Ceramic membranes permeability

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The construction of a commercial reactor, however, is difficult, and a sweep gas is required to shift the product composition away from equilibrium values. The achievable conversion also depends on the permeability of the membrane. Figure 7 shows the equilibrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equilibrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

P.M. Biesheuvel and H. Verweij, Ceramic Membrane Supports, Permeability, Tensile Strength and Stress , J. Membrane Sci. 156 141-52 (1999). 

The quantity of ambipolar conductivity is widely used for the analysis of -> electrolytic permeability of -> solid electrolytes, caused by the presence of electronic conductivity. Other important cases include transient behavior of electrochemical cells and ion-conducting solids, dense ceramic membranes for gas separation, reduction/ oxidation of metals, and kinetic demixing phenomena [iv]. In most practical cases, however, the ambipo-... 

Mechanical stability - Organic membranes contact and can undergo inelastic deformations under high pressures, leading to lower permeabilities. Ceramic membranes supported on robust materials such as stainless steel or structural ceramics can be expected to withstand very high pressures. 

Ceramic membrane is the nanoporous membrane which has the comparatively higher permeability and lower separation fector. And in the case of mixed gases, separation mechanism is mainly concerned with the permeate velocity. The velocity properties of gas flow in nanoporous membranes depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collision. The Knudsen number Kn Xydp is characteristic parameter defining different permeate mechanisms. The value of the mean free path depends on the length of the gas molecule and the characteristic pore diameter. The diffusion of inert and adsorbable gases through porous membrane is concerned with the contributions of gas phase diffusion and sur u e diffusion. 

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]. 

The openness (e.g., volume fraction) and the nature of the pores affect the permeability and permselectivity of porous inorganic membranes. Porosity data can be derived from mercury porosimetry information. Membranes with higher porosities possess more open porous structure, thus generally leading to higher permeation rates for the same pore size. Porous inorganic membranes, particularly ceramic membranes, have a porosity... 

Thermal and hydrothermal exposures can change the ix>re size and its distribution, porosity and tortuosity of a porous membrane which in turn influence the separation properties of the membrane such as permeability and permselectivity. Several ceramic membranes have been investigated for their responses to thermal and hydrothermal environments. 

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. 

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