(631a) Multi-Scale Modeling and Control of Autothermal Reactors for the Production of Hydrogen

The interest in economically efficient hydrogen production has been steadily increasing, and even more so recently, given the progress made in the development and implementation of fuel cell technologies. Autothermal reactors, combining exothermic and endothermic reactions are one of the most promising hydrogen production technologies, featuring in-situ heat generation, which allows for increased fuel efficiency and a compact size. From a design and operation point of view, autothermal reactors rely either on a constant, unidirectional flow, whereby the raw material for hydrogen production and the necessary fuel flow in different, parallel channels of the reactor (either in co-current or counter-current), or on flow reversal, in which case the catalyst bed within the reactor acts as a heat trap.

The majority of the research studies concerning the design and operation of autothermal reactors of either category investigate the steady state behavior of the system. However, in the context of integrating such reactors in larger systems that include fuel cells for power production, the transient operation of the autothermal reactor, enabling variable levels of hydrogen supply in response to varying power requirements to a fuel cell downstream becomes much more interesting. Thus, the availability of accurate, reliable and, at the same time, computationally efficient models is of great importance for dynamic analysis and control. Due to the inherent multiple-time scale behavior of autothermal reactors, their dynamic models are ill conditioned and challenging to simulate, and recent efforts have been aimed at deriving models of reduced dimensions that capture the salient dynamic features of the original system [1].

In this work, we initially approach the study of the dynamics of autothermal reactors in a generic manner. Using arguments from singular perturbation theory, we document the existence of two distinct time scales in their dynamic behavior, indicate the physical parameters that are at the origin of this dynamic feature and identify the variables that are associated with each time scale. Employing an asymptotic analysis, we derive reduced-order, nonlinear models for the dynamics in each time scale. In particular, we show that the derived slow model corresponds to generally accepted empirically derived simplified models for the class of reactors considered.

Subsequently, we present comparative dynamic simulation results using the rigorous model of two hydrogen production reactors, using, respectively, co-current and counter-current flow. We demonstrate that the corresponding reduced-order models capture the slow dynamics of the reactors very well. We compare the issues posed by the transient operation of each reactor, including the potential for reactor extinction under normal operating circumstances. Also, we interpret the mechanisms by which changes in production rate or disturbances in the operating parameters can cause the reactors to transition from an ignited to an extinguished state. We enumerate some of the challenges associated with the implementation of feedback control to alleviate the issues encountered in running the autothermal reactors considered. Finally, we demonstrate the implementation of linear and nonlinear model-based control algorithms and show that the transient operation of an autothermal reactor is improved by the implementation of feedback control.

[1] Gorbach, A., Eigenberger, G., Kolios, G., 2005. General approach for the reduction of detailed models for fast cycling processes. Ind. Eng. Chem. Res. 44, 2369--2381.