I
R. C. REID, A. B. REYNOLDS, D. T. MORGAN, G. W. BOND, Jr., J. E. SAVOLAINENI, and M. L. HYMANZ Massachusetts Institute of Technology Engineering Practice School, O a k Ridge, Tenn.
Vapor-Liquid Equilibria Dilute Nitric
Acid, Hydrochloric Acid, and Hydrochloric Acid-
Nitric Acid- Water Solution s )Design data for hydrochloric and/or nitric acid gas absorbers
IN
PROCESS development studies a t the Oak Ridge National Laboratory, it became necessary to obtain vapor-liquid equilibrium data on dilute, aqueous hydrochloric and nitric acid systems and on dilute hydrochloric acid-nitric acidwater solutions. A search of the literature did not yield sufficient reliable data, so an experimental study of these systems was initiated, the results of which are presented in this article. Such vapor-liquid equilibrium data may prove valuable in the design of hydrochloric and/or nitric acid gas absorbers and may provide as well a basis for the partial separation of chloride from nitrate by distillation of hydrochloric acid-nitric acid-water solutions. Both of these uses are important from an economic viewpoint, as acid absorbers may be used in the recovery of waste acid from process gases and removal of chloride ion from hydrochloric acidnitric acid-water dissolution solutions would permit the use of stainless steel equipment in further processing. All the work reported here is limited to dilute aqueous systems, below the azeotropic points for the pure acids and below the concentration range for the mixed acid where appreciable decomposition occurs. This latter limitation was necessary, as equilibrium data for hydrochloric acid-nitric acid-water solutions in concentration ranges where much decomposition occurs are meaningless because the concentration of the constituents is time dependent.
Experimental Equilibrium Stills. Both a modified Othmer still (8) and a Gillespie still (6) were used to obtain the vapor-liquid equilibrium data. The construction and
Present address, Oak Ridge National Laboratory, Union Carbide Nuclear Go., Oak Ridge, Tenn. 2 Present address, The Pfaudler Co., Rochester, N. Y.
operation of these stills are described in detail by Carney ( 2 ) . The Othmer still was heated by a n external heating coil and the Gillespie still by an internal heating coil. Both stills were thermally insulated, the Gillespie with asbestos and aluminum foil and the Othmer with aluminum foil only. Both stills were vented to the atmosphere, so the data reported are essentially isobaric. The maximum atmospheric pressure variation was only 8 mm. of mercury over a range from 739 to 747 mm. of mercury. Temperatures in the Gillespie still were measured with a calibrated thermometer inserted in the entrance to the disengagement chamber. Temperatures were not measured in the Othmer still, as it was attempted to minimize reflux. The possibility that some reflux occurred in the Othmer still is discussed in more detail later. All studies on hydrochloric acid-nitric acid-water solutions were limited to concentration ranges where decomposition was believed to be negligible. However, to ensure that very little decomposition did occur during the measurements, the atmospheric vent lines from the condensate condensers of both stills were passed through an acetone-dry ice cold trap which maintained a temperature of about -79' C. Any nitrosyl chloride C.) or chlorine (boiling point -5.5' (boiling point -35' C.) formed by the decomposition reaction (5) HNOa
+ 3 H C 1 4 NOCl + Cle + 2H20
would be condensed in the trap. Decomposition was assumed negligible if no condensation occurred in the cold trap during the measurement. Procedure. The Othmer and Gillespie stills were charged with approximately 375 and 175 ml. of solution, respectively. The solutions were boiled for 75 minutes, a t which time approximately eight to nine condensate receiver volumes had been circulated in each still. When necessary, Teflon boiling chips were used in the Othmer still to prevent bumping. T o verify that equilibrium had actually been attained in the 75 minutes of
