(72g) Control of Processes Whose Optimal Operating Conditions Are on the Saturation Edge

Hydrocarbon reforming can be used to produce economically hydrogen, a source for the proton exchange membrane fuel cell. The hydrogen-rich gas from hydrocarbon reforming reactors can have CO up to several thousand ppm level which poisons the anode catalyst of the fuel cell. The CO concentration need be removed below 10ppm. The preferential oxidation (PROX) is one of the simplest ways for this purpose. It uses the following oxidations:

     CO+0.5O₂ --> CO₂

     H₂+0.5O₂ --> H₂O

In above reactions, the CO oxidation should occur preferentially over the H₂ oxidation. The copper-cerium oxide catalyst of CuO-CeO₂ has been reported to have high activity and selectivity for CO oxidation. The CO PROX reactors with CuO-CeO₂ catalyst can remove CO from several thousand ppm to less than 10ppm. To design this PROX reactor, kinetics have been obtained by Lee and Kim (“Kinetics of CO and H₂ oxidation over CuO-CeO₂ catalyst in H₂ mixtures with CO₂ and H₂O,” Catal Today 2008;132;109-116). Rigorous simulations have been done by Kim et al. (“Preferential CO oxidation over CuO-CeO₂ in excess hydrogen: Effective factors of catalyst particles and temperature window for CO removal,” Int. J. Hydrogen Energy 2013;38;4429-4436). In this PROX reactor, limited O₂ should be added and O₂ should not exist in the exit stream for the optimal operations. A large amount of O₂ feed results in O₂ in the exit stream, reducing the efficiency of the PROX reactor. A small amount of O₂ feed results in high CO concentration in the exit stream and the objectives of the PROX reactor will not be met. The oxygen feed should be controlled on the edge just before O₂ appears in the exit stream.

There are several processes where optimal operational conditions are on the saturation edge as shown in the above PROX reactor. For example, the reactor temperature of polymerization reactor is given by the vaporization temperature of monomer and the optimal heat input is such that the reactor temperature should be on this saturation temperature edge. Additional heat increases the boil-up amount of monomer and the additional amount of cooling water should be needed. Some recycle systems with excess and limiting reagents give these control problems that maintain operating points on the saturation edges.

When the controlled variable is saturated, the variable has no information for feedback and the conventional control cannot be used. Control methods based on the sliding mode method and extremum seeking control method with coordinate system transformations are suggested and their performances are compared.