Comparison of Control Strategies for Dividing-Wall Columns

Abstract

Conventional ternary separations progressed via thermally coupled columns such as Petlyuk configuration to a novel design that integrates two distillation columns into one shell—a setup known today as dividing-wall column (DWC). The DWC concept is a major breakthrough in distillation technology, as it brings significant reduction in the capital invested as well as savings in the operating costs. However, the integration of two columns into one shell leads also to changes in the operating mode and ultimately in the controllability of the system. Although much of the literature focuses on the control of binary distillation columns, there are just a few studies on the controllability of DWC. In this work we explore the DWC control issues and make a comparison of various control strategies based on PID loops, within a multiloop framework (DB/LSV, DV/LSB, LB/DSV, LV/DSB) versus more advanced controllers such as LQG/LQR, GMC, and high order controllers obtained by H∞ controller synthesis and μ-synthesis. The controllers are applied to a dividing-wall column used in an industrial case study—the ternary separation of benzene−toluene−xylene. The performances of these control strategies and the dynamic response of the DWC is investigated in terms of product composition and flow rates, for various persistent disturbances in the feed flow rate and composition. Significantly shorter settling times can be achieved using the advanced controllers based on LQG/LQR, H∞ controller synthesis, and μ-synthesis.