A mixed salt decarbonization system and process

Abstract

The present invention provides a mixed salt decarbonization system and process, comprising a first absorption tower, a second absorption tower, a water scrubber, a liquid-liquid separator, an ammonia still, a first heat exchanger, a second heat exchanger, and a desorption tower. The gas outlet of the first absorption tower is connected to the gas inlet of the second absorption tower via a pipeline, the gas outlet of the second absorption tower is connected to the inlet of the water scrubber via a pipeline, the liquid outlet of the water scrubber is connected to the liquid inlet of the ammonia still via a pipeline, and the gas outlet of the ammonia still is connected to the spray inlet of the second absorption tower via a pipeline. The rich liquid outlets of the first and second absorption towers are connected to the inlet of a rich liquid circulation pump and a liquid-liquid separator via pipelines, respectively, and the rich liquid delivery pump is connected to the middle and upper spray inlets of the desorption tower via pipelines and the first and second heat exchangers, respectively. The present invention can reduce regeneration energy consumption, ammonia emissions and reboiler load, increase carbon dioxide pressure, and lower system operating costs.

Description

Technical Field

[0001] The invention relates to a mixed salt decarbonization system and process, belonging to the technical field of flue gas purification and carbon dioxide emission reduction. Background Art

[0002] Excessive emission of greenhouse gases is the main cause of global warming. The capture and separation of carbon dioxide is one of the important means of carbon emission reduction. The relatively mature conventional chemical absorption decarbonization technologies are the alcohol amine solution absorption method and the chilled ammonia process (CAP). The alcohol amine solution absorption method refers to a method of using a weak alkaline organic amine solvent as an absorbent to absorb hydrogen sulfide or carbon dioxide in the desulfurization and decarbonization chemical process, while CAP refers to a CO2 capture technology mainly developed by Alstom. This technology uses a mixed slurry of ammonium carbonate and ammonium bicarbonate as a recycled CO2 absorbent, which can achieve a decarbonization rate of 90%; among them, MEA (ethanolamine, monoethanolamine) / MDEA (methyldiethanolamine, Chemical absorption, using alcohol amine solutions such as amines as the primary absorbent, is the most widely used method in the market. However, due to high absorbent regeneration energy consumption, high reboiler loads, and high steam consumption, operating costs remain high. Compared with the MEA method, the chilled ammonia process, using ammonia water as the primary absorbent, offers a number of advantages: for example, ammonia water has a CO2 removal efficiency of up to 95%-98%, while the conventional MEA method is only around 90%. The CO2 absorption capacity can reach 1.2 kg (CO2) / kg (NH3), three times that of MEA solution, and it is free of equipment corrosion and oxidative degradation. At the same time, regeneration energy consumption is expected to be reduced by more than 50%, significantly reducing costs. However, ammonia carbon capture still has the following challenges: Due to the high vapor pressure of ammonia, the amount of gaseous ammonia evaporating from the top of the absorption tower is relatively large. Without additional scrubbing equipment, it is difficult to ensure that ammonia leakage meets emission standards. The slow hydrolysis rate of ammonium carbamate causes the carbonization degree of the absorbent to remain low for a long time, resulting in low ammonia utilization. In the late reaction stage, the carbonized ammonia water absorbs CO2 at a slow rate. Furthermore, the desorption energy consumption needs to be further reduced.

[0003] In order to make up for the above shortcomings of the refrigerated ammonia process, various improved technologies have emerged.

[0004] Prior art 1 related to the present invention:

[0005] Technical solution of prior art 1:

[0006] Chinese patent CN 1080144603A discloses a crystallization ammonia steam regeneration carbon capture system and method. This technology prevents the rich liquid discharged from the absorber from entering the regeneration tower directly. Instead, it uses a dissolution crystallization method using ethanol as a dissolution agent in a crystallizer to enhance the crystallization of the low-carbonized rich liquid. The system then enters a solid-liquid separation device, where the crystals are placed in the regeneration tower for heating. The liquid is then returned to the crystallizer and supplemented with a certain amount of ethanol to maintain the dissolution crystallization effect. The gas discharged from the regeneration tower passes through a nitrogen-carbon separation device, and the ammonia and the escaped and captured ammonia are fed into an absorbent regeneration device. The system then returns to the absorber to continue operating as an absorbent. This technology reduces the energy consumption of the desorption process by heating only the solid product in the regeneration tower, which is required to heat the large amount of water required for desorption of the rich liquid.

[0007] Disadvantages of the prior art 1:

[0008] This patented technology uses the principle that ammonium bicarbonate is insoluble in ethanol solution to perform rich liquid dissolution crystallization treatment, but has the following shortcomings: 1) Since the rich liquid is mostly water in addition to ammonium bicarbonate, in order to achieve the purpose of dissolution, a large amount of anhydrous ethanol solution needs to be added to increase the ratio of ethanol to water. The dissolution reaction is not an instantaneous reaction, and the crystallization process cannot be operated continuously. Compared with the original technology, this patented technology does not increase the carbonization time of the absorption liquid. On the contrary, the reaction time is greatly increased due to the addition of the crystallization process, and the economic efficiency of the process is greatly reduced; 2) The main components of the residual liquid after solid-liquid separation in the crystallizer are ammonia water and ethanol. Purification of ethanol requires additional equipment, such as distillation, membrane exchange, etc. Although dissolution crystallization saves energy consumption for desorption, it increases energy consumption for purification treatment, and the overall energy-saving performance of the overall process is not significantly improved; 3) The carbon dioxide gas at the top outlet of the regeneration tower is still maintained at normal pressure, and subsequent liquefaction treatment requires pressurization. This patented technology does not optimize this part of energy consumption.

[0009] The second prior art related to the present invention:

[0010] Technical solution of existing technology 2:

[0011] Chinese patent CN106693614A discloses a compact ammonia-based carbon capture system driven by an ammonia-water second-type absorption heat pump. This system compactly combines an ammonia-based carbon dioxide capture system with an ammonia-water second-type absorption heat pump. Ammonia gas at the top of the ammonia evaporation tower is first condensed to remove water. The second-type absorption heat pump is then used to exchange the calorific value of the ammonia gas at the top of the ammonia evaporation tower with a low-temperature heat source, increasing the calorific value of the ammonia gas. The ammonia gas then serves as a heat source for the reboiler in the desorption tower to heat the lean liquid, thereby saving energy consumption in the reboiler.

[0012] Disadvantages of the second prior art:

[0013] This method has the following technical deficiencies: (1) The patented technology still uses ammonia water as the absorbent, and the hydrolysis rate of ammonium carbamate in the absorption tower is slow, resulting in a low carbonization degree of the absorption liquid; (2) The amount of ammonia gas and water vapor produced at the top of the ammonia evaporation tower is small, and the energy consumption of the reboiler is large. The ammonia solution produced by condensation at the top of the tower is used as the working fluid, and the total amount of heat converted from the low-temperature waste heat source to the high-temperature heat source by the heat pump system is limited. If all of it is used in the reboiler, it still cannot meet the energy consumption under normal operation, and the reboiler still requires an external heat source.

