October 17, 2019, 02:15:46 PM
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Topic: Distillation - I can't get the boiling point - so much different than theory!!  (Read 11590 times)

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Offline 19kevin87

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I'm a new chemist and my boss wants me to find out the boiling point of the solvents from a mixture. Before the distillation is done in bulk in the big heater tank, I have to do it on a small scale in the lab to check the boiling point of the solvents.

From the theory, when a liquid starts to distillate, the temperature should be constant until all the liquid comes out and then the temperature starts rising again until the second liquid with the higher boiling point condenses.

The problem is the temperature is not constant when the first distillate condenses (temperature keeps rising even when the solvent is being distilled out) and then it fluctuates. After 140 C the temperature drops to 80 C and then rise to 140 C

I repeated the second time and the 1st distillate is around 70 C and 2nd distillate around 155 C..

So the bottom line is why the temperature is not constant even when it starts condenses??

Offline Borek

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From the theory, when a liquid starts to distillate, the temperature should be constant

No.

See for example http://www.solvent--recycling.com/fractional%20distillation.html
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Offline 19kevin87

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but the website you gave me is talking distillation in a different manner. I understand it has different boiling point but that is when the distillate is used again, meaning heating the distillate to get a new distillate with different boiling point because of its different composition in the mixture.

for my case, i didn't reheat my distillate. My problem is that the temperature fluctuates even when i'm collecting my distillate, which should be constant until all the solvent condenses and then continue to rise again until it reaches another solvent's boiling temperature

Offline billnotgatez

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Offline Borek

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but the website you gave me is talking distillation in a different manner. I understand it has different boiling point but that is when the distillate is used again, meaning heating the distillate to get a new distillate with different boiling point because of its different composition in the mixture.

No, you are using way too simplified model trying to understand what is happening. Temperature goes up during the distillation all the time. It doesn't go up with the same speed all the time, but there are no real plateaus with constant boiling temperature. Assuming you don't have an azeotrope boiling point of the mixture is always somewhere between boiling points of mixture components. When the mixture is dominated by more volatile component, its boiling point is lower, when it is dominated by the less volatile component, its boiling point is higher. Note that composition of your distillate is changing all the time (you are removing mostly lighter compound, so what is left contains higher and higher molar fraction of the heavier compound), so its boiling point changes as well.
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Offline typhoon2028

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Do you know what the components of the mixture are?  If you do you could do the following.

Make standards with known mixture amounts.  Measure refractive index and boiling point.

You could use a fractional column and record temperatures as material flows from tray to tray.  You could sample from the bottoms and record temperature every time a tray fills and starts to boil.

The bottoms will be changing boiling point and composition.  At first reflux of each tray, the temperature in the bottoms is the boiling point of the bottoms mixture.

Offline 19kevin87

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No, you are using way too simplified model trying to understand what is happening. Temperature goes up during the distillation all the time. It doesn't go up with the same speed all the time, but there are no real plateaus with constant boiling temperature. Assuming you don't have an azeotrope boiling point of the mixture is always somewhere between boiling points of mixture components. When the mixture is dominated by more volatile component, its boiling point is lower, when it is dominated by the less volatile component, its boiling point is higher. Note that composition of your distillate is changing all the time (you are removing mostly lighter compound, so what is left contains higher and higher molar fraction of the heavier compound), so its boiling point changes as well.

Thanks Borek for enlightening me! FYI, i'm distillating waste solvent which obtained from industries. My boss ask me to find the boiling point because he wants to know the identity of the solvent. Since you said that the boiling point changes all the time due to the changing composition, that means that I can't find out what is in my distillate right? Also my distillate would not be 100% of a particular solvent right?

So how do I find out the identity of the solvent in the mixture?

Anyone can help me out? thanks again.

Offline Yggdrasil

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The easiest way to identify the solvent is to run a GC-MS.

Offline billnotgatez

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Could you use a cheap Electrophoresis or is that not very effective on solvents?

http://en.wikipedia.org/wiki/Electrophoresis

Offline billnotgatez

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Quote
i'm distillating waste solvent which obtained from industries.

Can we then assume you know what is in the solvent, you just do want to determing the concentrations of each component?

Offline 19kevin87

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Can we then assume you know what is in the solvent, you just do want to determing the concentrations of each component?

because my company manages waste solvent from industries and the waste are usually a whole lot of solvents dumped in together..
But in the end my boss said is ok because the main purpose is just to separate the solvent like thinner and itself a mixture of solvents. We just need to know roughly at what temperature the mixture separates.

Well the main point is to recover the solvent so that i can be reused again in the industry.. There's not much need to obtain pure solvent because they will be using it for general purposes.



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