Green ammonia absorption cooling

Abstract

1. An ammonia synthesis process and plant comprising an ammonia synthesis converter and a downstream ammonia cooling system, the ammonia synthesis converter being arranged to receive an ammonia synthesis gas comprising hydrogen and nitrogen and produce an ammonia product gas stream and an off-gas ammonia stream; the ammonia cooling system comprising: an ammonia evaporator for evaporating an ammonia liquid stream and producing an ammonia vapor stream; an off-gas scrubbing unit for scrubbing the off-gas ammonia stream with the addition of water as a scrubbing agent and producing a water stream and an ammonia-depleted off-gas stream; an absorption cooling unit comprising water for cooling the ammonia vapor stream and collecting a condensed ammonia-water stream; a regeneration unit for producing from the condensed ammonia-water stream a purified water stream, the ammonia liquid stream, and an overhead ammonia gas stream.

Description

[Technical field]

[0001] The present invention relates to an ammonia refrigeration (refrigeration) unit for cooling ammonia product gas from an ammonia synthesis converter in an ammonia synthesis plant or process. The present invention also relates to an ammonia synthesis plant and method including an ammonia refrigeration system, where ammonia is produced from hydrogen by electrolysis utilizing electricity from renewable energy sources. [Background technology]

[0002] A conventional or traditional ammonia synthesis plant, as is well known in the art, typically includes a reforming section including a steam methane reforming unit and / or an autothermal reforming unit for catalytic steam reforming of a hydrocarbon feedstock, typically natural gas or a hydrocarbon feedstock derived from a renewable source, thereby producing a raw synthesis gas that is rich in hydrogen and combined with nitrogen to produce an ammonia synthesis gas. The ammonia synthesis plant has a downstream ammonia synthesis section, e.g., an ammonia synthesis loop, including an ammonia synthesis converter in which the ammonia synthesis gas is converted to an ammonia product gas stream.

[0003] In conventional ammonia synthesis plants, the so-called turndown ratio is about 100%. The turndown ratio, hereinafter also referred to as "turndown", is the ratio of operation at normal load, e.g., normal load of the upstream hydrocarbon feed, typically natural gas, used to produce ammonia synthesis gas and thus ammonia product gas, to what the ammonia synthesis plant can tolerate. Thus, in conventional ammonia synthesis plants, a turndown of e.g. 10% is not taken into account, since the natural gas feed is stationary and therefore the normal load, i.e., the turndown is 100%. However, in dynamic ammonia synthesis plants, such as green ammonia synthesis plants, where the hydrogen for the ammonia synthesis gas is produced by water or steam electrolysis powered by electricity from renewable energy sources such as solar power, wind power, hydroelectric power, etc., the power input from these power sources can vary greatly during the day and season, so that a turndown of e.g. 10% load occurs frequently and is therefore a very important issue to address.

[0004] In conventional natural gas-based ammonia synthesis plants or processes, an ammonia cooling circuit is usually employed to condense the produced ammonia. Thus, the produced ammonia is separated from the circulating ammonia synthesis gas by cooling / condensation. To avoid the accumulation of inerts in the plant, a small amount of ammonia-containing gas, i.e., an off-gas ammonia stream, is discharged from the plant, which is treated in a scrubber for environmental reasons, and the absorbed ammonia is recovered by distillation.

[0005] A well-known working principle in an ammonia refrigeration circuit is the use of so-called mechanical refrigeration, whereby ammonia is evaporated at low pressure, generating a low temperature, which is then utilized for cooling the main process stream of the upstream ammonia synthesis plant. The released ammonia vapor is compressed in a multi-stage ammonia compressor, for example a multi-stage centrifugal compressor driven by a steam turbine, and the compressed vapor is condensed by cooling water or air in an ammonia condenser. As a result, liquid ammonia is collected and reused for cooling. For example, in a standard, i.e. conventional, natural gas-based ammonia synthesis plant, there is usually an ammonia refrigeration compressor and an ammonia recovery unit consisting of two off-gas scrubbers and a distillation column.

[0006] Traditionally, cooling has been provided by multi-stage refrigeration compressors operating at multiple pressure levels, making the ammonia refrigeration circuit energy efficient at high loads but less efficient at low loads (or low turndown ratios) due to the complexity of the layout and compressor limitations. This is problematic when the plant load fluctuates widely and the compressor is steam-driven. At full plant load, the steam produced as a by-product in the ammonia plant produces enough steam to run the refrigeration compressor, but when the load drops this becomes impossible. Thus, the heat generated from the ammonia synthesis converter in a conventional ammonia synthesis plant is usually removed in a steam boiler, and the resulting superheated steam can be used to drive the multi-stage compressors of the ammonia refrigeration circuit with a steam turbine or to drive an alternator for electricity generation. However, the turndown of the steam turbine is lower than the turndown of the ammonia synthesis plant, for example its ammonia synthesis loop. Therefore, when the load of the ammonia synthesis plant is low (e.g. below 50%), an imbalance occurs between the steam produced in the ammonia synthesis plant and the steam used in the downstream ammonia refrigeration circuit, which changes how this steam is utilized by the steam turbine, forcing the steam turbine to be shut down and extra power to be used to condense the steam. Conventional steam turbines operate with superheated steam and therefore must include a steam superheater as part of the steam generation section, resulting in increased capital expenditures (CAPEX) and more complex operation.

[0007] WO2020057818A1 discloses an aqua / ammonia absorption refrigeration system and an improvement method thereof. A preferred application relates to an absorption refrigeration system for refrigerating process gas in an ammonia synthesis plant.

[0008] No. 4,376,758 discloses a process for the synthesis of ammonia from hydrocarbons, including cooling and purification. Off-gas containing as little as 0.48% ammonia is withdrawn at the top of an ammonia absorber. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] WO2020057818A1 [Patent Document 2] US4376758 Summary of the Invention [Problem to be solved by the invention]

[0010] It is an object of the present invention to provide a robust ammonia cooling system capable of withstanding fluctuating loads in ammonia synthesis plants with fluctuating power supplies, in particular in green ammonia synthesis plants, i.e. where the hydrogen required for the production of ammonia synthesis gas is obtained from electrolysis of water or steam powered by electricity from renewable energy sources.

