Process for the synthesis of ethanol from synthesis gas with recycle of off-gas

Abstract

This invention relates to the field of catalytic conversion of syngas and discloses a process for recycling tail gas from syngas to ethanol, comprising the following steps: S1, after purifying the syngas, the molar ratio of H2 to CO in the syngas is adjusted to 0.5-2 using an online gas analyzer to obtain purified syngas feedstock; S2, the purified syngas feedstock is fed into a fixed-bed reactor containing a molybdenum-based catalyst, and the reaction is carried out at a reaction temperature of 280-400℃, a reaction pressure of 5-15MPa, and a gas hourly space velocity of 3000-15000 h⁻¹. ‑1 Under certain conditions, syngas undergoes a one-step catalytic reaction with a molybdenum-based catalyst to produce reaction products, primarily ethanol with methanol and propanol as byproducts. This invention recycles most of the reaction tail gas after deep purification to the reactor for secondary catalytic reaction, achieving closed-loop reuse of unreacted syngas. Simultaneously, a crude alcohol flash evaporation unit is added to recover the effective components of low-carbon alcohols entrained in the flash vapor phase and reintegrate them into the tail gas purification system, reducing the loss of valuable products and improving the overall conversion rate of syngas.

Description

Technical Field

[0001] This invention relates to the field of catalytic conversion of syngas, and more particularly to a process for recycling tail gas from syngas to ethanol. Background Technology

[0002] Syngas is an important chemical feedstock gas obtained from carbonaceous raw materials such as coal, natural gas and biomass through gasification or reforming. It can be used to produce a variety of chemicals and liquid fuels, such as ethanol, methanol and low-carbon alcohols, through catalytic conversion.

[0003] Currently, in the syngas-to-ethanol process, due to the limitations of the thermodynamic equilibrium of the chemical reaction, the conversion rate of syngas passing through the catalyst bed in one pass is low. A large amount of unconverted H2 and CO are discharged with the tail gas, resulting in waste of raw materials. Furthermore, the product distribution is complex, generating mixed alcohols such as methanol and propanol, as well as hydrocarbon byproducts, in addition to ethanol, leading to a decrease in ethanol purity. In addition, a large amount of unreacted syngas is directly emitted or burned, which not only wastes resources but also increases energy consumption and carbon emissions. Summary of the Invention

[0004] The technical problem to be solved by this invention is that the existing technology has the disadvantages of low ethanol yield and decreased purity. To address this, we propose a process for recycling syngas tail gas to produce ethanol.

[0005] To achieve the above objectives, this application adopts the following technical solution: a process for recycling tail gas from syngas to ethanol, comprising the following steps: S1. After the syngas is purified, the molar ratio of H2 and CO in the syngas is adjusted to 0.5-2 using an online gas analyzer to obtain purified syngas feedstock. S2. The purified syngas feedstock is fed into a fixed-bed reactor containing a molybdenum-based catalyst, and the reaction is carried out at a temperature of 280-400℃, a pressure of 5-15MPa, and a gas space velocity of 3000-15000h. -1 Under certain conditions, the syngas undergoes a one-step catalytic reaction in the presence of a molybdenum-based catalyst to produce reaction products mainly composed of ethanol, with methanol and propanol as byproducts, while unreacted gaseous tail gas is also produced. S3. The reaction product obtained in step S2 is sent to a multi-stage condenser and cooled to 30-60°C before entering the first gas-liquid separator to separate the liquid crude alcohol product and the first tail gas. The liquid crude alcohol product is then sent to the mixed alcohol separation and purification section. S4. The first tail gas obtained in step S3 is sent into the second gas-liquid separator to remove alcohol droplets entrained in the tail gas. S5. The first tail gas treated in step S4 is passed sequentially through a molecular sieve adsorption tower and a pressure swing adsorption device to remove trace amounts of alcohol mist, low-carbon hydrocarbons, CO2 and N2 inert components, and obtain purified tail gas. S6. The purified exhaust gas obtained in step S5 is divided into two streams: the first stream is recirculated gas and the second stream is released gas. S7. Use online gas chromatography to detect the components of the circulating gas in real time, add fresh synthesis gas to the circulating gas, and recalibrate the molar ratio of H2 and CO in the circulating gas to 0.8-2; S8. After the H2 and CO are corrected, the recirculated gas is mixed with the purified syngas feedstock in step S1 and then returned to the fixed-bed reactor in step S2 to participate in the recirculation reaction. The volume ratio of the recirculated gas to the purified syngas feedstock is 0.3-1.8:1. At the same time, the recirculation ratio is dynamically adjusted according to the continuous running time of the device.