[0014] Therefore, providing a new mixed salt decarbonization system and process has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0015] In order to solve the above-mentioned shortcomings and deficiencies, an object of the present invention is to provide a mixed salt decarbonization system.

[0016] Another object of the present invention is to provide a mixed salt decarbonization process.

[0017] In order to achieve the above objectives, on the one hand, the present invention provides a mixed salt decarbonization system, wherein the mixed salt decarbonization system comprises: a first absorption tower, a second absorption tower, a water washing tower, a liquid-liquid separator, an ammonia still, a first heat exchanger, a second heat exchanger and a desorption tower;

[0018] The gas outlet of the first absorption tower is connected to the gas inlet of the second absorption tower through a pipeline, the gas outlet of the second absorption tower is connected to the inlet of the water scrubber through a pipeline, the liquid outlet of the water scrubber is connected to the liquid inlet of the ammonia still tower through a pipeline, and the gas outlet of the ammonia still tower is connected to the spray inlet of the second absorption tower through a pipeline;

[0019] The rich liquid outlet of the first absorption tower and the rich liquid outlet of the second absorption tower are respectively connected to the inlet of the rich liquid delivery pump through pipelines in sequence via the rich liquid circulation pump and the liquid-liquid separator. The outlet of the rich liquid delivery pump is respectively connected to the middle spray inlet and the upper spray inlet of the desorption tower through pipelines and via the first heat exchanger and the second heat exchanger.

[0020] As a specific embodiment of the mixed salt decarbonization system described above in the present invention, the first lean liquid outlet of the desorption tower is connected to the spray inlet of the first absorption tower through a pipeline in sequence via the first heat exchanger and the second lean liquid circulation pump, and / or the outlet of the rich liquid circulation pump is connected to the spray inlet of the first absorption tower through a pipeline.

[0021] As a specific embodiment of the mixed salt decarbonization system described above, the rich liquid outlet of the second absorption tower is connected to the inlet of the rich liquid circulation pump through a pipeline via a first flow regulating valve and a first check valve.

[0022] As a specific embodiment of the mixed salt decarbonization system described above, the rich liquid outlet of the first absorption tower is connected to the inlet of the rich liquid circulation pump through a pipeline, a second flow regulating valve, and a second check valve.

[0023] As a specific embodiment of the mixed salt decarbonization system described above in the present invention, the outlet of the rich liquid delivery pump is connected to the middle spray inlet of the desorption tower through a pipeline and in sequence via the fourth flow regulating valve and the first heat exchanger; the outlet of the rich liquid delivery pump is also connected to the upper spray inlet of the desorption tower through a pipeline and in sequence via the fifth flow regulating valve and the second heat exchanger.

[0024] As a specific embodiment of the mixed salt decarbonization system described above, the outlet of the rich liquid circulation pump is connected to the spray inlet of the first absorption tower through a pipeline, a third flow regulating valve, and a third check valve.

[0025] As a specific embodiment of the mixed salt decarbonization system described above in the present invention, the system further includes a first cooler, the first lean liquid outlet of the desorption tower is connected to the lean liquid inlet of the first cooler through a pipeline in sequence via the first heat exchanger and the second lean liquid circulation pump, and / or the outlet of the rich liquid circulation pump is connected to the lean liquid inlet of the first cooler through a pipeline, and the lean liquid outlet of the first cooler is connected to the spray inlet of the first absorption tower through a pipeline.

[0026] As a specific embodiment of the mixed salt decarbonization system described above in the present invention, the outlet of the rich liquid circulation pump is connected to the lean liquid inlet of the first cooler through a pipeline via a third flow regulating valve and a third check valve in sequence, and the lean liquid outlet of the first cooler is connected to the spray inlet of the first absorption tower through a pipeline.

[0027] As a specific embodiment of the mixed salt decarbonization system described above in the present invention, the system further includes a reboiler, which is arranged at the lower part of the desorption tower, and the reboiler is provided with a steam inlet, a steam outlet, a liquid inlet, a reboiler top outlet and a reboiler middle outlet. The second lean liquid outlet of the desorption tower is connected to the liquid inlet through a pipeline, and the top outlet of the reboiler is connected to the gas inlet of the desorption tower through a pipeline.

[0028] The composition of the lean liquid flowing out of the first lean liquid outlet and the second lean liquid outlet is the same, and the difference between the first lean liquid outlet and the second lean liquid outlet is only in the different setting positions. The present invention does not require the specific setting positions of the two. The setting positions of the two at the bottom of the desorption tower can be reasonably adjusted according to the actual on-site operation needs.

[0029] As a specific embodiment of the mixed salt decarbonization system described above in the present invention, the middle outlet of the reboiler is connected to the spray inlet of the second absorption tower through a pipeline in sequence via the second heat exchanger and the first lean liquid circulation pump, and / or the supernatant outlet of the liquid-liquid separator is connected to the spray inlet of the second absorption tower through a pipeline.

[0030] As a specific embodiment of the mixed salt decarbonization system described above in the present invention, the system also includes a second cooler, the middle outlet of the reboiler is connected to the lean liquid inlet of the second cooler through a pipeline in sequence via the second heat exchanger and the first lean liquid circulation pump, and / or the supernatant outlet of the liquid-liquid separator is connected to the lean liquid inlet of the second cooler through a pipeline, and the lean liquid outlet of the second cooler is connected to the spray inlet of the second absorption tower through a pipeline.

[0031] As a specific embodiment of the mixed salt decarbonization system described above in the present invention, the system further includes a third heat exchanger and a first condenser, the liquid outlet of the water scrubber is connected to the first inlet of the third heat exchanger through a pipeline, the first outlet of the third heat exchanger is connected to the liquid inlet of the ammonia still tower through a pipeline, the liquid outlet of the ammonia still tower is connected to the second inlet of the third heat exchanger through a pipeline, and the second outlet of the third heat exchanger is connected to the spray inlet of the water scrubber through a pipeline;

[0032] The gas outlet of the ammonia evaporation tower is connected to the inlet of the first condenser through a pipeline via a gas pressure reducing valve, the condensed water outlet of the first condenser is connected to the flushing water inlet of the ammonia evaporation tower through a pipeline, and the gas outlet of the first condenser is connected to the spray inlet of the second absorption tower through a pipeline.

[0033] As a specific embodiment of the mixed salt decarbonization system described above, the condensate outlet of the first condenser is connected to the flushing water inlet of the ammonia distillation tower through a pipeline via a circulating water pump.

[0034] As a specific embodiment of the mixed salt decarbonization system described above, the liquid outlet of the ammonia still is connected to the second inlet of the third heat exchanger via a pipeline via a third circulation pump.

[0035] As a specific embodiment of the mixed salt decarbonization system described above of the present invention, the system further includes a fourth heat exchanger and a second condenser, the liquid outlet of the water scrubber is further connected to the first inlet of the fourth heat exchanger through a pipeline, and the first outlet of the fourth heat exchanger is connected to the spray inlet of the water scrubber through a pipeline;

[0036] The gas outlet of the desorption tower is connected to the second inlet of the fourth heat exchanger through a pipe, the second outlet of the fourth heat exchanger is connected to the inlet of the second condenser through a pipe, and the condensate outlet of the second condenser is connected to the flushing water inlet of the desorption tower through a pipe.

