Process for the preparation of percarboxylic acid solutions

In a process for the continuous production of a substantially anhydrous solution of perpropionic acid in benzene which comprises contacting aqueous hydrogen peroxide with propionic acid in the presence of sulfuric acid as catalyst for the reaction to produce the perpropionic acid and water, extracting the reaction mixture with benzene for formation of a benzene phase rich in perpropionic acid and propionic acid and containing hydrogen peroxide, and an aqueous raffinate phase rich in hydrogen peroxide and catalyst, after-treating said benzene phase by extraction with water for formation of an after-treated benzene phase containing perpropionic acid, propionic acid, water, and a reduced amount of hydrogen peroxide, and aqueous phase containing hydrogen peroxide, recycling said aqueous phase containing hydrogen peroxide to the extraction of the reaction mixture for extraction thereof with benzene, dehydrating the after-treated benzene phase by azeotropic distillation to produce said substantially anhydrous solution of perpropionic acid in benzene and an aqueous phase containing hydrogen peroxide, distilling aqueous raffinate of the benzene extraction under reduced pressure to remove water therefrom and form a concentrated solution of hydrogen peroxide and catalyst, recycling said concentrated solution to said contacting of hydrogen peroxide and propionic acid, and introducing make-up hydrogen peroxide and propionic acid into said contacting step, the improvement which comprises:

• a. in said contacting step, the molar ratio of hydrogen peroxide:
• propionic acid being 0.8 - 1.4 :
• 1, the reaction temperature being 20.degree.-60.degree. C, the ratio of hydrogen peroxide:
• water by weight before the start of the reaction thereof with propionic acid based on 100% hydrogen peroxide being up to 1.2, and the concentration of sulfuric acid in the reaction mixture being 10 to 40% by weight;
• b. the extraction of the after-treatment being performed in at least three stages by passing the benzene phase serially through the three stages and in each stage extracting with water or water containing hydrogen peroxide, the aqueous phase containing hydrogen peroxide produced in said azeotropic distillation being introduced into the first state, and employed as an extracting agent therein, and aqueous phase containing hydrogen peroxide being withdrawn from the first stage, the extraction in the second and subsequent stages being carried out with fresh water which is introduced into the final stage and is withdrawn from the second stage as an aqueous phase containing hydrogen peroxide, and recycling the withdrawn aqueous phases from the first and second stages to the extraction of the reaction mixture for extraction thereof with benzene;
• c. employing aqueous hydrogen peroxide for the make-up of hydrogen peroxide, introducing at least part of the make-up aqueous hydrogen peroxide and at least part of the aqueous raffinate directly into the raffinate distillation, and in said raffinate distillation distilling off an amount of water about equal to the sum of the water in the aqueous hydrogen peroxide used as said make-up, the water formed during said contacting and the water introduced in said after-treatment, and withdrawing a sump product comprising a mixture of hydrogen peroxide, sulfuric acid, and water and recycling said sump mixture to said contacting, the raffinate distillation being performed in a rectification unit which at least in part consists essentially of a metal of the group tantalum, tantalum alloy, zirconium and zirconium alloy for reduction of decomposition of hydrogen peroxide, the residence time in the sump of the distillation column being 3-30 minutes, and the sump temperature of said raffinate distillation being 40.degree. - 120.degree. C;
• d. impurities accumulating in the process, and controlling the level of impurities by withdrawl of impurities from the process.