Graphical Method for Predicting Azeotropism and Effect of Pressure on

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1952-0006.ch004

Graphical Method for Predicting Azeotropism and Effect of Pressure on Azeotropic Constants L. H. HORSLEY The Dow Chemical

Co., Midland,

Mich.

Lecat (2) has devised an analytical method for determining azeotropic boiling points and compositions for certain related groups of binary systems. The method is based on the fact that the composition and boiling point of an azeotrope are related to the relative boiling points of the two components. Lecat thus obtained a series of equations of the form δ = a + \Δ\b+ Δ c C = d + Δe + Δ2f 2

where Δ = (boiling point of component A) — (boiling point of component B) |Δ| = difference in boiling point of A and B (absolute value of Δ) C = azeotropic composition in weight per cent A δ = difference in boiling point of azeotrope and the lower boiling component a, b, ....f = constants for a given series of related azeotropes such as methanol-hydrocarbons Note that Δ may be positive or negative; |Δ| is always positive. From a practical standpoint, for determining the azeotropic constants of a system, the plots of the above equations have been found more useful and are given in Figures 1 to 5 for forty-five systems for which data are available. Up to this time only the curve for ethanol-halide hydrocarbons has been published (1). Another use for this set of curves is for estimating the azeotropic boiling point and composition at pressures other than atmospheric. Consider the azeotrope methanol-benzene. Since the vapor pressure curves of methanol and benzene are known, the difference in boiling point, A , can be obtained at any pressure. From this value of A and the C - A curve for methanol-hydrocarbons the azeotropic concentration C at that pressure can be determined. For example, the effect of pressure on the methanol-benzene azeotrope is shown in Table I. Table I. Pressure, Mm. Hg 200 400 760 6,000 11,000

Boiling Point, ° C . Methanol Benzene 35 50 65 130 153

43 61 80 162 193

Effect of Pressure Azeotropic B o i l i n g P o i n t , ^_C. Found Calcd.

A >

° C.

23 39 55 125 150

-8 -11 -15 -32 -40

26 42 57 124 149

C, W e i g h t % Found Calcd. 30 33 39 54 64

34 36 40 55 63

A plot of A as a function of C from this table is shown in Figure 6. The experimental data are represented by the five points while the smooth curve is identical with the methanol-hydrocarbon curve in Figure 1. 321

AZEOTROPIC DATA Advances in Chemistry; American Chemical Society: Washington, DC, 1973.


rangeIN indicated areSERIES also 322 Similar curves and data for other systems over the pressure ADVANCES CHEMISTRY shown. In each case the curve is the same as the general curves of Figures 1 to 5, while the experimental points are for the particular system and for the pressure range indicated. In the same way, the 5-| A| curves of Figures 1 to 5 can be used to determine 5 and the azeotropic boiling point at any pressure from the value of | A| at that pressure. While the agreement between predicted and experimental values is far from perfect, the method has served as a valuable guide in estimating effect of pressure on azeotropic systems. It is recognized that it would be more convenient to be able to plot pressure instead of A as a function of C and 8. However, this would require a separate curve for each azeotrope, whereas the above method permits use of a single curve for a large group of systems.

Literature Cited (1) Lecat, Ann. soc. sci. Bruxelles, 55B, 43 (1935). (2) Lecat, Compt. rend., 183, 880 (1926); 184, 816 (1927); 189, 990 (1929); Ann. soc. sci. Bruxelles, 47B, 39, 87 (1927); 48B, 1, 105 (1928); 49B, 28, 119 (1929); 55B, 43, 253 (1935); 56B, 41 (1936); Atti accad. nazl. Lincei, (6) 9, 1121 (1929); Z. anorg. allgem. Chem., 186, 119 (1930).


GRAPHICAL METHOD FOR PREDICTING AZEOTROPISM

I I I I I I I I METHANOL - HYDROCARBONS


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C-A and 5-|A| Curves for Alcohol-Hydrocarbon, Glycol-Hydrocarbon, and Phenol-Hydrocarbon Systems C. Azeotropic composition in weight % first component 5. Boiling point of lower boiling component minus azeotropic boiling point |A|. Absolute difference in boiling points of components A . Boiling point of first component minus boiling point of second component


324

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C-A and 5-|A| Curves for Phenol-Hydrocarbon, Acid-Hydrocarbon, and Alcohol-Halide Hydrocarbon Systems



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Graphical Method for Predicting Azeotropism and Effect of Pressure on

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