Vanadium pentoxide vapor

Contaminants of this type include various combinations of vanadium, sulfur, and sodium compounds. Fuel ash corrosion is most hkely to occur when residual fuel oil (Bunker C fuel) is burned. In particular, vanadium pentoxide vapor (VjOj) reacts with sodium sulfate (Na2S04> to form sodium vanadate (NajO.bVjOj). The latter compound reacts with steel, forming a molten slag that runs off and exposes fresh metal to attack. Corrosion increases sharply with increasing temperature and the vanadium content of the fuel oil. If the vanadium content exceeds 150 ppm, the maximum tube wall temperature should be limited to 650 °C. Between 20 and 150 ppm V, the maximum tube wall temperature can be between 650 and 845 °C, depending on the sulfur content and the sodium-vanadium ratio of the fuel oil. With 5 to 20 ppm V, the maximum tube wall temperature can exceed 845 °C. 

Corrosion by fuel ash deposits can be one of the most serious operating problems with boiler and preheat furnaces. All fuels except natural gas contain certain inorganic contaminants that leave the furnace with products of combustion. In particular, vanadium pentoxide vapor (VjOj) reacts with sodium sulfate (NajSO ) to form sodium vanadate (NajO.tiVjOj). This compound wiU react with steel, forming a molten slag that runs off and exposes fresh metal to attack. 

The reaction uses a fixed-bed vanadium pentoxide-titanium dioxide catalyst which gives good selectivity for phthalic anhydride, providing temperature is controlled within relatively narrow limits. The reaction is carried out in the vapor phase with reactor temperatures typically in the range 380 to 400°C. 

Vapor-phase oxidation over a promoted vanadium pentoxide catalyst gives a 90% yield of maleic anhydride [108-31-6] (139). Liquid-phase oxidation with a supported palladium catalyst gives 55% of succinic acid [110-15-6] (140). 

Oxidation. Naphthalene may be oxidized direcdy to 1-naphthalenol (1-naphthol [90-15-3]) and 1,4-naphthoquinone, but yields are not good. Further oxidation beyond 1,4-naphthoquinone [130-15-4] results in the formation of ortho- h. h5 ic acid [88-99-3], which can be dehydrated to form phthaUc anhydride [85-44-9]. The vapor-phase reaction of naphthalene over a catalyst based on vanadium pentoxide is the commercial route used throughout the world. In the United States, the one phthaUc anhydride plant currently operating on naphthalene feedstock utilizes a fixed catalyst bed. The fiuid-bed process plants have all been shut down, and the preferred route used in the world is the fixed-bed process. 

Durene. The oxidation of durene (4) yields pyromeUitic acid [89-05-4] (17) or pyromeUitic dianhydride [89-32-7] (18) directly. The oxidation can be carried out with dilute nitric acid ia solution, with air and catalyst either ia the vapor phase over a soHd vanadium pentoxide-based catalyst or ia the... 

Oxidation. Benzene can be oxidized to a number of different products. Strong oxidizing agents such as permanganate or dichromate oxidize benzene to carbon dioxide and water under rigorous conditions. Benzene can be selectively oxidized in the vapor phase to maleic anhydride. The reaction occurs in the presence of air with a promoted vanadium pentoxide catalyst (11). Prior to 1986, this process provided most of the world s maleic anhydride [108-31 -6] C4H2O2. Currendy maleic anhydride is manufactured from the air oxidation of / -butane also employing a vanadium pentoxide catalyst. 

The process. A typical process for phthalic anhydride starts with mixing hor q-xylene vapor with excess preheated air in a ratio of 20 1 air to o-xylene by weight. The gaseous mixture is then fed to a reactor consisting of tubes packed with vanadium pentoxide catalyst on a silica gel. The reaction takes place at 800—1000°F. Like most oxidation reactions, this one is exothermic, and the heat of reaction must be removed from the tubes to maintain the reaction temperature. 

At present, almost all sulfuric acid is made by the contact process, which has been in use since 1831. The first step is exothermic air oxidation of SO2 catalyzed by vanadium pentoxide (V2O5) or platinum (reaction 10.5). The yield of SO3 is limited on the first pass to some 60% because the temperature rises to 600 °C or more usually, three more passes over the catalyst are made, and the yield can be increased to 98%. The SO3 vapor is then absorbed into 100% H2SO4 (reaction 10.6), and water is added to the resulting mixture of disulfuric (H2S207) and sulfuric acids (known as oleum) until the H2S2O7 is all hydrolyzed to H2SO4 (reaction 10.7). This obviates the aerosol problem. 

