High Temperature Sulphidation and Naphthenic Acid Corrosion modeling in Crude oil refining

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Master of Science in Engineering 16 November 2018High temperature sulphidic (HTS) and naphthenic acid corrosion (NAC) pose not only reliability, but also health, safety and environmental threats to crude oil distillation unit operations. Based on an oil refinery’s crude oil diet properties, materials of construction and operational conditions in the distillation units, these high temperature corrosion phenomena occur in varying severity. In a bid to improve profit margins, oil refineries may choose to process opportunity crude oils at the risk of aggravating these corrosion mechanisms. In order to predict corrosion rates due to HTS and NAC, an industry corrosion model, Predict CrudeTM, was used on a refinery’s atmospheric and vacuum distillation units. The model was based largely on empirical data from a 4-year joint industry program that collected NAC and HTS corrosion rate data. The model provided qualitative corrosion rates for different distillation product streams for three model cases, based on total acid number (TAN) namely crude blend TAN 0.35, TAN 0.5 and TAN 0.8. The corrosion rates produced by the model for the 0.35 and 0.5 TAN cases were assessed against the measured corrosion rates, using conventional (industrial ultrasonic thickness and profile radiography) plant inspection techniques. Good agreement between the model predicted corrosion rates for the 0.35 and 0.5 TAN cases and the practically observed corrosion rates was found. The 0.8 TAN case was used to assess upset (unusual) conditions only, as the plant did not process 0.8 TAN crude blends. The 0.8 TAN case model data also explained some of the higher than modelled corrosion rates that were measured. Thus, the model provided a closely accurate prediction of corrosion rates to be expected in the atmospheric and vacuum distillation units.E.R. 201