CN113108549B - 混合制冷剂集成氮膨胀循环的海上天然气液化系统 - Google Patents

混合制冷剂集成氮膨胀循环的海上天然气液化系统 Download PDF

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CN113108549B
CN113108549B CN202110438729.7A CN202110438729A CN113108549B CN 113108549 B CN113108549 B CN 113108549B CN 202110438729 A CN202110438729 A CN 202110438729A CN 113108549 B CN113108549 B CN 113108549B
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heat exchanger
compressor
inlet
outlet
liquefaction
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CN113108549A (zh
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金春鹤
辛元洙
封官宏
袁益龙
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Jilin University
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Jilin University
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0032—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/004—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by flash gas recovery
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
    • F25J1/005—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by expansion of a gaseous refrigerant stream with extraction of work
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Abstract

本发明公开了一种混合制冷剂集成氮膨胀循环的海上天然气液化系统，包括天然气液化管路和天然气液化装置，天然气液化装置包括混合制冷剂循环系统和氮膨胀过冷循环系统，本发明预冷和液化部分采用了混合制冷剂液化，深冷部分采用了氮膨胀制冷工艺，缩小了预冷和液化循环中的平均换热温差，提高了冷能利用率；混合制冷剂循环系统的制冷剂选用了氮气、甲烷、乙烷等轻烃类制冷剂混合物，提高了液化效率。相比于传统的氮膨胀液化系统，大幅提高了整个液化系统的液化效率，而且也比传统的SMR工艺相比提高了单线产能。此外，该液化系统设计可以在混合制冷剂循环系统里不使用丙烷制冷剂，降低海上用天然气液化工艺装置的安全性风险。

Description

混合制冷剂集成氮膨胀循环的海上天然气液化系统
技术领域
本发明涉及石油化工技术领域，更具体地，涉及一种混合制冷剂集成氮膨胀循环的海上天然气液化系统。
