Used oil undergoes a pretreatment step in the overall recycling process. The recycled oil is dewatered to get rid of its water content. The main tool used to dewater oil is gravity. The process involves allowing the natural separation of the oil from the water over time When the two have separated into an upper layer of oil and a bottom layer of water, the water is then drained from the oil and undergoes a treatment process to make it safe for disposal into the environment. Heat is also used in the dewatering process to speed it up by causing the water to evaporate. Once the water is separated out of the oil, it is then ready for the next step, filtering.
Debris and additives are filtered out of the recycled oil. Some of the additives extracted by filtering are also recycled for reuse. The used oil is placed in a reaction tank, where it is mixed with sulfuric acid and then heated.
A surfactant is added to the mixture, which is then left to separate into oil and a water-based, soap-like phase. This water-based part collects debris and contaminants, such as oil additives and metals, and then it is drained off at the bottom of the tank. The water is then evaporated off, and the waste is disposed of as solid matter. The oil is filtered again to remove finer particles of debris. The oil is then ready to be used as fuel, and can undergo further refining to produce different grades of fuel oils, from heavy fuels to lighter fuels for specialized burners.
Propane deasphalting (PDA) is the refining process that removes asphalt from the recycled oil. The used oil is fed into the deasphalting apparatus, where liquid propane is added from the bottom of the tank. As the propane rises through the oil, and the oil settles, it dissolves the lube oil elements, separating them out of the oil, and allowing them to collect at the bottom of the tank. The propane is then recycled by retrieving it from both the oil and the waste through vaporization. The vaporized propane is then collected, condensed and stored for future use in the PDA process.
Recycled oil can also undergo distillation to remove water, gases, petrol and solvents. The first type of distillation is atmospheric distillation, a process that separates these components from the oil based on their respective boiling points. As the recycled oil is heated in the distillation process up to 300 degrees Celsius, each component boils off and is collected for reuse.Another type of distillation is vacuum distillation, which further refines the oil to control its flash point, viscosity and carbon residue. The recycled oil undergoes heating at temperatures higher than 300 degrees Celsius, as the vacuum system works to reduce pressure build-up that, in turn, prevents destruction of the oil components. The oil is heated and distilled off into trays, as different grades of oil. The lightest is gas oil, which is distilled off at the highest level of the system. This is followed by light oil, medium oil and heavy oil. This process separates out the remaining components, called vacuum tower bottoms (VTB), the heaviest components in the oil, that were unaffected by the distillation process.