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One of the limitations of any group contribution method developed so far is the impossibility to distinguish among different isomers. This paper suggests a very simple procedure to extend the solution of group theory to mixtures containing isomers. The procedure suggested is based on theoretical considerations and is derived from phase equilibrium data for mixture of isomers in the dilute regions. A linear correlation is found between the surface area fraction (q) of the molecules and the ratio of the critical pressure over the critical temperature. Furthermore the investigation shows that the q values of the organic compounds which exhibit strong association phenomena should be substantially reduced: numerical values are reported depending upon the availability of experimental data.

This work proposes a method for the determination of data at infinite dilution and at very low concentration based on the isothermal measurement of the difference in pressure between two static cells containing the pure component and the mixture respectively. The method is particularly suitable for investigating binary systems whose components differ strongly in volatility, and when other methods such as gas liquid chromathography or ebulliometry fail.

The modelling of solid solubilities in supercritical fluids is usually performed by means of thermodynamic models based on cubic equations of state together with the use of correlations for estimating the solid properties. However, it was shown in the literature, that the sublimation pressure, very low for high molecular weight compound, is in many cases responsible of large deviations between experimental and calculated solubilities. In this work, the sublimation pressure of the solid compound is estimated by using experimental fusion data and liquid-vapour equilibrium properties obtained from the equation of state. The results provided by this method are compared with those obtained using sublimation pressures from literature. It is shown that this method leads to realistic values of binary interaction parameters k12 and allows also satisfactory predictions of the solubility of two solids in supercritical fluids.

The capability of existing models in extrapolating from infinite dilution properties to region far from the infinite dilution is investigated. Excess Gibbs energy models, group contribution models and equations of state are used for the prediction of the behavior of azeotropic and partially miscible systems. Only experimental infinite dilution activity coefficients are used to obtain parameters for the different models. The results obtained by all the models investigated are compared.

The paper deals with the possibility of predicting phase behavior of oil solutions in light hydrocarbon solvents. Such mixtures contain very many components: one or two well defined light alkanes (solvents) and one poorly defined continuous oil. Having significantly different molecular size, they can exhibit very non ideal phase behavior. We consider a heavy end (residue) which is similar to the feed for deasphalting processes. The fraction is characterized on the basis of Henry's constant data measured by gas liquid chromatography of light hydrocarbons in the heavy end. The heavy end is represented using pseudo-components. The Henry's constant gives all the information relative to the interactions between the light hydrocarbons (solvent) and the model components of the heavy oil. The procedure is tested on gas solubility data at low and high pressure. The perturbed hard chain equation (PHCT) coupled with flash algorithm is used for the calculations. As a result the mixtures considered are described by the thermodynamic model chosen through the pseudo-components used, thus allowing the optimization of deasphalting and solvent recovery processes.