(65d) DEM Simulation Studies on the Effect of Particle Size and Morphology on Mixing and Fluid Content Uniformity in a Double Cone Impregnator

Impregnation of active metals onto a porous support is an important step in the preparation of heterogeneous catalysts. In a typical impregnation process, metal solutions are sprayed over a particulate bed in a mixing vessel until the pore volume is reached. The inter-particle variability of the impregnated liquids inside the particles and metal content may significantly affect the activity and selectivity of the resulting catalyst. Current scale-up practices lead to poor fluid distribution and inhomogeneity in metal content. The aim of this work is to understand the dynamic behavior of the particles under the spray nozzle, which is essential for a desired content uniformity in the impregnation process.

Discrete Element Method (DEM) simulations coupled with an algorithm for the transfer of fluid to and between particles was used to investigate the impregnation process. The fluid transfer algorithm has been developed in our previous work [Powder Technology, 221, 57-69, 2012], and has been validated by geometrically equivalent experiments. In this work, using this DEM model, the effects of rotational speed, particle size and particle morphology were explored in order to achieve the best mixing and content uniformity in the particle bed. Mixing analysis and liquid distributions were used to investigate the propagation of the fluid throughout the particle bed. The mixing in the axial direction was found to be a function of the number of revolutions, and was independent on the rotational speed of the vessel. The content uniformity was characterized by the relative standard deviation of the fluid content from all the particles in the impregnated particle bed and was found to be better at lower rotational speeds. We also found that fluid content uniformity depends strongly on the rotational speed of the vessel, contrary to what is found in non-impregnated systems.

Since catalyst support particles come in a wide variety of sizes and shapes, it is with great interest that we investigate the behavior of different catalyst particles during the impregnation process. Both spherical and cylindrical particles were considered to determine the effect of particle size and particle morphology on the impregnated particle bed. Spherical particles were examined at various diameters, ranging from 2.5mm to 10mm; cylindrical particles were selected to examine various particle sizes and aspect ratios. The results show that the mixing time for cylinders is always shorter than that for spheres. This is consistent with work by Guo and Wassgren et al [Journal of Fluid Mechanics, 713, 1-26, 2012] where they found that cylinders have smaller fluctuating velocities and larger particle collision frequency, so they take less time to mix. In addition the mixing time in the axial direction for both spheres and cylinders followed an exponential function of the surface area to volume ratio (S/V) of the particle. Likewise the impregnation times as a function of S/V in the entire particle bed has the same trends than for the axial mixing times as a function of S/V. We found a power law correlation between axial mixing and fluid content uniformity suggesting that axial mixing in a double cone vessel is a dominating factor in achieving the fluid content uniformity in the particle bed.