Theoretical queston

[319]

If BP is ignited in a container that can withstand the pressure and is leak proof, will the pressure always remain in the container?

Sam

[FlaMtnBkr]

Probably some pressure, but I doubt it would be anywhere near peak or even average pressure. Once it cools and many of the combustion products cool from gases to solids, the pressure will significantly drop. That would be my guess at least.

[LambentPyro]

Well, pressure is created from the combustion of BP because of the gas and smoke particles that have nowhere to go. When BP is just sitting inside of the container, pressure is at equilibrium.

[hindsight]

Mumbles will correct me if I'm mistaken, but if solid BP combusts inside a closed container whose dimensions do not change with the combustion, the products result in a greater number of moles in the gaseous phase, therefore pressure increases. Also, in your theoretical example, is the container thermally isolated, or is the ending temperature the same as the starting temp?

 

Oops, just re-read FlaMtnBkr's post--sounds like we are saying the same thing.

Edited April 20, 2014 by hindsight

[pyrokid]

If there was a great enough amount of BP in the container to begin with, and it was impossible to rupture the container, wouldn't the deflagration of the BP be stopped at some point?

 

The way I see it the pressure in the system would prevent the further deflagration of the powder when the system is at equilibrium. The reaction products would have nowhere to go.

 

Right?

 

Would the reaction products simply occur in a different phase?

[hindsight]

Pyrokid-- Those were my thoughts, as well. Perhaps mass effect would inhibit further formation of product as soon as density of gas products reached a certain level shortly after ignition. The reaction in a different phase is interesting, but way above my level of understanding. This would be something amenable to computer modeling, perhaps.

Edited April 20, 2014 by hindsight

[Mumbles]

Given that one of the combustion products is nitrogen, the pressure should be higher than when you started. What would really happen is that the pressure would spike on combustion, and then if allowed to cool, it would slowly decrease until in thermal equilibrium. This will cause both a decrease in pressure from lowering the temperatures of the permanent gasses, as well as condensation and solidification of some of the other reaction products. It will still be higher than at the start though I would think. The only way for it to be lower is if one of the gasses already in the container (aka oxygen most likely) is used up more than it's replaced by another.

 

There are a lot of things to consider if you want the most accurate idea of what would happen. Most notably, the reaction pathway can change with increasing pressure and could also undergo a series of side reactions with some intermediately formed products. My gut feeling is that you'd end up with more solid products if you did this in a totally sealed container as opposed to doing it in the open. There is a driving force to keep the pressure lower, as well as much more intimate contact for a variety of side reactions to happen.

 

It would be interesting to see what happened. In addition to all the confinement issues, you'd also end up easily within the realm of supercritical CO2 and super critical water which, as pyrokid brought up, could add a whole new can of worms to this reaction.