The function 𝑓 is given here by the NH3 partial pressure: The NH3 absorption rate, i.e. the forward reaction rate in eq. ( ), is defined to have a first order dependence on both the NH3 partial pressure in the gas phase as well as the free absorption capacity (1 -𝜃 = 𝑥 ) as shown in eq. ( ), meaning 𝑀 = 1. However, since the NH3 sorption is an equilibrium reaction, the backwards reaction, i.e. the desorption, needs to be considered additionally. The desorption or backwards reaction rate is defined to have a first order dependence on the sorbent loading (eq. ( )).