A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification

Abstract

The utility model relates to a system of biogas co-production LNG and food grade LCO2 based on multistage purification, including desulfurization and deamination unit, first pressure boost unit, dehydration unit, membrane separation unit, liquefaction unit, second pressure boost unit and purification unit, the desulfurization and deamination unit adopts solid adsorbent composite bed to remove sulfide and ammonia gas in the biogas, and the pressure boost unit uses compressor to pressurize the biogas, the dehydration unit uses molecular sieve to adsorb moisture, and regenerates through TSA variable temperature desorption process, the membrane separation unit separates methane and CO2 using the difference of different gas molecular penetration rate, the liquefaction unit uses refrigerating unit to provide cold source for cold box, makes biological methane liquefaction into LNG, and provides cold source for CO2 low temperature rectification, the CO2 pressure boost unit uses compressor to pressurize CO2, and the purification unit uses rectification tower or separation tank to purify liquid CO 2； The system can produce liquefied natural gas and food grade liquefied carbon dioxide simultaneously, and realizes efficient utilization of biogas.

Description

Technical Field

[0001] This utility model relates to the field of biogas treatment technology, specifically to a system for the co-production of LNG and food-grade LCO2 from biogas based on multi-stage purification. Background Technology

[0002] Biogas, as a renewable energy source, is mainly composed of methane (CH4) and carbon dioxide (CO2), with methane typically comprising 50%-70% and carbon dioxide 30%-50%. It also contains small amounts of impurities such as hydrogen sulfide, ammonia, and water vapor. Currently, the main methods of utilizing biogas include direct combustion for power generation, purification to produce biogas, and liquefaction to produce LNG.

[0003] Traditional biogas treatment technologies mainly focus on methane recovery and utilization, while the carbon dioxide component, which constitutes a large proportion of biogas, is often directly emitted or simply separated and discarded. This not only wastes carbon resources but also exacerbates the greenhouse effect. How to efficiently recover methane from biogas while simultaneously achieving the resource utilization of carbon dioxide has become an important research direction in the field of biogas treatment.

[0004] In the prior art, CN111256431B discloses a pressurized biogas liquefaction system with mixed refrigerant and its operating method. This system includes a biogas pretreatment unit, a two-stage membrane separation unit, a mixed refrigerant refrigeration cycle unit, and a CO2 distillation and purification unit, enabling the separation and liquefaction of methane and carbon dioxide in biogas. While this technology can simultaneously produce liquid methane and liquid carbon dioxide, its desulfurization and dehydration process in the biogas pretreatment unit is relatively simple, making it difficult to meet the requirements for high-purity products, and it does not effectively recycle the regenerated gas.

[0005] CN114149838A discloses a biomass processing system capable of purifying biogas and carbon dioxide. The system includes a biomass feedstock pretreatment system, a biomass fermentation system, a biogas post-treatment system, a biogas purification system, and a carbon dioxide purification system. This system uses membrane separation technology to purify biogas, ensuring a methane purity of 97%, and purifies carbon dioxide using a cryogenic liquefaction distillation tower. However, the system employs conventional processes in the dehydration stage and does not consider the recycling of regenerated gas, thus its energy utilization efficiency needs improvement.

[0006] CN119464401A discloses a biogas fermentation, purification, liquefaction, and carbon capture process. This process employs a combination of wet-to-dry desulfurization, PSA pressure swing adsorption + MDEA decarbonization, molecular sieve dehydration, single-stage two-stage mixed refrigerant compression, and propane refrigeration compression to separate and liquefy CH4, H2S, and CO2 in biogas. While this process achieves efficient utilization of biogas, its decarbonization process is complex and energy-intensive, and the effective utilization of regenerated gas is not considered in the molecular sieve dehydration stage.

[0007] CN216106810U provides a biomass treatment system capable of purifying biogas and carbon dioxide. This system purifies biogas through a membrane separation device, avoiding the moisture increase problem associated with absorption purification. However, the system exhibits a relatively low methane recovery rate during membrane separation, and the pretreatment process before membrane separation is not optimized, affecting the overall efficiency of the system.

[0008] CN103421565B discloses a biogas decarbonization process and apparatus for simultaneous recovery of liquid CO2 via gas membrane separation. The process purifies biogas through primary and secondary gas separation membranes, and the gas from the permeate side of the primary membrane is pressurized by a CO2 compressor and then used to recover liquid CO2 via a distillation column. This process improves the CH4 recovery rate by returning the tail gas from the top of the distillation column and the gas from the permeate side of the secondary membrane to the primary membrane for further purification. However, this process does not delve deeply enough into the desulfurization and dehydration stages and does not fully consider the optimized utilization of cooling capacity.

