EPA-600/2-75-023
August 1975 Environmental Protection Technology Series
ABSORPTION
OF SULFUR DIOXIDE IN SPRAY COLUMN
AND TURBULENT CONTACTING
ABSORBERS
U.S. Environmental Protection Agency
Office of Research md Development
Washington, D. C. 20450
-------
EPA-600/2-75-023
ABSORPTION
OF SULFUR DIOXIDE IN SPRAY COLUMN
AND TURBULENT CONTACTING
ABSORBERS
by
C. Y. Wen and L. S . Fan
Department of Chemical Engineering
West Virginia University
Morgantown, West Virginia 26506
Grant No. R-800781.
ROAP No. 21ACY-041
Program Element No. 1AB013
EPA Project Officer: Robert H. Borgwardt
Industrial Environmental Research Laboratory
. Office, of Energy, Minerals, and Industry
Research Triangle Park, North Carolina 27711
Prepared for
U. S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, D. C. 20460
August 1975
-------
11
EPA REVIEW NOTICE
This report has been reviewed by the National Environmental Research
Center - Research Triangle Park, Office of Research and Development,
EPA, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These series are:
1. ENVIRONMENTAL HEALTH EFFECTS RESEARCH
2. ENVIRONMENTAL PROTECTION TECHNOLOGY
3. ECOLOGICAL RESEARCH
4. ENVIRONMENTAL MONITORING
5. SOCIOECONOMIC ENVIRONMENTAL STUDIES
6. SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS
9. MISCELLANEOUS
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to
develop and demonstrate instrumentation, equipment and methodology
to repair or prevent environmental degradation from point and non-
point sources of pollution. This work provides the new or improved
technology required for the control and treatment of pollution sources
to meet environmental quality standards.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
Publication No. EPA-600/2-75-023
-------
Ill
Table of Contents
Page
List of Figures v
List of Tables viii
Chapter
1. Introduction 1
1.1 Description of Scrubber-Holding Tank
Recycle System Using Limestone/Lime
Slurries 2
1.2 Scope of Investigation 2
2. Literature Review 6
2.1 General Review of Gas-Liquid Contactors . . 6
2.2 Review of Turbulent Contacting Absorbers . . 6
2.3 Review of the Chemistry of SC>2 Scrubbing
Using Limestone/Lime Slurries 21
2.3.1 Chemical Reaction in S02
Absorption 21
2.3.2 Oxidation and Solid Precipitation . . 23
3. Mathematical Model for SC>2 Absorption in
Spray and TCA Scrubbers 25
3.1 Introduction 25
3.2 Mathematical Models of Spray and TCA
Scrubbers 25
3.2.1 Scrubber Model 27
3.2.2 Mass Transfer Coefficients 34
3.2.3 Effect of Magnesium on SO- Scrubbing
Efficiency 54
-------
IV
Table of Contents (Continued)
Page
3.3 Simulation of SC>2 Scrubbing With
Limestone and Lime Slurries 58
4. Mechanism of S02 Scrubbing With
Limestone Slurries 65
4.1 Introduction 65
4.2 Reaction Mechanism 66
5. Analysis of pH for Lime Slurry at the
Outlet of the Scrubber 76
5.1 Introduction 76
5.2 Liquid Phase Material Balance for the
Scrubber 77
6. Conclusion and Discussion 86
Nomenclature 90
Bibliography 94
Appendices
A. Numerical Example of the Simulation of
EPA/RTP TCA Scrubber Using Lime Slurry
as the Scrubbing Medium 99
B. Some Comments on the Concentration of
Reactant, Cg, Calculated Via Equilibrium
Computer Program 104
-------
V
List of Figures
Figure Page
1.1 Configuration of Scrubber-Holding Tank
Recycle System Using Limestone/Lime Slurries
as the Scrubbing Medium 3
3.1 Schematic of TVA Shawnee Three-Bed TCA 28
5.2 Schematic of TVA Shawnee Spray Tower 29
3.3 Schematic of the EPA/RTP Research TCA Scrubber ... 30
3.4 Ratio of the Mass Transfer Resistances for
Spray Devices, R , as a Function of the pH of
the Scrubbing Slurry at the Inlet of the
Scrubber 37
3.5 An Idealized Stage of a Turbulent Contacting
Absorber 42
3.6 Typical Operating Data for the TVA Shawnee
TCA Showing the Dependence of the S0_ Removal
Efficiency on the Inlet S00 Concentration 46
3.7 The Ratio of the Gas Film to the Liquid Film
Mass Transfer Resistance as a Function of the
Inlet SO- Partial Pressure for the TVA Shawnee
TCA . . 7 48
3.8 The Pre-Exponential Function, As, in the
Expression for R, the Ratio of Resistances,
as a Function of the pH of the Inlet Slurry
for the TVA Shawnee TCA Without Spheres and
Spray Column. Low Magnesium Concentration
(Mg < 350 ppm) 50
3.9 The Pre-Exponential Function, Ap, in the
Expression for R, the Ratio of Resistances,
as a Function of the pH of the Inlet Slurry
for the Packed Section of TVA TCA and EPA TCA.
Low Magnesium Concentration (Mg < 350 ppm) 53
3.10 The-Effect of Magnesium in the Scrubbing
Slurry on the Ratio of the Gas to Liquid
Mass Transfer Resistances in the Spray
Section 55
-------
VI
List of Figures (Continued)
Figure Pa8e
3.11 The Effect of Magnesium in the Scrubbing Slurry
on the Ratio of the Gas to Liquid Mass Transfer
Resistances in the Packed Section .......... 56
3.12 Comparison of the Predicted and Observed S02
Removal Efficiencies for Spray-Type Devices
Using Limestone Slurry as the Scrubbing
Medium ....................... 60
3.13 Comparison of the Predicted and Observed S02
Removal Efficiencies for TCA Scrubbers
Using Limestone Slurry as Scrubbing Medium ..... 61
3.14 Comparison of the Predicted and Observed S02
Removal Efficiencies for TCA Scrubber Using
Lime Slurry as the Scrubbing Medium ......... 62
4.1 The Overall Ratio of the Mass Transfer
Resistances as a Function of the Arithmetic
Average of the S0~ Partial Pressure in the
Bulk Gas Phase Plotted as Suggested by
Equation (4-6) . Data from the TVA Shawnee
TCA. Low Magnesium Concentration in the
Limestone Slurry .................. 68
4.2 The Overall Ratio of Mass Transfer Resistance
as a Function of the Arithmetic Average of
the S02 Partial Pressure in the Bulk Gas
Phase Plotted as Suggested by Equation (4-6) .
Data from the TVA Shawnee TCA. High Magnesium
Concentration in the Limestone Slurry ........ 70
4.3 Idealization of the Concentration Profiles of
Species Important in the Transfer of S02
Across the Gas-Liquid Interface and the Chemical
Reaction in the Liquid Phase ............ 73
4.4 The Overall Ratio of Mass Transfer Resistances
as a Function of the Arithmetic Average of the
S02 Partial Pressure in the Bulk Gas Phase
Plotted as Suggested by Equation (4-6) . Data
from the TVA Shawnee Spray Column. Low Magne-
sium Concentration in the Limestone Slurry ..... 74
-------
VI 1
List of Figures (Continued)
Page
Figure
5.1 Comparison of the Predicted and Observed
Values of n for Lime Slurry at the Inlet
and Outlet of EPA/RTP TCA Scrubber 81
5.2 Comparison of the Predicted and Observed
Outlet Slurry pH for the Simulation of
EPA/RTP TCA Scrubber Using Lime Slurry
as the Scrubbing Medium 84
5.3 Comparison of the Predicted and Observed
S02 Removal Efficiencies for the Simulation
of EPA/RTP TCA Scrubber Using Lime Slurry
as the Scrubbing Medium 86
A.I Relationship Between n and pH for Lime
Scrubbing System for the Example Given
in Appendix B 102
-------
Vlll
List of Tables
Table
2.1 Classification DJ Gas-Liquid Contactors 7
2.2 Characteristics of Gas-Liquid Contactors 8
2.3 Data on Sulfur Dioxide Reduction Using Gas-Liquid
Contactors 12
3.1 Range of Data for the Spray Column and TCA
Operating Without Packing Spheres Used in
Constructing Figure 3.4 38
3.2 Range of Data for the EPA/RTP Research TCA Scrubber
Used in the Estimation of the Gas Side Mass
Transfer Coefficient for the Packed Section of
the TCA Scrubber 44
3.3 Range of Data for the TVA Shawnee TCA and EPA
In-House TCA Used in Determining A in Figure
3.9 ? 52
3.4 Range of Scrubber Operating Data Used in
Calculating the Effect of Magnesium on the
Pre-Exponential Factor, A 57
3.5 Summary of Equations Necessary for Simulating
the Performance of the TVA Shawnee TCA and
Spray Column and the EPA In-House TCA 59
3.6 Range of Data Used in Constructing Figure 3.14.
Data is for the EPA In-House TCA Using Lime
Slurries as the Scrubbing Medium 63
4.1 Comparison Between the Calculated Concentration
of the Species Which is Hypothesized to Instanta-
neously and Irreversibly React With Absorbed
S02 as H_SO in Limestone Slurries and Selected
Species Concentration Predicted from Equilibrium ... 71
5.1 Range of Data Used in Constructing Figures 5.1, 5.2
and 5.3. Data is for the EPA In-House TCA Using
Lime Slurries as the Scrubbing Medium 84
A. 1 Operating Conditions and Reference for the
Example Given in Appendix B. Data is for the
EPA In-House TCA Using Lime Slurries as the
Scrubbing Medium 104
-------
Chapter 1
Introduction
The presence of sulfur dioxide in flue gases occurs in
many common processes such as the combustion of heavy
oil and coal, smelting operations, sulfuric acid manufacture
and metallurgical processes. Because of the current concern over
impending energy shortages fuels with high sulfur content will be
increasingly used as important sources of energy in the future.
As a result, the environmental aspects of the burning of high
sulfur fuel and its associated SCL emission problems are being
given considerable attention by governments throughtout the
world.
Due to the EPA regulations on sulfur dioxide emissions
associated with stack gas, much effort has gone into studying the
removal of sulfur dioxide from these gases. Several processes for
the removal of sulfur dioxide from stack gases have been proposed
and studied in both small and large scale pilot-plant operations.
One of these processes is the wet scrubbing process which utilizes
recycled limestone or lime slurries as scrubbing liquid. It is
this process upon which the attention of this study is focused
In this chapter, a general description of the wet scrubbing
processes which utilize limestone/lime slurries as the scrubbing medium
is given and the scope of the study is defined.
-------
1.1 Description of Scrubber-Holding Tank Recycle System Using
Limestone/Lime Slurries
A simplified flow sheet for a wet scrubber-holding tank recycle
system is given in Figure 1.1. The absorption of sulfur dioxide is
carried out in the scrubber where intimate contact between the
upward flowing gas and downward flowing slurry is maintained. The
holding tank plays an important role in the recycle system by pro-
viding sufficient holding time for the scrubber effluent so that the
precipitation of calcium sulfite and calcium sulfate which are
produced by the reaction of sulfur dioxide and oxygen with limestone
or lime slurries will take place. These precipitants are then
purged from the system through the clarifier. It is in the make-
up tank that fresh limestone/lime slurries are provided for the
slurry recycle system.
Two of the most promising scrubbers for carrying out the sulfur
dioxide scrubbing with limestone/lime slurries appear to be the
turbulent contacting absorber (TCA) and spray tower. Several large
scale scrubber-holding tank recycle systems utilizing these
scrubbers have been built and a number of research projects on wet
scrubber systems have been sponsored by the EPA to test the
performance and reliability of these scrubbers.
1.2 Scope of Investigation
In this study, a mathematical model is developed which describes
the absorption of sulfur dioxide by limestone or lime slurries in
a scrubber. Parameters appearing in this model are evaluated from
-------
GAS OUT
GAS IN
SCRUBBER
LIMESTONE/LIME
SLURRIES
HOLDING
TANK
MAKE-UP TANK
CLARIFIER
SOLID
Figure 1.1 Configuration of Scrubber-Holding Tank Recycle System Using Limestone/Lime
Slurries as the Scrubbing Medium.
-------
experimental data obtained from the large scale turbulent contacting
absorber located at the TVA Shawnee power station and the EPA in-
house turbulent contacting absorber and spray column at Research
Triangle Park, North Carolina. Correlations are developed for the
gas film mass transfer coefficients and the overall coefficients in
the liquid film based on the hydrodynamic and chemical characteristics
of the system.
The scrubbing of sulfur dioxide from flue gas is simulated
utilizing the correlations developed in this study for the gas film
mass transfer coefficient and the ratio of mass transfer resistances.
The results are compared with the experimentally observed sulfur
dioxide removal efficiencies for widely differing size TCA scrubbers
and spray column scrubbers. It is demonstrated that the mathematical
model developed here can be used in the design and scale-up of the
process of absorbing S02 into limestone or lime slurries in spray
column and TCA scrubbers.
In light of the mathematical model and correlations developed
in this study the complex mechanism of SCL absorption into the
scrubbing slurry is analyzed and the key liquid phase reactions are
identified.
The rate of C02 transfer between the gas and liquid phases in
the scrubber effluent is also studied. Since the kinetics of
precipitation and dissolution of the calcium salts found in wet
scrubber processes are uncertain at this stage, the differential
material balances for the liquid phase in the scrubber cannot be
-------
solved. However, calculations based on assuming chemical equilibrium
in the liquid phase are shown to approximate the liquid phase analysis
reasonably accurately.
-------
Chapter 2^
Literature Review
2.1 General Review of Gas-Liquid Contactors
Gas-liquid contactors can be classified according to the geometry
of dispersed fluid, the relative direction of flow of two phases, the
type of operation, and the force utilized. A detailed description
of such classification has been shown in Table 2.1. In Table 2.2,
the various types of gas-liquid contactors commonly utilized in the
chemical industries are presented. Explanations for each type of
gas-liquid contactor are based on its hydrodynamic characteristics
and mass transfer behavior. For flue gas desulfurization systems,
various different types of gas-liquid contactors are being used.
Presently, a number of pilot plants are either being planned or under
construction. Summaries of their operations are provided by Deyitt
and ZadaL , 1974; and Ando'- •*, 1974. Other references relating
to the application of gas-liquid contactors on the desulfurization
processes are given in Table 2.3. It should be noted that Table
2.3 is provided according to the operational characteristics of
these contactors.
2.2 Review of Turbulent Contacting Absorbers
The turbulent contacting absorber (TCA)} first described
in 1959 , is a scrubber primarily intended to remove particulates
from a dust laden gas. TCA is a novel type of gas-liquid contacting
device which consists of non-flooding packings made of low density
-------
Table 2.1
Classification of gas-liquid
[2, 44, 45, 62]
contactors.
Geometry of
dispersed phase
Liquid drop, Gas bubble,
Liquid film, Liquid jet.
Relative direction
of flow of
two phases
Cocurrent flow, Countercurrent
flow, Crosscurrent flow
Type of operation
Batch system, Continuous system
Force utilized
Gravitational force, Centrifugal
force
-------
Characteristics of gas-liquid contactors.
