Properties of petroleum fractions of unknown composition, the so-called undefined petroleum fractions, are estimated using an approach based on group contribution methods. Using liquid density and molecular weight as experimental data and classical thermodynamics as framework, functional groups were assigned to each undefined fraction by minimizing its free energy. Thus, methods requiring molecular structures were used directly in phase equilibrium simulations of petroleum fluids. The proposed procedure also allows the critical properties of such undefined fractions to be calculated employing no specific correlations. The obtained results show reasonable accuracy concerning phase equilibrium experiments.

Application of an approach recently published [Carreoń-Calderoń et al. Ind. Eng. Chem. Res. 2012, 51, 14188− 14198] to heavy petroleum fluids allows calculation of critical properties and molecular pseudostructures of their undefined fractions. This approach is based on group contribution methods, where classical thermodynamics is used as framework to assign functional groups to each undefined fraction by minimizing its free energy. The calculated properties are employed to determine parameters entering into cubic equations of state and their respective mixing rules requiring molecular structures. Thus, phase equilibrium simulations of heavy petroleum fluids were performed employing neither specific correlations nor parameters, reducing the impact of fluid models over the thermodynamic characterization. The results are in agreement with experimental results of petroleum fluids heavier than 14°API.

-Hydrogen solubility in heavy undefined petroleum fractions is estimated by taking as starting point a method of characterization based on functional groups [Carreón-Calderón et al. (2012) Ind. Eng. Chem. Res. 51, 14188-14198]. Such method provides properties entering into equations of states and molecular pseudostructures formed by non-integer numbers of functional groups. Using Vapor-Liquid Equilibria (VLE) data from binary mixtures of known compounds, interaction parameters between hydrogen and the calculated functional groups were estimated. Besides, the incorporation of the hydrogen-carbon ratio of the undefined petroleum fractions into the method allows the corresponding hydrogen solubility to be properly estimated. This procedure was tested with seven undefined petroleum fractions from 27 to 6 API over wide ranges of pressure and temperature (323.15 to 623.15 K). The results seem to be in good agreement with experimental data (overall Relative Average Deviation, RAD < 15%).Résumé -Solubilité de l'hydrogène dans des fractions indéfinies du pétrole lourd à l'aide de la méthode de contribution de groupes -La solubilité de l'hydrogène dans des fractions indéfinies du pétrole lourd est estimée en prenant comme point de départ la méthode de contribution de groupes [Carreón-Calderón et al. (2012) Ind. Eng. Chem. Res. 51, 14188-14198]. Cette méthode fournit les paramètres d'entrée de l'équation d'état et une pseudo-structure moléculaire formée par des nombres non entiers pour les groupes fonctionnels. En utilisant les données de l'équilibre liquide-vapeur des mélanges binaires de composants connus, les paramètres d'interaction entre l'hydrogène et les groupes fonctionnels sont déterminés. En plus, l'incorporation du rapport hydrogène-carbone des fractions indéfinies du pétrole dans la méthode décrite permet d'estimer correctement la solubilité de l'hydrogène. Cette méthode a été testée avec sept fractions indéfinies de densité API de 6 à 27, et sur une large plage de pressions et de températures (323.15 à 623.15 K). Les résultats montrent un bon accord avec les données expérimentales (erreur moyenne relative globale < 15 %).

Classical nucleation theories of general application are taken as starting point to analyze the driving force for multicomponent gas hydrate nucleation from a single homogeneous phase. It is shown that the ratio between the specific surface energy and the critical radius of nucleation has a single value irrespective of the analyzed driving force expression. From this result, two driving force expressions for multicomponent gas hydrate nucleation are derived in the context of the so-called generalized nucleation theory, and it is demonstrated that the driving force for gas hydrates can be estimated using the same information given for the determination of the incipient formation points of the dispersed phase from a saturated phase.

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