Inorganic Membranes

Ceramic Membranes

Microporous ceramic membranes for ultrafiltration and microfiltration applications can be formed from a variety of metal oxides. Specifically, aluminium and titanium oxides are preferred precursors for the production of ceramic membranes. Because ceramic membranes are chemically inert and can be operated at high temperatures, these membranes offer some significant advantages over polymeric membranes. Pore diameters in ceramic membranes for ultrafiltration and microfiltration are in the 0.01 to 10 |im range and are typically made by a slip coating-sintering process. Other techniques, such as the sol-gel method, produce ceramic membranes with pores in the range of 10 to 100 A. In the slip coating-sintering process, a porous ceramic tube is made by pouring a dispersion of a coarse ceramic material and a binder into a mould. This mixture is then sintered at high temperature. The resulting porous tube is then coated with a mixture containing very small metal oxide particles and a binder; this mixture is called a slip suspension. Again, the mixture is sintered at high temperature to form a more finely porous layer. The slip-coating-sintering method can be used to make membranes with pore diameters between 100 to 200 A. More finely porous membranes can be fabricated by the sol-gel technique. First, the metal oxide, dissolved in alcohol, is hydrolyzed by addition of excess water. Then, the colloidal polymeric or inorganic hydroxide solution is cooled and coated onto a preformed microporous support made by the slip coating-sintering process. The coating must be dried very carefully to avoid cracking of the thin ceramic layer. The final step of the sol-gel method involves sintering of the coating at elevated temperature, typically between 500 and 800°C. In principle, membranes made by this process can be used in a variety of applications which require membranes that are stable in harsh environment and at elevated temperature. However, reproducibility of the membrane formation process on a large commercial scale is rather poor and the membrane costs are too high for these membranes to be used in any industrial separation process.

Metal Membranes

Metal membranes have been considered for a long time for gas separation applications, specifically hy drogen separation. Certain noble metals, for example palladium or palladium-silver and palladium-gold alloys, are permeable to hydrogen but essentially impermeable to all other gases. In the 1950s and 1960s, Union Carbide installed a pilot membrane system containing 25-| m-thick, isotropic palladium membranes. Because the hydrogen flux through these thick palladium membranes is quite low, the membranes had to be operated at about 400°C. Although the plant generated 99.9% hydrogen, commercialization of this process was economically not feasible because of the extremely high cost of the metal membrane (&$5000/m2). Furthermore, contaminants in the feed stream, such as hydrogen sulfide, poison the metal which results in a dramatic decline in hydrogen flux.

Anodic Membranes

Symmetric and asymmetric microporous membranes with a conical pore shape can be made from aluminium using an anodic oxidation process. Symmetric aluminium oxide membranes having a porosity of 65% and a pore size of about 200 nm can be made. The surface pores of asymmetric aluminium oxide membranes are about 25 nm. To prepare these membranes, a thin aluminium foil is anodically oxidized in an acid electrolyte, such as sulfuric or chromic acid, thereby forming an aluminium oxide. The unaffected fraction of the metal foil is subsequently removed using a strong acid. The pore size of membranes made by anodic oxidation is determined by the voltage and the acid type.

Carbon Membranes

Microporous carbon membranes can be made by compressing ultrafine carbon particles or by pyrolys-ing polymeric precursors. Degradation of the base polymer upon heating leads to carbonization. The pore size and porosity of the pyrolysed membranes depend primarily on the pyrolysis temperature and the pyrolysis atmosphere. Molecular sieve membranes made from pyrolysed polyacrylonitrile and polyimide as well as selective surface flow membranes made from polyvinylidene chloride-acrylate ter-polymer can have significantly better separation performance than polymeric membranes in gas separation applications. The pore sizes of microporous carbon membranes are typically in the 5 to 20 A range.

Glass Membranes

Isotropic glass membranes with a sponge structure can be made by thermal phase separation of an initially homogenous metal oxide mixture. Microporous glass membranes were produced by Corning (Vycor®), Schott, and PPG. Glass membranes are typically made as discs, tubes or hollow-fibres. To produce microporous glass membranes, a homogeneous melt consisting of 70 wt% SiO2, 23 wt% B2O3 and 7 wt% Na2O is formed between 1300 to 1500°C. Phase separation of the initially homogeneous glass melt occurs by lowering the temperature to about 800°C. One phase consists primarily of insoluble silicon dioxide. The other phase, rich in alkali borate, can be leached from the heterogeneous glass by treatment with a mineral acid. After removal of the alkali borate phase, a microporous silica membrane is formed.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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