Design And Control Of Self-Heat Recuperation Distillation Process

As one of the most widely used separation methods,distillation requires a great deal of energy that accounts for 40-60%energy requirement of the chemical and petrochemical industries and 3%energy consumption of the world.Energy savings of distillation processes have been continuously attracting extensive attention as the energy shortage problem becomes more and more serious in the world.The self-heat recuperation technology（SHRT）was developed based on the self-heat recuperation concept,and an energy-efficient self-heat recuperation distillation（SHRD）was proposed through combining the SHRT with distillation process.The SHRD process utilizes adiabatically compression to achieve heat exchange between streams,through which the low-temperature heat source can be recuperated,and energy requirement of the SHRD process can be dramaticlly reduced.However,more interests until recently have been only focused on the concept design of the SHRD,and the implementation methods for SHRD design in different separation systems are insufficient.Besides,the optimization and control of the SHRD processes have received little attention.If the SHRD configurations can be proposed according to different system features,as well as optimized and controlled,it would be meaningful for the application of SHRD processes.Based on the current research,this dissertation performs the SHRD modeling,optimization and control for three different separation mixtures,and the detailed contents are listed below:The rigorous models of basic distillation,vapor recompression distillation and SHRD were performed based on the separation of close-boiling n-butanol/iso-butanol mixture.The processes were optimized using total annual cost（TAC）as objective function.Then three performance indexes,namely TAC,thermodynamic efficiency and carbon dioxide（CO₂）emissions were calculated for the optimum processes.It shows that among the three processes,the SHRD design has the lowest TAC and CO₂ emissions,and the highest thermodynamic efficiency.The feed disturbances were analyzed for this promising SHRD sequence,based on which two control structures were proposed.Results show that the improved control strategy with dual-temperature control can handle the disturbances effectively for this SHRD system.The residue curve map was used to analyze the possibilities of separating ethanol/water mixture using benzene as entrainer.The steady-state models of conventional azeotropic distillation,self-heat recuperation azeotropic distillation（SHRAD）and extractive distillation were simulated.Then the optimization objective and design variables of the processes were analyzed,and a multiobjective genetic algorithm with constraints was implemented to obtain rigorous Pareto fronts of the three sequences.The TAC,thermodynamic efficiency and CO₂emissions of the design points were calculated based on the Pareto fronts.The calculation results show that there exists an optimum point in the Pareto fronts which has the lowest TAC and CO₂ emissions,as well as the highest thermodynamic efficiency.The SHRAD process has greater advantages in TAC,thermodynamic efficiency and CO₂ emissions aspects than the conventional design.Besides,the energy consumption of SHRAD process is lower than that of extractive distillation,and the thermodynamic efficiency of SHRAD process is also higher than that of the extractive distillation.The dynamic characteristics of the optimum SHRAD sequence had been studied and three control structures were proposed.The dynamic responses demonstrate that the product purities can be maintained near the original values,and the control effects are not inferior to those performed in the conventional process.It is proven that although the SHRAD system has complicated structure,it can operate normally with good controllability.Therefore,it is possible for the industrial application of the SHRAD design.The rigorous model for a conventional three-column reactive distillation was simulated for methyl acetate hydrolysis process.Then a self-heat recuperation reactive distillation（SHRRD）,which included the integrating of high-pressure（HP）and low-pressure（LP）columns,was proposed based on the operating characteristics of the conventional reactive distillation column.The two processes were optimized within TAC as objective function,and then the performance indexes were calculated.Results show that,compared with the conventional one,the SHRRD design has lower TAC and CO₂ emissions,and the thermodynamic efficiency is higher due to the self-heat recuperation between the HP and LP columns.Two control structures were proposed for the SHRRD design.After adding a temperature control loop to the LP column,the improved control structure can handle large feed disturbances and keep products at their desired purities,which show that this attractive SHRRD process could be operated normally with reasonable control.