US20260029193A1

US20260029193A1 - Liquefier control for transient heat loads

Info

Publication number: US20260029193A1
Application number: US19/278,103
Authority: US
Inventors: Joshua Light; Wyatt SCOTT; Michael A. Turney
Original assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Air Liquide Large Industries US LP
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Air Liquide Large Industries US LP
Priority date: 2024-07-24
Filing date: 2025-07-23
Publication date: 2026-01-29
Also published as: WO2026024848A2

Abstract

An apparatus and method for controlling a cryogenic liquefier during transient operation, the method comprising the steps of: providing a cryogenic liquefier, wherein the cryogenic liquefier comprises: a cryogenic liquefier configured to liquefy the cryogen gas at a temperature of approximately 20K, a cryogen feed flow conduit configured to deliver the cryogen gas to the cryogenic liquefier; a first refrigeration circuit, and a secondary refrigeration circuit, wherein the first refrigeration circuit comprises isenthalpic or near-isenthalpic expansion wherein the secondary refrigeration circuit comprises isentropic or near-isentropic expansion; switching from steady state operation to a transient operation; controlling an outlet temperature of liquid cryogen from the cryogenic liquefier by adjusting the refrigeration provided by the secondary refrigeration circuit.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Application Ser. No. 63/675,070 filed on Jul. 24, 2024, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD OF INVENTION

Embodiments of the invention relate to apparatuses and methods for improving the efficiency of hydrogen liquefaction unit. More particularly, the invention is particularly useful in addressing control issues related to transient heat loads, more particularly unexpected transient heat loads.