Table 2.2
[2, 7, 22, 37, 44, 45, 46, 53, 61, 62, 65, 68]
Type
of
Equipment
Wetted Wall
tower
Bubble
tower
Agitated
vessel with
sparger
Packed
tower
Wooden grid
packed
tower
Flow Mechanism
Dispersed
phase
(liquid)
gas
gas
gas
or
liquid
liquid
Geometry of
dispersed
phase
(film)
bubble
bubble
film
film
or
drop
Relative
flow
direction
cocurrent
or
count er-
current
cocurrent
or
counter-
current
cocurrent
or
counter-
current
counter-
current
or
cross-
current
Liquid
surface
renewal
slight
slight
slight
can be
obtained
with
specific
packings
excel-
lent
Flow
ratio
variable
variable
variable
variable
variable
can be
quite
low
Power Consumption
Phase in
which
power is
supplied
gas
gas
gas
gas
gas
Power
Consumption
effect on
efficiency
slight
slight
slight
slight
slight
Mass Transfer
Phase in which
resistance
predominates
gas or liquid
phase con-
trolling
liquid phase
controlling
gas or liquid
phase con-
trolling
liquid phase
controlling
oo
-------
Table 2.2 (Continued)
Type
of
Equipment
Bubble
cap
tower
Perforated
plate
tower
Perforated
plate
tower
without
downcomer
Rotational
current
tower
Flow Mechanism
Dispersed
phase
gas
gas
gas
gas
Geometry of
dispersed
phase
bubble
bubble
bubble
bubble
Relative
flow
direction
count er-
current
(cross-
current
on a
tray)
count er-
current
(cross-
current
on a
tray)
count er-
current
counter-
current
(cocurrent
on a tray)
.iquid
surface
renewal
sxcel-
lent
sxcel-
Lent
excel-
lent
excel-
lent
Flow
ratio
variable
can be
quite
low
variable
can be
quite
low
variable
can be
quite
low
variable
can be
quite
low
Power Consumption
Phase in
which
power is
supplied
gas
gas
gas
gas
Power
Consumption
effect on
efficiency
slight
slight
slight
slight
Mass Transfer
Phase in which
resistance
predominates
liquid phase
controlling
liquid phase
controlling
liquid phase
controlling
liquid phase
controlling
-------
Table 2.2 (Continued)
Type
of
Equipment
Spray
tower
Centrifugal
spray
scrubber
Cyclone
scrubber
Venturi
scrubber
Jet
scrubber
Centrifugal
contactor
Flow Mechanism
Dispersed
phase
liquid
liquid
liquid
liquid
liquid
liquid
Geometry of
Dispersed
phase
drop
drop
drop
drop
or
bubble
drop
drop
film
and jet
Relative
flow
direction
counter-
current
or
cocurrent
count er-
current
or
cross-
current
cocurrent
cocurrent
cocurrent
counter-
current
or
cocurrent
Liquid
surface
renewal
excel-
lent
good
slight
slight
good
excel-
lent
Flow
ratio
variable
high
high
high
high
variable
Power Consumption
Phase in
which
power is
supplied
liquid
gas
liquid
gas
or
liquid
liquid
gas
(or
equip-
ment
itself)
Power
Consumption
effect on
Efficiency
significant
significant
significant
significant
significant
significant
Mass Transfer
Phase in which
resistance
predominates
gas or liquid
phase
controlling
gas or liquid
phase
controlling
gas phase
controlling
gas or liquid
phase
controlling
gas or liquid
phase
controlling
gas or liquid
phase
controlling
-------
Table 2.2 (Continued)
Type
of
Equipment
Turbulent
contacting
absorber
Hydro-
Filter
scrubber
Flow Mechanism
Dispersed
Phase
liquid
(gas)
gas
(in
turbu-
lent
layer)
liquid
(other
portion)
Geometry of
dispersed
Phase
film
drop
(bubble)
bubble
drop
Relative
flow
direction
count er-
current
counter-
current
and
cocurrent
Liquid
surface
renewal
excel-
lent
fair
Flow
ratio
variable
variable
Power Consumption
Phase in
wh;Lch
power is
supplied
liquid
liquid
Power
Consumption
effect on
efficiency
significant
significant
Mass Transfer
Phase in which
resistance
predominates
gas phase
controlling
gas or liquid
phase
controlling
-------
Table 2.3
Data on sulfur dioxide reduction using gas-liquid contactors.
Type
of
contactor
Packed
tower
-
Wooden
grid packed
tower
Wooden
grid packed
tower
• •• i . •'
Wooden
grid packed
tower
Wooden
grid packed
tower
Wooden
grid packed
tower
Removal
process
Ammoniacal
liquor
process
Ammoniacal
liquor
process
\mmoniacal
liquor
process
Lime
process
Lime
arocess
Soda
process
SO vol %
Inlet
0.3
0.15
0.077
0.128
i
0.144
0.08
0.1
?
0.2
Outlet
0.06
0.018
(
0.038
0.017
0.01
i
0.04
0.006
0.05
i
0.1
Gas
flow
(kg/m hr)
2,100
4,750
i
10,000
4,200
5,000
i
10,500
11,000
4,400
i
13,500
Liquid
flow
(kg/m2hr)
4,000
i
6,700
2,800
i
5,500
55,800
4,800
»
13,600
32,000
3,000
Liquid
Gas
ratio
1.6
(
2.7
0.5
11.6
0.5
i
1.5
3.2
0.24
>
0.74
Pressure
loss
(mm aq)
<25
<25
<25
76
<25
Absorption
80
75
'
80
78
70
'
92
93
50
NOG
3.3
1.6
' •
2.3
3.4
30
i
110
2.4
0.75
i
0.95
(kgmole/
m hr atm)
95
i
250
100
i
350
680
24
143
i
325
Reference
[2]
[2]
[2]
[2]
[2]
[45]
Remarks
2" Raschig
ring
20 moles NH3/
100 moles H20
15.5 moles NH3/
100 moles H00
2
CaCO 10%
slurries
0.5N NaOH
solu.
N>
-------
Table 2.3 (Continued)
Type
of
contactor
Perforated
plate
tower
Spray
tower
Spray
tower
Cyclone
scrubber
Venturi
scrubber
Venturi
scrubber
Venturi
scrubber
Removal
process
Ammoniacal
liquor
process
Ammoniacal
S02 vol %
Inlet
0.3
liquor |
process
Lime
process
Soda
process
Ammoniaca!
liquor
process
Ammoniacal
liquor
process
Lime
process
0.3
0.08
i
0.17
0.1
i
0.17
1.4
i
1.6
0.13
i
0.14
Outlet
0.05
0.03
0.01
(
0.03
0.01
)
0.02
0.13
i
0.3
0.02
i
0.08
Gas
flow
2
(kg/m hr)
7,900
i
9,400
1,300
i
2,600
1,460
3,000
i
10,000
40
t
i/u
sec
40
i
\ Sll
DU
sec
, _ m
f\(\
ccir*
j c \*
Liquid
flow
(kg/m2hr)
2,500
i
3,200
1,700
7,600
Liquid
gas
ratio
0.26
t
0.37
1.3
5.5
0.26
i
1.52
0.4
0.9
;
2.2
0.4
i
2.1
Pressure
loss
(mm aq)
200
(
260
<50
145
(
380
300
i
500
330
i
620
Absorption
%
84
90
47
t
94
82
i
94
75
i
92
36
i
86
NOG
1.3
(
2.7
2.3
0.64
(
3.4
1.8
i
3.5
1.9
i
2.6
0.5
i
2.0
KOGa
(kgmole/
m hr atm)
220
t
260
15
>
35
11
240
i
750
1,400
i
4,300
550
i
700
500
)
2,200
Reference
[2]
[46]
[53]
[44]
[68]
[21
L ^* J
Remarks
6 stages
Ca(OH)2
6% slurries
0.6N Na»SO,
Z 6
C f\ 1 1 1
i>U 1U .
CaC03 10%
slurries
-------
Table 2.3 (Continued)
Type
of
contactor
Venturi
scrubber
Rotational
current
tower
Packed
tower
TCA
scrubber
Hydro -
filter
scrubber
Venturi
and after
scrubber
Removal
process
Soda
process
Ammonia
-water
Oxygen
-water
Lime-
stone
slurry
Na_CO_
2 3
solution
Na.CO_
£ j
solution
Na0CO_
2 3
Solution
S02 vol %
Inlet
1.5
0.2
i
0.23
0.09
i
0.12
0.12
0.06
i
0.12
Outlet
0.15
0.04
(
0.16
0.001
i
0.012
0.0012
i
0.023
0.005
i
0.041
Gas
flow
2
(kg/m hr)
40
I
1 Sll
oU
sec
1,995.8
i
2,620.8
7,459
i
14,918
4,787
(
7,196
1,794*
i
4,971
Liquid
flow
(kg/tAr)
7,308
(
11,232
48,060
!
101,196
Bottom
0-33,969
Top
0~12,071
7,962*
i
24,840
Liquid
gas
ratio
0.26
i
0.74
C.2
i
27.8
2.8
(
5.6
3.2
)
13.6
Bottom
0-7.09
Top
0-2.52
1.60
i
13.8
Pressure
loss
(mm aq)
300
i
500
2.54
i
50.8
53.34
(
317.5
114
i
312
61
i
330
Absorption
0,
'o
90
45
i
97
90
i
99
81
i
99
66
i
92
NOG
2.3
0.39
i
1.79
0.6
i
3.5
2.3
i
4.6
1.7
i
4.6
1.1
(
2.5
KOGa
(kgmole/
m hr atm)
520
i
650
621
i
1,732
55
i
425
104
J
417
301
i
1,228
16
i
101
Reference
[68]
TAI i
ol 1
L J
[6]
[29]
[38]
[29]
[38]
[29]
[38]
Remarks
Na2C03 solu.
1/2" intalox
packing
3 stages
5" packing/
stage
3/4" glass
packing
*only con-
sider that in
after scrubber
-------
Table 2.3 (Continued)
Type
of
contactor
TCA
scrubber
•
TCA
scrubber
Hydro-
filter
scrubber
Venturi
and after
scrubber
Venturi
and after
scrubber
Removal
process
Lime-
stone/
Lime-
stone
-MgO
slurries
Lime
slurry
Lime-
stone/
Lime-
stone
MgO
slurries
Lime-
stone/
Lime-
stone
-MgO
slurries
Lime
slurry
SO vol %
Inlet
0.19
(
0.31
0.24
i
0.28
0.18
i
0.36
0.22
,
0.34
0.15
0.35
Outlet
0.019
>
0.093
0.017
i
0.112
0.032
f
0.288
0.026
i
0.26
0.009
0.095
Gas
flow
(kg/m hr)
5,816
i
13,658
13,709
3,812
i
9,149
2,509*
i
6,490
5,408*
Liquid
flow
2
(kg/m hr)
49,428
>
119,520
113,400
;
136,800
Bottom
0~34,248
Top
0-11,509
0*
i
23,888
47,776*
Liquid
Gas
ratio
3.6
i
20.55
8.27
!
9.98
Bottom
0-8.98
Top
0-3.02
0
i
9.52
8.83
Pressure
loss
(mm aq)
45.7
i
254
152
(
381
167
i
381
15.2
i
330
84
96.5
Absorption
0.
'0
70
i
99
60
i
93
20
i
82
24
i
88
73
i
94
NOG
1.2
)
4.6
0.9
i
2.7
0.2
i
1.7
0.3
i
2.2
1.3
2.8
KOG3
(kgmole/
m hr atnf
42
i
382
75
i
225
28
i
577
6.1
t
115
57
122
Reference
[10]
;
[22]
[10]
i
[22]
[29]
i
[38]
[29]
i
[38]
[29]
[38]
Remarks
2~3 stages
10" packing/
stage
2-3 stages
10" packing/
stage
3/4" glass
packing
*only con-
sider that
in after
scrubber
*only con-
sider that
in after
scrubber
-------
Table 2.3 (Continued)
Type
* / r^**
of
contactor
Weir
scrubber
Counter-
current
tray
absorber
Removal
process
Lime
slurry
Lime-
stone
slurry
SCL vol %
Inlet
0.02
t
0.3
0.079
;
0.242
Outlet
0.002
0.03
0.0011
i
0.1
Gas
VJCL «J
flow
2
(kg/m hr)
30,000
SCFM
5,581
i
15,859
Liquid
flow-
(kg/in hr)
2,400
gpm
36,534
i
109,589
Liquid
gas
ratio
0.08
gal/
SCF
2.3
)
19.64
Pressure
loss
(mm aq)
25.4
25.4
i
254
Absorption
%
90
58
i
99
N
NOG
2.3
0.9
i
4.8
\f JJ
OG
\J \J
(kgmole/
3
m hr atm)
59
8Q 1
o •? x
Reference
[66]
[60]
L v v J
Remarks
Horizontal
Module
4 stages
2~3 stages
-------
17
spheres placed between retaining grids. Due to the counter-current
gas and liquid flows, the spheres are forced upward in a random,
turbulent motion, promoting intimate mixing between gas and liquid.
The effect of the turbulent action is two fold. First, gas and
liquid are brought together for a thorough interfacial contact.
Second, the moving spheres are continuously cleaned by the tumbling
action which effectively prevents solids build-up and thus eliminat-
ing channeling, plugging and fouling. ihe use of TCA permits much
greater gas and liquid velocities than are possible in conventional
scrubbers. Thus, a smaller tower may be employed for a given
operation.
In recent years a number of studies relating to hydrodynamic,
[491
mass and heat transfer in TCA have become available. (Kulbach1 ,
1961; Douglas et al, 1963; Douglas , 1964; Gel'perin et al
1965; , 1966; Blyakher et al . , 1967; Levsh et al.[50^, 1967,
, 1968; Gel'perin et al, 1968; Chen and Douglas \ 1968,
1969; Balabekov et al.'-3-', 1969; Khanna'-47-' , 1971; Barile and
Meyer'-5-', 1971; Borgwardt '•1°^ , 1972; O'neill et al.^57^, 1972; Tichy
et al.[631, 1972; Tichy and Douglasl64], 1973; Epstein t291, 1973;
Barile et al.-, 1974; McMichael et al.-, 1975; Kito et al
1975). However, the information on operational characteristics are
still fragmentary and the data reported are, in many instances,
contraditory.
In studying hydrodynamic phenomena of TCA, Gel'perin et al .
(1966) reported that liquid hold-ups, bed expansions, flow behavior
-------
18
and pressure drop are significantly affected by the liquid and gas
flow rates. Contrary to the usual practice of TCA grid arrangement
which provides large free area to allow large flow rate, in their
study, the free area of grids are rather small. Consequently the
pressure drop data which include large pressure drop across the
grids are not of practical interest. Levsh et al. , (1967)
presented a correlation for the low density disk packing and found
that pressure drop increases linearly with the gas flow rate.
F81
Blyakher , (1967) studied the pressure drop and separated the
effect of total pressure drop into components of contributions.
They are contributions due to the dry grid, the dry packing of the
bed, the liquid hold-up on the grid and the liquid hold-up of the
packing. Similar analysis for pressure drop was later presented by
Gel'perin et al.'-41-', (1968).
Liquid hold-up, bed expansion and minimum fluidization velocity
F231
in TCA were measured and correlated by Chen and Douglas , (1968).
The liquid hold-up was found to be independent of gas flow rate.
