Method of Cooling Boil-Off Gas and Apparatus Therefor

Abstract

A method of cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising at least the step of heat exchanging the BOG stream with a first refrigerant in a heat exchanger, the heat exchanger having an entry port and a warmer exit port, and comprising at least the steps of: (a) passing the first refrigerant into the entry port of the heat exchanger and into a first zone of the heat exchanger to exchange heat with the BOG stream, to provide a first warmer refrigerant stream; (b) withdrawing the first warmer refrigerant stream from the heat exchanger at an intermediate exit port between the entry port and the warmer exit port; (c) passing the first warmer refrigerant stream through an entry port located in a second zone of the heat exchanger that is warmer than the first zone (d) passing an oil-containing refrigerant stream through an entry port located in a second zone of the heat exchanger that is warmer than the first zone; (e) mixing the first warmer refrigerant stream and the oil-containing stream in the heat-exchanger to form a combined refrigerant stream; and (f) passing the combined refrigerant stream out of the heat exchanger through the warmer exit port.

Claims

1. A method of cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising at least the step of heat exchanging the BOG stream with a first refrigerant in a heat exchanger, the heat exchanger having an entry port and a warmer exit port, the method comprising the steps of: (a) passing the first refrigerant into the entry port of the heat exchanger and into a first zone of the heat exchanger to exchange heat with the BOG stream and to provide a first warmer refrigerant stream; (b) withdrawing the first warmer refrigerant stream from the heat exchanger at an intermediate exit port between the entry port and a warmer exit port; (c) passing the first warmer refrigerant stream through a second entry port located in the second zone of the heat exchanger, the second zone of the heat exchanger being warmer than the first zone (d) passing an oil-containing refrigerant stream through a third entry port located in a second zone of the heat exchanger; (e) mixing the first warmer refrigerant stream and the oil-containing stream in the heat-exchanger to form a combined refrigerant stream; and (f) passing the combined refrigerant stream out of the heat exchanger through the warmer exit port.

2. The method as claimed in claim 1 wherein the BOG stream is from a liquefied cargo tank in a floating vessel.

3. The method as claimed in claim 2 wherein the BOG stream is from a liquefied natural gas (LNG) cargo tank.

4. The method as claimed in claim 1 wherein the oil-containing refrigerant comprises a single mixed refrigerant (SMR).

5. The method as in claim 1 wherein the heat exchanger is a single liquefaction heat exchanger in a BOG liquefaction heat exchanger system.

6. The method as claimed in claim 1 wherein the heat exchanger is a liquefaction heat exchanger system comprising a multi-unit liquefaction heat exchange and two or more heat exchanger units, and the BOG stream and the first refrigerant pass through at least a coldest of the two or more heat exchanger units.

7. The method as claimed in claim 1 wherein the heat exchanger is a vertical or near vertical heat exchanger.

8. The method as claimed in claim 7 wherein the heat exchanger comprises a plate-fin heat exchanger or a printed circuit heat exchanger.

9. The method as claimed in claim 1 wherein the oil in the oil-containing refrigerant stream comprises compressor lubricating oil.

10. The method as claimed in claim 1 further comprising the step of expanding the first refrigerant prior to step (a).

11. The method as claimed claim 1 wherein the temperature of step (e) is higher than the temperature of the first zone in the heat exchanger.

12. The method as claimed claim 1 wherein the intermediate exit port is within the second zone of the heat exchanger.

13. (canceled)

14. The method as claimed claim 1 wherein the temperature of the second zone is warmer than the freezing temperature of the oil of the oil-containing refrigerant.

