Study On The Cooling Crystallization Process Of DL-methionine

Methionine on humans are the only sulfur-containing essential amino acids,which play an important role in the life of humans and animals.Methionine synthesis process has been relatively mature.But in the subsequent separation process,such as methionine continuous cooling crystallization separation occurs when the product size is too small,difficult to separate the floating and bulk density and other issues.In this paper,DL-methionine polymorphism and cooling crystallization process size and crystal form and other aspects were studied,in order to obtain a large grain size and a high bulk density product DL-methionine crystallization process.First,the polymorphism of methionine was studied.DL-methionine has three crystal forms of ?,? and ?,wherein the ?-form is obtained by reacting an aqueous solution of sodium methionine with a liquid acid such as hydrochloric acid,acetic acid,sulfuric acid or the like.The ?-type DL-Met can be obtained by cooling the crystal by methionine aqueous solution.DL-Met ? type found by chance in type a DL-Met crystallization process.Powder X-ray diffraction(XRD)and differential scanning calorimetry(DSC)were used to establish a qualitative and quantitative method for crystal crystallinity.Experimental analysis of the impact of DL-methionine nucleation factors,the results show that the temperature is the main factor of nucleation of specific crystal form.The solubility data of DL-methionine junction in pure water were determined by static method.The dissolution equilibrium properties of a and y in water were analyzed,the effects of the pH value of aqueous solution on the solubility of methionine by sulfuric acid and sodium hydroxide were also analyzed.The results show that the solubility of a is greater than that of y under different temperature conditions,which is consistent with the stability of crystal form.The amino acid dissolution ionization equilibrium model(Perturbed-chain SAFT model)can accurately describe and predict the degree of acid-base concentration and temperature of-COOH and-NH2 dissolved and ionized in methionine.With the increase of temperature,the amount of DL-methionine dissolved in water increased and the dissolution difference of the two crystal forms decreased.At the same temperature,the pH value increases,two crystal forms of DL-methionine dissolution amount of water first increases and then decreases to a minimum value,at the isoelectric point extremum i.e.CMet±.Then,The kinetic data of DL-methionine cooling crystallization were measured by continuous steady state method.The crystal nucleation kinetics and growth dynamics model of grain growth independent growth based on particle size balance were established.The growth rate equation:G=3.44×10-8?C-0.25;The nucleation rate equation:B=1.2×103 MT-1.15 NP 4.66?C1.89.At the same time,the factors that affect the system supersaturation are analyzed,that is,high saturation,rapid mixing,solid content is small(short residence time)will increase the rate of nucleation,a direct impact on methionine crystal product size and bulk density.Finally,the effects of residence time,stirring intensity,pH value and suspension density on the particle size distribution and crystal density of methionine were analyzed under the condition of continuous cooling crystallization.Combined with crystallization kinetics analysis,the shorter residence time,the pH value is the isoelectric point,the crystalline system in the suspended state(energy input)flow "moderate" can get higher quality products.