What is UNIFAC and when is it used?

UNIFAC (Universal Functional Activity Coefficient) is a predictive method for activity coefficients using group contributions - molecules are divided into functional groups with known interaction parameters. No experimental data needed for the specific system, only the group parameters. Useful when: no experimental VLE data available, screening many systems, and preliminary process design. Limitations: less accurate than fitted models (NRTL, UNIQUAC), poor for electrolytes and polymers, and group parameter availability varies. Modified UNIFAC (Dortmund) has improved accuracy.