Effects of temperature and pressure on gas-solid fluidization

Abstract

The objective of this paper is to review experimental and theoretical studies of gas-solid fluidization at elevated temperatures and pressures. The survey begins with the low velocity end of operations in the region between minimum fluidization velocity and minimum bubbling velocity and shows how correlations established at ambient temperature and pressure for these two quantities may be used to calculate their values at super-ambient conditions. The application of purely hydrodynamic fluid-bed stability criteria to account for the transition from the non-bubbling to the bubbling state is described and compared with the expected effect of interparticle forces on this transition. The effects of temperature and pressure on the dynamics of gas bubbles in powders of Groups A, B and D are considered next and areas of uncertainty in current theories of bubble motion are highlighted. Correlations for jet penetration are then discussed and recommendations made as to the most reliable of these. Circulating fluidized beds (CFBs) operated at high velocity are then considered and it is shown that many of the observed effects in these systems at superambient conditions can be accounted for in terms of changes in the value of the terminal fall velocity, u t , of the bed particles. The effects of changes in u t on entrainment, elutriation and choking are also considered. The effect of increased pressure in enhancing bed-to-surface heat transfer coefficients in beds of Group A powders is shown to be due to the suppression of bubbling while in beds of Group B materials the enhancement is through an increase in the gas convective component of the transfer coefficient. The small amount of work carried out on heat transfer in CFB combustors is reviewed. Pressure effects on the combustion of char in bubbling beds are considered in terms of an established two-phase theory model and it is concluded that the increased rate of solids burn-out at high presures is due to an increase in the value of the local Sherwood number thereby increasing the rate of mass transfer of oxygen to the surface of the burning particle. The important question of sintering leading to defluidization at elevated temperatures is then examined and attention drawn to the current lack of broadly based mechanistic models to account for and predict the phenomenon. The state of the art in the area of scaling relationships is reviewed and it is shown that while the scaling laws for bubbling beds are by now reasonably well established the same is not so for CFBs, indicating a major area for further work.