Sciencemadness Discussion Board

help with diminshed pressure distillation

0010110 - 20-2-2009 at 00:36

I have recently obtained a 24/40 ground glass distillation apparatus and most of the stuff i need to distill nitric acid under diminished pressure. There are a few things im am uncertain of. Don't worry about the safety precautions and handling methods of nitric acid for i am well aware of those; I have had some experience in handling red fuming nitric acid. The uncertainty i am having is what type of tubing to use when attaching the vacuum pump to the vacuum outlet on the apparatus. I am unsure if the common clear plastic tubing would with stand the nitric acid + the heat or to long. I doubt it will. The second thing I am unsure of if the vacuum pump i am using will be sufficient for distilling white nitric acid. The pumps the gauge average running read out says 25 inches of Hg. I'm not to sure about the type of measurement the gauge uses. Oh and i would also like to mention that i have two washing flasks to put between the vacuum and the apparatus. Any help would be greatly appreciated and Thank You in advance. :P

chemrox - 20-2-2009 at 17:08

Do NOT attach the vacuum adapter to the pump. If you hook directly to your pump you will quickly destroy the seals and rust the guts out of a mechanical pump distilling nitric acid. Please look up vacuum ditillation in Vogels or Organic Chem Survival Manual, threads here (UTFSE), etc. There are pressure conversion charts all over the web, use Google. As a minimum you must have a water trap if you use an aspirator or aspirator pump and you must have a KOH cylinder to protect you mechanical pump from acid. You should have a vapor trap and a vacuum release valve. A balast of 2-3 L is standard and a manometer or guage to assess vacuum, a manostat to control it is nice to have.

Globey - 22-2-2009 at 10:13

What chemrox said, plus, make sure the hose is thick walled enough as not to collapse under the reduced pressure. Even Tygon type seems to just barely work in a pinch, but I'd go for something more heavy duty. Butyl rubber car hose might be good. Also, I seem to remember that when distilling HNO3, you want a small amt of some reducer in the retort. Inevitably, your going to get some NOx near the end, but that doesn't mean that anything is impure.

DJF90 - 22-2-2009 at 12:39

Globey, I do not think you want a reducer in the retort. Nitric acid is a strong oxidiser and adding a reducing agent will lead to loss of yield (and possibly a dangerous scenario). Be aware that (conc.) nitric acid does not like rubber hence the need to distill it in a ground glass setup or all-glass retort. The only tubing not affected by conc. nitric acid is PFA/FEP/PTFE (consult a chemical resistance chart, there may be others), so use one of these if possible, otherwise just be careful and dont let your tubing corrode too much.

Contrabasso - 23-2-2009 at 11:27

You need to ensure that the pressure is low enough for the HNO3 to come over without decomposing, you then need to check that the condenser is adequate to catch all the vapour well before the outlet. Even use two condensers and/or use a small pump to recirculate iced water. Seriously consider an aspirator pump they are cheaper for when the acid kills them than a mechanical pump.

I have considered getting a bucket of water and a small pump to recirculate through an aspirator pump, but I need to consider whether the pressure achieved will allow the distilation to procede correctly.

Obviously if the vacuum is too great you will never actually condense the product (it'll go down the pump as vapour) If the vac is too little you risk the product decomposing.

Mr. Wizard - 23-2-2009 at 11:41

Somebody correct me if I'm wrong. The vapor pressure over a liquid is determined by the temperature of the liquid and vapor at equilibrium. So once the air , or other non acid vapor, over the boiling acid is removed couldn't you seal the system and maintain the low pressure by adequate cooling on the receiving side? The only problem would then be pressure resulting from breakdown products of the distillation or pressure rise due to inadequate cooling. Any thoughts on this?

brew - 23-2-2009 at 12:58

0010110, you could always contact a place that deals with vacuum pumps, used for distilation, as they usualy also sell the tubing that would be the right strength and size for your needs. 8mm(I.D.)size tubing I believe is the go. Also I'd consider, if this is your beginnings of using vacuum distillation, start of with something alot safer than Nitric acid. Learn the ins and outs of this type of procedures.

vulture - 23-2-2009 at 14:56

Quote:

Somebody correct me if I'm wrong. The vapor pressure over a liquid is determined by the temperature of the liquid and vapor at equilibrium. So once the air , or other non acid vapor, over the boiling acid is removed couldn't you seal the system and maintain the low pressure by adequate cooling on the receiving side? The only problem would then be pressure resulting from breakdown products of the distillation or pressure rise due to inadequate cooling. Any thoughts on this?


That is correct. Once the maximum pressure drop has been achieved equilibrium sets in and the pump is just maintaining the pressure. The bulk of corrosion occurs during startup, mostly because you are basically degassing your HNO3 first.

