Process and plant to decompose ammonia

Abstract

A process and a related plant for decomposing ammonia and synthesizing a hydrogen-rich process stream by a direct cooling performed downstream of the decomposition reactor with a stream consists of, or essentially consists of, nitrogen and / or ammonia, where the decomposition of ammonia takes place in a thermal or autothermal chemical reactor.

Description

[0001] Giovanni Manenti

[0002] PROCESS AND PLANT TO DECOMPOSE AMMONIA

[0003] DESCRIPTION

[0004] Scope

[0005] The present invention covers a process, and related plant, to decompose ammonia according to the reaction 2NH3i=»3H2+N2 and synthesize a process stream rich in hydrogen, where the decomposition of ammonia takes place in a thermal or autothermal chemical reactor.

[0006] Technical problem to be solved

[0007] The decomposition of ammonia into hydrogen and nitrogen is an endothermic process carried out in a chemical reactor conducted at a high temperature, usually in the presence of a catalyst. Such a decomposition process is nowadays a possible, major step of hydrogen economy and clean energy transition.

[0008] The decomposition reactor can be of thermal type, i.e. a reactor where the reaction heat is transferred to the ammonia stream to be decomposed by radiative and / or indirect convective heat exchange, or of autothermal type, i.e. a reactor where the reaction heat is obtained directly within the ammonia stream to be decomposed by oxidation of a portion of the ammonia itself.

[0009] The temperature of the effluent gas from the reactor can range from about 300°C up to 1100°C, depending on the type of reactor and catalyst. The pressure is usually more than 0.5 MPa(a) , often more than 3e5MPa(a) .

[0010] The effluent gas discharged from the decomposition reactor must be cooled for subsequent treatment operations .

[0011] However, the high temperatures and pressures, together with the high concentration of ammonia and hydrogen, make the effluent gas chemically aggressive relative to the steels used for heat exchangers; for example, the effluent gas can cause local overheating and corrosive phenomena such as hydrogen diffusion Giovanni Manenti cracks and nitriding . It is therefore often necessary to use expensive metal alloys with a high chromium and / or nickel content .

[0012] Therefore , the cooling of the high-temperature effluent gas , possibly by a reliable and relatively economic method, is a problem in the ammonia decomposition processes .

[0013] On the other hand, the use of process boilers located directly downstream of the decomposition reactor, based on the use of high- pressure boiling water as a refrigerant , require complex and expensive plant systems . Moreover , it should be also emphasized that the production of steam obtained from cooling of high- temperature process gases in the so-called "green" plants can be disadvantageous when electricity from renewable sources is available . Consequently, how to improve the energy efficiency of conventional ammonia decomposition plants , i . e . how to reduce possible production of steam by processing boiler or how to optimize heat exchange , is another problem arising in ammonia decomposition processes .

[0014] Scope of the invention

[0015] The present invention makes available a process for the decomposition of ammonia , and for the subsequent production of a hydrogen-rich stream, which is an alternative to conventional processes .

[0016] More specifically, the present invention makes available a process for the decomposition of ammonia where corrosion and overheating phenomena related to the heat exchangers , located downstream of the decomposition reactor , are mitigated or eliminated .

[0017] The present invention also makes available a process for reducing thermomechanical stress and size of the heat exchangers apt to cool the hot effluent gas and located downstream of the decomposition reactor . Giovanni Manenti

[0018] The present invention also makes available a process for eliminating one or more heat exchangers apt to cool the hot effluent gas and located downstream of the decomposition reactor .

[0019] The present invention also makes available a process to improve the energy efficiency of the ammonia decomposition plant , for example by decreasing the heat load and the related steam production of the process boiler downstream of the decomposition reactor .

[0020] Finally, the present invention makes available a plant , alternative to prior-art , for the decomposition of ammonia and the synthesis of a hydrogen-rich stream .

[0021] Prior-art

[0022] Patent documents Nos . EP4112540A1 , US3451783A, EP3430251B1 , WO2019038251A1 and W02024149889A1 describe processes for the decomposition of ammonia .

[0023] Doc . No . EP4112540A1 describes a process where ammonia is decomposed in two decomposition reactors , and the decomposition gas is subj ected to a direct cooling . A first hot decomposition gas , rich in hydrogen, exiting from the first reactor is subsequently cooled and then mixed with a second hot decomposition gas , rich in hydrogen, exiting from the second reactor . Therefore , as described in Fig . 1 and in paragraph 0038 of doc . No EP4112540A1 , the direct cooling of the second hot stream rich in hydrogen takes place using the first cold stream rich in hydrogen .

