Experimental Study On Cooling Crystallization Process Of Concentrated Citric Acid Solution

Citric acid is one of the most important organic acids, which was widely used in foodstuff, beverage, cosmetic, pharmacy and chemical industry. Citric acid is mainly produced through the process of ferment, purifying, concentration and crystallization. Because of the complex composition of the concentrated solution, there are a lot of problems faced in the practical crystallization process. It is a stubborn problem that domestic citric acid products are prone to be agglomerated in the process of deposition, transportation and utilization. In order to solve these problems, a systematic study of monohydrate citric acid cooling crystallization process was carried out.Firstly, effect of four mainly impurities in the citric acid concentrated solution were discussed. The experimental data showed that sulphuric acid, oxalic acid and aconite acid could increase the solubility of citric acid, while glucose can decrease the solubility of the citric acid in water. When the four impurities simultaneously existed, the solubility of the citric acid increased. Also, the effect of stirring speed and cooling rate on the meta-stable zone was discussed. The experimental data showed that the effect of cooling rate on the meta-stable zone has something to do with the stirring speed in the cooling process. With the increasing of the stirring speed, the meta-stabile zone is prone to narrow in a same cooling rate.The cooling crystallization kinetics of monohydrate citric acid in the concentrated solution was measured by batch dynamic method. The related secondary nucleation and crystal growth kinetic parameters were determined.Optimal cooling crystallization operation for monohydrate citric acid was obtained through observing the effect of initial concentration, cooling rate, temperature (adding seed), amount of seeds and stirring speed in the process of cooling crystallization. Based on the above study, optimum crystallization technology conditions were obtained. In such conditions, the purity was eligible and the quality of the products (crystal size and crystal size distribution) obtained was obviously better than ones on sale. Comparing the products of concentrated solution with and without impurities, the average crystal size, main size and median size of the crystals were reduced due to the existence of the impurities.The research can provide some useful guidance for the monohydrate citric acid industrial production.