JP2004189640A - Method for producing maleic anhydride - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing highly pure maleic anhydride without subjecting to a treatment for separating low-boiling substances. <P>SOLUTION: The method for producing maleic anhydride comprises feeding an aqueous solution containing crude maleic acid into an azeotropically dewatering tower and azeotropically dewatering it by using an azeotropic solvent azeotropically distillable with water to dewater the crude maleic acid to form maleic anhydride, wherein the azeotropic solvent used is an organic solvent having a maximum maleic acid dissolution concentration of at least 0.2 mass% at 20°C, the overhead pressure of the dewatering tower is at most 900 hPa, and the azeotropic solvent concentration of the bottom solution of the tower is at most 3,000 mass ppm. According to this method, it is possible to produce highly pure maleic anhydride without subjecting to a treatment for separating low-boiling substances or by a simplified treatment because the low-boiling substances containing the azeotropic solvent can be efficiently separated in the azeotropic dewatering step. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００６４】
に従って算出した最大溶解濃度が、本発明における最大溶解濃度である。最小添加量０．０１ｇ（０．０１３質量％）でも溶け残った場合を、不溶とした。最大溶解濃度は液温２０℃で測定する。
【００６５】
（参考例２：水に対する有機溶媒の最大溶解濃度の測定）
１００ｍｌの密閉できる蓋付のガラス容器に温度２０℃の溶媒８０ｇを入れ、スタラーで攪拌しながら対象とする有機溶媒を滴下し、目視で溶解状態を確認した。１時間攪拌しても溶け残りがあるときは、１回の滴下量をさらに少なくして再度上記の最大溶解濃度の確認を行なった。なお最小滴下量は０．０２ｇとした。このようにして、溶け残りがないと認められる最大滴下量（溶質質量）を求め、各溶媒８０ｇへの最大溶解濃度として算出した。具体的には、下記式
【００６６】
【数２】

【００６７】
に従って算出した最大溶解濃度が、本発明における最大溶解濃度である。最小滴下量０．０２ｇ（０．０２５質量％）でも溶け残った場合を、不溶とした。最大溶解濃度は液温２０℃で測定する。これらの値は本文中に記載した化学便覧やバイルシュタインに記載される当該溶媒の水への溶解性データとほぼ一致した。なお、本件の実施例は、８０ｇの溶媒に溶解する各溶質の最大溶解濃度を、上記の式に従い算出したものである。最大溶解濃度は液温２０℃で測定する。なお、２種以上の有機溶媒を混合して共沸脱水蒸留で使用する場合には、上記の方法によってその混合物の最大溶解濃度を測定する。
【００６８】
上記に従って評価した、ｏ−キシレン、メチルイソブチルケトン（ＭＩＢＫ）、ジイソプロピルケトン（ＤＩＰＫ）、ジイソブチルケトン（ＤＩＢＫ）、ジブチルーテル（ＤＢＥ）の最大溶解濃度データを表１に示す。
【００６９】
表１から、マレイン酸の最大溶解濃度や、水に対する最大溶解濃度は、ＭＩＢＫが他の物質より比較的高い数値を示した。ＤＩＰＫ、ＤＩＢＫとケトン基がより高い分子量の置換基になるに従ってＭＩＢＫよりも最大溶解濃度は低下してゆく傾向が見られ、ＭＩＢＫはマレイン酸を良好に溶解した。しかし、ｏ−キシレンはマレイン酸を溶解せず、かつ水へのこれらの物質での最大溶解濃度をほとんど示さない。
【００７０】
【表１】

【００７１】
（参考例３：少量成分の定量分析方法および検出限界）
（１）ＭＩＢＫと酢酸は、Ｊ＆Ｗ Ｓｃｉｅｎｔｉｆｉｃ社製 ＤＢ−５ カラムを用いたガスクロマトグラフィーで測定した。ＭＩＢＫの検出限界は５質量ｐｐｍ、酢酸の検出限界は１質量ｐｐｍであった。
【００７２】
（２）蟻酸は、ＧＬサイエンス社製Ｉｎｅｒｔｓｉｌ ＯＤＳ−３ カラムを用いた液体クロマトグラフィーで測定する。検出限界は２ｐｐｍであった。
【００７３】
（３）無水マレイン酸中のマレイン酸分は、以下の方法により測定する。
【００７４】
まず、５０ｍｌビーカーに測定サンプル（約１ｇ）を秤量して入れる。これにアセトン３０ｍｌ、ブロムフェノールブルー指示薬のエタノール溶液を加える。次いで、ビューレットを用いて、予め調製した０．１モルのｎ−エチルピペリジンのアセトン溶液で、黄色から青紫色に変わる点まで滴定し、その滴定量をＢ値とする。この滴定量から、次式によってマレイン酸質量％を算出する。
【００７５】
【数３】

【００７６】
なお、力値は以下の方法で求めた値である。
【００７７】
マレイン酸約０．２ｇを秤量してアセトンで３０ｍｌとする。上記のブロムフェノールブルー指示薬溶液を加え、０．１モルのｎ−エチルピペリジンのアセトン溶液で滴定し、その滴定量をＡ値とする。この滴定量から、次式によってマレイン酸質量％を算出する。
【００７８】
【数４】

