Carbon Dioxide Removal Unit

Abstract

This specification describes a method and apparatus for removing carbon dioxide from syngas. The apparatus includes an absorber, a stripper, and first, second, and third heat exchangers. The first and second heat exchangers together provide the reboil duty for the stripper and are heated by syngas and steam, respectively. A lean absorbent stream is removed from the stripper and cooled with water in the third heat exchanger before being returned to the absorber. This arrangement provides an efficient method for the recovery of CO2-rich absorbent.

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide removal unit for removing carbon dioxide from a synthesis gas stream. [Background technology]

[0002] In many processes, it is necessary to remove acid gases, such as CO2, from the product gas. This is typically accomplished by treating sour gas, which contains undesirable acid gases, with an absorbent, such as an amine, in an absorber. The absorbent chemically and / or physically removes the acid gases from the sour gas, producing a CO2-rich absorbent and a sweet gas (CO2-lean gas). The absorbent is typically regenerated so it can be reused in the absorber. Regeneration also releases CO2, which is typically compressed / dehydrated and transported for storage at an underground location or used for chemical production.

[0003] WO 2010 / 120527(A2) (Alstom Technology Ltd) describes a method for removing gaseous pollutants from a gas stream, comprising contacting the gas stream with a scrubbing solution to remove the gaseous pollutants from the gas stream by absorption into the scrubbing solution to produce a used scrubbing solution, and removing the gaseous pollutants from the used scrubbing solution to provide a regenerated scrubbing solution and a gas containing the removed pollutants. In the first regeneration stage, the gas containing the removed pollutants is cooled to reduce water vapor loss. In a preferred embodiment, the regenerated scrubbing solution is reboiled to further remove gaseous pollutants and provide a reboiled scrubbing solution. The reboiled scrubbing solution may be used for heat exchange with the regenerated scrubbing solution and the used scrubbing solution.

[0004] WO 2016 / 123386 A1 (Fluor Technologies Corporation) describes a method for capturing carbon dioxide from a gas mixture, including an absorber, a stripper, and a reboiler. The absorbent is a solvent system including a chemical solvent and a non-aqueous physical solvent. The stripper receives a CO2-rich solvent stream from the absorber and produces carbon dioxide and lean solvent through the use of a reboiler in fluid communication with a lower portion of the stripper. In the embodiment depicted in Figure 1 of this reference, steam is used for the reboil duty.

[0005] WO 2017 / 029145 A1 (Casale SA) describes a CO2 removal section for removing CO2 from hydrogen-containing synthesis gas, including an absorption section and a stripper. The stripper includes an upper zone through which the CO2-rich solution from the absorber is flashed to produce a first gas stream containing CO2 and a partially regenerated semi-lean solution, and a lower zone acting as a stripping zone through which the semi-lean solution is stripped to produce a second gas stream containing CO2 and a lean regenerated solution. The CO2 removal section includes a sealing means for separating the first and second gas streams, so that the first and second gas streams can be discharged separately from the stripper. A method for retrofitting a CO2 removal unit by introducing a sealing means is also described.

[0006] WO 2022 / 008876(A1) (Carbon Clean Solutions Limited) describes a method for regenerating a carbon dioxide-containing solvent by passing the carbon dioxide-containing solvent through a low-grade heat regenerator to form a carbon dioxide-lean solvent and passing the carbon dioxide-lean solvent through a low-grade heat reboiler. The term "low-grade heat" typically refers to a temperature range of 60°C to less than 120°C.

[0007] The energy efficiency of carbon dioxide removal units needs to be further improved. Summary of the Invention

[0008] The inventors herein describe a CO2 removal unit that more efficiently utilizes heat flow in a process that involves absorbing CO2 from a syngas in an absorber to generate a CO2-rich absorbent that is processed in a stripper to remove entrained CO2, producing lean absorbent and a CO2-rich overhead.

