Development of a Simplified Statistical Associating Fluid Theory for Fast Phase Equilibrium Calculations

Phase equilibrium requires that for each component the fugacity is the same across phases and that the system be in thermal and mechanical equilibrium. The fugacity of components can be calculated based either on deviation from ideal gas behavior using fugacity coefficients or on deviation from ideal solution behavior using activity coefficients. Deviation from ideal solution behavior is due to differences in intermolecular interactions. Although semi-empirical activity coefficient models (such as NRTL or UNIQUAC) are commonly used to calculate deviation from ideal solution behavior, in this work we propose to develop a simple but efficient way to calculate activity coefficients using an equation of state. One advantage of the proposed approach is that calculations can be up to two orders of magnitude faster than typical equation of state calculations since the density will be treated as an input and therefore, we do not iterate to determine the density at a given pressure. Consequently, rigorous statistical mechanics-based equations of state (such as SAFT or CPA) which are known to be computationally more expensive can be solved easier in process simulators and PVT modeling software to predict how molecular size, hydrogen bonding and multipolar interactions affect liquid phase non-ideality. The approach will enable us to produce two activity coefficient models, a general SAFT model (SAFT-AC) that is computationally fast and a model that simplifies SAFT to an extended Flory-Huggins model (SAFT-SAC) that includes associating interactions as well as compressibility effects.