A dmo production system and start-stop method

Abstract

The application discloses a DMO production system, which comprises two sets of DMO production subsystems, each of which comprises a DMO synthesis reaction unit, an MN regeneration unit and a purging unit, and the two sets of DMO production subsystems share one set of esterification tail gas recovery unit; two pipelines at the bottom outlet of the esterification tail gas recovery equipment 'non-condensable gas treatment tower' are respectively connected with the esterification tower inlets of the two sets of DMO production subsystems; the application also discloses a start-up and shutdown method for producing DMO, and the two sets of DMO production subsystems are staggered in the start-up and shutdown. The system and method of the application are more energy-saving and environment-friendly in the shutdown. In the shutdown, the two sets of DMO production subsystems are staggered in the shutdown, the methanol solution, in which nitrogen oxides purged and replaced are absorbed by the tower kettle of the non-condensable gas treatment tower, is connected to another set of running DMO production subsystem for utilization, so that the environmental protection accidents of yellow smoke caused by the nitrogen oxides discharge are avoided, the safety risks caused by long-time storage are avoided, the nitrogen oxides are recycled and utilized, and the energy-saving effect is achieved.

Description

Technical fields:

[0002] This invention relates to a DMO production system and process, belonging to the field of ethylene glycol production technology. Background technology:

[0004] Currently, there are two main routes for ethylene glycol production: the petroleum route and the coal route. The petroleum route, namely the ethylene oxide hydration method, is the most commonly used method for ethylene glycol production. It is technologically mature and widely applied, but its disadvantages include reliance on petroleum resources, high water and energy consumption, and high costs. The coal route includes direct production of ethylene glycol from syngas and indirect production from CO and H2 in syngas. The direct method has harsh conditions, high pressure requirements, low conversion rate, poor selectivity, and is difficult to implement, which restricts its industrial application. The principle of the indirect method for producing ethylene glycol is that raw coal is gasified in a gasification unit to obtain crude coal gas containing CO and H2. The crude coal gas enters a crude coal gas purification unit to separate CO and H2. The CO enters the dimethyl oxalate (DMO) production unit and reacts with methyl nitrite (MN) to produce DMO. The DMO reacts with H2 separated from the crude coal gas purification unit to produce ethylene glycol (EG) in the ethylene glycol unit. The indirect method has low cost, low energy consumption, low water consumption, and low emissions.

[0005] The production process of CO to DMO is as follows: CO reacts with methyl nitrite under normal pressure in the presence of a supported Pd / α-Al2O3 catalyst to produce dimethyl oxalate and NO. The main reaction equation is as follows:

[0006] 2CO + 2CH3ONO → (COOCH3)2 + 2NO

[0007] In this synthesis process, the main side reactions are: CO reacts with methyl nitrite (MN) to produce NO and dimethyl carbonate (CO(OCH3)2); methyl nitrite decomposes to produce NO, methyl formate (HCOOCH3), and methanol; and CO reacts with NO to produce N2 and CO2. The chemical equations for these side reactions are as follows:

[0008] CO + 2CH3ONO → 2NO + CO(OCH3)2

[0009] 4CH3ONO→4NO+ HCOOCH3+2CH3OH

[0010] 2CO + 2NO → N2 + 2CO2.

[0011] The above products, after being absorbed by methanol to obtain DMO, are further regenerated by MN and used as recycled process gas to react with the feedstock CO. The chemical equation for MN regeneration is as follows:

[0012] 4NO + O2 + 4CH3OH = 4CH3ONO + 2H2O.

[0013] As the system operates for longer periods, nitrogen oxides will continuously be lost. Nitric acid is added to the sodium hydroxide reaction tower to trigger a nitric acid reduction reaction, generating MN. Further reactions in the nitric acid reduction tower consume the residual nitric acid in the sodium hydroxide bottoms, producing MN. This method continuously replenishes the nitrogen oxides in the system. The nitric acid reduction reaction equation is as follows:

[0014] 3CH3OH + 2NO + HNO3 = 3CH3ONO + 2H2O

[0015] Currently, there are many studies on the DMO reaction. For example, Chinese patent applications with application numbers CN201010510033.2, CN201210542727.3, CN201510586904.1, and CN201710101381.6 all disclose methods for synthesizing DMO from CO. However, none of these methods disclose DMO production systems, meaning that no industrially usable DMO production system has been disclosed.

[0016] Inner Mongolia Rongxin Chemical Co., Ltd. is equipped with a production system with an annual output of 400,000 tons of ethylene glycol. It adopts a process route for the indirect synthesis of ethylene glycol from CO and H2 in syngas. After years of research and design, the ethylene glycol production system is now equipped with two parallel dimethyl oxalate (DMO) synthesis systems. The two DMO synthesis systems are operating in parallel and the operation is performing well.

