Effective Drying Zones and Nonlinear Dynamics in a Laboratory Spray Dryer

Spray drying is a widely used unit operation. However, there is not enough information regarding the design of spray dryer equipment. The aim of this research was the identification of spray drying zones using the concept of heat transfer units and to link this approach with results from CFD simulation in steady and nonsteady-state conditions. Experiments were carried out in a two-fluid nozzle laboratory co-current spray dryer. Three zones inside the chamber were found: first and second drying stage zones and a particle expansion stage. The earlier findings showed the highest transfer coefficients, which may suggest the presence of a highly turbulent flow. CFD analysis was performed to assess air and air-particle hydrodynamics, including nonlinear analysis and the effect of particles on the effective drying zones. Good agreement between assessed turbulence and transfer units was found. Drying zones had a high degree of air-particle recirculation, which could be characterized through nonlinear dynamics by evaluating Lyapunov coefficients and by the presence of attractors (related to fractal dimension of texture of reflected laser beam cropped images). While the transfer units approach is useful for construction of lumped models, CFD and experiments based on air-particle nonlinear trajectories give insight on understanding the turbulence that takes place inside the dryer. Both approaches may be useful and could complement each other for design purposes.