WO2020178045A1 - Method for producing ammonia - Google Patents

Abstract

The invention relates to a method for producing ammonia via the catalytic conversion of a nitrogen- and hydrogen-containing synthesis gas mixture, wherein in a start-up phase, when starting the reaction, the ammonia synthesis catalyst is heated in order to generate the required activation energy for the ammonia synthesis reaction, and wherein, according to the invention, a hot process gas is used for this heating, which arises in one of the process steps of the production method prior to the ammonia synthesis reaction (36). In addition, a hot process gas generated in a carbon monoxide conversion (11) of the system or alternatively a hot process gas occurring in a methanation unit (30) of the system can be used. With the invention, a start-up heater can be forgone, given that the required activation energy for the ammonia reaction is provided from available sources within the ammonia process.

Description

Process for the production of ammonia

The present invention relates to a method for producing ammonia by catalytic conversion of a nitrogen and hydrogen-containing synthesis gas mixture, the ammonia synthesis catalyst being heated in a start-up phase when the reaction is started in order to generate the required activation energy for the ammonia synthesis reaction, a hot process gas being used for this heating which is obtained in one of the process steps of the manufacturing process preceding the ammonia synthesis reaction.

An elementary component in the process of ammonia production, based on the well-known Haber-Bosch process, is the exothermic reaction of hydrogen and nitrogen to form ammonia in an approximate ratio of 3: 1. An iron-containing catalyst is usually used.

In order to start the exothermic ammonia reaction and thus be able to guarantee continued operation, the activation energy required for the reaction must first be provided. This is usually done by an externally heated start-up furnace. The heat to be provided in the start-up furnace is usually provided by burning hydrocarbons.

The operation and especially the starting of the start-up furnace turn out to be difficult in practice in many cases and are often fraught with problems. In addition, the start-up furnace only fulfills the task of warming up the ammonia synthesis catalyst and is then switched off again. The warm-up process of the ammonia synthesis catalyst takes several hours. There is no permanent use.

No. 4,171,343 A describes a start-up procedure for the synthesis of ammonia in a system which comprises a primary reformer, a secondary reformer, two carbon monoxide conversion units, a device for removing carbon dioxide, methanation and an ammonia synthesis converter. In this known method, the primary reformer and the secondary reformer are heated with steam and the device for low-temperature CO conversion is heated with preheated desulfurized hydrocarbon gas. In a subsequent phase, the process in the reformers is started up with preheated hydrocarbon gas and steam or oxygen and the gas from the reformers is used to preheat the high-temperature CO conversion. In addition, the device for removing carbon dioxide is preheated with exhaust gas from the low-temperature CO conversion and a proportion of gas from the high-temperature CO conversion. This is followed by a Process gas flow from the two reformers passed through both CO conversion units and through the device for removing CO ₂ for methanation in order to preheat them. Finally, the process gas flow from the methanation is fed into the preheated ammonia synthesis converter. In this process, the various parts of the system are heated up in a rather cumbersome manner in several phases. In this known system, no separate heat exchanger is provided by means of which the heat from the process gas from the high-temperature CO conversion unit could be used to directly heat the synthesis gas supplied to the ammonia unit without having to pass it through all the other system parts beforehand. The parts of the system are connected to one another in a purely sequential manner. In the warming-up phase, in principle in each step the downstream part of the installation is heated with a process gas previously generated in the upstream part of the installation.

US 2004/0219088 A1 describes a mini ammonia plant which is intended to be used for the in situ production of small amounts of ammonia. The system includes methanation and the process gas emerging from this is then brought to the process pressure required for ammonia synthesis by means of a compressor. Downstream of the compressor there is an electric gas heater (trim heater), which is also used as a preheater (start-up heater) in the start-up phase of the system. This preheater is located in the flow path between the compressor and a first ammonia converter and the process gas flowing out of the methanation always flows through it. In this known system, no separately flow-through line loop is provided, through which flow is only in the start-up phase. A heat exchange with a hot process gas, which occurs in other parts of the plant upstream of the methanation, for example after the CO conversion, is also not provided here. In addition to the "trim heater", the ammonia converters are preceded by a further heat exchanger, which, however, only provides for heat exchange with the hot product gases that arise during the ammonia reaction in the converters.

The object of the present invention is to provide an improved method for the synthesis of ammonia with preheating of the ammonia synthesis catalyst with the features of the type mentioned above, which makes it possible to save the start-up furnace in the ammonia synthesis and the necessary activation energy elsewhere from existing sources within the ammonia process.