[0009] In summary, existing technologies suffer from the following problems: 1) Traditional biogas treatment processes have low methane recovery rates, resulting in significant methane losses; 2) Carbon dioxide treatment in biogas is mostly limited to simple emissions or low-value utilization, failing to achieve high-value-added resource utilization; 3) Regenerated gas from dehydration units is typically emitted directly, failing to be effectively recycled; 4) The utilization of cooling energy during liquefaction is not optimized, and energy efficiency needs improvement; 5) There is a lack of an integrated system capable of simultaneously achieving efficient methane recovery and food-grade carbon dioxide production. Therefore, there is an urgent need to develop a high-efficiency, low-energy-consumption biogas treatment system to achieve high methane recovery rates and high-value utilization of carbon dioxide, while simultaneously addressing the issues of regenerated gas recycling and cooling energy optimization. Summary of the Invention

[0010] To address the technical problems of low methane recovery rate, insufficient carbon dioxide resource utilization, and low system integration efficiency in traditional anaerobic digestion processes, and to achieve the technical effects of efficient methane recovery, carbon dioxide resource utilization, and optimized system energy consumption, this utility model provides a system and method for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification.

[0011] The technical solution adopted by this utility model to solve its technical problem is as follows: A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification is provided, including a desulfurization and deammoniation unit, a first pressurization unit, a dehydration unit, a membrane separation unit, a liquefaction unit, a second pressurization unit, and a purification unit. Specifically, the desulfurization and deammoniation unit uses a solid adsorbent composite bed to remove sulfides and ammonia from the biogas; the pressurization unit uses a compressor to pressurize the biogas; the dehydration unit uses a molecular sieve to adsorb moisture from the biogas, regenerates it using a TSA temperature-dependent desorption process, and uses BOG as the regeneration gas heated to 180-300℃ for reverse regeneration of the molecular sieve; the membrane separation unit separates methane and CO2 by utilizing the different permeation rates of different gas molecules in the membrane module; the liquefaction unit uses a refrigeration unit to provide a cold source for the cold box, where biomethane is liquefied into LNG, and the refrigeration unit also provides a cold source for the low-temperature distillation of CO2; the second pressurization unit uses a compressor to pressurize the CO2; and the purification unit uses a distillation column or separator to purify liquid CO2. Specific content:

[0012] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification, the system comprising a desulfurization and ammonia removal unit, a first pressurization unit, a dehydration unit, a membrane separation unit, a liquefaction unit, a second pressurization unit, and a purification unit; wherein:

[0013] The desulfurization and ammonia removal unit consists of a blower, a desulfurization and ammonia removal tank, and a desulfurization filter F-101; the first pressurization unit consists of a biogas compressor, a cooler, and a first liquid separator; the dehydration unit consists of a molecular sieve dehydration tower group, a dust filter, a heater, a cooler, and a second liquid separator; the liquefaction unit consists of a cold box, a refrigeration system, and an LNG tank; the second pressurization unit consists of a CO2 compressor and a CO2 cooler; the purification unit consists of a CO2 distillation column and a CO2 heat exchanger, wherein:

[0014] Biogas is connected to the desulfurization tank via a blower; the output of the desulfurization tank is connected to the desulfurization filter, and the output of the desulfurization filter is connected to the biogas compressor; the output of the biogas compressor is connected to the first separatory tank via a cooler; one output of the separatory tank is connected to the molecular sieve dehydration tower assembly; the other output is connected to the wastewater treatment unit; simultaneously, the input of the molecular sieve dehydration tower assembly is connected in parallel to the second separatory tank via a cooler; the output of the second separatory tank is connected to the wastewater treatment unit.

[0015] The output end of the molecular sieve dehydration tower group is connected to the membrane separation unit through a filter; at the same time, the molecular sieve dehydration tower group is connected in parallel to the output end of the cold box through a heater;

[0016] One output of the membrane separation unit is connected to the CO2 distillation column via a CO2 compressor and a cooler in sequence; the CO2 distillation column is connected to the CO2 heat exchanger in parallel; the other output of the membrane separation unit is connected to the cold box; the output of the cold box is connected to an LNG tank; and a refrigeration system is connected to the cold box in parallel.

