Applications of the Design Methods

Distillation design methods are well established and described in detail in the traditional reference texts (see Kister, 1992). The original methods were formulated in the 1920s and 1930s such as the McCabe-Thiele (1925) and Ponchon-Savarit (1921, 1922) methods for binary mixtures, and the rigorous multicomponent analogues of the Lewis-Matheson (1932) and Thiele-Geddes (1933) procedures. Use of the latter trial-and-error methods emphasized the need for the incorporation of suitable numerical techniques to ensure that the solution (of number of stages) would actually converge, and also to reduce the time spent performing the calculations. Designers later came to rely on short-cut design methods, e.g. Fenske (1932), Underwood (1945, 1946, 1948), Smith-Brinkley (1960), etc., to provide 'ballpark' results before embarking upon the detailed rigorous calculations. Easier access to mainframe computers in the 1960s, and desktop machines in the late 1970s meant that the time required to perform the numerical calculations was reduced, and developments then centred on design methods which simplified the problem formulation, e.g. matrix manipulation techniques.

The arrival in the 1980s of general flowsheeting design packages such as HYSIM™ and PRO/II™ (replaced by HYSYS™ and PROTISS™ in the 1990s) shifted the design emphasis away from the develop ment of specific design methods to the use of commercially available packages which could provide quick and easy short-cut designs. These developments meant that the designer was liberated from tedious calculation but still required a sound knowledge of distillation principles and the ability to analyse the simulation results in order to avoid serious errors. Many papers have been published in the mid-1990s concerning the limitations of the simulation methods (e.g. Chemical Engineering Progress 91(6): 63-75, 1995; Chemical Engineering Education 31(1): 46-51, 1997), and the problems that can occur if too great an emphasis is placed upon their use with too little feedback from experienced designers (e.g. Chemical Engineering Progress 94(6): 63-77, 1998).

Other design packages are available, the most recent developments being the Computational Fluid Dynamics (CFD) modelling software. Such packages may be useful for modelling effects within distillation equipment rather than straightforward applications of the equilibrium stage calculations. CFD is discussed later in this article in relation to possible future developments.

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