Simultaneous multiphase flash and stability analysis calculations including hydrates

Abstract

We proposed an algorithm for phase equilibrium calculations involving hydrates. Our algorithm applies stability analysis simultaneously to flash calculations in order to handle scenarios differing in pressure, temperature and composition, through ranges presenting different number of phases that may also be in distinct physical states. Our work is based on the algorithms of Gupta et al. [Fluid Phase Equilibria, 65–89 (1991) 63] and Ballard and Sloan [Fluid Phase Equilibria 15–31 (2004b) 218], with modifications that contribute to both speed and robustness of these calculations. Our algorithm depends on fugacity coefficients that have to be calculated by suitable thermodynamic models using input variables from a given iteration. We treat pressure, temperature and guest components fugacities as input variables and present expressions for the calculations of fugacity coefficients for all components included in hydrate phases from these variables. We obtain these expressions through a combination of the hydrate phase thermodynamic model described by van der Waals and Platteuw with suitable thermodynamic modeling for the reference condition of the empty lattice of water. We applied the algorithm in P×T phase equilibrium diagram calculations involving liquid and vapor fluid phases, and ice and hydrates with sI and sII crystalline structures solid phases.