CN117715867A - Pressure control of an ammonia cooling circuit under varying loads - Google Patents
Pressure control of an ammonia cooling circuit under varying loads Download PDFInfo
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- CN117715867A CN117715867A CN202280048004.XA CN202280048004A CN117715867A CN 117715867 A CN117715867 A CN 117715867A CN 202280048004 A CN202280048004 A CN 202280048004A CN 117715867 A CN117715867 A CN 117715867A
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- vapor stream
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- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 title claims abstract description 924
- 229910021529 ammonia Inorganic materials 0.000 title claims abstract description 460
- 238000001816 cooling Methods 0.000 title claims abstract description 72
- 230000015572 biosynthetic process Effects 0.000 claims abstract description 94
- 238000003786 synthesis reaction Methods 0.000 claims abstract description 94
- 239000002826 coolant Substances 0.000 claims abstract description 57
- 238000000034 method Methods 0.000 claims abstract description 43
- 230000003134 recirculating effect Effects 0.000 claims abstract description 7
- 238000009420 retrofitting Methods 0.000 claims abstract description 7
- 239000000498 cooling water Substances 0.000 claims description 37
- 239000007789 gas Substances 0.000 claims description 27
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 19
- 239000001257 hydrogen Substances 0.000 claims description 19
- 229910052739 hydrogen Inorganic materials 0.000 claims description 19
- 239000007788 liquid Substances 0.000 claims description 18
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 16
- 238000005868 electrolysis reaction Methods 0.000 claims description 11
- 230000001105 regulatory effect Effects 0.000 claims description 10
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 8
- 230000005611 electricity Effects 0.000 claims description 7
- 238000000926 separation method Methods 0.000 claims description 5
- 238000001704 evaporation Methods 0.000 claims description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 238000010992 reflux Methods 0.000 claims description 4
- 238000010926 purge Methods 0.000 claims description 2
- 238000004064 recycling Methods 0.000 claims description 2
- 230000001276 controlling effect Effects 0.000 description 7
- 238000004519 manufacturing process Methods 0.000 description 4
- 238000005057 refrigeration Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 3
- 239000002184 metal Substances 0.000 description 3
- 238000002156 mixing Methods 0.000 description 3
- 238000011144 upstream manufacturing Methods 0.000 description 3
- 239000003054 catalyst Substances 0.000 description 2
- 230000006835 compression Effects 0.000 description 2
- 238000007906 compression Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- ZZUFCTLCJUWOSV-UHFFFAOYSA-N furosemide Chemical compound C1=C(Cl)C(S(=O)(=O)N)=CC(C(O)=O)=C1NCC1=CC=CO1 ZZUFCTLCJUWOSV-UHFFFAOYSA-N 0.000 description 2
- 239000006200 vaporizer Substances 0.000 description 2
- 238000013329 compounding Methods 0.000 description 1
- 238000009833 condensation Methods 0.000 description 1
- 230000005494 condensation Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- VZUGBLTVBZJZOE-KRWDZBQOSA-N n-[3-[(4s)-2-amino-1,4-dimethyl-6-oxo-5h-pyrimidin-4-yl]phenyl]-5-chloropyrimidine-2-carboxamide Chemical compound N1=C(N)N(C)C(=O)C[C@@]1(C)C1=CC=CC(NC(=O)C=2N=CC(Cl)=CN=2)=C1 VZUGBLTVBZJZOE-KRWDZBQOSA-N 0.000 description 1
- 238000010248 power generation Methods 0.000 description 1
- 230000001932 seasonal effect Effects 0.000 description 1
- 238000004904 shortening Methods 0.000 description 1
- 230000008016 vaporization Effects 0.000 description 1
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- Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
Abstract
The invention relates to a method for operating an ammonia cooling circuit of an ammonia synthesis device for preventing pressure changes in the cooling circuit due to fluctuating load or outlet temperature, wherein the method comprises: iii-1) adjusting the level of condensed ammonia in an ammonia condensing unit of the cooling circuit according to the pressure of the compressed ammonia vapor stream, wherein the ammonia condensing unit is a shell-and-tube heat exchanger; and/or iii-2) adjusting the flow of cooling medium to the ammonia condensing unit in dependence on the pressure of the compressed ammonia vapor stream of the cooling circuit; and/or iii-3) adjusting the temperature of the cooling medium directed to the ammonia condensing unit by recirculating a portion of the cooling medium back. The invention also relates to an ammonia cooling circuit arranged for carrying out the method, to the use thereof for retrofitting an ammonia synthesis plant to a green ammonia synthesis plant, and to an ammonia synthesis plant comprising the ammonia cooling circuit.
Description
The present invention relates to an ammonia cooling circuit of an ammonia synthesis device. Embodiments of the present invention relate to a method of operating an ammonia cooling circuit to reduce pressure variations therein caused by particularly fluctuating loads in a green ammonia synthesis plant, i.e. where the hydrogen required for ammonia synthesis is derived from electrolysis of water or steam powered by electricity generated by a renewable energy source. The invention also relates to an ammonia cooling circuit arranged for carrying out the method and to the use thereof for retrofitting an ammonia synthesis plant to a green ammonia synthesis plant.
In most current ammonia synthesis plants or processes, an ammonia cooling loop is typically used to cool the ammonia being produced. A well known principle of operation is to evaporate ammonia at low pressure, thereby creating a low temperature which in turn is used to cool the main process stream. The released ammonia vapor is then compressed in an ammonia compressor and the compressed vapor is condensed in an ammonia condensing unit by cooling water or air. The resulting liquid ammonia is then collected and reused for cooling.
