Contraction three-phase fluidized beds

The cocurrent gas-liquid-solid fluidized beds considered are those in which the liquid supports and completely wets the solid. The gas flow thus constitutes a perturbation of a liquid fluidized bed. Unlike a gas-solid or a liquid-solid fluidized bed, a gas-liquid-solid fluidized bed may either contract or expand when gas bubbles are introduced into the bottom of the bed. Considerable work has been done on deriving the criteria for the bed contraction and expansion. The most up-to-date work on this subject is by Epstein.28 He derived the criterion for initial contraction and expansion of three-phase fluidized beds. He suggested that if the quantity... 

As noted by the correlation of Kim et al.,58 any quantitative evaluation of the holdup characteristics of a three-phase fluidized bed must consider the phenomenon of bed contraction.137 The wakes of the bubbles rising in a three-phase fluidized bed consist of a particle-free liquid immediately below the bubble (commonly known as liquid wake ) and a lower region of particles and liquid which apparently moves with liquid (known as particulate region ). Particulate elutrition is caused by this particulate region, whereas bed contraction is caused by the liquid wakes. 

Epstein and co-workers (Bhatia and Epstein, 1974 Epstein, 1976 Epstein and Nicks, 1976 El-Temtamy and Epstein, 1978, 1979) have made very valuable contributions to the understanding of expansion/contraction characteristics of three-phase fluidized beds. Bhatia and Epstein (1974) proposed a wake model in which the solid contents of the liquid wakes behind the gas bubbles is negligible compared to the solid contents of the remaining... 

Prediction of Contraction/Expansion Behavior in Three-Phase Fluidized Beds... 

Joshi (1983) used a simpler form of this model for the prediction of contraction/expansion in three-phase fluidized beds. 

The phenomenon of expansion/contraction in three-phase fluidized beds was analyzed. A brief review of literature has been presented. Epstein and co-workers have made outstanding contributions in this area. An alternative method has been presented. A favorable comparison between the model predictions and the experimental observations was observed. 

On the basis of the generalized wake model of Bhatia and Epstein, " a criterion for the bed contraction was developed. In the generalized wake model, the three-phase fluidized bed is assumed fo consist of three regions, the gas bubble region, the wake region, and the liquid-solid fluidized region. Bed contraction will occur when the following criterion is satisfied, ij/ < 0 where... 

El Temtamy, S.A. Epstein, N. Contraction or expansion of three-phase fluidized beds containing fine/light solids. Can. J. Chem. Eng. 1979, 57 (4), 520-522. 

In gas-liquid-solid (three-phase) fluidized beds, solid particles are simultaneously contacted with both gas and liquid. The gas and liquid may flow cocurrently upward, or the liquid may descend, while the gas rises. The liquid usually forms the continuous phase in which the solid particles and gas bubbles are dispersed. The bubbles are larger when the particles are smaller, and bed contraction can occur when gas is introduced into a liquid-fluidized bed of fine particles. Higher pressures lead to smaller bubbles and increased gas hold-ups. 

Bubble dynamics and characteristics discussed above determine the hydrodynamic and heat and mass transfer behaviors in three-phase fluidization systems, which is important for better design and operation of three-phase fluidized beds. In this section, various hydrodynamic variables and transfer properties in three-phase systems are discussed. Specifically, areas discussed in the hydrodynamics section are minimum fluidization, bed contraction and moving packed bed phenomenon, flow regime transition, overall gas holdup and hydro-dynamic similarity, and bubble size distribution and the dominant role of larger bubbles. Later in this section, important topics covering transport phenomena will be discussed, which include heat and mass transfer and phase mixing. 

Epstein, N. and D. Nicks, "Contraction or Expansion of Three-Phase Fluidized Beds". Fluidization Technology, ed. D. Keairns 389-397 (Washington, Hemisphere Publishing Corporation 1976). 

Epstein, N, "Criterion for Initial Contraction or Expansion of Three-Phase Fluidized Bed" Can. J. Chem. Engng. 54 (1976) 259-263. 

Bed expansion and contraction are important phenomena in three-phase fluidization, since these affect the bed volume and the residence time(s) of all the phase(s). The phenomenon of bed contraction/expansion is also very closely linked to the problem of transition. The relationship between the expansion/contraction behavior and the transition will be clearly brought out later. 

For three-phase fluidization systems, two distinct phenomena pertaining to macroscopic hydrodynamic behavior—bed contraction and moving packed bed flow—are noted below. 

Jean RH, Fan LS. Letter to the editor Bed contraction criterion for three phase fluidization. Can J Chem Eng 65 351-352, 1987. 

Chiba T. Bed contraction of hquid-fluidised binary solid particles at complete mixing. In Yoshida K, Morooka S, eds. Proc Asian Conf on Fluidized Bed and Three-Phase Reactors. Tokyo Tokyo University, 1988, pp 385-392. 

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