Fixed bed catalytic conversion system

Abstract

The application relates to the technical field of catalytic reaction, and particularly discloses a fixed-bed catalytic conversion system which comprises a filter upper cover layer, a catalyst bed layer and a heat exchange structure. The filter upper cover layer is used for uniform flow, impurity trapping and dust filtering of raw material gas; the catalyst bed layer comprises a plurality of bed layers arranged along the gas flow direction, wherein the average particle size of the catalyst gradually decreases along the reaction gas flow direction, and the molar fraction of cesium contained in the catalyst in the total amount of alkali metal additives gradually increases along the reaction gas flow direction; the heat exchange structure is arranged between adjacent catalyst bed layers, and the temperature difference of the same section is controlled to be between 0-15 DEG C. The catalyst takes SiO2 as a carrier, V2O5 as an active component, and contains at least one of K, Na and Cs as an alkali metal additive. The system effectively improves the catalytic conversion efficiency and temperature control precision through the design of the gradient-distributed catalyst bed layer.

Description

Technical Field

[0001] This application relates to the technical field of catalytic reactions, and in particular to a fixed-bed catalytic conversion system. Background Technology

[0002] The catalytic oxidation of sulfur dioxide to sulfur trioxide is a core process in the sulfuric acid industry. This reaction is a strongly exothermic and reversible reaction, typically carried out industrially using a fixed-bed catalytic reactor. In a fixed-bed converter, this strongly exothermic reaction easily forms a localized high-temperature thermal peak at the inlet section (first bed), leading to the migration of active catalyst components and sintering of the support, thus accelerating deactivation. Simultaneously, in the final stage of the reaction, due to the low SO2 concentration, weak reaction driving force, and limitations imposed by chemical equilibrium, it is difficult to improve conversion efficiency. Furthermore, impurities such as arsenic (As), halogens (Cl, F), and dust often present in the feed gas can cause catalyst poisoning, pore blockage, and increased bed pressure drop.

[0003] Traditional converters typically load catalysts with uniform composition and physical properties into each bed and control temperature through inter-bed heat exchange or quenching. This method struggles to simultaneously optimize the thermal management of the first bed and the low-temperature activity of the last bed. For large-diameter radial flow reactors, there are also issues such as uneven gas flow distribution, radial temperature differences, and bypass flow, which affect isothermal operation and catalyst utilization efficiency. Summary of the Invention

[0004] To address the problem of localized overheating in existing fixed-bed catalytic conversion systems, this application provides a fixed-bed catalytic conversion system.

[0005] This application provides a fixed-bed catalytic conversion system, which adopts the following technical solution: A fixed-bed catalytic conversion system, comprising, in sequence along the direction of the reaction gas flow: The filter cover layer is used to equalize the flow of raw gas entering the system, capture impurities, and filter dust. The catalyst bed includes multiple beds arranged along the gas flow direction, wherein the average particle size of the catalyst in the bed decreases sequentially along the gas flow direction, the catalyst in the bed contains at least one of K, Na, and Cs as an alkali metal promoter, the molar fraction of cesium in the total amount of alkali metal promoter in the catalyst in the bed increases sequentially along the gas flow direction, and the bulk density of the catalyst in the bed increases sequentially along the gas flow direction. The heat exchange structure is set between adjacent catalyst beds to ensure that the temperature of the catalyst bed meets the preset temperature and to control the temperature difference |ΔTr| at the same cross section between 0 and 15 ℃.

[0006] By employing the above technical solution, gradient settings are implemented for the average particle size of the catalyst, the molar fraction of cesium in the total alkali metal promoter, and the catalyst packing density. Gradual changes in activity, alkali metal promoter composition, and pore structure are adopted in different zones (along the gas flow direction or along the bed height / radial) within the same converter or bed. Upstream, a heat-resistant, low-activity but highly thermally stable formulation with a larger particle size / higher macropore ratio is selected to reduce initial bed exothermics, lower the peak temperature, and reduce the risk of active component sintering and migration. Midstream uses conventional activity with standard particle size and macropore ratio to ensure effective gas diffusion and conversion, maintaining the reaction rate. Terminally, a cesium-promoted, low-temperature, high-activity formulation with a smaller particle size and higher macropore ratio is used to suppress the initial bed heat peak and thermal deactivation, maintaining a sufficient rate at low SO2 partial pressure and lower temperature. This achieves deep conversion and obtains a higher overall conversion rate without significantly increasing the pressure drop, while ensuring a high reaction rate in the terminal layer, especially maintaining effective catalytic activity at low temperatures and low SO2 concentrations.