[0017] Furthermore, the dehydration tower group uses molecular sieve towers for adsorption and dehydration, and is equipped with two or three towers for switching operation. The molecular sieves are mainly zeolites and their modified molecular sieves, and the regeneration adopts the TSA temperature-dependent desorption method.

[0018] Furthermore, in the liquefaction unit, the cold box combined with the refrigeration system liquefies methane. The methane is cooled to -90~-158℃ in the cold box. After the LNG is reduced to 0.03~0.6MPaG by the throttling valve, it enters the LNG tank to separate the gas phase BOG, which further improves the purity of LNG. The BOG is reheated in the cold box and then used as the regeneration gas of the dehydration unit. The liquid phase is transported out as LNG product.

[0019] Furthermore, the CO2 separated by the membrane separation unit is pressurized to 1~4MPa by a CO2 compressor, and after passing through a cooler, it enters a CO2 distillation column. After the medium at the bottom of the column is reheated, it enters a CO2 heat exchanger, and after being cooled and liquefied by a cold source in the refrigeration system, it enters the top of the CO2 distillation column. After the gas and liquid phases are separated by mass transfer, liquid CO2 at the bottom of the column meets food-grade requirements, and non-condensable gas at the top of the column is discharged at high altitude.

[0020] Furthermore, the cold box is a multi-flow plate-fin heat exchanger.

[0021] Furthermore, the refrigeration system is a mixed refrigerant refrigeration system or a cascade refrigeration system.

[0022] Beneficial effects

[0023] 1. Compared with traditional anaerobic digestion processes, the system of this invention achieves a high methane recovery rate (≥98%), which is significantly higher than the recovery efficiency of traditional processes, and effectively solves the problem of high methane loss rate in traditional processes;

[0024] 2. The CO2 purification process enables the high-value utilization of by-products, converting the CO2 directly emitted in traditional processes into food-grade liquid carbon dioxide, which reduces carbon emissions and creates economic value, solving the problem of carbon dioxide resource waste in traditional processes.

[0025] 3. This invention innovatively utilizes BOG cycle as regeneration gas for the dehydration unit, reducing the overall energy consumption of the system and achieving efficient utilization of energy within the system;

[0026] 4. The present invention uses a regenerative cold blowing process to extend the molecular sieve life, improve the system's operational stability and economy, and solve the problem of easy deactivation of molecular sieves in traditional processes;

[0027] 5. This invention achieves efficient and low-energy operation of the biogas treatment system through the organic integration of multi-stage purification processes, while simultaneously meeting the dual objectives of high methane recovery rate and carbon dioxide resource utilization. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a system structure for the co-production of LNG and food-grade LCO2 from biogas based on multi-stage purification, according to this utility model.

[0029] Figure 2 This is a schematic diagram of a method for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification according to this utility model.

[0030] Figure label:

[0031] P-101 Fan; V-101 Desulfurization and Ammonia Removal Tank; F-101 Desulfurization Filter; C-101 Biogas Compressor; E-101 Biogas Compressor Outlet Cooler; V-102 Biogas Compressor Outlet Separator; T-301A / B Dehydration Tower; F-301 Dust Filter; HE-301 Heater; E-301 Regenerated Gas Cooler; V-301 Regenerated Gas Separator; M-401 Membrane Separation Unit; ET-501 Cold Box; X-501 Refrigeration System; V-501 LNG Tank; C-601 CO2 Compressor; E-601 CO2 Compressor Outlet Cooler; E-602 CO2 Heat Exchanger; T-601 CO2 Distillation Column Detailed Implementation

[0032] The technical solution of this utility model will now be clearly and completely described with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of this utility model, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this utility model without creative effort are within the scope of protection of this utility model.

[0033] This invention provides a system for the co-production of LNG and food-grade LCO2 from biogas based on multi-stage purification, including a desulfurization and deammoniation unit, a first pressurization unit, a dehydration unit, a membrane separation unit, a liquefaction unit, a second pressurization unit, and a purification unit. The desulfurization and deammoniation unit uses a solid adsorbent composite bed to remove sulfides and ammonia from the biogas. The first pressurization unit uses a compressor to pressurize the biogas. The dehydration unit uses molecular sieves to adsorb moisture from the biogas. The membrane separation unit M-401 separates methane and CO2 by utilizing the different permeation rates of different gas molecules in the membrane module; the methane recovery rate is ≥98%.

