SYSTEM AND METHOD FOR SIMULTANEOUSLY PRODUCING HIGH-PURITY LIQUID NITROGEN AND HIGH-PURITY LIQUID CARBON DIOXIDE AT LOW COST BY USING FLUE GAS

Abstract

The present application belongs to the field of environmental protection, and in particular to a system and method for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas. The system provided by the present application comprises a flue gas boosting and cooling system, a CO.sub.2/N.sub.2 front-end separation system, a liquid CO.sub.2 preparation system and a high-purity liquid nitrogen preparation system The present application can maximize the use of flue gas to produce high-purity liquid nitrogen and liquid CO.sub.2 at low cost, and realize the synchronous resource utilization of carbon/nitrogen components.

Claims

1. A system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas, comprising: a flue gas boosting and cooling system, a CO.sub.2/N.sub.2 front-end separation system, a liquid CO.sub.2 preparation system and a high-purity liquid nitrogen preparation system; wherein the flue gas boosting and cooling system comprises a flue gas booster (1), a flue gas cooling tower (2), a water cooling tower (3), a low-temperature water pump (4) and a chiller unit (5), configured to pressurize, cool, dewater and wash raw flue gas; the CO.sub.2/N.sub.2 front-end separation system is configured to separate CO.sub.2 by using a pressure swing adsorption method, and comprises a plurality of pressure swing adsorption towers (13, 14), a switching flow path and a valve, a vacuum pump (15), a CO.sub.2 buffer device (16) and an adsorption tail gas buffer device (19); when the system is in operation, the plurality of pressure swing adsorption towers (13, 14) are configured to adsorb CO.sub.2 in the flue gas, and then the vacuum pump (15) depressurizes to desorb and obtain a CO.sub.2 product gas; the liquid CO.sub.2 preparation system comprises a CO.sub.2 booster (20), a CO.sub.2 purification tower (22), a CO.sub.2 condenser (24), a CO.sub.2 evaporator (23) and a CO.sub.2 subcooler (46), configured to further purify and liquefy the desorbed CO.sub.2 product gas to obtain a high-purity liquid CO.sub.2 product; the high-purity liquid nitrogen preparation system comprises a blower (47), a nitrogen booster (34), a nitrogen cooler (32), an expansion cooling system (31), a nitrogen liquefier (33), a gas-liquid separator (43) and a low-temperature separation system (42), configured to further purify and liquefy exhaust gas of the plurality of pressure swing adsorption towers (13, 14) to obtain a liquid nitrogen product; when the system is in operation, compression heat generated by compressing CO.sub.2 in the liquid CO.sub.2 preparation system is used to assist the operation of the plurality of pressure swing adsorption towers (13, 14); a part of fluid produced by the nitrogen liquefier (33) of the high-purity liquid nitrogen preparation system is drawn out to be used as a cold source for the liquid CO.sub.2 preparation system; dry exhaust gas produced by the high-purity liquid nitrogen preparation system is used in the water cooling tower (3) of the flue gas boosting and cooling system to recover cooling capacity; the flue gas boosting and cooling system, the CO.sub.2/N.sub.2 front-end separation system, the liquid CO.sub.2 preparation system and the high-purity liquid nitrogen preparation system, as well as various devices and equipments in each system, are connected through pipelines and valves.

2. The system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, wherein in the flue gas boosting and cooling system, the flue gas is pressurized to a required pressure by the flue gas booster (1), and the pressurized flue gas is sent to the flue gas cooling tower (2) for washing and cooling; the flue gas cooling tower (2) is structurally divided into two sections, and one part of ambient temperature circulating water is pumped into a middle and lower part of the flue gas cooling tower (2) through a first water valve (6), and another part of ambient temperature circulating water is pumped into the water cooling tower (3) through a second water valve (7); low-temperature water is pumped into an upper part of the flue gas cooling tower through a third water valve (8) and the low-temperature water pump (4), which is cooled by the dry nitrogen discharged from the high-purity liquid nitrogen preparation system and chiller unit (5); during the shutdown of the high-purity liquid nitrogen preparation system, a fourth water valve (9) is opened to cool the flue gas by circulating the low-temperature water in the upper part.

3. The system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, wherein in the CO.sub.2/N.sub.2 front-end separation system, inlets of the plurality of pressure swing adsorption towers (13, 14) are connected to the flue gas treated by the flue gas cooling tower (2) through a first gas valve (12) to continuously adsorb and capture CO.sub.2 in the flue gas; an inlet of the vacuum pump (15) is connected to the plurality of pressure swing adsorption towers (13, 14), and after the adsorption is completed, the vacuum pump (15) is used to desorb and regenerate the pressure swing adsorption towers that have been saturated with adsorption; an outlet of the vacuum pump (15) is connected to the CO.sub.2 buffer device (16), and desorbed gas is sent to the CO.sub.2 buffer device (16); a gas that is not adsorbed by the plurality of pressure swing adsorption towers (13, 14) is sent to the adsorption tail gas buffer device (19) through a second gas valve (18) as an adsorption tail gas.

