Sciencemadness Discussion Board

Distillation Under Reduced Pressure

anacharsis - 5-12-2010 at 14:09

I cannot for the life of me find an answer to exactly what it means in experiments as to the “0.3mm/Hg” in the following statement: “The residue was distilled at 105-110C at 0.3mm/Hg.”

I have a Rocker 600 vacuum pump that I use for filtration. The Rocker 600 has a dial with markings from 0 to 760mm/Hg. At it’s lowest performance, it runs at around the 240 marking. On it’s highest vacuum, it registers around 750. Now if atmospheric pressure is 760mm/Hg, I’m assuming that my pump, when it reads 240, means that it is pulling 240mm out of 1 atmosphere, or 240mm out of 760, or 760-240, or an atmospheric pressure reduced to 520mm/Hg. And when it reads 750 it is pulling 750mm out of 760mm – almost a complete vacuum.

So, does this mean that 0.3mm/Hg means that .3mm are being pulled out of 1 atmosphere or out of 760mm/Hg, and that the pressure is now 759.7mm. This would mean that a 0.3, or I’ve also seen 0.4, 0.2, 0.1 would be a very slight vacuum but apparently sufficient enough to reduce the boiling point of a material down to 105-110C or whatever.

Is this right? Any help would be appreciated.

entropy51 - 5-12-2010 at 14:23

You're correct about your pump. It is reading the differential pressure, the difference between the pump inlet pressure and atmospheric pressure.

10 mm is not an almost perfect vacuum. The pressure is 10/760 = 0.013 atmosphere.

0.3 mm is an absolute pressure, not a differential pressure. The pressure is 0.3/760 = 0.00039 atmosphere.

See here and the other hits from Googling "pressure measurement".

watson.fawkes - 5-12-2010 at 15:25

Quote: Originally posted by anacharsis  
“0.3mm/Hg”
To slightly pick at nits, the name of the unit is "0.3 mmHg", not "0.3mm/Hg". There's no "/". If you're using Google to do units conversion, as with the query "0.3 mmHg in atm=", it matters. Warning: sometime you see pressures quoted in mm or microns. These are mmHg and μmHg, respectively.

The unit comes from the use of mercury barometers. The unit of pressure "torr" is more-or-less a synonym for 1 mmHg, and named after Torricelli, who invented the barometer.

Magpie - 5-12-2010 at 16:55

Quote: Originally posted by anacharsis  

So, does this mean that 0.3mm/Hg means that .3mm are being pulled out of 1 atmosphere or out of 760mm/Hg, and that the pressure is now 759.7mm. This would mean that a 0.3, or I’ve also seen 0.4, 0.2, 0.1 would be a very slight vacuum but apparently sufficient enough to reduce the boiling point of a material down to 105-110C or whatever.


A pressure 0.3mmHg below atmospheric pressure would be a negligible vacuum and would not lower the boiling point a noticeable amount. Swings in the weather can change the atmospheric pressure by 10mmHg.

anacharsis - 5-12-2010 at 16:59

So does this mean that 0.3mmHg is 760mmHg reduced all the way down to 0.3mmHg, which would be an almost complete vacuum?

Magpie - 5-12-2010 at 17:01

yes

anacharsis - 5-12-2010 at 17:06

Thanks for the answer to this. My next question, then, is if I have a 24/40 distillation apparatus set up, with a vacuum receiving adapter on the end, how do I measure that the vaccum within the system is 0.3mmHg? And secondly, what kind of pump do you have to have to achieve this level of vacuum? Even my Rocker 600, which is a really nice pump, doesn't create a vacuum to 0.3mmHg. Thanks for the help.

