Residue Curves for Ternary Systems

The liquid-phase nonideality is so large that a heterogeneous azeotrope is formed. The molecules are so dissimilar that two liquid phases are formed. The composition of the vapor is 75.17mol% water at 1 atm. The compositions of the two liquid phases that are in equilibrium with this vapor are 43.86 and 98.05 mol% water, respectively. 

Residue curve analysis is quite useful in studying ternary systems. A mixture with an initial composition Xi(0) and X2(o is placed in a container at some fixed pressure. A vapor stream is 

OQiient Piopei methed Heniy e(npanents Ch n tlr)i 10 SrniHieri pproMh 

Notice that all the residue curves start at the lightest component (C4) and move toward the heaviest component (C6). In this sense, they are similar to the compositions in a distillation column. The light components go out to the top, and the heavy components go out at the bottom. We will show below that this similarity proves to be useful for the analysis of distillation systems. 

The generation of residue curves is described mathematically by a dynamic molar balance of the liquid in the vessel Muq and two dynamic component balances for components A and B. The rate of vapor withdrawal is V (moles per time). 

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As illustrated throughout this section, process simulators have extensive facilities for preparing phase-equilibrium diagrams T-x-y, P-x-y, x-y,... ), and residue curve maps and binodal curves for ternary systems. In addition, related but independent packages have been developed for the synthesis and evaluation of distillation trains involving azeotropic mixtures. These include SPLIT by Aspen Technology, Inc., and DISTIL by Hyprotech (now Aspen Technology, Inc., which contains MAYFLOWER developed by M.F. Doherty and M.F. Malone at the University of Massachusetts). 

FIG. 13-60 Residue curves for acetone-chloroform-methanol system suggesting a ternary saddle azeotrope. 

Figure 12.11 superimposes distillation lines and residue curves for the same ternary systems. Figure 12.11a shows the system n-pentane, n-hexane and n -heptane, which is a relatively wide boiling mixture. It can be observed in Figure 12.11a that there are significant differences between the paths of the distillation lines and the residue curves. By... 

Sketching Residue Curve Plots for Ternary Systems... 

Figure 7.24 Stability of residue curves for a ternary system in the vicinity of a binary azeotrope.

Figure 12.8 Residue Curve Map for Ternary System without Azeotrope...

Residue curves in batch distillation The illustrated forms of the Rayleigh equations (6.3.87) and (6.3.91) allow one to relate the change in the total number of moles in the residual liquid phase with the liquid-phase composition change in simple batch distillation with no reflux and total condensation. One would like to know how the liquid-phase composition changes with time. Visualization of this composition change pathway is going to be quite illustrative. For ternary systems, such visualizations are likely to be particularly useful. The topic of residue curve maps is briefly introduced here to that end. 

The residue curve maps for ternary mixtures having one binary minimum-boiling azeotrope are much more complicated. They are even more compiex when there are multiple azeotropes. See Doherty and Malone (2001) for illustrative figures. Siirola and Barnicki (1997) illustrate residue curves for a variety of systems. Complex properties of the residue maps are analyzed in detail in Doherty and Perkins (1978) and Van Dongen and Doherty (1984). 

The vapor is thea withdrawa from the stiH as distillate. The changing Hquid composition is most coavenieafly described by foUowiag the trajectory (or residue curve) of the overall composition of all the coexistiag Hquid phases. An exteasive amouat of valuable experimental data for the water—acetoae—chloroform mixture, including biaary and ternary LLE, VLE, and VLLE data, and both simple distillation and batch distillation residue curves are available (93,101). Experimentally determined simple distillation residue curves have also been reported for the heterogeneous system water—formic acid—1,2-dichloroethane (102). 

The simplest form of ternary RCM, as exemplified for the ideal normal-paraffin system of pentane-hexane-heptane, is illustrated in Fig. 13-58 7, using a right-triangle diagram. Maps for all other non-azeotropic ternary mixtures are qiiahtatively similar. Each of the infinite number of possible residue curves originates at the pentane vertex, travels toward and then away from the hexane vertex, and terminates at the heptane vertex. 

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

Figure A.4 Reactive residue curve maps for a ternary system containing inert, reaction A+B+/<- C+/.

Schematic DRDs are particularly useful in determining the implications of possibly unknown ternary saddle azeotropes by postulating position 7 at interior positions in the temperature profile. Also note that some combinations of binary azeotropes require the existence of a ternary saddle azeotrope. As an example, consider the system acetone (56.4°C), chloroform (61.2°C), and methanol (64.7°C) at 1-atm pressure. Methanol forms minimum-boiling azeotropes with both acetone (54.6°C) and chloroform (53.5°C), and acetone-chloroform forms a maximum-boiling azeotrope (64.5°C). Experimentally there are no data for maximum- or minimum-boiling ternary azeotropes for this mixture. Assuming no ternary azeotrope, the temperature profile for this system is 461325, which from Table 13-18 is consistent with DRD 040 and DRD 042. However, Table 13-18 also indicates that the pure-component and binary azeotrope data are consistent with three temperature profiles involving a ternary saddle azeotrope, namely, 4671325, 4617325, and 4613725. All three of these temperature profiles correspond to DRD 107. Calculated residue curve trajectories for the acetone-chloroform-methanol system at 1-atm pressure, as...

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