Petroleum Glossary

Many times we look forward to meanings and definitions for petroleum production and processing related terms. Quite some time back I had compiled my own little glossary of petroleum terms with the best available resources and here is what I came up with:

Aniline Point: It is defined as the temperature of a binary liquid solution at which equal parts of the two components of the solution are perfectly miscible. Although it is an arbitrary index (per ASTM D-611), the aniline point is of considerable value in the characterization of petroleum products. Aniline point gives a rough estimation of the aromatic content of where that value is important for functional requirements, as in the case of solvent power of naphthas & combustion characteristics of gasolines & diesel fuels.

API gravity: It is a means of classifying petroleum oils & is measured using hydrometers (per ASTM D-287 & D-1298) or pycnometers (per ASTM D-941 & D-1217). It is co-related to specific gravity at a standard temperature of 60°F by the following equation:
Degrees API = {141.5/sp. gr.} – 131.5

The specific gravities of petroleums usually range from about 0.8 (45.3° API) for the lighter crude oils to over 1.0 (10° API) for the heavier asphaltic crude oils.

Aviation gasoline: As the term indicates this is fuel for aircrafts. However, it is important to note that aviation gasoline itself may be classified for properties depending on the aircraft i.e. light civilian aircraft, heavy civilian & cargo aircraft & supersonic military aircraft. Most aircraft manufacturers provide specifications of aviation gasoline to be used for the aicrafts manufactured by them. However certain general characteristics for aviation gasolines are provided below point wise: a) Highest Octane numbers economically possible.

B) Narrow boiling ranges compared to conventional (automobile) gasoline, typically 38 to 170°C. c) Maximum heat content per unit mass (high proportion of combined hydrogen).

d) High chemical stability to withstand storage. e) No or extremely low content of olefins & small content of aromatics.


Barrel: It is a volume measure of petroleum feedstocks & one barrel equals 42 U.S. gallons (1 U.S. gallon = 3.7853 litres).

Cetane Number: It is the measure of the tendency of a diesel fuel to knock in a diesel engine. It is a scale based upon the ignition characteristics of two hydrocarbons, cetane (CH₃-(CH₂)₁₄-CH₃) & heptamethylnonane (molecular formula: C₁₆H₃₄). Cetane has a short delay period during ignition & is assigned a cetane number of 100, while heptamethylnonane has a long delay period & has been assigned a cetane number of 15. Thus cetane number is equivalent to the percentage of volume by cetane in the blend with heptamethylnonane, which matches the ignition quality of the test fuel (per ASTM D613). A higher cetane number for diesel fuels improves engine performance & decreases emissions.

Cloud Point: It is useful in estimating the relative amount of wax in an oil & is defined as the temperature at which paraffin wax or other solidifiable compounds present in a petroleum oil appear as a haze when the oil is chilled under definitely prescribed conditions (per ASTM D-2500, D-3117).

Coke: In petroleum terms, coke is the residue left by destructive distillation of petroleum residua. Coke formed in catalytic cracking operations is usually nonrecoverable, due to its use as a fuel for the cracking process. Though it has a number of end uses its chief use is in the manufacture of carbon electrodes for aluminum refining. Other uses include manufacture of carbon brushes, silicon carbide abrasives, structural carbon (pipes, Rashig rings etc.) & calcium carbide for acetylene.

Congealing Point: This is the temperature at which melted petrolatum (a kind of wax) ceases to flow when allowed to cool under definitely prescribed conditions (per ASTM D-938).

Dropping Point: This terminology is used for petroleum greases & is defined as the temperature at which the grease passes from a plastic solid to a liquid state, & begins to flow under the conditions of the test (per ASTM D-566, D-2265).

Flash Point: The flash point of petroleum or petroleum product is the temperature to which the product must be heated under the specified conditions of the method to give off sufficient vapor to form a mixture with air that can be ignited momentarily by a specified flame (per ASTM D-56, D-92, D-93).

Fire Point: It is the temperature to which the petroleum product must be heated under the prescribed conditions of the method to burn continuously when the mixture of vapor & air is ignited by a specified flame (per ASTM D-92).

Freezing Point: For pure or essentially pure hydrocarbons, the solidification temperature is the freezing point, the temperature at which a hydrocarbon passes from a liquid to a solid state (per ASTM D-1015, D-1016).

Gas Oil: It refers to the portion of crude petroleum boiling between the fuel oils (kerosene and/or stove oil) & the residuum. Maximum amount of cracking operations is carried out on gas oils.

