IT202300001395A1 - SYNTHESIS OF PROCESS GASES BY DIRECT NITROGEN COOLING - Google Patents

Description

PATENT APPLICATION FOR INDUSTRIAL INVENTION

Title: SYNTHESIS OF PROCESS GASES BY DIRECT NITROGEN COOLING

DESCRIPTION

Scope of application

The present invention is a method of synthesizing process gases comprising direct nitrogen quenching.

Specifically, the present invention relates to process gases discharged at high temperature from a chemical reactor in which the steam or carbon dioxide reforming process, or the adiabatic oxidation process with oxygen-containing gas, of hydrocarbons and/or ammonia and/or methanol has been conducted, for the subsequent production of hydrogen (H2), ammonia (NH3), methanol (CH3OH), Fischer Tropsch synthesis products, nitric acid (HNO3), hydrogen cyanide (HCN) or formaldehyde (CH2O).

The process gases of the present invention are typically synthesized and discharged from the corresponding reforming/oxidation reactors at operating temperatures above 500-550?C, often above 700-750?C, and at operating pressures above 0.12MPa(a), often above 0.20MPa(a), often above 0.45MPa(a), and often above 1.00MPa(a).

For subsequent chemical production, the method of synthesis of the process gases discharged at high temperature from the reformer/oxidation reactors requires that the gases be cooled and then subjected to further unit operations; therefore, the method of synthesis of the above-mentioned process gases includes a cooling operation downstream of the reformer/oxidation reactor. Sometimes, the cooling must be rapid (?quenching?) to stop residual chemical reactions.

The cooling operation of the above-mentioned process gases is conventionally carried out by indirect heat exchange in process boilers and/or heat exchangers installed downstream of the reforming/oxidation chemical reactors. Consequently, the process gas is cooled by high-pressure steam production, steam superheating or boiler feedwater preheating. High-temperature process gases containing carbonaceous or waxy solids are sometimes, alternatively, cooled by direct heat exchange with water, steam or hydrocarbons (quenching).

Purpose of the invention

Conventional indirect heat exchange cooling of process gases is a critical operation because process boilers and heat exchangers often operate under severe thermo-hydraulic and thermo-mechanical conditions and the process gas is often chemically aggressive towards steels. Furthermore, the adiabatic oxidation operation of hydrocarbons and/or ammonia and/or methanol also involves risks of temperature peaks of the process gas and therefore risks of overheating or damage to the equipment downstream of the oxidation reactor.

Conventional direct heat exchange cooling of process gases is also a critical operation because the cooling fluid mixed with the hot gas may undergo chemical reactions or change the chemistry of the synthesis, or because there is a high water consumption.

The process gas synthesis method with direct nitrogen cooling, object of the present invention, represents an alternative and effective technical solution in improving safety and productivity compared to synthesis methods based on conventional cooling. In other words, the present invention has the aim of providing an alternative synthesis method and, under specific operating conditions, an improvement over conventional synthesis methods.

The synthesis method described herein comprises a direct heat exchange operation between the process gas discharged at high temperature from the reforming/oxidation reactor and a cooler nitrogen stream; in other words, the synthesis method described herein comprises a mixing operation between the hot process gas to be cooled and a cold nitrogen stream.

Nitrogen, being chemically inert, does not attack steels and does not substantially modify the chemistry of the synthesis; furthermore, nitrogen can act as a chemical reagent if the process gas is used for the subsequent production of ammonia. However, it should be emphasized that mixing with nitrogen increases the volume of the process gas and reduces the partial pressure of the reacting chemical species.

The main advantages of the synthesis method of the present invention are:

- The use of a chemically inert cooling fluid and, therefore, not subject to consumption, not altering the chemical synthesis and not aggressive towards steels; - The speed of cooling of the process gas;

- The limited pressure drop related to mixing; - The possibility of controlling the synthesis of the process gas during transients by regulating the nitrogen flow rate;

- The possibility of dampening temperature peaks caused by malfunctions or transients of the oxidation reactor; - The possibility of mitigating the thermo-hydraulic and thermo-mechanical conditions of the process boilers or heat exchangers downstream of the chemical reactor;

- The possibility of decreasing the steam production in the process boiler or decreasing the thermal load on the heat exchangers;

- Mitigation of the chemical aggressiveness of the process gas;

- The possibility of compensating for a decrease in performance of the process boiler or heat exchangers.

State of the art

Mixing of cold gaseous reactants and hot gaseous reaction products is a technique sometimes used to control the temperature and yield of chemical reactions in a chemical reactor. When exothermic chemical reactions are conducted with the aid of a solid catalyst divided into catalyst beds, intermediate dilutions and cooling are sometimes performed by injecting cold reactants between the catalyst beds. Patent documents Nos. EP0026057 and EP0550525 describe synthesis processes and chemical reactors with catalytic bed chemical reactors with intermediate dilutions and cooling by injecting cold chemical reactants.

High temperature process gases containing significant soot, dust, carbon, or wax content are often subjected to direct cooling. Patent documents No. US5431703 and No. US4054424 describe two methods for rapidly cooling a process gas stream by mixing it with steam, or hydrocarbons, or mixtures of chemical reagents and inert gases.

Methods and devices for rapidly cooling a hot process stream by mixing it with a cold fluid are described in several patent documents, for example in Patent Nos. US3663645 and US5324486. These methods and devices specifically relate to process gases from hydrocarbon cracking, where the cold fluid for direct cooling is steam or hydrocarbon.

Finally, patent document No. EP0618282 teaches a method for mixing and tempering a hot chemical reactant stream by recycling cooled reaction products into a process boiler.

