CA2896284C

CA2896284C - Method for recycling methanol in the process of preparing dimethyl oxalate from synthesis gas

Info

Publication number: CA2896284C
Application number: CA2896284A
Authority: CA
Inventors: Weisheng Yang; Weimin Yang; Laibin He; Mujin Li; Song Hu
Original assignee: China Petroleum and Chemical Corp; Sinopec Shanghai Research Institute of Petrochemical Technology
Current assignee: China Petroleum and Chemical Corp; Sinopec Shanghai Research Institute of Petrochemical Technology
Priority date: 2014-07-03
Filing date: 2015-07-03
Publication date: 2022-06-21
Anticipated expiration: 2035-07-03
Also published as: PL413013A1; RU2015126483A; RU2015126483A3; RU2689569C2; CN105218306A; CN105218306B; AU2015203733B2; PL230738B1; CA2896284A1; AU2015203733A1

Abstract

A method for recycling methanol in a process of preparing dimethyl oxalate from synthesis gas is provided according to the present disclosure, comprising the steps of: i) a crude methanol stream containing methyl nitrite, methyl formate, and dimethyl carbonate is fed into a first separation column, and methyl nitrite is obtained at a top thereof and a crude methanol stream containing methyl formate and dimethyl carbonate is obtained at a bottom thereof; ii) the crude methanol stream containing methyl formate and dimethyl carbonate is fed into a second separation column, and methyl formate is obtained at a top thereof and a crude methanol stream containing dimethyl carbonate is obtained at a bottom thereof; and iii) the crude methanol stream containing dimethyl carbonate is fed into a dimethyl carbonate-methanol separation unit, and dimethyl carbonate and methanol stream to be recycled are obtained after separation. According to the method of the present disclosure, methyl formate accumulated in the system can be reduced, and thus the corrosion of the pipelines and the equipment can be alleviated. In addition, the purity of methanol can be improved, and thus the circulation volume thereof can be reduced, thereby the power consumption can be lowered. The present method has evident economical benefit.

Description

METHOD FOR RECYCLING METHANOL IN THE PROCESS OF PREPARING
DIMETHYL OXALATE FROM SYNTHESIS GAS
Technical field The present invention relates to the technical field of the preparation of dimethyl oxalate from synthesis gas, and in particular to a method for recycling methanol in the process of preparing dimethyl oxalate from synthesis gas.
Background of the Invention A gas-phase catalytic coupling reaction of CO at atmospheric pressure for preparing dimethyl oxalate (DMO) mainly comprises two steps, including a coupling reaction and an oxidative esterification reaction. The oxidative esterification reaction generates methyl nitrite (MN), which acts as oxygen carrier and intermediate in the coupling reaction. In turn, NO
produced in the coupling reaction is a reactant in the oxidative esterification reaction. As a result, coordination between the coupling reaction and the oxidative esterification reaction is a key to realizing a green and non-polluting circulatory system.
The equation for the coupling reaction is as follows:
2CH3ONO + 2C0---,(COOCH3)2 + 2N0 The equation for the oxidative esterification reaction is as follows:

