Example 05.15 Henry Constant for CO2 from Phase Equilibrium Data

problem:

Determine the Henry constant for carbon dioxide (1) in water with the help of phase equilibrium data at 50°C.

general

constants

and

definitions:

input data

Experimental data (partial pressure and liquid mole fraction of CO2):

Pure component vapor pressure of water at 50°C:

Virial coefficients:

Solution

Henry coefficients can be determined by the following equation:

where the fugacity of the gas f1 is often replaced by its partial pressure p1 at low pressures. In this example, the Virial equation will be used to calculate the real vapor phase behavior. The result will be compared to that of the simplified approach.

In the experimental data, only the partial pressure of CO2 is reported. As the mole fraction of water is nearly unity and the properties of the liquid mixture are not affected significantly by the small amount of dissolved CO2, the partial pressure of water can be assumed to be equal to the pure component vapor pressure. At high pressures, it will be affected by the Poynting factor. In this case the relative change in total pressure is very small due to the low vapor pressure of the solvent, so that this effect can be neglected.

Using the partial and total pressure, the composition of the vapor phase, the Virial coefficient of the vapor and the fugacity coefficient of CO2 can be calculated for all data points:

Now the ratio of the fugacity to the liquid mole fraction for CO2 can be regressed (linear equation) and plotted against the liquid mole fraction of CO2. Extrapolation to a mole fraction of zero yields the correct value of the Henry constant:

Both approaches give a nearly identical value of the Henry coefficient (equal to the intercept from the linear regression):

Nevertheless, only the correct form of the Henry's Law using the fugacity and not the partial pressure of the gas gives the approximately correct description of the gas solubility at higher pressures.