(35d) Novel Hybrid Distillation-Membrane Analysis Using Thermally Coupled Columns

A continuous membrane column involves the recycle of some permeated material, and operates in a counter-current manner. The assumption of constant molar overflow is not applicable here. To date, membrane columns have yet to find their place in industrial practice, even for simple configurations. The most probable reason for this is that the design of a membrane column is complex. Any membrane column, no matter how complex its configuration, can be divided into column sections. The Difference Point Equation (DPE) has previously been derived for constant molar overflow counter-current column sections. A more general form of the DPE has been derived to describe to retentate composition down the length of a section. For demonstration purposes, a simple Knudsen membrane is assumed. Using this as the permeation model, in conjunction with the DPE, allows one to graphically plot a column profile for a given initial condition. This allows one to not only configure simple columns, but complex configurations too. As an example, a thermally coupled membrane column will be discussed, using the same membrane throughout. Through simplifying assumptions, the system can be reduced to coupled column sections. Material balance constraints have to be satisfied in order for the configuration to be feasible. It was found that a thermally coupled membrane column is in fact not possible under any operating conditions. Although not a positive one, this is a very useful result, since both time and money can be saved by avoiding any further investigations. Another more useful and practical design is that of a membrane thermally coupled to a distillation column. In much the same way as before, this hybrid process was analyzed. Under certain conditions, there exists a region which makes such a configuration viable. An understanding into the behavior of both the membrane as well as the distillation profiles will allow one to avoid poor initial decisions and designs. Furthermore, this method introduces a novel approach to analyzing hybrid processes.