Beware of Bogus Roots with Cubic Equations of State


  • Ronald M. Pratt National University of Malaysia


One of the major strengths of the Peng-Robinson and similar equations of state is that it simultaneously calculates reliable saturated liquid and vapor volumes.  This makes these equations of state especially attractive tools in solving phase equilibria problems.  However, the Peng-Robinson equation of state, being cubic, will often give three real roots, and selecting the correct root(s) at the right time can be tricky.  This article addresses the problem in the context of a pure component, the case where undetected problems are most likely to arise.  Application is made to a pure component expanding through a Joule-Thomson apparatus exiting as a two-phase mixture.

Author Biography

Ronald M. Pratt, National University of Malaysia

Ronald M. Pratt is a lecturer in the engineering department at the National University of Malaysia. He obtained his BS in mathematics and in chemical engineering at the Colorado School of Mines, his MS in mathematics at the Fuxin Mining Institute in Liaoning Province, China, and his PhD in chemical engineering at the Colorado School of Mines. Research interests involve molecular dynamics and fractal modeling, and his teaching responsibilities have included undergraduate, graduate, and statistical thermodynamics courses and molecular simulation.






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