Modeling of an Industrial Fluidized-Bed Granulator for Urea Production

Abstract

The steady−state operation of a continuous industrial fluidized-bed granulator for urea production with multiple growth and cooling beds in series is modeled. The model is based on mass and energy balances, which account for the behavior of all the phases that coexist within the unit, as well as the bed hydrodynamics patterns. The granules growth that occurs through the deposition of droplets on urea seeds, followed by water evaporation and solidification of the urea present in the solution, is taken into account by considering the concentrated nature of the inlet urea solution. The proposed mathematical model successfully predicts industrial data for different plant capacities. The total urea dissolution heat is the most important thermal effect involved in the growth chambers. The granulator operation provides significant heat and mass transfer within the unit, with the water evaporation being almost complete and the outlet gas and particle temperatures being very similar.

Feedback control strategies for a continuous industrial fluidized-bed granulation process

“…ṁ melt k and x melt k are the urea solution mass flowrate atomized into chamber k and its water mass fraction, respectively. According to Bertín et al [52] it is accurate to assume that the urea solution water content evaporates immediately and completely; therefore the water mass balance can be neglected. The outlet solid mass flowrates are obtained by applying the Bernoulli equation:…”

“…The chambers are delimited by separating weirs with openings at the bottom that allow particle underflow by the principle of communicated vessels [52]. The fluidized-bed levels within each chamber have to remain below the height of the separating weirs to ensure that no solids will overflow, and over a minimum recommended height to guarantee that no drops will be sprayed over the fluidized beds in the growth chambers [47,48].…”

“…T k is the chamber temperature and, according to previous studies, can be accurately considered equal to the solid and air outlet temperatures [52]. For the granulator first chamber, T k − 1 and ṁ in k correspond to the seed temperature and mass flowrate, respectively.…”

“…It is worth to mention that according to one of the model hypothesis, the gaseous and liquid phases operate at pseudo-steady state. Thus, the mass and energy accumulations of air and liquid within the fluidized chambers are neglected [52]. The energy balance complete derivation can be found elsewhere [68].…”

Modeling of an Industrial Vibrating Double-Deck Screen of a Urea Granulation Circuit

“…The seeds grow through deposition of the melt droplets onto the solids surface, followed by urea solidification and water evaporation. 8 The granules that leave this size enlargement unit are cooled down and subsequently classified by double-deck screens into product, oversize, and undersize streams. The product is transported to storage facilities, while the oversize fraction is fed to crushers for size reduction.…”