SYSTEM AND METHOD FOR MAINTAINING PRESSURE IN AN INTERMEDIATE STORAGE TANK DURING LCO2 INJECTION AT AN LCO2 RECEIVING DEVICE

Description

FIELD OF THE INVENTION

[0001] The present invention generally relates to carbon capture and storage (CCS) technology, and more particularly to a system and method for pressure-maintenance of an intermediate liquefied CO 2 (LCO 2 ) storage tank during injection into a pipeline leading into a long-term LCO 2 storage facility of LCO 2 being withdrawn from the tank at a receiving terminal connected with the long term LCO 2 storage facility. The invention uses a slip stream of LCO 2 withdrawn from the warmer stream of LCO 2 being directed to a long-term LCO 2 storage from the terminal, which slip stream is fed into a tank heater in the intermediate storage tank for indirectly heating the contents of the tank.BACKGROUND ART

[0002] Captured and liquefied carbon dioxide can be transported in the liquid phase (LCO 2 ) from various locations and customers to CO 2 receiving terminals at which the CO 2 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 LCO 2 cargo transfer from ship.

[0003] A general description of an outline of a CCS chain can be found in Conceptualization of CO 2 Terminal for Offshore CCS Using System Engineering Process by Hyonjeong et al. Energies, 2019, 12, 4350.

[0004] Discharge from an intermediate LCO 2 tank of LCO 2 to be injected into a reservoir for long-term storage will cause the pressure in the intermediate storage tank to drop. In case of operating pressures near the triple point of CO 2 , it is necessary to ensure that the pressure in the tank does not drop sufficiently for dry ice to form. Standard liquefied gas terminals (e.g. LNG) counteract pressure drop by force-vaporising a portion of the liquefied gas and returning it as vapour to the tank, thereby replacing the emptied liquid volume. However, LCO 2 in CCS applications typically contains some water (e.g. 30-70 ppm by mol). The LCO 2 is mostly kept at temperatures below -20°C at the terminal, at which temperature CO 2 hydrates could potentially form. In the liquid phase of the LCO 2 , the concentration of water is insufficient to form hydrates, and thus poses no problem. However, if it is passed through a force-vaporiser, the same liquid composition (including the full water content) becomes vapour, which can form hydrates either in the vaporiser itself, which could cause blockages in the vaporiser, or, during a subsequent pressure let-down back to tank pressure, in the piping downstream of a corresponding expansion valve. This is particularly likely in the case of intermediate storage operation near the triple point.

[0005] It would be desirable to be able to provide for pressure-maintenance of an intermediate LCO 2 storage tank during discharge of LCO 2 from the tank so as to avoid formation of dry ice in the intermediate LCO 2 storage tank during LCO 2 discharge from the tank, while at the same time reducing the risk of undesired formation of hydrates from water contained in the LCO 2 .

[0006] It is an object of the present invention to provide a system and a method enabling the above.SUMMARY OF THE INVENTION

[0007] According to the invention, the above object has been achieved by using indirect liquid-to-liquid heat exchange using LCO 2 , wherein a slip stream of LCO 2 is withdrawn from a pressurised stream of LCO 2 in the system leading to an injection pipeline, which stream of LCO 2 has a higher temperature than that of the LCO 2 contained in the intermediate storage tank, and is forwarded to the intermediate storage tank for heat exchange with the LCO 2 contained therein, and, after the heat exchange, the cooled slipstream of LCO 2 is forwarded to the stream of LCO 2 being discharged from the intermediate tank to be pressurised, at a location downstream of a first pumping stage of the stream of LCO 2 being discharged from the intermediate tank.

[0008] Accordingly, in one aspect the invention relates to a system 110 configured to maintaining a desired pressure in a discharging intermediate LCO 2 tank 10 during discharging of LCO 2 from the tank thereby avoiding formation of dry ice in the intermediate LCO 2 storage tank during the LCO 2 discharge from the tank, which system avoids or reduces the risk of formation of hydrates in the system from water contained in the LCO 2 , comprising: an intermediate LCO 2 storage tank 10; an LCO 2 discharge conduit 41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62 , 73, 74, 30 connected to the intermediate LCO 2 storage tank 10 configured to discharge LCO 2 to be injected into a long-term CO 2 storage 1; a first LCO 2 pump 15 arranged along the discharge conduit 41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74, 30 ; an injection pump 17 arranged along the discharge conduit 41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62 , 73, 74, 30 downstream of the first LCO 2 pump 15; an LCO 2 heater 19 arranged along the discharge conduit 41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74 , 30 downstream of the LCO 2 injection pump 17; which system additionally comprises a tank heater 11 configured to receive and to transfer heat from a fluid stream being led through the tank heater 11 to the contents of the intermediate storage tank 10; an LCO 2 slip stream conduit 57; 67, 68, 66; 75, 79, 68, 66 arranged along the discharge conduit 41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43 , 61, 62, 73, 74 , 30 located downstream of the LCO 2 heater 19 configured to withdraw and convey, as the fluid stream, a slip stream in conduit 57; 66, 68, 67; 75, 79, 68, 66 to the tank heater 11 from the discharge conduit 41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43 , 61, 62, 73, 74, 30 ; and, a slip stream exit conduit 59; 69 configured to convey a fluid stream effluent from the tank heater 11 to the discharge conduit 41, 43, 51, 53, 55, 30; 41, 43, 61 , 62, 63, 65, 30; 41, 43, 61, 62, 73, 74 , 30 at a location downstream of the first LCO 2 pump 15.

