Studies On Reaction Mechanism And Kinetics Of Asphaltene Cracking

A pentane-insoluble asphaltene was processed by thermal cracking(A), thermal hydrocracking(B) and catalytic hydrocracking over NiMo/y-Al2O3(C) in a microbatch reactor at703K. The experimental data of asphaltene conversion fit second-order kinetics adequately, to give the apparent rate constants of1.704×10-2,2.435×10-2and9.360×10-2wt frac-1min-1for the three cracking processes respectively. A three-lump kinetic model is proposed and solved to evaluate rate constants of parallel reactions of asphaltenes to produce liquid oil (k1) and gas+coke (k3), and consecutive reaction from liquid to gas+coke (k2). The evaluated k1is1.697×10-2,2.430×10-2and9.355×10-2wt frac-1min-1, k2is3.605×10-2,2.426×10-2and6.347×10-3min-, and k3is6.934×10-5,5.416×10-5and4.803×10-5wt frac-1min-1for asphaltenes thermal cracking, thermal hydrocracking and catalytic hydrocracking, respectively.Analysis of selectivity shows that at the same level of asphaltene conversion, the selectivity of liquid products was ordered by C>> B>A, coke selectivity by A>>B>>C, and gas selectivity by A>B>C. In process A, a large amount of sulfur converted from the feed into coke; in process B, molecular hydrogen played a certain degree roles in inhibiting formation of coke with high sulfur content; and in process C, the catalyst effectively activated hydrogen molecules to hydrogenate the reactant and middle products, leading to a significant reduction of coke formation, and remarkable improvement of liquid stability, selectivity and quality (lower average molecular weight and sulfur content).Catalytic hydrocracking of asphaltene over NiMo/y-Al2O3was conducted at693K. A kinetic model with four lumps (asphaltene, liquid, gas and coke) containing five reaction paths was developed. The fitting of experimental data validated the suggested model and determined the rate constant of each individual reactions, to give the insight to the kinetic feature of asphaltene catalytic hydrocracking. The hydrocracking of asphaltene over NiMo/y-Al2O3at623-703K was investigated. The second order kinetic equation fits experimental data of asphaltene conversion adequately, giving the apparent activation energy to be144kJ/mol over the temperature range. Average molecular weight of liquid product was reduced significantly with increasing temperature.The coke formation in bitumen-derived asphaltene pyrolysis was investigated at703-823K. Coke yield increased with reaction time to approach an ultimate level which was reduced from47to41wt%with increasing reaction temperature from703-823K. H/C ratio of coke decreased with increasing reaction time and temperature. The experimental observation suggests a complex mechanism of carbonization, involving various reactions such as the polymerization of free radicals, cyclization of alkyl chains, dehydrogenation and aromatization of naphthenic rings, elimination of side chains on aromatic rings, condensation and pericondensation of aromatic rings. Various reactions play different roles in different steps of carbonization processes.