrane material for water transportation and the strong interactions between
Corresponding author. Tel.: +86 25 83172268; fax: +86 25 83172268. E-mail addresses: [email protected] (C. Yu), [email protected] (C. Zhong), yanmeiliu [email protected] (Y. Liu), [email protected] (X. Gu), [email protected] (G. Yang), [email protected] (W. Xing), [email protected] (N. Xu). Received 11 October 2011; Received in revised form 30 November 2011; Accepted 4 December 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.12.003
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EG molecules and the membrane. It is known that the elevated temperature could increase the water flux for pervaporation due to the higher partial pressure of water in the feed. However, most polymeric membranes cannot endure at high temperature due to low thermal stability, which generally required the operating temperature of <100 ◦ C. Recently, some researchers have devoted to develop highly stable polybenzimidazole (PBI)-based membranes for dehydration of EG, which also showed high separation factor up to 4500 (Wang et al., 2010, 2011). NaA zeolite membrane, as an inorganic membrane, has well-defined zeolitic pores with high hydrophilicity and the size of 0.42 nm. The membrane shows excellent permselectivity and flux for separation of water from organics (Kita et al., 1995). Moreover, NaA zeolite membrane can be operated at high temperature of >120 ◦ C. Industrial facilities based on NaA zeolite membrane have been constructed for dehydration of ethanol and isopropanol recently by Mitsui (Morigami et al., 2001), GFT-Inocermic and our group cooperating with Nanjing Jiusi Hi-Tech Co. Ltd. (China) (Yu et al., 2011). Recently, Nik et al. (2006) investigated NaA zeolite membrane for dehydration of EG in laboratory. A flux of 0.94 kg m−2 h−1 with separation factor of 1177 was achieved at 70 ◦ C for the feed of EG solution with 30 wt.% water, which was better than the separation results obtained on polymeric membranes. The results suggested that NaA zeolite membrane exhibits great potential in dehydration of EG and more extensive investigation on the process is necessary. Compared to other organic solvents, the achieved water flux for EG dehydration by NaA zeolite membrane was usually lower (Sommer and Melin, 2005; Nik et al., 2006). The intrinsic mechanism of the low flux is still unclear. On the other hand, since high water content has an effect on the structure of zeolite membranes (Li et al., 2007), it is vital to know the stability of NaA zeolite membranes used for dehydration of EG solution. In this work, therefore, we investigated pervaporation dehydration of EG using home-made tubular NaA zeolite membranes. We intended to optimize operation parameters for improving water flux through zeolite membranes and obtaining stable pervaporation process. A pilot-scale experiment for dehydration of EG was carried out to further evaluate the technical feasibility for industrial application.
2.
Experimental
2.1.
Membrane preparation
D8-Advance). The as-synthesized membranes consisted of a continuous NaA zeolite layer with a thickness of 10–15 m.
2.2.
Pervaporation
2.2.1.
Laboratory test
Dehydration of organic solvents was performed by pervaporation method based on NaA zeolite membranes. Fig. 1 shows a schematic diagram of the experimental setup used for laboratory test. A membrane module loaded with a NaA membrane tube (70 mm in length) was put in an oven for maintaining a constant operating temperature. A preheating loop connecting to the membrane module was used to heat up the feed flow, which was monitored by a thermocouple. The feed liquid with a constant flow rate of 80 mL min−1 driven by an advection pump was introduced through outside of membrane tube. The feed side pressure was maintained at 0.1 MPa (gauge pressure) to avoid feed liquid vaporizing at all experimental conditions. The pressure in permeate side was maintained below 100 Pa by evacuation with a vacuum pump throughout pervaporation operation. The permeate was condensed by two liquid nitrogen traps in parallel, allowing the experiments to be carried out in a continuous mode. The retentate was recycled back to the feed tank. The effective area of the membranes used in the experiments was 1.58 × 10−3 m2 . Different membranes were used for investigation of each operating parameter. The feed tank was maintained at constant compositions by compensating water. The compositions of the feed and permeate were analyzed by a gas chromatography (GC-8A, Shimadzu) equipped with a thermal conductivity detector (TCD) and a packed column of Parapak-Q. The membrane performance was evaluated by flux (J, kg m−2 h−1 ) and separation factor (˛), which were described respectively as J=
m Am t
(1)
and ˛=
YW /YE XW /XE
(2)
where m is the mass of the permeate, kg; Am the membrane area, m2 ; t the time interval, h; YW , YE , XW , XE the mass fractions of water and organics in the permeate and feed, respectively.
2.2.2. Home-made NaA zeolite membranes were hydrothermally synthesized on outer surface of porous mullite tubes by secondary growth method. The substrates had outer diameter of 12.8 mm, inner diameter 7.8 mm, average pore diameter 1 m, and porosity 40%. Prior to hydrothermal crystallization, the substrates were seeded with NaA zeolite particles (2–3 m) by a dip-coating technique. Hydrothermal crystallization was operated at 100 ◦ C for 3.5 h twice. Aluminosilicate solution for membrane synthesis was prepared by dissolving commercial chemicals of sodium aluminate and water glass in deionized water at room temperature. The composition of synthesis solution had the molar ratio of Al2 O3 :SiO2 :Na2 O:H2 O = 1:2:2:120. Detailed synthesis method was reported in our previous work (Liu et al., 2011). The membranes were characterized by scanning electron microscopy (FESEM, S-4800II, HITACHI) and X-ray diffraction (XRD, Bruker,
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Pilot test
A pilot-scale pervaporation facility (Fig. 2) with a total membrane area of about 3 m2 was built up for dehydration of EG. The layout of the pervaporation facility is shown in Fig. 3. The facility was equipped with 8 modules connected in series, each of which includes 13 pieces of tubular NaA zeolite membranes with 800 mm long. The pervaporation dehydration of EG/water mixtures was carried out in batch mode. The feed had an initial mass of 120 kg and water content of about 20 wt.%. The feed flow rate was controlled at about 100 kg m−2 h−1 . The feed was preheated by the retentate and then heated up to operating temperature by water steam. The heat loss for each module during pervaporation was compensated by introducing water steam through the jacket shell. The inlet and outlet temperatures for each module were monitored online. The pressure of the permeate side was kept at 1000–2000 Pa provided by a water ring vacuum pump coupled with a roots pump. The
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Fig. 1 – Schematic diagram of pervaporation apparatus used in laboratory.
temperature of the cooling agent in the permeate side was at 0–2 ◦ C.
3.
