Method of re-liquefying transported carbon dioxide

Abstract

A method of cooling and reliquefying a boil-off gas (BOG) stream 901 from a liquefied carbon dioxide cargo stored in a tank or tanks 101 on a floating transportation vessel, compring: compressing the

Description

FIELD OF INVENTION The present invention relates to a method of re-liquefying transported carbon dioxide. BACKGROUND TO THE INVENTION 5 In general, carbon dioxide is liquefied and then transported to destinations by carriers while stored in tanks. At present the number of existing liquefied CO2 (LCO2) carriers is small and the transportation vessels / ships are also small with liquid / liquefied CO2 being transported as a pure product for the food and beverage industries. During transportation, liquefied carbon dioxide is partially vaporized due to external heat 10 and generates boil-off gas (BOG). However, simply discharging BOG into the atmosphere is not desirable for economic and environmental reasons. Capturing carbon dioxide from external emitters, transporting larger quantities of liquefied carbon dioxide and reliquefying of the BOG is preferred for economic and environmental reasons. 15 SUMMARY OF THE INVENTION The present invention provides a method of cooling and reliquefying a boil off gas (BOG) stream from a liquefied carbon dioxide cargo stored in a tank or tanks on a floating transportation vessel, wherein the method comprises at least the steps of: compressing a BOG stream from the liquefied carbon dioxide cargo in one or more stages of BOG compression to provide a compressed BOG discharge stream; cooling the compressed BOG discharge stream against a carbon dioxide refrigerant stream to provide a cooled compressed BOG stream; and a warmer carbon dioxide refrigerant stream; compressing the warmer carbon dioxide refrigerant stream in one or more stages of refrigerant compression to provide a compressed carbon dioxide refrigerant discharge stream for subsequent use as the carbon dioxide refrigerant stream, which is operable to cool and at least partially condense the compressed BOG discharge stream; wherein the one or more stages of BOG compression and the one or more stages of refrigerant compression are provided by a single shaft multi-stage compression unit. The method may comprise compressing the BOG stream from the liquefied carbon dioxide cargo in two or more stages of BOG compression comprising at least a first stage and a final stage to provide the compressed BOG discharge stream. The method may comprise compressing the warmer carbon dioxide refrigerant stream in two or more stages of refrigerant compression comprising at least a first stage and a final stage to provide the compressed carbon dioxide refrigerant discharge stream for subsequent use as the carbon dioxide refrigerant stream, which is operable to cool and at least partially condense the compressed BOG discharge stream. The CO2 composition of the cargo CO2 and the refrigerant CO2 can influence the number of compression stages in each loop providing the compressed BOG discharge stream and a refrigerant loop providing the compressed carbon dioxide refrigerant discharge stream for subsequent use as the carbon dioxide refrigerant stream, which is operable to cool and at least partially condense the compressed BOG discharge stream. The method may further comprise cooling the compressed BOG streams after each stage of compression. The method may comprise cooling the carbon dioxide refrigerant streams after each stage of compression. The method may further comprise a reduction in pressure of the cooled compressed BOG stream prior to returning the cooled compressed BOG stream as re-liquefied carbon dioxide to the tank or tanks. The method may further comprise a reduction in pressure of the cooled compressed refrigerant stream prior to cooling the compressed BOG discharge stream against the carbon dioxide refrigerant stream. The method may further comprise further cooling the warmer carbon dioxide refrigerant stream, thereby reducing the temperature of the warmer carbon dioxide refrigerant stream before the step of compressing the warmer carbon dioxide refrigerant stream in one or more stages of refrigerant compression. The method may comprise topping-up the refrigerant stream. Topping up the refrigerant stream may include CO2 from the BOG stream being added directly to the refrigerant stream. Alternatively, or in addition, topping up the refrigerant stream may include CO2 from the BOG stream being added indirectly to the refrigerant 5 stream. Directly adding CO2 from the BOG stream to the CO2 refrigerant stream may be via oneway control and conduits fluidly connecting the BOG stream and the refrigerant stream, wherein CO2 from the BOG stream is added to the refrigerant stream by opening the valve in the event that CO2 from the refrigerant stream is lost. 10 Indirectly adding CO2 from the BOG stream to the CO2 refrigerant stream may comprise extracting CO2 from the BOG stream and extracting CO2 from the refrigerant stream and blending the CO2 extracted from the BOG stream with the CO2 extracted from the refrigerant stream returning a blended CO2 stream to the refrigerant stream in the event that CO2 from the refrigerant stream is lost. 15 The CO2 extracted from the BOG stream may comprise CO2 directly extracted from the tank or tanks. The method may include returning a blended vapour CO2 stream and a blended liquid CO2 stream to the refrigerant stream. The method may include reducing pressure of the extracted CO2 refrigerant stream. The method may include cooling of the returned blended vapour CO2 stream.The method may include reducing pressure of the blended liquid CO2 stream. The method may further comprise removing non-condensable materials from the cooled compressed BOG stream. The method may further comprise further cooling the carbon dioxide refrigerant stream using residual low temperature from the BOG stream from the liquefied carbon dioxide cargo. The present invention further provides a system for cooling and reliquefying a boil off gas (BOG) stream from a liquefied carbon dioxide cargo stored in tank or tanks on a floating transportation vessel, wherein the system comprises a single shaft multi-stage compression unit comprising at least two independent stages of compression, wherein at least one of the two independent stages of compression compress a BOG stream from the liquefied carbon dioxide cargo and at least one of the two