JP4162883B2 - Method for recovering ethylene carbonate - Google Patents

Description

【００１１】
触媒としては、慣用のエステル交換触媒を選択することができる。具体的には、均一系触媒として、トリエチルアミン等のアミン類、ナトリウム等のアルカリ金属、クロロ酢酸ナトリウムやナトリウムメチラート等のアルカリ金属化合物、及びタリウム化合物が、また不均一系触媒としては、官能基により変性したイオン交換樹脂、アルカリ金属又はアルカリ土類金属の珪酸塩を含浸した無定型シリカ類、アンモニウム交換Ｙ型ゼオライト、コバルトとニッケルとの混合酸化物等が、それぞれ例示できる。なお、均一系触媒を使用する場合は、逆反応の進行や副反応の発生を避けるために、蒸留分離に先立ち触媒を分離することが好ましい。
【００１２】
エステル交換の反応条件としては、反応温度は５０〜１８０℃で、メタノールとエチレンカーボネートの仕込みモル比は２〜２０とするのが一般的である。このモル比が２未満ではエステル交換の転化率が低下し、一方２０を超えると多量の未反応原料が系内に残留し、加熱・冷却等のエネルギーを多く要したり、リサイクル使用する場合の設備への負担が増す等の問題がある。
エステル交換反応は、バッチ反応によるもの（米国特許第３６４２８５８号明細書、特開昭５４−４８７１５号公報等）、反応釜の上部に蒸留塔を設置して連続的に生成したジメチルカーボネートを除去しながら反応を行うもの（米国特許第３８０３２０１号明細書等）、管状リアクターによる連続反応（特開昭６３−４１４３２号公報等）等、いずれの反応形式も適用可能である。
【００１３】
蒸留分離
前記式（１）で示されるエステル交換反応は平衡反応であって、エステル交換反応液中には、同式右辺の製品ジメチルカーボネート及び副生物エチレングリコールの他に、同式左辺のエチレンカーボネート及びメタノールが未反応のまま残存する。そこで、これらの成分を分離し、製品は取り出し、未反応原料は再使用するため、通常、蒸留分離が行われる。特に、低沸点成分である、未反応メタノール（沸点６４．７℃）及び製品ジメチルカーボネート（沸点９０．３℃）を分離した後のエステル交換反応液は、未反応エチレンカーボネート（沸点２４８．２℃）及び副生エチレングリコール（沸点１９７．３℃）からなり、該反応液から、未反応エチレンカーボネートを回収するのも、通常、蒸留分離による。
【００１４】
本発明は、蒸留分離により、エステル交換反応原料として十分な純度を有するエチレンカーボネートを回収する方法であり、同一蒸留塔に、複数種のエチレンカーボネート供給物を供給する。
すなわち、第１の供給物は、実質的にエチレングリコールのみを含むエチレンカーボネート供給物（Ａ）であり、通常、エチレンカーボネートとメタノールのエステル交換反応生成物から、未反応のメタノール及びジメチルカーボネートを分離した後の液状混合物が用いられる。なお、本発明において、実質的にエチレングリコールのみを含むとは、エチレングリコール以外の成分の含有量が５重量％以下、好ましくは１重量％以下のエチレンカーボネートとエチレングリコールの混合物を言う。
また、第２の供給物は、エチレングリコール以外に少なくとも水を含むエチレンカーボネート供給物（Ｂ）であり、通常、水の存在下、酸化エチレンと炭酸ガスとの反応により得られた生成物（以下、「ＥＣ化反応生成物」ともいう。）が用いられる。
【００１５】
