Granule breakage during drying processes

The drying of wet granules often involves an unwanted and uncontrolled size reduction. Current FDA PAT guidance stresses importance of process control and understanding. The aim of this study is to determine and understand the breakage phenomena during drying processes in order to control these processes. High shear granulated lactose granules with water as binding liquid were dried during variable periods. Subsequently the (partially) dried granules were exposed to agitation by the impeller and chopper in the granulator. Granule characterization revealed that the change in granule size of (partially) dried granules is dependent on water content and follows a three phase system characterized by a growth, plateau and breakage phase. The derived yield stress of the granules is a function of velocity. From this it is concluded that in the plateau phase above minimum water content, stress behavior of granules can be described with Rumpfs’ dynamic granule strength, whereas below minimum water content (breakage phase) granule strength is determined by the solid bridges. The extent and velocity of stress and water content of the granules during the process determine the size reduction phenomena.