Question

Which of the following happens when the gas velocity increases in fluidization?

• A. Bulk density of the bed increases and fluidization become less aggressive
• B. Bulk density of the bed decreases and fluidization become less aggressive
• C. Bulk density of the bed increases and fluidization become more aggressive
• D. Bulk density of the bed decreases and fluidization become more aggressive

Hint:

In fluidization, increasing gas velocity beyond the minimum fluidization velocity expands the bed (lowering bulk density) and enhances particle motion (more aggressive fluidization).

Answer 1

The Correct Option is D

Step 1: Understand fluidization and the effect of gas velocity.
Fluidization occurs when a gas (or liquid) passes upward through a bed of solid particles, suspending them against gravity. The gas velocity affects the behavior of the bed:
At low velocities, the bed remains packed (fixed bed).
At the minimum fluidization velocity (\( u_{mf} \)), the particles just begin to lift, and the bed transitions to a fluidized state.
As gas velocity increases beyond \( u_{mf} \), the bed expands, and the particles move more vigorously, leading to more aggressive fluidization (e.g., bubbling or turbulent regimes).
Step 2: Analyze the effect on bulk density.
Bulk density of the bed is the mass of particles per unit volume of the bed. In a fixed bed, the particles are closely packed, so the bulk density is high.
As gas velocity increases and the bed fluidizes, the bed expands (the height of the bed increases), and the particles become more spaced out. This expansion reduces the bulk density because the same mass of particles occupies a larger volume.
Step 3: Analyze the effect on fluidization aggressiveness.
Fluidization aggressiveness refers to the intensity of particle motion and mixing. At the onset of fluidization (\( u_{mf} \)), the bed behaves like a liquid with minimal particle motion.
As gas velocity increases, the fluidization becomes more aggressive: bubbles form (bubbling regime), particles move more vigorously, and at higher velocities, the bed may enter a turbulent or fast fluidization regime. This increased motion and mixing indicate more aggressive fluidization.
Step 4: Combine the effects.
Increasing gas velocity causes the bed to expand, decreasing the bulk density.
The fluidization becomes more aggressive due to increased particle motion and bubbling.
Step 5: Evaluate the options.
(1) Bulk density of the bed increases and fluidization become less aggressive: Incorrect, as bulk density decreases, and fluidization becomes more aggressive. Incorrect.
(2) Bulk density of the bed decreases and fluidization become less aggressive: Incorrect, as fluidization becomes more aggressive with increased velocity. Incorrect.
(3) Bulk density of the bed increases and fluidization become more aggressive: Incorrect, as bulk density decreases. Incorrect.
(4) Bulk density of the bed decreases and fluidization become more aggressive: Correct, as both effects align with increased gas velocity in fluidization. Correct.
Step 6: Select the correct answer.
When the gas velocity increases in fluidization, the bulk density of the bed decreases, and fluidization becomes more aggressive, matching option (4).