4. The system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, wherein in the liquid CO.sub.2 preparation system, an inlet of the CO.sub.2 booster (20) is connected to a gas in the CO.sub.2 buffer device (16) through a third gas valve (17), and a pressurized hot fluid is connected to the plurality of pressure swing adsorption towers (13, 14) through valves to improve regeneration and desorption efficiency of the pressure swing adsorption towers, and the hot fluid is cooled and returned to the CO.sub.2 evaporator (23) of the liquid CO.sub.2 preparation system through an eighth gas valve (21) as a heat source, the fluid is then further cooled and sent to the CO.sub.2 subcooler (46) for condensation and liquefaction, and then it is sent to a middle part of the CO.sub.2 purification tower (22) for low-temperature separation to obtain the high-purity liquid CO.sub.2 product at a bottom of the CO.sub.2 evaporator (23).

5. The system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, wherein a part of low-temperature fluid is drawn out from a middle and upper part of the nitrogen liquefier (33) of the high-purity liquid nitrogen preparation system and connected to a cold source inlet of the CO.sub.2 subcooler (46) through a fourth gas valve (44) to serve as a cold source to condense and liquefy the CO.sub.2 gas from the CO.sub.2 evaporator (23); after heat exchange in the CO.sub.2 subcooler (46), the low-temperature fluid is connected from a cold source outlet of the CO.sub.2 subcooler (46) to an inlet of the blower (47) of the high-purity liquid nitrogen preparation system through a fifth gas valve (25), and returned to the high-purity liquid nitrogen preparation system for recycling.

6. The system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, further comprising an external refrigerant circulation refrigeration system, wherein the external refrigerant circulation refrigeration system comprises a refrigerant compressor (26), a refrigerant cooler (27), a refrigerant heat regenerator (28) and a refrigerant throttle valve (29); a refrigerant outlet of the CO.sub.2 condenser (24) is connected to the refrigerant heat regenerator (28), a refrigerant flowing out of the refrigerant outlet of the CO.sub.2 condenser (24) is reheated by the refrigerant heat regenerator (28), pressurized by the refrigerant compressor (26), cooled by the refrigerant cooler (27), cooled by the refrigerant heat regenerator (28) and throttled by the refrigerant throttle valve (29), and then returns to a refrigerant inlet of the CO.sub.2 condenser (24) to complete the refrigerant circulation refrigeration.

7. The system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, wherein in the high-purity liquid nitrogen preparation system, gas from the adsorption tail gas buffer device (19) is combined with gas from the outlet of the CO.sub.2 subcooler (46) after boosted by the blower (47) through a sixth gas valve (30), and then connected to a hot-end inlet of the nitrogen liquefier (33) through a seventh gas valve (48), it is cooled by the nitrogen liquefier (33), and then sent to the low-temperature separation system (42) for separation and purification to obtain high-purity nitrogen; meanwhile, a part of the fluid extracted from the middle and upper part of the nitrogen liquefier (33) is sent to the CO.sub.2 subcooler (46) of the liquid CO.sub.2 preparation system through the fourth gas valve (44) to serve as a cold source; the high-purity nitrogen from the low-temperature separation system (42) is connected to a cold-end inlet of the nitrogen liquefier (33), reheated by the nitrogen liquefier (33), and then connected to an inlet of the nitrogen booster (34); gas from an outlet of the nitrogen booster (34) is cooled by the nitrogen cooler (32) and enters the expansion cooling system (31) for pressurization and expansion, and is connected to the hot-end inlet of the nitrogen liquefier (33) to provide the cooling capacity required by the nitrogen liquefier (33); low-temperature high-pressure gas from the nitrogen liquefier (33) is connected to the gas-liquid separator (43) through a high-pressure throttle valve (45); gas at the top of the gas-liquid separator (43) is reheated by the nitrogen liquefier (33) and returned to the inlet of the nitrogen booster (34); liquid at the bottom outlet of the gas-liquid separator (43) is sent to the low-temperature separation system (42); low-pressure exhaust gas from the low-temperature separation system (42) is reheated by the nitrogen liquefier (33) and is connected to the inlet of the water cooling tower (3) in the flue gas boosting and cooling system through a ninth gas valve (55) to cool circulating water; the low-temperature separation system (42) sends out a subcooled high-purity liquid nitrogen product.

8. The system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, wherein the flue gas booster (1) is a multi-stage centrifugal compressor or blower.

9. The system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, wherein the CO.sub.2 buffer device (16) and the adsorption tail gas buffer device (19) are separately selected as an air bag or a buffer tank.

10. A method for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas, wherein the flue gas is treated in the system for simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using flue gas according to claim 1, to obtain the liquid nitrogen product and the liquid carbon dioxide product.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solution in embodiments of the present application or the conventional technology, the following will briefly introduce drawings required for use in the embodiments or the conventional technology description. Apparently, the drawings described below are only embodiments of the present application. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative work.

[0040] FIG. 1 is a process flow framework diagram according to an embodiment of the present application.

[0041] FIG. 2 is a process flow schematic diagram according to an embodiment of the present application.

[0042] FIG. 3 is a process flow schematic diagram according to an embodiment of the present application.

