SYSTEM AND METHOD FOR OFFLOADING LCO2 FROM A SHIP TO AN INTERMEDIATE STORAGE AT AN LCO2 RECEIVING TERMINAL

Abstract

 A system 50 for offloading LCO₂ from an LCO₂ carrier ship 30 to an intermediate LCO₂ storage tank 40 at an LCOz receiving terminal 100 connected to a long term LCO₂ storage facility 120 is disclosed, which system avoids cross-contamination from cargo of one ship to another, wherein a slip stream of LCO₂ withdrawn from a main stream of LCO₂ being of-floaded to the terminal 100 is vaporised and returned to the LCO₂ carrier ship 30, and the vaporisation is used to providing cooling for complete or partial reliquefaction of a boil-off gas stream withdrawn from the intermediate LCO₂ storage tank 40, and wherein a resulting reliquefied fraction is returned to the intermediate LCO₂ storage tank 40. A corresponding method is also disclosed.

Description

FIELD OF THE INVENTION

[0001] The present invention generally relates to Cabon Capture and Storage (CCS) technology, and more particularly to a system for offloading liquid CO₂ (LCO₂) from an LCO₂ carrier ship to an intermediate LCO₂ storage tank at an LCO₂ receiving terminal connected to a long term LCO₂ storage facility, wherein cross-contamination from cargo of one ship to another, via common intermediate storage tanks, can be avoided. The invention also relates to a corresponding method. The invention uses a slip stream of LCO₂ withdrawn from a main stream of LCO₂ being offloaded to the terminal which is vaporised and returned to the LCO₂ carrier ship, and the vaporisation is used to providing cooling for complete or partial reliquefaction of a boil-off gas stream withdrawn from the intermediate LCO₂ storage tank, and a resulting reliquefied fraction is returned to the intermediate LCO₂ storage tank. The present invention also relates to a corresponding method.

BACKGROUND ART

[0002] Captured and liquefied carbon dioxide can be transported in the liquid phase (LCO₂) from various locations and customers to CO₂ receiving terminals at which the CO₂ will be temporarily stored in intermediate buffer storage tanks onshore, before being pumped in a dense phase via a pipeline and injected into an offshore long term storage reservoir. The purpose of the buffer storage is to allow for continuous injection to the long-term storage reservoir despite intermittent LCO₂ cargo transfer from ship.

[0003] A general description of an outline of a CCS chain can be found in Conceptualization of CO₂ Terminal for Offshore CCS Using System Engineering Process by Hyonjeong et al. Energies, 2019, 12, 4350. Said CCS chain description includes the return of CO₂ carriers to consider the vaporised CO₂ (VCO₂) in returning carriers. According to Hyonjeong et al., the same volume of VCO₂ must be loaded into the carrier's cargo tank when unloading LCO₂ from the carrier to the CO₂ terminal. The carrier's cargo tank is displaced by the VCO₂ of the storage tanks at the terminal, while LCO₂ fills the storage tanks. There are two reasons for loading VCO₂ into the carrier's cargo tank. The first is to allow the pressure and temperature of the cargo tank to be controlled during the unloading process. Constant pressure and temperature facilitate the process. The second reason is to prevent the rapid decrease in temperature due to Joule-Thomson cooling.

[0004] Thus, for example, during LCO₂ cargo transfer between ship and onshore intermediate storage, the displaced vapour in the onshore intermediate storage tank could be returned to the CO₂ carrier ship via a vapour return connection for pressure maintenance in both sets of tanks, i.e. of the carrier ship and of the intermediate storage of the CO₂ receiving terminal, respectively.

[0005] However, depending on origin and purity of the LCO₂ being received at the terminal, the vapour in the onshore storage tank or tanks may contain impurities. In the event that restrictions are imposed on the vapour quality of vapour being returned to the ship that are not met by the vapour composition in the onshore storage, such impurities could pose a substantial impediment to the functioning of the CCS chain. In particular, LCO₂ receiving terminals served by multiple customers, with CO₂ originating from various sources, could face challenges from a technical and commercial point with undesired cross-contamination caused by impurities contained in the vapour space of the onshore storage tanks when returned to the ships.

