Energy & Fuels 2004, 18, 465-469
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
Energy & Fuels, Vol. 18, No. 2, 2004
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 <10-3 Torr by a mechanical vacuum pump with a diffusion pump. Before a new experiment was performed, the pressure was re-set at the target condition. Each designed experimental condition was performed by measuring the concentration of reactants and products at least three times to ensure that the plasma-chemical reactions were in the steady state. Experimental Parameters and Conditions. The important parameters for a similar RF plasma-chemical reaction system were determined in previous studies. According to the model sensitivity analysis,16 the factorial fractional design method,17 and our experimental results,6 the most important factors that affect the conversion of reactants and selectivity of products should be the discharge power (P), the inlet CS2/ SO2 concentration ratio (R), and the SO2 feeding concentration ([SO2]). The operating pressure and total flow rate usually were only slightly affected. Therefore, the following comprehensive ranges of operating conditions were probed: P ) 15120 W, R ) 0-3.0, [SO2] ) 1%-2%, and at a total flow rate of 200 sccm (standard cm3/min) with N2 as the balanced gas. The operating pressure was set mainly at 30 Torr (4000 N/m2), unless another pressure was assigned, to generate a homogeneous glow discharge that would result in a large amount of energetic species and a relatively low effluent gas temperature. However, because a feed temperature of 150 °C could not increase the conversions of SO2 and CS2 in this RF reactor, the inlet temperature is fixed at room temperature (∼30 °C). Chemical Analysis. The components of the gaseous product were identified using gas chromatography (GC) equipment that was equipped for pulsed flame photometric detection (GC/ PFPD, using a model HP 6890 G. S. Q column, 30 m × 0.53 mm, Hewlett-Packard) and Fourier transform infrared (FTIR) spectrometry (model FTS-7, Bio-Rad) first, and then quantified mainly by the on-line FTIR equipment. A canister sampler (6 L) was used to sample the GC aliquots from the effluent gas mixture and then pressurized to slightly over 1000 Torr by adding pure N2 gas. For FTIR study, the analysis condition was at a resolution of 4 cm-1, a sensitivity of 1, and a scan number of 8, and calibration curves were achieved by comparing the response peak height at the same IR wavenumber. Solid samples called depositions are collected from the major discharge zone around electrodes and the downstream zone that exists between the electrodes and the exit of reactor. The chemical compositions of the deposition were analyzed via elemental analysis (Elementar/vario EL analyzer for carbon, (9) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data 1997, 26, 1329. (10) Singleton, D. L.; Cvetanovic, R. J. J. Phys. Chem. Ref. Data 1988, 17, 1377-1437. (11) Dean, A. M.; Bozzelli, J. W.; Ritter, E. P. Combust. Sci. Technol. 1991, 80, 63-85. (12) Saito, K.; Ueda, Y.; Ito, R.; Kakumoto, T.; Imamura, A. Int. J. Chem. Kinet. 1986, 18, 871-884. (13) Woiki, D.; Roth, P. Int. J. Chem. Kinet. 1995, 27, 547-553. (14) Glarborg, P.; Kubel, D.; Dam-Johnson, K.; Chiang, H. M.; Bozzelli, J. W. Int. J. Chem. Kinet. 1996, 28, 773-790. (15) Dagaut, P.; Voisin, D.; Cathonnet, M. Combust. Flame 1996, 106, 62-68. (16) Hsieh, L. T.; Lee, W. J.; Chen, C. Y.; Wu, Y. P. G.; Chen, S. J.; Wang, Y. F. J. Hazard. Mater. 1998, 63, 69-90. (17) Wang, Y. F.; Lee, W. J.; Chen, C. Y.; Hsieh, L. T. Environ. Sci. Technol. 1999, 33, 2234-2240.
