United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-058 June 1984
Project Summary
-—I
H
Buffer Additives for
Lime/Limestone Slurry
Scrubbing: Sulfite Oxidation with
Enhanced Oxygen Absorption
Catalyzed by Transition Metals
Richard K. Ulrich and Gary T. Rochelle
Sulfite oxidation was studied by
measuring the rate of enhanced oxygen
absorption across an unbroken interface
into solution containing sulfite (2-100
mM) and catalyst (0.01 to 100 mM) at
pH 4-6 and 50°C. Fe, Mn, Co, Cu, and
Cr ions were potent catalysts under
these conditions; Ni was inactive. At 10
mM, these catalysts gave pseudo-first
order (in oxygen) rate constants of 8.6,
43. 4.7, 95 and 11 sec, respectively.
Dry catalyst added in its upper valence
state (ferric, cupric, chromic) produced
high initial rates that fell in 1 to 6 hours
to steady state, while catalyst added in
its lower state (ferrous, manganous,
cobaltous) showed no high rates and
reached steady state in less than 1
minute. Ferric and ferrous eventually
resulted in the same rate. Fe was a
much stronger catalyst than Mn or Co,
but its rate was limited by Fe solubility
of about 0.01 mM at pH 5, resulting in
an enhancement factor of 2.4 at all
higher concentrations. Thiosulfate
(0.05-1mM) had a stronger inhibiting
effect and efficiency on Mn than on Fe.
EDTA was an effective inhibitor for Fe
at equal or greater concentrations.
Rates for Fe and Co increased with pH
from 4 to 5, while those for Mn were un-
changed. Strong positive Mn-Fe syner-
gisms were found to cause absorption
rates of up to five times those expected.
Dissolved Fe is probably the most
important indigenous catalyst in aque-
ous scrubbing for flue gas desulfuriza-
tion.
This Project Summary was developed
by EPA'5 Industrial Environmental
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
The liquid-phase sulfite oxidation
reaction involves adding an oxygen atom
supplied by dissolved oxygen to an S(4)
species, sulfite or bisulfite, to produce
S(6), sulfate or bisulfate. Th3 pH fixes the
distribution among protonated and
unprotonated species, but the reactants
and products are usually referred to as
simply "sulfite" and "sulfate." This
reaction is a very complex free radical
sequence involving a number of highly
reactive intermediate species. It is
catalyzed by micro-molar or higher
amounts of dissolved transition metal
ions having more than one valence state
and is very sensitive to impurities in
general. Recent studies indicate that the
reaction does not proceed at all in the
absence of a catalyst. The observed
kinetics are also sensitive to experimental
conditions so that there is no common
agreement on rate equation forms, rate
constants, or even reaction orders. This
widely varying behavior indicates that
different elementary reactions become
important at different conditions and
control the rate, contributing to the
complexity of the problem.
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The two main engineering uses of
sulfite oxidation are to determine mass
transfer characteristics in gas/liquid
contacting equipment, and to oxidize
calcium sulfite waste products in flue gas
desulfurization (FGD). Scientific interests
focus on the reaction's role in acid
precipitation and in free radical reactions
in general. Also, the sulfite oxidation
reaction has many features in common
with the solution-phase free-radical
oxidation of hydrocarbons, so there are
some opportunities for information cross-
over.
The purpose of this project was to study
sulfite oxidation under FGD conditions.
These conditions differ from those
usually used in other studies which tend
to be: (1) mass transfer characterization
projects at higher S(4) concentrations
and pH but lower catalyst levels, or (2)
acid precipitation studies at trace amounts
of S(4) and catalyst and lower pH. The
experiments were performed at pH 4 to
6 and 0.01-0.03 M sulfite to simulate
conditions in typical FGD oxidizing units.
The reaction occurred under heterogene-
ous conditions where oxygen was absorbed
out of air at 50°C into an agitated,
unsparged sulfite/catalyst solution typi-
cally containing 0.1 to 100 mM of Mn, Fe,
Co, Cu, Cr, or Ni. Under these conditions,
reaction kinetics were so fast that all of
the incoming oxygen was consumed in a
thin reaction zone at the gas/liquid
interface, and the bulk solution oxygen
concentration was maintained near zero.
