Glossary of Terms

Alkylate: The product of an alkylation reaction. It usually refers to the high-octane product from alkylation units. This alkylate is used in blending high-octane gasoline.

Alkylation: A refining process for chemically combining isobutane with olefin hydrocarbons (for example, propylene, butylenes) through the control of temperature and pressure in the presence of an acid catalyst, usually sulfuric acid or hydrofluoric acid. The product, alkylate, an isoparaffin, has high-octane value and is blended with motor and aviation gasoline to improve the anti-knock value of the fuel.

Aromatics: Hydrocarbons characterized by unsaturated ring structures of carbon atoms. The basic ring has six carbon atoms and is shaped like a hexagon. Some heavier aromatics with two or more hexagonal rings with common sides (polycyclic aromatics) are also present in gasoline; some are formed during combustion. Some aromatics are ozone forming; some are toxic. Benzene and polycyclics are toxic; xylenes and some of the more complex aromatics are active ozone formers. Commercial petroleum aromatics are benzene, toluene, and xylene.

Benzene: A hydrocarbon of the composition C6H6 and the initial member of the aromatic or benzene series. Its molecular structure is conceived as a ring of six carbon atoms with double linkage between each alternating pair and with hydrogen attached to each carbon atom. Benzene is a minor constituent of most crude oils and is produced mainly by the catalytic reforming of petroleum naphthas and from the various cracking processes. Benzene is a toxic compound.

Cut: A cut is a fraction of the charge stock separated by distillation. For example, kerosene is a cut of crude oil.

Normal Butane (C4H10): A normally gaseous straight-chain hydrocarbon. It is a colorless paraffinic gas that boils at a temperature of 31.1 degrees Fahrenheit. It is extracted from natural gas or refinery gas streams.

E200: Percent of fuel evaporated below 200 degrees Fahrenheit, as determined by ASTM distillation test D86.

E300: Percent of fuel evaporated below 300 degrees Fahrenheit, as determined by ASTM distillation test D86.

Isomerization: A refinery process which converts normal or straight chain hydrocarbons that have a poor octane quality into high-octane branch chain isomers. Thus, n-butane is converted into isobutane, etc.

NOx—Nitrogen Oxides: Chemical compounds containing nitrogen and oxygen; reacts with volatile organic compounds in the presence of heat and sunlight to form ozone. It also contributes to acid rain.

Octane Number: A number used to indicate gasoline’s antiknock performance in motor vehicle engines. The two recognized laboratory engine test methods for determining the antiknock rating, i.e., octane rating of gasoline, are the Research method and the Motor method. To provide a single number as guidance to the consumer, the antiknock index (R + M) /2, which is the average of the Research and Motor octane numbers, was developed.

Olefins: Olefins are highly reactive unsaturated compounds (that is, the carbon atoms in the molecule are able to accept additional atoms such as hydrogen or chlorine). Some are present in gasoline as a result of refinery manufacturing processes such as cracking. Some are created in the engine during combustion; most of these can be removed in the catalytic converter. They tend to be ozone formers and toxic.

Reformate: The product of the reforming process which runs at high temperature with a catalyst to convert paraffinic and naphthenic hydrocarbons into high octane stocks, primarily aromatics suitable for blending into finished gasoline.

Reid Vapor Pressure (RVP): A measure of product volatility, measured in pounds per square inch (psi). The higher the RVP, the more volatile a gasoline is and the more readily it evaporates.

T50: The temperature at which 50 percent of fuel has vaporized.

T90: The temperature at which 90 percent of fuel has vaporized.

Volatile Organic Compounds (VOCs): Organic compounds which participate in atmospheric photochemical reactions.