The effect of binder concentration in fluidized-bed granulation: Transition between wet and melt granulation

Abstract

According to the binder nature, fluidized-bed granulation (FBG) is usually classified as wet or melt granulation. In particular, the industrial urea granulation performed in fluidized beds, is often called “melt granulation” because a highly concentrated urea solution is used as binder (between 95–97 wt%) (Cotabarren et al., 2012). However, plant disturbances can cause changes in binder urea concentration and thus granulation can shift from melt to wet granulation and vice versa. In a previous investigation, the granulation system urea (seeds)–urea (binder) was extensively studied in a pilot-scale batch fluidized-bed granulator (Veliz Moraga et al., 2015). Besides, the effect of seed size, bed temperature, binder flowrate and fluidization and atomization air flowrates on process variables as well as on product properties were studied. The aim of this work is to analyze the effect of the binder urea concentration on the urea granulation performance and product properties. This concentration was varied from 87.5 wt% to 98 wt%, while the fluidization air velocity, urea melt flowrate, bed temperature set-point and atomization air flowrate were kept constant. The product properties (percentage of agglomerates and coated particles, crushing strength and moisture content) and granulation efficiency are discussed in terms of the transition from wet to melt granulation. The critical urea content was experimentally found; indeed, urea concentrations lower than the critical one dramatically affect the product quality. Finally, the criterion proposed by Villa et al. (2016) for predicting agglomerates formation is used to determine the minimum allowable binder urea concentration. The prediction is consistent with the trends experimentally observed, indicating the good capacity of the criterion to identify the boundary for agglomeration occurrence.