Method for preparing acetic acid from methanol via low-pressure carbonylation with high catalyst stability

Abstract

The present invention relates to the technical field of preparing acetic acid from methanol through low-pressure carbonylation and discloses a method for preparing acetic acid from methanol via low-pressure carbonylation with high catalyst stability, comprising the following steps: S1, in a reaction stage: introducing methanol or a reactive derivative thereof and CO into a reactor, adding a catalyst comprising a Group VIII metal, an iodomethane co-catalyst, an inorganic iodide, and an ionic liquid having an [NR1R2R3R4]+[CH3COO]- structure as a catalyst stabilizer, and enabling these materials to undergo a carbonylation reaction in the reactor to obtain a crude acetic acid product, wherein R1, R2, R3, and R4 are an alkyl group containing 4-9 carbons, an aryl group, an acyl group, or a combination thereof; S2, performing gas-liquid separation; S3, performing evaporation washing; S4, performing light removal treatment; and S5, performing purification treatment, so that when the reactants are in a reaction system having a low water content of 0.5-1.0%, the catalyst comprising the Group VIII metal remains stable, thereby reducing the energy consumption for a rear-end dehydration tower and a finished product tower and reducing production costs.

Description

A method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability Technical Field

[0001] This invention relates to the field of methanol low-pressure carbonyl synthesis of acetic acid technology, and specifically to a method for methanol low-pressure carbonyl synthesis of acetic acid with high catalyst stability. Background Technology

[0002] Acetic acid is an important basic organic chemical raw material, used to produce vinyl acetate monomer (VCM), cellulose acetate, acetic anhydride, terephthalic acid, chloroacetic acid, polyvinyl alcohol, acetate esters, and metal acetates, etc. It has wide applications in dyes, pharmaceuticals, pesticides, adhesives, and organic solvents, and is one of the fastest-growing and most important organic chemical products in recent years. The methanol low-pressure carbonylation synthesis method for acetic acid production can use coal as a raw material, offering high yield and low cost. It is an advanced technology for acetic acid production and is currently the most commonly used method.

[0003] In recent years, our company has continued to research existing technologies and optimize and upgrade their processes, striving for technological breakthroughs in product yield, product quality, reaction efficiency, production cost, and equipment optimization. On January 31, 2023, our company published a patent application with application number "202211316195.1" entitled "A Method for Low-Pressure Carbonylation Synthesis of Acetic Acid from Methanol with High Catalyst Stability." This method optimizes the structure of a single-stage reactor to ensure minimal internal temperature differences, resulting in more stable products. It also adds a two-stage reactor structure: the first stage is a fluid stirrer with an external circulation heat exchanger, serving as the main reactor; the second stage is a plug flow reactor to reduce backmixing, maintain a more ideal reaction rate, and improve the yield of acetic acid. To improve the alcohol conversion rate, an inter-stage heat exchanger was installed between the two reactors to maintain both reactors within the optimal reaction temperature range, thereby increasing the reaction rate. A washing device was also proposed to clean the gas after flash evaporation, reducing catalyst loss due to mist entrainment. An external heat exchanger was installed on the flash evaporator to reduce catalyst scaling loss through forced fluid flow. Furthermore, an integrated flash evaporation and washing tower was proposed, combining flash evaporation and gas washing functions in a small footprint, achieving low-cost, continuous, and stable acetic acid production. However, this method of low-pressure carbonylation synthesis of acetic acid from methanol still suffers from poor catalyst stability in low-water-content reaction systems, resulting in consistently high energy consumption in the distillation system. Summary of the Invention

[0004] The purpose of this invention is to provide a novel method for the low-pressure carbonylation synthesis of acetic acid from methanol with high catalytic stability, ensuring catalyst stability even under conditions of 0.5% to 14% water content. Specifically, the catalyst remains stable in a reaction system with a low water content of 0.5% to 1.0%.

[0005] This invention is achieved through the following technical solution:

[0006] A method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability includes the following steps:

[0007] S1, Reaction Section: Methanol or its reactive derivatives and CO are introduced into a reactor. A group VIII metal catalyst, iodomethane co-catalyst, inorganic iodide, and an ionic liquid with the structure of formula (I) are added as catalyst stabilizers to allow the materials to undergo a carbonyl synthesis reaction in the reactor, yielding crude acetic acid.

[0008] ,

[0009] Among them, R1, R2, R3, and R4 are alkyl, aryl, acyl, or combinations thereof containing 4 to 9 carbons;

[0010] S2, Gas-liquid separation: The crude acetic acid product generated in step S1 is transported to a gas-liquid separator for gas-liquid separation. The gas phase obtained after gas-liquid separation is sent to a high-pressure absorption tower and washed with liquid raw materials. The liquid phase obtained after washing is sent back to the reactor. The liquid phase obtained after gas-liquid separation is transported to the next stage for processing.