2N0 + 2CH OH + ¨02 2CH3ONO + H20

2 Date Recue/Date Received 2021-11-16 The methyl nitrite produced in the oxidative esterification reaction is circulated back to the coupling reaction.
The overall reaction equation is as follows:
2CH3OH + 2C0 + ¨1 0, (COOCH3)2 + H20 In the process of synthesizing dimethyl oxalate, a gas-phase catalytic coupling reaction between methyl nitrite and CO is performed on the catalyst, and dimethyl oxalate is generated. In the meantime, side reaction occurs and dimethyl carbonate (DMC) is generated. The main side reaction is as follows:
2CH3ONO + CO ¨> CO(0C113)2 2N0 Methyl nitrite might decompose and generate NO, methyl formate (MF), and methanol. The reaction equation is as follows:
4 CH3ONO ¨>4 NO+ C2H402+ 2 CH30 H
Under the condition of water presence, both methyl formate and dimethyl carbonate will experience hydrolysis reaction. The hydrolysis reaction of methyl formate produces formic acid and methanol. Formic acid is strongly corrosive, and thus might corrode the equipment. The hydrolysis reaction of dimethyl carbonate produces methanol and CO2. CO2 recycles and accumulates in the system, and is hard to be separated. The hydrolysis reaction equations are as follows:
CHOOCH3+H20 HCOOH+ CH3OH
CO(0C113)2 + H20 CO2+ 2CH3OH
In this case, the recycle of methyl nitrite from crude methanol, and the separation of methyl formate and dimethyl carbonate will significantly influence the safety and the economical efficiency of the production technology of ethylene glycol.
In the prior art, methods for recycling methanol from the process of preparing dimethyl oxalate through CO catalytic coupling reaction are provided. For example, according to literature CN102911059A, gas-phase and liquid-phase streams containing methyl nitrite are introduced into a rectifying column. The overhead stream of the rectifying column contains methyl nitrite, and the stream at the bottom thereof contains methanol and water. The plate number of the rectifying column is in a range of 10 to 50, the temperature of the bottom thereof is in a range of 50 to 200 C, the temperature of the top thereof is in a range of 10 to 100 C, the reflux ratio is in a range of 0.2 to 3.0, and the operation pressure is in a range of 50 to 400 KPa.
At present, a method for separating the azeotropic mixture of dimethyl carbonate and methanol includes the process selected from the group consisting of extractive distillation, membrane separation, and variable pressure distillation. According to literatures CN200610169592.5, CN200710064633, CN200710121912, CN200810145291, and CN201310034796, the mixture of dimethyl carbonate-methanol is separated through a membrane, because the membrane has different osmotic selectivities for methanol and dimethyl carbonate. However, the purity of the stream obtained cannot be proved to have high economical efficiency, and thus the stream, in many cases, should be further distilled.
According to literature CN201310098177, under atmospheric pressure, a dimethyl carbonate methanol mixture to be separated is fed to the extraction rectifying column from an intermediate section thereof, and an extraction agent (which is ethylene glycol) is fed from the top thereof, with a solvent ratio in a range of 1 to 3 and a reflux ratio being 2. High purity methanol is recovered from the top of the extraction rectifying column, and dimethyl carbonate and the extraction agent are recovered from the bottom thereof. The fraction at the bottom of the extraction rectifying column enters into an extraction agent recovery column, with a reflux ratio being 3.
Dimethyl carbonate is recovered from the top of the extraction rectifying column, and the extraction agent is recovered from the bottom thereof. The extraction agent can be recycled.
In literature CN101381309B, dimethyl carbonate is separated from the dimethyl carbonate-methanol mixture through a reduced pressure azeotropic distillation-high pressure azeotropic distillation twin column process. In addition, methanol is recycled.
However, in the prior art, there is no disclosure about the removal of methyl formate in the process of preparing ethylene glycol from, e.g., coal-based synthesis gas. In actual production equipment, the production of methyl formate cannot be avoided. In the process of preparing ethylene glycol from synthesis gas, because large amount of methanol is used, considering the economical efficiency, it must be recycled. If too much methyl formate is accumulated in the recycled methanol,

3 on the one hand, the methyl formate will decompose and produce formic acid which corrodes the equipment and the pipes, and on the other hand, the concentration of methanol will be reduced, and thus the circulation volume thereof will be increased, thereby the power consumption would be increased. Therefore, the separation of methyl formate is very crucial.
Summary of the Invention The objective of the present disclosure is to provide a method for recycling methanol in the process of preparing dimethyl oxalate from synthesis gas. The methanol recycled through the method according to the present disclosure has high purity, and can be directly circulated back to a dimethyl oxalate production process as a reactant or an absorbent therein, thereby the recycling efficiency of methanol can be improved. In addition, the method has the advantages of simple steps and low power consumption. In the meantime, according to the method, not only the corrosion of the process equipment and the pipelines caused by the accumulation of residual methyl formate in the methanol can be reduced, the quality of product ethylene glycol prepared downstream of the process of preparing dimethyl oxalate can also be improved.
According to the present disclosure, a method for recycling methanol in a process of preparing dimethyl oxalate from synthesis gas is provided, comprising the steps of:
i) feeding a crude methanol stream containing methyl nitrite, methyl formate, and dimethyl carbonate into a first separation column, and obtaining methyl nitrite at a top of the first separation column and a crude methanol stream containing methyl formate and dimethyl carbonate at a bottom thereof;
ii) feeding the crude methanol stream containing methyl formate and dimethyl carbonate into a second separation column, and obtaining methyl formate at a top of the second separation column and a crude methanol stream containing dimethyl carbonate at a bottom thereof;
and iii) feeding the crude methanol stream containing dimethyl carbonate into a dimethyl carbonate-methanol separation unit, and obtaining a dimethyl carbonate stream and a methanol stream after separation.
The method according to the present disclosure can also be interpreted as a method for treating waste in the process of preparing dimethyl oxalate from synthesis gas.
According to the method of waste disposal, methyl fonnate can also be recycled, thereby the quality of the recycled methanol can

