What precisely are a "tangent pinch" and "feed pinch" compared to true azeotropes?
Azeotropes famously vary on composition depending on pressure - and vary on their very existence depending on pressure. One of the highest profile
azeotropes, the low-boiling azeotrope of ethanol and water which at 1000 mbar is at 95,5% ethanol, shifts with pressure... to the point of vanishing
by migration to 100% ethanol at 93 mbar!
Now, water-acetic acid does not have an azeotrope at 1000 mbar... but does have a "tangent pinch" - on the water end. What does it mean? That dilute
acetic acid solutions are hard to concentrate because the vapour is nearly though not exactly liquid composition... but concentrated acetic acetic
acid solution should still be easy to concentrate to glacial because the tangent pinch is at water end, not acetic acid end?
That "tangent pinch" at 1000 mbar behaves as near azeotrope? Then does it actually convert to an azeotrope at some pressure other than 1000 mbar?
If a liquid mixture has a tangent pinch but no azeotrope at 1000 mbar, what features of the vapour pressure might suggest whether it converts to an
azeotrope at a lower or at a higher pressure, and predict a pressure at which the pressure at which an azeotrope appears looking from the side of data
without azeotrope?Rainwater - 16-8-2024 at 17:35
A tangent pinch is an area of a phase composition diagram that has a "tangent" like vector, referring to a mathematical tangent curve. https://chem.libretexts.org/Bookshelves/Physical_and_Theoret...
Find the liquid gas composition diagrams. Section 13.2.4
In short, the bigger the gap between those two lines, the easier it is to separate.
Here is a chart for acetic acid.
That is not a large difference between the liquid and gas phase at any point in time
Pay close attention to the 100c and 116c areas. These lines are almost touching. Meaning these two areas will require more seperation power than the
rest of the curves.
Quote:
That dilute acetic acid solutions are hard to concentrate because the vapour is nearly though not exactly liquid composition
..
That "tangent pinch" at 1000 mbar behaves as near azeotrope? Then does it actually convert to an azeotrope at some pressure other than 1000 mbar?...
since the commercial dehydration of acetic acid employs either distillation using an entrainer, or
fractional freezing,
and Wikipedia classes acetic acid:water mixtures as zeotropic,
I guess "no" is the answer.