However, the effect of both liquid flow rate and packing diameter
appear to be similar to that reported for the conventional fixed bed
absorbers. The minimum fluidization velocity was observed to vary
with packing diameter and liquid flow rate. In addition to these
variables the bed expansion correlation is also a function of gas
[241
flow rate Chen and Douglas1 J, (1969) studied the liquid mixing
behavior for TCA. Liquid mixing data in terms of Peclet number was
correlated as a function of the Peclet number in fixed bed, the ratio
-------
19
of packing diameter to column diameter and (u-u ,)/umf . Experimental
study for similar hydrodynamic parameters was also carried out by
Khanna'-47J(1971). in his investigation, wider liquid and gas flow
rates than that reported by Chen and Douglas"-23' 24-", (1968, 1969) were
examined. In addition, the gas-liquid interfacial area composed of
liquid drops, liquid film and gas bubble was considered and correlated
in terms of hydrodynamic properties.
C'neill et al.^ ^, (1972) classified the mode of gas-liquid
contacting operation in a tower containing fluidizing packings into
two types: type 1, fluidization in the absence of incipient flooding
and type 2, fluidization due to incipient flooding. TCA scrubber
utilizes very low density packings, hence, is classified as type 1.
In this mode of operation, the gas flow rate is increased at a
constant liquid flow rate until the upward force of liquid flow
balances the weight of packing plus the liquid hold up which is
equal to the total pressure drop. The gas velocity at this point is
identifying below the flooding velocity.
Static pressure within the expanded bed in TCA was measured by
Tichy et al. , (1972). They reported that the static pressure was
linearly proportional to the height of the bed. Using the pressure
drop as the criterion, Tichy and DouglasL , (1973) later classified
the TCA into three hydrodynamic regimes. These are static bed,
semi-mobile bed and fully mobile bed. In fully mobile bed, it is
divided into three regions, i.e., constant liquid hold-up region,
increasing liquid hold-up region and flooding region.
-------
20
, [27]
For the mass and heat transfer studies in TCA, Douglas et al.
(1965) investigated the absorption of C02 and S02 from dust-laden gas
by alkaline solution. The condensation of steam from the gas mixture
containing steam, air and H_S emitted from pulp digesters was observed
to significantly increase the mass transfer coefficient over that in
packed tower. Subsequently, the gas absorption data of NH^ by boric
F281
acid solution was reported by Douglas1 , (1964). The experimenta-
tion was conducted for a dehumidification and cooling of hot air
saturated with steam. It was found that the height of overall mass
transfer unit for the NH, absorption was approximately 1/2 to 1/3 of
that obtained in a packed bed. Furthermore, significant increase in
mass transfer was observed for the dehumidification and cooling system.
TCA used as a cooling tower for heated power-plant cooling water was
developed and analyzed by Barile and Meyer , (1971) and Barile et
f4l
al. , (1974). They revealed that a 20°F cooling range can be
easily achieved in a one foot packing bed.
The absorption of S02 by alkaline solutions and slurries in TCA
has recently been studied in a small scale scrubber (Borgwardt "• ^)
F29~38l
and a large scale scrubber (Epstein1 J). A significant improve-
ment in S02 removal efficiency was observed for TCA compared with
that using conventional packed tower and venturi scrubber. The
extensive data reported from these units were analyzed. Meanwhile, a
mathematical model was presented to describe the S09 absorption
process using limestone/lime slurries. (McMichael et al. , 1975).
Kito et al. , (1975) measured the gas-liquid interfacial area
-------
21
and gas side mass transfer coefficient is a TCA with stagnant liquid.
However, the liquid side mass transfer coefficient for TCA has so
far not been investigated.
2.3 Review of the Chemistry of SO Scrubbing Using Limestone/Lime
Slurries
2.3.1 Chemical Reaction in SO,, Absorption
Perhaps, the most difficult problem in S0_ scrubbing by limestone/
lime slurries is associated with the understanding of the chemical
reactions in the liquid phase. In addition to the presence of a number
of solid constituents numerous ionic and neutral species are also
present in the liquid phase. Examples of the major components present
are HS03 , S03=, CaS03, HS04 , MgS03> HS04 , ^SO^ S04=, Ca++, CaHC03+
HC03~, MgHC03+, C03=, H2C03, CaOH+, OH", H+, Mg++, CaC03> etc. An
attempt is made here to determine the key ionic or neutral reactions
associated with the S02 absorption in the overall scrubbing analysis.
Several reaction mechanisms have been proposed. However, considerable
additional information is needed to determine the key mechanism.
Boll'- , (1970) found from his mathematical model study that the
chemical reaction was approximately of the first order with respect to
both the partial pressure of S02 in equilibrium with the bulk liquid
and the surface area of the limestone. In view of the com-
plicated equilibrium phenomena involving multiple phases and chemical
reactions, Potts et al.^ ^, (1971) presented schematically the
interrelationship between gas, liquid and solid for the system
-------
22
containing CaO, S02, S03> C02 and H20. The complex processes of
chemical and diffusional steps, including gas diffusion and solid
dissolution, were explained by Slack et alJ59-', (1972). Experimental
performance of SO- absorption in limestone slurry in a laminar jet
absorber was studied by Bjerle et al. , (1972). It was shown that
the controlling reaction mechanism in the pH range between 8 to 9
was the irreversible and instantaneous reaction between total S02
and HCO ~ In addition, it was observed that the SO absorption
O ^
rates were practically equal for different scrubbing solutions
including 0.1 M HCO with pH of 8.3 and 0.1 M NaOH. However,
Vivian1- •", (1973) conducted an experiment in a short wet wall
column revealing that the chemical reaction was governed by the
irreversible and instantaneous reaction between SO- and the hydroxyl
ion forming bisulfite and sulfite ions. The liquid employed in his
experiment was lime slurry with pH ranging from 12 to 13. Deducing
from the mathematical model analysis, McMichael et al. *, (1975)
recently concluded that the absorbed SO- in form of H-SO reacts
instantaneously and irreversibly with OH , HCO ~ CaHCO + and MgHCO +
o 3 3
ions as it diffuses into the liquid phase. This view was shown to
be, in part, consistent with previous investigators (Bjerle, et al. ,
(1972); Vivian1- J, (1973)) and can partially explain the SO-
scrubbing data using limestone slurries. However, more studies are
necessary to clarify this conclusion.
-------
23
2-3.2 Oxidation and Solid Precipitation in the Slurries
One of the most important factors in optimizing the performance
of S02 scrubbing systems using limestone/lime slurries is control of
the slurry pH (Borgwardt L10~221 t Epstein t29~38l ) Differences in pH
affect the operation of a limestone/lime scrubber in two ways
(Cheremisinoff and Fellman^25-', (1974)): (1) Operating at a low pH
decreases the solubility of CaSO , therefore, promotes the formation
of calcium sulfate dihydrate, gypsum, precipitation. (2) Operating
at a high pH decreases the solubility of CaSO , therefore, promotes
the formation of calcium sulfite precipitation. Apparently, there
exists an optimum pH at which the precipitation of both sulfate and
sulfite becomes minimum. However, it should be noted that the solid
precipitation produced by calcium sulfite are soft and delicate and
easily altered mechanically, while the precipitation formed by
gypsum is characterized by hard and stubborn scale. The scale
seriously hinders the continuous operation of the tower by plugging
the surface of the grid. The source of gypsum is the oxidation of
calcium sulfite by dissolved oxygen. This reaction occurs in an
[221
aqueous solution and is favored at low pH (BorgwardtL , (1974)).
Only recently it was realized that subsaturation of calcium
sulfate dihydrate in the scrubbing liquid could be achieved in a
closed system. Although this finding was first reported by ICI (Imperial
Chemical Industries) in 1935. The results of Id's experience were
[221
recently confirmed in pilot tests conducted by Borgwardt1 J, (1974).
Operation with the scrubbing liquid unsaturated with respect to
dissolved calcium sulfate dihydrate presents an opportunity to reduce
-------
24
gypsum scaling. The subsaturation mode is achieved by the precipi-
tation of calcium sulfate occluded by the precipitant of calcium
sulfite. A solid solution formed by this mode is being identified.
F92l
(Borgwardt1*" ', (1974)). It is also found that the quantity of
calcium sulfate that can be occluded is directly related to the
sulfate activity in the scrubbing liquid and the sulfite precipita-
tion rate. Experimental data also show that O£ ion has the effect
of suppressing the sulfate activity while Mg ion could raise its
activity. Fly ash was found to have no significant effect on the
operations of SO scrubbing using limestone/lime slurries.
-------
25
Chapter 3
Mathematical Model for SO Absorption in Spray and TCA Scrubbers
3.1 Introduction
Among the wet processes using limestone slurries in scrubbing
SO- from flue gases, two of the most promising scrubbers for
carrying out the limestone scrubbing appear to be the turbulent
contacting absorber (TCA) and the spray column. Several large
scale scrubber-holding tank recycle systems have been built and
sponsored by the EPA to test the performance and reliability of
these scrubbers.
In this chapter experimental data from the large scale TCA
scrubber and spray column located at the TVA Shawnee power station
and the EPA TCA scrubber at Research Triangle Park, North Carolina,
are analyzed to obtain equations and correlations which can be
used to design and scale-up spray column and TCA scrubbers which
utilize limestone slurries to scrub SO- from flue gases.
In subsequent sections a mathematical model is developed which
describes the absorption of S0_ by limestone slurries in a scrubber;
parameters appearing in this model are evaluated from experimental
data and correlated in terms of the operating parameters; and the
experimentally observed S0_ removal efficiency is compared to the
efficiency computed from the model.
3.2 Mathematical Models of Spray and TCA Scrubbers
In the analysis and design of contacting devices used for the
-------
26
scrubbing of S02 from flue gas with limestone slurries the mass
transfer coefficients which describe the S02 fluxes in the gas
and liquid phases should be known. The gas film mass transfer
coefficients for physical absorption are relatively easily obtained
from experimental data; on the other hand, the liquid film coeffi-
cient for physical absorption is difficult to determine due to
chemical reactions (such as dissociation) which will occur upon
the absorption of a gas molecule into a liquid phase. In the
present case, where the mechanism of the absorption of SO into
limestone slurry is not well understood, it does not appear possible,
at the present time, to determine the liquid side mass transfer
coefficient for physical absorption for large scale scrubbers from
experimental data on scrubbing SCL from flue gases with limestone
slurries.
In this section a mathematical model for the spray column and TCA
scrubbers is developed. Gas side mass transfer coefficients which have
been previously reported in the literature or obtained from experi-
mental data in this study are presented for each column. The liquid
film resistance to the transfer of SO- into limestone slurries,
determined from experimental data, is correlated in terms of the
ratio of the gas and liquid film resistances. This ratio of the
resistances appears to be only a function of the magnesium concentra-
tion, pH of the slurry, SO- partial pressure in the gas phase and
type of scrubber within the accuracy of the experimental data.
-------
27
3.2.1 Scrubber Model
Scale drawings of the scrubbers to be considered in this study
are given in Figures 3.1, 3.2 and 3.3. In the TVA Shawnee TCA
scrubber, as shown in Figure 3.1, the scrubbing slurry is sprayed
from a single arrangement of nozzles at the top of the column and
falls through a series of grids and areas filled with "ping pong"
balls. Actually the TCA has also been operated without the spherical
packing pieces and in this situation the column is refered to as
the "TCA without packing spheres". Flue gas containing SCL in
concentrations up to 4500 ppm passes counter-currently to the
limestone slurry. The spray column with four spray headers shown
in Figure 3.2 is slightly more complicated than the TCA without
spheres in that the scrubbing slurry is sprayed at four levels.
However, as will be shown later, this situation is easily handled
and the column can be described mathematically in terms of the
mass transfer coefficients for the TCA without packing spheres.
In Figure 3.3 a scale drawing of the EPA/RTP TCA scrubber is given.
The TVA Shawnee TCA and spray column treat flue gas at a rate
equivalent to a 50 MW power station and have cross-sections which
are square (5 ft. edge) and circular (8 ft. dia.), respectively.
The EPA/RTP is approximately 9 inches in diameter.
For the TCA operating with packing spheres, the column is
divided into packed and unpacked sections; and mass transfer coeffi-
cients are used to describe the respective sections.
The molar flux of S02 across the gas-liquid interface of a packed
or unpacked section of a wet scrubber can be written in terms of the
-------
28
GAS OUT
CHEVRON
INLET KOCH
TRAY WASH
LIQUOR
\\^^\\
w|-
,r
\
STEAM
SPARGE
TAINING — <
?IDS
1 M b
1 IN *
V V/ V 1
<
I
Li ^ r- r-r-i i ICTMT
L TRAY WASh
A A A
~°~o~o~o~
°°oo0
0 00
_^
\po°o
o\o oo
O O O p.
ooo°*
°0°0°0§
\, /
^ LIQUOR
INLET
SLURRY
- K/I r^ m i c~ P A r^ k" i M r*,
\v\ {j t3 1 Lt r A Lx r\ 1 1 N O
SPHERES
200 c m
APPROX. SCALE
EFFLUENT SLURRY
Figure 3.1 Schematic of TVA Shawnee Three-Bed TCA (Epstein'-37^).
-------
CHEVRON
DEMI STEP
GAS IN
GAS OUT
A A A
A A A
A A A
\/ \/ V
X
^
29
DEMISTER WASH
DEMISTER WASH
NLET SLURRY
EFFLUENT
SLURRY
200 cm
> t
APPROX. SCALE
EFFLUENT SLURRY
1-iKure 3.2 Schematic of TVA Shawnee Spray Tower (Epstein * )
-------
30
GAS OUT
GAS IN
( TO DEMISTER )
A
oo°"
oog
00°
00
°00
^o
o
oo
00
"o"o"
OOo
080
00
OQO
88°
08°
°2*
o'oo
8|
oo
o°o
So
_p
L A
•\
^
INLET SLURRY
MOBILE PACKING
SPHERES
RETAINING
GRID
Y
EFFLUENT SLURRY
50 cm
APPROX. SCALE
Figure 3.3 Schematic of the EPA/RTP Research TCA Scrubber (Borgwardt
-------
31
gas or liquid side resistance as
S02 g S02 S02 L Ai A}
where k° is the liquid film mass transfer coefficient for physical
absorption, (cm/sec),
k is the gas side mass transfer coefficient for physical
o
2
absorption, (gmol/cm atm sec),
N<,_ is moles of S0? absorbed per unit time per unit interfacial
area, (gmol/cm sec),
, takes into account the reaction of the
diffusing H-SO with components found in the liquid phase.
The concentration of H2SO at the gas-liquid interface, CAi, can
be related to the partial pressure of S09 at the interface, Pcr,*, by
" OvJ A
Henry's law
P^" — HP f^-7T
nr*. ~ flVj. • IO —^J
S02 Ai
3
where H is the Henry's law constant, (atm cm /gmol).
This equation holds for sufficiently dilute solutions.
-------
32
The Henry's law constant, H, as a function of temperature has
been given by VivianL , (1973) as
In H = 17.360 - (3-3)
where T is the liquid temperature in degrees Kelvin.
An expression for the interfacial concentration of ^SO^,
can be obtained by substituting Equation (3-2) into (3-1).
kL
p i kgPSO? + H~ * PSO*
P ~ - (3-4)
k°
k + L ,
g „- +
where Pcn* is the partial pressure of SO- which could be maintained in
oU~ <£
equilibrium with the bulk liquid phase, (atm) .
Substitution of this equation into Equation (3-1) gives
. Nso2a (*' ""so., - V (3-5>
The rate of SO- absorption in a differential height of the scrubber ,
dz, can be written as
-G ^2 1 H ~* *
(P
P d a W° SO_ ' SO -
1 g L c. f.
