Home >> Science & Nature >> Science

How to Calculate the Vapor Pressure of a Propylene Propane & Butane Mixture

Vapor pressure is defined as an equilibrium pressure of a vapor with its corresponding liquid in a sealed system. As liquids evaporate into a vapor, the vapors have a tendency to condense back to a liquid. At certain temperatures, there is a corresponding vapor pressure where there is equilibrium between the evaporating liquid and the condensing vapor. Gas mixtures must use an approximation method with partial pressures. For each component and its concentration in the mixture, there will be a partial vapor pressure exerted. The total vapor pressure of a gas mixture is the sum of these partial pressures. In addition, the liquid temperatures of these gases are extremely low.

Things You'll Need

Pen
Paper
Calculator

Instructions

- 1
  Determine the gas mixture temperature and concentrations of the three gas components. For instance, assume the gas mixture has a constant temperature of -144 degrees Fahrenheit and the components have mass fractions of 0.25 for propylene, 0.30 for propane and 0.45 for butane.
- 2
  Determine the partial vapor pressure for each component at the corresponding temperature. This is done by referring to a reference such as the CRC handbook, listed below. For propylene, the vapor pressure at -144 degrees Fahrenheit is 0.68 pounds per square inch (psi), for propane it is 0.48 psi and for butane it is 0.03 psi.
- 3
  Determine the vapor pressure for the gas mixture using the formula Pmix = Σ xi pi where Pmix is the vapor pressure of the total mixture, xi is the component mass fraction and pi is the partial vapor pressure of each component (Σ means ̶0;sum of̶1;). This equates out to be 0.25 x 0.68 + 0.30 x 0.48 + 0.45 x 0.03 or a total vapor pressure of 0.3275 psi.

Science