[0034] The liquefaction unit utilizes a refrigeration unit to provide a cold source for the cold box, where biomethane is liquefied into LNG. Simultaneously, the refrigeration unit also provides a cold source for the cryogenic distillation of CO2. The second pressurization unit pressurizes the CO2 using a compressor. The purification unit purifies the liquid CO2 using a distillation column or separator. Figure 1 As shown, where:

[0035] The desulfurization and ammonia removal unit consists of a blower P-101, a desulfurization and ammonia removal tank V-101, and a desulfurization filter F-101.

[0036] The first pressurization unit consists of a biogas compressor C-101, a cooler E-101, and a first liquid separator V-102;

[0037] The dehydration unit consists of a molecular sieve dehydration tower group, a dust filter F-301, a heater HE-301, a cooler E-301, and a second separator V-301. The dehydration tower group of this invention adopts a two-tower or three-tower switching to realize the adsorption-regeneration-cold blowing cycle. The regeneration adopts the TSA variable temperature desorption process, in which the BOG is heated to 180-300℃ and the water in the molecular sieve is blown away in reverse to complete the desorption. Then, the unheated BOG is cold blown to complete the regeneration.

[0038] The liquefaction unit consists of a cold box ET-501, a refrigeration system X-501, and an LNG tank V-501.

[0039] The second booster unit consists of a CO2 compressor C-601 and a CO2 cooler E-601;

[0040] 1) The purification unit consists of a CO2 distillation column T-601 and a CO2 heat exchanger E-602. The C-601 compressor, E-601 / E-602 coolers, and T-601 distillation column are used in conjunction to produce liquid food-grade CO2, with non-condensable gas at the top of the column being discharged in compliance with standards. In this invention, the CO2 separated by the membrane module is pressurized to 1~4MPa by the C-601 CO2 compressor, and after passing through the air cooler or water cooler at the outlet of the E-601 compressor, it enters the T-601 CO2 distillation column. After reheating the medium at the bottom of the column, it enters the E-602 heat exchanger, and after being cooled and liquefied by a cold source in the X-501 refrigeration system, it enters the top of the T-601 CO2 distillation column. After mass transfer separation of the gas and liquid phases, liquid CO2 at the bottom of the column meets food-grade requirements, and non-condensable gas at the top of the column is discharged at high altitude. Specific details are as follows:

[0041] like Figure 2 As shown, biogas is connected to the sulfur deammoniation tank V-101 via blower P-101; the output of the sulfur deammoniation tank V-101 is connected to the desulfurization filter F-101, and the output of the desulfurization filter F-101 is connected to the biogas compressor C-101; the output of the biogas compressor C-101 is connected to the first liquid separator V-102 via cooler E-101; one output of the liquid separator V-102 is connected to the first dehydration tower T-301A and the second dehydration tower T-301B; the other output is connected to the wastewater treatment unit; simultaneously, the inputs of the first dehydration tower T-301A and the second dehydration tower T-301B are connected in parallel to the second liquid separator V-301 via cooler E-301; the output of the second liquid separator is connected to the wastewater treatment unit.

[0042] The output ends of the first dehydration tower T-301A and the second dehydration tower T-301B are connected to the membrane separation unit through filter F-301; at the same time, the output ends of the first dehydration tower T-301A and the second dehydration tower T-301B are connected in parallel to the output end of the cold box ET-501 through heater HE-301.

[0043] One output of the membrane separation unit M-401 is connected sequentially to the CO2 distillation column T-601 via the CO2 compressor C-601 and the CO2 compressor outlet cooler E-601; the CO2 distillation column T-601 is connected in parallel to the CO2 heat exchanger E-602; the other output of the membrane separation unit M-401 is connected to the cold box ET-501; the cold box output ET-501 is connected to the LNG tank V-501; and a refrigeration system X-501 is connected in parallel to the cold box. The specific implementation of this utility model is as follows:

[0044] Biogas is first sent to the desulfurization unit by the P-101 blower, and then enters the desulfurization and ammonia removal tank V-101. Sulfides and ammonia are removed by the desulfurization and ammonia removal molecular sieve, and then dust particles are removed by the F-101 desulfurization filter.

[0045] After entering the pressurization unit, the gas is pressurized to 1.5~6MPa by the C-101 biogas compressor. The gas is then cooled, separated into droplets, and then enters the dehydration unit.

