JP3539709B2

JP3539709B2 - Air separation equipment

Info

Publication number: JP3539709B2
Application number: JP32007797A
Authority: JP
Inventors: 篤宮本; 延尚菊地; 洋実木山
Original assignee: Air Water Inc
Current assignee: Air Water Inc
Priority date: 1997-11-20
Filing date: 1997-11-20
Publication date: 2004-07-07
Anticipated expiration: 2017-11-20
Also published as: JPH11153382A

Description

【０００１】
【発明の属する技術分野】
本発明は、圧縮動力を低減させ、かつ収率を増大させることのできる空気分離装置に関するものである。
【０００２】
【従来の技術】
一般に、窒素ガス（ＧＮ₂）や酸素ガス（ＧＯ₂）は、空気を原料とし、これを空気圧縮機で圧縮したのち、吸着塔に入れて圧縮空気中の水（Ｈ₂Ｏ）および炭酸ガス（ＣＯ₂）を除去し、さらに熱交換器を通して冷媒と熱交換させて超低温に冷却し、ついで精留塔で深冷液化分離して製品ガスを製造し、これを前記の熱交換器を通して常温近くまで昇温させるという工程を経て製造されている。このような装置として、図２に示す空気分離装置がある。
【０００３】
図２において、１は大気中の空気を取り込んで圧縮する空気圧縮機である。２は内部にモレキュラーシーブが充填された２個１組の吸着筒等の不純物除去手段であり、空気圧縮機１により圧縮された空気中の水および炭酸ガスを吸着除去する作用をする。なお、上記空気圧縮機１と吸着筒２の間には、ドレン分離器，フロン冷却器が設けられているが、図面ではこれを省略している。３は吸着筒２により水および炭酸ガスが吸着除去された圧縮空気を超低温に冷却する主熱交換器である。４は主熱交換器３により超低温に冷却された圧縮空気を高圧精留塔（第１精留塔）５の下部に気−液混相状態で送り込む供給パイプである。この供給パイプ４を通る圧縮空気の圧力は、例えば５kg／cm²Ｇ程度である。高圧精留塔５の内部では、供給パイプ４から送り込まれた圧縮空気のうち、液体圧縮空気６が底部に溜まり、ＧＮ₂が上部に滞留する。７は（装置外から液体窒素が供給される）液体窒素貯槽（図示せず）からの液体窒素（ＬＮ₂）を高圧精留塔５の上部に導入する導入パイプである。この導入パイプ７から導入された微量の液体窒素と低圧精留塔（第２精留塔）１０下部の凝縮器（コンデンサー）１２で液化された液体窒素の一部とが高圧精留塔５内を下方に流下し、下方から上昇してくる気体圧縮空気と向流的に接触し気体圧縮空気の高沸点成分（主として酸素）を液化する。このため、底部に溜まる液体圧縮空気６は酸素リッチになり、低沸点成分（主として窒素）が気化して上部に滞留する。そして、高純度化された窒素ガスは、第１製品窒素ガス取出パイプ８により高圧精留塔５外に取り出されて主熱交換器３に送られ、この主熱交換器３を通過する圧縮空気を冷却するとともに、それ自身が常温に昇温され製品窒素ガスとして装置外に送り出される。１０は低圧精留塔であり、高圧精留塔５の底部に溜まる液体圧縮空気６が膨脹弁１１ａ付き送給パイプ１１を経て送り込まれる。この低圧精留塔１０は、高圧精留塔５に比べて非常に低圧で（例えば高圧精留塔５が５kg／cm²Ｇ程度で、低圧精留塔１０が０．２kg／cm²Ｇ程度で）運転されている。この低圧精留塔１０には、その底部に凝縮器１２が設けられており、高圧精留塔５内から第１製品窒素ガス取出パイプ８内に取り出された窒素ガスの一部が第１還流用パイプ１３を介して導入される。この窒素ガスは低圧精留塔１０の底部に溜まる液体酸素（ＬＯ₂：純度９９．７％程度）１６を加温して液体酸素１６を気化させる働きをし、それ自身は液体酸素１６の冷熱によって液化し、その一部が流量調整弁１４ａ付き第２還流用パイプ１４を通って高圧精留塔５の上部に還流する。また、上記液体窒素の残部は、流量調整弁１５ａ付き分岐パイプ１５を通って低圧精留塔１０の上部に導入され還流液となる。１７は製品酸素ガス取出パイプであり、低圧精留塔１０の底部に溜まる液体酸素１６から気化した高純度の酸素ガスを取り出して主熱交換器３内に案内し、圧縮空気と熱交換させて常温にし製品酸素ガスとして装置外に送り出す作用をする。１９は第２製品窒素ガス取出パイプであり、低圧精留塔１０の上部に溜った窒素ガスを取り出して主熱交換器３に送り、圧縮空気を冷却するとともに、それ自身を常温に昇温させ製品窒素ガスとして装置外に送り出す作用をする。２１は排出用パイプであり、低圧精留塔１０の中央部に溜まる不純ガス分（排ＧＮ₂）を取り出して主熱交換器３内に案内し、圧縮空気を冷却したのち外部に放出する作用をする。図において、カッコ内に書かれた数字は、空気圧縮機１から吐出された圧縮空気の主熱交換器３通過量を１００％としたときの、各パイプ８，１７，１９，２１における概略通過量（％）を示している。
【０００４】
