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Author Topic: Tips for Overcoming a Poor House Vacuum?  (Read 4246 times)

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azmanam

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Tips for Overcoming a Poor House Vacuum?
« on: June 02, 2011, 02:36:08 AM »

I'm starting research at my new home and we have two choices for reduced pressure: high vac mechanical pump ... or the house vacuum line.  The high vac is good and does what it's supposed to.  The house vacuum... not so much.

I needed to vacuum distil my benzaldehyde (bp 178) through a short path to purify it for my reaction.  With the house vac it finally distilled at 135.  The anilne (bp 184) distilled at 141.  Quick nomograph conversion gives me a house vac pressure of ~250 mmHg.  This is not going to cut it.

At 250 mmHg, I cannot remove ethyl acetate on my rotovap.  I can't do quality, timely research if I have to turn the rotovap bath to 60 to remove EtOAc (and even when I do, it condenses in the *Ignore me, I am impatient* trap and cascades back into my flask.  Who knows what nasties are in the *Ignore me, I am impatient* trap of our (communal) rotovaps.)

Ok, complaining aside.  Does anyone have any tips when my two vacuum options are 250 mmHg or 0.1 mmHg?  I don't really want to have to go get liquid nitrogen to set up the high vac manifold to vac down my column fractions (we don't have house dry ice, plus the high vac manifold doesn't have a spinner).  Any tricks anyone knows to aid in my vacuum problems?
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Honclbrif

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #1 on: June 02, 2011, 03:15:11 AM »

I don't know if you'll be able to boost the house vac in any way. For applications between high vac and sink aspirator (i.e rotovap and difficult filtrations) my lab uses a diaphragm pump and its magic. I think its rated at around 75 torr and it will take off most common lab solvents without heat, but I set the bath to 35° to keep things from icing up. Set the bath to 50° and it will take off water, acetic acid, and even DMF (DMF goes very slowly, obviously). They have the added advantage of being very low maintenance and quite chemically resistant. You may be able to find a rebuilt on for a reasonable price.
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Dan

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #2 on: June 02, 2011, 06:50:04 AM »

Wow, that house vacuum sucks. I assume you have eliminated the possibility of leaks already?

How about a water aspirator? They're cheap and cheerful, though do use a fair bit of water. I've pulled off toluene at 40oC on a rotavap with a water aspirator, so they work.
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Honclbrif

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #3 on: June 02, 2011, 07:14:50 AM »

Was it a recirculating water aspirator? I've used one of them and it was pretty good too. It would definitely ice up the outside of an RBF of EtOAc. Don't know how they compare price wise to a diaphragm pump, but I would imagine they're probably cheaper. They do require a trickle of water through the basin to keep things fresh so you'll need a trap in front of it to keep solvents from going down the sink.
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azmanam

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #4 on: June 02, 2011, 09:47:27 AM »

Man, what I wouldn't give for a water aspirator at this point.  I suspect Dan was referencing a screw-into-the-faucet old school water aspirator.  I mentioned this in passing to my boss (as in 'ya know, even a water aspirator would pull better than the house...') but was shot down.  1) it wastes too much water and 2) if vapors don't get caught and re-condensed in the cold finger, we don't want solvents going down the drain.  So that's out.

If there are any leaks, it's between the tap and wherever the actual vacuum is.  There's only a short section of tubing from the tap to the rotovap.  It's definitely a building-wide problem.  All labs complain of the lack of adequate vacuum.

A diaphragm pump would be really nice.  I know that new they can be quite expensive, but I've never tried to look look for a used one.  We're a small-ish department in a small undergrad university, so we don't have oodles of funds.  If I ever get my own grant, maybe I'll buy one for myself.  :)

Today, we did ok with getting 'most' of the EtOAc off with the house vac, then pumping the last of the solvent off on the high vac manifold.  There was one column fraction that was ~125mL.  We left it on the rotovap over lunch (which is probably what I would have done in ideal circumstances, anyway) and most of the EtOAc was gone at ~45degC.  The high vac did ok with the rest.

I think I'm stuck making the best of the situation at hand.  If anyone knows any other tricks or workarounds, that'd be awesome.  Thanks for the advice so far :)
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Dan

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #5 on: June 02, 2011, 12:48:09 PM »

Was it a recirculating water aspirator?

Nope

I suspect Dan was referencing a screw-into-the-faucet old school water aspirator.

Yeah, one of those - they basically cost nothing, maybe £10, whereas a recirculating water aspirators are going to be £1000+ probably. Same goes for a vacuum controller for your high vac, and buying a second pump will be even more. Maybe this stuff is affordable second hand though? The only way I know of doing it on a budget is a basic water aspirator - but there are certainly issues with water volume as well as environmental concerns.

Is the house vac only for the one lab or several? Perhaps there is a valve open or partially open in another lab that nobody's noticed? Can you check the vacuum at the source, to see if the whole system is knackered or it is just a leak somewhere between the source and your lab?
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azmanam

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #6 on: June 02, 2011, 04:14:39 PM »

We had diaphragm pumps in our lab at grad school.  I don't remember the pressure exactly, but we measured with the manometer to low double digits mmHg and it worked quite nice.  We had chilled water recirculator (a garden pond water pump) which spiraled chilled water through the condenser part of the rotovap.  also worked quite nice.  Even with the efficient condensing, we had another vacuum flask sitting in the chilled water reservoir (the chilled water that recirculated through the condenser).  That collected quite a bit of solvent, too.  Even still, we noticed LOTS of solvent still being pulled through the diaphragm chambers. 

All that to say there is probably a real concern with solvent vapors passing through if I used water aspirator.

Perhaps I'll ask some questions about partially opened and forgotten vacuum taps.  I don't know where the source vacuum is, or how to check it's vacuum pressure, but maybe I'll ask some questions there too.  Being the new prof in town I feel hesitant to rock the boat.  Organic professors have been doing typical organic research in this building for decades.

I'll be sure to report back as I get new information.
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enahs

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #7 on: June 02, 2011, 04:38:45 PM »

What you want to do is go to a air-conditioner supply store and buy some vacuum pumps from there. You can easily get a 50-250 micron vacuum pump for $70.

Sure, they are not technically chemical rated. But when an equivalent chemical rated pump from Fisher cost >$1300 who really cares. Just figure out what the seals are made of and what solvents you should not use (you got a PhD in chemistry, you can figure that out).
Just hook it up through a series of traps and if it last more then 3 months, you win when it comes to cost.

I have had a $120, 25 micron vacuum pump from an air-conditioner supply store working in a Chemistry lab for over 2 years now. It goes through two traps that are submerged in a cooling tank (-10 C) and then another trap where I can use liquid nitrogen if I am really worried about the solvent. I have opened it up a couple of times to clean it and use some silicon grease on the seals.


Of course, where I am from we use a lot of AC, and so supply is high and cheap. And buy in the winter when prices for AC supply's are down too!
« Last Edit: June 02, 2011, 04:51:19 PM by enahs »
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fledarmus

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #8 on: June 13, 2011, 11:50:14 AM »

Check LinkedIn or your local chemical society for people that work in pharmaceutical research. With the amount of down-sizing that has gone on recently, you could probably find somebody with an unused vacuum pump that they would be willing to donate to a deserving academic lab
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azmanam

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Re: Tips for Overcoming a Poor House Vacuum?
« Reply #9 on: June 13, 2011, 12:00:25 PM »

Thanks for the tip - I will be follow up on many of these.

For now: run all columns in acetone/hexanes!
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