[0037] As a specific embodiment of the mixed salt decarbonization system described above, the liquid outlet of the water washing tower is connected to the first inlet of the third heat exchanger through a pipeline in sequence via the first circulation pump and the sixth flow regulating valve.

[0038] As a specific embodiment of the mixed salt decarbonization system described above, the liquid outlet of the water washing tower is further connected to the first inlet of the fourth heat exchanger through a pipeline via a first circulation pump.

[0039] As a specific embodiment of the mixed salt decarbonization system described above, the condensate outlet of the second condenser is connected to the flushing water inlet of the desorption tower through a pipeline via a second circulation pump.

[0040] In the present invention, the first absorption tower, the first cooler, the second absorption tower, the second cooler, the water washing tower, the liquid-liquid separator, the ammonia evaporation tower, the first heat exchanger, the second heat exchanger, the third heat exchanger, the first condenser, the fourth heat exchanger, the second condenser, the reboiler and the desorption tower are all conventional equipment.

[0041] On the other hand, the present invention also provides a mixed salt decarbonization process, wherein the mixed salt decarbonization process is implemented using the mixed salt decarbonization system described above, which comprises:

[0042] (1) In the first absorption tower, the pretreated CO2-containing flue gas is countercurrently contacted with the mixed salt absorbent and a chemical reaction occurs to remove the CO2 in the flue gas; the treated flue gas enters the second absorption tower, where the flue gas continues to countercurrently contact with the mixed salt absorbent. During the contact process, the residual CO2 in the flue gas reacts chemically with the mixed salt absorbent to further remove the CO2 in the flue gas;

[0043] (2) The flue gas obtained in step (1) enters a water washing tower, where the flue gas is countercurrently contacted with a water washing liquid for water washing, to obtain washed flue gas and a stream containing NH4 + aqueous solution;

[0044] (3) The NH4-containing + The aqueous solution enters the ammonia still, where ammonia and water are separated, and the resulting ammonia is sent to the spray layer of the second absorption tower for recycling;

[0045] (4) The bottom rich liquids of the first absorption tower and the second absorption tower are combined and sent to a liquid-liquid separator for separation. The bottom rich liquid obtained after separation is sent to a desorption tower after heat exchange and regenerated in the desorption tower to obtain high-concentration, high-pressure carbon dioxide.

[0046] As a specific embodiment of the mixed salt decarbonization process described above, in step (2), the water washing liquid used in the initial state is pure water, a dilute hydrochloric acid solution with a mass concentration of 1%, or a dilute sulfuric acid solution with a mass concentration of 1%.

[0047] As a specific embodiment of the mixed salt decarbonization process described above, step (3) further comprises:

[0048] The obtained ammonia gas is cooled in the second cooler and then sent to the spray layer of the second absorption tower for recycling.

[0049] As a specific embodiment of the mixed salt decarbonization process described above in the present invention, the process further includes: dividing the bottom rich liquid obtained in step (4) into two paths and allowing one of the rich liquid paths to undergo heat exchange with the lean liquid obtained in the desorption tower, and then sending the heat-exchanged rich liquid and the heat-exchanged lean liquid to the middle of the desorption tower for regeneration and the spray layer of the first absorption tower for recycling, and / or merging the bottom rich liquids of the first absorption tower and the second absorption tower and sending them to the spray layer of the first absorption tower for recycling.

[0050] As a specific embodiment of the mixed salt decarbonization process described above in the present invention, the process also includes: using a first cooler to cool the lean liquid after heat exchange and then sending it to the spray layer of the first absorption tower for recycling, and / or combining the rich liquids at the bottom of the first absorption tower and the second absorption tower, cooling them using the first cooler, and then sending them to the spray layer of the first absorption tower for recycling.

[0051] As a specific embodiment of the mixed salt decarbonization process described above in the present invention, the process also includes: using a reboiler to heat the tower body of the desorption tower, and heat-exchanging the lean liquid in the middle of the reboiler with another rich liquid, and then sending the rich liquid after the heat exchange and the lean liquid after the heat exchange to the upper part of the desorption tower for regeneration and the spray layer of the second absorption tower for recycling, and / or sending the supernatant obtained after separation in the liquid-liquid separator to the spray layer of the second absorption tower for recycling.

[0052] As a specific embodiment of the mixed salt decarbonization process described above, the bottom rich liquid obtained in step (4) is divided into two paths, wherein the split ratio of the rich liquid path for heat exchange with the lean liquid in the middle of the reboiler (i.e., the rich liquid path ultimately sent to the upper portion of the desorption tower) to the bottom rich liquid is 0.35-0.7. The split ratio is controlled by a fourth flow control valve and a fifth flow control valve, and the type of the split ratio, such as a volume split ratio or a mass split ratio, is also determined according to the flow type of the flow control valve.

[0053] In the present invention, the outlet temperature of the top of the desorption tower is relatively high and contains a large amount of water vapor. Directly cooling it through an external cooler will result in a large amount of heat loss. In this regard, the present invention adopts a rich liquid diversion process, that is, the bottom rich liquid obtained in step (4) is divided into two paths, and then heat exchanged with the lean liquid obtained from the desorption tower and the lean liquid in the middle of the reboiler, and then sent to the middle and upper parts of the desorption tower respectively. This can relatively simply and effectively recover this part of the heat at the top of the desorption tower. At the same time, since the temperature of the rich liquid after heating the lean liquid obtained from the desorption tower and the lean liquid in the middle of the reboiler is higher, it can increase the water vapor partial pressure after entering the desorption tower, thereby strengthening the driving force of CO2 separation and helping to reduce system energy consumption.

[0054] As a specific embodiment of the mixed salt decarbonization process described above in the present invention, the process also includes: using a second cooler to cool the lean liquid after heat exchange and then sending it to the spray layer of the second absorption tower for recycling, and / or using a second cooler to cool the supernatant obtained after separation in the liquid-liquid separator and then sending it to the spray layer of the second absorption tower for recycling.

[0055] Among them, the present invention does not make specific requirements on the temperature of the liquid returned to the first absorption tower after cooling by the first cooler and the temperature of the liquid returned to the second absorption tower after cooling by the second cooler. The temperature of the recycled liquid can be reasonably adjusted according to the actual on-site operation needs.

[0056] As a specific embodiment of the mixed salt decarbonization process described above, the process further comprises: firstly + The aqueous solution is divided into two paths and one of them contains NH4 + The aqueous solution is heat exchanged with the bottom liquid of the ammonia still, and then the aqueous solution after heat exchange is sent to the ammonia still and the bottom liquid after heat exchange is sent to the spray section of the water washing tower for recycling;

[0057] Among them, the present invention is to heat exchange the NH4 + There are no specific requirements for the temperature of the aqueous solution and the bottom liquid of the ammonia distillation tower. The temperatures of the two after heat exchange can be reasonably adjusted according to the actual needs of the site operation.

