A system and method for reducing the inventory medium in a supercritical CO2 pipeline during maintenance

Abstract

The present invention discloses a system and method for reducing the inventory of medium in a supercritical CO2 pipeline during maintenance. The reduction system includes a main pipeline transportation channel, a valve chamber connection system, a movable liquefaction and transfer system, a safety relief system for residual gas in the pipeline, and a safety monitoring system. Based on the basic physical properties of supercritical CO2 and the characteristics of pipeline maintenance conditions, aiming at the characteristics of large pipe volume and high medium inventory between the block valves of supercritical CO2 pipelines, from the perspectives of reducing emissions, efficient reinjection, and economic operation, through the system design and method of the present invention, effective emission reduction of CO2 in the target maintenance pipe section after shutdown is achieved, and further reference and inspiration are provided for subsequent large-scale CO2 pipeline transportation projects.

Description

Technical Field

[0001] The present invention belongs to the technical field of pipeline transportation, and particularly relates to a system and method for reducing the emission of the medium stored in a pipeline under the maintenance condition of a supercritical CO2 pipeline. Background Art

[0002] Under the background of the increasing demand for energy conservation, emission reduction and environmental protection in the energy and chemical industries day by day, large-scale capture and utilization of CO2 is an optimal way to effectively reduce carbon emissions. Among them, long-distance pipeline transportation is a key way to support the capture and utilization of CO2. The critical temperature of CO2 is about 31.1 °C, and the critical pressure is about 7.38 MPa. When transporting over a long distance through a pipeline, it is generally recognized that the supercritical transportation phase state mode with a pressure higher than the critical pressure is adopted to improve the economy of transportation. In addition, CO2 has the characteristic of solidifying at low temperatures. The triple point of pure CO2 is -56.6 °C and 0.518 MPa, which is a significant characteristic that differentiates CO2 from conventional hydrocarbon media.

[0003] In the pipeline transportation of conventional hydrocarbons, periodically performing in-pipeline inspections is a key means to effectively diagnose the internal state of the pipeline, and external maintenance also needs to be carried out regularly; when an abnormal situation occurs in the pipeline body, maintenance and replacement of the target pipe section are required, which requires stopping the transportation of the pipeline and emptying the medium in this section of the pipeline. Since CO2 itself comes from the capture link, and in a long-distance pipeline, the volume of the gas stored in the pipeline between the cut-off valves is large. Although direct venting is a relatively conventional treatment scheme for the medium stored in the pipeline, adopting this scheme for a CO2 pipeline goes against the original intention of carbon capture, and further exploration of more emission reduction-friendly schemes is needed, such as transporting by tanker trucks and reinjecting into the upstream and downstream pipelines.

[0004] However, there are few relevant achievements in the existing publicly reported literature on emission reduction measures under the maintenance condition of supercritical CO2 pipelines. Therefore, it is necessary to carry out relevant research to effectively reduce the CO2 in the target maintenance pipe section after stopping the transportation, and further provide reference for the subsequent large-scale CO2 pipeline transportation projects. Summary of the Invention

[0005] The purpose of the present invention is to provide a system and method for reducing the emission of the medium stored in a pipeline under the maintenance condition of a supercritical CO2 pipeline, which can efficiently and economically control the emission of the medium stored in a long-distance CO2 pipeline under the maintenance condition, aiming at the above-mentioned existing problems.

[0006] The main technical idea of the present invention is based on the basic physical properties of supercritical CO2 and the characteristics of pipeline maintenance conditions. Considering the large pipe volume (at least 8 km pipe length) and high medium inventory between the block valves of the supercritical CO2 pipeline, from the perspectives of reducing emissions, efficient reinjection, and economic operation, a valve chamber connection system, a mobile liquefaction and reinjection system, a pipeline residual gas safety relief system, a safety monitoring system, etc. are respectively set up to effectively control the medium emissions under pipeline maintenance conditions. Among them, the valve chamber connection system is set in the pipeline valve chamber and mainly includes a preset branch pipeline, a preset branch valve, a preset instrument, etc., which are used to provide a channel connecting the upstream pipeline and the downstream pipeline of the block valve after the pipeline stops transporting, create conditions for medium treatment and reinjection, and at the same time provide a main relief channel for the residual gas; the mobile liquefaction and reinjection system is a prefabricated mobile skid-mounted system, including pipelines, a small refrigeration cycle, heat exchange facilities, a booster pump, supporting valves, etc., which are used to liquefy the stored CO2 in the pipeline to be recovered and boost and reinject it into the downstream pipeline; the pipeline residual gas safety relief system is a prefabricated mobile skid-mounted system, including pipelines, control valves, a small vent riser, etc., which are used to directly conduct safety relief when the pressure of the stored CO2 in the pipeline is relatively low and liquefaction-boosting reinjection is no longer economical, so as to empty the residual CO2 in the pipeline; the safety monitoring system includes a pressure transmitter, a temperature transmitter, etc., which are used to monitor the processes of medium transfer, boosting, and venting in the pipeline.

