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Module 9: Packed beds

Lecture 33: Minimum fluidization velocity

Minimum fluidization velocity

The minimum fluidization velocity can be calculated by equating the pressure–drop across the fixed

packed–bed, calculated from Ergun's equation to that from the expression for fluidized bed under

particulate (smooth) conditions.

Let us calculate the pressure-drop from the 2nd expression:

Under fluidization conditions, pressure–drop equals effective weight of solid, as intraparticle forces

disappear and solids float in the bed exhibiting ‘liquid–like ‘behavior. For a fluidized bed of length of

L and bed-porosity of ,

Weight of solid-particles–buoyancy

Or

, etc.

where

R-call:

(Fig. 33a)

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Module 9: Packed beds

Lecture 33: Minimum fluidization velocity

At the minimum fluidization condition:

Apply Ergun's equation for ‘fixed–bed' at minimum fluidization condition or at the incipience of

fluidization:

, where superficial average velocity

at minimum fluidization state

Equate:

The above-equation is quadratic on (minimum fluidization velocity) and may be written in the

following form:

, where

For small particles

For large particles

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Module 9: Packed beds

Lecture 33: Minimum fluidization velocity

To avoid or reduce carryover of particles form the fluidized bed, keep the gas velocity between

. Recall

Terminal velocity, for low Reynolds number and,

for high Reynolds number

With the expressions for and known for small (viscous–flow) and large (inertial flow)

particles or Reynolds number, one can take the ratio of and :

For small

For spherical particles, and assuming

Therefore, a bed that fluidizes at 1cm/s could preferably be operated with velocities < 50 cm/s, with

few particles carried out or entrained with the exit gas.

For large

Or,

Therefore, operating safety margin in a bed of coarse particles is smaller and there is a

disadvantage for the use of coarse particles in a fluidized bed.

However, make a note that the operating particle size is also decided by the other factors such as

grinding cost, pressure-drop, heat and mass-transfer aspects.

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University:
Birla Institute of Technology and Science

Subject:
Fluid Mechanics

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18/04/2013