Liquid-Vapor Composition Curves of Acetic Acid and Water at Subatmospheric Pressures D. B. KEYES,University of Illinois, Urbana, Ill.
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N A previous paper' a method was described for determining the number of plates in a fractionating column necessary for the separation Of liquids' The method was applied to the concentration of aqueous solutions of acetic acid a t atmospheric pressure. The possibility that fewer plates would be required a t lower pressures has since been considered; this paper describes the method of obtaining data to be used for p l o t t i n g liquidvapor c o m p o s i t i o n curves of acetic acid and water a t subatmospheric pressures. From these curves the number of plates required for concentrati n g a c e t i c a c i d at t h e s e pressures may be found. The boiling points of aqueous acetic acid s o l u t i o n s a t subatmospheric pressures were d e t e r m i n e d . FIGURE1. EQUILIBRIUM LINE AND X = Y LINEAT ATMOSPHERIC PRES- The partial pressures SURE and hence t h e mole fractions of water and acetic acid were determined by the familiar nitrogen-bubbling method. The vapors carried over by the nitrogen were condensed in a trap cooled by solid carbon dioxide and acetone. The acetic acid contents were determined by titration with standard alkali and the water contents obtained by difference. The solutions whose vapor pressures were to be determined were maintained in a thermostat a t their boiling temperatures under reduced pressure. Thus the vapor pressures of the solutions a t their boiling points under reduced pressure were determined.
The results of the investigation are shown in Tables I and 11, and Figures 2 and 3. TABLE 1. B O I L W POINTS ~ OF AQmousA C ~ T IAC ~ S o I ~~m ~ o ~ s UNDER
REDUCED PRESSURES
ACETIC SOLN. ACID WATER Mole % Mole % 1 50.7 49.3 2 35.90 64.10 3 21.45 78.55 4 10.78 89.22 5 5.37 94.63 6 1.076 98.924 7 0.679 99.321 8 0.226 99.774
350 mm. H g O
c.
BOILING POINT 200 mm. H g 100 mm. H g
c.
82.0 80.50 80.3 79.3 78.6 78.0 77.5 77.1
69.0 67.5 66.9 66.1 65.4 64.8 64.4 64.1
c.
56.0 53.9 51.8 50.8 50.0 49.5 49.3 49.1
TABLE 11. LIQUID-VAPOR EQUILIBRIUM OF WATERw AQUEOUS ACETICACIDSOLUTIONS WATER IN
SOLN. 1 2 3 4 5 6 7
'
S
7
SOLN. Mole %
350 mm. H g
49.3 64.1 78.55 89.22 94.63 98.924 99.321 99.774
62.35 74.62 89.76 93.35 96.92 99.35 99.61 99.89
Mole %
WATERIN VAPOR 200 mm. H g 100 mm. H g M o l e yo Mole % 61.31 56.7 73.86 72.67 85.66 83.98 92.76 91.94 96.35 95.61 99.26 99.14 99.54 99.46 99.85 99.81
By the use of the method of Keyes, Soukup, and Nichols,' it may be seen from Figure 2 that more plates will be necessary in a column for the concentration of acetic acid a t sub-
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1 Keyea, D. B., Soukup, R.. and Nichols, W. A., IND. ENQ.CHEM.,20, 464 (1928).
1. d/3. FIGURE PHERIC
Mu< Pepceny Wufer -2
994
/p W K
L/gu/b', W8
EQUILIBRIUM LINESAT SUBATMOSPRESSURES FOR DILUTE SOLUTIONS
atmospheric than a t atmospheric pressures since the curves for the subatmospheric pressures fall between the curve of atmospheric pressure and the 45' line.
SUMMARY
M o / PBrcenrl wore/ r'n
70
Liquid ao
I (D
FIGURE 2. EQUILIBRIUM LINESAT SUBATMOSPHERIC PRESSURES Curves show decrease in water concentration in vapor a5 preBsure decreases.
The composition of the vapor of aqueous solutions of acetic acid varying in strength above 50 mole per cent of water has been determined a t their boiling points under pressures of 350, 200, and 100 mm. From these data the number of plates in a fractionating column necessary to concentrate acetic acid and water has been found to be appreciably more than a t atmospheric pressure. RECEIVED October 15, 1932. The experimental work of this paper warn carried out by R. V. Boyer. H . W. Nelson. E.Vacherlon, and J. W. Martin.
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