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 Thšming (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. Thšming, 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.