Results and discussion
Stability of NaA zeolite membranes in 3.1. pervaporation It is commonly accepted that pervaporation separation is an economic method for dehydration of EG solution with water content of up to 30 wt.% (Jehle et al., 1995). However, the structure of NaA zeolite membranes is apt to change in the
Fig. 2 – The pilot-scale pervaporation facility (membrane area about 3 m2 ) used for dehydration of EG.
solution with high water content (Li et al., 2007). Therefore, it is very important to determine appropriate operation conditions for dehydration of EG by NaA zeolite membranes. Fig. 4 shows the time dependences of EG dehydration over NaA zeolite membranes for the feeds with water contents of 50 wt.%, 30 wt.%, 20 wt.%, 15 wt.% and 10 wt.%. The pervaporation processes were operated at 100 ◦ C. High H2 O/EG separation factors of >5000 were achieved for all the feeds initially. The initial fluxes were found to increase with feed water contents, which was due to the increased driving forces through zeolite membranes. For the feeds with water content of <20 wt.%, the permeation fluxes tended to be stable after 20 h. The slight decrease in permeation flux at initial stage could be related to water concentration polarization since highly viscous EG would show an obvious resistance to water diffusion (Dotremont et al., 1994). For the feeds with water content beyond 30 wt.%, it was interesting to observe that continuous decline in permeation flux happened during the whole pervaporation processes. Meanwhile, the separation factors began to decrease after 20-h operation. The phenomena implied that the performance of membranes could decline when they were operated in the feeds with high water content. It was reported that the NaA zeolite membranes failed in long run stability in pervaporation at high water concentration, which was probably caused by the dissolution of amorphous-like materials in the zeolitic grain boundary (Li et al., 2007). XRD and SEM characterizations were used to examine the structures of NaA zeolite membranes before and after EG dehydration, which were shown in Figs. 5 and 6 respectively. It was observed from Fig. 5a that the fresh NaA zeolite membrane had typical characteristic peaks of NaA zeolite at 2 = 7.1◦ , 10.1◦ , 12.5◦ and 16.1◦ . After EG dehydration, the used membranes (Fig. 5b–f) kept the characteristic peaks. However, split peaks occurred at 7.1◦ for the membranes (Fig. 5e and f) used for high water content of EG solutions (≥30 wt.%). From the SEM image shown in Fig. 6a, clear grain boundaries were observed on the fresh membrane. No obvious change in the morphologies was found on the surfaces of the membranes used for low water content of feeds ≤20 wt.% (Fig. 6b–d). However, some cracks were observed on the membranes (Fig. 6e and f) used for high water content of feeds (≥30 wt.%). Especially, for the feed with water content of 50 wt.%, the NaA zeolite crystal grains of the used membrane could hardly be observed (Fig. 6f). These results suggested
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Fig. 3 – Layout of pervaporation facility in pilot test.
that the structures of NaA zeolite membranes changed during pervaporation dehydration of EG solutions with high water content of ≥30 wt.%, which was in accordance with separation results of EG dehydration. The structure change of NaA zeolite membranes could be attributed to hydrolysis reaction of some amorphous materials embedded in the membrane layer due to high water content. Initially, the hydrolyzed products could block the mouths of zeolitic pores and reduce the permeation flux. Similar phenomenon was also observed by Hasegawa et al. (2010), who investigated acid stability of NaA zeolite membranes. Further extension of operating time would result
Fig. 4 – Time dependences of pervaporation results over the membranes used for dehydration of EG with different feed water contents (operating temperature: 100 ◦ C).
in an increased permeation flux due to the amorphous substances leaving from membrane surface. At the same time, the separation selectivity of the membranes would drop largely. It was speculated that the hydrolysis reaction could be retarded by reducing temperature, which would improve hydrothermal stability of NaA zeolite membrane for high water content of feeds. Therefore, we investigated pervaporation performance of NaA zeolite membranes for dehydration of EG solution with high water content (30 wt.%) at 70 ◦ C. As shown in Fig. 7, a steady permeation flux with the separation factor of over 10,000 could be obtained after 25 h, which was obviously different from the results obtained at high operating temperature. Thus, in the process of EG dehydration, the membrane modules could be operated at low temperature for high feed water content. It was noted from Fig. 7 that the achieved permeation flux was still comparable at low operating temperature, which was because the high water content could provide remarkable driving force for water transport. To maintain a high stability of NaA zeolite membranes for dehydration of EG with initial water content of 30 wt.%, it was suggested that two operating temperatures could be used
Fig. 5 – XRD patterns of (a) a fresh membrane and (b–f) the used membranes for dehydration of EG with different feed water contents. Water contents: (b) 10 wt.%, (c) 15 wt.%, (d) 20 wt.%, (e) 30 wt.%, and (f) 50 wt.%.
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Fig. 6 – SEM images of (a) a fresh membrane and (b–f) the used membranes for dehydration of EG with different feed water contents. Water contents: (b) 10 wt.%, (c) 15 wt.%, (d) 20 wt.%, (e) 30 wt.%, and (f) 50 wt.%. for the pervaporation process. At the first stage, low operating temperature (e.g. 70 ◦ C) could be used for dehydrating EG solution from the water content of 30–20 wt.%. At the second stage, higher temperature (e.g. >100 ◦ C) could be adopted for dehydration of EG solution with water content of less 20 wt.%, which would be beneficial to obtain higher permeation flux.
Fig. 7 – Time dependence of pervaporation results over a NaA zeolite membrane for dehydration of EG with feed water content of 30 wt.% (operating temperature: 70 ◦ C).
3.2.
Effect of feed composition
Fig. 8 shows the effect of feed composition on the permeation flux and selectivity for dehydration of EG solutions by NaA zeolite membranes under 70 ◦ C and 120 ◦ C. The permeation flux was observed to increase with feed water content. Considerably low flux was achieved at 70 ◦ C while much higher flux
Fig. 8 – Effect of feed composition on permeation flux for dehydration of EG at the operating temperatures of 70 ◦ C and 120 ◦ C.
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Fig. 9 – Effect of feed composition on permeation flux for dehydration of ethanol at the operating temperatures of 70 ◦ C and 120 ◦ C. was obtained at 120 ◦ C. The permeation flux almost increased linearly with the feed water content during the testing range of feed water content (1.8–30 wt.%). The separation factor also increased with water content of feed. A flux of 4.03 kg m−2 h−1 with separation factor of >5000 was achieved for 20 wt.% feed water content at 120 ◦ C. Relative low separation factors achieved at low water content of feed could be due to low water adsorption amount at feed side, which resulted in more EG molecules diffusing through non-selective pores. Since EG has high viscosity and strong polarity compared with alcohols, we were curious about whether the pervaporation dehydration exhibited specific behavior. Thus, pervaporation dehydration of ethanol solutions was performed using NaA zeolite membranes for comparison. Fig. 9 shows the separation results operated at the similar temperatures and water content range to EG dehydration. It was observed that NaA zeolite membranes also showed high separation selectivity for dehydration of ethanol solution, which had a separation factor of H2 O/EtOH over 5000. Interestingly, the trends for permeation flux vs. feed water content were different between the two dehydration processes. For ethanol dehydration, the permeation flux increased very fast with increasing of water content before 15 wt.% while it tended to increase slowly afterwards. It has been shown that the permeation flux of EG dehydration had a linear increase with water content. The difference in the trends could be related with inherent properties of the solutions. Table 1 summarizes the properties of EG, ethanol and water, where EG shows much higher boiling point, viscosity and dipole moment than ethanol and water. Our previous work (Liu et al., 2011) has revealed that saturated adsorption of water on feed side of NaA zeolite membrane could be reached at high water Table 1 – Physical properties of EG, ethanol and water.a Properties Formula Molar mass (g mol−1 ) Viscosity (20 ◦ C, mPa s) Dipole moment (20 ◦ C, Debye) Kinetic molecular diameter (nm) Boiling point (1 atm, ◦ C) Saturated vapor pressure (kPa) 25 ◦ C 100 ◦ C a
EG
Ethanol
Water
C2 H4 (OH)2 62.07 21.13 2.28 0.45 197.3
C2 H5 OH 46.07 1.19 1.69 0.52 78.4
H2 O 18.02 0.89 1.85 0.26 100.0
0.011 2.097
7.92 224.5
3.17 101.32
Kinetic molecular diameters were obtained from the literature (Nik et al., 2006), some other parameters were obtained from the literature (Wang et al., 2011).
