Flow Configurations of Feed, Permeate and Retentate Streams

1 Flow Configurations of Feed, Permeate and Retentate Streams 

The flow patterns of the feed, permeate and retentate streams can greatly influence the membrane reactor performance. First of all, the crossflow configuration distinctly differs from the flow through membrane reactor. In addition, among the commonly employed crossflow arrangements, the relative flow direction and mixing technique of the feed and the permeate have significant impacts on the reactor behavior as well. Some of these effects are the results of the contact time of the reactant(s) or produces) with the membrane pore surface. 

As in the case of gas separation discussed in Chapter 7, which reaction component(s) in a membrane reactor permeates through the membrane determines if any gas recompression is required. If the permeate(s) is one of the desirable products and needs to be further processed downstream at a pressure comparable to that before the membrane separation, recompression of the permeate will be required. On the other hand, if the retentate(s) continues to be processed, essentially no recompression will be necessary. Recompressing a gas can be rather expensive and its associated costs can be pivotal in deciding whether a process is economical. 

Introducing a significant quantity of the carrier (or sweep) gas to the permeate side of the membrane, however, has two major implications. One is that it may necessitate the need to separate the permeate from the carrier gas downstream of the membrane reactor operation if the permeate is a valuable product. The other is the cost associated with the sweep gas. If chemically compatible, air can be used as the least expensive sweep gas available. 

Flow directions on feed and permeate sides. There are numerous possible combinations of the flow directions of the feed and permeate streams in relation to the membrane (and the support layer in the case of a composite membrane). 

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