Sensitivity of Process Design to Phase Equilibrium—A New Perturbation Method Based Upon the Margules Equation

Abstract

It is widely recognized by experts that the computer-based design of chemical processes depends strongly on the correlated thermodynamic and transport properties, and the effect of property uncertainties should be incorporated into the design. The most significant source of property uncertainties on process design is from the correlations of phase equilibrium. Many approaches have been proposed, but uncertainty analysis is not a routine component of today's industrial practice, mainly because education and awareness is lacking, and the proposed methods are difficult to apply. The purpose of this paper is to report a new approach to uncertainty analysis that is intuitively appealing and easy to apply in process simulation. The proposed approach focuses 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 pseudobinaries 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 proposed uncertainty analysis is performed by varying a single parameter for each component in the mixture. The utility of the proposed uncertainty method is demonstrated by application to two problems: (1) a propylene−propane superfractionator for which small changes in correlated relative volatilities have a large effect on the design of the distillation column, and (2) a dehexanizer column that separates a mixture containing many close-boiling components. It is demonstrated that the proposed analysis provides quantitative 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 is applicable to other models such as equations of state.

INES – An Interface Between Experiments and Simulation to Support the Development of Robust Process Designs

“…The activity coefficients at infinite dilution are varied by ±10 %. This modification is realized with the Margules approach as a consistent model for the uncertainty in the Gibbs excess [11]. In the right part of Fig.…”

CFD analysis of laboratory scale phase equilibrium cell operation

“…The predicted distillation column parameters, i.e., reflux ration, flows, and composition of the phases on the stages were different when using the three different VLE data sets. This indicates that the design results are highly sensitive to changes in the equilibrium data (Xin and Whiting, 2000;Whiting et al, 1999;Dong et al, 2005;Mathias, 2014;and Wakeham et al, 2001). This stresses the importance of accurate equilibrium data.…”

Uncertainty analysis of the CPA and a quadrupolar CPA equation of state – With emphasis on CO2

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