Ghetto Regulation of Vacuum?

DrugPhreak · Working Bee

I recently acquired a KNF vacuum pump that pulls 29.5" Hg, but I really don't want to buy the digital vacuum controller. Is there anyway that I could regulate what it pulls without such a device? Also, say you only have the boiling point of a substance at atmospheric pressure what formula could bee used to find out what its boiling point is @ 10 torr?

ApprenticeCook · DILLIGAF

Does the pump have a max vacuum cut-off? ie does it pump till it gets as far as it can then shut-off? or does it keep on keeping on?
If it cuts-off at the max vacuum there is your regulator, but if it just keeps on going you can do what i have, get a t-join in the vacuum line and put a screwgate vent to the atmosphere, open it ever so slightly to reduce the vacuum in the line, it works ok but its the best solution i have at the moment so if there is any better alternatives id like to hear them also.
As for distillation temps you can use the nomograph on rhodium to find the bp at whatever pressure using the bp at 760mmHg.

-AC

primathon · modified

Nomograph for Calculation of Boiling Points Under Vacuum
by Rhodium

Link: https://www.synthetikal.com/Rhodiums_pdfs/chemistry/equipment/nomograph.html

bio · Working Bee

This is an applet that converts vapor pressure to boiling point for you. It's pretty accurate I have found if you use the correct curve.

http://www.ch.cam.ac.uk/magnus/boil.html

To manually regulate the pressure you need a needle valve to introduce some bleed air close to the pump not into the system. Don't know what a "screw gate" is but the needle valve should be a high quality 10 turn tiny one say 1/8 max. If you can't afford or easily find one of these a needle valve (connected in reverse) used to regulate small gas appliances will work but much harder to adjust. Remember a needle valve works properly only in one direction so pressure types need to be connected in reverse. Many industrial suppliers carry needle valves specifiically made for vacuum or will order for you. The high vacuum versions are quite expensive but you don't need this for 5-10 torr.

Place your guage (closed end manometer) on the vacuum adaptor or as
close as possible and the bleed valve close to the pump using a short rated line at least 3/8 inch ID. It's a pain to adjust in the begining until you find the "sweet spot" and takes a few minutes to stabilize. With a good pump say 3cfm you will bleed way more air than an aspirator can pull and still get a better vacuum. Also try opening the gas ballast as this is usually all you need to do for about 1-10 torr . At least on a two stage 50micron rated 3cfm pump.

Beware to gradually increase the vacuum on a COLD system as a good
pump will almost instantly suck everything into the trap and itself. Sad

Also don't use it for solvents unless made for that or a dewar's in line.

DrugPhreak · Working Bee

Thanks for all the info! I really like that little applet. According to it B-asarone will boil at 132.1C @ 10 torr. Yep, it's one of those pumps that stays at a steady vacuum until you shut it off. I was about to purchase a new KNF pump and then I found a UN726.3FTP (http://knf.com/labpumps.htm) for $100 on a auction site so I couldn't pass up that deal even though 10 torr is still not strong enough. Crying or Very sad

I can't find what it was selling for new. Does anyone else have this pump or a similar one? I got the manual from KNF, but it wasn't put together that well IMO. I've been waiting to check the B-asarone content of this oil I have for the longest time and I found out that my aspirator was only pulling 5" Hg with the crappy water pressure! LOL! KNF says this pump needs a cold trap. What’s the best ghettoized cold trap? I was thinking of having a small filtering flask blown with two side arms. Then I could stopper it and place the flask in an acetone/ice/salt bath.

ApprenticeCook · DILLIGAF

5"Hg? man that is crap water pressure.... i can get 22"Hg easy with house pressure. Well best ghetto cold trap would be a dreshel bottle (gas washing bottle) stood up in a ice/salt bath, the acetone i dont think would make much difference, thats usually used more in a dry ice bath. But if you cant get the dreshel bottle and from what you said you seem to be able to make glass to design? make something similar.

Dreshel bottle: http://www.flaig.freeserve.co.uk/1114.jpg
The distillation apparatus or whatever its coming from is connected to the left which extends to the bottom of the bottle where you can have it exposed to the cooled surfaces or have the vapours washed through another substance, the vacuum pump is hooked up to the right hand side.

-AC

DrugPhreak · Working Bee

Yeah, that sure is crappy. I knew it was bad, but I didn't think it would be that bad. My other sink probably pulls at least 20" Hg, but the faucet would have to be replaced before an aspirator could be connected to it. I was about to do that, but even then the vacuum wouldn't be good enough to perform many distillations. Yep, I get most of my glass blown by a small glassblower to try and stay under the radar when procuring glassware. I do have a small Dreshel bottle that I use for gas scrubbing, but I've read that they shouldn't be used under high vacuum... although, I never tried to use it for a vacuum/cold trap before. Is this true? I was out of dry ice so I was trying to freeze B-asarone out of calamus oil with an acetone/ice/salt bath. It went down to -15C and nothing even started to freeze. Even though I will just distil it now I still want to see what temp B-asarone will freeze at, but yeah... for the cold trap an ice/salt bath should be fine though.

bio · Working Bee

For a cold trap all you need is a filter flask. Connect the pump to the side arm and the system to a dip tube thru a one-hole stopper. Coiling the dip tube increases surface area. If you also need a suckback trap then another filter flask would be needed and connected in the usual way which is opposite of the cold trap connection.

Ice/salt is not really cold enough to condense most of the vapors if you have a decent vacuum. Also a pump will never pull it's ultimate rated vacuum in practice on a normal distillation setup in practice. These ratings are a blankoff pressure reading which in the case of an oil pump would be the vapor pressure of the oil.

In use a system will always have some minute amount of leaks and out/off gassing at least in my experience. This is not to say a leak tight system is not attainable, it's just not practically very easily attainable. Anyone having done a lot of vacuum work on some of our rather crude hookups will attest to that.

DrugPhreak · Working Bee

Yeah, I didn't think that vacuum pump manufactures would be an exception when it comes to stating specs under the most ideal conditions. Is it really necessary to have a dip tube or glass tubing etc coming into the cold trap though? If it was placed in an acetone/dry ice bath do you think my idea of a double sidearm filtering flask would work ok? Those jars, which are in front on the intake and exhaust on some vacuums seem like they function in a similar fashion. Then I could just have the exhaust tube go into a small Erlenmeyer in another bath... or the same one.

bio · Working Bee

The idea is to maximize the surface area the vapor contacts before it reaches the pump, hence dip tube or in the case of a double side arm setup that would be ideal with one side arm at the bottom and useable for filtering too with bottom outlet for a hose. K....x makes a nice 4L one with removable plastic arm.

A metal trap w/ internal coil of course is much preferred for efficiency provided it's compatible.

DrugPhreak · Working Bee

Oh... OK. Originally, I was going to have this one made...

I'll have them make this one though...

One of these would be nice, but I try to keep my costs as low as possible...

bio · Working Bee

Yep, that's the idea. Probably want the bottom outlet closer to the bottom for filtering use. Look at the pic of the K.....x one. They are real handy when filtering large amounts without dismantling the thing. Just remember to close the valve when you walk away as the tank contents tend to end up in the pump. Sad

Yes, it's probably the stupidist thing I ever did, but I didn't mean too . Embarassed

Luckily it was the stuff on the paper that was needed.

If liquid level is a problem using as vapor trap just insert a piece a bent tubing so it's above the expected liquid level. Bubbling thru the liquid as mentioned above might not be such a good idea but have never done it. Think like a molecule under varying pressure and you will see what I mean!

BTW do molecules really think? always wondered about that. I do know electrons think though. Laughing