CONICET | Buscador de Institutos y Recursos Humanos

Granulation is a widely used process in fertilizer, food, pharmaceutical and mining industries to obtain products with attractive properties. Particularly, the melt or wet granulation uses a liquid binder that is sprayed onto the particulate phase to form bonds between particles or between the binder drops and solids. This process takes place in an agitated environment to promote the liquid dispersion and the granules growth and consolidation. Granulation can be carried out in different equipments, among others, high shear units, fluidized beds and rotary drums. The fluidized bed granulator (FDG) is a relatively new design, which combines the drum and fluidized bed granulation technologies. The FDG is a cylindrical horizontal rotating drum with internal lifters and has an inclined fluidized bed inside it where air is blown. This special design offers the very good heat and mass transfer rates provided by fluidization but with lower air flowrates. The FDG modeling is very complex; being the mathematical description of the particles circulation pattern one of the main difficulties. The granules inside the unit are mechanically raised by the lifters, sliding and rolling, up to the upper part of the drum from where they fall onto the fluidized bed for cooling or drying (i.e. melt or wet granulation, respectively). The slope of the fluidized bed surface allows the particles to flow down along the inclined perforated plate and fall into the lower part of the drum after being sprayed with the liquid binder. The lifters raise the granule coated with a new layer to be further cooled or dried. The same cycle is then repeated until the desired particle size is achieved. Potassium nitrate is a premium fertilizer (completely soluble in water and chlorine free) which is used in a wide range of high value agricultural applications (e.g., hydroponic and greenhouse environments). Nowadays, direct soil application is made by dissolution of KNO3 crystals in water or by spreading prills on the field. However, the KNO3 granules are harder and more crush resistant than the prills and its production allows certain product granulometry flexibility that makes feasible the physical mixtures of KNO3 with other granular fertilizers. In this work, a FDG of a capacity around 30 TPH for potassium nitrate production has been designed based on mathematical models and practical considerations. The drum dimensions, rotation speed and number of lifters have been selected from reported data for conventional rotating drum granulators. The fluidized bed table dimensions, its location and the flow and temperature of the fluidized air have been defined, through an iterative procedure, by rigorous solution of the mass and energy balances for the solid and gas phases in the fluidized bed and spraying zones. To this end, the solids lifter holdups at any angular position as well as the particle discharge flow onto the fluidized bed have been estimated. The fertilizer has solid-solid and liquid-solid phase transitions within the operating conditions of its production. Therefore, to correctly represent the thermal effects associated with the KNO3 melt solidification at 350 ºC and the solid-solid phase transition (from the room temperature orthorhombic phase II to a rhombohedral phase I when heated above 129 ºC), the corresponding transition kinetics have been determined by using differential scanning calorimetry and included in the present model. The developed rigorous mathematical representation is a useful tool to address the unit design and to further simulate and optimize the potassium nitrate granulation in FDGs.