Examples of prior art documents describing methods of synthesis of the process gases which are the subject of the present invention are the following patent documents:

- Docs. No. EP2723676B1 and No. EP3411327B1 describe high-temperature synthesis methods of H2/CO-rich process gases using adiabatic hydrocarbon oxidation reactors and by indirect heat exchange cooling in a waste heat boiler (or process gas boiler) installed downstream of the oxidation reactor and upstream of the water gas shift reactor. Based on these synthesis methods, the plants are equipped with cryogenic air separation units to obtain oxygen for adiabatic oxidation and nitrogen for the subsequent reaction with hydrogen to produce ammonia;

- Doc. No. GB837030 describes a method for the high-temperature synthesis of H2/CO-rich process gas using an adiabatic hydrocarbon oxidation reactor and direct heat exchange cooling with water downstream of the oxidation reactor and upstream of the water gas shift reactor;

- Doc. No. GB1306581 describes a method of high-temperature synthesis of a process gas rich in nitrogen oxides (NOx) by means of an adiabatic ammonia oxidation reactor and by indirect heat exchange cooling in a process boiler and heat exchangers installed downstream of the oxidation reactor and upstream of the scrubber and absorption columns;

- Doc. No. WO2015006548 describes a method for the high-temperature synthesis of a process gas rich in hydrogen cyanide (HCN) by means of an adiabatic oxidation reactor of an ammonia/methane mixture and by means of rapid cooling by indirect heat exchange in a first process boiler and then in a second process boiler installed downstream of the oxidation reactor and upstream of the absorption column;

- Doc. No. US4450301 describes a method of high-temperature synthesis of a formaldehyde-rich process gas (CH2O) using an adiabatic methanol partial oxidation reactor and by indirect heat exchange cooling in a process boiler or heat exchanger installed downstream of the oxidation reactor.

The available literature, however, does not teach a method for the synthesis of process gas, discharged at high temperature from a chemical reactor, including direct nitrogen quenching.

More precisely, the literature does not describe methods for synthesizing high-temperature process gases discharged from reforming/oxidation reactors rich in H2/CO for subsequent ammonia production, or rich in NOx for subsequent nitric acid production, or rich in HCN, or rich in CH2O, including direct cooling by mixing with a cooler nitrogen stream.

Brief description of the invention

According to the synthesis method described here, the process gas discharged at high temperature from the chemical reforming/oxidation reactor is subjected to direct heat exchange cooling with nitrogen, i.e. the hot process gas is mixed with a cooler nitrogen stream.

The cooling stream consists of nitrogen or consists mainly of nitrogen.

The synthesis method which is the object of the present invention refers to the following process gases:

- hydrogen (H2) and carbon monoxide (CO) rich gases synthesized in steam or CO2 reforming reactors of hydrocarbons or in adiabatic oxidation reactors with oxygen-containing gas of hydrocarbons, for the subsequent production of hydrogen (H2), ammonia (NH3), methanol (CH3OH) or olefins/paraffins/alcohols by Fischer Tropsch synthesis;

- gases rich in nitrogen oxides (NOx) synthesized in adiabatic oxidation reactors with oxygen-containing gas of ammonia, for the subsequent production of nitric acid (HNO3);

- gases rich in hydrogen cyanide (HCN) synthesized in adiabatic oxidation reactors with oxygen-containing gas of an ammonia/methane mixture, for the subsequent production of hydrogen cyanide;

- formaldehyde-rich gases (CH2O) synthesized in adiabatic partial oxidation reactors with oxygen-containing gas of methanol, for the subsequent production of formaldehyde.

H2- and CO-rich process gases synthesized in steam and/or CO2 reformers of hydrocarbons, or in adiabatic oxygen-containing gas oxidation reactors of hydrocarbons, are typically discharged at operating temperatures above 650-700?C, often above 800?C, and often above 950?C, and at operating pressures above 1.0MPa(a), often above 2.5MPa(a). Steam and/or CO2 reformers are operated in the presence of solid catalysts (flame or convective heat exchange reformers); adiabatic oxidation reactors may be operated in the presence of solid catalysts (autothermal reactors, secondary reformers or catalytic partial oxidation reactors) or in the absence of solid catalysts (partial oxidation or gasification reactors). Steam reforming proceeds according to the general reaction (considering methane as the hydrocarbon)

whereby methane and water form hydrogen

and carbon monoxide. Reforming with CO2 proceeds according to the general reaction (considering methane as the hydrocarbon)

on the basis of which methane and carbon dioxide

form hydrogen and carbon monoxide. Adiabatic oxidation with oxygen-containing gas proceeds according to the general partial oxidation reaction (considering methane as the hydrocarbon)

on the basis of which methane and oxygen form

hydrogen and carbon monoxide. The H2/CO-rich process gas discharged at high temperature typically has a mole percent of H2 greater than 20%, often greater than 30%, and often greater than 40%, and typically a mole percent of CO greater than 3%, often greater than 5%, and often greater than 10%.

It is emphasized that the general steam or CO2 reforming reactions or the general oxygen-containing gas adiabatic oxidation reactions of hydrocarbons, as described above, are referred for simplicity to methane. When reforming or adiabatic oxidation is carried out with higher hydrocarbons, the general reactions are not substantially modified: steam reforming of higher hydrocarbons proceeds according to the general reaction and adiabatic oxidation of hydrocarbons proceeds according to the general reaction

Furthermore, as an expert in the field knows, both steam or CO2 reforming and adiabatic oxidation with oxygen-containing gas of hydrocarbons also proceeds according to the secondary reaction of water gas shift (WGS).

Finally, as an expert in the field knows, steam reforming and CO2 reforming of hydrocarbons can be carried out simultaneously in the same reforming reactor; in other words, the reactants contain hydrocarbons, steam and CO2.

NOx-rich process gases synthesized in oxygen-containing adiabatic oxidation reactors of ammonia are typically discharged at operating temperatures above 700-750?C and at operating pressures above 0.12MPa(a), often above 0.25MPa(a). The reactors are operated in the presence of solid catalysts. Ammonia oxidation proceeds according to the general reaction whereby ammonia and oxygen react to form water and nitrogen monoxide; the process gas discharged at high temperature typically has a mole percent of NO in excess of 3%, often in excess of 5%, and often in excess of 7%.

HCN-rich process gases synthesized in oxygen-containing gas adiabatic oxidation reactors of an ammonia/methane mixture are typically discharged at operating temperatures above 750-800?C, often above 1000?C, and at operating pressures above 0.12MPa(a), often above 0.25MPa(a). The reactors are operated in the presence of solid catalysts. Oxidation of the ammonia/methane mixture proceeds according to the general reaction whereby ammonia, methane and oxygen form water and hydrogen cyanide. The conversion of ammonia is typically greater than 60%; typically the mole percent of HCN in the reactor exit process gas is greater than 3%, often greater than 5%.

CH2O-rich process gases synthesized in oxygen-containing gas adiabatic partial oxidation reactors of methanol are typically discharged at operating temperatures above 500-550?C and operating pressures above 0.12MPa(a), often above 0.25MPa(a). The reactors are operated in the presence of solid catalysts. The adiabatic partial oxidation of methanol proceeds according to the general reaction

where methanol and oxygen form water and formaldehyde. The conversion of methanol is usually greater than 60%; the mole percent of CH2O in the process gas exiting the reactor is typically greater than 3%, often greater than 5%.

