Dynamics And Control Of Reactive Distillation Columns With A Top-Bottom External Recycle

Reactive distillation integrates the reaction operation and separation operation into a single operating unit,which reduces equipment investment and operation costs compared with the conventional chemical process.This advantage makes the research of reactive distillation imperative.However,reactive distillation cannot be applied to all reactive systems.When dealing with the most unfavorable ranking of relative volatilities,the reaction section of the conventional reaction distillation column(CRDC)featured with the totally refluxed and totally reboiled operation mode and an intermediate product withdrawal is located at the bottom of the distillation column and the reactants feed from bottom and middle.The structure of CRDC makes the reaction operation and separation operation inconsonant,resulting in the poor control.For this case,a reactive distillation column with a top-bottom external recycle(RDC-TBER)can be used.In this structure,unreacted reactants(the relative volatility being the lightest and heaviest,tending to aggregate at the top and bottom of the RDC-TBER)are further reacted by returning to the reaction zone through the external recycle,thus greatly improving the thermodynamic efficiency of the system.However,RDC-TBER is equivalent to the CRDC’s structure refurbishment,which causes the dynamic characteristics and control scheme of RDC-TBER to remain unknown.Therefore,it is necessary to study this issue.This topic studied in detail the dynamic characteristics and control of RDC-TBER when dealing with the most unfavorable ranking of relative volatilities,with particular attention to the effect of external recycle on the system.In the course of the study,RDC-TBER is controlled.There are 6 operating variables for RDC-TBER:1.reflux flow rate,2.condenser heat duty,3.the heaviest reactant feed flow rate,4.the intermediate product flow rate,5.reboiler heat duty,6.external recycle flow rate;the first six operating variables control the level of condenser,tower pressure,the heaviest component composition on sensitive stage,the composition in the side product,and the level of reboiler,respectively.External recycle flow rate is used as an operating variable.Two different control schemes are proposed in this paper.They are:1.External recycle flow rate is controlled by flow rate,and it is fixed near steady-state value;2.External recycle flow rate controls the intermediate product composition.Studies have shown that,through the structure of the external recycle,the dynamic characteristics and controllability of the system have been significantly improved compared to CRDC.Two reactive distillation systems,executing a hypothetical quaternary reversible reaction and the lactic acid esterification with methanol,are employed to inspect the dynamics and controllability of the external recycle.In the same disturbance conditions(feed flow rate disturbance,feed component disturbance,product concentration disturbance),the control effect of RDC-TBER is better than that of CRDC.At the same time,in RDC-TBER,the control scheme of the concentration of the intermediate product in the external recycle flow rate control has a better control effect than the external recycle flow is constant.The solution has a better control effect than the external recycle flow rate using flow rate control.The studies in this paper demonstrate that RDC-TBER is very effective in improving the dynamic characteristics and control of the system in dealing with the most unfavorable ranking of relative volatilities.