Process for the gas recycling of maleic anhydride with high productivity and low carbon emissions

Abstract

A process and apparatus for the production of maleic anhydride by reacting a mixed gas containing molecular oxygen and a hydrocarbon, usually n-butane, over a suitable catalyst, wherein the maleic anhydride is produced in a tubular catalytic reactor, wherein after recovery of the maleic anhydride product by absorption with an organic solvent and after appropriate treatment steps, the gaseous effluent from the reactor is recycled back to the inlet of the catalytic reactor by compression.

Description

[0001] Overview This invention relates to a method for producing maleic anhydride by the catalytic oxidation of n-butane, characterized in that: a) Use pure oxygen as the oxidation medium; b) Extremely high yield and productivity; c) Generates an effluent stream rich in carbon dioxide, which can be used for carbon capture, utilization and storage (CCUS) or enhanced oil recovery (EOR). Technical Field

[0002] The embodiments disclosed herein generally relate to methods and apparatus for producing maleic anhydride by reacting a mixture of gases containing molecular oxygen and hydrocarbons (typically n-butane) over a suitable catalyst. More specifically, embodiments relate to the production of maleic anhydride in a tubular catalytic reactor, wherein after the maleic anhydride product is recovered by absorption with an organic solvent and after appropriate treatment steps, the gaseous effluent from the reactor is returned to the inlet of the catalytic reactor by compression and recycling.

[0003] Background and technical issues of the invention Maleic anhydride is an important raw material for the production of alkyd resins and unsaturated polyester resins. It is also a common intermediate for the chemical synthesis of butanediol (BDO), dimethyl succinate (DMS), γ-M butyrolactone (GBL), and tetrahydrofuran (THF). BDO can then be used alone or in combination with DMS to produce biodegradable plastics such as polybutylene terephthalate (PBAT) or polybutylene succinate (PBS).

[0004] It is produced by the partial oxidation of hydrocarbons (typically n-butane) over a vanadium-phosphorus-oxygen (VPO) catalyst contained in a tubular fixed-bed or fluidized-bed reactor. In both types of reactors, a large amount of exothermic reaction is removed along with the generation of steam.

[0005] In the reactor, except for the reaction of butane to maleic anhydride: C4H 10 + 3.5 O2→C4H2O3+ 4 H2O, Other side reactions will also occur, the main one being the combustion of butane to produce carbon monoxide and carbon dioxide: C4H 10 + 4.5 O2→4 CO+ 5 H2O, C4H 10 + 6.5 O2→4 CO2+ 5 H2O.

[0006] A small amount of butane is oxidized into organic acid byproducts, specifically acetic acid and acrylic acid.

[0007] All reactions are strongly exothermic. In a tubular fixed-bed reactor, the heat of reaction is appropriately removed by a circulating heat transfer medium (usually a mixture of molten salts), which then releases the heat into a steam generator.

[0008] In the conventional maleic anhydride method, the oxygen source is compressed air with an oxygen content slightly higher than 20 mol.

[0009] To reduce the risk of explosion, the reactor inlet gas typically contains only a small amount of feedstock, namely 1.5% to 2.0% butane by volume.

[0010] Given that the typical conversion of n-butane on VPO catalysts is 80% to 90%, this means that the maximum molar concentration difference of n-butane between the reactor inlet and outlet is in the range of 1.2% to 1.6%.

[0011] Unconverted butane is present in the reaction effluent. In a conventional one-through process, after maleic anhydride is recovered through absorption, the waste gas is sent to a thermal oxidizer. In the thermal oxidizer, both unconverted butane and carbon monoxide are burned into carbon dioxide, which is then released into the atmosphere after heat recovery.

[0012] Of course, incomplete conversion of butane not only leads to higher production costs, but also results in more carbon dioxide being released into the atmosphere.

[0013] US Patent 5,688,970 proposes to mitigate the above-mentioned drawbacks by recirculating a portion of the exhaust gas from the top of the absorber to the reactor inlet using a recirculating gas compressor, characterized by the use of a slightly higher pressure in the absorption of maleic anhydride and the exhaust gas recirculation.

[0014] While the method described in US 5,688,970 offers some economic and environmental advantages in principle, its effectiveness is limited by the oxygen content in the air. Considering that approximately 5 moles of oxygen are consumed per mole of n-butane, and that the efficiency of VPO catalysts decreases at lower oxygen concentrations under conventional process conditions, the maximum percentage of gas recirculation is limited to approximately 50% of the total gas, or 30-40% based on practical industrial experience, according to the patent text.

[0015] U.S. Patent 6,040,460 overcomes the aforementioned limitations by using oxygen or oxygen-enriched air as an oxidant. This patent teaches that by using butane concentrations above the explosion limit, by recovering butane from purge gas, and by using a carbon dioxide-rich gas source, the method provides excellent yields, higher productivity, reduced compression energy consumption, safer operation, and reduced environmental impact. However, in the examples and claims, the carbon dioxide concentration in the reaction mixture does not exceed 60% by volume.

[0016] Patent US 5,011,945 also relates to a high-yield and high-catalyst-productive process for maleic anhydride, which utilizes substantially pure oxygen and a circulating gas stream rich in carbon oxides, wherein the carbon monoxide concentration in the gas stream is higher than the carbon dioxide concentration. This method is further characterized by extremely high butane concentrations at the reactor inlet, low butane conversion per pass, the use of a VPO gradient catalyst, where reactivity is minimal closest to the reactor feed end, and the addition of a molybdenum-containing co-metal to the catalyst itself.

[0017] In this application, the circulating gas contains a high concentration of carbon monoxide, thus the method inherently carries the risk of runaway and deflagration at the inlet of the oxidation reactor.

[0018] US 4,987,239, US 5,126,463, US 5,179,215, US 5,262,547, US 5,278,319 and US 5,532,384 from BOC Group represent a series of applications dedicated to using pure oxygen or oxygen-enriched air in oxidation processes, including the production of maleic anhydride.

[0019] In particular, US 5,126,463 relates to a method for producing anhydrides, wherein carbon monoxide, present as a byproduct in the reactor effluent, is oxidized to carbon dioxide, and a portion of the gaseous effluent (mainly containing carbon dioxide) is recycled back to the reactor. However, this application is based solely on experimental tests of the selective oxidation of carbon monoxide to carbon dioxide, and the material balance of the maleic anhydride reaction is based on computer simulations, extrapolating laboratory reaction data to different reaction conditions, where the data becomes invalid. Specifically, this application neglects aspects related to extremely low selectivity, where, in the reaction of n-butane to maleic anhydride, when the oxygen content in the reaction gases is very low, primarily light organic acids are generated.

[0020] In all of the aforementioned applications and other related applications, the environmental impact of the large amounts of carbon dioxide emissions associated with maleic anhydride production has been ignored or at most mitigated by minor improvements in the efficiency of the proposed processes.

[0021] Therefore, it is desirable to provide new or improved methods to enable the industrial production of maleic anhydride from n-butane and oxygen in high yield and high catalyst / reactor productivity, while limiting or better avoiding any CO2 emissions into the environment.

[0022] In fact, the production of maleic anhydride by oxidation of n-butane is quite complex, especially when pure oxygen is used as the oxidation medium, which presents some problems and drawbacks.

[0023] First, while industrial experience with air is now widespread and solidified, knowledge of the use of different oxidizing gases remains quite limited, and industrial production is carried out by individual companies only for a specific period of time.