【００７９】
（実施例１）
図１に示す工程に従って無水マレイン酸を製造した。
【００８０】
まず、ベンゼンの接触気相酸化反応により排出された無水マレイン酸を含む反応ガスを、無水マレイン酸捕集器（１０）の出口ガス温度を６０℃にコントロールして粗製無水マレイン酸（１１）を捕集した。その後、この無水マレイン酸捕集器（１０）の出口ガスを水洗捕集器（２０）に導入した。ついで、水洗捕集器（２０）で該ガスを洗浄し次いで、粗マレイン酸含有水溶液を得た。該溶液のマレイン酸濃度は４２質量％であり、水分５８質量％、酢酸２０００質量ｐｐｍ、蟻酸７０質量ｐｐｍを含んでいた。この粗マレイン酸含有水溶液に対して共沸脱水溶媒としてＭＩＢＫを使用して共沸脱水を行った。
【００８１】
共沸脱水塔（４０）として、上段部分として濃縮部３２φ有堰５段、下段部分として回収部５０φ有堰１０段の蒸留塔を使用した。塔底から数えて第１０段目から粗マレイン酸含有水溶液を供給した。共沸溶媒はＭＩＢＫを使用し、マレイン酸水溶液の溶解水および無水化によって発生する水分に対して３．５質量倍を塔頂から供給した。塔頂での操作圧力７７０ｈＰａ、塔底温度を１９０℃にコントロールして８時間共沸脱水蒸留を行った。結果を表２に示す。系内組成が一定になった時点では塔底液温度１９０℃、また塔底液組成は、無水マレイン酸９６．９０質量％、マレイン酸１．２７質量％、フマル酸１．７７質量％、ＭＩＢＫ６００質量ｐｐｍであった。また蟻酸、酢酸は検出限界以下の質量ｐｐｍであった。粗マレイン酸含有水溶液中の低沸点物質である副生酸類の、塔底の粗無水マレイン酸液中での存在をほぼ無くすることができた。なお、共沸脱水塔（４０）内にはマレイン酸の析出はなかった。また、縮合物やマレイン酸や無水マレイン酸の重合物である黒色の固体物の析出も無かった。
【００８２】
次に、高沸点物質の分離工程（６０）として、デミスターを取り付けた空塔を用いて蒸留を行なった。塔頂での操作圧力を７０ｈＰａとし、共沸脱水塔（４０）の塔底液として得られた粗無水マレイン酸の供給量の９７質量％が留出するよう蒸留操作を行なった。続いて留出した粗無水マレイン酸液を用いて精製蒸留を行った。精製蒸留塔（８０）として、３２φ有堰１０段の蒸留塔を使用した。塔底に高沸分離工程（６０）の留出液である粗無水マレイン酸を供給し、塔頂から数えて第３段目から精製無水マレイン酸（６１）を抜き出した。塔頂での操作圧力６７ｈＰａ、塔底温度１４０℃、塔頂における還流比２．０、精製無水マレイン酸抜き出し９質量倍に対して塔頂への抜き出し量１質量倍の割合として、供給した粗無水マレイン酸の９７質量％が留出するようにして８時間蒸留を行った。結果を表３に示す。この精製無水マレイン酸は、ＭＩＢＫが検出限界量以下に減少していた。また蟻酸、酢酸は検出できなかった。他は有水分であるマレイン酸であり、無水マレイン酸純度は９９．８６〜９９．９１質量％の範囲内であった。凝固点が５２．７から５２．８℃の範囲内、溶融色が５であって、いずれもＪＩＳ規格（ＪＩＳ１３５９／１９８５、純度：９９．０質量％以上、凝固点：５２．０℃以上、溶融色：５０以下）を充分に満足するものであった。
【００８３】
（実施例２）
実施例１の共沸脱水蒸留において、塔頂操作圧を５００ｈＰａ、塔底温度を１７５℃とした以外は、実施例１と同様の条件として８時間の共沸脱水蒸留を行った。系内組成が一定になった時点での塔底組成は、無水マレイン酸９７．８８質量％、マレイン酸１．３５質量％、フマル酸０．７７質量％であり、ＭＩＢＫ、蟻酸、酢酸は検出限界以下の質量ｐｐｍであった。粗マレイン酸溶液中の共沸溶媒ＭＩＢＫおよび軽沸分である副生酸類の、塔底の粗無水マレイン酸液中での存在をほぼ無くすることができた。また蒸留塔内には縮合物やマレイン酸や無水マレイン酸の重合物である黒色固体物の析出は無かった。
【００８４】
次に共沸脱水蒸留の塔底液として得られた粗無水マレイン酸の高沸点分離蒸留および精製蒸留を実施例１と同様の条件で蒸留操作を行った。この精製無水マレイン酸中には、ＭＩＢＫ、蟻酸、酢酸は検出できなかった。他は有水分であるマレイン酸であり、無水マレイン酸純度は９９．８６〜９９．９１質量％の範囲内であった。凝固点が５２．７から５２．８℃の範囲内、溶融色が５であって、いずれもＪＩＳ規格（ＪＩＳ１３５９／１９８５、純度：９９．０質量％以上、凝固点：５２．０℃以上、溶融色：５０以下）を充分に満足するものであった。
【００８５】
（実施例３）
実施例１の共沸脱水蒸留において、塔頂から数えて１１段目の塔内温度を１５０℃にコントロールして８時間共沸脱水蒸留を行った以外は、実施例１と同様の条件として８時間の共沸脱水蒸留を行った。結果を表２に示す。系内組成が一定になった時点ではボトム温度１９０℃、また塔底組成は、無水マレイン酸９６．９２質量％、マレイン酸１．３０質量％、フマル酸１．７８質量％、ＭＩＢＫ検出限界以下の質量ｐｐｍ、またギ酸、酢酸も共に検出限界以下の質量ｐｐｍであった。共沸脱水塔内に供給された共沸溶媒および粗マレイン酸溶液中の軽沸分である副生酸類の、塔底の粗マレイン酸液中での存在をほぼ無くすることができた。また共沸脱水塔（４０）内には縮合物やマレイン酸や無水マレイン酸の重合物である黒色の固体物の析出はなかった。
【００８６】
次に共沸脱水蒸留の塔底液として得られた粗無水マレイン酸の高沸点分離蒸留および精製蒸留を実施例１と同様の条件で蒸留操作を行った。この精製無水マレイン酸中にはＭＩＢＫ、ギ酸、酢酸は検出できなかった。他は有水分であるマレイン酸であり、無水マレイン酸純度は９９．８６〜９９．９１質量％の範囲内であった。凝固点が５２．７から５２．８℃の範囲内、溶融色が５であって、いずれもＪＩＳ規格（ＪＩＳ１３５９／１９８５、純度：９９．０質量％以上、凝固点：５２．０℃以上、溶融色：５０以下）を充分に満足するものであった。
【００８７】
（比較例１）
実施例１の共沸脱水塔（４０）において、塔頂での操作圧力を常圧、塔底温度を１９０℃の蒸留条件とした以外は、実施例１と同様の条件として８時間の共沸脱水蒸留を行った。系内組成が一定になった時点での塔底液組成は、無水マレイン酸９５．１３６質量％、マレイン酸１．４７質量％、フマル酸１．７８質量％、ＭＩＢＫ１．４４質量％、蟻酸１５質量ｐｐｍ、酢酸１７３０質量ｐｐｍであり、共沸脱水塔内に供給された共沸溶媒および粗マレイン酸溶液中の低沸点物質である副生酸類の、塔底の粗無水マレイン酸液中での存在を無くすることができなかった。なお、蒸留塔内にはマレイン酸の析出はなかった。また、縮合物やマレイン酸や無水マレイン酸の重合物である黒色の固体物の析出も無かった。
【００８８】
次に、共沸脱水塔（４０）の塔底液として得られた粗無水マレイン酸を高沸点物質の分離蒸留（６０）を行なった後、精製蒸留塔（８０）に供給し、実施例１において塔頂における還流比を２．０とした以外は同様の条件で蒸留操作を行った。結果を表３に示す。この該精製蒸留塔（８０）から得た無水マレイン酸中にはＭＩＢＫは６４００ｐｐｍが検出された。また蟻酸、酢酸は検出できなかった。他は有水分であるマレイン酸であり、この精製無水マレイン酸の純度は、９９．１７〜９９．２２質量％の範囲内、凝固点が５２．４から５２．５℃の範囲内、溶融色が５であった。またＪＩＳ規格（ＪＩＳ１３５９／１９８５、純度：９９．０質量％以上、凝固点：５２．０℃以上、溶融色：５０以下）を満足するものであったが、ＭＩＢＫが精製無水マレイン酸中に検出された。
【００８９】
（比較例２）
実施例１の共沸脱水塔（４０）において、操作圧を常圧、塔底温度を１９０℃の蒸留条件とした以外は、実施例１と同様の条件として８時間の共沸脱水蒸留を行った。系内組成が一定になった時点での塔底液組成は、無水マレイン酸９５．１３６質量％、マレイン酸１．４７質量％、フマル酸１．７８質量％、ＭＩＢＫ１．４４質量％、蟻酸１５質量ｐｐｍ、酢酸１７３０質量ｐｐｍであり、共沸脱水塔内に供給された共沸溶媒および粗マレイン酸溶液中の低沸点物質である副生酸類の、塔底の粗無水マレイン酸液中での存在を無くすることができなかった。なお、共沸脱水塔（４０）内にはマレイン酸の析出はなかった。また、縮合物やマレイン酸や無水マレイン酸の重合物である黒色の固体物の析出も無かった。
【００９０】
次に、共沸脱水塔（４０）の塔底液として得られた粗無水マレイン酸を高沸点物質の分離蒸留（６０）を行なった後、精製蒸留塔（８０）に供給して精製した。精製蒸留塔（８０）において塔頂における還流比を７．０の条件とした以外は、実施例１と同じ条件で蒸留操作を行った。結果を表３に示す。該精製蒸留塔（８０）から得た精製無水マレイン酸中にはＭＩＢＫは２５０ｐｐｍが検出された。また蟻酸、酢酸は検出できなかった。他は有水分であるマレイン酸であり、この精製無水マレイン酸の純度は、９９．８４〜９９．８９質量％の範囲内、凝固点が５２．６から５２．７℃の範囲内、溶融色が５であった。またＪＩＳ規格（ＪＩＳ１３５９／１９８５、純度：９９．０質量％以上、凝固点：５２．０℃以上、溶融色：５０以下）を満足するものであったが、ＭＩＢＫが精製無水マレイン酸中に検出された。
【００９１】
（比較例３）
実施例１の共沸脱水塔（４０）において、操作圧を６７０ｋＰａ、塔底温度を１８０℃の蒸留条件とした以外は、実施例１と同様の条件として８時間の共沸脱水蒸留を行った。系内組成が一定になった時点ではボトム温度１８０℃、また塔底液組成は、無水マレイン酸９７．４２質量％、マレイン酸１．２９質量％、フマル酸０．８５質量％、ＭＩＢＫ４０２０質量ｐｐｍ、また蟻酸検出限界以下の質量ｐｐｍ、酢酸３８０質量ｐｐｍであり、共沸脱水塔内に供給された共沸溶媒および粗マレイン酸溶液中の軽沸物である酢酸の、塔底の粗無水マレイン酸液中での存在を無くすることができなかった。なお、共沸脱水塔（４０）内には縮合物やマレイン酸や無水マレイン酸の重合物である黒色の固体物の析出は無かった。
【００９２】
次に、共沸脱水塔（４０）の塔底液として得られた粗無水マレイン酸を高沸点物質の分離蒸留（６０）を行なった後、精製蒸留塔（８０）に供給して精製した。精製蒸留塔（８０）において塔頂における還流比を２．０および２０．０の条件とした以外は、実施例１と同じ条件で蒸留操作を行った。還流比を２．０とした場合には、該精製蒸留塔（８０）から得た精製無水マレイン酸中にはＭＩＢＫは３５００ｐｐｍが検出された。また蟻酸、酢酸は検出できなかった。他は有水マレイン酸であり、この精製無水マレイン酸の純度は、９９．８４〜９９．８９質量％の範囲内、凝固点が５２．６から５２．７℃の範囲内、溶融色が５であった。またＪＩＳ規格（ＪＩＳ１３５９／１９８５、純度：９９．０質量％以上、凝固点：５２．０℃以上、溶融色：５０以下）を満足するものであった。一方、還流比を２０．０とした場合には、該精製蒸留塔（８０）から得た精製無水マレイン酸中にはＭＩＢＫ、蟻酸、酢酸は検出できなかった。他は有水マレイン酸であり、この精製無水マレイン酸の純度は、９９．８６〜９９．９１質量％の範囲内、凝固点が５２．７から５２．８℃の範囲内、溶融色が５であった。いずれもＪＩＳ規格（ＪＩＳ１３５９／１９８５、純度：９９．０質量％以上、凝固点：５２．０℃以上、溶融色：５０以下）を充分に満足するものであった。ただし、精製蒸留塔における還流比を２０．０とする条件は経済的に不利であり工業生産として不可能な条件である。共沸脱水塔の塔底の粗無水マレイン酸液中の共沸溶媒ＭＩＢＫ濃度が４０２０質量ｐｐｍであった場合には、軽沸分の分離蒸留操作を省略すると、その後精製工程において共沸溶媒を検出限界以下とするためには、工業的に実施が困難な還流比の（８０）が必要であることを確認した。
【００９３】
（比較例４）
共沸溶媒にｏ−キシレンを使用して行った以外は、実施例１と同様の条件として共沸脱水蒸留を行った。６時間後系内組成がほぼ一定になったと考えられる時点での塔底組成は、無水マレイン酸９６．９９質量％、マレイン酸１．１９質量％、フマル酸１．８０質量％、ｏ−キシレン２００質量ｐｐｍ、蟻酸、酢酸は検出限界以下の質量ｐｐｍであった。ただし塔頂から数えて塔内第１１段から１４段にかけてマレイン酸結晶が析出して徐々に堆積が起こってゆき、塔内のトレーが閉塞していった。また蒸留塔内には縮合物やマレイン酸や無水マレイン酸の重合物である黒色の固体物の析出があり、これらも徐々に堆積が起こっていった。このため蒸留稼動は不安定さを増していったため、より長時間の安定稼動は不可能と考えられた。共沸溶媒にｏ−キシレンを使用して共沸脱水蒸留を減圧操作とすることで、塔底液中のｏ−キシレンは２００質量ｐｐｍと減少させることはできたが、マレイン酸が析出してくるため連続的な稼動は困難であることが確認された。
【００９４】
【表２】