[0011] S3. Evaporation and washing: The liquid phase obtained after separation in step S2 is passed into a flash evaporator for flash evaporation and washing. The gas phase obtained after flash evaporation and washing is washed with dilute acetic acid from the light phase of the downstream light phase removal section. The carbonylation catalyst, co-catalyst, and ionic liquid catalyst stabilizer material obtained after the treatment are sent back to the reactor. The gas phase stream is transported to the next section for processing.

[0012] S4, Lightweight treatment

[0013] The gas phase from step S3 is sent to a light component removal tower for separation, separating a top stream rich in methyl iodine, a side stream rich in acetic acid, and a bottom stream rich in heavy components. The liquid phase obtained after cooling the top stream in a heat exchanger enters a separator to obtain a light phase rich in water and a heavy phase rich in iodomethane. The heavy phase is returned to the reactor, and the light phase is sent to step S3 as washing liquid. The gas phase obtained after cooling is sent to a low-pressure absorption tower and washed with liquid feedstock. The liquid phase obtained after washing is returned to the reactor. The bottom stream rich in heavy components separated from the light component removal tower is returned to the reactor. The side stream rich in acetic acid separated from the light component removal tower is sent to the next stage for processing.

[0014] S5. Purification: The acetic acid-rich side stream from step S4 is sent to the product tower for further separation and purification to obtain the finished acetic acid and the propionic acid-rich byproduct stream.

[0015] Furthermore, a dehydration step is added between step S4 and step S5. The acetic acid-rich side stream obtained after step S4 is sent to a dehydration tower for further processing to obtain an acetic acid-rich stream and an aqueous dilute acetic acid stream. A portion of the aqueous dilute acetic acid stream and / or a portion of the light phase obtained in step S4 is used to wash the gaseous stream obtained after flash evaporation in step S3.

[0016] Furthermore, in step S1, the Group VIII metal catalyst is one or a combination of rhodium, iridium, and ruthenium; the inorganic iodide is one of lithium iodide, potassium iodide, and nickel iodide.

[0017] The R1, R2, R3, and R4 groups of the ionic liquid with formula (I) are all the same groups, some are the same groups, or are completely different groups; the ionic liquid is a single compound or a mixture of multiple compounds having the structure of formula (I).

[0018] Furthermore, the molar ratio of the ionic liquid to the group VIII metal catalyst is 5 to 60:1.

[0019] Furthermore, in step S1, the reactor includes a first-stage reactor and a second-stage reactor. The first-stage reactor is equipped with a fluid stirring mechanism. After the reactants are added to the first-stage reactor, a carbonyl synthesis reaction occurs under the catalytic conditions of the catalyst system. The product outlet at the top of the first-stage reactor is connected to the second-stage reactor, which allows the fluid to flow upwards without backmixing and transitions from a mixed flow to a plug flow. The mixture obtained after passing through the first-stage reactor is further transported to the second-stage reactor for further reaction, thus obtaining crude acetic acid.

[0020] The catalyst-containing material obtained after treatment in step S3, the heavy phase obtained in step S4, and the bottom stream of the light phase removal tower are all returned to a section of the reactor.

[0021] Furthermore, an inter-stage heat exchanger is provided between the first-stage reactor and the second-stage reactor, and the heat exchange medium in the inter-stage heat exchanger comes from boiler water.

[0022] Furthermore, in step S4, the gaseous stream obtained after the light phase removal tower is first cooled by a heat exchanger, and then the cooled liquid phase is sent to a separator for separation. The resulting heavy phase stream is sent back to a first-stage reactor for recycling, and the light phase stream rich in water and acetaldehyde is sent to the formaldehyde removal section.

[0023] Furthermore, the treatment method of the formaldehyde removal section is as follows: the light phase stream obtained after the separator is sent to the aldehyde concentration tower for treatment, and the resulting top stream rich in acetaldehyde and methyl iodine and bottom stream rich in water are sent to the extraction section, and the bottom stream rich in water is sent to the separator or reaction section.

[0024] The overhead stream rich in acetaldehyde and methyl iodine is sent to the extraction tower after being cooled by heat exchange. It is then extracted with demineralized water in the extraction tower. The resulting aldehyde-containing wastewater is sent to the wastewater treatment system for treatment, and the raffinate is returned to the separator or reaction section.

