A Kinetic Approach to the Pyrolysis of Naphthenic Acids

Abstract

Highly acidic crude oils are abundant at low cost; therefore, they are considered as good alternatives to typical crude oils, to provide an additional supply for the increasing demand for transportation fuels. However, they are extremely corrosive; therefore, the isolation of acids from crude oil and on-the-spot decomposition is a major concern. A wide variation of acidic compounds is present in these oils; most are known to be naphthenic acids (NAs). Isolated NAs can be processed in furnaces operating at different temperatures in the refinery/petrochemical sites. In this study, some aliphatic carboxylic acids are chosen as model compounds to rationalize the unimolecular decomposition of carboxylic acids via quantum mechanical tools. The methodology (B3LYP/6-311++G(2df,p)// B3LYP/6-31G(d)) to understand the role of competing reactions is established via comparison with experimental data for aliphatic acids. The same method 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 the acidic function on the ring are discussed. This study has tackled for the first time the unimolecular thermal decomposition mechanism of NAs elucidating the role of temperature in the branching ratios of decarboxylation versus dehydration. The results obtained from this study are expected to help in supplying reaction rates for detailed kinetic models and improving the optimization of the processes.