背景技术
天然气需求量的快速增加，开发深远海的天然气资源也得到了广泛的关注。浮式液化天然气生产储卸装置(Floating Liquefied Natural Gas，FLNG)是集天然气开采、处理、液化、存储、外输为一体的可移动海上天然气工厂，FLNG在深水海域的应用将有效地避免深水海域管道铺设所面临的技术难题，同时也为海上边际油气田的开发提供了经济有效的方案。天然气液化工艺是FLNG的核心技术。目前世界范围内实际投入商业生产的FLNG相对较少，因投入运行经验还不够，海上天然气液化技术有待工业化验证。
海上因为有波浪、空间相对不足等特殊作业环境，使得设计标准不同于陆上，比如平台的安全性、流程简洁性、紧凑性和模块化设计等。金海刚等研发的混合制冷与氮膨胀组合式海上天然气液化工艺，详见《天然气技术与经济》，2013年第7卷第3期，且上述技术方案也申请了发明专利，详见专利文献号CN10948706B；中国专利文献中公开了一种混合制冷剂预冷氮气膨胀的天然气液化系统及方法，详见专利文献号CN105737516A；中国专利文献中公开了一种混合制冷剂-膨胀剂制冷循环的天然气液化方法，详见专利文献号CN104315801A。由于FLNG上甲板空间的相对不足，要求液化流程尽可能减少易燃、易挥发性制冷剂的需求量和存储。丙烷是在混合制冷剂液化流程中，要添加的重要制冷剂之一。但考虑到丙烷的易燃且比空气稍重，容易在使用空间狭窄的甲板聚集给FLNG增加安全隐患。所以混合物制冷剂中丙烷必须要重视，万一泄露很容易聚集在平台之中，会增加潜在的安全隐患。氮膨胀制冷液化流程使用完全不可燃的氮气，该技术的优点是可以减少易燃组分的安全性问题，但液化效率以及运行费用方面比混合制冷剂流程低。
发明内容
针对现有技术中存在的问题，本发明的目的是提供一种混合制冷剂集成氮膨胀循环的海上天然气液化系统，用于解决传统氮膨胀液化效率相对低的问题，以及丙烷制冷剂在海上平台的安全性隐患的问题。
为实现上述目的，本发明采用如下技术方案：混合制冷剂集成氮膨胀循环的海上天然气液化系统，其特征在于，包括天然气液化管路和天然气液化装置，天然气液化装置包括混合制冷剂循环系统和氮膨胀过冷循环系统，其中：
天然气液化管路依次经过预冷多股流换热器、液化多股流换热器、主液化冷箱、第一JT阀和LNG分离罐；
混合制冷剂循环系统包括预冷多股流换热器、液化多股流换热器、第一MR压缩机、第一冷却器、第二MR压缩机、第二冷却器、第一气液分离罐、第三MR压缩机、MR增压泵、第三冷却器、第二气液分离罐、第二JT阀和第三JT阀，混合制冷剂循环系统使混合制冷剂在依次连接了第一MR压缩机、第一冷却器、第二MR压缩机、第二冷却器、第一气液分离罐、第三冷却器、第二气液分离罐、预冷多股流换热器和液化多股流换热器的制冷循环回路中循环，第三MR压缩机设置在第一气液分离罐的气相出口和第三冷却器的气相进口之间的连接管路上，MR增压泵设置在第一气液分离罐的液相出口和第三冷却器的液相进口之间的连接管路上；第二JT阀并接在预冷多股流换热器上；第三JT阀并接在液化多股流换热器上；
氮膨胀过冷循环系统包括主液化冷箱、第一N2压缩机、第一N2压缩机冷却器、第二N2压缩机、第二N2压缩机冷却器和N2膨胀机，第一N2压缩机、第一N2压缩机冷却器、第二N2压缩机和第二N2压缩机冷却器依次连接，第一N2压缩机的入口与主液化冷箱连接，第二N2压缩机冷却器的出口通过两个分支管路分别与主液化冷箱、预冷多股流换热器连接；主液化冷箱与预冷多股流换热器连接；N2膨胀机并接在主液化冷箱上。
