Patent Application
20250153093
The present patent application claims priority from Australian provisional patent application No. 2022900421 filed on 23 Feb. 2022, the contents of which should be understood to be incorporated into this specification by this reference.
The present invention relates to a vapour compression system that supplies the thermal energy requirement for a regeneration of a capture medium rich in a captured target gas. The invention is particularly applicable to an acid gas capture/removal process, for example carbon dioxide (CO2) capture, and it will be convenient to hereinafter disclose the invention in relation to that exemplary application. However, it is to be appreciated that the invention is not limited to that application and could be used in a variety of target gas capture and/or removal processes.
The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
The leading technology for CO2-capture uses a liquid absorbent, for example, an amine solution, that will absorb CO2 at low temperature and release CO2 at high temperature as a result of heat input. Conventionally, the heat requirement of the regeneration process will be met by steam that is for example extracted from a steam cycle in case of CO2-capture from power stations or provided by a separate boiler or heat available from another source.
A number of other prior CO2 capture processes use compression of the vapour stream from the desorber with subsequent heat recovery to lower the thermal energy requirement for CO2 desorption. A number of these process modifications incorporate compression of vapours from the loaded (rich) solutions or regenerated (lean) absorbent solutions, leading to a significant reduction in the heat requirement for the regeneration process.
WO2011/073672A1 describes one example of a system for the regeneration of a capture medium rich in a captured target gas that supplies the thermal energy required to heat the gas rich capture medium to dissociate into a gas rich vapour phase and a lean absorbent solution using a combination of thermal energy from vapour compression and a heat pump that recovers thermal energy from one or more sources of low-grade heat from several points of the capture process. Whilst vapour compression can be used to provide part of the thermal energy requirements for heating the rich absorbent solution in the reboiler, the process in WO2011/073672A1 requires the use of additional energy input to fully meet energy requirements of the reboiler.
Another example is described in United States patent publication No. 20160001223A1 which teaches a carbon dioxide recovery system that includes a regenerator arrangement which includes a compression driven energy recycling system connected to the top outlet of a regenerator. The compression energy recycle system improves the heat recovery efficiency by directly compressing the recovery gas discharged from the regenerator without subjecting the gas to condensation and separation of water vapour by the cooling process. However, the compression driven energy recycling system is not configured to supply all the energy requirements for the reboiler function at the base of the regenerator. Additional energy must be supplied to the reboiler by a steam heater/reboiler.
Thus, whilst previous process modifications, such as detailed in WO2011/073672A1 and US20160001223A1, show some energy savings through vapour compression and waste energy harvesting in the process, none of these processes provide a system that makes effective use of possible energy that can be derived from a vapour compression system. Ineffective use of that energy source requires these systems to source additional thermal energy from process lines or systems remote to the desorption process and equipment to complete the energy balance about the desorber.
It would therefore be desirable to provide a new or improved process for the regeneration of liquid absorbents that more effectively recovers heat from a vapour compression system for use in the regeneration of liquid absorbents.
A first aspect of the present invention provides a process of regeneration of a capture medium rich in a captured target gas comprising the steps of:
The present invention therefore provides a process/system for regeneration of capture mediums, such as liquid absorbents, used to capture a target as such as CO2, which includes more effective heat recovery from its vapour compression process. That heat recovery is utilised in the heating step of the process to facilitate the separation of the absorbed target gas from the capture medium into a gas containing vapour phase and a heated lean capture medium. As will be explained in more detail, depending on the process embodiment, that heat may be used to preheat the target gas rich capture medium and/or may be used to heat the capture medium in a desorber (for example using a reboiler) through which the target gas rich capture medium is heated via heat transfer within that desorber.
The vapour compression process of the present invention is based on a high vapour content of the gas stream leaving the absorption liquid regeneration process. This is counter to conventional regeneration process designs which typically endeavour to minimise the vapour content as it presents a loss of energy. Such prior energy efficient liquid regeneration process concepts are based on the minimisation of the water content of the wet CO2-product leaving the CO2-desorber. These regeneration process designs fail to recognise the full benefits of the vapour compression process with a high vapour content.
Nevertheless, it should be appreciated that the process of the present invention preferably does not have significantly high vapour content. At high steam fractions the total amount of gas/vapour becomes large. This leads to a high compression energy per amount of CO2 captured, but also a high amount of recoverable energy. The amount of recovered energy preferably should be such that it is sufficient to provide the thermal energy for the regeneration but no more than that. There is therefore a sweet spot in the range of steam fractions that can be used. It should be appreciated that the steam fraction is the fraction of water vapour (steam) in the overall vapour/gas mixture.