[0011] Another object of the present invention is to provide improved ammonia recovery of off-gas ammonia streams generated in ammonia synthesis plants. [Means for solving the problem]

[0012] These and other objectives are met by the present invention.

[0013] Accordingly, in a first aspect, the present invention relates to an ammonia synthesis plant comprising an ammonia synthesis converter and a downstream ammonia cooling system, the ammonia synthesis converter configured to receive an ammonia synthesis gas comprising hydrogen and nitrogen and to produce an ammonia product gas stream; wherein the ammonia synthesis plant is configured to produce an off-gas ammonia stream; The ammonia cooling system includes: an ammonia evaporator for evaporating the ammonia liquid stream and producing an ammonia vapor stream; - an off-gas scrubbing unit for scrubbing said off-gas ammonia stream under the addition of water as a scrubbing agent (absorbent) to produce a water stream and an ammonia-depleted off-gas stream; - an absorption cooling unit including water for cooling said ammonia vapor stream and collecting a condensed ammonia-water stream; from said condensed ammonia-water stream Purified water stream, the ammonia liquid stream; and overhead ammonia gas stream; Reproduction unit for generating: And, where: - the ammonia evaporator is positioned to receive the ammonia liquid stream and the heat exchange medium for evaporating the ammonia liquid stream to produce the ammonia vapor stream; the off-gas cleaning unit, preferably an off-gas scrubbing unit, is configured to receive the purified water stream from the regeneration unit and the off-gas ammonia stream as a scrubbing agent to produce the ammonia-depleted off-gas stream and the water stream; - said absorption cooling unit is preferably a mixing vessel containing water and is positioned to receive said water stream and said ammonia vapor stream, thereby producing and collecting said condensed ammonia-water stream; the absorption cooling unit further comprising a cooling device, such as a coil, positioned to receive a cooling medium, e.g., water, and an outlet for removing the condensed ammonia-water stream; the regeneration unit is a distillation column positioned to receive the condensed ammonia-water stream, i.e., an ammonia recovery distillation column; The regeneration unit includes an outlet for removing the purified water stream as a bottoms stream, an outlet for removing an overhead ammonia stream, and a separation unit, e.g., an ammonia recovery overhead drum, for separating an overhead ammonia gas stream and the ammonia liquid stream from the overhead ammonia stream.

[0014] As used herein, the term "comprise" also includes "comprise only" or "consist of". As used herein, the term "preferably" means "optionally", i.e. in any embodiment. As used herein, the term "first aspect" or "first aspect of the invention" refers to a plant (system, apparatus). The term "second aspect" or "second aspect of the invention" refers to a method (process). As used herein, the term "the present invention" or simply "invention" can be used interchangeably with the term "this application" or simply "application."

[0015] Further definitions are provided in relation to one or more of the embodiments below.

[0016] In one embodiment, the ammonia synthesis plant comprises a water or steam electrolysis unit for producing said hydrogen. Suitably, said electrolysis unit is powered by electricity from a renewable energy source such as solar, wind or hydropower. Optionally, said electrolysis unit is powered by electricity from thermonuclear power.

[0017] The water electrolysis unit is an alkaline / polymer electrolyte membrane electrolysis unit, i.e. an alkaline / PEM electrolysis unit (alkaline battery or polymer battery unit). For the purposes of the present invention, the term alkaline / PEM electrolysis unit means an alkaline and / or PEM electrolysis unit. The steam electrolysis unit is a solid oxide electrolysis cell unit (SOEC unit). Operation with SOEC units is usually carried out at high temperatures (700-800°C), which often offers advantages over alkaline / PEM electrolysis, as the latter operates at much lower temperatures, i.e. in the range of 60-160°C. It will be understood that alkaline / PEM electrolysis uses water and SOEC uses steam.

[0018] The term "green ammonia absorption cooling" will be understood to mean cooling of ammonia product gas in an ammonia cooling system where electrolysis of water or steam is performed to produce hydrogen.

[0019] The term "green ammonia plant" or "green ammonia synthesis plant" is also understood to mean an ammonia synthesis plant comprising a water or steam electrolysis unit for producing hydrogen for ammonia synthesis gas, the electrolysis unit being powered by electricity from renewable energy sources such as solar, wind, hydropower, etc. Optionally, said electrolysis unit can also be powered by electricity from thermonuclear power generation.

[0020] It will be understood that the ammonia refrigeration system includes a closed circuit and therefore may also be interpreted as, for example, an ammonia refrigeration circuit. The term "ammonia refrigeration circuit" can be used interchangeably with the term "ammonia refrigeration circuit."

[0021] The term "mechanical refrigeration" may be used interchangeably with the terms "compressor driven refrigeration circuit" or "compressor driven ammonia refrigeration."

[0022] The term "ammonia evaporator" is used interchangeably with the term "ammonia cooler" or simply "cooler." It will also be understood that the term "ammonia evaporator" or "cooler" refers to one or more ammonia evaporators, i.e., one or more coolers.

[0023] A similar interpretation may be applied to other units, for example the term "absorption cooling unit" means one or more absorption cooling units.

[0024] Preferably, the ammonia cooling system comprises multiple ammonia evaporators, i.e. multiple coolers, arranged in series, for example two coolers, which can condense the ammonia in the ammonia product gas, and the provision of multiple coolers can increase the energy efficiency of the plant or process.

[0025] "Off-gas scrubber" means a device that removes ammonia from the off-gas. The off-gas scrubber is preferably a wash or scrubbing tower, in which a scrubbing agent is used to scrub the ammonia present in the off-gas. It will be understood that "scrubbing agent" has the same meaning as "absorbent."

[0026] The term "regenerator unit" is used interchangeably with the term "regenerator."