Typically, the temperature and pressure variations in the ammonia cooling circuit are relatively small and are mainly caused by seasonal variations, as the load of the ammonia synthesis plant rarely deviates from the load close to full load. However, in green ammonia plants, hydrogen production by electrolysis and downstream green ammonia production are highly dependent on renewable energy sources, such as wind and solar energy, and such changes occur more frequently, for example about 20 changes per day. Temperature and pressure variations in an ammonia cooling loop are a direct consequence of the upstream fluctuating hydrogen load inherent in the intermittent nature of power generation when using, for example, wind or solar energy. In this case, the efficiency of the ammonia condenser in the ammonia cooling loop changes, even to the extent that stress fatigue may need to be considered in the design of the unit (i.e. the ammonia condensing unit). This has a significant impact on the risk of damaging the equipment (here in particular the ammonia condensing unit), equipment life, settling operations, in particular costs, especially on retrofit projects that require retrofitting (retrofitting) a conventional ammonia synthesis plant to a green ammonia synthesis plant.
WO 2020113011 discloses a method and apparatus for starting an air-cooled low charge packaged ammonia refrigeration system in stages, the system comprising an electrically operated valve on the condenser coil inlet, a main compressor discharge electrically operated valve, a bypass pressure regulating valve in the main compressor conduit, a check valve on the condenser outlet and speed control of the condenser fan. The cited reference thus relates to an air-cooled (non-evaporating) system which is not suitable for ammonia synthesis plants, the latter being essentially large in size, and the technical problem addressed in this reference mainly relates to eliminating the need for a separate oil pump which maintains the oil pressure during start-up.
US20200103149 also discloses an air-cooled ammonia refrigeration system and method. In some embodiments, an air-cooled ammonia refrigeration system includes: a plurality of air-cooled condensers, each condenser having a heat exchanger and at least one axial flow fan and having a first operating condition capable of condensing gaseous ammonia to form liquid ammonia; and an evaporator coupled to the air-cooled condenser. The system and method of this citation is not applicable to ammonia synthesis plants, as the latter are, as such, large in nature. Furthermore, to address the problems associated with the load in the system, this citation teaches providing a subcooler between the air-cooled condenser and the evaporator to reduce the temperature of the liquid ammonia, thereby reducing the load on the system.
It is therefore an object of the present invention to provide a robust ammonia cooling circuit capable of withstanding varying loads in ammonia synthesis plants, in particular in green ammonia synthesis plants, i.e. where the hydrogen required for ammonia synthesis is electrolytically derived from water or steam powered by electricity generated by renewable energy sources.
The present invention addresses this and other objects.
Thus, in a first general embodiment, the present invention is a method of operating an ammonia cooling circuit of an ammonia synthesis plant comprising an ammonia synthesis converter for producing an ammonia product gas stream, the method comprising:
i) Vaporizing an ammonia liquid stream in an ammonia vaporizer using (i.e., by providing) a heat exchange medium (e.g., an ammonia product gas stream) to produce an ammonia vapor stream, wherein the heat exchange medium is an ammonia product gas stream from an ammonia synthesis converter of an ammonia synthesis device;
ii) compressing the ammonia vapor stream in an ammonia compressor to produce a compressed ammonia vapor stream;
iii) Cooling the compressed ammonia vapor stream in an ammonia condensing unit using (i.e., by providing) a cooling medium such as cooling air or cooling water to produce a condensed ammonia stream and a cooling medium reflux;
iv) withdrawing a condensed ammonia stream and collecting it in an ammonia accumulator;
v) withdrawing the ammonia liquid stream from the ammonia accumulator;
wherein the method further comprises:
iii-1) adjusting the level of condensed ammonia in the ammonia condensing unit according to the pressure of the compressed ammonia vapor stream, wherein the ammonia condensing unit is a shell-and-tube heat exchanger with condensed ammonia passing on the shell side and a cooling medium (which is water (i.e. cooling water)) passing on the tube side.
It should be understood that the term "use" and the term "by providing" may be used interchangeably.
It should be understood that the cooling medium is water, which is used interchangeably with the term "cooling water".
It should be appreciated that the ammonia synthesis plant includes an ammonia synthesis converter for producing an ammonia product gas stream from an ammonia synthesis gas comprising hydrogen and nitrogen.
It should be understood that in the case where hydrogen is supplied by electrolysis of water or steam, the ammonia synthesis plant is a green ammonia synthesis plant. Thus, a green ammonia plant is defined herein as such an ammonia plant: wherein the hydrogen required for ammonia synthesis is provided by electrolysis of water or steam powered by electricity generated from renewable energy sources such as wind, solar or hydro.
The term "comprise" includes "comprising only", i.e. "consisting of.
The term "suitably" refers to an alternative, i.e. alternative embodiment.
The terms "invention (present invention)" or simply "invention" and "application (present application)" are used interchangeably.
The term "first aspect", for example "first aspect of the invention" refers to a method of operating an ammonia cooling circuit. The term "second aspect" refers to "ammonia cooling circuit". The term "third aspect" refers to the use of an ammonia cooling circuit for retrofitting an ammonia synthesis plant. The term "fourth aspect" refers to an ammonia synthesis device comprising an ammonia cooling loop.