[0024] In fact, all conventional processes using single-pass air consume a significant excess of oxygen relative to the total chemical consumption of O2 during operation. Typical oxygen content at the reactor outlet exceeds 12.5 mol%. Only 34%–35% of the oxygen entering the reactor is consumed during the reaction. Clearly, the VPO catalyst operates under conditions of significant oxygen excess throughout the entire length of the tubular reactor. Under these conditions, oxygen does not limit the reaction; it is merely a secondary parameter defining catalyst performance.

[0025] Conversely, in operations that employ gas circulation and use pure oxygen, operations are typically conducted with a minimal excess of oxygen due to the cost of oxygen itself.

[0026] Under conditions of low oxygen concentration, the inventors observed a significant increase in the production of light organic acids, particularly acetic acid.

[0027] The relevant stoichiometry for this reaction is as follows: C4H 10 + 2.5 O2→2 C2H4O2+ H2O.

[0028] As detailed in Comparative Example 2, data from a large industrial reactor (with over 30,000 tubes with an inner diameter of 21 mm and a length of 6 meters) showed that when the oxygen concentration at the reactor outlet was 2.5 mol%, the production of acetic acid was exceptionally high, four times or more higher than when using air and a large amount of excess oxygen.

[0029] The second aspect of the maleic anhydride production process involves the introduction of a converter that converts carbon monoxide into carbon dioxide, under conditions of gas recirculation and the generation of purge gas with a high concentration of carbon dioxide (for carbon capture, utilization and storage (CCUS)).

[0030] All maleic anhydride units using VPO catalysts produce some amount of carbon monoxide and carbon dioxide as undesirable byproducts. In air-based operations, carbon monoxide production is consistently higher than carbon dioxide production, with a CO / CO2 molar ratio of approximately 1.25.

[0031] Another surprising finding of this application is that the ratio of carbon monoxide to carbon dioxide produced in the maleic anhydride catalytic reactor is almost independent of the absolute concentration of these oxides and also independent of the CO / CO2 concentration ratio at the reactor inlet. In other words, regardless of the contribution of pure oxygen to the reaction, the molar ratio of oxides produced remains unchanged between single-pass air operation and recirculation operation.

[0032] Of course, based on the above statements, without introducing a CO converter, the circulating gas and purge gas from the device should be a mixture with a higher carbon monoxide content than carbon dioxide.

[0033] The difference between CO and CO2 is substantial in terms of the potential flammability of the reactant gases. In fact, carbon monoxide is flammable, with a very wide flammability concentration range, a lower flammability limit of 12 vol%, an upper flammability limit of 70 vol%, and a very low limiting oxygen concentration (LOC, the point at which combustion is impossible regardless of fuel concentration), equal to 5.5 vol%. All these values ​​refer to atmospheric pressure, 25°C, and air or air / nitrogen mixtures. Based on literature prediction methods used to calculate the flammability of various fuels and inert mixtures, the presence of both carbon monoxide and n-butane at the reactor inlet must be considered when assessing explosion risk.

[0034] In contrast, carbon dioxide is an inert gas; according to literature on fire dynamics, increasing the concentration of inert gases can be understood as treating them as thermal ballasts, which can quench the flame temperature to levels below which a flame cannot exist. Therefore, carbon dioxide is more effective as an inert medium than nitrogen because its molar heat capacity is more than 50% higher than that of nitrogen.

[0035] Therefore, the introduction of a converter that selectively converts carbon monoxide to carbon dioxide not only reduces the concentration of fuel (C4+CO) in the mixture, but also introduces a large amount of the compound, namely carbon dioxide, which has a much higher inertizing strength than nitrogen, the main inert gas present in conventional methods for producing maleic anhydride using air.

[0036] In summary, the introduction of the carbon monoxide selective converter is an important component that not only allows for the capture of carbon from the purge gas, but also provides safer reaction conditions with high concentrations of carbon dioxide and enables higher catalyst productivity.

[0037] Regarding the topic of selective oxidation of carbon monoxide, US2013 / 0131380 states that in the maleic anhydride process under gas recirculation conditions, the main problem to be solved is to convert CO to CO2 in a hydrocarbon-rich (butane) feed stream without oxidizing these hydrocarbons. With a significant increase in reaction temperature and a significant increase in the conversion rate of hydrocarbons present in the feed stream, the reaction can rapidly run out of control. Because the heat of combustion of butane is much higher than that of carbon monoxide, it is quite easy to initiate extremely dangerous and uncontrollable reactions.

[0038] The inventors of the aforementioned patent proposed solving this problem by using a fluidized bed reactor, which enables efficient heat transfer, uniform temperature of the catalyst within the reactor, and safe operation.

[0039] However, using fluidized beds is a complex, expensive, and difficult-to-operate solution.

[0040] On the same topic, US2005 / 0032628 proposes the use of noble metals Pt or Pd on a silica support with a continuous coating of molecular sieve material, which significantly increases the temperature interval between carbon monoxide oxidation and butane oxidation, up to 250°C or higher, compared to only 50°C for conventional catalysts without coating.

[0041] While the results are very interesting, there are also problems with the industrial production of such catalysts.

[0042] Based on the foregoing, there is a need for new methods and apparatus, which constitute the subject of this application for the production of maleic anhydride, wherein the apparatus configuration and operating parameters can simultaneously achieve safe operation, extremely high reaction yield and catalyst productivity, and produce a carbon dioxide-rich exhaust stream that can be used for carbon capture, utilization (e.g., for the food industry or as a structural unit for urea production) and storage (CCUS) or enhanced oil recovery (EOR). Summary of the Invention

[0043] The present invention aims to provide an improved and efficient method for producing maleic anhydride from n-butane, characterized by: (i) using pure oxygen as the oxidation medium; (ii) high yield and high catalyst productivity; and (iii) producing a carbon dioxide-rich exhaust stream that can be used for carbon capture, utilization and storage (CCUS) or enhanced oil recovery (EOR).

[0044] As described in the background section, in air-based operations, catalyst productivity is limited by safety considerations; the concentration difference of n-butane between the reactor tube inlet and outlet is at most in the range of 1.2 vol% to 1.6 vol% by volume.

[0045] To overcome this limitation, various existing methods propose operating at butane concentrations above the upper limit of flammability, accompanied by the presence of inert gases with high specific heat. This expands the safety conditions at the reactor inlet and allows for better management of the highly exothermic nature of the reaction.

[0046] Against this backdrop, the applicant company discovered that, as a logical consequence, reaction-driven operations conducted with high concentrations of reactant butane resulted in lower oxygen concentrations at the reactor outlet and an unexpected increase in the production of light organic acids, particularly acetic acid.

[0047] The problem this invention aims to solve is to find a solution that can improve catalyst productivity while limiting the increase in the production of light acids. This solution can also selectively convert carbon monoxide to carbon dioxide under conversion conditions that prevent runaway reactions, abnormal temperature increases, and oxidation of hydrocarbons present in the gaseous mixture.

[0048] As proposed in this invention, by using a tubular reactor with a selective oxidation catalyst, combined with diluting the selective catalyst with an inert material, and controlling the oxygen content at the CO converter inlet, effective cooling of the reactor can be achieved, thereby stabilizing and effectively controlling the reaction.

