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) admixing the first warmer refrigerant stream with an oil-containing refrigerant stream to provide a combined refrigerant stream; (d) passing the combined refrigerant stream into the heat exchanger through an entry port located in a second zone of the heat exchanger that is warmer than the first zone; (e) passing the combined refrigerant stream out of the heat exchanger through the warmer exit port. 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.

Claims

1. A method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising the steps of heat exchanging the BOG stream with the SMR in a single liquefaction heat exchanger in a vertical or near vertical BOG liquefaction heat exchanger, wherein the SMR is provided in an SMR recirculating system comprising 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) (1) separating the first SMR vapour stream to provide an oil-containing liquid-phase SMR stream and a SMR vapour stream, wherein the oil in the oil-containing liquid-phase SMR stream is compressor lubricating oil, and (2) passing the oil-based stream into an oil cooler and reinjecting the oil based stream back into the screw compressor; (d) passing the SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; (e)(1) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream, (2) passing the expanded lowest temperature SMR stream into a first zone of the liquefaction heat exchanger system, and (3) heat exchanging the expanded lowest temperature SMR stream against the BOG stream to provide a warmer SMR stream; (f) withdrawing the warmer SMR stream from the heat exchanger at an intermediate exit port, wherein the intermediate exit port is warmer than the first zone of the heat exchanger; (g) combining the warmer SMR stream with the oil-containing liquid-phase SMR stream to provide a combined refrigerant stream; (h) passing the combined refrigerant stream into the heat exchanger through an entry port located in a second zone of the heat exchanger that is warmer than the first zone; and (i) passing the combined refrigerant stream out of the heat exchanger through the warmer exit port.

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

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

4. A method as claimed in claim 1 wherein the liquefaction heat exchanger system comprises a multi-unit liquefaction heat exchange having at least two parallel heat exchanger units, and the BOG stream and the first refrigerant passes through at least the coldest of the heat exchanger units.

5. A method as claimed in claim 1 wherein the heat exchanger comprises a plate-fin heat exchanger.

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

7. A method as claimed in claim 1 wherein the temperature of the combined refrigerant stream of step (g) is higher than the temperature of the first zone in the heat exchanger.

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

9. A method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising the steps of: heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system between an entry port of the heat exchanger and a warmer exit port of the heat exchanger to provide a cooled BOG stream, wherein the SMR is provided in an SMR recirculating system comprising 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)(1) 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, and (2) passing the oil-based stream into an oil cooler and reinjecting the oil based stream back into the screw compressor; (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) (1) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream, (2) passing the expanded lowest temperature SMR stream into a first zone of the liquefaction heat exchanger system, and (3) heat exchanging the expanded lowest temperature SMR stream against the BOG stream to provide a warmer SMR stream; (h) withdrawing the warmer SMR stream from the liquefaction heat exchanger system at an intermediate exit port within a second zone of the liquefaction heat exchanger, wherein the second zone is warmer than the first zone of the liquefaction heat exchanger; (i) expanding the oil-containing liquid-phase SMR stream of step (e) to provide an at least partially expanded oil-containing refrigerant stream; (j) combining the warmer SMR stream of step (h) with the oil-containing refrigerant stream of step (i) to provide a combined refrigerant stream; (k) 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 (l) passing the combined refrigerant stream out of the liquefaction heat exchanger system through the warmer exit port.

10. A method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising the steps of: heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, providing an SMR recirculating system-comprising 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 an SMR vapour stream; (c) passing the oil-based stream into an oil cooler and reinjecting the oil based stream back into the at least one oil-injected screw compressor; (d) passing the SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; (e) (1) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream, (2) passing the expanded lowest temperature SMR stream into a first zone of the liquefaction heat exchanger system, and (3) heat exchanging the expanded lowest temperature SMR stream against the BOG stream to provide a warmer SMR stream; (f) withdrawing the warmer SMR stream from the heat exchanger at an intermediate exit port that is warmer than the first zone of the heat exchanger; (g) combining the warmer SMR stream with an oil-containing liquid-phase SMR stream to provide a combined refrigerant stream; (h) passing the combined refrigerant stream into the heat exchanger through an entry port located in a second zone of the heat exchanger that is warmer than the first zone; (i) passing the combined refrigerant stream out of the heat exchanger through the warmer exit port.

11. An apparatus for cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising a heat exchanger for heat exchange against the BOG stream operating according to the method of claim 1.

12. An apparatus for cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising a heat exchanger for heat exchange against the BOG stream and a refrigerant system operating according to the method of claim 9.

13. An apparatus for cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising a heat exchanger for heat exchange against the BOG stream and a single mixed refrigerant (SMR) system operating according to the method of claim 10.

