(58c) Reliable Extrapolation of Equation of State Thermodynamic Properties

Abstract:

Appropriate densities for vapor and liquid phases must be obtained during process simulation when using equations of state. For conditions of high pressure and high temperature, where such density roots may become identical, trivial K-values may occur which could cause the whole process simulation to fail. To avoid this type of failure, a strategy of obtaining a suitable pseudo density for an equation of state is developed. This strategy is generally applicable to any equation of state and has mathematical advantages over existing related works done by others.

Rigorous tests have also been done throughout the P-T diagram space (as indicated by the dots and dashed lines on the below plot). Reliable and appropriate phase equilibrium solutions had been obtained with less than five Newton iterations on average. For cases close to critical point, Newton iteration may go up to ten, with an artificial equilibrium solution obtained within the first five or less iterations (a pseudo density that is close to real still active on one or more of the phases). Usually a few more iterations are required to converge to the true solution (only true densities on existing phases).