capítulos de libros
Production of Granular Urea as Nitrogenous Fertilizer
Urea: Synthesis, Properties and Uses
NOVA Science Publishers
Lugar: Nueva york; Año: 2012; p. 1 - 63
The demand for urea is continuously growing and entwined with the need for fertilizers and animal feed additives. Industrially, urea is initially produced in liquid form as a concentrated solution. Then, it can be converted  into particulate material either  through granulation or prilling processes. Since  granules  have  better  attributes  than  prills,  nowadays  granulation  is  the  preferred Production route. Urea granulation is a multifaceted process that requires several operation units, which constitute the granulation circuit, to produce the solid form (granules) with the desired attributes. The main unit of the circuit is the granulator, where small urea particles known  as  seeds  are  continuously  fed  and  sprayed with  a urea  concentrated  solution. The seeds grow through deposition of the solution droplets onto the solids surface, followed by water evaporation and urea solidification. The granules that leave the size enlargement unit are size classified into product, oversize and undersize streams. The product is transported to  storage  facilities, while  the oversize  fraction  is  fed  to  crushers  for  size  reduction. The crushed oversize particles are then combined with the undersize granules and recycled back to the granulator as seeds. Focusing on urea, the advantages of granules over prills are discussed by exploring the physical properties of both  solid  forms. Then,  the  current  available  technologies  for urea granulation  are  presented  in  a  comparative manner. From  this  analysis,  the  fluidized-bed granulator appears as the most widely used equipment for granular urea production. Due to this  preference,  different  approaches  to  model  fluidized-bed  granulators  are  presented aiming  to give a comprehensive picture of  the  fundamental phenomena  that occurs within these granulation units. Special attention is placed on the granules growth mechanism, and its  proper  representation.  Although  coating  is  the  preferred  urea  growth  mechanism, unexpected  operating  situations may  favor  size  enlargement  by  agglomeration  that  is  an undesired  phenomenon.  Therefore,  based  on  experimental  data  obtained  in  a  pilot-scale fluidized-bed batch granulator for urea production, the influence of the operating variables on  both  granules  quality  and  growth  mechanisms  is  discussed.  Finally,  mathematical models for peripheral circuit units (crusher, cooler and screens) are presented. By coupling all the involved units, a complete granulation circuit simulator is reported. Steady-state and dynamics results obtained by means of the urea granulation simulator are provided to show the  influence  of  different  circuit  operating  variables  on  the  marketable  product  size distribution  and  the  plant  throughput. Summarizing,  this  chapter  gives  an  introduction  to the main features of the urea granulation process and remarks operation problems that face the granular urea production together with possible strategies to overcome them.