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J.S Tosh, Field J. H, Benson E. Haynes W.P1959
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Bureau of Mines
Report of Investigations 5484
EQUILIBRIUM STUDY OF THE SYSTEM POTASSIUM
CARBONATE, POTASSIUM BICARBONATE,
CARBON DIOXIDE, AND WATER
By J. S. Tosh, J. H. Field, H. E. Benson, and W. P. Haynes
C A L I F 0 R ft ! *
INSTITUTE f.2F !
JUL 2 - 1959 I
TECHNOLOGY '
GEOLOGIC
LfBWAHf *-
UNIVERSITY OF MICHIGAN
3 9015 07854 2415
United States Department of the Interior 1959
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The Library of Congress has cataloged this publication as follows:
Tosh, John S
Equilibrium study of the system potassium carbonate,
potassium bicarbonate, carbon dioxide and water, by J. S.
Tosh (and others. Washington] U. S. Dept. of the Interior,
Bureau of Mines, 1959.
ii, 23 p. illus. 27 cm. (U. S. Bureau of Mines. Report of inves-
tigations, 5484)
Bibliographical footnotes.
1. Potassium carbonates. 2. Carbon dioxide. 3. Chemical equi-
librium, r. Title. (Series)
[TN23.U43 no. 5484] Int 59-72
V. S. Dept. of the Interior. Library
for Library of Congress
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CONTENTS
Page
Summary 1
Introduction 1
Description of equipment 4
Operating procedure 5
Experimental results and discussion... 8
Correlation of data 18
ILLUSTRATIONS
Fig.
1. Rocking autoclave unit used in carbonate equilibrium
s tudies 3
2. Schematic of rocking autoclave unit 4
3. Autoclave and condenser assembly 5
4. Rocking autoclave unit and gas sampling manifold
used in carbonate equilibrium studies 6
5. Equilibrium pressure of carbon dioxide over 20-
percent equivalent potassium carbonate solution ... 13
6. Equilibrium pressure of carbon dioxide over 30-
percent equivalent potassium carbonate solution... 13
7. Equilibrium pressure of carbon dioxide over 40-
percent equivalent potassium carbonate solution... 14
8. Equilibrium pressure of carbon dioxide over 20-
percent equivalent potassium carbonate solution... 15
9. Equilibrium pressure of carbon dioxide over 30-
percent equivalent potassium carbonate solution... 15
10. Equilibrium pressure of carbon dioxide over 40-
percent equivalent potassium carbonate solution... 16
11. Equilibrium pressure of water vapor over 20-percent
equivalent potassium carbonate solution 17
12. Equilibrium pressure of water vapor over 30-percent
equivalent potassium carbonate solution 17
13. Equilibrium pressure of water vapor over 40-percent
equivalent potassium carbonate solution 18
14. Equilibrium pressure of water vapor over 20-percent
equivalent potassium carbonate solution 19
15. Equilibrium pressure of water vapor over 30-percent
equivalent potassium carbonate solution 20
16. Equilibrium pressure of water vapor over 40-percent
equivalent potassium carbonate solution 20
17. Variations of K with conversion for 40-percent
equivalent potassium carbonate solution 22
18. Variations of K with temperature and concentration.. 23
19. Comparison of constants with those of previous
inves tigators 23
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ii
TABLES
No. Page
1. Typical analyses of duplicate gas samples by mass
spectrometer 8
2. Equilibrium pressures of carbon dioxide and water
vapor over 20-percent equivalent potassium carbonate
solution 9
3. Equilibrium pressures of carbon dioxide and water
vapor over 30-percent equivalent potassium carbonate
solution 10
4. Equilibrium pressures of carbon dioxide and water
vapor over 40-percent equivalent potassium carbonate
solution. 11
5. Average values of K for 20- and 30-percent
concentrations 21
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EQUILIBRIUM STUDY OF THE SYSTEM POTASSIUM CARBONATE
POTASSIUM BICARBONATE, CARBON DIOXIDE, AND WATER-^
by
J. S. Tosh,^ J. H. Field,2/ H. E. Benson,^ and W. P. Haynes^
SUMMARY
An equilibrium study of the system potassium carbonate, potassium bicar-
bonate, carbon dioxide, and water has been conducted with solutions of 20-,
30-, and 40-percent equivalent potassium carbonate concentrations. These
equilibrium data cover the operating range of the hot carbonate scrubbing
system for removing carbon dioxide from gas mixtures. Such data are necessary
to establish the limits of purification and are used to design absorption and
regeneration towers.
Values of K in the relationship: K = (KHC03)2/(K2C03)PC02 we*e deter-
mined for higher temperatures of 70° to 140° C. and greater concentrations,
20 to 40 percent, than employed by previous investigators. For the 20- and
30-percent concentrations, K is constant at a given temperature; thus, once K
has been determined the equilibrium pressure of carbon dioxide can be calcu-
lated for any conversion. Variation of K values with changing conversion was
observed with the 40-percent solutions, so that equilibrium calculated pres-
sures are less reliable from average K values.
