Ceramic membranes materials selection

Another type of membrane is the dynamic membrane, formed by dynamically coating a selective membrane layer on a finely porous support. Advantages for these membranes are high water flux, generation and regeneration in situ abiUty to withstand elevated temperatures and corrosive feeds, and relatively low capital and operating costs. Several membrane materials are available, but most of the work has been done with composites of hydrous zirconium oxide and poly(acryhc acid) on porous stainless steel or ceramic tubes. 

The interest in ceramic membranes grew, together with the interest in membrane separation processes, due to their specific properties. They are chemically stable, can withstand high temperatures and are noncompressible. These characteristics made them the only materials available, which could withstand the harsh environment in the isotope separation. On the other hand, the brittleness of most materials is a problem and so is the selectivity. 

Ceramic membranes, a relatively new class of material, offer numerous advantages over conventional catalysts as support media. Th have a controlled, stable, pore-size distribution. This allows for selective removal of certain gas-phase products from the reaction zone and makes it possible to enhance the yield of products from thermocfynamically lim-... 

Several inorganic materials, especially the ceramic type, exhibit selective transport of species in the form of gas, ions or atoms through them and can be utilized as sensors. Sensors are those devices that convert nonelectrical signals into electrical ones. Although the applications are not directly related to separation processes, their underlying principles are the same as those governing the membranes used in traditional separation applications. Their uses as gas or chemical sensors deserve brief discussions here. 

Thermal shock resistance. Temperature swing as part of the normal cycles of operation or regeneration of the membranes or membrane reactors can lead to deleterious thermal shock. The materials for the various components in a membrane reactor should be carefully selected to impart good thermal sh k resistance. This is particularly important for high temperature reactions. Also listed in Table 9.5 is a summary of various membrane materials along with qualitative description of their resistance to thermal shock. Again, the available data apply to dense materials. While various metal oxides have been made into commercial inorganic membranes, they tend to be affected by thermal shock much more than other ceramic materials. 

Other ceramic sealing materials containing calcined colloidal particles are being pursued by the membrane manufacturers as the issue of sealing extremities of a membrane element continues to be one of the important materials selection and engineering challenges for inorganic membranes. 

Oxygen transport through the solid state can be caused by the difference in both electrical and chemical potentials. The last effect can be used for the development of oxygen-selective ceramic membranes. The demands of materials for such membranes are similar to the demands for solid electrolytes high density in order to prevent physical penetration of the gas and a high coefficient of oxygen... 

Some ceramic dense materials, such as Si02 [59], are also capable of being selectively permeated by hydrogen according to a solution-diffusion-desorption mechanism. The only difference with Pd-alloy membranes lies in the fact that hydrogen diffuses across silica membranes in a molecular form. 

The MF membranes are usually made from natural or synthetic polymers such as cellulose acetate (CA), polyvinylidene difiuoride, polyamides, polysulfone, polycarbonate, polypropylene, and polytetrafiuoroethylene (FIFE) (13). Some of the newer MF membranes are ceramic membranes based on alumina, membranes formed during the anodizing of aluminium, and carbon membrane. Glass is being used as a membrane material. Zirconium oxide can also be deposited onto a porous carbon tube. Sintered metal membranes are fabricated from stainless steel, silver, gold, platinum, and nickel, in disks and tubes. The properties of membrane materials are directly reflected in their end applications. Some criteria for their selection are mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and the cost of membrane material as well as manufacturing process. 

The considerations in this chapter were mainly prompted by the potential application of mixed-conducting perovskite-type oxides to be used as dense ceramic membranes for oxygen delivery applications, and lead to the following general criteria for the selection of materials... 

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