February, 1927
I S D U S T R I A L 9 S D E S G I S E E R I S G CHEMISTRY
205 -
Partial Pressures of Carbon Dioxide, Ammonia, and Water over the System Water-AmmoniaCarbon Dioxide-Ammonium Nitrate' By K. G. Clark and H. J. Krase FIXEDXITROGEN RESEARCH LABORATORY, BLREAU OF
I
SOILS, \TASHINGTOY, D C.
THE development of a continuous synthetic urea process being carried out a t this laboratory, it is necessary, in order to effect continuous working by ineans of standard equipment, to separate from each other the ammonia and carbon dioxide resulting from the decomposition of ammonium carbonate and carbamate. This separation can be accomplished by distillation from an aqueous solution containing the dissolved gases, provided that the partial pressures of carbon dioxide and ammonia differ sufficiently from each other to make such a separation practical and efficient. It is well known that the separation can be a t least partly effected by distillation from water solutions. The apparatus patented by Solvay2or that described by Wyld3 is designed to effect such a separation. The writers have not been able t o find quantitative data concerning the operation of these devices. However, their experience with such distillations leads them to belieJ-e that such a method of separationi. e., distillation from water solutions a t atmospht:ric pressure-is not very e f f i ~ i e n t . ~ Since the basis of such a separation is a knowledge of the partial vapor pressures of the solutions to be subjected to distillation, this paper is devoted to a discussion of the method of determination of the partial pressures and the results obtained.
The writers have substituted soda lime instead of caustic potash for carbon dioxide absorption and, instead of using calibrated leveling bottles, have employed a compensoineterb to measure the air passed through the solutions.
Method
Calculation of Results
The partial pressures of carbon dioxide, ammonia, and water have been determined over water solutions of ammonium carbonate b y Terres and Weisere6 Their method, with some modifications, has been adopted in this work and their data hare been included in order to afford a comparison with solutions containing ammonium nitrate in addition to the carbonate. This method consists briefly in passing a
The weights of carbon dioxide, ammonia, and water are converted into their corresponding volumes under standard conditions. The sum of these volumes and the volume of air passed through the bubblers is the total volume, from which the partial pressure may be directly calculated. The sum of the average barometric reading and the average pressure in the final bubbling tube is the total pressure.
stream of air through the solution in a thermostat, then determining the weight of carbon dioxide, ammonia, i n d water vapor in the air, after which the air volume is measured.
For e x a m p k in Experiment 48:
?;
1
8 4 6
Received October 5. 1926. U. S . Patent 618.429 (18941. Chcm. Age (London), 4,368 (1921). Krase and Hetheringtom, page 208, this issue. J. Gasbel., 68, 705 (1920).
Description of Apparatus
The apparatus is shown in Figure 1. A steady flow of air under constant pressure is obtained by means of the regulator and reservoir. The air is purified by passing through caustic potash and concentrated sulfuric acid and then enters a series of six bubbling tubes containing the solution of ammonium nitrate and carbonate. The sixth tube is provided with a manometer. The gas passes directly from this tube into a bubbler containing 0.5 N sulfuric acid, then through soda lime in Fleming bulbs, after which it is measured in a compensometer. I n the experiments a t 20" and 40" C., the bubbling tubes were of 60 cc. capacity, but a t higher temperatures 100-cc. tubes \\-ere found necessary so that larger volumes of solution could be used. By adjusting the compensoineter leveling bulb so that the manometer reading a t the sixth tube plus the barometer totaled 760 mm., the vapor pressure vias obtained under standard conditions.
Weight Weight Weight Weight
Total volume 6
c.,
of NH, = oGram , 0170 = 22, c c . at Mm. 760 of ~~0 = 0,0830 = 103.3 at 0 0 c., 760 of COZ = 0.1873 = 95.4a t 0 " C., 760 of Air = . . . = 5260.0 a t 0" C., 760 = 5481.1
Chem. &+ Met. E n f . , 28, 1104 (1920).
o,
= Per 409 cent = 1.885 = 1.743 = 95.964
=100.0
206
IXDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 19, No. 2
Mm. H g
Average barometric pressure = 756 Average manometric pressure = +4
-
Total pressure
= 760 Mm. Hg
Partial Partial Partial Partial
pressure pressure pressure pressure
NHe = 3 . 1 HzO = 14.3 CO? = 13.2 Air = 729.4 ~~
Total
= 760.0
Effect of O t h e r Ammonium Salts
I n order to compare the effect of ammonium sulfate and chloride on the system ammonia-carbon dioxide-water, with the effect due to ammonium nitrate, determinations of the partial pressures were made with these salts. The results are shown in Table IV, and compared with ammonium nitrate a t various concentrations. It will be seen that a 24.55 per cent ammonium chloride solution increases the difference between the vapor pressures of ammonia and carbon dioxide slightly more than a 29.45 per cent solution of ammonium nitrate, which produces a Table I-Partial
Results
Terres Of
1
NH3 wt.
