If Da is much greater than 1, which process dominates the overall rate?

If Da is much greater than 1, which process dominates the overall rate?

• A. Mass transfer dominates
• B. Reaction rate dominates
• C. Heat transfer dominates
• D. Diffusion dominates

Answer: (B)

Damköhler number compares how fast transport brings reactants to the reaction zone with how fast the chemical reaction proceeds. When this number is much greater than one, transport is effectively fast relative to the reaction, so the slowest step is the chemical reaction itself. In that case, the overall rate follows the intrinsic reaction rate rather than any mass or heat transfer limitation. So, the reaction kinetics dominate the overall rate in this regime. (If the Damköhler number were very small, then transport would be the bottleneck and diffusion/mass transfer would dominate.)

Damköhler number compares how fast transport brings reactants to the reaction zone with how fast the chemical reaction proceeds. When this number is much greater than one, transport is effectively fast relative to the reaction, so the slowest step is the chemical reaction itself. In that case, the overall rate follows the intrinsic reaction rate rather than any mass or heat transfer limitation. So, the reaction kinetics dominate the overall rate in this regime. (If the Damköhler number were very small, then transport would be the bottleneck and diffusion/mass transfer would dominate.)