この装置を用い、例えばつぎのようにして窒素ガスおよび酸素ガスを製造することができる。すなわち、まず空気圧縮機１により空気を圧縮し、ドレン分離器により圧縮された空気中の水分を除去し、フロン冷却器により冷却し、その状態で吸着筒２に送り込み、圧縮空気中の水および炭酸ガスを吸着除去する。ついで水および炭酸ガスが吸着除去された圧縮空気を主熱交換器３内に送り込んで超低温に冷却し、その状態で高圧精留塔５の下部に導入する。つぎにこの圧縮空気と、液体窒素貯槽から高圧精留塔５内に送り込まれた液体窒素および低圧精留塔１０から還流する液体窒素とを向流接触させて圧縮空気を精留し、窒素と酸素の沸点の差（酸素の沸点−１８３℃，窒素の沸点−１９６℃）により、圧縮空気中の高沸点成分である酸素を液化させ、窒素を気体として滞留させる。この窒素ガスを第１製品窒素ガス取出パイプ８から取り出して主熱交換器３に送り込み常温近くまで昇温させたのち、製品窒素ガスとして装置外に送り出す。この場合、液体窒素貯槽から高圧精留塔５内に導入される液体窒素は、圧縮空気液化用の寒冷源として作用し、それ自身は気化して製品窒素ガスの一部として第１製品窒素ガス取出パイプ８から取り出される。また、低圧精留塔１０の上部に溜まった窒素ガスを第２製品窒素ガス取出パイプ１９から取り出して主熱交換器３に送り込み常温近くまで昇温させたのち、製品窒素ガスとして装置外に送り出す。他方、高圧精留塔５の底部に溜った液体圧縮空気６を送給パイプ１１を経て低圧精留塔１０に送り込み、窒素を気化除去した液体酸素１６として低圧精留塔１０の底部に溜め、低圧精留塔１０底部の凝縮器１２を通る窒素ガスと熱交換させて気化させる。この気化させた酸素ガスを製品酸素ガス取出パイプ１７から取り出して主熱交換器３に送り込み常温近くまで昇温させたのち、製品酸素ガスとして装置外に送り出す。このようにして、高純度の窒素ガスと酸素ガスとが１台の装置により同時に得られる。
【０００５】
【発明が解決しようとする課題】
しかしながら、上記の空気分離装置では、運転圧力が約５kg／cm²Ｇ程度に設定された高圧精留塔５が下方に配置され、内部圧力が約０．２kg／cm²Ｇ程度に設定された低圧精留塔１０が上方に配置されている。このように、高圧精留塔５が低圧精留塔１０に比べて非常に高圧に設定されているのは、高圧精留塔５の上部に滞留する窒素ガスを第１還流用パイプ１３を介して低圧精留塔１０の凝縮器１２に送り込みここで液化させて還流させるため、および、高圧精留塔５の底部に溜まる液体圧縮空気６を送給パイプ１１を介して低圧精留塔１０に圧力差を利用して送り込むためである。したがって、上記の空気分離装置では、その最大の動力源である空気圧縮機１の圧縮動力が増大する。
【０００６】
本発明は、このような事情に鑑みなされたもので、圧縮動力を低下させ、かつ収率を増大させることのできる空気分離装置の提供をその目的とする。
【０００７】
【課題を解決するための手段】
上記の目的を達成するため、本発明の空気分離装置は、原料空気を圧縮する空気圧縮手段と、上記空気圧縮手段を経た圧縮空気を低温に冷却する主熱交換器と、上記主熱交換器により低温に冷却された圧縮空気を導入する第１精留塔と、上記第１精留塔から取り出された液体空気を導入する第２精留塔と、上記第２精留塔の下部に設けられた熱交換器と、上記第１精留塔から気体として窒素を取り出しこの窒素を上記主熱交換器を経由させることにより温度上昇させ製品窒素ガスとする製品窒素ガス取出パイプと、上記製品窒素ガス取出パイプを通る窒素ガスの一部を上記熱交換器に送るガスパイプと、上記ガスパイプに設けられ上記製品窒素ガス取出パイプを通る窒素ガスの一部を取り入れてこれを圧縮し上記熱交換器に送る窒素ガス圧縮手段と、上記熱交換器から延設され上記熱交換器内で生じた液化窒素を還流液として第１精留塔に戻す還流液パイプとを備え、上記第１精留塔と第２精留塔の相互の配設位置を、第１精留塔を上側に、第２精留塔をこの第１精留塔の下側に位置決めしたという構成をとる。
【０００８】
すなわち、本発明の空気分離装置は、従来例では高圧精留塔として下側に配設されていた第１精留塔を上側に配設し、従来例では低圧精留塔として上側に配設されていた第２精留塔を下側に配設している。このものでは、第１精留塔と第２精留塔をパイプ等で連結し、このパイプ等を介して、第１精留塔から取り出した液体空気を自然落下により第２精留塔に導入することができる。また、製品窒素ガス取出パイプを通る窒素ガスの一部を、製品窒素ガス取出パイプから取り出して窒素ガス圧縮手段により第２精留塔の熱交換器に導入し、この熱交換器を通過する間に、第１精留塔から導入した液体空気の冷熱を利用して液化したのち、還流液として第１精留塔に戻すことができる。これら自然落下や窒素ガス圧縮手段を利用することにより、両精留塔の圧力差を無くし、両精留塔の圧力を従来例の低圧精留塔の圧力と同程度の低圧にすることができる。したがって、空気圧縮機の吐出圧力を低下させることができ、これに伴い、その圧縮動力を低下させることができる。また、第１精留塔の圧力が低下すると、第１精留塔内の温度が低下し、窒素（Ｎ₂）と酸素（Ｏ₂）の相対揮発度が大きくなる。このため、第１精留塔内で窒素と酸素が分離しやすくなる。
【０００９】
また、本発明では、上記ガスパイプに、上記製品窒素ガス取出パイプを通る窒素ガスの一部を取り入れてこれを圧縮し上記熱交換器に送る窒素ガス圧縮手段を設けたため、この窒素ガス圧縮手段の圧縮動力が空気圧縮手段の圧縮動力に比べて大幅に小さいため、両圧縮手段の圧縮動力を合計しても、従来例の空気圧縮機の圧縮動力と比べて酸素製造原単位が約１０％削減可能となる。特に、第２精留塔で得られる製品酸素ガスの純度が比較的低純度（９３％程度）である場合には、窒素ガス圧縮手段の流量を少なくすることができ、動力削減の割合は大きくなる。
【００１０】
また、本発明では、上記熱交換器から、この熱交換器内で生じた液化窒素を還流液として第１精留塔に戻す還流液パイプを延設したため、窒素ガス圧縮手段の吐出圧力を利用して、熱交換器内で生じた液化窒素を第１精留塔に戻すことができる。
【００１１】
【発明の実施の形態】
つぎに、本発明の実施の形態を図面にもとづいて詳しく説明する。
【００１２】
