GB672319A - Purification of aqueous organic acids by distillation - Google Patents

Abstract

Aqueous aliphatic acids, particularly those obtained by Fischer-Tropsch synthesis, are freed from carbonyl impurities having boiling points close to those of the acids, by subjecting the crude material to azeotropic distillation with butyl acetate, iso-amyl acetate, amyl acetate, or dibutyl ether, whereby the carbonyl impurities are carried overhead in the water/entrainer azeotrope vapours. In the case of anhydrous acids, water must be added before the process can be effected. In an example, the aqueous product from a Fischer-Tropsch synthesis was stripped from low-boiling materials (including methanol, ethanol, propanol and butanol) in a fractionating column equivalent to about 20 theoretical plates. The stripped aqueous solution, which contained acetic acid (4.7 per cent), propionic acid (1.15 per cent), butyric acid (0.5 per cent), higher aliphatic acids (0.4 per cent), and aldehydes, ketones and esters, was charged with butyl acetate (3 ml. per 100 ml. of acid solution); and the mixture was distilled through a packed fractionating column surmounted by a condenser and separator. The butyl acetate layer of the condensate was returned to the column, and the water layer was withdrawn from the system. When the solution approached dehydration, recycling of butyl acetate was discontinued. The distillation was continued until the still temperature reached 100 DEG C. The entrainer was then removed from the anhydrous acids in the still by adding water (1 ml. per ml. of butyl acetate), and the fractionation was continued with the withdrawal of both condensate layers. The acids were then separated by fractionation. The acetic acid and propionic acid cuts were free from carbonyl compounds, and the butyric acid cut contained only a trace of carbonyls. In another example, the aqueous acids obtained by Fischer-Tropsch synthesis were distilled with iso-amyl acetate through a 30-plate Oldershaw column. In this case, all the iso-amyl acetate was removed as its water azeotrope; and the remainder of the water was withdrawn from the acids by distilling with ethyl acetate. The anhydrous acids were fractionated through a 60-plate Oldershaw column using a 20:1 reflux ratio. Sharps cuts of acetic, propionic, and n-butyric acids were obtained, all of which were carbonyl-free. Specifications 298,137, [Class 32], and 467,481 are referred to.