An empirical mixing model for mixing of reacting species in a fluid is proposed where the mixing process is divided into macromixing and micromixing. Macromixing is assumed to be instantaneous and it is modelled by dividing the injected fluid into a number Nd of identical droplets, each surrounded by the same amount of bulk fluid. Micromixing is modelled as a molecular diffusion process in each droplet. The model can be used for any fluid residence time distribution E(t). The model is solved by means of collocation polynomials and numerical integration of the set of coupled differential equations using a third order semi implicit Runge-Kutta method. The proposed model is used to simulate pilot plant experiments of the selective non-catalytic reduction of NO by NH3. The experiments have previously been simulated using an ideal plug flow reactor model without success. Combining the empirical kinetic model for the reactions with this mixing model gives satisfying results when choosing the mixing parameter corresponding to a mixing time of 20–30 ms.