Owen222
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Building a vacuum distiller
Hi guys,
I am at the design stage of building some equipment to distill under vacuum and I'd welcome some feedback or pointers (if I've missed something
obvious!). I will also be trying to do this as cheaply as possible (while not compromising on efficency (hopefully) or safety (definitely))
I'll also be updating this thread (with photos) as the build goes on so it can hopefully provide a resource for anyone attempting something similar in
the future!
Here is a rough diagram of the system:
1 = This will be a 5l Glass aspirator bottle for the starting liquid. The reason I've chosen this is that you can pick them up pretty cheaply (about
£15/$30 each), where as any other suitable lab glassware of this size and up seem to be at the hundreds of £s/$s.
The ones I've seen are made by Monax so as far as I am aware they will therefore be vacuum-proof. Any thoughts?
2 = This will be the vacuum pump. I've added it here on the diagram for the sake of clarity. I'm not really sure if it matters where on the system the
vacuum enters the process... does it???
The real question I have about this is the vacuum pump itself. While there appear to be some quite reasonable electric pump online around the
£250/$500 mark (I will need a constant and accurately measured vacuum so I'm not considering anything manually-operated) but I'm not sure if they are
suitable for this application.
I've read on various places online that as some of the steam from the boiling liquid will be unavoidably drawn-off by the pump that this could damage
it. Is this right? Some sources have said that only a water aspirator vacuum pump will be suitable as it won't be affected by moisture entering it.
These type of pumps are hugely more expensive so I'd rather not go down this route unless I have to...
3 = This will be a 'cold finger' chamber which will allow the steam to condensate. So that the process can be reliably monitored I would like this to
utilise a proper 'chiller' (rather than an aquarium pump and a bucket of iced water) but chillers seem very expensive. Are they any budget
alternatives? I've seen talk of re-purposing dehumidifing equipment but this looks a little too 'DIY'!
4 = This will be the chamber for the distillate to collect and will be the same type of vessel as in #1.
I'd love to hear any feedback or thoughts from you guys!!
Thanks for reading,
Owen.
PS - I am very aware of my country's laws regarding the distillation of certain liquids and will not be using this equipment to do anything illegal.
While building/testing the equipment I will only be boiling water under vacuum.
The ultimate aim *will* be to distill alcohol (so I will need to be able to do this using the equipment) however this will only be done once the
set-up is working correctly / safely and once all proper legal licences have been obtained.
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forgottenpassword
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Yes, it does matter where you attach the vacuum pump. It should be attached to 4, and 4 should be strongly cooled with salt/ice/water. If you were to
attach it where you have indicated, all of your distillate vapours will travel from 1 straight into the vacuum pump, rather than towards your
condenser (for which, you will need a more efficient condenser than a cold finger).
I assume that you mean a Buchner flask, rather than an aspirator - an aspirator has the spout at the bottom. Buchner flasks are designed to withstand
vacuum.
Water aspirator pumps are not hugely more expensive, they are much cheaper - and more suitable for your application. You will not need a strong vacuum
to distill ethanol.
By all means you can use your Buchner flask as the receiver, 4 in your diagram. The receiving flask will need strong external cooling.
[Edited on 11-8-2014 by forgottenpassword]
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Owen222
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Thanks for the reply... I really appreciate it!!
Sorry if I didn't explain clearly enough... I realised it wouldn't go on #1 but wasn't sure if it mattered where else along the chain it should go.
I've never seen any other vacuum or non-vacuum distilation set up where the 'receiving flask' also needs to be cooled so I hadn't figured this into
any of my plans. Will the distilate really be that hot?
Even if the cold finger is chilled to 0°C? Or are you referring to the surface area being too small? I am aware of other distillers using a cold
finger so I assumed this would suffice...
I was only considering aspirating flasks due to the price I could get them at and I would just use a suitable glass stopper the bottom spout. The
bottom spout would also be useful for draining the waste solution out and cleaning. The ones I'd seen were made by Monax so I'm pretty sure they are
vacuum-proof. I'll have a look into Buchner flasks as well though to see if they are cheaper/more suitable.
Quote: Originally posted by forgottenpassword  | | Water aspirator pumps are not hugely more expensive, they are much cheaper - and more suitable for your application. You will not need a strong
vacuum to distill ethanol. |
Could you provide a link to these cheap water aspirator pumps? All the ones I've seen are over £1,000/$2,000 (compared to the £300/$600 for standard
pumps)...
According to online calculators I will need around 42 Torr or 56 mbar in order to boil an azeotrope of 95% alcohol / 5% water at average UK winter
temperature.
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markx
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| Quote: Originally posted by Owen222 |
Could you provide a link to these cheap water aspirator pumps? All the ones I've seen are over £1,000/$2,000 (compared to the £300/$600 for standard
pumps)...