TABLE-US-00001 Description of reference numerals: 1: flue gas booster; 2: flue gas cooling tower; 3: water cooling tower; 4: low-temperature water pump; 5: chiller unit; 6: first water valve; 7: second water valve; 8: third water valve; 9: fourth water valve; 10: fifth water valve; 11: sixth water valve; 12: first gas valve; 13, 14: pressure swing adsorption tower; 15: vacuum pump; 16: CO.sub.2 buffer device; 17: third gas valve; 18: second gas valve; 19: adsorption tail gas buffer device; 20: CO.sub.2 booster; 21: eighth gas valve; 22: CO.sub.2 purification tower; 23: CO.sub.2 evaporator; 24: CO.sub.2 condenser; 25: fifth gas valve; 26: refrigerant compressor; 27: refrigerant cooler; 28: refrigerant heat regenerator; 29: refrigerant throttle valve; 30: sixth gas valve; 31: expansion cooling system; 32: nitrogen cooler; 33: nitrogen liquefier; 34: nitrogen booster; 35: ambient temperature water pump; 36: boosting end of first gas expansion; 37: first gas expansion cooler; 38: boosting end of second gas expansion; 39: second gas expansion cooler; 40: expansion end of second gas expansion; 41: expansion end of first gas expansion; 42: low-temperature separation system; 43: gas-liquid separator; 44: fourth gas valve; 45: high-pressure throttle valve; 46: CO.sub.2 subcooler; 47: blower; 48: seventh gas valve; 49: first nitrogen purification tower; 50: condenser of the first nitrogen purification tower; 51: second nitrogen purification tower; 52: condenser of the second nitrogen purification tower; 53: heat exchanger; 54: low-temperature circulation pump; 55: ninth gas valve.

DETAILED DESCRIPTION OF EMBODIMENTS

[0043] Hereinafter the technical solution in embodiments of the present application is clearly and completely described in conjunction with drawings in the embodiments of the present application. Apparently, the described embodiments are only part, not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

[0044] Reference is made to FIG. 1. The system for simultaneously producing high-purity liquid nitrogen and liquid carbon dioxide at low cost by using flue gas comprises at least: flue gas boosting and cooling system, CO.sub.2/N.sub.2 front-end separation system, liquid CO.sub.2 preparation system and high-purity liquid nitrogen preparation system. The system utilizes pressure swing adsorption technology and low-temperature separation technology, integrates the transfer and utilization of heating capacity and cooling capacity, and realizes the liquefaction and recovery of CO.sub.2 and N.sub.2 in flue gas.

[0045] Reference is made to FIG. 2. The flue gas boosting and cooling system comprises a flue gas booster (1), a flue gas cooling tower (2), a water cooling tower (3), a low-temperature water pump (4) and a chiller unit (5), configured to pressurize, cool, dewater and wash raw flue gas; the CO.sub.2/N.sub.2 front-end separation system is configured to separate CO.sub.2 by using a pressure swing adsorption method, and comprises a plurality of pressure swing adsorption towers (13, 14), a switching flow path and a valve, a vacuum pump (15), a CO.sub.2 buffer device (16) and an adsorption tail gas buffer device (19); when the system is in operation, the plurality of pressure swing adsorption towers (13, 14) are configured to adsorb CO.sub.2 in the flue gas, and then the vacuum pump (15) depressurizes to desorb and obtain CO.sub.2 product gas; the liquid CO.sub.2 preparation system comprises a CO.sub.2 booster (20), a CO.sub.2 purification tower (22), a CO.sub.2 condenser (24), a CO.sub.2 evaporator (23) and a CO.sub.2 subcooler (46), configured to further purify and liquefy the desorbed CO.sub.2 product gas to obtain a liquid CO.sub.2 product; the high-purity liquid nitrogen preparation system comprises a blower (47), a nitrogen booster (34), a nitrogen cooler (32), an expansion cooling system (31), a nitrogen liquefier (33), a gas-liquid separator (43) and a low-temperature separation system (42), configured to further purify and liquefy exhaust gas of the plurality of pressure swing adsorption towers (13, 14) to obtain a liquid nitrogen product; the flue gas boosting and cooling system, the CO.sub.2/N.sub.2 front-end separation system, the liquid CO.sub.2 preparation system and the high-purity liquid nitrogen preparation system, as well as various devices and equipments in each system, are connected through pipelines and valves.

[0046] Reference is made to FIG. 2. In the flue gas boosting and cooling system, the flue gas is pressurized to a required pressure by the flue gas booster (1), and the pressurized flue gas is sent to the flue gas cooling tower (2) for washing and cooling; the flue gas cooling tower (2) is structurally divided into two sections, and one part of ambient temperature circulating water is pumped into a middle and lower part of the flue gas cooling tower (2) through a first water valve (6), and another part of ambient temperature circulating water is pumped into the water cooling tower (3) through a second water valve (7); low-temperature water is pumped into an upper part of the flue gas cooling tower through a third water valve (8) and the low-temperature water pump (4), which is cooled by the dry nitrogen discharged from the high-purity liquid nitrogen preparation system and chiller unit (5). During the shutdown of the high-purity liquid nitrogen preparation system, a fourth water valve (9) is opened to cool the flue gas by circulating the low-temperature water in the upper part; the fifth water valve is a water supply valve (10), and the sixth water valve (11) is a drain valve.

