Method and apparatus for compressing a feed gas having a variable flow rate

Abstract

The energy efficiency and / or operational stability of a multi-stage compression system comprising a plurality of (N) centrifugal compressors compressing a raw gas having a variable flow rate is improved by: using a main circulation system to reversibly adjust the load on each compressor in response to changes in the flow rate of the raw gas to enable the centrifugal compressors to operate at a reduced capacity during periods when the flow rate is less than the total reduced capacity of all compressors, and, if necessary, using a local circulation system to avoid activating an anti-surge controller and switching one or more centrifugal compressors to a low power mode or a shutdown mode as required.

Description

Technical Field

[0001] The present invention relates to a method for improving energy efficiency and stability in a multi-stage compression system compressing a feed gas with a variable flow rate. In particular, the present invention relates to a method for saving power and preventing disturbances in the net compressed gas flow from the system that would otherwise be caused by the activation of an anti-surge controller. Background Art

[0002] A centrifugal compressor is a type of dynamic compressor in which gas is compressed by the mechanical action of rotating blades or impellers which impart velocity to the gas. Gas enters usually at the center of the impeller and is pushed out to the radial edges under the rotational motion, thereby delivering the gas at high velocity, impacting the casing. The velocity of the gas is converted into static pressure to deliver high pressure gas. These types of compressors are particularly suitable for handling large volumes of gas at a relatively low cost.

[0003] In order to properly compress process gases in centrifugal compressors, dry gas seals (or "DGS") are typically used to minimize any gas leakage. These dry gas seals contain two opposing sealing surfaces or sealing rings that are separated during normal operation of the centrifugal compressor compressing gas.

[0004] Typically, the gas used for compression is generated entirely using electricity generated from conventional energy sources (e.g., on-site gasoline, diesel, or hydrogen-powered generators, fuel cells), or electricity taken from local or national power grids. In this case, the centrifugal compressor is operated at maximum capacity to generate a net compressed product gas with the highest possible yield. The motor driving the impeller of the centrifugal compressor is therefore typically operated at a fixed speed (e.g., maximum speed). In these cases, the flow rate of the feed gas to the compression system is always maintained at a substantially constant maximum flow rate to maximize the output of the net compressed gas.

[0005] US5743715A relates to a process for balancing the load between compressors to ensure that the surge control lines of all compressors in the system are reached simultaneously. This document does not relate to how to compress gas with a variable flow rate, such as gas generated using renewable energy sources.

[0006] The inventors are not currently aware of any prior art that addresses the problems associated with compression of a feed gas having a variable flow rate over a wide range of flows.

[0007] In particular, the present invention relates to compressing a feed gas where the flow rate can be varied over a large range (e.g., 0 to 100% flow) over a relatively short time scale (e.g., 1 day), for example such as gas generated using electricity at least in part produced by one or more renewable energy sources (e.g., solar and / or wind power). Summary of the invention

[0008] According to a first aspect of the present invention, there is provided a process for operating a multi-stage compression system for compressing a feed gas having a variable flow rate,

[0009] The multi-stage compression system comprises a feed end, a plurality (N) of parallel centrifugal compressors, a product end and a main circulation system for circulating gas through the plurality (N) of centrifugal compressors, wherein each centrifugal compressor comprises an inlet, an outlet and a local circulation system having an anti-surge controller for circulating gas from the outlet to the inlet,

[0010] The process comprises:

[0011] (a) operating the first number (n) of centrifugal compressors at full load to compress the feed gas during a period in which the multi-stage compression system receives the feed gas at a flow rate equal to the total maximum capacity of the first number (n) of centrifugal compressors that produce net compressed gas;

[0012] (b) operating the first number (n) of centrifugal compressors at a minimum load for compressing the feed gas during a period in which the multi-stage compression system receives the feed gas at a flow rate ranging from less than a total maximum capacity of the first number (n) of centrifugal compressors to a total turndown capacity of the first number (n) of centrifugal compressors, the minimum load being determined based on the flow rate of the feed gas;

[0013] (c) circulating compressed gas using the primary circulation system as required to maintain the load of the first number (n) of centrifugal compressors above the point at which an anti-surge controller is activated during a period in which the multi-stage compression system receives the feed gas at a flow rate ranging from less than the total turndown capacity of the first number (n) of centrifugal compressors to greater than the total maximum capacity of the second number (n-1) of centrifugal compressors producing net compressed gas; and

[0014] (d) during a period in which the multi-stage compression system receives the feed gas at a flow rate equal to the total maximum capacity of the second number (n-1) of centrifugal compressors, unloading the centrifugal compressors so that the compressors enter a low power mode or an off mode in which the compressors generate no net compressed gas, while loading the remaining centrifugal compressors to maximum capacity,

[0015] wherein the process is reversible at any time, and wherein n is an integer equal to or less than N. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

[0017] Figure 1 is a simplified flow chart of the first embodiment of the present invention;

[0018] Figure 2 is a simplified flow chart of a second embodiment of the present invention;

[0019] Figure 3 is a simplified flow chart of a third embodiment of the present invention;

[0020] Figure 4 is a line graph providing an illustrative simulation example of the process of the present invention in the context of three centrifugal compressors arranged in parallel. DETAILED DESCRIPTION

[0021] In the following discussion of embodiments of the present invention, pressures given are absolute pressures unless otherwise stated.

[0022] The invention is particularly applicable to processes in which the variable flow of gas fed to the multi-stage compression system is the result of the generation of the gas using electricity generated at least in part from at least one renewable energy source. Preferably, this gas may be hydrogen generated by electrolysis of water.

[0023] A disadvantage of using renewable energy sources to generate gas for compression in centrifugal compressors is the inherent variation in the availability of energy, which may span from full power to no power over the course of a day. Although other energy sources (e.g., battery power or non-renewable energy sources) may be used to supplement the power when availability is low, it may still be insufficient to generate the maximum flow of gas for compression required to fully operate the centrifugal compressor or each centrifugal compressor.

[0024] For example, over the course of a day, the flow of gas used for compression can vary from maximum flow to very low flow, or no flow at all. Centrifugal compressors can tolerate some variation in the flow of gas used for compression, but not to the extent that is required if they are not shut down completely from time to time.

[0025] Centrifugal compressors are often equipped with a local circulation system with an anti-surge controller (e.g., an anti-surge valve controlled by a control system). This local circulation system protects the centrifugal compressor from the risk of operational damage caused by compressor surge. Surge occurs when the flow rate of the feed gas decreases beyond the point where the compressor can maintain operation at a stable impeller speed. Surge may cause hot gas to flow back through the compressor and / or severe pressure pulsations throughout the machine, possibly causing severe mechanical vibration and damage. Typically, anti-surge controllers are designed to be enabled before the compressor reaches the surge point. When enabled, these controllers quickly circulate gas through the centrifugal compressor, such as by opening a local circulation valve, to increase the compressor load and prevent surge. However, this is often at the expense of stability and controllability of the entire process and / or apparatus.

[0026] The anti-surge controller of the local circulation system can be activated at a flow rate of about 20% (preferably about 10%) above the flow rate at which the centrifugal compressor surges (sometimes referred to as the "surge line" in the art). When the flow rate of the raw gas to the compression system drops below this anti-surge controller activation point, the compressor anti-surge controller takes effect to protect the compressor. This protection measure can cause serious process disruptions to adjacent equipment and can cause the entire facility to trip in the most extreme cases.

[0027] In addition, activation of the anti-surge controller introduces a large amount of recycle gas into the system in a short period of time, thereby interrupting the flow of net compressed gas. This is undesirable because it may interrupt the steady flow of net compressed gas at the outlet of the system. The expression "net compressed hydrogen" refers to the total amount of compressed gas generated minus the total amount of gas recycled.

[0028] When a centrifugal compressor is shut down or shut down, the speed of the rotor or impeller is reduced until the opposing sealing surfaces of the dry gas seal (DGS) are no longer separated and in contact with each other. Therefore, frequently shutting down and shutting down the centrifugal compressor will accelerate the wear of the DGS. This shortens the life of the centrifugal compressor, thus requiring more frequent replacement or repair, which increases costs. Wear of the DGS also occurs when the centrifugal compressor is restarted or powered on.

[0029] Centrifugal compressors are at risk of damage every time they are started or shut down. In fact, the chances of compressor problems occurring at startup are higher than when they are shut down. In this regard, there are usually critical speeds that should be avoided. In addition, if compressors are shut down for a long period of time, they may be more susceptible to pitting and other types of corrosion, which can lead to stress corrosion cracking and, ultimately, failure of the impeller / compressor.

[0030] The above problems do not apply to centrifugal compressors compressing gas where the gas flow rate is substantially constant and maintained substantially above the centrifugal compressor's maximum turndown capacity or anti-surge control point. For example, these problems are not relevant to gas generated entirely using energy from a non-renewable grid because the flow of the feed gas is substantially at a maximum constant flow rate and the compressor is rarely shut down.

[0031] Thus, the inventors have recognized that there is a desire in the art to provide an improved method of operating a centrifugal compressor capable of compressing a feed gas having a variable flow rate over a wide range, such as gas generated at least in part using electricity produced from renewable energy sources.

[0032] The inventors have devised a process for operating a multi-stage compression system as described herein that reduces the number of centrifugal compressor shutdowns and thereby increases the life of the dry gas seals and reliability of the centrifugal compressor. Alternatively or additionally, the inventors have devised a process by which power can be conserved, such as so that the power can be used in other parts of the process, such as for generating feed gas and / or as part of a downstream process that consumes compressed gas. Alternatively, or additionally, the present invention can allow compression of feed gas with variable flow rates over a wide range without requiring unnecessary compressor shutdowns and / or excessive compressed gas cycling and / or using excess power and / or interfering with the flow of net compressed gas due to the activation of an anti-surge controller.

[0033] A multi-stage compression system is used to compress a feed gas having a variable flow rate, preferably in preparation for consumption in at least one downstream process.

[0034] The gas used for compression is typically generated using electricity at least in part produced from at least one renewable energy source, and may be any suitable gas. However, the process has particular application where the gas used for compression is hydrogen, such as hydrogen generated by electrolysis of water. This may be performed using a plurality of electrolysers.

[0035] In some embodiments, the process comprises generating hydrogen by electrolyzing water. Additionally or alternatively, the process may comprise feeding the compressed hydrogen to at least one downstream process for consumption in the downstream process.

[0036] Therefore, in some preferred embodiments, the process comprises:

[0037] Producing hydrogen by electrolysis of water;

[0038] compressing the hydrogen in a multi-stage compression system operating in accordance with the present invention; and

[0039] The compressed hydrogen is fed to at least one downstream process for consumption in the downstream process.

[0040] In some embodiments, at least some of the compressed hydrogen is used in the downstream process to produce ammonia and / or methanol, preferably to produce ammonia.

[0041] Centrifugal compression is particularly suitable for compressing large quantities of hydrogen at a relatively low cost, and therefore compression of hydrogen is particularly preferred and advantageous for the process of the present invention. Furthermore, hydrogen produced by electrolysis is even more suitable for centrifugal compression because it is "wet" and has a higher density, making centrifugal compression of the gas more efficient than compressing hydrogen not produced by electrolysis.

