In-line temperature measurement to improve the understanding of the wetting phase in twin-screw wet granulation and its use in process development

Wetting is the initial stage of wet granulation processes during which the first contact between the powder and the liquid occurs. Wetting is a critical step to allow granule growth and consolidation, but also to ensure uniform active pharmaceutical ingredient (API) distribution over all granule size fractions. A physical understanding of the wetting stage is therefore crucial to design a robust granulation process. In twin-screw granulation, wetting is physically separated from granule consolidation, growth, breakage and attrition. The present study used this particularity to investigate the wetting step in such a way that the fundamental mechanisms governing the wetting can be linked and understood. A modified granulator barrel was used allowing the collection of granules immediately after the wetting. A low drug-loaded pharmaceutical formulation containing a poorly soluble and poorly wettable API was used for this investigation. Granules obtained after the wetting zone were analysed for granule size distribution, API distribution over the different size fractions and granule temperature. It was found that “wetting efficiency” (i.e., fraction of powder being nucleated during the wetting stage) could be predicted using an energy balance based on in-line measurement of the granule temperature. Wetting efficiency could moreover be linked to final granule quality attributes (i.e., granule size distribution) at the outlet of the granulator. It was further demonstrated that granule growth and consolidation could only be achieved when complete wetting was achieved in the wetting zone of the granulator. This study suggested a methodology based on in-line temperature measurements to quickly determine wetting efficiency. The described methodology could therefore be used as a tool to gain more fundamental understanding of the wetting stage during twin-screw granulation as well as to define suitable formulation and process ranges for further granulation process development