United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-88/037 Nov. 1988
Project Summary
Effect of Relative Humidity and
Additives on the Reaction of
Sulfur Dioxide with Calcium
Hydroxide
Rosa Ruiz-Alsop and Gary T. Rochelle
Previous results with flue gas
desulfurization by spray drying of
Ca(OH)2 show that a significant
amount of SOj is removed in the bag
filters used to collect the solids. This
research program investigates the
reaction of SOz with Ca(OH)2 at
conditions similar to those of
commercial scale bag filters: 19-
74% relative humidity, 30.4-95 °C,
and 300-4000 ppm SO2. This study
was carried out in a bench-scale
fixed-bed reactor, with powder
reagent Ca(OH)2 dispersed in silica
sand. The gas phase was a mixture
of NZ, SOz, and water vapor. The
effects of Ca(OH)2 loading, tem-
perature, relative humidity, Inlet SOj
concentration, and additives were
investigated. Of the additives tried
(buffer acichf, and organic and
inorganic deliquescents), only the
deliquescent salts improved Ca(OH)2
reactivity toward SO2- The im-
provement depends on the type and
amount of salt and on the relative
humidity. The experimental data were
modelled by a shrinking core model
with zero order kinetics in SOz, using
an empirical correlation to account
for shape and surface roughness of
the Ca(OH)2 particles. The diffusion
coefficient of the SOj through the
product layer (De) increases linearly
with relative humdity and the amount
of additive, and the kinetic rate
constant (kg) increases exponentially
with relative humdity and the amount
of additive. De values ranging from
0.75E-9 to 1.20E-6 cm2/sec and ks
values ranging from 1.0E-9 to
8.23E-9 cm4/gmol sec in the model
simulated the experimental results.
This Project Summary was devel-
oped by EPA's Air and Energy Engi~
neering Research Laboratory. Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering Information at back).
Introduction
Flue gas desulfurization by spray
drying of a Ca(OH)2 slurry has become
increasingly important in recent years as
an alternative to the more traditional wet
lime or limestone scrubbing. During
spray drying, the SO2-containing flue
gas is contacted in the dryer with a finely
atomized aqueous solution or slurry of an
alkali (typically slaked lime or soda ash),
which absorbs and neutralizes the SC>2.
Simultaneously, the water is evaporated
from the slurry droplets leaving a solid
material which can be collected using
conventional solids collection equipment
such as bag filters or electrostatic
precipitators. Bag filters are preferred
because the unreacted Ca(OH)2 in the
solids reacts with SO2 in the bag niters,
producing additional SO2 removal. The
reaction between lime solids and S02
taking place in the ducts and bag filters
of a spray dryer system is the subject of
the present research.
-------
Most of the information available in the
literature for this reaction is reported
results of SO2 removal across the bag
filters of pilot and demonstration spray
dryer plants. These results are difficult to
interpret because the conditions of the
flue gas entering the bag filters depend
on the spray absorber behavior. Any
variable changes that affect the SOs
removal across the spray absorber will
change the concentration of SO2
entering the bag filters. Therefore, two
variables will have effectively been
changed in the bag filters and the
contribution of each cannot be isolated.
Two bench scale studies regarding the
reaction of interest have boen reported:
one at the Lund Institute of Tech-
nology-Sweden, and the other at EPA.
However, Vie characterization of this
reaction is far from complete.
The present study was carried out in a
bench-locale fixed-bed reactor, oper-
ated at conditions similar to those found
in bag filters of commercial spray drying
systems, and using powder reagent
Ca(OH)2 as the sorbent. The effects of
Ca(OH>2 loading, temperature, SOa
concentration, and relative humidity were
studied. Also, additives that improve
lime's reactivity toward SO2 were
identified.
Experimental Methods
The general design of the experimental
apparatus is given in Figure 1. A
simulated flue gas was synthesized by
combining Ng and SOg from gas
cylinders. The gas flow rates were
measured using rotameters. Water was
added to the system using a syringe
pump (Sage Instruments, Model 341 A)
and evaporated at 120*C in a stainless
steel chamber before mixing with the gas
stream.