Oxidation. The vapor-phase reaction of naphthalene over a catalyst based on vanadium pentoxide is the commercial route used throughout the world to funu plillialic anhydride. In the United Stales, the one phthalic anhydride plant currently operating on naphthalene feedstock utilizes a fixed catalyst bed, the preferred route worldwide. 

Another test method for the determination of mercury in coal (ASTM D-6414) involves (method A) solubilizing of the mercury in the sample by heating the sample at a specified temperature in a mixture of nitric and hydrochloric acids. The acid solutions produced are transferred into a vessel in which the mercury is reduced to elemental mercury. The mercury vapor is determined by flameless cold-vapor atomic absorption spectroscopy. An alternative method (method B) involved solubilization of the mercury by heating the sample in a mixture of nitric acid and sulfuric acid with vanadium pentoxide. The acid solution is then transferred into a vessel in which the mercury is reduced to elemental mercury. The mercury content is determined by flameless cold-vapor atomic absorption spectroscopy. However, mercury and mercury salts can be volatilized at low temperatures, and precautions against inadvertent mercury loss should be taken when using this method. 

Air is sufficient to oxidize the methyl groups of o-xylene, under the right conditions, like it is with p-, or w-xylene just described. However, here the similarity ends since commercial o-xylene oxidation is a vapor phase process [27]. ortf o-Xylene vapor, mixed with a large excess of air to ensure operation outside the explosive range, is fed to a reactor containing a supported vanadium pentoxide catalyst and heated to about 550°C. Using about a 0.1-second contact time under these conditions produces exit gases composed of phthalic anhydride, water, and carbon dioxide (Eq. 19.68). 

The oxidation of acetaldehyde may also be carried out at temperatures of 80° to 100° C. by means of oxidation towers either in the presence or absence of a catalyst.127 Under these conditions peracetic acid is normally decomposed as rapidly as it is formed. Ill towers wetted with acetic acid and containing catalysts such as vanadium pentoxide, uranium oxide, roasted ferroso-ferric oxide, etc., the reaction between the aldehyde vapors and air tS"very rapid and complete, and temperatures as low as 30° to... 

The relative proportions in which the products, chiefly benzaldehyde, benzoic acid, and anthraquinone, are obtained depends in a large measure on the temperatures to which the reaction mixture of toluene vapor and air is subjected. High temperatures, together with rapid rates of flow as well as high temperatures and mild catalysts, are conducive to bai-zaldehyde formation. With vanadium pentoxide catalysts oxidation of... 

In the vapor phase oxidation of benzene to maleic anhydride an active catalyst is necessary to force oxidation to rupture the ring without leading to complete destruction. Vanadium pentoxide or vanadium compounds such as tin vanadate have been successfully used for this purpose.26 In the oxidation of alkylated benzene compounds to benzaldehyde, benzoic acid, or phthalic anhydride, a milder form of catalyst is effective. The oxidation of naphthalene to naphthaquinone would also require a mild form of catalyst to prevent ring rupture caused by too severe oxidation. However, oxidation to phthalic anhydride may be realized under ordinary conditions by the use of such catalysts as have been found effective in benzene oxidation, i.e., oxides of the metals of the fifth and sixth groups of the periodic system, especially the oxides of vanadium and molybdenum. 

Ortho-xylene may be oxidized directly by air in vapor phase over vanadium pentoxide catalysts under conditions resembling those used in oxidation of naphthalene to phthalic anhydride. The stability of the cyclic anhydride structure of phthalic anhydride at the temperatures required and in the presence of oxidizing conditions is, of course, the distinctive feature. Since the oxidation of o-xylene to phthalic anhydride requires the theoretical interaction of only six atoms of oxygen relative to the nine required by naphthalene, the amount of heat generated per unit of product is less, and the volume of diluent gases in the product stream may be lower. Because of decreased formation of quinones and color bodies, product purification should be easier. Very little is available by way of information relative to commercial operating conditions. Some laboratory results of early work showed a maximum conversion to total acids of 18.2 per cent when commercial xylene was oxidized in vapor phase over unfused vanadium oxide catalyst. Recent work with o-xylene showed a conversion of 42.7 per cent to phthalic anhydride over unfused vanadium oxide catalyst and conversions up to 61.7 per cent to phthalic anhydride plus fi.6 per cent to maleic... 

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