| Ethylene is one of the most important chemical raw materials,and its production capacity can be used to measure the level of petrochemicals.At present,most of the ethylene is produced by steam cracking process.When ethylene is produced,side reactions can cause a layer of coke to adhere to the inner wall of the cracking furnace,leading to a rise in thermal resistance of the cracking furnace and a decrease in ethylene production capacity,even threatening process safety.Therefore,decoking operation is required after a certain period of time,which is accompanied by the process of feeding and unloading.The widely applied of decoking is steam-air decoking operation.The statistical parameters of furnace coil outlet temperature(COT)vary with time and the cracker may not be smooth when the operator operates during decoking,feeding and unloading,which can be called non-stationary operation.Since the reactions occurring in the cracking furnace during the steam-air decoking process are exothermic,the COT has the risk of rapid rise,so the control of COT is the core of the decoking process.The COT is controlled by a cascading PID fuel control loop,but the cracking furnace has dozens of tubes,so the COT distribution is sometimes scattered,and the tubes with high COT have a lower service life or even burned through.Once this happens,it is necessary to suspend the decoking until all COT are within the safe range,so it is especially important to control the COT distribution during the decoking process.The COT distribution control in the feeding and unloading process is also important,if the COT distribution is scattered in the feeding and unloading process,which leads to a high temperature of a tube and low flow of raw materials,the phenomenon of over-cracking will occur,which greatly shortens the production cycle,while the low temperature of the tube and high flow of raw materials will make the tube blockage,so once the COT is scattered,which leads to a large deviation of COT,it is necessary to suspend the feeding or unloading.Only by controlling the COT distribution can the decoking,feeding and unloading process be carried out smoothly.For the control of COT distribution in the decoking process,in this thesis PDF is applied to describe the COT distribution and wavelet neural network is used to approximate the PDF to obtain the weights and create a COT distribution model with the weights as the state variables.After dividing the decoking process into four stages and establishing a linear model for each stage,it is found that the structure of the COT distribution model of the decoking process can be regarded as nonlinear static and linear dynamic process and the Hammerstein model of the COT distribution of the decoking process is established,and the controller is designed with the MVC~3method,and the controller is solved by LMI with constraints.The effectiveness and stability of the controller have been verified with simulations and tests on a industrial ethylene cracker.In terms of COT distribution control,the feeding and unloading processes are considered as one process,i.e.,the feeding and unloading process.The rational square root model of COT distribution PDF is used to describe the change of COT distribution for the feeding and unloading process.The Hammerstein model of the COT distribution of the feeding and unloading process is established,and a minimum entropy control method of the COT distribution of the feeding and unloading process is proposed to minimize the entropy of the COT.Simulations were performed using industrial data,and the simulation results verify the effectiveness of the proposed method.In this thesis,a modeling and control method for the COT distribution of the non-stationary process of ethylene cracker is proposed,and an intelligent cracker control system for the decoking,feeding and unloading process has been developed and applied to realize the automation of the whole ethylene production process. |
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