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CN102506559A - 多段精馏制取高纯氮气空分工艺 - Google Patents

多段精馏制取高纯氮气空分工艺 Download PDF

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CN102506559A
CN102506559A CN2011103019576A CN201110301957A CN102506559A CN 102506559 A CN102506559 A CN 102506559A CN 2011103019576 A CN2011103019576 A CN 2011103019576A CN 201110301957 A CN201110301957 A CN 201110301957A CN 102506559 A CN102506559 A CN 102506559A
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nitrogen
air
rectifying column
rectifying
liquid nitrogen
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曹钧
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KAIFENG DONGJING AIR SEPARATION GROUP CO Ltd
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KAIFENG DONGJING AIR SEPARATION GROUP CO Ltd
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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
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04248—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion
    • F25J3/04284—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion using internal refrigeration by open-loop gas work expansion, e.g. of intermediate or oxygen enriched (waste-)streams
    • F25J3/0429—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion using internal refrigeration by open-loop gas work expansion, e.g. of intermediate or oxygen enriched (waste-)streams of feed air, e.g. used as waste or product air or expanded into an auxiliary column
    • F25J3/04303—Lachmann expansion, i.e. expanded into oxygen producing or low pressure column
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    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04006—Providing pressurised feed air or process streams within or from the air fractionation unit
    • F25J3/04078—Providing pressurised feed air or process streams within or from the air fractionation unit providing pressurized products by liquid compression and vaporisation with cold recovery, i.e. so-called internal compression
    • F25J3/04084—Providing pressurised feed air or process streams within or from the air fractionation unit providing pressurized products by liquid compression and vaporisation with cold recovery, i.e. so-called internal compression of nitrogen
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    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
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    • F25J3/04151—Purification and (pre-)cooling of the feed air; recuperative heat-exchange with product streams
    • F25J3/04187—Cooling of the purified feed air by recuperative heat-exchange; Heat-exchange with product streams
    • F25J3/04193—Division of the main heat exchange line in consecutive sections having different functions
    • F25J3/04206—Division of the main heat exchange line in consecutive sections having different functions including a so-called "auxiliary vaporiser" for vaporising and producing a gaseous product
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    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04248—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion
    • F25J3/04284—Generation of cold for compensating heat leaks or liquid production, e.g. by Joule-Thompson expansion using internal refrigeration by open-loop gas work expansion, e.g. of intermediate or oxygen enriched (waste-)streams
    • 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
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04406—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using a dual pressure main column system
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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
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
    • F25J3/04763—Start-up or control of the process; Details of the apparatus used
    • F25J3/04769—Operation, control and regulation of the process; Instrumentation within the process
    • F25J3/04812—Different modes, i.e. "runs" of operation
    • F25J3/04824—Stopping of the process, e.g. defrosting or deriming; Back-up procedures
    • 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
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/50—Processes or apparatus using separation by rectification using multiple (re-)boiler-condensers at different heights of the column
    • F25J2200/54—Processes or apparatus using separation by rectification using multiple (re-)boiler-condensers at different heights of the column in the low pressure column of a double pressure main column system
    • 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
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/02—Processes or apparatus using other separation and/or other processing means using simple phase separation in a vessel or drum
    • 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
    • F25J2245/00—Processes or apparatus involving steps for recycling of process streams
    • F25J2245/42—Processes or apparatus involving steps for recycling of process streams the recycled stream being nitrogen
    • 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
    • F25J2250/00—Details related to the use of reboiler-condensers
    • F25J2250/30—External or auxiliary boiler-condenser in general, e.g. without a specified fluid or one fluid is not a primary air component or an intermediate fluid
    • F25J2250/40—One fluid being air
    • 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
    • F25J2250/00—Details related to the use of reboiler-condensers
    • F25J2250/30—External or auxiliary boiler-condenser in general, e.g. without a specified fluid or one fluid is not a primary air component or an intermediate fluid
    • F25J2250/42—One fluid being nitrogen

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Separation By Low-Temperature Treatments (AREA)