[0046] The dehydration unit is implemented by a T-301 dehydration tower, which can be set to operate with two towers, one for adsorption and the other for regeneration / cold blowing. Alternatively, three molecular sieve towers can be set, one for adsorption, one for regeneration, and one for cold blowing. The following description focuses on two towers.

[0047] Adsorption: Moisture is removed by molecular sieve adsorption, which can remove moisture to 1 ppm, and obtain dry natural gas with qualified dew point. Then, after removing particulate impurities by F-301 dust filter, it enters the membrane separation unit.

[0048] Regeneration: After the molecular sieve becomes saturated with water, the TSA temperature-dependent desorption method is used, with BOG as the regeneration gas. The gas is heated to a high temperature of 180~300℃ by the HE-301 heater and passes through the molecular sieve bed. The water molecules adsorbed in the molecular sieve pores vaporize and leave the molecular sieve tower along with the regeneration gas. After being cooled by the E-301 regeneration gas cooler and separated by the V-301 regeneration gas separator, the gas phase is sent back to the inlet of the C-101 biogas compressor, and the liquid phase is discharged into the sewage buffer tank for transfer and treatment.

[0049] Cold blowing: After the hot regeneration is completed, the regenerated gas bypasses the heater and enters directly from the bottom of the molecular sieve tower, carrying away the heat in the molecular sieve tower. After being cooled by the E-301 regenerated gas cooler and separated by the V-301 regenerated gas separator, the gas phase is sent back to the inlet of the C-101 biogas compressor.

[0050] The membrane separation unit can utilize the different permeation rates of different components in biogas within the M-401 membrane module to separate most of the CO2 from biogas. CH4, as a biogas product, enters the cold box for liquefaction, with a methane recovery rate of up to 98%. The CO2 then enters the compressor for pressurization.

[0051] In the biogas liquefaction unit, biomethane is cooled to -90 to -158°C in the ET-501 cold box to liquefy LNG. After exiting the cold box, the LNG is depressurized to 0.03 to 0.6 MPaG through a throttling valve and enters the V-501 LNG tank for gas-liquid separation. The gaseous phase BOG is returned to the cold box for reheating and used as regeneration gas in the dehydration unit, while the liquid phase is transported as LNG product. The cold box is cooled by the X-501 refrigeration system.

[0052] The CO2 separated by the membrane module is pressurized to 1~4MPa by the C-601 CO2 compressor, and then enters the T-601 CO2 distillation column after passing through the E-601 compressor outlet air cooler or water cooler. After reheating the medium at the bottom of the column, it enters the E-602 heat exchanger, and is cooled and liquefied by a cold source in the X-501 refrigeration system before entering the top of the T-601 CO2 distillation column. After mass transfer separation of the gas and liquid phases, liquid CO2 at the bottom of the column meets food-grade requirements, and non-condensable gas at the top of the column is discharged at high altitude.

[0053] Example 1

[0054] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification includes a desulfurization and ammonia removal unit, a pressurization unit, a dehydration unit, a membrane separation unit, a liquefaction unit, a CO2 pressurization unit, and a purification unit.

[0055] The desulfurization and ammonia removal unit employs a solid adsorbent composite bed to remove sulfides and ammonia from biogas. This composite bed consists of multiple layers of different adsorbents: the bottom layer is a desulfurizing agent, primarily composed of iron oxide; the middle layer is activated carbon; and the top layer is a molecular sieve. Biogas enters from the bottom and passes sequentially through each layer of adsorbent. Hydrogen sulfide reacts with iron oxide to form iron sulfide, while ammonia is adsorbed by the activated carbon and molecular sieve. The composite bed is designed with a dual-tower structure, with one tower operating while the other is regenerated, ensuring continuous system operation. The regeneration process uses hot air purging, with the temperature controlled at 250-300℃, and the regeneration time is 8-12 hours.

[0056] The pressurization unit uses a compressor to pressurize the biogas. The compressor is a screw compressor, characterized by its compact structure, low vibration, and low noise. The compressor inlet pressure is atmospheric pressure, the outlet pressure is 0.8-1.2 MPa, and the compression ratio is 8-12. The compressor is equipped with a lubrication system, a cooling system, and a control system. The lubrication system uses forced lubrication, the cooling system uses water cooling, and the control system uses PLC control. The compressor speed is 1450-1750 rpm, the power is 250-350 kW, and the flow rate is 1000-1500 Nm³ / h.