In this description, the term "process gas" therefore refers to a process gas discharged at high temperature from a chemical reforming/oxidation reactor equivalent to those described above.

As an expert in the field knows, all the above general chemical reactions provide a comprehensive representation of the reaction mechanism; the actual chemical reaction mechanism may be more complex and proceed with multiple reactions in parallel and/or in series. For example, the general reaction of partial oxidation of methane to form a gas rich in H2/CO may proceed first according to a partial combustion reaction and then according to a reforming reaction and/or a reforming reaction with

As an expert in the field knows, the general chemical reactions above can be carried out in the presence of nitrogen; in particular, adiabatic oxidations can be carried out with a gas containing oxygen and nitrogen. Nitrogen, being inert in reforming/oxidation reactions, is intentionally not contemplated in the general reactions above.

The above process gases discharged at high temperature must be cooled in order to be subjected to subsequent unit operations and then used for the subsequent production of the final chemicals. According to the present invention, the process gases can undergo:

- cooling with direct heat exchange with nitrogen, without the aid of process boilers and/or heat exchangers,

- direct heat exchange cooling with nitrogen and indirect heat exchange cooling using process boilers and/or heat exchangers.

The hot process gas, according to the present invention, is mixed and cooled with a colder nitrogen stream, preferably under cryogenic conditions, coming from a cryogenic air separation unit. The cold nitrogen can be mixed in liquid or vapor phase. The mixing lowers the temperature of the process gas; by adjusting the temperature and flow rate of the nitrogen, the cooling of the process gas can be regulated.

If cooling is carried out with the aid of boilers and/or heat exchangers, direct cooling with nitrogen can be carried out upstream of the boilers or heat exchangers, downstream of them or in an intermediate position, i.e. between one exchanger and another.

The unit operation to which the process gas is subjected downstream of cooling can be of any type, for example a chemical conversion, a phase separation, a filtration, a rectification or distillation, an absorption or stripping.

Downstream of direct nitrogen cooling, if the percentage of nitrogen in the process gas is too high for subsequent chemical production, the excess nitrogen is separated; separation of nitrogen from the process gas can be performed by any physical method, for example by adsorption on solid material (zeolites) or condensation/washing under cryogenic conditions.

In this description, a stream or gas containing oxygen may correspond to atmospheric air, or to enriched atmospheric air, i.e. air having a higher percentage of oxygen than atmospheric air, or to pure oxygen, or to a mixture of oxygen and other compounds, such as nitrogen.

In support of the detailed description of the process gas synthesis method which is the subject of this invention, the following figures are reported:

- Fig.1, where a simplified process diagram is reported to carry out the method of the invention in accordance with a first embodiment;

- Fig.2, where a simplified process diagram is reported to carry out the method of the invention in accordance with a second embodiment;

- Fig.3, where a simplified process diagram is reported to carry out the method of the invention in accordance with a third embodiment;

- Fig.4, where a simplified process diagram is reported to carry out the method of the invention in accordance with a fourth embodiment;

- Fig.5, where a simplified process diagram is reported to carry out the method of the invention for the subsequent production of ammonia in accordance with a first embodiment;

- Fig.6, where a simplified process diagram is reported for carrying out the method of the invention for the subsequent production of ammonia in accordance with a second embodiment;

- Fig.7, where a simplified process diagram is reported for carrying out the method of the invention for the subsequent production of ammonia in accordance with a third embodiment;

- Fig.8, where a simplified process diagram is reported for carrying out the method of the invention for the subsequent production of ammonia in accordance with a fourth embodiment.

In the process diagrams shown here, the lines or currents connecting one or more units are indicated by arrows; in the description that follows, the terms lines or currents indicate both the fluid and the pipe or duct or means capable of making the fluid flow. The arrows indicate the direction of flow of the fluid.

In this description, the process gas pressure expressed in ?MPa(a)? equals 10<6>Pa absolute.

Detailed description of the invention

Fig.1 shows the process scheme of the method for synthesizing a process gas with direct nitrogen quenching according to a first embodiment.

The process scheme of Fig.1 comprises at least one stream of chemical reagents (1) to be fed into a first unit (U1) comprising at least one chemical reforming reactor with steam and/or CO2 or an adiabatic oxidation reactor with a stream containing oxygen. From the first unit (U1) a first stream of reaction products is extracted, i.e. a first stream of process gas (3a), having a temperature higher than 500?C. The first stream (3a) has a pressure higher than 0.12MPa(a), or higher than 0.20MPa(a), or higher than 0.45MPa(a).

The process diagram of Fig.1 comprises an air separation unit (ASU) into which a stream of atmospheric air (4) is introduced to obtain a stream of cold nitrogen (5) and a stream of oxygen (7). The stream of cold nitrogen (5) is provided with a flow regulation system (6), such as a flow meter and/or a valve. The stream of cold nitrogen (5) is in fluid communication with the first stream of process gas (3a) in a mixing zone or point (2) so as to form by mixing a second stream of process gas (3b), consisting of the first stream of process gas (3a) and the stream of cold nitrogen (5); the second stream of process gas (3b) therefore has a higher nitrogen content and a lower temperature than the first stream of process gas (3a).

The process diagram of Fig.1 includes a second unit (U2) into which the second process gas stream (3b) enters to be subjected to unit operations. Consequently, the first and second units (U1,U2) are connected to each other by the first and second process gas streams (3a,3b) placed in series.

The cold nitrogen stream (5) is preferably under cryogenic conditions; the cold nitrogen stream (5) can be mixed with the first process gas stream (3a) in liquid or vapor phase. The cold nitrogen stream (5) has equivalent or higher pressure and lower temperature than the first process gas stream (3a). Preferably, the cold nitrogen stream (5) has a temperature below 0?C, preferably below -50?C, and more preferably below -100?C.

Based on the embodiment of Fig.1, the method of the present invention substantially comprises the following operations:

- The separation of air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and nitrogen (5); - The introduction of at least one stream of chemical reagents (1) into the first unit (U1), comprising hydrocarbons and/or ammonia and/or methanol;

- Reforming with steam and/or CO2, or adiabatic oxidation with an oxygen-containing stream, in the first unit (U1) to synthesize the first process gas stream at high temperature (3a) and rich in H2/CO, or in NOx, or in HCN, or in CH2O;

- Cooling of the first stream of high temperature process gas (3a) by mixing it with colder nitrogen (5) to obtain a second stream of cooled process gas (3b) having a higher nitrogen content;

- The second process gas stream (3b) is fed into a second unit (U2) for carrying out unit operations.

