JP2003146983A - Method for recovering ethylene carbonate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an efficient method for producing dimethyl carbonate on the assumption of recovering ethylene carbonate having a purity capable of being utilized for an ester interchange reaction by distilling an ethylene carbonate mixture containing water, ethylene glycol, etc. SOLUTION: This method for recovering ethylene carbonate by feeding (A) an ethylene carbonate feed containing essentially only ethylene glycol and (B) an ethylene carbonate feed containing at least water except for the ethylene carbonate into an identical distillation column for performing continuous distillation by distilling water off from its top and taking out ethylene carbonate without containing water and having >=80 wt.% purity from its bottom is characterized by feeding the feed (B) at an upper stage than that of the feeding position of the feed (A), and drawing out a side stream vapor consisting mainly of ethylene glycol from a middle position of the feeding positions of both of the feeds.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for distilling a mixture containing ethylene carbonate and ethylene glycol to recover ethylene carbonate having a purity usable for a transesterification reaction. Particularly, in the step of producing dimethyl carbonate by transesterifying ethylene carbonate and methanol, carbon dioxide gas, water,
The present invention relates to a method for diluting a crude ethylene carbonate mixture containing ethylene glycol with a mixture of unreacted ethylene carbonate and by-product ethylene glycol in a transesterification reaction to reduce energy consumption when separating ethylene carbonate.

[0002]

2. Description of the Related Art A method for producing dimethyl carbonate includes
The methanol oxidative carbonylation method, urea methanolysis method, transesterification method and the like have been put to practical use or proposed. Among them, as a proposal for a method for producing dimethyl carbonate by transesterifying ethylene carbonate and methanol, a method by batch reaction (US Pat. No. 3,642,858, JP-A-54-4) is used.
8715, etc.), a distillation column is installed above the reaction kettle to carry out the reaction while continuously removing dimethyl carbonate (US Pat. No. 3,803,201, etc.), continuous reaction by a tubular reactor (special Kaisho 63-4
No. 1432, etc.), etc., but all of them are based on the premise that high-purity ethylene carbonate is used as a raw material.

When the intermediate product of the process for producing ethylene glycol from ethylene oxide and carbon dioxide gas via ethylene carbonate is used as a raw material for the transesterification method dimethyl carbonate production, carbon dioxide gas, water and ethylene are contained in the raw materials. Since it contains glycol, it is necessary to separate and purify ethylene carbonate before feeding it to the transesterification reactor. Water is removed because ethylene carbonate may be hydrolyzed in the presence of water, and ethylene glycol is removed because ethylene glycol is produced as a by-product of the transesterification reaction. This is because it works to return the raw material to the raw material system. For purification of ethylene carbonate, there are a method by distillation and a method by crystallization (Japanese Unexamined Patent Publication No. 7-89905), but there is a drawback that distillation requires energy cost and crystallization requires equipment cost.

When distilling and purifying ethylene carbonate containing ethylene glycol by distillation, since ethylene carbonate and ethylene glycol are azeotroped, usually while refluxing under reduced pressure, azeotropic distillation of ethylene carbonate and ethylene glycol is carried out from the top of the tower. Withdraw ethylene carbonate from the mixture and the bottom of the tower. The boiling point of ethylene carbonate under atmospheric pressure is 248 ° C, and that of ethylene glycol is 197 ° C.
Since the temperature of these vapors is high even under reduced pressure, heat can be recovered in the distillation condenser. However, when water is added to this, water distills from the top of the tower,
The temperature at the top of the column drops and heat cannot be recovered from the fluid extracted at the top of the column.

[0005]

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. Procurement cost, the latter as energy cost required for separation, has a problem that in any case, it increases the production cost of dimethyl carbonate.

That is, according to the present invention, an ethylene carbonate mixture containing carbon dioxide, water and ethylene glycol, which is obtained as an intermediate product of the step of producing ethylene glycol from ethylene oxide and carbon dioxide via ethylene carbonate, is distilled. Therefore, it is an object of the present invention to provide an efficient method for producing dimethyl carbonate on the premise that ethylene carbonate having a purity that can be used in a transesterification reaction is recovered.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted various studies in order to solve the above-mentioned problems, and as a result, when distilling and separating ethylene carbonate contained in a raw material, usually, The present inventors have found that energy consumption during the production of dimethyl carbonate can be reduced by taking out part of ethylene glycol contained in the liquid composition extracted from the column top as a sidestream vapor and recovering the condensation heat thereof.

