Sulfur Separation Factors Observed During
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NUCLEAR TECHNOLOGIES AND METHODS 111 In Poland, three big power stations use lignite in the ash and slag is depleted in the heavy isotope as an energy material: Bełchatów, Turów and Pąt- 34S in the coal combustion process. This is not clear nów. The solid samples (coal, ashes and slag) were why this difference between these results occurs, taken from the Turów and Pątnów Power Stations but probably it arises due to the combustion pro- to determine sulfur isotope ratio (34S/32S) in the cess conditions. Table 2. δ34S in different forms of sulfur in lignite and the products of its combustion from the Pątnów Power Station. coal combustion process. Each form of sulfur has The present and earlier data of sulfur isotope been prepared by extraction [5] of solid samples ratio in coal and lignite (including desulfurization and transform into stable compounds, which can process) indicate that it is possible to apply this be subsequently converted to gas phase (SO2) for method for further investigation of the migration mass spectrometric analysis [6] (Table 1 and 2). of sulfur compounds in ground waters and atmo- The received results for the Turów Power Sta- sphere. tion (Table 1) (δ34S range from -11.01 to 10.95‰) suggest that the sulfur isotope ratio, concerning References organic sulfur in the lignite, is the same as in the primary plant material. Sulfur is an essential con- [1]. Harter P.: Sulphate in the atmosphere. IEA Coal Re- stituent of the living cell. The plant can take up search Report ICTIS/TR30, London 1985, p.155. 2– SO4 or SO2 directly from the environment. The [2]. Chmielewski A.G., Wierzchnicki R., Derda M., Miko- sulfur organic compounds, resulting from assimi- łajczuk A.: Nukleonika, 47, 67 (2002). 2– 34 [3]. Krouse H.R.: Stable isotope studies of sulfur flows and latory of SO4 reduction are depleted in S. In the case of the Pątnów Power Station, the transformations in agricultural and forestry ecosystems. sulfur is depleted in 34S too (δ34S range from 7.23 IAEA, 1991, IAEA-SM-313/108. δ34 [4]. Chmielewski A.G., Derda M.: In: INCT Annual Re- to 22.6‰). S values in the slag and ash from port 2002. Institute of Nuclear Chemistry and Tech- the Turów Power Station are enriched in the heavier nology, Warszawa 2003, pp.122-123. 34 isotope S in the coal combustion process. The [5]. Westgate L.M., Anderson T.F.: Anal. Chem., 54, 2136 same effect was observed for the Bełchatów Power (1982). Station in earlier investigations [4]. The results δ34S [6]. Hałas S., Wolacewicz W.D.: Anal. Chem., 53, 686 for the Pątnów Power Station are opposite. Sulfur (1981). SULFUR SEPARATION FACTORS OBSERVED DURING ADSORPTION OF SO2 ON DIFFERENT SILICA GELS Agnieszka Mikołajczuk, Andrzej G. Chmielewski During this experimental work the sulfur isotope The low energy, about 50 kJ/mol, corresponds (34S/32S) separation factors were determined. The to a weak physical adsorption and the second about kinetic and equilibrium isotope effects were ob- 80 kJ/mol, to chemisorption [9]. The weak adsorp- served during the adsorption of sulfur dioxide on tion is related to interactions of sulfur dioxide and different types of silica gel sorbents. free sites on the surface, whereas a strong adsorp- Adsorption is also used in separation and puri- fication processes, including hazardous pollutant removal from flue gases. Sulfur dioxide is believed to be a major precursor of acid rains, therefore the control of sulfur dioxide emissions is a signifi- cant subject for research and development, as well as industrial implementation [1]. Many adsorbents were investigated for sulfur dioxide adsorption like carbonaceous adsorbents [2-4], metallic oxide sorbents of transition metals [5] or molecular sieves [6-7]. Process of sulfur di- oxide adsorption has been studied extensively. The products of surface reactions were analyzed from the point of view of removal efficiency and the fea- sibility of regeneration. It was found that sulfur dioxide on the activated carbon is adsorbed with Fig. Experimental adsorption isotherms of sulfur dioxide two adsorption energies [8]. on silica gels, T = 293 K, po = 870 hPa. 112 PROCESS ENGINEERING tion is connected with the presence of oxygen [8]. The sulfur separation factors were determined Raymundo-Pinero and co-workers suggested that during sulfur dioxide adsorption on silica gel samples. oxidation of SO2 to SO3 occurs in the 7 Å pores. At the beginning, in the experiments 4 g of silica gel The role of pore structure is not so well defined was used, the pressure of sulfur dioxide in a vacuum as the role of surface oxygenated groups [9]. Al- line was 870 