Computational Study on Removal of Naphthenic Acids from Petroleum-based Systems

Abstract

Naphthenic acids (NAs) are toxic compounds found in crude oil and products of petroleum processing including produced water. Reactions of hydroxyl radicals with NAs and ionic liquids (ILs) extraction have been reported to be effective in removing NAs from petroleum-based systems. However, the reaction and interaction mechanisms are not fully understood. In this thesis, a density functional theory study was conducted to explore mechanisms and kinetics of reactions between benzoic acid (BA), benzoate (BZ) and hydroxyl radicals as well as the extraction mechanisms of model NAs by ILs. The relationships between physicochemical properties of ILs and their intramolecular and intermolecular interactions were also investigated. The results show that all reaction pathways between BA, BZ and hydroxyl radicals involve the formation of pre-reactive complexes. The reaction rate constants for the addition reactions are highest for BZ in the aqueous phase, followed by BA in the aqueous phase, then by BA in the gas phase. For interactions between ILs and NAs, the main extraction mechanism by 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) for NAs without long alkyl chain is hydrogen bonding, whereas van der Waals interaction and hydrogen bonding are the dominant extraction mechanisms for NAs with long alkyl chain. With the incorporation of biodegradable substitutional groups, hydrogen bonding is still the main extraction mechanism, however, the interaction energy is higher than that of [BMIM][BF4]. More intermolecular hydrogen bonds occur when model NAs are absorbed by [BMIM][BF4] with -COOH or -COOCH3. Additionally, the interaction energy between model NAs and ILs with -COOH or -COOCH3 is higher than that with -OH, -NH2, or -OCH3. Aggregation behaviour and hydrogen bonds in ILs influence their densities and self-diffusion coefficients.