Recovery method of noble metal catalyst in process for synthesizing acetic acid through low-pressure methanol carbonylation

Abstract

The invention relates to a method for recovering a noble metal catalyst in a process for synthesizing acetic acid through low-pressure methanol carbonylation, which comprises the following steps: sampling and analyzing a rhodium-iodine catalyst solution in an acetic acid device reactor, determining the content of iodide ions in the rhodium-iodine catalyst solution, adding methanol which reacts with the iodide ions, and reacting with the methanol to obtain the noble metal catalyst. The content of methyl in the rhodium-iodine catalyst solution is increased; introducing nitrogen into the acetic acid device reactor to reduce the volume content of CO in the acetic acid device reactor so as to reduce the occurrence of CO intercalation reaction; and then synchronously raising the temperature and pressure of the acetic acid device reactor to set temperature and pressure, maintaining the set temperature and pressure for a certain period of time, reducing the pressure at the temperature and gradually relieving the pressure to normal pressure, cooling the rhodium-iodine catalyst solution to room temperature, standing, layering, filtering and recycling. By increasing the methyl content and reducing the CO volume content, rhodium is promoted to be converted into precious metal precipitation from a stable state of homogeneous reaction, and recovery of precious metal rhodium is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of precious metal catalyst recovery, and particularly relates to a method for recovering precious metal catalysts in a low-pressure methanol carbonylation process for synthesizing acetic acid. Background Art

[0002] In the process of synthesizing acetic acid by low-pressure methanol carbonylation, the catalyst system is a rhodium-iodine system, that is, the rhodium complex is a promoter and the iodide is a co-catalyst.

[0003] Since the new national standard for industrial glacial acetic acid has been implemented, when enterprises use the original acetic acid plant reactor to catalyze the reaction of rhodium-iodine catalysts to prepare acetic acid, more side reaction products will be produced, such as water gas and propionic acid. This not only causes the product quality to be on the verge of exceeding the standard, but also causes the device to be unable to operate at high load. At the same time, the selectivity of the rhodium-iodine catalyst in this system is poor, the effective components of acetic acid in the acetic acid equipment are low, the steam consumption is large, and the preparation cost of acetic acid products is high, so technological updating has become an inevitable trend. However, the price of catalyst rhodium has risen. If it is not recycled and reused, the production cost of acetic acid products will increase. However, the recovery of precious metal rhodium in the original catalyst system has become a major problem. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a method for recovering a precious metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid, wherein the method can reduce the volume content (partial pressure) of CO in the methanol carbonylation process for synthesizing acetic acid by increasing the methyl content in the precipitation conditions of the precious metal catalyst, thereby promoting the conversion of rhodium from a stable state of homogeneous reaction into a precious metal precipitation, and finally pouring out and filtering the precipitate to achieve the purpose of recovering the precious metal rhodium.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The first implementation scheme provided by the present invention is a method for recovering a precious metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid, comprising the following steps:

[0007] The rhodium-iodine catalyst solution in the reactor of the acetic acid device is sampled and analyzed to determine the content of iodine ions in the rhodium-iodine catalyst solution, and methanol is added to the rhodium-iodine catalyst solution under a constant pressure so that the methanol reacts with the iodine ions to increase the methyl content in the rhodium-iodine catalyst solution;

[0008] Then, nitrogen is introduced into the acetic acid unit reactor to reduce the volume content of CO in the acetic acid unit reactor and is used to replace the CO in the acetic acid unit reactor to reduce the occurrence of CO insertion reaction;

[0009] Then the temperature and pressure of the acetic acid unit reactor are simultaneously raised to the set temperature and pressure, and after maintaining the set temperature and pressure for a certain period of time, the pressure is reduced at this temperature and gradually released to normal pressure. The rhodium-iodine catalyst solution is cooled to room temperature and allowed to stand for stratification. The nitrogen is introduced to replace the dissolved CO in the acetic acid unit reactor and the rhodium-iodine catalyst solution, reducing the unstable [CH3Rh(CO)2I3] - The combination with carbonyl C=O accelerates the precipitation of rhodium.

