(350g) Assessment of Multicomponent Flux Models at Increased Rarefaction for Application in Heterogeneous Catalysis

Assessment of
Multicomponent Flux Models at Increased Rarefaction for Application in Heterogeneous
Catalysis

Lars Kiewidt (kiewidt@uni-bremen.de), Thomas Veltzke
(tveltzke@uni-bremen.de),

and Jorg Th?ming
(thoeming@uni-bremen.de)

Center for Environmental
Research and Sustainable Technology (UFT), University of Bremen, Leobener
Straµe, 28359 Bremen, Germany

Diffusive transport of multicomponent gas
mixtures in porous media and microchannels plays an important role in numerous
technical applications, for example in gas separation, fuel cells, and
heterogeneous catalysis. Furthermore, it often determines the integral behavior
and efficiency of the overall process. Understanding and modeling the
underlying transport mechanisms, for example molecular and Knudsen diffusion,
is thus key for further process intensification.

In this project, we focus on heterogeneous
catalysis. Figure 1a shows a schematic of counter diffusion in a catalyst pore:
Products E enter the pore and react to products P that leave the pore into the
bulk phase. In Fig. 1b the counter-diffusive process is modeled as a two bulb
diffusion cell connected by a tapered duct. Due to the counter-diffusive nature
of the process, multicomponent effects, for example osmotic diffusion, reverse
diffusion, and diffusion barriers, are likely to have a strong influence on the
performance of the catalyst, that is reactants might be pushed away from active
sites by leaving products. Further, entering reactants might hinder products
form leaving the pore and thus limit catalyst efficiency.

Figure 1: Counter-diffusion in a catalyst pore (a). Reactants E
enter pore and react to products P that leave the pore into the bulk phase. The
pore can be approximated by a two bulb diffusion cell connected by a tapered
duct (b).

In our previous study [1], we have already applied
the approach of a two bulb diffusion cell successfully, and demonstrated
analytically and experimentally that molecular multicomponent diffusion is
significantly slowed down in tapered ducts compared to molecular diffusion in uniform ducts. In this study we now investigate the
influence of increasing rarefaction, i.e., smaller ducts, on the counter-diffusive
transport of multicomponent gas mixtures. Under increasing rarefaction, the
influence of multicomponent effects should decrease because they arise from
molecule-molecule collisions of different species. Although Kerkhof's Binary Friction Model (BFM) [2] and Young's and
Todd's Cylindrical Pore
Interpolation Model (CPIM) [3], both successors of the widely applied Dusty Gas Model (DGM), were validated
for diffusion in porous plugs, and applied in real-world reaction-diffusion
problems [4], a systematic analysis of the contribution of molecular and
Knudsen diffusion, and the description of multicomponent effects with increasing
rarefaction is not available.

Therefore, we conducted multicomponent
counter diffusion experiments in a two bulb diffusion cell under increased
rarefaction, and analyzed the validity and accuracy of the above-mentioned
models. Further, we applied the models to simulate real-world
reaction-diffusion problems, and assessed their influence on the prediction of
catalyst efficiency.

References

[1]       T. Veltzke, L. Kiewidt, J.
Th?ming, Multicomponent gas diffusion in nonuniform tubes, AIChE J. 61 (2015)
1404–1412.

[2]       P. Kerkhof, A modified
Maxwell-Stefan model for transport through inert membranes: the binary friction
model, Chem. Eng. J. 64 (1996).

[3]       J.B.B. Young, B. Todd, Modelling
of multi-component gas flows in capillaries and porous solids, Int. J. Heat
Mass Transf. 48 (2005) 5338–5353.

[4]       J. Lim, J. Dennis, Modeling
reaction and diffusion in a spherical catalyst pellet using multicomponent flux
models, Ind. Eng. Chem. Res. 51 (2012) 15901–15911.