Put Your Column on the Map

Residue curve maps (RCMs) can be employed in a variety of ways, including system visualization, evluation of data, and process synthesis, modeling and troubleshooting

By Scott Barnicki

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Now let’;s put these facts together to generate one possible flow sheet alternative (Figure 4). Assume for the moment that the benzene-rich organic layer resulting from phase separation of the ternary azeotrope is available and is combined with the feed composition. The resulting mixture composition lies within Region I. This mixture can be distilled to give pure ethanol as the bottoms product (indirect split) and a composition close to the ternary azeotrope as the distillate. Once separated into organic and aqueous phases, we have regenerated the benzene-rich entrainer composition.

Distillation of the aqueous layer gives pure water as the bottoms product (indirect split) and a distillate composition close to a point on the feed-entrainer mixing balance line. The distillate from the water column can be combined with the original feed and the mixing balance adjusted accordingly. Although material balances must be confirmed by more detailed calculations, we have completed the design of the conceptual flow sheet. Other distillation sequences are possible. (A number of references [6, 7, 8, 9, 10] detail systematic methods of synthesizing distillation-based flow sheets using RCMs.)

Evaluation of data
Let’;s explore the use of p-xylene as an entrainer for the separation of water and acetic acid by azeotropic distillation. Literature data indicate the water/p-xylene and acetic acid/p-xylene form minimum-boiling binary azeotropes and the water/acetic acid is pinched but non-azeotropic. No information could be found on the existence of a ternary azeotrope. This, of course, does not mean that one does not exist. Using the method of sketching RCMs found in Perry’;s Handbook, 7th Edition [11], leads to the identification of three RCMs that are thermodynamically feasible for a system with binary minimum-boiling azeotropes between the highest-boiling pure component and each of the other two components. With this information, a simple experiment can be performed to determine which RCM is correct.

If no ternary azeotrope exists (Figure 6a), the p-xylene/water azeotrope will be the initial distillate no matter what composition is charged to the still pot because the  p-xylene/water azeotrope is the low-boiling node in each distillation region. If a minimum-boiling ternary azeotrope exists (Figure 6b), it will be the initial distillate for any composition charged to the still, as the ternary azeotrope is again the low-boiling node for both distillation regions. If a saddle azeotrope exists (Figure 6c), the composition of the distillate will depend upon the composition charged to the still pot. For two regions the water/p-xylene azeotrope is the low-boiling node, while the  p-xylene acetic acid azeotrope is the low-boiling node for the other two regions.

A ternary mixture consisting of 10 wt.%  p-xylene, 10 wt.% acetic acid and 80 wt.% water (chosen arbitrarily) was charged to a still pot equipped with a 1-in. inner diameter 3 3-in. tall vacuum-jacketed packed column with a timed reflux head set to 6:1 reflux ratio. The mixture was fractionated batch-wise until  p-xylene was exhausted from the still pot. All distillate cuts formed two liquid phases upon standing. The table gives the overall composition of the distillate cuts and final still pot composition.

For cuts 2-6, the data are consistent with the existence of a heterogeneous minimum-boiling ternary azeotrope, boiling point of 91°C, composition of roughly 1.5 wt.% acetic acid, 36 wt.% water and the remainder  p-xylene. Therefore, Figure 6b is the correct qualitative RCM for this system. This experiment also gave information about the boiling point of the azeotrope, the composition of the two phases formed by the azeotropic mixture and the density of the two phases. Activity-coefficient parameters can be estimated directly from the single azeotropic point [12].

A powerful tool
These examples have touched upon a few applications of RCMs in process synthesis, modeling, control and operation. Although some of the more academic research in this area can seem rather esoteric, the basic principles are quite simple. With a little practice, RCM analysis can become second nature and will be one of the first tools you turn to when tackling a complex distillation problem. 

Scott D. Barnicki is a research associate for Eastman Chemical Co., Kingsport, Tenn.


1. Kiva, V.N.; Hilmen. E.K.; Skogestad, S. Azeotropic Phase Equilibrium Diagrams.  Chem. Eng. Sci., 58, 1,903-1,953 (2003).

2. Horsley, L.H., Azeotropic Data – III, Advances in Chemistry Series 116, ACS, Washington, D.C., U.S.A. (1973).

3. Gmehling, J.; Menke, J.; Krafczyk, J.; Fisher, K., Azeotropic Data, Parts I-II, VCH Publishers, New York, U.S.A. (1994).

4. Barnicki, S. D. and Siirola, J. J., Enhanced Distillation, in Perry’;s Chemical Engineers’; Handbook, 7th Edition, McGraw-Hill, New York, U.S.A. (1997).

5. Doherty, M.F.; Malone, M.F., Conceptual Design of Distillation Systems, McGraw-Hill, Boston, U.S.A. (2001).

6. Barnicki, S. D. and Siirola, J. J., Separations Process Synthesis, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Wiley & Sons, New York, U.S.A. (1997).

7. Barnicki, S. D. and Siirola, J. J., Systematic Chemical Process Synthesis, in Formal Engineering Design Synthesis, Antonsson, E.K.; Cagan, J., eds. Cambridge University Press, U.S.A. (2001).

8. Pham, H.N.; Ryan, P.J.; Doherty, M.F., Design and Synthesis of Heterogeneous Azeotropic Distillation – III. Column Sequences. Chem Eng. Sci., 45, p. 1,845 (1990).

9. Urdaneta, R.Y.; Bausa, J.; Brüggemann, S.; Marquardt, W. Analysis and Conceptual Design of Ternary Heterogeneous Azeotropic Distillation Processes.  Ind. Eng. Chem. Res., 42, 3,602-3,611 (2003).

10. Doherty, M.F.; Malone, M.F., Conceptual Design of Distillation Systems, McGraw-Hill, Boston, U.S.A. (2001).

11. Barnicki, S. D. and Siirola, J. J., Enhanced Distillation, in Perry’;s Chemical Engineers’; Handbook, 7th Edition, McGraw-Hill, New York, U.S.A. (1997).

12. Poling, B.E.; Prausnitz, J.M.; O’;Connell, J.P. The Properties of Gases and Liquids, 5th Ed., McGraw-Hill, New York, (2001), page 8.70.

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