Patent Application
20250060154
The presently disclosed method relates to turndown of a nitrogen-based refrigeration system and process suitable for use in pre-cooling and/or liquefaction of a hydrogen feed stream.
Refrigerators for hydrogen liquefaction or hydrogen precooling typically utilize a nitrogen-based Reverse Brayton cycle refrigeration process with refrigerant recycling. The nitrogen-based Reverse Brayton cycle refrigeration process involves expanding a nitrogen refrigerant across one or more turbines and/or Joule Thomson valves. The low pressure exhaust streams and/or expanded stream is the used to cool the hydrogen feed stream. The resulting warmed low pressure refrigerant is then recycled and compressed in the refrigerant recycle loop. The purpose of the recycle loop is to conserve the nitrogen refrigerant molecules in order to minimize the required supply of nitrogen refrigerant from an external source.
most common commercial arrangement for hydrogen liquefaction typically includes a heat exchanger to cool the nitrogen refrigerant with the refrigeration provided in three stages. The first two stages of refrigeration utilize turbine/expanders, while the final stage of refrigeration typically employs Joule Thomson expansion to create a two-phase mixture from which liquid can be extracted. The two turbines are employed at different temperature levels along the heat exchanger and are typically referred to as the warm turbine/expander and cold turbine. Both the warm turbine/expander and the cold turbine/expander are each usually directly connected or coupled to a single stage booster compressor that extracts the energy of expansion. These booster compressors coupled to the turbines lower the amount of work that must be provided by the feed compressor and/or recycle compressor.
However, hydrogen liquefaction is a power intensive process and it is often desirable to have a large turndown capability to adjust liquid hydrogen production to current demand for such product and thus minimize the power consumption during such turndown events. Having a large and generally efficient turndown capability allows the hydrogen liquefaction and precooling systems to shed power when demand for liquid hydrogen product goes down, or when local power rates get too high. Turndown of the hydrogen liquefaction or hydrogen precooling process is accomplished by trying to maintain a constant pressure ratio and constant volume flow across the turbomachinery stages in the hydrogen liquefaction cycle and/or the hydrogen precooling cycle. Maintaining a generally constant pressure ratio and constant volume flow across the turbomachinery stages keeps the efficiency of those stages near their design points throughout the turndown event.
To accomplish constant volume, constant ratio, both the mass flow of the nitrogen refrigerant recycle loop, and the total head pressure are adjusted. As the hydrogen liquefaction and precooling systems are turned down, the head pressure and mass flow will both decrease. Since the density is decreased, yet the total volume of gas space is unchanged, nitrogen molecules must be removed from the recycle loop to accomplish the turndown. In addition, molecules must be added back to the loop when the plant is returned to full production.
To achieve the reduction of nitrogen refrigerant during turndown in conventional hydrogen liquefaction systems, the excess nitrogen refrigerant molecules simply are vented to the atmosphere or are withdrawn from the refrigeration process, compressed and then stored in external storage tanks. While there is little operating or capital costs associated with venting the excess nitrogen refrigerant, there is significant operating and capital costs associated with the external storage of the excess nitrogen refrigerant including nitrogen compressors, storage tanks, and the piping, valves, controls, etc. to couple the external nitrogen storage system with the hydrogen precooling and liquefaction system. Also, when the turndown event is completed and there is a need to ramp the hydrogen liquefaction and/or hydrogen precooling system back up to design conditions or normal operating mode, additional nitrogen refrigerant is needed to replace the vented nitrogen molecules or replace the externally stored nitrogen during turndown.
What is needed, therefore, is a means to mitigate the penalties associated with turndown and subsequent ramp-up of a hydrogen liquefaction and/or hydrogen precooling system. More specifically, there is a continuing need to reduce the operating and capital costs associated with turndown and subsequent ramp-up of a hydrogen liquefaction and/or hydrogen precooling system, in part, by eliminating the requirement for external compression and external storage of the nitrogen refrigerant during any such turndown.
