Conclusions
From these studies, it can be concluded that cocurrent
contacting of vapor and liquid streams is a very satisfactory method to utilize for the design of a multistage
vapor-liquid equilibrium unit.
A contactor, six in. in length, with an annular space
of %- or %-in., packed with Metex screen, will be 100%
efficient at throughputs up to 1000 grams per hr per
cm2, which corresponds, respectively, to 6 and 28 liters
of liquid per hr.
To realize this efficiency at higher throughputs, a longer
packed contactor can be utilized.
Nomenclature
=
more
b
=
less
E.F.
=
=
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volatile component
volatile component
enrichment factor (y0/yt)(x6/xa)
number of theoretical stages
a
y
=
x
=
a
=
concentration m vapor, mol %
concentration in liquid, mol %
relative volatility
Literature Cited
Fenske, M. R., Ind. Eng. Chem., 24, 482 (1932).
Gelus, Edward, Marpie, Stanley, Miller, . E., ibid., 41,
1757 (1949).
Herring, J. D., MS Thesis, Pennsylvania State University,
University Park, Pa., 1948.
Kirk, Norman, PhD Thesis, Pennsylvania State University, University Park, Pa., 1946.
McCormick, R. H., Barton, P., Fenske, M. R., AIChE
J., 8, 365 (1962).
Myers, H. S., Petrol. Refiner, 3, 175 (1957).
Prengle, H. W., Palm, G. F., Ind. Eng. Chem., 49, 1769
(1957).
Received for review August
Accepted March
=
14, 1970
15, 1971
Selection of Metal Oxides for
Removing SO2 From Flue Gas
Philip
S.
Lowell1, Klaus Schwitzgebel1, and Terry B. Parsons1
Tracor, Inc., Austin, Tex. 78721
Karl J. Sladek2
Department of Chemical Engineering, University of Texas at Austin, Austin, Tex. 78712
Oxides of 47 elements were evaluated for use as sorbents for removing SO2 from
flue gas, in processes based upon thermal regeneration of sorbent. Thermodynamic
analysis plus literature data were used to evaluate reaction paths for each oxide.
The most important sorbent requirements involve sulfite and sulfate decomposition behavior.
Sulfites were grouped according to the extent of disproportionation to sulfide and
sulfate during decomposition. Sulfate groups correspond to simple decomposition, decomposition to oxysulfates, and decomposition with valence change. Examination of all
the relevant data indicated that oxides of Al, Bi, Ce, Co, Cr, Cu, Fe, Hf, Ni, Sn,
Th, Ti, V, U, Zn, and Zr are most promising.
It
is widely recognized that the emission of sulfur oxides
from fossil fuel combustion causes a serious atmospheric
pollution problem. A variety of methods have been proposed for removing sulfur compounds either from fuels
before combustion or from flue gas afterward (7, 36).
Processes for treating flue gas include wet scrubbing, gasphase reaction to make a removable solid or liquid product,
sorption by a nonregenerable solid, and sorption by a
regenerable solid. A process of this last type, described
recently by Newell (30), employs sorption of sulfur oxides
on an alkalized alumina, which is an active form of NaA102,
and regeneration with reducing gas to produce H2S. The
cost of producing reducing gas is a major operating cost
item in this process.
1
2
Present address, Radian Corp., Austin, Tex. 78758
To whom correspondence should be addressed.
384
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
This paper covers dry metal oxide processes which produce S02, SO3, or H2S04 as by-product. It attacks the
problem of selecting from 47 pure metal oxides and an
additional group of binary metal oxides those which are
most promising for a thermally regenerable process. The
use of thermal, rather than chemical, regeneration should
result in operating cost advantages.
A flow diagram illustrating the type of process and
the sorbent requirements is given in Figure 1. Feed flue
gas containing S02, 02, N2, H20, and C02 enters the
sorber, which operates at temperature T$, and is contacted
with regenerated sorbent. For a coal-burning power plant,
typical flue gas S02 content would be 0.05 to 0.3%, and
typical 02 would be 2.0 to 3.5%. A lower limit of 100° C
is chosen for Ts to minimize corrosion and to prevent
plume droop and resulting localized pollution. The critical
sorption requirement is that the exit S02 content must
’
be 5 0.015%,
content is set
or that Pso„ á 0.00015 atm. The exit
arbitrarily at 0.028 atm.
