Technical Field

The invention relates to a carbon dioxide removal unit for the removal of carbon dioxide from a synthesis gas stream.

Background

In many processes it is necessary to remove acid gases, such as CO2, from a product gas. This is commonly achieved by treating the sour gas, containing the unwanted acid gas, with an absorbent such as an amine in an absorber. The absorbent chemically and/or physically removes the acid gas from the sour gas to produce a CO2-rich absorbent and sweet gas (CO2-lean gas). The absorbent is normally regenerated so it can be reused in the absorber. The regeneration also releases CO2 which is usually compressed/dehydrated and either transported for storage in underground locations or used for chemicals manufacturing.

W02010/120527A2 (Alstom Technology Ltd) describes a process for removing gaseous contaminants from a gas stream comprising: contacting the gas stream with a wash solution to remove gaseous contaminants from the gas stream by absorption into the wash solution to produce used wash solution; regenerating the used wash solution to remove gaseous contaminants from the used wash solution, to provide a regenerated wash solution and a gas comprising removed contaminants. In a first regeneration stage the gas comprising removed contaminants is cooled to reduce the loss of water vapor. In a preferred embodiment the regenerated wash solution is reboiled to further remove gaseous contaminants and provide a reboiled wash solution. The reboiled wash solution may be used for heat-exchange with regenerated wash solution and with used wash solution.

WO2016/123386A1 (Fluor Technologies Corporation) describes a process for carbon dioxide capture from a gas mixture, comprising an absorber, a stripper and a reboiler. The absorbent is a solvent system comprising a chemical solvent and a nonaqueous physical solvent. The stripper receives a COz-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 described in Figure 1 of this reference, steam is used for the reboiler duty

WO2017/029145A1 (Casale SA) describes a CO2 removal section for removing CO2 from a hydrogen-containing synthesis gas, comprising an absorption section and a stripper. The stripper comprises an upper zone where the COz-rich solution from the absorber is flashed to produce a first gaseous stream containing COz and a partially regenerated semi-lean solution, and a lower zone acting as a stripping zone where the semi-lean solution is stripped producing a second gaseous stream containing COz and a lean regenerated solution. The CO2 removal section comprises sealing means for isolating the first and second gaseous streams so that they can be exported separately from the stripper. A method of revamping a CO2 removal unit by introducing the sealing means is also described.

W02022/008876A1 (Carbon Clean Solutions Limited) describes a method for regenerating a solvent comprising carbon dioxide by passing the solvent comprising carbon dioxide 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 from 60 °C to less than 120 °C.

There is a need for further improvements in the energy efficiency of carbon dioxide removal units

Summary of the invention

The present inventors describe herein a CO2 removal unit which makes more efficient use of heat streams within the process. The process involves absorption of CO2 from a synthesis gas in an absorber to generate a CO2-rich absorbent. The CO2-rich absorbent is treated in a stripper to remove entrained CO2 and produce a lean absorbent and CO2-rich overheads.

A first important feature of the process is that the stripper is able to make use ofwaste heat from both synthesis gas and steam. In particular, CO2-rich absorbent is withdrawn from the stripper and subjected to heat exchange in a first heat exchanger using synthesis gas and in a second heat exchanger using steam. The heat exchangers may be arranged in series or in parallel The steam and synthesis gas together provide the reboiling duty for the stripper, which offers several advantages. The ability to provide the reboiling duty from steam means that the CO2 removal unit can function even if a synthesis gas stream is not available to provide the full reboiling duty, which is often the case in plants on start-up. This reduces CO2 emissions during start-up of the plant. Once the portion of the plant which produces synthesis gas is properly running the CO2 removal unit can switch so that the majority of the heating duty for the stripper is provided by synthesis gas.

A second important feature of the process is that lean absorbent from the stripper is used as a source of low grade heat and is treated by heat exchange to produce hot water and cooled lean absorbent. The hot water is typically produced at around 80 °C, and can be used in steam raising equipment. The steam thus raised, may be used as “process steam” e.g. forthe reforming and/or water-gas shift reactions, or can be used elsewhere on the plant (“steam export”). The lean absorbent stream therefore provides a significant amount of the heating duty for steam, which would otherwise have to be produced using high grade heat elsewhere in the plant. This arrangement therefore reduces overall energy consumption and associated costs.