进一步，所述预冷多股流换热器的第一入口通过天然气输送管道与天然气气源连接，预冷多股流换热器的第一出口连接液化多股流换热器的第一入口；液化多股流换热器的第一出口连接主液化冷箱的第一入口；主液化冷箱的第一出口与第一JT阀的进口连接；第一JT阀的出口与LNG分离罐的进口连接；LNG分离罐的气相出口通过BOG输送管路与主液化冷箱的第五入口连接，同时LNG分离罐的液相出口与LNG储罐连通。
进一步，混合制冷剂循环系统中，所述第一MR压缩机、第一冷却器、第二MR压缩机、第二冷却器和第一气液分离罐依次相连；第一气液分离罐的气相出口与第三MR压缩机的进口连通，第一气液分离罐的液相出口与MR增压泵的进口连通；第三MR压缩机的出口与第三冷却器的气体进口相连；MR增压泵的出口与第三冷却器的液体进口相连，第三冷却器的出口与第二气液分离罐连接；第二气液分离罐的液相出口连接预冷多股流换热器的第二入口；预冷多股流换热器的第二出口连接第二JT阀的入口；第二JT阀的出口连接预冷多股流换热器的第三入口连接；第二气液分离罐的气相出口连接预冷多股流换热器的第四入口，预冷多股流换热器的第四出口连接液化多股流换热器的第二入口连接；液化多股流换热器的第二出口连接第三JT阀的进口连接；第三JT阀的出口连接液化多股流换热器的第三入口，液化多股流换热器的第三出口连接预冷多股流换热器的第三入口；预冷多股流换热器的第三出口连接第一MR压缩机的进口。
进一步，氮膨胀过冷循环系统中，所述主液化冷箱的第二出口连接第一N2压缩机的入口，第一N2压缩机的出口与第一N2压缩机冷却器的入口连接；第一N2压缩机冷却器的出口与第二N2压缩机的入口连接，第二N2压缩机的出口与第二N2压缩机冷却器入口连接，第二N2压缩机冷却器的出口通过两个分支管道分别与预冷多股流换热器的第五入口、主液化冷箱的第三入口连接；主液化冷箱的第三出口和第四出口均与N2膨胀机的入口连接，N2膨胀机的出口与主液化冷箱的第二入口连接；主液化冷箱的第四入口与预冷多股流换热器的第五出口连接；主液化冷箱的第五出口与燃气管道连接，用于排出燃气。
进一步，所述预冷多股流换热器和液化多股流换热器均为板翅式换热器。
进一步，所述主液化冷箱为绕管式换热器。
进一步，所述N2膨胀机用于为第一N2压缩机提供部分或全部电能。
进一步，所述混合制冷剂包括氮气、甲烷、乙烷、丁烷中的两种或两种以上。
作为本发明的一种优选方案，所述混合制冷剂含有摩尔分率为10％～20％的甲烷、摩尔分率为45％～60％的乙烷、摩尔分率为10％～20％的丁烷、摩尔分率为10％～20％的戊烷和摩尔分率为3％～10％的氮气。
通过上述设计方案，本发明可以带来如下有益效果：本发明预冷和液化部分采用混合制冷剂循环系统，可根据项目工况条件和原料气组成，合理配置混合制冷剂组成和配比，提高了换热效率并降低了整个工艺流程的功耗，从而整个工艺系统能最优化低能耗运行；混合制冷剂中可以不使用丙烷的情况下能耗比现有氮膨胀液化高，能接近SMR单混合制冷剂流程的液化效率，而且比传统SMR提高单线LNG产能；同时混合制冷剂部分可以不使用丙烷制冷剂，消除了丙烷在海上平台中泄露可能性带来的安全隐患。
附图说明
此处的附图说明用来提供对本发明的进一步理解，构成本发明申请的一部分，本发明示意性实施例及其说明用于理解本发明，并不构成本发明的不当限定，在附图中：
图1是本发明天然气液化工艺流程示意图。
图中标记：1为预冷多股流换热器，2为液化多股流换热器，3为主液化冷箱，4为第一JT阀，5为LNG分离罐，6为第一MR压缩机，7为第一冷却器，8为第二MR压缩机，9为第二冷却器，10为第一气液分离罐，11为第三MR压缩机，12为MR增压泵，13为第三冷却器，14为第二气液分离罐，15为第二JT阀，16为第三JT阀，17为第一N2压缩机，18为第一N2压缩机冷却器，19为第二N2压缩机，20为第二N2压缩机冷却器，21为N2膨胀机。