The solution to the aforementioned object is provided by a method of the type mentioned at the beginning with the features of claim 1. According to the invention, it is provided that the hot process gas is passed through a device designed in the manner of a heat exchanger for preheating, which is integrated into a separate line circuit through which the synthesis gas supplied to the ammonia synthesis reaction does not flow during regular operation of the plant.

The general approach to solving the problem at hand is the use of hot process gas from one or more process steps preceding the ammonia synthesis. Since a process-side temperature of at least about 400 ° C. should be present to provide the necessary activation energy for ammonia synthesis, two sources from the previous process steps are particularly suitable for performing this task.

The method according to the invention preferably comprises at least one steam reforming and at least one carbon monoxide conversion downstream of the steam reforming, the hot process gas generated in the carbon monoxide conversion being passed downstream of this through the device designed as a heat exchanger for preheating, through which a colder gas flow is passed in countercurrent and is heated, which is used to warm up the ammonia synthesis catalyst in the ammonia converter.

The first possibility (variant 1) preferred in the context of the present invention is to use the hot process gas downstream of a carbon monoxide conversion, in particular downstream of the high-temperature carbon monoxide conversion (HTS) to warm up the ammonia synthesis catalyst. In terms of process technology, the carbon monoxide conversion is arranged after the steam reforming step.

In steam reforming, a feed gas stream containing hydrocarbons (usually natural gas) is reacted with steam in the primary reformer according to the reaction equation (1) listed below:

The task of the carbon monoxide conversion is to convert the carbon monoxide (CO) in the process gas to carbon dioxide and hydrogen by means of a catalytic reaction with water vapor, according to the following reaction equation (2):

(2) CO + H ₂ O C0 ₂ + H ₂

The aforementioned reaction is exothermic. A possibly possible modification of this variant of the method according to the invention provides that the hot process gas generated in the carbon monoxide conversion is divided downstream of it and only a partial flow of this hot process gas is used to warm up the ammonia synthesis catalyst in the ammonia converter.

The hot process gas which leaves the carbon monoxide conversion and is used for heating up the ammonia synthesis catalyst can in particular have a temperature of 400.degree. C. to 500.degree.

In the first variant of the method, the gas can thus be passed completely or only partially at a temperature of, for example, between about 400 ° C and 500 ° C through a heat exchanger, through which the relatively colder gas flows on the other side to warm up the ammonia synthesis catalyst.

There is usually a waste heat recovery behind the high-temperature carbon monoxide conversion, for example in the form of a boiler feed water preheater, which removes heat from the exothermic carbon monoxide conversion according to reaction (2) by preheating boiler feed water or other media. This is done in order to set the inlet temperature for the subsequent low-temperature carbon monoxide conversion (LTS) to approximately 200 ° C., for example. The heating of the boiler feed water serves in the further course of the energy use to generate high pressure steam with which various drive machines in the ammonia plant are driven after subsequent overheating. According to a preferred variant of the method according to the invention, the lack of energy for boiler feed water preheating due to the preheating of the ammonia synthesis catalyst can be provided in another way, for example by using an external auxiliary boiler, which in most applications is already present in an ammonia plant.

For the common ammonia production capacities between, for example, about 1000 and about 3300 tons per day, the energy to be provided for heating the ammonia synthesis catalyst ranges from about 7 to about 20 megawatts. In this variant of the invention, no noteworthy changes in the process management are necessary.

According to a second preferred variant of the invention, the method comprises at least one methanation and the hot process gas resulting from the methanation is wholly or only partially passed downstream of the methanation through the device designed as a heat exchanger for preheating, through which a colder Gas flow is passed and heated, which is used to warm up the ammonia synthesis catalyst in the ammonia converter.

An alternative possibility within the scope of the present invention to provide the corresponding activation energy for the catalyst in the ammonia synthesis is thus to use the exothermic reaction energy from the methanation. The task of methanation is to remove the residual traces of carbon monoxide (CO) and carbon dioxide (C0 ₂ ) from the process gas, which are harmful for the subsequent ammonia reaction. This is done by a catalytic reaction in which the CO and CO ₂ remaining in the process gas react with the hydrogen H ₂ in the process gas to form methane according to the following reaction equations (3) and (4). This reaction is exothermic and the amount of heat released can be influenced by the amount of C0 ₂ and CO in the process gas.

