lysander
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Vacuum distillation/desiccation apparatus
Various pieces of this have been asked before, but not with sufficient specificity. So here's another attempt:
I want to assemble a "hobbyist-level" vacuum distillation apparatus for extracting small volumes of "typical" solvents: water, alcohols, ketones, and
diols.
I already have standard lab glassware with 24/40 joints, 10mm barbed inlets.
How do I select a suitable pump?
I want a pump that can run (or hold vacuum) for extended periods (presumably days) to perform vacuum desiccation.
So far my research suggests that I should be looking at diaphragm pumps, but:
Are other features or specifications available in a low-three-figure pump that will make life easier for this application?
What kind of tubing do I want? I know that larger inner diameter is better, but I don't know if something special is used instead of regular
tubing to support the vacuum, or what the standard adapters and sizes are to come off the 10mm glass fittings.
What elements need to be part of such an apparatus, and how should they be configured in the apparatus? For example:
Filters: What are used, and when are they necessary?
Traps: Does this only refer to "cold traps," which I understand to simply be another condenser in front of the pump? If so, is it correct that
this would only help if I can cool the "trap" significantly below the temperature I'm using for the primary distillate condenser?
Manometers
Also, how do these answers change if I'm doing vacuum desiccation of the named solvents (i.e., where the only difference is I don't care to capture
the evaporate)? E.g., can I just let the system exhaust through the pump?
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Little_Ghost_again
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Quote: Originally posted by lysander  | Various pieces of this have been asked before, but not with sufficient specificity. So here's another attempt:
I want to assemble a "hobbyist-level" vacuum distillation apparatus for extracting small volumes of "typical" solvents: water, alcohols, ketones, and
diols.
I already have standard lab glassware with 24/40 joints, 10mm barbed inlets.
How do I select a suitable pump?
I want a pump that can run (or hold vacuum) for extended periods (presumably days) to perform vacuum desiccation.
So far my research suggests that I should be looking at diaphragm pumps, but:
Are other features or specifications available in a low-three-figure pump that will make life easier for this application?
What kind of tubing do I want? I know that larger inner diameter is better, but I don't know if something special is used instead of regular
tubing to support the vacuum, or what the standard adapters and sizes are to come off the 10mm glass fittings.
What elements need to be part of such an apparatus, and how should they be configured in the apparatus? For example:
Filters: What are used, and when are they necessary?
Traps: Does this only refer to "cold traps," which I understand to simply be another condenser in front of the pump? If so, is it correct that
this would only help if I can cool the "trap" significantly below the temperature I'm using for the primary distillate condenser?
Manometers
Also, how do these answers change if I'm doing vacuum desiccation of the named solvents (i.e., where the only difference is I don't care to capture
the evaporate)? E.g., can I just let the system exhaust through the pump? |
My first pump died when acetone got into the oil, personally I would try and keep as many contaminants out the pump as you can, maybe one exception is
a very old pump I have, I use hydraulic fluid as the oil and its a very simple pump. I regularly just empty all the oil out and replace as we buy
hydraulic oil by the barrel for the tractor, the pump didnt cost much and is built like a tank.
Alot of vacuum distillations can be done with a water aspirator, these can save using an expensive pump when there is a risk of damage by
contamination. But apart from that I cant help, I dont do many vacuum distillations as I dont seem to ever get the manometer set up correctly, I would
also like a perkin thingy as i like to be ale to change flasks out.
Vogel and other works have pretty good info on distillations but I can never work out how you connect the manometer correctly, I think it would make a
great you tube tutorial showing how to set up and actually use a manometer, I am pretty sure you dont start the distillation until you have closed off
the manometer so the solvent or whatever can get near the mercury, but many videos make this unclear
Dont ask me, I only know enough to be dangerous
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careysub
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As L_G_A says, for solvent removal (and pressures down to 30 mm or so) a water aspirator pump is the thing. Water contamination not such a problem.
Aspirators require a water trap to keep water from backing up into the distillation system.
The venerable Cason and Rapoport 1950 text "Laboratory Text in Organic Chemistry" (available cheaply through on-line used book sources) has some
excellent chapters on distillation equipment and technique.
Say they about protecting an oil pump:
"Solvent must be kept out of the pump, for a lower pressure than the vapor pressure of the oil cannot be obtained. When the pump will not give a low
pressure, or if corrosive materials are know to enter the pump, the oil should be changed. The pump should always be protected by a tower of flake
sodium hydroxide placed between the pump and the system."
The NaOH column would also dry the gas going to the pump.
