| With the increasingly tight supply of oil resources, petrochemical industry nowadays involves more and more heavy oil based applications. Heavy oil upgrading has been an important method for refineries to enhance the economic benefits and competitive capacity. Delayed coking has become a major processing technology for heavy oil and has the advantages of low investment and operating costs, handling different kinds of heavy oil etc.Delayed coking, however, is one of the most difficult units of refinery to operate and control, because of its strongly coupling, the semi-batch nature and its source complexity. Therefore, It is important and of practical significance to ensure smooth operation and improve the performance of control and management through the advanced control technology.The main contents of this thesis are as following:1. The technological process and conventional control system design for delayed coking units are introduced. Specially, Semi-batch nature of delayed coking units is analyzed and the relationship between continuous processes and batch processes is discussed in depth. A practical model for batch operation nature is proposed for on-line monitoring the operation status of batch processes.2. The operating conditions of delayed coking units are time-variant due to the processing different raw oil. Besides, delayed coking process is also subject to time-varying disturbance due to its semi-batch nature. Therefore, the poorly/less optimally tuned PID control performance is usually encountered in the delayed coking industry. The PID performance assessment in delayed coking units is important for maintaining and improving the control performance of PID controllers. For the time-invariant process under PID control, a two-layer structure method is provided, which enables performance analysis of PID controller by integrating both stochastic performance and deterministic performance of PID controller. For the process involving with time-varying disturbance, a PID trade-off curve is proposed, which represents the global optimum for abrupt disturbance and other representative disturbance. According to the trade-off curve, one can choose alterative benchmark to assess the PID control performance. The two developed performance assessment methods are applied to the delayed coking units.3. Focusing on the continuous/batch characteristics of the delayed cooking units, an event-driven iterative learning predictive control (ILMPC) is proposed to the delayed coking units, which is a class of continuous/batch processes characterized by the operations of batch processes generating periodic strong disturbances to the continuous processes. Thus, traditional regulatory controllers are unable to eliminate these periodic disturbances. ILMPC integrates the feature of iterative learning control (ILC) handling repetitive signal and the flexibility of model predictive control (MPC). By on-line monitoring the operation status of batch processes based on the model of batch operation characteristic, an event-driven iterative learning algorithm for batch repetitive disturbances is initiated and the soft constraints are adjusted timely as the feasible region is away from the desired operating zone. Besides, the choice of main controlled variables and manipulated variables, constraint conditions, the structure of controller etc are analyzed. The results of an industrial application show that the proposed ILMPC method is effective for the delayed coking units and improves the stability and enable operations closer to constraints of the delayed coking units.4. The economic assessment of advanced control for industrial processes is introduced. The minimum variance based performance assessment is coordinated into the compromise between the soft constrains adjustment of MPC and the economic performance of the system, in order to enhance the economic performance of advanced control system. The economic assessment is illustrated by an industrial furnace, which is a key unit for the delayed coking process.
|
PDF Full Text Request