High shear wet granulation (HSWG) is a globally important process that combines fine powders with a liquid binder to produce granules. The product granules have improved properties over fine powders including ease of handling, flowability, and dust reduction. An important example of HSWG is granulation of dry laundry detergent. Unlike many granulation processes that use a liquid binder, detergent granulation uses a semi-solid surfactant. The semi-solid surfactant has a non-zero yield stress that prevents it from penetrating into the powder bed by capillary action. The infinite drop penetration time forces the granulation of detergent to operate in the mechanical dispersion regime of the nucleation and wetting rate process.
In this study, two surfactants with different rheology are examined using regime separated granulation experiments. The regime separated experiments isolate the mechanical dispersion of the surfactants from the other rate process. The study examines the effects of impeller RPM and the temperature dependent rheology of the surfactants on the particle size distribution of the surfactant. It is demonstrated that the mechanical dispersion of surfactants can be modeled as a breakage process with the surfactant size distribution narrowing and mean size decreasing with increased mixing time. The experimental results are modeled using a 1D breakage only population balance model. The breakage kernel includes inputs from granulator conditions and material properties. The final result is a predictive model of mechanical dispersion that can be implemented into multi-dimensional models of the entire granulation process.