Nonspecific Membranes

Commercial utilization of membranes has relied almost exclusively on the use of nonspecific membrane materials. These materials have the ability to transport one component of a gas mixture in preference to a second. However, they are permeable to all components to at least some degree. Therefore, unlike palladium, which can act as a perfect separator, these membranes transport all stream components. The properties of both porous ceramics and polymeric membranes will be considered here.

Porous ceramics Porous ceramic membranes, with pore sizes ranging from a few nanometers to several microns, have been produced and are commercially available. These membranes separate by size exclusion. For the separation of gases and low molecular weight liquids, Knudsen diffusion is typically employed. For gases, Knudsen diffusion occurs for pore sizes of about 4-100 nm. For systems operating under the Knudsen diffusion regime, the separation of two molecules can be defined as:

Thus, high degrees of separation selectivity are only possible if the molecular weight difference between the two components is large. These materials have been employed for the separation of hydrogen from hydrocarbon streams. The ideal separation selectivity for hydrogen over butane, for example, is 5.8. Thus, a small but measurable separation can be achieved.

The microporous nature of these membranes allows them to have very high transport rates, as compared to nonporous palladium or ceramic materials. Yet, the separation achievable is limited.

Polymers The final category of membrane materials to be considered is polymers. Polymeric membranes are employed for the separation of gas streams, the recovery of organic vapour from air, the separation of mixtures of organic liquids and filtration of particles from aqueous streams. Nonporous polymeric membranes have been considered for membrane reactor applications. Nonporous polymeric membranes separate on the basis of sorption of the component into the polymeric matrix, diffusion across and desorption from the low pressure side.

Polymers offer several advantages when compared with the porous ceramics. For many gas pairs, the inherent selectivity of the polymeric membrane is substantially higher than that of a Knudsen diffusion-controlled ceramic. The polymeric membranes are also easier to prepare in high surface per volume modular configurations, resulting in a considerably lower price. Recent estimates put the price of commercial polymeric membranes at well under $10 per square foot membrane area, installed. In contrast, ceramics may cost 10 times as much.

Many applications of membrane reactors, including dehydrogenation of hydrocarbons, hydrogenations and partial oxidation reactions are high temperature reactions. Conventional polymeric membranes do not have either the chemical or thermal stability to be successfully used in these reactions. However, recent advances have provided materials that can be processed using conventional solvent-based techniques and are later cross-linked to provide the chemical and thermal resistance necessary for membrane reactor applications. New polyimide-based materials provide such characteristics.

Polyimides that are thermally stable to 300°C for extended periods have been reported. When incorporated in a membrane reactor for the dehydrogena-tion of butane, the system performance increased markedly. With no membrane, conversions of 22% were achieved. Following addition of the membrane to the integrated system, the conversion increased to over 30%.

The development of chemically stable polymeric materials provides an opportunity to influence liquidphase organic reactions.

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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