02
Spent sorbent is recycled to the regenerator, and is
contacted with purge gas, which may contain 02 or may
be 02-free. The regenerator is maintained at a higher
temperature, TR, to favor release of sulfur oxides. An
upper limit of 750° C is chosen as the maximum feasible
temperature for use with available materials of construction. The exit regenerator requirement is that Pso, +
0.01 atm, which is chosen so that S02, SO3, or
Pso,
sulfuric acid by-product production will be feasible.
These process requirements can be translated readily
into thermodynamic requirements on the sorbent. The
sorbent must be capable of reducing Ps0, to below 0.00015
100° C. As will be shown later, a large
atm for T
number of oxides obey this condition. The really critical
requirement is to obtain during regeneration a PSq, +
Pso, greater than 0.01 atm at a temperature below 750° C.
This will eliminate many oxides which form sulfites and
sulfates which are very stable. A good example is the
alkalized alumina sorbent (30), which forms a product that is too stable to be regenerated thermally.
Qualitatively, a thermally regeneradle sorbent must
undergo sorption with only a small negative standard
free energy change, aG°. Then, a moderate increase in
temperature will reverse the sign of AG° and make regeneration possible.
Before writing these requirements in terms of equilibrium constants, it is necessary to identify the important
reactions.
=
really consisted of rejecting metal oxides exhibiting
unfavorable behavior. The authors suggest that an oxide
which is unsuitable on the basis of its behavior in metaloxygen-sulfur reactions should be unsuitable also in the
more complicated case where reactions involving C02 and
H20 occur.
The chemistry of metal-oxygen-sulfur systems is quite
varied. For example, Cu forms compounds in both +1
and +2 valence states; in addition at least one oxysulfate,
CuSCh-CuO, is known. For sorption on Cu20, one could
envision the following reactions:
3/2 CuS04
Cu20 +1/2-^>2CuO +2S°2-
2
/
+
1/2 CuS
CuS03
(2)
^
+S02
-*-Cu2S03--'02» Cu2S04 ~1-·°
»
2 C11SO4
CuS04·CuO
For regeneration to Cu20
2 CUSO4
~S°3
»
CUSO4 CuO
·
~S°3
-
»
2
CuO-·--*»
Cu20
(3)
Reactions and Compounds
A convenient reaction on which to base the desired
process would be reversible sulfite formation; for a divalent
oxide
MeO + S02 % MeSOa
(1)
metal. However, the actual chemistry
is much more complicated since the flue gas contains
not only S02 but also substantial amounts of H20, C02,
and 02. Thus, besides sulfites and oxides, the reactions
could involve hydroxides and hydrates, carbonates, and
sulfates. It seemed worthwhile to base the present metal
oxide selection on reactions involving sulfur compounds.
Consequently, only reactions in metal-oxygen-sulfur systems were considered; hydroxides, hydrates, and carbonates were ignored. As will be shown later, the selection
where Me is
a
Feed Sorbent
2
CuS03 ~2S°2> 2 CuO—2°2» Cu20
(4)
Besides these bulk reactions, catalytic oxidation of S02
to S03 might occur as a side reaction during the sorption
process.
This example illustrates that the chemistry of metaloxygen-sulfur systems can involve the following features:
multiple oxidation states, sulfite formation, sulfate formation,
disproportionation of sulfites to sulfates and sulfides, sulfate
decomposition, sometimes via oxysulfates, and catalytic oxidation of S02.
To make meaningful predictions of sorption and regeneration behavior, it was necessary to determine the nature
of the actual reaction network for each oxide. This was
done by searching the literature on the descriptive chemistry of metal-oxygen-sulfur systems and by making predictions based on thermodynamic evaluation of alternate
reaction paths.