15. The method as claimed claim 1 further comprising the steps of providing a single mixed refrigerant (SMR) and heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, wherein the SMR is provided in an SMR recirculating system comprising at least the steps of: (a) compressing the SMR using at least one oil-injected screw compressor to provide a post-compression SMR stream; (b) separating the post-compression SMR stream to provide an oil-based stream and a first SMR vapour stream; (c) passing the first SMR vapour stream into the liquefaction heat exchanger system to cool the first SMR vapour stream and provide a cooled first SMR vapour stream; (d) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system; (e) separating the cooled first SMR vapour stream to provide an oil-containing liquid-phase SMR stream and an oil-free SMR vapour stream; (f) passing the oil-free SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; (g) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream, and to provide a warmer SMR stream; (h) withdrawing the warmer SMR stream from the liquefaction heat exchanger system at an intermediate exit port; (i) expanding the oil-containing liquid-phase SMR stream of step (e) to provide an at least partially expanded oil-containing refrigerant stream; (j) passing the warmer SMR stream of step (h) and the at least partially expanded oil-containing refrigerant stream of step (i) into the liquefaction heat exchanger system through separate entry ports located in a second zone of the liquefaction heat exchanger system that is warmer than the first zone; (k) combining the warmer SMR stream with the oil-containing refrigerant stream in the heat exchanger to provide a combined refrigerant stream; (l) passing the combined refrigerant stream into the liquefaction heat exchanger system through an entry port located in a second zone of the liquefaction heat exchanger system that is warmer than the first zone; and (m) passing the combined refrigerant stream out of the liquefaction heat exchanger system through the warmer exit port.

16. An SMR recirculating system for use with a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, wherein the SMR is provided in an SMR recirculating system comprising at least the steps of: (a) compressing the SMR using at least one oil-injected screw compressor to provide a post-compression SMR stream; (b) separating the post-compression SMR stream to provide an oil-based stream and a first SMR vapour stream; (c) separating the first SMR vapour stream to provide an oil-containing liquid-phase SMR stream and a SMR vapour stream; (d) passing the SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; (e) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass into a first zone of the liquefaction heat exchanger system for heat exchange against the BOG stream, and to provide a warmer SMR stream; (f) withdrawing the warmer SMR stream from the heat exchanger at an intermediate exit port; (g) passing the first warmer SMR stream through an entry port located in a second zone of the heat exchanger that is warmer than the first zone; (h) passing the oil-containing liquid-phase SMR stream through an entry port located in a second zone of the heat exchanger that is warmer than the first zone; (i) mixing the first warmer refrigerant stream and the oil-containing liquid-phase SMR stream in the heat-exchanger to form the combined refrigerant stream; and (j) passing the combined refrigerant stream out of the heat exchanger through the warmer exit port.

17. (canceled)

18. An apparatus for cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising: a heat exchanger for receiving the BOG stream and having an entry port and a warmer exit port, the heat exchanger further comprising a first refrigerant directed into the entry port and into a first zone within the heat exchanger to exchange heat with the BOG stream and to provide a first warmer refrigerant stream; an intermediate exit port between the entry port and the warmer exit port, the first warmer refrigerant stream directed through the intermediate exit port; a second entry port located in a second zone of the heat exchanger, the second zone of the heat exchanger being warmer than the first zone, and directing the first warmer refrigerant stream through the second entry port; and a third entry port located in a second zone of the heat exchanger, an oil-containing refrigerant stream directed into the third entry port, wherein the first warmer refrigerant stream and the oil-containing stream are mixed within the heat-exchanger to form a combined refrigerant stream, and the combined refrigerant stream is directed out of the heat exchanger through the warmer exit port.

19. The apparatus as claimed in claim 18 further comprising: a single mixed refrigerant (SMR) recirculating system having (a) at least one oil-injected screw compressor for compressing the SMR using to provide a post-compression SMR stream; (b) a separator for separating the post-compression SMR stream to provide an oil-based stream and a first SMR vapour stream; (c) directing the first SMR vapour stream into the heat exchanger to cool the first SMR vapour stream and provide a cooled first SMR vapour stream, and withdrawing the cooled first SMR vapour stream from the heat exchanger; (d) a separator for separating the cooled first SMR vapour stream to provide an oil-containing liquid-phase SMR stream and an oil-free SMR vapour stream; (e) directing the oil-free SMR vapour stream through the heat exchanger to provide a condensed SMR stream, and expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream; (f) directing the expanded lowest-temperature SMR stream through the heat exchanger for heat exchange against the BOG stream, and to provide a warmer SMR stream; (g) withdrawing the warmer SMR stream from the heat exchanger through the intermediate exit port; (h) expanding the oil-containing liquid-phase SMR stream to provide an at least partially expanded oil-containing refrigerant stream; (i) directing the warmer SMR stream and the at least partially expanded oil-containing refrigerant stream into the heat exchanger through separate entry ports located in the second zone of the heat exchanger; (j) combining the warmer SMR stream with the oil-containing refrigerant stream within the heat exchanger to provide a combined refrigerant stream; (k) passing the combined refrigerant stream into the heat exchanger through an further entry port located in the second zone of the heat exchanger; and (l) directing the combined refrigerant stream out of the heat exchanger through the warmer exit port.

