In this work, the scale-up of an ammonium sulphate crystallization process is examined by comparing the results of experiments in a 22 1 and in a 970 1 draft-tube baffled crystallizer. The CSD is monitored by using an on-line CSD measuring instrument which is based on Forward light scattering. Both crystallizer product CSDs exhibit a dynamic behaviour which lasts up to 20 h before a steady state is reached. The results showed that the start-up dynamic behaviour of the CSD in the two vessels is very similar in terms of wave amplitude and period. The dynamics of the system are modelled by solving a number of system mass and energy balances, as well as a population balance equation. The kinetics of crystal growth, attrition and secondary nucleation are considered. The simulation results show that both systems can be modelled with the same model framework and expressions for the kinetics processes.