INTRODUCTION
A new, improved process for removing carbon dioxide from gas mixtures by
hot solutions of potassium carbonate, has been developed by the Bureau of
M-fnp.s.5 6/ xhe process provides an effective and economical method for remov-
ing carbon dioxide from synthesis gas, a mixture of carbon monoxide and hydro-
gen, used in producing synthetic liquid fuels from coal. In this purification
1/ Manuscript completed January 7, 1959. Titles of publications cited herein
in parentheses are translations from the language in which they were
published.
7J Chemical engineer, Bureau of Mines, Region V, Pittsburgh, Pa.
3/ Acting chief, Gas Synthesis Section, Bureau of Mines, Region V, Pittsburgh,
Pa.
4/ Former chief, Gas Synthesis Section, Bureau of Mines, Region V, Pittsburgh,
Pa.
5/ Benson, H. E., Field, J. H., and Jimeson, R. M., CO2 Absorption Employing
Hot Potassium Carbonate Solutions: Chem. Eng. Prog., vol. 50, No. 7,
1954, pp. 352-364.
6/ Benson, H. E., Field, J. H., and Haynes, W. P., Improved Process for CO2
Absorption Uses Hot Carbonate Solutions: Chem. Eng. Prog., vol. 52, No.
10, 1956, pp. 433-438.
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process the acid gas is absorbed at elevated pressures at or near the tempera-
ture prevailing in the regenerator. The solution is depressurized and regener-
ated by conventional steam stripping. The need for heating the spent solution
between the absorption and regeneration steps is eliminated, and costs are
saved for both steam and heat-exchange equipment.
Equilibrium data for the system potassium carbonate-potassium bicarbonate-
carbon dioxide-water are not available at the operating conditions used in this
process. Several investigators ZrJ-Q/previously published equilibrium data for
carbon dioxide over solutions of potassium carbonate. All of these studies
used dilute solutions, 0.1 to 2 N at low temperatures, generally less than
100° C.
To evaluate the effectiveness of the pilot plant in removing carbon diox-
ide from gas mixtures, a stainless steel autoclave was operated concurrently
to provide the necessary equilibrium data. Partial pressures of carbon dioxide
and water vapor were first measured between 110° and 140° C. over solutions of
potassium carbonate and potassium bicarbonate equivalent to an original 40-
percent concentration of potassium carbonate. Equilibrium curves for these
data were published previously.11.' Subsequent measurements were made down to
70° C. and 0-percent conversion to extend these equilibrium curves. The ex-
panded data are included in this report. To broaden the scope of the original
study, equilibrium pressures over both 30-percent and 20-percent equivalent
potassium carbonate solutions were determined since these concentrations also
are of practical interest.
"Equivalent concentration of potassium carbonate" refers to a solution in
which only potassium carbonate and water are present. Thus a 40-percent equiv-
alent solution means a solution that would contain 40 grams of potassium car-
bonate and 60 grams of water if all the bicarbonate in the system were con-
verted back to carbonate.l^/
7/ Sieverts, A., and Fritzsche, A., (Potassium Carbonate Solutions and CO2. 1):
Ztschr. anorg. Chem., vol. 133, 1924, pp. 1-16.
8/ Walker, A. C, Bray, U. B., and Johnson, J., Equilibrium in Solutions of
Alkali Carbonates: Jour. Am. Chem., vol. 49, 1927, pp. 1235-1256.
9/ Brukner, B., and Wachtler, E., (Partial Pressures of Carbon Dioxide in
Aqueous Solutions of Alkali Carbonates and Bicarbonates): Ztschr.
Wirtschuftsgruppe Zuckerind., vol. 91, 1941, pp. 254-274.
10/ Dryden, I.G.C., Equilibrium Between Gaseous Carbon Dioxide and Hydrogen
Sulphide and Solutions of Alkali Carbonates, Bicarbonates and Hydro-
sulphides. Part I. Potassium Salts: Jour. Soc. Chem. Ind., vol. 66,
1947, pp. 59-62.
11/ Work cited in footnote 6, p. 1.
12/ A 40-percent potassium carbonate solution completely converted to bicar-
bonate contains 0 percent K2CO3, 51.4 percent KHCO3, and 48.6 percent
H20.
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FIGURE 1. - Rocking Autoclave Unit Used in Carbonate Equilibrium Studies.
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DESCRIPTION OF EQUIPMENT
Inert gas -
60-100 ps.i.g.
-M-
Pressure regulator
Flexible
connection
Bleed line
-Kr—
Vacuum
Open-end
manometer
Bleed line
Manostat
pressure
controller
The equilibrium study was conducted in the rocking autoclave shown in
figure 1. A schematic of the unit is given in figure 2. The autoclave con-
sists of a 3-inch schedule-
40 pipe of stainless steel
(Type 304) approximately 3
feet long, jacketed by a 4-
inch schedule 40-pipe of car-
bon steel. A cross-sectional
view of the autoclave-conden-
ser assembly is shown in fig-
ure 3. The assembly is
flanged on one end to facili-
tate charging and discharging.
