METHOD AND APPARATUS FOR COMPRESSING A GAS FEED WITH A VARIABLE FLOW RATE

Abstract

Energy efficiency and/or operational stability of a multistage compression system comprising a plurality (N) of centrifugal compressors that is compressing a gas feed having a variable flow rate is improved by adjusting reversibly the load on each compressor in response to changes in the flow rate of the gas feed using a main recycle system to enable operation of the centrifugal compressors at turndown capacity during periods when the flow rate is below total turndown capacity for all of the compressors, and if necessary, using the local recycle systems in order to avoid activation of anti-surge control, and switching one or more centrifugal compressors into low power mode or shutdown mode as required.

Claims

1. A process for operating a multistage compression system compressing a gas feed having a variable flow rate, said multistage compression system comprising a feed end, a plurality (N) of centrifugal compressors in parallel, a product end, and a main recycle system for recycling gas through the plurality (N) of centrifugal compressors, wherein each centrifugal compressor comprises an inlet, an outlet, and a local recycle system with anti-surge control for recycling gas from the outlet to the inlet, said process comprising: (a) during periods when the gas feed is received by the multistage compression system at a flow equal to the total maximum capacity of a first number (n) of centrifugal compressors producing net compressed gas, operating said first number (n) of centrifugal compressors at full load for compressing the gas feed; (b) during periods when the gas feed is received by the multistage compression system at a flow in a range from less than total maximum capacity of said first number (n) of centrifugal compressors to total turndown capacity of said first number (n) of centrifugal compressors, operating said first number (n) of centrifugal compressors at minimum load for compressing the gas feed, said minimum load being determined based on the flow of the gas feed; (c) during periods when the gas feed is received by the multistage compression system at a flow in a range from less than total turndown capacity of the first number (n) of centrifugal compressors to more than total maximum capacity for a second number (n−1) of centrifugal compressors producing net compressed gas, recycling compressed gas using the main recycle system as required to maintain the load of said first number (n) of centrifugal compressors above the point at which anti-surge controls are activated; and (d) during periods when the gas feed is received by the multistage compression system at a flow equal to the total maximum capacity for said second number (n−1) of centrifugal compressors, unloading a centrifugal compressor to put said compressor into a low power mode or shutdown mode in which said compressor produces no net compressed gas, while simultaneously loading the remaining centrifugal compressors to maximum capacity, wherein the process is reversible at any point, and wherein n is a whole number equal to or less than N.

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

3. The process according to claim 2, wherein the hydrogen gas is produced by electrolysis of water.

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

5. The process according to claim 1, wherein during periods specified in (b) the turndown capacity of each centrifugal compressor is defined as the minimum flow of gas that can be compressed by the centrifugal compressor without activation of its anti-surge control.

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

7. The process according to claim 1, wherein during periods specified in (b) the flow of the gas feed is distributed uniformly across all (n) centrifugal compressors at minimum load.

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

9. The process according to claim 1, wherein during periods specified in (d) the unloading of the centrifugal compressor comprises first reducing the flow of net compressed gas through said centrifugal compressor to zero using the local recycle system, and second reducing the load of said centrifugal compressor.

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

11. The process according to claim 1, wherein unloading to put a centrifugal compressor in said low power mode comprises reducing its rotor speed to within a range of from about 100 rpm to about 1500 rpm and operating such that it produces no net compressed gas.

12. The process according to claim 1, wherein during operation in said low power mode the centrifugal compressor is operating with a power of about 20% or less relative to maximum power and producing no net compressed gas.

13. A process for supplying compressed hydrogen gas for consumption in at least one downstream process, comprising: producing said hydrogen gas from electrolysis of water, compressing said hydrogen gas in a multistage compression system operated according to claim 1, and feeding said compressed hydrogen gas to at least one downstream process for consumption in said downstream process(es).

