GB917225A - Process for purifying sorbic acid - Google Patents

Abstract

Crude sorbic acid, especially the acid obtained by synthesis from crotonaldehyde and ketene, is purified by treating at a raised temperature with water containing a salt as electrolyte and having a pH value within the range of 0.5 to 7.0 and preferably 2.5 to 3.5 to dissolve out the sorbic acid, crystallising the sorbic acid from the aqueous medium and separating it from the mother liquor, and if desired, further purifying the sorbic acid so obtained by recrystallisation from an aqueous organic solvent. The initial extraction step should preferably be effected at a temperature not less than 80 DEG C., e.g., at 90 to 115 DEG C. Higher temperatures, e.g. up to 150 DEG C. may be used by working under superatmospheric pressure. Specified acids which may be used to acidify the aqueous extracting medium are sulphuric, phosphoric, benzene sulphonic and toluene sulphonic acids. Volatile acids such as hydrochloric and nitric acids may also be used but the acids of low volatility are preferred. Specified salts used as electrolytes are soluble alkali metal and alkaline earth metal salts of strong acids, especially strong mineral acids, e.g. sodium and potassium sulphates and phosphates, sodium and potassium chlorides, and potassium nitrate. It is preferred to use a salt of the same acid as that used to adjust the pH value of the extracting agent and if desired two or more acids or salts may be used. Suitable aqueous organic solvents for the optional final crystallisation step are aqueous alcohols and/or ketones, e.g. methanol, ethanol, propanol, glycol, glycol methyl ether, glycerine, acetone or methyl ethyl ketone, and the water content is preferably 30 to 70% by weight. The aqueous extracting agent is used in a proportion such as to enable the liquid, oily, and pitch-like constituents to settle out free from sorbic acid. Suitable apparatus for carrying out the process is described with reference to a drawing (not shown) and the extraction and recrystallisation may be carried out in a continuous manner. In an example the extraction is effected with water containing 1-3% by weight of sodium sulphate and acidified to pH 2.5 to 3.5 with sulphuric acid, the water used being preheated to a temperature near its boiling point and the purified sorbic acid being finally crystallised from an aqueous acetone solution containing 30% by weight of water. Specification 854,383 is referred to.