Partial Oxidation of o-Xylene to Phthalic Anhydride

In many cases, the external mass and heat transfer resistances are negligible because of the high gas flow rate that destroys the external resistances (see for example the ammonia converter, ch. 6, sec. 6.3.3, and the steam reformer, ch. 6, sec. 6.3.4). A counter example is the case of the partial oxidation of o-Xylene to phthalic anhydride, ch. 6, sec. 6.3.6, where external mass and heat transfer resistances must be taken into consideration for precise modelling of these reactors. 

As an illustrative example, we present in detail the different mechanisms and kinetic rate equations for the catalytic partial oxidation of o-Xylene to phthalic anhydride. 

For multiple reactions, whether reversible and cyclic such as the steam reforming of methane or irreversible, consecutive and parallel such as the partial oxidation of o-Xylene to phthalic anhydride, the situation becomes rather complex. The full practical implication of these findings has not been explored. Both the partial oxidation of... 

For more complex reaction networks, these component effectiveness factors are the correct numbers to indicate the effect of diffusion on the yield and selectivity for the different components. For this reason it is the components effectiveness factors formulation which should be used in connection with complex reaction networks. It will be shown that in these cases not only rj> I are possible but also ij <0 are possible for some intermediate components. This phenomenon will be discussed in sections 5.1.9 and 5.2.2, in connection with the highly endothermic steam reforming reaction, and the highly exothermic partial oxidation of o-Xylene to phthalic anhydride. 

Fixed bed reactors for the partial oxidation of o-Xylene to phthalic anhydride. 

Partial oxidations have not been as frequently studied. Harold and co-workers have simulated a partial oxidation reaction scheme in an 02-fed CMR. Bernstein et al. simulated methane partial oxidation, while Dixon et al. modelled partial oxidation of o-xylene to phthalic anhydride to examine temperature profiles. 

Dixon et al. simulated the partial oxidation of o-xylene to phthalic anhydride over a vanadium pentoxide catalyst supported on alumina, in a dense perovskite membrane tube. A non-isothermal model was used, which included the effect of temperature on the permeation rate. The competing reaction, complete oxidation to combustion products, is favored at higher temperatures. Comparisons were made to fixed bed reactors operated under the same conditions. For the fixed bed with inlet temperature 630 K, the usual hotspot near the front of the bed was seen, as shown in Figure 11. 

Kinetic and mechanistic investigations on the o-xylene oxidation over V205—Ti02 catalysts were carried out by Vanhove and Blanchard [335, 336] using a flow reactor at 450°C. Possible intermediates like o-methyl-benzyl alcohol, o-xylene-a,a -diol, toluic acid and phthalaldehyde were studied by comparing their oxidation product distribution with that of toluene. Moreover, a competitive oxidation of o-methylbenzyl alcohol and l4C-labelled o-xylene was carried out. The compounds investigated are all very rapidly oxidized, compared with o-xylene, and essentially yield the same products. It is concluded, therefore, that these compounds, or their adsorbed forms may very well be intermediates in the oxidation of o-xylene to phthalic anhydride. The ratio in which the partial oxidation products are formed appears to depend on the nature of the oxidized compounds, i.e. o-methylbenzyl alcohol yields relatively more phthalide, whereas o-xylene-diol produces detectable amounts of phthalan. This... 

This class of reactions, carried out in fluidized beds, involves parallel and series reactions, with reaction intermediates being the desired products. Industrial examples include partial oxidation of n-butane to maleic anhydride and o-xylene to phthalic anhydride. The vigorous solid mixing of fluidized beds is valuable for these reactions because they are highly exothermic. However, gas backmixing must be minimized to avoid extended gas residence times that lead to the formation of products of total combustion (i.e., CO2 and H2O). For this reason, fluidized bed catalytic partial oxidation reactors are operated in the higher velocity regimes of turbulent and fast-fluidization. 

