Maintaining reliability when processing opportunity crudes

A risk-based approach that considers crude oil corrosivity can allow refiners to safely and reliably process increasing levels of opportunity crude

CorrMat International

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Article Summary

Strong incentives exist in the low-margin, highly competitive petroleum refining business to process low-cost opportunity crudes. There is also a strong driver to reduce maintenance costs and maximise uptime in order to lower the per-barrel operating cost. The problem is that, if the refiner is not careful, these two objectives can conflict with each other. While some crudes of otherwise good quality are price advantaged because of transportation and logistical issues, one of the more common factors that drive the discounting of opportunity crudes is the presence of high concentrations of corrosive compounds such as sulphur and naphthenic acids. If the refinery operator does not understand how these feedstocks will affect the facility, there is a risk of excessive corrosion, leading to increased maintenance costs and expensive downtime.

While recent high-profile corrosion-related failures were not linked to opportunity crude processing, they illustrate the risk involved with high-temperature corrosion. Besides the cost of replacing corroded equipment, a fire resulting from the failure can drive up the cost of the failure exponentially. The potential for extensive, widespread equipment damage, loss of life and environmental damage, with resulting extended downtime for repair and increased scrutiny from regulatory agencies, make the cost of these sorts of events truly staggering.

While it might be tempting for refiners to forgo the risk of processing opportunity crudes altogether, the consequences of shutting down or selling unprofitable refineries is equally unappealing. In the difficult economic environment of recent years, many European and US refiners who could not or would not process opportunity crudes were forced to shut down or sell their facilities at depressed prices.

The keys to successfully processing opportunity crudes are in understanding the nature of the feedstocks and how they will process through the refinery, as well as in understanding where the greatest risks are, and then in developing appropriate capital and corrosion monitoring plans to manage those risks.

The localised concentrations of reactive sulphur and naphthenic acids, measured as the total acid number (TAN), in mg KOH/g oil, are the primary factors that control the high-temperature corrosivity of the oil. Historically, sulphidic and naphthenic acid corrosion are considered to be a concern starting at about 500°F and 450°F (260°C and 230°C), respectively. More recently, the industry has been trending towards a lower threshold temperature for sulphidation, and an increasing number of incidents of naphthenic acid corrosion at temperatures as low as 350°F (175°C) have been reported.

Besides occurring in the same areas of the crude and vacuum unit, the combination of sulphidic and naphthenic acid corrosion can be further complicated by the interactions between the two mechanisms. Sulphidic corrosion produces metal sulphide scales that limit the extent of attack by both reactive sulphur compounds and naphthenic acids. Naphthenic acids also attack the metal sulphide scales, thus impacting the ability of the scales to protect against both naphthenic acid and reactive sulphur attack. High shear stress due to process flow conditions is also known to accelerate the corrosion process.

As Figure 1 shows, high-temperature corrosion can be broken down into three distinct mechanisms: sulphur dominated, naphthenic acid dominated, and combined naphthenic and sulphidic corrosion. Corrosion based on the first two mechanisms is relatively easy to predict. The complex interactions between sulphur, naphthenic acids, shear stress and the different metallurgies, however, can make predicting actual corrosion rates very difficult when there are significant levels of both TAN and sulphur present. Industry’s efforts to develop models that combine the effects of sulphur and TAN have had limited success. What models exist are generally proprietary models developed by refiners using internally generated research.

Opportunity crude impacts
If significant amounts of opportunity crudes are to be processed with minimal costly capital upgrades and while still maintaining reliability, then better strategies for selecting feedstocks are required. These strategies need to move beyond traditional rules of thumb to take a risk-based engineering approach to selecting new opportunity crudes.

In addition to high-temperature corrosion, opportunity crudes can impair refinery reliability through decreased desalter performance, increased fouling and shortened hydrotreater run length. This article will focus on factoring the sulphur-dominated corrosion risk into the assessment process when evaluating new opportunity crudes. The general approaches recommended here are the same for naphthenic acid-dominated systems and for combined naphthenic acid/sulphur-dominated systems, but these systems are beyond the scope of this article because the use of proprietary corrosion models is required to estimate corrosion rates.

Crude oil selection
Traditionally, crude oil selection is made by the refinery planning and commercial functions. The refinery planning group assesses which crudes best fit in their configuration, and the commercial group goes out into the marketplace to procure those crudes at the lowest possible price.

Crude value is typically assessed using linear computer programs to estimate the actual expected refinery product yields based on the properties of the crude oil and the configuration of the refinery. The actual crudes purchased are the ones that generate the highest margins (the difference between the value of the products generated and the crude price). Longer-term reliability effects on the refinery, such as corrosion, are often not considered at all or are handled through some simplified rules of thumb.

For instance, wt% sulphur limits are typically set on refinery crude slates. These limits can be based on a number of factors, such as the sulphur-handling capacity of the amine system and sulphur plants, product specifications, emission limits, or corrosion. Trying to control refinery corrosion through a total crude oil sulphur specification is often not very effective for multiple reasons.

High-temperature sulphidic corrosion, often also referred to as sulphidation, typically starts to become a concern somewhere around 450-500°F (230-260°C) on carbon steel. The corrosion rate at any point in the process is a function of the process conditions (temperature and shear stress), the metallurgy and the concentration of reactive sulphur. The problem with using total crude oil sulphur to assess corrosion potential is that it does not necessarily represent the actual sulphur content at the location experiencing the corrosion.

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