Double-tower system and method for preparing oxalic acid by hydrolyzing dimethyl oxalate

Abstract

The invention relates to a double-tower system and method for preparing oxalic acid by hydrolyzing dimethyl oxalate, and aims to solve the technical problems of low dimethyl oxalate conversion rate, high energy consumption and complex process at present. The technical scheme is as follows: raw materials are preheated by a DMO preheater and then enter a pre-reactor with circulating mother liquor for pre-hydrolysis reaction; low-pressure steam supplements heat and desalted water, a gas-phase material extracted from the top of the tower is subjected to heat exchange through a mother liquor preheater and a methanol tower reboiler and enters a methanol tower for methanol refining, and a refined methanol product is extracted from the top of the methanol tower; and returning one part of the material extracted from the tower bottom to the reactive distillation tower as reflux liquid, and forming circulating mother liquor by the other part. The dimethyl oxalate conversion rate is high, the product quality is good, consumed water is supplemented in the reactive distillation column in the form of steam, water balance, heat coupling and waste heat utilization of the system are maintained, and the overall energy consumption is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of oxalic acid preparation, and particularly relates to a double-tower system and method for preparing oxalic acid by hydrolyzing dimethyl oxalate. Background Art

[0002] Oxalic acid is an important chemical product with a wide range of applications, including medicine, printing and dyeing, fine chemicals and other fields. There are many production processes for oxalic acid, mainly sodium formate method, polysaccharide oxidation method, carbon monoxide coupling method oxalate hydrolysis and glyoxal oxidation method. At present, the most widely used oxalic acid production process in China is the traditional sodium formate method. Since the sodium formate method requires the consumption of sulfuric acid and caustic soda and has lead pollution, it is gradually being replaced by other new processes. The synthesis of oxalate esters by carbon monoxide gas phase coupling method and then the production of oxalic acid by oxalate hydrolysis is not only low in production cost and environmentally friendly, but also can provide more flexible product solutions for enterprises producing ethylene glycol and oxalate esters, which is conducive to alleviating the current problem of overcapacity of ethylene glycol and the survival difficulties of related enterprises.

[0003] The currently disclosed or applied processes for producing oxalic acid by hydrolysis of dimethyl oxalate have the problems of high energy consumption, low conversion rate of dimethyl oxalate, low yield of oxalic acid, and complex process or equipment. When the conversion rate of dimethyl oxalate is low, unreacted dimethyl oxalate and intermediate product monomethyl oxalate will be included in the oxalic acid product during oxalic acid crystallization, resulting in a decrease in the quality of the oxalic acid product. Patent CN 104892389B discloses a process for preparing oxalic acid by continuous reactive distillation hydrolysis of dimethyl oxalate, in which dimethyl oxalate is directly introduced into a reactive distillation bulkhead tower in a molten state without pre-hydrolysis, which will result in a low conversion rate in the tower and complex equipment and process. Patent CN 104892400B discloses a combined process of catalytic synthesis of oxalic acid by intermittent reaction and continuous reaction distillation, and patent CN 105152916A discloses a process for synthesizing oxalic acid from oxalate. In both processes, the synthesis reaction, product separation and alcohol purification are realized in one tower, which has very high requirements for the tower and is difficult to realize in industrial production. In addition, it is difficult to realize energy coupling through equipment integration of these two processes, and the energy consumption is high. Patent CN110713437B discloses a device and preparation method for preparing oxalic acid by hydrolysis of oxalate, which has high energy consumption and low conversion rate of dimethyl oxalate. Summary of the invention

