Sciencemadness Discussion Board

How does a closed distillation not build up pressure

Yttrium2 - 9-12-2015 at 11:04

How can a closed distillation be done without pressure building up? I know everyone says don't do closed distillation but curiosity has got me.

if one section is heated, and the other is cooled
if in one section vapors are emitted, and in another vapors are condensed

then how does pressure develop?

how can closed distillation be done?



Praxichys - 9-12-2015 at 11:17

Assuming the apparatus is full of air when the distillation is started, the heating of that air as well as anything boiled out of the flask but not yet condensed will contribute to internal pressure. Any gas dissolved in the liquid and any gaseous decomposition products will also contribute.

Spherical and tubular glass does well under a vacuum because it has high compressive strength, and drawing a vacuum puts the glass in compression. On the other hand, pressurizing glassware exerts tensile load on the walls. Typical glass has a tensile strength about 20 times lower than its compressive strength.

Pressure also has an effect on the boiling point of compounds. Higher pressure will make the compound harder to boil, requiring more heat and thus risk of damaging the compound. On top of that, this effect will cause temperature readings to change and make it difficult to determine if the right compound is coming over.

As you can imagine, ground glass joints work well under low pressure (they get "sucked together"), but ordinary keck clips cannot withstand much separation pressure. I have had them separate after a chlorine generator got a little frisky, maybe 2 or 3 PSI.

If you want to "close off" a distillation, use a drying tube on the outlet or run the outlet tube into a scrubber/bubbler/suckback trap or something. What are you distilling? Do you need to keep the products in, or the environment out?

EDIT: Derp. I don't know.

[Edited on 9-12-2015 by Praxichys]

Yttrium2 - 9-12-2015 at 12:43

in the video titled Dissolving Lithium in Anhydrous Ammonia, by Applied Science on youtube, how come the pressure didn't build up enough to blow out the bungs?

wouldn't the heating of the N2 and the O2 within the vessel cause a pressure change via Charles Law?
"
Assuming the apparatus is full of air when the distillation is started, the heating of that air as well as anything boiled out of the flask but not yet condensed will contribute to internal pressure. Any gas dissolved in the liquid and any gaseous decomposition products will also contribute."
I'm going to have to say Praxichys is wrong, the air doesn't have to condense, atleast not to a liquid, to prevent pressure buildup. Extreme cold Charles law?

[Edited on 12/9/2015 by Yttrium2]

[Edited on 12/9/2015 by Yttrium2]

Texium - 9-12-2015 at 13:20

Quote: Originally posted by Yttrium2  
in the video titled Dissolving Lithium in Anhydrous Ammonia, by Applied Science on youtube, how come the pressure didn't build up enough to blow out the bungs?

wouldn't the heating of the N2 and the O2 within the vessel cause a pressure change via Charles Law?
"
Assuming the apparatus is full of air when the distillation is started, the heating of that air as well as anything boiled out of the flask but not yet condensed will contribute to internal pressure. Any gas dissolved in the liquid and any gaseous decomposition products will also contribute."
I'm going to have to say Praxichys is wrong, the air doesn't have to condense, atleast not to a liquid, to prevent pressure buildup. Extreme cold Charles law?
If you read what Praxichys said you would see that he said nothing about needing to condense the air. That would be ridiculous. All that he was saying was that the air in the apparatus expands when heated and needs somewhere to go. Usually in a distillation or a reflux setup, all of the air is pushed out of the system and the apparatus fills with vapor.

Yttrium2 - 9-12-2015 at 13:40

So, in the video, internal pressure does not build up because the receiving end is so cold?

Can someone please answer this question? I'm surprised the large amount of vapor doesn't cause an increase in pressure. Im guessing it's cause the receiving side is so cold...

Does this make sense?

Praxichys did say "but not yet condensed will lead to an increase in pressure"

Zts what are you saying is ridiculous and why? Are you saying condense as in condense to a liquid?

What this means is that when substance condenses pressure decreases. It is the inverse of what was said. It was still said.

Could we just jump straight to the point, can someone watch the video and explain why this closed distillation did not cause an increase in pressure? I didn't want this topic going to detritus, but I feel like I can't convey what I'm talking about without the video. Hopefully just because it the substance has illicit uses doesn't mean that my legitimate question gets shot down, or ignored

Amos - 9-12-2015 at 13:43

Well first off, if anything there would be a decrease in pressure as the ammonia entered the test tube, as it's condensing from a gas to a much more compact liquid.

Secondly, if you heat a container with only one exit and a stopper in that exit, the pressure simply rises with the temperature until the total force exerted on the stopper's exposed surface surpasses the combined force of gravity on the stopper and the friction between the stopper and the glass of the container. I suppose the other options are air escaping through tiny gaps or the glassware itself shattering, but you get it. At least, I hope you do.

