Sciencemadness Discussion Board

Vacuum Distillation Confusion

Nitrator - 30-9-2012 at 00:16

I'm confused about one small thing about vacuum distillation. This is probably a really stupid question, but I can't find the answer anywhere... Why doesn't the distillate get "sucked" out?

On a completely separate note, can you use a vacuum cleaner for vacuum distillation? I've never heard of anyone doing this...

[Edited on 9-30-2012 by Nitrator]

Doc B - 30-9-2012 at 02:19

The vacuum adaptor has an internal spout that delivers the distillate below the negative pressure aperture. The density of the distillate, being greater than the reduced atmosphere in the system, ensures it falls into the collection flask coupled with the vacuum adaptor design. The collection flask also has no heat applied to it and may in fact be chilled in an ice bath to reduce the volatility of its contents.

I often use a two necked flask with one neck fitted with a recovery bend, while the other has a condenser atop of which a in/outlet adaptor supplies the vacuum . Thereby forcing any distilled volatiles to overcome the chilled receptacle and ascend the condenser in order to exit the system.

It's also important to note that you purposefully introduce a minute leak via a long capillary type tube below the surface of the distilling solution. This establishes a rheological condition suitable to conduct reduced pressure distillation smoothly and effectively.

Doc B - 30-9-2012 at 02:21

As to your second question, the short answer is no.

Bot0nist - 30-9-2012 at 03:41

What Doc said...

And a vacuum cleaner can be rigged to work for vacuum filtrations, but cant provide the strong, consistant reduced pressure needed for distillation.

There is a recent thread on using household vacuums for filtration. Also see the thread on fridge compressors that member Peach put up. That is how many of use have obtianed our vacuum pumps suitable for reduced pressure distilations.

[Edited on 30-9-2012 by Bot0nist]

Nitrator - 30-9-2012 at 10:13

Ok, thanks so much. I never noticed the internal construction of vacuum adapters. I thought its was just a section with a hose connector leading away from the piece, like this: Attachment: Incorrect Perception of Vac Adaptor .pages (45kB)
This file has been downloaded 375 times

If it were like this, it wouldn't work, which is why I was confused.

As for the vacuum itself, how much negative internal pressure can a graham condenser withstand? Will regular distillation equipment be fine for most home vacuum distillations. If someone could advise as to what types of vacuum producers are acceptable that would be great. I.e., aspirator, fridge compressor, vacuum cleaner; I don't want to implode any glassware. (will be distilling HNO3 for the most part, so whatever will reduce the pressure enough eliminate decomposition)

Bot0nist - 30-9-2012 at 10:22

It will depend on the quality and condition of the condensor. Always test the setup before running the reaction for safety.

P.S. You can get red fuming nitric acid without vacuum, no problem. Do you specifically need white fuming for a sensitive nitration? I hear bubbling dry air through the RFNA will further oxidize the disolved NOxs, clearing the acid.

Nitrator - 30-9-2012 at 10:30

How do you test the condenser without breaking it?

And no, buying HNO3 is not an option; It's way cheaper to make. Yes, I could purify the RFNA, but it's still not as efficient as making pure HNO3 to begin with.

Hexavalent - 30-9-2012 at 10:55

Quote: Originally posted by Nitrator  
How do you test the condenser without breaking it?

What he means it to, say, load the distillation flask with water, for example, switch on the heat and ensure that the water completely condenses in the condenser with no vapour coming out of the end or any other problems.

Basically, what he means is check the system for faults with an inert, non-hazardous liquid before using it with something much more dangerous.

[Edited on 30-9-2012 by Hexavalent]

Bot0nist - 30-9-2012 at 11:03

I never said buy it. Just distill it at atmopheric pressure. I've done it dozens of times. How is bubbling air through RFNA harder than running a reduced pressure distillation? especially for someone who has never worked with a hard vacuum, or even owns a vacuum pump? RFNA distilled from a nitrate salt and concentrated sulfuric acid will work for nearly all applications.

Good luck with your Hoover, lol.

Endimion17 - 30-9-2012 at 11:09

I've never heard about a condenser imploding under high vacuum. Old, damaged flasks, yes. Condenser tubes, no.
If you want to reduce the pressure for distilling nitric acid, you better use a cold trap or your pump will be damaged.

But before attempting to do these things, I'd recommend reading a physics book. Your notion of "sucking" reminds me of the physics knowledge levels often seen in elemenatary school. No hard feelings, it's just my observation.

First elementary physics, then basic chemisty like boiling points and pressure, after that more complex things like boiling solutions, azeotropes, Raoult's law, etc.
You could skip the latter if you're having fun distilling water, but you're messing with nitric acid. Thinking that a vacuum "sucks stuff" is not something I'd put together with "boiling nitric acid". I immediately see horrific chemical burns.