[0004] The purpose of the present invention is to solve the above problems and provide a double-tower system and method for preparing oxalic acid by hydrolyzing dimethyl oxalate.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A double-tower system for preparing oxalic acid by hydrolyzing dimethyl oxalate, comprising a DMO preheater, a prereactor, a reaction distillation tower, a methanol tower, a methanol tower reboiler, a methanol tower condenser, an oxalic acid crystallization separation and drying device and a mother liquor preheater, wherein the inlet of the DMO preheater is externally connected to a dimethyl oxalate raw material through a pipeline, and the outlet is connected to the inlet of the prereactor, the outlet of the prereactor is connected to the inlet of the reaction distillation tower through a pipeline, the reaction distillation tower is externally connected to low-pressure steam, the bottom outlet of the reaction distillation tower is connected to the inlet of the oxalic acid crystallization separation and drying device through a pipeline, the bottom outlet of the reaction distillation tower is further connected to the inlet of the reaction distillation tower reboiler through a pipeline, the outlet of the reaction distillation tower reboiler is back-connected to the reaction distillation tower through a pipeline, one outlet of the oxalic acid crystallization separation and drying device is connected to the inlet of the mother liquor preheater, the outlet of the mother liquor preheater is connected to the inlet of the prereactor through a pipeline, and the other outlet of the oxalic acid crystallization separation and drying device outputs an oxalic acid product;

[0007] The bottom outlet of the methanol tower is connected to the top of the reactive distillation tower, the inlet of the methanol tower reboiler and the mother liquid preheater through pipelines, and the outlet of the methanol tower reboiler is connected back to the methanol tower through a pipeline;

[0008] The gas phase outlet at the top of the prereactor is connected to the gas phase inlet of the reaction distillation tower through a pipeline, the gas phase outlet at the top of the reaction distillation tower is connected to the hot side inlet of the mother liquor preheater through a pipeline, the hot side outlet of the mother liquor preheater is connected to the hot side inlet of the methanol tower reboiler through a pipeline, the hot side outlet of the methanol tower reboiler is connected to the inlet of the methanol tower through a pipeline, the gas phase outlet at the top of the methanol tower is connected to the inlet of the methanol tower condenser through a pipeline, and the outlet pipeline of the methanol tower condenser is divided into two routes, one route is connected back to the methanol tower, and the other route outputs the refined methanol product.

[0009] A method for preparing oxalic acid by hydrolyzing dimethyl oxalate comprises the following steps:

[0010] Step S1: the raw material dimethyl oxalate is preheated in a DMO preheater and then enters a pre-reactor. At the same time, the bottom material from the methanol tower is mixed with the crystal liquid of the oxalic acid crystallization, separation and drying device to obtain a circulating mother liquor, which is preheated in a mother liquor preheater and then enters a pre-reactor. The raw material dimethyl oxalate and the circulating mother liquor undergo a pre-hydrolysis reaction in the pre-reactor, and the reacted material is transported to a reactive distillation tower;

[0011] Step S2: the material is subjected to reactive distillation in a reactive distillation tower, so that dimethyl oxalate and the intermediate product monomethyl oxalate are further reacted and converted into oxalic acid, low-pressure steam is added to the reactor of the reactive distillation tower to supplement the heat required for reboiling the reactor, and this part of the steam is liquefied in the tower and used as the desalted water required to be supplemented for the system, and at the same time, a reactive distillation tower reboiler is set in the reactor of the reactive distillation tower to provide the heat required for reboiling the reactor, gaseous materials are extracted from the top of the tower, and oxalic acid-rich materials are extracted from the bottom of the tower and transported to an oxalic acid crystallization, separation and drying device to prepare an oxalic acid product;

[0012] Step S3: The gaseous material taken out from the top of the reaction distillation tower is firstly heat-exchanged in a mother liquor preheater, and then heat-exchanged in a methanol tower reboiler to provide the heat required for reboiling of the methanol tower kettle, and then enters the methanol tower for methanol refining. The refined methanol material taken out from the top of the methanol tower is condensed in a methanol tower condenser and divided into two streams, one of which is refluxed into the methanol tower, and the other is output as a refined methanol product. The material with a methanol content of ≤0.01wt% taken out from the bottom of the methanol tower is divided into two streams, one of which is returned to the top of the reaction distillation tower as the reflux liquid of the reaction distillation tower, and the other is mixed with the crystallization liquid from the oxalic acid crystallization, separation and drying device to form a circulating mother liquor.

[0013] Furthermore, in the circulating mother liquor of step S1, methanol ≤ 0.01 wt%, dimethyl oxalate ≤ 0.01 wt%, and monomethyl oxalate ≤ 0.001 wt%.