The present method may be characterized as a method of turndown of a nitrogen refrigeration system used in precooling/liquefaction of a hydrogen feed stream. The preferred method comprises the steps of: (a) receiving a hydrogen feed stream at a prescribed volumetric flow; (b) cooling the hydrogen feed stream in a first heat exchanger or first set of heat exchanger cores via indirect heat exchange with a low pressure gaseous recycle stream and a medium pressure gaseous recycle stream to yield a cooled, hydrogen feed stream; (c) cooling a high pressure nitrogen refrigerant stream in the first heat exchanger or first set of heat exchanger cores and diverting a first portion of the high pressure nitrogen refrigerant stream from within the first heat exchanger or first set of heat exchanger cores to yield a first diverted stream; (d) expanding the first diverted stream in a warm turbine/expander to yield a warm exhaust stream that forms a part of the medium pressure gaseous recycle stream at a temperature colder than the first diverted stream; (e) diverting a second portion of the high pressure nitrogen refrigerant stream from within the first heat exchanger or first set of heat exchanger cores to yield a second diverted stream, wherein the second diverted stream is at a temperature colder than the first diverted stream; (f) expanding the second diverted stream in a cold turbine/expander to yield a cold exhaust stream that forms another part of the medium pressure gaseous recycle stream at a temperature colder than the second diverted stream; (g) recycling the medium pressure gaseous recycle stream through the first heat exchanger or first set of heat exchanger cores to cool the high pressure nitrogen refrigerant stream and the hydrogen feed stream. The method also includes the steps of: (h) expanding the remaining portion of the of the high pressure nitrogen refrigerant stream in an expansion valve to yield a two-phase nitrogen stream; (i) separating the two-phase nitrogen stream in a phase separator to yield a liquid nitrogen stream and the low pressure gaseous recycle stream; and (j) recycling the low pressure gaseous recycle stream through the first heat exchanger or first set of heat exchanger cores to cool the high pressure nitrogen refrigerant stream and the hydrogen feed stream.
When operating in normal or design operating mode, referred to as a first operating mode, the method further comprises the steps of: (k) directing a prescribed volumetric flow of the liquid nitrogen from the phase separator to the second heat exchanger or second set of heat exchanger cores; and (l) precooling the cooled, hydrogen feed stream in a second heat exchanger or second set of heat exchanger cores via indirect heat exchange with the liquid nitrogen to yield a precooled hydrogen feed stream at a temperature of less than or equal to 80 Kelvin. However, when operating in a turndown mode or second operating mode, the method further comprises the steps of: (m) restricting the flow of the hydrogen feed stream to the first heat exchanger or first set of heat exchanger cores such that the flow is less than the prescribed volumetric flow of the hydrogen feed stream to the first heat exchanger or first set of heat exchanger cores; (n) restricting the flow of liquid nitrogen from the phase separator to the second heat exchanger or second set of heat exchanger cores via a control valve; (o) retaining a remaining portion of the liquid nitrogen in the phase separator; and (p) precooling the cooled, hydrogen feed stream in a second heat exchanger or second set of heat exchanger cores via indirect heat exchange with the restricted flow of liquid nitrogen to yield a precooled hydrogen feed stream at a temperature of less than or equal to 80 Kelvin. In the preferred embodiments, the phase separator is preferably designed to be oversized and capable of holding greater than 70% of the nitrogen refrigerant used in the refrigeration circuit
A third operating mode or ramp-up mode further includes the steps of: (q) increasing the flow of the hydrogen feed stream to the first heat exchanger or first set of heat exchanger cores such that the flow is greater than the flow of the hydrogen feed stream to the first heat exchanger or first set of heat exchanger cores in the second operating mode but less than the prescribed volumetric flow; (r) releasing additional flow of liquid nitrogen from the phase separator to the second heat exchanger or second set of heat exchanger cores via the control valve; and (s) precooling the cooled, hydrogen feed stream in a second heat exchanger or second set of heat exchanger cores via indirect heat exchange with the increasing flow of liquid nitrogen to yield a precooled hydrogen feed stream at a temperature of less than or equal to 80 Kelvin.
