Nonstationary phenomena in the low-temperature gas-phase catalytic oxidation of H2S with oxygen

Article abstract of DOI:10.1023/A:1012319614113

The conditions of the existence were determined and reasons were revealed for the appearance of periodic oscillations of the SO₂ concentration in the reaction products and warming temperature in the catalyst bed in the oxidation of H₂

Full text of DOI:10.1023/A:1012319614113

Kinetics and Catalysis, Vol. 42, No. 5, 2001, pp. 657–661. Translated from Kinetika i Kataliz, Vol. 42, No. 5, 2001, pp. 724–729.
Original Russian Text Copyright © 2001 by Kovalenko, Kundo, Novopashina, Kalinkin.
Nonstationary Phenomena in the Low-Temperature Gas-Phase
Catalytic Oxidation of H2S with Oxygen
O. N. Kovalenko, N. N. Kundo, V. M. Novopashina, and P. N. Kalinkin
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received December 22, 1999
Abstract—The conditions of the existence were determined and reasons were revealed for the appearance of
periodic oscillations of the SO2 concentration in the reaction products and warming temperature in the catalyst
bed in the oxidation of H2S with oxygen at temperatures below the dew point of sulfur on the V–Al–Ti oxide
catalyst. The formation of polysulfides during the reaction was experimentally found. It was proposed that the
adsorption of sulfur and polysulfides on the catalyst surface is responsible for the observed oscillatory pro-
cesses.
INTRODUCTION
high activity; however, SO2 was the main reaction prod-
uct for this catalyst. It was found that the vanadium-
containing catalyst is not sulfidized in the reaction. It
was assumed that the active oxygen of the catalyst is
retained under reaction conditions and the deep oxida-
tion of hydrogen sulfide can occur to form SO2. Cata-
lysts based on copper, zinc, and iron oxides are sul-
fidized in the reaction medium, which results in the
removal of active oxygen. Therefore, these catalysts are
exceeded in activity by the vanadium-containing cata-
lyst. However, the absence of active oxygen provides
their much higher selectivity in the formation of ele-
mental sulfur. The studies of the reaction kinetics on the
V–Al–Ti oxide catalyst at the ratio O2/H2S > 2.4 found
previously unknown periodic stepwise changes in the
SO2 concentration in the reaction products accompa-
nied by the corresponding changes in the catalyst tem-
perature. The oscillations of the concentrations of the
reacted substances and temperature were assumed to be
related to a periodic change in the character of the oxi-
dation and adsorption of reaction components.
The development of new efficient technologies of
sulfur extraction from dilute hydrogen sulfide–contain-
ing gases is urgent because environmental requirements
imposed on the content of sulfur compounds in natural,
fuel, and exhaust gases are becoming more rigid [1, 2].
Hydrogen sulfide oxidation at temperatures below
the dew point of sulfur is a promising direction in the
development of technologies for the fine purification of
gases containing up to 5% H2S [1, 3]. The oxidation of
H2S at temperatures below the dew point of sulfur pro-
vides, as a rule, a higher yield of sulfur; decreases
sharply the probability of side reactions of sulfur with
hydrocarbons, water, and oxygen to form toxic prod-
ucts; and prevents the removal of the formed sulfur
with reaction gases. At the same time, the process of
hydrogen sulfide oxidation in the sulfur condensation
regime is nonstationary. During the reaction, the sulfur
formed is accumulated on the catalyst surface and
decrease its activity. This results in the necessity of the
periodic regeneration of the catalyst. Therefore, most
studies of the catalytic oxidation of hydrogen sulfide in
a gas phase were carried out under stationary condi-
tions at temperatures higher than the dew point of sul-
fur where the stable operation of the catalyst is pro-
vided [1, 2, 4–6]. The kinetics of hydrogen sulfide oxi-
dation with oxygen under stationary conditions on the
Fe–Cr–Zn oxide catalyst was studied in detail [2, 4–6].
The reaction rate was found to be described by the
Langmuir–Hinshelwood equation: the reaction has the
first order with respect to hydrogen sulfide, and the
order with respect to oxygen varies from 0.5 to 0
depending on the oxygen content.
