Method and system for carbon dioxide liquefaction

Abstract

Disclosed is a method and system to for liquefying carbon dioxide. Herein a CO2 gas stream comprising, calculated on the basis of non-condensable matter, at least 90 mole % of CO2 is fed to a compression section, and compression so as to provide a liquid or supercritical fluid CO2 stream. The latter stream is divided into a CO2 process fluid, which is subjected to liquefaction, and a CO2 refrigerant fluid, which is subjected to a refrigeration cycle. The liquefaction comprises flashing the CO2 process fluid followed by separation of gaseous and liquid CO2. The latter is subjected to stripping with CO2. The former is liquefied, together with used CO2 stripping gas, by condensation. The cooling thereby required, is provided by vaporization of the CO2 refrigerant fluid. The gaseous CO2 resulting therefrom is returned as a feed to the compression stage.

Description

[0001] Title: METHOD AND SYSTEM FOR CARBON DIOXIDE LIQUEFACTION

[0002] Field of the Invention

[0003] The invention is in the field of carbon dioxide liquefaction. Particularly, the invention pertains to liquefying carbon dioxide as obtained from capturing carbon dioxide.

[0004] Background of the Invention

[0005] Today it is well-established that emissions of carbon dioxide (CO2) from flue gas, such as emissions resulting from the combustion of fossil fuel, as in power plants, fired heaters or cement production plants, present a major challenge to climate development. Accordingly, the capture of CO2 is pivotal to at least reducing effects such as global warming.

[0006] Various technologies for CO2 capture from flue gas exist, the most important of which being chemical absorption by means of a suitable CO2 - capturing liquid. Such liquids most typically are aqueous amine solutions, a prominent example being a 30% mono ethanol amine (MEA) solution. In this way a relatively pure carbon dioxide can be obtained, typically at least 80 mole %, more typically at least 90 mole %. It is desired to further purify and liquefy such captured CO2, so as to render it into a useful liquid CO2 product.

[0007] In order to liquefy gaseous carbon dioxide, it needs to be subjected to refrigeration. Conventionally, liquid ammonia is used as a refrigerant, but this has several drawbacks such as safety hazards and environmental impact upon leakage. Interestingly, also carbon dioxide itself is known as a refrigerant. It is therefore desired to liquefy carbon dioxide by means of a self-refrigerated cycle, i.e., employing only carbon dioxide itself as a working fluid for refrigeration.

[0008] A background technology in this respect is disclosed in CN106979664. Herein a raw carbon dioxide is subjected to various steps of purification by adsorption, and removal of non-condensable impurities. Eventually, a liquid carbon dioxide stream is obtained at the bottom of a cryogenic distillation column, and is divided into three streams. A first stream is subjected to throttling and cooling, and ends up as an evaporating coolant, providing a cold source for the cryogenic distillation column. A second stream is throttled and cooled, mixed with the evaporated carbon dioxide of the first stream, and therewith sent to a cryogenic heat exchanger as a cold source for the liquefaction of carbon dioxide. The third stream is removed as a high purity liquid carbon dioxide product.

[0009] The disclosed process has several drawbacks. It requires subjecting all of the carbon dioxide to all of the steps required to produce a high-purity liquid carbon dioxide, but recycles a large part thereof, i.e., said first and second streams, to the process merely as a cold source, rather than as a product. It would be desired to improve the process efficiency, by having only the actual product liquid carbon dioxide undergo all of the necessary process steps, and retrieve frigories from streams other than the product stream. Further, the use, as a cold source in this process, of the liquid carbon dioxide obtained from the liquefaction itself, which is at medium to low pressure, requires adopting a simple refrigeration loop. As this has a limited efficiency. It would be desired to allow employing alternative refrigeration loops that serve to obtain a higher efficiency.

[0010] Further background references on employing liquefied CO2 itself as refrigerant in a liquefaction process for CO2, include WO 2022 / 184794 and WO 20222 / 184646. The former document relates to improving operating instabilities upon cooling gas that is rich in compressed CO2. The latter reference seeks to optimize the heat exchanger in the described CO2 liquefaction process. Neither disclosure addresses an overall improvement of a self-refrigerated CO2 liquefaction process.

[0011] A further desire in the art relates to the new and continuing development of large-scale carbon capture facilities. This is expected to drive exponential growth in the liquid CO2 market, with expected expansion beyond the current main markets for liquid CO2, which are specialist sectors, such as the food and beverage industry, or pharmaceuticals. Considering the corresponding changes and scaling up of liquid CO2 demand, an issue is that no clear consensus or standard exists regarding the required liquid CO2 conditions of temperature and pressure and, in fact, with a greater diversity of end-users, also a diversity of liquid CO2 products is potentially demanded. It is accordingly desirable to have a liquefaction system capable of producing liquid CO2 at different pressure levels while maintaining high efficiency. This approach enables a single liquefaction facility to supply various purchasers in manner accommodating their specific requirements, and to adapt easily to future market changes or new standards.

[0012] Summary of the Invention

[0013] In order to better address one or more of the above-mentioned desires, the invention provides, in one aspect, a process for liquefying carbon dioxide, comprising:

[0014] - providing a CO2 gas stream comprising, calculated on the basis of non-condensable matter, at least 90 mole % of CO2;

[0015] - a compression stage comprising compressing the CO2 gas to a pressure at or above the critical pressure, thereby obtaining a liquid or supercritical fluid CO2 stream;

[0016] - splitting the supercritical fluid or liquid CO2 stream into a CO2 process fluid stream and a CO2 refrigerant fluid stream; - subjecting the CO2 refrigerant fluid stream to a refrigeration cycle comprising o expanding the CO2 refrigerant fluid stream so as to provide an expanded CO2 refrigerant fluid; o vaporizing the expanded CO2 refrigerant fluid so as to provide CO2 refrigerant vapor; o recycling the CO2 refrigerant vapor as CO2 gas input to the compression stage;

[0017] - subjecting the CO2 process fluid stream to liquefaction comprising: o a pre-cooling stage, comprising cooling the CO2 process fluid stream to a temperature in a range of from -10°C to 30°C, so as to provide a pre-cooled CO2 process fluid stream; o an expansion stage comprising subjecting the pre-cooled CO2 process fluid stream to flash evaporation so as to provide an expanded CO2 process fluid stream; o a separation stage comprising separating the expanded CO2 process fluid stream into CO2 flash vapor and residual CO2 liquid; o a stripping stage comprising subjecting the residual CO2 liquid to stripping with CO2 as a gaseous stripping medium so as to provide liquefied CO2 and stripping stage vapor; o a condensation stage comprising subjecting the stripping stage vapor to cooling, so as to form condensed CO2 fluid; wherein the cooling in the condensation stage comprises heat exchange with the vaporizing expanded CO2 refrigerant fluid as a coolant.

