RU2689569C2 - Methanol processing method in the process of producing dimethyl oxalate by gas synthesis - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to the technical field of producing dimethyl oxalate from synthesis gas and to a method of recycling methanol during synthesis of dimethyl oxalate from synthesis gas. Method includes steps i) feeding a crude methanol stream containing methyl nitrite, methyl formate and dimethylcarbonate, to a first separation column and producing methyl nitrite in an upper portion of a first separation column and a crude methanol stream containing methyl formate and dimethylcarbonate in a lower portion thereof; ii) feeding a crude methanol stream containing methyl formate and dimethyl carbonate to a second separation column and obtaining methyl formate in an upper portion of a second separation column and a crude methanol stream containing dimethyl carbonate in a lower portion thereof; and iii) feeding a crude stream of methanol containing dimethyl carbonate into an apparatus for separating dimethylcarbonate and methanol and after separation obtainment dimethylcarbonate and a stream of methanol to be reused.

EFFECT: technical result consists in reducing the amount of methyl formate accumulating in the system, thereby reducing corrosion of pipelines and equipment, improving purity of methanol, thereby reducing its circulating volume, id est reduced power consumption.

12 cl, 1 dwg, 4 tbl, 4 ex

Description

Cross reference to related application

This application claims priority based on Chinese patent application CN 201410314475.8 filed with the Chinese Patent Office on July 3, 2014, which is incorporated herein by reference.

Technical field

The present invention relates to the technical field of production of dimethyl oxalate from synthesis gas and, in particular, to a method for recycling methanol in the process of producing dimethyl oxalate from synthesis gas.

The level of technology

The gas-phase catalytic condensation reaction of CO at atmospheric pressure to obtain dimethyl oxalate (DMO) consists mainly of two stages, including the condensation reaction and the oxidative esterification reaction. In an oxidative esterification reaction, methylnitrite (MN) is formed, which acts as an oxidizing agent and an intermediate in the condensation reaction. In turn, the NO formed in the condensation reaction is a reagent in the oxidative esterification reaction. Thus, the consistency of condensation reactions and the oxidative esterification reaction is the key to creating an ecologically clean and non-polluting closed system.

The equation for the condensation reaction is given below:

The equation for the oxidative esterification reaction is given below:

Methyl nitrite obtained in the oxidative esterification reaction is sent back to the condensation reaction.

The general reaction equation is given below:

In the process of dimethyl oxalate synthesis, a gas-phase catalytic condensation reaction between methyl nitrite and CO is performed on the catalyst, and dimethyl oxalate is obtained. At the same time, a side reaction occurs with the formation of dimethyl carbonate (DMC). The main adverse reaction is shown below:

Methyl nitrite can decompose to form NO, methyl formate (MF) and methanol. The reaction equation is given below:

In the presence of water, methyl formate and dimethyl carbonate undergo hydrolysis reactions. In the hydrolysis reaction of methyl formate, formic acid and methanol are formed. Formic acid is highly corrosive and may therefore corrode equipment. In the reaction of hydrolysis of dimethyl carbonate, methanol and CO _{2 are formed} . CO _{2 is} recycled and accumulated in the system, and it is difficult to separate. The equations of hydrolysis reactions are given below:

In this case, the recycling of methyl nitrite from crude methanol and the separation of methyl formate and dimethyl carbonate significantly affect the safety and economic efficiency of the production technology of ethylene glycol.

In the prior art, methods have been proposed for recycling methanol in the process of producing dimethyl oxalate by the reaction of catalytic condensation of CO. For example, according to the literature source CN 102911059 A, gas-phase and liquid-phase streams containing methyl nitrite are fed to the distillation column. The upper stream of the distillation column contains methyl nitrite, and the stream in its lower part contains methanol and water. The number of plates in the distillation column is in the range from 10 to 50, the temperature in its lower part is in the range from 50 to 200 ° C, the temperature in its upper part is in the range from 10 to 100 ° C, the reflux ratio is in the range from 0 , 2 to 3.0, and the operating pressure is in the range from 50 to 400 kPa.

Currently, the method of separating azeotropic mixtures of dimethyl carbonate and methanol includes a process selected from the group consisting of extractive distillation, membrane separation and variable pressure distillation. According to literature sources CN 200610169592.5, CN 200710064633, CN 200710121912, CN 200810145291 and CN 201310034796, the mixture of dimethyl carbonate and methanol is separated using a membrane, since the membrane has different osmotic selectivity for methanol and dimethyl carbonate. However, it cannot be argued that the resulting purity of the stream has a high economic efficiency and, therefore, the specified stream in many cases must be further distilled.

