Modeling of Natural Draft urea Prilling Tower in Order to Decrease of Tendency Caking

Granulation is one of the drying processes in which a melt is sprayed to small size droplets through a fixed orifice system or a rotating bucket into a cooling medium which is usually air. During falling down through the uprising air, droplets will cool until the first outer layer starts to solidify. The solidified layer grows and the boundary of solid and melt moves towards the center as heat is converted to air. Fully solidified and dry particle are expected to settle on the floor of the tower.A mathematical model is proposed for the urea granulation tower of the Shiraz Petrochemical Complex. In this model the urea granulation process has been simulated by simultaneous solution of continuity, hydrodynamic, mass and heat transfer equations. The particle size distribution produced by the rotating bucket was determined by the urea product distribution obtained on the floor. Particle trajectory temperature and moisture distribution of the particles and the cooling air along the height of the tower were calculated from the mathematical model. The air temperature profile resulted from the model was compared to the plant data. The model predicted data were consistent with the plant data indicating the validity of model