Abstract
The equilibrium-stage concept has often been used in the design of distillation and absorption processes. Departure from equilibrium is accounted for by tray efficiency and height equivalent of a theoretical plate (HETP). However, inaccurate efficiencies or HETP predictions are accounted for by rule-of-thumb safety factors. Explicit consideration of mass transfer using a rate-based approach is found to be critical when mass-transfer driving force is very low. In this study, the effects of mass transfer on the design of an extractive distillation process for the separation of DMC/MeOH azeotrope are presented. Vapor-phase resistance is found to be dominant in the extractive distillation column. Moreover, HETP in the rectification section of the column is substantially higher than experimental data reported in the literature. This can be attributed to not only the high purity requirement of methanol but also more importantly the fact that overhead methanol is a saddle rather than an attractor in the residual curve map. Therefore substantial reduction in column height can be achieved by designing individual sections using the rate-based model. Use of packings with better mass-transfer characteristics can also substantially reduce the height of the rectifying section.