According to the literature source CN 201310098177, the mixture of dimethyl carbonate and methanol to be separated is fed at atmospheric pressure to an extraction distillation column in its middle section, and the extracting agent (which is ethylene glycol) is fed to its upper part with a solvent ratio ranging from 1 to 3 and a reflux ratio of 2. High purity methanol is recovered at the top of the extraction distillation column, and dimethyl carbonate and extractant are recovered at the bottom. The fraction from the lower part of the extraction distillation column enters the distillation column of the extracting agent with a reflux ratio of 3. Dimethyl carbonate is recovered in the upper part of the extraction distillation column, and the extracting agent is isolated in its lower part. Extractive agent can be reused.

In the literature source CN 101381309, dimethyl carbonate is isolated from a mixture of dimethyl carbonate and methanol using azeotropic distillation under reduced pressure and azeotropic distillation under high pressure, performed in a process with paired columns. In addition, methanol is recycled.

However, in the prior art there is no information about the removal of methyl formate in the process of producing ethylene glycol, for example, from coal synthesis gas. In modern production equipment, the formation of methyl formate cannot be avoided. Since a large amount of methanol is used in the process of producing ethylene glycol from synthesis gas, taking into account its economic efficiency, it must be reused. If too much methyl formate accumulates in the recycled methanol, then, on the one hand, methyl formate will decompose to form formic acid, which erodes equipment and pipes, and on the other hand, the concentration of methanol will decrease and, consequently, the amount of circulating methanol will increase which will increase and power consumption. Therefore, the release of methyl formate is very important.

Brief description of the invention

The present invention is to provide a method of recycling methanol in the process of obtaining dimethyl oxalate from synthesis gas. Methanol recycled using the method of the present invention has a high degree of purity and can be directly returned to the process of producing dimethyl oxalate as a reagent or absorbent, as a result of which the efficiency of methanol recycling can be improved. In addition, this method has the advantage of ease of stages and low power consumption. At the same time, according to this method, not only corrosion of process equipment and pipelines due to accumulation of residual methyl formate in methanol can be reduced, but the quality of the final ethylene glycol obtained at the stages of isolation and purification of dimethyl oxalate can be improved.

According to the present invention, a method is presented for recycling methanol in a process for producing dimethyl oxalate from synthesis gas, comprising the steps of:

i) feeding the crude methanol stream containing methyl nitrite, methyl formate and dimethyl carbonate to the first separation column and producing methyl nitrite in the upper part of the first separation column and the crude methanol stream containing methyl formate and dimethyl carbonate in its lower part;

ii) feeding the crude methanol stream containing methyl formate and dimethyl carbonate into the second separation column and producing methyl formate in the upper part of the second separation column and the crude methanol stream containing dimethyl carbonate in its lower part; and

iii) feeding the crude methanol stream containing dimethyl carbonate to a dimethyl carbonate and methanol separation unit and producing dimethyl carbonate and a methanol stream after separating.

The method according to the present invention can also be interpreted as a method of cleaning waste in the process of producing dimethyl oxalate from synthesis gas. In accordance with the method of waste disposal, methyl formate can also be recycled, whereby the quality of recycled methanol can be improved. Wastes according to the present invention generally refer to any stream containing methyl nitrite, methyl formate, dimethyl carbonate and methanol, obtained in the process of producing dimethyl oxalate from synthesis gas.

As described in the prior art section, in the prior art, it is unlikely that attention is paid to the removal of methyl formate from liquid waste (i.e., in the product stream of the process for producing dimethyl oxalate from synthesis gas from which dimethyl oxalate is removed) from synthesis gas, and therefore the by-product methyl formate remains in the recycled methanol. As methyl formate accumulates, formic acid is formed as a result of the decomposition of methyl formate, which eats away equipment and pipelines. At the same time, the methanol concentration decreases. Consequently, the circulating volume of methanol must be increased, which leads to an increase in energy consumption, or recycled methanol should be drained, for example, for additional purification. However, the author of the present invention discovered the problem described above and justified its importance. The author of the present invention through experiments and comparisons found that the release of methyl formate can significantly affect the economic efficiency of the process of producing dimethyl oxalate and the subsequent process of producing ethylene glycol. In addition, the selected methyl formate can also be recycled for future use and have a certain industrial value and economic benefits. Thus, the present invention can be implemented.

