CA2896290A1 - Method for producing dimethyl oxalate - Google Patents

Abstract

Disclosed is a method for producing dimethyl oxalate, comprising the following steps: step a): feeding, into a coupling reactor, a reaction material containing carbon monoxide and methyl nitrite, which react in the presence of a platinum-group metal catalyst, to obtain a dimethyl oxalate-containing gas-phase stream; and step b): feeding the dimethyl oxalate-containing gas-phase stream into a dimethyl oxalate separation column, and enabling counter-current contact of the dimethyl oxalate-containing gas-phase stream with a methanol-containing stream entering the separation column from a top thereof, so as to obtain crude methanol from the top of the column and a dimethyl oxalate product from a bottom of the column, wherein the dimethyl oxalate-containing gas-phase stream is not cooled before being fed into the dimethyl oxalate separation column. Further disclosed is a method for producing dimethyl oxalate, and dimethyl carbonate as a byproduct. The methods have the features of simple process steps, low energy consumption, high yield of dimethyl oxalate, and so forth.

Description

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METHOD FOR PRODUCING DIMETHYL OXALATE
Cross-reference to Related Applications The present application claims benefit of Chinese patent application CN
201410314462.0, filed on July 3, 2014 with the Chinese Patent Office, the entirety of which is incorporated herein by reference.
Field of the Invention The present disclosure relates to a method for producing dimethyl oxalate, and further relates to a method for producing dimethyl oxalate, and dimethyl carbonate as a byproduct.
Background of the Invention Dimethyl oxalate (DMO) is an important intermediate product of considerable significance in the chemical engineering industry. It can be used to produce oxalic acid through hydrolysis, and ethylene glycol through hydrogenation. Dimethyl oxalate can be synthesized substantially through two procedures. In one procedure, methanol and oxalic acid are used to produce dimethyl oxalate through esterification.
Such a procedure is subject to the defects of a large amount of wastewater emissions and severe environmental pollution. The other procedure is completed through a coupling reaction between carbon monoxide and methyl nitrite in the presence of a platinum catalyst. In recent years, due to rapid growth of the coal chemical industry, the latter procedure has drawn wide attention as an intermediate step in the production of ethylene glycol from coal through a synthesis gas. In such a procedure, a coupling reaction occurs between carbon monoxide, under the action of a supported catalyst of Pd/a-A1203, and methyl nitrite at atmospheric pressure, to generate dimethyl oxalate and nitrogen monoxide, wherein the main reaction equation is as follows:
2C0 +2CH3ONO,(COOCH3)2 +2NO .

, .
In such a synthesis procedure, the following side reactions substantially occur.
Carbon monoxide reacts with methyl nitrite to produce nitrogen monoxide and dimethyl carbonate (C3H603), wherein the methyl nitrite decomposes to generate nitrogen monoxide, methyl formate (C214402), and methanol, while carbon monoxide and nitrogen monoxide react with each other to produce nitrogen and carbon dioxide.
The equations of the above reaction are as follows:
CO+ 2 CH30 NO -> 2 NO+ C3H603 , 4 CH3ONO --> 4 NO+ C2H402 + 2 CH30 H , and

2 CO+ 2 NO -> N2 + 2 CO2 =
Currently, a pure dimethyl oxalate product can be obtained typically through absorption with methanol, followed by separation of methanol, dimethyl carbonate, and dimethyl carbonate with each other. That is, purification of dimethyl oxalate needs to be performed through an alcohol washing column and an alcohol recovery column. The pure dimethyl oxalate product obtained can be directly used as a product or as raw material for synthesis of ethylene glycol. Because an azeotropic phenomenon would occur between dimethyl carbonate and methanol, a liquid mixture of methanol and dimethyl carbonate obtained should go through a procedure of membrane separation, variable pressure rectification, or extractive distillation for separation, so as to obtain a pure dimethyl carbonate product.
UBE INDUSTRIES's patent application US 4453026A discloses reaction of carbon monoxide and methyl nitrite or ethyl nitrite in the presence of a platinum-group noble metal catalyst. The reaction products are condensed and separated to obtain a condensate and a non-condensable gas, wherein during a condensing step, a specific amount of methanol or ethanol is added, so as to prevent the dimethyl oxalate or diethyl oxalate from being mixed with the non-condensable gas, which would otherwise lead to crystallization. The condensate enters a primary rectifying column to generate a crude dimethyl oxalate or diethyl oxalate product.

CN 101993367A, CN 101993365A, CN 101993369A, CN 101993361A, CN
101492370A, and CN 101381309A each disclose performing gas-liquid separation on the reaction products of carbon monoxide and nitrites to obtain a gas-phase distillate and a liquid-phase distillate, and further performing separation and purification on the liquid-phase distillate containing oxalates to obtain a crude oxalate product.
CN 202643601U discloses separating dimethyl oxalate through a primary flash, a washing column, and dimethyl oxalate rectifying column. Crystallization of dimethyl oxalate occurs easily in the washing column due to low-temperature washing with methanol.
CN 101462961A discloses reacting carbon monoxide with methyl nitrite in contact with a platinum-group noble metal catalyst, to obtain a product containing both dimethyl oxalate and dimethyl carbonate. The product is fed into a condenser to contact with methanol and then condensed, thus generating a non-condensable gas, and a liquor condensate comprising dimethyl oxalate, dimethyl carbonate, methyl formate, and methanol. The liquor condensate is then fed to a distillation column to be distillated, to produce an azeotrope of the dimethyl carbonate and the methanol in a top of the column, and a stream containing the dimethyl oxalate in a bottom of the column. These process steps are complex. In addition, dimethyl oxalate, due to a relatively high condensation point thereof, will easily crystallize on a wall of the condenser, which would finally block the condenser.
To conclude the above, in the prior art, coupling products of carbon monoxide and methyl nitrite are all condensed before entering successive procedures.
The process steps are complex. Moreover, dimethyl oxalate easily crystallizes in a device and a pipe. Thus, heat preservation or heat tracing is required, in order to prevent blockage of the device and pipe by crystallized dimethyl oxalate. Meanwhile, crystallization of dimethyl oxalate in the device and the pipe also reduces the yield of dimethyl oxalate.

3 In addition, as discussed above, in the process of producing dimethyl oxalate by coupling reactions between carbon monoxide and methyl nitrite, dimethyl carbonate can be inevitably generated more often than not. As is well known, an azeotropic phenomenon occurs between dimethyl carbonate and methanol. Besides, methanol requires huge latent heat in vaporization thereof. Consequently, separation of dimethyl carbonate, especially low-concentrated dimethyl carbonate, from methanol requires complex process steps, long time, and high energy consumption.
Shanghai Coking & Chemical Corporation's patent application CN 101190884A
discloses a method for synthesizing dimethyl oxalate and a byproduct dimethyl carbonate. The method comprises first absorbing dimethyl carbonate along with dimethyl oxalate that are contained in a coupling reaction product with a large quantity of methanol, then separating methanol from dimethyl carbonate with dimethyl oxalate through extractive distillation, and finally separating dimethyl oxalate from dimethyl carbonate. Because the absorption step requires a large amount of methanol, a large amount of dimethyl oxalate is also required for extraction of the dimethyl carbonate from the methanol through extractive distillation.
Meanwhile, the large amount of methanol has to be recycled by being steamed out from a top of the column, which requires high energy consumption.
East China University of Science and Technology's patent application CN
101381309A discloses a method for separating low concentrated dimethyl carbonate through a double-column procedure during synthesis of dimethyl oxalate. The method comprises first separating methanol and dimethyl carbonate from dimethyl oxalate, and then separating methanol from dimethyl carbonate through variable pressure rectification. The problem of complex process steps and high energy consumption during separation of methanol from dimethyl carbonate still stay unsolved.
In a word, in the prior art, separation of dimethyl carbonate, either through variable pressure rectification or through extractive distillation, is subject to use of a large amount of methanol, thereby leading to complex process steps and high energy