本発明においては、上記供給物（Ａ）及び上記供給物（Ｂ）を、蒸留塔の所定位置に供給して連続蒸留を行い、塔頂から水を留出させ、塔底から水を含まない８０重量％以上、好ましくは９０重量％以上の純度のエチレンカーボネートを缶出させる。
しかして、塔頂からの留出物の組成は、両供給物の組成によって相違するが、供給物（Ｂ）として上記ＥＣ化反応生成物を用いた場合には、水の他に炭酸ガス及びエチレングリコールを含む留出物が得られるので、冷却凝縮して一部を蒸留塔に還流する。通常、凝縮液はエチレングリコール／エチレンカーボネートの共沸物で若干の水を伴い、非凝縮ベーパーは、水蒸気と炭酸ガスの混合物でエチレングリコール／エチレンカーボネートを同伴する。
【００１６】
この連続蒸留分離に際しては、エチレングリコールを主成分とする側流ベーパーを抜き出すことが必要であり、この高温ベーパーの抜き出しを行うことによって初めて、塔頂から水を留出させながらも、凝縮による熱回収が可能となり、エネルギー消費量の削減に有効である。
エチレングリコールは、前述の通り、水より沸点が高く、かつエチレンカーボネートより沸点が低いために、蒸留塔の濃縮部においてベーパー中に濃縮された部分が存在する。この部分の温度は塔頂に比べて高いので、ここから側流ベーパーを抜き出すことによりエチレングリコールを主成分とする温度の高いベーパーの凝縮熱を回収することが可能となる。
【００１７】
しかして、蒸留操作に際して、前記両供給物の供給位置及び側流ベーパーの抜き出し位置は、次の規準に従うことが必要である。
１）蒸気供給物（Ａ）の供給位置より上段の位置に、好ましくは理論段数で少なくとも２段上段の位置に、最も好ましくは塔頂位置に上記供給物（Ｂ）を供給すること。
２）両供給物の供給位置の中間の位置、好ましくは上記供給物（Ａ）の供給位置より、理論段数で少なくとも１段、上段の位置から、エチレングリコールを主成分とする側流ベーパーを抜き出すこと。
また、このようにして抜き出された、エチレングリコールを主成分とする側流ベーパーから、熱回収する方法は、特に制限はないが、例えば、上記側流ベーパーを、熱交換器で低温の液体と接触させることにより、凝縮熱を加熱源として利用する方法、上記側流ベーパーの凝縮熱で水蒸気を発生させ、それを加熱源として利用する方法等が好ましい。
【００１８】
蒸留分離に必要な段数（リボイラー、コンデンサーを除く塔本体）は、還流比によって異なるが、経済性を考慮すると５段以上が好ましく、１０段以上がさらに好ましい。還流比は０．０１〜１が好ましく、０．０１〜０．５がさらに好ましい。
蒸留塔の操作圧力は、運転温度の上昇により製品純度が低下するのを防止するため、大気圧以下とする。ただし、圧力を下げすぎると、分離効率、エネルギー効率が悪化するとともに側流ベーパーの温度が低下して熱回収が困難になるので、適切な圧力を選択する必要がある。適切な圧力範囲は２７ｋＰａ以下、好ましくは４〜１３ｋＰａの範囲である。
蒸留塔は、スルザーパッキング、メラパック、ＭＣパック等の規則充填物や、ＩＭＴＰ、ラシヒリング等の不規則充填物を充填した充填塔型式、泡鐘塔、シーブトレイ、バルブトレイ塔を用いた棚段塔型式等、いずれの型式を用いることもできる。
【００１９】
ＥＣ化反応
上記エステル交換反応の原料となるエチレンカーボネートは、通常、下記式（２）で表されるＥＣ化反応により、触媒の存在下、酸化エチレンと炭酸ガスから生成する。ＥＣ化反応触媒としては、ホスホニウム塩が活性が高くかつ再使用可能である点で好ましい。また、助触媒としてアルカリ金属炭酸塩を用いるのが、より好ましい。
【００２０】
【化２】

