Upsilon - 9-10-2013 at 14:24
I've been looking up fractional distillation, but I can't find any specific, helpful information. I'd like to better understand how they work, so
could someone please clearly explain how the process works? I would also like to build a distillation apparatus myself, as the premade ones that you
can buy aren't exactly economical. Does anyone know exactly what I'd need for one?
bfesser - 9-10-2013 at 14:58
Tubes.
Clearly you didn't try too hard.
<a href="https://en.wikipedia.org/wiki/Fractional_distillation" target="_blank">Fractional Distillation</a> <img
src="../scipics/_wiki.png" />
Next time, put in a bit more effort before opening a new thread.
WGTR - 9-10-2013 at 15:02
I'll explain it as clearly as I can; perhaps others can help out where my explanation is deficient.
Assume for a moment that you have two solvents that are soluble in each other, and they do not form azeotropes with each other, and you want to
separate them by distillation. This is a fairly simple sort of an example.
If you do a simple distillation, the gas above your boiling mixture will contain more vapor from your lower boiling point solvent than it will of your
higher boiling point one. When this vapor is condensed, the distillate will be richer in the lower boiling point solvent than the original solution
was. The separation of the two solvents by simple distillation will not be very good, however, unless the boiling points of the two solvents are very
far apart.
If you take your distillate (which has been condensed from the vapors of your original solution) and distill it again, the new distillate will contain
even greater concentration of your low boiling point solvent. In fact, if you do an infinite number of simple distillations, over and over again,
theoretically you would end up with a product that was 100% pure of your lowest boiling point solvent. I am simplifying this a bit.
What a fractional distillation column does, is that it combines several simple distillations in one column. A column with four "theoretical plates"
provides a distillate that is equivalent to four simple distillations.
So basically, a fractional distillation column saves a great deal of time and effort over the results obtained with multiple simple distillations.
[Edited on 9-10-2013 by WGTR]
Upsilon - 9-10-2013 at 15:10
In fact I did already read that Wikipedia article, and I didn't get much out of it.
Anyway, I understand the distillation column to an extent, but in other apparati there is a tube exiting the top of the column with input and output
for water. What is the purpose of this? Is it to condense the boiling gas back into a liquid?
WGTR - 9-10-2013 at 15:19
Once the solvent condenses at the top of the distillation column, it is still almost at its boiling point, so its vapor pressure is still very high.
The cooling condenser helps cool down your distillate so that its vapor pressure is lower (you lose less product).
Upsilon - 9-10-2013 at 15:23
Will this also work if the solute has a higher boiling point than water? Say, if I wanted to extract high purity acetic acid from household vinegar?
bfesser - 9-10-2013 at 15:42
At home, fractional distillation is hopeless for purifying ethanoic acid from vinegar. You're better off reacting
NaC<sub>2</sub>H<sub>3</sub>O<sub>2</sub> with H<sub>2</sub>SO<sub>4</sub>, and distilling
off the product. This has been covered on the forum before, try the search function.
Upsilon - 9-10-2013 at 15:55
I was afraid someone would say that. I've calculated the expenses of buying pure acetic acid vs the cost of distilling vinegar with baking soda and
sulfuric acid, and it turns out that it's 2-3 times more expensive to distill it yourself. Would I even be able to get a 70% solution through
distillation?
bfesser - 9-10-2013 at 16:14
Buy the anhydrous sodium acetate directly, rather than reacting sodium bicarbonate with acetic acid. Or, as you said, simply purchase the acetic
acid.