[0014] Furthermore, in step S1, the molar ratio of water to dimethyl oxalate in the pre-reactor feed is 15-30:1, the material residence time is 30-60 min, the operating pressure of the pre-reactor is 0.005-0.02 MPa, and the temperature is 85-95° C.

[0015] Furthermore, in step S2, the gaseous material extracted from the top of the reactive distillation tower contains 20-30 wt% methanol, and the oxalic acid-rich material extracted from the bottom of the tower is transported to the oxalic acid crystallization separation and drying device to prepare an oxalic acid product with a purity of ≥99.6 wt%.

[0016] Furthermore, in step S2, the reactive distillation tower is a packed tower with a total theoretical number of 30 to 50 plates, a top temperature of 95 to 105° C., a pressure of 0.005 to 0.02 MPa, a bottom temperature of 100 to 110° C., and a pressure of 0.01 to 0.03 MPa.

[0017] Furthermore, in step S3, the methanol tower is a packed tower with a total theoretical number of 25 to 45 plates, a top temperature of 40 to 55° C., a pressure of -0.04 to -0.06 MPa, a bottom temperature of 75 to 90° C., and a pressure of -0.035 to -0.055 MPa.

[0018] Furthermore, in step S3, the material extracted from the bottom of the methanol tower is divided into two streams, of which the stream returned to the top of the reactive distillation tower accounts for 30-80wt% of the total flow of the material extracted from the bottom of the tower.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The present invention has a high conversion rate of dimethyl oxalate and a good quality of oxalic acid product. The material with very low methanol content extracted from the bottom of the methanol tower is used as the reflux liquid of the reaction distillation tower at the top of the reaction distillation tower, so as to avoid using a large amount of material containing methanol extracted from the top of the reaction distillation tower for reflux, so that the methanol content in the upper section of the reaction distillation tower can be kept at a low level, and the equilibrium reaction can be promoted to proceed in the positive direction, thereby improving the conversion rate, and controlling the impurities such as dimethyl oxalate and oxalic acid monoester in the low-extracted oxalic acid solution of the reaction distillation tower at an extremely low level, so as to ensure the conversion rate of dimethyl oxalate and the index of the oxalic acid product;

[0021] 2. The water consumed by the entire system in the present invention is replenished in the form of steam in the reactor of the reaction distillation tower, which plays the role of replenishing water and providing reboiling heat for the reactor. The total replenishment amount is the same as the total consumption of the system. No excess process wastewater is discharged from the entire system, and the system water is balanced;

[0022] 3. The present invention optimizes and integrates the whole process, fully considers the coupling of heat in the production process and the recovery and utilization of waste heat, reduces the overall energy consumption, and thus reduces the production cost; fully utilizes the latent heat of the gas phase at the top of the reaction distillation tower, and the latent heat of the material can be used to preheat the circulating mother liquor. At the same time, the methanol tower adopts negative pressure operation to reduce the bottom temperature, and the remaining latent heat can be used as the heat source of the methanol tower reboiler, which greatly reduces energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a structural schematic diagram of the present invention;

[0024] In the figure: 1-DMO preheater; 2-prereactor; 3-reaction distillation tower; 4-reaction distillation tower reboiler; 5-methanol tower; 6-methanol tower reboiler; 7-methanol tower condenser; 8-oxalic acid crystallization separation and drying device; 9-mother liquor preheater. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0026] Example 1

[0027] like Figure 1As shown, a double-tower system for preparing oxalic acid by hydrolyzing dimethyl oxalate comprises a DMO preheater 1, a prereactor 2, a reaction distillation tower 3, a methanol tower 5, a methanol tower reboiler 6, a methanol tower condenser 7, an oxalic acid crystallization separation and drying device 8 and a mother liquor preheater 9, wherein the inlet of the DMO preheater 1 is externally connected to the dimethyl oxalate raw material through a pipeline, and the outlet is connected to the inlet of the prereactor 2, the outlet of the prereactor 2 is connected to the inlet of the reaction distillation tower 3 through a pipeline, the reaction distillation tower 3 is externally connected to low-pressure steam, the bottom outlet of the reaction distillation tower 3 is connected to the inlet of the oxalic acid crystallization separation and drying device 8 through a pipeline, the bottom outlet of the reaction distillation tower 3 is further connected to the inlet of the reaction distillation tower reboiler 4 through a pipeline, the outlet of the reaction distillation tower reboiler 4 is back-connected to the reaction distillation tower 3 through a pipeline, one outlet of the oxalic acid crystallization separation and drying device 8 is connected to the inlet of the mother liquor preheater 9, the outlet of the mother liquor preheater 9 is connected to the inlet of the prereactor 2 through a pipeline, and the other outlet of the oxalic acid crystallization separation and drying device 8 outputs the oxalic acid product;

