The Equilibrium Calculation functionality is used to compute the resulting equilibrium conditions for a mixture of a set of species and phases.

Right-click the relevant Thermodynamic System node (see Figure 6-8), Predefined System node, or the relevant External Thermodynamic System node, and select Equilibrium Calculation to start the Equilibrium Calculation Wizard.

The Equilibrium Calculation Wizard consists of the following steps:

The Equilibrium Calculation settings include three types of equilibrium functions.

Select one or more of the species available in the thermodynamic system and use the Add Selected button () to add them to the Selected species table. Click the Next button () to proceed to the next step in the wizard.

Use the Amount base unit list to use either kg or mol as base unit.

Select two Equilibrium conditions that define the current equilibrium, for example a given pressure and a temperature. These equilibrium conditions are used as arguments in the equilibrium functions, in addition to the composition (overall fractions of species).

The available equilibrium conditions are: Temperature, Pressure, Phase fraction, Energy (or Internal energy of formation), Enthalpy (or Enthalpy of formation), Specific volume, Density and Entropy (or Entropy of formation). For chemical reactions, it is recommended to use Enthalpy of formation, Entropy of formation, or Internal energy of formation, since they account for heat of reactions.

Selecting Phase fraction as one of the equilibrium conditions activates the Solution type input field. This can be used to indicate the direction of the desired solution, which is of great use especially near critical points. The options Undefined, Normal, or Retrograde define different directions for the search of the solution to the equilibrium equations. Using Normal means that the derivative of the vapor phase fraction with respect to temperature (at constant pressure and composition) is kept positive and the derivative of the vapor phase fraction with respect to pressure (at constant temperature and composition) is kept negative. Using Retrograde means that the opposite sign of the previous mentioned derivatives are enforced.

For a single species, the critical point is the highest pressure and temperature at which two phases (liquid and vapor) are distinguishable. However, for some multispecies systems, the critical point is a point between the dew point and the bubble point. In this case, the critical point does not represent the maximum pressure or the maximum temperature of vapor-liquid coexistence. This phenomenon is known as retrograde condensation. This means that under isothermal conditions, when the pressure decreases, some of the vapor condenses into liquid instead of expanding or vaporizing. An example of such system is formation of liquid hydrocarbons in a gas reservoir as the pressure decreases below the dew point pressure. In this case, setting Solution type to Normal or Retrograde may not be sufficient to distinguish between the two solutions.

Note that the Solution type setting is only available for a built-in thermodynamic systems. For external thermodynamic systems, the corresponding functionality needs to be supplied by the thermodynamic software provider. For instance, the COCO/TEA provider does not support the Normal or Retrograde options. In those cases, the Solution type should be Undefined.

Click the Next button () to proceed to the next step.

In this step, you can review all the functions including units and arguments. Click the Finish button () to exit the wizard and add functions at equilibrium state to the current thermodynamic system.

When creating an Equilibrium Calculation, the resulting functions are collected under the Mixture node. You can create new functions from an existing Mixture node. Right-click it and select Equilibrium Calculation to start the Equilibrium Calculation Wizard (see Figure 6-20).

Selecting an Equilibrium Calculation node displays the settings including the property functions, see Figure 6-22.

Can be used to evaluate and plot the equilibrium function for a range of argument values. First specify which of the included functions to plot. Then apply a Lower limit and Upper limit for each argument and click the Plot button ().