Question

For a highly exothermic, solid-catalysed reaction, what reactor configuration should be used to achieve high conversion?

• A. Fixed bed reactor
• B. Fluidised bed reactor followed by a fixed bed reactor
• C. Fixed bed reactor followed by a fluidised bed reactor
• D. Fluidised bed reactor

Hint:

Optimal multi-stage reactor configuration for highly exothermic reactions: - Stage 1 (Fluidized Bed): Handles the high heat release safely, controls hot spots, and achieves high intermediate conversion. - Stage 2 (Fixed Bed with cooling): Operates at lower temperatures to shift the equilibrium toward the products and maximize the final conversion.

Answer 1

The Correct Option is B

Concept: Designing reactors for highly exothermic, reversible, solid-catalyzed reactions requires balancing heat transfer limitations and thermodynamic equilibrium constraints:

• Kinetic / Heat Transfer Constraints: At low conversions (near the reactor inlet), the reaction rate is high, which releases a large amount of heat. This can create localized hot spots in a fixed bed reactor that can damage the catalyst. A fluidized bed reactor provides excellent fluid mixing and high heat transfer rates, which maintains a nearly isothermal temperature profile and helps control these hot spots.

• Thermodynamic Equilibrium Constraints: According to Le Chatelier's principle, the equilibrium conversion for an exothermic reaction decreases as temperature increases. To achieve high final conversions at high cumulative conversion stages, the temperature must be lowered to shift the chemical equilibrium toward the products.

Step 1: Analyzing the role of a Fluidized Bed Reactor first.
At the start of the reaction, the reactant concentrations are high, and a large amount of heat is released. Using a Fluidized Bed Reactor as the first stage provides excellent temperature control due to its high thermal conductivity and rapid mixing. This prevents localized overheating and allows the reaction to progress safely through the high-heat-release stage, achieving a significant intermediate conversion.

Step 2: Analyzing the role of a subsequent Fixed Bed Reactor stage.
As the reaction mixture progresses to higher conversions, the rate of heat release drops significantly because the reactant concentrations are lower. At this stage, the reaction becomes limited by thermodynamic equilibrium rather than heat removal. To achieve a high final conversion, the reaction mixture can be passed into a multi-stage Fixed Bed Reactor equipped with interstage cooling loops. Lowering the temperature between stages shifts the equilibrium curve toward the products, allowing the reaction to overcome the equilibrium limitations of the first stage and reach a high final conversion. Therefore, the optimal reactor configuration consists of a fluidized bed reactor followed by a fixed bed reactor.