The Thiele modulus is useful in predicting reactor behavior when what condition applies?

• A. Gas-liquid mass transfer is limiting
• B. Isothermal conditions prevail
• C. Solid catalyst particles are used
• D. Liquid phase reactions are slow

Answer: (C)

The Thiele modulus captures how fast a reaction happens inside a porous solid catalyst pellet compared to how fast reactants can diffuse through the pellet. It’s most relevant when solid catalyst particles with internal porosity are used, because internal diffusion can limit the overall rate. If diffusion inside the pellet is quick relative to the reaction (small modulus), the pellet behaves nearly uniformly and the actual rate is close to the intrinsic rate. If diffusion is slow relative to the reaction (large modulus), concentration gradients develop inside the pellet and the observed rate falls below the intrinsic rate, reflected by a lower effectiveness factor. This makes it a key tool for predicting reactor performance under internal diffusion control in solid catalyst particles.