Yttrium2 - 9-12-2015 at 13:50

Bump

Texium - 9-12-2015 at 17:00

Quote: Originally posted by Yttrium2  
Zts what are you saying is ridiculous and why? Are you saying condense as in condense to a liquid?
That's what the definition of condense is, yes.

Yttrium2 - 9-12-2015 at 17:09

Is this true? What's it called when gas is cooled and the molecules come closer together but not close enough to make a liquid?

Dr.Bob - 9-12-2015 at 19:07

Look at the "ideal gas law", try google or wikipedia. When you cool a gas, the volume decreases, when you heat it, it expands. If it condenses, that changes everything, as the volume of a liquid is MUCH less than the volume of a gas. It is rare to seal a reaction or distillation in a safe manner, unless using specific equipment designed for such pressure. Most reactions are done under a slight inert gas pressure, or under a vacuum, or with some method to regulate the pressure. Even a simple balloon of nitrogen works well, other gases are often added via a simple balloon, as that generates a small, positive pressure, with no chance of over pressuring the reaction.

aga - 10-12-2015 at 11:46

The question is silly.

In a sealed system, adding heat will increase pressure, even if it is just air in the system.

Practical application of this principle was first described by Hero of Alexandria in the 1st century AD.

Yttrium2 - 10-12-2015 at 12:30

How in the video dissolving lithium in anhydrous ammonia, the reaction was sealed?

How do you think the guy knew that it wouldn't cause a pressure buildup? It does not seem at all intuitive to me, it seems like there could be a lot of math used to show that there would be no pressure buildup, that is kind of beyond me. Since the system was being added too, it wasn't stagnant

How do you think the guy on applied science knew that the system wouldn't pop? Was it a lucky guess?

aga - 10-12-2015 at 12:50

Is it this one ?

https://www.youtube.com/watch?v=Eej6kG4NyFw

If so, the sheer Cold from the dry ice would enable any ammonia gas (-33 C) to condense rapidly, also any water in the system would further dissolve the ammonia gas immensely at such low temperatures ( ~47w% @ 0C, much more if @ -78C)

Given the Small scale and plastic tubing (which is flexible enough to expand slightly) there would be no serious risk of a pressure-induced explosion.

The pressure Would increase when heating was started, yet equilibriate as soon as the NH3 reacted, condensed or dissolved in the test tube.

If you'd have provided a link with the OP, this answer would have come sooner.

Edit :

The setup in the video isn't a distillation per-se.

It's a reaction between a solid metal and a gas - the lithium metal sits in the test tube and reacts with the ammonia gas from the ammonia generator in the vac flask.

[Edited on 10-12-2015 by aga]

Amos - 10-12-2015 at 13:02

Quote: Originally posted by Yttrium2  
How do you think the guy knew that it wouldn't cause a pressure buildup? It does not seem at all intuitive to me, it seems like there could be a lot of math used to show that there would be no pressure buildup, that is kind of beyond me.


That's because you haven't been in this field very long at all; just because it isn't intuitive to you doesn't mean that everyone trying to offer you an explanation is incorrect. The ammonia condenses to a liquid. This causes a decrease in pressure and pulls more ammonia in. End of story.

aga - 10-12-2015 at 14:02

Quote: Originally posted by Amos  
That's because you haven't been in this field very long at all; just because it isn't intuitive to you doesn't mean that everyone trying to offer you an explanation is incorrect. The ammonia condenses to a liquid. This causes a decrease in pressure and pulls more ammonia in. End of story.

Beginners in Chemistry, or anything else for that matter, have no experience, by definition.

Beginners in this Forum have no idea how advanced some of the other members are.

There was me, thinking i'd conquered some fundamentals, only to have that illusion shattered by blogfast25's Quantum Mechanics thread.

Then Darkstar weighed in to dump 100% pure Organic Chemistry reality into it.

On this forum, in my opinion, it is best to assume that you know next to nothing, as that is almost guaranteed to be the case when compared to some of the other members.

In any event, learn, learn more, continue to learn and enjoy it !

Yttrium2 - 10-12-2015 at 16:40

I suspected the cold receiver was what prevented a pressure build up. However, it seemed counter intuitive that this would work without developing pressure. I don't think it would be intuitive even for seasoned members. I've done a Charles law experiment before, a change in temperature did cause a large change in volume, however, more and more gas wasn't continually added to the system. How could this be intuitive?

Fulmen - 11-12-2015 at 11:56

I think this is an interesting question and there are several things one must factor in. First of all any trapped air will expand as it is heated according to [url=https://en.wikipedia.org/wiki/Charles's_law]Charles's law[/url]. The temperature will of course vary throughout the apparatus, and if the receptacle is cooled to blow ambient it can cancel out much or all expansion from the heating.
The vapor passing through the apparatus will also cause an increase in pressure. While it is condensed at the end there will always be some gas moving from the hot to the cold zone.