Sulfur dioxide oxidation kinetics

The chemical reactions involved in this process were outlined in the patent, which laid the basis for this industry. However, many improvements in practice have been achieved from a more detailed knowledge of the gas phase kinetics of the sulfur dioxide oxidation step, as well as from a better understanding of the gas-liquid equilibria associated with the hydration step. 

The reverse-flow chemical reactor (RFR) has been shown to be a potentially effective technique for many industrial chemical processes, including oxidation of volatile organic compounds such as propane, propylene, and carbon monoxide removal of nitrogen oxides sulfur dioxide oxidation or reduction production of synthesis gas methanol formation and ethylbenzene dehydration into styrene. An excellent introductory article in the topic is given by Eigenberger and Nieken on the effect of the kinetic reaction parameters, reactor size, and operating parameters on RFR performance. A detailed review that summarizes the applications and theory of RFR operation is given by Matros and Bunimovich. 

To date, there have been several unsuccessful attempts to fit these results to a simple model—for example, one based on a shrinking unreacted core or on reaction of a porous solid. The apparent role of water in the mechanism suggests that sulfur dioxide may be oxidized to sulfur trioxide on the surface and that sulfur trioxide diffuses through a product layer to react with calcium carbonate. This concept would be consistent with the similar kinetics observed for half- and fully calcined stone since the rate-determining step would presumably be the same in either case. This view is supported by the observation that reactivity in a fluidized bed decreases somewhat above about 850 °C because the thermodynamics of sulfur dioxide oxidation become less favorable. On the other hand, Borgwardt s observations with fully calcined stone (1) suggest that the decreased reactivity is caused by hard-burning of the stone. 

Mathur and Thodos Chem. Eng. Sci., 21 (1191), 1966] used the initial rate approach to analyze the kinetics of the catalytic oxidation of sulfur dioxide. They summarized the most plausible rate controlling steps for the reaction as ... 

The resulting products, such as sulfenic acid or sulfur dioxide, are reactive and induce an acid-catalyzed breakdown of hydroperoxides. The important role of intermediate molecular sulfur has been reported [68-72]. Zinc (or other metal) forms a precipitate composed of ZnO and ZnS04. The decomposition of ROOH by dialkyl thiophosphates is an autocata-lytic process. The interaction of ROOH with zinc dialkyl thiophosphate gives rise to free radicals, due to which this reaction accelerates oxidation of hydrocarbons, excites CL during oxidation of ethylbenzene, and intensifies the consumption of acceptors, e.g., stable nitroxyl radicals [68], The induction period is often absent because of the rapid formation of intermediates, and the kinetics of decomposition is described by a simple bimolecular kinetic equation... 

Brodzinsky, R., S. G. Chang, S. S. Markowitz, and T. Novakov, Kinetics and Mechanism for the Catalytic Oxidation of Sulfur Dioxide on Carbon in Aqueous Suspensions, J. Phys. Chem., 84, 3354-3358 (1980). 

Hoffmann, M. R., and D. J. Jacob, Kinetics and Mechanisms of Catalytic Oxidation of Dissolved Sulfur Dioxide in Aqueous Solution An Application to Nighttime Fog Water Chemistry, in SO2, NO, and N02 Oxidation Mechanisms Atmospheric Considerations, Acid Precipitation Series, Vol. 3, pp. 101-172 (J. I. Teasley, Series Ed.), Butterworth, Stoneham, MA, 1984. 

Hoffmann, M. R., On the Kinetics and Mechanism of Oxidation of Aquated Sulfur Dioxide by Ozone, Atmos. Environ., 20, 1145-1154(1986). 

Manganese(III) has been employed for the oxidation of aldoses, and a general mechanism for the oxidation has been proposed.167 The oxidation of hexoses, pentoses, hexitols, and pentitols by Mn(III), as well as by other cations, was proposed to proceed via a free-radical mechanism,168 as shown in Scheme 26. Oxidation of alditols produces the corresponding aldoses, which are further oxidized in the presence of an excess of oxidant to the lower monosaccharides and thence to formaldehyde, formic acid, and even carbon dioxide. The kinetics for the oxidation of aldoses and ketoses by Mn(III) in sulfuric acid medium have been reported.169... 

Gas phase oxidation of sulphur dioxide is kinetically inhibited and virtually impossible without a catalyst at any temperature. At ordinary temperatures the reaction is so slow that, in practical terms, it does not occur at all. Increasing the temperature increases the rate of reaction, but simultaneously the position of the equilibrium shifts unfavourably -away from sulphur trioxide and towards sulfur dioxide and oxygen. Without a catalyst, the temperature needed to make the system react at a practical speed is so high that a very poor conversion [i.e. very little SO3 production] is obtained."... 

Aurousseau et al. [109] electrochemically scrubbed S02-containing waste gas. The sulfur dioxide (0.7%) was dissolved in 0.5 M sulfuric acid, transported to the electrode, and finally oxidized at the graphite anode. The oxidation was limited by the transport of sulfur dioxide to the electrode as well as by poor reaction kinetics at the electrode. The use of three-dimensional electrodes was suggested to alleviate these problems. 

