- Email: [email protected]
Ultra-microporous silica membranes for He purification
Desalination 200 (2006) 89–91
Ultra-microporous silica membranes for He purification Camelia Barboiua, Alejandro Mourguesa, Béatrice Salaa*, Anne Julbeb, José Sanchezb, Serge de Perthuisc, Dominique Hittnerd a
Framatome-ANP, Centre Technique, BP13, 71380 Saint Marcel, France email: [email protected] b Institut Européen des Membranes (IEM), UMR 5635 CNRS-ENSCM-UMII, UMII-CC047, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France c Framatome-ANP, Centre Technique, BP13, 71380 Saint Marcel, France d Framatome-ANP Direction Projet et Ingéniérie (Bal: F 0653A) Tour AREVA, 1 place de la Coupole, 92084 Paris La Défense Cedex, France Received 22 October 2005; accepted 4 March 2006
1. Introduction The next generation of nuclear reactors (HTR – High Temperature Reactors) are modular and operate at 850°C, with helium as a heatconveying gas. A number or research programs on these reactors focused on the development of alternative technologies to the exiting ones, in order to gain either in process competitiveness or quality. The purification of the helium cooling system (300–500°C at 70 bars) with inorganic membranes is one of the considered potential improvements in relation with HTR development. The objective of this work was consequently to prepare, test and evaluate the potentialities of molecular sieve membranes for separating helium (Økinetic = 0.26 nm) from CO2 (Økinetic = 0.33 nm) and from molecules with higher kinetic diameter such as O2, N2, CO, CH4, Xe and Kr, at temperature higher than 300°C.
*Corresponding author.
Silica based ultramicroporous membranes were selected as the best candidates to fulfil these functions. Several techniques have been developed to process porous silica membranes. They include sol–gel synthesis [1,2], chemical leaching [3] and chemical vapor deposition or infiltration [4]. Among these, sol–gel processing attracts most attention due to its excellent processibility, its high versatility and its potential to control pore structure. Several keys to membrane production are recommended in the literature in order to maximize both the membrane selectivity and permeability: control pore sizes in the ultramicroporous range (below 0.7 nm) so that separation occurs by molecular sieving, maximize the pore volume, avoid cracks, pinholes, or other defects and minimize membrane thickness. Both repeated-coating and processing under clean room conditions are classically used to reduce silica membrane defects. But, although aiding in reducing defects and increasing selectivity, multi-step coating process reduces permeation flux by adding flow resistance.
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.253
90
C. Barboiu et al. / Desalination 200 (2006) 89–91
Because both high selectivity and high flux are equally important for industrial membrane applications, a specific sol formulation has been developed in order to prepare high flux defect free membranes for molecular sieving applications. The developed molecular sieve silica based membranes are designed to separate helium from other gases such as CO or CO2, with He permeance values close to (or higher than) 10–7 mol m–2s–1Pa–1, by a pore size exclusion mechanisms, where the larger molecules are either excluded or retarded in their diffusion through the membrane.
2. Experimental Ultramicroporous silica-based membranes (pore sizes less than 0.7 nm) have been prepared by the sol–gel route, using tetraethylorthosilicate (TEOS) as the silica precursor. The layer was deposited inside a tubular asymmetric alumina support (provided by PallExekia), with a mesoporous g -Al2O3 alumina inner layer. One critical challenge was to create large scale, reproducible, thin, and defect-free membranes. In order to reach these goals, the synthesis parameters (especially the sol composition and support pre-treatment) have been tuned to control the condensation of clusters during the silica network polymerization. A specific sol formulation, including namely an additive within the TEOS sol, has been developed. A Framatome-CNRS patent is pending, which describes the details of this new synthesis procedure. After the final heat-treatment, the membrane performance has been studied by single gas and gas mixture permeation at 300° C. The membrane morphology (thickness, homogeneity, infiltration) have been determined by Field-Emission Scanning Electron Microscopy (FE-SEM) and the absence of N2 adsorption was used as a first proof for the ultramicroporous structure of the membrane material.
