Apparatus and method for co-producing oxalic acid in a coal-to-ethylene glycol system
By introducing equipment such as hydrolysis kettles, condensers, distillation columns, and vacuum crystallizers into the oxalic acid production process, and by using corrosion-resistant materials and multiple tanks operating in parallel and alternately, the problems of high energy consumption, low conversion rate, and equipment blockage in oxalic acid production have been solved, achieving continuous production and cost reduction.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- HE NAN NENG YUAN JI TUAN YAN JIU ZONG YUAN YOU XIAN GONG SI
- Filing Date
- 2026-05-21
- Publication Date
- 2026-07-28
AI Technical Summary
Existing oxalic acid production processes suffer from high energy consumption, low conversion rate, low efficiency in separating methanol and oxalic acid, easy equipment blockage, difficulty in achieving continuous production, and high equipment investment costs.
The system employs a hydrolysis reactor, condenser, condensate tank, distillation column, vacuum crystallizer, vacuum system, crystal dehydration equipment, mother liquor tank, drying equipment, and storage and packaging system. It utilizes dimethyl oxalate as raw material and achieves continuous production of oxalic acid through series and parallel connections. Corrosion-resistant materials are used to reduce equipment costs.
This enables continuous production of oxalic acid, avoids equipment blockage, reduces equipment investment costs, and improves production efficiency and energy utilization.
Smart Images
Figure CN122461998A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of modern coal chemical technology, specifically relating to an apparatus and method for the co-production of oxalic acid in a coal-to-ethylene glycol system. Background Technology
[0002] Oxalic acid is an organic acid, the simplest dicarboxylic acid, also known as oxalic acid. It is a moderately strong acid, a mild protic acid, and plays a crucial role in organic synthesis. Oxalic acid is also a bidentate ligand, widely used as a precipitant and chelating agent in inorganic chemistry. Currently, the mainstream oxalic acid production process is the dimethyl oxalate (DMO) hydrolysis method. This process uses dimethyl oxalate, an intermediate product of coal-to-ethylene glycol, as raw material, and obtains oxalic acid through hydrolysis, distillation, crystallization, and drying. However, the traditional DMO hydrolysis method has two main problems: firstly, energy consumption during chemical production is high, with a large amount of reaction heat and steam condensate heat not being effectively utilized, resulting in significant energy waste; secondly, limited by chemical equilibrium, the reaction conversion rate is often low, and the separation efficiency between methanol and oxalic acid generated during hydrolysis is not high, leading to difficulties in subsequent purification and affecting product purity and production economics.
[0003] To address the aforementioned issues, Chinese patent CN222093296U discloses a heat recovery and utilization device in the process of hydrolyzing dimethyl oxalate to produce oxalic acid. The device includes a pre-reaction vessel, a reactive distillation column, and a methanol refining column. Dimethyl oxalate and water are mixed into a solution and pre-reacted in the pre-reaction vessel. After pre-reaction, the dimethyl oxalate solution is sent to the reactive distillation column for further reaction and purification. The oxalic acid solution obtained from the bottom of the reactive distillation column is sent to a crystallization device. The crude methanol gas exiting from the top of the reactive distillation column enters the methanol refining column for methanol purification. This device recovers a portion of the heat from the top of the methanol refining column to heat soft water; it also recovers the condensate from the reboilers of the reactive distillation column and the methanol refining column for use as water for the pre-hydrolysis in the pre-reaction vessel, effectively utilizing thermal energy. Simultaneously, it uses distillation technology to separate the methanol produced in the reaction from the system, breaking the equilibrium of the hydrolysis reaction to promote the forward reaction and achieving preliminary product separation. This solves the problems of low reaction conversion rate and difficulty in separating methanol and oxalic acid in traditional processes. However, this existing technology still has the following three shortcomings in practical applications:
[0004] (1) Lack of back-end crystallization and anti-clogging design: Oxalic acid crystals in the subsequent crystallization and solid-liquid separation sections are very easy to deposit and scale in the conveying pipelines, valves and pumps, which can lead to equipment blockage;
[0005] (2) Unable to achieve continuous production: Traditional crystallization methods, such as single-reactor intermittent cooling, result in intermittent production, making it difficult to meet the needs of large-scale continuous industrial production;
[0006] (3) High equipment investment cost: Oxalic acid solution is extremely corrosive at high temperatures. Existing technical solutions tend to use expensive zirconium or titanium materials to deal with corrosion, resulting in high investment costs for the overall equipment and pipelines of the project. Summary of the Invention
[0007] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is to provide an apparatus and method for the co-production of oxalic acid in a coal-to-ethylene glycol system, which prevents crystal blockage, realizes continuous production and reduces equipment investment, and meets the needs of large-scale industrial processes.