independent stages of compression compress a carbon dioxide refrigerant stream to provide a compressed carbon dioxide refrigerant discharge stream for use as a carbon dioxide refrigerant stream, which is operable to cool and at least partially condense the compressed BOG discharge stream; and wherein the system includes conduits that configure the system in an open CO2 cargo loop and a closed CO2 refrigerant loop. The system may include stacked pistons. The system may further comprise a connection to a BOG sequestration facility, wherein the system is configurable to prevent flow around the CO2 cargo loop and to prevent flow around the CO2 refrigerant loop and to permit flow of the BOG stream from the tank, through an open section of the CO2 cargo loop and through an open section of the CO2 refrigerant loop such that the BOG stream passes through the BOG compression stage or stages and the refrigerant compression stage or stages before exiting the 5 system via the connection to the sequestration facility. DRAWINGS Below, embodiments of the invention are described by way of example only, and with reference to the accompanying drawings in which: Fig. 1 shows a schematic diagram of a system of cooling, particularly re-liquefying, boil 10 off gas from a liquefied carbon dioxide cargo in a floating transportation vessel using a single shaft multi-stage compression unit; Fig. 1A shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied carbon dioxide cargo in a floating transportation vessel using a single shaft multi-stage compression unit and including a blending facility for topping up 15 refrigerant; Fig. 1B shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied carbon dioxide cargo in a floating transportation vessel using a single shaft multi-stage compression unit comprising two stage compression of BOG and a single stage compression of CO2 refrigerant; Fig. 1C shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied carbon dioxide cargo in a floating transportation vessel using a single shaft multi-stage compression unit comprising a single stage compression of BOG and two stage compression of CO2 refrigerant; 5 Fig. 2 shows a schematic diagram of a CO2 circuit / loop of Fig. 1, where CO2 BOG from the cargo / storage tank circulates, is reliquefied and returned to the cargo / storage tank; Fig. 3 shows a schematic diagram of the CO2 refrigerant circuit / loop of Fig. 1, where CO2 refrigerant circulates and provides cooling of the CO2 BOG gas before the CO2 BOG gas is returned; 10 Fig. 4A shows a representation of a single shaft, four stage compressor unit as used with the system illustrated in Fig. 1, Fig, 1A; Fig. 4B shows a representation of a single shaft, three stage compressor unit used with the system illustrated in Fig. 1B and 1C; and Fig. 5 shows an example of an indirect carbon dioxide refrigerant top-up system for use 15 with the system of Fig. 1A and 1C; Fig. 6 shows the schematic diagram of Fig. 1C including a condensate accumulator and associated pipework; Fig. 7 shows the schematic diagram of Fig. 1C including an additional heat exchanger in the refrigerant loop; and Fig. 8 shows the schematic diagram of Fig. 1C configured to send BOG directly to a BOG sequestration facility (permanent storage facility). DESCRIPTION The optimum way to transport large quantities of CO2 typically in quantities of 7,500 m3 or greater is in liquid / liquefied form at around 7-8 bar g and -50°C i.e., in a storage tank under low pressure and at low temperature. This low' pressure cargo tank option is economical for quantities in excess of 20,000 tonnes. The method and apparatus described below and with reference to the drawings seeks to provide an improved method and apparatus of re-liquefying CO2 boil of gas (BOG) whilst a large cargo of CO2 is being transported. An example of the apparatus and the method according to the present invention is illustrated in the drawings listed above and described more fully below. Fig. 1, 1A, 1B and 1C each represent a liquefied CO2 cargo storage tank 101 in a floating transportation vessel together with a multi-stage compression unit 10 (see Fig. 4A and 4B) that is illustrated schematically together with various cooling stages. The term multistage compression unit relates to a single compressor device 10, which in the present examples include a single shaft 10C, and four compression stages 102, 103, 106 and 107 (see Fig. 4A) or a single shaft 10C and three compression stages 102, 103, 106 (see Fig. 4B). The compressors and corresponding compression stages of the reliquefaction method described below are referenced 102, 103, 106, 107 in the figures to correspond with the compressor units of Fig. 4A and 4B. For each of the examples described, the method of CO2 reliquefaction, is arranged in 5 two distinct circuits / loops i.e., an open CO2 cargo loop (see 10A in Fig. 2) and a closed CO2 refrigerant loop (see 10B in Fig. 3). The liquefied CO2 cargo 101 may be pure CO2 (as used in the food and beverage industries), but most likely will be a composition of CO2 and other components e.g. H2O, N2, O2 etc. i.e., CO2 rich gas (for example >95 mol% of CO2). In this regard, the liquefied 10 cargo CO2 101 may include captured CO2, e.g., from various emitters, for environmental reasons. The composition of the CO2 rich gas will be dependent on the source of the gas e.g., captured gas. Examples of CO2rich gas specifications are provided in table 1 below. The composition of the boil-offgas 901 may differ from the liquefied CO2 cargo 101 due 15 to the nature of the components Table 1 Specification of CO2 CO2 Balance >95% >95% >95% H2O £30ppm <70ppm £30ppm £70ppm S30ppm N2 <2.4% <2.4% <2.4% O2 <10ppm <40ppm S10ppm <40ppm <10ppm H2 s500ppm <7500ppm ^50ppm <0.75ppm ^50ppm Table 1 (continued) Specification of CO2 Ar <0.4% <0.4% <0.4% CH4 < 1% <1% <1% CO <1200ppm s750ppm SlOOppm 2750ppm slOOppm O2 + N2 + H2 + Ar + CH4 + CO <2000ppm <4000ppm <4% NOx 51.5ppm S2.5ppm s10ppm S5ppm <10ppm In the illustrated examples, the storage / cargo tank 101 is connected by suitable pipework to a multi-stage compression unit. In the compression unit, each compression 5 stage 102, 103, 106, 107 is independent. Referring to Fig.1, the system as illustrated is arranged in two loops i.e., cargo loop and refrigerant loop (See Fig. 2 and Fig. 3) where two compression stages 102, 103 operate in the cargo loop and two compression stages 106, 107 operate in the refrigerant loop. In this example, both