A standard method was adopted for defining equilibrium
constants, K, for the several different types of reactions
involved. The convention was to write gas-solid reactions
for production of one mole of gas. For example, oxidation
of a 2+ to a 3+ oxide under sorption conditions (P0i
=
0.028 atm) is represented as
2 Me203 % 4
Figure
1. Process
flows and requirements
MeO + 02
(5)
This equation can represent either the 2+ to 3+ reaction
or the 3+ to 2+ reaction; the 02 is put on the right
to give a standard definition of K. At equilibrium, K
is equal to P0. (atm), where the standard state of gases
is 1 atm. It is assumed that the solids form separate
phases and solid activities are taken to be unity. The
2+ oxide is favored when K 3: 0.028 atm, and the 3+
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
385
is favored when
Vs
K
5 0.028, atm.
Me2(S03)3
-
y3
For
a
3+ sulfite reaction
(6)
Me203 + S02
To satisfy the sorption requirement, K must be 5 0.00015
atm, or log K must be 5 53.82. The regeneration requirement is, K s 0.01 atm, or log K S -2.00.
To find equilibrium constants as a function of temperature required data on standard entropies and heats
of formation as well as heat capacities and phase-transition
data. Compounds of interest were metal oxides, sulfites,
sulfates, and sulfides, plus a large group of stoichiometric
binary metal oxides, such as PbW04 and NaA102. After
obtaining all available information from the literature,
estimation procedures were developed to fill in the gaps.
The most important of these was an extension of Erdos’
method (8), which correlates heats of formation using
numerical coefficients for cations and anions in the solid.
Unknown standard entropies were estimated by Latimer’s
method (21), and heat capacities were predicted from
group contributions. The resulting thermodynamic data
base has been reported elsewhere (23). Some results of
equilibrium constant calculations will be presented in the
next section.
Reaction Networks
The complexities of the expected reaction networks were
described above. Having results of an extensive literature
survey and the thermodynamic data base, the features
of each reaction network were found as follows.
Oxides. For metals having multiple oxidation states,
the equilibrium constants for conversions of one oxide
into another under a flue gas atmosphere were calculated.
Results indicated the temperature ranges in which each
oxide was stable. Table I shows the results of all these
calculations.
Sulfite Formation. Formation and decomposition of
sulfites involves several competing reactions. Formation
of sulfites under flue gas ponditions occurs in competition
with sulfate formation. Once formed, a sulfite can decompose by a solid-state disproportionation to sulfate
and sulfide, or it can dissociate to give oxide and S02.
To determine the actual reaction path, both qualitative
information from the literature and quantitative thermodynamic calculations were used.
As a starting point, equilibrium constants were calculated for sulfite formation under flue gas conditions.
Table
1.
Stable Oxidation States at
Stable oxide
Other oxides considered
Ce02
CoO
Co304
Ce203
Co304
CoO
CuO
Cu20
FeO, Fe304
MnO, Mn203, Mn304
MnO, Mn304, Mn02
PbO
Pb02
SnO
Fe203
Mn02
Mn203
Pb02
PbO
Sn02
U03
U308, UO,
U02, U03
VO, V203, V204
W02
U308
V205
WO3
°
Calculations
386
'
were
P0,
=
0.028 atm
Temperatu re
0
range, Cc
25-750°
25-739°
739-750°
25-750°
25-750°
25-390°
390-750°
25-268°
268-750°
25-750°
25-544°
544-750°
25-750°
25-750=
performed for the range, 25° to 750° C.
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
Results, given in Table II, indicate that sulfite formation
the entire temperature range for the
alkali metals and some of the alkaline earths. Materials
such as MnS03 become unstable at temperatures between
100° and 400° C, while some sulfites, such as Fe2(S03)3,
are thermodynamically unstable in the entire temperature
range considered.
With these results on sulfite stability, the problem of
sulfite disproportionation was considered.
Sulfite Disproportionation. Results of calculations of aG°
for disproportionation to sulfate and sulfide are given in
the last column of Table II. In every case, the calculated
AG° was either positive or negative over the entire range
of 25° to 800° C; in no case did the sign of aG° change
in this interval.
These predictions can be broadened by including the
experimental results of Castellani-Bisi and co-workers (2,
3, 4, 5, 10), and others (9, 19, 34, 38), in which sulfites
heated in an inert atmosphere. Using both
were
experimental and theoretical results, sulfite behavior can
be classified in the following groups.
