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Maximizing Separation in Distillation: Heating more Slowly and Decreasing External Pressure

  1. Nov 3, 2014 #1
    I'm having trouble understanding why 1) Heating a solution more slowly would increase separation in a distillation and why 2) Lowering the pressure above the solution, i.e. with a vacuum, would decrease separation.

    Distillation depends on you collecting one component before the second component and condenses right? So if you decrease the time from when you hit the first boiling point and the second one, then youre decreasing separation.

    1) Heating it up slower increases separation?
    Is it that heating it slower, i.e. with less energy per unit time, means that it will stay at the first boiling point longer giving the vapor more time to condense and then evaporate again and recondense in the fractionating column? Versus if it was heated at a much higher rate/energy per second, then there wouldnt be time for as many condensation-evaporation cycles to occur BEFORE the second boiling point was reached and the distillation was over.

    Distillation depends on you collecting one component before the second component evaporates and condenses right?

    2) Why lowering the pressure above the solution would decrease the separation.
    As i understand it, lowering the external pressure will increase the vapor pressure for both of the components, thus the boiling point should go down for both of them. Does this decrease separation because they decrease in boiling point by different amounts?Because otherwise, if the separation in BP was the same just at a lower temperature i dont see how that would change anything.
  2. jcsd
  3. Nov 5, 2014 #2
    for 1) I think you've got it right.
    for 2)
    I am not 100% sure about this, but my guess is the difference in vapor pressures decreases as the pressure drops. The other argument though, is that you'll reach the higher boiling point faster meaning less time boiling the other component out.

    I hope this helps.
    Last edited: Nov 5, 2014
  4. Nov 5, 2014 #3
    Ah I see that makes logical sense. Thank you!
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