Specific Membranes

Certain materials have the ability to transport a single component with the complete exclusion of all others. Transport of hydrogen and oxygen through two of these materials is reviewed here.

Metals (hydrogen) Palladium and some of its alloys have the ability to transport hydrogen while completely excluding all other compounds. The permeation of hydrogen through metals is a multistep process involving:

• chemisorption of hydrogen on to the metal surface

• dissociation of the hydrogen

• dissolution of the atomic hydrogen from the surface into the bulk of the metal

• diffusion across the metal layer

• the desorption from the bulk of the metal to the surface

• reassociation

• desorption of molecular hydrogen from the metal surface

The diffusion of atomic hydrogen across the metal layer is typically the rate-limiting step. Therefore, the transport of the hydrogen can be modelled using a Fickian diffusion equation.

Alloys of palladium have proven effective for the transport of hydrogen. Pure palladium undergoes a phase transformation in the presence of hydrogen at moderate temperatures, and a density change ensues. Even though these changes are small, they are sufficient to produce a brittle, cracked and nonselective material after only a few cycles. Therefore, few of the membranes evaluated are pure palladium and most commercial metallic hydrogen purifiers are prepared from a palladium/silver alloy containing 23% silver.

A significant limitation to the use of palladium membranes is their strong susceptibility to poisoning by sulfur compounds and CO compounds frequently found in the hydrocarbon streams of interest. Research into the development of more resistant materials is underway. However, the improved resistance has so far been attained at the cost of permeability. This issue must be resolved before these membranes can be used in the chemical process industry.

Nonporous ceramics (oxygen) Ceramic membranes have been developed which will selectively transport hydrogen or oxygen. Hydrogen has been shown to be transportable through nonporous silicon dioxide. While the transport rates are extremely slow, the selectivity to hydrogen transport is infinite, just as in defect-free palladium. Silicon dioxide has the advantage of being more resistant to the presence of sulfur compounds that act as poisons for palladium. Nevertheless, the challenge of forming this material into extremely thin layers has limited its use. Perhaps as preparation techniques continue to improve, the use of this material for the highly selective transport of hydrogen will be re-examined.

Nonporous ceramics have also been used for the transport of oxygen. Oxide-conducting materials, in cluding stabilized zirconia, have been used in membrane reactors. These membranes can successfully transfer oxygen while barring the transport of all other compounds. Current limitations relating to the temperatures required for operation are discussed in subsequent sections.

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.

Get My Free Ebook

Post a comment