Phthalic anhydride operating conditions

Specifically, propylene oxide has reacted directly with maleic and phthalic anhydrides to produce unsaturated polyesters under these milder conditions (15, 16). This would certainly be a major first step toward simplifying the process and lowering the cost. Incidentally, use of propylene oxide in place of propylene glycol would also result in an additional saving of 1 cent per pound in total raw material cost as well (6). After polyesterification, the separate steps of cooling, dilution with styrene, catalysis, impregnation, gelation, and cure are a distinct operational and economic liability. 

The gas-phase air oxidation of hydrocarbons (o-xylene and/or naphthalene) to phthalic anhydride (PA) is carried out in multitubular reactors. The tubes are filled with a catalyst, whereas molten salts are recirculated in the intertubular space. This mode of operation makes it possible to carefully control conditions of the reaction, with the efficient removal of heat and the high yield of PA. The improvement in the design of the reactors and the introduction of a new generation of catalysts led to a considerable increase in productivity with a simultaneous decrease in the costs of production. Unfortunately, the intensification of the process diminished the lifespan of the catalysts. Based on the experience of PA manufacturers [1] the following data may be quoted (Table I) ... 

Air is sufficient to oxidize the methyl groups of o-xylene, under the right conditions, like it is with p-, or w-xylene just described. However, here the similarity ends since commercial o-xylene oxidation is a vapor phase process [27]. ortf o-Xylene vapor, mixed with a large excess of air to ensure operation outside the explosive range, is fed to a reactor containing a supported vanadium pentoxide catalyst and heated to about 550°C. Using about a 0.1-second contact time under these conditions produces exit gases composed of phthalic anhydride, water, and carbon dioxide (Eq. 19.68). 

According to the present method of operation, the products from the converter are passed continuously to a condenser where phthalic anhydride is first condensed in a relatively pure condition. The impurities having higher vapor pressures must be cooled further and condense in later sections. Without shutting down the converter the products are periodically... 

Exothermicity is theoretically 10,500 kJ/kg of phthalic anhydride, but, in practice, it amounts to nearly 17,000 kJ/kg, in view of the total combustion. In the high-temperature process, it amounts to 7100 kJ/h. 1 of catalyst The catalyst systems are based on supported vanadium pentoxide and titanium dioxide. Their life depends on the operating conditions and ranges between 2 and 3 years. Oxidation takes place in air. The air/hydro-carbon molar ratio is usually between 60 and 120, in order to operate beyond die low... 

Ortho-xylene may be oxidized directly by air in vapor phase over vanadium pentoxide catalysts under conditions resembling those used in oxidation of naphthalene to phthalic anhydride. The stability of the cyclic anhydride structure of phthalic anhydride at the temperatures required and in the presence of oxidizing conditions is, of course, the distinctive feature. Since the oxidation of o-xylene to phthalic anhydride requires the theoretical interaction of only six atoms of oxygen relative to the nine required by naphthalene, the amount of heat generated per unit of product is less, and the volume of diluent gases in the product stream may be lower. Because of decreased formation of quinones and color bodies, product purification should be easier. Very little is available by way of information relative to commercial operating conditions. Some laboratory results of early work showed a maximum conversion to total acids of 18.2 per cent when commercial xylene was oxidized in vapor phase over unfused vanadium oxide catalyst. Recent work with o-xylene showed a conversion of 42.7 per cent to phthalic anhydride over unfused vanadium oxide catalyst and conversions up to 61.7 per cent to phthalic anhydride plus fi.6 per cent to maleic... 

Dixon et al. simulated the partial oxidation of o-xylene to phthalic anhydride over a vanadium pentoxide catalyst supported on alumina, in a dense perovskite membrane tube. A non-isothermal model was used, which included the effect of temperature on the permeation rate. The competing reaction, complete oxidation to combustion products, is favored at higher temperatures. Comparisons were made to fixed bed reactors operated under the same conditions. For the fixed bed with inlet temperature 630 K, the usual hotspot near the front of the bed was seen, as shown in Figure 11. 

Operating MSR under novel process windows, the key performance parameters can be increased by a few orders of magnitude. A few examples are presented here. In the case of esterification of phthalic anhydride with methanol 53-fold higher reaction rate between 1 and 110 bar for a fixed temperature of 333 K was observed [14]. A multiphase (gas/liquid) explosive reaction of oxidation of cyclohexane under pure oxygen at elevated pressure and temperature (>200 C and 25 bar) in a transparent silicon/glass MSR increased the productivity fourfold. This reaction under conventional conditions is carried out with air [15]. Another example is for the synthesis of 3-chloro-2-hydroxypropyl pivaloate a capillary tube of 1/8 in. operated at 533 K and 35 bar, superheated pressurized processing much above the boiling point, allowedto decrease reaction time 5760-fold as compared to standard batch operation [16]. The condensation of o-phenylenediamine with acetic acid to 2-methylbenzimidazole in an MSR is an impressive example of the reduced reaction time from 9 weeks at room temperature to 30 s at 543 K and 130 bar [17]. 

Dias, C., Portela, M. and Bond, G. (1995). Oxidation of t>-Xylene to Phthalic Anhydride over V205/Ti02 1. Influence of Catalyst Composition, Preparation Method and Operating Conditions on Conversion and Product Selectivities, J. Catal., 157, pp. 344-352. 

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). 

It is observed that the flowrate of air is reduced by far more than 50% in the scaled-down case. This is because of the combination of the conpressor curve and the new pressure of Streams 5 and 6 after the naphthalene flowrate is scaled down by 50%. The total flowrate of Stream 8 is now 50.21 Mg/h, which is 30.6% of the original flowrate to the reactor. Given that the reactor was operating at five times minimum fluidization, the reactor is now in danger of not being fluidized adequately. Because the phthalic anhydride reaction is very exothermic, a loss of fluidization could result in poor heat transfer, which might result in a runaway reaction. The conclusion is that it is not recommended to operate at these scaled-down conditions. 

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