[0006] Preferably, in step S2, the fixed-bed reactor is equipped with a multi-stage temperature control module to control the maximum temperature rise of the reactor bed to ≤25℃.

[0007] Preferably, in step S5, the molecular sieve adsorption tower uses a polar molecular sieve to adsorb C1-C3 alcohols and low-carbon hydrocarbon impurities in the exhaust gas, and the pressure swing adsorption device removes CO2 and N2.

[0008] Preferably, in step S5, the overall removal rate of organic impurities in the purified exhaust gas is ≥98%.

[0009] Preferably, in step S6, the circulation ratio adopts a dynamic control mode: when the device operates continuously for 0-500 hours, the volume ratio of circulating gas to purified syngas raw material is 0.3-1:1; when the device operates continuously for 500-1000 hours, the volume ratio of circulating gas to purified syngas raw material is 1-1.8:1.

[0010] Preferably, the molybdenum-based catalyst is a supported molybdenum-based catalyst or a molybdenum sulfide-based catalyst, containing one or more second metal promoters selected from Cu, Fe, and Co.

[0011] Preferably, in step S6, the purge gas is collected and then incinerated for harmless treatment or recycled as a resource.

[0012] Preferably, in step S1, the syngas purification process includes dust removal, dry desulfurization and deoxygenation to remove dust, sulfides and oxygen impurities from the raw gas.

[0013] Preferably, in step S3, before the liquid crude alcohol product is sent to the mixed alcohol separation and purification section, it first enters a flash tank for reduced pressure flash evaporation. The flashed gas phase is pressurized by a compressor and then incorporated into the inlet of the molecular sieve adsorption tower in step S5 to recover the low-carbon alcohol components entrained therein.

[0014] Preferably, a tail gas heating heat exchanger is set between step S4 and step S5, using the reaction product gas prepared in step S2 as the heat source, to heat the first tail gas after the droplets are removed by the second gas-liquid separator to 80-150°C, and then send it into the molecular sieve adsorption tower.

[0015] The technical effects and advantages of this invention are as follows: In this invention, most of the reaction tail gas is recycled back to the reactor after deep purification to participate in the secondary catalytic reaction, realizing the closed-loop reuse of unreacted syngas. At the same time, a crude alcohol flash evaporation unit is added to recover the effective components of low-carbon alcohols entrained in the flash vapor phase and reintegrate them into the tail gas purification system, further reducing the loss of valuable products, improving the total conversion rate of syngas, avoiding the problem of direct emission of effective syngas in the prior art, reducing the consumption of raw material natural gas / coal, and reducing carbon emissions and pollutant emissions from tail gas incineration, which meets the low-carbon production requirements of the chemical industry.

[0016] In this invention, a second gas-liquid separator, a polar molecular sieve adsorption tower, and a pressure swing adsorption device are used to achieve a comprehensive removal rate of ≥98% for organic impurities in the tail gas. Alcohol mist, low-carbon hydrocarbons, and inert gases in the circulating gas are removed, avoiding long-term accumulation of impurities that interfere with the catalytic reaction. At the same time, a molybdenum-based composite catalyst is used to enhance the reaction pathway of syngas to ethanol, ultimately improving the selectivity of ethanol products by 10%-15% and significantly increasing the proportion of ethanol in the crude alcohol product, thereby reducing the energy consumption and equipment operating costs of the subsequent alcohol separation and purification section.

