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Author: Subject: Practical Vacuum Distillation
chemrox
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[*] posted on 3-2-2009 at 00:13


@Ozone: please post the whole apparatus (or I can tomorrow)



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Ozone
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[*] posted on 3-2-2009 at 19:31


OK, let's see.

I usually plan the fractionation ahead. The usual, 1 forerun, 1 fraction; two are left for either other fractions or to catch the remainder that comes over. The header joint is well greased which enables it to be turned easily. You can use collection flasks of various size as you see fit. The spiky bits on the side of each teat are for the connection of springs which should serve to keep the collector from falling off. I just use Keck clamps.

To the best of my knowledge, the rubber tubing on the bottom of the aspirator serves to "muffle" the water jet coming out of the bottom. This prevents it from spattering all over the place (from the side *and* when it strikes the bottom of the sink) and making a mess. I've not noticed a difference in vacuum either way.

OK, I was at a conference all day so I'll put together a sample apparatus for photography tomorrow. [Actually, tomorrow I have day-two of conference and I am judging science fair thursday. this would have to be done on Friday, then.]

I thought the BSA was a nice touch;).

Moooo,

O3



[Edited on 3-2-2009 by Ozone]

[Edited on 3-2-2009 by Ozone]




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chemrox
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[*] posted on 3-2-2009 at 22:20


You have to grease the shit out of the joint or put a teflon sleave on it. NB there's an inner part not shown that direct condensate to the lowest "udder." I intended to post the whole apparatus but my camera ran low on batteries. I will post the pic tomorrow to make this very useful piece of equipment clear to all. BTW this isn't the only kind of glassware that turns on an ST joined under vacuum. McLeod Gauges are often set up that way.



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[*] posted on 3-2-2009 at 22:41


I have seen these before although it was three pronged. I believe that the "cow" is attached at the end where one would normally put a receiving flask and at each udder is a small receiving flask. Distill under vacuum and the fraction you distill falls into the receiving flask pointing down. When another fraction begins to distill over you turn the cow and the fraction is collected in a separate flask.



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chemrox
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[*] posted on 4-2-2009 at 11:43
the whole cow setup




cow-pics 006.JPG - 81kB




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[*] posted on 4-2-2009 at 19:35


holy cow!
:D
How is that placed on the apparatus?
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Ozone
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[*] posted on 4-2-2009 at 20:36


I'll post a photograph of a whole assembled apparatus tomorrow evening.

That's a big-ass cow, Chemrox! My setup is a little different. Both of my cows, 24/40 125mL and a 14/20 25mL, are male at the header so I have to use a "gender-bender". I also do not have the drip tube. While nice, and I want one, mine works pretty well just by gravity (it's not as big, though).

Cheers,

O3




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[*] posted on 4-2-2009 at 21:04


Thats a pretty cool setup.

Iv seen little pig adaptors but Iv never seen a cow.:D
Do you ever have problems with different fractions ending up mixing trace amounts in each other from the sides of the receiver?
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Ozone
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[*] posted on 5-2-2009 at 18:16


Okay, here's the whole thing. I might put up a picture of a rig with a vertical condenser tomorrow.

You are going to get trace amounts anyway (this technique is not perfect). It is still MUCH easier to purify a fraction than the whole mix (even with the tube, "heart cutting" is difficult).

Cheers,

O3

[Edited on 5-2-2009 by Ozone]

Fractionator_01_small.jpg - 412kB




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Ozone
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[*] posted on 5-2-2009 at 18:21


A closeup of the vacuum adapter, gender bender and outfitted cow.

If you look closely at the vacuum adapter, you will see where the tube used to be. The 'bender was a tight fit and eventually, the tube broke off:o. Bummer.

O3

cow closeup_01_small.jpg - 284kB




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[*] posted on 10-2-2009 at 02:33
Practical Vacuum Distillation


Thanks everyone for such nice pics and good infos........................Now it may sound a little stupid.Can I use a hand vacuum pump like this one in pic

Hand%20Vacuum%20Pump.jpg - 22kB
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[*] posted on 10-2-2009 at 17:45


Those are for vacuum filtration not distillation. This thread is about practical vacuum distillation. It's simply not practical to stand a set up pumping away for hours only to pull a slight vacuum, get carpral tunnel and then have the thing break on you.

Get an aspirator or a vacuum pump.

I have an aspirator but no sink in my workspace so I need to totally rearrange my garage before installing a sink and finally vacuum distilling my nitric acid that has been in my freezer for the past three months.




