EPA-650/2-74-035
January 1974
Environmental Protection Technology Series
m.
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-------
EPA-650/2-74-035
EVALUATION OF EQUATIONS
FOR DESIGNING AMMONIACAL SCRUBBERS
TO REMOVE SULFUR OXIDES
FROM WASTE GAS
by
L.I. Griffin
Gas Cleaning and Metallurgical Processes Branch
Control Systems Laboratory
ROAP No. 21ACX-60
Program Element No. 1AB013
Prepared for
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, N. C. 27711
January 1974
-------
This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
-------
CONTENTS
Page
Introduction 1
Johnstone's Studies 2
Chertkov's Studies 6
Johnstone's Equations Recast for Simplicity 10
Limitations of Johnstone's Vapor Pressure Equations 15
Application of Johnstone's Recast Equations 16
References 18
Appendix A -- Johnstone's Recast Vapor Pressure
Equations 19
Fi gures
Fig.
1 The pH Values of Ammonium Sulfite-Bisulfite 8
Solutions
Tables
Table
1 Temperature Coefficients for Johnstone's 12
Vapor Pressure Equations
2 Acidity Factors for Johnstone's Equation 13
for Vapor Pressure of S02
3 Acidity Factors for Johnstone's Equation 14
for Vapor Pressure of NH3
iii
-------
INTRODUCTION
Scrubbing waste gas with ammoniacal solutions for removing
sulfur oxides has been practiced commercially in the United States,
Canada, Russia, Japan, Czechoslovakia and Romania during the last
several decades. These limited operations of varying duration were
prompted by serious local air pollution problems. In several
instances, scrubbing was discontinued when the processing operations
were modified to reduce pollution or when a cleaner fuel was supplied.
In recent years, interest in air pollution has been intensified
on a national and international scale. Ammonia scrubbing has many
potential advantages over competing waste gas cleaning processes.
This is particularly true if improvements expected are convincingly
demonstrated. Since the properties of ammoniacal solutions have
received considerable attention and careful laboratory measurements
have been made, a number of the tools needed for process improvements
studies are available in the literature. For example, H.F. Johnstone
and coworkers at the University of Illinois in the 1930's published
vapor pressure data for ammonia, sulfur dioxide, and water above
ammoniacal solutions along with the solution pH's. And, in the
1950's, B.A. Chertkov and his Russian countrymen reviewed and repeated
much of Johnstone's work.
The purpose of this paper is to comment on the accuracy and
limitations of the data obtained by both investigators, and to recast
Johnstone's vapor pressure equations so that they can be used more
easily in designing and controlling ammonia scrubbing processes for
removing sulfur oxides from waste gas.
-1-
-------
JOHNSTONE'S STUDIES
H. F. Johnstone's interest in controlling air pollution was
demonstrated by his extensive work on a number of processes for
scrubbing sulfur oxides from waste gases. Ammoniacal scrubbing,
among the first processes considered by Johnstone, was studied
very carefully and completely. In preparation for this work,
Johnstone") measured the vapor pressure of ammonia, sulfur
dioxide, and water above ammoniacal solutions of varying compositions
and pH's. As he surmised, vapor pressures were found to be functions
of temperature and composition while solution pH was a function
of composition alone. Theoretical considerations served as
the basis of Johnstone's correlations of his experimental vapor
pressure data. These considerations are discussed carefully
in Johnstone's paper and will not be repeated here. While
Johnstone's pH-composition relationship is empirical, it is
also plausible and appeal ingly simple. And, the degree of
correlation is quite good.
The equations used by Johnstone to correlate vapor pressures
that he measured are summarized below.
VP . M
SO
b°
2 C - 5 - nA
VPNH = N c. ( c " s " n (2)
NH3 2S - C + nA
^PSO ' ^^NH * ^H n are vaP°r pressures of the indicated compounds, in
2 3 2U millimeters of mercury, above ammoniacal solutions,
VP is the vapor pressure of pure water at the temperature of the
ammoniacal solution. VPW is in millimeters of mercury.
