Gas Fluidization Flow Regimes

Gas–solid fluidized beds can be operated in different hydrodynamic flow regimes ranging from fixed bed, delayed bubbling, bubbling, slugging, turbulent to fast fluidization, and pneumatic convey. It is important to differentiate these flow regimes because hydrodynamics, mixing, heat and mass transfer behaviour, and reactor performance all vary from regime to regime. This chapter discusses onset of bubbling and slugging fluidization and onset and termination of turbulent fluidization. The onset of fluidization occurs when the superficial gas velocity reaches the minimum fluidization velocity, where the particles in the bed become fully suspended by the fluid. As the gas velocity increases in small-diameter fluidized beds, bubble coalescence during their rise leads to the formation of larger bubbles. In bubbling fluidized beds, solids holdup inside gas bubbles tends to increase with increasing gas velocity. In gas–solid fluidized beds, pressure fluctuations are caused mainly by the eruption, formation, and splitting of gas bubbles.