[0024] Doc . No . US3451783A describes a process wherein ammonia is decomposed in a reactor and the hot decomposition gas , rich in hydrogen, is divided into two hot streams . One stream is firstly cooled and then mixed with the other hotter stream. Therefore , as described in Fig . 1 and in column 3 paragraphs 24 -49 of Doc . No . US3451783A, the direct cooling of the hot stream rich in hydrogen takes place using a cold stream rich in hydrogen . Giovanni Manenti

[0025] Doc . No . EP3430251B1 describes a process where the hot effluent gas from the thermal decomposition reactor is firstly indirectly cooled in a heat exchanger by preheating the ammonia stream to be decomposed, and then it is further indirectly cooled in a cooling unit .

[0026] Doc . No . WO2019038251A1 describes a process where the decomposition reactor is of autothermal type .

[0027] Doc . No . WO2024149889A1 , which is a prior-art document close to the present invention, describes a process wherein the hot effluent from thermal decomposition reactors is firstly cooled in an adiabatic decomposition reactor and then cooled by direct and / or indirect heat exchange , where the direct heat exchange takes place by mixing with water, steam or ammonia .

[0028] Brief description of the invention

[0029] The process here disclosed comprises a cooling operation, by direct heat exchange , of the hot and hydrogen-rich gaseous effluent exiting from a thermal or autothermal ammonia decomposition reactor .

[0030] The cooling takes place by mixing the high-temperature effluent with a cold stream of nitrogen and / or ammonia .

[0031] The direct cooling of the hot effluent is accomplished directly downstream of the decomposition reactor and upstream of any subsequent treatment operation on the hydrogen-rich stream .

[0032] The thermal ammonia decomposition reactor , described in this process and plant , is a reactor where the ammonia stream to be decomposed is subj ected to heating by indirect heat transfer, radiative or convective .

[0033] The autothermal ammonia decomposition reactor , described in this process and plant , is a reactor where a portion of the ammonia stream to be decomposed is oxidized with a gas containing oxygen in order to obtain heating directly within the stream . Giovanni Manenti

[0034] Consequently, thermal or autothermal reactors described here are not of adiabatic type and, preferentially, are of catalytic type .

[0035] The direct cooling obj ect of this invention applies to gaseous effluents discharged from thermal or autothermal reactors ; conversely, the direct cooling described in the prior-art document No . WO2024149889A1 applies to gaseous effluents discharged from adiabatic reactors .

[0036] The plant here disclosed is based on the installation of a cooling inj ection line directly downstream of the thermal or autothermal reactor .

[0037] The process and plant here disclosed are particularly advantageous for existing ammonia decomposition plants based on thermal or autothermal reactors ; the process and plant here disclosed allow a rapid and relatively economic implementation in existing plants to improve relevant performance or to mitigate / eliminate overheating or corrosion problems .

[0038] Attached Figs . 1 and 2 show two conceptual schemes of the process and relevant plant in accordance with, respectively, a first and a second preferential embodiment of the present invention .

[0039] Detailed description of the invention

[0040] The conceptual scheme of Fig . l comprises , in sequence , a reaction unit ( RU) , at least one cold stream ( 3 ) for direct cooling, and a treatment unit ( TU ) .

[0041] The reaction unit ( RU) comprises at least one thermal or autothermal reactor apt to the decomposition of ammonia at high temperature according to the reaction 2NH3i=»3H2+N2 .

[0042] The treatment unit (TU) comprises at least one equipment apt to perform a unit operation such as phase separation, chemical species separation, distillation / rectif ication or chemical Giovanni Manenti reaction; the treatment unit (TU) is therefore apt to treat the hydrogen-rich stream synthesized by the ammonia decomposition.

[0043] The cold stream (3) carries the cooling fluid.

[0044] According to Fig.l, a feed stream (1) rich in ammonia to be decomposed is fed into the reaction unit (RU) , whereas a hot gaseous effluent stream (2) , rich in hydrogen and obtained from the ammonia decomposition, exits the reaction unit (RU) . The temperature of the hot effluent stream (2) is above 300°C, preferably above 500°C, and even more preferably above 700°C. The pressure of the hot effluent stream (2) is higher than 2MPa(a) , preferably higher than 5MPa(a) , or higher than 8MPa(a) . Preferably, the hot effluent stream (2) has a molar hydrogen concentration higher than 20%, preferably higher than 30%, and even more preferably higher than 40%.