M and N are coefficients relating the vapor pressures to solution
temperature. As solution temperature increases, the respective vapor
pressures increase in response to increasing values of M and N.
-2-
-------
Solution composition is defined by the following ratios.
S = mols of S02 per 100 mols of H20.
C = mols of NH3 per 100 mols of H20.
Ca= C-nA. For solutions in which a strong acid is present,
which may be considered as completely ionized, such as
sulfuric acid, n and A are defined as
n = valence of acid ion
A = mols of strong acid per 100 mols of H20
Since composition in all cases is referred to 100 mols of
water, it is prudent to inquire whether the water referred to is
uncombined or "free" water or whether it is total water including
the water of constitution. According to Johnstone, the equations he
used in correlating vapor pressure data are based on total water including
water of constitution. The difference in calculated composition ratios
when "free" and total water are used can be significant as shown below.
Compounds
Contained
in Solution
(NH4)2S04
(NH4)2S03
NH4HS03
"Free" H20
TOTAL
Composition
Ratios
S
C
A
C*
Composition
of Solution
Determined
Analytically,
1
2
6
91
100
it
Mols Contained in Indicated
NH3 S02
Mols
2
4 2
6 6
12 8
Composition Ratios
Free" Water
8.79
13.19
1.10
10.99
S04=
1
1
Referenced to
Total Water
8.00
12.00
1.00
10.00
Compound
H20
1
2
6
91
100
-3-
-------
B.A. Chertkov, whose work is referred to later, employed Johnstone's
composition ratios and equations as well as another system for
expressing composition. To avoid confusion, composition ratios as
defined by Johnstone will be used exclusively throughout this report.
And, it should be remembered that Johnstone's composition ratios are
referenced to total water.
As noted earlier,the vapor pressure of both SO^ and N^ above
ammoniacal solutions increases with increasing solution temperature.
The coefficients, M and N - see Equations 1 and 2 - are related to
solution temperature through the following equations.
Iog10 M = 5.865-2369/T (4)
Iog10 N = 13.680-4987/T (5)
In the above expressions, T is expressed in degrees Kelvin. Values
of M and N are listed in Table 1 for 1°F increments ranging from
115 to 145°F. For this range, M doubles when the temperature is
increased by 24°F, and N doubles when the temperature is increased
by 12°F.
Johnstone notes that "some variations in the values of M and N
are to be expected as the total salt concentration is changed."
However, for the broad range of concentrations studied by Johnstone,
"a single value for each of the constants at any one temperature will
reproduce the actual vapor pressures within 10 percent."
Johnstone points out that "for the concentration range in which
we are interested and for the pH range between 4.5 and 6, it is
permissible to make simplifying assumptions" resulting in the composition
groupings or expressions functionally related to the vapor pressure of
S02 and NHg - see Equations 1 and 2. Johnstone cautions, however,
against using Equations 1 and 2 outside of the pH range listed above
"where the ionization of ammonium hydroxide or sulfurous acid,
respectively, can not be considered as being complete." After additional
related information is discussed, limitations on Equations 1 and 2 will
be considered further.
The above discussions have been concerned primarily with the
vapor pressures of S02 and NH3 above ammoniacal solutions. In addition,
Johnstone measured the vapor pressure of water above ammoniacal solutions.
-4-
-------
Johnstone found that the experimentally determined water vapor pressures
could be predicted with good accuracy from Raoult's law. He noted that
"it is perhaps somewhat surprising that the simple Raoult's law agrees
so well with the experimental data." In calculating the vapor pressure
of water above ammoniacal solutions containing sulfate, Johnstone used
Equation 3. When the solution contains no sulfate, A is zero and , .
Equation 3 reduces to the simplified expression reported by Johnstone.^1'
As noted earlier, Johnstone presented an empirical relationship
between the composition of ammoniacal solutions and pH. Discussion of
this correlation is being deferred until Chertkov's more complete data
on the same subject are presented so that comparisons can be made and
more meaningful conclusions reached.