That is, the gist of the present invention is to supply an ethylene carbonate feed (A) containing substantially only ethylene glycol and an ethylene carbonate feed (B) containing at least water in addition to ethylene glycol to the same distillation column. In the method for recovering ethylene carbonate, water is distilled from the top of the tower, and ethylene carbonate having a purity of 80% by weight or more containing no water is removed from the bottom of the tower. The above-mentioned feed (B) is fed to a position higher than the feed position, and a sidestream vapor containing ethylene glycol as a main component is withdrawn from a position intermediate between the feed positions of both feeds. It depends on the recovery method of. Another gist is that the feed (A) is a liquid mixture obtained by separating unreacted methanol and dimethyl carbonate from a transesterification reaction product of ethylene carbonate and methanol. Another gist is that the feed (B) is a product obtained by the reaction of ethylene oxide and carbon dioxide in the presence of water. Another point is that the heat of condensation of the sidestream vapor containing ethylene glycol as a main component, which is continuously extracted in the above-mentioned distillation operation, is used as a heating source. Still another gist resides in a method for producing dimethyl carbonate, which comprises using the ethylene carbonate obtained by the method for recovering ethylene carbonate as a raw material for the transesterification reaction of ethylene carbonate and methanol.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Transesterification reaction This reaction is a transesterification reaction of ethylene carbonate and methanol in the presence of a catalyst to produce dimethyl carbonate and ethylene glycol, which is represented by the following formula (1). To be done.

[0010]

[Chemical 1]

As the catalyst, a conventional transesterification catalyst can be selected. Specifically, as homogeneous catalysts, amines such as triethylamine, alkali metals such as sodium, alkali metal compounds such as sodium chloroacetate and sodium methylate, and thallium compounds,
As the heterogeneous catalyst, ion-exchange resins modified with functional groups, amorphous silicas impregnated with alkali metal or alkaline earth metal silicates, ammonium-exchanged Y-type zeolite, mixed oxides of cobalt and nickel, etc. Can be illustrated respectively. When a homogeneous catalyst is used, it is preferable to separate the catalyst before the distillation separation in order to avoid the progress of the reverse reaction and the occurrence of side reactions.

The transesterification reaction conditions are generally such that the reaction temperature is 50 to 180 ° C. and the molar ratio of methanol and ethylene carbonate charged is 2 to 20. If this molar ratio is less than 2, the conversion rate of transesterification 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 when recycled. There is a problem that the burden on the equipment increases. The transesterification reaction is a batch reaction (US Pat. No. 3,642,858, JP-A-54-4871).
No. 5, etc.), a distillation column is installed above the reaction kettle to carry out the reaction while continuously removing dimethyl carbonate (US Pat. No. 3,803,201, etc.), a continuous reaction by a tubular reactor (special Kaisho 63-41432
Any of the reaction formats, such as the publication, etc.) is applicable.

Distillation Separation The transesterification reaction represented by the above formula (1) is an equilibrium reaction, and in the transesterification reaction liquid, in addition to the product dimethyl carbonate and the by-product ethylene glycol on the right side of the formula, the left side of the formula Ethylene carbonate and methanol remain as unreacted. Therefore, these components are separated, the product is taken out, and the unreacted raw materials are 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, is unreacted ethylene carbonate (boiling point 24
8.2 ° C.) and by-product ethylene glycol (boiling point 197.
(3 ° C.), and the unreacted ethylene carbonate is usually recovered from the reaction solution by distillation separation.

The present invention is a method of recovering ethylene carbonate having a sufficient purity as a transesterification reaction raw material by distillation separation, and a plurality of kinds of ethylene carbonate feeds are supplied to the same distillation column. That is, the first feed is an ethylene carbonate feed (A) containing substantially only ethylene glycol, and usually unreacted methanol and dimethyl carbonate are separated from the transesterification reaction product of ethylene carbonate and methanol. The resulting liquid mixture is used. In the present invention, "substantially containing only ethylene glycol" means that the content of components other than ethylene glycol is 5% by weight or less,
Preferably it is a mixture of ethylene carbonate and ethylene glycol up to 1% by weight. The second feed is an ethylene carbonate feed (B) containing at least water in addition to ethylene glycol, and is usually a product obtained by the reaction of ethylene oxide and carbon dioxide in the presence of water (hereinafter , Also referred to as "EC-ized reaction product").