14. A method as claimed in claim 1 wherein the heat exchanger comprises a printed circuit heat exchanger.

15. The method as in claim 10 further comprising the steps of: (A) subsequent to step (d) withdrawing the SMR vapour stream from the liquefaction heat exchanger in the second zone to provide a first intermediate SMR vapour stream; (B) separating the first intermediate SMR vapour stream to form the oil-containing liquid-phase SMR stream and a second intermediate SMR vapour stream; and (C) passing the second intermediate SMR vapour stream into the liquefaction heat exchanger at an intermediate position that is cooler than the withdrawal of the SMR vapour stream in step (A), the second intermediate SMR vapour stream becoming the condensed SMR stream of step (d).

16. The method as claimed in claim 9 wherein the BOG stream is from a liquefied cargo tank in a floating vessel.

17. The method as claimed in claim 16 wherein the BOG stream is from a liquefied natural gas (LNG) cargo tank.

18. The method as claimed in claim 9 wherein the heat exchanger comprises a plate-fin heat exchanger.

19. The method as claimed in claim 9 wherein the temperature of the combined refrigerant stream of step (j) is higher than the temperature of the first zone in the heat exchanger.

20. The method as claimed in claim 9 wherein the temperature of the second zone is warmer than the freezing temperature of the oil of the oil-containing refrigerant.

Description

(1) 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:

(2) FIG. 1 is a schematic view of a prior art method of cooling a BOG stream using a prior art SMR system;

(3) FIG. 2 is a simplified schematic view of Area 200 of FIG. 1;

(4) FIG. 3 is a simplified schematic view of part of a method of cooling a BOG stream using an embodiment of the present invention;

(5) FIG. 4 is a schematic view of a method of cooling a BOG stream according to a first embodiment of the present invention;

(6) FIG. 5 is a schematic view of a method of cooling a BOG stream according to a second embodiment of the present invention;

(7) FIG. 6 is a schematic view of a method of cooling a BOG stream according to a third embodiment of the present invention; and

(8) FIG. 7 is a schematic view of a method of cooling a BOG stream according to a fourth embodiment of the present invention.

(9) Where relevant, the same reference numerals are used in different Figures to represent the same or similar feature.

(10) 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.

(11) 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.

(12) 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.

(13) 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.

(14) ‘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.

(15) 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.

(16) 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.

(17) 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 stream 52, to be merged externally with a oil-containing refrigerant stream 42, so as to form a combined refrigerant stream 54. This combined stream 54 is re-injected into heat exchanger 50 through an adjacent inlet or entry port 63 in zone B, to continue 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.

(18) 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), that oil cannot fall down into ‘zone A’ of the heat exchanger 50, where it would freeze.

(19) 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.

(20) 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.

(21) 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.

(22) 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.

(23) 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.

(24) 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.

(25) 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.

(26) 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.

(27) 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.

(28) 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.

(29) 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.

(30) 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.

(31) 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 stream 52, merged with the liquid-phase oil-containing SMR stream 42 to form a single or combined stream 54 outside of the heat exchanger 62. The combined stream 54 then enters the heat exchanger 62 through entry port 63, to continue 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.

(32) 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.

(33) 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.

(34) 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.

(35) 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.

(36) 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.

(37) 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.

(38) 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.

(39) 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.

(40) The warmer SMR refrigerant stream 67 can then be withdrawn through a suitable intermediate exit port to provide an external SMR stream 68.

(41) 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 merged with external SMR stream 68 to form a single or combined stream 69 outside of the heat exchanger 57. The combined stream 69 then enters the heat exchanger 57 through a suitable entry port, to continue 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.

(42) 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.

(43) 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.

(44) 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.

(45) As one example of a variant, FIG. 6 shows a method of cooling a boil-off gas stream from a liquefied gas tank according to a third embodiment of the present invention, much of which can be derived from the example of the second embodiment of the present invention shown in FIG. 5 and described in relation thereto hereinabove.

(46) In the same way as FIG. 5, a BOG stream 70 is provided from one or more LNG cargo tanks (not shown) and already compressed in a compressor (also not shown). Similarly, there is an SMR system 102, having an initial stream of SMR refrigerant gas 74 from a refrigerant receiver 51 sent through an oil-injected screw compressor 65 to provide a post-compression SMR stream 75, which can be separated and cooled to provide a cooler first vapour stream 80.

(47) In place of the single liquefaction heat exchanger system shown in FIG. 5, FIG. 6 shows a first liquefaction heat exchanger system 57A, and a second liquefaction heat exchanger system 57B. Optionally, the first and second liquefaction heat exchanger systems 57A and 57B are 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 57A and 57B being different, such as having a different size or capacity.

