(84b) Parameter Characterization for Drying of Porous Spherical Particles

Provided the porous particle to be dried can be described by the shell and shrinking core model and subject to several other assumptions, an easy means of characterizing the drying process utilizing an Excel spreadsheet is proposed. Typically in the drying process four major parameters are unknown, two related to the process, i.e., the heat and mass transfer coefficients, h and kG, and two which incorporate porous particle properties, i.e., NuIN and ShIN. It is shown how these 4 parameters may be determined experimentally from two or three drying runs. Once these parameters are known, predictions may be made for the effects of drying gas temperature and solvent partial pressure in the drying gas and also, with proper adjustments, particle diameter. Knowledge of h and kG, permits calculation of shell thermal conductivity and internal diffusion coefficient from the values determined for NuIN, and ShIN. If the particle characteristics are well known, then heat and mass transfer performance of the drying equipment and its operating conditions (e.g., fluid bed velocity) may be readily evaluated or vice versa. For NuIN/ShIN > 1.0, the mass mobility is relatively greater than thermal transport in the shell and it is shown that this condition leads to unexpected behavior in which the temperature at the wet core decreases with time although the surface temperature of the particle increases with time. The approach outlined can be tailored to either batch or continuous drying. If the ratio between heat and mass transfer coefficient, h/kG, can be assumed constant then the number of parameters which characterize the process reduces to only three.