Carbon dioxide liquefaction method and system

Abstract

A method and system for liquefying carbon dioxide is disclosed. Herein, a CO2 gas stream containing at least 90 mole % CO2, calculated on non-condensibles, is fed to a compression section and compressed to obtain a liquid or supercritical fluid CO2 stream. The supercritical fluid CO2 stream is divided into a CO2 process fluid (for liquefaction) and a CO2 refrigerant fluid (for a refrigeration cycle). The liquefaction process includes flashing the CO2 process fluid and then separating gaseous CO2 and liquid CO2. The liquid CO2 is stripped with CO2. The gaseous CO2 is liquefied by condensation together with spent CO2 stripping gas. The required cooling is achieved by vaporization of the CO2 refrigerant fluid. The gaseous CO2 thus produced is returned to the compression section as feed.

Description

Technical Field

[0001] This invention relates to the field of carbon dioxide liquefaction. Specifically, this invention relates to the liquefaction of carbon dioxide obtained from a carbon dioxide capture process.

[0002] background Today, emissions of carbon dioxide (CO2) in flue gas, such as those from the combustion of fossil fuels, including those from power plants, gas heaters, or cement production plants, are widely recognized as a major challenge to climate change. Therefore, CO2 capture is crucial for at least mitigating impacts such as global warming.

[0003] Several technologies exist for capturing CO2 from flue gas, the most important of which is chemical absorption using a suitable CO2 capture liquid. The most common such liquid is an amine solution, a typical example being a 30% monoethanolamine (MEA) solution. This yields relatively pure carbon dioxide, typically at least 80 mol%, and more commonly at least 90 mol%. Further purification and liquefaction of the captured CO2 are required to produce a usable liquid CO2 product.

[0004] To liquefy gaseous carbon dioxide, it needs to be cooled. Traditionally, liquid ammonia has been used as a refrigerant, but this has some drawbacks, such as safety hazards and environmental impacts from leaks. Interestingly, carbon dioxide itself is also a known refrigerant. Therefore, a self-cooling cycle is needed to liquefy carbon dioxide, that is, using only carbon dioxide itself as the working fluid for refrigeration.

[0005] CN106979664 discloses background technology in this regard. This technology purifies crude carbon dioxide through a multi-step process involving the adsorption and removal of non-condensable impurities. Ultimately, a liquid carbon dioxide stream is obtained at the bottom of a cryogenic distillation column and divided into three streams. The first stream, after throttling and cooling, serves as an evaporation coolant, providing a cold source for the cryogenic distillation column. The second stream, after throttling and cooling, mixes with the carbon dioxide evaporated from the first stream and is sent to a cryogenic heat exchanger, serving as a cold source for liquefied carbon dioxide. The third stream is discharged as a high-purity liquid carbon dioxide product.

[0006] The disclosed process has some drawbacks. It requires all carbon dioxide to undergo all the steps necessary to produce high-purity liquid carbon dioxide, but a significant portion of it (i.e., the first and second streams) is used merely as a cooling source rather than recycled as product. Ideal process efficiency can be achieved by subjecting only the actual product, liquid carbon dioxide, to all necessary process steps and recovering refrigerant from streams other than the product stream. Furthermore, since the medium-to-low pressure liquid carbon dioxide obtained during liquefaction is itself a cooling source, a simple refrigeration loop is required. However, the efficiency of this loop is limited. Therefore, alternative refrigeration loops are needed to achieve higher efficiency.

[0007] Further background references on the use of liquid CO2 itself as a refrigerant in CO2 liquefaction processes include WO2022 / 184794 and WO20222 / 184646. The former relates to improving operational instabilities in the cooling process of gases rich in compressed CO2. The latter aims to optimize the heat exchanger in the CO2 liquefaction process. Neither of these disclosures addresses overall improvements to the self-cooling CO2 liquefaction process.

[0008] Another need in this field is the development and continued development of large-scale carbon capture facilities. This is expected to drive exponential growth in the liquid CO2 market and expand beyond current major liquid CO2 markets to specialized sectors such as the food and beverage or pharmaceutical industries. Given the corresponding changes and expansion in the scale of liquid CO2 demand, there is currently no clear consensus or standard regarding the required temperature and pressure conditions for liquid CO2. In fact, as the end-user base becomes increasingly diversified, the demand for liquid CO2 products is likely to become more diversified as well. Therefore, an ideal liquefaction system should be able to produce liquid CO2 at different pressure levels while maintaining high efficiency. This approach allows a single liquefaction facility to supply in a way that meets the specific needs of different buyers and can easily adapt to future market changes or new standards.

[0009] Overview To better meet one or more of the above requirements, the present invention provides a method for liquefying carbon dioxide in one aspect, comprising: - Provide a CO2 gas stream, calculated based on noncondensable substances, wherein the CO2 gas stream contains at least 90 mol% CO2; - Compression stage, which includes compressing CO2 gas to a pressure equal to or above the critical pressure to obtain a liquid or supercritical fluid CO2 stream; - Separate the supercritical fluid or liquid CO2 flow into a CO2 process fluid flow and a CO2 refrigerant fluid flow; - The CO2 refrigerant fluid is placed in a refrigeration cycle, the refrigeration cycle comprising: o expands the CO2 refrigerant fluid flow to provide expanded CO2 refrigerant fluid; o vaporizes the expanded CO2 refrigerant fluid to provide CO2 refrigerant vapor; o The CO2 refrigerant vapor is input as CO2 gas and circulated to the compression stage; - Liquefying the CO2 process fluid includes: o Pre-cooling stage, including cooling the CO2 process fluid flow to a temperature range of -10°C to 30°C to obtain a pre-cooled CO2 process fluid flow; The expansion stage includes flash evaporation of the pre-cooled CO2 process fluid stream to provide an expanded CO2 process fluid stream. o Separation stage, including separating the expanded CO2 process fluid flow into CO2 flash vapor and residual CO2 liquid; The stripping stage includes stripping residual CO2 liquid using CO2 as a gaseous stripping medium to provide stripped liquefied CO2 and stripping stage vapor. The condensation stage includes cooling the stripping stage vapor to form condensed CO2 fluid; The cooling of the condenser stage includes heat exchange with a vaporized, expanding CO2 refrigerant fluid, which acts as a coolant.

