WO2026008712A1

WO2026008712A1 - Process for the valorization of hydrocarbon mixtures and of secondary streams of industrial processes through partial oxidation reaction and co2 separation technologies

Info

Publication number: WO2026008712A1
Application number: PCT/EP2025/068853
Authority: WO
Inventors: Luca Eugenio Riccardo BASINI; Gian Luca DEL FABBRO ARCOPINTO; Francesco BARBETTI; Emma Palo; Barbara MASCIOCCHI; Michele COLOZZI
Original assignee: Kt Tech SpA
Current assignee: Kt Tech SpA
Priority date: 2024-07-04
Filing date: 2025-07-02
Publication date: 2026-01-08
Anticipated expiration: 2027-01-04

Abstract

Process and plant for the production of ammonia and urea which use as feedstock a gaseous mixture deriving from various industrial processes, in particular metallurgical iron ore reduction processes. The process produces a loop that uses the short contact time catalytic partial oxidation (SCT-CPO) reaction to convert purge gases of the ammonia synthesis to synthesis gas which is combined with the feedstock to be treated. The oxygen for the partial oxidation reaction is obtained by electrolysis of water or through air separation. The CO2 contained in the feedstock and/or emitted in the process steps is used for the production of urea.

Description

PROCESS FOR THE VALORIZATION OF HYDROCARBON MIXTURES AND OF SECONDARY STREAMS OF INDUSTRIAL PROCESSES THROUGH PARTIAL OXIDATION REACTION AND CO₂ SEPARATION TECHNOLOGIES

DESCRIPTION

The present invention concerns a process for the conversion of gaseous hydrocarbon mixtures and/or residues of gaseous mixtures from industrial processes to synthesis gas (“syngas”) and subsequently to gaseous mixtures rich in H2 and N2 and to gaseous mixtures rich in CO2. The invention also concerns a plant for the implementation of such process. More particularly, the invention relates to a process in which gaseous mixtures containing hydrocarbons such as natural gas, but also gaseous mixtures emitted as by-products of various industrial processes, are used as feedstock, and to a plant for its implementation.

Background of the invention

The production of syngas is used to obtain, from primary hydrocarbon sources such as Natural Gas (NG), a large number of chemical products, fuels and fertilizers; moreover, synthesis gas is increasingly used in iron ore reduction processes. The production of methanol and its derivatives, of hydrocarbon fuels, of ammonia, of cast iron and of steel also causes the formation of purge gases and/or of by-products that are exploited above all to produce thermal energy through combustion processes. These processes also bring about significant greenhouse gas (GHG) emissions. For example, it has been calculated that in 2020 the global production of ammonia, which requires the intermediate production of syngas, caused, through the use of fossil fuels, around 450 Mt of CO2 emissions, equivalent to around 20% of the energy consumption of the chemical industry. If the ammonia industry were a country, it would be the 16^th largest producer of emissions in the world, between South Africa and Australia (Ammonia Technology Roadmap; IEA2020).

These considerations are also applied to other processes that use syngas, above all those that produce hydrocarbons, both with the Fischer-Tropsch process, and with refinery processes. In these processes, the syngas is used directly either to generate hydrogen in hydrotreatment processes of oil products or in the production of fuels. The emissions and energy consumptions of all these conversions via-syngas - if improved with the use of more efficient technologies also capable of using waste products - would bring noteworthy environmental and economic advantages.

Background art

Currently, the production of urea from ammonia is based on fossil fuels. ust over 70% of the production of ammonia takes place through the Steam Methane Reforming process, (hereinafter also SMR), which uses Natural Gas (hereinafter also NG), while most of the remaining production uses coal gasification.

A simplified diagram of a process for the production of ammonia and urea according to the prior art is shown in Fig. 1.

With reference to this figure, the essential process steps include a desulfurization step, a primary Steam Methane Reforming (hereinafter also “SMR”) reaction step, followed by a secondary AutoThermal Reforming (hereinafter also “ATR”) step, which uses air as oxidizer. In this way a synthesis gas containing 12 - 15% of CO is obtained. The majority of this CO is converted to CO2 and H2 through reaction with steam in one or more Water Gas Shift (hereinafter also WGS) reactors. The CO2 produced in the WGS step is removed from the synthesis gas typically with one or more Pressure Swing Adsorption/Desorption (hereinafter also PSA) units, and/or with chemical absorption/desorption units, for example using amine compounds. The small amounts of CO and CO2 remaining in the synthesis gas that partially deactivate the catalysts used for the ammonia synthesis reaction are converted to CP in a methanation reactor, thereby obtaining purification of the H2-N2 mixture, which is used for ammonia synthesis.

Ammonia synthesis typically takes place at 10 - 25 MPa and 350 - 550°C. Due to unfavorable equilibrium conditions, in the ammonia conversion reaction only 20 - 30% of the mixture of reagents is converted to ammonia per pass. Therefore, the ammonia produced in the gas delivered from the reactor is condensed and removed, while the mixture of unconverted reagents is recirculated and fed back to the ammonia synthesis reactor. To prevent the accumulation of inert molecules (mainly CH4 and Ar) in the recycling gas, a part of it must be constantly purged. The purge gas is usually used as fuel in the burners of the SMR stage.

Finally, the ammonia and the CO2 coming from the separation unit are used in the urea synthesis step.