*
In Equation (3-6) both § and P are functions of position in
the column and both depend on the mechanism of SCL absorption into
recycled limestone slurries. For a calcium carbonate, calcium sulfite,
carbon dioxide, sulfur dioxide and water system at equilibrium and a
pH greater than 4.7, the equilibrium SO- partial pressure of the
aqueous system will usually be much less than the S0_ partial pressure
-------
33
* *
found in the flue gas (i.e., P »P for pH>4.7). Thus P can
oU_ oU_ oU_
be ignored relative to P in Equation (3-6) to give
b(J
2
-G dPs°2 1 H
P dz ~ a $k°l PSCL (3"7)
T g T L 2
Equation (3-7) can be integrated over the height of the column
section (from Z to Z ) to give
1 H ~
where Kra = (r — + TjT and K,,a are the "average" enhancement factor and the "average*
u
overall mass transfer coefficient, respectively.
and AZ = Z? - Z-.
Large values of the enhancement factor, <(>, correspond to the gas
film being the dominating resistance to the transfer of S0_; and
small values of the enhancement factor indicate that the liquid film
resistance is important. If the gas and liquid film mass transfer
coefficients for physical absorption can be calculated, it is
possible to separate the liquid film mass transfer coefficient, k a,
lj
into a chemical reaction term (i.e., the enhancement factor, ^,)and
-------
34
the liquid film mass transfer coefficient for physical absorption, k°a.
However, for the Shawnee and EPA scrubbers being analyzed in this
study, it does not appear possible to calculate the liquid film mass
transfer coefficients for physical absorption from the data reported.
Thus, the separation of the liquid film mass transfer coefficient into
chemical reaction and mass transfer effects cannot be made at this
time. It is obvious that such a separation is desirable since it
may result in a simplification of the data and lead to a general
mathematical model for various scrubbers.
The second equation of Equation (3-9) can be arranged to give
R = rjr- = 2 (3-10)
Hk a , -— v '
g kga-KGa
The overall coefficient, K_a, can be calculated from experimental
data using the first equation of Equation (3-9). The gas film
mass transfer coefficient, k a, can be calculated by methods to be
o
discussed below.
The left hand side of Equation (3-10) (or the definition of R)
can be interpreted as the ratio of the gas film resistance to mass
transfer and the liquid film resistance. The larger this ratio the
more the SO transfer in the column is controlled by the gas film.
As will be shown below the ratio of resistances appears to be an
effective method of correlating liquid film resistances.
3.2.2 Mass Transfer Coefficients
The gas side mass transfer coefficient for physical absorption in
-------
35
the Shawnee TCA operating without packing spheres has been reported by
Epstein •* as
kSa = 0.00134 G°'8 L°'4 (3-11)
where a is the interfacial area available to mass transfer per unit
2 3
volume of the column, (cm /cm ),
2
G is the molar gas flow rate, (gmol/cm sec),
2
and L is the mass flow rate of the liquid phase, (g/cm sec).
This correlation is based on experiments on SO- absorption into sodium
carbonate solutions which had pH's in the range from 6.75 to 9.5.
At these high pH's the transfer of S0_ into these solutions can be
assumed to be gas film controlled. (Epstein , 1973)
The gas film mass transfer coefficient for the Shawnee TCA
operating without packing spheres can also be applied to the Shawnee
[381
spray column. Epstein1 J has reported data for a limestone deple-
tion run in the Shawnee spray column in which the initial pH of the
scrubbing slurry was 7.30. The Shawnee spray column with 4 spray
headers can be divided into 4 sections. The top section of the
column has a liquid flow rate of 1/4 L, the next a liquid flow rate
of 1/2 L and so on until in the bottom and fourth section the liquid
flow rate is equal to L. Assuming the same liquid flow rate
0 4
dependence (i.e., L " ) in the Shawnee spray column as in the Shawnee
TCA without spheres, Equation (3-9) can be written as
-------
36
4 . .4
= k a(G,L) Z (i) ^. (3-12)
fir . 4 i
i= 1
where Z is the length of the i— section and m and C^ are constants.
Using Epstein's limestone data at a pH of 7.30 and the above equation,
the gas film coefficient for the Shawnee spray column is calculated
to be k a = 1.65 x 10" gmole/cm atm sec at a gas flow rate of
6
2 2
G = 0.00548 gmole/cm sec and a total liquid rate of L = 0.652 g /cm sec.
The gas film coefficient calculated using Epstein's correlation
(Equation 3-11)) would give k a = 1.75 x 10 gmole/cm atm sec at the
O
same flow conditions. It can be seen that Epstein's correlation for
the gas film coefficient in the Shawnee TCA operating without packing
spheres can predict the gas film coefficient for the Shawnee spray
very accurately; and in the absence of more data it is assumed that
the gas film coefficient for both the Shawnee spray column 'and the
TCA without packing spheres can be given by Epstein's correlation
(Equation 3-11)).
The ratio of the mass transfer resistance of the gas film to
that of the liquid film can be calculated for the Shawnee TCA
without packing spheres and the Shawnee spray column with 4 spray
headers from the data of Epstein * •". The values of R obtained
for these scrubbers are shown in Figure 3.4 as a function of the
inlet pH. The range of operating conditions for the data shown in
Figure 3.4 are given in Table 3.1.
-------
37
i/)
or
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2 -
i
Rff'ERENCES AND RANGES OF
DATA GIVEN IN TABLE 3.1
TVA SHAWNEE DATA
• SPRAY COLUMN - "
4 HEADERS
A TCA WITHOUT
PACKING SPHERES
• TCA WITHOUT
PACKING SPHERES!
. WITH A p
CORRECTION
5.0
6.0
7.0
PH
Figure 3.4 Ratio of the Mass Transfer Resistances for Spray Devices, Rs
as a function of the pH of the Scrubbing Slurry at the Inlet
of the Scrubber.
-------
Table 3.1
Range of Data for the Shawnee Spray Column and TCA Operating Without Packing
Spheres Used in Constructing Figure 3.4
Equipment
and References
Spray Column
4 spray
headers [38]
TCA without
Packing
spheres "• *
pH
5.30
to
7.30
5.85
to
6.4
G
,gmole ^
1 2 J
cm sec
0.00548
0.00687
to
0.01378
L
( g 1
( 2 >
cm sec
0.652
0.886
to
2.86
Inlet
Slurry
Temperature
i
97 to 113
58 to 119
Inlet
Pso2
(ppm)
1750
to
3187
1850
tp
3300
Mg
Concentration
An /•• u n^ in slurry
AP (in. H20) (ppm)
not less
reported than
350
1.0 less
to than
5.8 350
00
-------
39
For some of the data used in Figure 3.4 very high pressure drops
along the length of the column with respect to the pressure drop which
would normally be expected at the same gas and liquid flow rates were
observed. This high pressure drop was probably due to scaling and
its effect on increasing the liquid hold-up in the scrubber. Also
at these high pressure drops a higher than normal SCL absorption
efficiency has observed. It was found that the high pressure drop
data could be made to agree with the normal pressure drop data in
Figure 3.4 if the gas film mass transfer coefficient was given by
kSa - 0.00134 G°'8 L0'4' (f£-)°'4 (3-13)
N
where AP is the pressure drop without scaling of the column,
(in. H20),
and AP is the observed pressure drop, (in.H-O).
This equation is exactly the same as Epstein's correlation (Equation
(3-11)) for the gas film coefficient for the spray column except it
includes the pressure drop correction.
The pressure drop without scaling, AP , can be correlatedL J
in terms of the gas and liquid flow rates as
APXT = 0.481 L°'6 G1'17 ZT (3-14)
N l
where Z is the total height of the spray section, (cm).
The ratio of mass transfer resistances, R, was correlated in terms
-------
40
of inlet slurry pH for several reasons:
1) The inlet slurry pH was readily available.
2) The amount of SO absorbed per liter of slurry is relatively
small and the slurry residence time fairly short and therefore, the
average condition of the slurry in the scrubber could be reflected
in its initial pH.
3) The inlet pH was used because this pH is more easily determined
in a recycle system than the pH of the outlet slurry from the scrubber.
In a recycle system with fairly large holding tank residence times the
liquid phase will approach equilibrium and under this condition the
composition of the liquid is fixed at a given pH, temperature and
partial pressure of C0_ above the holding tank. Many species in
limestone slurries are insensitive to the C0_ partial pressure and
very sensitive to the pH.
The effect of SO- partial pressure on the scrubbing efficiency
of limestone slurry has been recognized by Gleason^ ••, (1971] and
[541
Nannen et al. , (1974). These authors have noted that as the
partial pressure of S02 in the flue gas decreases the S0» transfer
becomes more gas film controlled. In accordance with this observa-
tion the ratio of the mass transfer resistance of the gas film to
that of the liquid film should increase with a decrease in S0_ partial
pressure in the flue gas. For the spray type devices discussed above,
the dependence of R on the S0_ partial pressure could not be
distinguished above the scatter of the data; nor could a dependence
of R on the hydrodynamics be established for the spray devices.
The scatter in the data in Figure 3.4 could be due in part to the
-------
41
fact that the raw data used to calculate the ratio of resistances, R,
was reported in ranges and the average of the reported ranges was used
to calculate the value of R.
In Figure 3.4 it can be seen that the extrapolation of the
straight line through the data points to a value of R /(R +1) equal
to one given a pH of 7.2 as the pH above which the S0_ absorption
is gas film controlled. This is in agreement with the assumption
that the S0_ absorption was gas film controlled at a pH of 7.3 in the
case of the limestone depletion run for the Shawnee spray column.
For the TCA operation with packing spheres the column must be
divided into two sections: one corresponding to the section filled
with packing spheres, and another where there are no packing spheres
and the column behaves much like a spray device. This situation is
illustrated in Figure 3.5. The reason for this separation is that
the packed and spray sections will have significantly different
contacting mechanisms and consequently different mass transfer
coefficients. Also this separation is desirable so that columns
with different packing heights can be compared.
An actual TCA scrubber can be made up of several stages such as
the one shown in Figure 3.5. In the development to follow it will be
assumed that the overall gas side mass transfer coefficients for the
spray sections and the packed sections are constant regardless of the
position of the section in the column and that the height of the
packed section in the fluidized state can be characterized by the
height of the packed section without gas flow. Chen and Douglas'- \
-------
42
EXPANDED
PACKING!
HEIGHT l
GRID
\
LIQUOR
I FLOW
PACK: KG
SPHERES
GRID
I
^HEIGHT OF
SPRAY SECTION
=UN-EXPANDED
HEIGHT OF PACKED
" SECTION
GAS FLOW
Figure 3.5 An Idealized Stage of a Turbulent Contacting Absorber.
-------
43
(1968) have reported bed expansion for TCA scrubbers; however, their
data was taken for gas velocities substantially lower than the gas
velocities of interest in this study.
The integrated mass balance on SO in the TCA scrubber can be
written as
in
r SQ P s
PI ln TUT = V ZP + V zs (3
so2
_ P _ s
where K~a, K~a are the overall gas side mass transfer coefficients in
u
the packed and spray sections of the TCA, respectively,
(gmol/cm atm sec),
and Z and Z are total height of the packed and spray sections of the
TCA scrubber, respectively, (cm) .
In terms of the ratio of the mass transfer resistance of the gas film
to that of the liquid film, Equation (3-15) can be written as
P in
S0 R s Rs
Zp * kg. ( Zs (3-16)
This follows from Equation (3-10) .
Equation (3-16) will be applied to the data reported by Borgwardt
(see Table 3.2 for references). These data were obtained from a
relatively small TCA scrubber which is shown in Figure 3.3 Only data
where the pH of the inlet scrubbing slurry is greater than or equal
to 6.6 were considered. At this pH the transfer of S02 in the packed
section is assumed to be gas film controlled. Examination of Figure
-------
Table 3.2
Range of Data for the EPA/RTP Research TCA Scrubber Used in the estimation of the Gas Side
Mass Transfer Coefficient for the Packed Section of the TCA Scrubber.
Equipment
and References
EPA/RTP Research
TCA Scrubber
pH
6.6
to
6.8
G
, gmole .,
( 2 J
cm sec
0.00577
to
0.01355
L
( g 1
( 2 J
cm sec
1.373
to
3.150
Z
P
(cm)
50.8
Inlet
"so,
(ppm)
~2520
Mg
Concentration
(ppm)
less
than
350
-------
45
3.4 shows that this is not the case for the spray section at a pH of
6.6, however, the value of the liquid film mass transfer coefficient
in the packed section will be much higher than in the spray section;
and thus, the ratio of resistances in the packed section will, in all
likelihood, be significantly larger than in the spray section.
Under the above assumption Equation (3-16) becomes
pin
SO R
P Z + ksa CT^~) Z (3-17)
PT p°«t g P g
S0
2
Thus using Borgwardt's data for pH - 6.6, the gas film mass transfer
coefficient for physical absorption, k a, can be computed from
o
Equation (3-17) . This coefficient as a function of the gas and
liquid flow rates is given by
kPa = 0.0352 G L0'35 (3-18)
o
The "range of data from which this coefficient was obtained is given
in Table 3.2. It was assumed in the above calculation that the gas
film mass transfer coefficient for the Shawnee spray column given
by Epstein (Equation (3-11)) was applicable to the spray section
of Borgwardt's TCA column.
Epstein has made several reliability runs for the Shawnee TCA
scrubber. In these long term tests the pH of the scrubbing slurry,
temperature and the liquid and gas flow rates are held almost constant.
However, the inlet partial pressure of SO- varied. As can be seen
in the example of Epstein's data given in Figure 3.6, a decrease
in the partial pressure of S0_ causes a corresponding increase in
-------
46
Ld
Q-
IE
UJ
CC
90
S5
e o
75
7 0
E
Q.
O,
y o
-7 V/>
O
O
3500
3000
2500
2000
1 5OO
O 2O 4O 60 80 1OO120140 160 1 &0 200220 24O
TE ST TIME , HOURS
Figure 3.6 Typical Operating Data for the TVA Shawnee TCA Showing
Remove
(Epst«
the Dependence of -the S02 Removal Efficiency on the
Inlet SO? Concentration. (Epstein^ J Run 525-2A).
-------
47
the S02 absorption efficiency. These data can be utilized to determine
the dependency of the ratio of resistances on the SCL partial pressure.
The ratio of the resistances is computed for the overall TCA column
with the spray and packed sections un-segregated using Equations
(3-9) and (3-10). The value of the gas film mass transfer coefficient
for physical absorption, k a, used in Equation (3-10) to calculate
&
the overall ratio was calculated from the following relationship:
overall Z Z
k a = ka ^ + k a -=?- (3-19)
g g Z g Z
s P
where k a and k a were evaluated from Equations (3-11) and (3-18)
o &
respectively.
The results of these calculations are shown in Figure 3.7. Here
R, the overall ratio of resistances for the Shawnee TCA, is shown as
a function of the inlet S0_ partial pressure. It can be seen in
this figure that the ratio of the resistances can be expressed as
R = A(pH) e"33°PS02 (3-20)
Equation (3-20) shows that the overall ratio of resistances is the
product of a pH function and a function of the S02 partial pressure.
The exponential term in Equation (3-20) follows from Figure 3.7 and
the pre- exponential pH function has been assumed; although, it is
not apparent in Figure 3.7 that the function A is a function of pH.
This relationship will be shown below.