【００９５】
【表３】

【００９６】
（結果）
共沸溶媒としてＭＩＢＫを使用して塔底液に残存するＭＩＢＫ濃度を３０００質量ｐｐｍ以下に制御した場合には、同時に蟻酸や酢酸などの低沸点物質の残存量も検出限界以下とすることができた。また、ＭＩＢＫ濃度を３０００質量ｐｐｍ以下に制御するには、単に共沸脱水塔における蒸留条件として、塔頂圧力を減圧条件である７７０ｈＰａ等とするだけでよく、極めて簡便であった。
【００９７】
共沸脱水塔においてＭＩＢＫ濃度が３０００質量ｐｐｍ以下である無水マレイン酸を精製蒸留すると、ＭＩＢＫの濃度を検出限界以下とすることができ、この際、同時に蟻酸や酢酸などの低沸点物質の残存量も検出限界以下とすることができた。しかも、共沸溶媒を含む蟻酸や酢酸などの低沸点物質の除去には、単に還流比を２．０に制御するのみで足り、極めて操作が簡便であった。
【００９８】
【発明の効果】
無水マレイン酸の製造工程において、粗マレイン酸含有水溶液を共沸脱水塔に供給し共沸脱水する際に、特定の有機溶媒を使用し減圧下で共沸脱水することで、該塔底液に残存する共沸溶媒濃度を３０００質量ｐｐｍと極めて低値にすることができ、同時に低沸点物質の濃度も低下させることができる。このため、従来必要とされた溶媒や低沸点物質の分離のための蒸留塔がなくても、高純度の無水マレイン酸を製造することができる。さらに次の工程である、３段以上の精製蒸留塔において、塔頂部から第２段以下から抜き出すことで、低沸点成分を除去できた極めて純度の高い無水マレイン酸を得る事ができる。すなわち、共沸溶媒および低沸点物質を除去するための蒸留分離工程を省くかもしくは簡略な装置としても、充分に高純度の無水マレイン酸を得ることができ、設備および稼動費用の低減を図ることができる。
【図面の簡単な説明】
【図１】図１は、本発明における無水マレイン酸の製造方法の好ましい製造方法の工程図である。
【符号の説明】
１…反応ガス、
１０…無水マレイン酸捕集器、
１１…粗製無水マレイン酸、
２０…水洗捕集器、
２１…リサイクル捕集水、
３０…濃縮装置、
４０…共沸脱水塔、
５０…油水分離槽、
５１…廃水、
６０…高沸点分離装置、
６１…残査、
７１…共沸溶媒、
８０…精製塔、
８１…精製無水マレイン酸
９０…溶媒回収塔。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing maleic anhydride including an azeotropic dehydration step in an azeotropic dehydration tower of a crude maleic acid-containing aqueous solution. The crude maleic acid-containing aqueous solution is supplied to an azeotropic dehydration column using an organic solvent having a dissolution concentration of 0.2% by mass or more or an organic solvent having a maximum dissolution concentration in water at 20 ° C of 0.3 to 5% by mass. The azeotropic dehydration column having an overhead pressure of 900 hPa or less and an azeotropic solvent concentration in the bottom liquid of the azeotropic dehydration column of 3000 mass ppm or less. It relates to a manufacturing method.
[0002]
[Prior art]
Maleic anhydride is obtained by oxidizing a raw material gas such as aliphatic hydrocarbon having 4 or more carbon atoms such as n-butane or benzene in a catalytic gas-phase oxidation reactor, and converting the obtained maleic anhydride or a gas containing maleic acid into anhydride. It is produced by a method of recovering and purifying as maleic acid. Further, since the washing water of the exhaust gas discharged when producing phthalic anhydride by the catalytic gas-phase oxidation reaction of naphthalene or o-xylene contains a considerable amount of maleic anhydride, the washing water is recovered. Is also produced by a method of purifying as maleic anhydride.
[0003]
In general, when producing maleic anhydride through a catalytic gas-phase oxidation reaction, the maleic anhydride obtained by the catalytic gas-phase oxidation reaction is purified as it is, or once it is collected in an aqueous solution and the crude There is a method of purifying a maleic acid-containing aqueous solution and dehydrating maleic acid to maleic anhydride to produce maleic anhydride.
[0004]
For example, in a method for producing maleic anhydride by subjecting a mixed gas of benzene and air to catalytic gas phase oxidation, the collected maleic acid (sometimes called crude maleic acid in the sense of unpurified maleic acid). An aromatic hydrocarbon solvent is disclosed as a solvent used when an aqueous solution containing is dehydrated through an azeotropic dehydration step. Specific examples of the aromatic hydrocarbon solvent include xylene, cymene, ethylbenzene, diethylbenzene, and dichlorobenzene (Patent Document 1).
[0005]
Generally, when maleic anhydride is obtained from the raw material gas by a catalytic gas phase oxidation reaction, the reaction gas discharged from the reactor includes a reaction product by-produced in the catalytic gas phase oxidation reaction together with maleic anhydride. The raw material gas itself contains impurities contained therein, and further contains low-boiling substances and high-boiling substances which are oxides of the impurities and the raw material compounds, as well as non-condensable gases. The low-boiling substance is, for example, acetic acid or formic acid. For this reason, in order to obtain a purified maleic anhydride having high purity, it is necessary to separate a low-boiling substance and a high-boiling substance together with the above-mentioned impurities, and further, it is necessary to separate the collected maleic acid into maleic anhydride. When the conversion step is an azeotropic dehydration step, it is necessary to remove the azeotropic solvent remaining in the crude maleic anhydride solution obtained after the azeotropic dehydration step, and to remove the impurities, low-boiling substances and high-boiling substances. Usually relies on a further distillation step. The above-mentioned crude maleic anhydride means maleic anhydride before being subjected to a purification step for obtaining a purified maleic anhydride having a higher purity. And Hydrocarbon Processing Sep. As disclosed in (1971), pp. 167-169, when the distillation step is a batch distillation, the distillation step can be performed in one distillation column which also serves as a solvent distillation step. A continuous distillation method is adopted.First, after passing through a multi-stage distillation column for separating and removing low boiling substances and azeotropic solvents, a multi-stage distillation for separating and removing high boiling substances to obtain purified maleic anhydride. It may be carried out by two or more distillation steps in a distillation column (Patent Documents 2 and 3). The above-mentioned distillation step may be referred to as a purification distillation step.
[0006]
[Patent Document 1]
Japanese Patent Publication No. 41-3172
[Patent Document 2]
JP-A-47-12316
[Patent Document 3]
JP-A-63-3137782
[0007]
[Problems to be solved by the invention]
As described above, conventionally, after the step of dehydrating a crude maleic acid-containing aqueous solution by azeotropic dehydration, the azeotropic solvent remaining in the crude maleic anhydride solution obtained as the bottom liquid from the azeotropic dehydration tower is removed. Or a step for removing low-boiling substances such as acetic acid and formic acid generated by the side reaction. For this reason, usually, after the azeotropic dehydration step, first, a distillation step is performed as a step of removing an azeotropic solvent and a low-boiling substance, and the bottom liquid obtained in the distillation step is further subjected to a purification distillation step to obtain a final product. In general, a high purity maleic anhydride is produced, and at least two multistage distillation column devices are required after the azeotropic dehydration step.
[0008]
In view of the above problems, the present invention further includes, after the azeotropic dehydration step in the production step of maleic anhydride including the azeotropic dehydration step, as a step for efficiently removing contained impurities and removing low-boiling substances. An object of the present invention is to provide a method for producing maleic anhydride which can omit a distillation step or use a simple apparatus.
[0009]
[Means for Solving the Problems]
The present inventors have studied in detail the types of by-products generated by the catalytic gas phase oxidation reaction of the raw material gas, the impurities generated in the production process of maleic anhydride, and the azeotropic dehydration conditions. When used as a boiling solvent and the concentration of the azeotropic solvent contained in the bottom liquid of the azeotropic dehydration column is 3000 mass ppm or less under reduced pressure conditions, the azeotropic solvent and low-boiling compounds are removed in the subsequent steps. The present inventors have found that high-purity maleic anhydride can be produced without a distillation apparatus or with a simpler distillation apparatus than before, and completed the present invention. That is, the above problem is solved by the following (1) to (5).
[0010]
(1) A crude maleic acid-containing aqueous solution is supplied to an azeotropic dehydration tower, and azeotropic dehydration is performed using an azeotropic solvent that azeotropes with water. In the method for producing maleic anhydride to obtain the maleic anhydride from the bottom liquid of the azeotropic dehydration tower,
An organic solvent having a maximum dissolution concentration of maleic acid at a temperature of 20 ° C. of 0.2% by mass or more as an azeotropic solvent,
A method for producing maleic anhydride, comprising setting the pressure at the top of an azeotropic dehydration column to 900 hPa or less, and further adjusting the concentration of the azeotropic solvent contained in the bottom liquid in the azeotropic dehydration column to 3000 mass ppm or less. .
[0011]
(2) As the azeotropic solvent, the azeotropic solvent further uses an organic solvent having a maximum dissolution concentration in water at a temperature of 20 ° C. of 0.3 to 5% by mass as physical properties. ) Described method.
[0012]
(3) Further, the bottom liquid of the azeotropic dehydration column is supplied to a purification distillation column having three or more theoretical distillation stages, and counted from the top of the purification distillation column to a second or lower theoretical distillation plate number. The method according to the above (1) or (2), further comprising a purification distillation step of purifying while extracting maleic anhydride from the position.
[0013]
(4) The method according to the above (3), wherein the distillation is performed at a reflux ratio of 0.1 to 10 in the purification distillation column.
[0014]
(5) The method according to any one of (1) to (4), wherein the azeotropic solvent is a ketone or an ester having a boiling point at a pressure of 1013 hPa in the range of 80 to 170 ° C. .
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention provides a crude maleic acid-containing aqueous solution, which is supplied to an azeotropic dehydration tower, using an azeotropic solvent that azeotropes with water, and by azeotropically dehydrating the contained maleic acid as maleic anhydride. A method for producing maleic anhydride from the bottom liquid of an azeotropic dehydration column, wherein an organic solvent having a maximum dissolution concentration of maleic acid at a temperature of 20 ° C. of 0.2% by mass or more is used as an azeotropic solvent. The production of maleic anhydride, wherein the pressure at the top of the azeotropic dehydration column is set to 900 hPa or less, and the concentration of the azeotropic solvent contained in the bottom liquid in the azeotropic dehydration column is 3000 ppm by mass or less. In a preferred embodiment of the present invention, an organic solvent having a maximum solubility in water at a temperature of 20 ° C. of 0.3 to 5% by mass is further used as the azeotropic solvent. A method for producing maleic anhydride, wherein the door.
[0016]
O-xylene, which is a typical example of a conventionally used azeotropic solvent, has a boiling point of 144.4 ° C., does not dissolve maleic acid, and has almost no solubility in water. When the crude maleic acid aqueous solution is subjected to azeotropic dehydration using this o-xylene as an azeotropic solvent, o-xylene remains together with crude maleic anhydride in the bottom liquid of the azeotropic dehydration tower, and It was necessary to use a multi-stage solvent separation distillation column in order to perform the separation. On the other hand, if the azeotropic solvent concentration in the bottom liquid is sufficiently low as 3000 ppm or less, and if it can be almost eliminated, it is not necessary to use a multi-stage solvent separation distillation column, but this is difficult with a conventional azeotropic solvent. It is. This is for the following reason.
[0017]
First, in order to make o-xylene in the bottom liquid of the azeotropic dehydration column 3000 ppm or less, the distillation temperature may be increased or the column may be depressurized and operated under azeotropic dehydration distillation conditions in which the separation temperature is reduced. Good. However, in order to prevent the loss of maleic anhydride having a boiling point of 204 ° C. from distilling from the top of the azeotropic dehydration tower and distill o-xylene, the temperature of the top of the azeotropic dehydration tower should be about 195 ° C. or less. Under ordinary atmospheric pressure conditions, it is impossible to sufficiently separate o-xylene from the bottom liquid. For this reason, about 2 to 20% by mass of o-xylene usually remains. In addition, under reduced pressure distillation conditions, the o-xylene concentration can be 3000 ppm or less, but it is the freezing point of maleic acid between the position where the crude maleic acid aqueous solution is introduced and the bottom of the azeotropic dehydration column. A phenomenon in which the temperature in the tower is lower than 130 ° C. occurs, and maleic acid may coagulate and precipitate to block the inside of the tower. However, in the present invention, by using an organic solvent in which the maximum dissolution concentration of maleic acid at a temperature of 20 ° C. is 0.2% by mass or more as an azeotropic solvent, water separation between the introduction position of the crude maleic acid aqueous solution and the bottom of the column is performed. Even when the temperature inside the azeotropic dehydration column where the dehydration reaction occurs is below the freezing point of maleic acid, no blockage of the column due to the precipitation of maleic acid occurs, and azeotropic dehydration can be performed even under reduced pressure. . As a result, the azeotropic solvent concentration in the bottom liquid can be reduced by appropriately selecting the conditions of the azeotropic dehydration step for purging the azeotropic solvent as a reduced pressure condition (taking the used azeotropic solvent out of the system). It is sufficiently low as 3000 ppm or less and can be almost completely eliminated. As a result, high-purity maleic anhydride can be produced only by performing a purification treatment for separating a high-boiling substance at the bottom of the column under reflux distillation conditions in which a small amount of a low-boiling substance is separated as a subsequent step.
[0018]
In a preferred embodiment of the present invention, an organic solvent having a maximum solubility of 0.3 to 5% by mass in water at a temperature of 20 ° C. as a further property of the azeotropic solvent is used as the azeotropic solvent. Maleic acid has a very high maximum solubility in water of 43.3% by mass even at 20 ° C., but hardly dissolves in a hydrophobic solvent. Within the column, a hydrophobic solvent and a maleic acid-containing aqueous solution in which maleic acid, which is hydrophilic, are dissolved in water are in a liquid-separated state, and maleic acid is precipitated from the portion of the tower where the hydrophobic solvent is mainly used. Occurs, leading to blockage in the tower.
[0019]
On the other hand, if a hydrophilic azeotropic solvent and water are used in the azeotropic dehydration step, the liquid will not be separated as described above in the column of the azeotropic dehydration tower, and will be a substantially uniform phase. For this reason, the hydrophobic portion is almost eliminated in the column, and the precipitation of maleic acid in the column is extremely reduced. Thus, even if the temperature in the azeotropic dehydration column where the above-mentioned liquid separation and dehydration reaction take place does not exceed the freezing point of maleic acid, clogging in the column due to precipitation of maleic acid does not occur, so that the azeotropic dehydration step can be performed even under reduced pressure conditions. it can. In addition, by appropriately selecting the azeotropic dehydration conditions, the azeotropic solvent concentration in the bottom liquid of the azeotropic dehydration column can be sufficiently low as 3000 ppm or less, and can be almost completely eliminated, and a small amount remains as a post-process. High-purity maleic anhydride can be produced only by performing a purification treatment for separating a high-boiling substance at the bottom of the column under reflux distillation conditions for separating the low-boiling substance.
[0020]
On the other hand, in Japanese Patent Publication No. 39-25550, azeotropic dehydration is carried out under reduced pressure as necessary using o-dichlorobenzene as an azeotropic solvent. Since the boiling point of o-dichlorobenzene is as high as 180 ° C. and the boiling point of maleic anhydride is close to 204 ° C., maleic anhydride is easily distilled at the top of the azeotropic dehydration tower. In this publication, in order to perform azeotropic dehydration while minimizing this loss, azeotropic dehydration is performed at a temperature as low as possible and at a reduced pressure above the dehydration reaction temperature. The maximum dissolution concentration is about 0.02% by mass at a temperature of 20 ° C., showing little solubility, and the above-mentioned o-dichlorobenzene also has a maximum dissolution concentration in water of about 0.01% by mass at a temperature of 20 ° C. Such a hydrophobic solvent. For this reason, if the operation is performed under an operating condition in which the concentration of the azeotropic solvent contained in the bottom liquid is 3000 ppm or less, the liquid composition in the tower becomes hydrophobic from the column in which the maleic acid has a freezing point or less, and the liquid in the column becomes hydrophilic. Maleic acid is gradually precipitated, and the blockage in the column progresses, making long-term operation difficult.
[0021]
Japanese Patent Publication No. 36-3818 discloses a method using anisole as an azeotropic solvent. Anisole has an affinity for water with a maximum solubility in water of 0.14% by mass at a temperature of 20 ° C, but has a low maximum solubility in water. A condensation reaction occurs with formaldehyde, which is a by-product of the phase oxidation reaction, and a resinous substance is generated to form a resinous substance. Therefore, substances that are chemically unstable and reactive in actual use, such as anisole, and substances that are reactive with maleic acid, maleic anhydride, and contained impurities and hinder the purification process are unsuitable. Therefore, as the azeotropic solvent used in the production method of the present invention, preferably, the method for producing maleic anhydride of the present invention is contained in a crude maleic acid-containing aqueous solution to be azeotropically dehydrated, maleic acid or an anhydride which is a target substance. It is selected from maleic acid or an azeotropic solvent that does not substantially react with the contained impurities contained.
