Sciencemadness Discussion Board

best aspirator for 24/40 glassware sizes?

coppercone - 13-5-2018 at 13:26

Can you guys help me choose an aspirator that can be used for 24/40 glassware?

I am inclined to pick a glass one, like the ones offered by sigma aldrich and ebay,
https://www.ebay.com/itm/Glass-Water-Aspirator-Pump-Humboldt...

https://www.sigmaaldrich.com/catalog/product/aldrich/z167940...

But the one sigma lists is only 100mmHg

This one, made of polypropylene ( i really wanted glass)
http://cynmar.com/vacuum-pumps-accessories/25570-FILTER-PUMP...

is rated for 20mmHg, and it has a specified flow rate.

Are there similar glass aspirators that can pull a low vacuum of 20mmHg? I intend to use water at 4 degrees Celsius. And what kind of flow rate is required (so I may know pump specification).

zed - 13-5-2018 at 14:32

Try the search engine. There have been numerous discussions about aspirators, and many personal recommendations.

https://www.youtube.com/watch?v=tYLlkTDstmo

Here, Nurdrage, gives us a tutorial.

As an aside: The aspirator has a limited evacuation speed. However, in some circumstances, it can be used in line, with a strong, vacuum storage vessel (like a stainless steel beer keg). And, the large volume of stored vacuum, can prove to be very useful.





[Edited on 13-5-2018 by zed]

coppercone - 13-5-2018 at 16:48

I did give it a search but it did not answer my questions for how much vacuum they can pull vs the flow rate vs the temperature. There is a thread from 2007 that is somewhat illuminating but it does not have the specific details I need.

I made a rather powerful one before using a steel aspirator and a heavy pump but it was not stable due to the heating of the pump. I want to know what the best design is. I would like to find a 20mmHg capable glass one that matches the plastic one I linked in the thread.

The biggest problem I have is finding specifications for an aspirator to buy, made of glass, that has things like flow curves.

[Edited on 14-5-2018 by coppercone]

Deathunter88 - 13-5-2018 at 23:16

This one is probably the best bang for your buck. https://www.fishersci.com/shop/products/thermo-scientific-na...

You can find it on ebay/amazon for around 15 bucks each.

zed - 14-5-2018 at 17:07

Glass? Well, it's a nice thought. But, I'm a giant, and I am rather clumsy. I break things.

Glass is just too fragile, for me to use, in that capacity. Metal for me! If'n corrosive gases eat my aspirator up.... after a while; fine.

It's a 10 to15 dollar item. I'll buy another one.

coppercone - 15-5-2018 at 16:51

I am having alot of trouble finding curves for an aspirator.

My market research tells me aspirators typically use about

5-20 LPM

and pressures above 8PSI (up to 25?)

But I can't find proper specifications on any of them:

http://spectrumlabs.com/labware/Aspirator.html - 25 PSI/unknown Flow, 60mmHg

https://www.sigmaaldrich.com/catalog/product/aldrich/z150762...

No data

https://cynmar.com/vacuum-pumps-accessories/25571-FILTER-PUM...

16mmHg (great)

No data for flow or pressure

https://cynmar.com/vacuum-pumps-accessories/25570-FILTER-PUM...

20mmHg 17LPM unknown pressure

https://www.thomassci.com/Equipment/Pumps/Vacuum/_/NALGENE-P...

7psi 7lpm unknown vacuum



Does anyone have real data?

How do these things operate? Do you basically need to reach a pressure that kinda platues out and it enters a flow regime?

I wish I had suction flow, vacuum level, water flow and water pressure for each one.

I want to specify a pump but the span is fucking ridiculous.
Given how expensive acid resistant pumps are, it would be worthwhile to buy every aspirator and the necessary flow meters and plumbings to categorize each of them, and probably still save money!

I guess a good question is: How much flow do you think is required for 24/40 scale work with a hot plate that maxes out at 1500W?

