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Comparison of SiO2-ZrO2-50% and commercial SiO2 membranes on the pervaporative dehydration of organic solvents
Desalination 193 (2006) 97–102
Comparison of SiO2-ZrO2-50% and commercial SiO2 membranes on the pervaporative dehydration of organic solvents Ane Urtiagaa*, Clara Casadoa, Masashi Asaedab, Inmaculada Ortiza a
Department of Chemical Engineering and Inorganic Chemistry, Universidad de Cantabria, Ave. Los Castros s/n, Santander, Cantabria, 39005, Spain Tel. +34 (94) 2201587; Fax +34 (94) 2201591; email: [email protected] b Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
Received 15 March 2005; accepted 7 October 2005
Abstract In this work several SiO2-ZrO2-50% tubular pervaporation (PV) membranes were prepared by the sol–gel and hot coating method. Their PV performance was investigated regarding the separation of water/isopropanol and water/acetone mixtures in terms of water flux and selectivity. The behaviour of these membranes was compared to the performance of two commercially available silica membranes (Pervatech BV, The Netherlands, and Pervap SMS, Sulzer, Germany) on the PV of water/isopropanol mixtures and a residual water/acetone mixture coming from the manufacture process of rubber antioxidants. An exponential dependence of the water flux with respect to the water activity in the feed was confirmed for both types of membranes. The value of the mass transfer parameter that describes the interaction of the permeating component, i.e. water, with the membrane has a very similar value for the commercial silica membranes and the prepared silica–zirconia membranes. Keywords: Pervaporation; Ceramic membranes; Silica; Zirconia
1. Introduction Organic solvents are very common in chemical industries and their purification and recycling is conventionally carried out by energy-intensive *Corresponding author.
processes, such as distillation. Pervaporation (PV) is an alternative to distillation for separating azeotropic or close-boiling component mixtures. Some inorganic membranes, based on hydrophilic zeolite and amorphous silica layers, have become commercially available in recent years [1–3].
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.10.019
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However, silica membranes show a questionable chemical stability when they are put in contact with hot aqueous mixtures [4]. They change their functional characteristics in the presence of humid atmosphere or water vapour [5,6]. Various techniques are being developed in order to improve the separation performance of existing microporous ceramic membranes. One of these techniques consists in doping with other oxides [7]. In this work, SiO2-ZrO2-50% membranes were prepared in the laboratory by the sol–gel and hot coating method and their performance in PV of isopropanol/water and acetone-water was evaluated. For comparison purposes, the performance of two commercially available silica membranes supplied by Pervatech BV (The Netherlands) and Sulzer Chemtech GmbH (Germany) is also shown in this study. 2. Experimental SiO2-ZrO2-50% membranes were prepared in the laboratory of the University of Hiroshima by the sol–gel method, which comprises the sol preparation and the sol coating on a porous substrate. Several colloidal sols had been prepared with tetra-ethoxy silane and zirconium tetra-nbutoxide as precursors, controlling the concentration (2.0, 1.5, 1.0, 0.8, 0.5 wt% alkoxides) by adding water and acid. One month after the sol preparation, sols were coated on a commercial tubular support made of alumina, kept at 180°C while contacting quickly a cloth wetted in the sol, and burnt at 450°C for 10–20 min. This operation was repeated several times with sols of decreasing particle size, i.e. wt% alkoxides, in order to obtain the membranes that have been tested in this work. The thickness and homogeneity of the coated layer was observed on the SEM photographs taken for a membrane sample prepared by the same method. The mean pore diameters measured with the permporometry technique [8] were less than 1 nm. The separation performance of these membranes was tested in the bench-scale plant at the laboratory