Abstract

一种多段精馏制取高纯氮气空分工艺，主要是利用低温方法加上采用多段精馏、等熵膨胀制冷循环及带有蒸发器的部分高纯氮气产品实现内压缩之制高纯氮气空分工艺。该方法能分段制取不同输出压力的氮气产品，合理分配精馏过程中的分离功即有效地减少制氮工艺流程中不可逆损失，能充分地利用工艺流程中的可利用的有用功，并使之转化为该制氮气工艺流程中所需要冷量，从而增加液氮产品的产量。该方法不仅能实现部分氮气产品内压缩，加上带有蒸发器的流程布置，能提供高压氮气和中压氮气等两种或两种以上压力等级的高纯氮气产品。不仅流程合理而且具有明显的节能效果，即有效地降低单位制氮能耗，其单位制氮能耗约为0.2164KW.h/Nm3N2，氮气提取率高可达71％，使高纯氮设备性能得到大幅度提高。

Description

多段精馏制取高纯氮气空分工艺
技术领域
本发明涉及一种低温法制取高纯氮气的空分工艺，特别是多段精馏制取高纯氮气空分工艺。它适用于低温法制取高纯氮气行业。
背景技术
目前，低温法制取高纯氮气行业大多是采用单级精馏，正流或返流等熵膨胀制冷循环的工艺进行制取高纯氮气的，该制氮工艺虽然具有流程简便、设备筒单且数量少、占地面积小，操作简单、运行稳定、氮气产品质量高等特点。但是，单级精馏制取高纯氮气的空分工艺有着分离耗功大，能耗高的固有缺陷。单位能耗在0.30-0.37KW.h/Nm3N2，氮气产品提取率低，仅为37-45％之间，且生产产品单一，仅能产出一种压力等级氮气产品，另外液氮产品产量较少。
近年来，国外出现了一种双段精馏、正流等熵膨胀制冷循环制取高纯氮气的空分工艺。该工艺虽然能提高氮气产品的提取率，降低氮气产品的单位能耗，使单位能耗可达0.25KW.h/Nm3N2左右，并能产出中、高压两种不同压力等级的高纯氮气产品，从而使设备的性能得到非常明显的提升。但是，如果随着用户对中、高压高纯氮气产品的输出压力的要求更高时，大量有用功不能充分利用，不可逆损失也会加大。中、高压高纯氮气产品的输出压力越高时，上述现象就愈突出。因此，需对产品氮气采用外压缩的方法，方能缓解所述的突出问题，但这又带来了设备增加、投资增大、外压缩设备选型困难，产品氮气加压后因加压过程污染而使其纯度降低，难以满足用户的要求等弊端。
发明内容
本发明的目的在于提供一种多段精馏制取高纯氮气空分工艺，它能够克服已有技术的不足，可有效地减少制取高纯氮气空分工艺流程中的不可逆损失，能充分地利用有用功，并使之转化为制取高纯氮气空分工艺流程所需的冷量，从而可增加液氮产品的产量，又能为部分氮气产品内压缩提供必要条件；并且能使能耗指标降低、节能效果好，产品质量高。
本发明的工艺流程包括如下工艺步骤：
<1>空气经压缩机压缩后并经预冷、纯化系统进行预处理后使之成为干燥的压缩空气，该干燥的压缩空气送入精馏系统的主换热器中与返流泠气体进行热交换，而该干燥的压缩空气自身得以冷却，冷却后的部分压缩空气送去1#膨胀机进行等熵膨胀，产生的部分冷量进入上精馏塔参加精馏，实现第一级正流等熵膨胀制冷循环。
<2>所述主换热器中的剩余的干燥压缩空气进一步冷却，在该主换热器内被冷却至饱和温度，送至蒸发器与加压后的液氮进一步换热，换热后送到下精馏塔参加精馏，在下精馏塔精馏过程中，下精馏塔顶部产生了高纯氮气，该高纯氮气与上精馏塔底部的1#冷凝蒸发器中的富氧液空换热而形成液氮，部分液化后的液氮作为回流液参加下精馏塔精馏，另一部分液氮经过冷器过冷后作为液氮产品送出精馏系统，剩余部分液氮经液氮泵加压后送到蒸发器中，与来自上述主换热器的空气进行换热得以汽化，汽化后的液氮再送到上述主换热器复热至常温，成为高压氮气产品送出精馏系统。在上精馏塔塔顶得到的高纯氮气，一部分从上精馏塔顶取出，经过冷器过冷，又经上述主换热器复热至常温后，成为中压氮气产品送出精馏系统。另一部分高纯氮气则到上精馏塔中的2#冷凝蒸发器中，与来自上精馏塔底部1#冷凝蒸发器并经过冷器过冷的富氧液空进行换热并冷凝、冷凝后的液氮作为回流液在上精馏塔参加精馏，而在上精馏塔塔顶的2#冷凝蒸发器中的富氧液空蒸发，该蒸发后的气体又经过冷器换热后，进入2#膨胀机进行等熵膨胀，产生冷量，该冷量经上述主换热器复热至常温后，送出精馏系统，去下一单元纯化系统再生纯化器，这样实现了第二级返流等熵膨胀制冷循环。