【００２１】
上記式（２）で表される付加反応は、非水系では高選択率で進行するが、反応速度が非常に遅く、実用的ではない。したがって、実際には、水の存在下に行うのが、反応速度を向上させることができて効果的である。しかし同時に、生成したエチレンカーボネートの一部が、下記式（３）で表される加水分解反応により、エチレングリコールとなり、エチレンカーボネートの含有率が低下する。
【００２２】
【化３】

【００２３】
ＥＣ化反応に用いる反応器の形式は、特に限定されるものではないが、底部から酸化エチレン、二酸化炭素、水及び触媒を供給し、頂部から生成するエチレンカーボネートを含む反応液と未反応の二酸化炭素等を留出させる、気泡塔型反応器を用いるのが効果的である。
この反応は、温度７０〜２００℃、好ましくは１００〜１７０℃、圧力４９０〜４９００ｋＰａ・Ｇ（ゲージ圧）、好ましくは９８０〜２９４０ｋＰａ・Ｇで行うのが一般的である。酸化エチレンに対する二酸化炭素及び水の供給量（モル比）は、それぞれ、通常０．１〜５及び０．１〜１０であるが、好ましくは、それぞれ０．５〜３及び０．５〜５とするのがよい。
また、この反応は発熱反応であるので、反応液の一部を外部に抜き出して、熱交換器により冷却した後で系内に返送するという、外部循環方式により反応温度を制御するのが好ましい。
【００２４】
ＥＧ製造への利用
前記蒸留分離に際して、塔頂から分取される、水とエチレングリコールを主成分とする低沸点成分、又は、エチレングリコールとエチレンカーボネートの共沸混合物は、エチレンカーボネートの加水分解によるＥＧの製造に利用することができる。
加水分解反応は、通常、１００〜１８０℃の温度、常圧〜１９６０ｋＰａ・Ｇの圧力で行われ、ＥＣ化反応触媒を含むエチレングリコール水溶液が得られ、加水分解反応副生物の二酸化炭素が気相で分離される。該水溶液は、水を塔頂から除去する脱水蒸留と、ＥＧとＥＣ化反応触媒を含む高沸点成分とを分離する精留とを組み合わせて行い、製品ＥＧが取得され、再利用可能成分が回収される。このようなＥＧ製造諸工程の採用は、経済性を確保するために望ましい。
【００２５】
【実施例】
（エチレンカーボネートの合成）
攪拌機付きオートクレーブに、水を４１重量％含む含水酸化エチレンを２２３４ｇ、触媒のトリブチルメチルホスホニウムアイオダイドを１１２ｇ、助触媒の炭酸カリウム４ｇを仕込み、元圧２１００ｋＰａの炭酸ガスを連続的に２０００ｋｇ／ｈで供給しながら、２０００ｋＰａ、１１０℃で３０分間反応させた。その後、反応液を８０℃まで冷却して５３ｋＰａに減圧し、主として未反応の炭酸ガスと酸化エチレンからなる気相を分離した。分離した気相は上記ＥＣ化反応に循環再利用することができる。一方、気相を分離した後の反応液は、分析したところ、エチレンカーボネートが４６重量％、エチレングリコールが３１重量％、水が１９重量％、触媒が４重量％で、未反応の酸化エチレンは０．１重量％未満であった。
【００２６】
（ＥＣ化触媒分離）
上記気相分離後の反応液は、２ｋＰａの減圧下で回分蒸留を行い、釜残として触媒を濃縮分離した。分離した触媒含有釜残は、前記ＥＣ化反応に循環再利用することができる。得られた留出液には、エチレンカーボネートが４７重量％、エチレングリコールが３２重量％、水が２０重量％含有されていた。これを、後記エチレンカーボネートの回収に際し、エチレンカーボネート供給物（Ｂ）として利用した。
【００２７】
（ジメチルカーボネートの合成）
メタノールとエチレンカーボネートを、モル比で４：１に調整し、四級アンモニウム塩構造がスペーサー基を介して架橋ポリスチレン鎖に結合したアニオン交換樹脂を充填した固定床反応器に通した。反応温度を８０℃、反応圧力を７００ｋＰａとし、ＬＨＳＶ＝２でエステル交換反応させたところ、エチレンカーボネート３８重量％、メタノール２８重量％、エチレングリコール１４重量％、ジメチルカーボネート２０重量％の反応液が得られた。
【００２８】
（ジメチルカーボネート反応液の蒸留分離）
上記のエステル交換反応液を、理論段数２０段の連続蒸留装置の１０段目に供給し、還流比１で蒸留分離を行い、塔頂からメタノールとジメチルカーボネートを留出させ、塔底からエチレンカーボネートとエチレングリコールの混合液を缶出させた。留出物からは、さらに未反応メタノールを回収し、製品ジメチルカーボネートを取得することができる。この缶出液の組成は、エチレンカーボネートが７３重量％、エチレングリコールが２７重量％であった。ジメチルカーボネートは１０重量ｐｐｍ未満で、メタノールは検出されなかった。この缶出液を、下記エチレンカーボネートの回収に際し、エチレンカーボネート供給物（Ａ）として利用した。
【００２９】
（エチレンカーボネートの回収）
図１の概念図に示す蒸留塔を用いて、エチレンカーボネートの回収を行った。図中、Ｓ１〜Ｓ６は、ストリーム番号を示し、○の中の文字は理論段数を示す。すなわち、理論段数１２段（コンデンサーとリボイラーを除く）の蒸留塔の、第１段に前記エチレンカーボネート供給物（Ｂ）を１２ｋｇ／ｈで、第４段に上記エチレンカーボネート供給物（Ａ）を６．４ｋｇ／ｈで供給した。
塔頂の圧力を８ｋＰａ、コンデンサー凝縮温度を５０℃、還流比を０．０５とし、塔頂留出の水を３４重量％含む塔頂液２．４ｋｇ／ｈ、塔底缶出のエチレンカーボネートを９５重量％含む１６０℃の塔底液９．５ｋｇ／ｈを連続的に抜きながら、第２段からエチレングリコールを７８重量％含む１２９℃の側流ベーパー５ｋｇ／ｈを抜き出した。コンデンサー非凝縮の塔頂ベーパー１．５ｋｇ／ｈは２５℃のベントコンデンサーで全凝縮した。操作条件と結果の詳細を、表−１と表−２に示した。
【００３０】
【表１】

