ceramic composite membranes

ceramic composite membranes

Desalination 258 (2010) 106–111 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m ...

384KB Sizes 78 Downloads 287 Views

Desalination 258 (2010) 106–111

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Pervaporation separation of n-octane/thiophene mixtures using polydimethylsiloxane/ceramic composite membranes Rong Xu, Gongping Liu, Xueliang Dong, Wanqin, Jin ⁎ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, China

a r t i c l e

i n f o

Article history: Received 19 September 2009 Received in revised form 16 March 2010 Accepted 18 March 2010 Available online 18 April 2010 Keywords: Pervaporation Desulfurization Composite membrane PDMS

a b s t r a c t Crosslinked polydimethylsiloxane (PDMS)/ceramic composite membranes were prepared and employed for desulfurization of model gasoline composed of n-octane and thiophene. The structural morphology and thermal stability of the composite membranes were characterized by scanning electron microscope (SEM) and thermogravimetric analysis (TGA). The pervaporation performances of the membranes under various crosslinking agent amounts, feed sulfur content, feed temperature, permeate pressure and feed flow rate were investigated. Experimental results indicated that 20% of the crosslinking agent amount was more preferable. Increase of sulfur content in feed resulted in a higher total flux but a lower sulfur enrichment factor. By increasing the feed temperature, the total flux increased while sulfur enrichment factor decreased. Low permeate pressure and high feed flow rate were beneficial to improve total flux and sulfur enrichment factor. At 303.15 K, the composite membrane exhibited high performance with the total flux of 5.37 kg m− 2 h− 1 and the corresponding sulfur enrichment factor of 4.22 for 400 μg g− 1 sulfur in feed under 210 Pa. Our results showed that the PDMS/ceramic composite membrane was a potential candidate to be used for sulfur removal from the gasoline. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Removal of sulfur from gasoline has received increasing attention with growing environmental awareness. Sulfur oxide emissions generated by vehicle engines may induce the formation of acid rain, decrease the efficiency of catalytic converters and promote corrosion of engine parts [1,2]. Therefore, many countries mandate more stringent regulations on sulfur content in gasoline. The specification in EU countries, requires a reduction in sulfur content of gasoline to 10 μg g− 1 by 2010 from the current 150 μg g− 1. Similarly tough regulations are also being implemented in the USA and China [3,4]. So developing cost-effective technology for gasoline desulfurization is an urgent task of the present society. Catalytic hydrodesulfurization (HDS) is a major method for gasoline desulfurization. But this conventional process requires high temperature, high pressure and high hydrogen consumption. Furthermore, HDS results in a significant loss in octane number due to the saturation of olefins and aromatics [5]. Pervaporation (PV) has been given much more attention as a promising and feasible technology for desulfurization of gasoline since Grace Davison Company offered the PV membrane-based S-Brane process in 2002 [6]. PV desulfurization

⁎ Corresponding author. Tel.: +86 25 83172266; fax: +86 25 83172292. E-mail address: [email protected] (Jin). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.03.035

demonstrates distinct advantages over conventional technologies such as low energy consumption, simple operation, and little reduction of octane number [7]. As is well known, the membrane is the core of the pervaporation technology for gasoline desulfurization. To date, the membrane materials for desulfurization are mainly focused on organic–organic composite membranes. Lin et al. [8-10] reported the desulfurization performance of crosslinked polyethylene glycol/polyethersulfone (PEG/PES) composite membranes for FCC gasoline desulphurization. Qi et al. [11-13] investigated the performance of polydimethylsiloxane/polyacrylonitrile (PDMS/PAN) composite membranes for sulfur removal from model gasoline. However, studies on gasoline desulfurization with ceramic-supported membranes have not been reported in the literatures. Compared to polymeric supports, ceramic supports exhibit superior chemical, thermal and mechanical stability, e.g., no swelling, no compaction and negligible transport resistance [14,15]. So, a high-performance membrane might be obtained by combination of polymeric layer and ceramic support [16,17]. In this work, we prepared crosslinked PDMS/ceramic composite membranes using tubular nonsymmetric ZrO2/Al2O3 as ceramic supports. This composite membrane was firstly utilized for pervaporation seperation of sulfur-containing compounds from model gasoline system (n-octane/thiophene). The effects of various factors including crosslinking agent amount, feed sulfur content, feed temperature, permeate pressure and feed flow rate on desulfurization performance were systematically investigated.