[0010] Then, the reactor of the acetic acid unit starts to increase temperature and pressure synchronously, and after maintaining the set temperature and pressure for a certain period of time, the reactor of the acetic acid unit maintains the temperature and reduces pressure and gradually releases the pressure to normal pressure. The rhodium-iodine catalyst solution is cooled to room temperature and allowed to stand for stratification, and the precipitate obtained by standing for stratification is filtered and recovered.

[0011] Preferably, when the CO volume content is less than 1%, the CO replacement operation is stopped.

[0012] Preferably, the amount of methanol added is 1.3 to 1.35 times the iodide ion content in the rhodium-iodine catalyst solution. - 1.3 to 1.35 times of the content is a guiding data. This is because CH3OH+HI—CH3I+H2O increases the methyl content in the system, which is beneficial to the precipitation of the catalyst in the rhodium-iodine catalyst solution.

[0013] Preferably, the acetic acid unit reactor is heated to 150°C at a heating rate of ≤30°C / h. While heating, the pressure of the acetic acid unit reactor is increased to 0.6MPa and maintained at this temperature and pressure for 1h, with a temperature range of 150±2°C and a pressure range of 0.6±0.05MPa, to keep the reaction going.

[0014] Preferably, when the rhodium-iodine catalyst solution is depressurized to normal pressure, the depressurized tail gas needs to be absorbed in the absorption zone, and the depressurization process needs to be maintained for 24 hours; after the rhodium-iodine catalyst solution is cooled to room temperature, the static stratification time is not less than 24 hours.

[0015] Preferably, before filtering, the supernatant after standing and stratifying is taken to analyze the content of dissolved rhodium in the rhodium-iodine catalyst solution. When the content of dissolved rhodium in the supernatant is ≤10ppm, the upper liquid and the precipitate in the reactor of the acetic acid device need to be separated, and the separated precipitate is filtered; when the content of dissolved rhodium in the supernatant is >10ppm, the above recovery method is repeated until the content of dissolved rhodium in the supernatant is ≤10ppm.

[0016] Preferably, when the content of dissolved rhodium in the supernatant is greater than 10 ppm and the above recovery method is repeated, the amount of methanol added is increased by 0.1 times the content of iodide ions in the reaction solution each time.

[0017] Preferably, before analyzing and sampling the rhodium-iodine catalyst solution in the acetic acid unit reactor, the rhodium-iodine catalyst solution in the acetic acid unit reactor needs to be cooled to room temperature, the liquid level of the acetic acid unit reactor is controlled at 50%, and the initial pressure of the acetic acid unit reactor is set to normal pressure.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The method for recovering the precious metal catalyst in the process of synthesizing acetic acid by low-pressure methanol carbonylation provided by the present invention can realize the recovery of the precious metal rhodium in the original catalyst system. Because the catalyst in the rhodium-iodine catalyst solution is rhodium triiodide, the co-catalyst is methyl iodide, and the catalyst active intermediate diiodide dicarbonyl rhodium [Rh(CO)2I2] is composed of rhodium, carbon oxide and iodine. - In the carbonylation reaction, [Rh(CO)2I2] - It undergoes a nucleophilic oxidative addition reaction with methyl iodide to produce an unstable hexacoordinated intermediate [CH3Rh(CO)2I3] - Then the methyl group in the intermediate structure turns to the adjacent Rh-CO coordination bond to form the acetyl intermediate [CH3CORh(CO)I3] - ,[CH3CORh(CO)I3] - Reaction with CO to convert into hexacoordinated [CH3CORh(CO)2I3] - , and then converted back into [Rh(CO)2I2] through reduction elimination reaction - , completing the catalytic cycle. The acetyl iodide produced during the reaction reacts with methanol or water to convert into acetic acid or methyl acetate, and methyl iodide or hydrogen iodide is generated at the same time. The hydrogen iodide reacts with methanol in the carbonylation reaction system and is converted into methyl iodide, which is then reused.