[0006] It would be desirable to enable pressure maintenance in both sets of tanks, i.e. of the carrier ship and of the intermediate storage of the CO₂ receiving terminal, while reducing, and preferably avoiding, the risk of contaminating an LCOz carrier tank at a ship in connection with offloading LCO₂ from the ship to an LCO₂ receiving terminal. Moreover, this should preferably be achieved in an energy efficient manner.

[0007] It is an object of the present invention to provide a system and a method enabling the above.

SUMMARY OF THE INVENTION

[0008] The present invention is based on using a slip stream of LCO₂, which is being withdrawn from a main stream LCO₂ being offloaded from a ship and imported to the terminal, for partially or completely reliquefying a stream of boil-off gas being withdrawn from an intermediate storage tank to which intermediate storage tank the LCO₂ main stream is being led, wherein the two streams are kept separate, and wherein a reliquefied fraction from the stream of boil-off gas is led back to the storage tank. The required cooling for reliquefaction of a fraction of the stream of boil-off gas is accomplished by subjecting the LCO₂ slip stream to a reduced pressure. The resulting cold partly vaporised LCO₂ slip stream, and the stream of boil-off gas are led to a heat exchanger for exchange of heat. After complete vaporisation of the LCO₂ slip stream, the resulting CO₂ vapour is led back to the ship in a vapour return conduit.

[0009] Accordingly, in one aspect the invention relates to a system 50 for offloading LCO₂ cargo from an LCO₂ carrier ship 30 to an intermediate LCO₂ storage tank 40 at an LCO₂ receiving terminal 100, which system avoids cross-contamination of cargo from one LCO₂ carrier ship 30 to another, said system comprising: an intermediate LCO₂ storage tank 40; a boil-off gas outlet 60 from the intermediate LCO₂ storage tank 40 configured to withdrawing boil-off gas from the intermediate LCO₂ storage tank 40; an LCO₂ cargo import conduit 1, 10 connected to the intermediate LCO₂ storage tank 40 configured to receive an LCO₂ cargo import stream from the LCO₂ carrier ship 30; a CO₂ vapour return conduit 4, 5, 6, 7, 8, 9 configured to be connected to the LCO₂ carrier ship 30 and to return from the system 50 CO₂ vapour to the LCO₂ carrier ship 30; an LCO₂ outlet 70 from the intermediate LCO₂ storage tank 40 configured to discharging LCO₂ from the intermediate LCO₂ storage tank 40; which system additionally comprises an LCO₂ slip stream conduit 2 connected to the LCO₂ cargo import conduit 1 configured to withdrawing a slip stream of LCO₂ from the LCO₂ cargo import conduit 1; a pressure let-down valve A connected to the LCO₂ slip stream conduit 2 configured to receiving the slip stream of LCO₂ having a first temperature 11, and to exiting an at least partly vaporised stream of LCO₂ having a second lower temperature T2; a partly vaporised stream conduit 3 connected to the pressure let-down valve A configured to receiving the at least partly vaporised stream of LCO₂ having a second lower temperature T2; a boil-off gas conduit 11 connected to the boil-off gas outlet 60; a first heat exchanger B connected to the partly vaporised stream conduit 3 and to the boil-off gas conduit 11, respectively, said heat exchanger being configured to receiving the at least partly vaporised stream of LCO₂ having the second lower temperature T2, to receiving the boil-off gas withdrawn from the boil-off gas outlet 60, to transferring heat from the boil-off gas to the at least partly vaporised stream of LCO₂ having the second lower temperature T2, to exiting a reliquefied fraction from the boil-off gas and to exiting an at least partly vaporised stream of LCO₂, respectively; a conduit 12, 15 connected to the intermediate LCO₂ storage tank 40 configured to receiving the exiting reliquefied fraction from the first heat exchanger B; wherein the CO₂ vapour return conduit 4, 5, 6, 7, 8, 9 is further configured to receive CO₂ vapour resulting from the at least partly vaporised stream of LCO₂ having the second lower temperature T2; and wherein a vapour return compressor F is arranged along the vapour return conduit 4, 5, 6, 7, 8, 9 configured to compressing the CO₂ vapour to be returned to the LCO₂ carrier ship 30.