[0018] In another aspect, the invention presents a system for liquefying carbon dioxide, said system comprising:

[0019] - a compression section (1) configured to compress a CO2 gas stream to a pressure at or above the critical pressure, said compression section having an inlet for a CO2 gas stream and an outlet for a liquid or supercritical fluid CO2 stream;

[0020] - a splitting section configured to split a supercritical fluid or liquid CO2 stream into a CO2 process fluid stream and a CO2 refrigerant fluid stream, said splitting section having an inlet in fluid communication with the outlet for a supercritical fluid or hquid CO2 stream of the compression section (1), an outlet for a CO2 process fluid stream and an outlet for a CO2 refrigerant fluid stream;

[0021] - a refrigeration expansion section configured to subject a supercritical fluid or liquid CO2 stream to expansion, said refrigeration expansion section having an inlet in fluid communication with the outlet for a CO2 refrigerant fluid stream of the splitting section, and having an outlet for expanded CO2 fluid comprising liquid CO2 and CO2 vapor;

[0022] - an evaporation section configured to subject a CO2 fluid comprising liquid CO2 and CO2 vapor to vaporization, said evaporation section comprising an inlet in fluid communication with the outlet for expanded CO2 fluid of the refrigeration expansion section, and an outlet for CO2 vapor in fluid communication with an inlet for CO2 gas of the compression section (1);

[0023] - a pre-cooling section (2) configured to subject a supercritical fluid or liquid CO2 stream to cooling, said pre-cooling section having an inlet in fluid communication with the outlet for a CO2 process fluid stream of the splitting section, and an outlet for pre-cooled liquid CO2;

[0024] - a liquefaction expansion section configured to subject liquid CO2 to flash evaporation, said liqu efaction expansion section having an inlet in fluid communication with the outlet for pre-cooled liquid CO2 of the precooling section, and an outlet for an expanded CO2 process fluid stream;

[0025] - a separation section configured to subject an expanded CO2 fluid to gas-liquid separation, said separation section having an inlet in fluid communication with the outlet for an expanded CO2 process fluid stream of the liquefaction expansion section, an outlet for flash vapor, and an outlet for residual CO2 liquid;

[0026] - a stripping section configured to subject liquid CO2 to stripping with CO2 as a gaseous stripping medium, said stripping section having in inlet in fluid communication with the outlet for residual CO2 liquid of the liquefaction expansion section, an inlet for gaseous CO2 stripping medium, an outlet for stripping stage vapor, and an outlet for stripped liquefied CO2;

[0027] - a condensation section configured to subject gaseous CO2 to cooling, said condensation section having an inlet in fluid communication with the outlet for flash vapor of the liquefaction expansion section and an inlet in fluid communication with the outlet for stripping stage vapor of the stripping section, and having an outlet for condensed CO2 fluid; wherein the condensation section is in thermal communication with the evaporation section.

[0028] Brief description of the drawings

[0029] FIG. 1 is a schematic drawing reflecting an embodiment of the process and system in accordance with the invention.

[0030] FIG.2 to FIG.5 are schematic drawings indicating alternative arrangements related to the stripping section.

[0031] Detailed description of the invention

[0032] In a broad sense, the invention is based on the judicious insight that the relatively high purity of carbon dioxide as obtained from a carbon capture process, enables the creation of a highly efficient self-refrigerated process to liquefy carbon dioxide. Herein a split between CO2 to be liquefied in the process, and CO2 to be used as a refrigerant, is made at a judicious early stage in the process. Thereby the CO2 refrigerant is subjected to a refrigeration cycle which judiciously is nested in the liquefaction process by not only employing the cooling effect of the refrigerant to a condensation step to obtain liquid CO2 in the liquefaction process, but also employing the compression section that initiates the liquefaction process, also as the compression section required in the refrigeration cycle.

[0033] The process starts with providing a CO2 gas stream comprising at least 90 mole % of CO2, calculated on the basis of non-condensable matter. Irrespective of the additional presence of water and condensable impurities, the CO2 gas stream thus comprises less than 15% of non-condensable impurities. The term “non-condensable impurities” is known in the field of CO2 liquefaction, and refers to gaseous impurities that do not condense under normal compression refrigeration conditions for CO2. Particularly, this refers to impurities that do not condense with water. Typically, non- condensable impurities comprise one or more of oxygen, nitrogen, and argon. Preferably, the CO2 gas stream comprises less than 10%, more preferably less than 5%, of non-condensable impurities. In the process of the invention, the amount of water and condensable impurities is not particularly critical. Typically this will be below 25 mole %, preferably of from 5 mole % to 10 mole %.

[0034] Preferably, the CO2 gas stream as defined above, comprises at least 80 mole % of CO2. Irrespective of the presence or absence of non- condensable impurities, the CO2 gas stream comprises preferably more than 90 mole % of CO2. More preferably, the CO2 gas stream comprises at least 95 mole % of CO2.

[0035] Whilst this stream can be of any source, it particularly applies to CO2 as obtained from an absorption-type carbon capturing process. This can be a process conducted elsewhere, with CO2 transported to a system according to the invention. This can also refer to an adjacent facility, such as a power plant, to capture CO2 from flue gas. It is also possible to have an integrated system the feed to which is raw CO2, and wherein the system comprises a CO2 purification section, such as an absorption section, upstream of the compression section applied in the present invention. In a first stage, the process comprises compressing the CO2 gas to a pressure at or above the critical pressure. With reference to the well-known phase diagram for CO2, the skilled person will understand that this will generally be a pressure of at least approximately 75 bar(g) and typically 100 bar (g). This is accomplished in a compression stage. Said stage comprises at least one compression step. It is preferred, as is customary in compression stages, to have compression followed by cooling. Preferably, the compression is conducted in a series of compression steps, each followed by a cooling step. In the cooling steps water will be allowed to condense, as will condensable impurities. Thus, water and other condensable impurities are subject to removal as a result of intercooling between compressors, followed by gasliquid separation. In a final step, aftercooling will preferably take place. It will be understood that in the event of a single compressor, the aftercooling takes place downstream of said single compressor. In the event of a plurality of compressors in series, the aftercooling takes place downstream of the final compressor, at the downstream end of said series of compressors.