[0058] The process also includes: decompressing and condensing the gas obtained from the top of the ammonia evaporation tower in sequence, then sending the obtained condensed water to the ammonia evaporation tower for recycling and sending the obtained ammonia gas to the spray layer of the second absorption tower for recycling.

[0059] As a specific embodiment of the mixed salt decarbonization process described above, the process further comprises: firstly making another path containing NH4 + The aqueous solution is heat exchanged with the high-concentration, high-pressure carbon dioxide and water vapor generated after regeneration in the desorption tower, and then the aqueous solution after heat exchange is sent to the spray section of the water washing tower for recycling, and then the carbon dioxide and water vapor after heat exchange are condensed to obtain condensed water and high-concentration, high-pressure carbon dioxide, and then the condensed water is sent to the desorption tower for recycling and the high-concentration, high-pressure carbon dioxide, that is, CO2 concentrated gas is discharged from the top of the condenser used for condensation.

[0060] Among them, the present invention is to heat exchange the NH4 + There are no specific requirements for the temperature of the aqueous solution and the high-concentration, high-pressure carbon dioxide and water vapor generated after regeneration in the desorption tower. The temperature of the two after heat exchange can be reasonably adjusted according to the actual needs of the on-site operation.

[0061] As a specific embodiment of the mixed salt decarbonization process described above, the mixed salt absorbent contains 30-55wt% of potassium carbonate, 5-10wt% of ammonium bicarbonate, 3-7wt% of amine reagent MDEA (N-methyldiethanolamine) and the balance water.

[0062] As a specific embodiment of the mixed salt decarbonization process described above, the alcohol amine reagent includes one or a combination of MDEA (N-methyldiethanolamine), MEA (monoethanolamine), DEA (diethanolamine) and DIPA (diisopropanolamine).

[0063] In the present invention, in the initial state, the mixed salt absorbent used in the first absorption tower and the second absorption tower is the same, but as the system operates, the return material components at the spray inlets of the first absorption tower and the second absorption tower are different, resulting in different concentrations of ammonium ions contained in the absorbents in the first absorption tower and the second absorption tower.

[0064] As a specific embodiment of the mixed salt decarbonization process described above in the present invention, the temperature of the first absorption tower is 20-30°C and the pressure is 0.08-0.15MPa; the temperature of the second absorption tower is 30-40°C and the pressure is 0.08-0.15MPa; the temperature of the desorption tower is 95-175°C and the pressure is 1-1.2MPa; the temperature of the water washing tower is 20-30°C and the pressure is 0-0.1MPa; the temperature of the ammonia evaporation tower is 120-130°C and the pressure is 0.18-0.22MPa.

[0065] As a specific embodiment of the mixed salt decarbonization process described above in the present invention, the temperature of the first absorption tower is 20-30°C and the pressure is 0.1 MPa; the temperature of the second absorption tower is 30-40°C and the pressure is 0.1 MPa; the temperature of the desorption tower is 95-175°C and the pressure is 1-1.2 MPa; the temperature of the water washing tower is 20°C and the pressure is 0.1 MPa; the temperature of the ammonia evaporation tower is 120°C and the pressure is 0.2 MPa.

[0066] As a specific embodiment of the mixed salt decarbonization process described above, the temperature of the reboiler is 140-170°C.

[0067] In the present invention, the pretreatment of the CO2-containing flue gas is a conventional technical means in the field, and those skilled in the art can select appropriate pretreatment means to treat the CO2-containing flue gas according to actual on-site operation needs.

[0068] In summary, compared with the prior art, the mixed salt decarbonization system and process provided by the present invention can achieve the following beneficial technical effects:

[0069] The mixed salt decarbonization process provided by the present invention uses an aqueous solution of a mixed salt mainly containing ammonium bicarbonate, potassium carbonate, etc. as an absorbent, has low regeneration energy consumption, and the desorption tower operates under conditions of 95-175°C and 10-12atm, producing high-concentration, high-pressure carbon dioxide with a volume concentration of over 99% and a pressure of up to 12atm, thereby reducing the energy consumption of subsequent carbon dioxide recompression and thus lowering the operating cost of the overall system.

[0070] The mixed salt decarbonization system and process provided by the present invention uses a water scrubber and an ammonia evaporator and controls their operating conditions, which can fully recycle the water and ammonia in the system, thereby reducing water consumption and ammonia escape.

[0071] The mixed salt decarbonization system and process provided by the present invention use a two-stage absorption tower including a first absorption tower and a second absorption tower, which can accelerate the reaction rate and reduce the reaction time; at the same time, the system and process also use a two-stage heat exchanger (i.e., a first heat exchanger and a second heat exchanger) and feed the rich liquid at different positions of the desorption tower (i.e., the upper part and the middle part of the desorption tower), which can make the temperature distribution in the desorption tower balanced while maximally recovering and utilizing the heat of the lean liquid, thereby reducing the load of the reboiler. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0073] Figure 1 This is a schematic structural diagram of the mixed salt decarbonization system provided in Example 1 of the present invention.

[0074] Description of main figures:

[0075] 10. Flue gas containing CO2;

[0076] 11. The first absorption tower;

[0077] 12. Second absorption tower;

[0078] 13. Water washing tower;

[0079] 14. Ammonia still;

[0080] 15. First condenser;

[0081] 16. Liquid-liquid separator;

[0082] 17. Desorption tower; 17-1. First lean liquid outlet of desorption tower; 17-2. Spray inlet of middle part of desorption tower; 17-3. Spray inlet of upper part of desorption tower;

[0083] 18. Reboiler; 18-1. Steam inlet; 18-2. Steam outlet; 18-3. Reboiler top outlet; 18-4. Reboiler middle outlet;

[0084] 19. Second condenser;

[0085] 20. Rich liquid circulation pump;

[0086] 21. First circulation pump;

[0087] 22. Circulating water pump;

[0088] 23. Second circulation pump;

[0089] 24. Rich liquid delivery pump;

[0090] 25. The first lean liquid circulation pump;

[0091] 26. Second lean liquid circulation pump;

[0092] 27. First heat exchanger; 27-1. Lean liquid inlet of heat exchanger; 27-2. Lean liquid outlet of heat exchanger; 27-3. Rich liquid inlet of heat exchanger; 27-4. Rich liquid outlet of heat exchanger;

[0093] 28. Second heat exchanger; 28-1. Lean liquid inlet of heat exchanger; 28-2. Lean liquid outlet of heat exchanger; 28-3. Rich liquid inlet of heat exchanger; 28-4. Rich liquid outlet of heat exchanger;

[0094] 29, third heat exchanger; 29-1, second outlet; 29-2, first inlet; 29-3, first outlet; 29-4, second inlet;

[0095] 30. Gas pressure reducing valve;

[0096] 31. First cooler; 31-1. Cooler lean liquid outlet; 31-2. Cooler lean liquid inlet;

[0097] 32. Second cooler; 32-1. Cooler lean liquid outlet; 32-2. Cooler lean liquid inlet;

[0098] 33. First flow regulating valve;

[0099] 34. First check valve;

[0100] 35. Second flow control valve;

[0101] 36. Second check valve;

[0102] 37. The third flow regulating valve;

[0103] 38. Third check valve;

[0104] 39. Fourth flow control valve;

[0105] 40. Fifth flow control valve;

[0106] 41. Sixth flow control valve;

[0107] 42. No CO2 flue gas;

[0108] 43. CO2 concentrated gas;

[0109] 44. The third circulation pump;

[0110] 45. The fourth heat exchanger. DETAILED DESCRIPTION

[0111] It should be noted that the term "comprise" and any variations thereof in the specification and claims of the present invention and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, technique, system, product or equipment that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, techniques, products or equipment.

