Interrupting runaway reactions: stop the stirring or keep stirring? 1

[guerky]

These are some perceptions and doubts that i have concerning a system where a runaway condition was achieved.

The most common safe stop condition in a reactor where dangerous exotermic reactions may develop is: stop the stirring and chill it down. Its kind of intuitive: if you hinder the mass transfer in the media you'll slow the reaction down, especially if you have a diluted media.
in the other hand, if we have a system where the relative concentrations of the problematic substances is very high (e.g: the solvent can decompose at certain conditions, or we have all the reagents added at one time in a batch reactor), the mass transfer will be a minor issue. Thus, the good temperature dissipation will be the major concern, and the stirring could be crucial in achieving this dissipation.

So, in terms of stirring, we have at least two very distinct scenarios (that in a viscous become even harder to analyse):
1) Diluted media, where the stirring would increase the heat exchange for chilling the reactor, but would favour the molecular colisions, which could cause a runaway in the entire system. In this case, the stirring would need to be stopped.
2) Concentrated media, where stopping the stirring would reduce the reaction rate in a first moment by hindering mass transfer, but since the media is saturated with the reagent the reaction would keep going, generating superheated zones where the runaway would start and spread. This scenario favours the stirring.

Some questions:

Is this analysis correct ? (qualitatively; i understand that determining it for sure would require a lot of kinetic data)

Could we use Damkhöler numbers of the system to state rules of thumb of when to stop stirring and when to keep the stirring?

Someone have some good reference material about controling runaway reactions?

Thank in advance.

 

[bimr]

It is a little more complicated than that. You have to evaluate temperatures and mass.

Stopping the mixer in your case 1) may gave rise to localized high concentrations causing excess heat.

See the linked paper:

https://www.icheme.org/communities/...Archive/S085 - Exothermic Reactors/S85-02.pdf

 

[Compositepro]

Where there is any possibility of a dangerous run-away reaction (often called an exotherm), there should be emergency procedures in place to respond safely. The exact procedures will depend on the circumstances. In my experience, the two most common approaches are to dump a quench medium into the mixer and keep stirring, or to dump the batch onto the floor, or a quench tank, or both.
A quench medium is the fastest way to reduce the batch temperature and it will reduce the reaction energy density (Joules/gram), which is the main factor in how dangerous an exotherm will be. The quench fluid should be soluble with the reactants and not too different in density. For epoxy resin mixes we used DOP or DMP (dioctylphthalate, dimethylphthalate). These have very high boiling points so they would tend not to boil as readily.

 

[Compositepro]

I will add that there are few cases where stopping the mixer would be appropriate. If the reaction is already starting to go out of control, then the reactants have already pretty well mixed. Stopping a mixer is a common cause for an exotherm because that essentially stops any cooling.

 

[Latexman]

Or "whomp" in a reaction killer and keep stirring.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.