Reliable Flash Calculations: Part 1. Nonsmooth Inside-Out Algorithms

• Published in: Industrial & engineering chemistry research Vol. 56; no. 4; pp. 960 - 973
• Main Authors: Watson, Harry A. J, Vikse, Matias, Gundersen, Truls, Barton, Paul I
• Format: Journal Article
• Language: English
• Published: American Chemical Society 01.02.2017
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/acs.iecr.6b03956

Dependable algorithms for nonideal vapor–liquid equilibrium calculations are essential for effective process design, simulation, and optimization. Inside-out algorithms [ Boston, J. ; Britt, H. Comput. Chem. Eng. 1978, 2, 109 ] for flash calculations serve as the basis for many of the algorithms used by process simulation software due to their robustness with respect to initialization and inexpensive computational cost. However, if the specified flash conditions imply a single-phase result, the conventional inside-out algorithms fail, as the solution is constrained to obey equilibrium relationships, which are only valid in the two-phase region. These incorrect results can be postprocessed to determine the true single-phase solution; however, such approaches either carry a high computational cost or are heuristic in nature and vulnerable to failure (or both). Such attributes are undesirable in a process simulation/optimization problem where many flash calculations must be performed for streams where the phase regime at the solution is not known a priori. To address this issue, this article presents modifications of the classical inside-out algorithms using a nonsmooth equation system in the inner loop to relax equilibrium conditions when necessary, allowing reliable convergence to single-phase results. Numerical results for simulations involving several common flash types and property packages are shown, highlighting the capability of the new nonsmooth algorithms for handling both two-phase and single-phase behavior robustly and efficiently.