zig
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help needed with vac distillation of a carbonyl
Hi all,
I've run into some annoying issues whilst trying to vac distill a high boiling carbonyl.
Once vacuum was applied and heating started, small volumes of lower boiling liquids expectedly did their thing and ghosted over. OK cool, that's what
I was expecting!
The carbonyl continued to be heated ~4hrs, with little indication of a boil on the horizon. Check the temp of the oil bath -DAMN!! 270C!! Meanwhile my
WORTHLESS distillation thermometer shows 70C.
Now I'm aware thermal lag exists between the heater and the heated, but seriously? That must not be right.
My pump has never given me problems performing similar operations in the past, and appeared to be working fine.
Any input, smart and kind humans?
[Edited on 28-1-2014 by zig]
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macckone
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Is your thermometer in the liquid or in the vapor space?
If little boiling is occurring a thermometer in the vapor space is
going to be cooler than the liquid by a considerable margin.
Do you have a vacuum gauge attached?
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zig
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Partial immersion thermometer was set in the vapor zone.
Vac source was attached to a manifold with pressure gauges. I hate to admit it but I've never much monitored the vacuum pressure (I know, I know..) as
I have never had an issue distilling similar high boiling organics
Quote: Originally posted by macckone  |
If little boiling is occurring a thermometer in the vapor space is
going to be cooler than the liquid by a considerable margin.
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So, the thermometer temp can have that large of a lag behind the solution temperature? I assume the thermometer begins to rise quickly as the bp is
approached, with higher quantities of hot/gaseous carbonyl molecules floatin around n stuff.
..so is it normal for the thermometer reading to stay stable (and low) during most of the heating?
On another note, is there any rule of thumb for the temp differential inside and outside the distillation flask? Sure would be nice to approximate the
solution temperature mid-distillation without removing the flask and taking its temp.
Sorry for the Spanish inquisition, I'm still a bit green on detailed lab procedures. Your help is hugely appreciated!
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zig
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After refreshing the vacuum oil, the gauge read that it was pulling 21inHg. Which equals, if my math is right, a measly dismal 533mmHg. Explains
things a bit.
I noticed that while the pump is running, the oil level in the window rises well past the fill line. Is this normal?
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BromicAcid
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As best I can I will help you. Take what I say with a grain of salt, I have done hundreds of distillations but it's inevitable that someone will have
a different experience or disagree with what I say. I've noticed that for some reason distillations are a very deep, almost religious experience and
few experts take well to a different point of view even if both can be correct.
That being said ... 
You mention your thermometer in the vapor space lagged significantly behind your bath temperature. That is to be expected, as macckone said, if your
material is not distilling then there are no vapors to hit your thermometer, remember a vacuum is a good insulator. If you have a well sealed setup
and you're pulling a good vacuum your head temp should not budge until something starts to distill from the system.
Do you have a column on your setup? I've had so many times distilling some high boiling compound with a column wrapped in heat tape where I had every
indication that the material was distilling. For hours with nothing coming over, only to remove a portion of the tape and see the vapor line was just
below the head and hidden by the heat tape. Once that first bit comes over, that's the hardest part and it's usually clear sailing from there.
Regarding temperature differential inside and outside the distillation flask, it depends on how fast you are heating and what your stirring is like.
Good stirring and holding temp everything will come to equilibrium unless you begin to distill material. Distillation will remove energy from the
system, in a good distillation an average of 10C differential is to be expected. Your results will of course vary based on a number of factors.
As for your vacuum pump, yes, 533 mm Hg is garbage. Is that the pump on itself or the pump on the setup? Did you have traps inline to catch any low
boilers? No, your pump should not be that full of oil (in my experience) and that could be the cause for some poor vacuum but usually it just means
that stuff will blow out the exhaust. This may or may not be your circumstance but usually when I see poor vacuum and overfull pump oil I think
someone sucked solvent to their pump. Solvent in the pump is a good way to kill your vacuum though usually not to that extent, then again it depends
on your pump.
Does the oil level suddenly bounce all over when you turn on your pump (when it is sealed)? Could mean that you have a leak somewhere and are sucking
air.
One step at a time. Drain your oil, refill. If oil is a valuable commodity then you can try to recycle it. Check the vacuum on the pump itself with
nothing attached. If good check to the setup. If everything vacuum checks then begin icing down traps and heating slowly. If you get to your target
boiling point, both pressure and temperature then go slowly, if your vacuum begins to get worse again then your compound may be decomposing, after all
some carbonyl compounds are unstable (wait, are you talking organometallic carbonyls?). Make sure you have good stirring to break the surface tension
and give good heat transfer and prevent bumping.