Additional definitions are provided below in connection with one or more embodiments.
The discharge pressure from the ammonia compressor (i.e. the pressure of the compressed ammonia vapor stream) depends on the condensation temperature of the ammonia vapor, which also depends on the cooling medium temperature in the ammonia condensing unit and its efficiency. The present invention enables varying the efficiency of the ammonia condensing unit and/or the flow rate and temperature of the cooling medium temperature, thereby reducing pressure variations.
It is therefore generally expected that a reduction of the load (hereinafter also referred to as "load") in an ammonia synthesis plant will suggest that the pressure of the cooling circuit is reduced by cooling as much as possible in the ammonia condensing unit to save compression energy, since the temperature of the condensed ammonia stream at the outlet of the ammonia condensing unit determines the pressure of the ammonia cooling circuit, which is the saturation pressure at that temperature, in contrast to the present invention aims to maintain the pressure in the cooling circuit, avoiding stress fatigue of the plant (ammonia condensing unit) and thus reducing the risk of plant life.
As used herein, the term "load" refers to the percentage of hydrogen feed to the ammonia synthesis unit for performing ammonia synthesis relative to normal operating load (100% load). It is assumed that the ammonia production capacity of the plant is proportional to the hydrogen feed used. It will thus be appreciated that a load of 10% means that the hydrogen for ammonia synthesis is 10% relative to normal operation. The hydrogen gas used comes from water (steam) electrolysis, as is well known in the art. Full load refers to 100% load and corresponds to normal operation of the ammonia synthesis plant.
As used herein, the term "normal operation" refers to a condition where the load is 100% (100% load) and the cooling water to the ammonia condensing unit is set to a typical value under summer daytime conditions, for example 29 ℃.
For the purposes of this application, normal operation may alternatively be considered operation of an ammonia synthesis plant in which the ammonia synthesis converter is operated at a load corresponding substantially to full capacity (100%) and thus varies by + -15%, e.g. a slight gap of + -10%, relative to the design value of the ammonia synthesis converter, i.e. the ammonia synthesis gas being directed therethrough is 85-115%, e.g. 90%, 95%, 100%, 105%, 110% of the design value of the converter.
The design value of the converter is suitably expressed in terms of Space Velocity (SV), for example the design value of the converter (reactor) is the space velocity over the first catalyst bed of the catalyst module, which is in the range 20000 to 50000Nm 3 /h/m 3 In the range of 25000 to 45000Nm, for example 3 /h/m 3 Within a range of (2). Thus, a low load of, for example, 10% means that the SV is in the range of, for example, 2500-4500Nm 3 /h/m 3 2500-4500h -1 Within a range of (2).
It will be appreciated that at varying loads, the reactor is operated at a load significantly different from the normal load (and thus from full capacity) with a variation of greater than + -15%, i.e. the process gas directed therethrough is for example 5-80% of the reactor design value, or for example 120% or 125% of the reactor design value.
The ammonia synthesis gas comprises hydrogen (H) 2 ) And nitrogen (N) 2 ) Suitably its molar ratio H 2 :N 2 3.
In carrying out the process according to the invention of step iii-1), the pressure variation is reduced or eliminated by controlling the efficiency of the ammonia condensing unit. This is achieved by, for example, providing a valve at the outlet of the ammonia condensing unit to adjust the level of condensed ammonia (i.e. the level of condensed ammonia in the shell side of the ammonia condensing unit). The valve is suitably adjusted by the liquid level in the ammonia condensing unit, depending on the discharge pressure of the ammonia compressor.
The ammonia condensing unit is a shell-and-tube heat exchanger, such as a U-tube type, and a straight tube type, such as a single pass tube side or a double pass tube side. The cooling medium is cooling water, for example cooling water from a cooling tower. For example, during normal plant operation (100% load, summer day conditions), the cooling water temperature is for example 29 ℃, but may increase to for example 32 ℃ in summer or decrease significantly at night, for example to for example 23 ℃ or even lower, resulting in temperature and pressure variations as well. Thus, even when operating at the same load (100% load), a higher or lower cooling water temperature may result in a significant change in the discharge pressure, i.e. in the cooling circuit, compared to normal operation, such that the maximum allowable pressure change exceeds the threshold required to avoid stress fatigue, as will become more apparent below.
For example, in the event that the load in the device is reduced to 10% relative to a normal 100% load, the discharge pressure and temperature change significantly, e.g., the pressure fluctuation is reduced to-10%, -20% or more relative to normal operation, resulting in a risk of stress fatigue, as further described above. These variations are reduced or eliminated by deliberately reducing the efficiency of the ammonia condensing unit relative to normal operation.
According to the invention, in step iii-1), the shell-and-tube heat exchanger is a shell-and-tube heat exchanger with condensed ammonia passing on the shell side and a cooling medium (which is water, i.e. cooling water) passing on the tube side.
In one embodiment, in iii-1), the level of ammonia condensed by the ammonia condensing unit is such that 70% or less, e.g. 65% or less, e.g. 60% or less, 50% or less, 40% or less, 30% or less, 20% or less, e.g. 10-75% of the surface area for heat exchange of the ammonia condensing unit is available relative to normal operation.
Thus, in case the ammonia condensing unit is, for example, a shell-and-tube heat exchanger having condensed ammonia in the shell side and cooling water in the tube side, 30% or more of the tubes (i.e. tube bundles) are intentionally covered with condensed ammonia relative to when operating under normal conditions. It should be appreciated that under normal conditions, the level of condensed ammonia in the ammonia condensing unit does not necessarily make 100% of the surface area for heat exchange available in the ammonia condensing unit. Under normal conditions, typically about 95% of the surface area for heat exchange is available.
