GB850176A - Purification of lower aliphatic acids - Google Patents

Abstract

Acetic, propionic, and butyric acids respectively containing small proportions of impurities are purified by heating the acid in the liquid phase to a temperature of at least 235 DEG C. but below that at which the acid decomposes to any substantial extent and then distilling off purified acid from the impure product. Preferably the acid is heated to between 235 DEG C. and 330 DEG C. and in the presence of a small proportion by weight, e.g. between 0,2 and 10% by weight, of an ionisable salt soluble in the acid, e.g. a sodium or potassium salt of the acid being treated. It is also desirable to have present a small proportion by weight of water, e.g. from 1 to 10% by weight based on the weight of the acid. The impure acids submitted to the heating process may be obtained by the liquid phase oxidation of lower aliphatic hydrocarbons containing up to 7 carbon atoms, e.g. propane or butane, such acids containing up to 2,000 parts per million of residual impurities such as aldehydes, hydroxycarbonylic compounds and ethylenically unsaturated compounds. To maintain the acids in the liquid phase a high superatmospheric pressure must be applied thereto and this may be the autogenous pressure developed by heating the acids in a closed vessel when the heating operation is carried out on a batch basis. The superatmospheric pressure may also be applied to the acids by means of a pump when the heating is carried out in a continuous manner by forcing the acids into and through a heating chamber. Suitable heating times are normally between 1 and 20 hours. During the heating period the impurities present decompose or condense to form high boiling compounds from which the fatty acids are separated by fractional distillation. A portion of the decomposition products is gaseous and may simply be vented from the acids during or after the heating period. Examples are given for the purification of acetic acid containing about 2,000 parts per million of impurities and which has been prepared by the liquid phase oxidation of propane and butane and which has first been purified by fractional distillation.