of the University of Hiroshima on the dehydration of synthetic mixtures of water/IPA at 75°C and water/acetone at 55°C. Two commercially available silica membranes were also characterised and the results were detailed in previous works [1,9]. These membranes were manufactured by Pervatech BV (Pervatech PVP, The Netherlands) and Sulzer Chemtech GmbH (Sulzer SMS, Germany), respectively. Both consist of an amorphous silica top layer over a tubular composite alumina porous support. In these cases, PV experiments were run in a specially built stainless steel bench-scale pilot plant at the University of Cantabria, using synthetic water/isopropanol mixtures and an industrial water/acetone waste effluent containing about 25 wt% water in acetone coming from the chemical production of rubber antioxidants. Water/isopropanol dehydration experiments shown in this study were performed at 70°C and water/acetone experiments at 50°C, that is, below their normal boiling points. 3. Results and discussion The structural characteristics of the SiO2-ZrO250% membranes are shown in Fig. 1. The microporous silica–zirconia layer obtained had a thickness of about 0.5 µm, as it is observed in Fig. 1a for a membrane sample. Fig. 1b shows the observed pore size distributions of yet another membrane sample prepared by the same sol–gel and hot coating after coating sols of decreasing particle size. The average pore size is lower as the particle size of the last sol coated is also lower. Fig. 2a shows the water flux as a function of water content in the feed for the SiO2-ZrO2-50% membranes during PV experiments of water/IPA at 75ºC and Fig. 2b — the water flux as a function of water content for the PV experiments of water/ acetone at 55ºC. The results obtained using the commercial silica membranes were included for comparison. It may be observed that SiO2-ZrO250% membranes give a higher water flux similar
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(b)
(a)
Fig. 1. SEM micrograph of (a) the cross section of the composite silica–zirconia membrane, and (b) the pore size distribution.
(a)
(b)
Fig. 2. Water flux as a function of water content in the feed observed at the PV experiments through SiO2-ZrO2-50% membranes and commercial SiO2 membranes, for the dehydration of (a) water/isopropanol and (b) water/acetone mixtures.
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than the commercial membranes, both for the isopropanol as for the acetone dehydration. The flux through membrane SiO2-ZrO2-50%(1) is the highest, which may be attributed to the smaller number of sol coatings that were necessary during the preparation procedure. Regarding the low water fluxes observed during the water/acetone PV experiments using the commercial membranes, it must be taken into account that for the characterisation of SiO2-ZrO2-50% membranes a synthetic binary mixture was used, whereas for the commercial membranes, an industrial water/ acetone effluent, having minor components that might interfere with the PV performance, was employed as feed mixture.
In order to give an idea of the separation behaviour of these membranes, permeate quality, represented by the water content in permeate, is plotted against the water content in the feed in Fig. 3. For water/isopropanol dehydration, shown in Fig. 3a, all membranes gave a water concentration higher than 97 wt%, thus providing good selectivity. Regarding the water/acetone dehydration, the water content in permeate data obtained at 50– 55°C are shown in Fig. 3b, revealing that the silica–zirconia membranes provided a slightly lower selectivity than in the dehydration of IPA, which might be attributed to the smaller size of the acetone molecule. Nevertheless, the water
(a)
(b)
Fig. 3. Permeate water content vs. feed water content through commercial and SiO2-ZrO2-50% membranes, for the dehydration of (a) water/isopropanol and (b) water/acetone mixtures.
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content in the permeate remained higher than 95 wt% till water in the feed is less than 5 wt%. It should be noted out that the commercial silica membranes yielded water contents in permeate higher than 99.5 wt%, which means that the organic content is low enough to permit further biological treatment [9] of the aqueous permeate. The compared results show that the feed can be efficiently dehydrated with the two types of membranes under study, although the dehydration process is more efficient using the silica–zirconia membranes since higher water fluxes are provided. However in the water/acetone separation the commercial silica membranes yields an
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aqueous permeate with homogenous organic content and better environmental characteristics than the prepared silica–zirconia membranes. In a previous work [10], it has been demonstrated that the water flux through the commercial silica membranes presented in this study could be predicted as an exponential function of the water activity in the feed solution. Fig. 4a shows an exponential relationship between the water flux and the water activity in the feed. A plot of ln Jw vs. water activity in the liquid mixture is presented in Fig. 4b, for the two SiO2-ZrO2-50% membranes and the commercial Pervatech membrane, on the dehydration of water/IPA mixtures. The linear
(a)
(b)
Fig. 4. Dehydration of water/IPA mixture through SiO2-ZrO2-50% membranes at 75ºC (a) evolution of water flux vs. water activity in feed and (b) ln water flux vs. water activity. Data obtained with the Pervatech PVP membranes are shown for comparison.
