Study On The Purification Technology Of Benzoic Acid

Benzoic acid is commonly used in chemical industry,medicine,food and other fields.At present,the refined process of benzoic acid in domestic and abroad has the defects of low purity,raw material loss and energy consumption.In 2017,the national standard for sodium benzoate(GB1886.184-2016),a food additive,added the requirement for phthalic acid.In order to obtain product that meets the requirement of standard G31886.184-2016,the content of phthalic acid,one material of producing benzoic acid,must be controlled.Therefore,it has significant meaning to develop a new process for the separation and purification of benzoic acid.This article aims to use melt crystallization technology to purity crude benzoic acid.The main research work of this paper is as follows:1.Determination of thermodynamic properties:Thermodynamic basic properties of benzoic acid,the melting point,melting enthalpy and the heat capacity at constant pressure,were measured by differential scanning calorimeter(DSC).Moreover,the specific heat capacity at contant temperature of benzoic acid solid and liquid were measured.Results show that the measured specific heat capacity at constant pressure of solid benzoic acid agreed with the literature,and the data of liquid benzoic acid was added at the same time.The results of SLE of benzoic acid-phthalic acid binary system(wt%benzoic acid=60%?100%)show that it is eutectic point system,which can be purified by melt crystallization.The SLE data were fitted by ideal solution model,Wilson equation and NRTL equation,and the temperature standard deviations were-2.03K,-0.13K and-0.10K respectively.It is shown that the results of ideal solution model was inaccurate,but Wilson equation and NRTL equation were satisfactory.The data of SLE and correlations were important basis of industrial production,recovery and purification and theoretical research.The determination of thermodynamic properties provided the foundation for calculating material balance and calorie balance.2.Crystal layer growth of benzoic acid static melt crystallization:Crystal layer growth rate is an important factor to production efficiency and impurities.The main factors that effect the crystal layer growth are the degree of undercooling and solid-liquid phase interface temperature,which were affected by the liquid temperature and the refrigerant temperature(the difference of temperature).It is significant to explore the influence of the solid-liquid phase interface temperature(the degree of undercooling or the difference of temperature)on the speed of crystal growth layer.3.Melt crystallization process development and optimizationThe effects of the initial condition of crystallization,the cooling rate,the final cooling temperature,the sweating rate and the sweating temperature on the purity,yield and productivity of benzoic acid were studied.Finally,a preferred benzoic acid melt crystallization process was developed.The benzoic acid product with an impurity content less than100 ppm can be obtained from the crude benzoic acid with an impurity content of more than 1000ppm,which can achieve purity requirements of food additive.There is without any solvent added,furthermore,the mother liquor can be mixed with raw material to achieve recycling production and be drained until the impurities accumulate to a certain degree,which can obtain high purity benzoic acid and achieve the the yield maximization.From those,it is important to develop the process in industrial production.