Delayed binary and multicomponent gas diffusion in conical tubes

• Mass transport is delayed by the conical geometry of tubes representing pores.
• Delay effect increases with ratio of inlet to outlet tube radius.
• Development and mathematical description of a novel steady-state diffusion cell.
• Comparability to the transient two-bulb-diffusion-cell.

Catalyst pores are typically non-uniform along their longitudinal axis, and the transport of gaseous reactants and products takes place in a somehow tapered confinement. In a previous study we observed a diffusion delay in single tapered pores by means of a transient two-bulb-diffusion-cell (Veltzke et al., 2015). Processes in heterogeneous catalysis, however, are typically operated under steady state conditions. Hence also the diffusion processes are non-transient and reactant species are permanently consumed while product species steadily emerge. To mimic steady-state multicomponent diffusion in a cone, we developed a novel two-pipe-diffusion-cell and described the mass transport by an analytical model.Here we can show that the delay effect, which is caused by volumetric changes in longitudinal direction, also exists for steady-state binary and multicomponent diffusion. It is experimentally confirmed that the diffusion hindrance increases with conicity of the test tube. Also the results are transferable to those of the transient two-bulb-diffusion-cell. The measurement of steady-state experiments, however, is much faster.

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