[0049] The objective of this invention is achieved by a method comprising the following steps: (a) A reaction gas mixture is fed into a tubular reactor unit containing a vanadium-phosphorus oxide catalyst, wherein the gas mixture comprises a vaporized stream rich in n-butane at a concentration above the corresponding upper limit of flammability in a highly inert atmosphere, a stream of high-purity oxygen, and a stream of carbon dioxide rich in gas that is recycled to the reactor via a gas compressor. (b) In the reaction tube of the tubular reactor unit, maleic anhydride is produced as the main product, carbon oxides and light organic acids as undesirable byproducts, and reaction water is produced. (c) Cooling the exhaust gas leaving the tubular reactor unit at a temperature of 380°C to 450°C by recovering the heat of reaction to generate steam at a single pressure value or at a reduced pressure value in multiple heat exchangers in series. (d) By using an organic solvent as the absorbent, maleic anhydride contained in the reaction gas is selectively recovered in an absorber in the form of a high-efficiency absorption tower. (e) Cool and wash the exhaust gas from the absorber with water to remove trace amounts of maleic anhydride and organic acid byproducts that were not removed from the absorber, thereby protecting the integrity of the gas compressor. (f) Purge a portion of the exhaust gas that has been washed with water to prevent the accumulation of inert substances and send it to the first thermal oxidizer, the first regenerative oxidizer or the first catalytic oxidizer to meet local regulations for atmospheric emission limits of carbon monoxide, hydrocarbons and nitrogen oxides. (g) The washed gas mixture is recirculated through a gas compressor to increase its pressure to the reaction value; (h) 35% to 60% of the compressed gas is fed into a second selective oxidizer, in which a suitable selective catalyst, along with controlled temperature and controlled oxygen content, promotes the safe conversion of most carbon monoxide into carbon dioxide, while essentially not oxidizing unreacted n-butane contained in the recycle gas. (i) The combined carbon dioxide-rich gas mixture is used as a diluent for new n-butane and pure oxygen and fed into the maleic anhydride tubular reactor unit.

[0050] As a preferred embodiment of the present invention, instead of purging and discharging the partially washed exhaust gas into the atmosphere as described in items (f) to (i) above, it is provided in the following items (f') to (f'): (f') The washed gas mixture is recirculated through a gas compressor to increase its pressure to the reaction value; (g') 40% to 60% of the compressed gas is fed into a selective oxidizer, where a suitable selective catalyst, along with controlled temperature and controlled oxygen content, promotes the safe conversion of most carbon monoxide into carbon dioxide, while essentially not oxidizing unreacted n-butane contained in the recycle gas. (h') purge some of the waste gas from the carbon monoxide converter to avoid the accumulation of inert substances, and combine the remaining part with the compressed gas that bypasses the CO converter; (i') The combined carbon dioxide-rich gas mixture is used as a diluent for new n-butane and pure oxygen and fed into the maleic anhydride tubular reactor unit. (j') The purge gas in item (h') is used as a carbon dioxide source and, after appropriate compression and purification steps, is used for carbon capture, utilization and storage (CCUS) or enhanced oil recovery (EOR). The method is described based on its unique characteristics as follows: i. Before injecting new butane and pure oxygen, the recirculated dilution gas is rich in carbon dioxide with a concentration higher than 85%; ii. The n-butane content at the inlet of the maleic anhydride reactor unit is controlled within 4% to 6%, and its conversion rate in the reactor is 35% to 50%; iii. The oxygen content at the inlet of the tubular reactor unit is controlled between 12 mol% and 16 mol%; iv. The mixture of butane, carbon dioxide, carbon monoxide, and water at the inlet of the tubular reactor unit is outside its flammability limit at its temperature and pressure; and v. Control the oxygen content at the outlet of the tubular reactor unit to a value below the limiting oxygen concentration (LOC, which is the percentage of oxygen below which the mixture is non-flammable regardless of the fuel concentration), but not below 4.0 mol.

[0051] Other aspects and advantages of the invention will become apparent from the following drawings, detailed description, non-limiting embodiments and appended claims. Attached Figure Description

[0052] The invention can be better understood through the accompanying drawings, which represent simplified versions of the prior art and the present invention; therefore, the scope of the drawings is only for clarification and the drawings themselves do not represent the full meaning of the invention. Figure 1 illustrates the conventional production processes for two different configurations of maleic anhydride, as follows: Figure 1A This is a block diagram illustrating the production of maleic anhydride from n-butane and air in a typical single-pass operation; Figure 1B This is a block diagram illustrating the production of maleic anhydride from n-butane and air or oxygen-enriched air in a partial exhaust gas recirculation unit. Figure 2 This is a block diagram of the method according to the present invention, in a full gas recirculation operation, using pure oxygen and using a carbon monoxide selective oxidation reactor, wherein the purge gas is discharged into the atmosphere after treatment; Figure 3 This is a block diagram of a preferred embodiment of the present invention, in a full gas recirculation operation, using pure oxygen and employing a carbon monoxide selective oxidation reactor, wherein purge gas is used for carbon capture. Detailed Implementation

[0053] Global demand for sustainable plastics, including biodegradable plastics, along with carbon neutrality goals, is changing the market landscape and production methods for maleic anhydride.

[0054] First, the maleic anhydride route has already taken the lead in butanediol production, and butanediol is one of the most important raw materials for novel bioplastics, particularly polybutylene terephthalate (PBAT). Furthermore, butanediol is combined with another maleic anhydride derivative, such as dimethyl succinate (DMS), to produce polybutylene succinate (PBS). The implied demand in the plastics market is entirely different in scale from the previous maleic anhydride market. Therefore, the production capacity of current and likely future maleic anhydride projects is typically much larger than it has been in recent years.

[0055] The second aspect relates to actual carbon neutrality goals, which presents unprecedented opportunities for the green or low-carbon transformation of the plastics industry today.

[0056] Unfortunately, existing technologies for producing maleic anhydride generate significant amounts of carbon dioxide emissions into the atmosphere due to their inherently low selectivity. This invention addresses the widespread need for a novel method that teaches an industrial-scale production of maleic anhydride from n-butane and oxygen in high yields and with high catalyst / reactor productivity, while limiting or better avoiding any carbon dioxide emissions into the environment.

[0057] Regarding improving catalyst and reactor productivity, this application proposes operating with the n-butane concentration at the reactor inlet above its upper flammability limit to achieve a significantly higher n-butane concentration gradient between the inlet and outlet of the maleic anhydride catalytic reactor than conventional methods. The novelty and inventiveness of this application lie in the incomplete utilization of oxygen used in the reaction, with the aim of limiting the production of light organic acids to economically acceptable levels. According to conventional practice in highly exothermic tubular reactors, oxygen conversion is controlled by adjusting the temperature of the heat transfer medium (i.e., the molten salt, typically a eutectic mixture of potassium nitrate, sodium nitrate, and nitrite) and recirculating it in the shell side of the tubular reactor.

[0058] In more precise terms, in the method according to the invention, considering that operation at high butane concentrations and oxygen concentrations below 20% favors high oxygen conversion, which would produce undesirable high levels of light organic acids, the catalyst temperature is controlled at a value lower than that of conventional methods, and the oxygen concentration at the reactor outlet is intentionally controlled below the limiting oxygen concentration (LOC, i.e., below which the mixture is non-flammable regardless of fuel concentration), but in any case not less than 4.0 mol.

