The terms “mixing” and “agitation” are sometimes used interchangeably. Mixing, refers to the ability to mix the components on a microscopic level, often called micromixing. Agitation, describes fluid that is in motion, not necessarily mixing (macromixing).
For successful mixing, we need both – micromixing (shaken) and macro-mixing (stirred) to get the mixing action throughout out the tank!
The different types of Sterimixer designs can be found in upstream, downstream and fill & finish applications:
Storage: the liquid is kept in constant movement in order to keep two or more different liquids mixed in a constant concentration and temperature throughout the vessel. A typical low-shear (see box) biopharmaceutical application in storage tanks, buffer and media hold, etc.
Heat transfer: the liquid needs to be in constant movement in order to achieve a constant temperature in heated (jacketed) vessels.
Mixing miscible liquids: the pumping action of the mixer mixes the liquids with the final purpose of having a homogeneous composition in the entire tank. Higher pumping rates will speed up mixing but might cause more shear and undesirable vortex forming. A balance between mixing time and allowable shear and vortex is sought.
Dissolving solids in liquid: similar to mixing miscible liquids, the pumping action helps dissolving and keeping the ingredients in solution. Solids that are hard to incorporate often require a higher degree of shear and the ability to form a vortex.
Suspensions: solids are kept in suspension in a liquid phase. Low shear mixer are typically used. The amount of transferred power required depends on the particle settling speed and the degree of suspension that is desired (e.g. off bottom or uniform suspension).
Dissolving gas in liquid: this is common in bioreactors where air is introduced and the mixer helps dispersing the bubbles in order to increase oxygen transfer. The degree of shear is related to the oxygen transfer rates required by the application.