Zero-emission pressure relief process system and method for offshore high-pressure dense-phase CO2 conveying pipeline

Abstract

The invention relates to a zero-emission pressure relief process system and method for an offshore high-pressure dense-phase CO2 conveying pipeline, and the system comprises a high-pressure dense-phase CO2 conveying subsea pipeline, the pipeline input end of the high-pressure dense-phase CO2 conveying subsea pipeline is connected with CO2 supercharging equipment of an offshore oil and gas field treatment platform, and the pipeline output end of the high-pressure dense-phase CO2 conveying subsea pipeline is connected with a CO2 injection wellhead of a wellhead platform of another offshore oil and gas field; the pipeline input end of the low-pressure oil-gas-water mixed transportation subsea pipeline is connected with a wellhead platform production wellhead of the offshore oil-gas field, and the pipeline output end of the low-pressure oil-gas-water mixed transportation subsea pipeline is connected with treatment platform oil-gas-water separation equipment of the offshore oil-gas field; and the pipeline input end of the dry gas output sea pipeline is connected with the treatment platform oil-gas-water separation equipment, and the pipeline output end of the dry gas output sea pipeline is connected with downstream receiving equipment. According to the method, it is guaranteed that the CO2 conveying pipeline can be subjected to safe pipe cleaning operation under the low pressure through pressure relief, and zero emission of released CO2 to the atmosphere is achieved.

Description

TECHNICAL FIELD

[0001] The application relates to a zero-emission pressure relief process system and method for a high-pressure dense-phase CO2 offshore pipeline, and belongs to the technical field of offshore oil and gas transmission. BACKGROUND

[0002] With the increasing number of CCS / CCUS projects in offshore oil and gas fields, corresponding submarine CO2 transmission pipelines need to be designed, constructed and operated. At present, offshore CO2 transmission pipelines generally adopt dense-phase transmission, and the transmission pressure needs to be kept above the critical pressure, generally higher than 8 Ma. For the condition of high CO2 injection pressure, the CO2 pipeline transmission pressure will also increase accordingly, reaching 40 MPa or even higher, which puts higher requirements on the design and operation of the CO2 transmission pipeline. The offshore CO2 pipeline is generally not discharged, but for the high-pressure (such as up to 40 MPa) CO2 pipeline, in the pigging condition, in consideration of operation safety, the pipeline pressure needs to be discharged to a lower pressure before pigging operation. If such high-pressure CO2 transmission pipeline needs to be discharged, the current technology generally adopts the scheme of direct discharge to the atmosphere, and the process flow is relatively simple, only needing to control the generation of dry ice during the discharge process, but a large amount of CO2 in the pipeline will be discharged to the atmosphere, which does not meet the requirements, and it is urgent to study a CO2 pipeline discharge process method that can reduce or even eliminate the discharge of CO2 to the atmosphere. SUMMARY

[0003] In view of the above technical problems, the application provides a zero-emission pressure relief process system and method for a high-pressure dense-phase CO2 offshore pipeline, which can ensure safe pigging operation of the CO2 transmission pipeline at a lower pressure through pressure relief, and realize zero discharge of discharged CO2 to the atmosphere, meet the environmental protection requirements, and realize the win-win of offshore “safe production” and “green environmental protection”.

[0004] To achieve the above purpose, the application adopts the following technical scheme:

[0005] A zero-emission pressure relief process system for a high-pressure dense-phase CO2 offshore pipeline, comprising:

[0006] A high-pressure dense-phase CO2 transmission submarine pipeline, the pipeline input end of the high-pressure dense-phase CO2 transmission submarine pipeline is connected with a CO2 booster device of an offshore oil and gas field treatment platform, and the pipeline output end is connected with a CO2 injection wellhead of a wellhead platform of another offshore oil and gas field;

[0007] A low-pressure oil-gas-water mixed transmission submarine pipeline, the pipeline input end of the low-pressure oil-gas-water mixed transmission submarine pipeline is connected with a production wellhead of a wellhead platform of an offshore oil and gas field, and the pipeline output end is connected with an oil-gas-water separation device of a treatment platform of the offshore oil and gas field;

[0008] A dry gas export sea pipeline, a pipeline input end of the dry gas export sea pipeline is connected to the oil-gas-water separation device of the processing platform, and a pipeline output end is connected to a downstream receiving device.