[0027] The term "load" refers to the percentage of hydrogen supply used to perform ammonia synthesis in an ammonia synthesis plant compared to the load during normal operation (100% load). The ammonia production capacity of the plant is assumed to be directly proportional to the hydrogen supply used. Thus, 10% load means that 10% of the hydrogen is used for ammonia synthesis compared to normal operation. The hydrogen used is from water (steam) electrolysis, as is well known in the art. 100% load corresponds to normal operation of an ammonia synthesis plant with a steady supply of natural gas.

[0028] As stated above, the term "turndown ratio", also referred to simply as "turndown", refers to the ratio of the normal loading of the upstream hydrocarbon feed, typically natural gas, used to produce ammonia synthesis gas and thereby ammonia product gas, to the normal loading that the ammonia synthesis plant can withstand. The term "normal load" or "normal operation" is also understood to refer to a condition where the load is 100%. Thus, 100% load, or normal load, corresponds to 100% turndown, and 10% load corresponds to 10% turndown.

[0029] The term turndown also applies to parts of an ammonia synthesis plant, such as units or equipment. Thus, the term turndown also applies, for example, to a steam turbine used to drive a compressor in a conventional ammonia refrigeration circuit. A turndown of, say, 50% of a steam turbine means the ratio of operation at normal load to what the steam turbine can tolerate.

[0030] It is now being found that in ammonia plants operating with widely varying loads and fast load variations, such as green ammonia plants operating solely on renewable energy, absorption cooling instead of mechanical refrigeration is advantageous because ammonia cooling systems can be designed with better turndown ratios than are available in compressor-operated cooling units and therefore can follow typical plant variations.

[0031] Absorption cooling according to the present invention is combined with off-gas scrubbing, e.g., off-gas absorption, resulting in a cooling system layout having a significantly fewer number of equipment units than a conventional compressor-driven refrigeration circuit plus a conventional off-gas scrubbing unit.

[0032] While the use of absorption cooling for refrigeration in conventional ammonia synthesis plants is known, for energy conservation reasons it has usually only been operated to produce an absorber ammonia-water solution of the required concentration. The present invention differs in that at least ammonia and water are separated into nearly pure components. Thus, the ammonia cooling system is suitable for both the production of purified water, i.e. ammonia-free water, and the production of ammonia liquor, some of which can be recovered as additional ammonia product.

[0033] The present invention therefore allows for the replacement of a conventional compressor-driven cooling circuit (i.e. mechanical refrigeration) and conventional off-gas absorption with one system.

[0034] The term "integration" as used herein means creating synergies in combining previously stand-alone units or systems, such as significantly reducing the number of resulting equipment units.

[0035] It will be appreciated that in one embodiment, the off-gas scrubbing unit (132) is an off-gas scrubbing unit, and the off-gas scrubbing unit (132) is also arranged to receive the overhead ammonia gas stream (139) from the regeneration unit (126), e.g., in admixture with the off-gas ammonia stream (143).

[0036] As mentioned above, the off-gas ammonia stream preferably also includes an overhead ammonia gas stream from the regeneration unit. Thus, in certain embodiments, means are also provided, such as a junction or mixing unit, for admixing the overhead ammonia gas stream into the off-gas ammonia stream. The combined stream is fed to an off-gas scrubbing unit. For purposes of this application, the term "junction" means "junction".

[0037] This achieves further integration. For environmental reasons, ammonia-containing off-gases, such as slip gas from an upstream ammonia synthesis plant or from the ammonia cooling system itself, need to be scrubbed. The ammonia-containing off-gases are scrubbed with pure water produced in the ammonia cooling system, more specifically in the bottoms stream of the regenerator, in order to recover the ammonia and minimize leakage of ammonia to the atmosphere. The water stream thus produced from the off-gas scrubbing unit is added to an absorption chiller, e.g. a mixing vessel, to recover ammonia as a product downstream and to send purified water back to the off-gas scrubbing unit.

[0038] Thus, in one embodiment, the absorption cooling unit is a mixing vessel containing water.

[0039] The ammonia present in the off-gas ammonia stream and the overhead ammonia gas stream is reduced to levels not obtainable in conventional plants or processes, resulting in the disposal of ammonia-containing streams to end users, which is highly undesirable. With the present invention, the ammonia content in the ammonia-depleted off-gas stream from the off-gas scrubbing unit is reduced to less than 10 ppmv, e.g., 7 or 5 ppmv ammonia content. The off-gas ammonia stream entering the off-gas scrubbing unit may, for example, contain 10-15% (mol %) ammonia and the overhead ammonia gas stream may, for example, contain 80-90% ammonia.

[0040] In an embodiment according to the first aspect of the present invention, in the ammonia cooling system, the heat exchange medium of an ammonia evaporator is the ammonia product gas stream.

[0041] This allows the ammonia cooling system to be fully integrated within the ammonia synthesis plant. Apart from green ammonia synthesis plants, which have inherent fluctuating power supplies due to the intermittent nature of renewable energy sources such as wind, solar, and hydroelectric power, the present invention is also suitable for cooling the ammonia product gas of any ammonia synthesis plant having a fluctuating power supply or a fluctuating hydrocarbon supply.

[0042] In an embodiment according to the first aspect of the present invention, in the ammonia cooling system, the regeneration unit comprises an upper section (I) and a lower section (II); The lower section (II) a power driven heating device and / or a reboiling unit positioned to receive the heat exchange medium; and an outlet for removing said purified water stream as a bottoms stream; Equipped with The upper section (I) an inlet for receiving a condensed ammonia-water stream; -Tower head, including: an outlet for removing said overhead ammonia stream; an ammonia condensing unit positioned to receive said overhead ammonia stream and a cooling medium, such as water, for producing a condensed overhead ammonia stream; a separation unit, preferably an ammonia recovery drum, positioned to receive said condensed overhead ammonia stream, said separation unit comprising an outlet for removing an ammonia liquid fraction and an outlet for removing said overhead ammonia gas stream; a reflux pump positioned to receive said ammonia liquid fraction and to direct a portion thereof to the top of said column as overhead reflux; means for removing a remaining portion of said ammonia liquid fraction as said ammonia liquid stream.