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relationship between the ln Jw and the water activity in the feed was confirmed. The average slope was 3.4, which agreed reasonably with the value of 3.3 previously obtained for the Pervatech PVP membrane [11]. Thus the mathematical model developed for the commercial silica membranes seems also valid for the SiO2-ZrO2-50% membranes prepared in the laboratory. 4. Conclusions The compared results showed that both water/ IPA and water/acetone feed mixtures could be efficiently separated by all the membranes under study, although the SiO2-ZrO2-50% membranes were more efficient since they provided higher water fluxes. On the other hand, in the case of the water/acetone separation, commercial silica membranes yielded an aqueous permeate with very low organic content in all the concentration range, and thus better environmental characteristics than the permeate obtained with the mixed oxide membranes. An exponential dependence of the water flux on the water activity in the feed was confirmed for both types of membranes and, thus, the flux data obtained with the silica–zirconia membrane fitted the mathematical model developed in previous works [10,11] for the commercial silica membranes. The value of the mass transfer parameter that describes the interaction of the permeating component, i.e. water, with the membrane has a very similar value for the silica membrane and the silica–zirconia membrane. Further results at different working temperatures and feed mixtures, seem to be needed to fully validate the model for the SiO2-ZrO2-50% membranes and
obtain the kinetic parameters that predict water flux through these membranes. Acknowledgements Financial support of the Spanish Ministry for Education and Science under project PPQ200300934 is gratefully acknowledged. One of the authors (C. Casado) also thanks the Ministry of Education and Science for the F.P.I. grant, including the opportunity for going on research stay at the University of Hiroshima. References [1] A.M. Urtiaga, C. Casado and I. Ortiz, Mat. Res. Soc. Symp. Proc. Materials Research Society, 752 (2002) 257–264. [2] A. Urtiaga, E.D. Gorri, C. Casado and I. Ortiz, Separ. Purif. Technol., 32 (2003) 207–213. [3] J.E. Elshof, C. Rubio Abadal, J. Sekulic, S. Doy Chowdhury and D.H.A. Blank, Microporous Mesoporous Mater., 65 (2003) 197–208. [4] M. Asaeda, J. Yang and Y. Sakou, J. Chem. Eng. Japan, 35 (2002) 365–371. [5] H.M. van Veen, Y.C. van Delft, C.W.R. Engelen and P.P.A.C. Pex, Separ. Purif. Technol., 22–23 (2001) 361–366. [6] Y.S. Lin, Separ. Purif. Technol., 25 (2001) 39–55. [7] J. Sekulic, M.W.J. Luiten, J.E. ten Elshof, N.E. Benes and K. Keizer, Desalination, 148 (2002) 19– 23. [8] T. Tsuru, T. Hino, T. Yoshioka and M. Asaeda, J. Membr. Sci., 186 (2001) 257–265. [9] A.M. Urtiaga, C. Casado, C. Aragoza and I. Ortiz, Separ. Sci. Technol., 38(14) 3473–3491. [10] I. Ortiz, D. Gorri, C. Casado and A. Urtiaga, J. Chem. Technol. Biotechnol., 80 (2005) 397–405. [11] C. Casado, A. Urtiaga, D. Gorri and I. Ortiz, Separ. Purif. Technol., 42 (2005) 39–45.
