GB760572A - Treatment of acid mixtures containing formic acid - Google Patents

Abstract

A fatty acid mixture containing formic acid in minor proportion, together with or in the absence of a minor proportion of water, is freed of said formic acid by treating said mixture with more acetic anhydride than the total amount required to react with any water originally present and with water produced by decomposition of formic acid, thereafter heating the mixture, preferably at a temperature in the range 100 DEG to 120 DEG C., for sufficient time to decompose the formic acid, adding to the product at least sufficient water to convert the remaining acetic anhydride to acetic acid and, if necessary, further heating to convert said acetic anhydride to acetic acid. The process may be applied to an acid mixture comprising a major proportion of acetic acid and a minor proportion of formic acid. In order to produce anhydrous acetic acid, the amount of water added to the product is just the amount needed to convert the remaining acetic anhydride to acetic acid. The process is particularly suitable for treatment of fatty acid mixtures comprising acids having up to 4 carbon atoms per molecule and up to 15 per cent by weight of formic acid. Such mixtures may be produced by partial aerial oxidation of hydrocarbons, such as paraffins or cycloparaffins with subsequent treatment of the product by azeotropic distillation, using for example hexane or toluene as an entrainer, to remove most of the formic acid, ketones, esters and water as an overhead product. The bottoms product so obtained contains acetic and propionic acids, together with formic acid and water. In an example, a mixture of acetic acid, formic acid and water is distilled in the presence of 45 per cent by weight of acetic anhydride using a reflux ratio in excess of 40:1, and the amount of water necessary to convert the acetic anhydride to acetic acid is added to the bottoms product. The Provisional Specification describes a broad process wherein formic acid is removed from acid mixtures by heating with acetic anhydride to decompose at least part of the formic acid. Said heating may be effected in a fractional distillation, acetic acid and excess acetic anhydride being recovered as separate fractions if required. Specifications 721,301 and 744,087 are referred to.