From its humble beginnings more than 3,000 years ago, lubrication technology has seen many phases of evolution. As it continues to evolve at an ever-increasing rate, base oil performance is making a larger contribution to finished lubricant performance.
This is Part 1 of a three-part series which describes the evolution of base oil technology and its impact on the lubricants industry. Part 1 addresses the past, Part 2 the present, and Part 3 the future.
Early lubrication began with animal fats and oils and slowly evolved to petroleum-based oils. Many generations of refining processes have since improved on Mother Nature. Early processes such as acid treating and solvent extraction improved the quality of base oils by removing some or most of the worst molecules from the oil. Later processes like hydrotreating, catalytic hydrocracking, catalytic dewaxing and modern wax hydroisomerization improved specific lubricating qualities.
Modern wax hydroisomerization, in particular, makes base oils with low impurities and typically water-white appearance. Now, about one-half of all base oils manufactured in North America are of such high quality.
Looking to the future, the trend is toward even higher base oil purity, higher viscosity index (VI), lower volatility and longer life. The distinction between heavily processed mineral oils and traditional synthetic oils will continue to blur.
Base oil technology has undergone many phases of evolution. In the first phase, animal fats were used as lubricants. Ancient inscriptions dating back to 1400 B.C. show beef and mutton fat (tallow) being applied to chariot axles. Little changed over the next 3,000 years except that the oils sometimes came from more exotic animals such as whales.
In 1852, petroleum-based oils became available. They were not widely accepted at first because they did not perform as well as many of the animal-based products. Raw crude did not make a good lubricant. The base oil industry was on the steep part of the learning curve.
But as the demand for automobiles grew, so did the demand for better lubricants. Lubricant manufacturers soon learned which crudes made the best lubricants. Then they improved on Mother Nature by refining the crude into narrow distillation cuts with varying viscosity. By 1923, the Society of Automotive Engineers classified engine oils by viscosity: light, medium and heavy. Engine oils contained no additives and had to be replaced every 800 to 1,000 miles.
In the 1920s, more lubrication manufacturers started processing their base oils to improve their performance. Three popular processing routes included:
Clay similar to kitty litter was used to soak up and remove some of the worst components in the petroleum base oil. These compounds were usually aromatic and highly polar compounds containing sulfur and nitrogen.
Concentrated sulfuric acid was used to react with the worst components in the base oil and convert them into a sludge that could be removed. Although this process effectively cleaned up the oil, it was expensive. This technology has virtually disappeared from North America due to environmental concerns about the acid and sludge.1
SO2 treating was a primitive extraction process that removed the worst components in the lube oil using a recyclable solvent. Unfortunately, the solvent was highly toxic. Although it also has been virtually phased out1, it was a useful stepping stone to conventional solvent extraction.
By approximately 1930, solvent processing emerged as a viable technology for improving base oil performance using a fairly safe, recyclable solvent. Most base oil producers in the world still use this process today.
Approximately half of the base oil in North America is currently manufactured using this route. Solvent refined base oils are commonly called Group I base oils which are characterized as those having less than 90 percent saturates (>10 percent aromatics) and more than 300 ppm sulfur. Table 1 shows all the base oil groups as defined by the American Petroleum Institute (API) Publication 1509.
The solvents and hardware used to manufacture solvent-refined base oils have evolved over time, but the basic strategy has not changed since 1930. The two main processing steps are:
Figure 1. Group 1 Solvent Extraction Process
Aromatics are removed by solvent extraction to improve the lubricating quality of the oil. Aromatics make good solvents but they make poor-quality base oils because they are among the most reactive components in the natural lube boiling range. Oxidation of aromatics can start a chain reaction that can dramatically shorten the useful life of a base oil.
The viscosity of aromatic components in a base oil also responds relatively poorly to changes in temperature. Lubricants are often designed to provide a viscosity that is low enough for good cold-weather starting and high enough to provide adequate film thickness and lubricity in hot, high-severity service. Therefore, when hot and cold performance is required, a small response to changes in temperature is desired.
