By Miyanishi M.
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Energy costs are high because of the number of process steps needed and the energy intensive nature of equipment such as refrigeration plant and solvent recovery systems. 4 tonnes fuel oil equivalent per tonne of base oil product are not uncommon. The costs of capital tend to relate to the age of the base oil plant. A brand new plant has to be financed and since base oil production plant is very expensive to build, depreciation and interest charges will be considerable. Most of the present day base oil plant is at least 15 years old and so by now is almost fully depreciated.
However, the process had severe drawbacks; it was very labour intensive and oils of high viscosity could not be filtered at low temperatures. Filtration efficiency could be greatly improved by diluting the oil with solvents such as naphtha, but the selectivity for wax removal was reduced. Improved solvent systems have been developed to give better de-waxing performance and important factors in the choice of solvent are: • • • • • • Good solubility of oil in the solvent and low solubility of wax in the solvent.
6 Solvent extraction Solvent extraction replaced acid treatment as the method for improving oxidative stability and viscosity/temperature characteristics of base oils. The solvent selectively dissolves the undesired aromatic components (the extract), leaving the desirable saturated components, especially alkanes, as a separate phase (the raffinate). e. to give good yields of high quality raffinate) solvent absorption power (to minimise the solvent/oil ratio) ease of separation of extract and raffinate phases ease of solvent recovery (boiling point must be below that of the raffinate or extract) desirable solvent properties (such as stability, safety, low toxicity, ease of handling, cost).