JP2004175729A

JP2004175729A - Method for treating treated liquid of wash column of ethylene production plant

Info

Publication number: JP2004175729A
Application number: JP2002344558A
Authority: JP
Inventors: Kenichiro Hiroto; 健一郎廣戸; Heigo Sato; 平吾佐藤; Tetsuhiro Yamauchi; 哲弘山内; Kazunari Takahashi; 和成高橋
Original assignee: Mitsubishi Chemical Corp
Current assignee: Mitsubishi Chemical Corp
Priority date: 2002-11-27
Filing date: 2002-11-27
Publication date: 2004-06-24

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the discharge of organic components centered on benzene and contained in a waste water to the outside of a system without worsening oil-water separation by efficient extraction by paying attention on a specific component. <P>SOLUTION: The method for treating a treated liquid of a wash column is the method for treating the waste water containing the treated liquid of the wash column by extraction and comprises washing a cracked gas of a naphtha introduced to a lower layer part of the wash column in an ethylene production plant by allowing the gas and an aqueous solution of sodium hydroxide introduced to the upper part of the wash column to countercurrently flow, discharging the treated liquid of the wash column containing the organic components contained in the cracked gas from the lower layer part, extracting and removing the organic components with a medium for the extraction from the waste water containing the treated liquid of the wash column, and discharging the waste water after removal to the outside of the system. The medium for the extraction is by-product oil of the ethylene production process, and the removal of the organic components by extraction is carried out by mixing the waste water with the medium for the extraction, subjecting the resultant mixture to oil drop-coagulating treatment, and subjecting the treated mixture to oil-water separation by an extraction vessel in the treating method. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
この発明は、エチレン製造プラントの洗浄塔において生じる処理液の処理方法に関する。
【０００２】
【従来の技術】
エチレン製造プロセスでは、一般的に触媒毒となる硫黄分をナフサの分解ガスからあらかじめ除去する作業が行なわれている。まず、ナフサを熱分解炉で分解し、次いで分留塔で気液を分離する。得られた気体、すなわち分解ガスを洗浄塔に送り硫黄分を除去する。
【０００３】
洗浄塔で固定化する硫黄分の除去プロセスは、図２に示すように、洗浄塔１１の下段部から導入される分解ガスａを、洗浄塔１１の中段部に導入される水酸化ナトリウム水溶液ｂと向流させることによって、分解ガスａに含まれる硫黄分を固定化し、次いで、排水ｅとして上記洗浄塔１１の最下段部から排出させ、そして、湿式酸化することにより硫酸ナトリウムに変化させて系外に排出するプロセスである。
【０００４】
しかし、排水ｅには硫黄分だけではなく、ナフサの分解時に生じる有機成分が遊離した状態や溶解した状態で含まれる。この排水ｅをそのまま湿式酸化すると、そのとき加えられる熱によって特に遊離した有機成分が異常反応し安定した運転が不可能となる。そのため、湿式酸化工程の前に排水ｅを油水混合機１２で抽出用媒体ｉと混合し、抽出槽１３へ送って、抽出済媒体と抽出した有機成分を含む液ｊを抽出排水ｋから分離して、排水ｅ中の遊離した有機成分及び溶存している有機成分の濃度を減らす。