[0036] The compression is conducted such that the CO2 gas becomes a supercritical CO2 fluid. Depending on the pressure reached, and the extent of aftercooling applied, the CO2 can either remain in the supercritical phase, but it is also possible that the cooling reduces the temperature of the CO2 to below the critical temperature, i.e., at conditions of pressure and temperature at which the CO2 will be in the liquid phase. Accordingly, the output of the compression stage is thus a liquid or supercritical fluid CO2 stream.

[0037] With reference to the system to which the invention also relates, the above-described compression stage is conducted in a compression section (1) configured to compress a CO2 gas stream to a pressure at or above the critical pressure. The compression section has an inlet for a CO2 gas stream and an outlet for a liquid or supercritical fluid CO2 stream.

[0038] Advantageously, the inlet for a CO2 gas stream is preferably in fluid communication with an outlet for CO2 gas of a facility for capturing carbon dioxide from flue gas. Such facilities generally comprise, inter alia, an absorption section, in which flue gas is contacted with an CC -absorbing liquid, and a regeneration section, in which said liquid is regenerated. The technology to remove CO2 from flue gas by means of chemical absorption has been well-established. A variety of such CC -absorbing liquids exists, all of which serving to present a suitable base that is capable of directly or indirectly binding carbon dioxide. Generally, this involves organic bases, typically amines. Alternatively, the CCh-absorbing liquid can be an inorganic solvent, such as potassium carbonate, particularly the “Hot Potassium Carbonate Process” (HPC) with which the skilled person is familiar. Suitable amines include diethanolamine (DEA), monoethanolamine (MEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and aminoethoxyethanol (Diglycolamine) (DGA).

[0039] Depending on the purity of the CO2 as fed to the compression stage, and the removal of condensable impurities upon cooling in between compression steps, the liquid or supercritical fluid CO2 stream obtained from the compression stage is suitable to provide a sufficiently pure CO2 product after stripping off the non-condensable impurities that may be present. Preferably, in order to still better secure the desired purity, further purification steps are included before the CO2 refrigerant fluid stream in the process of the invention is split off. This can be done by one or more drying steps and / or other purification steps with which the skilled person is familiar. In a preferred embodiment, in the event of a series of compression and intercooling steps, an additional purification is conducted after the penultimate intercooling and gas-liquid separation step, before the final compression and aftercooling steps. Preferably, in the compression stage, at least nearly all condensable impurities are removed, preferably to the extent that less than 50 ppm, preferably less than 30 ppm, of water remains.

[0040] Thus, with reference to the system to which the invention also relates, at least one purification unit is preferably included upstream of the final stage of the compression section. This unit particularly serves to further remove condensable impurities such as water, for example via adsorption on specific adsorbents (e.g. a molecular sieve). Advantageously, such unit can also be configured to remove other impurities that are possibly present, such as, e.g., SO2, which can be removed via adsorption with activated carbon, or ammonia. As desired, other water soluble compounds can be removed by washing the CO2 stream with water. The purification unit is conveniently located upstream of the refrigeration section of the plant, in order to better avoid the formation of ice in the system. In lieu of, but preferably in addition to, a purification unit after the penultimate stage of compression, the purification steps, and hence the corresponding purification units, can be divided over a number of purification units, each located at a different interstage of the compression section.

[0041] In a further generally preferred embodiment, during the compression stage the CO2 gas stream can be subjected to drying. This can be accomplished prior to the final compression stage, i.e. before the CO2 is either liquid, or supercritical but close to condensation to the liquid phase. The formation of a liquid CO2 phase would hinder any subsequent drying operations. Therefore, drying is preferably carried out at an intermediate pressure level, typically around 50-60 bar(g), which is believed to present a desirably balance between equipment size, pressure losses, and design pressure requirements, while avoiding any liquid CO2 at such drying steps.

[0042] Accordingly, in the system to which the invention also pertains, the compression section preferably comprises one or more units selected from the group consisting of purification units, drying units, and combinations thereof.

[0043] The inventors have realized that providing a CO2 gas stream of sufficiently high purity can be put to advantage in creating a selfrefrigeration loop. Said purity is reflected in the CO2 gas stream comprising least 90 mole % of CO2 calculated on the basis of non-condensable matter, thus comprising less than 15% of non-condensable impurities. Preferably the CO2 gas stream comprises more than 90 mole % of CO2 more preferably at least 95 mole % of CO2.

[0044] To this end, the process comprises splitting the supercritical fluid or liquid CO2 stream as obtained from the compression stage into a CO2 process fluid stream and a CO2 refrigerant fluid stream. From the CO2 process fluid stream a relatively large part can be directly processed, with removal of non-condensable impurities by stripping, into a liquid CO2 product. This involves expansion (typically controlled by an expansion valve or throttling valve) before stripping. After expansion, only a small fraction, referred to herein as flash vapor, is in the vapor phase. Accordingly, after said expansion most of the process CO2, referred to herein as the residual CO2 liquid, is allowed to remain in the liquid phase.

[0045] With reference to the system to which the invention also relates, the above-described splitting is conducted in a splitting section configured to split a supercritical fluid or liquid CO2 stream into a CO2 process fluid stream and a CO2 refrigerant fluid stream. The splitting section has an inlet in fluid communication with the outlet for a supercritical fluid or liquid CO2 stream of the compression section (1), an outlet for a CO2 process fluid stream and an outlet for a CO2 refrigerant fluid stream. Generally, the splitting section comprises an inlet for the supercritical fluid or liquid CO2 stream, a first outlet for the CO2 process fluid stream, a parallel, second outlet for the CO2 refrigerant fluid stream, and preferably controls to adjust the splitting ratio as desired.

