Equations for the UNIFAC method
- Last UpdatedSep 26, 2025
- 6 minute read
The UNIQUAC Functional-group Activity Coefficient (UNIFAC) method is a liquid activity coefficient (LACT) method. AVEVA Process Simulation uses a standard set of equilibrium calculations for most LACT methods. See the following sections for more information:
The main difference in the equilibrium calculations between the different LACT methods is the calculation of the activity coefficient.
Activity coefficient calculations
The UNIFAC method is based on the Universal Quasi-Chemical (UNIQUAC) model, which represents the excess Gibbs energy (and the logarithm of the activity coefficient) as a combination of two effects. We therefore use the activity coefficient of the Non-Random Two-Liquid (NRTL) equation:
Combinational term
We compute the combinational term, ln(giC), directly from the UNIQUAC equation by using the van der Waals area and volume parameter, which AVEVA Process Simulation calculates from the individual structural groups:
where
nc is the number of components
ng is the number of different groups in the mixture
is the lattice coordination number, which typically equals 10
nki is the number of functional groups of type k in molecule i
Rk is the volume parameter of functional group k
Qk is the area parameter of functional group k
xi is the mole fraction of component i in the liquid phase
Group volume and area parameters
We obtain the group volume and area parameters from the atomic and molecular structure.
where
Vwk is the van der Waals volume of group k
Awk is the van der Waals area of group k
Residual term
The following equation gives us the residual term, ln(giR):
where
Gk is the residual activity coefficient of group k in the mixture
Gki is the residual activity coefficient of group k in a reference solution that contains only molecules of group type i. This quantity is required so that giR → 1 as xi → 1.
Residual activity coefficient
The following equation gives us the residual activity coefficient. You use this equation to find both Gk and Gki.
The following equations give parameter qm and tmk:
where
amk is the binary interaction parameter for groups m and k
We assume that the binary energy interaction parameter amk is constant and not a function of temperature. We have incorporated a large number of interaction parameters between structural groups as well as parameters for group size and shape into the software.
Vapor-liquid equilibrium for Henry's solutes
We use the general vapor-liquid equilibrium calculations for all Henry's solvents. See Vapor-liquid equilibrium calculations for more information.
For molecular solutes (Henry's solutes), we use Henry's Law to model the equilibrium between the gaseous solute and the dissolved gas in the liquid phase:
where
Hi is the Henry's constant for component i in the mixed solvent
gi¥ is the infinite dilution (xi → 0) activity coefficient of molecular solute i in the mixed solvent
For Henry's solutes, we assume ideal behavior and set the activity coefficient (gi*) for all Henry's solute to one. This assumption simplifies the equilibrium equation between the gaseous solute and the dissolved gas to the following equation:
We use a simple additive mixing rule to calculate Hi:
where
A is the set of solvent components in the mixed solvent
XA is the mole fraction of solvent component A on a solute-free basis
HiA is the Henry's constant for component i in pure solvent A
You can include a pressure correction in the calculation of HiA. You use the Apply Henry's Law Pressure Correction using Brelvi O'Connell Model checkbox in the Equilibrium Options section of the Fluid Editor to turn on or turn off the pressure correction. When you select this checkbox, we use the following equation to calculate HiA:
where
PAsat is the saturation pressure of solvent A at the current temperature
viA¥ is the partial molar volume of molecular solute i at infinite dilution in pure solvent A
We use the Brelvi-O'Connell method[6] to calculate viA¥ as a function of characteristic volumes:
where
vCi is the characteristic volume from Brelvi-O'Connell[6] of component i
vCA is the characteristic volume from Brelvi-O'Connell[6] of solvent A
v0A is the liquid molar volume of pure solvent A calculated from the temperature-dependent property correlation for liquid density for pure solvent A
The temperature-dependent property correlations for liquid density are defined by the pure component (PURECOMP) data bank that the Fluid Type uses and by the local thermodynamic data overrides specified on the Temperature Dependent tab in the Component Data section of the Fluid Editor. Refer to Override temperature-dependent property data for more information.
You can also provide the characteristic volume data for components as temperature-dependent property data on the Temperature Dependent tab. If data is not available for a component, we fill the characteristic volume for the component (vCi) with the critical volume (Vci) data.
Changes to the Liquid Density method-override option in the Fluid Editor do not affect these calculations. See Effects of specifying thermodynamic method overrides for more information.