[0025] Furthermore, the gas phase obtained after treatment by the high-pressure absorption tower in step S2 and / or the gas phase obtained after treatment by the low-pressure absorption tower in step S4 are sent to the pressure swing adsorption tower for further treatment. The purified CO gas is then sent to a first-stage reactor as a raw material for the carbonyl synthesis reaction.

[0026] Furthermore, in step S5, the byproduct stream rich in propionic acid is sent to a propionic acid concentration tower for treatment, and then sent to a propionic acid product tower for treatment to obtain a propionic acid product liquid.

[0027] Furthermore, in step S5, the obtained acetic acid is processed by a deiodination system to obtain low-iodine acetic acid with an iodine content of less than 10 ppb. The deiodination system includes two-stage deiodination tanks.

[0028] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0029] I. This invention proposes a novel method for the low-pressure carbonyl synthesis of acetic acid from methanol. In this method, a quaternary ammonium ionic liquid catalyst stabilizer is introduced in the reaction stage S1. The quaternary ammonium cation can form an ionic compound with the active diiododicarbonyl rhodium. The added ionic liquid, possessing a specific structural formula (Ⅰ), can provide cations for the active component, such as rhodium, a Group VIII metal catalyst, and also serve as a solvent for the catalyst. According to the principle of "like dissolves like," this ionic liquid exhibits good solubility in the catalyst system, which is beneficial for maintaining catalyst stability. In this novel ionic liquid stabilizer, the acetate anion reacts with the cations in the acetic acid production system to form acetic acid or acetate salts, without introducing other impurity ions, thus simplifying the system and making post-processing more convenient.

[0030] Second, in this invention, the new method for low-pressure carbonyl synthesis of acetic acid from methanol exhibits good stability in the process of low-pressure carbonyl synthesis of acetic acid from methanol with a water content of 0.5-14%. In particular, when flash evaporation and / or evaporation processes are carried out at ultra-low water content of less than 1%, the catalyst can still remain stable. This results in the entire process of low-pressure carbonyl synthesis of acetic acid from methanol having lower energy consumption, less investment, and a wider range of process adjustment, providing favorable conditions for improving production efficiency and product quality. Attached Figure Description

[0031] Figure 1 is a structural diagram of the methanol low-pressure carbonyl synthesis acetic acid production system in Example 1.

[0032] Figure 2 is a structural diagram of another embodiment of a methanol low-pressure carbonyl synthesis acetic acid production system.

[0033] Figure 3 is a structural diagram of another implementation of a methanol low-pressure carbonyl synthesis acetic acid production system.

[0034] Figure 4 is a schematic diagram of the experimental apparatus for the low-pressure carbonyl synthesis of acetic acid from methanol.

[0035] The components are as follows: 1. First-stage reactor; 2. Methanol feed pipeline; 3. CO feed pipeline; 4. Second-stage reactor; 5. Inter-stage heat exchanger; 6. Gas-liquid separator; 7. Flash evaporator; 8. Evaporation heat exchanger; 9. Light weight removal tower; 10. Dehydration tower; 11. Product tower; 12. High-pressure absorption tower; 13. Low-pressure absorption tower; 14. Separator; 15. Aldehyde concentration tower; 16. Extraction tower; 17. Deiodization tank one; 18. Deiodization tank two; 19. Product acetic acid pipeline; 20. Waste acid collection pipeline; 21. Integrated flash evaporation and washing tower; 22. Washing tower; 23. Pressure swing adsorption tower; 24. Propionic acid concentration tower; 25. Propionic acid product tower; 26. Reactor; 27. Pipeline I; 28. Mother liquor pipeline I; 29. ​​Mother liquor pipeline II; 30. Product discharge pipeline; 31. Recovery pipeline. Embodiments of the present invention

[0036] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0037] Example 1

[0038] To facilitate public understanding of this solution, this embodiment uses a methanol low-pressure carbonyl synthesis acetic acid system currently in industrial operation as an example to further illustrate this solution.

[0039] A method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability, referring to Figure 1, includes the following steps:

[0040] Step 1, Reaction Section:

[0041] Methanol or its reactive derivatives and CO were fed into the reactor through the methanol feed line and CO feed line, respectively. Rhodium (800 ppm), water (3.2%), and [N(C5H)2]2% were added. 11 )2(C8H 17 )2] + [CH3COO] － A homogeneous mixture of ionic liquid, appropriate amount of iodomethane and lithium iodide, wherein the total amount of catalyst (including main catalyst, co-catalyst and ionic liquid) accounts for 35% of the total amount of materials, wherein the molar ratio of ionic liquid to rhodium is 23:1, and crude acetic acid is obtained.

