Sulfide promoted chronic fouling in a refinery: A broad phenomenon spanning a range of heat transfer surfaces and oil types

Fouling of metal heat-transfer surfaces employed in crude oil refining operations, driven by inorganic corrosion products, is not well understood. Here we employ a range of advanced analytical techniques, including TEM, SEM, FIB and XRD to systematically document the interdependent corrosion – fouling processes of three refinery metallurgies: a carbon steel, a (P91) 9Cr–1Mo steel, and a 347 stainless steel. We utilize two representative crude oil blends, testing at a metal surface temperature of 490°C and an oil bath temperature of 290°C. For the three metallurgies there is a mechanistic similarity of the fouling phenomenon, which begins with sulfidic corrosion of the metal surface and progresses to coking. It is observed that after 1h of testing, carbon steel samples actually fouled somewhat less than the P91, which was initially unexpected since the latter is considered a more sulfidic corrosion resistant alloy. TEM and SEM analyses demonstrate that there is poor adhesion of the sulfide layer on the carbon steel, which we hypothesize results in the metal surface effectively self-cleaning. Despite being the most resistant to sulfidic corrosion and to fouling, the stainless steel nevertheless forms a thin Fe–Cr–Mn rich inner sulfide and a thicker Fe-rich outer sulfide. We also examine the role of volatiles (reactor pressure), and demonstrate that fouling rates drop off with successive tests on the same oil batch. This is attributed to a gradual exhaustion of S-containing species that react at high temperature to sulfide the metal surface, which in turn catalyze the growth of the organic coke.