Process for capturing carbon dioxide from gaseous flows

Abstract

A process for recovering carbon dioxide from a carbon dioxide-rich gaseous feedstream, the process comprising: (a) an absorption step in which carbon dioxide is absorbed from the rich feedstream into an aqueous absorbent to produce a gaseous feedstream containing reduced carbon dioxide, called a “lean feedstream,” and an absorbent containing an increased level of carbon dioxide, called a “rich absorbent”; (b) a regeneration step in which the rich absorbent from the absorption step is subjected to a regeneration pressure lower than the absorption pressure and a regeneration temperature higher than the absorption temperature for a period of time such that carbon dioxide is desorbed from the absorbent and recovered, forming a lean absorbent containing less carbon dioxide than the rich absorbent, and returned to the absorption step; and (c) a transition step in which (i) the rich absorbent passing from the absorption step to the regeneration step is heated and depressurized in preparation for the regeneration step, and (ii) the lean absorbent passing from the regeneration step to the absorption step is cooled and pressurized in preparation for the absorption step, the transition step comprising transferring heat from the lean absorbent to the rich absorbent in one or more heat exchangers, the following improvements: (1) the regeneration step is at least i. a flash drum or in-line separator ii. A high-pressure regeneration step in which carbon dioxide is partially desorbed from the rich absorbent in either a separator, providing (A) a high-pressure carbon dioxide stream and (B) a semi-lean absorbent; ii. A low-pressure regeneration step in which further carbon dioxide is desorbed from a portion of the semi-lean absorbent in a regeneration column, providing (A) a low-pressure carbon dioxide stream and (B) a lean absorbent stream; and (2) the semi-lean absorbent from the high-pressure regeneration step is divided into a portion of the semi-lean absorbent sent to the low-pressure regeneration and a portion of the semi-lean absorbent returned to the absorption step through a transition step, and the absorption step The process is carried out in an absorption tower, and includes the following steps: (i) a rich feed flow is introduced into the absorption tower at the bottom and moves toward the top of the tower; (ii) a lean absorbent is introduced into the absorption tower at the top and moves toward the bottom of the tower in the opposite direction to the rich feed flow; (iii) a semi-lean absorbent is introduced into the absorption tower at a point between the rich feed flow and the lean absorbent flow and moves toward the bottom of the tower in the opposite direction to the rich feed flow; and (iv) the lean feed flow is recovered from the top of the tower and the rich absorbent is recovered from the bottom of the tower and sent to a regeneration process through a transition process.

Description

[Technical Field]

[0001] This invention relates to the field of chemical processes.

[0002] Introduction The reforming process produces synthesis gas from methane or other carbon and hydrocarbon feedstocks. The reformer may be a steam methane reformer, a self-heating reformer, or a partial oxidation reactor. Synthesis gas primarily contains hydrogen, carbon monoxide, and carbon dioxide. Many reforming processes also include a water-gas shift reactor applied after the reforming reactor, which converts most of the carbon monoxide in the synthesis gas into carbon dioxide and more hydrogen. See, for example, Mendes et al., "The Water Gas Shift Reaction: From Conventional Catalytic Systems to Pd-based Membrane Reactors - a Review," 5 Asia-Pac.J.Chem Eng.at 111-137 (2010). The water-gas shift reaction produces a gas stream containing high concentrations of carbon dioxide, such as a combination of hydrogen, carbon monoxide, and up to 60 mole percent carbon dioxide. The reforming process removes carbon dioxide from the gas stream to provide a concentrated hydrogen stream. The carbon dioxide removal process is generally performed after the water-gas shift process, but it can also be carried out in reforming processes that do not have a water-gas shift reactor.

[0003] Conventional carbon dioxide capture systems use an aqueous absorbent solution to capture carbon dioxide from a carbon dioxide-rich gaseous feed stream ("rich feed stream") in three steps. a) In the absorption process, the rich feed stream is brought into contact with the aqueous absorbent in the absorption tower under absorption temperature and pressure such that carbon dioxide is absorbed from the rich feed stream into the aqueous absorbent. The gaseous feed stream with reduced carbon dioxide, called the "lean feed stream," is recovered from the top of the absorption tower. The aqueous absorbent containing increased levels of carbon dioxide, called the "rich absorbent," is recovered from the bottom of the absorption tower. b) In the regeneration process, rich absorbent is supplied to the regeneration tower at a regeneration pressure lower than the absorption pressure and / or a regeneration temperature higher than the absorption temperature to desorb carbon dioxide gas. The desorbed carbon dioxide is recovered from the top of the regeneration tower and sent for further processing. The aqueous absorbent from which carbon dioxide has been desorbed is called a "lean absorbent" and is returned to the absorption process. c) In the transition process, (i) the rich absorbent passing from the absorption process to the regeneration process is heated and depressurized in preparation for the regeneration process, and (ii) the lean absorbent passing from the regeneration process to the absorption process is cooled and pressurized in preparation for the absorption process. Typically, heating and cooling are performed by passing the rich and lean absorbents through a primary heat exchanger that transfers heat from the lean absorbent to the rich absorbent, at least partially.

[0004] Examples of this process are illustrated in U.S. Patent Nos. 1,783,901, 5,853,680, 6,497,852, 8,303,685, 8,398,749, and 8,795,415.

[0005] Conventional processes produce a low-pressure carbon dioxide flow, which is often compressed to high pressures such as 75-150 bar by a series of compressors, allowing for efficient storage or transfer. Compressors are expensive, resulting in high capital costs and high energy consumption.

[0006] In improvements to conventional processes, the regeneration process is carried out in two stages: an initial high-pressure stage and a subsequent low-pressure stage. See, for example, International Application PCT Publication 2021 / 250083(A1) and Australian Patent No. 728167(B2). Low-pressure regeneration generates a low-pressure carbon dioxide flow with a pressure of 3 bar or less, similar to conventional processes. High-pressure regeneration recovers a significant portion of the carbon dioxide in the high-pressure flow of 3 bar or more. The high-pressure flow may omit one or two stages of compression so that the first one or two compression stages use smaller and less expensive compressors and less energy.

[0007] Despite these improvements, the overall process remains capital-intensive and energy-intensive. Further improvements to reduce capital and energy costs are desirable. [Overview of the project]

