- Email: [email protected]
water mixtures
Separation and Purification Technology 22-23 (2001) 689– 695 www.elsevier.com/locate/seppur
Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures A.W. Verkerk a,b,*, P. van Male a, M.A.G. Vorstman a, J.T.F. Keurentjes a a
Department of Chemical Engineering and Chemistry, Eindho6en Uni6ersity of Technology, Process De6elopment Group, PO Box 513, 5600 MB Eindho6en, The Netherlands b Department of Chemical Engineering and Chemistry, Eindho6en Uni6ersity of Technology, Dutch Polymer Institute, PO Box 513, 5600 MB Eindho6en, The Netherlands
Abstract In this paper, a set of performance data of a ceramic pervaporation membrane, provided by ECN, Petten, The Netherlands, is described. For the dehydration of alcohol/water mixtures, these membranes appear to combine high selectivities with high permeabilities, resulting in a high Pervaporation Separation Index (PSI). At 70°C the water flux and separation factor for the dehydration of isopropanol (water concentration varied between 1 and 7 wt.%) range from 0.45 to 2.8 kg/(m2 h), and 340–600, respectively. For the dehydration of n-butanol (water concentration varied between 1 and 5 wt.%) these values are between 0.4 and 2.3 kg/(m2 h) and 680– 1340, respectively. These flux values are high as compared with the ceramic pervaporation membranes described in the literature. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Dehydration; Ceramic membranes; Isopropanol; n-Butanol
Nomenclature D J K L p p 0i p* PSI xi
diffusion coefficient (m2/h) flux (kg/(m2 h)) Henry coefficient (1/Pa) thickness of the selective layer (m) pressure (Pa) vapour pressure of the pure component i at the concerning temperature (Pa) the equilibrium vapour pressure for a component in the vapour phase (Pa) pervaporation separation index (kg/(m2 h)) mole fraction of component i in the retentate (mol/mol)
* Corresponding author. Tel.: + 31-40-2474935; fax: + 31-40-2446104. E-mail address: [email protected] (A.W. Verkerk). 1383-5866/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 0 ) 0 0 1 8 5 - 4
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
690
yi qi z
mole fraction of component y in the permeate (mol/mol) loading, kg of component i adsorbed per kg of the selective layer (kg/kg) coordinate perpendicular to the membrane surface (m)
Greek letters h separation factor ( – ) ki activity coefficient of component i in the mixture (–) z density of the selective layer (kg/m3) Subscripts i, j p tot
component i and j, respectively permeate side total
1. Introduction Compared to distillation, pervaporation can often be considered a better candidate for the separation of close boiling, azeotropic or isomeric mixtures. These separations are difficult to achieve by conventional means [1]. As a broad range of mixtures can be separated using pervaporation, this opens the way to many different applications [2]. Examples are the production of high purity ethanol from potato mash [3] and the dehydration of isopropanol mixtures for recycling of cleaning agents in semiconductor and display industries [4]. For polymer pervaporation membranes, extensive research has been performed in finding an optimised membrane material having selective interaction with a specific component of the feed mixture to maximise the performance in terms of separation factor, flux and stability. However, the performance of these membranes can be largely influenced by changes in process conditions like concentration and temperature [5]. In this perspective, a stable multipurpose membrane, made of ceramics, could represent a major improvement. The interest in utilising such membranes in separations has increased, as ceramic membranes with narrow pore size distributions have become commercially available [6].
Inorganic membranes exhibit unique physical and chemical properties that are not (or only partially) shown by organic membranes. Inorganic membranes have better structural stability without the problems of swelling or compaction. Generally, they can withstand harsh chemical environments and high temperatures. Furthermore, the ceramic membranes are not liable to microbiological attack, and can be backflushed, steam sterilised or autoclaved [7]. In this paper, the synthesis of a novel ceramic pervaporation membrane is described, including a characterisation in terms of separation factor and flux. This has been done for the separation of water/isopropanol and water/n-butanol mixtures, thus giving an impression of the application potential of these membranes.
2. Theory The performance of a pervaporation membrane is usually expressed in the flux and separation factor. These parameters are commonly plotted as a function of concentration or mole fraction. In our view, however, it is more appropriate to plot the flux as a function of the driving force for transport.
