Naphthenic oil

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Crude oil is extracted from the bedrock before being processed in several stages, removing natural contaminants and undesirable hydrocarbons. This separation process produces mineral oil, which can in turn be denoted as paraffinic, naphthenic or aromatic. The differences between these different types of oils are not clear-cut, but mainly depend on the predominant hydrocarbon types in the oil. Paraffinic oil, for example, contains primarily higher alkanes, whereas naphthenic oils have a high share of cyclic alkanes in the mixture.

Contents

Classification

Crude oil appears in a host of different forms, which in turn determine how it should be refined. Classification of the crude oil can vary, because different actors have different starting points. For refineries, the interest has been primarily focused on the distribution between the distillation fractions: petrol, paraffin, gas oil, lubricant distillate, etc. Refiners look at the density of the crude oil – whether it is light, medium or heavy – or the sulfur content, i.e. whether the crude oil is “sweet” or “sour”.
The general classification of different kinds of crude oil is based on the guidelines drawn up by American Petroleum Institute (API), in which the properties can vary depending on, for example, hydrocarbon composition and sulfur content.

1. General Crude Oil Classification

Crude oil classification provides refiners with a rough guide to appropriate processing condition to reach desired products. Terminology like paraffinic, asphaltic, aromatic and naphthenic have been in use for a long time. With the progress of the science of the petroleum, addition of physical and chemical properties has been utilized to further enhance classification of crude oils. [1]

1.1 API gravity

Density has always been an important criterion of oils, generally an oil with low density is considered to be more valuable than an oil with higher density due to the fact that if contains more light fractions (i.e. gasoline). Thus, the API gravity or specific gravity is widely used for the classification of crude oils, based on a scheme proposed by the American Petroleum Institute (Table 1).
A high API value >30 means a light crude with paraffinic character; a low API value means a heavy crude with increasing aromatic character.

Table 1. Classifications of crude oil according to API gravity

Crude oil classification (°API)
Light Key Fraction 250-275 °CHeavy Key Fraction 275-300 °C
Paraffinic>40>30
Intermediate33-4020-30
Naphthenic<33<20

Low specific gravity ⇒ High °API value = paraffinic
High specific gravity ⇒ Low °API value = naphthenic

1.2 UOP characterisation factor, K factor

The UOP characterisation factor (Kw) (UOP 375-07 [2] ), is based on the observation that the specific gravities of the hydrocarbons are related to their H/C ratio (and thus to their chemical character) and that their boiling points are linked to their number of carbon atoms in their molecules. High values of Kw (13-12.5) indicate a predominately paraffinic character of its components; naphthenic hydrocarbons vary between 12-11 and values near 10 indicate aromatic character.

2. Chemical composition

The major types of hydrocarbons present in crude oils consist of 1) normal paraffins, 2) branched paraffins (iso-paraffins), 3) cycloparaffins (naphthenes) and 4) aromatics.

3. General Base Oil Classification

3.1 API base stock classification

According to API guidelines base stocks (the lubricant component, which are obtained after the crude oil is refined) are divided into five general categories.

Table 2. API base stock classifications [3] .*Viscosity Index (section 4.1).

GroupSulfur (mass %)Saturates (mass %)VI*
I>0.03<90≥80 – <120
II≤0.03≥90≥80 – <120
III≤0.03≥90≥120
IVAll polyalphaloefins
VAll others, e.g. esters, pale oil, naphthenics

Besides the above-mentioned properties, there are other methods that can be used to check if an oil is naphthenic or paraffinic. Among those:

3.2 Viscosity-gravity-constant (VGC)

The viscosity gravity constant is a mathematical relationship between the viscosity and specific gravity (ASTM D2501 [4] ) Paraffinic cuts have lower densities (and specific gravities) than naphthenic ones of about the same distillation range. VGC is of particular value in indicating a predominately paraffinic or cyclic composition. VGC is low for paraffinic crudes and high for naphthenic. [5] VGC is reported for base stocks and ranges from approximately 0.78 (paraffinic base stocks) to 1.0 (highly aromatic base stocks) and its value provides some guidance for the solvency properties of the oil. Like the results of the n-d-M method, VGC is usually reported for naphthenic products, but not for paraffinic ones.

