Composite PDMS membrane with high flux for the separation of organics from water by pervaporation

Journal of Membrane Science 243 (2004) 177–187

Composite PDMS membrane with high flux for the separation of organics from water by pervaporation Lei Lia,∗ , Zeyi Xiaob , Shujuan Tana , Liang Pua , Zhibing Zhanga a

Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Nanjing University, 210093 Nanjing, China b Department of Chemical Engineering Machinery, Sichuan University, Chengdu 610065, China Received 20 February 2004; accepted 12 June 2004 Available online 18 August 2004

Abstract The composite polydimethylsiloxane (PDMS) membranes supported by Cellulose acetate (CA) were prepared by pre-wetting method for the separation of methanol, ethanol, n-propanol and acetone from water. The experiments were carried out to investigate the effects of operating parameters on the pervaporation performance and the results showed that the membranes exhibited high total permeation flux. A resistance-in-series model was applied to analyze the transport of the permeants. It was found that boundary-layer mass transfer coefficient was proportional to Re0.5 and increased exponentially with temperature. The membrane mass transfer coefficient conformed to Arrhenius correlation and was independent of flow status. The estimated diffusivities of alcohols in membrane had a magnitude order of 10−10 m2 ·s−1 under a wider range of temperature, which was identical with those reported in the literature. © 2004 Elsevier B.V. All rights reserved. Keywords: Composite membrane; Pervaporation; Transport; PDMS; Pre-wetting

1. Introduction Among the major applications of pervaporation membrane processes, organic separation from organic/water mixtures is becoming more and more important. Especially there has been growing research interest in the application of pervaporation to biotechnology in areas such as recovery of ethanol from fermentation broths [1–4]. However, a major hurdle limits pervaporation commercialization, namely, the lack of proper membrane materials for this application. O’Brien et al. [2], after analyzing the fermentation–pervaporation processes of a commercial-scale fuel ethanol plant, concluded that such a coupling system could be cost-competitive if the performance of membranes was improved modestly so as to exhibit either the pervaporation total flux of 0.15 kg·m−2 ·h−1 or selectivity of 10.3 for ethanol to water. ∗ Corresponding author. Tel.: +86 25 8359 3772/6665; fax: +86 25 3317761. E-mail address: [email protected] (L. Li).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.06.015

For organic compound separation from water, many hydrophobic materials used for volatile organic compounds (VOCs) removal have been studied. The polydimethylsiloxane (PDMS) is among the most interesting and promising membranes and has been extensively investigated [5,6]. The preparation and pervaporation performance of composite PDMS membranes, consisting of a thin PDMS toplayer adhered on a porous support, has mainly been described in a variety of technical papers. The support materials mainly used were polyethersulfones [7–9], polyetherimides [10–12], polyimides [13], polyacrylonitriles [14,15], polyesters [16], and ceramics [17]. In addition, Several techniques have already been developed to prevent dilute PDMS solution from intruding pores in the support which may in a indirect way lead to high mass transfer resistance. The support pores could be filled with a nonsolvent [18,19] for the coating polymer, or with a solvent [16], before applying the coating solution. However, in the case of alcohols or ketone separation from water, it was recognized that composite PDMS membrane did not exhibit sufficient permeation

178

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187

flux due to the following reasons: (1) alcohols and ketone compounds, compared with aroma compounds or VOCs, are generally not very hydrophobic; (2) the proper choice of support materials which are perfect matches for the PDMS toplayer, has been not yet studied thoroughly and practically. In this study, by a pre-wetting method the PDMS was cast on the cellulose acetate (CA) microfiltration membranes, a type of highly hydrophilic material, to prepare the composite membranes. The pervaporation experiments were carried out to investigate the separation of methanol, ethanol, n-propanol and acetone from water. In addition, the mass transport of the alcohol and ketone compounds through the PDMS membrane was investigated and correlated using a resistance-in-series model.

Fig. 1. Scheme of resistance-in-series model.

where ko is the overall mass transfer coefficient, defined as: 2. Theory Based on the solution-diffusion model, the flux of the component i through membrane (from membrane surface to permeate side) can be expressed as: Ji =

Dm Ci,m (1 − pi,d /pei ) L

(1)

where Dm is the diffusion coefficient in the membrane, L the membrane thickness, Ci,m the permeate concentration in membrane at the liquid–membrane interface, pi,d the partial downstream pressure of the component i, and pei the equilibrium vapor pressure of the component i at aqueous–membrane interface. Meanwhile the general equation of mass transfer in liquid boundary layer can be generally given by: Ji = kl (Ci,b − Ci,w )

(2)

where kl is the boundary layer mass transfer coefficient, Ci,b the concentration of component i in liquid bulk and Ci,w the concentration in liquid at the liquid–membrane interface. It is necessary to note that Eq. (1) should be regarded as an approximation of the mass transfer across PDMS membrane by pervaporation, provided that the diffusion of permeants is assumed to be independent of their concentrations in membrane. This assumption is reasonable since the concentration of alcohols or ketone aqueous solution in this study is so low that the swelling of PDMS is regarded nearly negligible. Furthermore, Ci,m in Eq. (1) and Ci,w in Eq. (2) can conveniently be correlated with the partition coefficient of the permeant in both liquid and membrane: K = Ci,m /Ci,w [20]. In accordance with the resistance-in-series model as shown in Fig. 1, combining Eqs. (1) and (2) together generates the following expression for the average permeation flux: Ji =

