Static mixers as heat exchangers in supercritical fluid extraction processes

Abstract

The performance of a Kenics static mixer as a heat-transfer device for supercritical carbon dioxide (CO 2) flow is studied and compared with conventional tube-in-tube heat exchangers. Measurements were carried out at pressures ranging from 8 to 21 MPa, temperatures from 283 to 323 K, and mass flowrates from 2 to 15 kg/h. The corresponding Reynolds and Prandtl numbers, at bulk conditions, ranged between 10 3 and 2 × 10 4 and between 2 and 7, respectively. The temperature increase experienced by the supercritical CO 2 stream varied between 10 and 35 K. The heat fluxes obtained with the static mixer are one order of magnitude higher than the ones observed with a tube-in-tube heat exchanger for the same set of operating conditions. The heat-transfer enhancement is caused by the cross-sectional mixing of the fluid and to a lesser extent by conduction across the metallic mixing elements. Heat-transfer is also affected by temperature-induced variation of physical properties, especially in the pseudocritical region of the fluid. From the experimental data, a correlation was developed for convective heat-transfer to supercritical CO 2 in terms of the Nusselt number.