AAMC #3 : distillation

[EnginrTheFuture]

Item 112: Which of the following experimental modifications will most likely improve the degree of separation between limonene and carvone?

A. heating distillation flask at slower rate
B. Using a vacuum source that can achieve a lower pressure inside the distillation apparatus


I don't understand why having a lower pressure inside the apparatus would "narrow their difference in boiling temperature". Even so, narrowing this difference doesn't tell you the whole story I feel.

I was thinking along the lines of the maxwell Boltzmann distribution. If both T_boils are lowered, we lower solution temperature to the temp of the one with the lowest BP. Now both temperature distributions become much narrower because of the lowered solution temp and thus you have less "tail" extending out into the region of sufficient energy for boiling for the product we don't want to boil. I thought a broad distribution for both compounds in solution (in order to reach a high T_boil) would be bad news bears for separation compared to a lower temp, even if the t_boil for each was slightly closer.

Can anyone help me eliminate B or give me a better intuition for why A is right?

---

Members don't see this ad.

 

[syoung]

IIRC, heating it slower will allow for better separation between the two, especially if their Tboil is close. If they are far apart, lets say it was 80 and 100, then when you reach 80, it's easy to see all of the 80 is GONE.
But if it's 95 and 100, well if you heat it too fast, it will reach the 100 and so you'll get impure distillation. Whereas you heat it slower, you can better control its temperature and even maintain it close to the 95, thus allowing for more of the 95 substance to distill off without getting too much of the 100 one.

 

[MedPR]

Vacuum distillation is only used when the BP of something is so high that you would degrade it before actually boiling it. It isn't necessarily used to increase yield of one substance over another.

Heating slower allows you to maintain a certain temperature, thus preventing overheating and distilling both substances instead of just the more volatile one especially when the BP are already close together.. Carvone and Limonene have similar BPs right?

 

[EnginrTheFuture]

MedPR, Yup they are.


Thanks guys that clears up the concept well. Made something simple into something much harder than it needed to be

 

[V FIB]

Hi there. I'm reviving this thread/question .... I understand why the right answer is right, but in the explanations I've read about the wrong answers, it says that increasing the pressure will increase the boiling points (I understand why this so) and increase the difference between the boiling points of the two substances. The opposite things are true for lowering the pressure.

Can someone please explain why the difference between the two would be affected at all? Not getting it.

Thanks.

MedPR, Yup they are.


Thanks guys that clears up the concept well. Made something simple into something much harder than it needed to be

 

[manohman]

Hi there. I'm reviving this thread/question .... I understand why the right answer is right, but in the explanations I've read about the wrong answers, it says that increasing the pressure will increase the boiling points (I understand why this so) and increase the difference between the boiling points of the two substances. The opposite things are true for lowering the pressure.

Can someone please explain why the difference between the two would be affected at all? Not getting it.

Thanks.

Wanted to bump this because I had the same question.

Can anyone explain why

1) Heating it up slower increases separation?
Is it that heating it slower, i.e. with less energy per unit time, means that it will stay at the first boiling point longer giving the vapor more time to condense and then evaporate again and recondense in the fractionating column? Versus if it was heated at a much higher rate/energy per second, then there wouldnt be time for as many condensations BEFORE the second boiling point was reached and then youve basically just mixed the two?

Distillation depends on you collecting one component before the second component evaporates and condenses right?

2) Why lowering the pressure above the solution would decrease the separation.
As i understand it, lowering the external pressure will increase the vapor pressure for both of the components, thus the boiling point should go down for both of them. Does this decrease separation because they decrease in boiling point by different amounts?Because otherwise, if the separation in BP was the same just at a lower temperature i dont see how that would change anything.

 

[AwayFromReality]

Wanted to bump this because I had the same question.

Can anyone explain why

1) Heating it up slower increases separation?
Is it that heating it slower, i.e. with less energy per unit time, means that it will stay at the first boiling point longer giving the vapor more time to condense and then evaporate again and recondense in the fractionating column? Versus if it was heated at a much higher rate/energy per second, then there wouldnt be time for as many condensations BEFORE the second boiling point was reached and then youve basically just mixed the two?

Distillation depends on you collecting one component before the second component evaporates and condenses right?

2) Why lowering the pressure above the solution would decrease the separation.
As i understand it, lowering the external pressure will increase the vapor pressure for both of the components, thus the boiling point should go down for both of them. Does this decrease separation because they decrease in boiling point by different amounts?Because otherwise, if the separation in BP was the same just at a lower temperature i dont see how that would change anything.

1) You seem to have already answered the first question yourself. Heating it up slower allows you to stay at the lower boiling point (higher vapor pressure) thus more evaporation/condensation cycles would occur which means the final solution will be much more pure in terms of the more volatile (lower boiling point) solution.

2) Lowering the pressure above the solution will NOT change the individual vapor pressures of each solution but it will change the boiling point. Boiling point is defined as the point where the vapor pressure equals atmospheric pressure (in this case the pressure above the solution). So if we lower the pressure above the solution and vapor pressure stays at the same position, then the vapor pressure will equal the pressure above the solution a lot quicker therefore the boiling point is reached at a lower temperature.

Knowing this, the difference between the vapor pressure of liquid 1 and atmospheric pressure and the difference between the vapor pressure of liquid 2 and atmospheric pressure will decrease by the same ratio. Thus both of their boiling points will decrease but the boiling point (i.e the temp) difference between the two will decrease by a small margin making the difference a bit more narrower