Mixed-Oxide Sorbent for Moderate-Temperature Removal of Carbonyl Sulfide

Hai Zhao1,a, Youning Xu1 and Junqing Liu1 1Department of Energy and Power Engineering, Shenyang Institute of Engineering Liaoning, China

ahai365@126.com

Keywords: Mixed-Oxide Sorbent, Carbonyl Sulfide, Removal, Atmosphere.

Abstract. Fe-Mn-Ce oxides were used to remove carbonyl sulfide from syngas at moderate

temperature in this work. Tests showed that the sorbent exhibited a high reactivity and sulfur

capacity. At the same time, the sorbent exhibited better performance under cyclic operation.

Thermogravimetry was used to study reaction kinetics when gas contained different gases on the

desulfurization reaction. It was found that the apparent reaction activation energy was smaller in the

atmosphere of reaction gas contains hydrogen than that in the absence of hydrogen.

Introduction

H2S and COS are the common compounds which can be found in synthesis gas. Synthesis catalysts

are extremely sensitive to sulfur poisoning [1]. In general, H2S is easy to remove but the removal of

COS is difficult in that COS is rather inactive compared to H2S due to its neutrality. From the

viewpoint of energy-saving viewpoint, the desulfurization operation at lower temperature could

have better benefit [2]. Ayala and Abbasian have showed that, metal oxides such as Zn, Cu, Mn, Fe and

W have thermodynamic feasibility for the low-temperature desulfurization [3]

. Iron oxide is an

attractive sorbent for COS removal for it has high capacity, reactivity and good regenerability [4]

.

MnO is effective for COS removal because of its perfect sorbent utilization and high sulfur sorption

capacity [5]

. Moreover, CeO2-containing materials have been studied as structural and electronic

promoters used for catalysts. It has been shown that, addition of cerium oxide can improve the

catalyst redox properties of transitional metals.

In most cases, the composition of the syngas is different for the usage of the different gas. For

example, in the synthesis of methanol, CO and CO2 are both reactants and a module M = (H2 –

CO2)/ (CO + CO2) should be close to 2.0. For F–T syntheses, the desirable syngas composition is

characterized by a H2/CO ratio of about 2.0, whereas the optimal H2/CO ratio should be 1.0 for the

oxo-synthesis process. When such gases existed in desulfurization processes, the removal of COS

could be affected by these compounds.

In this paper, Fe-Mn-Ce oxides were used to remove COS from syngas at the temperature range

of 240-400 ℃, aiming at elucidating the feasibility of the mixed-oxides as a highly effective

sorbent. The influences of different gas compositions on the desulfurization performance were also

examined. In addition, the kinetic model of the sorbent and some behaviors were discussed.

Experimental

Sample and Tests. The sorbent was prepared by coprecipitation method. Desulfurization tests were

performed in a fixed-bed quartz reactor with a diameter of 1.2 cm placed in an electric furnace.

Three grams of sorbent was packed into the reactor. Exit gas concentrations were measured on-line

by a quadrupole mass spectrometry. Data from runs was considered to be completed until the COS

exit concentration was 10 mg/m3.

Kinetic Model of the Sample. The reaction in the particle is obey to kinetic model of shrinking

core reaction in the gas-solid non-catalyzing reaction based on the viewpoint of kinetic model of

shrinking core reaction. If the collectivity reactive domination is chemistry reaction, the relation of

ratio of reactivity and time is shown as below (1):

gF

tg

g

n

As

f

xxgtr

CK

t

t 1

0

)1(1)( −−===ρ

(1)

Advanced Materials Research Vols. 383-390 (2012) pp 5464-5469Online available since 2011/Nov/22 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.383-390.5464

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.2.10.23, Carnegie Mellon University, Pittsburgh, USA-26/10/14,08:26:09)

t is reaction time, tf is complete reaction time, ks is reaction velocity constant, CA is consistence

of A, n is reaction progression, ρ is density of solid, rg0 is radii of solid. Fg is the form gene of

particle.

Results and Discussion

Effects of the Desulfurization Temperature on the Sulfur Capacity. The sorbent was performed

at different temperature ranges of 240-400°C and Figure 1 presents the sulfur capacity curves of the

sorbent at different desulfurization temperatures.

Figure 1 showed that the sorbent exhibits a relatively low absorption capacity at 240 °C. The

absorption capacities of COS increase to 17% and 22 % at the temperatures of 280°C and 320°C,

respectively. It is clear that higher temperatures favor the breakthrough capacity whereas low

operation temperatures mean that the reaction

240 280 320 360 40012

14

16

18

20

22

24

Sulf

ur

capac

ity (

gS

/100g s

orb

ent)

Temperature (°C)

Fig.1 The desulfurization capacity curve obtained in the COS absorption at different

desulfurization temperatures (gas composition: 1.5% COS, 65 % H2, 15 % CO and 5 % CO2; P=1

atm; SV=1000 h-1)

rate is lower thus requiring larger sorbent volume. However, when the temperature is above 360°C,

the absorption capacity of COS no longer increases, suggesting that a further increase of the

desulfurization temperature has little effect on the sulfur capacity of the sorbent. The main reason is

higher temperatures maybe detrimentally affects the reaction of COS because Fe2O3 or Fe3O4 is

reduced to FeO in a stronger reduction atmosphere at high temperature.