[0009] A first key feature of the present process is the stripper's ability to utilize waste heat from both the syngas and steam. Specifically, CO2-rich absorbent is removed from the stripper and subjected to heat exchange in a first heat exchanger using syngas and a second heat exchanger using steam. The heat exchangers can be arranged in series or parallel. Steam and syngas together provide the reboil load for the stripper, which offers several advantages. The ability to provide the reboil load from steam means that the CO2 removal unit can function even when a syngas stream is not available to provide the full reboil load, which is often the case during plant startup. This reduces CO2 emissions during plant startup. Once the portion of the plant that produces syngas is operating properly, the CO2 removal unit can be switched over so that the majority of the heating load for the stripper is provided by the syngas.

[0010] A second key feature of the process is that lean absorbent from the stripper is used as a low-grade heat source and processed by heat exchange to produce hot water and cooled lean absorbent. Hot water, typically at about 80°C, can be used in the steam lifter. The steam thus generated may be used as "process steam," for example, for reforming and / or the water-gas shift reaction, or may be used elsewhere in the plant ("steam export"). The lean absorbent stream therefore provides a significant amount of heating duty for steam that would otherwise have to be generated using high-grade heat elsewhere in the plant. This arrangement therefore reduces overall energy consumption and associated costs.

[0011] In a first aspect, the present invention provides a method for removing carbon dioxide from a synthesis gas, comprising: contacting the syngas with an absorbent in an absorber (101, 201, 301) to produce a CO2-depleted syngas and a first CO2-rich absorbent stream (114, 214, 314); removing a first CO2-rich absorbent stream from the absorber and transferring the first CO2-rich absorbent stream to a stripper (102, 202, 302); removing one or more second CO2-rich absorbent streams (115, 215a, 215b) from the stripper and heating the one or more second CO2-rich absorbent streams (115, 215a, 215b) using a first heat exchanger (103, 203, 303) heated with synthesis gas (104a, 204a) and a second heat exchanger (105, 205, 305) heated with steam (106a, 206a) to separate the one or more second CO2-rich absorbent streams (115, 215a, 215b) into one or more absorbent-depleted CO2 streams (107a, 107b, 207a, 207b) and one or more CO2-depleted absorbent streams (108, 208a, 208b); returning the one or more absorbent-depleted CO2 streams and the one or more CO2-depleted absorbent streams to the stripper; removing a gaseous CO2-rich overhead stream from the stripper; removing a lean absorbent stream (109, 209, 309) from the stripper, cooling the lean absorbent stream by heat exchange with water (111, 211, 311) in a third heat exchanger (110, 210, 310) to produce a cooled lean absorbent stream (113, 213, 313), and returning the cooled lean absorbent stream to the absorber; The first heat exchanger and the second heat exchanger are arranged in series or in parallel.

[0012] The method is particularly suitable for integration into processes for producing decarbonized syngas (i.e., a mixture of hydrogen and carbon monoxide), H2, synthetic liquid fuels, or ammonia.

[0013] In a second aspect, the present invention provides an apparatus for removing carbon dioxide from a synthesis gas, comprising: an absorber (101, 201, 301) arranged to contact the syngas with an absorbent to produce a CO2-depleted syngas and a first CO2-rich absorbent stream (114, 214, 314); a stripper (102, 202, 304) positioned to receive the first CO2-rich absorbent stream from the absorber; a first heat exchanger (103, 203, 303) arranged to heat the second CO2-rich absorbent stream (115, 215a) from the stripper using the synthesis gas (104a, 204a) to separate the second CO2-rich absorbent stream into an absorbent-depleted CO2 stream (107a, 207a) and a CO2-depleted absorbent stream (108b, 208b), and arranged to return the absorbent-depleted CO2 stream and the CO2-depleted absorbent stream to the stripper; a second heat exchanger (105, 205, 305) arranged to heat the second CO2-rich absorbent from the stripper using steam (106a, 206a) to separate the CO2-rich absorbent into an absorbent-depleted CO2 stream (107b, 207b) and a CO2-depleted absorbent stream (108b, 208b), and arranged to return the absorbent-depleted CO2 stream and the CO2-depleted absorbent stream to the stripper; a third heat exchanger (110, 210, 310) positioned to receive the lean absorbent stream (109, 209, 309) from the stripper using water (111, 211, 311) to produce a cooled lean absorbent stream (113, 213, 313), and positioned to return the cooled lean absorbent stream to the absorber; The first heat exchanger and the second heat exchanger are devices arranged in series or in parallel.

