Question

The maximum amount of CO2 that could be absorbed in water at 1 atmosphere at room temperature (25 ͦ C). (ideall where on a phase diagram of CO2)

Answer 1

If we assume CO₂ is a simple gas we can apply Henry’s law that describes the equilibrium between vapor and liquid. Thus:

p_CO2 = K . x_CO2

where p_CO2 is the partial pressure of the gas in the bulk atmosphere (Pa), K is a constant (Pa) and x_CO2 is the equilibrium mole fraction of solute in liquid phase.

The solubility of CO₂ is temperature dependent, as shown in Table 1: Solubility of CO2 at a partial pressure for CO2 of 1 bar abs.

Table 1: Solubility of CO₂ at a partial pressure for CO₂ of 1 bar abs.

Temperature (oC)	0	10	20	30	40	50	80	100
Solubility (cm3 CO2/g water)	1.8	1.3	0.88	0.65	0.52	0.43	0.29	0.26

Furthermore, as stated above, CO₂ reacts with the water on dissolution and therefore one would expect that Henry’s law has to be modified.

However, according to Carrol and Mather a form of Henry’s law can be used for modeling the solubility of carbon dioxide in water for pressures up to about 100 MPa, as can be seen in Figure 1: Henry's Constant for Carbon Dioxide in Water - from Carroll et al.

They conclude that the Krichevsky-Kasarnovsky Equation, which can be derived from Henry’s Law, can be used to model the system CO₂-H₂O at temperatures below 100 ^oC. Thus in the range of interest, 20-35 °C, the Henry coefficient for CO₂ in water goes from 150 - 200 MPa/mole fraction

Applying the above to the conditions under investigation:

Temperature range: 20 – 35 °C
Pressure range: 80 – 90 bar

CO₂ concentration in gas phase: 1.3-1.7 mol%

The partial pressure of CO₂ in the gas phase is therefore in the range:

1.3/100 * 80 * 0.1 = 0.104 MPa

1.7/100 * 90 * 0.1 = 0.153 MPa

Applying Henry’s Law we calculate a CO₂ mole fraction in water in the range:

x_low = 0.104 / 200 = 0.00052

x_high = 0.153 / 150 = 0.00102
Converting mole fractions to concentrations:

At 20 °C the molar density of water = 998.21/18.02 = 55.39 mol/l

At 35 °C the molar density of water = 994.37/18.02 = 55.18 mol/l

Thus the CO₂ concentration range in water under these conditions is:

c_low = 0.00052 * 55.18 = 0.029 mol/l

c_high = 0.00102 * 55.39 = 0.056 mol/l