Currently, new production processes with greenhouse gas (hereinafter also GHG) emissions close to zero are emerging, among which those that use electrolysis, methane pyrolysis and methods and/or processes that consider both CO2 capture and storage. However, these methods are decidedly more costly than conventional methods, and CO2 storage is not always possible, particularly in contexts in which ammonia is produced.

With reference to the metallurgy sector, the iron and steel industry, which is one of the major industrial CO2 emitters, represents around 30% of the global industrial CO2 emissions (“Green Hydrogen-Based Direct Reduction for Low-Carbon Steelmaking”; K. Rechberger et Al.; Steel Research Int. 2020. 91, 2000110). More in particular, it has been reported that the iron ore reduction processes that take place with Blast Furnace (hereinafter BF), Direct Reduction (hereinafter DR) and Smelting Reduction (hereinafter SR) technologies have a large impact on the emissions of GHG and of other pollutants.

The main emissions in iron ore reduction processes that use BF technologies are linked to the production and to the consumption of coke, which causes emissions of particulate and of carcinogenic aromatic compounds in coke ovens that emit Coke Oven Gases (hereinafter also COG). Moreover, these plants consume a great deal of thermal energy.

The production of cast iron with BF technology is then followed by the production of steel in Basic Oxygen Furnaces (hereinafter also BOF), which also emit Basic Oxygen Furnace Gases (hereinafter also BOFG).

Another iron ore reduction method is Direct Reduction (hereinafter also DR), which does not require the use of coke but instead uses synthesis gas (“syngas”) mainly produced with steam reforming processes and with mixtures of reagents with a high CO2 content (Steam CO2 Reforming, hereinafter also SCR). However, these iron ore reduction plants also emit Direct Reduction Gases (hereinafter also DRG).

Finally, a further iron ore reduction method is the Smelting Reduction (hereinafter also SR) method. This technology is less common and even if it does not use coke, it requires the use of other carbonaceous materials and also emits gaseous mixtures (Smelting Reduction Gases, hereinafter also SRG), which are mainly used to produce thermal energy.

Table 1 includes compositions representative of the gases produced by processes that use blast furnace technology (Blast Furnace Gases, hereinafter also BFG), direct reduction (Direct Reduction Gases, hereinafter also DRG), and the gases mentioned above in relation to other COG, BOFG and SRG iron ore reduction technologies.

Table 1 GB 1035724 describes an ammonia synthesis process in which the gases emitted by direct reduction of the iron ores are used in ammonia synthesis. It does not describe re-use of the ammonia synthesis purge gas and production of urea.

Therefore, it would be desirable to obtain a process that, in addition to allowing use of the gases emitted in the metallurgy sector, in particular in iron ore reduction, in the ammonia-urea synthesis process, also allows reduction of the use of fossil resources and reduction or elimination of carbon oxide emissions.

Summary of the invention

An aspect of the present invention consists of a process for the synthesis of ammonia and urea by reaction of said ammonia and carbon dioxide in which:

- the process uses as feedstock a gaseous mixture including gases selected from the group consisting of: gaseous hydrocarbons, gases emitted as by-products of refinery processes, gases emitted as by-products of industrial processes other than refinery processes, gases emitted as by-products of iron ore reduction processes, gases derived from biomass fermentation; wherein the gases other than gaseous hydrocarbons comprise one or more of the following elements or compounds: hydrogen, nitrogen, carbon monoxide, carbon dioxide, said gaseous mixture is preliminarily treated to eliminate or reduce impurities selected from the group consisting of: sulfur compounds, aromatic organic compounds and particulate matter; characterized by comprising the following steps: a) treatment of said gaseous mixture constituting said feedstock with the water gas shift reaction, which increases the hydrogen content of the mixture; b) removal or reduction of the carbon dioxide content of the mixture treated in said step a), obtaining a stream containing carbon dioxide obtained by such removal; c) purification of hydrogen and nitrogen before the ammonia synthesis reaction, wherein said purification comprises the methanation reaction, d) synthesis reaction of ammonia through the reaction of hydrogen and nitrogen contained in said mixture, with the unreacted part constituting a recycling stream, a part of which is purged to avoid the accumulation of inert substances, e) short contact time catalytic partial oxidation (“SCT-CPO”) reaction of said purged part of said recycling stream by means of oxygen, obtaining synthesis gas ("syngas") having a content of hydrogen and carbon oxide greater than the content of hydrogen and carbon oxide of said purge stream; f) feeding said synthesis gas to said step a) and repeating the cycle of steps from a) to f); g) reaction of the ammonia obtained in said step e) with the carbon dioxide obtained in said step c), whereby urea is obtained.

According to a variant of the process, the gaseous mixture constituting the feedstock of the process is also subjected to short contact time catalytic partial oxidation (SCT-CPO) reaction for the production of synthesis gas, before being treated with water gas shift (WGS) reaction in said step a).

The oxygen used in the short contact time catalytic partial oxidation reaction of step e) can be pure oxygen or oxygen contained in air or contained in oxygen enriched air.

Another aspect of the invention relates to a plant for the production of urea from ammonia and carbon dioxide which uses as feedstock a gaseous mixture selected from the group consisting of gaseous hydrocarbons, gases emitted as by-products of industrial processes other than refinery processes, gases emitted as by-products of iron ore reduction processes, gases deriving from biomass fermentation, including:

A. a water gas shift (WGS) reaction reactor;

B. one or more units for the removal or reduction of the dioxide content from a gaseous mixture;

C. a methanation reactor;

D. an ammonia synthesis reactor,

E. a reactor for the synthesis of urea from ammonia and carbon dioxide; characterized by also comprising:

F. an SCT-CPO reactor connected to said ammonia synthesis reactor (D) and said water gas shift reaction reactor (A), configured to convert the purge gases of said ammonia synthesis reactor (D) to synthesis gas and to feed said synthesis gas to said water gas shift reaction reactor (A);

G. means for feeding carbon dioxide removed in said one more units (B) to said reactor for the synthesis of urea (E).

In the present description the terms “carbon dioxide” or the formula “CO2” are used without distinction, as they designate the same substance.