4 be improved. The waste according to the present disclosure generally refers to any stream containing methyl nitrite, methyl formate, dimethyl carbonate, and methanol produced in the process of preparing dimethyl oxalate from synthesis gas.
As stated in the technical background section, in the prior art, it is unlikely that the removal of methyl formate in the liquid waste (i.e., product stream of the process of preparing dimethyl oxalate from synthesis gas, from which dimethyl oxalate is removed), which is produced in the process of preparing dimethyl oxalate from synthesis gas, will be taken into consideration, and thus the byproduct methyl formate tends to remain in the recycled methanol. As the methyl formate accumulates, the decomposition of the methyl formate produces formic acid which corrodes the equipment and the pipelines. In the meantime, the concentration of methanol would be reduced.
Consequently, the circulation volume of methanol would be increased, rendering the power consumption to be increased, or the recycled methanol should be discharged, e.g. for further treatment. However, the inventor of the present disclosure found out the above problem, and made it an important issue. The inventor discovered through experiments and comparison that separating the methyl formate can significantly influence the economical efficiency of the production process of dimethyl oxalate and that of the subsequent production process of ethylene glycol. Moreover, the methyl formate separated can also be recycled for further use, and yield certain industrial value and economical benefit. The present disclosure can thus be realized.
In a preferred embodiment of the present disclosure, the methanol recycled in step iii) is directly circulated to an esterification reactor of the process of preparing dimethyl oxalate from synthesis gas as a reactant, or to the separation columns thereof as an absorbent. "Directly" means that the methanol recycled in step iii) does not need to be further purified, but rather directly be circulated to a synthesis reactor of the process of preparing dimethyl oxalate from synthesis gas as a reactant, or back to the separation columns thereof as an absorbent.
The methyl formate separated in said step i) can also be directly circulated back to a coupling reactor, in which dimethyl oxalate is prepared through CO catalytic coupling reaction.
According to the present disclosure, the synthesis gas can be any synthesis gas containing CO
and hydrogen. For example, the synthesis gas can be prepared from coal, natural gas, coke-oven gas,

5 blast furnace gas, tail gas of a calcium carbide furnace or petroleum.
Preferably, in the crude methanol stream containing methyl nitrite, methyl formate, and dimethyl carbonate in step i), a content of methyl nitrite is in a range of 0.1-10 wt%, preferably in a range of 0.5-8 wt%, a content of methyl formate is in a range of 0.1-25 wt%, preferably in a range of 0.5-25 wt%, a content of dimethyl carbonate is in a range of 0.1-28 wt%, preferably in a range of 0.5-24 wt%, and a content of methanol is in a range of 50-99 wt%, preferably in a range of 50-96 wt%.
In a preferred embodiment according to the present disclosure, an operating pressure by gage pressure at the top of the first separation column is in a range of 0-1.0 MPa, preferably in a range of 0.1-0.8 MPa, an operating temperature at the top thereof is in a range of 20-100 C, preferably in a range of 30-80 C, and an operating temperature at the bottom thereof is in a range of 50-140 C, preferably in a range of 86-129 C.
In the present disclosure, unless otherwise specified, the pressure refers to a gage pressure.
According to the present disclosure, the first separation column can be a packed column or a plate column. Preferably, a number of theoretical plates of the first separation column is in a range of 5-30, preferably in a range of 10-25.
In a preferred embodiment according to the present disclosure, an operating pressure by gage pressure at the top of the second separation column is in a range of 0.1-1.0 MPa, an operating temperature at the top thereof is in a range of 30-120 C, and an operating temperature at the bottom thereof is in a range of 60-140 C.
In a further preferred embodiment according to the present disclosure, an operating pressure by gage pressure at the top of the second separation column is in a range of 0.1-0.8 MPa, further preferably in a range of 0.15-0.5 MPa, an operating temperature at the top thereof is in a range of 56-109 C, and an operating temperature at the bottom thereof is in a range of 81-139 C, further preferably in a range of 91-129 C.