4 consumption.
Summary of the Invention One purpose of the present disclosure is to provide a new method for producing dimethyl oxalate, so as to solve the problems of complex process steps, easy blockage of devices and pipes by crystallization of dimethyl oxalate 1, high consumption of material and energy, and the like in the prior art during production of dimethyl oxalate.
The method of the present disclosure has the features of simple process steps, low energy consumption, high yield of dimethyl oxalate, etc.
Another purpose of the present disclosure is to provide a method for producing dimethyl oxalate, and meanwhile dimethyl carbonate as a byproduct. The method has the features of simple process steps and low energy consumption, and can produce high-purity dimethyl oxalate, and dimethyl carbonate as a byproduct. In addition, with this method, it will be unnecessary to separate dimethyl carbonate from methanol.
According to a first aspect of the present disclosure, a method for producing dimethyl oxalate is provided, comprising the following steps:
step a): feeding, into a coupling reactor, a reaction material containing carbon monoxide and methyl nitrite, which react in the presence of a platinum-group metal catalyst, to obtain a dimethyl oxalate-containing gas-phase stream; and step b): feeding the dimethyl oxalate-containing gas-phase stream into a dimethyl oxalate separation column, and enabling counter-current contact of the dimethyl oxalate-containing gas-phase stream with a methanol-containing stream entering the separation column from a top thereof, so as to obtain crude methanol from the top of the column and a dimethyl oxalate product from a bottom of the column, wherein the dimethyl oxalate-containing gas-phase stream is not cooled before being fed into the dimethyl oxalate separation column.
According to a preferred embodiment of the present disclosure, the dimethyl oxalate-containing gas-phase stream does not go through an alcohol washing column before being fed into the dimethyl oxalate separation column. That is, the dimethyl

5 oxalate-containing gas-phase stream does not have to be washed by an alcohol in any alcohol washing column.
As described above, a reaction stream flowing out from the coupling reactor is often condensed in the prior art. During such a step, dimethyl oxalate would be partially separated out, while the rest dimethyl oxalate flows into successive sections for further separation and purification. However, in such a process, dimethyl oxalate would easily crystallize in a device or a pipe. Moreover, the condensed dimethyl oxalate does not have a high purity. In addition, more often than not, the reaction stream flowing out from the coupling reactor needs to be washed by an alcohol in the prior art, which requires a large quantity of methanol.
In the method for producing dimethyl oxalate according to the present disclosure, however, the gas-phase stream from the coupling reactor enters directly into the dimethyl oxalate separation column for separation without being cooled. In addition, an alcohol washing step is omitted in the method of the present disclosure.
This not only eliminates a risk for dimethyl oxalate to be crystallized and precipitated, but also simplifies process devices and steps.
The platinum-group metal catalyst used in the method of the present disclosure is known in the art, and can be any proper catalyst used in catalyzing reactions between carbon monoxide and methyl nitrite to produce dimethyl oxalate.
According to the method provided by the present disclosure, the dimethyl oxalate product obtained in step b) generally has a purity higher than 99.85%.
According to a preferred embodiment of the present disclosure, the dimethyl oxalate separation column comprises: an absorbing and rectifying section, which is arranged between a feed inlet for the methanol-containing stream and a feed inlet for the dimethyl oxalate-containing gas-phase stream, and is provided with a column plate or a filler; and a stripping section, which is arranged between the inlet for the dimethyl oxalate-containing gas-phase stream and the bottom of the column, and is provided with a column plate or a filler.

6 Preferably, a height ratio of the absorbing and rectifying section to the stripping section is in the range from 0.2:1 to 5:1, more preferably 0.5:1 to 3:1.
Further preferably, the height ratio of the absorbing and rectifying section to the stripping section is in the range from 1:1 to 2:1. Tests have shown that the aforementioned ranges of height ratio enable better effects of absorption and separation of dimethyl oxalate and methanol.
In a preferred embodiment of the present disclosure, the top of the dimethyl oxalate separation column has a temperature in the range from 0 C to 60 C, preferably 25 C to 45 C, and a pressure in the range from 0.1 MPa to 0.3 MPa, preferably 0.15 MPa to 0.2 MPa. And preferably, the bottom of the dimethyl oxalate separation column has a temperature in the range from 161 C to 210 C, preferably 176 C to 195 C, and a pressure in the range from 0.1 MPa to 0.35 MPa, preferably 0.12 MPa to 0.24 MPa.
In the present disclosure, the pressures mentioned are all absolute pressures.
According to the method for producing dimethyl oxalate of the present disclosure, the filler can be structured or bulk packed, and the column plate can be in the form of a float valve tray, a sieve plate, a double pass tray, a bubble-cap tray, or a Thorman tray.
Preferably, the stripping section has a theoretical plate number in the range from 5 to 40.
According to the present disclosure, operation conditions of the coupling reactor include a reaction temperature in the range from 50 C to 200 C, preferably 60 C to 180 C, and a pressure in the range from 0.1 MPa to 2.0 MPa, preferably 0.1 MPa to 1.0 MPa.
Preferably, heat tracing is performed on a pipe arranged between the coupling reactor and the dimethyl oxalate separation column.
Preferably, heat tracing is performed on a discharge pipe arranged in the bottom

7 of the dimethyl oxalate separation column.
In the method for producing dimethyl oxalate provided in the present disclosure, carbon monoxide is first used to synthesize dimethyl oxalate. Raw material gasses containing carbon monoxide and methyl nitrate are fed into a reactor filled with a solid platinum-group metal catalyst for gas-phase catalytic reactions.
The coupling reactor can be preferably selected as a tubular, fixed bed reactor, which adopts circulating hot water for heat removal and produces steam as a byproduct.
The raw material gasses, before entering the reactor, are usually diluted with inert gasses such as nitrogen or carbon dioxide. The concentration of methyl nitrite in the raw material gasses may vary in a relatively large range. However, in order to obtain a proper reaction rate, the concentration of methyl nitrite in the raw material gasses cannot be lower than 3 vol%, and preferably be in the range from 5 vol%
to 30 vol%. The concentration of carbon monoxide in the raw material gasses may vary in a relatively large range also, generally in the range from 10 vol% to 90 vol%.
The reaction can be performed at a relatively low temperature and a relatively low pressure. The residence time of the gas-phase reactants in a catalyst bed is generally no more than 12 s, properly in the range from 0.2 s to 6 s.
In the method for producing dimethyl oxalate according to the present disclosure, a second step is separation of the dimethyl oxalate. In this step, the reaction product at an outlet of the coupling reactor directly enters the dimethyl oxalate separation column in the intermediate section thereof without being cooled, wherein counter-current contact is enabled between the reaction product and the methanol entering the column from the top thereof, to produce crude methanol and non-condensed gases from the top of the column, and the dimethyl oxalate product from the bottom of the column.
The absorbing and rectifying section is provided between the feed inlet of the methanol-containing stream and the feed inlet of the coupling reaction product stream, and is provided with the column plate or the filler, preferably a highly efficient and