[0017] However, the current two dimethyl oxalate synthesis systems have the following problems:

[0018] 1. Each system operates independently, and its shutdown and startup are unrelated to the other system, with no interconnection of materials. When shutting down, the process gas in the system is purged and replaced with nitrogen. Since the recovery situation of the other parallel system is not considered, the purged process gas goes to the non-condensable gas treatment tower to recover MN. After being treated by the non-condensable gas treatment tower, it goes to the incinerator or flare gas pipeline. On the one hand, the processing capacity is large, and on the other hand, the effective gas containing nitrogen oxides is lost in large quantities, and environmental accidents such as yellow smoke from the flare are prone to occur.

[0019] 2. When the system is started up, sodium nitrite is used as a raw material, and the reaction is due to excess nitric acid, which produces wastewater containing waste sodium nitrite and nitric acid, which is detrimental to the environment.

[0020] 3. Sodium nitrite has a high market price, making its use in automobiles costly;

[0021] 4. The preparation of sodium nitrite solution requires manual operation and is labor-intensive: Each time a single series is started, 8 tons of sodium nitrite (25kg / bag specification, a total of 320 bags) are used. It is necessary to manually load the sodium nitrite from the warehouse to the site, unload it manually, lift it to the top of the tank with a forklift, and then manually open the bags and pour it into the solution preparation tank to prepare the solution. The preparation process is labor-intensive, unintelligent and unfriendly to the operators.

[0022] 5. This method has a fast reaction time and a relatively violent reaction, resulting in relatively high safety risks. Summary of the Invention:

[0024] To solve the above-mentioned technical problems, the first objective of this invention is to provide a DMO production system, and the second objective is to provide a DMO start-up and shutdown method.

[0025] This invention is implemented by the following technical solution: a DMO production system, comprising two DMO production subsystems, each of which includes a DMO synthesis reaction unit, an MN regeneration unit, and a purging unit.

[0026] The DMO synthesis reaction unit includes a CO pipeline, a synthesis circulating gas heater, a synthesis reactor, an absorber precooler, and a DMO absorber. The synthesis reactor is a shell-and-tube heat exchanger reactor, and the tubes of the synthesis reactor are loaded with a Pd-based catalyst. The CO pipeline, the synthesis circulating gas heater, the tubes of the synthesis reactor, the absorber precooler, and the DMO absorber are connected in sequence. The upper inlet of the DMO absorber is connected to a methanol pipeline.

[0027] The MN regeneration unit includes a synthesis circulating gas compressor, a sodium sulfite reaction tower, a nitric acid reduction tower, an oxygen mixer, an esterification tower, and a synthesis circulating gas preheater. The outlet of the synthesis circulating gas compressor is connected to the lower inlet of the sodium sulfite reaction tower, the lower inlet of the nitric acid reduction tower, and the inlet of the oxygen mixer. The inlet of the oxygen mixer is also connected to the O2 pipeline and the NO pipeline. The upper inlet of the sodium sulfite reaction tower is connected to the nitric acid pipeline. The bottom outlet of the sodium sulfite reaction tower is connected to the upper inlet of the nitric acid reduction tower. The top outlets of the sodium sulfite reaction tower and the top outlets of the nitric acid reduction tower are both connected to the inlet of the esterification tower. The upper inlet of the esterification tower is connected to the methanol pipeline. The top outlet of the esterification tower of the two DMO production subsystems is also connected to the non-condensable gas treatment tower. The upper inlet of the non-condensable gas treatment tower is connected to the methanol pipeline. The bottom outlet of the non-condensable gas treatment tower is connected to the inlet of the esterification tower of the two DMO production subsystems.

[0028] The purging unit includes an emergency nitrogen storage tank and a purging tank.

[0029] The top outlet of the DMO absorber is connected to the inlet of the synthesis circulating gas compressor, and the outlet of the synthesis circulating gas preheater is connected to the inlet of the synthesis circulating gas heater; the emergency nitrogen storage tank is connected to the inlet of the synthesis reactor and the inlet of the oxygen mixer, respectively; the tube outlet of the synthesis reactor is also connected to the purge tank, and the purge tank is connected to the inlet of the non-condensable gas treatment tower.

[0030] Preferably, the DMO synthesis reaction unit further includes a steam drum, a demineralized water pipeline, and a 0.5MPa low-pressure steam pipeline. The demineralized water pipeline and the low-pressure steam pipeline are both connected to the inlet of the steam drum. The outlet of the steam drum is connected to the shell-side inlet of the synthesis reactor, and the shell-side outlet of the synthesis reactor is connected to the inlet of the steam drum.

