Patent ID: 7345207

Claim:
A process for preparing high-concentration formaldehyde solutions having a CH 2 O content of ≧50% by weight from an aqueous formaldehyde solution having a lower CH 2 O content by evaporation of part of this solution (partial evaporation), in which the aqueous formaldehyde solution is heated to an evaporation temperature T at which the gas phase becomes enriched in water relative to the liquid phase and the gas phase formed is taken off continuously or discontinuously, wherein the evaporation temperature T obeys the relationship: T[°C.]<T max [°C.] where T max ( c )= A+B× ( c/ 100)+ C× ( c/ 100) 2 +D ×( c/ 100) 3 and A=−68.759, B=−124.77, C=−12.851, D=−10.095, wherein a temperature which obeys the relationship T[°C.]>T min [°C.] where T min ( c )= A′+B ′×( c/ 100)+ C ′×( c/ 100) 2 +D ′×( c/ 100) 3 and where A′=+6.0156, B′=+52.918, C′=+49.699, D′=+34.286, is maintained in the aqueous formaldehyde solution at every point in the evaporator, where c is the instantaneous CH 2 O content of the aqueous formaldehyde solution during the evaporation in percent by weight and is from 20 to 99% by weight.