the cargo and the refrigerant loop contain a CO2 rich gas as the 10 circulating medium. The cargo loop involves circulating CO2boil offgas (BOG) stream 901 from the storage tank 101 and returning re-liquefied CO2 908 to the storage tank 101 to maintain the cargo pressure. In the cargo loop the BOG undergoes two stage compression from around 7 to 8 bar g 15 to 40 to 50.0 bar g. The refrigeration loop involves closed-loop circulation of CO2 (see Fig. 3), which undergoes two-stage compression from around 20.0 bar g to 140 - 150 bar g to provide a cold CO2 refrigerant stream that facilitates condensing BOG 906 before the reliquefied CO2 stream 908 returns to the storage / cargo tank 101. Referring first to the cargo loop (10A in Fig. 2), the CO2 BOG stream 901 (evaporation from the storage tank 101) passes through a two-stage compressor arrangement, where the BOG stream 901 is inducted into and compressed by a first (low pressure) compressor 102 to increase the pressure e.g. from around 7-8 bar g to 16 - 20 bar g, thereby delivering a modified CO2gas stream 902 to a first cooler / heat exchanger i.e. precooling heat exchanger 111. In this example, the CO2 gas stream 902 is cooled against a cooling stream of, for example, seawater, freshwater, air etc. The cooled gas stream 903 exits the heat exchanger 111 then undergoes a second (high pressure) compression stage via compressor 103, to further increase the pressure of the CO2 BOG stream 904 to around 40 to 50 bar g. The modified CO2BOG stream 904 exits the compressor 103 and passes through a second cooler / heat exchanger i.e., aftercooler heat exchanger 112 where the BOG stream 904 is cooled against a cooling stream, for example, seawater, freshwater, air etc. to produce a modified BOG stream 905. The CO2 BOG gas stream 905 exits the aftercooler 112 and passes through a discharge heat exchanger 104 (connected to the refrigerant loop 10B), where the discharge heat exchanger 104 cools the CO2 BOG stream 905 against a CO2 refrigerant stream 918. In Fig. 2 the heat exchanger 104 is shown to illustrate the cooling stage, but the heat exchanger 104 forms part of the CO2 refrigerant loop (10B in Fig. 3) and facilitates cooling and at least partial condensation of the BOG gas stream 906 in the cargo loop. In the cargo loop, a cooled and partially / wholly condensed CO2 stream 907 exits the discharge heat exchanger 104 and returns to the cargo tank 101 via valve 109 as reliquefied CO2 stream 908. The valve 109 maintains BOG gas stream 907 at the target pressure of 40 - 50 bar g and has the effect of reducing the pressure in stream 908 such that stream 908 will be at approximately tank pressure, subject to pressure drops in the piping system. Referring to the refrigeration loop (see Fig. 1 and Fig. 3), conventionally there would be a system for the BOG stream and a separate system for the refrigerant, but as illustrated and described above processing the BOG stream 901 to 908 (cargo loop) and processing the refrigerant stream 911 to 918 (refrigerant loop) is conducted using a single shaft multi-stage compressor unit. The refrigerant loop is a closed circulation loop containing substantially pure CO2. In the illustrated example, the refrigerant loop includes a valve 110 that facilitates topping-up the refrigerant loop directly from the CO2 cargo loop if there is a loss of refrigerant from the refrigerant loop. Top-up could be indirect (as discussed further below, with reference to Fig. 1A, 1C and 5)- Similar to the cargo loop, the refrigerant loop in Fig. 1 includes two-stage compression, which includes a first (low pressure) compressor 106 and a second (high pressure) compressor 107 to compress a CO2 refrigerant stream 912 from 20.0 bar g to 140 - 150 bar g. First stage compression compressed the CO2 refrigerant stream from 20 bar g to around 40 to 50 bar g. CO2 refrigerant gas 913 exiting the first compressor 106 passes through a first cooler / heat exchanger i.e. precooling heat exchanger 113. In this example, the CO2 gas stream 913 is cooled against a cooling stream of, for example, seawater, freshwater, air etc. The cooled CO2 refrigerant stream 914 exits the heat exchanger 113 then undergoes the second (high pressure) compression stage via compressor 107, to further increase the pressure of the CO2 refrigerant stream 914, 915 from around 40 to 50 bar g to around 140-150 bar g. The high-pressure CO2 refrigerant stream 915 exits the compressor 107 and passes through a second cooler / heat exchanger i.e., aftercooler heat exchanger 108 where the refrigerant stream 915 is cooled against a cooling stream, for example, seawater, freshwater, air etc. to produce a modified BOG stream 905. The aftercooler heat exchanger 108 may also partially condense the refrigerant stream 915 to provide the discharge refrigerant stream 916. The discharge refrigerant stream 916 is further cooled by heat exchanger 105, which uses residual low temperature from a returning heated refrigerant stream 911 (see below to provide refrigerant stream 917. Pressure of the CO2 refrigerant stream 917 exiting the aftercooler 108 is reduced over valve 114 to around 20.0 bar g before the resulting CO2 refrigerant stream 918 provides the cooling medium, via heat exchanger 104 for the CO2 cargo stream 906. The returning heated CO2 refrigerant 911 is the refrigerant that exits the heat exchanger 104 due to heat transfer between the CO2 cargo stream 906 and the CO2 refrigerant stream 918. The final stage of the refrigerant loop is where the heated CO2 refrigerant stream 911 is further heated by the heat exchanger 105, where the refrigerant stream 916 is cooled to normalize the temperature of the CO2 refrigerant. After which, further heated CO2 refrigerant stream 912 exits the heat exchanger 105 and passes to the first compression stage 103 thereby continuing the refrigerant cycle / loop. The heat exchanger 105 improves overall system efficiency by cooling the CO2 refrigerant stream 918 such that the cooled CO2 refrigerant stream facilitates cooling the CO2 cargo stream 906 to produce the partially / wholly condensed CO2 stream 907 that exits the discharge heat exchanger 104.The heat exchanger 105 also improves efficiency of the refrigerant loop by ensuring the temperature of the CO2 at the inlet of valve 114 is optimised. The heat exchangers 111, 112, 113, 108 using for example seawater, freshwater or air, may be any type of suitable heat exchanger, for example a shell and tube heat exchanger