Patent GB-668340-A (GB)
Title: Purifying unsaturated aliphatic aldehydes by extractive distillation and the unsaturated aliphatic aldehydes so produced
IPC: C07C45/78
Filed: 1949-09-14
Abstract: <PICT:0668340/III/1> A crude   b -unsaturated aliphatic aldehyde of low molecular weight, e.g. acrolein, such as is obtained by the catalytic oxidation of an olefine, is purified from saturated aldehydes and ketones having the same number of carbon atoms per molecule as the unsaturated aldehyde, by submitting it to extractive distillation at atmospheric or (preferably) higher pressures in the presence of sufficient water to maintain a single liquid phase in the distillation zone, and then submitting the bottoms, withdrawn from the distillation zone, to further extractive distillation using water as solvent, this second distillation being carried out at atmospheric or lower pressures.  The purified unsaturated aldehyde is withdrawn overhead from the second extractive distillation zone.  Crude acrolein, containing acetone, propionaldehyde, acetaldehyde, propylene, water and other impurities, emanating directly from a zone wherein propylene is undergoing catalytic oxidation, is cooled by injection of water through line 12 and by cooler 11, and is then passed into separator 13.  The gaseous phase from separator 13 is scrubbed with water in column 16 wherein a pressure of 125 p.s.i.g. is maintained; propylene and other fixed gases are eliminated from the top of column 16, while an aqueous solution containing acrolein and other carbonyl compounds, is withdrawn from the base of column 16, and is combined with the liquid phase from separator 13.  The combined liquid stream is introduced into stripping column 22 from which a vapour fraction comprising crude acrolein, containing acetaldehyde (6 per cent), propionaldehyde (2 per cent), acetone (2 per cent), and water (3-6 per cent), is withdrawn overhead while an aqueous solution containing formaldehyde and other organic compounds, is withdrawn at the base of the column.  The crude acrolein is fractionated in column 28, from which acetaldehyde is taken overhead. The crude acrolein from the base of column 28 is passed to extractive distillation column 38 which is supplied with water through line 41 to maintain 83 per cent of water on the trays in the column.  A pressure of 1436 mm and a temperature of 74 DEG C are maintained at the top of column 38, and it is operated at a reflux-ratio of 59 : 1 to take overhead a vapour mixture containing all the propionaldehyde in the feed to column 38, and also some of the acrolein.  This mixture is condensed, and that part not returned as reflux to column 38, is withdrawn from the system.  The dilute aqueous solution containing acrolein and acetone, withdrawn from the base of column 38, is introduced into column 55 wherein the second extractive distillation with water is effected; the water being introduced into the column via line 57 so as to maintain 83 per cent of water on the trays above the feed-inlet.  Column 55 is maintained under reduced pressure; the absolute pressure at the top of the column being 50 mm., and the temperature being -9.7 DEG C. The overhead from column 55 is substantially pure acrolein, containing only 0.4 per cent water.  The process may be applied to the separation of methacrolein from admixture with isobutyraldehyde, and methyl ethyl ketone. Specification 664,414 is referred to.