[0017] In this invention, the components of the circulating gas are monitored in real time by online gas chromatography, and fresh syngas is added precisely to correct the molar ratio of H2 and CO, so as to always maintain the reactor inlet gas at the optimal reaction ratio. At the same time, the ratio is dynamically controlled in stages. In the early stage of the device operation, the catalyst activity is high, and a low circulation ratio is used to avoid excessive gas flow rate causing space velocity imbalance. Attached Figure Description

[0018] The disclosure of this invention is illustrated with reference to the accompanying drawings. It should be understood that the drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. In the drawings, the same reference numerals are used to refer to the same parts: Figure 1 This is a schematic diagram of the framework of a synthesis gas to ethanol tail gas recycling process according to the present invention. Detailed Implementation

[0019] It is readily understood that, based on the technical solution of this invention, those skilled in the art can propose various interchangeable structural methods and implementations without altering the essential spirit of the invention. Therefore, the following detailed embodiments and accompanying drawings are merely illustrative examples of the technical solution of this invention and should not be considered as the entirety of the invention or as limitations or restrictions on the technical solution of this invention. Example 1

[0020] A process for recycling tail gas from syngas to ethanol includes the following steps: S1. After the syngas is purified by dust removal, dry desulfurization and deoxygenation, the molar ratio of H2 and CO in the syngas is adjusted to 1 by an online gas analyzer to obtain purified syngas feedstock. S2. The purified syngas feedstock is fed into a fixed-bed reactor containing a supported molybdenum-based catalyst (containing Cu). The reactor is equipped with a multi-stage temperature control module to control the maximum temperature rise of the reactor bed to ≤25℃, at a reaction temperature of 320℃, a reaction pressure of 10MPa, and a gas space velocity of 8000h⁻¹. -1 Under certain conditions, the syngas undergoes a one-step catalytic reaction in the presence of a molybdenum-based catalyst to produce reaction products mainly composed of ethanol, with methanol and propanol as byproducts, while unreacted gaseous tail gas is also produced. S3. The reaction product obtained in step S2 is sent to a multi-stage condenser and cooled to 45°C before entering the first gas-liquid separator to separate the liquid crude alcohol product and the first tail gas. Before the liquid crude alcohol product is sent to the mixed alcohol separation and purification section, it first enters the flash tank for reduced pressure flash evaporation. The flashed gas phase is pressurized by the compressor and then incorporated into the inlet of the molecular sieve adsorption tower in step S5 to recover the low carbon alcohol components entrained therein. S4. The first tail gas obtained in step S3 is sent into the second gas-liquid separator to remove alcohol droplets entrained in the tail gas. S5. A tail gas heating heat exchanger is set between step S4 and step S5. The reaction product gas prepared in step S2 is used as the heat source. The first tail gas after the droplets are removed by the second gas-liquid separator is heated to 120°C and then sent to the molecular sieve adsorption tower. The first tail gas passes through the polar molecular sieve adsorption tower and the pressure swing adsorption device in sequence. The polar molecular sieve adsorbs C1-C3 alcohols and low-carbon hydrocarbon impurities in the tail gas, and the pressure swing adsorption device removes CO2 and N2 to obtain purified tail gas. The comprehensive removal rate of organic impurities in the purified tail gas is ≥98%. S6. The purified exhaust gas obtained in step S5 is divided into two streams: the first stream is recirculated gas and the second stream is purge gas. The purge gas is collected and then incinerated for harmless treatment. S7. Use online gas chromatography to detect the components of the circulating gas in real time, add fresh synthesis gas to the circulating gas, and recalibrate the molar ratio of H2 and CO in the circulating gas to 1. S8. After the H2 and CO are corrected, the recirculated gas is mixed with the purified syngas feedstock in step S1 and then returned to the fixed-bed reactor in step S2 to participate in the recirculation reaction. The volume ratio of the recirculated gas to the purified syngas feedstock is 0.6:1. During the continuous operation of the device for 0-500 hours, the volume ratio of the recirculated gas to the purified syngas feedstock is dynamically adjusted within the range of 0.3-1:1. Example 2