3.3 n-d-M method

One way of obtaining compositional information on lubricating base oil is the n-d-M method (ASTM D3238 [6] ), an empirical method for determining the carbon type distribution by indicating the percentage of carbons in aromatic structure (%CA), the percentage of carbon in naphthenic structure (%CN) and the percentage of carbon in paraffinic structure (%CP). Development of the n-d-M method was the consequence of much preceding work relating composition refractive index (n), density (d), and molecular weight (M). [1]

3.4 VGC and refractivity intercept

If the viscosity, density, relative density (specific gravity) and refractive index [7] for a mineral oil is determined, the viscosity-gravity constant (VGC) and refractivity intercept (ri) can be calculated. Using the given values, the percent carbons (%CA, %CN, %CP) can be derived from a correlation chart, the ASTM D2140 method. [8] As with the n-d-M method, the results are normally reported for naphthenic oils.

4. Properties of Naphthenic Base Oils

Although several systems have been developed with the purpose for the classification of crude oils, they are usually mentioned as (1) paraffin base, (2) naphthene base (3) mixed base or (4) asphalt base. However, there appears to be no specific definition for these classifications. [9] Base oil specifications, as defined by the producer or the purchaser, largely encompass the physical properties required for the fluid; density, viscosity, viscosity index (VI), pour point and flash point, and solubility information from aniline point or viscosity-gravity constant (VGC). [10] Naphthenic base oils generally have intermediate VI's and very low pour points which make them useful in the manufacture of specialty lubricants. Dewaxing is normally not required due to the low quantities of linear paraffins (n-paraffins).

4.1 Viscosity index (VI)

Viscosity index (ASTM D2270 [11] ) is a measure of the extent of viscosity change with temperature; the higher the VI, the less the change. VI is calculated from viscosity measurements at 40°C and 100°C. The viscosities of paraffinic and naphthenic base oils have very different behavior with temperature change. Normally, paraffinic base oils have less viscosity variation (higher VI) than naphthenic base oil which display larger variation with temperature (lower VI). The low to intermediate VI make naphthenic base oils particularly suitable for specialty applications. [10]

4.2 Pour point

The pour point (ASTM D97 [12] ) measures the temperature at which a base oil no longer flows. For paraffinic base oils, pour points are usually between −12 °C and −15 °C, and are determined by operation of the dewaxing unit. The pour points of naphthenic base oils, generally devoid of wax content, may be much lower (down to <−70 °C). [10]

4.3 Aniline point

The aniline point (ASTM D611 [13] ) is of considerable value in the characterization of petroleum products. The aniline point measure of the ability of the base oil to act as a solvent and is determined from the temperature at which equal volumes of aniline and the base stock are soluble High aniline points (approximately 100°C or greater) imply a paraffinic base stock, while low aniline points (less than 100°C) imply a naphthenic or aromatic stock. [10]

4.4 Viscosity gravity constant (VGC)

VGC [4] is an indicator of base oil composition and solvency that is calculated from the density and viscosity. High values indicate higher solvency and therefore greater naphthenic or aromatic content. [10]

4.5 Refractive index (RI)

The refractive index can provide information of the composition of the base oil. Low RI values indicate paraffinic materials and high RI values indicate aromatic components. The RI value also increases with molecular weight. [14]

5. Naphthenic base oils, summary

6. Areas of application

Naphthenic oils have extraordinary low-temperature properties, high compatibility with many polymers and good solvent power. These are properties that make naphthenic oils particularly useful for the speciality oil market:

1. Transformer oils. Naphthenic oils have excellent cooling and insulating properties because of a low viscosity index. The good solubility of the oils is also important for enhanced compatibility with seals and gaskets, for example.

2. Process oils. Naphthenic oils are used in a large number of chemical processes due to their good solvent power. These include, for example, plasticizers in polymer-based formulations, rheology modifier in printing inks and carrier oil in anti-foaming agents. Naphthenic oils have been proven to be suitable in the tyre oils segment because of their low content of polycyclic aromatic hydrocarbons (PAHs), which are hazardous to health and the environment.

3. Lubricating oils. Base oils are needed to manufacture products such as greases and industrial lubricants. Naphthenic base oils are particularly suited as metalworking fluids. The main functions of the naphthenic oil in this case are cooling and lubrication, providing a balance between the two.

