A steady-state, three-dimensional, multiphase CFD modelling of a pilot-plant counter-current spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian-Lagrangian approach. The continuous phase turbulence is modelled using the differential Reynolds stress model. The drying kinetics is modelled using a single droplet drying model [1] which is incorporated into the CFD code using user-defined functions. Heat loss from the insulated tower wall to the surrounding is modelled by considering thermal resistances due to deposits on the inside surface, wall, insulation and outside convective film. For the particle-wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements.2
This work presents a novel procedure
to predict the airflow pattern
under different levels of deposition and Reynolds numbers for swirling-flow
industrial-scale spray-drying towers. It improves the accuracy in
the prediction of both the hydrodynamics and the effect of the deposition.
Initially, steady-state and transient simulations are compared, showing
that the model can be reduced to steady state for a certain mesh size.
The computational fluid dynamics (CFD) model is later calibrated using
the experimental swirl intensity values under different levels of
deposits that have reached a dynamic equilibrium. The model then is
validated for different Reynolds numbers of operation. Finally, the
validated model is applied to examine the vortex behavior and evaluate
the effect of the tower radius reduction. A limit of operation is
found for low Reynolds numbers, in terms of stability, and it is observed
that the momentum cannot be only modeled with the radius reduction.
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