The Mixing of a High and a Low Viscosity Liquid in a Kenics Static Mixer

Authors:

Robert L. Powell, Ph.D.

Thomas P. Bor

This paper examines the experimental performance of the Kenics Static Mixer when blending liquids with extreme viscosity differences, identifying distinct design requirements for various fluid combinations. It demonstrates that mixing a low-viscosity additive into a high-viscosity melt requires an edge-sealed design and a specific number of elements to prevent fluid bypassing. Conversely, when dispersing high-viscosity materials into low-viscosity streams, the process is primarily governed by achieving a critical shear rate rather than the length of the mixer.

Key Learnings

• Design Specificity: The effectiveness of a static mixer is highly dependent on whether a low-viscosity liquid is being added to a high-viscosity stream or vice versa.
• Edge Seal Requirement: When mixing a low-viscosity additive into a high-viscosity melt, an "edge seal" design (where elements are bonded to the wall) is mandatory to prevent the low-viscosity fluid from bypassing the mixing elements.
• Critical Shear Rate: For the reverse case—adding high-viscosity materials to low-viscosity streams—mixing only occurs once a "critical shear rate" is exceeded; below this threshold, the additive simply winds through the mixer without dispersing.
• Variable Dependency: * Low-into-High: Primarily dependent on the number of elements (length) and volume fraction.
• High-into-Low: Primarily dependent on the shear rate, with the number of elements being less critical once the shear threshold is met.
• Solubility and Interfacial Limits: Even with optimal mechanical mixing, chemical properties like solubility can limit the percentage of additive that can be successfully incorporated into a polymer phase.
• Injection Precision: To prevent product fluctuations, additives must be injected as near to the center of the stream as possible, as static mixers excel at radial mixing but provide very little back-mixing (longitudinal blending).