Experimental study of droplet vaporization in a fluidized bed

Abstract

luidized bed technology presents a wide range of applications in processes involving powders. Solid particles are suspended by the F fluidization gas and move freely throughout the bed. This ensures a good mixing of the particles, enables a homogeneous temperature and enhances heat and mass transfer ratios. Moreover, good mixing represents a great advantage when injecting a liquid into the bed since it ensures a good distribution of the droplets. Industrial applications where a liquid is sprayed into a fluidized bed include drying, agglomeration, coating as well as coking or catalytic cracking. The type of contact between the particles and the droplets will determine the quality of the process. Particles are usually heated in order to provide the energy needed for the vaporization of the liquid. Depending on the application, vaporization is expected to be very fast (in the range of a few milliseconds) or to be slow enough (several minutes) to allow the formation of liquid bridges that solidify between the particles.Agglomeration processes are based on the collision and coalescence of wet particles. Models have been developed to estimate the chances of agglomeration as wet particles collide (The relative strength of kinetic and viscous forces is expressed as a function of the Stokes number. For Stokes numbers greater than 1 (kinetic energy dominating), the coated particles will simply rebound without coalescence. On the other hand, Stokes numbers lower than 1 indicate that the colliding particles will coalesce. The critical value SP has to be established experimentally. It usually changes with time as particle size distribution, collision velocity or the liquid properties change.This model, already complex because of the time-evolving properties, still neglects an important parameter. As fluidized particles circulate in the bed, collisions between particles can cause the breakage of the granule. The same approach as for coalescence can be used to take the breakage into account (Tardos et al., 1997). To evaluate the chances of breakage of the wet agglomerate, a new Stokes number is introduced that compares kinetic forces to the energy needed to break the granule. However, the application is limited because of the complexity to estimate the strength of the wet granule or to extend the model to the case of particles where one of them is significantly larger than the other one.'Author to whom correspondence may be addressed. E-mail address: cbriens@The injection of liquid in a hot fluidized bed is used in many industrial processes. The larger droplets are likely to engulf particles and create agglomerates that should be avoided when a fast vaporization is expected. A new model is proposed to estimate the chances of agglomeration based on a critical bed temperature below which agglomerates form. This critical temperature for safe operating conditions is based on heat transfer between the particles and the liquid and takes the porosity of the solid into account The model could be applied to pure and multi-component mixt...