[0007] In some embodiments, the fixed-bed catalytic conversion system is an axial flow reactor, and the bulk density catalyst bed includes at least a first bed, a middle bed, and a last bed arranged along the direction of the reaction gas flow. The average particle size of the first bed catalyst is 6-10 mm, the average particle size of the middle bed catalyst is 4-6 mm, the average particle size of the last bed catalyst is 3-5 mm, the bulk density of the first bed catalyst is 0.6-0.7 kg / L, the bulk density of the middle bed catalyst is 0.7-0.8 kg / L, and the bulk density of the last bed catalyst is 0.8-0.9 kg / L.

[0008] In some embodiments, the proportion of cesium in the total molar number of alkali metal promoters in the first-bed catalyst with a bulk density Cs / (K+Na+Cs) = 0.00–0.20, the proportion of cesium in the total molar number of alkali metal promoters in the middle-bed catalyst with a bulk density Cs / (K+Na+Cs) = 0.10–0.45, and the proportion of cesium in the total molar number of alkali metal promoters in the last-bed catalyst with a bulk density Cs / (K+Na+Cs) = 0.20~0.60.

[0009] In some embodiments, the macropore volume fraction of the catalyst increases by 10% to 35% along the direction of the reaction gas flow, and the total pore volume of the catalyst is 0.3 to 1.2 mL·g. ~1 ; and / or, the specific surface area of ​​the first-bed catalyst is 50~120 m². 2 ·g ~1 The specific surface area of ​​the catalyst in the final bed is 80~200 m². 2 ·g ~1 .

[0010] In some embodiments, the catalyst uses SiO2 as a support, V2O5 as the active component, and the end-bed catalyst contains 0.2~0.8 wt% Cs2O.

[0011] In some embodiments, along the direction of the reaction gas flow, the filter cover layer sequentially includes: An inert distribution layer with a thickness of 10~50 mm is used, which may be made of inert ceramic spheres, inert rings or honeycomb ceramic components. The impurity trapping layer, with a thickness of 20–150 mm, is made of modified alumina or silica-alumina porous material with a pore volume of 0.4–1.0 mL·g. ~1 The median pore size is 8~20 nm; The dustproof layer, with a thickness of 10~30 mm, is made of multiple layers of metal wire mesh and / or small-sized inert rings.

[0012] In some embodiments, the heat exchange structure includes an interbed heat exchanger, a flow guide grid, and a support screen. The support screen is disposed between adjacent catalyst beds to support the catalyst of the upper bed and allow gas to pass through. The flow guide grid is disposed at the bottom of the support screen, and the interbed heat exchanger is disposed at the bottom of the support screen, so that the reaction gas is cooled from top to bottom through the interbed heat exchanger.

[0013] In some embodiments, the fixed-bed catalytic conversion system is a radial flow reactor, and the catalyst bed includes at least an outer ring region and a central region arranged along the gas flow direction. The outer ring region is filled with a first type of catalyst, and the central region is filled with a second type of catalyst. The average particle size of the first type of catalyst is larger than that of the second type of catalyst, and the wear resistance of the first type of catalyst is higher than that of the second type of catalyst.

[0014] In some embodiments, the first type of catalyst is an anti-wear molded block or a large-channel honeycomb block with an equivalent particle size of 8-10 mm; the second type of catalyst is a small-diameter extruded strip with an average particle size of 3-5 mm; and the catalyst packing volume ratio of the outer ring region to the central region is (0.3-0.7):(0.7-0.3).

[0015] In some embodiments, the radial flow reactor is also equipped with guide vanes or honeycomb straighteners to reduce bypass flow and ensure that the temperature difference of the radial cross section of the reactor meets the following condition: 0℃≤|ΔTr|≤15℃.

