Reliability of the correlation allowing the kij to switch from an alpha function to another one in hydrogen-containing systems

Due to its very low critical temperature, the reduced temperature of hydrogen can easily overcome 10 in chemical processes. As a consequence, the many alpha functions available in the literature – the parameters of which are determined at low temperature – lead to totally different aH2(T)aH2(T) values in the temperature range of industrial interest. As a consequence, the kij value used in the classical VdW1f mixing rules to correlate the VLE data of hydrogen-containing systems strongly depends on the selected equation of state (EoS) and/or alpha function. For hydrogen-containing systems, the kij may indeed vary from a large negative value to a large positive value by only changing the alpha function. This is a major concern since commercial process simulators generally provide a unique kij value which is assumed to be appropriate regardless of the equation of state and/or the alpha function. It was thus a challenge to check whether the relationship developed by our research group in 2010 – and aimed at calculating the kij for a desired thermodynamic model knowing the corresponding value for another one – could predict such large changes. This study concluded that such a correlation performed well and could thus be useful to simulate chemical processes in which H2 is a key component.