[0010] On the other hand, the present invention provides a system for liquefying carbon dioxide, the system comprising: - Compression section (1), configured to compress a CO2 gas stream to a pressure equal to or above the critical pressure, the compression section having an inlet for the CO2 gas stream and an outlet for a liquid or supercritical fluid CO2 gas stream; - A separation section configured to separate a supercritical fluid or liquid CO2 flow into a CO2 process fluid flow and a CO2 refrigerant fluid flow, the separation section having an inlet in fluid communication with the outlet of the supercritical fluid or liquid CO2 flow of the compression section (1), an outlet for the CO2 process fluid flow and an outlet for the CO2 refrigerant fluid flow; - A refrigeration expansion section configured to expand a supercritical fluid or liquid CO2 flow, the refrigeration expansion section having an inlet in fluid communication with the outlet of the CO2 refrigerant fluid of the separation section, and an outlet for expanding the CO2 fluid containing liquid CO2 and CO2 vapor; - An evaporation section configured to vaporize a CO2 fluid containing liquid CO2 and CO2 vapor, the evaporation section having an inlet in fluid communication with the outlet of the expanded CO2 fluid of the refrigeration expansion section, and an outlet for CO2 vapor in fluid communication with the CO2 gas inlet of the compression section (1); - A precooling section (2) configured to cool the supercritical fluid or liquid CO2 flow, the precooling section having an inlet in fluid communication with the outlet of the CO2 process fluid flow of the separation section, and an outlet for precooling liquid CO2. - A liquefaction expansion section configured for flash evaporation of liquid CO2, the liquefaction expansion section having an inlet in fluid communication with the precooled liquid CO2 outlet of the precooling section, and an outlet for the process fluid flow of the expanded CO2; - A separation section configured to perform gas-liquid separation of expanded CO2 fluid, the separation section having an inlet in fluid communication with the outlet of the expanded CO2 process fluid flow of the liquefaction expansion section, and an outlet for flash vapor and an outlet for residual CO2 liquid. - A stripping section configured to strip liquid CO2 using gaseous CO2 as the stripping medium, the stripping section having an inlet in fluid communication with the outlet of the residual CO2 liquid of the liquefaction expansion section, an inlet for the gaseous CO2 stripping medium, an outlet for the vapor of the stripping section, and an outlet for the stripped liquefied CO2. - A condensation section configured to cool the gaseous CO2, the condensation section having an inlet in fluid communication with the outlet of the flash gas of the liquefaction expansion section and an inlet in fluid communication with the steam outlet of the stripping section, and having an outlet for condensing the CO2 fluid; The condensation section is thermally connected to the evaporation section. Brief description of the attached diagram Figure 1 The diagram illustrates an implementation of the process and system according to the present invention.

[0012] Figures 2 to 5 The diagram illustrates other layout options related to the stripping section.

[0013] Detailed description In a broader sense, this invention is based on the insightful observation that the relatively high purity carbon dioxide obtained from carbon capture processes enables a highly efficient self-refrigeration process for liquefying carbon dioxide. In this process, in the early stages, the CO2 to be liquefied and the CO2 used as a refrigerant are cleverly separated. Thus, the CO2 refrigerant is placed in a refrigeration cycle cleverly nested within the liquefaction process, utilizing not only the cooling effect of the refrigerant to correspond to the condensation step in the liquefaction process to obtain liquid CO2, but also utilizing the compression section that initiates the liquefaction process as the compression section required for the refrigeration cycle.

[0014] The method first provides a CO2 gas stream containing at least 90 mol% CO2 (calculated based on non-condensable substances). Regardless of the presence of additional water and condensable impurities, this CO2 gas stream contains less than 15% non-condensable impurities. The term "non-condensable impurities" is known in the field of carbon dioxide liquefaction and refers to gaseous impurities that do not condense under conventional compression and refrigeration conditions of CO2. Specifically, it refers to impurities that do not condense with water. Typically, non-condensable impurities include one or more of oxygen, nitrogen, and argon. Preferably, the content of non-condensable impurities in the CO2 gas stream is less than 10%, more preferably less than 5%. In the process of the present invention, the content of water and condensable impurities is not particularly critical. Typically, this content is less than 25 mol%, preferably 5 mol% to 10 mol%.

[0015] Preferably, the CO2 gas stream described above contains at least 80 mol% CO2. Regardless of the presence of non-condensable impurities, the CO2 gas stream preferably contains more than 90 mol% CO2. More preferably, the CO2 gas stream contains at least 95 mol% CO2.

[0016] While the gas flow can originate from any source, it is particularly suitable for CO2 obtained through an absorption carbon capture process. This process can be carried out elsewhere, delivering the CO2 to the system according to the invention. This can also refer to adjacent facilities, such as power plants, for capturing CO2 from flue gas. Furthermore, integrated systems using pristine CO2 as feedstock can also be employed, comprising a CO2 purification section, such as an absorption section, located upstream of the compression section used in this invention.

[0017] In the first stage, the process involves compressing CO2 gas to a pressure equal to or above the critical pressure. Referring to the well-known CO2 phase diagram, those skilled in the art will understand that this pressure is typically at least about 75 bar (g) and typically 100 bar (g). This process is completed in a compression stage. This stage includes at least one compression step. As is customary in compression stages, compression is preferably performed first, followed by cooling. Preferably, compression is carried out in a series of compression steps, each followed immediately by a cooling step. In the cooling step, water and condensable impurities will condense. Therefore, water and other condensable impurities can be removed by cooling between compressors, followed by gas-liquid separation. In the final step, post-cooling is preferably performed. It should be understood that if there is only one compressor, post-cooling occurs downstream of that compressor. If multiple compressors are used in series, post-cooling occurs downstream of the last compressor, i.e., at the downstream end of the series of compressors.