Therefore, in the present invention, the steam fraction in the exit stream of target gas separator needs to be high enough to enable significant recovery of latent heat in addition to recovery of the sensible heat. The inventors have found that this requires a steam fraction of at least 0.8, preferably above 0.85, and more preferably above 0.9. The Inventors are not aware of any prior art arrangement that includes these unique and advantageous process conditions that utilise this particular steam fraction range in the exit stream of a target gas separator to provide more effective heat recovery for the vapour compression step (vapour compression energy recovery system). In embodiments, the gas containing vapour phase comprises the target gas and water vapour having a steam fraction of between 0.8 to 0.999. In embodiments, the gas containing vapour phase comprises the target gas and water vapour having a steam fraction of between 0.8 to 0.999, and more preferably a steam fraction of between 0.8 to 0.99, and yet more preferably between 0.9 and 0.99. In some embodiments, the target gas and water vapour have a steam fraction of between 0.9 to 0.999, and preferably between 0.95 to 0.999. In some embodiments, the target gas and water vapour have a steam fraction of between 0.8 to 0.995, preferably between 0.9 to 0.995. In embodiments, the gas containing vapour phase comprises the target gas and water vapour having a steam fraction of at least 0.9.
The required steam content of the gas containing vapour phase can be expressed as a steam to target gas ratio. In this context, the gas containing vapour phase comprising the target gas and water vapour has a steam/target gas ratio of preferably at least 4, more preferably at least 8, and yet more preferably at least 10. In embodiments, the steam/target gas ratio is preferably from 4 to 1000.
The vapour compression component of the process of the present invention relies on a high vapour content in the exhaust vapour stream which is subsequently compressed with heat recovery into the regeneration process that enables more effective heat recovery from that compressed vapour. It should be appreciated that external energy input is still required in the process/system of the present invention run the vapour compression system. Here, electricity can be used to run the compressors therein. The process therefore enables the use of electricity to run the thermal absorption liquid regeneration in a liquid absorbent-based CO2-capture process. This is particularly useful for direct air capture that can then be completely operated on renewable energy. This advantageously avoids the use of heat for liquid absorbent regeneration which might not be available in the process, an adjoining process, or the surrounding environment/industry.
The absorbed target gas can be separated from the target gas capture medium using any suitable process and process vessel. In embodiments, the step of separation of the absorbed target gas from the capture medium into a gas containing vapour phase occurs in one of: a flash vessel (or flash vaporisation vessel); or a desorption vessel. In this step, that separation or desorption process can therefore comprises one of: flash vaporisation, vaporisation, boiling, stripping, or a combination thereof.
The thermal balance of the absorption fluid regeneration/desorption process can be improved through heat exchange between various higher temperature process flows and lower temperature process flows in the system. In most absorption fluid regeneration systems, the thermal energy of the heated lean capture medium is used as a source of thermal energy to heat a lower temperature process flow/line, for example the target gas rich capture medium. In embodiments, the compressed vapour is used as a first source of thermal energy to provide energy required to heat the target gas rich capture medium, and the heated lean capture medium is used as a second source of thermal energy to provide energy required to heat the target gas rich capture medium. In this embodiment, the thermal energy required for the regeneration process is provided by thermal energy within the desorption process, and more particularly the vapour compression process. The first source of thermal energy and the second source of thermal energy preferably provides substantially all of the thermal energy required to heat the target gas rich capture medium. In this respect, the thermal energy required to heat the target gas rich capture medium is preferably provided substantially by, and in some embodiments consists solely of, the first source of thermal energy in combination with the second source of thermal energy.
The use of these different types of desorption processes and/or process vessels can lead to variations of the process of the present invention tailored to that process vessel.
For examples, in embodiments where the process includes a flash vaporisation vessel, the process of regeneration of a capture medium rich in a captured target gas may comprise the steps of:
In this embodiment, the process typically also includes the step of:
In some embodiments, the desorption vessel includes a reboiler, and the compressed vapour is used to heat the target gas rich capture medium in the reboiler. Here the compressed vapour is used to heat the capture medium in a desorber using the reboiler through which the target gas rich capture medium is heated via heat transfer within that desorber. The gas containing vapour phase could be used as the heat transfer medium/thermal working fluid within the reboiler to heat the capture medium therein (direct heat exchange) or could be used indirectly, where the gas containing vapour phase is used to heat another heat transfer medium/thermal working fluid that is then used within the reboiler to heat the capture medium therein (indirect heat exchange).