(3) CO + 3 H ₂ CH ₄ + H ₂ O

(4) CO ₂ + 4 H ₂ ^ CH ₄ + 2 H ₂ 0

In the process according to the invention, the hot process gas exiting from the methanation which can be used for heating the ammonia synthesis catalyst has a temperature of at least 390 ° C., preferably a temperature of at least 400 ° C., particularly preferably a temperature of at least 410 ° C. The hot gas emerging from the methanation can be used entirely or only partially in a heat exchanger in order to preheat the cold gas flowing on the other side of the heat exchanger to heat the ammonia synthesis catalyst. In this case, part of the heat of reaction from the methanation can be used for the start-up process of the ammonia synthesis cycle.

Usually, according to the state of the art, the gas exiting the methanation with a temperature of, for example, approx. 330 ° C is used in a gas-gas heat exchanger (GGT) to preheat the process gas entering the methanation to the required activation temperature of the methanation reaction. The required activation temperature for methanation is approximately 300 ° C. Due to the warming up of the ammonia synthesis catalyst would accordingly for the Setting the inlet temperature in the methanation, less energy is available.

In the method according to the invention, the process gas entering the methanation has a minimum temperature of 300 ° C, preferably a minimum temperature of 320 ° C, particularly preferably a minimum temperature of 330 ° C.

According to a further development of the method according to the invention, a start-up preheater connected upstream of the methanation in the flow path is preferably used for setting the inlet temperature of the process gas into the methanation. Since the exothermic reaction energy from the methanation is usually not sufficient to provide the required 300 ° C at the start of the methanation even in normal plant operation with a fresh catalyst in the methanation, a start-up preheater is connected upstream of the methanation reactor in a preferred design of the plant according to the invention. The start-up preheater provides the heat for setting the methanation inlet temperature in most cases through the condensation of high-pressure steam, which can be taken from the steam drum from the upstream process steps. This method can also be used if part of the heat from the methanation reaction is to be used to preheat the catalyst in the ammonia synthesis.

According to a preferred development of the method according to the invention, the hot process gas emerging from the methanation is at least partially passed through a gas / gas heat exchanger assigned to the methanation and used to preheat the process gas fed to the methanation.

A start-up preheater upstream of the methanation in the flow path can also be used to set the inlet temperature of the process gas in the methanation.

Particularly preferably, the process gas is first passed through a gas / gas heat exchanger and then through a start-up preheater before entering the methanation.

Under normal process conditions, the temperature at the methanation outlet is, for example, around 330 ° C to 340 ° C and is influenced by the carbon monoxide and carbon dioxide content in the gas entering the methanation. Under normal process conditions, the CO content of the inlet gas entering the methanation is, for example, about 0.3% by volume and the CO ₂ content there is, for example, about 0.05% by volume. The CO content is in the upstream (usually multi-stage) step of CO conversion and the C0 ₂ content is in the upstream step of the C0 ₂ - Cleaning stopped. The reactions of CO and C0 ₂ to methane and water taking place in the methanation according to the above reaction equations (3) and (4) usually lead to the above-mentioned outlet temperatures from the methanation of, for example, about 330 ° C to about 340 ° C.

According to a preferred development of the invention, the method comprises at least one method step for removing carbon dioxide (C0 ₂ scrubbing) from the process gas in a device provided for this purpose before the process gas is introduced into the ammonia converter and upstream of the methanation, with a partial flow of the process gas directly from the Methanation is supplied bypassing this facility for removing carbon dioxide. Since ideally a preheating temperature of, for example, about 380 ° C should be present for preheating the ammonia synthesis catalyst, an exit temperature from the methanation of around 400 ° C is advantageous, taking into account a moderate heat exchange surface in the heat exchanger between the methanation and ammonia catalyst preheater. In order to achieve the required exit temperature from the methanation, part of the process gas (preferably about 5 to 10% of the molar amount) is preferably passed around the C0 ₂ scrubbing so as to artificially reduce the C0 ₂ content in the process gas before the methanation to increase. As a result, the exothermic reaction in the methanation can be intensified and the outlet temperature from the methanation can be adjusted accordingly to the required temperature.

