The electrodeposition of asphaltenes and implications for asphaltene structure and stability in crude and residual oils

Abstract

Electrodeposition experiments have identified that colloidally stable asphaltene ‘micelles’ in crude oils possess little net charge and are apparently not affected by DC electric fields alone. However, upon destabilisation of the micelles (whether partial or total) with n-alkanes, electrodeposition occurs to extents which depend on the crude oil or residue, the alkane used and the relative dilution. Thus, at low dilutions, and hence incomplete destabilisation, the deposited material possesses a small net negative charge; higher dilutions result in a positively charged deposit. These findings are consistent with a structural model for asphaltene ‘micelles’ comprising negatively charged ‘peptising’ resins surrounding a positively charged central asphaltene core. In crude oils, the resins may be smaller and better able to stabilise the polar core, and are accordingly more difficult to displace. Electrical treatment therefore produces deposits with relatively weak charge. In thermally treated vacuum and visbroken residues, on the other hand, the deposits are highly positively charged and electrodeposition occurs much more readily, consistent with the reduced state of peptisation in these systems. This confirms earlier work indicating that the structure of asphaltene is thermally modified during distillation, a process which continues throughout visbreaking, resulting in lower degrees of asphaltene peptisation and the production of highly positively charged deposits. Flocculation between destabilised micelles is proposed to occur as a result of enhanced van der Waals' attractive forces as the distinction between resin-coated asphaltenes and the hydrocarbon oil is lost. This is reflected in an increase in the Hamaker interaction parameter as resins are stripped from the asphaltene.