Quantitative Analysis of Powder-Binder Agglomeration

Publication Reference: 
Author Last Name: 
J.D. Litster, K.P. Hapgood, S. Iveson and N. Page
Report Type: 
ARR - Annual Report
Research Area: 
Particle Formation
Publication Year: 
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This report summarises progress in IFPRl project 37 in 1999/2000. Karen Hapgood has completed her PhD (November, 2000) and much of this report summarises the significant results of her research on wetting and nucleation. Some very interesting in progress results on the dynamic mechanical properties of wet granules are also included.

A new model for predicting drop penetration time from powder and liquid binder properties is presented. This model takes account of the presence of macrovoids in loosely packed powder beds and introduces the concept of an effective porosity and effective pore size seen by liquid drops penetrating into the bed by capillary action. The effective pore size is smaller than the Kozeny model capillary size by 2 to 4 times for lactose powders and an order of magnitude for very fine zinc oxide and titanium dioxide powders. Model predicted penetration times are compared with experimental data for a wide range of binder and powder properties. Predictions are within an order of magnitude for all powders with good agreement for lactose and glass ballotini. This is much better than the existing literature models.

A simple model to predict the fraction of agglomerates formed in the spray zone as a dictionof dimensionless spray flux is developed using spatial statistics:

The equation is in very good agreement with both Monte Carlo simulations of drop coverage on a powder surface and experimental nucleation experiments.

The conceptual nucleation regime presented in report 37-02 is extended and compared with results From granulation experiments in 1 litre and 25 litre laboratory mixer granulators. Experiments confirm that the narrowest granule size distributions are produced in the drop controlled regime where there is both low dimensionless spray flux and short drop penetration time. The implications for granulator design and scale up are discussed.

The first set of results from detailed measurement of the dynamic mechanical properties of wet mass pellets are presented. Experiments with glass ballotini powders and a wide range of liquid binders confirm that peak flow stress is a strong function strain rate. All results can be collapsed onto a single correlation between dimensionless stress and capillary number. Similar results are shown for crushed silica powders. This work is the first step to develop a generalised correlation that relates wet mass constitutive properties to the formulation properties.

The main research goals for years 4 to 6 of the project are briefly discussed.