Butane

Also known as: C4, commercial butane, normal butane, nC4

Butane is one of the lightest liquid streams typically produced in a refinery. The butane molecule has four carbon atoms and 10 hydrogen atoms.

Butane is typically either blended into gasoline or LPG (in small volumes) or sold directly as a finished product. When blending into gasoline, butane is favored for its high octane but limited by its high vapor pressure. Because of vapor pressure limits, butane blending into gasoline is often completely eliminated in warmer months of the year, resulting in a seasonal glut of butane.

As a commercial finished product, butane is used as a home heating fuel, as cigarette lighter fluid, as a refrigerant gas, and as a propellant, but all of these require in fairly small volumes.

Butane is also frequently converted into isobutane for use as an alkylation feedstock. It is sometimes used as a refinery fuel, but this is typically its lowest-value end use and avoided where possible.

Butane production

Butane comes from many different process units in the refinery, as well as from outside sources such as natural gas plants (from separating NGLs) and from steam crackers (in the C4 raffinate).

Major sources of butane inside the refinery include:

• Atmospheric distillation - All crude oil grades yield some amount of butane when distilled. Typically, butane leaves the distillation tower in a wet gas stream that is sent to the saturated gas plant for separation from lighter gases (methane and ethane) that are then used for fuel
• FCC - In the FCC conversion process, large amounts of C4s are produced, including both saturated butane and unsaturated butylene. Often the unsaturated olefins are separated for use as feed to the alkylation unit
• Coker - Similar to the FCC, the coker conversion process generates mixed C4s containing saturated and unsaturated molecules. However, it is less common for coker C4s to have their olefins separated out as the percentage of unsaturated molecules is lower than from the FCC
• Reformer - Reformers will yield approximately 5% (by volume) of C4s in the conversion process

Butane (commercial) product quality

Commercial butane typically has the following specifications:
Vapor pressure: 31 psig at 70F; 59 psig at 100F; 97 psig at 130F
Specific gravity: 0.582 at 60F
Initial boiling point: 15F at 1 bar
Dew point: 24F at 1 bar
Specific heat: 0.549 Btu/lb at 60F; 2.299 kJ/kg at 15.6C
Lower limit of flammability: 1.9 vol% gas in air
Upper limit of flammability: 8.6 vol% gas in air
Latent heat of vaporization: 155 Btu/lb; 383.8 kJ/kg
Gross heating value (liquid): 21,170 Btu/lb; 49,241 kJ/kg
Gross heating value (gas): 3,350 Btu/ft3; 12,482 kJ/m3