Mass Transfer around Active Particles in Fluidized Beds

Abstract

Fluidized beds are extensively used in a number of gas-solid applications where significant heat and/or mass transfer rates are needed. The design and modelling of such processes requires the precise knowledge of the heat and mass transfer coefficients around immersed objects in the fluidized bed. Thus, it is not surprising that since the early spreading of the fluidized bed technology a considerable experimental and theoretical activity on this topic has been reported in the literature, mostly focused on the heat transfer coefficient. A more limited effort was dedicated to the estimation of the mass transfer coefficient, because of the inherent difficulty of measuring this quantity in the dense environment of a fluidized bed. Unfortunately, a fluidized bed is one of those cases where the analogy between heat and mass transfer does not hold, so that measured heat transfer coefficients cannot be used to estimate mass transfer rates under similar operating conditions. In fact, the bed particles represent an additional path to heat transfer around an immersed object, while they only result in a decrease of the available volume for gas mass transfer (except for the very particular case when the bed particles can adsorb one of the transferred components). Strictly speaking, an analogy exists between mass transfer and the gas-convective contribution to heat transfer in a fluidized bed. On the other hand, the particle-convective contribution to heat transfer (and also the radiative one, if relevant) has not an analogous mechanism in a mass transfer process. In this review paper we will focus our attention to the mass transfer coefficient around freely moving active particles in the dense phase of a fluidized bed. This case represents most situations of practical interest, whereas the case of a fixed object (with respect to a reference system bound to the reactor walls) is less frequently encountered in mass transfer problems, contrary to the heat transfer case. With active particle we mean a particle that is exchanging mass with the gas phase, because either a chemical reaction or a physical process (phase change) is taking place in or at the surface of the particle. Finally, we will mainly consider the case of mass transfer between the gas and one or few active particles dispersed in a fluidized bed of inert particles, as opposed to the case where the entire bed is made of active particles. This configuration is important for a number of processes like combustion and gasification of carbon particles, and most typically the inert particle size is smaller than the active particle size. In the next sections we will thoroughly review the experimental and theoretical work available in the literature on mass transfer in the dense phase of fluidized beds, showing the