-5-
-------
CHERTKOV'S STUDIES
Chertkov,^' like Johnstone, studied the properties of ammoniacal
solutions extensively. Both investigators measured the vapor pressure
of sulfur dioxide above artificially prepared solutions containing
ammonium sulfite and bisulfite. They found the measured values to be
in good agreement with Johnstone's equation based on theoretical
considerations. As opposed to artificially prepared solutions,
commercial sulfite solutions obtained by scrubbing waste gases contain
small percentages of sulfate resulting from oxidation and other causes.
If SO? is to be released by acidification of the sulfites, oxidation
to sulfate should be discouraged so as to maximize the S02 recovered.
However, since the scrubber solution will inevitably contain some
sulfate, both Chertkov and Johnstone measured the vapor pressures of
S02 above solutions containing sulfite, bisulfite, and sulfate.
Chertkov found the vapor pressure of S02 increased significantly
when (NH4)oS04 was added to solutions of ammonium sulfites. Chertkov
explains this by noting "if (NH^SO/j. -a completely dissociated salt
with a common cation is added to an ammonium sulfite-bisulfite solution,
the equilibrium for hydrolysis is displaced" toward "undissociated
ammonium hydroxide and sulfurous acid." The increased concentrations
of undissociated base and acid should "increase the vapor pressure of
S02 and NH3 above the solution."
Chertkov "assumed that the increase in S02 vapor pressure when
sulfate is added (to a sulfate free solution) is proportional to the
increase in the total concentration of salt in the solution." He
found that the S02 vapor pressures he measured above solutions of
ammonium sulfites and sulfate could be calculated as the product of
two factors. The vapor pressure of S02 above solutions of ammonium
sulfite, bisulfite, and sulfate is equal to the vapor pressure above
sulfate-free solution multiplied by the ratio of total salt concentration
after ammonium sulfate addition to the total concentration before
addition.
Johnstone, unlike Chertkov, found no statistically significant
effect of added ammonium sulfate on the vapor pressure of SC^ above
solutions containing all three salts. This finding is in accord with
Equation 1 derived by Johnstone from theoretical considerations.
Unfortunately, the S02 vapor pressure data obtained by Johnstone and
Chertkov above sulfite solutions containing ammonium sulfate do not
agree. But the data collected by each investigator are correlated
equally well when the appropriate equations are used. In this case,
it is necessary to make a choice - and hopefully the correct choice - so
that process calculations for air pollution control systems employing
ammoniacal solutions will not contain errors from using incorrect vapor
-6-
-------
pressure data. In the absence of convincing data, abstract considerations
are used in choosing between different procedures for calculating vapor
pressures.
A careful consideration of the work of both investigators suggests
that Johnstone's studies are more reliable than those reported by
Chertkov. Johnstone's vapor pressure equations for both S02 and NH3
were derived from sound theoretical considerations and are limited
by solution pH as noted. Chertkov's vapor pressure data are restricted
to SO^; no ammonia vapor pressures are reported. Also, Chertkov's
correlation for S0£ vapor pressure above solutions containing sulfate
is empirical with no theoretical foundation. Finally, Chertkov's studies
emphasized dilute solutions and solutions containing higher-than-normal
ratios of bisulfite to sulfite. These are not conditions of major
interest, and the results Chertkov obtained may reflect these atypical
conditions.
In addition to vapor pressures above ammoniacal solutions, both
Johnstone and Chertkov measured the pH of solutions of sulfite, bisulfite,
and sulfate. Johnstone'1) found that "over the entire range of concen-
trations (he) studied, there is a linear function of the sulfur dioxide-
ammonia ratio. The empirical equation
pH = 9.2 - 4.62 (S/C)
reproduces the observed values within 0.1 pH unit. Obviously, however,
the equation cannot be extrapolated to the bisulfite ratio." It is to
be noted that although Johnstone related pH to S/C by the equation re-
ported, his measured pH values of solutions containing sulfate require
S/Ca as the correlating ratio. Thus, the relationship
pH = 9.2 - 4.62 (S/CJ
a
is more rigorous and fits all of Johnstone's data. Chertkov also relates
pH to S/C . and his data are far more extensive than Johnstone's data.
a
Over the S/Ca range of 0.7 to 0.9, Chertkov^3' found a linear
relationship between pH and S/C represented by
3
pH = 8.88 - 4.0 (S/C )
a
As may be noted from Figure 1, the linear relationships of both investigators
obtained with artificially prepared solutions are in reasonable agreement.
