Sciencemadness Discussion Board - Question about pressure

Question about pressure

EvlRenne - 21-2-2021 at 13:21

Hello everyone!
I am sorry if my question is stupid, but I think better ask than sorry...
For my next experiment I need to work with a pressure vessel like this:

The conditions will be: ~130-150°C / ~8-10h under ~15psi.
So the question is - can I use magnetic stirring during the reaction? My concern is that physical stirring will add additional stress to the vessel which can lead to explosion.
I surfed the net and saw that in big-scale reactors overhead stirrer used (which is obvious if you need to stir 20+ L), on the other hand in Parr hydrogenation apparatus they shake the vessel.
I am confused.

DraconicAcid - 21-2-2021 at 13:29

15 psi is about 1 atmosphere, which isn't exactly high pressure.

njl - 21-2-2021 at 13:31

Is that glass or plastic? Either way stirring shouldn't be a problem. Now, do you mean that during the course of the reaction your vessel will be pressurized to 15 psi above atmospheric pressure, that you start with 15 psi (just about atmospheric pressure) and then allow the pressure to increase with temperature, or that 15 psi will be maintained throughout the reaction?

EvlRenne - 21-2-2021 at 13:56

Quote: Originally posted by njl

Is that glass or plastic? Either way stirring shouldn't be a problem. Now, do you mean that during the course of the reaction your vessel will be pressurized to 15 psi above atmospheric pressure, that you start with 15 psi (just about atmospheric pressure) and then allow the pressure to increase with temperature, or that 15 psi will be maintained throughout the reaction?

This is glass, reaction will start at atmospheric and build up to around 15psi above the atmospheric.

Twospoons - 21-2-2021 at 15:25

If adding a stirbar is going to break the vessel, then you are already operating too close to the failure point.
Pressure vessels have a rated working pressure that is well below the failure pressure, by a factor of at least three or more.

Texium - 21-2-2021 at 17:18

Such pressure bottles produced by reliable brands such as Ace and Chemglass are recommended for use up to 60psi. The tube shaped ones are even more rugged, with a recommended limit as high as 150psi. They are tested at 1.5 times that pressure.

Cheaper off brand ones may not see as rigorous quality assurance, but even so, 15 psi should be well below the limit. If you're still nervous, either shell out the cash to buy one from a reputable brand that you know has been tested, or get yourself a blast shield (which wouldn't be a bad thing to have in any case).

Edit: and in case it wasn't clear, using a magnetic stirbar in one of these should have no significant impact on its safety.

[Edited on 2-22-2021 by Texium (zts16)]

EvlRenne - 22-2-2021 at 03:36

Thank you, guys for answers.
Just in case I will use a shield around the vessel and will try to use stir bar along.

Fyndium - 26-2-2021 at 04:36

Small low velocity fragments are easily stopped even by thin obstacles, so containing a fragile vacuum or pressure vessel is easily done with plywood, plexiglas, etc. Tensile materials like steel pressure kettles tend to separate into large pieces, rendering them extremely dangerous even at low pressures.

Triflic Acid - 26-2-2021 at 07:28

I can't help but ask, what reaction are you planning to do with this reactor? If are doing something that could be done in metal, metal doesn't seem like a bad idea to me. Also, just as an added saftey precaution, you could get a small bucket or jar made of metal/thick plastic/wood/anything that doesn't make shrapnel and put your reaction vessel in it. Just put the stirrer underneath and you can stir the flask as well.

zed - 9-3-2021 at 21:26

Well, its hot. That changes the rules. What the final internal pressure will be at 150 C, is an important consideration.

In terms of the bottle itself.... Those are the good ones.

Flat bottomed bottles can withstand 3 to 5 atmospheres.

That bottle 5+.

Magnetic stirring? No problem! Unexpected pressure build-up.... Worthy of monitoring.

My opinion? Attach a pressure gauge. Perhaps a strong, small cross-section wire mesh, would make a good safety shield.

You may be starting at 15 PSI, but at 150 C, with a reaction going, Internal pressure could be hard to predict.

Glass bottles sometimes fail under pressure. It's bad.... Real bad. Pressure gauge.... good!

[Edited on 10-3-2021 by zed]

Fyndium - 10-3-2021 at 14:29

It's good to always note the case sensitivity of reaction material compatibilities in these instances. While glass is universally good for chemicals, there are multiple occasions where for example stainless steel will work great. I would say that bigger of an issue for an ordinary amateur is to obtain a suitable vessel for the reaction.

drwei - 10-3-2021 at 16:04

I’ve used the Chemglass sealed pressure vessels before for some cyclization reactions to prepare CAAC ligands and have always used a stir bar.

I haven’t seen it myself, but I’ve been told that when these fail it tends to be the teflon cap that goes and not the glass. You definitely shouldn’t have this problem with your reaction conditions though. Just make sure the cap is on tight to stop any solvent escaping.

It’s always a good idea to put these types of reactions behind a blast shield or sheet of polycarbonate too, just for good measure.