Process Optimization And Control Strategy Of Seperating Dimethyl Carbonate-methanol Azeotropic Mixture Using Special Distillation

It is difficult to separate the dimethyl carbonate(DMC)-methanol azeotrope mixture with traditional distillation for obtaining high purity products.In this paper,pressure swing distillation(PSD)and extractive dividing wall column(EDWC)were proposed for the separation of DMC-methanol system by Aspen Plus and Aspen Plus Dynamics.For the PSD separation technology,the optimum design for different heat integration processes were achieved by minimization the total annual cost(TAC)as the objective function,and the minimal TACs of no,partial and full heat integration pressure swing distillation are 323,892.509 $/y,259,093.925 $/y,248,353.238 $/y,respectively.Aiming at the problems of choosing the suitable sensitive plate caused by the uniform temperature distribution of the low-pressure column(LPC),a simple and effective scheme was put forward.The influences of the selection of the sensitive stage in the LPC on the dynamic responses in the different heat integrated processes were further studied on the base of rigorous steady-state simulation.The results showed that the composition-temperature cascade control structure have some superiorities in the aspect of handling the ±20% disturbance of feed flowrate and feed composition of the PSD with no and partial heat integration effectively.As to the best temperature control stage in the LPC,the stage 31 or 30 was selected as the control stage for the no heat integration PSD,however the stage 31 or10 was chosen for the partial heat integration PSD.For full heat integration PSD,thepressure-compensation temperature control structure with stage 29 in the LPC as the control stage should be adopted to achieve effective control.Extractive dividing wall column(EDWC)was proposed based on the extraction distillation(ED)process.EDWC simulated as a three-column model was studied in this paper,optimal operating parameters of the three-column model were obtained according to the optimization with sequential iterative method,and the corresponding TAC was 189,944.75 $/y.During the dynamic control system,aiming at the problems of choosing the suitable sensitive stage of the main column,the derivative slope criterion was used to screen the control stage location.The dynamic simulation results revealed that the improved control structures with vapor split ratio with stage 12 in the main column selected as the control stage showed effective control performance.As to the stage 5 in the main column selected as the control stage,control structure without vapor split ratio could maintain the product purities at their set points.And the control structure without vapor split ratio as a manipulated variable,because of its more practicality in industrial production,was chosen rather than the one with vapor split ratio.