Sciencemadness Discussion Board - Woes with reduced pressure distillation

Woes with reduced pressure distillation

Keras - 9-5-2023 at 11:02

Folks,

that looks like a newbie (tyro, if you like posh British English) problem, but I can’t get a reduced pressure distillation right. Every time I get severe bumping that floods part of my apparatus. It seems the pressure inside the flask is not steady and somehow alternates between highs and lows. I didn't find much info on the right way to carry out a reduced pressure distillation, so here are a few questions directed to those who have more practice than I do in this field:

1. Do you use a regular water aspirator directly connected to the exit hose of the distillation apparatus, or do you buffer the vacuum line with a flask in between?

2. Is it mandatory to use stirring to avoid bumping?

3. Is the use of vacuum grease mandatory?

4. Any other trick to iron out the process?

Thanks!

[Edited on 9-5-2023 by Keras]

BromicAcid - 9-5-2023 at 13:32

Sounds like you already know your problems.

1. Yes you can connect an aspirator directly to your setup. The vacuum should level off and the boiling point of your material will be dictated by your vacuum. It will take a bit to find that balance because your vacuum off your aspirator is dictated to some extend by the temperature of the water so it has to get to steady state. Also if you're not condensing all of your vapors or your receiver is not cooled to the point to keep them in then you'll also do a bit toward killing your vacuum.

2. Yes, stirring is pretty much mandatory. You're describing exactly the issues with bumping in your post, pot bumps, pressure goes up because a lot of volatile material is released at once. Pressure drops once the bump is done, material super-heats since it is now under better vacuum, material bumps - repeat ad-infinitum. You can also do a capillary sparge which is a good choice for some thicker materials. You need a real capillary though, thankfully they're not hard to make using a length of glass tubing and a torch but you'll need some way to connect it to your setup. If you have a large flask and you're just trying to strip solvents I've used just boiling stones in the past but you're not going to get consistent vacuum and you will get bumping.

3. Vacuum grease - it's going to help. Not only getting better vacuum but also help you to separate the joints later. There are a lot more silicone based greases at the hardware store than there were when I was doing chemistry at home. You might want to pick up some.

4. Assemble your setup. Connect your vacuum to your empty setup and pull your vacuum on it - find out what your maximum vacuum is which will give you a better idea what your distillation should be running at. When you add your material to distill, don't just pull it under full vacuum, pull it down slowly put a tee inline or crimp your tubing with your hand and open it slowly. Your material will likely off-gas, the temperature will drop if things start to distill, if that happens just keep slowly bringing it under vacuum. Make sure you have water flowing through your condenser. Once you've got it under full vac compare where it is vs where the setup was when it was empty. Make sure everything is still in place, i.e., if you've got thermometers that are held in palace by gaskets make sure nothing has slipped. Once you're satisfied then you can start heating - slowly. If you've got stirring or a capillary it should be smooth sailing in terms of bumping but that doesn't mean you're not going to run into odd head temps, multiple fractions, etc. along the way.

Lionel Spanner - 9-5-2023 at 14:11

I'd also suggest using as large a flask as practically possible, so if the mixture does bump, it'll still be contained within the flask.

In my experience, highly concentrated solutions of salts in water are particularly prone to bumping.

Rainwater - 9-5-2023 at 14:26

Things I've learned the hard way.

start with the reagents pre cooled well below the tempature you want them to boil at.
heat very slow until you have experience with how the reagents/solution act for a particular setup.
slowly apply vacuum without heating. When bubbles form, close the vacuum valve, slowly open it until the bubbles are under control or stop. (Outgassing)
when using hard vacuum, wrap the apparatus in a layer to catch shrapnel. I use steel bugscreen for windows.
your pressure will never get lower than the reagents vapor pressure at its current temperature.(see #1)
things expected during atmosphere pressure distillation may not happen under reduced pressure. Example, sulfuric acids white mist and nitric acids brown gas, will not be seen under reduced pressure.
the pressure in your receiving flask will be lower than in the boiling flask, causeing loss of distillate, keep it as cold as possible
appratus under vacuum is much, much easier to break (see #4)