In what is to follow it will be assumed that the ratio of the
mass transfer resistances of the gas film to that of the liquid film
has the form of Equation (3-20) for both the spray and packed
-------
48
ICE
1.0-
0.9
0.8
0.7
06
0.5
0.4
03
1.5
IN
CD
n
SHAWNEE TCA DATA( 31.32)
6 = 0.0094 gmd/cn£sec
L = 2.835 9 /cm2sec
INLET PH OF SLURRY
o 5.60 • 5.75
A 5.65 A 5.80
n 5.70 • 5.85
I
I
I
2.5
3.5
4.5
, atm
Figure 3.7 The Ratio of the Gas Film to the Liquid Film Mass Transfer
Resistance as a Function of the Inlet S02 Partial Pressure
for the TVA Shawnee TCA.
-------
49
sections of the TCA. For the Shawnee TCA operating without packing
spheres and the spray column with four spray headers the pH function,
A, is shown as a function of pH in Figure 3.8.
A new value of the gas film mass transfer coefficient for physical
p
absorption for the packed section of the TCA, k a, can be calculated
o
based on the A function for the spray section given in Figure 3.8.
The following equation can be used to calculate the gas film
p
coefficient for the packed section, k a
g
-! »^
P
f- m ^- = k> Z.. + k> ± Z_ (3-21)
T
so2
out
SO.
2
- V a 7 + VSa
— KdZj ^K.3
g P g
1
330Pso7
2
1+ A (pH)
i_— ^ ^
7
Zs
Again Borgwardt's data (see Table 3.2 for references) for the scrubbing
of SCL with limestone slurries having a pH greater than or equal to
p
6.6 can be used in conjunction with Equation (3-21) to obtain k a.
o
This coefficient can be expressed as a'function of the liquid and
gas flow rate as:
kPa = 0.00220 G'47 I/51 (3-22)
g
Since a new value of the gas film mass transfer coefficient has been
found, the functional dependence of the ratio of resistances on the
SO partial pressure should be re-evaluated in a manner similar to
that used to obtain the data in Figure 3.7. However, if R is
p
recomputed using the value of k a given by Equation (3-22) the slope
o
of the line through the plot of log R versus SO partial pressure is
-------
2.0
1.6
1.2
I/)
0,8
0.4
0
TVA SHAWNEE DATA
• SPRAY COLUMN -
4 HEADERS ( 3a )
A TCA WITHOUT
PACKING SPHERES
( 37)
. PH + 7.«92)-0.15
NOTE » As CAN BE EXTRAPOLATED
LINEARLY FROM PH = 6.G TO
As =0 AT PH=7.2 ;
SEE TABLE 3.1 FOR DATA RANGES
I
5.1
5.3
5.5
5.7
5.9
PH
6.1
6.3
6.5
6.7
Figure 3.8 The Pre-Exponenti al Function, As, in the lixpression for R, the Ratio of Resistance, as a
Function of the pi I of the Inlet Slurry for the TVA Shawnee TCA Without Spheres and Spray
Column. Low Magnesium Concentration (Mg < 350 ppm).
-------
51
approximately the same as that given in Figure 3.7 where R was computed
p
based on k a given by Equation (3-18). The magnitude of the two
&
values of R are slightly different however.
The pre-exponential pH function, A, for the packed section of
the TCA scrubber can be determined from the data reported by
Borgwardt and Epstein (references are given in Table 3.3) using the
following equation:
(3-23)
p in
G , S°2
PT ln pout -
S°2
kPa
330pso2
i , e
1 A
L P J
z +
p
r ksa
g
330Pso2
e
' A
s _
The gas film mass transfer coefficients for the packed section, k a,
&
and the spray section, k a, of the TCA can be determined by Equations
£»
(3-22) and (3-11) respectively. Other quantities appearing in
Equation (3-23) are evaluated from the experimental data.
The pre-exponential function for the packed section, A , is
shown in Figure 3.9. Although there is a substantial degree of
scatter, the trend of A as a function of the slurry pH can be seen.
Apparently, within the scatter of the data the function A is not
dependent on the gas and liquid flow rates. The scatter in Figure
3.9 is probably due in part to using the average value of data,
which is reported in ranges, as well as to the normal experimental
error associated with data taken from large scale equipment where
operating conditions and purity of reagents cannot be carefully
controlled. Also in the recycle scrubber-holding tank systems small
impurities introduced into the system, such as chlorine, which
-------
Table 3.3
Range of Data for the TVA Shawnee TCA and EPA In-House TCA Used in Determining Ap in Figure 3.9,
fgmple -, , g . Inlet Slurry Inlet PSO Total Height of
Equipment rm2c^p 2 Temperature (atml 2 Packing Spheres
and References pH cm sec (°F) x 103 (cm)
TVA Shawnee TCA
[29,30,31]
Epstein 32,36,37]
EPA In-House TCA
Borgwardtt12'13'18]
5.2
to
6.3
6.6
to
6.8
0.00692
to
0.01256
0.00577
to
0.01355
1.382
to
2.835
1.373
to
3.15
78
to
127
110
1.775
to
4.4
2.52
38.1
50.8
to
76.2
en
-------
a.
1.2
0.8
0.4
0.0
c
1 1 '
RANGE OF DATA AND OPERATING
CONDITION GIVEN IN TABLE 3.3
• TVA SHAWNEE TCA
A EPA IN-HOUSE TCA
• * t*e*** f
i i i
).1. 5.3 5.5 5.7
1 I I I
^ 1 Isl
-330D
Ap=-0. 51 7PH+3.41 :FOR PHi6.0
A-l _
Ap=0.308 ; FOR PH<6.0
• ^ •
5.9 6.1 6.3 6.5 6.7
PH
Figure 3.9 The Pre-Exponential Function, Ap, in the Expression for R, the Ratio of Resistances, as
a Function of the pH of the Inlet Slurry for the Packed Section of TVA TCA and EPA TCA.
Low Magnesium Concentration (Mg < 350 ppm).
(si
-------
54
originates from the coal and is absorbed in the scrubber, have a
tendency to build to very high concentration levels thus making a
complete analysis of the scrubbing system extremely difficult.
3.2.3 Effect of Magnesium on S02 Scrubbing Efficiency
The addition of magnesium oxide to the scrubbing slurry has two
major benefits to the operation of limestone wet scrubbers: 1) the
addition of magnesium can allow the scrubbing system to operate in
the unsaturated mode which prevents calcium sulfate scale formation
[221
(see Borgwardt ) and 2) the addition of magnesium improves
scrubbing efficiency of the system.
Since the composition of the slurry effects the scrubbing
efficiency of the scrubber only through the pre-exponential terms,
A and A , in the expression for R and R (see Equation (3-20)),
it was assumed that the effect of magnesium in the slurry on the
scrubbing efficiency could be associated with a change in the pre-
exponential factors, A and A . Thus correction factors, A and A ,
which would take into account the effect of magnesium on the pre-
exponential functions, A and A , respectively, were defined as
A. (without magnesium present)
i A^ (with magnesium present)C3-24)
where i = s or p for the spray and packed section, respectively.
These correction factors to the pre-exponential term for spray and
packed section are given in Figures 3.10 and 3.11, respectively.
The value of the pre-exponential factors without magnesium, A and
o
A , used in Equation (3-24) were calculated using the solid lines
-------
1.0
1
1 I I I 1 1 | 1 1
i I i I i i I | I I i i i i i
x
I I I 1 1 1 I 1
1 \
- RS = ( AP/AS) expc-sso PSO,) ^iv4 -
--
As = SO.lMg ' ;FOR
As = 1.0 > FOR Mg<:3
Mg2350 X,
50 ^s
A EPA/RTP TCA WITHOUT PACKING SPHERES
( SEE TABLE 3.4 FOR DATA RANGES AND REFERENCES)
CZD TVA SHAWNEE DATA? SPRAY COLUMN- 4 HEADERS AND
- TCA WITHOUT PACKING SPHERES ( SEE TABLE 3.1 FOR
DATA RANGES AND REFERENCES)
—
I
I I I I 1 1 I 1 1
10
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
102 103
—
< :
\r. "
A A ^XA"^
A_
1 1 1 1 1 1 1 1
1C
0.1 —
Mg , ppm
Figure 3.10 The Effect of Magnesium in the Scrubbing Slurry on tho Ratio of the Gas to Liquid Mass
Transfer Resistances in the Spray Section.
-------
56
I i | i i i i i
I I I I I I I I
A •
0.5
a.
3600
Ap = 1.0 ; FOR Mg-c3600
• TVA SHAWNEE TCA
A E PA /RTF TCA
( SEE TABLE 3.3 AND 3.4 FOR DATA
RANGE AND REFERENCES )
J I I I I I II
J i I ' I I M
1O
2
10
M
104
ppm
Figure 3.11 The effect of the Magnesium in the Scrubbing Slurry on the
Ratio of the Gas to Liquid Mass Transfer Resistances in the
Packed Section.
-------
Table 5.4
Range of Scrubber Operating Data Used in Calculating the Effect of Magnesium on the
Pre-Exponential Factor, A.
Equipment
and References
EPA/RTP Research
TCA Operating
without Packing
c . [15,16,20]
Spheres1 ' J
TVA Shawnee
TCA[33,34,35]
EPA/RTP Research
Tin 11 i e. IT
PH
5.3
to
6.4
5.6
to
6.1
G
,-gmole A
1 2 J
cm sec
0.0094
to
0.0107
0.0094
to
0.0107
L
C g )
cm sec
1.535
to
3.320
2.835
to
3.320
Inlet
Slurry
Temperature
125
116
to
126
Inlet
(ppm)
2520
to
2880
1900
to
3100
Height
of
Packing
(cm)
none
38.1
to
76.2
Magnesium
Concentration
(ppm)
470
to
16,700
450
to
15,000
TCA
-------
58
through the data points of Figures 3.8 and 3.9 respectively. Examina-
tion of Figures 3.10 and 3.11 reveals that the interphase transfer
of SO from the gas to the scrubbing slurries becomes gas film con-
trolled at high magnesium concentration.
3.3 Simulation of SO- Scrubbing With Limestone and Lime Slurries
Table 3.5 summarizes the correlations which can be used to
simulate the performance of the TCA and spray column in scrubbing
SO- from flue gases with limestone slurries. Based on these
correlations and the model presented in this study the SO- removal
efficiencies of the TVA Shawnee TCA and spray column and the EPA/
RTP TCA column can be computed fairly accurately and in most cases
within 5% accuracy. A comparison of the calculated and observed
S0_ removal efficiencies is shown in Figures 3.12 and 3.13 for the
spray and TCA devices, respectively, for limestone scrubbing of
SO from flue gases.
Figure 3.14 shows a comparison of the calculated and observed
S02 removal efficiencies for S0_ scrubbing with lime slurries.
In the case of lime slurries the pH variation across the scrubber
is substantial ranging, for example, from 8.0 at the inlet to 4.8
at the outlet. With this large change in pH it is not reasonable
to assume that the inlet slurry pH characterizes the slurry composi-
tion. However, it has been found that the lime scrubbing system
can be simulated fairly accurately by the model developed in this
study if the characteristic slurry pH is calculated based on the
log mean hydrogen ion concentration:
-------
59
Table 3.5
Summary of Equations Necessary for Simulating the Performance of the
TVA Shawnee TCA and Spray Column and the EPA In-House TCA
Spray Section
Packed Section
ka = 0.00134
o
L°'4 (3-11)
As -330 P in
P
P 47 fl
k a = 0.00220 G L
g
A _ in
R .
(3-22)
A given by solid line in
Figure 3.8 or by Equation
A given by solid line in
P Figure 3.9 or by Equations
A - = exp(-l.35 pH+7.82)-0.15
A given by solid line in
S Figure 3.10 or by Equations
-0.6682
A =50.1 Mg
'; for Mg>350
A = 1.0; for Mg<350
A " =-0.517 pH+3.41; for pH>6.0
A ~l = 0.308; for pH<6.0
A given by solid line in
P Figure 3.11 or by Equations
A =.2.2 x 107 Mg"2'065; for Mg>3600
A = 1.0; for Mg<3600
= kgaVa+RP}
P in
G . S°2.
P_, nout
T P
so2
K!5 Z + ife Zn (3-15)
b S bp
-------
90
80
_
UJ
cr
060
CO
h-
z
UJ
U5
U «
Q_
Q
40
y
<30
T
DATA FOR LIMESTONE SCRUBBING
• TV A SHAWNEE SPRAY COLUMN -
4 HEADERS ( 38 )
A TVA SHAWNEE TCA WITHOUT
PACKING SPHERES ( 37 )
• EPA/RTP TCA WITHOUT
PACKING SPHERES
( 1 5,1 6 . 20 )
20
2
5 % ERROR
1
1
1
0
30 40 50 60 70
OBSERVED PERCENT SO2 REMOVAL
80
Figure 3.12 Comparison of the Predicted and Observed S02 Removal
Efficiencies for Spray-Type Devices Using Limestone Slurry
as the Scrubbing Medium.
90
-------
61
100
LLl
cr
CM
o
90
80
z:
UJ
u
S70
Q_
Q
LLJ
u
O
60
50
40
40
DATA FOR LIMESTONE SCRUBBING
( SEE TABLES 3.3 AND 3.4 FOR RANGE
OF DATA AND REFERENCES )
• TVA SHAWNEE TCA
A EPA/ RTF TCA
r
5% ERROR
1
1
1
50 6O 70 80 90
OBSERVED PERCENT 5O2 REMOVAL
100
Figure 3.13 Comparison of the Predicted and Observed S02 Removal
Efficiencies for TCA Scrubbers Using Limestone Slurry
as the Scrubbing Medium.
-------
62
DATA FOR LIME SCRUBBING
{SEE TABLE 3.6 FOR RANGE
OF DATA AND REFERENCES)
A EPA/ RTF TCA
5 °/o ERROR
40
80
100
OBSERVED PERCENT SO,REMOVAL
Figure 3.14 Comparison of the Predicted and Observed SC>2 Removal
Efficiencies for TCA Scrubber Using Lime Slurry as the
Scrubbing Medium.
-------
Table 3.6
Range of Data Used in Constructing Figure 3.14. Data is for the EPA In-House TCA Using Lime
Slurries as the Scrubbing Medium
Equipment
and References
PH
inlet outlet
G
,gmole ,
I 2 J
cm sec
L 2 J
cm sec
Inlet
Slurry
Temperature
Inlet
Pso2
(ppm)
Height
of
Packing
(cm)
Magnesium
Concentration
in Liquid
(ppm)
EPA In-House TCA 5.7 4.4
Borgwardt
to to 0.0136
[19,21] 9.5 6.0
3.15
to
3.8
125
2430 50.8
to
2800
to
76.2
12
to
1150
ON
-------
64
pH - pH.
PH . 0.5,2 * log10 — - _ {3.24)
10 ln - 10 OUt
A procedure may be developed by which the pH of the outlet
slurry frora the scrubber nay be calculated from knowledge of the
inlet conditions of the scrubber. Then, the model developed in this
study for limestone systems may possibly be used to predict the
scrubbing efficiency of lime systems. A development of this
procedure is presented in Chapter 5.
As shown in Figures 5. 12, 5.15 and 3.14, the correlations
summarired in Table 5.5 are capable of predicting fairly accurately
the scrubbing efficiency of S00 scrubbers using limestone slurries
as well as lime slurries. These correlations can also provide
some insight into the mechanism of S0_ scrubbing.
