Aspirator station help

[MrJ]

[abolt]

I've used it to try distillation of sass, and nothing comes over until about 185 C, which makes for a really shitty vaccuum.

Are you using large blocks of ice in the reservoir?

Is the nipple of the aspirator slightly lower than the vacuum adaptor nipple?

Are you removing the vacuum tube before you turn off the water pump?

[MrJ]

Are you using large blocks of ice in the reservoir?
-does the reservoir temp really matter that much?

Is the nipple of the aspirator slightly lower than the vacuum adaptor nipple?
-i'd say the vaccuum adaptor nipple is maybe about 1 foot higher than the aspirator and it is connected with about 2 ft of hose

Are you removing the vacuum tube before you turn off the water pump?
-i usually turn off the vaccuum before disconnecting the tube

MrJ

[ApprenticeCook]

does the reservoir temp really matter that much?
- Yes. The pump will heat the water as it pumps it (esp after prolonged use) and the hotter water will lower the aspirator vacuum, ie as you keep running your water temp goes up and your vacuum goes down.
Ice the water, and put something over the intake to stop the ice from being sucked in.

i'd say the vaccuum adaptor nipple is maybe about 1 foot higher than the aspirator and it is connected with about 2 ft of hose
- Your prob is more than likely here.... the hose? what type? vaccuum hose? check when its running the tube is not crushing down under the vacuum... go buy vacuum hose if your not using it...

And you need to release the vacuum before turning off the pump, so in swims setup there is a tap which is opened to release the vacuum, then you turn off the water pump, otherwise unless you have a nrv (non return valve) the water at the output of the aspirator will be sucked in to the vacuum area.

Hope this helps...
-AC

[abolt]

Your prob is more than likely here.... the hose? what type? vaccuum hose? check when its running the tube is not crushing down under the vacuum... go buy vacuum hose if your not using it...

......also, lessen the length of your hose if possible by placing the unit closer to the vac nipple. Standard Rubber hoses do lessen the vacuum at the vacuum nipple. Therefore the longer the hose, the more vacuum is decreased.

UTFSE, and look for some excellent info posted by Osmium in relation to vacuum properties.

[Osmium]

> And you need to release the vacuum before turning off the pump, so in swims
> setup there is a tap which is opened to release the vacuum, then you turn
> off the water pump, otherwise unless you have a nrv (non return valve) the
> water at the output of the aspirator will be sucked in to the vacuum area.

I once flooded the whole setup of a finished distillation containing 100g of P2P by doing something stupid like that. Always let air in before turning off the water. Don't trust those valves.

[MrJ]

thx for the help guys.. but i think that stupid aspirator station is more trouble than it's worth!!!

friggin pump cost me a bundle... i think i'm gonna just get a friggin yellow jacket vaccuum....

i think i grew some white hair over this shit!!!

MrJ

[hypo]

> I once flooded the whole setup of a finished distillation containing
> 100g of P2P by doing something stupid like that.

with an aspirator? consider yourself lucky - separating the p2p and drying
it is a nuisance, but not more.
when my oil pump broke, she spilled her guts (nasty, dirty oil) all over
the collected product. that was a tragedy.  

[Sredni_Vashtar]

Not fair to UTFSE on this one - he has the nous to set the thing up and is asking for more advice.

Water temperature is important. The limiting vacuum of practical water-jet pumps (aspirators to Yanks) is the vapour pressure of water. (Because the pumping chamber is always contaminated with water splashes for Hypo etc.). If you use ice-cold water, you can achieve close to 5 mbar Hg with a good setup.

Your water reservoir is too large (if you are filling it to capacity). A bucket (2 gallons/ 10 L) is sufficient and easier to cool with ice.

Water pump horsepower is secondary - what matters is pressure and capacity. Most water-jet pumps require around 1.5 bar (45 foot head / 15 m head / 150 KPa) but some low flowrate lab. jobs take a higher pressure of above 3 bar (90 foot head / 30 m head / 300 KPa). Your pump has easily enough capacity, but has it the correct pressure? You could be over-pressuring your water-jet. If you greatly overpressure a water-jet pump, it will flood and your vacuum will die. You say that water is coming out of the vacuum inlet - maybe this is what is happening? Try restricting the water inlet to the water-jet pump with a valve or such and see what happens.

Ditch the bathtub and get a large bucket.

Vapour Pressure of water

Celsius  Pressure (mm Hg)
-10        2.15
0           4.58
5           6.54
10         9.21
11         9.84
12         10.52
13         11.23
14         11.99
15         12.79
20         17.54
25         23.76
30         31.8
37         47.07
40         55.3

Edit: easiest thing is for you to get a vacuum gauge and see what is really happening. Cheapest are automotive gauges but you can get cheap ones on auction sites that we know and love etc.

[abolt]

Your water reservoir is too large (if you are filling it to capacity). A bucket (2 gallons/ 10 L) is sufficient and easier to cool with ice.

I would say that ISwE, a larger reservoir volume with a larger volume of ice allows the returned water to cool down better and therefore provides a better, (and more importantly)constant vacuum.

Free Tip: Go to the outdoors/sporting store and buy some of those 20 litre water drums that people use for camping. They are usually very cheap. Look for the ones that have a rubber "O" ring that seals the screw on lid in order to make them "water tight". Fill them up with water and place them in the freezer for a couple of days and voila!, you have a great chunk of ice for your aspirator reservoir (~40-50 litre volume), which can maintain a pretty constant reservoir temp that lasts for several hours.