[0031] Preferably, the steam outlet of the steam drum is connected to the cold medium inlet of the synthesis cycle gas preheater, the cold medium outlet of the synthesis cycle gas preheater is connected to the inlet of the low-pressure steam condensate separator, the steam outlet of the low-pressure steam condensate separator is connected to the low-pressure steam compressor and the low-pressure steam air cooler respectively, the outlet of the low-pressure steam air cooler is connected to the inlet of the low-pressure steam air cooler condensate buffer tank, and the liquid outlets of both the low-pressure steam condensate separator and the low-pressure steam air cooler condensate buffer tank are connected to the inlet of the steam drum.

[0032] Preferably, a heating pipe is provided at the bottom of the DMO absorption tower, and the inlet of the heating pipe is connected to a low-pressure steam pipeline.

[0033] Preferably, the outlet of the synthesis gas compressor is connected in sequence to the gas cooler and the gas separator, the outlet of the gas separator is connected to the upper inlet of the DMO absorption tower, and the outlet of the gas separator is connected to the inlet of the oxygen mixer.

[0034] Preferably, the bottom outlet of the sodium sulfite reaction tower is connected to the inlet of the sodium sulfite reaction tower heater and the inlet of the reduction tower feed preheater, respectively; the outlet of the sodium sulfite reaction tower heater is connected to the reflux port of the sodium sulfite reaction tower; and the outlet of the reduction tower feed preheater is connected to the upper inlet of the nitric acid reduction tower.

[0035] Preferably, the bottom outlet of the nitric acid reduction tower is also connected to the inlet of the nitric acid reduction tower reboiler, and the outlet of the nitric acid reduction tower reboiler is connected to the inlet of the nitric acid reduction tower.

[0036] Preferably, the bottom outlet of the esterification tower is connected to the inlet of the preheater of the sodium sulfite reaction tower, and the outlet of the preheater of the sodium sulfite reaction tower is connected to the upper inlet of the sodium sulfite reaction tower.

[0037] Preferably, the bottom outlet of the esterification tower is also connected to the esterification tower reboiler cooler, and the outlet of the esterification tower reboiler cooler is connected to the inlet of the esterification tower.

[0038] The second objective of this invention is achieved by the following technical solution: a start-up and shutdown method for DMO production, wherein the two DMO production subsystems are staggered in their start-up and shutdown operations.

[0039] When the plant is shut down, the nitrogen in the emergency nitrogen storage tank is used to purge and replace the process gas in the synthesis reactor. The purging gas enters the non-condensable gas treatment tower through the purging tank. The methanol absorption liquid in the bottom of the non-condensable gas treatment tower is connected to the esterification tower of another operating DMO production subsystem for recovery.

[0040] While driving,

[0041] The first step involves transferring gaseous nitrogen oxides from another operating DMO production subsystem from the purge tank to the esterification tower in the DMO production subsystem to be started. Liquid nitrogen oxides from the other operating DMO production subsystem are then transferred through the non-condensable gas treatment tower's bottom outlet pipeline to the esterification tower in the DMO production subsystem to be started to establish a liquid level, and then from the esterification tower to the sodium hydroxide reaction tower and nitric acid reduction tower to establish a liquid level. A compression system in the DMO production subsystem to be started establishes a circulation of nitrogen oxides and methanol within the DMO production subsystem, providing initial NO and MN for the system. The cooled methanol solution collected by the circulating gas separator connected to the compressor outlet flows to the DMO absorber to establish a liquid level.

[0042] Step 2: Introduce CO feedstock gas from the CO pipeline of the DMO production subsystem to be started;

[0043] The third step is to add nitric acid from the nitric acid pipeline of the DMO production subsystem to be started, gradually increasing the nitrogen oxide content of the system.

[0044] Step 4: Connect the reactor of the DMO production subsystem to be started into the system;

[0045] Step 5: Introduce oxygen from the oxygen pipeline of the DMO production subsystem to be started, gradually increase the load, and the start-up is complete.

[0046] Advantages of this invention:

[0047] 1. The system and method of this invention are more energy-efficient and environmentally friendly during shutdown. During shutdown, the two DMO production subsystems are shut down in a staggered manner. The nitrogen oxides absorbed and purged by methanol in the bottom of the non-condensable gas treatment tower are then transferred to another operating DMO production subsystem for reuse. This avoids environmental accidents caused by yellow smoke from nitrogen oxide emissions or safety risks from long-term storage, and achieves energy-saving effects through the recycling of nitrogen oxides.

[0048] 2. The system and method of this invention reduce start-up costs. For a single DMO production system start-up, if sodium nitrite is used, it requires 8 tons of sodium nitrite (4500 yuan / ton) and 4.87 tons of nitric acid (1900 yuan / ton), resulting in a start-up cost of 45253 yuan. However, if the system of this invention is used for material cross-contamination and nitric acid is used as a raw material, producing the same amount of nitrogen oxides, it requires 4.87 tons of nitric acid (1900 yuan / ton) and 4.95 tons of methanol (2300 yuan / ton), resulting in a start-up cost of 20638 yuan. This represents a cost reduction of 24615 yuan for a single system start-up.