According to online calculators I will need around 42 Torr or 56 mbar in order to boil an azeotrope of 95% alcohol / 5% water at average UK winter
temperature. |
For example this one:
http://www.ebay.com/itm/Water-Aspirator-Vacuum-Filter-Pump-F...
I would suggest to build a closed loop aspirator station operated by a garden water pump. Meaning you have a container with water (say 10l capacity)
and the garden pump will draw water from the container, push it through the aspirator and dispence the water into the same container, hence creating a
closed loop system that draws vacuum through the aspirator and operates on the energy of the water pump. This setup will save you bigtime on water as
the aspirators can draw huge amounts of water that just goes to waste.
If the goal is to distill volatile solvents (alcohols, hydrocarbons, ketones) or corrosive substances (e.g. acids) or media that generates huge
amounts of water vapour, then a mechanical vacuum pump (if it is not entirely teflon coated inside) is not the weapon of choice. Any scrubbers that
one might use to prevent the harmful vapours from entering the pump will fail in time by saturation, human error or mechanical failure and the
contaminants will enter the pump. It is just a question of when and not if. Once the corrosive or solvent agent is inside the pump oil, the seals and
mating surfaces will say a swift "goodbye" and the unit is scrap. I've seen it happen many times and the only solution is to use an aspirator station
that is basically indestructible by solvents or corrosive agents. Furthermore, one can use a suitable neutralising solution in the aspirator container
to take care of acid vapours or any other contaminants.
Exact science is a figment of imagination.......
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Owen222
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| Quote: Originally posted by markx | For example this one:
http://www.ebay.com/itm/Water-Aspirator-Vacuum-Filter-Pump-F...
I would suggest to build a closed loop aspirator station operated by a garden water pump. Meaning you have a container with water (say 10l capacity)
and the garden pump will draw water from the container, push it through the aspirator and dispence the water into the same container, hence creating a
closed loop system that draws vacuum through the aspirator and operates on the energy of the water pump. This setup will save you bigtime on water as
the aspirators can draw huge amounts of water that just goes to waste. |
Yes, I definitely don't want to waste water but I was wanting something a little more precise. How difficult would it be to add some sort of means of
monitoring and adjusting the pressure of that type of garden pump? Would a garden pump be able to achieve and maintain 42 Torr / 56 mbar?
| Quote: Originally posted by markx | | If the goal is to distill volatile solvents (alcohols, hydrocarbons, ketones) or corrosive substances (e.g. acids) or media that generates huge
amounts of water vapour, then a mechanical vacuum pump (if it is not entirely teflon coated inside) is not the weapon of choice. Any scrubbers that
one might use to prevent the harmful vapours from entering the pump will fail in time by saturation, human error or mechanical failure and the
contaminants will enter the pump. It is just a question of when and not if. Once the corrosive or solvent agent is inside the pump oil, the seals and
mating surfaces will say a swift "goodbye" and the unit is scrap. I've seen it happen many times and the only solution is to use an aspirator station
that is basically indestructible by solvents or corrosive agents. Furthermore, one can use a suitable neutralising solution in the aspirator container
to take care of acid vapours or any other contaminants. |
That's exactly what I wanted to hear, thanks for confirming!
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markx
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| Quote: Originally posted by Owen222 |
Yes, I definitely don't want to waste water but I was wanting something a little more precise. How difficult would it be to add some sort of means of
monitoring and adjusting the pressure of that type of garden pump? Would a garden pump be able to achieve and maintain 42 Torr / 56 mbar?
|
Simple enough if you are at least mediocre in your mechanical inclination. Just add a pressure gauge to the water pump outlet to monitor the water
pressure and a vaccuum gauge to the aspirator vacuum output. That's about it. You can omit the water pressure gauge....it's a nice indicator, but does
not really tell you anything useful in terms of the vacuum side of the system. If you want to regulate the vacuum then add a ball valve to the output
of garden pump to restrict the flow of water into the aspirator....although I can already tell you that it is not an option you will be using much.
Most time you just run the aspirator at full throttle anyways.
The maximum vacuum an aspirator system can draw is limited by the vapour pressure of the working fluid at the temperature that it flows through the
aspirator (e.g. 32mbar for water at 25C). That is true if other factors are not the limiting ones: water pressure and particular aspirator design.
I'm confident that 56mbar can be achieved via a generic aspirator and impeller type garden pump that outputs inbetween 3-4bar of water pressure. But
you just have to test it out on your particular setup to get the final confirmation...
[Edited on 12-8-2014 by markx]
Exact science is a figment of imagination.......
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forgottenpassword
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| Quote: Originally posted by Owen222 | Thanks for the reply... I really appreciate it!!
Sorry if I didn't explain clearly enough... I realised it wouldn't go on #1 but wasn't sure if it mattered where else along the chain it should go.
I've never seen any other vacuum or non-vacuum distilation set up where the 'receiving flask' also needs to be cooled so I hadn't figured this into
any of my plans. Will the distilate really be that hot?