具体实施方式
为使得本发明的目的、特征、优点能够更加的明显和易懂，下面结合本发明的实施例中的附图，对本发明中的技术方案进行清楚完整地描述。显然，本发明不受下述实施例的限制，可根据本发明的技术方案与实际情况来确定具体的实施方式。为了避免混淆本发明的实质，公知的方法、过程、流程、元件和电路并没有详细叙述。在本发明的描述中，需要理解的是，术语“第一”、“第二”、“第三”、“第四”、“第五”仅用于描述目的，限定有“第一”、“第二”、“第三”、“第四”、“第五”的特征并不表示任何顺序、数量或者重要性，而只是用来区分不同的组成部分。
需要说明的是，本发明中LNG即液化天然气(Liquefied Natural Gas)；MR即混合制冷剂；BOG即蒸发气体(boil off gas)；JT阀即焦耳-汤姆逊节流膨胀阀；BOG输送管路即用于输送BOG的管路。
如图1所示，混合制冷剂集成氮膨胀循环的海上天然气液化系统包括天然气液化管路和天然气液化装置，天然气液化装置包括混合制冷剂循环系统和氮膨胀过冷循环系统，其中：
天然气液化管路依次经过预冷多股流换热器1、液化多股流换热器2、主液化冷箱3、第一JT阀4和LNG分离罐5；预冷多股流换热器1的第一入口通过天然气输送管道与天然气气源连接，预冷多股流换热器1的第一出口连接液化多股流换热器2的第一入口；液化多股流换热器2的第一出口连接主液化冷箱3的第一入口，主液化冷箱3的第一出口与第一JT阀4的进口连接；第一JT阀4的出口与LNG分离罐5的进口连接；LNG分离罐5的气相出口通过BOG输送管路与主液化冷箱3的第五入口连接，同时LNG分离罐5的液相出口与LNG储罐连通。
混合制冷剂循环系统用于对天然气进料的预冷和部分液化，混合制冷剂循环系统包括预冷多股流换热器1、液化多股流换热器2、第一MR压缩机6、第一冷却器7、第二MR压缩机8、第二冷却器9、第一气液分离罐10、第三MR压缩机11、MR增压泵12、第三冷却器13、第二气液分离罐14、第二JT阀15和第三JT阀16，第一MR压缩机6、第一冷却器7、第二MR压缩机8、第二冷却器9和第一气液分离罐10依次相连；第一气液分离罐10的气相出口与第三MR压缩机11的进口连通，第一气液分离罐10的液相出口与MR增压泵12的进口连通；第三MR压缩机11的出口与第三冷却器13的气体进口相连；MR增压泵12的出口与第三冷却器13的液体进口相连，第三冷却器13的出口与第二气液分离罐14连接；第二气液分离罐14的液相出口连接预冷多股流换热器1的第二入口，预冷多股流换热器1的第二出口连接第二JT阀15的入口；第二JT阀15的出口连接预冷多股流换热器1的第三入口连接；第二气液分离罐14的气相出口连接预冷多股流换热器1的第四入口，预冷多股流换热器1的第四出口连接液化多股流换热器2的第二入口连接，液化多股流换热器2的第二出口连接第三JT阀16的进口连接；第三JT阀16的出口连接液化多股流换热器2的第三入口，液化多股流换热器2的第三出口连接预冷多股流换热器1的第三入口；预冷多股流换热器1的第三出口连接第一MR压缩机6的进口；第二气液分离罐14分离出的液相混合制冷剂进入预冷多股流换热器1进行冷却，冷却后通过第二JT阀15节流降温后排出；第二气液分离罐14分离出的气相混合制冷剂依次通过预冷多股流换热器1和液化多股流换热器2进行冷却降温，冷却后通过第三JT阀16节流降温返回液化多股流换热器2，从液化多股流换热器2排出后与第二JT阀15排出的股流混合，混合后进入预冷多股流换热器1提供冷量，提供冷量后从预冷多股流换热器1排出返回第一MR压缩机6；
