High shear granulation : a study of blade-granule bed interactions

• Main Author: Chan, Ei Leen
• Other Authors: Salman, A. D. ; Hounslow, M. J.
• Published: University of Sheffield 2012
• Subjects: 531
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.564175

Wet granulation, a common unit operation in multiple industries, involves the production of "granules" which are assemblies of primary particles held together by interparticle bonds. Depending on the application, granules with the required attributes such as enhanced strength, flowability, dissolution properties or uniform composition, can be manufactured. It is widely accepted in granulation research that the wet granulation process is comprised of several competing rate processes that dictate the granule growth behaviour and ultimately the granule attributes. In the micro-scale models developed for these rate processes (granule coalescence, consolidation and breakage) and further mapping studies to link formulation, processing and equipment variables to the granule growth behaviour, where the most significant of such work being the "Granulation growth regime map" (Iveson and Litster, 1998b; Iveson et al., 2001b), a fundamental parameter is the external stress exerted on the granules during granulation. The external stress is exerted by the main agitator in the granulator and subsequently transmitted via inter-granule or granule-wall collisions in the system. This thesis studies and characterises the external stress in a high shear granulator, more specifically the impeller blade-granule bed stress. The reserach was divided into the following main parts: A novel, custom-built telemetric impeller stress sensor in the studied granulator was first developed for direct measurements of the instantaneous blade-bed stress. With this system, the steady-state blade-bed stresses were studied for a range of parameters including bed load, granule sieve size and granule/particle density for dry beds and liquid addition for wet beds. The bed surface velocity, measured using high speed recording and analysed with Particle Imaging Velocimetry (PIV), was used to represent the characteristic velocity of the dynamic bed. A correction factor was applied to the theoretical blade-bed stress equation derived based on the imparted inertial stress on continuous bed, which accounted for the increasing bed 'fluidisation' with increasing impeller speed. This enabled much improved predictions of the time-averaged blade-bed stress for the studied parameters, especially at high impeller speeds. The blade-bed stress behaviour during the granulation process was also studied while looking at the evolution of granule attributes. Further characterisation of the steady-state blade-bed stress was carried out by simulating the dynamic dry particle beds in the high shear granulator using the Discrete Element Method (DEM), a widely used simulation method for granular systems. Following the validation of the DEM simulation with the experiments, additional impeller speeds, particle/particle bed properties, impeller geometries and granulator scales were studied from the simulations. A modified correction factor was also applied in the blade-bed stress equation to account for different granulator scales and blade widths. Additionally for the bed characteristic velocity, it was also shown that the bed surface velocity is not the dominant factor for the stress over-prediction from theory with increasing impeller speeds, i.e. the increasing bed 'fluidisation' is the dominant factor. Finally, the temporal values of the steady-state blade-bed stress, bed surface velocity and bed height were studied in terms of the variability/fluctuations, for the different parameters/conditions studied in the experiments and simulations as previously mentioned. More importantly, the results were also related to the identified flow regimes of the granule/particle bed when the impeller speed or Froude number was varied.