The reactor was glass, 4 cm in
diameter, and 12 cm tall. The reactor
was packed with a mixture of silica sand
and Ca(OH)2 reactant in a weight ratio of
40:1. Sand avoids the channeling caused
by lime agglomeration. The silica sand
(between SO and 115 mesh) was
obtained from Martin Marietta Aggre-
gates.
The reactor was immersed in a water
bath that maintained system temperature
within 0.1 "C. Tubing upstream from the
reactor was heated to prevent the
condensation of moisture on the walls.
Before being analyzed, the gas was
cooled and the water vapor then
condensed out by cooling water and an
ice bath. The gas was analyzed for SO2
using » pulsed fluorescent SOg analyzer
Ice Bath
Figurt 1. Experimental apparatus.
(Thermoelectron Corporation Model 40),
and the SO? concentration was
continuously recorded. The SO2 analyzer
was calibrated using a calibration
standard (a mixture of 2000 ppm SOg
and N2). The reactor was equipped with
a bypass, to allow the bed to be pre-
conditioned and the gas flow stabilized at
the desired S02 concentration before
beginning the experimental run. Before
each experimental run, the bed was
humidified by flushing with pure N2, first
at a relative humidity of about 98% for 10
minutes, and then for 8 minutes at the
relative humidity at which the experiment
was to be performed. This humidification
simulated moisture conditions encoun-
tered in the bag filters where the solids
are originally slurry droplets.
The reaction time was normally 1 hr.
The raw data from each experimental run
were curves of SOa concentration
leaving the reactor versus time. These
curves were produced by the recorder of
the S02 analyzer.
The raw data from the experiments
were SO? concentrations from the reactor
as a function of time. By integration of
the SOa concentration over time and a
mass balance on the reactor, the average
fraction of Ca(OH)2 converted was
calculated at each time. As a backup, the
reacted solids were analyzed for sulfite
and hydroxide using acid/base and iodine
titrations.
Most of the experimental work involved
reagent grade Ca(OH)2 as a reactant.
Two batches of Ca(OH)2, identified as
Lime O and Lime A, were used. These
two batches of Ca(OH)g differ slightly in
particle size and BET surface area. The
slurrying and drying process caused a
slight decrease in the surface area of
Ca(OH)2.
An aqueous solution containing the
desired additive was prepared. This
-------
solution (5 ml) was than added to 1 g of
Ca(OH)2 and slurried. The sample was
placed in an oven to dry at 75°C for
about 14 hr, and then sieved to separate
the individual Ca(OH)2 particles prior to
being mixed with the silica and placed in
the reactor.
Model
The equations that describe the
absorption or a component from a
moving gas stream by a fixed solid in a
packed bed consist of two differential
equations obtained from material bal-
ances in the gaseous and solid phases.
Schematically:
dz
r»~ z=L
7"\ Cso,
I '
•hi.
Assuming uniform SOz and Ca(OH)2
concentrations in the r and 0 directions,
and neglecting SOg diffusion and
dispersion in the z direction as well as
the time derivative of the SOg
concentration, the following equations are
obtained:
Vmdcso2
Adz
= rrt
with boundary and initial conditions:
(1)
(2)
(3)
Att =
where:
vm
Cso.
A
volumetric flow rate of gas,
cm^/sec
concentration of SO2 in the
gas phase, gmol/cm3
cross sectional area of the
reactor, cm2
z = length of the reactor, cm
rSO2 = rate of disappearance of
gmol/cm3 sec
Clime = Ca(OH)2 concentration, gmo!/
cm3
t = time, sec
The rate equation for rso2 depends on
the model selected to represent the
kinetics of the reaction.