[0042] Operation of Centrifugal Compressors in Multistage Compression Systems

[0043] The present invention relates to a multi-stage compression system, which includes a feed end, a plurality of (N) parallel centrifugal compressors, a product end and a main circulation system for circulating gas through the plurality of (N) centrifugal compressors (or if one or more of the plurality of (N) centrifugal compressors are in low power mode or shutdown mode, through the number of centrifugal compressors that produce net compressed gas).

[0044] The main circulation system is used to circulate the gas through the multiple (N) centrifugal compressors. Therefore, the main circulation system circulates the gas of all centrifugal compressors at the same time, rather than individually. Therefore, the main circulation system can receive the gas discharged from the product end of the centrifugal compressor, and after appropriate decompression, feed this decompressed gas to the feed end of the centrifugal compressor. The main circulation system can receive compressed gas from each of the centrifugal compressors before or after the compressed gas is mixed into a single header line. After appropriate decompression, the decompressed gas can be separated from the header and fed to each of the centrifugal compressors. This allows all N compressors to operate with the same performance curve, thereby greatly simplifying load sharing between machines. The main circulation system is usually operated together with the speed controller of the compressor to modulate and maintain a constant suction pressure.

[0045] The use of a primary circulation system is particularly advantageous in the case of compression of wet hydrogen produced by electrolysis. Hydrogen produced by electrolysis of water is typically saturated with water, and during compression the water content of the hydrogen may change, for example due to a cooling step in the process. As the water content changes, the apparent molecular weight of the hydrogen also changes - which in turn changes the discharge pressure ratio at the product end of the individual compressors. Therefore, the use of a primary circulation system also allows the apparent molecular weight of the wet hydrogen to remain substantially constant across all of the plurality (N) of compressors.

[0046] N is an integer representing the total number of centrifugal compressors arranged in parallel in a multi-stage compression system implementing the process of the present invention, and this number may depend on the process requirements (eg, process scale, downstream processes, etc.).

[0047] Each centrifugal compressor includes an inlet, an outlet and a local circulation system with an anti-surge controller. The local circulation system is used to circulate gas from the outlet of the centrifugal compressor associated therewith to the inlet. Local circulation systems are known in the art and are mainly used to prevent surge of the compressor or during compressor unloading. When used, the local circulation system receives gas from the outlet of the centrifugal compressor and, after appropriate decompression, feeds the decompressed gas to the inlet of the centrifugal compressor. In some embodiments herein, the circulation system may be associated with more than one centrifugal compressor, such as more than one centrifugal compressor arranged in series, such as across multiple compression stages.

[0048] A pressure reducing member (e.g., a valve) may be used to reduce the pressure of the circulating gas to an appropriate level. In this regard, the appropriate level would be the inlet pressure of the compressor to which the gas is fed.

[0049] In some embodiments herein, a multi-stage compression system includes centrifugal compressors arranged in series as part of a multi-stage compression, and a local circulation system circulates gas from the outlet of an intermediate or final stage to the inlet of an initial stage. That is, when in use, the local circulation system receives gas from the outlet of a downstream stage of compression, and after appropriate decompression, feeds the decompressed gas to the inlet of an upstream stage of compression.

[0050] This allows the number of local circulation systems and associated valves to be reduced, thereby simplifying the design and operation of the compression system and reducing costs. However, in such an embodiment, it should be understood that no gas can be fed from the gas storage system to a point between the downstream and upstream stages.

[0051] As described above, when the flow to the compressor reaches the anti-surge control point (eg, a flow rate about 10% above the surge line), the local circulation system is quickly activated.

[0052] In the process of the present invention, the multi-stage compression system is operated so that it responds to changes in the flow rate of the raw gas at the inlet of the multi-stage compression system. The flow rate of this raw gas is variable, and certain operating modes of the centrifugal compressor as described herein will be enabled in response to certain changes in the flow rate of this raw gas. Operational changes occur within the system to accommodate changes in the flow rate of the raw gas.

[0053] The invention generally resides in a number of actions performed in response to said change in the flow rate of the raw gas, said actions being fully reversible at any point in the process dependent upon the change in the flow rate of the raw gas, for example when the change in the flow rate of the raw gas is reversed.

[0054] During the period specified in (a) – the maximum flow

[0055] During a period in which the multi-stage compression system receives feed gas at a flow rate equal to the total maximum capacity of a first number (n) of centrifugal compressors that produce net compressed gas, the process includes operating the first number (n) of centrifugal compressors at full load to compress the feed gas.

[0056] The first number (n) is the number of centrifugal compressors in the multi-stage compression system that are operated to generate net compressed gas, i.e., without stopping, shut down or in low power mode. Therefore, in the context of the present invention, (n) is an integer (1, 2, 3, ..., etc.) equal to or less than (N), the total number of centrifugal compressors used to implement the process. In the context of the present invention, the first number (n) may not be equal to zero, but in some cases it may still be desirable to shut down all centrifugal compressors or put them in idle mode (see below).

[0057] In other words, when the flow rate of the raw gas matches the total maximum capacity of the first number (n) of centrifugal compressors, the compressors are operated at maximum capacity to generate as much net compressed gas as possible.

[0058] As used herein, the term "total" refers to the sum of the maximum capacities of the (n) centrifugal compressors. The term "maximum capacity" refers to the 100% capacity of the compressor, i.e., the maximum amount of raw gas flow compressed by the compressor at 100% of its maximum power and rotor speed (i.e., full load). The "load" of a centrifugal compressor refers to the total flow of compressed gas generated (including any gas flow circulated by the main circulation system). For example, the load of a centrifugal compressor can be controlled by changing the rotor speed or adjusting the inlet guide vanes using a suitable VFD.

[0059] During the period specified in (b) – reduce the flow rate (turn down)

[0060] During a period in which the multi-stage compression system receives a feed gas at a flow rate ranging from less than a total maximum capacity of the first number (n) centrifugal compressors to a total turndown capacity of the first number (n) centrifugal compressors, the method includes operating the first number (n) centrifugal compressors at a minimum load for compressing the feed gas, the minimum load being determined based on the flow rate of the feed gas.

[0061] In addition to operating the centrifugal compressor at maximum capacity, the gas flow through the centrifugal compressor can be varied in several ways. For example, this can be achieved by changing the power and rotor speed of the impeller, or by adjusting the inlet guide vanes. However, the flow through the centrifugal compressor can only be reduced to a certain extent before the anti-surge controller is enabled. This process is known in the art as compressor "turndown". The capacity (or flow) that a centrifugal compressor turns down as much as possible, that is, compressing the minimum flow of gas without enabling any anti-surge controller, is called its turndown capacity (or maximum turndown). This point is usually substantially at or just above the anti-surge control point.

[0062] Therefore, the turndown capacity of each centrifugal compressor is defined herein as the minimum flow rate of gas that the centrifugal compressor can compress without activating its anti-surge controller.

[0063] When turned down, the compressor power is less than 100% relative to the maximum power (100%), but at the same time is about 60% or more, preferably about 70% or more, for example from 70% to 80%. This reduction in compressor power results in a reduction in rotor speed and therefore a corresponding reduction in the flow of net compressed gas at the product end of the compressor. In order to reduce the gas flow to the multi-stage compression system (at a constant discharge pressure), this generally requires a proportional reduction in the compressor power.

[0064] The term "total turndown capacity" is used herein to refer to the sum of the turndown capacities of the first number (n) of centrifugal compressors, ie, the ability to compress the flow of gas in a state in which all centrifugal compressors are turned down as much as possible (maximum turndown).

[0065] The minimum load is based on the flow rate of the raw gas, and therefore the minimum load refers to the minimum power and / or rotor speed of the compressor suitable for compressing all the flow of raw gas to generate the required discharge pressure. For example, if the flow rate of the raw gas is 85% of the full flow, the total minimum load of the centrifugal compressor during these periods will be 85% of the full flow.

[0066] Preferably, during these periods, the first number (n) of centrifugal compressors share the load approximately equally, so that the load on all compressors is the same, that is, the load on all centrifugal compressors is balanced, so that all compressors are approximately the same distance from the surge line. In practice, there may be small inherent fluctuations in the load between the compressors, but in this case, the load on all operating compressors will be as close to the level that can be achieved.

[0067] During the period specified in (c) – further reduction of flow (cycling)

[0068] During a period in which the multi-stage compression system receives feed gas at a flow rate ranging from less than the total turndown capacity of the first number (n) of centrifugal compressors to greater than the total maximum capacity of the second number (n-1) of centrifugal compressors producing net compressed gas, the process includes circulating the compressed gas using the main circulation system as required to maintain the load of the first number (n) of centrifugal compressors above the point at which the anti-surge controller is activated.

[0069] Thus, in other words, as the flow rate of the raw gas drops further (compared to the period specified in (b)) below the point at which the anti-surge controller within the centrifugal compressor is enabled, the primary circulation system circulates gas through the system. This has the effect of maintaining the first number (n) of loads above the anti-surge control point of the centrifugal compressor even as the flow rate of the raw gas drops further. Typically, the (n) centrifugal compressors are always operated at maximum turndown as far as safety considerations permit before any cycles are used to conserve as much power as possible.

[0070] This allows the multi-stage compression system to be operated in a manner that does not activate the compressor anti-surge controller. As described above, activation of the anti-surge controller interrupts the flow of net compressed gas at the output of the compression system, which in turn may be harmful to other processes (e.g., downstream processes that receive compressed gas for consumption). Therefore, by preventing damage to the compressor, the present invention allows for a more stable output flow of compressed gas, and / or more reliable compressor operation.

[0071] Operation of the first number (n) of centrifugal compressors as described above is only performed until the flow rate of feed gas to the multi-stage compression system reaches a point where the flow rate matches the maximum capacity of the (n-1) centrifugal compressors (i.e., one less compressor than is currently operating producing net compressed gas).

[0072] During the period specified in (d) – optimize (n-1) compressors

[0073] As the feed gas further decreases, the circulation volume required by the main circulation system increases (to maintain the load of the first number (n) of centrifugal compressors above the point at which the anti-surge controller is activated).

[0074] The compression of the circulating gas wastes electricity, and therefore it is often desirable to minimize the amount of gas circulated by the main circulation system as much as possible. It is therefore preferred that the amount of compressed gas circulated during the period specified in (c) is maintained at a minimum to save electricity. In practice, safety considerations and operational risks may require the use of slightly more than the minimum possible circulation where necessary.

[0075] Thus, the inventors have recognized that in a multi-stage compression system including multiple centrifugal compressors, the load of the centrifugal compressors can be optimized and the amount of circulation minimized. This can be achieved by dynamically and / or sequentially shutting down the compressors or placing them in a low power mode (sometimes referred to herein as "idling") while using the remaining compressors at high load with less (or no) circulation from the main circulation system.

[0076] During a period in which the multi-stage compression system receives feed gas at a flow rate equal to the total maximum capacity of the second number (n-1) of centrifugal compressors, the process includes unloading the centrifugal compressors to place the compressors into a low power mode or a shutdown mode in which the compressors generate no net compressed gas, while loading the remaining centrifugal compressors to maximum capacity.

[0077] This therefore allows the use of fewer centrifugal compressors to compress the raw gas, with little or no gas circulating through the main circulation system, thereby saving electricity and operating the system in a more efficient manner.

