Porous ceramic membranes

In this section a short introduction will be given on the synthesis of porous ceramic membranes by sol-gel techniques and anodization, carbon membranes, glass membranes and track-etch membranes. An extensive discussion will be given in Sections 2.3-2.S. 

Inorganic membranes commercially available today are dominated by porous membranes, particularly porous ceramic membranes which are essentially the side-products of the earlier technical developments in gaseous diffusion for separating uranium isotopes in the U.S. and France. Summarized in Table 3.1 are the porous inorganic membranes presently available in the market (Hsieh 1988). They vary greatly in pore size, support material and module geometry. 

Porous ceramic membrane layers are formed on top of macroporous supports, for enhanced mechanical resistance. The flow through the support may consist of contributions due to both Knudsen-diffusion and convective nonseparative flow. Supports with large pores are preferred due to their low resistance to the flow. Supports with high resistance to the flow decrease the effective pressure drop over the membrane separation layer, thus diminishing the separation efficiency of the membrane (van Vuren et al. 1987). For this reason in a membrane reactor it is more effective to place the reaction (catalytic) zone at the top layer side of the membrane while purging at the support side of the membrane. 

Furneaux, R. C. and M. C. Thornton. 1988. Porous "ceramic membranes produced from anodizing aluminium. Brit. Cer. Proc., Advanced Ceramics in Chemical Process Engineering Eds. B. C. H. Steele and D. P. Thompson, vol. 43, pp. 93-101. 

Hollow glass microbeads Porous ceramic membranes Microbeads coated by Ti02 particles Porous ceramic Ti02 and ZnO membranes prepared by sol-gel technique... 

Coronas, J. and Santamaria, J. (1999) Catalytic reactors based on porous ceramic membranes. Catalysis Today, 51, 377—389. 

Porous ceramic membranes for catalytic reactors - overview and new ideas. Journal of Membrane Science, 181, 3-20. 

In Chapter 10, the use of membranes for different applications are described. One of the possible membranes for hydrogen cleaning is an asymmetric membrane comprised of the dense end of a proton conduction perovskite such as BaCe0 95 Yb0 05O3 5 and a porous end to bring mechanical stability to the membrane. In this case, it is possible to take from the slurry, obtained by the acetate procedure, several drops to be released over a porous ceramic membrane, located in the spinning bar of a spin-coating machine. Thereafter, the assembly powder, thin film porous membrane is heated from room temperature up to 1573 K at a rate of 2K/min, kept at this temperature for 12 h, and then cooled at the same rate in order to get the perovskite end film over the porous membrane [50],... 

The in situ membrane growth technique cannot be applied using the zeolite-based ceramic porous membrane as support, under hydrothermal conditions in a solution containing sodium hydroxide. The high pH conditions will cause membrane amorphization and lead to final dissolution. Therefore, we tried to synthesize an aluminophosphate zeolite such as AlP04-5 [105] over a zeolite porous ceramic membrane. For the synthesis of the AlP04-5-zeolite-based porous membrane composite, the in situ membrane growth technique [7,13,22] was chosen. Then, the support, that is, the zeolite-based porous ceramic membrane, was placed in contact with the synthesis mixture and, subsequently, subjected to a hydrothermal synthesis process [18]. The batch preparation was as follows [106] ... 

FIGURE 10.6 XRD profile of the porous ceramic membrane covered with a AlP04-5 molecular sieve. 

Different ways of preparing high performance porous ceramic membranes have been developed [13, 14], but most of the membranes used in CMRs are obtained via sol-gel processes [13-15, 23—25]. 

IV. Development of porous ceramic membranes for a solar thermal water-splitting reactor, Int. J. Hydrogen Energy, 25 1043-1050 (2000). 

Much of the impetus for the awakened interest and utilization of inorganic membranes recently came hom a history of about forty or fifty years of some large scale successes of porous ceramic membranes for gaseous diffusion to enrich uranium in the military weapons and nuclear power reactor applications. In the gaseous diffusion literature, the porous membranes are referred to as the porous barriers. For nuclear power generation, uranium enrichment can account for approximately 10% of the operating costs (Charpin and Rigny, 1989]. 

