Axial dispersion in coiled tubular reactors : The Effect of Curvature at Low Dean Numbers

Deforming tubing (e.g., coiling) reduces axial dispersion in flow-injection systems and post-column reactors. An improvement is presented for a previous theoretical treatment of axial dispersion in coiled tubes. New experimental data are also presented for different coil radii. Changes were necessary because smaller-than-predicted dispersion improvements were observed. This contradicts earlier treatments that predict a linear dependence on the aspect ratio of the coil. Evidence is presented that this is explained by assuming a boundary layer thickness much smaller than the radius of the tube and proportional to the ratio of radial velocities. This is a special case of earlier theory and it predicts an almost curvature-independent change in dispersion. The inclusion of the critical Reynolds number, Re_c, to form a reduced parameter, (Re/Re_c), accounts for all observations, including some previously-defined empirical constants. The dispersion is found to increase as (Re/Re_c)^2/3Sc at low fluid velocities and to decrease as (Re^1/6_c/(Re/Re_c)^4/3Sc^0.08) at higher velocities. Experiments are reported for several coil radii and with a range of proteins and protein mixtures.