[0078] Although it is theoretically possible to reduce the circulation of the main circulation system to zero immediately using the remaining compressors, in practice such changes are ideally more gradual. For example, as the centrifugal compressors are unloaded and the load of the remaining compressors rises to maximum capacity, the main circulation system can gradually reduce the flow of circulating gas through the system to ensure a smoother transition during the load rebalancing between the centrifugal compressors. This will also help maintain a stable output of net compressed gas from the system and more safely prevent accidental activation of any anti-surge controller.

[0079] Thus, the present invention may allow the most efficient use of available electricity, since placing some centrifugal compressors in a low power mode or shutting down "frees up" available electricity, which may then be supplied to other parts of the process, such as generating gas (e.g. for an electrolyser to generate hydrogen), compressing gas (e.g. to supply electricity to operate the centrifugal compressors), or energy for downstream processes. Thus, the present invention is particularly useful in the context of renewable energy sources, where conserving available energy is important.

[0080] When used in the context of the present invention, the term "unloading" preferably includes firstly reducing the flow of net compressed gas through the centrifugal compressor to zero using the local circulation system, and secondly reducing the load of the centrifugal compressor. That is, the flow of net compressed gas can be reduced by increasing the amount of gas circulating through the centrifugal compressor by the local circulation system until all the fresh gas entering the inlet is replaced by the circulating gas. This advantageously allows the flow of net compressed gas through the system to be substantially unaffected by the process of reducing the load of the centrifugal compressor to place it in a low power mode (idling) or shutdown mode, and thus contributes to the stable operation of the net compressed gas through the system.

[0081] The centrifugal compressor can be unloaded to a standstill state by shutting it down, or by placing it in a low power mode (see below).

[0082] Placing the compressor in a low power mode is preferred to a complete shutdown because it reduces the number of shutdowns and / or restarts, thereby preventing excessive wear of the dry gas seals (DGS) within the centrifugal compressor. Alternatively or additionally, this also allows the process to react more quickly to a reversal of the feed gas flow because the time taken to bring the compressor out of the low power mode is less than the time taken to power up from a complete shutdown.

[0083] It will also be appreciated that the process may also be repeated in a cycle. That is, after the period specified in (d), once the compressor has been shut down, the first number (n) of compressors generating net compressed gas will be reduced by 1. In this way, the conditions of the period specified in (d) will be switched with the conditions of the period specified in (a), and the process may then be repeated, for example, if the flow of the feed gas further decreases, the period specified in (b), and then (c) and then (d) may again be applied to the new value of the first number (n). That is, due to the change in the first number (n) of centrifugal compressors, the sum of the total maximum capacity, the total turndown capacity and the flow rate when the anti-surge control point is enabled will change.

[0084] The process of the present invention is reversible at any time. If the process is reversed, the logic of the steps will simply be reversed. For example, when the process is performed in reverse, the process includes (d) loading the centrifugal compressors to remove the compressors from a low power mode or shutdown mode during a period in which the multi-stage compression system receives feed gas at a flow rate greater than the total maximum capacity of the second number (n) of centrifugal compressors, while reducing the load of the remaining centrifugal compressors to the total turndown capacity (when navigating from the period specified in (d) to the period specified in (c)). Figure 4is an illustrative example showing thresholds and conditions under which certain actions may be taken in a process, as well as resulting loads, cycle flows, etc.

[0085] Therefore, in addition, the process may further include, (e) during a period in which the multi-stage compression system receives feed gas at a flow rate greater than the total maximum capacity of the first number (n) of centrifugal compressors, if available, reloading an additional centrifugal compressor to bring the additional compressor out of a low power mode or shutdown mode until it begins to generate net compressed gas.

[0086] This allows the process to dynamically respond to changes in the flow rate of gas in the feed gas of the multi-stage compression system, such as changes in the amount of gas generated by the process driven by renewable energy. In this way, the process of the present invention provides a method of operating a multi-stage compression to compress a gas with a variable flow rate, which method, among other things, minimizes power consumption, provides a stable output of net compressed gas from the system, and / or improves compressor reliability.

[0087] In the event that the first number (n) is equal to one (i.e., there is only one centrifugal compressor operating to generate net compressed gas), the process may additionally include either continuing to operate the last centrifugal compressor at a load above its anti-surge control point using the main circulation system, or, alternatively, shutting down the compressor or placing the compressor in a low power mode. For example, whether the final centrifugal compressor continues to operate may depend on whether there is sufficient power to operate it. In some cases, it may be desirable to maintain at least one centrifugal compressor operating (n equals 1) to compress at least some of the gas. It should be understood that when the first number (n) is equal to 1, the cycle specified in (c) can be applied until the flow rate of the feed gas increases to reach the conditions of the cycle specified in (b).

[0088] Thus, where the first number (n) is equal to one, the process may include, during a period of raw gas received by the multi-stage compression system at a flow rate ranging from 0% flow to the maximum capacity of the centrifugal compressor to produce net compressed gas, circulating the compressed gas on demand using the main circulation system to maintain the load of the centrifugal compressor above the point at which the anti-surge controller is enabled.

[0089] In some cases, all centrifugal compressors may be shut down or put into low power mode, for example due to very low flow in the feed gas. In these cases, it will be appreciated that the periods specified in (a) to (d) will no longer apply until the centrifugal compressors are switched on or off low power mode and net compressed gas is produced again.

[0090] If all centrifugal compressors are off, the user (or controller) of the system can determine at what point the flow of feed gas is sufficient to justify turning the centrifugal compressors back on or out of low power mode. For example, this may depend on the available power for compression, and only when the flow of feed gas allows one centrifugal compressor to operate at maximum turndown without significant cycling is it considered energy efficient.

[0091] Thus, in some embodiments, the process includes, in the event that all centrifugal compressors are not generating net compressed gas, reloading the centrifugal compressors to bring the additional compressors out of a low power mode or shutdown mode until they begin generating net compressed gas during a period when the flow rate of the raw gas is equal to or greater than the total turndown capacity of at least one centrifugal compressor.

[0092] Low Power Mode / Idle

[0093] Based on the prior art, centrifugal compressors are usually shut down or shut down in response to a significant reduction in gas flow to the compression system, so as to be restarted once the gas flow increases sufficiently. However, in the context of the present invention, the centrifugal compressor can be operated in a "low power" mode (LP mode) instead.

[0094] Each centrifugal compressor typically includes at least one dry gas seal with opposing sealing surfaces inside it. Any dry gas seal suitable for centrifugal compressors can be used, and these are known in the art, including but not limited to single seals, tandem seals, and double opposing seals. In order to properly compress the process gas in the centrifugal compressor, a DGS can be used to minimize any gas leakage. These dry gas seals contain two opposing sealing surfaces or sealing rings, one is usually a rotating surface (sometimes called a "rotor"), and the other is a stationary surface (sometimes called a "stator"). The rotating surface has a lifting geometry designed into it so that when the rotating surface reaches a certain speed, it lifts off the stationary surface, creating a small gap so that the surfaces do not touch, which is used to minimize gas leakage.

[0095] Centrifugal compressors powered by a standard non-renewable grid will operate at a fixed speed (usually a maximum speed to provide the maximum amount of product gas). In these cases, the opposing seal faces of the dry gas seals separate rapidly and remain constant during compression of the gas while the compressor motor speed remains constant. Centrifugal compressors are rarely shut down, shut down, or restarted due to the continuous power supply from the grid.

[0096] When a centrifugal compressor with a dry gas seal shuts down, the motor speed drops to zero and the opposing seal faces begin to touch. The more often this happens, the more the opposing seal faces of the dry gas seal wear over time. This shortens the life of the dry gas seal, which means that the compressor needs to be serviced more frequently, increasing overall costs. More repairs to compressors in the system also result in interruptions to the entire process in which the repairs are performed, further complicating the operation of the process and increasing costs.

[0097] DGS are often used when compressing high pressure, low molecular weight, flammable, toxic and / or expensive gases. As DGS age, there are usually more leaks at the seals, resulting in more losses, which also have economic impacts.

[0098] In the low power mode, the centrifugal compressor or at least one centrifugal compressor is operated at a low amount of power sufficient to prevent contact of the opposing seal faces of the dry gas seals in the centrifugal compressor, and preferably without generating net compressed gas. Thus, the rotor speed of the centrifugal compressor is reduced, but not completely zero (i.e., the compressor is not shut down or stopped).

[0099] During the low power mode, the opposing seal faces (sometimes referred to in the art as "rings") are separated and not in contact. That is, the motor speed of the or at least one centrifugal compressor is reduced compared to the normal power mode, but the motor speed is high enough to exceed the so-called "lift-off" speed of the DGS so that these opposing seal faces remain separated from each other.

[0100] The opposing sealing surfaces typically have a rotating surface and a stationary surface. The rotating surface has a lifting geometry designed into it so that when the rotating surface reaches a certain speed, it lifts off the stationary surface. This creates a tiny gap with the non-contact surface, resulting in minimal gas leakage. Therefore, in the context of the present invention, "preventing contact" refers to the presence of such a tiny gap with the non-contact surface.

[0101] It should be understood that due to the presence of a non-zero rotor speed during the low power mode, the centrifugal compressor will operate in a manner that still generates compressed gas. However, this gas will be circulated from the product end of the compressor to the feed end. In other words, during the low power mode, there is no net compressed gas generation because only the cycle gas is compressed.

[0102] The amount of compressor power required to prevent contact between the relative sealing surfaces depends not only on the design of the centrifugal compressor, but also on the design of the dry gas seal. However, typically, a centrifugal compressor in low power mode will operate above this minimum power threshold to ensure that contact is prevented. In low power mode, the power of the centrifugal compressor is typically about 5% to about 20%, such as about 8% to about 15%, such as about 10%, of the maximum power of the compressor. The "lift-off" speed is the rotor speed (in rpm) required before the sealing surface of the DGS is out of contact, and depends at least in part on the design of the DGS and the manufacturer. In this regard, the manufacturer of a given DGS will indicate the lift-off speed of the DGS. However, it should be noted that the lift-off speed of a DGS of one manufacturer may be different from that of another manufacturer, even for a DGS of similar design. In addition, as the DGS ages and / or becomes contaminated, the lift-off speed may also change over time. With this in mind, the rotor speed during the low power mode is typically greater than, for example, at least two or even three times the lift-off speed indicated by the manufacturer to ensure that the sealing surfaces in the DGS do not contact. For example, if the lift-off speed of a given DGS is 300 rpm, the rotor speed of a compressor using this DGS during low power mode may be about 600 rpm or even 900 rpm.

[0103] It is within the skill of the artisan to determine by experimentation the appropriate rotor speed of the DGS in a centrifugal compressor operating in a low power mode. However, for purposes of illustration, the rotor speed during the low power mode will be less than the speed during the normal power mode (e.g., about 3000 rpm to about 3500 rpm), and may be in the range of from about 100 rpm to about 1500 rpm, for example, from about 200 rpm to about 1000 rpm, or from about 400 rpm to 900 rpm.

[0104] The rotor speed of the centrifugal compressor (or the power provided to the centrifugal compressor), for example, for switching between normal power mode and low power mode, can be manipulated using suitable means known to those skilled in the art, including but not limited to variable frequency drive (VFD) and mechanical drive. Other mechanical devices, such as a two-speed motor, may also be used.