or a plate-type heat exchanger. For high pressures, a printed circuit heat exchanger (PCHE) may also be viable. The CO2 heat exchangers 104, 105 may be any type of suitable heat exchanger, for example, a shell and tube heat exchanger, a plate-type heat exchanger, a PCHE, a plate and fin type heat exchanger etc. In the illustrated example, the pipework / conduits of the cargo loop and the refrigerant loop are independent with the exception of a branch 909, 910, which puts both loops in fluid communication via the valve 110. The valve 110, when open, facilitates topping up the CO2 refrigerant loop with CO2 directly from the cargo loop. Using the CO2 from the cargo / storage tank is beneficial. However, as noted above the cargo LCO2 may be CO2 rich, which means that the BOG stream / condensate 901 from the cargo / storage tank 101 may be rich in volatile components and therefore may not be suitable for direct top-up of the refrigerant loop. As such, an alternative source of CO2 for top-up purposes is illustrated in Fig. 1A, Fig. 1C and Fig. 5. Fig. 1A illustrates the system of Fig. 1 with the addition of an example of an indirect top-up system (referenced as 1000 in Fig. 5). The illustrated example also includes additional energy recovery via a heat exchanger 116. This ensures the temperature of the refrigerant CO2 within the system, as modified by the top-up, is optimised. Referring to Fig. 1A and Fig. 5, the indirect top-up system 1000, includes a receiver vessel 115. The receiver vessel 115 is configured to receive (and combine) clean / pure CO2 refrigerant 917 from the refrigerant loop and CO2 rich cargo 920 directly from the cargo / storage vessel 101. The resulting blended / combined CO2 contained in the receiver vessel 115 is therefore less rich in volatile components than the CO2 in the cargo loop and less pure than the CO2 in the refrigerant loop. In this example, the receiver vessel 115 feeds two blended CO2 refrigerant streams, i.e., a liquid stream 919 and a vapour stream 921 into the CO2 refrigerant loop. The first blended (liquid) CO2 stream 919 exits the vessel 115 and flows through the valve 114 where pressure in the CO2 stream 919 may be reduced and a CO2 refrigerant stream 918 exits the valve 114 as the cooling medium for the condenser / evaporator 104. The second blended (vapour) CO2 stream 921 exits the vessel 115 to the heat exchanger 116 and exits the heat exchanger 116 as CO2 refrigerant stream 922. The heat exchanger 116 acts to further cool the refrigerant stream 916 (exit stream from aftercooler 108) to normalise the temperature of the CO2 refrigerant as exit stream 916a. The stream 916a enters the heat exchanger 105 and exits as cooled refrigerant stream 916b. The pressure of the cooled refrigerant stream 916b is reduced to the level of the last compression stage (915) via valve 117 to feed clean / pure CO2 refrigerant 917 directly from the refrigerant loop to the receiver vessel 115. The CO2 refrigerant stream 922 exiting the heat exchanger 116, blends with the cooled CO2 refrigerant stream 914 exiting the heat exchanger 113. The blended / combined CO2 streams 922 and 914 produce CO2 refrigerant stream 914a which undergoes the second (high pressure) compression stage via compressor 107, to increase the pressure of CO2 refrigerant stream 914a entering the compressor 107 to provide an increased pressure exit CO2 refrigerant stream 915 at around 140 - 150 bar g. The high-pressure CO2 refrigerant stream 915 exits the compressor 107 and passes through the second cooler / heat exchanger i.e., aftercooler heat exchanger 108 where the refrigerant stream 915 is cooled against a cooling stream, for example, seawater, freshwater, air etc. to produce the discharge refrigerant stream 916. The aftercooler heat exchanger 108 may also partially condense the refrigerant stream 915 to provide the discharge refrigerant stream 916. As with Fig. 1 above, the discharge refrigerant stream 916 is further cooled by heat exchanger 105, which uses residual low temperature from a returning heated refrigerant stream 911 (see below) to provide condensed refrigerant stream 917. Pressure of the condensed CO2 refrigerant stream 917 exiting the receiving vessel 115 is reduced over valve 114 to around 20.0 bar g before the resulting CO2 refrigerant stream 918 provides the cooling medium, via heat exchanger 104 for the CO2 cargo stream 906. The returning heated CO2 refrigerant 911 is the refrigerant that exits the heat exchanger 104 due to heat transfer between the CO2 cargo stream 906 and the CO2 refrigerant stream 918. The final stage of the refrigerant loop is where the heated CO2 refrigerant stream 911 is further heated by the heat exchanger 105, where the refrigerant stream 916 is cooled to normalize the temperature of the CO2 refrigerant. After which, further heated CO2 refrigerant stream 912 exits the heat exchanger 105 and passes to the first compression stage 103 thereby continuing the refrigerant cycle / loop. The heat exchanger 105 improves the efficiency of the refrigerant loop 10B by ensuring the temperature of the CO2 at the inlet of valve 114 is optimised. The heat exchangers 111, 112, 113, 108 using for example seawater, freshwater or air, may be any type of suitable heat exchanger, for example a shell and tube heat exchanger or a plate-type heat exchanger. For high pressures, a printed circuit heat exchanger (PCHE) may also be viable. The CO2 heat exchangers 104, 105, 116 may be any type of suitable heat exchanger, for example, a shell and tube heat exchanger, a plate-type heat exchanger, a PCHE, a plate and fin type heat exchanger etc. Referring to Fig. 1B, in the illustrated example the storage / cargo tank 101 is connected by suitable pipework to a multi-stage compression unit. In this example, the compression unit includes three stages of compression, where each stage 102, 103, 106 is independent. Similar to Fig. 1 and 1A, as noted above, the system as illustrated is arranged in two loops i.e., cargo loop and refrigerant loop where two compression stages 102, 103 operate in the cargo loop and a single compression stage 106 operates in the refrigerant loop. Both the cargo and the refrigerant loops contain CO2 as the circulating medium. In this example the cargo loop involves circulating CO2 boil offgas (BOG) as described above with reference to Fig. 1, Fig. 1A and Fig. 2 i.e., two compression stages are used. The cargo loop involves circulating CO2boil offgas (BOG) stream 