[0049] 3. The system and method of this invention are more environmentally friendly during operation. Using this method, a single system can avoid generating 9.86 tons of waste salt—sodium nitrate—during a single operation, making it more environmentally friendly.

[0050] 4. The system and method of this invention make the operation safer and more user-friendly: When using this method to drive, there is no need for manual pouring of nitrite or manual preparation of nitrite solution, which are physically demanding operations. Nitric acid can be added remotely, which makes the process safer and more user-friendly.

[0051] 5. The system and method of this invention make the reaction process more stable and safer during startup: the reaction process during startup is no different from that during normal operation, which is more stable and safer than the method of producing nitrogen oxides by reacting sodium nitrite. Attached image description:

[0053] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0054] Figure 1 This is a schematic diagram of the DMO production system in Example 1.

[0055] In the diagram: 1. Synthesis reactor; 2. DMO absorber; 3. Sodium nitrate reaction tower; 4. Nitric acid reduction tower; 5. Esterification tower; 6. Non-condensable gas treatment tower; 7. CO pipeline; 8. O2 pipeline; 9. NO pipeline; 10. Nitric acid pipeline; 11. Methanol pipeline; 12. Synthesis circulating gas heater; 13. Absorber precooler; 14. Synthesis circulating gas compressor; 15. Oxygen mixer; 16. Synthesis circulating gas preheater; 17. Nitrogen storage tank; 18. Purge tank; 19. Steam drum; 20. Demineralized water pipeline; 21. Low-pressure steam pipeline; 22. Low-pressure steam condensate separator; 23. Low-pressure steam compressor; 24. Low-pressure steam air cooler; 25. Low-pressure steam air cooler condensate buffer tank; 26. Heating pipe; 27. Circulating gas cooler; 28. Circulating gas separator; 29. ​​Sodium nitrate reaction tower heater; 30. Reduction tower feed preheater; 31. Nitric acid reduction tower reboiler; 32. Esterification tower kettle cooler. Detailed implementation method:

[0057] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0058] Example 1:

[0059] This embodiment uses the modified DMO production system of Inner Mongolia Rongxin Chemical Co., Ltd. as an example, which includes two DMO production subsystems. Each DMO production subsystem includes a DMO synthesis reaction unit, an MN regeneration unit, and a purging unit.

[0060] The DMO synthesis reaction unit includes a CO pipeline 7, a synthesis circulating gas heater 12, a synthesis reactor 1, an absorber precooler 13, and a DMO absorber 2. The synthesis reactor 1 is a shell-and-tube heat exchanger reactor. The tubes of the synthesis reactor 1 are loaded with a Pd-based catalyst. The CO pipeline 7, the synthesis circulating gas heater 12, the tubes of the synthesis reactor 1, the absorber precooler 13, and the DMO absorber 2 are connected in sequence. The upper inlet of the DMO absorber 2 is connected to a methanol pipeline 11.

[0061] The MN regeneration unit includes a synthesis circulating gas compressor 14, a sodium sulfite reaction tower 3, a nitric acid reduction tower 4, an oxygen mixer 15, an esterification tower 5, and a synthesis circulating gas preheater 16. The outlet of the synthesis circulating gas compressor 14 is connected to the lower inlet of the sodium sulfite reaction tower 3, the lower inlet of the nitric acid reduction tower 4, and the inlet of the oxygen mixer 15. The inlet of the oxygen mixer 15 is also connected to O2 pipeline 8 and NO pipeline 9. The upper inlet of the sodium sulfite reaction tower 3 is connected to the nitric acid pipeline 10, and the bottom outlet of the sodium sulfite reaction tower 3 is connected to the nitric acid reduction tower 4. The top inlet is connected to the top outlet of the sodium sulfite reaction tower 3 and the top outlet of the nitric acid reduction tower 4, which are both connected to the inlet of the esterification tower 5. The top inlet of the esterification tower 5 is connected to the methanol pipeline 11, and the top outlet of the esterification tower 5 is connected to the inlet of the synthesis circulating gas preheater 16. The top outlet of the esterification tower 5 of the two DMO production subsystems is also connected to the non-condensable gas treatment tower 6. The top inlet of the non-condensable gas treatment tower 6 is connected to the methanol pipeline 11, and the bottom outlet of the non-condensable gas treatment tower 6 is connected to the inlet of the esterification tower 5 of the two DMO production subsystems.

[0062] The purging unit includes an emergency nitrogen storage tank 17 and a purging tank 18.

[0063] The top outlet of DMO absorber 2 is connected to the inlet of synthesis circulating gas compressor 14, and the outlet of synthesis circulating gas preheater 16 is connected to the inlet of synthesis circulating gas heater 12. Emergency nitrogen storage tank 17 is connected to the inlet of synthesis reactor 1 and the inlet of oxygen mixer 15, respectively. The tube side outlet of synthesis reactor 1 is also connected to purge tank 18, and purge tank 18 is connected to the inlet of non-condensable gas treatment tower 6.

