Geldart type

The phenomenon is represented by Figs. 17-40 and 17-41 for Geldart-type A and B solids, respectively (see beginning of Sec. 17). The initial efficiency of a particle size cut is found on the chart, and the parametric hue is followed to the proper overall solids loading. The efficiency for that cut size is then read from the graph. 

It is seen that for Geldart types A and B particles, fast fluidization requires superficial gas velocities approximately an order of magnitude greater than that for bubbling dense beds. In many applications of fast fluidization, the particles exiting top of the bed are captured by cyclones and recirculated for makeup injection at the bottom of the bed, hence this regime is also denoted as circulating fluidization, CFB. 

If the dense phase expands this immediately affects the reactor model because more gas will then flow via the favourable interstitial phase. Most models readily allow for this change given that the true division of flow can be predicted. Unhappily it has so far only been possible to measure this division experimentally at fairly low flow rates, well below those employed in commercial reactors. Certainly at velocities up to about 15 cm/s much more gas flows interstitially through Geldart type A powders than minimum fluidisation flow (17). 

Rowe, P. N., A Rational Explanation for the Behavior of Geldart Type A and B Powders when Fluidized, Dept. Chem. Biochem. Eng., Univ. Coll. London, April 1986. 

Catalysts for fluidized-bed reactors have to be spherical as well. The appropriate particle size fraction for gas-solid systems can be estimated after Geldart [1] from the density difference between soKd and gas. Most widely used catalysts for fluidized beds and risers are Geldart-type B powders with particle diameters ranging from 40 to 500 pm or solid densities between 1.4 X 10 and 4 x 10 kg/m, respectively. When fluidization is provided by a Kquid as in ebullated-bed reactors, the particle sizes may be substantially larger because of the higher buoyancy in these systems. However, all types of fluidized-bed catalysts must exhibit high mechanical stability because they are exposed to abrasion on reactor walls and internals, collisions between particles and shear forces exerted by the surrounding fluid. 

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