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Ind. Eng. Chem. Res. 2003, 42, 5946-5948
Thermodynamics of a New Cyclic Reaction System Involving BaS and BaSO4 for Converting Sulfur Dioxide to Elemental Sulfur Hong Yong Sohn† Department of Metallurgical Engineering, University of Utah, 135 S 1460 E RM 412, Salt Lake City, Utah 84112-0114
A new process for converting SO2 to elemental sulfur by a cyclic process involving CaS and CaSO4 was earlier developed through thermodynamic and experimental investigations. In this process, the raw material CaSO4 is reduced to produce CaS, which is used to reduce SO2 to elemental sulfur and produce CaSO4. The latter is then reduced to regenerate CaS. It has been shown that the reaction temperature for the reduction of SO2 to elemental sulfur is limited to approximately 1100 K for a reasonable degree of equilibrium conversion. It is the purpose of this paper to present the more favorable thermodynamic aspects of a similar cyclic process that involves BaS and BaSO4. It is shown that the reaction between SO2 and BaS is favorable to a much higher temperature of about 1400 K and the hydrogen reduction of BaSO4 to BaS is favorable in a much wider range of temperature. Gas streams containing high levels of sulfur dioxide are generated from nonferrous metal smelters, coalburning power plants, and integrated gasification combined cycle desulfurization units.1-4 Although a number of different methods of fixing sulfur dioxide, such as conversion to sulfuric acid or liquid sulfur dioxide, are available, reduction to elemental sulfur offers many attractive advantages. All of the existing processes for converting to elemental sulfur, however, either have been discontinued or have never been commercialized because of their complexities and/or costs. In a series of recent papers, Sohn and Kim5-7 described a new process for converting SO2 to elemental sulfur by a cyclic process involving CaS and CaSO4. In this process, the raw material CaSO4 is reduced to produce CaS, which is used to reduce SO2 to elemental sulfur and produce CaSO4. The latter is then reduced to regenerate CaS. Thus, the overall process does not generate a solid waste. It was also shown that no significant amounts of secondary gaseous pollutants are produced. A drawback for this reaction system is that the reaction temperature for the reduction of SO2 to elemental sulfur by CaS is limited to approximately 1100 K for a reasonable degree of equilibrium conversion.5 The rate at which this reaction can be carried out then is also limited accordingly. For example, at 1153 K and under the sulfur dioxide partial pressure of 25.8 kPa, about 60% of calcium sulfide powder was converted to calcium sulfate in 10 min,6 reducing the corresponding amount of sulfur dioxide to elemental sulfur. Although this rate is reasonably high, an even higher rate or a higher degree of SO2 conversion at the same temperature would certainly be advantageous. Thus, a large number of sulfide-sulfate systems, including those involving BaS, Cu2S, FeS, Na2S, Sb2S3, and ZnS, were studied to find a potential alternative to the CaSCaSO4 system. Among all of the systems analyzed, the BaS-BaSO4 system presented the most promising results, as described below. † Tel.: 801-581-5491. Fax: 801-581-4937. E-mail: hysohn@ mines.utah.edu.
Figure 1. Equilibrium composition for the reaction of 1 mol of BaS or CaS with 1 mol of SO2 at a total pressure of 1 atm: (a) BaS-SO2; (b) CaS-SO2.
Reduction of Sulfur Dioxide by Barium Sulfide The calculated equilibrium compositions for the BaSSO2 system, from a starting mixture of 1 mol of BaS and 1 mol of SO2, are shown at various temperatures in Figure 1a. The thermodynamic analysis was performed by the use of HSC chemical software developed
10.1021/ie030625g CCC: $25.00 © 2003 American Chemical Society Published on Web 10/21/2003
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5947 Table 1. Values of ∆G°f (kcal/mol) Used in the Equilibrium Calculations 800 K BaS8 BaSO49 CaS9 CaSO410-12 SO210 H2S9 H2O13
1000 K
1200 K
1400 K
1600 K
-104.778 -99.902 -94.621 -89.313 -83.997 -280.887 -259.466 -237.764 -216.123 -194.834 -109.918 -105.364 -100.610 -95.682 -90.804 -270.820 -249.552 -228.522 -207.763 -187.412 -72.489 -68.991 -65.513 -62.055 -58.614 -12.132 -9.827 -7.503 -5.174 -2.849 -48.640 -46.033 -43.363 -40.653 -37.915
Figure 3. Equilibrium composition for the reaction of BaSO4 or CaSO4 (1 mol) with H2 (3 mol) at a total pressure of 1 atm: (a) BaSO4-H2; (b) CaSO4-H2.
Figure 2. Equilibrium composition for the reaction of 1 mol of BaS or CaS, 1 mol of SO2, and 0.1 mol of H2 at a total pressure of 1 atm: (a) BaS-SO2-H2; (b) CaS-SO2-H2.