[0105] It will be appreciated that the control system may also be used to monitor and control the rotor speed or power output of the centrifugal compressor.

[0106] Renewable Energy

[0107] The method of the present invention comprises compressing a feed gas having a variable flow rate, such as a gas generated using electricity generated at least in part from at least one renewable energy source.

[0108] The operation of the compression system is often directed by gas generated using electricity from renewable energy sources (e.g., hydrogen from an electrolyzer). Typically, the power required to generate the gas for compression (e.g., using an electrolyzer) is much greater than the power required to run the compressor. When the gas supply is low or nonexistent, gas is often injected from a gas storage system.

[0109] To reduce the impact on the environment, it is preferred that the process is independent in terms of the power to generate the gas, and optionally to power the centrifugal compressor. Thus, it is preferred that all power requirements to generate the gas for compression and optionally for the centrifugal compressor are met using renewable energy sources, without using non-renewable energy sources to supplement the sources.

[0110] It will be appreciated that placing the centrifugal compressor or at least one centrifugal compressor in a low power mode when the available electricity generated from renewable energy sources is insufficient for normal operation of the multi-stage compression system risks reducing the net amount of compressed gas generated by the system. In such circumstances, it is preferred that the demand for compressed gas be met by feeding gas from a suitable gas storage system before considering the use of any non-renewable energy sources to generate more gas (or to power the centrifugal compressors).

[0111] Nevertheless, there may be situations where the demand for compressed gas cannot be met by the gas fed for compression (e.g. hydrogen from an electrolyser) or by the gas storage system. It is therefore envisaged that in some embodiments, electricity generated by one or more renewable energy sources may be supplemented by other sources during periods of particularly high demand for products, e.g. from a downstream process and / or during periods when renewable energy sources are only available below a threshold required to meet the process demand, or are not available at all, and the supply of gas from the gas storage system is insufficient to meet the demand.

[0112] Thus, in some embodiments, at least some additional electricity may be drawn from on-site battery storage and / or generated by one or more on-site gasoline, diesel or hydrogen powered generators (including fuel cells) and / or drawn from a local or national power grid.

[0113] However, there may be situations where the electricity generated by the renewable energy sources and the additional electricity is still insufficient for normal operation of the multi-stage compression system.

[0114] In these embodiments, the gas used for compression is generated using a centrifugal compressor, and the centrifugal compressor is optionally powered by

[0115] (i) electricity generated at least in part from at least one renewable energy source, and

[0116] (ii) electricity from on-site battery storage and / or electricity generated from one or more on-site gasoline, diesel or hydrogen powered generators.

[0117] Gas for compression

[0118] The feed gas for compression in the multi-stage compression system may comprise any gas suitable for compression in a centrifugal compressor with a variable flow rate. In the context of the present invention, the centrifugal compressor typically compresses gas generated using electricity at least partially from at least one renewable energy source.

[0119] The preferred gas for compression is hydrogen (preferably generated by electrolysis of water).Any suitable form of water electrolysis may be used, including alkaline water electrolysis and polymer electrolyte membrane (PEM) water electrolysis.

[0120] The water used for electrolysis is typically seawater that has been desalinated (possibly by reverse osmosis) and softened water.

[0121] The electricity required for electrolysis can be generated at least in part by any suitable renewable energy source. However, in some preferred embodiments, at least some of the electricity required for electrolysis is generated by renewable energy sources, including wind energy, solar energy, tidal energy and hydroelectric energy, or a combination of these energy sources, particularly wind energy and solar energy. The electricity generated from these sources can be used to power the electrolyzer.

[0122] Preferably, the process is independent in terms of power generation for the electrolysis. Thus, preferably all electricity requirements for the electrolysis are met using renewable energy sources.

[0123] However, it is contemplated that during periods when demand for products from downstream processes is particularly high and / or during periods when renewable energy sources are only available below the threshold required to meet demand or not available at all, the electricity generated by one or more renewable energy sources may be supplemented by other sources. In these cases, the additional electricity may be drawn from on-site battery storage and / or generated by one or more on-site gasoline, diesel or hydrogen powered generators (including fuel cells), and / or drawn from the local or national grid.

[0124] The electrolysis can be performed at any suitable scale, in some cases with a total capacity of less than 1 GW. However, in a preferred embodiment, the electrolysis has a total capacity of at least 1 gigawatt (GW). The maximum total capacity of the electrolysis is limited only by practical considerations, such as generating enough power from renewable energy sources to power multiple electrolyzers. Thus, the electrolysis can have a maximum total capacity of about 10 GW or more. The total capacity of the electrolysis can be, for example, from 1 GW to about 5 GW, such as from about 1.5 GW to about 3 GW.

[0125] Hydrogen is typically produced by electrolysis at a pressure slightly above atmospheric pressure (eg, about 1.3 bar). However, in some embodiments, electrolysis produces hydrogen at a slightly higher pressure (eg, up to about 3 bar).

[0126] Thus, hydrogen is often fed to the multi-stage compression system at a pressure of from atmospheric pressure to about 5 bar, such as from atmospheric pressure to about 3 bar, preferably from atmospheric pressure to about 1.5 bar, such as about 1.1 bar.

[0127] In some embodiments, the amount of hydrogen generated by the electrolyzer is variable, and thus during periods where insufficient hydrogen is generated by electrolysis, such as in situations where the flow rate of hydrogen is below the maximum turndown capacity of a single centrifugal compressor, gas may be fed to the multi-stage compression system from another source (e.g., a hydrogen storage system).

[0128] purification

[0129] In the preferred embodiment where the gas used for compression is hydrogen produced by electrolysis, it will be noted that hydrogen produced by electrolysis is typically saturated with water at 40° C. Therefore, such hydrogen often contains some residual oxygen, typically about 500 to about 1000 ppm (v). These impurities often must be removed, depending on the tolerances of any downstream processes.

[0130] In this regard, oxygen is a poison to conventional catalysts used in the Haber process. Thus, in embodiments where the downstream process is ammonia synthesis, the feed to the catalyst will contain less than about 10 ppm, typically less than about 5 ppm, of total oxygen, i.e., from any impurity source such as oxygen (O 2 ), water (H 2 O), carbon monoxide (CO), and / or carbon dioxide (CO 2 ) of oxygen atoms. Therefore, the feed will also be dry, i.e., not more than 1 ppm of water.

[0131] Downstream processes, such as refineries, that use conventional "grey" hydrogen (i.e., hydrogen derived from hydrocarbon or carbon-containing feed streams without carbon dioxide capture, such as by reforming natural gas) or "blue" hydrogen (i.e., hydrogen obtained in the same manner as grey hydrogen, but where some or all of the carbon dioxide associated with its generation is captured) have similar tolerances to oxygen and water. However, hydrogen liquefaction often has tighter specifications and requires no more than 10 ppb of water and 1 ppm of oxygen in the feed.

[0132] The compressed hydrogen produced by electrolysis is preferably purified before being fed to downstream processes. In this regard, residual oxygen in the compressed hydrogen can be converted to water by catalytic combustion of some of the hydrogen in the hydrogen to produce oxygen-depleted compressed hydrogen (containing no more than 1 ppm of O 2), which can then be dried to produce dry compressed hydrogen (containing no more than 1 ppm water) for use in downstream processes.

[0133] Multi-stage compression system

[0134] The multi-stage compression system is responsible for compressing the gas from the pressure at which the gas is produced to a high pressure. For example, in the case where at least some of the compressed gas is fed to at least one downstream process, the high pressure is generally a pressure at least slightly higher than the feed pressure of the downstream process.

[0135] It is readily understood that a "multi-stage" compression system has multiple compression stages that can be split between compressors in parallel and / or in series. The overall pressure ratio of each stage is generally in the range of about 1.5 to about 2.5 (e.g., about 2 to about 2.5) to limit the increase in the temperature of the compressed gas.

[0136] In a multi-stage compression system, coolers are typically required between adjacent stages ("intercoolers"), and a cooler is typically required after the final stage ("aftercooler") to remove the heat of compression from the compressed gas. Thus, in the context of this invention, a "stage" of compression refers to the portion of the compression system between the coolers.

[0137] The compressed hydrogen generated by the multi-stage compression system typically has a pressure of about 10 bar to about 50 bar. In some embodiments, the pressure of the compressed hydrogen is about 25 bar to about 35 bar, preferably about 30 bar. In other embodiments, the pressure of the compressed hydrogen is about 10 bar to about 12 bar, preferably about 11 bar.

[0138] In some embodiments, the multi-stage compression system has only a single section to compress the hydrogen to the desired high pressure. In other embodiments, the multi-stage compression system includes a first section and at least one additional section downstream of the first section.

[0139] In a particular embodiment, the multi-stage compression system has two sections, a first (low pressure or "LP") section compresses the hydrogen from a feed pressure to the multi-stage compression system to a first high pressure ranging from about 2 bar to about 6 bar, and a second (medium pressure or "MP") section compresses the hydrogen from the first high pressure to a final high pressure desired for downstream processes.

[0140] In some embodiments, after compression in the first section, the first high pressure of hydrogen may be in the range of about 2 bar to about 3 bar, such as 2.5 bar. In other embodiments, the first high pressure may be in the range of about 4 bar to about 6 bar, such as 5 bar.

[0141] In a preferred embodiment, a multi-stage compression system will include a phase separator upstream of each stage of compression to remove liquid water. For low pressure centrifugal compressors, the phase separator is often incorporated into the intercooler as a separate unit to potentially achieve capital and power benefits and simplify the system.

[0142] Downstream Process

[0143] In some embodiments, the compressed gas may be consumed in a downstream process, or in more than one downstream process arranged in parallel.

[0144] In the preferred embodiment where the gas used for compression is hydrogen, the downstream processes may include any process that currently uses "grey" or "blue" hydrogen. These processes include oil refining and steel making.

[0145] In a more preferred embodiment, at least some, such as all, of the compressed gas is hydrogen for use in the production of ammonia by the Haber (or Haber-Bosch) process, in which ammonia is produced by reacting a mixture of hydrogen and nitrogen over an iron-based catalyst at high temperature (typically about 400° C. to about 500° C.) and high pressure (typically in the pressure range from about 100 bar to 200 bar).

[0146] In other preferred embodiments, at least some, such as all, of the compressed gas is hydrogen used to generate methanol, such as by CO 2 hydrogenation.

[0147] In some embodiments, at least some, such as all, of the compressed gas is hydrogen for use in generating ammonia and / or methanol.

[0148] In other embodiments, at least some, such as all, of the compressed hydrogen is liquefied by cryogenic cooling.

[0149] In a further embodiment, a first portion of the compressed hydrogen is used to generate ammonia and a second portion of the compressed hydrogen is liquefied.

[0150] Return of stored gas

[0151] One drawback of using electricity generated from renewable energy sources (e.g., for gas generation) is the inherent fluctuations in energy availability, which in turn results in fluctuations in the flow of raw gas to the system. In some embodiments, in the present invention, this problem can be addressed (albeit temporarily) by providing a system for collecting and storing at least some, preferably all, of the excess gas generated during periods when production exceeds demand by downstream processes, and distributing the stored gas to the downstream processes during periods when demand exceeds production.

