Phase Equilibrium

Two phases are in equilibrium if the fugacities are the same: fiV = fiL. The vapour fugacity can be expressed in terms of the partial pressure, the fugacity coefficient and the Poynting factor. The liquid fugacity depends on the liquid composition, the saturation fugacity coefficient, the activity coefficient γi and the saturation pressure.

fiV = yi · p · φiV · Φi

fiL = xi · p_sat · γi · φiL_sat

The Poyinting factor Φi is to 1, which is a good assumption if the system pressure is not too high.

For some substances, namely ideal gases, the liquid fugacity is rather described using Henry's law. In this case fiL = xi ·Hi where Hi is the Henry coefficient of the substance i. The medium model has to supply the information for which component the equilibrium is calculated using Henry's law. The medium models also supply the values for the fugacity and activity coefficients. The medium models can rely on different correlations to calculate fugacity and activity coefficients. Some of these correlations can be found in Media.Correlations.

The VLE calculations are commonly carried out, for mixtures containing strongly polar compounds or electrolytes, with hybrid models, which use an activity coefficient model for the liquid phase and a fugacity coefficient model for the vapor phase:

The hybrid model is the best way to represent highly non-ideal liquid mixtures at low pressures.(Lit. [1], introduction part) For systems containing dissolved gases at low pressure and at small concentrations, use Henry's law. For highly non-ideal chemical systems at high pressures, use equations of state instead of activity models(not implemented).

fi0 is the reference fugacity and is commonly estimated with a saturation fugacity coefficient, the saturation pressure of the component at the system temperature and the Poynting correction for pressure (for pressures under 10 bar almost =1).

In case of existing supcritical substances at system temperature, the reference fugacity is replaced by Henry's law. (Lit. [2], p. 108)

Different classes are available to calculate the phase equilibrium:

IdealGasActivityCoeffLiquid: fugacity coefficient of the gas phase is set to one

IdealGasIdealLiquid: fugacity coefficients and activity coefficient are set to one

RealGasActivityCoeffLiquid: both gas and liquid phase are considered to behave non-ideal

RealGasIdealLiquid: activity coefficient is set to one


[1] Fluid Phase Equilibria 187 (2001) 397-405

[2] Gmehling, Kolbe, 1992, zweite Auflage, Thermodynamik


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ConstantKconstant value for K
H2O_CO2_MEAH2O and CO2-MEA Eq. for CO2 separation with MEA
H2O_CO2_MEA_startUpH2O and CO2-MEA Eq. for startUp CO2 separation with MEA
IdealGasActivityCoeffLiquidgas is ideal, non-ideality of liquid taken into account using an activity coefficient model
IdealGasIdealLiquidgas and liquid are ideal
RealGasActivityCoeffLiquidgas fugacity coefficient not equal to one, non-ideality of liquid taken into account using an activity coefficient
RealGasIdealLiquidgas fugacity coefficient not equal to one, liquid ideal
CO2_CO2_MEA_StartUpReboiler_newFormulationStartUp CO2-CO2-MEA eq. incl. reaction for Reboiler new formulation
CO2_CO2_MEA_StartUpReboiler_newFormulation_DesStartUp CO2-CO2-MEA eq. incl. reaction for Desorber new formulation

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