[0057] The dehydration unit uses molecular sieves to adsorb moisture from biogas and employs a TSA (Temperature Activated Gas) desorption process for regeneration. Boiled biogas (BOG) is heated to 180-300℃ to reverse the regeneration of the molecular sieves. The dehydration unit consists of three molecular sieve towers: Tower A, Tower B, and Tower C. These towers operate alternately, with one tower performing adsorption, one regeneration, and one cooling. The molecular sieve towers are 3-4 meters high and 0.8-1.2 meters in diameter, filled with type 3A molecular sieves to a height of 2.5-3.5 meters. The adsorption process is carried out at room temperature, a pressure of 0.8-1.2 MPa, and an adsorption time of 8-12 hours. During regeneration, the BOG gas is heated to 180-300℃ by a regeneration gas heater at a flow rate of 100-150 Nm³ / h, and the regeneration time is 6-8 hours. The cooling process uses room-temperature BOG gas at a flow rate of 100-150 Nm³ / h, and the cooling time is 2-4 hours. The regenerated and cooled gas is returned to the inlet of the biogas compressor. The regenerated gas heater uses electric heating, with a power of 30-50kW and a heating efficiency of 85-90%.

[0058] The membrane separation unit separates methane and CO2 by utilizing the different permeation rates of different gas molecules within the membrane module. The unit employs a polyimide hollow fiber membrane, and the membrane module consists of multiple membrane elements connected in parallel. To prevent membrane fouling, a refrigerated dryer, a precision filter, and an oil separator are installed before the membrane separation unit. The refrigerated dryer operates at 2-5℃, the filter has a precision of 0.1μm, and the oil separator uses activated carbon adsorption. The inlet temperature of the membrane separation unit is controlled at 35-45℃, and the pressure is 0.8-1.2MPa. The membrane module adopts a two-stage series structure. The permeate gas of the first-stage membrane module is mainly CO2, with a concentration of 85-95%. The permeate gas from the second-stage membrane module is returned to the inlet of the first-stage membrane module. The residual gas in the second-stage membrane module is high-purity methane, with a methane concentration of 95-98%, and a CH4 recovery rate of 97%.

[0059] The liquefaction unit utilizes a refrigeration unit to provide a cooling source for the cold box, where biomethane is liquefied into LNG. Simultaneously, the refrigeration unit also provides a cooling source for the cryogenic distillation of CO2. The cold box employs a multi-flow plate-fin heat exchanger made of aluminum alloy, characterized by high heat transfer efficiency and a compact structure. Multiple temperature zones are designed within the cold box, with temperatures progressively decreasing from -40℃ to -162℃. The refrigeration system uses a mixed refrigerant system, composed of nitrogen, methane, ethylene, propane, and isobutane, in proportions of 5%, 25%, 35%, 25%, and 10%, respectively. The refrigeration capacity is 100-150kW, with a refrigeration efficiency of 0.4-0.5. Methane gas is progressively cooled within the cold box, ultimately liquefying at -162℃ with a liquefaction rate of 95-98%. The liquefied LNG is stored in vacuum-insulated tanks with a volume of 50-100 m³, a storage temperature of -162℃, and a storage pressure of 0.12-0.15 MPa.

[0060] The second pressurization unit uses a compressor to pressurize CO2. The CO2 compressor is a reciprocating compressor with three stages, each with a compression ratio of 3-4. The first stage has an inlet pressure of 0.1-0.15 MPa and an outlet pressure of 0.3-0.5 MPa; the second stage has an inlet pressure of 0.3-0.5 MPa and an outlet pressure of 1.0-1.5 MPa; and the third stage has an inlet pressure of 1.0-1.5 MPa and an outlet pressure of 5.0-7.0 MPa. An intercooler is installed after each stage of compression to reduce the gas temperature to 30-40℃. The compressor has a power of 50-80 kW and a flow rate of 200-300 Nm³ / h.

[0061] The purification unit uses a distillation column to purify liquid CO2. The distillation column is 8-10 meters high and 0.6-0.8 meters in diameter, with 30-40 sieve plates spaced 0.2-0.25 meters apart. The operating pressure of the distillation column is 5.0-7.0 MPa, the top temperature is -30 to -20°C, and the bottom temperature is -5 to 5°C. The CO2 after the CO2 compressor outlet cooler is used as a heat source to reheat the liquefied CO2 at the bottom of the column. After exiting the top, it is cooled by a refrigeration system before entering the top of the column. The gas-liquid balance of the distillation column is regulated by a bypass before entering the bottom, with a bypass flow rate of 10-20% of the total flow rate. The purified liquid CO2 reaches a purity of 99.9%, meeting food-grade requirements. It is stored in a liquid CO2 storage tank with a volume of 30-50 m³, a storage temperature of -20°C, and a storage pressure of 2.0-2.5 MPa.

