COT Trajectory Optimization Based on Differential Dynamic Programming for Ethylene Decoking Process

During the cracking process of hydrocarbons, coke is inevitably generated on the internal surfaces of the coil. When the pressure drops and the surface temperature in the coil reaches the permissible limits, cracking production must be switched to decoking operation. In the decoking process, it not only releases large amounts of heat, but also involves extensive manual operation and frequent set-point adjustments, increasing labor load and safety risks. Coil outlet temperature (COT) trajectory tracking control is the critical issue in the decoking process. Because of the complex dynamic nonlinearity and uncertainty of the decoking process, the linear parameter varying (LPV) model based on model interpolation is applied to describe the nonlinearity and time-varying property of the decoking process in this work. Based on the identified LPV model and min-max optimization, a robust differential dynamic programming (RDDP) method is proposed and applied to the COT trajectory optimization in the decoking process. Simulation results and industrial applications have confirmed the feasibility and effectiveness of the proposed method.