Asymmetrical temperature control of a BTX dividing-wall distillation column

Abstract

On account of the intensive interaction between the prefractionator and main distillation column involved, achieving tight product purity control with a small set of temperature measurements represents a considerably challenging issue in the derivation of temperature inferential control systems for dividing-wall distillation columns (DWDCs). Although double temperature difference control (DTDC) scheme appears more effective than temperature control and temperature difference control schemes, the former should not always be considered as the best control decision under the premise of maximally three temperature measurements in each control loop. Instead of a symmetrical temperature control (STC) structure as the DTDC scheme, an asymmetrical temperature control (ATC) structure with each control loop involving a different number of temperature measurements can be derived and lead to not only a simplified control structure but also improved control system performance. In the current work, such possibility is highlighted with reference to a frequently studied benzene–toluene–o-xylene DWDC system. With the perfect maintenance of the four controlled product compositions on their specifications, the three kinds of controlled variables, i.e., temperature, temperature difference, and double temperature difference, are assessed with reference to all kinds of feed composition disturbances and the one with the minimum averaged variability is screened out for each control loop. In combination with the interaction analysis, three ATC schemes are finally derived. Closed-loop evaluations of these ATC schemes are carried out through in-depth comparison with the DTDC scheme and the former ones all show comparable transient responses and improved steady-state behaviors in comparison with the latter. Although the three ATC schemes result in relatively great steady-state discrepancies in the intermediate product purity, the drawback can be suppressed completely with the addition of two temperature measurements in the intermediate control loop. These findings demonstrate the feasibility and potential of adopting the ATC schemes for the tight inferential control of the DWDC.