GB567237A - Improvements in and relating to the production of hydroxyacetic acid - Google Patents

Abstract

<PICT:0567237/IV/1> A process for the production of substantially pure hydroxyacetic acid comprises evaporating within a period not exceeding two minutes, substantially the whole of the water from an aqueous solution of hydroxyacetic acid, said aqueous solution containing at least 40 per cent and preferably 50 per cent by weight of water. The aqueous solution of the acid may be obtained by the hydrolysis of a water miscible alkyl ester of the acid (methyl, ethyl, propyl and butyl esters specified) in the absence of added hydrolysis catalyst other than hydroxyacetic acid; during the hydrolysis the alcohol produced is distilled off leaving an aqueous solution of the acid containing at least 40 per cent of water. The rapid evaporation may be carried out by a "flash" evaporation process and a suitable apparatus is shown in the diagram. A mixture of a water-miscible ester of hydroxyacetic acid and water with or without some hydroxyacetic acid is heated at 40 DEG to 175 DEG C. in the hydrolysis vessel 1. The alcohol and water distil off into the column 2 and a reflux condenser 3 condenses the water which returns to the hydrolysis vessel via pipe 4. The resultant aqueous solution of hydroxyacetic acid is transferred by a pump 5 to the top of a falling film evaporator tube 6. The aqueous acid is passed into the evaporator tube from a distributer 8 and is held in contact with the walls of the tube by means of a wire spiral 7. The tube is steam heated in the annular space 9, steam condensate leaving at outlet 10. The hot acid becomes more and more concentrated as it passes downwardly through the tube. At the bottom of the tube it is substantially water free and molten. It is pumped from the tube through a steam-jacketed pump 16 into the solidifying chest 17 provided with two cylindrical cooled rolls 18. The molten acid is rapidly solidified and scraped off by the knives 22 and the acid crystals are discharged via the shoots 19. This process may be carried out at reduced pressure, for example, 400 mm. or less; in this case, vacuum may be applied at outlet 15 of the water vapour condenser 12. If pressures greater than 100 mm. are employed, it is preferable to pass a dry inert gas such as nitrogen or carbon dioxide through the evaporator tube; this may be introduced through pipe 11. The dehydration of the aqueous acid solution should be effected in from one to two minutes and preferably in less than 0.6 minutes to limit polymer formation. A table is given showing the equilibrium relationship between hydroxyacetic acid, its polymers and concentration in aqueous solution at 100 DEG C. The temperatures at which dehydration is carried out are preferably between about 78 DEG C. (the melting point of the acid) and 110 DEG C. The molten acid resulting from the rapid evaporation process may be solidified rapidly in less than 45 seconds and preferably less than 30 seconds, by pouring the molten acid over, on or through a chilled heat-conducting surface or through tubes, preferably in a dry atmosphere, to yield a microcrystalline hydroxyacetic acid. A coarse crystalline acid results from slow cooling of the molten acid, e.g. in crystallizing pans. Examples describe the hydrolysis of methyl hydroxy acetate, removal of the alcohol formed and dehydration at 100 DEG C. of the resulting acid solution containing 50 per cent by weight of water and the rapid cooling of the molten acid to yield microcrystalline hydroxyacetic acid; in one example, nitrogen is used as the dry inert gas.