For example, in embodiments where direct heat exchange with the gas containing vapour phase is intended, the process includes a desorber vessel/column having a reboiler which is used to heat the capture medium, typically lean capture medium at the bottom of the desorber vessel/column, the process of regeneration of a capture medium rich in a captured target gas may comprise the steps of:
In this embodiment, the process typically also includes the step of:
It should be appreciated that the compressing step can be directly applied to the gas containing vapour phase formed through separation of the absorbed target gas from the capture medium into a gas containing vapour phase, such as described above, and/or could be used to compress a vapour phase thermally associated with the gas containing vapour phase, to form a compressed vapour. In most processes, the compressing step is applied directly to the gas containing vapour phase or to a vapour phase thermally associated with the gas containing vapour phase. However, it should also be appreciated that it is possible for the process to include two parallel compression steps where both the gas containing vapour phase or to a vapour phase thermally associated with the gas containing vapour phase undergo the compressing step.
For indirect heat exchange, thermal working fluid is preferably heated by the gas containing vapour to produce the vapour phase thermally associated with the gas containing vapour phase. The working fluid is preferably selected as a medium capable of undergoing a phase change, i.e. it vaporises upon heat transfer. Then it can be it can be compressed with heat recovery via condensation. In these embodiments, the process further includes the step of heating a heat transfer fluid using the gas containing vapour phase to produce said a heated heat transfer fluid. Any suitable thermal working fluid can be used. In preferred embodiments, the thermal working fluid comprises a water rich phase capable of producing a steam/water vapour phase. Here the process further includes the step of heating the water rich phase using the gas containing vapour phase to produce said steam vapour phase. In these embodiments, the gas containing vapour phase is used to heat water/water vapour, which is then used within the reboiler to heat the capture medium therein. Again, whilst water/water vapour/steam is a convenient indirect thermal working fluid, it should be appreciated that other working fluids could equally be used.
In an example of this embodiment, the process includes a desorber vessel/column having a reboiler which is used to heat the capture medium, typically lean capture medium at the bottom of the desorber vessel/column, and the compressed vapour comprises steam which has been heated from the gas containing vapour phase. In these embodiments, the process of regeneration of a capture medium rich in a captured target gas may comprise the steps of:
In this embodiment, the process typically also includes the step of:
The water rich phase can be heated by the gas containing vapour phase using any number of heat exchange arrangements. In embodiments, the water rich phase is heated using:
In the thermal heat exchange embodiment, the compressed vapour is cooled by heat exchanging against water to heat said water to produce steam. The steam generated by heat exchanging is used to provide thermal energy to heat the target gas rich capture medium.
Similarly, the compressed vapour can be used to heat the target gas rich capture medium/capture medium using any number of heat exchange arrangements. Heat transfer can be through thermal heat exchange (sensible or latent heat exchange via conduction, convective or radiation thermal heat exchange). For latent heat exchange (phase change heat exchange)—this is preferably through the heat of condensation. In embodiments, wherein the compressed vapour is used to heat the target gas rich capture medium using at least one of:
Further thermal efficiencies can be provided by utilising heat from further process streams associated with the separation or desorption process vessels. In some embodiments, the process can further comprise the step of: using the condensed target gas and water phase as a further source of thermal energy for heating the target gas rich capture medium. In these embodiments, the condensed target gas and water phase composition may include a capture medium content. The process may further include the step of separating the capture medium from the target gas and water phase to form a separated capture medium. The separated capture medium is preferably mixed with the heated lean capture medium.
The steam phase does not necessarily need to be used for heat exchange with the target gas rich capture medium within a reboiler or other heat exchange vessel. In embodiments, the compressed vapour is used to heat the target gas rich capture medium by feeding the compressed vapour into the target gas rich capture medium. In these embodiments, the compressed vapour typically comprises the vapour phase thermally associated with the gas containing vapour, and in exemplary embodiments is water vapour/steam.
Here heat transfer is through direct mixing of the heated medium into the target gas rich capture medium. For example, the compressed vapour can be mixed with the capture medium in a process vessel to heat that capture medium. Where the process vessel is a desorber, the compressed vapour can be fed, mixed or injected into the capture medium within the desorber, or within a process volume fluidly connected to the desorber. Thus, in those embodiments where the vapour phase thermally associated with the gas containing vapour phase comprises a steam vapour phase, the steam vapour phase can be fed into the target gas rich capture medium, thereby directly heating the target gas rich capture medium.
