(94e) Scale up Studies of Dry Catalyst Impregnation for Improved Content Uniformity Using Simulations and Experiments

Dry catalyst impregnation of active metals onto a porous catalyst support is an important step in the preparation of heterogeneous catalyst. In a typical dry 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 desired content uniformity, and to develop a scale-up model for the dry impregnation process.

Four dimensionless numbers were considered to characterize the scale up of the system. All these quantities were derived taking into account that the system should have geometric and dynamic similarity. They were kept constant for different scales.

Π₁=L/D, Π₂=A_wetted/L², Π₃=Fr=Ω²D/2g, and Π₄=C_Q=Q/(Ω×D³), where L is the length of the cylinder, D is the diameter of the cylinder, A_wetted is the wetted area of the cylinder, Fr is the Froude Number, C_Q is the Flow Rate Number, Ω is the angular velocity (or rotational speed), and Q is the flow rate.

For the scale up study of our system, water impregnation was performed in 2 different cylindrical sizes: 1) small cylinder (D₁=19.3cm and L₁=29cm) and 2) large cylinder (D₂=40cm and L₂=60cm). Both systems have two spray nozzles located close to each other, generating a continuous wetted area at the center of the vessel. The rotational speed and spray rate were determined by keeping Fr and C_Q numbers constant for the 2 scales. The water content of the particles was compared for different times and locations. Relative standard derivation (RSD) is calculated from the axial water content, and is plotted as a function of the number of revolutions. It is shown that the two RSD curves collapse into one between the 2 different scales, indicating the dimensionless numbers are good scale-up parameters.

In addition, Discrete Element Method (DEM) simulations coupled with a novel algorithm allowing the transfer of metal solution to and between particles were used in combination with geometrically equivalent experiments to model dry catalyst impregnation. DEM simulations were performed for a cylindrical slice in three different sizes, varying from D=10cm, 20cm, and 30cm. Note that the slices are used in order to save computational time. Simulation results showed similar performance in all different size of the vessel and confirmed that the dimensionless numbers can be used to scale-up the impregnation process. We also performed water impregnation simulations in a cylindrical vessel with full length (L=29cm) and diameter (D=19.3cm), keeping a “one to one” size correspondence with experiments (i.e. we kept the exact geometrical dimensions and nozzle configuration as compared to the ones used in experiments) with the goal of validating the parameters chosen in the DEM simulations. These simulations are computationally very expensive due to the size of the vessel. Also notice that these simulations have axial dispersion as opposed to the cylindrical slice shapes. In addition, we explored the value of optimal Froude number. Rotational speeds were varied from 6rpm, 9rpm, 12rpm, and 15rpm. An optimal Fr value was found when the RSD is minimal, which gives the best content uniformity.