[0064] In this embodiment, the DMO synthesis reaction unit also includes a steam drum 19, a demineralized water pipeline 20, and a 0.5MPa low-pressure steam pipeline 21. The demineralized water pipeline 20 and the low-pressure steam pipeline 21 are both connected to the inlet of the steam drum 19. The outlet of the steam drum 19 is connected to the shell-side inlet of the synthesis reactor 1, and the shell-side outlet of the synthesis reactor 1 is connected to the inlet of the steam drum 19.

[0065] In this embodiment, the steam outlet of the steam drum 19 is connected to the cold medium inlet of the synthesis cycle gas preheater 16, the cold medium outlet of the synthesis cycle gas preheater 16 is connected to the inlet of the low-pressure steam condensate separator 22, the steam outlet of the low-pressure steam condensate separator 22 is connected to the low-pressure steam compressor 23 and the low-pressure steam air cooler 24 respectively, the outlet of the low-pressure steam air cooler 24 is connected to the inlet of the low-pressure steam air cooler condensate buffer tank 25, and the liquid outlets of the low-pressure steam condensate separator 22 and the low-pressure steam air cooler condensate buffer tank 25 are both connected to the inlet of the steam drum 19.

[0066] In this embodiment, a heating pipe 26 is provided at the bottom of the DMO absorption tower 2, and the inlet of the heating pipe 26 is connected to the low-pressure steam pipeline 21.

[0067] In this embodiment, the outlet of the synthesis gas compressor 14 is connected in sequence to the gas cooler 27 and the gas separator 28. The outlet of the gas separator 28 is connected to the upper inlet of the DMO absorption tower 2, and the outlet of the gas separator 28 is connected to the inlet of the oxygen mixer 15.

[0068] In this embodiment, the bottom outlet of the sodium nitrite reaction tower 3 is connected to the inlet of the sodium nitrite reaction tower heater 29 and the inlet of the reduction tower feed preheater 30, respectively, and the outlet of the sodium nitrite reaction tower heater 29 is connected to the reflux port of the sodium nitrite reaction tower 3; the outlet of the reduction tower feed preheater 30 is connected to the upper inlet of the nitric acid reduction tower 4.

[0069] In this embodiment, the bottom outlet of the nitric acid reduction tower 4 is also connected to the inlet of the nitric acid reduction tower reboiler 31, and the outlet of the nitric acid reduction tower reboiler 31 is connected to the inlet of the nitric acid reduction tower 4.

[0070] In this embodiment, the bottom outlet of the esterification tower 5 is connected to the inlet of the preheater of the sodium sulfite reaction tower 3, and the outlet of the preheater of the sodium sulfite reaction tower 3 is connected to the upper inlet of the sodium sulfite reaction tower 3.

[0071] In this embodiment, the bottom outlet of the esterification tower 5 is also connected to the esterification tower reboiler 32, and the outlet of the esterification tower reboiler 32 is connected to the inlet of the esterification tower 5.

[0072] DMO production system production principle:

[0073] Carbon monoxide feed gas from the CO purification cryogenic unit and synthesis cycle gas undergo a carbonylation reaction under certain temperature, pressure and catalyst conditions to produce dimethyl oxalate (COOCH3)2 and dimethyl carbonate (C3H6O3) as a byproduct. After the reaction gas is cooled, it is absorbed by methanol to obtain crude dimethyl oxalate (COOCH3)2.

[0074] The specific process is as follows:

[0075] The carbon monoxide feed gas from the purified CO cryogenic unit is split into two streams and enters two DMO synthesis subsystems respectively. After being mixed with compressed and preheated recirculating gas via a pipeline mixer, it enters the tube side of the synthesis recirculating gas heater 12. Heated by low-pressure steam (0.5 MPaG), it enters four parallel, symmetrically distributed synthesis reactors 1, each containing Pd-based catalysts. A carbonylation reaction occurs to synthesize dimethyl oxalate (COOCH3)2, with byproducts including dimethyl carbonate (C3H6O3) and methyl formate (C2H4O2). Synthesis reactor 1 is a tubular fixed-bed reactor, with the catalyst housed within the tubes. The shell-side medium is boiler water from the steam drum 19. Two reactors share one steam drum 19, with each reactor equipped with one operating and one standby steam drum 19 feedwater pump. Boiler feedwater enters the steam drum 19 through a pipeline network and then enters the reactor (water inlet at the top of the reactor) via a steam drum circulation pump for forced circulation. The heat generated by the reaction is removed by heating the shell-side water. The bed temperature is maintained within a certain range by adjusting the pressure of the steam drum 19, and the liquid level in the steam drum 19 is maintained by adjusting the boiler feedwater flow rate. Part of the by-product steam from the steam drum 19 goes to the shell side of the synthesis cycle gas preheater 16 to recover heat energy, and the gas-liquid condensate flows by gravity to the low-pressure steam condensate separator 22 for separation. The other part of the by-product steam entering the air cooler is first flowed by gravity to the low-pressure steam air cooler condensate buffer tank 25 after being cooled by air, and then split into two streams that flow back to the steam drum 19 by gravity.

