6 Refining Used Lubricating Oils
The waxy distillates and deasphalted oil require three further processing steps to
obtain acceptable base stock: (1) the viscosity–temperature relationship of the base
stock (improvement of the viscosity index) has to be enhanced by the removal of
aromatics to meet industry requirements for parafnic stocks; (2) oxidation resis-
tance and performance must be improved by the removal of aromatics, particularly
polynuclear aromatic compounds, nitrogen-containing constituents, and some of
the sulfur-containing compounds; and (3) the temperature at which the base stock
“freezes” due to the crystallization of wax must be lowered by wax removal so that
lubricated equipment can operate at winter temperatures.
After f