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Figure 1. Conversions of SO2 (CSO2) and CS2 (CCS2), relative to different SO2 feeding concentrations and CS2/SO2 ratios (R) at various discharge powers. hydrogen, nitrogen, and sulfur and and Heraeus/CHN-O rapid analyzer for oxygen), as well as the structures of the deposition were determined by X-ray diffraction (XRD) spectroscopy (model D/MAX III-V, Rigaku) with Cu KR radiation, over a scanning range of 5°-80° 2θ. Scanning electron microscopy (SEM) (Hitachi, model HF-2000) coupled with energy-dispersive X-ray analysis (EDX) (Noran, Model Voyager 1000) spectrometry, was also used to examine the appearances and morphologies of these depositions.
Results and Discussion Conversions of SO2 and CS2. The conversions of SO2 (CSO2) and CS2 (CCS2) were studied at various [SO2] and R values under various values of P. Figure 1A reveals that either exclusively SO2 or exclusively CS2 was difficult to convert, because the dissociated fragments would recombine in a plasma environment. The CSO2 and CCS2 conversions reached only 5.8% and 9.1%, respectively, at P ) 60 W, as well as 10.0% and 10.5%, respectively, when the value of P was increased to 120 W. However, the addition of CS2 improved the CSO2 and CCS2 conversions significantly. At [SO2] ) 2% with [CS2] ) 2% (that is, R ) 1), Figure 1B shows that the CSO2 conversion increased from 27.3% to 66.8% when P increased from 15 W to 120 W, whereas at R ) 2 (that is, [CS2] ) 4% and [SO2] ) 2%), the CSO2 conversion increased apparently from 26.5% to 98.2% (see Figure 1C). In regard to lower [SO2] values, such as [SO2] ) 1% at R ) 2, the CSO2 conversion increased rapidlys from 36.8% to 99.35%sas P increased from 15 W to 120 W (see Figure 1D). The results indicated that CSO2 and CCS2 were very sensitive to P, and P exhibited positive effects on CSO2 and CCS2. That should occur at a larger P values, to result in a greater plasma density, thus dissociating more molecular CS2 and SO2 until their complete conversion occurs. The results also expressed the influence of R on CSO2, especially under an applied power of <60 W: the CSO2 conversion was very low with R ) 0 (see Figure 1A), whereas the CSO2 conversion increased fairly well with R ) 1 (see Figure 1B) and significantly with R ) 2 (see Figure 1C, 1D). Moreover, a value of [SO2] ) 1% also resulted in a slightly higher CSO2 conversion than that of 2% [SO2] ) 2% to reveal that, at a same value of P, the close plasma density shared fewer SO2 molecules (a lower [SO2]) to provide more probabilities of dissociation. Because of the apparent influence on the CSO2 conversion by the inlet CS2/SO2 ratios (R values), the experi-
Conversion of SO2 and CS2 to Elemental Sulfur
Energy & Fuels, Vol. 18, No. 2, 2004 467 Table 1. Elemental Analysis Data of Sulfur Depositions
elemental content (wt %) sulfur carbon oxygen hydrogen nitrogen S/C ratio a
Figure 2. Conversions of SO2 (CSO2) and CS2 (CCS2), relative to different discharge powers and operating pressure at various CS2/SO2 ratios R at [SO2] ) 2%.
Figure 3. Comparisons of energy utilization efficiency for different applied powers (A) at R ) 2 and (B) for different inlet CS2/SO2 ratios at 30 W at [SO2] ) 2%.
mental conditions under two levels of power and two levels of operating pressure were examined. Figure 2 shows that the CSO2 conversion increases significantly as R increases, whereas the CCS2 conversion decreases as R increases, because a higher R value means a higher feeding concentration of CS2. It also shows that CCS2 is much greater than CSO2, because of the fact that CS2 is decomposed more easily than SO2, until R increases to 2, the approximately equal, higher CSO2 and CCS2 values are observed to reach 95.3% and 96.1%, respectively, at a power of 90 W with a pressure of 30 Torr (see Figure 2B), and 95.7% and 98.3%, respectively, at 90 W with a pressure of 10 Torr (see Figure 2C). When R increased to a value of 3, the CSO2 and CCS2 conversions both decrease. Hence, the better CSO2 and CCS2 conversions were obtained simultaneously under R ) 2 conditions. In addition, the CSO2 conversion at 30 W (see Figure 2A) was less than that at 90 W (Figure 2B), which revealed that a higher discharge power resulted in a higher decomposition fraction, because of the larger plasma density. However, the CSO2 conversion at 30 Torr (see Figure 2B) was just only slightly less than that at 10 Torr (see Figure 2C), which indicates that the operating pressure was not a very important parameter under such a homogeneous discharge condition; hence, most experimental conditions were operated at 30 Torr in this study. The definition of energy efficiency of CS2 (EECS2) and SO2 (EESO2) is the mass removed divided by the applied power (in units of mg/kJ). Figure 3A and 3B showed that more economic energy utilization was realized at P ) 30 W and under R ) 2 conditions; the EECS2 and EESO2 values were 4.48 and 11.61 mg/kJ, respectively.