[0047] Reference is made to FIG. 2. In the CO.sub.2/N.sub.2 front-end separation system, inlets of the plurality of pressure swing adsorption towers (13, 14) are connected to the flue gas treated by the flue gas cooling tower (2) through a first gas valve (12) to continuously adsorb and capture CO.sub.2 in the flue gas; an inlet of the vacuum pump (15) is connected to the plurality of pressure swing adsorption towers (13, 14), and after the adsorption is completed, the vacuum pump (15) is used to desorb and regenerate the plurality of pressure swing adsorption towers that have been saturated with adsorption; an outlet of the vacuum pump (15) is connected to the CO.sub.2 buffer device (16), and desorbed gas is sent to the CO.sub.2 buffer device (16); in order to improve the desorption and regeneration effect and the purity of the CO.sub.2 product, the hot fluid compressed by the CO.sub.2 booster (20) is used to heat the pressure swing adsorption towers (13, 14) that have been saturated with adsorption, and the cooled CO.sub.2 fluid is returned to the liquid CO.sub.2 preparation system; a gas that is not adsorbed by the plurality of pressure swing adsorption towers (13, 14) is sent to the adsorption tail gas buffer device (19) through a second gas valve (18) as an adsorption tail gas.

[0048] Reference is made to FIG. 2. In the liquid CO.sub.2 preparation system, an inlet of the CO.sub.2 booster (20) is connected to a gas in the CO.sub.2 buffer device (16) through a third gas valve (17), and a pressurized hot fluid is connected to the plurality of pressure swing adsorption towers (13, 14) through valves to improve regeneration and desorption efficiency of the pressure swing adsorption towers, and the hot fluid is cooled and returned to the CO.sub.2 evaporator (23) of the liquid CO.sub.2 preparation system through an eighth gas valve (21) as a heat source. The fluid is then further cooled and sent to the CO.sub.2 subcooler (46) for condensation and liquefaction. And then it is sent to a middle part of the CO.sub.2 purification tower (22) for low-temperature separation to obtain the high-purity liquid CO.sub.2 product at a bottom of the CO.sub.2 evaporator (23); if a food-grade CO.sub.2 product is required, a subsequent refining system can be set up to further remove other impurities in the liquid CO.sub.2 product.

[0049] Reference is made to FIG. 2. Apart of low-temperature fluid is drawn out from a middle and upper part of the nitrogen liquefier (33) of the high-purity liquid nitrogen preparation system and connected to a cold source inlet of the CO.sub.2 subcooler (46) through a fourth gas valve (44) to serve as a cold source to condense and liquefy the CO.sub.2 gas from the CO.sub.2 evaporator (23); after heat exchange in the CO.sub.2 subcooler (46), the low-temperature fluid is connected from a cold source outlet of the CO.sub.2 subcooler (46) to an inlet of the blower (47) of the high-purity liquid nitrogen preparation system through a fifth gas valve (25), and returned to the high-purity liquid nitrogen preparation system for recycling.

[0050] Reference is made to FIG. 2. An external refrigerant circulation refrigeration system is provided, the external refrigerant circulation refrigeration system comprises a refrigerant compressor (26), a refrigerant cooler (27), a refrigerant heat regenerator (28) and a refrigerant throttle valve (29); a refrigerant outlet of the CO.sub.2 condenser (24) is connected to the refrigerant heat regenerator (28), a refrigerant flowing out of the refrigerant outlet of the CO.sub.2 condenser (24) is reheated by the refrigerant heat regenerator (28), pressurized by the refrigerant compressor (26), cooled by the refrigerant cooler (27), cooled by the refrigerant heat regenerator (28) and throttled by the refrigerant throttle valve (29), and then returns to a refrigerant inlet of the CO.sub.2 condenser (24) to complete the refrigerant circulation refrigeration.

[0051] Reference is made to FIG. 2. In the high-purity liquid nitrogen preparation system, gas from the adsorption tail gas buffer device (19) is combined with gas from the outlet of the CO.sub.2 subcooler (46) after boosted by the blower (47) through the sixth gas valve (30), and then connected to a hot-end inlet of the nitrogen liquefier (33) through a seventh gas valve (48). It is cooled by the nitrogen liquefier (33), and then sent to the low-temperature separation system (42) for separation and purification to obtain high-purity nitrogen; meanwhile, a part of the fluid extracted from the middle and upper part of the nitrogen liquefier (33) is sent to the CO.sub.2 subcooler (46) of the liquid CO.sub.2 preparation system through the fourth gas valve (44) to serve as a cold source; the high-purity nitrogen from the low-temperature separation system (42) is connected to a cold-end inlet of the nitrogen liquefier (33), reheated by the nitrogen liquefier (33), and then connected to an inlet of the nitrogen booster (34); gas from an outlet of the nitrogen booster (34) is cooled by the nitrogen cooler (32) and enters the expansion cooling system (31) for pressurization and expansion, and is connected to the hot-end inlet of the nitrogen liquefier (33) to provide the cooling capacity required by the nitrogen liquefier (33); low-temperature high-pressure gas from the nitrogen liquefier (33) is connected to the gas-liquid separator (43) through a high-pressure throttle valve (45); gas at the top of the gas-liquid separator (43) is reheated by the nitrogen liquefier (33) and returned to the inlet of the nitrogen booster (34); liquid at the bottom outlet of the gas-liquid separator (43) is sent to the low-temperature separation system (42); low-pressure exhaust gas from the low-temperature separation system (42) is reheated by the nitrogen liquefier (33) and is connected to the inlet of the water cooling tower (3) in the flue gas boosting and cooling system through the ninth gas valve (55) to cool circulating water; the low-temperature separation system (42) sends out a subcooled high-purity liquid nitrogen product.