In a first aspect the invention relates to a process for the removal of carbon dioxide from a synthesis gas, comprising the steps of: contacting a synthesis gas and an absorbent in an absorber (101 , 201 , 301) to produce a CC>2-depleted synthesis gas and a first CC>2-rich absorbent stream (114, 214, 314); withdrawing the first COz-rich absorbent stream from the absorber and transferring the first CO2-rich absorbent stream to a stripper (102, 202, 302); withdrawing one or more second COa-rich absorbent stream(s) (115, 215a, 215b) from the stripper and heating the one or more second CO2-rich absorbent stream(s) to separate them into one or more absorbent-depleted CO2 stream(s) (107a, 107b, 207a, 207b) and one or more CO2-depleted absorbent stream(s) (108, 208a, 208b) 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); returning the one or more absorbent-depleted CO2 stream(s) and one or more CO2- depleted absorbent stream(s) to the stripper; withdrawing a gaseous CO2-rich overheads stream from the stripper; and withdrawing 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; wherein the first heat exchanger and second heat exchanger are arranged in series or in parallel.

The process is particularly suitable for integration into a process for producing decarbonised synthesis gas (i.e. a mixture of hydrogen and carbon monoxide), Hz, synthetic liquid fuels or ammonia.

In a second aspect the invention relates to an apparatus for the removal of carbon dioxide from a synthesis gas, comprising: an absorber (101 , 201 , 301) arranged to contact a synthesis gas with an absorbent to produce a CO2-depleted synthesis gas and a first CO2-rich absorbent stream (114, 214, 314); a stripper (102, 202, 304) arranged to receive the first CC -rich absorbent stream from the absorber; a first heat exchanger (103, 203, 303) arranged to heat a second COz-rich absorbent stream (115, 215a) from the stripper using synthesis gas (104a, 204a) in order 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 CC>2-depleted absorbent stream to the stripper; a second heat exchanger (105, 205, 305) arranged to heat a second CO2-rich absorbent from the stripper using steam (106a, 206a) in order to separate the COz-rich absorbent into an absorbent-depleted CO2 stream (107b, 207b) and a COz-depleted absorbent stream (108b, 208b), and arranged to return the absorbent-depleted CO2 stream and the CC>2-depleted absorbent stream to the stripper; a third heat exchanger (110, 210, 310) arranged to receive a lean absorbent stream (109, 209, 309) from the stripper and to cool the lean absorbent stream using water (11 1 , 211 , 311) to produce a cooled lean absorbent stream (113, 213, 313), and arranged to return the cooled lean absorbent stream to the absorber; wherein the first heat exchanger and second heat exchanger are arranged in series or in parallel.

In a third aspect the invention relates to a chemical plant for the production of decarbonised synthesis gas, H2, synthetic liquid fuels or ammonia, comprising an apparatus according to the second aspect.

Description of the Figures

Figure 1 is a flow diagram showing an arrangement in which the first and second heat exchangers are arranged in series. A synthesis gas and an absorbent are contacted in an absorber (101) to produce a CO2-depleted synthesis gas and a first COz-rich absorbent stream (114). The first COz-rich absorbent stream is sent from the absorber (101) to the 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 synthesis gas (104a) which is cooled to produce cooled synthesis gas (104b). Heat exchange between the synthesis gas (104a) and the second COz-rich absorbent stream (115) separates the first stream into an absorbent- depleted CO2 stream (107a) (which is typically a mixture of CO2 and steam) which is returned to the stripper, and a COz-depleted absorbent stream (108a) which 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 steam condensate (106b). Heat exchange between steam (106a) and CO2-depleted absorbent stream (116) further separates the COz-depleted absorbent into an absorbent-depleted CO2 stream (107b) and a COz-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 a cooled lean absorbent (113) is returned to the absorber. Figure 2 is a flow diagram showing an arrangement in which the first and second heat exchangers are arranged in parallel. A first COz-rich absorbent stream (214) is sent from the absorber (201) to the stripper (202). A second COz-rich absorbent stream (215a) is withdrawn from the stripper and fed to a first heat exchanger (203). The heating duty for the first heat exchanger is provided by synthesis gas (204a), which is cooled to produce cooled synthesis gas (204b). Heat exchange between the synthesis gas (204a) and the second COz-rich absorbent stream (215a) separates the second COz-rich absorbent stream into an absorbent-depleted COz stream (207a) and a COz-depleted absorbent stream (208a) which are returned to the stripper. A second COz-rich absorbent stream (215b) is withdrawn from the stripper and fed to a 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 steam (206a) and the second COz-rich absorbent stream (215b) further separates the COz-rich absorbent into an absorbent-depleted COz stream (207b) and a COz-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 a cooled lean absorbent (213) is returned to the absorber.