[0043] The term "reboil unit" may be used interchangeably with the term "reboiler."

[0044] The term "ammonia condensing unit" may be used interchangeably with the term "ammonia condenser."

[0045] The term "powered heating device" means a heating element that is powered by electrical power, thereby converting electricity or electrical energy into heat, for example, via the process of Joule heating.

[0046] The ammonia-water mixture is sent to the distillation column, and the ammonia at the top is condensed and used for cooling in the ammonia evaporator (chiller), so there is no overhead reflux, unlike the conventional ammonia absorption system, for example as shown in the attached FIG. 1, so that in the present invention, as shown in FIG. 2, overhead reflux can be realized, thereby realizing a distillation column with fewer trays, and at the same time, the purified water stream from the bottom of the distillation column can be used as a scrubbing agent (e.g., absorbent) in the off-gas washing unit.

[0047] Additionally, conventional ammonia absorption systems require the addition of significant amounts of demineralized water, whereas the present invention significantly reduces such requirements.

[0048] In an embodiment according to the first aspect of the invention, the ammonia synthesis plant further comprises one or more steam boilers arranged to receive the ammonia product gas stream upstream of said ammonia cooling system for recovering heat therefrom and generating steam, the plant being further arranged to supply at least a portion of said steam to said reboil unit or reboiler as a heat exchange medium.

[0049] The reboiler, as is well known in the art, is a heat exchanger, such as a tube-and-shell heat exchanger, that provides the heat necessary to boil the liquid from the bottom of the regenerator, which is preferably a distillation column, producing steam that is returned to the distillation column to drive the separation of the ammonia.

[0050] As a result, in the present invention, the reboiler load follows the ammonia production load, and therefore, balanced use of steam is possible even at any turndown level, particularly well below 100%, for example 10%, i.e., when the ammonia synthesis plant load is 10%. In other words, it is possible to balance the steam produced in the ammonia synthesis plant and the steam used in the ammonia cooling system at any operating load, for example between 10% and 100%.

[0051] Steam generated in the ammonia synthesis plant may suitably be supplied at a pressure level that matches the pressure levels required in the reboiler, thereby also allowing thermal efficiencies at least comparable to those achieved using mechanical refrigeration. Suitably the steam is medium pressure steam, for example 50 bar (264°C) steam.

[0052] Furthermore, when operating at low turndown rates below 50%, e.g., 30% or 10%, the availability of steam generated in ammonia synthesis to external users is limited, if possible. The present invention also solves this problem by linking an ammonia cooling system that accommodates absorption cooling with a dynamic ammonia synthesis gas plant, e.g., a green ammonia synthesis plant, having variable ammonia and steam production.

[0053] Furthermore, the present invention omits the use of a steam turbine connected to the operation of a conventional ammonia refrigeration circuit, and more specifically, there is no steam turbine present in an ammonia refrigeration system, as there is no centrifugal compressor, which is typically driven by such a steam turbine.

[0054] Instead of or in combination with the reboiler, a power-driven heating device is provided, which is preferably fitted into the regenerator at its bottom. This achieves greater flexibility, since the power required for the heating device can be supplied from renewable energy sources, as mentioned above, and if for any reason the steam reboiler requirements cannot be met, the power-driven heating device provides the heat required in the regenerator. Moreover, under transient conditions, such as during start-up, the regenerator can be operated only by the heating device driven by electricity.

[0055] In an embodiment according to the first aspect of the invention, in the ammonia cooling system, the ammonia evaporator further comprises an outlet for removing a purge ammonia stream, and the ammonia cooling system further comprises means, such as a junction unit or a mixing unit, upstream of the regeneration unit for combining the purge ammonia stream with the condensed ammonia-water stream.

[0056] An outlet is provided for removing a purge ammonia stream, i.e., liquid blowdown, so that water does not accumulate in the condenser.

[0057] Preferably, an ammonia recovery circulation pump directs the combined stream to a regeneration unit.

[0058] As an example, the condensed ammonia-water stream, after addition of a purge stream from the condenser, may be a stream containing, for example, 30-50% ammonia by weight, such as 35, 40, 45% ammonia by weight, the remainder being water.

[0059] In conventional systems, the purge ammonia stream from the condenser is sent directly to the distillation column, for example as shown in Figure 1. By combining the purge ammonia stream with the condensed ammonia-water stream, as in the present invention, a single pump is used to feed the ammonia-water to the regenerator, thereby improving energy efficiency. The purge stream from the ammonia evaporator contains not only ammonia but also water, which is thereby recovered, instead of being discarded as acid water, etc.

[0060] In an embodiment according to the first aspect of the invention, the ammonia refrigeration system further comprises: a heat exchanger, hereinafter also referred to as an aftercooler, upstream of the ammonia evaporator and arranged to receive the ammonia liquid stream and to receive a cooling medium, such as water, as a heat exchange medium, for cooling the ammonia liquid stream prior to entering the ammonia evaporator.

[0061] In an embodiment according to the first aspect of the invention, the regeneration unit of the ammonia cooling system further comprises a feed / effluent heat exchanger (F / E HEX) arranged to receive the condensed ammonia-water stream or a stream resulting from combining the purge ammonia stream with the condensed ammonia-water stream, which is arranged to receive the purified water stream as a heat exchange medium for pre-heating the condensed ammonia-water stream or the stream resulting from combining the condensed ammonia-water stream with the purge ammonia stream prior to entering the regeneration unit.

[0062] Preferably, the thus cooled purified water stream is further cooled in an additional heat exchanger, hereinafter also referred to as ammonia recovery cooler, which is thus preferably arranged to receive said purified water stream and a cooling medium, such as water as a heat exchange medium, for further cooling said purified water stream before it enters the off-gas scrubbing unit.