[0052] In the system provided by the present application, the flue gas booster (1) is a multi-stage centrifugal compressor or blower, which can be designed and selected according to the impurity content in the flue gas.

[0053] In the system provided by the present application, the specific number of the plurality of pressure swing adsorption towers (13, 14) can be designed and selected according to the flue gas treatment capacity and the impurity content in the flue gas.

[0054] In the system provided by the present application, the high-purity liquid nitrogen preparation system provides cooling capacity for the flue gas boosting and cooling system and the liquid CO.sub.2 preparation system, and provides matching cooling capacity according to the temperature requirements of each system, so as to achieve efficient utilization of the cooling capacity in the system and optimize the overall energy consumption.

[0055] In the system provided by the present application, the liquid CO.sub.2 preparation system can obtain industrial-grade liquid CO.sub.2, and a refining system can also be provided to produce food-grade liquid CO.sub.2.

[0056] In the system provided by the present application, the low-temperature separation system (42) in the high-purity liquid nitrogen preparation system can adopt a single nitrogen tower with single condenser purification process, or a double nitrogen tower with double condenser purification process.

[0057] In the present application, the specific steps of simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost using the flue gas in the system as shown in FIG. 2 are as follows. [0058] Step 1: The raw flue gas enters the inlet of the flue gas booster (1) in the flue gas boosting and cooling system. The pressurized flue gas is sent to the flue gas cooling tower (2) for washing and cooling, and water-soluble impurities in the flue gas are removed simultaneously. Ambient temperature circulating water is used in the lower section of the flue gas cooling tower (2), and one part of the ambient temperature circulating water is pumped into the middle and lower part of the flue gas cooling tower (2) through the first water valve (6) and an ambient temperature water pump (35), and another part of the ambient temperature circulating water is pumped into the water cooling tower (3) through the second water valve (7). Low-temperature water is pumped into the upper part of the flue gas cooling tower through a third water valve (8) and the low-temperature water pump (4), which is cooled by the dry nitrogen discharged from the high-purity liquid nitrogen preparation system and chiller unit (5). During the shutdown of the high-purity liquid nitrogen preparation system, the fourth water valve (9) is opened to cool the flue gas by circulating the low-temperature water in the upper part. The sixth water valve (11) is provided at the bottom of the flue gas cooling tower (2), and the circulating water is discharged from the flue gas cooling tower (2) through the sixth water valve (11); the fifth water valve (10) is a water supply valve, which replenishes water carried away by the nitrogen gas to the system. [0059] Step 2: The low-temperature flue gas at the outlet of the flue gas cooling tower (2) is connected to the inlets of the plurality of pressure swing adsorption towers (13, 14) in the CO.sub.2/N.sub.2 front-end separation system through the first gas valve (12), and the CO.sub.2 in the flue gas is continuously adsorbed, captured and separated by the principle of the pressure swing adsorption. The number of the pressure swing adsorption towers can be designed according to the treatment amount and concentration of the flue gas, and the pressure swing adsorption towers that have been saturated with adsorption are regenerated by reducing pressure and vacuum desorption. The inlet of the vacuum pump (15) is connected to the plurality of pressure swing adsorption towers (13, 14). After the adsorption is completed, the pressure swing adsorption towers (13, 14) are desorbed and regenerated by pressure reduction and vacuuming by a vacuum pump (15). The outlet of the vacuum pump (15) is connected to a CO.sub.2 buffer device (16). The concentrated CO.sub.2 product gas (the purity is preferably not less than 95%) after desorption is sent to the CO.sub.2 buffer device (16). In order to improve the regeneration effect and increase the concentration content of CO.sub.2, the hot fluid compressed by the CO.sub.2 booster (20) is used to heat the pressure swing adsorption towers (13, 14) during desorption and regeneration. The cooled CO.sub.2 fluid is returned to the liquid CO.sub.2 preparation system. The gas that is not adsorbed by the pressure swing adsorption towers (13, 14) is sent as adsorption tail gas (a nitrogen content is preferably about 90%) from the top of the pressure swing adsorption towers (13, 14) to the adsorption tail gas buffer device (19) through the second gas valve (18). [0060] Step 3: The gas in the CO.sub.2 buffer device (16) is connected to the inlet of the CO.sub.2 booster (20) through the third gas valve (17). The pressurized hot fluid is connected to the pressure swing adsorption towers (13, 14) through the valves. The compression heat of the compressor is used to improve the regeneration and desorption efficiency of the pressure swing adsorption towers (13, 14). At the same time, the hot fluid is cooled and returned to the CO.sub.2 evaporator (23) of the liquid CO.sub.2 preparation system through the eighth gas valve (21) to serve as a heat source. The fluid is then further cooled, and sent to the condenser in the CO.sub.2 subcooler (46) for liquefaction. And then it is sent to the middle part of the CO.sub.2 purification tower (22) for low-temperature separation to obtain the high-purity liquid CO.sub.2 product (the purity is preferably >99.9%) at the bottom of the CO.sub.2 evaporator (23), which is sent to the storage system. If food-grade CO.sub.2 products are required, a subsequent