Process and apparatus for compressing hydrogen gas in a centrifugal compressor

Abstract

A process for producing compressed hydrogen gas including: electrolysing water to produce hydrogen gas, and compressing the hydrogen gas in a multistage compression system including: a centrifugal compression stage and a recycle system for recycling a portion of the hydrogen gas from a product end to a feed end of the centrifugal compression stage; wherein hydrogen gas feed is fed to the feed end at a pre-determined feed temperature and pressure and mole fraction of water; wherein a portion of the hydrogen gas is removed from the product end, reduced in pressure in the recycle system to the pre-determined feed pressure and is then recycled to form at least part of the hydrogen gas feed to the centrifugal compression stage; and further including cooling hydrogen gas comprising the reduced pressure hydrogen gas such that the water mole fraction in the hydrogen gas feed is at the pre-determined water mole fraction.

Claims

1. A process for producing compressed hydrogen gas, the process comprising: electrolysing water to produce the hydrogen gas, and compressing the hydrogen gas in a multistage compression system to produce compressed hydrogen gas; the multistage compression system comprising: a centrifugal compression stage with a feed end and a product end, and a recycle system for recycling a portion of the hydrogen gas from the product end to the feed end of the centrifugal compression stage; wherein hydrogen gas feed is fed to the feed end of the centrifugal compression stage at a pre-determined feed temperature and pressure and having a pre-determined mole fraction of water; wherein a portion of the hydrogen gas is removed, as required, from the product end, reduced in pressure in the recycle system to the pre-determined feed pressure before any cooling step to provide reduced pressure hydrogen gas which is then recycled to form at least part of the hydrogen gas feed to the centrifugal compression stage; and wherein the process comprises cooling the hydrogen gas comprising the reduced pressure hydrogen gas such that the mole fraction of water in the hydrogen gas feed is at the pre-determined mole fraction of water.

2. The process according to claim 1, wherein the hydrogen gas comprising the reduced pressure hydrogen gas is cooled to a temperature within ±2° C. of the pre-determined feed temperature.

3. The process according to claim 1, wherein the pre-determined feed temperature is from 20 to 60° C.

4. The process according to claim 1, wherein the hydrogen gas feed to the centrifugal compression stage has an apparent molecular weight in a range from 2.2 to 3.5.

5. The process according to claim 1, wherein the apparent molecular weight of the hydrogen gas comprising the reduced pressure hydrogen gas after cooling is equivalent to an apparent molecular weight of the hydrogen gas feed.

6. The process according to claim 1, wherein the centrifugal compression stage is an initial stage of the multistage compression system.

7. The process according to claim 1, wherein the multistage compression system comprises a first section and at least one further section downstream of the first section.

8. The process according to claim 1, wherein the compressed hydrogen gas is consumed in at least one downstream process, and wherein during periods when more hydrogen gas is produced by the electrolysis than is required by the at least one downstream process(es), the process comprises feeding excess compressed hydrogen gas to storage, optionally after further compression; and wherein during periods when more hydrogen gas is required by the at least one downstream process(es) than is produced by the electrolysis, the process comprises withdrawing compressed hydrogen gas from the storage and, after suitable pressure reduction, feeding said hydrogen gas from the storage to a stage of the multistage compression system.

9. The process according to claim 8, wherein said hydrogen gas from the storage is fed to a stage of the multistage compression system by feeding it to an inter-cooler upstream of said stage.

10. The process according to claim 1, wherein the hydrogen gas comprising the reduced pressure hydrogen gas is cooled in the recycle system.

11. The process according to claim 1, wherein the hydrogen gas comprising the reduced pressure hydrogen gas is cooled in the hydrogen gas feed.

