A starting method for preparing maleic anhydride by n-butane oxidation

Abstract

This invention provides a start-up method for the oxidation of n-butane to maleic anhydride. The reactor is equipped with a two-stage molten salt temperature control system. The first stage begins with a temperature rise, introducing air at 30%-50% of the designed processing load and n-butane at 5%-30%. Both are then increased at a fixed rate, controlling the n-butane conversion rate to no more than 88%. The second stage of molten salt is then started, raising the temperature to 330-380°C. n-Butane is increased at 5-20% to 80-90% of the designed processing load. After stabilization, n-Butane is increased at a rate of 5-10% per week to the designed processing load, controlling the n-butane conversion rate to 80-90%. This invention, combined with a unique start-up method, reduces the number of active sites on the catalyst. While increasing the volume percentage of butane introduced into the reactor during the start-up phase, temperature adjustments are used to avoid excessive CO generation. x This also reduces excessive catalyst consumption at the reactor inlet during the start-up phase, prevents excessive catalyst temperature rise in the inlet section, reduces phosphorus loss rate from the catalyst, improves catalyst stability, and extends catalyst lifespan.

Description

Technical Field

[0001] This invention relates to a process for producing maleic anhydride from n-butane oxidation, and more particularly to a start-up method for producing maleic anhydride from n-butane oxidation. Background Technology

[0002] Maleic anhydride, also known as maleic anhydride, is an important chemical raw material produced by benzene oxidation or n-butane oxidation. Due to the presence of two carbonyl groups (C=O) conjugated with C=C double bonds in the maleic anhydride molecule, it exhibits reactive chemical properties and has wide applications in unsaturated polyester resins, lubricants, agricultural chemicals, coatings, inks, and surfactants. However, with the promotion of national environmental protection policies and the continued advancement of integrated industrial chain projects, both maleic anhydride and its downstream consumption will undergo changes. Looking at the development trend in the field of biodegradable plastics driven by environmental policies, the integrated development of maleic anhydride-1,4-butanediol-PBAT / PBS will become the mainstream development trend for new entrants to the maleic anhydride field. Therefore, the production technology of maleic anhydride becomes particularly crucial.

[0003] Currently, the main industrial production processes for maleic anhydride, based on raw materials, are benzene oxidation and n-butane oxidation. Initially, benzene oxidation was the most widely used, but due to environmental and cost factors, it has been gradually replaced by n-butane oxidation in recent years. n-Butane oxidation holds a leading position in maleic anhydride production, accounting for approximately 80% of the world's total production capacity. The selective oxidation of n-butane to maleic anhydride can be divided into fixed-bed, fluidized-bed, and moving-bed processes. Due to its high butane feedstock utilization, stable product quality, and simple operation, the fixed-bed process has become the primary method for maleic anhydride production. In the fixed-bed production process, industrial production uses an external circulating molten salt medium to remove the heat of reaction. Because of the strongly exothermic nature of the butane oxidation reaction, excessively high n-butane conversion rates can lead to localized hot spots in the catalyst bed, which are difficult to optimize and control. The presence of these hot spots negatively impacts catalyst performance, primarily manifested in increased production of byproducts such as carbon monoxide and carbon dioxide.

[0004] For the oxidation of n-butane to maleic anhydride, especially in the initial startup phase, due to the high initial catalyst activity and low butane concentration, butane over-oxidation leads to the production of carbon monoxide and carbon dioxide, characterized by high heat release and temperature rise. In molten salt reactors, the high-temperature molten salt transfers heat to the cooling medium through a molten salt cooler, thus removing heat and cooling the molten salt. Because the volume of molten salt in a single-stage molten salt reactor is large, the time required to reach the target molten salt temperature during the initial startup phase can be as long as tens of hours, during which time butane over-oxidizes to produce CO2. X The extremely poor selectivity of maleic anhydride leads to a waste of n-butane resources. Simultaneously, significant phosphorus loss from the catalyst occurs during this period, impacting its long-term stability. Furthermore, blockage of the molten salt loop during initial startup may cause reactor overheating.