[0009] The offshore high-pressure dense-phase CO2 transport pipeline "zero emission" pressure relief process system, preferably, a processing platform CO2 booster compressor, a processing platform CO2 launching cylinder and a wellhead platform CO2 receiving cylinder are sequentially arranged from upstream to downstream of the high-pressure dense-phase CO2 transport sea pipeline.

[0010] The offshore high-pressure dense-phase CO2 transport pipeline "zero emission" pressure relief process system, preferably, a processing platform oil-gas-water receiving cylinder and a wellhead platform oil-gas-water export launching cylinder are arranged on the low-pressure oil-gas-water mixed transport sea pipeline.

[0011] The offshore high-pressure dense-phase CO2 transport pipeline "zero emission" pressure relief process system, preferably, a processing platform wet gas booster compressor set, a dehydration device, a processing platform dry gas booster compressor set and a processing platform dry gas export launching cylinder are sequentially arranged from upstream to downstream of the dry gas export sea pipeline.

[0012] The offshore high-pressure dense-phase CO2 transport pipeline "zero emission" pressure relief process system further comprises a first throttling valve and a second throttling valve, the first throttling valve is arranged on a pipeline connecting the high-pressure dense-phase CO2 transport sea pipeline and the dry gas export sea pipeline, and the second throttling valve is arranged on a pipeline connecting the high-pressure dense-phase CO2 transport sea pipeline and the processing platform wet gas booster compressor set.

[0013] The second aspect of the present application provides an offshore high-pressure dense-phase CO2 transport pipeline "zero emission" pressure relief process method, comprising the following steps:

[0014] The booster facilities of the offshore oil and gas field processing platform and the high-pressure dense-phase CO2 transport sea pipeline are shut down, CO2 is discharged from the high-pressure dense-phase CO2 transport sea pipeline to the dry gas export sea pipeline with lower operating pressure, until the pressures of the two sea pipelines are consistent; if the pressure of the high-pressure dense-phase CO2 transport sea pipeline needs to be further reduced, the CO2 in the high-pressure dense-phase CO2 transport sea pipeline is discharged again to a position of a booster compressor set that meets the discharge pressure, and so on, if the pressure of the high-pressure dense-phase CO2 transport sea pipeline needs to be continuously reduced, the CO2 can be discharged to a lower operating pressure position in the processing process of the processing platform, until the high-pressure dense-phase CO2 transport sea pipeline is reduced to a pressure required for pigging, and the discharged CO2 is mixed, pressurized and exported in the processing process.

[0015] The offshore high-pressure dense-phase CO2 conveying pipeline "zero emission" pressure relief process method preferably continues to discharge the CO2 in the high-pressure dense-phase CO2 conveying sea pipeline to a position of the offshore oil and gas field processing platform booster compressor set that meets the discharge pressure, that is, according to the final pressure value required by the high-pressure dense-phase CO2 conveying sea pipeline to be discharged, a matching pressure access point is found in the booster compressor set of the offshore oil and gas field processing platform, the high-pressure dense-phase CO2 conveying sea pipeline is connected to the position through a pipeline and a throttle valve, and the throttle valve is opened until the pressure of the high-pressure dense-phase CO2 conveying sea pipeline is reduced to the pressure value required for pigging.

[0016] The offshore high-pressure dense-phase CO2 conveying pipeline "zero emission" pressure relief process method preferably requires that the initial discharge operation pressure of the high-pressure dense-phase CO2 conveying sea pipeline be higher than the pressure of the dry gas export sea pipeline, and dry ice is not generated during the pressure relief process.

[0017] The offshore high-pressure dense-phase CO2 conveying pipeline "zero emission" pressure relief process method preferably requires that the secondary discharge operation pressure of the high-pressure dense-phase CO2 conveying sea pipeline be higher than the operation pressure of the discharge access point of the booster compressor set of the offshore oil and gas field processing platform, and dry ice is not generated during the pressure relief process.

[0018] The offshore high-pressure dense-phase CO2 conveying pipeline "zero emission" pressure relief process method preferably, if dry ice is generated during the discharge process, a heater can be arranged upstream of the first throttle valve and the second throttle valve to increase the discharge temperature and prevent the generation of dry ice during the discharge process.