In a preferred embodiment of the present invention, the methanol recycled in step iii) is directly sent to the esterification reactor of the process for producing dimethyl oxalate from synthesis gas as a reactant or into the separation columns of the said process as an absorbent. “Directly” means that the methanol recycled in stage iii) does not require additional purification and can be directly sent to the synthesis reactor of the dimethyl oxalate production process from synthesis gas as a reactant or back to the separation columns of this process as an absorbent.

The methyl formate isolated at this stage i) can also be directly returned to the condensation reactor, in which dimethyl oxalate is obtained by the reaction of catalytic condensation of CO.

According to the present invention, synthesis gas can be any synthesis gas containing CO and hydrogen. For example, synthesis gas can be obtained from coal, natural gas, coke oven gas, blast furnace gas, tail gas calcining calcium carbide or oil.

Preferably, in the crude methanol stream containing methyl nitrite, methyl formate and dimethyl carbonate in step i), the methyl nitrite content is in the range of 0.1-10 wt. %, preferably in the range of 0.5-8 wt. %, the content of methyl formate is in the range of 0.1-25 wt. %, preferably in the range of 0.5-25 wt. %, the content of dimethyl carbonate is in the range of 0.1-28 wt. %, preferably in the range of 0.5-24 wt. %, and the methanol content is in the range of 50-99 mass. %, preferably in the range of 50-96 mass. %

In a preferred embodiment of the present invention, the working pressure on the manometer in the upper part of the first separation column is in the range of 0-1.0 MPa, preferably in the range of 0.1-0.8 MPa, the working temperature in its upper part is in the range of 20-100 ° C, preferably in the range of 30-80 ° C, and the operating temperature in its lower part is in the range of 50-140 ° C, preferably in the range of 86-129 ° C.

In the present description, unless otherwise specified, pressure refers to the pressure gauge.

According to the present invention, the first separation column may be a packed column or a plate column. Preferably, the number of theoretical plates of the first separation column is in the range of 5-30, preferably in the range of 10-25.

In a preferred embodiment of the present invention, the working pressure on the pressure gauge in the upper part of the second separation column is in the range of 0.1-1.0 MPa, the operating temperature in its upper part is in the range of 30-120 ° C, and the operating temperature in its lower part is in the range of 60-140 ° C.

In an additional preferred embodiment of the present invention, the working pressure on the pressure gauge in the upper part of the second separation column is in the range of 0.1-0.8 MPa, more preferably in the range of 0.15-0.5 MPa, the working temperature in its upper part is the range is 56-109 ° C, and the operating temperature in its lower part is in the range of 81-139 ° C, more preferably in the range of 91-129 ° C.

According to the present invention, the second separation column may be a packed column or a plate column with a number of theoretical plates in the range of 10-50, preferably in the range of 15-45.

In accordance with the method of the present invention, separation of dimethyl carbonate and methanol can occur in the apparatus for the separation of methanol and dimethyl carbonate as a result of a process selected from the group consisting of membrane separation, extraction distillation and variable pressure distillation. The separation technology of dimethyl carbonate and methanol is well known in the art. For example, in literature sources CN 200610169592.5, CN 200710064633, CN 200710121912, CN 200810145291 and CN 201310034796 a mixture of dimethyl carbonate and methanol is separated using a membrane, since the membrane has different osmotic selectivity for methanol and dimethyl carbonate. According to literature source CN 201310098177, the mixture of dimethyl carbonate and methanol to be separated is fed at atmospheric pressure to an extraction distillation column in its middle section, and the extracting agent (which is ethylene glycol) is fed to its upper part with the ratio of extracting agent to the liquid to be separated. , in the range from 1 to 3 and the reflux ratio is 2. High-purity methanol is recovered in the upper part of the distillation column, and dimethyl carbonate and the extracting agent from attract in its lower part. The fraction from the bottom of the distillation column enters the distillation column of the extracting agent with a reflux ratio of 3. Dimethyl carbonate is recovered in the upper part of the distillation column of the extracting agent, and the extracting agent is recovered in its lower part. Extractive agent can be reused. In the literature source CN 101381309 B, dimethyl carbonate is isolated from a mixture of dimethyl carbonate and methanol using azeotropic distillation under reduced pressure and azeotropic distillation under high pressure, performed in a process with paired columns.

