Design And Control Of Extractive Distillation And Pressure-swing Distillation Separation Sequences For Multiazeotropic System

There are many mixtures in the chemical industry that can form two or more azeotropes,these must be effectively separated to avoid environmental pollution and waste of resources.Extractive distillation and pressure-swing distillation are common methods for separating azeotropes,it is significant in the aspects of optimal design and dynamic control for the separation of multi-component systems with multiple azeotropes.In this paper,the design,simulation,optimization and control of extractive distillation and presssure-swing distillation are investigated,take methanol-acetonitrile-benzene and THF-ethanol-water ternary systems with mutliazeotropes as two examples.In the research about extractive distillation for separating a methanol-acetonitrile-benzene system,chlorobenzene is selected over aniline as a feasible solvent according to thermodynamic properties;Four conventional extractive distillation separation sequences and ten thermally coupled extractive distillation separation sequences are proposed on the basis the effect of solvent chlorobenzene has on the relative volatility of components in the system,and are optimized by the sequential iterative search method with the target of minimal total annual cost(TAC).The minimal TAC in the conventional extractive distillation separation sequences is 6.806×105$/a,that of the thermally coupled extractive distillation separation sequences is 5.834×105$/a and another promising thermally coupled extractive distillation separation sequence is 6.380×$/a.Finally,the control structures of the above three separate sequences are established to resist against two disturbances of feed flow and feed composition,and the effectiveness and robustness of these control structures are verified via the dynamic simulation on the platform of Aspen Dynamics.In the study on PSD for separating the THF-ethanol-water system,six conventional PSD separation sequences(WET,WTE,TEW,TWE,ETW and EWT)are proposed according to the pressure sensitivity analysis of azeotropes;Two PSD separation sequences(WET and TEW)with boundary-crossing are proposed based on the highly-curved distillation boundary nature;Furthermore,the above separation sequences are optimized and designed on the basis of minimal TAC via sequential iterative search procedure.The optimization results show that the TAC of the optimal conventional PSD separation sequence is 11.81×106$/a and that of the two crossing boundary PSD separation sequences are 12.27×106$/a and 12.13×106$/a,respectively,which can cause a reduction of 18.47%and 26.17%compared to the conventional sequence of the same product sequence,respectively.In addition,the various heat integrated schemes for the three PSD separation sequence are carried out and significantly reduced energy costs and economic costs are obtained.On the basis of steady state design,the dynamic control and operability of different heat integrated sequences is investigated.The results show that although the conventional PSD separation sequence(WTE)has better economical efficiency and thermal integration which can significantly save energy and economic costs,however the crossing boundary PSD separation sequences have better operational performance and controllability.The control performance of such thermal integration schemes is also weakened with the increase in integration degree and the reduction in freedom degree.