If it's not coming over at your desired vacuum and temperature don't keep hammering on it with more heat, decomposition does happen and it can kill
your product or kill your vacuum pump or... probably not kill you, at least if this is an established procedure. Only seen a runaway once during a
distill... twice... maybe three times during a distillation.
Edit: Forgot this tip. Insulation! If your compound really does distill in the 200C neighborhood, insulate your pot, insulate your column, insulate
your head. You may never get it over without lots of insulation because you will lose too much heat. I've seen plenty of people use just the
fiberglass insulation if needed though it makes a mess if you drop it in an oil bath.
[Edited on 1/29/2014 by BromicAcid]
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zig
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Holy molybdenum Mr. Acid! Truly you are ghe nunz bunz. I really appreciate you taking the time to type all that- you've hit me with some seriously
helpful knowledge.
I wasn't previously aware of a vacuums insulational properties. So THATS the deal with these vacuum-jacketed column things I keep seeing mentioned..I
hoped that their superiority was due to magic stored within the jacket.  Bitter
disappointment once again, damn!
I had no column in place. Short path. No insulation either..you mentioned fiberglass..what else might be used? Foil?
Anyway, I suffered a massive brainfart whilst typing up the op (though doctors say I'll be fine with water / rest). My current (new) vacuum which has
yet to please me in any fashion has not been used to distill solvents. My previous pump worked well on other high boiling crap, until it decided to
follow the bright white light a couple weeks ago. I never used cold traps because I got the thing for free, it was ancient, and I was in the market
for a new one.
That's to say there shouldnt be any solvent in my pump, but I'm reminded I need cold traps before trying anything too sheisty..
I'll update today after checking out the pump. Did I mention that after running for 30-45min, it starts to smoke up the house? That can't be a
feature..
[Edited on 29-1-2014 by zig]
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vmelkon
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| Quote: Originally posted by zig | I had no column in place. Short path. No insulation either..you mentioned fiberglass..what else might be used? Foil?
[Edited on 29-1-2014 by zig] |
You mean aluminum foil? How is that going to insulate?
I have used a old pair of pants. I cut it and wrapped the cloth around my glassware. Use water cotton cloth you have. Perhaps some tissue paper would
work.
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Crowfjord
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I've used socks before, wrapped and tied around the column and/or still head. In o-chem lab, they taught us to wrap with insulation, then cover that
with aluminum foil.
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zig
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Well now I know there's solvent in the resevoir- lots of fucking water.
Through some unholy miracle the unused red hose found its way into my water recirculation bucket. As all the vac / hosing / water circulation runs
under my workspace out of sight so in blissful ignorance I hit the switch and learned the sound of a waterboarded vacuum pump. Lotsa lessons learned
yesterday.
After dying out the manifold and tubes, I went out and got a slightly nicer vacuum (3 cfm vs. 2.5, 1/4 hp vs. 1/6), which seems to pull close to the
same lame-ass 533 mm Hg. It seems like the issue lies in the setup.
| Quote: Originally posted by BromicAcid |
Does the oil level suddenly bounce all over when you turn on your pump (when it is sealed)? Could mean that you have a leak somewhere and are sucking
air. |
The oil levels of both pumps did jolt strongly when powered on, but only briefly. This indicates a leak?
Is this done by simply closing both valves and running the pump connected centrally (normally), or does it need to be connected to the barb under the
gauge?
| Quote: Originally posted by BromicAcid |
If good check to the setup. If everything vacuum checks then begin icing down traps and heating slowly.
[Edited on 1/29/2014 by BromicAcid] |
With no apparent leaks, is there a way to sniff out the faulty connection? I do use corning high vacuum grease on all joints.
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zig
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Figured I'd provide closure for those curious.. after hours of defcon10, painstaking leakproofing the system, turns out the issue was a faulty gauge
reading an incorrect value. Problem solved by ditching the worthless manifold and measuring the vacuum strength via the bp of water.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and vapor, soot and flame,
poisons and poverty; yet among all these evils I seem to live so sweetly that I may die if I were to change places with the Persian king."
-Johann Joachim Becher
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testimento
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You really need to push into high vacuum level, since there is very little variation in boiling point at medium vacuum. At 500mbar water boils at 80c,
and 50mbar its 17 and 10mbar at -27c. When you've got a stuff that boils at 400C, you would need to apply 100 microbars (0.1mbar) in order to get it
over at 145C. I understand that oil industry uses high vacuum in large scale to distill heavy paraffins without problems.
Of course if your stuff is in boiling range like crude oil, you'll need to distill it in phases, first the lower boiling stuff and then the higher,
because the formed vacuum will be obviously so low your shorter chain stuff will evaporate into your pump. You'd use for eg. 10mbar for first fraction
and then 0.1mbar for the rest. This will be PITA if you haven't got good vacuum regulator, though.
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