[RickJames]

I have been messing around with distillation lately and I thought of something. I just want to know if there is any way that it would work. Basically, if I have a 20 psi valve attached to a boiler with different liquids in it, and only one of those liquids has enough pressure at that temperature to open the 20psi valve, would the valve only stay open until that liquid was gone, or would all the liquids come out until it's under 20 psi again? I hope that makes sense. I was thinking that maybe you could distill things based on pressure. Any help would be appreciated.

[fledarmus]

If you are trying to separate a mixture of volatile compounds, both of them will have a vapor pressure, and the vapor above your liquid will be a mixture of the components in your liquid. So no, you can't just use a gated pressure valve to remove only one component of a boiling mixture. However:

In general, the greater the difference in boiling points of the separate components of your mixture, the greater the difference in vapor pressures at the boiling point, and the more highly enriched the vapor will be in the lower boiling component (this generalization doesn't necessarily hold for azeotropes). This is usually seen as a problem in vacuum distillation - when you lower the pressure, you lower the boiling points, and usually you decrease the separation between the boiling points. This means that you get faster, low temperature distillation, but less efficient separation. It seems reasonable that increasing the pressure of your distillation would increase the separation in boiling points of your two mixtures, and that although you would have to increase the temperature at which you carried out your distillation, you might get better separation. I haven't seen any elevated pressure distillation systems, but I don't see any theoretical reasons that they wouldn't work the way you describe.

[curiouscat]

This is usually seen as a problem in vacuum distillation - when you lower the pressure, you lower the boiling points, and usually you decrease the separation between the boiling points. This means that you get faster, low temperature distillation, but less efficient separation.

The BP difference does decrease but the relative volatility (in most cases) increases I think. I'm not a 100% sure but I distinctly remember vacuum distillation being said to enhance separation factors.

Anyone know the underlying phenomenology?