Silica membranes hydrogen separation

Hwang, G.-J. et al., Separation of hydrogen from a H2-H20-HI gaseous mixture using a silica membrane, AIChE., 46, 92,2000. 

Pex, P.P.A.C. and Y.C. van Delft, Silica membranes for hydrogen fuel production by membrane water gas shift reaction and development of a mathematical model for a membrane reactor, in Carbon Dioxide Capture for Storage in Deep Geologic Formations—Results from the C02 Capture Project Capture and Separation of Carbon Dioxide from Combustion Sources, eds., D. Thomas, and B. Sally, Vol. 1, Chapter 17, 2005. 

In addition to the Pd-based membranes, microporous silica membranes for hydrogen permeation [8] can be produced by a special type of chemical vapor deposition [140] named chemical vapor infiltration (CVI) [141], A large amount of studies have been carried out on silica membranes made by CVI for hydrogen separation purposes [8,121], CVI [141] is another form of chemical vapor deposition (CVD) [140] (see Section 3.7.3). CVD involves deposition onto a surface, while CVI implies deposition within a porous material [141], Both methods use almost similar equipment [140] and precursors (see Figure 3.19) however, each one functions using different operation parameters, that is, flow rates, pressures, furnace temperatures, and other parameters. 

Much research has been performed on silica membranes produced by Chemical Vapour Infiltration for hydrogen separation purposes. In chapter 7, CVI experiments are described and a concise literature review is provided as well. Below some highlights will be presented briefly. 

Yoshino, Y., Suzuki, T., Nair, B.N., Taguchi, H., and Itoh, N., Development of tubular substrates, silica based membranes and membrane modules for hydrogen separation at high temperature, Journal of Membrane Science, 267, 8-17, 2005. 

Research on other types of materials for H2 separation has been motivated by relatively high cost of Pd and possible membrane degradation by acidic gases and carbon as summarized in Tsuru et al.76 These authors examined microporous silica membranes together with an Ni catalyst layer for SMR reaction. However, this type of membrane allows the permeation of hydrogen as well as other gases in reactants and products, which markedly reduces hydrogen selectivity and limits methane... 

Inorganic membrane development is still in progress [57] (see also Section 14.2.2). Microporous silica membranes have been developed at several universities and research institutes. Membrane selectivities of 15 and 20 for the separation of H2 from CO2 have been reported. Even higher selectivities for H2 arid CO, CH4 and N2 have been measured [20,57]. Most measurements reported in the literature have been performed on a laboratory scale. However, it has been shown that it is possible to upscale these microporous ceramic membranes to, at least, bench scale [31,57]. With other membranes such as noble (Pd) metal membranes and dense ceramic membranes very high and almost infinite selectivities for hydrogen are possible [58]. The permeation of these membranes is generally smaller than the permeation of microporous membranes. 

Hwang, G.-J., Onuki, K., Shimizu, S., and Ohya, H., Hydrogen separation in H2-H2O-HI gaseous mixture using the silica membrane prepared by chemical vapor deposition . Journal of Membrane Science, 162, 83-90, 1999. 

General criteria for selection of materials for the processing of hydrogen separation membranes are discussed. Performance and stability standards required for applications in high temperature membrane reactors have been focused. The correlations between pore structure and stability issues of membranes made of amorphous materials, specifically silica membranes are discussed in detail. 

Yoshida K, Hirano Y, Fujii H, Tsuru T, Asaeda M. Hydrothermal stability and performance of siLica-zirconia membranes for hydrogen separation in hydrothermal conditions. J Chem Eng Jpn. 2001 34(4) 523-30. 

State-of-the-art micro-porous membranes are based on silica, with sufficiently small pores, 2-10 A, to be selective towards hydrogen separation. One of the major problems with silica membranes under hydrothermal conditions is physical stability. Evaporation of silica-containing species is detrimental to long-term permselectivity and restricts the operation of these membranes to temperatures below 600 °C. Hydrogen permeances of >1 x 10 mol m s Pa with H2/CO2 permselectivity in the range 80-100 have, for instance, been measured with single deadend tubular micro-porous silica membranes for temperatures higher than 300 °C and with 4 bar pressure difference. These membranes were reported to be thermally stable for at least 2000 h at temperatures between 200 and 400 °C [50]. 

Microporous silica hydrogen permselective membranes have been extensively studied as a potentially more practical alternative to Pd membranes. Very recently, a comprehensive review was published, tackling various aspects of silica membrane synthesis, application and economics [63]. It was made evident that state-of-the-art silica membranes have good hydrogen flux and separation, as well as respectable thermal stability. However, the hydrothermal stability of a silica hydrogen permselective membrane is a key factor in determining its suitability for a commercial apphcation of membrane-assisted processes. 

Due to their ease of fabrication and scalability, silica membranes are good candidates for hydrogen separation, being also less expensive than metals and not susceptible to H2 embrittlement. Similarly, zeolite membranes have inherent chemical, mechanical, and thermal stability. 

It is worth noting that both Pd-aUoy and sUica-based membranes present some problem about material instability in the WGS environment. The Pd-aUoy membranes can be negatively affected by surface carbonization, sulfur poisoning, and hydrothermal embrittlement, whereas the amorphous silica-based membranes can show some degradation caused by the condensation reaction of sUanol in hydrothermal conditions (Tang et al., 2010). In particular, the siliceous MFI-type zeolite membranes, constituted by a crystalline microporous zeolite membrane, in recent years have been seen as attractive candidates for the WGS reaction because of the high-temperature hydrogen separation and for their intrinsic sulfur tolerance and hydrothermal stability. 

Gopalakrishnan, S., Costa, J. C. D. (2008). Hydrogen gas mixture separation by CVD silica membrane. Journal of Membrane Science, 323, 144—147. 

---