OXIDATION OF SO

The S was postulated to be in the singlet state in accordance with the results of earlier workers who found S( D) to be formed in the photolytic reaction (77). 

Reactions (65) and (66), followed respectively by reactions (45) and (59), adequately explain the formation of the final products of the thermal oxidation. Thus there is no need to postulate the occurrence of reaction (79). The photolysis of pure OCS indicates that reaction (78), with the S atoms in the singlet state, does indeed occur. 

The photochemical oxidation of SO2 was studied by several workers in the 1950 s. As the work has been reviewed by Leighton, we will only summarize the more important results. The product of the photo-oxidation is SO3, or, if water is present, H2SO4. Various workers have obtained quantum yields of SO3 ranging from 0.3 to 0.003. As the photo-oxidation is initiated by light of insuflfi-cient energy to break the OS-0 bond, it seems certain that the primary step is the formation of excited SO2 molecules. Presumably these excited species then react with O2 to form a peroxide which subsequently decomposes by various steps, e.g. 

The experimental results indicate that the rate of oxidation is approximately first order in [SO2] and independent of [O2] As Leighton has shown, the above mechanism can predict these results for suitable values of the various rate coefficients. However, the evidence at present is inconclusive, and the mechanism is tentative. 

The thermally initiated oxidation of SO2 has also been studied, Russell and Smith found that mixtures of SO2 and O2 which were allowed to stand over certain metal catalysts produced SO 3 even at room temperature. Flint and Lindsay have reported that at temperatures of 200-900 °C no oxidation occurs except that due to heterogenous decomposition catalyzed by silica above about 700 °C. Recently, Cullis et studied the system and concluded that an ap- 

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Currently, the most frequently used technology for SO reduction in the flue gas is a SO adsorbing additive. With such an additive, the following steps occur.Oxidation of SO to SO in the regenerator. 

Many reactions proceed much faster in the presence of a substance which is itself not a product of the reaction. This is the phenomenon of catalysis, and many life processes and industrial processes depend on it. Thus, the oxidation of SO, to S03 is greatly accelerated in the presence of V2O5 as a catalyst, and the commercial manufacture of sulfuric acid depends on this fact. 

One of the most important industrial chemical processes is the manufacture of sulfuric acid. A major step in this process is the oxidation of SO, with air or oxygen-enriched air in the reversible, exothermic reaction corresponding to equation (A) in Example 1-2 ... 

The optimal rate behavior with respect to T has important consequences for the design and operation of reactors for carrying out reversible, exothermic reactions. Examples are the oxidation of SO, to SO, and the synthesis of NH,. 

The synthesis of ammonia, N2 + 3H2 = 2NH3, like the oxidation of SO, (Section 1.5.4 and Figure 1.4), is an exothermic, reversible, catalytic reaction carried out in a multistage tubular flow reactor in which each stage consists of a (fixed) bed of catalyst particles. Unlike SO, oxidation, it is a high-pressure reaction (150-350 bar, at an average temperature of about 450°C). The usual catalyst is metallic Fe. 

In Section 5.3 for reversible reactions, it is shown that the rate of an exothermic, reversible reaction goes through a maximum with respect to T at constant fractional conversion /, but decreases with respect to increasing / at constant T. (These canchisians apply whether the reaction is catalytic or noncatalytic.) Both features are illustrated graphically in Figure 21.4 for the oxidation of SO, based on the rate law of Eklund (1956) ... 

Penkett, S. A. Oxidation of SO] and other atmospheric gases by ozone in aqueous solution. Nature Phys. Sci. 240 105-106, 1972. 

Data on four SOx catalyst systems show that the presence of combustion promoter causes an increase in the SOx Index at a regenerator temperature of 1250 F (Table II). This implies that, without combustion promoter, the rate-controlling step in SOx reduction is the oxidation of SO to SO (Equation 2 in Figure 1). The use of combustion promoter increases the rate of oxidation of SO to SO, thereby causing an increase in the SOx Index. 

It should be noted that the effectiveness of a combustion promoter decreases with Increasing regenerator temperature. The reason is that the rate of oxidation of SO to SO increases with temperature, while the SOx adsorptive capacity of the SOx catalyst decreases. Therefore, at some temperature, the rate of oxidation of SO to SO is fast enough, without combustion promoter, to supply all the SO which the SOx catalyst can accommodate. That temperature would vary for different SOx catalyst systems. For DA-250 + Additive R it is about 1425 F. 

