Structural Study of Asphaltenes from Iranian Heavy Crude Oil

Abstract

In the present study, asphaltene precipitation from Iranian heavy crude oil (Persian Gulf off-shore) was performed using <i>n<i/>-pentane (<i>n<i/>-C<sub>5<sub/>) and <i>n<i/>-heptane (<i>n<i/>-C<sub>7<sub/>) as light alkane precipitants. Several analytical techniques, each following different principles, were then used to structurally characterize the precipitated asphaltenes. The yield of asphaltene obtained using <i>n<i/>-pentane precipitant was higher than asphaltene precipitated with the use of <i>n<i/>-heptane. The asphaltene removal affected the <i>n<i/>-C<sub>5<sub/> and <i>n<i/>-C<sub>7<sub/> maltene fractions at temperatures below 204°C, as shown by the data obtained through the simulated distillation technique. Viscosity of heavy oil is influenced by the asphaltene content and behavior. The viscosity dependence of the test heavy oil on the shear rate applied was determined and the flow was low at y. above 25 s<sup>-1<sup/> . The reconstituted heavy oil samples were prepared by adding different amounts of asphaltenes to the maltenes (deasphalted heavy oil) and asphaltene effects were more pronounced at the low temperature of 25°C as compared with those at the higher temperatures. According to the power law model used in this study the flowability of the test heavy oil exhibited a pseudoplastic character. Structural results obtained from Fourier Transform InfraRed (FTIR) spectroscopy showed the presence of the different functional groups in the precipitated asphaltenes. For instance, the presence of different hydrocarbons (aliphatic, aromatic and alicyclic) based on their characteristics in the FTIR spectra was confirmed. Resins are effective dispersants, and removal of this fraction from the crude oil is disturbing to the colloidal nature of heavy oil; asphaltene flocculation and precipitation eventually occur. Appearance of pores in the Scanning Electron Microscopy (SEM) images was used as an indicator of the resin detachment. With the use of <sup>1<sup/>H and <sup>13<sup/>C Nuclear Magnetic Resonance (NMR) spectroscopy, two important structural parameters of the asphaltenes were determined. Namely, the aromaticity (fa) and the average number of carbon atoms per alkyl side chain (<i>n<i/><sub><i>carbon<i/><sub/>), where <i>f<sub>a<sub/><i/> for <i>n<i/>-C<sub>5<sub/> asphaltenes was lower (0.39) than that obtained with <i>n<i/>-C<sub>7<sub/> solvent (0.49). Additionally, the <i>n<i/><sub><i>carbon<i/><sub/> parameter values were 7.7 and 5.7 for <i>n<i/>-C5 and <i>n<i/>-C<sub>7<sub/> asphaltenes, respectively. Structural recognition of the oil constituents is the prerequisite of different techniques usable for heavy oil upgrading.