[0010] In a preferred embodiment of the system, the system additionally comprises means C, D, E configured to vaporising a remaining non-vaporised fraction of the at least partly vaporised stream of LCO₂ exiting the first heat exchanger B.

[0011] In yet a preferred embodiment of the system, the system comprises means G, 13, H, 14, configured to separate, compress, and direct a remaining gaseous fraction of the boil-off gas after partial reliquefaction thereof into a stream of compressed LCO₂ withdrawn from the intermediate LCO₂ storage tank 40 to be injected into a pipeline 110 connected to an underground long term storage facility 120.

[0012] In another aspect, the invention relates to a method for offloading LCO₂ cargo from an LCO₂ carrier ship 30 to an intermediate LCO₂ storage tank 40 at an LCO₂ receiving terminal 100, avoiding cross-contamination of cargo from one LCO₂ carrier ship 30 to another, said method comprising the following steps: i. receiving a main stream of LCO₂ from an LCO₂ carrier ship 30 and directing the main stream to an intermediate LCO₂ storage tank 40; ii. withdrawing LCO₂ from an LCO₂ outlet 70 from the intermediate LCO₂ storage tank 40; iii. withdrawing a stream of boil-off gas from a boil-off gas outlet 60 from the intermediate LCO₂ storage tank 40; iv. returning CO₂ vapour from the terminal 100 back to the LCO₂ carrier ship 30, which method additionally comprises the following steps: v. withdrawing from the main stream of LCOz a slip stream of LCO₂; vi. subjecting the withdrawn slip stream of LCO₂ having a first temperature T1 to a reduced pressure, thereby partly vaporising the withdrawn LCO₂ slip stream having a first temperature T1 so as to produce an at least partly vaporised LCO₂ slip stream having a second lower temperature T2; vii. subjecting the withdrawn stream of boil-off gas to heat exchange with the at least partly vaporised LCO₂ slip stream having the second lower temperature T2, thereby completely or partially liquefying the boil-off gas; viii. returning the resulting liquefied fraction to the intermediate LCO₂ storage tank 40; wherein, in step iv., the CO₂ vapour from the terminal 100 being returned to the LCO₂ carrier ship 30 comprises a stream of CO₂ vapour obtained from the slip stream of LCO₂ and is subjected to compression before being returned to the ship.

[0013] In a preferred embodiment of the method, the method additionally comprises a step ix. wherein a remaining non-vaporised fraction of the at least partly vaporised LCO₂ slip stream after heat exchange in step vii. is vaporised, and, and wherein, in step iv., the CO₂ vapour from the terminal being compressed and returned to the LCO₂ carrier ship 30 comprises the resulting CO₂ vapour from step ix.

[0014] In yet a preferred embodiment of the method, the method additionally comprises the step x., wherein, after heat exchange in step vii., a remaining non-liquefied gaseous fraction from the boil-off gas stream is separated and directed into a stream of LCO₂ being withdrawn from the intermediate LCO₂ storage tank 40 and conveyed to an underground long term storage facility 120.

[0015] The present invention provides a method and system providing vapour return for pressure support of LCO₂ carrier ship 30 tanks during liquid off-loading by generating vapours directly from the off-loading cargo at the terminal 100, eliminating the risk for potential off-spec vapour return from the onshore storage tank facility 40.

[0016] Also, the invention provides an energy efficient measure to reliquefy displaced vapour in the onshore storage tanks 40, by heat integration with the ship vapour return 4, 5, 6, 7, 8, 9.

[0017] The present invention provides a method for a force-vaporising a slip stream of the off-loaded LCO₂ cargo, providing vapour for pressure support to the ship tanks without ingress of any potential contaminants from the main onshore storage facility. The system also provides cooling duty for reliquefaction of displaced vapour in the onshore storage tanks, reducing the energy requirements for the BOG system.