図１は本発明の空気分離装置の一実施の形態を示している。この実施の形態では、空気圧縮機１，２個１組の吸着塔２，主熱交換器３等は、図２に示す空気分離装置と同様であり、同様の部分には同じ符号を付している。ただし、この実施の形態では、従来例の高圧精留塔（第１精留塔）５が上部塔３０として上側に配置され、従来例の低圧精留塔（第２精留塔）１０が下部塔３１として下側に配置されている。これら両塔３０，３１の内部圧力は、ともに０．２kg／cm²Ｇ程度である。また、上部塔３０の下部から下方に延びる流量調整弁３２ａ付き液体空気供給パイプ３２が下部塔３１に連結している。この液体空気供給パイプ３２は、上部塔３０の底部に溜まる液体圧縮空気６を自然落下により下部塔３１の上部に送り込む作用をする。一方、上部塔３０の上部から延びる第１製品窒素ガス取出パイプ８のうち、主熱交換器３より下流側の部分から循環用ガスパイプ３３が分岐している。このガスパイプ３３は、下部塔３１の底部の液体酸素１６中に配設された熱交換器１２に連結しており、主熱交換器３より上流側の部分に循環圧縮機（窒素ガス圧縮手段）３４が設けられている。この循環圧縮機３４は、第１製品窒素ガス取出パイプ８を通る製品窒素ガスの一部を取り入れて圧縮し、この圧縮窒素ガスを熱交換器１２に送り込み、この熱交換器１２で生じた還流液（液体窒素）を上部塔３０に戻す作用をする。この圧縮窒素ガスは、下部塔３１の底部の液体酸素（純度９９．７％程度）１６を加温して気化させる働きをし、それ自身は液体酸素１６の冷熱によって液化し、流量調整弁３５ａ付き還流液パイプ３５を通って上部塔３０の上部に還流する。この還流分は、導入パイプ７から導入される微量の液体窒素と同様、上部塔３０内における圧縮空気の精留に用いられる。また、上記液体窒素の残部は、流量調整弁３６ａ付き分岐パイプ３６を通って下部塔３１の上部に導入され酸素の精留に供される。図において、カッコ内に書かれた数字は、空気圧縮機１から吐出された圧縮空気の主熱交換器３通過量を１００％としたときの、各パイプ８，１７，１９，２１，３３における概略通過量（％）を示している。
【００１３】
この装置を用い、例えばつぎのようにして窒素ガスおよび酸素ガスを製造することができる。すなわち、まず空気圧縮機１により空気を０．７kg／cm²Ｇ程度に圧縮し、ドレン分離器により圧縮された空気中の水分を除去し、フロン冷却器により冷却し、その状態で吸着筒２に送り込み、圧縮空気中の水および炭酸ガスを吸着除去する。勿論、吸着筒２の吸着剤の性能上、圧力が３kg／cm²Ｇ程度必要な場合は、空気圧縮機１の吐出圧力を３kg／cm²Ｇ程度に上げなければならない。ついで水および炭酸ガスが吸着除去された圧縮空気を主熱交換器３内に送り込んで超低温に冷却し、供給パイプ４を通して気−液混相状態で上部塔３０の下部に導入する。この供給パイプ４を通る圧縮空気は、０．２kg／cm²Ｇ程度の圧力である。つぎにこの圧縮空気と、導入パイプ７から上部塔３０内に送り込まれた液体窒素および下部塔３１内の熱交換器１２から還流する液体窒素とを向流接触させて圧縮空気を精留し、窒素と酸素の沸点の差（酸素の沸点−１８３℃，窒素の沸点−１９６℃）により、圧縮空気中の高沸点成分である酸素を液化させ、窒素を気体として滞留させる。この窒素ガスを第１製品窒素ガス取出パイプ８から取り出して主熱交換器３に送り込み常温近くまで昇温させたのち、製品窒素ガスとして装置外に送り出す。この場合、液体窒素貯槽から上部塔３０内に導入される液体窒素は、圧縮空気液化用の寒冷源として作用し、それ自身は気化して製品窒素ガスの一部として第１製品窒素ガス取出パイプ８から取り出される。一方、下部塔３１には、その上部に、上部塔３０の底部に溜まった液体圧縮空気６を液体空気供給パイプ３２を介して自然落下により送り込む。また、第１製品窒素ガス取出パイプ８を通る製品窒素ガスの一部をガスパイプ３３を通して循環圧縮機３４に導入し、この循環圧縮機３４で４．６kg／cm²Ｇ程度に圧縮したのち再度主熱交換器３に通して超低温に（液化温度近くまで）冷却し、熱交換器１２に送り込む。送り込まれた圧縮窒素ガスを熱交換器１２で下部塔３１の底部に溜まった液体酸素１６と熱交換して略完全に液化したのち、液化窒素の一部を還流液パイプ３５を通して上部塔３０に送り込み、残部を分岐パイプ３６を通して下部塔３１に送り込む。そして、液体空気供給パイプ３２から送り込まれる酸素リッチな液体圧縮空気６と分岐パイプ３６から送り込まれる液体窒素とが還流液となり、下部塔３１内を下方に流下する。また、下部塔３１の底部に溜った液体酸素１６は熱交換器１２を通る圧縮窒素ガスと熱交換して気化し、下部塔３１内を上昇し、還流液と向流接触して精留され、高純度の窒素ガスが上部に滞留する。そして、製品酸素ガス取出パイプ１７から酸素ガスを取り出して主熱交換器３に送り込み常温近くまで昇温させたのち、製品酸素ガスとして装置外に送り出す。また、下部塔３１の上部に滞留する窒素ガスを第２製品窒素ガス取出パイプ１９から取り出して主熱交換器３に送り込み常温近くまで昇温させたのち、製品窒素ガスとして装置外に取り出す。このようにして、高純度の窒素ガスと酸素ガスとが１台の装置により同時に得られる。
【００１４】
このように、上記実施の形態では、従来例の高圧精留塔５を上部塔３０として上側に配設するとともに、従来例の低圧精留塔１０を下部塔３１として下側に配設しているため、上部塔３０で製造される液体圧縮空気６を下部塔３１に供給する場合に、自然落下でよい。また、循環圧縮機３４を用い、第１製品窒素ガス取出パイプ８を通る製品窒素ガスの一部を熱交換器１２に圧送し、還流液として上部塔３０に戻している。したがって、上部塔３０と下部塔３１との間に圧力差が不要となり、両塔３０，３１を低圧にすることができる。このため、空気圧縮機１の圧縮動力を低下させることができ、この空気圧縮機１と循環圧縮機３４の圧縮動力の合計を、従来例の空気圧縮機１の圧縮動力と比べて酸素製造原単位で約１０％削減できる。一方、上部塔３０の運転圧力が低下することで、上部塔３０内の温度が低下して窒素と酸素の相対揮発度が大きくなり、分離しやすくなる。
【００１５】
