Open and closed loop behavior of an industrial fluidized bed granulator

BERTÍN, DIEGO ESTEBAN; PIÑA, JULIANA; BUCALÁ, VERÓNICA

Lausanne

Taller; 5th International Granulation Workshop 2011; 2011

University of Sheffield

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 industrial granulator consists of a series of fluidized beds. Small urea particles (called seeds) enter the first chamber and flow under-currently from one chamber to another. Besides, a urea concentrated solution is sprayed from the bottom of the unit into the three first chambers in the form of tiny atomized drops. The granules grow through deposition of these droplets on the seeds material, followed by cooling and evaporation of the drops water content, which cause the solidification of the urea present in the solution. The bubbling nature of the fluidized bed, which is responsible for the strong solids mixing, promotes the repeatedly circulation of the granules through the spraying zone. Downstream the growth chambers, three fluidized bed cooling compartments are arranged to cool down the solids to temperatures lower than those reached in the growth chambers (somewhat higher than 100°C). Ambient air is used as the fluidizing gas, which is supplied to the granulator by a fan. The total fluidization air stream is divided into six streams, each to each chamber. The total and per chamber fluidization air flowrates are determined by the granulator total pressure drop, which is automatically controlled at the desired set-point. The distribution of the total air flowrate into the compartments can be manually controlled by manipulating control dampers. The narrow ranges of the variables that determine good operability of the industrial granulator require an efficient control to return the system rapidly to the desired operational point. A mathematical model to represent the non-steady-state operation of the granulator is developed in this work. Mass and energy balances are solved for all the fluidized beds to calculate the flowrates and temperatures of the streams that leave each chamber. The population balance equation is formulated assuming that coating is the only mechanism responsible for the particle size change and the particle size distribution is obtained for each chamber. A total pressure drop control scheme is developed though mechanical energy balances in order to link the bed pressure drop with the fluidization air mass flowrate supplied by the fan. Applying the developed model, the open and closed loop behaviour of the granulator is investigated with the aim of improving its operability. Within the set of analyzed process variables, those linked to the fluidized bed hydrodynamics (i.e., bed height, porosity, pressure drop) are the most affected by changed in the flowrates and temperatures of the inlet streams. Different control strategies are studied to maintain the bed height, pressure and temperature.