[0021] A process for recycling tail gas from syngas to ethanol includes the following steps: S1. After the syngas is purified by dust removal, dry desulfurization and deoxygenation, the molar ratio of H2 and CO in the syngas is adjusted to 1.5 by an online gas analyzer to obtain purified syngas feedstock. S2. The purified syngas feedstock is fed into a fixed-bed reactor containing a supported molybdenum-based catalyst. The supported molybdenum-based catalyst contains Co. The reactor is equipped with a multi-stage temperature control module to control the maximum temperature rise of the reactor bed to ≤25℃. The reaction is carried out at a reaction temperature of 340℃, a reaction pressure of 8MPa, and a gas space velocity of 6000h. -1 Under certain conditions, the syngas undergoes a one-step catalytic reaction in the presence of a molybdenum-based catalyst to produce reaction products mainly composed of ethanol, with methanol and propanol as byproducts, while unreacted gaseous tail gas is also produced. S3. The reaction product obtained in step S2 is sent to a multi-stage condenser and cooled to 40°C before entering the first gas-liquid separator to separate the liquid crude alcohol product and the first tail gas. Before the liquid crude alcohol product is sent to the mixed alcohol separation and purification section, it first enters the flash tank for reduced pressure flash evaporation. The flashed gas phase is pressurized by the compressor and then incorporated into the inlet of the molecular sieve adsorption tower in step S5 to recover the low carbon alcohol components entrained therein. S4. The first tail gas obtained in step S3 is sent into the second gas-liquid separator to remove alcohol droplets entrained in the tail gas. S5. A tail gas heating heat exchanger is set between step S4 and step S5. The reaction product gas prepared in step S2 is used as the heat source. The first tail gas after the droplets are removed by the second gas-liquid separator is heated to 100°C and then sent to the molecular sieve adsorption tower. The first tail gas passes through the polar molecular sieve adsorption tower and the pressure swing adsorption device in sequence. The polar molecular sieve adsorbs C1-C3 alcohols and low-carbon hydrocarbon impurities in the tail gas, and the pressure swing adsorption device removes CO2 and N2 to obtain purified tail gas. The comprehensive removal rate of organic impurities in the purified tail gas is ≥98%. S6. The purified exhaust gas obtained in step S5 is divided into two streams: the first stream is recirculated gas and the second stream is purge gas. The purge gas is collected and then incinerated for harmless treatment. S7. Use online gas chromatography to detect the components of the circulating gas in real time, add fresh synthesis gas to the circulating gas, and recalibrate the molar ratio of H2 and CO in the circulating gas to 1.2. S8. After the H2 and CO are corrected, the recirculated gas is mixed with the purified syngas feedstock in step S1 and then returned to the fixed-bed reactor in step S2 to participate in the recirculation reaction. The volume ratio of the recirculated gas to the purified syngas feedstock is 0.9:1. During the continuous operation of the device for 0-500 hours, the volume ratio of the recirculated gas to the purified syngas feedstock is dynamically adjusted within the range of 0.3-1:1. Example 3

[0022] A process for recycling tail gas from syngas to ethanol includes the following steps: S1. After the syngas is purified by dust removal, dry desulfurization and deoxygenation, the molar ratio of H2 and CO in the syngas is adjusted to 0.8 by an online gas analyzer to obtain purified syngas feedstock. S2. The purified syngas feedstock is fed into a fixed-bed reactor containing a supported molybdenum-based catalyst. The supported molybdenum-based catalyst contains Fe. The reactor is equipped with a multi-stage temperature control module to control the maximum temperature rise of the reactor bed to ≤25℃. The reaction is carried out at a reaction temperature of 300℃, a reaction pressure of 12MPa, and a gas space velocity of 5000h. -1 Under certain conditions, the syngas undergoes a one-step catalytic reaction in the presence of a molybdenum-based catalyst to produce reaction products mainly composed of ethanol, with methanol and propanol as byproducts, while unreacted gaseous tail gas is also produced. S3. The reaction product obtained in step S2 is sent to a multi-stage condenser and cooled to 50°C before entering the first gas-liquid separator to separate the liquid crude alcohol product and the first tail gas. Before the liquid crude alcohol product is sent to the mixed alcohol separation and purification section, it first enters the flash tank for reduced pressure flash evaporation. The flashed gas phase is pressurized by the compressor and then incorporated into the inlet of the molecular sieve adsorption tower in step S5 to recover the low carbon alcohol components entrained therein. S4. The first tail gas obtained in step S3 is sent into the second gas-liquid separator to remove alcohol droplets entrained in the tail gas. S5. A tail gas heating heat exchanger is set between steps S4 and S5. The reaction product gas prepared in step S2 is used as the heat source. The first tail gas after the droplets are removed by the second gas-liquid separator is heated to 140°C and then sent to the molecular sieve adsorption tower. The first tail gas passes through the polar molecular sieve adsorption tower and the pressure swing adsorption device in sequence. The polar molecular sieve adsorbs C1-C3 alcohols and low-carbon hydrocarbon impurities in the tail gas, and the pressure swing adsorption device removes CO2 and N2 to obtain purified tail gas. The comprehensive removal rate of organic impurities in the purified tail gas is ≥98%. S6. The purified exhaust gas obtained in step S5 is divided into two streams: the first stream is recirculated gas and the second stream is purge gas. The purge gas is collected and then incinerated for harmless treatment. S7. Use online gas chromatography to detect the components of the circulating gas in real time, add fresh synthesis gas to the circulating gas, and recalibrate the molar ratio of H2 and CO in the circulating gas to 1.5. S8. After the H2 and CO are corrected, the recirculated gas is mixed with the purified syngas feedstock in step S1 and then returned to the fixed-bed reactor in step S2 to participate in the recirculation reaction. The volume ratio of the recirculated gas to the purified syngas feedstock is 1.2:1. During the continuous operation of the device for 500-1000 hours, the volume ratio of the recirculated gas to the purified syngas feedstock is dynamically adjusted within the range of 1-1.8:1. Example 4