[0008] One aspect of the present invention is a process for recovering carbon dioxide from a gaseous feed stream containing a partial pressure of at least 1 bar of carbon dioxide, called a "rich feed stream," using an aqueous liquid absorbent called an "absorbent" that absorbs carbon dioxide, and this process is a) An absorption process carried out in an absorption tower, wherein a rich feed stream is brought into contact with an absorbent for a time such that carbon dioxide is absorbed from the rich feed stream to the aqueous absorbent at an absorption temperature of 100°C or less and an absorption pressure of at least 8 bar, thereby generating a gaseous feed stream containing reduced carbon dioxide, called a "lean feed stream," and an absorbent containing an increased level of carbon dioxide, called a "rich absorbent." b) A regeneration process in which the rich absorbent from the absorption process is subjected to a regeneration pressure lower than the absorption pressure and a regeneration temperature higher than the absorption temperature for a period of time such that carbon dioxide is desorbed from the absorbent and recovered, forming a lean absorbent containing less carbon dioxide than the rich absorbent, which is then returned to the absorption process. c) A transition step comprising (i) a rich absorbent passing from an absorption step to a regeneration step being heated and depressurized in preparation for the regeneration step, and (ii) a lean absorbent passing from a regeneration step to an absorption step being cooled and pressurized in preparation for the absorption step, the transition step comprising transferring heat from the lean absorbent to the rich absorbent in one or more heat exchangers, 1) The regeneration process is at least: (i) A high-pressure regeneration step is provided in which carbon dioxide is partially desorbed from the rich absorbent in a separator, which is either a flash drum or an in-line separator, at a temperature of 80 to 140°C and a pressure of 3 to 40 bar (absolute pressure), to provide a semi-lean absorbent containing (A) a high-pressure carbon dioxide stream having a pressure of at least 3 bar and (B) 90 percent or less of the carbon dioxide load of the rich absorbent. (ii) A low-pressure regeneration step in which, within the regeneration tower, further carbon dioxide is desorbed from a portion of the semi-lean absorbent at a temperature higher than the high-pressure regeneration temperature and a pressure lower than the high-pressure regeneration pressure, thereby providing (A) a low-pressure carbon dioxide flow having a pressure lower than the high-pressure carbon dioxide flow and (B) a lean absorbent flow containing 50 percent or less of the carbon dioxide load of the rich absorbent, (2) The semilean absorbent from the high-pressure regeneration stage is divided into a portion of the semilean absorbent that is sent to the low-pressure regeneration stage and a portion of the semilean absorbent that is returned to the absorption stage through the transition stage, (3) The absorption process is characterized by the following: (i) a rich feed flow is introduced into the absorption tower at the bottom and moves toward the top of the tower; (ii) a lean absorbent is introduced into the absorption tower at the top and moves toward the bottom of the tower in the opposite direction to the rich feed flow; (iii) a semi-lean absorbent is introduced into the absorption tower at a point between the rich feed flow and the lean absorbent flow and moves toward the bottom of the tower in the opposite direction to the rich feed flow; (iv) the lean feed flow is recovered from the top of the absorption tower; and (v) the rich absorbent is recovered from the bottom of the absorption tower.

[0009] A second aspect of the present invention is, further, according to the first aspect, • The rich feed stream enters the process at a temperature of at least 140°C. • The heat from the rich feed stream heats the rich absorbent at at least two points in the transition or regeneration process before the rich feed stream is introduced into the absorption tower.

[0010] A third aspect of the present invention is a hybrid aqueous absorbent according to the first or second aspect, wherein the aqueous absorbent further contains water, a water-miscible physical organic absorbent for carbon dioxide, and a water-miscible chemical organic absorbent for carbon dioxide.

[0011] High-pressure regeneration provides a high-pressure carbon dioxide flow, thereby reducing capital and energy costs during compression, as mentioned above. Performing high-pressure regeneration in a flash drum or in-line separator reduces capital costs compared to a high-pressure regeneration tower.

[0012] Flash drums and in-line separators also reduce the residence time of rich absorbents during high-pressure regeneration compared to high-pressure regeneration towers, thereby reducing the thermal exposure of the absorbent. Furthermore, by performing high-pressure regeneration at a pressure of 3-40 bar (absolute pressure) and a temperature of 90-140°C, a considerable amount of high-pressure carbon dioxide is recovered during the high-pressure regeneration stage, while thermal exposure and the load on the absorbent are minimized. High thermal exposure and load are known to degrade absorbents.

[0013] By recirculating a portion of the semi-lean absorbent to the absorption process without passing it through the low-pressure regeneration stage, the size and power consumption of the low-pressure regeneration tower are reduced, and the thermal exposure of the absorbent is reduced. Power consumption is also reduced by using heat from the rich feed flow to provide heat in the transition or regeneration stage.

[0014] Hybrid absorbents improve carbon dioxide release at medium temperatures during the high-pressure regeneration phase, enhance carbon dioxide absorption by the absorbent, thereby reducing the thermal exposure of the absorbent and improving process performance.

[0015] By combining these improvements, equipment costs, electricity consumption, and absorbent costs for the entire carbon capture process are significantly reduced. [Brief explanation of the drawing]

[0016] [Figure 1]Shows an apparatus for practicing the claimed invention, which produces three carbon dioxide streams, namely a high-pressure carbon dioxide stream, an intermediate-pressure carbon dioxide stream, and a low-pressure carbon dioxide stream, and recovers heat from the rich feed stream at two points in the regeneration step. [Figure 2] Shows an apparatus for practicing the claimed invention, which produces three carbon dioxide streams, namely a high-pressure carbon dioxide stream, an intermediate-pressure carbon dioxide stream, and a low-pressure carbon dioxide stream, and recovers heat from the rich feed stream at one point in the regeneration step and one point in the transition step. [Figure 3] Shows an apparatus for practicing the claimed invention, which produces three carbon dioxide streams, namely a high-pressure carbon dioxide stream, an intermediate-pressure carbon dioxide stream, and a low-pressure carbon dioxide stream, recovers heat from the rich feed stream at two points in the regeneration step, and recovers heat from the high-pressure carbon dioxide stream at one point in the transition step. [Figure 4] Shows an apparatus for practicing the claimed invention, which produces three carbon dioxide streams, namely a high-pressure carbon dioxide stream, an intermediate-pressure carbon dioxide stream, and a low-pressure carbon dioxide stream, recovers heat from the rich feed stream at two points in the regeneration step and one point in the transition step, and recovers heat from the high-pressure carbon dioxide stream at one point in the transition step. MODE FOR CARRYING OUT THE INVENTION

[0017] The process of the present invention recovers carbon dioxide from a gaseous feed stream using a liquid absorbent. In some embodiments, the feed stream may contain hydrogen, methane, ethane, propane, natural gas, ethylene, propylene, carbon monoxide, or other combustion products.

[0018] Before entering the absorption step, the feed stream contains carbon dioxide having a partial pressure of at least 1 bar, and is referred to as a "rich feed stream". In some embodiments, the partial pressure of carbon dioxide in the rich feed stream is at least 2 bar, or at least 3 bar, or at least 4 bar, or at least 5 bar. There is no critical limitation on the maximum partial pressure of carbon dioxide in the rich feed stream, however, a partial pressure exceeding 60 bar or 30 bar may be unnecessary. In some embodiments, the pressure is sufficiently low such that carbon dioxide does not liquefy.

[0019] In some embodiments, the rich feed stream contains at least 5 mole percent carbon dioxide, or at least 10 mole percent, or at least 15 mole percent, or at least 20 mole percent carbon dioxide. In some embodiments, the rich feed stream further comprises at least 40 mole percent, or at least 50 mole percent, or at least 60 mole percent hydrogen.

[0020] In some embodiments, the rich feed stream is generated by a water gas shift process. See, for example, Mendes et al., "The Water Gas Shift Reaction: From Conventional Catalytic Systems to Pd-based Membrane Reactors - a Review", 5 Asia-Pac. J. Chem Eng. at 111-137 (2010). The water gas shift process often - Hydrogen: at least 40 mole percent, or at least 50 mole percent, or at least 60 mole percent, or at least 70 mole percent. Up to 90 mole percent, or up to 80 mole percent, or up to 75 mole percent. - Carbon dioxide: at least 10 mole percent, or at least 15 mole percent. Up to 60 mole percent, or up to 50 mole percent, or up to 30 mole percent, or up to 25 mole percent. • Prepare a flow containing other impurities (e.g., unreacted hydrocarbons, carbon monoxide, nitrogen, sulfur oxides, nitrogen oxides, hydrogen sulfide, argon, and methanol) in amounts of 0 to 10 mole percent or 1 to 5 mole percent.