In the present description the terms “vapor phase” and “gas phase” are also used without distinction. In the present description the term “particulate” designates unburned carbon, typically an amorphous crystallographic structure deriving from incomplete combustion of organic matrices or from pyrolysis reactions.

In the present description the terms “comprising” and “containing” are used without distinction, the meaning of which does not exclude the presence of other elements, besides those defined after these terms. The term “consisting of’ is thus also included within this meaning. The terms “comprising” and “containing” have a broader meaning than “consisting of’, but do not exclude it.

Brief description of the drawings

The invention is now described hereunder with reference to the accompanying figures, wherein:

- Fig. 1 is a simplified diagram of a process for the production of ammonia and urea according to the prior art;

- Figs. 2-17 are simplified diagrams of different embodiments of the process for the production of ammonia and urea according to the invention;

- Figs. 18A and 18B are graphs of the enthalpy/temperature profiles obtained with (18A) low mass velocity, high contact time, tubular geometry of the catalyst bed and (18B) high mass velocity, short contact time, truncated-cone geometry of the catalyst bed; and

- Fig. 19 shows the main areas of an SCT-CPO reactor used in the method according to the invention.

Detailed description of the invention

With reference to the simplified diagram of the ammonia/urea process according to the prior art, shown in Fig. 1, the natural gas (NG) used in the steam methane reforming (SMR) process is first hydrodesulfurized to remove compounds containing sulfur, which are poisons for the catalysts. The desulfurized gas is then reacted in the SMR reactor, which uses a large heating furnace to supply the reaction heat [1], which is carried out inside a series of reforming tubes filled with a nickel-based catalyst. The synthesis gas produced by the SMR reactor is then reacted inside a subsequent air blown autothermal reforming (ATR) stage, named secondary reformer, to produce a mixture comprising carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2) and nitrogen (N2).

This mixture is sent to a water gas shift (WGS) reactor in which the carbon monoxide reacts with the water to form carbon dioxide and hydrogen [2], After removal of CO2, the mixture is further treated in a methanation reactor to eliminate residual CO and CO2 and the mixture obtained, in which H2 and N2 have a higher purity, is compressed and sent to the ammonia synthesis reactor (see Fig. 1). Ammonia synthesis [3] typically takes place at 400-500 °C and 10-30 MPa, in the presence of an iron-based catalyst.

The ammonia produced, together with unreacted H₂, N2 and CH₄, with argon (Ar) and with other impurities, is then cooled to condense it and separate it from the other gases. The unreacted hydrogen and nitrogen are then recycled and mixed with the new feedstock. To avoid the accumulation of impurities, a small part of the gas is eliminated from the ammonia loop and is often used as fuel in the SMR furnace.

Table 2 includes a typical composition of the purge gas of the ammonia synthesis reactor.

Table 2

The ammonia, together with the CO2, is used in the synthesis of urea at temperatures typically between 150 and 250 °C and pressures typically between 12 and 40 MPa depending on the reactions:

2 NH₃ + CO₂ NH₂CO₂NH₄ [4]

NH₂CO₂NH₄ NH2CONH2 + H2O [5]

In the first step ammonium carbamate (NH2CO2NH₄) is formed and in the following step this ammonium carbamate is dehydrated to obtain urea.

The present invention now provides innovative plant and process solutions to use both exhaust gases from other industrial processes, such as exhaust gases from iron ore reduction, and gases containing hydrocarbons produced from biomass in the production of ammonia and urea. Moreover, the process of the invention also allows use of the purge gas of the same ammonia synthesis loop. This result is obtained using the SCT-CPO reaction in a suitable reactor, which can be fed both with industrial exhaust gases and with the purge gas of the ammonia synthesis loop.

As mentioned above, the synthesis gas, also known as “syngas”, used for the synthesis of ammonia and urea, is produced mainly by steam methane reforming (SMR) of natural gas (NG), often combined with a secondary autothermal reforming (ATR) stage, as indicated in the European Commission document “Integrated pollution prevention and control - Reference document on the best available techniques for the large scale production of inorganic chemical products - Ammonia, acids and fertilisers, August 2007”.

The possibility of producing synthesis gas with a Short Contact Time - Catalytic Partial Oxidation (SCT-CPO) is also known.