[0152] In the context of the present invention, the use of a storage system may be particularly useful in situations where the first number (n) of compressors generating net compressed gas reaches 1 (i.e. only one compressor is operating and the remaining compressors are in low power mode or turned off). In the context of a process where gas is generated using renewable energy and the multi-stage compression system is powered by renewable energy, it may be more energy-efficient to shut down the final centrifugal compressor or put it in low power mode to avoid excessive circulation of gas and reduce energy consumption to save electricity.

[0153] Therefore, once the last centrifugal compressor has been unloaded, there will still be some gas flow in the feed gas to the multi-stage compression system, and the presence of a storage system allows this gas to be collected for later compression.

[0154] In some embodiments, the compressed gas may be stored without further compression. In these embodiments, the gas is stored at a maximum pressure of up to the pressure to which the gas was compressed in the multi-stage compression system, for example, up to about the feed pressure of a downstream process (of which there is only one) or about the feed pressure of one of the downstream processes (if there is more than one). In such embodiments, the compressed gas may be stored at a maximum pressure in the region of up to about 25 bar to about 30 bar.

[0155] However, the compressed gas may be further compressed before storage. In these embodiments, the compressed gas may be stored at a pressure of up to about 200 bar, or up to about 150 bar, or up to about 100 bar, or up to about 90 bar, or up to about 80 bar, or up to about 70 bar, or up to about 60 bar, or up to about 50 bar.

[0156] During periods when the level of demand for gas exceeds the level of production, compressed gas is withdrawn from storage and depressurized to produce depressurized gas.The pressure may be reduced in any conventional manner, in particular by passing the gas through a valve.

[0157] The pressure of the decompressed gas will depend on the pressure at the point in the multi-stage compression system to which the decompressed gas will be added.

[0158] In some embodiments, the depressurized gas may be fed to the final stage of the multi-stage compression system. In these embodiments, the depressurized gas will be at the inlet pressure of the feed to the final stage.

[0159] In other embodiments, the depressurized gas may be fed to an intermediate stage of a multi-stage compression system. In these embodiments, the depressurized gas will be at an inlet pressure at which the intermediate stage is fed.

[0160] The intermediate stage may be an intermediate stage within a compression section, or where there are two or more sections in a multi-stage compression system, may be an initial stage within a further compression section downstream of the first compression section. In these embodiments, the decompressed gas from the storage will be at the inlet pressure of the feed to the further compression section, i.e., the "inter-stage" pressure.

[0161] In further embodiments, the decompressed gas may be fed to the feed end of the multi-stage compression system, ie, the initial stage. In these embodiments, the decompressed gas will be the feed pressure of the multi-stage compression system, for example, about 1.1 bar.

[0162] During periods when demand outstrips production, processes may include:

[0163] reducing the pressure of compressed gas extracted from the storage to generate depressurized gas at an inlet pressure (first intermediate pressure) of a first stage of the multi-stage compression system; and

[0164] The depressurized gas is fed to the first stage.

[0165] In such an embodiment, once the pressure of the compressed gas in the reservoir drops to about the inlet pressure of the first stage, the method may include:

[0166] further reducing the pressure of the compressed gas extracted from the storage to generate a depressurized gas at an inlet pressure (a second intermediate pressure) of a second stage upstream of the first stage of the multi-stage compression system; and

[0167] The depressurized gas is fed to the second stage.

[0168] It should be understood that in the present context, the terms "first stage" and "second stage" do not refer to the relative positions of the stages in the multi-stage compression system in the downstream direction during normal operation. Rather, the terms are intended only to reflect the order in which the decompressed gas is fed to the stages of the multi-stage compression system during periods when demand exceeds production. The terms "first intermediate pressure" and "second intermediate pressure" should be interpreted accordingly as the first intermediate pressure being higher than the second intermediate pressure.

[0169] These embodiments may further include feeding the depressurized gas to other stages of the multi-stage compression system upstream of the first and second stages. In these further embodiments, the pressure of the compressed gas extracted from the storage is reduced to the inlet pressure of each stage.

[0170] In some preferred embodiments, the second stage is the initial stage of a multi-stage compression system.

[0171] It should be understood that in embodiments where the decompressed gas is fed to the second stage after the first stage, the gas flow to the first stage stops when the gas flow to the second stage begins. Generally speaking, the decompressed gas flow to a given compression stage stops when the decompressed gas begins to flow to another compression stage.

[0172] In some preferred embodiments, wherein during feeding of said decompressed gas to a stage, the centrifugal compressor upstream of said stage, or if more than one, at least one centrifugal compressor, is operated in said low power mode.

[0173] Since gas may be returned from storage to intermediate and / or initial stages of the multi-stage compression system, compressed gas may be stored at a pressure as low as a minimum of about 5 bar, and possibly even as low as a minimum of about 1.3 bar.

[0174] In embodiments where the compressed gas is further compressed prior to storage, another option is to feed the compressed gas extracted from storage directly to the downstream process after appropriate decompression until the storage pressure drops to the feed pressure of the downstream process. At this point, the pressure of the compressed gas extracted from the storage will be further reduced, and the reduced pressure gas is fed to a stage of the multi-stage compression system according to the present invention. However, these embodiments are not preferred, for example because of the additional capital expenditure of the high pressure storage system.

[0175] In the context of decompression of a storage system, the term "suitable" means that the pressure of the gas is reduced to an appropriate level taking into account the inlet pressures of the stages of the multi-stage compression system to which the decompressed gas is fed.

[0176] Compared to high pressure storage systems that only discharge to the feed pressure of the downstream process, these embodiments of the present invention enable the storage volume of gas to be reduced by using a multi-stage compression system already present in the process to recompress the gas from the storage when the storage pressure drops below the feed pressure. Thus, gas can continue to be drawn from the storage until the storage pressure drops to the minimum value of the feed pressure of the multi-stage compression system.

[0177] During periods when gas production is limited due to, for example, lack of power for the electrolyser, additional compression power is required, but this can be minimized by supplying the gas at the highest possible compressor interstage pressure given the storage pressure at a particular time. It also allows the maximum gas storage pressure to be equal to or lower than the feed pressure of any downstream process, eliminating any additional compression requirements for the stored gas.

[0178] It will be appreciated that the same volume of gas is stored in the same storage volume at the same maximum pressure, and that reducing the minimum storage pressure increases the volume of gas that is "releasable" from the storage, ie the available volume of stored gas.

[0179] However, the inventors have recognized that where gas is generated and then compressed in a multi-stage compression system for use in at least one downstream process, the releasable volume of stored gas can be increased by returning the gas from storage to a stage in the multi-stage compression system rather than directly to the downstream process, and that this arrangement reduces the total storage vessel volume required for the process.

[0180] For example, for a given mass of releasable gas, storing from a maximum pressure of 200 bar to a minimum pressure of 1.5 bar requires 15% less storage vessel volume than storing from a maximum pressure of 200 bar to a minimum pressure of 30 bar.

[0181] Similarly, for a given mass of releasable gas, storage from a maximum pressure of 100 bar to a minimum pressure of 1.5 bar requires 30% less storage vessel volume than storage from a maximum pressure of 100 bar to a minimum pressure of 30 bar.

[0182] Furthermore, for a given mass of releasable gas, storage from a maximum pressure of 50 bar to a minimum pressure of 1.5 bar requires 60% less storage vessel volume than storage from a maximum pressure of 50 bar to a minimum pressure of 30 bar.

[0183] Furthermore, storage from a maximum pressure of 30 bar to a minimum pressure of 1.5 bar is possible, compared to 30 bar to 30 bar where storage is not permitted.

[0184] Furthermore, although the total storage vessel volume increases as the maximum storage pressure decreases, the lower design pressure allows the vessel walls to be thinner and can reduce the overall capital cost of the storage system. Vessel thickness is often limited to a maximum value for reasons such as manufacturability, and in such cases, a lower design pressure will result in fewer vessels (although each vessel will be larger). Furthermore, the allowable stress of the vessel design can be increased below a specific vessel wall thickness, and if the lower design pressure allows the thickness to be below this threshold, the total vessel metal mass (and therefore the overall cost) can be reduced.

[0185] equipment

[0186] In a second aspect of the present invention, there is provided an apparatus for operating a multi-stage compression system for compressing a feed gas having a variable flow rate according to the process of the present invention, the apparatus comprising:

[0187] A multi-stage compression system, comprising a feed end, a plurality (N) of parallel centrifugal compressors, a product end, and a main circulation system for circulating gas through the plurality (N) of centrifugal compressors, wherein each centrifugal compressor comprises an inlet, an outlet, and a local circulation system having an anti-surge controller for circulating gas from the outlet to the inlet;

[0188] A control system is used to control the load of each centrifugal compressor and to control the circulation amount of the main circulation system and the local circulation system based on the flow rate of the raw gas according to requirements.

[0189] Power generation system

[0190] In some preferred embodiments, the apparatus comprises a power generation system for generating electricity from at least one renewable energy source, and wherein the gas for compression is generated at least in part using electricity generated from said power generation system.

[0191] Electricity for generating the compressed gas (and possibly for powering the or each centrifugal compressor of the multi-stage compression system) is generated from at least one renewable energy source, such as wind and / or solar energy.

[0192] In order to reduce the impact on the environment, it is preferred that the process is independent in terms of the generation of electricity to generate the gas used for compression (and optionally to power the centrifugal compressor). Therefore, it is preferred that the entire electricity demand is met using renewable energy sources, without using non-renewable energy sources to supplement the energy source. In this case, the demand for compressed gas is preferably met by feeding gas from a suitable storage system before considering the use of any non-renewable energy sources.

[0193] However, for example, there may not be enough gas available to feed from the storage system. Therefore, in some embodiments, the power generation system includes on-site battery storage and / or one or more on-site gasoline, diesel or hydrogen powered generators. Power from the battery storage and / or one or more on-site gasoline, diesel or hydrogen powered generators can be used to supplement additional power, or during periods when demand for products, such as from downstream processes, is particularly high, and / or during periods when renewable energy is only available below a threshold required to meet the process demand, or is not available at all.

[0194] In embodiments where wind energy is used to generate electricity, the power generation system will include a plurality of wind turbines. In embodiments where solar energy is used to generate electricity, the power generation system will include a plurality of photovoltaic cells or "solar cells."

[0195] Some embodiments will include multiple wind turbines and multiple photovoltaic cells.

[0196] The expression "in conductive communication" will be understood to mean that appropriate wires and / or cables will be used, together with any other relevant equipment, to connect the power generation system to the or each compressor in a safe and efficient manner.

[0197] In the context of the present invention, the or each centrifugal compressor may also be driven by a dedicated variable frequency drive, a mechanical drive or a dual speed motor.

[0198] In some preferred embodiments, the power generation system also generates electricity to power the centrifugal compressors of the multi-stage compression system and / or any downstream processes. Multi-stage compression system

[0199] The multi-stage compression system includes a plurality (N) of centrifugal compressors. A first number (n) of the centrifugal compressors are operated to generate net compressed gas, while the remaining centrifugal compressors are in a low power mode or shutdown state.

[0200] As described above, a multi-stage compression system typically includes a plurality of stages, each stage typically having a compression ratio in the range of about 2 to about 2.5. Intercoolers are typically provided between adjacent stages, and an aftercooler may be required after the final stage.