The present invention may also be characterized as a refrigeration system for precooling of hydrogen and liquefaction of nitrogen, the refrigeration system comprising: (i) an integral gear machine comprising a drive assembly, a bull gear, and a plurality of pinions arranged to drive four or more refrigerant compression stages of the refrigeration system and for receiving work produced by the at least two turbine/expanders of the refrigeration system; (ii) a refrigeration circuit configured to circulate a plurality of nitrogen streams including a high pressure nitrogen refrigerant stream and a hydrogen feed stream; (iii) an expansion valve disposed in the refrigeration circuit configured for expanding the high pressure nitrogen refrigerant stream to yield a two-phase nitrogen stream; (iv) a phase separator disposed within the refrigeration circuit and in fluid communication with the expansion valve and configured to receive the two-phase nitrogen stream and separate the two-phase nitrogen stream into a nitrogen liquid and a gaseous nitrogen stream; (v) a first heat exchanger or set of first heat exchange cores disposed within the refrigeration circuit and configured to cool the hydrogen feed stream and cool the high pressure nitrogen refrigerant stream via indirect heat exchange with exhaust streams from the at least two turbine/expanders and the gaseous nitrogen stream from the phase separator; (vi) a second heat exchanger or set of second heat exchange cores disposed within the refrigeration circuit and configured to receive the cooled hydrogen feed stream from the first heat exchanger or set of first heat exchange cores and precool the cooled hydrogen feed stream to a temperature of about 80 Kelvin or lower via indirect heat exchange with a liquid nitrogen stream received from the phase separator; and (vii) a control valve in fluid communication with the phase separator and configured to control the liquid nitrogen exiting the phase separator to the a second heat exchanger or set of second heat exchange cores in response to the flow of the hydrogen feed stream to the first heat exchanger or set of first heat exchange cores. The phase separator is sized to hold at least 70% by volume of the nitrogen in the refrigeration system. The refrigeration system may optionally include an ortho/para conversion catalyst vessel configured to treat the precooled hydrogen feed stream exiting the second heat exchanger or set of second heat exchanger cores. Alternatively, one or more of the heat exchange passages in the first or second heat exchangers may also contain ortho/para conversion catalysts.
In the preferred embodiments of the nitrogen-based refrigeration system, the refrigeration circuit is further configured to mix a nitrogen feed stream and a low pressure nitrogen recycle stream to form a nitrogen refrigerant stream and direct the nitrogen refrigerant stream to a nitrogen feed compressor. The compressed nitrogen refrigerant stream from the nitrogen feed compressor is then mixed with a nitrogen recycle stream and the mixed stream is directed to a nitrogen recycle compressor where it is further compressed. The further compressed nitrogen refrigerant stream is then directed to a warm booster compressor and a cold booster compressor to still further compress the nitrogen refrigerant stream and form the high pressure nitrogen refrigerant stream.
In many embodiments of the disclosed hydrogen precooling system and process, the first diverted stream is preferably less than or equal to about 40% by volume of the high pressure nitrogen refrigerant stream while the second diverted stream is greater than the volume of the first diverted stream. The first diverted stream is expanded in the warm turbine/expander to yield a warm exhaust stream at a temperature in the range of 150 Kelvin and 185 Kelvin while the second diverted stream is expanded in the cold turbine/expander to yield a cold exhaust stream at a temperature in the range of 85 Kelvin and 105 Kelvin. Both the warm exhaust stream and the cold exhaust stream are warmed to ambient temperatures in the first heat exchanger or first set of heat exchanger cores and recycled as the medium pressure nitrogen recycle stream.
In some preferred embodiments of the present system and method, one or more hydrogen return streams from a nearby hydrogen liquefaction plant may be circulated through the heat exchangers or sets of heat exchange cores. Such additional hydrogen streams might include a high pressure hydrogen refrigerant stream, a medium pressure hydrogen return stream, and a low pressure hydrogen return stream.
It is believed that the claimed invention will be better understood when taken in connection with the accompanying drawings in which:
Hydrogen Precooling with Nitrogen Liquefaction
Turning to
The nitrogen feed stream 12 is a purified and compressed gaseous nitrogen stream at a feed pressure preferably at or above 40 bar(a), and more preferably at a pressure of between about 40 bar(a) and 70 bar(a), most preferably at a pressure of about 55 bar(a). Nitrogen feed 12 is merged with the nitrogen recycle stream 72 and compressed in the nitrogen feed compressor 20 with the resulting compressed nitrogen stream 14 merged with recycle exhaust stream 59 and further compressed in a multi-stage nitrogen recycle compressor 25 to yield a gaseous nitrogen stream 28. The serially compression in feed compressor 20 and in the stages of the recycle compressor 25 may also include appropriate intercooling and/or aftercooling used to offset the temperature increases caused by the heat of compression. Such aftercooling may be accomplished by way of indirect contact with air, cooling water, chilled water or other refrigerating medium or combinations thereof.
