Endo
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Registered: 5-1-2006
Location: USA
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Fractional distillation at high altitude, predicting Azeotropic BP
Having done some fractional distillations here at 7300 ft I am trying to wrap my head around the decreased temperatures and how it affects a two
component mixture, ethanol/water and 1,4 Dioxane/ water when trying to get a good azeotropic fraction. I have been just doing my distillations blind
and capturing the fractions where temps hit a plateau. I figured it would be nice to be able to try to find a calculation that I could use to correct
my target temperature by looking up the barometric pressure on NOAA and converting it from in Hg.
I did some digging and found a couple pages and read through them trying to get my head around how a change in the overall pressure would affect the
azeotropic boiling point of the mixture.
Raoult's Law and ideal mixtures
And
Ideal fractional distillation
Vapor Pressure
Unfortunately in thinking through the details I am despairing of finding an easy conversion. Anybody out there have a simple process or know of a
correction table that works with mixtures of liquids?
Thanks!!
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happyfooddance
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Registered: 9-11-2017
Location: Los Angeles, Ca.
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The problem is that there is no easy conversion, because the underlying mechanics of even a binary azeotrope is so so so complex, and you are trying
to define it with two parameters (vapor pressure and temp.)
Not all azeotropes are even formed by the same dominant mechanics. It's best to look at the particular azeotrope to learn more about its
characteristics. For example, many azeotropes of water can be broken or shift as the boiling point of the mixture gets close to the freezing point of
water. This can be safely assumed to be because of the properties of water in the liquid phase at colder temps.
As for distilling @7200', that is a drop to about 23in Hg or about 580 torr, so will only change the boiling point a few degrees. Use a nomograph for
a crude (yet sometimes very effective) way of determining your unknowns from your knowns:
https://erowid.org/archive/rhodium/chemistry/equipment/pictu...
The ethanol water azeotrope disappears at around 17°C, so you will need a better vacuum source.
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