[0008] The technical solution adopted in this invention is as follows:
[0009] An apparatus for the co-production of oxalic acid in a coal-to-ethylene glycol system includes a hydrolysis reactor, a condenser, a condensate tank, a distillation column, a vacuum crystallizer, a vacuum system, crystal dehydration equipment, a mother liquor tank, drying equipment, and a storage and packaging system; it can utilize dimethyl oxalate, an intermediate product of coal-to-ethylene glycol, as raw material to achieve continuous and industrialized production of oxalic acid.
[0010] The hydrolysis reactor is connected to the dimethyl oxalate production unit of the coal-to-ethylene glycol system, allowing liquid dimethyl oxalate to be directly transferred from the coal-to-ethylene glycol system to the hydrolysis reactor. The hydrolysis reactor is equipped with inlets for dimethyl oxalate, demineralized water, and oxalic acid mother liquor, and features an electric stirrer. The bottom of the hydrolysis reactor has a steam inlet and a liquid phase outlet, while the top has a gas phase outlet. The gas phase outlet at the top of the hydrolysis reactor is connected to a condenser via a pipeline. The condenser is connected to a condensate tank, where the condensate obtained after condensing the gaseous substances is transported and stored. The condensate tank is connected to a distillation column; the material in the condensate tank is pumped to the distillation column.
[0011] Preferably, the number of hydrolysis reactors is 2-5, connected in series in the process flow. The hydrolysis reactors are connected by a bottom pump or by material overflow caused by a difference in liquid level. The top vapor outlets of all hydrolysis reactors share a condenser via pipelines, or each hydrolysis reactor has its own independent condenser. The oxalic acid mother liquor inlet of the first hydrolysis reactor in the process flow is connected to the mother liquor tank. The liquid outlet of the last hydrolysis reactor in the process flow is connected to the distillation column. The liquid material from the last hydrolysis reactor in the process flow is pumped to the distillation column.
[0012] The distillation column is equipped with a built-in reboiler, and has a top outlet and a bottom outlet. The top outlet of the distillation column is connected to a reflux tank through a top condenser; the bottom outlet of the distillation column is connected to a vacuum crystallizer through a pipeline.
[0013] Preferably, the hydrolysis vessel is made of glass-lined material or 316L material; the built-in reboiler is made of C-276 nickel-based alloy; and the distillation column body is made of glass-lined material, lined with PTFE, or 316L material.
[0014] The vacuum crystallization tank is equipped with an electric stirrer; a spiral baffle channel is provided at the top of the vacuum crystallization tank; the bottom of the vacuum crystallization tank is connected to the crystal dehydration equipment via a pipeline; the top of the vacuum crystallization tank is connected to a vacuum system via a pipeline; the vacuum system includes a vacuum pump and a vacuum spray tank, a water tank, and a heat exchanger connected in sequence.
[0015] Preferably, the number of vacuum crystallizers is 2-4, and they are arranged in parallel in the process.
[0016] Preferably, the vacuum crystallizer is provided with a compressed gas supply port, through which nitrogen, carbon dioxide or compressed air is supplied to pressurize the oxalic acid crystal slurry to the crystal dehydration equipment.
[0017] The crystal dehydration equipment is connected to the drying equipment and the mother liquor tank respectively. The drying equipment is connected to the storage and packaging system. The mother liquor tank is equipped with an electric stirrer and a steam coil heater.
