GB1567921A - Reactor for the production of formaldehyde - Google Patents

Description

(54) A REACTOR FOR THE PRODUCTION OF FORMALDEHYDE (71) We, BAYER AKTIENGESELLSCHAFT a body corporate organised under the laws of Germany of 509 Leverkusen, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to an apparatus for the production of formaldehyde.

Formaldehyde is produced by an air oxidation-dehydration process in the presence of a catalyst. In this process, methanol containing water is evaporated and passed over a silver catalyst together with a quantity of air whose oxygen content is sufficient for the oxidation of 70 to 80% of the methanol. Thus the methanol is oxidized or dehydrated according to the equations: CH3OH + I/202oCH20 + H20 CH3OH - CH2O+H2 As a result of the exothermic heat developed during the reaction, the reaction products, comprising N2, H2, CO2, CO, H20-vapour, CH30H- residue and CH20 are at a temperature of 600 to 7000C.

Since at this temperature the formaldehyde immediately decomposes again into CO + H2, in order to avoid unnecessary losses the reaction products should be cooled as quickly as possible to approximately 300"C. For this purpose there are used waste heat boilers of various designs.

Cooling. can take place through a vertically standing tube bundle cooler in which the reaction mixture flows through the tubes which in turn pass on the heat to the surrounding cooling water. This device has the disadvantage that vapour bubbles can form as a result of poor vapour lead-off under the upper tube plate, which by over-heating cause the destruction of the tube plate and the tubes. Furthermore, the heat exchange in the tube becomes increasingly worse as a result of the reduction of the flow rate consequent upon the decrease in volume of the reaction mixture by cooling in the direction of flow. A possible remedy would be to use tapered tubes, but these are very expensive to produce and to clean.

In the case of another type of cooling, the reaction mixture flows around the horizontal tubes of a tube bundle cooler, while the cooling water is pumped through the tubes in forced circulation. The tubes can be spiral shaped in the case of round cooler design or coil-shaped in the case of square cooler design. This apparatus is complicated by design, susceptible to faults because of the forced circulation, inaccessible for cleaning and expensive because of high energy consumption.

In addition it is known from steam boiler construction to arrange steam generators at an inclination to the horizontal and to achieve a natural circulation by means of a steam drum positioned at a high point.

An object of the invention is to provide a reliable apparatus which comprises single standard-members and permits optimum output, facilitating rapid cooling of the reaction products from 650"C to 300"C by intensive, indirect heat exchange and in which stress on material as a result of local over-heating is avoided.

According to the present invention there is provided an apparatus for the production of formaldehyde comprising a distribution chamber havmg at least one inlet for the reaction mixture, the lower limit of the distribution chamber being defined by a gas permeable catalyst bed arranged at an angle of from 10 to 35 to the horizontal, beneath which is located a heat exchanger having at least one outlet for the reaction products and a plurality of parallel cooling water tubes arranged parallel and directly below the catalyst bed, and the tubes being con nected in natural circulation with a steam drum arranged above the tubes by at least one down pipe and at least one riser pipe.

The advantages achieved with the invention are in particular that as a result of the inclination of the gas cooler, the steam produced during the cooling of the reaction mixture is led off quickly and safely in a natural circulation, so that undesirable side reactions on the product side and damage by over-heating on the steam side are avoided.

It was not foreseeable and therefore surprising that with an apparatus of the present invention a silver catalyst, despite its incandescent and therefore soft state at 650"C even after several weeks production under a changing load did not become degraded, so that reductions in output as a result of cracks or destruction because of local overheating did not take place.

According to a particular embodiment of the apparatus of the invention, the heat exchanger comprises straight cooling water tubes which extend between flat rectangular tube plates. The advantage of this design rests on the simple production, easy exchange facility and good cleaning capacity. Additionally, several devices can be coupled directly without difficulty to achieve a higher output.

According to a particular embodiment the interval, between the walls of adjacent cooling tubes decreases with increasmg distance from the catalyst bed. By the technically simple reduction of the tube intervals it is possible to orient the flowrates of the reaction mixture to achieve optimum heat transfer. In particular, by so doing, the effects occurring as a result of the volume reduction consequent upon the cooling of the reaction mixture can be compensated.

According to a further embodiment, the cooling water tubes extend between an inlet and an outlet chamber each of which is sub-divided into two sections, one down pipe being connected to the lower part of each inlet chamber section and a respective riser pipe being connected to the upper part of each outlet chamber section.

In addition to the sufficient supply and distribution of coolant, the short circuiting, of the flow is avoided by this embodiment.

An embodiment is shown diagrammatically in the drawing and described in more detail in the following.

The Figure shows a cross-section through the installation.

EXAMPLE The starting materials in vapour and gas form enter at a gas inlet pipe 1 and react after distribution in a funnel 2 in a catalyst bed 3 to form a gaseous reaction mixture containing formaldehyde at a temperature of approximately 600"C, which after cooling in a gas cooler 4 by flowing across cooling tubes 5 inclined at between 10 and 35 to the horizontal, leaves the reactor via a pipe 6. The cooling water flows out of the stream drum 7 via a down pipe 8 to a distributor 9, rises through connections 10 into inlet chamber sections 11 and flows with partial evaporation through the cooling tubes 5 to the corresponding opposite outlet chamber sections 14, and after leaving via connections 15 in the relevant riser pipes 16 passes into the steam drum 7 partially filled with liquid. The steam is removed via a pipe 17 to heat up the charge mixture. After the replenishment of the evaporated portion, the cooling water flows via the down pipe 8 back into the natural circulation.