901 from the storage tank 101 and returning re-liquefied CO2 908 to the storage tank 101 to maintain the cargo pressure. In this example, the CO2 BOG undergoes two-stage compression from around 7 to 8 bar g to 40 to 50 bar g. The refrigeration loop involves closed-loop circulation of CO2 (see Fig. 3), which undergoes single stage compression from around 20.0 bar g to 90.0 bar g to provide a cold CO2 stream 911 that is used to condense BOG 906 before the re-liquefied CO2 stream 908 returns to the storage / cargo tank 101. Referring first to the cargo loop, the CO2 BOG stream 901 (evaporation from the storage tank 101) passes through a two-stage compressor arrangement, where the BOG stream 901 is inducted into and compressed by a first (low pressure) compressor 102 to increase the pressure e.g. from around 7-8 bar g to 16 - 20 bar g. From there, a modified CO2gas stream 902 exits the first compressor 102 towards the first cooler / heat exchanger i.e. precooling heat exchanger 111. In this example, the CO2 gas stream 902 is cooled against a cooling stream of, for example, seawater, freshwater, air etc. The cooled gas stream 903 exits the heat exchanger 111 then undergoes a second (high pressure) compression stage via compressor 103, to further increase the pressure of the CO2 BOG stream 904 to around 40 to 50 bar g. The modified CO2 BOG stream 904 exits the compressor 103 and passes through a second cooler / heat exchanger i.e., aftercooler heat exchanger 112 where the BOG stream 904 is cooled against a cooling stream, for example, seawater, freshwater, air etc. to produce a modified BOG stream 905. The CO2 BOG gas stream 905 exits the aftercooler 112 and passes through a discharge heat exchanger 104 (connected to the refrigerant loop), where the discharge heat exchanger 104 coois the CO2 BOG stream 905 against a CO2 refrigerant stream 918. In the cargo loop, a partially / wholly condensed CO2 stream 907 exits the discharge heat exchanger 104 and returns to the cargo tank 101 via valve 109 as reliquefied CO2 stream 908. The valve 109 maintains BOG gas stream 907 at the target pressure of 40 - 50 bar g and has the effect of reducing the pressure in stream 908. Stream 908 will be at approximately tank pressure, subject to pressure drops in the piping system. In this example, the refrigeration loop involves closed-loop circulation of CO2 refrigerant, which undergoes compression from around 20.0 bar g to 90.0 bar g in a single stage compression to provide a cold CO2 stream 918 that is used to condense BOG 906 before the re-liquefied CO2 stream 908 returns to the storage / cargo tank 101. As noted above, with reference to Fig. 1 and Fig. 3, conventionally there would be a system for the BOG stream and a separate system for the refrigerant, but, as illustrated in Fig. 1B and described herein, processing the BOG stream 901 to 908 (via an open cargo loop) and processing refrigerant stream 911 to 918 (via a closed refrigerant loop) is conducted using a single shaft multi-stage compressor unit. In this example, the multistage compressor unit includes three independent stages of compression 102, 103, 106. The refrigerant loop is a closed circulation loop containing substantially pure CO2. In this example, the refrigerant loop includes single stage compression, which includes a single compressor 106 to compress a CO2 refrigerant stream 912 from 20.0 bar g to 90.0 bar g. CO2 refrigerant gas 915 exiting the compressor 106 passes over a cooler / heat exchanger i.e., aftercooler heat exchanger 108 where the refrigerant stream 915 is cooled against a cooling stream, for example, seawater, freshwater, air etc. to produce refrigerant stream 916. As with Fig. 1 above, the discharge refrigerant stream 916 is further cooled by heat exchanger 105, which uses residual low temperature from a returning heated refrigerant stream 911 to further cool the refrigerant stream 916 to provide cooled refrigerant stream 917. Pressure of the cooled CO2 refrigerant stream 917 exiting the aftercooler 105 is reduced over valve 114 to around 20.0 bar g before the resulting CO2 refrigerant stream 918 provides the cooling medium, via heat exchanger 104 for the CO2 cargo stream 906. The returning heated CO2 refrigerant 911 is the refrigerant that exits the heat exchanger 104 due to heat transfer between the CO2 cargo stream 906 and the CO2 refrigerant stream 918. The final stage of the refrigerant loop is where the heated CO2 refrigerant stream 911 is further heated by the heat exchanger 105, where the refrigerant stream 916 is cooled to normalize the temperature of the CO2 refrigerant. After which, further heated CO2 refrigerant stream 912 exits the heat exchanger 105 and passes to the first compression stage 106 thereby continuing the refrigerant cycle / loop. Referring to Fig. 1C, in the illustrated example the storage / cargo tank 101 is connected by suitable pipework to a multi-stage compression unit. In this example, the multi-stage compression unit includes three independent compression stages 102, 106, 107. Similar to Fig. 1, 1A and 18, the system as illustrated in Fig. 1C is arranged in two loops i.e., a cargo CO2 loop and a refrigerant CO2 loop (for illustration purposes see Fig. 2 and Fig. 3). In this example, a single compression stage 102 operates in the cargo CO2 loop and two compression stages 106, 107 operate in the refrigerant CO2 loop. In this example, both the cargo and the refrigerant loops contain CO2 as the circulating medium. The cargo loop involves circulating CO2boil offgas (BOG) stream 901 from the storage tank 101 and returning re-liquefied CO2 908 to the storage tank 101 to maintain the cargo volume. The CO2 refrigeration loop involves closed-loop circulation of CO2 (for illustration purposes see Fig. 3), which undergoes two stage compression from around 20.0 bar g to 140 - 150 bar g to provide a cold CO2 stream that is used to condense BOG 906 before the re-liquefied CO2 stream 908 returns to the storage / cargo tank 101. In this example, the CO2 BOG stream 901 (evaporation from the storage tank 101) passes through a single stage compressor arrangement, where the BOG stream 901 is inducted into and compressed by the compressor 102 to increase the pressure e.g. from around 7-8 bar g to 20 - 30 bar g, thereby delivering a modified CO2gas stream 902 to a cooler / heat exchanger 112. In this example, the CO2 gas stream 902 is cooled against a cooling stream of, for example, seawater, freshwater, air etc. A