【００３１】
【表２】

【００３２】
この側流ベーパーを熱交換器に供給し、８０℃の水と熱交換させ、その凝縮熱を回収した。このときの、側流ベーパーの凝縮熱は５ＭＪ／ｈであった。この凝縮熱で１０４ｋＰａの蒸気を発生させるとその量は２．１ｋｇ／ｈである。他方、リボイラー加熱に必要なエネルギーは１４ＭＪ／ｈであったので、リボイラーで消費した熱量の約３６％を１００℃の水蒸気として回収でき、この水蒸気を配管や装置類の加熱、保温に使用することができる。
【００３３】
なお、上記の操作で得られた塔底液の組成は、エチレンカーボネート９５重量％、エチレングリコール５重量％であり、ジメチルカーボネートの合成原料として、前記エステル交換反応に循環再使用できる。
【００３４】
【比較例】
実施例においてエチレンカーボネートの回収にかかる蒸留分離の目的はジメチルカーボネートの原料となるエチレンカーボネートを分離、回収することである。その場合、通常は供給液に含まれるエチレンカーボネート以外の成分を塔頂から抜き出すのが普通である。
そこで実施例と同じ操作で調製した２種類のエチレンカーボネート供給物（Ａ）、（Ｂ）を、実施例と同じ蒸留塔の同じ位置に供給して、塔底からエチレンカーボネートを９５重量％、エチレングリコールを５重量％含む缶出液９．５ｋｇ／ｈを抜き出し、その他の成分をすべて塔頂から抜き出した。操作条件は、側流ベーパーを抜かないことを除いて、実施例と同一である。操作条件と結果を表−３、表−４に示した。
【００３５】
【表３】

【００３６】
【表４】

【００３７】
塔頂液組成は、水２７重量％、エチレングリコール５８重量％、エチレンカーボネート１５重量％であった。リボイラー加熱に必要なエネルギーは１４ＭＪ／ｈであった。塔頂液の温度は５３℃なので、熱回収は不可能である。
【００３８】
【発明の効果】
以上説明したように、この発明によれば、エチレングリコール製造工程の中間ストリームとして得られる炭酸ガス、水、エチレングリコールを含有するエチレンカーボネート混合物、及びエステル交換反応の未反応エチレンカーボネートと副生エチレングリコールの混合物からエステル交換反応原料となるエチレンカーボネートを分離するにあたり、蒸留分離に要するエネルギーの一部を回収してエネルギー消費量を削減できるという効果が得られる。
【図面の簡単な説明】
【図１】 蒸留塔概念図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for recovering ethylene carbonate having a purity usable for transesterification by distilling a mixture containing ethylene carbonate and ethylene glycol.
In particular, in the step of producing dimethyl carbonate by transesterifying ethylene carbonate and methanol, a crude ethylene carbonate mixture containing carbon dioxide gas, water, and ethylene glycol is used to convert unreacted ethylene carbonate and by-product ethylene glycol in the transesterification reaction. The present invention relates to a method for reducing energy consumption when distilling together with a mixture to separate ethylene carbonate.
[0002]
[Prior art]
As a method for producing dimethyl carbonate, a oxidative carbonylation method of methanol, a methanolysis method of urea, a transesterification method and the like have been put to practical use or proposed.
Among these, as a proposal regarding a method for producing dimethyl carbonate by transesterification of ethylene carbonate and methanol, a method based on a batch reaction (US Pat. No. 3,642,858, JP-A-54-48715, etc.), reaction A distillation column is installed at the top of the kettle to perform the reaction while removing continuously formed dimethyl carbonate (US Pat. No. 3,803,201, etc.), continuous reaction using a tubular reactor (Japanese Patent Laid-Open No. 63-41432) Etc.), all of which are based on the premise that high-purity ethylene carbonate is used as a raw material.
[0003]
When using the intermediate product of the process of producing ethylene glycol from ethylene oxide and carbon dioxide via ethylene carbonate as a raw material for transesterification dimethyl carbonate production, carbon dioxide, water, and ethylene glycol are contained in the raw material. Therefore, it is necessary to separate and purify ethylene carbonate before supplying it to the transesterification reactor. The reason why water is removed is that ethylene carbonate may be hydrolyzed by the presence of water, and the reason why ethylene glycol is removed is because ethylene glycol is produced as a byproduct of the transesterification reaction. This is to return the material to the raw material system. For purification of ethylene carbonate, there are a distillation method and a crystallization method (Japanese Patent Laid-Open No. 7-89905), but distillation has the disadvantage that it requires energy costs and crystallization requires equipment costs.
[0004]
