Sciencemadness Discussion Board

Vacuum Distillation - Best Vacuum Pump? Will the glassware implode?

beerwiz - 6-2-2014 at 03:17

What is a good vacuum pump for less than $1000?

I'm trying to convert sodium bisulfate to sodium pyrosulfate. I tried distillation by heating it on a hot plate with a thermocouple and no vacuum and it wouldn't go past 205 Celcius. I need it to reach 315 Celcius with no vacuum. I guess I need to add insulation to the flask, add a sand bath and add vacuum to lower the high boiling point (315 C).

I'm now looking to add a vacuum. Which vacuum pump is the best for this? I've seen some that can pull a 0.001 mmhg vacuum, but I'm wondering if the heavy walled glass flasks, condensers, etc will be able to withstand the pressure. The glassware doesn't come with maximum negative or positive pressure specs. It just says heavy walled glass, will it implode or will it be okay?

Based on my calculations to lower the boiling temperature to 150 Celcius, the ultimate pressure will need to go down to 25 mmHg.

I've never attempted a vac distillation before. Any tips and pointers in the right direction would be appreciated.

It comes down to two questions:

1. What vacuum pump to use?
2. What kind of glassware to use to accomodate 25 to 1mmHg pressure without imploding?

[Edited on 6-2-2014 by beerwiz]

[Edited on 6-2-2014 by beerwiz]

testimento - 6-2-2014 at 04:43

1000 bucks is very high price for a pump. I've got mine for 70 bucks and it is even too powerful. Aspirator didn't cost more than 20 bucks. :) For a grand you're supposed to get regulated, PTFE-lined high vacuum pump.

If the glassware is of decent quality, flawless or free of cracks and ROUND, it will hold any vacuum very well. The pressure for absolute vacuum is about 1kg/cm2, so difference between high vacuum and aspirator vacuum in terms of stress is very small. Never use conical or edged glassware in vacuum processes, only round.

For vacuum distillation as it stands, apply vacuum first and then start the heating.

beerwiz - 6-2-2014 at 04:52

Quote: Originally posted by testimento  
1000 bucks is very high price for a pump. I've got mine for 70 bucks and it is even too powerful. Aspirator didn't cost more than 20 bucks. :) For a grand you're supposed to get regulated, PTFE-lined high vacuum pump.

If the glassware is of decent quality, flawless or free of cracks and ROUND, it will hold any vacuum very well. The pressure for absolute vacuum is about 1kg/cm2, so difference between high vacuum and aspirator vacuum in terms of stress is very small. Never use conical or edged glassware in vacuum processes, only round.

For vacuum distillation as it stands, apply vacuum first and then start the heating.


Your tips are very appreciated. I have a flask that is round but with a flat bottom. Is that a problem? Will it implode?

testimento - 6-2-2014 at 04:59

Flat bottom flasks are usually suitable for vacuum distillation and it has been taken into consideration at manufacturing phase by increasing the thickness. I have had no problem using flat bottom flasks in vacuum distillations.

If one is unsure and wants to use vac dist anyhow, one could put water into the flask and apply high vacuum and let it stand for a while. If it happens to implode, only water and glass shatter is ejected. This doesn't ensure the durability in long run of course and the pro setups always use plastic coated apparatus or apply vacuum behind fume hood protective cover. The power of implosion may be quite high since even small flasks endure total stresses of more than metric ton.

[Edited on 6-2-2014 by testimento]

Metacelsus - 6-2-2014 at 11:03

If it's quality glassware, it should stand up to heat and vacuum. However, if it's cheap, it may implode upon heating, or even with vacuum alone.

Ascaridole - 6-2-2014 at 13:04

Just remember temperature gradient! Whatever the hottest point on your glass to the coolest point on your glass should not exceed about 120ºC or thermal stress becomes a factor even for borosilicate. Having a flat bottom is not advisable at 25mm Hg. If performing a vacuum distillation you also need to consider a ballast for buffering the vacuum in the system and a method of accurately measuring your vacuum (McLeode, "U" manometer, Stick manometer, mechanical meter, etc.) Also if you get any type of oil diffusion, rotary vane, etc. type of pump with oil you need to get at least a dry ice trap to protect your oil from vapors.

If you still have questions about the glass Ace glass has a good reference section on their website.

Also I'm not sure a vacuum distillation of sodium bisulfate will yield pyrosulfate. I'm pretty sure the dehydration is not a kinetic process but a thermodynamic process. Was there a reference you could share?

beerwiz - 6-2-2014 at 21:48

Quote:

Also I'm not sure a vacuum distillation of sodium bisulfate will yield pyrosulfate. I'm pretty sure the dehydration is not a kinetic process but a thermodynamic process. Was there a reference you could share?


Appreciate the clarifications, thank you. About the NaHSO4, the reference is US Patent 6767528.

"2NaHSO4→Na2S2O7+H2O @ 240-250C"

[Edited on 7-2-2014 by beerwiz]

ZIGZIGLAR - 7-2-2014 at 00:46

Have you tried using a Nomograph to give you a better idea of what temperatures and pressures you can play with?

If you already have access to a heating device that can manage above 100c, then you can get away with a weaker vacuum.

Even cheap single stage vacuum pumps designed for portable refrigeration line purging can pull a decent vacuum. That being said, two stage pumps aren't that much more and can get you down to 0.015mmhg.

Where are you doing the distillations? Do you have access to a nitrogen or dry ice cold trap to protect the pump? You're likely to spend more on this than the pump itself to be honest haha

Another tip, vent to atmosphere prior to shutting the pump off or the shift in pressure will either suck volatiles into your pump case or contaminate your distillate with case oil.

Ascaridole - 7-2-2014 at 13:05

As I said before I'm pretty sure we are talking about a thermodynamic process not a kinetic process. The patent does indeed describe the process for converting the bisulfate into the pyrosulfate in great detail and even gives its deltaH = 20.4k-cal however it never mentions reduced pressure to increase reaction efficiency.

Pulling a vacuum produces a lower boiling point because of kinetic properties i.e.: partial pressure and gas law stuff. Decomposition is a bond breaking process in other words your heating something not to make its molecules move more freely and boil but to give them enough energy to break.

If by some chance your experiment should work it would imply that vacuum distilling sulfuric acid would lead to a possible route to free SO3 + H2O, an appealing approach for the home chemist but as I said, not likely. But hey it's science, who knows. Just be safe in your experiments.

Good luck

beerwiz - 14-2-2014 at 06:35

Quote: Originally posted by ZIGZIGLAR  
Have you tried using a [url=http://www.sigmaaldrich.com/chemistry/solvents/learning-center/nomo-assets.html]
Where are you doing the distillations? Do you have access to a nitrogen or dry ice cold trap to protect the pump? You're likely to spend more on this than the pump itself to be honest haha


I've looked at the Nomograph and determined the proper temps and pressures. The distillation will be done indoors so extra safety measures must be taken. I don't have a fume hood. I will use an ice trap. I made the mistake of putting a steel thermocouple in the reaction flask and the surface got mangled up and dissolved by the fumes dripping a green liquid/precipitate down the flask into the reaction.

[Edited on 14-2-2014 by beerwiz]

gravityzero - 14-2-2014 at 17:00

A nice pump can be had for under $200 new. Most name brand glassware is intended to support heavy vacuum and high heat.

I don't think you will experience any issues, unless using really cheap stuff.

The Rocker pump I have can pull vacuum to nearly 1inHg without causing any implosion risks.
I don't really have the need to go lower, but assume the vacuum costs would increase.

At this vacuum items that boil ~300C come over below 150C.