This work is aimed at studying the found nonsta-
tionary phenomena and revealing the conditions and
reasons for their appearance.
EXPERIMENTAL
The catalytic oxidation of H2S with oxygen was
studied in a flow-type reactor (outer diameter of
10 mm, height of 15 mm) with a fixed catalyst bed at
atmospheric pressure and 100–250°ë using model gas
mixtures containing 0.5–2 vol % H2S, oxygen, and
nitrogen. The é2/H2S ratio was varied from 0.6 to 10.
Previously [7], we studied the oxidation of H2S with The flow rate of the gas mixture was 60 l/h. A 3-g cat-
oxygen under the conditions of possible sulfur conden- alyst sample was taken as a fraction of 0.4−0.6 mm.
sation on the following three oxide catalysts recom- The heating of the catalyst during the reaction was
mended for H2S oxidation: Fe–Cr–Zn, Cu–Cr–Al, and detected by a thermocouple placed at the end of the cat-
V–Al–Ti. The V2O5–Al2O3–TiO2 catalyst exhibited alyst bed.
0023-1584/01/4205-0657$25.00 © 2001 MAIK “Nauka /Interperiodica”

658
[SO2], vol %
KOVALENKO et al.
oxidation. The chosen V–Ti–Al oxide catalyst is highly
T, °C
active at temperatures above the melting point of sulfur
(120°ë). We found that even at very short contact times
(0.003 s) the reaction rate depends on the catalyst grain
size. Therefore, virtually all experiments were carried
out in the region of internal diffusion. It is difficult to
perform H2S oxidation at temperatures below the melt-
ing point of sulfur because the catalyst rapidly loses its
activity due to sulfur deposition on its surface.
300
2.1
2.0
1.9
1.8
1.7
(‡)
1
2
250
Figure 1 shows the change in the characteristics of
the process with a change in the temperature of the ini-
tial gas mixture conveyed to the reactor.
200
6
0
1
2
3
4
5
It is seen in Fig. 1a that, at a temperature of the ini-
tial gas mixture of 203°ë and a temperature at the end
of the catalyst bed of 248°ë, the reaction occurs in the
stationary regime. After ~0.5 h after the beginning of
the experiment, the catalyst arrives at a stationary state.
During this time, a certain portion of sulfur compounds
(on a sulfur basis, 0.1 g sulfur per gram of catalyst) was
adsorbed on the catalyst. After the stationary state was
achieved, the conversion of H2S to SO2 was 100% and
the catalyst bed was heated to 45°ë.
The further decrease in the temperature of the initial
gas mixture to 130°ë and in the temperature at the end
of the catalyst bed to 163°ë resulted in the periodic
oscillations of the SO2 concentration in the exit gases and
of the temperature in the reactor. The temperature at the
end of the catalyst bed reached 300°ë at the instant of
inflammation, and the SO2 concentration increased by a
factor of 12 with respect to the concentration of con-
verted H2S and became equal to 25.3 vol % SO2 in the
exit gases (Fig. 1b). At the initial gas mixture composi-
tion 2 vol % H2S + 13 vol % é2, the amount of oxygen
in the gas phase is sufficient for the complete oxidation
of sulfur by the reaction
Time, h
[SO2], vol %
25
T, °C
300
(b)
250
200
150
100
20
8
~
~
2
1
6
4
2
0
2
4
6
8
Time, h
Fig. 1. Plots of the (1) SO concentration and (2) reaction
2
temperature vs. reaction time at an H S concentration in the
2
initial gas mixture of 2 vol %, an O /H S ratio of 5, and a
2
2
contact time of 0.2 s. Temperature of the initial gas mixture:
(a) 203 and (b) 130°C.
The components of the initial and final gas mixtures
were analyzed on two chromatographs connected in
series using a single sample. H2S, SO2, and H2O were
determined on a Tsvet-500 chromatograph using mod-
ified Porapak Q. O2 and N2 were determined on an
LKhM-8MD chromatograph using molecular sieves
NaX. In some experiments, hot exit gases were rapidly
dried on a trap packed with Mg(ClO4)2.
