INTRODUCTION:
Conventional lubricants are formulated based on mineral oils derived from petroleum. Mineral oils contain many classes of chemical components including aromatics, paraffins, naphthenes, sulfur and nitrogen species, etc, and its composition is determined primarily by the crude source. Un-additized mineral oils may be good for general purpose use, but are not optimized for any performance feature.
However, conventional mineral oils composed of Group II oils, using the latest refining and extraction techniques, offer advantages not seen in oils from the 1940’s through the 1970’s.
In order to set the stage, let us first review the basic API Groups of base oils and discuss their basic refining and crude oil processing techniques.
SECTION I: GROUP I THROUGH 3 BASE OILS.
solvent refining
[ DEFINITION OF WAX: Wax is a large hydrocarbon molecule that prevents oil from flowing at colder temperatures; paraffin, a flammable, whitish, translucent, waxy solid consisting of a mixture of saturated hydrocarbons, and obtained by distillation from petroleum or shale and used in candles, cosmetics, polishes, and sealing and waterproofing compounds; In chemistry, paraffin is used synonymously with Alkane, indicating hydrocarbons with the general formula CnH2n+2].
[ DEFINITION OF CATALYST: A Substance that participates in chemical reactions by Increasing the Rate of Reaction, yet the catalyst remains intact after the reaction is complete].
In the past, two-thirds of the base oil in North America were manufactured using solvent refining. Solvent refined base oils are commonly called Group I base oils which are characterized as those having less than 90% saturates (>10% aromatics) and more than 300 ppm sulfur.
The solvents and hardware used to manufacture solvent-refined base oils have changed over time, but the basic strategy has not changed since 1930. The two main processing steps are:
- Remove aromatics by solvent extraction.
- Remove wax by chilling and precipitation in the presence of a different solvent.
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 (V.I.). A higher V.I. indicates a smaller, more favorable response to temperature. Correspondingly, many turbine manufacturers have a minimum V.I. specification for their turbine oils. Base oil selection is key for meeting this specification because turbine oil additives do not normally contribute positively to the V.I. in turbine oil formulations.
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-SOL™. Phenol was another popular solvent but it is rarely used today due to environmental concerns. Solvent extraction typically removes 50-80% 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 freezing. 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 to -200C. Wax crystals form, precipitate, and are removed by filtration.
HYDROTREATING (PREDOMINATELY GROUP I)
Hydrotreating was developed in the 1950s and first used in base oil manufacturing in the 1960s by Amoco and others. It was used as an additional “cleanup” step added to the end of a conventional solvent refining process.
HYDROTREATING is a process for adding hydrogen to the base oil at elevated temperatures in the presence of catalyst to stabilize the most reactive components in the base oil, improve color, and increase the useful life of the base oil. This process removed some of the nitrogen and sulfur containing molecules but was not severe enough to remove a significant amount of aromatic molecules. Hydrotreating was a small improvement in base oil technology that would become more important later.
HYDROCRACKING (PREDOMINATELY GROUP II)
Hydrocracking is a more severe form of hydroprocessing. It is done by adding hydrogen to the base oil feed at even higher temperatures and pressures than simple hydrotreating. Feed molecules are reshaped and often cracked into smaller molecules. A great majority of the sulfur, nitrogen, and aromatics are removed. Molecular reshaping of the remaining saturated species occurs as naphthenic rings are opened and paraffin isomers are redistributed, driven by thermodynamics with reaction rates facilitated by catalysts. Clean fuels are byproducts of this process.
Chevron commercialized this technology for fuels production in the late 1950’s. In 1969 the first hydrocracker for Base Oil Manufacturing was commercialized in Idemitsu Kosan Company’s Chiba Refinery using technology licensed by Gulf. This was followed by Sun Oil Company’s Yabucoa Refinery in Puerto Rico in 1971, also using Gulf technology.
Group II base oils are differentiated from Group I base oils because they contain significantly lower levels of impurities (<10% aromatics, <300 ppm S). They also look different. Group II oils are so pure that they have almost no color at all. From a performance standpoint, improved purity means that the base oil and the additives in the finished product can last much longer. More specifically, the oil is more inert and forms less oxidation byproducts that increase base oil viscosity and react with additives.