[0019] The present application has the following advantages due to the above technical solutions:

[0020] The present application fully utilizes offshore low-pressure processing facilities (such as offshore dry gas export sea pipelines with lower pressure and offshore booster processing equipment) to receive and process the discharged CO2, realizes that all of the discharged CO2 of the offshore high-pressure dense-phase CO2 conveying pipeline enters the sealed low-pressure processing facilities and processes, ensures that the CO2 conveying pipeline can be safely pigged at a lower pressure through pressure relief, realizes zero emission of the discharged CO2 to the atmosphere, meets the environmental protection requirements, realizes the win-win of offshore "safe production" and "green environmental protection", and the like. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 It is a schematic diagram of the offshore high-pressure dense-phase CO2 conveying pipeline "zero emission" pressure relief process system of the present application.

[0022] In the drawings, the reference signs are as follows:

[0023] 01-high pressure dense phase CO2 transportation sea pipe; 02-low pressure oil-gas-water mixed transportation sea pipe; 03-dry gas transportation sea pipe; 04-CO2 launching cylinder of processing platform; 05-oil-gas-water collecting cylinder of processing platform; 06-dry gas launching cylinder of processing platform; 07-CO2 booster compressor of processing platform; 08-wet gas booster compressor set of processing platform; 09-dry gas booster compressor set of processing platform; 10-wellhead platform production wellhead; 11-wellhead platform CO2 injection wellhead; 12-CO2 collecting cylinder of wellhead platform; 13-oil-gas-water transportation launching cylinder of wellhead platform; 14-first throttle valve; 15-second throttle valve; 16-oil-gas-water separation equipment of processing platform; 17-dehydration device. DETAILED DESCRIPTION

[0024] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the present application are clearly and completely described below. Obviously, the described embodiments are some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the present application.

[0025] Unless otherwise defined, the technical terms or scientific terms used in the present application should be understood as the common meanings of the technical terms or scientific terms by those skilled in the art. The terms "first", "second", "third", "fourth" and similar words used in the present application do not represent any order, number or importance, but are only used to distinguish different components. The terms "including", "containing" and similar words mean that the components or objects before the words cover the components or objects listed after the words and their equivalents, and do not exclude other components or objects. The terms "connected" or "connected" and similar words are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect.

[0026] In order to facilitate the description, spatial relative terms can be used in the description to describe the relationship of one element or feature relative to another element or feature as shown in the drawings, such as "inner", "outer", "inside", "outside", "below", "above", etc. Such spatial relative terms are intended to include different orientations of the device in use or operation in addition to the orientation depicted in the drawings.

[0027] At present, the dense phase transportation is generally used for offshore CO2 transportation pipeline, and the transportation pressure needs to be kept above the critical pressure, generally higher than 8Ma, for the condition of high CO2 injection pressure, the CO2 pipeline transportation pressure will also increase accordingly, some up to 40MPa, even higher, which puts higher requirements on the design and operation of CO2 pipeline. Offshore CO2 pipeline is generally not discharged, but for high pressure (such as up to 40MPa) CO2 pipeline, in the pigging condition, considering the operation safety, the pipeline pressure needs to be discharged to a lower pressure before pigging operation, if the high pressure CO2 transportation pipeline needs to be discharged, the current technology generally adopts the scheme of direct discharge to the atmosphere, the process flow is relatively simple, only need to control the dry ice not to be produced in the discharge process, but will cause a large amount of CO2 in the sea pipeline to be discharged to the atmosphere, which does not meet the current requirements, and it is urgent to study a CO2 pipeline discharge process method which can reduce or even not produce CO2 discharge to the atmosphere.

[0028] Based on the above technical problems, the present application provides a kind of offshore high pressure dense phase CO2 transportation pipeline "zero emission" pressure relief process method, which can ensure that CO2 transportation pipeline can be operated safely at lower pressure by pressure relief, and also realizes zero emission of discharged CO2 to the atmosphere, meets the requirements of environmental protection, realizes the win-win of "safe production" and "green environmental protection" offshore.

[0029] The high pressure in the present application refers to not less than 40MPa, and the low pressure refers to not more than 3MPa.

