Let us consider the main methods used in turbine oil purification from various harmful impurities.
A 96% solution of sulfuric acid is used to remove unsaturated hydrocarbons, tar and nitrogen compounds from petroleum products. Its quantity is calculated based on the volume of oil to be purified (0.5-1% by weight). The result of sulfuric acid cleaning is the separation of oil: the top remains a cleaned layer with acid residues, and below is an acidic tar, outwardly resembling a black viscous mass.
To remove organic acids, phenols, hydrogen sulphide and mercaptans resort to cleaning with alkali (in most cases, caustic soda). It is also used to neutralize sulfuric acid, which was previously used to purify the oil. The substances obtained as a result of chemical reactions dissolve in water and can easily be removed together with an aqueous solution of alkali. This approach is still more often used in the purification of fuels.
Absorption of resins, sulfur compounds and organic acids is realized due to the use of special substances – adsorbents (bleaching clays or silica gel).
The method of selective purification provides for selective removal of oil components due to the different dissolving power of some reagents. In most cases phenol, furfural, liquid propane, nitrobenzene and the like can be used as a solvent. Some of these substances dissolve the undesirable component and are removed, while others dissolve hydrocarbons, and harmful impurities make precipitation. If the layers are separated correctly and the solvent is distilled off, then it can be reused.
Paraffins are hydrocarbons with a high pour point. The process of their removal is called dewaxing and is realized by cooling the oils and then separating the formed solid crystals.
Sulfur compounds are removed from the oils by hydrotreatment (treatment with hydrogen). High temperatures and pressures are required to efficiently flow the process. The compounds, which contain sulfur, are removed by binding with hydrogen to hydrogen sulphide.
In parallel, distillates are purified from organic acids and unsaturated hydrocarbons. The general scheme of hydrotreating is as follows. Contaminated oil, together with hydrogen, is heated to a temperature of 400 ° C, and then fed to a special reactor, maintaining a pressure of 50 kg / cm2. To accelerate the ongoing processes, an alumino-molybdenum catalyst is used.
Cleaning the oil in centrifuges is a simple, reliable and cheap way to get rid of mechanical impurities, water and tar. The disadvantage of centrifugation is the inability to provide the required grade of oil purity at an initially high degree of contamination.