[0028] The bottom outlet of the methanol tower 5 is connected to the top of the reaction distillation tower 3, the methanol tower reboiler 6 and the inlet of the mother liquid preheater 9 through pipelines, and the outlet of the methanol tower reboiler 6 is connected back to the methanol tower 5 through a pipeline;

[0029] The top gas phase outlet of the prereactor 2 is connected to the gas phase inlet of the reaction distillation tower 3 through a pipeline, the top gas phase outlet of the reaction distillation tower 3 is connected to the hot side inlet of the mother liquor preheater 9 through a pipeline, the hot side outlet of the mother liquor preheater 9 is connected to the hot side inlet of the methanol tower reboiler 6 through a pipeline, the hot side outlet of the methanol tower reboiler 6 is connected to the inlet of the methanol tower 5 through a pipeline, the top gas phase outlet of the methanol tower 5 is connected to the inlet of the methanol tower condenser 7 through a pipeline, and the outlet pipeline of the methanol tower condenser 7 is divided into two routes, one route is connected back to the methanol tower 5, and the other route outputs the refined methanol product.

[0030] A method for preparing oxalic acid by hydrolyzing dimethyl oxalate comprises the following steps:

[0031] Step S1: The raw material dimethyl oxalate is preheated in the DMO preheater 1 and then enters the pre-reactor 2. At the same time, the bottom material from the methanol tower 5 is mixed with the crystal liquid from the oxalic acid crystallization separation and drying device 8 to obtain a methanol content of 6.7e -3 wt%, dimethyl oxalate content 7.6e -3 wt%, monomethyl oxalate content 6.9e -4wt% of the circulating mother liquor is preheated in the mother liquor preheater 9 and then enters the pre-reactor 2. The total molar ratio of water and dimethyl oxalate in the feed is 15:1. The raw material dimethyl oxalate and the circulating mother liquor undergo a pre-hydrolysis reaction in the pre-reactor 2. The operating pressure of the pre-reactor 2 is 0.02MPa, the temperature is 95°C, the material residence time is 50min, and the reacted material is transported to the reactive distillation tower 3;

[0032] Step S2: the material is subjected to reactive distillation in a reactive distillation tower 3, so that dimethyl oxalate and the intermediate product monomethyl oxalate are further reacted and converted into oxalic acid. The reactive distillation tower 3 is a packed tower with a total theoretical number of 50 plates, a tower top temperature of 105°C and a pressure of 0.02MPa, a tower bottom temperature of 110°C and a pressure of 0.03MPa, and low-pressure steam is added to the reactor of the reactive distillation tower 3 to supplement the heat required for reboiling of the reactor. This part of the steam is liquefied in the tower and used as desalted water required to supplement the system. At the same time, a reactive distillation tower reboiler 4 is set in the reactor of the reactive distillation tower 3 to provide the heat required for reboiling of the reactor. A gaseous material containing 30wt% of methanol is extracted from the top of the reactive distillation tower 3, and an oxalic acid-rich material is extracted from the bottom of the tower and sent to an oxalic acid crystallization, separation and drying device 8 to prepare an oxalic acid product with a purity of 99.7wt%;

[0033] Step S3: The gaseous material taken out from the top of the reaction distillation tower 3 is firstly heat exchanged by the mother liquid preheater 9, and then heat exchanged by the methanol tower reboiler 6 to provide the heat required for reboiling of the methanol tower 5 kettle, and then enters the methanol tower 5 for methanol refining. The methanol tower 5 is a packed tower with a total theoretical number of 25 plates, a top temperature of 52°C, a pressure of -0.04MPa, a bottom temperature of 88°C, and a pressure of -0.035MPa. The refined methanol material taken out from the top of the methanol tower 5 is condensed by the methanol tower condenser 7 and divided into two streams, one of which is refluxed into the methanol tower 5, and the other is output as a refined methanol product. The methanol content of the methanol tower 5 bottom is 7.9e -3 The wt% material is divided into two streams, one of which accounts for 75wt% of the total flow and returns to the top of the reaction distillation tower 3 as the reflux liquid of the reaction distillation tower 3, and the other is mixed with the crystallization liquid from the oxalic acid crystallization separation and drying device 8 to form a circulating mother liquid.