[0063] The inclusion of an F / E HEX and preferably an ammonia recovery chiller allows for energy efficiencies to be maintained that are at least comparable to those that would normally be obtained if mechanical refrigeration were used instead, where such a heat exchanger would not normally be required.

[0064] In an embodiment according to the first aspect of the invention, the ammonia refrigeration system further comprises a conduit for removing a portion from said purge ammonia stream or from said ammonia liquid stream as additional ammonia product suitable for discharge as excess ammonia product.

[0065] Suitably the remaining portion of the ammonia liquid fraction at the top of the regeneration unit is removed as the ammonia liquid stream.

[0066] Whereas known (conventional) ammonia refrigeration circuits using ammonia absorption produce ammonia overhead from the regenerator which is condensed and collected in an overhead drum and then used for cooling, and the relatively water-rich regenerator bottoms stream is then used for absorption as a recycle stream (see, for example, FIG. 1), the present invention allows the overhead ammonia stream from the regenerator which is then condensed and collected in the overhead drum to be used not only for cooling, i.e., for ammonia evaporator, but also as part of the ammonia product pool, while mitigating issues with load swings and turndowns below 100%, especially for green ammonia synthesis plants (see FIG. 2).

[0067] Furthermore, whereas in conventional ammonia refrigeration circuits using ammonia absorption, a bottoms stream is produced in the regenerator that is typically about 25% by mass ammonia, 75% water rich, which is recycled to the absorber by combining with the ammonia vapor stream from the ammonia refrigerator, according to the present invention, the purified water bottoms stream from the regenerator according to the present invention has much more than 75% water, for example at least 99%, i.e. pure water. The pure water is advantageously used in the off-gas scrubber, avoiding the need to rely on an external source of pure water, such as using demineralized water.

[0068] In a second aspect, the present invention relates to a method for producing an ammonia product gas from an ammonia synthesis gas stream comprising hydrogen and nitrogen, said method also producing an off-gas ammonia stream, the method comprising the steps of: a) optional electrolysis of water or steam to produce said hydrogen; said electrolysis is preferably powered by electricity from renewable energy sources such as solar, wind or hydroelectric power; b) converting an ammonia synthesis gas into said ammonia product gas in an ammonia synthesis converter; and further comprising cooling the ammonia product gas according to the steps: c) evaporating an ammonia liquid stream in an ammonia evaporator and providing said ammonia product gas as a heat exchange medium to produce an ammonia vapor stream; d) feeding said off-gas ammonia stream to an off-gas scrubbing unit under the addition of a water stream and a purified water stream as a scrubbing agent (absorbent) to produce an ammonia-depleted off-gas stream; e) removing said water stream from said off-gas scrubber and feeding it together with said ammonia vapour stream to an absorption cooling unit under a supply of a cooling medium such as water to produce and collect a condensed ammonia-water stream; f) removing said condensed ammonia-water stream from said absorption cooling unit and feeding it to a regeneration unit, such as a distillation column, to produce said ammonia liquid stream, an overhead ammonia gas stream, and a purified water stream.

[0069] Compared to a conventional compressor-driven ammonia cooling circuit plus a conventional off-gas absorption unit, this combines absorption cooling with off-gas absorption, as per the first aspect of the invention, resulting in a much simpler process layout with a significantly lower number of equipment units and fewer process steps, thereby achieving high process and plant integration.

[0070] In methods of ammonia production that operate at widely varying loads and fast load variations, such as green ammonia plants powered solely by renewable energy, it has been found that providing absorption cooling rather than the conventional, typically more efficient compressor-driven ammonia cooling (mechanical refrigeration) provides a better Turntown ratio than can be achieved with mechanical refrigeration and tracks typical plant variations.

[0071] In an embodiment according to the second aspect of the invention, the method further comprises the steps of: g) feeding said overhead ammonia gas stream to said off-gas scrubbing system.

[0072] Slip streams within processes and plants that contain ammonia, such as the off-gas ammonia stream generated upstream to avoid inerts build-up during conversion of ammonia synthesis gas to ammonia product gas, and the overhead ammonia gas stream currently generated in the regeneration unit, are advantageously treated in an off-gas scrubbing unit, thereby also relieving the end user of the burden of finding a way to treat the overhead ammonia stream.

[0073] In an embodiment according to the second aspect of the invention, prior to step c), the ammonia product gas is first cooled by passing it through one or more steam boilers to produce steam, preferably medium pressure steam, and in step f), the regeneration unit comprises a power driven heating device or a reboiling unit, and the method further comprises supplying the steam to the reboiling unit.

[0074] Traditionally, when cooling of the ammonia product gas is provided by a multi-stage refrigeration compressor operating at multiple pressure levels, it has been found that ammonia cooling can be provided with high energy efficiency at high loads, but at low loads, complex processes and plant layouts result due to compressor limitations, including low efficiency at low loads. This becomes problematic when the plant load varies widely and the compressor is steam driven. At full plant load, steam produced as a by-product in the ammonia plant is generated in sufficient quantities to run the compressor for the ammonia refrigeration compressor, but at low loads, this is not possible.

[0075] Now, with the present invention, the steam produced can be conditioned to the steam required for use as a heat exchange medium, for example in the regenerator reboiler, avoiding the use of steam turbines required for compressor driven cooling.

[0076] Furthermore, by providing a power-driven heating device suitably fitted at the bottom in the regenerator, greater flexibility in the process is achieved. The power required for the heating device can be provided from renewable resources, and if for any reason the steam reboiler requirements cannot be met, the power-driven heating device provides the necessary heat in the regenerator. Moreover, as related to the first aspect of the invention, under transient conditions, such as during start-up, the regenerator can be operated only with the power-driven heating device.

[0077] In an embodiment according to the second aspect of the invention, in step c) a purge ammonia stream (liquid blowdown) is generated, and the method further comprises withdrawing said purge ammonia stream and combining it with said condensed ammonia-water stream.