Chertkov points out, however, that all the experimental data plotted in
Figure 1 "fit reasonably accurately around a curve of a shape character-
istic for such determinations-with sharp breaks at critical points
corresponding to the appearance of new compounds in solution, in this
-7-
-------
7.0
6.5
6.0
5.5
5.0
pH= 8.88- 4.0 S/Ca ( Chertkov)
pH= 9.2- 4.62 S/Ca (Johnstone)
0.5 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
S/C.,
Figure 1. The pH values of ammonium sulfite-bisulfite solutions.
-8-
-------
instance at S/Ca =0.5 and S/Ca =1.0." Chertkov notes further that "the
presence of fairly large amounts of ammonium sulfate and thiosulfate
and small amounts of inhibitor and light ash in the production liquor" -
see curve defined by points on Figure 1 -" has virtually no effect on
the pH, because of its high buffer effect. The total ammonium sulfite-
bisulfite concentration likewise has little practical effect on
solution pH." '
Thus, both Chertkov and Johnstone agree that acidity or pH of
ammoniacal solutions of sulfites and sulfate is a function of S/Ca.
The influence of other factors appears to be far less important.
-9-
-------
JOHNSTONE'S EQUATIONS RECAST FOR SIMPLICITY
Johnstone's equations for calculating the vapor pressure of S0
and NH^ have been extremely useful in studying ammoniacal scrubbing
of waste gases to remove sulfur oxides. Nearly forty years after
Johnstone published his vapor pressure equations, they still appear
to be the most complete and reliable. Despite wide acceptance of
Johnstone's equations, simplified forms of the relationships are
desirable. Simplified expressions should expedite use of the
equations and indicate by special groupings the specific influence
of important variables, e.g., solution acidity, on the individual
vapor pressures. This, of course, would provide a better under-
standing of the vapor pressure equations - a better understanding
that might be useful in extending process studies for controlling
pollution.
Johnstone's vapor pressure equations for S02 and N^ given
earlier are restated below.
VP • "
NH3
Since A refers_to sulfuric acid, in this case n is 2 matching the
valence of SOa". For this system, Equations 1 and 2 are more
specifically defined by:
VP™ = M (2S - C + 2A)2 (n
bU2 C - S - 2A
VP - N " S"2A) (?')
VPNH3 N 2S - C + 2A (2 }
The related quantities in Equations 1' and 2' are defined earlier and
are defined identically in the recast equations given in Appendix A.
Appendix A shows that Equations 1' and 2' may be expressed as follows.
VPSo2 = MSD (6)
VP = NCE (7)
-10-
-------
Both D and E are functions of S/Ca.
(2S/Ca - I)2
S/Ca (1 - S/Ca)
D=
E =
- S/Ca
2S/Ca - 1
Solution acidity is also a function of S/Ca, hence D and E are referred
to as acidity factors in Equations 6 and 7 and Tables 2 and 3. Thus,
the vapor pressures of both S02 and NH^ may be calculated as the product
of three factors - (1) an appropriate temperature coefficient, multiplied
by (2) the solution concentration of the component referred to in the
vapor pressure equation, multiplied by (3) an appropriate acidity factor.