[0018] The present invention achieves the advantage of providing CO₂ vapour return to an off-loading LCO₂ carrier ship 30, independent of the content in the onshore storage tanks, i.e. avoiding exposure of potential contaminants contained in the vapour space from previous off-loaded cargos, such as from other LCO₂ carrier ships 30.

[0019] Further embodiments and advantages of the invention will be apparent from the following detailed description and appended claims.

[0020] In the present disclosure, same reference numeral is used both to denote the conduit and the stream flowing therein. For example, reference numeral 2 is used to both denote the slip stream conduit, and also to denote the slip stream itself flowing in said conduit.

[0021] The term "long-term" as used herein denotes a storage intended to be permanent.

[0022] The present system and method may be combined with either one or both of the embodiments disclosed applicant's co-pending applications filed on even date herewith.

BRIEF DESCRIPTION OF THE ATTACHED DRAWING

[0023] Figure 1 shows an embodiment of the inventive system 50 as indicated by the dashed line implemented into an LCO₂ receiving terminal 100. The inventive system 50 in its most generic embodiment does not include C, D, E, which in a preferred embodiment can be arranged along the vapour return conduit 4, 5, 6, 7, 8, 9, and does also not include G and H, which in a preferred embodiment can be provided for separating out a remaining gaseous fraction after partial liquefaction of the boil-off gas from the reliquefied conduit 12, 15, and does also not include I, and J, which are units conventionally included in a conventional LCO₂ receiving terminal 100, in which the inventive system 50 has not been implemented.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The core of the invention is a process scheme that enables efficient heat integration between the cold closed-circuit vapour return stream to LCO₂ carrier ship 30 and the warmer displaced boil-off gas from the onshore storage tanks 40. This is achieved by using a slip stream of the off-loaded LCO₂ cargo, which serves a dual purpose:

• as the CO₂ pressure is let down to e.g. 7 barg, the temperature will drop to approximately -45°C, providing refrigeration for the boil-off gas displaced during filling of onshore storage tanks 40. This enables at least partial recovery of the boil-off gas, which is reliquefied and returned to the storage tank 40;
• the cold low pressure LCO₂ is vaporised by indirect heat exchange in the first heat exchanger B which enables a closed-circuit vapour return system, eliminating the risk for return of vapour potentially containing incompatible impurities from previous ships existing in the onshore storage tank facility.

[0025] In the above connection, a lower pressure will increase the cooling duty in the first heat exchanger B, but will also increase the compressor duty of vapour return compressor F.

[0026] Without CO₂ vapour return from the LCO₂ receiving terminal 100 to the LCO₂ carrier ship 30, e.g. in order to avoid cross-contamination of the tanks of the LCO₂ carrier ship 30, vapour for pressure maintenance on the ship could possibly alternatively be generated by force-vaporising liquid CO₂ cargo on the ship. Such solution is however considered inferior to the solution offered by the invention, since, i.a., with such solution, the displaced vapour in the intermediate LCO₂ storage tank or tanks 40 during filling will increase the sizing and energy requirements for the BOG system of the LCO₂ receiving terminal 100.

[0027] In the embodiment of the invention illustrated in FIG. 1, LCO₂ is offloaded from a ship 30 at pressures from e.g. 15-18 barg (denoted as medium pressure (MP), with a typical saturation temperature between -30°C and -20°C).

[0028] A slip stream 2 of the off-loaded cargo 1 is fed to a pressure let-down control valve A, where the pressure is let down to e.g. 6-10 barg (denoted as low pressure (LP), with a typical saturation temperature between -50°C and -40°C).

[0029] The flow rate of the slip stream 2 will be determined by the off-loading rate, and typically around 5% of the total flow will be required.

[0030] On the downstream side 3 of the pressure let-down valve A a two-phase LP LCO₂ stream is fed to the first heat exchanger B. In the embodiment in FIG. 1 the first heat exchanger B is depicted as a shell and tube heat exchanger but could alternatively be of another type, such as a plate or plate-fin heat exchanger. In the first heat exchanger B, the cold two-phase LP stream provides cooling of the vapour displaced 11 from the storage tank 40 during filling thereof. The displacement rate will depend on the filing rate 10, the injection rate 16, and the return of reliquefied boil-off gas 15.

