Heavy Component Description in the Kinetic Modeling of Hydrocarbon Pyrolysis

摘要

Since a very long time the use of complex kinetic schemes is diffused, with increasing frequency, in the modeling of a wide variety of chemical engineering problems such as gas and liquid phase pyrolysis, polymer degradation, oxidation and combustion processes. An example of such an extended kinetic scheme is present in SPYRO~(～R) which is adopted by the majority of ethylene producers for optimizing purposes. The dimensions and complexity of these detailed kinetic models impose and justify the adoption of proper simplification levels, coherent with the final aim of the model itself. Due to the huge amount of possible isomers already contained in the liquid pyrolysis feeds (naphthas and mainly gasoils), it is clearly convenient to lump a large number of real components into a properly selected number of equivalent components. Consequently, the corresponding elementary reactions are also grouped into equivalent or lumped reactions. This approach, already and successfully applied to the steam cracking process [Dente et al., 1970] has been recently improved through a better description of the most important classes of components contained in the feedstocks as well in the pyrolysis products when heavy liquid fractions are used. The new feedstock characterization as well as the latest extension of the kinetic scheme takes also advantage of the recent efforts in the modeling of liquid phase pyrolysis processes such as visbreaking and delayed coking of refinery residues [Bozzano et al., 1998, Dente et al., 1997]. General lines for the definition of the new component classes are presented and preliminary simulation results in practical examples are also reported. This improved description allows a better description and understanding both of cracking processes and of fouling phenomena in the cracking coils and in transfer line exchangers. Finally, this modeling activity finds strong analogies and mutual benefit with the modeling of combustion processes where the formation of polyaromatic hydrocarbons species (PAH), particulate and soot is of increasing concern and still remains a challenging problem.