Boiling point diagram, — x, equilibrium curve

While we have studied the properties of binary mixtures at constant temperature so far we shall now examine the behaviour of these mixtures at constant pressure. The conditions are those found in distillation, which is normally an isobaric process tending to establish equilibrium between the liquid and vapour phases. A boiling point diagram shows the boiling points and the equilibrium compositions of binary mix- 

Boiling point diagram for benzene-toluene solutions at 760 torr 

Florin [76] gives a method of calculating the boiling point diagram of an ideal multicomponent mixture. Only few points are determined but in addition the slopes of the curve at these points need to be calculated. 

The equilibrium curve reveals the relationship between the concentration of the liquid, Xb, and the corresponding vapour concentration, y, in equilibrium. Thus the equilibrium curve provides the basis for the calculation of separating stages by the McCabe-Thiele method [77] which is no doubt most commonly used due to its 

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From the boiling point diagram construct the equilibrium x-y curve for a total pressure of 760 mm Hg, Figure 8-19. 

Figure 14-12 A boiling point diagram for a solution of two volatile liquids, A and B. The lower curve represents the boihng point of a liquid mixture with the indicated composition. The upper curve represents the composition of the vapor in equilibrium with the boiling liquid mixmre at the indicated temperature. Pure liquid A boUs at a lower temperamre than pure hquid B hence, A is the more volatile liquid in this illustration. Suppose we begin with an ideal equimolar mixmre = Xg = 0.5) of liquids A and B. The point P represents the temperature at which this solution boils, Tj. The vapor that is present at this equilibrium is indicated by point Q (X = 0.8). Condensation of that vapor at temperature Ti gives a liquid of the same composition (point E). At this point we have described one step of simple distillation. The boiling liquid at point if is in equilibrium with the vapor of composition indicated by point S (X > 0.95), and so on.

FIG. 2-7 Enthalpy-concentration diagram for aqueous ammonia. From Thermodynamic and Physical Properties NH3-H20, Int Inst. Refrigeration, Paris, France, 1994 (88 pp.). Reproduced by permission. In order to determine equilibrium compositions, draw a vertical from any liquid composition on any boiling line (the lowest plots) to intersect the appropriate auxiliary curve (the intermediate curves). A horizontal then drawn from this point to the appropriate dew line (the upper curves) will establish the vapor composition. The Int. Inst. Refrigeration publication also gives extensive P-v-xtah es from —50 to 316°C. Other sources include Park, Y. M. and Sonntag, R. E., ASHRAE Trans., 96,1 (1990) 150-159 x, h, s, tables, 360 to 640 K) Ibrahim, O. M. and S. A. Klein, ASH E Trans., 99, 1 (1993) 1495-1502 (Eqs., 0.2 to 110 bar, 293 to 413 K) Smolen, T. M., D. B. Manley, et al.,/. Chem. Eng. Data, 36 (1991) 202-208 p-x correlation, 0.9 to 450 psia, 293-413 K)i Ruiter, J. P, 7nf. J. R rig., 13 (1990) 223-236 gives ten subroutines for computer calculations. 

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