SO2 oxidation

These catalysts contained promoters to minimise SO2 oxidation. Second-generation systems are based on a combined oxidation catalyst and particulate trap to remove HC and CO, and to alleviate particulate emissions on a continuous basis. The next phase will be the development of advanced catalysts for NO removal under oxidising conditions. Low or 2ero sulfur diesel fuel will be an advantage in overall system development. 

Performance criteria for SCR are analogous to those for other catalytic oxidation systems NO conversion, pressure drop, catalyst/system life, cost, and minimum SO2 oxidations to SO. An optimum SCR catalyst is one that meets both the pressure drop and NO conversion targets with the minimum catalyst volume. Because of the interrelationship between cell density, pressure drop, and catalyst volume, a wide range of optional catalyst cell densities are needed for optimizing SCR system performance. 

Finally, atmospheric chemical transformations are classified in terms of whether they occur as a gas (homogeneous), on a surface, or in a liquid droplet (heterogeneous). An example of the last is the oxidation of dissolved sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur in the gas phase, forming secondary products such as NO2 and O3 in the liquid phase, such as SO2 oxidation in liquid droplets or water films and as gas-to-particle conversion, in which the oxidized product condenses to form an aerosol. 

A related approach is to interface an industrial promoted catalyst with a solid electrolyte (Fig. 12.2). In this case the bulk of the commercial catalyst must be conductive. This concept has been already demonstrated for the case of NH3 synthesis on Fe-based promoted commercial catalysts (BASF S6-10 RED)16 and for the case of SO2 oxidation on V2O5-K2S2O7 based catalysts (Haldor-Topsoe VK-58).17... 

SO2 oxidation to H2SO4 on aerosols, in cloud droplets, and by gas phase reactions following attack by OH. 

Nevertheless, in all cases the conversion goes to equilibrium. As SO2 oxidation is exothermic, the equilibrium concentration decreases rapidly with increasing temperature [S.B. Rasmussen et al.,J. Power Plant Chem. 5 (2003) 360.]... 

Haure et al. (1989) also undertook experiments in which the liquid flow rate was periodically reduced rather than interrupted. Switching between time-average liquid velocities of 4.0 and 1.2 mm/s at s = 0.5 resulted in about a 10% increase in the time-average rate of SO2 oxidation over steady state. The rate improvement was independent of r over the 2 to 60 min range explored. This is considerably less than the increase when flow interruption is utilized. 

The evaluation of carriers and catalyst compositions showed that significantly higher SO2 oxidation activity could be achieved with Cs as a promoter under the operating conditions downstream the intermediate absorption tower as demonstrated by the results in Table 1, where the activity compared to the standard product is increased by more than a factor 2. This was clearly sufficient for the introduction of VK69 to the market as a new sulphuric acid catalyst. The activity results for different melt compositions were used to optimise the vanadium content and the molar ratios of K/V, Na/V. and Cs/V. However, the choice of Cs/V was not only a question of maximum activity, because of the significant influence of the Cs content on the raw material costs (the price of caesium is 50-100 times the price of potassium on a molar basis). Here, the economic benefits obtained by the sulphuric acid producer by the marginal activity improvement at high Cs content also had to be taken into account. 

Figure 13.2 Dependence of the intrinsic kinetic constant kc ofde-NO, and SO2 oxidation reactions (full squares and circles, respectively) on the V content of the catalyst. T = 350 °C, fede-NO, (Nmh ) fesOi-SOi (s )- Adapted from ref. [28]...

The catalyst must be designed to handle the abrasive environment where catalyst hnes are always present in the flue gas yet still perform with a low pressure drop typically below 5 inches of water column. It must also maintain activity continuously for a 5 year cycle, yet be selective enough to limit undesirable reactions like SO2 oxidation. The catalyst must also be able to withstand periodic blasts of steam or pressurized air coming from the soot blower system found in many of the newer FCCU SCR units. 

Horiuti s results (there was a war ) analyzed the SO2 oxidation case. Both Horiuti and Boreskov assumed that all reaction steps, except one of them, are reversible and fast. These steps are not obligatory adsorption steps. One reversible step, i.e. rate-determining one, is much slower than the rest of other steps. Using SO2 oxidation as an example and assuming power low kinetic expressions for the reaction rates, Boreskov showed that... 

For typical one-route linear mechanisms all the Horiuti numbers can be selected to be equal to 1. This is not necessarily true for non-linear reaction mechanism, e.g. for SO2 oxidation mechanism... 

An important feature of a reactor operating with reversing flows is a gradual decrease of temperature of the packed bed outlet that allows for higher conversion in an adiabatic catalyst bed than for steady-state performance of an exothermic reversible reaction such as SO2 oxidation or ammonia synthesis. Conventional operation can provide only the temperature rise along the adiabatic catalyst bed. 

Three commercial processes, namely the oxidation of volatile organic compounds (VOC) for purification of industrial exhaust gases, SO2 oxidation for sulfuric acid production, and NO reduction by ammonia, have employed the periodic flow reversal concept. 

If the SO2 and O2 concentrations are switched 180° out of phase so that S02 is absent from the reactor feed during one half cycle and O2 is absent in the other half cycle, Fig. 6 shows that F is less than 1 regardless of the cycle period. Forcing just the SO2 concentration at a constant 02 concentration also fails to enhance the rate of SO2 oxidation in a back-mixed reactor. Even though the experiments of Unni et al. (1973), discussed earlier, were performed under isothermal conditions and differentially so that they could have been simulated by Strots model, the strategy used by Unni was different from those investigated. Nevertheless, one of the experiments undertaken by Unni switched between a reactant mixture and a feed that did not contain SO2. This experiment exhibited F < 1. Strots model predicts this observation. 

A substantial literature, mainly in Russian, discusses the simulation of various industrial processes operating under flow reversal. Much of this work deals with SO2 oxidation. For the rate term, Russian researchers (Boreskov et al., 1982) used the expression... 

The role of water in SO2 oxidation over activated carbon is to react with the S03 formed to yield sulfuric acid. This removes SO3 from the catalyst... 

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