congresos y reuniones científicas
Analysis of the product granulometry, temperature and mass flow of an industrial multichamber fluidized bed urea granulator operating at unsteady state conditions
Sheffield, Inglaterra
Simposio; 9th International Symposium on Agglomeration and 4th International Granulation Workshop; 2009
Institución organizadora:
Particles Product Group-University of Sheffield
Within the wide variety of industries that utilize granulation to produce particles with defined properties, the fertilizer manufacture has an essential role in securing food supplies around the world. Urea in granular form is one of the most often used nitrogen-based fertilizers, being the global market conditions exceptionally favourable since 2007. In response to strong market fundamentals, the world urea capacity has been forecasted to expand from 136 Mt in 2008 to 192 Mt in 2011. In this context, knowledge improvements to operate more efficiently urea granulation plants will be extremely worthy. The industrial fluidized-bed granulator for urea production often has several growth chambers which allow obtaining narrower particle size distributions, homogenizing the particles residence time. A growth chamber is basically a bed of solids fluidized by air where small urea particles are fed continuously and a urea concentrated liquid solution is sprayed from the bottom of the unit. The granules grow through deposition of the tiny liquid drops onto the seeds, followed by solidification of the urea present in the solution through cooling and evaporation of the droplets water content. 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 to meet specific requirements for further processing of the granules. Currently, considerable trial and error is required to operate the industrial urea granulator. Its dynamics is complex and makes difficult to run the plant at steady state. Good operability of this key unit is essential to control several properties of the granules in order to avoid potential problems associated with the granulation circuit stability, urea storage, transportation and supply to the soil. Thus, the understanding of the fluidized-bed granulator dynamic behaviour is of great importance to produce granules with the desired attributes and achieve stable operations of the granulation circuit, preventing plant shut downs. In this work, a complete 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 together with the population balance equation (PBE). Since each granulator chamber can be modelled as a continuous stirred tank (CST), the multichamber granulator is represented by a series of CSTs. The PBE solution for non-steady state operations of continuous granulators with variable chamber solids mass has not been extensively studied. In fact, the granulators are usually modelled assuming constant bed mass to mimic the typical pressure drop control employed in industry. By using the population, mass and energy balances, the open-loop behaviour of the granulator is analyzed. With this aim, and to further develop efficient control strategies, different step disturbances are assayed. Based on the results of the performed sensitivity study, some guidelines are proposed in order to select manipulative variables to maintain the product quality (granulometry) and granulator stability (pressure drop, bed height and temperature) within the desired values.