Even if the cold finger is chilled to 0°C? Or are you referring to the surface area being too small? I am aware of other distillers using a cold
finger so I assumed this would suffice...
I was only considering aspirating flasks due to the price I could get them at and I would just use a suitable glass stopper the bottom spout. The
bottom spout would also be useful for draining the waste solution out and cleaning. The ones I'd seen were made by Monax so I'm pretty sure they are
vacuum-proof. I'll have a look into Buchner flasks as well though to see if they are cheaper/more suitable.
| Quote: Originally posted by forgottenpassword | | Water aspirator pumps are not hugely more expensive, they are much cheaper - and more suitable for your application. You will not need a strong
vacuum to distill ethanol. |
Could you provide a link to these cheap water aspirator pumps? All the ones I've seen are over £1,000/$2,000 (compared to the £300/$600 for standard
pumps)...
According to online calculators I will need around 42 Torr or 56 mbar in order to boil an azeotrope of 95% alcohol / 5% water at average UK winter
temperature. |
I suggested cooling for the same reason as I suggested not attaching the vacuum to 1. Usually people do no vacuum distill low boiling solvents, so it
is not surprising that you have not seen it mentioned. If your solvent is boiling near room temperature one one end, then it will boil near room
temperature at the other end.
What temperature do you intend to distill at?
An aspirator flask is not designed to operate under strong vacuum. Maybe it will hold, I don't know - I've never tried it. I have a couple of pyrex
aspirator flasks which are quite thick walled. You can try it and see, if you want.
A cheap aspirator pump like the one linked to is what I had in mind. Not only is it cheap, but it costs no money to run.
A cold finger has a limited surface area. Whilst it will work, it will lead to a slow rate of distillation compared to more traditional distillation
setups.
[Edited on 12-8-2014 by forgottenpassword]
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Fulmen
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Vacuum must be connected after the condenser. Simply put; what drives the vapor from the boiler to the condenser is a pressure difference. As the
liquid boils it expands, the vacuum sucks these vapors into the condenser. With the vacuum connected to the boiler the vapors will simply be sucked
out there.
As for cooling, it depends on the boiling point. You need to cool to well below this temperature to ensure full condensation. Consider the process at
atmospheric pressure, for ethanol/water you have a boiling point of around 75°C. Now imagine ethanol at 50-60°C, it's below the boiling point but
there will still be substantial amounts of vapor coming off. Same applies to a vacuum setup, by looking up vapor pressure for ethanol and using
atmospheric distillation as a reference you should be able to figure out the required condenser temperature.
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Owen222
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| Quote: Originally posted by markx |
Simple enough if you are at least mediocre in your mechanical inclination. Just add a pressure gauge to the water pump outlet to monitor the water
pressure and a vaccuum gauge to the aspirator vacuum output. That's about it. You can omit the water pressure gauge....it's a nice indicator, but does
not really tell you anything useful in terms of the vacuum side of the system. If you want to regulate the vacuum then add a ball valve to the output
of garden pump to restrict the flow of water into the aspirator....although I can already tell you that it is not an option you will be using much.
Most time you just run the aspirator at full throttle anyways.
The maximum vacuum an aspirator system can draw is limited by the vapour pressure of the working fluid at the temperature that it flows through the
aspirator (e.g. 32mbar for water at 25C). That is true if other factors are not the limiting ones: water pressure and particular aspirator design.
I'm confident that 56mbar can be achieved via a generic aspirator and impeller type garden pump that outputs inbetween 3-4bar of water pressure. But
you just have to test it out on your particular setup to get the final confirmation...
[Edited on 12-8-2014 by markx] |
Thanks!
Some really useful info there!!
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Owen222
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I intend to distill at around 15°C.
| Quote: Originally posted by forgottenpassword |
I suggested cooling for the same reason as I suggested not attaching the vacuum to 1. Usually people do no vacuum distill low boiling solvents, so it
is not surprising that you have not seen it mentioned. If your solvent is boiling near room temperature one one end, then it will boil near room
temperature at the other end. |
Yes this makes perfect sense!
I'm still puzzled why I haven't seen this in any other kind of set up (mostly I have seen set-ups using rotary evaporators which definitely don't cool
the distillate collecting vessel but I've not been able to find any info as to why this is...)
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Fulmen
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You are right, with the right setup cooling of the receptacle isn't necessary. Basically you need the inlet to be at the bottom of the condenser and
the vacuum connected to the top. This way any vapors coming off the receptacle will be sucked back into the condenser, the evaporation will also cool
the receptacle. Insulating it might be beneficial though.
BTW, don't forget that you need a fractional column for this.
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quantime
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Kugelror
http://en.wikipedia.org/wiki/Kugelrohr
I definitely want one.
[Edited on 15-8-2014 by quantime]
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Texium
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