In accordance with the method of the present invention, the crude methanol stream containing methyl nitrite, methyl formate and dimethyl carbonate is preferably fed to the first separation column in its upper part. Since methyl nitrite has a low boiling point, it can be relatively simple to isolate. The stream containing methyl nitrite is preferably fed to the first separation column in its upper part. In this case, the reflux condenser of the first separation column may be absent.

In accordance with the method of the present invention, the crude methanol stream containing methyl formate and dimethyl carbonate is preferably fed to the second separation column in its middle section.

Since the preparation of dimethyl oxalate from synthesis gas is a process preceding the process of producing ethylene glycol, the present invention additionally provides a method for recycling methanol in a method for producing ethylene glycol from synthesis gas, which includes the steps described above for the method of recycling methanol in the process for producing dimethyl oxalate from synthesis gas.

In addition, the present invention further provides a method for producing dimethyl oxalate from synthesis gas, comprising the steps of:

a) carrying out the reaction of methanol, oxygen and NO in the esterification reactor and obtaining a gas-phase reaction stream containing methyl nitrite and a liquid phase stream containing methanol, water, nitric acid, methyl nitrite, methyl formate and dimethyl carbonate, then obtaining a crude stream I of methanol containing methyl nitrite, methyl formate and dimethyl carbonate, after removing the heavy components from the liquid phase stream,

b) carrying out the reaction between CO isolated from synthesis gas and the gas-phase reaction stream obtained in step a) in the presence of a catalyst containing elements of the platinum group in a condensation reactor and obtaining a reaction stream containing dimethyl oxalate, methyl nitrite, methyl formate, dimethyl carbonate, NO and methanol,

c) isolating dimethyl oxalate from the reaction stream obtained in step b) and obtaining crude methanol stream II containing methyl nitrite, methyl formate and dimethyl carbonate, and a gas phase stream containing NO, then returning said gas phase stream to the esterification reactor in step a),

d) recycling methanol from crude methanol stream I obtained in step a) and / or crude methanol stream II obtained in step b), and optionally from any other crude methanol streams containing methylnitrite, methyl formate and dimethyl carbonate, which are formed in the process of producing dimethyl oxalate, using the aforementioned methanol recycling method of the present invention, and

e) returning the methanol obtained in step d) to the esterification reactor in step a) and / or to the absorption column of the dimethyl oxalate production process, which requires methanol.

In the process for producing dimethyl oxalate from synthesis gas according to the present invention, the conditions in steps a) and b) are generally accepted and well known in the art.

In accordance with the method of the present invention, a crude methanol stream containing methyl nitrite, methyl formate and dimethyl carbonate is fed to the first separation column (nitrite recovery column) and methylnitrite is obtained in the upper part of the first separation column. The crude methanol stream, from which methylnitrite is removed, is fed to the second separation column (light component removal column) and methylformate, which is the lighter component, is removed in the upper part of the second separation column. After removal of methyl formate, the crude methanol stream contains a small amount of dimethyl carbonate. After further separation, methanol is sent for further use, and dimethyl carbonate is discharged as a product.

In accordance with the method of the present invention, the amount of methyl formate accumulating in the system can be reduced, and therefore the likelihood of corrosion of pipelines and equipment can be reduced. The purity of the circulating methanol can be increased, and the volume of the circulating methanol can be reduced and, consequently, the power consumption can be reduced. The present invention has an obvious economic advantage and can achieve a favorable technical effect.

A brief description of the accompanying graphics

FIG. 1 is a flowchart of a process according to the present invention.

Detailed description of embodiments of the invention.

The present invention will be further described with reference to the accompanying graphic materials and embodiments, which in no case should not be interpreted as limiting the present invention.

As shown in FIG. 1, a crude stream I of methanol containing methyl nitrite, methyl formate and dimethyl carbonate is first fed to the first T-101 separation column and under the separation conditions methyl nitrite 2 in the upper part is isolated. Methyl nitrite 2 can be returned to a condensation reactor (not shown). The residue 3 from the bottom of the first separation column T-101 enters the second separation column T-102, and methyl formate 5 is extracted in the upper part of the second separation column T-102. Methyl formate 5 can be further used, for example, in the process of producing formic acid, formamide , High purity CO, etc. The residue 7 obtained from the second separation column T-102 contains a large amount of methanol and a small amount of dimethyl carbonate. A mixture of methanol and dimethyl carbonate is fed to an X-101 unit for the separation of methanol and dimethyl carbonate. Selected methanol 9 can be reused, for example, methanol can be fed to a previous reactor (not shown) to obtain dimethyl oxalate from synthesis gas. Dimethyl carbonate 8 is poured as a product.

