Distillation

[MedPR]

So TBR says this.

"Over three evaporation-condensation cycles, the system becomes richer and richer in the more volatile component until the ratio is roughly 27:1, which equates to a mixture that has 96.4% of the more volatile component"

Doesn't the more volatile component boil at a lower temperature? Thus it vaporizes more easily? So shouldn't the system be richer in the less volatile component because the more volatile component is being removed and transferred into the collection flask?

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[milski]

Yes, the more volatile compound boils first. Then you collect the vapor, condense it and do the cycle again on the condensed vapor. There is no point in re-boiling again the leftovers - they have much higher concentration of impurities than what you started with, since most of your pure compound has already evaporated.

 

[lkthlttr]

So TBR says this.

"Over three evaporation-condensation cycles, the system becomes richer and richer in the more volatile component until the ratio is roughly 27:1, which equates to a mixture that has 96.4% of the more volatile component"

Doesn't the more volatile component boil at a lower temperature? Thus it vaporizes more easily? So shouldn't the system be richer in the less volatile component because the more volatile component is being removed and transferred into the collection flask?

I think the use of the term "system" is what is confusing. I think if they used an alternate term like "the distillate" it would be clearer, as in "the distillate becomes richer and richer in the more volatile component". In my mind "the system" describes the entire apparatus from the boiling flask to the collecting flask, and so there would be no change in the ratio of the varying components at all, but maybe that's just me.

 

[MedPR]

I think the use of the term "system" is what is confusing. I think if they used an alternate term like "the distillate" it would be clearer, as in "the distillate becomes richer and richer in the more volatile component". In my mind "the system" describes the entire apparatus from the boiling flask to the collecting flask, and so there would be no change in the ratio of the varying components at all, but maybe that's just me.

Yea I agree. The distillate is what ends up in the collecting flask, right? So the more volatile one is the distillate?

 

[gettheleadout]

Yea I agree. The distillate is what ends up in the collecting flask, right? So the more volatile one is the distillate?

Yes and yes. For some reason my first thought seeing the OP was fractional distillation, but that's totally different.

 

[lkthlttr]

Yea I agree. The distillate is what ends up in the collecting flask, right? So the more volatile one is the distillate?

Yes. More volatile means boils at lower temperature which is the first to boil which is what is the major component found in the collecting flask.

Yes and yes. For some reason my first thought seeing the OP was fractional distillation, but that's totally different.

I actually think that it is talking about fractional distillation. When they are talking about "evaporation condensation cycles" I'm pretty sure that is referring to the fractionation column, which acts to separate out components based on their boiling points. The more volatile compound will condense and evaporate multiple times on it's progression through the fractionation column, and ultimately reach the collection flask much quicker than lower boiling components.

 

[gettheleadout]

I actually think that it is talking about fractional distillation. When they are talking about "evaporation condensation cycles" I'm pretty sure that is referring to the fractionation column, which acts to separate out components based on their boiling points. The more volatile compound will condense and evaporate multiple times on it's progression through the fractionation column, and ultimately reach the collection flask much quicker than lower boiling components.

I understand how fractionation works, but I don't understand what they're defining as a single evaporation-condensation cycle if this is the case...

 

[lkthlttr]

I understand how fractionation works, but I don't understand what they're defining as a single evaporation-condensation cycle if this is the case...

Oh. I just reread the question and I guess you're right. I had assumed that the evaporation-condensation cycle referred to the process of the evaporated liquid condensing on the fractionating column and then re-evaporating when the warm vapor from below warmed it again, you know like how they try to describe the "theoretical plates" of a fractionating column. If they're doing it in discrete steps of taking the distillate and performing a second (& third) distillation, then well I guess I don't know why you wouldn't just do a fractional distillation

 

[gettheleadout]

Oh. I just reread the question and I guess you're right. I had assumed that the evaporation-condensation cycle referred to the process of the evaporated liquid condensing on the fractionating column and then re-evaporating when the warm vapor from below warmed it again, you know like how they try to describe the "theoretical plates" of a fractionating column. If they're doing it in discrete steps of taking the distillate and performing a second (& third) distillation, then well I guess I don't know why you wouldn't just do a fractional distillation

Right? Haha

I vaguely recall the theoretical plates deal, but I always thought of the vapor movement and condensation within the column as continuous (i.e. not easily separated into cycles...)

 

[lkthlttr]

Right? Haha

I vaguely recall the theoretical plates deal, but I always thought of the vapor movement and condensation within the column as continuous (i.e. not easily separated into cycles...)