8 low resistant, structured or bulk packed filler. The absorbing section meanwhile functions as a rectifying section. A section between the feed inlet of the methyl oxalate-containing stream and the bottom of the column is the striping section, which can be in the form of a float valve tray, a sieve plate, a double pass tray, a bubble-cap tray, or a Thorman tray, or can be a filler.
After condensation at the top of the dimethyl oxalate separation column, a non-condensed gas is subject to subsequent treatment, while a condensed liquid is partially discharged as the crude methanol product for subsequent treatment, and partially recycled and to be mixed a feedstock of methanol to form the methanol-containing stream, which enters the dimethyl oxalate separation column.
The pipe from the outlet of the coupling reactor to the dimethyl oxalate separation column, and the discharge pipe arranged at the bottom of the dimethyl oxalate separation column are both preferably to be trace heated, preferably with steam, hot water, electricity, and the like, so as to prevent the dimethyl oxalate from being crystallized in the devices or in the pipes.
When the method for producing dimethyl oxalate provided by the present disclosure is used, cooling and alcohol washing devices are unnecessary, and absorption of the coupling product in a cooling device and rectification of the coupling product in a distillation device in the prior art can be completed in one dimethyl oxalate separation column, thereby reducing energy consumption and simplifying devices. Moreover, crystallization of the dimethyl oxalate in the cooling device can be prevented, thereby increasing yield of dimethyl oxalate. In addition, investment in equipment and floor space can be saved. Meanwhile, simplification of process steps renders costs of tracing heat significantly reduced. When the method for producing dimethyl oxalate according to the present disclosure is used, the yield of dimethyl oxalate can reach higher than 99.5%, with largely reduced energy consumption of a recovery system for dimethyl oxalate, which is a very obvious technical effect.
According to a second aspect of the present disclosure, a method for producing dimethyl oxalate, and dimethyl carbonate as a byproduct is provided, comprising the

