(202n) Control Systems for a Preferential CO Oxidation Reactor

Carbon monoxide (CO) in a H₂ rich mixture gas is a poison to the proton exchange membrane fuel cells (PEMFC). Preferential oxidation of CO (PROX) is one of the promising methods to reduce the CO concentration below 10 ppm in a H₂ rich mixture gas with a small amount of CO (1-2%) from the reformer. It has been reported to be realized via CuO-CeO₂ catalysts and a tubular reactor having high thermal conductivity. Although the oxidation of CO occurs preferentially over H₂, the oxidation of H₂ also occurs. Therefore, to minimize the consumption of H₂, a limited amount of O₂ should be fed. In this case, the conversion of CO increases first and then decreases as the reactor temperature increases. In the PROX process optimization to reduce CO concentration up to 10 ppm while minimizing the consumption of H₂, input multiplicities between the reactor temperature and CO conversion occur. These characteristics of a PROX reactor based on CuO-CeO₂ catalysts are verified from detailed reaction kinetics [1] and simulations [2]. The processes with input multiplicities have been reported to be very difficult to control. Implementing IMC controllers to processes with input multiplicities, the phenomena of "sudden destabilization" have been shown [3]. To avoid this, operations far from process conditions showing input multiplicities have been reported. However, it cannot be a solution to forgive the maximum efficiencies. To control processes with input multiplicities, control systems based on detailed nonlinear models are required. These models include physico-chemical models having minimum number of parameters and general black-box models. Various processes such as reaction, separation and biological processes show input multiplicities, especially, at the operation conditions of maximum efficiencies. For safeties of processes newly-developed and their competitiveness, this research is important. Especially, PROX which is dealt as an example of processes with input multiplicities has its own importance. The aims of this research are to establish control systems for processes with input multiplicities by concentrating on the optimizing control of PROX.

References

[1] H.C. Lee, D.H. Kim, “Kinetics of CO and H₂ Oxidation over CuO-CeO₂ Catalyst in H₂ Mixtures with CO₂ and H₂O,” Catalysis Today, 132, 109-116, 2008.

[2] D.H. Kim, D.R. Park, J. Lee, “Preferential CO Oxidation over CuO-CeO₂ in Excess Hydrogen: Effectiveness Factors of Catalyst Particles and Temperature Window for CO Removal,” Int. J. Hydrogen Energy, 38, 4429-4436, 2013.

[3] P.B. Sistu, B.W. Bequette, "Model Predictive Control of Processes with Input Multiplicities," Chem. Eng. Sci., 50, 921-936, 1995.