Fluidization

Fluidization is commonly defined as "the operation by which the fine solids are transformed into a fluid-like state through contact with a gas or liquid" [1]. 

Fluidized beds are known for their high heat and mass transfer coefficients, due to the high surface area-to-volume ratio of fine particles. Fluidized beds are used in a wide variety of industrial processes such reaction, drying, mixing, granulation, coating, heating and cooling. An example of an industrial application of fluidized beds is shown in Figure 1.

Figure 1. Industrial application of fluidized beds

As any other  solid-fluid processing system, fluidized beds have advantages and disadvantages. Some of them are listed below:

When trying to describe the operation of a fluidized bed, one main definition is the minimum fluidization velocity. This parameter is defined as "the superficial fluid velocity at which the upward drag force exerted by the fluid is equal to the apparent weight of the particles in the bed". 

Another common characteristic of fluidized beds is the bed expansion. When incipient fluidization is achieved, the fluid flowing upwards pushes the particles up and the separation distance between particles increases. This increases the void volume within the bed of particles and the bed is considered expanded. An schematic of an expanded bed is shown in Figure 2. 

Derivation of an equation that describes the flow of a fluid through a bed of particles can be done assuming the bed behaves as a group of tubes which follow a tortuous path.  The Carman-Koseny equation is good to describe laminar flow through a randomly packed bed of monosized spheres of diameter x [2].

xsv, the diameter of a sphere with the same surface to volume ratio, is used as a general form when non-spherical particles are used. This diameter is used since the drag force y proportional to the area and the number of particles is proportional to the volume. For turbulent flow this equation is:

For any flow regimes, the Ergun equation is used:  

At minimum fluidization the pressure drop of a packed bed and the one of a fluidized bed are the same. In laminar flow (Re* < 10) the pressure gradient increases linearly with superficial fluid velocity and independent of fluid density. Under turbulent flow (Re* > 2000) conditions, the pressure gradient increases as the square of the superficial fluid velocity and is independent of fluid viscosity. Where Re* = Re/(1-e).

When a fluid is passed upwards through a bed of particles the pressure loss in the fluid due to frictional resistance increases with increasing fluid flow. A point is reached when the upward drag force exerted by the fluid on the particles is equal to the apparent weight of the particles in the bed. The pressure drop of the gas flow is originated by the friction of the gas with “walls of tortuous channels” and by the drag force of moving particles [2].

From a plot of fluid pressure drop vs. superficial gas velocity key information can be obtained. Figure 3 shows an example of a fluidization plot [3]:

The first region, usually a straight line, represents the packed bed region. Here the solid particles do not move relative to one another and their separation is constant. Here the pressure drop versus gas velocity relationship is described by the Carman –Kozeny equation in the laminar flow regime and the Ergun equation in general.

The next section is ideally a horizontal region where the mass balance above applies. The superficial gas velocity at which the bed becomes fluidized is called minimum or incipient fluidization velocity. Umf increases with particle size and particle density (as bigger, heavier particles require higher gas flow to fluidize) and is affected by fluid properties [1-2]. 

 REFERENCES

1. Fluidization Engineering, Kunii and Levenspiel. Elsevier, 1991

2. Introduction to Particle Technology, Martin Rhodes. Wiley, 1998

3. http://www.vt1.tu-harburg.de

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