The motion of the vessel contents is influenced by the agitator and any baffles. The agitator, or impeller, provides the power to move the vessel contents. There are two types of impellers: radial-flow impellers, which move the liquids along the radius of the impeller, and axial-flow impellers, which move the liquids parallel to the axis of the agitator shaft. Without baffles, low-viscosity fluids tend to create vortices and swirl with poor mixing; baffles disrupt vortices and promote flow patters that lead to good mixing. With baffles in place either radial-flow or axial-flow agitators can provide good mixing for most chemical operations used in the chemical processing industry.1I confess that I didn't really know what a baffle was until this morning, when I saw the Wikipedia diagram. On the mixing front (pun?), this is something that I've seen, but not on the scale that Anderson refers to. Unsurpringly, agitation is something that is difficult, even on a scale as small as a 22-liter flask.
[later] Rapid distribution of reaction components does not take place on scale in conventional stirred reactors. For instance, based on pH measurement it may take 3-5 minutes for a reagent added at the top of a 500-gallon vessel to be evenly distributed throughout the solution. Mixing can be described as macromolecular mixing, which relates to the average composition of the vessel contents, and as micromolecular mixing, which considers the contacts of molecules on a molecular basis and any gradients that may result.2 Suitable micromolecular mixing can be difficult to perform on scale.
1. Oldshue, J.Y. Fluid Mixing Technology; McGraw-Hill: New York; 1983, 12-18.
2. Genck, W.J., "Crystallization's Forgotten Facet." Chem. Eng. 1997, 104(10), 94.
That is some strange vernacular. "Macromolecular" ? Isn't the most common use of this term one that refers to polymers or biopolymers or proteins or... some molecule with an unusually high aspect ratio?
ReplyDeleteIt doesn't surprise me that he thought he was free and clear using that term, as he doesn't apepar to discuss polymers at all in his book. That's probably a good thing, as the whole idea of "demixing" occuring upon agitation of polymer solutions would freak him out, and yet it is quite common. (Google 'polymer demixing' and 'flow-induced phase separation' for more info - you might even find my microcontribution to the field!)
Mixing - one of the seemingly simple yet horrifically complex things that keeps us process chemists in a job. Even more so when you go heterogeneous (biphasic, solids, or gases)
ReplyDeleteWe were working on a 75-L reactor, and the other chemist kept changing out the stir shaft...I ask "What seems to be wrong?" and he says "It keeps breaking in the middle!" So I look, grab a hammer, tap the post that the stir motor is connected to to straighten it, e voila! No more broken shafts...
ReplyDeleteI learned very little about these chemical reactor engineering concepts in grad school. When I finally picked up Organic Synthesis Engineering by LK Doraiswamy, I was amazed at just how little I understand about running (big)reactions!
ReplyDeleteDoes anybody know where to buy 5 or 10 gallon baffled reaction vessels that are plastic? I have a reaction to scale that contains HF so glass/steel will not work. Also, I assume the impeller should be wide relative to the reaction vessel diamter....at least 50% the diameter or so?
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