The absorption rate was measured, using
the pH-stat method, which allows solution
concentrations to remain constant indefi-
nitely while the absorption rate comes to
steady state.
The scope of this project was to
determine the important factors in fixing
the reaction rate (and therefore the
absorption rate) under FGD conditions
and to elucidate some aspects of the
reaction mechanism when common FGD
catalysts are present. The principal
solution effects that were considered
included catalyst concentration, S(4)
concentration, pH, and agitation rate.
Catalytic synergism was studied by
having two catalysts present simultane-
ously in the solution. The results of these
experiments were correlated to hypothe-
sized reaction steps with equations
describing simultaneous reaction and
mass transfer under these conditions in
order to judge the validity of proposed
reaction steps.
Theory
Both surface renewal and film theories
were used to model the processes
2
occurring in the system according to the
suitability of each to specific applications.
Reaction rate constants and reaction
orders were correlated with a Danckwerts
surface renewal equation that is genera-
lized to include any values of reaction
orders: 1/2
2
k, [S(4)jr [cat]P [O2],m
R=R<
1 +
m + 1
(Eq. 1)
where: R=the absorption flux,
gmols Oa/cm2 sec
R0=the physical absorption
flux = k0xfO2],, 1x10~9at
400 rpm agitation
kr=the homogeneous re-
action rate constant
m, n, p=homogeneous reaction
rate orders
k0x=the unenhanced liquid-
phase mass transfer
coefficient, 8.2x1 0"3
cm/sec at 400 rpm
The right-hand term in the brackets
dominates the unity at enhancement
factors above about three. One basis for
comparing catalysts was the first order
rate constant with respect to oxygen,
ki =k,[S(4)],1cat],pinEq. 1. Rearranging this
equation gives:
k, = _*°L. (E2 - 1 ) = 1 .81 (E2 - 1 ) sec"1
(Eq. 2)
where: E = the enhancement factor, R/R0
Another basis of comparison was estab-
lished so the absorption rate data from
these experiments could be converted to
a homogeneous reaction rate for compari-
son to literature values. The quantity rS4 is
defined as the average homogeneous
rate of S(4) consumption in the interfacial
reaction zone in gmols S(4)/liter sec and
is equal to twice the rate of oxygen
consumption, rox. roxisequaltoki times the
concentration of oxygen in the reaction
zone, [O2],/2:
rs* = 2ki([02l,/2) (E2 - 1) =
(2.71 x 10~4) (E5 - 1) M S(4)/sec
(Eq. 3)
Two coupled film theory models were
written for the interfacial region: one
describes the reaction zone at the
interface, the other describes transport
between the bulk solution and the
reaction zone.
Since the detailed mechanism of sulfite
oxidation is not known, a generalized free
radical mechanism was derived and
used in conjunction with equations
relating the oxygen absorption flux (R) to
the velocity of the homogeneous reactions
(rs*) to model the effects of changing
catalyst and inhibitor concentrations on
rs4 (and therefore on the observed
absorption rate). This generalized mecha-
nism involves some hypothesized initia-
tion and termination reactions (under-
stood to some extent), and uses a
generalized form for propagation reac-
tions, which are very numerous and
almost never known with any certainty in
sulfite oxidation. This model suggests
that the observed change in reaction
order with respect to Mn catalyst from 1
to 0.5 at 10 mM could be due to a change
from first to second order termination
kinetics as the free radical population in
the reaction zone increases. Including an
inhibition step in the model resulted in a
rate expression identical to the observed
relationship for inhibition by thiosulfate
(Eq. 4).
A film theory model was solved on a
computer to estimate the differences
between bulk and interface (reaction
zone) conditions during enhanced oxygen
absorption. It was found that, under FGD
conditions, significant lowering of pH and
S(4) concentration in the reaction zone
could occur. The cause of this increased
sensitivity at FGD conditions is the low
levels of S(4) which act as both a reactant
and, at the pH values of interest in this
project, a buffer. Reaction at the interface
can significantly decrease the interfacial
sulfite concentration, which depletes its
buffering ability and allows the interface
to achieve pH values that are up to two
units lower than the bulk pH of 5. At 10
mM, the S(4) mass transfer limited
enhancement factor would be about 33.