refining system can be set up to further remove other impurities in the CO.sub.2. At the same time, a part of the low-temperature fluid is drawn out from the middle and upper part of the nitrogen liquefier (33) of the high-purity liquid nitrogen preparation system and connected to the cold source inlet of the CO.sub.2 subcooler (46) through the fourth gas valve (44) to serve as a cold source to condense and liquefy the CO.sub.2 gas from the CO.sub.2 evaporator (23). After heat exchange in the CO.sub.2 subcooler (46), the low-temperature fluid is connected from the cold source outlet of the subcooler (46) to the inlet of the blower (47) of the high-purity liquid nitrogen preparation system through the fifth gas valve (25) and returned to the high-purity liquid nitrogen preparation system for recycling. At the same time, an external refrigerant circulation refrigeration system including a refrigerant compressor (26), a refrigerant cooler (27), a refrigerant heat regenerator (28) and a refrigerant throttle valve (29) is used to provide cooling capacity for the CO.sub.2 condenser (24). The refrigerant outlet of the CO.sub.2 condenser (24) is connected to the refrigerant heat regenerator (28). The refrigerant flowing out of the refrigerant outlet of the CO.sub.2 condenser (24) is reheated by refrigerant heat regenerator (28), pressurized by the refrigerant compressor (26), cooled by the refrigerant cooler (27), cooled by the refrigerant heat regenerator (28), throttled by the refrigerant throttle valve (29), and then returns to the refrigerant inlet of CO.sub.2 condenser (24) to complete the refrigerant circulation refrigeration. The refrigerant is preferably an environmentally friendly refrigerant. [0061] Step 4: The gas from adsorption tail gas buffer device (19) is combined with the gas from the outlet of CO.sub.2 subcooler (46) after boosted by the blower (47) through the sixth gas valve (30), and then connected to the hot-end inlet of nitrogen liquefier (33) through the seventh gas valve (48). It is cooled by the nitrogen liquefier (33), and then sent to the low-temperature separation system (42) for separation and purification to obtain high-purity nitrogen. At the same time, a part of the fluid extracted from the middle and upper part of nitrogen liquefier (33) is sent to the CO.sub.2 subcooler (46) of the liquid CO.sub.2 preparation system through the fourth gas valve (44) to serve as a cold source. The high-purity nitrogen from the low-temperature separation system (42) is connected to the cold-end inlet of the nitrogen liquefier (33), reheated by the nitrogen liquefier (33), and then connected to the inlet of the nitrogen booster (34). The gas from the outlet of the nitrogen booster (34) is cooled by the nitrogen cooler (32), and enters the expansion cooling system (31) for pressurization and expansion, and then connected to the hot-end inlet of the nitrogen liquefier (33) to provide the cooling capacity required by the nitrogen liquefier (33). The low-temperature high-pressure gas from the nitrogen liquefier (33) is connected to the gas-liquid separator (43) through the high-pressure throttle valve (45), the gas at the top of the gas-liquid separator (43) is reheated by the nitrogen liquefier (33) and returned to the inlet of the nitrogen booster (34), and the liquid at the bottom outlet of the gas-liquid separator (43) is sent to the low-temperature separation system (42); the low-pressure exhaust gas from the low-temperature separation system (42) is reheated by the nitrogen liquefier (33) and connected to the inlet of the water cooling tower (3) in the flue gas boosting and cooling system through the ninth gas valve (55) to cool the circulating water; the low-temperature separation system (42) sends out a super-cooled high-purity liquid nitrogen product (the purity is preferably >99.999%).

[0062] For the purpose of clarity, the present application is described in detail through the following examples.

Example 1

[0063] The specific process of simultaneously producing high-purity liquid nitrogen and high-purity liquid carbon dioxide at low cost by using the flue gas in the system shown in FIG. 3 is as follows:

[0064] The raw flue gas (at a temperature of about 50 C., a pressure of about 0.1 MPa, a flow rate of about 65000 Nm.sup.3/h, a molar composition of about 12% CO.sub.2, 73% N.sub.2, 8% H.sub.2O, 6%02, 0.984% Ar, 100 ppm CO, 50 ppm NO.sub.2, 10 ppm SO.sub.2) was sent to the inlet of the flue gas booster (1) in the flue gas boosting and cooling system, and pressurized to 0.57 MPa. The pressurized flue gas was sent to the flue gas cooling tower (2) for washing and cooling to about 12 C., and water-soluble impurities in the flue gas were removed simultaneously. Ambient temperature circulating water (about 32 C.) was used in the lower section of the flue gas cooling tower (2), and one part of the ambient temperature circulating water was pumped into the middle and lower part of the flue gas cooling tower (2) through the first water valve (6) and the ambient temperature water pump (35), and another part of the ambient temperature circulating water was pumped into the water cooling tower (3) through the second water valve (7). Low-temperature water is pumped into the upper part of the flue gas cooling tower through a third water valve (8) and the low-temperature water pump (4), which is cooled by the dry nitrogen discharged from the high-purity liquid nitrogen preparation system and chiller unit (5). During the shutdown of the high-purity liquid nitrogen preparation system, the fourth water valve (9) was opened to cool the flue gas by circulating the low-temperature water in the upper part. The sixth water valve (11) was provided at the bottom of the flue gas cooling tower (2), and the circulating water was discharged from the flue gas cooling tower (2) through the sixth water valve (11); the fifth water valve (10) was a water supply valve that replenished water carried away by the nitrogen gas to the system.

