Boiling and condensation curves for

Boiling and Condensation Curves for Completely Miscible Liquids. 

Fig. 21.12. Boiling point and condensation curves for solution of involatile solute, at constant pressure.

We shall consider first the constant pressure curves. The tangents to the boiling and condensation curves are given by equations (18.46) and (18.47), while the equations for the complete curves are given by (18.28) and (18.29) which may be written in this case (dp = 0) ... 

It then follows that for stable phases the slopes at corresponding points of the boiling point and condensation curves at constant pressure are both of the same sign. Similarly, for the slopes of the two coexistence curves at constant temperature. 

A liquidus curve is also called a bubble-point curve or a boiling-point curve. Other names for a vaporus curve are dew-point curve and condensation curve. These curves are actually cross-sections of liquidus and vaporus surfaces in a three-dimensional T-p-ZA phase diagram, as shown in Fig. 13.7 on the next page. In this figure, the liquidus surface is in view at the front and the vaporus surface is hidden behind it. 

The calculation of thermodynamic properties on the saturation line (boiling and condensation line) is carried out using the independent equation-of-com-pressibility curve. The standard deviation for reproducibility of initial experimental data for the set p, by the polynomial (0.21) amounts to 0.09%. Agreement is also good between the values of p and p", which are calculated by Eqs. (0.15) and (0.21), and the data of MEI (see Fig. 9). 

Repeat the boiling point determination with the following pure liquids (a) carbon tetrachloride, A.R. (77°) (6) ethylene dibromide (132°) or chlorobenzene (132°) (c) aniline, A.R. (184-6°) and (d) nitrobenzene, A.R. (211°). An air condenser should be used for (c) and (d). Correct the observed boiling points for any appreciable deviation from the normal pressure of 760 mm. Compare the observed boiling points with the values given in parentheses and construct a calibration curve for the thermometer. Compare the latter with the curve obtained from melting point determinations (Section 111,1). 

Determination of boiling points. Distillation method (Fig. II, 12, 1) for carbon tetrachloride (25 nil. distillation flask and small water condenser), and SiwoloboflF s method (Fig. II, 12, 2) for carbon tetrachloride, aniline and nitrobenzene. Calibration curve for thermometer. Determination of b.p. of unknown liquid. 

The shape of the coohng and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell-sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

The vap. press, curve of solid iodine is indicated by PO, Fig. 16 that of liquid iodine by 00 and the effect of press, on the m.p. of iodine by ON. At the triple point 0 these curves meet. Fig. 18 shows a similar curve for water. The curve PO thus represents the sublimation curve or hoar-frost line OC. the boiling or vaporization curve, i.e. the effect of press, on the b.p. of the liquid. The same phenomenon occurs with water, iodine, etc., and the principle involved is the same as indicated in the law represented by Clapeyron-Clausius equations with respect to the lowering of the m.p. by an increase of press. Consequently, if the vap. press, of iodine be less than that of th,e triple point, the solid does not melt, but rather sublimes directly without melting at the triple point at 114-15° (89 8 mm.) and A. von Richter at 116 1° (90 mm.). According to R. W. Wood, if the condensation of iodine vapour occurs above —60°, a black granular deposit is formed, but below that temp, a deep red film is produced. 

In conventional vapor phase molecular sieve operations, the operating temperature must be even higher and the operating pressure must be even lower than that required by the dew point curve for the high boiling constituents. These extra requirements are needed to prevent capillary condensation. Condensa-... 

In the area of the miscibility gap, we have liquid phases which show vapour pressure maxima. In this immiscible liquid phase system, the condensation curve therefore has a common point with the boiling curve and is called a heteroazeotrope. In this miscibility gap, the boiling temperature will be lower than for the pure compounds and the vapour phase in equilibrium will have a constant composition. 

For instance, Swann, Howard, and Reid27b passed air into petroleum hydrocarbons held in a still at about 750° F. (398.9° C.) under a pressure of 300 pounds per sq. in. (21.09 kgs. per sq. cm.) and analyzed the products contained in the aqueous layer. The apparatus used in the work consisted of a cylindrical steel still twenty-six feet high and four feet in diameter with a capacity of about 1000 gallons of oil. Heat exchangers wore provided to preheat the feed and water coils were used to control the reflux. The vapor product passed through a condenser to a receiver. The gas oil used in the work was a Midcontinent distillate of 38.3° B density at 60° F. (15.6° C.). The oil had an initial boiling point of 552° F. (288.9° C.) and an 80 per cent over point of 701° F. (371.7° G). A complete distillation curve for the oil up to 80 per cent is given. 

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