In the alternative variants for preheating the ammonia synthesis catalyst described herein, the duration of the preheating of the synthesis catalyst is a relevant influencing factor. In the cold state of the ammonia converter at the beginning of the preheating, the process gas temperature in the device for preheating (start-up heater) for the ammonia synthesis catalyst is, for example, about 50.degree. C. to about 80.degree. At an approximate rate of, for example, about 80 ° C. to about 100 ° C. per hour, the process of preheating the ammonia synthesis catalyst takes, on average, about 4 to about 6 hours, for example. During this period, the requirements for the start-up heater performance of the ammonia synthesis also change over time. The warmer the catalyst, the less heat has to be provided by the start-up heater for the ammonia synthesis. These changing requirements for the start-up heater for ammonia synthesis can be taken into account in different ways in the preferred process alternatives listed in the present application.

In the variant according to FIG. 1, the consequence of the continuously increasing inlet temperature of the ammonia synthesis gas in the preheater would be the ammonia synthesis the required performance of the heat exchanger between the high-temperature CO conversion and the downstream boiler feed water preheater, for example, and thus more heat is available again for the boiler feed water preheating. In this case, the additional heat required from an auxiliary boiler that may have to be used would decrease over time.

In the variant according to FIG. 2, it is not possible at this point to minimize the amount of process gas passed around the C0 ₂ scrubbing, since the C0 ₂ content at the entry into the methanation must be kept correspondingly high to the required temperature of, for example, about 380 ° C. for the ammonia converter catalyst. Since the performance of the preheater for ammonia synthesis decreases with increasing duration, the inlet temperature in the gas-gas exchanger increases and thus also the inlet temperature in the steam-operated preheater for methanation. The inlet temperature into the methanation should preferably not exceed a temperature of about 300 ° C. In the case of a process gas temperature of over 300 ° C upstream of the methanation preheater, a corresponding heat removal would have to take place in the preheater. This can be done, for example, by evaporating boiler feed water, whereby the steam generated in the process can be used for preheating purposes elsewhere in the process (for example in the process capacitor stripper).

The present invention also relates to a plant for the production of ammonia comprising at least one ammonia converter and units upstream of this in the flow path for generating and processing a synthesis gas to be fed into the ammonia converter comprising at least one reformer section, at least one carbon monoxide conversion downstream of this in the flow path and at least one device for preheating the ammonia synthesis catalyst in a start-up phase when starting the reaction in order to generate the necessary activation energy for the ammonia synthesis reaction, wherein according to the invention at least one device designed as a heat exchanger for preheating is provided as a device for preheating the ammonia synthesis catalyst, which device is arranged downstream of the carbon monoxide conversion and on the one hand is flowed through by a hot process gas and which on the other hand, on the input side in operative connection with at least s is a line for a colder gas stream of synthesis gas processed in the plant, in heat exchange with the hot process gas, and is in operative connection with the ammonia converter on the output side via at least one line. An essential element of the solution according to the invention is that the device for preheating the ammonia synthesis catalyst is integrated into its own separate line circuit, which can preferably be shut off via valves or the like, so that hot process gas, which is produced in a system part upstream of this device, is in a start-up phase can pass through this device and the synthesis gas fed to the ammonia converter in the start-up phase for the purpose of preheating by heat exchange can also be passed through this device. In normal operation of the plant, on the other hand, the above-mentioned separate line circuit can optionally be shut off and the synthesis gas generated in the plant can be fed directly to the ammonia synthesis reactor, since preheating is no longer necessary.

Another advantage of the invention is that the device for preheating can optionally be connected to different system parts in which hot process gases arise, for example with a carbon monoxide conversion or with a device for methanation. These are system parts that are usually present in an ammonia system and in which process gases with possibly different temperatures occur on the outlet side, so that the thermal energy stored in these process gases is targeted and variable for preheating the synthesis gas that is fed to the ammonia converter, as required, can be used.

The aforementioned separate line circuit for the device for preheating is preferably integrated into the line system of the plant in such a way that the synthesis gas can flow into this separate line circuit downstream of the synthesis gas compressor and is then fed to the ammonia synthesis reactor after being preheated in the device. This separate line circuit is generally only flowed through by the synthesis gas in the start-up phase of the plant, while this can flow through at least one alternative line from the synthesis gas compressor directly to the ammonia synthesis reactor in regular operation.

According to a preferred embodiment of the invention, the device for preheating is arranged downstream of a first carbon monoxide conversion of the system and a hot process gas generated in the first carbon monoxide conversion flows through it, the device for preheating being arranged upstream of a second carbon monoxide conversion of the system . In this variant of the design of the system, the heating of the process gas that occurs in the first carbon monoxide conversion can be used to preheat the synthesis gas before it gets into the ammonia converter. The first carbon monoxide conversion can work, for example, at a comparatively higher temperature, while the second carbon monoxide conversion arranged downstream of this is preferably a low-temperature CO conversion in which an additional water-gas shift reaction takes place. Systems for processing the synthesis gas for an ammonia reaction often include two such carbon monoxide conversions, which are connected in series and work at different temperatures.