Magpie - 5-12-2010 at 17:15

I will leave the answering of those questions to those who deal in such low pressures. Myself, I don't go below what my water aspirator can provide which is ~75mmHg, which I can measure with a cheap gage from the hardware store.

spirocycle - 5-12-2010 at 17:48

Quote: Originally posted by anacharsis  
Thanks for the answer to this. My next question, then, is if I have a 24/40 distillation apparatus set up, with a vacuum receiving adapter on the end, how do I measure that the vaccum within the system is 0.3mmHg? And secondly, what kind of pump do you have to have to achieve this level of vacuum? Even my Rocker 600, which is a really nice pump, doesn't create a vacuum to 0.3mmHg. Thanks for the help.


if you didn't know how to read the pressure on the experiment report, its pretty likely you dont know the pressure rating on your glass.
you can use a nomograph to adjust the boiling point for a pressure achievable by your vacuum or aspirator.
sigma aldrich has a nice interactive one online
you wouldn't want an implosion

anacharsis - 5-12-2010 at 18:45

Hey, thanks a lot for a great web site - the Sigma Aldrich nomograph. Let's say my glassware will support that level of vacuum, 0.3 mmHg, are you familiar with what kind of pump you would need that would go down to that level?

Justin - 5-12-2010 at 19:07

a good rotary vacuum pump will pull down to that, you can get the kind that are made for HVAC on ebay new for around 150USD, they will pull down to around something like the level your talking about, but honestly you don't need to be at exactly 0.3mmHg unless the compound your distilling will decompose above 110C. Look up the decomposition point of your compound and that will tell you what kind of vacuum you need to keep your compound intact.

[Edited on 12/6/2010 by Justin]

Ephoton - 6-12-2010 at 04:07

true justin but often glass setups that I have seen dont let the vac get there.

small leaks in things like your ground glass seals from not using the right

grease or from your thermometer input can make it near impossible to get

too 1 mmHG.

I have got down that far with a nice javac or other industrial vacuum but

the cheap ass monster vacs forget it.

ive had quite a few vacuums now and the cheap ones are exactly that.

cheap.

high vac takes a bit of effort if you ask me.

spirocycle - 6-12-2010 at 04:15

its usually not worth it either unless your compound will decompose, or boils at too high a temp

anacharsis - 6-12-2010 at 06:50

If the cheap vacuum pumps don't cut it, can you give me the name and/or exact type of pump that you're referring to when you say "a nice javac or other industrial vacuum ", that will will get down to 0.3mmHg. I want to be able to look into getting one. Thanks.

watson.fawkes - 6-12-2010 at 08:13

Quote: Originally posted by anacharsis  
If the cheap vacuum pumps don't cut it, can you give me the name and/or exact type of pump that you're referring to when you say "a nice javac or other industrial vacuum ", that will will get down to 0.3mmHg. I want to be able to look into getting one. Thanks.
From the bane of every US tool fetishist, Harbor Freight, here's an example of a cheap vacuum pump. Ultimate vacuum is listed at 75 micron, or 0.075 mmHg. That's a best case scenario. If you're looking for 0.3 torr, that's 300 micron, which isn't a lot of leeway for non-ideal situations.

But then again, I don't think your next step is to get a new pump immediately, but to measure the ultimate vacuum of your current pump. To do that, get a gauge and pump it down with nothing else in the system. At these levels, ordinary gauges may or may not work. By far the most common commercial use of this level of vacuum is refrigeration service, so it's a decent place to start. All you need at this point is a single gauge and some plumbing fittings. An easy, if not really cheap, way of getting such a gauge is to use a manifold gauge set such as this one from HF. At the least, you can see what such a gauge looks like.

Then, once you've measured ultimate vacuum, you can test your glass and see if any of the joints are out of spec and leak. I've heard tell that cone grinding on the tapers isn't as accurate or careful as it used to be.

Ephoton - 6-12-2010 at 08:39

I think thats the key too watson.fawkes :)

I think mostly its in the glass unless you have a vacuum that pulls a lot of
liters per minute.

The vacuum will pull to a graded micron no problems but if you have a leak you
then wish to have a vacuum that pulls a lot of liters to make up for this.

as for vacs :) go the shark it realy does look cool and it is made in
Australia (javac)


most decent vacuums I have used have cost around $1000 +



an addition to my post :



on looking at that vacuum watson it is truly crap.