[0007] Thus, the purpose of reducing the inventory of the medium in the supercritical CO2 pipeline under maintenance conditions is achieved.

[0008] The technical solution adopted by the present invention is: A system for reducing the inventory of the medium in a supercritical CO2 pipeline under maintenance conditions, which is characterized in that it includes a main pipeline transportation channel, a valve chamber connection system, a mobile liquefaction and transportation system, a pipeline residual gas safety relief system, and a safety monitoring system;

[0009] The valve chamber connection system includes an upstream branch pipe and a downstream branch pipe. One end of the upstream branch pipe is connected to the upstream pipeline of the main pipeline transportation channel, and the other end is connected to the upstream branch pipe block valve. One end of the downstream branch pipe is connected to the downstream pipeline of the main pipeline transportation channel, and the other end is connected to the downstream branch pipe block valve;

[0010] The mobile liquefaction and transportation system includes a connecting pipeline, a reliquefaction device, and a booster pump. The connecting pipeline is used to connect the upstream branch pipe block valve and the downstream branch pipe block valve. The reliquefaction device and the booster pump are sequentially arranged on the connecting pipeline. The reliquefaction device is used to liquefy the CO2 supplied upstream, and the booster pump is used to boost the liquefied CO2 supplied upstream and inject it into the downstream main pipeline;

[0011] The described pipeline residual gas safety relief system includes a vent shut-off valve and a supporting vent pipeline, which are used for safely relieving the low-pressure CO2 remaining after the medium transfer in the upstream main pipeline.

[0012] The described safety monitoring system is used for real-time monitoring of the pressure and temperature at each point of the pipeline.

[0013] For the supercritical CO2 pipeline inventory medium emission reduction system under maintenance conditions of the present invention, a bypass pipeline parallel to the re-liquefaction device is connected to the connecting pipeline, and a bypass shut-off valve is provided on the bypass pipeline.

[0014] For the supercritical CO2 pipeline inventory medium emission reduction system under maintenance conditions of the present invention, a first shut-off valve and a second shut-off valve are respectively provided at the upstream and downstream ends of the re-liquefaction device.

[0015] For the supercritical CO2 pipeline inventory medium emission reduction system under maintenance conditions of the present invention, a regulating valve is provided on the connecting pipeline between the first shut-off valve and the upstream branch shut-off valve. The upstream end of the bypass shut-off valve is connected to the connecting pipeline between the regulating valve and the first shut-off valve, and the downstream end of the bypass shut-off valve is connected to the connecting pipeline between the second shut-off valve and the booster pump.

[0016] For the supercritical CO2 pipeline inventory medium emission reduction system under maintenance conditions of the present invention, the movable liquefaction and transfer system further includes a post-pump safety valve, which is provided downstream of the booster pump, and the post-pump safety valve is connected to the upstream of the regulating valve through a pressure relief pipeline.

[0017] For the supercritical CO2 pipeline inventory medium emission reduction system under maintenance conditions of the present invention, the vent shut-off valve is provided downstream of the regulating valve and is used to connect the connecting pipeline and the vent pipeline.

[0018] For the supercritical CO2 pipeline inventory medium emission reduction system under maintenance conditions of the present invention, the safety monitoring system includes an upstream branch pressure transmitter and an upstream branch temperature transmitter provided on the upstream branch, a downstream branch pressure transmitter and a downstream branch temperature transmitter provided on the downstream branch, a first temperature transmitter and a first pressure transmitter provided between the downstream of the regulating valve and the upstream of the first shut-off valve, and a second pressure transmitter and a second temperature transmitter provided between the downstream of the second shut-off valve and the booster pump.

[0019] A method for reducing the inventory medium of a supercritical CO2 pipeline under maintenance conditions, characterized in that: the specific emission reduction method is as follows:

[0020] Before planned pipeline shutdown, reduce the pipeline operating pressure and reserve a pressure margin for downstream pipeline transfer and filling.

[0021] After the pipeline is shut down, connect and install the movable liquefaction and transfer system to the upstream branch cutoff valve and the downstream branch cutoff valve of the valve chamber connection system. Open the first cutoff valve, the second cutoff valve, and the bypass cutoff valve. Inject dry CO2 gas at a certain pressure into the pipeline in advance to establish sufficient back pressure and keep the vent cutoff valve closed.