In a variant of the system according to the invention, the device for preheating is arranged downstream of a methanation of the system and a hot process gas generated in the methanation flows through it. In this variant, instead of the hot process gas flow from the carbon monoxide conversion, the heat of a process gas flow from the methanation, which is usually part of an ammonia plant, is used.

The system according to the invention preferably further comprises at least one device for removing carbon dioxide from the process gas, which is preferably arranged upstream of the methanation, wherein at least one line is also provided through which the process gas can flow, bypassing the device for removing carbon dioxide, before the Process gas enters the methanation. As a rule, a so-called C0 ₂ scrub is present in ammonia plants, which has the task of at least largely removing the C0 ₂ generated during the previous CO shift. If you lead part of the process gas (preferably about 5 to 10% of the molar amount) around the C0 ₂ scrubbing, you can artificially increase the C0 ₂ content in the process gas before the methanation. As a result, the exothermic reaction in the methanation can be intensified and the outlet temperature from the methanation can be adjusted accordingly to the required temperature.

The system according to the invention preferably further comprises a line leading back from the device for preheating on the output side to a gas / gas heat exchanger of the methanation unit, wherein preferably a line through which hot process gas from the methanation flows also leads to the gas / gas heat exchanger. The gas / gas heat exchanger assigned to the methanation is used to preheat the process gas supplied to the methanation. If the process gas flowing in the aforementioned return line still has a sufficiently high temperature after flowing through the device for preheating, this heat can be used to heat the process gas in the gas / gas heat exchanger, which is fed to the methanation. On the output side with regard to the process gas stream that cools during the heat exchange, the gas / gas heat exchanger is preferably in operative connection with the ammonia converter via a line, with a synthesis gas compressor usually being connected upstream of the ammonia converter in order to bring the process gas to the pressure required for the ammonia reaction. Subsequently, the temperature-controlled synthesis gas either - in the regular operation of the plant - goes directly to the ammonia converter or - in the start-up phase, the process gas coming from the gas / gas heat exchanger first flows through the device for preheating described above and is then fed to the ammonia converter.

The present invention is explained in more detail below on the basis of exemplary embodiments with reference to the accompanying drawings. Show:

FIG. 1 shows a schematically simplified representation of an exemplary plant that can be used for the method according to the invention according to a first possible embodiment variant of the present invention;

FIG. 2 shows a schematically simplified representation of an exemplary plant that can be used for the method according to the invention, according to a second possible embodiment variant of the present invention.

The basic structure of a plant for the production of ammonia is explained below with reference to FIG. 1, in which, according to the invention, the catalyst for the ammonia reaction is preheated with the aid of a hot process gas from one of the upstream process steps. In FIG. 1, only the system parts of an ammonia system that are essential for understanding the present invention are shown. This usually comprises a reformer section 10, which can comprise different reformers, for example a combination of a primary reformer and a secondary reformer connected downstream of it. Alternatively, for example, an autothermal reformer or combinations of various reformers of the type mentioned can be used. If a primary reformer is used, a steam reforming of a hydrocarbon-containing feed gas stream, for example natural gas, takes place in this, with carbon monoxide and hydrogen being generated according to the reaction equation (5) shown below:

Following this reaction, if a secondary reformer is used, a further conversion of hydrocarbons in the presence of oxygen or air can take place be provided, whereby further hydrogen is generated according to the reaction equation (5 a) given below:

The synthesis gas generated in this way in the reformer section 10 is then fed via the line 12 to a first device 11 for converting carbon monoxide, also referred to as a water gas shift reaction, the task of which is to convert the carbon monoxide (CO) in the process gas by means of catalytic To convert the reaction with steam to carbon dioxide and hydrogen according to the following reaction equation (6):

(6) CO + H ₂ O CO ₂ + H ₂

The aforementioned reaction is exothermic. In many cases, two successive CO conversions are carried out, the first device, which is provided with the reference numeral 11 in FIG. 1, being a high-temperature carbon monoxide conversion (also called HTS). The process gas prepared in this way then reaches a boiler feed water preheater 18 during regular operation of the system via the line 14 branching off from the branch 13 and the open valve 15 and via the branch 16 into the line 17, in which excess heat from the heating of the process gas, for example in the Heat exchange can be used to preheat boiler feed water.

