Krishna Rajamani
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
ACS Omega. 2019 Feb 28;4(2):3409-3419. doi: 10.1021/acsomega.8b03480. Epub 2019 Feb 15.
The main focus of this article is on mixture separations that are driven by differences in intracrystalline diffusivities of guest molecules in microporous crystalline adsorbent materials. Such "kinetic" separations serve to over-ride, and reverse, the selectivities dictated by mixture adsorption equilibrium. The Maxwell-Stefan formulation for the description of intracrystalline fluxes shows that the flux of each species is coupled with that of the partner species. For -component mixtures, the coupling is quantified by a × dimensional matrix of thermodynamic correction factors with elements Γ ; these elements can be determined from the model used to describe the mixture adsorption equilibrium. If the thermodynamic coupling effects are essentially ignored, i.e., the Γ is assumed to be equal to δ , the Kronecker delta, the Maxwell-Stefan formulation degenerates to yield uncoupled flux relations. The significance of thermodynamic coupling is highlighted by detailed analysis of separations of five different mixtures: N/CH, CO/CH, O/N, CH/CH, and hexane isomers. In all cases, the productivity of the purified raffinate, containing the tardier species, is found to be significantly larger than that anticipated if the simplification Γ = δ is assumed. The reason for the strong influence of Γ on transient breakthroughs is traceable to the phenomenon of uphill intracrystalline diffusion of more mobile species. The major conclusion to emerge from this study is that modeling of kinetic separations needs to properly account for the thermodynamic coupling effects.
本文的主要关注点是由客体分子在微孔晶体吸附剂材料中的晶内扩散率差异驱动的混合物分离。这种“动力学”分离作用会超越并逆转由混合物吸附平衡所决定的选择性。用于描述晶内通量的麦克斯韦-斯蒂芬公式表明，每种物质的通量都与配对物质的通量相互耦合。对于n组分混合物，这种耦合由一个n×n维的热力学校正因子矩阵来量化，其元素为Γij；这些元素可以从用于描述混合物吸附平衡的模型中确定。如果基本忽略热力学耦合效应，即假设Γij等于克罗内克δij（δij），则麦克斯韦-斯蒂芬公式会退化，得到非耦合的通量关系。通过对五种不同混合物（N₂/CH₄、CO₂/CH₄、O₂/N₂、CH₄/C₂H₄和己烷异构体）的分离进行详细分析，突出了热力学耦合的重要性。在所有情况下，发现含有迁移较慢物质的纯化萃余物的生产率显著高于假设简化条件Γij = δij时预期的生产率。Γij对瞬态突破有强烈影响的原因可追溯到迁移性较强物质的晶内上坡扩散现象。这项研究得出的主要结论是，动力学分离的建模需要适当地考虑热力学耦合效应。