Multitubular reactors are mainly used in gas-phase partial oxidation processes, such as the air oxidation of light olefins, paraffins, and aromatics. Examples of chemistries where these reactors are used include the partial oxidation of methanol to formaldehyde, ethylene to ethylene oxide, ethylene and acetic acid to vinyl acetate, propylene to acrolein and acrylic acid, butane to maleic anhydride, isobutylene to methacrolein and methacrylic acid, and o-xylene to phthalic anhydride. An overview of the multitubular reactor process for the partial oxidation of n-butane to maleic anhydride is given here. 

Partial oxidation of hydrocarbons employing mixed metal oxides as catalysts comprises an economically important class of reactions for the upgrading of base feed stocks [3]. An illustrative example of it is the partial oxidation of o-xylene and/or naphthalene to phthalic anhydride (PA) with a world production of 3.2 million metric tons per year, industrially carried out in shell and tube reactors using air as the oxidizing agent [4]. 

As an example of selective molecular recognition with the imprinted silica 2, a Knoevenagel C-C bond-forming reaction was performed with a bifunctional reactant (Fig. 9). This type of a sequential reaction system is important in numerous industrial applications such as the dehydrogenation of butylene to butadiene or the partial oxidation of naphthalene or o-xylene to phthalic anhydride [44]. The ability to suppress the B to C reaction avoids production of undesired products in these cases. 

Catalytic partial oxidation of o-xylene and naphthalene is performed mostly in intensively cooled multi-tubular fixed bed reactors, but systems with a fluidized bed were also developed. Typically, V20s/Ti02 catalysts with K2SO4 or A1 phosphates as promoter are used. In fixed bed reactors, the conversion of both feedstocks per pass is around 90%, and the selectivity is in the range 0.86-0.91 mol PA per mol naphthalene and 0.78 mol per mol o-xylene. (Note that the selectivity would be 100%, if only the reactions according to Eqs. (6.13.1) and (6.13.2), respectively, would take place.) The active compounds are distributed on spheres of porcelain, quartz, or silicium carbide (shell catalyst). The thickness of the shell is only around 0.2 mm, and the diffusion paths for the reactants are short. By this means, the influence of pore diffusion is small, and the unwanted oxidation of phthalic acid anhydride to CO2 is suppressed compared to a catalyst with an even distribution of active compounds where the influence of pore diffusion would be much stronger (see Section 4.5.6.3 Influence of Pore Diffusion on the Selectivity of Reactions in Series ). Thus the intrinsic reaction rates are utilized for the modeling of a technical reactor (next Section 6.13.2). 

Reaction scheme (6) is typical in the oxidation of hydrocarbons (A, R, and S) in the presence of a large amount of oxygen. Therefore, the reactions become pseudo-first-order from the viewpoint of hydrocarbons, and the practically constant oxygen partial pressure can be included in the rate constants. The intermediate product, R, represents a partial oxidation product (such as phthalic anhydride in the oxidation of o-xylene or maleic anhydride in the oxidation of benzene), whereas S represents the undesirable byproducts (CO2, H2O). The triangle system (7) represents monomolecular reactions such as isomerizations A, for instance, can be 1-butene, which is subject to an isomerization to ds-2-butene and trflns-2-butene. 

Phthalic anhydride is produced by partial oxidation of o-xylene or naphthalene. Recently the path from naphthalene has been dramatically reduced due to a lack of availability of the raw material. Polynt has been producing PA since its foundation in 1955 based on proprietary technology and catalysts. 

The gas phase oxidation of naphthalene to phthalic anhydride over V2Os-based catalysts is one of the oldest successful partial oxidation processes and is still of industrial importance today. Common commercial catalysts are modified silica-supported V—K—S—O catalysts and catalysts similar to those used for benzene or o-xylene oxidation. Maximum phthalic anhydride yields of 80—85 mol. % (92—98 wt. %) at 350—400°C are reported. By-products are naphthoquinone (2—5%), maleic anhydride (2— 5%) and carbon oxides. 

Figure 11 Comparison of PFR and reactant-fed PBMR axial temperature profiles for o-xylene partial oxidation to phthalic anhydride, with different reactant inlet temperatures...

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