Over time, the re-refining processes are advancing and thus improving, especially from the aspects of yield and reduced environmental impacts. The most common of the generic systems are:
Acid-clay Re-refining
This ancient and once popular process, namely the acid-clay re-refining process, was first employed in the mid-1960s by many companies in United States, wherein large amounts of Sulphuric acid and clay were used to treat waste oils.
While the technology produced acceptable, although sub-standard, base oil, it also generated acid tar, oil saturated clay, and other hazardous waste by-products. Under increasing environmental pressure this technology has been banned in most countries, including many developing countries.
Vacuum Distillation/Evaporation Technology
Waste oils are chemically pre-treated to avoid the precipitation of contaminants which can cause corrosion and fouling of the equipment.
Pre-treated waste oils are distilled to separate off water and light hydrocarbons. The resulting water is treated and sent to a wastewater treating facility, while the light hydrocarbons are used at the plant as fuel or sold as a product.
The water-free oil then undergoes high vacuum distillation using a conventional vacuum column or in a thin film evaporator for separation of diesel fuel. Materials such as residues, metals, additive degradation products etc. are passed on to a heavy asphalt flux stream.
The distillate is hydro-treated at high temperature and pressure in the presence of catalyst to remove nitrogen, Sulphur, chlorine and oxygenated organic components. The used catalyst from the hydro-treating process is disposed off-site.
Under high vacuum, the hydro-treated oil is further fractionated into desired cuts which are used as components of motor, industrial and hydraulic oils. Residue from the vacuum distillation is used for producing road and roof bitumen.
SPECIFIC TECHNOLOGIES FOR WASTE OIL RECYCLING
KTI Process
The KTI (Kinetics Technology International) Process, also known as KTI Re-lube Technology, combines vacuum distillation and hydrogenation treatment to eliminate most of the polluting substances in used oil.
Basic Steps of the Process
Atmospheric distillation: This involves eliminating water and light hydrocarbons.
Vacuum distillation: The resultant product comes within the range of the lubricating oils. The working temperature should not exceed 250ºC.
Hydrogenation: Oils distilled in the previous stage are subjected to a hydrogenation treatment to eliminate the Sulphur, nitrogen and oxygen compounds. This stage is also used to improve the color and odor of the oil.
Fractionation: the hydrogenated oil is separated into different base oil fractions according to the specifications and necessary equipment of the product.
This technology accepts PCBs and other hazardous materials and gives an efficiency of 82% of high quality base oils. The waste produced in the vacuum distillation stage contains degraded additives, asphalt by-products, oxidized products and other impurities that have economic value. The first re-refinery based on this technology was established in Greece in 1992. At the present time, several plants of this type are in commercial operation, including plants in Tunisia and California.
The technology combines vacuum distillation and hydro-finishing to remove most of the contaminants from the waste oil. The basic stages of this process comprise atmospheric and vacuum distillation, hydro-finishing of the vacuum distillates, and fractionation. In this technology, an atmospheric distillation column removes water and light hydrocarbon fractions, while a first vacuum unit generates a product in the lubricating oil range. This process has no pre-treatment step, as it uses a special vacuum distillation that minimizes thermal stress by applying temperatures not in excess of 250°C in short bursts.11 These conditions favour thin-film evaporation. By using a wiped-film evaporator, the facility can take most types of feedstock.
The distilled oil is then mixed with hydrogen, heated and passed through a reactor to remove Sulphur, oxygen and nitrogen-containing compounds and improve the colour. The hydrogenated oil is finally stripped with steam or fractionated into different base oil fractions depending upon the product requirements and specifications. This technology accepts PCBs and other hazardous materials and produces high-quality base oils with a yield of about 82% Polluting by-products are minimized. The vacuum residue generated contains additives, asphalt extenders, oxidation/polymerisation products, metal and other impurities. The residue has commercial value.
Safety Kleen Technology
The Safety Kleen Process combines wiped-film vacuum distillation and fixed-bed catalytic hydro-treatment.
Dehydration
Water from waste oil is first removed through evaporation. The water is collected, treated to be chemically and biologically safe and then discharged. Water and light solvents are removed using an atmospheric flash drum.