When distilling and purifying ethylene carbonate containing ethylene glycol by distillation, ethylene carbonate and ethylene glycol azeotrope. Therefore, usually, while refluxing under reduced pressure, an azeotropic mixture of ethylene carbonate and ethylene glycol from the top of the tower, Remove ethylene carbonate from the bottom. Under normal pressure, ethylene carbonate has a boiling point of 248 ° C. and ethylene glycol has a temperature of 197 ° C. Even under reduced pressure, the temperature of these vapors is high, so that heat can be recovered in the distillation condenser. However, when water is added to this, water is distilled off from the top of the tower, so that the temperature at the top of the tower is lowered and heat recovery from the fluid extracted from the top of the tower becomes impossible.
[0005]
[Problems to be solved by the invention]
In the conventional method for producing dimethyl carbonate, it is necessary to procure high-purity ethylene carbonate as a raw material or to separate and purify ethylene carbonate prior to the transesterification reaction, the former being the raw material procurement cost and the latter being the energy required for separation. In all cases, there was a problem that the cost of manufacturing dimethyl carbonate increased.
[0006]
That is, the present invention is a method of distilling an ethylene carbonate mixture containing carbon dioxide gas, water, and ethylene glycol obtained as an intermediate product of a process of producing ethylene glycol from ethylene oxide and carbon dioxide via ethylene carbonate. An object of the present invention is to provide an efficient method for producing dimethyl carbonate on the assumption that ethylene carbonate having a purity usable for exchange reaction is recovered.
[0007]
[Means for Solving the Problems]
As a result of various investigations to solve the above problems, the inventors of the present invention usually include a liquid composition extracted from the top of a distillation column when the ethylene carbonate contained in the raw material is separated by distillation. The inventors have found that energy consumption during the production of dimethyl carbonate can be reduced by taking out a part of ethylene glycol as sidestream vapor and recovering its condensation heat, and have reached the present invention.
[0008]
That is, the gist of the present invention is that at least water other than ethylene carbonate feed (A) and ethylene glycol , which are liquid mixtures after separation of unreacted methanol and dimethyl carbonate from the transesterification product of ethylene carbonate and methanol. The ethylene carbonate feed containing (B) is fed to the same distillation column to carry out continuous distillation, water is distilled from the top of the column, and ethylene carbonate having a purity of 80% by weight or more not containing water is obtained from the bottom of the column. In the method for recovering ethylene carbonate to be discharged, the supply (B) is supplied to a position above the supply position of the supply (A), and ethylene glycol is supplied from an intermediate position between the supply positions of both supplies. consists in the recovery of ethylene carbonate, wherein the withdrawn side stream vapor mainly containing
Another gist is that the feed (B) is a product obtained by reaction of ethylene oxide and carbon dioxide in the presence of water. Another gist is to use, as a heating source, the heat of condensation of a sidestream vapor mainly composed of ethylene glycol, extracted continuously in the distillation operation. Yet another gist lies in a method for producing dimethyl carbonate, characterized in that ethylene carbonate obtained by the method for recovering ethylene carbonate is used as a raw material for a transesterification reaction between ethylene carbonate and methanol.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Transesterification reaction The reaction is an ester exchange reaction of ethylene carbonate and methanol in the presence of a catalyst to produce dimethyl carbonate and ethylene glycol, and is represented by the following formula (1).
[0010]
[Chemical 1]