[0016] Compared with the prior art, this application includes at least one of the following beneficial technical effects: 1. The design of the catalyst with a gradient decrease in average particle size along the gas flow direction optimizes the bed pore structure and reduces gas flow resistance while ensuring catalytic activity. In the radial flow reactor, the pressure drop is further reduced by the partitioning strategy of filling the outer ring with large-particle-size, highly wear-resistant catalyst and the central ring with small-particle-size, highly active catalyst. 2. Large-particle catalysts are used in the reactor inlet section (first bed) and co-filled with inert thermally conductive particles. Combined with the flow-guiding grid and heat exchange structure set in the bed, the heat of reaction can be efficiently dispersed and removed, and the temperature difference of the same cross section of the reactor can be controlled within the range of 0–15℃. This effectively avoids catalyst deactivation due to local sintering and improves operational safety and stability. 3. By setting up a three-layer filter cover, vapor impurities such as arsenic and halogens in the raw gas can be efficiently captured and dust can be filtered. This pretreatment measure effectively protects the downstream main catalyst, delays poisoning and pore blockage, and significantly extends the service life of the catalyst. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the axial flow fixed bed catalytic conversion system in Embodiment 1 of this application.

[0018] Figure 2 This is a flowchart of the axial flow fixed bed catalytic conversion system in Embodiment 1 of this application.

[0019] Figure 3 This is a schematic diagram of the structure and impurity collection mechanism of the filter cover layer in Embodiment 1 of this application.

[0020] Figure 4 This is a schematic diagram of the radial flow fixed bed catalytic conversion system in Embodiment 2 of this application. In the picture: 1. Gas filter cover layer; 2. Catalyst bed; 21. First bed; 22. Middle bed; 23. Last bed. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0022] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the term "and / or" in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Furthermore, the character " / " in this document, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0023] Example 1 Reference Figures 1 to 3This application provides a fixed-bed catalytic conversion system, which adopts an axial flow reactor structure and includes a filter cover layer 1, a catalyst bed layer 2 and a heat exchange structure in sequence along the direction of the reaction gas flow.

[0024] The filter cover layer 1 includes three sub-layers along the direction of the reaction gas flow: an inert distribution layer, a dirt trapping layer, and a dustproof layer. The thickness of the inert distribution layer is 10~50 mm, and it includes inert ceramic balls, inert rings, or honeycomb ceramic components. In this embodiment, the thickness of the inert distribution layer is preferably 20 mm, and it is composed of φ10 mm inert ceramic balls. It is used to uniformly distribute the raw material gas entering the system, suppress local scouring at the inlet, avoid gas flow concentration or scouring of the catalyst, and prevent local overheating. The impurity trapping layer has a thickness of 20–150 mm and comprises modified alumina or silica-alumina porous material with a pore volume of 0.4–1.0 mL·g⁻¹, a median pore size of 8–20 nm, and a specific surface area of ​​100–300 m² / g. The arsenic penetration capacity is ≥0.05 g As / g (simulated as AsCl₃); the chlorine penetration capacity is ≥0.02 g Cl / g. In this embodiment, the preferred thickness of the impurity trapping layer is 80 mm, composed of modified alumina porous material. The modified alumina is alumina loaded with alkali metal oxides (such as K₂O, Na₂O, CaO) or transition metal oxides (such as CuO, Fe₂O₃) through an impregnation method, used for the chemical adsorption of halogens (Cl, F) and arsenic-containing compounds. The preferred pore volume of the impurity trapping layer is 0.65 mL·g⁻¹. - ¹, the median pore size is preferably 12 nm, which can effectively capture vapor / fine particulate impurities such as As, Se, Cl, and F in the feed gas, protecting the downstream catalyst. The dustproof layer thickness is 10~30 mm, including multiple layers of metal wire mesh and / or small-sized inert rings. In this embodiment, the dustproof layer thickness is preferably 15 mm, supported by stainless steel woven mesh and composed of φ6 mm inert rings, realizing the dust filtration function and preventing fine powder from entering the downstream main catalyst layer, thereby avoiding bed blockage or performance degradation. The overall porosity of the filter cover layer 1 is 40–80%; the three-layer structure design of the filter cover layer 1 ensures that the feed gas is fully purified and uniformly distributed before entering the catalyst bed 2.

