WO2009149809A1

WO2009149809A1 - Method for producing formaldehyde

Info

Publication number: WO2009149809A1
Application number: PCT/EP2009/003498
Authority: WO
Inventors: Evin Hizaler Hoffmann; Leslaw Mleczko; Ralph Schellen; Stephan Schubert
Original assignee: Bayer Technology Services Gmbh
Priority date: 2008-05-29
Filing date: 2009-05-16
Publication date: 2009-12-17
Also published as: DE102008025887A1

Abstract

The invention relates to a method for producing formaldehyde by catalytic gas phase oxidation of methanol with oxygen, the reaction being carried out under adiabatic conditions on 10 to 60 catalyst beds that are arranged in series. The invention further relates to a reactor system for carrying out said method.

Description

Process for the preparation of formaldehyde

The present invention relates to a process for the preparation of formaldehyde by catalytic gas phase oxidation of methanol with oxygen, wherein the reaction is carried out in 10 to 60 successive reaction zones under adiabatic conditions, and a reactor system for carrying out the method.

Formaldehyde is generally prepared under the catalytic influence of metal oxides from gaseous methanol and oxygen in an exothermic, catalytic reaction according to formula (T):

CH ₃ OH (g) +0.5 O ₂ (g) → CH ₂ O (g) + H ₂ O; ΔH = -159 kJ / mol (T)

The formaldehyde produced by the reaction of formula (I) forms an essential starting material for many other syntheses in the chemical industry. In particular, formaldehyde is used in the production of dyes, pharmaceuticals and in textile finishing.

The removal and use of the heat of reaction is an important issue in the practice of formaldexhd synthesis. An uncontrolled increase in temperature can lead to permanent damage to the catalytic converter. It also exists at high

Temperatures the possibility of side reactions to form more or less large amounts of, inter alia, carbon monoxide, which is the next oxidation state of the

Represents formaldehyde. It is therefore advantageous to keep the temperature of the catalysts controlled in the course of the process at a level that underlies a rapid turnover

Minimization of side reactions and / or catalyst deactivation allows.

EP 1 551 544 (B1) discloses an apparatus and a method by means of which the reaction according to formula (I) can be carried out under pseudo-isothermal conditions. The method and apparatus are characterized by having multiple reaction zones within the apparatus, the reaction zones comprising heat exchange means and catalyst. Furthermore, to control selectivity and conversion of the process, each reaction zone is charged with a process gas of different composition and operated at a different temperature, which is considered to be pseudo-isothermal over the particular reaction zone. The process gases are divided into different proportions and also partially within the Reactor in a circle. Only parts of the process gas are withdrawn from the device. The device and the method carried out herein are disadvantageous, since the expenditure on equipment for the division of the process gases into partial streams whose controlled task in the individual reaction zones and the pseudo-isothermal procedure in these is very high. Furthermore, the three reaction zones disclosed by the circulation of the process gases in the reactor are not considered to be separate from each other, so that precise coordination of temperature and process gas composition in the individual reaction zones is only possible by complex control mechanisms, since a stable operating point initially set and it must be checked continuously.

US Pat. No. 2,504,402 discloses an apparatus and a process in which the reaction according to formula (I) is carried out in several successive adiabatic reaction zones, a heat exchange zone in each case between the individual adiabatic reaction zones. In addition, methanol is fed in each case before each reaction zone and / or heat exchange zone, the reaction zones each being operated in such a way that no methanol is present in the process gas downstream of the reaction zone. The procedure is disadvantageous because each reaction zone requires a quantitative conversion. This can either mean that the single amount of methanol that is fed into a reaction zone is very low, which would make the space-time yield of the process very low, or that larger amounts of methanol are introduced into a respective reaction zone, thus the expected temperatures of adiabatic reaction zones should be very high and thus the catalyst life should be low in these.

Other established processes use more than one reaction zone after which heat exchangers are installed to recool the product gas mixture after the exothermic reaction. In this way, the heat of reaction can be obtained at higher temperature levels, as well as increased sales can be achieved.

