Method for recovering precious metal catalyst in low-pressure methanol carbonylation process for synthesizing acetic acid

Abstract

The present invention relates to the recovery method of precious metal catalyst in low-pressure methanol carbonylation synthesis acetic acid process, comprise the steps:The rhodium iodine series catalyst solution in acetic acid unit reactor is sampled and analyzed, the content of iodide ion in rhodium iodine series catalyst solution is measured, the methanol reacted with iodide ion is added, to increase the methyl content in rhodium iodine series catalyst solution;Nitrogen is then passed into acetic acid unit reactor, CO volume content is reduced in acetic acid unit reactor, to reduce the generation of CO intercalation reaction;Then acetic acid unit reactor is synchronously heated and boosted to set temperature and pressure, and after maintaining a certain length of time under the set temperature and pressure, decompression at this temperature and gradually pressure relief to normal pressure, rhodium iodine series catalyst solution is cooled to room temperature and standing stratification filtration recovery.The present invention, by increasing methyl content, reduces CO volume content, prompts rhodium to be converted into precious metal precipitation precipitation from the stable state of homogeneous reaction, realizes the recovery of precious metal rhodium.

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 low-pressure methanol carbonylation process for synthesizing acetic acid, the catalyst system is a rhodium-iodine system, that is, a rhodium complex is a co-catalyst and an iodide is a co-catalyst.

[0003] With the implementation of the new national standard for industrial glacial acetic acid, companies are no longer using existing acetic acid plant reactors to produce acetic acid using rhodium-iodine catalysts. This produces a significant number of side products, such as water gas and propionic acid. This not only causes product quality to be near-substandard but also prevents the plant from operating at full capacity. Furthermore, the rhodium-iodine catalyst in this system exhibits poor selectivity, resulting in a low level of effective acetic acid components in the acetic acid plant, high steam consumption, and high production costs for acetic acid products. Therefore, technological upgrades are inevitable. However, the rising price of the rhodium catalyst, if not recycled and reused, will increase the production cost of acetic acid products. However, the recovery of the precious metal rhodium in the existing catalyst system has become a major issue. 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 the stable state of the homogeneous reaction to the precipitation of precious metals, 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 unit is sampled and analyzed to determine the iodide ion content in the rhodium-iodine catalyst solution. Methanol is then added to the rhodium-iodine catalyst solution under a constant pressure so that the methanol reacts with the iodide 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 to replace the CO in the acetic acid unit reactor, thereby reducing the occurrence of CO insertion reaction;

[0009] The acetic acid reactor is then heated and pressurized to the set temperature and pressure, maintained at this set temperature and pressure for a certain period of time, and then the pressure is gradually reduced to atmospheric pressure at this temperature. The rhodium-iodine catalyst solution is cooled to room temperature and allowed to stand for stratification. Nitrogen is introduced to displace dissolved CO in the acetic acid 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 begins 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 0.6 MPa and maintained at this temperature and pressure for 1 hour, with a temperature range of 150±2°C and a pressure range of 0.6±0.05 MPa, 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 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, it is allowed to stand for stratification for not less than 24 hours.

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

[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 such that the iodide ion content in the reaction solution is increased by 0.1 times 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 in 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 low-pressure methanol carbonylation process for synthesizing acetic acid provided by the present invention can realize the recovery of the precious metal rhodium in the original catalyst system. This is 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 hexacoordinate 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] - Reacts with CO to convert into hexacoordinated [CH3CORh(CO)2I3] - , and then converted back into [Rh(CO)2I2] - The catalytic cycle is completed. The acetyl iodide produced reacts with methanol or water to convert into acetic acid or methyl acetate, simultaneously generating methyl iodide or hydrogen iodide. The hydrogen iodide reacts with methanol in the carbonylation reaction system to convert it into methyl iodide, which can then be 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 diiodide dicarbonyl 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] - Reacts 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 causing the precious metal rhodium catalyst in the rhodium-iodine catalyst solution to precipitate, 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 low-pressure methanol carbonylation process for synthesizing acetic acid 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 a 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 scope of protection 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 making creative work are within the scope of protection of the present invention. The experimental methods described in the embodiments of the present invention are all conventional methods unless otherwise specified.

[0024] The inventors discovered that the conditions for stable operation of the methanol carbonylation process for synthesizing acetic acid 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 methanol carbonylation process for synthesizing acetic acid, it is necessary to meet the precipitation conditions, namely, high methyl content and low CO partial pressure. This can 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 fallout tank.

[0027] (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%, 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 times 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 acetic acid unit reactor and to displace the CO in the deactivation tank solution. When the CO volume content is less than 1%, the displacement operation is stopped. The introduction of nitrogen displaces the dissolved CO in the acetic acid unit reactor and the rhodium-iodine catalyst solution, thereby reducing 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℃ / h, and the temperature rises to 150℃. The pressure is increased simultaneously during the heating period, and the pressure is increased to 0.6MPa. After reaching the temperature and pressure, the pressure and temperature are maintained for 1 hour. The settlement tank is not emptied during the heating period.