This technology is described in numerous documents, such as the following patent documents: WO 2020/058859 Al, WO 2016/016257 Al, WO 2016/016256 Al, WO 2016/016253 Al, WO 2016/016251 Al, WO 2011/151082, WO 2009/065559, WO 2011/072877, US 2009/127512, WO 2007/045457, WO 2006/034868, US 2005/211604, WO 2005/023710. WO 97/37929. Partial oxidation reaction technology is also described in the following scientific literature: a) “Issues in H2 and synthesis gas technologies for refinery, GTL and small and distributed industrial needs”; Basini, Luca, Catalysis Today, 106 (1-4), p.34, Oct 2005; b) “Fuel rich catalytic combustion: Principles and technological developments in short contact time (SCT) catalytic processes”; Basini, L.; Catalysis Today, 117(4), 384-393; DOI: 10.1016/j.cattod.2006.06.043 Published: OCT 15, 2006; c) “Natural Gas Catalytic Partial Oxidation: A Way to Syngas and Bulk Chemicals Production / IntechOpen”; G. laquaniello, E. Antonetti, B. Cucchiella, E. Palo, A. Salladini, A. Guarinoni, A. Lainati and L. Basini; http://dx.doi.or /10.5772/48708; and d) “Short Contact Time Catalytic Partial Oxidation (SCT-CPO) for Synthesis Gas

Processes and Olefins Production”; L.E. Basini, A. Guarinoni, Ind. Eng. Chem. Res. 2013, 52, 17023-17037;

The SCT-CPO process performs a conversion of hydrocarbons, such as natural gas, to synthesis gas according to the following main reactions:

CH₄ + U O₂ = CO + H₂ [10]

CO + H2O = CO2 + H2 [6]

The reactions are catalyzed by suitable metal catalysts, on the hot surfaces of which the gaseous reagents collide for a few milliseconds. These conditions promote the formation of partial oxidation products, limiting the contribution of total oxidation reactions, which would form carbon dioxide instead of carbon monoxide and hydrogen. In the present description the term short contact time (or SCT) is not used as a limiting definition, as in the literature this technology is also defined only with the more general expression catalytic partial oxidation (or CPO).

Table 3 below indicates the main reactions involved in synthesis gas production processes, including the CPO reactions mentioned above.

Table 3

AH° 298 K [kJ/mole]

Steam - CO2 Reforming

CH₄ + H2O = CO + 3 H₂ 206 [6]

CO + H2O = CO₂ + H₂ -41 [7]

CH₄ + CO₂ = 2CO + 2 H₂ 247 [8]

Autothermal Reforming (ATR)

CH₄ + 3/2 O₂ = CO + 2 H2O -520 [10]

CH₄ + H2O = CO + 3 H₂ 206 [6]

CO + H2O = CO₂ + H₂ -41 [7]

Catalytic Partial Oxidation (CPO)

CH₄ + y₂ O₂ = CO + H₂ -36 [10]

CO + H2O = CO₂ + H₂ -41 [6]

The process of the present invention produces the synthesis of ammonia (NH3) and urea (NH2- CO-NH2) using: i. gaseous hydrocarbons, such as natural gas, ii. exhaust gases from other industrial processes such as those derived from iron ore reduction processes and from steel production processes, iii. mixtures containing hydrocarbons derived from refinery exhaust gases, iv. gases deriving from productions carried out in chemical and in electrical power plants; v. purge gas from the ammonia synthesis loop; vi. mixtures containing hydrocarbons derived from biomass fermentation, or biogas.

The gases other than gaseous hydrocarbons comprise one or more of the following elements or compounds: hydrogen, nitrogen, carbon monoxide, carbon dioxide.

It is interesting to note that the exhaust gases derived from other industrial processes and the gases containing hydrocarbons produced from biomass partially replace the use of other fossil fuels as feedstock and avoid greenhouse gas (GHG) emissions. Moreover, the use of the purge gas from the ammonia synthesis loop increases the yields of ammonia and urea. These objectives are obtained using short contact time - partial oxidation (SCT-CPO) reaction reactors.

Furthermore, the embodiments of the process described herein also comprise water gas shift (WGS) units and/or Sorption Enhanced Water Gas Shift (hereinafter also SEWGS) units, and electrolytic systems to contribute to the production of the oxidant flows required by SCT-CPO reactors and to balance the H2/N2 ratios in the ammonia synthesis reactor.

In all these embodiments the main objectives concern the use of CO2 contained in the industrial exhaust gases and the reduction of greenhouse gas (GHG) emissions.

Therefore, the process according to the invention includes unit operations performed in the following main equipment: i) NFE/Urea synthesis reactors; ii) SCT-CPO reactors; iii) WGS reactors and, in some cases, SEWGS reactors; iv) Electrolyzers for alkaline electrolysis (AE), proton exchange membrane electrolysis (PEME) and solid oxide electrolyzer cell (SOEC) electrolysis; v) optionally, air separation units (ASU) or vacuum pressure swing adsorption (VPSA) units. Moreover, in the process of the invention the purge gas coming from the ammonia synthesis loop is treated in the SCT-CPO reactor, with conversion of the purge gas to a stream that provides additional amounts of H2 and N2.

More in detail, the process of the invention allows treatment of the exhaust gases of iron ore reduction processes, i.e., COG, BOFG, BFG and alternatively DRG and SRG in (i) an H2/N2 rich synthesis gas and (ii) a CO2 rich stream that can be used for the production of ammonia and urea.

In the case of use of a feedstock consisting of exhaust gases such as BFG and BOFG, containing hydrocarbons, typically methane, in an amount less than 10% vol, or less than 5% vol, the process of the invention is represented with the embodiments of Figs. 2-9.

These embodiments use the SCT-CPO reaction and the SCT-CPO reactor mainly to convert the purge gas produced by the ammonia synthesis loop to syngas, to be treated together with the feedstock.

In an embodiment, pure oxygen obtained through hydrolysis of water or through air separation is used. As shown in the figures, the gases containing a small amount of hydrocarbons, such as BFG and BOFG, after a first section to remove the sulfur and a cleaning section, not illustrated, are treated in a WGS unit and in a PSA unit (Figs. 2, 3, 6 and 7), or in a SEWGS unit (Figs. 4, 5, 8 and 9) before reaching a methanation reactor, where the small amounts of CO and CO2 remaining in the synthesis gas are converted to CH4. The gas enriched in hydrogen and nitrogen is then sent to the ammonia synthesis reactor.