[0018] The compression process transforms CO2 gas into supercritical CO2 fluid. Depending on the pressure reached and the degree of post-cooling, CO2 can remain in a supercritical state, but cooling may also lower the CO2 temperature below the critical temperature, i.e., to the pressure and temperature conditions required for CO2 to be in a liquid state. Therefore, the output of the compression stage is either liquid or supercritical CO2 fluid.

[0019] Referring to the system of the present invention, the compression section is carried out in a compression section (1), which is configured to compress the CO2 gas stream to a pressure equal to or higher than the critical pressure. The compression section has an inlet for the CO2 gas stream and an outlet for the liquid or supercritical CO2 stream.

[0020] Advantageously, the inlet of the CO2 gas stream is preferably in fluid communication with the outlet of the CO2 gas from the apparatus used to capture carbon dioxide from flue gas. Such apparatuses typically include an absorption section and a regeneration section, in which the flue gas comes into contact with a CO2 absorbent; and in the regeneration section, the absorbent is regenerated. Techniques for removing CO2 from flue gas using chemical absorption are well-established. A variety of CO2 absorbents are available, all of which serve to provide a suitable alkali capable of directly or indirectly binding carbon dioxide. Typically, these absorbents are organic bases, typically amines. Alternatively, CO2 absorbents can also be inorganic solvents, such as potassium carbonate, particularly the "thermal potassium carbonate process" (HPC) familiar to those skilled in the art. Suitable amines include diethanolamine (DEA), monoethanolamine (MEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and aminoethoxyethanol (diethylene glycolamine) (DGA).

[0021] Depending on the purity of the CO2 fed into the compression stage and the removal of condensable impurities during cooling between compression steps, the liquid or supercritical fluid CO2 stream obtained from the compression stage can provide a sufficiently pure CO2 product after stripping to remove any non-condensable impurities that may be present. Preferably, to better ensure the required purity, a further purification step should be included in the process of the present invention before the separation of the CO2 refrigerant fluid stream. This can be achieved by one or more drying steps and / or other purification steps familiar to those skilled in the art. In a preferred embodiment, if there is a series of compression and intermediate cooling steps, additional purification is performed after the penultimate intermediate cooling and gas-liquid separation step and before the final compression and post-cooling step. Preferably, at least almost all condensable impurities are removed in the compression stage, preferably until the residual water content is less than 50 ppm, and more preferably less than 30 ppm.

[0022] Therefore, referring to the system of the present invention, it is preferable to provide at least one purification unit upstream of the last stage of the compression section. This unit is primarily used to further remove condensable impurities, such as water, for example, by adsorption onto a specific adsorbent (e.g., a molecular sieve). Advantageously, this unit can also be used to remove other possible impurities, such as SO2 (which can be removed by adsorption with activated carbon) or ammonia. If necessary, other water-soluble compounds can be removed by washing the CO2 stream with water. The purification unit is preferably located upstream of the refrigeration section of the apparatus to better prevent ice formation in the system. In addition to providing a purification unit after the penultimate stage of the compression section, the purification steps and corresponding purification units can be distributed among multiple purification units, each located between different stages of the compression section.

[0023] In another preferred embodiment, the CO2 gas stream can be dried during the compression stage. This can be done before the final compression stage, i.e., before the CO2 is in a liquid or supercritical but near-liquid-phase condensation state. The formation of liquid CO2 would hinder subsequent drying operations. Therefore, drying is preferably carried out at a moderate pressure level, typically around 50-60 bar (g), which is considered to achieve an ideal balance between equipment size, pressure loss, and design pressure requirements, while avoiding the presence of liquid CO2 during the drying step.

[0024] Therefore, in the system of the present invention, the compression section preferably includes one or more units selected from the purification unit, the drying unit, and combinations thereof.

[0025] The inventors recognized that providing a CO2 gas stream of sufficiently high purity is beneficial for constructing a self-cooling loop. This purity is manifested in the CO2 gas stream containing at least 90 mol% CO2 (calculated based on non-condensable substances), and therefore containing less than 15% non-condensable impurities. Preferably, the CO2 gas stream contains more than 90 mol% CO2, more preferably at least 95 mol% CO2.

[0026] To this end, the process involves separating the supercritical fluid or liquid CO2 stream obtained from the compression stage into a CO2 process fluid stream and a CO2 refrigerant fluid stream. A relatively large portion of the CO2 process fluid stream can be directly processed and stripped to remove non-condensable impurities, yielding the liquid CO2 product. This involves expansion prior to stripping (typically controlled by an expansion valve or throttle valve). After expansion, only a small portion (referred to herein as flash gas) remains in the gas phase. Therefore, after expansion, most of the process CO2 (referred herein as residual liquid CO2) remains liquid.

[0027] Referring to the system of this invention, the separation process is carried out in a splitting section configured to separate a supercritical fluid or liquid CO2 flow into a CO2 process fluid flow and a CO2 refrigerant fluid flow. This splitting section has an inlet in fluid communication with the outlet of the supercritical fluid or liquid CO2 flow from the compression section (1), an outlet for the CO2 process fluid flow, and an outlet for the CO2 refrigerant fluid flow. Typically, the splitting section includes an inlet for the supercritical fluid or liquid CO2 flow, a first outlet for the CO2 process fluid flow, a second outlet for the CO2 refrigerant fluid flow connected in parallel, and preferably, a control device for adjusting the separation ratio as needed.

[0028] CO2 refrigerant fluid is used as a refrigerant in a refrigeration cycle. As those skilled in the art know, in this cycle, typically, liquid refrigerant expands to form a mixture of liquid and vapor, which then evaporates. The resulting low-pressure refrigerant vapor is then compressed into high-pressure refrigerant vapor, which is then condensed to finally obtain liquid refrigerant. However, when CO2 is used as a refrigerant, the corresponding phase change is not strictly a phase change between liquid and vapor, but rather transcritical. Therefore, in a CO2 refrigeration cycle, the low-pressure refrigerant vapor is compressed to obtain a supercritical fluid. This supercritical fluid is then cooled and expanded below the critical temperature to obtain liquid refrigerant. Therefore, the method of the present invention includes: expanding a CO2 refrigerant fluid stream, typically to the range of 5 to 50 bar (g), more typically 7 to 30 bar (g), to obtain an expanded CO2 refrigerant fluid; evaporating the expanded CO2 refrigerant fluid to obtain CO2 refrigerant vapor; and feeding the CO2 refrigerant vapor as CO2 gas into the compression section of the cycle.