Light End Recovery
With the water removed, the waste oils go to tall distillation towers where light fuels are separated under atmospheric conditions and collected. The vacuum column/fuel stripper removes most of the fuel and heavier solvents. The vacuum distillation unit performs the combined functions of separating the lubricating oil from the heavy ends and generating multiple product streams. Chemically non-pretreated waste oil tends to foul heated surfaces over time, so thin-film evaporators are used. The lubricating oil fractions (cuts) are then hydro-treated over fixed beds of nickel-molybdenum catalyst. The hydro-treating is performed in stages to reduce catalyst fouling/poisoning and to enhance final product quality. This step improves thermal stability, colour and odour while reducing poly-nuclear aromatics and removing higher-boiling halogenates and polar compounds.
Fuel Stripping
The waste oils are now subjected to heating and vacuum distillation, thereby removing middle weight oils. By-product fuels can be used for industrial heating. The last steps include a kerosene stripper and base oil dryer. The aqueous by-product from the atmospheric distillation unit is sent for water treatment, while low-boiling hydrocarbon contaminants (light ends) recovered from all steps are combined for use as a fuel within the re-refinery. The boiler system had to be adequately designed due to the fuel having a relatively high chlorine content.
Vacuum Distillation
The waste oils are further distilled under higher vacuum and lube oil base stock/fractions are collected. By-products of the process are the residues, which can be used for industrial applications such as asphalt extenders. The by-product vacuum distillation residue is used as an asphalt extender or as fuel for industrial furnaces fitted with emission controls. The spent catalyst from the hydro treating step can be regenerated.
Hydro-treating for Removal of Final Impurities
The remaining oils are treated with hydrogen to remove Sulphur, nitrogen, chlorine, heavy metals and other impurities. This step also corrects any issues with odour, colour and corrosion performance.
The base oils recovered by the Safety Kleen re-refining process meet API standards for base oils. Lubricants made from Safety Kleen base stock include engine oils, gear lubricants, power transmission fluids, hydraulic oils, and industrial oils, all of which meet industry standards and specifications and therefore do not jeopardize warranties or the performance of equipment. Safety Kleen and its licensees have, worldwide, 262 branches, thirteen solvent recycling centres, three fuel blending facilities, and two waste oil re-refining plants.
In general, the technology does not generate unusable by-products. Safety Kleen operates more than ten plants re-refining more than 200 million gallons of used oil in the U.S. alone.
MTSP PROCESS
Baharan Palayesh Sabz Parsian with 10 years’ experience in re-refining used oils and manufacturing refinery lines uses
MTSP Process. This Iranian based company uses process including following steps:
Dehydration and Light End Recovery
Water from waste oil is first removed through evaporation. The water is collected, treated to be chemically and biologically safe and then discharged. Water and light solvents are removed using an atmospheric flash drum.
With the water removed, the waste oils go to tall distillation towers where light fuels are separated under atmospheric conditions and collected. The vacuum fuel stripper removes most of the fuel and heavier solvents. The vacuum distillation unit performs the combined functions of separating the lubricating oil from the heavy ends and generating multiple product streams. The hydro-treating is performed in stages to reduce catalyst fouling/poisoning and to enhance final product quality. This step improves thermal stability, colour and odour while reducing poly-nuclear aromatics and removing higher-boiling halogenates and polar compounds.
Fuel Stripping
The waste oils are now subjected to heating and vacuum distillation, thereby removing middle weight oils.
Vacuum Distillation
The waste oils are further distilled under higher vacuum and lube oil base fractions are collected. By-products of the process are the residues, which can be used for industrial applications such as asphalt extenders. The by-product vacuum distillation residue is used as an asphalt extender or as fuel for industrial furnaces fitted with emission controls. The spent catalyst from the hydro treating step can be regenerated.
For this step MTSP uses three methods to distil the oil. These three methods are thin film evaporation, wiped film evaporation and molecular evaporation.
Decolourization
At the end the final product gets decolourized using clay treatment, solvent or catalyst.