Description

[0109] Embodiments and an example of the present invention will now be described by way of example only and with reference to the accompanying schematic drawings in which:

[0110] FIG. 1 is a schematic view of a prior art method of cooling a BOG stream using a prior art SMR system;

[0111] FIG. 2 is a simplified schematic view of Area 200 of FIG. 1;

[0112] FIG. 3 is a simplified schematic view of part of a method of cooling a BOG stream using an embodiment of the present invention;

[0113] FIG. 4 is a schematic view of a method of cooling a BOG stream according to a first embodiment of the present invention;

[0114] FIG. 5 is a schematic view of a method of cooling a BOG stream according to a second embodiment of the present invention; and

[0115] FIG. 6 is a schematic view of a method of cooling a BOG stream according to another embodiment of the present invention.

[0116] Where relevant, the same reference numerals are used in different Figures to represent the same or similar feature.

[0117] FIG. 1 is a prior art arrangement described hereinabove, which requires an external refrigerant circuit and apparatus based on cascade 13 to achieve re-liquefaction of the compressed BOG using an SMR recirculating system and an oil-injected screw compressor 2.

[0118] FIG. 2 shows a simplified schematic view of Area 200 of FIG. 1, in which the first refrigerant stream 34 enters the exchanger 7 at its lowest temperature and into an area designated for illustration purposes only as a ‘zone A’, within which the temperature in the heat exchanger 7 is low enough that any compressor lubricating oil remaining in the stream 34 would freeze. However, stream 34 should not have sufficient oil content to cause a significant blockage or maloperation of the heat exchanger 7 by clogging.

[0119] This stream 34 is heated by the other hotter streams (such as the BOG stream in FIG. 1 from the exchanger 12, not shown) passing it in the exchanger 7, such that its enthalpy increases as it passes upwards through the exchanger 7, resulting in an increase in temperature and/or its vapour fraction.

[0120] At a warmer point in the exchanger 7 (i.e. within an illustrative warmer ‘zone B’), and at a location physically higher than the cold stream inlet port, the expanded oil-containing stream 42 (usually having a sufficient oil content that would cause clogging of the exchanger 7 if it were to freeze) is injected into the exchanger 7, and merged with the warmer stream to create a combined stream 28 that continues along the path of the original cold stream mentioned above.

[0121] ‘Zone B’ is sufficiently warm that oil in the injected stream 42 does not freeze. And if the arrangement shown in FIG. 2 is designed appropriately, there will be sufficient upwards velocity in the exchanger 7 at design conditions to ensure that any oil contained in stream 42 is always and only carried upwards in combined stream 28.

[0122] However, if the process is not operating at design conditions (for example, due to being operated at part-load, an external process disturbance, or being shut down), it is possible that the velocity of combined stream 28 will be lower than the terminal velocity of the oil particles introduced by oil-containing stream 42. Such an event would cause oil particles to fall down into ‘zone A’, where they would freeze and clog up the exchanger 7. This then requires shutting down the whole cooling process to access the heat exchanger 7 and physically or chemically remove the clogging oil and/or its solid components, causing unwanted delay and cost issues.

[0123] The present invention provides an alternative arrangement that avoids the abovementioned issue, by physically preventing the oil from being able to enter the coldest, typically the cryogenic, section of a heat exchanger.

