Propane Butane Mixtures - Evaporation Pressures - Boiling Points

It is common practice to use a mixture of propane (C3H8) and butane (C4H10) in essential oil hydrocarbon extraction practices.

Propane is more suited to colder environments since it boils at -44oF (-42oC) at atmospheric pressure. Butane boils at 33oF (0.6 oC) at atmospheric pressure. Adding propane will increase the pressure inside the system relative to butane under the similar conditions.

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

Vapor Pressure (psig
Mix Propane
(C3H8)
(%)
100 70 50 30 0
Butane
(C4H10)
(%)
0 30 50 70 100
Temp 
(oF)
-44 0 0 0 0 0
-30 6.8 0 0 0 0
-20 11.5 4.7 0 0 0
-10 17.5 9 3.5 0 0
0 24.5 15 7.6 2.3 0
10 34 20.5 12.3 5.9 0
20 42 28 17.8 10.2 0
30 53 36.5 24.5 15.4 0
40 65 46 32.4 21.5 3.1
50 78 56 41 28.5 6.9
60 93 68 50 36.5 11.5
70 110 82 61 45 17
80 128 96 74 54 23
90 150 114 88 66 30
100 177 134 104 79 38
110 204 158 122 93 47
  • 1 psi (lb/in2) = 144 psf (lbf/ft2) = 6894.8 Pa (N/m2) = 6.895x10-3 N/mm2 = 6.895x10-2 bar
Note that the boiling temperature is not the only parameter that influences on boiling of the propane butane mixture. Boiling requires heat and if the heat transferred to the liquid gas is limited - the liquid cools down and the evaporation is  reduced unless the pressure is also reduced. This process is called adiabatic cooling.
Propane Butane Mix Vapor Diagram - psig
Propane Butane Mixures - Evaporation Pressures - Boiling Points