[0059] The catalyst temperature depends on many factors, including the type of catalyst and, in particular, its degree of aging. With the same catalyst type and degree of aging, the average temperature will be 4°C to 12°C lower than conventional methods with lower n-butane inlet concentrations. This lower catalyst temperature represents another advantage of this method, as catalyst lifetime is generally longer when operating at lower temperatures.

[0060] Regarding the need for a method to avoid the release of carbon dioxide into the atmosphere, this new method first requires the use of oxygen instead of air as the oxidation medium, thereby avoiding the introduction of large amounts of nitrogen into the reaction loop. Furthermore, the method includes the use of a selective catalytic conversion reactor in the gas circulation loop to selectively oxidize carbon monoxide produced in the maleic anhydride reactor to carbon dioxide.

[0061] In this manner, (i) by avoiding the introduction of any nitrogen into the system; (ii) by selectively oxidizing carbon monoxide to carbon dioxide; (iii) by separating maleic anhydride in a solvent absorber; and (iv) by separating most of the reaction water and light organic acids in a scrubbing tower, the recycle gas will contain a high concentration of carbon dioxide. Therefore, the purge gas corresponding to the net generation of inert matter in the maleic anhydride reactor will also have a high concentration of CO2. This purge gas, instead of being released into the environment as in prior art methods, can be used for carbon capture, utilization, and storage after purification and compression steps adapted to the final application.

[0062] In existing methods, carbon dioxide emissions into the atmosphere are partly generated in the maleic anhydride reactor due to undesirable side reactions. Another, or even more significant, portion is generated in the thermal oxidizer, a result of the combustion of residual butane with additional fuel gases used to control combustion efficiency. As shown in Examples 1 and 4, another advantage provided by the present invention is a substantial reduction in total carbon dioxide production, attributed to the significant reduction or elimination of the portion of CO2 produced by the combustion of residual butane and fuel gases.

[0063] According to the method of the present invention, controlling the oxygen content at the outlet of the maleic anhydride reactor allows for the selective conversion of carbon monoxide into carbon dioxide. Under these conditions, runaway reactions, abnormal temperature increases, and oxidation of hydrocarbons present in the gaseous mixture can be avoided.

[0064] As described in more detail in the appended Example 4, the CO converter has a partial self-protection feature under controlled oxygen concentrations because even under runaway conditions, the oxygen content is only sufficient to burn a portion of the residual butane.

[0065] As proposed in this invention, the use of a tubular reactor with a selective oxidation catalyst, combined with dilution of the catalyst with an inert material and control of the oxygen content at the CO converter inlet, enables effective cooling of the reactor and stable and efficient control of the reaction.

[0066] The two main features of this invention—the control of oxygen content at the maleic anhydride reactor outlet and the use of a carbon monoxide selective oxidizer—cannot simply represent two independent aspects of the method, which are determined by different scopes. Rather, they should be considered as a whole with synergistic effects. On the one hand, the high concentration of carbon dioxide obtained through the carbon monoxide (CO) selective oxidizer provides safer conditions at the maleic anhydride reactor inlet, thus achieving, to some extent, higher oxygen concentrations and higher catalyst productivity. On the other hand, the control of oxygen concentration at the maleic anhydride reactor outlet and in the recirculated gas provides the minimum conditions for proper and safe carbon monoxide oxidation.

[0067] Figure 1 to Figure 3 The block diagram in the figure illustrates the method of the present invention and also shows the difference from the prior art method.

[0068] For clarity, Figure 1A , Figure 1B , Figure 2 and Figure 3 The same naming convention was used, as follows: Numbers that are multiples of 10 represent specific equipment or process units: 10 is a gas compressor used to compress fresh air or gas recirculation streams or mixtures of air and recirculated gas; 20 is a maleic anhydride tubular reactor using VPO catalyst; 30 is a cooling device used to recover heat from reactor effluent at temperatures above 400°C, up to 130°C to 180°C, and to generate high-pressure and / or medium-pressure steam. 40 is the absorption / stripping system, or simply absorber, used to recover maleic anhydride generated in the reactor and for its purification; 50 indicates the first oxidizer, namely a thermal oxidizer, regenerative oxidizer, or catalytic oxidizer, which is used to burn the residual amount of carbon monoxide and hydrocarbons contained in the exhaust gas or purge gas before releasing them into the atmosphere; 60 is a water cooling and scrubbing system for the recirculated exhaust gas from the absorber. Its purpose is to remove small amounts of maleic anhydride and organic acid byproducts that have not been removed from the absorber, thereby protecting the gas compressor. 70 indicates a second oxidizer, or selective oxidizer, in which a suitable selective catalyst, along with controlled temperature and controlled oxygen content, promotes the safe conversion of most carbon monoxide into carbon dioxide, while essentially not oxidizing unreacted n-butane contained in the recycle gas. Other numbers represent process material flows, as shown below: 1 is fresh air used as an oxygen source; 2 is a supplement to vaporized n-butane; 3 is the maleic anhydride flow produced; 4 is the purge gas emitted from the device and released into the atmosphere after treatment; 5 refers to the gas discharged from the compressor, air, or recirculated gas; 6 is the inlet gas of the maleic anhydride reactor; 7 is the outlet gas from the maleic anhydride reactor; 8 is the exhaust gas after cooling of the maleic anhydride reactor; 9 is the gas at the top of the maleic anhydride absorber; 11 is the section of gas that is purged to the oxidizer before being released into the atmosphere; 12. In operations with partial gas recirculation, the top gas portion is recirculated; 13 is the recirculated gas after being cooled and washed with water; 14 is the gas at the inlet of the recirculation compressor; 15 is fresh oxygen used as an oxidizing agent; 16 is a gas that bypasses the carbon monoxide converter; 17 is the inlet gas of the CO converter reactor; 18 is the outlet gas of the CO converter reactor; 19 is the gas recycled to the maleic anhydride reactor; 21 is the part of the CO converter that is recycled back to the maleic anhydride reactor; 22 is the part of the CO converter that has been purified and used as a CO2 source; Figure 1A This is a block diagram illustrating the production of maleic anhydride from n-butane and air in a typical single-pass operation. It represents, in a simplified form, the apparatus used in most existing maleic anhydride plants worldwide.

[0069] Fresh air 1 enters compressor 10, where its pressure is increased to balance the pressure drop across the maleic anhydride reactor and all other equipment and piping in the system until exhaust gases are released into the atmosphere. Before entering the reactor, compressed air 5 merges with vaporized and superheated n-butane stream 2. A mixture stream 6 (with a C4 concentration close to the lower flammability limit), controlled by an online analyzer (not shown), enters the tubular maleic anhydride reactor 20. Here, reaction conditions are controlled based on the results of an online analyzer at the outlet (not shown) and the catalyst temperature distribution along the pipeline, using molten salt temperature as the primary variable parameter. Reactor effluent 7, with temperatures between 400°C and 440°C, enters cooling system 30, which comprises multiple heat exchangers arranged in series / parallel.

[0070] The cooled reaction gas 8 (comprising nitrogen as the main component, plus residual oxygen, carbon oxides, maleic anhydride, organic acids, and reaction water) enters the absorption / stripping system or absorber 40 to recover the maleic anhydride produced in the reactor and for its purification. Material stream 3 represents the purified maleic anhydride product from the unit. Gas 9 from the absorption section (containing a certain amount of unconverted butane, carbon monoxide, and a small concentration of maleic anhydride and other organic acids in addition to nitrogen, carbon dioxide, and water) is sent to a thermal oxidizer, regenerative oxidizer, or catalytic oxidizer 50, and then discharged into the atmosphere (material stream 4).