[0020] Methanol is added to the rhodium-iodine catalyst solution to convert hydrogen iodide in the rhodium-iodine catalyst solution into CH3I, thereby increasing the methyl content in the rhodium-iodine catalyst solution. The increase in methyl content will convert the stable catalyst active intermediate diiododicarbonyl rhodium ([Rh(CO)2I2] - ) is converted into the unstable hexacoordinate intermediate [CH3CORh(CO)I3] - When methanol is added, the CO partial pressure is reduced, that is, the CO volume content in the system is reduced before the CO insertion reaction, thereby cutting off the [CH3CORh(CO)I3] - Reaction with CO to convert into hexacoordinated [CH3CORh(CO)2I3] - The subsequent transformation pathway makes the unstable intermediate [CH3CORh(CO)I3] - A complete decomposition reaction occurs, namely [CH3CORh(CO)I3]- →RhI3(solid)+2CO+CH 3- , thereby precipitating the precious metal rhodium catalyst in the rhodium-iodine catalyst solution, achieving the purpose of recovering the precious metal rhodium.

[0021] 2. The recovery rate of precious metal rhodium in the method for recovering precious metal catalyst in the process of synthesizing acetic acid by low-pressure methanol carbonylation provided by the present invention is 98.86% to 99%. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a reaction process mechanism diagram for the coal chemical acetic acid unit using rhodium triiodide as a catalyst and methyl iodide as a co-catalyst. DETAILED DESCRIPTION

[0023] The specific embodiments of the present invention are described in detail below, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of the present invention. The experimental methods described in the embodiments of the present invention are conventional methods unless otherwise specified.

[0024] The inventors have found that the conditions for stable operation of the process of synthesizing acetic acid by carbonylation of methanol are: high CO partial pressure, high iodide ion concentration, low dissolved rhodium concentration, low temperature, and low methyl content. Therefore, in order to recover the precious metal catalyst used in the process of synthesizing acetic acid by carbonylation of methanol, it is necessary to meet the conditions for its precipitation, that is, high methyl content and low CO partial pressure, so as to promote the conversion of dissolved rhodium from the stable state of homogeneous reaction to precious metal precipitation.

[0025] A method for recovering a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid comprises the following steps:

[0026] (1) Using the acetic acid plant reactor as a fall activation tank.

[0027] (2) The rhodium-iodine catalyst solution in the acetic acid unit reactor is cooled to room temperature, the liquid level in the acetic acid unit reactor is controlled at 50%, and the initial pressure of the acetic acid unit reactor is normal pressure.

[0028] (3) Manually sample and analyze the iodide ion content in the rhodium-iodine catalyst solution, and add methanol to the activation tank, during which the pressure of the activation tank is maintained at normal pressure; the amount of methanol added is 1.3 to 1.35 times the I content of the solution sampled and analyzed. The amount of methanol introduced is I - 1.3 to 1.35 times of the content is a guiding data. This is because CH3OH+HI—CH3I+H2O increases the methyl content in the system, which is beneficial to the precipitation of the catalyst in the rhodium-iodine catalyst solution.

[0029] (4) Nitrogen is introduced into the deactivation tank to reduce the volume content of CO in the reactor of the acetic acid unit, and is used to replace the CO in the deactivation tank solution. When the CO volume content is less than 1%, the replacement operation is stopped. The introduction of nitrogen is to replace the CO dissolved in the reactor of the acetic acid unit and the rhodium-iodine catalyst solution, and to reduce the unstable [CH3Rh(CO)2I3] - The combination with carbonyl C=O accelerates the precipitation of rhodium.