BACKGROUND OF THE INVENTION

Hydrogen liquefiers create refrigeration to remove heat from a stream of gaseous hydrogen (GH₂). To create refrigeration, a refrigerant gas (hydrogen or helium) is compressed, cooled, and then expanded, which results in lower temperatures. The expansion process can be near-isenthalpic expansion (i.e., expansion across a valve) or near-isentropic expansion (i.e., expansion across a turbine).
Various components can impact refrigeration production in the liquefier. Some non-limiting examples may include: inlet valves to turbines, turbine bypass valves, compressor suction and discharge pressures, compressor flow rate, valve that controls amount of refrigerant in the cycle. These components may be manipulated to control a specific temperature at the outlet of the liquefier.
It is preferable for the LH₂outlet temperature from the liquefier to be controlled to a consistent value in order to produce liquid hydrogen (LH₂) efficiently and to protect equipment in the refrigeration cycle. If the temperature gets too warm, then less LH₂is produced and GH₂may have to be vented or recycled. On the contrary, if the LH₂outlet temperature from the liquefier gets too cold, then it is possible that the rotating equipment is also getting too cold and may approach specific temperature set points that will shut down the plant to prevent equipment damage.
In the hydrogen liquefaction process, the feed hydrogen (ambient temperature) is first cooled to approximately 80° Kelvin (80K) in a cycle that uses nitrogen as the refrigerant. Next, the feed hydrogen is cooled to approximately 20° Kelvin (20K) in a cycle that uses hydrogen or helium as the refrigerant. The control challenges discussed herein are particularly relevant to the 20K coldbox, which is much more difficult to control.
In typical arrangements, the liquefier's outlet temperature can be controlled by setting the compressor flow rate, adjusting the pressure provided to the turbines based on manual feedback control, and any extra refrigerant in the hydrogen cycle is automatically removed from the refrigeration cycle based on a pressure set point. Typically the turbine inlet valves are opened all the way, the turbine bypasses are closed, and the compressor bypasses are closed.
These liquefiers typically operate at near steady state conditions. At steady state, the hydrogen inlet flow rate (heat load) is consistent, and minor changes are made to the pressure provided to the turbines or the compressor flow rate in order to maintain the outlet temperature. Due to the steady state, minor manipulations can effectively control the outlet temperature of the liquefier based on a feedback loop.
LH₂trailers are used to distribute LH₂from the liquefier to customers. During the process of loading LH₂into trailers, cryogenic vapor must be removed from the trailer. The liquefier can be designed to vent the cryogenic vapor to the atmosphere or recover the cryogenic vapor into the liquefaction process. It is generally preferable for liquefiers to recover this cryogenic vapor and use the GH₂to make more LH₂, since gaseous hydrogen molecules are economically valuable. In addition, these H₂molecules can either be recovered as warm (near ambient temperature) to the warm end of the liquefier or to the cold end of the liquefier if the gas to be recovered is cold. In addition to recovering the H₂molecules it is also desirable to recover the cold (refrigeration)
In one known method of recovering the H₂molecules, the cryogenic vapor is warmed and placed in a large, low pressure, GH₂storage container. The volume of the container is very large and has the ability to store all of the GH₂. The GH₂is slowly and consistently provided to the feed hydrogen circuit. Unfortunately, these large storage containers are expensive, which represents a large up-front cost for the production facility. Therefore, there has been interest in operating new liquefiers without them.
Another known method is operating without the large storage container and involves recovering the cryogenic vapor from a trailer back to the process. However, without a buffer to accumulate the GH₂in storage, the GH₂from the trailers is immediately introduced to the hydrogen liquefaction process. The large and quick increase of the flow of feed hydrogen to the liquefier causes the outlet temperature of the liquefier to increase. Correcting the temperature is difficult because the temperature is slow to react, and the system balance is delicate.
Moreover, the feedback controls described heretofore are best implemented for small manipulations (e.g., fine tuning). However, during a large and quick increase in the hydrogen feed flow rate, small manipulations of the liquefier are not sufficient to keep the liquefier outlet temperature in control, and the liquefier outlet temperature increases causing the liquefier to run inefficiently (venting H₂and using much higher specific power than design).
If the operator makes a large manipulation to cool outlet temperature, the outlet temperature feedback will not become apparent until too much cooling has been applied. This is because outlet temperature is a lagging indicator, which means the impact of a manipulation is not realized for a significant period of time. At this point, the temperature continues to decrease even if a warming manipulation is implemented. The large manipulations cause the liquefier to become unstable and lead to a large chance of tripping the plant. To avoid the risk of tripping the plant, the operator will often run inefficiently and make small changes until the plant is stable.
Similarly, during a large and quick decrease in feed hydrogen flow rate, the outlet temperature of the liquefier decreases rapidly. To avoid tripping the plant from being too cold, the operator makes large manipulations to warm the outlet temperature. Due to the lag in feedback, the operator typically warms the liquefier outlet temperature much more than needed, and the liquefier runs inefficiently for about six hours before the outlet temperature reaches the desired set point.
Existing methods of controlling the liquefier outlet temperature require feedback to determine the appropriate manipulation. The liquefier outlet temperature is slow to change from manipulations (lagging indicator), and the liquefier outlet temperature has significant momentum, which is difficult to overcome unless strong manipulations are made. Both of these characteristics typically cause the liquefier outlet temperature to swing when feedback control is attempted.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method that satisfies at least one of these needs.
In certain embodiments, the present invention is directed to methods and apparatuses for controlling a cryogenic liquefier, particularly during periods of transient heat load, that overcome the limitations of conventional control systems.
One aspect of the invention provides a method for controlling a cryogenic liquefier that has a refrigerant circuit with at least one expansion turbine. The method involves detecting a change in the heat load on the liquefier by measuring a leading indicator parameter. This parameter, such as the flow rate of a feed gas stream entering the liquefier, is measured upstream of the liquefier outlet and provides an early indication of a thermal disturbance. In response to detecting this change, a processor calculates a required adjustment to the refrigeration production. This calculation is based on a pre-determined relationship between the measured leading indicator and the amount of refrigeration needed to counteract the disturbance, and is performed before a significant deviation in the liquefier outlet temperature can occur. The method then involves proactively adjusting a refrigeration factor of the refrigerant circuit to implement the calculated adjustment.
In certain embodiments, the refrigeration factor is a function of the operating parameters of the at least one turbine, such as the differential pressure across the turbine system, the pressure ratio across the turbine system, or a work factor representative of the total work produced by the turbine system. The adjustment may be physically implemented by transmitting a control signal to an actuator, such as a refrigerant letdown valve positioned upstream of the turbines, thereby modifying the turbine inlet pressure.
Another aspect of the invention provides an apparatus for controlling a cryogenic liquefier. The apparatus includes at least one sensor configured to measure a leading indicator parameter of a heat load, a controller communicatively coupled to the sensor, and an actuator within the refrigerant circuit. The controller is programmed to receive a signal from the sensor indicating a change in the heat load, calculate a required adjustment to a refrigeration factor based on this signal, and transmit a control signal to the actuator to proactively implement the adjustment.
A key advantage of the present invention is its ability to maintain stable and efficient operation of the liquefier during large and rapid changes in heat load, such as those caused by the recovery of cryogenic vapor from a transport trailer. By preemptively matching refrigeration production to the anticipated heat load, the invention mitigates the temperature swings and instability associated with control systems that rely solely on the lagging feedback of the liquefier outlet temperature. This robust control eliminates the need for a large, expensive gaseous cryogen buffer storage tank, representing a significant capital cost saving.
According to an embodiment of the invention, there is a method for controlling a cryogenic liquefier during transient operation that includes the steps of: providing a cryogenic liquefier, wherein the cryogenic liquefier comprises: a cryogenic liquefier configured to liquefy the cryogen gas and cool to a temperature of approximately 20K, a cryogen feed flow conduit configured to deliver the cryogen gas to the cryogenic liquefier; a first refrigeration circuit, and a secondary refrigeration circuit, wherein the first refrigeration circuit comprises isenthalpic or near-isenthalpic expansion wherein the secondary refrigeration circuit comprises isentropic or near-isentropic expansion; switching from steady state operation to a transient operation; and controlling an outlet temperature of liquid cryogen from the cryogenic liquefier by adjusting the refrigeration provided by the secondary refrigeration circuit.
According to other optional features of the method:

- the step of controlling the outlet temperature further comprises manipulating a factor selected from the group consisting of: a differential pressure across a turbine system, a pressure ratio across the turbine system, rotating speed of the turbine system, a work factor across the turbine system;
- the method switches to the transient operation based upon a change in flow rate of cryogen gas in the feed flow conduit;
- the method switches to the transient operation based upon a determination that a cryogen storage vessel is entering a loading phase; and/or
- the method switches to the steady state operation based upon a determination that the cryogen storage vessel has finished its loading phase.

According to another embodiment of the invention, there is a method for controlling a cryogenic liquefier having a refrigerant circuit with at least one turbine that includes the steps of: detecting a change in a heat load on the cryogenic liquefier based on a measurement of a leading indicator parameter that is upstream of a liquefier outlet; calculating, using a processor, a required adjustment to refrigeration production in response to the detected change in the heat load; and proactively adjusting a refrigeration factor of the refrigerant circuit to effect the required adjustment to refrigeration production before a deviation in a liquefier outlet temperature caused by the change in the heat load occurs
According to other optional features of the method:

- the leading indicator parameter comprises a flow rate of a feed gas stream entering the cryogenic liquefier;
- the leading indicator parameter is determined from a heat balance calculation based on at least one of an inlet flow rate and an inlet temperature of a feed gas stream;
- the refrigeration factor is a function of at least an inlet pressure and an outlet pressure of the at least one turbine;
- the refrigeration factor is selected from the group consisting of: a differential pressure across the at least one turbine, a pressure ratio across the at least one turbine, and a work factor representative of total work produced by the at least one turbine;
- the step of proactively adjusting the refrigeration factor comprises transmitting a control signal to a refrigerant letdown valve positioned upstream of the at least one turbine;
- the change in the heat load is caused by introducing recovered cryogenic vapor from a transport trailer into a feed gas stream of the liquefier; and/or
- the method may also include using a feedback control loop based on the liquefier outlet temperature to provide a trim adjustment to the refrigeration factor.

According to another embodiment of the invention, there is an apparatus for controlling a cryogenic liquefier that may include a cryogenic liquefier comprising a refrigerant circuit with at least one turbine and an actuator configured to adjust a flow of refrigerant to the at least one turbine; at least one sensor configured to measure a leading indicator parameter of a heat load on the cryogenic liquefier, wherein the at least one sensor is positioned to measure the parameter upstream of a liquefier outlet; and a controller communicatively coupled to the at least one sensor and the actuator, the controller comprising a processor and a memory including computer-executable instructions that, when executed by the processor, cause the controller to: receive a signal from the at least one sensor indicating a change in the heat load; calculate, using a control algorithm, a required adjustment to a refrigeration factor of the refrigerant circuit; and transmit a control signal to the actuator to proactively implement the required adjustment to the refrigeration factor before the change in the heat load causes a deviation in a liquefier outlet temperature.
According to other optional features of the apparatus:

- the at least one sensor is a flow meter configured to measure a flow rate of a feed gas stream entering the cryogenic liquefier;
- the refrigeration factor is a function of at least an inlet pressure and an outlet pressure of the at least one turbine, and is selected from the group consisting of: a differential pressure, a pressure ratio, and a work factor; and/or
- the apparatus does not include a gaseous cryogen buffer storage tank for buffering recovered cryogenic vapor

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

The FIGURE provides an embodiment of the present invention.

