Visualization experimental study on NO₂ condensation process: insights into gas-fog-liquid-ice mode evolution

Pure nitrogen dioxide (NO₂) is a profitable feedstock for production of various chemicals. The condensation technology plays an essential role in NO₂ enrichment during production or resource recovery from industrial exhausts, which, however, remains challenging in process optimization due to the lack of thorough comprehension on NO₂ condensation behavior. In this work, the visualization experiments of NO₂ condensation on the vertical SS316 surface at varying temperatures, NO₂ mole fractions and volume flow rates in the absence and presence of non-condensable gas (NCG) were conducted. Three major NO₂ condensation modes, the fog mode, the droplet-streamlet-film (DSF) coexistence mode, and the freezing mode, have been identified. The formation process of NO₂ gas-fog-droplet-DSF mode as well as the occurrence frequency of streamlets (OFS) was quantified under different conditions. The higher OFS was found to contribute to greater overall heat transfer coefficient and condensate amount, showing the optimal parameters at each condition. Deeper insights into gas-fog, gas-liquid, and gas-ice phase transformations for NO₂ were gained, revealing the unique effects of dimerization of NO₂ to N₂O₄ on condensation. The dimerization triggers the facile formation (< 10 s) of N₂O₄ fog with increased intermolecular forces, the heterogeneity of heat and mass transfer in DSF mode with complex heat resistance network and condensate states, and the melting-thickening cycle of ice layer in the freezing process along with the release of dimerization reaction heat. The findings render a microscopic view of favorable heat and mass transfer towards optimal NO₂ condensation strategies in practical uses.