(48) FIG. 6 also shows the division of the optionally cooled BOG stream 71 into first and second cooled BOG streams 71A and 71B, and division of the cooler first vapour stream 80 into first and second cooler first vapour streams 80A and 80B. Each of these divisions can be carried out by a suitable splitting devices, valves or other units (not shown) known in the art.

(49) Optionally, the cooled BOG stream 71 and the cooler first vapour stream 80 are divided in a ratio that is comparable with the ratio of the size and/or capacity of the first and second liquefaction heat exchanger systems 57A and 57B in a respective and expected manner. For example, the ratio is 50:50 to create two equal streams respectively. Optionally, there can be variants thereof based on other ratios.

(50) FIG. 6 shows each of the first and second cooled BOG streams 71A and 71B passing through the respective liquefaction heat exchanger system 57A and 57B, whereby the cooled BOG streams 71A and 71B are condensed by the colder streams discussed herein in the SMR recirculating system 102, to leave the heat exchanger systems 57A and 57B as exit streams 73A and 73B respectively, to then be combined as a single return BOG stream 73, optionally returned back to the LNG cargo tank(s).

(51) FIG. 6 shows each of the cooler first vapour streams 80A and 80B passing into the respective first and second liquefaction heat exchanger systems 57A and 57B, wherein each refrigerant partial stream is cooled and at least partially condensed to a temperature which is higher than the solidification or freezing temperature of the oil. The cooled first SMR vapour streams 81A and 81B are withdrawn at an intermediate temperature along the liquefaction heat exchanger systems 57A and 57B respectively, to be combined to form a single cooled first SMR vapour stream 81, to then enter a vapour-liquid separator 58 in a manner shown and described in FIG. 5. The separator 58 provides an oil-containing liquid-phase SMR stream 82, generally comprising liquid and a residual oil amount, and an oil-free SMR vapour stream 84, which is then divided into part oil-free SMR vapour streams 84A and 84B to pass back into the respective first and second heat exchanger systems 57A and 57B at an intermediate temperature, optionally at a similar temperature to that at the withdrawal of the cooled first SMR vapour streams 81A and 81B.

(52) In the heat exchanger systems 57A and 57B, the oil-free SMR vapour streams 84A and 84B are cooled until they partly or wholly condense, leaving the heat exchanger systems 57A and 57B as condensed SMR streams 85A and 85B respectively. Thereafter, these streams can be combined to form a single condensed SMR stream 85, whose pressure is reduced via throttling a valve 61, to provide the lowest-temperature SMR stream 86. The lowest-temperature stream 86 is then divided into partial streams 86A and 86B, for return into the first and second heat exchanger systems 57A and 57B respectively, where they become warmer SMR refrigerant streams 67A and 67B respectively, thereby cooling the warmer streams in the heat exchanger systems 57A and 57B to provide the majority of the cooling duty therein.

(53) The warmer SMR refrigerant streams 67A and 67B can be withdrawn through suitable intermediate exit ports to provide first and second external SMR streams 68A and 68B, which can be combined into a single external SMR stream 68, to then be combined with an expanded stream 83 from the oil-containing liquid-phase SMR stream 82, to provide a combined stream 69 outside of the heat exchangers 57A and 57B. This combined stream 69 can then be divided into first and second streams 69A and 69B for return into the heat exchanger systems 57A and 57B respectively, leaving as post-cooling vapour streams 89A and 89B, which can be combined into a single return stream 89 to be returned to the refrigerant receiver 51.

(54) The ratio of each division or split of streams described above may be the same or different to the initial ratio division of the cooled BOG stream 71 and the cooler first vapour stream 80 described above.

(55) The temperatures of each of the streams described in relation to FIG. 6 can be wholly or substantially the same as those described in relation to FIG. 5.

(56) The benefit of FIG. 6 is to provide two heat exchanger systems 57A and 57B, thereby allowing 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.

(57) FIG. 7 shows a method of cooling a boil-off gas stream from a liquefied gas tank according to a fourth embodiment of the present invention, and using a single mixed refrigerant (SMR). FIG. 7 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.

(58) FIG. 7 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.

(59) Thus, FIG. 7 represents the provision of two separate liquefaction and separation systems 110A and 110B, which can be differentiated from the third embodiment shown in FIG. 6 which still uses a single vapour-liquid separator 58 to separate the cooled first SMR vapour stream 81 into a single oil-containing liquid-phase SMR stream 82 and an oil-free SMR vapour stream 84.

(60) FIG. 7 shows division of the cooler first vapour stream 80 in the same manner as shown in FIG. 6, 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. 7 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.

(61) 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. 7 provides the user with the advantages described in relation to the embodiment shown in FIG. 6, 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.

(62) 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.

(63) 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.