【０００５】
有機成分濃度を減らした抽出排水ｋを一度タンク１４に溜めたのち、湿式酸化槽１５へ送る。ここで空気ｌと水蒸気ｍを加えて硫黄分を硫酸ナトリウムに湿式酸化する。この時同時に抽出排水ｑに残っていた有機成分がガス化する。これらを気液分離塔１６へ送り、ｐＨ調整剤ｎを加えて中和させつつ気液を分離する。液体分は排水ｐとして処理され、気体分はガスｏとして大気へ放出される。ただし、抽出槽１３であらかじめ有機成分を抽出しているので、ガスｏに含まれて大気に放出される有機成分量を抑えることができる。しかしながら、抽出排水ｋに有機成分が含まれたまま湿式酸化すると、そのとき加えられる熱によって有機成分が反応気化され、有機成分を多く含んだガスが大気へ放出されて、環境上問題となる場合がある。
【０００６】
一方、洗浄塔１１の中部から上部では、中部から下部で起こる分解ガスａと水酸化ナトリウム水溶液ｂとの向流によって、霧状又は液滴状の混合物が発生するため、そのままではエチレンなどの有用なガス分ｄのみを取り出しにくい。そこで洗浄塔の上段部から水ｃを加えて向流させることにより、上記混合物を洗浄塔１１の中上段部で回収して、有用なガス分ｄのみを洗浄塔１１の最上段部から取り出す。回収した霧状または液滴状の回収液ｇは１９９８ＮＡＬＣＯ／ＥＸＸＯＮＥＴＨＹＬＥＮＥＳＹＭＰＯＳＩＵＭＴｏｋｙｏ，Ｊａｐａｎの中のＣａｕｓｔｉｃＳｙｓｔｅｍ（ＯｐｅｒａｔｉｏｎｏｆＣａｕｓｔｉｃＳｙｓｔｅｍｓＦｒｅｅＦｒｏｍＰｏｌｙｍｅｒ（ＲｅｄＯｉｌ）ＢｙＭｉｃｈａｅｌＷｅｉｓｍｉｌｌｅｒ）に記載されているように排水ｅと混合し排水ｈとなり、抽出工程経由で湿式酸化工程へ送られている。
【０００７】
しかしながら、この回収液ｇを排水ｅと混合した後、抽出工程で有機成分を除去しているが、ｃの水量が多いと排水ｈ中のアルカリ濃度が希釈され排水の密度が低下し、高比重抽出油を用いた場合ではｊとｋの比重差が縮まり油水分離を悪化させる問題を発生させる。このため油水分離性能を維持するためには、排水ｋとの比重差を増加させるため低比重の抽出油を選択あるいは購入することが必要となる。一般に図３、図４、表１に示すようにエチレンプラントで発生する各種副生熱分解油の比重とベンゼン濃度には相関があり、ベンゼン濃度が低い副生熱分解油は比重が１に近く油水分離には不向きである。従って従来は油水分離が良好な室温で比重が０．８〜０．９程度のベンゼン濃度が高い副生熱分解油を用いてきた。しかし特に近年排出基準が厳しくなり、従来のＣＯＤ、ＢＯＤなどの排出基準に加え、排出される成分の基準値が厳しく制限されてきているため排水ｋに含まれる有機成分は確実に低減させて有機成分が少ない排水として排出しなければならない。特にベンゼンは発ガン性物質でありその排出基準は近年厳しく制限されてきている。
【０００８】
【非特許文献１】１９９８ＮＡＬＣＯ／ＥＸＸＯＮＥＴＨＹＬＥＮＥＳＹＭＰＯＳＩＵＭＴｏｋｙｏ，Ｊａｐａｎの中のＣａｕｓｔｉｃＳｙｓｔｅｍ（ＯｐｅｒａｔｉｏｎｏｆＣａｕｓｔｉｃＳｙｓｔｅｍｓＦｒｅｅＦｒｏｍＰｏｌｙｍｅｒ（ＲｅｄＯｉｌ）ＢｙＭｉｃｈａｅｌＷｅｉｓｍｉｌｌｅｒ）
【０００９】
【発明が解決しようとする課題】
そこで本発明は、特定成分に着目した効率的な抽出により油水分離を悪化することなく、排水中に含まれるベンゼンを中心とした有機成分を系外に放出されるのを抑制することを目的とする。
【００１０】
【課題を解決するための手段】
本発明者等は上記課題を解決すべく鋭意検討した結果、抽出用媒体の種類及び油水分離工程の改善に着目し、ベンゼン含有濃度に上限値を設けること及び抽出用媒体と排水の油水分離で油滴凝集処理を織り込むことにより改善することを見出し、本発明を完成するに至った。即ち本発明の要旨は、エチレン製造プラントの洗浄塔の下段部に導入されるナフサの分解ガスを、上記洗浄塔の上部に導入される水酸化ナトリウム水溶液と向流させることによって、上記分解ガスを洗浄し、上記分解ガスに含まれる有機成分を含有する排水を上記洗浄塔の下段部から排出させ、該排水から抽出用媒体により上記有機成分を抽出除去し、除去後の排水を系外へ排出させるエチレン製造プラントの洗浄塔処理液の抽出処理方法であって、抽出用媒体がエチレン製造プロセスからの副生油であり、有機成分の抽出除去が、排水と抽出用媒体を混合し、次いで油滴凝集処理を行い、その後にデカンターにて油水分離を行うことにより行われることを特徴とする方法に存する。
【００１１】
【発明の実施の形態】
以下、本発明をより詳細に説明する。
本発明のエチレン製造プラントの洗浄塔処理液の抽出処理方法は、エチレン製造プラントの洗浄塔の下段部に導入されるナフサの分解ガスを、上記洗浄塔の上部に導入される水酸化ナトリウム水溶液と向流させることによって、上記分解ガスを洗浄し、上記分解ガスに含まれる有機成分を含有する洗浄塔処理液を上記洗浄塔の下段部から排出させ、該洗浄塔処理液を含む排水から抽出用媒体により上記有機成分を抽出除去し、除去後の排水を系外へ排出させるエチレン製造プラントの洗浄塔処理液を含む排水の抽出処理方法であって、抽出用媒体がエチレン製造プロセスからの副生油であり、有機成分の抽出除去が、排水と抽出用媒体を混合し、次いで油滴凝集処理を行い、その後に抽出槽にて油水分離を行うことにより行われることを特徴とする。
【００１２】
本発明において、洗浄塔処理液とは、洗浄塔内で水酸化ナトリウム水溶液によってナフサの分解ガスを洗浄した後の液を意味する。また、本発明（特許請求の範囲）における「排水」とは、洗浄塔処理液または洗浄塔処理液及び上記洗浄塔内で上記洗浄された分解ガスを水と向流させることによって発生する回収液との混合液（図１のＨ）を意味する。
【００１３】
