Oil-based liquid lubricants are composed of two (2) key ingredients: base oil and additive packages. Additive packages in a lubricant will tend to vary based on applications. This post will focus on the main ingredient, base oils. Base oils will comprise typically 80-99% of an oil-based lubricant. Before we look at the base oil in a finished lubricant, we have to understand how oil gets from a drilling site to a refinery and finally into your lubricant.
Extracting and Transporting Crude Oil
After crude oil is extracted from the ground at wellhead or platform drilling sites, it is transported via rail, ship, or pipeline. It is then stored at a terminal or hub. Oil & Gas Manufacturers then take the crude and refine it to meet product specifications. Some of these products include gasoline, heating oil, fuel oils, asphalt and road oil, and lubricants.
Separating the Crude Oil
The typical barrel of crude is 42 gallons. According to the American Petroleum Institute, only 0.5 gallons of crude oil in each barrel are used to make lubricants.
This is because lubricants require longer chain hydrocarbons. Most lubricants have hydrocarbon molecules with 26-40 carbon atoms. Crude oil is heated and vaporized then condensed; this process is known as distillation. The distilled oil is easily separated by hydrocarbon chain lengths. Shorter molecules rise to the top and longer molecules sink to the bottom. After the hydrocarbons with 26-40 carbon atoms are separated, they are sent to a refining process specific to lubricants.
Refining the Crude Oil
The distilled oil is refined using two different processes: extraction and conversion. Extraction involves four steps:
- Deasphalting: Takes residue at the bottom and separates into tar and compounds similar to lube distillates.
- Solvent Extraction: Remove most the aromatics and undesirable constituents of oil distillates. Results in neutral oil base stocks called reaffinates.
- Dewaxing: Reaffinates are dewaxed to produce a wax and dewaxed oil. The dewaxed oil becomes base stock for lubricants.
- Hydrofinishing: Changes polar compounds in oil by a chemical reaction. Oil becomes lighter in color and exhibits improved chemical stability.
The conversion process involves three steps and is becoming popular for refining. This process involves converting undesirable products into desirable products using hydrogen, heat, and pressure:
- Hydrocracking: Distillates are subjected to a chemical reaction at high temperatures and high pressure. The aromatic and naphthene rings are broken and joined using hydrogen to form an isoparaffin structure.
- Hydrodewaxing: A hydrogenation unit is used to deploy a catalyst that converts waxy normal paraffins to desirable isoparaffin structures.
- Hydrotreating: The first two processes broke chemical bonds, therefore it is necessary to saturate unsaturated molecules. By adding more hydrogen, the molecules saturate and become more stable and better able to resist oxidation
Classifying Refined Oil
This results in mineral oil base stocks that are classified into three different groups. API Groups I, II, and III are all mineral oil-based, but differ based the level of refining. The chart below gives a breakdown of each of the 3 mineral groups.
The key factors are: sulfur (%), saturates (%), and viscosity index. Some refer to Group III base stocks as “synthetic” due to the chemical processes they undergo. The natural chemical structure is altered from the natural structures found in mineral oil. However, the API classifies Group III oils as mineral because they originate from crude oil
The conversion process is more effective in reducing aromatic content, but the process is more expensive. Typically, the costs are passed along to end-users, but they get a higher quality base oil and better performance. Group II and Group III are becoming more prevalent as preferences shift towards conversion processes rather than extraction processes. Once refined, base stocks are blended with additives and additive packages to form a final product lubricant.