Base stock production
Group II and Group III base stocks are generally produced from distilled crude oil streams upgraded through high pressure catalytic processes involving hydrogen. The first step in the process involves hydrotreating and frequently hydrocracking. Hydrotreating saturates aromatic species in the feed stream and removes virtually all sulfur and nitrogen from the feed. Hydrocracking involves further conversion of hydrocarbons in the feed primarily to boost the viscosity index of the resultant base stock. Dewaxing and hydrofinishing via catalytic processes follows with the goal of converting wax species, preferably by isomerization, to reduce the pour point of the base stock, improve its low temperature fluidity, and to improve its oxidative stability. Group III base stocks differ from Group II in that they generally originate from high quality feeds and may require more severe processing.
Some Group III base stocks are produced by natural gas or coal conversion to synthesis gas, carbon monoxide and hydrogen, which is converted by the Fischer-Tropsch process and subsequently upgraded in a way similar to convention Group III base stocks.