The SEWGS unit uses a catalyst-adsorbent material that promotes the conversion of CO to EE and simultaneously adsorbs the CO2, promoting the shift of equilibrium toward the production of hydrogen. The adsorption of CO2 promotes the equilibrium of the reaction toward the production of hydrogen removing a product of the WGS reaction, and simultaneously allows the removal of CO2 from the syngas.

The references that describe the SEWGS technology mentioned herein are: (a) Cobden, Walspurger, Van Den Brink and Van Dijk, W02010/059055; (b) Van Dijk, Cobden, Walspurger and Dijkstra, WO2013/122467; (c) Vente, Sfakianakis and Cobden, W02020/025815.

The mixture rich in EE and N2 produced by the PSA and/or SEWGS unit is treated in a methanation reactor to convert the remaining CO and CO2 to CH4.

CO + 3H₂ CH₄ + H2O AH° = -206 kJ/mole [6] AH° = -165 kJ/mole [11]

This pass is necessary as CO and CO2 are poisons for the catalyst that produces ammonia, while CH4 acts as an inert gas in the ammonia synthesis loop.

The flow delivered from the methanation stage is first cooled, condensing and removing the steam produced in the methanation reaction, obtaining a low temperature saturated gas. The syngas must then be further treated in a drying unit to remove the saturated steam still present. The purified and dried syngas with an H2/N2 ratio between 2.2 and 3 v/v is sent to step a) of the process, so as to implement the ammonia synthesis loop.

The embodiments of the process represented in Figs. 6-9 are focused on the consumption of excess CO2 which, if it cannot be used in urea synthesis, is converted to synthesis gas with the SCT-CPO reaction and reactor, if necessary, using an additional stream containing hydrocarbons, preferably natural gas.

With regard to recycling of the purge gas in the ammonia synthesis loop, this recycling to the SCT-CPO reactor can be carried out either without removing the residual ammonia (Figs. 2, 4, 6 and 8), or removing the residual ammonia (Figs. 3, 5, 7 and 9). It must be observed that, in known ammonia synthesis processes, a part of the ammonia synthesis recirculation gas must be constantly purged to avoid the accumulation of inert gases, mainly methane and argon.

The purge gas essentially contains ammonia, nitrogen, hydrogen and inert gases (Table 2). The dimension of this purge stream controls the level of inert gases in the loop, keeping it at around 10-15% of the recirculating gas. In conventional plants, the purge gas is treated with water to eliminate the ammonia, before being used as fuel in steam methane reforming (SMR) burners or sent for hydrogen recovery.

In the process of the invention the purge gas is not used as fuel because there are no burners, as combustion is avoided in order to reduce the amount of emissions generated. The process thus converts the purge gas to a syngas mainly composed of H2, N2, CO and CO2 by SCT-CPO reaction.

It has been found that the use of a specific type of SCT-CPO reactor, as described below, allows recycling of the purge gas in the process.

The reactions in the SCT-CPO reactor convert methane and ammonia to hydrogen, nitrogen and carbon oxides that can be recirculated in the WGS or SEWGS unit, according to the different embodiments illustrated, to produce further fresh make-up syngas in the ammonia synthesis loop.

These characteristics are also exploited in the diagrams of Figs. 10-13, in which the SCT-CPO reactor is fed both with the purge gas of the ammonia synthesis loop, and with gaseous hydrocarbons such as NG, gases derived from biomass and from other industrial processes. In these cases, operation of the SCT-CPO unit precedes steps a) of WGS or SEWGS, and b) of PSA.

The embodiments of the process represented in Figs. 14-17 describe process diagrams analogous to those of Figs. 10-13, respectively, and are focused on consumption of the excess CO2 which, if it cannot be used in the synthesis of urea, is converted to synthesis gas with the SCT-CPO reaction and reactor, if necessary using an additional stream containing gaseous hydrocarbons (preferably natural gas).

The SCT-CPO reactor requires moderate preheating of the mixtures of reagents, and this preheating can be obtained by recovering the heat obtained from cooling of the synthesis gas produced, thus avoiding the use of burners and the associated CO2 emissions.

Moreover, pure oxygen, just as air or enriched air, can be used as oxidants in the SCT-CPO reactor to balance the H2/N2 ratio for ammonia synthesis. The adoption of specific characteristics for the SCT-CPO reactors allows synthesis gas rich in H2-N2 suitable for ammonia synthesis to be obtained.

Finally, the ammonia produced by the synthesis cycle can be used with the CO2 captured and separated in PSA or SEWGS units (according to the different diagrams proposed), to produce urea.

More particularly, Figs. 2 and 3 illustrate embodiments for the use of BFG and BOFG in the production of ammonia and urea.

The BFG and the BOFG, after a purification step, not illustrated, that removes compounds containing sulfur and other impurities, are converted to a gas rich in H2, N2 and CO2 through an initial WGS step (step a).

In the next step b) the CO2 produced is separated from the stream of H2 and N2 with a PSA unit. In step c) the mixture rich in H2 and N2 coming from step b) is subjected to the methanation reaction, to convert the remaining CO and CO2 to CPU (reactions [6] and [11] indicated above). In step d) the mixture of H2 and N2 is fed into the ammonia synthesis reactor.