A shrinking core model with kinetics of
zero order in S02 was chosen to fit the
experimental data. The shrinking (or
unreacted) core model assumes that the
reaction takes place at the exterior
surface of the particle. As the reaction
proceeds, the surface of reaction moves
into the interior of the solid, leaving
behind a layer of inert product. The
external radius of the particle remains the
same, assuming no shrinkage or swelling
of the product layer.
The shrinking core model was orig-
inally developed for the isothermal re-
action of spherical solid particles. The
Ca(OH)2 particles are non-spherical and
have a rough surface, so their surface
area is much higher than that of spherical
particles of the same volume. An em-
pirical expression was introduced to
account for the decrease in roughness as
the reaction progresses. A dimensionless
roughness parameter was defined as:
o =
(4)
where:
A = actual surface area of the lime,
mz/g
A0 = surface area of spherical particles
of equal mass, m/g
When the lime is unreacted, o can be
estimated as the ratio of the BET surface
area of the lime and the surface area
calculated from the Coulter Counter
particle size distribution, assuming
spherical particles. As the reaction
progresses, o should decrease and ap-
proach the limit o = 1.0 when all the lime
has reacted.
An empirical expression of the form:
o =
«*P(aXn +b) (5)
Ume
where Xi,me is the fraction of lime
unreacted, was used to describe the
change of roughness with the reaction.
Equation (5) must satisfy the condition:
At X,. = 1.0 =
lime o
o = BET area/A
o o
(6)
To force a to decrease more rapidly at
high values of X|ime and to simulate the
experimental results, the following
additional condition was imposed:
At X.. = 0.8 o = 2.0
lime
(7)
For the two batches of Ca(OH)2 used
in the experiments (Lime O and Lime A),
the constants a and b in Equation (5) took
slightly different values (a = 12.878 for
Lime O and a = 13.428 for Lime A, and
the values of b were -10.302 and -
10.742. respectively).
Using the roughness parameter o,
Equations (8) and (9) can be obtained for
the rate of disappearance of SC>2. as-
suming that chemical reaction (or S02
diffusion through the gas film and product
layer) is the rate controlling step.
_ _j_
32~V
lime s lime
(8)
(9)
1
V
v-1/3 .
Alim« '
r-g
S02
[
1
4nD NR(oo
e o
1/2
4nR2k No
f o
where:
V = reactor volume, cm3
R = radius of the particle, cm
N = number of Ca(OH)2 particles
Plime = Ca(OH)2 molar density,
gmol/cm3
= kinetic rate constant, cm/sec
'SO2 = S02 concentration at the gas
bulk gmol/cm3
De = diffusivity of SO2 through
product, cm2/sec
kg = mass transfer coefficient,
cm/sec
If the chemical reaction is slow, the rate
of disappearance of S02 will be given by
Equation (8). If the chemical reaction is
fast, all of the SOg that reaches the
surface of the core will be immediately
consumed and the concentration of S02
-------
at the surface of the unreacted core will
become zero. At these conditions the
rates of diffusion of the SOg through the
gas film and product layer will become
the limiting steps, and Equation (9) will
become important.
Of Equations (8) and (9), the one that
gives the lower rate of SOg dis-
appearance will determine the overall
kinetic rate. The parameters of the model
are kg, De, and ks. Equations (1) and (2)
-with the rate of disappearance of
SOa given by Equation (8) and (9)--
were integrated by assuming an average
particle size of the lime particles which
gave the same surface area as the
measured particle size distribution,
assuming spherical particles. The mass
transfer coefficient, kg, was not used as
an adjustable parameter because, at the
conditions at which the experiments were
performed, gas film diffusion is not likely
to be important. The only effect of gas
film diffusion is that it limits the rate
.expression (9) at the beginning of the
reaction where product layer resistance
is zero, as there is no product formed.
The value of kg (544 cm/sec) was
estimated using a Sherwood number of
2, corresponding to mass transfer from
spherical particles in a stagnant fluid.
