Method Of Cooling Boil Off Gas And An Apparatus Therefor

Abstract

A method of cooling a boil off gas stream (01) from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank (50) in a floating transportation vessel, said method comprising at least the steps of: compressing a boil off gas stream (01) from said liquefied cargo in two or more stages of compression comprising at least a first compression stage (65) and a final compression stage (75) to provide a compressed BOG discharge stream (06), wherein said first compression stage (65) has a first stage suction pressure and said final compression stage (75) has a final stage suction pressure; cooling the compressed BOG discharge stream (06) against one or more first coolant streams (202, 302) to provide a first cooled compressed BOG stream (08); providing a gaseous vent stream (51) from the first cooled compressed BOG stream (08); cooling the first cooled compressed BOG stream (08) against a second coolant stream (33) to provide a second cooled compressed BOG stream (35); expanding a portion of the second cooled compressed BOG stream (35) to the first stage suction pressure or below to provide a first expanded cooled BOG stream (33); using the first expanded cooled BOG stream (33) as the second coolant stream to provide a first expanded heated BOG stream (38); and cooling the gaseous vent stream (51) against the second coolant stream (33) to provide a cooled vent stream (53), wherein cooling of the first cooled compressed BOG stream (08) and cooling of the gaseous vent stream (51) occurs in a heat exchanger located adjacent to the liquefied cargo tank (50).

Claims

1. (canceled)

2. (canceled)

3. A method of cooling a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation vessel, said method comprising the steps of: compressing a boil off gas stream from said liquefied cargo in at least two stages of compression comprising at least a first compression stage and a second compression stage to provide a compressed BOG discharge stream, wherein said first compression stage has a first stage suction pressure and a first stage discharge pressure, and said second compression stage has a second stage suction pressure; cooling the compressed BOG discharge stream against at least one first coolant streams at a first cooling stage to provide a first cooled compressed BOG stream; cooling the first cooled compressed BOG stream against a second coolant stream at a second cooling stage to provide a second cooled compressed BOG stream; cooling the second cooled compressed BOG stream against a third coolant stream at a third cooling stage to provide a third cooled compressed BOG stream; providing a gaseous vent stream from the first cooled compressed BOG stream; cooling the gaseous vent stream against the second coolant stream at the second cooling stage to provide a cooled vent stream; cooling the cooled vent stream against the third coolant stream at the third cooling stage to provide a cooled vent stream; expanding a portion of the second cooled compressed BOG stream at the second cooling stage to a pressure between that of the first stage discharge pressure and the second stage suction pressure to provide a first expanded cooled BOG stream; expanding a portion of the third cooled compressed BOG stream at the third cooling stage to the first stage suction pressure or below to provide a second expanded cooled BOG stream; using the first expanded cooled BOG stream as the second coolant stream to provide a first expanded heated BOG stream; and using the second expanded cooled BOG stream as the third coolant stream to provide a second expanded heated BOG stream.

4. The method as in claim 3, wherein the liquefied cargo is selected from the group comprising: ethane, liquefied petroleum gas, liquefied petrochemical gas such as propylene and ethylene, and liquefied ammonia.

5. The method as in claim 3 wherein the liquefied cargo is ethane.

6. The method as in claim 5 wherein the liquefied ethane cargo comprises >0.1 mol % methane.

7. The method as in claim 6 wherein the liquefied ethane cargo comprises >0.5 mol % methane.

8. The method as in claim 3 comprising at least three stages of compression.

9. The method as in claim 3 comprising the further step of: separating the further cooled vent stream to provide a vent discharge stream and a cooled vent BOG return stream.

10. The method as in claim 9 comprising the further steps of: expanding the cooled vent BOG return stream to provide an expanded cooled vent BOG return stream; and passing the expanded cooled vent BOG return stream to a storage tank.

