Multi-objective Optimization Research Of Pressure Swing Distillation And Extractive Distillation Based On Economical Efficiency And Controllability

Chemical process design and control are two important issues,and it is necessary to establish a multi-objective optimization framework to integrate the design and control.This integrated process design approach relies on the fact that the achievable plant dynamic performance is a property inherent to process design.Pressure swing rectification(PSD)and extractive rectification(ED)are primary methods for the separation of industrial nonideal mixtures.In ED process,the use of ionic liquids instead of organic solvents can avoid environmental and safety problems related to the organic solvents.Up to now,the synthesis of PSD and ED processes mostly employs traditional design procedures,in which control factors are considered after the completion of process design that is conducted by considering steady state,economic objectives and process constraints.There is no study to evaluate the control property of PSD and ED processes during process design and optimization phase.In this paper,the optimization of PSD and ED processes are conducted to integrate design and control.Decision space is formed by the condition number(CN)and the total annual cost(TAC)of each design,and the CN and the TAC are used to evaluate the evaluation of the control properties and each design,respectively.Based on the second generation non-dominated sorting genetic algorithm,a multi-objective optimization strategy was established to deal with such optimization problem,and a corresponding joint optimization platform between Aspen plus and Matlab was established for performing process optimization.Based on established optimization strategy,PSD processes for the separation of ethanol/tetrahydrofuran(I)and methanol/chloroform(II)systems are designed and optimized,organic solvent based extractive distillation(OED)processes for the separation of I and methanol/acetonitrile(III)systems are designed and optimized,and ionic liquid based extractive distillation(ILED)processes for the separation of I,II and III systems are designed and optimized.Firstly,based on phase diagram tool,the thermodynamic feasibility analysis and the synthesis of PSD,OED and ILED processes are performed.Then the established multiobjective optimization platform is used to optimize these processes to determine free design variables of theses processes and to obtain the Pareto fronts in the decision space.The Pareto fronts offer trade-offs betwee the control properties and the design,namely,the reduction of TAC of these processes simultaneously increases their CN,which provides a trade-off decisionmaking space for the designs of PSD,OED and ILED processes.Simultaneously,the energy consumption and environmental impact potential(EIP)of all optimized designs on the Pareto front are also evaluated.Designs including a best design with smallest TAC(PT),a trade-off design between TAC and CN(PO)and a best design with smallest CN(PC)on the Pareto front are analyzed,and the results show that increasing the reflux ratio of distillation column can help to improve the control property of PSD,OED and ILED processes.But these increase meanwhilely lead to higher equipment investment cost and energy consumption,so TAC and EIP are also inceased.Then dynamic simulations are performed to compare the dynamic response performance of PT and PO designs under feed disturbance.The comparison results of the response of maximum deviation,final stability deviation and fluctuation show that the control property of PO is better than that of PT.In addition,considering the thermal stability of ionic liquid,the vacuum systems required for the recovery of the ionic liquid by vacuum operation in the ILED process are designed.The ILED processes has significant advantages such as lower energy consumption,better environmental benefits and economic benefit compared with the PSD and OED processes.Specificcally,the steam load and TAC of ILED process are significantly reduced,the reduction effect of the TAC is weakened when the costs related to vacuum operation are considered,and the economic advantage decreases as vacuum degree increases.Hence,it is necessary to consider the costs related vacuum operation when performs economic evaluation of ILED schemes.In the extractive distillation process,the amount of organic solvent is several times that of ionic liquid,and ionic liquid is more environmentally friendly,therefore ILED process is superior OED scheme.In addition,heat pump-assisted ILED configurations are designed to further reduce energy demand,and the results show that the innovative configurations have significant savings in energy consumption and TAC.