(470g) Modeling Transport and Reaction in Porous Catalyst Washcoat for Steam Methane Reforming in a Microchannel Reactor By CFD with Elementary Kinetics

Modeling transport and reaction in porous catalyst washcoat for steam methane reforming in a microchannel reactor by CFD with elementary kinetics

Chenxi Cao, Nian Zhang, Yi Cheng*

Department of Chemical Engineering, Tsinghua University, Beijing 100084, P.R.China

Email: yicheng@tsinghua.edu.cn

Towards an upcoming era of hydrogen economy, there is an increasing need in distributed hydrogen production for hydrogen fuelling stations, fuel cell systems and so forth. Currently, steam methane reforming (SMR) is the most widely used and cost-effective route for world-wide hydrogen production. The conventional fixed-bed based process for hydrogen production in centralized plants is unsuitable for distributed use for its drawbacks such as heat transfer and diffusional limitations, catalyst deactivation and large footprint, which has led to large research interests in the intensification of SMR using microchannel reactors. Micro-channel reactors have the advantages of highly efficient enhancement of heat and mass transfer, easy scale-up and fast startup times [1-2]. Thus, SMR in microchannel reactors have great potential in reducing capital cost and energy consumption and to realize a small-scale and compact application as required by distributed hydrogen production.

CFD simulation is a crucial tool for reactor design and process optimization, especially in the case of microchannel reactors in which experimental measurements of profile data within the reaction channel is difficult. CFD simulation  has been extensively used by researchers [3-6] for SMR in microchannel reactors with Ni and Rh as reforming catalyst, but the majority of which employ global kinetics and/or neglect the diffusional effects in the catalyst washcoat, For the millisecond SMR in microchannel reactors, effective usage of the interior surface of the washcoat may be important for Ni catalyst over which the intrinsic reaction rates of SMR is slower. Also, elementary reaction kinetics are preferred to global kinetics for ultra-fast reactions and can provide key information of the reaction path, rate-controlling step and surface coverage of key species on the catalyst throughout the washcoat. This work presents a comprehensive CFD modeling and simulation of a microchannel reactor, incorporating the elementary reaction kinetics of SMR over Ni and the modeling of the catalyst washcoat on the channel wall. Special focus is put on the transport and reaction in porous washcoat of the reforming channel.

The modeling and simulation is conducted in FLUENT 14.0 software linked with an external in-house stiff ODE solver for the elementary reaction kinetics network. An isotropic porous medium model is used to describe the momentum and heat transfer in the catalyst washcoat. Diffusion in the porous washcoat is described by an effective diffusivity model that takes into account both molecular diffusion and Knudsen diffusion. All properties used in the simulation are temperature dependent and no lumped model for heat and transfer is used. The model is quantitatively validated by our experimental data. The influence of catalyst thickness, porosity, pore diameter and operational factors such as WHSV on the reactor performance is investigated. Effectiveness factors and Damköhler number of the catalyst are determined under various conditions and allow for discussions of control regime and catalyst utilization ratio. It is revealed that the control regime shifts from reaction to diffusion when the washcoat thickness increases from 10 μm to 100 μm with the total catalyst loading kept constant under normal operating conditions. Porosity and pore diameter have minor effects on the reactor performance compared with catalyst thickness. Increasing the catalyst washcoat thickness from 25 μm to 50 μm with the catalyst loading per washcoat volume kept constant causes a more than 50 % drop in WHSV to achieve the same methane conversion of 90 %. It is also demonstrated that serious consideration should be taken on the diffusional effects in the catalyst washcoat when simulating heat coupling, as internal diffusion changes the overall reaction rates profile along the reaction channel and thus leads to different results in heat matching. Detailed reaction path analysis is further performed for on the basis of the presented simulation framework.

Keywords: Hydrogen production; Steam methane reforming; Micro-channel reactor; CFD with elementary reaction kinetics; porous catalyst washcoat

References

[1]  Tonkovich AY, Perry S, Wang Y, Qiu D, LaPlante T, Rogers WA. Microchannel process technology for compact methane steam reforming. Chem Eng Sci 2004;59:4819-24.

[2]  Johnson BR, Canfield NL, Tran DN, Dagle RA, Li XS, Holladay JD, et al. Engineered SMR catalysts based on hydrothermally stable, porous, ceramic supports for microchannel reactors. Catal Today 2007;120:54-62.

[3]  Stefanidis GD, Vlachos DG, Kaisare NS, Maestri M. Methane Steam Reforming at Microscales: Operation Strategies for Variable Power Output at Millisecond Contact Times. AlChE J 2009;55:180-91.

[4]  Zanfir M, Baldea M, Daoutidis P. Optimizing the Catalyst Distribution for Countercurrent Methane Steam Reforming in Plate Reactors. AlChE J 2011;57:2518-28.

[5]  Wang F, Zhou J, Wang GQ. Transport characteristic study of methane steam reforming coupling methane catalytic combustion for hydrogen production. Int J Hydrogen Energy 2012;37:13013-21.

[6]  Jeon SW, Yoon WJ, Baek C, Kim Y. Minimization of hot spot in a microchannel reactor for steam reforming of methane with the stripe combustion catalyst layer. Int J Hydrogen Energy 2013;38:13982-90.