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Air-Acetone condensation

  • 1.  Air-Acetone condensation

    Posted 12-10-2019 13:25
    Recently, I wanted to estimate at what temperature in a stream with mole fractions of 98.5 percent air and 1.5 percent acetone, acetone will condense completely at the pressure of 15 psia, and by using Aspen HYSYS, I get to know that the at -36 C acetone starts to condensate, and above that temperature it is vapor; however, I know that the boiling point for pure acetone is 56.67 C at the mentioned pressure. Another interesting point is that when I increased the mole fraction of the acetone to 10 percent the condensation start point for acetone increased to 1 C. I am not exactly sure that if it is related to the fact that since the overall heat coefficient is low at the first composition, and as I increase the acetone mole fraction and reduce the mole fraction of the air the overall heat coefficient rises.  Please guide me about this problem as I am totally confused.

    Elham Khalati IENG

  • 2.  RE: Air-Acetone condensation

    Posted 12-11-2019 20:51
    Hi Elham,
    The underlying concept here is vapor pressure and how it relates to partial pressure and phase change.  Vapor pressure of a liquid like acetone increases exponentially with temperature.  When the vapor pressure reaches the total pressure over the liquid, it can boil.  That's what happens with acetone under atmospheric pressure at 56.67 C.  The partial pressure of a gas in a mixture is the mole fraction of the gas times the total pressure.  When the partial pressure of a volatile compound exceeds its vapor pressure at a given temperature, it can condense.  With only 1.5% acetone in air, the temperature has to drop below -36 C, according to your Aspen run, for the vapor pressure to be low enough to allow condensation.  With 10% acetone in the air, the partial pressure is higher, so condensation can proceed against the higher vapor pressure that liquid acetone exerts at 1C.  I hope that helps.

    Stephen Lyke, PE
    Wilmington, DE