[0005] Regarding the application of catalysts in the oxidation of n-butane to maleic anhydride, domestic and international patents and literature have conducted relevant research. CN108101871A discloses a process method for the oxidation of n-butane to maleic anhydride. n-Butane, air, and recycled tail gas are mixed, and the resulting mixed reaction gas flows concurrently through two or more reaction zones connected in series, reacting with a vanadium-phosphorus-oxygen catalyst under oxidation reaction conditions. This method can fully utilize the catalyst's activity, especially in tail gas recycling processes, where it can compensate for the decrease in conversion rate caused by the decrease in oxygen content in the feedstock, reduce reaction hotspots, and improve product selectivity. However, it cannot reduce the initial by-product CO. X The problem is the large quantity.

[0006] The existing technologies mentioned above, when starting up, either improve the catalyst preparation method or modify the process method, but none of them solve the problems of high carbon monoxide and carbon dioxide generation and poor catalyst stability caused by large temperature difference between the catalyst bed and molten salt in the initial stage of the n-butane oxidation to maleic anhydride unit. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a start-up method for the oxidation of n-butane to maleic anhydride. The process method of this invention can reduce the problems of high COx by-products and low maleic anhydride selectivity in the initial stage of start-up, and achieve the goals of improving catalyst stability and extending the operating cycle of the unit.

[0008] To achieve the above-mentioned technical objectives, the present invention provides a start-up method for the oxidation of n-butane to maleic anhydride, comprising:

[0009] The reaction of n-butane oxidation to maleic anhydride is carried out in a tubular fixed-bed reactor. The catalyst is packed in the fixed bed. The upper and lower parts of the fixed-bed reactor are equipped with separate molten salt temperature control systems to achieve separate control of the temperature of the two catalyst beds.

[0010] (1) After loading the fixed-bed reactor with vanadium-phosphorus-oxygen catalyst, completing catalyst loading, air tightness inspection, reactor preheating, and dust removal, the reactor is preheated with air to raise the reactor temperature to 170-220℃. Air purging is then stopped, and nitrogen replacement is performed. Following the material flow direction, molten salt is first loaded into the reactor shell layer of the first catalyst bed that comes into contact with the material to achieve molten salt circulation. The catalyst bed temperature is controlled and raised to 340-390℃. Raw material air is then introduced, with the initial air intake being 30% of the designed processing load. 50%; After the air stabilizes, start introducing the raw material n-butane. The initial amount of n-butane introduced is 5%-30% of the design processing load, preferably 10%-25%. Increase the air introduction rate at an increase of 3-30%, preferably 5%-10% of the design processing load / h to reach the design processing load, and increase the amount of n-butane to 30-50% of the design processing load at an increase of 3-15%, preferably 3%-10% of the design processing load / h. Control the n-butane conversion rate at the reactor outlet to be no more than 88% by adjusting the reaction temperature.

[0011] (2) The reactor shell is filled with molten salt for the second catalyst bed that comes into contact with the material to achieve molten salt circulation and control the catalyst bed temperature to 330-380℃; the n-butane is gradually increased to 80-90% of the design processing load / h at a rate of 5-20%, preferably 8%-15%; the conversion rate of n-butane at the reactor outlet is controlled to 80-90% by adjusting the molten salt temperature of the second bed or both beds, wherein the molten salt temperature of the second bed is controlled to be 1-8℃ lower than that of the first bed, preferably 2-6℃; while the n-butane load is increased, phosphorus and water are added to the reaction system.

[0012] (3) After the processing load of n-butane reaches 80-90% of the design load and the reaction is stable, n-butane is gradually increased to the design processing load at a rate of 5-10% per week. The conversion rate of n-butane at the reactor outlet is controlled to 80-90% by adjusting the molten salt temperature of the second bed or both beds. The reaction conditions are adjusted to carry out the butane oxidation to maleic anhydride reaction within the design range.

[0013] Those skilled in the art should understand that the designed processing load refers to the reaction parameters designed for different reactor types and scales, which are reasonable designs that those skilled in the art can make based on site conditions.

[0014] Furthermore, when using air preheating, the hourly volumetric flow rate of air is controlled to be 500-2000 times the total catalyst loading volume, and the reactor heating rate is 3-8℃ / hour.

[0015] Furthermore, the nitrogen replacement involves adjusting the atmosphere of the catalyst bed, wherein the hourly volumetric flow rate of nitrogen is controlled to be 200-1000 times the total catalyst loading volume.