One example of this mixing heating embodiment of the process includes a desorber vessel/column which is heated by a steam vapour phase. In such embodiments, the process of regeneration of a capture medium rich in a captured target gas may comprise the steps of:
In this embodiment, the process typically also includes the step of:
The heated lean capture medium is used to heat the target gas rich capture medium using any number of heat exchange arrangements, including direct heat exchange forms and indirect heat exchange forms. In embodiments, the heated lean capture medium is used to heat the target gas rich capture medium using at least one of the following:
The compressing step can be conducted using any suitable compression arrangements and methods. The compressing step can therefore comprise a single compression stage or a multi-stage compression stage. Where the compressing step comprises a multi-stage compression stage, heat is preferably recovered between each compression stage using at least one intercooler located therebetween. Each compression stage preferably has a compression ratio of between 1.1 and 10. It should be appreciated that the compression ratio is dependent on the compressor used in the process (and thus commercially available). In the range of steam fractions 0.8-0.999 in the exit stream a higher compression ratio would generate more sensible heat that is most beneficial for lower zone range, whereas a low compression ratio would be most effective at the higher zone range. These considerations might be overruled by the requirement to deliver the target gas at elevated pressure in which case a high compression ratio is desirable regardless of the steam fraction. This will be the case if the target gas, for example CO2 is sent to be transported as supercritical liquid to a geological storage reservoir. In some embodiments, as some CO2 uses require compressed CO2 for compression, it can be advantageous to already start the compression at the CO2-desorber exit.
The absorption medium of the capture medium can comprise any suitable absorption fluid. In embodiments, the absorbent medium comprises at least one of:
It should be appreciated that the presence of significant amine vapours may result in co-absorption of CO2 and amines into the condensed water, which results in part removal of the CO2 due to the reaction with amine. However, the inventors have found that if the amine vapour pressure is small compared to the CO2 pressure, for example 10%, this loss is acceptable.
In embodiments, the absorbent medium comprises alkaline salts and salts of amino-acids. Exemplary examples include: Potassium, lithium and sodium salts of carbonate, phosphate, glycine, taurine, alanine, sarcosine, proline, lysine, methyltaurine, methionine, aminohexanoic acid, phenylalanine, glutamic acid, arginine aspartic acid, leucine, serine, threonine, glucosamine, dimethyl-glycine, methyl-amino-propionic acid, amino-butyric acid, pipecolic acid, preferably having concentrations between 0.005 and 5.0 M, more preferably between 0.5 and 2.5 M.
Finally, the target gas can comprise any desired gas that is capable of being absorbed in an absorption fluid, preferably an absorption liquid. The target gas typically comprises an acid gas, such as at least one of CO2, H2S, HCl, HF, SO2, SO3 or NOx. In many embodiments, the target gas comprises carbon dioxide (CO2).
A second aspect of the present invention provides a process for the removal and recovery of a target gas from a gas stream which comprises the steps of:
A third aspect of the present invention provides an apparatus for the regeneration of a capture medium rich in a captured target gas and the recovery of captured gas therefrom comprising:
As with the first aspect, the apparatus utilises a high steam fraction in the exit stream of the process vessel which is high enough to enable significant recovery of latent heat in additional to recovery of the sensible heat. The inventors have found that this requires a steam fraction of at least 0.8, preferably above 0.85, and more preferably above 0.9. In embodiments, the gas containing vapour phase comprises the target gas and water vapour having a steam fraction of between 0.8 to 0.999. In embodiments, the gas containing vapour phase comprises the target gas and water vapour having a steam fraction of between 0.8 to 0.999, and more preferably a steam fraction of between 0.8 to 0.99, and yet more preferably between 0.9 and 0.99. In some embodiments, the target gas and water vapour have a steam fraction of between 0.9 to 0.999, and preferably between 0.95 to 0.999. In some embodiments, the target gas and water vapour have a steam fraction of between 0.8 to 0.995, preferably between 0.9 to 0.995. In embodiments, the gas containing vapour phase comprises the target gas and water vapour having a steam fraction of at least 0.9.
Again, the steam content of the gas containing vapour phase can be expressed as a steam/target gas ratio. Here, the steam/target gas ratio of the gas containing vapour phase comprising the target gas and water vapour is preferably at least 4, more preferably at least 8, and yet more preferably at least 10. In embodiments, the steam/target gas ratio is preferably from 4 to 1000.
In embodiments, the apparatus further comprises: a heated lean capture medium supply conduit to supply heated lean capture medium to the at least one heating arrangement as a second source of thermal energy to heat the target gas rich capture medium. In these embodiments, the apparatus can be configured so that the first source of thermal energy and the second source of thermal energy provides the thermal energy required to heat the target gas rich capture medium to a temperature that facilitates the absorbed target gas to separate from the capture medium into a gas containing vapour phase comprising the target gas and water vapour. In embodiments, the supply of thermal energy to the heating arrangement is provided substantially by, preferably consists solely of, the heat recovered from the vapour recompression system in combination with the heat recovered from the heated lean capture medium.