3. Results The micrographs in Fig. 1 show the crosssections of two silica membranes deposited on the internal layer of an asymmetric alumina tubular support. The synthesis conditions were similar for both membranes, except that the membrane shown in Fig. 1a was prepared without any additive in the starting sol and the membrane in Fig. 1b was prepared with an additive. Clearly the homogeneity and quality of the membranes are very different. The additive provides plasticity to the membrane which limits the formation of cracks, and may even compensate the poor “finish-quality” of the support. The membrane thickness is typically in the range 100–500 nm, with a partial infiltration within the g -alumina supporting layer. The synthesized membranes enable to carry out gas separation up to 500° C under a transmembrane pressure lower than 8 bars. Helium permeance values close to 10–7 mol m–2 s–1Pa–1 and ideal selectivities a(He/CO2) of about 10 were obtained at 300° C. This tends to show that the additive in the starting sol yields a high ultramicroporous volume within the silica network. The performance of these membranes is now studied for mixture separation, before and after ageing cycles. Up-scaling of the developed membranes is possible either with modules made by assembling monotubes or with multichannel monoliths. A modeling of the separation processes achieved using such modules will be also presented, keeping a
b
Fig. 1. FE-SEM observation of silica based ultramicroporous membranes- (a) without any additive in the starting sol (b) with an additive in the starting sol.
C. Barboiu et al. / Desalination 200 (2006) 89–91
in mind that these He permselective membranes could also be efficiently applied to H2 separation from biogas or in hydrocarbon or coal reforming processes. Once demonstrated at a pilot plant scale, the developed membranes could potentially be licensed to industrial partners.
[2]
References
[4]
[1]
L.C. Klein and D. Gallagher, Pore structure of sol–gel silica membrane, J. Membr. Sci., 39 (3) (1988) 213–220.
[3]
91
C.J. Brinker, T.L. Ward, R. Sehgal, N.K. Raman, S.L. Hietala, D.M. Smith, D.W. Hua and T.J. Headley, Ultramicroporous silica-based supported inorganic membranes, J. Membr. Sci., 77 (1993) 165–179. R.P. Beaver, Method of producing porous hollow silica-rich fibers, US Patent 4,778,499 (1998). G.R. Gavalas, C.E. Megiris and S.W. Nam, Deposition of H2 permselective SiO2-films, Chem. Eng. Sci., 44 (9) (1989) 1829–1835.
Ultra-microporous silica membranes for He purification Camelia Barboiua, Alejandro Mourguesa, Béatrice Salaa*, Anne Julbeb, José Sanchezb, Serge de Perthuisc, Dominique Hittnerd a
Framatome-ANP, Centre Technique, BP13, 71380 Saint Marcel, France email: [email protected] b Institut Européen des Membranes (IEM), UMR 5635 CNRS-ENSCM-UMII, UMII-CC047, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France c Framatome-ANP, Centre Technique, BP13, 71380 Saint Marcel, France d Framatome-ANP Direction Projet et Ingéniérie (Bal: F 0653A) Tour AREVA, 1 place de la Coupole, 92084 Paris La Défense Cedex, France Received 22 October 2005; accepted 4 March 2006
1. Introduction The next generation of nuclear reactors (HTR – High Temperature Reactors) are modular and operate at 850°C, with helium as a heatconveying gas. A number or research programs on these reactors focused on the development of alternative technologies to the exiting ones, in order to gain either in process competitiveness or quality. The purification of the helium cooling system (300–500°C at 70 bars) with inorganic membranes is one of the considered potential improvements in relation with HTR development. The objective of this work was consequently to prepare, test and evaluate the potentialities of molecular sieve membranes for separating helium (Økinetic = 0.26 nm) from CO2 (Økinetic = 0.33 nm) and from molecules with higher kinetic diameter such as O2, N2, CO, CH4, Xe and Kr, at temperature higher than 300°C.
*Corresponding author.
Silica based ultramicroporous membranes were selected as the best candidates to fulfil these functions. Several techniques have been developed to process porous silica membranes. They include sol–gel synthesis [1,2], chemical leaching [3] and chemical vapor deposition or infiltration [4]. Among these, sol–gel processing attracts most attention due to its excellent processibility, its high versatility and its potential to control pore structure. Several keys to membrane production are recommended in the literature in order to maximize both the membrane selectivity and permeability: control pore sizes in the ultramicroporous range (below 0.7 nm) so that separation occurs by molecular sieving, maximize the pore volume, avoid cracks, pinholes, or other defects and minimize membrane thickness. Both repeated-coating and processing under clean room conditions are classically used to reduce silica membrane defects. But, although aiding in reducing defects and increasing selectivity, multi-step coating process reduces permeation flux by adding flow resistance.
