CONICET | Buscador de Institutos y Recursos Humanos

Depending on the granular product requirements, different processes of size enlargement are used in industry (Litster et al., 2004). Size enlargement involves a series of events through which small particles are converted into granules with desired properties (e.g. size, product appearance, particle moisture). The fertilizer industry requires the production of compounds that provide, to plants, one of the three essential nutrients: nitrogen, phosphorus and potassium, either individually or in combination. Urea fertilizers are the most important in providing nitrogen to crops and, in particular, the granulated urea is the most concentrated nitrogenous solid fertilizer. Therefore, it is widely used in the agricultural industry. The positive socio-economic impacts of this industry have been reflected in fertilizers demand, which increases as the world´s population does. Urea is initially produced in liquid form. Once urea is synthesized, it is usually converted into particulate material either through granulation or prilling. Since granules possess better attributes than prills, the granulation becomes the preferred process. The granulator is the key unit in the granulation circuit, in fact this stage it is essential to control several properties of the particulate matter that avoid potential problems associated with the storage, transportation and urea penetration into the soil (Reddy, 1998). Among the available granulators, the fluidized bed granulators are currently widely used. In this type of units, very small urea particles (usually called seeds) are continuously incorporated to the unit while a concentrated urea solution is sprayed inside the bed. The high heat and mass transfer rates provided by the fluidization air and the latent heat generated by the separation of the solution in its pure components facilitate the solidification/evaporation of tiny atomized drops of the urea solution onto the solid particles. Industrial granulators often have several growth chambers, which help to increase the solids residence time and produce narrower particle size distributions. The growing particles flow under currently from one chamber to another. Usually cooling chambers, where no urea solution is supplied, are placed downstream the growth beds. The purpose of these last chambers is to cool down the solids to temperatures lower than those reached in the growth chambers, which are somewhat higher than 100 °C. In this work, a dynamic model of a continuous industrial fluidized bed granulator for urea production is presented. Three growth and three cooling chambers in series are simulated. Non steady state mass and energy balances are solved for all the fluidized beds. Based on previous results (Bertín et al., 2007), complete evaporation of the water contained in the atomized drops as well as uniform temperature in each chamber are supposed. The understanding of the urea fluidized bed granulator dynamics is of great importance to produce granules with the desired attributes and to achieve stable operations of the granulation circuit to which belongs. The narrow ranges of the variables that determine good operability of the industrial granulator require thorough analysis (Knight, 2004). In view of this, parametric sensitivity studies were performed. Simulations under different disturbances in the inlet variables were conducted in order to evaluate their influence on the granulator open-loop behavior. Within the set of analyzed process variables, those linked to the fluidized bed hydrodynamics (i.e., bed height, porosity and pressure drop) were the most affected by changes in the flowrates and temperatures of the inlet streams. Different control strategies were studied to maintain the bed height, pressure and temperature. The bed temperature has to be maintained within certain limits to guarantee the granulation stability and product quality.

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