Thus, when the ammonia synthesis device is operated at varying loads, particularly below normal loads (100% load), the tubes may be "submerged" in the condensed ammonia to some extent, such that the compressed ammonia vapor stream entering the ammonia condensing unit is exposed to less cooling surface area than normal conditions, as many of the tubes are submerged in the condensed ammonia. For example, when operating at a lower load (e.g., 10% or 40% of normal load), a higher surface area will be less desirable in the ammonia condensing unit.
When operating an ammonia synthesis plant at, for example, 10% load, the discharge pressure from the ammonia compressor is reduced and the variation in discharge pressure (i.e., the pressure variation in the cooling circuit relative to normal operation) may be greater than the threshold required to avoid stress fatigue, resulting in a higher risk of plant stress fatigue. The pressure change may also be the result of a change in the condensed ammonia temperature (i.e. the outlet temperature of the ammonia condensing unit), for example due to the significantly lower temperature of the cooling water from the cooling tower used during the night or winter, while still operating under normal load. It has been found that the effect of the cooling water temperature variation may also be compounded by the effect of the load reduction, causing significant pressure variations in the ammonia cooling circuit, thereby putting the plant into stress fatigue.
Such pressure variations may in particular lead to the risk of reaching a maximum allowable pressure variation in the ammonia condensing unit of-15% of the design pressure for normal operation, i.e. when operating under normal load. For example, the design pressure for normal operation of an ammonia condensing unit is 25barg (25 bar above atmospheric). Operating at 10% load, for example at night with a cooling water temperature of 25 ℃, may result in a pressure change of-15% relative to the mechanical design pressure, i.e. 3.75barg (15% according to ASME standard ASME BPVC viii.2-2019, 25 barg), if for example the level of condensed ammonia in the ammonia condensing unit remains unchanged (such as when the load is 100% and the cooling water temperature is the daytime temperature (e.g. 29 ℃)). If the variation in mechanical design pressure is above this threshold of 3.75barg, i.e. greater than a-15% variation, for example-20%, the risk of equipment stress fatigue and equipment life is imminent.
By performing step i), the liquid level in the ammonia condensing unit is adjusted by exposing only 70% or less of the available surface area, the relative efficiency of the ammonia condensing unit is reduced relative to normal operation, thereby reducing or eliminating pressure variations.
As used herein, the term "relative efficiency" refers to the percentage of the available surface area of the ammonia condensing unit relative to normal operation. Thus, under normal operation, the relative efficiency of the ammonia condensing unit is 1 (100%).
Hereby a robust ammonia circulation loop is achieved which is able to withstand varying load and/or temperature conditions from day to night in an ammonia synthesis device, in particular when the ammonia synthesis device is retrofitted to a green ammonia synthesis device, or when a new green ammonia synthesis device is established. In a green ammonia plant, it is inevitable that the load is subjected to fluctuations due to the intermittent nature of renewable energy sources. In addition, green ammonia plants are often intended to be installed and operated in areas of the world where renewable energy (such as solar energy) availability is high, and thus also in areas where outdoor temperature diurnal variations are significant. For example, the significantly lower temperature at night results in challenges with significantly lower temperatures in the cooling water from the cooling tower, compounding the varying load that allows the variation of the design pressure in the ammonia condensing unit to reach values above the threshold of 3.75barg described above.
The term "robust" refers to the risk of achieving a stable pressure in the ammonia cooling circuit, regardless of, for example, load variations and/or external temperature variations, thereby preventing stress fatigue of the equipment and thereby shortening the life of the equipment (e.g., the life of the ammonia condensing unit).
As used herein, the term "ammonia cooling circuit" has the same meaning as "ammonia refrigeration circuit" commonly used in the art of ammonia synthesis.
In one embodiment, the method further comprises:
iii-2) adjusting the flow of cooling medium to the ammonia condensing unit in dependence on the pressure of the compressed ammonia vapor stream; and/or
iii-3) adjusting the temperature of the cooling medium directed to the ammonia condensing unit by recirculating a portion of the cooling medium back-flow.
Thus, by the present invention, a robust ammonia circulation loop can also be achieved according to step iii-2) by adjusting the flow of the cooling medium to the ammonia condensing unit, which is also suitably cooling water, thereby also providing a simple alternative that can be used alone or in combination with iii-1) or iii-3). In carrying out step iii-2), the flow of the cooling medium is regulated by, for example, providing a valve at the cooling medium inlet to the ammonia condensing unit, and it is regulated by the discharge pressure of the ammonia compressor. The flow rate is also adjusted to be high enough to prevent undesirable fouling in the ammonia condensing unit.
By the present invention, a robust ammonia cooling loop is also achieved by controlling the temperature in the ammonia condensing unit (i.e. the cooling medium temperature) via recirculation of a portion of the cooling medium back stream (i.e. the cooling medium after use in the ammonia condensing unit), according to step iii-3), thereby also providing a simple alternative that can be used alone or in combination with iii-1) or iii-2). The amount of recirculation is controlled, for example, by providing a valve in the cooling medium return flow before mixing with the incoming cooling medium, and it is regulated by the discharge pressure of the ammonia compressor. Since cooling water is used as the cooling medium, a pump is provided in the recirculation. It is also understood that the term "recirculating a portion of the cooling medium back-flow" refers to mixing with the incoming cooling medium, as described above.
