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.
Corrosion in crude overhead systems stems primarily from the presence of hydrogen chloride vapor present from hydrolysis of salts in the atmospheric crude distillation unit. The most common source of HCl is from the hydrolysis of calcium chloride and magnesium chloride salts at temperatures exceeding 250 oF (121 oC)1-7 though the HCl may also come from the decomposition of organic chloride species1,8- 10. HCl, being a light volatile gas, moves into the crude unit overhead condensing systems where it is readily absorbed into condensing water. Various remedies are used to mitigate the acidic attack from condensed water containing HCl, including neutralizing compounds like ammonia and organic amines2-4,6,8,11, film-forming inhibitors1,5-6,9, wash water systems1-2,6,11-12, and close control of temperature in the overhead circuit3,13-15.
Corrosion in crude distillation overhead systems is a common problem in refineries. The primary mechanism of concern in the overhead system is the acidic attack of unit metallurgy due to the presence of high concentrations of HCl in condensing water. Other corrosion and operational concerns include under-deposit corrosion and fouling usually resulting from unintended consequences of acid neutralization and / or inefficient desalter operations.
The presence of acid in the crude overhead system may result in particularly severe attack as the acids easily dissolve in condensing water resulting in local environments with pH values typically as low as pH 16,9-10.