GB833230A - An improved method of recovering the anhydrides of organic diacids from gases - Google Patents

Abstract

Anhydrides of dicarboxylic acids such as phthalic anhydride and maleic anhydride are recovered from gases by washing the gas with a solvent whereby a solution of the anhydride is formed; the solution is then distilled in the presence of an aliphatic hydrocarbon-fraction having a boiling-point lower than that of the solvent and within the temperature range of from 190 DEG to 250 DEG C. whereby the anhydride distils over with the hydrocarbon fraction yielding a distillate which, on cooling, separates into two phases-one of which is rich in the purified anhydride. The anhydride is preferably separated from this rich phase by distillation under reduced pressure. Suitable solvents are: pentachlorodiphenyl, tricresyl phosphate, dibutyl phthalate, ethyl tetrachlorophalate, heptachloropropane, hexachloropropane and naphthalene mono-chloride. In an example, gases at a temperature of 120 DEG C. and containing 40 g. of crude maleic anhydride per cubic metres are passed, at the rate of 1000 cu. m. per hour, through pipe 2 into the base of washing-column 1; and 500 litres per hour of pentachlorodiphenyl at 110 DEG C. are introduced through pipe 21 into the top of the column. The gases discharged through pipe 3 are almost completely free from maleic anhydride. The solution of maleic anhydride in pentachlorodiphenyl is fed through pipe 4 to bubble-plate column 5, the base of which is heated at 180 DEG C.; and a petroleum fraction of B.R. 205-215 DEG C. is introduced into the column through pipe 9 at the rate of 500 litres per hour. Vapours containing benzene and water-vapour leave the head of column 5, and are passed to condenser 6. The solution (anhydride, solvent, petroleum) withdrawn from the base of column 5 is passed through pipe 10 into rectifying column 11 which is heated, at the base, to 180 DEG C., while the head of the column is maintained at an absolute pressure of 200 mm. Hg. The vapours leaving column 11 are passed to evaporative-condenser 13, and the condensate is allowed to separate into two layers in separator 14 which is maintained at 80 DEG C. The upper layer is returned <PICT:0833230/III/1> to column 11 via pipe 16. The lower layer, which comprises maleic anhydride mixed with a small proportion of petroleum, is withdrawn through valve 15. This crude maleic anhydride is purified by distillation at 156 DEG C. under a pressure of 200 mm. The mixture of pentachlorodiphenyl and petroleum withdrawn from the base of column 11 is passed to superheater 18 which is heated to 180 DEG C. and maintained at an absolute pressure of 200 mm.; and the major part of the petroleum contained in the mixture is distilled off. The solvent then passes through superheaters 19 and 20, where the remainder of the petroleum is distilled off with the aid of a current of steam. The entrainment-steam required for superheaters 19 and 20 is supplied via pipe 22 from condenser 13. The vapours from superheaters 10 and 20 are fractionated to remove quinones in column 24, and they are then united with the vapours from superheater 18; the united vapours being passed to condenser 26. The condensed petroleum hydrocarbons are separated from the water in separator 27, and are returned to column 5 via pipe 9. In the case of phthalic anhydride, a hydrocarbon fraction boiling in the region of 245 DEG C. is used as the entraining liquid. Specification 832,619 is referred to.ALSO:Anhydrides of dicarboxylic acids such as maleic anhydride and phthalic anhydride are recovered from gases by washing the gas with a solvent whereby a solution of the anhydride is formed; the solution is then distilled in the presence of an aliphatic hydrocarbon fraction having a boiling point lower than that of the solvent and within the temperature range of from 190 DEG to 250 DEG C., whereby the anhydride distils over with the hydrocarbon fraction yielding a distillate which, on cooling, separates into two phases of which one is rich in the anhydride and from which the anhydride is subsequently recovered, preferably by distillation under reduced pressure. Suitable solvents are: pentachlorodiphenyl, tricresyl phosphate, dibutyl phthalate, ethyl tetrachlorophthalate, heptachloropropane, hexachloropropane, and naphthalene monochloride. Where the process is applied to phthalic anhydride, an hydrocarbon fraction boiling in the region of 245 DEG C. is preferably used as entraining liquid. When the process is used for maleic anhydride, it is advantageous to use a hydrocarbon fraction boiling in the range of 205-215 DEG C. Suitable apparatus for carrying out the method of invention is described (see Group III). Specification 832,619 is referred to.