[0034] The methanol content in the circulating mother liquid of step S1 can also be any value of ≤0.01wt%, the dimethyl oxalate content can also be any value of ≤0.01wt%, and the monomethyl oxalate content can also be any value of ≤0.001wt%;

[0035] The molar ratio of water to dimethyl oxalate in the feed of the pre-reactor 2 of step S1 can also be any value between 15 and 30:1, the material residence time can also be any value between 30 and 60 min, the operating pressure of the pre-reactor (2) can also be any value between 0.005 and 0.02 MPa, and the temperature can also be any value between 85 and 95° C.;

[0036] The methanol content of the gas phase material extracted from the top of the reaction distillation tower 3 in step S2 can also be any value between 20 and 30 wt %, and the purity of the oxalic acid product prepared by the oxalic acid crystallization separation and drying device (8) can also be any value ≥ 99.6 wt %;

[0037] The total theoretical plate number of the reactive distillation tower 3 can also be any value between 30 and 50, the tower top temperature can also be any value between 95 and 105° C., the pressure can also be any value between 0.005 and 0.02 MPa, the tower bottom temperature can also be any value between 100 and 110° C., and the pressure can also be any value between 0.01 and 0.03 MPa;

[0038] The total theoretical plate number of the methanol tower 5 in step S3 can also be any value between 25 and 45, the top temperature can also be any value between 40 and 55°C, the pressure can also be any value between -0.04 and -0.06MPa, the bottom temperature can also be any value between 75 and 90°C, and the pressure can also be any value between -0.035 and -0.055MPa.

[0039] In the step S3, the amount of material from the bottom of the methanol tower 5 returning to the top of the reactive distillation tower 3 may account for any value between 30 and 80 wt % of the total flow rate of the material extracted from the bottom of the tower.

[0040] In this embodiment, the conversion rate of dimethyl oxalate is 99.95 wt %.

[0041] Example 2

[0042] The difference between this embodiment and embodiment 1 is that:

[0043] The methanol content in the circulating mother liquor in step S1 is 5.3e -3 wt%, dimethyl oxalate content 5.8e -3 wt%, monomethyl oxalate content 5.1e -4 wt%, the total molar ratio of water and dimethyl oxalate in the feed is 20:1, the operating pressure of the pre-reactor 2 is 0.01MPa, the temperature is 88°C, and the material residence time is 35min;

[0044] In step S2, the total theoretical number of plates of the reaction distillation tower 3 is 43, the top temperature is 99° C., the pressure is 0.01 MPa, the bottom temperature is 104° C., the pressure is 0.016 MPa, the gas phase material extracted from the top of the reaction distillation tower 3 contains 26 wt % of methanol, and the purity of the oxalic acid product prepared by the oxalic acid crystallization separation and drying device 8 is 99.8 wt %;

[0045] In step S3, the total theoretical number of plates of the methanol tower 5 is 40, the top temperature is 48°C, the pressure is -0.05MPa, the bottom temperature is 84°C, the pressure is -0.046MPa, and the methanol content in the material taken out from the bottom of the methanol tower 5 is 6.6e -3 wt%, of which the reflux liquid returned to the top of the reaction distillation tower 3 accounts for 50wt% of the total flow;

[0046] In this embodiment, the conversion rate of dimethyl oxalate is 99.95 wt %.