o,oo
I
and
Grams Ntf, per li ter
PARTIAL PRESSURE
EXPT. "'NO3 % by
Pressures of N€L COz and HzO over Aqueous Solutions of (NI&)zdO. a&d NHhNOa
I
Weiser )
G./1. 10.08 20.39 30.81 41.47 52.32 63.36 74.60 86.16
COZ
NH3
Hz0
Cot
Sum
G./l. M m . H p Mm.Hg Mm.Hg M m . H g Temperature, 20' C. 13.04 26.38 39.87 53.67 67.71 82.00 96.54 111.33
1.8 3.6 5.1 7.1 8.8 10.6 12.3 14.2
16.8 16.3 15.6 15.4 14.8 14.2 13.8 13.3
2.7 3.7 4.4 4.9 5.2 5.4 0.J
5.2
21.3 23.6 25.1 27.4 28.8 30.2 31.3 33.0
1.9 3.1 4.4 4.3 5.9 1.9 3.1 4. I 6.0
13.7 14.3 14.4 14.2 14.2 12.5 12.7 12.6 12.8
11.8 13.2 16.6 14.4 16.8 18.8 22.8 26.9 29.4
27.4 30.6 35.4 32.9 36.9 33.2 38.6 44.2 48.2
29.8 29.30 28.43 28.45 28.80 49.20 49.15 47.42 45.75
8.32 16.68 30.10 33.10 46.36 7.42 15.64 30.56 44.66
0.00
10.08 20.39 30.81 41.47 52.32 63.36 74.60 86.16
13.04 26.38 39.87 53.67 67.71 82.00 96.54 111.33
55.6 55.0 54.2 53.1 52.0 51.0 50.2 49.3
4.8 9.7 14.5 19.4 24.4 29.5 34.6 39.5
9.6 17.6 22.0 24.0 25.3 26.6 27.4 28.0
70.0 82.3 90.7 96.5 101.7 107.1 112.2 116.8
29.5Sa 29.16O 29.45" 27.30a
7.70 16.17 33.10 44.86
10.56 18.36 44.96 62.22
45.0 42.4 51.5 46.3
5.6 12.6 14.4 17.6
58.9 68.1 86.4 97.2
109.5 123.1 152.3 161.1
7.30 49.45O 4 8 . 8 ~ 5 ~ 14.69 47.30a 31.24 45.80a 46.60
10.40 18.26 40.02 61.46
39.7 40.9 39.7 39.8
7.6 11.4 17.8 21.9
76.3 96.6 116.6 126.8
123.6 148.9 174.1 188.5
62.15" 61.05" 59.40n 57.75O.
7.30 15.57 31.90 42.31
13.26 27.56 45.38 68.60
88.4 112.4 139.0 157.3
126.5 161.5 195.0 218.1
0.00
10.08 20.39 30.81 41.47 52.32 83.36 74.60 86.16
29.5 8.6 12.6 36.5 19.2 36.8 38.5 22.3 Temperature, 60' C. 13.04 15.3 155.0 26.38 27.0 153.2 39.87 38.9 153.5 53.67 51.0 152.2 57.71 62.8 151.0 150.0 82.00 72.5 96.54 84.0 149.0 148.0 111.33 102.0
53.5 82.2 100.5 118.1 129.5 137.2 152.0 163.1
223.8 262.4 292.9 321.3 343.3 359.7 385.0 413.1
52A 53A 54A
29.78 29.50 28.82 27.15
7.77 15.72 22.80 47.62
10.19 20..43 29.80 61.93
14.5 21.9 28.6 50.0
132.5 142.5 134.2 148.0
243.4 307.0 334.0 359.0
390.4 471.4 496.8 557.0
55 56 57 58
49.67 48.95 47.57 46.02
7.98 15.72 30.32 46.95
8.22 17.35 37.23 58.83
16.7 27.7 45.5 63.3
110.8 122.5 112.8 113.5
361.0 389.5 420.0 439.5
488.5 539.7 578.3 616.3
47 48 49 49A 50A 43A 44A 45A 46A
Temperature, 40' C. Results
of Terres and Weiser
10 11 12D 13 7 4 5 6 14 15 16 17 Results of Terres and Weiser
51B
Calculated from specific gravity.
greater difference than ammonium sulfate. However, ammonium nitrate is the more soluble and 50 to 60 per cent solutions of this salt give a greater difference than a 25 per cent ammonium chloride solution, which is a saturated solution under these conditions. Discussion of R e s u l t s
The partial pressures of carbon dioxide, ammonia, and water over solutions containing ammonium carbonate and ammonium nitrate a t 20°, 40°, and 60" C. are given in Table I, and are plotted in Figures 2, 3, and 4. At 20" C., and especially a t the higher temperatures, the partial pressure of carbon dioxide is considerably greater in the presence of the ammonium nitrate than is shown by the determinations of
INDVSTRIAL A N D ENGINEERING CHEMISTRY
February, 1927
Terres and Reiser on aqueous solutions of ammonium carbonate. The results of these investigators are labeled "0 70?JH4x0a1' in the graphs. It will be seen from Figures 2, 3, and 4 that the ammonia vapor pressure curves are closely grouped together, indicating that the ammonia yapor pressure is very little influenced by increasing ammonium nitrate concentrations even up to
207
Table 111-Partial Pressures of NHa, COz, and Hz0 over Aqueous Solutions of NH4OH, (NHa)COs, and NHdNOa (Temperature, 20' C . )
-
--
EXPT. N H ~ N O I
NH3
PARTIAL PRESSURE
COZ
NHs
-._
Yo b y w f .