[0046] The CO2 refrigerant fluid stream is employed as a refrigerant in a refrigeration cycle. As familiar to the skilled person, in such cycle, in general, a liquid refrigerant is subjected to expansion so as to provide a mixture of liquid and vapor, which subsequently is subjected to evaporation, after which the resulting low-pressure refrigerant vapor is compressed into a high-pressure refrigerant vapor, which is then subjected to condensation to again provide liquid refrigerant. In the event of CO2 as a refrigerant, the corresponding phase changes are not strictly between liquid and vapor, but are transcritical. Thus, in a CO2 refrigeration cycle, a supercritical fluid is obtained after compression of the low-pressure refrigerant vapor. The supercritical fluid is subsequently cooled and expanded below the critical temperature so as to obtain the liquid refrigerant. Accordingly, the process of the invention comprises subjecting the CO2 refrigerant fluid stream to expansion, generally to a pressure in a range of from 5 to 50 bar(g), more typically 7 to 30 bar(g), so as to provide an expanded CO2 refrigerant fluid, vaporizing the expanded CO2 refrigerant fluid so as to provide CO2 refrigerant vapor, and recycling the CO2 refrigerant vapor as CO2 gas input to the compression stage.

[0047] With reference to the system to which the invention also relates, the CO2 refrigerant fluid stream is treated in a refrigeration expansion section, which is configured to subject a supercritical fluid or liquid CO2 stream to expansion. The refrigeration expansion section has an inlet in fluid communication with the outlet for a CO2 refrigerant fluid stream of the splitting section. The expansion as such is generally conducted in an expansion valve. With reference to the result of the expansion, the expansion section has an outlet for expanded CO2 fluid comprising liquid CO2 and CO2 vapor.

[0048] In order to enable the CO2 refrigerant fluid stream to be subjected to a refrigeration cycle, the system of the invention further comprises an evaporation section configured to subject a CO2 fluid comprising liquid CO2 and CO2 vapor to vaporization. This evaporation section comprises an inlet in fluid communication with the outlet for expanded CO2 fluid of the refrigeration expansion section, and an outlet for vaporized CO2 refrigerant. The latter outlet is in fluid communication with an inlet for CO2 gas of the compression section (1). Here too, advantageously, the vapor connection from the evaporation section to the compression section, comprises a heat exchanging section that is in thermal communication with the pre-cooling section in which the CO2 process fluid stream is treated. This allows recovery of frigories from the vaporized CO2 refrigerant, resulting in heated vaporized CO2 refrigerant to be returned to the compression stage.

[0049] The CO2 process fluid stream is treated in such a way as to result in a liquid CO2 product. To this end, the process comprises pre-cooling the CO2 process fluid stream. This is generally done to a temperature in a range of from -10°C to 30°C, preferably below 20°C, more preferably of from 0° to 20°C, resulting in a pre-cooled CO2 process fluid stream. In the expansion stage, the latter stream is subjected to flash evaporation, also known as adiabatic evaporation, or just as flashing. Generally, the pre-cooled CO2 process fluid stream thereby is depressurized to a pressure in a range of from 7 to 35 bar(g).

[0050] This results in the formation of a flash vapor, which is generally comprised in the range from 5 mole % to 25 mole %, such as 10-20 mole %. Said flash vapor comprises gaseous CO2 as well as possible gaseous noncondensable impurities that were present in the CO2 process fluid stream. In order to remove further non-condensable impurities from the residual CO2 liquid, the latter is subjected to stripping with CO2 as a gaseous stripping medium, typically in a stripping column. As a result, the remaining liquid can be obtained as a liquefied carbon dioxide product. After stripping, stripping stage vapor, comprising used CO2 stripping gas as well as the CO2 flash vapor and stripped non-condensable impurities, is subjected to condensation, thereby forming a condensed CO2 fluid, which generally is a two-phase stream mostly comprising liquid CO2 liquid, and a relatively small amount of vapor, which is concentrated with respect to non- condensable components. The vapor fraction preferably is low as possible, in order to minimize CO2 losses. Typically the vapor fraction is less than 5 mole %, preferably less than 1.5 mole %.

[0051] Generally the vapor, i.e. condensation stage vapor phase CO2 is separated off, and the resulting condensation stage liquid phase CO2 is refluxed to the stripping process. With reference to the liquefaction treatment described in the preceding paragraph, the system to which the invention also relates comprises the corresponding sections in which the treatment is conducted. This includes a pre-cooling section configured to subject a supercritical fluid or liquid CO2 stream to cooling. The pre-cooling section has an inlet in fluid communication with the outlet for a CO2 process fluid stream of the splitting section, and an outlet for pre-cooled liquid CO2. The latter outlet is in fluid communication with an inlet of a liquefaction expansion section, said section being configured to subject liquid CO2 to flash evaporation. In addition to an inlet in fluid communication with the outlet for pre-cooled liquid CO2 of the pre-cooling section, the liquefaction expansion section has an outlet for flash vapor and an outlet for residual CO2 liquid. The expansion section generally comprises an expansion valve as discussed above. The outlets for flash vapor and for residual CO2 liquid will generally be provided in connection with an expansion vessel that is in fluid communication with the expansion valve. In one embodiment, this expansion vessel (also referred to as a flash drum) is a separate vessel downstream of the expansion valve, and upstream of the stripping section in which the residual CO2 liquid that is subjected to stripping. In another embodiment, the expansion vessel is comprised in the stripping section, typically being the upper section of a stripping column. In such embodiment, the outlet for flash vapor of the liquefaction expansion section can also be an outlet for stripping gas from the stripping section. Similarly, the outlet for residual CO2 liquid of the liquefaction expansion section will in that event be comprised in the stripping section, typically allowing liquid to flow down from the upper part of a stripping column to a lower part thereof. The stripping section as such is configured to subject liquid CO2 to stripping with CO2 as a gaseous stripping medium. To this end the stripping section has in inlet in fluid communication with the outlet for residual CO2 liquid of the liquefaction expansion section, and an inlet for gaseous CO2 stripping medium. Further, the stripping section has an outlet for stripping stage vapor, and an outlet for stripped liquefied CO2. Advantageously, the inlet for gaseous CO2 stripping medium is in fluid communication with a source of CO2 vapor from the process itself, i.e., within the system of the invention. The gaseous CO2 stripping medium can be provided fully in the vapor phase, or as comprised in a biphasic CO2 fluid.