12. The process according to claim 1, wherein the multistage compression system comprises: a second centrifugal compression stage downstream of the centrifugal compression stage, the second centrifugal compression stage comprising a feed end and a product end, a second recycle system for recycling a portion of the hydrogen gas from the product end to the feed end of the second centrifugal compression stage; wherein the hydrogen gas feed is fed to the feed end of the second centrifugal compression stage at the pre-determined feed temperature and pressure and having the pre-determined mole fraction of water: wherein a portion of the hydrogen gas is removed, as required, from the product end of the second centrifugal compression stage, reduced in pressure in the second recycle system to the pre-determined feed pressure before any cooling step to provide reduced pressure hydrogen gas which is then recycled to form at least part of the hydrogen gas feed to the second centrifugal compression stage; and wherein the process comprises cooling hydrogen gas comprising reduced pressure hydrogen gas in the hydrogen gas feed to the second centrifugal compression stage such that the mole fraction of water in said hydrogen gas feed to the second centrifugal compression stage is at the pre-determined mole fraction of water.

13. An apparatus for producing compressed hydrogen gas, the apparatus comprising: a plurality of electrolysers arranged in parallel for electrolysing water to provide hydrogen gas; an electricity generation system for generating electricity to power the plurality of electrolysers, the electricity generation system being in electrically conductive communication with the plurality of electrolysers; a multistage compression system for compressing the hydrogen gas to provide a compressed hydrogen gas, the multistage compression system comprising: an inlet end, an outlet end and a centrifugal compression stage having a feed end and a product end, the inlet end being in fluid flow communication with the plurality of electrolysers via a feed header; and a recycle system in fluid flow communication with the product end and the feed end of the centrifugal compression stage, the recycle system comprising: a valve for reducing the pressure of recycled hydrogen gas to a pre-determined feed pressure before any cooling step to provide reduced pressure hydrogen gas; a cooler for cooling the hydrogen gas comprising the reduced pressure hydrogen gas in the recycle system and/or in the feed header to the centrifugal compression stage.

14. The apparatus according to claim 13, wherein the multistage compression system comprises: a second centrifugal compression stage downstream of the first centrifugal compression stage, the second centrifugal compression stage comprising a feed end and a product end and the feed end being in fluid flow communication with the product end of the first centrifugal compression stage via a feed header; and a second recycle system in fluid flow communication with the product end and the feed end of the second centrifugal compression stage, the second recycle system comprising: a valve for reducing the pressure of recycled hydrogen gas to the pre-determined feed pressure for the second centrifugal compression stage before any cooling step to provide reduced pressure hydrogen gas; a second cooler for cooling the hydrogen gas comprising the reduced pressure hydrogen gas in the feed header to the second centrifugal compression stage.

Description

EXAMPLES

(1) The invention will now be described by example only and with reference to the figures in which:

(2) FIG. 1 is a simplified flowsheet for a first example of the present invention;

(3) FIG. 2 is a simplified flowsheet for a second example of the present invention;

(4) FIG. 3 is a simplified flowsheet for a third example of the present invention.

(5) According to FIG. 1, feed 1 contains hydrogen gas supplied from electrolysers (and/or from storage), and is mixed with recycled hydrogen gas from stream 19 before being fed to phase separator S100. Phase separator S100 removes liquid water from the hydrogen gas to provide hydrogen gas feed 7 which is fed to the compressor stage K102 for compression.

(6) A recycle system (17, V110 and 19) collects hot compressed hydrogen gas from stream 9 at the product end of the compressor K102 as stream 17. In this system, the hot compressed hydrogen recycle stream 17 is reduced in pressure through the recycle valve V110 to the pre-determined feed pressure for the compression stage K102 before being cooled in recycle cooler E116 to the pre-determined feed temperature for the compression stage K102 to provide a cooled reduced pressure hydrogen gas stream 19. The temperature of stream 19 may also be controlled by bypassing a portion of the flow around cooler E116 and re-combining it with the cooled stream in varying degrees (not shown).

(7) This means that no liquid water has been condensed during cooling and no liquid water is present in stream 19. Stream 19 is then mixed with the feed 1 to provide mixed stream 3. The water content of mixed stream 3 and the feed to the compressor 7 is therefore unchanged when compared with feed 1.

(8) FIG. 2 describes an arrangement of the present invention which is an alternative to the arrangement shown in the flowsheet of FIG. 1.

(9) 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 embodiment of FIG. 2 from the process shown in FIG. 1.

