The next generation of hydrate prediction - II. Dedicated aqueous phase fugacity model for hydrate prediction

The van der Waals and Platteeuw [Adv. Chem. Phys. 2 (1959) 1] hydrate equation of state (EoS), coupled with the classical thermodynamic equation for hydrates, has been used in the prediction of hydrate formation for over 40 years. In Part I of this series [Fluid Phase Equilib. 194 (2002) 371], we proposed an alternative derivation of these equations using a different standard state. The new hydrate equations were shown to be simpler to use. The new approach allows for a full description of each phase via fugacity models. This is the second article in a series of four, providing a description of an aqueous phase fugacity model tailored specifically for the presence of mixed hydrate inhibitors such as salts and methanol in the aqueous phase. There are several fugacity models that can adequately describe the aqueous phase for systems of hydrocarbons and water. However, when salts, alcohols, and glycols are added to the system, most fugacity models fail to account for proper interactions. In this work, we present an aqueous phase fugacity model using a modified Helgeson [Geochim. Cosmochim. Acta 52 (1988) 2009] equation of state combined with a Bromley [AIChE J. 19 (1973) 313] activity model. The Helgeson equation of state describes hydrocarbon-water systems, while the Bromley activity model accounts for interactions within the aqueous phase when salts and methanol are added. The advantage of this model is that the aqueous phase fugacity can be described more accurately than with the approach of using cubic equations of state intended for hydrocarbons, so that, for example, salting out can be predicted. (C) 2003 Elsevier B.V All rights reserved.