[0009] In another aspect the invention refers to a method for maintaining pressure in a discharging intermediate LCO 2 tank during discharge of LCO 2 from the tank for avoidance of formation of dry ice in the intermediate LCO 2 storage tank during discharge, which system avoids or reduces the risk of formation of hydrates in the system from water contained in the LCO 2 , said method comprising the following steps: A discharging a stream of LCO 2 from an intermediate LCO 2 storage tank 10; B subjecting the stream of LCO 2 from step A to pumping in a first LCO 2 pump 15; C subjecting the stream of LCO 2 from step B to pumping in an injection pump 17; D subjecting the stream of LCO 2 from step C to heating in an LCO 2 heater 19, which method additionally comprises the following steps: E withdrawing a slip stream of LCO 2 from a heated stream of LCO 2 obtained in step D; F subjecting the slip stream of LCO 2 from step E to liquid-to-liquid heat exchange in a tank heater 11 with the liquid contents of the intermediate LCO 2 storage tank 10; and, G returning the slip stream of LCO 2 from step F to the stream of LCO 2 from step B.

[0010] The invention does not require excessive dehydration of the captured CO 2 further upstream in the CCS chain, such as at the liquefaction facility. According to the invention, contamination of either the LCO 2 within the storage tank or the heater's heating medium in the event of a tank-internal heater rupture is prevented.

[0011] Further embodiments and advantages of the invention will be apparent from the following detailed description and appended claims.

[0012] In the present disclosure, same reference numeral is used both to denote the conduit and the stream flowing therein. For example, reference numeral 41 is used to both denote the discharge stream conduit from the intermediate LCO 2 storage tank 10 (which is connected to the first pump 15), and also to denote the discharge stream itself flowing in said conduit.

[0013] The term "intermediate storage" as used herein denotes a storage intended to be temporary, prior to a long-term storage.

[0014] The term "long-term storage" as used herein denotes a storage intended to be permanent.BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

[0015] Figure 1 shows an embodiment of the inventive system 110 as indicated by the dashed line in its most generic embodiment, which system is configured to be implemented into an LCO 2 receiving terminal. Figure 2 shows another embodiment of the inventive system 110 as indicated by the dashed line, including a pressure let-down valve 21 arranged on the slip stream conduit 67, 68, 66 , and a complementary LCO 2 slip stream conduit 64, connecting the discharge conduit 41, 43, 61, 62, 63, 65, 30 with the slip stream conduit 67, 68, 66. Figure 3 shows yet another embodiment of the inventive system 110 as indicated by the dashed line, including a pressure let-down valve 21 arranged on the slip stream conduit 75, 79, 68, 66, and a recycle cooler 22 arranged on the discharge conduit 41, 43, 61, 62, 73, 74, 30, a recycle cooler conduit 80, 81 connected to the slip stream conduit 75, 79, 68, 66 , and to the recycle cooler 22. DETAILED DESCRIPTION OF THE INVENTION

