Radical species pyrolysis processes

A number of papers report investigations of the pyrolytic cleavage of aromatic hydrocarbons. The oxidation and pyrolysis of anisole at 1000 K have revealed first-order decay in oxygen exclusively via homolysis of the O—CH3 bond to afford phenol, cresols, methylcyclopentadiene, and CO as the major products.256 A study of PAH radical anion salts revealed that CH4 and H2 are evolved from carbene formation and anionic polymerization of the radical species, respectively.257 Pyrolysis of allylpropar-gyltosylamine was studied at temperatures of 460-500 °C and pressures of 10-16 Torr. The product mixture was dominated by hydrocarbon fragments but also contained SO2 from a proposed thermolysis of an intermediate aldimine by radical processes.258... 

Returning to the asphaltene/resid pyrolysis example, the mechanistic models of these processes are based on free radical chemistry. That is, the elementary steps which describe the reaction chemistry are written in terms of free radical species. Thus the development of a mechanistic model requires the time-dependent solution of an extensive set of material balance equations for every component as well as for each of the active centers. 

It is now firmly established that free radical reactions dominate the thermal degradation of hydrocarbon species (Benson, 1976 Dente etal., 1983 Poutsma, 2000 Savage, 2000). Only mechanistic radical kinetic schemes can provide reliable descriptions of the pyrolysis process. 

Because of the lower activation energy of their initiation reactions, the rising concentration of double bonds inside the polymer enhances the radical concentration and favours the pyrolysis process. This radical concentration is then used to evaluate the rate of formation and disappearance of all the species in the system. Thus, the production of the alkene Oj is simply expressed as... 

Detailed kinetic schemes also consist of several hundreds of species involved in thousands of reactions. Once efficient tools for handling the correspondingly large numerical systems are available, the extension of existing kinetic models to handle heavier and new species becomes quite a viable task. The definition of the core mechanism always remains the most difficult and fundamental step. Thus, the interactions of small unsaturated species with stable radicals are critical for the proper characterization of conversion and selectivity in pyrolysis processes. Parallel to this, the classification of the different primary reactions involved in the scheme, the definition of their intrinsic kinetic parameters, the automatic generation of the detailed primary reactions and the proper simplification rules are the important steps in the successive extension of the core mechanism. These assumptions are more relevant when the interest lies in the pyrolysis of hydrocarbon mixtures, such as naphtha, gasoil and heavy residue, where a huge number of isomers are involved as reactant, intermediate and final products. Proper rules for feedstock characterizations are then required for a detailed kinetic analysis. 

Ten types of elementary processes were defined, which included all the reaction types characteristic for the pyrolysis of alkanes. To speed up the process of reaction generation, five basic structural attributes were attached to each species. These attributes described the type of the species (molecule, radical), the length, the saturation, etc. The application of attributes enabled the sensible application of reaction types, since, for example, molecules do not take part in radical recombination reactions, and short radicals cannot take part in 1,4-isomerization. Where possible, rate parameters were taken from databases and others were estimated on the basis of ten rules which were applied according to the type of reaction and reaction partners. 

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