[0124] An illustration of the present invention is shown in FIG. 3. FIG. 3 shows the upwards-flowing first refrigerant stream 34 passing firstly into an illustrative temperature ‘zone A’ in a heat exchanger 50 through an entry port 49, then into an illustrative warmer temperature ‘zone B’, being withdrawn through an exit port 60 as a warmer refrigerant stream 52. The warmer refrigerant stream 52 is then passed back into the heat exchanger 50 via an entry port 43 in zone B.

[0125] Meanwhile, an oil-containing refrigerant stream 42 also enters the heat exchanger 50 via an entry port 63 in zone B, and then combined at a suitable junction point 47, being one or more pathways, so as to form a combined refrigerant stream 54. This combined stream 54 continues an upward flow, leaving the heat exchanger 50 through a warmer exit port 72 as a combined exit stream 28. Heat exchanger 50 is different from heat exchanger 7 in FIG. 2, in that heat exchanger 50 has the additional withdrawal port 60 to withdraw the first refrigerant stream 34.

[0126] In this way, should the flow of combined stream 54 within the heat exchanger 50 be too low, such that the stream's velocity is below that of the terminal velocity of the oil particles introduced in the oil-containing stream 42, the oil or oil particles cannot drop or fall down into ‘zone A’ of the heat exchanger 50, where it or they would freeze and cause issues and possible damage to the heat exchanger 50 as discussed above.

[0127] Compared to the arrangement in FIG. 2, the arrangement in FIG. 3 physically segregates the possible flow path of the oil from being able to enter the section of a heat exchanger where oil-freezing temperatures exist.

[0128] FIG. 4 shows a method of cooling a boil-off gas stream from a liquefied gas tank according to a first general embodiment of the present invention, and using a single mixed refrigerant (SMR), and comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, and wherein the SMR is provided in an SMR recirculating system.

[0129] In more detail, FIG. 4 shows a BOG stream 20 provided from one or more LNG cargo tanks (not shown) and already compressed in a compressor (also not shown). The BOG stream 20 is optionally ambient cooled in a first ambient heat exchanger 14, using a readily available cooling medium (e.g. seawater, freshwater, engine room cooling water, air) and/or a heat exchanger 12 using a partial stream 32 of the external refrigerant supplied via stream 30 from the refrigerant cascade 13. This optionally cooled (and compressed) BOG stream is then passed into a liquefaction heat exchanger system 62.

[0130] The liquefaction heat exchanger system 62 may comprise any form or arrangement of one or more heat exchangers able to allow heat exchange between two or more streams, optionally between multiple streams, and optionally having at least one stream running countercurrently to one or more other streams in a part or portion of the system, in particular between the BOG stream and one of the refrigerant. Any arrangement of more than one heat exchanger may be in series or in parallel or a combination of in series and in parallel, and the heat exchangers may be separate or conjoined or contiguous, optionally in a single cooled unit or box, and optionally in the form of one or more stages of providing the required heat exchange with the BOG stream to liquefy the BOG stream.

[0131] Liquefaction heat exchanger systems comprising more than one heat exchanger generally have a section, unit or stage being ‘warmer’ than another section, unit or stage, in the sense of the average temperature therein.

[0132] Some variants of suitable liquefaction heat exchanger systems are discussed and shown hereinafter. The skilled person can recognise other variants, and the invention is not limited thereby.

[0133] In the general liquefaction heat exchanger system 62 shown in FIG. 4, the cooled (and compressed) BOG stream is condensed by colder streams, and the condensed BOG stream leaves the exchanger system 62 via pipeline 21, and can be returned to the LNG cargo tanks.

[0134] In the SMR system, an initial stream of SMR refrigerant gas 22 from a refrigerant receiver 1 is sent to an oil-injected screw compressor 2. Oil-injected screw compressors are well known in the art, and not further described herein. Oil-injected screw compressors are well proven in industry and are cost-effective, especially for small scale or small volume compression, but are known to have the disadvantage that some, possibly even microscopic amounts, of the oil can become entrained in the gas passing through the compressor, and thus become a part of the gas discharge therefrom.

