Operation of Dividing-Wall Columns. 1. A Simplified Temperature Difference Control Scheme

Abstract

A simplified temperature difference control (STDC) scheme, consisting of two temperature and two temperature difference control loops, is proposed for the operation of dividing-wall columns (DWCs). The two temperature control loops are located, respectively, in the rectifying and stripping sections of the main distillation column and work to maintain the top and bottom product qualities. The two temperature difference control loops are arranged, respectively, in each side of the dividing wall with their temperature measurements arranged above and below the locations for introducing feed to the prefractionator and for withdrawing the intermediate product from the main distillation column. These special arrangements serve to keep a certain degree of separation of the fed mixture in the prefractionator and the purity of the intermediate product, tightening product quality control and rendering the STDC scheme with high robustness to the changes in the thermodynamic properties of the ternary mixtures separated. Separation of three ideal ternary mixtures of hypothetical components, A, B, and C, with different ease of separation indexes (ESIs) is chosen as illustrative examples to evaluate the proposed STDC scheme and thorough comparison is conducted with the currently available temperature control (TC) and temperature difference control (TDC) schemes. It is found that the STDC scheme is much superior to the TC scheme irrespective of the great changes in the ESIs. Although with relatively lower instrumentation cost and smaller engineering effort, it can generally lead to performance comparable with the TDC scheme. Application of the STDC scheme to the DWC separating a ternary mixture of ethanol, propanol, and butanol is also attempted and similar outcomes are obtained. Since the STDC scheme behaves in good accordance with the working principle of the DWC, it should be viewed as a general and effective strategy for the operation of the DWC.