What is distillation?Distillation operations are carried out in columns, that is, cylindrical pressure vesselswith large height to diameter ratios, containing various kinds of internals chosen andarranged appropriately to enable cost-effective separation of binary or multicomponent,mainly liquid, feed mixtures into desired pure products or specified fractions.As mentioned by Stichlmair and Fair in the preface of DistillationdPrinciplesand Practice, which focuses on understanding of the principles and their translationinto basic design practices, distillation columns are the workhorses of the petroleum,petrochemical, chemical, and related process industries [1]. A complementary, morepragmatic view is given by Kister in Distillation Design, with emphasis on the practicalaspects and reliability as encountered and required during the design of distillationcolumns [2].The outgoing point for design of a new distillation column is the top and/or bottomproduct specification (required purity and/or recovery) for a given feed mixture, withknown composition, flow rate, and thermal state at a given pressure and temperature.Upon choosing the operating pressure and assuming column pressure drop, detailedcolumn performance calculations deliver optimum stage and reflux requirements,all based on appropriate vaporeliquid equilibrium data. The next, column dimensioningstep is concerned with determining column diameter(s) and height for the chosentype of vaporeliquid contacting device. Critical positions are the top and bottomstage of rectification and stripping sections, and the column shell diameter is usuallybased on the stage with the maximum vapor load; and to complete the design calculationsproperly, one or more iterations on pressure drop are required.Upon completing stage and reflux requirement calculations, the internal flowrates of vapor and liquid as well as corresponding values of relevant physical properties(densities and viscosities of two phases and surface tension) are known perstage and serve as the basis for the determination of column diameter(s). However,the number of theoretical plates or equilibrium stages, which is by definition thenumber of times the mass transfer equilibrium has to be established betweenascending vapor and descending liquid streams above and below the feed to achievedesired separation, cannot be directly translated into column height. Namely, alongan operating distillation column the equilibrium between vapor and liquid is neverfully established. The column height required to produce the degree of mass transferequivalent to the given number of equilibrium stages will depend on the type andperformance characteristics of equipment (internals) chosen to generate interfacialarea, that is, to enable mass transfer to occur so that for each component the requirednumber of moles is transported accordingly, from vapor to liquid and reverse withinthe contact time available.