[0022]
The impurities contained in the azeotropic dehydration step of the present invention are, specifically, aldehydes such as formaldehyde, quinones such as benzoquinone and hydroquinone, fumaric acid, phthalic acid and the like, which are main impurities. Condensates and polymers. Under the operating conditions of the azeotropic dehydration step, it is preferable to select an azeotropic solvent having low reactivity or substantially no reactivity with the impurities contained in the azeotropic dehydration step of the present invention. An azeotropic solvent which reacts with maleic acid or maleic anhydride itself is of course unsuitable. This is because the yield of maleic anhydride obtained as a product decreases. In selecting an azeotropic solvent, it is necessary to make it inappropriate if there is any inconvenience, such as when actual harm such as blockage of the device can be confirmed or when the generated impurities cannot be separated from maleic anhydride in the product. Of course.
[0023]
In the present invention, the impurities contained in the azeotropic dehydration step of the present invention may also be referred to as low boiling substances or high boiling substances.
[0024]
In the present invention, the substance having a low boiling point means a substance having a boiling point lower than that of maleic anhydride in a standard state, and examples thereof include formic acid, acetic acid, acrylic acid, formaldehyde, acetaldehyde, p-benzoquinone, and water. In addition, a high-boiling substance refers to a substance having a boiling point higher than that of maleic anhydride in a standard state, and is exemplified by phthalic anhydride, fumaric acid, maleic acid or a composite polymer of maleic anhydride or maleic anhydride alone or with other impurities. it can. Further, the non-condensable gas refers to a substance that is gaseous in a standard state, and specific examples thereof include nitrogen, oxygen, air, propylene, propane, carbon monoxide, and carbon dioxide. Note that the above-mentioned standard state is a state where the atmospheric pressure is 1013 hPa (1 atm) and the temperature is 0 ° C. The low-boiling components to be separated in the present invention include impurities such as formic acid and acetic acid in addition to the azeotropic solvent. Most of these are substances with boiling points comparable to or lower than azeotropic solvents.
[0025]
In addition, it has been confirmed that impurities can be separated from the bottom liquid under the distillation conditions in which the azeotropic solvent in the bottom liquid is 3000 mass ppm or less. It can be used as an index that a distillation column for low-boiling separation is not required, or that even a very simple apparatus can provide purified maleic anhydride with sufficiently high purity.
[0026]
As such an azeotropic solvent that can be used in the present invention, the maximum dissolution concentration of maleic acid at a temperature of 20 ° C. (normal pressure) is preferably 0.2% by mass or more, more preferably 0.3% by mass or more, and furthermore Preferably 0.5% by mass or more, especially 1.0% by mass or more of an organic solvent, which reacts with maleic acid, maleic anhydride as a target substance and impurities contained in the azeotropic dehydration step of the present invention. Those with no properties can be widely used. However, if the maximum dissolution concentration of maleic acid as an organic solvent is too high, the maximum dissolution concentration in water often increases. For this reason, the amount of loss of the azeotropic solvent to the aqueous phase increases, and the amount of separated liquid recovered decreases, which is disadvantageous. The maximum dissolution concentration of maleic acid is preferably 0.2% by mass or more, more preferably 5.0% by mass or less, particularly 2.0% by mass or less. A boiling solvent can be selected.
[0027]
An organic solvent having a maximum dissolution concentration in water at a temperature of 20 ° C. (normal pressure) of 0.3 to 5% by mass, more preferably 0.5 to 4% by mass, particularly preferably 1 to 3% by mass, It is a preferred embodiment of the present invention to use as the azeotropic solvent maleic acid, maleic anhydride which is the target substance, and those which do not react with impurities contained in the azeotropic dehydration step of the present invention. In particular, if the maximum dissolution concentration in water is less than 0.3% by mass, oil-water separation between the water and the azeotropic mixture during azeotropic dehydration distillation is likely to occur, and on the other hand, more than 5% by mass. Although there is an effect of improving the oil-water separation state, when the organic solvent distilled off at the top of the column with water by azeotropic dehydration distillation is cooled and separated into water for recovery and utilization, the amount of loss to the aqueous phase is reduced. This is disadvantageous because the liquid separation recovery amount decreases. Although the maximum dissolution concentration varies depending on the pressure, the maximum dissolution concentration in the present invention is a value at normal pressure (1013 hPa). As the organic solvent used in the present invention, methylisobutyl ketone (1.Methyl butyl ketone) may be used based on the solubility of these organic solvents in water described in Chemical Handbook (issued by Maruzen Co., Ltd.) and Bayerstein Online (search by STN International). 91% by mass), diisopropyl ketone (0.43% by mass), 3-pentanone (4.6% by mass), 2-hexanone (1.75% by mass), 3-hexanone (1.57% by mass), 2- Heptanone (0.44% by mass), amyl acetate (0.25% by mass), and allyl acetate (2.8% by mass) can be exemplified.
[0028]
The azeotropic solvent used in the present invention has a maximum dissolution concentration of maleic acid at a temperature of 20 ° C. of 0.2% by mass or more, and a maximum dissolution concentration in water at a temperature of 20 ° C. of 0.3 to 5% by mass. % Of the organic solvent is more preferred. This is because the effect of preventing the clogging of the azeotropic dehydration tower is high, and the loss of the azeotropic solvent to the aqueous layer does not become too large.
[0029]
The organic solvent used in the present invention preferably has a boiling point at a pressure of 1013 hPa of 80 to 170 ° C, more preferably 100 to 160 ° C, and particularly preferably 110 to 150 ° C. When the boiling point is higher than 170 ° C., the boiling point is close to the boiling point of maleic anhydride, the rate of distilling off with the organic solvent increases, and it is difficult to distill off the organic solvent, and it is difficult to reduce the residual rate in the bottom liquid. Is not preferred. On the other hand, when the boiling point is lower than 80 ° C., the dehydration reaction temperature in the distillation column is decreased, and the dehydration rate is decreased, so that production may not be advantageously performed.
[0030]
The maximum dissolution concentration of maleic acid at a temperature of 20 ° C. used in the present invention is 0.2% by mass or more, and as an azeotropic solvent, the azeotropic solvent preferably further exhibits physical properties such as a maximum solubility in water at a temperature of 20 ° C. Examples of the organic solvent having a concentration of 0.3 to 5% by mass or a solvent having a boiling point of 80 to 170 ° C at a pressure of 1013 hPa include methyl isobutyl ketone, diisopropyl ketone, 3-pentanone, 2-hexanone, 3-hexanone, and 2-hexanone. There are ketones such as phthalone and esters such as amyl acetate and allyl acetate. In the present invention, it is particularly preferable to use ketones such as methyl isobutyl ketone, diisopropyl ketone and 2-hexanone and esters such as allyl acetate. As the azeotropic solvent used in the azeotropic dehydration step of the present invention, these ketones and esters are compatible with water as well as with maleic acid having a carboxylic acid group as a hydrophilic group. This is a preferred embodiment because the solubility of maleic acid in the boiling solvent is increased, and the separation of the low-boiling substance from maleic anhydride can be facilitated in the azeotropic dehydration step. Further, the above-mentioned solvent is also a preferred embodiment as an azeotropic solvent since it has no reactivity with maleic acid or maleic anhydride. In particular, ketones are preferred because of their excellent solubility in maleic acid. In addition, ketones satisfy the above conditions, are excellent in separation of low-boiling substances in the maleic anhydride production process, and have no reactivity with maleic acid or maleic anhydride, which is the target substance. It is also a form.
[0031]
The solubility of the maleic acid used in the present invention and the above-mentioned organic solvents having a specific range of solubility in water are formic acid, acetic acid, aldehydes and the like, particularly when producing maleic anhydride by catalytic vapor-phase oxidation of benzene. This is effective because the amount of by-products of quinones having low boiling points is large. However, these low-boiling substances are caused not only by benzene as a raw material but also by aliphatic hydrocarbons such as n-butane and by-products of phthalic anhydride production from naphthalene and o-xylene. Also occurs in the case of the production of maleic anhydride. Therefore, the method of the present invention is useful in a production method using any of the raw materials. For example, a maleic anhydride-containing gas obtained by a catalytic gas phase oxidation reaction of n-butane is washed with water, collected and collected. It is also effective when the method includes an azeotropic dehydration step using an organic solvent having
[0032]
In the step of azeotropically dehydrating the crude maleic acid-containing aqueous solution of the present invention, one of the above specific organic solvents may be used alone, or two or more thereof may be used in combination. In the present invention, even when two or more of the above-described specific organic solvents are used in combination, the maximum solubility concentration in water at a temperature of at least 20 ° C. is 0.3 to 5% by mass as a characteristic of the mixture of the organic solvents. Any mixture of organic solvents can be suitably used. More preferably, the maximum dissolution concentration of maleic acid at a temperature of 20 ° C. is 0.2% by mass or more, preferably 0.3% by mass, more preferably 0.5% by mass or more, particularly 1.0% by mass or more. Yes, more preferably, as a physical property of the mixture of the organic solvent, the maximum dissolution concentration in water at a temperature of 20 ° C. is 0.3 to 5% by mass, more preferably 0.5 to 4% by mass, and especially 1 to 3% by mass. % Of a mixture of organic solvents can be preferably used. Further, the mixture of the organic solvents is a mixture of organic solvents having an azeotropic point at a pressure of 1013 hPa of 70 to 100 ° C, more preferably 80 to 100 ° C, and particularly preferably 85 to 100 ° C. When the mixture of the organic solvent has the azeotropic point in the above range, the maximum dissolution concentration of maleic acid, the maximum dissolution concentration in water, to prevent the precipitation of maleic acid in the column, low formic acid, acetic acid, aldehydes and quinones. This is a preferred embodiment in the present invention because the boiling point substance can be easily separated.
[0033]
In the present invention, another organic solvent is used in combination with the organic solvent having the above-mentioned solubility characteristics (the characteristic in which the maximum dissolution concentration of maleic acid is in a specific range and the maximum dissolution concentration in water is in a specific range). In this case, the amount used is 1 to 40% by mass, more preferably 3 to 40% by mass of the amount of maleic acid used in the present case, the maximum dissolution concentration of the specific solvent or even the amount of the organic solvent having specific solubility characteristics in water. It is preferable to use it within the range of from 35 to 35% by mass, especially from 5 to 30% by mass. In addition, examples of the solvent having the above specific solubility characteristics include methyl isobutyl ketone, 3-pentanone, and 3-hexanone. The reason is that if more than 40% by mass of an organic solvent other than the organic solvent having specific solubility characteristics is used, the effect of separating low-boiling substances cannot be obtained. This is because the effect cannot be expected.
[0034]
The organic solvent other than the organic solvent having the above specific solubility characteristics is, specifically, an organic solvent having a low maximum solubility in water. Specifically, the solubility in water at a temperature of 20 ° C. is less than 0.2% by mass, more preferably less than 0.1% by mass, further preferably less than 0.08% by mass, and particularly preferably 0.05% by mass. % Organic solvent. In addition, by mixing and using such an organic solvent having a low maximum dissolution concentration in water, the amount of the specific organic solvent used can be reduced and an economical organic solvent can be selected.
[0035]
In addition, when a large amount of low-boiling substances remain in the bottom liquid and cannot be removed in a subsequent step, the combined use of an organic solvent that forms an azeotropic composition with low-boiling substances such as formic acid and acetic acid is more impure. This is an effective measure to obtain a maleic anhydride solution as a bottom liquid of an azeotropic dehydration column having a small amount of water. Other organic solvents other than the organic solvents having the specific solubility characteristics described above, specifically, organic solvents having a low maximum solubility in water, generally form an azeotropic composition with low boiling substances such as formic acid and acetic acid. It is an organic solvent that is easy to use. As such another organic solvent, any organic solvent having a preferable boiling point for azeotropic dehydration of the above-mentioned aqueous solution containing crude maleic acid can be used. Specifically, toluene, xylene, octane, cumene, mesitylene, ethylbenzene, hydrocarbons such as dimethylcyclohexane, isopropyl sulfide, sulfonated products such as allyl sulfide, o-dichlorobenzene, dibromoethane, 1-iodopropane and the like Examples thereof include halides.
[0036]
Hereinafter, an example of a preferred embodiment of the method for producing maleic anhydride of the present invention will be described with reference to FIG. In FIG. 1, 1 is a reaction gas, 10 is a maleic anhydride collector, 11 is crude maleic anhydride, 20 is a washing collector, 21 is recycled water, 30 is a concentrator, and 40 is an azeotrope. A dehydration tower, 50 is an oil / water separation tank, 51 is waste water, 60 is a high boiling point separator, 61 is a residue, 71 is an azeotropic solvent, 80 is a purification tower, 81 is a purified maleic anhydride, and 90 is a solvent recovery tower.
[0037]
First, a raw material gas is supplied to a catalytic gas phase oxidation reactor (not shown). The feed gas is not particularly limited as long as it produces maleic acid as a product by the catalytic gas phase oxidation reaction, and a known hydrocarbon used for producing maleic anhydride may be used as the feed material gas. it can. For example, benzene, butane (n-butane), hydrocarbons having 4 or more carbon atoms such as butenes (1-butene, 2-butene), butadiene (1,3-butadiene), or one of o-xylene and naphthalene Species or a mixture of two or more thereof can be exemplified.
[0038]
As the catalyst used in the reactor, a known catalyst can be used as long as it produces maleic acid or maleic anhydride, and an oxidation catalyst containing vanadium as a main component can be used. As such a catalyst, catalysts described in JP-A-5-261292, JP-A-5-262754, JP-A-5-262755, and JP-A-6-145160 are preferably used. it can.
[0039]
Since the catalytic gas phase oxidation reaction is literally an oxidation reaction, a molecular oxygen-containing gas is supplied together with the raw material gas. Air is usually used as such a molecular oxygen-containing gas, but air diluted with an inert gas, air enriched with oxygen, or the like can also be used.
[0040]
The reaction conditions may be conventionally known methods, but may be appropriately changed depending on the type of the oxidation catalyst used, the concentration of the feed material, the concentration of the molecular oxygen-containing gas, and the like. For example, the reaction is performed at a temperature of 300 to 600 ° C. using a vanadium-phosphorus catalyst. The reaction gas discharged from the catalytic gas phase oxidation reactor contains the reaction components by-produced together with maleic anhydride and the impurities contained in the raw material gas itself in the form as they are, and further contains the impurities and oxides of the raw material compounds. Low-boiling substances and high-boiling substances, as well as non-condensable gases.
[0041]
The composition of the reaction gas obtained by the catalytic gas phase oxidation reaction of benzene is generally maleic anhydride (hereinafter, “maleic anhydride” in the composition of the reaction gas includes maleic anhydride in terms of maleic anhydride). 2 to 5% by mass, 0.01 to 0.1% by mass of acetic acid, aldehyde, etc. as low-boiling substances excluding water vapor; 0.005 to 0.03% by mass of phthalic anhydride as high-boiling substances; Non-condensable gas and water vapor.
[0042]
Next, the reaction gas discharged from the reactor is supplied to the maleic anhydride collector (10). In the maleic anhydride collector (10), the maleic anhydride is cooled at a temperature higher than the melting point and lower than the boiling point, more preferably at 55 to 120 ° C, particularly preferably at 60 to 100 ° C, and a part of the maleic anhydride is converted into a liquid. Collect. Because of the residual vapor pressure of maleic anhydride, a large amount of maleic anhydride is also present in the reaction gas after cooling. For this reason, after performing such a collection step by cooling maleic anhydride, the exhaust gas after this step is supplied to a water-washing collector (20), and a large amount of maleic anhydride is converted into an aqueous solution. It is collected and recovered as a crude maleic acid-containing aqueous solution.
