Dispersion Coefficients of Supercritical Fluid in Fixed Beds

Abstract

The axial dispersion coefficient of hexachlorobenzene in supercritical carbon dioxide is investigated in a fixed-bed packed with glass beads. The on-line chromatographic pulse-response experiment is used in order to study the dynamics of a packed column under supercritical conditions. The radial dispersion is assumed negligible because of the packed column geometry. To estimate the axial dispersion coefficient, a pulse input of tracer/supercritical fluid (hexachlorobenzene/carbon dioxide) mixture is injected into the column and the effluent peak is analyzed using the moments of the chromatographic curve in the Laplace domain. The range of the operating conditions for temperature, pressure and flow rate of super critical fluid are 25-500 °C, 1200-4000 psia and 120-160 ml/hr, respectively. The experimental data indicate that the axial dispersion coefficient is a function of temperature, pressure and flowrate. The axial dispersion coefficient decrease with increasing temperature and increase with increasing pressure. This trend may be due to the increase of the density and viscosity of the super critical carbon dioxide. Further more, the axial dispersion coefficient increase with increasing interstitial velocity. These results suggest that the contribution by convection is more important than molecular diffusion under supercritical operations. In order to investigate the authenticity of the dynamic model as well as the extent of accuracy of the moment analysis which is used for parameter estimation, the experimental response peak is compared with the dimensionless theoretical (numerical solution) curve. The small deviation between the two curves is well within the range of experimental error of axial dispersion coefficient measurements.