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Ind. Eng. Chem. Res. 2007, 46, 2224-2230
RESEARCH NOTES Polydimethylsiloxane (PDMS)/Ceramic Composite Membrane with High Flux for Pervaporation of Ethanol-Water Mixtures Fenjuan Xiangli, Yiwei Chen, Wanqin Jin,* and Nanping Xu Membrane Science and Technology Research Center, Nanjing UniVersity of Technology, Xinmofan Road 5, Nanjing 210009, People’s Republic of China
The separation of ethanol-water could be cost-competitive using pervaporation in the production of renewable biomass ethanol, but the performance of the modified or unmodified polymeric membranes still is not satisfactory. For the purpose of improving the pervaporation performance of polymeric membranes, especially for flux, polydimethylsiloxane (PDMS) was deposited uniformly on the surface of tubular nonsymmetric ZrO2/Al2O3 porous ceramic supports. The as-prepared composite membranes were characterized by scanning electron microscopy (SEM), Fourier transform infrared-attentuated total reflectance (FTIR-ATR), and pervaporation experiments. The thickness of the PDMS layer formed atop the ZrO2 layer was on the order of 5-10 µm. In the ethanol-water pervaporation experiment, as the ethanol concentration increased, the total flux increased but the selectivity decreased. At the same time, with increasing operating temperature, the total flux of the composite membranes increased whereas the selectivity decreased. It was observed that the PDMS/ceramic composite membrane showed a great total flux of 19.5 kg m-2 h-1 and selectivity of 5.7 for ethanol to water at a feed temperature of 343 K under a pressure of 460 Pa in an ethanol concentration of 4.3 wt %. The total fluxes of the PDMS/ceramic composite membranes were superior to other reported PDMS membranes. 1. Introduction Renewable biomass energy has potential as the alternative energy source to fossil fuels. A typical biomass fermentation process produces 3-8 wt % ethanol and requires further purification up to >99.5 wt % as pure ethanol or gasohol.1 Conventional ethanol fermentation systems usually make use of batch operation and the concentration of ethanol is very low, because fermentation is inhibited by the ethanol that is produced. This leads to high cost directly in the production of biomass ethanol. Pervaporation is a promising membrane-based technique for the separation of liquid chemical mixtures, especially in azeotropic or close-boiling solutions. The advantage of this process lies in the energy savings, economic and environmental protection, and so on. Membrane pervaporation that removes ethanol from the fermentation can overcome the disadvantage of conventional ethanol fermentation systems. The pervaporation-bioreactor coupled process has been attracting the interest of researchers in biotechnology.2-6 In the process, two types of pervaporation membranesswater-permselective and ethanolpermselectivesare used to obtain pure ethanol. Numerous investigations have been conducted on the water-permselective membranes and industrial applications have been realized. However, with regard to the ethanol-permselective membranes, much fewer investigations have been performed. O’Brien et al.,4 after analyzing the cost of a commercial-scale fuel ethanol plant, noted that, with only minor improvements in either the total flux (0.15 kg m-2 h-1) or selectivity for ethanol to water (10.3), * To whom correspondence should be addressed. Tel.: +86-2583587211. Fax: +86-25-83587211. E-mail address:
[email protected].
a process that applies pervaporation for ethanol recovery from fermentation broths could be cost-effective. To date, the materials that have been used for ethanol-permselective membranes were mainly silicon-containing polymers, including polydimethylsiloxane (PDMS),7-10 poly[1-(trimethylsilyl)-1-propyne] (PTMSP),11-14 and their derivatives.15-20 Although the ethanol-water separation factor for PTMSP has been reported to be higher than that of PDMS, unfortunately, PTMSP membranes have been proven to deliver unstable performances, because their flux and selectivity each are reduced with time.6,18 The current benchmark hydrophobic material for pervaporation membrane is still PDMS,6 which has been known as a representative alcohol-permselective membrane material for aqueous solutions with low alcohol concentrations. A composite membrane generally has a thin dense skin layer on a porous support, and, thus, its flux can be increased. Composite PDMS membranes have been extensively investigated,21-24 but the performance of the modified or unmodified PDMS membranes cannot still meet the commercial requirement. Compared with polymeric supports, ceramic supports exhibit a high surface porosity and superior chemical, mechanical and heat structural stability, e.g., no swelling and compaction, while transport resistance in the support can be negligible.25-28 In this work, we reported the synthesis of a cross-linked PDMS layer atop a tubular nonsymmetric ZrO2/Al2O3 ceramic support for the purpose of improving the ethanol-permselective membrane performance. The as-prepared composite membrane showed a high flux and an acceptable selectivity. The effects of feed concentration and the operating temperature on the
10.1021/ie0610290 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2225 Table 1. Conditions Used To Prepare the Composite Membranes Concentration (wt %) membrane
PDMSa
TEOSb
dibutyltin dilauratec
M1 M2 M3
9.5 7.0 7.0
30 10 30
0.5 0.5 0.5
a Calculated as w b PDMS/(wPDMS + wn-heptane) × 100. Calculated as (wTEOS/ wPDMS) × 100. c Calculated as (wdibutyltin/wPDMS) × 100.