Internal pressures are indi-
cated on 4 gages covering the
range from 30 inches of mer-
cury vacuum to 1,000 p.s.i.g.
These gages are calibrated
periodically with a dead-
weight tester. The accuracy
is ± 0.1 p.s.i.g. between 30
inches of mercury vacuum and
60 p.s.i.g., and ± 0.25
p.s.i.g. between 60 and 300
p.s.i.g. The pressure at
equilibrium in these tests
did not exceed the range of
the 300 p.s.i.g. gage. A
shutoff valve is provided for
each gage to allow selection
of the appropriate pressure
range for best accuracy. The
pressure gages, their con-
necting lines, and all other
parts of the system that come
in contact with either the
gases in the autoclave or
(©
Thermometer
t
■*—( Condenser )*■—I
Cooling-water
c
i
Gos-sampling valve
Autoclave
4s Thermocouple wel
f
Liquid-sampling
valve
FIGURE 2. - Schematic of Rocking Autoclave Unit,
with the carbonate solutions are constructed of stainless steel.
Heat is applied through an electric winding on the outside of the jacket.
Desired temperatures are obtained by boiling water at appropriate pressures in
the jacket. Heat in excess of that required to maintain boiling is removed by
the condenser. For temperatures below 100° C, a vacuum pump and a manostat-
pressure controller are employed to maintain subatmospheric pressures. For
temperatures above the normal boiling point of water, elevated pressures are
maintained by adding inert gas from a controlled source. In this manner, the
temperature of the solution, measured by means of a chrome1-alumel thermocouple
and a Rubicon (type 2703) potentiometer, is controlled to ± 0.1° C.
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To jacket-pressure
controller
Cooling- water
exit ■"•—
Cooling-water
inlet
Pressure tap
^/Thermocouple well
Liquid-sampling
port
FIGURE 3. - Autoclave and Condenser Assembly.
A 1/3-hp. electric motor coupled with a 72:1 gear reducer rocks the auto-
clave condenser assembly through an arc of approximately 30° at a rate of 24
cycles per minute. The rocking motion conveyed to the autoclave insures uni-
form temperature in the contained charge, thorough dissolution of the solids,
and intimate contact between the liquid and the gas phase. These factors
assist in attaining equilibrium in a reasonable time.
When equilibrium is reached, as indicated by a constant pressure, samples
of the gas are drawn into a gas-sampling apparatus (fig. 4) through a length
of glass tubing connecting the apparatus to the gas-sampling port. The gas-
sampling valve is heated by means of an electrical tape heater. Its tempera-
ture is determined through a thermocouple attached to the valve body. By
maintaining the temperature of the valve at, or slightly above, the equilibrium
temperature, condensation of water vapor passing through the valve during sam-
pling is prevented. The gas-sampling apparatus consists of a glass manifold
to which are attached the sample bottles, a vacuum pump, a closed-end mercury
manometer for measuring the absolute pressure of the samples; and a 10-liter
bottle, which, because of its volume, ensures an efficient purge of the line
leading to the gas-sample bottles. With this equipment duplicate gas samples
are taken without stopping the rocking motion of the autoclave and therefore
without disturbing equilibrium conditions.
OPERATING PROCEDURE
Weighed amounts of commercial-grade calcined potassium carbonate!-/ and
13/ Typical chemical analysis of potassium carbonate (analysis supplied by
Harshaw Chemical Company): K2CO3 99.50, KOH 0.07, KC1 0.03, Fe203
0.0002, AS2O3 0.00002, Pb 0.0001, Na20 0.045 percent.
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FIGURE 4. - Rocking Autoclave Unit and Gas-Sampling Manifold Used in Carbonate
Equilibrium Studies.
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reagent-grade potassium bicarbonatelz' are charged to the autoclave. The fol-
lowing formulas, based on a mass balance of the reaction:
(K2C03 + co2 + H2° ^ 2KHC03),
are used to determine the weights of ingredients that make up a solution:
FC = (l-FVCC"1 + 0.3182 F),
Ffi = 1.4486/ [(CF)-1 + 0.3182],
Fw = [(1-C) - 0.1304 CF]/[(1 + 0.3182 CF)];
where:
Fq = weight fraction of potassium carbonate in converted solution,
Fg = weight fraction of potassium bicarbonate in converted solution,
Fjj = weight fraction of water in converted solution,
C = weight fraction of potassium carbonate in the original or unconverted
solution,
F = fraction of potassium carbonate converted to bicarbonate.
The weight fractions of ingredients thus determined yield a solution of
set concentration and conversion at room temperature. When the solution is
heated to equilibrium temperature, a portion of the bicarbonate decomposes,
liberating carbon dioxide. Therefore, the fraction of carbonate, converted to
bicarbonate under final equilibrium conditions, must be recalculated; this
takes into account the amount of carbon dioxide in the gas phase.
