Sciencemadness Discussion Board

vacuum

tom_servo - 13-11-2008 at 07:01

will something like this work for vacuum distillation and vacuum filtering
Quote:

KNF diaphragm liquid pumps are based on the principle of the oscillating displacement pump which is remarkably simple in design. The circular power from the motor is converted into vertical movement by an eccentric. This motion is then transferred to a diaphragm by means of a connecting rod which in conjunction with an inlet and outlet valve creates a pumping action. UNF 1.100 type liquid pump can be mounted in any position and can deliver 0.7 l/min and will operate against pressures of 30 mWg. Features Self-priming and excellent for pressure Sophisticated diaphragm technology and precise valve structures enable performances, depending on model, of up to 4 mWg suction and 60 mWg pressure.


basically what are the requirements for a pump to distill oils and filter solvents/oils

tom_servo - 13-11-2008 at 12:48

or how about a pump like this?
Found one for 75 bucks

vacuum

not_important - 13-11-2008 at 17:54

The first pump is too low in pumping rate for many filtering applications, and too little vacuum (giving about 500 mmHg) for filtering and vacuum distillation.

The second pump is good, but you'll need traps to protect the pump from solvents and acidic gases.

Aspirator pumps are good for filtering, solvent removal, and the like, they're not really affected by such nasties. An example is here http://tinyurl.com/595hjs but you can do better in regards to the price.

tom_servo - 13-11-2008 at 20:53

I was under the impression that these aren't good for vacuum distillation
something about water pressure changing?

HOw about making a pumping station with a spa jet or a pool pump?

not_important - 13-11-2008 at 22:22

Keeping stable pressure with an aspirator can be difficult, and stable pressure is needed for any vacuum distillations past simply separating a fairly high boiling or heat sensitive compound from stuff that is much less volatile. So indeed, for vacuum distillation you want a mechanical pump, and you'll want a cold trap or something else between the pump and the rest of the system if you want to avoid always changing the oil and possibly trashing the pump.

Suction filtering, solvent removal, evacuating desiccators, and the like, are not terrible sensitive to pressure fluctuations. For these an aspirator is just fine and it is much less likely to be damaged by stuff coming off the material being filtered or concentrated. You'll want a suck-back trap, but that's basically a heavy walled container with two connections, much simpler than cold traps.

tom_servo - 14-11-2008 at 07:17

ok awesome thank you

chemrox - 15-11-2008 at 00:38

Let's distinguish between aspirators and aspirator pumps. Aspirators run off the water supply. Aspirator pumps have a water reservoir and water pumps that pump the stored water through aspirators. If you have running water with decent steady flow such as most municipal supplies deliver you can use an aspirator for vacuum distillation at modest vacuums, rotovap, etc. The aspirator pumps are great for filtration and rotovap applications but not lengthy distillations due to the temp changes. Mechanical pumps come in to their own when higher vacuums are needed. With either aspirator or aspirator pump you need a water trap to protect against backflow and even then be careful.....with mechanical pumps you need a cold trap, a dessicator, particle filter and acid remover. A KOH column can take care of the particles and acids as well as moisture so that's two devices bewtween your system and the pump as a minimum plus you need a vacuum release cock in the line. I like two part traps so I can remove condensate with out taking the trap off the line completely but they introduce a potential source of leakage as do all the hoses and connections. For every hose connection you need a wire or cable tie. The less flexible hoses have lower vacuum loss but are harder to deal with. Added to all this gear is the possible desire to measure the vacuum objectively. For apsirators a manometer will do but for higher vacs a Mcloud Gauge or an electrical device is needed. Decent mechanical pumps are over $1k new but can be found used. Your needs will be driven by the amount of vacuum you need...a good aspirator pulls 20-30mm and a good mechanical pump with a reasonably tight setup can get you to under 1mm. Vacuums change during distillation just from the vapor pressure changes that occur during the process. With larger vessels a device called a manostat is sometimes employed. Vogels has a fairly good writeup on vacuum distillation as do other lab manuals.

Klute - 15-11-2008 at 06:18

I've been using an aspirator station as vacuum source for years, and have done numerous vacuum distn with it, with very satisfying results. Obviously, the vacuum ddoesn't go under 20-15mmHg with cold water, but it is stable enough to not have extreme variation of vapor temp. More than enough for most applications IMHO.