[0018] A method for co-producing oxalic acid in a coal-to-ethylene glycol system, implemented using the aforementioned coal-to-ethylene glycol system for co-producing oxalic acid, includes the following steps:
[0019] Step 1: Inject demineralized water and oxalic acid mother liquor into the first hydrolysis reactor to the designated level, start the stirrer, and introduce steam to heat the liquid in the reactor. Then, inject liquid dimethyl oxalate into the reactor and continuously replenish demineralized water, oxalic acid mother liquor, and steam. When the liquid level in the first hydrolysis reactor reaches the process requirement mark, introduce the material into the second hydrolysis reactor through the bottom of the reactor, start the stirrer, and continuously replenish demineralized water and steam. When the liquid level in the second hydrolysis reactor reaches the process requirement mark, introduce the material into subsequent hydrolysis reactors through the bottom of the reactor until it reaches the distillation column. The subsequent hydrolysis reactors are also continuously replenished with demineralized water and steam.
[0020] Meanwhile, methanol and water vapor escape from the top of the hydrolysis vessel, are condensed by the condenser to form a methanol-water solution, which is stored in the condensate tank and then pumped to the distillation column.
[0021] Preferably, in the first step, the temperature of the hydrolysis vessel is maintained at 50-100℃.
[0022] Step 2: Inside the distillation column, the liquid in the bottom of the column is heated by the built-in reboiler and turns into vapor that flows upward, providing the heat required for the entire distillation column; methanol is obtained at the top of the bottom of the column, part of which is returned to the distillation column through the reflux tank, and the other part is sent to the coal-to-ethylene glycol system for recycling; oxalic acid solution is obtained at the bottom of the bottom of the column and is sent to the vacuum crystallizer through pipeline;
[0023] Preferably, in the second step, the absolute pressure inside the vacuum crystallizer is maintained at no more than 15 kPa, so that the oxalic acid solution obtained from the distillation column can be transported to the vacuum crystallizer through pipeline using the negative pressure of the vacuum crystallizer.
[0024] Step 3: The oxalic acid solution from the distillation column bottom first enters the spiral baffle at the top of the vacuum crystallizer. Under vacuum conditions, the water in the oxalic acid solution evaporates into water vapor on the spiral baffle. Under stirring and high vacuum conditions, the water in the oxalic acid solution flowing down from the spiral baffle of the crystallizer continues to evaporate, achieving the effect of cooling and crystallization. When the temperature inside the vacuum crystallizer reaches the process requirements, nitrogen, carbon dioxide, or compressed air is added to the vacuum crystallizer through the compressed gas replenishment port to pressurize the oxalic acid crystal slurry to the crystal dehydration equipment.
[0025] Preferably, in the third step, based on multiple parallel vacuum crystallizers, each vacuum crystallizer adopts an intermittent operation mode, and independently performs feeding, crystallization, discharging and standby operations according to the time difference.
[0026] Preferably, in the third step, water vapor in the vacuum crystallizer is drawn to the vacuum spray tank by a vacuum pump, and then condensed by the water sprayed down from above the vacuum spray tank. The non-condensable gas is discharged to the outside by the vacuum pump. The sprayed water is collected in a water tank, and after being pressurized by a pump, part of the water is cooled by a heat exchanger and sent to the top of the vacuum spray tank for continued recycling, while the other part of the water is sent to the mother liquor tank. The water replenishment for the vacuum spray tank is demineralized water, which is replenished from the water pipeline between the heat exchanger and the vacuum spray tank.
[0027] Step 4: The oxalic acid crystal slurry from the vacuum crystallizer is separated from the mother liquor by a crystal dehydration device. The oxalic acid crystals are sent to a drying device, while the mother liquor is stored in a mother liquor tank. A steam coil heater in the mother liquor tank heats and keeps the mother liquor at a constant temperature. The generated steam condensate is discharged to the outside or to the mother liquor tank for use as system makeup water. At the same time, an electric stirrer in the mother liquor tank operates to prevent crystal formation and precipitation. The mother liquor in the mother liquor tank is pumped to the first hydrolysis reactor.
[0028] Step 5: After being dehydrated by the crystal dehydration equipment, the oxalic acid crystals continue to be dehydrated by the drying equipment to produce anhydrous oxalic acid or dihydrate oxalic acid, which is then sent to the warehousing and packaging system for packaging and storage.