6 According to the present disclosure, the second separation column can be a packed column or a plate column, with a number of theoretical plates being in a range of 10-50, preferably in a range of 15-45.
According to the method of the present disclosure, in the dimethyl carbonate-methanol separation unit, methanol and dimethyl carbonate can be separated through a process selected from a group consisting of membrane separation, extraction distillation, and variable pressure distillation.
The dimethyl carbonate-methanol separating technologies are well known in the art. For example, in literatures CN200610169592.5 , CN200710064633 , CN200710121912 CN200810145291 and CN201310034796, the mixture of dimethyl carbonate-methanol is separated through a membrane, because the membrane has different osmotic selectivities for methanol and dimethyl carbonate.
According to literature CN201310098177, under atmospheric pressure, a dimethyl carbonate-methanol mixture to be separated is fed to the extraction rectifying column from an intermediate section thereof, and an extraction agent (which is ethylene glycol) is fed from the top thereof, with a ratio of the extraction agent to the liquid to be separated being in a range of 1 to 3 and a reflux ratio being 2. High purity methanol is recovered from the top of the rectifying column, and dimethyl carbonate and the extraction agent are recovered from the bottom thereof. The fraction at the bottom of the rectifying column enters into an extraction agent recovery column, with a reflux ratio being 3.
Dimethyl carbonate is recovered from the top of the extraction agent recovery column, and the extraction agent is recovered from the bottom thereof. The extraction agent can be recycled. In literature CN101381309B, dimethyl carbonate is separated from the dimethyl carbonate-methanol mixture through a reduced pressure azeotropic distillation-high pressure azeotropic distillation twin column process.
According to the method of the present disclosure, the crude methanol stream containing methyl nitrite, methyl formate, and dimethyl carbonate is preferably fed into the first separation column from the top thereof. Because methyl nitrite has low boiling point, the separation thereof can be relatively easy. A stream containing methyl nitrite is preferably fed into the first separation column from the top thereof. In this case, a reflux unit of the first separation column can be omitted.
According to the method of the present disclosure, the crude methanol stream containing methyl formate and dimethyl carbonate is preferably fed into the second separation column from an

7 intermediate section thereof.
Since preparing dimethyl oxalate from synthesis gas is an upstream process of the process of preparing ethylene glycol, the present disclosure further provides a method for recycling methanol in the process of preparing ethylene glycol from synthesis gas, comprising the above steps of the method for recycling methanol in the process of preparing dimethyl oxalate from synthesis gas.
Moreover, the present disclosure further provides a method for preparing dimethyl oxalate from synthesis gas, comprising the steps of:
a) carrying out a reaction of methanol, oxygen and NO in an esterification reactor, and producing 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, subsequently obtaining a crude methanol stream I containing methyl nitrite, methyl formate, and dimethyl carbonate after removal of heavy components in the liquid-phase stream, b) carrying out a reaction between CO, which is separated from the synthesis gas, and the gas-phase reaction stream, which is obtained in step a), in the presence of a platinum group elements containing catalyst in a coupling reactor, and producing a reaction stream containing dimethyl oxalate, methyl nitrite, methyl formate, dimethyl carbonate, NO and methanol, c) separating dimethyl oxalate from the reaction stream obtained in step b), and obtaining a crude methanol stream II containing methyl nitrite, methyl formate, and dimethyl carbonate and a gas-phase stream containing NO, subsequently circulating said gas-phase stream back to the esterification reactor in step a), d) recycling methanol from the crude methanol stream I obtained in step a), and/or from the crude methanol stream II obtained in step b), and optionally from any other crude methanol streams containing methyl nitrite, methyl formate and dimethyl carbonate, which are produced in the process of preparing the dimethyl oxalate, using the abovementioned method for recycling methanol according to the present disclosure, and e) circulating the methanol obtained in step d) to the esterification reactor in step a), and/or to an absorption column of the process of preparing dimethyl oxalate, which absorption column requiring the methanol.