EP 1 251 951 (B1) discloses a device and the possibility of carrying out chemical reactions in the device, wherein the device is characterized by a cascade of reaction zones in contact with one another and heat exchanger devices which are arranged in a composite with one another. The process to be carried out herein is thus characterized by the contact of the various reaction zones with a respective heat exchanger device in the form a cascade. There is no disclosure as to the usability of the apparatus and method for the synthesis of formaldehyde from gaseous oxygen and methanol. Thus, it remains unclear how, starting from the disclosure of EP 1 251 951 (B1), such a reaction should be carried out by means of the device and the method carried out therein. Further, for the sake of uniformity, it is to be understood that the method disclosed in EP 1 251 951 (B1) is carried out in a device the same as or similar to the disclosure regarding the device. As a result, by the large-area contact of the heat exchange zones according to the disclosure with the reaction zones, a significant amount of heat takes place by heat conduction between the reaction zones and the adjacent heat exchange zones. The disclosure with regard to the oscillating temperature profile can therefore only be understood as meaning that the temperature peaks ascertained here would be stronger if this contact did not exist. Another indication of this is the exponential increase in the disclosed temperature profiles between the individual temperature peaks. These indicate that there is some heat sink of appreciable but limited capacity in each reaction zone which can reduce the temperature rise in it. It can never be ruled out that a certain dissipation of heat (eg by radiation) takes place. However, a reduction of the possible heat removal from the reaction zone would indicate a linear or degressive temperature gradient, since no further addition of educts is envisaged and thus, after an exothermic reaction, the reaction becomes slower and thus the heat of reaction produced would decrease. Thus, EP 1 251 951 (B1) discloses multi-stage processes in cascades of reaction zones from which heat in an undefined amount is removed by heat conduction. Accordingly, the disclosed method is disadvantageous in that accurate temperature control of the process gases of the reaction is not possible.

EP 0 820 345 (B1) discloses a process for the production of formaldehyde by catalytic gas phase oxidation of methanol with oxygen and an apparatus in which it can be carried out, comprising a plurality of adiabatic reaction zones. The revealed plurality is not limited. However, it is disclosed in the figures and exemplary embodiments that a maximum number of five adiabatic reaction zones can be present. It is further disclosed that heat exchange zones may be located between the adiabatic reaction zones. - A -

However, these heat exchange zones are connected to each other in such a way that they communicate with each other via a single heat exchange medium.

The disclosure of EP 0 820 345 (B1) is disadvantageous since, with the small number of adiabatic reaction zones and the central heat exchange zones, precise control and / or control of the temperature profile over the reaction zones is not possible. The reason for this is that a complex interconnection of the reaction zones with the heat exchange zones is forced, which in turn leads to a mutual influence of the reaction zones. The mutual influence is particularly disadvantageous in particular in the exothermic reaction according to formula (I), since with an adiabatic temperature increase in one of the maximum five reaction zones, which exceeds the previously calculated level, the coolant of the subsequent heat exchange zone is heated more strongly. This in turn leads to a lower cooling effect in the subsequent heat exchange zone, so that a disproportionate heating of the next but one reaction zone must be feared.

Based on the prior art, it would therefore be advantageous to provide a method that can be carried out in simple reaction devices and that allows accurate, simple temperature control, so that it allows high conversions with the highest possible purity of the product. Such simple reaction devices would be easily scaled up and are inexpensive and robust in all sizes.

For the catalytic gas phase oxidation of methanol with oxygen to formaldehyde, as described above, neither suitable reactors have yet been described, nor are suitable processes disclosed that permit them.

It is therefore an object to provide a process for the catalytic gas phase oxidation of methanol and oxygen to formaldehyde, which can be carried out under precise temperature control in simple reaction devices and thereby high conversions at high purities of the product allowed, the heat of reaction either in favor of the reaction or otherwise used.

It has surprisingly been found that a process for the preparation of formaldehyde from methanol and oxygen in the presence of heterogeneous catalysts, characterized in that it comprises 10 to 60 successive reaction zones with adiabatic conditions, is able to achieve this object. Methanol in the context of the present invention refers to a process gas which is introduced into the process according to the invention and which essentially comprises methanol. Essentially, in the context of the present invention, a proportion of more than 90% by weight. As a result, methanol present in the liquid phase is first vaporized before being fed to the process.

Oxygen, in the context of the present invention, refers to a process gas which is introduced into the process according to the invention and which essentially comprises oxygen. Preferably, oxygen is ambient air and therefore comprises a proportion of about 20% by volume of oxygen.

In addition to the essential components of the process gases methanol and oxygen, these can also include secondary components. Non-exhaustive examples of minor components that may be included in the process gases include nitrogen, dimethyl ether, carbon dioxide, carbon monoxide, and water.

