Xylene oxidation problem

Deactivation of catalysts is a major problem in o-xylene oxidation [6,7]. For this reaction, deactivation has been mainly attributed to the irreversible anatase - rutile transformation [2,3]. In fact, anatase was found to be the best support for vanadium pentoxide catalysts leading the presence of rutile to lower activities and selectivities [8,9], The anatase-rutile transition can take place at temperatures above 973 K [10] but it is known that the presence of vanadia promotes such transformation [11-14] which, in these conditions, can start at 773 K [14], Such temperatures are easily attained in industrial reactors due to the high exothermicity of o-xylene oxidation that can lead to the formation of temperature profiles lengthwise with pronounced maxima (hot spot) [1]. 

Finally, still in relation to the o-xylene oxidation process, " analyses of the practically relevant problem of how the foam packings should be best fitted into the reactor tubes are required. 

The original route from p-xylene was oxidation in the presence of nitric acid. But the use of nitric acid is always problematical. There are corrosion and potential explosion problems, problems of nitrogen contamination of the product, and problems due to the requirement to run the reactions at high temperatures. Just a lot of problems that all led to the development of the liquid air phase oxidation of p-xylene. Ironically the nitrogen contamination problem was the reason that the intermediate DMT route to polyester was developed, since that was easy to purify by distillation. Subsequently, DMT has secured a firm place in the processing scheme. 

A major problem associated with such autoxidations is that they are largely indiscriminate, i.e. they exhibit poor chemo- and regio- selectivities. They are synthetically useful only with relatively simple substrates containing one reactive position, e.g. the oxidation of toluene to benzoic acid or p-xylene to terephthalic acid. Any catalytic oxidation has to complete with this non-catalytic pathway. Moreover, the situation is further complicated by the fact that transition metal ions also catalyze autoxidations by mediating the decomposition of trace amounts of hydroperoxides into chain-initiating radicals, via the so-called Haber-Weiss mechanism ... 

The selective oxidation of hydrocarbons with dioxygen is of immense industrial importance [ 1 ]. A general problem in this area is to obtain high selectivi-ties, particularly at high substrate conversions. The reasons for this are twofold oxidation can occur at different C-H bonds in a molecule, leading to a low primary selectivity, and the initially formed product is often more reactive than the substrate and is oxidized further, ultimately to carbon dioxide and water, leading to low secondary selectivities. Hence examples of industrial processes tend to involve the oxidation of hydrocarbons in which one particular C-H bond is significantly more reactive, for example, cumene hydroperoxide from cumene, and/or the product is relatively stable towards further oxidation, for example, maleic anhydride from n-butane, phthalic anhydride from o-xylene... 

Beyond doubt, relatively little is known about the oxidation chemistry of aromatics, despite the increasing use of BTX (benzene, toluene and xylene) aromatics for improving the anti-knock behaviour of internal combustion engines. Further, the problem of polyaromatic hydrocarbons (PAH) associated with soot particles which originate particularly in diesel engines focuses attention on combustion-generated pollution. 

The problem of identifying and measuring ozone in the complex gaseous-aerosol mixture which is smog is formidable. There are such gases present as nitric oxide, nitrogen dioxide, sulfur dioxide, hydrocarbon vapors from methane up to probably xylene, plus such partial oxidation products as aldehydes, ketones, and acids. The search for a method of high specificity has been an intensive one. 

For intermediate reaction rates the use of the enhancement factor is not consistent with the standard approach of diffusional limitations in reactor design and may be somewhat confusing. Furthermore, there are cases where there simply is no purely physical mass transfer process to refer to. For example, the chlorination of decane, which is dealt with in the coming Sec. 6.3.f on complex reactions or the oxidation of o-xylene in the liquid phase. Since those processes do not involve a diluent there is no corresponding mass transfer process to be referred to. This contrasts with gas-absorption processes like COj-absorption in aqueous alkaline solutions for which a comparison with C02-absorption in water is possible. The utilization factor approach for pseudo-first-order reactions leads to = tfikC i and, for these cases, refers to known concentrations C., and C . For very fast reactions, however, the utilization factor approach is less convenient, since the reaction rate coefficient frequently is not accurately known. The enhancement factor is based on the readily determined and in this case there is no problem with the driving force, since Cm = 0- Note also that both factors and Fji are closely related. Indeed, from Eqs. 6.3.C-5 and 6.3.C-10 for instantaneous reactions ... 

This reaction model is fairly representative of the gas phase air oxidation of o-xylene into phthalic anhydride on VjOj catalysts. A represents o-xylene, B phthalic anhydride, and C the final oxidation products CO and COj, lumped together. The process conditions were already described in Sec. 11.5.b. The purpose of this example is mainly to check whether or not serious radial temperature gradients occur in such a reactor. For a better approximation of reality a reaction model is chosen that is closer to the true model than the one used in Sec. ll.S.b. In addition, it illustrates a yield problem, such as is often encountered in industrial practice. 

Exposure to styrene is the main occupational hygiene problem in reinforced plastics industry, where it is used as a crosslinking agent and solvent in unsaturated polyester resins. In addition, workers are exposed to acetone which is used as a clean-up solvent. Other solvents, such as methylene chloride, toluene, xylene, heptane (TLV 400 ppm, the Finnish OEL 300 ppm), methylcyclohexane (TLV and the Finnish OEL 400 ppm), and butyl acetate (TLV and the Finnish OEL 150 ppm) may also be used. Styrene is neurotoxic. Styrene is also a suspected carcinogen because it is metabolized via styrene-7,8-oxide. The TLV and the Finnish OEL of styrene is 20 ppm. Urinary mandelic acid concentration is the most common biological monitoring method for styrene. The ACGIH BEI is 800 mg/g creatinine and the FIOH BEI 3.2 mmol/1. 

For homogeneous sample counting the radioactive material must be soluble in the organic scintillation solvent (toluene, xylene, dioxane). Unfortunately most inorganic salts, hydrophilic substances, macromolecules (such as proteins, nucleic acid or polysaccharides) or biological tissues (muscle, bone, liver, brain) and body fluids (blood, plasma, urine, spinal fluid) are incompatible with the solubility characteristics of the liquid scintillant. To overcome these problems various useful methods for tissue preparation have been developed such as solubilisation by hydrolysis, wet oxidation, combustion. 

A solution to overcome this problem is the adoption of catalytic postreactors with the following duties (i) conversion of unreacted o-xylene to phthalic anhydride (ii) conversion of under-oxidation intermediates (o-tolualdheyde, phtalide) to phthalic anhydride and (iii) extensive destruction by deep oxidation of other... 

In order to see how these advantages could be realized in practice, the performance of a loop reactor was compared with that of a conventionally-built integral reactor.In this comparison the capability to handle actual industrial catalysts, the settling time of changing experimental conditions, the difficulty of the mathematical evaluation of the measured data were considered. The accuracy of the datas for scale up problems was checked in a pilot plant. For the reaction, the oxidation of o-xylene with a vanadiumpentoxide catalyst, an industrially important process, was chosen. 

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