Internally Staged Process

For small units, internal staging within a stack provides all the advantages of a multistage series process without requiring recirculating pumps or interconnecting piping (see Figure 7.37.8). In this design, the product stream makes several passes in series between a single set of electrodes, and therefore the degree of demineralization is higher than with a single pass. The process is continuous with a constant power demand.

Its advantages include a high degree of demineraliza-tion, no need for repressurizing pumps, and the need for only a single set of electrodes. This design has the following disadvantages:

A large membrane area is required per unit of product capacity.

The performance is sensitive to changes in flow rate. The pump must overcome a high pressure drop.

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