[0071] Figure 1B This is a block diagram illustrating the production of maleic anhydride from n-butane and air or oxygen-enriched air in a partially recirculated exhaust gas operation. It represents, in a simplified form, the equipment used in the few existing maleic anhydride plants worldwide.

[0072] Fresh air 1, along with recirculated airflow 13, enters compressor 10 as feed stream 14, where its pressure is increased to balance the pressure drop across the maleic anhydride reactor and all other equipment and piping in the system until exhaust gases are released into the atmosphere. Before entering the reactor, compressed gas mixture 5 is combined with a high-purity oxygen stream 15 (optional), and then with a vaporized and superheated n-butane stream 2. The feed stream 6, controlled by an online analyzer (not shown), enters tubular maleic anhydride reactor 20, with a C4 concentration close to the lower flammability limit. Here, reaction conditions are controlled based on the results of an online analyzer at the outlet (not shown) and the catalyst temperature distribution along the pipeline, using molten salt temperature as the primary variable parameter. Reactor exhaust gas 7, with temperatures between 400°C and 440°C, enters cooling system 30, which comprises multiple heat exchangers arranged in series / parallel.

[0073] Cooled reaction gases 8 (including nitrogen as the main component, plus residual oxygen, carbon oxides, maleic anhydride, organic acids, and reaction water) enter the absorption / stripping system or absorber 40 to recover and purify the maleic anhydride produced in the reactor. Material stream 3 represents the purified maleic anhydride product from the unit. Gas 9 from the absorption section 40 (containing a certain amount of unconverted butane, carbon monoxide, and a small concentration of maleic anhydride and other organic acids) is recycled as material stream 12 to reactor 20 via water cooling and scrubbing system 60 and gas compressor 10; the remainder, as material stream 11, is sent to thermal oxidizer, regenerative oxidizer, or catalytic oxidizer 50, and then discharged into the atmosphere as material stream 4.

[0074] Specifically, the recirculated gas 12 enters the water-cooled and scrubbing tower 60, where small amounts of maleic anhydride and organic acid byproducts are absorbed by the water, thereby protecting the gas compressor from corrosion. The scrubbed gas 13 represents the recirculated gas that merges with fresh air at the suction inlet of the compressor 10.

[0075] Figure 2 The block diagram of the improved method according to this application is in a full gas recirculation operation, using pure oxygen and a carbon monoxide selective oxidation reactor, wherein the purge gas is discharged into the atmosphere after treatment.

[0076] The recirculating gas stream 14, primarily containing carbon dioxide, enters compressor 10, where its pressure is increased to balance the pressure drop across the maleic anhydride reactor and all other equipment and piping in the system until the exhaust gas is released into the atmosphere. A portion of the compressed gas 5, as feed stream 17, enters selective oxidizer 70, where, using a suitable selective catalyst, coupled with controlled temperature and controlled oxygen content, the majority of carbon monoxide is safely converted to carbon dioxide, while the unreacted n-butane contained in the recirculating gas in the maleic anhydride reactor remains largely unoxidized. Feed stream 16 represents the portion of compressed gas that bypasses selective carbon monoxide oxidizer 70. As a general rule, any increase in the portion of gas bypassing the CO converter leads to an increase in the CO concentration in the recirculating gas, thus making the conversion reaction more unstable and prone to runaway conditions. Conversely, the size and capital cost of the conversion reactor will increase with a decrease in the bypassed feed stream. According to the method of the invention, the maximum percentage of bypassed feed is 65% of the total recirculating gas. The effluent 18 from the CO converter merges with the bypassed material stream 16; before entering the reactor, the mixture stream 19 merges with a high-purity oxygen stream 15, and then with a vaporized and superheated n-butane stream 2. The mixture stream 6, controlled by an online analyzer (not shown), enters the maleic anhydride tubular reactor 20, where the C4 concentration of this mixture stream exceeds the upper limit of flammability in an atmosphere containing a high concentration of carbon dioxide.

[0077] Here, reaction conditions are controlled based on the online analyzer at the outlet (not shown) and the catalyst temperature distribution along the pipeline, using molten salt temperature as the main variable parameter to maintain the oxygen concentration at the outlet above the preset value and not lower than 4 vol%. The oxidation reactor operates at 1400 h⁻¹. -1 Up to 2500h -1 The reactor operates within a range of volumetric hourly space velocities (GHSV), chosen to balance productivity and gas pressure drop. The oxidation catalyst is a vanadium-phosphorus mixed oxide (VPO) type catalyst. Specifically, catalysts developed to minimize the formation of light organic acids are preferred, such as Syndane 4122 LA or 4142 LA produced by Clariant. The reactor pressure can be selected from 1 bar g to 5 bar g, chosen to balance capital and operating costs.

[0078] Reactor effluent 7, with a temperature between 390°C and 440°C, enters a cooling system 30, which includes multiple heat exchangers arranged in series or parallel.

[0079] The cooled reaction gas 8 (comprising carbon dioxide as the main component, plus residual oxygen and n-butane, some carbon monoxide, maleic anhydride, organic acids, and reaction water) enters the absorption / stripping system or absorber 40 at a temperature of 130°C to 180°C to recover the maleic anhydride produced in the reactor and for its purification. Here, an organic solvent, typically dibutyl phthalate (DBP), is used as the selective absorbent for maleic anhydride in a closed-loop configuration. Material stream 3 represents the purified maleic anhydride product from the unit; its purity can be controlled at 99.8% to 99.9%, or even higher. Gas 9 from the absorption section (containing a certain amount of unconverted butane, carbon monoxide, and a small concentration of maleic anhydride and other organic acids) is fed into the cooling and scrubbing tower 60 at a temperature of 70°C to 75°C, where the organic acids are absorbed by water, and some of the reaction water is condensed. Gas 13 from the scrubbing tower is partially recycled to the reactor as material stream 14 via the recirculating compressor 10 at a temperature of 35°C to 45°C; the remaining portion is sent as material stream 11 to a thermal oxidizer, regenerative oxidizer, or catalytic oxidizer 50, and then discharged into the atmosphere as material stream 4.

[0080] Figure 3 This is a block diagram of a preferred embodiment of the present invention, in a full gas recirculation operation, using pure oxygen and employing a carbon monoxide selective oxidation reactor, wherein purge gas is used for carbon capture.

[0081] A recirculating gas stream 14, primarily containing carbon dioxide, enters compressor 10, where its pressure is increased to balance the pressure drop across maleic anhydride reactor 20 and all other equipment and piping in the system until the exhaust gas is released into the atmosphere. A portion of the compressed gas 5, as feed stream 17, enters selective oxidizer 70, where, using a suitable selective catalyst, coupled with controlled temperature and controlled oxygen content, the majority of carbon monoxide is safely converted to carbon dioxide, with minimal oxidation of unreacted n-butane contained in the recirculated gas; feed stream 16 represents the portion of compressed gas that bypasses selective carbon monoxide oxidizer 70. As a general rule, any increase in the portion of gas bypassing CO converter 70 leads to an increase in CO concentration in the recirculated gas, thus making the conversion reaction more unstable and prone to runaway conditions. Conversely, the size and capital cost of conversion reactor 70 will increase with a decrease in the bypassed feed stream. According to the method of the invention, the maximum percentage of bypassed feed is 60% of the total recirculated gas. The exhaust 18 from CO converter 70 is divided into two parts: the first part 21 merges with the bypass material stream 16, and the mixed material stream 19 represents the gas recirculated to the reactor; the second part, as material stream 22, represents the carbon dioxide-rich net purge gas for carbon capture and utilization.

