The Bubbling Fluidized Bed—BFB

We now look at the various contacting regimes in detail and see what predictions are available for each regarding reactor behavior. 

Pass gas upward through a bed of fine particles. For superficial (or inlet) gas velocities much in excess of this minimum the bed takes on the appearance of a boiling liquid with large bubbles rising rapidly through the bed. In this state we have the bubbling fluidized bed, BFB. Industrial reactors particularly for solid catalyzed gas-phase reactions often operate as bubbling beds with gas velocities Wq 5 30 u.  

Calculations show that the conversion in bubbling beds may vary from plug flow to well below mixed flow, see Fig. 20.6, and for many years the perplexing and embarrassing thing about this was that often we could not reliably estimate 

It was soon recognized that this difficulty stemmed from lack of knowledge of the contacting and flow pattern in the bed in effect, the bypassing of much of the solids by the rising bubble gas. This led to the realization that adequate prediction of bed behavior had to await a reasonable flow model for the bed. 

Since the bubbling bed represents such severe deviations from ideal contacting, not just minor ones as with other single-fluid reactors (packed beds, tubes, etc.), it would be instructive to see how this problem of flow characterization has been attacked. A wide variety of approaches have been tried. We consider these in turn. 

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In large-scale applications, fiuidized-bed boilers are now dominant on the market. Both bubbling fluidized beds (BFBs) and circulating fluidized beds (CFBs) are used for biofuel firing. BFBs were originally limited to a rather narrow specification of fuels but now flue-gas recycling is used in both types of boilers. Especially, CFBs are considered... 

FBC offers a technology that can be designed to burn a variety of fuels efficiently and in an environmentally acceptable manner with various forms. In FBC units, coal is combusted in a hot bed of sorbent particles that are suspended by combustion air that is blown in from below through a series of nozzles. Depending on the gas velocity in the bed, fluidized bed can be classified into bubbling fluidized bed (BFB) and circnlating fluidized bed (CFB). The fluidized bed can be operated at atmospheric (AFB) and elevated pressure (PFB). 

Gas plus catalyst soUd Usually BFB. For fast reactions, gas film diffusion may control and catalyst pore diffusion mass transfer may control if catalyst diameter >1.5 mm. Heat transfer heat transfer coefficient wall to fluidized bed is 20-40 X gas-wall at the same superficial velocity, h = 0.15-0.3 kW/m K. Nu = 0.5-2. Heat transfer from the bed to the walls U = 0.45 to 1.1 kW/m °C. from bed to immersed tubes U = 0.2 to 0.4 kW/m °C from solids to gas in the bed U = 0.017 to 0.055 kW/m °C. Fluidized bed usually expands 10-25 %. Backmix type reactor which increases the volume of the reactor and usually gives a loss in selectivity. Usually characterized as backmix operation or more realistically as a series of CSTR if the height/diameter > 2 Usually 1 CSTR for each H/D= 1. If the reactor operates in the bubble region, then much of the gas short circuits the catalyst so the overall apparent rate constant is lower by a factor of 10. 

It is seen that the Nusselt numbers for BFBs fall below those for convection from a single sphere, for Reynolds numbers less than 20. In fact, the magnitude of Nup for fluidized beds drops below the value of 2.0, which represents the lower limit of conduction heat transfer. The cause of this is the bubbling phenomenon. Low Reynolds numbers correspond to beds of fine particles (small flip and C/g), wherein bubbles tend to be clouded with entrained particles. This diminishes the efficiency of particle-gas contact below that represented by idealized plug flow, resulting in reduced values of Nup. As particle diameter increases (coarse particle beds), bubbles are relatively cloudless and gas particle contact improves. This is shown in Fig. 2 where the Nusselt numbers of fluidized beds are seen to increase with... 

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