GB693522A - Improvements in or relating to the isolation of amino acids - Google Patents

Abstract

In a method for the recovery of an aliphatic alpha-amino monocarboxylic acid from an alkaline aqueous solution of a metal salt of such acid, the solution is treated with a cation-exchange material consisting of an acidic cation exchange agent or an amino acid salt of such agent, the ion exchange material being admixed as a slurry with the solution in a proportion such as to bring the solution to a pH value between 4.5 and 8.5, separating the cation exchange material from the liquor, and crystallizing the amino acid from the liquor. The treatment is preferably carried out at above room temperature, e.g. at between 60 DEG C. and the boiling temperature of the alkaline solution. The portion of the amino acid remaining dissolved in the mother liquor from the crystallization of the amino acid may be recovered by contacting the mother liquor with the acidic form, i.e. the regenerated hydrogen-form, of the cation exchange agent whereby the amino acid is chemically absorbed to form a salt. The thus-treated ion exchange agent may then be used to treat a further batch of the alkaline aqueous solution of a metal salt of an amino acid so as to free the amino acid. The amino acid may be crystallized by cooling the treated liquor or may be recovered from the liquor by other ways, e.g. by adding an alcohol, e.g. methanol, ethanol, or propanol, to cause precipitation of the acid. The preferred ion exchange agents are those containing sulphonate or sulphonic acid radicals, e.g. sulphonated phenol-formaldehyde resins and sulphonated copolymers of monovinyl- and polyvinyl-aromatic hydrocarbons, e.g. of styrene and divinyl benzene or of ethylvinylbenzene and divinyl benzene. The alkaline aqueous amino acid salt solution may contain any water-soluble metal salt of the amino acid and may also contain soluble metal salts of acids as weak as or weaker than acetic acid in any concentration. Thus the solution may contain dissolved salts such as sodium (or potassium) carbonate or bicarbonate or sodium acetate. The solution may also contain a minor amount of one or more water-soluble salts of strong acids, e.g. NaCl, KCl, Na2SO4, or K2SO4, but the total amount of such salts should not exceed ten per cent of the amino acid salt. The treatment may be carried out batchwise or in a continuous manner. In examples: (1) a mixture of water and a granular acidic cation-exchange agent consisting of a sulphonated copolymer of styrene, ethyl vinyl benzene, and divinyl benzene is heated to 80-90 DEG C. and an aqueous liquor obtained by hydrolysis of 5-(beta-methyl - mercaptoethyl) hydantoin with an aqueous solution of sodium hydroxide and containing the sodium salt of dl-methionine and unreacted alkali, sodium carbonate, and minor amounts of other soluble inorganic salts such as sodium chloride, is then added to the aqueous slurry of the cation exchange agent to give a pH value of 7; the mixture is then filtered hot and the filtrate cooled to crystallize methionine which is filtered off. The cation exchange agent is then washed with ammonia solution and ammonia vaporized from the washings which are further treated as below. The cation exchange agent is then regenerated by treatment with dilute hydrochloric acid and washed with water. Another batch of the same alkaline hydrolysis liquor is then mixed with the above washings freed from ammonia, the mixture heated and then treated with the regenerated cation exchange agent to give a pH of 7 and the methionine crystallized from the mother liquor as before; (2) an aqueous hydrolysis liquor containing the sodium salt of dl-alanine and containing also sodium hydroxide and sodium carbonate is added to a slurry of water and the granular cation exchange material used in (1) at room temperature to give a pH value of 5. The mixture is then filtered, the filtrate concentrated by vacuum evaporation and methanol then added to precipitate crystalline alanine; (3) as in (2) except that the hydrolysis liquor contains the sodium salt of dl-alpha-amino butyric acid instead of the salt of dl-alanine; (4) as in (2) except that the amino acid salt present in the hydrolysis liquor is the sodium salt of dl-valine; (5) an alkaline hydrolysis liquor containing the sodium salts of l-isoleucine and d-alloisoleucine is treated with a granular acidic cation exchange agent similar to that used in (1) to give a slurry having a pH value of 7. The mixture is then filtered and the filtrate vacuum-evaporated to give an aqueous slurry which is then cooled to room temperature and diluted with methanol to yield a crystalline mixture of the free amino acids; (6) an aqueous sodium methionate solution containing also sodium carbonate and sodium hydroxide is added to an aqueous slurry of an acidic cation exchange agent as used in (1) to give a pH value of 7.5. The ion exchange agent is filtered off, washed with hot water and the washings added to the filtrate and the liquor is then cooled to crystallize methionine, the ion exchange agent is regenerated with dilute hydrochloric acid, washed with water, and the mother liquor from the methionine crystallization passed through a bed of the regenerated ion exchange agent and the latter then re-used as the cation exchange material for the first of the previous steps. This cycle of operations is repeated twice, crystalline methionine being recovered in each case. Other amino acids specified are glycine, leucine, and tryptophane.