In general, in the context of the present invention, process gases are understood as gas mixtures which comprise oxygen and / or methanol and / or formaldehyde and / or secondary components. In essence, however, process gases include oxygen and / or methanol and / or formaldehyde.

According to the invention, carrying out the process under adiabatic conditions essentially means that the reaction zone is neither actively supplied with heat nor heat withdrawn from the outside. It is well known that complete isolation against heat input or discharge is possible only by complete evacuation, excluding the possibility of heat transfer by radiation. Therefore, in the context of the present invention, adiabat means that no heat supply or removal measures are taken.

In an alternative embodiment of the method according to the invention, however, e.g. by isolation by means of well-known isolation means, e.g. Polystyrene insulating materials, or by sufficiently large distances to heat sinks or heat sources, wherein the insulating agent is air, a heat transfer can be reduced.

An advantage of the adiabatic driving method according to the invention of the 10 to 60 reaction zones connected in series with respect to a non-adiabatic driving mode is that no heat removal means need be provided in the reaction zones, which results in a considerable simplification of the construction. This results in particular simplifications in the manufacture of the reactor and in the scalability of the process and an increase in reaction conversions. In addition, the heat generated in the course of the exothermic reaction progress can be utilized in the single reaction zone to increase the conversion in a controlled manner.

Another advantage of the method according to the invention is the possibility of very accurate temperature control, due to the close staggering of adiabatic reaction zones. It can thus be set in each reaction zone advantageous in the reaction progress temperature.

The catalysts used in the process according to the invention are usually catalysts which consist of a material which, in addition to its catalytic activity for the reaction of the formula (I), is characterized by sufficient chemical resistance to oxygen under the conditions of the process and by a high specific surface area. Catalyst materials characterized by such chemical resistance to oxygen under the conditions of the process include, for example, catalysts comprising iron and molybdenum.

Specific surface area in the context of the present invention refers to the area of the catalyst material that can be reached by the process gases based on the mass of catalyst material used.

A high specific surface area is a specific surface area of at least 1 m ² / g, preferably of at least 2 m ² / g.

The catalysts of the invention are each in the reaction zones and can be used in all known forms, e.g. Fixed bed, fluidized bed, fluidized bed present.

Preferred are the forms of fixed bed and fluidized bed.

The fixed bed arrangement comprises a catalyst bed in the strict sense, ie loose, supported or unsupported catalyst in any form and in the form of suitable packings. The term catalyst bed as used herein also includes contiguous areas of suitable packages on a carrier. material or structured catalyst support. These would be, for example, to be coated ceramic honeycomb carrier with comparatively high geometric surfaces or corrugated layers of metal wire mesh on which, for example, catalyst granules is immobilized. As a special form of packing, in the context of the present invention, the presence of the catalyst in monolithic form is considered.

If a fixed-bed arrangement of the catalyst is used, the catalyst is preferably present in beds of particles with average particle sizes of 1 to 10 mm, preferably 1, 5 to 8 mm, particularly preferably 2 to 5 mm.

Particularly preferred in a fixed bed arrangement is a monolithic catalyst comprising iron and molybdenum.

When a catalyst in monolithic form is used in the reaction zones, in a preferred further development of the invention the monolithic catalyst is provided with channels through which the process gases flow. Usually, the channels have a diameter of 0.1 to 3 mm, preferably a diameter of 0.2 to 2 mm, more preferably from 0.5 to 1, 5 mm.

A monolithic catalyst with channels of the specified diameter is particularly advantageous, since this explosion protection can be ensured. This is done by absorbing the enthalpy through the wall of the monolith and thus suppressing further propagation of flames.

If a fluidized bed arrangement of the catalyst is used, the catalyst is preferably present in loose beds of particles, as have also previously been described for the fixed bed arrangement.

Beds of such particles are advantageous because the size of the particles have a high specific surface area of the catalyst material compared to the process gases oxygen and methanol and thus a high conversion rate can be achieved. Thus, the mass transport limitation of the reaction by diffusion can be kept low. At the same time, however, the particles are not yet so small that disproportionately high pressure losses occur when the fixed bed flows through. The ranges of the particle sizes given in the preferred embodiment of the process, comprising a reaction in a fixed bed, are thus an optimum between the achievable turnover from the reaction according to formula (I) and the generated pressure loss when carrying out the process. Pressure loss is coupled in a direct manner with the necessary energy in the form of compressor performance, so that a disproportionate increase in the same would result in an inefficient operation of the method.