Related Research Articles

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<span class="mw-page-title-main">Motor oil</span> Lubricant used for lubrication of internal combustion engines

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<span class="mw-page-title-main">Hydraulic fluid</span> Medium to transfer power in hydraulic machinery

A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backhoes, hydraulic brakes, power steering systems, automatic transmissions, garbage trucks, aircraft flight control systems, lifts, and industrial machinery.

<span class="mw-page-title-main">Synthetic oil</span> Lubricant consisting of artificially made chemical compounds

Synthetic oil is a lubricant consisting of chemical compounds that are artificially modified or synthesised. Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. The base material, however, is still overwhelmingly crude oil that is distilled and then modified physically and chemically. The actual synthesis process and composition of additives is generally a commercial trade secret and will vary among producers.

A visbreaker is a processing unit in an oil refinery whose purpose is to reduce the quantity of residual oil produced in the distillation of crude oil and to increase the yield of more valuable middle distillates by the refinery. A visbreaker thermally cracks large hydrocarbon molecules in the oil by heating in a furnace to reduce its viscosity and to produce small quantities of light hydrocarbons.. The process name of "visbreaker" refers to the fact that the process reduces the viscosity of the residual oil. The process is non-catalytic.

The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water: if its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks.

The viscosity index (VI) is an arbitrary, unit-less measure of a fluid's change in viscosity relative to temperature change. It is mostly used to characterize the viscosity-temperature behavior of lubricating oils. The lower the VI, the more the viscosity is affected by changes in temperature. The higher the VI, the more stable the viscosity remains over temperature fluctuations. The VI was originally measured on a scale from 0 to 100; however, advancements in lubrication science have led to the development of oils with much higher VIs.

Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil.

Light crude oil is liquid petroleum that has a low density and flows freely at room temperature. It has a low viscosity, low specific gravity and high API gravity due to the presence of a high proportion of light hydrocarbon fractions. It generally has a low wax content. Light crude oil receives a higher price than heavy crude oil on commodity markets because it produces a higher percentage of gasoline and diesel fuel when converted into products by an oil refinery.

Heavy crude oil is highly-viscous oil that cannot easily flow from production wells under normal reservoir conditions.

Microcrystalline waxes are a type of wax produced by de-oiling petrolatum, as part of the petroleum refining process. In contrast to the more familiar paraffin wax which contains mostly unbranched alkanes, microcrystalline wax contains a higher percentage of isoparaffinic (branched) hydrocarbons and naphthenic hydrocarbons. It is characterized by the fineness of its crystals in contrast to the larger crystal of paraffin wax. It consists of high molecular weight saturated aliphatic hydrocarbons. It is generally darker, more viscous, denser, tackier and more elastic than paraffin waxes, and has a higher molecular weight and melting point. The elastic and adhesive characteristics of microcrystalline waxes are related to the non-straight chain components which they contain. Typical microcrystalline wax crystal structure is small and thin, making them more flexible than paraffin wax. It is commonly used in cosmetic formulations.

<span class="mw-page-title-main">Asphaltene</span> Heavy organic molecular substances that are found in crude oil

Asphaltenes are molecular substances that are found in crude oil, along with resins, aromatic hydrocarbons, and saturates. The word "asphaltene" was coined by Boussingault in 1837 when he noticed that the distillation residue of some bitumens had asphalt-like properties. Asphaltenes in the form of asphalt or bitumen products from oil refineries are used as paving materials on roads, shingles for roofs, and waterproof coatings on building foundations.

<span class="mw-page-title-main">Naphthenic acid</span> Chemical compound

Naphthenic acids (NAs) are a mixture of several cyclopentyl and cyclohexyl carboxylic acids with molecular weight of 120 to well over 700 atomic mass units. The main fraction are carboxylic acids with a carbon backbone of 9 to 20 carbons. McKee et al. claim that "naphthenic acids (NAs) are primarily cycloaliphatic carboxylic acids with 10 to 16 carbons"[1], although acids containing up to 50 carbons have been identified in heavy petroleum. The term naphthenic acid has roots in the somewhat archaic term "naphthene" used to classify hydrocarbons. It was originally used to describe the complex mixture of petroleum-based acids when the analytical methods available in the early 1900s could identify only a few naphthene-type components with accuracy. Today "naphthenic" acid is used in a more generic sense to refer to all of the carboxylic acids present in petroleum, whether cyclic, acyclic, or aromatic compounds, and carboxylic acids containing heteroatoms such as N and S. Although commercial naphthenic acids often contain a majority of cycloaliphatic acids, multiple studies have shown they also contain straight chain and branched aliphatic acids and aromatic acids; some naphthenic acids contain >50% combined aliphatic and aromatic acids.

