Sulfur continued oxidation catalyst

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. 

The selectivity of the catalyst is of major importance in the case of chlorinated VOCs the oxidation products should not contain even more harmful compounds than the parent-molecule, for example, formation of dioxins should be avoided. In addition, the minimization of CI2 and maximization of HCl in a product gas should be achieved [61]. These are just a few examples of why researchers are continuing the search for VOC oxidation catalysts as well as new reactor concepts. The new possibilities include, for example, utilization of nanosized gold catalysts in the oxidation of sulfur-containing VOCs and microwave-assisted processes where combination of adsorption and oxidation is used in low-concentration VOC oxidation [62, 63]. 

The sulfidation mechanism was investigated by temperature-programmed sulfidation, as the oxidic catalyst was heated in a flow of H2S and ff2, and the consumption of IH S and ff2 and the evolution of H20 were measured continuously (13). ft was found that IH S is taken up and H20 given off, even at room temperature, indicating a sulfur-oxygen exchange reaction. This conclusion was confirmed by quick extended X-ray absorption fine structure (QEXAFS) studies (Fig. f, phase 2), which also demonstrated... 

Both the rate and tire equilibrium conversion of a chemical reaction depend on the tem-peraUire, pressure, and compositionof reactants. Consider,for example, the oxidation of sulfur dioxide to sulfur trioxide. A catalyst is required if a reasonable reaction rate is to be attained. Witli a vanadium pentoxide catalyst the rate becomes appreciable at about 573.15 K (300°C) and continues to mcrease at higher temperatures. On the basis of rate alone, one would operate tire reactorat the highest practical temperature. However, the equilibrium conversion to sulfur trioxide falls as temperature rises, decreasing from about 90% at 793.15 K (520°C) to 50% at about 953.15 K (680°C). These values represent maximum possible conversions regardless of catalyst or reaction rate. The evident conclusion is that both equilibrium and rate must be considered in the exploitation of chemical reactions for commercial purposes. Although reaction rates are not susceptible to thermodynamic treatment, equilibrium conversions are. Therefore, the purpose of this chapter is to detennine the effect of temperature, pressure, and initial composition on the equilibrium conversions of chemical reactions. 

In catalytic incineration, there are limitations concerning the effluent streams to be treated. Waste gases with organic compound contents higher than 20% of LET (lower explosion limit) are not suitable, as the heat content released in the oxidation process increases the catalyst bed temperature above 650 °C. This is normally the maximum permissible temperature to which a catalyst bed can be continuously exposed. The problem is solved by dilution-, this method increases the furnace volume and hence the investment and operation costs. Concentrations between 2% and 20% of LET are optimal, The catalytic incinerator is not recommended without prefiltration for waste gases containing particulate matter or liquids which cannot be vaporized. The waste gas must not contain catalyst poisons, such as phosphorus, arsenic, antimony, lead, zinc, mercury, tin, sulfur, or iron oxide.(see Table 1.3.111... 

Going around the reaction system in Fig. 16, the first problem are poisons for rhodium such as traces of sulfur compounds in the raw materials. 3 valent P-compounds as ligands are highly prone to oxidation according to PR3 + [O] -> 0=PR3. In a continuous process, even traces of peroxides in the starting olefin and traces of oxygen in the synthesis gas accumulate over the time, so meticulous purification steps are a must if ligand-modified rhodium catalysts are used. 

Rather unique multicomponent catalysts are those in which one of the components, being volatile, has to be replenished continuously. The oxidation of dilute hydrogen sulfide to elementary sulfur by air with active carbon as a catalyst is such an example. Small amounts of ammonia, if added to the gases entering the reactor, favor and complete this catalytic reaction the ammonia leaves the reactor unchanged (45). 

Caution is advised against using temperatures in excess of 315°C (600°F) since the hot hydrogen gas can reduce a substantial portion of the metal oxides in the catalyst. In addition, with respect to the sulfiding reaction, the reactivity of the reduced metals is lower than the reactivity of the metal oxides. The sulfiding process is continued until the sulfur content of the catalyst has reached a predetermined level, usually estimated by the level of hydrogen sulfide in the tail gas. 

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