In the present invention, the above-mentioned feed (A) and the above-mentioned feed (B) are fed to a predetermined position of a distillation column to carry out continuous distillation, distilling water from the top of the column and water from the bottom of the column. 80% by weight or more, preferably 90% by weight or more of pure ethylene carbonate containing no is removed. The composition of the distillate from the top of the column depends on the composition of both feeds, but when the above EC-ized reaction product is used as the feed (B), carbon dioxide and Since a distillate containing ethylene glycol is obtained, it is cooled and condensed, and a part thereof is refluxed in the distillation column. Normally, the condensate is an ethylene glycol / ethylene carbonate azeotrope with some water, and the non-condensing vapor entrains ethylene glycol / ethylene carbonate in a mixture of steam and carbon dioxide.

At the time of this continuous distillation separation, it is necessary to extract the sidestream vapor containing ethylene glycol as a main component, and the water is distilled from the top of the tower for the first time by extracting this high temperature vapor. The heat can be recovered by condensation, which is effective in reducing energy consumption. As described above, ethylene glycol has a higher boiling point than water and a lower boiling point than ethylene carbonate, so that there is a portion concentrated in the vapor in the concentration section 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 condensation heat of the vapor having a high temperature, which has ethylene glycol as a main component, by extracting the sidestream vapor from this.

In the distillation operation, however, it is necessary that the feed position of both feeds and the withdrawal position of the sidestream vapor follow the following criteria. 1) Feeding the above feed (B) to a position above the feed position of the vapor feed (A), preferably at least two theoretical upper stages and most preferably to the top position. 2) At least 1 in theoretical plate number from an intermediate position between the feed positions of both feeds, preferably the feed position of the above feed (A).
Extract the sidestream vapor containing ethylene glycol as the main component from the upper and lower positions. Further, the method of recovering heat from the sidestream vapor containing ethylene glycol as a main component thus extracted is not particularly limited,
For example, by contacting the sidestream vapor with a low temperature liquid in a heat exchanger, a method of utilizing the heat of condensation as a heating source, to generate steam by the heat of condensation of the sidestream vapor,
A method of utilizing it as a heating source is preferable.

The number of plates required for distillation separation (column main body excluding reboiler and condenser) depends on the reflux ratio,
Considering economic efficiency, the number of stages is preferably 5 or more, more preferably 10 or more. The reflux ratio is preferably 0.01 to 1,
0.01 to 0.5 is more preferable. The operating pressure of the distillation column is below atmospheric pressure in order to prevent the product purity from decreasing due to the increase in operating temperature. However, if the pressure is lowered too much, the separation efficiency and energy efficiency will deteriorate and the temperature of the sidestream vapor will decrease, making it difficult to recover heat.
Appropriate pressure should be selected. Suitable pressure range is 2
It is 7 kPa or less, preferably 4 to 13 kPa. Distillation columns include Sulzer packing, Melapack, MC
It is possible to use any type such as a packed tower type filled with an ordered packing such as a pack or an irregular packing such as INTP or Raschig ring, a tray tower type using a bubble bell tower, a sieve tray or a valve tray tower. .

EC Reaction The ethylene carbonate, which is a raw material for the above transesterification reaction, is usually produced from ethylene oxide and carbon dioxide gas in the presence of a catalyst by an EC reaction represented by the following formula (2). As an EC-forming reaction catalyst, a phosphonium salt is preferred because of its high activity and reusability. Further, it is more preferable to use an alkali metal carbonate as the co-catalyst.

[0020]

[Chemical 2]

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,
Not practical. Therefore, in practice, it is effective to carry out the reaction in the presence of water because the reaction rate can be improved. However, at the same time, a part of the produced ethylene carbonate is subjected to a hydrolysis reaction represented by the following formula (3),
It becomes ethylene glycol and the content rate of ethylene carbonate decreases.

[0022]

[Chemical 3]

The type of reactor used for the EC-forming 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 produced from the top and an unreacted reaction solution are added. It is effective to use a bubble column reactor that distills carbon dioxide and the like in the reaction. This reaction is carried out at a temperature of 70 to 200 ° C., preferably 10
It is general to carry out at 0 to 170 ° C. and a pressure of 490 to 4900 kPa · G (gauge pressure), preferably 980 to 2940 kPa · G. The supply amount (molar ratio) of carbon dioxide and water to ethylene oxide is usually 0.1 to 5 respectively.
And 0.1 to 10, but preferably 0.
5 to 3 and 0.5 to 5 are preferable. 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 liquid is extracted to the outside, cooled by a heat exchanger and then returned to the system.

