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Abstract:
Coated-wall flow tube reactors are frequently used to investigate gas uptake and heterogeneous or multiphase reaction kinetics under laminar flow conditions. Coating surface roughness may potentially distort the laminar flow pattern, induce turbulence and introduce uncertainties in the calculated uptake coefficient based on molecular diffusion assumptions (e.g., Brown/CKD/KPS methods), which hasn't been sufficiently addressed in previous applications. Here we investigate the influence of surface roughness and local turbulence on coated-wall flow tube experiments for gas uptake and kinetic studies. According to laminar boundary theory and considering the specific flow conditions in a coated-wall flow tube, we suggest using a critical height δc to evaluate turbulence effects in the design and analysis of coated-wall flow tube experiments. When a coating thickness εmax is larger than δc, the roughness elements of the coating may cause local turbulence and result in overestimation of the real uptake coefficient (γ). We collect εmax values in previous coated-wall flow tube studies and evaluate their roughness effects using the criterion of δc. In most cases, the roughness doesn't generate turbulence and has negligible effects. When turbulence is generated, the calculated effective uptake coefficient (γeff) can bear large difference compared to the real uptake coefficient (γ). Their difference becomes less for gas reactants with lower uptake (i.e., smaller γ), or/and for a smaller ratio of the coating thickness εmax/R0 to the flow tube radius R0, (εmax/R0). On the other hand, the critical height δc can also be adjusted by optimizing flow tube configurations and operating conditions (i.e., tube diameter, length and flow velocity), to ensure not only an unaffected laminar flow pattern but also a flexible residence time of gas reactants in flow tube reactors.
