Fluidizing gas velocity

Only Smith and Nienow (1983a) have reported a systematic investigation of the effects of gas velocity on granule size and on particle growth mechanisms. These authors used model materials (non-porous 

Reference Superficial gas velocity u (ms ) Relative gas u velocity mf Excess gas velocity u - u f (ms h 

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G. Critical strain energy release rate J/m J/m U Fluidization gas velocity cm/s ft/s... 

Increase solids mixing. Improve powder flowahihty of feed. Increase agitation intensity (e.g., impeller speed, fluidization gas velocity, or rotation speed). 

Measurements in large fluidized beds of fine particles indicate that bubble coalescence often ceases within a short distance above the gas distributor plate. Indications from density measurements or single bubble velocities are that bubble velocity Ug and diameter often reach maximum stable values, which are invariant with height or fluidizing gas velocity. 

Temperature can also affect fluidized beds if it causes the particles to sinter or become sticky. Siegell (1984) has shown that above a certain critical temperature, the fluidizing gas velocity must be increased above the minimum fluidization velocity in order to maintain fluidization in the bed. Siegell reported that the increase in velocity required to maintain fluidization was linearly proportional to the temperature increase. Increasing bedL/D, and smaller particle sizes, increased the tendency to agglomerate. 

This correlation predicts that the maximum vertical jet penetration into a fluidized bed varies with gas density to the 0.67 power, and decreases with increasing fluidizing gas velocity and increasing particle diameter. 

For specific values of the reactor height H and the reactor diameter D, the operating parameters are the feed temperature Tf, the wall temperature Tw, the catalyst injection rate qc, and the fluidizing gas velocity U0. The following table gives a base set of designparameters, operating variables, and chemical and physical properties for this UNIPOL reactor. 

The residence time is calculated based on the fluidizing gas velocity, assuming that the "free volume" (i.e. the volume of the expanded bed minus the volume of the sand) is fully utilized. At the temperature, total reactor gas flow rates, and sand bed volumes used, the residence time was about 0.5-1.0 sec. A typical operation began by washing the sand in 10% HNO3 and distilled water to remove impurities, such as iron, which may act as catalysts, and then calcined at 850° C for at least 12 hours to remove any sulfides and carbonates. The coal feed is then begun and pyrolysis products then exit the pyrolyser to a set of two cold traps fitted with cellulosic thimble filters maintained at 0° C. The outlet gas temperature after the first trap is 30-34° C. Much of the light char formed is entrained in the exit gas and carried into these traps, with most of it in the first trap. 

Primary air flow, supplied at the bottom of the bed, is insufficient for complete combustion of the fuel. The zone below the elevation of secondary-air jets is endothermic, cracking oil to yield carbon and fuel gas species. These burned in the upper, exothermic zone. Alumina product is withdrawn from a standpipe receiving solid from the upper zone, and burn-off of carbon in this zone is sufficient to yield a product that is acceptably white. Fluidizing-gas velocity being lower in the primary combustion zone than in the secondary, density is higher. Provision of the two zones accomplishes two purposes (1) affording a sufficient solid residence time in the primary zone and (2) reducing horsepower needed for air compression. 

Operating temperature Operating pressure Collapsed bed height Expanded bed height Average particle size of sand Density of solid sand particle Superficial fluidizing gas velocity... 

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