8 In the method for preparing dimethyl oxalate from synthesis gas according to the present disclosure, the conditions in steps a) and b) are conventional and well known in the art.
According to the method of the present disclosure, the crude methanol stream containing methyl nitrite, methyl formate, and dimethyl carbonate is fed into the first separation column (a nitrite recovery column), and methyl nitrite is obtained at the top of the first separation column. The crude methanol stream, from which methyl nitrite has been removed, is fed into the second separation column (a light component removal column), and methyl formate, which is a lighter component, is removed at the top of the second separation column. After methyl formate is removed, the crude methanol stream contains small amount of dimethyl carbonate. After further separation, the methanol is circulated for further use, and the dimethyl carbonate is withdrawn as a product.
According to the method of the present disclosure, methyl formate accumulated in the system can be reduced, and thus the possibility of corrosion of the pipelines and the equipment can be lowered. The purity of the circulating methanol can be increased, and the circulation volume of methanol can be reduced, and thus the power consumption can be reduced. The present disclosure has obvious economical benefit, and can achieve favorable technical effect.
Brief Description of the Accompanying Drawings Fig. 1 shows a flow diagram of a process for carrying out a method according to the present disclosure.
Detailed Description of the Embodiments The present disclosure will be further described in view of the accompanying drawings and the embodiments, which should in no case be construed as limitation to the present disclosure.
As shown in Fig. 1, a crude methanol stream I containing methyl nitrite, methyl formate, and dimethyl carbonate is fed into a first separation column T-101 first, and methyl nitrite 2 is recovered at a top thereof under separation conditions. The methyl nitrite 2 can be circulated back to a coupling reactor (not shown). A residue 3 from a bottom of the first separation column T-101 enters into a second separation column T-102, and methyl formate 5 is removed from a top of the second

9 separation column T-102. The methyl formate 5 can be further used, such as in the preparation process of formic acid, formamide, high-purity CO, and the like. A residue 7 obtained from the second separation column T-102 comprises large amount of methanol and small amount of dimethyl carbonate. A mixture of methanol and dimethyl carbonate is fed into a methanol-dimethyl carbonate separation unit X-101. Methanol 9 separated can be recycled, for example, the methanol can be fed into an upstream reactor (not shown) for preparing dimethyl oxalate from synthesis gas. Dimethyl carbonate 8 is withdrawn as a product.
Example 1 A crude methanol stream I containing methyl nitrite, methyl formate, and dimethyl carbonate is fed into a first separation column T-101. Methyl nitrite 2 is recovered at a top of the first separation column T-101, and circulated back to a coupling reactor. A residue 3 from a bottom of the first separation column T-101 is fed into a second separation column T-102, and methyl formate 5 is .. removed from a top of the second separation column T-102. A residue 7 from the bottom of the second separation column containing large amount of methanol and small amount of dimethyl carbonate is produced at a bottom of T-102. The residue 7 is fed into a methanol-dimethyl carbonate separation unit X-101, and methanol 9 is obtained after separation, and circulated back to a process of preparing dimethyl oxalate from synthesis gas and used as a reactant in a reactor thereof, or as an absorbent in a separation column thereof. Dimethyl carbonate 8 is withdrawn as a product. Specific operating conditions are as follows.
A feeding rate of the crude methanol stream I containing methyl nitrite, methyl formate, and dimethyl carbonate is 5000 kg/hr. In the crude methanol stream I, a content of methyl nitrite is 0.5 wt%, that of methyl formate is 0.5 wt%, that of dimethyl carbonate is 5 wt%, and that of methanol is 94 wt%.
A number of theoretical plates of the first separation column T-101 is 5, an operating pressure at a top of the first separation column is 0.1 MPa (by gage pressure, the same hereinafter), an operating .. temperature at the top thereof is 81 C, and an operating temperature at a bottom thereof is 85 C.
A number of theoretical plates of the second separation column T-102 is 35, an operating pressure at a top of the second separation column is 0.1 MPa, an operating temperature at the top = , thereof is 58 C, and an operating temperature at a bottom thereof is 94 C.
The composition of each of the main streams is shown in Table 1.
Table 1 Streams Parameters Temperature C 40 81 85 40 87 87 Pressure kPa 400 100 120 80 600 600 600 MN 0.50% 4.44% - -Content of MF 0.50% 3.82% 0.08% 95.47%
components Me0H 94.00% 84.86% 95.16% 4.31% 95.23% 99.95% <0.01%
wt%
DMC 5.00% 6.88% 4.76%
4.77% 0.05% >99.99%
Under normal operating conditions, a load of a reboiler in the first separation column T-101 is 0.324 MW. A load of a reboiler in the second separation column T-102 is 0.384 MW, and that of a condenser therein is 0.353 MW.
Example 2 The implementation of example 2 is the same as example 1, except for the specific operating conditions, which are as follows.
A feeding rate of a crude methanol stream I containing methyl nitrite, methyl formate, and dimethyl carbonate is 10000 kg/hr. In the crude methanol stream I, a content of methyl nitrite is 8 wt%, that of methyl formate is 20 wt%, that of dimethyl carbonate is 20 wt%, and that of methanol is 52 wt%.
A number of theoretical plates of the first separation column T-101 is 25, an operating pressure at a top of the first separation column is 0.7 MPa, an operating temperature at the top thereof is 30 C, and an operating temperature at a bottom thereof is 118 C.