Example 1

A crude stream I of methanol containing methyl nitrite, methyl formate and dimethyl carbonate is fed to the first separation column T-101. Methyl nitrite 2 is isolated in the upper part of the first separation column T-101 and returned to the condensation reactor. The residue 3 from the bottom of the first separation column T-101 is fed to the second separation column T-102, and in the upper part of the second separation column T-102 methyl formate is extracted 5. The residue 7 from the bottom of the second separation column containing a large amount of methanol and a small amount dimethyl carbonate, get in the bottom of the T-102. The residue 7 is fed to the X-101 unit for the separation of methanol and dimethyl carbonate, and after separation, methanol 9 is obtained, which is returned to the process for producing dimethyl oxalate from synthesis gas and used as a reagent in the reactor of the specified process or as an absorbent in the separation column of the specified process. Dimethyl carbonate 8 is poured as a product. The following are specific operating conditions.

The feed rate of the crude stream I of methanol containing methyl nitrite, methyl formate and dimethyl carbonate is 5000 kg / h. In the crude methanol stream I, the content of methyl nitrite is 0.5 wt. %, the content of methyl formate is 0.5 wt. %, the content of dimethylcarbonate is 5 mass. %, and the methanol content is 94 mass. %

The number of theoretical plates of the first separation column T-101 is 5, the working pressure in the upper part of the first separation column is 0.1 MPa (hereinafter, according to the gauge), the working temperature in its upper part is 81 ° C, and the working temperature in the lower its part is equal to 85 ° C.

The number of theoretical plates of the second separation column T-102 is 35, the working pressure in the upper part of the second separation column is 0.1 MPa, the working temperature in its upper part is 58 ° C, and the working temperature in its lower part is 94 ° C.

The composition of each of the main streams is presented in Table 1.

Under normal operating conditions, the load of the reboiler in the first separation column T-101 is 0.324 MW. The reboiler load in the T-102 second separation column is 0.384 MW, and the capacitor load in it is 0.353 MW.

Example 2

The implementation of example 2 is the same as in example 1, except for specific operating conditions, which are presented below.

The feed rate of the crude stream I of methanol containing methyl nitrite, methyl formate and dimethyl carbonate is 10,000 kg / h. In the crude methanol stream I, the content of methyl nitrite is 8 wt. %, the content of methyl formate is 20 mass. %, the content of dimethyl carbonate is 20 mass. %, and the methanol content is 52 mass. %

The number of theoretical plates of the first separation column T-101 is 25, the working pressure in the upper part of the first separation column is 0.7 MPa, the working temperature in its upper part is 30 ° C, and the working temperature in its lower part is 118 ° C.

The number of theoretical plates of the second separation column T-102 is 45, the working pressure in the upper part of the second separation column is 0.7 MPa, the working temperature in its upper part is 100 ° C, and the working temperature in its lower part is 129 ° C.

The composition of each of the main streams is presented in Table 2.

Under normal operating conditions, the load of the reboiler in the first separation column T-101 is 0.494 MW. The reboiler load in the T-102 second separation column is 1,100 MW, and the capacitor load in it is 1,137 MW.

Example 3

The implementation of example 3 is the same as in example 1, with the exception of specific operating conditions, which are presented below.

The feed rate of the crude stream I of methanol containing methyl nitrite, methyl formate and dimethyl carbonate is 10,000 kg / h. In the crude stream I of methanol, the content of methyl nitrite is 2 mass. %, the content of methyl formate is 1 mass. %, the content of dimethyl carbonate is 20 mass. %, and the methanol content is 77 mass. %

The number of theoretical plates of the first separation column T-101 is 15, the working pressure in the upper part of the first separation column is 0.5 MPa, the working temperature in its upper part is 43 ° C, and the working temperature in its lower part is 118 ° C.

The number of theoretical plates of the second separation column T-102 is 15, the working pressure in the upper part of the second separation column is 0.3 MPa, the working temperature in its upper part is 75 ° C, and the working temperature in its lower part is 105 ° C.

