Abstract
Heat and mass transfer to or from single active particles surrounded by inert (passive) particles in a fluidized bed has been investigated based on published correlations. Special emphasis is on the application of a proposal by Baskakov, further developed by Palchonok. This representation describes heat and mass transfer as a function of the size ratio of inert to active particles. Two limits have been chosen: the limit of small active particles, where the active and the inert particles are equal, and the limit of large active particles, where the influence of the size of the active particle has vanished. The presentation aims at finding a suitable relationship, describing the size ratio of inert to active particles on heat and mass transfer to/from particles in fluidized beds and to critically evaluate its usefulness. It seems that the agreement between available correlations is qualitative and only approximate estimations can be made. A generalized scheme for calculations is presented. The formulation is made for bubbling fluidization. A discussion is presented on its use in circulating fluidized bed applications for fuel conversion as well.
Highlights
Heat and mass transfer to or from active particles surrounded by inert particles in a fluidized bed has many applications, of which conversion of solid fuel is an important one
The Reynolds numbers can be converted into the Archimedes number, because the active particle is supposed to be in the particle phase most of the time subjected to minimum fluidization, or for larger particles, under the condition of maximum heat transfer
The formulation estimates the distribution of the heat or mass transfer between two limits, depending on the size of the inert particles in relation to the active particles by an empirical function f(di/da) =n, where n = 2/3 for heat transfer and n = 1 for mass transfer
Summary
Heat and mass transfer to or from active particles surrounded by inert (passive) particles in a fluidized bed has many applications, of which conversion of solid fuel (devolatilization, combustion or gasification) is an important one. This topic has been treated frequently in the literature, and many correlations for the determination of heat and mass transfer coefficients have been proposed, often starting from single-phase expressions. The procedure proposed by Baskakov et al [1] and subsequently elaborated by Palchonok [2,3], is just one of several alternatives, but it has not been sufficiently well documented and analysed. The only remark that should be done here is that, because of a misinterpretation, the work of Palchonok got an unfavourable evaluation by Di Natale et al the present topic area, fluidized beds of inactive bed particles containing a minor quantity of active particles whose size may differ from that of the inactive bed particles, the much-quoted work of Gunn [8] is not included in the present survey
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