9 following steps:
step a): feeding, into a coupling reactor, a reaction material containing carbon monoxide and methyl nitrite, which react in the presence of a platinum-group metal catalyst, to obtain a gas-phase stream containing both dimethyl oxalate and dimethyl carbonate;
step b): feeding the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate into an ester separation column, and enabling counter-current contact of the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate with a methanol-containing stream entering the ester separation column from a top of the column, and an extraction agent stream containing dimethyl oxalate and entering the ester separation column from an intermediate section thereof, so as to obtain crude methanol from the top of the column and a mixture containing both dimethyl oxalate and dimethyl carbonate from a bottom of the column; and step c): feeding the mixture into a dimethyl oxalate refining column, to obtain a dimethyl carbonate product from a top of the refining column and a dimethyl oxalate product from a bottom of the refining column, wherein the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate is not cooled before being fed into the dimethyl oxalate separation column.
The ester separation column refers to a separation column in bottom of which dimethyl oxalate and dimethyl carbonate are obtained.
Preferably, the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate does not go through an alcohol washing column before being fed into the ester separation column. That is, the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate does not have to be washed by an alcohol in any alcohol washing column.
According to a preferred embodiment of the present disclosure, the ester separation column comprises: an absorbing section, which is arranged between a feed inlet for the extraction agent stream and a feed inlet for the methanol-containing stream, and is provided with a column plate or a filler; an extracting section, which is arranged between a feed inlet for the gas-phase stream and the feed inlet for the extraction agent stream, and is provided with a column plate of a filler; and a stripping section, which is arranged between the feed inlet for the gas-phase stream and the bottom of the ester separation column, and is provided with a column plate or a filler.
The gas-phase product from the coupling reactor is fed into the ester separation column from between the extracting section and the stripping section thereof, moves upward to the extracting section, and get into counter-current contact with the liquid-phase dimethyl oxalate flowing downward. The liquid phase in the bottom of the extracting section flows downward to the stripping section and is separated therein, to obtain the mixture of dimethyl oxalate and dimethyl carbonate in the bottom of the column. The gas-phase stream in the top of the extracting section, after contacting with an extraction agent, moves upward to the absorbing section and gets into counter-current contact with methanol stream flowing downward from the top of the ester separation column. The methanol stream further absorbs dimethyl oxalate carried in the gas phase. Thus, a gas-phase stream substantially containing no dimethyl oxalate or dimethyl carbonate obtained from the top of the ester separation column can be recycled to an oxidation and esterification unit for regeneration of methyl nitrite.
Preferably, a height ratio of the absorbing section to the extracting section is in the range from 1:0.5 to 1:5, preferably 1: 1.5 to 1:3.5.
Preferably, a height ratio of the absorbing section to the stripping section is in the range from 1:0.2 to 1:5, preferably 1: Ito 1:2.
In the same way, the filler used in the ester separation column can be structured or bulk packed, and the column plate can be in the form of a float valve tray, a sieve plate, a double pass tray, a bubble-cap tray, or a Thorman tray.
Preferably, the stripping section has a theoretical plate number in the range from 5 to 40, more preferably from 10 to 25. In the prior art, merely methanol is used as an absorbing agent in a dimethyl oxalate separation column. The ester separation column of the present disclosure, however, is added with the extracting section, wherein dimethyl oxalate not only works as the extraction agent for separation of dimethyl carbonate and methanol, but also works as an absorbing agent for absorption of the gas-phase dimethyl oxalate. Addition of dimethyl oxalate as the absorbing agent enables the amount of methanol used to be reduced, and meanwhile ensures substantially complete absorption of the dimethyl carbonate and dimethyl oxalate that are contained in the gas-phase product from the coupling reactor. Thus, the liquid obtained in the bottom of the ester separation column will contain no methanol. This can reduce energy consumption and loss of methanol in the subsequent separation system.
According to the present disclosure, 50-90%, preferably 60-85%, and more preferably 60-70% of the dimethyl oxalate product obtained in step c) is recycled to the ester separation column as the extraction agent. Preferably, the extraction agent has a temperature in the range from 55 C to 210 C, more preferably 60 C to 150 C.
The dimethyl oxalate used as the extraction agent is cooled to a temperature in the above ranges before entering the ester separation column. A low temperature is favorable for reduction of the amount of dimethyl oxalate used as the extraction agent.
However, since the freezing point of dimethyl oxalate at atmospheric pressure is 54 C, too low a temperature thereof would lead to a risk of blockage of the pipe by crystallized dimethyl oxalate.
During the tests of the present disclosure, the influences of the concentration of dimethyl oxalate in the extraction agent upon the relative volatility of methanol to the dimethyl carbonate have been studied. It has been discovered that, in the method according to the present disclosure, a liquid phase in the extracting section of the ester separation column has a concentration of dimethyl oxalate equal to or higher than 20 mol%, e.g., 20-90%. Within the above ranges, an azeotropic point of methanol and dimethyl carbonate can be avoided, so that they can be readily separated from each other. The molar concentration of dimethyl oxalate in the liquid phase in the extracting section of the ester separation column can be adjusted through adjustment of flow of the extraction agent and adjustment of the flow of the methanol-containing stream added in the top of the column.
According to a preferred embodiment of the present disclosure, the top of the ester separation column has an operation pressure in the range from 0.1 MPa to 0.4 MPa, preferably 0.11 MPa to 0.25 MPa, and a temperature in the range from 0 C
to 60 C, preferably 20 C to 40 C. Too high an operation pressure in the ester separation column would be unsuitable, because a higher pressure in the column would lead to a higher temperature in the bottom of the column, which is unfavorable for stability of dimethyl oxalate in the bottom of the column. In the top of the ester separation column, however, positive pressure operation would be preferred since the operation pressure thereof is subject to restriction by the recycling gas system.
According to a preferred embodiment of the present disclosure, a volume ratio of the methanol-containing stream to the extraction agent stream fed into the ester separation column is in the range from 1:1 to 1:5, preferably from 1:1 to 1:3.
According to a preferred embodiment of the present disclosure, the dimethyl oxalate refining column has an operation pressure in the range from 0 MPa to 0.3 MPa, preferably 0.1 MPa to 0.2 MPa, and a temperature in the top thereof in the range from 20 C to 130 C, preferably 80 C to 110 C.
Preferably, the dimethyl oxalate refining column has a theoretical plate number in the range from 10 to 60, preferably 25 to 50, and a reflux ratio in the range from 2 to 200, preferably 5 to 50.
As a reactant stream in the method of the present disclosure, the reaction material comprises 5-40 mol% of carbon monoxide, 5-30 mol% of methyl nitrite, 1-10 mol% of nitrogen monoxide, 0.1-10 mol% of methanol, and inert gasses, such as nitrogen, as a balance. Preferably, the gas-phase material entering the coupling reactor comprises 10-30 mol% of carbon monoxide, 5-20 mol% of methyl nitrite, 2-8 mol%
of nitrogen monoxide, 1-8 mol% of methanol, and nitrogen as a balance. The methyl nitrite can be supplied by an oxidation and esterification device.
Preferably, the reaction temperature in the coupling reactor is in the range from 90 C to 150 C, and the reaction pressure therein is in the range from 0.1 MPa to 1 MPa. Preferably, the reaction temperature in the coupling reactor is in the range from 110 C to 130 C, and the reaction pressure therein is in the range from 0.2 MPa to 0.5 MPa. In the methods of two aspects according to the present disclosure, the reaction conditions in the coupling reactors can be the same. Due to strong exothermicity of the coupling reactions, a higher concentration of methyl nitrate would lead to stronger reactions. On the one hand, if reaction heat cannot be effectively removed, it would cause temperature runway in the reactor. On the other hand, too low a concentration of methyl nitrite in the reactor would increase the concentration of the inner gases therein, thereby increasing energy consumption of the system. Moreover, existence of nitrogen monoxide would restrain the reaction rate of coupling reactions.
Hence, it may be unfavorable to have too high a concentration of nitrogen monoxide in the coupling reactor. However, the concentration of nitrogen monoxide is correlated to the oxidation and esterification reaction, and therefore must be kept excessive to ensure complete reaction of oxygen. Because methanol is needed for regeneration of methyl nitrite in the oxidation and esterification reactor, the gas-phase stream from the oxidation and esterification reactor and entering the coupling reactor contains methanol at an amount that enables a gas-liquid equilibrium state. Although methanol may be unfavorable for the coupling reactions, cooling a product of the regeneration of methyl nitrite down to a lower temperature in order to reduce the amount of methanol contained in the coupling reactor would inevitably increase energy consumption of the system.
According to a preferred embodiment of the present disclosure, the coupling reactor is selected in the form of a tubular, fixed bed reactor. Reaction materials flow within the tubes, and saturated water flow between and among the tubes. The coupling reaction of carbon monoxide in preparation of dimethyl oxalate is a strong exothermic reaction. Reaction heat can be effectively released through the tubular, fixed bed reactor. Besides, vaporization of the saturated water also absorbs a large amount of heat, and meanwhile produces low-pressure stream as a by produce. Thus, the amount of circulating water used can be reduced, and the temperature of the water can be constantly maintained. At the same time, boiling water can provide a relatively large heat transfer coefficient and benefit removal of the reaction heat.
According to a preferred embodiment of the present disclosure, the ester separation column comprises a reboiler at the bottom and a condenser at the top =
thereof. With the condenser at the top, the amount of complementary methanol from outside and used for absorption can be reduced, while the reboiler at the bottom can remove the methanol contained in the liquid stream in the bottom of the extracting section. Thus, methanol can be avoided from being withdrawn along with dimethyl carbonate, which would otherwise cause methanol loss.
According to a preferred embodiment of the present disclosure, the liquid at the bottom of the ester separation column comprises 0.05-5%, preferably 0.1-3% of dimethyl carbonate, and methyl oxalate substantially as a balance.
In the method provided according the second aspect of the present disclosure, the absorbing section of the ester separation column, while being used for absorbing gas-phase dimethyl oxalate, can also be used as the rectifying section at the same time, which restricts movement of dimethyl oxalate toward the top of the column. The extracting section also plays a role of the absorbing section, wherein liquid-phase dimethyl oxalate first absorbs gas-phase dimethyl oxalate and dimethyl carbonate that are contained in a rising gas phase. This can significantly reduce the amount of methanol, the absorbing agent, used in the absorbing section. When the amount of methanol used is reduced, the loads of the reboiler and the condenser can both be reduced.
In the method for producing dimethyl oxalate, and dimethyl carbonate as a byproduct provided by the present disclosure, a cooling device and an alcohol washing device can be both omitted. And absorption of coupling products in a cooling device and rectification of the coupling products in a distillation device in the prior art can be completed in one ester separation column, thereby reducing energy consumption and simplifying devices. Besides, dimethyl oxalate can be prevented from being crystallized in the cooling device, thus improving yield of dimethyl oxalate. In addition, investment into equipment and floor space can be reduced.
Meanwhile, simplification of process steps also enables reduced costs in heat preservation. At the same time, the liquid-phase dimethyl oxalate used as the extraction agent and circulated in the system destroys the azeotropic balance between methanol and dimethyl carbonate, and thus facilitates separation of the methanol from the dimethyl carbonate. Subsequently, dimethyl oxalate and dimethyl carbonate can be separated from each other, thereby not only reducing energy consumption in separation of dimethyl carbonate, but also obtaining dimethyl oxalate and dimethyl carbonate products at required purities. In addition, since dimethyl carbonate is separated from methanol, the methanol flowing out of the separation column can be further used without being influenced by any accumulation of dimethyl carbonate.
When the technical solution for producing dimethyl oxalate, and dimethyl carbonate as a byproduct is used, the recovery rate of dimethyl oxalate can be higher than 99.5%, and the removal rate of dimethyl carbonate reaches higher than 99%, with significantly reduced amount of steam consumption in separation of dimethyl carbonate, which are rather beneficial technical effects.
Brief Description of the Drawings Fig. 1 schematically shows a flow chart of a method for producing dimethyl oxalate according to the present disclosure;
Fig. 2 schematically shows a flow chart of a method for producing dimethyl oxalate, and dimethyl carbonate as a byproduct according to the present disclosure;
and Fig. 3 shows distribution curves of the concentration of dimethyl oxalate and the relative volatility of methanol to dimethyl carbonate in an ester separation column.
Detailed Description of the Embodiments The present disclosure will be further explained through examples with reference to the accompany drawings. It should be understood that the scope of the present disclosure is not to be limited thereto.
As Fig. 1 shows, a nitrogen feedstock 1, a carbon monoxide feedstock 2, a methanol feedstock 3, and a methyl nitrite feedstock 4 are mixed, preheated, and then fed into a coupling reactor R-101 for coupling reaction therein. After the coupling reaction, the material stream 6 discharged out of the coupling reactor as reaction product directly enters a dimethyl oxalate separation column in an intermediate section thereof. A methanol 7 as an absorbing agent, and a reflux liquid 10 of the dimethyl oxalate separation column are mixed with each other to form a methanol-containing stream which enters a top of the dimethyl oxalate separation column. In an absorbing and rectifying section of the dimethyl oxalate separation column, countercurrent contact and thus reaction between the material stream 6 and the methanol-containing stream are enabled. A liquid phase that has absorbed dimethyl oxalate contained in the material stream 6 moves downward to enter a stripping section, wherein the dimethyl oxalate is separated, to obtain a dimethyl oxalate product 12 which is withdrawn from a bottom of the column. A gas 8 from the top of the dimethyl oxalate separation column is condensed through a condenser arranged in the top of the dimethyl oxalate separation column, whereby a non-condensed gas 9 is subject to subsequent treatment, while a crude methanol product 11 is partially withdrawn and partially works as the reflux liquid 10 of the dimethyl oxalate separation column. The pipes for the streams 6 and 12 are heat traced, so as to prevent the dimethyl oxalate from being crystallized in the device or pipes.
As shown in Fig. 2, a gas-phase raw material containing carbon monoxide and methyl nitrite enters a coupling reactor 21 through a pipe 25. A material discharged from the coupling reactor enters an ester separation column 22 in an intermediate section thereof between an extracting section 22b and a stripping section 22c through a pipe 26, wherein countercurrent contact is enabled between the material and dimethyl oxalate that enters a top of the extracting section 22b of the ester separation column through a pipe 33 and flows downward. Thus, a stream is obtained in a bottom of the extracting section 22b of the ester separation column and flows into the stripping section 22c of the ester separation column. After stripping is performed in the stripping section 22c, a liquid mixture of dimethyl oxalate and dimethyl carbonate is obtained in a bottom of the tripping section 22c of the ester separation column. The liquid mixture enters a refining column 23 through a pipe 29. A gas-phase stream obtained in a top of the extracting section 22b comes in countercurrent contact with a methanol stream entering a top of the absorbing section 22a of the ester separation column through a pipe 28 and flowing downward, wherein the methanol stream further absorbs dimethyl oxalate carried in the gas-phase stream. Thus, a gas-phase stream in a pipe 27 substantially containing no dimethyl oxalate or dimethyl carbonate in the top of the ester separation column enters an oxidation and esterification reactor for regeneration of methyl nitrite.
Dimethyl carbonate separated from a top of the dimethyl oxalate refining column 23 is withdrawn through a pipe 30. High-purity dimethyl carbonate is obtained from a bottom of the refining column 23, wherein the dimethyl oxalate stream partially enters a dimethyl oxalate condenser through a pipe 32, is condensed therein, and then recycled to the ester separation column 22 as the extraction agent, while the rest dimethyl oxalate is withdrawn through a pipe 31.
Example 1 A nitrogen feedstock 1, a carbon monoxide feedstock 2, a methanol feedstock 3, and a methyl nitrite feedstock 4 were mixed, preheated, and then fed into a coupling reactor R-101 at a flow rate of 30 t / h for coupling reaction therein. After the coupling reaction, a material stream 6 discharged out of the coupling reactor as reaction product directly entered a dimethyl oxalate separation column from an intermediate section thereof. A methanol 7 at a flow rate of 20 t / h, after being mixed with a reflux liquid 10 of the dimethyl oxalate separation column, entered the dimethyl oxalate separation column from a top thereof. A gas 8 in top of the column was condensed in a condenser arranged in the top of the dimethyl oxalate separation column, whereby a non-condensed gas 9 was subject to subsequent treatment, while a crude methanol product 11 was withdrawn. A dimethyl oxalate product 12 was withdrawn from a bottom of the column. The yield of dimethyl oxalate was higher than 99.99%.
The height of a bulk packed filler provided in the dimethyl oxalate separation column was 10 m; the theoretical plate number in the stripping section was 10;
and the height ratio of the absorbing and rectifying section to the stripping section was 2:1.
The operation temperature in the top of the column was 32 C, and the operation pressure in the top of the column was 0.14 MPa; while the operation temperature in the bottom of the column was 185 C, and the operation pressure in the bottom of the column was 0.185 MPa. A thermal load of a reboiler of the column was 4.0435 MW.