R. Xu et al. / Desalination 258 (2010) 106–111

2. Experimental 2.1. Materials The ceramic supports used were tubular asymmetric ZrO2/Al2O3 composite membranes (average pore size of top layer, 0.2 μm), supplied by Membrane Science & Technology Research Center in Nanjing University of Technology. They were 120 mm in length with the external diameter of 12 mm and the inner diameter of 8 mm. PDMS (viscosity, 5000 mPa s; M̅w = 60000) was purchased from Shanghai Synthetic Resin Company, China. Thiophene (99+%, extra pure) was supplied by Acros Organics. Tetraethylorthosilicate (TEOS), n-heptane, n-octane and dibutyltin dilaurate were of analytical grade from Sinopharm Chemical Reagent Co., Ltd, China and were used without further purification. 2.2. Membrane preparation The tubular ZrO2/Al2O3 supports were polished slightly with 600 mesh sand paper (991A, Starcke GmbH & Co. KG, Germany) to reduce the roughness of the support surface. After boiling for 30 min, the supports were cleaned and soaked for 12 h using deionized water. PDMS polymer mixed with 20% by weight of TEOS as crosslinking agent was dissolved in n-heptane to form a homogenous solution, then dibutyltin dilaurate as catalyst was added into the polymer solution. The solution was stirred at 303 K for 3 h, and then degassed under vacuum. Subsequently, the crosslinked PDMS solution was coated onto the outer surface of ceramic supports by a dip-coating method, which was similar to that in our previous work [18]. The membranes were dried for 24 h under room temperature, then introduced into an oven to remove any residual solvent and to complete crosslinking at 393 K for 12 h. All membranes in this work were prepared under identical conditions. 2.3. Membrane characterization 2.3.1. Scanning electron microscopy The surface and cross-section morphologies of the tubular-type composite membranes and the thicknesses of the organic separation layers were characterized by scanning electron microscopy (SEM) (QUANTA-2000). The dried composite membranes were fractured in liquid nitrogen and then sputtered with gold in vacuum.

107

2.3.2. Thermogravimetric analysis The thermal stability of the ceramic support, PDMS/ceramic composite membrane, and PDMS homogenous membrane were investigated by a thermogravimetric analyzer (TGA, STA 409 PC, NETZSCH, Germany). The samples with weight that ranged from 5 to 8 mg were heated from 313 to 1073 K at a heating rate of 10 K/min with a nitrogen flow of 15 mL/min. 2.4. Pervaporation experiments The schematic pervaporation apparatus is shown in Fig. 1. The membrane module was made of stainless steel, in which the effective membrane area was 32.78 cm2. The feed solution was continuously circulated from a feed tank through the tube side of the membrane module using a variable speed feed pump. Vacuum on the permeate side was monitored by a digital vacuum gauge. The permeated vapor was collected by turns in liquid nitrogen traps. About 2 h after starting the PV process, a mass transfer equilibrium was established and PV performance reached stable. At steady state, the weight of permeate collected in the cold trap was measured to obtain the total flux, J and the partial flux, Ji J=

M A×t

ð1Þ

and p

Ji = wi J

ð2Þ

where M is the total mass permeated, A the effective membrane area, t the experiment time interval, and wpi the weight fraction of component i in the permeate samples. The total sulfur contents of the feed and permeated samples were analyzed by a gas chromatography (GC-2014, SHIMADZU, Japan, equipped with FID). FFAP capillary column was used with the following dimensions: length 30 m, inside diameter 0.32 mm. The temperatures for injector, detector and oven were set at 180, 200 and 353 K, respectively. The sulfur enrichment factor, E, is defined as P

E=

w wF

Fig. 1. Schematic diagram of the pervaporation apparatus.