[0029] Referring also to the system of the present invention, the CO2 refrigerant fluid flow is processed in a refrigeration expansion section configured to expand the supercritical fluid or liquid CO2 flow. The inlet of the refrigeration expansion section is in fluid communication with the outlet of the CO2 refrigerant fluid flow in the separation section. The expansion process typically takes place in an expansion valve. Referring to the consequences of the expansion, the expansion section has an outlet for the expanded CO2 fluid containing both liquid CO2 and CO2 vapor.

[0030] To enable the CO2 refrigerant fluid flow to undergo a refrigeration cycle, the system of the present invention further includes an evaporation section configured to vaporize a CO2 fluid comprising liquid CO2 and CO2 vapor. The evaporation section includes an inlet in fluid communication with the expanded CO2 fluid outlet of the refrigeration expansion section, and an outlet for the vaporized CO2 refrigerant. This outlet is in fluid communication with the CO2 gas inlet of the compression section (1). Also advantageously, the vapor connection from the evaporation section to the compression section includes a heat exchange section in thermal communication with a precooling section, in which the CO2 process fluid flow is processed. This allows heat to be recovered from the vaporized CO2 refrigerant, thereby allowing the heated vaporized CO2 refrigerant to be returned to the compression stage.

[0031] The CO2 process fluid is processed to ultimately obtain liquid CO2 product. For this purpose, the process includes precooling the CO2 process fluid stream. This is typically carried out in a temperature range of -10°C to 30°C, preferably below 20°C, and more preferably in a temperature range of 0°C to 20°C, thereby obtaining a precooled CO2 process fluid stream. In the expansion stage, this stream undergoes flash evaporation, also known as adiabatic evaporation or flash evaporation. Typically, the precooled CO2 process fluid stream is thus depressurized to a pressure in the range of 7 to 35 bar (g).

[0032] This leads to the formation of a flash gas, typically ranging from 5 mol% to 25 mol%, for example, 10 mol% to 20 mol%. The flash gas contains gaseous CO2 and gaseous non-condensable impurities that may be present in the CO2 process fluid stream. To further remove non-condensable impurities from the residual CO2 liquid, the residual CO2 needs to be stripped using CO2 as a gaseous stripping medium (typically in a stripping tower). The remaining liquid can then be obtained as liquefied carbon dioxide. After stripping, the stripping stage vapor (containing used CO2 stripping gas, CO2 flash vapor, and stripped non-condensable impurities) is condensed to form a condensed CO2 fluid, which is typically a two-phase flow, mainly consisting of liquid CO2 and a small amount of vapor with a relatively high concentration of non-condensable components. To minimize CO2 loss, the vapor fraction is preferably as low as possible. Typically, the vapor fraction is less than 5 mol%, preferably less than 1.5 mol%.

[0033] Typically, the vapor (i.e., the condenser-stage gaseous CO2) is separated out, and the resulting condenser-stage liquid CO2 is refluxed back to the stripping process.

[0034] Referring to the liquefaction process described above, the present invention also relates to a system comprising a corresponding section for performing the process. This includes a precooling section configured to cool a supercritical fluid or liquid CO2 stream. The precooling section has an inlet in fluid communication with the outlet of the CO2 process fluid stream from the separation section, and an outlet for precooling the liquid CO2. This outlet is in fluid communication with the inlet of a liquefaction expansion section configured to flash the liquid CO2. In addition to the inlet in fluid communication with the precooled liquid CO2 outlet of the precooling section, the liquefaction expansion section also has an outlet for flash vapor and an outlet for residual liquid CO2. As described above, the expansion section typically includes an expansion valve. The flash vapor outlet and the residual liquid CO2 outlet are typically connected to an expansion vessel in fluid communication with the expansion valve. In one embodiment, the expansion vessel (also called a flash tank) is a separate vessel located downstream of the expansion valve and upstream of the stripping section, where the residual liquid CO2 is stripped. In another embodiment, the expansion vessel is located within the stripping section, typically at the top of a stripping tower. In this embodiment, the outlet of the flash vapor in the liquefaction expansion section can also serve as the outlet of the stripping gas in the stripping section. Similarly, the outlet of the residual liquid CO2 in the liquefaction expansion section is also located within the stripping section, typically allowing liquid to flow from the upper to the lower part of the stripping tower. The stripping section itself is configured to strip liquid CO2 using CO2 as the gaseous stripping medium. For this purpose, the stripping section has an inlet in fluid communication with the outlet of the residual liquid CO2 in the liquefaction expansion section, and an inlet for the gaseous CO2 stripping medium. Furthermore, the stripping section also has an outlet for stripping section vapor and an outlet for the stripped liquid CO2. Advantageously, the inlet of the gaseous CO2 stripping medium is in fluid communication with a CO2 vapor source from the process itself (i.e., within the system of the present invention). The gaseous CO2 stripping medium can be provided entirely in gaseous form or can be contained within a two-phase CO2 fluid.

[0035] Partially, stripping is performed via self-stripping of CO2 vapor present in the residual liquid CO2. Preferably, a certain amount of liquid CO2 is extracted from the stripped liquefied CO2, vaporized, and reused as the stripping medium, while the majority of the stripped liquefied CO2 is retained as the liquefied CO2 production stream. Those skilled in the art can determine the required amount of gas for CO2 stripping based on the amount of non-condensable impurities to be removed by stripping. As a guideline, for example, this amount is typically about 25% of the stripping section inlet flow rate to ensure that the concentration of non-condensable components in the liquefied CO2 is below 10 ppm. Most advantageously, the precooling section thus functions as a reboiling section of the stripping section. That is, the vaporization step of the portion of the stripped liquefied CO2 reused as the stripping medium includes: in the precooling section, heat exchange between the liquefied CO2 and the CO2 process fluid stream.