[0014] In a third aspect, the present invention relates to a chemical plant for producing decarbonized syngas, H2, synthetic liquid fuels or ammonia, the chemical plant comprising an apparatus according to the second aspect. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a flow diagram illustrating an arrangement in which first and second heat exchangers are arranged in series. Syngas and absorbent are contacted in an absorber (101) to produce a CO2-depleted syngas and a first CO2-rich absorbent stream (114). The first CO2-rich absorbent stream is sent from the absorber (101) to a stripper (102). A second CO2-rich absorbent stream (115) is withdrawn from the stripper and fed to a first heat exchanger (103). The heating duty for the first heat exchanger is provided by syngas (104a) being cooled to produce cooled syngas (104b). Heat exchange between the synthesis gas (104a) and the second CO2-rich absorbent stream (115) separates the first stream into an absorbent-depleted CO2 stream (107a) (typically a mixture of CO2 and steam) that is returned to the stripper, and a CO2-depleted absorbent stream (108a) that is sent to the second heat exchanger (105). The heating duty for the second heat exchanger is provided by steam (106a), which is cooled to produce a steam condensate (106b). Heat exchange between the steam (106a) and the CO2-depleted absorbent stream (116) further separates the CO2-depleted absorbent into an absorbent-depleted CO2 stream (107b) and a CO2-depleted absorbent stream (108b), which are returned to the stripper. A lean absorbent stream (109) is withdrawn from the stripper and sent to a third heat exchanger (110) where it is cooled using water (111) to produce hot water (112), and the cooled lean absorbent (113) is returned to the absorber. [Figure 2]1 is a flow diagram illustrating an arrangement in which first and second heat exchangers are arranged in parallel. A first CO2-rich absorbent stream (214) is sent from the absorber (201) to the stripper (202). A second CO2-rich absorbent stream (215a) is withdrawn from the stripper and fed to the first heat exchanger (203). The heating duty for the first heat exchanger is provided by the syngas (204a), which is cooled to produce cooled syngas (204b). Heat exchange between the syngas (204a) and the second CO2-rich absorbent stream (215a) separates the second CO2-rich absorbent stream into an absorbent-depleted CO2 stream (207a) and a CO2-depleted absorbent stream (208a), which are returned to the stripper. The second CO2-rich absorbent stream (215b) is withdrawn from the stripper and fed to the second heat exchanger (205). The heating duty for the second heat exchanger is provided by steam (206a), which is cooled to produce steam condensate (206b). Heat exchange between the steam (206a) and a second CO2-rich absorbent stream (215b) further separates the CO2-rich absorbent into an absorbent-depleted CO2 stream (207b) and a CO2-depleted absorbent stream (208b), which are returned to the stripper. A lean absorbent stream (209) is withdrawn from the stripper and sent to a third heat exchanger (210), where it is cooled using water (211) to produce hot water (212), and the cooled lean absorbent (213) is returned to the absorber. [Figure 3]1 is a flow diagram illustrating an arrangement in which a flash unit is present. A first CO2-rich absorbent stream (314) is sent from the absorber (301) to a flash unit (316). The flash unit separates a gaseous CO2 overhead stream (317) from a semi-lean absorbent stream (318) sent to the stripper (302). A CO2-rich absorbent (315) is withdrawn from the stripper and fed to first and second heat exchangers (303 / 305), which may be arranged in series or parallel (details not shown). In some embodiments, a gaseous CO2-rich overhead stream (319) from the stripper may be returned to the flash unit. A lean absorbent stream (309) is withdrawn from the stripper and sent to a third heat exchanger (310), where it is cooled using water (311) to produce hot water (312), and the cooled lean absorbent (313) is returned to the absorber.

[0016] In a preferred embodiment, the cooled lean absorbent (113, 213, 313) is further cooled by air cooling in a fifth heat exchanger before being returned to the absorber. DETAILED DESCRIPTION OF THE INVENTION

[0017] Any subheadings are included for convenience only and are not intended to limit the disclosure in any way.

