Quantitative Analysis of Powder-Binder Agglomeration

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Author Last Name: 
J.D. Litster, K.P. Hapgood, L. Liu, S. Iveson and N. Page
Report Type: 
Research Area: 
Particle Formation
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This report summarises progress in IFPRI project 37 in 1998/99, Significant progress has been made in studying (1) wetting and nucleation, and (2) consolidation and growth.

Ex-granulator spray flux studies are complete. This study focuses on the nucleation zone, which is the area where the liquid binder and powder surface come into contact and form the initial nuclei. An equipment independent parameter, dimensionless spray flux ‘I‘, is defined to characterise the most important process parameters in the nucleation process: solution flowrate, powder flux, and binder drop size. Experiments with red dye and image analysis demonstrate that changes in dimensionless spray flux correlate with a measurable difference in powder surface coverage, Nucleation experiments show that spray flux controls the size and shape of the nuclei size distribution. At low ‘I”, the system operates in the drop controlled regime, where one drop forms one nucleus and the nuclei size distribution is narrow. At higher ‘I”, the powder surface is cakes creating a broader size distribution. For controlled nucleation with the narrowest possible size distribution, it is recommended that the dimensionless spray flux be less than 0.1 to be in the drop- controlled regime.

Drop penetration studies are ongoing. A relatively simple drop penetration model accounts well for the effect of liquid properties on penetration time. However, the effect of powder bed structure is more complex and harder to predict quantitatively. This is the subject of ongoing work.

A new coalescence criteria for deformable granules has been developed. Granules may coalesce without plastic deformation (type I coalescence) or with plastic deformation (type II coalescence). The model helps explain a number of different observed granulation behaviours, identifies theoretically the key controlling groups for granule growth and gives a theoretical underpinning to the granulation regime map. The theoretical locations of the granule growth regime map boundaries proposed by Iveson and Litster (1998a) were also analysed leading to a better-quantified and improved regime map. Drum and mixer granulation data for a range of materials was used to validate the regime map.

The drum granulation data fitted the regime map well. However, the data from the large-scale mixers had Stokes deformation numbers Stdef which were several orders of magnitude too high. Hence the map is a useful tool for comparing the granulation behaviour of different materials in the same device. However, until we have a better understanding of the flow patterns and impact velocities in granulators, it cannot be used to compare different types of equipment, More work is required to better characterise the flow patterns and impact velocities in mixer type granulators.

Two new techniques have been developed to characterise the mechanics of wet powders and granules. These techniques will be used to characterise a wide variety of formulations in year 3 of the project.

The report outlines the research areas for 1999/2000 as well as discussing longer-term research goals.