Comparison of SiO2-ZrO2-50% and commercial SiO2 membranes on the pervaporative dehydration of organic solvents Ane Urtiagaa*, Clara Casadoa, Masashi Asaedab, Inmaculada Ortiza a
Department of Chemical Engineering and Inorganic Chemistry, Universidad de Cantabria, Ave. Los Castros s/n, Santander, Cantabria, 39005, Spain Tel. +34 (94) 2201587; Fax +34 (94) 2201591; email: [email protected] b Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
Received 15 March 2005; accepted 7 October 2005
Abstract In this work several SiO2-ZrO2-50% tubular pervaporation (PV) membranes were prepared by the sol–gel and hot coating method. Their PV performance was investigated regarding the separation of water/isopropanol and water/acetone mixtures in terms of water flux and selectivity. The behaviour of these membranes was compared to the performance of two commercially available silica membranes (Pervatech BV, The Netherlands, and Pervap SMS, Sulzer, Germany) on the PV of water/isopropanol mixtures and a residual water/acetone mixture coming from the manufacture process of rubber antioxidants. An exponential dependence of the water flux with respect to the water activity in the feed was confirmed for both types of membranes. The value of the mass transfer parameter that describes the interaction of the permeating component, i.e. water, with the membrane has a very similar value for the commercial silica membranes and the prepared silica–zirconia membranes. Keywords: Pervaporation; Ceramic membranes; Silica; Zirconia
1. Introduction Organic solvents are very common in chemical industries and their purification and recycling is conventionally carried out by energy-intensive *Corresponding author.
processes, such as distillation. Pervaporation (PV) is an alternative to distillation for separating azeotropic or close-boiling component mixtures. Some inorganic membranes, based on hydrophilic zeolite and amorphous silica layers, have become commercially available in recent years [1–3].
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.10.019
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A. Urtiaga et al. / Desalination 193 (2006) 97–102
However, silica membranes show a questionable chemical stability when they are put in contact with hot aqueous mixtures [4]. They change their functional characteristics in the presence of humid atmosphere or water vapour [5,6]. Various techniques are being developed in order to improve the separation performance of existing microporous ceramic membranes. One of these techniques consists in doping with other oxides [7]. In this work, SiO2-ZrO2-50% membranes were prepared in the laboratory by the sol–gel and hot coating method and their performance in PV of isopropanol/water and acetone-water was evaluated. For comparison purposes, the performance of two commercially available silica membranes supplied by Pervatech BV (The Netherlands) and Sulzer Chemtech GmbH (Germany) is also shown in this study. 2. Experimental SiO2-ZrO2-50% membranes were prepared in the laboratory of the University of Hiroshima by the sol–gel method, which comprises the sol preparation and the sol coating on a porous substrate. Several colloidal sols had been prepared with tetra-ethoxy silane and zirconium tetra-nbutoxide as precursors, controlling the concentration (2.0, 1.5, 1.0, 0.8, 0.5 wt% alkoxides) by adding water and acid. One month after the sol preparation, sols were coated on a commercial tubular support made of alumina, kept at 180°C while contacting quickly a cloth wetted in the sol, and burnt at 450°C for 10–20 min. This operation was repeated several times with sols of decreasing particle size, i.e. wt% alkoxides, in order to obtain the membranes that have been tested in this work. The thickness and homogeneity of the coated layer was observed on the SEM photographs taken for a membrane sample prepared by the same method. The mean pore diameters measured with the permporometry technique [8] were less than 1 nm. The separation performance of these membranes was tested in the bench-scale plant at the laboratory
of the University of Hiroshima on the dehydration of synthetic mixtures of water/IPA at 75°C and water/acetone at 55°C. Two commercially available silica membranes were also characterised and the results were detailed in previous works [1,9]. These membranes were manufactured by Pervatech BV (Pervatech PVP, The Netherlands) and Sulzer Chemtech GmbH (Sulzer SMS, Germany), respectively. Both consist of an amorphous silica top layer over a tubular composite alumina porous support. In these cases, PV experiments were run in a specially built stainless steel bench-scale pilot plant at the University of Cantabria, using synthetic water/isopropanol mixtures and an industrial water/acetone waste effluent containing about 25 wt% water in acetone coming from the chemical production of rubber antioxidants. Water/isopropanol dehydration experiments shown in this study were performed at 70°C and water/acetone experiments at 50°C, that is, below their normal boiling points. 