I would not use it nor recommend it.

it looks to be on par with the chinese knock offs like
the monster vac.

just check out some of the reports on the same page

you posted.

12in of Hg MAX! Review by Unhappy Customer


According to my vacuum gauge, this pump maxes out at around 12" of Hg... Far from being useful as a true vacuum pump.

(Posted on 9/20/10)
ok Review by Matt


pump pulls the expected -28 inhg in short order and thats good.

however the connecting rod bearing, or bearings failed almost immediately.

and yes the pump housing has been filled with the supplied oil from the beginning

failed in less then 15 minutes.

(Posted on 9/19/10)



I would recomend one of these :)
www.javac.com.au/docs/javac_shark.pdf

or buy a vac from these guys ive used three of there vacuums
and there one of the best ive used.
http://www.robinair.com/acsolutions/acvacuum/acvacuum.php


you dont want to skimp on a vacuum if your serious about
reduced pressure distillation.

fair enough if an aspirator will do the job but then why get
an oil pump if you can use an aspirator for the job.


ive made a lot of aspirator bucket type setups as well.

there all in the water pump and they can be very pricey too.
personally here I would go with an onga.


[Edited on 6-12-2010 by Ephoton]

anacharsis - 6-12-2010 at 12:09

Thanks for all the great info. I actually understand this now. I checked out the Shark pump, it looks really good and goes down to 10 microns, which being .01mmHg should be more than sufficient.

By the way, what got me into this is I was doing a synthesis of piperine and one part of the procedure was "When the reaction is complete, remove the reaction flask and distill off -4 mL of the CC4. Using a vacuum pump, distill the residue, which is methyl 4-bromo-2-buteuoate (93-102T @ 15 mmHg)..." So at that point, I just used a regular vacuum which (I now know) wasn't even close to 15mmHg and I ended up getting zero product. Will try it again when I get an appropriate pump.

Ephoton - 6-12-2010 at 12:20

ye experiance kills spelling

thats why our mods are some of the best around !!!!


best of luck :)



[Edited on 6-12-2010 by Ephoton]

Buying from the Horrible Fright

watson.fawkes - 6-12-2010 at 14:03

Quote: Originally posted by Ephoton  
on looking at that vacuum watson it is truly crap.

I would not use it nor recommend it.

it looks to be on par with the chinese knock offs like
the monster vac.
Oh, I have sympathy for the sentiment. HF is good for several things, one of which is not the criterion "works the first time, every time". It's almost certainly of Chinese make. The biggest problem with such things is consistency. Some of them are just fine; other aren't.

For precision mechanical things like a vacuum pump, it's best to consider HF as parts kit that comes pre-assembled, which might require rework to get something functional. As long as you're willing to accept that, it's a good bargain. If you don't know how to take apart a vacuum pump and debug it, spend more money and don't buy HF. For me, I don't think much anything about disassembling a gear box and re-packing grease, which, as I vaguely recall, I had to do on a HF angle grinder. Other people may be unable or unwilling to do that kind of thing.

For my money, if you want to learn about how a rotary vane pump works, you can consider the $100 for that pump the cost of tuition for a complete teardown. It will probably even work when you're done.

Ephoton - 7-12-2010 at 17:12

yes thats my point exactly.

why buy something to work with dangerous chemicals that you cant trust.

its like working with ether with an open flame in my eyes.

ill never buy a chinese pump again.

some chinese glass though is realy good and holds great vac :)

arsphenamine - 10-12-2010 at 12:38

Sometimes, you can get a Welch 1400 or 1405 2-stage rotary vane pump from eBay. The venerable Welch 1402 is probably overkill for the home experimenter.

It's @#$%&! heavy, too.

I would check the Welch Site to see if rebuild kits (vanes, seals, & gaskets) are available before committing to a purchase. This can add $100-200 above the acquisition price but is still preferable to buying new.

Solvent traps sometimes fail and solvent fouls the pump oil.
In a pinch, you can use single viscosity automotive oil such as 30W or 40W.