Group A. Sulfites of the alkali metals plus Ca, Sr,
and Ba disproportionate with little or no S02 evolution
when heated in an inert atmosphere. This result is in
accord with thermodynamic predictions since disproportionation is favorable while decomposition to oxide is not
favorable over the whole temperature range.
Group B. Sulfites of Mg, Zn, Cd, Pb, Mn, La, Sc,
Sm, and Y both disproportionate and decompose to the
oxide when heated in an inert atmosphere. Disproportionation is favored over the whole temperature range,
but decomposition to oxide begins below 800° C; the course
of the decomposition is controlled by the rates of the
two competing processes. Because of their chemical similarity to La and Sm, it would be expected that Ce and
the other lanthanides belong in this group.
Group C. Sulfites of Be, Th, and U show no thermodynamic tendency to disproportionate over the entire temperature range considered. This conclusion is based on
estimated thermodynamic properties; no experimental
work on sulfites of these metals could be found.
In summary, only the oxides of Be, Th, and U are
expected to be capable of performing as reversible sulfite
formers. The other sulfites are either too stable, too unstable, or exhibit disproportionation. However, these three
oxides can also form sulfates during sorption, since flue
gas contains oxygen. Sulfate reactions are considered next.
Sulfate Decomposition. Formation and decomposition of
sulfate appears to be the most likely reaction occurring
in an S02 sorption process. Thermodynamic calculations
indicated that nearly all the oxides of interest satisfy
the sorption requirement. The critical requirement is then
the sulfate decomposition temperature.
Since the literature on sulfates is so extensive, experimental values, rather than results of thermodynamic
calculations, were used in evaluating sulfate decomposition.
Literature values are collected in Table III. In cases
where duplicate studies have been reported, the authors
have chosen a preferred value; sources of additional values
are also listed. Some of the data have been taken from
a recent review (37).
It should be emphasized that the studies referred to
in Table III cover a variety of techniques. The most
common
technique is thermogravimetric analysis (tga),
in which the sample is heated in a gas stream and the
is favorable over
Table III. Decomposition of Sulfates
Table II. Behavior of Sulfites, Based
on Thermodynamic Calculations"
Sulfates which Decompose Directly to Oxide
Sulfite
Sulfite formation
°
temperature, C
Oxide*
log K
=
-3.82'
Ag20
BeO
315°
30°
Bi203
CaO
150=
CdO
CoO
FeO
MgO
MnO
,,
25=
185°
345°
200°
140°
120°
335°
UO,
ZnO
ZrO,
log K
590°
240°
255°
190°
160°
250°
NiO
PbO
ThO>
decomposition
°
temperature, C,
=
-2.00“
Disproportionation
favorable,
25-800°C
420='
75°
220°
750°
325°
340°
265°
230°
325°
75°
Yes
270°
445°
275°
Yes
Yes
210=
190°
440°
Decomposition
temperature
range,
Compound'
°
C
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
650-950°
690-760°
650-950°
781-810°
560-700°
550-650°
730-890°
300-587°
550-650°
A1,(S04)3
BeS04
Ce2(S04)3
Fe>(S04)3
Ga2(S04)3
Hf(SG4)2
NiS04
Sn(S04)2
Zr(S04)2
No
No
Method“
F
References'
37,
1
F, V 20
A
26, 40
F
S
35, 16, 22
37
S
11
s
15, 22
V
37
s
11
Sulfates which Decompose via Oxysulfates
Yes
Temperature range of calculations was 25° to 800° C
6Insufficient data available for reactions of Ce203, Ce02, Cr203,
Cs20, Ga203, GeO>, HfO», Ir02, La203, Mo03, Nb-Og, PdO, Re02,
Rh203, Sc203, Ta2Og, Ti02, W03, Y203. No sulfites or sulfates
Sulfite formation temperature
are known for As203, B203, Si02.
is <25° C for A1203, Fe203, Sb203, SnO>. “Sulfite formation and
temperatures are >800° C for BaO, K20, Li20,
decomposition
'
Na20, SrO. Reaction 2 Ag2S03 —> 2 Ag + S02 + Ag2S04 occurs
Decomposition
“
Solid product
Compound
Bi2 (S04) 3
1
at 100° C.
weight change is measured. Another technique is the static
method, in which the partial pressure and composition
of gas above the sample are measured in a closed system.
