Minimum Fluidization Velocity - Fluid Mechanics - Lecture Notes, Study notes for Fluid Mechanics. Birla Institute of Technology and Science

Fluid Mechanics

Description: The key points are: Minimum Fluidization Velocity, Packed Beds, Pressure–Drop, Ergun's Equation, Fluidization Conditions, Weight of Solid, Intraparticle Forces, ‘Liquid–Like ‘Behavior, Incipience of Fluidization, Superficial Average Velocity
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Module 9: Packed beds
Lecture 33: Minimum fluidization velocity
Minimum fluidization velocity
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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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