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How to Calculate Flammability of gas mixtureHelpful Member!(9) 

Leclerc (Chemical)
7 Oct 02 10:10
How can I calculate the flammability of a gas mixture from a knowledge of gas composition, and individual flammability limits?
Helpful Member!  TD2K (Chemical)
7 Oct 02 14:31
I came across one reference in “Explosions” by Springer-Verlag, Berlin Heidelberg, New York.

They claim (where Pn is the volume fraction of the individual components) that:

LEL =( P1 + P2 + ... Pn)/(P1/lel1 +P2/lel2 + ... Pn/leln)

P1 - vol fraction of first component
lel1 - lower explosive limit of component 1

For the UEL, the same equation is used except that lel is replaced with uel.

The book claims that this equation has been proven many times and gives a couple of references from German publications.
Leclerc (Chemical)
8 Oct 02 12:11
Thanks, TD2K.
the equation seems eminently plausible, so I will run with it whilst I investigate the reference.

The second part of my problem, which I forgot to add under Chem Eng.Process, but put under Combustion Engineering, deals with the addition of an inert (say N2) to the Gas/Air mixture. How do I calculate compositions of Gas/Air/Inert which are at the LEL and UEL?
Putting the question more visually, on a triangular diagram whose axes are Gas, Air, Inert, how do I draw the envelope delineating the flammable region?
TD2K (Chemical)
8 Oct 02 13:10
The best way I know is to get hold of the data from the Bureau of Mines on explosion limits of various gases in 02 enriched mixtures.  I had to do this for an atmospheric ethane tank that we were purging out and I needed to confirm that the procedure the contractor proposed would not take us through the flammability region (basically, purging with N2 partway and then switching to air but they proposed switching to air at a higher concentration that I was used to but insisted it was safe).

I'm a little fuzzy on the details (this was a few years ago and I did not bring the reference material back from Arabia).  I think I used the Bureau of Mines data to create a 3 phase diagram for AIR, N2 and ethane (I think the data was for O2, ethane and N2 but I converted it to air as I would be purging with air and it was easier to visually show this to management in this fashion).  I drew the flammability region on the curve and then could see how the purging would progress wrt the flammability region.  Basically, you looked at starting with 100% ethane moving along the straight line connecting it to the 100% N2 point.  When you switched to air purging, you then moved along the line to the 100% air and this was the area that I did not want to pass through the flammability region.

Rather hard to put into words.  Try playing around with it and if you still are stumped, I'll take another whack at it.
Helpful Member!(2)  Guidoo (Chemical)
11 Oct 02 7:21
Please check out "Rules of Thumb for Chemical Engineers" by Carl Branan, 2nd edition, pages 276-278.

Formula TD2K mentions is called LeChatelier's Law, and can also be written as:

Lm = 100/(x1/L1 + x2/L2 + ... xn/Ln)    %(vol)

where:
Lm = upper/lower flammability limit of gas mixture
Li = upper/lower flammability limit of component i
xi = concentration of component i in gas mixture

For combustible mixtures containing inert gases N2, CO2 as well as O2, following procedure can be followed:

- If O2 is present, the composition of the gas has to be corrected to render it "airless".
- Next, data from figures 1 and 2 on page 277 of Branan's book has to be used. These figures show flammable regions as a function of inert gas to flammable gas ratio for various combinations of flammable gases and inert gases.
- Finally Le Chatelier's law has to be applied.

Branan's book shows how this works with a detailed example.

Another possibility (for N2) I used myself is when you have literature data of flammable limits both in air and in pure oxygen. When plotting these upper and lower flammable limits in a composition triangle (with O2, N2 and the flammable gas in the three corners), and connecting the points by straight lines, you get a pretty good idea what the flammability envelope looks like.

Good luck!
dbachovchin (Chemical)
11 Oct 02 15:40
Limits of Flammability of Gases and Vapors  by HF Coward and GW Jones, US Bureau of Mines bulletin 503.

And Flammabilty Characteristics of Combustible Gases and Vapors, by M Zabetics Bureau of Mines buletin 627.

