Confusion on LEL and UEL 1

[koshyeng]

Our tank overhead vapor exhaust line has 15 mole% O2 and 0.09 mole% of acetone. With 0.09% of acetone how much O2 level will be required for an explosion to occur?

LEL and UEL of acetone is 3 and 13% respectively.

Is it Air (21%) minus UEL (13%) = 8% so O2 level should be less than 8%??? Plz help me out with these calculations.

Appreciate your help

Koshy

 

[djack77494]

Koshy,
The LEL (lower explosive limit) and UEL (upper EL) refer to the molar concentration of your combustible vapor in air. Thus, if you were to be releasing acetone vapors into the atmosphere, you would not reach a dangerous (from the viewpoint of an explosion) level until you got to 3 molar% acetone in the air. Though I've never thought of it in these terms before, I guess this defines the total explosive mixture as 3% acetone, (0.97 * 21) mole% oxygen, and (0.97 * 79) mole% nitrogen. The vapor mixture would remain explosive as the acetone concentration is further increased up until the point where it reaches 13 mole% acetone. Above that, the mixture is too rich and will not explode.
Doug

 

[Flareman]

Koshyeng
As djack77494 says, LEL and UEL are based on mixing the basic gas with air. The common misconception is that these numbers are absolute. They are not.
If you have a mixture which already contains oxygen in a container (vessel/pipe), the story is different.

Draw a vertical axis (bottom to top) = 0 - 100%. This is gas.
Draw a horizontal axis (L to R) = 0 - 100 % with the 79% point under the vertical axis. This is total nitrogen in the mixture.
Join 0% N2 to 100% gas. Mark it 100 - 0 % (bottom to top) = oxygen
Join 100% N2 to 100% gas point = RH sloping line
All gas lines are horizontal
All Oxygen line are parallel to the RH sloping line
All nitrogen lines are parallel to the LH sloping line.
Find the LEL and UEL and stoichiometric % on the vertical line
Draw a horizontal line through the LEL
Draw the stoichiometric ratio from the bottom RH corner through the stoic % on the vertical line
Draw from the intersect of stoic and LEL lines through the UEL
That's the total flammable envelope (theoretical) for all mixtures.
Find your mixture, inside or outside the envelope.
If you are already below the LEL, no chance of burning
If you are above the UEL, adding air makes it flammable (as you travel in a line to the 79% point on the bottom axis.
If you are in the envelope -- bang

Thre is a spreadsheet which does this for you at

http://www.geocities.com/flareman_xs

|main index|downloads| as well as a bit of an explanation in the paper "Making the Flare safe", at the same page.

Regards
David

 

[koshyeng]

Thank you Flareman. It gives me a good view of flammability envelope and I also went through NFPA code.

I couldn't get data on UFL of acetone in pure O2. Is there any calculations for this or an experimental data?

Thank You
Koshy

 

[Flareman]

Koshyeng
To some extent, all flammable limits are empirical because they are not absolutes. They are just a manifestation of conditions which will produce a flame temperature high enough to decompose the cold mixture adjacent to the flame.
Consequently, they are equipment and flame direction sensitive, which accounts for the wide variations in published data.
There are theoretical ways to predict the values too but all depend on some assumed heat transfer characteristics and are not really better than the empirical values.

I don't have specific data on Oxygen/Acetone mixtures but taking the gneral approach of my prior method, it would seem that mixtures in a range Oxygen/Acetone vol/vol from 0.9:1 through about 8:1 could be flammable per se (i.e: in the container). (Always use a big safety factor). The published auto-ignition temperature for acetone in oxygen is about 905 degF.

David