Design of a prilling device with controlled breakup of a particulate suspension jet into droplets

Abstract

Prilling is the industrial process in which thin jets of a molten substance break up into drops and the subsequent solidification of the droplets into solids due to cooling when they fall down in the prilling tower. Prilling is widely used for efficient production of a variety of materials such as, e.g., fertilizer grains, laundry detergent, and substances for drugs and foods.
While in most prilling applications the jet can be considered as a pure liquid, in some cases the molten substance may contain suspended particles. The present study is motivated by prilling of fertilizers composed of Urea or Ammonium Nitrate (AN) containing polyhalite particles. To date, the effects of suspended particles on jet breakup, drop formation and crystallization are not well-understood. In the first part of this report a literature review is given on prilling and the current understanding of the effect of suspended particles on this. It has been found in literature that a large concentration of suspended particles will increase the viscosity of the suspension but also promote breakup of the jet. Besides, the size of the particles also influences the viscosity and jet breakup, resulting in a decrease in the amount of smaller formed secondary droplets at larger suspended particle sizes, hence increasing monodispersity. The objectives of this MSc research project are to assess under what conditions high-quality fertilizer grains can be optimally prilled from polyhalite suspensions in molten urea or ammonium nitrate. An experimental setup was designed to assess how molten liquid jets of Urea or AN with suspended polyhalite particles can be optimally prilled for producing high-quality fertilizer grains. This design consists of melting and mixing the batch of materials, from where it will pump the particulate suspension through a vertical tube with a screw pump to a nozzle. Here, a jet will be formed. This will breakup into droplets, which will fall in an oil batch to increase cooling and hence fasten the solidification process. Unfortunately, the current design was always dripping and not able to form a jet due clogging and the formation of hard lumps, which decreased the flow rate. The droplets were however collected and analyzed. It was found in this thesis that a larger concentration of P4 particles in urea will increase the viscosity. When the particles are grinded more, the viscosity increased as well, however a direct relationship between particle size and viscosity has yet to be determined. The particles were also unequally distributed over the prills, because P4 tends to stick together. To obtain the best quality prills, it is recommended to use relatively large P4 particles in combination with larger nozzles of >1mm to decrease the desired prilling force and prevent clogging. Also melting