Currently, process intensification
using dividing-wall columns
(DWCs) is one of the most promising alternatives for reducing the
costs of the distillation process. However, for extractive distillation,
there are still questions regarding whether the DWC is a more economical
option than the conventional sequence (CS). Normally, extractive DWCs
are simulated with two thermally coupled columns (TCSs), and their
designs are usually obtained by setting the TCSs in different ways
that do not necessarily constitute an optimum design. Thus, this work
proposes a systematic procedure based on stage equilibrium for obtaining
an optimized DWC configuration, in terms of operability and design.
A strict comparison between the DWC configuration and the CS (also
optimized) was performed, and the best results in terms of the total
annual cost (TAC) were obtained for columns with a distinct number
of stages in each section of the wall; however, these columns did
not outperform the optimized conventional systems.
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.
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