[0030] (5) The temperature and pressure of the settlement tank begin to rise at a heating rate of ≤30°C / h until the temperature reaches 150°C. During the heating period, the pressure is simultaneously increased to 0.6MPa. After reaching the required temperature and pressure, the pressure and temperature are maintained for 1 hour. The settlement tank is not vented during the heating period.

[0031] (6) The temperature of the decompression tank is then maintained at 150°C to reduce the pressure in the decompression tank. The decompression causes the solution in the decompression tank to boil and the pressure is gradually reduced to normal pressure. The materials released in this process are mainly methyl iodide and methyl acetate. The decompressed exhaust gas is absorbed by the exhaust gas absorption zone. The decompression process needs to be maintained for 24 hours.

[0032] (7) Cool the rhodium-iodine catalyst solution in the activation tank to room temperature and allow it to stand to allow the precipitated rhodium and the clear liquid to separate completely. The standing time for separation should be no less than 24 hours.

[0033] (8) Take the supernatant liquid after standing in the fall activation tank and analyze the dissolved rhodium content.

[0034] (9) When the content of dissolved rhodium in the supernatant obtained by analysis is ≤10ppm, pour out the supernatant in the fall-out tank, clean out the precipitate, and filter it with filter paper; when the content of dissolved rhodium in the supernatant obtained by analysis is >10ppm, repeat steps 2-8, and increase the amount of methanol by 0.1 times the content of I each time.

[0035] The above-mentioned method steps are all capable of recovering precious metal catalysts. The following embodiment is the best scheme for the recovery of precious metal catalysts in the process of synthesizing acetic acid by low-pressure methanol carbonylation.

[0036] Example

[0037] A method for recovering a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid comprises the following steps:

[0038] (1) Using the acetic acid plant reactor as a fall activation tank.

[0039] (2) The rhodium-iodine catalyst solution in the acetic acid unit reactor is cooled to room temperature, the liquid level of the acetic acid unit reactor is controlled at 50%, the initial pressure of the acetic acid unit reactor is normal pressure, and the rhodium concentration in the rhodium-iodine catalyst solution is 700 ppm to 800 ppm.

[0040] (3) Manually sample and analyze the iodide ion content in the rhodium-iodine catalyst solution, and add methanol into the activation tank, during which the pressure in the activation tank is maintained at normal pressure; the amount of methanol added starts at 1.3 times the content of the solution sampled and analyzed I, and the maximum amount of methanol added is 1.35 times the content of the solution sampled and analyzed I.

[0041] (4) Nitrogen is introduced into the deactivation tank to reduce the volume content of CO in the reactor of the acetic acid unit, so as to replace the CO in the deactivation tank solution. When the volume content of CO is less than 1%, the replacement operation is stopped.

[0042] (5) The temperature and pressure of the settlement tank begin to rise at a heating rate of ≤30°C / h until the temperature reaches 150°C. During the heating period, the pressure is simultaneously increased to 0.6MPa. After reaching the required temperature and pressure, the pressure and temperature are maintained for 1 hour. The settlement tank is not vented during the heating period.

[0043] (6) The temperature of the decompression tank is then maintained at 150°C to reduce the pressure in the decompression tank. The decompression causes the solution in the decompression tank to boil and the pressure is gradually reduced to normal pressure. The materials released in this process are mainly methyl iodide and methyl acetate. The decompressed exhaust gas is absorbed by the exhaust gas absorption zone. The decompression process needs to be maintained for 24 hours.

[0044] (7) Cool the rhodium-iodine catalyst solution in the activation tank to room temperature and allow it to stand to allow the precipitated rhodium and the clear liquid to separate completely. The standing time for separation should be no less than 24 hours.

[0045] (8) Take the supernatant liquid after standing in the fall activation tank and analyze the dissolved rhodium content.

[0046] (9) When the content of dissolved rhodium in the supernatant obtained by analysis is ≤10ppm, pour out the supernatant in the fall-out tank, clean out the precipitate, and filter it with filter paper; when the content of dissolved rhodium in the supernatant obtained by analysis is >10ppm, repeat steps 2-8, and increase the amount of methanol by 0.1 times the content of I each time.