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chemrox
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[*] posted on 10-2-2009 at 18:05


get an aspirator or for a little more money an aspirator pump. Both have their uses. An apsirator pump has a water reservoir and a mechanical pump that pumps water through internal aspirators. No need to run up the water bill but the e bill suffers a bit.
@O3- ours aren't all that different. I went 250 on the flask for the cow and my receivers are 75 ml with 14/20 joints. I forgot to have him put spring holders on them so I need some 14/20 Klecks. I like having the flow director with the larger volume cow but at 125 you might not need it. I like having the white board right there where you work!! I have written on the cabinets and windows and parts of the floor...
I noticed you're using a vigeraux. I saw a dramatic improvement when I started using a Snyder column. The distillation I have planned is for something that boils around 168/0.3 mm so for that I'm using a short path with a 1" built-in vigeraux. I could show the vacuum station but I don't want her to despair.

[Edited on 10-2-2009 by chemrox]




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[*] posted on 10-2-2009 at 21:15


Yeah, mine's 24/40. I have ~ 1m worth of Snyder ball columns which are very nice to use--but I only really use them for solvent recovery. I find that it helps to drop some of the solvent into the top of the column (wet the column) prior to use. I suppose I could add a pic of some Snyders tomorrow.

I also have straight wall columns which I pack with steel wool, glass balls, etc. which perform better than the vigreaux. The vigreaux, however...is easy..and usually works very well. It's a great place to start.

Maybe we should address vacuum manifolds...later.

Nothing like the sound of chattering Snyder columns in the morning!

O3




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[*] posted on 11-2-2009 at 19:47


"Nothing like the sound of chattering Snyder columns in the morning!"

hehehe .. yeah that's the best!

I got a three cock manifold made for $45. I can't understand how Ace and Kontes even, charge so bloody much for those things...even with the high vac cocks. It probably took the glassblower about 10 minutes. I benefit from the guy (my glassblower) being really busy. When it doesn't make it impossible to get repairs on short notice that is. Like now. I'm working with a new guy I got through the U. We'll see how he does.

[Edited on 11-2-2009 by chemrox]




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[*] posted on 21-3-2009 at 17:36


I use vacuum setup based on rotary oil vacuum pump, it can provide vacuum up to 0.1 mm. Hg. However setups you shown in this topic look somekind unusual for me.

1. As far as i can see in provided setups nobody uses manometer or other pressure control. Use of pressure control is absolutely essential because pressure greatly affects boiling points, if pressure in system will be too low, even high boiling liquid can destill off just about room temperature and there will be great difficulty to condense it even in long condenser cooled by cold water stream. Before preforming vacuum destilation you must at first select work pressure range such that lower border pressure will provide high enough boiling point of reaction mixture to condense distillate efficently and high border pressure that allows to destill your mixture with selected heat source (e.g water or oil bath) at acceptable rate while not allowing destilled substances to undergo thermal decomposition (for example HNO3 is thermaly unstable). Apropriate pressure range can be selected from vapor pressure / temperature graph for distilled substance.

2. I don't see boiling capilary in your setups, and this is very strange, because it's absence makes vacuum destillation unstable and even dangerous. Under reduced pressure gasses that are dissolved in reaction mixture quickly escape from reaction mixture and seed gas bubbles esential for stable boiling no longer form - liquid can easily become overheated. This is absolutely inacceptable because overheated liquid mixture sooner or later will boil up with great violence, it can jump over to condenser outlet and in some cases even explosive boil up is possible (heat explosion). To solve this problem a boiling capilary is used in vacuum setups, this is thin glass tubbing with very small hole in the end that is passed almost to the bottom of distilation flask, then system is evacuated it passes stream of small air bubbles from atmosphere witch aid phase chance and allow liquid to have stable boiling. This additional air inlet is also used to fine tune pressure by means of gas valve connected to the top of the inlet capilary.

3. I don't see any trap devices between vacuum outlet and vacuum pump. If you are using water stream vacuum pump if pressure in water system will suddenly change water from the pump can easily jump into distillate flask and cause some nasty chemical reactions (especialy if destilled substances react violently with water). If you are using oil vacuum pump some uncondensed fumes can reach pump oil and dissolve in it to lower maximum vacuum, or if fumes are aggressive they can react with oil or corrode parts of vacuum pump. In usual setups there are trap flasks between vacuum outlet and vacuum pump. In case of water stream pump trap serve as emergency drain in case of water stream pressure fluctuations, and in case of oil pumps this trap flask is usualy strongly cooled (for example with solid CO2) to freeze any vapours not condensed by condenser and not allow them to enter pump oil or leave evacuated system.