[0112] In the present invention, terms such as "upper," "lower," "inner," "outer," "middle," "top," and "bottom" indicate positions or locations based on those shown in the accompanying drawings. These terms are intended primarily to better describe the present invention and its embodiments and are not intended to limit the devices, elements, or components indicated to specific positions, or to their construction or operation in a specific orientation.

[0113] Furthermore, some of the above terms may be used to express other meanings besides indicating a position or location. For example, the term "on" may also be used to indicate a dependency or connection in certain circumstances. Those skilled in the art will understand the specific meanings of these terms in the present invention based on the specific circumstances.

[0114] Furthermore, the terms "disposed" and "connected" should be interpreted broadly. For example, "connected" can mean a fixed connection, a removable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection, an indirect connection through an intermediary, or it can be internal communication between two devices, elements, or components. Those skilled in the art will understand the specific meanings of these terms in the present invention based on the specific circumstances.

[0115] The "range" disclosed in the present invention is given in the form of a lower limit and an upper limit. It can be one or more lower limits, and one or more upper limits respectively. A given range is defined by selecting a lower limit and an upper limit. The selected lower limit and upper limit define the boundaries of a particular range. All ranges defined in this way are combinable, i.e., any lower limit can be combined with any upper limit to form a range. For example, a range of 60-120 and 80-110 is listed for a particular parameter, and it is understood that a range of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​listed are 1 and 2, and the maximum range values ​​listed are 3, 4, and 5, then the following ranges can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5.

[0116] In the present invention, unless otherwise specified, the numerical range "ab" is an abbreviation for any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" are listed in this invention, and "0-5" is merely an abbreviation for these numerical combinations.

[0117] In the present invention, unless otherwise specified, all embodiments and preferred embodiments mentioned in the present invention can be combined with each other to form a new technical solution.

[0118] In the present invention, unless otherwise specified, all technical features and preferred features mentioned in the present invention can be combined with each other to form a new technical solution.

[0119] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the accompanying drawings and Examples. The following embodiments are part of the present invention, rather than all of the embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0120] Example 1

[0121] This embodiment provides a mixed salt decarbonization system, the structural diagram of which is shown in FIG. Figure 1 As shown, from Figure 1 As can be seen, the system includes:

[0122] a first absorption tower 11, a first cooler 31, a second absorption tower 12, a second cooler 32, a water scrubber 13, a liquid-liquid separator 16, an ammonia still 14, a first heat exchanger 27, a second heat exchanger 28, a third heat exchanger 29, a first condenser 15, a fourth heat exchanger 45, a second condenser 19, a reboiler 18, and a desorption tower 17;

[0123] wherein, the top gas outlet of the first absorption tower 11 is connected to the lower gas inlet of the second absorption tower 12 through a pipeline, the top gas outlet of the second absorption tower 12 is connected to the lower inlet of the water scrubber 13 through a pipeline, the lower liquid outlet of the water scrubber 13 is connected to the inlet of the first circulation pump 21 through a pipeline, the outlet of the first circulation pump 21 is connected to the inlet of the first tee through a pipeline, the first outlet of the first tee is connected to the first inlet 29-2 of the third heat exchanger 29 through a pipeline via a sixth flow regulating valve 41, the first outlet 29-3 of the third heat exchanger 29 is connected to the middle liquid inlet of the ammonia still 14 through a pipeline, the lower liquid outlet of the ammonia still 14 is connected to the second inlet 29-4 of the third heat exchanger 29 through a pipeline via a third circulation pump 44, and the second outlet 29-1 of the third heat exchanger 29 is connected to the upper spray inlet of the water scrubber 13 through a pipeline;

[0124] The top gas outlet of the ammonia still 14 is connected to the upper inlet of the first condenser 15 via a pipeline via a gas pressure reducing valve 30. The bottom condensed water outlet of the first condenser 15 is connected to the upper flushing water inlet of the ammonia still 14 via a pipeline via a circulating water pump 22. The top gas outlet of the first condenser 15 is connected to the lean liquid inlet 32-2 of the second cooler 32 via a pipeline. The lean liquid outlet 32-1 of the second cooler 32 is connected to the upper spray inlet of the second absorption tower 12 via a pipeline.

[0125] The second outlet of the first three-way valve is connected to the first inlet of the fourth heat exchanger 45 through a pipeline, and the first outlet of the fourth heat exchanger 45 is connected to the upper spray inlet of the water washing tower 13 through a pipeline;

[0126] The top gas outlet of the desorption tower 17 is connected to the second inlet of the fourth heat exchanger 45 through a pipeline, the second outlet of the fourth heat exchanger 45 is connected to the upper inlet of the second condenser 19 through a pipeline, and the bottom condensed water outlet of the second condenser 19 is connected to the upper flushing water inlet of the desorption tower 17 through a pipeline via the second circulation pump 23;

[0127] The lower rich liquid outlet of the first absorption tower 11 is connected to the first inlet of the second tee through a pipeline via the second flow regulating valve 35 and the second check valve 36. The lower rich liquid outlet of the second absorption tower 12 is connected to the second inlet of the second tee through a pipeline via the first flow regulating valve 33 and the first check valve 34. The outlet of the second tee is connected to the inlet of the rich liquid circulation pump 20 through a pipeline. The outlet of the rich liquid circulation pump 20 is connected to the liquid inlet of the liquid-liquid separator 16 through a pipeline. The bottom rich liquid outlet of the liquid-liquid separator 16 is connected to the inlet of the rich liquid delivery pump 24 through a pipeline. The outlet of the rich liquid delivery pump 24 is connected to the inlet of the third tee through a pipeline. The first outlet of the third tee is connected to the heat exchanger 27 through a pipeline via the fourth flow regulating valve 39. The rich liquid inlet 27-3 of the first heat exchanger 27 is connected, the rich liquid outlet 27-4 of the first heat exchanger 27 is connected to the middle spray inlet 17-2 of the desorption tower through a pipeline, the first lean liquid outlet 17-1 of the desorption tower is connected to the lean liquid inlet 27-1 of the first heat exchanger 27 through a pipeline, the lean liquid outlet 27-2 of the first heat exchanger 27 is connected to the cooler lean liquid inlet 31-2 of the first cooler 31 through a pipeline via the second lean liquid circulation pump 26, and the outlet of the rich liquid circulation pump 20 is also connected to the cooler lean liquid inlet 31-2 of the first cooler 31 through a pipeline in sequence via the third flow regulating valve 37 and the third check valve 38, the cooler lean liquid outlet 31-1 of the first cooler 31 is connected to the upper spray inlet of the first absorption tower 11 through a pipeline;