Something like distilling a liter of ether being heated by 1500W is the worst case (assuming no one can give me any numbers and I need to calculate something from first principles)

But I don't even know how much energy goes in from the hot plate to the glassware!!! :o

Sulaiman - 16-5-2018 at 00:56

Quote: Originally posted by coppercone  
I am having alot of trouble finding curves for an aspirator.
...
I guess a good question is: How much flow do you think is required for 24/40 scale work with a hot plate that maxes out at 1500W?
...
Something like distilling a liter of ether being heated by 1500W
...
But I don't even know how much energy goes in from the hot plate to the glassware!!! :o

Did you look at the link above provided by Deathunter88 ?
i.e. that link has the data that you want.
...
How quickly can you transfer heat to liquids in a flask by direct heating?
(I call it effective heating power)
Fundamentally it is determined by:
(difference in temperature across glass) x (thermal conductivity of glass) x (Area of heated glass) / (thickness of glass)

Recommendations for Borosilicate RBFs usually specify about 150oC maximum difference in temperature across the glass wall,
but less stress is better.
SO
the maximum heating rate is mostly determined by RBF size.

If you are going to distill 1l of diethyl ether you would be using a 2l flask,
which is lucky because I have measured the performance of a 24/28 2l flask with 1l water on a 1500W hotplate :D
I think I gave this data recently, not sure, here it is again;

1 litre of water in a 2 litre RBF, in a 5 litre aluminium pot with 2 litres of vegetable oil, on a 1500W hotplate.
With no insulation the water boiled off at a rate of 11.5 ml/minute,
that equates to an effective heating power of 432 Watts.

The hotplate did not operate at 1500W continuously as above 111oC the thermostat kept operating to limit the oil temperature to 1760C
The experiment was performed outdoors in England during January,
so Summer temperatures would give a higher effective heating measurement, and thermal insulation should be even better.

...
For 'vacuum' filtration the pump only has to operate long enough to evacuate the filter flask and seal the filter paper (if used),
a few seconds.
Whilst filtering the vacuum pump is mostly off, required only to compensate for any air leaking into the flask.

Similarly, reduced pressure distillation requires pumping down to the required pressure,
then compensating for any leaks.
If you have a trap before your pump then it also acts as a 'reservoir' for vacuum, allowing longer periods between pump operations.
I'm not that advanced yet, but a small air leak can be used, via a glass tube, to add bubbles to the boiling liquid to reduce/prevent bumping,
so the pump would have to deal with that.

P.S. I still don't have the confidence, gained by experience, to distil 1 litre of diethyl ether in a single batch :o

[Edited on 16-5-2018 by Sulaiman]

coppercone - 16-5-2018 at 09:04

Do you know how the actual curve looks like for an aspirator?

Like the mathematical model used to relate the parameters?

My idea is to use a electronic vacuum gauge to vary the power to a pump, to regulate vacuum. Do you know how linear the relationships are? If its exponential or something that might result in control system oddities that are difficult to work out.

At least that aspirator is a good starting point for catagorization if curves are not found.

I would need to make sure the pump that i choose has a flow and pressure curve that overlaps with variable aspirator operation and does not have any bizzare 'singularities' due to highly nonlinear functions so that the time constant is reasonable.

Based on what you measured it seems that using 1000w of heat entering the flask is a good safe number to base calculations on.

So the requirement is that the trap requires 1000w of cooling at its cold temperature to ensure minimal gas flow, and significantly less cooling if its part of a distillation and an adequate distillation column is used. Its kind of a bizzare requirement because you probobly don't want to use your aspirator apparatus as your primary condenser for something like driving off solvents, but it is a good redundancy incase there is a primary condenser failure so you dont have to deal with some kind of stupid situation involving a bunch of heavily contaminated water that has a high volume.

I figure the secondary condenser that isolates the water vapor from the cold trap and the reciving flask only needs maybe 100w of cooling (very overrated but should account for situations like insulation falling off the tubing and stuff.

This is starting to feel stupid but we dont have liquid nitrogen and co2 machines for private use typically. I would probobly reserve using ln2 from my dewar for very specific distillations. Eventually the guy at the welding store is gonna start getting curions whe you gothere every week. Maybe buying a big ass dewar is an option for some of us that prevents all the running around but its heavy on storage space and kind of an aspyxiation hazard..

