On the rheological behaviour of batch crystallisations

During batch processing the formation of a solid phase is often accompanied by a significant reduction in the mobility of a reaction mass as a “soft” solid is formed. Often the fluid becomes stagnant with a mobile zone or “cavern” around the agitator. This phenomenon was investigated using two in situ probes: (i) a particle size probe which detected the onset of crystallisation and (ii) a viscosity probe which detected the onset and extent of rheological problems. The viscosity probe measurement appears to correlate with the yield-stress of the slurry. A mechanistic analysis of the phenomenon suggests a new dimensionless number: the Nienow number (Ni), the ratio of turbulent stresses to the yield stress of a fluid. Complete mobility is generally achieved for Ni > 30. These systems scale with tip speed if super-saturation is controlled.