[Badger]

Hey everyone!

I did a lab where we seperated a mixture of heptane and m-xylene and we had pretty good GLC readings for the simple distillation we performed. However, when we did a GLC reading for the fractional distillation, we had a skewed print out and we didn't have time to obtain a better sample. For our results, we were able to calculate 80% mole heptane using simple distillation and only 60% mole heptane using fractional distillation (we had to guesstimate a little bit for this last calculation). Per these results, that would mean that the simple distillation was more effective but we're afraid to use them because of the error when using the GLC machine.

So, my question is, which should actually be more effective? Fractional distillation is supposed to yield a purer yield, but simple distillation is better for compounds where the individual portions of the mixture have wildly different boiling points (m-xylene and heptane are different by about 30-40 degrees Celsius if memory serves correctly).

Could anyone offer some help with this? Thanks!

[curiouscat]

I can't think why. A fractional should never be worse than a simple (I think).

One extreme worst case scenario might be if A + B was a azeotrope. In those cases fractional and simple would perform equally badly.

* If the distillation curve was getting pinched at a point fractional would be bad; yet I don't see it do worse than a simple distillation..

[Badger]

I can't think why. A fractional should never be worse than a simple (I think).

One extreme worst case scenario might be if A + B was a azeotrope. In those cases fractional and simple would perform equally badly.

* If the distillation curve was getting pinched at a point fractional would be bad; yet I don't see it do worse than a simple distillation..

That's exactly what I was thinking... but now I've started searching for articles and the literature keeps making it sound as if simple distillations will always be better for mixtures with radically different boiling points. I hadn't heard that before, and maybe its best I hadn't now. I'm second guessing myself!

[curiouscat]

That's exactly what I was thinking... but now I've started searching for articles and the literature keeps making it sound as if simple distillations will always be better for mixtures with radically different boiling points.

Which ones? Can you post?  I'm still surprised.

Yes, the relative utility of fractional dist. will go down for "easy" separations (say you had to strip Acetone from a high boiling component); but I can't see how it'd be worse than simple.

[orgopete]

There should not be any magic to distillation. Fractional distillation is the same as simple distillation repeated. If you redistilled your heptane a second (or third or nth) time, would it be more pure than the first?

While that is true, there are ways to improve or degrade the distillation efficiency. If the pot temperature is so high the distillation occurs before any equilibrium of the liquid-vapor can take place, then neither will be effective. Sometimes with small columns or samples, there can be a tendency to overheat fractional distillations reducing their efficiency. In practice, fractional distillations require more time to perform than simple distillations if they are to be effective.

[Badger]

Hey!

I'll get those articles posted in a little bit. I just have to finish up some work.

[curiouscat]

Hey!

I'll get those articles posted in a little bit. I just have to finish up some work.

Sure, thanks. I'm curious to see them.