Ci,b − Ci,w pi,d /pei = ko (Ci,b − Ci,w pi,d /pei ) 1/kl + L/Dm K

(3)

1 1 1 L 1 = + = + ko kl km kl Dm K

(4)

where km denotes the membrane mass transfer coefficient through the membrane. Eq. (4), known as the resistance-inseries model, has been widely used in the study of boundary layer mass transfer resistance effect in pervaporation of dilute organic–water binary mixtures [21–23]. For a sufficient low downstream pressure, the second term on the right hand of Eq. (3) becomes negligible. Then Eq. (3) reduces to: Ji = ko Ci,b

(5)

Eq. (5) can be used to derive the overall mass transfer coefficient from the experimentally measured flux Ji and feed concentration Ci,b . From Eq. (4), the boundary-layer mass transfer resistance (the intercept of the plot) and the permeability of the permeant (=Dm K, the reciprocal slope) in addition to the membrane resistance can be obtained form a plot of the permeation flux data versus the membrane thickness.

3. Experimental 3.1. Materials ␣,␻-Dihydroxypolydimethylsiloxane (PDMS) with an average molecular weight of 5000 was purchased from Shanghai synthetic resin Company, China. Tetraethylorthosilicate (TAOS), dibutyltin dilaurate, n-heptane, methanol, ethanol, n-propanol and acetone were obtained as analytical reagents from Shanghai Chemical Reagent Company, China. Distilled and deionized water was used. Cellulose acetate (CA) microfiltration membranes, with an average pore size of 0.5 ␮m, were from Shanghai Filter Company, China and used as supports.

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187

179

3.2. Preparation of composite PDMS membranes PDMS, crosslinking agent TAOS, and catalyst dibutyltin dilaurate were mixed according to a 10/1/0.2 weight ratio in n-heptane. Prior to coating, the CA support was laid and spread out on the surface of water in a basin. Excess water on the CA support surface was wiped off quickly with a filter paper. Directly afterwards, the PDMS solution was poured over the surface of support and the basin was put under a hood. The membrane system containing some crosslinked PDMS, after kept under ambient temperature for 2 h, was introduced into an vacuum oven at 60 ◦ C for 4 h to complete the crosslinking. With this technique, mass transfer resistance due to the intrusion of the PDMS solution into the porous substrate during fabrication of the composite membrane could be reduced. The composite membranes with skin layers of variable thickness could be achieved by controlling the concentration of PDMS solution or the coating amount. In this way, two composite membranes with skin layer thickness of 8 ␮m (1# membrane), and 16 ␮m (2# membrane) respectively were prepared for this study. 3.3. Scanning electron microscopy (SEM) Scanning electron microscopy was used to study the crosssectional morphology of the composite PDMS membrane and to measure the skin layer thicknesses. The composite PDMS membranes were fractured in liquid nitrogen. The fractured section was coated with a conductive layer of sputtered gold. The cross-section of the composite membrane was investigated using a Hitachi S-800 SEM. 3.4. Pervaporation Binary aqueous solutions containing methanol (5 wt.%), ethanol (5 wt.%), n-propanol (2 wt.%) and acetone (2 wt.%) were prepared as feed solutions for the experiments, respectively. Pervaporation experiments were carried out using a continuous set-up reported by Li et al. [24]. The composite membranes were fixed into a plate and frame pervaporation test cell, as shown in Fig. 2. The system provided an effective mass transfer area of 0.024 m2 . The distance between the parallel plates was 0.01 m. The liquid entered the feed compartment at the center of the cell and exited at a circular channel on the perimeter of the cell. The liquid flow rate was measured by means of rotameters. After a steady state was obtained, the permeating vapor was collected by two stages of cold traps. The first cold trap was provided with a refrigerant stream of −5 to −10 ◦ C, while the freezing medium in the second trap was liquid nitrogen. The downstream vacuity of membrane was kept at 266 Pa in all experiments. To keep the feed concentration almost constant, the collected permeant was added into the reservoir with 5 L feed solution in.

Fig. 2. Cross-section of pervaporation cell.

The experimental conditions were chosen in the following two series. One series of experiments was performed under constant feed temperature of 313 K and variable feed flow rate in the range of 5 to 120 L·h−1 , to investigate the effect of the liquid flow over the surface of membrane on the mass transfer. This enabled the determination of correlations of ko , kl and km to liquid flow status on the membrane. The other series of experiments was conducted under variable feed temperatures in the range of 298–323 K and fixed feed flow rate of 120 L·h−1 , to evaluate the effect of the temperature on the mass transfer with constant hydrodynamic conditions. This established the relationship between ko , kl , km and the feed temperature. Furthermore, the effect of the physicochemical properties of different compounds on mass transfer coefficients could also be elucidated by experimental measurements with the same methods for various diffusing compounds under constant feed temperature and Reynolds number. The permeation flux (J) at steady state was determined from the weight (W) of the collected permeant by using the following equation: J=

W At

(6)

where t is the experimental time interval for the pervaporation, and A the effective membrane surface area. A densimeter (DMA500, Anton Paar, Austria) was used to measure the liquid densities (the accuracy of 0.000001 g·ml−1 ) and determine the organic concentrations (the accuracy of 0.001 wt.%) by means of a standard curve of density versus concentration attached to the densimeter. The permselectivity of the membrane was calculated via the separation factor (α) defined as α=

Yo /Yw Xo /Xw

(7)

where X and Y are the weight fractions of species in the feed and permeate, respectively. Subscript “o” denotes organic compound and “w” water.