[0031] (6) Then, the temperature of the fall-activation tank is maintained at 150°C, and the pressure of the fall-activation tank is reduced. The pressure is reduced to boil the solution in the fall-activation tank, and the pressure is gradually reduced to normal pressure. The materials released in this process are mainly methyl iodide and methyl acetate. The depressurized exhaust gas is absorbed by the exhaust gas absorption zone. The pressure relief 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 for a long time to separate the precipitated rhodium and the clear liquid. The standing time should be no less than 24 hours.

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

[0034] (9) When the dissolved rhodium content 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 dissolved rhodium content 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 the precious metal catalyst. The following embodiment is an optimal solution for the method of recovering the precious metal catalyst in the low-pressure methanol carbonylation process for synthesizing acetic acid.

[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 fallout 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 to the activation tank, during which the pressure in the activation tank is maintained at atmospheric pressure; the amount of methanol added starts at 1.3 times the content of I in the solution sampling analysis, and the maximum amount of methanol added is 1.35 times the content of I in the solution sampling analysis.

[0041] (4) Nitrogen is introduced into the fall-activation tank to reduce the volume content of CO in the acetic acid unit reactor and to replace the CO in the fall-activation tank solution. When the CO volume content 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℃ / h, and the temperature rises to 150℃. The pressure is increased simultaneously during the heating period, and the pressure is increased to 0.6MPa. After reaching the temperature and pressure, the pressure and temperature are maintained for 1 hour. The settlement tank is not emptied during the heating period.

[0043] (6) Then, the temperature of the fall-activation tank is maintained at 150°C, and the pressure of the fall-activation tank is reduced. The pressure is reduced to boil the solution in the fall-activation tank, and the pressure is gradually reduced to normal pressure. The materials released in this process are mainly methyl iodide and methyl acetate. The depressurized exhaust gas is absorbed by the exhaust gas absorption zone. The pressure relief 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 for a long time to separate the precipitated rhodium and the clear liquid. The standing time should be no less than 24 hours.

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

[0046] (9) When the dissolved rhodium content 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 dissolved rhodium content 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 was verified that nitrogen gas at 500Nm 3 When the nitrogen is replaced by CO in the falling activation tank at a rate of 1 / h, 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. The comparison of the activation reaction time and the soluble rhodium in the solution is shown in Table 2:

[0052] Table 2 Comparison of activation reaction time and soluble rhodium in 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 prerequisite of CO content<1%, soluble rhodium>10PPm, and in order to increase the rate of recovery of rhodium catalyst, so the increment with 0.1 times of I ion content increased the methanol dosage, and repeated above-mentioned 3~6, until soluble rhodium content in the analytical solution was 8PPm, reached the setting condition and stopped falling, began to filter and reclaim catalyst. Now, the rate of recovery of rhodium catalyst was recovery 98.86%~99%.

[0055] The reaction mechanism 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 in the figure, 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 hexacoordinate 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] - Reacts with CO to convert into hexacoordinated [CH3CORh(CO)2I3] - , and then converted back into [Rh(CO)2I2] - The catalytic cycle is completed. The acetyl iodide produced reacts with methanol or water to convert into acetic acid or methyl acetate, simultaneously generating methyl iodide or hydrogen iodide. The hydrogen iodide reacts with methanol in the carbonylation reaction system to convert it into methyl iodide, which can then be 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 methyl content will convert the stable catalyst active intermediate diiodide dicarbonyl 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] - Reacts 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 causing the precious metal rhodium catalyst in the rhodium-iodine catalyst solution to precipitate, achieving the purpose of recovering the precious metal rhodium.

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

Claims

1. A method for recovering precious metal catalysts 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 unit is sampled and analyzed to determine the iodide ion content in the rhodium-iodine catalyst solution. Methanol is then added to the rhodium-iodine catalyst solution under a constant pressure so that the methanol reacts with the iodide 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 to replace the CO in the acetic acid unit reactor, thereby reducing the occurrence of CO insertion reaction; Then, the temperature and pressure of the acetic acid unit reactor 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 the same 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 a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid according to claim 1, wherein: When the CO volume content is less than 1%, the CO replacement operation is stopped.

3. The method for recovering a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid according to claim 1, wherein: 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 a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid according to claim 1, wherein: The acetic acid plant reactor was heated to 150° C. at a heating rate of ≤30° C. / h. While heating, the pressure of the acetic acid plant reactor was increased to 0.6 MPa and maintained at this temperature and pressure for 1 hour.

5. The method for recovering a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid according to claim 1, wherein: 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, it must be allowed to stand for stratification for no less than 24 hours.

6. The method for recovering a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid according to claim 1, wherein: Before filtration, the supernatant after standing and stratification needs to be analyzed for the dissolved rhodium content in the rhodium-iodine catalyst solution. When the dissolved rhodium content in the supernatant is ≤10ppm, the upper layer liquid and the precipitate in the acetic acid unit reactor need to be separated, and the separated precipitate is filtered; when the dissolved rhodium content in the supernatant is >10ppm, the above recovery method is repeated until the dissolved rhodium content in the supernatant is ≤10ppm.

7. The method for recovering a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid according to claim 6, wherein: 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 such that the content of iodide ions in the reaction solution increases by 0.1 times each time.

8. The method for recovering a noble metal catalyst in a low-pressure methanol carbonylation process for synthesizing acetic acid according to claim 1, wherein: 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 in the acetic acid unit reactor is controlled at 50%, and the initial pressure of the acetic acid unit reactor is set to atmospheric pressure.

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