After the solids have been added, the autoclave is assembled, evacuated
to about 5 mm. Hg to eliminate most of the air and leak-tested with inert gas
at 50 to 100 p.s.i.g. When the system is leakfree, the pressure is again re-
duced to approximately 5 mm. Hg. The quantity of water necessary to yield the
desired concentration is drawn into the autoclave through the gas-sampling
port. The rocking mechanism is started, the appropriate pressure set in the
steam jacket, and heat applied. When the desired temperature has been maintained
14/ Typical chemical analysis of potassium bicarbonate (Fischer certificate of
analysis lot 754945):
Percent Percent
Insoluble matter 0.007 Sulfate (SO4) 0.000
Normal carbonate 00 Calcium, magnesium and
Chloride (CI) 0002 NH4OH ppt 003
Nitrogen (N) 0003 Heavy metals (as Pb) 0001
Phosphate (PO4) 000 Iron (Fe) 0000
Sodium (Na) 02
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for at least one-half hour, gas samples are taken to be analyzed by the mass
spectrometer for carbon dioxide and water vapor. Since the gas in the auto-
clave contains water vapor, the pressure of the gas sample during the sampling
procedure must be kept below the saturated water-vapor pressure to prevent
condensation.
To sample the gas, the sampling apparatus is attached to the gas-sampling
port and evacuated to about 1 mm. Hg. The vacuum line is closed and the gas-
sampling valve throttled. This valve permits gas from the autoclave to flow
into the sampling apparatus. When the manifold pressure reaches 20 mm. Hg the
sampling valve is closed, the 10-liter bottle is isolated from the system,
and the pressure is again reduced. After two more purges, the sample bottles
are pressurized to 20 mm. Hg and removed from the manifold. The amount of gas
used in this procedure is approximately 200 cc. This volume has been found
adequate to purge the sample bottles effectively. Duplicate samples of gas
are taken for each test and ordinarily analyzed the same day to minimize air
contamination. For a particular analysis, the precision of the mass spectrom-
eter is approximately ±0.2 percent. To show the overall reproducibility of
the analytical results, typical analyses of two sets of duplicate samples are
shown in table 1, where each sample was analysed two times by the mass
spectrometer.
TABLE 1. - Typical analyses of duplicate gas samples
by mass spectrometer
Water,
volume-percent
Carbon dioxide,
volume-percent
Sample No.
83 A-l
51.1
52.6
48.9
47.4
83 A-2
51.0
50.0
49.0
85 A-l
59.0
58.7
50.0
41.0
85 A-2
59.4
58.9
41.3
40.6
41.1
EXPERIMENTAL RESULTS AND DISCUSSION
Equilibrium partial pressures of carbon dioxide and water vapor were de-
termined at 70° to 130° C. over 20- and 30-percent equivalent potassium carbon-
ate solutions, and at 70° to 140° C. over a 40-percent solution. These results
are summarized in the first five columns of tables 2 to 4.
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TABLE 2. - Equilibrium pressures of carbon dioxide and water vapor
over 20-percent equivalent potassium carbonate solution
("Conversion" means conversion of K2CO3 to KHCO3)
Equilibrium
Total
Equilibrium
pressure,
equilibrium
Temperature,
Conversion,
pressure,
water vapor,
pressure,
°C.
percent
CO2, p.s.i.
p.s.i.
p.s .i.a.
K
70
0
-
4.18
4.18
-
10.1
0.03
4.14
4.17
0.045
20.1
.17
3.96
4.13
.039
33.4
.56
3.89
4.45
.039
33.5
.46
4.12
4.58
.048
50.0
1.69
3.49
5.18
.038
66.9
4.30
3.15
7.45
.040
81.1
10.3
2.84
13.1
.044
90
0
-
9.23
9.23
-
10.0
0.08
9.47
9.55
0.019
20.0
.33
9.01
9.34
.020
33.2
1.05
8.89
9.94
.021
33.3
.87
9.15
10.0
.025
50.2
2.93
8.53
11.5
.022
67.3
8.30
6.84
15.1
.021
79.7
15.6
6.54
22.2
.025
110
0
-
17.8
17.8
-
10.0
0.10
17.8
17.9
0.014
20.2
.49
18.9
19.4
.013
33.5
1.83
18.4
20.3
.012
33.4
1.97
18.0
20.0
.011
33.4
1.95
18.6
20.5
.011
33.5
1.63
18.5
20.1
.013
49.6
4.69
17.4
22.1
.013
50.7
5.11
17.2
22.3
.013
67.2
13.7
14.9
28.6
.013
67.2
13.8
14.5
28.3
.013
82.4
28.6
15.1
43.7
.017
81.7
30.9
12.8
43.7
.015
130
0
-
33.5
33.5
-
10.0
0.17
33.8
34.0
0.0082
20.1
.79
35.8
36.6
.0081
33.3
2.45
35.0
37.4
.0085
33.4
2.18
35.2
37.3
.0096
50.0
7.41
33.2
40.6
.0084
66.9
19.0
30.2
49.2
.0087
66.6
20.2
29.1
49.3
.0081
79.2
40.4
26.7
67.1
.0091
78.8
41.8
25.3
67.1
.0085
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10
TABLE 3. - Equilibrium pressures of carbon dioxide and water vapor
over 30-percent equivalent potassium carbonate solution
("Conversion" means conversion of K2CO3 to KHCO3)
Equilibrium
Total
Equilibrium
pressure,
equilibrium
Temperature,
Convers ion
pressure,
water vapor,
pressure,
°C.
percent
CO2, p.s.i.
p.s.i.
p.s.i.a.