Fig.2 shows the process scheme of the method for synthesizing a process gas with direct nitrogen quenching according to a second embodiment.

The process diagram in Fig.2 is equivalent to that in Fig.1 except for the installation of a heat exchange unit (HE); in other words, the elements and the relative numbering of the process diagram shown in Fig.2 are equivalent to those of the process diagram shown in Fig.1. Therefore, for simplicity of description, the description of the process diagram in Fig.2 is partially omitted.

According to the embodiment of Fig.2, the heat exchange unit (HE) is installed between the first and second units (U1,U2) and, more specifically, is installed downstream of the mixing point (2). The heat exchange unit (HE) is connected to the first unit (U1) by means of the first and second process gas streams (3a,3b), placed in series, and is connected to the second unit (U2) by means of a third process gas stream (3c). The heat exchange unit (HE) is suitable for indirect heat exchange of the process fluid (3b) by using one or more auxiliary fluids. In other words, the heat exchange unit (HE) comprises one or more heat exchangers suitable for indirectly cooling the process gas (3b). Preferably, the heat exchange unit (HE) comprises a process boiler and, preferably, at least one other heat exchanger. The heat exchange unit (HE) is suitable for indirect heat exchange of the process gas (3b). heat exchange unit (HE) further cools the process gas to a temperature suitable for the unit operations to be carried out in the second unit (U2). More specifically, the heat exchange unit (HE) cools the second process gas stream (3b) to obtain the third process gas stream (3c) at a lower temperature. The cold nitrogen stream (5) is connected to the first process gas stream (3a) at the mixing point (2).

Based on the embodiment of Fig.2, the method of the present invention substantially comprises the following operations:

- The separation of air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and nitrogen (5); - The introduction of at least one stream of chemical reagents (1) into the first unit (U1), comprising hydrocarbons and/or ammonia and/or methanol;

- Reforming with steam and/or CO2, or adiabatic oxidation with an oxygen-containing stream, in the first unit (U1) to synthesize the first process gas stream at high temperature (3a) and rich in H2/CO, or in NOx, or in HCN, or in CH2O;

- Cooling of the first process gas stream (3a) by mixing it with colder nitrogen (5) to obtain a second process gas stream (3b) cooled and having a higher nitrogen content;

- Feeding the second process gas stream (3b) into a heat exchange unit (HE) for further cooling of the process gas by indirect heat exchange, thus obtaining a third, further cooled process gas stream (3c);

- The injection of the third stream of process gas (3c), exiting the heat exchange unit (HE), into a second unit (U2) for carrying out unit operations.

Fig.3 shows the process scheme of the method for synthesizing a process gas with direct nitrogen quenching according to a third embodiment.

The process diagram in Fig.3 is equivalent to that in Fig.2 except for the position of the heat exchange unit (HE); in other words, the elements and the relative numbering of the process diagram shown in Fig.3 are equivalent to those of the process diagram shown in Fig.2. Therefore, for simplicity of description, the description of the process diagram in Fig.3 is partially omitted.

According to the embodiment of Fig.3, the heat exchange unit (HE) is installed between the first and second units (U1,U2) and, more specifically, is installed upstream of the mixing point (2). The heat exchange unit (HE) is connected to the first unit (U1) by means of the first process gas stream (3a) and is connected to the second unit (U2) by means of the second and third process gas streams (3b,3c), placed in series. The heat exchange unit (HE) receives and cools the first process gas stream (3a), coming from the first unit (U1), and then discharges the second process gas stream (3b) at a lower temperature. The heat exchange unit (HE) is suitable for indirect heat exchange of the process fluid (3a) by using one or more auxiliary fluids. In other words, the heat exchange unit (HE) comprises one or more auxiliary fluids. heat exchangers for indirectly cooling the process gas (3a). Preferably, the heat exchange unit (HE) comprises a process boiler and, preferably, at least one further heat exchanger. The cold nitrogen stream (5) is connected to the second process gas stream (3b), exiting the heat exchange unit (HE), at the mixing point (2).

Based on the embodiment of Fig.3, the method of the present invention substantially comprises the following operations:

- The separation of air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and nitrogen (5); - The introduction of at least one stream of chemical reagents (1) into the first unit (U1), comprising hydrocarbons and/or ammonia and/or methanol;

- Reforming with steam and/or CO2, or adiabatic oxidation with an oxygen-containing stream, in the first unit (U1) to synthesize the first process gas stream at high temperature (3a) and rich in H2/CO, or in NOx, or in HCN, or in CH2O;

- The introduction of the first stream of process gas (3a) into the heat exchange unit (HE) for cooling the process gas by indirect heat exchange, thus obtaining a second stream of cooled process gas (3b);

- Cooling of the second process gas stream (3b) by mixing it with colder nitrogen (5) to obtain a third process gas stream (3c) further cooled and having a higher nitrogen content;

- The injection of the third stream of process gas (3c) into a second unit (U2) for carrying out unit operations.

Fig.4 shows the process scheme of the method for synthesizing a process gas with direct nitrogen quenching according to this embodiment.

The process diagram in Fig.4 is equivalent to that in Fig.2 or Fig.3 except that there are two heat exchange units (HE1,HE2); in other words, the elements and the relative numbering of the process diagram shown in Fig.4 are equivalent to those in the process diagram shown in Fig.2 or Fig.3. Therefore, for simplicity of description, the description of the process diagram in Fig.4 is partially omitted.

According to the embodiment of Fig.4, the two heat exchange units (HE1,HE2) are installed between the first and second units (U1,U2); more specifically, the first heat exchange unit (HE1) is installed upstream of the mixing point (2) and the second heat exchange unit (HE2) is installed downstream of the mixing point (2). The first heat exchange unit (HE1) is connected to the first unit (U1) by means of the first process gas stream (3a); the second heat exchange unit (HE2) is connected to the second unit (U2) by means of a fourth process gas stream (3d). The first heat exchange unit (HE1) is connected to the second heat exchange unit (HE2) by means of the second and third process gas streams (3b,3c), placed in series. The cold nitrogen stream (5) is connected to the second process gas stream (3b), exiting the first unit (U1). of heat exchange (HE1), at the mixing point (2). The first heat exchange unit (HE1) is adapted to the indirect heat exchange of the process fluid (3a) by using one or more auxiliary fluids. In other words, the first heat exchange unit (HE1) comprises one or more heat exchangers adapted to indirectly cool the process gas (3a). Preferably, the first heat exchange unit (HE1) comprises a process boiler or a heat exchanger; according to another preferential embodiment, the first heat exchange unit (HE1) comprises a process boiler and at least one other heat exchanger. The second heat exchange unit (HE2) is adapted to the indirect heat exchange of the process fluid (3c) by using one or more auxiliary fluids. In other words, the second heat exchange unit (HE2) comprises one or more heat exchangers adapted to indirectly cool the process gas (3c).

