Ammonia cracking system with enhanced energy efficiency and related process

Abstract

The present invention refers to a process and a system for producing hydrogen involving ammonia cracking, starting with the vaporization of a liquid ammonia feed into a gaseous stream. This stream may be pre-converted into a pre-cracked gas comprising ammonia, hydrogen, and nitrogen before being cracked in a reactor with a catalyst bed to produce a process gas containing hydrogen, nitrogen, and unreacted ammonia. The process gas is cooled and separated into a hydrogen-rich stream and a hydrogen-depleted off-gas stream. The cracking preferably occurs at temperatures between 750°C to 800°C and pressures of 25-45 bar g, with unreacted ammonia limited to up to 1 vol.%. The system includes an evaporator, optional pre-converter, ammonia cracking reactor, cooling section, and separation section, facilitating the conversion of ammonia into hydrogen efficiently.

Description

[0001] AMMONIA CRACKING SYSTEM WITH ENHANCED ENERGY EFFICIENCY AND RELATED PROCESS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to an ammonia cracking system and process, and more particularly, to an ammonia cracking system that eliminates the need for a recirculation water system.

[0004] BACKGROUND

[0005] Ammonia cracking is a critical industrial process used to produce hydrogen gas by decomposing ammonia into nitrogen and hydrogen. This process is essential for various applications, including fuel cells, chemical synthesis, and as a clean energy carrier. Traditional ammonia cracking systems often involve complex setups with multiple stages, including ammonia evaporation, cracking, and purification. These systems typically require additional components such as recirculation water systems and ammonia washing columns to manage ammonia slip and ensure the purity of the hydrogen produced. However, these additional components can increase both the capital and operational costs of the system, as well as complicate the overall process.

[0006] The need for more efficient and cost-effective ammonia cracking processes has become increasingly important as industries seek to reduce their environmental impact and improve energy efficiency. The integration of pressure swing adsorption (PSA) units in hydrogen purification has been a significant advancement, allowing for higher purity hydrogen production. However, managing ammonia slip which is the unreacted ammonia in the process gas obtained from the cracking step, remains a challenge, as excessive ammonia in the process gas can affect the performance of the PSA units and the quality of the hydrogen produced.

[0007] Therefore, there is a continuous demand for innovative solutions that can optimize energy recovery, reduce costs, and enhance the efficiency of ammonia cracking processes without compromising the quality of the hydrogen output.SUMMARY

[0008] The process for producing hydrogen by ammonia cracking may involve several steps. Firstly, a liquid ammonia feed stream is vaporized into a gaseous ammonia stream using an evaporator. Optionally, the gaseous ammonia stream may be pre-converted into a pre-cracked ammonia gas stream comprising ammonia, hydrogen, and nitrogen, in a pre-converter, which can include a catalyst bed. In the system of the present invention the optional pre-converter is preferably an adiabatic reactor comprising a catalyst bed. The pre-converter can alternatively be a heat exchange reactor in series or an electrical reactor.

[0009] The resulting gas is then cracked in an ammonia cracking reactor with at least one catalyst bed, to obtain a process gas comprising hydrogen, nitrogen, and unreacted ammonia. The process gas can be cooled in a cooling section and separated into a hydrogen-rich stream and a hydrogen-depleted off-gas stream in a separation section. The cracking temperature in the ammonia cracking reactor varies from 750 °C to 800 °C at a pressure of 25-45 bar g, with unreacted ammonia in the process gas up to 1 vol. %.

[0010] The process and system of the present invention eliminate the need for a recirculation water system, by instead feeding the cracked gas to the PSA, preferably directly without the need for a step or a section for removing ammonia from the process gas, thereby reducing capital (CAPEX) and operational costs (OPEX) while enhancing or at least not negatively affecting energy and hydrogen efficiency of the process according to the present invention.

[0011] DETAILED DESCRIPTION

[0012] The present disclosure provides a process and system for producing hydrogen through ammonia cracking, comprising an evaporator, an optional pre-converter, an ammonia cracking reactor, a cooling section, and a separation section, as well as means for circulating different recycle streams and uses thereof.

[0013] The process begins with the vaporization of a liquid ammonia feed stream into a gaseous ammonia stream, which may be pre-converted into a pre-cracked gas stream. The pre-converting step contributes to ensure that the ammonia comprising gas is adequately prepared for the cracking process, thereby improving the overall efficiency of hydrogen production. The pre-conversion alsocontributes to the reduction of unreacted ammonia in the final process gas, aligning with the system's goal of optimizing hydrogen purity and energy efficiency.

