Residue curve maps nonideal systems

Figure 3.10 Residue curve maps for nonideal ternary systems involving azeotropes.

An ideal mixture of n components requires a sequence of n - 1 conventional distillation columns (two product streams) to separate the components completely. The columns can be arranged sequentially without recycle between them. This picture changes when mixtures forming azeotropes must be separated. Nonideal systems sometimes require complex distillation arrangements involving more than n - 1 columns with recycle of material between the columns. For the analysis of such systems, we recommend the use of residue curve maps. We base the following summary on the excellent book by Doherty and Malone (1998), who pioneered the use of these techniques. 

FK>URE 23 A residue curve map for (a) a constant relative volali Iky system widi a = [5, 1 ] and (b) die nonideal acetone/benzene/chlorofonn system using the NRTL model atP= atm. 

FKvURK 2.17 Entire residue curve maps for (a) a constant relative volatility stem with a = [5,1 and (b) the nonideal acetone/benzene/chloroform system using die NRTL model at... 

FKjURK 2.18 Entire residue curve map for the nonideal acetone/ethanol/methanol system (with numbered nodes) using the NRTL model 3t P = atm. 

Residue curve maps have shown to provide valuable insights and design assistance for nonideal systems, particularly for reactive distillation. Transforming the composition variables according to Doherty s approach allows to define a reaction invariant space of... 

Residue curve (RCM) and distillation curve (DCM) maps are today standard tools for designing distillation systems dealing with nonideal mixtures involving azeotropes. A residue curve characterizes the evolution of the liquid composition in a vessel during a batchwise distillation experiment. The whole compositional space may be spanned by residue curves considering different initial mixture compositions. For nonreactive mixtures the RCM is obtained by solving the component dynamic material balance expressed by the following differential equation ... 

Another feature of a residue map we would like to illustrate is the representation of systems that form two liquid phases. In Fig, 6.3 we show how mixtures of vinyl acetate and water form two liquid phases with drastically different compositions. We can take advantage of this nonideality to help produce pure acetic acid from a single distillation column. In Fig. 6.4 we show how the net feed to a column can be changed by mixing the original feed with the vinyl acetate rich reflux. The new feed composition contains less acetic acid acid and water and more vinyl acetate. When we look at the residue curves that pertain to the new feed composition, we find that they move over areas with little water. Most of the feed water is rejected with the overhead vapors... 

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