Thus Oe and ks were the only param-
eters used to fit the experimental data
Depending on whether mass transfer or
chemical reaction is the controlling step,
only one of these parameters may be
important.
A computer program, with variable
step size in time and in distance along
the reactor, was written to model the
reaction. This program uses the IMSL
routine DGEAR to carry out the inte-
gration, and the IMSL routine ICSCCU to
provide interpolated values of lime con-
version needed for each time integration
step.
Results
The effects of relative humidity,
temperature, inlet ?Oa concentration,
and the amount of lime in the reactor on
the reaction of SO2 with powdered
reagent Ca(OH)2 were studied. The
experimental conditions are listed in
Tabtel.
Relative Humidity
Relative humidity was found to have a
dramatic effect on the rate of reaction of
SC>2 with Ca(OH)2 as illustrated by
Rgure 2. The full lines correspond to
experimental results at 2000 ppm inlet
and 66°C using 4 g of Lime 0.
Table 1. Experimental Conditions
Relative Humidify: 17-90°.b
500-4000 ppm
SO2 Inlet
Concentration:
Reactor
Temperature: 30.5-95°C
Nitrogen Flow Rate: 4600 cm3:min (O°C,
1 aim)
Amount of Lime: 1.0-4.0 g
Figure 2 shows that, for all relative
humidities, 100% of the SOa entering the
reactor is being removed during the first
1 or 2 min. of reaction, then the reaction
rate (represented by the slope of the
conversion versus time curves) de-
creases quickly at low relative humidities
but more slowly at high relative humid-
ities.
The broken lines in Figure 2 corre-
spond to the model prediction for these
experiments. Figure 3 shows the
dependence of the rate constant and
solid diffusion coefficient on relative
humidity. At high relative humidity, the
chemical reaction is the controlling step
and ks determines the rate of reaction.
25
20
•8 15
r
4 gLimeO
66°C
2000 ppm SO2
4.6 I/min H
D.>8.£-8
k, = 1.8E-9
Experimental
" Model
10
20 30
Time (minutes/
40
50
60
Figure 2. Effect of relative humidity on reaction rate. Lime 0.
100.00
B
20 30 40 50 BO 70
Relative Humidity l%l
Figure 3. Effect of relative humidity on model parameters.
-------
Any value of De greater than 8.0E-8
cm 'sec will give essentially the same
result. SOa diffusion through the product
layer becomes more important as the
relative humidity decreases. At 19%
relative humidity, Oe determines the rate
of adsorption of S02. At 19% relative
humidity, any value of ks greater than
1.0E-9 cm/sec will give essentially the
same results. Both De and ks are
affected by the relative humidity but. in
the range of relative humidities studied,
ks increases approximately linearly while
De increases exponentially. Because the
SOa diffusion coefficient changes more
rapidly than the kinetic constant, the
controlling mechanism changes when
the relative humidity is increased. The
strong effect of relative humidity on
reaction rate has also been reported by
other researchers.
Inlet SO2 Concentration
The effect of the inlst SOa concen-
tration on the reaction rate was found to
depend on the relative humidity. Figures
4 and 5 illustrate the effect of inlet SOa
concentration at 70 and 50% relative
humidity, respectively. The full lines in
these figures correspond to experimental
results, and the broken lines to the model
predictions. At 70% relative humidity, the
Ca(OH)2 conversion was practically
independent of the inlet SOa con-
centration as can be seen in Figure 4. At
lower relative humidity the reaction rate
is not affected by the SOa concentration
if the SOa concentration is high.
However, at lower levels of SOa tne
reaction rate is affected by the SOa
concentration as illustrated by Figure 5.
The observed effect of SOa c°n~
centratipn can be explained by assuming
that the reaction rate has zero order
kinetics in SOa, but at low relative
humidity and/or low SOa concentration,
SOa diffusion (instead of chemical
reaction) becomes the controlling step.
As can be seen from Figures 4 and 5,
the model predicts the SOa effect with
reasonable accuracy.
