Kinetic Modelling of Pyrolysis Processes in Gas and Condensed Phase

Abstract

Listen

Translate article

Abstract The main goal of this paper is to discuss the common features in the development and validation phases of the reaction schemes able to describe pyrolysis processes in both gas and condensed phases. The complexity of these systems is due not only to the large number of elementary reactions involved but also to the difficulty of properly characterizing the reacting mixtures. This is typical of the pyrolysis process when liquid feedstocks, such as naphtha and gasoil, are vaporized and cracked to produce ethylene and alkenes. However, it is also true for the refinery processes involved in upgrading heavy feeds and in the combustion of liquid feeds, such as gasoline, diesel or jet propulsion fuels. Consequently, apart from the analysis of the chemistry involved in the reacting systems, another important step is the characterization of these complex hydrocarbon mixtures. High-temperature radical reactions, typical of pyrolysis processes, are characterized by their modular and hierarchical structure. This feature means that pyrolysis reactions can be studied by starting from the simpler systems and then progressively extending the simulation capability of the model to more complex situations. High-temperature pyrolysis of large hydrocarbon species rapidly gives rise to small radicals and species, and their interactions are common ground shared by all pyrolysis systems. The interactions of small hydrocarbon species, such as hydrogen, methane, ethane and ethylene, together with their parent radicals are the true core of the kinetic scheme and they constitute the first hierarchical step in all the pyrolysis models. The same mechanisms and similar radical reactions are also extended to the kinetic modelling study of the pyrolysis of liquid and condensed phases. Using a similar approach, it is possible to deal with the kinetic modelling of carbon residue and carbonaceous deposit formation on pyrolysis coils, and, more generally, on the metallic walls of different process units. These pyrolysis and condensation reactions help explain the soot and carbon particle formation in combustion processes. Always a similar kinetic approach and the same lumping techniques are conveniently applied moving from the simpler system of ethane dehydrogenation to produce ethylene, up to the soot formation in combustion environments. A brief discussion on the mathematical modelling of steam cracking, visbreaking and delayed coking processes shows the practical and direct interest of these kinetic models. The thermal degradation of the plastics, of very great environmental interest, is a further and final application example of pyrolysis reactions in the condensed phase and concludes this kinetic analysis.