[0045] According to Fig.l, the cold stream (3) consisting or essentially consisting of nitrogen and / or ammonia is introduced into the hot effluent stream (2) at at least one mixing point (7) located directly downstream of the reaction unit (RU) . The cold stream (3) has a temperature lower than the temperature of the hot effluent stream (2) at the mixing point (7) . The temperature of the cold stream (3) is preferably below 50°C, more preferably below 0°C, and even more preferably below -50°C. Preferably, the cold stream (3) is in the liquid phase. Preferably the cold stream is nitrogen and / or ammonia under cryogenic conditions. Consequently, the cold stream (3) of the present invention has a chemical composition substantially different from the chemical composition of the cold stream (rich in hydrogen) described in prior-art documents No. EP4112540A1 and US3451783A. It is emphasized that the use of a cold stream (3) consisting or essentially consisting of nitrogen and / or ammonia to cool the hot effluent stream (2) is neither an obvious operation for an expert in the field nor an operation obviously deducible from prior-art documents Nos. EP4112540A1 and US3451783A. The non-obviousness arises from the fact that the cold stream (3) needs for pure or Giovanni Manenti essentially pure chemical species (nitrogen and / or ammonia) and in cold conditions, which preparation requires operations other than the decomposition of ammonia. In other words, the cold stream (3) of the present invention cannot be directly obtained by the ammonia decomposition operation.

[0046] According to Fig.l, the cold stream (3) is mixed with the hot effluent stream (2) to obtain a first cold effluent stream (4) , rich in hydrogen. The mixing of the two streams (2,3) corresponds to a direct cooling operation of the hot effluent stream (2) .

[0047] As per a preferential embodiment of the present process, the first cold effluent stream (4) is in gaseous phase; in other words, the cooling fluid of the cold stream (3) vaporizes completely if injected in liquid phase.

[0048] As per a preferential embodiment of the present process, the direct cooling lowers the temperature of the hot effluent stream (2) by at least 10°C, more preferably by at least 30°C, and even more preferably by at least 50°C.

[0049] According to Fig.l, the first cold effluent stream (4) is fed into the treatment unit (TU) ; a processed stream (6) rich in hydrogen exits from the treatment unit (TU) .

[0050] Therefore, according to the conceptual and preferential scheme of Fig.l, the process object of the present invention includes the following operations in sequence:

[0051] The introduction of an ammonia-rich feed stream (1) to be decomposed into a reaction unit (RU) ;

[0052] The decomposition of the ammonia in the reaction unit (RU) according to the reaction 2NH3i=»3H2+N2 by means of at least one thermal and / or autothermal reactor to obtain a hot gaseous effluent stream (2) , rich in hydrogen;

[0053] The injection of a cold stream (3) consisting of or essentially consisting of nitrogen and / or ammonia into the hot effluent stream (2) ; Giovanni Manenti

[0054] The direct cooling of the hot effluent stream (2) by mixing with the cold stream (3) to obtain a first cold effluent stream (4) , rich in hydrogen;

[0055] The introduction of the first cold effluent stream (4) into the treatment unit (TU) to perform a phase separation, a chemical species separation or a chemical reaction, to obtain a processed stream (6) rich in hydrogen.

[0056] It is emphasized that according to the present invention the gaseous effluent stream (2) existing from the reaction unit (RU) is directly discharged from a thermal or autothermal reactor.

[0057] The conceptual scheme reported in Fig .2 corresponds to the scheme reported in Fig.l except for the installation of a cooling unit (CU) placed between the reaction unit (RU) and the treatment unit (TU) and, more specifically, placed between the point of injection (7) of the cold stream (3) and the treatment unit (TU) . As a result, the description of Fig .2 is partially omitted by referring to the description of Fig.l for some details.

[0058] The cooling unit (CU) in Fig.2 receives the first cold effluent stream (4) , cools this stream further and discharges a second cold effluent stream (5) , rich in hydrogen. The second cold effluent stream (5) is fed into the treatment unit (TU) to obtain the processed stream (6) rich in hydrogen.

[0059] The cooling unit (CU) cools the first cold effluent stream (4) by means of indirect heat exchange, using heat exchangers, such as process boilers or superheaters, and auxiliary cooling fluids.

[0060] Consequently, in accordance with the conceptual and preferential scheme of Fig.2, the process covered by the present invention comprises the operations as described for Fig.l with the addition of the cooling operation by indirect heat exchange of the first stream of cold effluent (4) .