Based on the embodiment of Fig.4, the method of the present invention substantially comprises the following operations:

- The separation of air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and nitrogen (5); - The introduction of at least one stream of chemical reagents (1) into the first unit (U1), comprising hydrocarbons and/or ammonia and/or methanol;

- Reforming with steam and/or CO2, or adiabatic oxidation with an oxygen-containing stream, in the first unit (U1) to synthesize the first process gas stream at high temperature (3a) and rich in H2/CO, or in NOx, or in HCN, or in CH2O;

- The introduction of the first process gas stream (3a) into the first heat exchange unit (HE1) for cooling the process gas by indirect heat exchange, thus obtaining the second cooled process gas stream (3b);

- Cooling of the second process gas stream (3b) by mixing it with colder nitrogen (5) to obtain a third process gas stream (3c) further cooled and having a higher nitrogen content;

- The introduction and cooling of the third process gas stream (3c) by indirect heat exchange in the second heat exchange unit (HE2) to obtain a fourth process gas stream (3d) further cooled;

- The injection of the fourth process gas stream (3d) into the second unit (U2) for carrying out unit operations.

It is emphasized that the embodiments shown in Figs. 1-4, and the related synthesis methods including direct nitrogen cooling, are applicable to any process gas belonging to those described above, such as process gases rich in H2/CO for the subsequent production of H2/NH3/CH3OH or produced by Fisher Tropsch synthesis, or rich in NOx for the subsequent production of HNO3, or rich in HCN for the subsequent production of HCN, or in CH2O for the subsequent production of CH2O. In other words, the process gas exiting at high temperature from the first unit (U1) relating to Figs. 1-4 corresponds to one of the process gases described above and object of the present invention.

It is worth noting that, for all the embodiments of Figs.1-4, the oxygen (7) coming from the air separation unit (ASU) can be fed into the first unit (U1) to carry out the adiabatic oxidation reactions; in other words, based on a preferred embodiment, the oxygen stream (7) can be connected to the first unit (U1).

As one skilled in the art can understand, the cold nitrogen stream (5) can be mixed with the process gas stream (3a,3b,3c,3d) at multiple points; in other words, the synthesis method described herein can provide for more than one mixing point (2). It is therefore useful to note that the cold nitrogen (5), based on alternative and preferential embodiments of the present synthesis method, can be mixed with the hot process gas both upstream and downstream of a heat exchange unit (HE,HE1,HE2).

It is also worth noting that, according to an alternative embodiment of the present method, the mixing point (2) can be internal to the heat exchange unit (HE,HE1,HE2), i.e. the cold nitrogen (5) can be injected into a heat exchanger. Consequently, direct nitrogen cooling can be an operation carried out on a line or duct and/or in a heat exchanger.

As one skilled in the art may appreciate, the cold nitrogen (5) and oxygen (7), produced in the air separation unit (ASU), may be stored in tanks or vessels; therefore, in alternative embodiments of the present synthesis method, the cold nitrogen stream (5), or the cold nitrogen stream (5) and oxygen stream (7), are connected to tanks or vessels rather than being directly connected to the air separation unit (ASU) as shown in Figs. 1-4.

Fig.5 shows the process scheme of the method for synthesizing a process gas, with direct nitrogen cooling, useful for the subsequent production of ammonia in accordance with a first embodiment. More specifically, Fig.5 refers to a process scheme for producing ammonia by means of the adiabatic oxidation of hydrocarbons with oxygen, i.e. by means of an autothermal reforming reactor or a partial oxidation reactor or a catalytic partial oxidation reactor (U1).

The process diagram in Fig.5 includes at least:

- A first unit (U1) comprising at least one chemical reactor;

- A heat exchange unit (HE) comprising at least one heat exchanger;

- A second unit (U2) comprising at least one chemical reactor;

- A third unit (U3) comprising at least one piece of equipment for purifying the process gas;

- A fourth unit (U4) comprising at least one piece of equipment for purifying the process gas;

- A fifth unit (U5) comprising equipment for the production of ammonia;

- An air separation unit (ASU).

The first unit (U1) receives at least one stream of chemical reagents (1) and one stream of oxygen (7) coming from the air separation unit (ASU). The first unit (U1) is connected to the heat exchange unit (HE) by means of a first and a second stream of process gas (3a,3b) arranged in series, the heat exchange unit (HE) is connected to the second unit (U2) by means of a third stream of process gas (3c), the second unit (U2) is connected to the third unit (U3) by means of a fourth stream of process gas (3d), the third unit (U3) is connected to the fourth unit (U4) by means of a fifth stream of process gas (3e) and the fourth unit (U4) is connected to the fifth unit (U5) by means of a final synthesis gas stream (10). The air separation unit (ASU) is connected to the fifth unit (U5) by means of a final synthesis gas stream (10). connected to the first process gas stream (3a), at a mixing point (2), and to the fourth unit (U4) respectively by means of a first and a second stream of cold nitrogen (5a,5b). The first and second stream of cold nitrogen (5a,5b) are equipped with a flow regulation system, such as flow meters and/or valves (6a,6b).

Based on the embodiment of Fig.5, the synthesis method of the present invention substantially comprises the following operations:

- Separation of air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and cold nitrogen (5a,5b);

- The introduction of chemical reagents (1), including hydrocarbons, and oxygen (7) into the first unit (U1);

- Adiabatic oxidation with oxygen (7) of hydrocarbons, in the first unit (U1), to synthesize a first stream of process gas (3a) at high temperature and rich in H2/CO;

- Cooling of the first process gas stream (3a) exiting the first unit (U1) by mixing it with a first stream of cold nitrogen (5a), at the mixing point (2), to obtain a second process gas stream (3b) having a higher nitrogen content and a lower temperature than the first process gas stream (3a);

- Cooling of the second process gas stream (3b) in the heat exchange unit (HE), by indirect heat exchange with at least one auxiliary fluid, to obtain a third process gas stream (3c) further cooled;

- The injection of the third process gas stream (3c) into the second unit (U2) to carry out water gas shift reactions, according to the general reaction CO+H2O ?CO2+H2, and obtain a fourth process gas stream (3d) poor in, or free from, CO and rich in CO2;

- The injection of the fourth process gas stream (3d) into the third unit (U3) to remove CO2 and/or acid chemical species (8) and to obtain a fifth process gas stream (3e);

- The introduction of the fifth stream of process gas (3e) and, possibly, of a second stream of cold nitrogen (5b), preferably liquid, into the fourth unit (U4) to remove excess chemical species and/or oxygenated chemical species and/or impurities (9) and obtain a final synthesis gas stream (10) essentially made up of hydrogen and nitrogen, and with a molar ratio of approximately 3:1 between hydrogen and nitrogen;

- The final synthesis gas stream (10) is fed into the fifth unit (U5) to produce ammonia (11), according to the general reaction 3H2+N2 ?2NH3.