[0021] In some embodiments, the rich feed stream has a temperature of at least 150°C or at least 180°C. In some embodiments, the rich feed stream has a temperature of up to 220°C, up to 200°C, or up to 190°C. For example, the feed stream from a water-gas shift process may have a temperature of 180°C to 220°C.

[0022] Carbon dioxide is captured in two or more flows at different pressures. In some embodiments, carbon dioxide is recovered in two flows: a low-pressure flow having a pressure of 3 bar or less, and a high-pressure flow having a pressure of at least 3 bar. In some embodiments, carbon dioxide is recovered in three flows: a low-pressure flow with a pressure of 3 bar or less, a medium-pressure flow with a pressure higher than the low-pressure flow, and a high-pressure flow with a pressure higher than the medium-pressure flow.

[0023] This process uses three steps, as described above: an absorption step, a regeneration step, and a transfer step. Each process step uses an aqueous liquid absorbent called an "absorbent." In some embodiments, the absorbent contains one or more additives that enhance the absorption or desorption of carbon dioxide and are miscible with water.

[0024] Some additives to the absorbent may be chemical absorbents, meaning that carbon dioxide undergoes a reversible chemical reaction with the additive. In some embodiments, the additive is an organic amine. An example of an organic amine is one that satisfies formula 1, (1) NR a H b In the formula, each R is an independent organic part, a is the number of organic parts from 1 to 3, and b is the number of hydrogen atoms from 1 to 2.

[0025] The organic amine may be a primary amine (a=1 and b=2), a secondary amine (a=2 and b=1), or a tertiary amine (a=3 and b=0). In some embodiments, each R group is independently an alkyl group or an alkanol group. In some embodiments, each R group contains an average of at least one carbon atom or at least two carbon atoms. In some embodiments, each R group contains an average of up to six carbon atoms, or up to four carbon atoms, or up to three carbon atoms, or up to two carbon atoms. In some embodiments, the two R groups bond to each other to form a cyclic structure such as a piperidine, pyrrolidine, or piperazine structure. In some embodiments, the organic amine comprises two different R groups, such as at least one alkyl group and at least one alkanolamine group.

[0026] Examples of suitable organic amines include ethanolamine, diethanolamine, triethanolamine, n-methyldiethanolamine (MDEA), piperazine, and pyrrolidine. Suitable organic amines are commercially available.

[0027] Some additives to absorbents may be physical absorbents, meaning that the additive physically blends carbon dioxide with the absorbent but does not chemically react with it. Examples of physical absorbents include: Low molecular weight polyalkylene glycols and their monoethers, di(propylene glycol), tri(propylene glycol), di(ethylene glycol), tri(ethylene glycol), tetra(ethylene glycol), and their monomethyl ethers, monoethyl ethers, and mono-t-butyl ethers, such as methoxytriglycerin; • Cyclic sulfones such as sulfolanes, • Thiodiglycol, and Glycerin is one example.

[0028] Examples of polyalkylene glycols and their monoethers satisfy formula 2, (2) HO-(-CH2CHR 1 -O-) c -R 2 wherein R 1 and R 2 are each independently a hydrogen atom or an alkyl group, and c is the number of repetitions of the alkylene glycol unit.

[0029] · In some embodiments, each R 1 contains an average of no more than 5 carbon atoms, or no more than 3 carbon atoms, or no more than 2 carbon atoms, or no more than 1 carbon atom. In some embodiments, each R 1 is a methyl group or a hydrogen atom. In some embodiments, each R 1 is a hydrogen atom. · In some embodiments, each R 2 contains an average of no more than 6 carbon atoms, or no more than 4 carbon atoms, or no more than 3 carbon atoms, or no more than 2 carbon atoms. In some embodiments, each R 2 is a methyl group or a hydrogen atom. In some embodiments, each R 2 is a methyl group. · In some embodiments, c has an average of at least 1, or at least 2, or at least 2.5, or at least 2.8. In some embodiments, c has an average maximum of 8, or a maximum of 6, or a maximum of 5, or a maximum of 4, or a maximum of 3.5, or a maximum of 3.2.

[0030] Suitable physical absorbents are commercially available under the trademarks DOWANOL™, CARBITOL™, and UCAR SOL™.

[0031] In some embodiments, the absorbent includes both a chemical absorbent and a physical absorbent (referred to as a "hybrid absorbent"). For example, the absorbent is As mentioned above, the hybrid additive may contain polyalkylene glycol or its monoether. In some embodiments, the hybrid additive contains 10 to 50 weight percent of polyalkylene glycol or its monoether. • As mentioned above, one or more alkanolamines. In some embodiments, the hybrid absorbent additive contains 20 to 70 weight percent of alkanolamines. • Optionally, one or more cyclic amines as described above. In some embodiments, the hybrid absorbent additive contains 0 to 30 weight percent of cyclic amines.

[0032] Suitable hybrid additive packages are available commercially under the UCARSOL® trademark.

[0033] In some embodiments, the absorbent contains at least 15 weight percent, or at least 20 weight percent, or at least 24 weight percent of water, based solely on the weight of water and additives, and excluding any absorbed carbon dioxide. In some embodiments, the aqueous absorbent contains at least 25 weight percent, or at least 40 weight percent, or at least 50 weight percent, or at least 60 weight percent of additives, based solely on the weight of water and additives, and excluding any absorbed carbon dioxide.

[0034] Absorption process In the absorption process, a rich feed flow is brought into contact with the absorbent in an absorption tower where the absorbent flows countercurrently to the feed flow. The rich feed flow is introduced into the lower part of the tower and flows toward the top. Two absorbent flows are introduced into the absorption tower at two different points. The lean absorbent is introduced into the upper part of the tower and flows toward the bottom. The semi-lean absorbent is introduced into the tower between the lean absorbent and the rich feed flow and similarly flows toward the bottom of the tower.

[0035] Lean absorbents contain a lower carbon dioxide load than rich or semi-lean absorbents. ("Load" means the total moles of carbon dioxide in the absorbent divided by the total moles of amine.) In some embodiments, lean absorbents contain a carbon dioxide load of 0.2 moles or less, or 0.1 moles or less, or 0.08 moles or less per mole. There is no minimum desirable load for lean absorbents. The carbon dioxide load may be undetectable (essentially 0). In some embodiments, lean absorbents contain a carbon dioxide load of at least 0.01, or at least 0.02, or at least 0.05.

[0036] Semilean absorbents contain a lower carbon dioxide load than rich absorbents and a higher carbon dioxide load than lean absorbents. In some embodiments, semilean absorbents contain a carbon dioxide load of 0.5 or less, or 0.4 or less, or 0.3 or less. In some embodiments, semilean absorbents contain a carbon dioxide load of at least 0.05, or at least 0.1, or at least 0.15.