[0076] The gaseous products from synthesis reactor 1 enter the shell side of the precooler of DMO absorber 2. After being partially condensed by 60°C hot water, the gas-liquid mixture enters DMO absorber 2 from the middle and lower part. The circulating gas flows towards the top of DMO absorber 2. The small amount of dimethyl oxalate (COOCH3)2 entrained in the gas phase is effectively absorbed by methanol (there are two streams of methanol absorption: fresh methanol from the tank area and reflux methanol from the circulating gas separator) as the absorbent. The crude dimethyl oxalate (COOCH3)2 obtained from the bottom of the tower is sent to the subsequent refining process for refining.

[0077] The circulating gas from the top of DMO absorber 2, which does not contain dimethyl oxalate (COOCH3)2, is sent to the synthesis circulating gas compressor 14 for compression. A small portion is added to the synthesis circulating gas compressor 14 to replenish NO. The compressed gas then enters the sodium sulfite reaction tower 3 and the nitric acid reduction tower 4, respectively. The remaining portion is partially condensed by the circulating gas cooler 27 and then sent to the circulating gas separator 28. The separated liquid is pressurized and returned to DMO absorber 2 by the circulating gas separator pump. The separated circulating gas is mixed with oxygen from the air separation unit in the oxygen mixer 15 and then enters the esterification tower 5 for esterification regeneration. The regenerated circulating gas is sent from the top of the esterification tower 5 to the synthesis circulating gas preheater 16 for preheating in the tube side before being mixed with the raw material gas CO to complete one cycle.

[0078] The carbon monoxide feed gas contains inert gas. To avoid pressure increase caused by inert gas accumulation in the system, a small amount of circulating gas from the top of the esterification tower 5 is discharged to the non-condensable gas treatment tower 6.

[0079] Gases from the top of the sodium sulfite reaction tower 3 and the nitric acid reduction tower 4 enter the lower middle section of the esterification tower 5. Fresh methanol from the fresh methanol tank is sprayed at the top of the tower, while washed methanol from the non-condensable gas treatment tower 6 is sprayed in the upper middle section. The bottom liquid of the esterification tower 5 is sprayed in the middle section. The bottom of the esterification tower 5 is equipped with a bottom pump and a discharge pump. The former forces the bottom liquid to circulate through the bottom cooler 32, removing the heat of reaction via circulating water before returning it to the middle section of the esterification tower 5. The latter pressurizes the bottom liquid of the esterification tower 5, preheats it in the preheater of the sodium sulfite reaction tower 3, and sends it to the upper part of the sodium sulfite reaction tower 3, where it is mixed with nitric acid from the nitric acid tank before entering the tower. The circulating gas from the NO replenishment compressor is divided into two streams: one stream (approximately 2 / 3 of the total gas volume) enters the bottom of sodium nitrite reaction tower 3, where it mixes with the nitric acid and the liquid in the bottom of esterification tower 5 (containing methanol) and reacts with this circulating gas (containing NO) to form methyl nitrite; the other stream (approximately 1 / 3 of the total gas volume) enters the lower section of nitric acid reduction tower 4 and reacts with the liquid in the bottom of the sodium nitrite tower under the action of a catalyst to form methyl nitrite. The two streams of overhead gas from the top of sodium nitrite reaction tower 3 and nitric acid reduction tower 4 are mixed and enter the middle and lower section of esterification tower 5. The reaction in sodium nitrite reaction tower 3 needs to be completed under heating conditions. The liquid in the bottom of the tower is pressurized by the bottom pump of sodium nitrite reaction tower 3 and most of it is sent to the tube side of the sodium nitrite reaction tower heater 29. After being heated by saturated steam at 0.2 MPa, it is circulated to the middle and upper section of sodium nitrite reaction tower 3. A small portion is preheated by the preheater of the reduction feed tower and then enters nitric acid reduction tower 4.

[0080] To maintain the temperature of the nitric acid reduction tower 4, a nitric acid reduction tower reboiler 31 is installed in the tower bottom. It is a vertical thermosiphon with a heat source of 0.5 MPa saturated steam.

[0081] The original startup process used an NO generator to produce nitric oxide (NO) required for startup. The produced nitric oxide (NO) was mixed with the circulating gas from the synthesis circulating gas compressor 14 and then entered the oxygen mixer 15 to mix with oxygen. The mixed process gas entered the esterification tower 5 from the reactor. The improved scheme does not require this operation.

