Abstract

The Stefan column was designed in the 19^th Century to estimate binary gas diffusivities by placing a pure volatile liquid at the bottom overlaid with a stagnant gas. A steady gas sweep at the top removed the diffused species. Previous validation, by other research groups, of the Maxwell-Stefan multicomponent mass transport equations in the Stefan column prompted a study to test an azeotropic binary liquid as the source of both gaseous species, maintaining the composition at the liquid-gas interface constant. Azeotropic acetone (A)-n-hexane (B) mixtures were placed in a vertical glass pipette in preparation for an isothermal evaporation-diffusion experiment in atmospheric air (C). The pure solvents were also tested separately. The experimental interfacial descent, the solvents’ diffusivities in air, and their molar fluxes were provided as input to the Maxwell-Stefan equations to obtain the binary gas diffusivity D_AB,exp, which is unavailable in the literature. The pure solvent diffusivities in air showed good agreement with the Chapman-Enskog kinetic theory for low-density gases, but the experimental gas diffusivity of acetone in n-hexane exceeded consistently the theoretical predictions by at least two orders of magnitude. A possible explanation for this unexpected result is the high affinity between the solvent molecules in the liquid state, which undoubtedly affects their gas transport behavior. Further studies are necessary to elucidate this finding. This is the first attempt to use a liquid azeotrope in a Stefan column. By combining this approach with the Maxwell-Stefan transport equations, the gas diffusivity of acetone in n-hexane was obtained.

摘要

Stefan 柱是在 19 世纪设计的，^通过将纯挥发性液体置于底部覆盖有停滞气体来估计二元气体扩散率。顶部的稳定气体吹扫去除了扩散的物质。其他研究小组先前对 Stefan 柱中的 Maxwell-Stefan 多组分质量传输方程的验证促使一项研究测试共沸二元液体作为两种气态物质的来源，保持液-气界面的成分恒定。共沸丙酮 (A) -n将-己烷 (B) 混合物置于垂直玻璃移液管中，准备在大气 (C) 中进行等温蒸发扩散实验。纯溶剂也单独测试。将实验界面下降、溶剂在空气中的扩散率及其摩尔通量作为 Maxwell-Stefan 方程的输入，以获得文献中无法获得的二元气体扩散率 D _AB,exp。空气中的纯溶剂扩散率与低密度气体的 Chapman-Enskog 动力学理论非常吻合，但丙酮在n中的实验气体扩散率-己烷始终超过理论预测至少两个数量级。这种意外结果的一个可能解释是液态溶剂分子之间的高亲和力，这无疑会影响它们的气体传输行为。需要进一步的研究来阐明这一发现。这是在 Stefan 柱中使用液体共沸物的首次尝试。将该方法与Maxwell-Stefan输运方程相结合，得到了丙酮在正己烷中的气体扩散率。