[0025] Catalyst bed 2 comprises three beds arranged along the gas flow direction: a first bed 21, a middle bed 22, and a final bed 23. The average particle size of the catalyst in each bed decreases sequentially along the gas flow direction. The molar fraction of cesium in the total amount of alkali metal promoters increases sequentially along the gas flow direction. The bulk density of the catalyst in each bed also increases sequentially along the gas flow direction. The first bed 21 is packed with a heat-resistant V₂O₅ / K₂O / SiO₂ catalyst with an average particle size of 6–10 mm, a bulk density of 0.65 kg / L, and a specific surface area of ​​50–120 m²·g. -¹, the first bed 21 is co-filled with 5–30 vol% inert thermally conductive particles (SiC or α~Al2O3); the middle bed 22 is filled with conventional active V2O5 / K2O / SiO2 catalyst with an average particle size of 4–6 mm; the last bed 23 is filled with V2O5 / Cs2O / SiO2 catalyst with an average particle size of 3–5 mm and a specific surface area of ​​80–200 m²·g. - ¹ In the catalyst in the final bed 23, the proportion of cesium in the total molar number of alkali metal promoters, Cs / (K+Na+Cs), is 0.20~0.60. The design of the catalyst with the average particle size decreasing sequentially along the direction of the reaction gas flow effectively reduced the total pressure drop of the system from 18.6 kPa to 15.9 kPa.

[0026] Each catalyst bed uses SiO2 as a support, V2O5 as the active component, and contains at least one of K, Na, and Cs as an alkali metal promoter. The molar fraction of cesium in the total alkali metal promoter content increases sequentially along the reaction gas flow direction. In the middle bed 22 catalyst, the molar fraction of cesium in the total alkali metal promoter content, Cs / (K+Na+Cs), is 0.10~0.45; in the final bed 23 catalyst, the molar fraction of cesium in the total alkali metal promoter content, Cs / (K+Na+Cs), is 0.20~0.60, and the mass fraction of Cs2O is in the range of 0.2~0.8wt%. This gradient distribution significantly improves the catalytic activity, increasing the total conversion rate from 99.72% to 99.83%.

[0027] Along the direction of the reaction gas flow, the volume fraction of macropores (pore size greater than 50 nm) in the catalyst increases by 10% to 35%, with a total pore volume of 0.3 to 1.2 mL·g. - ¹. The specific surface area of ​​the first-bed catalyst 21 is 50~120 m²·g. - ¹, The specific surface area of ​​the catalyst in the final bed 23 is 80~200 m²·g - ¹, this gradient pore structure design optimizes the mass transfer of reactants in different bed layers.

[0028] The heat exchange structure is located between adjacent catalyst beds 2 and includes an interbed heat exchanger, a flow guide grid, and a support screen. The support screen, positioned between adjacent catalyst beds 2, supports the catalyst from the upper bed and allows gas to pass through. The flow guide grid is located at the bottom of the support screen to guide the airflow for uniform distribution. The interbed heat exchanger, located at the bottom of the support screen, cools the reactant gas from top to bottom. This heat exchange structure ensures that the temperature of each catalyst bed 2 meets the preset temperature requirements and controls the temperature difference |ΔTr| at the same cross-section between 0 and 15℃. The thermal peak of the first bed 21 (referring to the local high-temperature point formed by the conversion of residual energy into lattice thermal vibration when particles are incident on the crystal) is reduced by 32℃ compared to the configuration without a gradient.

[0029] This application also provides a catalyst loading method for a fixed-bed catalytic conversion system. This method achieves optimized configuration of the catalyst bed 2 through a reasonable layered loading strategy and regional configuration, thereby improving conversion efficiency and reducing pressure drop.