[0037] Lean and semi-lean absorbents absorb carbon dioxide from the rich feed stream, generating lean feed stream and rich absorbent. The rich absorbent is recovered at the bottom of the absorption tower, such as at or near the bottom of the tower. The lean feed stream exits the absorption tower at the top of the tower, such as at or near the top of the tower, and is recovered for further use or storage. The rich absorbent proceeds to a transition process, and then to a regeneration process.

[0038] The semi-lean absorbent is introduced into the absorption tower between the point where the rich feed flow is introduced and the point where the lean absorbent is introduced. In some embodiments, the semi-lean absorbent is introduced into the absorption tower at least 20 percent, or at least 30 percent, or at least 40 percent below the tower height below the lean absorbent flow, based on the distance between the lean absorbent introduction point and the rich feed flow introduction point. In some embodiments, the semi-lean absorbent is introduced into the absorption tower at up to 80 percent, or up to 70 percent, or up to 60 percent below the tower height below the lean absorbent flow, based on the distance between the lean absorbent introduction point and the rich feed flow introduction point.

[0039] The conditions within the absorption tower are selected to facilitate the absorption of carbon dioxide from the feed flow to the absorbent. In some embodiments, the absorption tower has packing and / or trays to increase contact between the rich feed flow and the absorbent. Examples of useful packing include random packing or structured packing.

[0040] The optimal pressure and temperature for the absorption process vary depending on the absorbent. As a general rule, it is known that increasing the pressure and decreasing the temperature promotes absorption.

[0041] The pressure inside the absorption tower (absorption pressure) is at least 8 bar. In some embodiments, the absorption pressure is at least 10 bar, or at least 12 bar, or at least 14 bar. In some embodiments, the absorption pressure is up to 100 bar, or up to 80 bar, or up to 70 bar, or up to 60 bar, or up to 50 bar.

[0042] The temperature inside the absorption tower (absorption temperature) is a maximum of 100°C. In some embodiments, the absorption temperature is a maximum of 90°C, or a maximum of 80°C, or a maximum of 70°C, or a maximum of 60°C, or a maximum of 55°C, or a maximum of 50°C. In some embodiments, the absorption temperature is greater than 0°C, or at least 10°C, or at least 20°C, or at least 30°C.

[0043] To maintain the desired absorption temperature, supplemental cooling may be necessary. For example, if the rich feed flow is hotter than the absorption temperature when approaching the absorption tower, it can be passed through one or more coolers to reduce its temperature. Lean and semi-lean absorbents are cooled in one or more heat exchangers during the transition process from the regeneration process to the absorption process, but may receive supplemental cooling before entering the absorption tower. Furthermore, an auxiliary cooler called an intercooler may be added to the absorption tower. A portion of the absorbent is removed from the tower, cooled in the intercooler, and then returned to the tower at a lower temperature. Similar results can be achieved by a pump-around process in which a portion of the rich absorbent leaving the absorption tower is separated from the main flow, cooled, and returned to the absorption tower at or below the semi-lean flow feed point.

[0044] The lean feed stream is recovered at or near the top of the absorption tower. In some embodiments, the absorption process captures at least 90 percent, or at least 95 percent, or at least 98 percent, or at least 99 percent, or at least 99.5 percent, or at least 99.8 percent of the carbon dioxide in the rich feed stream. While there is no maximum desirable capture of carbon dioxide, in some embodiments, the lean feed stream may retain at least 0.001 percent (99.999 mole percent capture) or up to 0.01 percent (99.00 mole percent capture) of the carbon dioxide in the rich feed stream.

[0045] The rich absorbent exiting the absorption tower contains a higher carbon dioxide load than the lean and semi-lean absorbents. In some embodiments, the rich absorbent contains a carbon dioxide load of at least 0.3, or at least 0.4, or at least 0.5. In some embodiments, the rich absorbent contains a carbon dioxide load of 1.0 or less, or 0.8 or less, or 0.7 or less. The temperature and pressure of the rich absorbent are approximately the same as the absorption temperature and absorption pressure.

[0046] The rich absorbent proceeds from the absorption process to the transition process and then to the regeneration process.

[0047] Regeneration process The regeneration process (desorption of carbon dioxide from the absorbent) is carried out in at least two stages, including high-pressure regeneration and low-pressure regeneration. In some embodiments, the regeneration process is carried out in three or more stages, including high-pressure regeneration, one or more medium-pressure regenerations, and low-pressure regenerations. It is known that increasing the temperature and decreasing the pressure promotes the desorption of carbon dioxide from the absorbent. Generally, each regeneration stage exposes the absorbent to a higher temperature, a lower pressure, or both, compared to the previous stage.

[0048] High-pressure and medium-pressure regeneration are flash distillations. Carbon dioxide in the rich feed stream is rapidly desorbed and vaporized by the pressure drop in the flash separator. Each flash separator is independently either a flash drum or an in-line separator. Unlike multi-stage distillation columns, flash separators do not have a gas-liquid contact area containing trays or packing to facilitate gas-liquid mass transfer. Rather, flash separators rely on gravity (flash drums and other conventional separators) or centrifugal force (in-line separators) to facilitate gas-liquid separation.

[0049] High-pressure regeneration is performed at a temperature of 80°C to 140°C and a pressure of 3 bar to 40 bar. The rich absorbent flow entering high-pressure regeneration is heated in the transition step, and therefore, in most embodiments, the temperature of high-pressure regeneration is higher than the absorption temperature. In some embodiments, the temperature of the rich absorbent entering high-pressure regeneration is at least 90°C or at least 100°C. In some embodiments, the temperature of the rich absorbent entering high-pressure regeneration is up to 130°C, up to 120°C, or up to 115°C.

[0050] In some embodiments, the pressure of the rich absorbent during high-pressure regeneration is at least 3 bar, or at least 5 bar, or at least 8 bar, or at least 10 bar, or at least 12 bar. In some embodiments, the pressure of the rich absorbent during high-pressure regeneration is up to 30 bar, or up to 20 bar, or up to 18 bar, or up to 16 bar. In most embodiments, the pressure of high-pressure regeneration is at or below the absorption pressure.

[0051] High-pressure regeneration generates a semi-lean absorbent and a high-pressure carbon dioxide flow. The high-pressure carbon dioxide flow has approximately the same temperature and pressure as the high-pressure regeneration conditions. The high-pressure carbon dioxide flow can be recovered and sent directly for compression or other use. Alternatively, heat may be recovered from the high-pressure carbon dioxide flow before it is sent, as described below for thermal integration and shown in Figures 3 and 4.

[0052] In some embodiments, one or more intermediate-pressure regenerations may be performed on the semilean absorbent after it has exited high-pressure regeneration and before it enters low-pressure regeneration. The intermediate-pressure regeneration is performed at a pressure lower than that of the high-pressure regeneration but higher than that of the low-pressure regeneration. In some embodiments, the pressure of the intermediate-pressure regeneration is at least 3 bar, or at least 4 bar, or at least 5 bar, or at least 6 bar. In some embodiments, the pressure of the intermediate-pressure regeneration is up to 15 bar, or up to 12 bar, or up to 10 bar, or up to 9, or up to 8 bar. For example, high-pressure regeneration may be performed at a pressure of 10-20 bar and intermediate-pressure regeneration at a pressure of 5-10 bar, or high-pressure regeneration may be performed at a pressure of 12-15 bar and intermediate-pressure regeneration at a pressure of 6-9.

