Research And Application On The Selection Of Process Control Structure Based On Energy Efficiency Analysis

In the modern industrial processes,most of the production processes are usually typical multi-input multi-output systems.There exit significant interactions between controlled variables and manipulated variables,which result in operating difficultly for SISO controllers and even undermining the normal operation of the system.As we know,due to the decentralized control system's easy maintenance characteristics,it is widely used in industrial processes.However,the performance of decentralized control system depends on heavily whether the coupled interconnections of the system are handled effectively.Therefore,we must consider the choice of control structure,that is,the choice of controlled variables and manipulated variables,and the paring problem between variables.At the same time,the choice of control structure cannot be only focused on the control loop analysis and control performance,the economic performance must be considered,for example,the energy efficiency and energy consumption.In this paper,the qualitative and quantitative methods of conventional control structure selection are studied in depth by combining with the energy efficiency index.And the problem of a binary distillation column is used as a case study to verify the effectiveness of the methods proposed.(1)The methods of control structure selection from domestic and international paper are summarized and classified.And then the method of control structure selection is divided into two categories: one is qualitative method,such as RGA and RGA improved methods,one based on Gramian's improved method,one based on energy efficiency analysis and other qualitative methods,As well as quantitative methods based on systematic optimization,controllability index optimization and advanced control optimization.The development and application of these two categories of methods are reviewed.(2)The RGA and Relative Exergy Array(REA)are combined in the paper,which consider the concept of controllability of relative gain array and the thermodynamic efficiency of the unit operation,and quantify the coupling of control structural loops and thermodynamic efficiency(i.e.,exergy efficiency).The control structure chosen can obtain both good operability and energy saving effect.The benzene-toluene distillation column is illustrated as an example.In this case,the steady state gain matrix and stead state exergy gain matrix of(LV,DV,LB)control structure of three direct control schemes are established by steady state simulation of the distillation column.And then combining the REA and RGA,controllability and energy efficiency are analyzed.Finally Aspen Dynamic is used to evaluate the three control structures.The case have shown that DV control scheme performs better in eliminating process coupling and energy efficiency utilization.(3)Considering that the RGA and REA are based on the steady-state gain of the system and the scope of application is limited.An integrated framework of process design and control based on steady-state inverse control process model is adopted.Assume that the process equipment parameters and operating parameters of the design are not considered,only the control structure is considered.At the same time,the energy efficiency index(i.e.,exergy loss)is introduced into the objective function,and the control structure selection problem is transformed into a mixed integer nonlinear optimization problem.This method is also applied to the benzene-toluene distillation column.Moreover,the control structure such as material balance control,distillation index control and stripping control index,the control effect of different control structures are analyzed deeply,focusing on the variation of internal state variables,and the influence of different disturbances.The energy efficiency of the case is analyzed and the optimized control structure selection is obtained.