A HIERARCHIC CONTROL STRUCTURE APPLIED TO AN AMMONIA TUBULAR REACTOR WITH COLD SHOT COOLING
Hierarchical control structures are useful in highly integrated processes, since they decompose a complex system in various subsystems, which may have different objectives (e.g. stabilization, performance, optimization) or scales of time. In this work, a hierarchical structure for the control of an ammonia reactor with cold-shot cooling, which is highly nonlinear and strongly coupled, is proposed. This hierarchical control system has three levels. The first level is composed of PI type controllers, and its main objective is stabilization. The second level is composed of an interpolated controller that provides the reference temperature for the control of the first bed of the reactor. Its objective is to extend the operating range of the controller and therefore, to improve the performance. The Third level is composed of an optimizer whose objective is to maximize the ammonia production. The results show that the hierarchical structure results in a robust and stable controller in a large operating range, keeping the control outputs on its reference while rejecting the disturbances.
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