CFD simulation of a pharmaceutical bubbling bed drying process at three different scales

• CFD approach is capable of simulating drying bubbling fluidized bed processes.
• Calculated solid moisture agreed well with different scale experimental data.
• The measurement of the critical moisture content of particles is essential.
• Dimensionless similarity linked with CFD is a reasonable approach for scale up.

Drying is one of the major unit operations in the manufacturing of solid pharmaceuticals. In many pharmaceutical processes, optimum design of the drying process will significantly enhance the rate and reliability of the production and, in turn, decrease the cost of the pharmaceutical products. In this study, gas–solid flow patterns, mixing, and drying of pharmaceutical particles for three different scale bubbling fluidized bed dryers were simulated using ANSYS/FLUENT Computational Fluid Dynamics (CFD) code. The simulations are based on the two-fluid granular model (Eulerian–Eulerian approach). The mathematical model that we used in this study is composed of the continuity, momentum, energy, and species transfer equations to simulate the flow pattern and heat and mass transfer processes in the pharmaceutical drying processes based on the bubbling fluidized beds.To extend FLUENT code capability to account for moisture transport and to simulate the drying process, needed user-defined scalar transport equations (UDS) were added to the code. The results of the CFD simulation were compared with the experimental data obtained at the laboratory scale (Duquesne University experiments) to validate and refine our CFD model. Then, the modified model was used to simulate the drying of the same material in Abbott Laboratories kilo- and 10-kilo-scale units. Our simulation results for solid particles drying as a function of dimensionless time showed that our CFD model along with the same dimensionless group can be used as a tool to scale up the drying process from experimental-scale to both kilo-scale and 10-kilo-scale fluidized bed dryers. In addition, the numerical simulation results compared well with the experimental data (performed by Duquesne University and Abbott Laboratories) on moisture removal rate and outlet gas temperature. This also could provide a very good demonstration of the CFD capability as a tool to be used in the design and scale-up of the drying processes.

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