Selectivity phthalic anhydride reactor

Johnsson et al. (1987) have shown examples of verification of a model for a 2.13 m diameter industrial phthalic anhydride reactor. Several bubbling bed models gave good overall prediction of conversion and selectivity when proper reaction kinetics were used. The results were shown to be quite sensitive to the bubble diameter. The comparison was a good check of the models for the reaction kinetics, but the reactor model required accurate bubble size estimates obtained from measurements of overall bed density in the reactor. 

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. 

The reactions are highly exothermic and very rapid. Consequently conventional practice in the design of fixed bed reactors for phthalic anhydride production has been based on the use of multitube reactors to ensure good heat transfer and good temperature control. These are required to ensure good selectivity. Often a thousand or more small diameter tubes may be... 

Extension of the Kunii-Levenspiel bubbling-bed model for first-order reactions to complex systems is of practical significance, since most of the processes conducted in fluidized-bed reactors involve such systems. Thus, the yield or selectivity to a desired product is a primary design issue which should be considered. As described in Chapter 5, reactions may occur in series or parallel, or a combination of both. Specific examples include the production of acrylonitrile from propylene, in which other nitriles may be formed, oxidation of butadiene and butene to produce maleic anhydride and other oxidation products, and the production of phthalic anhydride from naphthalene, in which phthalic anhydride may undergo further oxidation. 

The performance of a number of single oxides of transition metals was studied by Skorbilina et al. [295] using a differential reactor. As usual, o-tolualdehyde, phthalic anhydride and carbon oxides are the main reaction products. The initial selectivity with respect to partial oxidation products decreases in the order Co > Ti > V > Mo > Ni > Mn > Fe > Cu from 71% to 33%. The relatively high initial selectivities demonstrated by the deep oxidation catalysts (e.g. Co, Ni, Mn) indicates that the primary activation is probably the same for all these catalysts, while the differences that actually determine the character of the catalyst are connected with the stability of intermediates and products. 

It appears that in most cases the FFB is the reactor of choice whenever exothermic heat is great, or when selectivity is of importance, or when high capacity is the goal. Other processes, such as phthalic anhydride by oxidizing oxylene with air, and unsaturated aldehydes from an unsaturated olefine, are also reported to use FFB reactors. 

Conversion of n-pentane is defined as the number of moles of n-pentane converted to the number of moles of n-pentane feed to the reactor (in %mol). Selectivities to maleic and phthalic anhydrides are expressed as a fraction of moles of n-pentane converted into MA and PA (in %mol), respectively ... 

By increasing the void volume of the reactor, the homogeneous reaction diminishes the selectivity to partially oxidised products and, at the same time, increases the n-pentane conversion and significantly modifies the maleic to phthalic anhydride selectivity (see also Fig. 5). 

To obtain the total yield in relation to e-xylene, it is necessary to consider that of the phthalic anhydride production step, which barely exceeds 65 to 70 molar per cent in the gas phase oxidation (Von Heyden type reactor). At present no industrial plant employs this process, which is uneconomical in comparison with other methods, given the lack of selectivity of the o-xylene conversion step. 

Conversion of n-pentane is defined as the number of moles of n-pentane converted by the number of moles of n-pentane feed to the reactor (in % mol). Selectivities to maleic and phthalic anhydrides are expressed as the fraction of moles of n-pentane converted into AM and PM (in % mol.) respectively. Conversion and selectivity were measured after the time of activation of the precursor as described above (8 hours). The specific amount of n-pentane converted was calculated as the total moles of n-pentane converted by second divided by the BET surface area and the weight of the catalyst. Specific amounts of n-pentane converted into MA or PA were c culated in sii ar forms. 

The single- and multiple-turn plates are very effective in breaking bubbles. Although this kind of baffle is much more complex in structure, it is not easily eroded and can be used without maintenance for several years. Single- and multiple-turn plates have been successfully used to improve the synthetic yield and selectivity of phthalic anhydride and butadiene in pilot-scale fluidized bed chemical reactors. 

Figure 6.13.4 shows the strong influence of the diameter of the tubes on the maximum allowable cooling (= inlet) temperature. An internal diameter of more than 2.5 cm (standard value used in industrial reactors) would lead to a strong decrease of the maximum allowable gas inlet temperature with the result of a strong decrease in the o-xylene conversion for a tube length of 3 m (Figure 6.13.1). Conversely, smaller tubes (<2.5 cm) would also allow a safe operation for inlet temperatures of more than 400 °C but then the selectivity to phthalic anhydride would strongly decrease (Figure 6.13.1) and so also would the number of tubes needed (investment costs).

The work has shown that a strong exothermic reaction such as the o-xylene oxidation to phthalic anhydride can be operated in the explosion regime using a micropacked bed reactor, even with high adiabatic temperature rise of several 1000 K. An increase of the selectivity to total oxidation products was observed at higher o-xylene concentrations between 10 and 25 vol% o-xylene, which possibly was caused by the formation of a hotspot. 

What is the conversion of n hthalene and the selectivity to phthalic anhydride if die reactor operates at 600 K ... 

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). 

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