In a preferred embodiment of the process according to the invention, the conversion takes place in 12 to 50, more preferably 15 to 30 reaction zones connected in series.

A preferred further embodiment of the method is characterized in that the process gas emerging from at least one reaction zone is subsequently passed through at least one heat exchange zone downstream of said reaction zone.

In a particularly preferred further embodiment of the process, after each reaction zone is at least one, preferably exactly one heat exchange zone, through which the process gas leaving the reaction zone is passed.

The reaction zones can either be arranged in a reactor or arranged divided into several reactors. The arrangement of the reaction zones in a reactor leads to a reduction in the number of apparatuses used.

The individual reaction zones and heat exchange zones can also be arranged together in a reactor or in any combination of reaction zones with heat exchange zones in several reactors.

If reaction zones and heat exchange zones are present in a reactor, then in an alternative embodiment of the invention there is a heat insulation zone between them, in order to be able to obtain the adiabatic operation of the reaction zone.

In addition, each of the series-connected reaction zones can be replaced or supplemented independently of one another by one or more reaction zones connected in parallel. The use of reaction zones connected in parallel allows in particular their replacement or supplementation during ongoing continuous operation of the process. Parallel and successive reaction zones may in particular also be combined with one another. However, the process according to the invention particularly preferably has exclusively reaction zones connected in series.

The reactors preferably used in the process according to the invention can consist of simple containers with one or more reaction zones, as e.g. in Ullmann's Encyclopedia of Industrial Chemistry (Fifth, Completely Revised Edition, VoI B4, page 95-104, page 210-216), wherein in each case between the individual reaction zones and / or heat exchange zones heat insulation zones can be additionally provided.

In an alternative embodiment of the method, there is thus at least one heat-insulating zone between a reaction zone and a heat exchange zone. Preferably, there is a heat isolation zone around each reaction zone.

The catalysts or the fixed beds thereof are mounted in a manner known per se on or between gas-permeable walls comprising the reaction zone of the reactor. Particularly in the case of thin fixed beds, technical devices for uniform gas distribution can be provided in the flow direction in front of the catalyst beds. These can be perforated plates, bubble-cap trays, valve trays or other internals which cause a uniform entry of the process gas into the fixed bed by producing a small but uniform pressure loss.

In a particular embodiment of the process according to the invention, preference is given to using a molar excess of between 0 and 500% oxygen, based on the molar flow of methanol, before it enters the first reaction zone. By increasing the ratio of oxygen per methanol, on the one hand, the reaction can be accelerated and thus the space-time yield (amount of produced formaldehyde per mass of catalyst material) increased. On the other hand, a composition of the process gas is achieved which is further removed from the critical boundary composition for an explosion ,

In a further particularly preferred embodiment of the method, the inlet temperature of the process gas entering the first reaction zone is from 10 to 330 ^° C., preferably from 50 to 310 ^° C., particularly preferably from 100 to 290 ^° C. In yet another particularly preferred embodiment of the process, the absolute pressure at the inlet of the first reaction zone is between 1 and 4 bar, preferably between 1.1 and 3 bar, more preferably between 1.2 and 2.5 bar.

In a further particularly preferred embodiment of the process, the residence time of the process gas in a reaction zone is between 0.1 and 50 s, preferably between 0.2 and 20 s, particularly preferably between 0.5 and 10 s.

The methanol and the oxygen are preferably fed only before the first reaction zone. This has the advantage that the entire process gas can be used for the absorption and removal of the heat of reaction in all reaction zones. In addition, by such a procedure, the space-time yield can be increased, or the necessary catalyst mass can be reduced. However, it is also possible to meter in methanol and / or oxygen in the process gas as required before one or more of the reaction zones following the first reaction zone. In addition, the temperature of the conversion can be controlled via the supply of gas between the reaction zones.

In a preferred embodiment of the method according to the invention, the process gas is cooled after at least one of the reaction zones used, more preferably after each of the catalyst beds used. For this purpose, the process gas is passed after exiting a reaction zone through one or more of the above-mentioned heat exchange zones, which are located behind the respective reaction zones. These may be used as heat exchange zones in the form of heat exchangers known to those skilled in the art, e.g. Tube bundle, plate, Ringnut-, spiral, finned tube, micro-heat exchanger be executed. The heat exchangers are preferably microstructured heat exchangers.

