Fixed or Moving Beds

Batch Extractors. Coarse soHds are leached by percolation in fixed or moving-bed equipment. Both open and closed tanks (qv) having false bottoms are used, into which the soHds are dumped to a uniform depth and then treated with the solvent by percolation, immersion, or intermittent drainage methods. 

GAC may be used in fixed or moving beds and in downflow or upflow mode. Eixed beds are operated in downflow mode and as such, provide some amount of soflds filtration however, influent soflds concentration must be kept low (less than 5 mg/L suspended soflds) to prevent rapid plugging of the bed. Entered soflds are periodically removed by backwashing. Upflow beds are more tolerant of soflds because they are fluidized and expanded by the wastewater entering at the bottom. In moving beds, the flow is countercurrent and makeup, fresh carbon is added continuously at the top of the unit while an equal amount of spent carbon is removed from the bottom. 

Catalysis by solids depends on the amount of surface exposed to the fluid. Large specific surface is obtained with small particles, but primarily with highly porous structures. For instance, to achieve 1 m2/cc the diameter of a sphere must be reduced to 6(10-4) cm, but porous catalysts may have several hundred m2/cc. Practical limitations exist to the smallness of particles that can be used, such as pressure drop and entrainment. In fixed or moving beds, particle diameters are several millimeters, in fluidized beds they may be less than 0.1 mm. 

A number of other catalysts of both synthetic and natural clay types have been developed. Some of these catalysts have shown improvement in product distribution, but none have given appreciable octane benefit. For this reason, they have not been used commercially in fixed- or moving-bed units. 

Process Type Fixed or Moving Bed Fluidized Bed Suspended Bed... 

Fluidized beds, however, have some important disadvantages relative to packed (fixed or moving) bed... 

The dust present in the flue gas must be removed in the sulfidation unit. Dust can be removed only in fixed or moving beds of pellets by allowing it to accumulate in the interstitial space. This is not possible in a fluidized bed, providing one more reason why this bed is unacceptable. 

Packed bed regime (fixed or moving bed operation). Here the particle hold-up is typically in the region 0.5-0.7. The particle size suitable in the packed bed regime is usually larger than 1 mm because smaller particle sizes result in unacceptably high pressure drops. 

Type CAL, CPG, OL, SGL. [Calgon Carbon] Granular civbon for fixed or moving beds fw purification and dectd-orization [Pg.384]

Metal on acidified AbOa (fixed bed), zeolites Pt, Pt Re,orPt Ir on acidified AI2O3 (adiabatic, fixed or moving bed)... 

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