-------
65
Chapter 4
Mechanism of S02 Absorption In
Limestone Slurries
4.1 Introduction
Considerable discussion of the overall mechanism of 502 absorption
into limestone slurries has appeared in the literature and has been
recently reviewed by Nannen et al.f54!(1974). Most of the studies
reviewed by these authors were concerned with the overall reaction
by which S02 reacts with limestone (CaC03) to form CaSOs and CaS04-
More detailed studies of the absorption of S02 into alkaline solutions
have been carried out by Bjerle et alt7!(1972) and Viviant65J(1973).
Bjerle et alt7! experimentally studied the absorption of SO2 into
calcium carbonate slurried with pH's in the range of 8 to 9 in a
laminar jet absorber and found that the S02 absorption in calcium
carbonate slurry was greater than in water due to the instantaneous
reaction between absorbed S02 and the HC03~ ion. Furthermore Bjerle
et al.I7J observed that the S02 absorption rate into a slurry with a
pH of 8.3 and having HC03" ion concentration of 0.1 Molar was equal to
the S02 absorption rate into a 0.1 Molar NaOH solution. Vivianf65!
studied the absorption of SO2 into lime slurries with pH's in the range
from 12 ro 13 in a short wetted wall column. He concluded that the
SO2 absorption was enhanced by the instantaneous reaction between
absorbed SO2 and the hydroxyl ion. Based on the observations of Bjerle
et all7! and Vivianf65! it can be hypothesized that there would be a
point at which both hydroxyl and bicarbonate ions could have the same
-------
66
relative importance in the absorption S02 into a slurry.
4.2 Reaction Mechanism
By analyzing the correlations presented in the preceding chapter
possible mechanism for S02 absorption into limestone slurries, which
is consistant with the observations of Bjerle et al. J and Vivian1-
be identified. In this development, absorbed S02 as H2S03 is assumed
to react instantaneously and irreversibly with some unknown species
which has concentration of Cg in the bulk liquid phase.
H2S03 + YB ->• Products (4-1)
For this case, the enhancement factor, ((>, for S02 absorption in the
liquid film can be written as (see Hatta^ * (1932)) .
(4-2)
YDACAi
where DA and Dg are the molecular diffusivities of H2S03 and the unknown
species B in the liquid phase, respectively, (cm /sec) ,
and y is the stoichiometric factor.
Assuming the diffusivities are equal and the stoichiometric factor is
one, Equation (4-2) becomes
< = 1 + ^L_ (4-3)
CAi
Substituting this equation into Equation (3-10) gives
o
k.a -r-
L r, . ^R
Hkga cAi J
where the bar over the concentration indicates an average concentration.
-------
67
An expression for the concentration at the gas-liquid interface
can be obtained by substituting Equation (4-3) into Equation (3-1)
and utilizing Henry's law given by Equation (3-2):
where Ps02 ^s an average gas phase SC^ partial pressure.
Substituting Equation (4-5) into Equation (4-4) and subsequent
rearrangement gives
R+l ~ k£a + Hkga (k[a + Hgg0
Equation (4-6) shows that an estimate of the average concentration
of the unknown species, Cg, can be calculated from the slope of
a linear plot of 1/(R+1) versus I/PSQ->- In the discussion to follow
the average gas phase SC^ partial pressure was chosen as the arithmetic
average.
In Figure 4.1 the data reported for the Figure 3.7 for the TVA
Shawnee TCA scrubber has been re-plotted in manner suggested by
Equation (4-6). The liquid film mass transfer coefficient for physical
o
absorption kLa calculated from the intercept in Figure 4.1 is 0.0403
sec'1 and the concentration of the unknown species, CB, calculated
from the slope is 0.00053 gmol/liter.
As shown previously the presence of magnesium in the scrubbing
slurry has a significant effect on the S02 scrubbing efficiency (i.e.,
an increase in magnesium in the limestone slurry increases S02 scrubbing
efficiency above that for limestone alone with both slurries having the
-------
0.75
0.70
in
in
cu
'c
o
CD
•i—
•o
A
I
0.65
0.60
0.55
300
n
OO Total Mg < 350 ppm
5.60 < pH < 5.85
030
\ \ I I I I i I I I I I I I r
Operating Conditions and
references given in Table 4.1
Least Squares Fit of Data
j i A i i i
500
700
900
(Fso2'~'>
Figure 4.1 The Overall Ratio of the Mass Transfer Resistances as a Function of the Arithmetic
Average of the S02 Partial Pressure in the Bulk Gas Phase Plotted as Suggested by
Equation (4-6). Data from the TVA Shawnee TCA. Low Magnesium Concentration in the
Limestone Slurry.
1100
00
-------
69
same pH.) The high magnesium concentration data from the TVA Shawnec
TCA used in the construction of Figure 3.11 is presented in terms of
Equation (4-6) in Figure 4.2. Calculating the liquid film mass transfer
coefficient in the absence of chemical reaction, kLa, from the intercept
of Figure 4.2 gives a value of 0.0400 sec"1. It can be seen that the
o
agreement between the kLa calculated from the two sets of data (high
and low magnesium concentration) given in Figures 4.1 and 4.2 is
amazing. The concentration of the unknown species, Cg, for the high
magnesium case is found (from the slope given in Figure 4.2) to be
0.00145 gmol/liter.
It is of interest to compare the calculated concentrations of
the unknown specie, Cg, to the selected concentrations predicted from
equilibrium. The equilibrium calculations for limestone slurries
are based on Radian Corporation Equilibrium Program and have been
described in detail by Nelson^ •'(1974). In order to utilize the
equilibrium assumption the CC>2 partial pressure in equilibrium with
slurry and the pH must be specified.
In the absence of experimental data the C02 partial pressure
in equilibrium with the slurry is assumed to be equal to the flue
gas partial pressure of about 0.12 atm. The specified pH was chosen
as the inlet slurry pH to the scrubber.
Table 4.1 gives a comparison of the calculated value of the
concentration of the unknown species, CB, (i.e., concentration of the
species which reacts instantaneously with F^SOj) and the equilibrium
concentration of the bicarbonate ion. The agreement between the
two for the case of low magnesium is excellent. For the high magnesium
-------
1.0
0.8
0.6
0.4
0.2
0.0
600
4000 < Total Mg < 4500 ppm
5.8 < pH < 6.00
700
Operating Conditions and
references given in Table 4.1
800
900
atm
"1
Figure 4.2 The Overall Ratio of Mass Transfer Resistances as a Function of the Arithmetic Average
of the SC>2 Partial Pressure in the Bulk Gas Phase Plotted as Suggested by Equation (4-6)
Data from the TVA Shawnee TCA. High Magnesium Concentration in the Limestone Slurry.
-------
Table 4.1
Comparison Between the Calculated Concentration of the Species Which is Hypothesized to Instantaneously
and Irreversibly React with Absorbed S02 as H2SC>3 in Limestone Slurries and Selected Species Concentra-
tion Predicted from Equilibrium.
Scrubber Type
TVA Shawnee
TVA Shawnee
TVA Shawnee
l/l
Derating
Condition
Quantities
Calculated
Equation (4-6)
rt
6 O 8)
•H +J .H
f-l «) rH
rH (3 i— 1
•H <0 O
3 o i
o4 c S>
w o ^
u
and References
L(g/cm sec)
2
G(gmol/cm sec)
Total Mg
pH inlet
Figure Number
k°a (sec )
Ju
C (gmole/liter)
CHC03" +
CCaHC03
CMgHCO
CT = EC.
TrA
Epstetnt31'32'
2.835
0.0094
< 350
5.60 to 5.85
4.1
0.0403
0.000532
0.000530
0.000028
0.000015
0.000573
TCA [331
Epstein
2.835
0.0094
4000 to 4500
5.80 to 6.00
4.2
0.0400
0.001450
0.001160
0.000084
0.000258
0.001502
Spray Column
Epstein!-38]
0.652
0.00548
< 350
5.20 to 5.4
4.4
0.00544
0.000140
0.000220
0.000009
0.000005
0.000234
0.652
0.00548
< 350
5.60 to 5.8
4.4
0.00544
0.000430
0.000530
0.000028
0.000014
0.000573
0.652
0.00548
< 350
6.20 to 6.40
4.4
0.00544
0.00188
0.002000
0.000075
0.000043
0.002118
-------
72
case value of Cg is considerably higher than the equilibrium bicar-
bonate ion concentration. However, if the value of Cg is compared
to the sum of the bicarbonate and magnesium bicarbonate ion con-
centrations, good agreement is obtained. The calcium bicarbonate
ion would be expected to be chemically similar to the magnesium
bicarbonate ion. Therefore, the concentration Cg is hypothesized
to consist of the sum of four concentrations: (See Appendix B for comments)
CT = COH- + CHC03- + CMgHCO$ + GCaHC03+ (4~7)
As can be seen from Table 4.1 the agreement between this total concen-
tration, Cy, and the calculated value of Cg is very good. As defined
by Equation (4-7), C-p is also consistant with the observations of
Bjerle et al.'- •" and Vivian^ . An idealization of the concentra-
tion profiles in the vicinity of the gas-liquid interface is given
in Figure 4.3. The parallel, instantaneous irreversible reactions
occurring in the liquid phase would have the form:
H2S03 + OH" -»• H20 + HS03~
H2S03 + HC03~ -> H2C03 + HS03"
H2S03 + MgHC03+ -»• H2C03 + MgHS03+
H2S03 + CaHC03+ ->• H2C03 + CaHS03+
The data from the TVA Shawnee spray column used in constructing
Figure 3.4 has been re-plotted in Figure 4.4 in the manner suggested
by Equation (4-6). A comparison between the concentration, Cg,
calculated from this figure and the equilibrium total concentration,
C~, is given in Table 4.1; and it can be seen that two concentrations
are in fair agreement. One source of error in the agreement could
be that the liquid is sprayed into the spray column at four levels
-------
OJ
in
in
Q.
S_
C
o»
u
c
o
CJ
GASEOUS PHASE
BULK GAS I
PHASE
GAS-LIQUID INTERFACE
LIQUID PHASE
BULK LIQUID
PHASE
'HCO.
CMgHC03+
CCaHC03+
Distance Normal to the Gas-Liquid Interface
Figure 4.3 Idealization of the Concentration Profiles of Species
Important in the Transfer of S02 Across the Gas-Liquid
Interface and the Chemical Reaction in the Liquid Phase.
-------
74
0.80
0.75
0.70
0.65
in
in
Q}
c.
c
in
^—
o>
| 0.60
i
cz.
+
0.55
0.50
0.45
/
MM r i i i i | 14 i i i i i i i i i i i i i i i i i _
I " A~A~~ A—— — _^_ -
1 ^\ '-
^^"^a a
— °^n^ri -
D D^"^\^-
— -
-
- \ Least Squares Fit of Data Points
~~ \ ~
— \ -
_ \ Total Mg < 350 ppn
_ \ o o Inlet pH Range
Xntt00 A 5.2 - 5.4 I
— O\ D 5.6 - 5.8 ~
— O\ -
0§^° 0 6.2 - 6.4
: \ :
0 \
0 0 \
Operating conditions and _
_ references given in Table 4.1
i i i i 1 i I I I 1 I I i i i 1 I I I 1 I I 1 I 1 I I 1 I
200 300 400 500 600 700 80
(PA)"1, atnf1
Figure 4.4 The Overall Ratio of Mass Transfer Resistances as a Function
of the Arithmetic Average of the S02 Partial Pressure in the
Bulk Gas Phase Plotted as Suggested by Equation (4-6). Data
from the TVA Shawnee Spray Column. Low Magnesium Concentra-
tion in the Limestone Slurry.
-------
75
rather than all the liquid at the top of the column. Since both of
the mass transfer coefficients, k°a and k a, will vary with liquid
rate at the various stages of the scrubber, the slopes and inter-
cepts of Figure 4.4 are not strictly correct.
Another source of disagreement between the concentrations CD
D
and C given in Table 4.1 is the assumption of the pH and CO-
partial pressure of the slurry used in the equilibrium calculation
as described above. The use of the inlet slurry pH and the flue
gas C0_ partial pressure gives only rough estimates of these quan-
tities since the concentration C_ can be viewed as an average
D
concentration and it is not known at which position in the scrubber
this concentration applies.
-------
76
Chapter 5
Analysis of pH for Lime Slurry at the Outlet of the Scrubber
5.1 Introduction
It has been shown in Chapter 3 that the mathematical model, which
was developed to describe the absorption of S02 by limestone slurries,
can be used to simulate the lime slurry scrubber fairly accurately if
a characteristic slurry pH corresponding to the log mean hydrogen ion
concentration across the scrubber is utilized in the limestone correla-
tions. Hence, in order to be able to estimate the S02 scrubbing
efficiency in the lime system using the correlations developed for
limestone slurries, both the inlet and outlet pH of the scrubbing
slurry in the lime system must be known.
The purpose of this chapter is to develop a procedure by which
the pH of the outlet slurry from the lime scrubber and the SC>2 removal
efficiency of the lime scrubber can be calculated directly from
knowledge of the inlet condition of the slurry to the scrubber and
scrubber operating conditions. This procedure will make use of the
model developed for the limestone system.
For the TCA scrubber, heat transfer is known to occur very
rapidly (Barile and Meyer^, 1971; Barile et al.t4!, 1974); and
the heat capacity of the liquid phase is much greater than that of
the gas. Therefore, the system can be assumed to operate isothermally.
That is the variation in the liquid and gas temperature throughout the
system can be ignored for all practical purposes when the heat of
reaction generated in the system is negligibly small. Thus, in
-------
77
what is to follow, only the mass balances in the scrubber are consid-
ered .
5.2 Liquid Phase Material Balance for the Scrubber
The conservation of sulfur, carbon and calcium in the liquid in
the scrubber can be written as:
Sulfur
i i i i
= A. (5-1)
Carbon
t i i i
Mm + L [Cl • + L S . L [Cl - L S = A (5-2)
•~U2 m c,in L out c,out c
Calcium
L[Ca]. + L Sr . - L' [Ca] l/S_ = Ar (5-3)
L Jin Ca,in L Jout Ca,out Ca J
where:
MCQ is the absorption rate of SC>2 in the scrubber, mgmole/sec,
M^Q is the absorption rate of C02 in the scrubber, mgmole/sec,
[k]• is the total liquid phase concentration of species k in
the i—- stream of the scrubber, mgmole/liter,
S, . is the total concentration of species k as solid in the
k, i
i— stream of the scrubber, mgmole/liter,
A^ is the accumulation of species k as solid within the
scrubber, mgmole/sec,
and L is the liquid flow rate, liter/sec.
when:
i=in,out the subscript refers to the inlet and the outlet stream
of the scrubber, respectively,
and k=S,C,Ca the symbol k refers to sulfur, carbon and calcium,
respectively.
-------
78
The above equations apply to the steady state operation of the
scrubber system, that is, steady state with respect to the liquid
concentrations. A rate of scale formation is taken into account
through the use of the accumulation terms, A^.
Steady liquid concentrations may be inconsistent with the
steady build up of solids in the scrubber system. For example, it
i? known that the accumulation of scale in the scrubber can cause
an increase in the SC>2 removal which in turn can affect the liquid
compositions. However, this process normally takes place slowly
and it can be assumed that Equations (5-1) through (5-3) can be
applied at each small interval of time. This is equivalent to a
quasi-steady state assumption.
Subtracting Equation (5-3) from the sum of Equations (5-1) and (5-2)
gives
MS02 + Mco2 = L' Cnout - nin) (5-4)
where:
Vat = tslout + [Clout - tCalout
nin = [S]in - [C]in - [Ca]in
Here the fact that the solid calcium salts have a one to one correspond-
ence of calcium to sulfur or carbon has been used to eliminate the
solid concentrations appearing in Equations (5-1) through (5-3). It
can be noted that Equation (5-4) does not contain any solid composi-
tions.
