I'm looking for a reasonably accurate density of a mixture of two gases that act quite unlike ideal gases (generally steam and water-miscible solvents). We can get a first order estimate by taking a molar average but that can be an order of magnitude off, as in the case of water/ammonia.

Literature is the obvious place to start but I wasn't able to find any general sources and only a very few individual pairs in a narrow temperature/pressure range.

I would guess you should be able to derive an excess volume from fugacity but I haven't been able to find a formula that doesn't take even more difficult-to-measure inputs.

Standard modeling references like Refprop seem to use an ideal mixing model which isn't right.

I can also imagine measuring them myself or doing molecular dynamics modeling but these are both quite a bit more work than I was hoping for.

Are there any literature sources for this data that I couldn't find? Anyone have an idea of another way to calculate it?

  • $\begingroup$ Could you please give an example of a mixture and the expected results that would satisfy your needs? Furthermore, could you clarify why you cannot accept Refprop results even though Refprop is supposed to use a departure function to account for the departure from ideal mixing? $\endgroup$ – Faded Giant Aug 7 '15 at 13:02
  • $\begingroup$ @Loong According to Conde (which I think is a reputable source), the density of 75% molar concentration ammonia in water is 12.8 kg/m^3. If I do a simple molar average of the saturated vapor density of ammonia and water, I get 0.8. If I just use the ideal gas law, I get 0.6. Refprop reports 0.6. Based on that Reprop seems to be using a simpler method. $\endgroup$ – ericksonla Aug 10 '15 at 22:24

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