上記実施の形態において、空気圧縮機１と循環圧縮機３４の圧縮動力を下記の式（１）により算出し、図２に示す空気分離装置の空気圧縮機１の圧縮動力と比較した。この算出にあたって、上記実施の形態における空気圧縮機１の圧縮動力は、Ｐｓ：１ｋｇ／ｃｍ²ａｂｓ，Ｐｄ：１．７ｋｇ／ｃｍ²ａｂｓ，Ｑｓ：１０４（ウェットベース）ｍ³／ｍｉｎ，ｍ：１段，κ：１．４（このκの値は、以下の場合も同じである）に設定して得られた値であり、循環圧縮機３４の圧縮動力は、Ｐｓ：１．２ｋｇ／ｃｍ²ａｂｓ，Ｐｄ：５．６ｋｇ／ｃｍ²ａｂｓ，Ｑｓ：８７ｍ³／ｍｉｎ，ｍ：３段に設定して得られた値である。また、図２に示す空気分離装置の空気圧縮機１の圧縮動力を算出するにあたっては、Ｐｓ：１ｋｇ／ｃｍ²ａｂｓ，Ｐｄ：６．８ｋｇ／ｃｍ²ａｂｓ，Ｑｓ：１０４（ウェットベース）ｍ³／ｍｉｎ，ｍ：４段に設定した。その結果、上記実施の形態では、空気圧縮機１の圧縮動力が９７．３ｋｗで、循環圧縮機３４の圧縮動力が２８３ｋｗであり、その合計が３８０．３ｋｗであった。一方、図２に示す空気分離装置の空気圧縮機１の圧縮動力は３４９．１ｋｗであった。このとき、製品酸素の製造量はそれぞれ１９ｍ³／ｍｉｎと、１６ｍ³／ｍｉｎであるため、酸素製造原単位は（３８０．３／１９）／（３４９．１／１６）＝０．９２となり、上記実施の形態の圧縮動力は従来例の圧縮動力の約９２％に減少していることがわかる。
【００１６】
【式１】
Ｌａｄ＝（１／０．６１２）・Ｐｓ・Ｑｓ・（ｍκ／κ−１）・｛（Ｐｄ／Ｐｓ）^k-1/mk−１｝
Ｌａｄ：理論断熱圧縮動力〔ｋｗ〕
Ｐｓ：吸込ガスの絶対圧力〔ｋｇ／ｃｍ²ａ〕
Ｐｄ：吐出ガスの絶対圧力〔ｋｇ／ｃｍ²ａ〕
Ｑｓ：吸込状態に換算したガス量〔ｍ³／ｍｉｎ〕
ｍ：圧縮段数〔段〕
κ：ガスの断熱指数〔Ｃｐ／Ｃｖ，０℃，１ａｔｍ〕
【００１７】
なお、上記実施の形態では、外部からの寒冷源としてＬＮ₂を用いているが、これに限定するものではなく、ＬＯ₂等の極低温流体を用いていもよいし、膨張タービンを利用した空気分離装置にしてもよい。また、上記実施の形態では、製品ガスをすべてガスで取り出しているが、これに限定するものではなく、一部を液体で取り出してもよいし、酸素ガスのみを製造してもよい。
【００１８】
【発明の効果】
以上のように、本発明の空気分離装置によれば、従来例では高圧精留塔として下側に配設されていた第１精留塔を上側に配設し、従来例では低圧精留塔として上側に配設されていた第２精留塔を下側に配設している。このため、第１精留塔と第２精留塔をパイプ等で連結し、このパイプ等を介して、第１精留塔から取り出した液体空気を自然落下により第２精留塔に導入することができる。また、製品窒素ガス取出パイプを通る窒素ガスの一部を、製品窒素ガス取出パイプから取り出して窒素ガス圧縮手段により第２精留塔の熱交換器に導入し、この熱交換器を通過する間に、第１精留塔から導入した液体空気の冷熱を利用して液化したのち、還流液として第１精留塔に戻すことができる。これら自然落下や窒素ガス圧縮手段を利用することにより両精留塔の圧力差を無くし、両精留塔の圧力を従来例の低圧精留塔の圧力と同程度の低圧にすることができる。したがって、空気圧縮機の吐出圧力を低下させることができ、これに伴い、その圧縮動力を低下させることができる。また、第１精留塔の圧力が低下すると、第１精留塔内の温度が低下し、窒素（Ｎ₂）と酸素（Ｏ₂）の相対揮発度が大きくなる。このため、第１精留塔内で窒素と酸素が分離しやすくなる。
【００１９】
また、本発明では、上記ガスパイプに、上記製品窒素ガス取出パイプを通る窒素ガスの一部を取り入れてこれを圧縮し上記熱交換器に送る窒素ガス圧縮手段を設けたため、この窒素ガス圧縮手段の圧縮動力が空気圧縮手段の圧縮動力に比べて大幅に小さいため、両圧縮手段の圧縮動力を合計しても、従来例の空気圧縮機の圧縮動力と比べて酸素製造原単位で約１０％削減可能となる。特に、第２精留塔で得られる製品酸素ガスの純度が比較的低純度（９３％程度）である場合には、窒素ガス圧縮手段の流量を少なくすることができ、動力削減の割合は大きくなる。
【００２０】
また、本発明では、上記熱交換器から、この熱交換器内で生じた液化窒素を還流液として第１精留塔に戻す還流液パイプを延設したため、窒素ガス圧縮手段の吐出圧力を利用して、熱交換器内で生じた液化窒素を第１精留塔に戻すことができる。
【図面の簡単な説明】
【図１】この発明の一実施の形態を示す空気分離装置の構成図である。
【図２】従来例を示す空気分離装置の構成図である。
【符号の説明】
１空気圧縮機
３主熱交換器
６液体空気
８第１製品窒素ガス取出パイプ
１２熱交換器
３０上部塔
３１下部塔
３４循環圧縮機
３５還流液パイプ[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to an air separation device capable of reducing compression power and increasing yield.
[0002]
[Prior art]
In general, nitrogen gas (GN ₂ ) and oxygen gas (GO ₂ ) are obtained by using air as a raw material, compressing the raw material with an air compressor, and then putting the compressed air into an adsorption tower to supply water (H ₂ O) and carbon dioxide gas in the compressed air. (CO ₂ ) is removed, and further heat-exchanged with a refrigerant through a heat exchanger to cool to ultra-low temperature, and then cryogenically liquefied and separated in a rectification column to produce a product gas, which is passed through the heat exchanger at room temperature. It is manufactured through a process of raising the temperature to near. As such a device, there is an air separation device shown in FIG.