Utilization for EG production In the above-mentioned distillation separation, a low boiling point component containing water and ethylene glycol as a main component, or an azeotropic mixture of ethylene glycol and ethylene carbonate, which is collected from the top of the column, is It can be used for the production of EG by hydrolysis. The hydrolysis reaction is usually 10
It is carried out at a temperature of 0 to 180 ° C. and a pressure of atmospheric pressure to 1960 kPa · G to obtain an ethylene glycol aqueous solution containing an EC-forming reaction catalyst, and carbon dioxide as a by-product of the hydrolysis reaction is separated in the gas phase. The aqueous solution is combined with dehydration distillation for removing water from the top of the tower and rectification for separating EG and a high boiling point component containing an EC-forming reaction catalyst to obtain a product EG and recover a reusable component. To be done. Adoption of such EG manufacturing processes is desirable to ensure economic efficiency.

[0025]

Example (Synthesis of ethylene carbonate) In an autoclave equipped with a stirrer, 2234 g of hydrous ethylene oxide containing 41% by weight of water, 112 g of tributylmethylphosphonium iodide as a catalyst and 4 g of potassium carbonate as a cocatalyst were charged, and the original pressure was 2100 kPa. 20 carbon dioxide continuously
2000 kPa, 110 while supplying at 00 kg / h
The reaction was carried out at 30 ° C. for 30 minutes. Then, the reaction liquid was cooled to 80 ° C. and depressurized to 53 kPa, and a gas phase mainly composed of unreacted carbon dioxide gas and ethylene oxide was separated. The separated gas phase can be recycled and reused in the EC conversion reaction. On the other hand, the reaction solution after separating the gas phase was analyzed to show that ethylene carbonate was 46% by weight, ethylene glycol was 31% by weight, water was 19% by weight, the catalyst was 4% by weight, and unreacted ethylene oxide was It was less than 0.1% by weight.

(EC-catalyzed separation) The reaction liquid after the gas phase separation was subjected to batch distillation under a reduced pressure of 2 kPa, and the catalyst was concentrated and separated as a bottom residue. The separated catalyst-containing residue is E
It can be recycled and reused in the carbonization reaction. The obtained distillate contained 47% by weight of ethylene carbonate, 32% by weight of ethylene glycol and 20% by weight of water. When recovering ethylene carbonate, which will be described later,
Used as ethylene carbonate feed (B).

(Synthesis of dimethyl carbonate) A fixed bed in which methanol and ethylene carbonate were adjusted to a molar ratio of 4: 1 and an anion exchange resin having a quaternary ammonium salt structure bonded to a crosslinked polystyrene chain through a spacer group was filled. Passed through reactor. When the reaction temperature was 80 ° C., the reaction pressure was 700 kPa, and the transesterification reaction was performed at LHSV = 2, a reaction liquid of ethylene carbonate 38% by weight, methanol 28% by weight, ethylene glycol 14% by weight, and dimethyl carbonate 20% by weight was obtained. Was given.

(Distillation and Separation of Dimethyl Carbonate Reaction Liquid) The above transesterification reaction liquid was fed to the 10th stage of a continuous distillation apparatus having 20 theoretical plates, and separated by distillation at a reflux ratio of 1, and methanol was separated from the top of the column. Dimethyl carbonate was distilled off, and a mixed solution of ethylene carbonate and ethylene glycol was removed from the bottom of the column. Unreacted methanol can be further recovered from the distillate to obtain the product dimethyl carbonate. The composition of this bottoms was such that ethylene carbonate was 73% by weight and ethylene glycol was 27%.
% By weight. Dimethyl carbonate is 10 weight pp
Below m, no methanol was detected. This bottom solution was used as an ethylene carbonate supply material (A) when the following ethylene carbonate was recovered.

(Recovery of ethylene carbonate) Ethylene carbonate was recovered using the distillation column shown in the conceptual diagram of FIG. In the figure, S1 to S6 indicate stream numbers, and the characters in the circles indicate theoretical plate numbers. That is, in a distillation column having 12 theoretical plates (excluding a condenser and a reboiler), the ethylene carbonate feed (B) was 12 kg / h in the first stage, and the ethylene carbonate feed (A) was 6 in the fourth stage. It was supplied at 0.4 kg / h. The overhead pressure was 8 kPa, the condenser condensation temperature was 50 ° C., the reflux ratio was 0.05, the overhead liquid was 2.4 kg / h containing 34% by weight of water, and the ethylene carbonate was taken out from the bottom. 9
While continuously extracting 9.5 kg / h of a bottom liquid at 160 ° C containing 5% by weight, 5 kg / h of a sidestream vapor at 129 ° C containing 78% by weight of ethylene glycol was withdrawn from the second stage. Condenser non-condensing top vapor 1.5kg / h
Was totally condensed in a vent condenser at 25 ° C. Details of operating conditions and results are shown in Table-1 and Table-2.