[0128] The reboiler 18 is provided at the lower part of the desorption tower 17, and the reboiler 18 is provided with a steam inlet 18-1, a steam outlet 18-2, a liquid inlet, a reboiler top outlet 18-3 and a reboiler middle outlet 18-4. The second lean liquid outlet at the lower part of the desorption tower 17 is connected to the liquid inlet through a pipeline, and the reboiler top outlet 18-3 is connected to the lower gas inlet of the desorption tower 17 through a pipeline;

[0129] The middle outlet 18-4 of the reboiler is connected to the heat exchanger lean liquid inlet 28-1 of the second heat exchanger 28 through a pipeline, the heat exchanger lean liquid outlet 28-2 of the second heat exchanger 28 is connected to the cooler lean liquid inlet 32-2 of the second cooler 32 through a pipeline via the first lean liquid circulation pump 25, and the supernatant outlet of the liquid-liquid separator 16 is also connected to the cooler lean liquid inlet 32-2 of the second cooler 32 through a pipeline, and the cooler lean liquid outlet 32-1 of the second cooler 32 is connected to the upper spray inlet of the second absorption tower 12 through a pipeline;

[0130] The second outlet of the third three-way valve is connected to the rich liquid inlet 28-3 of the second heat exchanger 28 through a pipeline via the fifth flow regulating valve 40, and the rich liquid outlet 28-4 of the second heat exchanger 28 is connected to the upper spray inlet 17-3 of the desorption tower through a pipeline.

[0131] In this embodiment, the first absorption tower 11 and the second absorption tower 12 are both cylindrical packed towers, both filled with two layers of stainless steel wire mesh corrugated structured packing. The diameter ratio of the first absorption tower 11 and the second absorption tower 12 is 1:1, and the height ratio is 1:3.

[0132] In this embodiment, the liquid-liquid separator 16 is a cyclone separator.

[0133] Example 2

[0134] This embodiment provides a mixed salt decarbonization process, which is implemented using the mixed salt decarbonization system provided in Example 1. The process includes the following specific steps:

[0135] (1) The pretreated CO2-containing flue gas 10 (wherein the volume content of CO2 is 12%) enters from the lower part of the first absorption tower 11 and reacts chemically with the mixed salt absorbent in a countercurrent manner to remove the CO2 in the flue gas; the treated flue gas is demisted at the top of the tower and then enters the lower part of the second absorption tower 12 from the top of the first absorption tower 11. The flue gas continues to contact with the mixed salt absorbent in a countercurrent manner. During the contact process, the residual CO2 in the flue gas reacts chemically with the mixed salt absorbent to further remove the CO2 in the flue gas;

[0136] (2) The NH4 + The flue gas of the mist droplets leaves the top of the second absorption tower 12 and enters the lower part of the water scrubber 13. Pure water is sprayed from top to bottom in the water scrubber 13. In the water scrubber 13, the flue gas flows upstream in a countercurrent manner to be washed with water. The CO2-free flue gas 42 after washing is discharged into the atmosphere from the top of the water scrubber 13.

[0137] (3) The NH4-containing + Aqueous solution (i.e. NH4 washed down with water) + In the form of aqueous solution) after the first circulation pump 21 is divided into two paths and one path contains NH4 +The aqueous solution is heat-exchanged with the bottom liquid of the ammonia still in the third heat exchanger 29 to recover the heat of the bottom liquid of the ammonia still. The aqueous solution after heat exchange is then sent to the spray section of the ammonia still 14. After pressurization, temperature increase, and stripping in the ammonia still 14, ammonia and water are separated. The ammonia obtained after separation is decompressed by the gas pressure reducing valve 30, condensed to remove water by the first condenser 15, and cooled by the second cooler 32 before being sent back to the spray layer of the second absorption tower 12 for recycling. The condensed water obtained is sent to the ammonia still 14 for recycling. At the same time, the bottom liquid after heat exchange is sent to the spray section of the water scrubber 13 for recycling.

[0138] For the other road containing NH4 + The aqueous solution is first heat-exchanged with the high-concentration, high-pressure carbon dioxide and water vapor generated after regeneration in the desorption tower 17 in the fourth heat exchanger 45 to recover the heat of the CO2 gas at the top of the desorption tower, and then the aqueous solution after heat exchange is sent to the spray section of the water scrubber 13 for recycling, and then the carbon dioxide and water vapor after heat exchange are condensed in the second condenser 19 to obtain condensed water and high-concentration, high-pressure carbon dioxide, and then the condensed water is sent to the desorption tower 17 for recycling, and the high-concentration, high-pressure carbon dioxide, that is, the CO2 concentrated gas 43 is discharged from the top of the second condenser 19 used for condensation;

[0139] (4) The rich liquids at the bottom of the first absorption tower 11 and the second absorption tower 12 are combined and sent to the liquid-liquid separator 16 via the rich liquid circulation pump 20 for liquid-liquid separation. The supernatant obtained by separation is cooled by the second cooler 32 and sent to the spray layer of the second absorption tower 12 for recycling;

[0140] The bottom rich liquid obtained by separation is divided into two paths after passing through the rich liquid delivery pump 24, and one of the rich liquid paths is heat exchanged with the lean liquid obtained from the desorption tower 17 in the first heat exchanger 27. The rich liquid after heat exchange is then sent to the middle of the desorption tower for regeneration, and the lean liquid after heat exchange is sent to the first cooler 31 for cooling via the second lean liquid circulation pump 26, and then sent to the spray layer of the first absorption tower 11 for recycling. At the same time, the bottom rich liquids of the first absorption tower 11 and the second absorption tower 12 can be combined, cooled by the first cooler 31, and then sent to the spray layer of the first absorption tower 11 for recycling.

[0141] During the regeneration process, the reboiler 18 is used to heat the tower body of the desorption tower 17, and the lean liquid in the middle of the reboiler 18 is heat-exchanged with the other rich liquid in the second heat exchanger 28. The rich liquid after the heat exchange is then sent to the upper part of the desorption tower 17 for regeneration, and the lean liquid after the heat exchange is sent to the second cooler 32 for cooling through the first lean liquid circulation pump 25, and then sent to the spray layer of the second absorption tower 12 for recycling.

[0142] The rich liquids after heat exchange in the two paths are regenerated in the desorption tower 17 to produce high-concentration, high-pressure carbon dioxide and water vapor. The high-concentration, high-pressure carbon dioxide and water vapor are discharged from the top of the desorption tower 17, and after heat exchange in the fourth heat exchanger 45 and dehydration in the second condenser 19, high-concentration, high-pressure carbon dioxide, i.e., CO2 concentrated gas 43, is obtained.