[0042] Step 2, gas-liquid separation:

[0043] The crude acetic acid product generated in step S1 is transported to a gas-liquid separator for gas-liquid separation. The gas phase obtained after gas-liquid separation is sent to a high-pressure absorption tower and washed with liquid raw materials. The liquid phase obtained after washing is sent back to the reactor. The liquid phase obtained after gas-liquid separation is transported to the next stage for processing.

[0044] Step 3: Evaporation washing:

[0045] The liquid phase obtained after separation in step two is fed into an integrated flash evaporation and washing tower for flash evaporation and washing treatment. The gas phase obtained after flash evaporation and washing treatment is washed with light phase dilute acetic acid from the downstream light phase removal section. The material containing carbonylation catalyst, co-catalyst, and ionic liquid catalyst stabilizer obtained after treatment is sent back to the reactor. The gas phase stream is transported to the next section for further processing.

[0046] Step 4: Lightweight treatment:

[0047] The gas phase from step three is sent to a light component removal tower for separation, separating a top stream rich in methyl iodine, a side stream rich in acetic acid, and a bottom stream rich in heavy components. The liquid phase obtained after cooling the top stream in a heat exchanger enters a separator to obtain a light phase rich in water and a heavy phase rich in iodomethane. The heavy phase is returned to the reactor, and the light phase is sent to step three as washing liquid. The gas phase obtained after cooling is sent to a low-pressure absorption tower and washed with liquid feedstock. The liquid phase obtained after washing is returned to the reactor. The bottom stream rich in heavy components separated from the light component removal tower is returned to the reactor. The side stream rich in acetic acid separated from the light component removal tower is sent to the next stage for processing.

[0048] In this embodiment, the gaseous stream obtained after the light phase removal tower is first cooled by a heat exchanger, and then the cooled liquid phase is sent to a separator for stratification. The resulting heavy phase stream is sent back to a first-stage reactor for recycling, and the light phase stream rich in water and acetaldehyde is sent to the formaldehyde removal section.

[0049] Furthermore, the treatment method of the formaldehyde removal section is as follows: the light phase stream obtained after the separator is sent to the aldehyde concentration tower for treatment, and the resulting top stream rich in acetaldehyde and methyl iodine and bottom stream rich in water are sent to the extraction section, and the bottom stream rich in water is sent to the separator or reaction section.

[0050] The overhead stream rich in acetaldehyde and methyl iodine is sent to the extraction tower after being cooled by heat exchange. It is then extracted with demineralized water in the extraction tower. The resulting aldehyde-containing wastewater is sent to the wastewater treatment system for treatment, and the raffinate is returned to the separator or reaction section.

[0051] Step 5: Dehydration treatment:

[0052] The acetic acid-rich side stream obtained after step four is sent to a dehydration tower for further processing to obtain an acetic acid-rich stream and an aqueous dilute acetic acid stream. A portion of the aqueous dilute acetic acid stream and / or a portion of the light phase obtained in step four is used to wash the gaseous stream obtained after flash evaporation in step three.

[0053] Step 6: Purification process:

[0054] The acetic acid-rich side stream from step five is sent to the product tower for further separation and purification to obtain the finished acetic acid and the propionic acid-rich byproduct stream.

[0055] Furthermore, the resulting propionic acid-rich byproduct stream is sent to a propionic acid concentration tower for further processing, and then to a propionic acid product tower for further processing to obtain a propionic acid product liquid.

[0056] In this embodiment, the gas phase obtained after treatment by the high-pressure absorption tower in step two and / or the gas phase obtained after treatment by the low-pressure absorption tower in step four are sent to the pressure swing adsorption tower for further treatment. The purified CO gas is then sent to a first-stage reactor as a raw material for the carbonyl synthesis reaction.

[0057] In this embodiment, the final acetic acid is processed by a deiodination system to obtain low-iodine acetic acid with an iodine content of less than 10 ppb. The deiodination system includes two-stage deiodination tanks: a deiodination tank 1 and a deiodination tank 2.

[0058] In this embodiment, the reactor temperature was 192°C and the pressure was controlled at 2.90 MPa. After one month of operation, the rhodium content was sampled and detected at the reactor to be 790 ppm, and the loss rate of the rhodium catalyst was 1.25%.

[0059] Multiple batches of sampling revealed that under these process conditions, the loss rate of the rhodium catalyst remained stable at around 1.2%, with very little variation. This indicates that adding this catalyst system to the methanol-to-acetic acid low-pressure carbonylation process maintains catalyst stability under low water content conditions of 3.2%. Compared to the prior process, this reduces the energy consumption of both the dehydration tower and the product tower by approximately 10%.