-11-
-------
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-12-
-------
Table 2. ACIDITY FACTORS FOR JOHNSTONE'S EOUATION FOR VAPOR PRESSURE OF S02
s/ca
0.57
0.58
0.59
0.60
0.61
0.62
0.63
0.64
0.65
0.66
0.67
0.68
0.69
0.70
0.71
0.72
0.73
0.74
0.75
0.76
0.77
0.78
0.79
0.80
0.81
0.82
0.83
0.84
0.85
0.86
0.87
0.88
0.89
0.90
0.91
0.92
aAcidity factor, D
0
0.105
0.134
0.167
0.203
0.244
0.290
0.340
0.396
0.456
0.523
0.596
0.675
0.762
0.857
0.960
1.074
1.198
1.333
1.482
1.6^7
1.828
2.028
2.250
2.498
2.775
3.087
3.440
3.843
4.306
4.842
5.470
6.215
7.111
8. 210
9.587
1
0.108
0.137
0.170
0.207
0.249
0.295
0.346
0.401
0.463
0.530
0.603
0.683
0.771
0.867
0.971
1.085
1.211
1.348
1.498
1.664
1.847
2.0^9
2.274
2.524
2.805
3.121
3.478
3.887
4.356
4.900
5.538
6.297
7.211
8.334
9.744
2
o.m
0.140
0.174
0.211
0.253
0.300
0.351
0.407
0.469
0.537
0.611
0.692
0.780
0.877
0.982
1.097
1.224
1.362
1.514
1.681
1.866
2.070
2.297
2.551
2.835
3.154
3.517
3.930
4.406
4.959
5.608
6.380
7.313
8.460
9.905
3
0.113
0.143
0.177
0.215
0.258
0.305
0.356
0.413
0.476
0.544
0.619
0.700
0.789
0.887
0.993
1.110
1 .237
1.377
1.530
1.6Q9
1.885
2.092
2.321
2.578
2.865
3.189
3.556
3.975
4.458
5.019
5.680
6.466
7.417
8.590
-
4
0.116
0.147
0.181
0.219
0.262
0.310
0.362
0.419
0.482
0.551
0.627
0.709
0.799
0.897
1.004
1.122
1.250
5
0.119
0.150
0.185
0.223
0.267
0.315
0.367
0.425
0.489
0.558
0.634
0.718
0.808
0.907
1.016
1.134
1.264
1.391 1.406
1.546 1.563
1.717 1.735
1.905 1.925
2.114 2.136
2.346
2.605
2.895
3.223
3.595
4.020
4.510
5.081
5.752
6.553
7.523
8.722
-
2.370
2.632
2.926
3.258
3.635
4.066
4.563
5.143
5.826
6.641
7.631
8.858
-
6
0.122
0.153
0.188
0.228
0.271
0.320
0.373
0.431
0.496
0.566
0.642
0.726
0.818
0.918
1.027
1.147
1 .277
1.421
1.579
1.753
1.945
2.158
2.395
2.660
2.958
3.294
3.676
4.113
4.617
5.206
5.901
6.731
7.742
8.996
-
7
0.125
0.156
0.192
0.232
0.276
0.325
0.378
0.438
0.502
0.573
0.650
0.735
0.827
0.928
1.039
1.159
1.291
1.436
1.596
1.771
1.965
2.181
2.421
2.688
2.990
3.330
3.717
4.160
4.672
5.270
5.977
6.824
7.855
9.139
-
8
0.100
0.128
0.160
0.196
0.236
0.281
0.330
0.384
0.444
0.509
0.581
0.659
0.744
0.837
0.939
1.050
1.172
1.305
1.451
1.612
1.790
1.986
2.204
2.446
2.717
3.022
3.366
3.758
4.208
4.728
5.336
6.055
6.918
7.971
9.284
-
9
0.102
0.131
0.163
0.200
0.240
0.285
0.335
0.390
0.450
0.516
0.588
0.667
0.753
0.847
0.950
1.062
1.185
1.319
1.467
1.629
1.809
2.007
2.227
2.472
2.746
3.054
3.403
3.800
4.256
4.784
5.402
6.134
7.013
8.089
9.434
-
a (2S/Ca ' ])
Acidity factor, D = $/C (1 - S/C )
-13-
-------
Table 3. ACIDITY FACTORS FOR JOHNSTONE'S EQUATION FOR VAPOR PRESSURE OF NH.
s/cft
0.52
0.53
0.54
0.55
0.56
0.57
0.58
0.59
0.60
0.61
0.62
0.63
0.64
0.65
0.66
0.67
0.68
0.69
0.70
0.71
0.72
0.73
0.74
0.75
0.76
0.77
0.78
0.79
0.80
0.81
0.82
0.83
0.84
0.85
0.86
0.87
0.88
0.89
0.90
0.91
aAcid1ty f,
0
_ .