The composition of each of the main streams is presented in Table 3.

Under normal operating conditions, the load of the reboiler in the first separation column T-101 is 0.581 MW. The reboiler load in the second T-102 splitter column is 4,123 MW, and the capacitor load in it is 4.231 MW.

Example 4

The implementation of example 4 is the same as in example 1, except for specific operating conditions, which are presented below.

The feed rate of the crude stream I of methanol containing methyl nitrite, methyl formate and dimethyl carbonate is 25,000 kg / h. In the crude methanol stream I, the content of methyl nitrite is 4 wt. %, the content of methyl formate is 2 mass. %, the content of dimethyl carbonate is 10 mass. %, and the methanol content is 84 mass. %

The number of theoretical plates of the first separation column T-101 is 10, the working pressure in the upper part of the first separation column is 0.3 MPa, the working temperature in its upper part is 73 ° C, and the working temperature in its lower part is 95 ° C.

The number of theoretical plates of the second separation column T-102 is 30, the working pressure in the upper part of the second separation column is 0.15 MPa, the working temperature in its upper part is 58 ° C, and the working temperature in its lower part is 91 ° C.

The composition of each of the main streams is presented in Table 4.

Under normal operating conditions, the load of the reboiler in the first separation column T-101 is 1.164 MW. The reboiler load in the second T-102 splitter column is 3.776 MW, and the capacitor load in it is 3.863 MW.

Comparative Example 1

Technological stages and operating conditions of comparative example 1 can be the same as in example 2. In comparative example 1, the residue 3 from the bottom of the first separation column T-101 does not flow into the second separation column T-102, but directly goes to installation X- 101 for the separation of methanol and dimethyl carbonate. As a result, the content of methyl formate in the recycled solution of 8 methanol is 27.7 mass. %

Under such conditions, due to the high content of methyl formate in a solution of 8 methanol, it is not suitable for direct recycling and use in the reaction process. Instead, the methanol solution should be drained or further purified or disposed of as liquid waste. Even when recycling a solution of methanol can be used only a small part of it. At the same time, a large amount of fresh methanol is additionally required to meet production requirements. In addition, due to the high content of methyl formate, equipment may corrode. Ultimately, as methyl formate accumulates, methanol is to be discharged from the system.

Moreover, in this case, if it is impossible to isolate methyl formate in large quantities, the efficiency of the subsequent separation of methanol and dimethyl carbonate will be degraded.

Comparative Example 2

Technological stages and operating conditions of comparative example 2 may be the same as in example 4. In comparative example 2, the residue 3 from the bottom of the first separation column T-101 does not flow into the second separation column T-102, but directly enters the X- unit 101 for the separation of methanol and dimethyl carbonate. As a result, the content of methyl formate in the recycled solution of 8 methanol is 1.75 mass. %

Under these conditions, the amount of methanol returned to the reactor of dimethyl oxalate production from synthesis gas in comparative example 2 is greater than the amount of the corresponding methanol in example 4. As methyl formate accumulates over time, contaminated methanol ultimately needs to be unloaded . Consequently, a large amount of fresh methanol is required to meet production requirements. In addition, it was found that during the same period of operation, the equipment is more corrosive during operation in the mode according to comparative example 2 than in the mode according to example 4.

Although the present invention has been described in detail, those skilled in the art will recognize any modifications within the general spirit and scope of the present invention. It should be understood that various aspects, various implementation options, as well as the corresponding technical features mentioned herein, can be combined or partially or completely replaced with each other. In addition, specialists in this field of technology it is clear that the above description is only illustrative embodiments of the present invention, but it is not intended to limit the present invention.

List of Legend

T-101: first separation column

T-102: second separation column

X-101: installation for the separation of methanol and dimethylcarbonate,

1: crude methanol stream containing methyl nitrite, methyl formate and dimethyl carbonate,

2: recycled methyl nitrite,

3: the residue from the bottom of the first separation column,

4: gas leaving the top of the second separation column

5: methyl formate leaving in the upper part of the second separation column,

6: the upper countercurrent of the second separation column,

7: residue from the bottom of the second separation column,

8: recycled methanol solution, and

9: the final dimethyl carbonate.

