An improved Helmholtz energy model for non-polar fluids and their mixtures. Part 1: Application to non-polar pure fluids

• A Helmholtz energy model is presented for pure non-polar fluids.
• The Helmholtz energy is obtained from an extended corresponding model plus a temperature- and density-dependent correction.
• For 18 non-polar fluids the percentage AAD in pρT data was 0.175.
• Percentage AADs were 0.279 for vapour pressures. 1.563 for isobaric heat capacities and 0.638 for speeds of sound.
• Results compared favourably with the demanded accuracy for technical Helmholtz energy equations of state.

This is the first part of a series of two communications in which a Helmholtz energy model is developed and applied to the prediction of thermodynamic properties of non-polar fluids and their mixtures. In this first part, the application is concerned only with pure non-polar fluids.The Helmholtz energy model is based on previous work by the author and co-workers in which the Helmholtz energy of the fluid is represented as a contribution of two terms: one is an extended corresponding states model and the other is a correction term. In the case of single components, this correcting term is a function of temperature and density.In this study 18 fluids were considered, namely: the normal alkanes from ethane to octane, isobutane, ethylene, cyclohexane, benzene, toluene, nitrogen, carbon dioxide, carbon monoxide, oxygen and argon. Percentage absolute average deviations (AADs) were calculated with the following results: 0.175 for pρT data; 0.279 for saturation pressures; 0.168 and 0.324 for saturated-liquid and saturated-vapour densities, respectively; 1.364 and 1.563 for isochoric and isobaric heat capacities, respectively, and 0.638 for speeds of sound. This performance is, by and large, quite comparable with the demanded accuracy of modern technical Helmholtz energy models for fluids of industrial interest and practical applications in process design and simulation.