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Research Papers: Fundamental Issues and Canonical Flows

Joint Numerical–Experimental Investigation of Enhanced Chemical Reactivity in Microfibrous Materials for Desulfurization

[+] Author and Article Information
Ravi K. Duggirala

Mercedes-Benz Research &
Development India Pvt. Ltd.,
Bangalore 560 066, India

Christopher J. Roy

Professor
Aerospace & Ocean Engineering,
Virginia Polytechnic Institute and State University,
Blacksburg, VA 24061

Priyanka Dhage

Intel Corp.,
Hillsboro, OR 97124

Bruce J. Tatarchuk

Center for Microfibrous Material Manufacturing,
Auburn University,
Auburn, AL 36849

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received May 1, 2014; final manuscript received July 31, 2014; published online November 6, 2014. Assoc. Editor: Francine Battaglia.

J. Fluids Eng 137(3), 031204 (Nov 06, 2014) (10 pages) Paper No: FE-14-1236; doi: 10.1115/1.4028602 History: Received May 01, 2014; Revised July 31, 2014

Ultra-pure hydrogen is very much required for a healthy operation of proton exchange membrane (PEM) fuel cells. The concentration of sulfur in the fuel is an important controlling factor because it leads to pollution via sulfur oxides. H2S sorbent or catalysts coated on the particles that are in the order of 100 μm diameters entrapped into a high void volume carrier structure of sintered microfibers are observed to possess significantly higher heterogeneous reaction rates than packed beds of the small particle size. Fundamental reasons for this difference are investigated in this study to determine if such differences are caused by: (1) bed channeling, (2) microscale interstitial/interparticle velocity distributions, and/or (3) effect of presence of fibers. Since microscale fluid effects are not accounted for in traditional reaction engineering formulations, more rigorous approaches to the fluid flow, gaseous diffusion and surface reaction behaviors for a ZnO-based H2S sorbent have been undertaken using computational fluid dynamics (CFD). Simulation results have been compared with carefully prepared experimental samples of microfibrous materials. The experiments involved 14 wt.% ZnO/SiO2 at an operating temperature of 400 °C and a challenge gas consisting of 0.5 vol. % of H2S in H2 and were used to validate the CFD models (both geometric and species transport). These results show that CFD predictions of chemical conversion of H2S are within 10–15% of the experimentally measured values. The effects of residence time and dilution with void on the chemical conversion have been studied. Different microfibrous materials were modeled to study the effect of fiber diameter and fiber loading on the chemical conversion and pressure drop. It is observed that the dilution with void has a negative effect on the conversion; however, the addition of fibers not only compensated for the negative effect of dilution but also increased the reaction rate. The main goal of this study is to use CFD as a tool to design new materials with enhanced reactivity and reduced pressure drop. Our work suggests that new materials with enhanced chemical reactivity for a given pressure drop should be designed with fewer, larger diameter fibers. Our results show that the logs of reduction of H2S per pressure drop increased by a factor of six for the material with 8 μm diameter fibers with 3% volume fraction relative to a packed bed with same catalyst loading.

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References

Figures

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Fig. 1

Al2O3 particles entrapped in the matrix of 8 μm Ni fibers

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Fig. 2

ln((cinlet/coutlet)-1) versus time plot to measure log reduction

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Fig. 3

(a) Unit cell of the MFECS used in the simulations; (b) CFD model of the MFECS; and (c) SEM of a typical MFECS

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Fig. 4

Flow-field mesh: (a) triangular mesh on the fiber and particle walls and (b) triangular mesh on the top symmetry face with a prismatic boundary layer mesh shown around the particles and fibers

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Fig. 5

Comparison of log reduction of H2S obtained from simulations and experimental calculations (uncertainty in simulations is 3%)

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Fig. 6

Heterogeneous contacting efficiency of packed beds versus MFECS

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Fig. 7

3D Geometric model: (a) packed bed with 60 vol. % loading and (b) diluted bed with 20 vol. % loading

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Fig. 8

Effect of dilution: variation of log reduction with Reynolds number (uncertainty in simulations due to numerical approximations is ±3%)

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Fig. 9

3D Geometric model: (a) frozen bed with 20 vol. % loading and (b) MFECS with 20% catalyst and 3% of 2 μm fibers

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Fig. 10

Effect of fibers on chemical conversion (uncertainty in simulations due to numerical approximations is ±3%)

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Fig. 11

Velocity vectors (x and y component) extracted from a plane of the 3D simulation: (a) packed bed (60 vol. % loading); (b) frozen bed (20 vol. % loading); and (c) microfibrous material (20 vol. % loading and 1.5% of 4 μm fibers)

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Fig. 12

Variation of pressure drop with Reynolds number (uncertainty in simulations is 6%)

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Fig. 13

Comparison of heterogeneous contacting efficiency (ηHCE): (a) varying fiber diameter and (b) varying fiber loading

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