Certainly in reality that is the case. The idea of theoretical plates is just a conceptualization of the physical process and a way of comparing the separating efficiency of different chromatography/distillation columns.

 

[MedPR]

Bumping this since I'm still not completely getting it.

Yes, the more volatile compound boils first. Then you collect the vapor, condense it and do the cycle again on the condensed vapor. There is no point in re-boiling again the leftovers - they have much higher concentration of impurities than what you started with, since most of your pure compound has already evaporated.

So when we talk about multiple evaporation-condensation cycles, we aren't talking about the same system. We do one cycle to separate the most volatile component. Obviously there will be some of the lesser-volatile components, thus leaving us with a non pure distillate. Then we take the distillate and setup a completely new distillation apparatus and repeat? Then repeat and repeat and repeat?

And in regard to fractional distillation:

It's the same concept, just with beads or some other inert surface-area-enhancing solid in the column before the condenser. So the vapors get up into that column and go through cycles of condensing and re-evaporating. So basically using a fractionating column just saves you from dirtying more flasks and setting up multiple distillations of the distillate in addition to allowing you to separate liquids which are closer in BP (compared to simple distillation)?

Edit: So, besides the fact that the setup involves one more step for the lab student, why would you ever choose simple distillation over fractional distillation?

 

[pfaction]

I could be wrong, but I like to think of something close but different. Say, water and acetic acid. Let's say that water boils at 100, and acetic acid at 90.
Since acetic acid will boil first, the majority of it goes up. But, some water does too! The point of the liquid beads is that, IMO, the acetic acid will rise up in more quantity, but the heavier water will be more sluggish, not rising as fast or in the same quantity. Those molecules get adsorbed onto the beads, and fall down. Of course this is occuring to the acetic acid as well, but since they have more energy to escape, more do so.

 

[theseeker4]

Edit: So, besides the fact that the setup involves one more step for the lab student, why would you ever choose simple distillation over fractional distillation?

Some possibilities could be cost of the factional distillation apparatus, ability to discard leftovers rather than keeping everything in the system, ability to test the collected distillates and adjust distillation temperature/etc. for the next cycle, and so on. Wikipedia has a nice article and some good pictures that could help.

 

[MedPR]

Some possibilities could be cost of the factional distillation apparatus, ability to discard leftovers rather than keeping everything in the system, ability to test the collected distillates and adjust distillation temperature/etc. for the next cycle, and so on. Wikipedia has a nice article and some good pictures that could help.

Ok thanks. Could you also verify if the rest of that post is correct? I think I finally have it down and I don't want to be wrong!

 

[mcloaf]

Bumping this since I'm still not completely getting it.




So when we talk about multiple evaporation-condensation cycles, we aren't talking about the same system. We do one cycle to separate the most volatile component. Obviously there will be some of the lesser-volatile components, thus leaving us with a non pure distillate. Then we take the distillate and setup a completely new distillation apparatus and repeat? Then repeat and repeat and repeat?

And in regard to fractional distillation:

It's the same concept, just with beads or some other inert surface-area-enhancing solid in the column before the condenser. So the vapors get up into that column and go through cycles of condensing and re-evaporating. So basically using a fractionating column just saves you from dirtying more flasks and setting up multiple distillations of the distillate in addition to allowing you to separate liquids which are closer in BP (compared to simple distillation)?

Edit: So, besides the fact that the setup involves one more step for the lab student, why would you ever choose simple distillation over fractional distillation?

Yeah, you've got it.

As for simple distillation, it's just easier/much faster. If you are separating two liquids with very disparate boiling points there's no reason to waste your time with fractional distillation.

 

[MrNeuro]

Yeah, you've got it.

As for simple distillation, it's just easier/much faster. If you are separating two liquids with very disparate boiling points there's no reason to waste your time with fractional distillation.

you get a greater yield in a shorter amount of time w/ simple distillation. Another thing to note is we usually used fractional distillation when the boiling points of our two compounds were close, or if we wanted greater purity for chromatography, NMR etc etc.

plus cleaning those beads is pretty annoying

 

[MedPR]

Yeah, you've got it.

As for simple distillation, it's just easier/much faster. If you are separating two liquids with very disparate boiling points there's no reason to waste your time with fractional distillation.

you get a greater yield in a shorter amount of time w/ simple distillation. Another thing to note is we usually used fractional distillation when the boiling points of our two compounds were close, or if we wanted greater purity for chromatography, NMR etc etc.

plus cleaning those beads is pretty annoying

Ok thanks guys. One (two, kind of) techniques down, only about a dozen to go!