氮膨胀过冷循环系统用于对天然气全部液化和过冷提供冷能，氮膨胀过冷循环系统包括主液化冷箱3、第一N2压缩机17、第一N2压缩机冷却器18、第二N2压缩机19、第二N2压缩机冷却器20和N2膨胀机21，主液化冷箱3的第二出口连接第一N2压缩机17的入口，第一N2压缩机17的出口与第一N2压缩机冷却器18的入口连接；第一N2压缩机冷却器18的出口与第二N2压缩机19的入口连接，第二N2压缩机19的出口与第二N2压缩机冷却器20入口连接，第二N2压缩机冷却器20的出口通过两个分支管道分别与预冷多股流换热器1的第五入口、主液化冷箱3的第三入口连接；主液化冷箱3的第三出口和第四出口均与N2膨胀机21的入口连接，N2膨胀机21的出口与主液化冷箱3的第二入口连接；主液化冷箱3的第四入口与预冷多股流换热器1的第五出口连接；主液化冷箱3的第五出口与燃气管道连接，用于排出燃气。
所述预冷多股流换热器1和液化多股流换热器2均为板翅式换热器，所述主液化冷箱3为绕管式换热器。
所述第一N2压缩机17的部分或全部电能由N2膨胀机21来提供。
本发明中混合制冷剂的组分为氮气、甲烷、乙烷、丁烷为主，但不限于增加少量丙烷和戊烷制冷剂。
所述氮膨胀过冷循环系统中使用的制冷剂为氮气，但不限于增加少量甲烷制冷剂。
基于上述混合制冷剂集成氮膨胀循环的海上天然气液化系统，天然气液化具体方法包括如下过程：
S1：净化的天然气以绝对压力4.0MPa～7.5MPa、温度20℃～40℃进入预冷多股流换热器1预冷至-20℃～-50℃，预冷后进入液化多股流换热器2进一步冷却降温至-80℃～-120℃，随后进入主液化冷箱3深冷至-130℃～-158℃，最后通过第一JT阀4降温至-161℃后流入LNG分离罐5，分离出的BOG进入主液化冷箱3提供冷量后进入燃料系统或进行回收利用，LNG存储等待外运。
S2：混合制冷剂经第一MR压缩机6压缩至绝对压力0.5MPa～0.9MPa，压缩后经第一冷却器7冷却，第一冷却器7的冷源采用海水或空冷，之后继续经第二MR压缩机8压缩至绝对压力1.0MPaMPa～3.0MPa，压缩后经第二冷却器9冷却，随后通过第一气液分离罐10进行气液分离，气相混合制冷剂通过第三MR压缩机11继续增压至绝对压力3.0MPa～7.0MPa，液相混合制冷剂通过MR增压泵12增压至绝对压力3.0MPa～7.0MPa，绝对压力为3.0MPa～7.0MPa的液相混合制冷剂和绝对压力为3.0MPa～7.0MPa的气相混合制冷剂在第三冷却器13中混合，混合后通过第二气液分离罐14进行气液分离，第二气液分离罐14分离出的气相高压混合制冷剂依次通过预冷多股流换热器1和液化多股流换热器2冷却后通过第三JT阀16减压至绝对压力0.25MPa～0.45MPa获得-100℃～-125℃低温的气液两相混合流体，然后返回液化多股流换热器2提供冷量，提供冷量后从液化多股流换热器2排出返回预冷多股流换热器1；第二气液分离罐14分离出的液相高压混合制冷剂通过预冷多股流换热器1降温至-20～-50℃后，通过第二JT阀15减压至绝对压力0.25MPa～0.45MPa获得-40℃～60C低温流体，与从液化多股流换热器2返回的流股混合后一起进入预冷多股流换热器1提供冷量，提供冷量后从预冷多股流换热器1排出返回第一MR压缩机6，形成混合制冷剂循环(MRC)；