[0023] A process for recycling tail gas from syngas to ethanol includes the following steps: S1. After the syngas is purified by dust removal, dry desulfurization and deoxygenation, the molar ratio of H2 and CO in the syngas is adjusted to 1.2 by an online gas analyzer to obtain purified syngas feedstock. S2. The purified syngas feedstock is fed into a fixed-bed reactor containing a supported molybdenum-based catalyst, which contains Cu and Co. The reactor is equipped with a multi-stage temperature control module to control the maximum temperature rise of the reactor bed to ≤25℃, at a reaction temperature of 350℃, a reaction pressure of 9MPa, and a gas space velocity of 10000h. -1 Under certain conditions, the syngas undergoes a one-step catalytic reaction in the presence of a molybdenum-based catalyst to produce reaction products mainly composed of ethanol, with methanol and propanol as byproducts, while unreacted gaseous tail gas is also produced. S3. The reaction product obtained in step S2 is sent to a multi-stage condenser and cooled to 35°C before entering the first gas-liquid separator to separate the liquid crude alcohol product and the first tail gas. Before the liquid crude alcohol product is sent to the mixed alcohol separation and purification section, it first enters the flash tank for reduced pressure flash evaporation. The flashed gas phase is pressurized by the compressor and then incorporated into the inlet of the molecular sieve adsorption tower in step S5 to recover the low carbon alcohol components entrained therein. S4. The first tail gas obtained in step S3 is sent into the second gas-liquid separator to remove alcohol droplets entrained in the tail gas. S5. A tail gas heating heat exchanger is set between step S4 and step S5. The reaction product gas prepared in step S2 is used as the heat source. The first tail gas after the droplets are removed by the second gas-liquid separator is heated to 90°C and then sent to the molecular sieve adsorption tower. The first tail gas passes through the polar molecular sieve adsorption tower and the pressure swing adsorption device in sequence. The polar molecular sieve adsorbs C1-C3 alcohols and low-carbon hydrocarbon impurities in the tail gas, and the pressure swing adsorption device removes CO2 and N2 to obtain purified tail gas. The comprehensive removal rate of organic impurities in the purified tail gas is ≥98%. S6. The purified exhaust gas obtained in step S5 is divided into two streams: the first stream is recirculated gas and the second stream is purge gas. The purge gas is collected and then incinerated for harmless treatment. S7. Use online gas chromatography to detect the components of the circulating gas in real time, add fresh synthesis gas to the circulating gas, and recalibrate the molar ratio of H2 and CO in the circulating gas to 1.8. S8. After the H2 and CO are corrected, the recirculated gas is mixed with the purified syngas feedstock in step S1 and then returned to the fixed-bed reactor in step S2 to participate in the recirculation reaction. The volume ratio of the recirculated gas to the purified syngas feedstock is 1.5:1. During the continuous operation of the device for 500-1000 hours, the volume ratio of the recirculated gas to the purified syngas feedstock is dynamically adjusted within the range of 1-1.8:1. Example 5

