CO2 Saturation Properties
Carbon dioxide (CO2) is a naturally occurring compound found in the Earths atmosphere, produced through the decay of organic matter and the burning of fossil fuels. It is the most abundant greenhouse gas in the atmosphere, and is responsible for more than six-tenths of the current global warming trends. CO2 can be found in trace amounts in many fluids at natural conditions. Its solubility depends on the temperature and pressure of the medium. When a solution of CO2 is brought to a pressure higher than its saturating pressure, the dissolved gas comes out of solution, forming CO2 gas bubbles in the medium. The properties of the CO2 gas in saturated states, such as the critical pressure, isotherms, and critical temperature, determine the maximum pressure at which the CO2 is soluble in a medium.
The most important thermodynamic property of a saturated CO2 gas is its critical temperature and pressure. Though they vary slightly depending on the source of the CO2 and the temperature of the medium, altogether they were agreed to around Tc = 31.1 °C and Pc = 73.8 bar. In the presence of inert gas, the CO2 gas pressure is usually relatively low and the critical pressure of CO2 is never reached. Therefore, it is commonly assumed that the saturated pressure of the CO2 is proportional to the partial pressure of the CO2.
The isotherms (pressure-volume diagrams) of saturated CO2 gas at temperatures around its critical point can display surprisingly non-ideal behavior, in contrast to the Thomas-Fermi equation for ideal gases. This is due to large fluctuations in the gas-liquid interface near the critical point, resulting in an unusually high entropy of the mixture. This behavior is expected to deviate from the Thomas-Fermi equation more as the temperature gets closer to the critical temperature, and the pressure increases.
The enthalpy and entropy of CO2 also change depending on the state and thereby significantly influence the thermodynamic behavior of the substance. For example, at high temperatures and pressures above the critical point, the enthalpy of saturated CO2 gas is expected to be greater than the enthalpy of liquid CO2. This can cause a significant increase in the achievable yields of CO2-bearing products. Furthermore, a change in the entropy of CO2 due to its state change affects the Gibbs free energy and contributes significantly to the total thermodynamic potential of the CO2-bearing medium.
At elevated temperatures and pressures, the dissolution of CO2 in a liquid medium can form a Gibbs-Thomson fractionation mechanism, leading to strong fractionation effects in terms of differential solubility of CO2 in the liquid. This is because elevated temperatures increase the solubility of CO2 in liquid media, but higher solubility of CO2 in liquid results in its depletion near the interface in the form of a fractionating envelope.
In conclusion, the properties of CO2 in saturated states are determined by its critical pressure and temperature. Various other thermodynamic parameters, including enthalpy and entropy, also change significantly with pressure and temperature, and can significantly influence the fluid phase behavior of a CO2-bearing system. In particular, when dissolved in a liquid medium, CO2 can form a Gibbs-Thomson mechanism and cause strong fractionation effects.