[0018] Any aspect described with respect to a process is equally applicable to an apparatus, and vice versa.

[0019] absorber The role of the absorber is to remove CO2 present in the syngas by contacting the syngas with a liquid absorbent. The syngas fed to the absorber contains H2, CO, CO2, and H2O, and may contain unreacted hydrocarbons such as methane. CO2 removal by the absorbent can be achieved by chemical and / or physical absorption, and the term "absorbent" does not imply that the absorption is purely chemical adsorption. While a wide variety of absorbents can be used in the present invention, amine absorbents are particularly preferred. The absorbent may be a completely recycled absorbent (from the cooled lean absorbent stream), but is typically a mixture of recycled absorbent and fresh make-up absorbent.

[0020] The absorber produces a CO2-depleted syngas that can be sent for further processing (e.g., purification and / or compression) and a first CO2-rich absorbent stream that is sent for regeneration by the stripper, or by the flash unit and the stripper if a flash unit is present.

[0021] The CO2-depleted syngas can be used downstream to produce H2, synthetic liquid fuels, ammonia, or other chemicals.

[0022] flash unit The apparatus may include a flash unit located between the absorber and the stripper, the flash unit configured to separate a first CO2-rich absorbent stream from the absorber into a CO2 overhead stream and a semi-lean absorbent stream by a change in pressure, and configured to pass the semi-lean absorbent stream to the stripper, where it is separated into a gaseous CO2-rich overhead stream and a lean absorbent stream.

[0023] In some embodiments, the apparatus is configured to split the semi-lean absorbent stream from the flash unit into a first portion sent to the stripper and a second portion sent to the absorber, which can be beneficial because it reduces overall reboil energy requirements, reduces absorbent degradation, and provides flexibility when syngas operating conditions vary.

[0024] In a preferred embodiment, the semi-lean absorbent stream is heated by heat exchange with the lean absorbent stream from the stripper before entering the stripper, an arrangement that allows for more efficient heat utilization.

[0025] Stripper The role of the stripper is to separate the first CO2-rich absorbent stream, or the semi-lean absorbent stream if a flash unit is present, into a CO2-rich overhead stream and a lean absorbent stream. This is accomplished through the use of a first heat exchanger and a second heat exchanger. The first and second heat exchangers can be arranged in series or in parallel.

[0026] One or more second CO2-rich absorbent streams are removed from the stripper. The term "second" is used simply to distinguish this stream from the first CO2-rich absorbent stream described above in connection with the absorber. Typically, one second CO2-rich absorbent stream is removed when the heat exchangers are arranged in series, while two second CO2-rich absorbent streams are removed when the heat exchangers are arranged in parallel.

[0027] In a series configuration, the second CO2-rich absorbent stream is removed from the stripper to a first heat exchanger, where it is heated using syngas to separate it into an absorbent-depleted CO2 stream and a CO2-depleted absorbent stream. The absorbent-depleted CO2 stream is typically a mixture of CO2 and steam containing small or negligible amounts of absorbent and is returned to the stripper. The CO2-depleted absorbent stream is sent to a second heat exchanger, where it is heated using steam to generate a further absorbent-depleted CO2 stream and a further CO2-depleted absorbent stream, which are returned to the stripper. An exemplary configuration is shown in Figure 1. In a series configuration, it is also possible to place the second heat exchanger (using steam) first, followed by the first heat exchanger (using syngas). If the steam and syngas are at different temperatures, the heat exchanger with the lower temperature heat exchange medium should be placed first, followed by the heat exchanger using the higher temperature heat exchange medium.

[0028] In a parallel configuration, two second CO2-rich absorbent streams are removed from the stripper. The second CO2-rich absorbent stream is heated in a first heat exchanger using synthesis gas to separate it into an absorbent-depleted CO2 stream and a CO2-depleted absorbent stream, which are returned to the stripper. A similar process occurs in a second heat exchanger using steam. An exemplary configuration is shown in Figure 2.