The process vessel comprises any suitable process vessel in which a vapour phase can be separated from a liquid phase. In embodiments, the process vessel comprises one of: flash vaporisation vessel, vaporisation vessel, boiling vessel, desorption vessel or a combination thereof.
The heating arrangement comprises any suitable heat exchange vessel, system or process that enables heat to be transferred from the feed gas rich capture medium. Where the process volume includes the capture medium, the at least one heating arrangement preferably includes a heating arrangement configured to heat the feed gas rich capture medium in that process volume. That heating arrangement may comprise at least one heat exchanger. That heat exchanger can be at least one thermal (sensible heat) heat exchanger, condensing heat exchanger, or at least one mixing heat exchanger. Here heat transfer can be through thermal heat exchange (sensible or latent heat exchange via conduction, convective or radiation thermal heat exchange). For latent heat exchange (phase change heat exchange)—this is preferably through the heat of condensation. Alternatively, there can be thermal exchange through mixing.
In particular embodiments, the at least one heating arrangement includes a reboiler, preferably a condensing reboiler, configured to heat the target gas rich capture medium in the reboiler. The reboiler is typically fluidly connected to a desorption vessel or column and is used to heat capture medium within that desorption vessel or column.
The target gas rich capture medium is not necessarily heated within the process volume. In embodiments, the gas rich capture medium can be preheated before entering the process volume of the process vessel. In these embodiments, the at least one heating arrangement includes at least one preheating arrangement configured to heat the feed gas rich capture medium prior to the feed gas rich capture medium being fed into the process volume. In some embodiments, at least the heated lean capture medium is used to heat the target gas rich capture medium in the at least one preheating arrangement.
In some embodiments, the compressed vapour is used to heat the target gas rich capture medium in the at least one preheating arrangement. For example, in those embodiments the process vessel comprises a flash vaporisation vessel, and the compressed vapour is used to heat the target gas rich capture medium in the at least one preheating arrangement. In these embodiments, the heated lean capture medium is typically also used to heat the target gas rich capture medium in the at least one preheating arrangement.
As with the first aspect, it should be appreciated that the vapour recompression system can be directly applied to the gas containing vapour phase formed in the at least one heating arrangement, such as described above, and/or could be used to compress a vapour phase thermally associated with the gas containing vapour phase, to form a compressed vapour. In most processes, the vapour recompression system is applied directly to the gas containing vapour phase or to a vapour phase thermally associated with the gas containing vapour phase. However, it should also be appreciated that it is possible for the process to include two parallel the vapour recompression systems where both the gas containing vapour phase or to a vapour phase thermally associated with the gas containing vapour phase undergo the compression.
As explained for the first aspect of the present invention, a secondary thermal working fluid can be used to heat the target gas rich capture medium. In these embodiments, that secondary thermal working fluid is heated by the gas containing vapour phase. In an example embodiment, the vapour phase thermally associated with the gas containing vapour phase comprises a steam vapour phase. Here, the apparatus can further include an indirect heat exchanger arrangement configured to heat a water rich phase using the gas containing vapour phase to produce said steam vapour phase. The indirect heat exchanger can comprise any suitable heat exchange configuration, such as at least one heat exchanger. That heat exchanger can be at least one thermal (sensible heat) heat exchanger, at least one condensing heat exchanger, or at least one mixing heat exchanger. Here heat transfer can be through thermal heat exchange (sensible or latent heat exchange vis conduction, convective or radiation thermal heat exchange). For latent heat exchange (phase change heat exchange)—this is preferably through the heat of condensation. Alternatively, there can be thermal exchange through mixing.
In embodiments, the at least one heating arrangement includes:
The at least one heating arrangement can include two (or more) preheating arrangements. In embodiments, the at least one heating arrangement includes:
Where the condensed target gas and water phase includes a capture medium content, the apparatus may further include a separator configured to separate the capture medium from the target gas and water phase to form a separated capture medium. In some embodiments, the separated capture medium is mixed with the heated lean capture medium.
