Clean method for preparing D,L-methionine

摘要

The present invention discloses a clean method for preparing a D,L-methionine comprising the steps of: preparing a potassium cyanide solution using a crystallized mother solution containing potassium carbonate as an absorbing liquid to absorb hydrocyanic acid, then reacting the potassium cyanide solution with 3-methylthio propionaldehyde and an ammonium bicarbonate solution at 50-150° C. for 3-15 minutes so as to obtain a 5-(β-methylthioethyl)glycolyurea solution, then bring the 5-(β-methylthioethyl)glycolyurea solution to a temperature of 140-220° C. and subjecting to a saponification reaction for 2-5 minutes, after the completion of the saponification, reducing the temperature to 0-40° C., extracting with an organic solvent, neutralizing the water phase with CO2 and crystallizing, then filtering, washing, and drying to obtain an acceptable D,L-methionine product; bring the crystallized D,L-methionine mother solution from filtration to a temperature to 110-160° C. to remove CO2, which are all then circulated and used as a hydrocyanic acid-absorbing liquid. The process route of the present invention is a route suitable for a continuous and clean production, substantially without producing waste water and waste gas.

主权项

1. A clean method for preparing D,L-methionine, comprising the following steps:
(1) preparing a 5-(β-methylthioethyl)glycolyurea reaction fluid by using 3-methylthio propionaldehyde, KCN and NH4HCO3 solutions as raw materials for continuous reaction in the tubular reactor that is gradually heated; (2) decompressing the 5-(β-methylthioethyl)glycolyurea reaction fluid to separate NH3 and CO2 for delivery to another tubular reactor for decomposition and saponification to obtain a saponification reaction fluid; (3) eliminating NH3 and CO2 produced during saponification through desorption of the saponification fluid to obtain a D,L-methionine potassium solution; (4) obtaining a NH4HCO3 solution through water spray and absorption of NH3 and CO2 as separated in steps (2) and (3), and directly using use the NH4HCO3 solution as a material for preparation of the 5-(β-methylthioethyl)glycolyurea reaction fluid in step (1); (5) cooling down the D,L-methionine potassium solution in step (3), and then using an organic solvent for reversed and continuous extraction; directly using an organic solvent recycled through distillation of an organic solvent layer for reversed and continuous extraction; raffinate produced by distillation being treated as waste fluid; (6) delivering water that is produced through counter-current and continuous extraction in step (5) to a continuous crystallizer, simultaneously delivering CO2 gas to the continuous crystallizer for acidification to control PH value of reaction fluid in the continuous crystallizer at 6-9, and then obtaining a crystallization intermixture; (7) separating solid and liquid in the crystallization intermixture to obtain crude D,L-methionine product and a crystallization mother solution; washing crude D,L-methionine product with water for solid-liquid separation to obtain a filter cake of D,L-methionine and washing filtrate; (8) drying the filter cake of D,L-methionine under the protection of inert gas to obtain D,L-methionine product; (9) combining the crystallization mother solution and the washing filtrate from step (7) for delivery to a decomposition column to fully decompose KHCO3 into K2CO3 for release of CO2 gas; released CO2 gas that is subjected to compression is delivered to continuous crystallizer for acidification and crystallization; (10) using the K2CO3 containing mother solution as obtained in step (9) for absorption of HCN gas to prepare a KCN solution that is the material for preparation of 5-(β3-methylthioethyl)glycolyurea in step (1).