[0022] Open the upstream branch cutoff valve and slowly open the regulating valve. The medium in the upstream pipeline quickly fills into the movable liquefaction and transfer system. Close the first cutoff valve and the second cutoff valve. Open the downstream branch cutoff valve and slowly open the booster pump to establish a pressurization channel for the upstream pipeline, the bypass cutoff valve, the booster pump, and the downstream pipeline.

[0023] When the CO2 pressure in the upstream pipeline drops to the bubble point pressure, open the first cutoff valve and the second cutoff valve, close the bypass cutoff valve, and turn on the re-liquefaction device to cool and re-liquefy the incoming gas-liquid two-phase CO2. After the re-liquefied CO2 is pressure-lifted by the booster pump, it is injected into the downstream pipeline.

[0024] When the pressure in the upstream pipeline drops to 1.8 MPa.g - 2.0 MPa.g, close the first cutoff valve, the second cutoff valve, the re-liquefaction device, the booster pump, and the downstream branch cutoff valve, and turn off the movable liquefaction and transfer system. At the same time, open the vent cutoff valve to connect the upstream pipeline, the upstream branch cutoff valve, the regulating valve, the vent cutoff valve, and the vent pipeline, and start the pipeline residual gas safety relief system to discharge the residual CO2 in the upstream pipeline.

[0025] In the method for reducing the inventory medium in the supercritical CO2 pipeline during maintenance, when the bypass cutoff valve is in the open state, the re-liquefaction device is in the closed state. During the operation process, monitor and analyze the physical properties of CO2 in the upstream pipeline through the first temperature transmitter and the first pressure transmitter. When the CO2 turns into a gas-liquid two-phase state, close the bypass cutoff valve and turn on the re-liquefaction device. At the same time, monitor the pressurized pressure through the downstream branch pressure transmitter and the downstream branch temperature transmitter. When the pressurized pressure exceeds the design pressure of the downstream pipeline, the downstream pipeline should be linked to open the external discharge operation of the medium. After reducing the system pressure of the downstream pipeline, continue the transfer operation of CO2 in the upstream pipeline.

[0026] The method for reducing the inventory medium in the supercritical CO2 pipeline during maintenance conditions of the present invention detects the parameters of the second pressure transmitter and the second temperature transmitter during the liquefaction and pressurization process. When the temperature of the second temperature transmitter is lower than -15°C, the refrigerant circulation rate of the re-liquefaction device is adjusted; when the temperature of the second temperature transmitter is lower than -18°C, the re-liquefaction device and the booster pump are immediately shut down for troubleshooting; when the pressure of the second pressure transmitter exceeds the design pressure of the downstream pipeline, the external discharge operation of the medium should be linked to the downstream pipeline to reduce the system pressure of the downstream pipeline, and then the transfer operation of CO2 in the upstream pipeline is continued.

[0027] Compared with the prior art, the positive effects of the present invention are as follows: Based on the basic physical properties of supercritical CO2 and the characteristics of pipeline maintenance conditions, aiming at the characteristics of large pipe volume (at least 8 km pipe length) and high medium inventory between the cut-off valves of supercritical CO2 pipelines, from the perspectives of reducing emissions, efficient reinjection, and economic operation, through the system design and method of the present invention, effective emission reduction of CO2 in the target maintenance pipe section after shutdown is achieved, and further reference and inspiration are provided for the subsequent large-scale CO2 pipeline transportation projects.

[0028] Specifically manifested as:

[0029] (1) Scientifically designed

[0030] Aiming at the transportation medium and operation characteristics of supercritical CO2 pipelines, the present invention proposes methods such as reducing the pipeline operating pressure in advance, reserving transfer margin, re-liquefying and transferring to the downstream main line, and discharging residual medium from the perspective of reducing carbon emissions during the maintenance process, to clean the CO2 in the pipeline inventory of the main line pipeline to be truncated and maintained, ensuring that the transfer ratio is above 96%, and significantly reducing a large amount of carbon emissions caused by the maintenance of long-distance, high-pressure, supercritical CO2 pipelines. Further, compared with the conventional operation concept of pipe section venting during planned maintenance of conventional hydrocarbon pipelines, the method proposed by the present invention is based on the principle of "prioritizing transfer and scientific venting", which better meets the development needs of the dual-carbon goal.

[0031] (2) Good economy

[0032] Based on the occurrence frequency of planned maintenance, the present system sets up a movable liquefaction and reinjection system, which greatly reduces the investment in setting up fixed facilities at the station, and can also meet the applications of different supercritical CO2 pipeline systems. That is to say, by setting up a set of movable liquefaction and reinjection systems, the operation and maintenance of different supercritical CO2 pipelines can be satisfied. Further, considering the transfer efficiency comprehensively, the present system selects carbon steel material to replace stainless steel material, which greatly controls the system investment.

