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The Influence of porosity on droppowder impact and contact angle

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					      097 The Influence of porosity on drop/powder impact and contact
                           angle during penetration
                   – validity of CDA and DDA predictions.
                                       1            2            2                  1                 1
                 H. Charles-Williams • K. Flore • H. Feise • M.J.Hounslow • A.D.Salman
 1
     Department of Chemical and Process Engineering, University of Sheffield, Mappin Street, Sheffield, S1
                                  3JD, UK . , a.d.salman@sheffield.ac.uk
                 2
                   BASF Aktiengesellschaft, GCT/T - L540, 67056 Ludwigshafen, Germany

Granulation is used in industry as a size enlargement process where formulations can be premixed, and
stored without issues of in-homogeneity. The study of this process is usually broken down into three set of
rate processes; nucleation, consolidation and growth, and breakage. Nucleation is the first step in
granulation during which a liquid binder is brought into contact with the powder, it wets the powder, and
granule nuclei are formed.

Selection of the right powder-liquid combination is paramount. Where penetration times are longer the
process may move from being drop controlled to the undesirable mechanical dispersion regime leading to
a broad granule size distribution. The drop penetration time can vary over several orders of magnitude
and penetration behaviour on loosely packed porous beds is quite complex and highly dependent on the
microstructure of the bed (Hapgood et al., 2002). A combined drop penetration and nuclei growth model
would provide a more complete picture of drop penetration kinetics and nuclei morphology (Iveson et al.,
2001) and dramatically reduce the need for pilot scale work in determining the suitability of a new
formulation.

There are two limiting cases for the penetration of a droplet into a porous material, these are highlighted in
Figure 1. In the constant drawing area (τCDA) the contact area is constant ensuring a maximum number of
pores available for drainage, whereas, in the decreasing drawing area (τ DDA) case the contact angle of the
drop remains constant, and contact area decreases as the volume decreases. This is considered to be 9x
longer than CDA penetration (Hapgood et al., 2002, Marmur, 1988).




 Figure 1: Limiting cases of drop penetration on a porous surface, a) constant drawing area case, b) decreasing drawing
                                             area case (Hapgood et al., 2002).



There are a number of models currently used that predict the drop penetration times based on these
limiting cases. Recent modification to standard equations made by Hapgood (Hapgood et al., 2002) have
yielded estimates closer to the real times but are still based on these limiting cases and obtain an
accuracy of typically up to ± 1s.

In this work, the shape of a water drop impacting and subsequently penetrating into a powder bed of fine
grade lactose is studied in terms of drop apex height, spreading diameter and apparent contact angle to
determine the applicability of the CDA and DDA penetration based drop models.
Results presented indicate a transition from CDA to DDA type penetration in terms of contact area
variation at about 50% of the total penetration time. However, a fairly constant decrease in contact angle
during drop penetration is observed. It is proposed that a model that incorporates this transitional
behaviour will yield far more accurate predictions of penetration time.
                                             REFERENCES

HAPGOOD, K. P., LITSTER, J. D., BIGGS, S. R. & HOWES, T. (2002) Drop penetration into porous powder beds.
     Journal of Colloid and Interface Science, 253, 353-366.

IVESON, S. M., LITSTER, J. D., HAPGOOD, K. & ENNIS, B. J. (2001) Nucleation, growth and breakage
      phenomena in agitated wet granulation processes: a review. Powder Technology, 117, 3-39.

MARMUR, A. (1988) The Radial Capillary. Journal of Colloid and Interface Science, 124, 301-308.

				
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