S. KHAN and N. M. TUKUR, Saudi Aramco, Dhahran, Saudi Arabia
Used oils generated through industrial and non-industrial (e.g., locomotives) sources are an industry segment that needs global attention for enhanced circularity. Used oils have the potential to benefit the decarbonization of an operating asset’s portfolio to achieve climate goals.
Proper incentivization schemes, stakeholders and influencers—including governments, policy makers, used oil recyclers, re-refiners and environmental entities—can orchestrate new business models to translate circular strategies into competitive advantages, company resilience and improved revenue models.
The improved circularity of used oils will contribute to improved resource efficiency through prolonged functionality and increased secondary use as feed streams to produce fresh lubricants and products for sustained environmental benefits. Capital injections are warranted to add new infrastructure for waste used oils collection, storage, logistics and reprocessing. Therefore, the relationship between maximizing waste used oils collection, recycling, reuse and repurposing and the resulting environmental benefits is clear. However, the inconsistent quality supply of used oils is a serious challenge.
This article will examine the basic tenets of used oils, including typical composition, environmental implications, global generation from lubricants consumption, and challenges related to used oils recycling for a better and cleaner environment.
Used oil. According to Basel Convention technical guidelines,1 the term “used oil†is defined as:
“…any semi-solid or liquid used product consisting totally or partially of mineral oil or synthesized hydrocarbons (synthetic oils), oily residues from tanks, oil-water mixtures and emulsions.â€
Used oils originate from industrial and non-industrial sources after being used for lubrication, hydraulics, heat transfer, electrical insulating (dielectric) or other purposes. Used oils’ original characteristics change during use while still exhibiting their inherent lubricating and heating value benefits. The pickup of incidental materials make the waste used oils unsuitable for continued direct use without treatment.
Synthetic oils cover a wide range of chemicals and will include hydrocarbon esters, phosphate esters, glycols, chlorinated hydrocarbons, silicon oils and synthetic hydrocarbons. These synthetic oils are similar in composition to those found in petroleum-based oils, considering that basic hydrogen and carbon compounds are combined using a chemical process. Synthetic or non-conventional lube base oils also include polyalphaolefins, hydrocracked and gas-to-liquids (GTL) base oils. These nonconventional lube base oils feature superior performance and allow for extended drain intervals due to low-viscosity and high-viscosity indexes for new-generation automatic transmissions fluids.
Used oil components and byproducts. Typically, used oil is comprised of slightly more than half base oil, fuel oil, heavy extract and associated water. FIG. 1 shows a typical compositional breakdown of waste used oil.1
Base oil recovery from used oil is the core business for used oil re-refiners, which target maximized yields for reprocessed base oil that is blended with proprietary additives to finally produce reprocessed lubricant oil products.
In the re-refining process, the waste used oil feedstock is distilled into lighter fractions—such as light solvent, diesel grade fuel oil, light and heavy extract oil, base oil and residual water—utilizing boiling point differences. The used oil composite byproduct as a blend of light solvent, fuel oil and light extract oil will typically constitute 35% of the influent stream and could be a potential quality candidate for heavy fuel oil diluent, substituting conventional cutter stocks such as kerosene and diesel refined products. The residual heavy extract oil from the re-refining of used oil is not a distilled fraction of interest due to high level of metals, among other factors.
FIG. 2 provides a simplified presentation of used oil refining feedstock, products/byproducts and the used oil composite byproduct stream.
The consistent quality supply of composite byproducts from used oil is a major attraction if it is to be utilized as fuel oil diluent.
Used oil statistical data (global and regional generation and circularity). Based on statistical data,2 global lubricant consumption was calculated to be ~36.1 MM metric tpy in 2017. However, the demand for lubricants has been forecast to reach 38.1 MM metric tpy by 2028.3 The breakdown by region of lubricant demand in 2017 is shown in FIG. 3.