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

15. An apparatus for operating a multistage compression system compressing a gas feed having a variable flow rate according to claim 1, said apparatus comprising: a multistage compression system comprising a feed end, a plurality (N) of centrifugal compressors in parallel, a product end, and a main recycle system for recycling gas through the plurality (N) of centrifugal compressors, wherein each centrifugal compressor comprises an inlet, an outlet, and a local recycle system with anti-surge control that recycles gas from the outlet to the inlet; a control system for controlling the load of each centrifugal compressor and for controlling the amount of recycling by the main recycle system and local recycle system, as required, based on the flow of the feed gas.

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

17. The apparatus according to claim 15, wherein the gas for compression is hydrogen gas, the apparatus comprising: a plurality of electrolysers for producing said hydrogen gas, wherein the electrolysers are powered at least in part by electricity generated from said electricity generation system, and wherein said feed end of said multistage compression system is in fluid flow communication with said plurality of electrolysers.

18. The apparatus according to claim 15, comprising at least one downstream processing unit for consuming compressed gas, said downstream processing unit(s) being in fluid flow communication with said outlet end of said multistage compression system.

19. The apparatus according to claim 15, comprising: a storage system for storing compressed gas, said storage system being in fluid flow communication with said outlet end of said multistage compression system and at least one compressor of said multistage compression system; and a second control system for controlling pressure and flow of compressed gas from said multistage compression system to said storage system and for controlling pressure and flow of compressed gas from said storage system to said compressor(s) of said multistage compression system based on the flow of the gas feed to the multistage compression system.

Description

EXAMPLES

[0271] The invention will now be described by example only and with reference to the figures in which:

[0272] FIG. 1 is a simplified flowsheet for a first embodiment of the present invention;

[0273] FIG. 2 is a simplified flowsheet for a second embodiment of the present invention;

[0274] FIG. 3 is a simplified flowsheet for a third embodiment of the present invention;

[0275] FIG. 4 is a line graph providing a illustrated simulated example of the process of the present invention in the context of three centrifugal compressors arranged in parallel.

[0276] According to FIG. 1, hydrogen is produced at about atmospheric pressure by electrolysis of water in a plurality of electrolyser units indicated generally by reference numeral 2.

[0277] The electricity required to power the electrolysers 2 is generated at least in part by renewable energy sources (not shown) such as the wind and/or the sun. In some embodiments, however, at least some additional electricity may be taken from onsite battery storage and/or generated from one or more onsite petrol-, diesel- or hydrogen-powered generator(s), including fuel cells and/or taken from a local or national grid (not shown).

[0278] A stream 4 of hydrogen gas is removed from the electrolysers 2 at a pressure just over atmospheric pressure (e.g. about 1.1 bar) and is fed a multistage compression system 100 to produce a stream 36 of compressed hydrogen gas. In this example, the multistage compression system 100 comprises three centrifugal compressors, 10, 12 and 14, that are arranged in parallel.

[0279] Stream 4 has recycled hydrogen gas added to it, as required, to form combined stream 6, which is then fed to header 8 before being compressed in parallel compressors 10, 12, and 14. Compressed hydrogen gas from each of the centrifugal compressors 10, 12, and 14 is fed to header 28 and forms the combined stream 30 of compressed hydrogen gas. Combined stream 30 may have optionally have gas removed from it for recycling, before being fed as stream 36 to a downstream stage of compression (not shown) or at least one downstream process (not shown).

[0280] The multistage compression system 100 includes a main recycle system 32 which removes gas from combined stream 30 and, after suitable pressure reduction in valve 34, feeds it to the inlet of the multistage compression system by combining it with stream 4 to form stream 6.

[0281] Each centrifugal compressor 10, 12, and 14 also has an associated local recycle system 16, 18 and 20, with valves 22, 24, and 26 respectively, each local recycle system has anti-surge control. Each recycle system removes compressed gas from the product end and, after suitable pressure reduction with a valve (22, 24, 26), feeds it to the feed end of the associated centrifugal compressor.

[0282] 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 multistage compression system and accordingly controls the load of the centrifugal compressors 10, 12 and 14. The valve of the main recycle system (34) and the valves of the local recycle systems 22, 24 and 26 are also electrically connected to the control system, such that the amount of recycling by the recycle systems, as well as the amount of recycling by the main recycle system, is controlled to implement the process of the present invention as required.

