[Cracking Question Bank] What are the parameters that affect hydrocarbon cracking, and how do they influence it?

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This post was last edited by Guan Gongyu on 2015-10-19 at 15:06. What are the parameters that affect hydrocarbon cracking, and how do they influence it? The yield of products obtained from hydrocarbon cracking is closely related to the properties of the cracking feedstock. For the same pyrolysis feedstock, the yield of the products obtained from pyrolysis depends on the process parameters of the pyrolysis reaction. The main process parameters of the pyrolysis process are: pyrolysis depth, pyrolysis temperature, residence time, and hydrocarbon partial pressure, which will be explained below respectively. (1) Pyrolysis depth: Pyrolysis depth refers to the extent to which the pyrolysis reaction proceeds, and it is primarily determined by the pyrolysis temperature and residence time. At the same pyrolysis temperature, the pyrolysis depth increases as the residence time lengthens. For the same residence time, the degree of pyrolysis increases as the pyrolysis temperature rises. Therefore, at the same cleavage depth, there can be various combinations of cleavage temperature and residence time. (2) Pyrolysis temperature: Based on the analysis of the radical reaction mechanism, within a certain temperature range, increasing the pyrolysis temperature helps to improve the yields of ethylene and propylene obtained from the primary reaction. It can also be seen from the chemical equilibrium of the cracking reaction that increasing the cracking temperature facilitates the reaction leading to the formation of ethylene and relatively reduces the reaction that results in the loss of ethylene, thereby enhancing the selectivity of cracking. However, when the cracking reaction reaches chemical equilibrium, the yield of olefins is very low, and the cracking products will mainly be hydrogen and carbon. Therefore, the reaction that produces olefins through cracking must be controlled within a certain range of cracking depth. Under the same residence time conditions, different pyrolysis feedstocks require different pyrolysis temperatures. The lower the molecular weight of the feedstock for cracking, the higher the cracking temperature required ； The higher the hydrogen content in a lightweight pyrolysis feedstock, the higher the pyrolysis temperature required for its pyrolysis. (3) Residence time: The residence time of the material in a tubular pyrolysis furnace refers to the time it takes for the pyrolysis feedstock to pass through the radiation section coil. However, since the pyrolysis reaction in the radial coil occurs under non-isothermal and variable-volume conditions, it is difficult to accurately calculate its true residence time; therefore, various assumptions are often made to estimate it and to simply describe the residence time, such as equivalent residence time, apparent residence time, and average residence time. Under the condition of ensuring a certain degree of pyrolysis depth, there can be various combinations of pyrolysis temperature and residence time. Therefore, for the cracking reactions used to produce olefins, the cracking temperature and residence time are a set of interrelated and inseparable parameters. High temperature and short residence time are the key factors in altering the product yield. (4) Hydrocarbon partial pressure: The hydrocarbon partial pressure refers to the partial pressure of gaseous hydrocarbons in the material fed into the cracking furnace tubes. The feed to a tubular cracking furnace is a mixture of hydrocarbons and water vapor; it is thus the proportion of hydrocarbons in this mixture that is considered. From the perspective of chemical equilibrium, the hydrocarbon cracking reaction is a process in which the number of molecules increases. For the dehydrogenation reaction, reducing the pressure favors the formation of ethylene and hinders the occurrence of secondary reactions. From a kinetic perspective, although a low hydrocarbon partial pressure reduces the rates of both the primary and secondary reactions, its effect on the rate of the secondary reaction is much greater than that on the primary reaction, thereby increasing the relative rate of the primary reaction with respect to the secondary reaction. In summary, for a given pyrolysis feedstock, maximizing the utilization of the facility means striving to achieve the optimal combination of high temperature, short residence time, and low hydrocarbon partial pressure, while ensuring an appropriate degree of pyrolysis. This yields the most optimal product yield distribution and ensures a reasonable coking removal cycle. Correct +10. Other moderators, please assist with scoring