11. The method as in claim 3 wherein the step of cooling the compressed BOG discharge stream against at least one first coolant stream to provide a first cooled compressed BOG stream comprises: pre-cooling the compressed BOG discharge stream against a pre-cooling coolant stream as a first coolant stream to provide a pre-cooled compressed BOG stream; and cooling the pre-cooled compressed BOG stream against a first refrigerant stream as a first coolant stream to provide the first cooled compressed BOG stream.

12. The method as in claim 11 wherein the pre-cooling coolant stream is selected from the group comprising: a seawater stream, an ambient air stream and a refrigerant stream.

13. The method as in claim 11 wherein the first refrigerant stream is selected from the group comprising propane and propylene.

14. The method as in claim 3 wherein stages of compression are the compression stages of a multi-stage compressor.

15. An apparatus to cool a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank in a floating transportation vessel, said apparatus comprising: a compression system to compress the boil off gas stream comprising at least two stages of compression comprising at least a first compression stage and a final compression stage to provide a compressed BOG discharge stream, at least one first heat exchanger to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream; at least one second heat exchanger adjacent to the liquefied cargo tank to further cool the first cooled compressed BOG stream and a gaseous vent stream against a coolant stream to provide a second cooled compressed BOG stream and a cooled vent stream.

16. An apparatus to cool a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank in a floating transportation vessel comprising, the apparatus comprising: a compression system to compress the boil off gas stream comprising at least two compression trains, each train comprising at least two stages of compression comprising at least a first compression stage and a second compression stage to provide a compressed BOG discharge stream, wherein said first compression stage has a first stage suction pressure and a first stage discharge pressure, and said second compression stage has a second stage suction pressure; a first cooling stage comprising at least one first heat exchanger to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream; a second cooling stage comprising at least one second heat exchanger to cool the first cooled compressed BOG stream to provide a second cooled compressed BOG stream using a second coolant stream having a pressure between that of the first stage discharge pressure and the second stage suction pressure; and a third cooling stage comprising at least one third heat exchanger to cool the second cooled compressed BOG stream to provide a third cooled compressed BOG stream using a third coolant stream having first stage suction pressure or below.

17. The apparatus as in claim 16 further comprising: a first passage to pass a first expanded heated BOG stream provided from the passage of the second coolant stream through the second cooling stage into a second compression train, and a second passage to expand a second expanded heated BOG stream provided from the passage of the third coolant stream through the third cooling stage to the first stage suction pressure or below, and to pass the further expanded heated BOG stream into a first compression train.

18. The apparatus as in claim 17 further comprising: a third passage to expand at least a portion of a first expanded heated BOG stream provided from the passage of the second coolant stream through the second cooling stage, to a pressure between that of the first stage discharge pressure and the second stage suction pressure, and to pass the further expanded heated BOG stream into a first compression train, and a fourth passage to pass at least a portion of a second expanded heated BOG stream provided from the passage of the third coolant stream through the third cooling stage into a second compression train.

19. (canceled)

20. (canceled)

21. The apparatus as in claim 20 further comprising: a controller able to control the passage of the first expanded heated BOG stream from the second cooling stage into the first compression train or the second compression train or both, and to control the passage of the second expanded heated BOG stream after the third cooling stage into the first compression train or the second compression train or both.

22. (canceled)

23. (canceled)

24. (canceled)

25. The method of cooling a boil of gas stream as in claim 3, wherein the cooling of the first cooled compressed BOG stream and cooling of the gaseous vent stream occurs in a heat exchanger located adjacent to a tank retaining the liquefied cargo.

26. The method of cooling a boil of gas stream as in claim 25, wherein cooling of the first cooled compressed BOG stream and the cooling of the gaseous vent stream take place in the heat exchanger.