[0135] In FIG. 4, compressing the initial SMR stream 22 using the one oil-injected screw compressor 2 provides a post-compression SMR stream 23, which enters a first oil separator 3, optionally having a filter, which separates the post-compression SMR stream 23 to provide an oil-based stream 25 and a first SMR vapour stream 24. Most of the oil is removed in the separator 3 typically by gravity and/or filtration. The recovered oil-based stream 25 is drained into a pipeline where pressure differences or an optional oil pump 4 passes the oil to oil cooler 5 cools the oil, which is then re-injected as stream into compressor 2.

[0136] The first SMR vapour stream 24 is mostly oil-free, but does contain some degree of oil carryover. The first SMR vapour stream 24 is cooled in a second ambient heat exchanger 6 using a readily available cooling medium (e.g. seawater, freshwater, engine room cooling water, air), and further cooled in another cooler 11 using the separate circuit 13 to pass to separator 8. Separator 8 provides vapour stream 26, which passes into the liquefaction heat exchanger 62, as a first refrigerant, where the refrigerant is cooled and at least partially condensed.

[0137] Meanwhile, the vapour—liquid separator 8 provides a bottom liquid-phase SMR stream 29, generally comprising liquid and a residual oil amount. Thereafter, the pressure of the oil-containing liquid-phase SMR stream 29 can be reduced by a flash valve 9, resulting in some vaporisation and an associated reduction in temperature to provide an at least partly vaporised, liquid-phase oil-containing SMR stream 42.

[0138] In FIG. 4, the first refrigerant vapour stream 26 is cooled until it partly or wholly condenses, leaving the heat exchanger system 62 as a condensed SMR stream 27. Thereafter, the pressure is reduced via throttling valve 10, leading to partial vaporisation and temperature reduction to provide the expanded lowest-temperature SMR stream 34. The expanded lowest-temperature SMR stream 34 is the coldest SMR refrigerant stream in the SMR system, having a temperature below the oil-freezing or oil-solidification temperature of the oil in the oil-injected screw compressor 2.

[0139] The expanded lowest-temperature SMR stream 34 is sent back into heat exchanger 62 through entry port 49, where it vaporises as it heats up, and in doing so, cools the warmer streams in the heat exchanger system 62 to provide the majority of the cooling duty. The warmer SMR refrigerant stream can then be withdrawn through port 60 to provide warmer SMR refrigerant stream 52, which is then passed back into the heat exchanger 62 via an entry port 63. Meanwhile, the liquid-phase oil-containing SMR stream 42 also passes into the heat exchanger via a suitable and separate entry port (not shown), and then combined or merged with the warmer SMR refrigerant stream 52 at a suitable location or locations within the heat exchanger 62 to form a single or combined stream 54. The combined stream 54 continues its passage through and out of the heat exchanger system 62 through the warmer exit port 72, leaving as a post-cooling vapour stream 28, to be returned to refrigerant receiver 1.

[0140] FIG. 5 shows a method of cooling a boil-off gas stream from a liquefied gas tank according to a second general embodiment of the present invention, and using a single mixed refrigerant (SMR), and comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, and wherein the SMR is provided in an SMR recirculating system. In comparison with FIG. 4, the method shown in FIG. 5 does not require an external cascade 13. For clarity purposes, not all of the entry and exit ports of the heat exchangers shown in FIGS. 4-7 have been specifically labelled.

[0141] FIG. 5 shows a BOG stream 70 provided from one or more LNG cargo tanks (not shown) and already compressed in a compressor (also not shown). The BOG stream 70 is optionally ambient cooled in a first ambient heat exchanger 64, using a readily available cooling medium (e.g. seawater, freshwater, engine room cooling water, air). This optionally cooled (and compressed) BOG stream 71 is then passed into a liquefaction heat exchanger system 57 (typically a brazed aluminium plate-fin heat exchanger), where the cooled (and compressed) BOG stream 71 is condensed by the colder streams discussed herein before in the SMR recirculating system 101, to leave the exchanger system 57 via pipeline 73, and optionally return back to the LNG cargo tanks.

[0142] In the SMR system 101, an initial stream of SMR refrigerant gas 74 from a refrigerant receiver 51 is sent to an oil-injected screw compressor 65. Oil can become entrained in the gas passing through the compressor, and thus become a part of the gas discharge therefrom.