[0033] (3) Promote technological development

[0034] The solution proposed by this system plays an important role in engineering guidance and reference. The key process system configurations such as the system depressurization operation before shutdown, the reliquefaction and transfer of the stored medium after shutdown, and the discharge of the residual medium are satisfied, which promotes the development of technical concepts and technological progress in this field. Brief Description of the Drawings

[0035] The present invention will be described by way of specific embodiments with reference to the accompanying drawings, wherein

[0036] Figure 1 is a schematic structural diagram of the present invention.

[0037] Reference numerals in the drawings: 1 is the upstream pipeline, 2 is the main line block valve, 3 is the downstream pipeline, 4 is the upstream branch pressure transmitter, 5 is the upstream branch temperature transmitter, 6 is the upstream branch block valve, 7 is the upstream branch, 8 is the downstream branch pressure transmitter, 9 is the downstream branch temperature transmitter, 10 is the downstream branch block valve, 11 is the downstream branch, 21 is the connecting pipeline, 22 is the regulating valve, 23 is the first temperature transmitter, 24 is the first pressure transmitter, 25 is the first block valve, 26 is the reliquefaction device, 27 is the second block valve, 28 is the second pressure transmitter, 29 is the second temperature transmitter, 30 is the booster pump, 31 is the bypass block valve, 32 is the safety valve after the pump, 33 is the pressure relief pipeline, 41 is the vent block valve. Detailed Description of the Preferred Embodiments

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and illustrated in the accompanying drawings here can be arranged and designed in various different configurations.

[0039] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the present invention claimed, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0040] It should be noted that, without conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0041] It should be noted that similar reference numerals and letters denote similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0042] In the description of the embodiments of the present invention, it should be noted that the indicated orientation or positional relationship is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the invention product is usually placed during use, or the orientation or positional relationship commonly understood by those skilled in the art, or the orientation or positional relationship in which the invention product is usually placed during use. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention. In addition, the terms "first" and "second" are only used for distinguishing descriptions and cannot be construed as indicating or implying relative importance.

[0043] In the description of the embodiments of the present invention, it should also be noted that unless otherwise clearly specified and limited, the terms "set" and "connect" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a direct connection or an indirect connection through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances; the drawings in the embodiments are used to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Usually, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0044] As Figure 1 shown, a system for reducing the inventory medium in a supercritical CO2 pipeline during maintenance conditions, characterized in that it includes a main pipeline transportation channel, a valve chamber connection system, a movable liquefaction and transportation system, a pipeline residual gas safety relief system, and a safety monitoring system. By setting up this system, the inventory medium in the supercritical CO2 pipeline during maintenance conditions can be effectively transported to achieve the purpose of CO2 emission reduction.

[0045] Among them, the main pipeline transportation channel includes the existing upstream pipeline 1, the main pipeline cut-off valve 2, and the existing downstream pipeline 3, which are used to provide the main pipeline transportation channel and the main pipeline cut-off function for supercritical CO2, and the main pipeline transportation channel is a fixed configuration of the existing supercritical CO2 pipeline.

[0046] The valve chamber connection system includes an upstream branch pipe 7 and a downstream branch pipe 11. One end of the upstream branch pipe 7 is connected to the upstream pipeline 1 of the main pipeline transportation channel, and the other end is connected to the upstream branch pipe cut-off valve 6. One end of the downstream branch pipe 11 is connected to the downstream pipeline 3 of the main pipeline transportation channel, and the other end is connected to the downstream branch pipe cut-off valve 10. By setting up a bypass branch pipe, the basic conditions for transporting or emptying the medium in the main pipeline are provided.