180

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187

4. Results 4.1. SEM micrograph of composite PDMS-CA membrane The morphology of the composite PDMS-CA membrane used in this study is presented in Fig. 3. It is evident from the picture that the PDMS top layer is tightly and properly cast on the top of the CA substrate. It can also be found that less intrusion of PDMS into the micropores of the substrate occurs as a result of the fabrication method used in this study.

Fig. 5. Plot of total flux as a function of feed flow rate (T = 313 K); data for the following membranes: (solid line)–1# , (dashed line)—2# ; (♦) acetone.

4.2. Pervaporation of the organic–water mixtures 4.2.1. Effect of feed flow rate The effect of feed flow rate on the total flux and selectivity for methanol (ethanol, n-propanol, acetone)–water mixtures are presented in Figs. 4–7, respectively. With increasing flow rate, both total flux and selectivity increase. In addition, the effect of flow rate on the pervaporation performance of acetone–water mixture is more evident than three alcohol–water systems. Fig. 6. Plot of selectivity as a function of feed flow rate (T = 313 K); data for the following membranes: (solid line)—1# , (dashed line)—2# ; () methanol; (♦) ethanol.

4.2.2. Effect of temperature The effect of feed temperature on the total flux and selectivity for methanol (ethanol, n-propanol, acetone)–water mixtures are revealed in Figs. 8–10, respectively. The total flux of four systems all increase monotonically with increasing temperature because the mobility of permeating molecules are enhanced both by the temperature and by the higher mobility of the polymer segments. However, the effects of temperature on the pervaporation performance of the four mixtures are distinctly various for different systems. The selectivity of ethanol to water increases with increasing temperature. In contrast, those of acetone and methanol to water descend. Besides, that of n-propanol to water remains nearly invariable. Our results are evidently different from those in the literature. Fig. 3. Cross-section of composite PDMS membrane by SEM.

Fig. 4. Plot of total flux as a function of feed flow rate (T = 313 K); data for the following membranes: (solid line)—1# , (dashed line)—2# ; () methanol; (
) n-propanol; (♦) ethanol.

Fig. 7. Plot of selectivity as a function of feed flow rate (T = 313 K); data for the following membranes: (solid line)—1# , (dashed line)—2# ; () npropanol; (♦) acetone.

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187

Fig. 8. Plot of total flux as a function of temperature (F = 120 L·h−1 ); data for the following membranes: (solid line)—1# , (dashed line)—2# ; () methanol; (
) acetone; (+) n-propanol; (♦) ethanol.

Fig. 9. Plot of selectivity as a function of temperature (F = 120 L·h−1 ); data for the following membranes: (solid line)—1# , (dashed line)—2# ; () n-propanol; (♦) acetone.

Molina et al. [25] found that separation index of methanol, ethanol, iso-propanol increased by increasing temperature on CMG-OM-010 and 1060-SULZER PDMS membranes. Vankelecom et al. [26] investigated from their experiments that selectivity of ethanol tended to decrease with increasing temperature using PDMS membranes. The rational explanation of those phenomena is as follows: diffusion and solubility of penetrating components changes significantly with temperature, which depends on many factors such as different organics, different membrane-preparation method, and different supports of composite membranes and so on. The further study is being carried on.

181

4.2.3. Comparison of pervaporation performance For comparison purpose, the pervaporation performances of different PDMS membrane reported by other research groups for separating ethanol–water mixture are listed in Table 1. It can be seen that the total flux of the composite PDMS-CA membranes is remarkably higher than those of most PDMS supported and unsupported membranes reported in the literature while the composite PDMS-CA membranes in this study keep reasonably and acceptably selective for ethanol. In the case of the separation of ethanol from 5 wt.% ethanol solution at feed temperature of 313 K under 266 Pa, the composite PDMS-CA membrane with skin layer thickness of 8 ␮m in this study has a total flux of 1300 g·m−2 ·h−1 and a selectivity of 8.5 for ethanol to water, which has in practice reached and exceeded the requirement of the membranes for the use in cost-competitive fermentation–pervaporation processes, assumed that thermodynamic effects of other components in fermentation broths on activity coefficients and coupling effects will not severely affect pervaporation performance. Thus, it is evident that the composite PDMS-CA membrane prepared in this study has excellent practical industrial application future. Based on the comparison, it is believed that the excellent pervaporation performance of the composite PDMS-CA membrane depends on both the material type of the substrate and the preparation method. Brief qualitative explanation is given as follows. On one hand, CA is a type of highly hydrophilic material. By the pre-wetting method, the pores of the CA substrate prior to coating are full of water due to the capillary effect. This can mitigate mass transfer resistance caused by the intrusion of the PDMS solution into the porous substrate during the fabrication of the composite membrane. Meanwhile thin and defect-free dense PDMS skin layer can be assured to coat on CA support. On the other hand, hydrogen-bonding force exists between the O–H in the CA and oxygen atom in the PDMS, which causes the toplayer and substrate to adhere tightly. These effects both lead to a perfect match between CA support materials and the PDMS toplayer, which consequentially gives rise to high total flux and acceptable selectivity of PDMS composite membranes. 4.3. Analysis of transport 4.3.1. Definition of Reynolds number In the present work, the liquid on the surface of the membrane underwent a circular dipole flow pattern, as shown in Fig. 11. Reynolds number is redefined for this special pattern as follows: Re =

Fig. 10. Plot of selectivity as a function of temperature (F = 120 L·h−1 ); data for the following membranes: (solid line)—1# , (dashed line)—2# ; () methanol; (♦) ethanol.