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
The separation factor, h, is defined as follows: h=
yi /xi yj /xj
(1)
in which y and x are the fractions of component i and j in the permeate and retentate, respectively. The total flux, Jtot, is the sum of the fluxes (S Ji,) of the components in the mixture. The flux of component i can according to Fick’s law be written as: Ji = zDi
dqi dz
(2)
with z the density of the selective layer (kg/m3), Di the diffusion coefficient (m2/s), qi the loading (mass of component i adsorbed per mass of the selective layer (kg/kg)) and z (m) the coordinate perpendicular to the membrane surface. The diffusion coefficient is a function of temperature and may also be a function of q. If the loading of component i is proportional to the partial pressure, pi, of that component, we may write: (3)
q=Kp
in which K is the Henry coefficient (1/Pa). Substitution of q in Eq. (2) then gives: Ji = zDi Ki
dpi dz
(4)
If Di is assumed to be independent of q, integration of Eq. (4) at constant temperature over the thickness of the selective layer, L, results in the flux, which then is proportional to (p *i −pi,p)/L. In which, p *i is the equilibrium vapour pressure for component i at the feed and pi,p the equilibrium vapour pressure at the permeate side. If the pressure of the permeate is small compared to the equilibrium vapour pressure at the retentate side, the driving force for transport equals the equilibrium vapour pressure: p*i = ki xi p 0i
measure for the driving force, which will give a better insight in the behaviour of the membrane itself.
3. Experimental The solvents isopropanol and n-butanol, both pro analysi, were obtained from Merck (Darmstadt, Germany). The membrane performance was measured with the set-up depicted in Fig. 1. The tubular ceramic pervaporation membrane (4) consisted of several support layers of h- and g-alumina. The 200 nm permselective top layer, of the outer wall of the tube, was of amorphous silica [8]. The tubular ceramic pervaporation membrane was placed in a glass vessel with heating jacket (1) in dead-end configuration. This vessel was filled with the alcohol/water mixture, which was stirred with a propeller-type stirrer with a diameter of 5 cm at a stirring speed of 600 rpm. The temperature in the vessel was kept constant, measured with a Pt100 (3). A vacuum pump (Edwards RV5) (8) provided the vacuum. The permeate pressure was controlled with a needle valve (10) and measured with an ATM 100 mbar absolute pressure transmitter (AE sensors) (9). Liquid nitrogen was used as a cooling agent for the cold traps (5, 6 and 7). The connection from the membrane to the cold traps 5 and 6 was thermostated to prevent condensation. The compositions of the feed and permeate were analysed using an automated
(5)
in which ki is the activity coefficient of component i in the liquid mixture ( – ), xi is the mole fraction of component i in the mixture (mol/mol) and p 0i is the vapour pressure of the pure component i (Pa). Now the flux can be plotted as a function of this
691
Fig. 1. Laboratory scale pervaporation set-up.
692
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
Fig. 2. The water flux for dehydration of alcohol/water mixtures using a ceramic pervaporation membrane compared with those of Van Gemert and Cuperus [2], indicated by *.
Karl –Fischer titration apparatus (Mitsubishi, model CA-100) and a refractive index measurement (Euromex Refractometer RF 490) at 25°C, respectively. Dehydration of alcohol/water mixtures was performed at 70°C. The alcohol/water mixture was set to a 1 wt.% water concentration and dehydration of the mixture was started. After several measurements of flux and determination of the corresponding retentate and permeate compositions, the water concentration was set to 2 wt.% and dehydration of the mixture was started again, etc. The pressure of the permeate was smaller than 1 mbar during all experiments. Values of the equilibrium vapour pressures were calculated using the Wilson equation for the activity coefficients.