boiling time, solutions of identical initial composition were boiled for 75 and 240 minutes. Comparison of the liquid and vapor compositions from these runs showed that 75 minutes was sufficient time for the attainment of equilibrium. Studies were also made to determine if any entrainment was occurring during boiling. Both stills were first rinsed with distilled water until no chloride could be detected in the wash with silver nitrate solution. The stills were then charged with a 20% by weight solution of potassium chloride and boiled for 75 minutes a t the same rate as used in the equilibrium runs. Samples of the vapor condensate were then analyzed for chloride concentration. The Gillespie still condensate analyzed 8 p.p.m. of chloride and the Othmer still 3 p.p.m. of chloride. Based on these experiments, entrainment was assumed negligible. Analysis of Still Liquid and Condensate. Specific gravities of the condensate and pot liquid were determined on a Moore-Westphal balance. After measurement of the specific gravity, the acid samples were analyzed for both total acid and chloride concentration. Nitrate concentrations were obtained by difference. The total acid concentration was determined by titration with standard sodium hydroxide. For acid concentrations greater than 0.3M a Beckman automatic titrator was used; for concentrations lower than 0.3M manual titration was used, as microburets were required. The chloride concentration was determined by potentiometric silver nitrate titrations, using calomel and silver-silver chloride electrodes. At concentrations less than 0.02M microburets were used. The accuracy of the above analyses has been estimated by the Special Analysis Laboratory of Oak Ridge National Laboratory to range between 2 and 5%, the higher concentration ranges being the more accurate. Duplicate samples sent to the laboratory checked within f15y0 for concentrations lower than 0.02M in total acid and within f2% for concentrations greater than 2M. VOL. 49, NO. 8
AUGUST 1957
1307
6oF
40
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1
02
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>-
O----PRESENT
-
008 006
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// 1
d /
O---- O T b M E R S T I L L D A T A LEGEND S T I L L DATA
I A----GILLESPIE
4 I 1
LORD R A L E I G H 110, PHIL MAG
c
P 0 0 0 1 l I
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2
1
4
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l 6
l
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X , M O L E PER C E N T H N 0 3 , L I Q U I D
Figure 3.
1308
-4
WORK, O T H U E R S T I L L
AL: 0
II
Equilibrium data for nitric acid-water at atmospheric pressure
INDUSTRIAL AND ENGINEERING CHEMISTRY
34
plotted in Figure 3 and compared with data from Carpenter and Babor ( 3 ) , Berl and Samtleben ( I ) , and Jaulmes (7). The dilute acid results reported by Carpenter and Babor and by Berl and Samtleben indicate a higher vapor con-
VAPOR-LIQUID E Q U I L I B R I A
-
HCI, 0.38
06
TO 6.9 MOLE PER CENT 04
0.4
t01
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008
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006
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-
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0 0.1’2 L
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*- 0.04-
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0.01
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-
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r
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0006 -
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0004
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Y/X D----
FOR H C I OVER
BOILING HCI-HN03-HzO Y I X FOR PURE HCI-HzO
0.004
001
0008
0.002 -
-
-
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0 001 0
acid-water systems .
ri
4.
centration than found in the present work. At higher liquid concentrations all the data agree closely. The equilibrium apparatus used by these two groups of investigators was similar to that of Lord Raleigh. A batch distillation from a simple still was used with little attempt to prevent small amounts of entrainment. Since even very minute amounts of entrainment at these low concentrations can cause greatly exaggerated vapor compositions, it is not surprising that simple batch distillation may yield inaccurate results. The low vapor compositions reported by Jaulmes are more difficult to explain. A steam distillation technique was employed and the curves of vapor composition us. time of distillation were extrapolated back to zero time. This extrapolated “zero-time” vapor composition was divided by the liquid composition to give distribution coefficients (y/x) as a function of x . These distribution coefficients were nearly constant from about 0.02 to 4 mole yo. Jaulmes’ results agree well with those reported here up to about 1 mole % nitric acid in the liquid; above this value the divergence is apparent.
