The Study Of Purification Of Benzoic Acid

At present in China, many large enterprises produce the benzoic acid using tolueneoxidation, and the cobalt salt to be the metal catalyst. With the adjustment of nationallaws and regulations, for the main product caprolactam is significantly reduced due tolower level, now the market of industrial grade acid is already saturated, variousmanufacturers of low profits, but with the development of technology, the demand ofhigh-quality pharmaceutical grade benzoic acid and feed grade has a sharp rise, theprice is high and the profit has a large space. Therefore, new methods developed toachieve high-quality benzoic acid becomes especially important.Crude benzoic acid was made using toluene oxidation, because it contains a lot ofimpurities, so in this paper, we take a vacuum distillation of crude benzoic acid, andobtain Benzoic acid material with higher purity. In this paper we study the refinedtechnology of the benzoic acid material. As follows:(1) In this paper, we use dilution crystallization method to purify the benzoic acid.It shows, after purifying in this method, the benzoic acid has a significant improvement inphysical properties. We also studied the optimal experimental conditions, we found thatwhen the dissolution temperature was55℃, crystallization temperature was25℃, stirringspeed was600r/min, dilution ratio was1:4, crystallization time was4min, dryingtemperature was90℃, we achieved the optimal purity and recovery of the benzoic acid,the purity can reach99.5-99.9%, the recovery can reach nearly70%. And in this methodwe achieved the goal of producing high purity benzoic acid.(2) In this paper we put forward a new way to purify the benzoic acid with doublefalling film crystallization. Control the temperature of the crystalline tower in113-119℃,the benzoic acid take a recycle crystallzation in this tower, when the crystallization rateget50-75%, stop the process; Then control the temperature of the crystalline tower in120-125℃, timing of sampling, after6h the sweat content get99.2-99.4%（GC）, stop it.Then keep rise the temperature to130-135℃, melt out all the material, we achieve the high quality benzoic acid that can be used in medicine and feed. The recovery is close to50%. The benzoic acid that have no reaction can be used to form benzonitrile and sodiumbenzoate, greatly reduce the cost of production of high-quality acid.The method has a simple purification process, a high degree of automation, a strongability of controling, and can achieve any purify rate in98.5-99.9%of benzoic acid, atthe same time, this method has a high production efficiency, Just a few hours will be ableto complete, low cost, and no wastewater discharge, suitable for industrial production.Compared with dilution crystallization method, it is not only getting a High yieldand high purity benzoic acid, but also adapting to amplify production with Higherefficiency and lower production costs. At first, the method of double falling filmcrystallization is without using any solvent, realized the source of the pollutant emissionsreduction, more environmental. The second, the benzoic acid after crystallizationoperation can be used recycling, when its purify rate is lower than99.7%（GC）, it can bewithout any treatment as a raw material for the production of benzonitrile. The sweatobtained during sweating (99.7~98.5%) was used directly in the production of sodiumbenzoate. In this way, significantly reducing the cost of production of high-puritybenzoic acid. In dilution crystallization method, the acid dissolved in the solvent waste.