Figure 3 is a flow diagram showing an arrangement in which a flash unit is present. A first COz- rich absorbent stream (314) is sent from the absorber (301) to the flash unit (316). The flash unit separates out a gaseous CO2 overheads stream (317) and a semi-lean absorbent stream (318) which is sent to the stripper (302). COz-rich absorbent (315) is withdrawn from the stripper and fed to the first and second heat exchangers (303/305) which may be arranged in series or parallel (detail not shown). In some embodiments a gaseous COz-rich overheads 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 a cooled lean absorbent (313) is returned to the absorber.

In preferred embodiments 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

Any sub-headings are included for convenience only and are not intended to limit the disclosure in any way.

Any aspect being described in connection with the process is equally applicable to the apparatus and vice versa.

Absorber The role of the absorber is the remove CO2 present in a synthesis gas by contacting the synthesis gas with a liquid absorbent. The synthesis gas fed to the absorber comprises H2, CO, CO2, H2O and may contain unreacted hydrocarbons such as methane. The removal of CO2 by the absorbent may be achieved by chemical and/or physical absorption and the term “absorbent" is not intended to imply that the absorption is purely chemisorption. A wide variety of absorbents may be used in the present invention, but amine absorbents are particularly preferred. The absorbent may be entirely recycled absorbent (from the cooled lean absorbent stream) but is typically a mixture of recycled absorbent and fresh make-up absorbent.

The absorber produces a COz-depleted synthesis gas which can be sent for further treatment (e.g. purification and/or compression) and a first COz-rich absorbent stream. The first CO2-rich absorbent stream is sent for regeneration by the stripper, or where a flash unit is present, by the flash unit and stripper.

The CO2-depleted synthesis gas may be used downstream to produce H2, synthetic liquid fuels, ammonia or other chemicals.

Flash unit

The apparatus may include a flash unit located between the absorber and the stripper. The flash unit is arranged to separate the first CCk-rich absorbent stream from the absorber into a CO2 overheads stream and a semi-lean absorbent stream by pressure change, and is arranged to send the semi-lean absorbent stream to the stripper where it is separated into a gaseous CO2- rich overheads stream and a lean absorbent stream.

In some embodiments the apparatus is arranged to split the semi-lean absorbent stream from the flash unit into a first portion which is sent to the stripper and a second portion which is sent to the absorber. This can be beneficial because it reduces the overall reboiling energy requirement, reduces absorbent degradation, and offers flexibility in case of variation in syngas operating conditions.

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. This arrangement allows for more efficient heat usage.

Stripper The role of the stripper is to separate the first COz-rich absorbent stream, or the semi-lean absorbent stream in the case where the flash unit is present, into a COz-rich overheads stream and a lean absorbent stream This is achieved by the use of a first heat exchanger and a second heat exchanger. The first and second heat exchangers may be arranged in series or in parallel.

One or more second COz-rich absorbent stream(s) are withdrawn from the stripper. The term “second” is simply to differentiate this stream from the first COz-rich absorbent stream described previously in connection with the absorber. Typically, one second COz-rich absorbent stream is withdrawn when the heat exchangers are arranged in series, while two second COz-rich absorbent streams are withdrawn when the heat exchangers are arranged in parallel.

In a series arrangement a second COz-rich absorbent stream is withdrawn from the stripper to the first heat exchanger where it is heated using synthesis gas, in order to separate it into an absorbent-depleted CO2 stream and a COz-depleted absorbent stream. The absorbent-depleted CO2 stream, which is typically a mixture of CO2 and steam containing low or negligible amounts of absorbent, is returned to the stripper. The COz-depleted absorbent stream is sent to the second heat exchanger where it is heated using steam, in order to generate a further absorbent- depleted CO2 stream and a further COz-depleted absorbent stream, which are returned to the stripper. An exemplary arrangement is shown in Figure 1. In a series arrangement it is also possible for the second heat exchanger (using steam) to be arranged first, followed by the first heat exchanger (using synthesis gas). If the steam and syngas are at different temperatures then the heat exchanger with the cooler heat exchange medium should be arranged first, followed by the heat exchanger using the hotter heat exchange medium.