[0047] The movable liquefaction and transfer system includes a connecting pipeline 21, a small re-liquefaction device 26, and a booster pump 30. The connecting pipeline 21 is used to connect the upstream branch cutoff valve 6 and the downstream branch cutoff valve 10, providing a medium re-liquefaction and pressurization path. It is made of carbon steel. The re-liquefaction device 26 and the booster pump 30 are sequentially arranged on the connecting pipeline 21. The re-liquefaction device 26 is a small integrated system of cyclic expansion refrigeration, including modules such as refrigerant compression, expansion refrigeration, and working medium-refrigerant heat exchange, which is used to provide cooling capacity for the CO2 supplied upstream and liquefy it. The subcooling temperature is controlled at 2-4°C. Further, preferably, the refrigerant is propane, and the lowest temperature of the CO2 after heat exchange shall not be lower than -20°C. A bypass pipeline parallel to the re-liquefaction device 26 is connected to the connecting pipeline 21, and a bypass cutoff valve 31 is arranged on the bypass pipeline. The bypass cutoff valve is a manual ball valve, made of carbon steel, and is arranged on the bypass pipeline of the small re-liquefaction device. It is opened when the upstream medium is still in the liquid phase. First cutoff valves 25 and second cutoff valves 27 are respectively arranged at the upstream and downstream ends of the re-liquefaction device 26. Both the first cutoff valve and the second cutoff valve are manual ball valves, made of carbon steel, and are arranged on the main path of the connecting pipeline 21, used to control the activation and isolation of the small re-liquefaction device. A regulating valve 22 is arranged on the connecting pipeline 21 between the first cutoff valve 25 and the upstream branch cutoff valve 6. The regulating valve is electrically regulated, used to control the CO2 transfer and discharge flow in the upstream main line. The upstream end of the bypass cutoff valve 31 is connected to the connecting pipeline 21 between the regulating valve 22 and the first cutoff valve 25, and the downstream end of the bypass cutoff valve 31 is connected to the connecting pipeline 21 between the second cutoff valve 27 and the booster pump 30. The booster pump 30 is arranged at the end of the connecting pipeline, electrically controlled, preferably a reciprocating booster pump, made of carbon steel, used to pressurize the liquid CO2 supplied upstream and inject it into the downstream main line pipeline. Further, the booster pump 30 should meet a wide range of inlet pressure and outlet pressure fluctuation ranges. The movable liquefaction and transfer system is used to connect with the valve chamber connection system to form a movable integrated system. When the upstream pipeline needs to be shut down for maintenance, it efficiently transfers the medium in the upstream pipeline and inputs it into the downstream main line, minimizing the CO2 venting volume in the upstream main line to the greatest extent.

[0048] Wherein, the movable liquefaction and transfer system further includes a safety valve 32 after the pump. The safety valve 32 after the pump is arranged downstream of the booster pump 30. The safety valve 32 after the pump is connected to the upstream of the regulating valve 22 through a pressure relief pipeline 33. The set pressure of the safety valve after the pump is 1.05 times the design pressure of the downstream main line. The pressure relief pipeline is connected to the safety valve after the pump to send the pressure relief medium to the upstream of the regulating valve.

[0049] The described pipeline residual gas safety relief system is integrally installed on the movable liquefaction and transfer system, and specifically includes a vent shut-off valve 41 and a supporting vent pipeline for safely discharging the low-pressure CO2 remaining after the transfer of the medium in the upstream main pipeline.

[0050] Specifically, the vent shut-off valve 41 is arranged downstream of the regulating valve 22, normally closed, made of carbon steel, a manual ball valve, used to connect the connecting pipeline 21 and the vent pipeline. The supporting vent pipeline includes a vent pipe and a vent riser, both of which are in a movable and on-site assembly mode, used to introduce the CO2 to be discharged into a safe location and height for discharge.

[0051] The safety monitoring system includes temperature transmitters and pressure transmitters arranged in the entire emission reduction system, used to monitor the pressure and temperature at each point of the pipeline in real time.

[0052] Specifically, the safety monitoring system includes an upstream branch pipe pressure transmitter 4 and an upstream branch pipe temperature transmitter 5 arranged on the upstream branch pipe 7, used to monitor the node temperature and pressure during the whole process of pipeline operation. At the same time, the upstream branch pipe 7 is used to provide a flow channel for the transfer and discharge of the main pipeline medium, and is made of carbon steel; the upstream branch pipe shut-off valve 6 is a manual ball valve, made of carbon steel, normally closed, and the downstream of the valve is blocked by a blind flange; a downstream branch pipe pressure transmitter 8 and a downstream branch pipe temperature transmitter 9 arranged on the downstream branch pipe 11, used to monitor the node temperature and pressure during the whole process of pipeline operation. At the same time, the downstream branch pipe 11 is used to provide a flow channel for the transfer and discharge of the main pipeline medium, and is made of carbon steel; the downstream branch pipe shut-off valve 10 is a manual ball valve, made of carbon steel, normally closed, and the upstream of the valve is blocked by a blind flange; a first temperature transmitter 23 and a first pressure transmitter 24 arranged between the downstream of the regulating valve 22 and the upstream of the first shut-off valve 25, used to measure the medium temperature and pressure within this range; a second pressure transmitter 28 and a second temperature transmitter 29 arranged between the downstream of the second shut-off valve 27 and the booster pump 30, used to measure the medium temperature and pressure within this range.