[0082] Example 2:

[0083] This embodiment illustrates the start-up and shutdown methods of the DMO production system after its renovation by Inner Mongolia Rongxin Chemical Co., Ltd.

[0084] (1) When the system is shut down for maintenance, the two DMO production subsystems are shut down and started up in a staggered manner. When shutting down, the nitrogen in the emergency nitrogen storage tank 17 is used to purge the process gas of the synthesis reactor 1. The medium in the synthesis reactor 1 is treated through the purging tank 18 and the non-condensable gas treatment tower 6. After methanol absorption, the methanol absorbent liquid in the tower bottom is sent to another operating DMO production subsystem for recovery. During the reactor purging process, the pressure of the purging tank 18 is ensured to be less than 0.42 MPa. The purging is carried out in small amounts and multiple times until the replacement is qualified. The methanol absorbed in the tower bottom of the non-condensable gas treatment tower 6 is used to recover the nitrogen oxides purged and replaced and then sent to another operating DMO production subsystem for use, so as to achieve the goals of environmental protection and energy saving.

[0085] (2) Start-up with nitric acid as raw material: The DMO production subsystem to be started is connected to another operating DMO production subsystem through the system vent pipeline to introduce NO, and nitric acid is used as raw material to supplement the nitrogen oxides in the system, further increasing the nitrogen oxide content in the system and achieving smooth start-up;

[0086] Taking the start-up of DMO synthesis A as an example: When DMO synthesis A is shut down and cut off for maintenance, DMO synthesis B is running normally. After DMO synthesis A completes maintenance and meets the start-up conditions according to the normal start-up procedure, the first step is to transfer the NO from the running DMO synthesis B unit to the esterification tower 5 of the DMO synthesis A system to be started, through the vent gas pipeline at the top of esterification tower 5 and the bottom liquid of non-condensable gas treatment tower 6. This establishes liquid levels in each tower of DMO synthesis A, thereby establishing a NO cycle in DMO synthesis A and providing initial NO and MN for DMO synthesis A, without needing to add NO to the system through NO pipeline 9; the second step is... The first step involves introducing CO feedstock gas. The second step involves adding nitric acid from sodium nitrite reaction tower 3, operating according to routine parameters to gradually increase the nitrogen oxide content in the system. The principle is: HNO3 + 2NO + 3CH3OH → 3CH3ONO + 2H2O. Nitrogen oxides—methyl nitrite (MN)—are obtained by adding nitric acid. The third step involves introducing the nitrogen oxides into the system from synthesis reactor 1. The principle is: 2CO + 2CH3ONO = (COOCH3)2 + 2NO. This reaction converts methyl nitrite into NO. Through the third and fourth steps, the nitrogen oxide concentration in the system is replenished and increased using nitric acid feedstock. The fifth step involves introducing oxygen, gradually increasing the load, and completing the start-up process.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A DMO production system, characterized in that, It includes two DMO production subsystems, each of which includes a DMO synthesis reaction unit, an MN regeneration unit, and a purging unit. The DMO synthesis reaction unit includes a CO pipeline, a synthesis circulating gas heater, a synthesis reactor, an absorber precooler, and a DMO absorber. The synthesis reactor is a shell-and-tube heat exchanger reactor, and the tubes of the synthesis reactor are loaded with a Pd-based catalyst. The CO pipeline, the synthesis circulating gas heater, the tubes of the synthesis reactor, the absorber precooler, and the DMO absorber are connected in sequence. The upper inlet of the DMO absorber is connected to a methanol pipeline. The MN regeneration unit includes a synthesis circulating gas compressor, a sodium sulfite reaction tower, a nitric acid reduction tower, an oxygen mixer, an esterification tower, and a synthesis circulating gas preheater. The outlet of the synthesis circulating gas compressor is connected to the lower inlet of the sodium sulfite reaction tower, the lower inlet of the nitric acid reduction tower, and the inlet of the oxygen mixer. The inlet of the oxygen mixer is also connected to the O2 pipeline and the NO pipeline. The upper inlet of the sodium sulfite reaction tower is connected to the nitric acid pipeline. The bottom outlet of the sodium sulfite reaction tower is connected to the upper inlet of the nitric acid reduction tower. The top outlets of the sodium sulfite reaction tower and the top outlets of the nitric acid reduction tower are both connected to the inlet of the esterification tower. The upper inlet of the esterification tower is connected to the methanol pipeline. The top outlet of the esterification tower of the two DMO production subsystems is also connected to the non-condensable gas treatment tower. The upper inlet of the non-condensable gas treatment tower is connected to the methanol pipeline. The bottom outlet of the non-condensable gas treatment tower is connected to the inlet of the esterification tower of the two DMO production subsystems. The purging unit includes an emergency nitrogen storage tank and a purging tank. The top outlet of the DMO absorber is connected to the inlet of the synthesis circulating gas compressor, and the outlet of the synthesis circulating gas preheater is connected to the inlet of the synthesis circulating gas heater. The emergency nitrogen storage tank is connected to the inlet of the synthesis reactor and the inlet of the oxygen mixer, respectively. The tube-side outlet of the synthesis reactor is also connected to the purge tank, and the purge tank is connected to the inlet of the non-condensable gas treatment tower. The bottom outlet of the sodium sulfite reaction tower is connected to the inlet of the sodium sulfite reaction tower heater and the inlet of the reduction tower feed preheater, respectively. The outlet of the sodium sulfite reaction tower heater is connected to the reflux port of the sodium sulfite reaction tower. The outlet of the reduction tower feed preheater is connected to the upper inlet of the nitric acid reduction tower.

