- Oxidation of o-Xylene to Phthalic Anhydride
There are several factors in favor of its use: the number of carbon atoms rcmains the same in the product and feedstock - unlike naphthalene, there is no oxidative degradation - and the evolution of heat is reduced due to the lower oxygen requirement. Despite this fact, many plants have been so constructed that either o-xylene or naphthalene can be used.
The current o-xylene oxidation processes can be divided into two main groups:
1. Gas-phase oxidation with a fixed- or fluidized-bed catalyst based on V2O5
2. Liquid-phase oxidation with dissolved metal salt catalysts
In commercial processes, o-xylene is generally oxidized in the gas phase. Two widely used processes were developed by BASF and Chemische Fabrik von Heyden (now Wacker Chemie). In 1989, the world capacity for phthalic anhydride made by the BASF and Von Heyden processes was about 1.0×106 tonnes per year and more than 1.5×106 tonnes per year, rcspectively.
In both processes, o-xylene (95% pure) is oxidized at 375-410°C with an excess of air over V2O5 catalysts arranged in multitube reactors with about 10000 tubes.
The BASF catalyst consists of a mixture of V2O5 and TiO2 with promoters such as Al and Zr phosphates which are distributed on spheres of, for example, porcelain, quartz, or silicium carbide, which have a smooth surface and are largely pore-free (shell catalysts). Phthalic anhydride is obtained with a selectivity of 78 % (based on o-xylene) and, after a two-stage distillation, with a purity of at least 99.8 %. The byproducts include o-toluic acid, phthalide, benzoic acid, and maleic anhydride, as well as CO2 from the total oxidation.
Improved enlarged reactors now have an output of 40-50000 tonnes per year phthalic anhydride per unit. In a new development in the Von Heyden process, specially constructed tube reactors are used which, using a salt melt for cooling, allow an exact control of the temperature profile, and thus a higher loading of the air with o-xylene (60 g/m3 vs. 44 g/m3). The more intense heat generation leads to substantial energy savings.
Other firms such as Nippon Shokubai and Alusuisse have also realized new technologies for a higher o-xylene/air ratio.
Other fixed-bed processes were developed by Ftalital (now Alusuisse), Japan Gas (now Mitsubishi Gas), Pechiney-Saint Gobain, Rhône-Progil, Ruhröl (now Hüls), and Scientific Design.
The catalyst selectivities in the fluidized- and fixed-bed processes are almost equal. However, since the danger of explosion in the fluidized bed is considerably less, a lower excess of air can be used. As a result, part of the phthalic anhydride can be removed above its melting point, i. e., as a liquid. This type of isolation offers marked technological advantages over crystal deposition.
o-Xylene can also be oxidized in the liquid phase with air in processes developed by, for example, Rh8ne-Progil. Soluble acetates or naphthenates of Co, Mn, or Mo are generally used with cocatalysts containing bromine. Carboxylic acids, mainly acetic acid, are added as solvents. The oxidation is conducted at about 150 °C. The phthalic acid is removed as a solution in acetic acid, separated in crystalline form by cooling, dehydrated to the anhydride and distilled. The selectivity is reported to be 90%.
Although other firms such as Hüls and Standard Oil of Indiana have also developed liquid-phase processes for phthalic anhydride manufacture, this technology has not been used commercially.
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