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Fig. 10 – Temperature dependences of water flux (Jw ) for dehydration of EG with different feed water contents. content of 30 wt.% for dehydration of ethanol solution. The stable tendency of permeation flux for ethanol dehydration could be due to the saturated adsorption on the feed side for the water content of 30 wt.%. For EG dehydration, however, saturated adsorption was not reached at the similar water content, which could be due to the strong polarity of EG (Table 1). As a high polar solvent with two hydroxyl groups in each molecule, EG can form a hydrogen-bonded network with the polar active sites of the zeolites on the surface of the membrane due to the strong interactions between the hydroxyl groups and the surface silanol groups (Sekulic´ et al., 2005a,b). Thus, the competitive adsorption from EG molecules would be stronger than that from ethanol molecules. Comparing the permeation fluxes for the two separation systems, EG dehydration showed lower permeation flux than ethanol dehydration at the same operating conditions, as shown in Figs. 8 and 9. This could be explained by that high adsorption amount of EG on feed side would lead to low adsorption amount of water and thus low driving force for water transport, which would affect water permeation flux. Moreover, the higher viscosity and smaller kinetic diameter of EG could also have a negative effect on the water permeation flux. Due to high viscosity, the EG molecules would exert high transport resistance on water diffusion in the solution. On the other hand, the EG molecules could be easier to penetrate into zeolitic channels because of smaller kinetic diameter, which would affect water molecules to diffuse through the channels.
3.3.
Effect of temperature
Fig. 10 shows the effect of temperature on water flux of NaA zeolite membrane for dehydration of EG at different feed compositions. The Arrhenius plots for water flux vs. temperature showed linear relationship, indicating it was an activated process for water permeating through NaA zeolite membrane. According to the well-accepted solution-diffusion model, the permeation of a component through NaA zeolite membrane is dominated by adsorption-diffusion mechanism. The elevated operating temperature would result in increase of diffusivity but decrease of adsorption coefficient. In our previous work (Guo et al., 2011), molecular simulation was used to investigate temperature dependence of water flux for ethanol dehydration. It was found that water diffusion was enhanced at elevated temperatures while no obvious variation in water adsorption amount. This was due to the fact that the partial pressure of water in ethanol increased while the adsorption coefficient for water over zeolite surface decreased.
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Fig. 11 – Apparent activation energy of water (Ea,w ) for dehydration of EG with different feed water contents. Therefore, high operating temperature was beneficial to obtain high water flux for pervaporation dehydration. To determine apparent activation energy for water permeation, the Arrhenius equation was used for description of water permeation flux as follows,
E a,w
Jw = J0 exp −
(3)
RT
where J0 is the standard water flux related to the membrane, kg m−2 h−1 , Ea,w the apparent activation energy of water, kJ mol−1 . Fig. 11 shows the apparent activation energies at different feed water contents based on the experimental data from Fig. 10. It was found that the apparent activation energy varied from 43 to 38 kJ mol−1 with the increase of feed water content from 1.8 to 23.8 wt.%. Table 2 illustrates the reported apparent activation energies of water for dehydration of alcohols by NaA zeolite membranes, which are similar to the values obtained in our work. It is known that the apparent activation energy is related to the inter-molecular interaction and molecule/zeolite framework interaction. The similar apparent activation energies for dehydration of different system implied that the interactions between water molecules and zeolite membranes made a main effect on water permeation. However, a decrease in apparent activation energy at different feed water contents (Fig. 11) suggested that the interactions between water and EG molecules also affected the water permeation. It was very possible that the EG molecules in membrane channels could have an important effect on water diffusion. Since the water partial pressure varied at different operating temperatures, we figured water permeation flux vs. water partial pressure difference across membrane (pw ) at fixed feed water contents, as shown in Fig. 12. The water partial pressure in the feed was calculated by Wilson equation, and the permeate partial pressure of water was assumed to be the total pressure at the permeate side. It was interesting to
Table 2 – Apparent activation energies of water permeation (Ea,w ) for organics dehydration by NaA zeolite membranes. Solutions
Ethanol/water Ethanol/water Methanol/water EG/water
Water content (wt.%) 2.5–59 10 10 1.8–23.8
Ea,w (kJ mol−1 )
Ref.
41–43 35 43 38–43
Titus et al. (2006) Okamoto et al. (2001) Okamoto et al. (2001) This work
Fig. 12 – Water permeation flux (Jw ) as a function of water partial pressure difference across membrane (pw ) for dehydration of EG with different water content. find that the water flux increased linearly with pw under the fixed feed water contents. The results indicated that water permeance (Jw /pw ) through membrane remained constant. As derived by ten Elshof et al. (2003) and Sekulic´ et al. (2005a,b), the water permeance for multicomponent mixture could be described by the following equation, eff
Fw =
Jw Hw Dw = pw L
(4)
where Hw is the adsorption coefficient of water, kg m−3 Pa−1 , eff Dw an effective diffusivity of water, m2 s−1 , and L the thickness of the NaA zeolite membrane layer, m. As temperature increases, the adsorption coefficient of water (Hw ) decreases eff while the diffusion coefficient (Dw ) increases, which results in a balanced permeance. The result is different from that for polymeric membranes reported by Wang et al. (2011), where a decrease in water permeance with temperature was achieved. This is probably due to the different adsorption behaviors between polymeric membranes and NaA zeolite membranes. For pervaporation dehydration by NaA zeolite membranes, the water adsorption amount in the zeolite membrane is hardly influenced by temperature under the investigated operating conditions (Guo et al., 2011). The fitted values of water permeances through NaA zeolite membrane at different water contents were shown in Table 3. It was found that the permeance increased from 3.36 × 10−9 to 1.12 × 10−8 kg m−2 s−1 Pa−1 when the feed water content increased from 1.8 wt.% to 23.8 wt.%. This could be explained by that the frictional effect between water molecules and EG molecules becomes weaker at high water content, resulting in larger diffusivity for water transport in zeolitic pores according to Maxwell–Stefan theory (Krishna and van den Broeke, 1995; Krishna and Wesselingh, 1997). On the other hand, the adsorption coefficient of water (Hw ) is only Table 3 – Fitted permeances of water (Fw ) under different feed contents. Water content in feed (wt.%)
Fw (kg m−2 s−1 Pa−1 )
Coefficient correlation (R2 )
1.8 4.4 10.4 17.1 20.0 23.8
3.36 × 10−9 5.08 × 10−9 5.9 × 10−9 8.81 × 10−9 9.53 × 10−9 1.12 × 10−8
0.9990 0.9875 0.9865 0.9916 0.9829 0.9961
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Fig. 13 – Time dependences of the feed water content of EG solution during the pervaporation dehydration in batch mode. temperature dependent. Thus, high water permeance could be achieved at high feed water content.
3.4.