[0053] The working principle of the present invention is:

[0054] (1) Under normal operating conditions, in order to improve the transportation efficiency as much as possible, supercritical phase is used to transport CO2. Under this transportation condition, the density of CO2 can reach 850 kg / m 3 or more. In the pipe section between the main pipeline shut-off valves, the actual pipe inventory of CO2 is relatively large. This pipe inventory of CO2 can be considered for transfer to avoid all the CO2 specially collected upstream from being directly discharged into the atmosphere before planned maintenance.

[0055] (2) When a planned maintenance of a certain section of pipeline is required, all the medium in this section of pipeline needs to be emptied. Before the pipeline stops transporting, the system operating pressure is actively reduced through operation, but the CO2 under the operating pressure still remains in the supercritical state. The purpose of this operation is to reduce the pressure of the downstream pipeline and provide a greater available pressure margin for the CO2 transportation proposed in this application.

[0056] (3) After the pipeline stops transporting, a movable liquefaction and transportation system is installed in the downstream valve chamber (or station) adjacent to the maintenance section of the pipeline. When the medium pressure in the upstream pipeline still remains in the dense phase state, it is directly injected into the downstream pipeline after being pressurized by a booster pump, and the CO2 is stored in the downstream pipeline by utilizing the compressibility of CO2 itself. When the medium pressure in the upstream pipeline drops to the gas-liquid two-phase state, the medium is first introduced into the re-liquefaction system, and the liquefied subcooled CO2 is injected into the downstream pipeline after being pressurized. When the CO2 pressure in the upstream main pipeline drops to around 1.8 - 2.0 MPa.g, since the corresponding liquefaction temperature is close to -20 °C, which exceeds the low-temperature limit of the carbon steel material of the system, further re-liquefaction and reinjection are not recommended; at the same time, at this pressure, the density of CO2 is about 40 kg / m 3 or so, and the remaining CO2 in the pipeline accounts for less than 4% of the initial transportation period, and the transportation rate has reached 96%. For this part of the remaining CO2, it is treated by direct venting.

[0057] The present invention also provides a method for reducing the inventory medium during the maintenance of a supercritical CO2 pipeline, and the specific reduction method is as follows:

[0058] Step 1: During normal transportation, the operating pressure of the supercritical CO2 pipeline is controlled above 10 MPa.g, and the transportation temperature is close to the soil temperature. Under the planned shutdown and maintenance condition, the upstream export pump, intermediate booster pump, main pipeline block valves at various places, etc. are orderly closed, and the main pipeline realizes safe shutdown. Further, to improve the operation efficiency of the subsequent steps, before the planned shutdown, the pipeline operating pressure is reduced to around 8 MPa.g to reduce the medium mass in the pipeline section to be maintained and reserve the available transportation and filling pressure margin for the downstream pipeline.

[0059] Step 2: After the pipeline stops transporting, since at least the pipeline medium between two block valves needs to be emptied during the upstream main pipeline maintenance, and the emptying volume is large, most of the inventory medium is considered to be transported to the adjacent downstream main pipeline. For this purpose, the movable liquefaction and transportation system is connected and installed with the upstream branch block valve and the downstream branch block valve of the valve chamber connection system, and the first block valve, the second block valve, and the bypass block valve are opened. Dry CO2 gas with a pressure of 2.2 MPa.g - 2.5 MPa.g is pre-injected into the pipeline to establish sufficient back pressure to avoid severe low temperature after directly introducing the CO2 from the upstream main pipeline, and the vent block valve is kept closed.

[0060] Step 3: Open the upstream branch pipe shut-off valve and slowly open the regulating valve. The medium in the upstream pipeline is quickly filled into the mobile liquefaction and transfer system. Since the pressures of the upstream and downstream pipelines are consistent and CO2 has certain compressible characteristics, close the first and second shut-off valves, open the downstream branch pipe shut-off valve, and slowly start the booster pump to establish a booster channel for the upstream pipeline, bypass shut-off valve, booster pump, and downstream pipeline.

[0061] In this step, when the bypass shut-off valve is in the open state, the reliquefaction device is in the closed state. During the operation, the physical state of CO2 in the upstream pipeline is monitored and analyzed by the first temperature transmitter and the first pressure transmitter. When CO2 is converted into gas-liquid two-phase, the bypass shut-off valve is closed and the reliquefaction device is opened. At the same time, the pressure after pressurization is monitored by the downstream branch pressure transmitter and the downstream branch temperature transmitter. When the pressure after pressurization exceeds the design pressure of the downstream pipeline, the downstream pipeline should be linked to open the discharge operation of the medium, and after reducing the system pressure of the downstream pipeline to 8MPa.g, the CO2 transfer operation of the upstream pipeline is continued.