[0065] The low-temperature flue gas at the outlet of the flue gas cooling tower (2) was connected to the inlets of the plurality of pressure swing adsorption towers (13, 14) in the CO.sub.2/N.sub.2 front-end separation system through the first gas valve (12), and the CO.sub.2 in the flue gas was continuously adsorbed, captured and separated. The adsorber that has been saturated with adsorption was regenerated by reducing pressure and vacuum desorption. The inlet of the vacuum pump (15) was connected to the plurality of pressure swing adsorption towers (13, 14). After the adsorption was completed, the pressure swing adsorption towers (13, 14) were vacuumed to less than 10 kPa by the vacuum pump (15) to achieve desorption and regeneration of the adsorption tower. The outlet of the vacuum pump (15) was connected to the CO.sub.2 buffer device (16). After desorption, the CO.sub.2 product gas with a purity of not less than 95% was sent to the CO.sub.2 buffer device (16). The hot fluid compressed by the CO.sub.2 booster (20) was used to heat the regenerating pressure swing adsorption towers (13, 14), and the cooled CO.sub.2 fluid was returned to the liquid CO.sub.2 preparation system. The gas that is not adsorbed by the adsorber was sent as the adsorption tail gas (nitrogen content is about 90%) from the top of the pressure swing adsorption towers (13, 14) to the adsorption tail gas buffer device (19) through the second gas valve (18).

[0066] The gas (at a flow rate of about 8000 Nm.sup.3/h) in the CO.sub.2 buffer device (16) was connected to the inlet of the CO.sub.2 booster (20) through the third gas valve (17), and the pressurized hot fluid (about 2.4 MPaA) was connected to the pressure swing adsorption towers (13, 14) through the valves. The compression heat of the compressor was used to improve the regeneration and desorption efficiency of the pressure swing adsorption towers (13, 14). At the same time, the hot fluid was cooled and returned to the CO.sub.2 evaporator (23) of the liquid CO.sub.2 preparation system through the eighth gas valve (21) to serve as a heat source. The fluid is then further cooled, and sent to the CO.sub.2 subcooler (46) for condensation and liquefaction (about 16 C.). And then it was sent to the middle part of the CO.sub.2 purification tower (22) for low-temperature separation, to obtain the high-purity liquid CO.sub.2 product with a purity of more than 99.9% at the bottom of the CO.sub.2 evaporator (23), which was sent to the storage system. If food-grade CO.sub.2 products were required, a subsequent refining system could be set up to further remove other impurities in the CO.sub.2. At the same time, a part of the fluid (about 30 C.) was drawn out from the middle and upper part of the nitrogen liquefier (33) of the high-purity liquid nitrogen preparation system and connected to the cold source inlet of the CO.sub.2 subcooler (46) through the fourth gas valve (44). After heat exchange in the CO.sub.2 subcooler (46), the fluid was connected from the cold source outlet of the CO.sub.2 subcooler (46) to the inlet of the blower (47) of the high-purity liquid nitrogen preparation system through the fifth gas valve (25). After being pressurized by the blower (47), the fluid was returned to the high-purity liquid nitrogen preparation system for recycling. At the same time, an external refrigerant circulation refrigeration system including a refrigerant compressor (26), a refrigerant cooler (27), a refrigerant heat regenerator (28) and a refrigerant throttle valve (29) was provided. An environmentally friendly refrigerant R507 was compressed to about 1.8 MPa by the refrigerant compressor (26), cooled to about 37 C. after heat exchange with water in the refrigerant cooler (27), cooled to below 8 C. after heat exchange with the gas coming out of the evaporator in the refrigerant heat regenerator (28), throttled by the refrigerant throttle valve (29), and entered the CO.sub.2 condenser (24) of the CO.sub.2 purification tower (22) to absorb heat and evaporate. The rising gas at the top of the tower was condensed in the CO.sub.2 condenser (24). After evaporation, the refrigerant was connected to the refrigerant heat regenerator (28) through the refrigerant outlet of the CO.sub.2 condenser (24), and was reheated by the refrigerant heat regenerator (28), pressurized by the refrigerant compressor (26), cooled by the refrigerant cooler (27), cooled by the refrigerant heat regenerator (28), and throttled by the refrigerant throttle valve (29) and then returned to the refrigerant inlet of CO.sub.2 condenser (24), to complete the refrigerant circulation refrigeration system and provide cooling capacity for the condenser of the CO.sub.2 purification tower.

