Sciencemadness Discussion Board

vacuum distillation hole?

Db33 - 2-12-2016 at 20:14

ive seen alot of videos where, when someone is doing a vacuum distillation they talk about using a needle or something to poke a hole in one of the stoppers to create a little place for air to get in? Is this something thats suppose to be done on all vacuum distillations or only for certain circumstances? I thought a vacuum distillation was suppose to be completely sealed with no air or anything allowed to enter the system.

Phosphor-ing - 2-12-2016 at 20:31

Vacuum distillations require control of the vacuum. If you just sealed the system and pulled a vacuum the boiling point would continue to drop as the pressure did. This could result in some serious bumping. Allowing air in in a controlled fashion allows you to adjust the vacuum pressure.

violet sin - 2-12-2016 at 20:51

Now corect me if I'm wrong, but it can be either front to back or inline right? Meaning you technically could relieve some vacuum from the boiling flask, receiving flask or after all glassware on the vac line( in non-atmosphere sensitive materials). Trickier if boiling a flamable solvent.. prob dont want a ton of 02 and flamable vapor in a heated disty set. There must be a standard practice in that case. The point is just to make sure you don't pull too far down, as to have adjustable control within a range of applied vacuum, and subsequently boiling pt.

Same idea on a blower for a metal melting furnace. Vent some pressure onel it's way to the fuel in that case instead of sending full air all the time. Moderation of applied heat without having to swing full on/full off in a narrow window.

Sulaiman - 3-12-2016 at 03:16

if the reaction is sensitive to air or its components then the 'leak' can be after the vacuum takeoff at the receiver

but if the air does not interfere with the reagents, or the undesired part of the air (e.g. CO2, O2, H2O) can be removed,
then from what I have read,
the best is an air-leak via a tube that is beneath the surface of the liquid in the pot, as this controlls pressure and minimises 'bumping'.
e.g. https://erowid.org/archive/rhodium/chemistry/equipment/disti...

XeonTheMGPony - 3-12-2016 at 07:29

you can ballast the vacuum pump at the inlet, I simply loosen the conection at the vac pump to control my vacuum level.

How ever sweeping the vapors into the condenser can be an advantage, to achieve this I simply do not grease one of the joints at the still head so a small amount or air is sucked in and sweeps the distillate into the condenser.

For air free atmospheres it is trickier set up and Dry oxygen free nitrogen is used as the sweeping gas, but one at home could probably make a pure co2 atmosphere and use CaCl drying tube, I have don this with the age old baking soda acid method and a balloon as a gas resavour.

True of all distillations less heat is more! In other words start slow and steady, increase heat in small stages for good separation.

[Edited on 3-12-2016 by XeonTheMGPony]

BromicAcid - 3-12-2016 at 08:03

If you cannot stir/agitate your mixture or even use boiling stones a capillary can be introduced into your pot subsurface usually with an inert gas to help agitate the mixture. To make a capillary tube get a piece of glass tubing to fit a Torion or other tight sealing system and heat until red then draw it out to make two halves. Break off just the very tip of one of those halves to make a section with a hole smaller than a human hair. You will be amazed how large the bubbles are from that small of a hole.

Put that sub-surface and off you go. If you're putting a hole in a stopper somewhere in the system you're going to impact what the best vacuum you can achieve is without the commensurate benefit in agitation. In other words, worthless. If you do need to control your vacuum from the pump, you should admit air/nitrogen to your system after your traps and before your vacuum pump. In any instance where you are putting gas into the system (either through capillary, hole in stopper, or just unknown leak in the system) you are making a headache for yourself. I say this because you have a much greater chance of sweeping your product past your condenser/receiver or sweeping whatever solvent you are trying to collect in your traps straight to your pump. Then you're going to need to change your pump oil much sooner than later.

For me, whenever I need a vacuum distillation, the best vacuum is usually my goal and to admit gas into the the system and cause a poor vacuum as a result (even with the capillary your ultimate vacuum is impacted by several tenths of a mm Hg) would be a horrible idea. However I have also run reactions where it is a necessity to 'sparge' gas during the distillation to break adducts or removed complexed hydrogen chloride.

Essentially, there are reasons you were you might want to introduce gas subsurface, but there has to be a need and there will always be tradeoffs.

Magpie - 3-12-2016 at 11:38

One of the problems with these discussions is that of different goals among the commenters. Using a vacuum pump to exhaust an air-conditioner is a totally different goal than using it for a vacuum distillation.

In the first case capacity is important (time =money). This is important to air conditioning technicians. And that is what these cheap rotary vane oil lubricated/sealed pumps are designed for - the backyard mechanic wanting to evacuate and recharge his automobile air-conditioner.