[0143] In this embodiment, the mixed salt absorbent is a mixed salt solution consisting of 30 wt% potassium carbonate, 7 wt% ammonium bicarbonate, 3 wt% MDEA and the balance water;

[0144] The temperature of the first absorption tower is 30°C and the pressure is 0.1 MPa; the temperature of the second absorption tower is 30°C and the pressure is 0.1 MPa; the CO2 loading capacity of the rich liquid at the lower outlet of the first absorption tower 11 is 0.37 mol / mol rich liquid, and the CO2 loading capacity of the rich liquid at the lower outlet of the second absorption tower 12 is 0.70 mol / mol rich liquid;

[0145] The ratio of the rich liquid volume flow rate through the second flow regulating valve 35 to the rich liquid volume flow rate through the third flow regulating valve 37 is 4:1;

[0146] The sixth flow regulating valve 41 is linked to the alkalinity of the aqueous solution inside the water scrubber 13. When the alkalinity of the aqueous solution inside the water scrubber 13 is greater than 5.7 mg / L, the valve is fully opened; when the alkalinity is less than 3.8 mg / L, the valve is closed.

[0147] The ratio of the rich liquid volume flow rate through the fifth flow regulating valve 40 to the rich liquid volume flow rate through the fourth flow regulating valve 39 is 7:13;

[0148] The operating temperature of reboiler 18 is 160°C and the load is 1.84MJ / kgCO2;

[0149] The temperature of the ammonia still 14 is 120°C, the pressure is 0.2 MPa, and the load is 0.14 MJ / kg CO2;

[0150] The temperature of the water washing tower 13 is 20°C and the pressure is 0.1MPa;

[0151] The operating temperature of the desorption tower 17 is 140° C. and the pressure is 1.0 MPa.

[0152] With the above settings, the NH3 content in the purified gas at the top of the water scrubber 13, i.e., the CO2-free flue gas 42, is less than 3 ppm, and the residual CO2 content in the purified gas is only 0.2% (dry volume);

[0153] After dehydration, the second condenser 19 obtains high-pressure carbon dioxide with a volume concentration greater than 99% and a pressure of up to 10 atm, namely, CO2 concentrated gas 43.

[0154] Example 3

[0155] This embodiment provides a mixed salt decarbonization process, which is also implemented using the mixed salt decarbonization system provided in Example 1. The difference between the process and Example 2 is that:

[0156] In this embodiment, the mixed salt absorbent is a mixed salt solution consisting of 40 wt% potassium carbonate, 10 wt% ammonium bicarbonate, 5 wt% MDEA and the balance water;

[0157] The CO2 loading capacity of the rich liquid at the lower outlet of the first absorption tower 11 is 0.40 mol / mol rich liquid, and the CO2 loading capacity of the rich liquid at the lower outlet of the second absorption tower 12 is 0.54 mol / mol rich liquid;

[0158] The ratio of the rich liquid volume flow rate through the second flow regulating valve 35 to the rich liquid volume flow rate through the third flow regulating valve 37 is 5:1;

[0159] The ratio of the rich liquid volume flow rate through the fifth flow regulating valve 40 to the rich liquid volume flow rate through the fourth flow regulating valve 39 is 9:11;

[0160] The operating temperature of the desorption tower is 150°C and the pressure is 1.2 MPa;

[0161] The operating temperature of reboiler 18 is 170°C and the load is 1.78MJ / kgCO2;

[0162] The pressure of the ammonia evaporation column 14 is 0.22 MPa, and the load of the ammonia evaporation column 14 is 0.08 MJ / kgCO2.

[0163] With the above settings, the NH3 content in the purified gas at the top of the water scrubber 13, i.e., the CO2-free flue gas 42, is less than 3 ppm, and the residual CO2 content in the purified gas is only 0.5% (dry volume);

[0164] After dehydration, the second condenser 19 obtains high-concentration, high-pressure carbon dioxide with a volume concentration greater than 95% and a pressure of up to 12 atm, namely, CO2 concentrated gas 43.

[0165] In summary, the mixed salt decarbonization process provided by the embodiments of the present invention uses an aqueous solution of a mixed salt containing mainly ammonium bicarbonate, potassium carbonate, etc. as an absorbent, has low regeneration energy consumption, and the desorption tower operates at 95-175°C and 10-12 atm, producing high-concentration, high-pressure carbon dioxide with a volume concentration of over 99% and a pressure of up to 12 atm. This reduces the energy consumption of subsequent carbon dioxide recompression, thereby reducing the operating cost of the overall system.

[0166] The mixed salt decarbonization system and process provided by the embodiment of the present invention uses a water scrubber and an ammonia still and controls their operating conditions to fully recycle the water and ammonia in the system, thereby reducing water consumption and ammonia escape.

[0167] The mixed salt decarbonization system and process provided in the embodiment of the present invention uses a two-level absorption tower including a first absorption tower and a second absorption tower, which can accelerate the reaction rate and reduce the reaction time; at the same time, the system and process also use a two-level heat exchanger (i.e., a first heat exchanger and a second heat exchanger) and feed the rich liquid at different positions of the desorption tower (i.e., the upper and middle parts of the desorption tower), which can make the temperature distribution in the desorption tower balanced while maximally recovering and utilizing the heat of the lean liquid, thereby reducing the load of the reboiler.

[0168] The above description is merely a specific embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, substitutions of equivalent components, or equivalent changes and modifications made within the scope of patent protection, should still fall within the scope of this patent. Furthermore, the technical features of this invention may be freely combined with one another, with other technical inventions, and with other technical inventions.

Claims

1. A mixed salt decarbonization system, characterized in that: The mixed salt decarbonization system includes: a first absorption tower, a second absorption tower, a water washing tower, a liquid-liquid separator, an ammonia still, a first heat exchanger, a second heat exchanger and a desorption tower; The gas outlet of the first absorption tower is connected to the gas inlet of the second absorption tower through a pipeline, the gas outlet of the second absorption tower is connected to the inlet of the water scrubber through a pipeline, the liquid outlet of the water scrubber is connected to the liquid inlet of the ammonia still tower through a pipeline, and the gas outlet of the ammonia still tower is connected to the spray inlet of the second absorption tower through a pipeline; The rich liquid outlet of the first absorption tower and the rich liquid outlet of the second absorption tower are respectively connected to the inlet of the rich liquid delivery pump through pipelines in sequence via the rich liquid circulation pump and the liquid-liquid separator, and the outlet of the rich liquid delivery pump is respectively connected to the middle spray inlet and the upper spray inlet of the desorption tower through pipelines and via the first heat exchanger and the second heat exchanger; The system further includes a third heat exchanger and a first condenser, the liquid outlet of the water scrubber is connected to the first inlet of the third heat exchanger through a pipeline, the first outlet of the third heat exchanger is connected to the liquid inlet of the ammonia still tower through a pipeline, the liquid outlet of the ammonia still tower is connected to the second inlet of the third heat exchanger through a pipeline, and the second outlet of the third heat exchanger is connected to the spray inlet of the water scrubber through a pipeline; The gas outlet of the ammonia still tower is connected to the inlet of the first condenser through a pipeline via a gas pressure reducing valve, the condensed water outlet of the first condenser is connected to the flushing water inlet of the ammonia still tower through a pipeline, and the gas outlet of the first condenser is connected to the spray inlet of the second absorption tower through a pipeline; The system further includes a fourth heat exchanger and a second condenser, the liquid outlet of the water scrubber is further connected to the first inlet of the fourth heat exchanger through a pipeline, and the first outlet of the fourth heat exchanger is connected to the spray inlet of the water scrubber through a pipeline; The gas outlet of the desorption tower is connected to the second inlet of the fourth heat exchanger through a pipe, the second outlet of the fourth heat exchanger is connected to the inlet of the second condenser through a pipe, and the condensate outlet of the second condenser is connected to the flushing water inlet of the desorption tower through a pipe.