In some cases, solid magnesium salts may also be present in
the scrubbing slurry. However, experimental evidence shows that the
-------
79
amount of solid magnesium salts are usually insignificant in compari-
son with the amount of solid calcium salts in the scrubbing liquid.
Therefore, consideration of the magnesium material balance in the
scrubber appears unnecessary.
A further simplification of Equation (5-4) is possible if it
is noted that the C07 absorption rate, M™ , can be ignored for
lime slurries. This follows from the fact that the magnitude of
the C02 absorption rate, MCQ , evaluated from Equation (5-4) is
usually less than 10% of the S02 absorption rate, MSQ (Borgwardt I19'21 J ,
1974). Consequently, Equation (5-4) for the lime system approximately
reduces to
i
MS02 = L (TW - nin) C5'5}
In utilizing Equation (5-5) to calculate the scrubbing efficiency
and effluent slurry pH of the lime scrubber, this equation must be
solved simultaneously along with the expressions developed in Chapter
3 for computing the scrubbing efficiency and the rate of S02 absorp-
tion, Mso • By the methods discussed in Chapter 3, the rate of S02
absorption, MSQ_, in the lime scrubber is determined if the inlet
and outlet slurry pH, inlet S02 partial pressure and operating and
scrubber parameters are specified. In the operation of a scrubber,
which has no recycle loops, only the inlet slurry pH, inlet S02
partial pressure and operating and scrubber parameters may be
specified. Therefore, the S02 absorption rate can be calculated
a priori if the outlet slurry pH from the scrubber can be found.
Mathematically this may be expressed as
-------
80
= MQO ^PH J (5~6)
7 i>0-> r OUt
Equations (5-5) and (5-6) give the relationships between the
quantities Mso^, PHout' r'in and nout ' Therefore> in order to simulate
the scrubber, two more relationships must be established or the
specification of two variables is necessary. A fortunate circumstance
occurs which makes the establishment of two additional relationships
a simple matter. The value of n can be related to the pH through
chemical equilibrium. Thus,
in
and nout = E(pHout) (5-8)
In these equations the symbol E is an operator which relates n to
pH through chemical equilibrium. It remains to be shown that the
value of n calculated by chemical equilibrium using the experimental
pH can closely approximate the experimental value of n- This is
done in Figure 5.1. It can be seen from this figure that the agree-
ment between the calculated value of n(or ~cai) and the experimental
value of r, (or r\e xp) for the slurry at the inlet and outlet of the
scrubber is fairly good. The equilibrium relationship between n and
pH was calculated using the equilibrium program developed by Radian
Corporation. This calculation has been described in detail by
Nel sent56!, (1974).
To summarize, the lime scrubber can be simulated by the follow-
ing sequence of steps:
1) Inlet conditions and operational and scrubber parameters are
set.
-------
81
50
30
10
O)
+j
OJ
o
en
E
O!
ZJ
O
re
o
rn
Data for Lime'Scrubbing
(see Table 4.1 for Operating
Data and References)
1.0
0.8
for the inlet Slurry of the Scrubber
for the Outlet Slurry of the Scrubber, ,
I
j i
I i
0.8 1.0
3 5 7 10
n, experimental, mgmole/liter
30
50
f-'i^ure 5.1 Comparison of the Predicted and Observed Values of n for
Lime Slurry at the Inlet and Outlet of EPA/RTP TCA Scrubber.
-------
82
2) The value of n- is computed through chemical equilibrium
Equation (5-7) from pH. which is specified.
3) Relationships (5-5), (5-6) and (5-8) are solved simultaneously.
Utilizing the above sequence of steps, the lime scrubber data of
Borgwardt (see Table 5.1 for references) has been simulated. (see
Appendix A for example of simulation). The results of this simulation
are summarized in Figures 5.2 and 5.3. In Figure 5.2, a comparison
between the observed and calculated outlet slurry pH from the scrubber
is given; and in Figure 5.3 the observed and calculated SCL removal
efficiencies are compared. It can be seen that the agreement of the
calculated and observed outlet pH or SCL removal efficiency is good.
Thus, through simple material balances and the observations that n
can be related to pH of the lime slurry through chemical equilibrium
and that CCL absorption rate for lime scrubber system is negligible
compared to the S0_ absorption rate, a procedure has been developed
by which the SCL scrubbing efficiency and outlet slurry pH of a
scrubber utilizing lime slurry can be calculated from correlations
developed for the limestone scrubbing system.
-------
Table 5.1
Range of Data Used in Constructing Figures 5.1, 5.2 and 5.3. Data is for the EPA In-House TCA
Using Lime Slurries as the Scrubbing Medium
Equipment
and References
G
!• gmole
inlet outlet
L-
f g 1
1 2 J
cm sec
Inlet
Slurry
Temperature
Inlet
Pso2
(ppm)
Height
of
Packing
(cm)
Magnesium
Concentration
in Liquid
(ppm)
EPA In-House TCA 5.7 4.5 3.15
Borgwardt'-19-' to to 0.0136 to
9.5 6.0 3.8
125
2430
to
2800
50.8
to
76.2
12
to
1150
00
OJ
-------
84
7.0
6.8
6.6
6.4
•o 6.2
o>
2 6.0
3
<_>
"•O CO
O 3 -O
r.
2 5.6
4-5
3
C
5.4
5.2
5.0
4.8
4.6
I I I I I I
Data for Lime Scrubbing
(see Table 5.1 for Operating
Data and References)
A EPA/RTP Research TCA
5% Error
, - / I I I I I I I I I I I I
44 1 1 1 1 1 1 1 1 1 1
4.4 4.6 4.8 5.9 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.
(pHL...,.!,^, experimental
o * £. O • * %/ •'_ _ , — _-_ _ , .
(pH)outlet> experimental
Figure 5.2 Comparison of the Predicted and Observed Outlet Slurry
pH for the Simulation of EPA/RTP TCA Scrubber Using
Lime Slurry as the Scrubbing Medium.
0
-------
85
•o
0)
o
(T3
(J
O)
u
CU
03
O
CU
CM
O
CO
100
90
80
70
60
50
40
T
T
T
Data for Lime Scrubbing
(see Table 5.1 for Operating
Data and References)
A EPA/RTP Research TCA
5% Error
40 50 60 70 80 90
S09 removal efficiency, experimental
100
Figure 5.3 Comparison of the Predicted and Observed SO- Removal
Efficiency for the Simulation of EPA/RTP TCA Scrubber
Using Lime Slurry as the Scrubbing Medium.
-------
86
Chapter 6
Conclusions and Discussion
In this study a mathematical model which can simulate both large
and small scale TCA scrubbers used for the scrubbing of S02 from flue
gases by limestone, limestone-magnesium oxide and lime slurries has
been proposed. The parameters which appear in this model have been
evaluated from experimental data. The gas film mass transfer coeffi-
cients for the spray and packed sections of the TCA were obtained from
the literature and calculated from experimental data, respectively.
The liquid film resistances for both the spray and packed section
were calculated from experimental data and correlated in terms of the
ratio of the mass transfer resistance in the gas film to that in the
liquid film as a function of inlet pH, magnesium concentration of the
scrubbing slurry and inlet partial pressure of SC>2 in the flue gas.
Within the accuracy of the experimental data, the ratio of the mass
transfer resistances appears to be independent of the gas and liquid
flow rates. The temperature dependence of the ratio of resistances
was not determined since the experimental data were available only
in a narrow range of temperature.
For the data analyzed the chloride concentration in scrubbing
liquid ranged from essentially zero to 10,000 parts per million.
There did not appear to be any relationship between chloride concentra-
tion and S02 scrubbing efficiency except through the influence of
chloride on the slurry pH.
-------
87
Although many assumptions were made in the analysis of the lime-
stone scrubbers and scatter of the data can be seen in the derived
correlations, these assumptions appear to have given rise to a satis-
factory method of simulating the performance of both small and large
scale TCA and spray column scrubbers in removing S02 from flue gases
using limestone, limestone-magnesium oxide or lime slurries as the
scrubbing medium. In most cases the calculated SC>2 removal efficiency
was within 5% of the experimentally observed efficiency.
The scatter in the data in the figures presented in this study
arise from several sources. Probably the most important of these is
that many of the data are taken from large scale units where the
operating conditions and purity of reagents are difficult to control.
Also a substantial number of the data were reported as averages over
long periods of operation or reported in ranges and the averages over
these ranges were utilized in the analysis.
The liquid film mass transfer resistance was correlated in terms
of the ratio of the gas to liquid film resistance because it did not
appear possible to separate the liquid film mass transfer coefficient
into the enhancement factor and the liquid film mass transfer coeffi-
cient for physical absorption. A possible mechanism for 862 absorp-
tion into limestone slurries has been proposed in this study in which
the absorbed SC>2 as ^803 reacts instantaneously and irreversibly in
parallel with OH", CaHC03+, MgHC03+, and HC03" ions. The physical
liquid mass transfer coefficient can be calculated, as shown in this
study, by this mechanism. However, sufficient data are not presently
available to make this approach practical.
-------
88
The discussion of the mechanism of SC>2 absorption into limestone
magnesium oxide slurries has revealed:
1} that the absorbed SC>2 as H2S03 could possibly react instantan-
eously and irreversibly with OH~, HC03~, CaHC03+ and MgHCC^"1" in
parallel reactions.
2) a method by which the liquid phase mass transfer coefficient
for physical absorption can be calculated.
The mechanism of S02 absorption reported here, with refinement,
could be used to model the S02 scrubbing efficiency of scrubbers using
limestone-magnesium oxide slurries. However, one complicating factor
is that the physical liquid film mass transfer coefficient must be
known at gas and liquid flow rates encountered in actual scrubbing
operations. However, with all the data analyzed in this study only
one value of k?a for one pair of gas and liquid rates could be
obtained for the TCA scrubber.
For a scrubber which utilizes lime slurries as the scrubbing
medium, the S02 removal efficiency is shown to be a function of the
log mean hydrogen concentration across the scrubber. Therefore,
efficiency of the lime scrubber cannot be calculated simply from
knowledge of the inlet condition as in the case of the limestone
scrubber. A simple procedure has been developed in this study by
which the lime system can be simulated with only the inlet conditions
being specified. This procedure utilizes the limestone correlations
and is based on the observations that the C02 absorption rate in the
lime scrubber system is very small and that the value of n can be
-------
89
related to pH of the lime slurry through equilibrium. The procedure
has been shown to give reasonable estimates of the S02 scrubbing
efficiency for the TCA scrubber utilizing lime slurries.
-------
Nomenclature
90
A
Ak
Ap, As
CA
CM
CM
CB
D,
H
Mi
Specific interfacial area available to
mass transfer
Pre-exponential factor in the expression
for R defined by Equation (3-20)
Accumulation of species k as solid
within the scrubber
Pre-exponential factor in the expression
for R for the packed and spray sections,
respectively
H2S03 concentration in the bulk liquid
phase
Interfacial H2S03 concentration
Average interfacial ^SO concentration
Concentration of species which react
instantaneously and irreversibly with
H2S03
Sum of the OH~, CaHCC^*, MgHC03+ and
HC03~ ions
Molecular diffusivity of specie B in
the liquid phase
An operator, defined by Equations
(5-7) and (5-8)
Molar gas flow rate based on corss-
sectional area of the scrubber
Molar gas flow rate based on the free
projected area of the grid
Henry's law constant
Total liquid phase concentration of
species k in the i"* stream of the
scrubber
Gas film mass transfer coefficient for
physical absorption
era *•
Diraensionless
mgmole/sec
Dimensionless
gmol/cm^
gmol/cm^
gmol/cnr
gmol/cm
gmol/cm^
cm /sec
gmol/cm2sec
gmol/cm^sec
atm cm /gmol
mgmole/liter
gmol/cm atm sec
-------
Nomenclature (continued)
91
k°
kL
kga
Kga
,5
kga
overall
kpa
M
'S02
AP
AP
N
Liquid film mass transfer coefficient
for physical absorption
Average overall gas side mass transfer
coefficient
Overall gas side mass transfer coeffi-
cient for the packed section of the TCA
Gas side mass transfer coefficient for
the packed section
Overall gas side mass transfer coeffi-
cient for the spray section of the TCA
Gas side mass transfer coefficient for
the spray section
Gas film mass transfer coefficient
defined by Equation (3-19)
Liquid flow rate based on cross-
sectional area of the scrubber
Liquid flow rate
Absorption rate of C02 in the
scrubber
Absorption rate of S02 in the
scrubber
Molar flux of S02
Pressure drop over the length of the
scrubber
Pressure drop without scaling
Partial pressure of S02 in the bulk gas
phase
Average partial pressure of S02 in the
bulk gas phase
Interfacial partial pressure of S02
cm/sec
gmol/cm atm sec
gmol/cm atm sec
gmol/cm^atm sec
gmol/cm atm sec
gmol/cm atm sec
gmol/cm atm sec
g/cm sec
liter/sec
mgmole/sec
mgmole/sec
gmol/cm sec
in H20
in H20
atm
atm
atm
-------
Nomenclature (continued)
92
S02
OUt
S02
S0
PT
R
R
u
"mf
U
BN
z
AZ
ZT
Inlet partial pressure of S02 in the
bulk gas phase
Outlet partial pressure of S02 in the
bulk gas phase
Partial pressure of S02 which could be
maintained in equilibrium with the
bulk liquid phase
Total pressure
Ratio of the gas to liquid film mass
transfer resistances
Average ratio of mass transfer
resistances in TCA
The value of R in the packed and
spray sections, respectively
Total concentration of species k
as solid in the i^S- stream of the
scrubber
Liquid temperature
Gas mass velocity based on cross-
sectional area of the scrubber
Gas mass velocity u at the minimum
fluidization condition
Linear velocity at the bottom of
the scrubber with scale present
Linear velocity at the bottom of
the scrubber without scale present
Height measured from gas inlet
Difference between height Z2 and Zj
Total height of the transfer region
Height of the i— position
atm
atm
atm
atm
Dimensionless
Dimensionless
Dimensionless
mgmole/liter
g/cm sec
2
g/cm sec
cm/sec
cm/sec
cm
cm
cm
cm
-------
93
Height of the packed section in the TCA
Height of the spray section in the TCA
Greek Symbols
*
Y
> A
Enhancement factor for mass transfer in
the liquid film due to chemical reaction
Average enhancement factor
Stoichiometric factor
Magnesium correction factor for As and A
i - [Ca]i
cm
cm
Dimensionless
Dimensionless
Dimensionless
Dimensionless
mgmole/liter
-------
94
Bibliography
1. Ando, J., paper presented in Symposium on Flue Gas Desulfuriza-
tion, Atlanta, Georgia, (1974).
2. Atsukawa, M., Sangyo Kankyo Kogaku (Japan), 44, 23 (1965).
3. Balabekov, 0. S., Tarat Romankov, E. Ya., and Mikhalev, M. F.,
Trans. Zhur. Prikladnoi Khim., 4^, (7), 1540, (1969).
4. Barile, R. G., Dengler, J. L. and Hertwig, T. A., A.I.Ch.E.
Symposium Series, 70, (138), 154, (1974).
5. Barile, R. G., and Meyer, D. W., Chem. Eng. Progr. Symp. Ser.,
67_, (119), 134, (1971).
