面向氙气分离七元环STT分子筛膜构筑与传质的基础研究

Membrane separation could efficiently separate CO2, generated by human tissue in the closed-circuit anesthesia system, from xenon (Xe). This technique exhibits great potential to break the “supply ceiling” of xenon from the cryogenic distillation of air. It has proven that the competitive adsorption between Xe and CO2 plays a key role in the permeation flux and selectivity of the zeolite membrane. Herein, an all-silica STT zeolite membrane supported on ceramic hollow fibers is proposed for CO2/Xe separation. STT zeolite is featured with a 2D channel in xz planes: the first pathway (PW1) lies along the [001] direction and has an aperture size of 0.37×0.53 nm; the second pathway (PW2), with an aperture size of 0.24×0.35 nm, is interconnected with the first pathway. It is proposed to improve the membrane selectivity and permeation flux by tuning the aperture size and surface property. The all-silica STT zeolite membrane would be prepared by a phase transition from an amorphous silica layer to a crystalline structure. The incorporation of other atoms, involving Al, Ge, or B, would be achieved by solid reaction at high temperature, targeting a weak interaction with Xe. The electrical density would be tuned by ions exchange for an enhanced CO2/Xe adsorption selectivity. Based on the gas permeation, a mechanism would be proposed to explain the effects of competitive adsorption on separation performance of STT zeolite membranes for CO2/Xe separation. Finally, the CO2 and Xe transfer property would be described by the Maxwell-Stafen equation. All of these efforts would be beneficial to the membrane material design and real application of Xe purification.

氙气(Xe)是一种优良的麻醉剂，在欧洲已实现商业应用；Xe产能不足是制约其全球推广的关键性瓶颈。膜法在线脱除人体麻醉过程中呼出的CO2能够实现Xe高效循环套用，为Xe供应开辟新途径。如何消除分子间竞争吸附和上坡扩散，提升膜通量，是膜分离技术能否推向实际应用的关键。本项目提出构筑双级孔全硅STT分子筛膜用于CO2/Xe分离，通过膜孔道和表面性质调控，提升膜选择性和通量。项目提出原位相转化法在中空纤维载体上制备膜分离层；然后通过高温固相反应在分子筛膜表面引入骨架杂原子以构筑弱氙气吸附表面，研究表面性质与分离性能之间的匹配关系；采用先骨架原子杂化后阳离子交换的方法调控传质孔道荷电性，提高CO2/Xe吸附选择性；开展气体膜分离过程研究，建立二元竞争吸附气体分子在分子筛膜中传质数学模型，揭示气体分子尺寸和竞争吸附与分子筛膜传质过程的相互作用规律。

面向医疗麻醉过程中氙气在线分离，项目开展了STT分子筛无氟合成配方研究、高选择性STT分子筛膜制备条件优化、模拟麻醉氙气在线净化与分离分子筛膜技术研究、高湿度环境中分子筛膜稳定性评测等工作。研究出无氟合成STT分子筛新配方，在四通道α-Al2O3中空纤维载体上合成出高质量STT分子筛膜，发现提升晶化温度、延长晶化时间均可促进七元环传质孔道的构筑，从而制备出以七元环为主导的高选择性STT分子筛膜。对于模拟麻醉氙气分离与净化，STT分子筛膜的CO2渗透性为1.04 × 10-7 mol m-2 s-1 Pa-1，CO2/Xe理想选择性为480，并将其与DD3R、高硅CHA等传统分子筛膜进行对比。DD3R分子筛膜的CO2渗透性为1.5 × 10-7 mol m-2 s-1 Pa-1，CO2/Xe理想选择性为560；高硅CHA分子筛膜的CO2渗透性和CO2/Xe选择性分别为8.6×10-8 mol m-2 s-1 Pa-1和1070。为进一步探讨CO2和Xe分子在不同构型分子筛孔道中传质机理的研究提供了基础数据。利用吸附穿透实验和分子模拟等技术手段，研究了CO2和Xe分子在DD3R分子筛膜孔道的竞争吸附-扩散行为，揭示出强吸附特性Xe分子对快扩散CO2分子传质的阻碍作用机制。对于模拟等摩尔CO2/Xe进料，相较于单组分CO2渗透性，STT、DD3R和高硅CHA分子筛膜的CO2渗透性分别下降了46%、50%和14%。CHA分子筛笼内CO2分子的传质受Xe阻碍作用较弱，CO2渗透性较单组分渗透性仅下降了14%；高硅CHA分子筛膜在水蒸气环境中亦具有优异的稳定性，已实现连续稳定运行30天以上。针对高湿度的氙气麻醉尾气，系统研究了DD3R分子筛膜的分离性能，CO2渗透性为2.0×10-8 mol m-2 s-1 Pa-1，CO2/Xe选择性为67±12，已完成300小时的连续稳定运行测试。优化设计分子筛膜在线净化与回收Xe过程，开展技术经济分析，分离运行成本约为1.5美元/Nm3，远低于商品化Xe的市场价格（8万美元/Nm3）。此外，将项目STT分子筛膜拓展应用于焦炉煤气制氢，H2渗透性达2.9×10-8 mol m-2 s-1 Pa-1，H2/CH4选择性为53.8；拓展应用于天然气提氦，He渗透性达4.7×10-8 mol m-2 s-1 Pa-1，He/CH4选择性为273。

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