JP2557099B2 - Method for separating dimethyl carbonate - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for separating organic carbonate, and more particularly to a method for efficiently separating methanol and dimethyl carbonate from a mixture of methanol and dimethyl carbonate.

[Conventional technology]

As a method for producing dimethyl carbonate, methanol is used as a suitable catalyst, for example, copper salt (Japanese Patent Publication No. 45-11129, USP436047).
7, USP4604242, JP 58-33857, JP 60-58739,
JP-B-52-45693, JP-B-52-46927) and white metal-copper salt (USP3114762, JP-B-61-8816, JP-B-61-43338, USP)
4361519, Japanese Examined Patent Publication No. 55-185542) and the like, a method of reacting with carbon monoxide and oxygen, or a method of reacting carbon dioxide with ethylene oxide, propylene oxide, etc. to form a carbonate, and transesterifying it with methanol. Manufacturing method (EP0119840, Japanese Patent Publication No. 61-4381),
Alternatively, a method of reacting with urea or urethane (USP283
4799, USP4327035) etc.

In these methods, methanol is commonly used in excess, so that the reaction solution is obtained as a methanol solution of dimethyl carbonate and by-produced water. In the case of industrially producing dimethyl carbonate, it is necessary to isolate and purify dimethyl carbonate from this mixture and at the same time to recycle unreacted methanol to the reactor. As is well known, this three-component system has two azeotropic relationships. Since it contains, it cannot be separated simply by using the boiling point difference. The two azeotropic relationships are the two-component system of methanol and dimethyl carbonate (70% by weight of methanol under normal pressure,
It exists in a binary system of dimethyl carbonate and water).

Therefore, when the reaction liquid is separated by distillation, an azeotropic mixture of methanol and dimethyl carbonate is separated from the top of the column, and dimethyl carbonate and water are collected from the bottom of the column. As is well known, the column bottom liquid obtained here is separated at an appropriate temperature, and the upper layer liquid and the lower layer liquid are respectively distilled, whereby dimethyl carbonate and water can be isolated. However, since the azeotrope at the top of the column forms a homogeneous phase, it must be returned to the reactor as it is, unless special treatment as described below is performed. In that case, a large amount of dimethyl carbonate is circulated in the reactor, which lowers the production efficiency. Therefore, various methods for breaking the azeotropic distillation of methanol and dimethyl carbonate have been considered so far.

First, when distillation is used as an operation for separating methanol and dimethyl carbonate, a method for isolating dimethyl carbonate by distilling under normal pressure with an entrainer immiscible with methanol (JP-A-54-41820).
Alternatively, a method of performing extractive distillation using water as a solvent (Japanese Patent Publication No. 56-17).
333), both of which can finally isolate methanol, but the former requires a large amount of energy to separate methanol from the entrainer, and the latter requires large energy to separate water from methanol. Not. As another method, there is a method of distilling at a pressure of 10 atm, a bottom temperature of 150 ° C, and a top temperature of 142 ° C (Japanese Patent Publication No. 59-3463). In this method, it is described that the overhead azeotropic composition is 95% in methanol (probably a molar concentration), but methanol cannot be isolated.

As a method of separating methanol and dimethyl carbonate by an operation other than distillation, a method of selectively adsorbing and concentrating a carbonate from an alcohol solution of a dialkyl carbonate using a hydrophobic zeolite (JP-A-60-106505),
There is a method (USP3803201) of solid-liquid separation by cooling a mixture of methanol and dimethyl carbonate to several tens of degrees below freezing, but the former has problems with low adsorption concentration and desorption method, and the latter has problems with cooling. It seems to be difficult to put into practical use.

Therefore, at present, there is no choice but to recycle the mixture having an azeotropic composition into the reaction system by the distillation method.

[Problems to be Solved by the Invention]

However, if it is industrialized, the designer naturally tries to assemble a recovery system in order to minimize the cost, but as long as the known separation direction is used, the azeotropic mixture of methanol and dimethyl carbonate will be generated due to the problems as described above. There is no choice but to recycle it into the reaction system. Therefore, the concentration of dimethyl carbonate in the reaction solution is higher than that in the case where methanol is isolated and recycled.

Although dimethyl carbonate is generally a stable compound, part of it hydrolyzes when heated with water in the presence of a catalyst, so the higher the concentration of dimethyl carbonate in the reactor, the more dimethyl carbonate per unit volume. In addition to lowering the yield of carbon monoxide, when methanol is reacted with carbon monoxide and oxygen, the selectivity of carbon monoxide to dimethyl carbonate decreases, leading to an increase in the carbon monoxide usage rate.

As described above, since it is not desirable that the concentration of dimethyl carbonate in the reaction solution is high from the viewpoint of the reaction, it is desired to isolate unreacted methanol from dimethyl carbonate and recycle it to the reaction system.

