A process 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.
Process for preparing high-concentration gaseous formaldehyde having a molar CH.sub.2O:H.sub.2O ratio of .gtoreq.0.6 from an aqueous formaldehyde solution by evaporation of at least part of the solution, in which the aqueous formaldehyde solution is heated to a vaporization temperature T and the gas phase formed is taken off, wherein the evaporation temperature T obeys the relationship: T[.degree. C.].gtoreq.T'.sub.min[.degree. C.] where T'.sub.min(c)=A+B.times.(c/100)+C.times.(c/100).sup.2+D.times.(c/100).sup- .3 and A=+68.759, B=+124.77, C=-12.851, D=-10.095, where c is the instantaneous CH.sub.2O content of the aqueous formaldehyde solution during the evaporation in percent by weight and is from 20 to 99% by weight.