[0029] The beneficial effects obtained by adopting the above technical solution are as follows:
[0030] (1) The present invention uses nitrogen, carbon dioxide or compressed air to pressurize and transport oxalic acid crystal slurry. When the slurry is discharged from the vacuum crystallizer, nitrogen, carbon dioxide or compressed air is introduced to establish pressure and directly pressurize the crystal slurry to the centrifuge. This avoids the deposition of crystals in pipe bends, valves and pump chambers, solves the blockage problem and reduces the maintenance workload of cleaning the pipes.
[0031] (2) The present invention adopts a parallel alternating operation mode of multiple crystallizers, and sets up multiple parallel vacuum crystallizers, which are in the feeding, crystallizing, discharging and standby states according to the time difference, thereby perfectly connecting the continuous distillation at the front end with the intermittent crystallization at the back end, ensuring that the device can continuously produce oxalic acid products in a long cycle and stably, and realizing continuous production.
[0032] (3) The present invention adopts a graded anti-corrosion material substitution strategy. The hydrolysis kettle is made of glass enamel, the distillation column body is lined with PTFE, and the key heat-receiving components are made of C-276 alloy. Non-metallic materials (glass enamel, PTFE) replace expensive special metals (such as zirconium), which greatly reduces equipment costs while ensuring corrosion resistance. Attached Figure Description
[0033] Figure 1 This is a schematic diagram of an apparatus and method for co-producing oxalic acid in a coal-to-ethylene glycol system according to the present invention.
[0034] Figure label:
[0035] 1. Hydrolysis vessel; 2. Condenser; 3. Condensate tank; 4. Distillation column; 5. Top condenser; 6. Reflux tank; 7. Vacuum crystallizer; 8. Vacuum spray tank; 9. Vacuum pump; 10. Water tank; 11. Heat exchanger; 12. Crystal dehydration equipment; 13. Mother liquor tank; 14. Drying equipment. Detailed Implementation
[0036] The technical solution of the present invention will now be described more clearly and completely with reference to the accompanying drawings.
[0037] like Figure 1 As shown, this invention discloses an apparatus for the co-production of oxalic acid in a coal-to-ethylene glycol system, comprising a hydrolysis reactor 1, a condenser 2, a condensate tank 3, a distillation column 4, a vacuum crystallizer 7, a vacuum system, a crystal dehydration device 12, a mother liquor tank 13, a drying device 14, and a storage and packaging system. It can utilize dimethyl oxalate, an intermediate product of coal-to-ethylene glycol, as a raw material to achieve continuous and industrialized production of oxalic acid. Specific implementation steps are illustrated below:
[0038] Demineralized water and mother liquor were injected into the first hydrolysis reactor 1 to 40% of the liquid level. The stirrer was started, and the steam valve was opened to heat the liquid in the reactor to 95°C. The dimethyl oxalate valve was then opened to inject dimethyl oxalate liquid into the hydrolysis reactor 1. Demineralized water, mother liquor, and dimethyl oxalate were continuously added to the hydrolysis reactor 1 according to the process feed ratio. When the liquid level reached 85%, the bottom pump of the first hydrolysis reactor 1 was turned on to introduce oxalic acid solution into the second reactor. Three hydrolysis reactors 1 were connected in series. The hydrolysis reactor 1 was made of glass-lined material, and steam was heated using a coil with perforated openings. The coil was made of C-276 material.
[0039] When the liquid level in the second hydrolysis vessel 1 reaches 40%, start the electric stirrer, add a certain amount of demineralized water, and turn on the bottom steam coil to maintain the temperature inside the vessel at 95°C; when the liquid level in the second hydrolysis vessel 1 reaches 85%, turn on the bottom pump of the second hydrolysis vessel 1 to introduce oxalic acid solution into the third vessel.
[0040] When the liquid level in the third hydrolysis vessel 1 reaches 40%, start the electric stirrer, add a certain amount of demineralized water, and open the bottom steam coil to maintain the temperature inside the vessel at 95°C; when the liquid level in the third hydrolysis vessel 1 reaches 85%, turn on the bottom pump of the third hydrolysis vessel 1 to introduce oxalic acid solution into the distillation column 4.