[0011]
As the catalyst, a conventional transesterification catalyst can be selected. Specifically, amines such as triethylamine, alkali metals such as sodium, alkali metal compounds such as sodium chloroacetate and sodium methylate, and thallium compounds as homogeneous catalysts, and functional groups as heterogeneous catalysts Examples thereof include ion-exchange resins modified by the above, amorphous silicas impregnated with alkali metal or alkaline earth metal silicates, ammonium-exchanged Y-type zeolite, mixed oxides of cobalt and nickel, and the like. When a homogeneous catalyst is used, it is preferable to separate the catalyst prior to distillation separation in order to avoid the progress of the reverse reaction and the occurrence of side reactions.
[0012]
As reaction conditions for transesterification, the reaction temperature is generally 50 to 180 ° C., and the molar ratio of methanol to ethylene carbonate is generally 2 to 20. If this molar ratio is less than 2, the transesterification conversion rate decreases, while if it exceeds 20, a large amount of unreacted raw material remains in the system, which requires a large amount of energy such as heating and cooling, or is used for recycling. There are problems such as increasing the burden on equipment.
The transesterification reaction is based on a batch reaction (US Pat. No. 3,642,858, JP-A-54-48715, etc.), and a distillation column is installed at the top of the reaction kettle to remove continuously formed dimethyl carbonate. Any type of reaction can be applied, such as those in which the reaction is carried out (US Pat. No. 3,803,201, etc.) and continuous reaction in a tubular reactor (JP-A-63-41432, etc.).
[0013]
Distillation separation The transesterification reaction represented by the formula (1) is an equilibrium reaction, and in the transesterification reaction solution, in addition to the product dimethyl carbonate and the by-product ethylene glycol on the right side of the formula, The left side ethylene carbonate and methanol remain unreacted. Therefore, these components are separated, the product is taken out, and the unreacted raw material is reused. Therefore, distillation separation is usually performed. In particular, the transesterification reaction liquid after separating unreacted methanol (boiling point 64.7 ° C.) and product dimethyl carbonate (boiling point 90.3 ° C.), which are low boiling components, was unreacted ethylene carbonate (boiling point 248.2 ° C.). ) And by-product ethylene glycol (boiling point: 197.3 ° C.), and it is usually by distillation separation that unreacted ethylene carbonate is recovered from the reaction solution.
[0014]
The present invention is a method for recovering ethylene carbonate having sufficient purity as a raw material for transesterification by distillation separation, and a plurality of types of ethylene carbonate feeds are supplied to the same distillation column.
That is, the first feed is an ethylene carbonate feed (A) that contains substantially only ethylene glycol, and usually separates unreacted methanol and dimethyl carbonate from the transesterification product of ethylene carbonate and methanol. The resulting liquid mixture is used. In the present invention, “substantially containing only ethylene glycol” means a mixture of ethylene carbonate and ethylene glycol in which the content of components other than ethylene glycol is 5% by weight or less, preferably 1% by weight or less.
The second feed is an ethylene carbonate feed (B) containing at least water in addition to ethylene glycol, and usually a product obtained by reaction of ethylene oxide and carbon dioxide in the presence of water (below) , Also referred to as “EC conversion reaction product”).
[0015]
In the present invention, the above feed (A) and the above feed (B) are supplied to a predetermined position of a distillation column to perform continuous distillation, and water is distilled from the top of the column, and no water is contained from the bottom of the column. Ethylene carbonate having a purity of 80% by weight or more, preferably 90% by weight or more is decanted.
Thus, the composition of the distillate from the top of the column varies depending on the composition of both feeds. However, when the above EC reaction product is used as the feed (B), carbon dioxide and water in addition to water. Since a distillate containing ethylene glycol is obtained, it is cooled and condensed, and a part thereof is refluxed to the distillation column. Usually, the condensate is an ethylene glycol / ethylene carbonate azeotrope with some water, and the non-condensing vapor is accompanied by ethylene glycol / ethylene carbonate with a mixture of water vapor and carbon dioxide.
[0016]
In this continuous distillation separation, it is necessary to extract a side stream vapor containing ethylene glycol as a main component, and it is not until the high temperature vapor is extracted that water is distilled from the top of the tower and the heat generated by condensation is removed. Recovery is possible and effective in reducing energy consumption.
As described above, ethylene glycol has a boiling point higher than that of water and lower than that of ethylene carbonate. Therefore, there is a portion concentrated in the vapor in the concentration portion of the distillation column. Since the temperature of this portion is higher than that at the top of the column, it is possible to recover the heat of condensation of the vapor having a high temperature mainly composed of ethylene glycol by extracting the side flow vapor therefrom.
[0017]
Therefore, in the distillation operation, it is necessary to follow the following criteria for the supply position of the both feeds and the extraction position of the side flow vapor.
1) Supply the feed (B) to a position above the supply position of the steam feed (A), preferably at least two theoretical stages, and most preferably to the top position.
2) A sidestream vapor containing ethylene glycol as a main component is extracted from a position intermediate between the supply positions of both the supplies, preferably at least one theoretical plate from the supply position of the supply (A). thing.
Further, there is no particular limitation on the method of heat recovery from the sidestream vapor mainly composed of ethylene glycol extracted in this manner. For example, the sidestream vapor is cooled with a low-temperature liquid using a heat exchanger. A method of using the heat of condensation as a heat source by contacting with water, a method of generating water vapor with the heat of condensation of the sidestream vapor and using it as a heat source, and the like are preferable.
[0018]
The number of stages required for distillation separation (column main body excluding reboiler and condenser) varies depending on the reflux ratio, but is preferably 5 or more, more preferably 10 or more in consideration of economy. The reflux ratio is preferably from 0.01 to 1, and more preferably from 0.01 to 0.5.
The operation pressure of the distillation column is set to atmospheric pressure or lower in order to prevent the product purity from being lowered due to an increase in operating temperature. However, if the pressure is lowered too much, the separation efficiency and energy efficiency are deteriorated, and the temperature of the sidestream vapor is lowered to make it difficult to recover the heat. Therefore, it is necessary to select an appropriate pressure. A suitable pressure range is 27 kPa or less, preferably 4 to 13 kPa.
Distillation towers are packed with regular packings such as sulzer packing, melapack, MC pack, etc., packed towers packed with irregular packings such as IMTP, Raschig rings, etc. Any type can be used.
[0019]
EC conversion reaction Ethylene carbonate, which is a raw material for the transesterification reaction, is usually generated from ethylene oxide and carbon dioxide in the presence of a catalyst by an EC conversion reaction represented by the following formula (2). As the EC conversion reaction catalyst, a phosphonium salt is preferable because of its high activity and reusability. It is more preferable to use an alkali metal carbonate as a promoter.
[0020]
[Chemical 2]