I amalso wondering about what vacuum variation can do for backflow. If you drop pressure to prevent bumping/superboiling, you may suck in water into your trap past the condenser.

I am not sure if there is a good way to remove the water from the water trap.

The three o options i thought of so far
1)tollerate this behavior. make some kind of special trap with a 'airlock' of valves that can jetison excessive water periodically. One of these valves needs to open smoothly to prevent sudden vacuum variations that can disturb something sensitive like rotary band distillation or reflow, since the volume of the trap needs to be probobly three times larger then the volume of your setup

2) add some kind of check valve. Best i can think of is a swagelok check valve. I need to think about what a check valve means for the control system

3) add some kind of bleed. I need to think about this. You are adding extra gas to the system.

In a rigid system, you would need to run the aspirator at full capacity and drown it with gas.

This means that your pump runs at maximum so the theoretical ware is heaviest and power consumption is highest.

Since we are humans we can adjust the maximum pressure and trim it with a bleed valve.

To do anything more then that, to have dynamic adjustment with minimum consumption of bleed gas (which could be argon) you would need to make a more complicated control system so both pump flow and bleed are controlled by some kind of intelligent algorithm.

The nice thing about bleed is that it should do alot to prevent bumping if the gas is introduced into your boiling flask through a capillary tube. I dont know if its motion through the liquid can decrease purity of a distillation due to the increased turbidity and aresol generation (ojtop of the boiling that occurs). I expect this to be small but if the bleed gas flow is heavy you would probobly not want to introduce all of it in the boiling flask.

But due to the fact that the vacuum level in the flask will be regulated by a controller, if you do want to use a bleed capillary tube, you should probobly have a control system on it to maintain constant flow regardless of the pressure of the reactor.

This would be useful for distilling air sensitive chemicals. But, can enoigh water vapor be trapped from the aspirator without liquid nitrogen to make use of its vacuumevels for things like reduced pressure distillations or reactions?

Can you add a tertiary bleedvalve as not to effect flow in the condenser area too much but to prevent diffusion? Of the water vapor?

Can someone explain the dynamics of the aspirator water vaper as backflow in the system? Its hardto imsgine it will go anywhere so long there is fluid flow from the gasses or vapors traveling through the glassware...

My understanding is that if i had an aspirator hooked up to a flask with nothing in it, and it ran for long enough, the water vapor would reach some kind of concentration on the volume of the flask as there is basically some small kind ofwater surface being exposed to the interior..

But if something is present in the flask, and it is being evacuated, the water vapor will form some kind of conce tration salient in the flow path. Is this correct? And the higher thr flow is, the shorter the ingress of the water vapor at significant concentrations is?

Does some kind of strict layer or boundary form in systems with flow? How does diffusion occur in flow streams? Can someone show me some kind of function, preferably illustrated, to show the concentration of water vapor diffusion (if this is the correct word) in a fluid stream?

I imagine its small because like, we dont worry too much about air back tracking into our propane tanks, but i assume it also depends on some kind of gas/vapor solubility constants? Can it be thought of in regards to hydroscopic behavior?

coppercone - 16-5-2018 at 09:08

Also i say the word flow but i dont really know how it behaves in gasses. Is the water vapor concentration backflow (for lack of better words) different in gradient with turbulant and laminar flow?

streety - 16-5-2018 at 18:00

Are you trying to build a continuous flow system or a batch system? I assume it would be a batch system so I would avoid bleeding off liquid from the cold trap and instead just use a suitably sized cold trap.

If you want to precisely control the pressure it might be more efficient to complement the argon bled into the boiling vessel with a second valve close to the aspirator bleeding in air. Controlling the power of the pump would be more challenging.

What are you planning to use this with? Depending on the temperature you need for the cold trap adapting a freezer or water cooler might be an option. If you are going to the effort of avoiding all contamination of the aspirator water would a vacuum pump be a better choice?

zed - 16-5-2018 at 18:33

Well, sometimes a thousand Watts is a thousand watts, and sometimes it isn't.