[0052] Partly, the stripping is self-stripping by virtue of CO2 vapor present in the residual CO2 liquid. Preferably, a desirable amount of liquid CO2 is withdrawn from the stripped liquefied CO2, vaporized, and reused as a stripping medium, the major part of the stripped liquefied CO2 remaining as a liquefied CO2 production stream. The skilled person will be able to determine the amount of CO2 stripping gas needed, depending on the amount non-condensable impurities to be removed with stripping. By way of guidance, as an example, such amount is typically around 25% of the flow rate at the inlet of the stripping section, in order to achieve concentrations of non-condensable components below lOppm in the liquefied CO2. Most advantageously, the pre-cooling thereby works as a reboiling section for the stripping section. I.e., the step of vaporizing the part of the stripped liquefied CO2 to be reused as a stripping medium, comprises heat exchange of said liquefied CO2 with the CO2 process fluid stream in the pre-cooling section.

[0053] The system of the invention comprises a condensation section configured to subject gaseous CO2 to cooling. The condensation section has an inlet in fluid communication with the outlet for flash vapor of the liquefaction expansion section and an inlet in fluid communication with the outlet for stripping stage vapor of the stripping section. These inlets can refer to one and the same inlet serving both purposes, or to different inlets. The condensation section has an outlet for condensed CO2 fluid, in fluid communication with a gas-liquid separation section, such as a knock-out drum. Advantageously, the condensation section is in thermal communication with the evaporation section. Accordingly, the system allows the stream of CO2 refrigerant fluid to act as a coolant for the part of the CO2 process fluid that still has to be liquefied. Depending on the degree of purity of the initial CO2 fed to the process, the gaseous CO2 subjected to condensation will contain amounts of non-condensed impurities. Accordingly, the condensed CO2 fluid is preferably subjected to gas-liquid separation, preferably employing a knockout drum. The liquid CO2 can be treated and obtained separately, but preferably it is returned, typically by reflux to the stripping column, to the stripping stage.

[0054] The condensation of the gaseous CO2 obtained from the expansion and stripping stages, requires cooling. In accordance with the invention, this is provided for by the aforementioned refrigeration cycle to which the CO2 refrigerant fluid is subjected. Accordingly, the cooling in said condensation stage comprises heat exchange with the vaporizing expanded CO2 refrigerant fluid as a coolant.

[0055] The process of the invention is applicable to different types of CO2 refrigeration cycles, including a single-loop vapor compression cycle. Preferably, a dual-pressure CO2 refrigeration cycle is employed. This increases the efficiency of the liquefaction process, as it displays a higher performance coefficient (COP) than a single refrigeration loop.

[0056] In the dual-pressure cycle, schematically illustrated in FIG. 1, instead of flashing refrigerant CO2 directly to the pressure level required at the condenser, refrigerant CO2 is initially depressurized to an intermediate pressure level (typically 40-70 bar(g)), typically via a throttling valve (initial throttling valve V3 in FIG.l). The CO2 vapors that are consequently formed are separated in a flash drum, and recycled so as to contribute as a coolant to the pre-cooling stage. Liquid CO2 refrigerant fluid of intermediate pressure as obtained from the initial depressurization, is further flashed, typically via a further throttling valve (final throttling valve V4 in FIG.l). The resulting expanded CO2 refrigerant fluid is then used as a cooling medium in the condensation stage, where it is vaporized, and recycled to the compression stage, again also via heat exchange in the pre-cooling stage. This dual pressure embodiment further improves the heat integration within the process and the overall process efficiency.

[0057] Preferably, the system of the invention is configured for carrying out the aforementioned dual-pressure cycle. In that event, the refrigeration expansion section comprises an initial expansion unit, such as an expansion valve, configured to subject liquid CO2 refrigerant fluid to initial depressurization, said initial expansion unit having an inlet in fluid communication with the outlet for a CO2 refrigerant fluid stream of the splitting section and having an outlet for CO2 refrigerant fluid of intermediate pressure as obtained from initial depressurization; a gas-liquid separation unit, such as a flash drum, configured to separate CO2 refrigerant fluid of intermediate pressure into liquid CO2 of intermediate pressure and CO2 vapor of intermediate pressure, said gas-liquid separation unit having an inlet in fluid communication with the outlet for CO2 refrigerant fluid of intermediate pressure of the initial expansion unit, having an outlet for liquid CO2 of intermediate pressure and having an outlet for CO2 vapor of intermediate pressure in fluid communication with an inlet for CO2 gas of the compression section; a further expansion unit, such as a further expansion valve, configured to subject liquid CO2 refrigerant fluid of intermediate pressure to further expansion, said further expansion unit having an inlet in fluid communication with the outlet for liquid CO2 of intermediate pressure of the gas-liquid separation unit and an outlet for expanded CO2 fluid comprising liquid CO2 and CO2 vapor, the latter outlet being in fluid communication with an inlet of the evaporation section. Advantageously, the vapor connection from the further expansion unit to the compression section, comprises a heat exchanging section that is in thermal communication with the pre-cooling section in which the CO2 process fluid stream is treated. This allows recovery of frigories from the expanded CO2 refrigerant vapor of intermediate pressure, resulting in heated expanded CO2 refrigerant vapor to be returned to the compression stage. The process of the invention in general employs a beneficial level of heat integration. The CO2 refrigerant vapor stream employed as a coolant in the condensation of gaseous CO2 comprising flash vapor and stripping gas, still has a relatively low temperature, typically in a range of above -50°C and below 0°C, such as -45°C to -10°C. The same holds, in the applicable embodiment, for the CO2 vapor that result from intermediate depressurization in the aforementioned dual-pressure cycle. Both of these streams, in fact being used coolant streams, can be further employed as a coolant in pre-cooling the CO2 process stream that is to be liquefied. Thus any available frigories in either or both of said used coolant streams can be exploited. This allows recycling refrigerant CO2 back to the compression step at higher temperature, such as a temperature in a range of from 20°C to 50°C, preferably at least 30°C, such as 30°C to 40°C, without wasting any frigorie. Similarly, in the event that the liquefied CO2 obtained from stripping is subjected to further expansion, the frigories available in the vapor CO2 stream resulting therefrom are recovered in the pre-cooling stage as well.