[0143] In FIG. 5, compressing the initial SMR stream 74 using the one oil-injected screw compressor 65 provides a post-compression SMR stream 75, which enters an oil separator 53, optionally having a filter, which separates the post-compression SMR stream 75 to provide an oil-based stream 76 and a first SMR vapour stream 79. Most of the oil is removed in the separator 53 typically by gravity and/or filtration. The recovered oil-based stream 76 is drained into a pipeline where pressure differences or an optional oil pump 66 passes the oil to stream 77, and an oil cooler 55 cools the oil, which is then re-injected as stream 78 into compressor 65.

[0144] The first SMR vapour stream 79 is mostly oil-free, but does contain some degree of oil carryover. The first SMR vapour stream 79 is cooled in a second ambient heat exchanger 56 using a readily available cooling medium (e.g. seawater, freshwater, engine room cooling water, air) to provide a cooler first vapour stream 80. Depending on the composition and pressure of the refrigerant, as well as on the temperature achieved in the second ambient heat exchanger 56, some condensation of the SMR may start to occur.

[0145] The cooler first vapour stream 80 passes into the liquefaction heat exchanger system 57, where the refrigerant is cooled and at least partially condensed. The temperature to which it is cooled is higher than the solidification temperature of the oil. The cooled first SMR vapour stream 81 is withdrawn at an intermediate temperature along the liquefaction heat exchanger system 57, and enters a vapour—liquid separator 58. In the separator 58, an oil-containing liquid-phase SMR stream 82, generally comprising liquid and a residual oil amount, can be drained via as stream 82.

[0146] In the separator 58, an oil-free (or essentially oil-free) SMR vapour stream 84 is sent into the heat exchanger system 57. In FIG. 5, the oil-free SMR vapour stream 84 enters the heat exchanger system 57 at an intermediate temperature, optionally at a similar temperature to that at the withdrawal of the cooled first SMR vapour stream 81. In the heat exchanger system 57, this oil-free SMR vapour stream 84 is cooled until it partly or wholly condenses, leaving the heat exchanger system 57 as a condensed SMR stream 85. Thereafter, the pressure is reduced via throttling valve 61, leading to partial vaporisation and temperature reduction to provide the expanded lowest-temperature SMR stream 86. The expanded lowest-temperature SMR stream 86 is the coldest SMR refrigerant stream in the SMR system 101, having a temperature below the oil-solidification temperature of the oil in the oil-injected screw compressor 65.

[0147] The expanded lowest-temperature SMR stream 86 is sent back into heat exchanger system 57, where it becomes a warmer SMR refrigerant stream 67 as it heats up, and in doing so, cools the warmer streams in the heat exchanger system 57 to provide the majority of the cooling duty.

[0148] The warmer SMR refrigerant stream 67 can then be withdrawn through a suitable intermediate exit port to provide an external SMR stream 68, and then returned back into the heat exchanger 57 via a suitable entry port at or near the same level or location as the intermediate exit port.

[0149] Meanwhile, the pressure of the oil-containing liquid-phase SMR stream 82 can be reduced by a flash valve 59, resulting in some vaporisation and an associated reduction in temperature. The SMR system 101 is designed such that this lower temperature is still above the solidification temperature of the oil. The expanded stream 83 is passed into the heat exchanger 57 via a suitable inlet port, and then is merged with the returned SMR stream to form a single or combined stream 69 inside the heat exchanger 57. The combined stream 69 continues passage through and out of the heat exchanger system 57, leaving as a post-cooling vapour stream 89, to be returned to refrigerant receiver 51.

[0150] The liquefaction heat exchanger system shown in FIG. 5 may comprise any form or arrangement of one or more heat exchangers able to allow heat exchange between two or more streams, optionally between multiple streams, and optionally having at least one stream running countercurrently to one or more other streams in a part or portion of the system, in particular between the BOG stream and one of the refrigerant. Any arrangement of more than one heat exchanger may be in series or in parallel or a combination of in series and in parallel, and the heat exchangers may be separate or conjoined or contiguous, optionally in a single cooled unit or box, and optionally in the form of one or more stages of providing the required heat exchange with the BOG stream to liquefy the BOG stream.

