Mechanical dispersion of semi-solid binders in high-shear granulation

Granulation is an important industrial process used to produce many foods, medicines, consumer products, and industrial intermediate products. This thesis focuses on high shear wet granulation with the specific case study of detergent manufacture using a high shear pin mixer. The key rate process in detergent manufacturing was determined to be the mechanical dispersion of the semi-solid surfactant binder. The pin mixer and mechanical dispersion utilized experiments, population balance models, and discrete element method (DEM) models. The mechanical dispersion of the surfactant binder was studied using a lab scale 6 liter pin mixer. An experimental method was developed to isolate mechanical dispersion from the other rate processes of granulation. Experiments were conducted over a range of impeller speeds, mixing times, and surfactant injection temperatures. Two surfactants where used each with different yield stresses. The yield stresses of both surfactants were characterized using uniaxial compression tests and extrapolated to the impact speeds observed in the pin mixer. Using the yield stress to calculate the Stokes deformation number revealed that the breakage of surfactant would occur at all impact conditions in the pin mixer. The mechanical dispersion results demonstrated that the rate process could be modeled as a breakage process. The results determined that the key parameter governing the mechanical dispersion of paste was the number of revolutions of the impeller. This implies that impaction or sudden stress from the impeller is the mechanism that causes nuclei breakage. The results of the mechanical dispersion experiments were then used to develop a mechanistic semi-empirical model. Because the results indicated that breakage should occur for every impact with the impeller, the model was based on particle impact efficiency between the impeller and nuclei. The impact efficiency was described in a way similar to particle gas filtration where the Stokes number is the characteristic dimensionless group. The population balance model was breakage only and was able to accurately predict the full size distributions of the surfactant nuclei. The results showed that the model was able to accurately account for the effect of tip speed and number of revolutions. This was found by fitting the simulation to a single impeller speed and then predicting the size distributions by varying only the velocity input. Finally, a DEM unit shear cell was developed to understand the transmission of stress from a bulk material to a single large particle of interest similar to surfactant nuclei. The simulation examined the effect of both shear rate, placement of the large particle, and the material properties. The results determined that the material properties used in the simulation had a much greater effect on the shear profile and stress in the shear cell than the effect of the macroscopic shear rate. Using the von Mises yield criteria, the results demonstrated that the shear cell transmitted more stress to the large particle than the yield stress characterized experimentally from the surfactant. The results indicate that the surfactant should break in shear within the pin mixer. Mechanical dispersion has been successfully modeled for the case of detergent granulation in the pin mixer. The combined results demonstrate that mechanical dispersion of surfactant can be modeled as a breakage process. The number of impeller orations and the Stokes number are key parameters to accurately describe and model the simulation. The surfactant should break apart due to both impact and shear within the granulator