Theories of chromatographic efficiency applied to expanded beds

Various quantities such as plate height (HETP), number of plates (N), axial dispersion coefficient (Dax) and Bodenstein number (Bo) are used to describe the efficiency of, and dispersion in chromatographic columns. Different quantities highlight different aspects of the performance. Due to the expansion of expanded-bed columns, the information contained in some of these quantities is not the same for expanded beds as for packed beds. In this article the mentioned quantities are described and discussed both theoretically and related to experimental data. It is concluded that they are often used in a confusing way. Quantities modified to be more informative when comparing beds of different expansions are developed (NEB=N/expansion2 and HETPEB=HETP·bed expansion) and recommendations of which quantity to use in what situation are given.