[0201] The multi-stage compression system further comprises a main circulation system for circulating gas through the plurality (N) of centrifugal compressors. Each centrifugal compressor also comprises a local circulation system for circulating gas from an outlet to an inlet of the compressor, and the local circulation system comprises an anti-surge controller.

[0202] The stages of the multi-stage compression system may be arranged in at least two compression sections, a first section and another section downstream of the first section.

[0203] Each section may include one or more compression stages, and associated coolers. A phase separator may also be included upstream of each compression stage to remove liquid from the hydrogen to be compressed.

[0204] In a specific embodiment, the multi-stage compression system has two sections, a first (low pressure or "LP") section compresses hydrogen from a feed pressure to the multi-stage compression system to a first high pressure, and another (medium pressure or "MP") section compresses hydrogen from the first high pressure to a final high pressure desired by a downstream process.

[0205] The LP section may have one or more, for example two, compression stages, and the MP section may have two or more, for example 3 or 4, compression stages.

[0206] The number of compressors used will depend on the total capacity of the process. For example, for a process with a total electrolyser capacity of 2.2GW (for generating hydrogen), the multi-stage compression system may have 8 to 10 compressors. The skilled person will appreciate that processes with higher total capacities will require a greater number of compressors.

[0207] The compressor in the LP section may be appropriately enlarged, for example by 10%, to accommodate the loss of the machine. Additionally or alternatively, the multi-stage compression system may include a spare compressor in the LP or MP section, which will be switched in to replace another machine that has failed in the relevant section.

[0208] Control System

[0209] The apparatus includes a control system for controlling the load of each centrifugal compressor and for controlling the circulation amounts of the main circulation system and the local circulation system based on the flow rate of the raw gas according to requirements.

[0210] In an embodiment of the power generation system in which there is an indication of the flow rate of the raw gas, the power generation system generates electricity from at least one renewable energy source. However, as described above, in some embodiments, the power generation system also includes an on-site battery storage and / or generates electricity by one or more on-site gasoline, diesel or hydrogen-powered generators. In such an embodiment, the device includes a control system for switching the centrifugal compressor or each centrifugal compressor between normal power mode and low power mode based on the level of electricity generated by the at least one renewable energy source and on-site battery storage of the power generation system and / or by one or more on-site gasoline, diesel or hydrogen-powered generators as required.

[0211] It will be appreciated that the control system is in electrical communication with the or each centrifugal compressor in the multi-stage compression system.

[0212] A control system implements the process of the present invention.

[0213] Therefore, the control system is configured to:

[0214] (a) operating a first number (n) of centrifugal compressors at full load to compress the feed gas during a period in which the multi-stage compression system receives feed gas at a flow rate equal to the total maximum capacity of the first number (n) of centrifugal compressors that produce net compressed gas;

[0215] (b) operating the first number (n) of centrifugal compressors at a minimum load for compressing the feed gas during a period in which the multi-stage compression system receives feed gas at a flow rate ranging from less than a total maximum capacity of the first number (n) of centrifugal compressors to a total turndown capacity of the first number (n) of centrifugal compressors, the minimum load being determined based on the flow rate of the feed gas;

[0216] (c) during a period in which the multi-stage compression system receives feed gas at a flow rate ranging from less than a total turndown capacity of a first number (n) of centrifugal compressors to greater than a total maximum capacity of a second number (n-1) of centrifugal compressors producing net compressed gas, circulating the compressed gas on demand using the primary circulation system to maintain the load of the first number (n) of centrifugal compressors above the point at which the anti-surge controller is activated; and

[0217] (d) during a period in which the multi-stage compression system receives the feed gas at a flow rate equal to the total maximum capacity of the second number (n-1) of centrifugal compressors, unloading the centrifugal compressors so that the compressors enter a low power mode or an off mode in which the compressors generate no net compressed gas, while loading the remaining centrifugal compressors to maximum capacity,

[0218] In other words, the control system simply monitors the gas flow in the feed gas of the multi-stage compression system and then sends a signal to each centrifugal compressor to operate according to the process described herein.

[0219] Thus, the control system dictates the most efficient manner to operate the centrifugal compressors of the multi-stage compression system without excessively shutting down the centrifugal compressors, and / or conserving power, thereby allowing more power to be "freed up" for use in other parts of the process, such as gas production, or any downstream processes, and / or allowing a steadier output of net compressed gas from the system by avoiding enabling an anti-surge controller.

[0220] Electrolyzer

[0221] In some preferred embodiments, the gas used for compression is hydrogen, preferably generated by electrolysis of water. Thus, in such embodiments, the apparatus comprises a plurality of electrolyzers for generating hydrogen, wherein the feed end of the multi-stage compression system is in fluid flow communication with the plurality of electrolyzers. The electrolyzers are powered at least in part by electricity generated from the electricity generation system.

[0222] The electrolysis of water may be provided by a plurality of electrolysis units or "cells." Each unit or cell may be referred to as an "electrolyzer."

[0223] The plurality of electrolyzers typically have a total capacity of at least 1 GW, but in some cases the capacity may be less than 1 GW. The maximum total capacity of the electrolyzers is limited only by practical considerations, such as generating sufficient power from renewable energy sources to power the plurality of electrolyzers. Thus, the electrolyzers may have a maximum total capacity of 10 GW or more. The total capacity of the electrolyzers performing electrolysis may be from 1 GW to 5 GW, such as from about 1.5 GW to about 3 GW.

[0224] The plurality of electrolytic cells are often composed of a large number (eg, hundreds) of individual cells grouped into "modules" that also include process equipment such as pumps, coolers, and / or separators, etc., and these module groups are typically arranged in separate components.

[0225] Each module typically has a maximum capacity of at least 10 MW (eg 20 MW), and each component typically has a total capacity of at least 100 MW (eg 400 MW).

[0226] Any suitable type of electrolyser may be used with the present invention. In this regard, there are three conventional types of electrolyser - alkaline electrolyser, PEM electrolyser and solid oxide electrolyser - and each of these types of electrolyser is theoretically suitable for use with the present invention.

[0227] Alkaline electrolyzers operate by converting hydroxide ions (OH - ) is transported from the cathode to the anode through an electrolyte to produce hydrogen on the cathode side. Electrolyzers using liquid alkaline solutions of sodium hydroxide or potassium hydroxide as electrolytes are commercially available. Commercial alkaline electrolyzers are typically operated at a temperature ranging from about 100°C to about 150°C.

[0228] In a PEM electrolyzer, the electrolyte is a solid plastic material. Water reacts at the anode to form oxygen and positively charged hydrogen ions. Electrons flow through an external circuit, and the hydrogen ions selectively pass through the PEM to the cathode. At the cathode, the hydrogen ions combine with electrons from the external circuit to form hydrogen gas. PEM electrolyzers are typically operated in a temperature range of about 70°C to about 90°C.

[0229] Solid oxide electrolyzers use solid ceramic materials as electrolytes that selectively conduct negatively charged oxygen ions (O 2- ). Water at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions. The oxygen ions pass through the solid ceramic membrane and react at the anode to form oxygen gas and produce electrons for the external circuit. Solid oxide electrolyzers must be operated at a sufficiently high temperature for the solid oxide membrane to function properly, for example, at about 700°C to about 800°C.

[0230] Due to the lower operating temperatures, the use of alkaline electrolyzers and / or PEM electrolyzers is generally preferred.

[0231] The plurality of electrolytic cells may be arranged in at least two parallel groups. In these embodiments, the apparatus comprises:

[0232] a first header to collect hydrogen from each electrolyzer in each group; and

[0233] a second header that collects hydrogen from the first header and feeds the hydrogen to a feed end of the multi-stage compression system;

[0234] In some embodiments, wherein the apparatus further comprises a storage system for storing compressed hydrogen, the apparatus further comprises a conduit for feeding the compressed hydrogen from the storage system to the second header after suitable decompression.

[0235] These embodiments of the invention may use any suitable water source. However, in embodiments where seawater is used to generate water for electrolysis, the apparatus will further comprise at least one unit (or apparatus) for desalination and softening of seawater.

[0236] Purification system

[0237] In some embodiments, where there are downstream processes that cannot tolerate the levels of water and oxygen inherently present in the compressed hydrogen generated by electrolyzing water, the apparatus may include a purification system in which the compressed hydrogen is purified.

[0238] The purification system typically includes a "deoxygenation" unit in which oxygen is removed by catalytic combustion of the hydrogen to produce water and oxygen-depleted compressed hydrogen.

[0239] The oxygen-depleted gas may then be dried in a dryer, such as an adsorption unit, such as a temperature swing adsorption (TSA) unit, to generate dry compressed hydrogen for use in downstream processes.

[0240] Downstream processing unit

[0241] In some embodiments, the apparatus includes at least one downstream processing unit for consuming compressed gas, the downstream processing unit being in fluid flow communication with the outlet end of the multi-stage compression system.

[0242] The downstream processing unit may be any unit that utilizes a gas, such as hydrogen, as a feedstock.

[0243] Examples of suitable downstream processing units include oil refineries, steel manufacturing facilities, ammonia synthesis facilities, or hydrogen liquefaction facilities. In some embodiments, the ammonia synthesis facility is arranged in parallel with the hydrogen liquefaction facility.

[0244] In a particularly preferred embodiment, the downstream processing unit comprises an ammonia synthesis plant (for example using the Haber-Bosch process) and / or a methanol synthesis plant (for example using CO 2 Hydrogenation).

[0245] Storage System

[0246] In some embodiments, the apparatus includes a storage system for storing compressed gas, the storage system being in fluid flow communication with the outlet end of the multi-stage compression system and with at least one compressor of the multi-stage compression system.

[0247] Storage systems typically include multiple pressure vessels and / or piping segments connected to a common inlet / outlet header.

[0248] The pressure vessel may be spherical, e.g. up to about 25 m in diameter, or "bullet-shaped", i.e. a horizontal vessel up to about 12 m in diameter with a large L / D ratio (typically up to about 12:1).

[0249] Salt domes can also be used if the geological conditions of the site allow.

[0250] In some embodiments, the apparatus includes a second control system that controls not only the pressure and flow of compressed gas from the multi-stage compression system to the storage system, such as during periods when gas production exceeds demand, but also controls the pressure and flow of compressed gas to the multi-stage storage system, such as during periods when gas demand exceeds production.

[0251] It should be understood that the second control system may be integrated with the control system described above with respect to the centrifugal compressor, or may be separate.

[0252] In some embodiments, the second control system will simply seek to maintain the pressure of the gas in the downstream header of the downstream process. Thus, in order to continuously provide a fixed amount of gas to the downstream process, the pressure controller will be maintained on the exhaust header feeding the downstream process.

[0253] If the pressure in the discharge header exceeds the required feed pressure (e.g., because more gas is available than is consumed by downstream processes), the pressure can be relieved by opening a valve in the feed line to the storage.

[0254] Once the pressure in the discharge header drops to the required feed pressure, the valve in the feed line to the reservoir will close.

[0255] If the pressure in the discharge header drops below the required feed pressure (eg because less gas is available than is consumed by downstream processes), the pressure is increased by opening a valve in a first return line from storage to the first stage in a multi-stage compression system.

[0256] The valve in the first return line will remain open until the pressure in the discharge header exceeds the required feed pressure, indicating that the gas production level has returned to the required level, at which time the valve will close, or until the pressure in the storage vessel drops to approximately the inlet pressure of the first stage of the multi-stage compression system fed by the first return line.

