Vanadium catalysts catalyst operation

Oxidation of methanol to formaldehyde with vanadium pentoxide catalyst was first patented in 1921 (90), followed in 1933 by a patent for an iron oxide—molybdenum oxide catalyst (91), which is stiU the choice in the 1990s. Catalysts are improved by modification with small amounts of other metal oxides (92), support on inert carriers (93), and methods of preparation (94,95) and activation (96). In 1952, the first commercial plant using an iron—molybdenum oxide catalyst was put into operation (97). It is estimated that 70% of the new formaldehyde installed capacity is the metal oxide process (98). 

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

Catalysts. In industrial practice the composition of catalysts are usuaUy very complex. Tellurium is used in catalysts as a promoter or stmctural component (84). The catalysts are used to promote such diverse reactions as oxidation, ammoxidation, hydrogenation, dehydrogenation, halogenation, dehalogenation, and phenol condensation (85—87). Tellurium is added as a passivation promoter to nickel, iron, and vanadium catalysts. A cerium teUurium molybdate catalyst has successfliUy been used in a commercial operation for the ammoxidation of propylene to acrylonitrile (88). 

Catalyst cost constitutes 15-20% of the capital cost of an SCR unit therefore, it is essential to operate at temperatures as high as possible to maximize space velocity and thus minimize catalyst volume. At the same time, it is necessary to minimize the rate of oxidation of S02 to S03, which is more temperature sensitive than the SCR reaction. The optimum operating temperature for the SCR process using titanium and vanadium oxide catalysts is about 38CM180oC. Most installations use an economizer bypass to provide flue gas to the reactors at the desired temperature during periods when flue gas temperatures are low, such as low-load operation. 

CAT-OX [Catalytic oxidation] An adaptation of the Contact process for making sulfuric acid, using the dilute sulfur dioxide in flue-gases. A conventional vanadium pentoxide catalyst is used. Developed by Monsanto Enviro-Chemical Systems, and operated in Pennsylvania and Illinois in the early 1970s. 

SNAM (2) An ammoxidation process for converting propylene to acrylonitrile. The catalyst is based on molybdenum/vanadium or bismuth, operated in a fluidized bed. Operated in Europe in 1968. 

Under FCCU operating conditions, almost 100% of the metal contaminants in the feed (such as nickel, vanadium, iron and copper porphyrins) are decomposed and deposited on the catalyst (2). The most harmful of these contaminants are vanadium and nickel. The deleterious effect of the deposited vanadium on catalyst performance and the manner in which vanadium is deposited on the cracking catalyst differ from those of nickel. The effect of vanadium on the catalyst performance is primarily a decrease in catalyst activity while the major effect of nickel is a selectivity change reflected in increased coke and gas yields (3). Recent laboratory studies (3-6) show that nickel distributes homogeneously over the catalyst surface while vanadium preferentially deposits on and reacts destructively with the zeolite. A mechanism for vanadium poisoning involving volatile vanadic acid as the... 

Finally a 200 B/D 80 tall demonstration unit was used for studying operating parameters, equipment, process variations, and finally catalysts, catalyst treatments, and varying feedstocks. Much of the 200 B/D work has been described in published reports.(8-10) In brief, satisfactory runs with excellent yields and good conversion were made on catalysts containing as much as 12,300 ppm of nickel plus vanadium.(A)... 

In contrast to the aforementioned binary oxides, V2Os has a stronger oxidation power and is able to attack hydrogen attached to the aromatic nucleus. Sometimes attention is drawn to the importance of a layer structure in the catalyst or to geometric factors (e.g. Sachtler [270]). Unexpectedly, however, very effective vanadium-based catalysts exist which operate in the molten state, indicating that a fixed structure is not important. The catalytic activity of molten oxide phases seems to occur exclusively in the oxidation of aromatic hydrocarbons over V2Os-based catalysts, such systems have not been reported for the selective oxidation of olefins. 

The oxidation of propane and of propene to acrylic acid has also been investigated 87,106,256-261). Vanadium phosphate catalysts that show good performance for the oxidation of C3 hydrocarbons and vanadium phosphate catalysts that are active and selective for C4 and C5 hydrocarbon oxidation have several differences in their structure and operating conditions. 

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. 

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