The compositions of feedstocks fed into the reactor and those of the main streams were shown in Table 1.
Table 1 Stream Parameter Temperature, 'V 110 120 40 20 20 185 Pressure, MPa 0.33 0.21 0.95 0.14 0.14 0.185 N2 44.30% 44.30% 0 50.15% 180 ppm CO 19.08% 10.17% 0 11.51% 171 ppm 0 NO 5.23% 15.07% 0 17.06% 134 ppm 0 CO2 0 30 ppm 0 34 ppm 307 ppm Methyl 29.74% 9.72% 0 11.01% 1.03% 0 nitrite Content of Methyl composition 0 0 0 0 346 ppm 0 formate by weight Methanol 1.65% 1.7% 99.78% 9.59% 94.40% 0 Dimethyl 0 0.57%
0.217% 0.627% 4.48% 0.144%
carbonate Water 0 0 50 ppm 0 14.7 ppm 16 ppm Dimethyl 0 18.41% 0 0 0 >99.85%
oxalate Example 2 The steps of Example 1 were repeated except that compositions of the feedstocks and parameters of the columns were different from those of Example 1.
The height of a bulk packed filler provided in the absorbing and rectifying section of the dimethyl oxalate separation column was 15 m; the theoretical plate number of the stripping section was 20; and the height ratio of the absorbing and rectifying section to the stripping section was 1.5:1. The operation temperature in the top of the column was 29 C, and the operation pressure in the top of the column was 0.12 MPa; while the operation temperature in the bottom of the column was 178 C, and the operation pressure in the bottom of the column was 0.15 MPa. A thermal load of a reboiler of the column was 3.680 MW. The yield of dimethyl oxalate was higher than 99.99%.

The compositions of the main streams were shown in Table 2.
Table 2 Stream Parameter Temperature, C 110 120 40 15 15 178 Pressure, MPa 0.31 0.21 0.95 0.11 0.11 0.15 N2 44.30% 44.30% 0 140 ppm 50.63% 0 CO 15.61% 6.59% 0 90 ppm 7.53%

NO 8.71% 18.79% 0 150 ppm 21.47%

CO2 0 50 ppm 0 0 60 ppm 0 Methyl 29.74% 9.22% 0 0.88% 10.52% 0 nitrite Content of Methyl composition 0 730 ppm 0 490 ppm 830 ppm 0 formate by weight Methanol 1.65% 1.72% 99.77% 91.96% 8.85% 0 Dimethyl 0 0.88% 0.22% 7.07% 0.89%
0.14%
carbonate Water 0 0 50 ppm 10 ppm 0 20 ppm Dimethyl 0 18.41% 0 0 0 >99.85%
oxalate Example 3 The steps of Example 1 were repeated except that compositions of the feedstocks and parameters of the columns were different from those of Example 1.
The height of a bulk packed filler provided in the absorbing and rectifying section of the dimethyl oxalate separation column was 20 m; the theoretical plate number of the stripping section was 30; and the height ratio of the absorbing and rectifying section to the stripping section was 1.35:1. The operation temperature in the top of the column was 34 C, and the operation pressure in the top of the column was 0.16 MPa; while the operation temperature in the bottom of the column was 187 C, and the operation pressure in the bottom of the column was 0.2 MPa. A thermal load of a reboiler of the column was 4.801 MW. The yield of the dimethyl oxalate was higher than 99.99%.