2. The DMO production system according to claim 1, characterized in that, The DMO synthesis reaction unit also includes a steam drum, a demineralized water pipeline, and a low-pressure steam pipeline. The demineralized water pipeline and the low-pressure steam pipeline are both connected to the inlet of the steam drum. The outlet of the steam drum is connected to the shell-side inlet of the synthesis reactor, and the shell-side outlet of the synthesis reactor is connected to the inlet of the steam drum.

3. The DMO production system according to claim 2, characterized in that, The steam outlet of the steam drum is connected to the cold medium inlet of the synthesis cycle gas preheater. The cold medium outlet of the synthesis cycle gas preheater is connected to the inlet of the low-pressure steam condensate separator. The steam outlet of the low-pressure steam condensate separator is connected to the low-pressure steam compressor and the low-pressure steam air cooler, respectively. The outlet of the low-pressure steam air cooler is connected to the inlet of the low-pressure steam air cooler condensate buffer tank. The outlets of both the low-pressure steam condensate separator and the low-pressure steam air cooler condensate buffer tank are connected to the inlet of the steam drum.

4. The DMO production system according to claim 2, characterized in that, A heating pipe is installed at the bottom of the DMO absorption tower, and the inlet of the heating pipe is connected to a low-pressure steam pipeline.

5. The DMO production system according to claim 1, characterized in that, The outlet of the synthesis gas compressor is connected in sequence to the gas cooler and the gas separator. The outlet of the gas separator is connected to the upper inlet of the DMO absorption tower, and the outlet of the gas separator is connected to the inlet of the oxygen mixer.

6. The DMO production system according to claim 1, characterized in that, The bottom outlet of the nitric acid reduction tower is also connected to the inlet of the nitric acid reduction tower reboiler, and the outlet of the nitric acid reduction tower reboiler is connected to the inlet of the nitric acid reduction tower.

7. The DMO production system according to claim 1, characterized in that, The bottom outlet of the esterification tower is connected to the inlet of the preheater of the sodium sulfite reaction tower, and the outlet of the preheater of the sodium sulfite reaction tower is connected to the upper inlet of the sodium sulfite reaction tower.

8. The DMO production system according to claim 1, characterized in that, The bottom outlet of the esterification tower is also connected to the esterification tower reboiler cooler, and the outlet of the esterification tower reboiler cooler is connected to the inlet of the esterification tower.

9. The start-up and shutdown method for producing DMOs using a DMO production system according to any one of claims 1-8, characterized in that, The two DMO production subsystems are scheduled to stop and start in a staggered manner. When the plant is shut down, the nitrogen in the emergency nitrogen storage tank is used to purge and replace the process gas in the synthesis reactor. The purging gas enters the non-condensable gas treatment tower through the purging tank. The methanol absorption liquid in the bottom of the non-condensable gas treatment tower is connected to the esterification tower of another operating DMO production subsystem for recovery. While driving, The first step involves transferring gaseous nitrogen oxides from another operating DMO production subsystem from the purge tank to the esterification tower in the DMO production subsystem to be started. Liquid nitrogen oxides from the other operating DMO production subsystem are then transferred through the non-condensable gas treatment tower's bottom outlet pipeline to the esterification tower in the DMO production subsystem to be started to establish a liquid level, and then from the esterification tower to the sodium hydroxide reaction tower and nitric acid reduction tower to establish a liquid level. A compression system in the DMO production subsystem to be started establishes a circulation of nitrogen oxides and methanol within the DMO production subsystem, providing initial NO and MN for the system. The cooled methanol solution collected by the circulating gas separator connected to the compressor outlet flows to the DMO absorber to establish a liquid level. Step 2: Introduce CO feedstock gas from the CO pipeline of the DMO production subsystem to be started; The third step is to add nitric acid from the nitric acid pipeline of the DMO production subsystem to be started, gradually increasing the nitrogen oxide content of the system. Step 4: Connect the reactor of the DMO production subsystem to be started into the system; Step 5: Introduce oxygen from the oxygen pipeline of the DMO production subsystem to be started, gradually increase the load, and the start-up is complete.

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