PRO/II-Keyword Manual

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Thermodynamics calculations typically constitute a significant portion of the computational effort of a flowsheet simulation. In particular, much time is spent on determining rigorous K-values during flash calculations. To reduce this overhead, a simple local thermodynamic method can be used in a perturbation during the flowsheet optimization.
Therefore, during a flowsheet optimization, the base case is converged using rigorous thermodynamics calculations. The converged state properties (temperature, pressure and composition) of the various units are used to generate the local thermodynamic model, which is then used for all perturbation calculations for the current optimization cycle. The premise is that each perturbation calculation represents only a small departure of the state variables from their base case value and hence the local models should provide an adequate representation of the thermodynamic behavior.

• TP models are found to be adequate for streams whose properties are not strong functions of composition. • TPX models are found to be adequate for streams whose properties are strong functions of liquid composition. • TPXY models are found to be adequate for streams whose properties are strong functions of liquid and vapor composition.