The compositions of the main streams were shown in Table 3.
Table 3 Stream Parameter Temperature, C 110 110 40 15 15 189 Pressure, MPa 0.31 0.21 0.95 0.14 0.14 0.20 N2 46.86% 46.87% 0 190 ppm 53.42%

CO 17.51% 9.42% 0 160 ppm 10.74%

NO 9.35% 18.38% 0 180 ppm 20.95%

CO2 0 50 ppm 0 0 6 ppm 0 Methyl 24.49% 6.12% 0 0.72% 6.96% 0 nitrite Content of Methyl composition 0 600 ppm 0 490 ppm 680 ppm 0 formate by weight Methanol 1.77% 1.83% 99.77% 92.58% 7.17% 0 Dimethyl 0 0.72% 0.22% 6.58% 0.67%
0.14%
carbonate Water 0 0 50 ppm 10 ppm 0 20 ppm Dimethyl 0 16.58% 0 0 0 > 99.85%
oxalate Example 4 The steps of Example 1 were repeated except that compositions of the feedstocks and parameters of the columns were different from those of Example 1.
The height of a bulk packed filler provided in the absorbing and rectifying section of the dimethyl oxalate separation column was 25 m; the theoretical plate number of the stripping section was 40; and the height ratio of the absorbing and rectifying section to the stripping section was 1.25:1. The operation temperature in the top of the column was 36 C, and the operation pressure in the top of the column was 0.18 MPa; while the operation temperature in the bottom of the column was 192 C, and the operation pressure in the bottom of the column was 0.22 MPa. A thermal load of a reboiler was 4.769 MW. The yield of dimethyl oxalate was higher than 99.99%.
The compositions of the main streams were shown in Table 4.

Table 4 Stream Parameter Temperature, C 110 110 40 15 15 192 Pressure, MPa 0.31 0.21 0.95 0.16 0.16 0.22 N2 46.86% 46.86% 0 220 ppm 53.97% 0 CO 17.52% 9.42% 0 190 ppm 10.85%

NO 9.35% 18.38% 0 200 ppm 21.17% 0 CO2 0 50 ppm 0 0 60 ppm 0 Methyl 24.49% 6.12% 0 0.83% 7.03% 0 nitrite Content of Methyl composition 0 600 ppm 0 560 ppm 0.07% 0 formate by weight Methanol 1.77% 1.83% 99.78% 92.03% 6.27% 0 Dimethyl 0 0.72% 0.22% 7.02% 0.63%
0.14%
carbonate Water 0 0 50 ppm 10 ppm 0 20 ppm Dimethyl 0 16.59% 0 0 0 >99.85%
oxalate Comparative Example 1 The device disclosed in CN 202643601U (the entirety of which is incorporated herein for reference) was used. A coupling reaction product was first cooled through a heat exchanger, wherein a part of dimethyl oxalate was condensed. A gas phase and a liquid phase from the heat exchanger both entered a gas-liquid separator, wherein dimethyl oxalate that can be directly used was obtained in a bottom of the gas-liquid separator. The rest dimethyl oxalate contained in a non-condensed gas phase from the gas-liquid separator was fed into an absorbing column, to be absorbed by methanol. A
resulting liquid from the absorbing column was finally separated through distillation to obtain dimethyl oxalate.
The gas-liquid mixture was cooled down to 60-70 C in the heat exchanger. The height of a packed filler in the absorbing column was 25 m. The amount of methanol used for absorption was the same as the amount of the methanol that entered the top of the dimethyl oxalate separation column in Example 4. For other conditions, reference can be made to CN 202643601U. The theoretical plate number in the refining column of the dimethyl oxalate was 40. A required thermal load of a reboiler was 11.849 MW, which was obviously higher than the thermal load of the reboiler of Example 4, i.e., 4.769 MW.
Example 5 A gas-phase feedstock containing carbon monoxide and methyl nitrite entered a coupling reactor 21 through a pipe 25, and a coupling reaction product entered an ester separation column 22 in an intermediate section thereof between an extracting section 22b and a stripping section 22c through a pipe 26, and came into countercurrent contact with dimethyl oxalate as an extraction agent that entered a top of the extracting section 22b of the ester separation column and flowed downward. A
liquid stream obtained in a bottom of the extracting section 22b of the ester separation column flowed into the stripping section 22c of the ester separation column, wherein after stripping was performed, a liquid mixture of dimethyl oxalate and dimethyl carbonate was obtained in the bottom of the tripping section 22c. This liquid mixture entered a refining column 23 through a pipe 29. A gas-phase stream obtained from the top of the extracting section 22b came into countercurrent contact with methanol entering a top of an absorbing section 22a of the ester separation column through a pipe 28 and flowing downward, wherein the methanol further absorbed dimethyl oxalate carried in the gas phase. As a result, a gas-phase stream in pipe 27 substantially containing no dimethyl oxalate or dimethyl carbonate from the top of the ester separation column entered an oxidation and esterification reactor for regeneration of methyl nitrite.
Dimethyl carbonate separated from a top of a refining column 23 was withdrawn through a pipe 30. High-purity dimethyl carbonate was obtained in a bottom of the refining column 23, wherein the dimethyl oxalate stream partially entered a dimethyl oxalate condenser (e.g., a heat exchanger) through a pipe 32, was condensed therein, and then recycled to the ester separation column 22 as an extraction agent, while the rest dimethyl oxalate was withdrawn through a pipe 31.
The temperature in the coupling reactor was 120 C, and the reaction pressure , .
thereof was 0.3 MPa.
The ratio of the absorbing section of the ester separation column to the extracting section thereof was 1:3, and the theoretical plate number of the stripping section was

10. The operation pressure in the ester separation column was 0.16 MPa. The operation temperature in the top of the column was 38 C, while the operation temperature in the bottom of the column was 177 C. The volume ratio of the extraction agent stream (dimethyl oxalate) to the absorbing agent stream (methanol) was 1.2:1.
As to the dimethyl oxalate refining column, it had a theoretical plate number of 40, a reflux ratio of 6, an operation pressure of 0.12 MPa, and operation temperatures of 95 C in the top thereof and 176 C in the bottom thereof. The ratio of the dimethyl oxalate circulated as the extraction agent to the dimethyl oxalate product withdrawn out was 1.3:1. The dimethyl oxalate as the extraction agent was cooled to 80 C
through the heat exchanger.
In this example, the relationship between the concentration of dimethyl oxalate and the relative volatility of methanol to dimethyl carbonate in the ester separation column was studied, with the result as shown in Fig. 3, which indicates distribution curves of the concentration of dimethyl oxalate and the relative volatility of methanol to dimethyl carbonate in the ester separation column. In Fig. 3, the values as indicated in the position of theoretical plates, from small to large, respectively represent the theoretical plate numbers successively arranged from the top to the bottom of the ester separation column. In the extracting section which was located below the theoretical plates of the feed inlet of the extraction agent, when the concentration of dimethyl oxalate in the liquid phase was increased to 28 mol%, i.e., the molar concentration of dimethyl oxalate in the extracting section of this example, dimethyl oxalate obviously functioned as an extraction agent, such that the relative volatility of methanol to dimethyl carbonate was increased from 0.7 as in the absorbing section (above the theoretical plates of the feed inlet of the extraction agent) to 1.8, crossing an azeotropic point where the relative volatility of methanol to dimethyl carbonate is 1.
At this point, methanol cannot be separated from dimethyl carbonate. The dimethyl carbonate was altered from a light component which formed azeotropy with methanol to a heavy component, and moved toward the bottom of the column, while the methanol moved toward the top of the column, such that the dimethyl carbonate could be readily separated from the methanol.
The recovery rate of dimethyl oxalate was higher than 99.99%, and the removal rate of dimethyl carbonate was 99.6%. The loads of the reboilers in the ester separation column and the dimethyl oxalate refining column were 7.667 MW and 1.256 MW, respectively.
3.0 The compositions of the feedstocks into the reactor and those of the streams in the pipes were as shown in Table 5.
Table 5 Stream in pipe Parameter .