[0078] The purge stream from the ammonia evaporator contains not only ammonia but also water, which is recovered rather than being discarded as acid water, etc. Furthermore, by taking out the purge ammonia stream, water is not allowed to accumulate in the cooler.

[0079] In an embodiment according to the second aspect of the invention, the process further comprises diverting a portion of the ammonia liquid stream or the purge ammonia stream (i.e., liquid blowdown from the condenser) as additional ammonia product, thereby utilizing either stream as a bleed of excess ammonia.

[0080] This method allows not only cooling of the ammonia product gas, but also the production of additional valuable ammonia products, instead of simply having a closed ammonia refrigeration circuit, for example the ammonia liquid stream, part of which is diverted as additional ammonia product, has a purity of at least 99.5% by mass NH3.

[0081] It will be appreciated that any of the embodiments and associated advantages according to the first aspect of the invention may be used in conjunction with the second aspect of the invention, and vice versa. [Brief description of the drawings]

[0082] FIG. 1 shows a prior art ammonia refrigeration system, more specifically, a prior art ammonia refrigeration circuit with absorption cooling.

[0083] FIG. 2 illustrates an absorption cooling type ammonia refrigeration system according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0084] Referring to both figures, the ammonia refrigeration circuit utilizes absorption cooling. The basic principle is to absorb ammonia vapor from the ammonia refrigerator into a liquid ammonia-water mixture and remove the heat of absorption by water cooling. The resulting condensed ammonia-water mixture is sent to the regenerator (distillation column) where the ammonia overhead is condensed and used for cooling (Figures 1 and 2) and optionally also as additional ammonia product (Figure 2), while the bottoms product from the regenerator is used either directly for ammonia absorption (Figure 1) or as a pure water stream in the off-gas scrubbing unit (Figure 2).

[0085] With specific reference to prior art FIG. 1, an ammonia refrigeration circuit 10 is shown. Ammonia liquid stream 1 from an ammonia recovery drum 12 passes through a first exchanger 14 upstream of an ammonia evaporator (chiller) 16, using ammonia vapor stream 7 as heat exchange medium. The resulting ammonia liquid stream 1' is then evaporated in a cooler 16 using ammonia product gas 5 from an ammonia synthesis converter of an ammonia synthesis plant (not shown), thereby producing ammonia vapor stream 7. After feeding to be cooled in the heat exchanger 14, the ammonia vapor stream 7' is combined with a water-rich bottoms stream 25', e.g. 25% ammonia / 75% water by weight, from a regenerator (distillation column) 26, the upper section I of which is arranged with a number of trays 26'. The ammonia-water mixed stream is then led to an absorber 18, which consists of a cooling device 18' and a mixing vessel 18''. Cooling water 39 in chiller 18' removes the heat of absorption, thereby producing a condensed ammonia-water stream 15, which is fed to mixing vessel 18". Condensed ammonia-water stream 19 is withdrawn from mixing vessel 18", transported via pump 22, heated in heat exchanger 24 using bottoms stream 25 from regenerator 26 as the heat exchange medium, and fed to regenerator 26 as stream 21. Purge ammonia stream (liquid blowdown) 29 is transported via a separate pump 20 to the top of regenerator 26.

[0086] The regenerator 26 includes an upper section I and a lower section II. The lower section is provided with a reboiler 28 for boiling the bottoms liquid 33 and generating steam 35 for driving the distillative separation. The reboiler 28 is typically driven by steam 37 generated by a steam turbine (not shown). An overhead ammonia stream 23 is withdrawn and directed to an ammonia condenser 30 using a cooling medium 41 such as water. The thus condensed ammonia stream 27 is directed to an ammonia recovery drum 12 from which said ammonia liquid stream 1 is withdrawn, thereby closing the ammonia refrigeration circuit.

[0087] If mechanical refrigeration of the same load were used, a centrifugal compressor driven by a condensing steam turbine would be required. The compressor replaces the heat exchanger 14, the absorber 18 with its cooling unit 18' and mixing vessel 18'', the heat exchanger 24, the reboiler 28, the regenerator 26, the pump 22 and the pump 20.

[0088] In the present invention, in the cooler processing nearly pure ammonia, ammonia vapor boils at low pressure, thereby cooling and partially condensing the ammonia product gas (optionally ammonia synthesis gas) on the tube side. The ammonia vapor is sent to an absorber, preferably a mixing vessel containing mostly water, where it is absorbed in water. The condensed ammonia and water mixture is then sent to a regenerator, preferably a distillation column. Pure ammonia, i.e., 99% by mass or more ammonia, for example 99.5% by mass or more ammonia, is recycled to the cooler as an ammonia liquid stream. Discharge of excess ammonia is also preferably made from the cooler purge stream, i.e., liquid blowdown from the cooler. Purified water from the regenerator is sent to an off-gas scrubbing unit to remove ammonia in the off-gas to trace levels, for example less than 10 ppmv ammonia. From the bottom of the off-gas scrubbing unit, water is sent to a mixing vessel.

[0089] 2, in accordance with an embodiment of the present invention, an ammonia cooling system 100 is shown. Ammonia liquid stream 101″, diverted from an overhead reflux stream 101′ in a downstream regenerator 126, passes through a heat exchanger 114 (e.g., an aftercooler using cooling water 131 as the heat exchange medium) and is placed in an upstream ammonia evaporator (chiller) 116. The thus-cooled ammonia liquid stream 101″ is then cooled to 500° C. iv is then vaporized in cooler 116 using ammonia product gas 105 from an ammonia synthesis converter of an ammonia synthesis plant (not shown). This produces ammonia vapor stream 107 which is directed to absorber 118, which includes a cooling unit 118' and a mixing vessel 118''. Cooling water 139 in cooler 118' removes the heat of absorption, thereby producing a condensed ammonia-water stream 119 which is withdrawn from mixing vessel 118'', transported via pump 122, heated by feed / discharge heat exchanger 124 using bottoms stream 125 from regenerator 126 as the heat exchange medium, and fed to regenerator 126 as stream 121. The thus cooled produced water stream 125' is further cooled in ammonia recovery cooler 144, for example using cooling water 147, to produce stream 125''. The purge ammonia stream (liquid blowdown) 129 is suitably combined with the condensed ammonia-water stream 119 from the absorber 118 before being transported to the regenerator 126 via pump 122. Discharge of excess ammonia 129' is also preferably provided from the cooler purge stream 129.

