Question

In continuous fluidization,

• A. there is no entrainment of solids
• B. solids are completely entrained
• C. velocity of the fluid is small
• D. pressure drop is less than that of batch fluidization

Hint:

Remember the sequence of fluidization regimes as fluid velocity increases:
Fixed Bed \( \rightarrow \) Minimum Fluidization \( \rightarrow \) Bubbling/Slugging Bed \( \rightarrow \) Turbulent Bed \( \rightarrow \) Fast/Continuous Fluidization (complete entrainment) \( \rightarrow \) Pneumatic Transport.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks about the characteristic behavior of solids in a continuous fluidization system.
Fluidization is a process where solid particles are transformed into a fluid-like state through contact with a gas or liquid moving upward.
This question tests understanding of the different regimes of fluidization, moving from minimum fluidization to pneumatic transport.

Step 2: Key Formula or Approach:
As the superficial velocity of the fluid (\( u_0 \)) increases, the bed passes through several distinct regimes:
1. Fixed Bed: \( u_0 \lt u_{mf} \) (where \( u_{mf} \) is the minimum fluidization velocity).
2. Particulate/Bubbling Fluidization: \( u_{mf} \le u_0 \lt u_t \) (where \( u_t \) is the terminal settling velocity of the particles).
3. Continuous Fluidization / Fast Fluidization: \( u_0 \gt u_t \).
In this fast fluidization or continuous fluidization regime, the fluid drag is greater than the gravitational force on individual particles, causing them to be carried out of the bed.

Step 3: Detailed Explanation:

• Batch Fluidization: In batch fluidization, the fluid velocity is maintained above the minimum fluidization velocity but below the terminal velocity of the particles.
The particles remain confined within the column, and there is negligible entrainment.

• Continuous Fluidization: When the fluid velocity exceeds the terminal settling velocity of the particles, the upward drag force overcomes gravity and buoyancy.
The particles are swept out of the column with the fluid, which is referred to as complete entrainment.
To maintain continuous operation, these entrained solids must be separated from the fluid stream (typically using a cyclone separator) and recycled back to the bottom of the fluidization column.
This is the fundamental principle behind circulating fluidized bed reactors (CFBR) and pneumatic transport.


Step 4: Final Answer:
In continuous fluidization, the fluid velocity is high enough that the solids are completely entrained by the moving fluid.