In attempting to model batch granulation processes, we have found that a model based on aggregation terms alone does not suffice. Microstructural characterization of granules shows that both the particle size and the porosity evolve during the process, resulting in time-dependent aggregation and fragmentation behavior. In order to accurately model batch granulation processes, therefore, a multidimensional population balance on at least two granule properties, such as size and porosity, accounting for both aggregation and fragmentation, will need to be framed.