(578c) Counter-Current Autothermal Reactors For Hydrogen Generation: Modeling And Dynamic Analysis

Increasing environmental and political pressure to mitigate the effects of global warming has fostered a growing interest in renewable and clean energy resources. Amongst others, Ethanol- and Hydrogen-based transportation and energy generation solutions have been developed. While Ethanol can be derived from US-based resources such as corn via well known processes, the quest for efficient and economical hydrogen sources continues. A very promising technology in this field are autothermal reactors that combine endothermic hydrogen production reactions and exothermic reactions that create the thermal energy necesary to support hydrogen generation, occurring in parallel channels. Such devices tend to be compact and would lend themselves to on-board or on-site hydrogen generation for transportation/energy production applications.

Such applications typically assume that hydrogen can be produced at a variable rate that can satisfy the changing demands of a downstream user or device, therefore placing some dynamic requirements on the autothermal reactors, in addition to specifying steady-state performance criteria. In our prior work [1], we have investigated the dynamics of unidirectional autothermal reactors featuring hydrogen generation via endothermic methane steam reforming and water gas shift, occurring in co-current with the exothermic catalytic combustion of methane. We demonstrated that such systems exhibit a two time scale behavior. Using singular perturbation arguments, we traced the origin of the fast dynamics of the fluid phase and the slow dynamics of the reactor walls to the difference in heat capacity between the solid and the gas phase. Also, it was shown that, under certain (encountered) operating conditions, reactor extinction is possible and we explained the mechanisms by which this phenomenon occurs. Moreover, we proved that these phenomena cannot be avoided in the absence of a control system and proposed a simple control strategy to improve dynamic performance.

In the present paper, we focus on autothermal hydrogen generation using counter-current flow reactors. Such reactors pose distinct challenges, and we employ a case study to demonstrate that simply reversing the fuel flow in a reactor designed for co-current operation results in a drop in conversion and poor dynamic performance. Based on the simulation results and physical considerations, we explain the aforementioned findings, and propose a reactor redesign based on modifying the catalyst distribution that improves both steady-state conversion and provides for a more robust operation during load changes.

Drawing on a previously introduced model-reduction technique [1], we derive a control-relevant reduced-order model of the reactor and use it to synthesize a state observer for predicting the conversion and product distribution of the reactor, parameters that are seldom available on-line due to the slow turnaround time of analytical equipment [2].

All the novel developments proposed in the paper are illustrated through simulations using a first-principles detailed model of an autothermal reactor for hydrogen generation.

References [1] Baldea, M. and Daoutidis, P., Dynamics and Control of Autothermal Reactors for the Production of Hydrogen, Chem. Eng. Sci., 64, 2007, 3218-3230. [2] Görgün, H., Arcak, M., Varigonda, S. and Bortoff, S.A., Observer designs for fuel processing reactors in fuel cell power systems, Int. J. Hydrogen Energy, 30, 2005, 447-457