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GATE 2025 Chemical Engineering, Q23 · Reaction Engineering

The correct answer is D, σ²_θ monotonically decreases with N.

Question

Choose the CORRECT statement that describes the dependence of the variance (σθ2)(σ²_θ) of the residence time distribution (RTD) with respect to the number of tanks (N) in the Tanks-in-Series model of non-ideal reactors.

  • A. σθ2σ²_θ monotonically increases with N
  • B. σθ2σ²_θ first increases and then decreases with N
  • C. σθ2σ²_θ first decreases and then increases with N
  • D. σθ2σ²_θ monotonically decreases with N

Official answer: σθ2σ²_θ monotonically decreases with N

Why

GATE 2025 official key: (D). σθ2=σ²_θ = 1/N for the Tanks-in-Series model, so variance decreases monotonically as N increases (approaching plug flow as N→∞).N→\infty ).

Explanation cross-checked for consistency. Final answer matches the official IIT answer key.

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