Multicomponent distillation equilibrium curve

Figure 8-2 illustrates a typical normal volatility vapor-liquid equilibrium curve for a particular component of interest in a distillation separation, usually for the more volatile of the binary mixture, or the one where separation is important in a multicomponent mixture. 

Identifying mislocated feed points The feed point should be where the g-line intersects the equilibrium curve. This rule works well for binary distillation (Sec, 2,2,5), but not so well for multicomponent distillation (Sec, 2.3.7). In Fig, 2,22 it works well the x-y diagram... 

Identify mislocated feed. For binary distillation the feed point should be where the q-line intersects the equilibrium curve. For multicomponent distillation this may or may not be the case. So for multicomponent distillation feed location, key ratio plots and d/b plots are preferred and discussed next. 

The separation of a binary mixture by distillation may be represented in two-dimensional space while n-dimensional space is required to represent the separation of a multicomponent mixture (i > 2). The graphical method proposed by McCabe and Thiele9 for the solution of problems involving binary mixtures is presented in a subsequent section. The McCabe-Thiele method makes use of an equilibrium curve which may be obtained from the boiling-point diagram."... 

Integral condensation in which the liquid remains in equilibrium with the uncondensed vapour. The condensation curve can be determined using procedures similar to those for multicomponent flash distillation given in Chapter 11. This will be a relatively simple calculation for a binary mixture, but complex and tedious for mixtures of more than two components. 

Feed analyses in terms of component concentrations are usually not available for complex hydrocarbon mixtures with a final normal boiling point above about 38°C (100°F) (n-pentane). One method of handling such a feed is to break it down into pseudo components (narrow-boiling fractions) and then estimate the mole fraction and K value for each such component. Edmister [Ind. Eng. Chem., 47,1685 (1955)] and Maxwell (Data Book on Hydrocarbons, Van Nostrand, Princeton, N.J., 1958) give charts that are useful for this estimation. Once K values are available, the calculation proceeds as described above for multicomponent mixtures. Another approach to complex mixtures is to obtain an American Society for Testing and Materials (ASTM) or true-boiling point (TBP) curve for the mixture and then use empirical correlations to construct the atmospheric-pressure equilibrium-flash curve (EFV), which can then be corrected to the desired operating pressure. A discussion of this method and the necessary charts are presented in a later subsection entitled Petroleum and Complex-Mixture Distillation. ... 

The RCM technique has been considered a powerful tool for the flowsheet development and preliminary design of conventional multicomponent, nonideal separation processes (Fien and Liu, 1994). It represents a good approximation to actual equilibrium behavior and allows the designer to perform feasibility analysis of separation processes where nonideal and azeotropic mixtures are involved. Traditionally, nonreactive residue curves have been used to predict the liquid-composition trajectories in continuous distillation... 

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