Pilot-scale test for EG dehydration
A pilot-scale pervaporation dehydration of EG solution was performed on a facility with about 3 m2 membrane area. 120 kg of EG solution with water content of about 20 wt.% was dehydrated in batch mode. The water content of the feed was continuously declining during pervaporation. Fig. 13 shows time dependences of water content in the feed at three operating temperatures (80 ◦ C, 100 ◦ C and 120 ◦ C). The solution could be successfully dehydrated to less than 1 wt.% at the temperature of 120 ◦ C during 20 h, which suggested the technical feasibility of pervaporation for EG dehydration. Comparably, the pervaporation separation at 100 ◦ C required longer operating time to reach the water content target due to relatively lower permeation flux at the temperature. At 80 ◦ C, the pervaporation separation showed very low efficiency for dehydration of the EG with low water content. This was because that low operating temperature caused not only the low diffusivity of water through zeolitic pores but also very limited driving force due to the low partial pressure of water. Fig. 14 shows the water flux and water content in permeate for the pilot-scale pervaporation separation at different feed water content and operating temperature. The water flux increased with the increase of feed water content. The elevated operating temperature was beneficial to improve the permeation flux. The trends were similar to those obtained in lab test. However, the permeation fluxes achieved in the pilotscale experiment were obviously lower than those in lab test. For dehydration of EG with feed water content of about 20 wt.% at 120 ◦ C, the achieved water fluxes were 4.03 kg m−2 h−1 for lab test and 3.12 kg m−2 h−1 for pilot-scale experiment. The difference should be attributed to scale-up effect for the pervaporation process. For the pilot-scale experiment, the operating temperature and flowing state could be relatively non-uniform in the membrane modules compared with labscale test, which would reduce the separation efficiency of NaA zeolite membranes. Besides, the absolute pressure in permeate was controlled at 1000–2000 Pa for pilot-scale operation, which was higher than that for lab test (<100 Pa). Thus, the reduced driving force in pilot-scale operation could also make a contribution to lower permeation flux. It seems that the temperature and concentration polarization of the large module in pilot-scale experiment were more severe than short single channel module used in lab test. To improve the module
Fig. 14 – Water flux and the water content in permeate vs. feed water content at three operating temperatures (80 ◦ C, 100 ◦ C and 120 ◦ C). efficiency, further optimization of module design and operation conditions is necessary. It can be seen that the water content obtained in permeate was 88–98 wt.%, which was lower than those obtained in lab test. This was due to relatively low performance for NaA zeolite membranes produced in large scale compared to those prepared in laboratory. However, the achieved water content in permeate was acceptable by industrial production of EG.
4.
Conclusions
NaA zeolite membranes exhibited high separation performance for dehydration of EG/water mixtures. A flux of 4.03 kg m−2 h−1 with separation factor over 5000 was achieved for 20 wt.% feed water content at 120 ◦ C. The inherent properties (polarity, viscosity and kinetic diameter) of EG lead to a relative low water permeation flux for EG dehydration compared with ethanol dehydration. The EG solution with high water content of 20–30 wt.% could result in low stability of NaA zeolite membranes for pervaporation at high temperatures (>100 ◦ C). To improve the membrane stability, lower operating temperatures could be used for dehydration of the EG solution with high water content. A pilot-scale test was successfully demonstrated for dehydration of EG, which proved the technical feasibility of the pervaporation process.
Acknowledgments This work was support by the National Basic Research Program of China (2009CB623403), National High-tech R&D Program of China (2009AA034802), National Natural Science Foundation of China (21176117, U0834004), Science & Technology Support Program (Industry) of Jiangsu Province of China (BE2008141), the Natural Science Foundation of the Jiangsu Higher
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Education Institutions (09KJA530002), the Priority Academic Program Development of Jiangsu Higher Education Institutions and 333 High-Level Personnel Training Project in Jiangsu Province.
References Dogan, H., Hilmioglu, N.D., 2010. Chitosan coated zeolite filled regenerated cellulose membrane for dehydration of EG/water mixtures by pervaporation. Desalination 258, 120. Dotremont, C., van den Ende, S., Vandommelea, H., Vandecasteele, C., 1994. Concentration polarization and other boundary layer effects in the pervaporation of chlorinated hydrocarbons. Desalination 95, 91. Du, J.R., Chakma, A., Feng, X., 2008. Dehydration of EG by pervaporation using poly(N,N-dimethylaminoethyl methacrylate)/polysulfone composite membranes. Sep. Purif. Technol. 64, 63. Feng, X., Huang, R.Y.M., 1996. Pervaporation with chitosan membranes. I. Separation of water from EG by a chitosan/polysulfone composite membrane. J. Membr. Sci. 116, 67. Guo, R., Hu, C., Li, B., Jiang, Z.Y., 2007. Pervaporation separation of EG/water mixtures through surface crosslinked PVA membranes: coupling effect and separation performance analysis. J. Membr. Sci. 289, 191. Guo, S.Y., Yu, C.L., Gu, X.H., Jin, W.Q., Zhong, J., Chen, C.L., 2011. Simulation of adsorption, diffusion, and permeability of water and ethanol in NaA zeolite membranes. J. Membr. Sci. 376, 40. Hasegawa, Y., Nagase, T., Kiyozumi, Y., Hanaoka, T., Mizukami, F., 2010. Influence of acid on the permeation properties of NaA-type zeolite membranes. J. Membr. Sci. 349, 189. Hinchliffe, A.B., Porter, K.E., 2000. A comparison of membrane separation and distillation. Chem. Eng. Res. Des. 78, 255. Hyder, M.N., Chen, P., 2009. Pervaporation dehydration of EG with chitosan-poly(vinyl alcohol) blend membranes: effect of CS-PVA blending ratios. J. Membr. Sci. 340, 171. Jehle, W., Staneff Th Wagner, B., Steinwandel, J., 1995. Separation of glycol and water from coolant liquids by evaporation, reverse osmosis and pervaporation. J. Membr. Sci. 102, 9. Kita, H., Horii, K., Ohtoshi, Y., Tanaka, K., Okamoto, K., 1995. Synthesis of a zeolite NaA membrane for pervaporation of water/organic liquid mixtures. J. Mater. Sci. Lett. 14, 206. Krishna, R., van den Broeke, L.J.P., 1995. The Maxwell–Stefan description of mass transport across zeolite membranes. Chem. Eng. J. 57, 155. Krishna, R., Wesselingh, J.A., 1997. The Maxwell–Stefan approach to mass transfer. Chem. Eng. Sci. 52, 861. Li, Y.S., Zhou, H., Zhu, G.Q., Liu, J., Yang, W.S., 2007. Hydrothermal stability of LTA zeolite membranes in pervaporation. J. Membr. Sci. 297, 10. Lipnizki, F., Field, R.W., Ten, P.K., 1999. Pervaporation-based hybrid process: a review of process design, applications and economics. J. Membr. Sci. 153, 183.
Liu, Y.M., Yang, Z.Z., Yu, C.L., Gu, X.H., Xu, N.P., 2011. Effect of seeding methods on growth of NaA zeolite membranes. Micropor. Mesopor. Mater. 143, 348. Morigami, Y., Kondo, M., Abe, J., Kita, H., Okamoto, K., 2001. The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane. Sep. Purif. Technol. 25, 251. Naidu, Y., Malik, R.K., 2011. A generalized methodology for optimal configurations of hybrid distillation-pervaporation processes. Chem. Eng. Res. Des. 89, 1348. Nik, O.G., Moheb, A., Mohammadi, T., 2006. Separation of EG/water mixtures using NaA zeolite membranes. Chem. Eng. Technol. 29, 1340. Okamoto, K., Kita, H., Horii, K., Tanaka, K., 2001. Zeolite NaA membrane: preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures. Ind. Eng. Chem. Res. 40, 163. ´ J., ten Elshof, J.E., Blank, D.H.A., 2005a. Separation Sekulic, mechanism in dehydration of water/organic binary liquids by pervaporation through microporous silica. J. Membr. Sci. 254, 267. ´ J., ten Elshof, J.E., Blank, D.H.A., 2005b. Selective Sekulic, pervaporation of water through a nonselective microporous titania membrane by a dynamically induced molecular sieving mechanism. Langmuir 21, 508. Sommer, S., Melin, T., 2005. Performance evaluation of microporous inorganic membranes in the dehydration of industrial solvents. Chem. Eng. Process. 44, 1138. ´ J., Chowdhury, S.R., Blank, ten Elshof, J.E., Abadal, C.R., Sekulic, D.H.A., 2003. Transport mechanisms of water and organic solvents through microporous silica in the pervaporation of binary liquids. Micropor. Mesopor. Mater. 65, 197. Titus, M.P., Llorens, J., Tejero, J., Cunill, F., 2006. Description of the pervaporation dehydration performance of A-type zeolite membranes: a modeling approach based on the Maxwell–Stefan theory. Catal. Today 118, 73. van Hoof, V., van den Abeele, L., Buekenhoudt, A., Dotremont, C., Leysen, R., 2004. Economic comparison between azeotropic distillation and different hybrid systems combining distillation with pervaporation for the dehydration of isopropanol. Sep. Purif. Technol. 37, 33. Wang, Y., Gruender, M., Chung, T.S., 2010. Pervaporation dehydration of ethylene glycol through polybenzimidazole (PBI)-based membranes. 1. Membrane fabrication. J. Membr. Sci. 363, 149. Wang, Y., Chung, T.S., Neo, B.W., Gruender, M., 2011. Processing and engineering of pervaporation dehydration of ethylene glycol via dual-layer polybenzimidazole (PBI)/polyetherimide (PEI) membranes. J. Membr. Sci. 378, 339. Yu, C.L., Liu, Y.M., Chen, G.L., Gu, X.H., Xing, W.H. Pretreatment of isopropanol solution from pharmaceutical industry and pervaporation dehydration by NaA zeolite membranes. Chin. J. Chem. Eng., 19(6), 2011.
