Group I and Group II base oils (BO) are conventional mineral oils obtained from heavy petroleum fractions using various solvents. The most sought-after paraffinic feedstocks give a high yield of products with a high viscosity index, although they also contain a lot of paraffins. For some applications, naphthenic-base oils are preferable, since they give a high yield of products with a medium or low viscosity index, a very low paraffin content and a low pour point, which is inherent in their nature.
Production of Group I and II base oils takes place in several stages:
1. To produce base oils with a high and medium viscosity index, selective solvent extraction of vacuum distillates obtained by vacuum distillation is used. Selective solvents remove colored, unstable, low-index components from the base oil. The presence of aromatic compounds reduces base oil quality, because they are among the most reactive components of the oil within the boiling range. Oxidation of aromatics can initiate a chain reaction that can significantly shorten the service life of the base oil. The process consists of direct contact of the vacuum distillate fraction with a solvent such as phenol, furfural or N-methyl-2-pyrrolidone. The resulting product is usually called "raffinate".
2. The next process, which lowers the pour point, is dewaxing. The oil is mixed with a blend of two solvents (methyl ethyl ketone and toluene). The resulting oil solution is cooled to a temperature of -6 …-12°C. At this temperature paraffin crystals precipitate and are separated by filtration, and the solvent is distilled off from the oil. The result is a dewaxed oil with improved properties: a lower pour point and a higher viscosity index. The by-product is paraffin slack wax, which subsequently serves as feedstock for catalytic hydrocracking, from which high-quality base oils can be obtained.
3. Adsorption or hydrotreating. After it the oil becomes lighter in color, so this process is sometimes called decolorizing. Bleaching clay or crystalline aluminosilicates, zeolites with uniform porosity, are used as adsorbents. By selecting zeolites with pores of a specific size, selective adsorption of particular compounds can be carried out: resinous and asphaltene substances, alkenes and polycyclic arenes. Adsorbent purification is usually carried out after selective refining and dewaxing. Hydrotreating is carried out by the action of hydrogen on petroleum fractions in the presence of a catalyst. Unsaturated and aromatic molecules of the base oil are converted into saturated ones. At the same time, desulfurization and removal of nitrogen-containing compounds take place. Mild hydrotreating is also usually used to reduce the color and odor of the oil.
That is all for now. We will cover the production of Group III and IV base oils in our upcoming publications.
