Question 8.6: Fluidized Bed A fluidized bed consists of spherical catalys......

A fluidized bed is a vessel or chamber in which solid particles are suspended in an upward flow of a gas (Figure 8.4). The buoyant solid particles therefore behave as though they were in a liquid state. Used extensively in the chemical industry, fluidized beds provide excellent mixing, heat transfer, and mass transfer characteristics. They are used in catalytic reactions where powdered or pelleted catalysts have a high specific surface area. They are also used in furnaces in which coal is combusted in a hot bed of ash or sand through which air is passed. Fluidization permits lower temperatures to be used, thereby avoiding the production of polluting oxides of nitrogen.

The behaviour of a fluidized bed depends on the particle size and the fluidizing gas velocity. When fine particles are fluidized at low gas velocities, the bed expands, but without the formation of bubbles. At higher velocities, at the bubbling regime, there are three distinct zones in the bed: The grid zone is located at the bottom of the bed and corresponds to gas penetrating the bed. It is dependent on the types of grid used. The bubbling zone is where bubbles grow by coalescence and rise to the bed surface where they break. The freeboard zone is where some particles are carried above the bed surface and are elutriated from the system while others are returned to the bed (Figure 8.5).

The process of fluidization involves the suspending of solid particles in an upward flow of a fluid. In particulate fluidization involving liquids, each particle behaves individually and collides with others, yet remains a certain distance apart. As the velocity of fluidization is increased, the bed expands. It is used in the backwashing of filter beds and ion exchange resin beds. In aggregated fluidization involving gases, similar conditions exist up to the point of incipient fluidization. At higher velocities, flow passes through the bed in the form of bubbles, and the bed expands due to the volume of the bubbles.

The behaviour of a fluidized bed depends on the particle size and the gas velocity. When fine particles are fluidized, the bed expands but without the formation of bubbles. This bubbleless regime disappears, however, when the gas velocity is increased above the bubbling velocity.

The behaviour of a fluidized bed depends on the particle size and the gas velocity. When fine particles are fluidized, the bed expands but without the formation of bubbles. This bubbleless regime disappears, however, when the gas velocity is increased above the bubbling velocity.

The support grid has the property of giving a good distribution for fluid flow without generating any appreciable pressure drop. The pressure drop across the support grid can therefore usually be neglected and is usually independent of bed conditions (Figure 8.6).

Zone I: At low flow rates the bed particles remain in position; therefore, bed depth and voidage remain constant since the pressure drop is proportional to the velocity. At point A, the bed starts to expand.

Zone II: At higher fluid velocities, the upward drag of fluid on the particles reduces the particle weight and the particles start to move apart. Interparticulate and wall friction retains the bed in its original position. Under some conditions where particle-to-wall friction is small, the bed may rise as a piston or plug flow. The pressure drop passes through a maximum at B, and then falls slightly to an approximately constant value. This change is due to an increase in the bed voidage and the pressure drop rises more slowly.

Zone III: The particles eventually move sufficiently far apart that they are just touching. Friction disappears, and the pressure drop falls back to a uniform value at the point of incipient fluidization. If the velocity increases further, the pressure drop will remain constant because it is generated by the weight of the bed, which is also constant (line C-D).

Zone IV: Raising the velocity of the gas further may increase the pressure drop due to the friction between the wall of the container but may, however, decrease due to elimination of the particles in the bed, which is time dependent. This condition would be avoided for fluidization because the entire bed would be elutriated from the container. This phenomenon is used in conveying solids in pneumatic conveyors such as grain elevators.

Decreasing the fluid velocity allows the bed to contract where the particles are just touching on each other corresponding to the minimum fluidizing velocity. The voidage has reached the maximum stable value for a fixed bed. Decreasing the fluidizing velocity further allows the bed to be reformed at a pressure across the bed less than that prior to fluidization.

A bed of very small particles will begin to fluidize when the pressure drop across the bed begins to equal the weight of the bed per unit area:

{\frac{\Delta p}{L}}=(1-e)(\rho_{P}-\rho)g          (8.23)

The pressure drop over the bed is given by

{\frac{\Delta p}{L}}={\frac{150\mu{{U}}(1-e)^{2}}{(\varphi d_{p})^{2}e^{3}}}+{\frac{1.75\rho{{U}}^{2}(1-e)}{\varphi d_{p}e^{3}}}        (8.24)

Known as the Ergun equation after Turkish-born American chemical engineer Sabri Ergun (1918–2006), the incipient point of fluidization corresponds to the highest pressure drop at the minimum fluidization velocity. At the point of fluidization it may be assumed that each spherical particle touches six others in the form of a cube arrangement, such that the voidage is

e=1{\mathrm{-}}\pi/6=0.476          (8.25)

From Equations 8.23, 8.24, and 8.25,

\frac{150(1-0.476)^{2}\times0.002\times U}{1\times0.476^{3}\times0.0015^{2}}+\frac{1.75(1-0.476)\times800\times U^{2}}{1\times0.476^{3}\times0.0015} \\ =9.81\times(1-0.476)(3000-800)                  (8.26)

Solving gives a fluidization velocity of 0.0157 ms ^{–1} .