(205c) From the Van Der Waals to a Universal Group Contribution Equation of State

For the development of the various processes in the chemical, pharmaceutical, food, gas processing and petrochemical industry reliable models for the prediction of pure component properties, phase equilibria and excess properties are required

Powerful predictive gE-models, such as ASOG, UNIFAC and modified UNIFAC for sub-critical conditions are already available for a long time. The development of gE-mixing rules for equations of state allowed the usage of the group contribution concept also for systems with supercritical compounds. Group contribution equations of state have the great advantage, that at the same time other thermophysical properties, such as densities, heat capacities, vapor pressures, heats of vaporization etc. are obtained, which are required as additional information in the g-j-approach. In the different process simulators the group contribution equation of state PSRK was implemented. It is a powerful tool for the prediction of vapor-liquid equilibria (VLE) over a large pressure and temperature range. Using an electrolyte model, PSRK can even be applied for systems with strong electrolytes. However, weaknesses of the model exist, e.g. poor results are obtained for liquid densities, asymmetric systems, excess enthalpies, which are mainly caused by the use of original UNIFAC method and the SRK equation of state.

To overcome these weaknesses, a new group contribution equation of state, called (VTPR), has been developed in our group. In this group contribution equation of state, the results for liquid densities have been significantly improved by using a volume-translated Peng-Robinson equation of state and improved mixing rules for the parameter a and b. By fitting temperature dependent group interaction parameters simultaneously to different thermodynamic properties using the Dortmund Data Bank (DDB) reliable parameters are obtained for this new group contribution equation of state, which also can be applied for systems with strong electrolytes and polymers. To extend the range of applicability a procedure was developed, which directly allows the application of the already available mod. UNIFAC (Dortmund) parameters.

In the lecture after an outline about the important steps during the development of equations of state, starting from the van der Waals equation of state published 1873, typical results of VTPR for pure components as well for the mixtures and possible applications of industrial interest will be presented, e.g. the effect of strong electrolytes on the various phase equilibria, the influence of the real behavior (Kj) on the chemical conversion and the enthalpy of reaction (DhR), etc..