With an increasing focus on resid upgrade and bottom of the barrel conversion over the years due to economic advantages, refiners are increasingly processing heavy crudes that yield higher residue content. These crudes are also higher in salt, metal and sulfur content. Often in these cases, there are issues with resid quality in terms of metal content such as Iron (Fe), Calcium (Ca), Sodium (Na) etc. The presence of sulfur/H2S also poses problems that interfere in chemical treatment for removal of some of the mentioned metals above. Several refineries hence blend these heavier crude oils with lighter, sweeter feed-stocks to help meet resid quality.
Crude oils containing calcium naphthenates are a good example for the issue at hand. These crudes originate from various areas. Some of them are from West Africa (Kuito, Doba), the North Sea (Heidrun, Captain), China (Shengli, Bohai Bay) and Indonesia (Serang). The problems posed by high calcium crudes are emulsion stability within the desalter, scaling problems caused by high levels of calcium in the desalter effluent water and corrosion due to a low pH. The high metal content generally makes the crudes conductive which results in high amperage draws of the desalters. Doba crude is high in acids as well as metals which poses further problems during processing. Hence the best way in which to process this crude is with crudes of high API and low sulfur content.
There are several issues that co-exist and it becomes difficult to develop a distinctly clear strategy in resolving these issues. There are metals removal programs that work well under controlled environment. These products that are also acidic in nature pose corrosion issues and often this characteristic limits treat rates. This paper will discuss in detail various issues related with resid quality when processing such type of crude oils in a refinery. There are interesting case studies available where these additives have provided good resid upgrading opportunities due to better performance.
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