まず、エチレン製造プロセスにおける、上記処理液の処理方法に関する発明について説明する。
エチレンの製造条件、洗浄塔の運転条件、後術する以外の洗浄塔処理液の廃水処理条件等は、通常のエチレン製造プロセスにおける条件を用いればよい。
熱分解炉から出てきた気液混合ガスを分留し得られる様々な副生油は、精製され、ベンゼン、トルエン、キシレンなどの有効成分回収工程へ供給されたり、燃料あるいは再度熱分解に供給したり、本発明に使用する抽出用媒体などとして供される。熱分解条件やプロセスにより組成は多少変化があるが、基本的な組成はプロセスにより大差はない。一方、熱分解炉から出てきた気液混合ガスを分留し、液体成分を取り除いたナフサの分解ガスＡは、図１に示すように、洗浄塔１の下段部へ送られる。洗浄塔１では、中段部から水酸化ナトリウム水溶液Ｂを導入して、下部から中部で分解ガスＡと向流させることにより、分解ガスＡ中の主に硫化水素からなる硫黄分を反応させて、硫化ナトリウムや水硫化ナトリウムにする。これらの硫化ナトリウムや水硫化ナトリウムは、洗浄塔処理液Ｅとして洗浄塔１の最下段部から排出される。洗浄塔処理液Ｅは他に、反応しきらなかった水酸化ナトリウムと、ナフサの分解ガスに含まれる有機成分とを含有している。
【００１４】
なお、本発明における副生油とは、熱分解炉からでてきた気液混合ガスに含まれる常温常圧で液体となる成分を主体としたものである。これらの副生油は、図３に示したガソリン塔から得られる軽質燃料油（ＬＦＯ）、分解燃料油（ＥＨＥ）、パンオイル（Ｐａｎｏｉｌ）、クエンチ塔から得られる重質分解ガソリン（Ｈ−ＴＣＲ）、コンプレッサ−から得られる中質分解ガソリン（Ｍｉｄ−ＴＣＲ）、脱ブタン塔から得られる分解ガソリンなどである。
【００１５】
洗浄塔１内において、下部の向流によって霧状または液滴状になり上昇した処理液と分解ガスの混合物は、洗浄塔１の上段部から水Ｃを加えて、洗浄塔１の上段部から回収される。
回収液Ｇは、排水Ｅと同様に有機成分を含んだままである。この回収液Ｇと洗浄塔処理液Ｅを混合後、排水Ｈとして油水混合機２で抽出用媒体Ｉと混合し、次に油滴凝集装置７を経由して抽出分離槽である抽出槽３へ送って、抽出用媒体と抽出した有機成分の混合液である抽出済媒体と抽出された有機成分を含む液Ｊを抽出排水Ｋから分離して、抽出排水Ｋが含有する有機成分の濃度のうち特にベンゼン濃度を元の排水Ｈよりも少なくする。この時使用する抽出用媒体Ｉはコスト削減の観点からエチレン製造プロセスで発生した副生熱分解油の中から選択する。従来は抽出排水中の残留有機成分の総量を下げるために油水分離が簡単な、比重が０．８近い抽出用媒体を用いていた。しかし、排水中の特定成分であるベンゼン除去の観点から、抽出用媒体中のベンゼン濃度は、なるべくゼロに近い方が好ましい。このため図４のベンゼン濃度と比重の関係から、従来油水分離が困難であった１に近い比重の油、好ましくは比重０．８８以上の油、より好ましくは比重０．９以上の油が好ましい。
【００１６】
本発明における油滴凝集処理とは、積極的に油滴を凝集させて分離する処理を意味し、濾過課程でろ材と油水とのぬれの差を利用し油滴を凝集させる方法、または遠心力を加えて油滴の分離速度を加速させる方法、または凝集剤を添加し油滴を成長させ分離する方法などが挙げられる。エネルギ−原単位や添加剤の除去の問題から、新たな物質を用いたり駆動力を必要としないものが望ましい。通常コアレッサ−や遠心力を生かし微少な油滴や水滴を凝集させ粒径を大きくし油水分離性能を上げる遠心分離器などである。一般にはコアレッサ−が好ましい。抽出排水Ｋは一旦タンク４に溜めてから、湿式酸化槽５に送る。
【００１７】
湿式酸化槽５では、タンク４から送られてきた抽出排水Ｑに空気Ｌと水蒸気Ｍを加え、抽出排水Ｋ中の硫化ナトリウムや水硫化ナトリウムを硫酸ナトリウムに変化させる。このとき同時に、抽出排水Ｋに含まれる有機成分がガス化するが、抽出槽３で有機成分濃度を減少させているため、ガス化する量は抑えられる。
湿式酸化槽５を経た後は気液分離塔６へ送り、ｐＨ調整剤Ｎを加えて中和しつつ、ガス化した微量の有機成分を含む気体成分Ｏと、硫酸ナトリウムが含まれる液体成分Ｐとに分離する。気体成分Ｏは大気中に放出されるが、抽出槽３であらかじめ排水Ｈ中の有機成分濃度を減少させているため、この気液分離塔６での有機成分の大気への放出量は抑えられる。また、液体成分Ｐは活性汚泥処理などを経由して工業排水として処理される。
【００１８】
抽出用媒体Ｉはコスト削減の観点からエチレン製造プロセスで発生した副生油の中から選択する。ナフサの分解ガスにベンゼンが含まれている場合、この抽出用媒体に含まれるベンゼンの濃度は５重量％以下であることが望ましく、より好ましくは３重量％以下である。ベンゼンはナフサの分解によって生じる有機成分のなかでも、特に環境上問題とされることが多く、大気への排出量は他の有機成分よりもさらに少ないことが望ましい。抽出用媒体Ｉに含まれるベンゼンの濃度が高すぎると、抽出排水Ｋに残留するベンゼンの量が無視できない量になり、場合によっては抽出作業を行なう前よりもベンゼン濃度が上がってしまう事もありうる。これに対し、抽出用媒体Ｉ中のベンゼン濃度を低くしておけば、抽出槽３において、より効率よくベンゼンを回収することができる。しかし図４、表１に示すようにベンゼン濃度が低い副生油は、比重が１に近く従来油水分離が困難であったため活用されてこなかった。本発明者らは、系外に放出する排水中のベンゼン削減を重要な課題と考え、この油水分離改善のために問題を解決するため油滴凝集装置を抽出工程内に織り込むことにより、ベンゼン濃度削減と油水分離改善という二律相反する問題を解決し本発明とした。
【００１９】