[0030] First, a filter cover layer 1 is installed at the reactor inlet. This cover layer adopts a three-layer structure: the top layer is an inert distribution layer, 20 mm thick, filled with 10 mm diameter inert ceramic balls for uniformly distributing the feed gas flow; the middle layer is a dirt-catching layer, 80 mm thick, filled with modified γ-Al₂O₃ material with a pore volume of 0.65 mL / g, a median pore size of 12 nm, and an arsenic penetration capability of over 0.07 g / g, effectively removing impurities from the feed; the bottom layer is a dustproof layer, 15 mm thick, composed of stainless steel woven mesh support and 6 mm diameter inert rings, preventing dust from entering the catalyst bed 2. This three-layer cover layer structure effectively protects the downstream catalyst and extends its service life.

[0031] Next, catalysts with different average particle sizes and alkali metal additive compositions are sequentially layered along the airflow direction to form a partitioned gradient catalyst bed 2. The first bed 21 is filled with a heat-resistant V2O5 / K2O / SiO2 catalyst, and SiC particles are mixed in. The particle size of the heat-resistant V2O5 / K2O / SiO2 catalyst is 6~8mm. In this embodiment, the V2O5 content can be 7wt%, the K2O content can be 14wt%, and the SiO2 content can be 75wt%. It also contains a small amount of other additives (such as Na2O, CaO, P2O5, Sb2O3, etc.). The first bed 21 is filled with SiC particles accounting for 20 vol% and has a bulk density of 0.78 kg / L. This configuration has excellent heat resistance and can withstand the high inlet temperature conditions. The second bed is filled with conventional active V2O5 / K2O / SiO2 catalyst with a particle size of 4~6 mm, which provides good catalytic activity under medium temperature conditions. The last bed 23 is filled with V2O5 / Cs2O / SiO2 catalyst with a particle size of 3~4 mm. The molar ratio of Cs / (K+Na+Cs) is 0.45. The addition of cesium promoter significantly improves the low-temperature activity and ensures the deep conversion of SO2 in the exhaust gas.

[0032] In a preferred embodiment, the catalyst configuration can be further optimized when the feed composition fluctuates. When the SO2 concentration fluctuates randomly within the range of 8.5 ± 1.5 vol%, the molar ratio of Cs / (K+Na+Cs) in the final bed 23 is increased to 0.45~0.55, and the macropore volume fraction in the middle bed 22 is increased to 25~30%. This configuration can effectively cope with feed fluctuations and maintain stable conversion performance.

[0033] Using the above-described filling method, with a furnace gas composition of SO2 = 9.0 vol%, O2 = 11 vol%, H2O = 0.2 vol%, and the remainder being N2, the inlet temperature is 420°C and the space velocity is 4000 h⁻¹. - ¹When the first bed 21 thermal peak was 32°C lower than the comparative example without gradient and without inert co-fill, the total pressure drop decreased from 18.6 kPa to 15.9 kPa, the activity retention rate increased by about 12% after 1000 h, and the total conversion rate increased from 99.72% to 99.83%.

[0034] The implementation principle of this embodiment is as follows: The filter cover layer 1 disperses and reorganizes the gas entering the reactor, achieving an initial uniform distribution of gas across the entire cross-section of the reactor. This creates uniform initial conditions for the subsequent catalytic reaction. Then, utilizing the synergistic effect of physical and chemical adsorption, trace amounts of vaporized poisons such as arsenic (As), selenium (Se), chlorine (Cl), and fluorine (F) in the raw material gas are efficiently removed, fundamentally cutting off the main pathway of catalyst poisoning. Finally, a mechanical filtration barrier intercepts dust particles entrained in the gas, preventing them from entering the main catalyst bed 2 and causing pore blockage. The catalyst bed 2 significantly improves the effective thermal conductivity of the bed, allowing the heat released by the reaction to be rapidly conducted to the heat exchange structure through the solid particle network, rather than accumulating locally to form heat peaks. This effectively inhibits the migration of V2O5 active components and the sintering of the SiO2 support.