Because the PPR is operated as an adiabatic reactor, the strongly exothermic oxidation reaction (iii) causes a temperature wave traveling through the bed, giving rise to a peak outlet temperature in the initial period of the acceptance cycle, as. shown in Fig. 26. In this figure, the temperature profile predicted by a mathematical model developed at the Shell laboratory in Amsterdam is compared with the profile measured in an industrial reactor to be described later. During the initial oxidation period, the copper is not yet active for reaction with sulfur oxides, so there is a slip of sulfur in the initial period, as can be seen in Fig. 27. It can be inferred that the sulfur dioxide concentration profile of the effluent of the industrial reactor is in close agreement with the profile predicted on the basis of a kinetic model developed at the Shell laboratory in Amsterdam. 

Leitenberger s work (1939) is concerned with the oxidation of sulfur dioxide in adiabatic beds and tubular reactors. Using the Boresskow-Slinko kinetic equation he calculated the optimal tem-6... 

Example 5 Here, we maximize the conversion in the catalytic oxidation of sulfur dioxide in fixed-bed reactors, which has been investigated by Lee and Aris (1963). The reaction and the kinetics are described as follows ... 

The anodic oxidation of benzene produces a mixture of polyphenylene compounds. This oligomerization can be performed in acetonitrile [21] or in liquid sulfur dioxide [22]. Mixed coupling between naphthalene and alkyl benzenes has also been demonstrated (Table 1, numbers 12-16). The relative yield of mixed coupling products increases with the basicity of the alkyl benzene with mesitylene 19%, with tetramethylbenzene 42%, and with pentamethylbenzene 64% of mixed coupling products are obtained. This suggests an electrophilic reaction between naphthalene cation radicals and alkylbenzenes. The mixed coupling reaction of phenanthrene with anisole has been studied kinetically. The results indicate that initially a complex PA is formed between the phenanthrene radical cation and anisole, followed by an electron transfer from the complex. The resulting PA" -anisole complex then decomposes to the product [23]. 

Hedges, S.W. Yeh, J.T. Kinetics of sulfur dioxide uptake on supported cerium oxide sorbents. Environ. Progr. 1992, II (2), 98-103. 

The kinetics of the electro-oxidation of graphite in molten KHSO4 to volatile compounds (carbon dioxide, carbon monoxide, and traces of sulfur dioxide) was studied in the range 180—320° C. The faradaic yield for carbon dioxide (4F per mol of carbon dioxide) is about 90%. The rds is the desorption of oxygen-containing intermediate species [91]. 

Example 13-5 Using the one-dimensional method, compute curves for temperature and conversion vs catalyst-bed depth for comparison with the experimental data shown in Figs. 13-10 and 13-14 for the oxidation of sulfur dioxide. The reactor consisted of a cylindrical tube, 2.06 in. ID. The superficial gas mass velocity was 350 lb/(hr)(ft ), and its inlet composition was 6.5 mole % SO2 and 93.5 mole % dry air. The catalyst was prepared from -in. cylindrical pellets of alumina and contained a surface coating of platinum (0.2 wt % of the pellet). The measured global rates in this case were not fitted to a kinetic equation, but are shown as a function of temperature and conversion in Table 13-4 and Fig. 13-13. Since a fixed inlet gas composition was used, independent variations of the partial pressures of oxygen, sulfur dioxide, and sulfur trioxide were not possible. Instead these pressures are all related to one variable, the extent of conversion. Hence the rate data shown in Table 13-4 as a function of conversion are sufficient for the calculations. The total pressure was essentially constant at 790 mm Hg. The heat of reaction was nearly constant over a considerable temperature range and was equal to — 22,700 cal/g mole of sulfur dioxide reacted. The gas mixture was predominantly air, so that its specific heat may be taken equal to that of air. The bulk density of the catalyst as packed in the reactor was 64 Ib/ft. ... 

Hoffmann, M. R. and Edwards, J. 0. "Kinetics and Mechanism of the Oxidation of Sulfur Dioxide by Hydrogen Peroxide in Acidic Solution," J. Phys. Chem. 1975, 7, 2096-2098. 

Larson, T. V., Horike, N. R., and Halstead, H. "Oxidation of Sulfur Dioxide by Oxygen and Ozone in Aqueous Solution A Kinetic Study with Significance to Atmospheric Processes,"... 

Additionally, in a microreactor the intrinsic kinetics and deactivation behavior of SCR catalysts is studied with flows up to 1.5m h . In both test facilities it is possible to vary all process parameters temperature, the ammonia to nitric oxide feed ratio, the nitric oxide and sulfur dioxide concentrations, the space velocity, and the catalyst geometry. These techniques provide information for somewhat small areas and therefore should always be performed to complement bench- or laboratory-scale activity and selectivity measurements. 

Faust, B. C., M. R. Hoffmann, and D. W. Bahnemann (1989), Photocatalytic Oxidation of Sulfur Dioxide in Aqueous Suspensions of a-Fe203, J. Phys. Chem., 93, 6371-6381. Gonzalez, A. C., and M. R. Hoffmann (1989a), The Kinetics and Mechanism of the Catalytic Hydrolysis of Nitropheny Acetates by Metal Oxide Surfaces, J. Phys. Chem., submitted. 

Oxidation of sulfite to sulfate can proceed by mechanisms other than those involving oxygen from air. The reaction kinetics and the mechanics are not yet understood, but our experiments show that oxygen-free sulfite in lime/limestone slurries, exposed to sulfur dioxide, slowly decomposes under process conditions. In fact, our experiments indicate that auto-redox reactions of sulfur oxyacids can occur in all coal desulfurization systems, including coal-gasification systems and that impurities present in commercial flue gas systems are capable of catalyzing the reaction under process conditions. 

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