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Conversion of Sulfur Dioxide and Carbon Disulfide to Elemental Sulfur under Plasma-Induced Conditions Cheng-Hsien Tsai,*,† Ya-Fen Wang,‡ Minliang Shih,‡ and Yi-Wen Luo§ Department of Chemical Engineering, National Kaohsiung University of Applied Sciences, No. 415, Chien-Kung Road, Kaohsiung 807, Taiwan, Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy and Science, Tainan, 717, Taiwan, and Department of Health Care Administration, Chung-Hwa College of Medical Technology, Tainan, 717, Taiwan Received May 6, 2003. Revised Manuscript Received October 27, 2003
CS2 and SO2 appear in industrial processes such as the Claus reaction; however, few studies have focused on both simultaneous reactions. In this study, the conversion of CS2 and SO2 into elemental sulfur proceeded efficiently, using a radio-frequency plasma-induced system. The main experimental parameters were as follows: the feeding concentration of SO2, discharge power (P), inlet CS2/SO2 ratio (R), and system pressure at room temperature. The conversions were sensitive to either P or R. When P was increased from 15 W to 120 W, the SO2 conversion only increased from 27.3% to 66.8% at [SO2] ) 2% and R ) 1, whereas conversion apparently increased from 26.5% to 98.2% when R ) 2 ([CS2] ) 4%). Interestingly, the optimum operating condition was observed at R ) 2 to reach larger conversions for both SO2 and CS2, simultaneously. At R ) 2, the stoichiometric ratio for C/O is 1, providing oxygen as a prior sink for the C atom, and resulting in CO as the major gaseous product; the selectivity of S1 was 0.989 at 90 W, because most S atoms formed into elemental sulfur. The purity of sulfur reached 98.5%, and the X-ray diffraction patterns indicated great amounts of S8 structure.
Introduction Sulfur dioxide (SO2) is the chief precursor of acid rain; its emission ultimately leads to the formation of sulfate aerosol participates, which results in highly acidic precipitation and has a significant cooling effect on the climate.1,2 To reduce the emissions, a high concentration of SO2 is reduced commonly by an effective reducing agent, such as methane (CH4), carbon monoxide (CO), or/and hydrogen gas (H2), hydrogen sulfide (H2S), and carbon sources.3,4 In regard to the treatment of carbon disulfide (CS2), combustion, biological gas desulfurization processes, and catalyst oxidation are usually utilized. CS2 and SO2 exist in environments such as a Claus plant. CS2 can form in the thermal stage, which also yields SO2 in the Claus process, which is used worldwide for the conversion of H2S to elemental sulfur.5 However, for the consequent conversion of CS2 in the reductive tail gas, catalytic treatment is needed and is enhanced by the reaction of CS2 with SO2.5 In addition, from our * Author to whom correspondence should be addressed. E-mail:
[email protected]. † National Kaohsiung University of Applied Sciences. ‡ Chia-Nan University of Pharmacy and Science. § Chung-Hwa College of Medical Technology. (1) Bates, T. S.; Lamb, B. K.; Guenther, A.; Dignon, J.; Stoiber, R. E. J. Atmos. Chem. 1992, 14, 315-337. (2) Lelieveld, J.; Heintzenberg, J. Science 1992, 258, 117-120. (3) Murthy, K. S.; Rosenberg, H. S.; Engdahl, R. B. J. Air Pollut. Control Assoc. 1976, 26, 851-855. (4) Ratcliffe, C. T.; Pap, G. Fuel 1980, 59, 237-243. (5) Clark, P. D.; Dowling, N. I.; Huang, M. Appl. Catal. B 2001, 31, 107-112.
previous studies, odorous CH3SH and dimethylsulfoxide (DMS) also yielded CS2 with SO2, and both contents became quite comparable under oxygen-lean conditions (inlet O2/CH3SH ratio of ∼1 and inlet O2/DMS ratio of 1-2) in an radio-frequency (RF) plasma reactor.6,7 Sequential elemental sulfur was recovered from CS2 under the partial-oxidation discharge conditions, by accompanying the SO2 in the effluents.8 Hence, the conditions of the co-existence of SO2 with CS2 obviously occur not only in a Claus reaction but also in a plasmalysis reaction and do need further treatment. So far, the discharge process used to yield elemental sulfur from the high concentrations of SO2 and CS2 has not been studied. Therefore, the objective of this study is to demonstrate the RF plasma potential with a dry, single removal process to convert SO2 and CS2 simultaneously to yield elemental sulfur at the feeding condition of room temperature. The important parameters and operation conditions for a RF plasmalysis conversion process that have not been studied are reported in this study. Experimental Section Experimental Apparatus. The experimental equipment is similar to that of our previous studies.6,7 The flow rates of SO2, evaporated CS2, and carrier gas (N2) were adjusted using (6) Tsai, C. H.; Lee, W. J.; Chen, C. Y.; Liao, W. T. Ind. Eng. Chem. Res. 2001, 40, 2384-2395. (7) Tsai, C. H.; Lee, W. J.; Chen, C. Y.; Tsai, P. J.; Fang, G. C.; Shih, M. Plasma Chem. Plasma Process. 2003, 23, 141-157. (8) Tsai, C. H.; Lee, W. J.; Chen, C. Y.; Liao, W. T.; Shih, M. Ind. Eng. Chem. Res. 2002, 41, 1412-1418.
10.1021/ef030105y CCC: $27.50 © 2004 American Chemical Society Published on Web 02/11/2004
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a mass flow controller (Brooks, model 5850 E). These gases flowed into a mixer and then were introduced into a vertical cylindrical glass reactor (inner diameter of 4 cm, with a height of 15 cm). The plasma reactor consisted of two wrapped external copper electrodes (height of 5.5 cm) connected by a matching network (Matchbox PFM) that was coupled to a 13.56 MHz RF generator (model PFG 600 RF, Fritz Huttinger Elektronik GmbH). A thermocouple was set at the center of the cross section of the reactor at the rear of the after-glow discharge zone, to measure the temperature of the effluents. To clean up the contaminants and check the overall system for leakage, the system was pumped until the pressure was