K
70
0
-
3.67
3.67
-
0
-
3.99
3.99
-
10.0
0.04
3.90
3.94
0.056
20.0
.17
3.53
3.70
.063
33.2
.51
4.07
4.58
.068
49.4
1.97
3.59
5.56
.051
65.2
4.44
3.43
7.87
.057
64.8
4.46
3.31
7.77
.055
81.0
12.0
3.32
15.3
.058
90
0
-
8.58
8.58
-
0
-
8.13
8.13
-
10.0
0.09
8.35
8.44
0.026
19.9
.35
8.36
8.71
.030
33.0
.91
8.63
9.54
.038
48.9
3.45
8.30
11.8
.028
64.1
8.18
6.94
15.1
.028
78.6
19.6
6.26
25.9
.029
110
0
-
15.2
15.2
-
0
-
15.4
15.4
-
10.0
0.14
17.9
18.0
0.017
10.0
.13
17.8
17.9
.018
19.8
.59
16.8
17.4
.017
20.0
.73
17.4
18.1
.014
32.8
1.65
17.6
19.3
.020
48.3
5.50
16.2
21.7
.017
62.5
13.4
14.3
27.6
.016
62.5
13.1
14.6
27.8
.016
76.1
28.4
14.1
42.5
.017
130
0
-
30.2
30.2
-
0
-
30.6
30.6
-
10.0
0.19
31.4
31.6
0.012
10.0
.19
30.8
31.0
.013
19.8
.86
32.0
32.9
.012
32.6
2.42
33.8
36.2
.013
47.3
9.03
32.0
41.1
.009
61.5
18.0
29.1
47.2
.011
61.0
19.6
30.6
50.2
.010
73.5
38.4
26.9
65.3
.010
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11
TABLE 4. - Equilibrium pressures of carbon dioxide and water vapor
over 40-percent equivalent potassium carbonate solution
("Conversion" means conversion of K2CO3 to KHCO3)
Equilibrium
Total
Equilibrium
pressure,
equilibrium
Temperature,
Conversion,
pressure,
water vapor,
pressure,
°C.
percent
C02, p.s.i.
p.s.i.
p.s.i.a.
K
70
10.1
0.04
3.42
3.46
0.085
20.1
.20
3.66
3.86
.078
33.4
.61
3.30
3.91
.084
42.2
1.32
2.89
4.21
.070
90
10.0
0.08
7.41
7.49
0.042
20.0
.34
7.68
8.02
.045
33.3
1.22
7.26
8.48
.041
41.9
2.49
6.97
9.46
.036
110
0
-
15.6
15.6
-
10.0
0.13
14.2
15.6
0.027
20.0
.71
15.7
16.4
.021
29.5
1.42
15.7
17.1
.026
29.6
1.61
15.6
17.2
.023
33.4
2.32
15.0
17.3
.021
35.1
2.09
15.6
17.7
.027
40.3
4.14
14.2
18.3
.019
44.9
4.76
15.0
19.8
.022
48.5
7.89
13.4
21.3
.017
55.4
9.40
15.5
24.9
.021
58.5
8.43
14.0
22.4
.028
60.1
14.2
12.5
26.7
.018
63.8
11.6
16.6
28.2
.027
66.4
20.8
11.6
32.4
.018
71.4
26.0
12.7
38.7
.019
75.9
31.5
13.3
44.8
.021
80.9
50.5
11.0
61.5
.019
85.0
72.7
10.1
82.8
.018
87.4
69.7
16.4
86.1
.024
120
0
-
21.9
21.9
-
29.5
1.81
21.7
23.5
0.021
29.5
1.82
22.2
24.0
.020
34.9
3.40
21.5
24.9
.016
40.2
3.52
21.5
25.0
.022
44.6
6.11
21.2
27.3
.017
48.2
7.66
19.7
27.4
.017
55.1
12.5
20.0
32.5
.016
58.8
18.5
17.5
36.0
.013
62.6
20.9
17.5
38.4
.014
70.6
33.6
15.3
48.9
.014
74.4
42.5
15.3
57.8
.014
79.1
65.9
13.2
79.1
.012
83.5
86.1
15.2
101.3
.013
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12
TABLE 4. - Equilibrium pressures of carbon dioxide and water vapor over
40-percent equivalent potassium carbonate solution (Con.)