A number of theoretical plates of the second separation column T-102 is 45, an operating pressure at a top of the second separation column is 0.7 MPa, an operating temperature at the top thereof is 100 C, and an operating temperature at a bottom thereof is 129 C.
The composition of each of the main streams is shown in Table 2.
Table 2 Stream Parameters Temperature C 40 30 118 40 129 129 Pressure kPa 400 700 720 680 600 600 MN 8.00% 98.42%
Content of MF 20.00% 1.06% 21.68% 90.43% 0.03% 0.04%
components Me0H 52.00% 0.44% 56.56% 9.57% 71.36% 99.56% <0.01%
wt%
DMC 20.00% 0.09% 21.76%
28.61% 0.40% >99.99%
Under normal operating conditions, a load of a reboiler in the first separation column T-101 is 0.494 MW. A load of a reboiler in the second separation column T-102 is 1.100 MW, and that of a condenser therein is 1.137 MW.
Example 3 The implementation of example 3 is the same with example 1, except for the specific operating conditions, which are as follows.
A feeding rate of a crude methanol stream I containing methyl nitrite, methyl formate, and dimethyl carbonate is 10000 kg/hr. In the crude methanol stream I, a content of methyl nitrite is 2 wt%, that of methyl formate is 1 wt%, that of dimethyl carbonate is 20 wt%, and that of methanol is 77 wt%.
A number of theoretical plates of the first separation column T-101 is 15, an operating pressure at a top of the first separation column is 0.5 MPa, an operating temperature at the top thereof is 43 C, and an operating temperature at a bottom thereof is 118 C.
A number of theoretical plates of the second separation column T-102 is 15, an operating pressure at a top of the second separation column is 0.3 MPa, an operating temperature at the top thereof is 75 C, and an operating temperature at a bottom thereof is 105 C.
The composition of each of the main streams is shown in Table 3.
Table 3 Stream Parameters Temperature C 40 43 118 40 105 105 Pressure kPa 400 500 520 280 600 600 MN 2.00% 96.66%
Content of MF 1.00% 0.29% 1.02% 90.91%
components Me0H 77.00% 2.50% 78.57% 9.04% 79.36% 99.74% <0.01%
wt%
DMC 20.00% 0.55% 20.41% 0.04% 20.64% 0.26% >99.99%
Under normal operating conditions, a load of a reboiler in the first separation column T-101 is 0.581 MW. A load of a reboiler in the second separation column T-102 is 4.123 MW, and that of a condenser therein is 4.231 MW.
Example 4 The implementation of example 4 is the same with example 1, except for the specific operating conditions, which are as follows.
A feeding rate of a crude methanol stream I containing methyl nitrite, methyl formate, and dimethyl carbonate is 25000 kg/hr. In the crude methanol stream I, a content of methyl nitrite is 4 wt%, that of methyl formate is 2 wt%, that of dimethyl carbonate is 10 wt%, and that of methanol is 84 wt%.

