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.