Since the oxidation of SO to SO is a step in the operation of SOx catalysts, an increase in oxygen concentration should favor the reaction, and thereby increase the efficiency of SOx catalysts. The evidence indicates that this occurs. Baron, Wu and Krenzke (9) have shown that an increase in excess oxygen from 0.9% to 3.4% resulted in a 20% reduction in SOx emissions. This was for a steam-deactivated catalyst in a laboratory unit at 1345 F (no combustion promoter). 

Sulfur Dioxide. The formation of SO2 from CH3S and CH3SO was previously discussed. SO2 is currently understood to be oxidized in the gas phase by OH radicals in a series of steps leading to sulfuric add (H2SO4) ana is thus of major importance to add deposition processes. The chapter by Anderson et al. (this volume) describes new experimental studies of the mechanism of the oxidation of SO . 

The oxidation of SO in rain drops by means of O3 and H2O2 is consequently the dominant non-catalytic reaction. It is interesting to note that the HiOi-oxidation is rather insensitive to decrease of pH. These reactions due to their rates are probably quite capable of producing enough sulfate in rain water to account for observed levels. 

Figure 17.20. Control of temperature in multibed reactors so as to utilize the high rates of reaction at high temperatures and the more favorable equihbrium conversion at lower temperatures, (a) Adiabatic and isothermal reaction hnes on the equihbrium diagram for ammonia synthesis, (b) Oxidation of SO, in a lour-bed reactor at essentially atmospheric pressure, (c) Methanol synthesis in a tour bed reactor by the ICI process at 50 atm not to scale 35% methanol at250 C, 8.2% at300°C, equihbrium concentrations.

NR = nonreactive toward hydrocarbons PO = oxidation of phosphines to phosphine oxides MF — peroxometallacyclic adduct formation with cyanoalkenes NSE —nonstereoselective epoxidation SE=stereoselective epoxidation AE = asymmetric epoxidation HA- hydroxylation of alkanes HB=hydroxylation of arenes OA = oxidation of alcohols to carbonyl compounds K = ketonization of Lermina 1 aikenes SO oxidation of SO to coordinated SO4 MO = metallaozonide formation with carbonyl compounds 1 = oxidation of isocyanides to isocyanates. 

V Oj and other VO -based compounds are widely used in a variety of industrially important chemical processes such as the oxidation of SO to SO during the... 

Figure 5. Effect of l adsorption onto n-type MoS, on the photoelectrochemical oxidation of 1 M SO, in 6 M H Ok. In the absence of I (top), no dark or photooxidation of SO, occurs. In the presence of I mM / (bottom) the mediated oxidation of SO, occurs at a potential corresponding to the onset for T oxidation. The electrode (0.07 cm ) was irradiated at 632.8 nm ( 40 mW/cm ). Data are from...

It is well known that SO anions stimulate corrosion of steel surfaces by preventing an in situ formation of iron oxides, which may impede the diffusion processes involved in the corrosion reactions. These sulfate anions are either formed in the atmosphere by oxidation of SO or by direct reaction with the steel surface in the presence of water to form so-called sulfate nests. The latter transformation may take place at the unprotected metal surface or possibly, at least in principal, after SO2 has permeated the organic film and arrived at the metal support. The diffusion of SO anions through organic coatings seems... 

Fig. 13-9 Radial temperature profiles in a fixed-bed reactor for the oxidation of SO with air...

Sulfate ion i.s the chemical component usually present in highest concentration in the submieron atmospheric aerosol. Almo.st all of the sulfate results from the atmospheric oxidation of SO either by homogeneous gas-phase reactions or by aerosol- or droplet-phase reactions. Reaction with the hydroxyl radical OH is thought to be the major ga.s-phase mechanism. Many solution-phase processes are possible, including reaction with dissolved HiO and reactions with 0 catalyzed by dissolved metals such as Fe and Mn (Seinfeld and Pandis, 1998). 

As a contribution to a subproject of CMD of EUROTRAC-2, scientists from ICHF studied experimentally the effect of a-pinene and cis-verbenol on the rate of S(IV) oxidation catalyzed by Fe, as well as the ozone-affected autoxidation of aqueous SO2 in the presence of calcium. The investigations were aimed at clarifying the mechanism of the aqueous phase oxidation of SO(IV) simultaneously by O3 and O2, calcium acting as condensation nuclei. Inhibition of the S(IV) autoxidation in the atmosphere by secondary terpenic compounds was also studied... 

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