| Based on a detailed review of literatures about simulation and analysis of status and development trends of ethylene pyrolysis furnace, a comprehensive numerical simulation and operating optimizations were done in this paper after combining heat transfer between furnace and coil with the details of decoking process. The main points of this thesis are as follows:(1). The mathematic model of heat transfer between furnace and coils was built on the basis of zone methods. The calculation program for direct heat transfer areas was edited and used to calculate and compare with the data from book, the results had a good agreement with the reference.(2). The industrial ethane pyrolysis furnace was simulated by coupling heat transfer between furnace and coils as well as reactions in coils The plug flow reactor model and the shrinking core model and zonal approach method were used to describe the fluid flow, gas–solid heterogeneous reactions of decoking in coils and heat transfer in furnace, respectively, a general model, can fully and almost accurately describe heat transfer between furnace and coils in decoking process, was obtained. Because of an agreement of the simulation results with the plant data, the correctness of the coupled model and calculation program can be certified. The profiles of temperature, coke thickness, heat flux and so on can be acquired from simulation, these results revealed the basic characters of heat and mass transfers as well as their inherent and interactional rules that provides basises for the optimization of decoking operation.(3). There are some enhanced back-mixing of fluid in bend sections of the coil, and therefore, in the tube elbow, a CSTR model was used to describe fluid mixing, that is, PFR and CSTR in series was used to describe the coil in decoking process with the coupling method.(4) Adjusting tube inlet gas flow and temperature with invariable furnace operating conditions, simulations were done and results were:①under the condition of inlet temperature 580℃, 670℃, external tube skin temperature limitation 1100℃, the biggest security decoking air flow rate curve was gained and under these curves are safe operating zones.②A fitting equations in order to forecast external tube skin maximum temperature when the inlet temperature at 620℃.the maximum calculated value error of fitting equation with the simulated value is 15℃. This fitting equation can help decoking procedure optimization and ensure safety to the pyrolysis furnace which adopt the inlet temperature 620℃and the unchanged gas flowrate.(5). Considering the operation demand of waste heat boiler that is just behind of colis, a basic steps for design of decoking procedure was proposed, a new optimization method was put forward by adjusting coil inlet process gas flowrate and/or CIT once COT was not satisfied.Using the new approach, decoking procedures as well as simulation results could be obtained simultaneously. At the same usage of fuel and air in furance, the new decoking procedure derived from the above method gave approximately the same decoking time with the plant present operation, but total consumptions of steam and air was less than the plant present, residue coke is less than that of the plant present procedure, CO2 volume fraction at coil outlet and the ratio of steam and air were in the control range, the temperatures on external tube skin were below the up limitation of coil material.(6). Keeping pyrolysis furnace operation as well as gas flow rate and the temperature at coil inlet no changes from the beginning to the end, a complex optimization was used to get the shortest decoking time, an optimum operation condition was obtained, in the meantime the maximum external tube skin temperature is less than the uplimit of tolerance of coil materialThis work provided valuable fundamentals for developing an advanced on-line coke-burning procedure for the ethylene production and directing the design of a novel pyrolysis process. |
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