Under steady-state conditions equation ( ) can be transformed into equation , with the total catalyst mass within the reactor volume 𝑚 cat and the conversion rate Δ𝑛̇𝑖 of the species 𝑖, which is calculated from the effective surface flux density 𝐽 𝑖 e (𝑦 = 𝑟 p ) via equation . For derivation see appendix. The catalyst pellet is characterized by the mean radius 𝑟 p , the catalyst porosity 𝜀 (ratio of pore volume to spherical pellet volume) and the skeletal density 𝜌 p .