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Limiting Oxygen Concentration for an Isomerate Storage Tank

Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
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.

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

Wikipedia is your friend - LOC

Good luck,
Latexman

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
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?

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

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

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank


Latexman is right.

The logic of such an estimation is given by:

LOC = [moles Fuel/(moles Fuel+moles Air)](moles O2/moles Fuel)
= LFL (moles O2/moles Fuel)

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
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 ??

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

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

Good luck,
Latexman

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
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.

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

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

Good luck,
Latexman

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

What college course is this for?

Good luck,
Latexman

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

Flammability Limit

Good luck,
Latexman

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

To Bandit1422:

Stoichiometry.

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
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(xi/LELi))

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
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.

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

Quote (Bandit1422)

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

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank


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 Cst and UFL = 3.50 Cst; where Cst is volume % of fuel in fuel plus air.

Combustion stoichiometry shows:

CmHxOy + z O2 → mCO2 + x/2 H2O

where z = m + x/4 - y/2.
z is moles O2/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.

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

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

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
I read through the thread that you provided and got a little higher value based on the calculations i was referred to. I have found that toluene's experimental LEL is around 1.26 and you multiple that by the 9 decided from the stoicmetric conversion and you get around 11.4 compared to the 9.5 discussed in the thread. This created some confusion as well as the basis for using these calculations:

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

How do they differ and which is preferred?

Finally so essentially applying a safety factor of 40% would equate to (LOC-(LOCx.4))

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank


My intention was ≤0.4 LOC.

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

Quote (Bandit1422)

I have found that toluene's experimental LEL is around 1.26 and you multiple that by the 9 decided from the stoicmetric conversion and you get around 11.4 compared to the 9.5 discussed in the thread. This created some confusion . . .
Most of the time, different estimation methods give different results. The BEST value comes from test data in a qualified lab. On safety issues it boils down to, are you willing to bet your life and livelihood on an estimate. If not, have it measured.

Good luck,
Latexman

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
Im going to assume that the reason for the large safety factor would be because the level of variation in comparison with the hazard associated. Is there any background (book references etc...) providing supplemental information in using a 40% safety factor?

RE: Limiting Oxygen Concentration for an Isomerate Storage Tank


In addition.
The same book by Crowl & Louvar gives an example of how to estimate the LFL and HFL of a mixture of air with flammable gases, based on Le Chatelier's empirically derived equation to determine if a certain mixture is flammable or not.
LFLmix = 1÷Σ (yi/LFLi)
and
UFLmix = 1÷Σ (yi/UFLi)
Where

yi = mol fraction of the flammable gas i in the mixture with air
LFLi = LFL of the flammable gas i in the mixture
UFLi = UFL of the flammable gas i in the mixture

The authors advise us that empirical formulas like these have their limitations. Therefore, I suggest following Latexman's advice.

BTW, the lowest (roughly) estimated LOC doesn't necessarily go with the lowest LFL. Take, for example:

Metane: LFL = 5.0. LOC = 5.0×2 = 10.0
Hexane: LFL = 1.1. LOC = 1.1×9.5 = 10.45
Ethylene: LFL = 2.7. LOC = 2.7 ×3 = 8.1
Heptane: LFL = 1.1. LOC = 1.1× 11 = 12.1
Acetone: LFL = 2.5. LOC = 2.5×4 = 10.0



RE: Limiting Oxygen Concentration for an Isomerate Storage Tank

(OP)
If you would be interested in helping i came up with a Limited Oxygen Concentration Calculator for this isomerate material based on Estimated (Stoichiometric) Calculations, Experimental Calculations I was able to find, and calculations using LeChateleirs Mixing Rule being that the mixture is typically constant. I could send it to either one of you for a quick review. If not thanks I greatly appreciate your help in assissting me to understand this information.

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