[FilosofumRex]

I use Aspen-Hysys which is extremely expensive for non-commercial users, and would love to have a Pythonic alternative.

That said, there's a long tail of features needed to make it practical (mole/mass/vol frac conversion, unit conversions, chemical Rxn, Steam tables etc.)

[Saloc]

I would say it depends on the application: For energy stuff, ideal gas + water/steam might be sufficient. Rolling a lightweight process simulation framework (e.g. on top of gaspype and pint) easily competes with the costs for an aspen license. For other applications, that requires solving multiple phases of mixtures, the tradeoff shift quite allot.