UNIFAC — UNIQUAC Functional-group Activity Coefficient method

The UNIQUAC Functional-group Activity Coefficient (UNIFAC) method was developed in 1975 by Fredenslund, Jones, and Prausnitz[1]. This method estimates activity coefficients based on the group contribution concept following the Analytical Solution of Groups (ASOG) model proposed by Derr and Deal[2] in 1969. Interactions between two molecules are assumed to be a function of group-group interactions. Although there are thousands of chemical compounds of interest in chemical processing, the number of functional groups is much smaller. Group-group interaction data are obtained from reduction of experimental data for binary component pairs.

The UNIFAC-CI option adds more UNIFAC group-group interaction parameters to the existing collection. These parameters predict vapor-liquid equilibria (VLE). The UNIFAC-CI methodology is based on atom-atom connectivity indices and interactions[3], which we use in turn to predict missing UNIFAC group-group interaction parameters. This methodology uses only currently available experimental data. For systems that have existing experimental data, the accuracy of the VLE predicted with the original UNIFAC parameters is similar to that predicted with the UNIFAC-CI parameters. However, in this implementation, the UNIFAC-CI option predicts only the missing UNIFAC parameters. This allows the UNIFAC model to calculate the VLE for systems for which it could not calculate before.

The UNIFAC method requires the vapor pressure, van der Waals area, and van der Waals volume for each component to successfully complete its calculations.

If AVEVA Process Simulation cannot find component property data in any of the custom or default data banks for the Fluid Type, it uses the UNIFAC data to calculate property information for that component.