What describes a fluidized bed reactor?

• A. A reactor in which the contents are perfectly mixed with no concentration gradients along the length.
• B. A batch vessel used for fermentation.
• C. A fixed-bed reactor in which the catalyst remains stationary.
• D. In this type of reactor, a fluid (gas or liquid) is passed through a solid granular material (usually a catalyst) at high enough speeds to suspend the solid and cause it to behave as though it were a fluid.

Answer: (D)

Fluidized beds operate by suspending solid catalyst particles with an upward flow of gas or liquid. The fluid velocity is high enough to overcome the weight of the particles, causing them to behave like a fluid with vigorous mixing and good contact between reactants and catalyst. This suspension also enhances heat and mass transfer, since the particles are continuously re-ordered and dispersed, preventing channels and hot spots. This description matches the idea of a solid granular material being held in suspension by a flowing fluid.

In contrast, a perfectly mixed batch vessel would imply uniform concentration throughout over time but not the mechanical suspension of solids. A batch fermentation vessel is typically a different, finite-time process not designed for sustaining suspended solids in a continuous flow. A fixed-bed reactor has the catalyst stationary and does not achieve the dynamic suspension and fluid-like behavior characteristic of a fluidized bed.