In the refining industry, the business need to work with opportunity crudes, an essential element of current day reality, has meant that refinery operators face substantial risk of corrosion damage in both the atmospheric unit and the vacuum unit. The atmospheric unit, also called the crude distillation unit (CDU), is often exposed to increased levels of chlorides and sulfur species, leading to substantial corrosion issues related to the species that manifest in crude overhead operations. These corrosion issues include: (a) under deposit corrosion due to sublimating species such as ammonium chloride, (b) aqueous corrosion due to hydrochloric acid (HCl) or acidic sulfur species, and (c) fouling issues related to the build-up of sublimating species. Often, the principal cause for these problems may be correlated to high chloride content in the crude coming out of the desalter or inadequate controls to ensure conditions for fractionation operations above aqueous dew point. The difficulty in predicting and assessing the contribution and severity of these corrosion / fouling problems stems from the significant complexity of the chemistry involved and the inadequate documented experience correlating speciation of contaminants from the crude feed to the corrosion problem. The purpose of this paper will be to: (a) review published literature to characterize and classify speciation related to the types of impurities encountered in opportunity crudes; and (b) describe and categorize published case studies of corrosion in crude unit overhead operations. In doing so, the authors will attempt to delineate the primary corrosion problems encountered in CDU overhead systems, parametric components or species in the crude that drive corrosion, and gaps in technology that require additional study.
Key words: crude overhead, salt hydrolysis, HCl, ammonium chloride, dew point, salt formation, fouling, neutralizer, ammonia, amine, film-forming inhibitors