("Conversion" means conversion of K2CO3 to KHCO3)
Equilibrium
Total
Equilibrium
pressure,
equilibrium
Temperature,
Convers ion,
pressure,
water vapor,
pressure,
°C.
percent
CO2, p.s.i.
p.s.i.
p.s.i.a.
K
130
0
-
29.8
29.8
-
10.0
0.20
27.7
27.9
0.017
19.9
1.36
29.1
30.5
.011
29.2
3.94
29.5
33.4
.009
34.6
6.03
28.1
34.1
.009
40.2
6.61
27.7
34.3
.012
54.0
20.5
23.6
44.1
.009
56.8
19.5
20.7
40.2
.011
58.0
30.6
17.5
48.1
.007
61.7
29.8
20.9
50.7
.009
64.4
33.2
21.0
54.2
.010
68.5
48.0
17.5
65.5
.009
72.6
57.4
18.1
75.5
.009
77.6
80.1
18.3
98.4
.009
81.4
104.4
17.3
121.7
.009
82.8
99.4
17.7
117.1
.011
140
0
-
40.0
40.0
-
29.2
3.13
41.6
44.7
0.0108
29.5
3.75
39.3
43.1
.0095
34.5
7.58
36.4
44.0
.0069
40.2
7.43
38.7
46.1
.0104
43.8
14.6
33.7
48.3
.0066
47.3
15.2
36.6
51.8
.0079
53.6
25.9
29.9
55.8
.0067
56.6
41.6
22.1
63.7
.0049
58.6
44.1
20.1
64.2
.0051
71.0
72.3
20.5
92.8
.0064
75.6
95.3
20.6
116.0
.0065
77.2
134.0
7.95
142.0
.0052
Figures 5 to 7 show partial pressures of carbon dioxide plotted as a func-
tion of the amount of conversion of the original carbonate to bicarbonate.
Since the pressure of carbon dioxide approaches zero over a solution containing
only carbonate, but is high over a completely converted solution, that is, 100-
percent bicarbonate , the semilogarithmic plots form reverse S-shaped curves
that approach the 0-percent ordinate asymptotically, and rise sharply as the
100-percent ordinate is approached.
The equilibrium data point out the advantage of a split-flow operation to
obtain a high degree of carbon dioxide removal. In this type of operation a
portion of the total solution is regenerated more thoroughly and then cooled
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0 2 40 6 60 100
PERCENT OF K2C03 CONVERTED TO KHCO3
FIGURE 5. - Equilibrium Pressure of Carbon Dioxide
Over 20-Percent Equivalent Potassium
Carbonate Solution.
0 80 40 80 80 100
PERCENT OF K2C03 CONVERTED TO KHCO3
FIGURE 6. - Equilibrium Pressure of Carbon Dioxide
Over 30-Percent Equivalent Potassium
Carbonate Solution.
u>
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14
200
UJ
cr
z>
CO
CO
UJ
cr
a.
X
o
o
CD
cr
<
before being admitted to the
top of the absorption column.
Both steps effectively lower
the equilibrium pressure of
carbon dioxide over the solu-
tion.
The partial pressures
of carbon dioxide are also
shown in a Clausius-Clapeyron-
type plot, where the logarithm
of pressure is plotted as a
function of reciprocal abso-
lute temperature for selected
levels of constant conversion.
For the 20-percent solution
(fig. 8) and the 30-percent
solution (fig. 9)JJL' families
of straight parallel lines
were obrained. When the data
for the 40-percent solution
were similarly plotted in
figure 10, the lines were
slightly curved rather than
straight. These plots offer
a convenient method for ex-
trapolating the carbon diox-
ide pressure over a wide range
of temperatures. Once the
pressure at one temperature is
known, the pressure at any
temperature for the same level
of conversion can readily be
obtained by drawing a line
through the point parallel to
the existing lines.
.03
Water-vapor pressures
are shown in figures 11 to
13. Plots of the 20-percent
(fig. 11) and 30-percent (fig.
12) data are quite similar.
In both instances the vapor
pressures either remain near-
ly constant or increase slightly to a maximum at approximately 20-to 40-percent
conversion before starting to decline. A plot of the 40-percent data (fig. 13)
0 20 40 60 80 100
PERCENT OF K2C03 CONVERTED TO KHC03
FIGURE 7. - Equilibrium Pressure of Carbon Dioxide Over
40-Percent Equivalent Potassium Carbonate
Solution.
15/ The pressures of carbon dioxide plotted in figures 8 to 10 are taken from
the smoothed curves in figures 5 to 7.
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FIGURE 8.
Equilibrium Pressure of Carbon Dioxide Over
20-Percent Equivalent Potassium Carbonate
Solution.
