Question

Minimum fluidization velocity in a fluidized bed corresponds to the condition when:

• A. Particles start to dissolve
• B. Pressure drop equals weight of particles per unit area
• C. Flow becomes turbulent
• D. Bed height becomes zero

Hint:

Logic Tip: It's a simple tug-of-war. Gravity pulls down, fluid pushes up. "Incipient fluidization" happens exactly when the upward push perfectly cancels out the downward weight.

Answer 1

The Correct Option is B

Concept:
Fluidization occurs when a solid particulate bed is transformed into a fluid-like state through contact with a gas or liquid. This is achieved by blowing the fluid upwards through the solid particles.

Step 1: At low fluid velocities, the fluid merely seeps through the empty spaces between the stationary solid particles. This is a packed bed.

Step 2: As the upward fluid velocity increases, the fluid exerts an increasing upward drag force on the individual particles. This upward force manifests as a pressure drop across the bed.

Step 3: The primary force keeping the particles stationary is gravity, pulling them downward. This is the total weight of the particle bed.

Step 4: Minimum fluidization velocity ($U_{mf}$) is the exact speed where the upward force (pressure drop caused by the fluid) perfectly balances the downward force (the weight of the particles).

Step 5: At this specific velocity, the particles are no longer resting on each other; they become suspended in the fluid flow. Therefore, the pressure drop across the bed equals the effective weight of the bed per unit cross-sectional area.