(555d) Pseudo-Distributed Feed Configurations for Catalytic Plate Microchannel Reactors

Pseudo-Distributed Feed Configurations for Catalytic Plate Microchannel Reactors

Richard Pattison and Forrest Estep and Michael Baldea

Department of Chemical Engineering

The University of Texas at Austin, 1 University Station C0400, Austin, TX 78712

email: mbaldea@che.utexas.edu

Catalytic plate microchannel reactors (CPRs) are one of the most successful implementations of process intensification. They have been recognized as important contributors in the path towards monetizing stranded and associated natural gas resources through small scale gas-to-liquids (GTL) processing at the well head [1-2]. Microchannel architectures minimize transport limitations and result in devices with high surface-area-to-volume ratios; typical CPR dimensions are an order of magnitude smaller than those of conventional equipment with comparable processing capacity [3].

Despite their economic benefits, microchannel reactors pose significant control and operational challenges. Steam methane reforming, a very likely first step in the GTL process, is highly endothermic. When carried out in CPRs, reforming is supported by the exothermic combustion of methane, with the two reactions occurring in alternate channels. Synchronizing heat generation and consumption along the reactor is particularly challenging and, if not properly addressed, could result in temperature hot spots that can compromise the integrity of the reactor [4-6]. In conventional reactors, these issues are typically addressed by judiciously distributing the feed streams along the reactor to modulate the rate of heat generation and consumption. Physical considerations (including the lack of availability of distributed actuators at the required small dimensions) limit the application of this strategy to microchannel reactor stacks.

In this paper, we propose a novel approach for controlling the axial temperature profiles in microchannel reactors. Specifically, we propose using a segmented catalyst macromorphology, whereby the catalyst coating in the microchannels alternates between catalytically active and inactive (“blank”) segments. We introduce an optimization-based strategy for simultaneously selecting a parameterized temperature profile that guarantees high conversion and reduced maximum temperatures, and the optimal locations, lengths, and number of segments along the reactor length that ensures close tracking of the optimal temperature profile.

We demonstrate the effectiveness of the proposed optimization approach using a detailed 2-dimensional autothermal steam methane reforming CPR model. Compared to a base-case reactor with non-segmented catalyst geometry, the resulting reactor exhibits less variability in the axial temperature profile, a significant reduction in the maximum steady state reactor temperature, and higher conversions in both the reforming and combustion channels. To further analyze the results, we demonstrate that the temperature and conversion profiles obtained using the optimal segmented catalyst morphology are very similar to those obtained in a hypothetical reactor with multiple, axially distributed feed locations.

Furthermore, the operation of the reactor demonstrates improved dynamic robustness compared to the base-case when subject to composition and flow rate disturbances in the reforming channel. A feedback-feedforward linearizing controller is implemented that adjusts the flow rate to the combustion channel to compensate for these fluctuations. The maximum reactor temperature throughout a disturbance sequence is significantly reduced, and temperature variability along the reactor is minimal.

References

[1] Ogugbue, C.; Chukwu, G.; Khataniar, S. Economics of GTL technology for gas utilization. Hydrocarbon Economics and Evaluation Symposium, Dallas, TX, 2007.

[2] Hall, K.R. A new gas to liquids (GTL) or gas to ethylene (GTE) technology. Catalysis today 2005, 106, 243-246.

[3] Zanfir, M.; Gavriilidis, A. Catalytic combustion assisted methane steam reforming in a catalytic plat reactor. Chem. Eng. Sci. 2003, 58, 3947-3960.

[4] Baldea, M.; Daoutidis, P. Dynamics and Control of Autothermal Reactors for the Production of Hydrogen. Chem. Eng. Sci. 2007, 62, 3218-3230.

[5] Zanfir, M.; Gavriilidis, A. Influence of flow arrangement in catalytic plate reactors for methane steam reforming. Chem. Eng. Res. & Des. 2004, 82, 252-258.

[6] Vaccaro, S.; Malangone, L.; Ciambelli, P. Modelling of a catalytic micro-reactor coupling endothermic methane reforming and combustion. Int. J. of Chem. Reactor Eng. 2010, 8.