This work presents a new dynamic modelling approach for calcium looping systems that allows explicit sorbent deactivation and purge/makeup. These are common in plant operations, but often neglected in modelling. This model adopts a Monte Carlo approach, tracking merely 100 particles between the carbonator and the calciner for 60 reaction cycles with the particle residence time in each reactor determined stochastically. The simulated results agree well with the experimental data in both the transient and steady-state stages. This model provides a promising approach to predicting the dynamic behaviour of calcium looping systems under relatively realistic conditions at low computational cost. The stochastic description of a multiphase reactor via the Monte Carlo process can be widely adapted in various scenarios.