Separation theory in thermal diffusion columns.

The equipment used in thermal diffusion is generally categorized into two kinds, i.e. static cells (or single stage convective free cells) and thermogravitational columns (or Clausius-Dickel columns). In the static cells which were used in the early work of thermal diffusion, attempts have been made to eliminate convective currents. Since the concentration gradient at steady state is such that the flux due to ordinary diffusion just counterbalances that resulting from thermal diffusion, the separation obtained in those cells is very small and of no practical use. A more detailed study of the mechanism of separation in the Clausius-Dickel column finds that the convective currents in the column actually have two conflicting effects, the desirable cascading effect and the undesirable remixing effect. It is believed that proper adjustment of the convective strength might improve the separation. Consequently, based on this idea, some improved columns have been devised. Typically, thermal diffusion columns are designed with columns in series parallel arrangement to obtain larger scale or higher purity product, or even to reduce the time needed to reach steady state operation. For simplicity, the separation theory is considered first. (from paper)