[0043]
A conventionally known method can be adopted as the collecting conditions of the water-washing collector (20). Water can be used as the collecting liquid, but a part of water or an aqueous solution generated in the maleic acid concentration and dehydration steps can also be used as a part of the collecting liquid. The tower top temperature is preferably low in order to improve the collection rate of maleic anhydride, and the cooling tower attached to the water-washing collector (20) is used to reduce the tower top temperature to 10%. -90 ° C, more preferably 20-60 ° C. If the temperature is lower than 10 ° C., the solubility of the maleic acid decreases and crystals are precipitated, which causes an increase in the pressure loss of the water-washing collector (20) and a decrease in the stage efficiency of the collecting tower (20) due to the deterioration of the dispersibility of the liquid. . In addition, excessive cooling energy is required. On the other hand, if the temperature exceeds 90 ° C., the collection rate of maleic anhydride decreases.
[0044]
The reaction gas is collected in the water-washing collector (20) so that the concentration of the maleic acid in the bottom liquid is 10 to 80% by mass, more preferably 20 to 60% by mass. Into the column from above, and brought into countercurrent contact with a maleic anhydride-containing gas to collect maleic anhydride. When the maleic acid concentration exceeds 80% by mass, the collection temperature must be raised to 90 ° C. or higher to prevent the precipitation of maleic acid, and the collection rate decreases. The amount of water distilled in the concentration / dehydration step of the aqueous solution increases, which is uneconomical.
[0045]
In addition, unlike FIG. 1, all of the reaction gas is supplied to the water-washing collector (20) without collecting the maleic anhydride in a liquid state by the maleic anhydride collector (10). And may be collected as a crude maleic acid-containing aqueous solution.
[0046]
Next, the crude maleic acid-containing aqueous solution is supplied to the azeotropic dehydration tower (40). At this time, the crude maleic acid-containing aqueous solution is obtained by collecting the reaction product gas obtained by subjecting benzene to catalytic vapor phase oxidation with water, and contains formic acid, acetic acid, acrylic acid, formaldehyde, acetaldehyde, p-benzoquinone. And the like, which are effective and preferable in that they can remove low-boiling substances such as. The crude maleic acid-containing aqueous solution may be concentrated by distilling water at a stage before being supplied to the azeotropic dehydration tower (40). As such a concentrator (30), a thin-film evaporator can be used in addition to a conventionally known tower such as a plate tower, a wet wall tower, and a spray tower. As such a concentrator, a thin film evaporator is preferable.
[0047]
As such an azeotropic dehydration tower (40), a known tower such as a tray tower, a packed tower, a wet wall tower, a spray tower, or the like can be used. The azeotropic dehydration tower (40) is usually preferably a tray column or a packed column, similarly to the water-washing and collecting device (20). In this case, it is preferable to use a distillation column having three or more theoretical plates, more preferably 4 to 20 plates, and particularly preferably 5 to 15 plates. If the number of stages is less than three, the contact time between maleic acid and the azeotropic solvent is insufficient, and the dehydration rate is reduced. Further, the distillation of maleic anhydride to the top of the column increases, and the loss of maleic anhydride increases. It is sufficient that the number of stages is necessary and sufficient so as not to cause such an adverse effect. If the number of stages is too large, equipment cost increases, which is uneconomical.
[0048]
In the present invention, when dehydrating crude maleic acid contained in a crude maleic acid-containing aqueous solution by azeotropic dehydration, an organic solvent having a maximum dissolution concentration of maleic acid at a temperature of 20 ° C of 0.2% by mass or more is used as an azeotropic solvent. The azeotropic solvent used is more preferably characterized in that the azeotropic solvent further exhibits a physical property in which the maximum dissolution concentration in water at a temperature of 20 ° C. is 0.3 to 5% by mass. Examples of such an organic solvent include the various organic solvents described above, and it is particularly preferable to use methyl isobutyl ketone (abbreviated as MIBK). In addition, since the azeotropic dehydration conditions vary depending on the azeotropic solvent used, for example, when MIBK is used as the azeotropic solvent, the azeotropic dehydration tower (40) is provided with 1 mass of the crude maleic acid-containing aqueous solution. To supply 3 to 5 mass of MIBK. In general, the pressure at the top of the column (absolute pressure) is 900 Pa or less, preferably 100 to 900 Pa, more preferably 200 to 900 hPa, and particularly preferably 300 to 800 hPa. If the pressure is less than 100 hPa, the size of the vacuum apparatus becomes large, which is uneconomical. On the other hand, if it exceeds 900 hPa, it may be difficult to reduce the azeotropic solvent concentration of the bottom liquid to 3000 ppm or less. The tower top temperature is 50 to 150 ° C, more preferably 60 to 130 ° C. If the temperature is lower than 50 ° C., the condenser becomes large. On the other hand, if the temperature exceeds 150 ° C., not only polymerization of maleic acid is liable to occur in the column, but also maleic anhydride is more frequently distilled off at the top of the column. Loss increases.
[0049]
The bottom temperature is 140 to 195 ° C, more preferably 150 to 190 ° C. When the temperature is lower than 140 ° C., the dehydration reaction of maleic acid becomes slow, and the dehydration of maleic acid becomes insufficient, leading to a decrease in the yield of maleic anhydride. On the other hand, if the temperature exceeds 195 ° C., the boiling point of maleic anhydride is approached, so that distillation is not preferred, and decomposition and polymerization are liable to occur. In order to achieve such azeotropic dehydration conditions, the type and amount of the azeotropic solvent, the water concentration in the crude maleic acid-containing aqueous solution, the change in the raw material supply stage, the reflux ratio of the condenser attached to the top of the column, the number of columns, Temperature, pressure, and other conditions may be adjusted.
[0050]
As the azeotropic solvent used for the azeotropic dehydration, the azeotropic solvent (71) recovered in the production process of maleic anhydride may be reused. Further, as shown in FIG. 1, the azeotropic solvent is not limited to the case where it is directly supplied to the azeotropic dehydration tower (40), and may be mixed with the raw material and supplied into the azeotropic dehydration tower (40).
[0051]
In order to stabilize the continuous operation of the azeotropic dehydration distillation, more specifically, to eliminate the fluctuation of the temperature inside the column and to operate it more stably, the temperature control should be performed in the recovery part of the azeotropic dehydration column more than the bottom liquid temperature. More preferably, the temperature is controlled below １ ／ of the theoretical number of distillation stages in the recovery section, counting from the position of introduction of the crude maleic acid aqueous solution in the azeotropic dehydration column toward the bottom of the column. Particularly preferred. For example, when the temperature at the bottom of the column is 180 ° C. and the theoretical number of distillation stages in the recovery section is 10 and the temperature is controlled at the sixth stage counted from the top, it is preferable to control the temperature between about 150 and 155 ° C. When the temperature is controlled at the eighth stage from the top under the same conditions, it is preferable to control the temperature between about 170 and 175 ° C. This is because an effect of further preventing dirt and clogging in the temperature azeotropic dehydration tower can be expected. By controlling the temperature of the azeotropic dehydration distillation according to the method described herein, when the temperature at the bottom of the column is controlled, the temperature control fluctuates by about 4 to 10 ° C. The range of the temperature control swing can be up to. If the control temperature range in the azeotropic dehydration tower is too wide or disturbed, the substance causing the dirt and clogging will also extend to the lower part of the recovery section of the azeotropic dehydration tower, causing dirt and clogging in the tower The part will also extend to the above part. For this reason, the stability of the conditions in the azeotropic dehydration tower is impaired, and it becomes a cause that long-term continuous operation becomes difficult.
[0052]
In the present invention, it is preferable that an oil / water separation tank (50) is attached to the top of the azeotropic dehydration tower (40) during the azeotropic dehydration to reflux the azeotropic solvent of the overhead distillate. When an organic solvent having a maximum dissolution concentration in water at a temperature of 20 ° C. of 0.3 to 5% by mass is used as an azeotropic solvent, the organic solvent is contained in the aqueous phase side of the overhead liquid. In order to make the process more economical, it is an advantageous method to collect and use this organic solvent in an azeotropic dehydration tower. The aqueous phase of the top distillate is supplied to the solvent recovery column (90) to distill the dissolved organic solvent, separated from the top of the column and sent to the oil / water separation tank (50). And used as an azeotropic solvent. Also, from the bottom of the column, it is obtained as recovered water containing no organic solvent. As the solvent recovery tower (90), conventionally known tray towers, packed towers, wet wall towers, spray towers and the like can be used.
[0053]
The bottom liquid of the azeotropic dehydration tower (40) is supplied to a high-boiling-point separation device (60) to separate high-boiling substances, and then to perform purification in a subsequent step. As a result, phthalic anhydride, fumaric acid, high-boiling point polymer, condensate, and the like by-produced by the catalytic gas phase oxidation reaction are discharged as a residue (61).
[0054]
Here, the high boiling point separation device (60) may be a continuous type or a batch type, and may be a conventionally known plate column, a packed tower, a wet wall tower, a spray tower or the like, or a batch type rotary retention tank. A type evaporator or a thin film evaporator can be preferably used. Further, the above-mentioned types of apparatuses can be used alone or in combination of plural types in series or in parallel. In the case of a continuous method, it is preferable to use two thin film evaporators in series. As shown in FIG. 1, the maleic anhydride (11) may be supplied to the high-boiling point separator (60) together with the bottom liquid of the azeotropic dehydration tower (40) for purification.
[0055]
Next, in the present invention, in addition to maleic anhydride, very little low-boiling impurities and low-boiling components of an azeotropic solvent are contained in the gas component from which the high-boiling substance discharged from the high-boiling point separation device is removed. Since there is no solvent separation column, the product is supplied to the purification column (80) for purification without providing a solvent separation column, and the distilled maleic anhydride (81) can be obtained as a product. However, if the separation of impurities in the purification tower is considered to be insufficient, a very simple low-boiling-point component separation device can be installed. Examples thereof include a distillation apparatus having a theoretical plate number of one stage or less, such as a retention tank type evaporator, a thin film evaporator, a wet wall tower, and a spray tower. The low boiling components are distilled off using these devices, and the higher purity maleic anhydride solution obtained from the bottom can be led to the purification column.
[0056]
As for the distillation conditions of the high boiling point separation device (60), when a distillation column is used, distillation can be performed under conventionally known distillation conditions, for example, a column top pressure (absolute pressure) of 1 to 40 kPa, more preferably 3 to 40 kPa. ２０20 kPa. The operation is carried out at a tower top temperature of 70 to 170C, more preferably 90 to 140C. In addition, the amount of distillate can be appropriately determined according to the allowable amount of high boiling point impurities in the product. The reflux ratio is usually in the range of 0.1 to 10, more preferably 6 or less, still more preferably 5 or less, more preferably 0.3 to 5, and particularly preferably 0.3 to 5. When a thin film evaporator is used as the high boiling point separator (60), the temperature is generally set to 80 to 200 ° C, more preferably 110 to 180 ° C.
[0057]
In the present invention, purified maleic anhydride having a sufficiently high purity is supplied to a purification column having three or more distillation stages and is extracted from the position of the second stage or lower in the number of distillation stages counted from the top of the purification column. Obtainable.
[0058]
In the case where the purified maleic anhydride is withdrawn from the top of the tower, the reflux ratio must be very high in order to separate the low-boiling components in order to separate the low-boiling components in order to purify the crude maleic anhydride in which the low-boiling components remain. It needs to be higher. When the low boiling point component is separated in the solvent separation tower, the reflux ratio in the purification tower can be 5 or less, more preferably about 0.2 to 2, but if the solvent separation tower is eliminated In other words, it is necessary to increase the reflux ratio to an economically inoperable level of about 20 to 30 or more. On the other hand, in the present invention, the azeotropic dehydration conditions can be selected by using the above-mentioned organic solvent as the azeotropic solvent as the azeotropic dehydration conditions so that the concentration of the low-boiling substance containing the azeotropic solvent can be reduced. Therefore, the reflux ratio in the purification tower can be kept within an economic range.
[0059]
A distillation method of extracting purified maleic anhydride from the top of the purification tower can also be selected when low-boiling substances can be almost completely separated and removed during the production operation. However, in the actual production operation, the ratio of the low-boiling substance varies depending on the reaction conditions and the like. If low-boiling substances increase during continuous operation and cannot be separated and removed in the azeotropic dehydration column until the concentration becomes sufficiently low, separation and purification are economical with a distillation method in which purified maleic anhydride is extracted from the top of the column. It may not be possible to respond within the operating range of the reflux ratio. For this reason, in order to make the reflux ratio more economically advantageous, a method of extracting purified maleic anhydride from the middle stage of the purification distillation column is more preferably selected. In particular, when the number is counted from the top of the column to the second stage or less of the theoretical number of distillation stages, even when a slightly low-boiling substance remains, it is possible to obtain a sufficiently high-purity maleic anhydride in which the amount is reduced. As the purification column, it is preferable to use a distillation column having three or more theoretical plates, more preferably 4 to 25 theoretical plates, and particularly 5 to 20 theoretical plates. Further, by extracting purified maleic anhydride from the top of the purification tower from the second or lower stage of the number of distillation stages, sufficiently high purity maleic anhydride can be obtained. The reflux ratio is preferably in the range of 0.1 to 10, more preferably in the range of 0.2 to 7, more preferably in the range of 0.3 to 5, and particularly preferably in the range of 0.3 to 3. preferable. The distillation conditions are preferably such that the absolute pressure at the top is 2 to 40 kPa and the temperature at the top is 80 to 170 ° C. Other known conditions can be adopted.
[0060]
In the present invention, since the specific organic solvent is used as the azeotropic solvent in the azeotropic dehydration step, the azeotropic solvent concentration at the bottom of the azeotropic dehydration tower can be 3000 ppm by mass or less. The products obtained from the boiling point separation column have extremely low contents of azeotropic solvents and low boiling substances. Conventionally, in addition to maleic acid in the gas component discharged from the high boiling point separation device, when an azeotropic solvent remains, it was necessary to supply the azeotropic solvent to a solvent separation tower or the like to separate the azeotropic solvent. However, in the present invention, this step can be omitted or a very simple apparatus can be used.
[0061]
【Example】
Hereinafter, the present invention will be described specifically with reference to examples.
[0062]
(Reference Example 1: Measurement of maximum dissolution concentration of maleic acid in the solvent)
80 g of a solvent at a temperature of 20 ° C. was placed in a 100 ml hermetically sealed glass container with a lid, and maleic acid of a special grade of reagent of the maximum dissolved concentration measurement item was added while stirring with a stirrer, and the dissolved state was visually checked. If there was any remaining undissolved even after stirring for 1 hour, the amount added at a time was further reduced and the above-mentioned maximum dissolved concentration was confirmed again. The minimum addition amount was 0.01 g. In this way, the maximum addition amount (mass of solute) at which no undissolved residue was found was calculated and calculated as the maximum dissolved concentration in 80 g of each solvent. Specifically, the following equation
[0063]
(Equation 1)