[0062] Step 4: Since the CO2 pressure in the upstream pipeline drops to the bubble point pressure, it is impossible to directly perform the boosting operation through the booster pump. Open the first block valve and the second block valve, close the bypass block valve, and start the reliquefaction device to cool and reliquefy the inflowing gas-liquid two-phase CO2. The subcooling temperature is controlled at 2 to 4°C. The reliquefied CO2 is pressurized by the booster pump and then injected into the downstream pipeline.

[0063] During the liquefaction and pressurization process, detect the parameters of the second pressure transmitter and the second temperature transmitter. When the temperature of the second temperature transmitter is lower than -15°C, adjust the refrigerant circulation volume of the reliquefaction device; when the temperature of the second temperature transmitter is lower than -18°C, immediately shut down the reliquefaction device and the booster pump to troubleshoot; when the pressure of the second pressure transmitter exceeds the design pressure of the downstream pipeline, the downstream pipeline should be linked to open the discharge operation of the medium, and after reducing the system pressure of the downstream pipeline, continue the CO2 transfer operation of the upstream pipeline.

[0064] Step 5: When the pressure of the upstream pipeline is reduced to 1.8MPa.g~2.0MPa.g, the corresponding liquefaction temperature under this pressure is lower than -20℃, and the density of CO2 gas phase is about 4% of the initial supercritical transport. The above steps have achieved a medium transfer rate of about 96%, so no further liquefaction and transfer are considered. Close the first shut-off valve, the second shut-off valve, the reliquefaction device, the booster pump and the downstream branch shut-off valve, close the movable liquefaction and transfer system, and at the same time, open the venting shut-off valve, connect the upstream pipeline, the upstream branch shut-off valve, the regulating valve, the venting shut-off valve and the venting pipeline, start the pipeline residual gas safety release system, and release the residual CO2 in the upstream pipeline.

[0065] During the venting process, the venting speed is controlled by a regulating valve to prevent the detection parameter of the first temperature transmitter from dropping below -20°C. When the pressure transmitter of the upstream branch pipe indicates that the pressure is close to 0 MPa.g, it means that the CO2 in the upstream main line has been basically vented. After injecting nitrogen for replacement using the interface of the adjacent upstream station, subsequent maintenance work can be carried out.

[0066] The present invention is not limited to the foregoing specific embodiments. The present invention extends to any new feature or any new combination disclosed in this specification, as well as any new combination of the steps of any new method or process disclosed.

Claims

1. A system for reducing the inventory medium in a supercritical CO2 pipeline during maintenance, characterized in that: It includes a main line transportation channel, a valve chamber connection system, a movable liquefaction and transfer system, a pipeline residual gas safety relief system, and a safety monitoring system; The valve chamber connection system includes an upstream branch pipe (7) and a downstream branch pipe (11). One end of the upstream branch pipe (7) is connected to the upstream pipeline (1) of the main line transportation channel, and the other end is connected to the upstream branch pipe cut-off valve (6). One end of the downstream branch pipe (11) is connected to the downstream pipeline (3) of the main line transportation channel, and the other end is connected to the downstream branch pipe cut-off valve (10); The movable liquefaction and transfer system includes a connection pipeline (21), a reliquefaction device (26), and a booster pump (30). The connection pipeline (21) is used to connect the upstream branch pipe cut-off valve (6) and the downstream branch pipe cut-off valve (10). The reliquefaction device (26) and the booster pump (30) are arranged on the connection pipeline (21) in sequence. The reliquefaction device (26) is used to liquefy the CO2 supplied upstream, and the booster pump (30) is used to boost the liquefied CO2 supplied upstream and then inject it into the downstream main line pipeline. A bypass pipeline parallel to the reliquefaction device (26) is connected to the connection pipeline (21). A bypass cut-off valve (31) is arranged on the bypass pipeline. A first cut-off valve (25) and a second cut-off valve (27) are respectively arranged at the upstream and downstream ends of the reliquefaction device (26). A regulating valve (22) is arranged on the connection pipeline (21) between the first cut-off valve (25) and the upstream branch pipe cut-off valve (6). The upstream end of the bypass cut-off valve (31) is connected to the connection pipeline (21) between the regulating valve (22) and the first cut-off valve (25), and the downstream end of the bypass cut-off valve (31) is connected to the connection pipeline (21) between the second cut-off valve (27) and the booster pump (30); The pipeline residual gas safety relief system includes a vent cut-off valve (41) and a supporting vent pipeline, which are used to safely relieve the low-pressure CO2 remaining after the medium transfer in the upstream main line pipeline; The safety monitoring system is used to monitor the pressure and temperature at each point of the pipeline in real time.

