Web Release Date: May 11,
Reliable Computation of High-Pressure Solid-Fluid Equilibrium
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Department of Chemical Engineering, 182 Fitzpatrick Hall, University of Notre Dame, Notre Dame, Indiana 46556, and Escola de Química, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ 21949-900, Brazil
Received for review September 1, 1999
Revised manuscript received November 1, 1999
Accepted November 6, 1999
Abstract:
The calculation of solid-fluid equilibrium at high pressure is important in the modeling and design of processes that use supercritical fluids to selectively extract solid solutes. We describe here a new method for reliably computing solid-fluid equilibrium at constant temperature and pressure or for verifying the nonexistence of a solid-fluid equilibrium state at the given conditions. Difficulties that must be considered include the possibility of multiple roots to the equifugacity conditions and multiple stationary points in the tangent plane distance analysis done for purposes of determining global phase stability. Somewhat surprisingly, these issues are often not dealt with by those who measure, model, and compute high-pressure solid-fluid equilibria, leading in some cases to incorrect or misinterpreted results. It is shown here how these difficulties can be addressed by using a methodology based on interval analysis, which can provide a mathematical and computational guarantee that the solid-fluid equilibrium problem is correctly solved. The technique is illustrated with several example problems in which the Peng-Robinson equation of state model is used. However, the methodology is of general purpose and can be applied in connection with any model of the fluid phase.
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