Determination of Naphthenic Acid Profile in Ghana’s Jubilee Oil Using Gas Chromatography-Mass Spectrometry

Abstract

Crude oil is the life-blood of the global economy. Its importance stems from the fact that it is a base product for a wide variety of goods [Drugs, Plastics, Liquefied Petroleum Gas (LPG)]. The oil discovery (over 3 billion barrel reserves in hydrocarbon and gas), about 60 km offshore between the Deepwater Tano and Cape Three Points Block in South western Ghana is a valuable natural asset and it has the potential of boosting the Ghanaian economy. During petroleum processing, various waste products are generated. One of such products is Naphthenic acids (NA). Naphthenic acids are organic acids naturally occurring in crude oil and a constituent of waste associated with oil refinery. Naphthenic acids serve as biomarkers for identification of the source of crude oil. The presence of Naphthenic acid in the aquatic environment causes toxic effects due to their weak biodegradable nature; the toxicity of Naphthenic acids depends on the class of Naphthenic acids present in the crude oil. The study assessed the profile of Naphthenic acids in Ghana’s Jubilee crude oil using Low Resolution Electron Impact – Gas Chromatography Mass Spectrometry (LREI-GCMS) after isolation of Naphthenic acids in the Jubilee oil by a modified Kupchan’s Partitioning Process. The Mass Spectrometric (MS) Work Station Software was used for the identification of the Naphthenic acids present in the Jubilee crude oil. The quality of the Jubilee oil was also evaluated through the use of some key physico-chemical parameters [Total Acid Number (TAN), Sulphur Content, Viscosity, Pour Point, Flashpoint, Water Content and Densities] based on the American Standards for Testing and Materials (ASTM, 2007). The Total Acid Number was determined by Colorimetric Titration (ASTM D974); Sulphur Content by X-ray Fluorescent Spectrometry (ASTM D4294); Pour Point by the use of the SETA cloud and Pour Point Refrigerator Technique (ASTM D97); Viscosity by Gravity Timed Method (ASTM D445); Density by the Hydrometer Method (ASTM D1298); Flashpoints by the Pensky-Martens Closed Cup Method (ASTM D93); and Water Content by Distillation (ASTM D95). The results for the physico-chemical analysis revealed that, the Jubilee crude has a Flashpoint of > 80.5 ⁰C, Density of 36.55 ⁰API, Pour point of -15 ⁰C and Sulphur content of 0.168 wt%. The Total Acid Number (TAN) for the Jubilee crude oil was 0.58 mg KOH/g crude; Viscosity of 3.899 cSt at 50 ⁰C and a negligible Water content. Based on National Fire Protection Association (NFPA 30) and American Petroleum Institute (API) classification standards, the results for the physico-chemical parameters indicates that Ghana’s Jubilee is combustible, light and sweet crude with relatively high Acid content, low Pour point and Viscosity. The analysed (using MS Work Station Software) LREI-GCMS chromatogram identified two Naphthenic acids, a couple of homologues belonging to the monocyclic ring family (𝒛=−𝟐). The mz⁄ peaks of these acids were found at 168.1 and 184.1. These masses correspond to molecular formulas (𝐶10𝐻17𝑂2)− and (𝐶18𝐻17𝑂2) respectively. The Naphthenic acids were identified as Metaethyl-3-cyclopentylpropanoic acid, (𝐶10𝐻17𝑂2)− and Metaethyl-3-cyclopentylbutanoic acid, (𝐶11𝐻20𝑂2)