pervaporation performance of the PDMS/ceramic composite membranes were investigated. 2. Experimental Section 2.1. Materials. R,ω-Dihydroxypolydimethylsiloxane (PDMS), with an average molecular weight of 5000, was purchased from Shanghai Synthetic Resin Company, China. Tetraethylorthosilicate (TEOS), n-heptane, dibutyltin dilaurate, and ethanol were obtained as analytical reagents from Shanghai Chemical Reagent Company, China. The ceramic supports that were used were tubular nonsymmetric ZrO2/Al2O3 membranes, with an average pore size of 0.2 µm, which were manufactured by Membrane Science & Technology Research Center at the Nanjing University of Technology. They were 75 mm in length with the external diameter of 12 mm and the inner diameter of 7.5 mm. 2.2. Preparation of Composite Membranes. The tubular ZrO2/Al2O3 supports were polished with fine sandpaper (A35PM, Riken Corundum Co., Ltd., Japan) slightly, to make the PDMS polymer adhere to the support well and form the thin and defectfree PDMS layer. The ceramic supports were washed for 5 min by ultrasonic, then were cleaned and soaked for 24h using pure water. The pores of the supports were filled with water to prevent the penetration of a coating polymer solution into the pores and excess water on the support surface was wiped off with filter paper before dip-coating the polymer solution. PDMS polymer was dissolved in n-heptane, according to various PDMS concentrations of 9.5, 7.0, and 7.0 wt %, then the crosslinking agent (TEOS) and catalyst (dibutyltin dilaurate) were added into the polymer solution. The ratios of PDMS/TEOS/dibutyltin dilaurate were fixed at 100/30/0.5, 100/10/0.5, and 100/30/0.5, respectively. The three polymer solutions were stirred at room temperature for 10 min and became coating precursors by degassing. The cross-linked PDMS layers were formed on the outer surface of the ceramic supports for 1 min by a dip-coating method. The conditions of preparing the composite membranes are summarized in Table 1. After the coated composite membranes were cured at room temperature for 24 h, they were dried to remove the residual solvent in an oven at 120 °C for 12 h. The composite membranes derived from different PDMS/ TEOS/dibutyltin dilaurate ratios were denoted M1, M2, and M3, as shown in Table 1. 2.3. Characterization. The surface and cross-sectional morphologies of the PDMS/ceramic membranes and the thicknesses of PDMS layers were characterized by scanning electron microscopy (SEM) (QUANTA-2000). Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) (NEXUS870) was applied to detect the presence of cross-linking PDMS compounds on the surfaces of the PDMS/ceramic composite membranes. 2.4. Pervaporation Experiment. A pervaporation experiment was conducted using a homemade apparatus, as shown in Figure 1. It was composed of three parts, which were upstream at atmospheric pressure, downstream at a vacuum pressure of ∼460 Pa in all experiments, and the pervaporation cell. The feed tank
Figure 1. Schematic diagram of the apparatus used for pervaporation. Legend: 1,2smembranes; 3, 4sfeed tanks; 5,6svacuum gauges; 7,8s condenser pipes; 9,10,11,12scold traps; 13sbuffer bottle; 14svacuum pump; 15sthermocouple; and 16sconstant temperature tank.
was maintained at constant temperature by controlling the water bath. The downstream component consisted of a vacuum pump, a vacuum regulator to control the permeation side pressure, and two-stage cold traps. The first stage was kept at 3 °C with a cryostat, and the second cold traps to collect permeation was frozen by liquid nitrogen. The ethanol concentrations in the feed and the permeation sides were measured using gas chromatography (GC-8A, SIGMA, Japan) equipment that was coupled with a thermal conductivity detector, using a Φ3 m × 2 m stainless steel packed column and helium as the carrier gas. The injector temperature was maintained at 150 °C, and the detector temperature was 140 °C. The permeation flux (J) at steady state is calculated using the following equation:
J)
W At
(1)
where W is the weight of the collected permeation, A the effective area of the membrane, and t the permeation time for the pervaporation. The permselectivity of the composite membranes is expressed by a separation factor R, which is defined as
) RHEtOH 2O
YEtOH/YH2O XEtOH/XH2O
(2)
where YEtOH, YH2O, XEtOH, and XH2O are the weight fractions of ethanol and water in the permeation and feed sides. 3. Results and Discussion 3.1. Characterization of Composite Membranes. Figure 2 shows SEM images of tubular-type PDMS/ZrO2/Al2O3 composite membranes. The active PDMS layer was coated uniformly on the porous surface of the ceramic supports. Figures 2a, 2c, and 2e are the surfaces of the three PDMS/ceramic composite membranes (M1, M2, and M3). It can be obsevred that the surfaces of the three membranes had great differences in regard to structure and morphology. Among the three membranes, M1 was the smoothest and densest. Figures 2b, 2d, and 2f show the different morphologies of the cross sections of M1, M2, and M3. The thicknesses of the top PDMS layers were all M3 > M1 and the order was vice versa for the selectivities among the three membranes. The total flux of M2 was from 12 kg m-2 h-1 to 20 kg m-2 h-1 in the range of 3-10 wt % ethanol at 333 K, which was very high, compared to the reported literature, and the separation factors was from 7.6 to 4.4. The performance difference between the three membranes was the result of the different overall transport resistance. The polymer concentration can determine the thickness of dense layer, which is the major contribution to the overall transport resistance. In our work, due to the different polymer concentration and crosslinking degree, the top PDMS layer of the composite membrane M1 was the thickest and densest, while the top layer of M2 was thinnest and loosest among the three membranes. It could be thought
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Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007
Figure 5. Effect of temperature on the pervaporation performance of the PDMS/ceramic composite membranes: (a) total flux and (b) selectivity. Ethanol concentration ) 4.3 wt %; permeation side pressure ) 460 Pa.