2. The mixed salt decarbonization system according to claim 1, characterized in that: The first lean liquid outlet of the desorption tower is connected to the spray inlet of the first absorption tower through a pipeline via the first heat exchanger and the second lean liquid circulation pump in sequence, and / or the outlet of the rich liquid circulation pump is connected to the spray inlet of the first absorption tower through a pipeline.

3. The mixed salt decarbonization system according to claim 1 or 2, characterized in that: The system also includes a first cooler, and the first lean liquid outlet of the desorption tower is connected to the lean liquid inlet of the first cooler through a pipeline via a first heat exchanger and a second lean liquid circulation pump in sequence, and / or the outlet of the rich liquid circulation pump is connected to the lean liquid inlet of the first cooler through a pipeline, and the lean liquid outlet of the first cooler is connected to the spray inlet of the first absorption tower through a pipeline.

4. The mixed salt decarbonization system according to claim 1, characterized in that: The system also includes a reboiler, which is arranged at the lower part of the desorption tower. The reboiler is provided with a steam inlet, a steam outlet, a liquid inlet, a reboiler top outlet and a reboiler middle outlet. The second lean liquid outlet of the desorption tower is connected to the liquid inlet through a pipeline, and the reboiler top outlet is connected to the gas inlet of the desorption tower through a pipeline.

5. The mixed salt decarbonization system according to claim 4, characterized in that: The middle outlet of the reboiler is connected to the spray inlet of the second absorption tower through a pipeline in sequence via the second heat exchanger and the first lean liquid circulation pump, and / or the supernatant outlet of the liquid-liquid separator is connected to the spray inlet of the second absorption tower through a pipeline.

6. The mixed salt decarbonization system according to claim 4 or 5, characterized in that: The system also includes a second cooler, the middle outlet of the reboiler is connected to the lean liquid inlet of the second cooler through a pipeline via a second heat exchanger and a first lean liquid circulation pump in sequence, and / or the supernatant outlet of the liquid-liquid separator is connected to the lean liquid inlet of the second cooler through a pipeline, and the lean liquid outlet of the second cooler is connected to the spray inlet of the second absorption tower through a pipeline.

7. A mixed salt decarbonization process, characterized in that: The mixed salt decarbonization process is implemented by using the mixed salt decarbonization system according to any one of claims 1 to 6, which comprises: (1) In the first absorption tower, the pretreated CO2-containing flue gas is countercurrently contacted with the mixed salt absorbent and a chemical reaction occurs to remove the CO2 in the flue gas; the treated flue gas enters the second absorption tower, where the flue gas continues to countercurrently contact with the mixed salt absorbent. During the contact process, the residual CO2 in the flue gas reacts chemically with the mixed salt absorbent to further remove the CO2 in the flue gas; (2) The flue gas obtained in step (1) enters a water washing tower, where the flue gas is countercurrently contacted with a water washing liquid for water washing, to obtain washed flue gas and a stream containing NH4 + aqueous solution; (3) The NH4-containing + The aqueous solution enters the ammonia still, where ammonia and water are separated, and the resulting ammonia is sent to the spray layer of the second absorption tower for recycling; (4) The bottom rich liquids of the first absorption tower and the second absorption tower are combined and sent to a liquid-liquid separator for separation. The bottom rich liquid obtained after separation is sent to a desorption tower after heat exchange and regenerated in the desorption tower to obtain high-concentration, high-pressure carbon dioxide.

8. The mixed salt decarbonization process according to claim 7, characterized in that: The process further includes: dividing the bottom rich liquid obtained in step (4) into two paths and making one of the rich liquid paths heat exchange with the lean liquid obtained in the desorption tower, and then sending the heat-exchanged rich liquid and the heat-exchanged lean liquid to the middle of the desorption tower for regeneration and the spray layer of the first absorption tower for recycling, and / or merging the bottom rich liquids of the first absorption tower and the second absorption tower and sending them to the spray layer of the first absorption tower for recycling.

9. The mixed salt decarbonization process according to claim 7, characterized in that: The process also includes: using a reboiler to heat the tower body of the desorption tower, and exchanging heat between the lean liquid in the middle of the reboiler and the rich liquid in another path, and then sending the rich liquid and the lean liquid after the heat exchange to the upper part of the desorption tower for regeneration and the spray layer of the second absorption tower for recycling, and / or sending the supernatant obtained after separation in the liquid-liquid separator to the spray layer of the second absorption tower for recycling.

10. The mixed salt decarbonization process according to claim 7, characterized in that: The process also includes: firstly + The aqueous solution is divided into two paths and one of them contains NH4 + The aqueous solution is heat exchanged with the bottom liquid of the ammonia still, and then the aqueous solution after heat exchange is sent to the ammonia still and the bottom liquid after heat exchange is sent to the spray section of the water washing tower for recycling; The process also includes: decompressing and condensing the gas obtained from the top of the ammonia evaporation tower in sequence, then sending the obtained condensed water to the ammonia evaporation tower for recycling and sending the obtained ammonia gas to the spray layer of the second absorption tower for recycling.

11. The mixed salt decarbonization process according to claim 7 or 10, characterized in that: The process also includes: firstly making another path containing NH4 + The aqueous solution is heat exchanged with the high-concentration, high-pressure carbon dioxide and water vapor generated after regeneration in the desorption tower, and then the aqueous solution after heat exchange is sent to the spray section of the water washing tower for recycling, and then the carbon dioxide and water vapor after heat exchange are condensed to obtain condensed water and high-concentration, high-pressure carbon dioxide, and then the condensed water is sent to the desorption tower for recycling.

12. The mixed salt decarbonization process according to claim 7, characterized in that: The mixed salt absorbent comprises 30-55 wt% of potassium carbonate, 5-10 wt% of ammonium bicarbonate, 3-7 wt% of an alcohol amine reagent and the balance of water.

13. The mixed salt decarbonization process according to claim 7 or 12, characterized in that: The temperature of the first absorption tower is 20-30°C and the pressure is 0.08-0.15MPa; the temperature of the second absorption tower is 30-40°C and the pressure is 0.08-0.15MPa; the temperature of the desorption tower is 95-175°C and the pressure is 1-1.2MPa; the temperature of the water washing tower is 20-30°C and the pressure is 0-0.1MPa; the temperature of the ammonia evaporation tower is 120-130°C and the pressure is 0.18-0.22MPa.

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