It appears that the vapour pressure of propane and propylene vary considerably (ie by maybe 40psi +, propylene being higher). I have a situation with a liquid LPG burner where the supply is supposedly 95%+ propane. I am having premature vapourisation problems because I think that there may be a significant proportion of propylene in the mix. Hence to keep it a liquid prior to the nozzle exit I think I will need to raise the supply pressure considerably.

Would anyone have a graph of vapour pressure Vs temperature for propane/propylene mixtures?

Thanks in advance.

Rod Nissen.
Combustion & Engineering Diagnostics

Vapor pressure is a pure component property that is a function of temperature only. For a mixture, you can have partial pressures of each component, which when added yield the total pressure of the mixture. Therefore, a mixture does not really have a vapor pressure. You are looking for a graph of total pressure vs. temperature for the mixture. This is a little bit nit picky and academic, but it may aid in your search.

Good luck,
Latexman

I don't have a chart, but it isn't too hard to apply Raoult's law to a known mixture. Your LPG supplier should be able to provide you with at least an average assay of your mixture, and you can make a spreadsheet to generate a chart. In my area of the US, the LPG suppliers leave a bit of propane and ethylenes in there too. You could do the same with your propylene (assuming that's the only other thing in there).

Here's a blog I did a while back on this issue, although it's for the much more demanding application of grilling beef:

http://www.kele.com/templates/blog.aspx?id=5336&am...

Good on ya,

Goober Dave

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Thanks for he info.
There is plenty of info and graphs on propane butane mixtures, whose individual vapour pressures vary considerably. We have issues here in Australia where commercial propane may contain up to 60% propylene. This has caused problems with how the gas burns in vapour burners (ie soot formation). Now it loks like it could have a bearing on liquid LPG burners. I was just hoping there may have been a similar graph to the propane/butane mix already.

If not will try your recommendations.

Rod Nissen.
Combustion & Engineering Diagnostics

if you know (or may estimate) the composition of the mixture a bubble point calc. will give the saturation pressure, from bubble point to dew point you go from 0 to 100% vaporization,
there are VLE charts but I prefer to use a software (in my case Prode Properties with Excel), a EOS as SRK or PR should give accurate estimates.

I wonder how relevant this detailed data is to solving your problems. A tank of LPG will always be very close to its bubble point regardless of its actual composition. Piping that is above the liquid level in the tank will always contain some vapor unless you use a pump which is located below the tank.
Up to 60% propylene in commercial propane? Wow, that really surprises me. Yes, I would expect sooting in a burner designed for propane. You need an adjustable air mixture on the burner.

knowing the operating conditions inside the tank one can guess the composition of the liquid mixture (at equilibria) as that with bubble point at those conditions, vapor phase can be more complex when incondensables are present, once they have been displaced one can calculate gas composition from liquid composition at specified conditions.
I agree with compositepro about the percentage of propylene...

My concern is vapourisation happening down at the burner. The supply pump takes liquid from the bottom of the tank. To properly function the pressurised liquid must travel to the end of the lance, only turning to vapour at the noxzzle exit where it burns. I think I am experiencing part vapourisation within the lance, which displaces the higher heating value/spec vol liquid. Today I increased the supply pump pressure fron 180psi to 220psi on test. So far operation has improved with no flame failures. Still testing. Also hoping for a propane supply of 95-98% at next fill for comparison.

Rod Nissen.
Combustion & Engineering Diagnostics

I've never seen a burner that uses liquid propane directly. The propane must vaporize at some point before it burns. If heat isn't added during vaporization the temperature can drop to as low as -40F and any moisture in the propane will freeze.

Liquid propane, butane or mixture burners are quite common in the Asphalt industry ranging from 15 to 100MM Btu/hr. The vapourisation happens at the exit from the nozzle (ideally) when released into the furnace or drier space which is obviously at a low (atmospheric or below) pressure.If there is moisture in the propane of course it will freeze......there should not be or we would be in trouble. However you can get icing around the nozzle but that is the cooling effect freezing atmospheric moisture.

Rod Nissen.
Combustion & Engineering Diagnostics

Dear op9 Hello/Good evening,
The 'apparent icing' may actually be "LPG hydrate formation" generally looking similar to ice;

if the point local conditions favor this physical bondage of moisture with LPG!
Hopefully this also indicates another possibility of ice like appearance 'in near vicinity of' or 'around' burner tips.

Best Regards
Qalander(Chem)

I think that the propylene was added to the propane to increase the flamme temperature for flame cutting operations. Get hold of the suplier as he may be able to help you. I believe that the term for this mixtures is MAPP gas.

Dear chicopee Hello/Good Morning,

Your inference seems reasonable to me as, in all fairness

'propylene' should be costlier than 'straight run LPG' mixtures commonly available/marketed!

Best Regards
Qalander(Chem)