[0031] Ideally, in a case of operation wherein the two-phase LP LCO₂ stream has been fully vaporised after heat exchange 4 in the first heat exchanger B, the resulting CO₂ vapour is returned to the ship after compression of the CO₂ vapour stream 7 in a vapour return compressor F.

[0032] Following heat exchange in the first heat exchanger B in a case of operation wherein the partly vaporised LP LCO₂ stream 4 has not been fully vaporised, the stream is further vaporised in a second heat exchanger C, using the warm discharge stream 8 of vapour from the vapour return compressor F. In the inventive system, first and second heat exchangers B and C, respectively, could be referred to as cross heat exchangers.

[0033] In case the heat content of the boil-off stream 11 and of the compressor discharge stream 8 are not sufficient to completely vaporise the LP LCO2, so that a partly vaporised LCO₂ stream 5 remains after the second heat exchanger C, a dedicated vaporiser D is provided. The vaporiser D could be heated electrically, by ambient sea water or other suitable heat sources available at the terminal 100.

[0034] From the vaporiser D the LP CO₂ 6 is preferably fed to a knock-out drum E to ensure that any remaining liquid droplets are not found in the suction flow 7 to the vapour return compressor F.

[0035] The vapour return compressor F will increase the pressure to be compatible with the ship cargo tank pressure. The compressor discharge 8 is cooled by cross-heat exchange in heat exchanger C, providing heat for vaporisation of a remaining liquid phase in the LP LCO2 stream 4 as described above.

[0036] Vapour 9 is returned to ship 30 at pressure and temperature compatible with the MP cargo at a rate that equals the volumetric flow of off-loaded liquid 1.

[0037] As shown in the embodiment of the invention illustrated in FIG. 1, the offloaded LCOz is split into two process sections. As described above, the LP LCO2 2 is providing vapour for the displaced liquid in the ship cargo tanks 30 in a closed circuit 2, 3, 4, 5, 6, 7, 8, 9. The major part of the imported LCO₂ 1 is transferred 10 at MP conditions to the intermediate storage tank(s) 40.

[0038] The storage tank or tanks 40 are operated at MP condition, and liquid CO₂ is transferred 16 to injection pump or pumps I. In the embodiment of FIG. 1, the injection pump I is shown as a single pump but could also be configured in series with booster pumps. The pump(s) provide sufficient head for dense phase CO₂ to be exported to a pipeline 110 and subsequently injected into a reservoir 120 for permanent storage. Downstream of the injection pump 17, an injection heater J may be arranged to avoid sub-zero temperatures entering the pipeline at the landfall 19. The injection heater J could for example be heated electrically, by ambient sea water or other suitable heat sources available at the terminal.

[0039] The export flow rate 16 is likely to be lower than cargo transfer rate 10, hence, as the level in the intermediate storage tank 40 increases vapour will be displaced. To control pressure, the displaced vapour 11 will need to be processed, to avoid CO₂ emission to atmosphere by vent.

[0040] The displaced vapour 11 at MP conditions will be reliquefied against the colder LP LCO₂ stream 3 in a heat exchanger B. Ideally, in a case of operation wherein the BOG and/or displaced vapour 11 can be completely reliquefied, the reliquefied BOG or reliquefied displaced vapour can be returned to intermediate storage tank 40 via conduit 12, 15. In a case of operation wherein a gaseous fraction remains 12 after cooling in the vapour return heat exchanger B, the partly reliquefied vapours 12 are fed to a two-phase separator G, where reliquefied BOG/reliquefied displaced vapour is returned 15 to the intermediate storage tank 40. The gas phase 13, enriched with non-condensable impurities, is fed to a boil-off gas compressor H and the effluent from the compressor is injected into high-pressure LCO₂ in the export conduit 18, where it will be dissolved, complying with single phase flow assurance requirements.

