Experimental evaluation of parameters affecting the coating performance of urea seeds in a prototype bottom external mixing spray two-fluid nozzle fluidized bed granulator

Abstract

A comparative study was carried out on coating and coalescence mechanisms of urea seeds by molten urea, using external mixing two-fluid nozzle fluidized bed granulator (FBG). In this regard, the variables influencing a new parameter defined as coating performance (CP) of urea granules were modeled. The variables included of spraying and atomizing air pressures, masses of loaded seeds and molten urea, and mass flow rate of fluidization air. Considering all interactions between the variables, the optimum conditions for obtaining the maximum 60% CP was determined. Furthermore, according to cumulative size distributions analyses, the conditions leading to proper size guide number (SGN) and uniformity index (UI) was characterized. The SEM observations showed that, in most cases, the porous initial urea seeds were coated by a compact molten urea layer. Considering the dimensionless Stokes numbers, the extent of fluidized particles’ velocity leading to domination of coating or coalescence was verified.