[0041] Each of the three hydrolysis reactors 1 is equipped with a condenser 2 at its top. The vapor from the top of the three hydrolysis reactors 1 enters the corresponding condenser 2 through the top outlet and is condensed into a methanol-water solution, which is collected in a condensate tank 3 and then pumped to a distillation column 4.
[0042] The heat source for distillation column 4 is an internal reboiler in the reboiler, with the tubes made of C-276 material. The column body is lined with PTFE, and the packing is made of 316 material. High-purity methanol is obtained at the top of the column; part of it is refluxed, and part is collected and sent to the ethylene glycol unit for reuse. Oxalic acid aqueous solution is obtained from the reboiler and pumped to vacuum crystallizer 7.
[0043] Four vacuum crystallizers 7 are connected in parallel. The four vacuum crystallizers 7 are made of glass-lined material and operate with a time difference: one is receiving material, one is cooling and crystallizing, one is discharging material, and one is on standby. First, vacuum pump 9 evacuates the vacuum crystallizers 7 to 5 kPa (absolute pressure). The oxalic acid aqueous solution from the bottom of distillation column 4 enters the upper spiral baffle channel of the vacuum crystallizer 7. Under vacuum conditions, the water in the oxalic acid solution evaporates into water vapor. After entering the lower part of the vacuum crystallizer 7, under stirring and vacuum conditions, some of the water continues to evaporate into water vapor. The water vapor enters the vacuum spray tank 8, is cooled by the spray water, and flows down into the water tank 10. The water in the water tank 10 is pressurized by a pump; part of it is sent to the mother liquor tank 13 for reuse, and part is sent to the vacuum spray tank 8 for use after passing through the heat exchanger 11. When discharging material from the vacuum crystallizer 7, nitrogen is first added to break the vacuum, and nitrogen is continuously added to the tank. Under the pressure of nitrogen, the oxalic acid slurry is forced to the crystal dehydration device 12.
[0044] Inside the crystal dehydration device 12, the slurry is separated into hydrated oxalic acid crystals and oxalic acid mother liquor. The oxalic acid crystals are sent to the subsequent drying device 14 for drying, while the mother liquor enters a mother liquor tank below the crystal dehydration device 12. A heating coil and a stirrer are installed in the mother liquor tank to maintain the mother liquor temperature at 60°C, preventing crystal precipitation and blockage of the equipment and pipelines. According to the process material ratio, the mother liquor is pumped to the first hydrolysis reactor 1.
[0045] Hydrous oxalic acid crystals are dried by heating in drying equipment 14 to obtain oxalic acid dihydrate. Oxalic acid dihydrate is then sent to a warehousing and packaging system for packaging and storage.
[0046] This invention uses dimethyl oxalate from coal-based ethylene glycol enterprises as raw material. These enterprises can utilize this process to produce oxalic acid products, thereby enriching their product chain and enhancing their risk resistance and profitability. The container equipment of this invention can be made of glass-lined, PTFE-lined, or 316L stainless steel, replacing expensive titanium alloys and nickel alloys, thus reducing overall investment. The steam heating section uses a coil form that directly connects to the oxalic acid solution, simplifying the equipment type and reducing investment. The distillation column 4 can recover methanol and ensure complete hydrolysis of dimethyl oxalate. The reboiler of the distillation column 4 is built-in, which greatly reduces the size and weight of the equipment, reducing reboiler investment while not affecting the overall heat supply of the distillation column 4. The crystallization system utilizes vacuum dehydration to achieve both cooling and dehydration, simplifying the equipment structure and reducing investment. Because the crystallization system uses a pressure-resistant cylindrical structure, compressed gas can be used to transport the slurry, reducing the risk of slurry system blockage and equipment damage. The hydrolysis kettle 1 operates in series, and the vacuum crystallizer 7 operates alternately in parallel, enabling continuous production. The process and equipment of this invention are simple overall, with low investment, continuous production capability, and are also more environmentally friendly and energy-efficient.
