GB687855A - Improvements in or relating to a process of purifying aliphatic acids by distillation - Google Patents

Abstract

<PICT:0687855/III/1> Mixtures of lower aliphatic acids obtained by hydrogenation of carbon monoxide are purified from esters, aldehydes, ketones and similar impurities by distilling the mixture with a small proportion of water to take overhead the water-azeotropes of the impurities. A crude acid mixture containing acetic, propionic, butyric and higher acids, is introduced through line 51 into fractionator 52 which is maintained under sub-atmospheric pressure; the pressure in the reboiler being about 500 mm. A higher acid fraction (butyric and above), is withdrawn from fractionator 52 via line 58, while a fraction comprising acetic and propionic acids, is withdrawn overhead at about 100 DEG C. This over head fraction is condensed, and the condensate it passed to column 61 via line 60. Water (about 1 lb. per 8 or 9 lbs. of acid-mixture), is introduced into column 61 through line 62. The water azeotropes of the contaminants are taken overhead, and condensed to give two liquid phases which are separated in separator 66. The water layer is returned to column 61; some through line 62 and the remainder through line 68. If desired an aqueous solution of an oxidizing agent, such as potassium permanganate, peracetic acid, or sodium hydrochlorite, may be introduced into column 61 through line 56. The purified mixed acids are withdrawn from the base of column 61, and introduced into column 71 from which acetic acid is taken overhead. An oxidizing agent may be introduced, as a dry powder, into column 71 via line 57. A side stream containing impurities is withdrawn through line 59. The bottoms from column 71 are introduced into column 77 wherein propionic acid is separated from heavier components under subatmospheric pressure. The bottoms from column 77 are recycled via line 50 to fractionator 52. The heavier acids withdrawn from fractionator 52 via line 58, may be distilled under reduced pressure in a batch still, to take butyric acid overhead. If the original acid mixture was derived synthetically using an iron catalyst, the residues in the batch still will comprise iron salts, which may be distilled with dilute sulphuric acid to recover the organic acids as their water-azeotropes. In a modification, the crude acid mixture containing acetic, propionic, butyric and higher acids is purified by azeotropic distillation with water, without prior separation into two fractions.