With reference to the embodiment of Fig.5, an expert in the field can understand that the synthesis method which is the object of the present invention provides that the cold nitrogen (5a,5b) is dosed at the mixing point (2) and in the fourth unit (U4) so that the synthesis operations are optimised; in other words, the total quantity of nitrogen present in the final synthesis gas stream (10) is obtained by regulating the quantity of nitrogen relating to the first and second streams (5a,5b).

Fig.6 shows the process scheme of the method for synthesizing a process gas, with direct nitrogen cooling, useful for the subsequent production of ammonia in accordance with a second embodiment. More specifically, Fig.6 refers to a process scheme for producing ammonia by adiabatic oxidation with an oxygen/nitrogen mixture of hydrocarbons, i.e. by means of an autothermal reforming reactor or a partial oxidation reactor or a catalytic partial oxidation reactor (U1).

The process diagram of Fig.6 is equivalent to that of Fig.5 except for the installation of a third nitrogen stream (5c); in other words, the elements and the relative numbering of the process diagram shown in Fig.6 are equivalent to those of the process diagram shown in Fig.5. Furthermore, the main operations or units of Fig.6 are equivalent to those of Fig.5. Therefore, for simplicity of description, the description of the process diagram of Fig.6 and the relative method are partially omitted.

In accordance with the embodiment of Fig.6, a third nitrogen stream (5c), obtained from the air separation unit (ASU), is connected to the oxygen stream (7); the third nitrogen stream (5c) is provided with a third flow regulation system (6c). The third nitrogen stream (5c) is mixed with the oxygen stream (7) to obtain an oxygen/nitrogen mixture (12). The oxygen/nitrogen mixture (12) is fed into the first unit (U1) to perform the adiabatic oxidation of the hydrocarbons. The possibility of mixing oxygen and nitrogen allows the temperature and yield of the adiabatic oxidation reaction to be adjusted. As a person skilled in the art can understand, the total amount of nitrogen present in the final synthesis gas stream (10) is obtained by adjusting the amount of nitrogen relative to the first, second and third streams (5a,5b,5c). It is worth noting that the nitrogen in the third stream (5c) preferably has a higher temperature than the first and second nitrogen streams (5a,5b) to promote the oxidation reaction. Preferably, the temperature of the oxygen/nitrogen stream (12) is higher than 150?C and, preferably, higher than 250?C.

Fig.7 shows the process scheme of the method for synthesizing a process gas, with direct nitrogen cooling, useful for the subsequent production of ammonia in accordance with a third embodiment. More specifically, Fig.7 refers to a process scheme for producing ammonia by the adiabatic oxidation of hydrocarbons with air, i.e. by means of an autothermal reforming reactor or secondary reforming reactor (U1).

The process diagram of Fig.7 is equivalent to that of Fig.5 except for the injection of air (4b) instead of oxygen in the first unit (U1) and the installation of a sixth unit (U6); in other words, the elements and the relative numbering of the process diagram shown in Fig.7 are equivalent to those of the process diagram shown in Fig.5. Furthermore, the main operations or units of Fig.7 are equivalent to those of Fig.5. Therefore, for simplicity of description, the description of the process diagram of Fig.7 and the relative method are partially omitted.

According to the embodiment of Fig.7, the synthesis method of the present invention substantially comprises the same operations as those of Fig.5 except for the operation of introducing air (4b) into the first unit (U1), to carry out the adiabatic oxidation of the hydrocarbons, and the operation of methanation in the sixth unit (U6), installed between the third and fourth units (U3,U4). Consequently, a first stream of air (4a) is introduced into the air separation unit (ASU) while a second stream of air (4b) is introduced into the first unit (U1). The sixth unit (U6) receives the fifth stream of process gas (3e), exiting from the third unit (U3); in the sixth unit (U6) a second stream of process gas (3e) is introduced into the air separation unit (ASU). the methanation operation is carried out, according to the general reactions CO+3H2 ?CH4+H2O and CO2+4H2 ?CH4+2H2O, to completely or almost completely eliminate the carbon oxides from the process gas, forming methane and water, in the presence of a solid catalyst. From the sixth unit (U6) a sixth stream of process gas (3f) exits and is fed into the fourth unit (U4) for the purification of the process gas and for the possible injection of a second stream of cold nitrogen, (5b), preferably liquid, in direct contact with the process gas (3f).

As a person skilled in the art can appreciate, the embodiment of Fig.7 can be easily modified by injecting the oxygen stream (7) into the second air stream (4b) to obtain oxygen-enriched air to be injected into the first unit (U1); consequently, the first unit (U1) carries out an adiabatic oxidation of the hydrocarbons by means of enriched air.

Fig.8 shows the process scheme of the method for synthesizing a process gas, with direct nitrogen cooling, useful for the subsequent production of ammonia according to a fourth embodiment. More specifically, Fig.8 refers to a process scheme for producing ammonia by steam and/or CO2 reforming of hydrocarbons.

The process diagram in Fig.8 includes at least:

- A first unit (U1) comprising at least one chemical reactor;

- A heat exchange unit (HE) comprising at least one heat exchanger;

- A second unit (U2) comprising at least one chemical reactor;

- A third unit (U3) comprising at least one piece of equipment for purifying the process gas;

- A fourth unit (U5) comprising equipment for the production of ammonia;

- An air separation unit (ASU).