[0090] The regenerator 126 comprises an upper section I, which contains a number of trays 126', and a lower section II. At the bottom, a reboiler 128 is provided for boiling the bottom liquid 133 and generating steam 135 for driving the distillative separation. The reboiler 128 is driven by steam 137, which is preferably generated in a steam boiler for cooling the ammonia product gas leaving the ammonia converter, and is then fed to the ammonia product gas 105. The lower section II of the regenerator 126 may also be equipped with a power-driven heating device (not shown), for example fitted at the bottom of the column. The upper section I comprises an overhead section: an overhead ammonia stream 123 is withdrawn and directed to an ammonia condenser 130 with the aid of a cooling medium 141, such as water. The overhead ammonia stream 127 thus condensed is directed to a separation unit 112, preferably an ammonia recovery drum, from which an ammonia liquid fraction 101 is taken and also an overhead ammonia gas stream 139 is taken. A reflux pump 134 is positioned to deliver ammonia liquid fraction 101, 101', a portion of which is directed to the top of regenerator 126 as overhead reflux stream 101''. The remaining portion of the ammonia liquid fraction is utilized as ammonia liquid stream 101''.

[0091] In the lower section II of the regenerator 126, the reboiler 128 is driven by steam 137 produced in one or more steam boilers (not shown) that recover heat from the ammonia product gas stream. This allows the steam produced in the ammonia synthesis plant, e.g., an upstream ammonia synthesis loop, to be balanced with the steam used in the ammonia refrigeration system 100 at any operating load between 10-100% and any corresponding turndown. No steam turbine is required. Additionally, separation unit 112 at the top of the regenerator 126 provides ammonia liquid streams 101''', 101' for use in the ammonia refrigeration system 16. iv Apart from the excess ammonia discharge, a stream 101 is used as additional product ammonia. v can be obtained.

[0092] Further, instead of recycling the bottoms stream 25 from the regenerator 26 to the absorber 18 of FIG. 1, a bottoms purified water stream 125, 125', 125'', containing at least 99% by mass water, e.g., 99.98 mol% or more H2O, e.g., 100 mol% H2O with less than 50 ppmv NH3, is preferably conducted together with an overhead ammonia gas stream 139 to an off-gas scrubbing unit 132 for removing ammonia from an off-gas ammonia stream 143 produced in an ammonia synthesis plant. The purified water stream 125'' is thus used as a scrubbing agent. From the off-gas scrubbing unit 132, e.g., an off-gas scrubber, a water stream 117 is removed as a bottoms stream and then sent to the absorber 118 together with the ammonia vapor stream 107 from the cooler 116. From the off-gas scrubbing unit 132, a clean off-gas stream is removed, i.e., an ammonia-depleted off-gas stream 145 with less than 10 ppmv NH3, e.g., less than 7 ppmv.

Claims

1. 1. An ammonia synthesis plant comprising an ammonia synthesis converter and a downstream ammonia cooling system (100), wherein the ammonia synthesis converter is configured to receive an ammonia synthesis gas comprising hydrogen and nitrogen and produce an ammonia product gas stream (105); and the ammonia synthesis plant is configured to generate an off-gas ammonia stream (143); Here, the ammonia cooling system (100) comprises: - ammonia liquid stream (101, 101', 101''', 101 ｉｖ an ammonia evaporator (116) for evaporating said ammonia product gas stream (105) to produce an ammonia vapor stream (107), the heat exchange medium of said ammonia evaporator (116) being said ammonia product gas stream (105); - an off-gas scrubbing unit (132) for scrubbing said off-gas ammonia stream (143) under the addition of water as a scrubbing agent to produce a water stream (117) and an ammonia-depleted off-gas stream (145); - an absorption cooling unit (118) containing water for cooling said ammonia vapor stream (107) and collecting a condensed ammonia-water stream (119); from said condensed ammonia-water stream (119, 121) a purified water stream (125, 125', 125''); The ammonia liquid stream (101, 101', 101''', 101 ｉｖ )and an overhead ammonia gas stream (139); a reproduction unit (126) for generating Including, And here, - the ammonia evaporator (116) evaporates the ammonia liquid stream (101, 101', 101''', 101 ｉｖ ), and the ammonia liquid stream (101, 101', 101''', 101 ｉｖ ) to generate said ammonia vapor stream (107); - said off-gas cleaning unit (132), preferably an off-gas scrubbing unit, is arranged to receive the purified water stream (125, 125', 125'') from said regeneration unit as a scrubbing agent and said off-gas ammonia stream (143) to produce said ammonia-depleted off-gas stream (145) and said water stream (117); - said absorption cooling unit (118), preferably a mixing vessel (118'') containing water, is arranged to receive said water stream (117) and said ammonia vapour stream (107) thereby producing and collecting said condensed ammonia-water stream (119), said absorption cooling unit further comprising a cooling device (118'), e.g. a coil, arranged to receive a cooling medium (139), e.g. water, and an outlet for removing said condensed ammonia-water stream (119); said regeneration unit (126) is a distillation column arranged to receive said condensed ammonia-water stream (119, 121), said regeneration unit (126) comprising an outlet for removing said purified water stream (125) as a bottoms stream, an outlet for removing an overhead ammonia stream (123), and a separation unit (112) for separating said overhead ammonia gas stream (139) and said ammonia liquid stream (101) from said overhead ammonia stream (123), The ammonia synthesis plant.

2. 2. The ammonia synthesis plant of claim 1, comprising a water or steam electrolysis unit for producing the hydrogen, the electrolysis unit being powered by electricity from a renewable energy source, e.g. solar, wind or hydroelectric.

