Am I Mixing or Dispersing? What’s the Difference?
INDCO customer service representatives and engineers provide assistance with mixing equipment selection daily. A common discussion with customers is the determination of whether a given application calls for a mixer or a disperser. Mixing and dispersion processes both entail rotating equipment within a vessel to process liquid-based products. In the following paragraphs we will provide definitions, discussion of flow vs. shear, mixing and dispersion equipment differences and examples of each application type.
“Mixing” is the creation of a homogeneous material through the intermingling of individual ingredient “particles” occurring at the trailing edge of a rotating impeller. Both mixing (also referred to as blending) and dispersion processes technically meet that definition. The differentiator is whether a given product requires a “flow-driven” or a “shear-driven” process to achieve the desired results.
Material flow is provided throughout the vessel by the size, type and rotational speed of the impeller. A flow-driven process is generally one where two or more liquids are easily blended, a soluble solid is effectively dissolved with gentle motion or settled solids are easily put back into suspension. Examples include brine solutions, dissolved sugars, and blends of chemicals having similar physical properties. Marine style propellers, hydrofoil impellers and axial flow turbines are common impeller styles used in flow-driven applications. The impeller size and speed are selected based on the material properties, such as viscosity.
The amount of shear generated within a blending application can also be increased with impeller size and speed adjustments. This is useful for ingredients that require more than gentle agitation to blend but not to the extent where dispersion is a must. Low viscosity applications often provide the designer options to choose a direct drive high speed mixer paired with a small impeller or a higher torque, slower speed model with a gearbox and larger impeller. As viscosity increases the need for a gear driven model also increases.
Shear-driven processes, such as dispersion, are those where violent action at the impeller’s edge is required to achieve the desired outcome. Dispersion is a high-speed process utilizing a sharp blade-type impeller and requires more horsepower than simple agitation for similar batch sizes. A deep vortex with visibility to the top surface of the blade is desirable. One point that is commonly misunderstood is that for true dispersion a high tip speed at the end of the blade is critical to success. Since dispersion often includes solids into liquids, oils or other higher viscosity materials a high horsepower motor is necessary to provide both the necessary speed with high torque. Simply putting a dispersion blade on a mixer designed for blending does not provide the results of a disperser. Examples of dispersion applications are the addition of liquid or powder pigments to inks and paints, combining oils with lighter base liquids to produce lotions, creams and other similar products.
Call our engineers at (800) 851-1049 to help you select the proper mixer or disperser for your application or to meet your exact specifications. INDCO is located at 4040 Earnings Way, New Albany, IN 47150.