DETAILED DESCRIPTION

While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.
It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
In experimentations with an existing liquefier, the inventors have attempted to control the liquefier outlet temperature by manipulating the refrigerant letdown valve (JT valve that provides isenthalpic expansion); however, this method proved ineffective in managing transient heat load disturbances.
Certain embodiments of the present invention provide a control method and apparatus for managing transient heat loads in a cryogenic liquefier, particularly a hydrogen liquefier operating at approximately 20K. A key feature of certain embodiments of the invention is the departure from traditional control systems, which rely on the lagging indicator of liquefier outlet temperature, to a proactive control scheme that relies on leading indicators of the heat load. This allows the system to adjust refrigeration production in anticipation of a thermal disturbance, rather than in reaction to it, thereby maintaining stability and efficiency.
The heat load on the liquefier can change dramatically, for example, when boil-off gas from a transport trailer being loaded with LH₂is recovered and introduced back into the process. Without a large buffer tank, this influx of relatively warm gas directly enters the liquefier's feed circuit, causing a rapid increase in the heat load. An embodiment of the present invention measures a leading indicator of this change, such as the flow rate of the feed stream entering the liquefaction unit, or calculates the required duty based on a real-time heat balance.
Based on this leading indicator, a controller calculates the necessary change in refrigeration production. This refrigeration is primarily generated by the near-isentropic expansion of a refrigerant (e.g., hydrogen or helium) across one or more turbines. The amount of refrigeration, or work, produced by these turbines can be characterized by a “refrigeration factor.” The controller adjusts this factor to meet the anticipated heat load.
In certain embodiments of the invention, it is possible to more tightly control the liquefier outlet temperature by manipulating a factor based on the feed flow and/or a heat balance.

Refrigeration Factors

In various embodiments, this refrigeration factor may be defined in several ways:

1. dP Factor

The differential pressure across the turbines will hereafter be referred to as the dP factor.
$dP factor = Turbine outlet pressure - Turbine inlet pressure$
In order to maintain the dP factor, the turbine inlet pressure may be controlled by manipulating the pressure set point of the refrigerant letdown valve.
$Pressure setpoint for refrigerant letdown valve = dP factor + Turbine o utlet pressure$
The flow through the refrigeration circuit as well as the pressure drop across the turbines of the refrigeration circuit are characterized by the dP factor. Because the flow and the pressure drop determine the refrigeration produced with a specific configuration of equipment, the dP factor determines the amount of refrigeration that is produced by the liquefier. When the liquifier outlet temperature changes, the dP factor can be manipulated to provide more cooling or less cooling. Due to the consistent hydrogen feed flow rate of previous liquefiers, the liquefiers can operate in a steady state with feedback control and minimal disturbances of the outlet temperature.

2. Pr Factor

The pressure ratio across the turbines will hereafter be referred to as the Pr factor.
$\Pr factor = Turbine outlet pressure / Turbine inlet pressure$
In order to maintain the Pr factor, the turbine inlet pressure should be controlled by manipulating the pressure set point of the refrigerant letdown valve.
$Pressure setpoint for refrigerant letdown valve = Turbine outlet pressure / dP ratio$
The specific refrigeration produced by the turbines can be characterized by the Pr factor. Because the specific refrigeration produced determines the total refrigeration produced with a specific configuration of equipment, the Pr factor determines the amount of refrigeration that is produced by the liquefier. When the liquifier outlet temperature changes, the Pr factor can be manipulated to provide more cooling or less cooling. Due to the consistent hydrogen feed flow rate of previous liquefiers, the liquefiers can operate in a steady state with feedback control and minimal disturbances of the outlet temperature.
The Pr factor can also be the Turbine inlet pressure/Turbine outlet pressure.

3. Work Factor

The work factor is a factor that represents the total amount of work produced by the turbines.
It is a more comprehensive factor representing the total work produced by the turbines, which accounts for both flow rate and specific work.
Flow rate is a function of the square root of differential pressure, system restriction, and inlet density. System restriction is a characteristic that can be modified by system components.
Differential pressure is readily determined by pressure sensors, and a fixed differential pressure may be used in certain circumstances. Inlet density, being a function of temperature and pressure, depends primarily on pressure when the inlet temperature is nearly constant. A fixed density may also be assumed in some circumstances.
Efficiency or specific work is a function of the pressure ratio. The efficiency or specific work can also be a function of similar parameters:

- Pr
- Pr{circumflex over ( )}0.5
- Pr{circumflex over ( )}2
- Pr{circumflex over ( )}3
- Pr{circumflex over ( )}4

As a non-limiting example, the work factor may be represented by the following equation:
$work factor = \sqrt{P_{i n}^{2} - (P_{i n} * P_{out})} * \frac{P_{i n}}{P_{out}}$

- P_in=pressure at the inlet of the turbine system
- P_out=pressure at the outlet of the turbine system