[0016] Furthermore, when the first catalyst bed is heated, its heating rate is controlled to be 2-10℃ / hour.

[0017] Furthermore, when the second catalyst bed is heated, its heating rate is controlled to be 2-10℃ / hour.

[0018] The vanadium-phosphorus-oxygen catalyst used in this invention is known to those skilled in the art and can be prepared in-house or using commercially available catalysts. Vanadium-phosphorus-oxygen catalysts inevitably experience phosphorus loss during use, therefore, phosphorus replenishment is performed during the n-butane oxidation process. Typically, phosphorus replenishment during startup involves adding an organic solvent containing a phosphorus source.

[0019] Furthermore, the phosphorus and water replenishment performed during the start-up phase of this invention involves mixing the phosphorus source and water into the raw materials and feeding them together.

[0020] Furthermore, the total makeup water volume of the reactor is 1.5-4.5v% of the total gas feed volume of the reactor, and the total phosphorus supplementation of the reactor is 2-50ppm.

[0021] Furthermore, the phosphorus source used for phosphorus supplementation is at least one of trimethyl phosphate and triethyl phosphate.

[0022] Furthermore, in the reactor for the oxidation of n-butane to maleic anhydride, the reactants are fed from the bottom and discharged from the top.

[0023] Furthermore, the volume ratio of the first catalyst bed to the second catalyst bed is 1:4 to 1:1.

[0024] Furthermore, the reaction conditions for the oxidation of butane to maleic anhydride are: reaction temperature 365-450℃, pressure 0.1-0.5MPa, and n-butane mixed gas space velocity 600-2500h⁻¹. -1 The concentration of n-butane is 1.0-1.9% (volume percentage).

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention adopts a fixed-bed reactor with two independent molten salt heating systems and a unique start-up method. On the one hand, it reduces the amount of catalyst used during the start-up stage, reduces the number of active sites of the catalyst, and increases the volume percentage of butane introduced into the reactor during the start-up stage. The molten salt temperature is adjusted quickly and sensitively, making it easy to control the conversion rate of n-butane and avoid generating too much COx. At the same time, it slows down the excessive consumption of catalyst at the reactor inlet during the start-up stage, avoids excessive temperature rise of the catalyst in the inlet section, reduces the phosphorus loss rate in the catalyst, improves the stability of the catalyst, and extends the service life of the catalyst.

[0027] (2) The start-up method of the present invention adopts a fixed-bed reactor with two independent molten salt heating systems. The heating temperature of each molten salt system can be adjusted flexibly and timely according to the selectivity and yield of maleic anhydride, thereby reducing the generation of excessive by-product COx due to untimely adjustment of the temperature of the entire bed and improving the selectivity of maleic anhydride.

[0028] (3) The present invention uses a fixed-bed reactor with two independent molten salt heating systems, which can meet technical and product requirements and reduce energy consumption at low molten salt temperature and circulation rate.

[0029] (4) The start-up process involved in the method of the present invention is simple and controllable, and there is no risk of catalyst bed overheating. The product adjustment time is short.

[0030] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the reactor apparatus used for the oxidation of n-butane to maleic anhydride according to the present invention. Detailed Implementation

[0032] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0033] use Figure 1 The illustrated fixed-bed reactor for the oxidation of n-butane to maleic anhydride is a tubular reactor 1, 9000 mm high and 6000 mm in diameter. It contains 20,000 tubes, each 21 mm in diameter, filled with a vanadium-phosphorus-oxygen catalyst. The reactor is divided into two equal beds: a first bed 2 and a second bed 3. The reaction temperature is controlled by separate molten salt pumps I 4 and II 5. The feedstock (butane), air stream 6, and supplementary phosphorus and water stream 7 are all bottom-feeded, first contacting the first bed 2 and then the second bed 3. The feedstock exits from the top of the reactor. Based on the reactor scale, the total design processing load is: a feedstock air flow rate of 76000 kg / h and a feedstock n-butane flow rate of 2900 kg / h.