by Outokumpu Research Oy, which is based on the principle of the Gibbs free-energy minimization. The Gibbs free-energy data used in this and subsequent calculations are listed in Table 1. The amounts of all other species considered including sulfur species S3S8 were negligible. It is seen that when sulfur dioxide gas is reacted with an excess amount of barium sulfide, the solid product will be barium sulfate and the gaseous product will be essentially pure sulfur up to a temperature of about 1350 K. The product barium sulfate can be reduced to barium sulfide without any other environmental problems, as will be explained later. For comparison, the corresponding equilibrium compositions for the CaS-SO2 system are shown in Figure 1b. It is seen that the equilibrium conversion of SO2 to elemental sulfur in the BaS-SO2 system remains high up to a higher temperature (about 1400 K) than that in the CaS-SO2 system (below 1100 K). Therefore, the SO2
reduction reaction can be carried out at a higher temperature with BaS, presenting the possibility of greater reaction rates. Some sulfur dioxide streams may contain low levels of water vapor or hydrogen. Thus, an additional calculation was performed with the above feed with an additional 0.1 mol of hydrogen, as shown in parts a and b of Figure 2 respectively for BaS and CaS. It is seen that the hydrogen is consumed to reduce SO2 to produce water vapor but also generates a small amount of hydrogen sulfide. The content of hydrogen sulfide relative to sulfur is substantially lower at the higher temperatures (at ∼1350 K) at which the BaS-SO2 system can be operated, providing an additional advantage over the CaS-SO2 system (at ∼1050 K). In either case, after sulfur is separated by condensation, the remaining gas that might also contain unreacted SO2 could be recycled to the feed stream, scrubbed, or fed to a sulfuric acid plant, if one is available nearby. Any oxygen that might be contained in the feed stream would oxidize barium sulfide to barium sulfate. Hydrogen Reduction of Barium Sulfate The barium sulfate produced from the above-mentioned reduction of sulfur dioxide by barium sulfide to elemental sulfur must, in turn, be reduced to regenerate barium sulfide. Several reducing agents could be used, but hydrogen would be most convenient. Thus, in the previous work with the CaS-SO2 system,5,7 we inves-
5948 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
tigated the hydrogen reduction of calcium sulfate. The BaS-SO2 system, which has a number of advantages for the reduction of sulfur dioxide to elemental sulfur, as mentioned above, will work only if the produced barium sulfate can be reduced to regenerate barium sulfide. Equilibrium calculations for the overall H2-BaSO4 system at various temperatures were performed using the above-mentioned HSC software. The calculated data presented in Figure 3a indicate that BaS is the main product and the side reactions that produce BaO, SO2, or H2S do not occur to appreciable extents, much less even at higher temperatures than the corresponding side reactions in the H2-CaSO4 system shown in Figure 3b. Thus, this presents the possibility of greater reaction rates for barium sulfate reduction to barium sulfide, as was the case for the reaction of sulfur dioxide by barium sulfide compared with that by calcium sulfide. On the basis of these favorable results of thermodynamic analyses, experimental work on the rates and other aspects of the sulfur dioxide reduction through the BaS-BaSO4 system will be carried out in this laboratory. Literature Cited (1) Dalton, S. M. Flue Gas Desulfurization Design in the U.S.: Additives and Materials of Construction. In Symposium Series on Desulfurization 3; Kyte, W. S., Ed.; Chameleon Press: London, 1993; pp 67-77. (2) Friedman, L. J. Production of liquid SO2, sulfur and sulfuric acid from high strength SO2 gases. In Sulfur Dioxide Control in Pyrometallurgy; Chatwin, T. D., Kikumoto, N., Eds.; TMS: Warrendale, PA, 1981; pp 205-220. (3) Asteljoki, J. A.; Bailey, L. K.; George, D. B.; Rodolff, D. W. Flash Converting-Continuous Converting of Copper Mattes. J. Met. 1985, 37 (5), 20-23.
(4) Kwong, V.; Meissner, R. E. Rounding up Sulfur. Chem. Eng. 1995, 102, 74-83. (5) Sohn, H. Y.; Kim, B.-S. A Novel Cyclic Process Involving CaS and CaSO4 for Converting Sulfur Dioxide to Elemental Sulfur without Generating Secondary Pollutants. 1. Determination of Process Feasibility. Ind. Eng. Chem. Res. 2002, 41, 3081-3086. (6) Sohn, H. Y.; Kim, B.-S. A Novel Cyclic Process Involving CaS and CaSO4 for Converting Sulfur Dioxide to Elemental Sulfur without Generating Secondary Pollutants. 2. Kinetics of the Reduction of Sulfur Dioxide by Calcium Sulfide Powder. Ind. Eng. Chem. Res. 2002, 41, 3087-3091. (7) Sohn, H. Y.; Kim, B.-S. A Novel Cyclic Process Involving CaS and CaSO4 for Converting Sulfur Dioxide to Elemental Sulfur without Generating Secondary Pollutants. 3. Kinetics of the Hydrogen Reduction of Calcium Sulfate to Calcium Sulfide. Ind. Eng. Chem. Res. 2002, 41, 3092-3096. (8) Kosolapovoi, T. G. Svoistva, Polysenie Prim. Tugoplavkih Soedin. 1986, 928. (9) Barin, I. Thermochemical Data of Pure Substances; VCH Verlags Gesellschaft: Weinheim, Germany, 1989. (10) Barin, I. Thermochemical Data of Pure Substances; VCH Verlags Gesellschaft: Weinheim, Germany, 1993; Parts I and II. (11) Barin, I.; Knacke, O.; Kubaschewski, O. Thermochemical Properties of Inorganic Substances, Supplement; SpringerVerlag: Berlin, 1977; p 861. (12) Barin, I.; Knacke, O.; Kubaschewski, O. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin and New York, 1973 and 1977 (Supplement). (13) JANAF Thermochemical Tables, 3rd ed.; Chase, M. W., et al., Eds.; American Chemical Society: Washington, DC, 1985; Vol. 14, pp 1-1856.
Received for review July 25, 2003 Revised manuscript received September 29, 2003 Accepted October 1, 2003 IE030625G