(10) In contrast to FIG. 1, in FIG. 2 there is no cooler in the recycle system, one is instead located in the hydrogen gas feed to cool mixed feed 3. Thus, in this example the reduced pressure recycled hydrogen gas in stream 19 is not cooled until after it is mixed with feed 1 as combined stream 3. This example is particularly suited to centrifugal compression stages which are an intermediate or final stage of compression, where the cooler can act as both an “inter-cooler” and cooler for the hot recycled hydrogen gas in stream 19.

(11) FIG. 3 describes an identical recycling system around a stage as in FIG. 1, but further describes an example where this is used in conjunction with a downstream stage in series with the arrangement shown in FIG. 2.

(12) The same numerical references have been used to denote features of the flowsheet in FIG. 3 that are common to the flowsheets of FIGS. 1 and 2. The following is a discussion of the features that distinguish the embodiment of FIG. 3 from the processes shown in FIGS. 1 and 2.

(13) In this example where there is another downstream stage in series, the second reduced pressure recycle hydrogen gas stream 23 from the downstream second stage (not shown) may be mixed with the hot discharge stream 9 from the upstream first stage K102 before being cooled in the upstream stage inter-cooler E106 as an alternative to a separate recycle cooler.

(14) This arrangement avoids changes in water concentration during recycling of the compressor stages. This would not be the case for conventional compression systems, where the recycle around a stage (K102) is always taken after the cooling (E106) and liquid water removal (S100) steps, which would decrease the water concentration in the recycle feed.

(15) The system depicted in FIG. 3 will now be explained in more detail with reference to the following Example which contains simulated data. This data was generated using a computer simulation package, Aspen Plus with version no. 10.

Example

(16) TABLE-US-00002 Stream 1 3 5 7 9 10 11 15 Temp., ° C. 40.0 40.0 40.0 40.0 140.9 141.0 40.0 40.0 P, bar 1.10 1.10 1.10 1.10 2.54 2.54 2.50 2.50 Molar Flow, kmol/hr 321.6 1072.0 1072.0 1072.0 1072.0 1043.0 1043.0 1030.6 Vap. Fract. 1.00 1.00 1.00 1.00 1.00 1.00 0.99 1.00 Mol. fract. Water 0.0671 0.0671 0.0671 0.0671 0.0671 0.0412 0.0412 0.0296 Relative humidity 100% 100% 100% 100% 5% 3% 100% 100% Mol. wt. 3.090 3.090 3.090 3.090 3.090 2.675 2.675 2.490 Stream 17 19 23 52 60 Temp., ° C. 140.9 40.0 141.0 40.0 P, bar 2.54 1.10 2.54 2.50 Molar Flow, kmol/hr 750.4 750.4 721.4 0.0 12.4 Vap. Fract. 1.00 1.00 1.00 0.00 Mol. fract. Water 0.0671 0.0671 0.0296 1.0000 Relative humidity 5% 100% 2% Mol. wt. 3.090 3.090 2.490

(17) The above Example illustrates the use of a low pressure recycle cooler to avoid the condensation of water that occurs when gas is cooled at high pressure. The feed 1 is saturated with a relative humidity of 100% but with a flow at 30% of full flow. Due to the reduced flow, the recycle system is activated so that the compression stage operates at 100% flow. The reduction in pressure in valve V110 prior to cooling in the cooler E116 means that no water is condensed before the recycle stream 19 is remixed with the feed 1. This example also illustrates cooling of the recycle stream 23 fed from a downstream stage in the inter-cooler E106.

(18) It can be seen that from the above example, that the present invention demonstrates a way to recycle hydrogen gas around a stage without substantially reducing the mole fraction of water in the hydrogen gas feed to a centrifugal compression stage. This in turn results in the hydrogen gas feed having an apparent molecular weight which maintains optimal performance of the centrifugal compressor.

(19) While the invention has been described with reference to the preferred embodiments depicted in the figures, it will be appreciated that various modifications are possible within the spirit or scope of the invention as defined in the following claims.

(20) 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”.

(21) 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.