[0024] A process for recycling tail gas from syngas to ethanol includes the following steps: S1. After the syngas is purified by dust removal, dry desulfurization and deoxygenation, the molar ratio of H2 and CO in the syngas is adjusted to 0.6 by an online gas analyzer to obtain purified syngas feedstock. S2. The purified syngas feedstock is fed into a fixed-bed reactor containing a supported molybdenum-based catalyst. The supported molybdenum-based catalyst contains Fe and Co. The reactor is equipped with a multi-stage temperature control module to control the maximum temperature rise of the reactor bed to ≤25℃. The reaction is carried out at a reaction temperature of 280℃, a reaction pressure of 14MPa, and a gas space velocity of 12000h. -1 Under certain conditions, the syngas undergoes a one-step catalytic reaction in the presence of a molybdenum-based catalyst to produce reaction products mainly composed of ethanol, with methanol and propanol as byproducts, while unreacted gaseous tail gas is also produced. S3. The reaction product obtained in step S2 is sent to a multi-stage condenser and cooled to 55°C before entering the first gas-liquid separator to separate the liquid crude alcohol product and the first tail gas. Before the liquid crude alcohol product is sent to the mixed alcohol separation and purification section, it first enters the flash tank for reduced pressure flash evaporation. The flashed gas phase is pressurized by the compressor and then incorporated into the inlet of the molecular sieve adsorption tower in step S5 to recover the low carbon alcohol components entrained therein. S4. The first tail gas obtained in step S3 is sent into the second gas-liquid separator to remove alcohol droplets entrained in the tail gas. S5. A tail gas heating heat exchanger is set between steps S4 and S5. The reaction product gas prepared in step S2 is used as the heat source. The first tail gas, after the droplets are removed by the second gas-liquid separator, is heated to 80°C and then sent to the molecular sieve adsorption tower. The first tail gas passes through the polar molecular sieve adsorption tower and the pressure swing adsorption device in sequence. The polar molecular sieve adsorbs C1-C3 alcohols and low-carbon hydrocarbon impurities in the tail gas, and the pressure swing adsorption device removes CO2 and N2 to obtain purified tail gas. The comprehensive removal rate of organic impurities in the purified tail gas is ≥98%. S6. The purified exhaust gas obtained in step S5 is divided into two streams: the first stream is recirculated gas and the second stream is purge gas. The purge gas is collected and then incinerated for harmless treatment. S7. Use online gas chromatography to detect the components of the circulating gas in real time, add fresh synthesis gas to the circulating gas, and recalibrate the molar ratio of H2 and CO in the circulating gas to 0.8. S8. After the H2 and CO are corrected, the recirculated gas is mixed with the purified syngas feedstock in step S1 and then returned to the fixed-bed reactor in step S2 to participate in the recirculation reaction. The volume ratio of the recirculated gas to the purified syngas feedstock is 0.4:1. During the continuous operation of the device for 0-500 hours, the volume ratio of the recirculated gas to the purified syngas feedstock is dynamically adjusted within the range of 0.3-1:1.

[0025] The process performance of Examples 1 to 5 was tested, and the CO single-pass conversion rate, total alcohol selectivity, ethanol selectivity, and maximum temperature rise of the catalyst bed were measured. The test results are shown in the table below: CO single-pass conversion rate (%) 36 33 39 37 34 Total alcohol selectivity (%) 70 73 66 69 71 Ethanol selectivity (%) 53 49 56 51 50 Maximum temperature rise of the bed (°C) 18 20 22 19 16 As shown in the table above, the CO single-pass conversion rate of each embodiment is ≥33%, with the highest value of 39% achieved in Example 3 under the conditions of reaction temperature 300℃, pressure 12.0MPa and recycle ratio 1.2:1. The total alcohol selectivity of all embodiments was ≥66%, indicating that tail gas recirculation did not have an adverse effect on the product composition. The ethanol selectivity of all embodiments was ≥49%, with the highest being 56% in Example 3, indicating that the supported molybdenum-based catalyst with Fe promoter was more conducive to ethanol production under conditions of 300°C and high cycle ratio. The maximum temperature rise of the bed in each embodiment was ≤22℃, which proves that the multi-stage temperature control module effectively avoids the sintering deactivation problem caused by local overheating of the catalyst bed, demonstrating the versatility and stability of the technical solution of the present invention under different process conditions.