[0082] Depending on the end use, it should undergo purification steps (not shown in the figure) to reduce the negative impact of some impurities on pipeline transportation, geological storage, and / or enhanced oil recovery.

[0083] Before entering reactor 20, the mixed recirculated material stream 19 merges with the high-purity oxygen stream 15, and then with the vaporized and superheated n-butane stream 2. The mixed material stream 6, controlled by an online analyzer (not shown), enters the maleic anhydride tubular reactor 20 in an atmosphere containing a high concentration of carbon dioxide, where the C4 concentration of the mixed material stream exceeds the upper limit of flammability. Here, reaction conditions are controlled based on the outlet online analyzer (not shown) and the catalyst temperature distribution along the pipeline, using the molten salt temperature as the primary variable parameter to maintain the outlet oxygen concentration above a preset value and not less than 4 vol%. The oxidation reactor operates at 1400 h⁻¹. -1 Up to 2500 h -1 The reactor operates within a range of volumetric hourly space velocities (GHSV), chosen to balance productivity and gas pressure drop. The oxidation catalyst is a vanadium-phosphorus mixed oxide (VPO) type catalyst. Specifically, catalysts developed to minimize the formation of light organic acids are preferred, such as Syndane 3122 LA or 3142 LA produced by Clariant. The reactor pressure can be selected from 1 bar g to 5 bar g, chosen to balance capital and operating costs.

[0084] Reactor effluent 7, with a temperature between 390°C and 440°C, enters a cooling system 30, which includes multiple heat exchangers arranged in series or parallel.

[0085] The cooled reaction gas 8 (comprising carbon dioxide as the main component, plus residual oxygen and n-butane, some carbon monoxide, maleic anhydride, organic acids, and reaction water) enters the absorption / stripping system 40 at a temperature of 130°C to 180°C to recover the maleic anhydride produced in the reactor and for its purification. Here, an organic solvent, typically dibutyl phthalate (DBP), is used as the selective absorbent for maleic anhydride in a closed-loop configuration. Specifically, a purge stream 22 still containing a certain amount of unconverted n-butane can be sent to an absorption tower (not shown), where lean organic solvent from the maleic anhydride vacuum stripper (included in process unit 40) can be used to recover a large portion of this n-butane by absorption. The solvent from this tower is then used in the main maleic anhydride absorber of unit 40, where n-butane is desorbed and recovered to the recirculated gas stream 9 due to the lower operating pressure and contact with the exhaust gas from the maleic anhydride reactor. In this way, the n-butane (n-C4) absorber achieves a dual purpose: reducing butane consumption and realizing the first step of purification of carbon dioxide-rich gas to be utilized or stored.

[0086] Material stream 3 represents the purified maleic anhydride product from the unit; its purity can be controlled at 99.8% to 99.9%, or even higher. Gas 9 from the absorption section (containing a certain amount of unconverted butane, carbon monoxide, and a small concentration of maleic anhydride and other organic acids) is fed into the cooling and scrubbing tower 60 at a temperature of 70°C to 75°C, where the organic acids are absorbed by water and some of the reaction water is condensed. Gas 14 from the scrubbing section is recycled back to the reactor at a temperature of 35°C to 45°C via the recirculation compressor 10.

[0087] The present invention has been described in great detail through the following embodiments, but in any case, these embodiments should not be construed as limiting the scope of the invention itself or its implementation.

[0088] Comparative Example 1 An industrial-grade maleic anhydride factory according to Figure 1B The proposed solution uses fresh air without adding oxygen and operates under partial exhaust gas recirculation conditions.

[0089] The reactor at 1800 h -1 Operating under the gas volume spacetime velocity.

[0090] The reactor tube has an inner diameter of 21 mm and a length of 6 meters. Reactor cooling and control are achieved through the circulation of molten salt.

[0091] The catalyst was Clariant's SYNDANE 3122 LA type catalyst.

[0092] After cooling, the reactor effluent is fed into a high-efficiency absorber, where dibutyl phthalate is used as a selective organic solvent to recover maleic anhydride. The exhaust gas from the top of the absorber is scrubbed with water and then partially recirculated back into the reactor via a centrifugal compressor. The portion of the exhaust gas that is not recirculated is sent to a thermal oxidizer and then released into the atmosphere.

[0093] The gas composition at the reactor inlet and reactor outlet is shown in Table 1.

[0094] Table 1

[0095] The total amount of VPO catalyst is 52.8 tons.

[0096] The catalyst has a production rate of 0.106 kg / h maleic anhydride per kg of catalyst.

[0097] The amount of carbon dioxide emitted into the environment from the thermal oxidizer chimney is approximately 74,000 MT / Y, equivalent to 1.64 tons of CO2 emitted for every ton of maleic anhydride produced; more than 50% of the carbon dioxide comes from the combustion of unconverted butane, as well as the combustion of some additional natural gas required for combustion efficiency. The CO2 concentration in the exhaust gas is 7.7% by weight.

[0098] Comparative Example 2 According to Figure 1B In the maleic anhydride plant, a reduced amount of fresh air is used and pure oxygen is added, with a gas recirculation ratio of 0.92, defined as: recirculated gas flow rate / (recirculated gas + purge gas) flow rate.

[0099] This condition is not much different from total recirculation operation (RR=1), but the reduced amount of fresh air is sufficient to introduce a large amount of nitrogen into the system, thereby diluting the total carbon oxides (CO+CO2) to below 30 vol.

[0100] The butane concentration at the reactor inlet is much higher than in conventional units. The low oxygen concentrations at both the reactor inlet and outlet ensure operational safety.

[0101] The reactor at 2400 h -1 Operating under high gas volume spacetime velocities.

[0102] The temperatures at the reactor inlet and outlet were 140 min and 436 min, respectively.

[0103] The reactor piping has an inner diameter of 21 mm and a length of 6 meters. Cooling and control of the reactor are achieved through the circulation of molten salt.

[0104] After cooling, the reactor effluent is fed into a high-efficiency absorber, where dibutyl phthalate is used as a selective organic solvent to recover maleic anhydride.

[0105] The exhaust gas from the absorber is washed with water and then recirculated back to the reactor via a centrifugal compressor. A portion of the washed gas is purified by a thermal oxidizer before being released into the atmosphere.

[0106] The gas composition at the reactor inlet and reactor outlet is shown in Table 2.

[0107] Table 2

[0108] Data from the factory proves two key points: a. Under operating conditions with low oxygen concentration at the reactor outlet, the generation of light organic matter is very high. At an O2 concentration of 2.4 vol%, the total generation of light acids exceeds 10 mol% of the maleic anhydride generation, or about 7% by weight.

[0109] b. The molar ratio of CO / CO2 generated is approximately 1.25, a value very similar to that under conditions of low or no gas recirculation.