4. Results and discussion
4.1. Membrane performance Fig. 2 shows the water flux of isopropanol/water and n-butanol/water mixtures plotted as a function of the equilibrium partial water vapour pressure, p*. This Figure also includes the data of Van Gemert and Cuperus [2] for comparison. These data will be discussed in the next section. From Fig. 2 it becomes clear that for the iso-
propanol/water mixture the water flux increases from 0.45 up to 2.8 kg/(m2 h) upon an increase of the water content from 1 to 7 wt.%. For n-butanol/water mixtures ranging from 1 to 5 wt.% the water flux increases from 0.4 up to 2.3 kg/(m2 h). From both plots we see a fairly linear relation, so the assumption that the product of Henry coefficient and diffusion coefficient (Eq. (4)) is constant seems to be plausible. That the Henry coefficient of water is fairly constant for an isopropanol/water mixture on silica can be concluded from the results of Wolf and Schlu¨nder [9]. The difference between the water flux of isopropanol and n-butanol at the same driving force, respectively, can possibly be ascribed to a different influence of propanol and butanol on the adsorption behaviour of water at the amorphous silica. Furthermore, the degree of concentration polarisation may be different in the two alcohol mixtures or the cross-diffusion effect may have an influence. In Fig. 3 the selectivities are plotted as a function of the water concentration. For isopropanol the selectivities vary between 340 and 690. For n-butanol these values are between 680 and 1340. The selectivities vary, because the membrane does not seem to be stabilised at the beginning of a run at a new concentration. During each run, when the concentration decreases, the separation factor shows an increase which reaches a rather stable value at the highest water fractions. It can, there-
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
693
fore, be assumed that after sufficient time the separation factor reaches a constant value of 600 and 1300 for isopropanol and n-butanol, respectively, slightly increasing with decreasing water content.
factors in the separation process, a Pervaporation Separation Index (PSI) [10] can be defined as a measure of the separation ability of a membrane:
4.2. Comparison with literature data
When we relate our membrane performances with earlier cited polymer pervaporation membranes, interesting comparisons can be made (see Table 1). Our study shows a PSI of 1200 (up to 1800 after stabilisation), whereas most other membranes used show significantly lower values of PSI. The only membranes having a higher PSI are restricted in temperature. This obviously counts for all the polymer membranes. Furthermore, all the polymer membranes, except the CMC-CE-02, show a decrease in PSI, with increasing temperature. For n-butanol we find a PSI of 1920. On butanol/water mixtures hardly any literature is available. A siloxane –phosphazene copolymer used by Roizard et al. [16] shows a PSI of 18 for a n-butanol (7.7 wt.%)/water mixture at 40°C.
The data in Figs. 2 and 3 by Van Gemert and Cuperus [2], indicated by methanol*, ethanol*, and isopropanol*, are used for comparison. These mixtures have also been separated with a ceramic pervaporation membrane at 70°C (methanol/water at 60°C). The water fluxes plotted this way also show a fairly linear relation. Nevertheless, the water fluxes for isopropanol mixtures are substantially lower as compared to the ECN membrane. A reason for the higher water flux of the membrane used in this study lies in the very thin selective layer. Unfortunately, Van Gemert and Cuperus [2] do not report the thickness of the selective layer of their ceramic membrane. The separation factor of the ceramic membrane by Van Gemert and Cuperus [2] towards water/isopropanol is around 600, comparable with our data for isopropanol. The separation ability of a membrane can be expressed in terms of permeation and separation factor. Usually there is a trade-off between these two factors; i.e. when one factor increases, the other decreases. As both of them are important
Fig. 3. The separation factor for dehydration of alcohol/water mixtures using a ceramic pervaporation membrane compared with those of Van Gemert and Cuperus [2]. Symbols are the same as those used in Fig. 2.
PSI = J*toth
(6)
5. Concluding remarks The ceramic pervaporation membranes show very good membrane performances for the separation of alcohol/water mixtures. It is expected that even significantly higher fluxes, with similar selectivities, can be achieved at higher temperatures. To predict the membrane performance more has to be known about the transport mechanism through the membrane. Diffusion and adsorption data are needed for this purpose. Because ceramic membranes can withstand high temperatures and harsh environments, dehydration of high boiling organic solutions, like DMSO and DMF could be performed. Other interesting applications can be in chemical reactions, limited by their thermodynamical reaction equilibrium, by the selective removal of one of the products [17]. It can be expected that ceramic pervaporation membranes can be a highly interesting tool for industry, provided they can be produced cheap at a large scale.