[0062] This system can simultaneously produce liquefied natural gas (LNG) and liquefied carbon dioxide (LCO2). LNG production is 500-800 kg / h with a purity of 95-98%; LCO2 production is 300-500 kg / h with a purity of 99.9%. The total energy consumption of the system is 0.8-1.2 kWh / Nm³ biogas, with an energy utilization rate of 85-90%.

[0063] Example 2

[0064] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification is basically the same as that in Example 1, except that the compressor is a reciprocating compressor.

[0065] The reciprocating compressor adopts a four-cylinder double-acting structure with a cylinder diameter of 200-250mm, a piston stroke of 150-200mm, and a speed of 300-400rpm. The compressor inlet pressure is atmospheric pressure, the outlet pressure is 0.8-1.2MPa, and the compression ratio is 8-12. The compressor is equipped with a lubrication system, a cooling system, and a control system. The lubrication system uses forced lubrication, the cooling system uses water cooling, and the control system uses PLC control. The compressor power is 250-350kW, and the flow rate is 1000-1500Nm³ / h.

[0066] Example 3

[0067] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification is basically the same as that in Example 1, except that the compressor is a centrifugal compressor.

[0068] The centrifugal compressor adopts a single-shaft, three-stage structure with an impeller diameter of 300-400mm and a rotational speed of 10,000-15,000 rpm. The compressor inlet pressure is atmospheric pressure, the outlet pressure is 0.8-1.2 MPa, and the compression ratio is 8-12. The compressor is equipped with a lubrication system, a cooling system, and a control system. The lubrication system uses forced lubrication, the cooling system uses water cooling, and the control system uses PLC control. The compressor power is 250-350kW, and the flow rate is 1000-1500 Nm³ / h.

[0069] Example 4

[0070] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification is basically the same as that in Example 1, except that the dehydration unit adopts a two-tower switching operation.

[0071] The dehydration unit consists of two molecular sieve towers, tower A and tower B, which operate alternately, one for adsorption and the other for regeneration and cooling. The molecular sieve towers are 3-4 meters high and 1.0-1.5 meters in diameter, filled with type 3A molecular sieves to a height of 2.5-3.5 meters. The adsorption process takes place at room temperature, at a pressure of 0.8-1.2 MPa, for 12-16 hours. During regeneration, BOG gas is heated to 180-300℃ by a regeneration gas heater at a flow rate of 100-150 Nm³ / h, for 8-10 hours. The cooling process uses room temperature BOG gas at a flow rate of 50-80 Nm³ / h, for 4-6 hours. The regenerated and cooled gas is returned to the inlet of the biogas compressor.

[0072] Example 5

[0073] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification is basically the same as that in Example 1, except that the regenerated gas heater uses a heat transfer medium heating method.

[0074] The heat transfer oil heater uses thermal oil as the heat transfer medium, with a temperature of 300-350℃ and a flow rate of 5-8 m³ / h. The thermal oil is heated by an electric heater with a power of 50-70 kW. The heat exchange area of ​​the heat transfer oil heater is 5-8 m², and the heat exchange efficiency is 80-85%. BOG gas is heated to 180-300℃ in the heat transfer oil heater for the regeneration of the molecular sieve.

[0075] Example 6

[0076] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification is basically the same as that in Example 1, except that the membrane material is a cellulose acetate membrane.

[0077] The membrane separation unit uses cellulose acetate membranes, and the membrane module consists of multiple membrane elements connected in parallel. To prevent membrane fouling, a refrigerated dryer, a precision filter, and an oil separator are installed before the membrane separation unit. The refrigerated dryer operates at a temperature of 2-5℃, the filter has a precision of 0.1μm, and the oil separator uses activated carbon adsorption. The inlet temperature of the membrane separation unit is controlled at 30-40℃, and the pressure is 0.8-1.2MPa. The membrane module adopts a three-stage series structure. The permeate gas of the first and second stage membrane modules is mainly CO2, with a concentration of 90-98%. The permeate gas of the third stage membrane module is returned to the inlet of the first stage membrane module. The residual gas of the third stage membrane module is high-purity methane, with a methane concentration of 98-99%, and the CH4 recovery rate reaches 99%.

