(193ag) Validation of Surrogate Mixtures by Thermodynamic Evaluation of Phase Equilibria

Complex multicomponent systems like crude oil or fuels tend to raise problems in estimation of physical properties, calculation of thermodynamic equilibria or combustion simulations incorporating chemical kinetic models. One approach for handling such systems is the introduction of substitute mixtures comprising a significantly reduced number of components compared to the original multicomponent system, known as surrogates. With surrogates, the original mixture is replaced by a few key components, aiming at maximum compliance of the surrogate mixture properties compared to the original system. Compared to the established pseudocomponent approach for characterisation of multicomponent mixtures, the use of real components allows application of rigorous thermodynamic methods and models.

Selection of representative key components for surrogates

The selection of representative key components for surrogate mixtures in many cases turns out to be empirical. In order to improve selection, a new approach has been developed and validated, based upon a statistic optimisation algorithm. With the resulting surrogate component set, physical properties as well as phase equilibria can be calculated using the full band width of thermodynamic methods and models like the activity coefficient approach.

Evaluation of surrogate mixtures by SLE and VLE calculations

With the isofugacity criterion, solid-liquid-equilibrium (SLE) and vapor-liquid-equilibrium (VLE) calculations can be applied for validation of surrogate mixture properties. Demonstrated with an example from fuels specification, crucial properties, relevant in production, blending and handling, can be estimated. SLE calculations can be applied for estimation of the cloud point from which the cold flow properties like pour point or cold filter plugging point can be derived by correlations; VLE calculations can be used for estimation of boiling characteristics which are the basis for simulation of rectification processes.