Mixing Effects in a Multi-Stage Mixer Column

Authors:

James Y. Oldshue

Frank Hodgkinson

Jean-Claude Pharamond

This paper analyzes the performance and scale-up of multi-stage mixer columns used in liquid-liquid extraction, with emphasis on the Oldshue-Rushton column design. It compares mixing effects, mass transfer rates, and stage efficiency across various extraction systems such as mixer-settlers, rotating disc, and York-Scheibel columns. The study highlights how proper impeller design, shear control, and residence time optimization lead to improved dispersion, throughput, and overall extraction performance.

Key Learnings

• Multi-stage mixer columns play a crucial role in improving liquid-liquid extraction efficiency.
  
• The Oldshue-Rushton column design provides effective mixing and mass transfer through controlled shear and dispersion.
  
• Proper impeller selection and rotational speed are vital for optimizing stage efficiency and minimizing phase separation issues.
  
• Scale-up of extraction columns requires understanding the relationship between shear rate, residence time, and throughput capacity.
  
• Excessive shear can cause emulsion formation, while insufficient mixing reduces mass transfer performance.
  
• Comparative analysis of systems like mixer-settlers, rotating discs, and York-Scheibel columns helps identify optimal designs for specific industrial applications.
  
• Pilot plant testing and batch break-time studies are essential for predicting full-scale column performance.
• Controlling column geometry and interstage mixing enhances process stability and solvent extraction efficiency.