LIST OF REFERENCE SIGNS USED

[0041] 

1, 10

LCO₂ cargo import conduit

2

LCO₂ slip stream conduit

3

partly vaporised LCO₂ stream conduit

4, 5, 6, 7, 8,

9 CO₂ vapour return conduit

11

boil-off gas conduit

12, 13, 14

remaining gaseous fraction conduit (from boil-off gas)

12, 15

reliquefied fraction conduit

16

conduit connecting bottom outlet 70 with injection pump I

17

conduit connecting outlet from injection pump with injection heater J

30

LCO₂ carrier ship

40

intermediate LCO₂ storage tank

50

system for offloading LCO₂ cargo (from an LCO₂ carrier ship)

60

boil-off gas outlet (from intermediate LCO₂ storage tank)

70

LCO₂ outlet (from intermediate LCO₂ storage tank)

100

LCO₂ receiving terminal

110

pipeline

120

underground long term storage facility

T1

first temperature (of slip stream)

T2

second lower temperature (of partly vaporised LCO₂ stream)

A

pressure let-down valve

B

first heat exchanger

C

second heat exchanger

D

vaporiser

E

knock-out drum

F

vapour return compressor

G

two-phase separator

H

boil-off gas compressor

I

injection pump

J

injection heater

Claims

1. A system (50) for offloading LCO₂ cargo from an LCO₂ carrier ship (30) to an intermediate LCO₂ storage tank (40) at an LCO₂ receiving terminal (100), which system avoids cross-contamination of cargo from one LCO₂ carrier ship (30) to another, said system comprising:

- an intermediate LCO₂ storage tank (40);

- a boil-off gas outlet (60) from the intermediate LCO₂ storage tank (40) configured to withdrawing boil-off gas from the intermediate LCO₂ storage tank (40);

- an LCO₂ cargo import conduit (1, 10) connected to the intermediate LCO₂ storage tank (40) configured to receive an LCO₂ cargo import stream from the LCO₂ carrier ship (30);

- a CO₂ vapour return conduit (4, 5, 6, 7, 8, 9) configured to be connected to the LCO₂ carrier ship (30) and to return from the system (50) CO₂ vapour to the LCO₂ carrier ship (30);

- an LCO₂ outlet (70) from the intermediate LCO₂ storage tank (40) configured to discharging LCO₂ from the intermediate LCO₂ storage tank (40);

characterized in additionally comprising,

- an LCO₂ slip stream conduit (2) connected to the LCO₂ cargo import conduit (1) configured to withdrawing a slip stream of LCO₂ from the LCO₂ cargo import conduit (1);

- a pressure let-down valve (A) connected to the LCO₂ slip stream conduit (2) configured to receiving the slip stream of LCO₂ having a first temperature (T1), and to exiting an at least partly vaporised stream of LCO₂ having a second lower temperature (T2);

- a partly vaporised stream conduit (3) connected to the pressure let-down valve (A) configured to receiving the at least partly vaporised stream of LCO₂ having a second lower temperature (T2);

- a boil-off gas conduit (11) connected to the boil off gas outlet (60);

- a first heat exchanger (B) connected to the partly vaporised stream conduit (3) and to the boil-off gas conduit (11), respectively, said heat exchanger being configured to receiving the at least partly vaporised stream of LCO₂ having the second lower temperature (T2), to receiving the boil-off gas withdrawn from the boil-off gas outlet (60), to transferring heat from the boil-off gas to the at least partly vaporised stream of LCO₂ having the second lower temperature (T2), to exiting a reliquefied fraction from boil-off gas and to exiting an at least partly vaporised stream of LCO₂, respectively;

- a reliquefied fraction conduit (12, 15) connected to the intermediate LCO₂ storage tank (40) configured to receiving the exiting reliquefied fraction from the first heat exchanger (B);

- wherein the CO₂ vapour return conduit (4, 5, 6, 7, 8, 9) is further configured to receiving CO₂ vapour resulting from the at least partly vaporised stream of LCO₂ having the second lower temperature (T2); and

- a vapour return compressor (F) arranged along the vapour return conduit (4, 5, 6, 7, 8, 9) configured to compressing the CO₂ vapour to be returned to the LCO₂ carrier ship (30).