In a parallel arrangement two second COz-rich absorbent streams are withdrawn from the stripper. The second COz-rich absorbent stream is heated in the first heat exchanger using synthesis gas, in order to separate it into an absorbent-depleted COz stream and a COz-depleted absorbent stream, which are returned to the stripper. An analogous process occurs in the second heat exchanger using steam. An exemplary arrangement is shown in Figure 2.

Although referred to herein as an “absorbent-depleted CO2 stream” this stream will typically contain significant amounts of steam, and steam may be the major component of this stream The separation of the second COz-rich absorbent stream(s) into absorbent-depleted CO2 stream(s) and COz-depleted absorbent stream(s) may also be referred to as “reboiling”, with syngas and steam together providing the reboiling duty.

The first and second heat exchangers provide the heat duty for regenerating the COz-rich absorbent using synthesis gas and steam respectively. The relative proportion of heat duty provided by the first and second heat exchangers may be changed over time. For example, at start-up the second heat exchanger may provide most of the heat duty while at steady state most of the heat duty is provided by the first heat exchanger.

During steady operation syngas provides the majority of the heating duty for the stripper, therefore is preferred that if the first and second heat exchangers are arranged in series then the order is: first heat exchanger (heated by syngas) followed by second heat exchanger (heated by steam) This arrangement allows the amount of steam to be controlled to meet the remaining reboiling duty.

The first and second heat exchangers may be located within or outside of the stripper and the term “withdrawing COz-rich absorbent from the stripper” should be understood accordingly. It is preferred that the heat exchangers are located outside of the stripper because this arrangement is less complex and more reliable.

The temperature of the synthesis gas and steam fed to the first and second heat exchangers should be high enough to cause desorption of the CO2 from the COz-rich absorbent. Typically the synthesis gas fed to the first heat exchanger is at least 100 °C, such as 120 to 200 °C, preferably 130 to 170 °C. Typically the steam fed to the first heat exchanger is at least 100 °C, such as 120 to 200 °C, preferably 120 to 130 °C

A COz-rich overhead stream is removed from the top of the stripper. These overheads may be sent for further processing downstream, e.g. CO2 compression and dehydration optionally followed by CO2 transport and storage. Where a flash unit is present the CCh-rich overhead stream is preferably returned to the flash unit.

Return to absorber

A lean absorbent stream is withdrawn from the stripper, typically from the bottom of the stripper. This lean absorbent stream needs to be cooled before being returned to the absorber because absorption in the absorber is more efficient when the absorbent is cool. The lean absorbent stream is therefore cooled in a third heat exchanger using water. The water provided to the third heat exchanger is at a temperature less than that of the lean absorbent stream, and is typically provided at around 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 demineralised water. The products from the third heat exchanger are a cooled lean absorbent stream and a hot water stream

The use of water to cool the lean absorbent stream reduces the duty on cooling unit for the absorber recycle. The use of low grade heat from the lean absorbent stream to produce a hot water stream improves the overall plant efficiency. The hot water stream can subsequently be used to generate steam, which may be used as reactant for steam reforming and/or water-gas shift reactions elsewhere on the plant, e g. in a steam methane reformer (SMR), autothermal reformer (ATR) or water-gas shift unit, or may be exported e.g. “steam export”. Steam would otherwise have to be produced using high grade heat from elsewhere.

In some embodiments the apparatus comprises a fifth heat exchanger arranged to further cool the cooled lean absorbent stream from the third heat exchanger by air cooling. This is preferable in order to cool the absorbent as much as possible before it is returned to the absorber. A further benefit of cooling the lean absorbent stream in the fifth heat exchanger using water is that may allow for a smaller air cooler to be used. This reduces operating costs compared to an arrangement where cooling of the lean absorbent is achieved entirely by air cooling.

In some embodiments, where a flash unit is present, the apparatus comprises a fourth heat exchanger arranged to receive lean absorbent from the stripper and semi-lean absorbent from the flash unit. Lean absorbent from the stripper is cooled by heat exchange with semi-lean absorbent from the flash unit.

Example

A simulation was carried out on the arrangement shown in Figure 2. The lean absorbent stream (209) withdrawn from the stripper had very low levels of residual CO2. Cooling of the lean absorbent stream in heat exchanger (210) generated a hot water stream (212) at temperature of 79 °C which can be used for steam export.