[0003]
In FIG. 2, reference numeral 1 denotes an air compressor that takes in and compresses air in the atmosphere. Reference numeral 2 denotes an impurity removing means such as a set of two adsorption cylinders, each of which is filled with a molecular sieve, and functions to adsorb and remove water and carbon dioxide in the air compressed by the air compressor 1. Note that a drain separator and a CFC cooler are provided between the air compressor 1 and the adsorption cylinder 2, but these are omitted in the drawings. Reference numeral 3 denotes a main heat exchanger for cooling the compressed air, from which water and carbon dioxide gas has been removed by adsorption, to an extremely low temperature. Reference numeral 4 denotes a supply pipe for sending compressed air cooled to an extremely low temperature by the main heat exchanger 3 to a lower part of a high-pressure rectification tower (first rectification tower) 5 in a gas-liquid mixed phase state. The pressure of the compressed air passing through the supply pipe 4 is, for example, about 5 kg / cm ² G. Inside the high-pressure rectification tower 5, of the compressed air sent from the supply pipe 4, the liquid compressed air 6 accumulates at the bottom and GN ₂ stays at the top. Reference numeral 7 denotes an introduction pipe for introducing liquid nitrogen (LN ₂ ) from a liquid nitrogen storage tank (not shown) (to which liquid nitrogen is supplied from outside the apparatus) into the upper part of the high-pressure rectification column 5. A small amount of liquid nitrogen introduced from the introduction pipe 7 and part of the liquid nitrogen liquefied in the condenser (condenser) 12 below the low-pressure rectification column (second rectification column) 10 are mixed in the high-pressure rectification column 5. Flows downward, and comes into countercurrent contact with the gas compressed air rising from below to liquefy the high boiling point component (mainly oxygen) of the gas compressed air. For this reason, the liquid compressed air 6 accumulated at the bottom becomes oxygen-rich, and the low-boiling components (mainly nitrogen) are vaporized and stay at the top. Then, the highly purified nitrogen gas is taken out of the high-pressure rectification tower 5 by the first product nitrogen gas extraction pipe 8 and sent to the main heat exchanger 3, and the compressed air passing through the main heat exchanger 3 While cooling itself to room temperature and sent out of the apparatus as product nitrogen gas. Reference numeral 10 denotes a low-pressure rectification tower, into which liquid compressed air 6 collected at the bottom of the high-pressure rectification tower 5 is fed via a supply pipe 11 with an expansion valve 11a. The low-pressure rectification tower 10 has a very low pressure compared to the high-pressure rectification tower 5 (for example, the high-pressure rectification tower 5 has a pressure of about 5 kg / cm ² G, and the low-pressure rectification tower 10 has a pressure of about 0.2 kg / cm ² G). In) driving. The low pressure rectification column 10 is provided with a condenser 12 at the bottom thereof, and a portion of the nitrogen gas extracted from the high pressure rectification column 5 into the first product nitrogen gas extraction pipe 8 is subjected to a first reflux. Through the use pipe 13. The nitrogen gas serves to heat liquid oxygen (LO ₂ : purity: about 99.7%) 16 accumulated at the bottom of the low-pressure rectification column 10 to vaporize the liquid oxygen 16, and to cool and heat the liquid oxygen 16 itself. And a part thereof is returned to the upper part of the high-pressure rectification column 5 through the second reflux pipe 14 with the flow control valve 14a. The remainder of the liquid nitrogen is introduced into the upper part of the low-pressure rectification column 10 through the branch pipe 15 with the flow control valve 15a and becomes a reflux liquid. Reference numeral 17 denotes a product oxygen gas extraction pipe, which extracts high-purity oxygen gas vaporized from liquid oxygen 16 stored at the bottom of the low-pressure rectification column 10 and guides it into the main heat exchanger 3 to exchange heat with compressed air. It works at normal temperature and sends it out of the device as product oxygen gas. Reference numeral 19 denotes a second product nitrogen gas extraction pipe, which takes out nitrogen gas collected in the upper part of the low-pressure rectification column 10 and sends it to the main heat exchanger 3 to cool the compressed air and raise the temperature of itself to room temperature. It acts to send it out of the device as product nitrogen gas. Reference numeral 21 denotes a discharge pipe for taking out an impurity gas (discharge GN ₂ ) accumulated in the central portion of the low-pressure rectification tower 10 and guiding it to the main heat exchanger 3 to cool the compressed air and discharge it to the outside. do. In the figure, the numbers written in parentheses indicate the approximate passage through the pipes 8, 17, 19, and 21 when the amount of compressed air discharged from the air compressor 1 passes through the main heat exchanger 3 as 100%. The amount (%) is shown.
[0004]
Using this apparatus, nitrogen gas and oxygen gas can be produced, for example, as follows. That is, first, air is compressed by the air compressor 1, moisture in the compressed air is removed by the drain separator, cooled by the Freon cooler, and then sent to the adsorption column 2 in that state. Adsorb and remove carbon dioxide. Next, the compressed air from which water and carbon dioxide have been adsorbed and removed is sent into the main heat exchanger 3 to be cooled to an extremely low temperature, and then introduced into the lower part of the high-pressure rectification column 5 in that state. Next, the compressed air is rectified by bringing the compressed air into liquid nitrogen sent from the liquid nitrogen storage tank into the high-pressure rectification tower 5 and the liquid nitrogen refluxed from the low-pressure rectification tower 10 in countercurrent contact to rectify the compressed air. Oxygen, which is a high-boiling component in compressed air, is liquefied by the difference between the boiling points of oxygen (boiling point of oxygen -183 ° C, boiling point of nitrogen -196 ° C), and nitrogen is retained as a gas. This nitrogen gas is taken out of the first product nitrogen gas extraction pipe 8, sent to the main heat exchanger 3, and heated to near normal temperature, and then sent out of the apparatus as product nitrogen gas. In this case, the liquid nitrogen introduced into the high-pressure rectification tower 5 from the liquid nitrogen storage tank acts as a cold source for liquefaction of compressed air, and is itself vaporized and becomes the first product nitrogen gas as a part of the product nitrogen gas. It is taken out from the take-out pipe 8. Further, the nitrogen gas collected in the upper part of the low-pressure rectification tower 10 is taken out from the second product nitrogen gas extraction pipe 19, sent to the main heat exchanger 3, heated to near normal temperature, and sent out of the apparatus as product nitrogen gas. . On the other hand, the liquid compressed air 6 collected at the bottom of the high-pressure rectification column 5 is sent to the low-pressure rectification column 10 via the feed pipe 11 and stored as liquid oxygen 16 from which nitrogen has been removed by vaporization at the bottom of the low-pressure rectification column 10. It is vaporized by heat exchange with nitrogen gas passing through a condenser 12 at the bottom of the low-pressure rectification column 10. The vaporized oxygen gas is taken out from the product oxygen gas take-out pipe 17, sent to the main heat exchanger 3, heated to near normal temperature, and sent out of the apparatus as product oxygen gas. In this way, high-purity nitrogen gas and oxygen gas can be obtained simultaneously by one apparatus.