2. The system (50) of claim 1, additionally comprising

- means (C, D, E) arranged along the CO₂ vapour return conduit (4, 5, 6, 7, 8, 9) configured to vaporising a remaining non-vaporised fraction of the at least partly vaporised stream of LCO₂ exiting the first heat exchanger (B).

3. The system (50) of claim 1 or 2, additionally comprising

- an injection pump (I) connected via a conduit (16) to the LCO₂ outlet (70) from the intermediate LCO₂ storage tank (40) configured to receiving LCO₂ withdrawn from the intermediate LCO₂ storage tank (40), and to ejecting a stream of compressed LCO₂ to be injected into a pipeline (110) connected to an underground long term storage facility (120); and

- a remaining gaseous fraction conduit (12, 13, 14) and means (G,H) arranged along said remaining gaseous fraction conduit (12, 13, 14) configured to receiving, separating from the reliquefied fraction conduit (12, 15), and compressing, a remaining gaseous fraction exiting the first heat exchanger (B) resulting from the boil-off gas, and to injecting the resulting compressed gaseous fraction of the boil-off gas into a stream of compressed LCO₂ to be injected into a pipeline (110) connected to an underground long term storage facility (120).

4. A method for offloading LCO₂ cargo from an LCO₂ carrier ship (30) to an intermediate LCO₂ storage tank (40) at an LCO₂ receiving terminal (100), avoiding cross-contamination of cargo from one LCO₂ carrier ship (30) to another, said method comprising the following steps:

i.) receiving a main stream of LCO₂ from an LCO₂ carrier ship (30) and directing the main stream to an intermediate LCO₂ storage tank (40);

ii.) withdrawing LCO₂ from an LCO₂ outlet (70) from the intermediate LCO₂ storage tank (40);

iii.) withdrawing a stream of boil-off gas from a boil-off gas outlet (60) from the intermediate LCO₂ storage tank (40);

iv.) returning CO₂ vapour from the terminal (100) back to the LCO₂ carrier ship (30),

characterized in additionally comprising the following steps:

v.) withdrawing from the main stream of LCOz a slip stream of LCO₂;

vi.) subjecting the withdrawn slip stream of LCO₂ having a first temperature (T1) to a reduced pressure, thereby partly vaporising the withdrawn LCO₂ slip stream having a first temperature (T1) so as to produce an at least partly vaporised LCO₂ slip stream having a second lower temperature (T2);

vii.) subjecting the withdrawn stream of boil-off gas to heat exchange with the at least partly vaporised LCO₂ slip stream having the second lower temperature (T2), thereby partly or completely liquefying the stream of boil-off gas into a liquefied fraction;

viii.) returning the resulting liquefied fraction to the intermediate LCO₂ storage tank (40);

wherein, in step (iv.), the CO₂ vapour from the terminal (100) being returned to the LCO₂ carrier ship (30) comprises a stream of CO₂ vapour obtained from the slip stream of LCO₂ and is subjected to compression before being returned to the ship.

5. The method for offloading LCO₂ cargo from an LCO₂ carrier ship (30) to an intermediate LCO₂ storage tank (40) at an LCO₂ receiving terminal (100) of claim 4, additionally comprising the following steps:

ix.) vaporising a remaining non-vaporised fraction of the at least partly vaporised LCO₂ slip stream after heat exchange in step (vii);

wherein, in step (iv.), the CO₂ vapour from the terminal being compressed and returned to the LCO₂ carrier ship (30) comprises the resulting CO₂ vapour from step (ix.).

6. The method of claim 4 or 5, additionally comprising the step of:
x.) after heat exchange in step (vii.) separating and directing a remaining non-liquefied gaseous fraction from the boil-off gas stream into a stream of LCO₂ being withdrawn from the intermediate LCO₂ storage tank (40) and conveyed to an underground long term storage facility (120).

Drawing