S3：氮气制冷剂经第一N2压缩机17压缩至绝对压力0.9MPa～2MPa，压缩后经第一N2压缩机冷却器18冷却，第一N2压缩机冷却器18的冷源采用海水或空冷，之后继续经第二N2压缩机19压缩至绝对压力4.5MPa～7.5MPa，压缩后经第二N2压缩机冷却器20冷却，随后高压制冷剂分为两路，其中大部分直接进入主液化冷箱3，另一部分(大约3％～10％)首先进入预冷多股流换热器1降温至-20℃～-50℃后进入主液化冷箱3，通过主液化冷箱3降温至-37℃～-47℃之后这两路制冷剂混合，并一起经N2膨胀机21减压至绝对压力0.25MPa～0.4MPa获得-161℃～-163℃低温后，返回到主液化冷箱3提供冷量，形成氮膨胀过冷循环。
所述混合制冷剂(MR)组成中，甲烷摩尔分率为10％～20％、乙烷摩尔分率为45％～60％、丁烷摩尔分率为10％～20％、戊烷摩尔分率为10％～20％、氮气摩尔分率为3％～10％。
本发明公开了一种混合制冷剂集成氮膨胀循环的海上天然气液化系统，包括天然气液化管路和天然气液化装置，天然气液化装置包括混合制冷剂循环系统和氮膨胀过冷循环系统，本发明预冷和液化部分采用了混合制冷剂液化，深冷部分采用了氮膨胀制冷工艺，缩小了预冷和液化循环中的平均换热温差，提高了冷能利用率；混合制冷剂循环系统的制冷剂选用了氮气、甲烷、乙烷等轻烃类制冷剂混合物，提高了液化效率。相比于传统的氮膨胀液化系统，该天然气液化系统在预冷和液化制冷循环部分用了混合制冷剂大幅提高了整个液化系统的液化效率，而且也比传统的SMR工艺相比提高了单线产能20％左右。此外，该液化系统设计可以在混合制冷剂循环系统里不使用丙烷制冷剂，降低海上用天然气液化工艺装置的安全性风险。
本发明除了适用于海上天然气液化之外，还能适用于陆上中小规模的天然气液化厂。
以上说明对本发明只是简要说明性的，而非限制性的，本领域专业技术人员的理解，在不脱离全力要求所限定的范围情况下，可做出适当修改或变化，但都将落入本发明的保护范围之内。

Claims (6)

1.混合制冷剂集成氮膨胀循环的海上天然气液化系统，其特征在于，包括天然气液化管路和天然气液化装置，天然气液化装置包括混合制冷剂循环系统和氮膨胀过冷循环系统，其中：
天然气液化管路依次经过预冷多股流换热器(1)、液化多股流换热器(2)、主液化冷箱(3)、第一JT阀(4)和LNG分离罐(5)；
混合制冷剂循环系统包括预冷多股流换热器(1)、液化多股流换热器(2)、第一MR压缩机(6)、第一冷却器(7)、第二MR压缩机(8)、第二冷却器(9)、第一气液分离罐(10)、第三MR压缩机(11)、MR增压泵(12)、第三冷却器(13)、第二气液分离罐(14)、第二JT阀(15)和第三JT阀(16)，混合制冷剂循环系统使混合制冷剂在依次连接了第一MR压缩机(6)、第一冷却器(7)、第二MR压缩机(8)、第二冷却器(9)、第一气液分离罐(10)、第三冷却器(13)、第二气液分离罐(14)、预冷多股流换热器(1)和液化多股流换热器(2)的制冷循环回路中循环，第三MR压缩机(11)设置在第一气液分离罐(10)的气相出口和第三冷却器(13)的气相进口之间的连接管路上，MR增压泵(12)设置在第一气液分离罐(10)的液相出口和第三冷却器(13)的液相进口之间的连接管路上；第二JT阀(15)并接在预冷多股流换热器(1)上；第三JT阀(16)并接在液化多股流换热器(2)上；
氮膨胀过冷循环系统包括主液化冷箱(3)、第一N2压缩机(17)、第一N2压缩机冷却器(18)、第二N2压缩机(19)、第二N2压缩机冷却器(20)和N2膨胀机(21)，第一N2压缩机(17)、第一N2压缩机冷却器(18)、第二N2压缩机(19)和第二N2压缩机冷却器(20)依次连接，第一N2压缩机(17)的入口与主液化冷箱(3)连接，第二N2压缩机冷却器(20)的出口通过两个分支管路分别与主液化冷箱(3)、预冷多股流换热器(1)连接；主液化冷箱(3)与预冷多股流换热器(1)连接；N2膨胀机(21)并接在主液化冷箱(3)上；