If you're planning to do alot of HNO3 distillation a PTFE/ETFE membrane pump might be worthwile. They can be had for $200 used and 100mbar is adequate for distilling HNO3.

What's inside your washing flasks? Have you provided safeguards against it being sucked into the pump or the apparatus? Note that water vapor is already capable of destroying a standard rotary vane pump.

[Edited on 23-2-2009 by vulture]

[Edited on 24-2-2009 by vulture]

chemrox - 23-2-2009 at 18:32

@Globey- I'd love to find some of that 2" butyl car hose with a 1/4 or 3/8 id. That is nice stuff for vacuum hookups but I can't find it anywhere.

Arrhenius - 23-2-2009 at 23:41

Quote:
Originally posted by Mr. Wizard
Somebody correct me if I'm wrong. The vapor pressure over a liquid is determined by the temperature of the liquid and vapor at equilibrium. So once the air , or other non acid vapor, over the boiling acid is removed couldn't you seal the system and maintain the low pressure by adequate cooling on the receiving side? The only problem would then be pressure resulting from breakdown products of the distillation or pressure rise due to inadequate cooling. Any thoughts on this?


I think this was misleadingly responded to. In theory this might work, but as you heat your pot of HNO3, its vapor pressure goes up, and the cooling is not so efficient as to condense 100% of the vapor. In practice, you have to keep pumping on it otherwise the pressure works its way back up to the point that your distillation stops, whether your distillation be in a rotovap or a fractionation setup.

I would not count on your wash bottles to save your pump. A dry ice trap is pretty standard, but this too will not save it. I'm not familiar with distilling HNO3, but is an aspirator an option? This would surely save you a lot of troubles.



[Edited on 23-2-2009 by Arrhenius]

panziandi - 24-2-2009 at 02:40

I'm sure some of my old texts say to use an aspirator.

biotech7 - 24-2-2009 at 05:56

i performed pressure vs B.P. of nitric acid simulation with ACDLab.
i hope it's useful to 0010110.

B.R.

1.gif - 13kB

DJF90 - 24-2-2009 at 06:18

There is a nomograph tool on the Sigma Aldrich website you might find useful: http://www.sigmaaldrich.com/chemistry/solvents/learning-cent...

gsd - 24-2-2009 at 07:43

Quote:
Originally posted by Mr. Wizard
Somebody correct me if I'm wrong. The vapor pressure over a liquid is determined by the temperature of the liquid and vapor at equilibrium. So once the air , or other non acid vapor, over the boiling acid is removed couldn't you seal the system and maintain the low pressure by adequate cooling on the receiving side? The only problem would then be pressure resulting from breakdown products of the distillation or pressure rise due to inadequate cooling. Any thoughts on this?


AND the pressure built-up due to ingress of air thru' joints. This is practically unavoidable. Even if the quantity leaked in is very small, at low pressures it results in significant volumetric load on the pump.

gsd

Arrhenius - 24-2-2009 at 09:38

Oh ya, you definitely need an aspirator. You just need to get below the point where the acid significantly decomposes, right?? Should get you there, and there's no need for traps etc. :)

DJF90 - 24-2-2009 at 14:44

You still need a trap to prevent suckback of water into your freshly distilled conc. nitric acid. Otherwise you can have a potentially dangerous situation on your hands. Some aspirators have "check valves" but I would still put in a trap to be safe.

panziandi - 24-2-2009 at 14:47

Often those valves are unreliable!

DJF90 - 24-2-2009 at 14:51

Finding out it fails in the middle of a nitric acid distillation is definately not good timing, hence the suggestion for a trap ;)

Arrhenius - 24-2-2009 at 19:29

Ahh, oui oui... trap for water. I threw a gauge on an aspirator today and got down to 400torr with ease, which is sufficient I should think.

0010110 - 2-3-2009 at 22:59

I appreciate all the advice. I think I have decided to just purchase a couple small plastic water jet aspirators. I really don't think my mechanical pump is such a good idea. I tried a few vacuum distillations with just water and the pump seemed to be having a hard time keeping a vacuum. it constantly jumped from high vac to low. there was no leak in the system so i figured the pump just isn't cut out for it.

watson.fawkes - 3-3-2009 at 08:23

Quote:
Originally posted by 0010110
I really don't think my mechanical pump is such a good idea. I tried a few vacuum distillations with just water and the pump seemed to be having a hard time keeping a vacuum. it constantly jumped from high vac to low. there was no leak in the system so i figured the pump just isn't cut out for it.
The leak is in the pump. It's burping, is my guess, although it's hard to say much for certain without knowing what kind of pump. Although even from what you've said, I'd worry about whether your oil level is adequate and whether the oil has sufficient viscosity (either from being old or being the wrong kind of oil).