[0058] Without wishing to be bound by theory, the inventors believe that the foregoing is made possible by virtue of a synergy resulting from the manner in which the process of the invention employs splitting the supercritical fluid or liquid CO2 stream obtained from compression, into a CO2 process fluid stream and a CO2 refrigerant fluid stream. Since a large part of the process CO2 is in the liquid phase after the liquefaction expansion, relatively low flow rates of CO2 refrigerant are required to complete the liquefaction (typically, less than 50% of the process CO2 mass flow rate). In addition, the process CO2 is at a high-pressure level and thus in a dense phase (either liquid or supercritical). Accordingly, the process CO2 has a high heat capacity and maintains a temperature typically high enough to generate enough CO2 vapor for stripping upon heat exchange with the pre-cooling section, even if the used refrigerant CO2 vapor streams are heated back to the aforementioned higher temperature before being recycled.

[0059] In addition to the foregoing, it is noted again that the process CO2 is compressed to a high pressure level (typically 100 bar(g)), exceeding its critical point. In this way, it can first be liquified at a relatively high temperature level by cooling it with cooling water, cold process streams and any other low-grade cooling medium that may be present, such as the used coolant streams discussed above.

[0060] The stripping stage can be handled in various ways. In one embodiment, schematically illustrated in FIG. 2, the expanded CO2 process fluid stream as obtained from the expansion stage, is subjected to stripping, after which the resulting liquid and gaseous fractions are treated as described above.

[0061] In another embodiment, the expanded CO2 process fluid stream is initially subjected to gas-liquid separation. The liquid resulting therefrom is subjected to stripping, typically by feeding it to an upper section of a stripping column. The gas resulting from said gas-liquid separation is employed as a stripping gas in the stripping of the liquid CO2, typically by feeding it to a lower section of a stripping column. Accordingly, in this embodiment, schematically illustrated in FIG. 3, the stripping stage comprises subjecting the expanded CO2 process fluid stream to gas-liquid separation so as to obtain a stripping stage liquid and a stripping stage gas, subjecting the stripping stage liquid to stripping with CO2 as a gaseous stripping medium, thereby employing the stripping stage gas as a stripping medium.

[0062] In the event that the expanded CO2 process fluid stream is subjected to gas-liquid separation prior to being subjected to stripping, this can also be done in different ways. In one such embodiment, schematically illustrated in FIG. 4, the gas liquid separation is carried out together with the gas-liquid separation of the condensed CO2 fluid resulting from condensation of gaseous CO2 obtained from the stripping step. To this end, the expanded CO2 process fluid stream is fed to the condensation section (preferably to one and the same condenser) in which said gaseous CO2 is subjected to condensation.

[0063] In a still further embodiment, the expanded CO2 process fluid stream is initially subjected to gas-liquid separation, with the gas resulting therefrom being employed as a stripping gas in the stripping of liquid CO2, as mentioned above. In this embodiment, schematically illustrated in FIG. 4, the liquid resulting from said gas-liquid separation is fed to the condensation section to end up, as discussed above, in gas-liquid separation together with the condensed CO2 fluid, and is thereafter subjected to stripping.

[0064] Combinations of the aforementioned embodiments are also possible. E.g., part of the expanded CO2 process fluid stream can be subjected to stripping directly, and part can be subjected to gas-liquid separation first.

[0065] In a first embodiment of the invention, the stripped liquefied carbon dioxide obtained from the stripping stage is the actual product of the liquefaction process.

[0066] In a second embodiment of the invention, which is represented in FIG.l, the liquefied CO2 production stream is subjected to a post-treatment in which it is expanded, preferably flashed, to a final pressure level, and subjected to gas-liquid separation. An advantage hereof is that the pressure applied during stripping, i.e., particularly the operating pressure of a stripping column, is decoupled from the final pressure of the liquid CO2. This allows a further degree of freedom to optimize the stripping stage, if desired. Also, operating the column at a higher pressure will effectively result in a higher temperature in the condensation section. As a result, the refrigerant CO2 can be at a higher temperature. This, in turn, allows to reduce the extent of the depressurization of the refrigerant, therefore advantageously reducing consumption at the compressor. The step of expanding the liquefied CO2 will further allow to achieve a higher degree of flexibility in terms of pressure level of the final CO2 product. Thus a desirable product variation can be obtained, as well as production at different specifications for different end-uses, such as different uses of the CO2 in adjacent plants. Different pressure levels (and therefore different temperatures) can be achieved by changing the pressure drop of the expansion, typically by operating a throttling valve. To the extent that this results in minor variations in the flow rates in the system, such can be managed without difficulty, e.g., via a compressor turndown in the compression step and / or appropriate design margins in other equipment employed in the process.

[0067] As illustrated in, but not limited by, FIGI, the process and system of the invention thus preferably can be provided in a configuration that allows to produce CO2 at different pressure (and therefore temperature) levels. This is possible because the final pressure of the liquid product is decoupled from the operating pressure of the stripping column (3), by means of valve V2. Product CO2 (21) is depressurized and flashed through V2 to form biphasic stream (22). Said stream is separated into its vapor (24) and liquid (23) fractions, by means of the expansion vessel (6). The pressure level of (6) can be changed independently from the pressure of the stripping column (3), via a dedicated pressure control (not shown in the drawing, but it is customary). Vapor (24) is recycled to the compressor (1) through the heat exchange system (2), to recover its frigories. Changing the pressure of the expansion vessel (6) causes a change in the flow rate of the vapor (24) separated. Due to the configuration of the system and the high-degree of thermal integration, however, this change in flow rate of (24) does not impact significantly the system efficiency and the operation of the compressor (1), and can be performed within the operating margins of the unit.