Claims
1. A device for the co-production of oxalic acid in a coal-to-ethylene glycol system, characterized in that, Includes hydrolysis kettle (1), condenser (2), condensate tank (3), distillation column (4), vacuum crystallizer (7), vacuum system, crystal dehydration equipment (12), mother liquor tank (13), drying equipment (14), and storage and packaging system; The hydrolysis reactor (1) is connected to the dimethyl oxalate production unit of the coal-to-ethylene glycol system; the hydrolysis reactor (1) is provided with inlets for dimethyl oxalate, demineralized water and oxalic acid mother liquor and is equipped with an electric stirrer; the bottom of the hydrolysis reactor (1) is provided with a steam inlet and a liquid phase outlet, and the top is provided with a gas phase outlet; the top gas phase outlet of the hydrolysis reactor (1) is connected to a condenser (2) through a pipeline; the condenser (2) is connected to the condensate tank (3); the condensate tank (3) is connected to the distillation column (4); The distillation column (4) is equipped with a built-in reboiler. The distillation column (4) has a top outlet and a bottom outlet. The top outlet of the distillation column (4) is connected to the reflux tank (6) through a top condenser (5). The bottom outlet of the distillation column (4) is connected to the vacuum crystallizer (7) through a pipeline. The vacuum crystallizer (7) is equipped with an electric stirrer; a spiral baffle channel is provided on the upper part of the vacuum crystallizer (7); the bottom of the vacuum crystallizer (7) is connected to the crystal dehydration equipment (12) through a pipeline; the top of the vacuum crystallizer (7) is connected to the vacuum system through a pipeline; the vacuum system includes a vacuum pump (9) and a vacuum spray tank (8), a water tank (10), and a heat exchanger (11) connected in sequence. The crystal dehydration device (12) is connected to the drying device (14) and the mother liquor tank (13) respectively. The drying device (14) is connected to the storage and packaging system. The mother liquor tank (13) is equipped with an electric stirrer and a steam coil heater.
2. The apparatus for co-producing oxalic acid in a coal-to-ethylene glycol system according to claim 1, characterized in that, The number of hydrolysis vessels (1) is 2-5, and they are connected in series in the process. The hydrolysis vessels (1) are connected by bottom pumps or by material overflow through liquid level difference. The top gas phase outlets of all hydrolysis vessels (1) share a condenser (2) through pipelines or each hydrolysis vessel (1) is independently equipped with a condenser (2). The oxalic acid mother liquor injection port of the first hydrolysis vessel (1) in the process is connected to the mother liquor tank (13), and the liquid phase outlet of the last hydrolysis vessel (1) in the process is connected to the distillation column (4).
3. The apparatus for co-producing oxalic acid in a coal-to-ethylene glycol system according to claim 2, characterized in that, The hydrolysis vessel (1) is made of glass-lined material or 316L material; the built-in reboiler is made of C-276 nickel-based alloy; the distillation column (4) is made of glass-lined material, lined with PTFE or 316L material.
4. The apparatus for co-producing oxalic acid in a coal-to-ethylene glycol system according to claim 3, characterized in that, The number of vacuum crystallizers (7) is 2-4, and they are arranged in parallel in the process.
5. The apparatus for co-producing oxalic acid in a coal-to-ethylene glycol system according to claim 4, characterized in that, The vacuum crystallizer (7) is equipped with a compressed gas supply port. Nitrogen, carbon dioxide or compressed air are supplied through the compressed gas supply port to pressurize the oxalic acid crystal slurry to the crystal dehydration equipment (12).