Fρ πbµ

(8)

where F denotes volumetric flow rate of the feed, ρ the density, b the thickness of the cell and µ the dynamic viscosity. This definition of Re is similar to that in the work of Bandini et al. [37].

182

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187

Table 1 Prevaporation performance of different PDMS supported and unsupported membranes Membrane

Membrane thickness (␮m)

Ehtanol concentration in the feed (wt.%)

Temperature (◦ C)

J (g·m−2 ·h−1 )

α

Ref.

PDMS-PS graft copolymer supported on a PES PDMS-PS block copolymer PDMS-PPP graft copolymer PDMS PDMS PDMS-PI graft copolymer PDMS-PS IPN supported membrane PDMS composite membrane PDMS coated on silicate membrane PDMS/PEI/PPP composite membrane 1060 Sulzer PDMS commercial membrane PDMS supported on a CA

20 39 30 40 – 20 15 2 30 40 – 8

10 10 7.0 11.9 8 6.6 10 5 5 5 13 5

60 25 30 25 30 48 60 30 30 40 40 40

130 27 19 14 25 32 160 1130 110 270 800 1300

6.2 6.2 40 7.1 10.8 6.6 5.5 4.2 37 3.7 2.1 8.5

[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [25] This study

coefficients are distinctly different for four organics. The effect is much significant for acetone while it is negligible for methanol. As for alcohol homologues (methanol, ethanol and n-propanol), the effect of Re on ko is becoming more significant with increasing number of carbon atoms. This seems owing to the increase of hydrophobicity of the membrane and molar volume of alcohols with increasing carbon atoms. The curve of ko versus feed temperature under fixed flowrate is illustrated in Fig. 13. It can be seen that ko is linearly dependent on feed temperature. Fig. 11. Dipole flow on the surface of membrane.

4.3.2. Overall mass transfer coefficient A plot of ko , obtained from the Eq. (5), against Re at constant feed temperature is shown in Fig. 12. A primarily fitting to experimental data shows that the increasing Reynolds numbers produced higher permeating rates of organics. It implies that the mass transfer is influenced by organic diffusion through the liquid boundary layer adjacent to the membrane. These results are consistent with those from pervious studies of Urtiaga et al. [38], Psaume et al. [39], and Lipski and Cˆote [21] using PDMS membranes for the removal of VOC from aqueous solution. Furthermore, Fig. 12 also demonstrates that the effects of liquid flow in the module on the overall mass transfer

Fig. 12. Plot of overall mass transfer coefficient as a function of Re (T = 313 K); data for the following membranes: (solid line)—1# , (dashed line)—2# ; () methanol; (
) ethanol; (+) n-propanol; (♦) acetone.

4.3.3. Liquid boundary and membrane mass transfer There have existed several semi-empirical mass transfer correlations [40,41] to estimate the boundary layer mass transfer coefficient as a function of the Reynolds number in pervaporation. For laminar flow along a flat-channel cell, a general correlation can be expressed directly in terms of the boundary layer mass transfer coefficient as
 b c D kl = aRe Sc (8) = a × 10−6 Reb dh where a, b, c and a are constants particular to each module and to the flow regime, D is diffusivity related to properties of the fluid and dh is the hydraulic diameter of the membrane cell. Various values for constants a, b and c can be found in the literature. For fixed module geometry and dilute solution,

Fig. 13. Overall mass transfer coefficient versus feed temperature (F = 120 L·h−1 ); data for the following membranes: (solid line)—1# , (dashed line)—2# ; (+) methanol; (
) ethanol; (♦) n-propanol; () acetone.

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187

Fig. 14. Liquid film and membrane mass transfer coefficient as a function of Re for membranes of different skin layer thickness (T = 313 K; data for acetone) (♦) kl ; (
) km (1# ); () km (2# ).

Fig. 15. Liquid film and membrane mass transfer coefficient as a function of Re for membranes of different skin layer thickness (T = 313 K; data for methanol) (♦) kl ; (
) km (1# ); () km (2# ).

the exponent b should be independent of type of the particular VOC. Figs. 14 and 15 show the curves for the boundary layer and membrane mass transfer coefficients against Re (data for methanol and acetone). The boundary layer mass transfer data are well-fitted by the power law relationship as shown in both figures. Values for exponent b of 0.5161 and 0.5142 are obtained for methanol and acetone, respectively. To test the validity of the method further, the experiments were repeated with two other organics, i.e., ethanol and npropanol. The results with all four organics are consistent as shown in Table 2, yielding an average b value of 0.50. The b value obtained in this work is identical with those in works of Urtiaga et al. [42], Jiang et al. [43] and Smart et al. [44]. In all cases the observed behaviors clearly indicate that there is an appreciable concentration polarization confined in a thin layer close to the membrane. Consequently the boundary layer effect at a given membrane thickness becomes more significant at lower Reynolds numbers. This is in agreement with the result from Urtiaga et al. [45].