I have never successfully distilled a material that required seriously high temperatures, over a stir plate, either with a vacuum or without it.... Regardless of Wattage. And, Oil baths have not worked well for me.

But, a well fitting heating mantle of lesser Wattage, because of superior heat transfer, always seems to get the job done.

As for the reliability of Vacuum aspirators; well, they are balky. Historically, we controlled water flow through the aspirator manually, via adjusting the tap. All is well, until someone in an nearby room, flushes a toilet .... etc.. Then, abruptly ...Back up

coppercone - 17-5-2018 at 05:55

I want it for general purpose lab use for 24/40 wcale chemistry that has to deal with like 2 liters of reactants or solvents max.

I thought precision control might be useful because maybe you can do things like control reaction dynamics i.e. adjusting the evaporation of some kind of passive chemical in the reaction by varying pressure to maintain finer thermodynamic control etc.

The purpose of a cold trap bleed would be because i think the control system could behave in situations that lead to quick pressure changes in the reactor or distillation that makes the water flow change so that the pressure in the flask pulls in the water in the stream

I guess you can restrict the time constants to disallow abrupt pressure changes but there is always the possibility of the flask running out of gas and being under a sealed vacuum where i guess it would need to be really slow to increase pressure unless of course the bleed valve is made mandatory for the aparatus to work

I am kind of paranoid about a check valve clogging up if a sublimation is done.

And i guess what im trying to do is determine a useful build specification. If its not too hard then i might as well put in the extra features to be capable of handling continuous reaction process but i feel that requires much larger scale to make sense.

I did things like make ether from h2so4 etc, fuming hno3, some organic reactions fsuitable for perfumes, etc but i am interested in all types of chemistry be it organic or inorganic and very interested in instrumentation design and making generally useful high end lab tools that are reliable and decrease user anxiety and increase repeatability to make science easier...

I renember my hno3 being distilled over fuming sulfuric acid and my control system was poking a hole in the aspirator hose and using a double gloved hand to control bumping and drip rate.. I found the process highly stressful and primitive even eith a bump flask.

I wanna see what i can do, i have a vacuum pump already but i justfind the aspirator interesting and cool and

I seek to better understand the backflow of moisture process. What should i study to get a physics understanding of it?

I dont like using a vacuum pump unless i got a dry ice or ln2 cold trap.

The eorst distillation i did was vacuum h2so4 using a aspirator that was kinda harrowing.

I think my vacuum distilled hno3 over h2so4 for a single run was like 94 percent conc, up from either 70 percent or vacuum fortified 70 percent that i distilled once before with a small vigrux column

My measurement of concentration was with a volumetric flask at 25ml and a 0.001 gram analytical balance using equations and constants i pulled from the crc handbook

[Edited on 17-5-2018 by coppercone]

streety - 18-5-2018 at 17:31

If the intention is to develop a system that allows you to perform reactions you currently are unable to do it might be useful to write them out in as much detail as possible. You can then develop a list of features.

If the subject of vacuum systems itself interests you I would suggest doing what you were contemplating earlier in the thread, buy several aspirators and fully characterize them. It would not be excessively expensive with a no-frills pump and would be interesting data. You could even consider designing your own aspirator based on what you learn.

coppercone - 19-5-2018 at 07:29

well the purpose of conferring with you guys is to kinda future proof the thing, its a significant investment (with my time being the #1 expense always with custom engineering, unless its made of gold), so this thing should be all it can be

if I make a a nice enclosure for it and engineer it all compact, then it turns out this entire swath of crap is unavailable because I missed some 200$ part, it would be a crisis.

But yes, a system block diagram is in the works

[Edited on 19-5-2018 by coppercone]

zed - 19-5-2018 at 12:51

You can't possibly foresee every potential problem. For me, it leads to overthinking, and paralysis.

Build a simple system, and improve it as need arises. Measure it, if you must.

Shoot first, for acquiring an aspirator that actually works. Some models....don't!