Effect of Solvent Content and Heat Integration on the Controllability of Extractive Distillation Process for Anhydrous Ethanol Production

Considering that an analysis of both dynamic behavior and controllability of an extractive distillation process is important, and also taking into consideration the absence of studies in literature related to the influence of heat integration and solvent content on the controllability of this process, this work aims to investigate such aspects, or more specifically, investigate the dynamic responses in the presence of feed disturbances when operating at high, medium, and low solvent content. The production of anhydrous ethanol as the main product with ethylene glycol as solvent was the case study in this work, and a control scheme based on recent studies was used to perform this study. It was verified that the solvent content in the extractive section has an influence on top product composition when feed flow rate and composition disturbances are introduced in the azeotropic mixture feed stream; the system with high solvent content presented the highest offsets. The results also showed that the presence of heat integration causes considerable changes in the temperature of the recycled solvent stream when disturbances occur in the system feed flow rate, affecting the purity of the top product. A modification in the control scheme was proposed for the system with high solvent content, significantly improving the control of ethanol purity. The new control scheme includes two temperature control loops in the extractive column and a temperature control loop in the recycled solvent stream, using a bypass stream around the heat exchanger. It was also verified that the dimensions of the extractive column and reflux vessel are smaller when the system is configured to operate with a high solvent content.