Figure 5.12 shows a general scheme of solvent dewaxing that uses stage-wise refrigeration of the feedstock after it is mixed with the solvent. The lowest temperature used in the refrigeration cascade depends on the desired pour point of the lube oils base stock product. Upon refrigeration, wax compounds solidify to form crystals. Wax crystals are carried in the solvent to a rotary filter where wax is separated on a filter cloth covering the rotating drum. The layer of wax (filter cake) on the drum is scraped from the filter by a blade and carried away in a solvent stream to a steam-stripping unit to recover and recycle the solvent separated from the wax product. The wax product, called slack wax, can be used to make paraffin wax for candles, microwax used in the cosmetics industry, and petrolatum for petroleum jelly. The dewaxed oil from the filtration unit is also steam stripped to recover the solvent to produce the lube oil base stock.
Figure 5.12. Solvent dewaxing process.
The two principal solvents used in solvent dewaxing units are methyl ethyl ketone (MEK) and propane. Although the majority of dewaxing units in the U.S. refineries use MEK), some advantages of using propane as a solvent compared to MEK include the following :
· Propane is used both as a diluent and as a refrigerant
· Lower capital investment
· Refrigeration energy savings
· Higher filtration rates
· Rejection of asphaltenes and resins in the feed
· Higher VI than ketone dewaxing