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Pervaporation dehydration of ethylene glycol by NaA zeolite membranes Congli Yu, Chao Zhong, Yanmei Liu, Xuehong Gu ∗ , Gang Yang, Weihong Xing, Nanping Xu State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, PR China
a b s t r a c t Home-made NaA zeolite membranes were used for pervaporation dehydration of ethylene glycol (EG)/water mixtures. Hydrothermal stability of the membranes in pervaporation was investigated for industrial application purpose. The membranes exhibited good stability for water content of less than 20 wt.% at 100 ◦ C. The reduction of operating temperature was effective to improve membrane stability for operating at high feed water content (e.g. 30 wt.%). The influence of feed water content and operating temperature on dehydration of EG was extensively investigated. A permeation flux of 4.03 kg m−2 h−1 with separation factor of >5000 was achieved at 120 ◦ C for the separation of the solution with 20 wt.% water content. A pilot-scale pervaporation facility with membrane area of 3 m2 was built up for dehydration of EG with the water content of 20 wt.%, which showed technical feasibility for industrial application. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Ethylene glycol; NaA zeolite membranes; Pervaporation; Dehydration
1.
Introduction
Ethylene glycol (EG) is one of the major chemicals, which can be used as precursors to polyester, non-volatile antifreeze, plasticizer, etc. The conventional synthesis route via hydrolysis of ethylene oxide uses excess water to improve EG yield, which requires an extra dehydration process to acquire pure product. Multistage evaporation and distillation are commonly employed for the dehydration, which are highly energy-consuming due to high boiling point of EG. In the past decades, pervaporation technique based on membrane separation has gained increasing attention in many chemical processes. Since only a small amount of permeate has to be vaporized, the separation process can save more energy compared with conventional separation techniques, such as distillation and adsorption (Lipnizki et al., 1999; Hinchliffe and Porter, 2000; van Hoof et al., 2004; Naidu and Malik, 2011). The separation efficiency of pervaporation mainly depends on sorption equilibrium and mobility of components through membrane channels, which is almost independent of vapor–liquid equilibrium associated with feed mixture. For EG
∗
dehydration, pervaporation has been considered as a promising technique with water content of less than 30 wt.% (Jehle et al., 1995). Polymeric membranes have been investigated for dehydration of EG by several groups. Feng and Huang (1996) first demonstrated pervaporation dehydration of EG by using chitosan membranes. A permeation flux of 0.3 kg m−2 h−1 and the water content of >92 wt.% in permeate were achieved at 35 ◦ C for the feed of 90 wt.% EG. Since then, various polymeric membranes were employed for dehydration of EG, such as surface crosslinked PVA membranes (Guo et al., 2007), poly (N,N-dimethylaminoethyl methacrylate)/polysulfone composite membranes (Du et al., 2008), chitosan-poly (vinyl alcohol) blend membranes (Hyder and Chen, 2009), chitosan coated zeolite filled regenerated cellulose membranes (Dogan and Hilmioglu, 2010), etc. Although continuous progress has been made on water selectivity for polymeric membranes, the achieved water fluxes were still modest for practical utilization. The low water flux is related with the inherent property of polymeric membrane material for water transportation and the strong interactions between
Corresponding author. Tel.: +86 25 83172268; fax: +86 25 83172268. E-mail addresses: [email protected] (C. Yu), [email protected] (C. Zhong), yanmeiliu [email protected] (Y. Liu), [email protected] (X. Gu), [email protected] (G. Yang), [email protected] (W. Xing), [email protected] (N. Xu). Received 11 October 2011; Received in revised form 30 November 2011; Accepted 4 December 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.12.003
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EG molecules and the membrane. It is known that the elevated temperature could increase the water flux for pervaporation due to the higher partial pressure of water in the feed. However, most polymeric membranes cannot endure at high temperature due to low thermal stability, which generally required the operating temperature of <100 ◦ C. Recently, some researchers have devoted to develop highly stable polybenzimidazole (PBI)-based membranes for dehydration of EG, which also showed high separation factor up to 4500 (Wang et al., 2010, 2011). NaA zeolite membrane, as an inorganic membrane, has well-defined zeolitic pores with high hydrophilicity and the size of 0.42 nm. The membrane shows excellent permselectivity and flux for separation of water from organics (Kita et al., 1995). Moreover, NaA zeolite membrane can be operated at high temperature of >120 ◦ C. Industrial facilities based on NaA zeolite membrane have been constructed for dehydration of ethanol and isopropanol recently by Mitsui (Morigami et al., 2001), GFT-Inocermic and our group cooperating with Nanjing Jiusi Hi-Tech Co. Ltd. (China) (Yu et al., 2011). Recently, Nik et al. (2006) investigated NaA zeolite membrane for dehydration of EG in laboratory. A flux of 0.94 kg m−2 h−1 with separation factor of 1177 was achieved at 70 ◦ C for the feed of EG solution with 30 wt.% water, which was better than the separation results obtained on polymeric membranes. The results suggested that NaA zeolite membrane exhibits great potential in dehydration of EG and more extensive investigation on the process is necessary. Compared to other organic solvents, the achieved water flux for EG dehydration by NaA zeolite membrane was usually lower (Sommer and Melin, 2005; Nik et al., 2006). The intrinsic mechanism of the low flux is still unclear. On the other hand, since high water content has an effect on the structure of zeolite membranes (Li et al., 2007), it is vital to know the stability of NaA zeolite membranes used for dehydration of EG solution. In this work, therefore, we investigated pervaporation dehydration of EG using home-made tubular NaA zeolite membranes. We intended to optimize operation parameters for improving water flux through zeolite membranes and obtaining stable pervaporation process. A pilot-scale experiment for dehydration of EG was carried out to further evaluate the technical feasibility for industrial application.
2.
Experimental
2.1.
Membrane preparation
D8-Advance). The as-synthesized membranes consisted of a continuous NaA zeolite layer with a thickness of 10–15 m.
2.2.
Pervaporation
2.2.1.
