EVALUATION OF THE PERFORMANCE OF MULTISTAGE MEMBRANE SEPARATION CASCADES

The minimization of the work consumption in a multistage membrane sepa ration cascade operating in isoenthalpic conditions was investigated w ith regard to the splitting factor of each stage, for the assigned val ues of the number of stages, the retentate and permeate pressures, and the flow pattern. The mathematical model of the membrane separation c ascade is presented, and five possible different flow patterns togethe r with system variance are discussed. Under isoenthalpic conditions th e work needs are due to the compression of the permeated stream, and t he work losses to the generation of entropy on the mixing of recycled streams. A three-component mixture (NH3, H-2, N-2) and a polyethylene membrane were used as the standard testing system; the results of the optimization for three, five, seven, or nine stages cascades, using as the initial splitting factor values 0.5, 1.0, and 1.5 for all stages, are discussed. Increasing the number of stages increases the membrane surface area and the total work consumption owing to the increase in recycled stream flow. In the case under examination, both the efficien cy index and the specific efficiency decrease for cascades of more tha n five stages. Therefore membrane staging seems to be unattractive fro m the point of view of both capital investment and operating costs, at least for more than a certain number of stages, depending on the spec ific problem.