They have all the gases, mixtures, etc.
Leclerc (Chemical)
14 Oct 02 6:22
thanks once again, people.
I did use the Bulletins way back in the early '70s for a similar job, but I thought there these days there would have been a more immediate form of the data. It appears that I was wrong. Never mind, I am perfectly happy with the bulletins, and I hope to have copies of the USB documents to hand within a few days.
regards, Leclerc.
junnip (Mechanical)
2 Jul 03 7:34
I have also this same problem and I now know how to calculate gas mixture which contains nitrogen. But the problem is how to calculate flammability limits of gas mixture which contains water vapour?
Helpful Member!  25362 (Chemical)
2 Jul 03 9:17
Nitrogen and carbon dioxide are geenrally used for inerting purposes and to reduce the MOC in a flammable vapour mix. Steam (water vapour) is sometimes used for the same purpose.
junnip (Mechanical)
2 Jul 03 14:18
Yes, but how can I calculate flammability limits if gas mixture contains for example 30 vol-% CH4, 30 vol-% H2O, 30 vol-% N2 and 10 vol-% CO ?
25362 (Chemical)
6 Jul 03 7:51
It is well known that with additions of inerts such as nitrogen the flammable range gets smaller, up to a point where the mixture is no longer flammable at room
conditions.

For methane/air the internet tells us this happens for mixtures containing 21% CO2, or 35% N2, or 48% He, or 26% water vapour by volume.  

To junnip, the example mixture you gave doesn't include oxygen or any other oxidant, thus at room conditions it wouldn't be flammable, right ?

I think every potentially flammable mixture should have its LFL and UFL checked in a specialized lab to be sure of its safe handling.

FWIW one may find the following site, on MOC with examples, of interest:

http://www.ntron.com/tb1.htm
kenvlach (Materials)
7 Jul 03 5:50
This subject: LEL, UEL, Le Chatelier's Rule, the US BurMines documents, as well as the effects of increasing temperature or total pressure or adiabatic compression is covered in Perry's Chemical Engineers' Handbook, 7th Edn., pages 26-53 to 26-55.
Helpful Member!(4)  Flareman (Petroleum)
7 Jul 03 11:07
I posted a page which will estimate the flammable envelope for you, together with the effect of a diluent, based on published data for the flammable gas.
http://www.geocities.com/flareman_xs/Gases/Flammability_calc.html
I just posted it today so there has been no feed-back yet.  If it needs 'tweaking' let me know.

David
moquiton (Civil/Environmental)
18 Feb 05 23:07
Hello everybody!
i have read you questions and answers and i still have the doubt of how calculate the flammability limits of a mixture with more than one inert gas (CH4 CO2 N2 air).

I have a diagram of Methane and these inert gases but i don´t know how i can take both into account?

thanks every body

moquiton (Civil/Environmental)
18 Feb 05 23:36
Is there a method or formula with which we can obtain the flammabily limits of  whatever flammable gases + whatever non-flammable gases + air, in aproximatively way?

Could you recommend me any book for flammability limits of flammable gases + non-flammable gases (i am interesting when there are some non-flammable gases in the mixture) + air?

thanks again
25362 (Chemical)
19 Feb 05 2:57
I haven't seen procedures to estimate the "corrected" flammability limits upon adding inert gases of many sorts to flammable mixtures.

Although LFL/UFL are based on fuel in air at 25oC and 760 mm Hg, consider that in the process of inerting a combustible mixture one reduces the concentration of oxygen (the key oxidant) below a minimum "MOC".

For many (but not all) gases the MOC is ~10%, and for many dusts ~8%. I don't think there are procedures to estimate the flammability limits by adding inert gases of many kinds.

A control point of common use is 4% below the MOC, ie, 6% oxygen when the MOC=10%.

The MOC is expressed as % oxygen in the mixture of air + fuel. Below the MOC the reaction cannot generate sufficient energy to heat the entire mixture (including the inerts) as needed for flame propagation.