本発明において、排水と抽出用媒体との混合はラインミキサ−で通常行われ、このときの排水流量：抽出用媒体の流量比は１：０．０５から１：５０であり、好ましくは１：０．１から１：５である。抽出用媒体の割合が多すぎると、十分な油水分離を行うために抽出槽の滞留時間を得ようとすると、抽出槽が大きくなる問題や抽出処理後の抽出済媒体の廃油処理量が多くなりコスト的に不利となる。一方少なすぎると油分と水との接触効率が低下し、排水中のベンゼンや油分の除去効率が低下するため不利である。
【００２０】
混合後の液温度は、室温から１５０℃であり、望ましくは３０℃から１００℃である。液温度が高いと油分の水への溶解度が上昇するため、ベンゼンや油分の除去効率が低下する。液温度が低すぎると液の粘度が上昇し、油水分離効率が悪化する。
本発明における油水分離は、室温から１５０℃の範囲で常圧から１ＭＰａの範囲で行われる。油水分離時の温度は上記記述のように液温度が高いと油分の水への溶解度が上昇するため、ベンゼンや油分の除去効率が低下し、また液温度が低すぎると液の粘度が上昇し、油水分離効率が悪化するため、操作温度は３０℃から１００℃がより望ましい。抽出槽は高圧になると機器建設費の面で不利になるため、操作圧力は低い方が望ましい。しかし抽出用媒体あるいは排水の気化抑制のため、低圧で加圧することがより好ましい。従って操作圧力は微加圧から０．３ＭＰａがより望ましい。抽出槽の滞留時間は短いと油水分離の性能が低下し、また長すぎると装置の建設費増大を招くため滞留時間は１０分から５時間が望ましい。滞留時間は３０分から２時間がより好ましい。
【００２１】
なお、エチレン製造プラントとは、単にエチレンのみを製造するプラントだけではなく、メタンやエタン等の燃料ガス、プロピレン等の樹脂材料、ガソリン、燃料油等といった、石油や石炭、天然ガス等を原料とする他の有機化学製品を生産するプラントを含めた総称である。
本発明におけるエチレン製造プラントの洗浄塔とは、分解ガス中に含まれる硫化物をアルカリ水溶液と接触し、硫化ナトリウム、水硫化ナトリウムなどの形態で分解ガスから除去するためのアルカリ洗浄塔を示している。
【００２２】
また、洗浄塔における最上段部、上段部、中上段部、中段部、下段部、最下段部という区別や、上部、中部、下部という範囲は、基本的には上から上記の順番であるが、それらは絶対的なものではなく、その区別や範囲における相対的な目安である。
【００２３】
【実施例】
実施例１
図１に示すエチレン製造プロセスの一部であるソ−ダ洗浄塔の缶出から発生する排水Ｅ及び上部水洗部より得られた回収液Ｇの混合液をＨ液とする。排水Ｅ中のベンゼン濃度は２４８ｗｔｐｐｍであり、排水Ｅの比重は１．０７（１５／４℃）であった。回収液Ｇ中のベンゼン濃度は２０７ｗｔｐｐｍであり、比重は１．００（１５／４℃）であった。Ｈ液中のベンゼン濃度は２２５ｗｔｐｐｍで、比重は１．０４（１５／４℃）であった。
【００２４】
このＨ液６０ｍｌに対しベンゼン含有量が０ｗｔ％で比重が１．００（１５／４℃）ある（ｉ）液を３０ｍｌ混合し、３０℃で１時間攪拌後した。攪拌後速やかに遠心分離器で３０００Ｇ３０秒処理後、２００ｍｌのメスシリンダ−に遠心後の混合液を入れ静置し水相（１）と油相を分離した。この静置で油水分離が完全に終了する時間を油水界面の変化で求めた。静置後１分で分離した。また得られた水相（１）中のベンゼン濃度を測定すると１１ｗｔｐｐｍであった。
【００２５】
比較例１
実施例１における（ｉ）液の代わりにベンゼン含有量が４２ｗｔ％である（ｉｉ）液を用いた以外は実施例１と同様に行った。得られた水相（２）液中のベンゼン濃度は１５３ｗｔｐｐｍであった。また油水分離に要する時間は静置後１分であった。
【００２６】
比較例２
実施例１で混合後の遠心分離器処理を行わなかったこと以外は実施例１と同様に行った。得られた水相（３）液中のベンゼン濃度は１０ｗｔｐｐｍであった。また油水分離に要する時間は静置後２０分であった。
【００２７】
比較例３
比較例１で混合後の遠心分離器処理を行わなかったこと以外は比較例１と同様に行った。得られた水相（４）液中のベンゼン濃度は１４８ｗｔｐｐｍであった。また油水分離に要する時間は静置後３分であった。
【００２８】
【発明の効果】
本発明により、特定成分に着目した効率的な抽出により油水分離を悪化することなく、排水中に含まれるベンゼンを中心とした有機成分を系外に放出されるのを抑制できる。
【００２９】
【表１】

【００３０】
【表２】

【００３１】
【表３】

【００３２】
表３中、Ｃ○類とは、当該炭素数（○と同一）で表に記載成分を除いたものの合計である。
【図面の簡単な説明】
【図１】エチレン製造プラントの、洗浄塔処理液の処理プロセスの例を示す模式図
【図２】従来のエチレン製造プラントにおける洗浄塔処理液の他の処理プロセスの例を示す模式図
【図３】エチレンプラントで副生する熱分解油の発生位置を示す図
【図４】副生油の比重とＢｚ濃度の関係を示す図
【符号の説明】
１，１１：洗浄塔
２，１２：油水混合機
３，１３：抽出槽
４，１４：タンク
５，１５：湿式酸化槽
６，１６：気液分離塔
７．：油滴凝集装置
Ａ，ａ：ナフサ分解ガス
Ｂ，ｂ：水酸化ナトリウム水溶液
Ｃ，ｃ：水
Ｄ，ｄ：分解ガス中の有用なガス分
Ｅ，ｅ：洗浄塔処理液
Ｇ，ｇ：回収された霧状物又は液滴状物を含む回収液
Ｈ，ｈ：排水（洗浄塔処理液と回収液の混合排水）
Ｉ，ｉ：抽出用媒体
Ｊ，ｊ：抽出済媒体と抽出された有機成分を含む液
Ｋ，ｋ：抽出排水
Ｌ，ｌ：空気
Ｍ，ｍ：水蒸気
Ｎ，ｎ：ｐＨ調整剤
Ｏ，ｏ：湿式酸化時に気化した成分を含むガス
Ｐ，ｐ：湿式酸化処理後の排水
Ｑ，ｑ：抽出排水[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for treating a treatment liquid generated in a washing tower of an ethylene production plant.
