Packed Bed/ Fixed Bed Fluidzation

Packed Bed Fluidzation

A fixed or packed bed refers to a bed of stationary particles residing on a perforated grid, through which a gas flows. A fixed layer of small particles or objects arranged in a vessel to promote intimate contact between gases, vapors, liquids, solids, or various combinations thereof; used in catalysis, ion exchange, sand filtration, distillation, absorption, and mixing. 

Є = Void fraction of the bed

X = Sphericity

µ = Dynamic viscosity

Pg = Density of the gas

U = Superficial gas velocity

Principles of Fluidization

Principles of Fluidization:-

1.       Fixed bed

2.      Expanded bed

3.      Incipient bed

4.      Bubbling bed

5.      Slag flow

6.      Turbulent regime

7.       Fast bed

8.     Transport bed (Pneumatic Transportation)

Fluidization & It's Characteristics

Fluidization:-

Fluidization is defined as:

 “An operation through which fine solids are transformed into a fluid like state through contact with either a gas or a liquid”.

A roasting process in which finely divided solids are suspended in a rising current of air (or other fluid), producing a fluidized bed; used in the calcination of various minerals, in Fischer-Tropsch synthesis, and in the coal industry. Under the fluidized state, the gravitational pull on granular solid particles is offset by the fluid drag on them. Thus the particles remain in a semi-suspended condition. A fluidized bed displays characteristics similar to those of a liquid.

Fig: A fluidized bed demonstrates all the characteristics of a fluid.

Characteristics of Fluidization:-

1. The static pressure at any height is approximately equal to the weight of bed solids per unit cross section above that level.
2. The bed surface maintains a horizontal level, irrespective of how the bed is titled; also the bed assumes the shape of the vessel.
3. The solids from the bed may be drained like a liquid through an orifice at the bottom or on the side.
4. An object denser than the bulk of the bed will sink, while one lighter than the bed will float. Thus, a steel ball sinks in the bed, while a light shuttlecock floats on the surface.
5. Particles are well mixed, and the bed maintains a nearly uniform temperature throughout its body when heated.

Handbook of Coal Analysis

Handbook of Coal Analysis
               by
      James G. Speight




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