Wednesday, May 23, 2012

Process Wednesday: directionality of agitators can be important!

In passing recently, I heard that it might matter which direction (clockwise/counter-clockwise) an agitator was stirring in a reactor. I was a little surprised by this, but looking through the Handbook of Industrial Mixing, I see that it indeed matters (emphasis mine):
In addition to suspending solids off the tank bottom, a process may require homogenous suspension throughout the bulk. An additional axial flow impeller, perhaps an up-pumping one, may be needed at a higher level for this purpose... 
When using axial flow impellers, the mixer rotation can be reversed to create up-pumping action. This pumping mode can be effective for some systems, such as entrainment of floating solids and gas dispersion. The up-pumping flow provides an effective mechanism for incorporating lighter solids on the liquid surface near the wall. This avoids the need for creating a vortex, which can cause air entrainment and mechanical vibrations (see Chapter 10). For gas dispersion, the up-pumping impeller is generally used at the bottom along with a down-pumping impeller at the top. Such a configuration can provide good gas-holding capacity and prevent mechanical vibrations caused by opposite flows resulting from a down-pumping impeller at the bottom. 
So apparently there's "up-pumping" and "down-pumping" (parallel to the agitator shaft, I assume) and depending on the relative density of things inside your reactor (gases, solids, liquids, etc.), you might want to do either one. Worth remembering, I suppose.

5 comments:

  1. It's from the direction of the pitch of the agitator blades. Just think of a propeller on an airplane or boat. It doesn't work if you have 90 degree blades or a retreat blade.

    Sorry if that's obvious. I didn't realize it until I started using a pitched blade agitator myself.

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    1. It wasn't immediately obvious in the conversation that I was having, but yes, it makes a good bit of sense.

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  2. Good stirring is vital for a number of reasons, some are, keeping suspensions suspended (obvious). Good heat transfer from the reactor walls to the medium and vice versa. Obtaining the correct crystal modification and particle size. However to much stirring can also cause problems, it can degrade your material, especially if you are using a polymer supported reagent.
    The correct stirrer configuration (and speed) should be examined in the lab first, important if you have to change production sites.
    This happened to me at my pilot plant and production site everything went well. Moving abroad to a new production site screwed everything up as they did not have the same equipment. So it was back to the lab for me.

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  3. Is this not exactly the same reason why reversing the direction of your ceiling fan blades and running it in the winter will save your heating bill?

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  4. Of course I forgot. The direction of stirring affects the enantiomeric excess. As described by Cheaters and Steal some years ago.

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