[0151] Liquefaction heat exchanger systems comprising more than one heat exchanger generally have one section, unit or stage being ‘warmer’ than another section, unit or stage, in the sense of the average temperature therein.

[0152] Variants of suitable liquefaction heat exchanger systems are known, including the liquefaction heat exchanger system comprising two heat exchangers. The skilled person is aware of other variations possible within the scope of the present invention.

[0153] For example, in place of the single liquefaction heat exchanger system shown in FIG. 5, first and second parallel liquefaction heat exchanger systems could be used, each using the method of cooling a BOG stream as per the present invention, and possibly based on splitting either the cooler first vapour stream 80 or the expanded lowest-temperature SMR stream 86 in FIG. 5 or a similar stream, to form split streams for each of the heat exchangers. The first and second liquefaction heat exchanger systems could be the same or similar, i.e. having the same or similar size and/or capacity; but the present invention extends to the first and second liquefaction heat exchanger systems being different, such as having a different size or capacity.

[0154] The benefit of two heat exchanger systems allows the user to better accommodate the heat exchanger systems, especially within a confined or restricted space or spacing on a cargo vessel, and/or to help share the load, loading duty, cooling, cooling duty required, especially where there may be variation thereof due to variants in the amount or nature of the BOG stream to be reliquefied.

[0155] FIG. 6 shows a method of cooling a boil-off gas stream from a liquefied gas tank according to a another embodiment of the present invention, and using a single mixed refrigerant (SMR). FIG. 6 shows a BOG stream 70 provided from one or more LNG cargo tanks, and requiring reliquefaction in a manner described in relation to the first, second and third embodiments of the present invention described herein above.

[0156] FIG. 6 shows two liquefaction and separation systems 110A and 110B. Each system is based on the portion of FIG. 5 labelled 110, and encompassing the liquefaction heat exchanger system 57, the vapour-liquid separator 58, and the streams and pipelines associated therewith within the boundary of 110.

[0157] Thus, FIG. 6 represents the provision of two separate liquefaction and separation systems 110A and 110B.

[0158] FIG. 6 shows division of the cooler first vapour stream 80, and division of the optionally cooled and compressed BOG stream 71, into split BOG stream 71A and 71B, with each of the split streams entering into respective first and second liquefaction and separation systems 110A and 110B. FIG. 6 also shows the resultant liquefied BOG streams 73A and 73B being provided by the first and second liquefaction and separation systems 110A and 110B, which streams can then be combined into a single return BOG stream 73 as described above.

[0159] The temperatures and/or operations of each of the first and second liquefaction and separation systems 110A and 110B can be the same or different to that shown for the system 110 in FIG. 5. The embodiment of the present invention shown in FIG. 6 provides the user with the advantages described herein above, in particular the allowance of some variants in the positioning and/or location of the liquefaction and separation systems, and/or variants in the capacity of each of the liquefaction and separation systems 110A and 110B, typically due to variants in the supply of BOG to be reliquefied.

[0160] The skilled person can see that the present invention can be provided by the use of more than two units, stages, frames etc., such as the use of more than two liquefaction heat exchangers, more than two vapour-liquid separators, etc., so as to provide the most efficient overall method for cooling the BOG to be provided thereto, whilst only requiring the provision of one SMR refrigerant system.

[0161] The present invention is a modification of a refrigerant cycle for BOG cooling, and LNG re-liquefaction in particular, that allows the use of a cost-efficient oil-injected screw compressor in the refrigerant system. The present invention is also able to accommodate the possibility of different flows or flow rates of the first refrigerant stream and the oil-containing refrigerant stream, such that there is reduced or no concern by the user of the process in relation to possible oil freezing and clogging of the heat exchanger caused by variation of the flow or flow rate of the oil-containing refrigerant stream. Furthermore, the need to regularly carry out maintenance of heat exchangers to remove frozen oil, especially in parts of the heat exchanger which are typically the coldest, or at least operating at a temperature below the oil condensation point, is reduced or removed entirely.