hPa. The experimental data are shown though it is believed that the developed porosity is in Table 1. important to “store” sulfuric acid as a product of The results presented in Table 2 demonstrate oxidation, large pores decrease the conversion of that the silica gel SG 1 has the best properties for Table 1. Properties of silica gels. SO2 to SO3, which is accompanied by a decrease in the separation of sulfur isotope in such conditions; a total sorption capacity. On the other hand, when the silica gel mass, temperature and gaseous sul- small pores are present sulfuric acid is strongly fur dioxide pressure over the adsorbent. The gas bonded to the surface [10]. phase was enriched in the heavy sulfur isotope. In this work, the most important point was the Sulfur dioxide in a gas phase contained 4.49‰ iso- sulfur separation factor. During the experimental tope 34S more when compared with sulfur dioxide work adsorption isotherms of sulfur dioxide on silica adsorbed on the silica gel SG 1, while the time gels were determined (Fig.). According to the litera- adsorption was 5 min. Sample SG 1A contained Table 2. Sulfur isotope fractionation factors between sulfur dioxide in gas phase and sulfur dioxide adsorbed on silica gel. Mass of silica gel samples was 4 g, po = 870 hPa, standard deviation of measurements – 0.02‰. ture data [1], the adsorption isotherms are described CoCl2, it was the reason why the sulfur separation by the Freundlich model or deactivation model factor was smaller by ca. 1.33‰ than that obtained which suggests a significant decrease of activity of for SG 1. Sulfur dioxide adsorbed on silica gel was the sorbent with time with respect to probable enriched in the 32S. The separation factor was the changes in pore structure, in the active surface area smallest in the case when sulfur isotopes 34S and and active site distribution of the sorbent (silica gel). 32S were separated on SG 4. Properties of silica gels are shown in Table 1. During the experimental work, the time of ad- The adsorption of sulfur dioxide increases from sorption of sulfur dioxide on silica gels was changed silica gel SG 1A to SG 4. Silica gels SG 1 and SG 1A from 5 min to 600 h. After 1 month, the system had similar physical properties because SG 1A ad- attained the isotope equilibrium state. These sepa- ditionally contained CoCl2 incorporated in its struc- ration data are shown in Table 3, which refer to ture as an indicator. Sulfur dioxide adsorption on the results completed last year [11]. Table 3. Sulfur isotope fractionation factors between sulfur dioxide in gas phase and sulfur dioxide adsorbed on silica gel. Weight of silica gels SG 1, SG 1A, SG 2 were 8 g; mass of silica gels SG 3 and SG 4 was 4 g; po = 870 hPa; standard deviation was 0.02‰. –4 SG 1A decreased by about 5x10 g SO2/g gel com- Mass of silica gel SG 3 and 4 samples used was pared with gel SG 1. only 4 g due to the small volume of vacuum line. NUCLEAR TECHNOLOGIES AND METHODS 113 Probably, the isotope separation factor will be References higher if the mass of silica gel will be bigger. When the systems attained equilibrium state, the sepa- [1]. Kopac T., Kocabas S.: Chem. Eng. Proc., 41, 223-230 ration factor measured was almost the same for (2002). both gels. Sulfur dioxide in the gas phase was en- [2]. Bagreev A., Bashkova S., Bandosz T.J.: Langmuir, 18, riched in the isotope 32S. 1257-1264 (2002). Sulfur separation factors were low in the equi- [3]. Bashkova S., Bagreev A., Locke D.C., Bandosz T.J.: librium state (below 1). When the adsorption time Environ. Sci. Technol., 35, 3263-3269 (2001). [4]. Lee Y.-W., Park J.-W., Choung J.-H., Choi D.-K.: was 5 min, the separation factor was higher for each Environ. Sci. Technol., 36, 1086-1092 (2002). sample of silica gels and the highest was for sample [5]. Lin Y.S., Deng S.G.: Sep. Purif. Technol., 13, 65 (1998). SG 2 (Table 3). Initially, on silica gel was adsorbed [6]. Kopac T., Kayamakci E., Kopac T.: Chem. Eng. 32 SO2 and the gas phase was enriched in the heavy Commun., 164, 99-110 (1998). sulfur isotope. When the system has reached equi- [7]. Kopac T.: Chem. Eng. Proc., 38, 45-53 (1999). librium state sulfur dioxide which contained iso- [8]. Davini P.: Carbon, 39 (9), 1387-1393 (2001). tope 34S was predominantly adsorbed on silica gel. [9]. Raymundo-Pinero E., Cazola-Amorós D., Salinas- The pressure/temperature kinetic process has -Martinez de Lecea C., Linares-Solano A.: Carbon, much more advantages over equilibrium process 38 (3), 335-344 (2000). [10]. Bagreev A., Rahman H., Bandosz T.J.: Environ. Sci. in regard to possible applications. Silica gels SG 1, Technol., 34, 4587-4592 (2000). SG 1A and SG 2 may be used as adsorbents for [11].