[0002]
[Prior art]
In the ethylene production process, generally, an operation of previously removing a sulfur component, which is a catalyst poison, from a naphtha decomposition gas is performed. First, naphtha is decomposed in a pyrolysis furnace, and then gas and liquid are separated in a fractionation tower. The obtained gas, that is, the cracked gas is sent to a washing tower to remove the sulfur content.
[0003]
As shown in FIG. 2, the process of removing the sulfur content fixed in the washing tower is performed by converting the decomposition gas a introduced from the lower part of the washing tower 11 into an aqueous sodium hydroxide solution b introduced into the middle part of the washing tower 11. Countercurrent to fix the sulfur content in the decomposition gas a, and then discharge it as waste water e from the lowermost part of the washing tower 11, and convert it to sodium sulfate by wet oxidation This is the process of discharging to the outside.
[0004]
However, the wastewater e contains not only sulfur but also organic components generated during the decomposition of naphtha in a free state or a dissolved state. If the waste water e is wet-oxidized as it is, the organic components liberated particularly by the heat applied at that time react abnormally, and stable operation becomes impossible. Therefore, the wastewater e is mixed with the extraction medium i by the oil-water mixer 12 before the wet oxidation step, sent to the extraction tank 13, and the liquid j containing the extracted medium and the extracted organic component is separated from the extraction wastewater k. Thus, the concentration of the free organic component and the dissolved organic component in the waste water e is reduced.
[0005]
The extraction wastewater k having a reduced organic component concentration is once stored in a tank 14 and then sent to a wet oxidation tank 15. At this time, sulfur is wet-oxidized to sodium sulfate by adding air 1 and steam m. At this time, the organic components remaining in the extraction wastewater q are gasified at the same time. These are sent to the gas-liquid separation tower 16, and the gas-liquid is separated while adding and neutralizing the pH adjuster n. The liquid component is treated as wastewater p and the gas component is released to the atmosphere as gas o. However, since the organic component is extracted in advance in the extraction tank 13, the amount of the organic component contained in the gas o and released to the atmosphere can be suppressed. However, when wet oxidation is performed with the organic component contained in the extraction wastewater k, the organic component is reacted and vaporized by the heat applied at that time, and a gas containing a large amount of the organic component is released to the atmosphere, which poses an environmental problem. There is.
[0006]
On the other hand, from the middle to the upper part of the washing tower 11, a counter-current of the decomposition gas a and the aqueous sodium hydroxide solution b generated from the middle to the lower part generates a mist-like or droplet-like mixture. It is difficult to take out only the gas amount d. Then, by adding water c from the upper part of the washing tower and causing it to flow countercurrently, the above-mentioned mixture is recovered in the upper middle part of the washing tower 11, and only useful gas components d are taken out from the uppermost part of the washing tower 11. The collected liquid g in the form of a mist or droplets can be collected from the Nautical System in the NALCO / EXXON ETHYLENE SYMPOSIUM Tokyo, Japan, as described in the Operation System of a Custom System, from a free system in the case of a free-from-air system. The wastewater is mixed with the wastewater e to form wastewater h, which is sent to the wet oxidation step via the extraction step.
[0007]
However, after the recovered liquid g is mixed with the wastewater e, the organic components are removed in the extraction step. However, when the amount of water of c is large, the alkali concentration in the wastewater h is diluted, the density of the wastewater is reduced, and the specific gravity is increased. In the case where the extracted oil is used, the specific gravity difference between j and k is reduced, which causes a problem of deteriorating oil-water separation. Therefore, in order to maintain the oil-water separation performance, it is necessary to select or purchase a low specific gravity extracted oil in order to increase the specific gravity difference from the drainage k. Generally, as shown in FIGS. 3 and 4 and Table 1, there is a correlation between the specific gravity of various by-product pyrolysis oils generated in an ethylene plant and the benzene concentration, and the by-product pyrolysis oil having a low benzene concentration has a specific gravity close to 1. Not suitable for oil-water separation. Therefore, conventionally, a by-product pyrolysis oil having a specific gravity of about 0.8 to 0.9 and a high benzene concentration at room temperature where oil-water separation is favorable has been used. However, especially in recent years, emission standards have become stricter, and in addition to the conventional emission standards for COD, BOD, etc., the standard values of the emitted components have been severely restricted. Must be discharged as wastewater containing less components. In particular, benzene is a carcinogen and its emission standards have been severely restricted in recent years.
