Sciencemadness Discussion Board

problem with vacuum distillation set up

Furboffle - 30-10-2012 at 19:14

I just bought this vacuum pump at harbor freight today:

http://www.harborfreight.com/25-cfm-vacuum-pump-98076.html

I'm trying to use it for vacuum distillation, I connected the pump to a 500ml filtration flask with 200ml of water. after a few minutes maybe like 10 tiny air bubbles formed on the side of flask, disappeared and that was it. I emptied it and tried with 200ml of methanol, it managed to suck out a tiny amount of dissolved air, it bubbled very lightly for about 1 minute. after that it went dead.

what am I missing. I've tried this with an aspirator and managed to violently remove dissolved air from water or methanol. checking with a gauge I managed to hit about 150torr via the aspirator. it also caused the flask to get very cold which was expected. the vacuum pump was gauged at a maximum of 185 torr and no noticeable temperature drop. the vacuum is said to reach maximum of 10 pascals which is about .075 torr. thats no where near what I'm getting from it. is there a proper way to arrange the whole setup to increase vacuum?

I tried searching the forum for info but found little help. only thing I saw was letting it run 10 minutes so that it warms up will help it pull maximum vacuum. after running 15 minutes mine made no increase vacuum from 185 torr.

I'm hoping to do some research with indole compounds and the intermediate products are heat sensitive, so boiling is an issue, I could leave it to evaporate but the work needs to progress rapidly from step to step. intermediate products quickly break down from ambient heat, light, air, etc... so vacuum distillation is quite essential in purification to move on to the next step...

any advice on this would be great cause I feel like this should be quite adequate for boiling water LET ALONE METHANOL...

watson.fawkes - 30-10-2012 at 19:35

How did you prepare the pump for operation? By what means is it connected to the flask?

Magpie - 30-10-2012 at 19:46

I have a pump just like yours and it has been working fine for me. But I don't use it for filtrations. I only have used it for vacuum distillations. IIRC I'm getting down below a torr on a sealed system. Measure the vacuum on the filter flask with nothing in it.

I wouldn't use it where I would be drawing a lot of condensable vapors (like water and methanol) through the pump. This would contaminate the oil in the pump. Usually a cold trap is used ahead of the pump to remove such condensables.

Furboffle - 30-10-2012 at 22:01

Quote: Originally posted by watson.fawkes  
How did you prepare the pump for operation? By what means is it connected to the flask?


I have a hose barb screwed on to the inlet valve on the pump. Then a rubber hose connected directly from there to a 500ml filtration flask with a rubber stopper plugging the top...
I feel like this should work.

I plan on getting a cold trap to connect between flask and pump, but I mean just for testing the strength of vacuum without a cold trap Shouldn't make a difference as far as it working, correct?

watson.fawkes - 31-10-2012 at 03:32

Quote: Originally posted by watson.fawkes  
How did you prepare the pump for operation? By what means is it connected to the flask?
Quote: Originally posted by Furboffle  
I have a hose barb screwed on to the inlet valve on the pump. Then a rubber hose connected directly from there to a 500ml filtration flask with a rubber stopper plugging the top.
So you answered the second question but not the first. Did you do anything but open the box and take out the pump?

Furboffle - 31-10-2012 at 06:43

Quote: Originally posted by watson.fawkes  
So you answered the second question but not the first. Did you do anything but open the box and take out the pump?

sorry didn't quite catch that aspect of the question. to prepare the pump I filled the oil to the fill line then connected it and cranked it up. so aside from adding oil yeah pretty much just took it out of the box. is there more prep work?
actually regarding the oil: when I turn it on it seems to shoot out oil vapor as soon I turn it on is that normal? its only filled to the line so there isn't excess oil in it.. mig

Magpie - 31-10-2012 at 08:39

Quote: Originally posted by Furboffle  

actually regarding the oil: when I turn it on it seems to shoot out oil vapor as soon I turn it on is that normal?


If you run the pump against the atmosphere it will suck its maximum, ie, 2.5cfm, and all that air will form an aerosol with the pump oil and spew out the exhaust breather cap. This is normal. When you run it against a closed system like your plugged filtration flask this will stop as the air flow will be near zero cfm. Try it, and measure the vacuum with your gage at the same time.

Furboffle - 31-10-2012 at 14:11

Quote: Originally posted by Magpie  
Quote: Originally posted by Furboffle  

actually regarding the oil: when I turn it on it seems to shoot out oil vapor as soon I turn it on is that normal?


If you run the pump against the atmosphere it will suck its maximum, ie, 2.5cfm, and all that air will form an aerosol with the pump oil and spew out the exhaust breather cap. This is normal. When you run it against a closed system like your plugged filtration flask this will stop as the air flow will be near zero cfm. Try it, and measure the vacuum with your gage at the same time.


Ok this helped me find the issue. It was causing mist to come out regardless of being connected to a sealed flask or not. After examining the hose barb I screwed on to the inlet I noticed there was some leakage, the mist coming out of the air exhaust, a little bit was being sucked into the joint where the barb screwed on to the inlet. I need plumbing tape to form a proper seal. But after removing that attachment and connecting a slightly bigger hose directly to the inlet and to the flask, boiling occurred instantly and the misting stopped and I'm now in business.

[Edited on 31-10-2012 by Furboffle]

chemrox - 1-11-2012 at 21:05

I highly recommend reading a chem lab manual written for lab technicians. I can give you some references via u2u tomorrow. I believe most people here already have such materials. Anyway, reading the sections on how a vacuum station is set up can be helpful to you and the life of your pump. If magpie can get 1 torr from such a pump I would expect you to get maybe 10 torr. But you must set it up. You need a trap and filter as well as a vacuum control. All connections should be wired, clamped or zipped. By the way 150 for an aspirator is terrible. Is your water warm or something? I'm about to post a related question.