GB763339A - Improvements in recovery of maleic and phthalic acid anhydrides - Google Patents

Abstract

Phthalic or maleic anhydrides are recovered from dilute gaseous mixtures containing either one or both anhydrides by scrubbing only the maleic or only the phthalic anhydride from the said gaseous mixture with dibutyl phthalate, stripping the anhydride from solution in a first fractionating zone having a separation efficiency equivalent to 4 to 7.5 theoretical plates, a pressure in the range 10-35 mm. Hg. at the top of said zone and 40-60 mm. Hg. at the bottom, a temperature in the range 75-175 DEG C. at the top and 110-225 DEG C. at the bottom, and then subjecting the partially stripped liquor to further stripping in a second zone having a separation efficiency equivalent to 2-4 theoretical plates, a pressure in the range 13-25 mm. Hg. at the top and 20-60 mm. Hg. at the bottom, a temperature in the range 85-195 DEG C. at the top and 210-250 DEG C. at the bottom, wherein the stripping ratio of each zone is within the range 1.0 to 0.2, recycling the anhydride product of the second zone to the first zone, and recycling the stripped solvent to the absorption step. The fractionating zones may be tray columns, the first having 10-20 trays and the second 5-10 trays, or packed columns. The absorber is a cylindrical tower containing 30 trays. For phthalic anhydride recovery, the absorber temperature should be within the range 70-120 DEG C., e.g. 70-85 DEG C. at the top and 110-120 DEG C. at the bottom, and the flow rate of upwardly flowing gas to downwardly flowing liquid should be 20-40 mols. of gas per mol. of liquid. For maleic anhydride recovery, the temperature in the absorber should be within the range of 40 DEG to 80 DEG C. and the relative flow rate should be 40-60 mols. of gas per mol. of liquid. Detailed examples describe the separation of phthalic anhydride from a gaseous mixture obtained by catalytic oxidation of naphthalene or o-xylene, and the recovery of maleic anhydride obtained by catalytic oxidation of benzene. To separate a gaseous mixture of maleic and phthalic anhydrides, two systems in series may be used. The maleic anhydride is first absorbed (lower temperature) and then stripped out of the solvent. The gas is then passed to a second absorber, working at higher temperatures, to absorb phthalic anhydride which is then stripped out of the solvent using the above conditions. Alternatively, using a single system, the gaseous mixture may be processed to recover maleic anhydride then reprocessed to recover phthalic anhydride. When the feed contains more than 2 per cent by volume of anhydride, it is advantageous to cool the gas to liquefy a portion of the anhydride leaving a feed gas containing not more than 2 per cent by volume of anhydride.ALSO:Phthalic or maleic anhydrides are recovered from dilute gaseous mixtures containing either one or both anhydrides by scrubbing only the maleic or only the phthalic anhydride from the said gaseous mixture with dibutyl phthalate, stripping the anhydride from solution in a first fractionating zone having a separation efficiency equivalent to 4 to 7.5 theortical plates, a pressure in the range 10.35 mm. Hg. at the top of said zone and 40-60 mm. Hg. at the bottom, a temperature in the range 75-175 DEG C. at the top and 110-225 DEG C. at the bottom, and then subjecting the partially stripped liquor to further stripping in a second zone having a separation efficiency equivalent to 2-4 theoretical plates, a pressure in the 13-25 mm.Hg. at the top and 26-60 mm.Hg. at the bottom, a temperature in the range 85-195 DEG C. at the top and 210-250 DEG C. at the bottom, wherein the stripping ratio of each zone is within the range 1.0 to 0.2, recycling the anhydride product of the second zone to the first zone, and recycling the stripped solvent to the absorption step. The fractionating zones may be tray columns, the first having 10-20 trays and the second 5-10 trays, or packed columns. The absorber is a cylindrical tower containing 30 trays. For phthalic anhydride recovery, the absorber temperature should be within the range 70-120 DEG C. e.g. 70-85 DEG C. at the top and 110-120 DEG C. at the bottom, and the flow rate of upwardly flowing gas to downwardly flowing liquid should be 20-40 mols of gas per mol of liquid. For maleic anhydride recovery, the temperature in the absorber should be within the range of 40 DEG to 80 DEG C. and the relative flow rate should be 40-60 mols of gas per mol of liquid. Detailed examples describe the separation of phthalic anhydride from a gaseous mixture obtained by catalytic oxidation of naphthalene or o-xylene, and the recovery of maleic anhydride obtained by catalytic oxidation of benzine. To separate a gaseous mixture of maleic and phthalic anhydrides, two systems in series may be used. The maleic anhydride is first absorbed (lower temperatures) and then stripped out of the solvent. The gas is then passed to a second absorber, working at higher temperatures, to absorb phthalic anhydride which is then stripped out of the solvent using the above conditions. Alternatively, using a single system, the gaseous mixture may be processed to recover maleic anhydride then reprocessed to recover phthalic anhydride. When the feed contains more than 2 per cent by volume of anhydride, it is advantageous to cool the gas to liquefy a portion of the anhydride leaving a feed gas containing not more than 2 per cent by volume of anhydride.