Propane Butane Mixture - Evaporation Pressure

It is common to distribute a mixture of propane (C₃H₈) and butane (C₄H₁₀) (LPG) for combustion purposes.

Propane is more suited to colder environments since it evaporates at -44 ^oF (-42 ^oC) at atmospheric pressure. Butane evaporates at 33 ^oF (0.6 ^oC) at atmospheric pressure.

Vapor pressures for mixtures of the two products relative to atmospheric pressure (gauge pressure) are indicated below.

• propane vapor pressure (abs)
• butane vapor pressure (abs)

• 1 psi (lb/in²) = 144 psf (lb_f/ft²) = 6894.8 Pa (N/m²) = 6.895×10^-3 N/mm² = 6.895×10^-2 bar

Note that the evaporation temperature is not the only parameter that influences on evaporation of the propane butane mixture. Evaporation requires heat and if the heat transferred to the liquid gas is limited - the liquid cools down and the evaporation is  reduced.

• Larger consumers requires heat exchangers fueled with hot water, electric heater or combustion of the propane butane mix itself to supply the evaporation heat.
• Smaller consumers requires propane butane mix containers with efficient heat transfer from the surroundings. By example - composite containers provides less heat transfer compared to steel containers and may cause problems at lower surroundings temperatures.  

Propane Butane Mix Vapor Diagram - psig

Propane Butane Mix Vapor Diagram - bar

Butane Vapor Pressure (abs)