ð3Þ

108

R. Xu et al. / Desalination 258 (2010) 106–111

where wFand wP refer to the weight fractions of thiophene in the feed and permeate samples, respectively. 3. Results and discussion 3.1. SEM images of the composite membrane Fig. 2 shows the SEM micrograph of the PDMS/ZrO2/Al2O3 composite membrane. The active PDMS layer is coated on the surface of tubular ceramic supports uniformly. The surface of the composite membrane is dense and defect-free (Fig. 2a), and three-layer structures (Al2O3 layer, ZrO2 layer, and PDMS layer) can be observed clearly (Fig. 2b). The PDMS layer is well adhered to the porous ceramic support layer and has a thickness of about 8 μm. 3.2. Thermogravimetric analysis The thermal stability of the ceramic support, PDMS/ceramic composite membrane, and PDMS homogenous membrane were analyzed by TGA under nitrogen atmosphere, and the resulting thermograms are shown in Fig. 3. The ceramic support exhibited excellent thermal stability. The PDMS homogenous membrane

Fig. 3. TGA curves of PDMS homogenous membrane, PDMS/ceramic composite membrane, and the ceramic support.

exhibited a degradation at round 550 K, whereas the PDMS/ceramic composite membrane started to decompose at about 600 K. Taking the temperature at 1% weight loss (Td) to evaluate the thermal stability of the membranes, the composite membrane (Td = 613 K) exhibited better thermal stability than the PDMS homogenous membrane (Td = 561 K). A possible explanation for retardation on thermal degradation is formation of an enhanced interface structure between the PDMS layer and the thermally stable ceramic support layer. 3.3. The effect of crosslinking agent amount on membrane performance In order to enhance the selectivity and restrict the swelling behavior of the active PDMS layer, the effect of crosslinking agent amount on membrane performance was investigated. From Fig. 4, the sulfur enrichment factor increased by increasing the amount of crosslinking agent as expected, while the total flux decreased. When the crosslinking agent was added to the PDMS solution, the chemical connection occurred between macromolecules and reticular spatial structure formed, which was favorable for swelling resistance of membranes in gasoline. And then the mobility of macromolecules and chain segments weakened with the interchain free volume lessened, which accounted for the decrease of permeation flux. However, because of thiophene species having stronger affinity to the membrane, the permeation flux of the thiophene species decreased

Fig. 2. SEM images of the tubular-type PDMS/ZrO2/Al2O3 composite membrane: (a) surface of membrane, and (b) cross section of membrane.

Fig. 4. Effect of crosslinking agent amount on flux and sulfur enrichment factor (sulfur content in feed: 400 μg g− 1, feed temperature: 303.15 K, feed flow rate: 30 L/h, permeate pressure: 210 Pa).

R. Xu et al. / Desalination 258 (2010) 106–111

109

Fig. 5. Effect of sulfur content in feed on total flux and sulfur enrichment factor (feed flow rate: 30 L/h, permeate pressure: 210 Pa).

Fig. 7. Arrhenius plots between partial flux and feed temperature.

more slowly than that of hydrocarbon species. Hence, the sulfur enrichment factor increased with the addition of crosslinking agent. Meanwhile, excessive addition of crosslinking agent solution brought a higher viscosity solution, which was hardly to be coated on the support layer uniformly. So the amount of crosslinking agent at 20% should be more practical due to the trade-off between permeation flux and sulfur enrichment factor.

3.4.2. Feed temperature The feed temperature is an important factor that will affect the membrane performance for desulfurization. As shown in Fig. 6, when the temperature increased, the total flux increased, whereas the sulfur enrichment factor decreased. Increase of the feed temperature accelerated the mobility of the polymer chains and generated larger available free volume within the membrane for diffusion. In addition, higher temperature resulted in higher vapor pressure difference which would enhance the transport driving force. On the other hand, improvement of the swelling degree of the polymer membrane weakened difference of solubility and diffusion velocity and resulted in more n-octane transport, thus causing the decrease of sulfur enrichment factor. Subsequently, the excessive swelling of the active PDMS layer might be restricted partly in the area of the PDMS-ceramic support interface due to an enhanced organic–inorganic structural stability [15]. So the change degree of sulfur enrichment factor weakened. Comparison of the partial flux of thiophene and n-octane is helpful to further understanding above results. The temperature dependence of the partial flux was described by Arrhenius relationship [20]:

3.4. Effects of operating parameters on pervaporation performance 3.4.1. Sulfur content in feed Fig. 5 illustrates the effect of sulfur content on pervaporation performance for desulfurization. A higher feed sulfur content resulted in a higher total flux but a lower sulfur enrichment factor. As the sulfur content increased, the thiophene molecules dissolved more and more in the polymer chains, leading to the extensive swelling of the membrane. Consequently, both the thiophene and the n-octane components permeated through the membrane easily, and total flux increased. The increasing swelling of the membrane would weaken difference of sorption and diffusion characteristics of the individual component, and the octane molecules could diffuse more easily through swollen membrane. Therefore, the sulfur enrichment factor declined. Nevertheless, the total flux and sulfur enrichment factor changed slightly when sulfur content reached 600 μg g− 1, which may be due to the occurrence of swelling balance and the existence of confined deformation effect [19].

with Ai as the pre-exponential factor, Epi the activation energy of permeation for component i, T the feed absolute temperature, and R the gas constant. Fig. 7 presents the relationship between logarithm

Fig. 6. Effect of feed temperature on total flux and sulfur enrichment factor (sulfur content in feed: 400 μg g− 1, feed flow rate: 30 L/h, permeate pressure: 210 Pa).

Fig. 8. Effect of permeate pressure on total flux and sulfur enrichment factor (feed temperature: 303.15 K, sulfur content in feed: 400 μg g− 1, feed flow rate: 30 L/h).

  Epi Ji = Ai exp − RT

ð4Þ

110

R. Xu et al. / Desalination 258 (2010) 106–111

3.5. Comparison of pervaporation performance with literatures

Fig. 9. Effect of feed flow rate on total flux and sulfur enrichment factor (feed temperature: 303.15 K, sulfur content in feed: 400 μg g− 1, permeate pressure: 210 Pa).

of partial flux and reciprocal of absolute temperature. According to the slope, the activation energy values were calculated to be Eoctane= 29.22 kJ mol− 1, Ethiophene= 22.06 kJ mol− 1, which indicated that the transport of octane through the membrane was more sensitive to the temperature. 3.4.3. Permeate pressure A vapor pressure difference across the membrane is the crucial driving force for pervaporation process. Fig. 8 shows the effect of permeate pressure on the membrane performance for desulfurization. As the permeate pressure increased, the total flux decreased significantly because there was a reduction of driving force for transport of components. In contrast, the sulfur enrichment factor declined gently. Similar results were reported by other researchers [8,21]. The above results indicated that relatively high vacuum (low permeate pressure) was beneficial to improve both total flux and sulfur enrichment factor. 3.4.4. Feed flow rate The effect of feed flow rate on pervaporation performance for desulfurization is presented in Fig. 9. Total flux and sulfur enrichment factor both increased slightly with increasing the feed flow rate. The increase of feed flow rate was favorable to reduction of concentration polarization and thickness of liquid boundary layer. The mass transfer resistance of the boundary layer was lowered. Meanwhile, a reduction of concentration polarization meant that thiophene concentration near the membrane surface was close to that in the bulk, which could enhance sorption and swelling of thiophene in the membrane. Consequently, both total flux and sulfur enrichment factor rose slightly.