[0030] As shown in Figure 1 The offshore high pressure dense phase CO2 transportation pipeline "zero emission" pressure relief process system involved in the present application comprises:

[0031] High pressure dense phase CO2 transportation sea pipeline 01, the pipeline input end of high pressure dense phase CO2 transportation sea pipeline 01 is connected with CO2 booster equipment of offshore oil and gas field treatment platform, and the pipeline output end is connected with wellhead CO2 injection wellhead 11 of another offshore oil and gas field wellhead platform;

[0032] Low pressure oil-gas-water mixed transportation sea pipeline 02, the pipeline input end of low pressure oil-gas-water mixed transportation sea pipeline 02 is connected with production wellhead 10 of offshore oil and gas field wellhead platform, and the pipeline output end is connected with oil-gas-water separation equipment 16 of treatment platform of offshore oil and gas field;

[0033] Dry gas export sea pipeline 03, the pipeline input end of dry gas export sea pipeline 03 is connected with oil-gas-water separation equipment 16 of treatment platform, and the pipeline output end is connected with downstream receiving equipment.

[0034] Further, as shown in Figure 1As shown, from upstream to downstream of the high-pressure dense-phase CO2 delivery sea pipeline 01, a treatment platform CO2 booster compressor 07, a treatment platform CO2 ball launching cylinder 04 and a wellhead platform CO2 ball collecting cylinder 12 are sequentially arranged; the low-pressure oil-gas-water mixed delivery sea pipeline 02 is provided with a treatment platform oil-gas-water ball collecting cylinder 05 and a wellhead platform oil-gas-water external delivery ball launching cylinder 13; from upstream to downstream of the dry gas external delivery sea pipeline 03, a treatment platform wet gas booster compressor set 08, a dehydration device 17, a treatment platform dry gas booster compressor set 09 and a treatment platform dry gas external delivery ball launching cylinder 06 are sequentially arranged.

[0035] Further, as shown, Figure 1 Further, the system further comprises a first throttling valve 14 and a second throttling valve 15, the first throttling valve 14 is arranged on a pipeline connecting the high-pressure dense-phase CO2 delivery sea pipeline 01 and the dry gas external delivery sea pipeline 03, and the second throttling valve 15 is arranged on a pipeline connecting the high-pressure dense-phase CO2 delivery sea pipeline 01 and the treatment platform wet gas booster compressor set 08.

[0036] The main working process of the process system is as follows: CO2 is boosted to a delivery pressure at the offshore oil and gas field treatment platform, delivered to the wellhead platform CO2 injection wellhead 11 through the high-pressure dense-phase CO2 delivery sea pipeline 01 for oil displacement, and the oil-gas-water mixture produced from the wellhead platform production wellhead 10 is collected and delivered to the offshore oil and gas field treatment platform through the offshore low-pressure oil-gas-water mixed delivery sea pipeline 02 for treatment, and the qualified gas is delivered to the dry gas external delivery sea pipeline 03 through the booster compressor for external delivery, etc.

[0037] Specifically, the high-pressure dense-phase CO2 is boosted to 40 MPa by the treatment platform CO2 booster compressor 07, and then delivered from the treatment platform to the wellhead platform CO2 injection wellhead 11 for oil displacement, and the low-pressure oil-gas-water mixed delivery sea pipeline 02 collects and delivers the oil-gas-water stream from the wellhead platform to the treatment platform for treatment, and the delivery pressure is 3 MPa; the oil-gas-water stream is separated, boosted and dehydrated at the treatment platform, the qualified dry gas is compressed and boosted to 10 MPa by the treatment platform dry gas booster compressor set 09, and then delivered through the dry gas external delivery sea pipeline 03.

[0038] Further, before the pigging operation of the high-pressure dense-phase CO2 delivery sea pipeline 01 is required, the operating pressure of the high-pressure dense-phase CO2 delivery sea pipeline 01 needs to be reduced to 3 MPa, which is the delivery pressure of the low-pressure oil-gas-water mixed delivery sea pipeline 02, to ensure low-pressure safe pigging.

