Dynamics And Control Of Pressure-Swing Distillation Process For Azeotropic Systems Of Methanol/Ethanol-Tetrahydrofuran

The separation using pressure swing distillation (PSD) rose the attention of researchers due to the advantages of no third component introduced and high purity. Design parameters and equipment conditions of PSD with heat integration were determined based on the optimal steady state simulation from our previous studies. Dynamics of PSD process were analyzed using simulation software of Aspen Plus Dynamics to determin the schemes with strong anti-disturbance ability.Two peaks were found in the temperature slope curve for the methanol-THF system with feed composition of 75mol%THF. Anti-disturbance ability of control schemes with two different temperature control stage was investigated, the composition/temperature cascade control structure can handle ±20% feed flow and feed composition disturbances well. The selection of the temperature control stage has certain influence on the dynamics, mainly on the way of variation of the purity of the target product when disturbances occur. The product purity has a fluctuation but it can promote the balance of product purity when theoretical stage with smaller value of the temperature slope was choosed. The anti-disturbance ability of the control schemes was influenced due to the high coupling of full heat integration process, which the disturbance of the former column can easily transfer to the latter one for the system with feed composition of 25mol%THF. Results reveal that pressure-compensated temperature control shows the advantage of strong controllability for process of heat integration with the decreased degree of freedom.Systems of ethanol-THF with feed composition 90mol%THF,50mol%THF and 10mol%THF were studied. In the case of 90mol%THF, temperature control stage was determined based on the slope criterion and the sensitivity criterion. More robust control was achieved when two temperature controllers are added into the LPC to controll the reflux and reboiler heat input, respectively. In the case of 50mol%THF, QR2/F control structure can not improve the control effectively, but the time dealing with disturbances was shortened and a quicker balance was achieved when QR2/F structure was added on the basis of pressure -temperature compensation control structure. In the case of 10mol%THF, the pressure-temperature compensation control structure can handle feed flow and feed composition disturbances well.Dynamics and control structures of PSD studied in this paper have certain significance for realizing the automation of industrial production and the improvement of the control level for the seperation of methanol-THF and ethanol-THF systems. The results can be used to guide the general rule of dynamic characteristics in the separation of two binary azeotropic systems using PSD.