These methods can produce quite different results, since
in the former method the partial pressure of gaseous products above the sample approaches zero, while in the
latter method, it approaches the equilibrium value. Other
methods use still different conditions; for example, in some
cases, samples have been heated in air. While the information in the tables comes from a variety of sources, some
comparisons and conclusions are still possible. The most
important of these is the fact that sulfates can be
categorized as to the path of the decomposition reaction.
The parts of Table III refer to the following groups.
Group A. Sulfates of the alkali metals, alkaline earths,
plus Al3', Ce3', Fe3', Ga3', Hf4', Sc3', Sn4", Th4', and
Zr4' exhibit simple decomposition directly to the corresponding metal oxide. Nine of these show some decomposition below 800° C.
Group B. Sulfates in this group decompose ultimately
to the corresponding oxide, but with intermediate formation of oxysulfates. This group includes sulfates of Bi3',
Cd2', Ce4', Cr3', Cu2', La3+, Pb2', Ti4', V5', Y3', and
Zn2'. While complete decomposition may require very
high temperatures, partial decomposition to oxysulfate
may occur at much lower temperatures, as for example
with Bi2(S04)3.
Group C. This group decomposes with oxidation and
includes sulfates of Co2', Fe2+, Mn2', Sn2', U4', V3',
and VO2'. The only sulfate in this group which decomposes
via an oxysulfate is U(S04)2. The placement of sulfates
in this group is rather arbitrary, since the formation of
a higher oxide depends strongly on the temperature and
atmosphere.
Bi203·
2 Bi2(S04)3
2
Bi,03·
Bi2(S04)3
3 Bi203·
Bi2(S04)3
CdS04
CdS04-2 CdO
Ce(S04)2
Ce(S03)2
Cr2(S04)3
CuS04
temperature
°
range, C
Method
Bi203·
2 Bi,(S04)3
2 Bi203·
Bi2(S04)3
3 Bi203·
Bi2(S04)3
Bi203
425°
F
24
555°
F
24
620°
F
24
860°
F
24
CdS04-2 CdO
CdO
853-870°
F
F
Ce(S03)2
CeO,
Cr20(S04)2
400-860°
31, 23,
31, 23,
22
22
22
22, 14,
33
22, 14,
33
26
26
25, 18
25, 18
37
> 1065°
CuO-CuS04
460-640°
700-840°
A
A
A
A
CuO-CuS04
CuO
840-935°
A
La2(S04)3
La202S04
PbS04
La202S04
La203
890-1096°
1300-1400°
860-960°
A
A
A
A
PbO-PbS04
PbO
TiOS04
PbO-PbS04
Ti(S04)2
TiOS04
TiO.
V,Og-nSOg“
Y2(S04)3
Y202S04
ZnS04
ZnO-2 ZnS04
References'
V2Og
Y202S04
Y203
ZnO-2 ZnS04
ZnO
>860°
>960°
150°
430-600°
100-200°
920-1124°
1124-1248°
610°
740=
F
F
A
A
A
A
25
25
25,
25,
37
37
26
26
25, 17, 33
25, 17, 33
Sulfates which Decompose with Oxidation
CoS04
FeS04
CoO, Co304
MnS04
Mn304
Sn02
SnS04
U(S04)2
U02S04
V2(S04)3
voso4
Fe203
uo,so4
U02, UsOs,
U03
2 4, , »
V2Og
680-930°
603-810°
S
880-1100°
320-597°
300-500°
500-700°
A
V
380-408°
431-553°
F
S
s
V
s
12,
35,
32,
22,
22
1, 22,
38, 39
6, 13
37
37
37
37
37
Other sulfates in this class are BaS04, CaS04 , Cs:,S04, K2SO,
Li2S04, MgSOi, Na2S04, Rb2S04, Sc2(S04)3, SrS04, Th(S04)2;
b
decomposition temperatures of these are > 800° C. F, sample under
flowing inert gas, V, sample under vacuum, A, sample decomposed
in air, S, static experiment. 'First reference listed is source of
data given here; others give additional data. Further references
on some of these are given by Stern and Weise (37). Jn = 2,
“
3, or 4.
