(98h) Effect of the Characteristic Dimension of Catalytic Wall Microchannels and Microslits on the Performance of Microreactors Coupling the Methane Steam Reforming and Combustion Reactions: A CFD Simulation Study

G. Arzamendi¹,
I.
Uriz¹, P.M. Diéguez¹, M. Montes², M.A. Centeno²,
J.A. Odriozola³, and L.M. Gandía^1,*

¹Departamento
de Química Aplicada. Universidad Pública de Navarra, Campus de Arrosadía s/n,
E-31006
Pamplona.

Spain

²Grupo de Ingeniería Química, Departamento de Química
Aplicada, Facultad de Ciencias Químicas de San Sebastián, UPV/EHU, Paseo Manuel
de Lardizábal 3, 20018 San Sebastián,
Spain

³Instituto de Ciencia de Materiales de Sevilla, Centro
Mixto CSIC-Universidad de Sevilla, Avda. Américo Vespucio 49, 41092
Sevilla, Spain

^*Corresponding author. E-mail address: lgandia@unavarra.es (L.M. Gandía)

Both H₂ and syngas
(synthesis gas, a mixture of H₂ and CO) are expected to play an
increasingly important role in our energetic system. H₂/syngas
production technologies can be integrated in Natural Gas Combined Cycle plants for
precombustion CO₂ capture, in advanced systems as the Integrated
Gasification Combined Cycle with or without capture of CO₂ for
future coal-based power plants and, of course, for synthetic liquid fuels
production through the gas-to-liquids (GTL) and coal/biomass-to-liquids
(CTL/BTL) processes [1]. Syngas can be used also as fuel for high-temperature
solid oxide and molten carbonate fuel cells [2].

Steam reforming of natural gas in
conventional packed-bed reactors is the preferred technology for commercial
large-scale H₂ and syngas manufacture. However, for small-scale
stationary or mobile/portable applications as well as production offshore or in
remote areas, microreaction technology offers a convenient solution [3-5].

In this work, a Computational Fluid
Dynamics (CFD) simulation study on the thermal integration of the endothermic steam
reforming of methane (SRM) and exothermic methane combustion reactions in
microstructured devices is presented. In a previous study on this system, the
effects on the performance of catalytic wall microchannels of the gas streams
space velocities, SRM catalyst loading and steam-to-carbon (S/C) ratio were
investigated [6]. In this work, the effect of the characteristic dimension (d = 0.35, 0.70, 1.40 and 2.80 mm) of square
microchannels of 20 mm
of length on the microrreactor performance is shown. Moreover, microslides are
also considered as a new geometry with very different aspect ratio compared
with square microchannels. The characteristic dimension of microslides has been
also varied between 0.35 and 2.80 mm whereas their length has been kept at 20 mm. CFD models of these
geometries have been developed with commercial ANSYS^® CFX software
considering both parallel and cross flow arrangement. It has been assumed that
thin layers of typical Ni and Pd catalysts for the SRM and methane combustion
reactions, respectively, have been deposited onto the walls of the
microchannels or microslides. The loadings were established at 2-4 mg/cm²
and 1 mg/cm² for the Ni and Pd catalysts, respectively. Heterogeneous
catalytic reactions (SRM, water-gas shift-WGS, and methane combustion in air)
have been modeled considering the inner walls as sources of products and sinks
of reactants. Kinetic expressions of the relevant catalytic reactions have been
taken from the literature [6]. The feedstreams consisted of methane and steam
with S/C = 2 and 2 wt.% methane in air for the SRM and combustion reactions; the
inlet temperature was set at 600ºC for both streams.

Steady state simulations have
evidenced a marked influence of the characteristic dimension of both channels
and slides on the microrreactor performance. For example, at GHSV of 10000 h^-1
the SRM methane outlet conversion decreases according to 99.9, 99, 96 and 87%
as the microchannel characteristic dimension increases as 0.35. 0.70, 1.40 and 2.80 mm, respectively. In
contrast, the selectivity was almost unaffected, resulting a reformate with a H₂/CO
ratio close to 3.8. To take into account the very different surface-to-volume
ratio associated to each dimension the performance was compared also on a WHSV
basis. Although the differences decreased the performance still improved with
the decrease of the characteristic dimension. For example, at WSHS of 13542 h^-1
the microchannels with d = 0.35 mm yielded 93% methane
conversion while those with d = 2.80 mm gave 87%. An
analysis of the results has shown that mass transport limitations became an
issue for the largest characteristic size as evidenced by concentration
profiles as the ones that can be seen in Figure 1. Therefore, decreasing the
characteristic dimension has the advantages of a higher surface-to-volume ratio
and a lower influence of transport limitations.

When comparing the two geometries
considered in this work it is found that the performance of the microchannels
is better than that of the microslides; however, the differences are small, of
the order of 3-6 percent units of methane conversion at the reactor exit. This
is due to the improved heat transfer characteristics and surface-to-volume
ratio of microchannels compared with the microslides. Nevertheless, the
microslides can be manufactured more easily and at lower cost so they are an
interesting option to develop steam methane microreformers.

Literature cited

1. Wei
W, Kulkarni, P, Liu K. in: Hydrogen and Syngas Production and Purification
Technologies (Eds.: Liu K, Song C, Subramani V). AIChE ? John Wiley & Sons
Inc.,
Hoboken, NJ. 2010, p. 451-485.

2. Song
C. in: Hydrogen and Syngas Production and Purification Technologies (Eds.: Liu
K, Song C, Subramani V). AIChE ? John Wiley & Sons Inc.,
Hoboken, NJ.
2010, p. 1-13.

3. Tonkovich
AY, Perry S, Wang Y, Qiu D, LaPlante T, Rogers WA. Microchannel process
technology for compact methane steam reforming. Chem.
Eng. Sci. 2004;59:4819-4824.

4. Tonkovich
ALY, Yang B, Perry ST, Fitzgerald SP, Wang Y. From seconds to milliseconds to
microseconds through tailored microchannel reactor design of a steam methane
reformer. Catal. Today 2007;120:21-29.

5. Subramani V, Sharma P, Zhang L,
Liu K. in: Hydrogen and Syngas Production and Purification Technologies (Eds.:
Liu K, Song C, Subramani V). AIChE ? John Wiley & Sons Inc.,
Hoboken, NJ.
2010, p. 14-126.

6. Arzamendi G, Diéguez PM, Montes
M, Odriozola JA, Falabella Sousa-Aguiar E, Gandía LM. Methane steam reforming
in a microchannel reactor for GTL intensification: A computational fluid
dynamics simulation study. Chem.
Eng. J. 2010;154:168-173.