Of course, bigger reservoirs require more containers of ice.

[biotechdude]

If using an aspirator, ensure that the hose connections are really secure (use clamps etc).  Otherwise after a few hours they will be be hot, expand and pop off and spray shit everywhere.  Its not cool to be woken at 2am and lying in bed thinking "what's that hissing sound??.."

[WizardX]

Add anti-freeze to the water so you are able to cool the water close to 0 oC to get better vacuum.

The vapour pressure of water @ 5 oC is 6.54 mmHg. If you go 2 x 6.54 mmHg = 13.08 mmHg of vacuum (approx) at the vaccuum adaptor nipple.

The vapour pressure of water @ 0 oC is 4.58 mmHg. If you go 2 x 4.58 mmHg = 9.16 mmHg of vacuum (approx) at the vaccuum adaptor nipple.

[goiterjoe]

Not only can you hold several gallons of water in a cooler, but the insulation will also help prevent the ice from melting as fast.  I always used coolers to hold the water and ice for my condensers as well.

[abolt]

Add anti-freeze to the water so you are able to cool the water close to 0 oC to get better vacuum.

If you are going to utilise anti freeze, I suggest you use an enclosed system that minimises the inhalation and contact with any "misting" that may occur.

Pharmacology:

Ethylene glycol is rapidly absorbed once it is ingested and is then widely distributed into body tissues. Peak blood levels are generally seen in 1-4 hours. Exposure to the skin and lungs may cause irritation but does not cause the systemic toxicity in the way that methanol does. Lethal quantities in adults are considered to be 100 ml, but in children much less may cause serious cardiac, renal, and CNS toxicity.

Ethylene glycol itself is relatively nontoxic. After absorption the unchanged compound undergoes glomerular filtration and passive reabsorption. It is then broken down into metabolites that are highly toxic and cause the associated findings of ethylene glycol toxicity. Ethylene glycol is converted to glycoaldehyde by alcohol dehydrogenase. This is the rate limiting step of a reaction in the liver that continues to breakdown the glycoaldehyde into glycolate, glyoxylate, and oxylate (see below).

Inhalation:
 
Vapor inhalation is generally not a problem unless heated or misted. Exposure to vapors over an extended time period has caused throat irritation and headache. May cause nausea, vomiting, dizziness and drowsiness. Pulmonary edema and central nervous system depression may also develop. When heated or misted, has produced rapid, involuntary eye movement and coma.

http://www.embbs.com/cr/alc/alc7.html

[Sredni_Vashtar]

The viscosity of water increases significantly close to zero, add ethylene glycol and it will increase further. A powerful pump will handle this but will maybe double the heating effect on the water from work done.

That's a good point about ethylene glycol, it is acutely and chronically toxic. Propylene glycol is harmless by comparison but more viscous.

I think pure water is best:

You can use antifreeze and refrigeration to get subzero coolant temperatures, but for what? Say you wanted to cool down to -5 C: you'd need around 15% ethylene glycol by mass. This would bring the vapour pressure down to around 3 mbar Hg, but the viscosity would be 3 times as much as water at 0 C. Is it worth it for 2 mbar extra vacuum? Plus, you'll need a refrigeration method (old chest freezer etc.). 5 mbar is good enough for most needs. Better to concentrate on getting a very reliable 5 mbar.

Converting an old fridge to cool the pump water might be a good idea though. No more ice. I used to have a 1HP garden pump and that melted a lot of ice.

[WizardX]

Antifreeze solutions for automotive (automotive radiator fluid) contains about 30 to 50% w/v of ethylene glycol in water with some corrosion inhibitors.

A concentration of 15-20% w/v is all we need, because at 5 oC water is still liquid.

If viscosity of water is an issue, you can minimize the viscosity by making a 10% w/v ethylene glycol solution and test the freezing point by placing it in your home freeze. Take periodic temperature measurments to see what temperature it freezes at? Increase or decrease the ethylene glycol concentration as desired.

[Sredni_Vashtar]

Wizard, the point I was trying to make is that I don't think sub-zero cooling is worth the hassle. You need to add antifreeze and the combined effect of adding anitfreeze and the low water temperature increases the viscosity of the water so much that you need several times more work pumping: this heats the water, so you need much more cooling. All this to get from 5 mbar Hg to 3 mbar Hg.

If you just float loads of ice in circulating water, you can cool it to almost freezing point and enjoy 5 mbar vapour pressure.

If you look at commercial recirculating aspirator stations, they are often like this:

10 L water capacity
150 W water pump
2 aspirators
Optional refrigerated cooling



This goes with what I have been saying: you want as low a power water pump as possible to do the job - anything more wastefully heats the water. If you calculate from flow and pressure requirements, a couple of aspirators need less than 150 W. High pressure, low flowrate pumps are hard to find, so everyone uses massive 0.75 H.P. jobs for a single aspirator and consequently melts loads of ice. Insulating the system helps too.

Refrigerated cooling is a complicated: difficult to break up an old fridge/freezer and suspend the cooling element in the water. I was thinking: use a separate cooling circuit. First, get your recirculating aspirator station working well, then if you want to avoid ice, add a cooling circuit.

Get a fridge/freezer and place a bucket of antifreeze solution inside and use an aquarium pump to circulate the antifreeze through a cooling worm in the aspirator water. Haven't tried this, but a domestic freezer would have have enough power to cool the water, maybe a fridge would too (with fridges, constant workload is the issue - overheating the compressor).