Other Gas Separation Applications

Potentials for inorganic membranes in other gas separation applications are enormous in varieties. Significant technological breakthroughs and development time, however, are required to realize those potential benefits. 

Separation of methanol and methyl tert-butyl ether (MTBE) is an important step in the production of MTBE which is a fast growing chemical. The separation by pervaporation using silica membranes has been modestly successful [van Gcmert and Cuperus, 1995J. A maximum separation factor of about 19 can be obtained. 

AI2O3 (10-20) -196 0.23- 3.2 3.2x105 Itaya etal. [1984] anodic oxide 

On the other hand, actual binary mixture tests using porous alumina and glass membranes show separation factor values for helium recovery from oxygen that are lower than what Knudsen diffusion provides, as indicated in Table 7.15. Only Koresh and Soffer [1983a 1983b] show an ideal separation factor of 20 to 40 with a low permeability of 1.2x10 barrers when molecular sieve membranes with a reported pore diameter of 0.3 to 0.5 nm are used. 

Membranes have been suggested for use in the separation of helium from natural gas [Spillman, 1989] which typically contains 85% methane and 10% ethane. While no test data using real or simulate natural gas is available, there is some information on the separation of helium and ethane using alumina and silica membranes [Havredaki and Petropoulos, 1983], Table 7.16. Clearly, Knudsen diffusion is dominant in the these limited tests. Thus no promising separation performance has been demonstrated. 

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Counterflow modules are always more efficient than crossflow modules, but the advantage is most noticeable when the membrane selectivity is much higher than the pressure ratio across the membrane and a significant fraction of the most permeable component is being removed from the feed gas. This is the case for air-dehydration membrane modules, so counterflow capillary modules are almost always used. With most other gas-separation applications, the advantage offered by counterflow designs does not offset the extra cost of making the counterflow type of module, so they are not widely used. 

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