cooled gas stream 905 exits the heat exchanger 112 and passes through a discharge heat exchanger 104 (connected to the refrigerant loop), where the discharge heat exchanger 104 cools the CO2 BOG stream 906 against a CO2 refrigerant stream 918. In the illustrated example, the refrigerant loop includes a valve 110 that facilitates direct topping-up the refrigerant loop from the cargo loop if there is a loss of refrigerant from the refrigerant loop. This example also includes an example of indirectly topping-up the refrigerant loop. This is done via a receiving vessel 115, which is configured to receive and blend CO2 from the cargo loop with CO2 received from the refrigerant loop and to return blended / combined CO2 into the refrigerant loop. As noted above with reference to Fig. 1 and Fig. 5 the blended CO2 is delivered as a vapour stream and a liquid stream for processing within the refrigerant loop. In this example, the refrigerant loop includes two stage compression, which includes a first (low pressure) compressor 106 and a second (high pressure) compressor 107. The first (low pressure) compressor 106 compresses the CO2 refrigerant stream 912 from 20.0 bar g to around 140-150 bar g. CO2 refrigerant gas 913 exiting the first compressor 106 passes through a first cooler / heat exchanger i.e. precooling heat exchanger 113. In this example, the CO2 gas stream 913 is cooled against a cooling stream of, for example, seawater, freshwater, air etc. The cooled CO2 refrigerant stream 914 exits the heat exchanger 113. As noted above, the second blended (vapour) CO2 stream 921 exits the vessel 115 to the heat exchanger 116 and exits the heat exchanger 116 as CO2 refrigerant stream 922. The CO2 refrigerant stream 922 exiting the heat exchanger 116, blends with the cooled CO2 refrigerant stream 914 exiting the heat exchanger 113. The blended / combined CO2 streams 922 and 914 produce CO2 refrigerant stream 914a which undergoes the second (high pressure) compression stage via compressor 107, to increase the pressure of CO2 refrigerant stream 914a entering the compressor 107 to provide an increased pressure exit CO2 refrigerant stream 915 at around 140 - 150 bar g. The high-pressure CO2 refrigerant stream 915 exits the compressor 107 and passes through a second cooler / heat exchanger i.e., aftercooler heat exchanger 108 where the refrigerant stream 915 is cooled against a cooling stream, for example, seawater, freshwater, air etc. to produce an exit stream 916. The aftercooler heat exchanger 108 may also partially condense the refrigerant stream 915 to provide the discharge refrigerant stream 916. As noted above, a refrigerant top-up system (as illustrated in Fig. 5) is included. As such the discharge refrigerant stream 916 enters heat exchanger 116, which acts to further cool the refrigerant stream 916 (exit stream from aftercooler 108) to normalise the temperature of the CO2 refrigerant as exit stream 916a. The stream 916a enters the heat exchanger 105 and exits as cooled refrigerant stream 916b. The pressure of the cooled refrigerant stream 916b is reduced to the level of the last compression stage via valve 117 to provide clean / pure CO2 refrigerant 917 directly from the refrigerant loop to the receiver vessel 115. Refrigerant stream 916a is condensed by heat exchanger 105, which uses residual low temperature in a returning refrigerant stream 911 to provide condensed refrigerant stream 916b. The pressure of the cooled refrigerant stream 916b is reduced to the level of the last compression stage via valve 117 to provide clean / pure CO2 refrigerant 917 directly from the refrigerant loop to the receiver vessel 115. The first blended (liquid) CO2 stream 919 exits the vessel 115 and flows through the valve 114 where pressure in the CO2 stream 919 may be reduced and a CO2 refrigerant stream 918 exits the valve 114 as the cooling medium for the condenser / evaporator 104. Returning heated CO2 refrigerant 911 is the refrigerant that exits the heat exchanger 104 due to heat transfer between the CO2 cargo stream 906 and the CO2 refrigerant stream 918. The final stage of the refrigerant loop is where the heated CO2 refrigerant stream 911 is further heated by the heat exchanger 105, where the refrigerant stream 916 is cooled to normalize the temperature of the CO2 refrigerant. After which, heated CO2 refrigerant stream 912 exits the heat exchanger 105 and passes to the first compression stage 103 thereby continuing the refrigerant cycle / loop. The example illustrated in Fig. 6 includes the components as illustrated in Fig. 1B and described above with the addition of a condensate accumulator 1500 included in the cargo loop. The condensate accumulator 1500 and associated pipework being located in the flow path between the heat exchanger 104 and the valve 109. The cargo loop of each of Fig.1, Fig.lA, Fig. 1B and Fig. 1C have in common a partially / wholly condensed CO2 stream 907 exiting the discharge heat exchanger 104 and returning to the cargo tank 101 via the valve 109 as a reliquefied CO2 stream 908. Accordingly, it should be appreciated the addition of the condensate accumulator 1500 and associated pipework, as described further below is exemplified using the layout of Fig. 1B but applies to each of Fig.1, Fig. 1A, Fig.1 B and Fig. 1C, The condensate accumulator 1500 is provided to permit removal of non-condensable materials 1502. In the illustrated example, the condensate accumulator 1500 intercepts the partially / wholly condensed CO2 stream 907 as it exits the discharge heat exchanger 104 before the partially / wholly condensed CO2 stream 907 is returned to the cargo tank 101 via the valve 109. The condensate accumulator 1500 acts as a separator or diverter and facilitates removal of non-condensable materials 1502 from the cargo loop. Hydrogen, argon, nitrogen, oxygen, carbon monoxide, methane being examples of materials, which at low concentrations materials may be soluble in liquefied CO2 to some degree, but in higher concentrations may be unable to condense under the same conditions where pure (or nearly pure) CO2 condenses. The non-condensable materials 1502 are discharged from the condensate accumulator 1500 towards a valve 1503, which facilitates transporting the non-condensable materials to atmosphere or to another location on the vessel in which the system is deployed. An example is transporting to and from an on-board treatment facility (e.g. an incinerator) to remove flammable or toxic