Again, the steam vapour phase does not necessarily need to be used for heat exchange with the target gas rich capture medium within a reboiler or other heat exchange vessel. In embodiments, the compressed vapour is used to heat the target gas rich capture medium by feeding the compressed vapour into the target gas rich capture medium. In such embodiments, the process vessel comprises a desorption vessel, and the at least one heating arrangement comprises at least one mixing heat exchanger configured to heating the target gas rich capture medium by feeding the compressed vapour into the target gas rich capture medium in the process volume of the desorption vessel or a volume fluidly connected to said process volume. Here heat transfer is through direct mixing of the heated medium into the target gas rich capture medium. In embodiments, the vapour phase thermally associated with the gas containing vapour phase comprises a steam vapour phase, and the steam vapour phase is fed into the target gas rich capture medium in the process volume, or a volume fluidly connected to the process volume of the desorber, thereby directly heating the target gas rich capture medium.
The process vessel is preferably operated at atmospheric or above atmospheric pressure. In embodiments, the process vessel has an operating pressure of at least 1 bara, preferably at least 1.5 bara.
The vapour recompression apparatus can have any suitable configuration. In embodiments, the vapour recompression apparatus comprises a plurality of compressors in series. In embodiments, the vapour recompression apparatus comprises a single stage compressor or a multi-stage stage compressor. Each compressor preferably has a compression ratio of between 1.1 and 10. In embodiments where the vapour recompression apparatus comprises a multi-stage stage compressor, the apparatus further comprises at least one intercooler located between each compressor to recover heat therefrom. The vapour recompression system and compressors thereof can be driven by any suitable power or energy source. Preferably, the vapour recompression system is driven from a source of electrical power. That electrical power can have any generation source. However, renewable (green) energy generation sources (for example wind, solar, wave power etc) can provide particular advantageous efficiencies for the process of the present invention. Moreover, renewable energy does not produce significant (if any) CO2-emissions. As renewable energy is increasingly and very conveniently produced as electricity there is also a preference to use electricity only to drive the thermal regeneration.
In embodiments, the apparatus further comprises a condenser to condense and recover capture medium vapour from the compressed vapour phase. In some embodiments, the apparatus further comprises a condenser to condense and recover water vapour from the vapour phase thermally associated with the gas containing vapour phase. In some embodiments, the apparatus includes a separator configured to target gas from water from the gas containing vapour phase after passing through the at least one heating arrangement.
The apparatus of this third aspect of the present invention can be fluidly connected to an absorption apparatus configured to absorb a target gas from a source gas stream into a capture medium, producing a target gas rich capture medium which is fed to the regeneration apparatus for regeneration of capture medium and recovery of absorbed target gas therefrom. This provides an overall target gas capture and recovery apparatus and process. In other embodiments, the present invention provides an apparatus for removal of a target gas from a source gas stream comprises an absorption apparatus fluidly upstream of a regeneration apparatus in accordance with any one of the preceding claims. The source gas stream preferably includes a target gas selected from at least one of CO2, H2S, HCl, HF, SO2, SO3 or NOx, preferably CO2. For example, the source gas stream could be at least one of: flue gas from a combustion apparatus for the burning of carbonaceous fuels, such as flue gas from a thermal power plant.
A fourth aspect of the present invention provides a process according to the first aspect of the present invention, comprising feeding the gas rich capture medium through a regeneration apparatus in accordance with the third aspect of the present invention having a heating arrangement to heat the gas rich capture medium and thereby cause captured gas to dissociate into a gas rich vapour phase; wherein the compressed vapour is used to supply thermal energy to the heating arrangement.
The process and apparatus of the present invention can be used for Direct Capture from Air and CO2 capture from power plants. At the low CO2 liquid loadings resulting from a Direct Air Capture process, the desorption conditions would be such that high steam fractions are encountered at typical regeneration temperature between 10° and 120° C.
High steam fractions are encountered when liquid absorbents are regenerated deeply and CO2-loadings are limited in the absorption process, e.g. by operation at high liquid flow rates. Use of the present invention may enable smaller absorber towers to be used in CO2-capture from coal or gas fired power plants as a result of the higher driving forces in the absorber. This will lead to smaller absorber and CAPEX advantages.
A further aspect of the present invention therefore provides a thermal power plant comprising a combustion means to burn carbonaceous fuel and generate steam provided a thermal power plant comprising a combustion means to burn carbonaceous fuel and generate steam, and a regeneration apparatus in accordance with the third aspect of the present invention. In some embodiments, the thermal power plant comprises a gas removal apparatus in accordance with the third aspect of the present invention and a flue gas outlet fluidly connected to supply flue gas directly from the combustion apparatus as a source gas stream to the removal apparatus.
