GB716181A - Improvements in or relating to the manufacture of formaldehyde from methane - Google Patents

Abstract

Formaldehyde is obtained by catalytic oxidation of methane using as catalyst nitric oxide or a substance which will yield nitric oxide under the conditions of reaction by passing methane, or gas containing methane, admixed with oxygen and with the catalyst through a heated reaction zone in which the ratio of surface to volume is maintained at above 2.0 cm-1, such zone comprising a vessel consisting, or lined with, a non-metallic refractory material, and then separating formaldehyde from the gases leaving the reaction zone. The residual gas after removal of the formaldehyde is preferably recycled together with fresh methane and oxygen, to the reaction zone. The process is preferably operated with a feed gas containing 50-85 per cent by volume of methane (preferably 70-80 per cent of methane) and with an oxygen concentration of 10-16 per cent by volume and using 0.1-1.0 per cent by volume of nitric oxide. The nitric oxide can be fed as such to the feed gas, or equivalent quantities of compounds which produce nitric oxide in the heated reaction zone, such as nitric acid, methyl nitrite or nitromethane can be fed as vapour or spray into the feed gas. Examples of suitable refractory materials are silica, alumina, aluminosilicate glass and silicon carbide and the reactor is preferably constructed by dividing the vessel lengthwise into a number of evenly spaced compartments by means of parallel plates. In examples (1) mixtures of methane, oxygen, nitrogen and a small amount of nitric oxide are fed to silica tubes of various lengths and diameters contained in a furnace at 680 DEG C. The production rate and concentration of formaldehyde for tubes having a surface to volume ratio of 3.2 cm.-1 to 11.8 cm.-1 are tabulated and comparative results are given when operating with a tube of surface to volume ratio 1.9 cm.-1; (2) using the same furnace as in (1) a similar gaseous mixture is fed to (a) an unpacked silica tube of surface to volume ratio 2.5 cm.-1, (b) the same tube packed with silica chips so that the overall surface to volume ratio is 8.6 cm.-1 and (c) a narrower unpacked silica tube having a surface to volume ratio of 8.9 cm.-1, and the production rates of formaldehyde at optimum contact time are compared; (3) as in (1) except that the gaseous mixture is passed through (a) an alumina tube of surface to volume ratio 4.1 cm.-1, (b) a silicon carbide tube of surface to volume ratio 5.0 cm.-1 and (c) an aluminosilicate combustion glass of surface to volume ratio 4.8 cm.-1. A comparative example is also given wherein the tube is of silver-plated glass having a surface to volume ratio of 4.2 cm.-1, the yield of formaldehyde in this case being very low; (4) a gas mixture consisting of 80 per cent methane, 10 per cent oxygen and the remainder consisting mainly of nitrogen and carbon dioxide and p small amounts of carbon monoxide and hydrogen is preheated to 640 DEG C. then mixed with nitric oxide (0.4 per cent by volume) and passed into a reactor consisting of a rectangular steel vessel lined with silica plates and divided lengthwise by means of thin silica plates into evenly spaced compartments giving a surface to volume ratio of 6.0 cm.-1 and the production rate and concentration of formaldehyde at 0.25 sec. contact time are given. Specifications 520,840, 664,741 and 716,180 are referred to.