Wednesday, June 8, 2011

Process Wednesday: minimum stirrable volume

How much distance between the blades and the bottom of
the reactor? Probably doesn't matter, right?
Photo credit:
Reading through Kilomentor's archives, one finds a treasure trove of information for the young process chemist. This morning, the concept of 'minimum stirrable volume':
The minimum stirrable volume is exactly what the name teaches. It is the minimum volume of liquid needed in the reactor so that the turning stirrer paddles effectively stir the reactor contents. To some extent this minimum stirrable volume depends upon what is being stirred and what will happen during the initial stage of the reaction of interest. For example, a homogeneous solution in which an exothermic refluxing occurs at the beginning of the reaction may only need the moving blades to touch the liquid surface because gas evolution and thermal convection are going to move the homogeneous liquid phase around. At the other extreme, if zinc powder, tin granules or magnesium turning need to be swept up off the bottom of the reactor, the immersion of the stirrer blades probably needs to be complete and the stirring rapid. 
The reason chemists, who are more accustomed to working in the laboratory tend to forget the minimum stirring volume constraint is that two of the most common laboratory stirrer types are the magnetic stirrer and the crescent bladed overhead mechanical type. The magnetic stir bar on the bottom of the flask so that the minimum stirrable volume is very small while the crescent bladed stirrer is very often set up with its curved edge nestled up against the bottom of the round bottomed flask in which it is installed so that again the minimum stirrable volume is close to zero.
It's been a while since I used a stir bar in any of my reactions; that being said, it still influences my thinking quite a bit. It only takes one situation in which you note that your reaction volume doesn't reach the bottom of the agitator blades to remind you of this concept. It's funny how forgetting little details like 'minimum stirrable volume' can really ruin your day...

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