In step e) the purge gas of the ammonia synthesis reactor is sent to an SCT-CPO unit. It has been observed that the purge gas can be used directly in the SCT-CPO reactor (Fig. 2) or used in the SCT-CPO reactor after separation of the residual ammonia by scrubbing (Fig. 3). In both cases, use of the SCT-CPO reactor allows the methane and the ammonia of the purge gas to be converted to H2 and N2, which are used in step f) as make-up gas of the feedstock, producing the ammonia synthesis loop.

The oxygen required by the SCT reactor can be supplied by electrolysis of steam and/or through an air separation unit (ASU). Electrolysis of steam also produces a stream of H2 that is used to balance the H2/N2 ratio useful for the ammonia synthesis cycle.

In step g) of urea synthesis, the ammonia produced by the synthesis loop and the CO2 coming from the separation unit are used.

Figs. 4 and 5 describe embodiments of the process for converting BOFG and BFG to ammonia and urea with the use of the SEWGS unit to separate the CO2 in step b).

Subsequently, the CO2 is separated from the stream of H2-N2 in an SEWGS unit, increasing conversion to H2 thanks to capture and separation of the CO2.

In the embodiments of the process with SEWGS unit (Figs. 4, 5, 8, 9, 12, 13, 16, 17), it is possible to locate the desulfurization section downstream of the SEWGS unit, as this technology can operate with catalysts and temperature conditions in which sulfur poisoning does not occur. Positioning of the desulfurization section downstream of the SEWGS unit offers the following advantages: (i) the SEWGS unit is capable of adsorbing acid compounds (such as H2S) and removing them from the stream of H2-N2; (ii) the desulfurization section can be designed for a lower capacity due to the removal of CO2 and to the lower composition of sulfur in the stream of H2-N2. The stream of H2-N2 thus obtained is compressed and used in the ammonia synthesis loop to produce ammonia, according to step d).

In step e) the purge gas of the ammonia synthesis loop is sent directly (Fig. 4) to an SCT-CPO reactor or scrubbed to remove the residual ammonia (Fig. 5) before recycling to the SCT-CPO reactor. Therefore, the use of the SCT-CPO reactor allows the methane and ammonia of the purge gas to be converted to H2 and N2, which can be used as make-up gas in the ammonia synthesis loop in step f).

The streams of pure oxygen can be supplied to the SCT-CPO reactor by electrolysis of steam and/or through an air separation unit (ASU). Electrolysis of water or steam also produces a stream of H2 which is used to balance the H2/N2 ratio useful for ammonia synthesis.

In the urea synthesis step g), the ammonia produced by the synthesis loop in step d) and the CO2 coming from the separation unit of step b) are used.

Figs. 6 and 7 show embodiments that include, in addition to the process units of Figs. 2 and 3, the possibility of re-using any excess CO2 emissions coming from step b) in the SCT-CPO reactor, producing in step e) additional feedstock for the ammonia and urea reactors.

Figs. 8 and 9 show embodiments that include, in addition to the process units of Figs. 4 and 5, the possibility of re-using any excess CO2 emissions in the SCT-CPO reactor, producing additional feedstock to use in step f) to increase the ammonia and urea production loop.

Figs. 10 and 11 illustrate embodiments of the process for the use of gaseous hydrocarbons (preferably natural gas) with the possible addition of exhaust gases such as BFG, COG, BOFG, DRG, SRG and biogas, in which, after a cleaning section (not shown), they are sent in a step ao) to an SCT-CPO reactor, also using a stream of O2 generated by the steam electrolyzer and/or by an ASU.

The synthesis gas produced is then sent to the WGS step a), then to step b) of CO2 removal by PSA, then to step c) of purification by methanation and finally fed to the ammonia synthesis reactor in step d).

In step e) the purge gas from the ammonia synthesis loop is sent directly (Fig. 10), or after an ammonia scrubbing section (Fig. 11), to the same SCT-CPO reactor. The stream of ammonia and CO2 coming from the separation unit is used in the urea synthesis step.

Figs. 12 and 13 describe embodiments of the process for the use of gaseous hydrocarbons, preferably natural gas, with the possible addition of exhaust gases such as BFG, COG, BOFG, DRG, SRG and biogas that, after a cleaning section (not shown), are sent, in step ao), to an SCT- CPO reactor also using the stream of O2 generated by a steam electrolyzer or by an ASU.

The synthesis gas produced is then sent to the SEWGS step a), then to step b) of CO2 removal by PSA, then to step c) of purification by methanation and finally fed to the ammonia synthesis reactor in step d).

The purge gas from the ammonia synthesis loop is sent directly (Fig. 12), or after an ammonia scrubbing section (Fig. 13), to the same SCT-CPO reactor, according to step e).

The streams of ammonia and CO2 coming from the separation unit are used in the urea synthesis step g).

Figs. 14 and 15 include, in addition to the process units of Figs. 10 and 11, re-use of any excess CO2 emissions in the SCT-CPO reactor, producing additional feedstock for the synthesis of ammonia and urea.

Also Figs. 16 and 17 show embodiments that include, in addition to the process units of Figs. 12 and 13, the use of any excess CO2 emissions in the SCT-CPO reactor, producing additional feedstock for the ammonia and urea reactors.

Another aspect of the invention concerns a reactor for carrying out the SCT-CPO reaction.

It was found that the use of SCT-CPO reactors and of specific operating conditions improves the possibility of converting a stream rich in CO2 to a syngas rich in H2 and CO.