3. Results and discussion The structural characteristics of the SiO2-ZrO250% membranes are shown in Fig. 1. The microporous silica–zirconia layer obtained had a thickness of about 0.5 µm, as it is observed in Fig. 1a for a membrane sample. Fig. 1b shows the observed pore size distributions of yet another membrane sample prepared by the same sol–gel and hot coating after coating sols of decreasing particle size. The average pore size is lower as the particle size of the last sol coated is also lower. Fig. 2a shows the water flux as a function of water content in the feed for the SiO2-ZrO2-50% membranes during PV experiments of water/IPA at 75ºC and Fig. 2b — the water flux as a function of water content for the PV experiments of water/ acetone at 55ºC. The results obtained using the commercial silica membranes were included for comparison. It may be observed that SiO2-ZrO250% membranes give a higher water flux similar
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(b)
(a)
Fig. 1. SEM micrograph of (a) the cross section of the composite silica–zirconia membrane, and (b) the pore size distribution.
(a)
(b)
Fig. 2. Water flux as a function of water content in the feed observed at the PV experiments through SiO2-ZrO2-50% membranes and commercial SiO2 membranes, for the dehydration of (a) water/isopropanol and (b) water/acetone mixtures.
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than the commercial membranes, both for the isopropanol as for the acetone dehydration. The flux through membrane SiO2-ZrO2-50%(1) is the highest, which may be attributed to the smaller number of sol coatings that were necessary during the preparation procedure. Regarding the low water fluxes observed during the water/acetone PV experiments using the commercial membranes, it must be taken into account that for the characterisation of SiO2-ZrO2-50% membranes a synthetic binary mixture was used, whereas for the commercial membranes, an industrial water/ acetone effluent, having minor components that might interfere with the PV performance, was employed as feed mixture.
In order to give an idea of the separation behaviour of these membranes, permeate quality, represented by the water content in permeate, is plotted against the water content in the feed in Fig. 3. For water/isopropanol dehydration, shown in Fig. 3a, all membranes gave a water concentration higher than 97 wt%, thus providing good selectivity. Regarding the water/acetone dehydration, the water content in permeate data obtained at 50– 55°C are shown in Fig. 3b, revealing that the silica–zirconia membranes provided a slightly lower selectivity than in the dehydration of IPA, which might be attributed to the smaller size of the acetone molecule. Nevertheless, the water
(a)
(b)
Fig. 3. Permeate water content vs. feed water content through commercial and SiO2-ZrO2-50% membranes, for the dehydration of (a) water/isopropanol and (b) water/acetone mixtures.
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content in the permeate remained higher than 95 wt% till water in the feed is less than 5 wt%. It should be noted out that the commercial silica membranes yielded water contents in permeate higher than 99.5 wt%, which means that the organic content is low enough to permit further biological treatment [9] of the aqueous permeate. The compared results show that the feed can be efficiently dehydrated with the two types of membranes under study, although the dehydration process is more efficient using the silica–zirconia membranes since higher water fluxes are provided. However in the water/acetone separation the commercial silica membranes yields an
101
aqueous permeate with homogenous organic content and better environmental characteristics than the prepared silica–zirconia membranes. In a previous work [10], it has been demonstrated that the water flux through the commercial silica membranes presented in this study could be predicted as an exponential function of the water activity in the feed solution. Fig. 4a shows an exponential relationship between the water flux and the water activity in the feed. A plot of ln Jw vs. water activity in the liquid mixture is presented in Fig. 4b, for the two SiO2-ZrO2-50% membranes and the commercial Pervatech membrane, on the dehydration of water/IPA mixtures. The linear
(a)
(b)
Fig. 4. Dehydration of water/IPA mixture through SiO2-ZrO2-50% membranes at 75ºC (a) evolution of water flux vs. water activity in feed and (b) ln water flux vs. water activity. Data obtained with the Pervatech PVP membranes are shown for comparison.