[Edited on 22-3-2009 by Engager]




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[*] posted on 25-3-2009 at 01:48


Since I intend vacuum distilling and use of a rotary evaporator with an oil pump as well (<1mbar), I was wondering if I could replace the dry ice/acetone cold trap with an 60 cm graham condensor. (water on the outside) This would be cooled with an refrigerated circulator bath, this one http://www.capovani.com/dp/cat/107/63160/iinfo.cfm?LCl=986&a... to be more precise. It would be running with water/EG at -5 till -8 degrees celcius. Is this sufficient to prevent solvent from reaching the pump oil? The rotavap/liebig cooler would also be cooled with this temperature.



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[*] posted on 25-3-2009 at 02:54


That all depends on what you intend to distill. Look up a graph on the BP of the solvent with pressure plotted in, and cross-check that with the temperature of the condensing water and the efficiency of your condenser.
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[*] posted on 3-4-2009 at 22:30


Quote: Originally posted by Gruson  
Since I intend vacuum distilling and use of a rotary evaporator with an oil pump as well (<1mbar), I was wondering if I could replace the dry ice/acetone cold trap with an 60 cm graham condensor. (water on the outside) This would be cooled with an refrigerated circulator bath, this one http://www.capovani.com/dp/cat/107/63160/iinfo.cfm?LCl=986&a... to be more precise. It would be running with water/EG at -5 till -8 degrees celcius. Is this sufficient to prevent solvent from reaching the pump oil? The rotavap/liebig cooler would also be cooled with this temperature.


Remember that although something may be well below its boiling point for a given pressure it still may have some, even significant, vapour pressure and as such will eventually end up in your pump.
For most common solvents this is not really problematic as your can bleed them from your oil easily enough (assuming a vane pump here). Especially if using a fully synthetic vac pump oil. However for distillations of more reactive substances you really want to avoid any contamination into your pump and -8 ain't gonna cut it even for moderate vacuums. That is why the dry ice cold finger is standard good practise, however its annoying for the home experimenter because you must have the dry ice around and it has this silly habit of subliming away and warming up the earth.

I posted recently in relation to small domestic benchtop ice cream makers being an excellent alternative to the dry ice cold finger. They run continuously (i leave mine on all week), sit at around -38 and negate the need for the consumable dry ice. I just fill mine with ethanol and immerse the trap into it.




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[*] posted on 6-4-2009 at 00:11


That is an excellent idea. I presume you have the icemaker with built-in compressor? Which type exactly? (I believe you have a "Nemox"?) I can get my hands on dry ice, but that would be a 70 km drive. Okay for one particular synthesis, but I was looking for a solution on the long term. Does ethanol has the heat capacity needed for efficiently cooling the cold trap?



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[*] posted on 6-4-2009 at 00:55


Quote: Originally posted by Gruson  
That is an excellent idea. I presume you have the icemaker with built-in compressor? Which type exactly? (I believe you have a "Nemox"?) I can get my hands on dry ice, but that would be a 70 km drive. Okay for one particular synthesis, but I was looking for a solution on the long term. Does ethanol has the heat capacity needed for efficiently cooling the cold trap?


The reason i leave it on all week is it has about a 20min warm-up time (ha funny to describe it as this) before it is at it lowest temperature. This solution is not as good as dry ice obviously, but many more times convenient and permanent. I use ethanol because i have it and didn't want to use any solvents potentially aggressive to the polymers of the unit, whatever they may be. If your unit is fully cooled it works fine.





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[*] posted on 6-4-2009 at 18:36


A great OTC method I've adapted uses a Reynolds Handi-Vac, hand held vacuum pump, and have had decent results. Not the same as my carbon vane, but it seems to be able to deal with the organic vapors so far, and is handy and easy to grab real quick. Maybe not for critical sensitive work, but surely to speed things along which otherwise are slow, but not necessarily all that heat sensitive without it.
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[*] posted on 7-4-2009 at 08:20


What about the crushed ice with calcium chloride hexahydrate in ratio 2:3, i read that it achieves temp. about -40 degrees celsius...would that be sufficient (for example for DCM)? And also calcium chloride is quite cheap.

[Edited on 7-4-2009 by marksev1]
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[*] posted on 19-7-2009 at 20:26


Would this be the right thread to ask questions about boiling butane (or ammonia) from water using Mazzei Injectors? I can start a new thread if I need to. I want to know if such aspirators are capable of the task?

Here's the information on these injectors:
http://www.mazzei.net/products/injector_info.htm

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[*] posted on 19-7-2009 at 22:09


Is your goal to purify the water, or to recover the butane/ammonia?

For simply removing the gas, bubbling air through the water might do the job, use a packed column with counterflow of air and water.
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