Operation Improvement And Dynamic Flexibility Analysis Of Chemical Process Systems Under Uncertainty

Chemical processing plants are usually complex dynamic systems which need to be smoothly controlled during operation in the presence of uncertainties and most of the materials are inflammable, explosive, toxic, hazard. Abnormal situations occur from time to time. Therefore, operation safety of the processing industry has to be guaranteed for the security of the country and the people’s life. These uncertainties can correspond to variations either in external parameters, such as quality of the feedstreams, product demand, environmental conditions, or to internal process parameters such as transfer coefficients, reaction constants, physical properties. The uncertainties may lead to the occurrence of the accident. In order to effectively handle these uncertainties, chemical plants must have the flexibility to achieve feasible operation over a given range of the above stated uncertain conditions. A lot of research work has focused on the common assumption that the process operates at steady state for any value of the uncertain parameters. Although this assumption is true for most of the operating life of a continuous process, there are certain situations in which the results of the steady state flexibility analysis are of little help. Such situations include startup or shutdown of a process; transitions from one operating point to another, the role of the control system will eliminate or minimize the effect of disturbances. Furthermore, the steady state analysis cannot be applied directly to batch processes because of their inherently dynamic nature. As was noted above, it is meaningful to investigate the flexibility analysis of the dynamic system to improve the safety and the operation quality of the chemical processes. Firstly, for the production quality and operation safety of chemical batch process, an operation optimization strategy is proposed based on the critical operation point. Fistly, the optimal operation point is found using nonlinear programming algorithm (Sequential Quadratic Programming Method), than search separatly according to the different variation situations of the uncertainty parameters between the optimal operation point and boundary points using the golden section search strategy. By this method, the safety and the quality of the desired product for the batch process is improved. Two typical chemical batch processes were used to demonstrate the effectiveness of the proposed approach which provides a simpler and quicker search strategy for the operation optimiztion and flexibility analysis of chemical process system under uncertainty.Secondly, it is considered that the response of the control system may be influenced by the time delay of the process. Real chemical processes are usually dynamic and subject to uncertainty in either internal or external parameters. In this respect, flexibility analysis is particularly important to the design and operation of chemical processes. However, there is a challenge in flexibility analysis to cope with inner time delay which can induce unpredictable effect. This chapter formulates dynamic systems with time delay as differential difference equation (DDE) systems, meanwhile devises a modified finite element collation method to carry out flexibility analysis. The proposed method is combined with the linear quadratic regulator (LQR) and Lagrange polynomial for the optimal solution of control variables and state variables respectively. The algorithm is investigated on two typical chemical processes with time delay. Results illustrate that the modified finite element collation method possesses a high confidence with regards to the robustness.Finally,a sensitivity concept and sensitivity index to the effects of time delay is proposed for study of dynamic process system flexibility. An integrated analysis method of flexibility and controllability for operation safety of dynamic process system with time delay is proposed and demonstrated here. First, effects of time delay for dynamic system flexibility analysis and the system sensitivity are tested with a third-order and a second-order system, combined with the controllability analysis and controller optimization and simulation. Finally, a typical chemical reactor case study based dynamic flexibility analysis and process controllability is demonstrated taking account the effect of time delay in system operation, and tested the system sensitivity of the effects of time delay. It provides a more accurate analysis of flexibility for the operation of dynamic process system, so the operation safety and stability of the real process system operation can be guaranteed effectively.