[0016] According to the present invention a desired pressure in a discharging intermediate LCO 2 storage tank 10 can be maintained during discharging of LCO 2 from the tank 10, thereby avoiding formation of dry ice in the intermediate LCO 2 storage tank during the LCO 2 discharge from the tank. The invention also avoids or reduces the risk of formation of hydrates in the system from water contained in the LCO 2 . Since the LCO 2 in CCS applications, and hence also in the intermediate LCO 2 storage tank 10, typically contains some water, it is desirable to prevent the full liquid composition from becoming vapour. According to the present invention pressure in the storage tank 10 is maintained during injection by heating the liquid in the intermediate LCO 2 storage tank to sustain its vapour pressure (bubble point pressure) at the required tank pressure, effectively increasing the heat ingress into the tank in a controlled way. The source of heating is a slipstream of the warmed, pressurised LCO 2 that is injected into a pipeline 2 leading into a long-term CO 2 storage 1, which is introduced as an indirect heat source to a tank heater 11, either as an immersed heater (such as a tube bundle, coil bundle, plate pack, or similar), or as an external heater (such as a jacket or coil on the external tank surface). This has the advantage that the heat exchange medium is the LCO 2 itself, and will thus not be prone to freezing when it heats the tank contents. In the case of an immersed heater, any rupture will not cause undesirable contamination of the LCO 2 in the storage tanks, as the fluid on both sides of the tank heater is the same. The LCO 2 used for heating is additionally kept in the liquid phase, and thus avoids the risk of hydrate formation itself.

[0017] After heat exchange in tank heater 11, the cooled slipstream of LCO 2 59; 69 exiting the tank heater is returned to the stream of LCO 2 being discharged from the intermediate tank at a location downstream of a first pumping stage 15. Because of temperature approaches in the heat exchanger 11, the cooled recycled LCO 2 must always be at least slightly warmer than the LCO 2 in the storage tank 10. If the at least slightly warmer cooled recycled LCO 2 stream is let down to exactly tank pressure, it will flash (partly vaporise), which should not be sent to a pump suction as this will typically cause pump cavitation. But if the slightly warmer cooled recycled LCO 2 is let down to a somewhat higher pressure, e.g. after a first pumping stage 15, it can remain liquid, which allows for subsequent further pumping.

[0018] With reference to the drawings, the slip stream is taken downstream of the LCO 2 heater 19 from conduit 55 as shown in Fig. 1, 65 as shown in Fig. 2, and 74 as shown in Fig. 3, respectively.

[0019] With reference to Fig. 1, an embodiment is shown wherein LCO 2 is pumped in two 15, 17 (or more - not shown) stages, and then heated prior to entering the pipeline 2. A slipstream of the warmed, high-pressure LCO 2 to be injected into the pipeline is recycled to a heater to warm the liquid in the storage tank, itself being cooled, and returned to an intermediate point at a higher pressure than the storage tank, such as downstream of the first pumping stage.

[0020] With reference to Fig. 2, another embodiment is shown wherein the LCO 2 pressure of the withdrawn slip stream is reduced prior to entering the tank heater, and the LCO 2 stream used as the heating medium is part-bypassed via complementary LCO 2 slip stream conduit 64 to allow control of the temperature the LCO 2 entering the let-down valve, in order to ensure that the heating LCO 2 passing through the tank heater does not vaporise as a result of the pressure reduction. Accordingly, in embodiments wherein temperature control is not required, e.g. depending on the performance characteristics of the LCO 2 heater 19, and / or the temperature of the heated effluent 65, a complementary LCO 2 slip stream conduit 64 may be omitted.

[0021] With reference to Fig. 3, yet another embodiment is shown wherein temperature control of the stream 68 entering the let-down valve is effected by part-cooling with high-pressure, cold LCO 2 62 upstream of the LCO 2 heater 19. LIST OF REFERENCE SIGNS USED

[0022] 1underground long-term storage facility 2pipeline 10intermediate LCO 2 storage tank 11tank heater 15first LCO 2 pump on LCO 2 discharge conduit 17injection pump 19LCO 2 heater 21pressure let-down valve 22recycle cooler 57LCO 2 slip stream conduit 67, 68, 66LCO 2 slip stream conduit 75, 79, 68, 66LCO 2 slip stream conduit 41, 43, 51, 53, 55, 30LCO 2 discharge conduit 41, 43, 61, 62, 63, 65, 30LCO 2 discharge conduit 41, 43, 61, 62, 73, 74, 30LCO 2 discharge conduit 59slip stream exit conduit from tank heater 64complementary LCO 2 slip stream conduit 69slip stream exit conduit from tank heater 80, 81recycle cooler conduit 110system