2
4 TOTAL
6 6 ACID, MOLE P E R C E N T
10
12
14
Figure 5. Effect of nitrate concentration on distribution of nitrate between vapor and liquid for hydrochloric acidnitric acid-water and nitric acid-water systems
It is believed that vapor-liquid equilibrium data taken with a carefully operated Gillespie still are more accurate than those taken either by simple batch distillation or by vapor entrainment techniques; for this reason the curve in Figure 3 is drawn through the Gillespie still data points. HCI-HNOrH20 Solutions. Experimental results for liquid-vapor equilibrium of hydrochloric acid-nitric acidwater solutions are presented in Table I and Figures 4 to 7 . A modification of a technique presented by Crooks and others (4) for low chloride hydrochloric acid-nitric acid-water solutions was used in preparing the plots. Crooks obtained a smooth curve by plotting the logarithm of the distribution coefficient ( y / x ) against the mole fraction of nitric acid in the solution or essentially against the total acid in the solution, as the chloride concentrations used in his work were very small. Accordingly, in the present investigation semilog plots ofy/x for each acid us. total acid in the liquid were used to correlate the data. This technique worked very well for hydrochloric acid. When y / x for hydrochloric acid is plotted against total acid
in the liquid (Figure 4), the data for both hydrochloric acid-water and hydrochloric acid-nitric acid-water solutions plot together and yield an approximate straight line, although a slight curvature can be noted in the hydrochloric acidnitric acid-water curve. This type of plot was not applicable for nitric acid data correlation. Various means were tried to correlate the data, but none was completely satisfactory. One of the best methods, discussed below, qualitatively, and to some extent quantitatively, describes the change in the distribution coefficients of nitric acid as a function of hydrochloric acid concentration. In this correlation method, y / x for nitric acid was plotted against the total acid concentration (analogous to the hydrochloric acid correlation), A considerable scatter of points resulted. There were, however, definite trends if groups of points of similar liquid nitric acid content were considered. Four such groups may be found from Table I and these data may be plotted as shown in Figure 5. Lines of constant liquid nitric acid concentration have been drawn through these data. All the lines VOL. 49, NO. 8
AUGUST 1957
1305,
LEGEND
32
t
HCI-HNO~-HZO
30
02
c
00 0 8
I
I
I
I
I
I
3
2
I
I 4
I
I
X. MOLE PER CENT
Figure 6.
Effect
1
5 “02,
1 - 1
I
6
7
I
I
I
8
b
’
LlQUlO
of liquid nitric acid concentration on A
are nearly parallel and this fact forms the basis of the proposed correlation. An arbitrary base line was drawn on Figure 5 (dashed line), the remainder of the data from Table I were plotted, and perpendicular distances from each point to the base line were measured and plotted as a function of liquid nitric acid concentration. These distances, noted here as A, when plotted again liquid nitric acid concentration, yielded a curve with but little scatter (Figure 6). This “smoothing” curve may be used to plot the final correlation in Figure 7, where the distribution coefficient is shown as afunction of total acid concentration for vari-
/
Figure 7. Effect of nitrate concentration on distribution of nitrate between vapor and liquid for hydrochloric acidnitric acid-water and nitric acid-water systems
ous integral values of liquid nitric acid concentration. The A increments for pure nitric acid water solutions are also shown in Figure 6. The two curves diverge below about 4. mole 7 0 nitric acid but converge above this concentration. The divergence at low concentrations indicates that in this region extrapolation of any constant nitric acid liquid line will not intersect the pure nitric acid-water curve at the correct concentration. No reason for this anomaly is offered, although the accuracy of the nitric acid analyses in the low concentrations is not good, as they were calculated by subtracting potenti-
ometrically measured chloride from total acid--i.e., by the difference of two numbers, both of which were often larger than the resulting nitric acid concentration. Figure 7 is an empirical correlation of the experimental data but should be sufficiently accurate for engineering design purposes, Literature Cited (1) Rerl, E., Samtleben, O., Z. angew, Chem. 35, 201 (1922). ( 2 ) Carney, T. P., “Laboratory Frac-
tional Distillation,” pp. 186 -96, Macmillan, New York, 1949.