[0053] In some embodiments, the semi-lean absorbent is heated before entering intermediate-pressure regeneration. For example, as described below for thermal integration, the heat exchanger may heat the semi-lean absorbent with heat from the rich feed flow. In some embodiments, the temperature of the semi-lean absorbent entering intermediate-pressure regeneration is at least 80°C, or at least 90°C, or at least 95°C, or at least 100°C. In some embodiments, the temperature of the semi-lean absorbent entering intermediate-pressure regeneration is up to 140°C, or up to 130°C, or up to 125°C. For example, high-pressure regeneration may be carried out at a temperature of 90–120°C, and intermediate-pressure regeneration may be carried out at a temperature of 100–130°C.

[0054] Intermediate-pressure regeneration generates an intermediate-pressure carbon dioxide stream and a stream of semi-lean absorbent with reduced carbon dioxide content. The pressure of the intermediate-pressure carbon dioxide stream is approximately equivalent to the pressure of the intermediate-pressure regeneration. The intermediate-pressure carbon dioxide stream is recovered for use or storage. In many embodiments, it is sent to be compressed to a higher pressure.

[0055] The semi-lean absorbent from high-pressure or medium-pressure regeneration is split into two flows that proceed to different applications. A portion of the semi-lean absorbent is sent to low-pressure regeneration, and a portion of the semi-lean absorbent is returned to the absorption process via a transition step. As previously mentioned, the semi-lean absorbent returned to the absorption process is introduced into the absorption tower between the lean absorbent and the rich feed stream. In some embodiments, at least 2 percent, or at least 4 percent, or at least 8 percent, or at least 12 percent of the semi-lean absorbent is sent to low-pressure regeneration. The remaining semi-lean absorbent (up to 98 percent, or 96 percent, or 92 percent, or 88 percent) is returned to the absorption process via a transition step. In some embodiments, up to 80 percent, or up to 60 percent, or up to 50 percent, or up to 40 percent, or up to 30 percent, or up to 26 percent, or up to 20 percent of the semi-lean absorbent is sent to low-pressure regeneration. The remaining semi-lean absorbent (at least 20 percent, or 40 percent, or 50 percent, or 60 percent, or 70 percent, or 74 percent, or 80 percent) is returned to the absorption process via a transition step.

[0056] In low-pressure regeneration, carbon dioxide is further desorbed as a gas from the semi-lean absorbent.

[0057] The low-pressure regeneration process is carried out in a regeneration tower. Known towers, such as those equipped with packing or trays, can be used. In some embodiments, the absorbent and carbon dioxide stream move countercurrently through the tower. For example, the semi-lean absorbent is introduced from the middle to the top of the tower, the lean absorbent is recovered at the bottom of the tower, and the low-pressure carbon dioxide stream is recovered near the top of the tower.

[0058] The conditions in the low-pressure regeneration process are selected to promote the desorption of carbon dioxide from the absorbent without significantly degrading the absorbent. Low-pressure regeneration is performed at a regeneration temperature at least the same level as that of high-pressure and medium-pressure regeneration, and at a regeneration pressure lower than that of high-pressure and medium-pressure regeneration.

[0059] In some embodiments, the low-pressure regeneration pressure is a maximum of 5 bar, or a maximum of 4 bar, or a maximum of 3 bar, or a maximum of 2 bar, or a maximum of 1 bar. In some embodiments, the low-pressure regeneration pressure is at least 0.5 bar, or at least 0.8 bar, or at least 0.9 bar, or at least 1 bar.

[0060] In some embodiments, the low-pressure regeneration temperature is at least 90°C, or at least 100°C, or at least 110°C. In some embodiments, the regeneration temperature is up to 150°C, or up to 140°C, or up to 135°C. In some embodiments, the temperature during low-pressure regeneration may be kept below the maximum temperature to reduce the thermal exposure of the absorbent.

[0061] To maintain a suitable temperature in the regeneration tower, the semi-lean absorbent may receive supplemental heat before or during the regeneration process. Additional heat can be supplied by known means such as a reboiler, heating jacket, or heating coil. In one embodiment, the reboiler heats a portion of the lean fluid flow leaving the regeneration process and recirculates it back into the regeneration process. In some embodiments, the additional heating within the reboiler is provided, all or partly, using heat from the rich feed flow.

[0062] In some embodiments, some water in the absorbent may evaporate during the low-pressure regeneration process. Since water can act as a stripping gas, evaporation may be desirable. A condenser may be used to avoid loss of vaporized absorbent. The condenser may be located at or near the top of the regeneration column.

[0063] Carbon dioxide is recovered from the regeneration process as a "low-pressure carbon dioxide stream" with a pressure approximately the same as the regeneration pressure inside the regeneration tower.

[0064] In some embodiments, at least 40 percent, or at least 50 percent, or at least 60 percent, or at least 70 percent, or at least 80 percent, or at least 85 percent of the carbon dioxide recovered in this process is recovered in the high-pressure and medium-pressure flows. The remaining carbon dioxide (up to 60 percent, or up to 50 percent, or up to 40 percent, or up to 30 percent, or up to 20 percent, or up to 15 percent) is recovered in the low-pressure flow. There is no desirable maximum recovery rate in the high-pressure and medium-pressure carbon dioxide flows, but in some embodiments, recovering more than 95 percent of the carbon dioxide in the high-pressure and medium-pressure flows, and less than 5 percent in the low-pressure flow, may be inefficient.

[0065] Transition process During the transition process, the absorbent moves back and forth between the absorption process and the regeneration process.

[0066] As mentioned above, the regeneration process is carried out at a higher temperature than the absorption process. Rich absorbents exiting the absorption process are heated in the transition process in preparation for regeneration. Lean and semi-lean absorbents exiting the regeneration process are cooled in the transition process in preparation for the absorption process. At least part of the heating and cooling is achieved by a heat exchanger that transfers heat from the lean and semi-lean absorbents to the rich absorbents. Examples of suitable heat exchangers include shell and tube exchangers, plate and flame exchangers, or plate and shell exchangers.

[0067] In some embodiments, the rich absorbent in the transition process is separated into at least two separate flows. The first flow of the rich absorbent receives heat from the lean absorbent in a first heat exchanger. The second flow of the rich absorbent receives heat from the semi-lean absorbent in a second heat exchanger.

[0068] In some embodiments, the rich absorbent in the transition process is separated into at least three distinct flows. A first flow of rich absorbent receives heat from the lean absorbent in a first heat exchanger. A second flow of rich absorbent receives heat from the semi-lean absorbent in a second heat exchanger. A third flow of rich absorbent receives heat from the rich feed flow in a third heat exchanger.

[0069] After leaving the heat exchanger, the separated rich absorbent streams are recombined. In some embodiments, an auxiliary heater may provide additional heating to the rich absorbent downstream of the heat exchanger. In some embodiments, an auxiliary cooler may provide additional cooling to the lean and semi-lean absorbents downstream of the heat exchanger.

