Present Status and Potential of Group Contribution Methods for Process Development

The synthesis, design and optimization of unit operations or whole plants in the chemical, pharmaceutical, food, gas processing and petrochemical industry can be performed with the help of process simulators. However, since often 60-80% of the total costs arise in the separation step, a reliable knowledge of the thermophysical properties, in particular the phase equilibrium behavior is required for the simulation.

The models for excess Gibbs energy g^E and equations of state allow the calculation of the phase equilibrium behavior of multicomponent systems using binary information alone. But often the required experimental data of the binary systems are missing. Since the measurement of the missing phase equilibria is expensive and very time-consuming, a reliable predictive thermodynamic model with a large range of applicabilities would be most desirable.

A prerequisite for the development of a powerful predictive method is a large database. Therefore, published pure component properties, phase equilibrium data, and excess properties are continuously stored in a computerized form using the Dortmund Data Bank (DDB) already since 1973. At the same time, experimental techniques were built up to measure systematically the missing data.

The information stored in the DDB was used to develop group contribution methods such as UNIFAC. The weaknesses of the UNIFAC method were removed with the development of Modified UNIFAC. In combination with electrolyte models, the influence of strong electrolytes on the phase equilibrium behavior was taken into account.

But g^E models and group contribution methods in contrast to equations of state show the disadvantage that they can not be applied to supercritical compounds. The introduction of g^E-mixing rules in equations of state led to group contribution equations of state such as PSRK or VTPR.

In this lecture the continuous development, present status and potential of contribution methods for the development of chemical processes and typical results for industrial applications will be shown.