In the embodiment illustrated in
The high pressure nitrogen refrigerant stream 39 is then cooled in at least one heat exchange section E1. A first portion of the cooled refrigerant stream 39 is diverted as a first diverted stream 52 and expanded in the warm turbine/expander 50 configured to expand the first diverted stream 52 to generate refrigeration. The first diverted stream 52 is preferably in the range of 20% to 60% and more preferably about 40% by volume of the high pressure nitrogen refrigerant stream 39 and is expanded in the warm turbine/expander 50 down to a pressure in the range of 1.3 bar(a) to 10.0 bar(a). The exhaust stream 54 of the warm turbine/expander is preferably at a temperature in the range of 150 Kelvin to 185 Kelvin and more preferably at about 170 Kelvin and warmed to ambient temperatures the heat exchange sections E2 and E1 while cooling the compressed refrigerant stream 39 and precooling the hydrogen feed stream 40 also traversing the heat exchange sections E2 and E1. recycled to the multi-stage recycle compressor 25.
The compressed refrigerant stream 39 continues to cool in heat exchanger section E2 and E3 to a temperature in the range of 150 Kelvin to 185 Kelvin and more preferably at about 176 Kelvin when a second portion of the cooled refrigerant stream 39 is diverted as a second diverted stream 56. The second diverted stream is preferably in the range of about 70% to 95% by volume of remaining cooled refrigerant stream 39 or in the range of between about 35% to 70% of the original high pressure nitrogen refrigerant stream 39. The second diverted stream is expanded in cold turbine/expander 55 configured to expand the second diverted stream 56 to generate additional refrigeration. The exhaust stream 58 of the cold turbine/expander 55 exits the cold turbine/expander at a temperature in the range of between 85 Kelvin and 105 Kelvin and more preferably at a temperature of about 97 Kelvin and is warmed to ambient temperatures in heat exchange sections E3, E2, and E1 while cooling the compressed refrigerant stream 39 and also precooling the hydrogen feed stream 40 traversing the same heat exchange sections E3, E2, and E1. The warming exhaust stream 58 from the cold turbine/expander 55 may be merged at some point with exhaust stream 54 of the warm turbine/expander 50 and the warmed streams is then recycled as recycle stream 57 to the recycle compressor 25.
The remaining third portion 62 is cooled in heat exchange section E3 and then expanded in a Joule Thomson expansion valve 60 with the resulting two-phase nitrogen stream 64 directed to the phase separator 70. The two-phase nitrogen stream 64 discharged from the Joule Thompson expansion valve 60 is a two-phase mixture in the range of 15% to 25% vapor phase and 85% to 75% liquid phase. The phase separator 70 separates the vapor nitrogen from the liquid nitrogen with the resulting vapor nitrogen stream 72 recycled to the nitrogen feed compressor 20 via heat exchange sections E4, E3, E2, and E1 where it precools the hydrogen feed stream. The resulting liquid nitrogen stream 74 also exits the phase separator 70.
A first part of the liquid nitrogen stream is optionally taken as liquid nitrogen product stream 76 while a second part of the liquid nitrogen stream 75 is directed to heat exchange section E5 where it is boiled against the cooled hydrogen feed stream to fully precools the hydrogen and yield a precooled hydrogen stream 45. Although not shown, external liquid can also be added to the process at the phase separator 70 if needed. After boiling, the two-phase nitrogen stream 78 leaving heat exchange section E5 is sent back to the phase separator 70. The low pressure vapor stream 72 exiting the phase separator 70 is warmed back to ambient temperatures through warming in heat exchange sections E1, E2, E3, and E4.
In the present system and method for hydrogen precooling with nitrogen liquefaction, there are or may be four distinct hydrogen streams in the process, including a hydrogen feed stream 40, a low pressure hydrogen return stream 42 that come from an adjacent hydrogen liquefaction process, a medium pressure hydrogen return stream 43 that also comes from the adjacent hydrogen liquefaction process, and a high pressure hydrogen stream 44.
