Extra Column Dispersion - Low Dispersion Connecting Tubes > Page 30
It is seen from figure 10, that at low linear velocities, where there is little radial mixing, (H) increases as (u) increases. It is also seen that the coiled tubes 1 and 2, of larger radii, give a greater dispersion than that in the smaller radii tubes, 3 and 4. Radial mixing is seen to commence at higher velocities and subsequently (H) starts to decrease as (u) increases. At those velocities where radial mixing dominates, dispersion becomes virtually independent of the linear velocity (u). It is also seen that the maximum value of (H), for any particular coil, occurs at differing values of (u) which depend on the combined values of the tube radius and the coil aspect ratio (f). It would appear that, the higher the aspect ratio of the coil, the lower the velocity at which the dispersion is at a maximum.,
Although the straight tube theory of Golay has not been applied to coiled tubes, his equation can qualitatively explain the shape of the curves in figure 12. According to Golay, at low velocities longitudinal diffusion dominates and controls the dispersion. As the velocity approaches the optimum, the resistance to mass transfer term becomes dominant and, consequently, (H) rapidly increases. However, at high velocities, where radial flow is generated, the effective diffusivity of the solute dramatically increases, which, as a consequence, reduces the resistance to mass transfer to almost zero, accompanied by a corresponding dramatic reduction in (H). At exceedingly high velocities, the remaining contribution to dispersion is longitudinal diffusion, the magnitude of which is now hardly significant and, as a result, (H) becomes inconsequential.
The effect of the radius of the coils on dispersion in coiled tubes was examined experimentally by Scott and Simpson (10).