Normally when one of the two performance indicators of a porous ceramic membrane for gas separation (i.e., separation factor and permeability) is high, the other is low. It is, therefore, necessary to m e a compromise that offers the most economic benefit Often it is desirable to slightly sacrifice the separation factor for a substantial increase in the permeation flux. This has been found to be feasible with a 5% doping of silica in an alumina membrane [GaBui et al., 1992]. 

Yeast can be separated from an ethanol fermentation broth by porous ceramic membranes with backflushing [Matsumoto et al., 1988]. Tubular alumina membranes with a nominal pore diameter of 1.6 pm were demonstrated to be effective for this application with a maximum permeate flux of 1,1(X) IVhr-m with backflushing. The permeate flux increases with increasing feed rale (or crossflow velocity) and TMP and with decreasing yeast concentration. Various backflushing techniques were investigated and the reverse flow of filtrate (instead of air) either by pressure from the permeate side... 

The idea of a probe utilizing a porous ceramic membrane that is permeable to the gas to be measured but not to the molten nonferrous metal has been proposed [De SchuUer et al 1991]. The probe contains a tube which is closed off at its immersion extremity by the membrane. A cover is fitted to the end of the tube which melts when immersed in the molten metal. A vacuum is created inside the cover. By measuring the pressure in the tube after the membrane is immersed and the vacuum applied, the hy ogen content having diffused through the membrane is thus determined. The preferred ceramic membrane is made of alumina. 

Shown in Table 9.7 are some examples of incorporating catalysts into porous ceramic membranes. Both metal and oxide catalysts have been introduced to a variety of ceramic membranes (e.g., alumina, silica, Vycor glass and titania) to make them catalytically active. The impregnation/heat U eatment procedures do not appear to show a consistent cause-and-effeci relationship with the resulting membrane permeability. For example, no noticeable change is observed when platinum is impregnated into porous Vycor glass... 

Microporous membranes. While dense metal or metal oxide membranes possess exceptionally good peimselectivities, their permeabilities are typically lower than those of porous inorganic membranes by an order of magnitude or more. Commercial availability of porous ceramic membranes of consistent quality has encouraged an ever... 

C. Porous Ceramic Membranes Exhibiting Specific Magnetic... 

Coronas J., Menendez M. and Santamaria J., Methane oxidative coupling using porous ceramic membrane reactors. Part II. Reaction studies, Chem. Engng. Sci. 49 2015 (1994). Coronas J., Menendez M. and Santamaria J., Development of ceramic membrane reactors with non-uniform permeation pattern. Application to methane oxidative coupling, Chem. Eng. Sci. 49 4749 (1994). 

Kitao S., Ishizaki M. and Asaeda M.. Permeation mechanism of water through fine porous ceramic membrane for separation of organic solvcnt/watcr mixtures. Presented at ICIM 91, Montpellier, France, (1991), p. 175. 

Lafaiga D., Santamaria J. and Menendez M., Methane oxidative coupling using porous ceramic membrane reactors. Part I. Reactor development, Chem. Eng. ScL 49 2005 (1994). Sloot H.J., Smolders C.A., van Swaaij W.P.M. and Versteeg G.F., High-temperature membrane reactor for catalytic gas-solid reactions, AIChE J. J5 887 (1992). 

Basic mechanisms involved in gas and vapor separation using ceramic membranes are schematized in Figure 6.14. In general, single gas permeation mechanisms in a porous ceramic membrane of thickness depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collisions. In membranes with large mesopores and macropores the separation selectivity is weak. The number of intermolecular collisions is strongly dominant and gas transport in the porosity is described as a viscous flow that can be quantified by a Hagen-Poiseuille type law ... 

In summary, one can see that separation selectivity for gas and vapor molecules depends on the category of pores (mesopores, supermicropores, and ultramicropores) and on the related transport mechanisms. Either size effect or preferential adsorption effect (irrespective of molecular dimension) is involved in selective separation of multicomponent mixtures. The membrane separation selectivity for two gases is usually expressed either as the ratio between the two pure gas permeation fluxes (ideal selectivity) or between each gas permeation flux measured from the mixture of the two gases (real selectivity). More detailed information on gas and vapor transport in porous ceramic membranes can be found in Ref. [24]. 

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