[0067] The gas (a flow rate of about 51,000 Nm.sup.3/h, a pressure of about 0.5 MPa) from the adsorption tail gas buffer device (19) was combined with the gas (a flow rate of about 10,000 Nm.sup.3/h) from the outlet of the CO.sub.2 subcooler (46) of the liquid CO.sub.2 preparation system after boosted by the blower (47) through the sixth gas valve (30), and then connected to the first inlet of the nitrogen liquefier (33) through the seventh gas valve (48). About 51,000 Nm.sup.3/h of the gas was connected to the low-temperature separation system (42) through the first outlet of the nitrogen liquefier (33), cooled to about 178 C. by the nitrogen liquefier (33) and then sent to the first nitrogen purification tower (49) for separation and purification. At the same time, about 10,000 Nm.sup.3/h of fluid was extracted from the flow path and sent to the CO.sub.2 subcooler (46) of the liquid CO.sub.2 preparation system through the second outlet (about 30 C.) of the nitrogen liquefier (33) and the fourth gas valve (44) to serve as a cold source. A high-purity nitrogen (a purity was >99.999%) was obtained at the top of the first nitrogen purification tower (49), and the high-purity nitrogen was connected to the second inlet of the nitrogen liquefier (33), and reheated to about 37 C. by the nitrogen liquefier (33), then connected to the inlet of the nitrogen booster (34) through the second outlet of the nitrogen liquefier (33). The first outlet (about 3.0 MPaA, 40 C., 65000 Nm.sup.3/h) of the nitrogen booster (34) was connected to the third inlet of the nitrogen liquefier (33), and the high-purity nitrogen was cooled to about 9 C. by the nitrogen liquefier (33), and connected from the third outlet of the nitrogen liquefier (33) to the inlet of the expansion end of second gas expansion (40) to serve as an expansion gas source to expand and provide cooling capacity. The expanded gas was connected to the fourth inlet of the nitrogen liquefier (33), and reheated to about 37 C. by the nitrogen liquefier (33), then returned to the inlet of the nitrogen booster (34) through the fourth outlet. The second outlet (about 3.0 MPaA, 40 C., 110,000 Nm.sup.3/h) of the nitrogen booster (34) was connected to the inlet of the boosting end of first gas expansion (36), and the gas was pressurized to about 6.9 MPa at a temperature of 40 C. through the boosting end of first gas expansion (36), the first gas expansion cooler (37), the boosting end of second gas expansion (38), and the second gas expansion cooler (39) in sequence, and then connected to the fifth inlet of the nitrogen liquefier (33); a part of the gas was connected to the inlet of the expansion end of first gas expansion (41) through the fifth outlet (about 79,000 Nm.sup.3/h, 90 C.) of the nitrogen liquefier (33) to serve as an expansion gas source to expand and provide cooling capacity. The expanded gas was connected to the first inlet of the gas-liquid separator (43); a part of the gas passed through the sixth outlet of the nitrogen liquefier (33) and was connected to the second inlet of the gas-liquid separator (43) through the high-pressure throttle valve (45). The first outlet of the gas-liquid separator (43) was connected to the sixth inlet of the nitrogen liquefier (33). The gas was reheated in the nitrogen liquefier (33), and then returned to the inlet of the nitrogen booster (34) through the seventh outlet of the nitrogen liquefier (33). The second outlet of gas-liquid separator (43) outputed a high-purity liquid nitrogen product that was sent to the heat exchanger (53). The high-purity liquid nitrogen was cooled in the heat exchanger (53) and then output as a product. In order to improve the low-temperature separation efficiency, the separation system of this example adopted a double-tower double-condenser process. The liquid air obtained at the bottom of the first nitrogen purification tower was supercooled to about 180 C. by the heat exchanger (53) and throttled to about 2.92 MPaA and then sent to the condenser of the first nitrogen purification tower (50) as a cold source to cool the rising gas at the top of the first nitrogen purification tower. After absorbing heat and evaporating in the condenser of the first nitrogen purification tower (50), the gas was sent to the second nitrogen purification tower (51) for further separation and purification, and high-purity liquid nitrogen was obtained at the top of the second nitrogen purification tower (51), the high-purity liquid nitrogen was pressurized to about 0.8 MPa by a low-temperature circulation pump (54) and then pumped back to the top of the first nitrogen purification tower. The liquid air obtained at the bottom of the second nitrogen purification tower was supercooled to about 185 C. by a heat exchanger (53), and throttled to about 1.3 MPa and then sent to the condenser of the second nitrogen purification tower (52) as a cold source to cool the rising gas at the top of the first nitrogen purification tower. The waste gas obtained after absorbing heat and evaporating in the condenser of the second nitrogen purification tower (52) was reheated by a heat exchanger (53) and then connected to the seventh inlet of the nitrogen liquefier (33), and reheated to about 37 C. by the nitrogen liquefier (33), then connected to the inlet of the water cooling tower (3) in the boosting and cooling system through the eighth outlet and the ninth gas valve (55) for cooling the circulating water.

[0068] In this example, the purity of the obtained high-purity liquid nitrogen product is greater than 99.999%.

[0069] The above is only a preferred example of the present application. It should be pointed out that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.