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
387
Group D. Sulfates of Pd, Ag, and the noble metals
decompose to give the free metal; these are not included
in Table III.
Decomposition temperatures of sulfates cover a wide
range, as shown in Table III. Metals having sulfates
which show some decomposition with SO2 or SO3 evolution
below 800° C are Ag, Al, Be, Bi, Ce, Co, Cr, Cu, Fe,
Ga, Hf, Ni, Sn, Ti, V, U, Zn, and Zr.
Results of this sulfate decomposition survey are useful
in selecting materials which can undergo regeneration
within the specified range of temperature. As mentioned
earlier, sulfate will probably be formed with all oxides,
either directly during sorption, or by way of sulfite oxidation or disproportionation during sorption, or as one of
the regeneration reactions. Some insight into direct sulfate
formation or sulfite oxidation during sorption can be gained
by examining the catalytic properties of metal oxides in
S02 oxidation.
Catalytic Oxidation. Adsorption and oxidation of S02
on an oxide surface can be regarded as a precursor to
bulk sulfate formation. These surface processes have been
widely studied in investigations of catalysts for S02 oxidation. The catalytic process on oxides should involve the
following steps: sorption of SO2 to form a sulfite, oxidation
of sulfite to sulfate, and sulfate decomposition with evolution
of S03.
Requirements for a good catalyst can be compared to
those for a good S02 sorbent. For the catalyst, all three
steps are surface reactions and occur at the same temperature. For the S02 sorbent, the first two steps should
occur
as bulk reactions converting much of the sorbent
to sulfate during sorption at Ts. The last step should
occur during regeneration to oxide at a higher temperature.
With this in mind, the literature on S02 oxidation
catalysts was surveyed; results are summarized in Table
IV. Since Pso, was so high in these experiments, it is
difficult to compare these results with those in Table
III. At least some sulfate was formed in most cases.
Table IV. Behavior of Oxides in
Oxide
Catalytic
activity
relative
investigated,
to Pt
"C
SO2
Oxidation0
Temperature
BÍ2O3
None
500-700°
Ce02
CuO
Slight
Moderate
600-700°
450-750°
Fe203
High
500-700°
Mn02
Slight
Pb02
None
450-700°
500-700°
SnO,
Slight
400-750°
Ti02
Moderate
500-750°
U02
None
500-750°
V205
High
425-700°
Transformation of
catalyst to sulfate
Sulfate formed below
600° C, evolved S03 above
600° C
Some sulfate formed
Much CuSO< formed
below 650° C and
deactivated catalyst
Not reported,
probably none
Much MnSC>4 formed
Catalyst completely
converted to PbS04
Small amounts of
SnS04, Sn(S04)2 formed
Small amount of
T1OSO4 formed
Sulfate formed
below 700° C, evolved
S03 above 700° C, U02S04
found after test
Not reported,
probably
°
Results of Neumann (27, 28, 29)
388
.
Feed
was
none
7% S02 in air.
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
Evidently, the sulfate decomposition temperature
was
active
below the operating temperature for the more
oxides, and was above the operating temperature for the
less active oxides.
The reactions of a sorbent going through the process
shown in Figure 1 can now be summarized as follows.
The sorbent recycled from the regenerator is an oxide
(or oxysulfate). Flue gas feed to the sorber contains oxygen
and this may cause a change in oxidation state; favored
oxidation states have been listed in Table I.
Sorbent can then react with S02 to form sulfite (Table
II). However, sorbent can also react with S02 and oxygen
to form sulfate. This may occur as a bulk reaction in
which all or most of the sorbent is converted to sulfate,
or it may be confined to a thin surface layer because
of simultaneous S03 desorption. A few results have been
presented in Table IV.
Spent sorbent enters the regenerator and is contacted
with purge gas. At regeneration temperature, which is
higher than that of the sorber, equilibrium is shifted
toward complete dissociation to an oxide or oxysulfate.
