A lubricant usually consists of a base fluid, generally of petroleum origin, combined with additive chemicals
that enhance the various desirable properties of the base fluid. Base fluids are essentially obtained from two
main sources: the refining of petroleum crude oil and the synthesis of relatively pure compounds with properties
that are suitable for lubricants.
Petroleum lubricating oils are made from the higher boiling portion of the crude oil that remain after removal of
the lighter fractions. Crude oils contain varying amounts of compounds of sulfur, nitrogen and oxygen, metals
such as vanadium and nickel, water and salts. All of these materials can cause problems in refining or
subsequent product applications. The manufacture of the lube base stocks from crude oil involves a series of
subtractive processes to remove these undesirable components, resulting in a base oil that meets performance
requirements. The manufacture of the lube base oils involves following processes.
Vacuum distillation process separates the atmospheric residue mixture into a series of fractions
representing different molecular weight ranges or viscosity ranges from 90-100 neutral to the 500
neutrals.(the neutral number is the SUS viscosity at 100℉) The residue contains the heavier base oils
such as the bright stocks. (150 to 250 SUS at 210℉) The latter is separated from asphaltenes and resins
prior to introduction into the extraction process.
Extraction process involves removal of impurities in the base oils like aromatics, polars, sulfur and
nitrogen compounds. Especially, aromatics 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. Conventionally, solvent(furfural) extraction
was adopted as the purification process, in which aromatics are removed by feeding the raw lube
distillate (vacuum gas oil) into a solvent extractor where it is countercurrently contacted with a solvent.
The resulting product is usually referred to as raffinate. Hydrocracking is a more recent form of
purification process. It is done by adding hydrogen to the base oil feed at higher temperatures and
pressures. Feed molecules are reshaped and often cracked open into smaller molecules. A great majority
of sulfur, nitrogen and aromatics are removed. This massive reforming process produces molecules that
have improved viscometrics and thermal and oxidative stability than product from solvent extraction
The next step in the lube base oil manufacture is the dewaxing process. Solvent dewaxing process
utilizes dewaxing solvents like MEK(methyl-ethyl-ketone), which is one of the most popular ones, to be
mixed with the waxy oil. The mixture is then cooled to a temperature 10 to 20 below the desired pour
point. The wax crystals are then removed from the oil by filtration. More desirable alternatives to the
solvent dewaxing are i) catalytic dewaxing and ii) wax hydroisomerization. Catalytic dewaxing removes
long n-paraffins and waxy side chains from other molecules by catalytical cracking them into smaller
molecules. The wax hydroisomerization process, more advanced form of the catalytic dewaxing process,
isomerizes n-paraffins and other molecules with waxy side chains into branched molecules with very
desirable quality as lube base oils rather than cracking them away.
The final process in the manufacturing of lube base oil is hydrofinishing to improve color and
thermal/oxidative stability of base oil. In hydrofinishing process, hydrogen is added to base oil at an
elevated temperature in the presence of catalyst. By reaction of hydrogen with some remained sulfur
and/or nitrogen containing molecules, these sulfur/nitrogen containing compounds are decomposed into
smaller molecules to improve product color and stabilities
Another source of lubricant base fluids is the synthetic route. Traditionally, synthetics was defined as “A
product prepared by chemical reaction of lower molecular weight materials to produce a fluid of higher
molecular weight designed to provide certain predictable properties.” Currently, there are two types of synthetic
base oils commercially available.(PAO and hydrocracked base oil) Until mid 1990, Polyalphaolefins(PAO)
were the most widely used conventional synthetic lube base fluid in the US and Europe. They are made by
combining two or more decent molecules into an oligomer, or short chain length polymer. Because PAOs are
all-hydrocarbon structures and wax-free, they have low pour points, usually below -40°C, very high viscosity
indexes and good thermal stability. But because of limited availability of raw material, PAO production was
limited to very specific application. However, since mid 1990s, new type of synthetic base oil, hydrocracked
VHVI(very high viscosity index) base stocks like S-OIL's Ultra-S, have come to use because of commercial
availability. Because VHVI base oil is made by chemically converting the petroleum based molecules into a
PAO-like molecular structure, it shows quite similar properties to PAO at much less cost. Hence S-OIL’s Ultra-
S series will make an excellent economical alternative for PAOs for applications like crankcase engine oils,
gear and power train lubricants and some industrial lubricants requiring very high quality standard and extended
Understanding the Differences in Base Oil Groups
Tags: industrial lubricants
Almost every lubricant used in plants today started off as just a
base oil. The American Petroleum Institute (API) has categorized base oils into five categories (API 1509,
Appendix E). The first three groups are refined from petroleum crude oil. Group IV base oils are full synthetic
(polyalphaolefin) oils. Group V is for all other base oils not included in Groups I through IV. Before all the
additives are added to the mixture, lubricating oils begin as one or more of these five API groups.
Group I base oils are classified as less than 90 percent saturates, greater than 0.03 percent sulfur and with a
viscosity- index range of 80 to 120. The temperature range for these oils is from 32 to 150 degrees F. Group I
base oils are solvent-refined, which is a simpler refining process. This is why they are the cheapest base oils on
Group II base oils are defined as being more than 90 percent saturates, less than 0.03 percent sulfur and with a
viscosity index of 80 to 120. They are often manufactured by hydrocracking, which is a more complex process
than what is used for Group I base oils. Since all the hydrocarbon molecules of these oils are saturated, Group II
base oils have better antioxidation properties. They also have a clearer color and cost more in comparison to
Group I base oils. Still, Group II base oils are becoming very common on the market today and are priced very
close to Group I oils.
Group III base oils are greater than 90 percent saturates, less than 0.03 percent sulfur and have a viscosity index
above 120. These oils are refined even more than Group II base oils and generally are severely hydrocracked
(higher pressure and heat). This longer process is designed to achieve a purer base oil. Although made from
crude oil, Group III base oils are sometimes described as synthesized hydrocarbons. Like Group II base oils,
these oils are also becoming more prevalent.
The Changing Use of Base Oils
A recent study on the use of base oils in today’s plants in comparison to a little more than a decade ago found a
dramatic change has occurred. Present-day Group II base oils are the most commonly used base oils in plants,
making up 47 percent of the capacity of plants in which the study was conducted. This compared to 21 percent
for both Group II and III base oils just a decade ago. Currently, Group III accounts for less than 1 percent of the
capacity in plants. Group I base oils previously made up 56 percent of the capacity, compared to 28 percent of
the capacity in today’s plants.
Group IV base oils are polyalphaolefins (PAOs). These synthetic base oils are made through a process called
synthesizing. They have a much broader temperature range and are great for use in extreme cold conditions and
high heat applications.
of lubrication professionals use both synthetic
and mineral-based lubricants in their plant,
according to a recent poll at
Group V base oils are classified as all other base oils, including silicone, phosphate ester, polyalkylene glycol
(PAG), polyolester, biolubes, etc. These base oils are at times mixed with other base stocks to enhance the oil’s
properties. An example would be a PAO-based compressor oil that is mixed with a polyolester. Esters are
common Group V base oils used in different lubricant formulations to improve the properties of the existing
base oil. Ester oils can take more abuse at higher temperatures and will provide superior detergency compared
to a PAO synthetic base oil, which in turn increases the hours of use.
Remember, whichever base oil you choose, just be sure it is appropriate for the application, temperature range
and conditions in your plant.