In the present application, the heat exchange medium of the ammonia evaporator in step i) is the ammonia product gas stream from the ammonia synthesis converter of the ammonia synthesis plant. Thus, the ammonia cooling circuit is integrated downstream of the ammonia synthesis converter of the ammonia synthesis device. The ammonia product gas thus cooled is suitably separated into product ammonia and an ammonia product gas recycle stream that is directed to an ammonia synthesis converter of an ammonia synthesis process or plant and which may additionally be used to cool the ammonia product gas prior to entering the ammonia evaporator.
In one embodiment, step i) is performed in a plurality of ammonia evaporators, for example in two ammonia evaporators arranged in series or in parallel.
In one embodiment, the ammonia vapor stream is directed to a knock-out drum for producing said ammonia vapor stream prior to performing step ii), i.e. ammonia vapor compression. The ammonia compressor is thus protected because any droplets in the ammonia vapor stream are removed.
In one embodiment, an ammonia liquid purge stream is withdrawn from an ammonia evaporator and directed to an ammonia separation unit, such as a flash evaporator, for producing a separated ammonia vapor stream. In a specific embodiment, the separated ammonia vapor stream is fed directly to the ammonia compressor in step ii) or is combined with said ammonia vapor stream from the knock-out pot, i.e. before step ii) is performed. The ammonia compressor is thus still protected because any droplets in the ammonia vapor stream are removed in the ammonia separation unit (e.g., flash vessel) and in the knock-out pot as described above. The ammonia separation unit may also be arranged to receive the ammonia product gas and to discharge as a bottom effluent liquid ammonia product.
In one embodiment, step ii) is performed in a multi-stage ammonia compressor (e.g., a two-three stage ammonia compressor).
Suitably, the separated ammonia vapor from the ammonia separation unit (e.g., flash vessel) is sent to the first stage of a multi-stage ammonia compressor.
It should be understood that throughout this application, the term "discharge pressure of the ammonia compressor" refers to the most downstream compressor when a multi-stage ammonia compressor is used. It is also understood that when referring to a pressure change in the ammonia cooling circuit, it refers to the discharge pressure from the ammonia compressor, which is the same as the pressure in the ammonia condensing unit.
In a particular embodiment, step ii) is performed in a multi-stage ammonia compressor using an inter-stage knock-out pot. Likewise, the knock out pot is provided to protect the ammonia compressor by removing any droplets in the ammonia vapor stream.
Under normal operating conditions, the load of the ammonia synthesis plant rarely deviates from full load (100% load), so the risk of ammonia compressor surge is generally low. It has also been found that load variations can be further reduced or eliminated by specific anti-surge control of the ammonia compressor.
The risk of surge in ammonia compressors increases when the load changes and decreases to, for example, 40% or 10% of normal load. As is well known in the art, surge is a serious technical problem in compressors, the flow through the compressor is reversed by surge, and surge must be prevented to avoid damaging the compressor.
Anti-surge systems use instrumentation surrounding the compressor, including a surge controller that algorithmically monitors the performance of the compressor to monitor when its operation is approaching a surge condition. For example, the flow rate, pressure and temperature of a suction line carrying ammonia vapor and a discharge line carrying compressed ammonia vapor are fed into a surge controller, and when a surge condition is reached, gas from the discharge line is recirculated through a flow control valve to the suction line to maintain a forward flow in the ammonia compressor.
During normal operation, no anti-surge system is needed, but when the load changes and decreases to e.g. 10%, the lower capacity of the ammonia compressor can therefore be managed by a speed controller in the compressor motor and/or by using an anti-surge system as described above. However, this results in recirculation of the hot ammonia vapor upstream of the ammonia compressor with the consequent risk of the temperature of the metal parts therein reaching a temperature variation higher than 28 ℃, which requires removal of the compressor or compressor parts for stress fatigue analysis.
Thus, in another embodiment, step ii) further comprises providing an anti-surge system in the ammonia compressor, such as an anti-surge valve, i.e. a recoil valve, and optionally a flow regulating valve; and further recycling a further part of the compressed ammonia vapor stream, i.e. a dedicated process recycle stream, e.g. up to 10% of its flow, e.g. up to 1% of its flow, through an anti-surge valve and an optional flow regulating valve of the ammonia compressor.
Thus, a continuous flow may be achieved to prevent the risk of stress fatigue in the ammonia compressor or metal parts therein, as the flow is controlled in order to keep the temperature variation of the metal parts of the ammonia compressor below 28 ℃, from the almost closed position to the open position of the anti-surge valve. Although the anti-surge system is dedicated to the protection of ammonia compressors, the dedicated process recycle stream can further protect additional equipment, such as piping and anti-surge coolers for the final stage of the ammonia compressor, in a simple manner.
Thereby, the robustness of the ammonia cooling circuit is further enhanced by preventing the risk of stress fatigue in the equipment (not only the ammonia condensing unit, but also the additional equipment).
