How important is the base oil in the selection of lubricants?

Contrary to popular belief, there is no doubt that the type of base oil and the refining method used must be considered when selecting lubricants, particularly for unique or unusual applications. In order to understand why base oils exhibit different qualities, it is necessary to briefly describe the various refining processes.

Acid/Clay Refining

This process involves the treatment of crude oil distillates with sulfuric acid or oleum. It initially improves the color and aging tendencies while increasing density and viscosity index. Today, the use of acid/clay refining processes is limited to the manufacturing of some white oils and sulfonates along with the regeneration of some waste oils. This process has, to a large degree, been replaced by more modern refining methods. This is due to the fact that the process produces large quantities of acid sludge, which is very difficult to dispose of without a negative effect on the environment.

Solvent Refining

This is the term for the removal of most of the aromatics and undesirable constituents of oil distillates by liquid extraction. Common and suitable extractants are phenol, furfural and sulfur dioxide. Furfural is used extensively as the extractant for the refining of paraffinic oils. The resulting base stocks are raffinates (referred to as neutral oils) and an extract fluid that is rich in aromatic content, which is highly sought after as process oils and fuel oils.

After solvent extraction, the raffinates are de-waxed to improve low-temperature fluidity and then hydrofinished to improve the color and stability. The final quality of the base oil is determined by the severity of the application of temperatures and pressures in the hydrofinishing process. The base oils are now ready to be selectively blended with the appropriate additives.

Catalytic Hydrogenation

Also called hydrotreating, this refining process subjects the distillates to a chemical reaction with hydrogen in the presence of a catalyst as high as 420 degrees C and pressures up to 3,000 psi. Hydrotreating processes are the favored methods used by many base oil refiners and lubricant manufacturers. This is due to the small material losses involved. When hydrotreating is performed, more than 90 percent of the aromatic content is converted to hydrocarbons.

The principle of all hydrogen-treating processes is quite similar. The crude distillate is preheated to temperatures between 150 and 420 degrees C and then brought together with hydrogen or a hydrogen-enriched gas through a fixed bed reactor. The oil reacts with the hydrogen in the presence of a catalyst to control and speed up the reactions.

There is no question about the higher purity of hydrotreated base oils, but they do have some disadvantages. Some additive types cannot be effectively blended with these base oils because they drop out of solution. That is, hydrotreated base oils cannot retain their solubility for some chemicals, and thus additive retention may be seriously affected.

In addition, because severely hydrotreated base oil contains almost no aromatics, these oils must be fortified with seal swell agents in the additive package. Solvent-refined base oils, on the other hand, retain some aromatics, which are “natural” seal swell agents.