[0034] Example 1

[0035] After completing the catalyst loading and airtightness check, the reactor temperature was preheated from room temperature to 200°C using air. During preheating, the hourly volumetric flow rate of air was 1000 times the total catalyst loading volume, and the reactor heating rate was 4°C / hour. Subsequently, air purging was stopped, and nitrogen replacement was performed. The hourly volumetric flow rate of nitrogen was 400 times the total catalyst loading volume. After nitrogen replacement was completed, molten salt was loaded into the reactor shell layer of the first bed that came into contact with the material, and molten salt circulation was achieved. The catalyst bed temperature was then raised to 370°C at a heating rate of 6°C / hour.

[0036] After the temperature of the first bed is constant, air at 50% of the designed processing load is introduced into the reactor at a rate of 38,000 kg / h. Then, n-butane at 15% of the designed processing load is introduced into the reactor at a rate of 435 kg / h to start the reaction. After the reaction stabilizes, the air is increased to the designed processing load at a rate of 3,800 kg / h, while the n-butane throughput is increased to 30% of the designed processing load at a rate of 174 kg / h. The n-butane conversion rate at the reactor outlet is controlled to be no more than 85% by adjusting the molten salt temperature. After the reaction stabilizes, the composition of the reaction products is analyzed by gas chromatography to obtain the n-butane conversion rate, product selectivity, and yield at this point. The results are shown in Table 1.

[0037] The reactor shell of the second bed, which comes into contact with the material, is filled with molten salt and molten salt is circulated. The temperature of the catalyst bed is raised to 365℃ at a rate of 6℃ / hour. After the bed temperature stabilizes, the n-butane throughput is increased to 80% of the design processing load at a rate of 232 kg / h. Phosphorus and water are added to the reaction system, with the water volume controlled at 2.5% of the total reactor feed volume and the phosphorus concentration maintained at 10-25 ppm. The conversion rate of n-butane at the reactor outlet is controlled to 84±2% by adjusting the molten salt temperature of the second bed or both beds. After stabilization, the reaction temperature of the first bed is 393.5℃ and the reaction temperature of the second bed is 390.2℃.

[0038] After the n-butane processing load reached 80% of the design load and the reaction stabilized, the subsequent n-butane processing load was gradually increased at an increase rate of 290 kg / week to the design processing load. The conversion rate of n-butane at the reactor outlet was controlled to 84±2% by adjusting the molten salt temperature of the second bed. After the start-up was completed, a long-term evaluation of 500 h was carried out. Then, the composition of the reaction products was analyzed by gas chromatography. The conversion rate and product selectivity of n-butane at this time, as well as the hot spot temperature of the reactor bed, are shown in Table 2.

[0039] Example 2

[0040] After the catalyst loading and airtightness check of the tubular fixed-bed reactor for the oxidation of n-butane to maleic anhydride are completed, the reactor temperature is preheated from room temperature to 210°C using air. During preheating, the hourly volumetric flow rate of air is controlled at 1500 times the total catalyst loading volume, and the reactor temperature rise rate is 6°C / hour. Subsequently, air purging is stopped, and nitrogen replacement is performed. The hourly volumetric flow rate of nitrogen is controlled at 700 times the total catalyst loading volume. After nitrogen replacement is completed, molten salt is loaded into the reactor shell layer of the first bed that comes into contact with the material, and molten salt circulation is achieved. The catalyst bed temperature is then raised to 380°C at a temperature rise rate of 8°C / hour.

[0041] After the temperature of the first bed is constant, air is introduced into the reactor at a rate of 30-400 kg / h, which is 40% of the designed processing load. Then, n-butane at 15% of the designed processing load is introduced to start the reaction at a rate of 435 kg / h. After the reaction stabilizes, the air is increased to the designed processing load at a rate of 60-80 kg / h, while the n-butane throughput is increased to 40% of the designed processing load at a rate of 116 kg / h. The n-butane conversion rate at the reactor outlet is controlled to be no more than 85% by adjusting the molten salt temperature. After the reaction stabilizes, the composition of the reaction products is analyzed by gas chromatography to obtain the n-butane conversion rate, product selectivity, and yield at this point. The results are shown in Table 1.

[0042] Subsequently, molten salt was loaded into the reactor shell of the second bed, which is in the first contact with the material, and molten salt circulation was implemented. The catalyst bed temperature was then raised to 372°C at a rate of 8°C / hour. After the bed temperature stabilized, the n-butane throughput was increased to 85% of the design processing load at an increase rate of 348 kg / h. Phosphorus and water were added to the reaction system, with the water addition controlled at 2.8% of the total reactor feed volume, and the phosphorus concentration maintained at 10–25 ppm. Simultaneously, the conversion rate of n-butane at the reactor outlet was controlled to 84 ± 2% by adjusting the molten salt temperature of the second bed or both beds. After stabilization, the reaction temperature of the first bed was 396.5°C, and the reaction temperature of the second bed was 392.0°C.