所述预冷多股流换热器(1)的第一入口通过天然气输送管道与天然气气源连接，预冷多股流换热器(1)的第一出口连接液化多股流换热器(2)的第一入口；液化多股流换热器(2)的第一出口连接主液化冷箱(3)的第一入口；主液化冷箱(3)的第一出口与第一JT阀(4)的进口连接；第一JT阀(4)的出口与LNG分离罐(5)的进口连接；LNG分离罐(5)的气相出口通过BOG输送管路与主液化冷箱(3)的第五入口连接，同时LNG分离罐(5)的液相出口与LNG储罐连通；
混合制冷剂循环系统中，所述第一MR压缩机(6)、第一冷却器(7)、第二MR压缩机(8)、第二冷却器(9)和第一气液分离罐(10)依次相连；第一气液分离罐(10)的气相出口与第三MR压缩机(11)的进口连通，第一气液分离罐(10)的液相出口与MR增压泵(12)的进口连通；第三MR压缩机(11)的出口与第三冷却器(13)的气体进口相连；MR增压泵(12)的出口与第三冷却器(13)的液体进口相连，第三冷却器(13)的出口与第二气液分离罐(14)连接；第二气液分离罐(14)的液相出口连接预冷多股流换热器(1)的第二入口；预冷多股流换热器(1)的第二出口连接第二JT阀(15)的入口；第二JT阀(15)的出口连接预冷多股流换热器(1)的第三入口连接；第二气液分离罐(14)的气相出口连接预冷多股流换热器(1)的第四入口，预冷多股流换热器(1)的第四出口连接液化多股流换热器(2)的第二入口连接；液化多股流换热器(2)的第二出口连接第三JT阀(16)的进口连接；第三JT阀(16)的出口连接液化多股流换热器(2)的第三入口，液化多股流换热器(2)的第三出口连接预冷多股流换热器(1)的第三入口；预冷多股流换热器(1)的第三出口连接第一MR压缩机(6)的进口；
氮膨胀过冷循环系统中，所述主液化冷箱(3)的第二出口连接第一N2压缩机(17)的入口，第一N2压缩机(17)的出口与第一N2压缩机冷却器(18)的入口连接；第一N2压缩机冷却器(18)的出口与第二N2压缩机(19)的入口连接，第二N2压缩机(19)的出口与第二N2压缩机冷却器(20)入口连接，第二N2压缩机冷却器(20)的出口通过两个分支管道分别与预冷多股流换热器(1)的第五入口、主液化冷箱(3)的第三入口连接；主液化冷箱(3)的第三出口和第四出口均与N2膨胀机(21)的入口连接，N2膨胀机(21)的出口与主液化冷箱(3)的第二入口连接；主液化冷箱(3)的第四入口与预冷多股流换热器(1)的第五出口连接；主液化冷箱(3)的第五出口与燃气管道连接，用于排出燃气。
2.根据权利要求1所述的混合制冷剂集成氮膨胀循环的海上天然气液化系统，其特征在于：所述预冷多股流换热器(1)和液化多股流换热器(2)均为板翅式换热器。
3.根据权利要求1所述的混合制冷剂集成氮膨胀循环的海上天然气液化系统，其特征在于：所述主液化冷箱(3)为绕管式换热器。
4.根据权利要求1所述的混合制冷剂集成氮膨胀循环的海上天然气液化系统，其特征在于：所述N2膨胀机(21)用于为第一N2压缩机(17)提供部分或全部电能。
5.根据权利要求1所述的混合制冷剂集成氮膨胀循环的海上天然气液化系统，其特征在于：所述混合制冷剂包括氮气、甲烷、乙烷、丁烷中的两种或两种以上。
6.根据权利要求1所述的混合制冷剂集成氮膨胀循环的海上天然气液化系统，其特征在于：所述混合制冷剂含有摩尔分率为10％～20％的甲烷、摩尔分率为45％～60％的乙烷、摩尔分率为10％～20％的丁烷、摩尔分率为10％～20％的戊烷和摩尔分率为3％～10％的氮气。
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