The first unit (U1) receives at least one stream of chemical reagents (1). The first unit (U1) is connected to the heat exchange unit (HE) by means of a first and a second stream of process gas (3a,3b) placed in series, the heat exchange unit (HE) is connected to the second unit (U2) by means of a third stream of process gas (3c), the second unit (U2) is connected to the third unit (U3) by means of a fourth stream of process gas (3d), the third unit (U3) is connected to the fourth unit (U5) by means of a fifth stream of process gas (3e) and by means of a final synthesis gas stream (10) placed in series. The air separation unit (ASU) is connected to the second unit (U2). connected to the first process gas stream (3a) and to the fifth process gas stream (3e) respectively by means of a first and a second cold nitrogen stream (5a,5b) and respectively in a first and a second mixing point (2a,2b). The first and the second cold nitrogen stream (5a,5b) are provided with a flow regulation system, such as flow meters and/or valves (6a,6b).

Based on the embodiment of Fig.8, the synthesis method of the present invention substantially comprises the following operations:

- Separation of air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and cold nitrogen (5a,5b);

- The introduction of chemical reagents (1), including hydrocarbons, steam and/or CO2, into the first unit (U1);

- Reforming of hydrocarbons with steam and/or CO2, in the first unit (U1), to synthesize a first stream of process gas at high temperature (3a) and rich in H2/CO; - Cooling of the first stream of process gas (3a) exiting the first unit (U1) by mixing it with a first stream of cold nitrogen (5a), in a first mixing point (2a), to obtain a second stream of process gas (3b) having a higher nitrogen content and a lower temperature than the first stream of process gas (3a);

- Cooling of the second process gas stream (3b) in the heat exchange unit (HE), by indirect heat exchange with at least one auxiliary fluid, to obtain a further cooled third process gas stream (3c);

- The injection of the third process gas stream (3c) into the second unit (U2) to carry out water gas shift reactions and obtain a fourth process gas stream (3d) poor in, or free from, CO and rich in CO2;

- The injection of the fourth process gas stream (3d) into the third unit (U3) to remove CO2, acid chemical species, impurities and excess chemical species (8) and to obtain a fifth process gas stream (3e) essentially consisting of hydrogen or hydrogen and nitrogen; - The possible mixing of the fifth process gas stream (3e) with a second nitrogen stream (5b), at the second mixing point (2b), to obtain a final synthesis gas stream (10) essentially consisting of hydrogen and nitrogen, and with a molar ratio of approximately 3:1 between hydrogen and nitrogen;

- The final synthesis gas stream (10) is fed into the fourth unit (U5) to synthesize ammonia (11).

With reference to the embodiment of Fig.8, an expert in the field can understand that the synthesis method of the present invention provides that the cold nitrogen (5a,5b) is dosed at the mixing points (2a,2b) so that the synthesis operations are optimised; in other words, the total quantity of nitrogen present in the final synthesis gas stream (10) is obtained by adjusting the quantity of nitrogen relating to the first and second streams (5a,5b). It should be noted that, for the embodiment of Fig.8, the nitrogen relating to the second stream (5b) preferably has a higher temperature than that of the first nitrogen stream (5a); it should be underlined that the second nitrogen stream (5b) preferably has the purpose of adjusting the composition of the final synthesis gas (10) rather than cooling the process gas (3e).

For the embodiments of Figs.5-8, the second nitrogen stream (5b) is shown in dashed lines. It is worth noting that the second nitrogen stream relating to Figs.5-8 is optional. The second nitrogen stream (5b) relating to Figs.5-8 is useful for adjusting the composition of the final synthesis gas (10) and/or for carrying out a final purification of the process gas in cryogenic conditions. In the latter case, the second nitrogen stream (5b) is in cryogenic conditions and, preferably, in the liquid state so as to carry out a scrubbing of the process gas and eliminate the unwanted residual chemical species (for example, methane, water, carbon oxides, non-condensables).

According to an alternative embodiment of the method of the present invention and relating to the embodiments of Figs.5-8, the hot process gas discharged from the first unit (U1) is cooled by mixing with cold nitrogen downstream of the heat exchange unit (HE); in other words, the mixing point (2) is located downstream of the heat exchange unit (HE).

In accordance with an alternative embodiment of the method of the present invention and relating to the embodiments of Figs.5-8, the hot process gas discharged from the first unit (U1) is cooled by multiple mixings with cold nitrogen; in other words, the hot process gas is mixed with cold nitrogen by multiple and distinct mixing points.

It is worth noting that, for all embodiments relating to Figs. 1-8, the first unit (U1) where the reforming/oxidation operation takes place, may comprise more than one reforming/oxidation reactor. For example, as an expert in the field will know, for the production of hydrogen, ammonia, methanol or Fischer Tropsch synthesis products, a steam/CO2 reforming reactor and an adiabatic oxidation reactor with oxygen-containing gas arranged in series or in parallel are sometimes used.

As one skilled in the art can understand, for all embodiments of the synthesis method described herein, additional equipment and/or units may be inserted between one unit and another, such as heat exchangers, phase separators, compressors or pumps, scrubbing or rectification columns, without modifying the inventive concept of the present invention.

As one skilled in the art can understand, the units (U1,U2,U3,U4,U5,U6,HE,H31,HE2,ASU) described in the embodiments of the process gas synthesis method may include various equipment, such as tanks, heat exchangers, rectification columns, phase separators, rotating machines, without modifying the inventive concept of the present invention.

The cooling operation of the process gas, discharged at high temperature from the reforming/oxidation reactor, by direct contact with cold nitrogen has the following characteristics:

- Process gas cooling is rapid and occurs with limited pressure drop;

- Nitrogen is an inert chemical species, therefore it does not substantially modify the synthesis chemistry and does not attack construction steels;

- The cooling nitrogen can be subsequently removed from the process gas by a physical separation method; - If the process gas is used for subsequent ammonia production, the cooling nitrogen is used as a chemical reagent in the ammonia production unit (U5).

In accordance with the detailed description above, it is therefore clear that the method for synthesizing a process gas by direct nitrogen cooling, which is the subject of the present invention, allows obtaining the advantages already set out. In particular, in the case of existing production plants, the method which is the subject of the present invention offers the following specific advantages:

- In the event of temperature peaks or uncontrolled reactions in the first unit (U1), where the process gas is synthesized at high temperature, direct cooling with nitrogen allows to mitigate the temperature of the process gas entering the subsequent unit (HE,U2) and therefore to safeguard it from possible overheating and/or damage. In this case, the method of the present invention has an emergency or safeguard cooling function;

- The thermo-hydraulic and thermo-mechanical conditions of the equipment included in the heat exchange unit (HE), i.e. the heat exchangers, are mitigated, and therefore the equipment works in less severe conditions and its operating life can be extended. In this case, the method of the present invention can be used for a modernization and/or an enhancement of the existing plant;

- If the heat exchange unit (HE) includes process boilers, the steam production can be decreased. In this case, the method of the present invention can be used for a modernization of the existing plant;

- If a heat exchanger designed to cool the process gas by indirect heat exchange fails or suffers a decrease in performance, direct cooling with nitrogen makes it possible to partially or totally compensate for the lack of heat exchange by the heat exchanger. In this case, the method of the present invention has an emergency or temporary cooling function.

