Phthalic anhydride tubular reactors

Figure 13.5 shows a flowsheet for the manufacture of phthalic anhydride by the oxidation of o-xylene. Air and o-xylene are heated and mixed in a Venturi, where the o-xylene vaporizes. The reaction mixture enters a tubular catalytic reactor. The heat of reaction is removed from the reactor by recirculation of molten salt. The temperature control in the reactor would be diflficult to maintain by methods other than molten salt. 

Ortho-xylene (A) is oxidised to phthalic anhydride (B) in an ideal, continuous flow tubular reactor. The reaction proceeds via the complex consecutive parallel reaction sequence, shown below. The aim of the reaction is to produce the maximum yield of phthalic anhydride and the minimum production of waste gaseous products (C), which are CO2 and CO. 

This chapter contains a discussion of two intermediate level problems in chemical reactor design that indicate how the principles developed in previous chapters are applied in making preliminary design calculations for industrial scale units. The problems considered are the thermal cracking of propane in a tubular reactor and the production of phthalic anhydride in a fixed bed catalytic reactor. Space limitations preclude detailed case studies of these problems. In such studies one would systematically vary all relevant process parameters to arrive at an optimum reactor design. However, sufficient detail is provided within the illustrative problems to indicate the basic principles involved and to make it easy to extend the analysis to studies of other process variables. The conditions employed in these problems are not necessarily those used in current industrial practice, since the data are based on literature values that date back some years. 

So far, no reference has been made to the presence of more than one phase in the reactor. Many important chemicals are manufactured by processes in which gases react on the surface of solid catalysts. Examples include ammonia synthesis, the oxidation of sulphur dioxide to sulphur trioxide, the oxidation of naphthalene to phthalic anhydride and the manufacture of methanol from carbon monoxide and hydrogen. These reactions, and many others, are carried out in tubular reactors containing a fixed bed of catalyst which may be either a single deep bed or a number of parallel tubes packed with catalyst pellets. The latter arrangement is used, for exjimple, in the oxidation of ethene to oxiran (ethylene oxide)... 

Another reactor system which has several attractive features for heat removal is the tubular, heat-exchange reactor. Good temperature control can be achieved in the tubular reactor if the coolant approximates an isothermal heat sink. Light gas recycle can be reduced significantly compared to fixed-bed systems. Tubular reactors have been used for Fischer-Tropsch reactions and for synthesis of methanol and phthalic anhydride, for example. 

Comment. The classical industrial process comprises cyclizing o-bcnzoylbenzoic acid (from reaction of phthalic anhydride with benzene) with acid using a solvent or a ball mill process. It is difficult to see how the Bram process could be made competitive. All solid support processes would seem to require pumping a solid slurry through a tubular microwave reactor. 

The industrial production of phthalic anhydride is a.o. based on the direct oxidation of naphthalene with air in cooled tubular reactors filled with catalyst. The reaction scheme can be simplified [15,19] to ... 

Lurgi 01 Gas Chemie GmbH Phthalic anhydride O-xylene, naphthalene Multi-tubular reactor oxidizes o-xylene at high yield with maximum heat recovery for export HP steam 110 1998... 

Multibed tubular reactors to study o-Xylene oxidation have been used by McLean (in Wainwright and Foster, 1979) the catalyst is similar to that used by Boag (in Wainwright and Foster, 1979). The network differs in that McLean shows that the phthalic anhydride is formed almost exclusively from phthalide whereas Boag has indicated that this step does not occur. 

Multi-tubular reactor for o-xylene oxidation to phthalic anhydride. Deggendorfer Werft, from Cmehlingand Brehm (1996). 

To simulate such a multi-tubular reactor we use the reaction scheme given by Scheme 6.13.1, which is fairly representative for the catalytic gas-phase air oxidation of o-xylene to phthalic anhydride (Froment and Bischoff, 1990). For simplification, we assume that only CO2 (and steam) and not CO (and hydrogen) are formed as unwanted by-products. 

The chapter presents a brief overview of the current research on V205/Ti02 catalysts for o-xylene oxidation to phthalic anhydride at Clariant. Phthalic anhydride is produced in tubular, salt-cooled reactors with a capacity of about 5 Mio to per annum. There is a rather broad variety of different process conditions realized in industry in terms of feed composition, air flow rate, as well as reactor dimensions which the phthalic anhydride catalyst portfolio has to match. Catalyst active mass compositions have been optimized at Clariant for these differently realized industry processes utilizing artificial neural networks trained on high-throughput data. Fundamental pilot reactor research unravelling new details of the reaction network of the o-xylene oxidation led to an improved kinetic reactor model which allowed further optimizing of the state of the art multi-layer catalyst system for maximum phthalic anhydride yields. 

While the experimental results reported above were all collected at the laboratory scale, one proof-of-concept at an industrial scale has been recently reported in the open literature, involving a campaign of o-xylene oxidation runs in a tubular pilot reactor loaded with washcoated conductive (aluminium) honeycomb catalysts and operated under representative conditions for the industrial production of phthalic anhydride (PA). 

Let us discuss this question through a practical example invol ving simulation of a tubular fixed bed catalytic reactor for the synthesis of phthalic anhydride(PA). 

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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