Impact of inert on flammability of submicron aerosol 1

[slowpoke]

How might one determine the flashpoint change of a submicron aerosol of a liquid with a defined flash point due to a given addition of an inert gas?

 

[owg]

I think that the various flash point tests are for liquids. I don't know of any flash point tests that generate and test a submicron aerosol. I think they just test the vapour from a small test vessel. You may need to deal with lower explosion limit (LEL) rather than flash point. The nitrogen vendors may be able to supply you with references or papers on the subject. Look for Minimum Oxygen Content (MOC) for a given gas to render it non flammable. Sorry I can't be of more help.

HAZOP at

http://www.curryhydrocarbons.ca

 

[25362]

For suspensions with drop diameters less than 0.01 mm the LFL is considered virtually the same as the vapour.

However, mechanically formed mists are apparently more dangerous. I read that in experiments with larger drops (about 0.2 mm) the LFL was found to be less than 1/10 the normal LFL value. This is an important factor when inerting in the presence of mists.

With droplets having diameters of 0.6 mm up to 1.5 mm flame propagation is considered impossible. However, disturbances than can shatter large drops can create hazardous conditions.

Follow owg's advice.

 

[slowpoke]

Gracias to 25362
My interest is with aerosols in the 0.3 to 10 micron range. Where might I find elaboration on the rationale for their 'normal' LFL?

 

[unclesyd]

There was a lot of work done with Therminol and Dowtherm vapors or mists. A lot of this work concerned the droplet or mist size in a cloud. This work was done in connection of the damage expected with the ignition of these materials. With the right conditions you had an explosion, actually detonation, in that one of the main variables was the droplet size.
I’ll see if I can find the author of the work and if there was anything published.
As mentioned in previous posts the inert gas producers did have a lot of information on inerting.
Also check Texas A&M.
Also again check on nebulizers. There was a lot of work done on creating different droplet sizes and dispersions by eans of nebulizers. A lot of the work was with increasing flame efficiency.

 

[Flareman]

The LFL of a vapor is really an empirical determination describing the leanest condition which will propagate a flame. When the gas burns it releases transient radicals which radiate heat into the unburned materials and help them to break up for conversion. This means that the flame burns upward faster than it burns downward and the LFL reflects that difference. LFLs are often given for upward, horizontal, and downward propagation. Effectively the each LFL tends to describe a(n almost) constant flame temperature condition, so a good way to make an estimate of the inert effect is to check the normal adiabatic flame temperature for the gas/air mix or your gas/inert air mix and adjust them so that the come out the same. A quick estimate allows you to look at the ratio of excess air to gas at the published LFL and use your inert to erode that ratio. Nitrogen replaces nitrogen (vol) 1:1 but H2O is about 1.3:1 and CO2 is about 1.8:1 (as a function of their specific heats at temperature).

The LFL is usually published at 25 degC conditions but if you have a higher temperature it will be less than that, in roughly linear proportion to the temperature difference between 25 degC and the adiabatic flame tmperature I mentioned above.

Always bear in mind that this is an estimate so use a factor of safety. I have previous put this estimation on the web as an Excel spreadsheet at

http://www.geocities.com/flareman_xs/Gases/Flammability_calc.html.

[You will need IE5 and Excel on your PC]


David