I. Cachadiña

CALCULATION OF THE VAPORIZATION ENTHALPY OF NONPOLAR FLUIDS AT THE STANDARD TEMPERATURE

ABSTRACT Different analytical models were used to calculate the enthalpy of vaporization of nonpolar fluids at the standard temperature of 298.15 K. The models considered were some general classical expressions, three group contribution models that allow the property to be calculated at any temperature, another group contribution model specifically designed for calculations at the standard temperature, and finally a molecular model proposed by our research group. The results for 42 nonpolar fluids are compared with the value obtained in the correlation proposed in the DIPPR project.

Comparison of Predictive Correlations for the Normal Boiling Density of Nonpolar Fluids

Seven empirical correlations for the calculation of the normal boiling density of non polar pure fluids are studied for their accuracy and applicability. One of the correlations is specific only for the normal boiling density, five are based on the corresponding states method (these need the critical parameters and acentric factor as inputs), and the other is a new kind of correlation that uses the Lennard-Jones molecular parameters and the acentric factor as input data. As reference, we take the value for the normal boiling density given by the specific correlation functions proposed in the DIPPR project for 252 nonpolar fluids, grouped into 17 families. In view of the results, we recommend the use of the Yamada-Gunn expression because of its simplicity and because other, less simple, models represent no clear improvement over its accuracy. We also recommend the use of the molecular-parameter model because it gives very good results for some families of fluids.

Prediction of the enthalpy of vapourisation for anhydrides, formates, acetates, propionates, butyrates, esters, and ethers

Four analytical correlations based on the use of the corresponding states principle were used to calculate the enthalpy of vapourisation of fluids. Three of these correlations require as inputs the critical temperature and the acentric factor. The fourth requires a molecular Lennard–Jones parameter and the acentric factor. Results for 184 polar and non-polar fluids grouped into 9 families are compared with the values accepted by the Design Institute for Physical Property (DIPPR) project. Recommendations are given for the use of each model and for the choice of the adequate model for each family of fluids.

Changing the teaching/learning procedures in physics for agricultural engineering. A case study

The subject ‘Physical Fundamentals of Engineering’ for agricultural engineers in the University of Extremadura has long had high rates of students not attending classes, not presenting for examinations and, finally, failing the subject. During the 2007 and 2008 courses, the teaching/learning procedures were strongly modified. Analysis of the proportion of students taking final examinations and of the final marks shows that the new procedure leads to a satisfactory progress in most of the objectives, including a general improvement in their marks and pass rates. In particular, it was found that the new procedure needs to be maintained by the teachers for at least two courses for the greatest success.

Application of Hard-Sphere and Hard-Disk Equations of State to the Weeks-Chandler-Andersen Reference System

Perturbation theories in statistical mechanics are based on the separation of the contributions of repulsive and attractive intermolecular forces. The former can be modeled by an appropriate modification of the hard-sphere or hard-disk properties through a scaling procedure, for which a temperaturedependent molecular diameter is needed. In this chapter, we analyze such a scaling procedure when applied to the well-known Weeks–Chandler–Andersen perturbation theory. Calculations are performed not only for the whole range of temperatures and densities, but attention is also focused on certain zones of the phase diagram of special interest. The best choices for the combination of equations of state and molecular diameters are given, following accuracy and/or simplicity criteria.

Evaluación de Modelos de Predicción de Densidades Líquidas de Saturación de Aldehídos, Cetonas y Alcoholes

Se ha disenado un programa de ordenador que permite estudiar la validez de expresiones empiricas para predecir la densidad de liquidos saturados puros. El programa incluye una base de datos con valores aceptados de literatura a fin de comparar los calculados con los modelos. Para cada dato, el programa indica la desviacion absoluta y porcentual. Para cada fluido y para cada familia de fluidos se lista el intervalo de temperaturas y se obtienen desviaciones absolutas y porcentuales, indicando a que temperatura se produce la maxima desviacion. Ademas se generan graficas que permiten visualizar los datos junto a las predicciones de los modelos. Se ha estudiado la validez y exactitud de ocho expresiones analiticas, basadas en el principio de estados correspondientes, para predecir la densidad liquida de saturacion de aldehidos, cetonas y cuatro familias de alcoholes.