The partial pressure of SOa above the
solution is predicted to increase because
of this lower pH at the surface. A pH 5 and
1O mM sulfite solution would have a SOa
partial pressure of 20 ppm; but, at an
enhancement factor of 15, it could rise
almost to levels that would be expected in
flue gas from some coal-fired boilers(500
ppm).
Results and Discussion
The measured oxygen absorption rates
for single catalysts (Figure 1) were
correlated with Eq. 2 and 3 to obtain the
kinetic information in Table 1. The slopes
of the lines in Figure 1 equal half the
catalyst reaction order, so Mn shifts from
first-to half-order at 10 mM, possibly due
to a change in termination reactions at
the higher reaction rates; Co tends
toward first order, and Ni shows too little
enhancement to determine a slope.
Under the same conditions, Fe catalyst
showed apparent zero order kinetics at
E=2.4 at pH 5, making it the most potent
catalyst below 1 mM. The homogeneous
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70
Figure 1.
Catalyst Concentration. M
Effect of Mn, Co, Fe, and Ni catalyst concentration on the enhancement factor at
10 mM S(4)and pH 5.
oxidation rates in the reaction zone were
on the order of 1 mM S(4)/liter sec.
Table 1. Comparison of Catalytic A ctivities
During Enhanced Oxygen Absorp-
tion into 10 mM S(4) and3OO mM
S(6) at pH 5, 50°C, and 400 RPM
Agitation
mM catalyst £a *," rs«c
5
10
50
0.1
0.5
5
10
50
5
10
50
10
50
10
10
Mn
Mn
Mn
Fe
Fe
Fe
Fe
Fe
Co
Co
Co
Ni
Ni
Cr
Cu
2.5
5.0
7.5
3.1
3.1
2.6
2.4
2.4
1.5
1.9
3.2
1.1
1.2
2.7
7.3
9.5
43
99
16
16
10
8.6
8.6
2.3
4.7
17
11
94
0.0011
0.0050
0.0120
0.0019
0.0019
0.0012
0.0010
0.0010
0.0003
0.0005
0.0020
0.0013
0.0110
"£ = enhancement factor.
b*i = first order fin Oz) rate constant, sec"1
Crs4 = average homogeneous oxidation rate in
the reaction zone, gmols S(4)/liter sec.
Comparisons were made between rs4
values in Table 1 and homogeneous
reaction rates from published rate expres-
sions extrapolated to the conditions of
this project. The expressions were
evaluated at 5 mM Mn or Co and 0.1 mM
Fe (the lower level for iron was used since
experiments indicate that the catalyst is
solubility-limited above this point) and
were found to give rate values higher
than those reported here for Mn and Co
but seemed to be of the right order of
magnitude for Fe. Most of these expres-
sions came from experiments performed
at pH values comparable to those in this
project, but they were mostly at lower
sulfite and catalyst concentrations in
order to obtain rates low enough to use
homogeneous reactors. The agreement for
Fe catalyst may be the result of the
published rate expressions' being extra-
polated less for this case.
The distribution of catalyst valence
states is at steady state since the catalyst
ion (reduced during initiation) reoxidized
by dissolved oxygen in the reaction zone
or by other oxidizing species in solution.
Experiments, involving adding catalysts
to the solution in different valence states,
and literature data support the idea that
the upper valence state of the metal ion is
the catalytic agent, acting by removing an
electron from an S(4) ion to create a free
radical chain reaction in the reaction
zone.
The apparent zero-order kinetics of Fe
catalyst were due to solubility limitations
on a catalytically active ferric species.
