Process and apparatus for preparing pure formaldehyde

Abstract

A process and an apparatus for the post-purification in particular of pyrolysis gases for preparing pure formaldehyde, hemiformal being pyrolyzed in an evaporator, the resulting formaldehyde vapor being passed through a condensed liquid component collected in the bottom area of the evaporator, the condensed liquid component being passed out as a falling film situated on the inner wall through a cooled outlet tube disposed in the bottom area of the evaporator and the formaldehyde vapor being passed through the cooled outlet tube in cocurrent and the liquid component and the formaldehyde being taken off separately at the outlet tube end remote from the evaporator.

1 . A process for the post-purification in particular of pyrolysis gases for preparing pure formaldehyde, having the process steps of: 
 pyrolyzing hemiformal in an evaporator,    passing the resulting formaldehyde vapor through the condensed liquid component collected in the bottom area of the evaporator,    passing out the condensed liquid component through a cooled outlet tube disposed in the bottom area of the evaporator, so that the condensed liquid component flows down as a falling film on the inner wall of the cooled outlet tube,    passing the formaldehyde vapor in cocurrent through the cooled outlet tube,    taking off the formaldehyde vapor at the outlet tube end remote from the evaporator, and    taking off the condensed liquid component at the outlet tube end remote from the evaporator.    
     
     
         2 . A process for the post-purification of pyrolysis gases comprising minor components for preparing pure formaldehyde, having the process steps of: 
 pyrolyzing of hemiformal in an evaporator,    passing the resulting formaldehyde vapor through the condensed liquid component collected in the bottom area of the evaporator, which is low in minor components,    if appropriate precooling the vapor, so that some of the vaporized alcohol present after elimination of formaldehyde from the hemiformal is condensed,    taking off some of the condensed liquid component which is low in minor components from the bottom area of the evaporator,    taking off some of the condensed liquid component which is low in minor components from the bottom area of the evaporator,    passing out the other portion of the condensed liquid component through a cooled outlet tube disposed in the bottom area of the evaporator, so that this portion of the condensed liquid component flows down as a falling film on the inner wall of the cooled outlet tube.    passing the formaldehyde vapor in cocurrent through the cooled outlet tube,    taking off the formaldehyde vapor at the outlet tube end remote from the evaporator, and    taking off the other portion of the condensed liquid component, which after flowing through the outlet tube has a higher content of minor components, at the outlet tube end remote from the evaporator    
     
     
         3 . The process as claimed in  claim 1  or  2 , wherein the condensed liquid component runs out spontaneously (by gravity) into the outlet tube.  
     
     
         4 . The process as claimed in  claim 1  or  2 , wherein the condensed liquid component is transported into the outlet tube using a transport apparatus.  
     
     
         5 . The process as claimed in one of the preceding claims wherein the pyrolysis temperature is between 100 and 200° C.  
     
     
         6 . The process as claimed in one of the preceding claims, wherein the temperature of the cooled outlet tube is between −20° C. and +40° C.  
     
     
         7 . The process as claimed in one of the preceding claims, wherein the process pressure is in the vacuum range.  
     
     
         8 . An apparatus for the post-purification in particular of pyrolysis gases for preparing pure formaldehyde having an outlet tube ( 5 ) which is disposable in the bottom area of an evaporator ( 1 ), the outlet tube ( 5 ) being coolable and having a takeoff ( 11 ) for taking off the vapors formed in the evaporator ( 1 ).  
     
     
         9 . The apparatus as claimed in  claim 8 , wherein the evaporator ( 1 ) is a thin-film evaporator.  
     
     
         10 . The apparatus as claimed in  claim 8  or  9  wherein the evaporator ( 1 ) is constructed in the bottom area ( 15 ) for collecting the condensed liquid component.  
     
     
         11 . The apparatus as claimed in  claim 10 , wherein an apparatus for transporting the condensed liquid component into the outlet tube ( 5 ) is disposed in the collection area ( 15 ) of the evaporator ( 1 ).  
     
     
         12 . The apparatus as claimed in  claim 11 , wherein the transport apparatus is a screw conveyor.  
     
     
         13 . The apparatus as claimed in one of  claims 8  to  12 , wherein the takeoff ( 11 ) for taking off the vapors is disposed at the outlet tube ( 5 ) end remote from the evaporator ( 1 ).  
     
     
         14 . The apparatus as claimed in one of  claims 8  to  13 , wherein the outlet tube ( 5 ) is of double-wall construction.  
     
     
         15 . The use of formaldehyde purified according to one of the preceding claims for preparing polymers oligomers, dyestuffs and fertilizers, and for treating seeds.  
     
     
         16 . The use as claimed in  claim 15 , wherein the polymer is polyoxymethylene and the oligomer is trioxane.