[0068] The post-treatment flashing of the liquefied CO2 obtained from stripping will result in a post treatment CO2 vapor stream, which can be exploited to further recover frigories in the process, preferably in the precooling stage. As a result of heat exchange in the pre-cooling stage, said post-treatment CO2 vapor stream becomes heated, and is recycled to the compression stage as a heated post-treatment CO2 vapor stream.

[0069] In sum, the invention concerns a method and system to for liquefying carbon dioxide. Herein a CO2 gas stream comprising at least 90 mole % of CO2, calculated on the basis of non-condensable matter, less than 15% of non-condensable impurities, such as comprising, irrespective of the presence or absence of non-condensable impurities, at least 90 mole % of CO2, is fed to a compression section, and compression so as to provide a liquid or supercritical fluid CO2 stream. The latter stream is divided into a CO2 process fluid, which is subjected to liquefaction, and a CO2 refrigerant fluid, which is subjected to a refrigeration cycle. The liquefaction comprises flashing the CO2 process fluid followed by separation of gaseous and liquid CO2. The latter is subjected to stripping with CO2. The former is liquefied, together with used CO2 stripping gas, by condensation. The cooling thereby required, is provided by vaporization of the CO2 refrigerant fluid. The gaseous CO2 resulting therefrom is returned as a feed to the compression stage.

[0070] The invention is further illustrated with reference to the drawings. It will be understood that the drawings do not limit the invention. E.g., the invention is not limited to the specific types of equipment and specific plant systems as shown. The figures schematically show equipment parts and process streams relating to embodiments of the invention.

[0071] FIG.l presents a process scheme reflecting a process and system in accordance with a preferred embodiment of the invention in which a dual pressure refrigeration cycle is employed, condensed CO2 is subjected to gasliquid separation, and liquefied CO2 obtained from the stripping section is subjected to further expansion.

[0072] FIG.2 to FIG.5 are schematic drawings indicating alternative arrangements related to the stripping section, as discussed hereinbefore. In the drawings, the reference signs, indicating equipment and streams, have the following meaning:

[0073] 1. Compression section;

[0074] 2. Pre-cooling section;

[0075] 3. Stripping section (depicted as a stripping column);

[0076] 4. Condensation section;

[0077] 5. Condenser knockout drum;

[0078] 6. Expansion vessel;

[0079] 7. Intermediate flash drum;

[0080] 8. Purification unit;

[0081] 9. Raw CO2;

[0082] 10. Compressed CO2 stream;

[0083] 11. Process CO2;

[0084] 12. Pre-cooled process CO2;

[0085] 13. Expanded CO2 process fluid;

[0086] 13V = CO2 flash vapor;

[0087] 13L = residual CO2 liquid;

[0088] 14. Stripping stage vapor;

[0089] 15. Condensed CO2 fluid;

[0090] 16. Condensation stage vapor phase CO2;

[0091] 17. Condensation stage liquid phase CO2;

[0092] 18. Stripped liquefied CO2 stream from stripper outlet;

[0093] 19. Liquefied CO2 withdrawn from (18) for reuse as a stripping medium;

[0094] 20. Stripping Medium;

[0095] 21. Liquefied CO2 production stream;

[0096] 22. Flashed liquefied CO2;

[0097] 23. Liquid Product CO2;

[0098] 24. Post-treatment CO2 vapor stream;

[0099] 25. Heated post-treatment CO2 vapor stream refrigerant CO2;

[0100] 26. CO2 refrigerant fluid;

[0101] 27. Expanded CO2 refrigerant fluid; 28. Heated expanded CO2 refrigerant vapor;

[0102] 29. Liquid CO2 refrigerant;

[0103] 30. Intermediate pressure CO2 refrigerant fluid;

[0104] 31. Expanded CO2 refrigerant fluid; 32. Vaporized CO2 refrigerant;

[0105] 33. Heated vaporized CO2 refrigerant;

[0106] VI Expansion stage throttling valve;

[0107] V2 Post-treatment throttling valve;

[0108] V3 Dual-pressure refrigeration cycle initial throttling valve; V4 Dual-pressure refrigeration cycle final throttling valve.

Claims

Claims1. A process for liquefying carbon dioxide, comprising:- providing a CO2 gas stream comprising, calculated on the basis of non-condensable matter, at least 90 mole % of CO2;- a compression stage comprising compressing the CO2 gas to a pressure at or above the critical pressure, thereby obtaining a liquid or supercritical fluid CO2 stream;- splitting the supercritical fluid or liquid CO2 stream into a CO2 process fluid stream and a CO2 refrigerant fluid stream;- subjecting the CO2 refrigerant fluid stream to a refrigeration cycle comprising o expanding the CO2 refrigerant fluid stream so as to provide an expanded CO2 refrigerant fluid; o vaporizing the expanded CO2 refrigerant fluid so as to provide CO2 refrigerant vapor; o recycling the CO2 refrigerant vapor as CO2 gas input to the compression stage; subjecting the CO2 process fluid stream to liquefaction comprising: o a pre-cooling stage, comprising cooling the CO2 process fluid stream to a temperature in a range of from -10°C to 30°C, so as to provide a pre-cooled CO2 process fluid stream; o an expansion stage comprising subjecting the pre-cooled CO2 process fluid stream to flash evaporation so as to provide an expanded CO2 process fluid stream; o a separation stage comprising separating the expanded CO2 process fluid stream into CO2 flash vapor and residual CO2 liquid; o a stripping stage comprising subjecting the residual CO2 liquid to stripping with CO2 as a gaseous stripping medium, saidmedium optionally comprising the CO2 flash vapor, so as to provide stripped liquefied CO2 and stripping stage vapor; o a condensation stage comprising subjecting the stripping stage vapor, optionally together with the CO2 flash vapor to cooling, so as to form condensed CO2 fluid; wherein the cooling in the condensation stage comprises heat exchange with the vaporizing expanded CO2 refrigerant fluid as a coolant.

2. A process according to claim 1, wherein the CO2 gas stream provided to the process, is obtained from an absorption-type carbon capturing process.

3. A process 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. A process according to any one of the preceding claims, wherein the pre-cooling stage comprises cooling the CO2 process fluid stream to a temperature in a range of from 0°C to 20°C.