25
20
a
70% RH
4 g Lime 0
4.61/minNi
66°C
2150 ppm
1500 ppm
1060 ppm
Experimental
Model: De > r.4£-8
k, = 1.5E-9
0 10 20 30 40
Time (minutes!
Figure 4, Effect of SO2 concentration, high relative humidity.
50
60
25
20
a
5 10
I
50% RH
4 gLimeO
4.61/minNt
66°C
4000 ppm
2050 ppm _ ^. *•
1100 ppm
D. = 1.4E-8
, = 7.55-9
0 10 20 30 40
Time fminutesl
Figure 5. Effect of SOi concentration, moderate relative humidity.
50
60
Amount of CafOHfe In the
Reactor
The effect on the average Ca(OH)a
conversion of changing the amount of
Ca
-------
keeping all other variables constant. The
relative humidity was 74%. so in this
region the reaction is expected to be
kinetically controlled.
The broken lines in Figure 7 are the
model predictions for the experiments
run at the two different temperatures. At
the conditions at which the experiments
were performed, the reaction rate is
kinetically controlled, so ks is the only
important adjustable parameter in the
model. By using the values of ks given
by the model, an apparent activation
energy of 2.9 kcal/gmol can be estimated
for Ca(OH)2. This value of activation
energy is somewhat lower than the value
of 6 kcal/gmol reported by other sources
for this reaction.
A very weak dependence of the
reaction rate with temperature was also
reported by other researchers.
Additives
Two organic acids (adipic acid and
glycolic acid) and three organic de-
liquescents (ethylene glycol. triethyl-
ene glycol, and monoethanolamine) were
selected as test additives for Ca(OH)2.
All of them proved to be detrimental to
the reaction of SOa with Ca(OH)2-
A number of deliquescent salts were
also tested as additives at 74 and 54%
rela'ive humidity. The beneficial effect of
the salts depends on the type and
amount of salt and the relative humidity.
At high relative humidity (74%), all the
deliquescent salts tried v,ere successful
in increasing the reactivity of the
Ca(OH)2 toward 862. At a lower relative
humidity (54%), some of the salts did not
perform as well, and some, such as
Ca(N03>2, did not have any beneficial
effect.
The water activity over saturated
solutions of the salts (70°C and 1 atm) is
about equal to the fractional relative
humidity of the gaseous phase that
would be in equilibrium with a saturated
solution of the salt at that temperature. If
one of these deliquescent salts is
contacted with a gaseous phase of
relative humidity greater than the water
activity, the salt will capture water from
the gas phase and become a solution.
This tendency to capture water has been
extensively documented in the literature
by studies of the growth of salt
containing aerosols as a function of the
atmospheric relative humidity. From data
in the literature, it is clear that (based on
deliquescence alone) most of the salts
tested, specifically NaNO3 and all the
chlorides except LiCI, should not have
any beneficial effect at 54% relative
I
n
!
20
18
16
14
12
10
8
6
4
2
0
€
c
.0
5
I
1000 ppm SO.
50% RH
4.6 1/minNi
66°C
Figure 6.
0 10 20 30
Time Iminutesl
Effect of Ca(OHl2 loading, 1000 ppm SO*
SO
60
25
20
IS
r
74% RH
500 ppm SOi
1 g Lime A
4.6 I/mm Ni
64.4°C
Experimental
Model: D« > 8.E-8
1O 20 30 40
Time (minutes)
SO 60
Figure 7. Effect of reactor temperature. Lime A.
humidity. Nevertheless, these salts are
among the ones that behave the best at
that moderate relative humidity. A
possible explanation for this finding
would be a hysteresis phenom-enon;
this will be discussed later.
