F. Cuadros

Determination of Lennard-Jones interaction parameters using a new procedure

For theoretical and chemical engineering applications, accurate and, if possible, simple models of molecular interactions are needed. We have recently proposed a new procedure for determining Lennard-Jones interaction parameters for fluids, forcing agreement between the values of the pressure obtained from empirical equations of state and those obtained from computer simulations. In this work we obtain new intermolecular Lennard-Jones parameters for non-polar molecules, taking into account their deviation from the spherical shape by means of an acentric factor. Our procedure could help to connect the microscopic and macroscopic worlds and it will be progressively implemented in order to obtain a better representation of other substances and mixtures of chemical interest.

Molecular thermodynamic models for the vapor-liquid equilibrium of nonpolar fluids

We propose simple expressions giving the main vapor-liquid properties for 42 nonpolar fluids. These expressions are molecular models based on a perturbative procedure, where the Lennard-Jones (LJ) system is taken as reference, the perturbed expressions being simple polynomial functions of the temperature for a given substance. The molecular parameters used, which are the only input needed in the molecular models, are the two LJ parameters, related to molecular size and the molecular interaction intensity, and the acentric factor, related to the molecular shape. The proposed molecular models are extrapolatable outside the temperature range used in the fit and seem to be easily applicable to other nonpolar fluids.

Energy-environmental benefits and economic feasibility of anaerobic codigestion of Iberian pig slaughterhouse and tomato industry wastes in Extremadura (Spain).

Anaerobic digestion of Iberian pig slaughterhouse and tomato industry wastes, as well as codigestion operations from such residues, are reported to achieve 54-80% reduction in Chemical Oxygen Demand and 6-19 N m(3)/m(3) substrate methane production. Furthermore, 0.79-0.88 m(3)water/m(3) substrate is seen to be recovered after the above mentioned operations, which might be used as irrigation water, and 0.12-0.21 m(3)agricultural amendment/m(3) substrate with 91-98% moisture content. The present paper also reports on the economic feasibility of both an anaerobic codigestion plant operating with 60% slaughterhouse wastes/40% tomato industry wastes (optimal ratio obtained in previous laboratory-scaled experiments), and an anaerobic digestion plant for Iberian pig slaughterhouse waste. Payback times are reported as 14.86 and 3.73 years, respectively.

Application of hard sphere equations of state to the Weeks–Chandler–Andersen reference system

Six analytical expressions (four of which have been published recently) for the equation of state of a hard sphere system, together with two different expressions for the molecular diameter, have been tested in order to reproduce molecular dynamics results for pressure and potential energy of the Weeks–Chandler–Andersen reference system for the Lennard-Jones potential. The best choices for the combination of equations of state and molecular diameters in the calculation of those properties are given. It is shown how that choice may differ for different ranges of temperatures and densities, and how there is not a direct relation between the simplicity or complexity of the analytical expressions and their accuracy.

A new procedure for determining Lennard-Jones interaction parameters

Abstract Because the connection factors between theoretical and experimental results of thermodynamic quantities are given through the molecular interaction parameters, for chemical engineering applications it is necessary to use exact values of these parameters for a determined molecular interaction model. Because the results of computer simulation may be considered ‘exact’ for a determined intermolecular potential, it makes an excellent tool for investigating this connection. The purpose of the present work is to propose a procedure for determining interaction parameters for fluids by forcing agreement between the values of pressure obtained from empirical Equations of state in phase space regions where we are sure they are most exact and those obtained from computer simulation.

An extensive study of the Helmholtz free energy of Lennard-Jones fluids using WCA theory

Abstract On the basis of results from molecular dynamics and using Weeks-Chandler-Andersen theory, a simple and exact analytical expression for the Helmholtz free energy of a system of particles interacting through the Lennard-Jones potential is obtained for a wide range of densities and temperatures. The values obtained from this expression and its derivative, the potential energy, are compared with existing published values, and are in reasonable agreement. The combination of accuracy with simplicity—only five parameters are used — could be very useful in theoretical and chemical engineering applications, where straightforward mathematical handling of thermodynamic properties is an important requirement. The high temperature approximation for the radial distribution function and for the Helmholtz free energy are compared with results from exact calculation. The results show this approximation to work well for the radial distribution function at high densities only, but to be good even at low densities for the Helmholtz free energy.

Mathematical modeling of the VLE curve of Lennard-Jones fluids. Application to calculating the vapour pressure

Abstract In this Letter, we present a straight forward analytical way to obtain the vapour–liquid equilibrium (VLE) curve for Lennard-Jones fluids, based on the symmetrical properties of the derivatives of the densities in the vapour and liquid phases, ρ V and ρ L .

Accurate vapour-liquid equilibrium calculations of simple fluids

Abstract From computer simulation data of 3D Lennard-Jones fluids, we present very simple analytical equations of pressure and chemical potential as functions of the temperature and density. By using the standard thermodynamic procedure of a liquid-vapour equilibrium; i.e. pressure and chemical potential in the gas phase to be equal to the corresponding values in the liquid phase at a given temperature, we have been able to determine the vapour-liquid coexistence curve. Despite the simplicity of our method, we obtain more precise results than those published in the literature because we use simultaneously both equalities: for pressures and for chemical potentials.

Modification of the soave equation of state suggested by using computer simulation

Abstract A suitable function of the form f ( P , υ, T ) = 0, known as the equation of state, can be used to evaluate many important properties of pure substances and mixtures. At present, no single equation of state exists that is equally suitable for all the properties for any large range of substances. Even for substances with similar physicochemical properties, the equations of state are only valid in some particular range of temperatures and densities. Because the powerful technique of computer simulation yields results that can be considered as “exact” for a given model, such simulation data can be used for a comparison with the results of a determined equation of state. In this work, molecular dynamics simulation data for a Lennard—Jones system are compared with the predictions given by the empirical Soave equation of state. The results lead to the proposal of a modification of the Soave equation of state at high densities.

Chemical potential for simple fluids from equations of state

The chemical potential for both the hard-sphere and the Lennard-Jones systems is calculated from analytical equations of state proposed in the literature and results are compared with data obtained by different authors from computer simulations. Eighteen equations of state for hard spheres, some of them recently published, were considered, and for the Lennard-Jones system three semi-empirical equations and two semi-theoretical ones, one of the latter being proposed by us in a previous work. Good agreement, except for particular expressions and/or particular temperature or density ranges, is found for most cases and for both hard-sphere and Lennard-Jones fluids. Our results indicate that the use of expressions which are more complex than the Carnahan—Starling equation for hard spheres or than our proposal for Lennard-Jones fluids is not needed to obtain the chemical potential accurately.