Looking for a suitable, adjustable vacuum pump..

[Rabidreject]

So I’m looking for a vacuum pump that will reliably pull a vacuum of 640 mm hg.
I’m totally new to vacuum distillation and am trying to make use of my benzaldehyde without having to get nitroethane as I can’t find it.
I need a decent vacuum pump for a Buchner anyway - I’m not sure if they are all adjustable or not but it would be useful to use the same pump for reactions if I’m going to buy one….
Can anyone point me in the correct direction ? EBay? AliExpress? Whatever really makes just need to be able to pull 640 mmhg and no one seems to list it as that unit.
I did a conversion on one on eBay (from China anyway so I’d get it from AliExpress probably).
Thanks! I really need your help!

 

[Frit Buchner]

So. VACUUM IS CONFUSING TO A LOT OF PEOPLE. 640mm/hg is about 2.5 psi of vacuum. Meaning it is 2.5 psi lower tha air pressure at sea level (14.7 psi) it's not really a vacuum it's just lower than atmospheric pressure which is 750 mm/hg..1 atmosphere 14.7 psi 750 Pascale 1 torrent. All the same this small amount of vacuum can be achieved with a 12v Dc automotive vacuum pump that operates flop up headlights or windshield washer mechanisms. You can get one for $25-30.
The next pump up the list is a single stage rotory vane vacuum pump. They vacuum bubbles out of epoxy are 1/4 to 1/3 hp and can produce 15+ times the vacuum you say need. They can be had for $50-70 on Ebay Amazon ect. Bigger single stage pumps are available, double stage rotary vanes and other designs are available starting from 20 times what you need to 50 or more times what you need. Get a cold trap or you'll be changing the oil every time you use it. Generally the automotive type pumps don't require a trap, can do pressure or vacuum, don't require oil and are very small. Each type has some pros and cons

 

[Rabidreject]

Funnily enough, I met a chemist on here and he said exactly the same thing. I also watched a video on pootube on it and that furthered my understanding.
I have used a hand vacuum pump before but it broke.
I ended up going with a pump that is basically two in one, positive pressure as well as negative - probably for repairing cars (well that’s what it says).
It says its max pressure is 7 bar.
To be honest, i was aiming to follow the synth for racemic amphetamine, which utilises a vacuum distillation at around 650mm hg I think, so as you say it’s a really small vacuum (or negative pressure, as you quite rightly pointed out.).
Honestly it seems like a strange vacuum to pull because as you quite rightly pointed out - 650mm hg is not that far below/above ) atmospheric pressure.
I think the biggest thing I realised is it’s not a 1 size fits all scenario for pumps in a lab…multiple different types are needed - especially in if we are talking about getting a rotovap or a GCMS or something….
I also realise I need to buy a cold trap to put between the thing I’m pulling a vacuum on and the pump - just in case. I think i have my head around it all now

 

[Rabidreject]

By the way, thanks for replying to my post, I’m pretty new to posting on here and have been a long time lurker - ever since I found you at least! It has also lead (no pun intended) to me meeting at least 1 person who actually knows what they are doing and I find very inspiring….

 

[Frit Buchner]

By the way, thanks for replying to my post, I’m pretty new to posting on here and have been a long time lurker - ever since I found you at least! It has also lead (no pun intended) to me meeting at least 1 person who actually knows what they are doing and I find very inspiring….

RabidrejectThe electric pumps being able to push is handy sometimes, I'll tell you how. Vacuum reduces the boiling temp and pressure raises it. At 2 atmospheres (14.7 psi above ambient) water boils at 250f instead of 212. There are a few things you can do with that. The best thing in my opinion is making a supersaturated solution for crystallization. You are dissolving something in minimum hot solvent. It starts to crystallize. Perfect, let it boil solvent off for a minute. Put your air line on and pressurize it. Now the solvent can get hotter . The crystals redissolve and you cut the heat. That's it , supersaturated solution with no guessing or anything letvit start precipitating and pressurize it. Probably save yourself a round of vacuuming off solvent after you first harvest.

 

[OrgUnikum]

For filtration and such a used dentist pump is best, they are cheap and far better quality then the Chinese 60 $ crap. Ebay or better classifieds. Those stand some abuse all the others would have died long ago.
For real vacuum distillation you need a two-stage rotary vane oil-sealed pump with ballast valve. A used 50 years old lab pump is better then a new HVAC pump.

You put a T-connector and a needle valve in the suction line to regulate vacuum. 5 $.

You do not neeed a gauge. Mechanical ones are useless below 100 mbar and digital ones are too expensive and not robust all. A soft touch on the needle valve is all you need.

And ALWAYS first establish a stable vacuum and then apply heat. ALWAYS.

 

[Frit Buchner]

I'm not a chemist. A single stage pump can boil water at -50f, what do you need so much vacuum to do? I there a scenario where a self funded clandestine drug chemist could make use of more vacuum? Can standard glassware support more vacuum?

 

[blacky2340]

I don't know about y'all but my vacuum pump cost less than 20 euros. Just rip a compressor of an old fridge drain the freon and hook it up to a socket. It lasted much longer and still alive than my rotary vane pump.

 

[TheVacuumGuy]

I just purchased a heavy duty high pressure water pump (no aliexpress or amazon). Using a large plastic box and appropriate hosing, it's recirculating water.
Then at the outlet, an aspirator is placed. So the combination can also scrub gas and pump liquids if needed. Can get a high enough pressure on the aspirator to let water boil at 25 celsius. When i put some strips of icecubes in box, the vacuum will be lower and water can boil at 10/15, and it's chemical resistant as well (at least for bromine, which i tested). Adjusting is as easy as turn the knob of the power supply up/down.

 

[blacky2340]

I just purchased a heavy duty high pressure water pump (no aliexpress or amazon). Using a large plastic box and appropriate hosing, it's recirculating water.
Then at the outlet, an aspirator is placed. So the combination can also scrub gas and pump liquids if needed. Can get a high enough pressure on the aspirator to let water boil at 25 celsius. When i put some strips of icecubes in box, the vacuum will be lower and water can boil at 10/15, and it's chemical resistant as well (at least for bromine, which i tested). Adjusting is as easy as turn the knob of the power supply up/down.

TheVacuumGuyCan it beat a 20 eur fridge compressor that wont get stuck like some other pumps do when solvents get in there?

 

[TheVacuumGuy]

Can it beat a 20 eur fridge compressor that wont get stuck like some other pumps do when solvents get in there?

blacky2340Pricewise: no. But I haven't used a fridge compressor so can't tell.

 

[blacky2340]

Pricewise: no. But I haven't used a fridge compressor so can't tell.

TheVacuumGuyTry it, you'll be surprised

 

[yuiopjkl]

You just need 640 mmhg
Use (12V R385 DC Diaphragm Pump)
It's cheap and you can get more pressure than you need.
You can also use it to pump water into the condenser