Crystallization Process Of D,L-methionine

Methionine is one of the indispensable amino acids for human and animals.It can regulate the metabolic process and is widely used in feed,food,medicine,cosmetics and other industries.Methionine crystals are generally scaly or powdery crystals,with problems like low bulk density,poor morphology,easy collapse,poor solid-liquid separation,and low yield.In view of the problems in the methionine crystallization purification technology,it is of great significance to study the crystallization process.Taking D,L-methionine(D,L-Met)as the object of study,the thermodynamics and kinetics of D,L-Met crystallization were systematically studied.Firstly,the relationship between the solubility of D,L-Met at 294.05 K to 347.05 K with the temperature and pH value was studied by static method.Experiments show that the solubility of D,L-Met increases with increasing temperature,and as the pH value changes toward greater acidity or alkalinity.The activity coefficient model was used to estimate the dissolution enthalpy of D,L-Met.Comparing the calculated value of the enthalpy of dissolution with the experimental value shows that the Wilson model can better represent the dissolution thermodynamics of D,L-Met,and the calculated value of the enthalpy of dissolution is 10.6751 kJ·mol-1.Then the turbidity method was used to study the influence of different operating conditions on the metastable zone of cooling crystallization,and the relationship between the metastable zone and the kinetics of nucleation was discussed.A series of super-solubility curves of spontaneous nucleation were obtained from measuring the crystallization starting temperature of D,L-Met solution with corresponding saturation temperature from 304.27 K to 341.95 K at three different stirring speeds(250 rpm,350 rpm,450 rpm)and three different cooling rates(0.3 K/min,0.5 K/min,0.7 K/min).The results show that as the saturation temperature decreases,the stirring rate decreases,and the cooling rate increases,the width of the metastable zone becomes wider.The Nyvlt equation,Kubota equation,self-consistent Nyvlt-like equation and classical three-dimensional nucleation theory(3D CNT)equation were used to estimate the nucleation kinetic parameters.Finally,taking the rate of crystal mass deposition per unit volume as the crystal growth rate,the effects of crystal size,stirring speed,crystallization temperature on the crystallization kinetics were studied by the desaturation method.The growth rate curves obtained from the experiment of different seed crystal sizes show that D,L-Met grows independently of the particle size in the aqueous solution.As the stirring speed increases,the surface reaction becomes the control step.Therefore,the growth order under stirring speed greater than 350 rpm is basically the same,and as the stirring speed decreases from 350 rpm,the growth order also decreases.Since the activation energy of the surface reaction process is greater than the activation energy of the mass transfer process,the crystal growth process tends to be controlled by mass transfer at a higher crystallization temperature.Therefore,when the temperature is lower than 298.95 K,the crystal growth order is the same,but as the temperature increases,the growth order decreases.According to the growth rate constant obtained from 289.25 K to 298.95 K,the activation energy of the surface reaction process can be estimated to be about 86.31 kJ·mol-1.