The method for synthesizing a process gas by direct nitrogen cooling, as conceived and described, is in any case susceptible to numerous modifications and variations, all of which can be traced back to the same inventive concept. Furthermore, all the details can be replaced with technically equivalent elements.

The scope of protection of the present invention is defined by the appended claims.

Example

The chemical composition and typical operating conditions of a process gas discharged from an autothermal reforming reactor, i.e. a reactor for the adiabatic oxidation with oxygen of a hydrocarbon feedstock in the presence of a solid catalyst, for the subsequent production of ammonia, are reported below:

According to a conventional synthesis method, downstream of the adiabatic oxidation reactor the process gas is conventionally cooled by at least one shell and tube process gas boiler (PGB), with the tube side gas and shell side water boiling typically at 320?C. After conventional cooling, the process gas is subjected to a unit operation.

The conventional synthesis method is modified in accordance with the present invention; more specifically, this example consists of inserting a mixing point between the oxidation reactor and the PGB to directly quench the hot process gas with cold nitrogen.

The nitrogen available for direct cooling of the process gas is at a temperature of -140?C and a pressure of 3.0MPa(a); the cooling nitrogen is in gas phase and is mixed with the process gas upstream of the PGB.

Assuming a hot process gas flow rate exiting the reactor of 10000kg/h and mixing carried out under ideal thermodynamic and fluid dynamic conditions, direct nitrogen cooling allows to obtain approximately the operating parameters shown in the following Graph 1.

Specifically, Graph 1 shows the profile of the steam production in the PGB (?Steam Prod in PGB?), of the process gas temperature at the inlet of the PGB (?Temp gas inlet PGB?) and of the heat flux at the inlet of the PGB tubes (?Heat flux inlet PGB?) as the flow rate of cold nitrogen (on the x-axis) mixed with the hot process gas exiting the reactor varies.

The three operating parameters are 100% without injection and direct nitrogen cooling, i.e. in the case of a conventional synthesis method. As shown in Chart 1, based on the synthesis method of the present invention, direct nitrogen cooling allows to decrease the three operating parameters. For example, by injecting and mixing a cold nitrogen flow rate equal to 25% (2500kg/h) of the hot process gas flow rate, it is possible to reduce the heat flux and the gas temperature at the PGB inlet by more than 10% and reduce the steam production in the PGB by almost 10%. Consequently, the thermo-mechanical and thermo-hydraulic operating conditions of the PGB are mitigated because:

- the metal temperatures of the tubes at the entrance of the PGB decrease significantly,

- the vapor content in the PGB decreases and the two-phase cooling of the water improves significantly.

A skilled person in the art can understand that the numerical example described here is, for example, convenient in the case of a PGB having a portion of exchanger tubes out of use. In this case, at the nominal load of the plant, the PGB works in more critical thermo-hydraulic and thermo-mechanical conditions since the total flow rate of the hot process gas passes through only a portion of the exchanger tubes. By using the synthesis method object of the present invention, i.e. by realizing a direct cooling with nitrogen upstream of the PGB, it is possible to mitigate the operating conditions of the PGB and continue to operate the plant without risk of further damage to the PGB tubes.

Claims (11)

PATENT APPLICATION FOR INDUSTRIAL INVENTION Title: SYNTHESIS OF PROCESS GAS BY DIRECT COOLING WITH NITROGEN CLAIMS 1. Method of synthesis of a process gas, for the subsequent production of at least one of the chemical substances among hydrogen, ammonia, methanol, olefins/paraffins/alcohols from Fischer Tropsch synthesis, nitric acid, hydrocyanic acid, formaldehyde, comprising the following operations: - The introduction of at least one stream of chemical reagents containing hydrocarbons and/or ammonia and/or methanol into a first unit (U1); - The reforming with steam and/or CO2, or the adiabatic oxidation with gas containing oxygen, of said hydrocarbons and/or ammonia and/or methanol to synthesize the process gas at high temperature in the first unit (U1); - The cooling of the process gas; - The introduction of the cooled process gas into a second unit (U2) for further unit synthesis operations; said method being characterized in that said cooling comprises at least one operation of mixing the process gas with a cooler stream consisting, or essentially consisting, of nitrogen. 2. Method according to claim 1, wherein the process gas exiting said first unit (U1) contains hydrogen (H2) and carbon monoxide (CO) having a molar concentration of at least 20% and 3% respectively, or nitrogen oxides (NOx) having a total molar concentration of at least 3%, or hydrogen cyanide (HCN) having a molar concentration of at least 3%, or formaldehyde (CH2O) having a molar concentration of at least 3%. 3. A method according to claim 1 or 2, wherein said reforming or adiabatic oxidation proceeds according to at least one of the following general chemical reactions: 4. Method according to claim 2 or 3, wherein the process gas exiting said first unit (U1) has a temperature equal to or greater than 500?C. 5. Method according to claim 2 or 3, wherein the process gas exiting said first unit (U1) has a pressure equal to or greater than 250,000 Pa absolute. 6. Method according to any of claims 1 to 5, wherein said cooling also comprises an indirect heat exchange operation by means of at least one heat exchanger. 7. The method of claim 6, wherein said at least one heat exchanger is a process boiler cooling the process gas by boiling pressurized water. 8. A method according to any of claims 1 to 7, wherein said nitrogen stream is produced in an air separation unit. 9. The method according to claim 4 or 5, wherein in said second unit (U2) the process gas is subjected to the water gas displacement chemical reaction to convert carbon monoxide (CO) into carbon dioxide (CO2). 10. A method according to claim 9, comprising downstream of said second unit (U2) the following further operations: - removing carbon monoxide, carbon dioxide, water and hydrocarbons from the process gas; - preparing a mixture consisting of, or essentially consisting of, hydrogen and nitrogen with a molar ratio of approximately 3:1; - producing ammonia. 11. The method of claim 1, wherein said oxygen-containing gas is atmospheric air, or atmospheric air enriched in oxygen, or a mixture of oxygen and nitrogen, or oxygen.