Laboratory test
Dehydration of organic solvents was performed by pervaporation method based on NaA zeolite membranes. Fig. 1 shows a schematic diagram of the experimental setup used for laboratory test. A membrane module loaded with a NaA membrane tube (70 mm in length) was put in an oven for maintaining a constant operating temperature. A preheating loop connecting to the membrane module was used to heat up the feed flow, which was monitored by a thermocouple. The feed liquid with a constant flow rate of 80 mL min−1 driven by an advection pump was introduced through outside of membrane tube. The feed side pressure was maintained at 0.1 MPa (gauge pressure) to avoid feed liquid vaporizing at all experimental conditions. The pressure in permeate side was maintained below 100 Pa by evacuation with a vacuum pump throughout pervaporation operation. The permeate was condensed by two liquid nitrogen traps in parallel, allowing the experiments to be carried out in a continuous mode. The retentate was recycled back to the feed tank. The effective area of the membranes used in the experiments was 1.58 × 10−3 m2 . Different membranes were used for investigation of each operating parameter. The feed tank was maintained at constant compositions by compensating water. The compositions of the feed and permeate were analyzed by a gas chromatography (GC-8A, Shimadzu) equipped with a thermal conductivity detector (TCD) and a packed column of Parapak-Q. The membrane performance was evaluated by flux (J, kg m−2 h−1 ) and separation factor (˛), which were described respectively as J=
m Am t
(1)
and ˛=
YW /YE XW /XE
(2)
where m is the mass of the permeate, kg; Am the membrane area, m2 ; t the time interval, h; YW , YE , XW , XE the mass fractions of water and organics in the permeate and feed, respectively.
2.2.2. Home-made NaA zeolite membranes were hydrothermally synthesized on outer surface of porous mullite tubes by secondary growth method. The substrates had outer diameter of 12.8 mm, inner diameter 7.8 mm, average pore diameter 1 m, and porosity 40%. Prior to hydrothermal crystallization, the substrates were seeded with NaA zeolite particles (2–3 m) by a dip-coating technique. Hydrothermal crystallization was operated at 100 ◦ C for 3.5 h twice. Aluminosilicate solution for membrane synthesis was prepared by dissolving commercial chemicals of sodium aluminate and water glass in deionized water at room temperature. The composition of synthesis solution had the molar ratio of Al2 O3 :SiO2 :Na2 O:H2 O = 1:2:2:120. Detailed synthesis method was reported in our previous work (Liu et al., 2011). The membranes were characterized by scanning electron microscopy (FESEM, S-4800II, HITACHI) and X-ray diffraction (XRD, Bruker,
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Pilot test
A pilot-scale pervaporation facility (Fig. 2) with a total membrane area of about 3 m2 was built up for dehydration of EG. The layout of the pervaporation facility is shown in Fig. 3. The facility was equipped with 8 modules connected in series, each of which includes 13 pieces of tubular NaA zeolite membranes with 800 mm long. The pervaporation dehydration of EG/water mixtures was carried out in batch mode. The feed had an initial mass of 120 kg and water content of about 20 wt.%. The feed flow rate was controlled at about 100 kg m−2 h−1 . The feed was preheated by the retentate and then heated up to operating temperature by water steam. The heat loss for each module during pervaporation was compensated by introducing water steam through the jacket shell. The inlet and outlet temperatures for each module were monitored online. The pressure of the permeate side was kept at 1000–2000 Pa provided by a water ring vacuum pump coupled with a roots pump. The
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Fig. 1 – Schematic diagram of pervaporation apparatus used in laboratory.
temperature of the cooling agent in the permeate side was at 0–2 ◦ C.
3.
Results and discussion
Stability of NaA zeolite membranes in 3.1. pervaporation It is commonly accepted that pervaporation separation is an economic method for dehydration of EG solution with water content of up to 30 wt.% (Jehle et al., 1995). However, the structure of NaA zeolite membranes is apt to change in the
Fig. 2 – The pilot-scale pervaporation facility (membrane area about 3 m2 ) used for dehydration of EG.
solution with high water content (Li et al., 2007). Therefore, it is very important to determine appropriate operation conditions for dehydration of EG by NaA zeolite membranes. Fig. 4 shows the time dependences of EG dehydration over NaA zeolite membranes for the feeds with water contents of 50 wt.%, 30 wt.%, 20 wt.%, 15 wt.% and 10 wt.%. The pervaporation processes were operated at 100 ◦ C. High H2 O/EG separation factors of >5000 were achieved for all the feeds initially. The initial fluxes were found to increase with feed water contents, which was due to the increased driving forces through zeolite membranes. For the feeds with water content of <20 wt.%, the permeation fluxes tended to be stable after 20 h. The slight decrease in permeation flux at initial stage could be related to water concentration polarization since highly viscous EG would show an obvious resistance to water diffusion (Dotremont et al., 1994). For the feeds with water content beyond 30 wt.%, it was interesting to observe that continuous decline in permeation flux happened during the whole pervaporation processes. Meanwhile, the separation factors began to decrease after 20-h operation. The phenomena implied that the performance of membranes could decline when they were operated in the feeds with high water content. It was reported that the NaA zeolite membranes failed in long run stability in pervaporation at high water concentration, which was probably caused by the dissolution of amorphous-like materials in the zeolitic grain boundary (Li et al., 2007). XRD and SEM characterizations were used to examine the structures of NaA zeolite membranes before and after EG dehydration, which were shown in Figs. 5 and 6 respectively. It was observed from Fig. 5a that the fresh NaA zeolite membrane had typical characteristic peaks of NaA zeolite at 2 = 7.1◦ , 10.1◦ , 12.5◦ and 16.1◦ . After EG dehydration, the used membranes (Fig. 5b–f) kept the characteristic peaks. However, split peaks occurred at 7.1◦ for the membranes (Fig. 5e and f) used for high water content of EG solutions (≥30 wt.%). From the SEM image shown in Fig. 6a, clear grain boundaries were observed on the fresh membrane. No obvious change in the morphologies was found on the surfaces of the membranes used for low water content of feeds ≤20 wt.% (Fig. 6b–d). However, some cracks were observed on the membranes (Fig. 6e and f) used for high water content of feeds (≥30 wt.%). Especially, for the feed with water content of 50 wt.%, the NaA zeolite crystal grains of the used membrane could hardly be observed (Fig. 6f). These results suggested
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Fig. 3 – Layout of pervaporation facility in pilot test.
that the structures of NaA zeolite membranes changed during pervaporation dehydration of EG solutions with high water content of ≥30 wt.%, which was in accordance with separation results of EG dehydration. The structure change of NaA zeolite membranes could be attributed to hydrolysis reaction of some amorphous materials embedded in the membrane layer due to high water content. Initially, the hydrolyzed products could block the mouths of zeolitic pores and reduce the permeation flux. Similar phenomenon was also observed by Hasegawa et al. (2010), who investigated acid stability of NaA zeolite membranes. Further extension of operating time would result
Fig. 4 – Time dependences of pervaporation results over the membranes used for dehydration of EG with different feed water contents (operating temperature: 100 ◦ C).
in an increased permeation flux due to the amorphous substances leaving from membrane surface. At the same time, the separation selectivity of the membranes would drop largely. It was speculated that the hydrolysis reaction could be retarded by reducing temperature, which would improve hydrothermal stability of NaA zeolite membrane for high water content of feeds. Therefore, we investigated pervaporation performance of NaA zeolite membranes for dehydration of EG solution with high water content (30 wt.%) at 70 ◦ C. As shown in Fig. 7, a steady permeation flux with the separation factor of over 10,000 could be obtained after 25 h, which was obviously different from the results obtained at high operating temperature. Thus, in the process of EG dehydration, the membrane modules could be operated at low temperature for high feed water content. It was noted from Fig. 7 that the achieved permeation flux was still comparable at low operating temperature, which was because the high water content could provide remarkable driving force for water transport. To maintain a high stability of NaA zeolite membranes for dehydration of EG with initial water content of 30 wt.%, it was suggested that two operating temperatures could be used
Fig. 5 – XRD patterns of (a) a fresh membrane and (b–f) the used membranes for dehydration of EG with different feed water contents. Water contents: (b) 10 wt.%, (c) 15 wt.%, (d) 20 wt.%, (e) 30 wt.%, and (f) 50 wt.%.
