Extracting CO₂ from the atmosphere via direct air capture (DAC) provides a pathway to counteract the rising CO₂ concentration in the atmosphere. Processes using amine functionalized solid sorbents have attracted considerable attention, as they exhibit high affinity toward CO₂ at atmospheric concentrations. The process is significantly influenced by the mass transfer kinetics of adsorption, and accurate quantification is crucial for improving process models and DAC systems. In this study, we addressed this critical issue by quantifying the mass transfer kinetics of three amine functionalized structured sorbents: two alumina pellets with unimodal (TRI@unimodal) and bimodal (TRI@bimodal) pore size distributions, and a honeycomb mullite/alumina monolith (TRI@monolith). A modeling framework was developed to enable the use of a commercial volumetric sorption device to measure sorbent mass transfer kinetics, and to distinguish them from resistances within the device. The measurements revealed distinct mass transfer regimes, with pore diffusion playing a significant role in the bimodal pellets, whereas a surface resistance introduced by the functionalization procedure dominated in the unimodal pellets. The device was unable to capture the pore diffusion in the monolith due to instrument resistances limiting this regime. A self-limiting diffusion behavior previously reported in literature was identified in the amine layer, which decreased diffusion with increasing CO₂ uptake. We estimate kinetic parameters for all three sorbent materials for use in a widely used linear driving force (LDF) model adapted for amine functionalized sorbents. The parameter describing the mass transfer in the gas phase is nearly five times larger for TRI@bimodal than for TRI@unimodal. For the mass transfer in the amine layer, the parameter increases progressively from TRI@monolith to TRI@unimodal to TRI@bimodal. The results highlight the importance of pore structure and functionalization procedure to improve DAC sorbents.

中文翻译：

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超稀条件下胺功能化结构接触器中 CO2 的质量传递


通过直接空气捕获 （DAC） 从大气中提取 CO₂ 提供了一种途径来抵消大气中不断上升的 CO₂ 浓度。使用胺官能化固体吸附剂的工艺引起了相当大的关注，因为它们在大气浓度下对 CO₂ 表现出高亲和力。该工艺受吸附传质动力学的显著影响，准确定量对于改进工艺模型和 DAC 系统至关重要。在这项研究中，我们通过量化三种胺功能化结构吸附剂的质量传递动力学来解决这一关键问题：两个具有单峰 （TRI@unimodal） 和双峰 （TRI@bimodal） 孔径分布的氧化铝颗粒，以及一个蜂窝状莫来石/氧化铝整体 （TRI@monolith）。开发了一个建模框架，以便使用商用体积吸附装置来测量吸附剂传质动力学，并将其与装置内的阻力区分开来。测量揭示了不同的传质机制，孔隙扩散在双峰颗粒中起着重要作用，而功能化程序引入的表面电阻在单峰颗粒中占主导地位。由于仪器电阻限制了这种状态，该装置无法捕获整体中的孔扩散。在胺层中发现了先前文献中报道的自限性扩散行为，该行为随着 CO₂ 摄取的增加而减少扩散。我们估计了所有三种吸附剂材料的动力学参数，用于广泛使用的线性驱动力 （LDF） 模型，该模型适用于胺功能化吸附剂。 描述 TRI@bimodal 在气相中传质的参数几乎是 TRI@unimodal 的五倍。对于胺层中的传质，该参数从 TRI@monolith 逐渐增加到 TRI@unimodal 再到 TRI@bimodal。结果强调了孔结构和功能化程序对改进 DAC 吸附剂的重要性。