A Kinetic Approach to the Pyrolysis of Naphthenic Acids

Highly acidic crude oils are abundant at low cost; therefore,theyare considered as good alternatives to typical crude oils, to providean additional supply for the increasing demand for transportationfuels. However, they are extremely corrosive; therefore, the isolationof acids from crude oil and on-the-spot decomposition is a major concern.A wide variation of acidic compounds is present in these oils; mostare known to be naphthenic acids (NAs). Isolated NAs can be processedin furnaces operating at different temperatures in the refinery/petrochemicalsites. In this study, some aliphatic carboxylic acids are chosen asmodel compounds to rationalize the unimolecular decomposition of carboxylicacids via quantum mechanical tools. The methodology (B3LYP/6-311++G(2df,p)//B3LYP/6-31G(d)) to understand the role of competing reactions is establishedvia comparison with experimental data for aliphatic acids. The samemethod is applied to NAs as well, and the effects of chain length,number of rings, and type of rings as well as the location of theacidic function on the ring are discussed. This study has tackledfor the first time the unimolecular thermal decomposition mechanismof NAs elucidating the role of temperature in the branching ratiosof decarboxylation versus dehydration. The results obtained from thisstudy are expected to help in supplying reaction rates for detailedkinetic models and improving the optimization of the processes.