Effect of Radial Temperature Profiles on Yields in Steam Cracking K. M. Van Geem, G. J. Heynderickx, and G. B. Marin Laboratorium voor Petrochemische Techniek, Ghent University, B-9000 Gent, Belgium Radial temperature profiles during steam cracking result in radial nonuniformities in the product yields due to radial variations in the concentration of the radicals. The effect of using a 1-D or a 2-D reactor model on the calculated product yields is evaluated for the cracking of ethane. With a 2-D reactor model the simulated ethylene yield decreases. Ethylene formed at the high-temperature zone near the hot wall diffuses to the center where secondary reactions are favored, generating C 3 and C 4 olefins. This effect is confirmed by the calculation of a reactor of a Kellogg Millisecond Furnace. In this small-diameter reactor the 1-D behavior is more pronounced, resulting in higher ethylene yields at comparable conversions. The effect of the radial gradients on the coking rate calculated with a fundamental kinetic coking model based on elementary reaction steps is even more pronounced. Only when the coke model is coupled to a 2-D reactor model, a good agreement with the reference data is observed. In order to obtain accurate simu- lation results the more detailed 2-D reactor model is required, even if this increases the computational effort. © 2004 American Institute of Chemical Engineers AIChE J, 50: 173–183, 2004 Keywords: thermal cracking, two dimensional reactor model, simulation, coke formation, ethune cracking Introduction Tubular reactors are used in industry for important processes such as steam cracking and polymerization. Their analysis and design is frequently based on 1-dimensional models, that is, considering gradients only in the axial direction. Semiempirical correlations can approximate the average of radial concentra- tion and temperature profiles from more-dimensional models (Sundaram and Froment, 1979), but do not provide any infor- mation on the importance and consequences of the nonunifor- mities for the reactor performance. The latter is of particular importance for the endothermic steam-cracking process. The trend toward high-severity cracking (Plehiers and Froment, 1991) demands higher heat fluxes, higher process-gas temper- atures, and shorter residence times. Higher heat fluxes amplify the radial temperature gradients and make the 1-dimensional plug-flow model insufficient (Froment, 1992). Furthermore, a radial temperature gradient implies that the conditions prevail- ing at the process gas– coke interface on the one hand, and the conditions in the center of the reactor coil on the other hand may differ appreciably (De Saegher, 1996). Coke formation at the interface conditions (Sundaram et al., 1981) or at averaged conditions as calculated with a 1-dimensional model will dif- fer. It was shown before (Heynderickx et al., 1992) that cir- cumferential nonuniformities in flux and temperature, due to the shadow effects in the furnace, also result in nonuniform coking rates and coke layers. The radial temperature profile can have a significant effect on the calculated reactant concentra- tions and more in particular on those of the gas-phase radicals at the internal wall of the reactor tubes (Reyniers et al., 1994). The implementation of more dimensional models for complex reactions in a tubular reactor seems to be inevitable. Sundaram and Froment (1979, 1980) coupled a global kinetic model for the cracking of ethane to a two-dimensional (2-D) reactor model. Their results confirm the existence of important radial- temperature gradients. For molecular species, on the other Correspondence concerning this article should be addressed to G. B. Marin at guy.marin@rug.ac.be. © 2004 American Institute of Chemical Engineers AIChE Journal 173 January 2004 Vol. 50, No. 1