However, if the spent sorbent contains sulfite, disproportionation to sulfide and sulfate may occur (Table II).
Most likely, spent sorbent will be sulfate or oxysulfate;
decomposition of these has been summarized in Table
III.
Selection of Potential Sorbents
The purpose of selecting oxides as potential sorbents
at this point was to provide a rational basis for the future
development of processes of the type shown in Figure
1.
Considerable future sorbent development will be
required, involving many factors not discussed here, such
as sorption rate and sorbent stability during long term
sorption-regeneration cycling. As the development work
will be quite expensive, it appeared most economical to
proceed as far as possible using thermodynamic analyses
plus literature data, as has been done here.
At the outset, oxides of 47 elements were considered.
While this list is rather extensive, the authors believed
that consideration of a complete list was the best way
toward unambiguous solution of the present selection problem.
The elements considered and those having oxides
selected as potential sorbents are presented in Figure 2.
The general philosophy of selection was to eliminate only
those materials for which some definite unfavorable behavior could be cited. Initially, oxides of Ba, Ca, Cd,
Cs, K, La, Li, Mg, Mn, Na, Rb, Sc, Sr, and Y, were
eliminated mainly because of unfavorable sulfate decomposition temperatures. Th02 was retained because of possible favorable sulfite behavior. Oxides of Ag, As, B, and
Si were eliminated because of unfavorable chemistry (see
Table II and discussion of Group D sulfates). Be was
eliminated mainly because of the toxicity of its compounds,
and Ga, Ge, Ir, Pd, Re, and Rh, because of high cost.
Since M0O3 has a relatively high vapor pressure at elevated
temperatures, it was eliminated. Experimental data
showing reasonable sulfate decomposition temperatures for
oxides of 15 of the remaining 20 elements have been
given in Table III, and these 15 plus Th are the ones
which were selected. No decision was possible for oxides
of Nb, Sb, Ta, and W.
In summary, the 16 potential sorbents represent the
authors’ best judgment based on all the available informa-
Period la lia
llla\IVa\Va\Vla Vlb
\
1
H
j
1
1
2
1
GROUP
VIII
1
1
i
1
m
1
1
1
\!b llb\lllb\IVb\Vb Vlb
1
1
1
1
1
5
6
7
1
1
1
1
1
3
3
Tc
III I
Os
Fr
1
\I
\
i
c
1
1
3
4
1
a
1
i
|
H He
n
Ü¡ P
1 Mas
pt Au Hg 77
11
Vllb O
O
F Ne
S Cl Ar
Se
Br Kr
I Xe
Po At Rn
Sb Te
n 11
Ra A c
LANTHANIDE E
ser,es
ACTINIDE
SERIES
11
I"
Nd Pm Sm Eu Gd Tb Dy Ho Er Tm yb Lu
Apq 111
111
Np Pu Am Cm Bk Cf Es Fm Md
Lw
Figure 2. Selection of oxides
Horizontal and vertical cross-hatched, oxides considered; vertical
hatched oxides selected
tion. The information developed here should provide a
useful framework for design of future experimental
development. Some study of sorption kinetics on these
materials has been carried out and will be reported elsewhere.
Binary Metal Oxides
Many well-defined stoichiometric oxides such as CaCr204
known. The existence of these compounds indicates
that a negative standard free energy change occurs upon
reaction of the constituent oxides—e.g., CaO + Cr203.
Because of this aG°, one would expect a binary metal
oxide to exhibit different behavior towards sulfur oxides
than either of its single constituent oxides.
The reaction of CaCr204 with S03 can be represented
by
are
CaS04 + Cr203 ^ CaCr204 + S03
(7)
where the S03 is written on the right-hand side in
accordance with the convention described previously. This
is only one possible representation of the sorption process
and assumes that the sorption product exists as two distinct phases, rather than as a solution or a compound
(Cr203-CaS04). Unfortunately experimental information
on mixed sulfate-oxide pairs is not available.
This equation represents the process as an oxide displacement. Going toward the right, Cr203 displaces S03,
while the opposite occurs
in the reverse
direction.