In the warm-up phase of the plant, on the other hand, according to the invention, the synthesis gas mixture leaving the high-temperature CO conversion 11 is fed to the junction 13 via the open valve 45 and the line 44 as hot process gas to the device 39 for preheating, which is countercurrent to the cooler synthesis gas is flowed through, which is fed to the ammonia converter 36 in order to preheat this synthesis gas in the start-up phase and warm-up phase of the system in the device 39. The function of this device 39 for preheating is explained in more detail below.

The following first explains the other components of the system shown in FIG. 1 and their function in regular operation. After flowing through the boiler feed water preheater 18, the process gas passes via line 19 into a second CO conversion, a so-called low-temperature CO conversion 20, in which, at a lower temperature, another water gas shift reaction according to the above equation (2) expires. The process gas is then fed via line 21 to a further heat exchanger 22 in which heat is recovered. The process gas is then fed via line 23 to a device 24 in which a C0 ₂ scrub is carried out, which serves to at least partially remove the carbon dioxide previously formed in the water-gas shift reaction from the gas mixture. Via line 25, the process gas thus processed first reaches a gas / gas heat exchanger 26, then via line 27 to a start-up preheater 28 for methanation and is then fed to methanation 30 via line 29.

In the methanation, the residual proportions of carbon dioxide and possibly carbon monoxide still contained in the process gas are removed by conversion into methane in accordance with the reaction equations (7) and (8) already mentioned in the application.

(7) CO + 3 H ₂ CH ₄ + H ₂ O

(8) CO ₂ + 4 H ₂ CH ₄ + 2 H ₂ O

Since this reaction takes place exothermically, the process gas is reheated in the methanation 30 and the heat of the heated process gas can be used for a heat exchange by transferring the gas mixture from the methanation 30 to the gas / gas heat exchanger 26 via the line 31 and thus leads in countercurrent to the process gas flow, which is fed to the gas / gas heat exchanger 26 via line 25 and then reaches methanation 30 via lines 27 and 29. After flowing through the gas / gas heat exchanger 26, the process gas flows through the line 32 to a synthesis gas compressor 33, by means of which it is brought to the process pressure provided for the reaction and can then be used in the regular operation of the system via the line 34 and the open valve 35 are fed directly to the ammonia converter 36, in which the synthesis gas containing nitrogen and hydrogen is converted to ammonia.

In the start-up phase of the plant, however, it is necessary to preheat the synthesis gas because the catalyst requires a certain minimum temperature for the ammonia reaction. In this case, therefore, the valve 35 is initially closed and the valve 38 is opened instead, so that the synthesis gas is first fed via the line 37 to the device for preheating 39, through which it flows in countercurrent to the hot process gas, which from the high-temperature CO conversion 11 is supplied via the open valve and line 44 to the device for preheating 39 coming. In this way, in the start-up phase, the compressed synthesis gas supplied via the line 37 is preheated in the device 39 and can then be supplied to the ammonia converter 36 via the line 40 and the opened valve 41. For this warm-up, the CO conversion 11 used heated gas stream, that is, heat that was generated in the process itself and it is therefore no longer necessary to use a separate burner-operated heating device for preheating the synthesis gas fed to the ammonia reactor 36.

The line 37, the device 39 for preheating and the line 40 form their own separate and lockable line circuit through which the process gas coming from the synthesis gas compressor 33 can optionally flow in the start-up phase. In regular operation of the plant, if no preheating of the synthesis gas upstream of the ammonia reactor is required, the device for preheating 39 can be bypassed by closing the valve 45 and thus the process gas coming from the high-temperature CO conversion 11 via the branch and the opened valve 15 can flow directly through line 14 into line 17 in order to then go through the process cycle described above. The separate line circuit can also be bypassed for the colder synthesis gas downstream of the synthesis gas compressor 33 by closing the valve 38 and opening the valve 35 when the synthesis gas no longer needs to be preheated.

An alternative exemplary embodiment of the present invention is explained below with reference to FIG. The same parts of the system are denoted in FIG. 2 with the same reference symbols as in FIG. 1 and the parts of the system already described there and their function are not described again here. Rather, only the differences to the previously described variant are presented here. The process gas coming from the high-temperature CO converter 11 is fed directly to the line 17 and thus reaches the boiler feed water preheater 18. The device for preheating 39 is therefore not flowed through in the variant of FIG. 2 at this point in the system.