The Asia-Pacific region was the largest consumer of lubricants with 43% of global annual lubricants demand at 15.5 MM metric tpy, followed by the Americas (27%, 9.6 MM metric tpy), Europe (19%, 6.9 MM metric tpy), Africa (6%, 2.2 MM metric tpy) and the Middle East (5%, 1.8 MM metric tpy).
In 2017, the automotive sector was the key contributor fueling global lubricants demand, including and not limited to engine oils, gear oils, transmission fluids, etc. The sector accounted for more than half of global lubricants demand, supported by increasing demand for consumer vehicles in the Asia-Pacific region, particularly in China and India. Interestingly, the Asia-Pacific region represented about 40% of the 2017 world market for automobiles in terms of volume, and the region is expected to remain a core driver for automobile and lubricants demand.
Globally, the demand in 2017 only marginally increased by 0.56% vs. 2007 consumption of 35.9 MM metric tpy. Lubricants demand in the Asia-Pacific region and the Middle East increased by 8% and 1%, respectively, over that 10-yr period (2007–2017), and this can be attributed to increased motorization and industrialization. Conversely, demand decreased by 4% in North America, 1% in Latin America and 2% in Europe. Importantly, the decline in lubricants demand for Europe and the Americas may be attributed to the use of higher performance lubricants increasing the intervals between oil changes, as well as the increased use of electric vehicles (EVs). EVs normally require less maintenance than internal combustion engine (ICE) vehicles fueled by gasoline or diesel due to fewer fluids to change and far fewer moving parts.
The global generation of used oil is estimated based on the global consumption of lubricants. According to the U.S. Department of Energy (DOE),4 global used oil consumption in 2017 was equivalent to ~11.65 MM metric tpy, which represented only ~32.3% of total global consumption. This indicated that most used oil may have been burned as fuel, dumped on land, or simply lost due to the low efficiency of collection systems.
Environmental implications. It is known that used oil poses a carcinogenic risk to people.5 The main carcinogenic agents found in waste used oil are polycyclic aromatic hydrocarbons (PAHs) with 3–7 rings, such as benzo(a)pyrene, benz(a)anthracene and chrysene. These chemical constituents—and many more like them—are formed during the combustion cycle in the engine and, therefore, are found in high volumes in waste used oil streams. Importantly, PAHs are also present in crude oil and can be present in unused base fluids produced in manufacturing facilities.
TABLE 1 provides a summary of analytical analyses conducted on re-refined used oils samples in previous studies and reveals substantially elevated levels of total PAHs content against a virgin reference oil sample: 7.5 mg/kg in the virgin reference oil sample vs. 22 mg/kg–126 mg/kg in re-refined used oil.
Note: Interpreting the carcinogenic risk based on the analytical PAH results in TABLE 1 is difficult. No evidence of an increase in skin cancer is manifested based on previous studies, provided sensible precautionary measures of personal hygiene and handling are taken.
The effectiveness of used oils enhanced circularity is pivotal to minimizing associated environmental impacts. Used oil recycling has two key options:
It is important to consider the adequate treatment of used oils prior to repurposing the recycled used oil as an energy source in industrial boilers or furnaces to take full advantage of any potential environmental benefits. Utilizing recycled waste used oils without adequate treatment poses environmental concerns due to harmful chemical constituents in the flue gas, such as PAHs, polychlorinated biphenyls (PCBs), polychlorinated terphenyls (PCTs), dioxins and heavy metals, among others. Typical refinery processes, such as hydrogenation and solvent extraction, may be used as treatment methods. Otherwise, a much higher thermal environment within the combustion chamber will be required to ensure complete oxidative destruction of PAHs, PCBs and PCTs. The utilization of waste used oil in cement production could be a satisfactory disposal method from an environmental perspective.
Biodegradability of used oils. Biodegradation is the breakdown of a chemical by organisms and is further categorized into two distinct levels: “primary biodegradability†and “ultimate biodegradability.†Primary biodegradation is achieved when the loss of one or more active groups renders the molecule inactive regarding a particular function. Ultimate biodegradation is the complete breakdown to CO2, H2O and mineral salts (PO4, SO4, NO3, etc.), and is therefore referred to as mineralization.