COz
Hz0
Sum
M m . Hg M m . H 2 M m . Hg 6.4 11.9 28.7 1.2 12.5 35,s 0.4 16.9 49.8 0.2 11.6 46.5
31 32 33 34
28.85 29.80 30.07 30.50
G./1. 50.70 51.60 50.90 48.86
35 36 37 38
48.10 48.70 49.40 50.10
52.06 52.10 52.02 52.00
64.74 37.82 18.90 6.94
9.2 25,s 32.5 45.9
12.5 1.6, 0.i 0.0
9.4 10.1 11.0 10.2
31.1 37.5 44.2 56.1
39 40 41 42
57.9 59.3 60.2 61.0
55.12 51.80 50.60 49.00
64.80 39.40 20.80 6.60
12.7 27.7 41.6 48.7
13.0 2.1 0.5 0.0
10.5 9.7 9.8 9.8
36.2 39.5 51.9 58.5
G / l . Mm. Hg 59.34 10.4 36.58 22.1 17.90 32.5 6.56 34.7
a constant temperature of 20" C. is given in Table I11 and is plotted in Figure 5. The ammonia vapor pressure is seen t o drop off with increasing carbon dioxide in solution, due to the formation of ammonium carbonate. This curve illustrates the equilibrium conditions which would be obtained in a scrubbing tower working with this particular solution. That the acidic property of these solutions is typical of other ammonium salts, a t least those salts of strong acids, is +hewn in Table IV. Table IV-Effect
of Other Ammonium Salts PARTIAL PRESSURE
TEMP.
SHa
COz
ADDED
SALT
c.
63 per cent in the experiments a t 40" C. The water vapor curves show the expected lowering, due to the added salt. I n the behayior of the carbon dioxide vapor pressure curves we see the greatest effect of the ammonium nitrate. It is evident that in general a t any temperature the carbon dioxide pressure increases with the amount of nitrate in solution. This illustrates the surprising acidity of ammonium nitrate solutions, a fact in support of which other evidence is available, especially that dealing with the ammonia system of acids and bases. The amount of ammonia and water vapor carried over a t various temperatures by a stream of carbon dioxide is given in Table 11. As was to be expected, a t the lower temperatures ammonium bicarbonate was first formed, which a t a higher temperature (between 40" and 60" C.) is no longer stable in solution.
40a 40
G./1. 30.81 33.10
C./I. 39.87 44.96
40
31.24
40.02
40
31.90
45.38
40
30.05
38.62
40
31.65
37,26
a
70
COz
XHa c o ~ ~ f f ~ Hz0 H 3
Mm. Hg Mm. Hg M m . Hg .Mm. H g
0.0 22.0 29.45 86.4 NHaNOj 47.30 116.6 NHINO; 59.40 139.0 NHPNOI 29.55 53.0 (NHdzSOn 24.55 97.2 iTHaC1
14.5 14.4
7.5 72.0
54.2 51.5
17.8
98.8
39.7
19.2
119.8
36.8
19.4
33.6
48.0
15.8
81.4
48.0
Results of Terres and Weiser.
Fl'q. 5.
",,(4 -
Parfialpressures o f h YO over aqueous solufions of N H , O H ~ [ N ~ I ~ C ~ NU, 50 qms/hfer and NH, NO,
Temp. 20"C.
Table 11-Partial Pressures of NHa and H20 over an Aqueous Solution of ~ ( N H ~ Z C and O ~ N H I N O ~ ,Determined by Bubbling COz through Solution
I !
COMPOSITION OF SOLUTION
EXPT.TEMP. Beginning of Detn.
60-1 60-2 60-3 60-4
I
20 40 60 65
1
I
31.30 31.30 31.30 31.30
38.60 38.60 38.60 38.60
48.75 48.75 48.75 48.75
1
End ol Detn.
PARTIAL PRESSURE
26.41a 41.6Sa 49.05 0.1 1 31.15 52.44 48.45 ! 8 . 3 30.68 32.82 4 9 . 0 5 6 0 . 9 1 29.70 30.34 49.30 I 7 4 . 0
18.4 40.9 122.0 167.5
I
Crystals of ( N H ~ H C O Jformed in the solution chamber is t h a t of the clear solution above the (NHdHCOs crystals
Analysis
The gas composition from these solutions expressed in per cent by volume is plotted in Figure 2 of the paper by Krase and H e t h e r i n g t ~ n . ~ The change in the partial pressures of ammonia, carbon dioxide, and water with incieasing earbon dioxide content a t
Conclusions
The results presented above indicate that a much more complete separation of carbon dioxide from ammonia can be effected by distillation from aqueous solutions containing an ammonium salt of a strong acid than from solutions of ammonium carbonate alone, because of the greater difference in the partial pressure of the gases to be separated. The practical results obtained from such distillations are presented in another paper.4
