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I was wondering if vacuum distillation can seperate azeotropic mixtures.

Consider the ethanol/water system. At 95.6% ethanol content, the system forms an azeotrope because the vapor composition matches the liquid composition:

enter image description here

As one can see, no further seperation occurs in the condensed ethanol phase (going to the right of the diagram).

One of my questions is: If we start with an 50/50 (v/v) ethanol/water mixture, will eventually all the ethanol boil off, i.e will the starting flask consist completely of water after distillation finished? I get that the received ethanol will only contain 95.6% ethanol, but will the starting mixture contain 100% water after ethanol is boiled off (going to the left of the diagram).

And another question is: At lower pressures, do the vapor composition curve and liquid composition curve intersect or is fractional distillation of ethanol/water possible? At pressures of down to 10 mbar?

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  • $\begingroup$ Sorry I meant 10 mbar, that’s what membrane pumps in our lab use $\endgroup$
    – Mäßige
    Apr 19, 2023 at 15:29
  • $\begingroup$ Vapor pressure at 20 Deg C is about 20 mbar.// Ethanol content would approach 0% asymptotically. // Position of azeotrop composition is pressure dependent, cannot tell in which way. $\endgroup$
    – Poutnik
    Apr 19, 2023 at 16:25
  • $\begingroup$ You will receive 95.6% ethanol only using reactivation column, otherwise the distillate composition will reflect the partial vapor pressure ratio over the liquid, according the phase diagram. $\endgroup$
    – Poutnik
    Apr 20, 2023 at 7:07

1 Answer 1

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One of my questions is: If we start with an 50/50 (v/v) ethanol/water mixture, will eventually all the ethanol boil off, i.e will the starting flask consist completely of water after distillation finished? I get that the received ethanol will only contain 95.6% ethanol, but will the starting mixture contain 100% water after ethanol is boiled off (going to the left of the diagram).

Practically yes. Imagine you register the temperature along the distillation. After almost all the ethanol has gone to the other flask, you will register that the tempereature will reach a plateau around $ T \approx 100 \; ^\circ\pu{C} $. This means that the initial flask has now pure water, if the experiment is performed at ambient pressure.

And another question is: At lower pressures, do the vapor composition curve and liquid composition curve intersect or is azeotropic distillation of ethanol/water possible? At pressures of down to 10 mbar?

I will try your claim by the two following elements:

  1. The gas phase is modelled by the Soave-Redlick-Kwong EOS.
  2. The liquid phase is modelled by the NRTL model. There are the results in Txy diagrams. I will zoom on purpose the values of $ x_\pu{ethanol} \in (0.8,1.0) $, to see the evolution of the minimum azeotrope more clearly:

enter image description here enter image description here enter image description here enter image description here

Thus, the azeotrope disappears as the pressure decreases. However, working at this values of pressure, is industrially discourarged. As a first rule, you will always avoid running distillation columns in vacuum, because of the energetic costs that it will demand. However, from a thermodynamic point of view, you are correct.

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  • $\begingroup$ Wow thanks a lot for this answer :) $\endgroup$
    – Mäßige
    Apr 20, 2023 at 6:25
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    $\begingroup$ This is this first time I've seen a liquid-vapor equilibrium diagram for a mixture at different pressures, and it's presented really well. Thank you for this contribution! $\endgroup$ Apr 21, 2023 at 10:39
  • $\begingroup$ @NicolauSakerNeto My pleasure, thanks for the compliment. $\endgroup$ Apr 22, 2023 at 0:02

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