Liquid Mixing and Agitation Equipment

Shaken or Stirred

What is Liquid Mixing and Agitation?

The terms “mixing” and “agitation” are sometimes interchangeable. Mixing refers to the ability to combine the components on a microscopic level, often called micro mixing. Agitation describes fluid that is in motion, but not necessarily mixing (macro mixing). For successful mixing, we need both – micro mixing (shaken) and macro mixing (stirred) to achieve the mixing action throughout the tank!

Where in the mixing process would you find Sterimixers and Sterivalves?

Numerous Sterimixer designs can be found in upstream, downstream and fill & finish applications including:
  • 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. For example, a typical low-shear biopharmaceutical application in storage tanks, buffer and media hold.
  • Heat transfer: the liquid needs to be constantly moving in order to achieve a constant temperature in heated (jacketed) vessels.
  • Mixing miscible liquids: the pumping action of the mixer combines the liquids with the objective being the creation of a homogeneous composition in the tank. Higher pumping rates will speed up mixing but might produce more shear and the formation of an undesirable vortex. A balance between mixing time and acceptable shear/vortex is required.
  • Dissolving solids in liquid: similar to mixing miscible liquids, the pumping action helps to dissolve and keep the ingredients in a liquid state. 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 mixers 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-the-bottom or uniform suspension).
  • Dissolving gas in liquid: this is common in bioreactors where air is introduced and the mixer helps disperse the bubbles in order to increase oxygen transfer. The degree of shear is related to the oxygen transfer rates required by the application.
 

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Solids Suspension

Solids are kept in suspension in a liquid phase. Low shear mixers are typically used. The amount of transferred power required depends on the particle settling velocity, the viscosity of the liquid, the percent solids and the degree of suspension that is desired (e.g. off-the-bottom or uniform).

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Liquid/Liquid Blending

Low shear mixers are used when the liquids are miscible. The pumping action of the mixer combines 2 or more liquids to create a homogeneous mixture in the tank. If the liquids are immiscible then, some degree of shear is required.  Quite often, a balance between flow and shear is used.

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Heat Transfer

The liquid needs to be constantly moving in order to achieve a constant temperature in vessels with heating or cooling capabilities.  A low shear mixer is used to circulate the liquid across the heat transfer surface.

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Dissolving Solids in Liquid

Similar to mixing miscible liquids, the mixer pumping action helps to dissolve the solids. Floating solids often require the formation of a vortex plus some shear to be fully incorporated.  Sinking solids must be suspended to be fully incorporated and some solids require shear to be incorporated. All solids will reach a saturation point, where no additional solids can be incorporated.

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Dissolving Gas in Liquid

This is a common application in bioreactors, where a gas is introduced and the mixer disperses the bubbles to increase the oxygen transfer. The required degree of shear is related to the oxygen transfer rate required by the application.

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Storage

The liquid is kept in constant movement using a low shear mixer in order to maintain homogeneity of the concentration and temperature of the liquid throughout the vessel. Typical storage applications in the biopharmaceutical industry are buffer and media hold.

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Flow vs. Shear

High pumping, low shear impellers are used in general blending applications and where the product can easily be damaged, such as cell culture. Low pumping, high shear impellers are used to create emulsions and incorporate powders that tend to float or agglomerate.

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Viscosity

Viscosity is a property of fluids that characterizes their resistance to flow. A higher viscosity fluid is more difficult to pump than a lower viscosity fluid, leading to longer blend times. Some fluids “act” less viscous when shear is applied to them, often referred to as the apparent viscosity of a fluid. Viscosity is a very important factor to consider when sizing a mixer.

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Vortex or not?

A vortex is a flow pattern formed by the mixer for the purpose of drawing down solids that would otherwise float. In a great number of cases, a vortex is not desirable because it can result in air entrainment and incomplete mixing.

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