Microporous silica membranes hydrogen separation

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. 

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. 

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. 

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. 

Membrane separators offer the possibility of compact systems that can achieve fuel conversions in excess of equilibrium values by continuously removing the product hydrogen. Many different types of membrane material are available and a choice between them has to be made on the basis of their compatibility with the operational environment, their performance and their cost. Separators may be classified as (i) non-porous membranes, e.g., membranes based on metals, alloys, metal oxides or metal—ceramic composites, and (ii) ordered microporous membranes, e.g., dense silica, zeolites and polymers. For the separation of hot gases, the most promising are ceramic membranes. 

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