Many researchers have investigated desulfurization performance by pervaporation. Table 1 presents the pervaporation performance of different composite membranes reported recently. It can be found that the sulfur enrichment factor of PEG membrane is higher than that of PDMS membrane. According to solubility parameter theory, compared to the solubility parameter of PDMS, the solubility parameter of PEG is more proximate to that of thiophene, which then performed larger affinity to thiophene in PV separation process [9]. However, in fluid catalytic cracking (FCC) gasoline, the flux would increase and the sulfur enrichment factor would decrease due to more severe swelling of the polymeric membranes (see the date of ref. [10] in Table 1). In addition, some transition-metal ion exchanged zeolite (e.g. Ni2+Y zeolite, AgY zeolite) as an adsorptive filler was incorporated into polymeric membranes by related researchers, which led to a significant increase in flux (see the date of ref. [23] in Table 1). However, compared to the performance of those composite membranes using polymeric supports presented in Table 1, the ceramic-supported PDMS membrane exhibits a higher permeation flux and an acceptable sulfur enrichment factor. The main reason for this high flux is that the porous ceramic support possesses a high porosity and a low mass transfer resistance in PV process. A similar conclusion was drawn in our previous work [18]. In practical application, this high flux could greatly meet the requirements of commercial applications of membrane desulfurization technology. 4. Conclusions Crosslinked PDMS/ceramic composite membranes were utilized for removal of sulfur impurities out of model gasoline by pervaporation process. Experimental results demonstrated that the total flux increased with the increase of feed temperature and sulfur content, while the sulfur enrichment factor decreased. A relatively low permeate pressure was in favor of the total flux and sulfur enrichment factor. Meanwhile, both the total flux and sulfur enrichment factor increased slightly with increasing the feed flow rate. At 303.15 K, the total flux of the composite membrane reached to 5.37 kg m− 2 h− 1, with the corresponding sulfur enrichment factor of 4.22 for 400 μg g− 1 sulfur in feed under 210 Pa. Our study indicated that the crosslinked PDMS/ceramic composite membranes had a good sulfur removal efficiency and were potential candidates to be used for practical desulfurization application. Acknowledgments This work was supported by the National Basic Research Program of China (No. 2009CB623406), National Natural Science Foundation of China (No. 20990222), the “Six Kinds of Important Talents”

Table 1 Pervaporation performance for desulfurization of different composite membranes. Membranes

Feed

Thickness (μm)

CAa amount (%)

T (K)

Sb content (μg g− 1)

J (kg m− 2 h− 1)

E

Reference

PEG/PES PEG/PES PEG/PEI PDMS/PAN PDMS-Ni2+Y/PS PDMS/PEI PDMS/ceramic PDMS/ceramic

Model mixtures FCC gasoline Heptane/ethyl thioether n-octane/thiophene n-heptane/thiophene n-heptane/thiophene n-octane/thiophene n-octane/thiophene

10 16 6 11

18 17 19

358 373 353 303 303 323 303 303

1200 900 300 1387 500 500 400 997

0.4 3.37 1.8 1.5 3.26 0.7 5.37 6.34

5.13 3.63 6.7 4.9 4.84 4.8 4.22 3.84

[9] [10] [22] [11] [23] [24] This work This work

11 8 8

20 20

Model mixtures: thiophene, n-heptane (35 wt.%), cyclohexane (10 wt.%), 1-butylethylene (40 wt.%), toluene (15 wt.%). a CA: crosslinking agent. b S: sulfur.

R. Xu et al. / Desalination 258 (2010) 106–111

Program of Jiangsu Province (No. 2007007) and the Natural Science Foundation of Jiangsu Province (BK2009021) of China. References [1] C.S. Song, An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel, Catal. Today 86 (2003) 211–263. [2] M.A.B. Siddiqui, S. Ahmed, A.M. Aitani, C.F. Dean, Sulfur reduction in FCC gasoline using catalyst additives, Appl. Catal., A 303 (2006) 116–120. [3] J.A. Kocal and T.A. Brandcold, Removal of sulfur-containing compounds from liquid hydrocarbon streams, US Patent 6368495, 2002. [4] H.H. Yi, J.M. Hao, X.L. Tang, Atmospheric environmental protection in China: current status, developmental trend and research emphasis, Energ. Policy 35 (2007) 907–915. [5] R.Y. Zhao, C.L. Yin, H.J. Zhao, C.G. Liu, Synthesis, characterization, and application of hydotalcites in hydrodesulfurization of FCC gasoline, Fuel Process. Technol. 81 (2003) 201–209. [6] L.S. White, R.F. Wormsbecher and M. Lesemann, Membrane separation for sulfur reduction, US Patent 6896796, 2005. [7] X.J. Zhao, G. Krishnaiah, T. Cartwright, S-Brane technology brings flexibility to refiners' clean fuel solutions, NPRA Annual Meeting, San Antonio, TX, 2004. [8] L.G. Lin, G. Wang, H.M. Qu, J.R. Yang, Y.F. Wang, D.Q. Shi, Y. Kong, Pervaporation performance of crosslinked polyethylene glycol membranes for deep desulfurization of FCC gasoline, J. Membr. Sci. 280 (2006) 651–658. [9] L.G. Lin, Y. Kong, H.M. Qu, J.R. Yang, Y.F. Wang, D.Q. Shi, Selection and crosslinking modification of membrane material for FCC gasoline desulfurization, J. Membr. Sci. 285 (2006) 144–151. [10] Y. Kong, L.G. Lin, Y.Z. Zhang, F.W. Lu, K.K. Xie, R.K. Liu, L. Guo, S. Shao, J.R. Yang, D.Q. Shi, Studies on polyethylene glycol/polyethersulfone composite membranes for FCC gasoline desulphurization by pervaporation, Eur. Polym. J. 44 (2008) 3335–3343. [11] R.B. Qi, C.W. Zhao, J.D. Li, Y.J. Wang, S.L. Zhu, Removal of thiophenes from n-octane/ thiophene mixtures by pervaporation, J. Membr. Sci. 269 (2006) 94–100. [12] R.B. Qi, Y.J. Wang, J.D. Li, S.L. Zhu, Sulfur removal from gasoline by pervaporation: the effect of hydrocarbon species, Sep. Purif. Technol. 51 (2006) 258–264.