2.75
K X 1.000
2.95
-4 —
-.6
-.8 —
- 1.0
2.45
2.55 2.65 2.75
l/T°K X 1,000
2.65
2.95
FIGURE 9. - Equilibrium Pressure of Carbon Dioxide Over
30-Percent Equivalent Potassium Carbonate
Solution.
t-n
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16
2 4
2 0
-. 12 —
-.8
2.7 28
l/T °K x 1.000
FIGURE 10. - Equilibrium Pressure of Carbon Dioxide Over 40-Per-
cent Equivalent Potassium Carbonate Solution.
shows the same ten-
dency for the pres-
sures to hold steady
to approximately 30-
or 40-percent con-
version, followed by
a rapid rate of de-
cline between 30-
and 60-percent con-
version.
The water-vapor
pressures are also
plotted in a Claus-
ius-Clapeyron form
(figs. 14 to 16).W
If a vapor is assum-
ed to obey the ideal
gas law, this equa-
tion can be inte-
grated to yield log
P = -L/2.303 RT + C,
where P is vapor
pressure, L is the
heat of vaporiza-
tion, and C is a
constant. When log
P is plotted as a
function of recip-
rocal temperature,
the slope of the
line is:
- L/2.303 R or L = - (slope x 2.303 x 1.987).
The Clausius-Clapeyron equation normally applies to equilibrium between
two phases of a pure component; the same method of calculation was used for
the water vapor data. A heat of vaporization of approximately 9,300 calories
per gram mole. (980 B.t.u. per pound) was obtained, which is close to the
average value between 70° and 130° C. obtained from the steam tables. For the
more highly converted 40-percent solutions, as shown in figure 16, the heat
of vaporization apparently is not constant since the slope of the line begins
to change when about 50-percent of the potassium is in the form of bicarbonate
This may be due to an increased concentration of salts formed in the solution
when one molecule of carbonate reacting with carbon dioxide and water forms 2
molecules of bicarbonate.
16/ The pressures of water vapor plotted in figures 14 to 16 are taken from
the smoothed curves in figures 11 to 13.
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0 6 40 60 60 100
PERCENT OF K2C03 CONVERTED TO KHC03
FIGURE 11. - Equilibrium Pressure of Water Va-
por Over 20-Percent Equivalent
Potassium Carbonate Solution.
40
0 80 4 60 60 100
PERCENT OF K2C03 CONVERTED TO KHC03
FIGURE 12.-Equilibrium Pressure of Water Va-
por Over 30-Percent Equivalent
Potassium Carbonate Solution.
Genera
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18
45
O.
UJ
cr
z>
to
in
UJ
cr
Q.
(T
o
Q_
<
>
i
cr
LlI
\-
<
10
90 °C.
-A-
0 20 40 60 80 100
PERCENT OF K2C03 CONVERTED TO KHC03
FIGURE 13. - Equilibrium Pressure of Water Vapor Over
40-Percent Equivalent Potassium Carbon-
ate Solution.
CORRELATION OF DATA
McCoyiZ/ first deter-
mined, for dilute solutions
of sodium carbonate at con-
stant temperature, that
equilibrium in the system
can be expressed as a rela-
tionship between the partial
pressure of carbon dioxide
in the gas phase, the cation
concentration, the concen-
trations of carbonic acid,
normal carbonate, and bicar-
bonate in solution. Later
Sieverts and Fritzschei^/
worked with more concentrated
solutions at higher tempera-
tures. They derived a modi-
fied expression, relating the
concentration of carbonate
and bicarbonate, and the par-
tial pressure of carbon di-
oxide to a constant, K. The
basis for the derivation of
this constant is the rela-
tionship between the effec-
tive concentration or activ-
ity of carbonic acid in solu-
tion and (1) the first and
second ionization constants
of carbonic acid and (2) the
partial pressure of carbon
dioxide in the vapor phase.
For the reaction
K2C03 + H2O + CO2
2KHC0
3'
constant K is expressed as;
K = (KHC03)2/(K2C03)PC02.
The concentrations of
KHCO3 and K2CO3 are given in
gram-moles per liter of solu-
tion, and the partial pressure
17/ McCoy, H. N., Equilibrium in the System Sodium Carbonate, Sodium Bicarbon-
ate, Carbon Dioxide and Water: Am. Chem. Jour., vol. 29, 1903, p. 437.
18/ Work cited in reference 7, p. 2.
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19
1.7
1.5
.4 —
1.2 —
UJ
or
=>
to
1 1
if)
UJ
or
Q_
10
UL
o
0.
<
>
0.9
or
UJ
<
0.8
£
o
o
0.7
0.4
1 1 1 1
~\ ~~
\\
\^\
X^\
V\
\^\
\\xx
Conversion of K2C03 to KHCO3 \\ \\
5 *\ \>
_ ^ vs. \N ~~
— I \\
1 1 1 1 \
0.6 —
0.5
2.45
FIGURE 14.
2.55
2.65
l/T
2.75
K X 1,000
2.85
2.95
Equilibrium Pressure of Water Vapor Over 20-Percent
Equivalent Potassium Carbonate Solution.
of carbon dioxide is expressed in mm. Hg. Once K has been determined, the
equilibrium pressure of carbon dioxide can be calculated for any conversion.