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Fig. 6 – SEM images of (a) a fresh membrane and (b–f) the used membranes for dehydration of EG with different feed water contents. Water contents: (b) 10 wt.%, (c) 15 wt.%, (d) 20 wt.%, (e) 30 wt.%, and (f) 50 wt.%. for the pervaporation process. At the first stage, low operating temperature (e.g. 70 ◦ C) could be used for dehydrating EG solution from the water content of 30–20 wt.%. At the second stage, higher temperature (e.g. >100 ◦ C) could be adopted for dehydration of EG solution with water content of less 20 wt.%, which would be beneficial to obtain higher permeation flux.
Fig. 7 – Time dependence of pervaporation results over a NaA zeolite membrane for dehydration of EG with feed water content of 30 wt.% (operating temperature: 70 ◦ C).
3.2.
Effect of feed composition
Fig. 8 shows the effect of feed composition on the permeation flux and selectivity for dehydration of EG solutions by NaA zeolite membranes under 70 ◦ C and 120 ◦ C. The permeation flux was observed to increase with feed water content. Considerably low flux was achieved at 70 ◦ C while much higher flux
Fig. 8 – Effect of feed composition on permeation flux for dehydration of EG at the operating temperatures of 70 ◦ C and 120 ◦ C.
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Fig. 9 – Effect of feed composition on permeation flux for dehydration of ethanol at the operating temperatures of 70 ◦ C and 120 ◦ C. was obtained at 120 ◦ C. The permeation flux almost increased linearly with the feed water content during the testing range of feed water content (1.8–30 wt.%). The separation factor also increased with water content of feed. A flux of 4.03 kg m−2 h−1 with separation factor of >5000 was achieved for 20 wt.% feed water content at 120 ◦ C. Relative low separation factors achieved at low water content of feed could be due to low water adsorption amount at feed side, which resulted in more EG molecules diffusing through non-selective pores. Since EG has high viscosity and strong polarity compared with alcohols, we were curious about whether the pervaporation dehydration exhibited specific behavior. Thus, pervaporation dehydration of ethanol solutions was performed using NaA zeolite membranes for comparison. Fig. 9 shows the separation results operated at the similar temperatures and water content range to EG dehydration. It was observed that NaA zeolite membranes also showed high separation selectivity for dehydration of ethanol solution, which had a separation factor of H2 O/EtOH over 5000. Interestingly, the trends for permeation flux vs. feed water content were different between the two dehydration processes. For ethanol dehydration, the permeation flux increased very fast with increasing of water content before 15 wt.% while it tended to increase slowly afterwards. It has been shown that the permeation flux of EG dehydration had a linear increase with water content. The difference in the trends could be related with inherent properties of the solutions. Table 1 summarizes the properties of EG, ethanol and water, where EG shows much higher boiling point, viscosity and dipole moment than ethanol and water. Our previous work (Liu et al., 2011) has revealed that saturated adsorption of water on feed side of NaA zeolite membrane could be reached at high water Table 1 – Physical properties of EG, ethanol and water.a Properties Formula Molar mass (g mol−1 ) Viscosity (20 ◦ C, mPa s) Dipole moment (20 ◦ C, Debye) Kinetic molecular diameter (nm) Boiling point (1 atm, ◦ C) Saturated vapor pressure (kPa) 25 ◦ C 100 ◦ C a
EG
Ethanol
Water
C2 H4 (OH)2 62.07 21.13 2.28 0.45 197.3
C2 H5 OH 46.07 1.19 1.69 0.52 78.4
H2 O 18.02 0.89 1.85 0.26 100.0
0.011 2.097
7.92 224.5
3.17 101.32
Kinetic molecular diameters were obtained from the literature (Nik et al., 2006), some other parameters were obtained from the literature (Wang et al., 2011).
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Fig. 10 – Temperature dependences of water flux (Jw ) for dehydration of EG with different feed water contents. content of 30 wt.% for dehydration of ethanol solution. The stable tendency of permeation flux for ethanol dehydration could be due to the saturated adsorption on the feed side for the water content of 30 wt.%. For EG dehydration, however, saturated adsorption was not reached at the similar water content, which could be due to the strong polarity of EG (Table 1). As a high polar solvent with two hydroxyl groups in each molecule, EG can form a hydrogen-bonded network with the polar active sites of the zeolites on the surface of the membrane due to the strong interactions between the hydroxyl groups and the surface silanol groups (Sekulic´ et al., 2005a,b). Thus, the competitive adsorption from EG molecules would be stronger than that from ethanol molecules. Comparing the permeation fluxes for the two separation systems, EG dehydration showed lower permeation flux than ethanol dehydration at the same operating conditions, as shown in Figs. 8 and 9. This could be explained by that high adsorption amount of EG on feed side would lead to low adsorption amount of water and thus low driving force for water transport, which would affect water permeation flux. Moreover, the higher viscosity and smaller kinetic diameter of EG could also have a negative effect on the water permeation flux. Due to high viscosity, the EG molecules would exert high transport resistance on water diffusion in the solution. On the other hand, the EG molecules could be easier to penetrate into zeolitic channels because of smaller kinetic diameter, which would affect water molecules to diffuse through the channels.
3.3.
Effect of temperature
Fig. 10 shows the effect of temperature on water flux of NaA zeolite membrane for dehydration of EG at different feed compositions. The Arrhenius plots for water flux vs. temperature showed linear relationship, indicating it was an activated process for water permeating through NaA zeolite membrane. According to the well-accepted solution-diffusion model, the permeation of a component through NaA zeolite membrane is dominated by adsorption-diffusion mechanism. The elevated operating temperature would result in increase of diffusivity but decrease of adsorption coefficient. In our previous work (Guo et al., 2011), molecular simulation was used to investigate temperature dependence of water flux for ethanol dehydration. It was found that water diffusion was enhanced at elevated temperatures while no obvious variation in water adsorption amount. This was due to the fact that the partial pressure of water in ethanol increased while the adsorption coefficient for water over zeolite surface decreased.
