Effective Derivation of Asymmetrical Temperature Control Schemes for Dividing-Wall Distillation Columns

Abstract

The potential of adopting asymmetrical temperature control (ATC) schemes to tightly control dividing-wall distillation columns (DWDCs) has yet to be fully explored due to the following two problems. First, the inference accuracy toward the controlled product purity of the candidate controlled variables (CCVs) synthesized for each control loop was not high enough. Second, the determination of the most appropriate CCV for each control loop lacked a sufficient theory basis. To alleviate these two problems, two measures are specially adopted in the current work, thereby forming a systematic method for deriving the ATC schemes. First, the creations of a double temperature difference (DTD) and temperature difference (TD) are reformed with the aid of a recently defined performance metric, the averaged absolute variation magnitudes (AAVMs). Namely, DTD is generated according to a novel method based on the AAVMs, rather than the conventional method based on the sensitivity analysis and SVD analysis, and TD is modified in form through multiplying the temperature of the reference stage by a coefficient that equals the ratio of the AAVMs of the sensitive stage and the reference stage. Then, a sequential search procedure is proposed to help determine the most appropriate CCV for each control loop from all of the synthesized CCVs with a combined consideration of the inference accuracy to the controlled product purity and the interaction between control loops. In terms of the operation of a benzene–toluene–o-xylene DWDC, the proposed method is assessed by means of a thorough comparison between the derived ATC scheme and the double temperature difference control scheme. In comparison with the latter, the former displays not only relatively better transient responses but also smaller steady-state deviations in the three controlled product qualities. These findings demonstrate the effectiveness and feasibility of the method proposed for deriving ATC schemes.