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# Vapor phase concentration

## Vapor phase concentration

(OP)
Is raoults law valid for very low concentration of solute in aqueous solutions?
I have 9000 ppm (w) of Acetone in Water and I am trying to determine vapor phase concentration of Acetone at 120F and 760 mmHg.

So I converted in to mole fractions ( Xac: 0.00046 and Xwater: 0.9996) With pure vapor pressures of Acetone and water at 120F, I calculated partial vapor pressures of each component.

From my calcs, partial vapor pressure of water is higher than that of Acetone which didn't make sense.

Thanks.

### RE: Vapor phase concentration

Acetone in water is not very close to an ideal solution, so you may want to try to find some activity coefficients. If nothing else, consider using activity coefficients at infinite dilution. That may be better than assuming ideal solution (activity coefficient = 1), which is what Raoult's law does,

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Vapor phase concentration

(OP)
I used activity coeff. with UNIFAC models.
Is it right to say since total vapor pressure is less than system pressure, there will be no vapor phase?

This is an open pit, so I believe I need to look into calculating evaporation rates of mixtures. Any guidance or reference for determining evaporation rates for mixtures?

### RE: Vapor phase concentration

See table 13-2 in Perry Chem Engg Handbook 7th edition for the Wilson, van Laar or Margules constants to predict low pressure partial vapor pressures for the non ideal acetone - water binary.

### RE: Vapor phase concentration

Pls note these correlations give you the equilibrium closed system vapor pressure for acetone, and not the open system vapor pressure. The open system vapor pressure in the air sweeping across this open pit will be diffusion controlled, and will be somewhat dependant on the closed system partial pressure for acetone at the operating temp.
If you are after the concentration of acetone in the wind sweeping across this pit, try asking the process safety engineer in your Company to build a model of this diffusion process in Phast or similar simulators.

Yes, there will be a vapor phase, but its total closed partial pressure will be less than 1.0atm, so it wont be boiling.

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