Catalysts sulfur dioxide oxidation

Reproducible measurements of absolute activity for sulfur dioxide oxidation catalysts are very difficult to obtain for a number of reasons, including the fact that the reaction is extremely fast. In addition, there are differences in techniques and reporting methods used by the various workers. Pulse microreactors have been used to study quantities of these catalysts as small as 500 mg (83). 

Several other reports have also shown the importance of effective catalyst wetting on the performance of a bench-scale trickle-bed reactor. Hartman and Coughlin37 concluded that for sulfur dioxide oxidation in qojjntercurrejQt trickle-bed reactor packed with carbon particles, the catalyst was not completely wet at low liquid flow rates (of the order of 5 x 10 4 cm s-1). Sedricks and Kenney86 found that, during catalytic hydrogenation of crotonaldehyde in a cocurrent trickle-bed reactor, liquid seeped. into dry palladium-on-alumina... 

The laboratory scale physical model of the catalytic sulfur dioxide oxidation is a 0.05 m-diameter reactor containing 3 mm-diameter pellets of catalyst over a height of 0.15 m. The bed is flushed through at 430 °C by a gas flow that contains 0.07 kmol S02/kmol total gas, 0.11 kmol 02/kmol total gas and 0.82 kmol N2/kmol total gas. The gas spatial velocity is 0.01 m/s. 

DOCs also convert sulfur dioxide to sulfur trioxide, which forms sulfuric acid droplets or solid sulfate particles. These add to the amount of particulates emitted and can put an engine out of compliance. One approach to this problem is to lower fuel sulfur from the 1994 level to 0.01 wt% or less. A second approach is to develop a catalyst that oxidizes HC and CO but not sulfur dioxide. Newly developed catalysts that make almost no sulfate at temperatures as high as 400 C have brought this approach a step closer to reality. A third approach concerns catalyst placement. Sulfur dioxide oxidizes above 350-400°C over a EXX, while hydrocarbons do so below this temperature. A DOC could be located to have an inlet temperature favoring HC conversion. 

The reason for limiting the temperature in sulfur dioxide oxidation is based on two factors excessive temperatures decrease the catalyst activity, as just mentioned, and the equilibrium yield is adversely affected at high temperatures. This last point is the important one in explaining the need to maintain the temperature level in the dehydrogenation of butene. Still other factors, such as physical properties of the equipment, may require limiting the temperature level. For example, in reactors operated at very high temperatures, particularly under pressure, it may be necessary to cool the reactor-tube wall to preserve the life of the tube itself. 

We have examined commercial vanadium catalysts in sulfur dioxide oxidation at 400 and 470° using whole pellets (mean diameter 5.88 nun.) and two sizes of crushed pellets (diameters 2.36 and 1.14 mm.). 

Iron modified zeolites and ordered mesoporous oxides have been studied as catalysts for the sulfur dioxide oxidation in sulfur rich gases. Both zeolitic materials and mesoporous oxides show very good activity in this reaction. Other than solid state or incipient wetness loaded MCM-41 materials, the zeolites do not show an initial loss of activity. However, they loose activity upon prolonged exposure to reaction conditions around 700°C. The zeolitic samples were analyzed via X-ray absorption spectroscopy, and the deactivation could be related to removal of iron from framework sites to result in the formation of hematite-like species. If the iron can be stabilized in the framework, these materials could be an interesting alternative to other iron based catalysts for the commercial application in sulfur rich gases. 

The first supported molten salt catalyst systems date from 1914, where BASF filed a patent on a silica-supported V20s-alkali pyrosul te sulfur dioxide oxidation catalyst [48], which even today - as a slightly modified catalyst system - is still the preferred catalyst for sulfuric acid production [49]. However, it took many years to realize in the 1940s [50,51], that the catalyst system actually was a molten salt SLP-type system which is best described by a mixture of vanadium alkah sulfate/hydrogensulfate/pyrosulfate complexes at reaction conditions in the temperature range 400-600 °C with the vanadium complexes playing a key role in the catalytic reaction [49]. 

The essential radicals are formed from phenolic groups in the lignosulfonate molecules by oxidation with hydrogen peroxide in the presence of a catalyst. Out of a number of catalysts, sulfur dioxide (SO2) has been proven to be the most effective [13]. A 50%... 

Lee, J.K. Hudgins, R.R., and Silveston, P.L., Sulfur dioxide oxidation in a periodically operated trickle-bed Comparison of activated carbon catalysts. Environ Prog., 15(4), 239-244 (1996). 

Sulfur dioxide oxidizes the remaining hydrogen sulfide and both are converted to elemental sulfur at 300 °C in the presence of an alumina catalyst ... 

Most catalysts supported on titanium dioxide reach an optimum NOX reduction temperature that depends on the catalyst composition and the treated gas. Activity then declines as the secondary reactions compete for the armnonia reductant and sulfur dioxide oxidation becomes excessive. Typical operation is in the range 300°-425°C although zeolite catalysts operate from 300°-600°C. ... 

Catalysts may therefore be designed for nse in specific duties. For power plant, the design must balance the reaction rates of NOX reduction and sulfur dioxide oxidation in the restricted range of temperature of flue gas leaving the boiler, or at the dust and sulfur dioxide removal stages. A low activity catalyst that reaches maximum NOX reduction between, say 380°-400°C, can be more efficient than a catalyst that is more active between 300°-350°C because, overall, it produces less sulfur trioxide at the fixed operating temperature. ... 

Conventional SCR and sulfuric acid catalysts are used. Nitrogen oxides are removed with maximum conversion at about 390°C, following dust removal, with a typical promoted vanadium pentoxide/titania catalyst. Sulfur dioxide is... 

Most nonmetallic elements (except nitrogen, oxygen, chlorine, and bromine) are oxidized to their highest state as acids. Heated with concentrated acid, sometimes ia the presence of a catalyst, sulfur, phosphoms, arsenic, and iodine form sulfuric, orthophosphoric, orthoarsenic, and iodic acid, respectively. SiHcon and carbon react to produce their dioxides. 

Rhenium oxides have been studied as catalyst materials in oxidation reactions of sulfur dioxide to sulfur trioxide, sulfite to sulfate, and nitrite to nitrate. There has been no commercial development in this area. These compounds have also been used as catalysts for reductions, but appear not to have exceptional properties. Rhenium sulfide catalysts have been used for hydrogenations of organic compounds, including benzene and styrene, and for dehydrogenation of alcohols to give aldehydes (qv) and ketones (qv). The significant property of these catalyst systems is that they are not poisoned by sulfur compounds. 

Oxidation of sulfur dioxide in aqueous solution, as in clouds, can be catalyzed synergistically by iron and manganese (225). Ammonia can be used to scmb sulfur dioxide from gas streams in the presence of air. The product is largely ammonium sulfate formed by oxidation in the absence of any catalyst (226). The oxidation of SO2 catalyzed by nitrogen oxides was important in the eady processes for manufacture of sulfuric acid (qv). Sulfur dioxide reacts with chlorine or bromine forming sulfuryl chloride or bromide [507-16 ]. 

Another sulfur dioxide appHcation in oil refining is as a selective extraction solvent in the Edeleanu process (323), wherein aromatic components are extracted from a kerosene stream by sulfur dioxide, leaving a purified stream of saturated aHphatic hydrocarbons which are relatively insoluble in sulfur dioxide. Sulfur dioxide acts as a cocatalyst or catalyst modifier in certain processes for oxidation of o-xylene or naphthalene to phthaHc anhydride (324,325). 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

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