[0039] Further, according to simulation, if the high-pressure dense-phase CO2 transportation sea pipe 01 is directly released from 40 MPa to 3 MPa, the throttling low temperature caused by high pressure difference will produce dry ice, in order to ensure that dry ice does not block the pipe during the pressure relief process, in combination with the field process, the high-pressure dense-phase CO2 transportation sea pipe 01 needs to be released in two times: the first time, the throttling valve 14 on the pipeline connecting the high-pressure dense-phase CO2 transportation sea pipe 01 and the dry gas external transportation sea pipe 03 is opened, until the high-pressure dense-phase CO2 transportation sea pipe 01 is reduced to 10 MPa, consistent with the pressure of the offshore dry gas external transportation sea pipe 03, and the minimum temperature of the high-pressure dense-phase CO2 transportation sea pipe 01 during the pressure relief process is 9.8℃, which will not produce dry ice; the second time, after 5 hours of the first time, the operating temperature of the high-pressure dense-phase CO2 transportation sea pipe 01 is heated and raised after heat exchange with seawater (25.6℃), the second throttling valve 15 on the pipeline connecting the high-pressure dense-phase CO2 transportation sea pipe 01 and the inlet of the intermediate compressor of the wet gas booster compressor set 08 of the processing platform is opened, until the high-pressure dense-phase CO2 transportation sea pipe 01 is reduced to 3 MPa, consistent with the transportation pressure of the low-pressure oil-gas-water mixed transportation sea pipe 02, and the minimum temperature of the high-pressure dense-phase CO2 transportation sea pipe 01 during the pressure relief process is 2.3℃, which will not produce dry ice.

[0040] Further, if dry ice is produced during the release process, a heater can be arranged upstream of the first throttling valve 14 and the second throttling valve 15 to raise the release temperature and prevent dry ice from being produced during the release process.

[0041] Further, the pigging operation of the high-pressure dense-phase CO2 transportation sea pipe 01 is recommended to be performed in summer, so that after the initial pressure relief, the sea pipe with a low operating temperature (9.8℃) caused by CO2 release can be heat-exchanged with seawater with a high temperature (30℃), and then the secondary pressure relief is performed, so that the generation of dry ice during the release process is minimized.

[0042] The present application fully utilizes offshore low-pressure processing facilities (such as the dry gas external transportation sea pipe 03 with a lower pressure and offshore booster processing equipment) to receive and process the released CO2, realizes that all the released CO2 of the offshore high-pressure dense-phase CO2 transportation pipe enters the sealed low-pressure processing facility and process, ensures that the CO2 transportation pipe can be safely pigged at a lower pressure through pressure relief, realizes zero emission of the released CO2 to the atmosphere, meets the environmental protection requirements, realizes the win-win of "safe production" and "green environmental protection" offshore.

[0043] It should be pointed out finally that the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit the same; and although the present application has been described in detail with reference to the foregoing embodiments, it should be appreciated by those skilled in the art that the technical solutions recorded in the foregoing embodiments can be modified, or some technical features thereof can be replaced equivalently; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A zero-emission pressure relief process system for offshore high-pressure dense-phase CO2 transport pipelines, characterized in that, include: High-pressure dense phase CO2 transport pipeline (01), the pipeline input end of the high-pressure dense phase CO2 transport pipeline (01) is connected to the CO2 booster equipment of the offshore oil and gas field processing platform, and the pipeline output end is connected to the CO2 injection wellhead (11) of the wellhead platform of another offshore oil and gas field. Low-pressure oil, gas and water mixed transport subsea pipeline (02), the pipeline input end of the low-pressure oil, gas and water mixed transport subsea pipeline (02) is connected to the production wellhead (10) of the wellhead platform of the offshore oil and gas field, and the pipeline output end is connected to the oil, gas and water separation equipment (16) of the processing platform of the offshore oil and gas field. Dry gas export pipeline (03), the pipeline input end of the dry gas export pipeline (03) is connected to the oil-gas-water separation equipment (16) of the processing platform, and the pipeline output end is connected to the downstream receiving equipment.

2. The "zero-emission" pressure relief process system for offshore high-pressure dense-phase CO2 transport pipelines according to claim 1, characterized in that, From upstream to downstream of the high-pressure dense-phase CO2 transport subsea pipeline (01), a CO2 booster compressor (07) for the processing platform, a CO2 launching tube (04) for the processing platform, and a CO2 receiving tube (12) for the wellhead platform are arranged sequentially.