The aniline point of an oil is defined as the minimum temperature at which equal volumes of aniline and lubricant oil are miscible, i.e. form a single phase upon mixing.

Heavy oil production is a developing technology for extracting heavy oil in industrial quantities. Estimated reserves of heavy oil are over 6 trillion barrels, three times that of conventional oil and gas.

Bitumen froth treatment is a process used in the Athabasca oil sands (AOS) bitumen recovery operations to remove fine inorganics—water and mineral particles—from bitumen froth, by diluting the bitumen with a light hydrocarbon solvent—either naphthenic or paraffinic—to reduce the viscosity of the froth and to remove contaminants that were not removed in previous water-based gravity recovery phases. Bitumen with a high viscosity or with too many contaminants, is not suitable for transporting through pipelines or refining. The original and conventional naphthenic froth treatment (NFT) uses a naphtha solvent with the addition of chemicals. Paraffinic Solvent Froth Treatment (PSFT), which was first used commercially in the Albian Sands in the early 2000s, results in a cleaner bitumen with lower levels of contaminates, such as water and mineral solids. Following froth treatments, bitumen can be further upgraded using "heat to produce synthetic crude oil by means of a coker unit."

Base oils are used to manufacture products including lubricating greases, motor oil and metal processing fluids. Different products require different compositions and properties in the oil. One of the most important factors is the liquid’s viscosity at various temperatures. Whether or not a crude oil is suitable to be made into a base oil is determined by the concentration of base oil molecules as well as how easily these can be extracted.

<span class="mw-page-title-main">SAE J300</span> Standard for engine oil

SAE J300 is a standard that defines the viscometric properties of mono- and multigrade engine oils, maintained by SAE International. Key parameters for engine oil viscometrics are the oil's kinematic viscosity, its high temperature-high shear viscosity measured by the tapered bearing simulator, and low temperature properties measured by the cold-cranking simulator and mini-rotary viscometer. This standard is commonly used throughout the world, and standards organizations that do so include API and ILSAC, and ACEA.

In thermodynamics, the Volume Correction Factor (VCF), also known as Correction for the effect of Temperature on Liquid (CTL), is a standardized computed factor used to correct for the thermal expansion of fluids, primarily, liquid hydrocarbons at various temperatures and densities. It is typically a number between 0 and 2, rounded to five decimal places which, when multiplied by the observed volume of a liquid, will return a "corrected" value standardized to a base temperature.

References

  1. 1 2 van Nes, K.; van Westen, H. A. (1951). Aspects of the Constitution of Mineral Oils. New York: Elsevier.{{cite book}}: CS1 maint: multiple names: authors list (link)
  2. ↑ "UOP_375" . Retrieved 1 March 2021.
  3. ↑ "API" (PDF). Retrieved 27 February 2021.
  4. 1 2 "ASTM_D2501" . Retrieved 2 March 2021.
  5. ↑ Hill, J. B.; Coats, J. B. (1928). "The Viscosity-Gravity Constant of Petroleum Lubricating Oils". Ind. Eng. Chem. 20, 6 (6): 641. doi:10.1021/ie50222a025.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ↑ "ASTM_D3238".
  7. ↑ "ASTM_D1218".
  8. ↑ "ASTM_D2140".
  9. ↑ Avilino, S (1994). Lubricant Base Oil and Wax Processing. New York: Marcel Dekker.
  10. 1 2 3 4 5 Lynch, T. R (2008). Process Chemistry of Lubricant Base Stocks. Boca Ranton: CRC Press.
  11. ↑ "ASTM_D2270".
  12. ↑ "ASTM_D97".
  13. ↑ "ASTM_D611".
  14. ↑ Speight, J. G. (1999). The Chemistry and Technology of Petroleum. New York: Marcel Dekker.
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