Refiners are continuously challenged with processing crudes that have increased concentrations of sulfur and naphthenic acid. Very often the original equipment metallurgy was not designed for such corrosive conditions, and therefore substantial upgrades are required in the high temperature areas (greater than 428 oF (220 oC)) where this type of corrosion will show up. This paper will present new concepts for the evaluation of critical TAN (Total Acid Number) and sulfur level based on the comprehensive evaluation of high temperature corrosion risk. A model consisting of public and proprietary data has been developed for this assessment. The primary factors evaluated include TAN / sulfur content, materials of construction, and temperature. Utilization of this model enhances the predictability of corrosion due to TAN and sulfur. Historical data of corrosion monitoring is used to validate the accuracy of the model. Minimum wall thickness required for pressure vessels and piping is also calculated. Then, high risk parts of distillation units are identified. Critical operational TAN and sulfur level of a crude distillation unit is determined by the identified maximum allowable TAN and sulfur concentration of any one circuit. This advanced knowledge will allow for increased reliability of crude distillation units where this is implemented.
One of the worldwide trends in crude oil refining industry is the purchasing and processing of crudes with increasingly higher TAN and sulfur content. Such “opportunity” crudes are intentionally purchased with great discounts. Since the cost of crude oil accounts for approximately 90% of the economics, any money that can be saved on crude purchases can potentially provide an economic savings over industry competition. On the other hand, inherently corrosive crude oil could etch refineries' profit margin by significantly increasing the corrosion rate, and thereby reduce the reliability of the equipment. Therefore, processing corrosive crudes with confidence is a great challenge in the competitive market. Naphthenic acid (NAC) and sulfidation corrosion (SC) attack refinery equipment that operates in the high temperature range of 425 oF (218 oC) up to about 750oF (400 oC). Over the last 60 years many efforts had been made to understand and predict NAC and SC1-8. However, precisely predicting NAC and SC rates based on common knowledge is still a challenging task. Below are some rules of thumb and useful facts regarding NAC and SC:
_ There are two different corrosion prediction tools for SC. The modified McConomy curves are used for SC in H2-free streams (which is the default type SC discussed in this paper) and the Couper- Gorman curves that are used for SC in H2/H2S-containing streams9,10.
_ SC in H2-free streams is thought to occur at temperatures above 446 oF (230 °C). High temperature SC is typically a general corrosion. The protective iron sulfide scale that forms from SC will partially inhibit NAC7.
_ Limited plant experience shows that the modified McConomy curves generally overestimate SC rate. However, the modified McConomy curves could underestimate SC rate at some specific side cuts or locations.