183

For the involved organic compounds, the relative contribution of the boundary layer and membrane mass transfer coefficient to the overall transfer coefficient is obviously different for various organics. For example, in the range of Reynolds number investigated, methanol transfer is limited by the membrane mass transfer resistance whereas acetone transfer under the same hydrodynamic conditions depends on both the mass transfer resistance. The effect of membrane thickness is particularly important for methanol. Our experimental results are similar with those of Brookes and Livingston [20]. They argued that the percentage of each mass transfer resistance in overall resistance is highly dependent on partition coefficient K, one of the important thermodynamic property of organic compound in a binary phase system of liquid and membrane. It is demonstrated clearly that the analytical method based on the resistance-in-series model is easy of access to determining the dominant resistance for various organic systems and any given set of conditions. The method also provides an optimizing tool for the practical pervaporation process. The data for the membrane mass transfer coefficient represent two horizontal straight lines in Figs. 14 and 15, which show the independence of the diffusion in membrane of flow status on membrane surface. It is in accordance with the definition of km in Eq. (4). Estimation of the boundary layer thickness for all organics in this work can be obtained from the definition: kl = Di,b /Lb , where Di,b denotes the diffusion coefficient of organic compound in the boundary layer and Lb is the thickness of the boundary layer. The values Di,b can be obtained by Wilke–Chang correlation [46]. A plot of Lb against Re is shown in Fig. 16. It can be seen that Lb gradually decreases to a stable state with a rise of Re. The liquid film mass transfer on membrane surface is substantially dependent on the diffusion of ethanol in water. According to Wilke–Chang correlation, the viscosity of a liquid is an exponentially decreasing function of temperature, causing the diffusivity to increase exponentially with temperature. Fig. 17 gives the dependence of the boundary layer mass transfer coefficient on temperature. As can be seen, the boundary layer mass transfer coefficient does increase exponentially with temperature at constant flow status. This observation explains well the dependence of kl on temperature.

Table 2 Values for constants in Eq. (8) by data fitting Compound

a

b

Methanol Ethanol n-Propanol Acetone

0.3171 0.3294 0.2685 0.4932

0.5161 0.4903 0.4765 0.5142

Fig. 16. Liquid boundary layer thickness versus Re (T = 313 K) (♦) methanol; (
) ethanol; (×) n-propanol; () acetone.

184

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187 Table 4 Values for constants in Eq. (10) by data fitting

Fig. 17. kl vs. feed temperature (F = 120 L·h−1 ) (
) methanol; (+) ethanol; () n-propanol; (♦) acetone. Table 3 Values for constants in Eq. (9) by data fitting Compound

c

d

R2

Methanol Ethanol n-Propanol Acetone

1E−08 0.0016 1E−08 0.0005

0.0669 0.0267 0.0647 0.0336

0.9141 0.9576 0.9857 0.9844

A fitting with much high correlation coefficient is carried out for formulating the relationship of kl and T: kl = 10−6 c exp(dT )

(9)

Under the present experiment, values of c and d in Eq. (9) are obtained from fitting data for four organics and shown in Table 3. According to Eq. (4), the membrane mass transfer coefficient km is essentially dependent on the diffusivity Dm in membrane and the partition coefficient K in liquid–membrane bi-phase system. The conventionally accepted viewpoint is that both the diffusivity and the partition coefficient conform to Arrhenius-type correlations, such as Scatchard– Hildebrand activity-solubility equation for partition coefficient and the equation used by Watson et al. [47,48] for the diffusivity. Hence product of Dm and K should certainly be an Arrhenius equation as well. Fig. 18 reveals the Arrheniustype curves of the splitter membrane transfer coefficient. The fitting generates a quite typical Arrhenius equation:
 f −6 km = 10 e exp − (10) T

Compound

e

f

R2

Methanol Ethanol n-Propanol Acetone

49297 1E+10 1E+7 2E+7

3166.4 6948.1 4550.3 4409.5

0.9124 0.9529 0.9878 0.9783

Values of e and f in Eq. (10) for four organics investigated are given in Table 4, by fitting to the data published in our previous work [49]. It is necessary to point out that the values for constants shown in Table 4 are only the results from data fitting that can not be assumed to have any concrete physical meaning. However, it is advantageous for us to find the correlation above from the experimental results quickly. The boundary layer and membrane mass transfer coefficients take on such temperature-dependent behaviors that the analytical method based on the resistance-in-series model seems to be thought of a reasonable and promising technique to conveniently determine both two coefficients. A comprehensive experimental project is being conducted so as to further validate this technique. 4.3.4. Diffusion coefficient in silicone rubber There have been several studies involving the measurement of the diffusion coefficient of ethanol in PDMS membranes. Watson and Payne [47] measured the diffusion coefficient of ethanol in PDMS membrane to be 1.5 × 10−10 m2 ·s−1 at 1 wt.% ethanol concentration and feed temperature of 80 ◦ C, while the result obtain by LaPack et al. [50] in PDMS membrane filled with fumed silica was 0.4 × 10−10 m2 ·s−1 at concentration lower than 0.1% and feed temperature of 25 ◦ C. Doig et al. [51] gave diffusivity for ethanol in PDMS membrane of 6 × 10−10 m2 ·s−1 at feed temperature of 25 ◦ C. In terms of Eq. (4), the parameter Dm K can be calculated by product of km and δ, which characterizes the solubility–diffusion of solutes in membrane. Thus, by the estimation of K, the diffusion coefficient of ethanol is derived from the experiments to compare with the valves in the literature. Silicone rubber is a type of polymer in the rubbery state and so this enables the equilibrium partitioning of ethanol between the liquid and membrane phases to be modeled using Hildebrand solubility parameter theory. Brooks and Livingston [20] gave the following equation for low organic compound activity (less than 0.4): ln K =

