Sulfur dioxide oxidation rate model

It is partly the fault of statistics that experimenters have misconstrued the value of the number and precision of data points relative to the value of the location of the points. The importance of the location of the data in the model specification stage can be seen from Fig. 1, which represents literature data (M3) on sulfur dioxide oxidation. The dashed and solid lines represent the predicted rates of two rival models, and the points are the results of two series of experimental runs. It can be seen that neither a greater number of experimental points nor data of greater precision will be of major assistance in discriminating between the two rival models, if data are restricted to the total pressure range from 2 to 10 atm. These data simply do not place the models in jeopardy, as would data below 2 atm and greater than 10 atm total pressure. This is presumably the problem in the water-gas shift reaction, which is classical in terms of the number of models proposed, each of which adequately represent given sets of data. 

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. 

Sintering machines prepare large nodules (lumps) from beneficiated iron ore fines, and iron oxide containing dusts recycled from particulate emission control equipment. It is estimated that uncontrolled operation would discharge particulate at the rate of about 0.3% of the mass of sinter produced, or about 2,700 kg from a machine producing 900 tonnes of sinter per day. Cyclones can decrease the particulate emission to about one-quarter of these levels. It is also possible to use the sintering machine as a roaster to enable sulfur removal from sulfur-containing iron ores. This produces a more amenable ore, but it also produces sulfur dioxide in the waste gas stream. No emission-rate data for sulfur dioxide in sinter plant exhaust gas is available, since this has not normally been recovered. However, a mathematical model which enables estimation has been described [13]. 

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

Even complex chemical reaction mechanisms can be separated into several definite elementary reactions, i. e. the direct electronic interaction process between molecules and/or atoms when colliding. To understand the total process B-fot example the oxidation of sulfur dioxide to sulfate - it is often adequate to model and budget calculations in the climate system to describe the overall reaction, sometimes called the gross reaction, independent of whether the process A Bis going via a reaction chain A C D E. .. Z B. The complexity of mechanisms (and thereby the rate law) is significantly increased when parallel reactions occur A X beside A- C,E- X beside E F. Many air chemical processes are complex. If only one reactant (sometime called an educt) is involved in the reaction, we call it a unimolecular reaction, that is the reaction rate is proportional to the concentration of only one substance (first-order reaction). Examples are all radioactive decays, rare thermal decays (almost autocatalytic) such as PAN decomposition and all photolysis reactions, which are very important in air. The most frequent are... 

Susnow RG, Dean AM, Green WH, Peezak P, Broadbelt LJ (1997) Rate-based constraction of kinetic models for complex systems. JPhy Chem 101 3731-3740 Tanaka N, Rye DM, Xiao Y, Lasaga AC (1994) Use of stable sulfur isotope systematics for evaluating oxidation reaction pathways and in-cloud-scavenging of sulfur dioxide in the atmosphere. Geophys Res Let 21 1519-1522... 

In the case of washout of sulfur dioxide (SO2), a precursor of acid rain, the high solubility and the chemical reactivity of aqueous SO2 result in nonattainment of equilibrium. Thus, semiempirical models have been proposed for the SO2 concentration in air beneath the rain-forming cloud. These models lump together all processes affecting SO2 removal from air (i.e., dissolution into water droplets, hydration, oxidation, and ionization). A first-order decay constant for the SO2 concentration. A, varies with the rainfall characteristics (rainfall rate and size of raindrops). Boubel et al. (1994) suggest a value of A equal to... 

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