[0110] The reactor's productivity was 0.124 kg / h maleic anhydride per kg of catalyst, which was 17% higher than Comparative Example 1, even under extremely low oxygen concentration conditions. Of course, the higher productivity was mainly related to the higher gas space velocity.

[0111] Adding fresh air to the exhaust gases allows unreacted butane to burn and carbon monoxide to burn off. The amount of carbon dioxide emitted into the environment from the thermal oxidizer's chimney is approximately 86,000 MT / Y, equivalent to 1.94 tons of CO2 emitted for every ton of maleic anhydride produced. The CO2 concentration in the exhaust gases is 30% by weight.

[0112] Embodiment 3 of the present invention This embodiment relates to operation in a pilot-scale reaction apparatus according to the present invention.

[0113] The reactor consists of a single pipe, the same dimensions as a typical industrial reactor pipe: an inner diameter of 21 mm and a length of 6 meters. The pipe is surrounded by molten salt circulating at a controlled temperature.

[0114] The gas discharged from the reactor is passed through a water scrubber to absorb maleic anhydride and light organic acids. After scrubbing, the gas is partially purified to remove accumulated inert gases (mainly carbon oxides) and then recycled back into the reactor using a laboratory diaphragm compressor.

[0115] At the compressor outlet, fresh oxygen and vaporized butane are introduced under flow control to achieve the desired reagent conditions in the reaction unit.

[0116] Since the pilot plant lacks a carbon monoxide selective converter, operation at high carbon dioxide concentrations is simulated by introducing a high-purity (95%) CO2 stream.

[0117] The test reactor was tested at 1750 h. -1 Up to 2200 h -1 Operating under the gas volume spacetime velocity.

[0118] The data in Table 3 is from 1750h. -1 The following operations were performed.

[0119] Table 3

[0120] Under the conditions of the present invention, particularly when the oxygen concentration at the reactor outlet is higher than the limiting oxygen concentration and higher than 4 vol%, Example 3 of the present invention shows the following advantages: I) The concentration difference of n-butane is extremely high: approximately 2.4 vol%, much higher than that of Comparative Example 1 (Δ = 1.4 vol%) and Comparative Example 2 (Δ = 1.7 vol%). II) The high Δ conversion rate translates into extremely high catalyst productivity, at 0.152 kg / h maleic anhydride per kg of catalyst. Compared to Example 1, the catalyst productivity is increased by 42%.

[0121] This means that, under the conditions of this invention, the practical inherent limitation of maleic anhydride production in the largest maleic anhydride reactor (approximately 70,000 MT / Y to 75,000 MT / Y) can be exceeded to reach the target of 100,000 MT / Y. III) The amount of hyaluronic acid produced, while still higher than that of a single-pass operation, was reduced by 40% compared to Example 2. This figure is acceptable considering the other advantages offered by such an operation.

[0122] Embodiment 4 of the present invention according to Figure 3 Experimental data from the pilot plant were derived into a large industrial reactor, and a carbon monoxide selective reactor was introduced at the outlet of the recirculation compressor.

[0123] Considering the current largest maleic anhydride tubular reactor (with 39,500 pipes with an inner diameter of 25 mm and a catalyst bed of 5,500 mm, corresponding to 106.6 m...), 3 The catalyst achieved a 99% recovery rate of maleic anhydride and a gas volume space velocity of 1830 h⁻¹ in the absorption / stripping / purification section. -1 Under these conditions, the yield of the corresponding maleic anhydride with a purity of 99.9% exceeds 12,500 kg / h or 100,000 MT / Y. The productivity of a single reactor is increased by 35% compared to existing technologies.

[0124] The device produces 13,600 kg / h of exhaust gas, of which carbon dioxide accounts for 94% by weight. After compression and purification, the gas can be used for carbon capture and utilization. The carbon dioxide / maleic anhydride production ratio is 1:1 by weight. The carbon dioxide production is 40% lower than in Example 1.

[0125] The gas composition at the inlet and outlet of the maleic anhydride reactor is shown in Table 4. Except for the complete disappearance of nitrogen (nitrogen entered the circulation as an impurity in the carbon dioxide injection in Example 3) and a significant reduction in carbon monoxide, these values ​​are very similar to those in Table 3 of Example 3, both of which are made up by a higher concentration of carbon dioxide.

[0126] Table 4

[0127] More interestingly, the conditions at the inlet and outlet of the carbon monoxide selective oxidizer are shown in Table 4A.

[0128] Table 4A Carbon monoxide selective oxidizer

[0129] This data is based on the use of a Pt / Pd catalyst supported on Al2O3 in a tubular vertical reactor to produce low / medium pressure steam. To avoid the risk of runaway conditions leading to butane combustion, the reactor has several safety options: First, under the conditions taught in this invention, the converter possesses partial self-protection characteristics when the oxygen concentration is controlled; this is because even in a runaway state, the oxygen content only allows for partial combustion of residual butane, as the oxygen concentration is below the stoichiometric value for complete combustion of CO + C4. In other words, in a runaway state, the maximum heat of reaction that needs to be removed is far less than the heat released at higher oxygen levels. These conditions form the basis for the correct use of the following safety procedures: The catalyst is inertized to suppress hot spot temperature; Use the minimum heat transfer surface required for a catalyst volume greater than the catalyst volume; Using a selective catalyst, butane is converted at a significantly higher temperature than carbon monoxide; the pressure of the generated steam is controlled to regulate the temperature at the reactor outlet.

Claims

1. A method for producing maleic anhydride by catalytic oxidation of n-butane and pure oxygen, wherein, The method includes the following steps: (a) A reaction gas mixture is fed into a tubular reactor unit containing a vanadium-phosphorus oxide catalyst, wherein the gas mixture comprises a vaporized stream rich in n-butane at a concentration above the corresponding upper limit of flammability in a highly inert atmosphere, a stream of high-purity oxygen, and a stream of carbon dioxide rich in gas that is recycled to the reactor via a gas compressor. (b) In the reaction tube of the tubular reactor unit, maleic anhydride is produced as the main product, carbon oxides and light organic acids as undesirable byproducts, and reaction water is produced. (c) Cooling the exhaust gas leaving the tubular reactor unit at a temperature of 380 to 450 degrees Celsius by recovering the heat of reaction to generate steam at a single pressure value or at a reduced pressure value in multiple heat exchangers in series. (d) By using an organic solvent as the absorbent, maleic anhydride contained in the reaction gas is selectively recovered in an absorber in the form of a high-efficiency absorption tower. (e) Cool and wash the exhaust gas from the absorber with water to remove trace amounts of maleic anhydride and organic acid byproducts that were not removed in the absorber, thereby protecting the gas compressor. (f) Purge a portion of the exhaust gas that has been washed with water to prevent the accumulation of inert substances and send it to the first thermal oxidizer, the first regenerative oxidizer or the first catalytic oxidizer to meet local regulations for atmospheric emission limits of carbon monoxide, hydrocarbons and nitrogen oxides. (g) The washed gas mixture is recirculated through a gas compressor to increase its pressure to the reaction value; (h) 35% to 60% of the compressed gas is fed into a second selective oxidizer, in which a suitable selective catalyst, along with controlled temperature and controlled oxygen content, promotes the safe conversion of most carbon monoxide into carbon dioxide, while essentially not oxidizing unreacted n-butane contained in the recycle gas. (i) The combined carbon dioxide-rich gas mixture is used as a diluent for new n-butane and pure oxygen and fed into the maleic anhydride tubular reactor unit. Its features are: i. Before injecting new butane and pure oxygen, the recirculated dilution gas is rich in carbon dioxide with a concentration higher than 85%; ii. The n-butane content at the inlet of the tubular reactor unit is controlled within 4% to 6%, and its conversion rate in the same reactor is 35% to 50%; iii. The oxygen content at the inlet of the tubular reactor unit is controlled between 12 mol% and 16 mol%; iv. The content of butane, carbon dioxide, carbon monoxide and water mixed at the inlet of the tubular reactor unit is outside the flammability limit at its temperature and pressure; v. Control the oxygen content at the outlet of the tubular reactor unit to a value below the limiting oxygen concentration (LOC, i.e., below which the mixture is non-flammable regardless of the fuel concentration), but not below 4.0 mol.