[0035] Example 2 Reference Figure 4 In a preferred embodiment, the fixed-bed catalytic conversion system can also employ a radial flow reactor structure. The catalyst bed 2 includes an outer ring region and a central region arranged along the gas flow direction. The outer ring region is filled with a first type of catalyst, and the central region is filled with a second type of catalyst. The average particle size of the first type of catalyst is larger than that of the second type of catalyst, and the first type of catalyst has higher wear resistance than the second type of catalyst. The first type of catalyst is a molded block catalyst or a large-channel honeycomb block catalyst (such as σ-ring, perforated extruded block) with wear resistance, having an equivalent particle size of 8-10 mm and high wear resistance; the second type of catalyst is a small-particle-size extruded strip catalyst with an average particle size of 3-5 mm. The catalyst loading volume ratio of the outer ring region to the central region is (0.3-0.7):(0.7-0.3), preferably 0.45:0.55 in this embodiment. The radial flow reactor is equipped with a honeycomb rectifier plate to reduce bypass flow and keep the radial cross-section temperature difference of the reactor ≤15℃, preferably ≤10℃.

[0036] This application also provides a catalyst loading method for a fixed-bed catalytic conversion system, employing a differentiated zone loading strategy for radial flow reactors. A first-type catalyst with a larger average particle size and higher wear resistance is loaded in the outer ring zone, specifically using σ-ring type wear-resistant molded blocks with an equivalent particle size of 8-10 mm. This zone accounts for 45% of the total reactor volume. The large-particle-size catalyst possesses excellent mechanical strength and wear resistance, capable of withstanding the scouring effect of radial flow. Simultaneously, the larger pore structure facilitates radial gas flow and reduces pressure drop. A second-type catalyst with a smaller average particle size and higher activity is loaded in the central zone, using 3-5 mm extruded strip catalysts with increased Cs promoter content. This zone accounts for 55% of the total reactor volume. The small-particle-size catalyst provides a larger specific surface area and higher catalytic activity, achieving efficient conversion in the central region of the reactor.

[0037] Honeycomb rectifier plates are installed at the top and bottom of the radial flow reactor to ensure uniform airflow distribution and reduce dead zones and short circuits. Thermocouple arrays and fiber optic DTS temperature monitoring systems are installed at the top and bottom of the bed to achieve real-time temperature monitoring. The inlet temperature is regulated by the pre-heat exchanger to maintain a radial temperature difference |ΔTr| ≤ 10°C, ensuring uniform temperature distribution within the reactor.

[0038] Using the above filling method, at 5000h - ¹At space velocity, the total pressure drop is reduced by approximately 15%, and the SO2 leakage at the end is reduced by 25%. Under fluctuating feed conditions, the conversion rate fluctuation is halved, the peak SO2 in the tail discharge is reduced by 35%, and the peak temperature of the first bed 21 is reduced by approximately 40°C compared to the control, significantly improving the system's stability and conversion efficiency.

[0039] The implementation principle of this application embodiment is as follows: the airflow flows radially from the outer ring to the center, the inlet velocity is the highest, the scouring force is the strongest, the airflow converges and flows out in the central area, ensuring that the gas and the catalyst are in full contact, and achieving a high degree of uniformity in reaction progress and temperature on the radial cross section.

[0040] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A fixed-bed catalytic conversion system, characterized in that, Along the direction of the reaction gas flow, the following are included in sequence: The filter cover layer (1) is used to equalize the flow of raw gas entering the system, capture impurities and filter dust; The catalyst bed (2) includes multiple beds arranged along the gas flow direction, wherein the average particle size of the catalyst in the bed decreases sequentially along the gas flow direction, the catalyst in the bed contains at least one of K, Na, and Cs as an alkali metal promoter, the molar fraction of cesium in the catalyst in the bed increases sequentially along the gas flow direction, and the packing density of the catalyst in the bed increases sequentially along the gas flow direction. A heat exchange structure is provided between adjacent catalyst beds (2) to ensure that the temperature of the catalyst bed (2) meets the preset temperature and the temperature difference |ΔTr| of the same cross section is controlled between 0℃ and 15℃.