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.253
90
C. Barboiu et al. / Desalination 200 (2006) 89–91
Because both high selectivity and high flux are equally important for industrial membrane applications, a specific sol formulation has been developed in order to prepare high flux defect free membranes for molecular sieving applications. The developed molecular sieve silica based membranes are designed to separate helium from other gases such as CO or CO2, with He permeance values close to (or higher than) 10–7 mol m–2s–1Pa–1, by a pore size exclusion mechanisms, where the larger molecules are either excluded or retarded in their diffusion through the membrane.
2. Experimental Ultramicroporous silica-based membranes (pore sizes less than 0.7 nm) have been prepared by the sol–gel route, using tetraethylorthosilicate (TEOS) as the silica precursor. The layer was deposited inside a tubular asymmetric alumina support (provided by PallExekia), with a mesoporous g -Al2O3 alumina inner layer. One critical challenge was to create large scale, reproducible, thin, and defect-free membranes. In order to reach these goals, the synthesis parameters (especially the sol composition and support pre-treatment) have been tuned to control the condensation of clusters during the silica network polymerization. A specific sol formulation, including namely an additive within the TEOS sol, has been developed. A Framatome-CNRS patent is pending, which describes the details of this new synthesis procedure. After the final heat-treatment, the membrane performance has been studied by single gas and gas mixture permeation at 300° C. The membrane morphology (thickness, homogeneity, infiltration) have been determined by Field-Emission Scanning Electron Microscopy (FE-SEM) and the absence of N2 adsorption was used as a first proof for the ultramicroporous structure of the membrane material.
3. Results The micrographs in Fig. 1 show the crosssections of two silica membranes deposited on the internal layer of an asymmetric alumina tubular support. The synthesis conditions were similar for both membranes, except that the membrane shown in Fig. 1a was prepared without any additive in the starting sol and the membrane in Fig. 1b was prepared with an additive. Clearly the homogeneity and quality of the membranes are very different. The additive provides plasticity to the membrane which limits the formation of cracks, and may even compensate the poor “finish-quality” of the support. The membrane thickness is typically in the range 100–500 nm, with a partial infiltration within the g -alumina supporting layer. The synthesized membranes enable to carry out gas separation up to 500° C under a transmembrane pressure lower than 8 bars. Helium permeance values close to 10–7 mol m–2 s–1Pa–1 and ideal selectivities a(He/CO2) of about 10 were obtained at 300° C. This tends to show that the additive in the starting sol yields a high ultramicroporous volume within the silica network. The performance of these membranes is now studied for mixture separation, before and after ageing cycles. Up-scaling of the developed membranes is possible either with modules made by assembling monotubes or with multichannel monoliths. A modeling of the separation processes achieved using such modules will be also presented, keeping a
b
Fig. 1. FE-SEM observation of silica based ultramicroporous membranes- (a) without any additive in the starting sol (b) with an additive in the starting sol.
C. Barboiu et al. / Desalination 200 (2006) 89–91
in mind that these He permselective membranes could also be efficiently applied to H2 separation from biogas or in hydrocarbon or coal reforming processes. Once demonstrated at a pilot plant scale, the developed membranes could potentially be licensed to industrial partners.
[2]
References
[4]
[1]
L.C. Klein and D. Gallagher, Pore structure of sol–gel silica membrane, J. Membr. Sci., 39 (3) (1988) 213–220.
[3]
91
C.J. Brinker, T.L. Ward, R. Sehgal, N.K. Raman, S.L. Hietala, D.M. Smith, D.W. Hua and T.J. Headley, Ultramicroporous silica-based supported inorganic membranes, J. Membr. Sci., 77 (1993) 165–179. R.P. Beaver, Method of producing porous hollow silica-rich fibers, US Patent 4,778,499 (1998). G.R. Gavalas, C.E. Megiris and S.W. Nam, Deposition of H2 permselective SiO2-films, Chem. Eng. Sci., 44 (9) (1989) 1829–1835.