Temperature, C 100 27 177 40 176 80 Pressure, IVIPa 0.30 0.14 0.19 0.12 0.14 0.14 N2 36.10% 39.43% 0.00 0.00 0.00 0.00 CO 20.05% 13.84% 0.00 0.00 0.00 0.00 NO 4.30% 13.52% 0.00 0.00 0.00 0.00 Content of Methyl 34.96% 20.24% 0.00 0.00 0.00 0.00 compositio nitrite n by Methanol 4.59 % 12.97 % 0.01 0.92 % 0.00 0.00 weight Dimethyl 0.00 237 ppm 1.47% 99.08% 7 ppm 7 ppm carbonate Dimethyl oxalate 0.00 0.00 98.52% 0.00 > 99.9% > 99.9%
Example 6 The steps were the same as those in Example 5, with different reaction conditions and parameters of columns.
The reaction temperature in the coupling reactor was 120 C, and the reaction -- pressure thereof was 0.3 MPa.
The height ratio of the absorbing section of the ester separation column to the extracting section thereof was 1 :1.5, and the theoretical plate number of the stripping section was 25. The operation pressure in the ester separation column was 0.16 MPa.
The operation temperature in the top of the column was 38 C, while the operation temperature in the bottom of the column was 180 C. The volume ratio of the extraction agent stream (dimethyl oxalate) to the absorbing agent stream (methanol) was 2.2:1.
As to the dimethyl oxalate refining column, it had a theoretical plate number of 40, a reflux ratio of 8.5, an operation pressure of 0.12 MPa, and operation temperatures of 96 C in the top thereof and 176 C in the bottom thereof. The ratio of the dimethyl oxalate as an extraction agent to the dimethyl oxalate product withdrawn out was 2.3:1. The dimethyl oxalate as the extraction agent was cooled to 100 C
through the heat exchanger. The concentration of dimethyl oxalate in the liquid phase in the extracting section of the ester separation column was 42 mol%.
The recovery rate of dimethyl oxalate was higher than 99.99%, and the removal rate of dimethyl carbonate was 99.5%. The loads of the reboilers in the ester separation column and the dimethyl oxalate refining column were 8.945 MW and 1.543 MW, respectively.
The compositions of the feedstocks into the reactor and those of the streams in the pipes were shown in Table 6.
Table 6 Stream in pipe Parameter Temperature, C 100 27 180 40 176 100 Pressure, MPa 0.30 0.14 0.19 0.12 0.14 0.14 N2 36.10% 39.43% 0.00 0.00 0.00 0.00 CO 20.05% 13.84% 0.00 0.00 0.00 0.00 NO 4.30% 13.52% 0.00 0.00 0.00 0.00 Methyl Content of34.96% 20.24% 0.00 0.00 0.00 0.00 nitrite composition Methanol 4.59% 12.97% 0.00 0.00 0.00 0.00 by weight Dimethyl 0.00 299 ppm 1.07% > 99.9% 5 ppm 5 ppm carbonate Dimethyl 0.00 0.00 98.93% 0.00 > 99.9% > 99.9%
oxalate Example 7 The steps were the same as those in Example 5, with different compositions of feedstocks, reaction conditions, and parameters of columns.
The reaction temperature in the coupling reactor was 130 C, and the reaction pressure thereof was 0.4 MPa.
The ratio of the absorbing section of the ester separation column to the extracting section thereof was 1:2, and the theoretical plate number in the stripping section was 15. The operation pressure in the ester separation column was 0.2 MPa. The operation temperature in the top of the column was 42 C, while the operation temperature in the bottom of the column was 186 C. The volume ratio of the extraction agent stream (dimethyl oxalate) to the absorbing agent stream (methanol) was 1.8:1.
As to the dimethyl oxalate refining column, it had a theoretical plate number of 30, a reflux ratio of 8.8, an operation pressure of 0.20 MPa, and operation temperatures of 113 C in the top thereof and 193 C in the bottom thereof.
The ratio of the dimethyl oxalate as an extraction agent to the dimethyl oxalate product withdrawn out was 1.9:1. The dimethyl oxalate as the extraction agent was cooled to 60 C through the heat exchanger. The concentration of dimethyl oxalate in the liquid phase in the extracting section of the ester separation column was 36 mol%.
The recovery rate of dimethyl oxalate was higher than 99.99%, and the removal rate of dimethyl carbonate was 99.5%. The loads of the reboilers in the ester separation column and the dimethyl oxalate refining column were 9.459 MW and 1.741 MW, respectively.
The compositions of the feedstocks into the reactor and those of the streams in the pipes were shown in Table 7.
Table 7 Stream in pipe Parameter Temperature, C 100 25 186 40 193 80 Pressure, MPa 0.4 0.14 0.23 0.20 0.22 0.22 N2 47.21% 51.85% 0.00 0.00 0.00 0.00 CO 16.86% 9.98% 0.00 0.00 0.00 0.00 NO 4.52% 14.33% 0.00 0.00 0.00 0.00 Methyl Content of 27.56% 11.20% 0.00 0.00 0.00 0.00 nitrite composition Methanol 3.86% 12.64% 0.00 0.00 0.00 0.00 by weight D imethyl 0.00 355 ppm 1.34% > 99.9% 7 ppm 7 ppm carbonate Dimethyl oxalate 0.00 0.00 98.66% 0.00 > 99.9%
> 99.9%
Comparative Example 2 The same scale and similar reaction conditions of Example 5 above, and the device disclosed in CN 101190884A (the entirety of which is incorporated herein for reference) were adopted. A coupling reaction product was absorbed with a large quantity of methanol in an alcohol washing column, to obtain a liquid in a bottom of the column containing 40 wt% of methanol, 1.1 wt% of dimethyl carbonate, and 58.9 wt% of dimethyl oxalate. An alcohol recovery column had a total theoretical plate number of 80, and a reboiler load of 28.134 MW. An ester separation column had the same number of theoretical number as that in Example 5, and a reboiler load of 1.872 MW. Such reboiler loads were obviously higher than the reboiler loads of the ester separation column and the dimethyl oxalate refining column of Example 5 of the present disclosure.
In addition, the investment in equipment according to CN 101190884A is higher than the investment in equipment for carrying out the method according to the present disclosure.
Although the present disclosure has been explained in detail, modifications within the spirit and scope of the present disclosure would be apparent for those skilled in the art. Moreover, it should be understood that various aspects, and parts of different, specific embodiments recited in the present disclosure, and various features as listed can be combined or partially or completely exchanged with each other. In addition, those skilled in the art can understand that, the descriptions above merely constitute exemplary implementing manners of the present disclosure, but are not intended to limit the present disclosure.
List of references R-101. coupling reactor;
C-101. dimethyl oxalate separation column;
D-102. reflux tank of the dimethyl oxalate separation column;
1. nitrogen feedstock;
2. carbon monoxide feedstock;
3. methanol feedstock;
4. methyl nitrite feedstock;
5. material fed into to the coupling reactor;
6. material stream discharged out of the coupling reactor;
7. methanol feedstock as an absorbing agent;
8. gas from a top of the dimethyl oxalate separation column;
9. non-condensed gas from the top of the dimethyl oxalate separation column;
10. reflux liquid of the dimethyl oxalate separation column;