[0064]
Is the maximum dissolved concentration in the present invention. A case in which even the minimum addition amount of 0.01 g (0.013 mass%) remained undissolved was regarded as insoluble. The maximum dissolution concentration is measured at a liquid temperature of 20 ° C.
[0065]
(Reference Example 2: Measurement of maximum dissolution concentration of organic solvent in water)
80 g of a solvent at a temperature of 20 ° C. was placed in a 100 ml hermetically sealed glass container with a lid, and the target organic solvent was added dropwise while stirring with a stirrer, and the dissolved state was visually checked. If there is any undissolved portion after stirring for 1 hour, the above-mentioned maximum dissolved concentration was confirmed again by further reducing the amount of one drop. The minimum drop amount was 0.02 g. In this way, the maximum dropping amount (mass of solute) at which no undissolved residue was found was calculated and calculated as the maximum dissolution concentration in 80 g of each solvent. Specifically, the following equation
[0066]
(Equation 2)

[0067]
Is the maximum dissolved concentration in the present invention. In the case where it remained dissolved even with the minimum drop amount of 0.02 g (0.025% by mass), it was regarded as insoluble. The maximum dissolution concentration is measured at a liquid temperature of 20 ° C. These values almost coincided with the solubility data in water of the solvent described in the Chemical Handbook and Beilstein described in the text. In the present example, the maximum dissolution concentration of each solute dissolved in 80 g of the solvent was calculated according to the above formula. The maximum dissolution concentration is measured at a liquid temperature of 20 ° C. When two or more organic solvents are mixed and used for azeotropic dehydration distillation, the maximum dissolution concentration of the mixture is measured by the above-described method.
[0068]
Table 1 shows the maximum dissolved concentration data of o-xylene, methyl isobutyl ketone (MIBK), diisopropyl ketone (DIPK), diisobutyl ketone (DIBK), and dibutyl ether (DBE) evaluated according to the above.
[0069]
From Table 1, the maximum dissolution concentration of maleic acid and the maximum dissolution concentration in water showed that MIBK was relatively higher than other substances. As DIPK, DIBK and the ketone group became higher molecular weight substituents, the maximum dissolution concentration tended to be lower than that of MIBK, and MIBK dissolved maleic acid well. However, o-xylene does not dissolve maleic acid and shows little maximum concentration of these substances in water.
[0070]
[Table 1]