[0014] The resulting gas stream (preferably pre-cracked) is further processed in an ammonia cracking reactor to obtain a process gas comprising hydrogen, nitrogen and unreacted ammonia (or ammonia slip). Cracking temperature is preferably comprised between 750 and 800 degrees Celsius, preferably between 770 and 790 degrees Celsius. The process of the present invention is more efficient the higher the cracking temperature. The flue gas of the ammonia cracker or ammonia cracking reactor is the stream that results from the combustion of the fuel with air in the furnace. The reason is that increasing the cracking temperature decreases the ammonia slip, hence the amount of NH3 that needs to be diverted to the selective catalytic reduction (SCR) to limit NOx emissions. As the NH3 demanded by the SCR increases, more raw NH3 needs to be diverted but also the temperature of the flue gas u.s. the air and fuel heating coils is decreased, thereby reducing the ability to recover heat in the waste heat section. The reasonable percentage of ammonia slip is therefore between 0.5% and 0.8% volume.

[0015] The process gas is cooled in a cooling section, comprising one or more heat exchangers, optimizing energy recovery by utilizing thermal energy from the process gas.

[0016] Subsequently, the process gas is separated into a hydrogen-rich stream and a hydrogen-depleted off-gas stream across a separation section, preferably comprising one or more pressure swing adsorption units(s) or PSA(s). Increasing the efficiency of the hydrogen separation unit(s) (HSU) doesn’t always help to achieve the purpose of the invention. If the hydrogen recovery in the PSA is below ca. 80%, then the duty delivered by the tail-gas exceeds the vendor requirements for the ammonia cracking reactor, e.g. a tubular cracker. The optimal hydrogen recovery is in the range 80 - 87% (volume) for some preferred embodiments and 95-98% (volume) for others.

[0017] The process and system of the present invention eliminate the need for a recirculation water system, by instead feeding the cracked gas to the PSA, thereby reducing capital (CAPEX) and operational costs (OPEX) while enhancing or at least not negatively affecting energy and hydrogen efficiency of the process according to the present invention.

[0018] The system for producing hydrogen by ammonia cracking according to the present invention comprises an evaporator, an optional pre-converter, an ammonia cracking reactor, a cooling section, and a separation section. A pre-heated ammonia feed stream can be provided to theevaporator where it is gasified into an ammonia stream, which may optionally be delivered to a pre-converter, with or without a catalyst bed and converted into a pre-cracked ammonia gas stream before being cracked in an ammonia cracking reactor and converted into a process gas. The process gas is then cooled in a cooling section and separated into a hydrogen-rich stream and a hydrogen-depleted off-gas stream in a separation section.

[0019] The system according to the present invention can accommodate higher ammonia slip levels than known systems for a similar purpose, allowing for increased ammonia content in the process gas directed to the PSA unit(s), without the inconveniences commonly associated to ammonia slip, as excessive ammonia in the process gas can affect the performance of the PSA units and the quality of the hydrogen produced.

[0020] The ammonia cracking reactor is preferably a fired tubular reactor, but can be any kind of suitable reactor, such that the efficiency of the ammonia cracking process or the state of equipment are not compromised, i.e. negatively affected and an ammonia slip between 0,5% and 0,8% is achieved. More than one reactor may be used for ammonia cracking according to the present invention. The ammonia cracking reactor may be designed to accommodate the use of hydrogen-depleted off-gas streams as fuel, thereby enhancing the energy efficiency of the system. Additionally, the reactor may be capable of utilizing external streams comprising methane, ammonia, or other suitable compositions as fuel. The ammonia cracking process is preferably performed industrially in fired tubular crackers and the system may be optimized for energy recovery by utilizing thermal energy from the process gas.

[0021] The use of a Ni-based catalyst in the ammonia cracking reactor is preferred in some embodiments to facilitate the efficient conversion of ammonia into hydrogen and nitrogen, ensuring that the process gas is adequately prepared for the subsequent separation step.

[0022] The cooling step and section are crucial for managing the thermal energy of the process gas. The cooling section preferably comprises one or more heat exchangers, to collect heat from the process gas. This collected heat may then be utilized in the gasification of a liquid ammonia stream, thereby enhancing the energy efficiency of the system.

[0023] The process gas, having undergone cooling, is prepared for subsequent separation into a hydrogenrich stream and a hydrogen-depleted off-gas stream. The cooling step optimizes the energy recovery within the system, as it allows for the thermal energy from the process gas to beeffectively harnessed and reused. This approach contributes to the overall efficiency and sustainability of the ammonia cracking process, thereby reducing operational costs and improving hydrogen production efficiency.

