Petroporphyrins: Their Purification and Impact on Asphaltene Aggregation and Interfacial Phenomena

Petroporphyrins contribute to many of the challenges encountered when producing, transporting, and refining heavy crude oil and bitumen. They are the source of heavy metals which poison catalysts and which may facilitate the aggregation, deposition, and emulsion formation exhibited by asphaltenes. Here they are extracted and enriched to ultra-high purities from several sources: an Athabasca Bitumen, a Canadian northern tier crude, and a North American heavy crude. Petroporphyrins are characterized by UV-Visible (UVVis) spectroscopy, X-ray fluorescence (XRF) spectroscopy, and mass spectrometry. The majority of the petroporphyrins are simple etio (407 nm Soret band) or DPEP (410 nm Soret band) types, but some are more functionalized compounds with highly broadened and shifted Soret bands.

Small-angle Neutron Scattering (SANS) is used to characterize the aggregation of pure vanadyl petroporphyrins (VOPPs) as well as of asphaltenes mixed with either VOPPs or synthetic porphyrins. Simple VOPPs generally form small nanoaggregates, likely dimers and trimers, and have low solubility in toluene and form precipitates. More complex VOPPs can form large, asphaltene-like nanoaggregates in toluene. Less functionalized porphyrins are shown to inhibit asphaltene aggregation while ore functionalized porphyrins, potentially having one or more intermolecular interaction sites outside of the central macrocycle, tend to promote aggregation.

Vanadyl petroporphyrins purified from a North American crude oil vacuum residue are used in experiments probing the interfacial phenomena of the petroporphyrins as well as asphaltenes-petroporphyrin mixtures. Water-oil emulsion stability as a proxy for film strength is measured using the centrifugal method. Interfacial tension and elasticity of asphaltene and petroporphyrins films are recorded over tens of hours using an interfacial dilational rheometer. Film thicknesses of asphaltene films with and without petroporphyirns are estimated by fitting multiple models to small-angle neutron scattering curves. UV-Visible spectrophotometry is used to compare petroporphyrin content in the interfacial and bulk material of emulsions. Petroporphyrins are shown to be highly surface active and have significant effects on asphaltene film development. Highly-surface-active petroporphyrins form monolayers at the interface with very low tension high dilatational elasticity. However, this high elasticity does not translate to film strength.