Condensation Separation of NO₂with Dimerization Reaction in the Presence of Noncondensable Gas: Critical Assessment and Model Development

Pure nitrogen dioxide (NO₂) has significant economic value and is widely used in many fields, for which condensation technology plays an important role in separation and purification. However, developing cost-effective NO₂condensers remains challenging due to the lack of precise theoretical guidelines and comprehensive understanding of NO₂condensation process. In this work, NO₂condensation at various inlet surface subcoolings, mole fractions of noncondensable gas (NCG), and Re numbers was studied with a visualization experimental system. The influential rules of each parameter on heat transfer coefficients (HTCs) and the NO₂condensate state as the coexistence of droplet, streamlet and film were revealed. A substantial underestimation of experimental data by the classical heat and mass transfer analogy (HMTA) model was quantified. The large discrepancy was found to originate from the uniqueness in heat transfer, mass transfer, and condensate state caused by NO₂dimerization during condensation. A modified HMTA model was developed considering the release heat of dimerization reaction and the promotion of mass transfer by an increased NO₂concentration gradient within the diffusion layer which contribute to improvements of HTCs by ∼6 and ∼49%, respectively. The correction of liquid film roughness regarding potential heterogeneity of dimerization was proposed as a function of the key parameters, contributing to the improvement of HTCs by ∼150%. An accurate theoretical formula for HTCs prediction within an error of ±25% was finally derived, providing the key step for success in practical applications.