[0008]
[Non-Patent Document 1] 1998 NALCO / EXXON ETHYLENE SYMPOSIUM Tokyo, Japan, Custom System (Operating of Customer Systems Free-from-A-Wilmer) (Red Oil Boiler)
[0009]
[Problems to be solved by the invention]
Accordingly, an object of the present invention is to suppress the release of organic components such as benzene contained in wastewater out of the system without deteriorating oil-water separation by efficient extraction focusing on specific components. I do.
[0010]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to solve the above problems, and as a result, focused on the type of extraction medium and the improvement of the oil-water separation process, setting an upper limit for the benzene content concentration and separating the extraction medium and the wastewater from oil-water. It has been found that the improvement is achieved by incorporating the oil droplet aggregation treatment, and the present invention has been completed. That is, the gist of the present invention is that the decomposition gas of naphtha introduced into the lower part of the washing tower of the ethylene production plant is counter-flowed with the aqueous sodium hydroxide solution introduced into the upper part of the washing tower, whereby the decomposition gas is produced. After washing, the wastewater containing the organic component contained in the decomposition gas is discharged from the lower part of the washing tower, the organic component is extracted and removed from the wastewater by an extraction medium, and the wastewater after the removal is discharged outside the system. A method for extracting a treatment liquid of a washing tower of an ethylene production plant, wherein the extraction medium is a by-product oil from the ethylene production process, and extraction and removal of organic components is performed by mixing wastewater with the extraction medium, and then extracting the oil. The method is characterized by performing a droplet aggregation treatment and thereafter performing oil-water separation in a decanter.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail.
The method for extracting a treatment liquid for a washing tower of an ethylene production plant according to the present invention comprises the steps of: dissolving a naphtha gas introduced into a lower part of a washing tower of an ethylene production plant with an aqueous sodium hydroxide solution introduced into an upper part of the washing tower. By countercurrent, the decomposed gas is washed, and a treatment liquid for a washing tower containing an organic component contained in the decomposed gas is discharged from a lower portion of the washing tower, and extracted from a wastewater containing the treatment liquid for the washing tower. A method for extracting and removing the organic components by a medium and discharging the wastewater after the removal to the outside of the system, the method comprising the step of extracting wastewater containing a treatment liquid for a washing tower of an ethylene production plant, wherein the extraction medium is a by-product from the ethylene production process. It is oil, and the extraction and removal of organic components are performed by mixing wastewater and extraction medium, then performing oil droplet aggregation treatment, and then performing oil-water separation in the extraction tank. To.
[0012]
In the present invention, the treatment solution for the washing tower means a solution after the decomposition gas of naphtha is washed with an aqueous solution of sodium hydroxide in the washing tower. In the present invention (claims), the term “drainage” refers to a treatment liquid for a washing tower or a treatment liquid for a washing tower and a recovery liquid generated by causing the decomposed gas washed in the washing tower to flow in countercurrent to water. (H in FIG. 1).
[0013]
First, an invention relating to a method for treating the above-mentioned treatment liquid in an ethylene production process will be described.
The ethylene production conditions, the operation conditions of the washing tower, the wastewater treatment conditions of the washing tower treatment liquid other than the post-treatment, and the like may be the same as those in a normal ethylene production process.
Various by-product oils obtained by fractionating the gas-liquid mixed gas coming out of the pyrolysis furnace are refined and supplied to the process of recovering active components such as benzene, toluene, and xylene, or supplied to fuel or re-pyrolysis. Or as an extraction medium used in the present invention. The composition varies somewhat depending on the thermal decomposition conditions and the process, but the basic composition does not vary much depending on the process. On the other hand, the naphtha-decomposed gas A from which the gas-liquid mixed gas discharged from the thermal decomposition furnace is fractionated and the liquid component is removed is sent to the lower stage of the washing tower 1 as shown in FIG. In the washing tower 1, an aqueous sodium hydroxide solution B is introduced from the middle stage, and countercurrently flows from the lower portion with the decomposed gas A from the lower portion, thereby reacting a sulfur component mainly composed of hydrogen sulfide in the decomposed gas A, Change to sodium sulfide or sodium hydrosulfide. These sodium sulfide and sodium hydrosulfide are discharged from the lowermost part of the washing tower 1 as a washing tower treatment liquid E. The washing tower treatment liquid E further contains sodium hydroxide that has not reacted completely and an organic component contained in the decomposition gas of naphtha.
[0014]
The by-product oil in the present invention is mainly composed of a component which becomes liquid at normal temperature and normal pressure contained in the gas-liquid mixed gas discharged from the pyrolysis furnace. These by-product oils are light fuel oil (LFO), cracked fuel oil (EHE), pan oil (Panoil) obtained from the gasoline tower shown in FIG. 3, and heavy cracked gasoline (H-TCR) obtained from the quench tower. ), Cracked gasoline obtained from a compressor (Mid-TCR), cracked gasoline obtained from a debutanizer, and the like.
[0015]
In the cleaning tower 1, the mixture of the processing liquid and the decomposition gas that has been formed into a mist or a droplet by the countercurrent at the lower part and rises is added with water C from the upper part of the cleaning tower 1, and from the upper part of the cleaning tower 1. Collected.
