Pyrolysis of a binary mixture of complex hydrocarbons: Reaction modeling

Abstract

A reaction model was developed for the pyrolysis of binary mixtures of compounds whose reactions can be apportioned into three or fewer parallel chains coupled by chain transfer. A representative application of this model to the pyrolysis of mixtures of the asphaltene model compounds n-pentadecylbenzene (PDB) and n-tridecylcyclohexane (TDC) illustrates its utility. The model, along with its associated rate constant estimates, quantitatively correlated the experimental temporal variations of the product yields from the individual pyrolyses of PDB and TDC. Model results for binary mixtures of PDB and TDC showed that the pyrolysis rates for both compounds were accelerated by the addition of the second compound. For instance, the pyrolysis of PDB at an initial concentration of 10-4M proceeded at a 36% higher rate in the presence of TDC at an equal concentration. The rate of TDC pyrolysis at [TDC] = 10-4M, on the other hand, increased more than nine-fold upon the addition of PDB at a concentration of 10-4M. Unlike the accelerated reaction rates, product selectivities were largely insensitive to the presence of the second compound. These results are consistent with and can be explained on the basis of the influence of concentration on the relative kinetics of bimolecular and unimolecular propagation, chain transfer, and termination steps. The model results also lead to the identification of quantitative criteria for determining when an added compound can act as a rate accelerator. Finally, this study permits speculation into the effects of interactions between alkylaromatic and alkylnaphthenic moieties in asphaltenes on the pyrolysis pathways, products, and kinetics.