Multiple Steady States In Intermediate Entrainer Separation Process

Multiple steady states in intermediate entrainer separation process are studied in asystematic manner. On the basis of∞/∞analysis and sensitivity analysis tool inAspen Plus, two stable steady state solution branches can be easily traced. As for theunstable steady state solution branch, sensitivity analysis cannot always work well,even if pretty well estimates have been provided.“Design Specs” in Aspen Plus hasbeen proved to be effective in finding unstable steady state solution. For columns withfinite number of trays operating at finite reflux, the numerically constructed productscomposition bifurcation diagrams show erratic behaviors in the vicinity of two turningpoints. Qualitative and quantitative comparisons between steady state simulationresults and∞/∞analysis perdition results for this intermediate entrainer separationprocess have been made. It has been found that the predicted presence of internal statemultiplicity by∞/∞analysis does not exist in reality. For this intermediateentrainer separation process, when product quality specifications are specified, locatethe design point or operating point on the unstable steady state solution branch maygreatly cut the cost, thus it is desirable to find out a suitable control structure tostabilize this more economical operating point. Three control structures usingconventional PI controllers have been presented. By dynamic simulation, it turned outthat the conventional control structure cannot handle feed flow rate disturbances wellas a result of the pressure effect. To overcome the adverse effect of pressurefluctuation, differential temperature control and pressure—compensated temperaturecontrol have been proposed. The dynamic simulation results reveal that both controlstructure work pretty well and show somewhat similar dynamic performances, evenfor feed flow rate and feed composition disturbances. Comparison of dynamicperformances between differential temperature control and pressure—compensatedtemperature control are also made.