Diffusion of a solute in dilute solution in a supercritical fluid
Diffusion of a solute in dilute solution in a supercritical fluid
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超临界流体中溶质在稀溶液中的扩散
DOI：
10.1098/rspa.1991.0035
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发表时间：
1991
期刊：
影响因子：
--
通讯作者：
S. E. Coleby
中科院分区：
文献类型：
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作者：
A. Clifford;S. E. Coleby
回顾了超临界流体（高于其临界温度和压力的流体）稀溶液中溶质的二元扩散系数行为的实验证据。在极低稀释度下的测量，特别是通过泰勒色散技术进行的测量表明，在高于临界温度几度的恒温下，密度和扩散系数的乘积表现出从零密度到远高于临界密度的小、连续且平缓的变化。然而，在较高但仍然很低的浓度（例如摩尔分数约为 10-3）下进行的一些测量显示关键区域的系数降低。基于非平衡热力学的方程被放入这样的形式中，其中临界区域中的二元扩散系数的行为，但不是非常接近临界点，可以使用状态方程来检查。使用混合物的范德华状态方程计算二氧化碳溶液中的萘，以表明系数“异常”降低的形式和数量级，尤其是其对浓度的依赖性。这些表明即使在萘摩尔分数为 4.0 x 10-4 或更低且温度比纯溶剂的临界温度高 1、3 和 9 K 时，也具有显着效果。此外，讨论了临界区域中超临界流体中溶质相对于空间或细胞固定坐标的通量。由于该区域中萘等溶质的偏摩尔体积较大且为负，因此可以导致定义扩散系数的参考系快速移动，通常朝浓度源移动。因此，在临界区域中，相对于空间固定坐标的溶质通量进一步显着减少。因此，扩散系数的降低和重心运动的结合可以导致关键区域中溶质传质的显着减少，并且可能是实验中观察到的有时非常大的扩散异常的解释。
The experimental evidence for the behaviour of the binary diffusion coefficient for a solute in dilute solution in a supercritical fluid (a fluid above its critical temperature and pressure) is reviewed. Measurements at very low dilution, particularly by the Taylor dispersion technique, indicate that, at constant temperature a few degrees above the critical temperature, the product of density and the diffusion coefficient exhibits a small, continuous and undramatic variation from zero density to well above the critical density. However, some measurements made at higher, but still very low concentrations (e.g. with mole fractions around 10-3), show a lowering of the coefficient in the critical region. The equations, based on non-equilibrium thermodynamics, are put into a form in which the behaviour of the binary diffusion coefficient in the critical region, but not very close to the critical point, may be examined using an equation of state. Calculations for naphthalene in solution in carbon dioxide are carried out using the van der Waals equation of state for mixtures to indicate the form and order of magnitude of the ‘anomalous’ lowering of the coefficient, and especially its dependence on concentration. These indicate a substantial effect even at naphthalene mole fractions of 4.0 x 10-4 or less and a temperatures 1, 3 and 9 K above the critical temperature of the pure solvent. In addition the flux of a solute in a supercritical fluid in the critical region with respect to space or cell-fixed coordinates is discussed. Because of the large and negative partial molar volumes of solutes like naphthalene in this region, the frames of reference, according to which the diffusion coefficients are defined, can be caused to move rapidly, commonly towards the source of concentration. Thus fluxes of solute with respect to space-fixed coordinates are further substantially reduced in the critical region. The combination of the lowering of the diffusion coefficient and barycentric motion can therefore cause a very significant reduction of solute mass transfer in the critical region and may be the explanation of the sometimes very large diffusion anomalies observed experimentally.