[0005]
[Problems to be solved by the invention]
However, in the above-mentioned air separation device, the high-pressure rectification column 5 whose operating pressure is set to about 5 kg / cm ² G is arranged below, and the internal pressure is set to about 0.2 kg / cm ² G. A low-pressure rectification column 10 is arranged above. The reason why the high-pressure rectification column 5 is set at a very high pressure as compared with the low-pressure rectification column 10 is that the nitrogen gas accumulated at the upper part of the high-pressure rectification column 5 is passed through the first reflux pipe 13. Into the condenser 12 of the low-pressure rectification column 10 for liquefaction and reflux, and the liquid compressed air 6 collected at the bottom of the high-pressure rectification column 5 to the low-pressure rectification column 10 via the feed pipe 11. This is for feeding using a pressure difference. Therefore, in the above-described air separation device, the compression power of the air compressor 1, which is the largest power source, increases.
[0006]
The present invention has been made in view of such circumstances, and an object of the present invention is to provide an air separation device capable of reducing compression power and increasing yield.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, an air separation device of the present invention comprises an air compression unit for compressing raw air, a main heat exchanger for cooling compressed air that has passed through the air compression unit to a low temperature, and the main heat exchanger. A first rectification column for introducing compressed air cooled to a low temperature, a second rectification column for introducing liquid air taken out of the first rectification column, and a lower portion of the second rectification column A product heat exchanger, a product nitrogen gas extraction pipe for taking out nitrogen as a gas from the first rectification column and passing the nitrogen through the main heat exchanger to raise the temperature to produce product nitrogen gas. A gas pipe that sends a part of the nitrogen gas passing through the gas extraction pipe to the heat exchanger, and a part of the nitrogen gas that is provided in the gas pipe and passes through the product nitrogen gas extraction pipe, is compressed, and compressed to the heat exchanger. Sending nitrogen gas compression Stage and, a reflux pipe liquefied nitrogen occurring within the heat exchanger extending from the heat exchanger back into the first rectification column as a reflux liquid, the first rectification column and a second rectification The first rectification column is positioned above the first rectification column, and the second rectification column is positioned below the first rectification column.
[0008]
That is, in the air separation device of the present invention, the first rectification column, which was disposed on the lower side as a high-pressure rectification column in the conventional example, is disposed on the upper side, and in the conventional example, the first rectification column is disposed on the upper side as a low-pressure rectification column. The second rectification column, which has been used, is disposed below. In this apparatus, the first rectification tower and the second rectification tower are connected by a pipe or the like, and the liquid air taken out of the first rectification tower is introduced into the second rectification tower via the pipes by natural fall. can do. Further, a part of the nitrogen gas passing through the product nitrogen gas extraction pipe is taken out from the product nitrogen gas extraction pipe, introduced into the heat exchanger of the second rectification column by the nitrogen gas compression means , and passed through the heat exchanger. Then, after liquefaction using the cold heat of the liquid air introduced from the first rectification column, it can be returned to the first rectification column as a reflux liquid. By utilizing these natural falling and nitrogen gas compression means , the pressure difference between the two rectification columns can be eliminated, and the pressure of both rectification columns can be reduced to the same level as that of the conventional low-pressure rectification column. . Therefore, the discharge pressure of the air compressor can be reduced, and the compression power can be reduced accordingly. Further, when the pressure in the first rectification column decreases, the temperature in the first rectification column decreases, and the relative volatility of nitrogen (N ₂ ) and oxygen (O ₂ ) increases. Therefore, nitrogen and oxygen are easily separated in the first rectification column.
[0009]
Further, in the present invention, the above gas pipe, due to the provision of the nitrogen gas compression means for sending to the heat exchanger to compress it incorporates a portion of the nitrogen gas through the product nitrogen gas take-out pipe, the nitrogen gas compression means Is significantly smaller than the compression power of the air compression means, so that even if the compression powers of both compression means are totaled, the oxygen production unit consumption is about 10% as compared with the compression power of the conventional air compressor. It can be reduced. In particular, when the purity of the product oxygen gas obtained in the second rectification column is relatively low (about 93%), the flow rate of the nitrogen gas compression means can be reduced, and the rate of power reduction is large. Become.
[0010]
Further, in the present invention, from the heat exchanger, because of the extended the reflux pipe for returning the liquid nitrogen produced in the heat exchanger to the first rectification column as a reflux liquid, the discharge pressure of the nitrogen gas compression means The liquefied nitrogen generated in the heat exchanger can be returned to the first rectification column by utilizing it.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described in detail with reference to the drawings.