WHAT WE CLAIM IS: 1. An apparatus for the production of formaldehyde comprising a distribution chamber having at least one inlet for the reaction mixture, the lower limit of the distribution chamber being defined by a gas permeable catalyst bed arranged at an angle of from 10 to 35 to the horizontal, beneath which is located a heat exchanger having at least one outlet for the reaction products and a plurality of parallel cooling water tubes arranged parallel and directly below the catalyst bed, the tubes being connected in natural circulation with a steam drum arranged above the tubes by at least one down pipe and at least one riser pipe.

2. An apparatus as claimed in claim 1, wherein the cooling water tubes are straight and extend between flat rectangular tube plates.

3. An apparatus as claimed in claim 1 or 2, wherein the interval between adjacent cooling water tubes decreases with increasing distance from the catalyst bed.

4. An apparatus as claimed in any preceding claim, wherein the cooling water tubes extend between an inlet and an outlet chamber, each of which is sub-divided into two sections, one down pipe being connected to the lower part of each inlet chamber section and a respective riser pipe being connected to the upper part of each outlet chamber section.

5. An apparatus substantially as herein described with reference to the accompany-1 ing drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. nected in natural circulation with a steam drum arranged above the tubes by at least one down pipe and at least one riser pipe. The advantages achieved with the invention are in particular that as a result of the inclination of the gas cooler, the steam produced during the cooling of the reaction mixture is led off quickly and safely in a natural circulation, so that undesirable side reactions on the product side and damage by over-heating on the steam side are avoided. It was not foreseeable and therefore surprising that with an apparatus of the present invention a silver catalyst, despite its incandescent and therefore soft state at 650"C even after several weeks production under a changing load did not become degraded, so that reductions in output as a result of cracks or destruction because of local overheating did not take place. According to a particular embodiment of the apparatus of the invention, the heat exchanger comprises straight cooling water tubes which extend between flat rectangular tube plates. The advantage of this design rests on the simple production, easy exchange facility and good cleaning capacity. Additionally, several devices can be coupled directly without difficulty to achieve a higher output. According to a particular embodiment the interval, between the walls of adjacent cooling tubes decreases with increasmg distance from the catalyst bed. By the technically simple reduction of the tube intervals it is possible to orient the flowrates of the reaction mixture to achieve optimum heat transfer. In particular, by so doing, the effects occurring as a result of the volume reduction consequent upon the cooling of the reaction mixture can be compensated. According to a further embodiment, the cooling water tubes extend between an inlet and an outlet chamber each of which is sub-divided into two sections, one down pipe being connected to the lower part of each inlet chamber section and a respective riser pipe being connected to the upper part of each outlet chamber section. In addition to the sufficient supply and distribution of coolant, the short circuiting, of the flow is avoided by this embodiment. An embodiment is shown diagrammatically in the drawing and described in more detail in the following. The Figure shows a cross-section through the installation. EXAMPLE The starting materials in vapour and gas form enter at a gas inlet pipe 1 and react after distribution in a funnel 2 in a catalyst bed 3 to form a gaseous reaction mixture containing formaldehyde at a temperature of approximately 600"C, which after cooling in a gas cooler 4 by flowing across cooling tubes 5 inclined at between 10 and 35 to the horizontal, leaves the reactor via a pipe 6. The cooling water flows out of the stream drum 7 via a down pipe 8 to a distributor 9, rises through connections 10 into inlet chamber sections 11 and flows with partial evaporation through the cooling tubes 5 to the corresponding opposite outlet chamber sections 14, and after leaving via connections 15 in the relevant riser pipes 16 passes into the steam drum 7 partially filled with liquid. The steam is removed via a pipe 17 to heat up the charge mixture. After the replenishment of the evaporated portion, the cooling water flows via the down pipe 8 back into the natural circulation. WHAT WE CLAIM IS:

1. An apparatus for the production of formaldehyde comprising a distribution chamber having at least one inlet for the reaction mixture, the lower limit of the distribution chamber being defined by a gas permeable catalyst bed arranged at an angle of from 10 to 35 to the horizontal, beneath which is located a heat exchanger having at least one outlet for the reaction products and a plurality of parallel cooling water tubes arranged parallel and directly below the catalyst bed, the tubes being connected in natural circulation with a steam drum arranged above the tubes by at least one down pipe and at least one riser pipe.

2. An apparatus as claimed in claim 1, wherein the cooling water tubes are straight and extend between flat rectangular tube plates.

3. An apparatus as claimed in claim 1 or 2, wherein the interval between adjacent cooling water tubes decreases with increasing distance from the catalyst bed.

4. An apparatus as claimed in any preceding claim, wherein the cooling water tubes extend between an inlet and an outlet chamber, each of which is sub-divided into two sections, one down pipe being connected to the lower part of each inlet chamber section and a respective riser pipe being connected to the upper part of each outlet chamber section.

5. An apparatus substantially as herein described with reference to the accompany-1 ing drawing.