Multi-objective optimization and composition control of Sargent dividing-wall distillation columns

Abstract

Sargent dividing-wall distillation columns (DWDCs) can significantly reduce the energy consumption required for separating quaternary mixtures. However, their effective design and strict product purity control can be challenging to achieve, which severely limits their development and application in the industry. To address this problem, a systematic study on their design and control was conducted in this article based on the case of separating a quaternary mixture of methanol, ethanol, 1-propanol, and n-butanol. Firstly, a multi-objective optimization framework was proposed, in which the total annual cost (TAC), CO2 emission, and condition number (CN) were taken as objective functions, and NSGA-III and TOPSIS algorithms were employed to search for the optimal design of the Sargent DWDC. To avoid the convergence difficulty issue of the rigorous model of the Sargent DWDC, a Random Forest surrogate model was used in the optimization process to predict its steady-state and dynamic performance accurately and rapidly. Secondly, a novel composition control structure, consisting of seven control loops, was proposed to achieve strict product purity control of the Sargent DWDC. Three composition difference control loops, located at the prefractionator and the middle column, are responsible for maintaining their composition profile shapes and thus mitigating the impact of feed disturbances on the main column. Additionally, four composition control loops, located at the main column, are responsible for strictly controlling the purities of the distillate, upper sidestream, lower sidestream, and bottom products. Dynamic simulation results showed that the Sargent DWDC can be operated stably under the proposed composition control structure for various feed composition and feed flow rate disturbances. These results not only confirmed the capability of the proposed composition control structure to strictly maintain product purities in the Sargent DWDC but also validated the effectiveness of the proposed multi-objective framework in achieving an energy-efficient and controllable Sargent DWDC.