27. A method of cooling a boil off gas stream as in claim 3 wherein the pressure of the second coolant stream is between that of the first stage discharge pressure and the second stage suction pressure at a second cooling stage within the step of cooling of the first cooled compressed BOG stream against a second coolant stream, and the third coolant stream has a first stage suction pressure within the step of cooling the second cooled compressed BOG stream against the third coolant stream the third cooling stage; and further comprising the step of passing the first expanded heated BOG stream and the second expanded heated BOG stream into at least one compression stage at a pressure that is between the first stage discharge pressure and the second stage suction pressure, that is at or below the first stage suction pressure, or that is between the first stage discharge pressure and the second stage suction pressure and also at or below the first stage suction pressure.

Description

[0113] Embodiments of the invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:

[0114] FIG. 1 shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to one embodiment of the invention.

[0115] FIG. 2 shows a schematic diagram of a system for cooling, particularly re-liquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to another embodiment of the invention.

[0116] FIG. 3 shows a schematic diagram of a system for cooling, particularly re-liquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to another embodiment of the invention.

[0117] Floating re-liquefaction systems draw the vapor, also known as boil off gas, from one or more storage tanks and pass the boil off gas to a compressor in which it is compressed such that the compressed vapor can be cooled and condensed against one or more coolants as the heat sink/refrigerant. For instance, seawater may be used to pre-cool, typically de-superheat, the compressed vapour in an open cycle pre-cooling circuit. The pre-cooled compressed vapour can then be further cooled against a refrigerant in a closed cycle refrigerant circuit.

[0118] The method and apparatus disclosed herein seeks to provide an improved method and apparatus of re-liquefying BOG. An embodiment of the method and apparatus according to the present invention is disclosed in FIG. 1. Where appropriate, identical stream and component names, and the same reference numerals as those in FIG. 1 have been used for corresponding streams and components in the remaining Figures.

[0119] FIG. 1 shows a liquefied cargo storage tank (50) in a floating transportation vessel, such as an ethane or LPG carrier. A liquefied ethane cargo may comprise other components, such as a proportion of methane.

[0120] In order to cool, particularly re-liquefy, evaporated cargo from the storage tank (50), a boil off gas stream (01), comprising evaporated cargo, is passed to a compression system (60) having two or more stages of compression. The boil off gas stream (01) may have a pressure (the “BOG pressure”) in the range of from above 0 to 500 kPa gauge. The compression system (60) may be a multi-stage compressor comprising two or more stages. By “multi-stage compressor” it is meant that each compression stage in the compressor is driven by the same drive shaft, sometimes termed a compression train. Alternatively, the compression system (60) may comprise independently driven compressors for each of the stages of compression. When the compression system (60) is a multi-stage compressor, it is typically a reciprocating compressor.

[0121] The embodiment of FIG. 1 shows a compression system (60) having a first stage (65) and a second stage (70) and a third and final stage (75), although the method and apparatus described herein is also applicable to compressors having two stages or more than three stages. The first stage (65) and final stage (75) of compression provide low and high pressure streams respectively at their discharge.

[0122] The compression system (60) compresses the boil off gas stream (01) to provide a compressed BOG discharge stream (06). The compressed BOG discharge stream (06) may have a pressure (the “final stage pressure”) in the range of from 1.5 to 3.2 MPa or above, eg. up to 6 MPa.

[0123] The compressed BOG discharge stream (06) is cooled in one or more first heat exchangers (200, 300) against one or more first coolant streams (202, 302) to provide first cooled compressed BOG stream (08).

[0124] In the embodiment of FIG. 1, the compressed BOG discharge stream (06) can be passed to a pre-cooling heat exchanger (200) as one of the one or more first heat exchangers. The compressed BOG discharge stream (06) is pre-cooled against a pre-cooling coolant stream as one of the one of more first coolant streams. The pre-cooling coolant stream (202) may be an air or a water stream, such as an ambient air or seawater stream. The pre-cooling heat exchanger (200) may be a shell and tube heat exchanger or a plate heat exchanger. The pre-cooling heat exchanger may de-superheat the compressed BOG discharge stream (06). The pre-cooling heat exchanger (200) provides a pre-cooled compressed BOG stream (07) and heated pre-cooling coolant stream (204). Typically, the seawater used as the pre-cooling coolant would have a temperature of +36° C. or below, more typically +32° C. or below.

