Fluidized-bed bioreactors: What is the primary function of the disengagement (freeboard) zone at the top of a fluidized bed?

Introduction / Context:
Fluidized beds suspend solid particles in an upward fluid stream. The top zone is engineered to minimize particle carryover so that the bed remains stable and material is conserved. Understanding this function is important for reactor design and scale-up.

Given Data / Assumptions:

• Three regions: distributor, fluidized bed, disengagement/freeboard.
• Concern: entrainment of fine or partially fluidized particles into the outlet.
• Goal: retain catalyst/biocatalyst beads or carriers inside the reactor.

Concept / Approach:
The disengagement zone provides a larger cross-sectional area and lower superficial velocity, allowing entrained particles to decelerate, settle back, and disengage from the gas/liquid. This reduces particle loss and stabilizes the inventory. It may incidentally reduce splashing/foaming, but that is not its principal design objective.

Step-by-Step Solution:
Recognize entrainment as the key risk at the reactor outlet.Expand area (or provide internals) to reduce local velocities.Allow particles to disengage and return to the bed by gravity.Outcome: reduced particle loss and steadier bed height.

Verification / Alternative check:
Material balance around the reactor shows lower solids loss and less fines capture in downstream equipment when an adequate disengagement zone is provided.

Why Other Options Are Wrong:
Reducing particle velocity alone (b) is a means, not the primary function; the objective is solids retention.

Foaming control (c) can be a secondary benefit but is not the main purpose.

All of the above (d) overstates secondary effects; the key function is solids disengagement.

Common Pitfalls:

• Undersizing the freeboard, leading to chronic carryover.
• Ignoring particle size distribution changes that affect entrainment.

Final Answer:
Reduce loss of particles from the reactor by allowing entrained solids to disengage and fall back