Base Oil Hydrotreating Process

Lubricant base oils are produced in a series of steps which are designed to enhance certain desirable properties. For paraffinic oils, these include viscosity index, oxidation resistance, thermal stability and low temperature fluidity.

Starting from petroleum crude oil, the typical process for making a base oil is as follows:

  • Separation of lighter boiling materials, such as gasoline, diesel, etc.

  • Distillation to isolate desired base oil viscosity grades

  • Selective removal of impurities, such as aromatics and polar compounds

  • Dewaxing to improve low temperature fluidity

  • Finishing to improve oxidation resistance and heat stability

In general, both solvent refined and hydrotreated base oils are manufactured this way, but differ in the type and severity of processes used.

Before reviewing how base oil is manufactured, the American Petroleum Institute’s (API) Base Oil Classification system should be explained. As shown below, the API system classifies base oils into five major groups. While these groups were originally intended to be used for formulation of engine oils, their definition is widely understood and usage has expanded beyond this area.

Base Oil Characteristics
API Group
Sulfur Wt, %
 Saturates Wt, %  Viscosity Index VI  Manufacturing Method
 I  >0.03  <90  80-119  Solvent Refined
 II  ≤ 0.03  ≥90  80-119  Hydroprocessed
 III  ≤ 0.03  ≥90  120+  Severely Hydroprocessed
 IV  Polyalphaolefins (PAOs)  Oligomerization
 V  Other Base Oils  Various

Although not officially recognized by the API, the following additional terms are often used in the industry.

Base Oil Characteristics
API Group
Sulfur Wt, %
 Saturates Wt, %  Viscosity Index VI  Manufacturing Method
 II+  ≤ 0.03  ≥90  110-119  Hydroprocessed
 III+  ≤ 0.03  ≥90  130+  Severely Hydroprocessed

Group I, or conventional base oils are manufactured by solvent refining and contain more than 0.03 wt % sulfur and/or less than 90 wt % saturates. Group I oils continue to be used in a number of applications due to their solvency and performance characteristics.

Group II and III base oils are manufactured by what the API calls hydroprocessing or severe hydroprocessing. With sulfur content of less than or equal to 0.03 wt % and saturates content of more than or equal to 90 wt %, they are more pure than Group I base oils. Many of the Petro-Canada Lubricants products are produced using a special high severity version of hydroprocessing, called severe hydrotreating, to produce Group II, II+, III and III+ base oils.

Initially, lighter products such as gasoline, diesel, etc., are separated from crude oil by atmospheric distillation. The heavier material is then charged to a vacuum distillation tower, where lubricant fractions of specific viscosity ranges are taken off. These fractions are then treated individually in a solvent extraction tower. A solvent such as furfural is mixed with the fractions and extracts about 70-85% of the aromatic material present. The solvent extracted lube fraction is then dewaxed by chilling to a low temperature, which removes much of the wax. This improves the low temperature fluidity of the product. Finally, the dewaxed lube fractions can be finished to improve their color and stability, depending on the application requirements. One common method of finishing is mild hydrofinishing. The API classifies the products of solvent refining as Group I base oils.

Solvent Refining / Extraction Process


The severe hydrotreating process works to eliminate aromatic and polar compounds by reacting the feedstock with hydrogen in the presence of a catalyst at high temperatures and pressures.

Several different reactions occur in this process, the principal ones being:

  • Removal of unwanted or undesirable polar compounds containing sulfur, nitrogen and oxygen

  • Conversion of aromatic hydrocarbons to saturated cyclic hydrocarbons

  • Breaking up of heavy polycyclo-paraffins into lighter, saturated hydrocarbons

These reactions take place at temperatures as high as 400 C/752 F, pressures around 3000 psi and in the presence of a catalyst. The hydrocarbon molecules that are formed are very stable and this makes them ideal for base oils used for lubricant blending. They are classified by the API as Group II base oils.

There are two stages in the severe hydrotreating process. The first stage removes unwanted polar compounds and converts the aromatic components to saturated hydrocarbons. After separation into desired viscosity grades by vacuum distillation, batches of waxy lube base oil are chilled and dewaxed. These are then passed through a second stage high pressure hydrotreater for additional saturation. This final step maximizes stability by removing the last traces of aromatic and polar molecules, producing a water-white and 99.9% pure base oil.

Two-Stage Severe Hydrotreating Process


In addition to severe hydrotreating, the hydroisomerization process employs a specialized catalyst to selectively isomerize wax (mixture of long chain n-paraffins) to high VI, low pour point, isoparaffinic base oil. The process yields base oils with higher VIs and improved yields, compared to previous conventional dewaxing techniques. The process is capable of producing base oil with a VI above 130. More often, it is set up to produce high viscosity index (Group II+ and III) base fluids with VIs ranging from 115 to 127. A further process feature is the flexibility it offers to produce base oils with pour points lower than -25 C/-13 F.

Hydroisomerization is employed in conjunction with severe hydrotreating to produce superior base oils that have:

  • High Viscosity Index (VI)

  • Low Volatility

  • Excellent Oxidation Resistance

  • High Thermal Stability

  • Excellent Low Temperature Fluidity

  • Low Toxicity

These features give performance characteristics very similar to lubricants formulated with polyalphaolefin (PAO), the most common type of synthetic.

Two-Stage Severe Hydrotreating/ Hydroisomerization Process


Base oils are fundamental building blocks of finished lubricants. Their composition and physical properties are influenced by the refining technology used. Formulators will choose base oils with characteristics suited to the end application, and match them with specially selected additives to optimize performance of finished products.

Color Often used as a visual indicator of purity, as it is usually related to the amount of aromatics present. Severely hydrotreated base oils are clear and colorless.
Viscosity Index (VI) Is a measure of the rate of change of viscosity with temperature. Severely hydrotreated base oils have high VIs so they 'thin out' less at high temperatures yet remain pumpable (or fluid) at low temperatures.
Oxidation Resistance The ability to resist chemical degradation caused by oxygen and /or increasing temperatures. Severely hydrotreated base oils respond very well to anti-oxidants resulting in excellent resistance to oxidation and long lubricant life in finished products.
Thermal Stability The ability to resist permanent changes to physical and chemical properties caused by heat. Severely hydrotreated base oils have very good resistance to heat.
Carbon Residue The amount of insoluble residue produced due to thermal stress. Severely hydrotreated base oils produce low amounts of carbon residue.
Demulsibility The ability of a lubricating oil to separate from water. Severely hydrotreated base oils separate readily from water.
Low Toxicity The degree to which a substance harms a living organism. Severely hydrotreated base oils have low toxicity, due to a virtual absence of impurities.
The degree of biodegradability in a lubricant is measured by calculating its rate of conversion to carbon dioxide by living organisms. Severely hydrotreated base oils have good biodegradability characteristics.