The preferred application of the process and apparatus of the present invention is in CO2-capture from ambient air (Direct Air Capture-DAC), where the liquid regeneration will result in the production of high vapour content in the CO2-product. However, it should be appreciated that the present invention can be used for:
The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:
The present invention provides a process, apparatus and system for regeneration of capture mediums, such as liquid absorbents, used to capture a target as such as CO2, in which the thermal energy required for the regeneration process is provided by thermal energy within the desorption process, and more particularly the vapour compression process. The present invention typically forms part of a target gas absorption-desorption process which includes an absorber 100 such as illustrated in
The following description relates to the capture of carbon dioxide. However, it should be appreciated that other target gases could equally be used such as H2S, HCl, HF, SO2, SO3 or NOx which can absorbed by a suitably matched absorption fluid/liquid.
The regeneration of aqueous absorption liquids used in absorption systems such as shown in
One type of regeneration system is shown in
The lean absorbent in line 316 that is not introduced into the desorber reboiler 315, is recycled back to the absorption column 110 (
CO2 released from the absorbent, water vapour and minor amounts of absorbent form a gas containing vapour phase, which is withdrawn from the desorber column 310 through a gas withdrawal line 350. It should be appreciated that the water vapour withdrawn through line 350, and the condensed water removed in the desorption column 310, may comprise minor amounts of absorbent. The water and water vapour from this line 350 therefore typically include water and water vapour including minor amounts of absorbent, where appropriate. The processing of that gas containing vapour phase from gas withdrawal line 350 depends on the particular embodiment of the present invention described below.
In conventional regeneration systems, the lean absorbent in the reboiler 315 is typically heated by means of electricity, or a heating medium, such as steam. Generally, the provision of thermal energy via electricity, for example using resistive or inductive heating, is considered wasteful and electrically driven heat pumps are preferred because of their higher efficiency. In such a heat pump application, gases are compressed, resulting in a temperature increase with heat recovery via condensation of the liquids from the compressed gases.
The present invention proposes the use of vapour compression systems which more effectively recover thermal energy from that system needed for regeneration of the aqueous absorption liquids and CO2-desorption. Two systems are proposed:
The starting point for both the direct and indirect vapour compression systems is to analyse the thermal energy requirement per unit mass of CO2 released, the so-called specific reboiler duty (MJ/kg CO2), in more detail. This thermal energy requirement has the following three contributions:
The minimisation of the thermal energy requirement is an important objective in CO2-capture technology development work involving extensive evaluation of different absorption liquids, considering their intrinsic characteristics, such as vapour-liquid equilibria and reaction enthalpies, and process conditions such as regeneration temperature and pressure in the CO2-desorber. In particular, as the amount of steam leaving the CO2-desorber represents a loss of energy, process and design optimisation focuses on the minimisation of the steam/CO2 ratio in the stream exiting the CO2-desorber. The Applicant's U.S. Pat. No. 10,040,023, “Process and apparatus for heat integrated liquid absorbent regeneration through gas desorption” is an example of how the thermal energy requirement can be minimised by new process and equipment designs.
As discussed in the background, a number of prior art processes have been developed that use compression of the CO2-stream from the desorber with subsequent heat recovery to lower the thermal energy requirement for CO2 desorption. However, none of these processes include a CO2-desorption process in which all the thermal energy required is provided by energy transfer from the regeneration stage/desorption stage within that process. This is because energy efficient liquid regeneration process concepts are based on the minimisation of the water content of the wet CO2-product leaving the CO2-desorber. Such processes fail to recognise the full benefits of the vapour compression process.
In the present invention, the steam content in the CO2-product in the vapour compression process is high (within the steam fractions discussed below) so to achieve high recovery of energy via the condensation of water to supply the overall heat requirement of the capture process. This is in contrast to the common approach of minimisation of steam content in the exit stream from the CO2-desorber in prior art regeneration processes.
As noted above, latent heat contained in the exit gas stream from the CO2-desorber can be recovered and reused in the regeneration of the absorption liquid and CO2-desorption through two different pathways means:
In addition to heat exchange between the lean liquid stream and rich liquid stream (for example as shown using heat exchanger 312 in
As illustrated in
This first concept is akin to a Mechanical Vapour Recompression (MVR) process in which low pressurisation and high rates of evaporation are used. MVR is used in industrial applications such as the production of freshwater from seawater and in liquid and solids dehydration.
This concept (system 300A) includes the previously described desorber column 310 and desorber reboiler 315 and processes the gas containing vapour phase withdrawn from the desorber column 310 through a gas withdrawal line 350. As illustrated in
Compression of the steam/CO2 stream is advantageous for most subsequent storage and utilisation options which often require CO2 at elevated pressure, for example in excess of 100 bar, if CO2 is used for geological storage. Such high pressures are not needed for other cases where CO2 is used as reactant to produce carbonates or fuels or where CO2 is used to promote plant growth.
