Process for separating trioxane from an aqueous mixture
DESCRIPTION 
     The preparation of trioxane gives rise to an azeotropic mixture consisting essentially of trioxane, water and formaldehyde. From this mixture trioxane is extractively separated with the aid of an azeotrope-former such as methylene chloride or benzene. In a subsequent distillation the azeotrope-former is recovered and returned to the extractive distillation. In this process large amounts of azeotrope-former need to be used and recovered with high energy consumption. Disposal of unavoidable boilable emissions is costly, since methylene chloride and benzene are classified as dangerous pollutants. 
     It is an object of the present invention to provide a remedy. The invention achieves this object by extracting water from the mixture by pervaporation and separating the water-depleted mixture (retentate) by rectification into trioxane and an azeotropic mixture of trioxane, water and formaldehyde. 
     The pervaporation can be carried out at temperatures of from 70°to 120° C. using a membrane of polyvinyl alcohol at pressures of from i to 3 bar on the retentate side and from 30 to 150 mbar on the permeate side. 
     The advantages of the process of the invention are essentially that no additional component (such as azeotrope-former) is required, thus causing the emission problem to disappear and lowering the energy consumption (lower temperature level). Furthermore, the pervaporation unit occupies less space than the extractive distillation equipment at a comparable throughput. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     An embodiment of the invention will now be more particularly described by way of example with reference to the figure. 
    
    
     An aqueous mixture (line 1) containing 65% by weight of trioxane, 27.5% by weight of water and 7.5% by weight of formaldehyde is treated in a pervaporation unit 2, comprising a membrous 3 of polyvinyl alcohol from GFT (Gesellschaft fur Trenntechnik mbH, D-6680 Neunkirchen/Saar), to remove water (line 4). 
     The separation is carried out at 90° C. The pressure is 1 bar on the retentate side 5, (50 mbar) on the permeate side 6. The permeate obtained is water and the retentate obtained is a mixture of 84% by weight of trioxane, 10% by weight of formaldehyde and 6% by weight of water (line 7). The retentate is separated in a rectification column 8 at atmospheric pressure into pure trioxane (bottom product, line 9) and an azeotropic mixture of trioxane, formaldehyde and water (top product, line 10).