The recovery liquid G still contains an organic component similarly to the wastewater E. After mixing the recovered liquid G and the treatment liquid E in the washing tower, they are mixed as the drainage H with the extraction medium I in the oil-water mixer 2 and then to the extraction tank 3 as an extraction separation tank via the oil droplet aggregation device 7. The liquid J containing the extracted medium and the extracted organic component, which is a mixture of the extraction medium and the extracted organic component, is separated from the extraction wastewater K, and the concentration of the organic component contained in the extraction wastewater K In particular, the benzene concentration is made lower than the original wastewater H. The extraction medium I used at this time is selected from the by-product pyrolysis oil generated in the ethylene production process from the viewpoint of cost reduction. Conventionally, an extraction medium with a specific gravity close to 0.8 has been used, in which oil-water separation is easy to reduce the total amount of residual organic components in the extraction wastewater. However, from the viewpoint of removing benzene, which is a specific component in wastewater, the benzene concentration in the extraction medium is preferably as close to zero as possible. For this reason, from the relationship between the benzene concentration and the specific gravity in FIG. 4, an oil having a specific gravity close to 1 which has conventionally been difficult to separate oil and water, preferably an oil having a specific gravity of 0.88 or more, more preferably an oil having a specific gravity of 0.9 or more is preferable .
[0016]
The oil droplet agglomeration treatment in the present invention means a process of actively aggregating and separating oil droplets, and a method of agglomerating oil droplets by utilizing the difference in wetting between a filter medium and oil water in a filtration process, or a centrifugal force. To increase the separation speed of oil droplets, or a method of adding a coagulant to grow and separate oil droplets. In view of the problem of energy consumption and removal of additives, it is desirable to use a new substance or one that does not require a driving force. Usually, it is a centrifugal separator or the like that utilizes a coalescer or a centrifugal force to aggregate small oil droplets and water droplets to increase the particle size and improve oil-water separation performance. Generally, a coalescer is preferred. The extraction wastewater K is temporarily stored in the tank 4 and then sent to the wet oxidation tank 5.
[0017]
In the wet oxidation tank 5, air L and steam M are added to the extraction wastewater Q sent from the tank 4, and sodium sulfide and sodium hydrosulfide in the extraction wastewater K are changed into sodium sulfate. At this time, at the same time, the organic components contained in the extraction wastewater K are gasified. However, since the concentration of the organic components is reduced in the extraction tank 3, the amount of gasification can be suppressed.
After passing through the wet oxidation tank 5, it is sent to a gas-liquid separation tower 6, which is neutralized by adding a pH adjuster N, and a gaseous component O containing a small amount of gasified organic components and a liquid component P containing sodium sulfate. And separated into Although the gas component O is released into the atmosphere, since the concentration of the organic component in the wastewater H is previously reduced in the extraction tank 3, the amount of the organic component released into the atmosphere in the gas-liquid separation tower 6 can be suppressed. . Further, the liquid component P is treated as industrial wastewater via activated sludge treatment or the like.
[0018]
The extraction medium I is selected from the by-product oil generated in the ethylene production process from the viewpoint of cost reduction. When benzene is contained in the decomposition gas of naphtha, the concentration of benzene contained in the extraction medium is preferably 5% by weight or less, more preferably 3% by weight or less. Benzene is often considered to be particularly environmentally harmful among the organic components generated by the decomposition of naphtha, and it is desirable that the amount of benzene emitted into the atmosphere is even lower than that of other organic components. If the concentration of benzene contained in the extraction medium I is too high, the amount of benzene remaining in the extraction effluent K becomes a non-negligible amount, and in some cases, the benzene concentration may be higher than before the extraction work. sell. On the other hand, if the benzene concentration in the extraction medium I is reduced, benzene can be more efficiently recovered in the extraction tank 3. However, as shown in FIG. 4 and Table 1, by-product oil having a low benzene concentration has not been utilized because the specific gravity is close to 1 and oil-water separation was difficult in the past. The present inventors consider the reduction of benzene in wastewater discharged out of the system as an important issue, and incorporated an oil droplet aggregating device into the extraction process to solve this problem in order to improve oil-water separation. The present invention solves two conflicting problems of reduction and improvement of oil-water separation.
[0019]
In the present invention, the mixing of the waste water and the extraction medium is usually performed in a line mixer, and the ratio of the flow rate of the waste water to the flow rate of the extraction medium is 1: 0.05 to 1:50, preferably 1: 5. 0.1 to 1: 5. If the ratio of the extraction medium is too large, there is a problem that the extraction tank becomes large in order to obtain sufficient residence time in the extraction tank in order to perform sufficient oil / water separation, and the amount of waste oil processed by the extracted medium after the extraction processing increases. It is disadvantageous in terms of cost. On the other hand, if the amount is too small, the efficiency of contact between oil and water decreases, and the efficiency of removing benzene and oil in wastewater decreases, which is disadvantageous.
[0020]
The liquid temperature after mixing is from room temperature to 150 ° C, preferably from 30 ° C to 100 ° C. If the liquid temperature is high, the solubility of the oil in water increases, so that the efficiency of removing benzene and the oil decreases. If the liquid temperature is too low, the viscosity of the liquid increases, and the oil-water separation efficiency deteriorates.
The oil-water separation in the present invention is performed in a range from room temperature to 150 ° C. and in a range from normal pressure to 1 MPa. As described above, the oil-water separation temperature increases the solubility of oil in water when the liquid temperature is high, so that the efficiency of removal of benzene and oil decreases, and when the liquid temperature is too low, the viscosity of the liquid increases. Since the oil-water separation efficiency deteriorates, the operation temperature is more preferably from 30 ° C to 100 ° C. If the pressure in the extraction tank is high, it is disadvantageous in terms of equipment construction costs, so that a lower operating pressure is desirable. However, it is more preferable to pressurize at a low pressure in order to suppress vaporization of the extraction medium or wastewater. Therefore, the operating pressure is more preferably from 0.3 MPa to slightly pressurized. If the residence time of the extraction tank is short, the performance of oil / water separation is reduced, and if it is too long, the construction cost of the apparatus is increased. Therefore, the residence time is preferably 10 minutes to 5 hours. The residence time is more preferably from 30 minutes to 2 hours.
