(189f) Modeling of An Industrial Vibrating Double-Deck Screen of a Urea Granulation Circuit

Granulation is a key particle size enlargement process, widely used in the pharmaceutical, food, mining and fertilizer industries (Adetayo et al., 1995). Approximately 60% of the products in the chemical industry are produced in granular form (Balliu, 2005). Granulation converts fine particles and/or atomizable liquids (suspensions, solutions or melts) into granular material with more desirable properties than the original feed. The granulation process is considered as one of the most significant advances in the fertilizers industry, providing products with higher resistance and lower tendency to caking and lump formation. About 40% of the world?s population depend indirectly on fertilizers for their daily bread (Balliu, 2005). On a worldwide basis urea is the most popular solid nitrogen fertilizer and its use grows much more rapidly than that of other products (Fertilizer Manual, 1998). Urea granulation is a complex operation that cannot be carried out in a single unit; it is rather achieved by a combination of process units with specific functions constituting the granulation circuit (Figure 1). The main unit is the granulator, where small urea particles known as seeds (generally product out of specification) are continuously introduced and sprayed with a concentrated solution of the fertilizer. The seeds grow through deposition of the fertilizer solution droplets on the solids surface followed by water evaporation (Bertin et al., 2007). There are different types of granulators such as fluidized beds and fluidized drums. The granules that leave the enlargement size unit are classified in double-deck screens into product, oversize and undersize streams. The product is transported to storage facilities, while the oversize fraction is fed to a double-roll crusher for size reduction. The crushed oversize particles are then combined with the undersize granules and returned to the granulator as seeds (Cotabarren et al., 2008). Generally, in fertilizer granulation plants only a small fraction of the material leaving the granulator is in the specified product size range; therefore high recycle ratios are common. The characteristics of the recycle, which are the consequence of what has happened previously in the granulator, influence what will happen later on in that unit. Thus, cycling surging and drifting of particles make take place. In extreme cases, these periodical oscillations coupled with the large dead time, can result in plant shut down or permanent variations in the plant capacity as well as product quality (Adetayo et al., 1995). To minimize these problems it is necessary to have a fundamental understanding of the effects of the recycling of material, which is nearly always indispensable in the granulation process, on the behavior of the circuit. Many authors found that the operation of the screening and crushing section has a decisive influence on the recycle stream and hence on the circuit stability (among others, Zhang et al., 2000; Heinrich et al., 2003; Drechsler et al., 2005; Radichkov et al., 2006). In view of this and the recognized important role of the plant simulation to predict and optimize the granulation circuit operation (Adetayo et al., 1995; Wildeboer, 1998; Drechsler et al., 2005), reliable models for all the process units should be available. Recently, a validated mathematical model for the industrial double-roll crusher of a urea granulation circuit has been reported (Cotabarren et al., 2008). As for the crusher, the screen modeling requires the knowledge of some parameters that have to be determined from experimental data. Two different approaches, the kinetic and probabilistic, have been used for modeling screening operations. Karra (1979) developed a mathematical model for describing the performance of a vibrating screen, based in terms of the oversize partition curve. This is a curve that represents the percentage of material recovered in the oversize stream, normalized respect to the fifty percent passing size (d50). Hence, the d50 or cut size, is defined as the particle size for which the oversize partition curve has a value of 0.5. The cut size is a parameter affected by a combination of operating variables and material conditions such us the screen feed rate, location of the deck, throughfall aperture (h) percent of oversize (%>h), half-size under (%<0.5h) and near-mesh (0.75hh), half-size under (%<0.5h) and near-mesh (0.75h