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This paper describes the design and application of an optimized control system, based on adaptive predictive expert control methodology, to the sulfur recovery units (SRUs) of the Pemex Refinery in Cadereyta, and presents comparative results obtained in relation to those of the conventional control system operating the plant. The sulfur recovery process represents the final stage in the refinery production chain, and is responsible for removing the sulfur content, usually in the form of hydrogen sulfide (H 2 S) waste gas generated by other refinery processes. The main goal is minimizing the sulfur content in the gasses released into the atmosphere to comply with emission level requirements. This goal can be obtained by a precise control of the ratio between hydrogen sulfide and sulfur dioxide (SO 2 ) in the tail gas, which determines, through a Claus reaction, the maximum sulfur recovery. The SRUs operating in Cadereyta were connected to a common amine acid gas collector and a common ammonia acid gas collector. Optimized control strategies were designed to control the pressure on both acid gas collectors and the flow rate of air entering each SRU. The results show that the H 2 S : SO 2 ratio in the tail gas is considerably more stable under optimized control, particularly in the presence of abrupt changes in the acid gas load. The peaks observed in this ratio under conventional control disappear under optimized control showing a significant improvement in the process operation. Optimized control strategies for pressure in the acid gas common headerThe OCS for pressure in the acid gas common header is shown in Figure 5. Experience shows that pressure in the acid gas lines for the various SRUs is very close and representative of the pressure in the acid gas common header. For this reason, a good estimate of the pressure in the acid gas common header is the maximum pressure in the gas lines. As can be seen on the left side of Figure 5, this maximum value enters as process variable (PV) in the master ADEX controller for the acid gas pressure. This controller receives the measurement of the acid gas flow rates produced by the various plants of the refinery, as perturbations to be taken into account in the predictive model. Figure 5. Diagram of the optimized control strategies for pressure in the acid gas common header.The control signal produced by the master ADEX controller for the acid gas pressure is a master signal, which determines the total acid gas flow rate that needs to be absorbed by the set of all SRUs. This total flow rate of acid gas is distributed by a 'load manager', which sends the set point to the acid gas flow rate ADEX controller of each SRU. The distribution is made to equalize the percentage of acid flow rate treated by each SRU, calculated in relation to the maximum and minimum of the range of variation of these flow rates in each SRU.On the right hand side of Figure 5, it can be seen how the ADEX flow controllers determine the position of the corresponding flow control valve for each SRU. It can...

Hybrid systems are those that inherently combine discrete and continuous dynamics. This paper considers the hybrid system model to be an extension of the discrete automata associating a continuous evolution with each discrete state. This model is called the hybrid automaton. In this work, we achieve a mathematical formulation of the steady state and we show a way to obtain the initial conditions region to reach a specific limit cycle for a class of uncoupled and coupled continuous-linear hybrid systems. The continuous-linear term is used in the sense of the system theory and, in this sense, continuous-linear hybrid automata will be defined. Thus, some properties and theorems that govern the hybrid automata dynamic behavior to evaluate a limit cycle existence have been established; this content is explained under a theoretical framework.

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