本发明采用上述技术方案，由于进行多段精馏、双级等熵膨胀制冷循环及带蒸发器内压缩制取氮气空分工艺，能分段制取不同输出压力的高纯氮气产品，合理分配精馏过程中的分离功，即有效地减少制取高纯氮气过程中的不可逆损失，充分地利用工艺流程中的可利用的有用功，使之转化为制取高纯氮气空分工艺流程中所需要的冷量，从而可增加液氮产品的产量，还能为氮气产品的内压缩提供必要条件，实现部分液氮产品内压缩，加上带有蒸发器的流程布置，不仅解决了产品外压缩所造成的设备增加、投资增大、外压设备选型困难、产品氮气加压后纯度降低等缺陷，而且还降低了单纯采用提高加工空气压力所带来的空压机排压高，能耗增大、设备承压等级增加、设备成本增加等缺点。若用已有技术来获得下列高压氮气产品时，高压氮气输送压力为：1.4MPa(A)，产量：1000N3/h，纯度：含氧量小于5PPM，则空压机排压要达到1.49MPa(A)，而本发明采用带蒸发器内压缩方法来达到上述同样指标时，空压机排压则为1.2MPa(A)，空压机能耗降低约256KW。本发明与已有技术相比，节能效果显著。本发明可有效地降低单位制氮能耗及提高氮气提取率，其单位制氮能耗约为0.2164KW.h/Nm3N2，而氮气提取率可达71％，能使高纯氮设备性能得到大幅度提高，并能显著提高液氮产量，与已有技术相比液氮产量可为已有技术的2倍以上。同时还能提供高压氮气及中压氮气两种或两种以上压力等级的氮气产品。并且，与已有技术相比仅需增加两台液氮泵、两台膨胀机及蒸发器，设备投资及占地面积增加较少，设备压力等级也得以降低。同时操作简单，液氮、高压氮气及中压氮气的产量均可调节，变工况能力强。
附图说明
附图为多段精馏制取高纯氮气空分工艺流程原理图。
具体实施方式
下面结合附图详细描述本发明的具体实施方式。
本发明采用多段精馏、双级等熵膨胀制冷循环及带蒸发器内压缩工艺制取高纯氮气。现以双段精馏为例，其具体工艺流程如下：
<1>空气经空气过滤及压缩系统15压缩后，进入预冷系统16预冷，再进入纯化系统17纯化后，得干燥的压缩空气，把该压缩空气送入精馏系统的位于冷箱5中的主换热器1中，与返流冷气体进行热交换，而该干燥的压缩空气自身得以冷却，当干燥压缩空气被冷却到＜15℃时，一部分取出去位于冷箱5中的1#膨胀机2进行绝热膨胀，产生的部分冷量进入上精馏塔8参加精馏，实现第一级正流等熵膨胀制冷循环；
<2>剩下的压缩空气在上述主换热器1中继续被冷却，当被冷却至饱和温度并送至位于冷箱5中的蒸发器4与加压后的液氮进一步换热，换热后送到位于冷箱5中的下精馏塔11参加精馏，所述的饱和温度是随着工艺流程所需压力的变化而变化的，现以饱和温度-110K为例，经过下精馏塔11的精馏，在该下精馏塔11的塔顶产生了高纯氮气，而在塔底产生了富氧液空，该富氧液空从下精馏塔11中取出经过冷器14过冷后，送到位于冷箱5中的上精馏塔8中参加精馏，下精馏塔11塔顶得到的高纯氮气与上精馏塔8底部1#冷凝蒸发器9中的富氧液空进行换热，该富氧液空蒸发成为上精馏塔8中的上升蒸汽，而高纯氮气被液化为液氮，其中一部分液氮作为回流液回流到下精馏塔11中参加精馏，而另一部分液氮经过冷器14过冷后作为液氮产品，经冷箱5中的液氮量筒10计量后送出精馏系统，进入贮存系统12，剩余部分液氮经液氮泵6加压送到蒸发器4中，与来自所述主换热器1中的空气进行换热得以汽化，液化后的液氮再送到主换热器1中复热至常温，得高压氮气产品，并送出精馏系统到用户18。
而来自所述1#膨胀机2的膨胀空气及来自所述下精馏塔11的富氧液空，进入所述上精馏塔8中参加精馏后，在该上精馏塔8的塔顶得到高纯氮气，而在塔底得到高富氧液空。该高富氧液空中的一部分从上精馏塔8底部取出，经过冷器14过冷后，送到上精馏塔8顶部的2#冷凝蒸发器7中作为冷源。而上精馏塔8顶部的高纯氮气，一部分从该塔内取出，经过冷器14过冷后，进入主换热器1复热至常温，得中压氮气产品，送出精馏系统到用户18。另一部分高纯氮气送到所述2#冷凝蒸发器7中，与来自上精馏塔8底部1#冷凝蒸发器2并经过冷器14过冷的高富氧液空进行换热并冷凝，换热冷凝后作为回流液回上精馏塔8参与精馏，而该2#冷凝蒸发器7中的高富氧液空与上精馏塔8顶部的高纯氮气换热并汽化后，送入过冷器14换热，而后，进入位于冷箱5中的2#膨胀机3进行绝热膨胀，膨胀后的高富氧空气进入主换热器1复热后并送出精馏系统，去纯化系统17再生纯化器，从而实现第二级返流等熵膨胀制冷循环。液体排放槽13为设备停车时，剩余液体排放之用。

