Fluid Mixing, Heat Transfer and Scale-up

Author:

J.Y. Oldshue, M.S, Ph.D.

This article explores the principles and practices of fluid mixing in industrial processes, focusing on impeller design, flow patterns, and shear rates. It discusses heat transfer in mixing systems, including comparisons between flat-blade turbines and propeller impellers, and presents experimental data to guide scale-up from pilot to plant-scale operations. Key considerations for successful mixing include power input, flow per unit volume, and maintaining uniformity in chemical, solid, and gas-liquid processes.

Key Learnings

• Understanding fluid motion in mixing vessels requires consideration of impeller pumping capacity, flow patterns, and shear rates.
• Effective mixer design depends on the specific mixing goals, including uniformity, mass transfer, or chemical reaction rates.
• Heat transfer efficiency varies with impeller type, speed, diameter, and positioning, with flat-blade turbines and propellers showing different performance characteristics.
• Scale-up from pilot to plant-scale systems requires careful adjustment of variables such as impeller speed, diameter, power per unit volume, and flow characteristics to maintain geometric and hydraulic similarity.
• Turbulent and laminar shear rates must be measured and controlled to avoid damage to solids or catalysts and ensure consistent process performance.
• Baffle and coil placement in tanks can significantly impact flow patterns, turbulence intensity, and heat transfer coefficients.
• Proper selection and calibration of measurement tools, such as hot-wire velocity probes, are essential for accurate flow and shear characterization.
• Mass transfer and oxygen absorption in liquid/gas systems depend on flow distribution, superficial velocity, and impeller configuration.
• Maintaining the optimal impeller-to-tank size ratio is crucial for achieving process efficiency while minimizing power consumption.
• Process-specific considerations, such as chemical reaction speed or solid suspension requirements, must guide scale-up decisions.