CATALYTIC DEWAXING AND WAX HYDROISOMERIZATION GROUP III
[ DEFINITION: ISODEWAXING™: A patented process developed by Chevron which includes the catalytic hydroprocessing steps of Hydrocracking, Hydroisomerization, and Hydrotreating to produce Group III oils].
[ DEFINITION: isomerization; the chemical process by which a compound is transformed into any of its isomeric forms, i.e., forms with the same chemical composition but with different structure or configuration and, hence, generally with different physical and chemical properties; the process by which one molecule is transformed into another molecule which has exactly the same atoms, but the atoms have a different arrangement.
The first catalytic dewaxing and wax hydroisomerization technologies were commercialized in the 1970s. Shell used wax hydroisomerization technology coupled with solvent dewaxing to manufacture extra high V.I. base oils in Europe. Exxon and others built similar plants in the 1990s. In the U.S., Mobil used catalytic dewaxing in place of solvent dewaxing, but still coupled it with solvent extraction to manufacture conventional oils.
Catalytic dewaxing was a desirable alternative to solvent dewaxing especially for conventional neutral oils, because it removed n-paraffins and waxy side chains from other molecules by catalytically cracking them into smaller molecules. This process lowered the pour point of the base oil so that it flowed at low temperatures, like solvent dewaxed oils. Hydroisomerization also saturated the majority of remaining aromatics and removed the majority of remaining sulfur and nitrogen species.
Chevron was the first to combine catalytic dewaxing with hydrocracking and hydrofinishing in their Richmond, California base oil plant in 1984. This was the first commercial demonstration of an all-hydroprocessing route for lube base oil manufacturing.
In 1993, the first modern wax hydroisomerization process was commercialized by Chevron. This was an improvement over earlier catalytic dewaxing because the pour point of the base oil was lowered by isomerizing (reshaping) the n-paraffins and other molecules with waxy side chains into very desirable branched compounds with superior lubricating qualities rather than cracking them away. Hydroisomerization was also an improvement over earlier wax hydroisomerization technology, because it eliminated the subsequent solvent dewaxing step, which was a requirement for earlier generation wax isomerization technologies to achieve adequate yield at standard pour points. Modern wax hydroisomerization makes products with exceptional purity and stability due to extremely high degree of saturation. They are very distinctive because, unlike other base oils, they typically have no color.
By combining three catalytic hydroprocessing steps (Hydrocracking, Hydroisomerization, Hydrotreating), molecules with poor lubricating qualities are reshaped into higher quality base oil molecules. Pour point, V.I., and oxidation stability are controlled independently.
All three steps convert undesirable molecules into desirable ones, rather than have one, two, or all three steps rely on subtraction.
Among the many benefits of this combination of processes is greater crude oil flexibility; that is, less reliance on a narrow range of crude oils from which to make high-quality base oils. In addition, the base oil performance is exceptionally favorable and substantially independent of crude source, unlike solvent-refined base oil.
So base oils with a “conventional” V.I. (80-119) are Group II. Base oils with an “unconventional” V.I. (120+) are Group III. Group III oils have also been called unconventional base oils (UCBOs) or very high V.I. (VHVI) base oils.
Modern Group III oils have greatly improved oxidation stability and low temperature performance. Consequently, many group I or II plants are now being upgraded to enable them to make the modern hydroisomerized Group III oils.
Modern Group III oils today can be designed and manufactured so that their performance closely matches PAOs in most commercially finished lube applications.
From a processing standpoint, modern Group III base oils are manufactured by essentially the same processing route as modern Group II base oils. Higher V.I. is achieved by increasing the temperature or time in the hydrocracker. This is sometimes collectively referred to as the “severity.” Alternatively, the product V.I. could be increased simply by increasing the feed V.I., which is typically done by selecting the appropriate crude.
SUMMARY OF SECTION I: So up to this point, we see that Group I to III base oils (excepting GTL, below) result from a succession of steps defined by the severity of processing and the catalyzation of crude oil. I.e., the “reshaping of molecules via catalytic action.”
GAS-TO-LIQUID (GTL) BASE OILS:
The API classifies GTL base oils as Group III or unofficially it has been called, “Group III+.” It is this author’s view that the GTL process results in a “synthesized” oil and should be given a separate API classification as they do PAO, or moved to the Group V classification. A separate, future debate can address this issue and will not be further discussed here in this white paper.