Fig. 18. Arrhenius curves of the membrane mass transfer coefficient vs. 1/T; (
) methanol; (×) n-propanol; () ethanol; (♦) acetone.

vi vi ((δi − δw )2 − (δi − δm )2 ) − RT vw

(11)

where δm , δi and δw denote solubility parameter for membrane, organic compound and water, respectively; ␯i and νw are molar volume of organic compound and water, respectively. The required parameters for the experimental systems can be obtained in the literature [52]. They are: νe =

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187

58.5 × 10−6 m3 ·mol−1 , νw = 18 × 10−6 m3 ·mol−1 , δw = 36.6 MPa0.5 , δe = 26.6 MPa0.5 , δm = 17 MPa0.5 . Thus, Eq. (11) can be solved to give a value of K of 0.04 at 25 ◦ C for ethanol. So the diffusion coefficient of ethanol in PDMS membrane at 5 wt.% ethanol concentration and feed temperature of 25 ◦ C is 2.875 × 10−10 m2 ·s−1 , which is at the same order of magnitude as values reported in the literature. In analogous way, values for Dm of 5.72 × 10−10 m2 ·s−1 and 1.312 × 10−10 m2 ·s−1 can be obtained for methanol (298 K) and n-propanol (303 K), respectively.

5. Conclusions Composite PDMS-CA membranes with thin-film toplayer were prepared by a pre-wetting method to separate methanol, ethanol, n-propanol and acetone from water. The effects of operating parameters on the pervaporation performance were investigated. It was found that the composite PDMSCA membranes had remarkably higher permeation flux than those PDMS supported and unsupported membranes reported in literature and was reasonably and acceptably selective for ethanol. A resistance-in-series model was used to obtain the mass transfer coefficients in the separation of four organics abovementioned from water. It showed that the overall mass transfer coefficient increased linearly with temperature, and is exponentially related to Reynolds number. The boundary layer mass transfer coefficient appeared in a power law relationship to Reynolds number, and changed linearly with temperature. The membrane mass transfer coefficient was revealed to conform to an Arrhenius correlation and to be independent of flow status. In addition, the relative importance of the contributory mass transfer resistances for the overall pervaporation rates of organics through the composite PDMS membranes was investigated clearly in this study. The prediction of K led to a rough estimation of diffusivity for alcohols in membrane, which gave values of Dm for alcohols at a magnitude order of 10−10 m2 ·s−1 under a wider range of temperature.

Acknowledgements The present work was supported financially by National 985 Project of PR China (No. 985XK-015).

Nomenclature A b Ci,b

effective membrane surface area (m2 ) thickness of the membrane cell (m) organic compound concentration in bulk feed solution (g·ml−1 )

Ci,m Ci,w D Di,b Dm dh F J Ji W K kl km ko pi,d pei L Lb T Xo Xw Yo Yw

185

organic compound concentration in membrane phase at equilibrium (g·ml−1 ) organic compound concentration in aqueous phase at equilibrium (g·ml−1 ) diffusivity related to properties of the fluid (m2 ·s−1 ) aqueous phase diffusion coefficient of organic compound (m2 ·s−1 ) diffusion coefficient of organic compound in silicone membrane (m2 ·s−1 ) hydraulic diameter of the membrane cell (m) volumetric flow rate of the feed (m3 ·h−1 ) permeation flux at steady state (g·m−2 ·h−1 ) permeation flux of component i (g·m−2 ·h−1 ) weight of the permeant collected (g) membrane/aqueous phase partition coefficient boundary layer mass transfer coefficient (m·s−1 ) membrane mass transfer coefficient (m2 ·s−1 ) overall mass transfer coefficient (m·s−1 ) partial downstream pressure of the component i (Pa) equilibrium vapor pressure of the component i at aqueous-membrane interface (Pa) membrane thickness (␮m) boundary layer thickness (␮m) temperature (K) weight fractions of organic in the feed weight fractions of water in the feed weight fractions of organic in the permeate weight fractions of organic in the permeate

Greek letters α separation factor δe solubility parameter for ethanol (MPa0.5 ) δm solubility parameter for membrane (MPa0.5 ) solubility parameter for organic compound δi (MPa0.5 ) δw solubility parameter for water (MPa0.5 ) ρ density of the feed (kg·m−3 ) µ dynamic viscosity of the feed (Pa·s) νe molar volume of ethanol (m3 ·mol−1 ) νi molar volume of organic compound (m3 ·mol−1 ) molar volume of water (m3 ·mol−1 ) νw

References [1] K. Kargupta, S. Datta, S.K. Sanyal, Analysis of the performance of a continuous membrane bioreactor with cell recycling during ethanol fermentation, Biochem. Eng. J. 1 (1998) 31. [2] D.J. O’Brien, L.H. Roth, A.J. McAloon, Ethanol production by continuous fermentation–pervaporation: a preliminary economic analysis, J. Membr. Sci. 166 (2000) 105.