111

[13] R.B. Qi, Y.J. Wang, J.D. Li, C.W. Zhao, S.L. Zhu, Pervaporation separation of alkane/ thiophene mixtures with PDMS membrane, J. Membr. Sci. 280 (2006) 545–552. [14] Y.K. Hong, W.H. Hong, Influence of ceramic support on pervaporation characteristics of IPA/water mixtures using PDMS/ceramic composite membrane, J. Membr. Sci. 159 (1999) 29–39. [15] T.A. Peters, C.H.S. Poeth, N.E. Benes, H.C.W.M. Buijs, F.F. Vercauteren, J.T.F. Keurentjes, Ceramic-supported thin PVA pervaporation membranes combining high flux and high selectivity; contradicting the flux-selectivity paradigm, J. Membr. Sci. 276 (2006) 42–50. [16] J.D. Jou, W. Yoshida, Y. Cohen, A novel ceramic-supported polymer membrane for pervaporation of dilute volatile organic compounds, J. Membr. Sci. 162 (1999) 269–284. [17] Y. Zhu, R.G. Minet, T.T. Tsotsis, A continuous pervaporation membrane reactor for the study of esterification reactions using a composite polymeric/ceramic membrane, Chem. Eng. Sci. 51 (1996) 4103–4113. [18] F.J. Xiangli, Y.W. Chen, W.Q. Jin, N.P. Xu, Polydimethylsiloxane (PDMS)/Ceramic Composite Membrane with High Flux for Pervaporation of Ethanol–Water Mixtures, Ind. Eng. Chem. Res. 46 (2007) 2224–2230. [19] Y.W. Chen, F.J. Xiangli, W.Q. Jin, N.P. Xu, Organic–inorganic composite pervaporation membranes prepared by self-assembly of polyelectrolyte multilayers on macroporous ceramic supports, J. Membr. Sci. 302 (2007) 78–86. [20] M. Xiao, J.T. Zhou, Y. Tan, A.L. Zhang, Y.H. Xia, L. Ji, Treatment of highlyconcentrated phenol wastewater with an extractive membrane reactor using silicone rubber, Desalination 195 (2006) 281–293. [21] N. Rajagopalan, M. Cheryan, Pervaporation of grape juice aroma, J. Membr. Sci. 104 (1995) 243–250. [22] J. Chen, J.D. Li, J.X. Chen, Y.Z. Lin, X.G. Wang, Pervaporation separation of ethyl thioether/heptane mixtures by polyethylene glycol membranes, Sep. Purif. Technol. 66 (2009) 606–612. [23] B. Li, D. Xu, Z.Y. Jiang, X.F. Zhang, W.P. Liu, X. Dong, Pervaporation performance of PDMS-Ni2+Y zeolite hybrid membranes in the desulfurization of gasoline, J. Membr. Sci. 322 (2008) 293–301. [24] C.W. Zhao, J.D. Li, R.B. Qi, J. Chen, Z.K. Luan, Pervaporation separation of n-heptane/ sulfur species mixtures with polydimethylsiloxane membranes, Sep. Purif. Technol. 63 (2008) 220–225.