[0024] The process gas is separated into at least a hydrogen-rich stream and a hydrogen-depleted off-gas stream, across a separation section. This separation step is preferably performed across at least one Pressure Swing Adsorption (PSA) unit, which can effectively isolate the hydrogen-rich stream from the remaining components. The hydrogen-depleted off-gas stream, which may still contain valuable energy content, can be redirected as fuel for the ammonia cracking reactor, thereby enhancing the overall energy efficiency of the system. Additionally, an external stream comprising one or more of natural gas (including methane), ammonia, and / or other suitable compositions may be introduced as fuel to support the ammonia cracking process.

[0025] The separation section comprises one or more pressure swing adsorption units(s) or PSA(s). This separation step may be crucial for isolating hydrogen from the process gas, allowing for its subsequent use or storage. The hydrogen-depleted off-gas stream may be used as fuel in the ammonia cracking reactor, further enhancing the system's energy efficiency. Additionally, an external stream comprising methane, ammonia, or other suitable compositions may be used as fuel in the ammonia cracking reactor to support the cracking process.

[0026] To further optimize the separation process, a step for reducing the ammonia content in the process gas may be incorporated upstream, utilizing processes such as recirculation water, ammonia washing, scrubbing, quenching or other.

[0027] The process and system of the present invention advantageously allow for the accommodation of higher ammonia slip levels in the PSA unit(s), as the ammonia removal process step is no longer necessary, leading to reduced capital expenditures and operational costs. The integration of these steps contributes to improve energy efficiency and enhance hydrogen purity.

[0028] In a first preferred embodiment of the present invention, wherein the feed or fuel to the ammonia cracking reactor can comprise or contain i) ammonia, ii) process gas and / or natural gas (including methane), approximately 80 to 87% recovery of hydrogen gas is achieved in preferably one single PSA unit. The hydrogen obtained thereby has a low CO2 footprint, since carbon emissions duringthe process are low since a significant part of the heating duty is supplied by the PSA off gas and also due to less equipment and process steps being needed.

[0029] In a second preferred embodiment of the present invention, wherein the feed or fuel comprises mostly natural gas (including methane), approximately 95 to 98% recovery of hydrogen gas is achieved in preferably two PSA units. The efficiency of the ammonia cracking process, when measured for example in terms of specific ammonia consumption (i.e. the tons of ammonia required to produce one ton of H2), will be higher when compared to standardly known processes and plants, where a step for removing ammonia from the process gas is required. To some extent, the carbon emissions can be lower when using this configuration, when compared to said standardly known processes and plants.

[0030] In another preferred embodiment of the present invention, ammonia consumption is not higher than 7.2 kg ammonia / kg hydrogen obtained.

[0031] In another preferred embodiment of the present invention, the cracking temperature in the ammonia cracking reactor is between 770 °C and 790 °C, most preferably 785 °C.

[0032] In another preferred embodiment of the present invention, the process gas comprises unreacted ammonia ranging from 0.45 vol. % to 0.95 vol. %, most preferably from 0.54 vol. % to 0.84 vol. %.

[0033] In another preferred embodiment of the present invention, the process gas may comprise up to 75% hydrogen and up to 25% nitrogen.

[0034] During the cooling step, one or more heat exchangers collect heat from the process gas. At least part of the heat collected during the cooling step may be used in the gasification of a liquid ammonia stream.

[0035] In another preferred embodiment of the present invention, the residual or flue gas from the ammonia cracking reactor can be used to preheat some of the feed, air and fuel stream(s).In another preferred embodiment of the present invention, at least part of the process gas may be used as fuel in the ammonia cracking reactor.

[0036] In another preferred embodiment of the present invention, at least part of the hydrogen-depleted off-gas stream from one or more PSA units may be used as fuel in the ammonia cracking reactor.

[0037] In another preferred embodiment of the present invention, an external stream containing natural gas (including methane), ammonia, or other suitable composition can be used as fuel in the ammonia cracking reactor.

[0038] Optionally, a step for reducing the ammonia content in the process gas may be added upstream to the separation step, which can include one or more of recirculation water, ammonia washing, scrubbing, quenching or other.

[0039] In another preferred embodiment of the present invention, a Ni-based catalyst is used in the ammonia cracking step and reactor. Other catalysts can be used, as Fe / Co and others, such that they show reasonable activity within the preferred cracking temperature range.

[0040] The pre-converter may be an adiabatic reactor, a heat exchange reactor in series HTER-s or an e-REACT.

[0041] The cooling section may comprise one or more heat exchangers.

[0042] The separation section may include one or more PSA units.