It is generally determined experimentally. If data are not available, the MOC is estimated using stoichiometry of the combustion reaction and the LFL. This procedure works for many hydrocarbons (acetylene is one exception) and organic molecules (ethylene oxide is another exception).

Following this procedure,  

MOC = (LFL)(Moles O2/Moles fuel)

In a way, having the experimental MOC one could estimate backwards the LFL.

Take, for example, toluene whose MOC has been measured to be 9.5. Since the stoichiometric ratio is 9, the LFL would be 9.5/9 = 1.1, vs the published LFL=1.2 .

BTW, Steam is not always recommended for inerting, because its condensation, if the conditions allow it, would bring back the oxygen concentration into the flammable region.

Some published experimental MOC I've found:

methane, 12; propane, 11.5; ethylene oxide, 0; benzene, 11.4; toluene, 9.5; H2S, 7.5; acetylene, 0; hydrogen, 5.

As it can be seen from these data, not all estimations are even in the "ballpark". It appears that the MOC also changes with pressure and temperature.

When googleing around just ask for minimum oxygen concentration for combustion, and you'll find a plethora of pertinent articles. For example;

http://www.banksengineering.com/about_flame_arrestors_and_detona.htm  
 
moquiton (Civil/Environmental)
19 Feb 05 10:08
25362

So, there is a MOC, that represent the minimum concentration of oxygen for igniting a mixture.Is there also a maximum oxygen concetration? Could be represented by the next formula?

Max Oxygen Conc. = (UFL)(Moles O2/Moles fuel)

Thank you
25362 (Chemical)
19 Feb 05 13:53
To moquiton, some sources also call the MOC a "maximum oxygen concentration", depending on what they mean by that.

Regarding you question: do you mean an oxygen-enriched atmosphere ?
moquiton (Civil/Environmental)
19 Feb 05 16:40
To 25362, that is what I want to say: a mixture in an oxygen-enriched atmosphere.

Thanks

Flareman (Petroleum)
21 Feb 05 13:48
There is relatively recent research by Wierzba and others from University of Calgary which covers this in a strongly technical manner for those so inclined....
Journal Inst Energy Sept 1996 V69 pp 122-130
Journal of Energy Resources technology 1994 V116 p 181
Transactions of ASME 1999, V121 p 74

The upper limit is a little complex to estimate because it is a factor of the energy content of the intermediate radicals which are released by partial compbustion.  The low limt is a breeze to estimate because it is only a factor of the thermal properties of the components.

All gases have a limiting heat transfer characteristic at the low limit which inhibits flame propagation.  This is a feature of the radiant properties and adiabatic flame temperature.  As the radiant properties at the limit are largely independent of inert content, we only need to take account of the varying heat-sink ability as we change the inert from Nitrogen (=Air) to something else.  This means replacing the excess nitrogen at the low limit with an amount of Nitrogen which has the same heat sink propeties as the actual inert.

For a given mixture of Flammable in inert an estimate of the true low limit concentration of flammable within the mixture [CLF] and of the complete mixture in Air may be obtained by use of the familiar Le Chatelier procedure modified by the Nitrogen equivalent of the Inert portion.
      NE   = S { NEi * I%i }
where I%i  = fractional quantity of ith Inert component    
      NEi  = nitrogen equivalent of ith Inert component
      CLF  = Sigma {   F%n     }  /  Sigma { F%n / CLn }       = low limit of Flammable portion in Air
      CLM  = Sigma { I%i + F%n }  /  Sigma { F%n / CLn }       = low limit conc of total Mixture in Air
where F%n  = fractional quantity of nth Flammable component
      CLn  = lower limit of nth Flammable component

There are relatively few common Inerts in gas mixtures and the following suggestions are made for values of NE
N2 = 1
H2O = 1.35
CO2= 1.82
SO2 = 2.1
He + Ar = 0.65

Remember that this is an estimate and use an approriate factor of safety depending on whether you are looking to be flammable or non-flammable.

