170 Refining Used Lubricating Oils
Viscolube had been working with the propane extraction process under licence
from the Institut Français du Pétrole (IFP) since the early 1970s (Audibert 2006) to
remove the asphalt fraction, later developing its own thermal deasphalting (TDA)
process, and this formed the second step in the process.
The embodiment in Figure 7.5 shows the used oil entering a static mixer together
with a basic reagent such as a 30% solution of sodium hydroxide (NaOH), the alkali
addition being controlled by a pH measurement at the bottom of the preash column
at a preferred value of between 10.8 and 11.2. This stream is then heated to a temper-
ature between 120°C and 140°C (248°F–284°F) before entering the preash column.
This ...