Gasoline: It is essentially a complex mixture of hydrocarbons that boils below 180°C or, at most, below 200°C. The hydrocarbon constituents in this boiling range are those that have 4 to 12 carbon atoms in their molecular structure. In general the hydrocarbons in gasoline can be paraffins (including cycloparaffins), olefins & aromatics.

Kerosene: It is a straight-run petroleum fraction with an approximate boiling range of 205 to 260°C. It is also defined as a refined petroleum distillate which has a flash point above 25°C and which is suitable as an illuminant when burned in a wide lamp.

Knocking: It is the tendency of a fuel/air mixture to detonate in internal combustion engines. It is thought to be due to autoignition of part of the highly compressed air/fuel mixture in front of the flame. If knocking is severe it can result in loss of power & damage to the pistons & bearings of the IC engine.

Liquefied Petroleum Gas (LPG): It is a mixture of propane & butane & finds extensive use as cooking gas in bottled form or in piped form. It can also be used as a motor fuel & is a much cleaner fuel for automobiles from the environmental emission viewpoint. Special handling measures are required because of high pressures & extreme inflammability.

Liquefied Natural Gas (LNG): This is liquefied form of the natural gas obtained from well heads & predominantly contains methane along with small quantities of ethane, propane & butane. It is an excellent automobile fuel when used as a compressed gas (CNG), which burns cleanly & environmental emissions from CNG are less harmful compared to conventional gasoline or diesel.

Lubricating Oils: Lubricating oils are differentiated from other fractions of crude oil by their usually high (>400°C) boiling point, as well as their high viscosity. Fractions of crude oil suitable for production of lubricating oils are characterized by presence of hydrocarbons containing 25 to 40 carbon atoms per molecule. Another distinguishing feature is the presence of higher amounts of normal & branched (including cyclo-) paraffins compared to the lower boiling petroleum fractions.

Middle Distillates: The kerosene & light gas oil fractions which are usually the last of the fractions separated by distillation at atmospheric pressure are collectively called middle distillates.

Naphtha: Petroleum naphtha is actually a generic term which is applied to refined, partly refined, or unrefined petroleum products. In the strictest sense of the term, not less than 10% of the material should distill below 175°C, while not less than 95% of the material should distill below 240°C under standard distillation conditions (per ASTM D86).

Octane Number (per ASTM D2699 & ASTM D2723): It is defined as the percentage of isooctane (2,2,4-trimethylpentane) in a mixture with normal heptane, which as a sample fuel has the same knocking characteristics as the gasoline being tested. In the octane number scale, isooctane has an octane number of 100 indicating excellent anti-knock properties while n-heptane has an octane number zero indicating poor anti-knock properties.

Pour Point: The pour point of a petroleum oil is the lowest temperature at which the oil will pour or flow under definitely prescribed conditions when it is chilled without disturbance at a standard rate (per ASTM D-97).

Reforming: It is a process for conversion of naphthas into more volatile products of higher octane number via simultaneous combination of polymerization, cracking, dehydrogenation, & isomerization.

Saybolt Universal Viscosity (per ASTM D-88): It is the time in seconds required for the flow of 60 ml of petroleum from a container, at constant temperature, through a calibrated orifice.

Smoke Point: This is a test developed by the Institute of Petroleum (British) for determining the burning behavior of a fuel that can be burned in a lamp. Thus, it is defined as the height in millimeters of the flame that can be produced in a standard lamp without causing smoking.

Sour Oil: Petroleum fractions containing sulfur compounds (hydrogen sulfide &/or mercaptans) and readily recognizable by their odor are called "sour".

Straight-Run: This term is used for most petroleum fractions & it means fractions which have been directly obtained after distillation of the crude oil & have not undergone any other process.

Sweetening: It is defined as any process employed for the removal of hydrogen sulfide & mercaptans from petroleum fractions & petroleum fractions that are free of obnoxious sulfur compounds either naturally or because of treatment are called "sweet". However, a sweet fraction may contain sulfur compounds that have no odor.

List of References::
1. The Chemistry and Technology of Petroleum – James G. Speight
Publisher: Marcel Dekker, Inc.
2. Petroleum Refinery Engineering – W. L. Nelson
Publisher: McGraw-Hill Kogakusha Ltd.
3. Chemical Process Industries – R. Norris Shreve & Joseph A. Brink, Jr.
Publisher: McGraw-Hill International Book Company

I would invite readers of this blog to contribute more glossary terms. Let us make it the most comprehensive glossary of all in the petroleum industry to learn and appreciate. Looking forward to your contributions dear friends.