In a fluidized-bed bioreactor, how is bulk mixing and particle suspension primarily achieved?

Introduction / Context:
Fluidized-bed reactors suspend solid particles (e.g., immobilized cells or catalysts) by flowing fluid upward at velocities above the minimum fluidization velocity. Proper fluidization yields excellent contact and mixing with low shear compared to stirred beds.

Given Data / Assumptions:

• Bed of particles with a defined size and density distribution.
• Upward fluid flow (liquid and/or gas) through distributor plates or spargers.
• No mechanical impeller in the bed.

Concept / Approach:
When the drag force from the upward flow balances the net weight of particles, the bed expands and behaves like a fluid. Mixing is therefore driven primarily by the rising fluid stream, bubble-induced circulation (if gas present), and particle–fluid interactions—not by gravity or molecular diffusion alone.

Step-by-Step Solution:

Verification / Alternative check:
Pressure-drop vs. velocity curves show a plateau at fluidization; visual diagnostics confirm suspension and circulation arise from upward flow.

Why Other Options Are Wrong:

• Gravity alone cannot suspend particles; it opposes fluidization.
• Diffusion is far too slow to mix macroscopic beds.
• “Both equally” misstates roles; upward flow is decisive.
• Acoustic mixing is not standard in fluidized beds.

Common Pitfalls:
Operating below minimum fluidization so the bed channels instead of mixing.

Final Answer:
Upward movement of the incoming fluidizing feed stream.