[0047] Example 3

[0048] The difference between this embodiment and embodiment 1 is that:

[0049] The methanol content in the circulating mother liquor in step S1 is 3.4e -3 wt%, dimethyl oxalate content 4.9e -3 wt%, monomethyl oxalate content 4.5e -4 wt%, the total molar ratio of water and dimethyl oxalate in the feed is 30:1, the operating pressure of the pre-reactor 2 is 0.005MPa, the temperature is 85°C, and the material residence time is 20min;

[0050] In step S2, the total theoretical number of plates of the reaction distillation tower 3 is 30, the tower top temperature is 95° C., the pressure is 0.005 MPa, the tower bottom temperature is 100° C., the pressure is 0.01 MPa, the gas phase material extracted from the top of the reaction distillation tower 3 contains 20wt% methanol, and the purity of the oxalic acid product prepared by the oxalic acid crystallization separation and drying device 8 is 99.8wt%;

[0051] In step S3, the total theoretical number of plates of methanol tower 5 is 45, the top temperature is 43°C, the pressure is -0.058MPa, the bottom temperature is 76°C, the pressure is -0.055MPa, and the methanol content in the material taken out from the bottom of methanol tower 5 is 6.3e -3 wt%, of which the reflux liquid returned to the top of the reaction distillation tower 3 accounts for 30wt% of the total flow;

[0052] In this embodiment, the conversion rate of dimethyl oxalate is 99.96 wt %.

Claims

1. A double tower system for preparing oxalic acid by hydrolysis of dimethyl oxalate, characterized in that: The invention comprises a DMO preheater (1), a prereactor (2), a reaction distillation tower (3), a methanol tower (5), a methanol tower reboiler (6), a methanol tower condenser (7), an oxalic acid crystallization separation drying device (8) and a mother liquor preheater (9), wherein the inlet of the DMO preheater (1) is connected to a dimethyl oxalate raw material via a pipeline, and the outlet is connected to the inlet of the prereactor (2), the outlet of the prereactor (2) is connected to the inlet of the reaction distillation tower (3) via a pipeline, the reaction distillation tower (3) is connected to a low-pressure steam, and the bottom outlet of the reaction distillation tower (3) is connected to the dimethyl oxalate raw material via a pipeline. The reaction distillation tower (3) is connected to the inlet of the oxalic acid crystallization separation and drying device (8) through a pipeline, the bottom outlet of the reaction distillation tower (3) is also connected to the inlet of the reaction distillation tower reboiler (4) through a pipeline, the outlet of the reaction distillation tower reboiler (4) is connected back to the reaction distillation tower (3) through a pipeline, one outlet of the oxalic acid crystallization separation and drying device (8) is connected to the inlet of the mother liquor preheater (9), the outlet of the mother liquor preheater (9) is connected to the inlet of the pre-reactor (2) through a pipeline, and the other outlet of the oxalic acid crystallization separation and drying device (8) outputs the oxalic acid product; The bottom outlet of the methanol tower (5) is connected to the top of the reaction distillation tower (3), the methanol tower reboiler (6) and the inlet of the mother liquid preheater (9) through pipelines, and the outlet of the methanol tower reboiler (6) is connected back to the methanol tower (5) through a pipeline; The gas phase outlet at the top of the pre-reactor (2) is connected to the gas phase inlet of the reaction distillation tower (3) through a pipeline, the gas phase outlet at the top of the reaction distillation tower (3) is connected to the hot side inlet of the mother liquor preheater (9) through a pipeline, the hot side outlet of the mother liquor preheater (9) is connected to the hot side inlet of the methanol tower reboiler (6) through a pipeline, the hot side outlet of the methanol tower reboiler (6) is connected to the inlet of the methanol tower (5) through a pipeline, the gas phase outlet at the top of the methanol tower (5) is connected to the inlet of the methanol tower condenser (7) through a pipeline, and the outlet pipeline of the methanol tower condenser (7) is divided into two routes, one of which is connected back to the methanol tower (5) and the other is used to output the refined methanol product.

