Conversion of Carbonyl Sulfide Removal Using a Low-Temperature Discharge Approach

(1)

Tsai et al., Aerosol and Air Quality Research, Vol. 7, No. 2, pp. 251-259, 2007

Conversion of Carbonyl Sulfide Using a Low-Temperature

Discharge Approach

Cheng-Hsien Tsai

1∗

, Ping-Szu Tsai

1

, Chih-Ju G. Jou

2

, Wei-Tung Liao

3 1

Department of Chemical and Material Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan.

2

Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 811, Taiwan.

3

Department of Chemical and Material Engineering, Southern Taiwan University of Technology, Tainan 710, Taiwan

Abstract

Carbonyl sulfide (COS) are usually yielded from the petrifaction industry or steel-making plants. In this study, a low-temperature radio-frequency (RF) plasma approach was used to

destruct COS for removing sulfur. The results showed that at an inlet O2/COS molar ratio of 3,

the removal efficiency of COS reached 98.4% at 20 W and 4000 N/m2, with the major product

being SO2 with small amounts of sulfur deposition. The removal efficiency of COS was lower in

the H2-containing condition than in the O2-containg one. However, when H2 was added into the

COS/N2 mixtures, the products, including major elemental sulfur with CS2 as a minor product,

were easily collected and recovered.

Keywords: Carbonyl sulfide; RF plasma; Destruction; Acid rain; Sulfur.

∗

Corresponding author. Tel: +886-7-381-4526; Fax: +886-7-383-0674 E-mail address: [email protected]

(2)

INTRODUCTION

Carbonyl sulfide (COS) is an odourless, tasteless and colourless polar gas molecule with a boiling point of -50.2℃, which is relatively different from other sulfur-containing impurities of hydrocarbon feedstocks (Adewuyi and Carmichael, 1987). The major emission sources of COS are the conversion of fossil fuels, steel-making plants, and waste landfills. In addition, during the high temperature stage of the Claus process, the formation of COS results from hydrocarbons being present in the flue gas according to the following reactions.

CO2 + H2S = COS + H2O (1) CH4 + SO2 = COS + H2O + H2 (2)

However, COS that is emitted into the atmospheric environment will not only contribute to the

formation of SO2 and promote photochemical reactions, but will also have an effect on the

climate (Lelieveld and Heintzenberg, 1992).

The tail gas from Cluas Plants is usually incinerated and COS and CS2 are converted to the

harmful SO2. CaO is then used as an absorbent to produce CaSO4 (Borgwardt et al., 1987),

though it is not economically viable. An alternative method of reducing the levels of COS involves hydrogenation, which takes advantage of the hydrogen present in the Claus process via

reaction (3) by using a Co-Mo-Al2O3 type catalyst (Tong et al., 1992; Rhodes, et al., 2000).

COS + 4H2 = H2S + CH4 + H2O (3)

In addition, the removal of COS can be carried out using SO2 to oxidize COS to carbon dioxide

and elemental sulfur (SO2 + 2COS = 2CO2 + S) (Rhodes et al., 2000). Recently, among the

catalytic methods, COS hydrolysis has been recognized as the promising process due to the mild reaction conditions and higher conversion via the reaction (4) (Zhang et al., 2004).

COS + H2O = H2S + CO2 (4)

Odorous and toxic H2S formed from reactions (3) and (4) need to be further removed using the

Claus reaction (2H2S + SO2 = 3S + 2H2O) to produce elemental sulfur (Clark et al., 2001).

However, some problems that are caused by the catalytic methods, such as the reduction of catalytic activity, the poison of catalysts, and product selectivities, can be further improved.

So far, the discharge process used to recover sulfur from a high concentration of COS has not been studied. A 13.56 MHz radio frequency (RF) source is commonly used in industry for generating energetic electrons (1-10 eV) to drive electron impact dissociation and penning

(3)

CONCLUSIONS

In this study, high concentration of COS was successfully destructed and converted into other

sulfur-containing products by adding H2 or O2, or without an additive, utilizing a low-pressure RF

discharge approach. The important finding is that no H2S was detected, which is very different

from the traditional catalytic removal process, including COS hydrogenation (COS + 4H2 = H2S

+ CH4 + H2O) and hydrolysis of COS and CS2 (COS + H2O → H2S + CO2). The discharge

approach replaced major product H2S with elemental sulfur that can be easily recovered with or

without H2, and can avoid the requirement of the sequential Claus reaction in order to convert

H2S into elemental sulfur. However, due to the relatively low pressure, this approach is not currently practical.

ACKNOWLEDGMENTS

The authors would like to thank the National Science Council in Taiwan for financially supporting this research work (Grant NSC 92-2211-E-151-002).

REFERENCES

Adewuyi, Y.G. and Carmichael, G.R. (1987). Kinetics of Hydrolysis and Oxidation of Carbon Disulfide by Hydrogen Peroxide in Alkaline Medium and Application to Carbonyl Sulfide. Environ. Sci. Technol. 21: 170-177.

Borgwardt, R.H.; Bruce, K.R. and Blake, J. (1987). An Investigation of Product-layer Diffusivity for Calcium Oxide Sulfation. Ind. Eng. Chem. Res. 26: 1993-1998.

Clark, P.D.; Dowling, N.I. and Huang, M. (2001). Conversion of CS2 and COS over Alumina and

Titania under Claus Process Conditions: Reaction with H2O and SO2. Appl. Catal. B-Environ.

31: 107-112.

Lelieveld, J. and Heintzenberg, J. (1992). Sulfate Cooling Effect on Climate through in-cloud Oxidation of Anthropogenic. Science 258: 117-120.

Rhodes, C.; Riddel, S.A.; West, J.; Williams, B.P. and Hutchings, G.J. (2000). The Low-Temperature Hydrolysis of Carbonyl Sulfide and Carbon Disulfide: a Review. Catal. Today 59: 443-464.

Roth, J.R. (1995) Industrial Plasma Engineering, Vol. 1: Principles, Institute of Physics Publishing: Bristol and Philadelphia.

Tong, S.; Dalla Lana, I.G. and Chuang, K.T. (1992). Appraisal of Catalysts for the Hydrolysis of Carbon Disulfide. Can. J. Chem. Eng. 70: 516-522.

(4)

Tsai, C.H.; Lee, W.J.; Chen, C.Y.; Liao, W.T. and Shih, M. (2002). Formation of Solid Sulfur by Decomposition of Carbon Disulfide in the Oxygen-Lean Cold Plasma Environment. Ind. Eng. Chem. Res. 41: 1412-1418.

Tsai, C.H.; Lee, W.J.; Shih, M.; Chen, C.Y. and Tsai, P.J. (2004). Conversion of SO2 into Elemental Sulfur by Using the RF Plasma Technique. AIChE J. 50: 524-529.

Wang, Y.F.; Tsai, C.H.; Shih, M.; Hsieh, L.T. and Chang, W.C. (2005). Direct Conversion of Methane into Methanol and Formaldehyde in an RF Plasma Environment I: A Preliminary Study. Aerosol Air Qual. Res. 5: 204-210.

Zhang, Y.; Xiao, Z. and Ma, J. (2004). Hydrolysis of Carbonyl Sulfide over Rare Earth Oxysulfides. Appl. Catal. B-Environ. 48: 57-63.

Received for review, March 4, 2007 Accepted, May 4, 2007