Frequent variation of crude oil processed in most refineries have increased the need to predict accurately the potential corrosivity of these crudes toward the distillation units. The results of an industry survey on the subject presented herein, indicate that numerous empirical, analytical and experimental tools are used by the refining industry to assess and predict in a timely manner the corrosivity of crude oils. The applicability and relevance to the plant of each predictive method are discussed.
High temperature crude corrosivity problems have been investigated since the 1950?s, mostly because of their severity and economic impact on refining operations [?I Despite the progress made in elucidating the role of different parameters on the corrosion process, present knowledge does not yet make it possible to predict and understand the high temperature corrosivity of crude oils toward distillation units. The uniqueness in process conditions, materials of construction and blend processed in each refinery and especially the frequent variation in crude or blend processed increases the problem of correlating corrosion of a unit to a certain type of crude oil. In addition, a large number of parameters influences the high temperature crude corrosion process. The total acid number (TAN) or neut number, velocity, temperature, sulfur content, and materials of construction are known to be the controlling variables.
REFINING INDUSTRY SURVEY
A survey of the refining industry on crude corrosivity assessment tools was performed by sending a survey form to individuals working on the subject in their respective company. The survey consisted of classifying several empirical, analytical and experimental methods as very predictive, predictive, somewhat-predictive, non-predictive or unknown.
The results of the survey (Table 1) show that the industry relies primarily on previous experience and the TAN of the crude and its fractions. The analysis of sulfur compounds was next followed by laboratory tests and flow modeling which ranked last.
Despite the usefulness of all the methods included in the survey, the refining industry uses primarily the most easily available tools such as previous experience and standard analysis data found in crude assays. Such a way of predicting crude corrosivity does not indicate necessarily a lack of understanding of the corrosion process but underlines a critical lack of resources, especially time, in dealing with urgent technical problems.
Economics and products yield, rather than corrosion, is the top priority in opportunity crude purchasing. Typically, a 500,000 bbl tanker is docked close to a refinery. The price of its crude is from $1.00 to $3.00 less than the current market. A profit of $500,000 to 1,500,000 can be made rapidly since an average refinery process about 200,000 bbl/day. If consulted at all, the corrosion engineer is given typically a few weeks to decide on how to process the opportunity crude safely. At best, the corrosion engineer has a crude assay with TAN and sulfur analysis of cuts available. The decision on how to process the crude (blending ratio, inhibitor, process conditions changes) is then based on previous experience, if any, and simple rules of thumbs using TA
