Join the Department of Chemical and Biological Engineering for a fall seminar on Wednesday, September 26 from 3:15–4:30 p.m. in Perlstein Hall Auditorium. Peter K. Kilpatrick, provost and senior vice president for academic affairs, will deliver a lecture titled “Ultrapure Petroporphyrins, their Molecula Properties and their Role in Impacting Water-in-Oil Emulsions Stabilized by Asphaltenes.”

Petroporphyrins are the heavy metal (V, Ni) containing hydrocarbon molecules in petroleum, accounting for as much as 0.2-0.4% of heavy petroleum. As such, they are the origin of many of the challenges associated with crude oil upgrading due to catalyst poisoning.  They are also central in impacting the self association of asphaltenes and the related adsorption of asphaltenic aggregates in interfaces, including pipeline deposition, fouling, and emulsion stabilization.  Here we report the purification and characterization from a North American heavy crude residue of a very wide variety of vanadyl petroporphyrins with intense Soret bands ranging from 405-423 nm. The purity and identity of these materials was determined through metal analysis by x-ray fluorescence spectroscopy; porphyrin Soret, alpha, and beta band absorbance by UV/visible spectrophotometry; and molecular weight distribution and porphyrin types by LDI-TOF and FT-ICR mass spectrometry. These porphyrins, purified from the vacuum resid, comprise at least 10 differing porphyrin base types (including DPEP, Etio, and Rhodo forms and their combinations, with and without thiophenic pendant groups).

We have utilized small-angle neutron scattering (SANS) to investigate the aggregation behavior of these petroporphyrins, measuring nanoaggregate size of several different petroporphyrins in toluene and 60-40 toluene-hexane mixtures at multiple concentrations and temperatures. We have also used PFGSE-NMR to estimate nanoaggregate size to corroborate and complement our SANS results. Additionally, SANS has allowed us to measure the thickness of the petroporphyrin films that form at oil/water interfaces by emulsifying water/toluene mixtures with dissolved petroporphyrin, allowing the films to form, then washing away residual petroporphyrin in solution and measuring the scattering profile of the remaining petroporphyrin films, which can be used to estimate its thickness.

Using an oscillating drop rheometer, we have measured the tension and elasticity of the isolated petroporphyrins and have seen that despite significant variation in molecular structure, the tension and elasticity of petroporphyrin interfacial films is remarkably consistent, with tensions steadying at around 15 mN/m (about 10 mN/m less than asphaltenes) and elasticities tending to plateau around 40 mN/m (about 15 mN/m greater than asphaltenes). Interestingly, while asphaltenes exhibit molecular rearrangement at the interface, indicated by a decrease in elasticity beginning sometime between 24 and 48 hours, we have observed no such behavior with petroporphyrins. Even after several days, the elasticity continues to slowly increase while the tension remains fairly consistent.

Finally, we have performed these same characterizations (aggregate size, film thickness, interfacial tension and rheology) with asphaltenes as well, both with and without added petroporphyrins of several different classes, to better understand the relationship between these two families of hydrocarbons.