Permeation Rate and Permeate Recovery

The permeation rate and its profile along the membrane reactor length can substantially determine the reactor performance. Moreover, to increase the reaction conversion, a lower permeate concentration on the permeate side is often adopted. This has implications on energy consumption and downstream separation costs. These issues will be addressed here. 

They have also modeled a perfectly mixed membrane reactor for the same reaction. For this membrane configuration, the C2 selectivity is essentially 1(X)% for a jo value of less than 0.2 and drops off rapidly with increasing The yield becomes greater as the relative oxygen permeation rate increases. The yield reaches a maximum and then decreases with the permeation rate. There is only a limited range of Jo in which the yield can reach beyond 20%. 

Permeate recovery. To achieve high conversions, it is often desirable to maintain a very low permeate partial pressure which leads to an increase in the permeation rate. Vacuum or a sweep gas is usually employed to attain a low permeate pressure. Vacuum adds some energy cost while the use of a sweep gas may require further downstream processing for the recovery of the permeate (if it contains the desired species) or the separation of the permeate from the sweep gas. The use of a condensable gas or vapor as the sweep gas will facilitate the recovery of the permeate. For example, steam can be condensed at a relatively lower temperature and easily separated from many other gases. However, the issue of hydrothermal stability of the membrane, discussed in Chapter 9, can be critical. Air, on the other hand, is a convenient sweep or carrier gas to use because 

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