Manufacture of formaldehyde

Claim 1

In a process for the manufacture of formaldehyde by oxidative dehydrogenation of methanol in admixture with water vapor and air and in the presence of a silver catalyst at elevated temperature, the improvement which comprises purifying the methanol and supplying the purified methanol, water vapor and air mixture to the catalyst for said oxidative dehydrogenation which comprises: evaporating the methanol and water in a rectifying plate column in which said crude methanol and water as liquids are fed to the top and the air to the bottom of the column; carrying out rectification by refluxing liquid by overflow from an upper plate assembly to a lower plate assembly to a lower plate assembly and from the lowermost plate assembly to a liquid bottoms while supplying heat substantially only to said liquid bottoms to evaporate methanol for conduction with air and water vapor as a countercurrent gas upwardly through said column; directing the liquids fed to each plate assembly as an individual rectifying zone such that the liquids flow first over a central bubble cap platform area, then to a lower surrounding sieve plate joined to said platform and finally to an annular retaining and overflow zzone on a second plate joined below each sieve plate from which the liquid is conducted to the next lower rectifying zone or to said liquid bottoms, the coutercurrent gas flowing upwardly through each sieve plate for intimate contact with the liquid flowing therein; withdrawing a liquid bottoms fraction from said plate column as an impure waste material substantially free of methanol; and recovering a vaporous fraction from the top of said column consisting essentially of gaseous methanol, water vapor and air as a feed stream for said oxidative dehydrogenation.