Preparation of formaldehyde

Abstract

Abstract This invention relates to an improved process for the preparation of formaldehyde by oxidising methanol by air in the presence of a silver catalyst and is characterised in that the gases leaving the installation after separation of the condensable products, viz. water, methanol and formaldehyde, are at least partly reinjected in the cold state into the gaseous mixture leaving the catalyst layer. The present invention relates to the preparation of formaldehyde by the known process of oxidising methanol by air in the presence of a silver catalyst. In this known preparation, a gaseous mixture of methanol and air is passed at a suitable temperature over a silver catalyst. The reaction products leave the catalyst layer at a temperature between approximately 500*C and 650*C. These gases also contain the methanol which has not reacted, as well as the desired formaldehyde, water, nitorgen, carbon monoxide, carbon dioxide and hydrogen. It was proved nitrogen, long time ago that in such a gaseous mixture, the temperature of which is between approximately 500*C and 600*C, a certain number of chemical reactions are liable to develop which are considered as parasitic with respect to the main formation reaction of the formaldehyde and which it is generally desired to minimise or suppress. To this end, various methods have been advocated which can be summed up more particularly in the use of an indirect heat exchanger and in the use of atomization of water or a diluted solution of formol. Naturally, it was the aim of the various known methods to lower the temperature of the gases leaving the catalyst layer as quickly as possible to a value lower than 350*C - 450*C. These methods of treating hot gases have various drawbacks and it is an object of the invention to avoid or minimise these drawbacks as far as possible. According to the present invention, a method of preparing formaldehyde by oxidising methanol by air in the presence of a silver catalyst is characterised in that the gases leaving the plant after separation from the condensable products (water, methanol and formaldehyde) ARE AT LEAST partly reinjected in the cold state into the gaseous mixture leaving the catalyst layer. The reinjected gases contain, more particularly, nitrogen, carbon dioxide, carbon monoxide and hydrogen. These gases are referred to as ''''tail'''' gases, and are cold, since they are obtained after treating the gaseous mixture leaving the catalyst layer by known methods having as an object, the condensation or absorption of the condensable chemical products contained in this gaseous mixture. They are, on the other hand, only of small value and are usually rejected to atmosphere. However, used according to the invention, they allow the yield from a plant for preparing the formaldehyde to be substantially improved. This improvement in the yield is carried out following a complex mechanism which has not yet been entirely elucidated. It is caused certainly partly by the rapid cooling of the gaseous mixture leaving the catalyst layer, in which the synthesis of the formal dehyde is carried out: for this reason, it will be apparent that the reinjection of the tail gases must be effected as near as possible to the last layer of the catalyst bed and that it must be ensured, if maximum improvement is to be obtained, that a good distribution of the gases injected into the gaseous mixture leaving the catalyst is effected. But it is also certain that the improvement of yield is caused partly by the composition of the said tail gas. In order to obtain maximum improvement of yield, it has been found, on the other hand, that it was necessary to inject a quantity of tail gas into the gaseous mixture leaving the catalyst layer, representing per molecule of formol (formaldehyde) prepared approximately 1.7 times the quantity of tail gases produced. This is why, according to the invention, such a quantity of tail gas will be made to circulate and be reinjected into the gaseous mixture and that the gaseous circuit will be cleared from the excess gases produced as the reaction takes place. Amongst the other advantages of the method according to the invention may be mentioned: 1. The increase in the volume of inert gas resulting from the recirculation enables the evaporation of a large part of the water and methanol situated at the base of the first absorption tower, which is a particularly interesting advantage when ureaformaldehyde concentrates are to be produced by absorption by means of solutions of urea. 2. This same increase of the gaseous volume leaving the reactor facilitates thermal recuperation of the reaction heat by exchange, whilst avoiding premature condensation in the exchangers, on account of the partial decrease in pressure of the condensable part. 3. The invention can be adapted immediately to methods using a circuit operating at a pressure lower than atmospheric pressure throughout the line with synthesis of transport for the gases situated at the tail of the installation. Only the calibre of the compressor or ventilator used is to be increased.

Inventors

• 👤 R Maux

Bibliographic Details