"The most common error in the use of the oil pump is that of allowing solvent vapors to reach the pump, as a result of overlooking the effect of
Raoult's law* in reducing the partial pressure of a solvent from a solution in some higher-boiling material. When the mole fraction of solvent is
reduced to a value such as 0.01, the vapor pressure of the solvent int the mixture is only 1/100 of its value when alone... Before an oil pump can be
connected to a system containing a mixture in which a solvent is present, the pressure must first be reduced as low as possible with a water pump, as
the bath is raised to at least as high a temperature as will be later used for distillation with the oil pump. These conditions are maintained until
the pressure in the system ceases to drop and is as low as the water pump will produce."
They go on to suggest that a cold trap (acetone-dry ice) is an alternative to the solvent removal process described above, but "not always feasible" -
but in any case the best positive protection the pump can have.
The venerable ice-and-salt mixture (1 part salt to 2 parts ice) can provide freezing down to -18 C. 1 part CaCl2 to 0.61 parts ice/water produces -39
C; 1 part CaCl2 to 0.70 parts ice/water the temperature can go as low as -55 C, but I wouldn't count on going this low as the mixture ratio is
critical. More convenient than dry ice.
I have wondered whether an activated charcoal filter would be useful after the hydroxide acid filter, or an upstream water absorber (CaCl2) might
prolong the life of the NaOH by limiting is moisture absorption.
*It states that the partial vapor pressure of each component of an ideal mixture of liquids is equal to the vapor pressure of the pure component
multiplied by its mole fraction in the mixture.
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lysander
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OK, water aspirator sounds adequate and great, but how should I drive it? I'm assuming I don't want to just connect to the pressurized municipal
water supply and send a constant stream down the drain ... or maybe I do? My utility cost is $.01/gallon. Something like this claims peak vacuum of 28.5"Hg running 1.75 gallons/minute. I guess $1/hour of vacuum could be reasonable, especially saving the cost/trouble
of traps and filters.
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Little_Ghost_again
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Registered: 16-9-2014
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High pressure pump and second hand lab tap? then use a water container and just pump it in circles.
Dont ask me, I only know enough to be dangerous
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lysander
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Yeah, I guess any water pump and reservoir would be the alternative.
BTW, I have seen two mentions of "capillary bubblers" and warnings about bump and explosion risks in their absence, but haven't found any further
information on what that component is or how it works. Can anyone explain or point me to the right reference or terms?
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Sulaiman
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I recently bought one of those cheap Chinese HVAC rotary vane vacuum pumps,
(£80, relatively cheap but still to me a non-trivial ammount ... don't tell my wife how much it cost!)
It is a dual stage unit with a Chinese rating of 0.3 pa ultimate vacuum,
chosen for chemistry, physics/electronics and de-gassing of silicone rubber to finish off my solar panels,
a single stage 5pa pump would have been £20 cheaper
and for chemistry use probably a better buy because
at 611 pa water boils at 0C so a water/ice bath could not condense the water vapour.
On reflection, for vacuum filtration and distillation
I will probably mostly continue to use my small piston/diaphragm vacuum pump
that is adequate for the job, corrosion resistant and cheap
(£12, Chinese via eBay).(I bought two, I 'killed' one by trying two in series)
If I did not have the small pump I would go for a water aspirator,
not too high vacuum, corrosion resistant,
and with running (not pump recirculated) water, fume disposal.
My lab/shed does not have running water, my reason for not using a water aspirator.
You will probably want a small aquarium water pump to circulate ice water through your condenser,
anything colder becomes expensive
(e.g. one 10kg block of dry ice would cost me £35)
I set mine up so that the receiving flask is in the same ice/water as the circulating pump.
So, in summary, my suggestion is;
if you have running water, go for a corrosion resistant water aspirator
if not then a small diaphragm pump is a good choice
or if you can afford a rotary vane vacuum pump, and oil changes, and the running cost of traps, go for it.
P.S. I have read about but not used a capillary bubbler to reduce 'bumping'
there are two types,
air leak, when distilling under vacuum a (usually) glass tube drawn to a fine point allows air to leak into the system in the body of the boiling
liquid,
or a capillary tube sealed at one end with air in the tube is put in the distillation pot.
either introduces gas/vapour bubbles that allow easy change from liquid to vapour of the distillate.
'boiling chips' add surface area and some add vapour pockets to ease the liquid/vapour transition
P.P.S. anyone got a design for a one-piece vacuum filter/dessicator
or would it be impractical ?
[Edited on 13-9-2015 by Sulaiman]
[Edited on 13-9-2015 by Sulaiman]
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