depositions at major discharge zone (N ) 3)a
depositions at downstream zone (N ) 3)a
98.50 1.13 <0.10 0.17 0.10 87.2
98.60 1.05 <0.10 0.14 0.11 93.9
N represents the number of samples.
However, under these conditions, the CSO2 and CCS2 conversions were only 69.5% and 76.2%, respectively. Solid Product Patterns. The reaction of a mixture of inlet CS2/SO2 ratio R ) 2 with various P values were performed to evaluate the product patterns. The components of solid samples were analyzed by an elemental analyzer and are listed in Table 1. The results showed that the depositions collected from the major discharge zone and the downstream zone were composed almost entirely of elemental sulfur, and the purities of sulfur were >98.5% with trace atoms of carbon (<1.13%), hydrogen (<0.17%), and nitrogen (<0.11%), whereas the oxygen content was below the detection limit (see Table 1). Hence, the formation pathways of elemental sulfur for these two depositions should be similar and were evidenced by the similar XRD patterns. The XRD patterns of depositions gained from the major discharge zone and the downstream zone are consistent with that from the Joint Committee for Powder Diffraction Standards standard (JCPDS File Card No. 83-2284), to conduct to the structural characteristics of these two aggregates contained high contents of S8. In regard to morphology, observation of the deposition aggregates via SEM showed that they were remarkably different. The SEM photomicrographs show that the depositions at the major discharge zone were in the form of fine spherical aggregates (Figure 4A), whereas the depositions collected from the downstream zone were in the form of thin, long, leaf-shaped aggregates ∼100 nm thick (Figure 4B). The difference in the configurations of the aggregate should be due to their intrinsic differences associated with the formation pathways. In the major discharge zone, elemental sulfur first generated from this plasma process was in a gaseous form, and then rapidly diffused to and/or recombined, attached, and aggregated on the wall to form polysulfur via the heterogeneous wall reaction. However, the growth of sheetlike polysulfur suggests that it may be formed through parts of the former aggregate being melted away, because of the higher wall temperature, as a result from the electrodes, then flowed, condensed or quenched, and combined with gaseous sulfur. However, the deposition processes are complicated and the measurement of intermediates that form in the plasmachemical reaction process is difficult for the identification of polymerization mechanisms at this stage, which warrants the need for further confirmation in the future. To understand the qualitative analysis for its completeness, the quantification data were checked through overall carbon mass balance of the gaseous products. The results showed that the carbon mass balance was reduced within 3.0% to reveal that almost all major
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Tsai et al.
Figure 6. Selectivities of elemental sulfur (SS) and CO (SCO) (A) for various CS2/SO2 ratios at 90 W and (B) for different discharge powers at CS2/SO2 ) 2.
Figure 4. SEM photomicrographs of depositions collected from (A) the major discharge physical zone and (B) the downstream zone. (Conditions: [SO2] ) 2%, inlet CS2/SO2 ratio R ) 2, at P ) 90 W.)
