Heat and mass transfer

Abstract

Introduction A great number of processes carried out in spouted beds require the application of different modes of heat and/or mass transfer. We may distinguish among the following modes: Heat transfer, mass transfer, simultaneous heat and mass transfer – between fluid and particles, Heat transfer between wall and bed, and Heat transfer between submerged object and bed. For each mode, transfer mechanisms are examined; then experimental findings and, in some cases, theoretical studies are discussed. Between fluid and particles Transfer mechanisms and models Quite often, the basic assumption for analysis of heat or mass exchanged between fluid and particles is that heat is transferred to the particles under conditions of external control, neglecting heat transmission within the particles. For heat transfer in the absence of mass transfer, this is justified when the particle heat transfer Biot number is sufficiently small (e.g., 2 . For simultaneous heat and mass transfer, such as when the particles are well wetted at the surface, the average temperature at the particle surfaces is substantially uniform, and external control again prevails. Assuming plug flow conditions through the bed, the axial fluid temperature distribution can be described by a dimensionless function. Because the spouted bed consists of two distinct regions, with the average gas velocity in the spout being one or two orders of magnitude greater than that in the annulus, the decline of gas temperature in these two zones is quite different; it is slight in the spout and considerable in the annular zone.