Flammability limits: acids which associate in the vapour phase.

[Leclerc]

When flammability limits for, for example, acetic acid, are published, they are, as usual, quoted as % v/v. I assume that the v/v approach is used because it is dilution which limits flammability by failing to sustain burning, rather than something based upon stoichiometric considerations.

I have been trying to calculate whether vapours in equilibrium with mixtures containing acetic acid are flammable.

My start point has been:
(i) LFL for acetic acid/air is ~4 % v/v.
(ii) Flashpoint of acetic acid is 39 deg. C.

I take pure acetic acid at 39 deg. C, and then calculate the composition of acectic acid/ air in equilibrium. This gives me 7.87 % v/v acetic acid in air. I assume that this is higher than ~4 %v/v because, although the VLE method is fairly sophisticated, it is based upon vapour pressure, which is a liquid property.

Independently, I calculate that the average molecular weight of associated and non-associated acetic acid species in vapour at 30 deg. C is 107.8.

I take the results of my first calculation (7.87% v/v unassociated acetic acid, etc.) and change into mass units. I then change again into volume units using the higher molecular weight of 107.8, which gives me a figure of 4.5% v/v acetic acid species in air. This is not far away from the 4 %v/v LFL.

Is this approach correct? Is the similarity of 4% and 4.5% more than coincidence? Is there a simpler way of doung this calculation? Can anybody confirm that, for acetic acid, the published figures for acetic acid refer to the % volume of associated and non-associated species?

This is not homework. If it were I would change courses.

 

[Latexman]

Why did you use 30 C in one case and 39 C in the other?

Good luck,
Latexman

 

[StoneCold]

At 39C I calculate the vapor pressure of Acetic Acid as 33mm Hg. So that means that the maximum v/v% in an open space (like a room) could be 33/760x100 = 4.34%. So you would be just above the LEL of 4 if there is no ventilation in the room and assuming you have enough acetic acid in an open container to reach the equilbrium volume and you wait long enough.

Lot's of conditions have to be met to get there.

I don't understand your 7.87% conversion to 4%????

Regards
StoneCold

 

[Leclerc]

to Latexman: I make a typo: both tempeatures should be 39 deg. C.

to Stonecold:
My physprops package calculates the vapour pressure of acetic acid as 0.045 bar at 39 deg. C, which could give me 0.045/1.013x100=4.44%v/v acetic acid in air; similar to your figure.

However, my physprops package also calculates the composition of acetic acid/air in equilibrium with 100 acetic acid liquid at 1 bara and 39 deg. C as 7.87%. It also tells me that acetic acid in the vapour associates, and that the average molecular weight of the monomer and dimer is 107.8.

I think what is happening is that at 39 deg. C, assuming no association, acetic acid would evaporate into air until the back pressure, the partial pressure of acetic acid in the vapour causes an equal rate of condensation of acetic acid from the vapour into liquid. This happens when the partial pressure is 4.34% of total pressure.

When there is association, more acetic acid evaporates into the vapour where some proportion associates, reducing the number of molecules. the evaporation continues until the partial pressure of acetic acid monomer and dimer causes an equal rate of condensation of acetic acid from the vapour into liquid. This happens when the partial pressure of acetic acid species is 4.34 % of total pressure.

Although the partial pressure (and partial volumes) of acetic acid species is the same in both cases, the weight of acetic acid species is markedly different.

This is fine until one requires to calculate how much acetic acid one needs to evaporate into air before the UFL is reached. The UFL is quoted as being between 4 and 5 %v/v acetic acid in air.

I am assuming the the UFL should be redefined as "being between 4 and 5 %v/v of monomer and dimer acetic acid, which is equivalent to 7.87% v/v of acetic acid monomer if it didn't associate."

7.87 x average molwt of monomer plus dimer/molwt monomer = 4.34

 

[25362]

Here are some notes:

1. The published (Kirk-Othmer) open cup flash point of acetic acid is 57 deg C.

2. Published (CRC) values LFL~4% and HFL~20%, are generally given at 25 deg C and atm. pressure. At 25 deg C the degree of association would be near 90%, meaning an average MW ~109.2, with 81.8% molar proportion of dimer.

3. LFL values for compounds with the general composition
C_mH_xO_y, can be approximated from stoichiometry as follows:

LFL = 0.55(100)[÷](4.76m+1.19x-2.38y+1)​

The HFL is estimated to be 6.36 times the LFL.

Applying this formula to acetic acid, results would be
LFL_dimer = 2.7%, LFL_monomer = 5.2%. Averaging (by Le Chatelier equation) LFL_mix = 3%.

4. Thus, I'd say Leclerc is right in assuming the tabulated LEL/HFL values refer to the mix of dimer and monomer at the particular temperature. BTW, Leclerc's last formula, if true, should be reversed to suit results.

 

[Leclerc]

Thanks, Gents for your replies and advice. As usual, most things which at first appear to be coincidence, later on are proved not to be so.

Leclerc.