(99a) Barriers and Challenges to the Selective Removal of Vanadium From the Asphaltene Fraction

Most crude oils contain traces of vanadyl porphyrins within their asphaltene fraction. Although these metals are only present in trace quantities, they have a significant detrimental impact on crude oil processing units; therefore, their selective removal is highly desirable. The fact that the majority of the vanadium is contained within the highly aromatic, highly polar asphaltene fraction also poses additional roadblocks to their selective removal. This fraction has been shown to associate/aggregate significantly in most (if not all) solvents. Recent work on the nature and possible mechanisms of the molecular association of asphaltenes in solution can be extended to help elucidate the molecular interactions occurring between asphaltenes and metalloporphyrins, and hence the nature of the inclusion of metalloporphyrins within the asphaltene fraction. Solubility measurements with simple vanadyl porphyrins showed that they fit the operational definition for asphaltenes, exhibiting negligible solubility in n-heptane and orders of magnitude higher solubility in toluene. Further studies were also carried out using a stirred diffusion cell. These experiments determined that the size of the asphaltene aggregates in toluene at 25°C and 1 g/L were between 5 and 9 nm. These aggregated structures were extremely stable, remaining intact for periods of >7 days. Neither an increase in temperature nor a decrease in concentration (to 0.1 g/L) reduces the size of these structures below 5 nm, and the origin of the asphaltenes (Athabasca, Safaniya, Venezuela) did not have a significant impact. A significant amount (>85%) of the native petroporphyrins present in the asphaltene fraction remain bound within the above aggregated structures and hence are not susceptible to selective removal. The native petroporphyrins that are free to diffuse appear to be larger than model vanadyl porphyrins as indicated by pore hindrance effects. Neither an increase in temperature nor a decrease in concentration results in an increase in the amount of petroporphyrins available for diffusion. The selective separation of metalloporphyrins is clearly hampered by gaps in the basic understanding of the aggregation behavior of the asphaltene fraction, as well as gaps in understanding metalloporphyrin properties and behavior in solution.