Fluidization Regimes for Gas-Solid Suspension Flow

Most gas-solid systems experience a range of flow regimes as the gas velocity is increased. Several important gas-solid fluidization regimes for the chemical process industry are sketched in Fig 10.1. In dense fluidized beds regions of low solid density may be created. These gas pockets or voids are frequently referred to as bubbles. 

The main characteristics of the pertinent gas-solid flow regimes are [144, 

Smooth fluidization In fine particle A beds, a limited increase in gas flow rate above minimum fluidization can result in smooth, progressive expansion of the bed. Bubbles do not appear as soon as the minimum fluidization state is reached. There is a narrow range of velocities in which uniform expansion occurs and no bubbles are observed. Such beds are called a particulate fluidized bed, a homogeneously fluidized bed, or a smoothly fluidized bed. However, this regime does not exist in beds of larger particles of 13 30 B and D, in these cases bubbles do appear as soon as minimum fluidization is reached. 

The model predictions of the transition borders between the different gas-solid flow regimes shown in Fig 10.1 are still not reliable. The borders are generally sharp and fairly well correlated for the minimum fluidization (mf) and minimum bubbling (mb) transitions, whereas the transitions at higher gas superficial velocities are diffuse and poorly understood. 

The general flow regime diagram shown i Fig 10.2 illustrates the progression of changes in behavior of a bed of solids as the gas velocity is progressively increased. The letters A, B, C and D refer to the Geldart classification of solids. 

Elach of the fluidization regimes has characteristic solids concentration profiles. A plot of the profile showing the solids concentration versus the height above the distributor for the bubbling bed regime of fluidization takes a pronounced s-shape. 

With increasing gas velocity the s-shape profile becomes less pronounced and is 

Dense phase fluidization Gas fluidized beds are considered dense phase fluidized beds as long as there is a clearly defined upper limit or surface to the dense bed. The dense-phase fluidization regimes include the smooth fluidization, bubbling fluidization, slugging fluidization, and turbulent fluidization regimes. In a dense-phase fluidized bed the particle entrainment rate is low but increases with increasing gas velocity. 

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Fluidization Regimes for Gas-Solid Suspension Flow INCREASING U. g... 

A regime map of Fo versus the solid volume fraction, ap, for various gas-solid flows was presented by Hunt (1989), as shown in Fig. 4.3. Hunt (1989) suggested that except when Fo > 1 and ap > 0.1, use of the pseudocontinuum model is inappropriate. Thus, from Fig. 4.3, it can be seen that the pseudocontinuum model is applicable to packed beds, incipient fluidized beds, and granular flows, whereas it is not applicable to pneumatic transport flows, dilute suspensions, bubbling beds, and slugging fluidized beds [Glicksman and Decker, 1982 Hunt, 1989]. 

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