[0043] After the n-butane processing load reached 85% of the design load and the reaction stabilized, the subsequent n-butane processing load was gradually increased to the design processing load at an increase rate of 232 kg / week. The n-butane conversion rate at the reactor outlet was controlled to 84±2% by adjusting the molten salt temperature of the second bed. After the start-up was completed, a long-term evaluation of 500 h was carried out. Then, the composition of the reaction products was analyzed by gas chromatography. The n-butane conversion rate and product selectivity at this time, as well as the hot spot temperature of the reactor bed, are shown in Table 2.

[0044] Example 3

[0045] After the catalyst loading and airtightness check of the tubular fixed-bed reactor for the oxidation of n-butane to maleic anhydride are completed, the reactor temperature is preheated from room temperature to 220°C using air. During preheating, the hourly volumetric flow rate of air is controlled at 1800 times the total catalyst loading volume, and the reactor temperature rise rate is 8°C / hour. Subsequently, air purging is stopped, and nitrogen replacement is performed. The hourly volumetric flow rate of nitrogen is controlled at 200 times the total catalyst loading volume. After nitrogen replacement is completed, molten salt is loaded into the reactor shell layer of the first catalyst bed that comes into contact with the material to achieve molten salt circulation, and the catalyst bed temperature is raised to 385°C at a temperature rise rate of 10°C / hour.

[0046] After the temperature of the first catalyst bed is constant, air is introduced into the reactor at a rate of 38,000 kg / h, which is 50% of the designed processing load. Then, n-butane at a rate of 580 kg / h is introduced to start the reaction. After the reaction stabilizes, the air is increased to the designed processing load at a rate of 7,600 kg / h, while the n-butane flow rate is increased to 50% of the designed processing load at a rate of 232 kg / h. The n-butane conversion rate at the reactor outlet is controlled to be no more than 85% by adjusting the molten salt temperature. After the reaction stabilizes, the composition of the reaction products is analyzed by gas chromatography to obtain the n-butane conversion rate, product selectivity, and yield at this point. The results are shown in Table 1.

[0047] Subsequently, molten salt was loaded into the reactor shell of the second catalyst bed, which is in the initial contact with the material, and molten salt circulation was implemented. The catalyst bed temperature was then raised to 380℃ at a rate of 6℃ / hour. After the bed temperature stabilized, the n-butane throughput was increased to 90% of the design processing load at an increase rate of 290 kg / h. Phosphorus and water were added to the reaction system, with the water addition controlled at 3.0% of the total reactor feed volume, and the phosphorus concentration maintained at 10–25 ppm. Simultaneously, the conversion rate of n-butane at the reactor outlet was controlled to 84±2% by adjusting the molten salt temperature of the second bed or both beds. After stabilization, the reaction temperature of the first bed was 401.2℃, and the reaction temperature of the second bed was 395.8℃.

[0048] After the n-butane processing load reached 90% of the design load and the reaction stabilized, the subsequent n-butane processing load was gradually increased at an increase rate of 145 kg / week to the design processing load. The conversion rate of n-butane at the reactor outlet was controlled to 84±2% by adjusting the molten salt temperature of the second bed. After the start-up was completed, a long-term evaluation of 500 h was carried out. Then, the composition of the reaction products was analyzed by gas chromatography. The conversion rate and product selectivity of n-butane at this time, as well as the hot spot temperature of the reactor bed, are shown in Table 2.

[0049] Example 4

[0050] After the catalyst loading and airtightness check of the tubular fixed-bed reactor for the oxidation of n-butane to maleic anhydride are completed, the reactor temperature is preheated from room temperature to 170°C using air. During preheating, the hourly volumetric flow rate of air is controlled at 2000 times the total catalyst loading volume, and the reactor temperature rise rate is 4°C / hour. Subsequently, air purging is stopped, and nitrogen replacement is performed. The hourly volumetric flow rate of nitrogen is controlled at 1000 times the total catalyst loading volume. After nitrogen replacement is completed, molten salt is loaded into the reactor shell layer of the first catalyst bed that comes into contact with the material, and molten salt circulation is achieved. The catalyst bed temperature is then raised to 390°C at a temperature rise rate of 8°C / hour.

