Fluidized beds flow regimes

Andres, M. B., Chen, Z., Grace, J. R., Elnashaie, S. S. E. H., Jim Lim, C., Rakib, M., et al. (2009). Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors a simulation study. Chemical Engineering Science, 64, 3598—3613. Ayturk, M. E., Kazantzis, N. K., Ma, Y. H. (2009). Modeling and performance assessment of Pd- and Pd/Au-based catalytic membrane reactors for hydrogen production. Energy Environmental Science, 2, 430—438. 

Mahecha-Botero, A., Chen, Z., Grace, J.R. et al. (2009) Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors A simulation study. Chemical Engineering Science, 64, 3598-3613. 

A standpipe may operate in two basic flow regimes depending on the relative velocity of the gas to the solids packed bed flow and fluidized bed flow. 

Standpipes generally operate in three basic flow regimes packed bed flow, fluidized bed flow, and a dilute-phase flow called streaming flow. 

As fluidized beds are scaled up from bench scale to commercial plant size the hydrodynamic behavior of the bed changes, resulting, in many cases, in a loss of performance. Although there have been some studies of the influence of bed diameter on overall performance as well as detailed behavior such as solids mixing and bubble characteristics, generalized rules to guide scale-up are not available. The influence of bed diameter on performance will differ for different flow regimes of fluidization. 

Gas jets in fluidized beds were reviewed by Massimilla (1985). A more recent review is by Roach (1993) who also developed models to differentiate three jet flow regimes jetting, bubbling and the transition. However, most of the data were from jets smaller than 25 mm. The discussion here will emphasize primarily large jets, up to 0.4 m in diameter, and operation at high temperatures and high pressures. The gas jets can also carry solids and are referred to as gas-solid two-phase jets in this discussion. 

At gas velocities higher than those used for BFBs we successively enter the turbulent (TB), fast fluidized (FF), and the pneumatic conveying (PC) regimes. In these contacting regimes solids are entrained out of the bed and must be replaced. Thus in continuous operations we have the CFB, shown in Fig. 20.1. Flow models are very sketchy for these flow regimes. Let us see what is known. 

Reactors with moving solid phase Three-phase fluidized-bed (ebullated-bed) reactor Catalyst particles are fluidized by an upward liquid flow, whereas the gas phase rises in a dispersed bubble regime. A typical application of this reactor is the hydrogenation of residues. 

The operation of fluidized-bed reactors can be seen as the transition region between con-tinuous-stined tank and packed-bed reactors. In a fluidized bed, a bed of solid particles is fluidized by the upward flow of the gas or liquid stream, which may be inert or contain material relevant to the reaction. The several fluidization regimes are shown in Figure 3.51. 

For the fluidized bed process the bed expansion as a consequence of an increase in linear flow rate has to be considered. In a simplified picture diffusive transport takes place in a boundary layer around the matrix particle which is frequently renewed, this frequency being dependent on velocity and voidage, as long as convective effects, e.g. the movement of particles are neglected. Rowe [74] has included these considerations into his correlation for kf in fluidized beds, which is applicable for a wide range of Reynolds numbers, including the laminar flow regime where fluidized bed adsorption of proteins takes place (Eq. 19). The exponent m is set to 1 for a liquid fluidized bed, a represents the proportionality factor in the correlation for packed beds (Eq. 18) and is assumed as 1.45. 

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