Dry impregnation of active metals onto a porous catalyst support is an important step in the preparation of 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.
DEM simulations were conducted in a full length cylinder vessel of 30cm long and 20cm in diameter to study the effect of several parameters: rotation speed, particle size and shape. Identical experiments were done under the same conditions: 2 rotation speeds (9 rpm and 25 rpm) and spray rate of 1 L/hr. Axial water distribution is compared and good agreement is found between experiments and simulations. High water content is observed in the center due to insufficient mixing. Using DEM simulations we predict a more homogeneous axial water distribution at higher rotation speeds (50 rpm). The calculation of axial dispersion coefficient shows that particle dispersion is faster along the axis at higher rotation speeds, resulting a more uniform water distribution.
Two dimensionless numbers are used to characterize the scale up of the system: Froude number (Fr) and Spray Flux number (Ψ). The Froude number is defined as the ratio of inertial force to gravitational force. The calculation of Fr gives the correlation between rotation speed (ω) and vessel size (R). Spray Flux number measures the density of spray droplet in the spray zone. The location and area of spray zone can significantly affect the overall content uniformity. Results in the cylindrical vessel show that a homogeneous liquid distribution with optimal particle mixing is obtained by minimizing the spray rate and Ψ for an optimal Fr number. The rules are further examined in the double cone geometry, where three different rotation speeds (7rpm, 15rpm, and 25rpm) are compared with three different spray rates (1.5L/hr, 3L/hr, and 5L/hr). It is found that keeping Fr number at the optimal value and lower spray rates increase the content uniformity of the resulting particles.
Using the cylindrical vessel, the effect of particle size on the overall mixing and metal content uniformity was also studied. The optimal conditions previously ascertained (7rpm of rotation speed and 0.3 L/hr of flow rate) were used as a baseline for comparing different particle sizes (4.7mm, 3mm, and 2.4mm). Results show that in the smaller particle system, mixing is slower, which leads to a poor content uniformity. Furthermore, mixing and content uniformity are studied for mixtures of different particle sizes. The binary system is composed of 50% of small particles (2.4mm) and 50% of large particles (4.7mm). Tertiary system has 1/3 of each of the three particle sizes. In the particle mixtures, small particles tend to pass through the interstices of the large particles when they are flowing, and heavy particles tend to roll down the sloped bed surface, resulting in particle segregation and poor content uniformity.
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