A Soft Approach to Modeling Thermodynamic Properties of Helium and Hydrogen

The effort to update The Properties of Gases and Liquids covers a broad range of compounds and conditions, including some peculiar molecules like helium and hydrogen at their saturation conditions. These compounds are often written off to "quantum" effects, but that does not eliminate the need to estimate their properties in many engineering applications that often require process simulation. These engineering applications require timely and accurate estimates that preclude quantum computations. This presentation shows that how to describe the peculiar properties of Helium and Hydrogen by adapting a classical potential function. The most important and challenging properties are the highly negative acentric factors. We show that a "softened" potential function can characterized these acentric factors. This softening is achieved by reducing the 12-6 of the Lennard-Jones potential to 9-6, and scaling the effective value of epsilon for the repulsive part of the potential to match the measured acentric factors. Although we should not take the physical basis of this potential model too seriously, it does have implications for properties like Henry's constant at room temperature that are far from the saturation conditions of these quantum fluids. What are these implications and are they reasonable? This requires considering peculiar extrapolations, like the solution properties of effective hard spheres in solutions of chain fluids at packing fractions around 0.6, while the highest stable packing fraction for pure hard spheres is 0.494. We show how to infer those properties by extrapolating the solution behavior at finite concentrations. We conclude with a set of guidelines to which EOS descriptions of these fluids must conform for meaningful engineering applications.