[0043] The ammonia cracking reactor is preferably a fired tubular reactor but can be any type of suitable reactor, such that the efficiency of the ammonia cracking process or the state of equipment are not compromised, i.e. negatively affected and an ammonia slip between 0,5% and 0,8% is achieved.EXAMPLE

[0044] A typical composition of the process gas leaving the cracker is as follows:

[0045] Ha: 74.0-74.5 vol.%

[0046] Na: 24.0-24.8 vol.%

[0047] NIL: 0.5-0.8 vol.%

[0048] HaO: 0.5-1.0 vol.%

[0049] The water present in the process gas is intentionally added to prevent corrosion during transportation and storage. For reference, commercial-grade ammonia typically contains 0.2-0.5 wt.% FLO.

[0050] It should be noted that no additional water is introduced into the process upstream of the separation (washing) step described below. The purpose of this separation step is to reduce the amount of unconverted ammonia in the process gas.

[0051] An example composition is:

[0052] IL: 74.8 vol.%

[0053] Na: 24.9 vol.%

[0054] NH3: 1300-1500 ppm

[0055] HaO: 0.09 vol.%

Claims

CLAIMS1. A process for producing hydrogen by ammonia cracking, comprising:Vaporization of a liquid ammonia feed stream (1) into a gaseous ammonia stream (2), in an evaporator,Optionally, pre-converting said gaseous ammonia stream (2) into a pre-cracked ammonia gas stream (3) comprising ammonia, hydrogen and nitrogen, in a pre-converter, optionally comprising a catalyst bed,Cracking a gas stream (2, 3) in an ammonia cracking reactor comprising a catalyst bed, to obtain a process gas (4) comprising hydrogen, nitrogen and unreacted ammonia,Cooling said process gas (4) in a cooling section, andSeparating the process gas into a hydrogen-rich stream (5) and a hydrogen depleted off gas stream (6), in a separation section,characterized in that,Cracking temperature in the ammonia cracking reactor is between 750°C to 800°C ; and Unreacted ammonia in the process gas is up to 1 vol. %.

2. Process according to claim 1 wherein the cracking temperature in the ammonia cracking reactor is between 770 °C and 790 °C.

3. Process according to any one of claims 1 to 2 wherein the process gas (4) comprises unreacted ammonia from 0.45 vol. % to 0.95 vol. %, preferably from 0.54 vol. % to 0.84 vol. % or from 1300 to 1500 ppm.

4. Process according to any one of claims 1-3 wherein the process gas comprises up to 75% H2 and up to 25% N2, preferably 74,8 vol.% H2 and 24,9 vol.% of N2.

5. Process according to any one of claims 1-4 wherein during the cooling step, one or more heat exchangers collect heat from the process gas (4).

6. Process according to any one of claims 1-5 wherein at least part of the heat collected during the cooling step is consumed in the vaporization of a liquid ammonia stream (1).

7. Process according to any one of claims 1-6 wherein the flue gas from the ammonia cracking reactor is used to preheat at least one of the feed, air and fuel.

8. Process according to any one of claims 1-7 wherein at least part of the process gas (4) is used as fuel in the ammonia cracking reactor.

9. Process according to any one of claims 1-8 wherein the separation step is performed in at least one PSA unit(s).

10. Process according to any one of claims 1-9 wherein at least part of the hydrogen depleted off gas stream (6) from one or more PSA unit(s) is used as fuel in the ammonia cracking reactor.

11. Process according to any one of claims 1-10 wherein an external stream comprising methane and / or ammonia and / or other suitable composition is used as fuel in the ammonia cracking reactor.

12. Process according to any one of claims 1-11 wherein a step for reducing the ammonia content in the process gas (4) can be added upstream to the separation step, comprising one or more of recirculation water, ammonia washing, scrubbing and / or quenching.

13. Process according to any one of claims 1-12 wherein a Ni-based catalyst is used in the ammonia cracking step and reactor.

14. System for producing hydrogen by ammonia cracking according to the process of claims 1-13, said system comprising:a) Evaporator;b) Optional pre-converter;c) Ammonia cracking reactor;d) Cooling section;e) Separation section;Wherein an pre-heated ammonia feed stream (1) is provided to the evaporator where it is vaporized into an ammonia gas stream (2) which may optionally be delivered to a pre-converter optionally comprising at least one catalyst bed and converted into a pre-cracked ammonia gas stream (3) before being cracked in an ammonia cracking reactor and converted into a process gas (4) , said process gas being cooled in a cooling section and directly fed to a separation section, into a hydrogen rich stream (5) and a hydrogen depleted off gas stream (6), bypassing an optional ammonia removing section.

15. System according to claim 14, wherein the pre-converter is an adiabatic reactor.

16. System according to any one of claims 14-15, wherein the cooling section comprises one or more heat exchanger(s).

17. System according to any one of claims 14-16 wherein the separation section comprising one or more PSA unit(s).

18. System according to any one of claims 14-17 wherein the ammonia cracking reactor is a fired tubular reactor.