7.833
5.750
4.500
3.667
3.071
2.625
2.278
2.000
1.773
1.583
1.423
1.286
1.167
1.063
0.971
0.889
0.816
0.750
0.690
0.636
0.587
0.542
0.500
0.462
0.426
0.393
0.362
0.333
0.306
0.281
0.258
0.235
0.214
0.194
0.176
0.158
0.141
0.125
0.110
1
_
7.565
5.598
4.402
3.598
3.021
2.586
2.247
1.975
1.752
1.566
1.408
1.273
1.156
1.053
0.962
0.881
0.809
0.744
0.685
0.631
0.582
0.537
0.496
0.458
0.423
0.390
0.359
0.331
0.304
0.279
0.255
0.233
0.212
0.193
0.174
0.156
0.139
0.123
0.108
2
_
7.313
5.452
4,308
3.532
2.972
2.549
2.217
1.951
1.732
1.549
1.394
1.261
1.145
1.043
0.953
0.874
0.802
0.738
0.679
0.626
0.578
0.533
0.492
0.454
0.419
0.387
0.356
0.328
0.301
0.276
0.253
0.231
0.210
0.191
0.172
0.154
0.138
0.122
0.107
3
_
7.076
5.314
4.217
3.468
2.925
2.512
2.188
1.927
1.712
1.533
1.380
1.248
1.134
1.034
0.945
0.866
0.795
0.732
0.674
0.621
0.573
0.529
0.488
0.451
0.416
0.383
0.353
0.325
0.299
0.274
0.251
0.229
0.208
0.189
0.170
0.153
0.136
0.120
0.105
4
9.917
6.853
5.182
4.130
3.406
2.878
2.476
2.160
1.904
1.693
1.516
1.366
1.236
1.123
1.024
0.937
0.859
0.789
0.725
0.668
0.616
0.568
0.525
0.484
0.447
0.412
0.380
0.350
0.322
0.296
0.272
0.249
0.227
0.206
0.187
0.168
0.151
0.135
0.119
0.104
ctor, E
5
9.500
6.643
5.056
4.045
3.346
2.833
2.441
2.132
1.881
1.674
1.500
1.352
1.224
1.113
1.015
0.929
0.851
0.782
0.720
0.663
0.611
0.564
0.520
0.480
0.443
0.409
0.377
0.347
0.320
0.294
0.269
0.246
0.225
0.204
0.185
0.167
0.149
0.133
0.117
0.102
6
9.115
6.444
4.935
3.964
3.288
2.789
2.407
2.104
1.858
1.655
1.484
1.338
1.212
1.103
1.006
0.920
0.844
0.776
0.714
0.657
0.606
0.559
0.516
0.477
0.440
0.406
0.374
0.345
0.317
0.291
0.267
0.244
Q.223
0.202
0.183
0.165
0.148
0.131
0.116
0.101
7
8.759
6.257
4.819
3.886
3.231
2.747
2.374
2.077
1.836
1.637
1.469
1.325
1.201
1.092
0.997
0.912
0.837
0.769
0.708
0.652
0.601
0.555
0.512
0.473
0.436
0.403
0.371
0.342
0.314
0.289
0.265
0.242
0.220
0.200
0.181
0.163
0.146
0.130
0.114
0.100
8
8.429
6.079
4.708
3.810
3.176
2.705
2.341
2.051
1.815
1.619
1.453
1.312
1.189
1.082
0.988
0.904
0.830
0.763
0.702
0.647
0.596
0.550
0.508
0.469
0.433
0.399
0.368
0.339
0.312
0.286
0.262
0.240
0.218
0.198
0.179
0.161
0.144
0.128
0.113
-
9
8.121
5.910
4.602
3.737
3.123
2.665
2.309
2.025
1.794
1.601
1.438
1.299
1.178
1.072
0.979
0.897
0.823
0.756
0.696
0.642
0.592
0.546
0.504
0.465
0.429
0.396
0.365 '
0.336
0.309
0.284
0.260
0.237
0.216
0.196
0.178
0.160
0.143
0.127
0.111
-
Acidity factor, E
- S/C
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LIMITATIONS OF JOHNSTONE'S VAPOR PRESSURE EQUATIONS
According to Johnstone, his vapor pressure equations for S02
and NHg should not be used outside the pH range of 4.5 to 6.0.
