Let us look into the listed methods in more detail. If large particles and water are the largest impurities in the oil, settling may suffice. The process only requires a container which is filled with the contaminated oil. After several hours of settling, the gravity will pull the impurities to the bottom of the container. The main drawback of this method is the long duration of the process.
Centrifuging is also used to remove solid particles and water from oil. Centrifuges rely on the centrifugal force, instead of gravity. The method does not require much time, but the degree of purification may not be satisfactory if the oil is highly contaminated.
Filtration is a part of every modern oil recycling system. Filters can remove water and dispersed particles of almost any size. The product range of the modern filtration equipment is quite wide: filter presses, band, drum and disk vacuum filters. Textiles, cardboard and paper are used as filtration materials.
Coagulation is used if the oil contains particles of colloidal size. This process involves artificially enlarging contaminant particles by adding coagulants to the oil. These can be surfactants, electrolytes and some hydrophilic high molecular compounds. Coagulation efficiency is determined by the current oil temperature, as well as intensity and duration of agitation.
None of the above methods can remove aging products and acidic compounds. Only adsorbents have this capability, therefore, full oil recycling is impossible without them.
Adsorbents are natural or synthetic materials with high retaining capacity. They absorb water and aging products on contact with oil. Bleaching clay, silica gel, alumina silicates and zeolites are among the most commonly used sorbents. Adsorption processing is usually the final stage of oil recycling.
Extraction is the process of treating the oil with special solvents to extract three impurities: particulate matter, water and concentrated organic compounds. The main limitation of this method is the high cost of the solvents.
Acids and alkali can also be employed for oil recycling. In the former case, it is usually sulfuric acid, which itself is a hazardous material. Besides, the process results in formation of acidic tar bottoms, which is difficult to safely dispose of with the known methods.
Hydrogenation is the process of treating the oil by hydrogen in the presence of a catalyst. Unsaturated hydrocarbons are transformed into saturated hydrocarbons, which facilitate removal of impurities and improves stability of the oil. The technology involves high capital and operating costs.
The review clearly shows that complete oil recycling is mostly impossible using only one single process. Therefore, oil recycling systems must combine various approaches, with many factors influencing the oil for the best results. The specific sequence and the list of the methods used is selected based on the specifics of the oil.