Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, NY 13244, USA.
J Chromatogr A. 2010 Jun 25;1217(26):4454-62. doi: 10.1016/j.chroma.2010.04.043. Epub 2010 Apr 22.
The supercritical fluid (SCF) chromatographic technique based on the Taylor dispersion theory has been widely applied in determination of diffusion coefficients of various organic compounds in SCFs. This study was aimed to understand impacts of mobile phase mean velocity (MPMV) and column orientation on diffusion coefficient measurements. The benzene/SCCO(2) system was investigated. Experiments were carried out at 40 and 60 degrees C and 9-15 MPa over a wide range of CO(2) densities at varying MPMV and repeated in two column orientations, vertical and horizontal. It was found that both MPMV and column orientation significantly affected measurements of diffusion coefficients in SCFs. When the column was installed vertically, apparent diffusion coefficients obtained at relatively low CO(2) density (<580 kg/m(3)) increased with increasing MPMV over the entire velocity ranges. This results in a conclusion that diffusion coefficients cannot be accurately determined under these conditions using a vertically installed column. Under all other conditions, as MPMV increased, apparent diffusion coefficients initially increased, then remained constant, and finally increased again. The initial increase of apparent diffusion coefficients was associated with significant decline of curve-fitting errors, which indicates that the buoyancy effects are non-negligible and will cause larger errors. Accordingly, a new generalized D(12)-U diagram comprised of three regions is proposed. Column orientation affected diffusion coefficient measurements mainly by enhancing or weakening the buoyancy effects. When the column was installed vertically, the buoyancy effects were enhanced, leading to lower apparent diffusion coefficients, especially when CO(2) density was relatively low. In addition, it was found that when CO(2) density was below approximately 580 kg/m(3), diffusion coefficients obtained when the column was horizontally installed were higher than those obtained when the same column was vertically installed. When CO(2) density was above that value, opposite outcomes resulted. Finally, the horizontal orientation of a diffusion column is recommended for diffusion coefficient measurements by the SCF chromatographic technique, especially when densities of SCFs are relatively low.
基于泰勒分散理论的超临界流体（SCF）色谱技术已广泛应用于测定各种有机化合物在 SCF 中的扩散系数。本研究旨在了解流动相平均速度（MPMV）和柱方向对扩散系数测量的影响。研究了苯/SCCO 2 体系。在 40 和 60°C 下，在 9-15 MPa 范围内，在不同的 MPMV 和两种柱方向（垂直和水平）下，在 CO 2 密度变化范围内进行了实验。结果表明，MPMV 和柱方向均显著影响 SCF 中扩散系数的测量。当柱垂直安装时，在相对较低的 CO 2 密度（<580 kg/m 3 ）下，在整个速度范围内，获得的表观扩散系数随 MPMV 的增加而增加。这得出的结论是，在这些条件下，使用垂直安装的柱子无法准确确定扩散系数。在所有其他条件下，随着 MPMV 的增加，表观扩散系数最初增加，然后保持不变，最后再次增加。表观扩散系数的初始增加与曲线拟合误差的显著下降有关，这表明浮力效应不可忽略，会导致更大的误差。因此，提出了一个新的包含三个区域的通用 D 12 - U 图。柱方向主要通过增强或减弱浮力效应来影响扩散系数的测量。当柱垂直安装时，浮力效应增强，导致表观扩散系数降低，特别是当 CO 2 密度相对较低时。此外，还发现当 CO 2 密度低于约 580 kg/m 3 时，当柱水平安装时获得的扩散系数高于同一柱垂直安装时获得的扩散系数。当 CO 2 密度高于该值时，结果相反。最后，建议在 SCF 色谱技术中使用扩散柱的水平方向进行扩散系数测量，特别是当 SCF 的密度相对较低时。
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