The report also shows the values of
the diffusion coefficient of SO2 through
the product layer (De), and the kinetic
constant (ks) that can be used to
simulate the experimental runs using salt
additives. De values ranging from 1.5E-
B to 120E-8 (cm2/sec) and ks values
from 1.5E-9 to 8.23E-9 (cm4/gmol sec)
were used in the simulation of the salt
experiments. Depending on tha amount
and type of salt added, chemical reaction
or SOg diffusion can become the
controlling step. A reasonably good
agreement was found between the
predictions of the model and the
experimental results. The maximum
percentage of error between the
experimental data and predicted values
-------
_
I
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c |5
+
iu
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is
0123456789 10
Mole % NaCI
Figun 9. Effect of NaCI on model parameters.
was below 10% In most cases; \n a few
exceptions, it was below 20%.
The in'Lance of the salt concentration
on the 802 reaction rate is illustrated by
Figure 6. The salts used were NaCI and
NaNOa in concentrations ranging from 1
to IS mole%. The experiments were
carried out at a relative humidity of 54%,
and a reactor temperature of 66°C.
Figure 8 shows that the conversion
increases with increasing concentration
of additive until about 10 mole%. After
this the curve levels off. The optimum
concentration of additive is then about 10
mole% for 1:1 electrolytes like NaCI and
NaN03.
The deliquescent salts affected the
model parameters similarly to the relative
humidity: the diffusion coefficient
increased more rapidly than the kinetic
constant by the addition of the salts.
Figure 9 illustrates the effect of NaCI on
the model parameters. The kinetic
constant increased linearly when NaCI
concentration was increased from 0 to 10
mole%. while the diffusion coefficient
increased exponentially after a sharp
increase from 0 to 1 mole% NaCI.
As mentioned earlier, before each
experimental run the fixed bed was
prehumidified by flushing with pure N2 at
a relative humidity of about 98% for 10
minutes before flushing with Ng at the
relative humidity at which the experiment
was to be performed.
This prehumidification could be why
some of the salts were still effective at a
relative humidity lower than the one
predicted from equilibrium consider-
ations. Due to hysteresis it is possible
that, when the relative humidity was
lowered to the experimental conditions
after the prehumidification, some excess
water remained in the solids. Strong
hysteresis effects have been reported in
NaCI aerosols.
To check if hysteresis was responsible
for the beneficial effect of some salts at
low relative humidities, experimental runs
were made omitting the prehumidification
step. Table 2 shows the results obtained
at 54 and 17.4% relative humidity with
and without prehumidification of the bed
at 98% relative humidity. The additives
used were NaCt. NaNO3, and KCI. At
54% relative humidity, even when some
decrease of the Ca(OH)2 conversion was
found without the prehumidification, the
results with additives were still far
superior to those with the pure Ca(OH)£.
Hysteresis then, cannot explain all of the
beneficial effect observed at 54% relative
humidity. At 17.4% relative humidity, all
the beneficial effect with NaCI appears to
be due to prehumidification of the bed;
i.e., due to a hysteresis phenomenon.
Conclusions
As shown earlier, the relative humidity
of the gaseous phase is the most
important variable in the reaction of SO?
with Ca(OH)2 solids. This result agrees
with results reported ii the literature for
S02 removal in the bag filters of spray
dryer pilot and commercial plants. The
other variables tested (i.e., temperature,
amount of Ca(OH>2, and SO; con-
centration) have less impact on the
reaction rate. The different effect of SOg
concentration at low and high relative
humidities c?n be explained by assuming
that the reaction has zero order kinetics
in S02 and that, at low relative humidity,
the reaction rate is mass transfer
controlled while at high relative
humidities the reaction is controlled by
reaction kinetics.
-------
Effect of Prehuman/cation of the Bed at 98% flH on CafOH)2 Reactivity
500 ppm SOj, 1.0 g Ca(OH)2 A, 4.6 llmin (O'C, 1 aim) W2
Average Ca(OH)2 Conversion after 1 hr 1%)
Additive (Mole%)
54% AH
66'C
Prehumidified
Yes
No
17.4% RH
95'C
Prehumiaified
Yes
None
70S NaCI
10% N«M03
fOKKC/
11.2
27.0»
27.2s
37.3
-
23.2
23.7
19. 3
4.0
9.7
779
3.4
-
4.0
-
—
humidity: the diffusion coefficient in-
creases more rapidly than the rate
constant. When increasing amounts of
the same salt (NaCI) were added, ks
increased linearly and De nearly ex-
ponentially. Depending on the amount
and type of salt additive, chemical
reaction or gas diffusion can be the
controlling mechanism.