[0030]

[Table 1]

[0031]

[Table 2]

This sidestream vapor was fed to a heat exchanger,
The heat of condensation was recovered by exchanging heat with water at 0 ° C. At this time, the heat of condensation of the sidestream vapor was 5 MJ / h. When steam of 104 kPa is generated by this heat of condensation, the amount is 2.1 kg / h. On the other hand, the energy required to heat the reboiler was 14 MJ / h, so about 36% of the heat consumed by the reboiler can be recovered as 100 ° C steam, and this steam should be used to heat and heat pipes and equipment. You can

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 recycled and reused in the transesterification reaction as a raw material for the synthesis of 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 which is a raw material of dimethyl carbonate. In that case, components other than ethylene carbonate contained in the feed liquid are usually withdrawn from the top of the column.
Then, two kinds of ethylene carbonate feeds (A) and (B) prepared by the same operation as in the example were fed to the same position in the same distillation column as in the example, and 95 wt% of ethylene carbonate and ethylene were fed from the bottom of the column. A bottom solution (9.5 kg / h) 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 example, except that the sidestream vapor is not removed. The operating conditions and results are shown in Table-3 and Table-4.

[0035]

[Table 3]

[0036]

[Table 4]

The composition of the top liquid is 27% by weight of water, 58% by weight of ethylene glycol, and 15% by weight of ethylene carbonate.
Met. The energy required to heat the reboiler is 14M
It was J / h. Since the temperature of the overhead liquid is 53 ° C, heat recovery is impossible.

[0038]

As described above, according to the present invention, carbon dioxide obtained as an intermediate stream in the ethylene glycol production process, water, an ethylene carbonate mixture containing ethylene glycol, and unreacted ethylene carbonate in the transesterification reaction. In separating ethylene carbonate, which is a raw material for transesterification reaction, from a mixture of ethylene glycol and by-product, there is an effect that energy consumption can be reduced by recovering a part of energy required for distillation separation.

[Brief description of drawings]

[Figure 1] Conceptual diagram of distillation column

Claims (9)

[Claims] 1. An ethylene carbonate feed (A) containing substantially only ethylene glycol and an ethylene carbonate feed (B) containing at least water in addition to ethylene glycol are fed to the same distillation column for continuous distillation. ,
In a method for recovering ethylene carbonate in which water is distilled from the top of the tower and ethylene carbonate having a purity of 80% by weight or more and containing no water is removed from the bottom of the tower, the ethylene carbonate is collected at a position above the feed position of the feed (A). The above-mentioned feed (B) is fed, and from a position intermediate between the feed positions of both feeds,
A method for recovering ethylene carbonate, which comprises extracting a sidestream vapor containing ethylene glycol as a main component. 2. The ethylene carbonate according to claim 1, wherein the feed (B) is fed to a position at least two theoretical stages above the feed position of the feed (A) and above. Recovery method. 3. The method for recovering ethylene carbonate according to claim 1, wherein the feed (B) is fed to the top of a distillation column. 4. A sidestream vapor containing ethylene glycol as a main component is withdrawn from at least one theoretical plate and a position above the feed position of the feed (A). The method for recovering ethylene carbonate according to any one of 1. 5. The feed (A) is a liquid mixture obtained by separating unreacted methanol and dimethyl carbonate from a transesterification reaction product of ethylene carbonate and methanol. 4. The method for recovering ethylene carbonate according to any one of 4 above. 6. The feed (B) is a product obtained by the reaction of ethylene oxide and carbon dioxide gas in the presence of water, according to any one of claims 1 to 5. The method for recovering ethylene carbonate described. 7. The heat of condensation is utilized as a heating source by bringing the sidestream vapor into contact with a low temperature liquid in a heat exchanger, as claimed in any one of claims 1 to 6. Method for recovering ethylene carbonate. 8. The method for recovering ethylene carbonate according to claim 1, wherein steam is generated by the heat of condensation of the sidestream vapor and is used as a heating source. 9. A dimethyl compound, characterized in that the ethylene carbonate obtained by the method for recovering ethylene carbonate according to any one of claims 1 to 6 is used as a raw material for the transesterification reaction of ethylene carbonate and methanol. Method for producing carbonate.