ErgoloidMesylate - 9-5-2023 at 17:43

Quote: Originally posted by Keras

Folks,

that looks like a newbie (tyro, if you like posh British English) problem, but I can’t get a reduced pressure distillation right. Every time I get severe bumping that floods part of my apparatus. It seems the pressure inside the flask is not steady and somehow alternates between highs and lows. I didn't find much info on the right way to carry out a reduced pressure distillation, so here are a few questions directed to those who have more practice than I do in this field:

3. Is the use of vacuum grease mandatory?

[Edited on 9-5-2023 by Keras]

There is a special teflon sleeve that is ribbed that seals joints for vacuum issues.

Keras - 9-5-2023 at 22:36

Thanks a bunch to all for your respective input, very instructive. ♡

Meanwhile, I found two interesting resources about vacuum distillation on the net, which can serve as further reference: A ‘reference’ page and a nice video.

They both state that bumping is inevitable, and they use a Claisen adapter to somehow isolate the flask from the condenser, so that the product won’t splash into the condenser. Also, they use magnetic stirring since boiling chips won’t work. TBH, I never used boiling chips, I use finely ground (~dust) glass. It doesn’t really work in that case either. The problem is, my heating mantle is quite bulky and I’m not sure stacking it over my hotplate/magnetic stirrer would work. I’m probably ‘doomed’ to use the hot plate/stirrer alone (which hopefully won’t pose a problem since heating is reduced). I've ordered a glass blower made Claisen adapter for the microscale kit I use.

Besides, I should definitely use grease (I’m not used to working with it, since most of my glassware has Rodaviss joints which – at least they claim – doesn’t require them).

Also, it’s interesting to know in what order the things should be set up (like, say, applying heat last to let the volatile solvents boil off first and let cool before the vacuum is broken).

ErgoloidMesylate - 10-5-2023 at 05:49

Quote: Originally posted by Keras

Thanks a bunch to all for your respective input, very instructive. ♡

Meanwhile, I found two interesting resources about vacuum distillation on the net, which can serve as further reference: A ‘reference’ page and a nice video.

They both state that bumping is inevitable, and they use a Claisen adapter to somehow isolate the flask from the condenser, so that the product won’t splash into the condenser. Also, they use magnetic stirring since boiling chips won’t work. TBH, I never used boiling chips, I use finely ground (~dust) glass. It doesn’t really work in that case either. The problem is, my heating mantle is quite bulky and I’m not sure stacking it over my hotplate/magnetic stirrer would work. I’m probably ‘doomed’ to use the hot plate/stirrer alone (which hopefully won’t pose a problem since heating is reduced). I've ordered a glass blower made Claisen adapter for the microscale kit I use.

Besides, I should definitely use grease (I’m not used to working with it, since most of my glassware has Rodaviss joints which – at least they claim – doesn’t require them).

Also, it’s interesting to know in what order the things should be set up (like, say, applying heat last to let the volatile solvents boil off first and let cool before the vacuum is broken).

Since you apparently enjoy grease in your solvents, mentioning the problems you are experiencing, and the ease of solutions, and affordability of solutions, knowing this can greatly limit any chemistry experiments you carry out, since you are an experimenter, it is inexcusable you persist somewhat beligerantly at doing it half ass and incorrectly, having been given the helpful info you asked for, almost as if you are saying FU guys, i will continue to do things quite retardedly.

[Edited on 10-5-2023 by ErgoloidMesylate]

ErgoloidMesylate - 10-5-2023 at 06:25

Quote: Originally posted by ErgoloidMesylate

Quote: Originally posted by Keras

To help you since you don't want to buy a special sleeve, go buy some teflon tape, wrap the male joint and snug it into the female joint, then wrap teflon tape around the outside, that should improve your vacuum.

Keras - 10-5-2023 at 06:30

Oh yes, I do have thin teflon tape, that could do the trick too. Nice idea