Radiant Section Simulation And Crossover Section Structure Optimization Of Cracking Furnace

Ethylene production and technology are crucial to the development of the petrochemical industry. The core unit for the ethylene industry is always the cracking furnace. In China, the ethylene production capacity and output exceeded17million tons in2012, and the current maximum production capacity of a single cracking furnace arrived at150000tons annually. Within the period of the Twelfth Five-Year Plan, China will develop ethylene production with large scale. With the development and technological progress of ethylene industry, researches on the optimization of largescale cracking furnace have become particularly important. This paper used computational fluid dynamics (short for CFD) to simulate cracking furnace, analyzed its operational features, and optimized the working conditions.Radiant section of cracking furnace is commonly consisted of hydrocarbon cracking reaction zone inside tubes and combustion heat release zone outside tubes. In this paper, all hydrocarbon cracking reactions were translated into heat flux by the calculating program of hydrocarbon cracking reaction, so the coupling simulation of inside and outside of tubes had been achieved. Fuel gas combustion is the key step of heat transfer in the radiant section of cracking furnace. The computational accuracy of the combustion simulation is crucial to the research on cracking furnace, particularly in optimizing ethylene production equipment. This paper studied some combustion models of the radiant section simulation. The results showed that the finite rate/eddy dissipation combustion model is highly suitable for the simulation of cracking furnace burner because of the vigorous mixing of fuel gas and air, the high jet velocity, as well as the large volume of radiant section.The mode and proportion of heat in the burners of the cracking furnace directly determine the distribution of flue gas temperature field in the radiant section. The results showed that the joint heat of the hearth and wall burners at a ratio of7:3is the optimal heat condition of the cracking furnace with the processes of the2-1type tubes, naphtha material, and propylene and ethylene at a ratio of0.5. The air preheater installed into the hearth burner enables energy conservation in the cracking furnace presently. Given that the heat sources and temperature ranges vary in different ethylene plants, this paper studied five common air-preheated temperature levels to simulate the temperature fields of radiant section. The results showed that the optimal air-preheated temperature is about353.15K.A cracking furnace with double radiant sections, including one convection section was considered as the direction of development in a largescale cracking furnace. This paper studied the newly technological processes of a largescale cracking furnace containing double radiant sections to provide theoretical guidance for the optimization of operational conditions. The results showed that the average flue gas temperature of the radiant section exit under the100%processing capacity was10K-15K higher than that under the75%capacity in the different cracking processes. Under this operational condition, the status was stable and safe. The average flue gas temperature of the radiant section exit in the cracking process was70K-75K higher than that under coking when the double radiant sections were under the cracking and coking. Given that the flue gas temperature during the cracking is much higher than that during the coking, the flue gas of the two radiant sections flowing into the convection section produces more heat transfer. This phenomenon affected the heat demand of tubes in the convection section. The research results showed that the arc-shaped structure of the crossover section would considerably slow down the degree of mixing of flue gas flowing from both radiant sections, which met the heat demand of cracking and coking processes.Through numerical simulation and data analysis, the thesis determined the operational status of cracking furnace had been grasped. This paper also obtained data which were not easily measured in industrial production, and established a method for optimizing the operational conditions of cracking furnace. The results will give out theoretical significance in the production and optimization of cracking furnace, and provide a reference of the important parameters for the design of the new cracking furnaces.