Production of phthalic anhydride PA from naphthalene

The main application for naphthalene is traditionally the production of phthalic anhydride. TTie oxidation of naphthalene to phthalic anhydride has undergone noticeable changes in recent years. Important stages in this development were mercury catalysis and the introduction of fixed-bed and fluidized-bed processes (see Chapter 7.1.1). Since there are no basic differences between the gas-phase oxidation of naphthalene and of o-xylene to produce phthalic anhydride, it is possible to use one plant with suitable modifications for both feedstock materials (dualfeed plants). 

In contrast to the fixed-bed reactor, where the contact time in high-temperature processes is 0.2 sec. and in low-temperature processes 3 sec., the contact time in the fluidized bed is 10 to 20 sec. A further difference is the lower air/naphthalene ratio, which is 10 1 to 12 1 in the fluidized bed, whereas in fixed-bed processes values of 15 1 to 30 1 are usual. 

In contrast to the fixed-bed process, naphthalene is fed to the reactor in liquid form at the base of the catalyst bed spontaneous evaporation then takes place by contact with the hot catalyst particles. Air is introduced below the naphthalene feed through a distribution plate. The temperature in the reaction zone is between 340 and 380 °C and the operating pressure is around 4 bar. 

Any catalyst which becomes entrained in the reaction gases is separated by special filters and returned to the reactor. Up to 60% of the phthalic anhydride can be separated from the reaction product in liquid form by cooling. The remainder is condensed as solid. The uncondensable portions of the reaction product are partially removed by washing the tail gases are incinerated. 

Hie crude phthalic anhydride is purified by vacuum distillation. The yield is around 87-88 wt%, based on the naphthalene feed, and has a crystallizing point of 130.5-130.8 °C corresponding to a purity of 96-97%. 

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