The Chemistry of a Standard Propane Tank

How can we determine the mass and volume of liquid and vapor phase propane in a standard propane tank?

Given a total of 20lbm of propane at 25 ∘C, what is the total heat required to maintain the tank as 5lbm of propane vapor flows out?

Answer:

A 20lbm propane tank at 25 degrees Celsius contains around 10.60lbm of liquid propane and 9.40lbm of vapor propane. When 5lbm vapor propane flows out of the tank, a heat of about 95,000 Btu is required to maintain the tank's temperature.

Explanation:

To answer these two-part questions about a standard propane tank, we'll first need to use the information provided and the relevant physical and chemical properties of propane.

(a) Propane in liquid form has a specific gravity of about 0.493 at 25 degrees Celsius, which means that a 20lbm (pound-mass) propane tank contains around 20lbm / 0.493 ≈ 40.57 liters of liquid propane. Given that the internal volume of the tank is 21.5 liters, this suggests ~19.07 liters is occupied by propane vapor. Since the mass of propane in the vapor phase is directly proportional to its volume, this means the mass of the liquid propane is approximately 10.60lbm and the mass of the vapor phase propane is 9.40lbm.

(b) When 5lbm of propane vapor flows out of the tank, it will undergo a phase transition from gas to liquid. The heat of vaporization of propane is approximately 19,000 Btu/lbm. Therefore, the heat required to maintain the remaining propane at 25˚C would be 5lbm * 19,000 Btu/lbm = 95,000 Btu.