[0021]
The addition reaction represented by the above formula (2) proceeds with high selectivity in a non-aqueous system, but the reaction rate is very slow and is not practical. Therefore, in practice, it is effective to increase the reaction rate in the presence of water. However, at the same time, part of the produced ethylene carbonate becomes ethylene glycol by the hydrolysis reaction represented by the following formula (3), and the content of ethylene carbonate decreases.
[0022]
[Chemical 3]

[0023]
The type of the reactor used for the EC conversion reaction is not particularly limited, but ethylene oxide, carbon dioxide, water and a catalyst are supplied from the bottom, and a reaction liquid containing ethylene carbonate generated from the top and unreacted dioxide. It is effective to use a bubble column reactor that distills carbon or the like.
This reaction is generally performed at a temperature of 70 to 200 ° C., preferably 100 to 170 ° C., and a pressure of 490 to 4900 kPa · G (gauge pressure), preferably 980 to 2940 kPa · G. Carbon dioxide and water supply amounts (molar ratio) to ethylene oxide are usually 0.1 to 5 and 0.1 to 10, respectively, preferably 0.5 to 3 and 0.5 to 5 respectively. It is good to do.
Further, since this reaction is an exothermic reaction, it is preferable to control the reaction temperature by an external circulation system in which a part of the reaction solution is extracted outside, cooled by a heat exchanger and then returned to the system.
[0024]
Use for EG production In the distillation separation, a low boiling point component mainly composed of water and ethylene glycol, or an azeotrope of ethylene glycol and ethylene carbonate, collected from the top of the column, is ethylene carbonate. It can be used for the production of EG by hydrolysis of.
The hydrolysis reaction is usually carried out at a temperature of 100 to 180 ° C. and a pressure of normal pressure to 1960 kPa · G to obtain an ethylene glycol aqueous solution containing an EC conversion reaction catalyst. Separated by The aqueous solution is a combination of dehydration distillation that removes water from the top of the column and rectification that separates EG and high-boiling components containing an EC reaction catalyst to obtain product EG and recover reusable components. Is done. The adoption of such various EG manufacturing processes is desirable for ensuring economic efficiency.
[0025]
【Example】
(Synthesis of ethylene carbonate)
An autoclave equipped with a stirrer was charged with 2234 g of hydrous ethylene containing 41% by weight of water, 112 g of tributylmethylphosphonium iodide as a catalyst, and 4 g of potassium carbonate as a cocatalyst. Carbon dioxide with an original pressure of 2100 kPa was continuously added at 2000 kg / h. While feeding, the reaction was carried out at 2000 kPa and 110 ° C. for 30 minutes. Thereafter, the reaction solution was cooled to 80 ° C. and decompressed to 53 kPa, and a gas phase mainly composed of unreacted carbon dioxide and ethylene oxide was separated. The separated gas phase can be recycled for the EC conversion reaction. On the other hand, the analysis of the reaction liquid after separating the gas phase revealed that ethylene carbonate was 46% by weight, ethylene glycol was 31% by weight, water was 19% by weight, catalyst was 4% by weight, and unreacted ethylene oxide was It was less than 0.1% by weight.
[0026]
(EC catalyst separation)
The reaction solution after the gas phase separation was subjected to batch distillation under reduced pressure of 2 kPa, and the catalyst was concentrated and separated as the residue in the kettle. The separated catalyst-containing kettle residue can be recycled and reused in the EC conversion reaction. The obtained distillate contained 47% by weight of ethylene carbonate, 32% by weight of ethylene glycol, and 20% by weight of water. This was utilized as an ethylene carbonate feed (B) in recovering ethylene carbonate described later.
[0027]
(Synthesis of dimethyl carbonate)
Methanol and ethylene carbonate were adjusted to a molar ratio of 4: 1 and passed through a fixed bed reactor filled with an anion exchange resin in which a quaternary ammonium salt structure was bonded to a crosslinked polystyrene chain via a spacer group. A transesterification reaction was performed at 80 ° C., a reaction pressure of 700 kPa, and LHSV = 2 to obtain a reaction solution of 38% by weight of ethylene carbonate, 28% by weight of methanol, 14% by weight of ethylene glycol, and 20% by weight of dimethyl carbonate. It was.
[0028]
(Distillation separation of dimethyl carbonate reaction liquid)
The above transesterification reaction solution is supplied to the 10th stage of a continuous distillation apparatus having a theoretical plate number of 20 and subjected to distillation separation at a reflux ratio of 1, distilling methanol and dimethyl carbonate from the tower top, and ethylene carbonate from the tower bottom. And a mixed solution of ethylene glycol was taken out. Unreacted methanol can be further recovered from the distillate to obtain product dimethyl carbonate. The composition of the bottoms was 73% by weight of ethylene carbonate and 27% by weight of ethylene glycol. Dimethyl carbonate was less than 10 ppm by weight and no methanol was detected. The bottoms were used as ethylene carbonate feed (A) in the recovery of the following ethylene carbonate.
[0029]
(Recovery of ethylene carbonate)
Ethylene carbonate was recovered using the distillation tower shown in the conceptual diagram of FIG. In the figure, S1 to S6 indicate stream numbers, and the characters in the circles indicate the number of theoretical plates. That is, in a distillation column having 12 theoretical plates (excluding the condenser and reboiler), the ethylene carbonate feed (B) is 12 kg / h in the first stage and the ethylene carbonate feed (A) is 6 in the fourth stage. Supplied at 4 kg / h.
The top pressure is 8 kPa, the condenser condensation temperature is 50 ° C., the reflux ratio is 0.05, the top liquid 2.4 kg / h containing 34% by weight of water distilled from the top of the tower, While 9.5 kg / h of the bottom liquid at 160 ° C. containing 95% by weight was continuously withdrawn, 5 kg / h of side flow vapor at 129 ° C. containing 78% by weight of ethylene glycol was withdrawn from the second stage. Non-condensed tower top vapor 1.5 kg / h was completely condensed in a 25 ° C. vent condenser. Details of the operating conditions and results are shown in Tables 1 and 2.
[0030]
[Table 1]