The hydrogen feed stream 40 is a gaseous hydrogen stream that will ultimately become the liquid hydrogen product. The hydrogen feed stream 40 is preferably a purified and compressed gaseous hydrogen stream at a pressure in the range of 10 bar(a) to 40 bar(a) and is substantially free of hydrocarbons and other impurities. This hydrogen feed stream 40 is then further compressed in a hydrogen compressor to a pressure preferably in the range of 40 bar(a) to 70 bar(a). In the illustrated embodiment, the hydrogen feed stream 40 is cooled in heat exchanger sections E1, E2, and E3 against the turbine exhaust streams 54, 58, the low pressure nitrogen vapor stream 72 exiting the phase separator 70, and the hydrogen recycle streams (i.e. the low pressure hydrogen return stream 42 at a pressure range of 1.3 bar(a) to 2 bar(a) at a temperature of about 80 Kelvin and the medium pressure hydrogen return stream 43 at a pressure range of 2 bar(a) to 9 bar(a) at a temperature of about 80 Kelvin).
The hydrogen feed stream 40 is further cooled in heat exchanger section E4 against the low pressure nitrogen vapor stream 72 exiting the phase separator 70 and the hydrogen recycle streams and then still further cooled in heat exchanger section E5 against a liquid nitrogen stream 75 exiting the phase separator 70. Upon exiting E5, the cooled hydrogen feed stream 45 is directed to an ortho/para conversion vessel 48 filled with catalyst. The cooled hydrogen stream 45 will warm slightly in the catalyst filled vessel 46, so the catalyst treated stream 47 it is returned to heat exchange section E5 where it is re-cooled to a temperature of about 80 Kelvin. The final heat exchange passage 49 in heat exchange section E5 may also be optionally filled with an ortho/para conversion catalyst. Alternate contemplated embodiments may utilize ortho/para conversion catalysts in heat exchange passages within heat exchange sections E3 and E4 to reduce or minimize the need for the ortho/para conversion vessel 48.
As indicated above, the high pressure hydrogen stream 44 is preferably at a pressure in the range of 40 bar(a) to 70 bar(a) and more preferably at a pressure of about 55 bar(a). The high pressure hydrogen stream 44 is directed to and through heat exchange sections E1, E2, E3, E4, and E5 where it is cooled to a temperature of about 80 Kelvin. This high pressure hydrogen stream 44 is not contacted with ortho/para conversion catalyst since the high pressure hydrogen stream 44 is to be used for refrigeration in the adjacent hydrogen liquefaction process and recycled back through the present system.
Compander Machine with 2 Turbine/Expanders and 4 Refrigerant Compression Stages
The present system and method for precooling hydrogen with concurrent nitrogen liquefaction combines the above-described process design with an integrated compression-expansion system. The compression-expansion system combines the recycle compressor, booster compressors, warm turbine/expander and cold turbine/expander into a single machine driven by a single motor.
Linde Inc., a member of the Linde Group of Companies, has also developed a portfolio of integral gear machines that combine compression stages and high efficiency turbine/expanders having up to four pinions in what is referred to as an integral gear ‘bridge’ machine or BriM. Linde's ‘bridge’ machines are conventionally used in hydrogen/syngas plants as well as air separation plants and typically come in different frame sizes, for example between about 90 mm and 180 mm frame sizes. Design studies have examined applications of the Linde ‘bridge’ machines to operatively couple a plurality of radially inflow turbines and centrifugal refrigeration compression stages in liquefaction systems but such applications have not yet been commercialized.
The Linde ‘bridge’ machines come fully packaged or integrated with appropriate PLC controllers, control valves, safety valves, oil system, etc. and can be easily outfitted with intercoolers and/or aftercoolers. The hardware constraints and limitations of the Linde ‘bridge’ machines are typically a function of bull gear and driver assembly size. In general, the Linde ‘bridge’ machine drivers pertinent for the present system and method spans the range of about 4 MW to 20 MW with associated maximum pinion speeds in the range of 20,000 to 50,000 rpm. Furthermore, the maximum power imparted to any given pinion or any given turbine-compression stage pairing is preferably limited to less than 50% and in some cases to about 35% of the total ‘bridge’ machine driver power.