Silica Silica
This study
10 mm, PVA binding 1 mm, PVA binding
[2]
Chitosan, cross-linked, PS support 10 mm, no PVA binding 50
[14]
Sodium alginate
0.9 1.6
Chitosan (cross-linked) Chitosan/PS-composite
[13]
[15]
6.0
Carboxymethylated poly(vinyl alcohol)
70
70
70
50 50
30 30
80 0.15 0.40
0.5
0.11
[12]
65 55
CMC-CE-01 CMC-CE-02
[11]
Flux, kg/(m2 h) 10 wt.%
Membrane type or material
Reference
T (°C)
2.1
0.3
1.0
0.4 0.7
5.0
0.09 0.27
0.20
0.055 0.09
Flux, kg/(m2 h) 5 wt.%
600
500
2500
350/350 250/350
7000
1100/2000 350/800
1800/3700
370/520 800
h (–) 10/5 wt.%
1250
150
2500
300/140 400/350
40 000
160/180 140/200
900/900
80/30 70
PSI kg/(m2 h) 10/5 wt.%
After stabilisation PSI = 1800
Going from 50 to 70°C, h decreases tenfold
PSI is roughly the same at 60°C
PSI drops with increasing temp. PSI increases with temperature
Comments
Table 1 Overview of fluxes and selectivities of various pervaporation membranes in the system water/isopropanol at 5 and 10 wt.% water
694 A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
Acknowledgements We would gratefully thank P.P.A.C. Pex from ECN, Petten the Netherlands for providing the membranes. Furthermore, we thank the Dutch Polymer Institute for their financial support. Finally, L.J.P van den Broeke and E.L.V. Goetheer, both from the Eindhoven University of Technology, are acknowledged for very fruitful discussions.
References [1] S.K. Ray, S.B. Sawant, J.B. Joshi, V.G. Pangarkar, Ind. Eng. Chem. Res. 36 (1997) 5265. [2] R.W. van Gemert, F.P. Cuperus, J. Membr. Sci. 105 (1995) 287. [3] N.G. Grobben, G. Eggink, F.P. Cuperus, H.J. Huizing, Appl. Microbiol. Biotechnol. 39 (1993) 494. [4] Y.M. Lee, S.Y. Nam, S.Y. Ha, J. Membr. Sci. 159 (1999) 41. [5] R.M. Waldburger, F. Widmer, Chem. Eng. Technol. 19 (1996) 117.
.
695
[6] F.M. Velterop, Book of Abstracts, vol. 2, Euromembrane 99, Leuven, 19 – 22 September, 1999, p. 118. [7] H.P. Hsieh, R.R. Bhave, H.L. Fleming, J. Membr. Sci. 39 (1988) 221. [8] H.M. van Veen, Y.C. van Delft, C.W.R. Engelen, P.P.A.C. Pex, Book of Abstracts, vol. 2, Euromembrane 99, Leuven, 19 – 22 September, 1999, p. 209. [9] H.E. Wolf, E.-U. Schlu¨nder, Chem. Eng. Process. 38 (1999) 211. [10] R.Y.M. Huang, X. Feng, Sep. Sci. Tech. 28 (1993) 2035. [11] R. Atra, G. Vatai, E. Bekassy-Molnar, Chem. Eng. Process. 38 (1999) 149. [12] S.Y. Nam, H.J. Chun, Y.M. Lee, J. Appl. Polym. Sci. 72 (1999) 241. [13] M. Ghazali, M. Nawawi, R.Y.M. Huang, J. Membr. Sci. 124 (1997) 53. [14] R.Y.M. Huang, R. Pal, G.Y. Moon, J. Membr. Sci. 160 (1999) 17. [15] R.Y.M. Huang, R. Pal, G.Y. Moon, J. Membr. Sci. 160 (1999) 101. [16] D. Roizard, R. Clement, P. Lochon, J. Kerres, G. Eigenberger, J. Membr. Sci. 113 (1996) 151. [17] W.J.W. Bakker, I.A.A.C.M. Bos, W.L.P. Rutten, J.T.F. Keurentjes, M. Wessling, Proceedings of the International Conference on Inorganic Membranes, Nagano, Japan, 1998, p. 448.
Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures A.W. Verkerk a,b,*, P. van Male a, M.A.G. Vorstman a, J.T.F. Keurentjes a a
Department of Chemical Engineering and Chemistry, Eindho6en Uni6ersity of Technology, Process De6elopment Group, PO Box 513, 5600 MB Eindho6en, The Netherlands b Department of Chemical Engineering and Chemistry, Eindho6en Uni6ersity of Technology, Dutch Polymer Institute, PO Box 513, 5600 MB Eindho6en, The Netherlands
Abstract In this paper, a set of performance data of a ceramic pervaporation membrane, provided by ECN, Petten, The Netherlands, is described. For the dehydration of alcohol/water mixtures, these membranes appear to combine high selectivities with high permeabilities, resulting in a high Pervaporation Separation Index (PSI). At 70°C the water flux and separation factor for the dehydration of isopropanol (water concentration varied between 1 and 7 wt.%) range from 0.45 to 2.8 kg/(m2 h), and 340–600, respectively. For the dehydration of n-butanol (water concentration varied between 1 and 5 wt.%) these values are between 0.4 and 2.3 kg/(m2 h) and 680– 1340, respectively. These flux values are high as compared with the ceramic pervaporation membranes described in the literature. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Dehydration; Ceramic membranes; Isopropanol; n-Butanol
Nomenclature D J K L p p 0i p* PSI xi
diffusion coefficient (m2/h) flux (kg/(m2 h)) Henry coefficient (1/Pa) thickness of the selective layer (m) pressure (Pa) vapour pressure of the pure component i at the concerning temperature (Pa) the equilibrium vapour pressure for a component in the vapour phase (Pa) pervaporation separation index (kg/(m2 h)) mole fraction of component i in the retentate (mol/mol)
* Corresponding author. Tel.: + 31-40-2474935; fax: + 31-40-2446104. E-mail address: [email protected] (A.W. Verkerk). 1383-5866/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 0 ) 0 0 1 8 5 - 4
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
690
yi qi z
mole fraction of component y in the permeate (mol/mol) loading, kg of component i adsorbed per kg of the selective layer (kg/kg) coordinate perpendicular to the membrane surface (m)
Greek letters h separation factor ( – ) ki activity coefficient of component i in the mixture (–) z density of the selective layer (kg/m3) Subscripts i, j p tot
component i and j, respectively permeate side total
1. Introduction Compared to distillation, pervaporation can often be considered a better candidate for the separation of close boiling, azeotropic or isomeric mixtures. These separations are difficult to achieve by conventional means [1]. As a broad range of mixtures can be separated using pervaporation, this opens the way to many different applications [2]. Examples are the production of high purity ethanol from potato mash [3] and the dehydration of isopropanol mixtures for recycling of cleaning agents in semiconductor and display industries [4]. For polymer pervaporation membranes, extensive research has been performed in finding an optimised membrane material having selective interaction with a specific component of the feed mixture to maximise the performance in terms of separation factor, flux and stability. However, the performance of these membranes can be largely influenced by changes in process conditions like concentration and temperature [5]. In this perspective, a stable multipurpose membrane, made of ceramics, could represent a major improvement. The interest in utilising such membranes in separations has increased, as ceramic membranes with narrow pore size distributions have become commercially available [6].
Inorganic membranes exhibit unique physical and chemical properties that are not (or only partially) shown by organic membranes. Inorganic membranes have better structural stability without the problems of swelling or compaction. Generally, they can withstand harsh chemical environments and high temperatures. Furthermore, the ceramic membranes are not liable to microbiological attack, and can be backflushed, steam sterilised or autoclaved [7]. In this paper, the synthesis of a novel ceramic pervaporation membrane is described, including a characterisation in terms of separation factor and flux. This has been done for the separation of water/isopropanol and water/n-butanol mixtures, thus giving an impression of the application potential of these membranes.
2. Theory The performance of a pervaporation membrane is usually expressed in the flux and separation factor. These parameters are commonly plotted as a function of concentration or mole fraction. In our view, however, it is more appropriate to plot the flux as a function of the driving force for transport.
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
The separation factor, h, is defined as follows: h=
yi /xi yj /xj
(1)
in which y and x are the fractions of component i and j in the permeate and retentate, respectively. The total flux, Jtot, is the sum of the fluxes (S Ji,) of the components in the mixture. The flux of component i can according to Fick’s law be written as: Ji = zDi
dqi dz
(2)
with z the density of the selective layer (kg/m3), Di the diffusion coefficient (m2/s), qi the loading (mass of component i adsorbed per mass of the selective layer (kg/kg)) and z (m) the coordinate perpendicular to the membrane surface. The diffusion coefficient is a function of temperature and may also be a function of q. If the loading of component i is proportional to the partial pressure, pi, of that component, we may write: (3)
q=Kp
in which K is the Henry coefficient (1/Pa). Substitution of q in Eq. (2) then gives: Ji = zDi Ki
dpi dz
(4)
If Di is assumed to be independent of q, integration of Eq. (4) at constant temperature over the thickness of the selective layer, L, results in the flux, which then is proportional to (p *i −pi,p)/L. In which, p *i is the equilibrium vapour pressure for component i at the feed and pi,p the equilibrium vapour pressure at the permeate side. If the pressure of the permeate is small compared to the equilibrium vapour pressure at the retentate side, the driving force for transport equals the equilibrium vapour pressure: p*i = ki xi p 0i
measure for the driving force, which will give a better insight in the behaviour of the membrane itself.