In a second general embodiment according to the first aspect, the invention is a method of operating an ammonia cooling circuit of an ammonia synthesis plant comprising an ammonia synthesis converter for producing an ammonia product gas stream, the method comprising:
i) Evaporating the ammonia liquid stream in an ammonia evaporator using a heat exchange medium to produce an ammonia vapor stream, wherein the heat exchange medium is an ammonia product gas stream from an ammonia synthesis converter of an ammonia synthesis device;
ii) compressing the ammonia vapor stream in an ammonia compressor to produce a compressed ammonia vapor stream;
iii) Cooling the compressed ammonia vapor stream in an ammonia condensing unit using a cooling medium to produce a condensed ammonia stream and a cooling medium reflux;
iv) withdrawing a condensed ammonia stream and collecting it in an ammonia accumulator;
v) withdrawing the ammonia liquid stream from the ammonia accumulator;
wherein the method further comprises:
-adjusting the flow of cooling medium to the ammonia condensing unit in dependence of the pressure of the compressed ammonia vapor stream; and/or
-adjusting the temperature of the cooling medium led to the ammonia condensing unit by recirculating a portion of the cooling medium back-flow.
The associated benefits are further described above in connection with the corresponding embodiment according to the first general embodiment.
In a second aspect, the invention also comprises an ammonia cooling circuit arranged for carrying out the method according to any of the above embodiments.
The ammonia cooling circuit includes:
-an ammonia evaporator arranged to receive the ammonia liquid stream and to provide an ammonia vapor stream, the ammonia evaporator being arranged to receive the ammonia product gas stream from the ammonia synthesis converter of the ammonia synthesis device as a heat exchange medium;
-an ammonia compressor arranged to receive the ammonia vapor stream and to provide a compressed ammonia vapor stream;
-an ammonia condensing unit arranged to receive the compressed ammonia vapor stream and to provide a condensed ammonia stream, the ammonia condensing unit being a shell-and-tube heat exchanger arranged to receive a cooling medium, the cooling medium being water, i.e. cooling water, wherein the condensed ammonia passes on the shell side and the cooling water passes on the tube side;
-an ammonia accumulator arranged to receive the condensed ammonia stream and to provide an ammonia liquid stream.
The ammonia cooling loop does not include a subcooler between the air condensing unit condenser and the ammonia evaporator, the subcooler being arranged to remove heat from the ammonia liquid stream flowing from the air-cooled condenser to the ammonia evaporator.
In a third aspect, the invention comprises the use of an ammonia cooling circuit according to the second aspect of the invention for retrofitting an ammonia synthesis plant to a green ammonia synthesis plant. The ammonia cooling circuit of the present invention is thus suitably retrofitted to an ammonia cooling circuit of an ammonia synthesis plant that is retrofitted (retrofitted) to a green ammonia synthesis plant, i.e. an ammonia synthesis plant in which the hydrogen required for ammonia synthesis is provided by water or steam electrolysis powered by renewable energy sources such as wind, solar or hydraulically generated electricity.
In a fourth aspect, the invention comprises an ammonia synthesis device, suitably a green ammonia synthesis device, comprising an ammonia cooling loop arranged for carrying out the method according to any of the above embodiments, the green ammonia synthesis device being defined as such an ammonia synthesis device: wherein the hydrogen required for ammonia synthesis is provided by water or steam electrolysis powered by renewable energy sources such as wind, solar or hydraulically generated electricity.
Accordingly, the ammonia synthesis device suitably comprises:
-an ammonia cooling circuit according to the second aspect of the invention;
-an ammonia synthesis converter arranged to receive an ammonia synthesis gas comprising hydrogen and nitrogen and to provide an ammonia product gas stream;
-an electrolysis unit arranged to receive water or steam and to provide said hydrogen.
It will be appreciated that in the case where the electrolysis unit is powered by electricity generated from a renewable energy source such as wind, solar or hydro, the ammonia synthesis plant is a green ammonia synthesis plant.
Any of the embodiments of the first aspect of the invention and associated advantages may be used in combination with the second, third and fourth aspects of the invention.
Fig. 1 shows a typical arrangement of an ammonia cooling circuit under normal operation.
Fig. 2 shows an embodiment with a varying load according to the invention, wherein the ammonia cooling circuit is operated by controlling the level of ammonia condensed in the ammonia condensing unit.
Fig. 3 shows an embodiment according to the invention with a varying load, wherein the ammonia cooling circuit is operated by mixing a part of the back flow of the cooling medium with the cooling medium in the ammonia condensing unit and/or by controlling the flow of the cooling medium.
Referring to FIG. 1, an ammonia cooling circuit under normal operation (100% load, typical conditions during summer day, where the cooling water temperature is, for example, 29 ℃) is shown. An ammonia cooling circuit is disposed downstream of the ammonia synthesis converter in the ammonia synthesis device. From the ammonia compressor 10 in which the drive unit 10' is arranged, a compressed ammonia vapor stream 1 is produced. The discharge pressure (i.e., the pressure of the ammonia vapor stream 1) is typically about 13barg under normal operating conditions. In the ammonia condensing unit 20, the compressed vapor stream is cooled to produce a condensed ammonia stream 2 at about the same pressure as the upstream pressure (i.e., about 13 barg). For cooling, a cooling medium (cooling water 3), here cooling water at 29 ℃, from, for example, a cooling tower, enters the ammonia condensing unit 20 and leaves as a cooling medium return 4. The condensed ammonia stream 2 is collected in an ammonia accumulator 30 and then recycled as ammonia liquid stream 5. The ammonia liquid stream 5 is vaporized at low pressure in an ammonia vaporizer 40 using ammonia product gases 8, 9 from an ammonia synthesis converter (not shown) as heat exchange medium. The resulting ammonia vapor stream 6 is directed to a knock out pot 50 to remove any droplets, then discharged as ammonia vapor stream 7 and finally compressed in an ammonia compressor 10, closing the circuit. Since the load (100% load, i.e. normal operation) is stable over a longer period of time, there is no dedicated means for controlling the pressure in the cooling circuit.