In addition, recycling a part of the dimethyl carbonate purified by the reaction to the reaction system is not preferable because it not only requires extra energy and space but also affects the quality of the product to some extent.

To reiterate, in order to achieve high reaction results (activity and selectivity), it is desirable to recycle unreacted methanol to the reaction system without containing dimethyl carbonate, but at present, it is economical to use methanol and dimethyl carbonate. There is no separate method and it has to be recycled as an azeotropic mixture.

[Means for solving the problem]

It is well known that the azeotropic composition moves due to changes in pressure, but the azeotropic relationship between methanol and dimethyl carbonate is 70/30 at normal pressure according to the measurement by the inventors.
The weight ratio (azeotropic temperature 64 ° C.), while the concentration of methanol increases sharply as the pressure increases, reaching 79/21 weight ratio (azeotropic temperature 104 ° C.) under 4 atmospheres. As the pressure further increases, the increase slows down, but when it reaches 10 atm, it becomes 88/12 weight ratio (azeotropic temperature 138 ° C) and at 15 atm 93/7 weight ratio (azeotropic temperature 155 ° C). (For other pressures, see the reference experiment below).

By increasing the pressure in this way, an azeotropic composition richer in methanol and less in dimethyl carbonate can be obtained.However, the higher the pressure, the closer the volatility of methanol and dimethyl carbonate becomes, especially in the vicinity of the azeotropic point. Differences in liquid composition are reduced and separation becomes difficult. Therefore, it should be noted that there is a certain upper limit even when trying to obtain methanol having a low dimethyl carbonate content by increasing the operating pressure of the distillation column.

However, the present inventors have found that pure methanol can be easily obtained from the bottom of the column by atmospheric distillation of an overhead azeotropic mixture obtained by performing distillation under a slight pressure. Since the relative volatility of methanol and dimethyl carbonate is close to 1, there is no significant temperature difference from the top to the bottom of the column, but it was confirmed that pure methanol can be stably obtained by maintaining an appropriate flow ratio. Was done.

As a result of diligent studies based on these results, the present inventors have shown that
The present invention has been completed.

That is, the present invention is a method for separating methanol and dimethyl carbonate from a liquid containing methanol and dimethyl carbonate, wherein two distillation columns are used in combination and the first column is operated at a higher pressure than the second column. The separation method is provided.

 Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a process diagram showing a preferred embodiment of a process for separating methanol and dimethyl carbonate according to the method of the present invention. In the case of a crude reaction liquid obtained by reacting a feed liquid to the process, the crude reaction liquid contains It contains dimethyl carbonate, which is a product, water, a trace amount of reaction by-products, and unreacted methanol. Usually, it is charged as it is into the first distillation column, but in some cases, low boiling impurities may be removed in advance, or water may be removed by adsorption or other methods. Here, the process will be described for a mixture of methanol, dimethyl carbonate, and water, but the same treatment can be performed when water is not contained or when trace impurities are contained.

That is, the crude reaction liquid containing methanol, dimethyl carbonate, and water is supplied from the line 1 to the vicinity of the middle stage of the first distillation column. This first column operates at a higher pressure than the second column. An azeotropic mixture of methanol and dimethyl carbonate containing almost no water under the operating pressure is obtained from the top of the first column, and a mixture of dimethyl carbonate and water (containing almost no methanol) is obtained from the column bottom 3. The bottom liquid is sent to the next purification step and dimethyl carbonate as a product and water are separated by a known method. The top distillate of the first column is subsequently fed to the second distillation column operated at a lower pressure than the first column by line 2, and an azeotropic mixture under the pressure is obtained from the top of the column. Almost pure methanol is obtained from the bottom 5 of the column. The azeotrope at the top of the column passes through line 4 and is recycled to the stage close to the composition of the first distillation column.

Here, the operating pressure of each of the first and second columns can theoretically be operated as long as there is a pressure difference. Exists.

First, regarding the operating pressure difference between the two distillation columns, if the difference is too small, the recycle flow rate between the two columns will be too large, resulting in a loss of economic efficiency and a problem in operation stability. Industrially, a significant difference in azeotropic composition occurs with a pressure difference of 1 atm or more,
It is more preferable to operate with a pressure difference of 3 atm or more.

Next, regarding each operating pressure, this is the optimum setting when there is a condition according to the heat source and refrigerant used,
Basically, it is preferable to carry out the first tower at 2 atm or more and the second tower at normal pressure. In this case, since the top vapor temperature of the first tower exceeds 80 ° C and the bottom temperature of the second tower is 70 ° C or less, it is possible to cover the heating source of the second tower with the top vapor of the first tower. is there.
Even more preferably, the first tower may be operated at 4 atm or higher and the second tower at normal pressure. In this case, the top vapor temperature of the first tower is 100
Since the bottom temperature of the second column is above 70 ° C and exceeds 70 ° C, it is easy to cover the heating source of the second column with the top vapor of the first column.