[0029] Although referred to herein as the "absorbent-depleted CO2 stream," this stream typically contains a significant amount of steam, and steam may be the major component of this stream. The separation of the second CO2-rich absorbent stream into an absorbent-depleted CO2 stream and a CO2-depleted absorbent stream may also be referred to as "reboiling," with the syngas and steam together providing the reboil duty.

[0030] The first and second heat exchangers provide heat loads for regenerating the CO2-rich absorbent using syngas and steam, respectively. The relative proportions of heat load provided by the first and second heat exchangers may be varied over time. For example, during start-up, the second heat exchanger may provide the majority of the heat load, and during steady state, the majority of the heat load may be provided by the first heat exchanger.

[0031] During steady-state operation, the synthesis gas provides the majority of the heating load for the stripper, so if the first and second heat exchangers are arranged in series, the order is preferably the first heat exchanger (heated by synthesis gas) followed by the second heat exchanger (heated by steam). This arrangement allows the amount of steam to be controlled to meet the remaining reboil load.

[0032] The first and second heat exchangers may be located inside or outside the stripper, and the term "removing the CO2-rich absorbent from the stripper" should be understood accordingly. It is preferred that the heat exchangers be located outside the stripper, as this arrangement is less complex and more reliable.

[0033] The temperatures of the syngas and steam supplied to the first and second heat exchangers must be high enough to cause desorption of CO from the CO2-rich absorbent. Typically, the syngas supplied to the first heat exchanger is at least 100°C, e.g., 120-200°C, preferably 130-170°C. Typically, the steam supplied to the first heat exchanger is at least 100°C, e.g., 120-200°C, preferably 120-130°C.

[0034] A CO2-rich overhead stream is removed from the top of the stripper. These overheads may be sent for further downstream processing, such as CO2 compression and dehydration, optionally followed by CO2 transport and storage. If a flash unit is present, the CO2-rich overhead stream is preferably returned to the flash unit.

[0035] Return to absorber A lean absorbent stream is removed from the stripper, typically from the bottom of the stripper. Because absorption in the absorber is more efficient when the absorbent is cold, this lean absorbent stream needs to be cooled before being returned to the absorber. Therefore, the lean absorbent stream is cooled in a third heat exchanger using water. The water supplied to the third heat exchanger is at a lower temperature than the lean absorbent stream, typically about 20°C (e.g., 10-35°C), and is heated to about 80°C (e.g., 70-90°C) by heat exchange with the lean absorbent stream. The water is preferably demineralized water. The products from the third heat exchanger are a cooled lean absorbent stream and a hot water stream.

[0036] The use of water to cool the lean absorbent stream reduces the load on the cooling unit for the absorber recycle. Using low-grade heat from the lean absorbent stream to generate a hot water stream improves overall plant efficiency. The hot water stream can then be used to generate steam, which can be used as a reactant for steam reforming and / or water-gas shift reactions elsewhere in the plant, for example, in a steam methane reformer (SMR), autothermal reformer (ATR), or water-gas shift unit, or can be exported, for example, as a "steam export." Otherwise, steam must be generated using high-quality heat from elsewhere.

[0037] In some embodiments, the apparatus includes a fifth heat exchanger arranged to further cool the cooled lean absorbent stream from the third heat exchanger by air cooling. This is preferred to cool the absorbent as much as possible before it is returned to the absorber. An additional advantage of using water to cool the lean absorbent stream in the fifth heat exchanger is that it may be possible to use a smaller air cooler. This reduces operating costs compared to arrangements in which cooling of the lean absorbent is achieved entirely by air cooling.

[0038] In some embodiments, when a flash unit is present, the apparatus includes a fourth heat exchanger positioned to receive lean absorbent from the stripper and semi-lean absorbent from the flash unit, where the lean absorbent from the stripper is cooled by heat exchange with the semi-lean absorbent from the flash unit. [Example]

[0039] Simulations were performed with the configuration shown in Figure 2. The lean absorbent stream (209) removed from the stripper had very low levels of residual CO. Cooling of the lean absorbent stream in heat exchanger (210) generated a hot water stream (212) at a temperature of 79°C that could be used for steam export.

[0040] [Table 1]

[0041] [Table 2]

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