S + O2 = SO2.
Probably, the active oxygen of the catalyst participates
in the reaction, and V5+ is reduced to lower oxidation
states at the instant of inflammation. In the next cycle
before the new inflammation, the catalyst is reoxidized.
A decrease in the temperature of the initial gas mix-
ture to the sulfur solidification temperature ceases the
oscillatory processes and simultaneously decreases the
catalyst activity due to sulfur deposition (Fig. 2).
We also found that the appearance and character of
oscillations can be controlled by changing the O2/H2S
molar ratio or temperature of the initial gas mixture. In
particular, a change in the temperature of the initial gas
mixture during the reaction can induce the transition
from a stationary regime of the process to a nonstation-
ary regime and vice versa. Figure 3 shows that a
decrease in the temperature of the initial gas mixture
during the reaction from 200 to 120°ë resulted in the
inflammation regime, and the subsequent temperature
Results were obtained as plots of the conversion of
H2S (XH S ), concentration of SO2 in the reaction prod-
2
ucts, and temperature in the catalyst bed vs. time.
The IK-27-40 industrial catalyst with the composi-
tion (wt %) 0.5CaO/5V2O5/30Al2O3/64.5TiO2 pre-
pared by the procedure described in [8] was used as the
catalyst. The specific surface area of the catalyst did not
exceed 135 m2/g.
RESULTS AND DISCUSSION
1. Influence of Temperature and O2/H2S Ratio
on the Course of the Reaction
We studied the influence of the temperature and increase to 200°ë resulted again in the regime of stable
oxygen concentration on the character of H2S oxida- operation. The data obtained at a constant temperature
tion. Figures 1–4 demonstrate the conditions of H2S of the gas mixture but with a change in the O2/H2S
KINETICS AND CATALYSIS Vol. 42 No. 5 2001

NONSTATIONARY PHENOMENA
659
molar ratio showed the behavior described below (Fig. 4).
[H2S], [SO2], vol %
2
T, °C
At the oxygen to hydrogen sulfide ratio O2/H2S = 2 and
the temperature much lower than the dew point of sul-
fur, sulfur was adsorbed on the catalyst, resulting in a
decrease in the hydrogen sulfide conversion in a certain
time interval. An increase in the oxygen concentration
sharply increased the temperature in the catalyst bed by
150°ë; adsorbed sulfur compounds were oxidized to
SO2; and the SO2 concentration in the gas phase
increased by a factor of 5, as compared with the H2S
concentration in the reaction mixture. After the
adsorbed sulfur compounds were removed, the catalyst
temperature decreased to the initial level. The cycle of
oscillations of the concentration of formed SO2 and
temperature can be repeated by a repeated change in the
O2/H2S ratio.
200
1
150
1
0
3
2
100
6
0
1
2
3
4
5
Time, h
Fig. 2. Plots of the (1) H S concentration, (2) SO concen-
2
2
tration, and (3) reaction temperature vs. reaction time at an
H S concentration in the initial gas mixture of 2 vol %, an
2
O /H S ratio of 5, and a contact time of 0.2 s. Temperature
2
2
of the initial gas mixture 100°C.
2. Region of Existence of Oscillations
The region of the existence of oscillations depends
first on the temperature. At a temperature of the gas
mixture of 200°ë or higher, the oxidation rate of H2S is
high and oxidation occurs predominantly to SO2 [2].
Therefore, reduced sulfur species are not accumulated
in considerable amounts on the catalyst surface. By
contrast, at temperatures below the melting point of
sulfur, the sulfur formed is condensed to cover the cat-
alyst surface. The temperature below 120°ë is insuffi-
cient for sulfur ignition and catalyst regeneration. At
intermediate temperatures of the gas mixture of
130−150°ë, sulfur adsorption resulted periodically in
ignition. Polysulfides formed by incomplete H2S oxida-
tion or the reaction of H2S with adsorbed sulfur can ini-
tiate a self-accelerating oxidation reaction resulting in
ignition. The é2/H2S ratio similarly affects the adsorp-
tion and ignition of sulfur compounds. An increase in
this ratio leads to the fast oxidation of adsorbed sulfur
species to SO2. Low é2/H2S ratios result in catalyst
deactivation due to sulfur deposition. Intermediate
é2/H2S ratios result in the appearance of the oscillatory
mode.
