2023 AIChE Annual Meeting
Mixing of Viscous, Non-Newtonian Fluids
Many viscous fluids processed in industry exhibit non-Newtonian rheology. These industries include paints, personal products such as shampoo and detergents and food products such as ketchup and mayonnaise. In some cases, the end use properties of the products are highly dependent on their rheologies. For example, the sensation of eating some foods is partly a result of its texture or âmouth feelâ and the of performance of coatings will depend on how they flow when they are applied to a surface.
In this seminar the measurement of viscosity will be discussed with the interpretation of the data. Then the application of this information to the design of mixing equipment, where necessary, will be presented.
The power drawn by an impeller operating in the laminar regime is proportional to the viscosity of the fluid (as is pressure drop in laminar pipe flow) but, if the fluid is non-Newtonian, its viscosity will be determined by the shear rate that the impeller imparts to it and this must be taken into account in the sizing to the agitator drive.
Another process where this behavior must be taken into account is the blending of non-Newtonian fluids. The apparent viscosity of the fluid will be low near the impeller, where velocity gradients are high, and high near the vessel wall where velocity gradients are low and the difference in viscosity in these regions must be accounted for in order to make an accurate estimate of the blend time.
Finally, there is a class of fluids which exhibit a yield stress, meaning that a minimum level of shear must be applied before the fluid will move. Examples include toothpaste and high solids loading slurries. The rules used to size mixing equipment for these fluids will also be discussed.