Cooling and seeding effect on supersaturation and final crystal size distribution (CSD) of ammonium sulphate in a batch crystallizer

Different cooling policies (natural (NCP), linear (LCP), controlled (CCP), as well as a proposed cooling policy; impulse changes in natural cooling policy (IC-NCP)) and seeding policies (with two different mean seed size and various seed loadings) were employed to investigate the effect of cooling and seeding on the final CSD of ammonium sulphate grown in a batch-cooling crystallizer. The crystallizer was equipped with an on-line density meter for the estimation of supersaturation. By increasing the amount of the seeds, the supersaturation peak was reduced and a uni-modal final CSD was obtained, irrespective of the cooling policy. At high seed loading (Cs), greater than theoretical critical seed loading (Cs,ctheo) predicted by the seed chart, the cooling policy had no significant effect on the final CSD. However, at low Cs, controlled cooling policy was needed in order to ensure uni-modal and narrow final CSD with large mean size. With the help of a simple on-off control, the IC-NCP was applied to overcome the mathematical complexities associated with the controlled cooling policy. All CSD results of the IC-NCP mode were between the linear and controlled cooling policies, and closer to the controlled cooling profile.