[0070] As mentioned above, the regeneration process is performed at a lower pressure than the absorption process. In the transition process, pumps raise the pressure of the lean and semi-lean absorbents to the absorption pressure before they enter the absorption process. In some embodiments, multiple pumps can be used to increase the pressure in stages. The pumps can be located upstream or downstream of the heat exchanger, or both. Examples of suitable pumps include centrifugal pumps, positive displacement pumps, regeneration turbine pumps, axial flow pumps, and ejector pumps.

[0071] Similarly, during the transition process, one or more pressure reducing valves may reduce the pressure of the rich absorbent from the absorption pressure before the rich absorbent enters the regeneration process. Examples of suitable pressure reducing valves include globe valves, diaphragm valves, gate valves, and needle valves.

[0072] Compression of carbon dioxide flow The process of the present invention generates high-pressure carbon dioxide flows, low-pressure carbon dioxide flows, and optionally one or more medium-pressure carbon dioxide flows. In some embodiments, these flows are compressed to a pressure suitable for use, storage, or transport. In some embodiments, the target pressure of the recovered carbon dioxide is at least 40 bar, or at least 45 bar, or at least 50 bar, or at least 55 bar, or at least 60 bar. In some embodiments, the target pressure of the recovered carbon dioxide is up to 200 bar, or up to 150 bar, or up to 100 bar, or up to 80 bar. In some embodiments, the carbon dioxide is a liquid or supercritical fluid at the target pressure and 25°C.

[0073] Compression may be performed by using a series of two or more compressors to gradually increase the pressure of carbon dioxide to a target pressure. The number of compressors depends on the target pressure and the efficiency of the compressors. In some embodiments, each compressor increases the pressure of carbon dioxide by at least 1.5 times, or at least 2.0 times, or at least 2.5 times. In some embodiments, each compressor increases the pressure of carbon dioxide by up to 5 times, or up to 4 times, or up to 3 times.

[0074] In some embodiments, compression is carried out using at least two compression stages, or at least three compression stages, or at least four compression stages. In some embodiments, compression is carried out using eight or fewer compression stages, or seven or fewer compression stages, or six or fewer compression stages, or five or fewer compression stages. For example, if the low-pressure carbon dioxide flow is 1 to 2 bar, the first compression stage can increase the pressure to 3 to 5 bar, the second compression stage can increase the pressure to 6 to 13 bar, the third compression stage can increase the pressure to 15 to 30 bar, and the fourth compression stage can increase the pressure to 40 to 80 bar.

[0075] Intermediate-pressure and high-pressure carbon dioxide flows can be introduced into the compression process at a higher stage than low-pressure carbon dioxide flows. As a result, lower-stage compressors can be smaller and use less energy. For example, if the compression process uses the four stages mentioned above, a high-pressure carbon dioxide flow of 12-15 bar can be introduced into the third or fourth compression stage, and an intermediate-pressure carbon dioxide flow of 6-9 bar can be introduced into the third compression stage. If the low-pressure flow contains only 5-15% carbon dioxide, the capacity and power consumption of the first and second compression stages can be reduced by more than 80%.

[0076] Thermal integration As described, the rich feed stream may enter the process at a temperature of at least 150°C, or at least 180°C, or at least 200°C, such as from a water-gas shift process. Since the absorption temperature is lower than the temperature of the rich feed stream, the rich feed stream must be cooled before entering the absorption process. The heat from the rich feed stream may be useful for heating the absorbent in the regeneration process, the transition process, or both.

[0077] In particular, the following points in this process can recover and utilize heat from the rich feed flow. • Inside the reboiler that heats the contents of the low-pressure regeneration tower, • In the heat exchanger, the rich absorbent is heated before high-pressure regeneration. • In the heat exchanger where the semilean absorbent is heated before medium-pressure regeneration, and • Heating the semi-lean absorbent inside the heat exchanger before low-pressure regeneration.

[0078] In some embodiments, heat is recovered from the rich feed flow at at least two of these locations. In some embodiments, heat is recovered from the rich feed flow at at least three of these locations. In some embodiments, heat is recovered from the rich feed flow at at least four of these locations. In some embodiments, one of the locations where heat is recovered is a reboiler for a low-pressure regeneration tower. In some embodiments, heat is recovered in the reboiler before high-pressure regeneration. In some embodiments, heat is recovered in the reboiler before medium-pressure regeneration. In some embodiments, heat is recovered in the reboiler before both high-pressure and medium-pressure regeneration.

[0079] In addition, the high-pressure carbon dioxide flow exiting the high-pressure regeneration can have a temperature of 140°C. It may be advantageous to recover heat from this flow before sending it to compression. This heat can be advantageously recovered into the rich absorbent using a heat exchanger before high-pressure regeneration.

[0080] Reference to the drawing: Figure 1: Figure 1 illustrates an example of the process of the present invention.

[0081] Column C1 is an absorption column. Column C1 has (a) an outlet for rich absorbent at the bottom of the column, (b) an inlet for rich feed flow at the lowest point of the gas-liquid contact section, including packing or a tray to facilitate gas-liquid mass transfer, (c) an inlet for semi-lean absorbent about 50% above the contact section of the column, (d) an inlet for lean absorbent at the top of the contact section, and (e) an outlet for lean feed at the top of the column. Column C1 contains Raschig Super Rings #2 packing. Optionally, a water wash may be included, which is an additional contact section above the lean feed point designed to recover the solvent from the gas by contacting the gas with water, and the solvent is returned to the solvent system at some point.

[0082] Column C2 is a regeneration column. Column C2 has (a) an outlet for lean absorbent at the bottom of the column, (b) an inlet for semi-lean absorbent around or near the top of the contact section, and (c) an outlet at the top of the column for recovering desorbed carbon dioxide, with a condenser recapturing any evaporated absorbent. A reboiler (H5) at the bottom of Column C2 takes a portion of the lean absorbent, heats it, and returns it to Column C2 to maintain the regeneration temperature. Column C2 contains 2-inch random packing.

[0083] S1 is a high-pressure flash drum. S2 is a medium-pressure flash drum. Each flash drum has an inlet for rich absorbent, a top outlet for carbon dioxide, and a bottom outlet for a liquid line for partially desorbed absorbent.

[0084] The rich feed stream contains approximately 25 mole percent hydrogen, approximately 74 mole percent carbon dioxide, and less than 1 mole percent each of water, methane, and carbon monoxide. The rich feed stream has a temperature of approximately 154°C at the start of the process.

[0085] The rich feed flow enters the system through line L1. Line L1 carries the rich feed flow through heat exchanger H5 and then H4, lowering the temperature of the rich feed flow to below 40°C, and then introduces the rich feed flow near the bottom of absorption tower C1. In some embodiments, the rich feed may also undergo auxiliary cooling through a cooler (not shown) before entering absorption tower C1.

[0086] Lean absorbent (containing a carbon dioxide load of 0.1 or less) enters absorption tower C1 near the top via line L6 at a temperature of 40°C or less. Semi-lean absorbent (containing a carbon dioxide load of 0.1 to 0.5) enters absorption tower C1 near the middle via line L5b at a temperature of 40°C or less. In some embodiments, absorption tower C1 has auxiliary cooling (not shown). Carbon dioxide is absorbed from the rich feed stream into the semi-lean and lean absorbent within tower C1, generating a rich absorbent (containing at least 0.4 carbon dioxide load) and a lean feed stream (containing less than 1 mole percent carbon dioxide). The lean feed stream exits from the top of tower C1 and is recovered.