[0060] Example 2

[0061] The only difference between this embodiment and Embodiment 1 is that:

[0062] In step one, the reactor is a preferred embodiment, specifically including a primary reactor and a secondary reactor. Referring to Figure 2, the primary reactor is equipped with a fluid stirring mechanism. After the reactants are added to the primary reactor, a carbonyl synthesis reaction occurs under the catalytic conditions of the catalyst system. The product outlet at the top of the primary reactor is connected to the secondary reactor, which allows the fluid to flow upwards without backmixing and transitions from a mixed flow to a plug flow. The mixture obtained after passing through the primary reactor is further transported to the secondary reactor for further reaction, thus obtaining crude acetic acid.

[0063] The catalyst-containing material obtained after treatment in step three, the heavy phase obtained in step four, and the bottom stream of the light phase removal tower are all sent back to a section of the reactor.

[0064] In addition, an inter-stage heat exchanger is provided between the first-stage reactor and the second-stage reactor, and the heat exchange medium in the inter-stage heat exchanger comes from boiler water.

[0065] In this embodiment, the reactor temperature was 192°C and the pressure was controlled at 2.90 MPa. After one month of operation, the rhodium content was sampled and detected at the reactor to be 790 ppm, and the loss rate of the rhodium catalyst was 1.25%.

[0066] Multiple batches of sampling revealed that under these process conditions, the loss rate of the rhodium catalyst remained stable at around 1.2%, with very little variation. This indicates that adding this catalyst system to the methanol-to-acetic acid low-pressure carbonylation process maintains catalyst stability under low water content conditions of 3.2%, reducing energy consumption in both the dehydration tower and the product tower by approximately 10%.

[0067] Compared with Example 1, this embodiment uses a superior reaction equipment structure to replace the original ordinary reactor. Investigation revealed that the reactor used in this invention contains [N(C5H]... 11 )2(C8H 17 )2] + [CH3COO] － The ionic liquid catalyst system, under low water content (3.2%), maintained the stability of the rhodium catalyst (Group VIII metal catalyst) in various methanol-to-acetic acid synthesis systems at low pressure carbonylation, with a loss rate consistent with that of the rhodium catalyst in Example 1. Furthermore, introducing a new methanol-to-acetic acid synthesis system at low pressure carbonylation allows for more ideal reaction conditions, thereby increasing the reaction rate.

[0068] Example 3

[0069] The only difference between this embodiment and embodiment 2 is that:

[0070] In step one, the water content is 0.5%. Acetic acid is used to make up for the reduced water content. The process conditions are controlled according to the following requirements. After one month of operation, the rhodium content of the catalyst is tested.

[0071] In this embodiment, during the methanol low-pressure carbonyl synthesis of acetic acid process, the flash evaporation temperature was controlled at 138°C and the pressure was controlled at 0.10 MPa when separating the product liquid and the catalyst. The rhodium content before feeding was 800 ppm. After one month of operation, the rhodium content in the reaction system was measured to be 782 ppm, and the loss rate of the rhodium catalyst was 2.25%.

[0072] Multiple batches of sampling revealed that under these process conditions, the loss rate of the rhodium catalyst remained stable at around 2.25%, with minimal variation. This indicates that adding this catalyst system to the methanol-to-acetic acid low-pressure carbonylation process maintains catalyst stability under low water content (0.5%), reducing energy consumption in both the dehydration tower and the product tower by approximately 17%.

[0073] Example 4

[0074] The only difference between this embodiment and Example 2 is that the water content is 8%, the amount of acetic acid is reduced accordingly, the other components remain unchanged, the process conditions are controlled according to the following requirements, and the rhodium content of the catalyst is tested after one month of operation.

[0075] In this embodiment, during the methanol low-pressure carbonyl synthesis of acetic acid process, the flash evaporation temperature was controlled at 137°C and the pressure was controlled at 0.12 MPa when separating the product liquid and the catalyst. The rhodium content before feeding was 800 ppm. After one month of operation, the rhodium content in the reaction system was measured to be 790 ppm, and the loss rate of the rhodium catalyst was 1.25%.

[0076] In industrial production, higher water content in the reactor makes it easier to ensure the stability of Group VIII metal catalysts. However, as the water content in the reactor increases, the energy consumption of the downstream dehydration tower and product tower gradually increases. Furthermore, in industrial production, the water content is generally controlled below 9%. Therefore, this embodiment uses a water content of 8% as an example to examine the energy consumption of the dehydration tower and product tower under this process condition.

[0077] In this embodiment, the unit energy consumption of the dehydration tower and the product tower is 1.0t. The energy savings of the dehydration tower and the product tower in other embodiments are based on this embodiment.