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relationship between the ln Jw and the water activity in the feed was confirmed. The average slope was 3.4, which agreed reasonably with the value of 3.3 previously obtained for the Pervatech PVP membrane [11]. Thus the mathematical model developed for the commercial silica membranes seems also valid for the SiO2-ZrO2-50% membranes prepared in the laboratory. 4. Conclusions The compared results showed that both water/ IPA and water/acetone feed mixtures could be efficiently separated by all the membranes under study, although the SiO2-ZrO2-50% membranes were more efficient since they provided higher water fluxes. On the other hand, in the case of the water/acetone separation, commercial silica membranes yielded an aqueous permeate with very low organic content in all the concentration range, and thus better environmental characteristics than the permeate obtained with the mixed oxide membranes. An exponential dependence of the water flux on the water activity in the feed was confirmed for both types of membranes and, thus, the flux data obtained with the silica–zirconia membrane fitted the mathematical model developed in previous works [10,11] for the commercial silica membranes. The value of the mass transfer parameter that describes the interaction of the permeating component, i.e. water, with the membrane has a very similar value for the silica membrane and the silica–zirconia membrane. Further results at different working temperatures and feed mixtures, seem to be needed to fully validate the model for the SiO2-ZrO2-50% membranes and
obtain the kinetic parameters that predict water flux through these membranes. Acknowledgements Financial support of the Spanish Ministry for Education and Science under project PPQ200300934 is gratefully acknowledged. One of the authors (C. Casado) also thanks the Ministry of Education and Science for the F.P.I. grant, including the opportunity for going on research stay at the University of Hiroshima. References [1] A.M. Urtiaga, C. Casado and I. Ortiz, Mat. Res. Soc. Symp. Proc. Materials Research Society, 752 (2002) 257–264. [2] A. Urtiaga, E.D. Gorri, C. Casado and I. Ortiz, Separ. Purif. Technol., 32 (2003) 207–213. [3] J.E. Elshof, C. Rubio Abadal, J. Sekulic, S. Doy Chowdhury and D.H.A. Blank, Microporous Mesoporous Mater., 65 (2003) 197–208. [4] M. Asaeda, J. Yang and Y. Sakou, J. Chem. Eng. Japan, 35 (2002) 365–371. [5] H.M. van Veen, Y.C. van Delft, C.W.R. Engelen and P.P.A.C. Pex, Separ. Purif. Technol., 22–23 (2001) 361–366. [6] Y.S. Lin, Separ. Purif. Technol., 25 (2001) 39–55. [7] J. Sekulic, M.W.J. Luiten, J.E. ten Elshof, N.E. Benes and K. Keizer, Desalination, 148 (2002) 19– 23. [8] T. Tsuru, T. Hino, T. Yoshioka and M. Asaeda, J. Membr. Sci., 186 (2001) 257–265. [9] A.M. Urtiaga, C. Casado, C. Aragoza and I. Ortiz, Separ. Sci. Technol., 38(14) 3473–3491. [10] I. Ortiz, D. Gorri, C. Casado and A. Urtiaga, J. Chem. Technol. Biotechnol., 80 (2005) 397–405. [11] C. Casado, A. Urtiaga, D. Gorri and I. Ortiz, Separ. Purif. Technol., 42 (2005) 39–45.