ABSTRACT
The current trend in the refining industry is to utilize the heavier and higher molecular weight feedstocks which are more difficult to process. Sulfur occurs naturally in crude oils and must be removed during the refining process to meet the product specifications. Crude oils containing high sulfur content, frequently termed as ‘‘sour’’ crudes, require more extensive processing than ‘‘sweet’’ crudes which contain little or no dissolved hydrogen sulfide, mercaptans, and other sulfur compounds (Gary and Handwerk, 2001). Crude oil is first distilled in an atmospheric distillation unit to remove gases, gasoline, naphthas, kerosene, and light gas oil. The remaining liquid from the bottom of the crude oil distillation tower is flashed and evaporated in a vacuum. Porphyrins forming complexes with metals, such as vanadium and nickel, appear throughout the entire boiling range of the crude oil but tend to concentrate in the heavier fractions and in nonvolatile residues. Some metallic constituents were found in higher boiling distillates (Speight, 1999). The extraction solvent removes the condensed ring aromatics and polar molecules from the lube oil distillates. The solvent-rich fraction, after evaporation of solvent, is an aromatic-rich extract used as fuel, cracker feed, and process oils (Mang, 2001). The solvent dewaxing process removes the waxy type molecules from the lube oil distillates. Slack wax can be used as feed to slack wax hydroisomerization or, after a deoiling process, can be used to produce petroleum wax. The typical yields of various petroleum fractions in conventional refining of a crude oil, suitable to produce lube oil base stocks, are shown in Table 3.1.