186

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187

[3] M. Di Luccio, C.P. Borges, T.L.M. Alves, Economic analysis of ethanol and fructose production by selective fermentation coupled to pervaporation: effect of membrane costs on process economics, Desalination 147 (2002) 161. [4] D.J. O’Brien, G.E. Senske, M.J. Kurantz, J.C. Craig, Ethanol recovery from corn fiber hydrolysate fermentations by pervaporation, Bioresource Technol. 92 (2004) 15. [5] C.B. Almquist, S.-T. Hwang, The permeation of organophosphorus compounds in silicone rubber membranes, J. Membr. Sci. 153 (1999) 57. [6] C.K. Yeom, H.K. Kim, J.W. Rhim, Removal of trace VOCs from water through PDMS membranes and analysis of their permeation behaviors, J. Appl. Polym. Sci. 73 (1999) 601. [7] Y. Chen, T. Miyano, A. Fouda, T. Matsuura, Preparation and gas permeation properties of silicone-coated dry polyethersulfone membranes, J. Membr. Sci. 48 (1990). [8] J.D. Le Roux, D.R. Paul, Preparation of composite membranes by a spin coating process, J. Membr. Sci. 74 (1992) 233. [9] A. Fouda, Y. Chen, J. Bai, T. Matsuura, Wheatstone bridge model for the laminated polydimethylsiloxane/polyethersulfone membrane for gas separation, J. Membr. Sci. 64 (1991) 263. [10] C.A. Page, A.E. Fouda, R. Tyagi, T. Matsuura, Pervaporation performance of polyetherimide membranes spin- and dip coated with polydimethylsiloxane, J. Appl. Polym. Sci. 54 (1994) 975. [11] S.-G. Li, K.V. Peinemann, A novel method for the preparation of composite membranes for gas separation, in: W.R. Bowen, R.W. Field, J.A. Howell (Eds.), Euromembrane (Bath) Proceedings, 1995, I 353. [12] J. Bai, A.E. Fouda, T. Matsuura, J.D. Hazlett, A study on the preparation and performance of polydimethylsiloxane-coated polyetherimide membranes in pervaporation, J. Membr. Sci. 88 (1992) 100. [13] A. Fouda, J. Bai, S.Q. Zhang, O. Kutowy, T. Matsuura, Membrane separation of low volatile organic compounds by pervaporation and vapor permeation, Desalination 90 (1993) 209. [14] K. Ebert, A. Bezjak, K. Nijmeijer, M.H.V. Mulder, H. Strathmann, The preparation of composite membranes with a glassy top layer, in: W.R. Bowen, R.W. Field, J.A. Howell (Eds.), Euromembrane (Bath) Proceedings, 1995, I 237. [15] J.M.S. Henis, M.K. Tripodi, Composite hollow fiber membrane for gas separation: the resistance model approach, J. Membr. Sci. 8 (1981) 233. [16] F.-J. Tsai, D. Kang, M. Anand, Thin-film-composite gas separation membranes: on the dynamics of thin film formation mechanism on porous substrates, Sep. Sci. Tech. 30 (7–9) (1995) 1639. [17] M.E. Rezac, W.J. Koros, Preparation of polymer–ceramic composite membranes with thin defect-free separating layers, J. Appl. Polym. Sci. 46 (1992) 1927. [18] B. Bikson, J.K. Nelson, Composite membranes and their manufacture and use, AP 4826599 (1989). [19] S.E. Williams, B. Bikson, J.K. Nelson, R.D. Burchesky, Composite membranes for enhanced fluid separation, EP 0,286,091 B1 (1994). [20] P.R. Brookes, A.G. Livingston, Aqueous–aqueous extraction of organic pollutants through tubular silicone rubber membranes, J. Membr. Sci. 104 (1995) 119. [21] C. Lipski, P. Cˆote, The use of pervaporation for the removal of organic contaminants from water, Environ. Prog. 9 (1990) 254. [22] B. Raghunath, S.-T. Hwang, General treatment of liquid-phase boundary layer resistance in the pervaporation of dilute aqueous organics through tubular membranes, J. Membr. Sci. 75 (1992) 29. [23] J.G. Wijmans, A.L. Athayde, R. Aaniels, J.H. Ly, H.D. Kamaruddin, I. Pinnau, The role of boundary layers in the removal of volatile organic compounds from water by pervaporation, J. Membr. Sci. 109 (1996) 135. [24] L. Li, Z.Y. Xiao, Z.B. Zhang, S.J. Tan, Mass transfer kinetics of pervaporation by using a composite silicone rubber membrane: (I) the convective transport on membrane surface, J. Chem. Ind. Eng. 53 (11) (2002) 1169 (in Chinese).

