Internal Mixing: A Practical Investigation of the Flow and Temperature Profiles during a Mixing Cycle

Abstract

Abstract From the results of mixing trials with a highly instrumented BR Banbury and biconical rotor rheometry of mixed batches, a detailed analysis of flow and mixing characteristics in the region of a rotor wing has been undertaken. An ‘angled spreader blade’ analogy of the rotor wing is proposed as being a viable basis for mathematical modelling. A one-dimensional flow analysis is used, in which power-law flow behavior and isothermal conditions are assumed. Dispersive mixing, which depends on the stress levels generated during mixing, is shown to occur throughout the entire mass of material swept in front of the rotor wing and not simply at the rotor tip. In addition, the stress levels depend more strongly on batch temperature than on rotor speed. High rotor speeds tend to lead to reduced stress levels as a result of the associated rapid rise in batch temperature, although choosing an appropriate fill factor can minimize temperature rise by promoting efficient heat transfer to the cooling water. During each rotor revolution, the rotor wing collects a mass of material from the reservoir between the rotors. This mass of material is then progressively reduced by leakage flow under the rotor tip and flow around the end of the wing, until the revolution is completed by the return of a residue to the reservoir. The flow around the end of the rotor is shown to be consistently greater than the leakage flow, although the ratio can be influenced by both fill factor and rotor speed. At high rotor speeds and low fill factors, it appears that material is retained in the regions of the side frames of the mixer and may give batch inhomogeneity through poor distribution mixing.