Ind. Eng. Chem. Res. 2002, 41, 3087-3091
3087
A Novel Cyclic Reaction System 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 Hong Yong Sohn* and Byung-Su Kim† Department of Metallurgical Engineering, University of Utah, 135 S 1460 E RM 412, Salt Lake City, Utah 84112-0114
The process feasibility for converting sulfur dioxide to elemental sulfur by the cyclic reaction involving calcium sulfide and calcium sulfate was established in part 1 of this series. In this part, detailed experimental results are presented on the kinetics of the reaction between calcium sulfide and sulfur dioxide, which produces elemental sulfur and calcium sulfate. The experiments were carried out in the temperature range of 973-1153 K under sulfur dioxide partial pressures between 9 and 60 kPa by the use of a thermogravimetric analysis technique. As an example, about 60% of the calcium sulfide powder from the hydrogen reduction of fresh calcium sulfate was converted in 10 min at 1153 K under a sulfur dioxide partial pressure of 25.8 kPa. The reactivity decreased somewhat during the first five cycles of reaction and regeneration but remained largely intact thereafter up to the tenth cycle. A pore-blocking model was found to fit the reaction rate. The reaction is first order with respect to sulfur dioxide partial pressure and has an activation energy of 24-32 kcal/mol (101-134 kJ/mol) at different reaction cycles. 1. Introduction The process feasibility for converting sulfur dioxide to elemental sulfur by the cyclic reaction involving calcium sulfide and calcium sulfate was established in part 1 of this series.1 The advantages of this new process have been discussed therein. It was shown that the reactions take place at reasonable rates. For an optimal design of any process, however, a detailed investigation of the kinetics under sufficiently wide ranges of the process variables is necessary. Thus, the kinetics of the two component reactions comprising the overall process have been studied. This paper is concerned with the results for the reaction of sulfur dioxide with calcium sulfide, which reduces the former to elemental sulfur and oxidizes the latter to calcium sulfate. This reaction can be represented by1
CaS(s) + 2SO2(g) ) CaSO4(s) + S2(g)
(1)
The intrinsic kinetics reported in this paper was determined by eliminating the effects of external mass transfer and diffusion between the particles. The reduction kinetics of calcium sulfate with hydrogen gas is reported in part 3 of this series.2 2. Experimental Section Experiments were carried out in a thermogravimetric analysis (TGA) apparatus described in part 1 of this series.1,3 Calcium sulfide particles were obtained by reducing fresh calcium sulfate particles as the starting material, and the reaction product calcium sulfate was * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 801-581-5491. Fax: 801-5814937. † Present address: Korea Institute of Geoscience & Mineral Resources, Daejon, Korea 305-350.
reduced by hydrogen to regenerate calcium sulfide, which was used to react with sulfur dioxide in the subsequent cycle. The reaction cycle was repeated to the tenth cycle. The reactivity of fresh reagent-grade calcium sulfide particles was also determined as a reference, but the product calcium sulfate from this reaction was not used further. The calcium sulfate powder (99.5%) obtained from Aldrich Chemical Co. was of