[0283] Although not shown for brevity, the multistage compression system 100 typically comprises inter-coolers between stages of compression and after-coolers after a final stage. There may also be phase separators upstream of each stage of compression to remove liquid from the stream entering the compressors.

[0284] FIG. 2 depicts a second embodiment of the present invention. The same numerical references have been used to denote features of the flowsheet in FIG. 2 that are common to the flowsheet of FIG. 1. The following is a discussion of the features that distinguish the first embodiment of FIG. 2 from the process shown in FIG. 1.

[0285] According to FIG. 2, the multistage compression system 200 has two stages of compression depicted, a first stage 201, and a second stage 202.

[0286] The compressed gas from header 28 forms combined stream 30 which is then fed to header 48 of the second stage 202. The second stage comprises the same features as the first stage 201 from FIG. 1, including three centrifugal compressors 50, 52, and 54 with the associated local recycle systems and valves.

[0287] In FIG. 2 the main recycle system recycles compressed gas from the outlet of the second stage (stream 70) and, after suitable pressure reduction using valve 34, feeds it to the inlet to the first stage as stream 6. Stream 76 contains net compressed gas and is fed to a downstream stage of compression (not shown) or at least one downstream process (not shown).

[0288] Stream 80 shows where the addition of compressed gas at an appropriate pressure from a suitable storage system may be added, e.g. when there is particularly low flow of gas from the electrolysers 2, and/or where demand from the downstream process (not shown) cannot be met by the electrolysers 2 alone. Stream 80 may add gas from storage by feeding it to an inter-stage point (stream 30) between stages 201 and 202.

[0289] FIG. 3 depicts a third embodiment of the present invention. The same numerical references have been used to denote features of the flowsheet in FIG. 3 that are common to the flowsheet of FIG. 2. The following is a discussion of the features that distinguish the first embodiment of FIG. 3 from the process shown in FIG. 2.

[0290] Regarding FIG. 3, a simplified design of the multistage compression system depicted in FIG. 2 is shown. In this figure, the multistage compression system 300 still comprises two stages, 301 and 302. However, the local recycle systems 16, 18, and 20 receive gas from the outlet of a compressor (50, 52, or 54) in the second stage 302 and, after suitable pressure reduction with valves (22, 24, or 26), feed reduced pressure gas to the inlet of a different, yet corresponding compressor in series within the first stage 301.

[0291] Thus, compared with the arrangement in FIG. 2, this arrangement has three less recycle systems and three less valves required. It therefore allows for a simpler, more cost effective design of the multistage compression system that is simpler to operate. However, note that no gas can be fed from storage to an interstage.

[0292] FIG. 4 is a line graph which illustrates an example of how a multistage compression system such as the one depicted in FIG. 1, may be operated according to the process of the present invention. This data has been generated using Microsoft Excel and may not precisely reflect observation in a real world example.

[0293] In this example, there are three (N) centrifugal compressors each with a turndown capacity of 80%. For simplicity, this example assumes a linear decrease in flow of the gas feed starting from a flow of 100% over 100 hours at a rate of 1% per hour. This graph does not show any local recycle gas flow, or part of the unloading phase of the centrifugal compressors. In reality, flow of the gas feed would be expected to fluctuate widely over this time period rather than decrease steadily. However, the example is intended to merely illustrate the process.

[0294] At reference numeral 400 (0 hours), the gas feed flow is 100% of the capacity of all three (n) centrifugal compressors which are producing net compressed gas (shown with line 410). The first number (n) of centrifugal compressors is therefore 3. The gas feed flow is equal to the total maximum capacity of the three centrifugal compressors (100%) and so all three compressors are at full load (100%) compressing all of the gas feed. This corresponds to the periods specified in (a) according to the invention.

