Nicolas Riesco

Viscosity of liquid mixtures: The Vesovic-Wakeham method for chain molecules

New expressions for the viscosity of liquid mixtures, consisting of chain-like molecules, are derived by means of Enskog-type analysis. The molecules of the fluid are modelled as chains of equally sized, tangentially joined, and rigid spheres. It is assumed that the collision dynamics in such a fluid can be approximated by instantaneous collisions. We determine the molecular size parameters from the viscosity of each pure species and show how the different effective parameters can be evaluated by extending the Vesovic-Wakeham (VW) method. We propose and implement a number of thermodynamically consistent mixing rules, taking advantage of SAFT-type analysis, in order to develop the VW method for chain molecules. The predictions of the VW-chain model have been compared in the first instance with experimental viscosity data for octane-dodecane and methane-decane mixtures, thus, illustrating that the resulting VW-chain model is capable of accurately representing the viscosity of real liquid mixtures.

Thermodynamics of mixtures containing ethers. Part I. DISQUAC characterization of systems of MTBE, TAME or ETBE with n-alkanes, cyclohexane, benzene, alkan-1-ols or alkan-2-ols. Comparison with Dortmund UNIFAC results

Binary mixtures of methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME) or ethyl tert-butyl ether (ETBE) and n-alkanes, cyclohexane, benzene, alkan-1-ols or alkan-2-ols are characterized in terms of DISQUAC. n The corresponding interaction parameters are reported. Systems with isomeric monooxaalkanes (linear or branched) and n-alkanes are characterized by the same QUASICHEMICAL (QUAC) interaction parameters. n The DISPERSIVE (DIS) parameters are larger for those mixtures with tertiary-alkyl ethers. It is remarkable that in systems with alkan-1-ols, the first and third QUAC parameters are kept constant. The same trend is observed n in many other alcoholic solutions previously studied. Mixtures with alkan-1-ols or alkan-2-ols differ only n in the DIS parameters, which are larger for those systems with alkan-2-ols. Vapour–liquid equilibria (VLE), molar excess enthalpies (HE), logarithms of activity coefficients at infinite dilution (ln γi∞) or solid–liquid equilibria (SLE) are correctly described by DISQUAC. HE values of ternary systems, including compounds considered in this work, are also represented by DISQUAC using binary parameters only. A complete comparison between DISQUAC calculations and those obtained from the Dortmund version of UNIFAC using the parameters available in the literature is included. DISQUAC improves results, except for VLE of mixtures including n-alkanes, which are slightly better represented by UNIFAC. This may be attributed to the fact that the empirical combinatorial term used in UNIFAC is more suitable, particularly for those systems with components very different in size. DISQUAC also improves results obtained from the ERAS model for HE of alcoholic solutions (where association is expected). Thermodynamic properties are analysed in terms of the effective dipole moments (). The importance n of structural effects is remarked.

Calculation of the transport properties of a dilute gas consisting of Lennard-Jones chains

The transport properties in the dilute gas limit have been calculated by the classical-trajectory method for a gas consisting of chain-like molecules. The molecules were modelled as rigid chains consisting of spherical segments that interact through a combination of site-site Lennard-Jones 12-6 potentials. Results are reported for shear viscosity, self-diffusion, and thermal conductivity for chains consisting of 1, 2, 3, 4, 5, 6, 7, 8, 10, 13, and 16 segments in the reduced temperature range of 0.3-50. The results indicate that the transport properties increase with temperature and decrease with chain length. At high temperatures the dependence of the transport properties is governed effectively by the repulsive part of the potential. No simple scaling with chain length has been observed. The higher order correction factors are larger than observed for real molecules so far, reaching asymptotic values of 1.019-1.033 and 1.060-1.072 for viscosity and thermal conductivity, respectively. The dominant contribution comes from the angular momentum coupling. The agreement with molecular dynamics calculations for viscosity is within the estimated accuracy of the two methods for shorter chains. However, for longer chains differences of up to 7% are observed.

Reference Correlation of the Viscosity of Cyclohexane from the Triple Point to 700 K and up to 110 MPa

A new correlation for the viscosity of cyclohexane is presented. The correlation is based upon a body of experimental data that has been critically assessed for internal consistency and for agreement with theory. It is applicable in the temperature range from the triple point to 700 K at pressures up to 110 MPa. In the dilute gas region, at pressures below 0.3 MPa, the correlation is valid up to 873 K. The overall uncertainty of the proposed correlation, estimated as the combined expanded uncertainty with a coverage factor of 2, varies from 0.5% for the viscosity of the dilute gas and of liquid at ambient pressure to 5% for the viscosity at high pressures and temperatures. Tables of the viscosity generated by the relevant equations, at selected temperatures and pressures and along the saturation line, are provided.

Reference Correlation of the Viscosity of para-Xylene from the Triple Point to 673 K and up to 110 MPa

A new correlation for the viscosity of para-xylene (p-xylene) is presented. The correlation is based upon a body of experimental data that has been critically assessed for internal consistency and for agreement with theory. It is applicable in the temperature range from the triple point to 673 K at pressures up to 110 MPa. The overall uncertainty of the proposed correlation, estimated as the combined expanded uncertainty with a coverage factor of 2, varies from 0.5% for the viscosity of the dilute gas to 5% for the highest temperatures and pressures of interest. Tables of the viscosity generated by the relevant equations, at selected temperatures and pressures and along the saturation line, are provided.

Prediction of Heavy Oil Viscosity in Current Reservoir Simulators

An accurate estimate of oil mobility is essential for reliable exploitation of oil and gas reservoirs, and for development of new technologies, particularly with the industry’s increasing reliance on unconventional resources such as heavy oil. The viscosity of heavy oil decreases rapidly with increasing temperature and with increasing concentrations of light components, and can vary over 2-3 orders of magnitude during typical exploration and productions operations. The accurate prediction of the viscosity is therefore difficult, and simple extrapolation of the current methods used in the petroleum industry for predicting the fluid viscosity in conventional reservoirs is fraught with difficulties. This paper presents a comprehensive analysis of the use of a simple mole-average power law that is based on the Arrhenius equation and used as the default method in some widely used thermal reservoir simulators to predict mixture viscosity. Predictions based on the equation are compared to a set of accurate benchmark data that are based on the best available experimental data for hydrocarbon mixtures. These data cover a temperature range of -175°C to 200°C, and extend to high pressures. The accuracy of the data is better than 5%, sufficient for validation purposes. We summarize the conditions under which the simple power law provides reasonable estimates of viscosity, identify the ranges of pressure and temperature and composition for which large deviations can be expected, and discuss the implications of using the equation to predict heavy-oil viscosity. We demonstrate that the current practice of fitting scarce experimental data and using the viscosity of the heavy fraction as an adjustable parameter is problematic, and that extrapolation in temperature or to similar mixtures is highly unreliable.

CHAPTER 8:Dense Fluids: Viscosity

This chapter reviews recent advances in modelling and predicting the viscosity of dense fluids. As no complete theory exists for dense fluids, a number of different approximate approaches represent the current state of the art. We summarize the recent developments of two approaches (known as the Assael–Dymond and Vesovic–Wakeham approaches) that are based on kinetic theory, as well as the Friction Theory approach based on macroscopic considerations, and examine their ability to predict the viscosity of dense fluids and fluid mixtures.