Technology Brief

This bulletin provides a step-by-step look at a typical multi-shaft mixer application which demonstrates the system's processing flexibility in terms of mixing, heating, cooling and deaerating batch material at different viscosity stages.

Mixer flexibility in terms of viscosity range

Multi-shaft mixers comprised of two or more independently-driven agitators working in tandem are well-known for their wide ranging uses in multi-viscosity mixing applications. In such processes, the batch material undergoes many changes in viscosity, perhaps starting out as a thin liquid that gradually becomes more viscous with the addition of solid raw materials and/or lowering of temperature. Some applications will reach a viscosity peak mid-processing and finish as a low-viscosity fluid. In all these stages, the mixer must be capable of delivering good turnover and effective heat transfer, impart the required shear input and quickly deaerate batch product, as necessary.

Most multi-shaft mixer configurations consist of a low speed anchor agitator which compliments one or two high speed devices, such as a sawtooth disc-style disperser blade or a rotor/stator assembly. The disperser and rotor/stator are particularly effective at fast powder wet-out, dispersion and particle or droplet size reduction. However, both designs are inherently limited in terms of viscosity range. On its own, a disperser blade will produce acceptable flow patterns for products up to 50,000 cP; the rotor/stator up to around 10,000 cP. The anchor agitator therefore performs the crucial role of promoting bulk flow, encouraging viscous product to move towards the high speed devices and constantly removing batch material from the vessel walls to optimize heat transfer. In combination, all three agitators can handle formulations as high as a million centipoise.