Figs. 18 and 19 show the characteristics of an SCT-CPO reactor according to the invention (Fig. 18B) and of a prior art reactor (Fig. 18 A).

The thermochemical properties of the reaction environment produced in an SCT-CPO reactor useful for treating the mixtures described above in relation to the ammonia synthesis process can be discussed considering the system composed by the equations [1-3], [6-8] and [12-13], CH₄ + 2O₂ CO₂ + 2H₂O AH° = -803 kJ/mol [12]

CH₄ + ’A O₂ CO + 2 H₂ AH° = -36 kJ/mol [13]

CO + H2O CO₂ + H₂ AH° = - 41 kJ/mol [7]

CH₄ + CO₂ 2CO + 2 H₂ AH° = +247 kJ/mol [8]

CH₄ + H2O 3H₂ + CO AH° = +206 kJ/mol [6]

The exothermic total oxidation reaction [12] has the highest probability of being located at the beginning of the catalyst bed, while the endothermic steam-CO2 reforming reactions [1] and [8] and the slightly exothermic WGS reaction or the RWGS reaction [2] have the highest probability of occurring in the subsequent zone. It must be noted that by increasing the reaction temperature above 830°C, the reaction [2] is shifted to the left side and a slightly endothermic RWGS reaction together with the steam-CCh reforming reaction [1] and [8] is favored.

Moreover, it has been found that the extent and the location of these reactions is greatly influenced by physical and chemical factors. Total combustion [12] proved to be the most competitive reaction on noble-metal based catalysts (Rh, Ru, Ir, Pt, Pd) at "low temperature" (T<750°C) and at high O2 partial pressure. These are typically the conditions that are produced at the beginning of the catalyst beds in tubular reactors operating with “high” contact times (in excess of 1 s). In these cases, it has been found that the thermal profiles of the reaction environments are determined by the highly exothermic reaction [8] with a smaller contribution of the reactions [13] and [2], followed by the highly endothermic steam-CCh reforming reactions [1] and [8], These conditions give rise to very high axial temperature gradients. Moreover, the energy release associated with total combustion also determines the propagation of heterogeneous reactions in the gaseous phase, giving rise to non-selective free radical chemistry that leads to the formation of unsaturated molecules and soot.

It has in fact been found that with tubular reactors, catalytic partial oxidation reactions cannot be carried out at high pressure, as the reaction [12] is not controllable and propagates the reactions in the gaseous phase with the risk of ignition of the flame and production of free radical reactions that lead to the formation of unsaturated hydrocarbons and soot.

It has instead been found that the temperatures of the solid catalyst can reach values exceeding 1000°C, while the gas remains relatively cold, when using a geometry of the reaction environment that allows the contact time at the inlet of the catalyst bed to be reduced to a few milliseconds and that allows expansion of the reaction volume when the temperatures and the molar flow increase due to advance of the reaction. This effect is obtained by adopting a truncated-cone shaped geometry of the catalyst bed, together with geometrical characteristics of the catalyst that allow the pressure drop inside the reaction zone to be reduced.

Consequently, it has been found that in these conditions of short contact time, methane conversion depends mainly on the O2/C ratios, while it remains almost unchanged by the addition of steam and CO2. This addition instead changes the H2/CO ratios in the synthesis gas produced, clearly indicating that the reactivity is mainly determined by direct partial oxidation [13] and by the RWGS reaction [2],

Figs. 18A and 18B show qualitative images of the enthalpy/temperature profiles obtained with (18 A) low mass temperature, high contact time, tubular geometry of the catalyst bed and (18B) high mass velocity, short contact time, truncated-cone shaped geometry of the catalyst bed. Fig. 19 shows the main zones of the SCT-CPO reactor, including a truncated-cone shaped reaction zone. These zones comprise:

- an inlet and mixing zone having a cylindrical shape;

- a first pre-heating zone of the heat shield having a cylindrical shape;

- a reaction zone including a catalyst, having a truncated cone shape with a radius Ri of the inlet section and a radius R2 of the outlet section and a height L;

- a second heat shield zone having a cylindrical shape;

- an exit zone from the reactor having a cylindrical shape;

- the external angle a of the inlet section of the truncated cone shaped reaction zone is less than 90°;

The external angle a shown in Fig. 19 is preferably less than 80°, more preferably between 75° and 30°.

The other geometrical characteristics, namely: i. the inlet radius of the truncated cone Ri, ii. the outlet radius of the truncated cone R2, iii. the height of the truncated cone L, and iv. the filling of the catalyst bed are designed to allow pressure drop (PD) values in the catalyst bed between 0.1 and 10 ATM, preferably between 0.5 and 5 ATM.

For this purpose, the R1/R2 ratios are between 0.9 and 0.1, preferably between 0.8 and 0.4. Moreover, fillings of the catalyst bed that minimize the pressure drop conditions are preferred, using pellet or monolithic structures and combinations thereof.

Furthermore, the existence of a non-thermal equilibrium between the gaseous and the solid phase was explained considering that the chemical value generated on the surfaces, and emitted by irradiation, is absorbed and dissipated much better by the solid phase than by the gaseous phase and is transferred along the catalyst bed from the hotter points toward the colder points, equalizing the surface temperatures of the solid.

