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.

本研究针对实验室规模填料塔中采用氢氧化钠水溶液（aqueous sodium hydroxide，NaOH）脱除空气中低浓度碘蒸气的过程展开探究，同时采用实验与建模两种研究方法。实验旨在测定总容积传质系数（overall volumetric mass transfer coefficient，KGaeff），并考察气体流速、液体流速、氢氧化钠浓度以及填料类型的影响。研究结果表明，KGaeff与有效传质界面面积强烈依赖于气相雷诺数（Reynolds number，ReG），而在多数工况下液相参数对其影响极小。不过，当气相雷诺数较高且氢氧化钠浓度较低时，液体流速会逐渐对KGaeff产生影响，这表明传质控制机制发生了转变。研究人员开发了针对KGaeff的关联式，并将其集成至一维有限差分求解器中以模拟吸收过程。预测得到的出口碘浓度与实验结果吻合良好，偏差不超过10%。相阻力分析显示，当ReG≥25且[OH⁻]≤0.25 M时，传质过程从气相控制转变为气液混合控制。此外，孔隙率与KGaeff呈负相关关系，这凸显了选择可最小化气相阻力的填料材料的重要性。上述研究结果凸显了气相流体力学与氢氧根浓度在优化碘脱除工艺中的关键作用。本研究所得结论对于核与放射化学设施中用于控制放射性碘排放的碱性洗涤塔的设计与运行具有重要参考价值。