In a further embodiment of the method, steam is generated during cooling of the process gas in the heat exchange zones by the heat exchanger.

Within this further embodiment, it is preferable in the heat exchangers, which include the heat exchange zones, to carry out evaporation on the side of the cooling medium, preferably partial evaporation.

Partial evaporation, in the context of the present invention, refers to evaporation in which a gas / liquid mixture of a substance is used as the cooling medium is used and in which even after heat transfer in the heat exchanger is still a gas / liquid mixture of a substance.

The carrying out of evaporation is particularly advantageous because in this way the achievable heat transfer coefficient from / to process gases on / from the cooling / heating medium becomes particularly high and thus efficient cooling can be achieved.

The export of a partial evaporation is particularly advantageous because the absorption / release of heat by the cooling medium thereby no longer results in a temperature change of the cooling medium, but only the gas / liquid balance is shifted. This has the consequence that over the entire heat exchange zone, the process gas is cooled to a constant temperature. This in turn safely prevents the occurrence of temperature profiles in the flow of process gases, thereby improving control over the reaction temperatures in the reaction zones and, in particular, preventing the formation of local overheating by temperature profiles.

In an alternative embodiment, instead of evaporation / partial evaporation, a mixing zone can also be provided upstream of the entrance of a reaction zone in order to standardize the temperature profiles in the flow of process gases which may arise during cooling by mixing transversely to the main flow direction.

In a likewise preferred embodiment of the process, the reaction zones connected in series are operated at an average temperature increasing or decreasing from reaction zone to reaction zone. This means that within a sequence of reaction zones, the temperature can be both increased and decreased from reaction zone to reaction zone. This can be adjusted, for example, via the control of the heat exchange zones connected between the reaction zone. Further options for setting the average temperature are described below.

The thickness of the flow-through reaction zones can be chosen to be the same or different and results according to laws generally known in the art from the residence time described above and enforced in each case in the process

Process gas quantities. The mass enforceable according to the invention with the method Streams of product gas (formaldehyde), which also results in the process gas quantities to be used, are usually between 0.01 and 25 t / h, preferably between 0.1 and 20 t / h, more preferably between 1 and 10 t / h.

The maximum outlet temperature of the process gas from the reaction zones is usually in a range from 260 ° C to 400 ° C, preferably from 280 ⁰ C to 380 ⁰ C, particularly preferably from 300 ⁰ C to 350 ⁰ C. The control of the temperature in the

Reaction zones are preferably carried out by at least one of the following measures:

Dimensioning of the adiabatic reaction zone, control of heat dissipation between the

Reaction zones, addition of gas between the reaction zones, molar ratio of the starting materials / excess of oxygen used, addition of inert gases, in particular

Nitrogen, carbon dioxide, before and / or between the reaction zones.

The composition of the catalysts in the reaction zones according to the invention may be identical or different. In a preferred embodiment, the same catalysts are used in each reaction zone. However, it is also advantageous to use different catalysts in the individual reaction zones. Thus, especially in the first reaction zone, when the concentration of the reaction educts is still high, a less active catalyst can be used and in the further reaction zones the activity of the catalyst can be increased from reaction zone to reaction zone. The control of the catalyst activity can also be carried out by dilution with inert materials or carrier material. Also advantageous is the use of a catalyst in the first and / or second reaction zone, which is particularly stable against deactivation at the temperatures of the process in these reaction zones.

0.1 kg / h to 10 kg / h, preferably 0.5 kg / h to 8 kg / h, particularly preferably 1 kg / h to 5 kg / h of formaldehyde can be produced by the process according to the invention per 1 kg of catalyst.

The inventive method is thus characterized by high space-time yields, combined with a reduction of the apparatus sizes and a simplification of the apparatus or reactors. This surprisingly high space-time yield is made possible by the interaction of the inventive and preferred embodiments of the new method. In particular, the interaction of staggered, adiabatic reaction zones with intermediate heat exchange zones and the defined Residence times enable precise control of the process and the resulting high space-time yields, as well as a reduction in the by-products formed, such as dimethyl ether, dimethoxymethane and carbon monoxide.

