Patent GB-716180-A (GB)
Title: Improvements in or relating to the production of formaldehyde
Filed: 1950-11-22
Abstract: In a process for the production of formaldehyde by the direct oxidation of an aliphatic hydrocarbon having not more than four carbon atoms in the molecule, e.g. methane, the whole or part of the heat that can usefully be recovered from the formaldehyde-containing gases leaving the reaction zone is recovered by cooling said gases by passing them over the surface of a non-metallic refractory material having a surface to volume ratio of at least 2 cm.-1, whereby substantially the whole of the heat removed from the gases is transferred to the refractory material itself, and the heat contained in the refractory material is then used for some useful purpose, e.g. for re-heating the cooled gases after separation of formaldehyde therefrom.  The non-metallic refractory material may be in the form of a tube made of such material, or of a metal tube lined with such material, or of either type of tube packed with pieces of such material.  The refractory material may also be in the form of a moving bed of pebbles but not in the form of particles sufficiently small to become suspended in the gas during its passage over them. In many cases two regenerators may be used each consisting of a vessel made of or lined with the refractory material and packed with such material whereby its surface to volume ratio is at least 2.0 cm.-1, each vessel being alternately heated by the hot gases leaving the reaction zone and cooled by the cooled gases from the following cycle, suitable valves presenting a minimum of metallic surface being provided to change the gas paths through the two vessels.  When the refractory material is in the form of a moving bed of refractory pebbles through which the hot gases are passed the residual cooled gases after separation of the formaldehyde therefrom may be re-heated by contact with the heated pebbles and the cooled pebbles reused for cooling the hot gases from the reaction zone.  Instead of removing the whole of the useful heat contained in the reaction gases the gases may be arranged to leave the packed vessel or pebble heat exchanger at 250-350 DEG  C. and then passed to a conventional tube and shell heat exchanger before final cooling and recovery of formaldehyde.  The stripped gases may then be returned to the opposite phase of the tube and shell heat exchanger and thence to a second vessel lined and filled with refractory material which has been heated by the reaction gases in a previous cycle, or to the appropriate zone of the pebble heat exchanger.  The balance of heat required to bring the gases to the reaction temperature may be supplied either by further heating of the gases or by external heating of the reactor.  Comparative results are given when formaldehyde containing gases at 500 DEG  C. and at 550 DEG  C. are cooled by passing through an empty combustion glass tube of high alumina content and of surface to volume ratio of 2.3 cm.-1 and such tube modified by the insertion of one or more concentric glass tubes to give a surface to volume ratio of 3.6, 4.3 and 5.5 cm.-1, the contact time in each case being one second.