In order to maintain the work factor, a controller can manipulate the pressure set point on the pressure control loop of the refrigerant letdown valve.
Manipulate pressure setpoint for refrigerant letdown valve pressure controller so that
$\sqrt{P_{i n}^{2} - (P_{i n} * P_{out})} * \frac{P_{i n}}{P_{out}} = requested work factor$
The flow through the refrigeration circuit as well as the specific refrigeration produced across the turbines of the refrigeration circuit may be characterized by the work factor. Because the flow and the pressure drop determine the refrigeration produced with a specific configuration of equipment, the work factor determines the amount of refrigeration that is produced by the liquefier. When the liquefier-outlet-temperature changes, the work factor can be manipulated to provide more or less cooling. Due to the consistent hydrogen feed flow rate of previous liquefiers, the liquefiers can operate in a steady state with feedback control and minimal disturbances of the outlet temperature.
In a preferred embodiment, the controller manipulates this chosen factor by adjusting the set point of an actuator in the refrigerant circuit, such as a refrigerant letdown valve upstream of the turbines. By adjusting this valve, the controller modifies the inlet pressure to the turbines (Pin), thereby changing the chosen factor and proactively matching the refrigeration production to the heat load.
Inlet pressure of the turbines can also be modified by adjusting outlet pressure of the refrigerant recycle compressor(s).

Factors Based on Flow Rate

In certain situations, the feed flow rate can change quickly within a hydrogen liquefier. For example, when a trailer starts the loading process, there is a large and quick increase in hydrogen feed flow rate, which is synonymous with a large and quick increase in the heat load on the liquefier. Meanwhile the refrigeration production remains constant. This imbalance of heat causes the outlet temperature to increase.
When the trailer is finished loading, there is a large and quick decrease in hydrogen feed flow rate, which is synonymous with a large and quick decrease in the heat load on the liquefier. Meanwhile the refrigeration production remains constant. This imbalance of heat causes the outlet temperature to decrease.
The large and quick change in hydrogen feed flow rate causes the outlet temperature of the liquefier to be impacted greatly when the dP parameter is only changed based on feedback control. The dP parameter can be implemented with a new control logic based on feed flow in order to provide sufficient refrigeration for the change in feed flow through the liquefier.

Factor Based on Heat Balance

When the feed flow changes quickly, the heat load is not equivalent to the refrigeration produced. The outlet temperature changes when the heat transfers are not balanced, which causes the outlet temperature to vary.
A heat balance can be performed on the 20K coldbox. In this non-limiting example, there are three streams: H₂feed circuit, refrigeration circuit, and low-pressure H₂circuit. The refrigeration circuit and the low-pressure H₂circuit absorb heat from the H₂feed circuit in order to meet the specified outlet temperature.
$Heat balance equation : Δ H_{F} = Δ H_{L P} + Δ H_{R C}$

- ΔH_F=heat removed from feed stream
- ΔH_LP=heat absorbed by low-pressure H₂circuit
- ΔH_RC=heat absorbed by refrigeration circuit

The heat that needs to be removed from the H₂feed circuit can be calculated from thermodynamic data. The heat absorbed from the low-pressure H₂circuit can also be calculated from thermodynamic data. The heat absorbed by the refrigeration circuit is a function of a factor.
$Δ H_{F} = Δ H_{L P} + Δ H_{R C} (dP parameter)$
A factor can be manipulated with innovative control logic of certain embodiments of the present invention to produce sufficient refrigeration to allow heat to be absorbed into the refrigeration circuit. This will prevent the outlet temperature from changing when the heat balance is changing.