3. 3. An ammonia synthesis plant according to claim 1, wherein the off-gas cleaning unit (132) is an off-gas scrubbing unit, the off-gas cleaning unit (132) being further arranged to receive the overhead ammonia gas stream (139) from the regeneration unit (126), for example in admixture with the off-gas ammonia stream (143).

4. 4. The ammonia synthesis plant of claim 3, wherein the ammonia cooling system (100) further comprises means for add-mixing the overhead ammonia gas stream (139) with the off-gas ammonia stream (143), e.g., a splicing or mixing unit.

5. 5. The ammonia synthesis plant according to claim 1, wherein the absorption cooling unit (118) is a mixing vessel (118'') containing water.

6. In the ammonia cooling system (100), the regeneration unit (126) comprises an upper section (I) and a lower section (II), The lower section (II) comprises: a power driven heating device and / or reboiling unit (128) arranged to receive the heat exchange medium (137); and an outlet for removing said purified water stream (125) as a bottoms stream; Equipped with The upper section (I) - an inlet for receiving a condensed ammonia-water stream (119, 121); - A tower head containing: an outlet for removing an overhead ammonia stream (123); an ammonia condensing unit (130) arranged to receive said overhead ammonia stream (123) and a cooling medium (141), e.g., water, to produce a condensed overhead ammonia stream (127); a separation unit (112) arranged to receive said condensed overhead ammonia stream (127), preferably an ammonia recovery drum, comprising an outlet for removing an ammonia liquid fraction (101) and an outlet for removing said overhead ammonia gas stream (139); a reflux pump (134) arranged to receive said ammonia liquid fraction (101) and direct a portion thereof to the top of said column as an overhead reflux stream (101''); - means for removing the remaining portion of said ammonia liquid fraction as said ammonia liquid stream (101'''); Equipped with The ammonia synthesis plant according to any one of claims 1 to 5.

7. 7. The ammonia synthesis plant of claim 6, further comprising one or more steam boilers positioned upstream of the ammonia cooling system (100) to receive the ammonia product gas stream (105) for recovering heat therefrom and generating steam, wherein the plant is further configured to supply at least a portion of the steam as a heat exchange medium (137) to the reboil unit (128).

8. 8. An ammonia synthesis plant according to any one of claims 1 to 7, wherein in the ammonia cooling system (100), the ammonia evaporator (116) further comprises an outlet for removing a purge ammonia stream (129), and wherein the ammonia cooling system (100) further comprises means, upstream of the regeneration unit (126), for combining the purge ammonia stream (129) with the condensed ammonia-water stream (119), e.g. a joining unit or a mixing unit.

9. 9. The ammonia synthesis plant according to any one of claims 1 to 8, wherein the regeneration unit (126) of the ammonia cooling system (100) further comprises a feed / effluent heat exchanger (F / E HEX) 124 arranged to receive the condensed ammonia-water stream (119) or a stream (121) resulting from combining a purge ammonia stream (129) with the condensed ammonia-water stream (119), for receiving the purified water stream (125) as a heat exchange medium for pre-heating the condensed ammonia-water stream (119) or the stream (121) resulting from combining the condensed ammonia-water stream (119) with the purge ammonia stream (129) before entering the regeneration unit (126).

10. The ammonia refrigeration system (100) may generate additional ammonia product (129', 101''') from the purge ammonia stream (129) or from the ammonia liquid stream (101''') suitable for discharge as excess ammonia product. ｖ The ammonia synthesis plant according to any one of claims 1 to 9, further comprising a conduit for extracting a portion of the ammonia synthesis product as a reaction product.

11. 1. A process for producing an ammonia product gas (105) from an ammonia synthesis gas stream comprising hydrogen and nitrogen, comprising: The process also includes producing an off-gas ammonia stream (143); The method comprises: a) an optional electrolysis step of water or steam to produce said hydrogen; said electrolysis is preferably powered by electricity from a renewable energy source, e.g. solar, wind or hydropower; b) converting an ammonia synthesis gas into said ammonia product gas (105) in an ammonia synthesis converter; Including, The method comprises the steps of: c) subjecting an ammonia liquid stream (101, 101', 101''', 101') to an ammonia evaporator (116) for producing an ammonia vapor stream (107) while supplying said ammonia product gas (105) as a heat exchange medium. ｉｖ evaporating the condensed water; d) feeding said off-gas ammonia stream (143) to an off-gas scrubbing unit (132) with the addition of a purified water stream (125, 125', 125'') as a scrubbing agent to produce a water stream (117) and an ammonia-depleted off-gas stream (145); e) removing said water stream (117) from said off-gas scrubbing unit (132) and feeding it together with said ammonia vapor stream (107) under a supply of cooling medium (139), e.g. water, to an absorption cooling unit (118) to produce and collect a condensed ammonia-water stream (119); f) removing the condensed ammonia-water stream (119) from the absorption cooling unit (118) and feeding it to a regeneration unit (126) and regenerating the ammonia liquid stream (101, 101', 101''', 101 ｉｖ ), producing an overhead ammonia gas stream (139) and a purified water stream (125); The method further comprising cooling the ammonia product gas according to

12. 12. The method of claim 11, further comprising the step of: g) feeding said overhead ammonia gas stream (139) to said off-gas scrubbing unit (132).

13. 13. The method of any one of claims 11-12, wherein prior to step c), the ammonia product gas (105) is first cooled by passing it through one or more steam boilers to produce steam, and wherein in step f), the regeneration unit (126) comprises a power driven heating device or reboil unit (128), the method further comprising supplying the steam (137) to the reboil unit (128).

14. The method according to any one of claims 11 to 13, wherein in step c) a purge ammonia stream (129) is generated, the method further comprising withdrawing said purge ammonia stream (129) and combining it with said condensed ammonia-water stream (119).

15. The method according to any one of claims 11 to 14, further comprising converting a portion of the ammonia liquid stream (101'') or the purge ammonia stream (129) into an additional ammonia product (101 ｖ , 129′).

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