GB864069A - Process for the manufacture of aromatic carboxylic acids - Google Patents

Abstract

<PICT:0864069/IV (b)/1> In a process for the manufacture of aromatic carboxylic acids by the oxidation of aromatic compounds with dilute nitric acid (e.g. of 10 to 30% strength), the reaction is carried out by passing the reaction-mixture, at elevated temperature and under a pressure of at least 80 atm., through a horizontal tube; and the conditions are so chosen that the reaction-product, i.e. the required carboxylic acid, does not separate out from the reaction mixture. The process, which is particularly advantageous for making terephthalic acid and p-nitrobenzoic acid, enables high yields of the product per unit of space and time to be obtained; and the temperature-control is facilitated, e.g. by arranging heating and cooling jackets at different sections along the reaction tube. The tube may be repeatedly doubled back on itself to form a series of horizontal sections arranged one above the other. In order to keep the residence-time as low as possible, the velocity of flow of the reaction mixture through the tube should be at least five times the internal radius of the tube multiplied by the number of alkyl carbon atoms per molecule in the compound to be oxidised. The reaction may be initiated by bringing the material to be oxidised into contact with nitric acid which has been heated to about 170 DEG C., and ensuring that the heat of reaction raises the temperature of the reaction-mixture to above 230 DEG C. In Fig. 2 is shown a reaction tube wherein the heat of reaction is utilized to preheat the reaction-mixture by working on a countercurrent principle. When the reaction is complete, the pressure of the reaction-mixture is reduced to atmospheric pressure, preferably via a stage of intermediate pressure e.g. 15 atm., in the apparatus shown in Fig. 4. The reaction mixture is discharged from reaction tube 15 through a valve; and the mixture is cooled and diluted by pumping water, at a temperature of 100 DEG -200 DEG C., into it through pipe 14. The mixture then enters vessel 10 which is maintained at the intermediate pressure by means of the valve 13 connected to condenser 12. A suspension of well-developed crystals is formed in vessel 10; and after a residence-time of about 15 minutes, the suspension is discharged to atmospheric pressure through valve 18. The particle-size and purity of the crystals depends on the pressure and residence-time in vessel 10, the particle size being larger the higher the pressure (see Group III). In Example 1, p-xylene at room temperature and nitric acid of 15% strength at a temperature of 170 DEG C. were pumped through a tube having an internal diameter of 24 mm. and an overall length of 42 metres. The pressure was maintained at 100 atm., and the temperature at 250 DEG C. Upon expansion, terephthalic acid crystallized out with a purity of 95%. In Example 2, o-chlorobenzoic acid is obtained by oxidation of o-chlorotoluene. Example 3 relates to the oxidation of 2-chloro-4-nitrotoluene yielding 2-chloro-4-nitrobenxoic acid. In Example 5, p-nitrobenzoic acid is obtained by oxidation of p-nitrotoluene. The process may also be used for the preparation of isophthalic acid and of p-chlorobenzoic acid. Terephthalic acid may be obtained by oxidation of p-chloromethyl toluene. A list of other suitable starting materials is given. Specification 698,157 is referred to.ALSO:In a process for the manufacture of aromatic carboxylic acids by the oxidation of organic compounds with dilute nitric acid, the reaction being carried out by passing the reaction-mixture, at elevated temperature and under a pressure of at least 80 atmospheres, through a horizontal tube whereby the product is obtained in solution (see Group IV(b)), the carboxylic acid is obtained in the form of pure and readily-filterable crystals by discharging the solution into a vessel, preferably fitted with a stirrer and reflux-condenser, and first partially releasing the pressure to 1 to 50 atm. (gauge) and then, after a residence-time of 15 seconds to 60 minutes, during which the desired crystals are formed, releasing the pressure to atmospheric conditions. Example 6 describes the preparation of crystals of p-nitrobenzoic acid having a predominant particle size of 40 microns. Example 7 describes the preparation of crystalline terephthalic acid having an average particle size of 10-30 microns. For a given residence time, the particle size of the crystals depends on the pressure established in the crystallizing-vessel, the particle size being larger the higher the pressure. Specification 698,157 is referred to.