Today’s crude petroleum oil dates back millions of years. There are two main theories for the origin of crude oil. One suggests that crude oil comes from carbon deposits deep in the earth; the other suggests it was created from the remains o tiny animals and plants that settled with mud and silt.
Over millions of years of intense pressure and heat, this organic matter turned into what is now known as crude oil. Regardless of crude oil’s origins, humankind has found many ways to harness this dark, thick, stinky substance.
The composition of crude oil is complex. It contains individual hydrocarbons or hydrocarbon compounds. Hydrocarbons are organic compounds that are entirely composed of hydrogen and carbon atoms.
Crudes have varying amounts of elemental compounds such as sulfur, nitrogen, oxygen and metals such as nickel or vanadium. Water-containing salts also can be found in crude oil. Many of these inherent compounds of crude petroleum must be removed to make the oil usable.
A refining process removes the materials that inhibit the use of crude oils. As an example, sulfur must be removed to meet environmental regulations. The many materials found in crude oil add complexity to the refining process, meaning there is a higher cost to removing these materials.
Unprocessed crude can be used, but there are limited applications for it, such as in power plants and some internal combustion engines. Most often crude is distilled into different fractions. Fractions are batches of a particular substance, in this case, different molecules of hydrocarbons.
Anyone who has driven past an oil refinery will recall that these plants appear to be very large mazes of piping and other large units with smoke stacks scattered about. The complex maze consists of piping, distillation units, furnaces, hydrocrackers and a number of other units needed to refine crude oil. All of these components are necessary to separate the hundreds of different types of hydrocarbon molecules into simpler, more usable forms.
A detailed discussion of the oil refining process is beyond the scope of this article, but Figure 1 does a good job of illustrating the process. As crude oil begins the refining process, it enters a distillation tube. From there the separated molecules enter additional treatment centers to be further broken down into usable oils and substances such as sulfur, butanes, jet fuel, kerosene, diesel oil, fuel oil, petroleum coke, asphalt and gasoline.
The refining of crude oils can produce a variety of lubricant types of varying quality and viscosity grades. These lubricants can be refined to maximize their beneficial characteristics and minimize those that are not desirable; however, it’s an expensive process.
Base Oil Categories
The American Petroleum Institute (API) developed a classification system for base oils that focuses on the paraffin and sulfur content and degree of saturation of the oil. The saturate level indicates the level of molecules completely saturated with hydrogen bonds, leaving them inherently un-reactive.
There are five groups in the classification system, ranging from Group I – Group V. Figure 2 details the five groups by their manufacturing process, saturate and sulfur level and their viscosity index (VI). General group characteristics are listed below.
Base Oil Characteristics
Group I Characteristics
Group I base oils are the least refined of all the groups. They are usually a mix of different hydrocarbon chains with little uniformity. While some automotive oils use these stocks, they are generally used in less-demanding applications.
Group II Characteristics
Group II base oils are common in mineral-based motor oils. They have fair-to-good performance in the areas of volatility, oxidation stability, wear prevention and flash/fire points. They have only fair performance in areas such as pour point and cold-crank viscosity.
Group III Characteristics
Group III base oils consist of reconstructed molecules that offer improved performance in a wide range of areas, as well as good molecular uniformity and stability. These synthesized materials can be used in the production of synthetic and semi-synthetic lubricants.
Group IV Characteristics
Group IV base oils are made from polyalphaolefins (PAO), which are chemically engineered synthesized base stocks. PAOs offer excellent stability, molecular uniformity and improved performance.
Group V Characteristics
Group V is a catch-all category. Most base oils in this category are chemically engineered base oils that do not fall into any of the categories previously mentioned. Typical examples of Group V stocks are esters, polyglycols and silicone, but the category also includes vegetable oils and other base oils that don’t fit under the other categories. As with Group IV stocks, Group V stocks tend to offer performance advantages over Groups I -III.
Defining Mineral Oil Properties
Mineral oils are generally classified as paraffinic and naphthenic. The difference between paraffinic stocks and naphthenic stocks is one of molecular composition, resulting in inherent solvent differences between the two types of stock.
Paraffinic oils are characterized by straight chains of hydrocarbons where the hydrogen and carbon atoms are connected in a long linear composition, similar to a chain.
The wax matter within the paraffinic stock results in these elements turning to solids at low temperatures; therefore, untreated paraffinic stocks do not have good cold-temperature performance and consequently, the pour point of paraffinic stocks is higher. For a paraffinic stock to flow at low temperatures, the heaviest waxes must be removed and usually pour-point depressants are necessary.
Paraffinic stocks display good high temperature performance with high oxidation stability and high flash/fire points. Paraffinic stocks also have a high viscosity index (VI), meaning they exhibit high viscosity stability over a range of temperatures.
Naphthenic oil stocks are much like paraffinic stocks in that they contain only hydrocarbons. However, naphthenic stocks differ and are characterized by a high amount of ring hydrocarbons, where the hydrogen and carbon atoms are linked in a circular pattern. Conventionally, when the paraffinic carbon content of oil is less than 55-60 percent, the oil is labeled as naphthenic.
Naphthenic crudes contain little to no wax and therefore remain liquid at low temperatures; however, they thin considerably when heated. Naphthenic stocks generally have a low VI. These stocks have higher densities than paraffinic stocks, and they have greater solvency abilities than their paraffinic counterparts. Because naphthenic stocks contain little wax, they display lower pour points than paraffinic stocks. These stocks are also volatile and have a lower flash point.
Because naphthenic crude contains degradation products that are soluble in oils, they present fewer problems with the formation of sludge and deposits. Due to the performance characteristics of naphthenic oils, they are generally used in applications where low pour points are required and the application temperature range is narrow.
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