components before discharge to atmosphere, or during loading transferring non-condensable materials 1502 back to the shore terminal or another facility. The example illustrated in Fig. 7 includes the components as illustrated in Fig. 1B and described above with the addition of an additional heat exchanger 1600 included in the refrigerant loop. The heat exchanger 1600 and associated pipework is included to improve efficiency of the system by providing a further cooling stream to optimize the temperature at the inlet of the valve 918 with outlet flow towards the heat exchanger 104 in the refrigerant loop. It should be appreciated the addition of the additional heat exchanger and associated pipework as described further below applies to each of Fig.1, Fig. 1A, Fig.1 B and Fig. 1C, but is exemplified using the layout of Fig. 1B. In this example, the refrigeration loop involves closed-loop circulation of CO2 refrigerant, which undergoes compression from around 20.0 bar g to 90.0 bar g in a single stage compression to provide a cold CO2 stream 918 that is used to condense BOG 906 before the re-liquefied CO2 stream 908 returns to the storage / cargo tank 101. In this example, the refrigerant loop includes single stage compression, which includes a single compressor 106 to compress a CO2 refrigerant stream 912 from 20.0 bar g to 90.0 bar g. CO2 refrigerant gas 915 exiting the compressor 106 passes over a cooler / heat exchanger i.e., aftercooler heat exchanger 108 where the refrigerant stream 915 is cooled against a cooling stream, for example, seawater, freshwater, air etc. to produce refrigerant stream 916. The discharge refrigerant stream 916 is further cooled by heat exchanger 105, which uses residual low temperature from a returning heated refrigerant stream 911 to further cool the refrigerant stream 916 to provide cooled refrigerant stream 917. In this example, the cooled refrigerant stream 917 is further cooled by the additional heat exchanger 1600, which uses residual low temperature from the CO2 BOG stream 901 (evaporation from the storage tank 101) to further cool the refrigerant stream 917 to provide a cooled (optimized temperature) refrigerant stream 917A to the inlet of valve 114. The refrigerant stream 917A exits the valve 114 as the cooling medium for the condenser / evaporator 104. Pressure of the cooled CO2 refrigerant stream 917A exiting the heat exchanger 1600 is reduced over valve 114 before the resulting CO2 refrigerant stream 918 provides the cooling medium, via heat exchanger 104 for the CO2 cargo stream 906. As with Fig. 1B, heated CO2 refrigerant 911 exits the heat exchanger 104 due to heat transfer between the CO2 cargo stream 906 and the CO2 refrigerant stream 918? The final stage of the refrigerant loop is where the heated CO2 refrigerant stream 911 is further heated by the heat exchanger 105, where the refrigerant stream 916 is cooled to normalize the temperature of the CO2 refrigerant. After which, further heated CO2 refrigerant stream 912 exits the heat exchanger 105 and passes to the first compression stage 106 thereby continuing the refrigerant cycle / loop. The example illustrated in Fig. 8 includes the components as illustrated in Fig. 1B and described above but represents a reconfiguration of the system which represents an alternative use of the compressors 102, 103 and 106 for direct injection of BOG to a BOG sequestration facility (permanent storage facility). This example is applicable to floating storage and injection applications. In this example the valves 109 and 114 are closed thereby preventing flow around the cargo loop and refrigerant loop respectively. In this example, the direct injection process utilizes the branches 909, 910 and valve 110, which are described above as facilitating putting both the refrigeration loop and the cargo loop in fluid communication for the purpose of topping up the CO2 refrigerant loop with CO2 directly from the cargo loop. In this example, the branches 909, 910 and the valve 110 facilitate flow of BOG from a section of the cargo loop into the refrigerant loop and thereby utilizing a section of the refrigerant loop to process the BOG, such that a direct flow path (injection path) is created from the tank 101 to a conduit / pipework 1700 connecting the system to a permanent storage facility. The CO2 BOG stream 901 (evaporation from the storage tank 101) passes through a two-stage compressor arrangement located in the cargo loop, where the BOG stream 901 is inducted into and compressed by a first (low pressure) compressor 102 to increase the pressure e.g. from around 7-8 bar g to 16-20 bar g. From there, the modified CO2 BOG gas stream 902 exits the first compressor 102 towards the first cooler / heat exchanger 111 i.e. precooling heat exchanger 111. In this example, the CO2 BOG gas stream 902 is cooled against a cooling stream of, for example, seawater, freshwater, air etc. The cooled gas stream 903 exits the heat exchanger 111 then undergoes the second (high pressure) compression stage via compressor 103, to further increase the pressure of the CO2 BOG stream 904 to around 40 to 50 bar g. The further modified CO2 BOG stream 904 exits the compressor 103 and passes through the second cooler / heat exchanger 112 i.e., aftercooler heat exchanger 112 where the BOG stream 904 is cooled against a cooling stream, for example, seawater, freshwater, air etc. to produce a modified BOG stream 9O5.The modified BOG stream 905 is transported to a leg of the refrigerant loop via a fully open valve 110, thereby minimizing pressure changes across the valve. From there the BOG stream 912A exiting the valve enters a third 5 compression stage, where the compressor 106 compresses the BOG stream 912A from 40-50 bar g to 90.0 - 120 bar g to produce a further modified BOG stream 915A that exits the compressor 106. The BOG stream 915A passes over a further cooler / heat exchanger i.e., the aftercooler heat exchanger 108 where the BOG stream 915A is cooled against a cooling stream, for example, seawater, freshwater, air etc. It should be io appreciated, with reference to If we Fig. 1, which includes two compression stages in each of the cargo loop and the refrigerant loop much higher pressures in the final BOG stream could be achieved. The final BOG stream 1702 is then exported from the refrigerant loop part of the system e.g. to a permanent storage facility via suitable pipework 1700. 15 Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.