2. A method for preparing oxalic acid by hydrolyzing dimethyl oxalate using the double-tower system for preparing oxalic acid by hydrolyzing dimethyl oxalate according to claim 1, characterized in that: The steps include: Step S1: the raw material dimethyl oxalate is preheated in a DMO preheater (1) and then enters a pre-reactor (2). At the same time, the bottom material from the methanol tower (5) is mixed with the crystal liquid from the oxalic acid crystallization separation and drying device (8) to obtain a circulating mother liquor, which is preheated in a mother liquor preheater (9) and then enters the pre-reactor (2). The raw material dimethyl oxalate and the circulating mother liquor undergo a pre-hydrolysis reaction in the pre-reactor (2), and the reacted materials are transported to a reactive distillation tower (3); Step S2: the material is subjected to reactive distillation in a reactive distillation tower (3) so that dimethyl oxalate and the intermediate product monomethyl oxalate are further reacted and converted into oxalic acid, low-pressure steam is added to the reactor of the reactive distillation tower (3) to supplement the heat required for reboiling the reactor, and this part of the steam is liquefied in the tower and used as desalted water required to be supplemented for the system. At the same time, a reactive distillation tower reboiler (4) is provided in the reactor of the reactive distillation tower (3) to provide the heat required for reboiling the reactor, gaseous materials are extracted from the top of the tower, and oxalic acid-rich materials are extracted from the bottom of the tower and transported to an oxalic acid crystallization, separation and drying device (8) to prepare an oxalic acid product; Step S3: The gaseous material taken out from the top of the reaction distillation tower (3) is firstly heat exchanged in a mother liquid preheater (9), and then heat exchanged in a methanol tower reboiler (6) to provide the heat required for reboiling in the bottom of the methanol tower (5), and then enters the methanol tower (5) for methanol refining. The refined methanol material taken out from the top of the methanol tower (5) is condensed in a methanol tower condenser (7) and divided into two streams, one of which is refluxed into the methanol tower (5) and the other is output as a refined methanol product. The material with a methanol content of ≤0.01wt% taken out from the bottom of the methanol tower (5) is divided into two streams, one of which is returned to the top of the reaction distillation tower (3) as the reflux liquid of the reaction distillation tower (3), and the other is mixed with the crystallization liquid from the oxalic acid crystallization separation and drying device (8) to form a circulating mother liquor.

3. The method for preparing oxalic acid by hydrolyzing dimethyl oxalate according to claim 2, characterized in that: In the circulating mother liquid of step S1, methanol ≤ 0.01 wt%, dimethyl oxalate ≤ 0.01 wt%, and monomethyl oxalate ≤ 0.001 wt%.

4. The method for preparing oxalic acid by hydrolyzing dimethyl oxalate according to claim 2, characterized in that: In the step S1, the molar ratio of water to dimethyl oxalate in the feed of the pre-reactor (2) is 15-30:1, the material residence time is 30-60 min, the operating pressure of the pre-reactor (2) is 0.005-0.02 MPa, and the temperature is 85-95° C.

5. The method for preparing oxalic acid by hydrolyzing dimethyl oxalate according to claim 2, characterized in that: In the step S2, the gaseous material extracted from the top of the reaction distillation tower (3) contains 20-30 wt% methanol, and the oxalic acid-rich material extracted from the bottom of the tower is transported to the oxalic acid crystallization separation and drying device (8) to prepare an oxalic acid product with a purity of ≥99.6 wt%.

6. The method for preparing oxalic acid by hydrolyzing dimethyl oxalate according to claim 2, characterized in that: The reactive distillation tower (3) in step S2 is a packed tower with a total theoretical number of 30 to 50 plates, a top temperature of 95 to 105° C., a pressure of 0.005 to 0.02 MPa, a bottom temperature of 100 to 110° C., and a pressure of 0.01 to 0.03 MPa.

7. The method for preparing oxalic acid by hydrolyzing dimethyl oxalate according to claim 2, characterized in that: The methanol tower (5) in step S3 is a packed tower with a total theoretical number of 25 to 45 plates, a top temperature of 40 to 55°C, a pressure of -0.04 to -0.06 MPa, a bottom temperature of 75 to 90°C, and a pressure of -0.035 to -0.055 MPa.

8. The method for preparing oxalic acid by hydrolyzing dimethyl oxalate according to claim 2, characterized in that: In the step S3, the material extracted from the bottom of the methanol tower (5) is divided into two streams, of which the stream returned to the top of the reactive distillation tower (3) accounts for 30-80 wt% of the total flow of the material extracted from the bottom of the tower.

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