Motionless Mixers for Viscous Polymers

Authors:

S. J. Chen

Alan R. MacDonald

This document examines the application of motionless mixers in polymer processing, specifically highlighting how helical elements within a pipe facilitate radial mixing and flow division without moving parts. It demonstrates that these devices achieve ideal plug-flow characteristics and significantly improve heat transfer coefficients by up to 300% compared to empty pipes. By eliminating temperature and velocity gradients, the technology ensures uniform product quality and precise control over additives in highly viscous systems.

Key Learnings

• Mechanical Simplicity: Motionless mixers operate without moving parts or external power, utilizing only the pressure drop of the flowing fluid to achieve high-efficiency mixing.
• Mixing Mechanisms: The technology relies on two simultaneous actions: flow division, which geometrically splits the stream into S = 2ⁿ strata, and radial mixing, which continuously rotates and inverts the fluid from the center to the pipe wall.
• Plug-Flow Characteristics: These mixers exhibit a high Peclet number (averaging 145), meaning they closely approximate ideal plug-flow; this ensures a narrow residence-time distribution and prevents polymer degradation.
• Thermal Homogenization: The continuous radial inversion eliminates transverse temperature gradients, which is critical for polymers, where a variance of even 10°C can significantly alter product quality and tensile strength.
• Enhanced Heat Transfer: The units function as efficient heat exchangers by reducing the wall-film effect, increasing the inside heat-transfer coefficient by 250% to 300% compared to empty pipes.
• Design Trade-offs: Effective system design requires balancing the number of mixer elements (typically 18 to 24 for uniform blending) against the resulting pressure drop and residence time.
• Industrial Benefits: Implementation leads to improved fiber dyeability, better sheet-thickness control, reduced waste, and lower capital investment compared to dynamic cooling equipment like secondary extruders.