Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Pirri on 07/03/2016 05:00:21
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I'm trying to calculate the amount of a chemical, that if turned into vapor, would be sufficient to hit 100% Lower Explosive Limit (LEL), assuming no ventilation.
So, for example, if I was using Ether the LEL is 1.9%. If I was using 100ml of Ether in a room with a volume of 500m3 how would I calculate the %LEL for that volume? and also detemine what amount could be used before reaching the LEL.
I have been trying to find the answer everywhere and while I can find formulas to calculate the LEL of a mixure I can find an example of the more practical calculation of its concentration
Cheers
K
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if I was using Ether the LEL is 1.9%. ....in a room with a volume of 500m3 how would I calculate the %LEL for that volume?
1.9% of 500m3 = 0.019 x 500m3 = 9.5m3 of Ether vapor = 9500 liters of Ether vapor
At normal room temperature and pressure, 1 "mole (http://en.wikipedia.org/wiki/Mole_(unit))" of a gas occupies about 24.7 liters of volume.
So 9500 liters requires 9500/24.7 moles = 385 moles.
also determine what amount could be used before reaching the LEL.
I assume you are talking about Diethyl Ether (http://en.wikipedia.org/wiki/Diethyl_ether), (C2H5)2O? (There is a whole family of chemicals called "ethers".)
This chemical has a molecular mass of 76 (you calculate this by adding up the mass of each element, as you find in the periodic table).
This means that 1 "mole" of ether weighs 76 grams.
So 385 moles requires 29.2 kg of Ether. This is a lot more than just a small 100ml bottle!
Warning: Do not try this experiment at home! Diethyl Ether is extremely flammable, it is chemically reactive and can burst into flames on contact with things that you would not consider hot. It can also make people unconscious (for many years it was used as an anaesthetic).
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That calculation is all very well, but imagine what happens in reality.
You spill some ether, or put it in a dish or whatever and eventually it will evaporate and produce (roughly) the LEL.
So you might imagine that, if you didn't have that much ether, you couldn't reach the LEL. Well, near the bottle or dish or whatever, there's a lot more than the LEL.
Also, ether vapour will probably kill you at about a tenth of the LEL
Why bother to do the calculation?
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I am trying to understant the process of calculating what the physical amount is that is represented by the %LEL in theory. I chose Diethyl Ether because I had found a SOP for a lab prac that included calcuating if the amount fell below the %LEL and did not understand how they came to their results. The room size and %LEL came from their procedure. An amended version of the document is included below:
From the MSDS :TWA 400ppm (1210 mg/m3), STEL 500 ppm (1520 mg/m3), lower explosion limit 1.9%. S.G 0.71...
The lower explosion limit of 1.9% is equivalent to a concentration of 23.3 gm/m3 so explosion is not a hazard.
(The molecular weight of air approximates to 30 so a cubic metre of air weighs
1000 1230
------- x30 =1230 gm so 1.9% represents a concentration of ------ x 1.9
24.4 100
= 23.3 gm/m3 )
There is however an ignition hazard from a source such as flame, sparks or even static electricity.
In my work space people are starting to use Ethanol and Acetone, nowhere near as dangerous as Diethyl Ether but they are flammable. They are using ethanol in such a way that it is evaopating into the air. I would like to be able to calculate that the quantities they are using are not an explosion risk and I had been planning on working on a worst case scenario of no ventilation.
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Well before they are an explosion risk, they are a toxic risk.