[0295] From 1 to 19 hours, as the flow of the gas feed reduces below 100% the three (n) centrifugal compressors are turned down accordingly to match the flow of the gas feed (from 1 to 20 hours). This corresponds to the periods specified in (b) according to the invention.

[0296] At reference numeral 401 (20 hours) the flow of the gas feed reaches the total maximum turndown capacity of the three compressors (80%, shown with line 440). That is, just above the point at which anti-surge control for all three centrifugal compressors is activated. In order to prevent anti-surge controls from activating, the main recycle system starts to introduce recycled gas through the (n) centrifugal compressors in order to maintain the load just above the point at which anti-surge control is activated. From 20 to 33 hours, the flow of the gas feed drops further below this point, and so the more recycled gas flow is introduced by the main recycle system to compensate. This corresponds to the periods specified in (c) according to the invention.

[0297] At reference numeral 402 (33 hours) the flow of the gas feed is equal to the total maximum capacity for a 2 centrifugal compressors, i.e. the second number (n−1), centrifugal compressors (66%, shown with line 420). Therefore, at this point one centrifugal compressor is unloaded and put into low power mode or shut down, whilst the remaining two centrifugal compressors are simultaneously loaded to maximum capacity (66% of total flow for two compressors, shown with line 420). At this point (at reference numeral 403), given the load for the two remaining compressors is now above the anti-surge control points for both compressors (54%) (shown as a line with reference numeral 450), there is no longer any need to recycle gas using the main recycle system.

[0298] At reference numeral 403 the amount of recycled gas drops to zero, which ideally would be the most efficient way to operate the system. However, it will be appreciated that in practice the change may be more gradual to prevent shocks to the flow of the system and to accommodate more gradual changes in load for centrifugal compressors.

[0299] The process then repeats as above, but for n=2 compressors, since now there are only two compressors in operation producing net compressed gas (with the other in low power or shutdown).

[0300] Between reference numerals 403 and 404 the gas feed flow allows for turndown of the two compressors without recycling. At 404, recycling is required to maintain the load of two (n) compressors above their anti-surge control points (shown with line 450). At reference numeral 405, the gas feed flow reaches the total maximum capacity for one compressor, since (n−1)=(2−1)=1. At this point one of the two compressors is unloaded and shut down or put in low power mode, and the last remaining compressor is operated at maximum load for one compressor (33%, shown with line 430) to compress the flow of the gas feed (33%).

[0301] From reference numeral 406 to 407 the final compressor can be turned down in line with the flow of the gas feed. At 407 however, the flow of the gas feed reaches just above the anti-surge control point (27%, shown with line 460) for the final (n) compressor and recycling is required by the main recycle system to ensure the load of the final (n) compressor is maintained above its anti-surge control point (27%), with more recycled gas being added the lower the gas feed flow drops.

[0302] FIG. 4 demonstrates how the multistage compression system of FIG. 1 can be operated in a way that it dynamically responds to the changes in gas feed flow, including sequentially shutting compressors down or putting them in a low power mode whilst simultaneously balancing the load of the remaining compressors with less (or no) recycling from the main recycle system.

[0303] It can be seen from FIG. 4 that the line for amount of flow of net compressed gas tracks the flow of the gas feed from the electrolysers, without requiring unnecessary recycling, shutting down all compressors, or unnecessarily wasting electricity. This demonstrates how the process of the invention is able to operate centrifugal compressors safely by maintaining their load just above the anti-surge control lines whilst still efficiently compressing a gas feed that has a variable flow ranging anywhere from 100% flow down to 0% flow.

[0304] The preceding 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 teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any other of the examples.

[0305] In this specification, unless expressly otherwise indicated, the word “or” is used in the sense of an operator that returns a true value when either or both of the stated conditions are met, as opposed to the operator “exclusive or” which requires only that one of the conditions is met. The word “comprising” is used in the sense of “including” rather than to mean “consisting of”.

[0306] All prior teachings above are hereby incorporated herein by reference. No acknowledgement of any prior published document herein should be taken to be an admission or representation that the teaching thereof was common general knowledge in Australia or elsewhere at the date thereof.