Claims (1)

1.一种多段精馏制取高纯氮气空分工艺，其特征在于该工艺包括如下步骤：
<1>将经压缩、预冷、纯化后的干燥压缩空气，送入精馏系统中的主换热器(1)中与返流泠气体进行热交换，该干燥压缩空气自身得以冷却，冷却后的部分压缩空气送去1#膨胀机(2)进行等熵膨胀，产生的部分冷量进入上精馏塔(8)参加精馏，从而实现第一级等熵膨胀制冷循环；
<2>所述主换热器(1)中的剩余干燥压缩空气进一步冷却，在该主换热器(1)中被冷却至饱和温度，送至蒸发器(4)中与加压后的液氮进一步换热后，再送到下精馏塔(11)参加精馏，在下精馏塔(11)精馏过程中，于下精馏塔(11)的顶部产生高纯氮气，该高纯氮气在1#冷凝蒸发器(9)中与上精馏塔(8)底部的富氧液空进行换热而形成液氮，部分被液化后的液氮作为回流液参加下精馏塔(11)精馏，另一部分液氮经过冷器(14)过冷后作为液氮产品送出精馏系统，剩余部分液氮经液氮泵(6)加压后送到蒸发器(4)中，与来自所述主换热器(1)中的空气进行换热得以汽化，汽化后的液氮再送到该主换热器(1)中复热至常温，成为高压氮气产品送出精馏系统，而上精馏塔(8)的塔顶得到的高纯氮气，一部分从上精馏塔(8)塔顶取出，经过冷器(14)过冷，又经所述主换热器(1)复热至常温后，成为中压氮气产品送出精馏系统，而另一部分高纯氮气到上精馏塔(8)中的2#冷凝蒸发器(7)中，与来自上精馏塔(8)底部1#冷凝蒸发器(9)并经过冷器(14)过冷的富氧液空换热并冷凝，冷凝后的液氮作为回流液在上精馏塔(8)中参加精馏，而在上精馏塔(8)中的2#冷凝蒸发器(7)中的富氧液空蒸发，该蒸发后的气体又经过冷器(14)换热后，送入2#膨胀机(3)进行等熵膨胀，产生冷量，该冷量经所述主换热器(1)复热至常温后，送出精馏系统，去纯化系统(17)再生纯化器，从而实现第二级等熵膨胀制冷循环。
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