As shown in
The main advantage of the BriM arrangement is to decouple the speed and power of the warm and cold turbine-boosters. The turbomachinery components or stages paired on each pinion can be chosen to optimize their rotational speed. While the illustrated embodiment in
Also, the power of a booster compressors and turbines are not required to match even if paired on the same pinion. The work produced in any of the turbines is directly supplied as shaft power to the relevant booster stage on that pinion. Thus, if the turbine produces greater power than a booster compressor or compression stage coupled to the same pinion, the pinion will just put any excess power back into the bull gear and lower the overall power requirement required of the motor. If, on the other hand, the booster compressor or compression stage requires higher power than the turbine coupled to the same pinion can provide, the pinion will need to receive additional power from the motor via the bull gear.
When combined with highly optimized wheels, the BriM 80 can be much more efficient than a traditional solution of direct connection or coupling between the warm turbine/expander and warm booster compressor and between the cold turbine/expander and cold booster compressor. In other words, a key feature of the BriM 80 is the avoidance of the direct coupling between the turbines and their associated boosters. This allows the speed and power of each turbomachinery stage to be the optimal speed and diameter for the overall process. Since the turbines are connected to the integrated system, their power is just directly adsorbed by the system and the motor then adjusts as needed to supply the remaining required power. When the turbines are set to their ideal conditions, this also minimizes the pinch in the heat exchangers. This optimizes the refrigeration balance between the cold and warm turbines and saves power.
The technical advantages associated with the illustrated BriM arrangements compared to the conventional designs is that the BriM machine and all of the wheels can be designed to operate under ideal conditions. Also, since the speeds of the turbines and booster compressors are no locked or fixed together, the speed of the turbines and the speed of the compressors will be independently optimized. Finally, the power of the booster compressors is variable and not constrained or dictated by the work produced by the turbines. This is because each compressor stage and turbine stage is connected to the motor through the bull gear.
The utilization of the BriM allows for a more optimized process cycle since the speed and power of the turbomachinery stages are no longer coupled. This means the flow and pressure ratio through the turbines can be set to minimize the pinch in the heat exchangers. This minimizes the power needed to drive the nitrogen recycle loop. In the conventional hydrogen precooling process or conventional nitrogen liquefaction process the flow through the cold turbine/expander is often forced to be too high to prevent the coupled booster from running in an undesirable low efficiency state. This additional flow through the cold turbine/expander and at the cold end of the process creates an unnecessary inefficiency.
As detailed in the Example below, the illustrated configuration saves about 750 kW power for hydrogen precooling in a 50 ton per day (TPD) hydrogen liquefaction plant or about 14% power savings compared to the traditional hydrogen precooling process cycles that employ coupled turbine boosters. Similar power savings can be realized when also using the design to produce liquid nitrogen as a coproduct together with hydrogen precooling or to produce liquid nitrogen as the sole product. This magnitude of potential power savings yields a more efficient process with a lower total cost of ownership relative to the traditional hydrogen precooling process cycles that employ coupled turbine boosters.
A number of computer simulations were run to characterize the comparative performance of the hydrogen precooling system and process as shown in
The system and process referred to as
In one such computer simulation, referred to as Case 1, the optimized designs of the respective hydrogen precooling systems were simulated and referred to as the ‘Design’ case and wherein the
As shown in the Tables above, the above-described system and method provides a significant improvement in power consumption over a comparative conventional hydrogen precooling system. At design conditions (Case 1), the system and method offers a 13.7% improvement in relative power to handle the precooling of set feed flow of hydrogen at about 25535 Nm3/h. The improvement in relative power increases in situations where the system is run at excess flow conditions of about 109% of design flow or 27777 Nm3/h.
Likewise in turndown conditions, where the hydrogen feed flow is reduced, the system and method demonstrates improvements of 4.7% at a 50% turndown rate or 12768 Nm3/h and 6.3% at a 75% turndown rate or 6364 Nm3/h. Even at maximum turndown conditions, the system and method shows relative power improvements on the order of 5% compared to some prior art hydrogen precooling systems discussed above.
While the above-described simulations contemplate a hydrogen precooling process, alternate contemplated embodiments would also demonstrate superior performance compared to the prior art conventional systems. In one such alternate embodiment, the refrigeration cycle is used to precool hydrogen and co-produce some liquid nitrogen. The liquid nitrogen is produced by removing only a portion of the liquid nitrogen from the bottom of the phase separator. Producing liquid nitrogen product stream by removing some of the liquid nitrogen from the bottom of the phase separator would require the nitrogen recycle loop be designed larger to accommodate the additional refrigeration load.
