(52g) DEM Simulation Scale-Up Study of Dry Impregnation Processes

In the manufacturing of heterogeneous catalysts, the impregnation of active metals onto a porous catalyst support is a crucial preparation step that may significantly affect the activity and selectivity of the resulting catalysts particles.  In a typical dry impregnation (pore filling or incipient wetness impregnation) process, metal solutions are sprayed over a particulate bed in a mixing vessel until the pore volume is reached.  Current scale-up practices lead to poor fluid distribution and a wider particle size distribution.  There are two issues to consider when scaling up the process: the balance between inertial and gravitational forces and the size and magnitude of the spray flow pattern as a function of system scale.

In this work, we use a Discrete Element Method (DEM) and work with two dimensionless numbers, Froude number (Fr) and Spray Flux number ( Ψ ), to characterize the scale up of the system.  Froude number measures the ratio of inertial force to gravitational force.  A good balance between the inertial and gravitational forces is important to obtain a given particle motion.  The Spray Flux number measures the density of spray droplet (or flux) in the spray zone.  The calculation of each dimensionless quantity converts specific operation parameters into a term value.

DEM simulations are performed in a simple cylindrical geometry whose width covers the entire spraying zone.  We vary the vessel size, the rotation speed and the spray flow rate and the location of the nozzles.  Our results show that for smaller Froude numbers the powder bed exhibits a flat surface profile, thus facilitating a better control of the size of the wetted area.  However if the Froude number is too low, the system does not show good mixing.  On the other hand, if the Fr number is too large the surface profile is two-angled and particle collision and attrition are more likely to occur.  We found an optimal value for Fr number that gives the best mixing profile and particle dispersion, and also displays a flat surface profile.  When the optimal Fr number is considered across various scales, an optimal Spray Flux number ( Ψ ) is also determined based on a given Fr number.  We observe that the location and area of the spraying zone can significantly affect the overall content uniformity.  Smaller Spray Flux numbers tend to give more homogeneous liquid distribution during and after the total liquid is depleted, while larger Spray Flux numbers result in greater inhomogeneities of the fluid content and poorer particle mixing in the powder bed.  Hence, a homogeneous liquid distribution with optimal particle mixing is obtained by minimizing the spray rate and Ψ for an optimal Fr number.  In larger scale blenders, multiple nozzles can be used and placed evenly along the axis to increase the spraying area and to keep the Spray Flux number low.