Vacuum distillation of azeotropic mixtures

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](https://i.stack.imgur.com/AtYF2.png)](https://i.stack.imgur.com/AtYF2.png)


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?





> 
> 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](https://i.stack.imgur.com/2ejH0.jpg)](https://i.stack.imgur.com/2ejH0.jpg)
[![enter image description here](https://i.stack.imgur.com/KJAUf.jpg)](https://i.stack.imgur.com/KJAUf.jpg)
[![enter image description here](https://i.stack.imgur.com/gXDOk.jpg)](https://i.stack.imgur.com/gXDOk.jpg)
[![enter image description here](https://i.stack.imgur.com/ieSrU.jpg)](https://i.stack.imgur.com/ieSrU.jpg)


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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