[0125] The pre-cooling heat exchange/exchanger (200) is optional in the method and apparatus disclosed herein. It is advantageous because it reduces the cooling duty of the subsequent cooling steps. However, is it not an essential aspect, such that in an alternative embodiment, the compressed BOG discharge stream (06) can be passed directly to the discharge heat exchanger (300). In such circumstances, the cooling capacity of the discharge heat exchanger (300) would have to be increased to compensate for the absence of pre-cooling.

[0126] The pre-cooled compressed BOG stream (07) can then be passed to a discharge heat exchanger (300) as another of the one or more first heat exchangers. The discharge heat exchanger (300) cools the pre-cooled compressed BOG stream (07) against a first refrigerant stream (302) as another of the one or more first coolant streams. The discharge heat exchanger (300) provides a first cooled compressed BOG stream (08) and a heated first refrigerant stream (304).

[0127] The first refrigerant stream (302), discharge heat exchanger (300) and heated first refrigerant stream (304) may be part of a first refrigerant system (not shown). Such a first refrigerant system may further comprise a first refrigerant compressor to compress the heated first refrigerant stream (304) to provide a compressed first refrigerant stream, a first refrigerant cooler to cool the first refrigerant to provide a cooled compressed first refrigerant stream and a first refrigerant expansion device to expand the cooled compressed first refrigerant stream to provide the first refrigerant stream (302). The first refrigerant system may be a closed system. The first refrigerant may comprise one or more organic compounds, particularly hydrocarbons and fluorinated hydrocarbons such as propane, propylene, difluoromethane and pentafluoromethane, including the fluorinated hydrocarbon mixture R-410A, as well as one or more inorganic compounds such as ammonia.

[0128] Where the cooled compressed BOG stream (08) is not fully condensed, there is a gaseous vent stream also provided, either from the discharge heat exchanger (300) as stream (51a), and/or from the discharge receiver (305) as stream (51b). Whilst FIG. 1 shows the two streams (51a, 51b) as separate, such streams may be provided separately or combined or without any distinction, depending upon the nature and construction of the discharge heat exchanger (300) and the discharge receiver (305). The provision of these stream or streams is known in the art. The gaseous vent stream (51) may comprise both ‘non-condensable’ components and ‘in-condensable’ components. The in-condensable components are generally considered to be components which cannot practically ever by compressed and condensed within the confines and operating parameters of a particular floating transportation vessel BOG cooling system, and primarily relate to nitrogen.

[0129] The first cooled compressed BOG stream (08) is then second cooled. This can be achieved by passing the first cooled compressed BOG stream (08) to a heat exchanger (20). The cooling of the first cooled compressed BOG stream (08) is against a second coolant stream to provide a second cooled compressed BOG stream (35). The action of the second coolant is to provide a second cooled compressed BOG stream (35). A portion of this stream (35) is expanded to the first stage suction pressure or below to provide a first expanded cooled BOG stream (33).

[0130] The cooling of the gaseous vent stream (51) using the same heat exchanger (20) can condense a portion of the components of the boil off gas which could not be condensed in the discharge heat exchanger (300) against the first refrigerant such as propane or propylene. The so formed cooled vent stream (53) is typically an at least partly condensed stream.

[0131] It is a particular feature of the embodiment of the present invention that cooling of the first cooled compressed BOG stream (08) and cooling of the gaseous vent stream (51) takes place in a heat exchanger (20), which is located adjacent to the liquefied cargo tank (50). In a conventional system, heat exchangers are typically located adjacent to the compressor(s), with the storage tank located remotely and connected via long pipelines. Locating the heat exchanger adjacent to the cargo tank in the present invention offers an improvement in performance by reducing the heat transfer from the surroundings (which are significantly warmer than the returning cooled BOG) into the BOG stream (11). The reduced heat transfer has the effect of improving overall performance in two ways—less cooling is ‘lost’ to the surroundings and the accumulation of methane in the BOG vapour is reduced.

