An Experimental Investigation of Mass Transfer and Flow Resistance in the Kenics Static Mixer

Authors:

W. David Morris

Paul Misson 

This paper investigates the mass transfer and flow resistance characteristics of the Kenics static mixer under laminar flow conditions using a naphthalene evaporation technique. The experimental results demonstrate that the alternating helical inserts significantly enhance mass transfer by inducing secondary flow, though this improvement comes at the cost of increased pressure drop compared to straight tubes. These findings suggest that the device is a highly effective geometry for applications requiring efficient diffusion with minimal physiological damage, such as in blood oxygenators or desalination membranes.

Key Learnings

• Significant Mass Transfer Enhancement: The Kenics static mixer, which uses alternating right- and left-hand helical inserts, produces a substantial increase in mass transfer from the tube walls compared to a standard straight circular tube.
• Unique Axial Profile: Unlike a conventional tube where mass transfer decays exponentially as the boundary layer thickens, the Kenics mixer exhibits a sharp increase in the Sherwood number over the first few elements, reaching a peak at the second or third element before gradually declining.
• Influence of Reynolds Number: Under laminar conditions, the level of mass transfer enhancement is highly dependent on the Reynolds number, whereas in a standard tube, this influence is usually normalized by the downstream location.
• Flow Resistance Penalty: The improvement in mass transfer is accompanied by an increase in flow resistance (pressure drop). Generally, the proportional increase in flow resistance is greater than the corresponding gain in mass transfer.
• Correlation Development: The study suggests that the Seider-Tate equation can be modified with a linear coefficient based on the number of helical elements (N) to accurately predict the mean Sherwood number: Shₘ = 1.86(1 + 0.322N) (...).
• Medical and Industrial Utility: Due to its ability to induce secondary flow and circulate fluid from the core to the walls, the mixer is identified as an ideal geometry for blood oxygenators and desalination membranes, where minimizing diffusion resistance is critical.