At the concentrations and temperatures previously investigated by Sieverts
and FritzscheJ^-' and Dryden,2£/ K was independent of the conversion to bicar-
bonate but varied with temperature.
K values for the solutions and temperatures used in this investigation
are given in the last columns of tables 2 to 4. For the 20- and 30-percent
19/ Work cited in reference 7, p. 2.
20/ Work cited in reference 10, p. 2.
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20
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21
concentrations, K was constant at a given temperature for each concentration.
In table 5 the average K values for each temperature are summarized. For the
40-percent concentration K does not remain constant at 110° C. and higher but
tends to decrease with increasing conversion to bicarbonate. Thus the equil-
ibrium pressure of carbon dioxide rises more rapidly with conversion of the
40-percent concentration than for the more dilute solutions. The variation of
K with conversion at the 40-percent concentration is illustrated in figure 17.
K values used to prepare this figure are calculated from the smoothed equilib-
rium pressure curves of figure 7 to minimize variations from the individual
tests. The variation in K occurs at the 40-percent concentration because there
is greater deviation from ideality of the ionization constants of carbonic
acid - one of the bases for deriving K - as the solution becomes less dilute.
TABLE 5. - Average values of K for 20- and 30-percent concentrations
Average percent
deviation from
arithmetic mean
Temperature,
Number
of tests
Arithmetic
mean of K
Mean error of
arithmetic mean
20
-percent con
centrations
70
7
0.042
.022
.013
.0086
± 0.001
.001
.0003
.0001
± 7.8
90
7
8.4
7.7
4.4
110
12
130
9
30
-percent con
centrations
70
7
6
9
8
0.058
.030
.017
.011
± 0.004
.003
.001
.001
± 7.6
9.4
5.9
90
110
130
11.4
21/
Dryden— related K to the temperature by plotting the logarithm of K as
a function of reciprocal absolute temperature. Figure 18 shows such a plot
for the data obtained in this investigation. The curves for the 20- and 30-
percent concentrations follow the same trend; log K decreases with increasing
temperaturej the rate of decrease becomes smaller with an increase in tempera-
tures . The data for 40-percent concentrations are not given in this plot
since, as shown in figure 17, K is not constant for all conversions.
In figure 19, log K values obtained by previous investigators for solu-
tions of potassium salts up to 2N concentration are plotted with the values
obtained in this study for 20-percent (3.44N) concentration. Drydenz__'
21/ Dryden, I. G. C, Equilibrium Between Gaseous Carbon Dioxide and Hydrogen
Sulphide and Solutions of Alkali Carbonates, Bicarbonates and Hydrosul-
phides. Part I. Potassium Salts: Jour. Soc. Chem. Ind., vol. 66, 1947,
pp. 59-62.
22/ Work cited in reference 21.
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22
.024
.020 —
0 15 —
.010 —
005
obtained lower values
than Sieverts and
Fritzsche23/ at 30° to
60° C. Because the
slopes of the curves
differ appreciably, if
Dryden's curve is ex-
tended to higher tem-
peratures , it crosses
that of Sieverts and
Fritzsche at about 75°
C, indicating that
further extrapolation
would give higher
values for Dryden above
this temperature. Dry-
den's curve between 0°
and 30° C. shows a lin-
ear change of the loga-
rithm of K with recip-
rocal temperature.
However, there is a de-
viation from the linear
relationship from 30°
to 60° C. The data for
20-percent concentration
between 70° and 130° C.
shows a slight upward
slope, and very nearly
lines up with Dryden's
data between 0° and 30°
C. This would indicate
that the change in slope
exhibited by Dryden's
data between 30° and 60°
C. is not consistent.
20 40 60 80
PERCENT OF K2C03 CONVERTED TO KHC03
100
FIGURE 17. - Variations of K With Conversion for 40-
Percent Equivalent Potassium Carbon-
ate Solution.
23/ Sieverts, A., and Fritzsche, A. (Potassium Carbonate Solutions and CO;
1): Ztschr. anorg. Chem., vol. 133, 1924, pp. 1-16.
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.02
006
J L
2.4 2.5 2.6 2.7 2.8
l/T'K X 1,000
2.9 3.0
FIGURE 18. - Variations of K With Temperature and
Concentration.
.2
i
.06
.06
.04
.02
.006 -
0.1
.08
.06
.04
.01
.008
a-20 percent solution
â–¡ - 30 percent solution
.01
.006
.004
2.3 2.5
2.7
D Sieverts and Fritzsche 2.0N
O Dryden 2.0N
X Brukner ond Wbchtler O.IN
A This study 3.44 N
J i L
2.9 3.1
l/T °K x 1,000
3.3 3.5 3.7
FIGURE 19. - Comparison of Constants With Those
of Previous Investigators.
I NT„ - BU . 0 F M I NES .P GH. .PA. 252
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