[0021]
In addition, the ethylene production plant is not limited to a plant that produces only ethylene, but also uses petroleum, coal, natural gas, etc., such as fuel gas such as methane and ethane, resin materials such as propylene, gasoline, and fuel oil. Is a generic term that includes plants that produce other organic chemical products.
The washing tower of the ethylene production plant in the present invention refers to an alkali washing tower for contacting the sulfide contained in the decomposed gas with an aqueous alkali solution and removing the sulfide from the decomposed gas in the form of sodium sulfide, sodium hydrosulfide, etc. I have.
[0022]
Also, in the washing tower, the uppermost part, upper part, middle upper part, middle part, lower part, the distinction of the lowermost part, and the range of upper part, middle part, lower part are basically the above order from the top, , They are not absolute, they are relative measures in their distinction and scope.
[0023]
【Example】
Example 1
A liquid mixture of waste water E generated from the bottom of the soda washing tower, which is a part of the ethylene production process shown in FIG. 1, and the recovered liquid G obtained from the upper washing section is referred to as H liquid. The benzene concentration in the wastewater E was 248 wtppm, and the specific gravity of the wastewater E was 1.07 (15/4 ° C). The benzene concentration in the recovery liquid G was 207 wtppm, and the specific gravity was 1.00 (15/4 ° C.). The benzene concentration in the H solution was 225 wtppm, and the specific gravity was 1.04 (15/4 ° C.).
[0024]
To 60 ml of the H solution, 30 ml of the solution (i) having a benzene content of 0 wt% and a specific gravity of 1.00 (15/4 ° C.) was mixed and stirred at 30 ° C. for 1 hour. Immediately after stirring, the mixture was centrifuged at 3000 G for 30 seconds, and the mixed solution after centrifugation was placed in a 200 ml measuring cylinder and allowed to stand to separate an aqueous phase (1) and an oil phase. The time at which the oil-water separation was completely completed in this standing was determined by the change in the oil-water interface. Separated one minute after standing. When the benzene concentration in the obtained aqueous phase (1) was measured, it was 11 wtppm.
[0025]
Comparative Example 1
Example 1 was repeated except that the liquid (ii) having a benzene content of 42 wt% was used instead of the liquid (i). The benzene concentration in the obtained aqueous phase (2) solution was 153 wtppm. The time required for oil-water separation was 1 minute after standing.
[0026]
Comparative Example 2
Example 1 was repeated in the same manner as in Example 1 except that the centrifugal separator treatment after mixing was not performed. The benzene concentration in the obtained aqueous phase (3) liquid was 10 wtppm. The time required for oil-water separation was 20 minutes after standing.
[0027]
Comparative Example 3
The same operation was performed as in Comparative Example 1 except that the centrifugal separator treatment after mixing was not performed in Comparative Example 1. The benzene concentration in the obtained aqueous phase (4) solution was 148 wtppm. The time required for oil-water separation was 3 minutes after standing.
[0028]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, it can suppress that organic components centering on benzene contained in wastewater are discharged | emitted out of a system, without deteriorating oil-water separation by efficient extraction which paid attention to a specific component.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
[Table 3]

[0032]
In Table 3, C ○ is the total of the carbon number (same as ○) excluding the components described in the table.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of a process for treating a cleaning tower treatment liquid in an ethylene production plant. FIG. 2 is a schematic diagram showing another example of a treatment process for a cleaning tower treatment solution in a conventional ethylene production plant. FIG. 4 is a diagram showing the position of generation of pyrolysis oil produced as a by-product in an ethylene plant.
1, 11: washing tower 2, 12: oil-water mixer 3, 13: extraction tank 4, 14: tank 5, 15: wet oxidation tank 6, 16: gas-liquid separation tower : Oil droplet aggregator A, a: Naphtha decomposition gas B, b: Sodium hydroxide aqueous solution C, c: Water D, d: Useful gas content in decomposition gas E, e: Cleaning tower treatment liquid G, g: Recovery Liquids H, h containing waste mist or droplets: wastewater (mixed wastewater of cleaning tower treatment liquid and recovered liquid)
I, i: extraction medium J, j: liquid K containing extracted medium and extracted organic components, k: extraction wastewater L, l: air M, m: water vapor N, n: pH adjuster O, o: Gas containing components vaporized during wet oxidation P, p: wastewater after wet oxidation treatment Q, q: extracted wastewater

Claims

The decomposed gas of naphtha introduced into the lower part of the washing tower of the ethylene production plant is counter-flowed with the aqueous sodium hydroxide solution introduced into the upper part of the washing tower to wash the decomposed gas. The washing tower treatment liquid containing the contained organic component is discharged from the lower stage of the washing tower, and the organic component is extracted and removed from the wastewater containing the washing tower treatment liquid by an extraction medium. A method for extracting wastewater containing a treatment liquid from a washing tower of an ethylene production plant, wherein the extraction medium is a by-product oil from the ethylene production process, and the extraction and removal of organic components is performed by removing the wastewater and the extraction medium. Mixing, then performing oil droplet aggregation treatment, and then performing oil-water separation in an extraction tank.

The method according to claim 1, wherein the benzene concentration in the extracted oil, which is a by-product oil of the ethylene production plant, is 5 wt% or less.