〔The invention's effect〕

The method of the present invention has made it possible to separate methanol and dimethyl carbonate from a liquid containing methanol / dimethyl carbonate forming an azeotrope without applying a large energy load. Therefore, dimethyl carbonate that accompanies unreacted methanol recycled to the reaction system is eliminated, and it has become possible to improve the reaction yield and selectivity.

In addition, the present invention can also be used in the case of separating methanol and dimethyl carbonate from a mixture of unreacted methanol and dimethyl carbonate after various transesterification reactions using dimethyl carbonate as a raw material.

〔Example〕

Some of the embodiments of the present invention are shown below to aid understanding of the person who uses the present invention, but the present invention is not limited thereto.

Reference experiment (Measurement of azeotropic data) 1) 250 g each of methanol and dimethyl carbonate (DMC) were placed at the bottom of a stainless steel perforated plate column with an inner diameter of 28 mm and 20 plates, and total reflux operation was performed by changing the column top pressure. The upper 10 stages of the column were heated to the same temperature and periodically sampled to confirm that the top composition was constant, and then the composition was taken as the azeotropic composition under that pressure.

 The measurement results are shown in Table 1 (symbol * 1).

2) The pressure-resistant autoclave with a volume of 1 installed in the thermostat was degassed, then a mixed solution of methanol and dimethyl carbonate (DMC) was charged therein, and the temperature was raised to a predetermined pressure while stirring. After stabilizing, the gas phase and the liquid phase were sampled and the gas-liquid equilibrium was measured.

The azeotropic composition obtained as a result is shown in Table 1 (symbol * 2).

Example 1 An atmospheric pressure mixture consisting of 70% by weight of methanol and 30% by weight of dimethyl carbonate at the 20th level from the top of a stainless perforated plate column having an inner diameter of 28 mm and a plate number of 40, operated at a top pressure of 6 atm and a reflux ratio of 5. Charge the boiling mixture at a flow rate of 300 g / h and
It was stabilized so that the temperature of the stage was 155 ° C. This time,
As an overhead distillate, an azeotropic mixture of 82.5% by weight of methanol and 17.5% by weight of dimethyl carbonate was obtained at 254.5 g / hr, and 45.5 g of dimethyl carbonate containing no methanol was obtained at the bottom of the column. At this time, the top temperature was 118 ° C and the bottom temperature was 16 ° C.
It was 7 ° C.

The overhead distillate was passed through an intermediate tank to an inner diameter of 40 m
A glass Oldershaw distillation column with m and 40 plates was continuously charged, and the flow rate was controlled at a reflux ratio of 5 under normal pressure to make it steady. At this time, 106 g of pure methanol containing no dimethyl carbonate per hour was obtained from the bottom of the column, and 148.5 g of azeotrope containing 70% by weight of methanol and 30% by weight of dimethyl carbonate per hour was obtained from the top of the column. At this time, the top temperature was 64 ° C and the bottom temperature was
It was 66 ° C.

Example 2 58% by weight of methanol, 35% by weight of dimethyl carbonate, 20% from the top of the stainless steel perforated plate column of Example 1,
A mixture consisting of 7% by weight of water was charged at a flow rate of 150 g / h,
Continuous operation was performed while charging a mixed liquid consisting of 70% by weight of methanol and 30% by weight of dimethyl carbonate at a flow rate of 120 g / hr from the top of the tower to a temperature of 130 ° C at the 10th from the bottom.
It became steady so that. At this time, as the overhead distillate, methanol 82.5% by weight, dimethyl carbonate 17.5% by weight
The water-free azeotropic mixture was obtained at 207 g / h, and 83% by weight of methanol-free dimethyl carbonate, water
63 g of a 17% by weight liquid was obtained every hour. The top temperature at this time is 118
℃, the bottom temperature was 140 ℃.

This overhead distillate was passed through an intermediate tank to obtain Example 1
Was continuously charged to the 20th stage from the top of the Aldershaw distillation column made of glass, and the flow rate was controlled at a reflux ratio of 5 under normal pressure to make it steady. At this time, 87 g of pure methanol containing no dimethyl carbonate per hour was obtained from the bottom of the column, and 120 g of azeotrope containing 70% by weight of methanol and 30% by weight of dimethyl carbonate per hour was obtained from the top of the column. At this time, the tower top temperature was 64 ° C and the tower bottom temperature was
It was 66 ° C.

[Brief description of drawings]

FIG. 1 is a process diagram showing a preferred embodiment of a process for separating methanol and dimethyl carbonate according to the method of the present invention. 1: Crude reaction liquid supply line 2: First tower top distillate supply line 3: First tower bottom 4: Second tower top distillate supply line 5: Second tower bottom

Claims (1)

(57) [Claims] 1. A method for separating methanol and dimethyl carbonate from a liquid containing methanol and dimethyl carbonate, wherein two distillation columns are used in combination and the first column is operated at a higher pressure than the second column. And the separation method.