Hydrocarbon compounds, such as surfactants, that exhibit demonstrable water solubility are degraded by biological mechanisms utilizing these compounds as an energy source, so they are removed from the environment.6 Looking into the natural systems, microbes such as bacteria, fungi, etc., produce emulsifiers that render insoluble materials accessible for degradation. A literature survey reveals that oil does degrade, and the addition of essential nutrients, such as PO4, SO4, NO3, etc., can accelerate the degradation process by indigenous microorganisms in a short time and at low temperature.
Furthermore, hydrocarbon oil does biodegrade in aerobic conditions, albeit slowly. The environmental degradation of lubricating oil is less easily demonstrated due to its complex hydrocarbon nature. Importantly, the hydrophobic nature of oil inhibits the rate of biodegradation. The biodegradability of used oil is an area requiring further evaluation.
Challenges of re-refining used lube oil. As earlier highlighted, only about one-third of lubricating oil used is collected for recycling and only 27% of the collected volumes are directed for re-refining back into base oil. The remaining used lubricating oil is assumingly sub-optimally utilized as fuel in the furnaces, or simply burned.
Globally, re-refining used oil into base oil is the preferred trend due to the environmental benefits, which include reduced land and marine pollutants, lower CO2 emissions (up to 50%) and savings in primary natural resources through used oil enhanced circularity. Despite the benefits of the re-refining process, key challenges limiting its growth in developing countries include:
Takeaways. This article provides a holistic overview on the basic compositional attributes of used oil, the global generation from lubricants consumption, key challenges associated with its enhanced circularity, and its environmental benefits.
Global lubricant consumption was quantified to be approximately 36.1 MM metric tpy in 2017, with demand forecast to reach 38.1 MM metric tpy by 2028. Expectedly, the Asia-Pacific region is the largest consumer of lubricants, with 43% of the global annual demand at 15.5 MM metric tpy, which is supported by increased motorization and industrialization, especially in China and India. Vehicular electrification will continue to impact the demand for lubricants—particularly in developed geographies such as Europe, the Americas and Australia—due to the use of higher performance lubricants and electric automotive technology. Not all consumed lubricating oils are fully recovered globally.
Globally, key stakeholders and influencers alike (including governments, policy makers, used oil recyclers and re-refiners, and environmental authorities) are encouraged to exert united efforts to orchestrate new business models. The aim is to enable and further promote circular strategies into competitive advantages, improved resilience and sustainable revenue models, with proper incentivization schemes that highlight industry’s shared goals to sustainably protect the environment. Maximizing waste used oils collection, recycling, reusing and repurposing, etc., has clear environmental benefits. The inconsistent quality supply of used oils, and the lack of incentivization schemes and regulatory mandates are identified among the serious challenges inhibiting the enhanced circularity of used oils on a sustainable basis. HP
LITERATURE CITED
SHAHREYAR KHAN is an Engineering Specialist with Saudi Aramco’s Process & Control Systems department. He has extensive global work experience over two decades in the downstream conventional oil refining, shale wet-side processes and fuels quality, process design, technical services, operations support, R&D, capital projects, and industrial investment appraisals and commercial negotiations. Khan is a Chartered Professional Chemical Engineer (Engineers Australia) and earned a Registered PE Queensland (RPEQ) title. He also received a Master in Project Management degree from the University of Sydney, Australia.
NASIRU M. TUKUR is an Engineering Consultant with Saudi Aramco’s Process & Control Systems department. Dr. Tukur earned his PhD in chemical engineering from the University of Manchester, UK, and an MS degree in the same specialty from the King Fahd University of Petroleum & Minerals (KFUPM) in Dhahran, Saudi Arabia. He has more than 30 yr of experience in the refining and petrochemical industries, as well as academia. He previously taught chemical engineering at KFUPM and worked with SABIC Engineering & Project Management as a Senior Process Engineer in Jubail Industrial City before joining Saudi Aramco in 2010.
