Pressure Developed by Combustion of Propane..... 3

[CharlesRLowery]

All,

If I have a closed cylinder of known dimensions(Height, diameter, volume,etc....) that contains a known mixture of propane and air, How can I figure out what pressure will remain in the cylinder after the mixture is ignited....

 

[IRstuff]

You have x moles of propane, y moles of air --> z moles of CO2, i moles of H2O, j moles of air, etc.

TTFN

FAQ731-376

 

[Flareman]

Charles
Is this a theoretical problem or a real world issue?
If it's the latter, there are things called explosions and detonations to think about which involve flame temperature, dynamics and container conductivity as well as molar balance.
How close to a stoichiometric mixture will you be? Where and what is the ignition source?
Look into NFPA codes 68 and 69 which address how to design for explosive conditions.

David

 

[CharlesRLowery]

(Flareman)

I am working on a personal project and I want to establish a baseline before I start building prototype after prototype.

I only want to generate a pressure, the volume of the container can be considered as fixed.

I will most likely have a spark ignition source and I will be using the C3H8+5O2 mixture.{C3H8+(5)O2 => (4)H2O+(3)CO2)}

I think I'll have to pull out the chem and thermo books.

 

[FOURe]

In my opinion, you do have the initial conditions defined.

Assuming that ideal and complete combustion takes place, one can calculate the flame temperature of the combustion products for the given stoichiometry.

Then, one can use the ideal gas state equation to find out what the pressure should be: pV = mRT

http://www.engineering-4e.com

 

[25362]

Charles,

In your first query you mention air, therefore your reaction should include 18.8 N[sub]2[/sub] on both sides of the equation.

The initial state would show 24.8 moles; the final, 25.8 moles. Thus, the pressure increase due to the number of moles would be small.

The pressure increase will in fact be the result of thermal energy due to the blast.

My suggestion: read section 6 of Crowl & Louvar's Chemical Process Safety: Fundamentals with Applications Prentice Hall, where detonations and deflagrations inside vessels are explained.

 

[CharlesRLowery]

Thank you all, I am a machine design guy and it'll just be easier to go out in the shop, build it, and measure it....

 

[IRstuff]

Well, the issue is whether we're talking about an explosion or the end result of the reaction.

The former is not something to be trivially bandied about, as fly shards of a broken cylindrical container will definitely ruin someone's day.

TTFN

FAQ731-376

 

[psafety]

Propane
Pmax = 7.9 bar; NFPA 68, 2007 pg 62

 

[CharlesRLowery]

I have a lot of experience with pressures up to 30,000 psi, no broken cylinder worries here. From some research it seems that the largest aspirated liquid fuel cylinder combustion pressure is around 600psi. I'll design the cylinder to accomodate.

- It seems that it would take forever to calc this out I haven't found any methods that will Yield the desired result. I'll post results if anyone's interested.

 

[IRstuff]

Combustion is quite different than a static pressure.

TTFN

FAQ731-376

 

[FOURe]

Here is a URL that contains the combustion products flame temperature, which is ~ 4,580 [F] or 2,825 [K].

http://glassalchemy.com/index.php/resources/articles/propane-or-methane

Now, it should not be difficult to find out what the pressure is for the given geometry ...

http://www.engineering-4e.com