[SO2], vol %
T, °C
300
200°C
120°C
25
15
200°C
2
250
200
150
100
~
10
2
~
1
1
0
2
4
6
8
Time, h
Fig. 3. Plots of the (1) SO concentration and (2) reaction
2
temperature vs. reaction time at an H S concentration in the
2
2
initial gas mixture of 2 vol %, an O /H S ratio of 5, and a
2
contact time of 0.2 s. The temperature of the initial gas mix-
ture changed during the reaction.
The results of the experiment compared to the ther-
mogravimetric analysis data and published data on the
ignition of bulk sulfur are collected in the table.
3. Reasons for the Appearance of Oscillatory Processes
The reason for the ignition of the catalyst bed should
be revealed to explain the mechanism of oscillatory
processes. At a concentration of H2S in the initial gas
mixture of 2 vol % and temperatures up to 200°ë, the
formed sulfur is almost completely condensed in cata-
lyst pores [9]. It was assumed that the ignition of ele-
mental sulfur condensed in catalyst pores can be a step
responsible for the heating of the catalyst bed. To con-
firm this assumption, we experimentally determined
the ignition temperature of sulfur adsorbed on the cata-
lyst during the reaction. The experiment was carried out
in a flow-type reactor. The experimental conditions for
the determination of the ignition temperature were sim-
ilar to those of preceding experiments but the initial gas
mixture did not contain hydrogen sulfide.
The tabulated data show that the dispersion of sulfur in
the catalyst pores decreases the ignition temperature of
sulfur, depending on the oxygen content of the gas mix-
ture, by 105°ë (11 vol % é2) and 200°ë (21 vol % é2),
as compared to the ignition of bulk sulfur. However, the
temperature observed at the beginning of sulfur igni-
tion in the reaction is lower by 140°ë than that in a mix-
ture without hydrogen sulfide.
Based on these data, we cannot explain the appear-
ance of oscillatory processes observed in the reaction
by only the ignition of sulfur localized on the catalyst
surface. The results obtained show that sulfur adsorbed
on the catalyst in the reaction medium exhibited a
higher reactivity. At least partially bound sulfur species
KINETICS AND CATALYSIS Vol. 42 No. 5 2001

660
KOVALENKO et al.
O2/H2S = 2 O2/H2S = 10 O2/H2S = 2
~
~
~
100
95
90
85
80
75
Solvent
Solvent
1
3
~
0
1
2
3
4
10
8
2
H2Sn
~~
~
~
6
O2/H2S = 10
Simple
gases
H2S
4
O2/H2S = 2
O2/H2S = 2
2
2
0
0
1
2
3
4
Unknown
peak
300
250
200
150
100
1
O2/H2S = 10
O2/H2S = 2
O2/H2S = 2
240 180
120
Time, s
60
0
3
Fig. 5. Chromatograms of (1) the reaction gas mixture,
(2) hydrogen polysulfide extracted with diethyl ether, and
(3) solvent.
0
1
2
3
4
Time, h
Fig. 4. Plots of the (1) H S conversion ( X
), (2) SO con-
2
2
H2S
reactions. For example, in aqueous neutral and alkaline
solutions, polysulfide ions are readily oxidized with
oxygen to sulfur at room temperature [10]. The oxida-
tion of H2S in solutions can have a self-accelerating,
autocatalytic character due to the accumulation of more
reactive intermediate polysulfides. Similar processes
can occur during gas-phase oxidation on the catalyst
surface. The amount of polysulfides and the length of a
polysulfide chain increase during oxidation, which is
accompanied by an increase in the reactivity. When
polysulfides are accumulated in a concentration higher
centration, and (3) reaction temperature vs. reaction time at
an H S concentration in the initial gas mixture of 2 vol %, a
2
temperature of the initial gas mixture of 145°C, and a con-
tact time of 0.2 s. The O /H S molar ratio changed during
2
2
the reaction.
with a higher reactivity are formed, most likely on the
catalyst surface along with elemental sulfur.