The system area with the boiler feed water preheater 18, the low-temperature CO converter 20, the heat exchanger 22 for heat recovery and the C0 ₂ scrubber 24 is designed in the same way as in the variant previously described with reference to FIG. In contrast to this, however, a line 46 which can be shut off via a valve 47 is provided, which branches off from line 23 upstream of the C0 ₂ scrubbing so that the C0 ₂ scrubbing can be bypassed and the process gas through the line 46 shown in dashed lines directly into the Line 25 can flow, which leads to the gas / gas heat exchanger 26. The line 25 can be shut off via a further valve 48. When the valve 47 in the bypass line 46 is closed, the process gas can, however, be fed directly to the C0 ₂ scrubbing system via the line 23. When the valve 48 is open, the process gas enters the gas / gas heat exchanger 26 and then, after preheating, flows into the methanation 30 as described above. In contrast to the variant described above, the process gas flow that leaves the methanation 30 in the embodiment according to FIG , used in the start-up phase of plant operation for preheating the synthesis gas in device 39. For this purpose, a line 49, which can be used optionally and is therefore shown in dashed lines, is provided, which leads from methanation 30 to the device for preheating 39 and which can optionally be shut off via a further valve 50. This hot process gas flow from the methanation is used here to preheat the synthesis gas compressed by the synthesis gas compressor 33, which flows in the start-up phase of the system via the line 37 through the device for preheating 39 and then, after preheating, feed it to the ammonia converter 36 via the line 40 to preheat the catalyst there.

In the variant according to FIG. 2, a further line 51, shown in broken lines and which can be shut off via a valve 52, is provided, which allows the somewhat cooled process gas to be returned to the methanation gas / gas heat exchanger 26 after flowing through the device 39 for preheating with the valve 52 open . After the gas / gas heat exchanger 26 has flowed through, the process gas can be fed to the synthesis gas compressor 33 via line 32 and then, depending on whether preheating is provided in the start-up phase, either directly to the ammonia converter 36 via line 34 or passed via the line 37 through the device 39 for preheating and then fed to the ammonia converter 36 via the line 40. An essential difference in the variant according to FIG. 2 is that instead of the process gas after the high-temperature CO conversion 11, a hot process gas from the methanation 30 is used for preheating in the device 39. Since the process gas from line 49 cools down as it flows through device 39 for preheating, it is returned via line 51, this line 51 opening into line 31 at a branch 54 upstream of gas / gas heat exchanger 26. It is thus also possible to mix the process gas stream, which has been heated in the methanation 30 and which then flows via the line 31 to the gas / gas heat exchanger 26, with the process gas from the line 51 in order, if necessary, to optimize the temperature setting at this point. List of reference symbols