[0078] Example 5

[0079] The difference compared to Example 2 is as follows:

[0080] Step five, i.e., the dehydration tower, is eliminated. Since the water content in the reaction system is reduced, the dehydration tower can be removed without affecting the normal operation of the system. Eliminating the dehydration tower also saves on equipment investment costs.

[0081] Example 6

[0082] The difference compared to Example 2 is as follows:

[0083] In step three, a traditional flash evaporator + evaporation heat exchanger + washing tower is used instead of an integrated flash evaporation washing tower, as shown in Figure 3. The liquid phase obtained after separation in step two is passed into the flash evaporator and, in conjunction with the evaporation heat exchanger, performs flash evaporation and evaporation treatment on the material. The gas phase obtained after flash evaporation and evaporation is then washed using dilute acetic acid from the downstream light phase removal section via the washing tower.

[0084] In this embodiment, the reactor temperature was 192°C and the pressure was controlled at 2.90 MPa. After one month of operation, the rhodium content was sampled and detected at the reactor to be 790 ppm, and the loss rate of the rhodium catalyst was 1.25%. The loss rate of the metal catalyst was the same as in Example 2, indicating that this process is also applicable to the traditional flash evaporator + evaporation heat exchanger + washing tower production system, and can also ensure the stability of the catalyst.

[0085] Example 7

[0086] To further investigate the method for low-pressure carbonyl synthesis of acetic acid from methanol according to the present invention, this embodiment uses a laboratory simulation apparatus to investigate the effect of process conditions on group VIII metal catalysts.

[0087] Referring to Figure 4, in this embodiment, acetic acid is synthesized using the production system shown in Figure 4 in the methanol low-pressure carbonyl synthesis process. The production system includes a reactor 26, a flash evaporator 7, and an evaporation heat exchanger 8. The reactor 26 is connected to a methanol feed line 2 and a CO feed line 3. The reactor 26 is connected to the feed inlet of the flash evaporator 7 through line I 27. The flash evaporator 7 is connected to the reactor 26 through a mother liquor line I 28 and to the evaporation heat exchanger 8 through a mother liquor line II 29. The top of the flash evaporator 7 is connected to a product discharge line 30. The crude product evaporated in the flash evaporator 7 is sent to the distillation process for further processing through the product discharge line 30. The reactor 26 is also connected to a recovery line 32.

[0088] The reactant raw material, methanol, is fed into reactor 26 via methanol feed line 2. The reactant raw material, carbon monoxide, is fed into reactor 26 via CO feed line 3. Reactants containing 800 ppm rhodium, 3.2% water, and 8% [N(C5H)] are added to reactor 26. 11 )2(C8H 17 )2] + [CH3COO] － A homogeneous mixture of ionic liquid, appropriate amount of iodomethane, and lithium iodide is used. The total amount of catalyst (including main catalyst, co-catalyst, and ionic liquid) accounts for 35% of the total material. After the reaction in reactor 26 is completed, the material is transported to flash evaporator 7 through pipeline I27. Flash evaporator 7, in conjunction with evaporation heat exchanger 8, performs flash evaporation of the material to separate the product from the catalyst. The material containing catalyst and ionic liquid is returned to reactor 26 through mother liquor pipeline I28 to achieve catalyst recycling. The gas phase (product gas) obtained after flash evaporator 7 is sent to the distillation system for further processing through product discharge pipeline 30. The light components (including water, iodomethane, methyl acetate, etc.) obtained after distillation are returned to reactor 26 through recovery pipeline 32. Evaporation heat exchanger 8 assists in accelerating the separation of materials by flash evaporator 7. After one week of stable operation, the rhodium content in the reaction system is measured.

[0089] In this embodiment, during the methanol low-pressure carbonyl synthesis of acetic acid process, the flash evaporation temperature was controlled at 138°C and the pressure was controlled at 0.10 MPa when separating the product liquid and the catalyst. The rhodium content before feeding was 800 ppm. After running for one week, the rhodium content in the reaction system was measured to be 784 ppm, and the loss rate of the rhodium catalyst was 2.00%.

[0090] In actual production, considering factors such as reaction rate, product quality, product yield, and production cost, the temperature of flash evaporation should be controlled within the range of 135~143℃ and the pressure within the range of 0.10~0.30MPa.

[0091] Multiple batches of sampling revealed that under these process conditions, the loss rate of the rhodium catalyst remained stable at around 2.00%, with very little variation. This indicates that adding this catalyst system to the methanol-to-acetic acid low-pressure carbonylation process maintains stability even with a low water content of 3.2%. Furthermore, the low water content in the methanol-to-acetic acid low-pressure carbonylation process reduces the energy consumption of the distillation system.

