Consider a first order catalytic reaction in a porous catalyst pellet.

Given $R$ – characteristics length of the pellet; $D_{e}$ – effective diffusivity; $k_{c}$ – mass transfer coefficient; $k_{1}$ – rate constant based on volume of the catalyst pellet; $C_{s}$ – concentration of reactant on the pellet surface.

The expression for Thiele modulus is

1. $\frac{k_{c}R}{D_{e}}$
2. $R\sqrt{\frac{k_{1}}{D_{e}}}$
3. $R\sqrt{\frac{k_{1}C_{s}}{D_{e}}}$
4. $R\sqrt{\frac{D_{e}}{k_{1}}}$