A typical re-refining process consists of the following steps:
Pre-processing: Filtration of waste oil and dehydration through decantation method.
Dehydration: Separation of water and light hydrocarbons using distillation method.
Diesel Separation: Separation of diesel fraction (C7 – C12) through distillation method.
Asphalt Separation: Separation of asphalt fraction using thin-film evaporator. Requires high temperature and vacuum.
Final Treatment: After the separation of water, light hydrocarbons, and asphalt fraction, the waste oil undergoes chemical processing through solvent extraction or hydrogenation method.
Fractionation: Obtaining different fractions of base oil based on differences in boiling points.
Distillation is the process of separating components in a liquid mixture by taking advantage of the differences in their boiling points, achieved through partial vaporization followed by condensation.Distillation can be classified into various categories based on different parameters. Depending on the mode of operation, it can be continuous-batch; atmospheric-vacuum; based on feed stream, binary system-multicomponent system; based on the number of product streams, single stream-multiple streams; and based on column internals, structured column-packing column, among other classifications.
In processes such as the refining and recovery of base oils from crude petroleum, the most common types of distillation encountered are vacuum distillation in packed columns and atmospheric water removal in flash evaporators. Different types of packing materials are shown in the figure below.
This process is a liquid-liquid extraction process, using propane as the solvent. The process is based on the principle that propane does not dissolve high-viscosity substances while dissolving low-viscosity substances. Since asphalt is the highest viscosity fraction, it forms sediment during extraction, remaining insoluble in propane and separating from the mixture.
All impurities in the waste mineral oil, along with high molecular weight components with low volatility, are separated as bottom product in the thin film evaporator. Achieving the highest possible waste oil recycling requires reaching high temperatures and the lowest vacuum values. To prevent oil degradation at high temperatures, the residence time within the equipment must also be minimized. The thin film evaporator used in this process is vertically cylindrical in structure, with a rotating shaft inside that creates a thin oil film layer on the surface. The rotation of the shaft generates a turbulent flow regime, causing the oil film layer to reach high temperatures. The volatile components vaporize and are removed as an overhead stream. Subsequently, they are condensed by passing through a condenser.
In a short path evaporator, the condenser is designed to be inside the equipment body. Classic thin film evaporators and short path evaporators have different designs. As a result, while the classic types can only be reduced to a minimum operating pressure of 1 mbar, the internal pressures of short path evaporators can be lowered to as low as 0.001 mbar due to their distinct designs.
Solvent extraction is typically based on the separation of aromatics, naphthenes, and impurities within waste oil using a solvent. This separation process occurs in three steps. In the first step, the solvent and waste oil are brought into contact. In the second step, the two resulting phases (solvent-rich phase, extract, and solvent-lean phase, raffinate) are separated. Finally, these phases undergo distillation for the purpose of solvent recovery.
Catalytic hydrogenation has long been recognized as a modern and successful refining process. The process is based on the principle of contacting the oil fraction with a solid catalyst in a pressurized hydrogen environment. The flexibility of the process, achieved through the selection of suitable catalysts and operating conditions, allows for its applicability across a wide range of product outputs, from the lightest to the heaviest fractions. Operating conditions may vary within a framework determined by the nature of the required reactions and the oil fractions subject to hydrogenation.
Similar to the hydrogenation process used in petroleum refining, the objective of hydrogenation in waste oil recovery is also the stabilization of unwanted components. Removal of sulfur compounds and an increase in the saturated hydrocarbon content can lead to the production of products meeting the Class 2, Type 2 quality of the TS 13369 base oil standard. The reactions achieved through hydrogenation are provided below.
Bleaching is a finishing step carried out in waste oil recovery processes. The primary objectives of this process are as follows:
- Removal of residual impurities after dehydration and refining,
- Lightening of the dark color,
- Elimination of odors.
Bleaching, a relatively straightforward process, is conducted in three steps. Oil, which has been stripped of water, light hydrocarbons, and asphaltic residues, is introduced into a reactor to come into contact with clay at a concentration of 2-5% by mass. The mixture is left to complete the adsorption process with the aid of mixers in a tank kept at the required temperature. The duration of this process can vary depending on the type of clay used, mixing efficiency, and feedstock characteristics. The duration is determined based on preliminary tests. Strong acids present in the composition of activated clay react with impurities and additives in the oil, forming insoluble sulfates. After the adsorption process, the slurry-like material left to settle in the reactor tank undergoes phase separation after some time. The sulfates settling to the bottom of the reactor tank and waste clay are sent for disposal, while the mixture suspended in the upper portion is pumped through filter-press layers for filtration. An example process flow diagram is provided below.