[0087] The rich absorbent exits tower C1 through line L2 at a temperature of 40°C or lower. Line L2 is divided into two separate lines labeled L2a and L2b. Line L2a carries a portion of the rich absorbent into line L3 through heat exchanger H1, where a portion of the rich absorbent receives heat from the lean absorbent. Line L2b carries a portion of the rich absorbent into line L3 through heat exchanger H2, where a portion of the rich absorbent receives heat from the semi-lean absorbent.

[0088] Line L3 delivers the rich absorbent through an auxiliary heater, where the rich absorbent is heated to a temperature of at least 110°C, and then into the flash drum S1. The pressure inside the flash drum S1 is 10-20 bar, and high-pressure carbon dioxide is released from the rich absorbent flow at that pressure. The high-pressure carbon dioxide is recovered from the top of the flash drum S1.

[0089] The semi-lean absorbent exits the flash drum S1 via line L4. Line L4 carries the semi-lean absorbent through heat exchanger H4, where it receives heat from the rich feed flow. Line L4 then carries the heated semi-lean absorbent into flash drum S2. The pressure inside flash drum S2 is 3-10 bar, and medium-pressure carbon dioxide is released from the semi-lean absorbent flow at that pressure. The medium-pressure carbon dioxide is recovered from the top of flash drum S2. The absorbent exiting flash drum S2 is semi-lean absorbent containing reduced levels of carbon dioxide with a load of 0.05-0.3.

[0090] The semi-lean absorbent exits the flash drum S2 through line L5. Line L5 branches into lines L5a and L5b. Line L5a carries a portion of the semi-lean absorbent into regeneration tower C2. Line L5b carries a portion of the semi-lean absorbent through heat exchanger H2, where a portion of the semi-lean absorbent transfers heat to the rich absorbent flow. From heat exchanger H2, the semi-lean absorbent is pressurized to absorption pressure, cooled to absorption temperature, and returned to absorption tower C1 near its center.

[0091] A portion of the semi-lean absorbent enters the regeneration tower C2 via line L5a. The temperature inside the regeneration tower is at least 100°C, and the pressure inside the regeneration tower is 6 bar or less. Low-pressure carbon dioxide desorbs from the semi-lean absorbent at that pressure, producing a lean absorbent and low-pressure carbon dioxide stream. The low-pressure carbon dioxide exits the tower C2 from the top and is recovered. The lean absorbent exits the tower C2 via line L6. The side stream of lean absorbent is heated in the heat exchanger H5 by the heat from the rich absorbent stream and returned to tower C2 to maintain the heat inside tower C2.

[0092] The remaining lean absorbent is transported by line L6 through heat exchanger H1, where it is heated into a rich absorbent. From heat exchanger H1, the lean absorbent is pressurized to the absorption pressure, cooled to the absorption temperature, and returned to the absorption tower C1 near the top of the tower.

[0093] Figure 2: Figure 2 illustrates another embodiment of the process of the present invention, including an alternative thermal integration. Figure 2 proceeds similarly to Figure 1, except that: Line L2 branches into three separate lines L2a, L2b, and L2c, each carrying a rich absorbent. Lines L2a and L2b proceed similarly to Figure 1. Line L2c carries a portion of the rich absorbent through a heat exchanger H3, where a portion of the rich absorbent is heated by the rich absorbent flow, and then into line L3.

[0094] Line L1 carries the rich supply flow from heat exchanger H5 through heat exchanger H3 and then into tower C1. Optionally, an auxiliary cooler may be added after heat exchanger H3 and before the rich supply flow enters tower C1 for further cooling. Unlike Figure 1, heat exchanger H4 is absent, and line L1 does not pass through heat exchanger H4.

[0095] Figure 3: Figure 3 illustrates another embodiment of the process of the present invention, including an alternative thermal integration. Figure 3 proceeds similarly to Figure 1, except that: Line L2 branches into three separate lines L2a, L2b, and L2c, each carrying a rich absorbent. Lines L2a and L2b proceed similarly to Figure 1. Line L2c carries a portion of the rich absorbent through a heat exchanger H3, where a portion of the rich absorbent is heated by a high-pressure carbon dioxide flow, and then into line L3.

[0096] Line L7 receives a high-pressure carbon dioxide flow from the flash drum S1. Line L7 then transports the high-pressure carbon dioxide flow through a heat exchanger H3, where the high-pressure carbon dioxide flow heats the rich absorbent flow, to a location where the high-pressure carbon dioxide is recovered.

[0097] Figure 4: Figure 4 illustrates another embodiment of the process of the present invention, including an alternative thermal integration incorporating improvements from Figures 1, 2, and 3. Figure 4 proceeds similarly to Figure 1, except that: Line L2 branches into three separate lines L2a, L2b, and L2c, each carrying a rich absorbent. Lines L2a and L2b proceed similarly to Figure 1. Line L2c carries a portion of the rich absorbent through heat exchanger H3, then heat exchanger H6, and then into line L3.

[0098] Line L1 carries a rich feed flow through three different heat exchangers, and then into column C1. First, the rich feed flow passes through heat exchanger H5, as shown in Figure 1. Second, the rich feed flow passes through heat exchanger H4, as shown in Figure 1. Third, the rich feed flow passes through heat exchanger H6, where the rich feed flow heats the rich absorbent flow. Optionally, an auxiliary cooler (not shown) may further cool the rich feed flow after heat exchanger H6 and before the rich feed flow enters column C1.

[0099] Line L7 receives a high-pressure carbon dioxide flow from the flash drum S1. Line L7 then transports the high-pressure carbon dioxide flow through a heat exchanger H3, where the high-pressure carbon dioxide flow heats the rich absorbent flow, to a location where the high-pressure carbon dioxide is recovered. [Examples]

[0100] The following examples illustrate specific embodiments of the present invention, but do not limit the broadest scope of the invention.

[0101] Figure 1 illustrates a carbon dioxide capture system according to Example 1 of the present invention. Figure 2 illustrates a carbon dioxide capture system according to Example 2 of the present invention. Figure 3 illustrates a carbon dioxide capture system according to Example 3 of the present invention. Figure 4 illustrates a carbon dioxide capture system according to Example 4 of the present invention.

[0102] As a comparative example (CE1), we model Example 1 from Iijima et al. (Australian Patent No. 728167 2001).

[0103] Each embodiment recovers carbon dioxide from the rich feed stream, which is the output of the water-gas shift process. The rich feed stream is shown in Table 1.

[0104] [Table 1]

[0105] Each example uses an absorbent containing 25 wt percent water and 75 wt percent UCARSOL® Hybrid920 hybrid absorbent blend, based solely on the weight of water and hybrid absorbent blend, excluding any dissolved carbon dioxide. The lean absorbent entering the absorption process is at a temperature of 30°C and a pressure of 26.1 bar. The flow rates are shown in Table 3.

[0106] Table 2 lists the details of each recovery system in IE1 to IE4. As a comparative example (CE1), we model Example 1 from Iijima et al. (Australian Patent No. 728167 (2001)).

