A Review of the Alpha Functions of Cubic Equations of State for Different Research Systems

Abstract

Alpha functions affect the predictive accuracy of cubic equations of state for the thermodynamic properties. The alpha functions usually aimed at predicting the specific compounds. This article reviewed various alpha functions for the prediction of five kinds of compounds—non-polar and weakly polar compounds, polar compounds, heavy hydrocarbons, reservoir fluids and natural gases, and water. Most alpha functions in polynomial and exponential forms can predict the thermodynamic properties of non-polar and weakly polar compounds. The alpha functions for the polar compounds usually have more coefficients or terms of variables in forms. Some alpha functions for heavy hydrocarbons are similar in forms to that for the simple fluid, some are generalized with the re-defined acentric factor, and the others are modified by introducing the triple temperature and normal boiling temperature into the functions. For the complex reservoir fluids and natural gases, the alpha functions are set as the characteristic constants of pseudo-components—pseudo-three phase temperature, pseudo-normal boiling temperature, pseudo-critical temperature, and pseudo-molecular weights—as the variables. The alpha functions for water are modified by adding more terms of reduced temperature into the functions to improve the predictive accuracy of vapor pressure. According to the properties of each kind of compounds, the universal alpha functions should be proposed for the simple fluid, and the specific alpha functions should be dedicatedly explored for the complex fluid and water.