Effects of Gas Composition. As hydrogen and carbon oxide are the main compounds found in

most feed gases, the desulfurization experiments were performed in various composition of feed

gas. The result is shown in Figure 2.

0 5 10 15 20 25

H2: 80%, COS: 1.5%

H2: 65%, CO: 15%,

COS: 1.5%

H2: 40%, CO: 40%

COS: 1.5%

H2: 65%, CO: 15%,

CO2: 5%, COS: 1.5%

sulfur capacity / %

Fig.2 Effect of gas composition on the desulfurization capacity of the sorbent (Balance N2;

P=1 atm; T=325 °C; SV=1000 h-1)

Advanced Materials Research Vols. 383-390 5465

As shown in Figure 2, hydrogen has a positive effect on the sulfur capacity. When hydrogen

concentration was 80%, the sorbent exhibited the highest sulfur capacity indicated the positive

effect of hydrogen is more pronounced and the positive effect can be explained by the desulfidation

reaction shown as equations (2)-(5). The results showed that high hydrogen concentration favored

the right side of the reactions and promoted the desulfurization reaction.

Figure 2 suggested that carbon monoxide has a negative effect on desulfurization performance

of the sorbent. The effect of carbon monoxide on desulfurization performance can be explained by

reaction equations (2), (3) and (4). Reaction equation 4 shows that when H2 is present,

COS can be catalyzed by iron oxide and manganese oxide and converted to H2S subsequently H2S

will be captured by the metal oxides. H2S is not shown until the sorbent is deactive in the

experiment. When the gas to be desulfurized contains carbon monoxide, it is expected that

increasing the concentration of carbon monoxide favors the left direction reaction for equation (5)

and will inhibit desulfurization progress.

3 4 2 2Fe O +3COS+4H 3FeS+3CO+4H O� (2)

2 3 2 2Fe O +2COS+3H 2FeS+2CO+3H O� (3)

2 2FeO+COS+H FeS+CO+H O� (4)

2 2COS+H CO+H S� (5)

It also can be seen from Figure 2, carbon dioxide only appears to have a small negative effect on

the desulfurization efficiency probably as a result of competitive adsorption. For one part, CO2 is

rather inactive than CO. For another, there is small amount of water content in the reactor and the

hydrolysis is not main reaction during the COS removal process.

Kinetic Mode. There are many kinds of kinetic models such as retract model, improved retract

model and equivalent particle model [6]

. Retract model, improved retract model and equivalent

particle model are mainly applied in the aspect of desulfurization reaction process.

Figure 3 shows it is the same to analyse of kinetic data in the kinetic model of shrinking core

reaction or in the modified kinetic model of shrinking core reaction.

0 5 10 15 20

0.0

0.2

0.4

0.6

0.8

1.0

ratio of desulfurizing reactivity/X

time/min

Fig.3 Fitting curves of COS conversion versus time for desulfurizer using different kinetic

models(■: data; —: retract model; - - -: improved retract model)

Reaction Activation Energy in Syngas. Tests were carried out at 200～400 °C using a gas

composition of 12.27g·m-3

COS, 50 % H2 and balance N2.

According to data of COS in the same concentration under different temperatures, reaction

activation energy Ea and pre-exponential factor k0 can be calculated.

)exp()( 10tRT

Ek

dt

dx a−=→ (6)

Do logarithmic transformation to both sides, get

RT

Ek

dt

dx a−=→ 10t ln)ln( (7)

5466 Manufacturing Science and Technology, ICMST2011

-2.20 -2.15 -2.10 -2.05 -2.00 -1.95 -1.90 -1.85 -1.80 -1.75

-5.7

-5.6

-5.5

-5.4

-5.3

-5.2

-5.1

-5.0

-4.9

-4.8

ln(dx/dt)

t-0

ln(-1/RT)(*104)

Fig.4 The solution of Ea and k’0 by least squares

0 200 400 600 800 1000 1200

0.00

0.05

0.10

0.15

0.20

TG/m

g

time/s

Fig.5 TG versus time at different temperatures at 7.36g·m-3 (■: 400℃; ⊙: 380℃; ▽: 320℃; ▲: 280℃)

According to the beginning rates, the equation above is fitted using least square method and

then Figure 4 can be gotten.

It can get that Ea=12.36 kJ·mol-1

, k0=0.02470g-1

·m3·s

-1

Kinetics equation is written as follows:

32)1()314.8

12360exp(0247.0 xC

Tdt

dxA −−=

(8)

Then:

])1(1)[314.8

12360exp(

15.40 31

0

xTC

tA

−−= (9)

Kinetic Experiments under Atmosphere without Hydrogen. In order to investigate the

influence of hydrogen in desulfization reaction, reactive gas without hydrogen was used under the

same reaction condition.