Implementation

In order to better control the process, the refrigeration production can be adjusted with the current heat load. The current heat load on the liquefier can be simplified to the hydrogen feed flow rate into the 20K coldbox, which is a parameter that is easily available for operations. A more accurate heat load can be determined by calculating the amount of heat that needs to be extracted from molecules entering the 20K coldbox in order to reach the desired outlet temperature. This is accessible because the inlet flow and temperature are known and the desired outlet temperature is known.
The refrigeration production at specific factors is known due to past performance. The factor determines the amount of refrigeration. This expansion process creates the refrigeration that has produced known cooling performance in previous periods. For example, the pressure set point of the refrigerant letdown valve upstream of the turbines can be manipulated to control the factor and set the refrigeration production. In order to maintain the desired outlet temperature, the factor should be adjusted to match the heat load. This change in refrigeration can be made before the outlet temperature of the liquefier changes, which allows for much more stable control of the liquefier. The outlet temperature can also be monitored to provide a feedback loop and allow minor adjustments to the dP parameter.
In certain embodiments, said innovative control of the factor and in effect the refrigerant letdown valve is the preferred method to control refrigeration in the liquefier because the method is effective and efficient. The factor controls the refrigeration cycle by changing the flow and pressure drop of refrigerant through expanders. When the flow and pressure drop changes, the amount of refrigeration produced will change.
Prior art methods typically use a similar scheme that controls the inlet pressure to the turbines, but their systems do not account for the outlet pressure of the turbines. This is because these liquefiers employ the use of a large gaseous hydrogen storage tank, which acts as a buffer, thereby enabling them to operate at near steady state. These types of plants can control the pressure based on a feedback scheme.
However, embodiments of the present invention seek to eliminate the need for any additional gaseous hydrogen buffer storage, which results in large shifts in feed flow and heat load. Thus, the innovative control of the dP parameter as described herein is much better suited for embodiments of the present invention.
Therefore, in certain embodiments the control system utilizes logic that anticipates the required process operating conditions ahead of time. The relationship between the heat load (as determined by the leading indicator) and the required refrigeration factor is established based on known thermodynamic principles and past performance data of the liquefier. When a change in the leading indicator is detected (e.g., a sudden increase in feed flow), the controller uses this pre-determined relationship to calculate a new target value for the refrigeration factor and immediately implements the change. This proactive adjustment counteracts the thermal disturbance before it can cause a significant deviation in the liquefier outlet temperature. A traditional feedback loop based on the outlet temperature can still be used in parallel to make minor, “trim” adjustments to the factor, ensuring long-term precision
In addition to superb control, using the factor is the most efficient temperature control method. In certain embodiments, all the compression power can be utilized in the refrigeration process, and the system will not vary to an inefficient operating point. Other methods of controlling refrigeration-turbine inlet valves, turbine bypasses, and recycle compressor-require additional work and energy consumption by the recycle compressor. Maintaining the minimum work required by recycle compressor decreases the power consumed by the recycle compressor and results in more efficient liquefaction.
The FIGURE provides a simplified schematic diagram of a cryogenic liquefier, generally designated as 10, in which an embodiment of the present invention may be implemented. A gaseous feed stream 2 enters the coldbox of the cryogenic liquefier, where this gaseous feed stream 2 is cooled within the coldbox through indirect heat exchange via first heat exchanger 15 and second heat exchanger 25 and exits as a two-phase stream via a feed flow outlet 26. This outlet stream enters a flash vessel 30, which separates the liquid and vapor phases. The liquid phase is the LH₂produced, which is sent to a storage sphere 40. The vapor phase, referred to as the feed flash LP stream 32, is a low-pressure stream that is recycled back into first and second heat exchangers 15, 25 to recover its cooling potential before being sent to the suction of a compressor 50.
Refrigeration is provided to the cryogenic liquefier 10 using two different refrigeration cycles. First refrigeration cycle 60 is preferably a nitrogen refrigeration cycle, and second refrigeration cycle 65 is preferably a hydrogen or helium refrigeration cycle. First refrigeration cycle 60 provides the “warm temperature” cooling, while the second refrigeration cycle 65 provides the “cold temperature” cooling, which is the cooling needed to liquefy the gaseous feed stream 2. As used herein, “warm temperature” cooling is cooling that reduces the temperature of the gaseous feed stream 2 to a temperature just above that in which impurities within gaseous feed stream 2 would freeze. As a non-limiting example, if the gaseous feed stream 2 comprises hydrogen, then the warm temperature cooling is configured to cool the gaseous feed stream 2 to a temperature between 80K and 100K, plus or minus a few degrees Kelvin.
In the embodiment shown, first refrigeration cycle 60 includes a pair of turboboosters, a JT valve, and a flash vessel, all of which are used to provide the necessary compression and expansion for refrigeration provisions. First refrigeration cycle 60 further includes recycle compressors that are configured to compress the refrigerant within first refrigeration cycle 60. Those of ordinary skill in the art will recognize that the first refrigeration cycle 60 is not limited to the setup shown in the FIGURE, rather, any known refrigeration cycle that is capable of providing the warm temperature cooling may be used.
