Patent GB-769565-A (GB)
Title: Process for recovering concentrated formic acid from its dilute aqueous solutions
IPC: C07C51/48
Filed: 1954-03-11
Abstract: Formic acid is extracted from its dilute aqueous solutions, which may also contain alkali metal salts, with an extraction agent consisting of a water-soluble ketone or a mixture of such a ketone and a hydrophobic auxiliary liquid, such as a hydrocarbon or a halogen derivative thereof, the resinification normally produced by the action of high percentage formic acid on ketones being prevented by conducting the extraction with such a proportion of ketone and of hydrophobic liquid (if used) that after the extraction agent and water entrained thereby have been driven off from the extract containing the formic acid, a formic acid remains, the concentration of which is below that of the azeotropic mixture of formic acid and water, i.e. below 77 per cent, and this formic acid is then dehydrated by azeotropic distillation with a suitable entraining agent other than a ketone.  When a hydrophobic auxiliary liquid is used it is preferred that the proportion thereof be such that a formic acid with a concentration below 70 per cent remains after the extraction agent and entrained water have been driven off.  The process may be carried out continuously, the extraction agent after separation in the second stage being recycled through the first two stages and the entraining agent for the azeotropic dehydration of formic acid removed from the third stage being recycled for use in the third stage, fresh acid solution being constantly introduced into the extraction stage and high percentage formic acid leaving the dehydration column at the base. In examples: (1) an aqueous solution containing formic acid, sodium sulphate and a small amount of sodium formate, is extracted at 30 DEG  to 35 DEG  C. in an eight-hour current extraction with a mixture of methyl ethyl ketone (92.5 parts by volume) and benzene (7.5 parts by volume), 100 parts by volume of the solvent mixture being used per 100 parts by volume of the acid solution.  The solvent mixture together with some entrained water is separated without resinification phenomena from the extract containing the formic acid by means of fractional distillation and the 60.4 per cent formic acid remaining in the sump is then dehydrated in known manner by azeotropic distillation with n-propyl formate; (2) as in (1) except that the extracting solvent is a mixture of acetone (75 parts by volume) and methylene chloride (25 parts by volume) the formic acid remaining in the sump having a concentration of 40.6 per cent.ALSO:Formic acid is extracted from its dilute aqueous solutions, which may also contain alkali metal salts, with an extraction agent consisting of a water-soluble ketone or a mixture of such a ketone and a hydrophobic auxiliary liquid, such as a hydrocarbon or a halogen derivative thereof, the resinification normally produced by the action of high percentage formic acid on ketones being prevented by conducting the extraction with such a proportion of ketone and of hydrophobic liquid (if used) that after the extraction agent and water entrained thereby have been driven off from the extract containing the formic acid, a formic acid remains the concentration of which is below that of the azeotropic mixture of formic acid and water, i.e. below 77 per cent, and this formic acid is then dehydrated by azeotropic distillation with a suitable entraining agent other than a ketone.  When a hydrophobic auxiliary liquid is used it is preferred that the proportion thereof be such that a formic acid with a concentration below 70 per cent remains after the extraction agent and entrained water have been driven off. The process may be carried out continuously, the extraction agent after separation in the second stage being recycled through the first two stages and the entraining agent for the azeotropic dehydration of formic acid removed from the third stage being recycled for use in the third stage, fresh acid solution being constantly introduced into the extraction stage and high percentage formic acid leaving the dehydration column at the base.  In examples (1) an aqueous solution containing formic acid, sodium sulphate and a small amount of sodium formate is extracted at 30 to 35 DEG C. in an eight hour countercurrent extraction with a mixture of methyl ethyl ketone (92.5 parts by volume) and benzene (7.5 parts by volume), 100 parts by volume of the solvent mixture being used per 100 parts by volume of the acid solution.  The solvent mixture together with some entrained water is separated without resinification phenomena from the extract containing the formic acid by means of fractional distillation and the 60.4 per cent formic acid remaining in the sump is then dehydrated in known manner by azeotropic distillation with n-propyl formate; (2) as in (1) except that the extracting solvent is a mixture of acetone (75 parts per volume) and methylene chloride (25 parts per volume) the formic acid remaining in the sump having a concentration of 40.6 per cent.