[0257] In the latter case, not only is the valve in the first return line closed, but also the valve in the second return line from the storage to the second stage (upstream of the first stage) in the multi-stage compression system is opened to continue feeding gas from the storage back to the downstream process.

[0258] This type of control system may be referred to as a "split range" control system.

[0259] Aspects of the invention include:

[0260] #1. A process for operating a multi-stage compression system for compressing a feed gas having a variable flow rate,

[0261] The multi-stage compression system comprises a feed end, a plurality (N) of parallel centrifugal compressors, a product end and a main circulation system for circulating gas through the plurality (N) of centrifugal compressors, wherein each centrifugal compressor comprises an inlet, an outlet and a local circulation system having an anti-surge controller for circulating gas from the outlet to the inlet, and the process comprises:

[0262] (a) operating the first number (n) of centrifugal compressors at full load to compress the feed gas during a period in which the multi-stage compression system receives the feed gas at a flow rate equal to the total maximum capacity of the first number (n) of centrifugal compressors to produce net compressed gas;

[0263] (b) operating the first number (n) of centrifugal compressors at a minimum load for compressing the feed gas during a period in which the multi-stage compression system receives the feed gas at a flow rate ranging from less than a total maximum capacity of the first number (n) of centrifugal compressors to a total turndown capacity of the first number (n) of centrifugal compressors, the minimum load being determined based on the flow rate of the feed gas;

[0264] (c) circulating compressed gas using the primary circulation system as required to maintain the load of the first number (n) of centrifugal compressors above the point at which an anti-surge controller is activated during a period in which the multi-stage compression system receives the feed gas at a flow rate ranging from less than the total turndown capacity of the first number (n) of centrifugal compressors to greater than the total maximum capacity of the second number (n-1) of centrifugal compressors producing net compressed gas; and

[0265] (d) during a period in which the multi-stage compression system receives the feed gas at a flow rate equal to the total maximum capacity of the second number (n-1) of centrifugal compressors, unloading the centrifugal compressors so that the compressors enter a low power mode or an off mode in which the compressors generate no net compressed gas, while loading the remaining centrifugal compressors to maximum capacity,

[0266] wherein the process is reversible at any time, and wherein n is an integer equal to or less than N.

[0267] #2. A process according to #1, wherein the gas used for compression is hydrogen.

[0268] #3. The process according to #2, wherein the hydrogen is generated by electrolyzing water.

[0269] #4. A process according to any of #1 to #3, wherein the gas used for compression is generated at least in part using electricity produced from at least one renewable energy source.

[0270] #5. The process according to any one of #1 to #4, wherein during the period specified in (b), the turndown capacity of each centrifugal compressor is defined as the minimum flow rate of gas that the centrifugal compressor can compress without activating its anti-surge controller.

[0271] #6. The process of any of #1 to #5, wherein during the period specified in (b), the turndown capacity of each centrifugal compressor is 60% or more of the maximum gas flow through the centrifugal compressor.

[0272] #7. The process according to any one of #1 to #6, wherein during the period specified in (b), the flow rate of the feed gas is evenly distributed over all (n) centrifugal compressors at minimum load.

[0273] #8. A process according to any one of #1 to #7, wherein during the period specified in (c), the amount of compressed gas circulated is maintained at a minimum to save electricity.

[0274] #9. The process according to any one of #1 to #8, wherein during the period specified in (d), the unloading of the centrifugal compressor comprises firstly reducing the flow of net compressed gas through the centrifugal compressor to zero using the local circulation system, and secondly reducing the load of the centrifugal compressor.

[0275] #10. The process of any of #1 to #9, wherein placing a centrifugal compressor in a low power mode comprises reducing a rotor speed of the centrifugal compressor to a speed that is still sufficient to prevent contact between opposing seal faces of dry gas seals within the centrifugal compressor.

[0276] #11. A process according to any of #1 to #10, wherein unloading to place the centrifugal compressor in said low power mode comprises reducing its rotor speed to a range of about 100 rpm to about 1500 rpm and operating it so that it generates no net compressed gas.

[0277] #12. The process of any of #1 to #11, wherein during operation in the low power mode, the centrifugal compressor operates at a power of about 20% or less relative to maximum power and generates no net compressed gas.

[0278] #13. A process for supplying compressed hydrogen for consumption by at least one downstream process, comprising:

[0279] The hydrogen is generated by electrolyzing water,

[0280] compressing the hydrogen in a multi-stage compression system operating according to any of #1 to #12, and

[0281] The compressed hydrogen is fed to at least one downstream process for consumption in the downstream process.

[0282] #14. A process according to any of #2 to #13, wherein at least some of the compressed hydrogen is used to generate ammonia and / or methanol in the downstream process.

[0283] #15. An apparatus for operating a multi-stage compression system for compressing a raw gas having a variable flow rate according to #1, the apparatus comprising:

[0284] A multi-stage compression system, comprising a feed end, a plurality (N) of parallel centrifugal compressors, a product end, and a main circulation system for circulating gas through the plurality (N) of centrifugal compressors, wherein each centrifugal compressor comprises an inlet, an outlet, and a local circulation system having an anti-surge controller for circulating gas from the outlet to the inlet;

[0285] A control system is used to control the load of each centrifugal compressor and to control the circulation amount of the main circulation system and the local circulation system based on the flow rate of the raw gas according to requirements.

[0286] #16. The device according to #15, comprising:

[0287] A power generation system for generating electricity from at least one renewable energy source, and wherein the gas used for compression is at least partially generated using electricity generated from the power generation system.

[0288] #17. The apparatus according to #16, wherein the gas used for compression is hydrogen, the apparatus comprising:

[0289] A plurality of electrolyzers for generating the hydrogen,

[0290] wherein the electrolyser is powered at least in part by electricity generated from the electricity generation system, and

[0291] Wherein the feed end of the multi-stage compression system is in fluid flow communication with the plurality of electrolytic cells.

[0292] #18. The apparatus of any of #15 to #17, comprising at least one downstream processing unit for consuming compressed gas, said downstream processing unit being in fluid flow communication with said outlet end of said multi-stage compression system.

[0293] #19. The apparatus according to any of #15 to #18, comprising:

[0294] a storage system for storing compressed gas, the storage system being in fluid flow communication with the outlet end of the multi-stage compression system and at least one compressor of the multi-stage compression system; and

[0295] A second control system is used to control the pressure and flow of compressed gas from the multi-stage compression system to the storage system, and to control the pressure and flow of compressed gas from the storage system to the compressor of the multi-stage compression system based on the flow of the raw gas to the multi-stage compression system.

[0296] according to Figure 1 , hydrogen is produced at about atmospheric pressure by electrolyzing water in a plurality of electrolyzer units generally indicated by reference numeral 2.

[0297] The electricity required to power the electrolyzer 2 is generated at least in part by renewable energy sources (not shown), such as wind and / or sun. However, in some embodiments, at least some additional electricity may be drawn from on-site battery storage and / or electricity generated by one or more on-site gasoline, diesel or hydrogen powered generators (including fuel cells) and / or from a local or national power grid (not shown).

[0298] A stream 4 of hydrogen is removed from the electrolyser 2 at a pressure slightly above atmospheric pressure (eg, about 1.1 bar) and fed to a multi-stage compression system 100 to generate a stream 36 of compressed hydrogen. In this example, the multi-stage compression system 100 includes three centrifugal compressors 10, 12 and 14 arranged in parallel.

[0299] As required, recycled hydrogen is added to stream 4 to form a combined stream 6, which is then fed to header 8 before being compressed in parallel compressors 10, 12 and 14. Compressed hydrogen from each of the centrifugal compressors 10, 12 and 14 is fed to header 28 and forms a combined stream 30 of compressed hydrogen. Combined stream 30 may optionally have gas removed therefrom for recycling before being fed as stream 36 to a downstream stage of compression (not shown) or to at least one downstream process (not shown).

[0300] The multi-stage compression system 100 comprises a main circulation system 32 which removes gas from the combined stream 30 and, after suitable pressure reduction in valve 34 , feeds it to the inlet of the multi-stage compression system by combining it with stream 4 to form stream 6 .

[0301] Each centrifugal compressor 10, 12 and 14 also has an associated local circulation system 16, 18 and 20 (with valves 22, 24 and 26 respectively), each with an anti-surge controller. Each circulation system removes compressed gas from the product end and feeds the compressed gas to the feed end of the associated centrifugal compressor after appropriate pressure reduction using valves (22, 24, 26).

[0302] Each centrifugal compressor 10, 12 and 14 is electrically connected to a control system indicated by reference numeral 40. The control system 40 monitors the amount of gas flow to the multi-stage compression system and accordingly controls the load of the centrifugal compressors 10, 12 and 14. The valves of the main circulation system (34) and the valves of the local circulation systems 22, 24 and 26 are also electrically connected to the control system so that the circulation amount of the circulation system and the circulation amount of the main circulation system are controlled to implement the process of the present invention as required.

[0303] Although not shown for simplicity, the multi-stage compression system 100 typically includes an intercooler between compression stages and an aftercooler after the final stage. There may also be a phase separator upstream of each compression stage to remove liquid from the flow entering the compressor.

[0304] Figure 2 A second embodiment of the present invention is described. The same reference numerals are used to denote Figure 2 In the flowchart in Figure 1 The following are the differences between Figure 2 The first embodiment of Figure 1 Discussion of the features of the process shown.

[0305] according to Figure 2 , the multi-stage compression system 200 has two compression stages as shown, a first stage 201 and a second stage 202.

[0306] The compressed gas from the header 28 forms the combined flow 30 which is then fed to the header 48 of the second stage 202. The second stage includes Figure 1 The first stage 201 has the same features as the first stage 201, including three centrifugal compressors 50, 52 and 54 and associated local circulation systems and valves.

[0307] exist Figure 2 In the embodiment of the present invention, the main circulation system circulates the compressed gas (stream 70) from the outlet of the second stage and feeds it to the inlet of the first stage as stream 6 after appropriate pressure reduction using valve 34. Stream 76 contains the net compressed gas and is fed to a downstream stage of compression (not shown) or to at least one downstream process (not shown).

[0308] Stream 80 illustrates the situation where the addition of compressed gas at a suitable pressure from a suitable storage system may be added (e.g. when the flow of gas from electrolyser 2 is particularly low), and / or where the demand from a downstream process (not shown) cannot be met by electrolyser 2 alone. Stream 80 may add gas from storage by feeding the gas to an interstage point between stages 201 and 202 (stream 30).

[0309] Figure 3 A third embodiment of the present invention is depicted. The same reference numerals are used to denote Figure 3 In the flowchart in Figure 2 The following are the differences between Figure 3 The first embodiment of Figure 2 Discussion of the features of the process shown.

[0310] about Figure 3 , showing Figure 2A simplified design of a multi-stage compression system is shown. In this figure, the multi-stage compression system 300 still includes two stages, 301 and 302. However, the local circulation systems 16, 18 and 20 receive gas from the outlet of the compressor (50, 52 or 54) in the second stage 302, and after appropriate decompression using valves (22, 24 or 26), the decompressed gas is fed to the inlet of different but corresponding compressors in series in the first stage 301.