[0012]
FIG. 1 shows an embodiment of the air separation device of the present invention. In this embodiment, an air compressor 1, a set of two adsorption towers 2, a main heat exchanger 3 and the like are the same as those of the air separation device shown in FIG. 2, and the same parts are denoted by the same reference numerals. ing. However, in this embodiment, the high-pressure rectification tower (first rectification tower) 5 of the conventional example is disposed on the upper side as the upper tower 30, and the low-pressure rectification tower (second rectification tower) 10 of the conventional example is located at the lower part. It is arranged on the lower side as a tower 31. The internal pressure of both towers 30 and 31 is about 0.2 kg / cm ² G. In addition, a liquid air supply pipe 32 with a flow control valve 32 a extending downward from a lower part of the upper tower 30 is connected to the lower tower 31. The liquid air supply pipe 32 acts to send the liquid compressed air 6 accumulated at the bottom of the upper tower 30 to the upper part of the lower tower 31 by natural fall. On the other hand, of the first product nitrogen gas extraction pipe 8 extending from the upper part of the upper tower 30, a circulation gas pipe 33 branches from a portion downstream of the main heat exchanger 3. This gas pipe 33 is connected to the heat exchanger 12 disposed in the liquid oxygen 16 at the bottom of the lower tower 31, and a circulating compressor (nitrogen gas compression means) is provided at a portion upstream of the main heat exchanger 3. 34 are provided. The circulating compressor 34 takes in and compresses a part of the product nitrogen gas passing through the first product nitrogen gas extraction pipe 8, sends the compressed nitrogen gas to the heat exchanger 12, and generates the reflux gas generated in the heat exchanger 12. It acts to return the liquid (liquid nitrogen) to the upper tower 30. The compressed nitrogen gas serves to heat and evaporate the liquid oxygen (purity of about 99.7%) 16 at the bottom of the lower tower 31, and liquefy itself by the cold heat of the liquid oxygen 16, and the flow control valve 35 a Then, the liquid is refluxed to the upper part of the upper tower 30 through a reflux liquid pipe 35. This reflux is used for the rectification of the compressed air in the upper tower 30 in the same manner as the trace amount of liquid nitrogen introduced from the introduction pipe 7. The remainder of the liquid nitrogen is introduced into the upper part of the lower tower 31 through the branch pipe 36 with the flow control valve 36a and supplied to the rectification of oxygen. In the figure, the numbers in parentheses indicate the pipes 8, 17, 19, 21, and 33 when the amount of compressed air discharged from the air compressor 1 passing through the main heat exchanger 3 is 100%. The approximate passing amount (%) is shown.
[0013]
Using this apparatus, nitrogen gas and oxygen gas can be produced, for example, as follows. That is, first, air is compressed to about 0.7 kg / cm ² G by the air compressor 1, water in the compressed air is removed by the drain separator, and the air is cooled by the Freon cooler. To adsorb and remove water and carbon dioxide in the compressed air. Of course, when the pressure of about 3 kg / cm ² G is required for the performance of the adsorbent of the adsorption cylinder 2, the discharge pressure of the air compressor 1 must be increased to about 3 kg / cm ² G. Next, the compressed air from which water and carbon dioxide have been adsorbed and removed is sent into the main heat exchanger 3 to be cooled to an extremely low temperature, and is introduced into the lower part of the upper column 30 through the supply pipe 4 in a gas-liquid mixed phase state. The compressed air passing through the supply pipe 4 has a pressure of about 0.2 kg / cm ² G. Next, the compressed air is brought into countercurrent contact with the liquid nitrogen fed into the upper tower 30 from the introduction pipe 7 and the liquid nitrogen refluxed from the heat exchanger 12 in the lower tower 31 to rectify the compressed air, Oxygen, which is a high-boiling component in compressed air, is liquefied by the difference between the boiling points of nitrogen and oxygen (boiling point of oxygen -183 ° C, boiling point of nitrogen -196 ° C), and nitrogen is retained as a gas. This nitrogen gas is taken out of the first product nitrogen gas extraction pipe 8, sent to the main heat exchanger 3, and heated to near normal temperature, and then sent out of the apparatus as product nitrogen gas. In this case, the liquid nitrogen introduced into the upper tower 30 from the liquid nitrogen storage tank acts as a cold source for liquefaction of compressed air, and is itself vaporized and becomes the first product nitrogen gas extraction pipe as a part of the product nitrogen gas. 8 On the other hand, the liquid compressed air 6 collected at the bottom of the upper tower 30 is sent into the lower tower 31 by gravity through a liquid air supply pipe 32. In addition, a part of the product nitrogen gas passing through the first product nitrogen gas extraction pipe 8 is introduced into a circulating compressor 34 through a gas pipe 33, compressed to about 4.6 kg / cm ² G by the circulating compressor 34, and then re-used. It is cooled to an extremely low temperature (to near the liquefaction temperature) through the heat exchanger 3 and sent to the heat exchanger 12. The sent compressed nitrogen gas is heat-exchanged with the liquid oxygen 16 collected at the bottom of the lower tower 31 by the heat exchanger 12 to substantially completely liquefy, and then a part of the liquefied nitrogen is passed through the reflux liquid pipe 35 to the upper tower 30. The remaining part is sent to the lower tower 31 through the branch pipe 36. Then, the oxygen-rich liquid compressed air 6 sent from the liquid air supply pipe 32 and the liquid nitrogen sent from the branch pipe 36 become a reflux liquid and flow downward in the lower tower 31. The liquid oxygen 16 accumulated at the bottom of the lower tower 31 is vaporized by heat exchange with the compressed nitrogen gas passing through the heat exchanger 12, rises in the lower tower 31, comes into countercurrent contact with the reflux liquid, and is rectified. , High-purity nitrogen gas stays at the top. Then, oxygen gas is taken out from the product oxygen gas take-out pipe 17, sent to the main heat exchanger 3, heated to near normal temperature, and sent out of the apparatus as product oxygen gas. Further, the nitrogen gas remaining in the upper part of the lower tower 31 is taken out from the second product nitrogen gas take-out pipe 19, sent to the main heat exchanger 3 and raised to near normal temperature, and then taken out of the apparatus as product nitrogen gas. In this way, high-purity nitrogen gas and oxygen gas can be obtained simultaneously by one apparatus.
[0014]
As described above, in the above embodiment, the conventional high-pressure rectification tower 5 is provided on the upper side as the upper tower 30, and the conventional low-pressure rectification tower 10 is provided on the lower side as the lower tower 31. Therefore, when the liquid compressed air 6 produced in the upper tower 30 is supplied to the lower tower 31, the liquid compressed air 6 may be naturally dropped. In addition, a part of the product nitrogen gas passing through the first product nitrogen gas extraction pipe 8 is pumped to the heat exchanger 12 using the circulation compressor 34 and returned to the upper tower 30 as a reflux liquid. Therefore, there is no need for a pressure difference between the upper tower 30 and the lower tower 31, and the pressure in both the towers 30 and 31 can be reduced. For this reason, the compression power of the air compressor 1 can be reduced, and the sum of the compression power of the air compressor 1 and the compression power of the circulating compressor 34 is compared with the compression power of the air compressor 1 of the conventional example. The unit can be reduced by about 10%. On the other hand, when the operating pressure of the upper tower 30 decreases, the temperature in the upper tower 30 decreases, the relative volatility of nitrogen and oxygen increases, and separation becomes easier.