Considering the price of a mecanical vacuum pump, and all the equipement that goes with it (manifold, cold trap, dewar for LN2, manometer, etc etc), an aspirator station is much more pratical for the home-chemist, especially a newbee.

The vacuum can be used for distn (you can routhly change the vacuum by changing the water flow with a tap, never diminish the flow while connected to a setup to avoid suck-back though), vacuum filtering, dessicators, etc and no worries about the vapors that pass in the aspirator, you can disgard the water regularily. Adding some Na2CO3 to the water will prevent rusting caused by acidic water (thanks to Bio for this tip), and a little Oxone will prevent algae formation and bad smells.

There are several setups displayed on the forum, have a look at the constructions, which are pretty easy to do with minimal investement. And it will last years (over 4-5 now for me)

Ullmann - 15-11-2008 at 09:00

Personally the cheapest and most convenient I know of is a fridge pump cut off an old fridge... It cost nothing and works for years at 20 mmHg...

497 - 15-11-2008 at 13:18

Quote:

It cost nothing and works for years at 20 mmHg


Wow do they really do that good? I always heard they'd make like 150mmHg at best. I guess I'll have to go try one out.

Would it be possible to use two (or more) of them in a multiple stage arrangement to get a better vacuum? Considering how easy to get they are, this might be helpful.

Edit:
According to this the standard freezer/refrigerator compressors will get down to "a few 10s of torr" while a "rotary piston" type compressor from an air conditioner will get down to 1mmHg or so. Not bad at all! Too bad air conditioners are not common where I am...

[Edited on 15-11-2008 by 497]

chemrox - 15-11-2008 at 18:09

@Klute Good tip about the Na2CO3 but my aspirator pump has Al parts and a plastic reservoir so Na2CO3 would be bad news. It is as you say a good alternative to a mechanical pump station with all the attendant paraphenalia and consumables. A large, thick walled Ehrlenmeyer water trap is essential .. these pumps really push water up the lines when shut off if the vacuum isn't released first.

@Ullmann- I'm impressed. How much flow do you get through that pump? Or how long will it take to reach ultimate vacuum in a 1L flask? Any idea how much electricity it uses?

Ullmann - 19-11-2008 at 10:10

Quote:
@Ullmann- I'm impressed.


How much flow do you get through that pump? I do not know
Or how long will it take to reach ultimate vacuum in a 1L flask? I do not know
Any idea how much electricity it uses? Same than a fridge with the door open

It works very good to do buchner filtration in 1 liter flask and to distill the apparatus will be in vacuo in less than a minute... It is convenient... Place a fan next to it for cooling the surface of the pump otherwise oil will go out of the tube and the pump will heat to more than 80°C then...

I had tried to put them in stages but the vacuum was not very enhanced, to distill at less than 10 mmHg go buy a two stages vac pump for fridge and compressor repair... its much cheaper than labo pump and they pull to a high vacuum...

Klute - 19-11-2008 at 14:38

I also had good results with fridge compresors, but the vacuum was slightly weaker than the aspirator (with cold water). They do deliver a more stable vacuum though.

I used a oil trap at the air outlet, to avoid spraying oil mist everywhere: a large "cone" with small holes at the top, the oil mist would deposit itself on the sides, and slowly fall back into the tube when the compresr wa shut off.

But the ones I had lasted much less, a year or two max before problem started setting in, they are not meant to be used in a open system, and the oil/pump must be pretty sensible to solvent vapors and/or humidity. Also you can't keep them on for two long, as they heat up enormously as Ullman said, and finish going into thermal protection. I bypassed the thermal protection once as it kept going off every 5 min when I had just taken it off the fridge, but the compressor died a few months after from over-heating I guess.

There are a lot of different sizes and models out there, so it's hard to give a definate answer on the air flow, but max vacuum cnabe obtained in a matter of 10-15 with 1l flasks with the ones I had.

They do remain an excellent OTC vacuum source, especially as they are FREE. Just feels bad to throw all that pentane out in the atmosphere when cutting them off...