Packed bed absorption of dilute iodine vapor from air to aqueous caustic soda: mass transport modelling and validation

This study investigates the removal of low-concentration iodine vapor from air using aqueous sodium hydroxide (NaOH) in a laboratory-scale packed column, employing both experimental and modelling approaches. Experiments were conducted to determine the overall volumetric mass transfer coefficient (KGaeff) and assess the influence of gas flow rate, liquid flow rate, NaOH concentration, and packing type. Results showed that KGaeff and the effective interfacial area were strongly dependent on the gas-phase Reynolds number (ReG), while liquid-phase parameters had minimal impact under most conditions. However, at high gas Reynolds numbers and low NaOH concentrations, the liquid flow rate progressively affected KGaeff, indicating a transition in the controlling mass transfer regime. A correlation for KGaeff was developed and integrated into a one-dimensional finite difference solver to simulate the absorption process. The predicted outlet iodine concentrations agreed with experimental results, with deviations within 10%. Phase resistance analysis revealed a shift from gas-phase-controlled to mixed gas-liquid controlled mass transfer at ReG ≥ 25 and [OH]≤ 0.25 M. Additionally, porosity was found to inversely affect KGaeff, underscoring the importance of selecting packing materials that minimize gas-phase resistance. These findings highlight the critical role of gas-phase hydrodynamics and hydroxide concentration in optimizing iodine removal. The insights gained are valuable for the design and operation of caustic scrubbers intended to control radioactive iodine emissions in nuclear and radiochemical facilities.