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Fig. 11 – Apparent activation energy of water (Ea,w ) for dehydration of EG with different feed water contents. Therefore, high operating temperature was beneficial to obtain high water flux for pervaporation dehydration. To determine apparent activation energy for water permeation, the Arrhenius equation was used for description of water permeation flux as follows,
E a,w
Jw = J0 exp −
(3)
RT
where J0 is the standard water flux related to the membrane, kg m−2 h−1 , Ea,w the apparent activation energy of water, kJ mol−1 . Fig. 11 shows the apparent activation energies at different feed water contents based on the experimental data from Fig. 10. It was found that the apparent activation energy varied from 43 to 38 kJ mol−1 with the increase of feed water content from 1.8 to 23.8 wt.%. Table 2 illustrates the reported apparent activation energies of water for dehydration of alcohols by NaA zeolite membranes, which are similar to the values obtained in our work. It is known that the apparent activation energy is related to the inter-molecular interaction and molecule/zeolite framework interaction. The similar apparent activation energies for dehydration of different system implied that the interactions between water molecules and zeolite membranes made a main effect on water permeation. However, a decrease in apparent activation energy at different feed water contents (Fig. 11) suggested that the interactions between water and EG molecules also affected the water permeation. It was very possible that the EG molecules in membrane channels could have an important effect on water diffusion. Since the water partial pressure varied at different operating temperatures, we figured water permeation flux vs. water partial pressure difference across membrane (pw ) at fixed feed water contents, as shown in Fig. 12. The water partial pressure in the feed was calculated by Wilson equation, and the permeate partial pressure of water was assumed to be the total pressure at the permeate side. It was interesting to
Table 2 – Apparent activation energies of water permeation (Ea,w ) for organics dehydration by NaA zeolite membranes. Solutions
Ethanol/water Ethanol/water Methanol/water EG/water
Water content (wt.%) 2.5–59 10 10 1.8–23.8
Ea,w (kJ mol−1 )
Ref.
41–43 35 43 38–43
Titus et al. (2006) Okamoto et al. (2001) Okamoto et al. (2001) This work
Fig. 12 – Water permeation flux (Jw ) as a function of water partial pressure difference across membrane (pw ) for dehydration of EG with different water content. find that the water flux increased linearly with pw under the fixed feed water contents. The results indicated that water permeance (Jw /pw ) through membrane remained constant. As derived by ten Elshof et al. (2003) and Sekulic´ et al. (2005a,b), the water permeance for multicomponent mixture could be described by the following equation, eff
Fw =
Jw Hw Dw = pw L
(4)
where Hw is the adsorption coefficient of water, kg m−3 Pa−1 , eff Dw an effective diffusivity of water, m2 s−1 , and L the thickness of the NaA zeolite membrane layer, m. As temperature increases, the adsorption coefficient of water (Hw ) decreases eff while the diffusion coefficient (Dw ) increases, which results in a balanced permeance. The result is different from that for polymeric membranes reported by Wang et al. (2011), where a decrease in water permeance with temperature was achieved. This is probably due to the different adsorption behaviors between polymeric membranes and NaA zeolite membranes. For pervaporation dehydration by NaA zeolite membranes, the water adsorption amount in the zeolite membrane is hardly influenced by temperature under the investigated operating conditions (Guo et al., 2011). The fitted values of water permeances through NaA zeolite membrane at different water contents were shown in Table 3. It was found that the permeance increased from 3.36 × 10−9 to 1.12 × 10−8 kg m−2 s−1 Pa−1 when the feed water content increased from 1.8 wt.% to 23.8 wt.%. This could be explained by that the frictional effect between water molecules and EG molecules becomes weaker at high water content, resulting in larger diffusivity for water transport in zeolitic pores according to Maxwell–Stefan theory (Krishna and van den Broeke, 1995; Krishna and Wesselingh, 1997). On the other hand, the adsorption coefficient of water (Hw ) is only Table 3 – Fitted permeances of water (Fw ) under different feed contents. Water content in feed (wt.%)
Fw (kg m−2 s−1 Pa−1 )
Coefficient correlation (R2 )
1.8 4.4 10.4 17.1 20.0 23.8
3.36 × 10−9 5.08 × 10−9 5.9 × 10−9 8.81 × 10−9 9.53 × 10−9 1.12 × 10−8
0.9990 0.9875 0.9865 0.9916 0.9829 0.9961
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Fig. 13 – Time dependences of the feed water content of EG solution during the pervaporation dehydration in batch mode. temperature dependent. Thus, high water permeance could be achieved at high feed water content.
3.4.
Pilot-scale test for EG dehydration
A pilot-scale pervaporation dehydration of EG solution was performed on a facility with about 3 m2 membrane area. 120 kg of EG solution with water content of about 20 wt.% was dehydrated in batch mode. The water content of the feed was continuously declining during pervaporation. Fig. 13 shows time dependences of water content in the feed at three operating temperatures (80 ◦ C, 100 ◦ C and 120 ◦ C). The solution could be successfully dehydrated to less than 1 wt.% at the temperature of 120 ◦ C during 20 h, which suggested the technical feasibility of pervaporation for EG dehydration. Comparably, the pervaporation separation at 100 ◦ C required longer operating time to reach the water content target due to relatively lower permeation flux at the temperature. At 80 ◦ C, the pervaporation separation showed very low efficiency for dehydration of the EG with low water content. This was because that low operating temperature caused not only the low diffusivity of water through zeolitic pores but also very limited driving force due to the low partial pressure of water. Fig. 14 shows the water flux and water content in permeate for the pilot-scale pervaporation separation at different feed water content and operating temperature. The water flux increased with the increase of feed water content. The elevated operating temperature was beneficial to improve the permeation flux. The trends were similar to those obtained in lab test. However, the permeation fluxes achieved in the pilotscale experiment were obviously lower than those in lab test. For dehydration of EG with feed water content of about 20 wt.% at 120 ◦ C, the achieved water fluxes were 4.03 kg m−2 h−1 for lab test and 3.12 kg m−2 h−1 for pilot-scale experiment. The difference should be attributed to scale-up effect for the pervaporation process. For the pilot-scale experiment, the operating temperature and flowing state could be relatively non-uniform in the membrane modules compared with labscale test, which would reduce the separation efficiency of NaA zeolite membranes. Besides, the absolute pressure in permeate was controlled at 1000–2000 Pa for pilot-scale operation, which was higher than that for lab test (<100 Pa). Thus, the reduced driving force in pilot-scale operation could also make a contribution to lower permeation flux. It seems that the temperature and concentration polarization of the large module in pilot-scale experiment were more severe than short single channel module used in lab test. To improve the module
Fig. 14 – Water flux and the water content in permeate vs. feed water content at three operating temperatures (80 ◦ C, 100 ◦ C and 120 ◦ C). efficiency, further optimization of module design and operation conditions is necessary. It can be seen that the water content obtained in permeate was 88–98 wt.%, which was lower than those obtained in lab test. This was due to relatively low performance for NaA zeolite membranes produced in large scale compared to those prepared in laboratory. However, the achieved water content in permeate was acceptable by industrial production of EG.
4.
Conclusions
NaA zeolite membranes exhibited high separation performance for dehydration of EG/water mixtures. A flux of 4.03 kg m−2 h−1 with separation factor over 5000 was achieved for 20 wt.% feed water content at 120 ◦ C. The inherent properties (polarity, viscosity and kinetic diameter) of EG lead to a relative low water permeation flux for EG dehydration compared with ethanol dehydration. The EG solution with high water content of 20–30 wt.% could result in low stability of NaA zeolite membranes for pervaporation at high temperatures (>100 ◦ C). To improve the membrane stability, lower operating temperatures could be used for dehydration of the EG solution with high water content. A pilot-scale test was successfully demonstrated for dehydration of EG, which proved the technical feasibility of the pervaporation process.
Acknowledgments This work was support by the National Basic Research Program of China (2009CB623403), National High-tech R&D Program of China (2009AA034802), National Natural Science Foundation of China (21176117, U0834004), Science & Technology Support Program (Industry) of Jiangsu Province of China (BE2008141), the Natural Science Foundation of the Jiangsu Higher
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Education Institutions (09KJA530002), the Priority Academic Program Development of Jiangsu Higher Education Institutions and 333 High-Level Personnel Training Project in Jiangsu Province.
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