11. crude methanol product;

12. liquid stream from a bottom of the dimethyl oxalate separation column (dimethyl oxalate product);
21. coupling reactor;
22. ester separation column;
22a. absorbing section;
22b. extracting section;
22c. stripping section;
23. dimethyl oxalate refining column;
24. dimethyl oxalate condenser;
25. material inlet pipe of the coupling reactor;
26. material outlet pipe of the coupling reactor;
27. gas-phase pipe in a top of the ester separation column;
28. methanol inlet pipe;
29. liquid pipe in a bottom of the ester separation column;
30. dimethyl carbonate pipe in a top of the dimethyl oxalate refining column;

31. outlet pipe for dimethyl oxalate products;
32. pipe for circulation of dimethyl oxalate; and 33. pipe for circulation of cooled dimethyl oxalate.

Claims (20)

Claims

1. A method for producing dimethyl oxalate, comprising the following steps:
step a): feeding, into a coupling reactor, a reaction material containing carbon monoxide and methyl nitrite, which react in the presence of a platinum-group metal catalyst, to obtain a dimethyl oxalate-containing gas-phase stream; and step b): feeding the dimethyl oxalate-containing gas-phase stream into a dimethyl oxalate separation column, and enabling counter-current contact of the dimethyl oxalate-containing gas-phase stream with a methanol-containing stream entering the separation column from a top thereof, so as to obtain crude methanol from the top of the column and a dimethyl oxalate product from a bottom of the column, wherein the dimethyl oxalate-containing gas-phase stream is not cooled before being fed into the dimethyl oxalate separation column.

2. The method according to claim 1, wherein the dimethyl oxalate-containing gas-phase stream does not go through an alcohol washing column before being fed into the dimethyl oxalate separation column.

3. The method according to claim 1 or 2, wherein the dimethyl oxalate separation column comprises:
an absorbing and rectifying section, which is arranged between a feed inlet for the methanol-containing stream and a feed inlet for the dimethyl oxalate-containing gas-phase stream, and is provided with a column plate or a filler; and a stripping section, which is arranged between the inlet for the dimethyl oxalate-containing gas-phase stream and the bottom of the column, and is provided with a column plate or a filler.

4. The method according to claim 3, wherein a height ratio of the absorbing and rectifying section to the stripping section is in the range from 0.2:1 to 5:1.

5. The method according to claim 4, wherein the height ratio of the absorbing and rectifying section to the stripping section is in the range from 1:1 to 2:1.

6. The method according to claim 3, wherein the top of the dimethyl oxalate separation column has a temperature in the range from 0 °C to 60 °C, and a pressure in the range from 0.1 MPa to 0.3 MPa; and/or wherein the bottom of the dimethyl oxalate separation column has a temperature in the range from 161 °C to 210 °C, and a pressure in the range from 0.1 MPa to 0.35 MPa.

7. The method according to claim 3, wherein the filler is structured or bulk packed, and the column plate is in the form of a float valve tray, a sieve plate, a double pass tray, a bubble-cap tray, or a Thorman tray; and / or wherein the stripping section has a theoretical plate number in the range from to 40.

8. The method according to claim 1 or 2, wherein operation conditions of the coupling reactor include a reaction temperature in the range from 50 °C
to 200 °C and a pressure in the range from 0.1 MPa to1.0 MPa.

9. The method according to claim 1 or 2, comprising:
heat tracing a pipe arranged between the coupling reactor and the dimethyl oxalate separation column; and / or heat tracing a discharge pipe arranged in the bottom of the dimethyl oxalate separation column.

10. A method for producing dimethyl oxalate, and dimethyl carbonate as a byproduct, comprising the following steps:
step a): feeding, into a coupling reactor, a reaction material containing carbon monoxide and methyl nitrite, which react in the presence of a platinum-group metal catalyst, to obtain a gas-phase stream containing both dimethyl oxalate and dimethyl carbonate;
step b): feeding the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate into an ester separation column, and enabling counter-current contact of the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate with a methanol-containing stream entering the ester separation column from a top of the column, and an extraction agent stream containing dimethyl oxalate and entering the ester separation column from an intermediate section thereof, so as to obtain crude methanol from the top of the column and a mixture containing both dimethyl oxalate and dimethyl carbonate from a bottom of the column; and step c): feeding the mixture into a dimethyl oxalate refining column, to obtain a dimethyl carbonate product from a top of the refining column and a dimethyl oxalate product from a bottom of the refining column, wherein the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate is not cooled before being fed into the dimethyl oxalate separation column.

11. The method according to claim 10, wherein the gas-phase stream containing both dimethyl oxalate and dimethyl carbonate does not go through an alcohol washing column before being fed into the ester separation column.

12. The method according to claim 10 or 11, wherein the ester separation column comprises:
an absorbing section, which is arranged between a feed inlet for the extraction agent stream and a feed inlet for the methanol-containing stream, and is provided with a column plate or a filler;
an extracting section, which is arranged between a feed inlet for the gas-phase stream and the feed inlet for the extraction agent stream, and is provided with a column plate of a filler; and a stripping section, which is arranged between the feed inlet for the gas-phase stream and the bottom of the ester separation column, and is provided with a column plate or a filler.

13. The method according to claim 12, wherein a height ratio of the absorbing section to the extracting section is in the range from 1: 0.5 to 1:5; and/or wherein a height ratio of the absorbing section to the stripping section is in the range from 1:0.2 to 1:5.

14. The method according to claim 12, wherein the filler is structured or bulk packed, and the column plate is in the form of a float valve tray, a sieve plate, a double pass tray, a bubble-cap tray, or a Thorman tray; and / or wherein the stripping section has a theoretical plate number in the range from to 40.

15. The method according to claim 12, wherein 50-90% of the dimethyl oxalate obtained in step c) is recycled to the ester separation column as the extraction agent;
and / or wherein the extraction agent has a temperature in the range from 55 °C
to 210 °C.

16. The method according to claim 15, wherein a liquid phase in the extracting section of the ester separation column has a concentration of dimethyl oxalate equal to or higher than 20 mol%.

17. The method according to claim 12, wherein the top of the ester separation column has an operation pressure in the range from 0.1 MPa to 0.4 MPa, and a temperature in the range from 0 °C to 60 °C.

18. The method according to claim 12, wherein a volume ratio of the methanol-containing stream to the extraction agent stream fed into the ester separation column is in the range from 1:1 to I :5.

19. The method according to claim 10 or 11, wherein the dimethyl oxalate refining column has an operation pressure in the range from 0 MPa to 0.3 MPa, a temperature in the top thereof in the range from 20 °C to 130 °C; and/or wherein a theoretical plate number in the range from 10 to 60, and a reflux ratio in the range from 2 to 200.

20. The method according to claim 12, wherein the reaction material comprises 5-40 mol% of carbon monoxide, 5-30 mol% of methyl nitrite, 1-10 mol% of nitrogen monoxide, 0.1-10 mol% of methanol, and inert gasses as a balance.