(311c) Impregnation of Metal Solutions in Catalytic Porous Particles and Axial Dispersion Studies in Rotating Drum Using Experiments and DEM Simulations

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 and to optimize the process variables to achieve the best mixing and fluid uniformity in the particle bed.

Impregnation experiments were conducted in a rotating drum of D=20cm and L=30cm. The vessel was placed on a roller and the rotation speed was set at 9 rpm. γ-Alumina spheres of d=4.9mm contained a surface area of 215 m²/g and a pore volume of 0.6 cc/g. Each experiment was conducted with 1.75kg of alumina spheres. Nickel nitrate hexahydrate was dissolved in aqueous media and used as the metal precursor. The spray rate was set at 3.5 L/hr. After the metal solution has been sprayed, mixing of impregnated particles was continued for additional 10 minutes. During the impregnation process, higher metal content per unit mass is observed in the center of the vessel than at the edges of the vessel. This inhomogeneity levels off with additional rotations. The dispersion of material in the axial direction is the most critical factor determining the final uniformity of metal content in the particle bed. The axial dispersion coefficients are also calculated experimentally for the cases of both metal transfer and water transfer in the rotating drum.

In addition we developed a novel Discrete Element Method (DEM) model for the transfer of metal solutions from the nozzle to the particles and between particles. When a droplet of metal solution comes into contact with a catalyst support particle, the droplet penetrates into the pores. The deposition of the metal ions takes place during the equilibration of the liquid phase inside the pores and solid surface of the pores. The adsorption mechanism can be modeled by a Langmuir isotherm, which depicts a relationship between the extent of adsorption of the surface (or the fraction of the adsorption sites occupied) and the molar concentration of the solution. Solving the Langmuir model coupled with the mass balance, the equilibrated absorbed metal and the concentration of metal in the liquid phase can be calculated. Simulation results were compared with experiments with identical setup. Results obtained from our model show good agreement in the metal content in particles along the axis of rotation with experiments, and therefore it was demonstrated that a novel metal transfer algorithm incorporated into DEM could be used to accurately model catalyst impregnation.