The main experimental observations on the thermochemical properties of the environments of the SCT-CPO reactors, carried out after optimization of the characteristics of the reaction environment, are summed up as follows:

- the temperature of the solid phase increases considerably at the beginning of the bed and the temperature profiles are leveled through radiative and conduction mechanisms in axial and radial direction;

- temperature differences originate between the gaseous phase and the solid phase; - part of the surface temperatures are higher than the adiabatic temperatures;

- the temperatures of the gas are always lower than the adiabatic temperatures and increase gradually from the inlet to the outlet of the bed.

It has also been found that part of the reaction heat is transferred toward the inflowing reagents inside the first heat shield and in this way pre-heating of the reagents inside the reactor is also obtained.

An SCT-CPO reactor with the aforesaid characteristics is optimal for conduction of the process according to the invention.

Claims

1. Process for the synthesis of ammonia and urea by reaction of said ammonia and carbon dioxide in which:

- the process uses as feedstock a gaseous mixture including gases selected from the group consisting of: gaseous hydrocarbons, gases emitted as by-products of refinery processes, gases emitted as by-products of industrial processes other than refinery processes, gases emitted as by-products of iron ore reduction processes , gases derived from biomass fermentation ; wherein the gases other than gaseous hydrocarbons comprise one or more of the following elements or compounds: hydrogen, nitrogen, carbon monoxide, carbon dioxide;

- said gaseous mixture is preliminarily treated to eliminate or reduce the impurities selected from the group consisting of: sulfur compounds, aromatic organic compounds and particulate matter; characterized by comprising the following steps: a) treatment of said gaseous mixture constituting said feedstock with the water gas shift reaction, which increases the hydrogen content of the mixture; b) removal or reduction of the carbon dioxide content of the mixture treated in said step a), obtaining a stream containing carbon dioxide obtained by such removal or reduction; c) purification of hydrogen and nitrogen before the ammonia synthesis reaction, where said purification comprises the methanation reaction, d) ammonia synthesis reaction through the reaction of hydrogen and nitrogen contained in said mixture, with the unreacted part constituting a recycling stream, a part of which is purged to avoid the accumulation of inert substances, e) short contact time catalytic partial oxidation reaction ("SCT-CPO") of said purged part of said recycling stream by means of oxygen, obtaining synthesis gas ("syngas") having a content of hydrogen and carbon oxide greater than the content of hydrogen and carbon oxide of said purge stream; f) feeding said synthesis gas to said step a) and repeating the cycle of steps from a) to f); g) reaction of the ammonia obtained in said step e) with the carbon dioxide obtained in said step c), whereby urea is obtained.

2. Process according to claim 1, characterized in that said water gas shift (WGS) reaction of said step a) is a Sorption Enhanced Water Gas Shift (SEWGS) reaction.

3. Process according to one or more of the previous claims, characterized in that a part of said carbon dioxide obtained in said step b) is subjected to the SCT-CPO reaction in the reactor in which said step e) of catalytic partial oxidation of said purge stream is carried out.

4. Process according to one or more of the previous claims, characterized in that said step e) is carried out using oxygen chosen from: pure oxygen, oxygen contained in air, oxygen contained in air enriched in oxygen.

5. Process according to claim 4, characterized in that said step e) is carried out using pure oxygen produced by electrolysis of water or steam, with also production of hydrogen which is used to balance the H2/N2 ratio of the mixture fed to said step d) of ammonia synthesis.

6. Process according to one or more of the previous claims, characterized in that said gaseous mixture constituting the feedstock of the process contains less than 10% vol of methane, preferably less than 5% vol of methane.

7. Process according to one or more of the previous claims, characterized in that said gaseous mixture constituting the feedstock of the process is subjected to the short contact time catalytic partial oxidation (SCT-CPO) reaction for the production of synthesis gas in a step aO), before being treated with the water gas shift reaction in said step a).

8. Process according to one or more of the previous claims, characterized in that said removal or reduction of the carbon dioxide content of the treated mixture of said step b) is achieved by physical and/or chemical adsorption/desorption of said carbon dioxide.

9. Plant for the production of urea from ammonia and carbon dioxide which uses as feedstock a gaseous mixture including gases selected from the group consisting of gaseous hydrocarbons, gases emitted as by-products of refinery processes, gases emitted as by-products of industrial processes other than refinery processes, gases emitted as byproducts of iron ore reduction processes, gases derived from biomass fermentation, including:

A. a water gas shift (WGS) reaction reactor;

C. a methanation reactor;

D. an ammonia synthesis reactor,

E. a reactor for the synthesis of urea from ammonia and carbon dioxide; characterized by also comprising: F. an SCT-CPO reactor connected to said ammonia synthesis reactor (D) and said water gas shift reaction reactor (A), configured to convert the purge gases of said ammonia synthesis reactor (D) to synthesis gas and to feed said synthesis gas to said water gas shift reaction reactor (A);

G. means for feeding carbon dioxide removed in said one or more units (B) to said reactor for the synthesis of urea (E).

10. Plant according to claim 9, characterized in that said SCT-CPO reactor (F) comprises:

- an inlet and mixing zone having a cylindrical shape;

- a first pre-heating zone of the heat shield having a cylindrical shape;

- a reaction zone having a truncated cone shape with a radius Ri of the inlet section and a radius R2 of the outlet section and a height L;

- a second heat shield zone having a cylindrical shape;

- an exit zone from the reactor having a cylindrical shape; wherein the external angle a of the inlet section of the truncated cone shaped reaction zone is less than 90°.

11. Plant according to claim 10, characterized in that said radii Ri and R2 of said truncated cone shaped reaction zone have an R1/R2 ratio between 0.9 and 0.1, preferably between 0.8 and 0.4.