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Creating liquid isobutane from r600a

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PRIDES
#1
Nov19-13, 06:56 PM
P: 3
There is a recent project I have been working on that requires the use of Isobutane as a propellant/solvent, so I have come up with a way to study the chemical compatibility of isobutane:

I have a pressure chamber that I built from transparent PVC and opaque PVC with a tire valve as an inlet for air and isobutane:
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I use r600a refrigerant for isobutane (420 grams, 720 liters)
I can put solid objects into the top of the chamber.
I use anti-air loss tire valve connectors.

The procedure is as follows:
1. connect r600a refrigerant canister to pressure chamber and wait a little bit.
2. remove the connection quickly.
3. connect air compressor to the pressure chamber and pressurize to 50 psi.
4. optional: remove the connection quickly.

I know that the vapor pressure of Isobutane is about 43 psi, so I should see a liquid in the chamber, but I don't. Please this is making no sense to me.
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etudiant
#2
Nov19-13, 08:56 PM
PF Gold
P: 866
Why do you expect liquid isobutane when the bulk of the pressure is created by the air compressor?
It seems unlikely that the isobutane has close to 43 psi partial pressure in your setup, rather based on your setup, which equilibrates the isobutane/air mix at atmospheric pressure (15psi), you have maybe 7-8 psi partial pressure of isobutene, the rest air. So there is far too little isobutene in your setup for anything to liquefy.
I would also note that there needs to be a way to remove the latent heat of condensation if you plan to see a liquid. Remember that your setup is in some ways a bit like a refrigerator, it cannot work unless it has some way to reject heat.
PRIDES
#3
Nov19-13, 09:33 PM
P: 3
Quote Quote by etudiant View Post
Why do you expect liquid isobutane when the bulk of the pressure is created by the air compressor?
It seems unlikely that the isobutane has close to 43 psi partial pressure in your setup, rather based on your setup, which equilibrates the isobutane/air mix at atmospheric pressure (15psi), you have maybe 7-8 psi partial pressure of isobutene, the rest air. So there is far too little isobutene in your setup for anything to liquefy.
I would also note that there needs to be a way to remove the latent heat of condensation if you plan to see a liquid. Remember that your setup is in some ways a bit like a refrigerator, it cannot work unless it has some way to reject heat.
Ah ok! I think I understand now. So the air is containing most of the pressure. I've done some thinking and math and I don't think I could create liquid isobutane in the container just by pressure. But if I waited until it was 10 F outside, and went outside and just poured the isobutane in the container, could I then keep it liquid by pressurizing it while it was liquid, and then bringing it into room temperature to carry out my observations, or would I need to carry them out in the cold? Would it have a different chemical compatibility properties in the cold?

etudiant
#4
Nov20-13, 07:38 AM
PF Gold
P: 866
Creating liquid isobutane from r600a

Well, if you start with liquid isobutane almost filling the container at 10*F, there will be ample pressure to keep your sample liquid at room temperature as well. No need to pump it up to start with, that will just bump up your system pressure more, for no purpose.
You should in any case be careful about putting much pressure into PVC tubing. It does not really have much strength. Please wear good safety goggles while working with this setup, pressurized PVC bursts rather than leak slowly. Also, make sure there are no flames or spark sources around, isobutane is a great fuel.
The chemical reactions that you are investigating will simply happen more slowly at lower temperatures, but they do take place. Isobutane is a pretty stable compound though, so it would be a surprise to see it become involved in chemical reactions. Do you perhaps have some solvent effects in mind on seals or lubricants?
PRIDES
#5
Nov20-13, 07:57 AM
P: 3
Quote Quote by etudiant View Post
Well, if you start with liquid isobutane almost filling the container at 10*F, there will be ample pressure to keep your sample liquid at room temperature as well. No need to pump it up to start with, that will just bump up your system pressure more, for no purpose.
You should in any case be careful about putting much pressure into PVC tubing. It does not really have much strength. Please wear good safety goggles while working with this setup, pressurized PVC bursts rather than leak slowly. Also, make sure there are no flames or spark sources around, isobutane is a great fuel.
The chemical reactions that you are investigating will simply happen more slowly at lower temperatures, but they do take place. Isobutane is a pretty stable compound though, so it would be a surprise to see it become involved in chemical reactions. Do you perhaps have some solvent effects in mind on seals or lubricants?
Well I need to use isobutane as a solvent, so I need to know what it can dissolve, and I know its not really a solvent, but chemical compatibility charts suggest that it has a bad rating with EPDM definitely, but charts have some conflicting data concerning other plastics such as Nylon:

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But of course, I'm not a chemist, so I don't know exactly what the bad rating means, whether it be that isobutane dissolves the plastic, or it has a bad reaction with it.
etudiant
#6
Dec27-13, 09:13 PM
PF Gold
P: 866
Quote Quote by PRIDES View Post
Well I need to use isobutane as a solvent, so I need to know what it can dissolve, and I know its not really a solvent, but chemical compatibility charts suggest that it has a bad rating with EPDM definitely, but charts have some conflicting data concerning other plastics such as Nylon:

Attachment 64122

But of course, I'm not a chemist, so I don't know exactly what the bad rating means, whether it be that isobutane dissolves the plastic, or it has a bad reaction with it.
A bad rating in this context means that it would dissolve/infiltrate/swell the item getting put into the isobutane. Note that temperature and pressure will impact solubility, but unless you heat the materials a lot, chemical reactions are very unlikely between plastics and isobutane.
Isobutane is a simply hydrocarbon, just contains hydrogen and carbon, so it is a non polar solvent, good for most hydrocarbons. If you add polar groups to the hydrocarbons, from oxygen atoms for instance, or if you polymerize the material under test, it will not dissolve nearly as easily. Nylon is an example of such an oxygen containing material, plus it is polymerized, so it is pretty insoluble in isobutane.
There are solubility tables that can help you get the answer to your question or at least give you a decent basis for estimating.


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