Another object of the invention is a reactor system for the reaction of methanol and oxygen to formaldehyde, characterized in that it feeds (Z) for a process gas comprising methanol and oxygen or for at least two process gases, of which at least one methanol and at least one oxygen and 10 to 60 reaction zones (R) connected in series in the form of fixed beds of a heterogeneous catalyst, heat insulation zones (I) in the form of insulating material and between these heat exchange zones (W) in the form of plate heat exchangers being connected to the reaction zones. and discharges for the process gases are connected and include the supply and discharge lines for a cooling medium.

The reactor system may also comprise 12 to 50, preferably 15 to 30 reaction zones in the form of fixed beds.

The insulating material of the heat insulating zones is preferably a material having a

Wärmeleitkoeffizient λ less than or equal to 0.08 Particularly preferred are about

Polystyrene, polyurethanes, glass wool or air.

The present invention will be elucidated with reference to the drawings without, however, being limited thereto.

1 shows a schematic representation of an embodiment of the reactor system according to the invention, the following reference numerals being used in the figures:

Z: supply line (s) R: reaction zone (s)

I: thermal insulation zone (s)

W: heat exchange zone (s) 2 shows reactor temperature (T), methanol conversion (U) and formaldehyde selectivity (Y) over a number of 23 reaction zones (S) with downstream heat exchange zones (according to Example 1).

3 shows reactor temperature (T), methanol conversion (U) and formaldehyde selectivity (Y) over a number of 16 reaction zones (S) with downstream heat exchange zones (according to Example 2).

The present invention will be further illustrated by the following Examples 1 and 2, without being limited thereto.

Examples

Example 1 :

In this example, the process gas flows through a total of 23 fixed catalyst beds of iron and molybdenum, ie through 23 reaction zones. Each after a reaction zone is a heat exchange zone in which the process gas was cooled before it enters the next reaction zone. The process gases used at the beginning of the first reaction zone are methanol and air, the volume flow of the air being set so that an excess of 400% of the oxygen based on methanol is present at the beginning of the first reaction zone. The absolute inlet pressure of the process gas directly in front of the first reaction zone is 1, 1 bar. The length of the fixed catalyst beds, ie the reaction zones, is always 0.1 m. The activity of the catalyst used changes from reaction zone to reaction zone (see Table 1) (the activity of the last reaction zone was normalized to 100%). There is no replenishment of process gases before the individual catalyst stages. The residence time in the entire system is 1.2 seconds.

Table 1: Activity of the catalyst in the reaction zones according to Example 1

The results are shown in FIG. Here, the individual reaction zones are listed on the x-axis, so that a spatial course of developments in the process is visible. The temperature of the process gas is indicated on the left y-axis. The temperature profile across the individual reaction zones is shown as a thick, solid line. The right-hand y-axis shows the total methanol conversion and the selectivity of formaldehyde. The course of the conversion over the individual reaction zones is shown as a thick dashed line. The course of selectivity, as a thin solid line.

It can be seen that the inlet temperature of the process gas before the first reaction zone is about 285 ⁰ C. Due to the exothermic reaction to formaldehyde under adiabatic conditions, the temperature in the first reaction zone rises to about 300 ⁰ C, before the process gas is cooled in the downstream heat exchange zone again. The inlet temperature before the next reaction zone is about 285 ° C. By exothermic adiabatic reaction, it rises again to about 300 ⁰ C. The sequence of heating and cooling continues. The inlet temperatures of the process gas upstream of the individual reaction zones essentially do not change during the course of the process. This is advantageous, since in the course of the reaction a constant temperature is thus used on average, which enables optimum selectivity of formaldehyde.

A method according to this specific embodiment requires the specified overhead of reaction and heat exchange zones, but this has no linear economic effects due to the similarity of the components. There is obtained a conversion of methanol of 99 mol%. The selectivity is kept constant at 95.5 mol%. The space-time yield obtained, based on the mass of catalyst used, is 2.67 kg _Fθ rm _a i _d e _hy ci / kg C _at h.

Example 2:

In this example, the process gas flows through a total of 16 reaction zones, ie over 16 fixed catalyst beds of iron and molybdenum. Each after a reaction zone is a heat exchange zone in which the process gas is cooled before it enters the next reaction zone. The process gases used at the outset are, like the inlet pressure before the first reaction zone, identical to those of Example 1. The length of the catalyst stages, ie the reaction zones, is shown in Table 2. The activity of the catalyst is in the first reaction zone 100% in all other is set by dilution with catalytically inactive material has an activity of 5.7%. This is achieved after the first reaction zone oscillating, in a temperature window between 280 ⁰ C and 320 ⁰ C, on which the process settles. There is no replenishment of gas before the individual catalyst stages. The residence time in the entire system is 1.1 seconds.