At lower and higher pH's, Johnstone notes that some of the
assumptions made in deriving the equations are no longer valid and
the accuracy of the equations is less reliable. Johnstone's
equations, as recast in Appendix A, show that vapor pressures of
S02 and NH3 are functions of solution temperature, concentration of
S02 and NH3 in solution, and S/Ca. While pH is primarily a function
of S/Ca, other factors exert a small influence. Thus, different
investigators obtain slightly different relationships between pH
and S/Ca, e.g., Johnstone and Chertkov - See Figure 1. If Johnstone's
relationship between pH and S/Ca is valid, a pH of 6.1 (only 0.1
pH unit higher than the upper pH limit recommended) is consistent
with a S/Ca value of 0.67. While this S/Ca should be acceptable,
the accuracy of calculated vapor pressure for lower values of S/Ca
and higher pH's will be more questionable.
While Johnstone's vapor pressure equations should be reliable/-}
within the pH range of 4.5 to 6.0, R.A. Berdyanskaya and coworkers^ '
found that accurate SO? vapor pressures could not be calculated from
Johnstone's equation when S/Ca is above 0.87. According to
Berdyanskaya, "the simplified equation for calculating the equilibrium
partial pressure of S02 over ammonium sulfite-bisulfite solution,
proposed by Johnstone, gives large errors when applied to solutions
in which more than 85% of the dissolved S02 is bound in the form of
bisulfite (S/C > 0.87), especially at temperatures over 80°." But,
according to Johnstone, the upper limit on S/Ca should be well above
0.87. Nevertheless, the more restrictive upper limitation on S/Ca of
0.87 is recommended in calculating S02 vapor pressures. While this
more conservative limitation on S/Ca may not be entirely justified,
it imposes few, if any, practical limitations on development of waste
gas scrubbing processes employing ammoniacal solutions.
In summary, vapor pressures for S02 and NH3 calculated from
Johnstone's equations are more reliable when the acidity factors lie
within the S/Ca range of 0.67 to 0.87. Outside this range, the
acidity factors are more questionable, and the vapor pressures are
less reliable.
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APPLICATION OF JOHNSTONE'S RECAST EQUATIONS
As noted earlier, Johnstone's vapor pressure equations in recast
simplified form provide a better understanding of the factors controlling
the vapor pressures so that process results may be optimized. Secondly,
the recast equations expedite vapor pressure calculations. Process
optimization calculations using the recast equations are outside the
scope of this report; however, an example of calculations using the
simplified vapor pressure equations is outlined below.
Consider the top stage of an ammoniacal scrubber to remove SO?
from waste gas. If vapor-liquid equilibrium is established and 200 ppm
of S02 escape with the waste gas, calculate S, Ca, and C in the top
stage liquor and the ammonia loss in ppm in the waste gas. Assume
the following conditions prevail in the top stage.
Conditions
Temperature, °F =130
S/Ca = 0.75
A = 0.1S
Calculations for the Example
VPSQ2 = MSD
VPsQ,, = 20° PPm = 0.152mm of Hg
M at 130°F = 0.0430 See Table 1
D for 0.75 S/Ca = 1.333 See Table 2
S - 0.152 = 2 65
0.0430 X 1.333
S/Ca = 0.75
Ca = S/0.75 = 2.65/0.75 = 3.533
A = 0.1S = 0.1 X 2.65 = 0.265
C = C + 2A = 3.533 + 2 X 0.265 = 4.063
a
-16-
-------
= NCE
N at 13QQF = 0.0287 See Table 1
C = 4.063
E for 0.75 S/Ca = 0.500 See Table 3
Vp. = 0.0287 X 4.063 X 0.500
NH
3 = 0.0583 mm of Hg = 76.7 pptn
Calculation of the example using Johnstone's equations in their original
form is considerably more time consuming than the simplified procedure
outlined above.