[0078] Example 7

[0079] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification is basically the same as that in Example 1, except that the refrigeration system adopts a cascade refrigeration system.

[0080] The cascade refrigeration system consists of three refrigeration cycles: a propane precooling cycle, an ethylene intermediate-temperature cycle, and a methane cryogenic cycle. The propane cycle has an evaporation temperature of -35 to -30°C and a condensation temperature of 40 to 45°C; the ethylene cycle has an evaporation temperature of -95 to -90°C and a condensation temperature of -30 to -25°C; and the methane cycle has an evaporation temperature of -165 to -160°C and a condensation temperature of -90 to -85°C. The compressor power for each cycle is 30-40kW, 40-50kW, and 50-60kW, respectively. The total cooling capacity of the cascade refrigeration system is 120-150kW, and the refrigeration efficiency is 0.45-0.55.

[0081] Example 8

[0082] A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification is basically the same as that in Example 1, except that the CO2 purification unit uses a separator tank.

[0083] The CO2 purification unit uses a separation tank, which is 3-4 meters high and 1.0-1.2 meters in diameter. The working pressure of the separation tank is 5.0-7.0 MPa, and the working temperature is -30 to -20℃. CO2 gas condenses into liquid in the separation tank; non-condensable gases are discharged from the top of the tank, and liquid CO2 is discharged from the bottom. A demister is installed inside the separation tank to remove entrained droplets. The separated liquid CO2 has a purity of 99.5%, meeting food-grade requirements. It is stored in a liquid CO2 storage tank with a volume of 30-50 m³, a storage temperature of -20℃, and a storage pressure of 2.0-2.5 MPa.

[0084] It should be noted that Examples 1, 2, 3, 4, 5, 6, 7, and 8 are all systems and methods for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification.

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this utility model, and not to limit it. Although this utility model has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this utility model.

Claims

1. A system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification, characterized in that: The system includes a desulfurization and ammonia removal unit, a first pressurization unit, a dehydration unit, a membrane separation unit, a liquefaction unit, a second pressurization unit, and a purification unit; wherein: the desulfurization and ammonia removal unit consists of a blower, a desulfurization and ammonia removal tank, and a desulfurization filter; the first pressurization unit consists of a biogas compressor, a cooler, and a first liquid separator; the dehydration unit consists of a molecular sieve dehydration tower assembly, a dust filter, a heater, a cooler, and a second liquid separator; the liquefaction unit consists of a cold box, a refrigeration system, and an LNG tank; The second pressurization unit consists of a CO2 compressor and a CO2 cooler; the purification unit consists of a CO2 distillation column and a CO2 heat exchanger, wherein: biogas is connected to the sulfur deammoniation tank via a blower; the output end of the sulfur deammoniation tank is connected to the desulfurization filter, and the output end of the desulfurization filter is connected to the biogas compressor; the output end of the biogas compressor is connected to the first separating tank via a cooler; one output end of the separating tank is connected to the molecular sieve dehydration tower group; the other output end is connected to the wastewater treatment unit; simultaneously, the input end of the molecular sieve dehydration tower group is connected to... The second separating tank is connected in parallel; the output end of the second separating tank is connected to the wastewater treatment unit; the output end of the molecular sieve dehydration tower group is connected to the membrane separation unit through a filter; at the same time, the molecular sieve dehydration tower group is connected in parallel to the output end of the cold box through a heater; one output end of the membrane separation unit is connected to the CO2 distillation tower in sequence through a CO2 compressor and a cooler; the CO2 distillation tower is connected in parallel to the CO2 heat exchanger; the other output end of the membrane separation unit is connected to the cold box; the output end of the cold box is connected to an LNG tank; at the same time, a refrigeration system is connected in parallel to the cold box.

2. The system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification according to claim 1, characterized in that, The dehydration tower group uses molecular sieve towers for adsorption and dehydration, and can be set up to switch between two or three towers. The molecular sieves are mainly zeolites and their modified molecular sieves. Regeneration is carried out using the TSA temperature-switching desorption method.

3. The system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification according to claim 2, characterized in that, The cold box is a multi-flow plate-fin heat exchanger.

4. The system for co-producing LNG and food-grade LCO2 from biogas based on multi-stage purification according to claim 1, characterized in that, The refrigeration system is a mixed refrigerant refrigeration system or a cascade refrigeration system.

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