[0071]
(Reference Example 3: Quantitative analysis method for small components and detection limit)
(1) MIBK and acetic acid were measured by gas chromatography using a DB-5 column manufactured by J & W Scientific. The detection limit of MIBK was 5 ppm by mass, and the detection limit of acetic acid was 1 ppm by mass.
[0072]
(2) Formic acid is measured by liquid chromatography using an Inertsil ODS-3 column manufactured by GL Sciences. The detection limit was 2 ppm.
[0073]
(3) The maleic acid content in maleic anhydride is measured by the following method.
[0074]
First, a measurement sample (about 1 g) is weighed and placed in a 50 ml beaker. To this, 30 ml of acetone and an ethanol solution of bromophenol blue indicator are added. Next, using a burette, titration is performed with a 0.1 mol solution of n-ethylpiperidine prepared in advance in acetone to a point at which the color changes from yellow to bluish purple, and the titer is defined as the B value. From this titration amount, the maleic acid mass% is calculated by the following equation.
[0075]
[Equation 3]

[0076]
The force value is a value obtained by the following method.
[0077]
About 0.2 g of maleic acid is weighed and made up to 30 ml with acetone. The above bromophenol blue indicator solution is added, and titration is performed with an acetone solution of 0.1 mol of n-ethylpiperidine, and the titer is defined as an A value. From this titration amount, the maleic acid mass% is calculated by the following equation.
[0078]
(Equation 4)