10 reformer section

11 High temperature CO conversion

12 Line / active connection

13 junction

14 management

15 valve

16 junction

17 management

18 Boiler feed water preheater

19 management

20 Low temperature CO conversion

21 management

22 Heat exchangers for heat recovery

23 management

24 C0 ₂ laundry

25 management

26 Gas / gas heat exchangers

27 Leadership

28 Start-up preheaters for methanation

29 Leadership

30 methanation

31 management

32 line

33 Syngas Compressor

34 Leadership Valve

Ammonia converter

management

Valve

Device for preheating in the start-up phase / heat exchanger

management

Valve

management

Valve

management

Valve

management

Valve

Valve

management

Valve

management

Valve

Valve

Junction

Claims

Claims 1. A method for the production of ammonia by catalytic conversion of a nitrogen and hydrogen-containing synthesis gas mixture, wherein in a start-up phase when starting the reaction, the ammonia synthesis catalyst is heated in order to generate the necessary activation energy for the ammonia synthesis reaction, a hot process gas being used for this heating is obtained in one of the process steps of the production method upstream of the ammonia synthesis reaction, characterized in that the hot process gas is passed through a device (39) designed in the manner of a heat exchanger for preheating, which is integrated into a separate line circuit (37, 40) which is part of the regular Operation of the plant is not traversed by the synthesis gas supplied to the ammonia synthesis reaction. 2. The method according to claim 1, characterized in that the method comprises at least one steam reforming (10) and at least one carbon monoxide conversion (11) downstream of the steam reforming, wherein the hot process gas generated in the carbon monoxide conversion (11) downstream of this through the device (39) designed as a heat exchanger for preheating is passed through which a colder gas flow (37) is passed and heated in countercurrent, which is used to warm up the ammonia synthesis catalyst in the ammonia converter (36). 3. The method according to claim 2, characterized in that the hot process gas generated in the carbon monoxide converter (11) is divided downstream of this (13) and only a partial flow (44) of this hot process gas for heating the ammonia synthesis catalyst in the ammonia converter (36) is used. 4. The method according to any one of claims 1 to 3, characterized in that the hot process gas leaving the carbon monoxide conversion and used for heating the ammonia synthesis catalyst has a temperature of 400 ° C to 500 ° C. 5. The method according to any one of claims 1 to 4, characterized in that the method comprises at least one methanation (30) and the hot process gas resulting from the methanation downstream of the methanation is passed completely or only partially through the device for preheating (39) designed as a heat exchanger, through which a colder gas flow (37) is passed and heated in countercurrent, which is used to warm up the ammonia synthesis catalyst in the ammonia converter (36). 6. The method according to claim 5, characterized in that the process gas entering the methanation has a temperature of at least 300 ° C, preferably a temperature of at least 320 ° C, particularly preferably a temperature of at least 330 ° C. 7. The method according to any one of claims 5 or 6, characterized in that the hot process gas emerging from the methanation is passed at least partially through a gas / gas heat exchanger (26) assigned to the methanation and is used to preheat the process gas fed to the methanation. 8. The method according to claim 7, characterized in that a start-up preheater (28) connected upstream of the methanation (30) in the flow path is used for setting the inlet temperature of the process gas into the methanation. 9. The method according to claim 8, characterized in that the process gas before entering the methanation (30) is first passed through the gas / gas heat exchanger (26) and then through the start-up preheater (28). 10. The method according to any one of claims 5 to 9. Characterized in that this at least one method step (24) for removing carbon dioxide from the process gas in a device (24) provided for this purpose before the process gas is introduced into the ammonia converter (36) and upstream the methanation (30) and a partial flow of the process gas is fed to the methanation, bypassing this device (24). 11. The method according to any one of claims 5 to 10, characterized in that the process gas exiting the methanation has a minimum temperature of 390 ° C, preferably a minimum temperature of 400 ° C, particularly preferably a minimum temperature of 410 ° C. 12. The method according to any one of claims 5 to 11, characterized in that at least one device for lowering the temperature of the process gas in the Flow path is provided before entry into the methanation, in particular a start-up preheater (28) connected upstream of the methanation (30) being used for this purpose. 13. Plant for the production of ammonia comprising at least one ammonia converter (36) and units upstream of this in the flow path for generating and processing a synthesis gas to be introduced into the ammonia converter, comprising at least one reformer section (10), at least one carbon monoxide conversion (11) downstream of this in the flow path and at least one device for preheating the ammonia synthesis catalyst in a start-up phase when starting the reaction, in order to generate the required activation energy for the ammonia synthesis reaction, characterized in that at least one device (39) designed as a heat exchanger for preheating is provided as the device for preheating the ammonia synthesis catalyst, which is arranged downstream of the carbon monoxide conversion (11) and on the one hand a hot process gas flows through it and which, on the other hand, is in operative connection with at least one line (37) on the inlet side stands for a colder gas stream of synthesis gas processed in the plant, in heat exchange with the hot process gas, and is in operative connection with the ammonia converter (36) on the output side via at least one line (40). 14. Plant according to claim 13, characterized in that the device for preheating (39) is arranged downstream of a first carbon monoxide conversion (11) of the system and is flowed through by a hot process gas generated in the first carbon monoxide conversion (11) and the Device for preheating (39) is arranged upstream of a second carbon monoxide conversion (20) of the system. 15. Plant according to claim 13, characterized in that the device for preheating (39) is arranged downstream of a methanation (30) of the plant and a hot process gas generated in the methanation flows through it. 16. Plant according to claim 15, characterized in that it comprises at least one device (24) for removing carbon dioxide from the process gas, which is preferably arranged upstream of the methanation (30), wherein Furthermore, at least one line (46) is provided through which the process gas can flow, bypassing the device (24) for removing carbon dioxide, before the process gas reaches the methanation. 17. Plant according to claim 15 or 16, characterized in that it further comprises a line (51) leading back from the device for preheating (39) on the outlet side to a gas / gas heat exchanger (26) of the methanation unit, preferably also one of hot process gas from the methanation through which line (31) leads to the gas / gas heat exchanger (26). 18. Plant according to claim 17, characterized in that the gas / gas heat exchanger (26) is in operative connection on the output side via a line (32) with the ammonia converter (36).