Production and Use of Phthalic Anhydride Overview

Phthalic anhydride (PA, CgH403), the anhydride of phthalic acid, is widely used in the chemical industry as an important organic intermediate for the production of plasticisers ( 60%), unsaturated polyester resins (19%), and alkyd resins (14%), and also for fine chemicals ( 7%) such as dyes, insecticides, and pharmaceuticals. The values given in brackets are the mean values of Japan, USA, and Western Europe use in 1997 (Weissermel and Arpe, 2003). The current worldwide PA production is 4.5 million toimes (2005). 

PA is produced by catalytic oxidation either of ortho-xylene (CgHio) or of naphthalene (CioHg)  

Up to the beginning of the 1960s PA was mainly produced from naphthalene, that is, on the basis of tar from coke making. In the 1970s, the demand for PA increased. Simultaneously, blast furnace coke production decreased due to a reduction of steel production and increased efficiency of the blast furnace process. In 1960, 750 kg coke was needed per tonne of pig iron compared to 500 kg since 1970 (Peters and Reinitzhuber, 1994, see Fig. 6.5.17). This led to o-xylene becoming an economically attractive alternative feedstock, and to a shift from coal to crude oil based PA synthesis. Today, more than 85% of PA production worldwide is based on o-xylene. 

Unwanted by-products of PA production by catalytic oxidation of these two hydrocarbons (HCs) are CO2 and water. For naphthalene, a minimum of two mol of CO2 per mol PA is already formed according to the stoichiometry [Eq. (6.13.2)] whereas for o-xylene CO2 formation is only the result of the unwanted catalytic combustion of o-xylene and PA (Section 6.13.2). The effect of stoichiometry and combustion to CO2 (which is higher for o-xylene) leads in total to a phthalic anhydride yield related to the mass of the feed (naphthalene or o-xylene, O2 and N2 not counted) of 100-110 wt% compared to the theoretical maximum values according to the stoichiometries of 116 wt% (naphthalene) and 140 wt% (o-xylene). 

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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