Patent GB-780535-A (GB)
Title: Purifying acetic acid by distillation
Filed: 1955-05-27
Abstract: <PICT:0780535/III/1> Crude acetic acid is purified by subjecting it to fractional distillation in two main stages each of which comprises two columns connected in series; lower boiling impurities are distilled off from the acetic acid in both stages, using one of the columns of each stage, and acetic acid is distilled off from higher boiling impurities in the other columns.  The process is particularly applicable to acetic acid prepared by oxidation of butane; and it yields a product of satisfactory "permanganate time," e.g. 1.75 hours.  The permanganate time may be increased even more by distilling off acetic acid from high boiling impurities in the presence of potassium permanganate after distilling off low boiling impurities in the second stage.  When water and formic acid are present in the crude acetic acid they can be removed by an azeotropic distillation preferably carried out between the two main stages.  In an example, crude acetic acid was passed through pipe 11 into column 12 which, as with the following columns, was operated under reflux. Lower boiling impurities and some water were taken off through pipe 13; and the acetic acid was drawn off from the base of column 12 and introduced into the middle section of column 15, which was operated at a base-temperature of 155 DEG  C.  Higher boiling impurities were withdrawn through pipe 16, while acetic acid together with water and formic acid were taken off overhead and introduced into column 18. This column was operated at a head temperature of 102 DEG  C., and a mixture containing water with a small proportion of acetic and formic acids distilled off through pipe 19.  The acetic acid leaving the base of column 18 was taken through pipe 21 into column 22, into which a stream of pentane was introduced through pipe 23.  An azeotrope of pentane, water and formic acid was taken off through pipe 24; the azeotrope was separated into two liquid phases and the pentane phase was returned to column 22 through pipe 23.  Acetic acid was withdrawn from the base of column 22 and was passed to column 26 which was operated at a reflux ratio of 10 : 1, a head temperature of 117 DEG  C. and a base temperature of 120 DEG  C.  A mixture containing some acetic acid, a small amount of water and a small amount of lower boiling impurities distilled over from the head of column 26. Acetic acid withdrawn from the base of column 26 was finally distilled in column 29 which was supplied through pipe 31 with a solution of potassium permanganate in acetic acid and water. The base of column 29 was maintained at 120 DEG  C., and a mixture of acetic acid and higher boiling impurities was withdrawn therefrom and returned via pipe 32 to column 15.  The purified acetic acid obtained at the head of column 29 had a concentration of 99.6 per cent by weight and a permanganate time of 2 hours.  In a modified embodiment, columns 12 and 15 are interchanged, so that the crude acetic acid is first distilled off from the higher boiling impurities in column 15 and is then passed to column 12 from which the low boiling impurities are distilled off.  In another embodiment, the azeotropic distillation is carried out before any of the higher boiling impurities are removed, i.e. the crude acid is passed first into column 12, then into columns 18 and 22, and then to column 15. Specification 727,078 is referred to.