[0036] The system of the present invention includes a condensation section for cooling gaseous CO2. The condensation section has an inlet in fluid communication with the flash outlet of the liquefaction expansion section and an inlet in fluid communication with the outlet of the stripping stage vapor of the stripping section. These inlets may be the same inlet serving both purposes or refer to different inlets. The condensation section has an outlet for condensing CO2 fluid in fluid communication with a gas-liquid separation section (e.g., a separator). Preferably, the condensation section is in thermal communication with an evaporation section. Therefore, the system allows the flow of CO2 refrigerant fluid to act as a coolant for the unliquefied portion of the CO2 process fluid.

[0037] Depending on the initial purity of the CO2 supplied to the process, the condensed gaseous CO2 will contain a certain amount of uncondensed impurities. Therefore, it is preferable to perform gas-liquid separation on the condensed CO2 fluid, preferably using a separator. The liquid CO2 can be processed and collected separately, but it is preferable to return it to the stripping stage, typically via reflux to the stripping tower.

[0038] The condensation of gaseous CO2 obtained from the expansion and stripping stages requires cooling. According to the invention, the aforementioned refrigeration cycle is used to achieve this objective, and the CO2 refrigerant fluid undergoes this cycle. Therefore, cooling in the condensation stage involves heat exchange with the vaporized, expanded CO2 refrigerant fluid (as a coolant).

[0039] The process of this invention is applicable to different types of CO2 refrigeration cycles, including single-loop vapor compression cycles. Preferably, a dual-pressure CO2 refrigeration cycle is used. Because its coefficient of performance (COP) is higher than that of single-loop refrigeration, the efficiency of the liquefaction process can be improved.

[0040] exist Figure 1 In the schematic dual-pressure cycle, the refrigerant CO2 does not flash directly to the pressure level required by the condenser, but first passes through the expansion valve ( Figure 1 The initial throttling valve (V3) in the system depressurizes the refrigerant CO2 to an intermediate pressure level (typically 40-70 bar (g)). The resulting CO2 vapor is separated in a flash tank and recycled to provide coolant as a pre-cooling stage. The intermediate-pressure liquid CO2 refrigerant fluid obtained from the initial depressurization is typically passed through another throttling valve (V3). Figure 1 The final throttling valve (V4) in the process performs further flash evaporation. The resulting expanded CO2 refrigerant fluid is then used as the cooling medium in the condensing stage, where it is vaporized and circulated again through the heat exchange of the precooling stage to the compression stage. This dual-pressure structure further improves the thermal integration and overall process efficiency in the process.

[0041] Preferably, the system configuration of the present invention is used to perform the above-described dual-pressure cycle. In this configuration, the refrigeration expansion section includes an initial expansion unit (e.g., an expansion valve) for initially depressurizing the liquid CO2 refrigerant. The initial expansion unit has an inlet in fluid communication with the CO2 refrigerant outlet of the separation section, and an outlet for the intermediate-pressure CO2 refrigerant obtained after the initial depressurization. A gas-liquid separation unit (e.g., a flash tank) is configured to separate the intermediate-pressure CO2 refrigerant fluid into intermediate-pressure liquid CO2 and intermediate-pressure CO2 vapor. The gas-liquid separation unit has an inlet in fluid communication with the outlet of the intermediate-pressure CO2 refrigerant fluid from the initial expansion unit, an outlet for the intermediate-pressure liquid CO2, and an outlet for the intermediate-pressure CO2 vapor in fluid communication with the inlet of the CO2 gas in the compression section. A further expansion unit, such as a further expansion valve, is configured to further expand the intermediate-pressure liquid CO2 refrigerant fluid. The further expansion unit has an inlet in fluid communication with the outlet of the intermediate-pressure liquid CO2 from the gas-liquid separation unit, and an outlet for expanding the CO2 fluid (containing liquid CO2 and CO2 vapor), which is in fluid communication with the inlet of the evaporation section. Advantageously, the vapor connection from the further expansion unit to the compression stage includes a heat exchange section in thermal communication with a precooling section, in which the CO2 process fluid flow is processed. This allows refrigerant to be recovered from the expanded CO2 refrigerant vapor at intermediate pressure, thereby returning the heated expanded CO2 refrigerant vapor to the compression stage.

[0042] The process of this invention typically employs a favorable thermal integration. The CO2 refrigerant vapor stream, used as a coolant for condensing gaseous CO2 (including flash vapor and stripping gas), still has a relatively low temperature, typically above -50°C and below 0°C, for example, -45°C to -10°C. In suitable embodiments, the CO2 vapor generated by intermediate decompression in the aforementioned dual-pressure cycle is also applicable. Both of these fluids are essentially coolant streams and can be further used as coolants for precooling the CO2 process stream to be liquefied. Therefore, any available cooling capacity of any one or both of these used coolant streams can be utilized. This allows the refrigerant CO2 to be circulated back to the compression step at higher temperatures (e.g., in the range of 20°C to 50°C, preferably at least 30°C, for example, 30°C to 40°C) without wasting any cooling capacity. Similarly, if the liquefied CO2 obtained from stripping undergoes further expansion, the available cooling capacity in the resulting CO2 vapor stream can also be recovered in the precooling section.

[0043] Unwilling to be bound by theory, the inventors believe that the above results are achieved because the process of this invention utilizes the synergistic effect of splitting the compressed supercritical fluid or liquid CO2 stream into a CO2 process fluid stream and a CO2 refrigerant fluid stream. Since most of the process CO2 remains in the liquid phase after liquefaction and expansion, only a relatively low CO2 refrigerant flow rate is required to complete liquefaction (typically less than 50% of the process CO2 mass flow rate). Furthermore, the process CO2 is under high pressure and therefore in a high-density phase (liquid or supercritical). Thus, the process CO2 has a high heat capacity, and even if the used refrigerant CO2 vapor stream is heated back to the aforementioned higher temperature before being recycled, it can maintain a temperature that is typically high enough to generate sufficient CO2 vapor for stripping during heat exchange in the precooling section.

