Question

In a cyclone separator, particles are separated due to:

• A. Gravity and buoyancy
• B. Centrifugal force and drag
• C. Electrostatic attraction
• D. Magnetic force

Hint:

The "cut size" (\( d_{pc} \)) of a cyclone is the particle size separated with \( 50\% \) efficiency.
Decreasing the cyclone diameter increases the centrifugal force, which lowers the cut size and improves separation efficiency for finer particles.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary physical forces responsible for separating solid particles from a gas stream in a cyclone separator.
This is a standard particle-separation unit operation in chemical engineering.

Step 2: Key Formula or Approach:
A cyclone separator uses a gas flow pattern to generate rotational motion without moving parts.
As the fluid rotates, particles are subjected to an outward centrifugal force and an inward fluid drag force.
The balance between these forces determines whether a particle is separated or carried out with the clean gas.

Step 3: Detailed Explanation:

• Working Principle: The gas-solid mixture enters tangentially at high velocity into the cylindrical chamber of the cyclone.
This creates a double vortex fluid flow pattern.
The outer spiral flows downward, carrying heavier particles, while the inner spiral flows upward, carrying clean gas.

• Centrifugal Force: The circular motion generates a high centrifugal force, which accelerates the particles radially outward toward the wall:
\[ F_c = \frac{m \cdot v_{\theta}^2}{r} \]
where \( m \) is the particle mass, \( v_{\theta} \) is the tangential velocity, and \( r \) is the radial position.

• Drag Force: As the gas moves radially inward toward the central vortex, it exerts an inward drag force on the particles:
\[ F_d = 3 \cdot \pi \cdot \mu \cdot d_p \cdot (v_{r,f} - v_{r,p}) \]

• Separation Mechanism: For larger or denser particles, the outward centrifugal force overcomes the inward drag force.
These particles hit the wall, lose kinetic energy, and slide down into the collection hopper under gravity.
Smaller particles, where drag dominates over centrifugal force, remain entrained in the gas stream and exit through the top vortex finder.


Step 4: Final Answer: The separation of particles in a cyclone separator is driven by the balance between centrifugal force and fluid drag.