[0015]
In the above embodiment, the compression power of the air compressor 1 and the circulating compressor 34 was calculated by the following equation (1) and compared with the compression power of the air compressor 1 of the air separation device shown in FIG. In this calculation, the compression power of the air compressor 1 in the above embodiment is Ps: 1 kg / cm ² abs, Pd: 1.7 kg / cm ² abs, Qs: 104 (wet base) m ³ / min, m: One stage, κ: 1.4 (the value of κ is the same in the following cases) is a value obtained by setting, and the compression power of the circulating compressor 34 is Ps: 1.2 kg / cm ² abs, Pd: 5.6 kg / cm ² abs, Qs: 87 m ³ / min, m: values obtained by setting to 3 steps. In calculating the compression power of the air compressor 1 of the air separation device shown in FIG. 2, Ps: 1 kg / cm ² abs, Pd: 6.8 kg / cm ² abs, Qs: 104 (wet base) m ³ / Min, m: set to 4 stages. As a result, in the above embodiment, the compression power of the air compressor 1 was 97.3 kw, the compression power of the circulation compressor 34 was 283 kw, and the total was 380.3 kw. On the other hand, the compression power of the air compressor 1 of the air separation device shown in FIG. 2 was 349.1 kw. At this time, since the production amounts of the product oxygen are 19 m ³ / min and 16 m ³ / min, respectively, the oxygen production basic unit is (380.3 / 19) / (349.1 / 16) = 0.92, It can be seen that the compression power of the above embodiment is reduced to about 92% of the compression power of the conventional example.
[0016]
(Equation 1)
Lad = (1 / 0.612) · Ps · Qs · (mκ / κ−1) · {(Pd / Ps) ^{k−1 / mk} −1}
Lad: Theoretical adiabatic compression power [kw]
Ps: Absolute pressure of suction gas [kg / cm ² a]
Pd: absolute pressure of discharged gas [kg / cm ² a]
Qs: gas amount converted to the suction state [m ³ / min]
m: Number of compression stages [stage]
κ: Gas adiabatic index [Cp / Cv, 0 ° C., 1 atm]
[0017]
In the above embodiment, LN ₂ is used as a cold source from the outside. However, the present invention is not limited to this, and a cryogenic fluid such as LO ₂ may be used, or air using an expansion turbine may be used. It may be a separating device. Further, in the above embodiment, all the product gas is taken out by gas. However, the present invention is not limited to this, and a part may be taken out by liquid or only oxygen gas may be produced.
[0018]
【The invention's effect】
As described above, according to the air separation apparatus of the present invention, the first rectification column, which was disposed on the lower side as the high-pressure rectification column in the conventional example, is disposed on the upper side, and the low-pressure rectification column in the conventional example. The second rectification column disposed on the upper side is disposed on the lower side. Therefore, the first rectification tower and the second rectification tower are connected by a pipe or the like, and the liquid air taken out of the first rectification tower is introduced into the second rectification tower by natural fall via the pipe or the like. be able to. Further, a part of the nitrogen gas passing through the product nitrogen gas extraction pipe is taken out from the product nitrogen gas extraction pipe, introduced into the heat exchanger of the second rectification column by the nitrogen gas compression means , and passed through the heat exchanger. Then, after liquefaction using the cold heat of the liquid air introduced from the first rectification column, it can be returned to the first rectification column as a reflux liquid. By utilizing the natural fall or the nitrogen gas compression means , the pressure difference between the two rectification columns can be eliminated, and the pressures of the both rectification columns can be reduced to the same level as the pressure of the conventional low-pressure rectification column. Therefore, the discharge pressure of the air compressor can be reduced, and the compression power can be reduced accordingly. Further, when the pressure in the first rectification column decreases, the temperature in the first rectification column decreases, and the relative volatility of nitrogen (N ₂ ) and oxygen (O ₂ ) increases. Therefore, nitrogen and oxygen are easily separated in the first rectification column.
[0019]
Further, in the present invention, the above gas pipe, due to the provision of the nitrogen gas compression means for sending to the heat exchanger to compress it incorporates a portion of the nitrogen gas through the product nitrogen gas take-out pipe, the nitrogen gas compression means Is significantly smaller than the compression power of the air compression means. Therefore, even if the compression powers of both compression means are summed up, the unit of oxygen production is about 10% as compared with the compression power of the conventional air compressor. It can be reduced. In particular, when the purity of the product oxygen gas obtained in the second rectification column is relatively low (about 93%), the flow rate of the nitrogen gas compression means can be reduced, and the rate of power reduction is large. Become.
[0020]
Further, in the present invention, from the heat exchanger, because of the extended the reflux pipe for returning the liquid nitrogen produced in the heat exchanger to the first rectification column as a reflux liquid, the discharge pressure of the nitrogen gas compression means The liquefied nitrogen generated in the heat exchanger can be returned to the first rectification column by utilizing it.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an air separation device showing an embodiment of the present invention.
FIG. 2 is a configuration diagram of an air separation device showing a conventional example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Air compressor 3 Main heat exchanger 6 Liquid air 8 First product nitrogen gas extraction pipe 12 Heat exchanger 30 Upper tower 31 Lower tower 34 Circulating compressor 35 Reflux liquid pipe

Claims

Air compression means for compressing the raw air, a main heat exchanger for cooling the compressed air passing through the air compression means to a low temperature, and a first rectification column for introducing the compressed air cooled to a low temperature by the main heat exchanger A second rectification column for introducing the liquid air taken out of the first rectification column, a heat exchanger provided below the second rectification column, and a gas from the first rectification column as a gas. The nitrogen is taken out, the temperature is increased by passing the nitrogen through the main heat exchanger, and the product nitrogen gas extraction pipe is turned into product nitrogen gas, and part of the nitrogen gas passing through the product nitrogen gas extraction pipe is sent to the heat exchanger. A gas pipe for sending , a part of nitrogen gas provided in the gas pipe and passing through the product nitrogen gas extracting pipe, compressing the nitrogen gas and sending it to the heat exchanger; and a nitrogen gas compressing means extending from the heat exchanger, Raw in heat exchanger Liquefied nitrogen and a reflux pipe for returning to the first rectification column as reflux liquid was, the arrangement position of mutual of the first rectification column and a second rectification column, in the upper first rectification column, An air separation device, wherein a second rectification column is positioned below the first rectification column.