In another alternate embodiment, the refrigeration cycle with BriM machine integration can be modified to produce only a liquid nitrogen product stream. This alternate embodiment would remove and hydrogen streams from the process and require a slightly reworked evaporator at the cold end of the process. However, the turbomachinery configurations and BriM machine integration would remain the same and all of the benefits and advantages would apply to stand alone liquid nitrogen production.
Based on the comparative data shown in the Tables above, the illustrated system and process for hydrogen precooling also has a very large turndown range while maintaining high efficiency. This is preferably accomplished by simultaneously lowering the mass flow of the nitrogen in the recycle loop and the total head pressure while maintaining a constant pressure ratio across the turbines. When done correctly, each turbomachinery stage will see the same volume flow and pressure ratio as the design conditions. This means each stage will be at or near the same operating point on its performance curve and the efficiency of each stage will stay nearly constant throughout the turndown process. The only limit to turndown is once the process hits the point where the medium pressure nitrogen recycle pressure would become sub-ambient pressures or the process operating parameters encounter turbomachinery design limitations.
Unlike conventional hydrogen liquefaction systems and methods, the present system and method does not vent the excess nitrogen nor withdraw the excess nitrogen from the refrigeration process, compress the withdrawn nitrogen stream and store the same in external nitrogen storage tank. Rather, the excess nitrogen is accumulated within the phase separator 70 during turndown mode and released back to the recycle loop during turn-up mode (i.e. when ramping up liquid hydrogen production). In the present embodiments, the phase separator 70 is oversized and designed/configured to be capable of holding greater than 70% of the nitrogen refrigerant used in the refrigeration circuit during normal operating modes.
More specifically, the present hydrogen liquefaction system and method is configured to operate in three modes including normal production mode, turndown mode, and ramp-up mode. In the first operating mode or normal production mode, a prescribed flow of the hydrogen feed stream is cooled in the first heat exchanger (or first set of heat exchanger cores) and second heat exchanger (or second set of heat exchanger cores) while a prescribed flow of the liquid nitrogen is directed from the phase separator 70 to the second heat exchanger E5 or second set of heat exchanger cores where the liquid nitrogen completes the precooling of the hydrogen feed stream via indirect heat exchange to yield a precooled hydrogen feed stream. The prescribed flow of liquid nitrogen from the phase separator 70 to the second heat exchanger E5 or second set of heat exchanger cores is controlled via control valve 77 to precool the cooled, hydrogen feed stream to a temperature of less than or equal to about 80 Kelvin.
In the second operating mode, or turndown mode, the flow of the hydrogen feed stream is restricted via turndown valve 41 and the corresponding flow of liquid nitrogen from the phase separator 70 to the second heat exchanger E5 or second set of heat exchanger cores is also restricted via a control valve 77. The restricted flow of liquid nitrogen is less than the prescribed volumetric flow of the liquid nitrogen in the first operating mode and may be set in response to the restricted flow of the hydrogen feed stream such that the precooled hydrogen stream is reduced to a temperature of less than or equal to 80 Kelvin.
In the third operating mode, or ramp-up mode, the flow of the hydrogen feed stream is preferably increased by adjusting the turndown valve 41 to a flow greater than the restricted flow of the hydrogen feed stream from the second operating mode but less than the prescribed flow of the hydrogen feed stream from the first operating mode. The corresponding flow of liquid nitrogen in this third operating mode is also adjusted or controlled via control valve 77 such that additional flow of liquid nitrogen is released from the phase separator 70 to the second heat exchanger or second set of heat exchanger cores compared to the second operating mode. The flow of liquid nitrogen from the phase separator to the second heat exchanger or second set of heat exchanger cores is preferably set in response to the restricted flow of the hydrogen feed stream such that the precooled hydrogen stream is precooled to a temperature of less than or equal to about 80 Kelvin.
While the present refrigeration system and method for turndown of a nitrogen-based refrigeration system and process has been described with reference to a preferred embodiment, it is understood that numerous additions, changes, and omissions can be made without departing from the spirit and scope of the present invention as set forth in the appended claims.