2. The method according to claim 1, wherein, The catalytic reaction of oxidation to maleic anhydride was carried out at 1400 h. -1 Up to 2500 h -1 It is carried out at airspeed.

3. The method according to claim 1, wherein, The catalytic reaction for oxidation to maleic anhydride was carried out at temperatures ranging from 370°C to 450°C.

4. The method according to claim 1, wherein, Volatile organophosphorus compounds are added to the gaseous reaction feed mixture to control the catalyst activity.

5. The method according to claim 1, wherein, The oxidation catalyst is a vanadium-phosphorus mixed oxide (VPO) type catalyst, which has been developed to minimize the formation of light organic acids.

6. The method according to claim 1, wherein, The tubular reactor unit for producing maleic anhydride has an enhanced heat transfer system, serving as a molten salt circulation or a steam generation surface area, or both; under the conditions of this method, higher productivity of the reactor can be achieved compared to prior art methods.

7. The method according to claim 1, wherein, The second selective catalytic oxidizer used to convert most of the carbon monoxide into carbon dioxide is a fixed-bed tubular reactor, which, under temperature-controlled conditions, ensures that the CO conversion rate is not less than 80% and the butane conversion rate is not more than 5%.

8. The method according to claim 7, wherein, The hot spot temperature inside the second selective catalytic oxidizer tube is controlled by using a selective oxidation catalyst mixed with an inert material.

9. The method according to claim 8, wherein, The exothermic reaction is removed by circulating high-pressure water or a different heat transfer medium in the shell side of the fixed-bed tubular reactor, or, as a preferred option, by generating steam under controlled pressure.

10. A method for producing maleic anhydride by catalytic oxidation of n-butane and pure oxygen, wherein, After appropriate compression and purification steps, purge gas is used as a carbon dioxide source for carbon capture, utilization, and storage (CCUS) or enhanced oil recovery (EOR); wherein the method includes the following steps: (a) A reaction gas mixture is fed into a tubular reactor unit containing a vanadium-phosphorus oxide catalyst, wherein the gas mixture comprises a vaporized stream rich in n-butane at a concentration above the corresponding upper limit of flammability in a highly inert atmosphere, a stream of high-purity oxygen, and a stream of carbon dioxide rich in gas that is recirculated to the same first tubular reactor unit by a gas compressor. (b) In the reaction tube of the tubular reactor unit, maleic anhydride is produced as the main product, carbon oxides and light organic acids as undesirable byproducts, and reaction water is produced. (c) Cooling the exhaust gas leaving the tubular reactor unit at a temperature of 3808 to 450 degrees by recovering the heat of reaction to generate steam at a single pressure value or at a reduced pressure value in multiple heat exchangers in series. (d) By using an organic solvent or water as the absorbent, maleic anhydride contained in the reaction gas is selectively recovered in an absorber in the form of a high-efficiency absorption tower. (e) Cool and wash the exhaust gas from the absorber with water to remove trace amounts of maleic anhydride and organic acid byproducts that were not removed in the absorber, thereby protecting the gas compressor. (f) The washed gas mixture is recirculated through a gas compressor to increase its pressure to the reaction value; (g) 40% to 60% of the compressed gas is fed into a selective oxidizer, where a suitable selective catalyst, in conjunction with controlled temperature and controlled oxygen content, promotes the safe conversion of most carbon monoxide into carbon dioxide, while essentially not oxidizing unreacted n-butane contained in the recycle gas. (h) Purge some of the exhaust gas from the carbon monoxide converter to avoid the accumulation of inert substances, and combine the remaining part with the compressed gas that bypasses the CO converter; (i) The combined carbon dioxide-rich gas mixture is used as a diluent for new n-butane and pure oxygen and fed into the maleic anhydride tubular reactor unit. (j) Using the purge gas in item (h) as a source of carbon dioxide, after appropriate compression and purification steps, for carbon capture, utilization and storage (CCUS) or enhanced oil recovery (EOR). Its features are: i. Before injecting new butane and pure oxygen, the recirculated dilution gas is rich in carbon dioxide with a concentration higher than 85%; ii. The n-butane content at the inlet of the tubular reactor unit is controlled within 4% to 6%, and its conversion rate in the reactor is 35% to 50%; iii. The oxygen content at the inlet of the tubular reactor unit is controlled between 12 mol% and 16 mol%; iv. The content of butane, carbon dioxide, carbon monoxide and water mixed at the inlet of the tubular reactor unit is outside the flammability limit at its temperature and pressure; v. Control the oxygen content at the outlet of the tubular reactor unit to a value below the limiting oxygen concentration (LOC, i.e., below which the mixture is non-flammable regardless of the fuel concentration), but not below 4.0 mol.

11. The method of claim 10, wherein, The catalytic reaction of oxidation to maleic anhydride was carried out at 1400 h. -1 Up to 2500 h -1 It is carried out at airspeed.

12. The method according to claim 10, wherein, The catalytic reaction for oxidation to maleic anhydride was carried out at temperatures ranging from 370°C to 450°C.

13. The method according to claim 10, wherein, Volatile organophosphorus compounds are added to the gaseous reaction feed mixture to control the catalyst activity.

14. The method of claim 10, wherein, The oxidation catalyst is a vanadium-phosphorus mixed oxide (VPO) type catalyst, which has been developed to minimize the formation of light organic acids.

15. The method according to claim 10, wherein, The tubular reactor unit used for the production of maleic anhydride has an enhanced heat transfer system, serving as a molten salt circulation or a steam generation surface area, or both; under the conditions of this method, a higher reactor productivity can be achieved compared to prior art methods.

16. The method of claim 10, wherein, The selective catalytic oxidizer used to convert most of the carbon monoxide into carbon dioxide is a fixed-bed tubular reactor, which, under temperature-controlled conditions, ensures that the CO conversion rate is not less than 80% and the butane conversion rate is not more than 5%.

17. The method according to claim 16, wherein, The hot spot temperature inside the selective catalytic oxidizer tube is controlled by using a selective oxidation catalyst mixed with an inert material.

18. The method according to claim 17, wherein, The exothermic reaction is removed by circulating high-pressure water or a different heat transfer medium in the shell side, or, as a preferred option, by generating steam under controlled pressure.

19. The method according to claim 10, wherein, The method includes an absorption step in which a large portion of the unconverted n-butane is recovered from the purge gas described in items h) and j) by using the lean organic solvent of item d) as the absorbent, wherein the pressure of the C4 absorber is higher than the pressure of the maleic anhydride absorber of item d).

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