[0311] Therefore, with Figure 2 This arrangement has three fewer recirculation systems and three fewer valves required than the arrangement in FIG. It therefore allows a simpler, more cost-effective design of a multistage compression system that is simpler to operate. However, note that no gas can be fed from the storage chamber to the interstage.

[0312] Figure 4 is a graph showing how a process according to the invention may be operated e.g. Figure 1 An example of a multi-stage compression system such as the one shown in . This data was generated using a spreadsheet and may not accurately reflect observations in a real-world example.

[0313] In this example, there are three (N) centrifugal compressors, each with a turndown capacity of 80%. For simplicity, this example assumes that the flow rate of the raw gas decreases linearly over 100 hours at a rate of 1% per hour from 100% flow. This figure does not show any local recycle gas flow, or part of the unloading stage of the centrifugal compressor. In practice, it is expected that the flow rate of the raw gas will fluctuate greatly during this period, rather than steadily decrease. However, this example is intended only to illustrate the process.

[0314] At reference number 400 (0 hours), the raw gas flow is 100% of the capacity of all three (n) centrifugal compressors that produce net compressed gas (shown as line 410). Therefore, the first number (n) of centrifugal compressors is 3. The raw gas flow is equal to the total maximum capacity (100%) of the three centrifugal compressors, and therefore all three compressors compressing all the raw gas are at full load (100%). This corresponds to the period specified in (a) according to the present invention.

[0315] From 1 hour to 19 hours, when the flow rate of the raw gas drops below 100%, the three (n) centrifugal compressors are adjusted down accordingly to match the flow rate of the raw gas (from 1 hour to 20 hours). This corresponds to the cycle specified in (b) according to the present invention.

[0316] At reference number 401 (20 hours), the flow rate of the raw gas reaches the total maximum turndown capacity of the three compressors (80%, shown as line 440). That is, just above the point at which the anti-surge controllers of all three centrifugal compressors are enabled. In order to prevent the anti-surge controllers from being enabled, the main circulation system begins to introduce recycle gas through the (n) centrifugal compressors to maintain the load just above the point at which the anti-surge controllers are enabled. From 20 hours to 33 hours, the flow rate of the raw gas further drops below this point, and therefore the main circulation system introduces more recycle gas flow to compensate. This corresponds to the period specified in (c) according to the present invention.

[0317] At reference number 402 (33 hours), the flow rate of the raw gas is equal to the total maximum capacity (66%, shown as line 420) of the two centrifugal compressors (i.e., the second number (n-1) of centrifugal compressors). Therefore, at this point, one centrifugal compressor is unloaded and placed in a low power mode or shut down, while the remaining two centrifugal compressors are simultaneously loaded to maximum capacity (66% of the total flow rate of the two compressors, shown as line 420). At this point (at reference number 403), assuming that the load of the two remaining compressors is now above the anti-surge control point (54%) of the two compressors (shown as a line with reference number 450), it is no longer necessary to circulate the gas using the main circulation system.

[0318] At reference numeral 403, the amount of circulating gas drops to zero, which would ideally be the most efficient way to operate the system. However, it should be understood that in practice, this change may be more gradual to prevent shocks to the system's flow and to accommodate more gradual changes in the centrifugal compressor's load.

[0319] The above process is then repeated, but for n = 2 compressors, since now only two compressors are in operation generating net compressed gas (the other is at low power or shut down).

[0320] Between reference numerals 403 and 404, the raw gas flow allows the two compressors to be turned down without cycling. At 404, cycling is required to maintain the load of the two (n) compressors above their anti-surge control points (shown as line 450). At reference numeral 405, the raw gas flow reaches the total maximum capacity of one compressor, since (n-1) = (2-1) = 1. At this point, one of the two compressors is unloaded and shut down or placed in a low power mode, and the last remaining compressor is operated at the maximum load of one compressor (33%, shown as line 430) to compress the flow of raw gas (33%).

[0321] From reference number 406 to 407, the final compressor can be turned down depending on the flow of the raw gas. However, at 407, the flow of the raw gas has just reached above the anti-surge control point (27%, shown as line 460) of the final (n) compressor, and the main cycle system requires cycling to ensure that the load of the final (n) compressor is maintained above its anti-surge control point (27%), the more recycle gas is added, the lower the raw gas flow drops.

[0322] Figure 4 Shows Figure 1 How a multi-stage compression system can be operated in a manner that dynamically responds to changes in raw gas flow, including sequentially turning down compressors or placing them in a low power mode while balancing the load on the remaining compressors under conditions of less (or no) circulation from the main circulation system.

[0323] from Figure 4 As can be seen in the graph, the line of net compressed gas flow tracks the flow of the feed gas from the electrolyzer without requiring unnecessary cycles, shutting down all compressors, or unnecessary waste of electricity. This demonstrates how the process of the present invention can safely operate a centrifugal compressor by maintaining its load just above the anti-surge control line, while still efficiently compressing feed gas with any variable flow from 100% flow to 0% flow.

[0324] The foregoing description has been presented to illustrate and describe examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teachings. It should be understood that any feature described with respect to any one example may be used alone or in combination with other features described, and may also be used in combination with any feature of any other example, or in combination with any combination of any other examples.

[0325] In this specification, unless explicitly stated otherwise, the word "or" is used in the sense of an operator that returns a true value when either or both of the conditions are met, rather than the operator "exclusive or" which requires only one of the conditions to be met. The word "comprising" is used in the sense of "including", rather than meaning "consisting of".

[0326] All prior teachings mentioned above are hereby incorporated herein by reference. Acknowledgment of any previously published document in this document is not to be taken as an acknowledgment or representation that its teachings were common general knowledge in Australia or elsewhere at that time.

Claims

1. A process for operating a multi-stage compression system for compressing a feed gas having a variable flow rate, The multi-stage compression system comprises a feed end, a plurality of centrifugal compressors connected in parallel, a product end and a main circulation system for circulating gas through the plurality of centrifugal compressors, wherein each centrifugal compressor comprises an inlet, an outlet and a local circulation system having an anti-surge controller for circulating gas from the outlet to the inlet, the number of the plurality of centrifugal compressors connected in parallel is N, and the process comprises: (a) operating a first number of centrifugal compressors at full load to compress the feed gas during a period in which the multi-stage compression system receives the feed gas at a flow rate equal to the total maximum capacity of the first number of centrifugal compressors that produce net compressed gas, the first number being n; (b) operating the first number of centrifugal compressors at a minimum load for compressing the feed gas during a period in which the multi-stage compression system receives the feed gas at a flow rate ranging from less than a total maximum capacity of the first number of centrifugal compressors to a total turndown capacity of the first number of centrifugal compressors, the minimum load being determined based on the flow rate of the feed gas; (c) circulating compressed gas using the primary circulation system as required to maintain the load of the first number of centrifugal compressors above the point at which the anti-surge controller is enabled during a period in which the multi-stage compression system receives the feed gas at a flow rate ranging from less than the total turndown capacity of the first number of centrifugal compressors to greater than the total maximum capacity of a second number of centrifugal compressors producing net compressed gas, the second number being n-1; as well as (d) during a period in which the multi-stage compression system receives the feed gas at a flow rate equal to the total maximum capacity of the second number of centrifugal compressors, unloading the centrifugal compressors to place the compressors into a low power mode or an off mode in which the compressors generate no net compressed gas, while loading the remaining centrifugal compressors to maximum capacity, wherein the process is reversible at any time, and wherein n is an integer equal to or less than N.

2. The process according to claim 1, wherein the gas used for compression is hydrogen.

3. The process of claim 2, wherein the hydrogen is generated by electrolysis of water.

4. The process of claim 1, wherein the gas used for compression is generated at least in part using electricity produced from at least one renewable energy source.

5. The process of claim 1 , wherein during the period specified in (b), the turndown capacity of each centrifugal compressor is defined as the minimum flow rate of gas that the centrifugal compressor can compress without activating its anti-surge controller.

6. The process of claim 1 wherein during the period specified in (b), the turndown capacity of each centrifugal compressor is 60% or more of the maximum gas flow through the centrifugal compressor.

7. The process of claim 1, wherein during the period specified in (b), the flow rate of the feed gas is evenly distributed across all centrifugal compressors at minimum load.

8. The process of claim 1, wherein during the period specified in (c), the amount of compressed gas circulated is maintained at a minimum to conserve electricity.

9. The process of claim 1 , wherein during the period specified in (d), unloading the centrifugal compressor comprises firstly reducing the flow of net compressed gas through the centrifugal compressor to zero using the local circulation system, and secondly reducing the load of the centrifugal compressor.

10. The process of claim 1, wherein placing a centrifugal compressor in a low power mode comprises reducing a rotor speed of the centrifugal compressor to a speed that is still sufficient to prevent contact of opposing seal faces of dry gas seals within the centrifugal compressor.

11. The process of claim 1 wherein unloading to place the centrifugal compressor in the low power mode comprises reducing its rotor speed to within the range of 100 rpm to 1500 rpm and operating it such that it generates no net compressed gas.

12. The process of claim 1, wherein during operation in the low power mode, the centrifugal compressor operates at a power of 20% or less relative to maximum power and generates no net compressed gas.

13. A process for supplying compressed hydrogen for consumption by at least one downstream process, comprising: The hydrogen is generated by electrolyzing water, compressing the hydrogen in a multi-stage compression system operating according to claim 1, and The compressed hydrogen is fed to at least one downstream process for consumption in the downstream process.

14. A process according to claim 13, wherein at least some of the compressed hydrogen is used to produce ammonia and / or methanol in the downstream process.

15. An apparatus for a multi-stage compression system for compressing a feed gas having a variable flow rate in accordance with the process operation according to claim 1, the apparatus comprising: A multi-stage compression system, comprising a feed end, a plurality of centrifugal compressors connected in parallel, a product end, and a main circulation system for circulating gas through the plurality of centrifugal compressors, wherein each centrifugal compressor comprises an inlet, an outlet, and a local circulation system having an anti-surge controller for circulating gas from the outlet to the inlet, and the number of the plurality of centrifugal compressors connected in parallel is N; A control system is used to control the load of each centrifugal compressor and to control the circulation amount of the main circulation system and the local circulation system based on the flow rate of the raw gas according to requirements.

16. The device according to claim 15, comprising: A power generation system for generating electricity from at least one renewable energy source, and wherein the gas used for compression is generated at least in part using electricity generated from the power generation system.

17. The apparatus according to claim 16, wherein the gas for compression is hydrogen, the apparatus comprising: A plurality of electrolyzers for generating the hydrogen, wherein the electrolyser is powered at least in part by electricity generated from the electricity generation system, and The feed end of the multi-stage compression system is in fluid flow communication with the plurality of electrolytic cells.

18. The apparatus of claim 15, comprising at least one downstream processing unit for consuming compressed gas, the downstream processing unit being in fluid flow communication with an outlet end of the multi-stage compression system.

19. The apparatus according to claim 15, comprising: a storage system for storing compressed gas, the storage system being in fluid flow communication with an outlet end of the multi-stage compression system and at least one compressor of the multi-stage compression system; as well as A second control system is used to control the pressure and flow of compressed gas from the multi-stage compression system to the storage system, and to control the pressure and flow of compressed gas from the storage system to the compressor of the multi-stage compression system based on the flow of raw gas to the multi-stage compression system.

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