Otilio Hernández-Garduza

Modeling of microemulsion phase diagrams from excess Gibbs energy models

Abstract Projects on tertiary oil recovery by means of microemulsions have been mainly concerned with, first, the ability of a microemulsion to dissolve oil and water simultaneously and, second, the attainment of very low interfacial tensions. Therefore, the design and analysis of chemical flooding processes for enhanced oil recovery must be based on calculations of phase equilibria for these systems, which are composed of water (brine), oil, surfactant and co-surfactant (usually an alcohol). Consequently, the understanding of phase behavior of these systems is of fundamental importance to the development of any surfactant-based chemical flooding process. The purpose of this work was to give a thermodynamic analytical representation of the phase diagram of microemulsion systems similar to those used in enhanced oil recovery. The algorithms presented for the calculation of multiphase liquid equilibria and the methods for the estimation of the excess Gibbs energy model interaction parameters were successfully tested for the representation of experimental multiphase liquid equilibrium data of an oil−brine−surfactant−alcohol model system. In addition, to represent effectively the phase diagram of this system, an empirical expression was introduced into the selected excess Gibbs energy model to account for the specific role of the surfactant in these complex systems.

Vapor pressures of pure compounds using the Peng-Robinson equation of state with three different attractive terms

Abstract Accurate representation of pure compounds vapor pressures is required to increase the robustness of equations of state when predicting phase equilibria for mixtures. Using cubic equations of state, this representation largely depends on improving the temperature-dependent attractive term of the equation of state (EOS) to cover data from the triple to critical points. With this purpose, the Peng–Robinson equation of state is applied with three different attractive terms: Mathias, Mathias–Copeman, and Carrier–Rogalski–Peneloux. Experimental vapor pressures for 311 pure compounds (9000 experimental values) have been fitted. The studied compounds include nitrogen compounds, oxides, sulfides, chlorides, oxyhalides, inorganic compounds, alkanes, cycloalkanes, alkenes, alkadienes, alkynes, aromatic hydrocarbons, halogenated alkanes, halogenated cycloalkanes, halogenated alkenes, halogenated aromatic hydrocarbons, alcohols, ethers, aldehydes, ketones, alkanoic acids, esters, phenols, heterocyclic oxygen compounds, heterocyclic nitrogen compounds, hydrocarbon nitrogen compounds, and sulfur compounds. Overall average absolute deviations of 0.416, 0.214 and 0.276% have been found for the resulting Peng–Robinson–Mathias (PRM), Peng–Robinson–Mathias–Copeman (PRMC), and Peng–Robinson–Carrier–Rogalski–Peneloux (PRCRP) equations of state, respectively.

Equation of state associated with activity coefficient models to predict low and high pressure vapor–liquid equilibria

Abstract A simple and thermodynamically consistent method is presented to establish an equation of state for mixtures by using activity coefficient model parameters. All current solution models such as NRTL, van Laar, UNIFAC, or any other thermodynamic model can be used. The main feature of the method presented is that only a single scaling factor value determined at a given reference temperature is required to predict the vapor–liquid equilibria in a wide range of temperature and pressure. The performance of the method is tested on the prediction of the vapor–liquid equilibria at low, moderate, and high pressures for six binary systems (methanol−benzene, acetone−water, methanol−acetone, methanol−water, ethanol−water, and 2-propanol−water) and a ternary system (acetone−water−methanol). For comparison, vapor–liquid equilibrium calculations were carried out with the Wong and Sandler method by using the PRSV equation of state associated with the van Laar and scaling factors. On the whole, it is found that at high pressures both methods give similar predictions but at low pressures the proposed method gives sometimes better results than that of Wong and Sandler method.

Generalization of composition-dependent mixing rules for multicomponent systems: prediction of vapor–liquid and liquid–liquid equilibria

Abstract A consistent generalization to mixing rules depending on the composition proposed by Adachi and Sugie, Panagiotopoulos and Reid, and Stryjek and Vera for the two-parameter mixing rule and by Schwartzentruber and Renon for the three-parameter mixing rule is presented. The invariance problem and dilution effect shortcomings pointed out by Michelsen and Kistenmacher when the original mixing rules are applied to multicomponent mixtures, are avoided by the generalized mixing rules. The proposed mixing rules involving their respective excess function models associated with a cubic equation of state (PRSV or PRCRP), were used on the representation of binary vapor–liquid (hydrocarbon–hydrocarbon, acetone–alcohol, acetone–water, alcohol–water, and alcohol–hydrocarbon) and liquid–liquid (hydrocarbon–water) equilibrium data. The binary interaction parameters of the model were used to test the performance of the generalization on the prediction of ternary vapor–liquid (acetone–methanol–water, acetone–ethanol–water and hexane–ethanol–benzene) and liquid–liquid (water–methanol–benzene, water–ethanol–hexane and water–1-propanol–benzene) equilibria. In addition, it is shown that a more satisfactory prediction of the ternary vapor–liquid and liquid–liquid equilibria can be obtained by using a limit form of the generalized three-parameter excess function model.

An efficient flash procedure using cubic equations of state

Abstract A robust and highly efficient procedure to solve the isothermal two-phase flash problem at critical and sub-critical conditions is proposed in this work. From a calculated initialization, an accelerated successive substitution method is applied until either the solution is achieved or the change in the Gibbs free energy becomes positive, i.e. the Gibbs energy increases. From this point, an unconstrained optimization method is applied where the Hessian is kept positive definite and a line search method is implemented to guarantee a decrease in the Gibbs function. Using cubic equations of state to calculate the required thermodynamic properties has broadly tested the approach. Three hydrocarbon binary systems and a five-component mixture are used in this work to show the robustness of this procedure.

Étude des modèles thermodynamiques pour représenter des mélanges contenant des hydrocarbures, de l’eau et des alcools: I. Traitement général et représentation des équilibres liquide–vapeur et des enthalpies de mélange des systèmes binaires sous basse pression

Abstract The aim of this work was to indicate some thermodynamic models based on the formalism ‘Equation of state–Excess function’ able to represent thermodynamic properties of complex systems. First, we have developed the characteristics of the corrected Peng–Robinson equation of state for pure components as well as the conditions of its extension to mixtures by using excess function models such as those of van Laar, NRTL, Adachi–Sugie, Schwartzentruber–Renon and that of the ‘Surrounded Molecule’. Afterwards, we have studied the simultaneous representation of vapor–liquid equilibria and mixing enthalpies of binary systems containing non-polar, polar or associated compounds by using a temperature-dependence of the parameters for all models considered. The obtained results allow thus to establish what classes of compounds need the use of excess functions depending of one, two or three binary interaction parameters.