Claims (21)

1. The method of recycling methanol in the process of obtaining dimethyl oxalate from synthesis gas, which includes the stage: i) feeding the crude methanol stream containing methyl nitrite, methyl formate and dimethyl carbonate to the first separation column and producing methyl nitrite in the upper part of the first separation column and the crude methanol stream containing methyl formate and dimethyl carbonate in its lower part, ii) feeding the crude methanol stream containing methyl formate and dimethyl carbonate into the second separation column and producing methyl formate in the upper part of the second separation column and the crude methanol stream containing dimethyl carbonate in its lower part, and iii) feeding the crude methanol stream containing dimethyl carbonate to a plant for separating dimethyl carbonate and methanol and producing dimethyl carbonate and methanol stream after separating. 2. The method according to claim 1, characterized in that the pressure on the pressure gauge in the upper part of the first separation column is in the range of 0-1.0 MPa, the working temperature in its upper part is in the range of 20-100 ° C, the working temperature in its lower part is in the range of 50-140 ° C; and / or the number of theoretical plates in the first separation column is in the range of 5-30. 3. The method according to claim 1, characterized in that the working pressure on the pressure gauge in the upper part of the second separation column is in the range of 0.1-1.0 MPa, the working temperature in its upper part is in the range of 30-120 ° C, and working temperature in its lower part is in the range of 60-140 ° C. 4. The method according to claim 3, characterized in that the working pressure on the pressure gauge in the upper part of the second separation column is in the range of 0.1-0.8 MPa, the working temperature in its upper part is in the range of 56-109 ° C, and working temperature in its lower part is in the range of 81-139 ° C. 5. The method according to claim 1 or 3, characterized in that the second separation column is a packed column or a tray column with a number of theoretical plates in the range of 10-50. 6. The method according to claim 1, characterized in that in an apparatus for separating dimethyl carbonate and methanol, separation of methanol and dimethyl carbonate takes place as a result of a process selected from the group consisting of membrane separation, extraction distillation and variable pressure distillation. 7. The method according to claim 1, characterized in that the crude methanol stream containing methyl nitrite, methyl formate and dimethyl carbonate is fed to the first separation column in its upper part. 8. The method according to claim 1, characterized in that the crude methanol stream containing methyl formate and dimethyl carbonate is fed to the second separation column in its middle section. 9. The method according to claim 1, characterized in that the synthesis gas is obtained from coal, natural gas, coke oven gas, blast furnace top gas, tail gas after calcining calcium carbide or oil. 10. The method according to claim 1, characterized in that in the crude stream of methanol containing methyl nitrite, methyl formate and dimethyl carbonate, the content of methyl nitrite is in the range of 0.1-10 wt.%, The content of methyl formate is in the range of 0.1-25 wt. %, the content of dimethyl carbonate is in the range of 0.1-28 wt.% and the methanol content is in the range of 50-99 wt.%. 11. The method according to claim 1, characterized in that the methanol recycled in step iii) is directly sent to the esterification reactor of the process for producing dimethyl oxalate from synthesis gas as a reactant or into the separation columns of the said process as an absorbent. 12. A method of producing dimethyl oxalate from synthesis gas, comprising the steps of: a) carrying out the reaction of methanol, oxygen and NO in the esterification reactor and obtaining a gas-phase reaction stream containing methyl nitrite and a liquid phase stream containing methanol, water, nitric acid, methyl nitrite, methyl formate and dimethyl carbonate, then obtaining a crude stream I of methanol containing methyl nitrite, methyl formate and dimethyl carbonate, after removing the heavy components from the liquid phase stream, b) carrying out the reaction between CO isolated from synthesis gas and the gas-phase reaction stream obtained in step a) in the presence of a catalyst containing elements of the platinum group in a condensation reactor and obtaining a reaction stream containing dimethyl oxalate, methyl nitrite, methyl formate, dimethyl carbonate, NO and methanol, c) isolating dimethyl oxalate from the reaction stream obtained in step b) and obtaining crude methanol stream II containing methyl nitrite, methyl formate and dimethyl carbonate, and a gas phase stream containing NO, then returning said gas phase stream to the esterification reactor in step a), d) recycling methanol from crude methanol stream I obtained in step a) and / or crude methanol stream II obtained in stage b) using methanol recycling method in accordance with any one of claims 1 to 11, and e) returning the methanol obtained in step d) to the esterification reactor in step a) and / or to the absorption column of the dimethyl oxalate production process, which requires methanol.

Images