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REFERENCES
(1) Johnstons, H.F. "Recovery of Sulfur Dioxide from Waste Gases:
Equilibrium Partial Vapor Pressure over Solutions of the Ammonia-
Sulfur Dioxide-Water System." Ind. Eng. Chem., 27(5), May 1935,
pp 587-593.
(2) Chertkov, B.A. and Dobromyslova, N.S. "The Influence of Traces
of Sulfate on the Partial Pressure of SO? over Ammonium Sulfite-
Bisulfite Solutions." J. Appl. Chem. USSR, 37(8), August 1964,
pp 1707-1711.
(3) Chertkov, B.A., Puklina, D.L., and Pekareva, T.I. "The pH
Values of Ammonium Sulfite-Bisulfite Solutions." J. Appl. Chem.
USSR, 32(6), 1959, pp 1417-1419.
(4) Berdyanskaya, R.A., Golyand, S.M., Chertkov, B.A. "On the Partial
Pressure of S02 Over Ammonium Sulfite - Bisulfite Solutions.11
J. Appl. Chem. USSR, (32), 1959, pp 1978-1984.
-18-
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APPENDIX A
JOHNSTONE'S RECAST VAPOR PRESSURE EQUATIONS
(2S - C + 2A)2
VPsn (mm) = M (T)
bU2 C - S - 2A
(2S - Ca)2
— ^ . J, I.U1-JUL-
4S2 - 4SCa + C 2
*
C 2(4S2/Ca2 - 4S/C. + 1)
u a _ a o
ca(i - s/ca)
C.(2S/C.-1)2
MS
1 - S/Ca
(S/Ca)(Ca)(2S/Ca - I)2
s/ca(i - s/ca)
(2S/C - I)2
s/ca(i - s/ca)
MSD (6)
C(C - S - 2A)
VPNH (mm) • N (21)
NH3 2S - C + 2A
C, - S
ZS - C
a
cd - s/c
= NC
C(2S/Ca -
a a
1 - S/C,
= NC
2S/C - 1
a
NCE (7)
-19-
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 REPORT NO.
EPA-650/2-74-035
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Evaluation of Equations for Designing Ammoniacal
Scrubbers to Remove Sulfur Oxides from Waste Gas
5. REPORT DATE
January 1974
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
L.I. Griffin
9. PERFORMING ORGANIZATION NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
EPA, ORD, NERC-RTP
Control Systems Laboratory
Gas Cleaning and Metallurgical Processes Branch
Research Triangle Park. NC 27711
1AB013: ROAP 21ACX-60
11. CONTRACT/GRANT NO.
NA--In-house Report
12. SPONSORING AGENCY NAME AND ADDRESS
NA--In-house Report
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The report reviews the work of H. F. Johnstone in i835 and of B. A. Chertkov in
the 1950's , related to laboratory vapor pressure-temperature measurements of
sulfur dioxide, ammonia, and v.ater above ammoniacal solutions. It indicates
that, although Johnstone and Chertkov are in general agreement, their measurements
lead to different conclusions in several instances. The report sugges.ts resolutions
of the differences noted: the relationship recommended should provide a reliable
basis for designing ammoniacal scruboers for removing sulfur oxides from waste
gas. As for other absorbents , design data for ammoniacal scrubbers must
include detailed knowledge of solution properties. Ammonium sulfite and bisulfite
are far more soluble in water than other sulfites. This constitutes an important
scrubber credit that has prompted careful study of ammoniacal solutions.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
Air Pollution
Ammonium Compounds
Scrubbers
Desulfurization
Exhaust Gases
Flue Gases
Measurement
Vapor Pressure
Sulfur Oxides
Ammonia
Water
Air Pollution Control
Stationary Sources
Ammoniacal Solutions
H. F. Johnstone
B.A. Chertkov
13B
7B, 7C
7A
7D
2 IB
14 B
13. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
23
20. SECURITY CLASS (This page)
Unclassified
22..PRICE
EPA Form 2220-1 (9-73)
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