The first German edition of the book“Fluid dynamics of packed columns with modern random and structured packings for gas/liquid systems” was published in 1991. It sold out within a few years. Added to this were numerous enquiries, in particular within the industry, prompting me to publish a second, extended edition. A packed column remains the core element of any diffusional separation process. This underlines the need for basic design principles for packed columns, which enhance the design process by making it more accurate and reliable. The SBD (suspended bed of droplets) model introduced in the first German edition of the book was well received by the experts and is now used by a large number of com- nies in the industry, as it offers improved reliability in the—fluid dynamic design of packed columns. For the purpose of facilitating the design process, the SBD model was in- grated into the simulation programme ChemCAD. The software programme FDPAK, which is available for Windows, has certainly contributed to the widespread use of the SBD model. The programme is very user-friendly and the calculation results are p- sented in tabular as well as graphic form, showing—flood load, pressure drop and hold-up diagrams in the entire operating range.