Analysis of Local Recycle for Membrane Pervaporation Systems

We present an analysis of local recycle in membrane pervaporation processes. Local recycle (reheating and recycling part of the retentate from a membrane unit back to the inlet) can mitigate the problem of temperature drop in pervaporation units. The excess fluid acts as a thermal carrier, increasing the temperature. However, a trade-off occurs because this recycle also decreases the concentration of the more permeable species on the retentate side of the membrane. We present a method based on dimensional analysis that can be used to quickly determine whether local recycle around a single membrane unit is desirable. We show that membrane modules can be classified into one of three types: local recycle is not desirable, local recycle is desirable with an intermediate recycle ratio, and local recycle is desirable with the maximum possible recycle ratio. The method is illustrated using three case studies, two of which are based on hybrid distillation/pervaporation processes. The results indicate that the correct application of internal recycle can significantly improve efficiency and reduce cost.