Control of a Column/Pervaporation Process for Separating the Ethanol/Water Azeotrope

The pervaporation process features a liquid feed and retentate and a vapor permeate. The phase change produces a temperature decrease as the retentate flows through the unit. Since flux rates decrease with decreasing temperature, the conventional pervaporation unit consists of several membrane modules in series with interstage heating. The vapor permeate must be condensed for recovery and recycle, and refrigeration is usually required. Hybrid systems of distillation columns and pervaporation units are frequently used in situations where distillation alone is impossible or very expensive. Despite the many papers dealing with pervaporation, the issue of dynamic control seems to be almost completely unexplored. That is the purpose of this paper. A hybrid column/pervaporation process is studied that is designed to produce 99.77 wt % ethanol from a feed stream of ethanol/water mixture with composition near the azeotrope. The control objective is to maintain the purity of the ethanol product retentate stream in the face of disturbances in feed flowrate and feed composition. There are two possible manipulated variables: permeate pressure and retentate temperature. Permeate flux is increased by decreasing permeate pressure or increasing retentate temperature. A simplified dynamic pervaporution model is developed that captures the essential features of the process using energy and component balances along with overall pervaporation performance relationships. Dynamic simulations are used to demonstrate the effectiveness of a control structure that uses a cascade composition/temperature structure. A simple process modification is shown to improve controllability.