Definitions

Definitions of Words and Terms Used in the Gas Processing Industry

absorber
A tower or column that provides contact between natural gas being processed and a liquid solvent.
absorption
The operation in which one or more components in the gas phase are transferred to (absorbed into) a liquid solvent.
acid gas

The hydrogen sulfide and/or carbon dioxide contained in, or extracted from, gas or other streams.
adsorbent
A solid substance used to remove components from natural gas being processed.
adsorption
The process by which gaseous components are adsorbed on solids because of their molecular attraction to the solid surface.

API Gravity
An arbitrary scale expressing the relative density of liquid petroleum products. The scale is calibrated in degrees API, calculated by the following formula:


associated gas
Gaseous hydrocarbons occurring as a free-gas phase under original oil-reservoir conditions of temperature and pressure. atmospheric pressure The pressure exerted on the earth by the earth’s atmosphere. A pressure of 760 mm of mercury, 29.92 inches of mercury, or 14.696 psia is used as a standard for some measurements. State regulatory bodies have set other standards for use in measuring the legal volume of gas. Atmospheric pressure may also refer to the absolute ambient pressure at any given location.
blowdown
The act of emptying or depressuring a vessel. This may also refer to discarded material, such as blowdown water from a boiler or cooling tower.
bubble point
The temperature at a specified pressure at which the first stable vapor forms above a liquid.
compressibility factor
A factor, usually expressed as "Z," which gives the ratio of the actual volume of gas at a given temperature and pressure to the volume of gas when calculated by the ideal gas law.
compression ratio
The ratio of the absolute discharge pressure from a compressor to the absolute intake pressure. Also applies to one cylinder of a reciprocating compressor and one or more stages of a rotating compressor.
condensate
The liquid formed by the condensation of a vapor or gas; specifically, the hydrocarbon liquid separated from natural gas because of changes in temperature and pressure when the gas from the reservoir was delivered to the surface separators. In a steam system it may be water that is condensed and returned to the boilers.
critical density
The density of a substance at its critical temperature and critical pressure.
critical pressure
The vapor pressure of a substance at its critical temperature. 
critical temperature
For a pure component, the maximum temperature at which the component can exist as a liquid.
dew point
The temperature at any given pressure, or the pressure at any given temperature, at which liquid initially condenses from a gas or vapor. It is specifically applied to the temperature at which water vapor starts to condense from a gas mixture (water dew point), or at which hydrocarbons start to condense (hydrocarbon dew point).
distillation
The process of separating materials by successively heating to vaporize a portion and then cooling to liquefy a part of the vapor. Materials to be separated must differ in boiling point and/or relative volatility.
dry gas
(1) Gas whose water content has been reduced by a dehydration process. (2) Gas containing little or no hydrocarbons commercially recoverable as liquid product. Gas in this second definition preferably should be called lean gas.
flash point
The lowest temperature at which vapors from a hydrocarbon liquid will ignite. See ASTM D-56.
fractionation
Generally used to describe separation of a mixture of hydrocarbons into individual products based on difference in boiling point and/or relative volatility.
gas constant (R)
The constant multiplier in the Ideal Gas Law. Numerically, R=PV/T, if V is the volume of one mole of an ideal gas at temperature T and pressure P.
gas injection
The injection of natural gas into a reservoir to maintain or increase the reservoir pressure or reduce the rate of decline of the reservoir pressure.
gas lift
A method for bringing crude oil or water to the surface by injecting gas into the producing well bore.
gas-oil ratio (GOR)
The ratio of gas to liquid hydrocarbon produced from a well. This may be expressed as standard cubic feet of gas per barrel of stock tank liquid.
gas processing
The separation of constituents from natural gas for the purpose of making salable products and also for treating the residue gas to meet required specifications.
gas processing plant
A plant which processes natural gas for recovery of natural gas liquids and sometimes other substances such as sulfur.
hydrate
A solid material resulting from the combination of a hydrocarbon with water under pressure.
Joule-Thomson effect
The change in gas temperature which occurs when the gas is expanded at constant enthalpy from a higher pressure to a lower pressure. The effect for most gases at normal pressure, except hydrogen and helium, is a cooling of the gas.
lean gas
(1) The residue gas remaining after recovery of natural gas liquids in a gas processing plant. (2) Unprocessed gas containing little or no recoverable natural gas liquids.
LNG (liquefied natural gas)
The light hydrocarbon portion of natural gas, predominately methane, which has been liquefied.
LP-gas (liquefied petroleum gas)
Predominately propane or butane, either separately or in mixtures, which is maintained in a liquid state under pressure within the confining vessel.
LRG (liquefied refinery gas)
Liquid propane or butane produced by a crude oil refinery. It may differ from LP-gas in that propylene and butylene may be present.
LTX (low temperature extraction unit)
A unit which uses the cooling of a constant enthalpy expansion to increase liquid recovery from streams produced from high pressure gas condensate reservoirs. Also called LTS (low temperature
separation) unit.
Mcf
An abbreviation for one thousand cubic feet of gas.
MMcf
An abbreviation for one million cubic feet of gas.
mercaptan
Any of a homologous series of compounds of the general formula RSH. All mercaptans possess a foul odor.
natural gas
Gaseous form of petroleum. Consisting predominately of mixtures of hydrocarbon gases. The most common component is methane.
NGL (natural gas liquids)
Natural gas liquids are those hydrocarbons liquefied at the surface in field facilities or in gas processing plants. Natural gas liquids include ethane, propane, butanes, and natural gasoline.
recovery
That percent or fraction of a given component in the plant feed which is recovered as plant product.
recycle
Return of part of a process stream to a point upstream from where it was removed to enhance recovery or control.
reflux
In fractionation, the portion of condensed overhead returned to the column to enhance achievable purity of the overhead product.
reflux ratio
A way of giving a relative measurement to the volume of reflux. Usually referred either to the feed or overhead product.
relief system
The system for safely relieving excess pressure to avoid exceeding equipment design pressure.
sour gas
Gas containing undesirable quantities of hydrogen sulfide, mercaptans, and/or carbon dioxide. It also is used to refer to the feed stream to a sweetening unit.
specific gravity
The ratio of the mass of a given volume of a substance to that of another equal volume of another substance used as standard. Unless otherwise stated, air is used as the standard for gases and water for liquids, with the volumes measured at 60°F and standard atmospheric pressure.
sweet gas
Gas which has no more than the maximum sulfur and/or CO2 content defined by (1) the specifications for the sales gas from a plant; (2) the definition by a legal body. Also, the treated gas
leaving a sweetening unit.