Optimization And Realization Of Manipulated Variable Reference Trajectory In Process Transition Of Distillation Column

The chemical process system is generally composed of multiple operation units,including the flow process,the heat transfer process and the mass transfer process.Thus,chemical systems are mostly complex processes with nonlinear characteristics such as large time-delay and thermal inertia.In recent years,with the upsized and comprehensive production scale and the high automated and continuous production modes,the chemical industry has put forward higher requirements for the optimization and control of process systems.Especially for the transition process with a wide-range change in working conditions,it is difficult to achieve a smooth transition of working conditions with the conventional process control strategy due to the complexity of the chemical system mechanism.Therefore,it is necessary to study the corresponding countermeasures,that is,the process transition strategy.In this paper,the transition strategy is systematically analyzed and discussed from three aspects: the continuity,the dynamic characteristics and the coordinated application with the control system.(1)Aiming at the shortcomings of the existing solution approach for transition problems,that is,the transition strategy obtained by the dynamic optimization method based on time discretization and numerical approximation does not have the continuity with physical significance,the analytical method based on frequency domain is proposed in this paper.This method is different from the traditional time-domain solution idea.The basic principle is to convert the time-domain transition problem into the frequency-domain transition problem equivalently.Then,the frequency domain model is used to solve the differential and algebraic equations because of its simplicity,and the analytical computation to the complex transition problem is realized,thereby obtaining the continuous transition strategy.In addition,the proposed method satisfies the endpoint constraints of the transition process during the identification of the transfer function,which averts adding constraints in succeeding calculations and reduces the computational complexity.(2)Although the frequency domain-based analytical method can address the problem of continuity for transition strategy,it sacrifices some dynamic characteristics in the calculation.However,process transition is the study of dynamic process for the system,thus accurate dynamic characteristics are very important.In this regard,based on the analysis idea of frequency domain,a frequency domain-based two-step optimization approach is further presented.The method integrates the superiorities of the analytical way and the optimization way.Firstly,the continuous transition strategy with physical meaning is obtained by the analytical method.Subsequently,all the parameters in the continuous transition function are corrected through parameter optimization on this basis,and then the transition strategy with compound characteristics such as continuity and separate dynamic is achieved.When the process transition occurs,the obtained transition strategy with compound characteristics causes less fluctuation,and the transition effect has been improved.Furthermore,the frequency domain-based two-step optimization approach can also be quickly used for solving different transition problems of the same plant through modifying the parameters.(3)In fact,the simple transition strategy is to obtain the optimal transition trajectory of the manipulated variable,that is,the reference trajectory of the manipulated variable.And there is no function of feedback,so it cannot correct for deviation caused by interferences.Therefore,it is necessary to coordinate the process transition strategy with the process control system,and then realizes the process transition through a closed-loop control structure.In view of the poor anti-interference ability of the existing conventional feedback coordination scheme,a feedforward-feedback coordination scheme that introduces the reference trajectory of manipulated variables is developed.This scheme relieves the pressure of the feedback control system by adding the reference trajectory of the manipulated variable,and achieves the two requirements of process transition and disturbance elimination separately.The reference trajectory of the manipulated variable is mainly employed to realize the large-scale transition of the operating conditions,while the other disturbances are eliminated by the feedback control.Hence,the presented coordination scheme can fast respond and compensate for the adverse effects brought by different actions,which is of stronger anti-interference ability and wider application range.Additionally,a corresponding analysis-based solution is developed for the proposed coordination scheme as well.The method makes full use of the known conditions,and the corresponding feedforward control law and feedback controller setpoint are derived according to the reference trajectory of the manipulated variables acquired above.The solution process is simple and convenient,which avoids the defects of the optimization-based solution such as complex operations.Finally,the feasibility and superiority of the above-mentioned process transition strategies and the coordination scheme have been effectively verified through the simulation study of the ethylene distillation column with fluctuating feed.