Optimization And Control Of Separation For Toluene-ethanol Azeotropic System With Special Distillation

In this paper, two methods of pressure swing distillation (PSD) and extractivedistillation (ED) are applied to separate toluene-ethanol azeotropic system. Theresearches for steady state economic optimization and control structure of the twoprocesses are carried out with Aspen Plus and Aspen Dynamics, respectively.For PSD, the global optimization method using the TAC as the objectivefunction is developed, a sequential iterative search is used to calculate and obtain theoptimum PSD economic cases with the no heat integration, the partial and the fullyheat integration between high-pressure column (HPC) and low-pressure column(LPC) operated in different pressures. By comparing the optimal economic cases ofevery kind of PSD, it is found that the equipment costs in the no heat-integrated PSDis the most, the operation cost in the partial heat-integrated PSD is the minimum,total annual cost (TAC) in the fully heat-integrated PSD is the minimum. The threePSD processes with different control structure are established step by step anddynamic performances are tested and analyzed using Aspen Dynamics software. Theresults reveal that no heat-integrated PSD and partial heat-integrated PSD canhandle flow and composition disturbances well by adding control structures of thecomposition/temperature cascade and a pressure-compensated temperature,respectively. While in the fully heat-integrated PSD, the developed control structurecan handle only a relatively small flow disturbance because of the highly couplingof the two columns and the decrease in controllable degree of freedom.For ED, equal relative volatility curve is plotted using Flash2and the relativevolatility of the two key components under different feed ratios is studied. N-butyl benzene (NBB) was selected as a suitable extractive solvent. A method of globaloptimal iteration is developed for this process, and the optimal design parametersand operating conditions for the ED process are calculated through iterativecomputation. Dynamic state simulation and analysis for several control structuresare proceeded. The responses results reveal that control structure with fixed refluxratio cannot handle feed composition disturbances well. The control structure withfixed R/F and QR/F ratio can not only deal with feed composition disturbances quitewell but also has a faster response speed for feed flow disturbance, while it cannotmaintain purity of top product to the desired values. A final control structurecontrolling top reflux and reboiler heat input simultaneously is developed by addingan additional temperature controller, which makes the whole ED process can becontrolled more robust.By the comparison for the economic efficiency and controllability of the twoseparation processes, it is found that the ED process has the minimum TAC and abetter control performance for the case with the same feed composition disturbances.