Claims

1. A method of cooling and reliquefying a boil off gas (BOG) stream from a liquefied carbon dioxide cargo stored in a tank or tanks on a floating transportation vessel, wherein the method comprises at least the steps of:compressing a BOG stream from the liquefied carbon dioxide cargo in one or more stages of BOG compression to provide a compressed BOG discharge stream;cooling the compressed BOG discharge stream against a carbon dioxide refrigerant stream to provide a cooled compressed BOG stream; and a warmer carbon dioxide refrigerant stream;compressing the warmer carbon dioxide refrigerant stream in one or more stages of refrigerant compression to provide a compressed carbon dioxide refrigerant discharge stream for subsequent use as the carbon dioxide refrigerant stream, which is operable to cool and at least partially condense the compressed BOG discharge stream;wherein the one or more stages of BOG compression and the one or more stages of refrigerant compression are provided by a single shaft multi-stage compression unit.

2. The method of claim 1, wherein compressing the BOG stream from the liquefied carbon dioxide cargo is in two or more stages of BOG compression comprising at least a first stage and a final stage to provide the compressed BOG discharge stream.

3. The method of claim 1 or 2, wherein compressing the warmer carbon dioxide refrigerant stream is in two or more stages of refrigerant compression comprising at least a first stage and a final stage to provide the compressed carbon dioxide refrigerant discharge stream for subsequent use as the carbon dioxide refrigerant stream, which is operable to cool and at least partially condense the compressed BOG discharge stream.

4. The method of any preceding claim, further comprising cooling the compressed BOG streams after each stage of compression.

5. The method of any preceding claim, further comprising cooling the carbon dioxide refrigerant streams after each stage of compression.

6. The method of any preceding claim, further comprising reducing pressure of the cooled compressed BOG stream prior to returning the cooled compressed BOG stream as re-liquefied carbon dioxide to the tank or tanks.

7. The method of any preceding claim, further comprising reducing pressure of the cooled compressed refrigerant stream prior to cooling the compressed BOG discharge stream against the carbon dioxide refrigerant stream.

8. The method of any preceding claim further comprising a further cooling step, which further cools the warmer carbon dioxide refrigerant stream, thereby reducing the temperature of the warmer carbon dioxide refrigerant stream before the step of compressing the warmer carbon dioxide refrigerant stream in two or more stages of refrigerant compression.

9. The method as claimed in any preceding claim, comprising topping-up the carbon dioxide refrigerant stream.

10. The method as claimed in claim 9, wherein topping up the refrigerant stream includes carbon dioxide from the BOG stream being added directly to the refrigerant stream.

11. The method as claimed in claim 9 or 10, wherein topping up the refrigerant stream includes carbon dioxide from the BOG stream being added indirectly to the refrigerant stream.

12. The method as claimed in claim 9, 10 or 11, wherein directly adding carbon dioxide from the BOG stream to the carbon dioxide refrigerant stream is via one-way control and conduits fluidly connecting the BOG stream and the refrigerant stream, wherein CO2 from the BOG stream is added to the refrigerant stream by opening the valve in the event that CO2 from the refrigerant stream is lost.

13. The method as claimed in any of claims 9 to 12, wherein indirectly adding carbon dioxide from the BOG stream to the carbon dioxide refrigerant stream comprises extracting carbon dioxide from the BOG stream and extracting carbon dioxide from the refrigerant stream and blending the carbon dioxide extracted from both the BOG stream with the carbon dioxide extracted from the refrigerant stream to produce a blended carbon dioxide stream and returning the blended carbon dioxide stream to the refrigerant stream in the event that CO2 from the refrigerant stream is lost.

14. The method as claimed in claim 13, wherein the carbon dioxide extracted from the BOG stream comprises carbon dioxide extracted directly from the tank or tanks.

15. The method as claimed in any of claims 13 to 14, further comprising reducing pressure of the extracted carbon dioxide refrigerant stream.

16. The method as claimed in any of claims 13 to 15, further comprising reducing pressure of the blended liquid carbon dioxide stream.

17. The method as claimed in any of claims claim 13 to 16, comprising returning a blended vapour carbon dioxide stream and a blended liquid carbon dioxide stream to the refrigerant stream.

18. The method as claimed in claim 17, comprising cooling the returned blended vapour carbon dioxide stream.

19. The method as claimed in any preceding claim, further comprising removing noncondensable materials from the cooled compressed BOG stream.

20. The method as claimed in any preceding claim, comprising further cooling the carbon dioxide refrigerant stream using residual low temperature from the BOG stream from the liquefied carbon dioxide cargo.

21. A system for cooling and reliquefying a boil off gas (BOG) stream from a liquefied carbon dioxide cargo stored in tank or tanks on a floating transportation vessel, wherein the system comprises a single shaft multi-stage compression unit comprising at least two independent stages of compression, wherein at least one of the two independentstages of compression compress a BOG stream from the liquefied carbon dioxide cargo and at least one of the two independent stages of compression compress a carbon dioxide refrigerant stream to provide a compressed carbon dioxide refrigerant discharge stream for use as a carbon dioxide refrigerant stream, which is operable to cool and at5 least partially condense the compressed BOG discharge stream; and wherein the system includes conduits that configure the system in an open CO2 cargo loop and a closed CO2 refrigerant loop.

22. The system of claim 21, further comprises stacked pistons.

23. The system of claim 21, further comprising a connection to a BOG sequestration10 facility, wherein the system is configurable to prevent flow around the CO2 cargo loop and to prevent flow around the CO2 refrigerant loop and to permit flow of the BOG stream from the tank, through an open section of the CO2 cargo loop and through an open section of the CO2 refrigerant loop such that the BOG stream passes through the BOG compression stage or stages and the refrigerant compression stage or stages15 before exiting the system via the connection to the sequestration facility.

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