Assuming that this representation is correct, aG° for the
above process can be predicted from a knowledge of AG°
for the reaction of Cr203 with CaO, plus free energy data
on the individual oxides. As mentioned earlier, aG° for
formation of binary metal oxides was included in the
thermodynamic data base developed for this work.
Using these data, several sets of calculations were performed. For Equation 7, log K at 750° C was predicted
to be +4.3, compared with -10.5 predicted for decomposition of CaS04 alone. As another example, log K at 750° C
for decomposition of NiS04 + V205 to S03 and NiV206
was
predicted to be +0.5, as compared with -2.8 for
NiS04 alone. Finally, for decomposition of Na2S04 + A1203
to 2 NaA102 and S03, log K at 750° C was predicted
to be -11.1, compared with -20.3 for Na2S04 alone.
cross-
These calculations indicate that the addition of a second
oxide may affect quite substantially the equilibrium constant of a given sulfate decomposition. It should be
emphasized, however, that calculations were based upon
the assumption of separate phases and upon estimates
of thermodynamic properties, in most instances. The next
step would appear to be to test the predicted effect in
an experimental study of the decomposition of sulfateoxide mixtures. Until experimental work has been performed, it will not be possible to reach a general conclusion
on the applicability of binary oxides.
Conclusions
The objective of this work was to identify potential
sorbents for removal of sulfur oxides from flue gas. To
do this, it has been necessary to establish as well as
possible the chemistry of metal-oxygen-sulfur systems.
Published data furnished the basic facts and thermodynamic correlations extended these to previously unknown cases. Judicious combinations of reported data and
thermodynamic calculations made it possible to predict
reaction paths for many cases. Examination and comparison of compound behavior have led to useful chemical
generalizations. Results have been used to select 16 potentially useful single oxide sorbents. Binary oxides constitute
an additional class of possible sorbents; selection of potentially useful binaries will require experimental work.
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Received for review September
Accepted March
9, 1970
4, 1971
The work upon which this article is based was performed pursuant
to contract PH-86-68-68 with the National Air Pollution Control
Administration, Environmental Health Service, Public Health Service, Department of Health, Education, and Welfare, Leon Stankus,
Project Officer.
Continuous Precipitate Flotation
of Chromium(lll) Hydroxide
Robert B. Grieves1 and Richard W. Lee
Department of Chemical Engineering, University of Kentucky, Lexington, Ky. 40506
T he separation of ions from aqueous solution by precipitation followed by flotation of the precipitate was shown
to be a feasible process by several investigators, including
Baarson and Ray (1964), Grieves and Bhattacharyya
(1969), Mahne and Pinfold (1969), Rubin (1968), and
Skrylev and Mokrushin (1961). Precipitate flotation of
the first kind involves collection of charged precipitate
particles by a surface-active agent of opposite charge.
Precipitate flotation of the second kind (Mahne and Pinfold, 1968) eliminates the use of a surfactant; instead
it relies on the interaction between a hydrophilic, cyclic
organic reagent and a metal ion to form a hydrophobic
precipitate which is floatable.
A three-stage process of reduction of acid chromate
(HCrOl) with NaHSCL, followed by precipitation of
1
To whom correspondence should be addressed.
390
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
Cr(III) with NaOH, followed by batch flotation with an
anionic surfactant (sodium dodecylsulfate), provided 97%
removal of chromium from aqueous suspensions 0.93 mM
in Cr (Bhattacharyya et al., 1971). The process was
strongly pH-dependent: below pH 6.3, soluble chromium
species became appreciable, and the flotation results
tended to parallel the decrease in the fraction of Cr present
as precipitate; above pH 9.7, the charge of the precipitate
was reversed, as indicated by surface potential measurements. The optimum pH range for flotation was 7.0 to
8.8. For suspensions with a doubled Cr concentration
(1.86 mill), the optimum pH range was lowered and narrowed to 6.3 to 6.5, indicating modifications in particle
surface characteristics. In a batch process, 87% flotation
was achieved at a 0.093-mol dodecylsulfate per mol Cr
ratio.
Virtually any precipitate flotation process of commercial