High-sulfur bitumen amplifies the effect of polyphosphoric acid

Abstract

This study examines the effect of polyphosphoric acid (PPA) in bitumens with low-sulfur content and high-sulfur (10% sulfur) content. The recently lowered limit on the sulfur content of marine fuel will result in increased sulfur content of the bitumen remaining at the bottom of the barrel. Since the effect of PPA on bitumen's performance highly depends on bitumen’s composition, we hypothesize that high-sulfur bitumen, with an increased content of sulfoxide due to aging, will have higher intermolecular interactions with PPA leading to increase in bitumen’s elasticity. We used computational modeling and laboratory experiments to examine this hypothesis. Based on the results, under laboratory aging, the content of sulfoxide is increased for bitumen samples containing sulfur; a greater increase is observed for high-sulfur bitumen. Our molecular modeling, showed that H-bonding and H-transfer interactions of PPA’s terminal OH with sulfoxide’s polar group (S = O) are responsible for the effective interaction between the two components. The H-transfer interaction has a considerable binding energy (-70.0 kcal/mol), although it is not dominant. H-bonding interactions, with lower binding energy (−(17–22) kcal/mol), are dominant. The high concentration of sulfoxides and dominant contribution of H-bonding interactions can provide an impressive improvement in bitumen’s performance. Upon the introduction of PPA to each bitumen, high–sulfur bitumen showed about 22% increase in percent recovery, while low-sulfur bitumen showed about 12% increase in percent recovery. A higher percent recovery is an indicator of enhanced elasticity. This was also evidenced in a significantly higher complex shear modulus in high-sulfur bitumen compared to low-sulfur bitumen.