Hydrated Fe does not have a sufficient
oxidizing potential (0.77 V) to remove an
electron from sulfite (0.89 V) but FeOOH
might (0.908 V). Fe scans at 0.001 to 0.1
mM (Figure 2) showed a change of
reaction order from one to zero at some
pH-dependent Fe concentration. These
breakpoints were due to the catalytically
active species' reaching a solubility limit
at 0.01 mM total Fe at pH 5. The solubility
of FeOOH is much lower (about lO^mM)
and, if it is in fact the active species, its
concentration reaches this point at 0.01
mM total Fe. These breakpoints shifted to
higher concentrations at lower pH due to
the increased solubility of the ferric
species. The reaction rate with Fe catalyst
was higher at higher pH because the
catalyst regeneration reaction (ferrous
oxidation to ferric) is base-catalyzed. Red-
brown Fe precipitates were visible at total
Fe concentrations above 0.5 mM at pH 5.
The amounts of these solids were
observed to increase with time and pH,
and were found to be catalytically
inactive.
FGD systems operating under the
conditions of this project with Fe catalysis
would be expected to show enhancement
factors that are independent of pH. A
decrease in pH retards the Fe-catalyzed
oxidation reaction (Figure 2), but the
increase in total dissolved S{4) concen-
tration, together with the observed half-
order dependency on S(4), results in
equal offsetting effects.
Thiosulfate was found to inhibit the Mn
and Fe catalyzed rate by acting as a free
70
• i
| I I I L
70-5
Iron Concentration. M
10-*
Figure 2.
Effect of low Fe concentration on the enhancement factor at 30 mM S(4) and various
pH values.
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radical scavenger. It was degraded only in
the presence of sulfite oxidation in a few
hours, and was consumed at a rate
proportional to the remaining thiosulfate
concentration raised to the three-halves
power. Thiosulfate additions of 0.05 to
0.15 mM to Mn systems completely
recovered to the uninhibited rate in 1 to 3
hours. No significant induction time
followed the addition of thiosulfate, and
the absorption rate recovered linearly
from the lowered value. The absorption
rate was inversely proportional to the
square-root of the thiosulfate concentra-
tion (Figure 3), and this rate form was also
successfully modelled by the generalized
radical mechanism. At 30 mM Mn, 10
mM S(4), and pH 5:
E = 6.17
(1 + 46.8 [S2O3"2, mM])1/2
(Eq. 4)
Thiosulfate required between one and
two orders of magnitude more concentra-
tion to have the same percentage of
retardation on Fe catalyzed systems than
on those with Mn. Also, each thiosulfate
ion prevented the oxidation of about 40
sulfite ions with Mn, but prevented only
about 3 with Fe. Therefore, the radicals
that react with thiosulfate must be
generated more rapidly by Fe catalyst
than by Mn.
EDTA (ethylenediamine-tetraacetic acid)
retarded the reaction by chelating metal
ions, thereby sterically hindering their
catalytic action. The Fe/EDTA complex
seemed to be totally inactive. With 5 mM
Fe giving obvious formation of a precipit-
ate, an equivalent amount of EDTA was
required. Therefore, EDTA would not be
an effective inhibitor in systems with an
excess of iron salt solids.
The rate of oxygen absorption into
sulfite/catalyst solutions under FGD
conditions was measured as a function of
the agitation rate (Figure 4), to estimate
the liquid-phase mass transfer coefficient
and the interfacial contact area, using a
method that has the advantage of not
requiring detailed prior knowledge of the
reaction kinetics. The bottom curve is the
physical absorption MnesinceO.1 mMMn
is enough to deplete the bulk-phase
oxygen, but not enough to cause enhance-
ment (Figure 1). The upper absorption
rate curves were made with enough
catalyst to cause enhancement at low
agitation rates, but E falls to unity when a
catalyst's line converges with the physical
absorption line. The calculated values of
k0> and A were found to be much too large
and small, respectively, at high agitation
intensities, perhaps due to a slowing
down of the local reaction rate. This effect
has been observed by other investigators
0.14
0.12 —
O.JO —
0.08 —
0.06 —
0.04 —
0.02 —
0.00
— 3
0.02 0.04 0.06 0.08
Thiosulfate Concentration, mM
0.10
Figure 3. Decrease of enhancement due to adding thiosulfate at 30 mM Mn. 10 mM S(4),
and pH 5.
and is hypothesized to occur because the
average length of time that fluid elements
spend at the surface is shorter than the
induction time of the free radical reaction
in the elements: the average rate is less
than that which would be observed at
steady state.
