The Simulation And Control Of Decoking Process Of Ethylene Cracking Furnace

Ethylene is the most important raw materials of chemical production. There is a huge demand in the world.98% ethylene is produced by tube steam thermal cracking method. Coke may deposits on the inner wall of the radiant section of cracking furnace and transfer line exchanger during the ethylene cracking process because of the secondary reaction. This coke will cause the decreasing of the heat transfer efficiency, increasing of fuel consumption, increasing of the outer wall temperature, and shortening of the lifespan of furnace tube. When the cracking furnace run to a certain time, it need to be stop production and changed to decoking. The commonly decoking operation spends long time. It shortens the time for production. Large amount of steam, hot air and fuel are consumed in the decoking process. The cost of the cracking operation is increased. Therefore, the research of rapid decoking is very import. It can prolong the running time and bring huge economic benefits for ethylene industrial production.The technology of decoking in pipeline cracking furnace is analyzed in this work, and decoking by steam air mixtures is chose. Through the analysis of the physical and chemical process in decoking, such as heat transmission, mass transmission, combustion and turbulent flow, a one-dimensional heterogeneous piston flow model is established. This model is a distributed parameter model. The outlet temperature of the furnace tube, concentration, pressure, and the thickness of coke are functions of time and space. Model form is nonlinear hyperbolic partial differential equations. The analysis and calculation of distributed parameter model is very difficult. The implicit finite difference method is used to solve equations. Simulation results of the decoking are compared with those of the real process, and then the validity of the model is validated.Combining with mechanism model, expert experience and actual experiments, automatic closed-loop control algorithm of Ethylene cracking furnace is put forward. Control variables of decoking are optimized by the feedback of CO2 density and its rate of change. Before change control variables, a simulation is performed to ensure the safety and smooth of decoking. By means of the experiment in pilot plant, the algorithm is proved can speed up the decoking. In order to realize the continuous control of decoking, iterative learning algorithm was used to control the process of decoking. CO2 concentration of ideal decoking process is used as a reference trajectory, and control variables are adjusted in real time by the error of CO2 concentration. The simulation results proved that this method can achieve fast and stable decoking process.Finally, an automatic rapid decoking control system with self-owned license for the ethylene cracking furnace is developed for industrial decoking operation. OPC technology is adopted to realize the real-time communication of control software and the pilot plant controller. This soft can realize automatic rapid decoking control of the ethylene cracking furnace, including condition monitoring, automatic control, transfinite alarm, trend display, data storage, and other functions. The feasibility and effectiveness of proposed software are verified in an industrial ethylene cracking furnace pilot plant.