The use of renewable energy sources to produce ammonia will provide fluctuations in the feed gas flow rate of hydrogen over the course of a day, resulting in fluctuating loads relative to normal operation, producing many and possibly also abrupt pressure fluctuations in the ammonia recycle loop. Furthermore, significant changes between diurnal temperatures result in significant changes in cooling water temperature (as this cooling water is typically from a cooling tower), thereby also significantly affecting the outlet temperature of the ammonia condensing unit, and thus the discharge pressure of the compressed ammonia stream. These are solved, i.e. reduced or even eliminated, by the method according to the invention.
Thus, with reference to fig. 2, by controlling the efficiency of the ammonia condensing unit 20, i.e. how much surface area is available in the ammonia condensing unit (shell-and-tube heat exchange unit) for heat exchange, pressure variations (pressure variations of the discharge pressure of the compressed ammonia stream 1) are reduced or eliminated. As shown, the level of ammonia condensed on the shell side of the ammonia condensing unit 20 is adjusted by providing a valve 20' at the outlet of the ammonia condensing unit 20. The valve is suitably regulated by the discharge pressure of the ammonia compressor (as indicated by PC 20' "), alternatively also by the liquid level in the ammonia condensing unit 20 (as indicated by LC 20").
In the event that the load in the plant is reduced to, for example, 10% relative to the normal 100% load, and this may be further compounded with the night reduced cooling water temperature, significant changes in the discharge pressure and temperature occur, which are reduced or eliminated by deliberately reducing the efficiency of the ammonia condensing unit. This is achieved by: the level of condensed ammonia in the ammonia condensing unit 20 is adjusted so that by deliberately covering the tubes in the ammonia condensing unit with condensed ammonia, the surface area for heat exchange therein is less than that available when operating under normal conditions (normal operation). The cooling medium 3 is cooling water supplied from a cooling tower.
Referring to fig. 3, the pressure variation (pressure variation of the discharge pressure of the compressed ammonia stream 1) is reduced or eliminated by adjusting the flow rate of the cooling medium to the ammonia condensing unit 20 and/or by controlling the temperature of the cooling medium 3 to the ammonia condensing unit 20. Thus, in the former method, the pressure variation of the discharge pressure of the compressed ammonia stream 1 is reduced or eliminated by adjusting the flow of the cooling medium 3 (e.g., air or water) to the ammonia condensing unit 20. Through valve 20 v The flow is appropriately regulated according to the discharge pressure of the ammonia compressor (as represented by PC 20' "). In the latter method, a recirculation flow 4 'from the cooling medium return flow 4 is provided, so that the temperature of the cooling medium 3' led to the ammonia condensing unit 20 is adjusted. Through valve 20 iv The recycle stream is appropriately adjusted according to the discharge pressure of the ammonia compressor (as represented by PC 20' "). Since cooling water is used as the cooling medium, a pump (not shown) for recirculating flow 4' is provided.
Examples
The following table shows the temperature and pressure changes of the ammonia cooling circuit of the ammonia synthesis device with respect to normal operation (100% load, daytime cooling water inlet temperature of e.g. 29 ℃) at different loads, the load of the ammonia condensing unit using cooling water as cooling medium, and the relative efficiency of the ammonia condensing unit at varying loads and nighttime cooling water inlet temperatures. In general, the pressure change is a result of a change in the temperature of the condensed ammonia (i.e., the outlet temperature of the ammonia condenser) caused by a lower cooling water inlet and/or an oversized ammonia condensing unit due to the low load.
The values in the table for a relative efficiency of 100% essentially correspond to the operation of the cooling circuit according to fig. 1, and therefore the load resulting in pressure variations and the variation of the cooling water temperature are not properly reduced to avoid reaching a threshold of-15% of the maximum allowable pressure variation of 100% load with respect to the mechanical design pressure of the ammonia condensing unit, i.e. 15% (3.75 barg) of 25 barg.
For example, if the ammonia production is changed to 40% duty and the night cooling water temperature is reduced to 23 ℃ and the relative efficiency of the ammonia condensing unit is kept unchanged (100%) by not reducing the available surface area for heat exchange therein, a threshold of-15% is reached (13.1-3.75 = 9.3 barg). However, as described with respect to fig. 2, by partially "flooding" the tubes (tube bundles) of the ammonia condensing unit, the relative efficiency is 60% and the pressure variation is only-5% and is therefore acceptable.
Watch (watch)
* Basic condition
** According to one embodiment of the invention, the ammonia condensing unit is a shell-and-tube heat exchanger with condensed ammonia outside the tube and cooling water inside the tube by partially "flooding" the tube in the ammonia condensing unit
*** According to ASME BPVC viii.2-2019, the maximum allowable pressure variation of the ammonia condensing unit is 15% of the mechanical design pressure, i.e. 15% of 25 barg=3.75 barg
**** Relative efficiency: percentage of available surface area of the ammonia condensing unit relative to normal operation with cooling water at 29 ℃ and 100% load.