But for vacuum distillation capacity hardly matters. How much air are you going to evacuate from a well-sealed system? The condensable vapor is, well, mostly all going to be condensed at the condenser and leaves the system as a liquid. So where is the need for capacity?

I never introduce air into my system unless to prevent/eliminate bumping, and only after first trying magnetic stirring. To introduce a tiny stream of bubbles I will install an ebulliator as described above by BromicAcid.

Going for the best vacuum possible allows distillation at the lowest temperature possible. That's why you do it: to lower the temperature and prevent thermal degradation of the distillate.

Introducing a lot of air into my system causes my pump to exhaust a lot of oil mist. That makes a mess and consumes oil. I minimize the ebulliator air/Ar intoduction.

Chemetix - 3-12-2016 at 12:13

When doing higher molecular weight distillations at higher temperatures with vacuum, I've found it practical to use a really fine capillary to inlet argon. No nasty oxygen to have it's wicked way with the organics and a sweep gas when you want some lighter stuff to come over. A fine capillary can still let you get down to 1x10^-2 mmHg. and prevent bumping. Some broken bits of glass in the neck of the still head works as a mist trap and given the colour of what I've seen stick to it, I can say it makes a difference to the final product.

Db33 - 3-12-2016 at 16:13

i meant to say that i HAVE a magnetic stirrer/hotplate and a heating mantle/stirrer so it would always have magnetic stirring. In THAT case, where im not using the air for agitation or boiling stones, THEN how likely is it that i need to introduce a tiny hole for air to get in? Magpie says they never do so im wondering if its fine if i do without it as well.

Magpie - 3-12-2016 at 16:23

An ebulliator, like described by BromicAcid, is a tiny hole that, using a glass tube, introduces a tiny stream of air (or inert gas) below the liquid surface in the pot. I have never introduced air at any other location.

Sulaiman - 3-12-2016 at 21:52

if I use my little -80 kpa vacuum pump (20 to 25 kpa abs.) with no air leak,
water boils and/or condenses at 60 to 65 C,
and the b.p. of most solvents is well below ambient,
if I use my rotary pump with no air leak
most of my products would not condense in an ice-water condenser
and go rampaging towards my precious,
so too little pressure is a problem for me,
hence the NEED for an air leak (or very good pump control, which would ).
so consider the b.p. (condensation temperature) of your product vs. condenser temperature and vapour presure
i.e. cryogenic condensers required for lower pressure/b.p. hence not tried by me.
I also have no experience with high m.w. compounds.
I just know what I can't do with my present abilities :(

[Edited on 4-12-2016 by Sulaiman]

BromicAcid - 4-12-2016 at 08:58

If you're using an air leak to control vacuum your setup should be:


System ---> Traps ----> Air Leak ---> Manometer ---> Pump

You didn't mention where your air leak (I usually call this a bleed) is located but the issue is when you have:

System / Air ---> Traps ---> Manometer ---> Pump

Because then you can sweep vapors to your precious pump oil and you make your traps / condenser work overtime.

Magpie - 4-12-2016 at 09:35

Quote: Originally posted by Sulaiman  
...if I use my rotary pump with no air leak
most of my products would not condense in an ice-water condenser
and go rampaging towards my precious,
so too little pressure is a problem for me,
hence the NEED for an air leak (or very good pump control, which would ).


Why would you use vacuum distillation in this situation?

Sulaiman - 4-12-2016 at 12:30

Why would you use vacuum distillation in this situation?
_____________________________________________
I wanted to reduce b.p. and I should have said 'reduced pressure' rather than vacuum.
. mainly because I'm inexperienced :)
. partly because I had a new pump
. partly just to see what happens

I have no vacuum control other than on/off for my rotary,
traps (Al bottles with oily steel wool to catch particles and liquids)
a simple 0 to -1 atm bourdon gauge, valves, piping etc.

forgot: the sub-surface air inlet needs a valve
I think that it does more than agitation,
I read that the bubbles provide a liquid/gas interface that allows easier vapourisation, reducing bumping.

Magpie - 4-12-2016 at 14:47

I have no way to control the vacuum when using my rotary vane pump either, other than to bleed air in ahead of the pump. But I hate doing this because of the nuisance oil mist that is generated. Normally I just let the pump do its thing and pull its best vacuum.

The few times I've wanted to decrease the vacuum is to match that published in hand books where they also give the temperature. This is a way to identify your product, ie, if your bp is the same as the handbook value at the reduced pressure (often 5-20mmHg) then that's good identification information.

The Sigma-Aldrich nomograph for T vs P usually is pretty good for this, but sometimes it's off some. Try it sometime ;):
http://www.sigmaaldrich.com/chemistry/solvents/learning-cent...

[Edited on 4-12-2016 by Magpie]

Sulaiman - 4-12-2016 at 15:40

:D I have :D