The lubricants industry expresses this response as the viscosity index (VI). A higher VI indicates a smaller, more favorable response to temperature. Petroleum distillates were solvent extracted to the extent necessary to match the quality (VI) of the distillate from high-quality Pennsylvania-grade crude. By definition, this distillate had a VI of 100. Distillate from low-quality Louisiana-grade crude defined the 0 point on the VI scale.
Aromatics are removed by feeding the raw lube distillate (vacuum gas oil) into a solvent extractor where it is contacted countercurrently with a solvent. Popular choices of solvent are furfural, n-methyl pyrrolidone (NMP) and DUO-SOLTM. Phenol was another popular solvent but it is rarely used today due to environmental concerns.
Solvent extraction typically removes 50 percent to 80 percent of the impurities (aromatics, polars, sulfur and nitrogen-containing species). The resulting product of solvent extraction is usually referred to as a raffinate.
The second step is solvent dewaxing. Wax is removed from the oil to keep it from crystallizing in the customer’s sump or crankcase at low temperatures.
Wax is removed by first diluting the raffinate with a solvent to lower its viscosity to improve low-temperature filterability. Popular dewaxing solvents are methyl-ethyl ketone (MEK)/ toluene, MEK/methyl-isobutyl ketone or (rarely) propane. The diluted oil is then chilled to -10°C to -20°C. Wax crystals form, precipitate and are removed by filtration.
Dewaxing lowers the pour point (freezing point) of the base oil. Base oil pour point targets are set in the marketplace by competitive forces that balance the cost of incremental dewaxing with the cost of incremental additives commonly used to depress the pour point of finished lubricants. These competitive forces drive most manufacturers to a common set of targets.
Naphthenic base oils were also available but not normally used in mainstream lubricants. Naphthenic oils are made from crude oil distillates that are more nearly Louisiana-grade, rather than Pennsylvania-grade. They are rich in cyclo-paraffins which are also called naphthenes. Naphthenic oils are sometimes called pale oils due to their pale color. They are rich in aromatics which gives them a solvency advantage for certain types of additives.
The paraffin content in these oils is low so they have an inherently low pour point and do not require solvent dewaxing. But they have a low VI relative to paraffinic oils so their lubricating performance and oxidation stability are relatively poor. They are normally used in specialty applications such as metal working fluids and transformer oils.
Over the next several decades, the solvent refining process did not change much. Finished oil quality improved mainly due to the addition of additives. Additives began to be widely used in 1947 when the API began to categorize engine oils by severity of service: regular, premium and heavy-duty. Additives were used to extend the life only in premium and heavy-duty oils.
Regular engine oils were unadditized and were intended for 80 to 100 hours of low-severity service. Premium engine oils were additized to extend the life of passenger car engine oils. Heavy-duty engine oils were more heavily additized to extend the engine oil life in more severe applications such as commercial trucks and construction equipment.
In 1950, multigrade oils were first introduced which were additized with polymers to enhance the VI of the oil which improved the hot and cold performance of the oil (Figure 2).
Figure 2. Additives Hit the Scene
For several more decades, the lubricants industry continued to rely heavily on additive technology to improve the performance of finished oils. Lubricant quality improved significantly only when the additive chemistry improved. This was the only viable strategy until a significant improvement in base oil technology was available.
Part 2 will explain how new hydroprocessing technologies opened the door for significant improvements in base oil quality and finished lubricant performance.
This is a modified version of the article “The Evolution of Base Oil Technology” which appeared in STP 1407 -Turbine Lubrication in the 21st Century, copyright ASTM International.
1. National Petroleum Refiners Association. “1999 Lubricating Oil and Wax Capacities of Refiners and Re-Refiners in the Western Hemisphere.” January 1999.
2. Kramer, D., Lok, B. and Krug, R. (2001). “The Evolution of Base Oil Technology.” Turbine Lubrication in the 21st Century, ASTM STP #1407, W. Herguth and T. Warne, Editors. American Society for Testing and Materials, West Conshohocken, PA.