Dynamic simulation and optimization of a urea granulation circuit

COTABARREN, IVANA; PIÑA, JULIANA; BUCALÁ, VERÓNICA; ROMAGNOLI, JOSÉ

Roma, Italia

Conferencia; 9th International Conference on Chemical and Process Engineering,; 2009

AIDIC, Italian Association of Chemical Engineering.

Granulation is a key particle size enlargement process, widely used in the pharmaceutical, food, mining and fertilizer industries. This operation converts fine particles and/or atomizable liquids (suspensions, solutions or melts) into granular material with desired properties. 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. 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. 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 and urea solidification. 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 double-roll crushers for size reduction. The crushed oversize particles are then combined with the undersize granules and returned to the granulator as seeds. 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 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. To minimize these problems it is necessary to have a fundamental understanding of the effects of the recycling of material on the behavior of the granulation circuit.      In view of this, the relatively high current installed world urea capacity and its forecasted expansion (from 136 Mt in 2008 up to 192 Mt in 2011), the dynamic modeling and simulation to optimize the urea granulation circuits operation will play an important role in this fertilizer economy. In the present work, previous validated models against industrial data for the crusher, screen and granulator units are integrated into the dynamic circuit flowsheet by means of an equation-oriented modeling system (gPROMS Model Builder Environment). The complete circuit dynamic model is then used to carry out sensitivity analyses by studying the effect of different operating variables on the particle size distributions and mass flowrates of the product and recycle streams. Once the critical variables are identified, the gPROMS optimization tools are used to maximize the plant production maintaining the marketable product granulometry.