[0051] After the temperature of the first catalyst bed is constant, air is introduced into the reactor at a rate of 22800 kg / h, which is 30% of the designed processing load. Then, n-butane at 15% of the designed processing load is introduced to start the reaction at a rate of 435 kg / h. After the reaction stabilizes, the air is increased to the designed processing load at a rate of 3800 kg / h, while the n-butane flow rate is increased to 50% of the designed processing load at a rate of 145 kg / h. The n-butane conversion rate at the reactor outlet is controlled to be no more than 85% by adjusting the molten salt temperature. After the reaction stabilizes, the composition of the reaction products is analyzed by gas chromatography to obtain the n-butane conversion rate, product selectivity, and yield at this point. The results are shown in Table 1.

[0052] Subsequently, molten salt was added to the reactor shell of the second catalyst bed, which is in the initial contact with the material, to achieve molten salt circulation. The catalyst bed temperature was then raised to 380℃ at a rate of 8℃ / hour. After the bed temperature stabilized, the n-butane throughput was increased to 80% of the design processing load at an increase rate of 435 kg / h. Phosphorus and water were added to the reaction system, with the water volume controlled at 2.5% of the total reactor feed volume and the phosphorus concentration maintained at 10–25 ppm. Simultaneously, the conversion rate of n-butane at the reactor outlet was controlled to 84±2% by adjusting the molten salt temperature of the second bed or both beds. After stabilization, the reaction temperature of the first bed was 395.8℃, and the reaction temperature of the second bed was 393.0℃.

[0053] After the n-butane processing load reached 80% of the design load and the reaction stabilized, the subsequent n-butane processing load was gradually increased at a rate of 290 kg / week to the design processing load. The conversion rate of n-butane at the reactor outlet was controlled to 84±2% by adjusting the molten salt temperature of the second bed. After the start-up was completed, a long-term evaluation of 500 hours was carried out. Then, the composition of the reaction products was analyzed by gas chromatography. The conversion rate and product selectivity of n-butane at this time, as well as the hot spot temperature of the reactor bed, are shown in Table 2.

[0054] Comparative Example 1

[0055] The reaction was carried out using a tubular fixed-bed reactor of the same specifications as that of this invention, but with only one stage of molten salt. The total design processing load of the reactor was: a feed air flow rate of 76,000 kg / h and a feed n-butane flow rate of 2,900 kg / h. After catalyst loading and airtightness checks were completed, the reactor temperature was preheated from room temperature to 220°C using air. During air preheating, the hourly volumetric flow rate of air was controlled at 1,800 times the total catalyst loading volume, and the reactor heating rate was 8°C / hour. Subsequently, air purging was stopped, and nitrogen purging was performed. The hourly volumetric flow rate of nitrogen was controlled at 500 times the total catalyst loading volume. After nitrogen purging was completed, molten salt was loaded into the reactor shell to achieve molten salt circulation, and the catalyst bed temperature was raised to 385°C at a heating rate of 8°C / hour. After the temperature was constant, air was initially introduced into the reactor at 50% of the design processing load, followed by n-butane at 20% of the design processing load to initiate the reaction. After the reaction stabilized, the air was increased to the design processing load at a rate of 10% of the design processing load / h, while the amount of n-butane was increased to 50% of the design processing load at a rate of 8% of the design processing load / h. After the reaction stabilized, the composition of the reaction products was analyzed by gas chromatography to obtain the conversion rate of n-butane, product selectivity, and yield at this point. The results are shown in Table 1.

[0056] Phosphorus and water were initially added to the reaction system, with the water addition controlled at 3.0% of the total reactor feed volume, and the phosphorus concentration maintained at 10–25 ppm. Subsequently, the n-butane throughput was increased at a rate of 10% / h to 90% of the design processing load, while the conversion rate of n-butane at the reactor outlet was controlled at 80–90% by adjusting the molten salt temperature. After the n-butane throughput reached 90% of the design load and the reaction stabilized, the n-butane throughput was gradually increased at a rate of 5% / week to the design processing load, and the conversion rate of n-butane at the reactor outlet was controlled at 80–90% by adjusting the molten salt temperature. After startup, a 500-hour long-term evaluation was conducted, and the composition of the reaction products was analyzed by gas chromatography. The n-butane conversion rate, product selectivity, and reactor bed hot spot temperature results are shown in Table 2.

