(574b) New Vapor Pressure Prediction with Improved Thermodynamic Consistency Using the Riedel Equation

Joseph W. Hogge¹, Neil F. Giles, Richard L. Rowley, Thomas A. Knotts, W. Vincent Wilding

¹Department of Chemical Engineering, Brigham Young University, USA

*Joseph Hogge: joehogge@byu.edu

Scientific topic: Thermodynamics and Transport Properties

Thermophysical Properties and Phase Behavior

 

Keywords: Multi-Property Optimization, Vapor Pressure, Heat Capacity, Heat of Vaporization

Abstract

Vapor pressure, enthalpy of vaporization, liquid heat capacity, and ideal gas heat capacity for pure compounds between the triple point and critical point are important properties for process design and optimization. Unfortunately, the literature is filled with poor and inconsistent data or is void of experimental data for leading-edge compounds common in today’s chemical industries. As such, there is high demand to develop methods to assess the accuracy and consistency of experimental data and correctly predict properties when such data are not available.

This work focuses on improving the thermodynamic consistency and accuracy of vapor pressure, enthalpy of vaporization, liquid heat capacity, and ideal gas heat capacity since these can drastically affect the safety and profitability of chemical processes. This is done by improving the temperature correlation of the vapor pressure because it is related to the other properties through rigorous thermodynamic relationships involving temperature derivatives. For many years, the Riedel equation has been considered an excellent and simple choice among vapor-pressure correlating equations [1], but this work shows that it requires modification of the final coefficient to provide thermodynamic consistency with thermal data [2]. Specifically, Riedel’s equation set the final coefficient to a constant value of 6, but this severely restricts the ability of the model to produce accurate liquid heat capacity data below the normal boiling point.

This presentation will show 1) newly created predictive correlations where this final coefficient is changed from 1 to 6 in integer steps, and 2) how these result in improved prediction of vapor pressures for compounds with limited or no experimental data. The methodology followed is that originally proposed by Riedel [3] but the last coefficient was chosen based on its ability to simultaneously represent vapor pressure and liquid heat capacity. This procedure improves the fit to liquid heat capacity data by 5-10% (average absolute deviation), while maintaining the fit of vapor pressure data similar to other prediction methods. This procedure also greatly improves the ability to predict low-temperature vapor pressure by using liquid heat capacity data. This feature of the work suggests that previous understanding of the temperature dependence of low-temperature vapor pressure was incorrect and that new paradigms need to be developed.

References

1. Velasco, S., F.L. Roman, J.A. White, and A. Mulero, A predictive vapor-pressure equation. Journal of Chemical Thermodynamics, 2008. 40(5): p. 789-797.

2. Hogge, J.W., N.F. Giles, T.A. Knotts, R.L. Rowley, and W.V. Wilding, The Riedel vapor pressure correlation and multi-property optimization. Fluid Phase Equilibria, 2016. 429: p. 149-165.

3. Riedel, L., Eine neue universelle Dampfdruckformel Untersuchungen über eine Erweiterung des Theorems der übereinstimmenden Zustände. Teil I. Chemie Ingenieur Technik, 1954. 26(2): p. 83-89.