[25] J.M. Molina, G. Vatai, E.B. Molnar, Comparison of pervaporation of different alcohols from water on CMG-OM-010 and SULZER membranes, Desalination 149 (2002) 89. [26] I.F.J. Vankelecom, D. Depre, S.D. Beukelaer, J.B. Uytterhoeven, Influence of zeolites in PDMS membranes: pervaporation of water/alcohol mixtures, J. Phys. Chem. 99 (1995) 13193. [27] S. Takegemi, H. Yamada, S. Tusujii, Pervaporation of ethanol/water mixture using novel hydrophobic membrane containing polydimethylsiloxane, J. Membr. Sci. 75 (1992) 93. [28] K. Okamoto, A. Butsuen, S. Nishioka, K. Tanaka, H. Kita, S. Asakawa, Pervaporation of water–ethanol mixtures through polydimethylsiloxane block-copolymer membranes, Polym. J. 19 (1987) 734. [29] Y. Nagase, S. Mori, K. Matsui, Chemical modification of poly (substituted-acetylene) IV. Pervaporation of organic liquid–water mixture through poly(1-phenyl-1-propyne)/polydimethylsiloxane graft copolymer membrane, J. Appl. Polym. Sci. 37 (1989) 1259. [30] Z. Changliu, L. Moe, X. We, Separation of ethanol–water mixtures by pervaporation-membrane separation process, Desalination 62 (1987) 299. [31] K. Ishibara, K. Matsui, Pervaporation of ethanol–water mixture through composite membrane composed of styrene-fluoroalkyl acrylate graft copolymers and cross-linked polydimethylsiloxane membrane, J. Appl. Polym. Sci. 34 (1987) 437. [32] T. Kashiwagi, K. Okabe, K. Okita, Separation of ethanol from ethanol/water mixtures by plasma-polymerized membranes from silicone compounds, J. Membr. Sci. 36 (1988) 353. [33] L. Liang, E. Ruckenstein, Pervaporation of ethanol–water mixtures through polydimethylsiloxane–polystyrene interpenetrating polymer network supported membranes, J. Membr. Sci. 114 (1996) 227. [34] I. Blume, J.G. Wijmans, R.W. Baker, The separation of dissolved organics from water by pervaporation, J. Membr. Sci. 49 (1990) 253–286. [35] T. Ikegami, H. Yanagishita, D. Kitamoto, H. Negishi, K. Haraya, T. Sano, Concentration of fermented ethanol by pervaporation using silicalite membranes coated with silicone rubber, Desalination 149 (2002) 49. [36] M.O. Galindo, A.I. Clar, I.A. Miranda, A.R. Greus, Characterization of poly(dimethylsiloxane)–poly(methyl hydrogen siloxane) composite membrane for organic water pervaporation separation, J. Appl. Poly. Sci. 81 (2001) 546. [37] S. Bandini, A. Saavedra, G.C. Sarti, Vacuum membrane distillation: experiments and modeling, AIChE J. 43 (1997) 398. [38] A.M. Urtiaga, E.D. Gorri, J.K. Beasley, I. Ortiz, Mass transfer analysis of the pervaporation separation of chloroform from aqueous solutions in hollow fiber devices, J. Membr. Sci. 156 (1999) 275. [39] R. Psaume, P. Aptel, Y. Aurelle, J.C. Mora, J.L. Bersillon, Pervaporation: importance of concentration polarization in the extraction of trace organics from water, J. Membr. Sci. 36 (1988) 373. [40] H.O.E. Karlsson, G. Tragardh, Pervaporation of dilute organic-waters mixtures. A literature review on modeling studies and application to aroma compound recovery, J. Membr. Sci. 76 (1993) 121. [41] G. Charbit, F. Charbit, C. Molina, Study of mass transfer limitations in the determination of waste waters by pervaporation, J. Chem. Eng. Jpn. 30 (1997) 382. [42] A.M. Urtiaga, E.D. Gorri, I. Ortiz, Modeling of the concentration–polarization effects in a pervaporation cell with radial flow, Sep. Purif. Technol. 17 (1999) 41. [43] J.-S. Jiang, D.B. Greenberg, J.R. Fried, Pervaporation of methanol from a triglyme solution using a Nafion membrane. 2. Concentration–polarization, J. Membr. Sci. 132 (1997) 263. [44] J. Smart, V.M. Starov, R.C. Schucker, D.R. Lloyd, Pervaporation extraction of volatile organic compounds from aqueous system with use of a tubular transverse flow module. Part II. Experimental results, J. Membr. Sci. 143 (1998) 159.

L. Li et al. / Journal of Membrane Science 243 (2004) 177–187 [45] A. Urtiaga, E.D. Gorri, I. Ortiz, Mass-transfer modeling in the pervaporation of VOCs from diluted solutions, AIChE J. 48 (2002) 572. [46] C.R. Wilke, P.C. Chang, Correlation of diffusion coefficients in dilute solutions, AIChE J. 1 (1955) 264. [47] J.M. Watson, P.A. Payne, A study of organic compound pervaporation through silicone rubber, J. Membr. Sci. 49 (1990) 171. [48] J.M. Watson, G.S. Zhang, P.A. Payne, The diffusion mechanism in silicone rubber, J. Membr. Sci. 73 (1992) 55. [49] L. Li, Z.Y. Xiao, Z.B. Zhang, S.J. Tan, Mass transfer kinetics of pervaporation by using a composite silicone rubber membrane. (I)

187

The convective transport on membrane surface, J. Chem. Ind. Eng. 53 (11) (2002) 1169 (in Chinese). [50] M.A. LaPack, J.C. Tou, V.L. McGuffin, C.G. Enke, The correlation of membrane permselectivity with Hildebrand solubility parameters, J. Membr. Sci. 86 (1994) 263. [51] S.G. Doig, A.T. Boam, A.G. Livingston, D.C. Stuckey, Mass transfer of hydrophobic solutes in solvent swollen silicone rubber membranes, J. Membr. Sci. 154 (1999) 127. [52] D.R. Lide, Handbook of Chemistry and Physics, 71st ed., CRC, Boca Raton, FL, 1991.