Purification Of Benzoic Acid With High Purity

Benzoic acid is an important organic raw material and product, and is widely used to produce medical intermediate, food additive, cosmetic and chemical product, such as phenol and caprolactam. Liquid-phase oxidation of toluene by air is the main process to produce benzoic acid now. However, the benzoic acid product is yellowish with some irritating smell due to many impurities in product, which limits its application. Therefore, it is necessary to purify benzoic acid product from liquid-phase oxidation of toluene.This work is to study the purification of benzoic acid which is produced by liquid-phase oxidation of toluene with air. Gas chromatograph-mass spectrometer (GC-MS) was used to identify the impurities of raw benzoic acid from liquid-phase oxidation of toluene by air. Gas chromatograph (GC) was used to analyze the main impurities of purified benzoic acid product quantitativly by standard curve method. Benzyl acetate, benzoic alcohol and biphenyl were identified as main impurities.According to the analysis result of impurities, two practicable methods were proposed to refine benzoic acid product from liquid-phase oxidation of toluene. One is solventing-out crystallization with acetic acid-water. Another is adsorption stripping by using aromatic adsorbent (SD300) and activated carbon as adsorbent. These two methods can both remove the impurities, yellow color and irritating smell from raw benzoic acid, and achieve the purity above 99.99% of benzoic acid purified. The effect of factors such as solventing-out temperature, ratio of solvent to anti-solvent, crystallizing time and stir speed etc in the solventing-out crystallization were carefully investigated. It was found that the highest recovery rate and purity could be obtained at the condition of solventing-out temperature of 50°C-25°C, ratio of solvent to anti-solvent of 1:5, crystallizing time of 1 min, stir speed of 480r/min and drying time of 1.5 hour at 90°C. In the process of adsorption stripping, the influence of factors like temperature and mass of adsorbent etc were clarified. It was showed that the highest recovery rate and purity were achieved at temperature of 70°C and adsorbent of 4g in 500ml benzoic acid solution for SD300, and at temperature of 60°C and adsorbent of 2g in 500ml benzoic acid solution for activated carbon. At last, regeneration experiments of these two adsorbents showed that the adsorbents could be reused to refine the benzoic acid after treatment. The experimental results in this work lay a foundation for the future industrial application of benzoic acid purifying.