Residence time distribution of glass ballotini in isothermal swirling flows in a counter-current spray drying tower

Abstract

Abstract:The particle residence time in counter-current spray drying towers has a significant influence on the moisture content of the powder exiting the tower. Therefore, the reliability of predictions of residence time by numerical methods is highly desirable. A combined experimental and computational fluid dynamics investigation is reported for the prediction of the residence time distributions of glass beads with a narrow size range of 300-425 m in a counter-current tower with isothermal swirling flows of air. The particle-wall collision is taken into account using a rough-wall collision model. Overall, a reasonably good agreement is obtained between the measurements and predictions. Consideration of wall roughness results in greater axial dispersion of particles in the tower compared to a smooth wall assumption. The rough particle-wall collision is important for a reliable prediction of residence time distributions. In addition, analysis of the results infers that the clustering effect of particles on drag and particle-particle interactions are important and should be investigated in a future study.

The Use of Optimized Restitution Coefficients to Improve Residence Time Prediction in Computational Fluid Dynamics-Discrete Parcel Method Models for Counter-Current Spray Dryers

“…Figure depicts the expected flow structures in the chamber under the conditions studied: a central recirculation region forms at the bottom of the cylindrical chamber and an upward jet that extends to the exit. The axial velocity shows a minimum in the midpoint of the radius and increases toward the wall, whereas the tangential velocity shows a high intensity at the bottom and a Rankine vortex at the top with a decreasing swirl intensity as the flow moves through the chamber. ,, Bearing this structure in mind, two injection heights above the recirculation area radial positions at the center and in the midpoint of the radius are used to understand how the central jet would affect the initial dispersion and establish the validity of the assumption of a constant sedimentation velocity across the cylindrical chamber taken in 1D PFR models of this unit …”

“…A rigorous computational fluid dynamics (CFD) modeling platform to describe spray dryers should undergo a sequential validation. − One must ensure that a proper reproduction of an isothermal single-phase flow is available before incorporating the momentum transfer in multiphase isothermal systems . Then, one can describe heat and mass transfer including an equation of state, the energy equation, and single droplet drying models.…”

“…These shortcuts explained the inability of models to describe this operation accurately and underscored the need to conduct a rigorous validation of the general modeling platform. Following on the work of Francia et al , on single-phase fluid dynamics, the focus in experimentation moved onto studying particle dispersion and residence time . Our previous work provided a validated model for the isothermal single-phase flow .…”

“…To the best of the authors’ knowledge, only a recent study has characterized the dispersion independently from drying. Ali et al . studied the RTD of glass beads in a cold isothermal chamber.…”

Numerical Study on Spray Drying Process: Effect of Nonuniform Temperature Field and Interaction between Droplets on Evaporation Rates of Individual Droplets

“…[21][22][23][24][25] Recently, the CFD studies that considered the collision between droplets (or particles) and deposition on a spray dryer wall have increased, and the importance of the boundary condition between a particle and wall has been discussed. [26][27][28][29][30] Previous studies have primarily focused on a single droplet drying or the overall fluid and droplet drying behaviors inside a spray dryer. Normally, in the spray drying process, numerous sprayed droplets form a tear-shaped droplet aggregate (group), which is then dried in a non-uniform temperature field.…”