With respect to the second refrigeration cycle 65, the embodiment shown in the FIGURE includes a high pressure refrigerant stream 56 that is cooled via indirect heat exchange in first heat exchanger 15. Once cooled therein, the high pressure refrigerant stream 56 may be slightly cooled second heat exchanger 25, before being expanded in cold turbine 41. Following expansion in cold turbine 41, the resulting refrigerant stream is preferably split into at least two streams, more preferably three streams, with first stream 42 being used for isenthalpic or near-isenthalpic expansion via JT valve 51, and second stream(s) 44 being used for isentropic or near-isentropic expansion via turbines 43, 45.
The first stream 42 is at least partially condensed within the second heat exchanger 25 and withdrawn at a colder location than the second stream 44, before being pressure reduced across JT valve 51 to about atmospheric pressure and introduced to liquid/gas separator 53. The gaseous portion is preferably combined with feed flash LP stream 32 and re-warmed in second heat exchanger 25 and first heat exchanger 15 to form low pressure refrigerant 49. Liquid refrigerant is withdrawn from the bottom of liquid/gas separator 53, and then recycled back to the liquid/gas separator 53, thereby acting as a thermosiphon
The second stream 44 is subsequently sent to at least one expansion turbine, represented collectively as the turbine system 43, 45. The expansion of the refrigerant across the turbine system 43, 45 produces “Turbine work,” which generates the primary cooling for this stage of the process.
The medium-pressure refrigerant exiting the turbine system 43, 45, designated as the MP refrigerant stream 46, is returned through first and second heat exchangers 15, 25 for warming to the suction of a second stage of compressor 55.
In certain embodiments, the control method of the present invention focuses on managing the work produced by this turbine system 43, 45. The controller is configured to calculate a refrigeration factor based on the pressure at the inlet of the turbine system 43, 45 (related to the HP refrigerant stream) and the pressure at the outlet of the turbine system 43, 45 (related to the MP refrigerant stream 46). The pressures and flow rates of the refrigerant stream at various can be measured using sensors A, B, C, and D. The communication connection lines have not been shown from sensors C and D and controller 110 solely for purposes preventing overcrowding in order to maintain clarity in the FIGURE. Similarly, controller 110 is also configured to adjust pressure drop across turbines 41, 43, and 45; however, the communication connection lines are also omitted in order to not unduly burden the FIGURE.
In an additional embodiment, the controller 110 can be configured to manipulate this chosen factor by adjusting the set point of an actuator in the refrigerant circuit, such as a refrigerant letdown valve G upstream of the turbines 43, 45. By adjusting this valve, the controller modifies the inlet pressure to the turbines (Pin), thereby changing the chosen factor and proactively matching the refrigeration production to the heat load. While the FIGURE does not show a control valve upstream of turbine 43, Applicants note that a control valve is preferably disposed on this line as well. This control valve was only omitted on the FIGURE due to available space in the drawing. Similarly, the communication connection line from refrigerant letdown valve G has been omitted in order to not unduly burden the FIGURE.
In operation, boil-off gas from the LH₂storage sphere 40, designated as the LP sphere gas stream 48, can also be recovered. This stream is returned to the first and second heat exchangers 15, 25 to recover its cold energy before being sent, along with LP refrigerant 49 to the suction of compressor 50.
It is the sudden introduction of similar recovered gas streams from transport trailers 100 into the main Feed flow inlet 2 via line 102 that creates the transient heat loads managed by the present invention. In certain embodiments, controller 110 is configured to monitor temperature and flow rates of feed flow inlet 2. Controller 110 may also be further configured to receive input data such as temperature and flow rate of LH₂stream via sensor E, as well as temperature and flow of recovered gas streams 102 from transport trailers 100 using sensor F. Those of ordinary skill in the art will recognize that sensor F can be located downstream of the mixing point with the feed gas stream 2, with any appreciable difference to the previously read flow rate and temperature being attributed to GH₂from trailers 100 via line 102.
In another optional embodiment, direct control of the refrigeration factor is achieved by manipulating the operating parameters of the refrigerant recycle compressor system 50, 55 rather than a dedicated letdown valve G. In this configuration, the controller 110, in response to a detected change in the leading indicator, is configured to adjust the loading of the refrigerant compressor. This adjustment directly alters the compressor's discharge pressure, which in turn establishes the inlet pressure (Pin) to the turbine system 41, 43, 45. By manipulating the compressor to set the turbine inlet pressure, the controller proactively adjusts the differential pressure, pressure ratio, or work factor across the turbines to match the required refrigeration. In such an embodiment, upstream valves like the refrigerant letdown valve G may not be used for active modulation and can instead be maintained in a fixed or manual position, serving primarily as a safety device to prevent turbine overspeed.
Once the gaseous hydrogen from the trailers 100 is finished being evacuated to an acceptable level, fresh liquid hydrogen 104 is loaded into transport trailers 100.
While the invention was primarily developed for hydrogen liquefaction, the principles could be applied to other cryogenic liquefaction processes (e.g., helium, natural gas, CO₂) that experience similar transient heat loads.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device.
The singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. The terms about/approximately a particular value include that particular value plus or minus 10%, unless the context clearly dictates otherwise.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

