[R0b0t1]

> one property i've relying on is that the second virial coefficient minimum density is always lower than the saturated gas density.. until a threshold near criticality.

A (or even the) phenomenologically useful reason anyone ever cares about supercriticality in industry is to use a chemical as a solvent. This seems like a precise statement of what makes it so. You know anything else that would be related?

[longemen3000]

A lot of new work on supercritical fluids is the ability of calculate properties for those. normally we don't access to those conditions, so it is tricky to measure, but there is always new work on correlations capable of explaining the anomalies present on near critical and supercritical fluids.

On the viscosity front, there is this new framework named Entropy Scaling, that postulates that scaled viscosity ≈ f(residual entropy)^(2/3).

There is also new work on calculation of interfacial tensions from pure equations of state,that are relevant because you could have a two phase liquid-liquid mix and you want to transport something from one liquid phase to another. Also, as the article says, over the critical point, there is no surface tension, because there is only one phase. (do not take this phrase literally, because that only happens in a pure compound, as soon as you have a mixture, the surface tension will depend on all elements of the mixture)

Furthermore, some fluid mixtures present something called UCST (upper critical solution temperatures) and LCST (lower critical solution temperatures). imagine if you lower the temperature of and oil-water mixture and suddenly they start to mix. and if you keep lowering the temperature, they unmix again!, and of course, not every fluid presents those properties, but we need to understand those who do. and specially, calculate those points. Few if any industrial thermodynamic calculators can do this, because you require fourth order accurate derivatives (can this be solved with automatic differentiation like what they do with neural networks?, of course, but you need a differentiation-ready equation of state)

On more macro properties, supercritical cycles are useful. you can start from a liquid and end with a gas without boiling, if you pressurize your liquid over the critical pressure (isothermically) then lower the temperature (isobarically) and finally releasing pressure (isothermically). one advantage of doing that is that you don't produce bubbles or any kinetic artifacts that are present during a normal phase transition.

[bernulli]

There’s plenty of applications beyond using them as solvents, e.g. fuel injection in jet engines, Diesel, or rockets; cooling in power plants or rockets; drilling; power cycles, etc.

[R0b0t1]

Can you explain? As I understand it supercriticality is just something you need to deal with, not that you necessarily want.

[bernulli]

Regardless of why you get to supercritical conditions, once you have them, that's plenty of reason to care about what supercriticality does in industry.

You want high pressures to achieve a high efficiency of a process (gas turbine, rocket, Diesel), then it's more of a side effect that the pressure is supercritical - I think that's what you allude to.

However, you may also want your fuel jet to mix more efficiently, so absence of phase equilibrium with surface tension may be advantageous.

You may also want a working fluid in a power cycle where expansion through a turbine does not end in subcritical spray that destroys your turbine blades.

You may also want to have a heat exchanger that does not have a 'boiling crisis', i.e. a subcritical vapor film that drastically reduces your heat flux when you exceed a certain limit temperature: think about it, the situation when your system gets unnaturally hot, and you really want to get rid of the heat, is when the heat transfer collapses. Now, there are subtleties about whether or where this can still happen at supercritical conditions, but let's just say for high enough pressures, a distinct phase transition no longer occurs (somewhere beyond 3 to 10 times the fluid critical pressure).