Vapor-liquid equilibrium maximum boiling

The data in Tables I-XVI (see Appendix for all tables) show the isobaric vapor-liquid equilibrium results at the boiling point for potassium, ammonium, tetramethylammonium, tetraethylammonium, tetra-n-propylammonium, and tetra-n-butylammonium bromides in various ethanol-water mixtures at fixed liquid composition ratios. The temperature, t, is the boiling temperature for all solutions in these tables. In all cases, the ethanol-water composition was held constant between 0.20 and 0.35 mole fraction ethanol since it is in this range that the most dramatic salt effects on vapor-liquid equilibrium in this particular system should be observed. That is, previous data (12-15,38) have demonstrated that a maximum displacement of the vapor-liquid equilibrium curve by salts frequently occurs in this region. In the results presented here, it should be noted that Equation 1 has been modified to... 

Figure 14.11 The critical locus for (xiQHp + x2C6F6), a system with a maximum boiling azeotrope. In (A), the circles represent the critical points (a and b) of pure components (1) and (2) the solid lines represent (vapor + liquid) equilibrium for the pure substances the dashed line is the critical locus, and the short-dashed line represents the azeotrope composition, which intersects the critical locus at point c. (B) shows the intersection of the (vapor + liquid) equilibrium lines with the critical locus.

Figure 8-7 Minimum boiling azeotrope (top), and maximum boiling azeotrope (bottom). Data from Vapor-Liquid Equilibrium Data Collection, J. Gmeliling, U. Onken, and W. Arlt, DECHEMA. 

Let us first consider binary mixtures which we shall term ordinary by this is meant that the liquid components dissolve in all proportions to form homogeneous solutions which are not necessarily ideal and that no complications of maximum or minimum boiling points occur. We shall consider component A of the binary mixture A-B as the more volatile, i.e., the vapor pressure of pure A at any temperature is higher than the vapor pressure of pure B. The vapor-liquid equilibrium for each pure substance of the mixture is of course its vapor-pressure-temperature relationship, as indicated in Fig. 7.1. For binary mixtures an additional variable, concentration, must likewise be considered. Mole fractions are the most convenient concentration terms to use, and throughout this discus-... 

An important system in distillation is an azeotropic mixture. An azeotrope is a liquid mixture which when vaporized, produces the same composition as the liquid. The VLE plots illustrated in Figure 11 show two different azeotropic systems one with a minimum boiling point and one with a maximum boiling point. In both plots, the equilibrium curves cross the diagonal lines. 

Many liquid mixtures exhibit azeotropes at intermedrate concentrations such that the liquid and its equilibrium vapor have the same composition. No separation of (his concentration is possible by partial vaporization. A binary mixture may have a minimum bailing azeotrope, where the boiling temperature of the azeotrope is less than that of the pare components, or a maximum boiling azeotrope, where (he boiling temperature is higher (han that of the pare components. About 90% of die known azeotropes are of the minimum variety. 

Moody, F. J. 1975, Maximum Discharge Rate of Liquid-Vapor Mixtures from Vessels, in Non-equilibrium Two-Phase Flows, R. T. Lahcy, Jr., and G. B. Wallis, Eds., ASME, New York. (3) Moore, F. D., and R. B. Mesler, 1961, The Measurement of Rapid Surface Temperature Fluctuations during Nucleate Boiling of Water, AIChE J. 7 620-624. (2)... 

Suppose that, under the pressure P, we may observe a condition of indifferent equilibrium where the liquid mixture and the saturated vapor have the same composition f the boiling-point 0 of the mixture of composition is maximum or minimum among the boiling-points which the liquid mixture may have under constant pressure P. 

Comment 2 Suppose that our feed composition had been a little higher, say xp. No matter how many steps we took, we could never get a liquid composition higher than that at the intersection of the operating and equilibrium curves. In that case, any solution for the specified L/V ratio is impossible. Instead the L/V ratio would have to be decreased, which will rotate the operating line clockwise about the specified point. This will raise the liquid composition at the intersection point and make the separation possible. It turns out there exists maximum L/V, or a minimum vapor rate V which allows a particular bottoms composition xp to be produced from a given feed composition xp and liquid flowrate L. This minimum V/L is called the minimum boil-up ratio. 

Figures 3 and 4 are the predicted profiles of vapor and liquid composition along the column with 43 ml of catalyst and a reflux flow rate of 22 g/tnin. It is important to note that both the liquid and vapor concentration profiles for acetone in the column are relatively high and hence it is favorable for the formation of DAA. The equilibrium constants calculated from the equilibrium conversion data [9,10] are given in Figure 5, which indicates that at 54 °C, the Ac conversion at equilibrium conversion is only 4.3 wt %. In order to carry out the aldol condensation of acetone in the CD column, the temperature at the reaction zone of the CD column will be near the boiling point of Ac in order to maintain liquid vapor equilibrium. Our CD experimental results show that a maximum concentration of 55 wt% of DAA concentration was obtained which clearly exceeds the equilibrium conversion. The aldol condensation of Ac to produce DAA is an excellent example to demonstrate that in situ separation in a CD column results in an increased yield for equilibrium limited reactions.

The appearance of azeotropic points has important consequences for the distillation of the mixtures concerned. First let us consider a system with a boiling point maximum (Fig. 14.23). A liquid mixture having the composition x boils at temperature Ti and its corresponding vapor is enriched by the more volatile component B (xf). If the vapor is removed continuously from equilibrium by simple distillation, meaning by condensation in a receiver, the composition of the... 

For the mixture isopropyl ether/isopropyl alcohol, positive deviations from Raoult s law occur. They are so strong that the total pressure reaches a maximum for a certain concentration. The vapor pressure maximum corresponds to a boiling temperature minimum as it is depicted in the middle row. Boiling curve and dewpoint curve contact each other in the so-called azeotropic points. Here the mole fractions of vapor y and liquid have the same value which also follows from the equilibrium diagram below. In the azeotropic point A the equilibrium curve intersects the diagonal Pa = To the left of point H the mole fraction of compo-... 

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