[0132] In the arrangement shown in FIG. 1, the heat exchanger (20) to cool the first cooled compressed BOG stream (08) and the gaseous vent stream (51) is located closer to the liquefied cargo tank (50) than the compressors (60) and the cooling of the compressed BOG discharge stream (06) against one or more first coolant streams to provide a first cooled compressed BOG stream (08). In this way, the efficiency of the overall arrangement shown in FIG. 1 can reduce the heat loss of the ‘colder’ parts of the arrangement, compared to those parts wherein the BOG is at a relative higher temperature.

[0133] FIG. 1 also shows the cooled vent stream (53) optionally being passed to a vent stream separator (150), such as a gas/liquid separator. The vent stream separator (150) provides a vent discharge stream (55) being wholly or substantially the in-condensable components, which is typically a vapour stream, and a cooled vent BOG return stream (57), which is typically a condensed stream, comprising those components of the boil off gas which were condensed in the heat exchanger. The pressure of the vent discharge stream (55) may be reduced, for instance to a pressure appropriate for return to the storage tank (50), for storage elsewhere or for venting.

[0134] Optionally, separator (150) could be a discrete separator device, or be integrated into the tank, or utilise an existing feature of the tank such as the “emergency pump removal column”.

[0135] The cooled vent BOG return stream (57) may be passed through a vent return stream pressure reduction device (58), such as a Joule-Thomson valve or expander, to provide an expanded cooled vent BOG return stream (59). The expanded cooled vent BOG return stream (59) can be passed to the storage tank (50), for instance by addition to the expanded cooled BOG return stream (36).

[0136] FIG. 2 shows another method and apparatus of the present invention. In common with FIG. 1, FIG. 2 shows a liquefied cargo storage tank (50) from which a boil off gas stream (01), comprising evaporated cargo, is passed to a compression system (60), having three stages of compression being a first stage (65), a second and intermediate stage (70) and a third and final stage (75).

[0137] The compression system (60) provides a compressed BOG discharge stream (06) which can be cooled against one or more first coolant streams (202, 302) at a first cooling stage (400) to provide a first cooled compressed BOG stream (08). The first cooled compressed stream (08) is then cooled against a second coolant stream at a second cooling stage (410) to provide a second cooled compressed BOG stream (34). Stream (34) is then cooled against a third coolant stream at a third cooling stage (42) to provide a third cooled compressed BOG stream (35).

[0138] A gaseous vent stream (51) is provided from the first cooled compressed BOG stream (08) in the same way as shown in FIG. 1, and cooled against the second coolant stream at the second cooling stage (410) to provide a cooled vent stream (52). The cooled vent stream (52) is then cooled against the third coolant stream at the third cooling stage (420) to provide a cooled vent stream (53).

[0139] In FIG. 2, a portion of the second cooled compressed BOG stream (34) is expanded to a pressure between that of the first stage discharge pressure and the second stage suction pressure at the second cooling stage (410) to provide a first expanded cooled BOG stream (34a).

[0140] Meanwhile, a portion of the third cooled compressed BOG stream (35) is expanded to the first stage suction pressure or below at the third cooling stage (420) to provide a second expanded cooled BOG stream (35a).

[0141] The first expanded cooled BOG stream (34a) is then used as the second coolant stream to provide a first expanded heated BOG stream (39), which can be returned into the compression system (60) between the first stage (65) and the second stage (70).

[0142] And the second expanded cooled BOG stream (35a) is then used as the third coolant stream to provide a second expanded heated BOG stream (38), which can be returned into the compression system (60) before be first stage (65).

[0143] Overall, the user can then tune the amounts of the first and second expanded cooled BOG streams (34a, 35) to improve, preferably to maximise, the overall efficiency of the cooling configuration, and so to reduce the energy consumption required, which may vary over time.