3. Experimental The solvents isopropanol and n-butanol, both pro analysi, were obtained from Merck (Darmstadt, Germany). The membrane performance was measured with the set-up depicted in Fig. 1. The tubular ceramic pervaporation membrane (4) consisted of several support layers of h- and g-alumina. The 200 nm permselective top layer, of the outer wall of the tube, was of amorphous silica [8]. The tubular ceramic pervaporation membrane was placed in a glass vessel with heating jacket (1) in dead-end configuration. This vessel was filled with the alcohol/water mixture, which was stirred with a propeller-type stirrer with a diameter of 5 cm at a stirring speed of 600 rpm. The temperature in the vessel was kept constant, measured with a Pt100 (3). A vacuum pump (Edwards RV5) (8) provided the vacuum. The permeate pressure was controlled with a needle valve (10) and measured with an ATM 100 mbar absolute pressure transmitter (AE sensors) (9). Liquid nitrogen was used as a cooling agent for the cold traps (5, 6 and 7). The connection from the membrane to the cold traps 5 and 6 was thermostated to prevent condensation. The compositions of the feed and permeate were analysed using an automated
(5)
in which ki is the activity coefficient of component i in the liquid mixture ( – ), xi is the mole fraction of component i in the mixture (mol/mol) and p 0i is the vapour pressure of the pure component i (Pa). Now the flux can be plotted as a function of this
691
Fig. 1. Laboratory scale pervaporation set-up.
692
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
Fig. 2. The water flux for dehydration of alcohol/water mixtures using a ceramic pervaporation membrane compared with those of Van Gemert and Cuperus [2], indicated by *.
Karl –Fischer titration apparatus (Mitsubishi, model CA-100) and a refractive index measurement (Euromex Refractometer RF 490) at 25°C, respectively. Dehydration of alcohol/water mixtures was performed at 70°C. The alcohol/water mixture was set to a 1 wt.% water concentration and dehydration of the mixture was started. After several measurements of flux and determination of the corresponding retentate and permeate compositions, the water concentration was set to 2 wt.% and dehydration of the mixture was started again, etc. The pressure of the permeate was smaller than 1 mbar during all experiments. Values of the equilibrium vapour pressures were calculated using the Wilson equation for the activity coefficients.
4. Results and discussion
4.1. Membrane performance Fig. 2 shows the water flux of isopropanol/water and n-butanol/water mixtures plotted as a function of the equilibrium partial water vapour pressure, p*. This Figure also includes the data of Van Gemert and Cuperus [2] for comparison. These data will be discussed in the next section. From Fig. 2 it becomes clear that for the iso-
propanol/water mixture the water flux increases from 0.45 up to 2.8 kg/(m2 h) upon an increase of the water content from 1 to 7 wt.%. For n-butanol/water mixtures ranging from 1 to 5 wt.% the water flux increases from 0.4 up to 2.3 kg/(m2 h). From both plots we see a fairly linear relation, so the assumption that the product of Henry coefficient and diffusion coefficient (Eq. (4)) is constant seems to be plausible. That the Henry coefficient of water is fairly constant for an isopropanol/water mixture on silica can be concluded from the results of Wolf and Schlu¨nder [9]. The difference between the water flux of isopropanol and n-butanol at the same driving force, respectively, can possibly be ascribed to a different influence of propanol and butanol on the adsorption behaviour of water at the amorphous silica. Furthermore, the degree of concentration polarisation may be different in the two alcohol mixtures or the cross-diffusion effect may have an influence. In Fig. 3 the selectivities are plotted as a function of the water concentration. For isopropanol the selectivities vary between 340 and 690. For n-butanol these values are between 680 and 1340. The selectivities vary, because the membrane does not seem to be stabilised at the beginning of a run at a new concentration. During each run, when the concentration decreases, the separation factor shows an increase which reaches a rather stable value at the highest water fractions. It can, there-
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
693
fore, be assumed that after sufficient time the separation factor reaches a constant value of 600 and 1300 for isopropanol and n-butanol, respectively, slightly increasing with decreasing water content.
