A Simplified Conceptual Design Methodology for Double-Feed Extractive Distillation Processes

Shortcut methods have an important role in the conceptual design of distillation processes because they rapidly provide numerical and qualitative constraints for important design parameters such as minimum reflux ratio, number of stages, process feasibility, possible splits, and, specifically for extractive distillation, minimum solvent feed ratio and maximum reflux ratio. The majority of past research in this area has dealt with single feed columns, with very little emphasis being placed on double feed extractive distillation processes.

In this work, a simplified method using reversible distillation models has been applied to the conceptual design of double feed extractive distillation columns. The proposed methodology uses the boundary value method (BVM) and the constant molar overflow (CMO) assumption for modeling the extractive column. The primary novel contribution of this work is the use of continuation methods for calculating the curves that determine the feasibility region and all the design parameters that can be derived from such a calculation.  Combining the continuation methods with the column equations yields a simple method for calculating the pinch curves that border the feasibility region. We have found that these extractive distillation pinch curves, in addition to determining the feasibility region, are also useful for calculating an approximate minimum feed ratio independent of other parameters, as well as accurately pinpointing the minimum and maximum reflux ratios. Applications on extractive distillation systems allow us to conclude that the method is reliable, fast, and much easier to implement than other methods presented in the literature.