Figure 5. Mole fraction of products, relative to (A) different inlet CS2/SO2 ratios at P ) 90 W and (B) various discharge powers at CS2/SO2 ) 2 for [SO2] ) 2%.
products were found and the reduction of mass should be caused by the formation of depositions. Gaseous Product Distributions. The major gaseous products detected were CO, CO2, and OCS. Their yields were based on the mole fraction of the effluent and are shown in Figure 5. The results showed that the product compositions apparently are affected by different R values at P ) 90 W (Figure 5A) and by various P values at R ) 2 (see Figure 5B) at [SO2] ) 2%. Considerable quantities of elemental sulfur (expressed as S1, as calculated by the sulfur mass balance) were observed. Figure 5A shows that, as R increased
from 0 to 3, the sulfur products were presented mainly in the form of elemental sulfur and its molar fractions (total sulfur, expressed as S1) were consistently increased from 0.17% to 10.9%, accompanied by an increase in [OCS], from 0% to 0.13%. The low [OCS] value was observed because almost all the C atoms react with O atoms to form mainly CO and, hence, resulted in the inhibition of OCS formation. This inference can be confirmed by the result that the [CO] value was increased significantly from 0% to 3.37%. However, the [CO2] value was only in the range of 0%-0.07%, which indicates that carbon (provided by CS2) that initially reacted primarily with oxygen, which was yielded from the decomposition of SO2, were more likely to react to form CO as the value of R increased; even though the R value was <1, higher [CO] values still were observed, because of the high thermodynamic stability of CO. With increased discharge power from P ) 15 W to P ) 120 W, Figure 5B presented that the total sulfur concentration [S1] increased from 2.66% to 9.85%, the [CO] concentration increased from 0.70% to 3.59%, the [CO2] content was elevated from 0.022% to 0.052%, whereas the [OCS] value decreased from 0.46% to 0.1%. The variation in [OCS] indicated that a higher [OCS] was used around a higher R value under lower P conditions, which was unfavorable, in regard to increased conversion. Hence, the yield of OCS was restricted not only by the formation of precursors, but also by the depletion of OCS via the reoxidation reaction (OCS + O f CO + SO) and the re-dissociation of the weaker OCdS bond.9 Effect of CS2/SO2 and Power on Sulfur Selectivity. The selectivities of sulfur (SS ) [S1]/([S1] + [OCS])) and CO (SCO ) [CO]/([CO] + [CO2])) were examined. SS remained in the range of 0.982-0.988, regardless of the R values at 90 W (Figure 6A), which shows that, under a higher P value (such as 90 W), SS does not vary with R, which indicates that [CO] does not dominate the formation rate of OCS, perhaps because of the easily redissociation of OCS. The inference is matched with the SS value, which increases from 0.859 to 0.990 as P increases from 15 W to 120 W at R ) 2 (Figure 6B). In regard to the SCO value, Figure 6A shows that SCO is highly and positively dependent on the value of R and increased from 0.714 to 0.980 as R increased from 0.3 to 3. In addition, Figure 6B indicates that SCO is affected only somewhat by P and is 0.971-0.994, because at R ) 2, the stoichiometry atomic C:O ratio is 1:1; that is, the theoretical carbon-containing product under this
Conversion of SO2 and CS2 to Elemental Sulfur
condition is CO, which results in inhibition of the formation of CO2 and OCS and the recombination of CS2 and SO2. The results also show that when P was >30 W with R ) 2 or higher, SS can attain a value as high as 0.95 (see Figure 6B). In summary, the experimental results suggest that the utilization of the radio-frequency (RF) plasma technique is beneficial to convert simultaneously high concentrations of sulfur dioxide (SO2) and carbon disulfide (CS2) to elemental sulfur at an inlet CS2/SO2 ratio of R ) 2 with a higher discharge power P. By combining the homogeneous and heterogeneous reac-
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tions, the overall conversion reaction, at R ) 2, [SO2] ) 2%, and with a power of P ) 90 W applied at 30 Torr in the RF plasma-induced system, is expressed as follows: SO2 + 2CS2 f solid depositions (4.79S1) + gaseous products (1.75CO + 0.038CO2 + 0.058OCS). Acknowledgment. This research was supported by funds from National Science Council, Taiwan, through Grant No. NSC 91-2211-E-006-025. EF030105Y