Claims

We claim:

1. A method for controlling a cryogenic liquefier during transient operation, the method comprising the steps of:

providing a cryogenic liquefier, wherein the cryogenic liquefier comprises: a cold end heat exchanger configured to liquefy the cryogen gas at a temperature of approximately 20K, a cryogen feed flow conduit configured to deliver the cryogen gas to the cryogenic liquefier; a first refrigeration circuit, and a secondary refrigeration circuit, wherein the first refrigeration circuit comprises isenthalpic or near-isenthalpic expansion wherein the secondary refrigeration circuit comprises isentropic or near-isentropic expansion;

switching from steady state operation to a transient operation; and

controlling an outlet temperature of liquid cryogen from the cryogenic liquefier by adjusting the refrigeration provided by the secondary refrigeration circuit.

2. The method as claimed in claim 1, wherein the step of controlling the outlet temperature further comprises manipulating a factor selected from the group consisting of: a differential pressure across a turbine system, a pressure ratio across the turbine system, rotating speed of the turbine system, a work factor across the turbine system.

3. The method as claimed in claim 1, wherein the method switches to the transient operation based upon a change in flow rate of cryogen gas in the feed flow conduit.

4. The method as claimed in claim 1, wherein the method switches to the transient operation based upon a determination that a cryogen storage vessel is entering a loading phase.

5. The method as claimed in claim 4, wherein the method switches to the steady state operation based upon a determination that the cryogen storage vessel has finished its loading phase.

6. A method for controlling a cryogenic liquefier having a refrigerant circuit with at least one turbine, the method comprising:

detecting a change in a heat load on the cryogenic liquefier based on a measurement of a leading indicator parameter that is upstream of a liquefier outlet;

calculating, using a processor, a required adjustment to refrigeration production in response to the detected change in the heat load; and

proactively adjusting a refrigeration factor of the refrigerant circuit to effect the required adjustment to refrigeration production before a deviation in a liquefier outlet temperature caused by the change in the heat load occurs.

7. The method of claim 6, wherein the leading indicator parameter comprises a flow rate of a feed gas stream entering the cryogenic liquefier.

8. The method of claim 6, wherein the leading indicator parameter is determined from a heat balance calculation based on at least one of an inlet flow rate and an inlet temperature of a feed gas stream.

9. The method of claim 6, wherein the refrigeration factor is a function of at least an inlet pressure and an outlet pressure of the at least one turbine.

10. The method of claim 9, wherein the refrigeration factor is selected from the group consisting of: a differential pressure across the at least one turbine, a pressure ratio across the at least one turbine, and a work factor representative of total work produced by the at least one turbine.

11. The method of claim 6, wherein the step of proactively adjusting the refrigeration factor comprises transmitting a control signal to a refrigerant letdown valve positioned upstream of the at least one turbine.

12. The method of claim 6, wherein the change in the heat load is caused by introducing recovered cryogenic vapor from a transport trailer into a feed gas stream of the liquefier.

13. The method of claim 6, further comprising using a feedback control loop based on the liquefier outlet temperature to provide a trim adjustment to the refrigeration factor.

14. An apparatus for controlling a cryogenic liquefier, the apparatus comprising:

a cryogenic liquefier comprising a refrigerant circuit with at least one turbine and an actuator configured to adjust a flow of refrigerant to the at least one turbine;

at least one sensor configured to measure a leading indicator parameter of a heat load on the cryogenic liquefier, wherein the at least one sensor is positioned to measure the parameter upstream of a liquefier outlet; and

a controller communicatively coupled to the at least one sensor and the actuator, the controller comprising a processor and a memory including computer-executable instructions that, when executed by the processor, cause the controller to:

i) receive a signal from the at least one sensor indicating a change in the heat load;

ii) calculate, using a control algorithm, a required adjustment to a refrigeration factor of the refrigerant circuit; and

iii) transmit a control signal to the actuator to proactively implement the required adjustment to the refrigeration factor before the change in the heat load causes a deviation in a liquefier outlet temperature.

15. The apparatus of claim 14, wherein the at least one sensor is a flow meter configured to measure a flow rate of a feed gas stream entering the cryogenic liquefier.

16. The apparatus of claim 14, wherein the refrigeration factor is a function of at least an inlet pressure and an outlet pressure of the at least one turbine, and is selected from the group consisting of: a differential pressure, a pressure ratio, and a work factor.

17. The apparatus of claim 14, wherein the apparatus does not include a gaseous cryogen buffer storage tank for buffering recovered cryogenic vapor.