Preparation of formaldehyde

摘要

Formaldehyde is prepared by (a) passing a mixture of methanol and water with a concentration of between 50 and 90% by weight, based on the total weight of the two substances, of methanol into a packed column which possesses a packing having a total layer thickness of not less than 50 cm and a total surface area of not less than 0.5 cm2 per cm3 of packing, a liquid circulation of from 15 to 90 g/min per g/min of methanol fed to the column, and a concentration of from 10 to 45% by weight of methanol in the recycle liquid, the temperature at the point of entry of the recycle liquid in the column being from 50 DEG to 86 DEG C. and the bottom temperature being 10 DEG -25 DEG C. lower than this, i.e. from 40 DEG to 70 DEG C., (b) separating off methanol and water from the column by stripping with air, an inert gas and/or exit gas, the throughput being from 0.5 to 3 tonnes of methanol and water per hour per m2 of column cross-section, (c) converting the gaseous mixture of methanol, water, air, inert gas and/or exit gas at a space velocity of from 0.5 to 3 tonnes of methanol per m2 of catalyst bed cross-section per hour, in the presence of a silver catalyst at from 550 DEG to 750 DEG C. and (d) finally, cooling and absorbing the hot reaction gases, and using the resulting heat of absorption and, if desired, the heat of reaction and/or the heat of condensation partially or completely for heating the recycle liquid, methanol containing impurities being used if desired and, after the stages (a) and (b) and before the stages (c) and (d), the measures of a stage (e) being carried out, wherein the gaseous mixture formed is passed through two layers of wire filter having a wire diameter of from 0.1 to 0.5 mm and a free layer volume of from 90 to 99.5 vol. %, based on the total volume of one wire layer, at a flow rate of from 7 to 13 m/sec in the first layer, and from 1 to 4 m/sec in the second layer. The formaldehyde obtainable by the process of the invention is a disinfectant, tanning agent, reducing agent and a useful starting material for the preparation of synthetic resins, adhesives and plastics.