6. A method for co-producing oxalic acid in a coal-to-ethylene glycol system, implemented based on the apparatus for co-producing oxalic acid in a coal-to-ethylene glycol system as described in claim 5, characterized in that, Includes the following steps: Step 1: Inject demineralized water and oxalic acid mother liquor into the first hydrolysis vessel (1) to the specified level, start the stirrer, and introduce steam to heat the liquid in the vessel. Then inject liquid dimethyl oxalate into the vessel and continuously replenish demineralized water, oxalic acid mother liquor and steam. When the liquid level of the first hydrolysis vessel (1) reaches the process requirement scale, introduce the material into the second hydrolysis vessel (1) through the bottom of the vessel, start the stirrer, and continuously replenish demineralized water and steam. When the liquid level of the second hydrolysis vessel (1) reaches the process requirement scale, introduce the material into the subsequent hydrolysis vessel (1) through the bottom of the vessel until the distillation column (4). The subsequent hydrolysis vessel (1) also continuously replenishes demineralized water and steam. Meanwhile, methanol and water vapor escape from the top of the hydrolysis vessel (1), are condensed by the condenser (2) to form a methanol-water solution, which is stored in the condensate tank (3) and then pumped to the distillation column (4). Step 2: Inside the distillation column (4), the liquid in the bottom of the column is heated by the built-in reboiler and forms steam that flows upward, providing the heat required for distillation of the entire distillation column (4); methanol is obtained at the top of the bottom of the column, and part of it is returned to the distillation column (4) through the reflux tank (6), while the other part is sent to the coal-to-ethylene glycol system for recycling; oxalic acid solution is obtained at the bottom of the bottom of the column and is sent to the vacuum crystallizer (7) through the pipeline. Step 3: The oxalic acid solution transported from the bottom of the distillation column (4) first enters the spiral baffle at the top of the vacuum crystallizer (7). Under vacuum conditions, the water in the oxalic acid solution evaporates into water vapor on the spiral baffle. Under stirring and high vacuum conditions, the water in the oxalic acid solution flowing down from the spiral baffle of the crystallizer continues to evaporate, achieving the effect of cooling and crystallization. When the temperature inside the vacuum crystallizer (7) reaches the process index requirements, nitrogen, carbon dioxide or compressed air is added to the vacuum crystallizer (7) through the compressed gas replenishment port, and the oxalic acid crystal slurry is pressurized to the crystal dehydration equipment (12). Step 4: The oxalic acid crystal slurry transported from the vacuum crystallizer (7) is separated from the mother liquor by the crystal dehydration equipment (12). The oxalic acid crystals are sent to the drying equipment (14), while the mother liquor is stored in the mother liquor tank (13). The mother liquor is heated and kept warm by the steam coil heater in the mother liquor tank (13). The generated steam condensate is discharged to the outside or discharged to the mother liquor tank (13) for use as system makeup water. At the same time, the electric stirrer in the mother liquor tank (13) works to prevent the formation of crystals and precipitation. The mother liquor tank (13) is pumped to the first hydrolysis kettle (1). Step 5: After being dehydrated by the crystal dehydration equipment (12), the oxalic acid crystals continue to be dehydrated by the drying equipment (14) to generate anhydrous oxalic acid or dihydrate oxalic acid, which is then sent to the storage and packaging system for packaging and storage.
7. The method for co-producing oxalic acid in a coal-to-ethylene glycol system according to claim 6, characterized in that, In the first step, the temperature of the hydrolysis vessel (1) is maintained at 50-100℃.
8. The method for co-producing oxalic acid in a coal-to-ethylene glycol system according to claim 6, characterized in that, In the second step, the absolute pressure inside the vacuum crystallizer (7) is maintained at no more than 15 kPa, and the oxalic acid solution obtained from the distillation column (4) can be transported to the vacuum crystallizer (7) through pipeline using the negative pressure of the vacuum crystallizer (7).
9. The method for co-producing oxalic acid in a coal-to-ethylene glycol system according to claim 6, characterized in that, In the third step, based on multiple parallel vacuum crystallizers (7), each vacuum crystallizer (7) adopts an intermittent operation mode, and independently performs feeding, crystallization, discharging and standby operations according to the time difference.
10. The method for co-producing oxalic acid in a coal-to-ethylene glycol system according to claim 6, characterized in that, In the third step, the water vapor in the vacuum crystallizer (7) is drawn to the vacuum spray tank (8) by the vacuum pump (9), and then condensed by the water sprayed down from the top of the vacuum spray tank (8). The non-condensable gas is discharged to the outside through the vacuum pump (9). The sprayed water is collected in the water tank (10). After being pressurized by the pump, part of the water is cooled by the heat exchanger (11) and sent to the top of the vacuum spray tank (8) for continued recycling. The other part of the water is sent to the mother liquor tank (13). The water replenishment of the vacuum spray tank (8) is demineralized water, which is replenished from the water pipeline between the heat exchanger (11) and the vacuum spray tank (8).