Sciencemadness Discussion Board - Distillation of drain opener and the use of Krytox LVP vacuum grease.

Distillation of drain opener and the use of Krytox LVP vacuum grease.

Kiwichemicali - 12-9-2012 at 01:47

Last weekend I distilled drain opener.
Vacuum of 100mbar was applied to keep the temperature within ranges I feel comfortable with.
All joints were greased with Krytox LVP.
To prevent bumping I threw a few PTFE stirring bars in the source RBF. That works great because the stirring bar surface is rough enough. PTFE boiling chips don't work because they start to float when the drain opener heats up.
An Hempel column was used as condenser, the sulfuric acid vapour is hot enough to condense using passive cooling.
Distillation went well, no foaming. The drain opener I used wasn't black but did contain impurities. When heated it turned light brown and during the distillation it shifted to light yellow.
At first some water came over. When the sulfuric acid started to drip I stopped the distillation to throw away the first distillate. The difference between the condensing of water and the condensing of sulfuric acid is easy to see. End result was clear concentrated sulfuric acid.
The only thing I was disappointed about was the chemical inertness of the Krytox LVP. It did withstand the cooled acid but not the hot vapours. It turns black, however I was still able to separate the joints when the distillation was done but grease wasn't greasy anymore on the joints that came into contact with the hot vapours.
If anyone has suggestions for a better grease? Maybe one of the other Krytos greases?
I'm not to happy with PTFE sleeves because it's difficult to use them and still get a leak free setup.

Kiwi.

woelen - 12-9-2012 at 03:00

You now have some conc. sulphuric acid. Use that as 'grease' for sealing the joints. This is used in many types of distillation where very corrosive chemicals are distilled.

Kiwichemicali - 13-9-2012 at 01:02

Thanks, I'm aware of the possibility to use concentrated sulfuric acid as a lubricant. However that doesn't work for the distillation of the acid itself.

Kiwi

woelen - 13-9-2012 at 01:36

Why doesn't that work for the distillation of the acid itself? The lubricant assures that the joints to not get stuck together and it assures that no vapors can escape. Especially in your situation this may work well, because you already have some concentrated acid. If you want to distill more, then you can sacrifice a very small amount of the acid you have now as a lubricant.

Heuteufel - 13-9-2012 at 07:09

I really cant recommend concentrated sulfuric acid as "grease"! Already at room temperature its viscosity is much too low for it to be a good grease. At elevated temperatures sulfuric acid is not any more viscous, than plain water. Result: the joints tend to leak and are difficult to seperate afterwards. If you use a good quality of silicon based grease (I like the high-viscosity grease from Baysilone®) you will have no problems.

[Edited on 13-9-2012 by Heuteufel]

Mailinmypocket - 13-9-2012 at 08:36

Quote: Originally posted by Heuteufel

I really cant recommend concentrated sulfuric acid as "grease"! Already at room temperature its viscosity is much too low for it to be a good grease. At elevated temperatures sulfuric acid is not any more viscous, than plain water. Result: the joints tend to leak and are difficult to seperate afterwards. If you use a good quality of silicon based grease (I like the high-viscosity grease from Baysilone®) you will have no problems.

[Edited on 13-9-2012 by Heuteufel]

I have used a VERY small amount of sulfuric to make an airtight seal in glass joints while distilling bromine and nitric acid, none has ever seeped out... It does require more careful handling though.

High quality silicone grease is good and safe, I have only had problems with SO3, in which case conc. sulfuric is best.

Bot0nist - 13-9-2012 at 10:28

I have used conc. H2SO4 for grease in many HNO3 distillations at woelen's behest. It works like a charm to seal the joints. It's viscosity is not an issue, IMO.

watson.fawkes - 13-9-2012 at 11:03

Quote: Originally posted by Heuteufel

Already at room temperature its viscosity is much too low for it to be a good grease. At elevated temperatures sulfuric acid is not any more viscous, than plain water.

Viscosity is more of a requirement when the ground glass seals are out of tolerance than when they mate well. In an ideal situation, the average height of the gap between mating surfaces is only a few microns. It's why you only need a very little bit of sealant on high-quality joints. On the other hand, if the mating surfaces are out of conicity (the property of being shaped like an ideal cone), you could get gaps 100 times as large. In those situations the viscosity of a sealant matters much more. As always, the larger ΔP is, the more force on the sealant there is, so what might work for atmospheric distillation, say, might not work for vacuum distillation.

Evidently taper grinding tools used to be standard in trade and available mounted on handles for field honing operations (touch up). I haven't seen them for sale as new stock, however.

My guess as to why sulfuric acid works well as a sealant is how hygroscopic it is. It would expand a bit at either or both the leading and trailing edge of the seal, providing a bit of mechanical force. Diffusion through a microns-thick fluid layer, however, is quite slow, so an almost-steady state is quickly reached and the seal has long stability.

woelen - 13-9-2012 at 22:39

I have no experience with vacuum distillation, but with normal air-pressure distillations I have really good experience with H2SO4. I used it for bromine and HNO3, but also for higher boiling compounds, especially apolar solvents. I never had joints which were stuck to each other. I use VERY little amounts. Just enough to wet the joint, you can easily see whether all of the joint is wetted or not.

I also have used paraffin-butter for sealing joints. This also works well, but it is more work to cleanup. With conc. H2SO4 I just rinse my glassware with tap water and do a final rinse with a small amount of distilled water and then allow the glassware to dry. In this way it remains perfectly clean and no residues remain on and near the joints.

Kiwichemicali - 14-9-2012 at 06:20

Quote: Originally posted by woelen

Why doesn't that work for the distillation of the acid itself? The lubricant assures that the joints to not get stuck together and it assures that no vapors can escape. Especially in your situation this may work well, because you already have some concentrated acid. If you want to distill more, then you can sacrifice a very small amount of the acid you have now as a lubricant.

I did try it in the distillation I described above. Unfortunately it doesn't work for the relative high temperatures that are needed when distilling sulfuric acid at 100mbar. At room temperature the joints move fine when 'greased' with concentrated sulfuric acid but when the distillation was finished they were stuck and I was lucky I could take them apart after some heating.

Kiwi

[Edited on 14-9-2012 by Kiwichemicali]

hissingnoise - 14-9-2012 at 07:24

A couple of glindemann rings on the cone might be more suitable for vacuum work than a viscous liquid . . .

Endimion17 - 14-9-2012 at 07:55

I never had any problems with sulphuric acid lubrication for atmospheric pressure distillations. It works fine, but the point is to use very little near the top of the joint. It's hygroscopic, and the smear increases in size as the distillation progresses. If the joints are even and high quality, it's fine.
Vacuum distillation is another thing. Even with high quality joints, air gets inside at elevated temperatures because the viscosity drops down. I'd recommend fluorinated greases for such distillations, if the material is very corrosive.

Regarding the distillation of the acid itself, I just hope no one will try to run water through the cooler. Distillation produces roughly 98% acid which boils at around 337 °C. The temperature difference is greater than 150 degrees, which is roughly the maximum recommended tolerance for borosilicate coolers. Using water is not neccessary and increases the chance of the expensive piece of glassware breaking into pieces, not to mention how dangerous it would be for the worker.
Air cooler is enough, as well as an empty Liebig cooler, too.