3. The "zero-emission" pressure relief process system for offshore high-pressure dense-phase CO2 transport pipelines according to claim 2, characterized in that, The low-pressure oil, gas and water mixed transport subsea pipeline (02) is equipped with an oil, gas and water receiving tube (05) for the processing platform and an oil, gas and water external transport tube (13) for the wellhead platform.

4. The "zero-emission" pressure relief process system for offshore high-pressure dense-phase CO2 transport pipelines according to claim 3, characterized in that, From upstream to downstream of the dry gas export pipeline (03), the processing platform wet gas booster compressor unit (08), dehydration device (17), processing platform dry gas booster compressor unit (09), and processing platform dry gas export launcher (06) are arranged sequentially.

5. The "zero-emission" pressure relief process system for offshore high-pressure dense-phase CO2 transport pipelines according to claim 4, characterized in that, It also includes a first throttling valve (14) and a second throttling valve (15). The first throttling valve (14) is installed on the pipeline connecting the high-pressure dense phase CO2 transmission subsea pipeline (01) and the dry gas export subsea pipeline (03). The second throttling valve (15) is installed on the pipeline connecting the high-pressure dense phase CO2 transmission subsea pipeline (01) and the wet gas booster compressor unit (08) of the processing platform.

6. A "zero-emission" pressure relief process for a marine high-pressure dense-phase CO2 transport pipeline according to claim 5, characterized in that, Includes the following steps: Shut down the pressurization facilities of the offshore oil and gas field processing platform and the high-pressure dense phase CO2 transmission pipeline (01), and release CO2 from the high-pressure dense phase CO2 transmission pipeline (01) to the dry gas export pipeline (03) with a lower operating pressure until the pressure of the two pipelines is the same; if the pressure of the high-pressure dense phase CO2 transmission pipeline (01) needs to be further reduced, the CO2 in the high-pressure dense phase CO2 transmission pipeline (01) is released a second time to the pressurization compressor unit to meet the release pressure, and so on. If the pressure of the high-pressure dense phase CO2 transmission pipeline (01) needs to be further reduced, the CO2 can be released to a lower operating pressure position in the processing process of the processing platform until the high-pressure dense phase CO2 transmission pipeline (01) is reduced to the pressure required for pipeline cleaning. The released CO2 is mixed with the gas in the processing process for processing, pressurization and export.

7. The "zero-emission" pressure relief process for offshore high-pressure dense-phase CO2 transport pipelines according to claim 6, characterized in that, The CO2 in the high-pressure dense phase CO2 transmission pipeline (01) is further released to the location where the pressure of the booster compressor unit of the offshore oil and gas field processing platform meets the release pressure. That is, according to the final pressure value that the high-pressure dense phase CO2 transmission pipeline (01) needs to release, a matching pressure access point is found in the booster compressor unit of the offshore oil and gas field processing platform. The high-pressure dense phase CO2 transmission pipeline (01) is connected to this location through pipelines and throttle valves. The throttle valves are opened until the pressure of the high-pressure dense phase CO2 transmission pipeline (01) is reduced to the pressure value required for pipeline cleaning.

8. The "zero-emission" pressure relief process for offshore high-pressure dense-phase CO2 transport pipelines according to claim 6, characterized in that, The initial venting pressure of the high-pressure dense-phase CO2 transmission pipeline (01) must be higher than the pressure of the dry gas transmission pipeline (03), and no dry ice will be generated during the venting process.

9. The "zero-emission" pressure relief process for offshore high-pressure dense-phase CO2 transport pipelines according to claim 6, characterized in that, The secondary venting operation pressure of the high-pressure dense-phase CO2 transmission pipeline (01) needs to be higher than the venting access pressure of the booster compressor unit of the offshore oil and gas field processing platform, and no dry ice will be generated during the depressurization process.

10. The "zero-emission" pressure relief process for offshore high-pressure dense-phase CO2 transport pipelines according to claim 6, characterized in that, If dry ice is generated during the venting process, a heater can be installed upstream of the first throttle valve (14) and the second throttle valve (15) to increase the venting temperature and prevent dry ice from being generated during the venting process.

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