5. A process according to any one of the preceding claims, wherein the condensation stage comprises subjecting the condensed CO2 fluid to separation of condensation stage gas phase CO2 and condensation stage liquid phase CO2, and returning the liquid phase CO2 to the stripping stage.

6. A process according to any one of the preceding claims, wherein the expanded CO2 refrigerant fluid is obtained by a two-step process comprising depressurizing the refrigerant CO2 stream to an intermediate pressure level of 40-70 bar(g) under formation of intermediate pressure flash vapor and an intermediate pressure CO2 refrigerant liquid, separating the intermediate pressure flash vapor from the intermediate pressure CO2 refrigerant liquid, and further expanding the intermediate pressure CO2 refrigerant liquid so as to provide the expanded CO2 refrigerant fluid, wherein the process further comprises recycling the intermediate pressure flash vapor to the pre-cooling stage, as a coolant for the CO2 process fluid stream.

7. A process according to any one of the preceding claims, comprising employing the expanded CO2 process fluid stream as a liquid phase in the condensation stage so as to become comprised 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 together with subjecting the condensed CO2 fluid to separation as defined in claim 5, and sending the residual CO2 liquid to the stripping stage together with the condensation stage liquid phase CO2.

8. A process according to any one of the claims 1 to 6, comprising employing the residual CO2 liquid as a liquid phase in the condensation stage, and sending it to the stripping stage after further gas-liquid separation together with subjecting the condensed CO2 fluid to separation as defined in claim 5.

9. A process according to any one of the preceding claims, comprising subjecting the stripped liquefied carbon dioxide to expansion and gas-liquid separation, thereby allowing to provide different batches of liquefied carbon dioxide at different pressures.

10. A system for liquefying carbon dioxide, said system comprising- a compression section (1) configured to compress a CO2 gas stream to a pressure at or above the critical pressure, said compression section having an inlet for a CO2 gas stream and an outlet for a liquid or supercritical fluid CO2 stream;- a splitting section configured to split a supercritical fluid or liquid CO2 stream into a CO2 process fluid stream and a CO2 refrigerant fluid stream, said splitting section having an inlet in fluid communication with the outlet for a supercritical fluid or liquid CO2 stream of the compression section (1), an outlet for a CO2 process fluid stream and an outlet for a CO2 refrigerant fluid stream;- a refrigeration expansion section configured to subject a supercritical fluid or liquid CO2 stream to expansion, said refrigeration expansion section having an inlet in fluid communication with the outlet fora CO2 refrigerant fluid stream of the splitting section, and having an outlet for expanded CO2 fluid comprising liquid CO2 and CO2 vapor;- an evaporation section configured to subject a CO2 fluid comprising liquid CO2 and CO2 vapor to vaporization, said evaporation section comprising an inlet in fluid communication with the outlet for expanded CO2 fluid of the refrigeration expansion section, and an outlet for CO2 vapor in fluid communication with an inlet for CO2 gas of the compression section (1);- a pre-cooling section (2) configured to subject a supercritical fluid or liquid CO2 stream to cooling, said pre-cooling section having an inlet in fluid communication with the outlet for a CO2 process fluid stream of the splitting section, and an outlet for pre-cooled liquid CO2;- a liquefaction expansion section configured to subject liquid CO2 to flash evaporation, said liqu efaction expansion section having an inlet in fluid communication with the outlet for pre-cooled liquid CO2 of the precooling section, and an outlet for an expanded CO2 process fluid stream;- a separation section configured to subject an expanded CO2 fluid to gas-liquid separation, said separation section having an inlet in fluid communication with the outlet for an expanded CO2 process fluid stream of the liquefaction expansion section, an outlet for flash vapor, and an outlet for residual CO2 liquid;- a stripping section configured to subject liquid CO2 to stripping with CO2 as a gaseous stripping medium, said stripping section having in inlet in fluid communication with the outlet for residual CO2 liquid of the liquefaction expansion section, an inlet for gaseous CO2 stripping medium, an outlet for stripping stage vapor, and an outlet for stripped liquefied CO2;- a condensation section configured to subject gaseous CO2 to cooling, said condensation section having an inlet in fluid communication with the outlet for flash vapor of the liquefaction expansion section and an inlet in fluid communication with the outlet for stripping stage vapor of the stripping section, and having an outlet for condensed CO2 fluid;wherein the condensation section is in thermal communication with the evaporation section.

11. A system according to claim 10, wherein the inlet for a CO2 gas stream of the compression section is in fluid communication with an outlet for a CO2 gas stream of a facihty for capturing carbon dioxide from flue gas, said facility comprising an absorption section configured to contact the flue gas with a CC -absorbing liquid, said absorption section comprising an outlet for a CC>2-containing gas stream in fluid communication with said inlet for a CO2 gas stream of the compression section.

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

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

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

15. A system according to any one of the claims 10 to 13, wherein the separation section is comprised in a stripping column of the stripping section.

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

17. A system according to any one of the claims 10 to 16, configured for carrying out the process of claim 6, wherein the refrigeration expansion section comprises:- an initial expansion unit configured to subject liquid CO2 refrigerant fluid to initial depressurization, said initial expansion unit having an inlet in fluid communication with the outlet for a CO2 refrigerant fluid stream of the splitting section and having an outlet for CO2 refrigerant fluid of intermediate pressure as obtained from initial depressurization;- a gas-liquid separation unit configured to separate CO2 refrigerant fluid of intermediate pressure into liquid CO2 of intermediate pressure and CO2 vapor of intermediate pressure, said gas-liquid separation unit having an inlet in fluid communication with the outlet for liquid CO2 refrigerant fluid of intermediate pressure of the initial expansion unit, having an outlet for liquid CO2 of intermediate pressure, and having an outlet for CO2 vapor of intermediate pressure in fluid communication with an inlet for CO2 gas of the compression section;- a further expansion unit, configured to subject liquid CO2 refrigerant fluid of intermediate pressure to further expansion, said further expansion unit having an inlet in fluid communication with the outlet for liquid CO2 of intermediate pressure of the gas-liquid separation unit, and having an outlet for expanded CO2 fluid comprising liquid CO2 and CO2 vapor, the latter outlet being in fluid communication with an inlet of the evaporation section.

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