Separation of benzoic acid residue by flash-vacuum distillation-melt crystallization

A large amount of residue including benzoic acid, benzyl benzoate and fluorenone can form in the production of benzoic acid by oxidizing toluene. To recycle the resources and reduce secondary pollution, the treatment of benzoic acid residue was carried out to obtain the three purified materials by flash-vacuum distillation combination method, and the influence of the operating parameters, such as the top pressure, reflux ratio and top and bottom temperatures was investigated to obtain the best operating conditions. The experimental results show that the benzoic acid purity can reach 97% through distillation under the following conditions: the top pressure is 1600 Pa, the bottom temperature is 190-200°C, the top temperature is 130-135°C, and reflux ratio is 5:1. The best operating conditions for benzyl benzoate distillation column are: the top pressure is 400 Pa, bottom temperature is 250-260°C, the top temperature is 150-160°C, and reflux ratio is 5:1. The benzyl benzoate purity can reach 95% and the fluorenone purity can reach 92%. When refined by melt crystallization, the benzoic acid purity can be improved up to 99.6%, and the purities of benzyl benzoate and fluorenone are both above 95%.