[0047] Through experiments, it is verified that nitrogen gas at 500Nm 3 When the nitrogen is replaced by CO in the fall activation tank at 10000 rpm, the relationship between the nitrogen replacement time and the CO content is shown in Table 1:

[0048] Table 1 Relationship between nitrogen replacement time and CO content

[0049]

[0050]

[0051] The initial amount of methanol added was 1.3 times the iodide ion content, and the comparison of the activation reaction time and the soluble rhodium in the solution is shown in Table 2:

[0052] Table 2 Comparison of the activation reaction time and the soluble rhodium in the solution

[0053] Precipitation reaction time Rhodium concentration (PPm) time Rhodium concentration (PPm) 4 hours 210 8 hours 34 9 hours 36 12 hours 34 13 hours 14 15 hours 35 17 hours 15 19 hours 15 21 hours 23 23 hours 20 25 hours 19 29 hours 19

[0054] As shown in Table 2, when methanol was added with 1.3 times of iodide ion content, under the premise of CO content<1%, soluble rhodium>10PPm, and in order to increase the recovery of rhodium catalyst, the methanol dosage was increased with the increment of 0.1 times of I ion content, and above-mentioned 3~6 was repeated until soluble rhodium content was 8PPm in the analytical solution, reaching the setting condition to stop falling and start filtering to recover catalyst. At this moment, the recovery of rhodium catalyst was 98.86%~99%.

[0055] The reaction mechanism of the reaction process of the coal chemical acetic acid unit using rhodium triiodide as a catalyst and methyl iodide as a co-catalyst is as follows:

[0056] like Figure 1 As shown, the catalyst in the rhodium-iodine catalyst solution is rhodium triiodide, the co-catalyst is methyl iodide, and the catalyst active intermediate diiodide dicarbonyl rhodium [Rh(CO)2I2] is composed of rhodium, carbon monoxide, and iodine. In the carbonylation reaction, [Rh(CO)2I2] - It undergoes a nucleophilic oxidative addition reaction with methyl iodide to produce an unstable hexacoordinated intermediate [CH3Rh(CO)2I3] - Then the methyl group in the intermediate structure turns to the adjacent Rh-CO coordination bond to form the acetyl intermediate [CH3CORh(CO)I3] - ,[CH3CORh(CO)I3] - Reaction with CO to convert into hexacoordinated [CH3CORh(CO)2I3] - , and then converted back into [Rh(CO)2I2] through reduction elimination reaction - , completing the catalytic cycle. The acetyl iodide produced during the reaction reacts with methanol or water to convert into acetic acid or methyl acetate, and methyl iodide or hydrogen iodide is generated at the same time. The hydrogen iodide reacts with methanol in the carbonylation reaction system and is converted into methyl iodide, which is then reused.

[0057] When methanol is added to the rhodium-iodine catalyst solution, the hydrogen iodide in the rhodium-iodine catalyst solution is converted into CH3I, thereby increasing the methyl content in the rhodium-iodine catalyst solution. The increase in the methyl content will convert the stable catalyst active intermediate diiododicarbonyl rhodium ([Rh(CO)2I2] - ) is converted into the unstable hexacoordinate intermediate [CH3CORh(CO)I3] -When methanol is added, the CO partial pressure is reduced, that is, the CO partial pressure in the system is reduced before the CO insertion reaction, thereby cutting off the [CH3CORh(CO)I3] - Reaction with CO to convert into hexacoordinated [CH3CORh(CO)2I3] - The subsequent transformation pathway makes the unstable intermediate [CH3CORh(CO)I3] - A complete decomposition reaction occurs, namely [CH3CORh(CO)I3] - →RhI3(solid)+2CO+CH3 - , thereby precipitating the precious metal rhodium catalyst in the rhodium-iodine catalyst solution, achieving the purpose of recovering the precious metal rhodium.