Claims

1. A system (110) configured to maintaining a desired pressure in a discharging intermediate LCO2 tank (10) during discharging of LCO2 from the tank thereby avoiding formation of dry ice in the intermediate LCO2 storage tank during the LCO2 discharge from the tank, which system avoids or reduces the risk of formation of hydrates in the system from water contained in the LCO2, comprising: - an intermediate LCO2 storage tank (10); - an LCO2 discharge conduit (41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74, 30) connected to the intermediate LCO2 storage tank (10) configured to discharge LCO2 to be injected into a long-term CO2 storage (1); - a first LCO2 pump (15) arranged along the discharge conduit (41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74, 30); - an injection pump (17) arranged along the discharge conduit (41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74, 30) downstream of the first LCO2 pump (15); - an LCO2 heater (19) arranged along the discharge conduit (41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74, 30) downstream of the LCO2 injection pump (17); characterized in additionally comprising, - a tank heater (11) configured to receive and to transfer heat from a fluid stream being led through the tank heater (11) to the contents of the intermediate storage tank (10); - an LCO2 slip stream conduit (57; 67, 68, 66; 75, 79, 68, 66) arranged along the discharge conduit (41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74, 30) located downstream of the LCO2 heater (19) configured to withdraw and convey, as the fluid stream, a slip stream in conduit (57; 67, 68, 66; 75, 79, 68, 66) to the tank heater (11) from the discharge conduit (41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74, 30); and, - a slip stream exit conduit (59; 69) configured to convey a fluid stream effluent from the tank heater (11) to the discharge conduit (41, 43, 51, 53, 55, 30; 41, 43, 61, 62, 63, 65, 30; 41, 43, 61, 62, 73, 74, 30) at a location downstream of the first LCO2 pump (15).

2. The system of claim 1, additionally comprising - a pressure let-down valve (21) arranged along the slip stream in conduit (67, 68, 66).

3. The system of claim 2, additionally comprising: - a complementary LCO2 slip stream conduit (64) arranged along discharge conduit (41, 43, 61, 62, 63, 65, 30) at a location downstream of the injection pump (17) and upstream of the LCO2 heater (19) configured to withdraw a complementary slip stream from the discharge conduit (41, 43, 61, 62, 63, 65, 30) and to connect the latter with the slip stream in conduit (67, 68, 66) at a location upstream of the pressure let-down valve (21).

4. The system of claim 1, additionally comprising - a pressure let-down valve (21) arranged along the slip stream in conduit (75, 79, 68, 66); and, - a recycle cooler (22) arranged along the discharge conduit (41, 43, 61, 62, 73, 74, 30) at a located downstream of the injection pump (17) and upstream of the LCO2 heater (19) configured to receive a partial stream from the slip stream; - a recycle cooler inlet conduit (80) arranged along the slip stream conduit (75, 79, 68, 66) at a location upstream of the pressure let-down valve (21) configured to connect the slip stream conduit (75, 79, 68, 66) with an inlet of the recycle cooler (22); and, - a recycle cooler outlet conduit (81) arranged along the slip stream conduit (75, 79, 68, 66) at a location downstream of the recycle cooler inlet conduit (80), wherein the partial stream from the slip stream is being led through the recycle cooler (22).

5. A method for maintaining pressure in a discharging intermediate LCO2 tank during discharge of LCO2 from the tank for avoidance of formation of dry ice in the intermediate LCO2 storage tank during discharge, which system avoids or reduces the risk of formation of hydrates in the system from water contained in the LCO2, said method comprising the following steps: A) discharging a stream of LCO2 from an intermediate LCO2 storage tank (10); B) subjecting the stream of LCO2 from step (A) to pumping in a first LCO2 pump (15); C) subjecting the stream of LCO2 from step (B) to pumping in an injection pump (17); D) subjecting the stream of LCO2 from step (C) to heating in an LCO2 heater (19) characterized in additionally comprising the following steps: E) withdrawing a slip stream of LCO2 from a heated stream of LCO2 obtained in step (D); F) subjecting the slip stream of LCO2 from step (E) to liquid-to-liquid heat exchange in a tank heater (11) with the liquid contents of the intermediate LCO2 storage tank (10); and, G) returning the slip stream of LCO2 from step (F) to the stream of LCO2 from step (B).

6. The method of claim 5, additionally comprising the following steps: H) subjecting the slip stream of LCO2 from step (E) to a reduced pressure in a pressure let-down valve (21); I) withdrawing a complementary LCO2 slip stream from step (C); and, J) combining the complementary LCO2 slip stream from step (I) with the slip stream of LCO2 from step (E) before entering to step (H).

7. The method of claim 5, additionally comprising the following steps: H) subjecting the slip stream of LCO2 from step (E) to a reduced pressure in a pressure let-down valve (21); K) withdrawing a partial stream from the LCO2 slip stream from step (C); L) subjecting the partial stream of LCO2 from step (K) to liquid-to-liquid heat exchange in a recycle cooler (22) with the stream of LCO2 from step (C) before entering to step (D); and, M) combining the partial stream of LCO2 from step (L) with the remaining slip stream resulting from step (K) before entering to step (H).