GB814119A - Dehydrating formaldehyde by distillation and partial condensation - Google Patents

Abstract

<PICT:0814119/III/1> Substantially anhydrous formaldehyde is obtained by vaporizing an aqueous solution of formaldehyde, and subjecting the vapours at an absolute pressure of not less than one atmosphere to a two-stage partial condensation process whereby condensates which are rich in water are obtained; formaldehyde gas is withdrawn from the second stage and further dehydrated by cooling it below 0 DEG C. so that partial polymerization takes place whereby the remaining water separates out with the solid polymer. The apparatus shown comprises a steam-heated vaporizer 2, two water-cooled condensers 4, 9, connected in series, and a pair of shell-and-tube heat-exchangers 14, 15, which are connected in parallel. Aqueous formaldehyde, containing 50 to 60 per cent by weight of formaldehyde, is pumped through vaporizer 2 which is heated to about 135-165 DEG C. The solution vaporizes at a temperature of about 115 DEG C. and a pressure of 2 p.s.i.g. The line connecting the vaporizer-outlet to the first condenser 4 is heated by steam in a jacket so that the vapours enter the condenser at a temperature of about 125 DEG C. Partial condensation is effected; and the mixture of condensate and vapour, at about 65 DEG C., so obtained is led into receiver 6. The vapour is conducted via a steam-jacketed line 8 so that it is raised to about 105 DEG C. before it enters the second condenser 9, where further condensation takes place. The second-stage condensate which has a temperature of about 35 DEG C., is separated from the uncondensed vapours in receiver 11. The vapours, heated to about 80 DEG C., are led into one or other of the heat-exchangers 14, 15, which is cooled by heptane supplied at a temperature of - 20 DEG C. About 15 per cent of the vapours forms a solid polymer product on the inside surface of the cooled tubes, and in so doing, a large proportion of the remaining water is removed from the vapours. The vapour withdrawn through outlet 16 contains about 99.5 per cent H.CHO, and may, if desired, be immediately polymerized to high molecular weight polymer. The condensate withdrawn through outlet 12 may be recirculated by mixing it with the feed stream entering vaporizer 2. The condensate withdrawn through outlet 7 may be concentrated and then recirculated. In normal operation only one heat-exchanger is used while the other is cleaned out. Cleaning may be accomplished by blowing steam through the partially-plugged tubes which causes the deposited solid to dissolve forming aqueous formaldehyde.ALSO:Substantially anhydrous formaldehyde is obtained by vaporizing an aqueous solution of formaldehyde, passing the vapours through two condensers, joined in series, to condense out the bulk of the water-vapour, cooling the gas withdrawn from the second condenser to a temperature below 0 DEG C. so that part of the formaldehyde polymerizes whereby the remaining water-vapour separates out with the solid polymer, and withdrawing anhydrous formaldehyde in the gaseous state from the polymerization zone. In an example, an aqueous solution, containing 50 to 60 per cent by weight of formaldehyde, is vaporized; and the vapours are passed, at a pressure of 2 p.s.i.g. and an initial temperature of 125 DEG C., through two condensers in series, each of which is provided with a steamjacket (see Group III). The two stages of water-condensation are effected at 65 DEG C. and 35 DEG C. respectively. The vapours leaving the second condenser are heated to about 80 DEG C., and led to a tubular heat-exchanger cooled by heptane which is supplied at - 20 DEG C. About 15 per cent of the vapours form a solid polymer on the insides of the tubes. The vapour withdrawn from the outlet of the heat-exchanger comprises 99.5 per cent H.CHO.