GB832619A - Improvements in or relating to a method of separating phthalic anhydride entrained in gases - Google Patents

Abstract

<PICT:0832619/III/1> Gases containing entrained phthalic anhydride, such as the hot gases obtained from the synthesis of phthalic anhydride by catalytic oxidation, are washed with a liquid aliphatic hydrocarbon or a mixture of liquid aliphatic hydrocarbons having a boiling-point higher than 200 DEG C., whereby the anhydride passes from the gas into the liquid hydrocarbon to form a solution; this solution is distilled using a fractionating column and a vapour mixture comprising the anhydride and hydrocarbon leaves the head of the column; the vapour is condensed and the condensate is cooled to a temperature at which it separates into two phases-one of which is rich in anhydride. A petroleum fraction boiling between 260 DEG and 265 DEG C. may be used as washliquid. Penta- or tetradecane may also be used. The distillation is preferably carried out under reduced pressure. The anhydride-rich phase of the condensate may be separated in either the liquid state or the solid state. A gas, at a temperature of 140 DEG C., containing 50 grams of finely-divided phthalic anhydride per cubic meter is passed through pipe 1 into the lower part of plate-type column A; and a mixture of straight-chain hydrocarbons, boiling between 260 and 265 DEG C., is introduced into the top of the column through pipe 3. The temperature in column A is maintained at 140 DEG C. The gases leaving the column through pipe 2 are practically free from phthalic anhydride; these gases are cooled to recover any hydrocarbon carried over. The solution leaving column A through pipe 4 contains 9% phthalic anhydride; and this solution is distilled in column C, the base of which is maintained at a temperature of 180 DEG C. while the head of the column is maintained under an absolute pressure of 60 mm. Hg. The vapour from the head of the column contains 25% phthalic anhydride, and is condensed in condenser C1, which is maintained at 130 DEG C. The condensate flows into decanter D, which is also maintained at 130 DEG C., where it separates into two liquid layers. The lower layer, which comprises phthalic anhydride containing a small amount of dissolved hydrocarbon, is withdrawn from decanter D and is stripped in plate-type column E, which is heated to 180 DEG C and is maintained at an absolute pressure of 50 mm. Hg. Substantially pure phthalic anhydride is withdrawn from the base of column E. The hydrocarbon vapour leaving the head of column E is condensed in condenser C2; and the condensate is returned to the decanter. The hydrocarbon layer from decanter D is returned to column C as reflux. Any naphthoquinones, present in the initial gas, accumulate together with hydrocarbons in the base of column C. The naphthoquinones are separated from the hydrocarbons in refrigerator F; and the hydrocarbons are then pumped back to the top of column A after being re-heated to 140 DEG C.ALSO:Gases containing entrained phthalic anhydride, such as the hot gases obtained from the synthesis of phthalic anhydride by catalytic oxidation, are washed with a liquid aliphatic hydrocarbon or a mixture of liquid aliphatic hydrocarbons having a boiling point higher than 200 DEG C., whereby the anhydride passes from the gas into the liquid hydrocarbon to form a solution; this solution is distilled using a fractionating column and a vapour mixture comprising anhydride and hydrocarbon leaves the head of the column; the vapour is condensed and the condensate is cooled to a temperature at which it separates into two phases, one of which is rich in phthalic anhydride. A petroleum fraction boiling between 260 DEG and 265 DEG C. may be used as wash-liquid, Pentaor tetra-decane may also be used. The distillation is preferably carried out under reduced pressure. The anhydride-rich phase of the condensate may be separated in either the liquid state or the solid state. Suitable apparatus for carrying out the method of the invention is described (see Group III).