Detailed Examination of Energy Flows and Entropy Generation in Low Pressure Binary Distillation Columns

Abstract

Variations of the heat flows and entropy generation for binary distillation systems are investigated with varying feed composition, feed condition, and feed stage for a fixed number of stages and product purities, along with the effects of imposed irreversibilities. For ambient feed and products, both full system and column-only results were obtained using AspenPlus V9 for the systems benzene–toluene and methanol–water at one bar pressure. An optimum feed stage exists at the maximum driving force composition. Reboiler heat input is strongly correlated with reflux rate and with feed enthalpy. While system heat input is linearly correlated with entropy generation, the relation of reboiler heat to entropy generation depends on the selected material basis. Full system energy input and entropy generation are minimized for cold feed, though they are maximized when considering only the column. Entropy generation for heat transfer over a 5 K temperature difference is equivalent to tray efficiencies reduced to 60%.