[0044] In addition to the above, it should be noted again that the process CO2 is compressed to a high pressure level (typically 100 bar (g)) above its critical point. Thus, it can first be liquefied at a relatively high temperature by cooling water, cold process fluids, and any other low-grade cooling media that may be present (such as the used coolant stream discussed above).

[0045] The stripping stage can be carried out in various ways. In one implementation method (e.g.) Figure 2 (Illustratively shown), the expanded CO2 process fluid stream obtained from the expansion stage is stripped, and then the resulting liquid and gas fractions are processed according to the method described above.

[0046] In another embodiment, the expanded CO2 process fluid flow first undergoes gas-liquid separation. The resulting liquid is stripped, typically fed to the upper part of a stripping tower. The gas produced by gas-liquid separation is used as stripping gas to strip liquid CO2, typically fed to the lower part of the stripping tower. Therefore, in this embodiment (e.g.) Figure 3 (Illustratively shown), the stripping stage includes: gas-liquid separation of the expanded CO2 process fluid flow to obtain stripping stage liquid and stripping stage gas; stripping the stripping stage liquid using CO2 as a gaseous stripping medium; and using the stripping gas as the stripping medium.

[0047] If the expanded CO2 process fluid stream undergoes gas-liquid separation before stripping, different methods can be used. In one embodiment (e.g.) Figure 4 (Illustratively shown), gas-liquid separation is performed together with the gas-liquid separation of the condensed CO2 fluid obtained by condensing the gaseous CO2 obtained in the stripping step. For this purpose, the expanded CO2 process fluid is fed into the condensation section (preferably into one identical condenser) where the gaseous CO2 is condensed.

[0048] In another embodiment, the expanded CO2 process fluid stream is first subjected to gas-liquid separation, and the resulting gas is used as stripping gas to strip liquid CO2, as described above. In this embodiment (e.g.) Figure 4 (Illustratively shown), the liquid obtained from the gas-liquid separation is sent to the condensation section, where it is finally separated from the condensed CO2 fluid, as described above, and then stripped.

[0049] The above-described embodiments can also be used in combination. For example, part of the expanded CO2 process fluid flow can be directly stripped, while another part can undergo gas-liquid separation first.

[0050] In the first embodiment of the present invention, the stripped liquefied carbon dioxide obtained from the stripping stage is the actual product of the liquefaction process.

[0051] In the second embodiment of the present invention (e.g.) Figure 1 As shown, the liquefied CO2 production stream undergoes post-processing, namely expansion (preferably flash evaporation) to the final pressure level and gas-liquid separation. The advantage of this is that the pressure applied during stripping, particularly the operating pressure of the stripping tower, is decoupled from the final pressure of the liquid CO2. This allows for more flexible optimization of the stripping stage as needed. Furthermore, operating the stripping tower at higher pressures effectively increases the temperature of the condensation section. Therefore, the refrigerant CO2 can be at a higher temperature. This, in turn, reduces the degree of refrigerant decompression, thereby effectively reducing compressor energy consumption. The expansion step of liquefied CO2 further increases the flexibility of the final carbon dioxide product pressure level. Thus, a variety of desired products can be obtained, as well as different specifications of products produced for different end uses (e.g., different uses of CO2 in adjacent plants). Different pressure levels (and thus different temperatures) can be achieved by changing the pressure drop during the expansion process (typically by operating a throttle valve). If this results in small changes in system flow, these can be easily controlled, for example, by reducing the compressor load in the compression step and / or allowing appropriate design margins for other equipment used in the process.

[0052] like Figure 1As shown (but not limited to), the process and system of the present invention are preferably configured to produce CO2 at different pressure levels (and thus different temperatures). This is because the final pressure of the liquid product is decoupled from the operating pressure of the stripping tower (3) via valve V2. The product CO2 (21) is depressurized by V2 and flashed to form a two-phase flow (22). This two-phase flow is separated into vapor (24) and liquid (23) by an expansion vessel (6). The pressure of the expansion vessel (6) can be changed independently of the pressure of the stripping tower (3) by a dedicated pressure control device (not shown in the figure, but this is conventional). The vapor (24) is circulated to the compressor (1) via the heat exchange system (2) to recover its cooling capacity. Changing the pressure of the expansion vessel (6) will cause a change in the flow rate of the separated vapor (24). However, due to the structure of the system and the high degree of thermal integration, this change in the flow rate of the vapor (24) will not have a significant impact on the system efficiency and the operation of the compressor (1), and can be carried out within the operating margin of the unit.

[0053] The liquefied CO2 obtained from stripping undergoes post-treatment flash evaporation to generate a post-treatment CO2 vapor stream. This vapor stream can be used for further recovery of cold energy from the process, preferably in the pre-cooling stage. This is due to the heat exchange in the pre-cooling stage. The post-treatment CO2 vapor stream is heated and circulated back to the compression section as a heated post-treatment CO2 vapor stream.

[0054] In summary, this invention relates to a method and system for liquefying carbon dioxide. A CO2 stream containing at least 90 mol% carbon dioxide (calculated based on non-condensable substances) and less than 15% non-condensable impurities (containing at least 90 mol% CO2 regardless of the presence of non-condensable impurities) is fed into a compression stage and compressed to provide a liquid or supercritical fluid CO2 stream. This fluid is separated into a CO2 process fluid (for liquefaction) and a CO2 refrigerant fluid (for the refrigeration cycle). The liquefaction process involves flashing the CO2 process fluid, followed by separating gaseous and liquid CO2. The latter is stripped with CO2. The former, along with the used CO2 stripped gas, is liquefied by condensation. The required cooling is achieved by vaporizing the CO2 refrigerant. The resulting gaseous CO2 is returned as feed to the compression stage.