Bot0nist - 30-9-2012 at 11:19

My sentiments exactly, Endi. Crawl > Walk > Run.

Nitrator - 30-9-2012 at 11:39

I understand basic physics quite well. Nevertheless, I understand your concern. I know that a vacuum doesn't actually suck, but it's easier to use this unscientific term. Suction is a real thing, but is often incorrectly visualized as a pull from the vacuum instead of the correct view of a push from a higher pressure to equalize a region of lower pressure. I completely understand how a vacuum works. And based on this understanding, I'm afraid that my precious/expensive distillation setup may be crushed by the atmospheric pressure. I just wanted to make sure that a regular distillation setup could withstand the atmospheric pressure.

p.s. I have a thorough understanding of chemistry as well, and I am completely aware of all the hazards accompanied with all forms of HNO3.

Hexavalent - 30-9-2012 at 11:52

Quote: Originally posted by Nitrator  
I have a thorough understanding of chemistry as well

Then you shouldn't even be asking this extremely simple question if you have a 'thorough understanding of chemistry as well'.

Standard distillation apparatus will hold a vacuum, how strong will depend on the age, composition, temperature, thickness and more factors of the glass: you provide no such details. Personally, I'm sure it would be fine as long as you don't allow the vacuum to get too high.

Lambda-Eyde - 30-9-2012 at 12:11

Quote: Originally posted by Hexavalent  
Personally, I'm sure it would be fine as long as you don't allow the vacuum to get too high.

The laws of physics won't obey your personal opinion. There's a very little difference in the force pushing the glass from the outside at 1 torr vs. 0.01 torr. Jointed lab glassware is constructed to handle a pressure difference of >1 atm (which of course is impossible when working at atmospheric pressure).

The exception is large round bottom flasks (though I don't understand exactly why - anyone who does is welcome to explain it) and scratched flasks/condensers/adapters.

[Edited on 30-9-2012 by Lambda-Eyde]

Nitrator - 30-9-2012 at 12:12

@ Hexavalent
First of all, the issue of "glass withstanding a vacuum" is a physics issue, not a chemistry issue. So perhaps you need to freshen up on that yourself.
Second, I recognized that it was a simple question when I asked it, thereby implying that I could easily understand it, but simply had a "brain freeze." And remember, neither of my questions had to do (primarily) with chemistry, but had to do with physics. The "understanding of chemistry" quote was in response to an attack on my knowledge of chemistry, which was based on my supposed lack of knowledge of physics. As I have made it clear that I understand physics, the chemistry claim against me is invalid.
Third, I did in fact give details regarding the distillation apparatus. One can easily infer from my post that it is a typical borosillicate glass setup with a graham condenser instead of a liebig. The spiralled glass inside a graham condenser is thinner than a liebig, so I just wanted to make sure it was okay. It's just a safety precaution. It makes pretty good sense to be certain that your $125 dollar setup can withstand a vacuum. Fortunately, the other posters cleared any doubts I had, whereas your post have not presented any new information and have only criticized me. Plus your claims take my words out of context. If you dont have anything intelligent or helpful to say, then please refrain from saying anything.
Best regards.

Nitrator - 30-9-2012 at 12:14

Thank you Lambda-Eyde for your helpful comment. That is exactly what I'm looking for. :)

hissingnoise - 30-9-2012 at 12:28

The perceived advantages of vacuum dist. are, IMO, somewhat overstated related to HNO<sub>3</sub> dist.!
RFNA is easily lightened by bubbling in dry air or oxygen; any remaining yellowness is removed by dropping in a 'pinch' of urea and stirring . . .

Nitrator - 30-9-2012 at 12:34

Although I will be using it primarily for HNO3, I will distill other things as well.
Yes, RFNA can have the nitric oxides removed, BUT you still lose product during the distillation as NO2/N2O4 floats away. I have distilled HNO3 multiple times and the yield is considerably low (70%). Even if you convert the nitric oxides to HNO3, you are still lower than the yield produced by vacuum distillation. PLus, I'd like to do one step and be done with it. It's just all around better.

[Edited on 9-30-2012 by Nitrator]

Nitrator - 30-9-2012 at 12:58

A note to everyone who says to just blow air through it.
If I wanted to change RFNA into WFNA, than this would obviously make sense. However, this is not my goal. My goal is to get the most nitric acid from a constant amount of sulfuric acid and a nitrate salt. Vacuum distillation offers the highest yield. This is what I'm looking for, to be efficient. RFNA with oxides removed will not be as much as WFNA. An aspirator is relatively cheap, so I'll probably get one shortly. Plus, if I have the necessary vacuum equipment, I can distill many other compounds as well. So drop the air blowing/urea please.