[0031]
[Table 2]

[0032]
This sidestream vapor was supplied to a heat exchanger, and heat exchange was performed with water at 80 ° C., and the heat of condensation was recovered. At this time, the condensation heat of the sidestream vapor was 5 MJ / h. When 104 kPa of steam is generated by this heat of condensation, the amount is 2.1 kg / h. On the other hand, the energy required for reboiler heating was 14 MJ / h, so about 36% of the heat consumed by the reboiler can be recovered as water vapor at 100 ° C, and this water vapor can be used for heating and heat insulation of pipes and devices. Can do.
[0033]
The composition of the bottom liquid obtained by the above operation is 95% by weight of ethylene carbonate and 5% by weight of ethylene glycol, and can be reused in the transesterification reaction as a synthesis raw material for dimethyl carbonate.
[0034]
[Comparative example]
In the examples, the purpose of distillation separation for recovery of ethylene carbonate is to separate and recover ethylene carbonate as a raw material for dimethyl carbonate. In that case, it is usual to extract components other than ethylene carbonate contained in the feed liquid from the top of the column.
Therefore, two types of ethylene carbonate feeds (A) and (B) prepared by the same operation as in the example were supplied to the same position in the same distillation column as in the example, and 95 wt% ethylene carbonate from the bottom of the ethylene, 9.5 kg / h of bottoms containing 5% by weight of glycol was withdrawn, and all other components were withdrawn from the top of the column. The operating conditions are the same as in the examples except that the side flow vapor is not removed. The operating conditions and results are shown in Table-3 and Table-4.
[0035]
[Table 3]

[0036]
[Table 4]

[0037]
The top liquid composition was 27% by weight of water, 58% by weight of ethylene glycol, and 15% by weight of ethylene carbonate. The energy required for reboiler heating was 14 MJ / h. Since the temperature of the top liquid is 53 ° C., heat recovery is impossible.
[0038]
【The invention's effect】
As described above, according to the present invention, carbon dioxide gas obtained as an intermediate stream in the ethylene glycol production process, water, an ethylene carbonate mixture containing ethylene glycol, and unreacted ethylene carbonate and by-product ethylene glycol in a transesterification reaction In separating ethylene carbonate, which is a raw material for transesterification, from this mixture, it is possible to recover part of the energy required for distillation separation and reduce the energy consumption.
[Brief description of the drawings]
[Figure 1] Conceptual diagram of distillation tower

Claims (8)

Ethylene carbonate feed (A) which is a liquid mixture after separation of unreacted methanol and dimethyl carbonate from the transesterification product of ethylene carbonate and methanol, and ethylene carbonate feed (B) containing at least water in addition to ethylene glycol ) Is supplied to the same distillation column, water is distilled from the top of the column, and ethylene carbonate having a purity of 80% by weight or more containing no water is discharged from the bottom of the column. In this case, the feed (B) is supplied to a position above the supply position of the supply (A), and a sidestream vapor containing ethylene glycol as a main component from a position intermediate between the supply positions of the two supplies. A method for recovering ethylene carbonate, wherein 2. The method for recovering ethylene carbonate according to claim 1, wherein the feed (B) is fed to a position of at least two theoretically upper stages from the feed position of the feed (A). The method for recovering ethylene carbonate according to claim 1 or 2, wherein the feed (B) is fed to the top of a distillation column. The side flow vapor which has ethylene glycol as a main component is withdrawn from the position of at least one theoretical plate number and the upper stage from the supply position of the feed (A). The method for recovering ethylene carbonate as described in 1. The ethylene carbonate according to any one of claims 1 to 4 , wherein the feed (B) is a product obtained by a reaction between ethylene oxide and carbon dioxide in the presence of water. Collection method. The method for recovering ethylene carbonate according to any one of claims 1 to 5 , wherein the heat of condensation is utilized as a heating source by contacting the sidestream vapor with a low-temperature liquid in a heat exchanger. . The method for recovering ethylene carbonate according to any one of claims 1 to 5 , wherein water vapor is generated by the condensation heat of the sidestream vapor and used as a heating source. A method for producing dimethyl carbonate, wherein the ethylene carbonate obtained by the method for recovering ethylene carbonate according to any one of claims 1 to 5 is used as a raw material for a transesterification reaction between ethylene carbonate and methanol.

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