IGS — Improved Grayson-Streed method

The Improved Grayson-Streed (IGS) method is an extension of the Grayson-Streed method to support a rigorous three-phase calculation for hydrocarbon systems with an aqueous phase. See GS — Grayson-Streed method for more information.

For hydrocarbon-water mixtures, the Grayson-Streed and Erbar-modified Grayson-Streed1 methods accurately predict the phase behavior of the hydrocarbon-rich phase, but do not do as well in predicting the water-rich phase. To improve the predictions for the water-rich phase, IGS uses a separate set of solubility parameters in the water-rich phase and improved liquid fugacity curves for N2, H2S, H2O, CO, CO2, and O2 compared to the original work.

In addition, it was found that the Grayson-Streed (GS) liquid fugacity coefficient for the simple fluid decreases rapidly as Tr increases above 2.5, and can in fact become negative. Therefore, IGS replaces the liquid fugacity coefficient for the simple fluid with that for hydrogen at reduced temperatures of 2.5 and greater.

Compared to the IGS implementation in SIM4ME Thermo, we have modified the IGS method to match the GS logic. This improves performance and robustness while maintaining the IGS method's advantages when predicting three-phase conditions.