[0092] As can be seen from the above, the loss of metal catalyst is more obvious when using laboratory equipment compared with industrial operating equipment, but it can still remain stable. This is related to the production system involved in the industrial production of acetic acid through low-pressure carbonyl synthesis of methanol.

[0093] Example 8

[0094] The only difference between this embodiment and embodiment 7 is that:

[0095] In step one, add 5% [N(C6H)] based on the total mass of the solution. 13 )3(C6H5)] + [CH3COO] － 10% of [N(C4H9)2(C9H 19 (C3H7CO) + [CH3COO] － An ionic liquid mixture with 2% water content was used. The amount of acetic acid was reduced accordingly, while the other components remained unchanged. After one week of operation, the rhodium content of the catalyst was measured to be 788 ppm, and the loss rate of the rhodium catalyst was 1.50%.

[0096] It is evident that the stability of the catalyst can be maintained regardless of whether a single ionic liquid or a mixture of multiple ionic liquids is added.

[0097] Example 9

[0098] The only difference between this embodiment and Example 7 is that the rhodium content is increased to 1600 ppm, while the other components remain unchanged. After one week of operation, the rhodium content of the catalyst was measured to be 1591 ppm.

[0099] It is evident that the addition of ionic liquids can maintain the stability of the catalyst even at high catalyst concentrations.

[0100] Example 10

[0101] This embodiment uses the laboratory simulation apparatus as in Example 7, and controls the reaction conditions according to Table 1. The ionic liquid in the catalyst system is a single [N(C5H)] ionic liquid. 11 )2(C8H 17 )2] + [CH3COO] －The remaining process conditions are the same as in Example 7. After one week of stable operation, the content of metal catalyst in the reaction system is detected, and the detection results are shown in Table 1.

[0102] Table 1

[0103]

[0104] As shown in Table 1, in the catalyst system for the low-pressure carbonyl synthesis of acetic acid from methanol, the Group VIII metal catalyst can be selected from one or a combination of rhodium, iridium, and ruthenium. Under low water content conditions, the metal catalyst can maintain stability, and it can even remain stable under ultra-low water content conditions of 0.5%.

[0105] Comparative Example 1

[0106] The only difference between this comparative example and the scheme in Example 7 is that...

[0107] No [N(C5H)] is added to the reaction system. 11 )2(C8H 17 )2] + [CH3COO] － The ionic liquid was replenished with acetic acid to maintain its mass. After one week of operation, the rhodium content of the catalyst was measured to be 396 ppm, and the catalyst loss rate was 50.5%.

[0108] Comparative Example 2

[0109] The only difference between this comparative example and Example 8 is that [N(C6H] is not added. 13 )3(C6H5)] + [CH3COO] － , [N(C4H9)2(C9H 19 (C3H7CO) + [CH3COO] － An ionic liquid mixture was used to replenish the mass of the ionic liquid and the reduced mass of water using acetic acid. After one week of operation, the rhodium content of the catalyst was measured to be 358 ppm, and the catalyst loss rate was 55.2%. This indicates that under this reaction system, the loss rate of the catalyst-rhodium ratio is significantly increased, and the stability of the catalyst system is significantly reduced.

[0110] As can be seen from the above examples, without the addition of the ionic liquid catalyst stabilizer in this scheme, especially under low water content conditions, the catalyst loss is significant.

[0111] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability, characterized in that, Includes the following steps: S1, Reaction Section: Methanol or its reactive derivatives and CO are introduced into a reactor. A group VIII metal catalyst, iodomethane co-catalyst, inorganic iodide, and an ionic liquid with the structure of formula (I) are added as catalyst stabilizers to allow the materials to undergo a carbonyl synthesis reaction in the reactor, yielding crude acetic acid. ， Among them, R1, R2, R3, and R4 are alkyl, aryl, acyl, or combinations thereof containing 4 to 9 carbons; S2, Gas-liquid separation: The crude acetic acid product generated in step S1 is transported to a gas-liquid separator for gas-liquid separation. The gas phase obtained after gas-liquid separation is sent to a high-pressure absorption tower and washed with liquid raw materials. The liquid phase obtained after washing is sent back to the reactor. The liquid phase obtained after gas-liquid separation is transported to the next stage for processing. S3. Evaporation and washing: The liquid phase obtained after separation in step S2 is passed into a flash evaporator for flash evaporation and washing. The gas phase obtained after flash evaporation and washing is washed with dilute acetic acid from the light phase of the downstream light phase removal section. The carbonylation catalyst, co-catalyst, and ionic liquid catalyst stabilizer material obtained after the treatment are sent back to the reactor. The gas phase stream is transported to the next section for processing. S4, Lightweight treatment The gas phase from step S3 is sent to a light component removal tower for separation, separating a top stream rich in methyl iodine, a side stream rich in acetic acid, and a bottom stream rich in heavy components. The liquid phase obtained after cooling the top stream in a heat exchanger enters a separator to obtain a light phase rich in water and a heavy phase rich in iodomethane. The heavy phase is returned to the reactor, and the light phase is sent to step S3 as washing liquid. The gas phase obtained after cooling is sent to a low-pressure absorption tower and washed with liquid feedstock. The liquid phase obtained after washing is returned to the reactor. The bottom stream rich in heavy components separated from the light component removal tower is returned to the reactor. The side stream rich in acetic acid separated from the light component removal tower is sent to the next stage for processing. S5. Purification: The acetic acid-rich side stream from step S4 is sent to the product tower for further separation and purification to obtain the finished acetic acid and the propionic acid-rich byproduct stream.

2. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 1, characterized in that: A dehydration step is added between step S4 and step S5. The acetic acid-rich side stream obtained after step S4 is sent to a dehydration tower for further processing to obtain an acetic acid-rich stream and an aqueous dilute acetic acid stream. A portion of the aqueous dilute acetic acid stream and / or a portion of the light phase obtained in step S4 are used to wash the gaseous stream obtained after flash evaporation in step S3.

3. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 1, characterized in that: In step S1, the Group VIII metal catalyst is one or a combination of rhodium, iridium, and ruthenium; the inorganic iodide is one of lithium iodide, potassium iodide, and nickel iodide. The R1, R2, R3, and R4 groups of the ionic liquid with formula (I) are all the same groups, some are the same groups, or are completely different groups; the ionic liquid is a single compound or a mixture of multiple compounds having the structure of formula (I).

4. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 1, characterized in that: The molar ratio of the ionic liquid to the group VIII metal catalyst is 5~60:

1.

5. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 1, characterized in that: In step S1, the reactor includes a first-stage reactor and a second-stage reactor. The first-stage reactor is equipped with a fluid stirring mechanism. After the reactants are added to the first-stage reactor, a carbonyl synthesis reaction occurs under the catalytic conditions of the catalyst system. The product outlet at the top of the first-stage reactor is connected to the second-stage reactor, which allows the fluid to flow upwards without backmixing and transitions from a mixed flow to a plug flow. The mixture obtained after passing through the first-stage reactor is further transported to the second-stage reactor for further reaction, thus obtaining crude acetic acid. The catalyst-containing material obtained after treatment in step S3, the heavy phase obtained in step S4, and the bottom stream of the light phase removal tower are all returned to a section of the reactor.

6. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 5, characterized in that: An interstage heat exchanger is installed between the first-stage reactor and the second-stage reactor, and the heat exchange medium in the interstage heat exchanger comes from boiler water.

7. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 5, characterized in that: In step S4, the gaseous stream obtained after the light phase removal tower is first cooled by a heat exchanger, and then the cooled liquid phase is sent to a separator for separation. The resulting heavy phase stream is sent back to a first-stage reactor for recycling, and the light phase stream rich in water and acetaldehyde is sent to the formaldehyde removal section.

8. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 7, characterized in that, The treatment method of the formaldehyde removal section is as follows: the light phase stream obtained after the separator is sent to the aldehyde concentration tower for treatment, and the resulting top stream rich in acetaldehyde and methyl iodine and bottom stream rich in water are sent to the extraction section, and the bottom stream rich in water is sent to the separator or reaction section. The overhead stream rich in acetaldehyde and methyl iodine is sent to the extraction tower after being cooled by heat exchange. It is then extracted with demineralized water in the extraction tower. The resulting aldehyde-containing wastewater is sent to the wastewater treatment system for treatment, and the raffinate is returned to the separator or reaction section.

9. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 7, characterized in that: The gas phase obtained after treatment in the high-pressure absorption tower of step S2 and / or the gas phase obtained after treatment in the low-pressure absorption tower of step S4 are sent to the pressure swing adsorption tower for further treatment. The purified CO gas is then sent to a first-stage reactor as a raw material for the carbonyl synthesis reaction.

10. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 1, characterized in that: In step S5, the resulting propionic acid-rich byproduct stream is sent to a propionic acid concentration tower for treatment, and then to a propionic acid product tower for further treatment to obtain a propionic acid product liquid.

11. The method for low-pressure carbonyl synthesis of acetic acid from methanol with high catalyst stability according to claim 10, characterized in that: In step S5, the obtained acetic acid is processed by a deiodination system to obtain low-iodine acetic acid with an iodine content of less than 10 ppb. The deiodination system includes two-stage deiodination tanks.

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