that the transport resistances in the top PDMS layer may be of the order of
M2 < M3 < M1 3.2.2. Effect of Feed Temperature. Generally, the permeation flux increases as the operating temperature increases, but the selectivity usually decreases. The relationship between the temperature and the permeation flux usually follows the Arrhenius equation.32 The effect of the temperature in the range of 313-343 K was shown in Figure 5 for composite membranes M1-M3. An increase in the feed temperature from 313 K to 343 K resulted in an approximately four-fold increase in the total fluxes for the three membranes, but it had little effect on the separation factor. The variation of the total flux with temperature was determined to follow an Arrhenius relationship:
J ) J0 exp
( ) -Ea RT
(3)
where J is the total flux, J0 the exponential factor, Ea the apparent activation energy of permeation, R the gas constant, and T the feed temperature. Ea, which is determined from the slopes of the ln J vs 1/T plots, is shown in Figure 5a. For membranes M1-M3, the Ea values are 40.94, 39.19, and 41.98 kJ/mol, respectively, which are almost equal, because of their similar slopes. Transport through the membrane is produced by maintaining a lower pressure on the permeate side of the membrane than that on the feed side. When the temperature is increased, the difference in vapor pressure across the membrane for the component transport is increased. According to the transport theory by the solution-diffusion model,33 the increased difference in vapor pressure at higher temperatures is responsible for the higher permeation flux. The membrane-solute-solvent coupling interaction had a great effect on the transport process
Figure 6. Effect of operating time on the pervaporation performance of the PDMS/ceramic composite membranes: (a) total flux and (b) selectivity. Ethanol concentration ) 4.3 wt %; temperature ) 333 K; permeation side pressure ) 460 Pa.
of sorption and diffusion. The effect of transport is related to not only the polymer properties, such as free volume, chain mobility, degree of cross-linking, and so on, but also the properties of the permeating components, such as the polarity of the permeating components, molecular size, molecular shape, temperature and concentration of the permeating component, and so on. When the temperature was increased, the rubbery PDMS swelled and the polymer segments had more free volume and chain mobility. The ethanol molecules and the water molecules then diffused quickly through the free volume of the PDMS membrane. As a result, at higher temperatures, the diffusion rate of the two permeating molecules increased, leading to high permeation fluxes. However, the diffusion rate of the water was larger than that of the ethanol and the effect of the diffusion was more than that of the sorption through the rubbery PDMS membrane. The effect caused the selectivity to decrease. 3.2.3. Effect of Operating Time. Generally, some polymer membranes have the disadvantage of chemical, mechanical, and heat structural instability. The stability of composite membranes with a thin dense skin layer on a porous support generally can be improved. In our work, the effect of operating time on the flux and the selectivity of the composite membranes was also studied. As shown in Figure 6, the total fluxes and selectivities changed slightly with time over a period of 10 h. The total fluxes reached steady state in a short time. The fluxes were ∼5.6, 12.2, and 7.9 kg m-2 h-1 and the selectivities were 8.8, 6.0, and 6.8 for membranes M1, M2, and M3, respectively. Because of the existence of the ceramic supports, the three-dimensional degree of swelling of the rubbery PDMS polymer was reduced. Thus, the ceramic support would be beneficial to the good performance of the composite membrane. 3.3. Comparison of Pervaporation Performance with Literature. Many research groups have become interested in separating ethanol from water using pervaporation. Table 2 gives
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2229 Table 2. Pervaporation Performance of Different PDMS Membranes membranea PDMS-PI PDMS-PS/PESF PDMS PDMS-PPP PDMS-PS/IPN silicalite-PDMS PDMS/PVDF PDMS/CA M2 M2 M2 M3 M3
thickness (µm) 20 20 30 15 20 104 2
