Granulation rate processes

The Washburn test requires some specialized testing equipment to perform, and an alternate test is the drop penetration time test which is more directly related to the wetting of spray drops into a granulating powder (Hapgood et al., 2003). A drop of known volume is gently placed onto a small powder bed with porosity Sf, and the time taken for the drop to completely sink into the powder bed is measured. The drop penetration time, tp, is given by  

The Washburn test and drop penetration time test are closely related, but the latter is simpler to perform as a screening and investigation test when developing or troubleshooting a granulation process. 

If either criterion is not met, powder mixing and shear characteristics will dominate this is the mechanical dispersion regime. Viscous or poorly wetting binders are slow to flow through the powder pores and form nuclei. Drop coalescence on the powder surface (also known as pooling ) may occur and create a very broad nuclei size distribution. In the mechanical dispersion regime, the liquid binder can only be dispersed by powder shear and agitation. 

Consolidation is the term used to describe the increase in granule density caused by closer packing of primary particles as liquid is squeezed out as a result of collisions. Consolidation can only occur whilst the binder is still liquid. Consolidation determines the porosity and density of the final granules. Factors influencing the rate and degree of consolidation include particle size, size 

This Stokes number is different from that used in cyclone scale-up in Chapter 9. The cyclone scale-up Stokes number Stkgo incorporates the dimensionless ratio of particle size to cyclone diameter, i.e.  

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Granulation Rate Processes and Effect of Operating Variables. 20-75... 

They can process plastic, sticky materials and can spread viscous binders. That is, they can operate in the mechanical dispersion regime of wetting and the deformable regime of growth (see Granulation Rate Processes ). 

Scale-up and Operation As discussed above, scale-up of high-shear mixers is difficult due to complex powder flow patterns, wide variations in shear rates among mixers, and competition among granulation rate processes. Ideally, the following should be preserved with scale-up. 

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