Study On The Separation Process Of Methionine Saponification Solution With Evaporation

The final step in the process of methionine production by the hydantoin method is the conversion of a sodium methionine into methionine by carbonization reaction.Since the saponification solution contains sodium carbonate with a relatively high concentration,direct carbonization reaction to produces methionine,the sodium carbonate will simultaneously react with carbon gas to form sodium bicarbonate crystals,thereby decrease the purity of the methionine crystal product.In order to improve the quality of the methionine carbonized crystal product,it is necessary to remove a part of sodium carbonate before the carbonization crystallization of the methionine saponification solution,and reduce the sodium ion content to less than 3%in the solution,avoiding the precipitation of sodium bicarbonate during the carbonation crystallization process of the methionine saponification solution.In this paper,the big difference of solubility between the sodium methionate and sodium carbonate at the high temperature range was used to remove sodium carbonate by evaporation crystallization.The phase equilibrium of sodium methionine and monohydrate sodium carbonate in the aqueous solution under the temperature range from 50℃ to 90℃ was experimentally investigated.The effect of temperature and the concentration of sodium methionine on the solubility of monohydrate sodium carbonate was experimentally determined.The theoretical removal rate of sodium carbonate can be determined from thermodynamics data.Based on the experimental crystallization thermodynamics data,this paper used intermittent dynamic method to study the evaporation crystallization kinetics of monohydrate sodium carbonate in methionine saponification solution.And based on the measured crystallization kinetic data,nucleation and growth kinetics equation of monohydrate sodium carbonate in methionine saponification solution was obtained.According to the obtained crystallization kinetic model,the factors affecting the nucleation rate and growth rate of monohydrate sodium carbonate in the methionine saponification solution can be determined.At the same time,it can also provide data support for the design of the crystallizer,and the development and optimization of the crystallization process.Based on the crystallization thermodynamics and crystallization kinetics data,this paper studied the evaporation condition to removal the sodium carbonate of methionine saponification solution.The effects of the stirring rate,evaporation rate and the addition amount of the seed crystal on the crystal size of monohydrate sodium carbonate crystal were investigated,and the optimized process conditions for reducing the mother liquor entrainment of monohydrate sodium carbonate crystal was determined.The concentration ratio of the methionine saponification solution was determined by examining the different evaporation rates.Through orthogonal test,the optimized process condition of the evaporation crystallization was determined based on the k value analysis.The optimal experimental conditions were as follows:stirring rate was 250 rpm,seed crystal addition was 6%,evaporation rate was 0.26 g/min,and the evaporation temperature was 70℃.The optimized process conditions were verified by experiments.The sodium methionate entrainment in the monohydrate sodium carbonate crystal product obtained under the optimized process conditions is 7.05%,the removal rate of sodium carbonate is 72.73%,and the average crystal size is 447.74 μm.