[0144] FIG. 3 shows another method and apparatus of the present invention.

[0145] FIG. 3 shows a liquefied cargo storage tank (50) from which a boil off gas stream (01), comprising evaporated cargo, is passed to a compression system (60) comprising two or more compression trains (60a, 60b), wherein each train comprises two or more stages of compression. Each train comprises at least a first compression stage (65, 65′) and a second compression stage (70,70′) to provide a combined compressed BOG discharge stream (06).

[0146] The compressed BOG discharge stream (06) is cooled in one or more first heat exchangers (200, 300) to provide a first cooled compressed BOG stream (08) as described hereinbefore.

[0147] The first cooled compressed BOG stream (08) is cooled against a second coolant stream having a pressure between that of the first stage discharge pressure and the second stage suction pressure at a second cooling stage (410) to provide a second cooled compressed BOG stream (34). The second cooled compressed BOG stream (34) is cooled against a third coolant stream having a pressure between that of the first stage discharge pressure and the second stage suction pressure at a third cooling stage (420) to provide a third cooled compressed BOG stream (35).

[0148] Optionally, a gaseous vent stream (51) is provided from the first cooled compressed BOG stream (08) as described hereinbefore, and is also cooled against the second coolant stream at the second cooling stage (410) to provide a cooled vent stream (52). The cooled vent stream (52) is cooled against the third coolant stream at the third cooling stage (420) to provide a cooled vent stream (53).

[0149] A portion of the second cooled compressed BOG stream (34) is expanded at the second cooling stage (410) to a pressure between that of the first stage discharge pressure and the second stage suction pressure to provide a first expanded cooled BOG stream (34a). The first expanded cooled BOG stream (34a) is then used as the second coolant stream to provide a first expanded heated BOG stream (39).

[0150] Meanwhile, a portion of the third cooled compressed BOG stream (35) is expanded at the third cooling stage (420) to provide a second expanded cooled BOG stream (35a). The second expanded cooled BOG stream (35a) is used as the third coolant stream to provide a second expanded heated BOG stream (38).

[0151] The first expanded heated BOG stream (39) and the second expanded heated BOG stream (38) are passed into one or more of the two or more compression trains (60a, 60b), at either a pressure between that of the first stage discharge pressure and the second stage suction pressure, or a pressure at or below the first stage suction pressure, or both. These streams can be passed along passages and through valves (22) to achieve the desired expanded pressure for delivery into the desired part of the compression system (60) to improve, preferably to maximise, the efficiency of the cooling configuration.

[0152] In one example, the first expanded heated BOG stream (39) from the second cooling stage (410) can be passed into the second compression train (60b), and the third coolant stream (35a) can be expanded after the third cooling stage (420) to the first stage suction pressure or below to provide the second expanded heated BOG stream (38), which may be passed into the first compression train (60a).

[0153] In another example, at least a portion of the second coolant stream (34a) may be expanded after the second cooling stage (410) to a pressure between that of the first stage discharge pressure and the second stage suction pressure to provide the first expanded heated BOG stream (39), which may be passed into the first compression train (60a), and at least a portion of the third coolant stream (35a) after the third cooling stage (420) may be passed into the second compression train (60b).

[0154] The passing of the coolant streams into the various compression trains can be organised by a controller (not shown), able to operate the required valves to balance the required flows.

[0155] Optionally, a number of identical sets of compressors and heat exchangers are provided. OPEX & CAPEX benefit can be obtained by operating the compressor systems in an integrated fashion via the provision of valves or gates to direct the cooled BOG flow appropriately. The units can operate independently if required, depending on the cooling capacity required.

[0156] The person skilled in the art will understand that the invention can be carried out in many various ways without departing from the scope of the appended claims. For instance, the invention encompasses the combination of one or more of the optional or preferred features disclosed herein.