Mixing in Viscous Liquids

Authors:

C.K. Coyle

H.E. Hirschland

B.J. Michel

J.Y. Oldshue

This study investigates the mixing performance of helical impellers in both Newtonian and pseudoplastic viscous fluids, focusing on blend time, turnover time, and power consumption. Experiments examined the effects of impeller geometry, pitch ratio, number of flights, impeller speed, fluid viscosity, and tank dimensions on mixing efficiency. Key findings include the negligible effect of viscosity above 1,500 centipoise on blend time, the beneficial impact of inner flights for pseudoplastic fluids, and the trade-offs between impeller configuration, power consumption, and mixing performance.

Key Learnings

• Blend time in viscous fluids is inversely proportional to impeller speed and largely independent of viscosity above 1,500 centipoise.
• Turnover time is approximately one-third of blend time for both Newtonian and pseudoplastic fluids.
• Inner flights on helical impellers significantly reduce blend time for pseudoplastic fluids but have minimal effect on Newtonian fluids.
• Two outer flights can achieve the same blend time at a lower impeller speed but may require higher torque.
• Pitch ratio affects mixing efficiency: a 0.5 ratio performs better in pseudoplastic fluids compared with 0.9.
• Liquid depth-to-tank diameter ratio (Z/T) impacts power consumption and blending efficiency; lower Z/T ratios reduce power requirements but may increase vessel cost.
• Shear rate from the impeller is critical for calculating power consumption, especially in pseudoplastic fluids.
• Helical impellers generally provide better large-scale mixing and can be preferred over axial flow turbines when uniformity and lower speed operation are needed.
• Visual observation of blend time is a reliable and consistent method for evaluating mixing performance compared with analytical point measurements.
• Open impellers can match helical impellers for certain viscosity ranges but may differ in heat transfer, scale of mixing, and uniformity.