The kinetic constant was a very weak
function of temperature. The estimated
activation energy was 2.9 kcal/gmol.
Average of two nxperimsntal runs.
Most of the deliquescent salts tried
effectively increased Ca(OH)a reactivity
toward SOj. The extent of the beneficial
effect was a function of the type of salt.
the salt concentration, and the relative
humidity. Some salts are effective at a
lower relative humidity than would be
predicted from their deliquescent
properties. Hysteresis due to pre-
humidification of the bed appears to be
partially responsible for this behavior, but
it cannot explain all of the reactivity
improvement observed at 54% relative
humidity. An alternate explanation
proposed is that the chlorides and
NaNOa modify the properties of the
product CaSOs 1/2H2O layer that is
formed as the reaction takes place.
thereby facilitating the access of the SO?
to the unreacted Ca(OH)z which remains
in the interior of the particle. NaCI and
CaCfe have been reported to enhance
the SOz reactivity of limestones in
fluidized-bed combustion by affecting
the pore structure of the lime during
calcination, which then increases the
extent of sulfation of the limestone.
The only previous modelling effort for
this reaction used an integral shrinking
core model with only reaction kinetics to
explain the dependence of reaction rate
on lime conversion. That effort's
experimental data fit this integral
shrinking core model only after a certain
lime conversion had been reached. The
sharp decrease in reaction rate observed
at initial times was attributed to a
decrease in surface roughness, but no
attempt was made to correlate this
decrease in surface roughness with lime
conversion. The model neglected the
effects of SOz diffusion through the
product layer, and the SOg concentration
and Kme concentration profiles in the
fixed bed reactor. All of these factors will
be more important at initial times, when
the SOa removal is higher, so it is not
surprising that the experimental data
could be fit only at later times.
The simple model presented here
seems able to predict with reasonable
accuracy the effect of all the process
variables tested and explain the trends
observed in the experimental data. The
experimental data was estimated to have
±11% experimental error, so that most
predictions of the model are well within
the range of experimental error. The
values of the diffusion coefficient used in
the modelling (from 0.75E-9 to 1.20E-
6} seem reasonable for diffusion of SOg
in a solid, as they are of the same order
of magnitude of the diffusivities of gases
in polylmers.
The relative humidity affects both the
diffusion coefficient of SO2 and the
kinetic constant. In the range of relative
humidity studied (19 to 74%), the kinetic
constant increased linearly with relative
humidity, while the diffusion coefficient
increased exponentially. This depend-
ence of the parameters on the relative
humidity leads to a change in the
reaction controlling mechanism as the
relative humidity decreases. At high
relative humidity and/or high SOa
concentration, the reaction rate is
kinetically controlled and the reaction
rate is independent on the SOg level. At
low relative humidity and/or low SOz
concentration, the controlling step is the
diffusion of the S02 through the
CaS03-1/2H2O product layer. At these
conditions the overall reaction rate
becomes affected by the SO?
concentration which is the driving force
for diffusion.
The addition of deliquescent salts
increases the diffusion coefficient and
the kinetic constant similarly to relative
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R. Ruiz-Alsop and G. T. Rochelle are with the University of Texas at Austin,
Austin, TX 78712.
Charles B. Sedman is the EPA Project Officer (see below).
The complete report, entitled "Effect of Relative Humidity and Additives on the
Reaction of Sulfur Dioxide with Calcium Hydroxide," (Order No. PB 88-234
1741 AS; Cost: $25.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-88/037
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