[0055] The present invention will be further described with reference to the accompanying drawings. It should be understood that the drawings do not limit the invention. For example, the invention is not limited to the specific type of equipment and specific equipment systems shown. The drawings schematically illustrate equipment components and process flows related to embodiments of the present invention.

[0056] Figure 1 A process flow diagram according to a preferred embodiment of the present invention is shown, wherein a dual-pressure refrigeration cycle is employed, the condensed CO2 is subjected to gas-liquid separation, and the liquefied CO2 obtained from the stripping section is further expanded.

[0057] Figures 2 to 5 This is a schematic diagram illustrating other layout options related to the stripping section, as described above.

[0058] In the diagram, the reference symbols indicating devices and flows have the following meanings: 1. Compression section; 2. Pre-cooling section; 3. Stripping section (represented by a stripping tower in the diagram); 4. Condensation section; 5. Condenser separator tank; 6. Expansion container; 7. Intermediate flash evaporator; 8. Purification unit; 9. Crude CO2; 10. Compress the CO2 stream; 11. Process CO2; 12. Pre-cooling process for CO2; 13. Expanding CO2 process fluid; 13V = CO2 flash vapor; 13L = residual CO2 liquid; 14. Stripping-grade steam; 15. Condensing CO2 fluid; 16. Condensation stage vapor phase CO2; 17. Condensation stage liquid phase CO2; 18. The liquefied CO2 stream stripped from the stripping tower outlet; 19. The liquefied CO2 extracted from (18) is reused as a stripping medium; 20. Stripping medium; 21. Liquefied CO2 production flow; 22. Flash evaporation of liquefied CO2; 23. Liquid product CO2; 24. Post-treatment CO2 vapor stream; 25. Heated post-treatment CO2 vapor stream refrigerant CO2; 26. CO2 refrigerant fluid; 27. Expanding CO2 refrigerant fluid; 28. Heated, expanding CO2 refrigerant vapor; 29. Liquid CO2 refrigerant; 30. CO2 refrigerant fluid at intermediate pressure; 31. Expanding CO2 refrigerant fluid; 32. Vaporize CO2 refrigerant; 33. Heated vaporized CO2 refrigerant; V1 expansion stage throttle valve; V2 aftertreatment throttle valve; V3 dual-pressure refrigeration cycle initial throttling valve; V4 dual-pressure refrigeration cycle final throttling valve.

Claims

1. A method for liquefying carbon dioxide, comprising: - Provide a CO2 gas stream, calculated based on noncondensable substances, wherein the CO2 gas stream contains at least 90 mol% CO2; - Compression stage, which includes compressing CO2 gas to a pressure equal to or above the critical pressure to obtain a liquid or supercritical fluid CO2 stream; - Separate the supercritical fluid or liquid CO2 flow into a CO2 process fluid flow and a CO2 refrigerant fluid flow; - The CO2 refrigerant fluid is placed in a refrigeration cycle, the refrigeration cycle comprising: o expands the CO2 refrigerant fluid flow to provide expanded CO2 refrigerant fluid; o vaporizes the expanded CO2 refrigerant fluid to provide CO2 refrigerant vapor; o The CO2 refrigerant vapor is input as CO2 gas and circulated to the compression stage; - Liquefying the CO2 process fluid includes: o Pre-cooling stage, including cooling the CO2 process fluid flow to a temperature range of -10°C to 30°C to obtain a pre-cooled CO2 process fluid flow; The expansion stage includes flash evaporation of the pre-cooled CO2 process fluid stream to provide an expanded CO2 process fluid stream. o Separation stage, including separating the expanded CO2 process fluid flow into CO2 flash vapor and residual CO2 liquid; The stripping stage includes stripping residual CO2 liquid using CO2 as a gaseous stripping medium, wherein the stripping medium optionally contains CO2 flash vapor to provide stripped liquefied CO2 and stripping stage vapor. o Condensation stage, including the stripping stage vapor, optionally together with CO2 flash vapor, is cooled to form condensed CO2 fluid; The cooling of the condenser stage includes heat exchange with a vaporized, expanding CO2 refrigerant fluid, which acts as a coolant.

2. The method according to claim 1, wherein the CO2 gas stream provided to the method originates from an absorption carbon capture process.

3. The method according to claim 1 or 2, wherein the compression stage comprises one or more steps selected from the group consisting of: purifying the CO2 gas stream, drying the CO2 gas stream, and combinations thereof.

4. The method according to any one of the preceding claims, wherein the precooling stage comprises cooling the CO2 process fluid stream to a temperature in the range of 0°C to 20°C.

5. The method according to any one of the preceding claims, wherein the condensation stage comprises separating condensed CO2 fluid into condensation stage gaseous CO2 and condensation stage liquid CO2, and returning the liquid CO2 to the stripping stage.

6. The method according to any one of the preceding claims, wherein the expanded CO2 refrigerant fluid is obtained by a two-step process, the two-step process comprising depressurizing the refrigerant CO2 stream to an intermediate pressure level of 40-70 bar (g), simultaneously forming intermediate pressure flash vapor and intermediate pressure CO2 refrigerant liquid; separating the intermediate pressure flash vapor and the intermediate pressure CO2 refrigerant liquid; and further expanding the intermediate pressure CO2 refrigerant liquid to obtain the expanded CO2 refrigerant fluid, wherein the method further comprises recirculating the intermediate pressure flash vapor as a coolant to the precooling stage for use in the CO2 process fluid stream.

7. The method according to any one of the preceding claims, comprising using an expanded CO2 process fluid stream as the liquid phase of a condensation stage, such that it is contained in the condensed CO2 fluid, wherein the separation stage comprises separating the expanded CO2 process fluid stream into CO2 flash vapor and residual CO2 liquid, performing the separation as described in claim 5 on the condensed CO2 fluid, and sending the residual CO2 liquid together with the liquid phase CO2 of the condensation stage to the stripping stage.

8. The method according to any one of claims 1 to 6, comprising using the residual CO2 liquid as a liquid phase in the condensation stage and sending it to the stripping stage after further gas-liquid separation, while simultaneously performing the separation of the condensed CO2 fluid according to claim 5.