[0057] Comparative Example 2

[0058] The reaction was carried out using a tubular fixed-bed reactor of the same specifications as that of this invention, but with only one stage of molten salt. The total design processing load of the reactor was: a flow rate of 76,000 kg / h for the feed air and a flow rate of 2,900 kg / h for the feed n-butane. After catalyst loading and airtightness checks were completed, the reactor temperature was preheated from room temperature to 170°C using air. During air preheating, the hourly volumetric flow rate of air was controlled at 2,000 times the total catalyst loading volume, and the reactor heating rate was 4°C / hour. Subsequently, air purging was stopped, and nitrogen purging was performed. The hourly volumetric flow rate of nitrogen was controlled at 1,000 times the total catalyst loading volume. After nitrogen purging was completed, molten salt was loaded into the reactor shell to achieve molten salt circulation, and the catalyst bed temperature was raised to 390°C at a heating rate of 8°C / hour. After the temperature was constant, air was initially introduced into the reactor at 30% of the designed processing load, followed by n-butane at 15% of the designed processing load to initiate the reaction. After the reaction stabilized, the air was increased to the designed processing load at a rate of 5% of the designed processing load / h, while the amount of n-butane was increased to 50% of the designed processing load at a rate of 5% of the designed processing load / h. After the reaction stabilized, the composition of the reaction products was analyzed by gas chromatography to obtain the conversion rate of n-butane, product selectivity, and yield at this point. The results are shown in Table 1.

[0059] Phosphorus and water were initially added to the reaction system, with the water addition controlled at 2.5% of the total reactor feed volume, and the phosphorus concentration maintained at 10–25 ppm. Subsequently, the n-butane throughput was increased at a rate of 15% / h to 80% of the design processing load, while the conversion rate of n-butane at the reactor outlet was controlled at 80–90% by adjusting the molten salt temperature. After the n-butane throughput reached 80% of the design load and the reaction stabilized, the n-butane throughput was gradually increased at a rate of 10% / week to the design processing load, and the conversion rate of n-butane at the reactor outlet was controlled at 80–90% by adjusting the molten salt temperature. After startup, a 500-hour long-term evaluation was conducted, and the composition of the reaction products was analyzed by gas chromatography. The n-butane conversion rate, product selectivity, and reactor bed hot spot temperature results are shown in Table 2.

[0060] Table 1 presents the activity evaluation results of the first stage of start-up for the examples and comparative examples. Table 2 presents the activity evaluation results of the catalysts after start-up and operation for 500 hours in the examples and comparative examples.

[0061] Table 1

[0062]

[0063] As shown in Table 1, during the start-up phase, the butane conversion rate of this invention can be maintained at 84-89%, while that of the comparative example is as high as about 95%. This is because the butane concentration is low at the initial start-up stage. Under the same start-up molten salt conditions, the catalyst in the comparative example has more active sites, resulting in a faster reaction and a larger bed temperature rise. This makes it impossible to control the conversion rate at an optimal level, easily leading to excessively high conversion rates. It is well known to those skilled in the art that higher butane conversion rates result in lower selectivity, generate more heat, cause excessively high bed temperature rises, lead to rapid phosphorus loss from the catalyst, and affect the stability of subsequent reactions.

[0064] Table 2

[0065]

[0066]

[0067] As can be seen from the results in Tables 1 and 2, the start-up method of this invention exhibits high selectivity and yield of maleic anhydride in the initial stage, and produces CO2 as a byproduct. X The selectivity is low, and after the butane has been running at full load for a period of time, it is known that the catalyst of this start-up method has high selectivity and weight yield of maleic anhydride, and the hot spot temperature of the bed is low, which will effectively extend the catalyst's operating time.

