Desulfurizing Heavy Oils

SUCCESS IN applying today's sulfur-removal methods to heavy crude oil depends primarily on the oil's concentration of polycyclic aromatic hydrocarbons, not on the nature of the oil's sulfurous molecules, according to a study conducted at ConocoPhillips (Environ. Sci. Technol., DOI: 10.1021/es0720309). That observation, which contradicts conventional wisdom in fuel processing, deepens understanding of desulfurization chemistry and may open the valve to using alternative feedstocks to produce low-sulfur transportation fuels.

To reduce the sulfur content of gasoline and diesel fuels to the low parts-per-million level, as mandated in several countries to decrease harmful pollutants, oil manufacturers routinely use cleanup technologies such as hydrodesulfurization. In that process, sulfur compounds in fuels react with hydrogen in the presence of Co-Mo- or Ni-Mo-based catalysts to produce volatile H₂S, which is removed from the feed material.

Years of investigations have shown that the effectiveness of such desulfurization methods in treating light crude oils is limited by the reactivity of hard-to-remove sulfur compounds such as dibenzothiophenes. As a result, scientists focus on the structures and other properties of those compounds to optimize sulfur-removal catalysts and processes. But as high-quality feedstocks are gradually depleted, fuel suppliers are turning to heavier crude oils. Thus far, those oils, which are complex mixtures that are difficult to analyze, aren't yielding to desulfurization-optimizing methods.

"We all assumed that the conventional wisdom in light oils would apply to heavy oils. But it doesn't work that way," says Tushar V. Choudhary, who conducted the study with Stephen Parrott and Byron Johnson at ConocoPhillips.

On the basis of newly developed analytical methods, pilot-scale hydrotreating tests, and kinetics investigations of several heavy oils, the ConocoPhillips researchers conclude that three-ringed and larger polycyclic aromatic hydrocarbons, such as anthracene, reduce the effectiveness of heavy-oil desulfurization by blocking the access of sulfur compounds to catalysts' active sites.

"Now that they found the culprits, researchers can begin designing new catalysts or conditions to eliminate them," says Daniel E. Resasco, a catalysis specialist at the University of Oklahoma, Norman.