Table 2: Length of the reaction zones according to Example 2

It can be seen that the inlet temperature of the process gas before the first reaction zone is about 175 ° C. By utilizing the reaction enthalpy of the later reaction zones in the first heat exchange zone and by the exothermic reaction to formaldehyde under adiabatic conditions, the temperature rises to about 320 ⁰ C before entering the second reaction zone, before the process gas is cooled in the downstream heat exchange zone. The inlet temperature before the next reaction zone is again about 280 ⁰ C. By exothermic adiabatic reaction, it rises again to about 320 ⁰ C. The sequence of heating and cooling continues. The inlet temperatures of the process gas upstream of the individual reaction zones increase with increasing number of reaction zones and settle around a mean value of about 320 ° C.

Another feature of the operation of the reaction zones under adiabatic conditions is shown in FIG. If one observes in particular the shape of the temperature profile within the reaction zones 10 to 16 and the shape of their temperature profile, it can be seen that the slope of the temperature increase over the reaction zone decreases. This shows the essential property of the process that no significant heat sink is present in the reaction zones.

It is a conversion of 99.0% of the input of the first reaction zone used methanol, calculated from the remaining mass output of the last reaction zone obtained. The selectivity is about 95.8% with respect to formaldehyde. The space-time yield achieved based on the mass used catalyst

2.66 kg formaldehyde / kgKath.

Claims

Patentansprflche

1. A process for the preparation of formaldehyde from methanol and oxygen in the presence of heterogeneous catalysts, characterized in that it comprises 10 to 60 successive reaction zones with adiabatic conditions.

2. The method according to claim 1, characterized in that the conversion in 12 to

50, preferably 15 to 30 successive reaction zones happens.

3. The method according to claim 1 or claim 2, characterized in that the inlet temperature of the entering into the first reaction zone process gas 10 to 33O ⁰ C, preferably from 50 to 31O ⁰ C, more preferably from 100 to 29O ⁰ C.

4. The method according to claim 1 to 3, characterized in that the absolute pressure at the entrance of the first reaction zone is between 1 and 4 bar, preferably between 1, 1 and 3 bar, more preferably between 1, 2 and 2.5 bar.

5. The method according to any one of the preceding claims, characterized in that the residence time of the process gas in all reaction zones together between 0.1 and

50 s, preferably between 0.2 and 20 s, more preferably between 0.5 and 10 s.

6. The method according to any one of the preceding claims, characterized in that the catalysts comprise iron and molybdenum.

7. The method according to any one of the preceding claims, characterized in that the

Catalysts are present in a fixed bed arrangement.

8. The method according to claim 7, characterized in that the catalysts are present as monoliths.

9. The method according to any one of claims 1 to 6, characterized in that the catalysts are in fluidized bed arrangement.

10. The method according to claims 7 or 9, characterized in that the catalysts are present in beds of particles having average particle sizes of 1 to 10 mm, preferably 1, 5 to 8 mm, particularly preferably from 2 to 5 mm.

11. The method according to any one of the preceding claims, characterized in that after at least one reaction zone is at least one heat exchange zone through which the process gas is passed.

12. The method according to claim 11, characterized in that after each reaction zone at least one, preferably a heat exchange zone is located, through which the process gas is passed.

13. The method according to any one of the preceding claims, characterized in that there is at least one heat insulation zone between a reaction zone and a heat exchange zone.

H. Method according to claim 13, characterized in that around each

Reaction zone is a heat insulation zone.

15. Reactor system for carrying out a method according to one of the preceding claims, characterized in that it feeds (Z) for a process gas comprising methanol and oxygen or for at least two process gases, of which at least one methanol and at least one oxygen and 10 to 60 in a row connected reaction zones (R) in the form of fixed beds of a heterogeneous catalyst, which are between the reaction zones heat insulation zones (I) in the form of insulating material and between these heat exchange zones (W) in the form of plate heat exchangers with the reaction zones via inlets and outlets for the process gases are connected and include the supply and discharge lines for a cooling medium.