Claims

1. A start-up method for the oxidation of n-butane to maleic anhydride, comprising: The reaction of n-butane oxidation to maleic anhydride is carried out in a tubular fixed-bed reactor. The catalyst is packed in the fixed bed. The upper and lower parts of the fixed-bed reactor are equipped with separate molten salt temperature control systems to achieve separate control of the temperature of the two catalyst beds. (1) After the fixed-bed reactor is filled with vanadium-phosphorus-oxygen catalyst, the reactor is preheated with air to raise the temperature to 170-220℃. Air purging is stopped and nitrogen replacement is carried out. Molten salt is first loaded into the reactor shell layer of the first catalyst bed that comes into contact with the material along the material flow direction to achieve molten salt circulation. The temperature of the catalyst bed is controlled to rise to 340-390℃. Raw material air is introduced. The initial amount of air introduced is 30%-50% of the design processing load. After the air stabilizes, n-butane is introduced as the raw material. The initial amount of n-butane introduced is 5%-30% of the design processing load. The air introduction rate is increased at a rate of 3-30% of the design processing load / h to reach the design processing load. The amount of n-butane is then increased to 30-50% of the design processing load at a rate of 3-15% of the design processing load / h. The n-butane conversion rate at the reactor outlet is controlled to be no greater than 88% by adjusting the reaction temperature. (2) The reactor shell is filled with molten salt for the second catalyst bed that comes into contact with the material to achieve molten salt circulation and control the catalyst bed temperature to 330-380℃; n-butane is gradually increased to 80-90% of the design processing load by increasing the rate of n-butane by 5-20% / h. The conversion rate of n-butane at the reactor outlet is controlled to 80-90% by adjusting the molten salt temperature of the second bed or both beds. The molten salt temperature of the second bed is controlled to be 1-8℃ lower than that of the first bed. While the n-butane load is increased, phosphorus and water are added to the reaction system. (3) After the processing load of n-butane reaches 80-90% of the design load and the reaction is stable, n-butane is gradually increased to the design processing load at a rate of 5-10% per week. The conversion rate of n-butane at the reactor outlet is controlled to 80-90% by adjusting the molten salt temperature of the second bed or both beds. The reaction conditions are adjusted to carry out the butane oxidation to maleic anhydride reaction within the design range.

2. The starting method according to claim 1, characterized in that, In step (1), the initial amount of n-butane introduced is 10%-25% of the designed processing load.

3. The starting method according to claim 1, characterized in that, In step (1), after the introduction of air and raw materials, the air introduction rate is increased at a rate of 5%-10% of the design processing load / h to reach the design processing load, and the amount of n-butane is increased to 30-50% of the design processing load at a rate of 3%-10% of the design processing load / h.

4. The starting method according to claim 1, characterized in that, In step (2), the n-butane is gradually increased from 8%-15% of the designed processing load / h to 80-90% of the designed processing load.

5. The starting method according to claim 1, characterized in that, In step (2), the temperature of the molten salt in the second bed is controlled to be 2-6℃ lower than that in the first bed.

6. The starting method according to claim 1, characterized in that, In step (1), when air preheating is used, the hourly volumetric flow rate of air is controlled to be 500-2000 times the total catalyst loading volume, and the heating rate of the reactor is 3-8℃ / hour.

7. The starting method according to claim 1, characterized in that, In step (1), when the first catalyst bed is heated, its heating rate is controlled to be 2-10℃ / hour; in step (2), when the second catalyst bed is heated, its heating rate is controlled to be 2-10℃ / hour.

8. The starting method according to claim 1, characterized in that, The phosphorus and water supplementation involves mixing the phosphorus source and water into the raw material and feeding them together. The phosphorus source used for phosphorus supplementation is at least one of trimethyl phosphate and triethyl phosphate.

9. The starting method according to claim 1, characterized in that, The total makeup water volume of the reactor is 1.5-4.5% of the total gas feed volume of the reactor, and the total phosphorus supplementation of the reactor is 2-50 ppm.

10. The starting method according to claim 1, characterized in that, The volume ratio of the first catalyst bed to the second catalyst bed is 1:4 to 1:

1.

11. The starting method according to claim 1, characterized in that, The reaction conditions for the oxidation of butane to maleic anhydride are as follows: reaction temperature 365-450℃, pressure 0.1-0.5MPa, and n-butane gas space velocity 600-2500h⁻¹. -1 The concentration of n-butane is 1.0-1.9% by volume.

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