[0107] [Table 2]

[0108] Table 3 shows the operation of each recovery system in a steady state. In Table 3, separator 1 is high-pressure regeneration, separator 2 is medium-pressure regeneration, and regenerator is low-pressure regeneration. Each system recovers more than 99% of the available carbon dioxide. Each system recovers high-pressure, medium-pressure, and low-pressure carbon dioxide flows. Absorbent temperature, absorbent load, and power consumption are recorded for each system. The embodiment of the present invention achieves similar results to the comparative example using an inexpensive flash drum instead of an expensive high-pressure tower. The embodiment of the present invention uses less power than the comparative example. The embodiment of the present invention exposes the absorbent to less heat and lower load levels than the comparative example. Heat and high loads are known to degrade absorbents.

[0109] [Table 3]

Claims

1. A process for recovering carbon dioxide from a gaseous feed stream having a carbon dioxide partial pressure of at least 1 bar, called a "rich feed stream," using an aqueous liquid absorbent called an "absorbent" that absorbs carbon dioxide, (a) An absorption step in which a rich feed stream is brought into contact with the aqueous absorbent for a time such that carbon dioxide is absorbed from the rich feed stream to the aqueous absorbent in the absorption tower at an absorption temperature of 120°C or less and an absorption pressure of at least 8 bar, thereby generating a gaseous feed stream containing reduced carbon dioxide, called a "lean feed stream," and an absorbent containing an increased level of carbon dioxide, called a "rich absorbent." (b) A regeneration step in which the rich absorbent from the absorption step is subjected to a regeneration pressure lower than the absorption pressure and a regeneration temperature higher than the absorption temperature for a period of time such that carbon dioxide is desorbed from the absorbent and recovered, thereby forming a lean absorbent containing less carbon dioxide than the rich absorbent, which is returned to the absorption step. (c) A transition step comprising: (i) the rich absorbent passing from the absorption step to the regeneration step being heated and depressurized in preparation for the regeneration step; and (ii) the lean absorbent passing from the regeneration step to the absorption step being cooled and pressurized in preparation for the absorption step, the transition step comprising transferring heat from the lean absorbent to the rich absorbent in one or more heat exchangers, (1) The regeneration process is at least: i. A high-pressure regeneration step in which carbon dioxide is partially desorbed from the rich absorbent in a separator, which is either a flash drum or an in-line separator, at a temperature of 80°C to 140°C and a pressure of 3 to 40 bar, to provide (A) a high-pressure carbon dioxide stream having a pressure of at least 8 bar and (B) a semi-lean absorbent containing 90 percent or less of the carbon dioxide load of the rich absorbent. ii. A low-pressure regeneration step in which, within the regeneration tower, further carbon dioxide is desorbed from a portion of the semi-lean absorbent at a temperature higher than the temperature of the high-pressure regeneration and a pressure lower than the pressure of the high-pressure regeneration, thereby providing (A) a low-pressure carbon dioxide flow having a pressure lower than the high-pressure carbon dioxide flow and (B) a lean absorbent flow containing 50 percent or less of the carbon dioxide load of the rich absorbent. (2) The semi-lean absorbent from the high-pressure regeneration stage is divided into a portion of the semi-lean absorbent that is sent to the low-pressure regeneration stage and a portion of the semi-lean absorbent that is returned to the absorption stage through the transition stage. (3) The absorption step is characterized by (i) the rich supply flow being introduced into the absorption tower at the bottom of the tower and moving toward the top of the tower, (ii) the lean absorbent being introduced into the absorption tower at the top of the tower and moving toward the bottom of the tower in the opposite direction to the rich supply flow, (iii) the semi-lean absorbent being introduced into the absorption tower at a point between the rich supply flow and the lean absorbent flow and moving toward the bottom of the tower in the opposite direction to the rich supply flow, (iv) the lean supply flow being recovered from the top of the tower, and (v) the rich absorbent being recovered from the bottom of the tower.

2. The process according to claim 1, wherein in the transition stage, the flow of the rich absorbent is divided into at least a first flow and a second flow, the first flow of the rich absorbent is heated by the lean absorbent through a first heat exchanger, the second flow of the rich absorbent is heated by the semi-lean absorbent through a second heat exchanger, and the separated flows of the rich absorbent are combined again.

3. The process according to claim 1, wherein the regeneration step further includes an intermediate pressure regeneration step in which, after the semilean absorbent has left the high-pressure regeneration step and before the semilean absorbent enters the low-pressure regeneration step, carbon dioxide is partially desorbed from the semilean absorbent in a separator, which is either a flash drum or an in-line separator, at a temperature of 80°C to 140°C and a pressure of 3 to 15 bar, thereby providing (A) an intermediate pressure carbon dioxide stream having a pressure of 3 to 15 bar and (B) a semilean absorbent with reduced carbon dioxide.

4. The rich supply flow enters the process at a temperature of at least 150°C, and the heat is transferred to the following four points in the process: (a) Inside the reboiler that heats the contents of the low-pressure regenerating tower, (b) In the heat exchanger that heats the rich absorbent before the high-pressure regeneration, (c) Inside the heat exchanger in which the semilean absorbent is heated before the medium-pressure regeneration, and (d) The process according to claim 3, wherein the semilean absorbent is recovered from the rich feed stream in at least two of the following: a heat exchanger that heats the semilean absorbent before the low-pressure regeneration.

5. The process according to claim 4, wherein heat is recovered from the rich feed flow in the reboiler (a) and in at least one of the heat exchangers (b) to (d).

6. The process according to claim 4, wherein heat is recovered from the rich feed flow in (a) the reboiler, (b) the heat exchanger, and (c) the heat exchanger.

7. The process according to claim 4, wherein heat is further recovered from the high-pressure carbon dioxide stream in a heat exchanger that heats the rich absorbent before the high-pressure regeneration.

8. The process according to claim 4, wherein the separator in the high-pressure regeneration and the medium-pressure regeneration is a flash drum, respectively.

9. The process according to claim 4, wherein the separator in the high-pressure regeneration and the medium-pressure regeneration are each in-line separators.

10. The process according to claim 4, wherein the high-pressure regeneration is performed at a pressure of 8 to 20 bar, the medium-pressure regeneration is performed at a pressure of 5 to 10 bar, and the low-pressure regeneration is performed at a pressure of 0.5 to 5 bar.

11. The process according to claim 4, wherein the high-pressure regeneration is performed at a pressure of 12 to 15 bar, the medium-pressure regeneration is performed at a pressure of 6 to 9 bar, and the low-pressure regeneration is performed at a pressure of 1 to 3 bar.

12. The process according to claim 4, wherein the semi-lean absorbent is introduced into the absorption tower from 30% to 70% of the tower height below the lean absorbent flow, based on the distance between the lean absorbent introduction point and the rich supply flow introduction point.

13. The process according to claim 11, wherein 70 to 95% of the carbon dioxide recovered in the process is recovered in the high-pressure carbon dioxide stream or the medium-pressure carbon dioxide stream.

14. The process according to any one of claims 1 to 13, wherein the absorbent is a hybrid aqueous absorbent containing water, a physical organic absorbent for carbon dioxide, and a chemical organic absorbent for carbon dioxide.

15. The process according to claim 14, wherein the chemical absorbent comprises an organic amine, and the physical solvent comprises a low molecular weight polyalkylene glycol and its monoether, a cyclic sulfone, a thiodiglycol, or glycerin.

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