Multiple Steady-states Analysis And Control In Entrainer-enhanced Reactive Distillation Process

Entrainer-enhanced reactive distillation is the process of adding entrainers to the reaction distillation system,which can effectively solve the problem that the reaction conversion is limited by the formation of the azeotrope of the reactants with the partial product during the reaction distillation and the high temperature of reaction section caused by the high boiling point component and other issues.However,the introduction of entrainers,will inevitably lead to more non-linear system,and even multi-steady-state phenomenon,to the distillation of the operation and control of many problems.When the system has multiple steady states,for a special disturbance,the profile of the column may jump from a desired steady state to an undesired steady state,causing the irregular behavior of the distillation column,and calling for higher demand to the design of the control scheme for the distillation column.N-butyl acetate is an important solvent in the chemical industry.In this paper,the production of n-butyl acetate was studied by using entrainers to carry out systematic multistable analysis and control.Firstly,the Aspen Plus software sensitivity analysis tool was used to analyze the six key operating parameters of the systems,such as reboil ratio,butanol feed rate,reflux ratio,bottom flow rate,operating pressure and reboiler duty.Through the bifurcation diagram to the purity of product,we found that the input or output multiple steady states occur in entrainer-enhanced reactive distillation process under certain operating conditions,with a severe nonlinear dynamic behavior.Secondly,the steady-state and dynamic simulation of the internal temperature and composition profile of the distillation column are analyzed based on the nonlinear wave theory.The multiple steady-state phenomena are explained from qualitative analysis and quantitative calculation.The results show that the non-monotonic change of the reaction conversion rate or wave velocity with the wave position is the main reason for the process of steady state.Finally,the dynamic control performance of the three control structures is investigated by Aspen Dynamics software.Through the dynamic simulation,it is found that the control structure that the sensitivity tray temperature is controlled by the ratio of the two reactants shows great superiority in resisting the feed disturbance and maintaining the stability of the system.In addition,the temperature difference control structure can significantly reduce the steady-state tolerance and overshoot of the product purity in entrainer-enhanced reactive distillation,can better handle the feed flow and composition disturbance,compared with conventional temperature control,achieving better control effect.