Analysis of optimal control problems and plant debottlenecking for urea granulation circuits

Abstract

The optimal control of a complete urea granulation circuit is studied in this work. The dynamic circuit operation is modeled by means of population, mass, and energy balances for the equipment of an industrial urea granulation flowsheet. The objective for the optimal control of the granulation circuit is to drive the process from a given steady-state (corresponding to the nominal capacity of a large-scale plant) to another one, by maximizing the throughput during the transient. To this aim, realistic operating constraints are taken into account and, based on an analysis of the active physical limitations, different optimal control variables are selected (i.e., urea melt flow rate, granulator discharge area, and granulator fluidization air temperature and mass flow rate). The results provide new promising operating points, which can be achieved by tracking the calculated optimal time-trajectories for the selected control variables, and give insights into the main plant bottlenecks.