Minimum Reflux Mode Fractionation Classes

Knowledge about the distillation process regularities under minimum reflux is the background of the distillation theory, and this mode analyzing is the most important stage of the distillation column design. 

As it has been already mentioned above, at minimum reflux a column has infinite number of steps (N = oo) (i.e., the trajectory passes through one or more stationary points of the bundle). In the column, these stationary points will correspond to the so-called zones of constant concentrations that are identical at adjacent trays. 

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Generally speaking, for the first and second fractionation classes under the minimum reflux mode, the points of compositions in the zones of constant concentrations (i.e., stationary points of the trajectory bundles) should be arranged at the trajectories of reversible distillation built for the product points. It follows from the conditions of the material balance and the phase equilibrium in the zones of constant concentrations. Figure 2.11b illustrates the partially reversible process (it is reversible in the colunm parts that are from the constant concentration zones for the minimum reflux mode up to the column ends). 

The conducted analysis of product points evolution, depending on R for ideal mixtures, determines a number of the important qualitative regularities of the minimum reflux mode the existence of three classes of fractioning, the availability of one or two zones of constant concentrations in each section of the column, feasibility of various splits by means of a corresponding choice of two parameters of the mode -otR and D/F. 

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