Claims (12)
1. A method of operating an ammonia cooling circuit that is an ammonia cooling circuit of an ammonia synthesis device that includes an ammonia synthesis converter for producing an ammonia product gas stream, the method comprising:
i) Evaporating the ammonia liquid stream in an ammonia evaporator using a heat exchange medium to produce an ammonia vapor stream, wherein the heat exchange medium is an ammonia product gas stream from an ammonia synthesis converter of the ammonia synthesis device;
ii) compressing the ammonia vapor stream in an ammonia compressor to produce a compressed ammonia vapor stream;
iii) Cooling the compressed ammonia vapor stream in an ammonia condensing unit using a cooling medium to produce a condensed ammonia stream and a cooling medium reflux;
iv) withdrawing the condensed ammonia stream and collecting it in an ammonia accumulator;
v) withdrawing the ammonia liquid stream from the ammonia accumulator;
wherein the method further comprises:
iii-1) adjusting the level of condensed ammonia in the ammonia condensing unit according to the pressure of the compressed ammonia vapor stream, wherein the ammonia condensing unit is a shell-and-tube heat exchanger with condensed ammonia passing on the shell side and a cooling medium passing on the tube side, the cooling medium being water, i.e. cooling water.
2. The method of claim 1, wherein in iii-1) the level of condensed ammonia in the ammonia condensing unit is such that 70% or less of the surface area of the ammonia condensing unit for heat exchange is available relative to normal operation.
3. The method of any of claims 1-2, further comprising:
iii-2) adjusting the flow of cooling medium to the ammonia condensing unit according to the pressure of the compressed ammonia vapor stream; and/or
iii-3) adjusting the temperature of the cooling medium directed to the ammonia condensing unit by recirculating a portion of the cooling medium back.
4. A process according to any one of claims 1-3, wherein prior to performing step ii) the ammonia vapor stream is directed to a knock-out pot for producing the ammonia vapor stream.
5. The process of any one of claims 1-4, wherein an ammonia liquid purge stream is withdrawn from the ammonia evaporator and directed to an ammonia separation unit, such as a flash evaporator, for producing a separated ammonia vapor stream.
6. The method of claim 5, wherein the separated ammonia vapor stream is fed directly to the ammonia compressor in step ii) or combined with the ammonia vapor stream from the knock-out pot.
7. The method of claim 6, wherein the separated ammonia vapor stream is fed directly to the ammonia compressor in step ii) or is combined with the ammonia vapor stream from the knock-out pot, i.e., prior to performing step ii).
8. The method of any of claims 1-7, wherein step ii) further comprises providing an anti-surge system, such as an anti-surge valve, i.e., a recoil valve, and optionally a flow regulating valve, in the ammonia compressor; and further recycling a further portion of the compressed ammonia vapor stream, i.e. a dedicated process recycle stream, e.g. up to 10% of its flow, e.g. up to 1% of its flow, through the anti-surge valve of the ammonia compressor and the optional flow regulating valve.
9. A method of operating an ammonia cooling circuit that is an ammonia cooling circuit of an ammonia synthesis device that includes an ammonia synthesis converter for producing an ammonia product gas stream, the method comprising:
i) Evaporating the ammonia liquid stream in an ammonia evaporator using a heat exchange medium to produce an ammonia vapor stream, wherein the heat exchange medium is an ammonia product gas stream from an ammonia synthesis converter of the ammonia synthesis device;
ii) compressing the ammonia vapor stream in an ammonia compressor to produce a compressed ammonia vapor stream;
iii) Cooling the compressed ammonia vapor stream in an ammonia condensing unit using a cooling medium to produce a condensed ammonia stream and a cooling medium reflux;
iv) withdrawing the condensed ammonia stream and collecting it in an ammonia accumulator;
v) withdrawing the ammonia liquid stream from the ammonia accumulator;
wherein the method further comprises:
-adjusting the flow of cooling medium to the ammonia condensing unit in dependence of the pressure of the compressed ammonia vapor stream; and/or
-adjusting the temperature of the cooling medium led to the ammonia condensing unit by recirculating a portion of the cooling medium back-flow.
10. An ammonia cooling circuit arranged for carrying out the method according to any one of claims 1-9.
11. Use of the ammonia cooling circuit according to claim 10 for retrofitting an ammonia synthesis device to a green ammonia synthesis device defined as such: wherein the hydrogen required for ammonia synthesis is provided by power water or steam electrolysis from renewable energy sources such as wind, solar or hydro-generated electricity.
12. An ammonia synthesis device comprising:
-an ammonia cooling circuit according to claim 10;
-an ammonia synthesis converter arranged to receive an ammonia synthesis gas comprising hydrogen and nitrogen and to provide an ammonia product gas stream;
-an electrolysis unit arranged to receive water or steam and to provide said hydrogen.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DKPA202100759 | 2021-07-14 | ||
| DKPA202100993 | 2021-10-18 | ||
| DKPA202100993 | 2021-10-18 | ||
| PCT/EP2022/068753 WO2023285240A1 (en) | 2021-07-14 | 2022-07-06 | Control of pressure in an ammonia cooling circuit at varying loads |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CN117715867A true CN117715867A (en) | 2024-03-15 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN202280048004.XA Pending CN117715867A (en) | 2021-07-14 | 2022-07-06 | Pressure control of an ammonia cooling circuit under varying loads |
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| Country | Link |
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| CN (1) | CN117715867A (en) |
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2022
- 2022-07-06 CN CN202280048004.XA patent/CN117715867A/en active Pending
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