( 3 ) Carpenter, C. D., Babor, J., Chem. M e t . Eng. 26, 443 (1922); 27, 121 (1922).
Table 1.
Vapor-Liquid Equilibria for Hydrochloric Acid-Nitric Acid-Water Solutions
Liquid Phase Mole % Mole % HC1 HNOi
Vapor Phase
l_____l____.
Run
Pressure,
NO.
Mm. Hg
1-G 2-G 3-G 4-G 5-G 6-G 7-G 8-G 9-G
10-G 11-G 12-G 13-G 14-G 15-6 16-G 17-G 18-G 19-G 20-G 21-G 22-G 23-G 24-G
Temp., O
c.
100.2 101.5 102.0 103.0 103.8 103 5 101.7 101.7 102.1 102.9 103.5 103.9 100.7 101.2 102.0 103.0 104.1 101.8 102.8 103.2 104.8 103.9 104.1 104.2
739.5 740.2 741.6 742.0 742.5 742.5 740.7 740.7 739.7 739.7 740.7 739.6 741.7 744.6 743.8 742.7 741.5 740.7 740.0 741.9 742.2 741.9 744.0 742.2
I
SP
.
gr.
1.025 1.035 1.044 1.054 1.062 1.070 1.073 1.045 1.050 1.058 1.069 1.066 1.052 1.062 1.061 1.082 1.100 1.076 1.098 1.107 1.119 1.123 1.126 1.136
2.16 3.22 4.28 5.36 6.34 6.88 6.17 3.40 4.30 5.02 5.59 6.15 0.381 0.832 2.25 2.98 3.62 0.382 0.388 0.850 0.385 0.665 1.10 0.488
~
1 3 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.48 0.49 0.46 0.47 0.53 0.72 1.31 0.99 0.90 0.86 1.25 0.90 2.89 3.26 2.45 3.32 3.81 4.34 5.67 6.00 6.97 7.07 6.99 8.11
SP. gr.
0.997 0.998 1.002 1.002 1.012 1.019 0.998 0.998
1.000 1.004 1.010 1.012 0.997 0.998 0.999 1.005 1.115 0.998 1.002 1.004 1.005 1.010 1.010 1.010
Mole % Mole % HC1 HNOI 0.033 0.108 0.249 0.560 1.21 1.81 2.22 0.177 0.315 0.558 0.985 1.27 0.009 0.033 0.127 0.389 0.825 0.027 0.468 0.152 0.087 0 244 0.318 0.174 e
0.013 1.015 0.036 0.058 0.135 0.254 0.381 0.036 0.059 0.083 0.225
0.192 0.042 0.073 0.094 0.281 0.532 0.125 0.234 0.329 0.398 0.537 0.737 0.628
(4) Crooks, R. C., Wilson, R. Q., Bearse, A. E., Filbert, R. B., Jr., “Cornposition of Vapors from Boiling Nitric Acid Solutions,” Battelle Memorial Institute Rept. BML-978, (1953). (5) Ephraim, F., “Inorganic Chemistry,” pp. 71 1-21, Interscience Publishers, New York, 1954 (tr. by P. C. L,. Thorne and E. R. Roberts). ( 6 ) Gillespie, D. T., I N D . ENG. C H E M . , ANAL.ED. 18. 575 (19461. ( 7 ) Jaulmes, P., J: chim‘. phys. 31, 227 (1934). (8) Othmer, D. F., Anal. (.hem. 20, 763 (1948). ( 9 ) Othmer, 2). F., IND.END. CHEM.20, 743 (1928). (10) Raleigh, Lord, Phil. Mug. IV, Ser. 6, 521 (1902). RECEIVED for review September 12, 1956
ACCEPTED January 30, 1957
Abstracted from KT-230, Massachusetts Institute of Technology Engineering Practice School report written under Subcontract 70 with Union Carbide Nuclear Co., Oak Ridge, Tenn.