[0058] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for recovering a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid, characterized in that: The steps include: The rhodium-iodine catalyst solution in the reactor of the acetic acid device is sampled and analyzed to determine the content of iodine ions in the rhodium-iodine catalyst solution, and methanol is added to the rhodium-iodine catalyst solution under a constant pressure so that the methanol reacts with the iodine ions to increase the methyl content in the rhodium-iodine catalyst solution; Then, nitrogen is introduced into the acetic acid unit reactor to reduce the volume content of CO in the acetic acid unit reactor and is used to replace the CO in the acetic acid unit reactor to reduce the occurrence of CO insertion reaction; Then, the temperature and pressure of the reactor of the acetic acid unit are simultaneously increased to the set temperature and pressure, and after maintaining the set temperature and pressure for a certain period of time, the pressure is reduced at this temperature and gradually released to normal pressure. The rhodium-iodine catalyst solution is cooled to room temperature and allowed to stand for stratification, and the precipitate obtained by standing for stratification is filtered and recovered.

2. The method for recovering precious metal catalyst in the process for synthesizing acetic acid by low-pressure methanol carbonylation as claimed in claim 1, characterized in that: When the CO volume content is less than 1%, the CO replacement operation is stopped.

3. The method for recovering precious metal catalyst in the process for synthesizing acetic acid by low-pressure methanol carbonylation as claimed in claim 1, characterized in that: The amount of methanol added is 1.3 to 1.35 times the iodide ion content in the rhodium-iodine catalyst solution.

4. The method for recovering precious metal catalysts in the process for synthesizing acetic acid by low-pressure methanol carbonylation as claimed in claim 1, characterized in that: The acetic acid unit reactor is heated to 150° C. at a heating rate of ≤30° C. / h. While heating, the pressure of the acetic acid unit reactor is increased to 0.6 MPa and maintained at this temperature and pressure for 1 hour.

5. The method for recovering precious metal catalyst in the process for synthesizing acetic acid by low-pressure methanol carbonylation as claimed in claim 1, characterized in that: When the rhodium-iodine catalyst solution is depressurized to normal pressure, the depressurized tail gas needs to be absorbed by the absorption zone, and the depressurization process needs to be maintained for 24 hours; after the rhodium-iodine catalyst solution is cooled to room temperature, the static stratification time is not less than 24 hours.

6. The method for recovering precious metal catalyst in the process for synthesizing acetic acid by low-pressure methanol carbonylation as claimed in claim 1, characterized in that: Before filtering, the supernatant after standing and stratifying needs to be analyzed for the content of dissolved rhodium in the rhodium-iodine catalyst solution. When the content of dissolved rhodium in the supernatant is ≤10ppm, the upper liquid and the precipitate in the reactor of the acetic acid device need to be separated, and the separated precipitate is filtered; when the content of dissolved rhodium in the supernatant is >10ppm, the above recovery method is repeated until the content of dissolved rhodium in the supernatant is ≤10ppm.

7. The method for recovering precious metal catalyst in the process for synthesizing acetic acid by low-pressure methanol carbonylation as claimed in claim 6, characterized in that: When the content of dissolved rhodium in the supernatant is greater than 10 ppm, the above recovery method is repeated, and the amount of methanol added is increased by 0.1 times the content of iodide ions in the reaction solution each time.

8. The method for recovering precious metal catalyst in the process for synthesizing acetic acid by low-pressure methanol carbonylation as claimed in claim 1, characterized in that: Before analyzing and sampling the rhodium-iodine catalyst solution in the acetic acid unit reactor, the rhodium-iodine catalyst solution in the acetic acid unit reactor needs to be cooled to room temperature, the liquid level of the acetic acid unit reactor is controlled at 50%, and the initial pressure of the acetic acid unit reactor is set to normal pressure.

Images