A number of theoretical plates of the first separation column T-101 is 10, an operating pressure at a top of the first separation column is 0.3 MPa, an operating temperature at the top thereof is 73 C, and an operating temperature at a bottom thereof is 95 C.
A number of theoretical plates of the second separation column T-102 is 30, an operating pressure at a top of the second separation column is 0.15 MPa, an operating temperature at the top thereof is 58 C, and an operating temperature at a bottom thereof is 91 C.
The composition of each of the main streams is shown in Table 4.
Table 4 Stream Parameters Temperature C 40 73 95 40 91 91 91 Pressure kPa 400 200 220 130 400 400 400 MN 4.00% 60.76% 0.01%
Content of MF 2.00% 8.08% 1.57% 96.36%
components Me0H 84.00% 27.20% 88.00% 3.63% 89.40% 99.88% <0.01%
wt%
DMC 10.00% 3.96% 10.43%

10.60% 0.12% >99.99%
Under normal operating conditions, a load of a reboiler in the first separation column T-101 is 1.164 MW. A load of a reboiler in the second separation column T-102 is 3.776 MW, and that of a condenser therein is 3.863 MW.
Comparison example 1 The operating steps and operating conditions of comparison example 1 can be made reference to example 2. In comparison example 1, a residue 3 from a bottom of a first separation column T-101 does not enter a second separation column T-102, but rather directly enters into a methanol-dimethyl carbonate separation unit X-101. As a result, in recycled methanol solution 8, a content of methyl formate is 27.7 wt%.

Under such circumstances, due to the large content of methyl formate therein, the methanol solution 8 is not suitable to be directly recirculated and used in the reaction process. Instead, the methanol solution should be discharged, or further treated, or disposed as liquid waste. Even if the methanol solution is recirculated, only a small part thereof can be reused. In the meantime, large amount of fresh methanol is additionally required, so that the production requirements can be satisfied. In addition, due to large content of methyl formate, the equipment is apt to be corroded. As methyl formate accumulates, waste methanol would have to be discharged at last.
Furthermore, in this case, if the high content methyl formate fails to be separated, subsequent separation efficiency of methanol and dimethyl carbonate will be influenced.
Comparison example 2 The operating steps and operating conditions of comparison example 2 can be made reference to example 4. In comparison example 2, a residue 3 from a bottom of a first separation column T-101 does not enter into a second separation column T-102, but rather directly enters into a methanol-dimethyl carbonate separation unit X-101. As a result, a content of methyl formate in recycled methanol solution 8 is 1.75 wt%.
Under such circumstances, the amount of methanol circulated back to the reactor of the process of preparing dimethyl oxalate from synthesis gas of comparison example 2 is larger than that of example 4. As methyl formate accumulates over time, waste methanol would have to be discharged at last. Consequently, large amount of fresh methanol is required, so that the production requirements can be satisfied. In addition, it is discovered that within the same service time, the equipment suffers from more obvious corrosion when being operated in a mode according to comparison example 2 than in a mode according to example 4.
Although the present disclosure has been described in detail, any modifications within the spirit and scope of the present disclosure will be apparent for those skilled in the art. It should be understood that various aspects, different embodiments, as well as the respective technical features mentioned herein may be combined, or partially or completely exchanged with one another. In addition, those skilled in the art can understand that the above descriptions merely constitute exemplary implementing manners of the present disclosure, but are not intended to limit the present disclosure.

List of Reference Signs T-101: first separation column 1-102: second separation column X-101: methanol-dimethyl carbonate separation unit, 1: crude methanol stream containing methyl nitrite, methyl formate and dimethyl carbonate, 2: recycled methyl nitrite, 3: residue from a bottom of the first separation column, 4: overhead gas from the second separation column, 5: methyl formate from the top of the second separation column, 6: top reflux of the second separation column, 7: residue from the bottom of the second separation column, 8: recycled methanol solution, and 9: product dimethyl carbonate.