Sulfur atoms linked to form polysulfide chains are
known to exhibit a high reactivity toward oxidation than a certain limit, the oxidation process can be inten-
Ignition temperatures of bulk sulfur and sulfur adsorbed on the catalyst in different gas atmospheres
Sample
Sulfur adsorbed on catalyst
Composition of gas mixture, vol %
Ignition temperature, °C
2H2S + 11O2 + 87N2
145
285
190
390
The same
11O2 + 89N2
The same (thermogravimetric analysis data)
Bulk sulfur [1]
air
air
KINETICS AND CATALYSIS Vol. 42 No. 5 2001

NONSTATIONARY PHENOMENA
661
sified to result in the ignition and desorption of the ratio O2/H2S ≥ 2; temperature of the initial gas mixture
accumulated sulfur compounds in the form of SO2.
100–150°C. The appearance and character of oscilla-
tions can be controlled by changing the O2/H2S ratio or
the temperature of the initial gas mixture during the
reaction.
(2) The formation of hydrogen polysulfides as inter-
mediate products of H2S oxidation was experimentally
established.
(3) A reason for the ignition of adsorbed sulfur-con-
taining products can be the self-accelerating reaction of
polysulfides with active oxygen in the vanadium-con-
taining catalyst.
An additional factor indicative of the existence of
intermediate compounds during H2S oxidation in the
gas phase was the detection of an unidentified peak
directed opposite to that of H2S, which could belong to
polysulfides, in the chromatographic analysis of the
reaction products. The rapid drying of hot exit reaction
gases followed by direct chromatographic analysis
(without separation of components) is the necessary
condition for the detection of the peak.
To identify the peak, we synthesized hydrogen
polysulfide by the acid decomposition of ammonium
polysulfide and the extraction of the resulting H2Sn with
diethyl ether according to the procedure published in
[11]. The chromatograms of hydrogen polysulfides, the
solvent, and an unknown peak are presented in Fig. 5.
The shape and the emergence time of the peak of hydro-
gen polysulfide are consistent with those of the peak to
be identified. The data obtained confirm the participa-
tion of hydrogen polysulfide as an intermediate in the
low-temperature gas-phase oxidation of H2S.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, project no. 98-03-32312a.
REFERENCES
1. Grunval’d, V.R., Tekhnologiya gazovoi sery (Technology
of Gaseous Sulfur), Moscow: Khimiya, 1992.
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soedineniya prirodnykh gazov i neftei (Sulfur-Contain-
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1989.
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Ust. Razv., 1997, vol. 5, p. 129.
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retained on the catalyst surface due to the interaction
with basic sites of the catalyst and can partially be des-
orbed to the gas phase under the action of acidic com-
ponents contained in the gas mixture (H2S and SO2).
Polysulfide oxidation can be an autocatalytic reac-
tion. During oxidation, the polysulfide chain grows to
form polysulfides that are more reactive in oxidation
and contain more sulfur atoms. This can result in the
autocatalytic character of the oxidation and in the igni-
tion of adsorbed sulfur compounds.
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CONCLUSIONS
Based on the studies, the following conclusions can
be drawn:
(1) The conditions were determined for the exist-
ence of periodic oscillations of the concentration of
SO2 as a reaction product and of the temperature of H2S
oxidation with oxygen at temperatures below the dew
point of sulfur on the V–Al–Ti oxide catalyst: molar
10. Kundo, N.N. and Keier, N.P., Kinet. Katal., 1970,
vol. 11, no. 1, p. 91.
11. Nekrasov, B.V., Kurs obshchei khimii (General Chemis-
try), Moscow: Goskhimizdat, 1952, p. 295.
KINETICS AND CATALYSIS Vol. 42 No. 5 2001