Kiwichemicali - 1-10-2012 at 02:55

Quote: Originally posted by hissingnoise  
The perceived advantages of vacuum dist. are, IMO, somewhat overstated related to HNO<sub>3</sub> dist.!
RFNA is easily lightened by bubbling in dry air or oxygen; any remaining yellowness is removed by dropping in a 'pinch' of urea and stirring . . .

This weekend I distilled about 650ml nitric acid. I always use a vacuum because it saves me time and less product is wasted because of evaporation during the air bubbling process.
The whole operation was a great success. Only one distillation was needed and I only had to bubble dry air through the acid for about half an hour.
I was surprised by the quality of the acid, density was 1.51. Since I couldn't believe my eyes I determined en redetermined twice but every time the result was the same. Even tried double distilled water to check for measurement errors but that too gave me the correct value.


Hexavalent - 1-10-2012 at 07:43

Quote: Originally posted by Nitrator  
@ Hexavalent
First of all, the issue of "glass withstanding a vacuum" is a physics issue, not a chemistry issue.

Also, I have helped you identify what a 'test run' with your glassware is, an important step before using any unfamiliar apparatus with hazardous chemicals.

Who knows, from your posts we can't identify unambiguously whether or not you can see any imperfections in your glass, whether or not the entire assembly is professional, proper equipment and more. These can, and often do, make a difference. Don't get defensive - if you disagree with a claim, then that's fine as long as you can back it up. We are only here to help.

Besides, what level of vacuum were you looking for?

Nitrator - 1-10-2012 at 21:27

Thanks for trying to help, but I know what a test run is; I've done it before. I thought he was referring to a way to test the setup's ability to withstand a vacuum, not simply test if it can withstand regular distillation.

Vacuum level and imperfections have already been addressed. All I wanted was a simple answer to a simple question. Just wanted to know if typical glass distill could handle a near complete vacuum. Once I received that information, I decided what is best for me, since I know the condition of my setup. There is no need for me to go into a tedious description of my setup.

I'm not defensive, and I also backed up my claim. So I'm not really sure what your problem is. (I have a theory but it's rather complex. lol)

I hope you will read the entire thread with an objective mind. You will see that I was simply asking a few questions and people were answering me. As the don't know my background, they were concerned about my safety since I was asking relatively simple questions. I explained to them that they had nothing to worry about, and that was that. Then some punk kid comes in here, talk'n about how clueless I am with no basis for his claims. Naturally, I chewed the punk out. The punk's response?: "I have no real argument so I'll just say some big (irrelevant) words and top it off with the 'ol 'Don't be so defensive!' garbage." Weak man... weak.
The purpose of this forum is to try to help people. But I can read you like a book. You one of those who tries to show off how smart they are, and you do this primarily by belittling others. I've seen quite a bit of ur post on here and on youtube. So, try to act mature and genuinely help people;) Best regards

hissingnoise - 2-10-2012 at 01:44

Pretty schizoid post . . .

tetrahedron - 2-10-2012 at 02:38

the vacuum doesn't remove any product? suppose you don't introduce air or an inert gas through a capillary tube..then you wouldn't need to apply suction the whole time, but only at the start to remove the atmosphere..after starting the distillation, the only species present in the atmosphere will be your product, but this will condensate on the cooler side. or what's the problem with this scheme?

hissingnoise - 2-10-2012 at 04:40

When distilling heat-sensitive compounds, the applied vacuum must be sustained and constant as possible.
Variations in pressure will lead to some decomposition and loss.
Depressing the b.p. also decreases dew-point and the receiver temp. must be lowered accordingly to prevent loss of product to the vacuum.

Panache - 5-10-2012 at 06:28

if you're distilling acids under vacuum using a rotary vane pump made of tool steel (ie not teflon), in addition to the cold trap a sodium hydroxide trap should be used, in the order such as pump---NaOH trap----coldtrap----manifold-----system.
i have found that the most effective way of constructing such a trap is simply to fill a glass canister with large uniform naOH pellets made in the following way.
Heat a volume of NaOH to 320C in a porcelain dish of sorts that has some sort of pouring lip. when uniformly hot and dehydrated, gently pour the caustic into a large chilled polypropylene beaker of dry toluene or xylene placed on the floor from bench height. pour off the non-polar and dry the pellets in a dessicator under vacuum. the pellets thus produced are hard, uniform and pack well, but be careful to use them only for the acid distillations/filtrations, hell can unfold should you not monitor the trap and replace/renew it as needed.

A poorer cousin to this method involves rolling large glass marbles in molten caustic and using these in the trap.
a rich aunt to this method involves a teflon pump and no trap.

molten caustic will leave your skin devasted, your eyes useless and your penis pierced, don't use molten caustic whilst naked, or in anyway not fully clothed and protected. seems a silly disclaimer but dick cheney shot his hunting buddy in the face so i guess humans need sometimes to be told simple things (such as 'don't shoot your hunting buddy in the face as it will injure them' or 'wear suitable protection when handling molten caustic')