2. The fixed-bed catalytic conversion system according to claim 1, characterized in that: The fixed-bed catalytic conversion system is an axial flow reactor. The catalyst bed (2) includes at least a first bed (21), a middle bed (22), and a last bed (23) arranged along the direction of the reaction gas flow. The average particle size of the catalyst in the first bed (21) is 6-10 mm, the average particle size of the catalyst in the middle bed (22) is 4-6 mm, the average particle size of the catalyst in the last bed (23) is 3-5 mm, the bulk density of the catalyst in the first bed (21) is 0.6-0.7 kg / L, the bulk density of the catalyst in the middle bed (22) is 0.7-0.8 kg / L, and the bulk density of the catalyst in the last bed (23) is 0.8-0.9 kg / L.

3. The fixed-bed catalytic conversion system according to claim 1, characterized in that: The catalyst uses SiO2 as a support and V2O5 as the active component, and the end-bed catalyst contains 0.2~0.8 wt% Cs2O.

4. A fixed-bed catalytic conversion system according to claim 3, characterized in that: In the first-bed catalyst, the proportion of cesium in the total molar amount of alkali metal promoters, Cs / (K+Na+Cs), is 0.00~0.20; in the middle-bed catalyst, the proportion of cesium in the total molar amount of alkali metal promoters, Cs / (K+Na+Cs), is 0.10~0.45; and in the final-bed catalyst, the proportion of cesium in the total molar amount of alkali metal promoters, Cs / (K+Na+Cs), is 0.20~0.

60.

5. A fixed-bed catalytic conversion system according to claim 2, characterized in that: Along the direction of the reaction gas flow, the macropore volume fraction of the catalyst increases by 10% to 35%, and the total pore volume of the catalyst is 0.3 to 1.2 mL·g. ~1 ; and / or, the specific surface area of ​​the first-bed catalyst is 50~120 m². 2 ·g ~1 The specific surface area of ​​the end-bed catalyst is 80~200 m². 2 ·g ~1 .

6. A fixed-bed catalytic conversion system according to claim 1, characterized in that: Along the direction of the reaction gas flow, the filter cover layer (1) comprises, in sequence: An inert distribution layer with a thickness of 10-50 mm is provided, wherein the inert distribution layer is made of inert ceramic spheres, inert rings or honeycomb ceramic components. The impurity trapping layer, with a thickness of 20–150 mm, is made of modified alumina or silica-alumina porous material with a pore volume of 0.4–1.0 mL·g. ~1 The median pore size is 8~20 nm; The dustproof layer, with a thickness of 10~30 mm, is made of multiple layers of metal wire mesh and / or small-sized inert rings.

7. A fixed-bed catalytic conversion system according to claim 1, characterized in that: The heat exchange structure includes an interbed heat exchanger, a flow guide grid, and a support screen. The support screen is disposed between adjacent catalyst beds (2) to support the catalyst of the upper bed and allow gas to pass through. The flow guide grid is disposed at the bottom of the support screen, and the interbed heat exchanger is disposed at the bottom of the support screen. The reaction gas is cooled from top to bottom through the interbed heat exchanger.

8. A fixed-bed catalytic conversion system according to claim 1, characterized in that: The fixed-bed catalytic conversion system is a radial flow reactor. The catalyst bed includes at least an outer ring region and a central region arranged along the gas flow direction. The outer ring region is filled with a first type of catalyst, and the central region is filled with a second type of catalyst. The average particle size of the first type of catalyst is larger than that of the second type of catalyst, and the wear resistance of the first type of catalyst is higher than that of the second type of catalyst.

9. A fixed-bed catalytic conversion system according to claim 8, characterized in that: The first type of catalyst is an anti-wear molded block or a large-channel honeycomb block with an equivalent particle size of 8~10 mm; the second type of catalyst is a small-particle-size extruded strip with an average particle size of 3~5 mm; the catalyst filling volume ratio of the outer ring region to the central region is (0.3~0.7):(0.7~0.3).

10. A fixed-bed catalytic conversion system according to claim 8, characterized in that: The radial flow reactor is also equipped with guide vanes or honeycomb straighteners to reduce bypass flow and ensure that the temperature difference of the radial cross section of the reactor meets the following condition: 0℃≤∣ΔTr∣≤15℃.

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