9. The method according to any one of the preceding claims, comprising expanding and gas-liquid separation of stripped liquefied CO2, thereby enabling the provision of different batches of liquefied carbon dioxide at different pressures.

10. A system for liquefying carbon dioxide, the system comprising: - Compression section (1), configured to compress a CO2 gas stream to a pressure equal to or above the critical pressure, the compression section having an inlet for the CO2 gas stream and an outlet for a liquid or supercritical fluid CO2 gas stream; - A separation section configured to separate a supercritical fluid or liquid CO2 flow into a CO2 process fluid flow and a CO2 refrigerant fluid flow, the separation section having an inlet in fluid communication with the outlet of the supercritical fluid or liquid CO2 flow of the compression section (1), an outlet for the CO2 process fluid flow and an outlet for the CO2 refrigerant fluid flow; - A refrigeration expansion section configured to expand a supercritical fluid or liquid CO2 flow, the refrigeration expansion section having an inlet in fluid communication with the outlet of the CO2 refrigerant fluid of the separation section, and an outlet for expanding the CO2 fluid containing liquid CO2 and CO2 vapor; - An evaporation section configured to vaporize a CO2 fluid containing liquid CO2 and CO2 vapor, the evaporation section having an inlet in fluid communication with the outlet of the expanded CO2 fluid of the refrigeration expansion section, and an outlet for CO2 vapor in fluid communication with the CO2 gas inlet of the compression section (1); - A precooling section (2) configured to cool the supercritical fluid or liquid CO2 flow, the precooling section having an inlet in fluid communication with the outlet of the CO2 process fluid flow of the separation section, and an outlet for precooling liquid CO2. - A liquefaction expansion section configured for flash evaporation of liquid CO2, the liquefaction expansion section having an inlet in fluid communication with the precooled liquid CO2 outlet of the precooling section, and an outlet for the process fluid flow of the expanded CO2; - A separation section configured to perform gas-liquid separation of expanded CO2 fluid, the separation section having an inlet in fluid communication with the outlet of the expanded CO2 process fluid flow of the liquefaction expansion section, and an outlet for flash vapor and an outlet for residual CO2 liquid. - A stripping section configured to strip liquid CO2 using gaseous CO2 as the stripping medium, the stripping section having an inlet in fluid communication with the outlet of the residual CO2 liquid of the liquefaction expansion section, an inlet for the gaseous CO2 stripping medium, an outlet for the vapor of the stripping section, and an outlet for the stripped liquefied CO2. - A condensation section configured to cool the gaseous CO2, the condensation section having an inlet in fluid communication with the outlet of the flash gas of the liquefaction expansion section and an inlet in fluid communication with the steam outlet of the stripping section, and having an outlet for condensing the CO2 fluid; The condensation section is thermally connected to the evaporation section.

11. The system of claim 10, wherein the inlet of the CO2 gas stream of the compression section is in fluid communication with the outlet of the CO2 gas stream of a facility for capturing carbon dioxide from flue gas, the facility including an absorption section configured to contact the flue gas with a CO2 absorbent liquid, the absorption section including an outlet of a CO2-containing gas stream in fluid communication with the inlet of the CO2 gas stream of the compression section.

12. The system of claim 10 or 11, wherein the compression section comprises one or more units selected from purification units, drying units, and combinations thereof.

13. The system according to any one of claims 10 to 12, comprising a condensation stage separation section configured to perform gas-liquid separation on condensed CO2 fluid, the condensation stage separation section having an inlet, a vapor outlet, and a liquid outlet in fluid communication with an outlet of the condensed CO2 fluid of the condensation stage, wherein the liquid outlet is in fluid communication with a liquid inlet of the stripping stage.

14. The system according to any one of claims 10 to 13, wherein the outlet of the flash vapor of the separation section is in fluid communication with the inlet of the stripping gas of the stripping section, and the outlet of the residual CO2 liquid of the separation section is in fluid communication with the inlet of the liquid CO2 of the stripping section or the inlet of the liquid CO2 of the condensation section.

15. The system according to any one of claims 10 to 13, wherein the separation section is contained in a stripping tower of the stripping section.

16. The system according to any one of claims 10 to 15, comprising a production expansion section having an inlet in fluid communication with an outlet of liquefied CO2 from the stripping section and an outlet of liquefied CO2 fluid for expansion, the production expansion section further comprising a production separation section configured to perform gas-liquid separation on the expanded CO2 fluid, the production separation section having an inlet in fluid communication with an outlet of liquefied CO2 fluid from the expansion section, an outlet for vapor, and an outlet for liquefied CO2 product.

17. The system according to any one of claims 10 to 16, configured to implement the method of claim 6, wherein the cooling expansion section comprises: - An initial expansion unit configured to initially depressurize a liquid CO2 refrigerant fluid, the initial expansion unit having an inlet in fluid communication with the outlet of the CO2 refrigerant fluid flow of the separation section, and an outlet for an intermediate pressure CO2 refrigerant fluid obtained from the initial depressurization. - A gas-liquid separation unit configured to separate an intermediate-pressure CO2 refrigerant fluid into intermediate-pressure liquid CO2 and intermediate-pressure CO2 vapor, the gas-liquid separation unit having an inlet in fluid communication with the outlet of the intermediate-pressure liquid CO2 refrigerant fluid of the initial expansion unit for the outlet of the intermediate-pressure liquid CO2, and an outlet having an outlet in fluid communication with the inlet of the CO2 gas of the compression section for the intermediate-pressure CO2 vapor; - A further expansion unit configured to further expand the intermediate-pressure liquid CO2 refrigerant fluid, the further expansion unit having an inlet in fluid communication with the outlet of the intermediate-pressure liquid CO2 of the gas-liquid separation unit, and an outlet for an expanded CO2 fluid containing liquid CO2 and CO2 vapor, the outlet of the expanded CO2 fluid being in fluid communication with the inlet of the evaporation section.

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