Refinery operators face increasingly complex challenges in managing integrity of process units and assets – driven by the goal to achieve operational excellence and maximize asset performance while minimizing costs and maintaining the highest safety standards. Naphthenic acids (NAP) and organic sulfur compounds (OSC) present in crude oils pose serious hot oil corrosion problems in oil refineries, especially with the increase in processing of lower-quality, opportunity crudes. In oil at 400-750F (204-400C), simultaneous naphthenic acid plus sulfidation reactions (SNAPS) remove iron (Fe) from steel surfaces. In Corrosion/2021, the authors introduced a molecular mechanistic model that quantifies and predicts SNAPS corrosion rates. The current paper describes the molecular basis of the model's reactions, kinetics, and mass transport to vessel walls. Unlike empirical industry correlation models, the new model takes a radically different, molecular view of hot oil corrosion. The model captures concurrent reaction rates among carbon steel, NAP, OSC, and corrosion products. Competitive adsorption between NAP and OSC and the role of a nano-particulate barrier layer result in para-linear algorithms. Factors are applied to compensate for the reduction of iron surface availability from alloys. Inputs include typical refinery operating / process parameters, oil composition and properties commonly available in a refinery. The model predicts instantaneous and average corrosion rates as well as cumulative metal loss while incorporating reactions simultaneously generating and depleting a "barrier layer". This is accomplished by including additional steps in the NAP and OSC reaction mechanisms. The effect of flow conditions on corrosion kinetics is determined in terms of mass transport or accelerated mass transport of reactive species toward the vessel walls.
Sulfur and acidic impurities in crude oils pose serious hot oil corrosion problems in crude distillation units (CDU) and associated vacuum distillation units (VDU), especially with the increase in processing of low-quality, opportunity crudes.1-4 In the range of 200-400°C, reactive sulfur compounds cause sulfidation corrosion of ferritic carbon and chrome steels in CDU, VDU, and front ends of downstream units operating at hot oil temperatures.5-7 Over the same temperature range, naturally occurring carboxylic acids in crudes can be so aggressive that higher alloy, austenitic stainless steels containing >2.5% Mo are required for processing high acid oils.8-11 Although sulfidation and acid corrosion occur over the same temperature range, they differ in two significant ways. Sulfidation forms an iron sulfide solid that is semi resistant to further corrosion and relatively insensitive to flow velocity. Acids form oil soluble organic salts that can be washed away especially in areas of high turbulence.12-14