[0026] The technical scope of this invention is not limited to the content described above. Those skilled in the art can make various modifications and variations to the above embodiments without departing from the technical concept of this invention, and all such modifications and variations should fall within the protection scope of this invention.

Claims

1. A process for recycling tail gas from syngas to ethanol, characterized in that, Includes the following steps: S1. After the syngas is purified, the molar ratio of H2 and CO in the syngas is adjusted to 0.5-2 using an online gas analyzer to obtain purified syngas feedstock. S2. The purified syngas feedstock is fed into a fixed-bed reactor containing a molybdenum-based catalyst, and the reaction is carried out at a temperature of 280-400℃, a pressure of 5-15MPa, and a gas space velocity of 3000-15000h. -1 Under certain conditions, the syngas undergoes a one-step catalytic reaction in the presence of a molybdenum-based catalyst to produce reaction products mainly composed of ethanol, with methanol and propanol as byproducts, while unreacted gaseous tail gas is also produced. S3. The reaction product obtained in step S2 is sent to a multi-stage condenser and cooled to 30-60°C before entering the first gas-liquid separator to separate the liquid crude alcohol product and the first tail gas. The liquid crude alcohol product is then sent to the mixed alcohol separation and purification section. S4. The first tail gas obtained in step S3 is sent into the second gas-liquid separator to remove alcohol droplets entrained in the tail gas. S5. The first tail gas treated in step S4 is passed sequentially through a molecular sieve adsorption tower and a pressure swing adsorption device to remove trace amounts of alcohol mist, low-carbon hydrocarbons, CO2 and N2 inert components, and obtain purified tail gas. S6. The purified exhaust gas obtained in step S5 is divided into two streams: the first stream is recirculated gas and the second stream is released gas. S7. Use online gas chromatography to detect the components of the circulating gas in real time, add fresh synthesis gas to the circulating gas, and recalibrate the molar ratio of H2 and CO in the circulating gas to 0.8-2; S8. After the H2 and CO are corrected, the recirculated gas is mixed with the purified syngas feedstock in step S1 and then returned to the fixed-bed reactor in step S2 to participate in the recirculation reaction. The volume ratio of the recirculated gas to the purified syngas feedstock is 0.3-1.8:

1. At the same time, the recirculation ratio is dynamically adjusted according to the continuous running time of the device.

2. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: In step S2, the fixed-bed reactor is equipped with a multi-stage temperature control module to control the maximum temperature rise of the reactor bed to ≤25℃.

3. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: In step S5, a polar molecular sieve is selected for the molecular sieve adsorption tower to adsorb C1-C3 alcohols and low-carbon hydrocarbon impurities in the exhaust gas, and the pressure swing adsorption device removes CO2 and N2.

4. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: In step S5, the overall removal rate of organic impurities in the purified exhaust gas is ≥98%.

5. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: In step S6, the circulation ratio adopts a dynamic control mode: when the device operates continuously for 0-500 hours, the volume ratio of circulating gas to purified syngas raw material is 0.3-1:1; when the device operates continuously for 500-1000 hours, the volume ratio of circulating gas to purified syngas raw material is 1-1.8:

1.

6. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: The molybdenum-based catalyst is a supported molybdenum-based catalyst or a molybdenum sulfide-based catalyst, containing one or more second metal promoters selected from Cu, Fe, and Co.

7. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: In step S6, the purge gas is collected and then incinerated for harmless treatment or recycled as a resource.

8. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: In step S1, the syngas purification process includes dust removal, dry desulfurization and deoxygenation to remove dust, sulfides and oxygen impurities from the raw gas.

9. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: In step S3, before the liquid crude alcohol product is sent to the mixed alcohol separation and purification section, it first enters the flash tank for reduced pressure flash evaporation. The gas phase generated by flash evaporation is pressurized by the compressor and then incorporated into the inlet of the molecular sieve adsorption tower in step S5 to recover the low-carbon alcohol components entrained therein.

10. The method for recycling syngas tail gas to ethanol according to claim 1, characterized in that: A tail gas heating heat exchanger is set between step S4 and step S5. The reaction product gas prepared in step S2 is used as the heat source to heat the first tail gas after the droplets are removed by the second gas-liquid separator to 80-150°C, and then send it into the molecular sieve adsorption tower.

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