| Résumé | Inorganically driven fouling of metal heat-transfer surfaces employed in crude oil refining operations is not well understood. The object of this study is twofold: First, we systematically elucidate the time-dependent mechanism of the interrelated carbonaceous and sulfidic build up that occurs at high temperatures on a metal surface (540 °C metal temperature, 250°C oil bath temperature). Second, we demonstrate that additions of 0.5, 1.3 and 5.7 vol% thiophene (C4H4S) cause a 2×, 10×, and 20× reduction in the fouling factor after a 1400 min exposure. Analytical techniques including TEM, SEM-EDX, FIB, Auger electron spectroscopy and XRD were employed to detail the fouling phenomenology for a heated stainless steel wire immersed in atmospheric bottoms fraction crude oil, exposed for 1-1400 min. A key microstructural observation is the transformation of the wire's as-received near-surface textured austenitic grain structure into a micron scale (e.g. ∼10 μm at 1400 min) highly porous inner-sulfide/chromium oxide bilayer composite. Additionally, we observe significant localized sulfidic attack into the bulk of the metal. During testing, an iron sulfide (pyrrhotite Fe(1 - x )S) corrosion product forms almost instantaneously at the metal surface, followed by coke formation around its periphery at longer times. This temporal sequence, combined with the observation that the thicker regions of the foulant are clearly associated with detached plumes of the sulfide, leads us to argue that the sulfide is essential for promoting organic fouling. This is brought about by the sulfide's action as a potent dehydrogenation catalyst that drives the transformation of pitch to coke. We hypothesize that the tremendous fouling inhibition effect of the thiophene originates from its adsorption onto the sulfide surfaces, thereby blocking the dehydrogenation reactions. |
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