[0079]
(Example 1)
Maleic anhydride was produced according to the process shown in FIG.
[0080]
First, the reaction gas containing maleic anhydride discharged by the catalytic gas phase oxidation reaction of benzene was used to control the outlet gas temperature of the maleic anhydride collector (10) to 60 ° C. to produce crude maleic anhydride (11). Collected. Thereafter, the outlet gas of the maleic anhydride collector (10) was introduced into the water-washing collector (20). Next, the gas was washed with a water-washing collector (20), and a crude maleic acid-containing aqueous solution was obtained. The maleic acid concentration of the solution was 42% by mass, and contained 58% by mass of water, 2000% by mass of acetic acid, and 70% by mass of formic acid. This crude maleic acid-containing aqueous solution was subjected to azeotropic dehydration using MIBK as an azeotropic dehydrating solvent.
[0081]
As the azeotropic dehydration tower (40), a distillation column having an enrichment section of 32φ with a weir of 5 stages as an upper section and a recovery section of 50φ with a weir of 10 stages as a lower section was used. A crude maleic acid-containing aqueous solution was supplied from the tenth stage counted from the bottom of the column. As an azeotropic solvent, MIBK was used, and 3.5 mass times of the dissolved water of the maleic acid aqueous solution and the water generated by dehydration were supplied from the top of the tower. The azeotropic dehydration distillation was performed for 8 hours while controlling the operation pressure at the top of the column at 770 hPa and the bottom temperature at 190 ° C. Table 2 shows the results. When the composition in the system became constant, the bottom liquid temperature was 190 ° C., and the bottom liquid composition was 96.90% by weight of maleic anhydride, 1.27% by weight of maleic acid, 1.77% by weight of fumaric acid, MIBK600 It was mass ppm. Also, formic acid and acetic acid were at the mass ppm below the detection limit. The presence of by-product acids, which are low-boiling substances in the aqueous solution containing crude maleic acid, in the crude maleic anhydride solution at the bottom of the column could be almost eliminated. No maleic acid was precipitated in the azeotropic dehydration tower (40). In addition, there was no precipitation of a black solid that was a condensate or a polymer of maleic acid or maleic anhydride.
[0082]
Next, as a high boiling point substance separation step (60), distillation was carried out using an empty column equipped with a demister. The operation pressure at the top of the column was set to 70 hPa, and a distillation operation was performed so that 97% by mass of the supply amount of the crude maleic anhydride obtained as the bottom solution of the azeotropic dehydration column (40) was distilled off. Subsequently, the purified maleic anhydride solution was subjected to purification distillation. As the purification distillation column (80), a distillation column having 10 stages of a 32φ weir was used. Crude maleic anhydride as a distillate in the high boiling separation step (60) was supplied to the bottom of the column, and purified maleic anhydride (61) was extracted from the third stage counted from the top of the column. The operating pressure at the top of the column was 67 hPa, the bottom temperature was 140 ° C., the reflux ratio at the top was 2.0, and the amount of crude maleic anhydride withdrawn at the top was 9 times by mass, and the amount of crude maleic anhydride was 1 mass at the top. Distillation was carried out for 8 hours so that 97% by weight of maleic anhydride was distilled off. Table 3 shows the results. In this purified maleic anhydride, MIBK was reduced below the detection limit. Formic acid and acetic acid could not be detected. The other was maleic acid having water content, and the purity of maleic anhydride was in the range of 99.86 to 99.91% by mass. The freezing point is in the range of 52.7 to 52.8 ° C., the melt color is 5, and all are JIS standards (JIS 1359/1985, purity: 99.0% by mass or more, freezing point: 52.0 ° C. or more, melt color : 50 or less).
[0083]
(Example 2)
In the azeotropic dehydration distillation of Example 1, azeotropic dehydration distillation was performed for 8 hours under the same conditions as in Example 1 except that the operation pressure at the top was 500 hPa and the temperature at the bottom was 175 ° C. When the composition in the system became constant, the bottom composition was 97.88% by weight of maleic anhydride, 1.35% by weight of maleic acid and 0.77% by weight of fumaric acid, and MIBK, formic acid and acetic acid were detected. The mass ppm was below the limit. The presence of the azeotropic solvent MIBK and the light-boiling by-products in the crude maleic acid solution in the crude maleic anhydride solution at the bottom of the column could be almost eliminated. There was no precipitation of a condensate or a black solid, which was a polymer of maleic acid or maleic anhydride, in the distillation column.
[0084]
Next, high-boiling-point separation distillation and purification distillation of the crude maleic anhydride obtained as the bottom liquid of the azeotropic dehydration distillation were carried out under the same conditions as in Example 1. MIBK, formic acid, and acetic acid could not be detected in the purified maleic anhydride. The other was maleic acid having water content, and the purity of maleic anhydride was in the range of 99.86 to 99.91% by mass. The freezing point is in the range of 52.7 to 52.8 ° C., the melt color is 5, and all are JIS standards (JIS 1359/1985, purity: 99.0% by mass or more, freezing point: 52.0 ° C. or more, melt color : 50 or less).
[0085]
(Example 3)
In the azeotropic dehydration distillation of Example 1, the azeotropic dehydration distillation was performed under the same conditions as in Example 1 except that the azeotropic dehydration distillation was performed for 8 hours while controlling the temperature in the eleventh column from the top of the column to 150 ° C. A time azeotropic dehydration distillation was performed. Table 2 shows the results. When the composition in the system becomes constant, the bottom temperature is 190 ° C., and the bottom composition is 96.92% by weight of maleic anhydride, 1.30% by weight of maleic acid, 1.78% by weight of fumaric acid, below the MIBK detection limit. , And both formic acid and acetic acid were below the detection limit by mass ppm. The presence of the azeotropic solvent supplied into the azeotropic dehydration column and the by-product acids as light boiling components in the crude maleic acid solution in the crude maleic acid solution at the bottom of the column could be almost eliminated. In the azeotropic dehydration tower (40), there was no precipitation of a condensate or a black solid which was a polymer of maleic acid or maleic anhydride.
[0086]
Next, high-boiling-point separation distillation and purification distillation of the crude maleic anhydride obtained as the bottom liquid of the azeotropic dehydration distillation were carried out under the same conditions as in Example 1. MIBK, formic acid, and acetic acid could not be detected in the purified maleic anhydride. The other was maleic acid having water content, and the purity of maleic anhydride was in the range of 99.86 to 99.91% by mass. The freezing point is in the range of 52.7 to 52.8 ° C., the melt color is 5, and all are JIS standards (JIS 1359/1985, purity: 99.0% by mass or more, freezing point: 52.0 ° C. or more, melt color : 50 or less).
[0087]
(Comparative Example 1)
In the azeotropic dehydration column (40) of Example 1, azeotropic distillation for 8 hours was carried out under the same conditions as in Example 1 except that the operating pressure at the top was set to normal pressure and the bottom temperature was set to distillation conditions of 190 ° C. Dehydration distillation was performed. When the composition in the system became constant, the composition of the bottom liquid was 95.136% by weight of maleic anhydride, 1.47% by weight of maleic acid, 1.78% by weight of fumaric acid, 1.44% by weight of MIBK, and 15% of formic acid. Ppm of acetic acid, 1730 mass ppm of acetic acid, and by-products which are low-boiling substances in the azeotropic solvent and crude maleic acid solution supplied to the azeotropic dehydration column, in the crude maleic anhydride solution at the bottom of the column. I couldn't lose my presence. Note that no maleic acid was precipitated in the distillation column. In addition, there was no precipitation of a black solid that was a condensate or a polymer of maleic acid or maleic anhydride.
[0088]
Next, the crude maleic anhydride obtained as the bottom liquid of the azeotropic dehydration tower (40) was subjected to separation distillation of high-boiling substances (60), and then supplied to the purification distillation tower (80). , A distillation operation was carried out under the same conditions except that the reflux ratio at the top of the column was 2.0. Table 3 shows the results. In the maleic anhydride obtained from the purification distillation column (80), 6400 ppm of MIBK was detected. Formic acid and acetic acid could not be detected. The other is maleic acid which is water-containing, and the purity of this purified maleic anhydride is in the range of 99.17 to 99.22% by mass, the freezing point is in the range of 52.4 to 52.5 ° C, and the molten color is It was 5. In addition, it satisfied the JIS standard (JIS 1359/1985, purity: 99.0% by mass or more, freezing point: 52.0 ° C. or more, melt color: 50 or less), but MIBK was detected in the purified maleic anhydride. Was.
[0089]
(Comparative Example 2)
In the azeotropic dehydration column (40) of Example 1, azeotropic dehydration distillation was performed for 8 hours under the same conditions as Example 1 except that the operating pressure was normal pressure and the distillation column temperature was 190 ° C. Was. When the composition in the system became constant, the composition of the bottom liquid was 95.136% by weight of maleic anhydride, 1.47% by weight of maleic acid, 1.78% by weight of fumaric acid, 1.44% by weight of MIBK, and 15% of formic acid. Ppm of acetic acid, 1730 mass ppm of acetic acid, and by-products which are low-boiling substances in the azeotropic solvent and crude maleic acid solution supplied to the azeotropic dehydration column, in the crude maleic anhydride solution at the bottom of the column. I couldn't lose my presence. No maleic acid was precipitated in the azeotropic dehydration tower (40). In addition, there was no precipitation of a black solid that was a condensate or a polymer of maleic acid or maleic anhydride.
[0090]
Next, the crude maleic anhydride obtained as a bottom liquid of the azeotropic dehydration tower (40) was subjected to separation distillation (60) of a high-boiling substance, and then supplied to a purification distillation tower (80) for purification. The distillation operation was performed under the same conditions as in Example 1 except that the reflux ratio at the top of the purification distillation column (80) was set to 7.0. Table 3 shows the results. 250 ppm of MIBK was detected in the purified maleic anhydride obtained from the purification distillation column (80). Formic acid and acetic acid could not be detected. The other is maleic acid which is water-containing, and the purity of this purified maleic anhydride is in the range of 99.84 to 99.89% by mass, the freezing point is in the range of 52.6 to 52.7 ° C, and the molten color is It was 5. In addition, it satisfied the JIS standard (JIS 1359/1985, purity: 99.0% by mass or more, freezing point: 52.0 ° C. or more, melt color: 50 or less), but MIBK was detected in the purified maleic anhydride. Was.
[0091]
(Comparative Example 3)
In the azeotropic dehydration column (40) of Example 1, azeotropic dehydration distillation was performed for 8 hours under the same conditions as Example 1 except that the operating pressure was 670 kPa and the bottom temperature was 180 ° C. . When the composition in the system became constant, the bottom temperature was 180 ° C., and the bottom liquid composition was 97.42% by weight of maleic anhydride, 1.29% by weight of maleic acid, 0.85% by weight of fumaric acid, and 4020% by weight of MIBK. Of acetic acid, which is less than the detection limit of formic acid and 380 mass ppm of acetic acid, which is supplied to the azeotropic dehydration column and which is a light boiler in the crude maleic acid solution. The presence in the acid solution could not be eliminated. In the azeotropic dehydration tower (40), there was no precipitation of a condensate or a black solid that was a polymer of maleic acid or maleic anhydride.
[0092]
Next, the crude maleic anhydride obtained as a bottom liquid of the azeotropic dehydration tower (40) was subjected to separation distillation (60) of a high-boiling substance, and then supplied to a purification distillation tower (80) for purification. The distillation operation was performed under the same conditions as in Example 1 except that the reflux ratio at the top of the purification distillation column (80) was 2.0 and 20.0. When the reflux ratio was 2.0, 3500 ppm of MIBK was detected in the purified maleic anhydride obtained from the purification distillation column (80). Formic acid and acetic acid could not be detected. The other is aqueous maleic acid. The purity of the purified maleic anhydride is in the range of 99.84 to 99.89% by mass, the freezing point is in the range of 52.6 to 52.7 ° C, and the melt color is 5. there were. Further, it satisfied the JIS standard (JIS 1359/1985, purity: 99.0% by mass or more, freezing point: 52.0 ° C. or more, melt color: 50 or less). On the other hand, when the reflux ratio was 20.0, MIBK, formic acid, and acetic acid could not be detected in the purified maleic anhydride obtained from the purification distillation column (80). The other is aqueous maleic acid. The purity of the purified maleic anhydride is in the range of 99.86 to 99.91% by mass, the freezing point is in the range of 52.7 to 52.8 ° C., and the melt color is 5. there were. All of them satisfied the JIS standard (JIS 1359/1985, purity: 99.0% by mass or more, solidification point: 52.0 ° C. or more, melt color: 50 or less). However, the condition where the reflux ratio in the purification distillation column is 20.0 is economically disadvantageous and is impossible for industrial production. When the concentration of the azeotropic solvent MIBK in the crude maleic anhydride solution at the bottom of the azeotropic dehydration column was 4020 mass ppm, the separation distillation operation of the low boiling point was omitted, and the azeotropic solvent was subsequently purified in the purification step. It has been confirmed that a reflux ratio of (80), which is industrially difficult to carry out, is necessary in order to make it below the detection limit.
[0093]
(Comparative Example 4)
Azeotropic dehydration distillation was performed under the same conditions as in Example 1 except that o-xylene was used as the azeotropic solvent. After 6 hours, when the composition in the system is considered to be substantially constant, the bottom composition is 99.99% by mass of maleic anhydride, 1.19% by mass of maleic acid, 1.80% by mass of fumaric acid, o-xylene 200 mass ppm and formic acid and acetic acid were mass ppm below the detection limit. However, the maleic acid crystals were precipitated from the 11th stage to the 14th stage in the column, counting from the top, and the deposition gradually occurred, and the tray in the column was clogged. Further, in the distillation column, black solids, which were condensates and polymers of maleic acid and maleic anhydride, were deposited, and these were gradually deposited. For this reason, the distillation operation increased instability, and it was considered impossible to perform stable operation for a longer time. By using o-xylene as the azeotropic solvent and performing azeotropic dehydration distillation under reduced pressure, o-xylene in the bottom liquid could be reduced to 200 mass ppm, but maleic acid was precipitated. Therefore, it was confirmed that continuous operation was difficult.
[0094]
[Table 2]

[0095]
[Table 3]

[0096]
(result)
When the MIBK concentration remaining in the bottom liquid is controlled to 3000 mass ppm or less using MIBK as an azeotropic solvent, the residual amount of low-boiling substances such as formic acid and acetic acid can be reduced to the detection limit or less at the same time. Was. Further, in order to control the MIBK concentration to 3000 ppm by mass or less, the distillation conditions in the azeotropic dehydration column may be simply adjusted to 770 hPa or the like, which is a reduced pressure condition, which is extremely simple.
[0097]
By purifying and distilling maleic anhydride having a MIBK concentration of 3000 mass ppm or less in an azeotropic dehydration tower, the MIBK concentration can be reduced to a detection limit or less, and at the same time, the residual amount of low boiling substances such as formic acid and acetic acid remains. Was also below the detection limit. Moreover, the removal of low-boiling substances such as formic acid and acetic acid containing an azeotropic solvent only requires controlling the reflux ratio to 2.0, which is extremely simple.
[0098]
【The invention's effect】
In the production process of maleic anhydride, when a crude maleic acid-containing aqueous solution is supplied to an azeotropic dehydration column to perform azeotropic dehydration, a specific organic solvent is used for azeotropic dehydration under reduced pressure, whereby The concentration of the remaining azeotropic solvent can be made extremely low as 3000 mass ppm, and at the same time, the concentration of the low-boiling substance can be reduced. Therefore, high-purity maleic anhydride can be produced without a conventionally required solvent or distillation column for separating low-boiling substances. In the next step, ie, a three-stage or more purification distillation column, by extracting from the top of the column from the second or lower stage, extremely pure maleic anhydride from which low boiling components can be removed can be obtained. That is, even if a distillation separation step for removing an azeotropic solvent and a low boiling point substance is omitted or a simple apparatus is used, sufficiently high-purity maleic anhydride can be obtained, and reduction in equipment and operation costs can be achieved. Can be.
[Brief description of the drawings]
FIG. 1 is a process chart of a preferred method for producing maleic anhydride in the present invention.
[Explanation of symbols]
1 ... reaction gas,
10 ... maleic anhydride collector,
11: Crude maleic anhydride,
20 ... Washing collector,
21 ... Recycled collected water,
30 ... concentrator,
40 ... azeotropic dehydration tower,
50 ... oil-water separation tank,
51 ... wastewater,
60 high-boiling point separator,
61 ... Residual,
71 ... azeotropic solvent,
80 ... refining tower,
81 ... Purified maleic anhydride
90 ... Solvent recovery tower.

Claims (5)

A crude maleic acid-containing aqueous solution is supplied to an azeotropic dehydration tower, and azeotropic dehydration is performed using an azeotropic solvent that azeotropes with water to dehydrate the crude maleic acid contained therein to form maleic anhydride. In the method for producing maleic anhydride to obtain the maleic anhydride from the bottom liquid of the column,
An organic solvent having a maximum dissolution concentration of maleic acid at a temperature of 20 ° C. of 0.2% by mass or more as an azeotropic solvent,
A method for producing maleic anhydride, comprising setting the pressure at the top of an azeotropic dehydration column to 900 hPa or less, and further adjusting the concentration of the azeotropic solvent contained in the bottom liquid in the azeotropic dehydration column to 3000 mass ppm or less. . 2. The method according to claim 1, wherein the azeotropic solvent is an organic solvent having a maximum dissolution concentration in water at a temperature of 20 [deg.] C. of 0.3 to 5% by mass as a property further exhibited by the azeotropic solvent. . Further, the bottom liquid of the azeotropic dehydration column is supplied to a purification distillation column having three or more theoretical distillation stages, and anhydrous from the second or lower stage of the theoretical number of distillation plates counted from the top of the purification distillation column. 3. The method according to claim 1, further comprising a purification distillation step of purifying while extracting maleic acid. The method according to claim 3, wherein distillation is performed in the purification distillation column at a reflux ratio of 0.1 to 10. The method according to any one of claims 1 to 4, wherein the azeotropic solvent is a ketone or an ester having a boiling point at a pressure of 1013 hPa in the range of 80 to 170 ° C.

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