2. The supercritical CO2 pipeline maintenance working condition tube storage medium emission reduction system according to claim 1, characterized in that: The movable liquefaction and transfer system further includes a safety valve (32) after the pump. The safety valve (32) after the pump is arranged downstream of the booster pump (30), and the safety valve (32) after the pump is connected to the upstream of the regulating valve (22) through a pressure relief pipeline (33).

3. The supercritical CO2 pipeline maintenance working condition pipe inventory medium emission reduction system according to claim 2, characterized in that: The vent cut-off valve (41) is arranged downstream of the regulating valve (22) and is used to connect the connection pipeline (21) and the vent pipeline.

4. The supercritical CO2 pipeline maintenance condition pipe inventory medium emission reduction system according to claim 3, characterized in that: The safety monitoring system includes an upstream branch pipe pressure transmitter (4) and an upstream branch pipe temperature transmitter (5) arranged on the upstream branch pipe (7), a downstream branch pipe pressure transmitter (8) and a downstream branch pipe temperature transmitter (9) arranged on the downstream branch pipe (11), a first temperature transmitter (23) and a first pressure transmitter (24) arranged between the downstream of the regulating valve (22) and the upstream of the first cut-off valve (25), and a second pressure transmitter (28) and a second temperature transmitter (29) arranged between the downstream of the second cut-off valve (27) and the booster pump (30).

5. A method for reducing emissions of the stored medium in a supercritical CO2 pipeline during maintenance, according to claim 4, characterized in that: The specific emission reduction method is as follows: Before planned pipeline shutdown, reduce the pipeline operating pressure and reserve a pressure margin for downstream pipeline transfer and filling. After pipeline shutdown, connect and install the upstream and downstream branch line block valves of the movable liquefaction and transfer system and the valve chamber connection system, open the first block valve, the second block valve and the bypass block valve, inject dry CO2 gas at a certain pressure into the pipeline in advance, establish sufficient back pressure, and keep the vent block valve closed. Open the upstream branch line block valve, slowly open the regulating valve, and the medium in the upstream pipeline quickly fills into the movable liquefaction and transfer system. Close the first block valve and the second block valve, open the downstream branch line block valve, and slowly open the booster pump to establish a pressurization channel for the upstream pipeline, bypass block valve, booster pump and downstream pipeline. When the CO2 pressure in the upstream pipeline drops to the bubble point pressure, open the first block valve and the second block valve, close the bypass block valve, and start the re-liquefaction device to cool and re-liquefy the incoming gas-liquid two-phase CO2. After the re-liquefied CO2 is pressure-lifted by the booster pump, it is injected into the downstream pipeline. When the pressure of the upstream pipeline drops to 1.8MPa.g - 2.0MPa.g, close the first block valve, the second block valve, the re-liquefaction device, the booster pump and the downstream branch line block valve, and close the movable liquefaction and transfer system. At the same time, open the vent block valve to connect the upstream pipeline, upstream branch line block valve, regulating valve, vent block valve and vent pipeline, and start the pipeline residual gas safety relief system to discharge the residual CO2 in the upstream pipeline.

6. The emission reduction method according to claim 5, characterized in that: When the bypass block valve is in the open state and the re-liquefaction device is in the closed state, during the operation process, monitor and analyze the physical properties of CO2 in the upstream pipeline through the first temperature transmitter and the first pressure transmitter. When the CO2 turns into a gas-liquid two-phase state, close the bypass block valve and open the re-liquefaction device; at the same time, monitor the pressurized pressure through the downstream branch line pressure transmitter and the downstream branch line temperature transmitter. When the pressurized pressure exceeds the design pressure of the downstream pipeline, the downstream pipeline should be linked to open the external discharge operation of the medium. After reducing the system pressure of the downstream pipeline, continue the transfer operation of CO2 in the upstream pipeline.

7. The emission reduction method according to claim 5, characterized in that: During the liquefaction and pressurization process, detect the parameters of the second pressure transmitter and the second temperature transmitter. When the temperature of the second temperature transmitter is lower than -15°C, adjust the refrigerant circulation volume of the re-liquefaction device; when the temperature of the second temperature transmitter is lower than -18°C, immediately close the re-liquefaction device and the booster pump for troubleshooting; when the pressure of the second pressure transmitter exceeds the design pressure of the downstream pipeline, the downstream pipeline should be linked to open the external discharge operation of the medium. After reducing the system pressure of the downstream pipeline, continue the transfer operation of CO2 in the upstream pipeline.

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