6. Berkowitz, J. B., Evaluation of Problems Related to Scaling in
Limestone Wet Scrubbing, report prepared by ADL for the EPA,
April, (1973).
7. Bjerle, I., Bengtsson, S. and Farnkviet, K. Chem. Eng. Sci., 27,
1853 (1972).
8. Blyakher, I. G., Zhivaikin, L. Ya and Yurovskaya, N. A., Inter-
national Chem. Eng., 7_ (3), 485, (1967).
9. Boll, R. H., paper presented at Lime/Limestone Wet Scrubbing
Symposium, Pensadola, Florida, (March 16-20, 1970).
10. Borgwardt, R., Limestone Scrubbing of SO? at EPA Pilot Plant,
Report No. 1, (August, 1972).
11. Borgwardt, R., ibid., Report No. 2 (September, 1972).
12. Borgwardt, R., ibid., Report No. 3 (October, 1972).
13. Borgwardt, R., ibid., Report No. 4 (November, 1972).
14. Borgwardt, R., ibid., Report No. 6 (January, 1973).
15. Borgwardt, R., ibid., Report No. 7 (February, 1973).
16. Borgwardt, R., ibid., Report No. 11 (June, 1973).
17. Borgwardt, R., ibid., Report No. 12 (July, 1973).
18. Borgwardt, R., ibid., Report No. 14 (January, 1974).
19. Borgwardt, R., ibid., Report No. 15 (February, 1974).
-------
95
Bibliography (Continued)
20. Borgwardt, R., ibid., Report No. 16 (June, 1974).
21. Borgwardt, R., ibid., Report No. 17 (July, 1974).
22. Borgwardt, R., paper presented at the EPA Flue Gas Desulfurization
Symposium, Atlanta, Georgia (November 4-7, 1974).
23. Chen, B. H. and Douglas, W. J. M.,Can. J. Chem. Eng., 46, 245,
(1968). —
24. Chen, B. H., and Douglas, W. J. M., Can. J. Chem. Eng., 47, 113,
(1969).
25. Cheremisinoff, P. N., and Fellman, R. T.,Power Eng., 54, (October,
1974) .
26. Devitt, T. W., and Zada, F. K., paper presented in Symposium on
Flue Gas Desulfurization, Atlanta, Georgia, (1974).
27. Douglas, H. R., Snider, I. W. A. and Tomlinson, II, G. H.,
Chem. Eng. Prog., 59_, (12), 85, (1963).
28. Douglas, W. J. M., Chem. Eng. Prog., 60_, (7), 66, (1964).
29. Epstein, M., EPA Alkali Scrubbing Test Facility at the TVA
Shawnee Power Plant, Bechtel progress report prepared for the
EPA for July 1, 1973 to August 1, 1973 (August 31, 1973).
30. Epstein, M., ibid., progress report for October 1, 1973 to
November 1, 1973 (November 30, 1975).
31. Epstein, M., ibid., progress report for December 1, 1974 to
January 1, 1974 (January 31, 1974).
32. Epstein, M., ibid., progress report for January 1, 1974 to
February 1, 1974 (February 28, 1974).
33. Epstein, M., ibid., progress report for May 1, 1974 to June
1, 1974 (June 30, 1974).
34. Epstein, M., ibid., progress report for June 1, 1974 to July
1, 1974 (July 31, 1974).
35. Epstein, M., ibid., progress report for July 1, 1974 to
August 1, 1974 (August 31, 1974).
-------
96
Bibliography (Continued)
36. Epstein, M., EPA Alkali Scrubbing Test Facility: Limestone Wet
Scrubbing Test Results, report prepared by Bechtel for the EPA
(January, 1974).
37. Epstein, M., EPA Alkali Scrubbing Test Facility: Sodium Carbonate
and Limestone Test Results, report prepared by Bechtel for the EPA
(August, 1973).
38. Epstein, M., Sybert, L., Wang, S. C., Leivo, C. C., Princiotta,
F- T., paper presented at the 66th Annual Meeting of the
A.I.Ch.E., Philadelphia, Pennsylvania, (November, 1973).
39. Gel'perin, N. I., Savchenko, V. I., Ksenzenko, B. I., Grishko,
V. Z., and Dianov, E. A., Khimicheskoe Promyshlennost, 11,
(1965).
40. Gel'perin, N. I., Grisko, V. Z., Savchenko, V. I., and
Shchedro, V. M., Chem. Petrol. Eng., (1), 36, (1966).
41. Gel'perin, N. I., Savchenko, V. I., and Grisko, V. Z., Teor.
Osn, Khim. Tekhnol., 2^ (1), 76, (1968).
42. Gleason, R. J., paper presented at the Second International
Lime/Limestone Wet Scrubbing Symposium, New Orleans, Louisiana
(November 8-12, 1971).
43. Hatta, S., Techol. Reports Tohoku Imp. Univ., 10, 119 (1932).
44. Johnstone, H. F., and Silox, H. E., Ind. Eng. Chem., 39, 808,
(1947).
45. Johnstone, H. F., and Single, A. D., Ind.Eng. Chem., 29, 288,
(1937).
46. Johnstone, H. F., and Williams, G. C., Ind. Eng. Chem., 31,
993, (1939).
47. Khanna, R. T., Ph.D. Thesis, McGill Univ., Canada, (1971).
48. Kito, M., Shimada, M., Sakai, T. Sugiyama, S., and Wen, C. Y.,
paper to be presented at Engineering Foundation Conferences,
California, (June, 1975).
49. Kulbach, A. W.j Chem. Eng. Progr. Symp. 'Ser., 57_, No. 35, (1961).
50. Levsh, I. P. Krainev, N. I., and Niyazov, M. I., Uzb. Khim. Zh.,
5_, 72, (1967).
-------
97
Bibliography (Continued)
51. Levsh, I. P. Krainev, N. I., and Niyazov, M. I. International
Chem. Eng., 8_, (4), 610, (1968).
52. McMichael, W. J., Fan, L. S. and Wen, C. Y., paper presented
at A.I.Ch.E. Meeting, Houston, Texas, (March 16-20, 1975).
53. Nakagawa, S., Seisan to Gijutsu (Japan), 5_, 33, (1964).
54. Nannen, L. W., R. E. West and F. Kreith, J. Air. Poll. Control
Association, 24, 29 (1974).
55. "New Floating Bed Scrubber Won't Plug", Chem. Eng., 66, 106,
(December 14, 1959).
56. Nelson, R. D., Jr., M.S. Thesis, West Virginia University,
Morgantown, West Virginia (1974).
57. O'neill, B. K., Nicklin, D. J., Morgan, N. J., and Leung, L. S.,
Canadian J. Chem. Eng., 50, 595, (1972).
58. Potts, J. M., Slack, A. V., and Hatfield, J. D., paper presented
at Second International Lime/Limestone Wet Scrubbing Symposium,
New Orleans, LA, (November 8-12, 1971).
59. Slack, A. V., Falkenberry, H. L., and Harrington, R. E., J. Air
Poll. Control Association, 2.2, 159, (1972).
60. Strom, S. S., and Downs, W., paper presented at A.I.Ch.E. Meeting,
Philadelphia, Pennsylvania, (November 11-15, 1973).
61. Takahashi, T., and Akagi, Y., Memoirs of the School of Eng.,
Okayama Univ., _3, 51, (1968).
62. Takahashi, T. and Fan, L. T., Removal of Sulfur Dioxide from
Waste and Exhaust Gases, Report 12, Institute for Systems Design
and Optimization, Kansas State University, Manhattan, Kansas,
(1969) .
63. Tichy, J., Wong, A., and Douglas, W. J. M., Can. J. Chem. Eng.,
5£, 215, (1972).
64. Tichy, J., and Douglas, W. J. M., Can. J. Chem. Eng., 51, 618
(1973).
65. Vivian, J. E., The Absorption of S02 into Lime Slurries: An
Investigation of Absorption Rates^ and.Kinetics, report prepared
for the HEW Dept., (September, 1973).
-------
98
Bibliography (Continued)
66. Weir, Jr., A., paper presented in Symposium on Flue Gas Desulfuri-
zation, Atlanta, Georgia, (1974).
67. Wen, C. Y., Wet Scrubber Study, report prepared by West Virginia
University for the EPA, Report No. 35 (December, 1973).
68. Yagisawa, and Hayashi, Ryuan, (Japan), 14, 167, (1961).
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99
Appendix A
Numerical Example of the Simulation of EPA/RTP TCA Scrubbers Using
Lime Slurry as the Scrubbing Medium
The simulation of the lime scrubber consists of the following
sequence of steps:
1) Inlet conditions and operation and scrubber parameters are set.
2) The value of n- is computed through chemical equilibrium
Equation (5-7) from pH. which is specified.
3) Relationships (5-5), (5-6) and (5-8) are solved simultaneously.
A numerical example is provided in this Appendix to carry out the
simulation of the EPA/RTP TCA scrubber using lime slurry as the
scrubbing medium. This simulation follows the sequence of steps given
above. The inlet conditions and operation and scrubber parameters are
given in Table B.I. The value of n is computed through chemical
equilibrium computer program (NelsonL , 1974) by specifying the
following experimental values for parameters:
Temperature = 125°F
Maximum CO- pressure = 0.12 atm
Total Sulfate concentration in the liquid phase = 37.25 mgmole/
liter
Total Magnesium concentration in the liquid phase = 22.IS
mgmole/liter
Total Chloride concentration in the liquid phase =0.0 mgmole/liter
and desirable pH.
The value of n as a function of pH computed from the above
described chemical equilibrium computer program is shown in Figure
B.I where the inlet conditions for pH and n are shown. A series
of trial and error procedures are involved in solving
-------
Table /\. ]
Operating Conditions and Reference for the Example Given in Appendix B. Data for the BPA In-House
TCA Using Lime Slurries as the Scrubbing Medium
Equipment
and References
PH
inlet outlet
G
,gmole ,
1 2
cm sec
L
J.
1 2
cm sec
Inlet
Slurry
Temperature
EPA In-House TCA
Inlet
(ppm)
Height
of
Packing
(cm)
Magnesium
Concentration
in Liquid
(ppm)
Borgwardt
[19]
6.1 4.9 0.0136
3.15
125
2800
76.2
546
o
o
-------
101
Equations (5-5), (5-6) and (5-8) simultaneously to obtain solutions of
^SO ' nout anc* PHout' Tnese procedures in terms of numerical solutions
are shown in the subsequent development for this example.
1) Assume PHout (4-9) to calculate SO., removal efficiency
(Sb°o) via scrubber model as summarized in Table 3.5.
f
2) Compute the value for M /L (9.52 mgmole/liter) from the
bU2
S0? removal efficiency as obtained from step (1) and then,
calculate r. (35.52 mgmole/liter) via Equation (5-5). Sub-
sequently, pH (4.80) can be found via Equation (5-5). Sub-
Figure B.I. New value of S0? removal efficiency (85.5%)
can, therefore, be calculated based on the new value of
pH (4.80).
v out *• J
5) Compare pH (4. SO) with the assumed value of pH (4.90)
to check if it meets the desirable convergent criterion.
The same check applies to the SO., removal efficiency.
4) If both pH and S0? removal efficiency meet the
desirable convergent criteria, the calculation is completed.
Otherwise, iteration proceeds from step (1) with new
value (4.80) replaced for pH .
These procedures reveal the following results for this
example:
pH . = 4.80
f out
S0? removal efficiency = 85.5%.
where the experimental observed values for this example are:
=4.90
S0_ removal efficiency = 81%.
-------
100
80
60
40
S-
-------
103
The agreement between the simulated and experimental values of pH and
SCL removal efficiency appears fairly good.
-------
104
Appendix B
Some Comments on the Concentration of Reactant, Cg, Calculated Via
Equilibrium Computer Program"
In order to utilize the equilibrium computer program (Nelson ,
1974), three major parameters must be specified. They are slurry pH,
total sulfate concentration in the liquid phase and maximum CCL
partial pressure. For limestone recycle slurry scrubbing system, an
examination of the experimental data reveals that the variations of
slurry pH and total sulfate concentration in the liquid phase across
the scrubber are, in general, less than 15% (Borgwardt ,
1972~1974). In addition, the maximum CC- partial pressure for the
slurry in the scrubber appears most likely to be the flue gas CO,,
partial pressure. Thus, through an equilibrium calculation only a
slight variation of concentration profile for each individual
specie across the scrubber can be expected.
Examining the concentration of total carbonate in the liquid
phase obtained from the equilibrium calculation and the experimental
observation, it is shown that a reasonable agreement can be achieved.
HCO which is identified as the major reactant specie contributes
significantly towards the total carbonate concentration in the
liquid phase. Consequently, it appears that the concentration of
HCO can be approximated from an equilibrium calculation.
It is concluded that only a slight variation of concentration
for the total carbonate in the liquid phase across the scrubber exists.
The major reactant specie, HCO,, can be approximated from the equili-
brium calculation. The use of the inlet slurry condition specified
-------
105
for the equilibrium calculation shown in this study offers convenient
methods in determining the concentrations of reactant species via
equilibrium calculation. (also see page 40 for reasons for using
inlet slurry pH in the correlation of mass transfer coefficient).
-------
106
TECHNICAL REPORT DATA
(Please read la&vctions on the reverse before completing)
1. REPORT NO.
EPA-600/2-75-023
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Absorption of Sulfur Dioxide in Spray Column and
Turbulent Contacting Absorbers
5. REPORT DATE
August 1975
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
C.Y. Wen and L.S. Fan
9. PERFORMING OR9ANIZATION NAME AND ADDRESS
West Virginia University
Department of Chemical Engineering
Morgantown, West Virginia 26506
10. PROGRAM ELEMENT NO.
1AB013; ROAP 21ACY-041
11. CONTRACT/GRANT NO.
Grant R-800781
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research.and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final; 6/74 - 6/75
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
10 ABSTRACTThe report'gives results of an analysis of experimental data, from both
small and large scale turbulent contacting absorbers (TCA) and spray columns used
in the wet scrubbing of SO2 from flue gases, to obtain gas film mass transfer coef-
ficients and overall coefficients in the liquid film which includes chemical reaction in
the liquid film. Recycled limestone, limes tone-magnesium oxide, and lime scrubbing
slurries were investigated. Gas film coefficients for the spray and TCA scrubbers
were calculated from data on SO2 scrubbing with sodium carbonate solutions. Overall
mass transfer resistances in the liquid phase were correlated for both scrubbers in
terms of the ratio of the gas film and liquid film mass transfer resistances. The ratio
of the resistances was found to be a function of only the scrubber type, inlet SO2 par-
tial pressure in the gas phase, slurry pH, and magnesium concentration of the scrub-
bing slurry. Specifically, it was found that the ratio of the gas and liquid film mass
transfer resistances (or the fraction to which SO2 removal is gas film controlled)
increases with increasing slurry pH and magnesium concentration and decreasing SO2
partial pressure. Correlations for the gas film mass transfer coefficient and the ratio
of mass transfer resistances are shown to predict fairly accurately the experimentally
observed SO2 removal efficiencies.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Sulfur Dioxide
Absorption
Calcium Oxides
L-imestone
Magnesium Oxides
Calcium Carbonates
Sodium Hydroxide
Slurries
Scaling
Flue Gases
Des ulfur ization
Mathematical Models
Air Pollution Control
Stationary Sources
Turbulent Contacting
Absorbers (TCA)
Spray Towers
13B
07B
11G
11F
21B
07A, 07D
12A
13. DISTRIBUTION STATEMEN1
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
114
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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