Other cautions - if you use the flammable limits in tables they are based on 25 degC for the gas and the air.  Flammable limits get closer together as the temperature goes up towards the Auto Ignition temp and get a little fuzzy close the the AIT.  However, if the air is always ambient, the gas temperature itself has less of an effect.  It's all about total energy in the mixture.
Check the Wierzba ASME paper.

If you have already tried my excel sheet (noted above) and it doesn't work let me know.

Happy calculating
Helpful Member!  mbeychok (Chemical)
28 Feb 05 17:55
Laclerc:

It might be useful to read this tutorial from a Professor Ludlow at Arizona State University:

web.umr.edu/~dludlow/classes/che258/Flammability.ppt

I am not sure if it will help you, but I hope that it does.

Milton Beychok
(Contact me at www.air-dispersion.com)

Guidoo (Chemical)
1 Mar 05 4:14
I wonder if Leclerc is still interested, after 2 1/2 years...
Leclerc (Chemical)
1 Mar 05 5:16
Interested? I am always interested! I, personally, was pleased to receive some useful replies within the first week which got me going, for which I posted my thanks.

Since then I have been happy to see the thread unravel, but I suspect that Guidoo is wondering how long should a thread run? I don't know what the protocol is here: does the thread initiator remain a "Threadminder" and has he a responsibility to stop a thread along the lines of, "This correspondence is at an end: Threadminder"?

I now know that I have one duty, of which Guidoo has gently reminded me, and that is to regularly thank contributors. I apologise for not doing so in recent years.

As to ending the thread, who am I to do this deed? All I can do is to remind people (as if they need reminding!) that if a contribution looks and feels like a new topic, then start a new thread.

thank you
Leclerc
alunjames (Materials)
11 Jul 05 0:42
On the premise that all threads relating to LEL and MOC gone prior are accurate.

How does one calculate the MOC of Carbon Monoxide?
IRstuff (Aerospace)
11 Jul 05 1:05
There is nothing that prevents a thread from running ad infinitum or even ad naseum, particularly if there is something new to be added.

The only thing to be vigilant about are people that hijack old threads with off-topic questions or people that are simply reviving old threads for some sort of self-aggrandization.  

In general, if the new additions are not contributions, but separate questions, the poster should be gently reminded to RF their post and start a new thread, so as to not confuse others with unrelated material.

TTFN



25362 (Chemical)
11 Jul 05 1:34

The calculated value from the stoichiometric reaction would be 12.5*0.5 = 6.25% vol of oxygen. Where 12.5 is the published LEL, and 0.5 represents the moles of oxygen per mol of CO.

However, published MOC values call for 5.5%, which is lower than the "estimated" MOC.

Although in theory, below the MOC, the reaction couldn't generate enough heat to get a self propagating flame, it should be applied with a sizable safety factor as been repeatedly said in this and other threads.
Flareman (Petroleum)
11 Jul 05 15:31
I'm always leery of saying MOC.  Does it mean minimum or maximum?

The Lower explosive limit for CO (upward propagation) is very roughly 12.8% CO in 87.2% air which gives an oxygen figure of 18.3% for the lean condition (ie. too much air to burn).
Similarly at the UEL of 74% (upward propagation) the air is 26% = 5.5% O2 (too rich to burn).

These are the experimental limits at which, some reaction will almost/just propagate through a mixture (in the right conditions)

Apply a factor of safety.
If you want it to burn, apply a factor of safety of 1.5 to the air/gas ratio to find
- the richer mixture at LEL (O2= 17.2%)
- the leaner mixture at UEL (O2 = 7.3%)

If you want it to NOT BURN use
- 0.25* LEL (O2 = 20.3%)
- O2 at UEL - 3% (O2 = 4.3%)
alunjames (Materials)
11 Jul 05 19:47
Using purely stoichometric theory is the MOC (min) for Deuterium 3.3%?????

Big thanks to Mr Beychock for the link to the ludlow article.

Excellent insight on the theoretical determination of Flammability limits and MOC.

Has made it quite straight forward for me to spreadsheet MOC's using the chemical composition.

Am still very wary of Acetylene  and Ethylene Oxide as I understand they "misbehave". Are there any/many others that don't obey the MOC convention?

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