Characterization of the paraffinic hydrocarbon fraction of Nigerian bitumen using multivariate analytical techniques.

Akhigbe, Godwin Ehimengbale (2015)



Analyses of organics, metals and physical properties of the paraffinic hydrocarbon fraction of Nigerian bitumen were carried out with the view to establishing the characteristics which will aid the exploitation and exploration of this natural resource. Bitumen samples were extracted from the oil sands obtained from Southwestern Nigeria using toluene and then deasphalted using n-pentane, while paraffinic hydrocarbons were eluted by column chromatography using n-hexane as eluents. The organic components of the paraffinic hydrocarbon fraction were investigated using Fourier transform infrared (FT-IR) spectrometry and Gas chromatography. The metal concentrations were determined usingAtomic absorption spectrophotometer. The physical properties (flash point, carbon residue, refractive index and colour) were determined using standard methods. The results revealed that paraffinic hydrocarbons in the bitumen had a mean value percentage of 35.128 ± 2.729. The Infrared spectra of the paraffinic hydrocarbon fraction showed mainly the presence of C-H (CH3) and C-H (CH2) functional groups, indicating high purity of the saturate fraction. Thirty-one different organic compounds which were mainly saturates of high molecular weight (> C9) were identified and quantified by Gas Chromatograph. Pentadecane had the highest concentration with a mean value of 3369.744 ± 1375.669 mg/l, while undecane had the least concentration (7.541 ± 4.057 mg/l). The concentration of total organics was highest in B2 (26747.448 mg/l), while B1 (13121.459 mg/l) was the least. The paraffinic fraction had a mean carbon preference index (CPI) value of 1.035, indicating high maturity of the fossil fuel. The principal component analysis using the organic concentrations as variable showed close clustering, and correlation with close eigen values, indicating that the paraffinic hydrocarbon compounds had similar chemical properties, common sources, and/or maturation age. This study revealed that Mn had the highest mean concentration (4.746 ± 1.096 mg/l), while Co (0.394 ± 0.859 mg/l) had the least. The concentrations of Mn, Cu, Co, Zn, Fe, V, Cr and Ni in the paraffinic hydrocarbon fraction were generally low compared to other fractions of Nigerian bitumen. The t-test values for the metal concentrations indicated no significant difference (except Mn) for all the bitumen fractions, while concentrations of the metals in the paraffinic fraction and Nigerian crude oil showed significant difference except Mn, Cr and Fe. The cluster analysis of the metals showed two groups which are fairly well correlated indicating similar sources and/or chemical affinity. It was observed that V/Ni ratio of the paraffinic hydrocarbon fraction was 0.10 – 1.88 (1.12 ± 0.685), indicating that Nigerian bitumen matures with the age of the producing field. The V/Ni ratio of the paraffinic fraction was close to that of Nigerian bitumen 0.45 – 2.28 (1.12) which was higher than the average value (0.16) obtained for Nigerian crude oil. The V/(V+Ni) ratio 0.09 – 0.65 (0.50 ± 0.226) obtained in this study is also similar to 0.31 – 070 (0.50) obtained for Nigerian bitumen. The colour of the paraffinic hydrocarbon fraction ranged from colourless to off-white. This study concluded that the paraffinic hydrocarbon fraction of Nigerian bitumen was a useful source of petrochemicals, refineries feedstock and fuel.