factors in the separation process, a Pervaporation Separation Index (PSI) [10] can be defined as a measure of the separation ability of a membrane:
4.2. Comparison with literature data
When we relate our membrane performances with earlier cited polymer pervaporation membranes, interesting comparisons can be made (see Table 1). Our study shows a PSI of 1200 (up to 1800 after stabilisation), whereas most other membranes used show significantly lower values of PSI. The only membranes having a higher PSI are restricted in temperature. This obviously counts for all the polymer membranes. Furthermore, all the polymer membranes, except the CMC-CE-02, show a decrease in PSI, with increasing temperature. For n-butanol we find a PSI of 1920. On butanol/water mixtures hardly any literature is available. A siloxane –phosphazene copolymer used by Roizard et al. [16] shows a PSI of 18 for a n-butanol (7.7 wt.%)/water mixture at 40°C.
The data in Figs. 2 and 3 by Van Gemert and Cuperus [2], indicated by methanol*, ethanol*, and isopropanol*, are used for comparison. These mixtures have also been separated with a ceramic pervaporation membrane at 70°C (methanol/water at 60°C). The water fluxes plotted this way also show a fairly linear relation. Nevertheless, the water fluxes for isopropanol mixtures are substantially lower as compared to the ECN membrane. A reason for the higher water flux of the membrane used in this study lies in the very thin selective layer. Unfortunately, Van Gemert and Cuperus [2] do not report the thickness of the selective layer of their ceramic membrane. The separation factor of the ceramic membrane by Van Gemert and Cuperus [2] towards water/isopropanol is around 600, comparable with our data for isopropanol. The separation ability of a membrane can be expressed in terms of permeation and separation factor. Usually there is a trade-off between these two factors; i.e. when one factor increases, the other decreases. As both of them are important
Fig. 3. The separation factor for dehydration of alcohol/water mixtures using a ceramic pervaporation membrane compared with those of Van Gemert and Cuperus [2]. Symbols are the same as those used in Fig. 2.
PSI = J*toth
(6)
5. Concluding remarks The ceramic pervaporation membranes show very good membrane performances for the separation of alcohol/water mixtures. It is expected that even significantly higher fluxes, with similar selectivities, can be achieved at higher temperatures. To predict the membrane performance more has to be known about the transport mechanism through the membrane. Diffusion and adsorption data are needed for this purpose. Because ceramic membranes can withstand high temperatures and harsh environments, dehydration of high boiling organic solutions, like DMSO and DMF could be performed. Other interesting applications can be in chemical reactions, limited by their thermodynamical reaction equilibrium, by the selective removal of one of the products [17]. It can be expected that ceramic pervaporation membranes can be a highly interesting tool for industry, provided they can be produced cheap at a large scale.
Silica Silica
This study
10 mm, PVA binding 1 mm, PVA binding
[2]
Chitosan, cross-linked, PS support 10 mm, no PVA binding 50
[14]
Sodium alginate
0.9 1.6
Chitosan (cross-linked) Chitosan/PS-composite
[13]
[15]
6.0
Carboxymethylated poly(vinyl alcohol)
70
70
70
50 50
30 30
80 0.15 0.40
0.5
0.11
[12]
65 55
CMC-CE-01 CMC-CE-02
[11]
Flux, kg/(m2 h) 10 wt.%
Membrane type or material
Reference
T (°C)
2.1
0.3
1.0
0.4 0.7
5.0
0.09 0.27
0.20
0.055 0.09
Flux, kg/(m2 h) 5 wt.%
600
500
2500
350/350 250/350
7000
1100/2000 350/800
1800/3700
370/520 800
h (–) 10/5 wt.%
1250
150
2500
300/140 400/350
40 000
160/180 140/200
900/900
80/30 70
PSI kg/(m2 h) 10/5 wt.%
After stabilisation PSI = 1800
Going from 50 to 70°C, h decreases tenfold
PSI is roughly the same at 60°C
PSI drops with increasing temp. PSI increases with temperature
Comments
Table 1 Overview of fluxes and selectivities of various pervaporation membranes in the system water/isopropanol at 5 and 10 wt.% water
694 A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
A.W. Verkerk et al. / Separation/Purification Technology 22-23 (2001) 689–695
Acknowledgements We would gratefully thank P.P.A.C. Pex from ECN, Petten the Netherlands for providing the membranes. Furthermore, we thank the Dutch Polymer Institute for their financial support. Finally, L.J.P van den Broeke and E.L.V. Goetheer, both from the Eindhoven University of Technology, are acknowledged for very fruitful discussions.
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