Claims

1. A method for recycling methanol in a process of preparing dimethyl oxalate from synthesis gas, comprising the steps of:
i) feeding a crude methanol stream containing methyl nitrite, methyl formate, and dimethyl carbonate into a first separation column, and obtaining methyl nitrite at a top of the first separation column and a crude methanol stream containing methyl formate and dimethyl carbonate at a bottom thereof, ii) feeding the crude methanol stream containing methyl formate and dimethyl carbonate into a second separation column, and obtaining methyl formate at a top of the second separation column and a crude methanol stream containing dimethyl carbonate at a bottom thereof, and iii) feeding the crude methanol stream containing dimethyl carbonate into a dimethyl carbonate-methanol separation unit, and obtaining a dimethyl carbonate stream and a methanol stream after separation.

2. The method according to claim 1, wherein an operating pressure by gage pressure at the top of the first separation column is in a range of 0-1.0 MPa, an operating temperature at the top thereof is in a range of 20-100 °C, an operating temperature at the bottom thereof is in a range of 50-140 °C; and/or a number of theoretical plates of the first separation column is in a range of 5-30.

3. The method according to claim 1, wherein an operating pressure by gage pressure at the top of the second separation column is in a range of 0.1-1.0 MPa, an operating temperature at the top thereof is in a range of 30-120 °C, and an operating temperature at the bottom thereof is in a range of 60-140 °C.

4. The method according to claim 3 wherein an operating pressure by gage pressure at the top of the second separation column is in a range of 0.1-0.8 MPa, an operating temperature at the top thereof is in a range of 56-109 °C, and an operating temperature at the bottom thereof is in a range of 81-139 °C.

5. The method according to claim 1 or claim 3, wherein the second separation column is a packed column or a plate column, with a number of theoretical plates being in a range of 10-50.

6. The method according to claim 1, wherein in the dimethyl carbonate-methanol separation unit, methanol and dimethyl carbonate are separated through a process selected from a group consisting of membrane separation, extraction distillation, and variable pressure distillation.

7. The method according to claim 1, wherein the crude methanol stream containing methyl nitrite, methyl formate, and dimethyl carbonate is fed into the first separation column from the top thereof.

8. The method according to claim 1, wherein the crude methanol stream containing methyl formate and dimethyl carbonate is fed into the second separation column from an intermediate section thereof.

9. The method according to claim 1, wherein the synthesis gas is prepared from coal, natural gas, coke-oven gas, blast furnace gas, tail gas of a calcium carbide furnace, or petroleum.

10. The method according to claim 1, wherein in the crude methanol stream containing methyl nitrite, methyl formate, and dimethyl carbonate, a content of methyl nitrite is in a range of 0.1-10 wt%, that of methyl formate is in a range of 0.1-25 wt%, that of dimethyl carbonate is in a range of 0.1-28 wt%, and that of methanol is in a range of 50-99 wt%.

11. The method according to claim 1, wherein the methanol recycled in step iii) is directly circulated to an esterification reactor of the process of preparing dimethyl oxalate from synthesis gas as a reactant, or to the separation columns thereof as an absorbent.

12. A method for preparing dimethyl oxalate from synthesis gas, comprising the steps of:
a) carrying out a reaction of methanol, oxygen and NO in an esterification reactor, and producing 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, subsequently obtaining a crude methanol stream I containing methyl nitrite, methyl formate, and dimethyl carbonate after removal of heavy components from the liquid-phase stream, b) carrying out a reaction between CO, which is separated from the synthesis gas, and the gas-phase reaction stream, which is obtained in step a), in the presence of a platimum group elements containing catalyst in a coupling reactor, and producing a reaction stream containing dimethyl oxalate, methyl nitrite, methyl formate, dimethyl carbonate, NO, and methanol, c) separating dimethyl oxalate from the reaction stream obtained in step b), and obtaining a crude methanol stream II containing methyl nitrite, methyl formate, and dimethyl carbonate and a gas-phase stream containing NO, subsequently circulating said gas-phase stream back to the esterification reactor in step a), d) recycling methanol from the crude methanol stream I obtained in step a), and/or from the crude methanol stream II obtained in step b), and optionally from any other crude methanol streams containing methyl nitrite, methyl formate, and dimethyl carbonate, which are produced in the process of preparing the dimethyl oxalate, using a method for recycling methanol according to any one of claims 1 to 11, and e) circulating the methanol obtained in step d) to the esterification reactor in step a), and/or to an absorption column of the process of preparing dimethyl oxalate, which absorption column requiring the methanol.