Limiting Oxygen Concentration for an Isomerate Storage Tank 3

[Bandit1422]

I am looking for a basis for LOC (same as Minimum Oxygen Concentration) when applying nitrogen purge to an Isomerate storage tank. Can anyone explain to me the tactics in developing this? I understand it is based on LEL and have seen variance based on pressure, altitude, etc.... Can someone help me gain a little better understanding of this? As well i understand a common practiced safety factor is 25% of the LEL.

 

[25362]

I hope one of these threads may contain the answer your query:

thread135-15647
thread135-115960
thread124-53646
thread798-255535
thread798-33615
thread798-144582
thread816-162978

 

[Latexman]

Wikipedia is your friend - LOC

Good luck,
Latexman

 

[Bandit1422]

Thank you for the information thus far but in research I have not been able to find out how they calculate the percentages based on the products (ex. propane, butane, etc...) such as that listed in the wikipedia page provided. I am more looking for how one would determine the percent nitrogen purge needed for isomerate in a storage tank. And was as well wondering if pressure or volume of the tank could make a difference on this?

 

[25362]

Thread798-66873

 

[Latexman]

The estimating method I was taught (a long time ago) for N2 was - it is the stoichiometric amount of oxygen at complete combustion at the LFL. For example:

C3H8 + 5O2 ----> 3CO2 + 4H2O

LFL C3H8 = 2.2% in air

LOC = 2.2 x 5 = 11.0% (11.5% by lab data)

Good luck,
Latexman

 

[25362]

Latexman is right.

The logic of such an estimation is given by:

LOC = [moles Fuel/(moles Fuel+moles Air)](moles O[sub]2[/sub]/moles Fuel)
= LFL (moles O[sub]2[/sub]/moles Fuel)

 

[Bandit1422]

Thanks guys...............Say your working with a UVCB compound thats compositions consists of multiple hydrocarbons. I played around with a couple simple equations to start with but when I used Latexmans theory it came out with a high percent of O2.

Idk how applicable this is but just for simplicity I took to basic hydrocarbons Methane and Propane

C3H8 + CH4 + 10 02 -----> 2 CO2 + 6 H20

In this scenario your multiplier would be 10

So LOC = LEL of this compound x 10 ??

 

[Latexman]

No. Do each compound seperately. Then, pick the lowest LOC.

Good luck,
Latexman

 

[Bandit1422]

Oh ok can you assist me in determining LFL for chemicals...........

As stated by 25362 LEL= (moles of O2/moles of fuel) so in terms of propane its LEL = (5 moles of O2/ ????) where do you determine the moles of fuel being that wouldnt it just be one mole of fule sorry my chemistry has left my brain at the moment.

 

[Latexman]

Look for LFL on the MSDS for each chemical. If you don't have an MSDS, Google for one.

Good luck,
Latexman

 

[Latexman]

What college course is this for?

Good luck,
Latexman

 

[Latexman]

Flammability Limit

Good luck,
Latexman

 

[25362]

To Bandit1422:

Stoichiometry.

 

[Bandit1422]

This is a work related situation. We have a spherical tank that in the oncoming months will be filled with an Isomerate product and I was trying to assist someone in determining the proper amount of nitrogen to put on the vessel in order to eliminate the explosion hazard. This could be presented in Minimum Oxygen Concentration then applying a safety factor.

In regards to LEL once i have determined LELs for all of the chemicals in Isomerate would I then use Le Chatelier's mixing rule? LEL(Mix)=1/(Sum(x[sub]i[/sub]/LEL[sub]i[/sub]))

 

[Bandit1422]

I found this example in some text

C7H16 + 11 O2 = 7 CO2 + 8 H2O
Air = 11/ 0.21 = 52.38 moles air /mole of C7H16 at stoichiometric conditions

Could either of you explain the stoicometry used to develop the LEL from this equation. This will provide with a good example which I could use to approach the other chemicals.

 

[Latexman]

In regards to LEL once i have determined LELs for all of the chemicals in Isomerate would I then use Le Chatelier's mixing rule? LEL(Mix)=1/(Sum(xi/LELi))

If the concentrations will remain consistent over time, yes. If the concentrations vary naturally or there may be some change in the future that would change the composition, I'd recommend evaluating all the combustible species separately and choosing the lowest LOC, but that's me. Depending on how big your safety factor is, it may not matter anyway.

Good luck,
Latexman

 

[25362]

From the book by Crowl and Louvar, Chemical Process Safety: Fundamentals with Applications - Prentice Hall, the flammability limits for many hydrocarbons could be roughly estimated (although experimental determinations are always recommended) using the following equations:

LFL = 0.55 C[sub]st[/sub] and UFL = 3.50 C[sub]st[/sub]; where C[sub]st[/sub] is volume % of fuel in fuel plus air.

Combustion stoichiometry shows:

C[sub]m[/sub]H[sub]x[/sub]O[sub]y[/sub] + z O[sub]2[/sub] [→] mCO[sub]2[/sub] + x/2 H[sub]2[/sub]O
​

where z = m + x/4 - y/2.
z is moles O[sub]2[/sub]/mole fuel. The result is then:

LFL = 55[÷](4.76m + 1.19x - 2.38y + 1)​

UFL = 350[÷](4.76m + 1.19x - 2.38y +1)​

In your example:

m =7; x = 16; y = 0

LFL = 55[÷](4.76[×]7 + 1.19[×]16 +1) = 1.03 vs actual 1.1%
UFL = 350[÷](4.76[×]7 + 1.19[×]16 +1) = 6.6 vs 6.7% actual.

 

[Bandit1422]

Thank you this helped alot. Would you apply a safety factor (say like 25%) to the LOC calculations to ensure efficiency?

 

[25362]

Refer to thread798-33615.

I've seen plants using 40% of the MOC and even less with ketones.