[0061] In accordance with an alternative and preferential embodiment of this process and related system, not shown in the figures, the Giovanni Manenti cold stream (3) is fed into the cooling unit (CU) ; in other words, the mixing between the cold stream (3) and the hot effluent stream (2) takes place either within a heat exchanger or between two heat exchangers .

[0062] It is emphasized that the nitrogen and / or ammonia of the cold stream (3) mix with the nitrogen and ammonia of the hot effluent stream (2) , so the process disclosed here does not substantially change the chemistry of the decomposition process. However, it must be underlined that, in accordance with a preferential embodiment of the present process and in accordance with the purposes of the plant itself, the cold stream (3) consists or essentially consists of nitrogen.

[0063] It is emphasized that the ammonia of the cold stream (3) is preferably feed ammonia (1) , obtained from the feed stream (1) or from a point upstream of the feed stream (1) . The amount of ammonia that may be injected by the cold stream (3) is preferably minimized .

[0064] It should be noted that the cold stream (3) is carried by a pipe that can be easily joined to the hot effluent stream (2) or to a heat exchanger of the cooling unit (CU) . The injection of the cold fluid into the hot fluid is preferably carried out with an injection device, such as a nozzle or distributor.

[0065] The process and related plant described here, as per above details, presents the following advantages:

[0066] The process and the plant can be implemented with a relatively economic and rapid service activity in existing plants ;

[0067] Potential corrosion or overheating phenomena in conventional heat exchangers can be mitigated or eliminated by injecting the cold stream (3) and pre-cooling the hot effluent ( 2 ) ; Giovanni Manenti

[0068] The energy balance of the plant can be improved by exploiting the low enthalpy of nitrogen and / or liquid ammonia and reducing any steam production;

[0069] Nitrogen and / or liquid ammonia from the cold stream ( 3 ) can be pumped, rather than compressed .

[0070] The process and relevant plant described here are subj ect to further modifications and variations , all attributable to the same inventive concept , as well as to include additional equipment and devices without changing the essence of the invention .

[0071] The scope of protection of the present invention is described in the attached claims .

Claims

Giovanni ManentiPROCESS AND PLANT TO DECOMPOSE AMMONIACLAIMS1. Process for decomposing ammonia, according to the decomposition reaction 2NH3i=»3H2+N2 , including:The introduction of an ammonia-rich feed stream to be decomposed (1) into a reaction unit (RU) ;The decomposition of ammonia in the reaction unit (RU) to obtain a hot gaseous effluent stream (2) , rich in hydrogen; The cooling of the hot effluent stream (2) , directly downstream of said decomposition, to obtain a first or a first and second cold effluent stream (4,5) , rich in hydrogen;The introduction of the first or second cold effluent stream (4,5) into a treatment unit (TU) ;Phase separations, chemical species separations and / or conduction of one or more chemical reactions in said treatment unit (TU) , to obtain a processed stream (6) , rich in hydrogen; where the hot effluent stream (2) , at exit of said reaction unit (RU) , is directly obtained by means of a decomposition reaction conducted with a heat transfer in thermal or autothermal reactors, where said cooling includes a direct heat exchange operation by mixing said hot effluent stream (2) with a colder cooling stream (3) , said process being characterized in that said cooling stream (2) consists of, or essentially consists of, nitrogen and / or ammonia.

2. Process as per claim 1, where the cooling includes an indirect heat exchange operation by means of one or more heat exchangers .

3. Plant for decomposing ammonia, according to the decomposition reaction 2NH3i=»3H2+N2 , including: an ammonia-rich feed stream to be decomposed (1) ; a reaction unit (RU) for the decomposition of ammonia;Giovanni Manenti a hot effluent stream (2) discharged from the reaction unit (RU) , rich in hydrogen and obtained by decomposition of ammonia; a treatment unit (TU) to perform a phase separation, and / or a chemical species separation and / or chemical reactions, on hydrogen-rich streams; where said reaction unit (RU) comprises one or more thermal or autothermal reactors from which said hot effluent stream (2) is directly discharged, said plant includes a cooling stream (3) entering the plant at a mixing point (7) located directly downstream of said reaction unit (RU) , where said cooling stream (3) is apt to inject a cold fluid, consisting of or essentially consisting of nitrogen and / or ammonia, into said hot effluent stream (2) .

4. Plant as per claim 3, including one or more heat exchangers located between said reaction (RU) and treatment (TU) units.

Images