Coupling activity coefficient models, Henry constant equations, and equations of state to calculate vapor-liquid and solid-liquid equilibrium data

The sour gas sequestration problems require the knowledge of the gas solubility in brines. Phase equilibrium involving very diluted species as dissolved gas molecules in water or in electrolytic solutions are commonly modeled by a heterogeneous approach needing an activity model coupled to a Henry's constant for the aqueous phase and an equation of state (EOS) for the vapor phase. Because models contain assumptions and parameters fitted in specific conditions, it is necessary to ensure that the coupling of these different elements is consistent. In this work, after having separately tested three activity models (the Pitzer model, a Bromley model parameterized by Jager and the extended Uniquac model) on a freezing-point water depression calculation in the presence of thermodynamic inhibitors, different couplings with Henry's constant (Dhima, Duan, and Shock and Helgeson equations) and with two EOS (Soave-Redlich-Kong and Peng-Robinson) are carried out. The comparison of these different associations highlights the role of interaction parameters, Henry's constant and EOS on the CH4 and CO2 solubility calculation in electrolytic solutions. It is shown that if it is naturally preferable to use the same conditions as those used for the fitting of the activity model parameters to get the best reliability in predictions, a good accuracy can also be obtained by coupling rigorously different approaches. (c) 2006 Elsevier B.V. All rights reserved.