The phenomenon of catalytic synergism
was studied by measuring the rate with
two catalysts in the solution simultan-
eously. Strong positive interactions were
found for the Mn-Fe couple, and strong
negative interactions for the Mn-Cu pair.
Some combinations of Mn and Fe were
strong enough to cause the rate to reach
the S(4) mass transfer limit at E=33, so no
kinetic information could be developed on
faster rates. Adimensionless "synergism
coefficient" was defined as the ratio of
the absorption rate observed with both
catalysts present to the absorption rate
which would be expected from surface
renewal theory if the catalysts did not
interact. The separate rates cannot be
added linearly (as can homogeneous
rates) because of the effects of mass
transfer considerations, so a more
involved approach is necessary. This
synergism coefficient was found to be as
high as 5 for Mn-Fe, and as low as 0.1 for
Mn-Cu.
Conclusions
Results of the study led to several
conclusions:
(1) Fe, Mn, Co, Cr, and Cu are potent
catalysts under FGD conditions, but their
relative activities depend on the specific
conditions. Significant enhancement
factors could be possible in actual
scrubbers at Mn and Fe concentrations
typically observed. Fe alone could give
enhancements of 2 to 5 depending on pH
and S(4) concentration, while Fe-Mn
synergisms could result in enhancements
as high as 30. FGD units operating under
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these conditions with Fe catalysis would
exhibit enhancement factors that are
independent of pH.
(2) The upper state of multivalent
transition metal ions initiates the free
radical reaction chain by removing an
electron from a sulfite or bisulfite ion. The
catalytic mechanisms of Fe and Mn are
quite different. Fe generates free radicals
at a faster rate. Therefore it comes as no
surprise that organic acid degradation is
less severe in the presence of Mn.
(3) Some catalytically active ferric ion is
solubility-limited, resulting in apparent
zero order kinetics above 0.01 mM total
Fe at pH 5. Precipitated Fe solids are
inactive.
(4) 0.05 to 0.15 mM thiosulfate
effectively inhibits the reaction catalyzed
by Mn, but has less effect on Fe systems.
Oxidation in FGD systems can be effective-
ly inhibited by thiosulfate, although it
degrades rapidly, especially with Fe
catalysis.
(5) EDTA inhibits Fe catalysis at equal
or higher concentrations by chelating the
ferrous and ferric ions into an inactive
complex. EDTA would not be an effective
catalyst in FGD systems, since enough
EDTA would have to be added to complex
all of the dissolved and precipitated iron.
(6) Significant reductions in pH and
[S(4J] can occur in the interfacial reaction
zone under FGD conditions due to the
combination of high absorption fluxes
and low bulk S<4) levels.
(7) Oxygen absorption from air into
sulfite/catalyst solutions under FGD
conditions as a method of mass transfer
characterization can lead to overestima-
tion of the liquid-phase coefficient and
underestimation of contact area, perhaps
due to reduced reaction rates at higher
agitation intensities. The enhancement
factors measured in this project may not
be directly applicable to a real scrubber
because of changes in kox and variation of
the heterogeneous reaction kinetics with
agitation.
II
II
1*0.5
0.1
I
I
I
100
10 u
^
200 400 600 800
Agitator Speed, rpm
1000
1100
Figure 4.
Effect of agitator speed on the total oxygen absorption rate, Mn andMn-Fe synergistic
catalysts at pH 5.
R. K. Ulrich and G. T. Rochelle are with the University of Texas, A ustin, TX78712.
J. David Mobley is the EPA Project Officer (see below).
The complete report, entitled "Buffer Additives for Lime/Limestone Slurry
Scrubbing: Sulfite Oxidation with Enhanced Oxygen Absorption Catalyzed by
Transition Metals," (Order No. PB84-189 950; Cost: $17.50, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/981
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