The relation between conversion rates and time under different temperatures is shown in Figure

5, while COS concentration is 7.36 g·m-3

.

Advanced Materials Research Vols. 383-390 5467

-2.20 -2.15 -2.10 -2.05 -2.00 -1.95 -1.90 -1.85 -1.80 -1.75

-2.8

-2.7

-2.6

-2.5

-2.4

-2.3

ln(dx/dt)

t-0

ln(-1/RT)(*104)

E’a=21.92 kJ·mol-1, '

1ln k =-0.9182

It can be calculated that:

k’0=0.01810 g-1

·m3·s

-1

Correlation coefficient of least square method R=0.9529, thus

E’a=21.92 kJ·mol-1

, k'0=0.01810 g-1

·m3·min

-1

Kinetics equation can be represented:

32)1()

314.8

21920exp(0181.0 xC

Tdt

dxA −−=

(10)

Then:

])1(1)[314.8

21920exp(

132.55 31

0

xTC

tA

−−= (11)

0 1 2 3 4 5 6 7 8 90

2

4

6

8

10

Fresh sorbent

R1

R2

R3

CO

S c

once

ntr

atio

n (

mg/m

3)

Reaction time ( h)

Fig.4 COS breakthrough curves for sorbent in 3 sulfidation/regeneration cycles (gas composition: 1.5% COS, 65 % H2, 15 % CO and 5 % CO2; P=1 atm; T=325 °C; SV=1000 h-1)

Compared Equation (9) with (11), as can be seen that when hydrogen exists, reaction activation

energy is 12.36 kJ·mol-1

; when gas without hydrogen, reaction activation energy is 21.92 kJ·mol-1

.

It is concluded that desulfurization reaction rate can be increased when hydrogen exists.

Regeneration of the Sorbent. The regeneration of the sorbent is crucial to commercial use.

Regeneration tests were carried out at 800 °C using a gas with 5% O2, 15% steam and the balance

N2.

Figure 6 illustrates the breakthrough curves of COS using fresh and regenerated sorbents after

the first (R1), the second (R2), and the third (R3) regeneration cycles. The efficiency of the sorbent

decreases as the number of cycles increases but the prebreakthrough COS concentration in the

5468 Manufacturing Science and Technology, ICMST2011

outlet gas is still very low, the measurements show that below 1 mg/m3 COS could be obtained in

the effluent gases. Although the breakthrough time decreased, however, the efficiency of

regeneration remained as high as 75% after three cycles.

Conclusions

The following conclusions are drawn from this work.

(1) Results reveal that the sorbent can remove COS from 1.5% to less than 1 mg/m3

in

appropriate conditions. The sulfur capacity of the sorbent increases from 17% to 22 % as the

temperature increases from 240 °C to 320 °C

(2) The gas composition influences the sorbent performance during desulfurization processes. H2

has a positive effect, while CO has a negative effect. In addition, CO2 has a slight negative effect on

the desulfurization reaction. This result can be explained via desulfurization and hydrogenation

reactions.

(3) When reaction gas contains hydrogen, reaction activation energy is 12.36 kJ·mol-1

. When

reaction gas doesn’t contain hydrogen, the reaction activation energy is 21.92 kJ·mol-1

.

References

[1] Peter, W., Nicola, C., Younga, John, W., Colin, R., Graham, J. H. and C.N. Satterfield, 1991,

Carbonyl sulphide hydrolysis using alumina catalysts, Catalysis Today, vol.49,pp.99-104

[2] Kouichi, M., Kazuhiro, M., Tomohiko, I., Tomoyuki, Y., Hiroyuki, N., and Kenji, H.,

Simultaneous Removal of COS and H2S from Coke Oven Gas at Low Temperature by Use of

an Iron Oxide. Ind. Eng. Chem. Res. 1992, vol.31, pp.415-419

[3] Ayala, R., Abbasian, J., 1995. Advanced low-temperature sorbents. In: McDanniel, H.M., et al.,

(Eds.), Proceedings of the Coal-Fired Power Systems ’95. Review Meeting, DOE/METC-

95/1018, pp. 407

[4] Racid B. Slimane, Javad Abbasian. Utilization of metal oxide-containing waste materials for

hot coal gas desulfurization, Fuel Processing Technology, 2001,vol. 70,pp. 97-113

[5] Wakker, J. P., Gerritsen, A. W. and Moulijin, J. A., 1993, High Temperature H2S and COS

Removal with MnO and FeO on γ-Al2O3 Acceptors, Ind. Eng. Chem. Res., vol.32, pp. 139-149

[6] Y. G. Pan, J. F. Perales, E. Velo, L. Puigjaner. Kinetic behaviour of iron oxide sorbent in hot

gas desulfurization. J Fuel; 2005, vol.84,pp.1105-1109.

Advanced Materials Research Vols. 383-390 5469

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