(203a) Process Sensitivity to Phase Equilibrium – A New Perturbation Method Based Upon the Margules Equation

Process Sensitivity to Phase Equilibrium – A New Perturbation Method Based Upon the Margules Equation

Paul M. Mathias

Fluor Corporation

Aliso Viejo, CA 92698

Paul.Mathias@Fluor.com

It is widely recognized that the design of chemical processes depends on the estimated thermodynamic and transport properties, and the effect of property uncertainties should be incorporated into the design.  The most important source of uncertainties on process design is the estimates of phase equilibrium.  Many approaches have been proposed, but uncertainty analysis is not a routine component in today’s industrial practice – because the available methods are cumbersome to apply.  The purpose of this presentation is to report a new approach to uncertainty analysis that is intuitively appealing and easy to apply in process simulation.

The proposed approach focusses on activity coefficients, and is based upon the simplification that for the purpose of perturbation, the liquid mixture can be treated as a set of pseudo-binaries described by the Margules equation.  The resulting perturbation in the activity coefficient of component i goes to zero when its mole fraction approaches unity, and also is relatively small when its estimated activity coefficient is close to unity (i.e., near-ideal systems are likely to be modeled more accurately than highly nonideal systems).  In practice, the uncertainty analysis is performed by varying a single parameter for each component.

The utility of the proposed uncertainty method is demonstrated by application to several problems: (1) the textbook problem of separating the acetone-chloroform-benzene ternary mixture; (2) superfractionation for which small changes in predicted relative volatility have a large effect on the distillation design; and (3) a dehexanizer column that separates a mixture containing many close-boiling components.  It is demonstrated that the proposed analysis provides insight into the effect of property uncertainties and helps to quantify the safety factors that need to be imposed upon the design.

While the proposed method is applied to activity-coefficient models, the same idea can easily be used for other models such as equations of state.