The indirect process is based on the transfer the latent heat in the exit gas stream from the CO2-desorber via condensation to an incoming water stream that will be converted into steam. The latent heat from the high steam fraction in the desorber exit can thus be converted into pure steam that is subsequently compressed with heat recovery via condensation by the following pathways:
This concept (system 300B) also includes the previously described desorber column 310 and desorber reboiler 315 and processes the gas containing vapour phase withdrawn from the desorber column 310 through a gas withdrawal line 350. As illustrated in
As illustrated in
The process operation in both direct and indirect process will be most beneficial at high steam content of the CO2 product in the desorber as this will lead to the highest recovery of latent heat from the stream fraction. In this regard, each of the processes illustrated in
A variety of liquid absorbents can be used. The preferred absorption liquids will be:
Further optimisation for different absorption liquids and under varying process conditions will entail a trade-off between power consumption for compression and lean loading requirement. A low lean loading will be beneficial for the CO2-absorption process, resulting in higher mass transfer and therefore smaller equipment. However, this entails a deeper degree of absorbent regeneration that results in a higher specific power consumption.
The presence of amine vapours will result in co-absorption of CO2 and amines into the condensed water, which results in part removal of the CO2 due to the reaction with amine. If the amine vapour pressure is small compared to the CO2 pressure, for example 10%, this loss is acceptable.
The compression arrangements in the systems shown in
It should be appreciated that further process steps or stages may also be included in the concepts described above. For example, the target gas and water/water vapour phase from line 250 (
The following examples were developed using a process model developed in ProTreat® simulator—a process simulator tool for gas treating available from Optimized Gas Treating, Inc. (OGT), Buda, Texas, United States of America.
An example of absorbent regeneration system based on concept 1 (
The process model was developed in ProTreat® discussed above based in the process flow diagram provided in
The specific power consumption for the vapour compression (efficiency=0.8) is equal to 1.94 MWh/tonne CO2.
A second example of the process of concept 1 (
In this example under similar process conditions as described above for Example 1, the H2O vapour fraction of the vapour stream from the flash operation was lower (0.835) together with a lower compression ratio (1.5 bar to 2.2 bar) and a lower degree of regeneration (lean liquid loading=0.088 mol/mol) was achieved. In addition, an additional heat exchanger 313 is included transferring latent heat from the condensed vapour stream 275 to the rich solution 140 from the absorber 110 (
The overall process resulted in a lower compression energy requirement of 1.12 MWh/tonne CO2.
It is considered that further optimisation for different absorption liquids and under varying process conditions will entail a trade-off between power consumption for compression and lean loading requirement. A low lean loading will be beneficial for the CO2-absorption process, resulting in higher mass transfer and therefore smaller equipment. However, this entails a deeper degree of absorbent regeneration that results in a higher specific power consumption.
A process modelling example is provided where the exhaust from the desorber is compressed to 10 bar via a two-stage adiabatic compression process following the process illustrated in
The compression process of the process in
Table 1 indicates where the heat from the compression process exceeded the reboiler heat requirement—thus not providing a result for a single case but rather serving to provide the minimum steam ratio range. It was assumed that there was negligible heat requirement for heating up the absorption liquid in the desorber. Referring to Table 1, it can be seen that at higher steam fraction the specific compressor power consumption increases but also the amount of recoverable heat. At the higher inlet temperature of 120° C. and higher exit pressure of 2 bar the specific power consumption is lower than for 100° C./1 bar. The amount of heat recovery is slightly lower.
9.98
21.22
19.90
H = 10.75
P = 0.672
H = 20.05
H = 22.34
P = 0.843
P = 1.34
H = 94.26
H = 103.7
H = 115.1
P = 2.16
P = 4.21
P = 6.72
H = 174.0
H = 189.4
H = 208.2
H = 231.1
P = 0.95
P = 4.32
P = 8.43
P = 13.4
H = 873.2
H = 950.3
H = 1044.4
H = 1159
P = 4.75
P = 21.6
P = 42.1
P = 67.2
Table 1 also shows the specific energy requirement for regeneration of the absorption liquids, the so-called reboiler duty. In the calculation, the regeneration energy is calculated as the sum of the reaction enthalpy requirement for CO2-desorption and the latent heat of evaporation for water. In this relatively simple analysis, the heat requirement for heating up the absorption liquid after exchanging heat with the regenerated solution is ignored, assuming an ideal temperature approach for the heat exchange. The reboiler duty thus calculated is a minimum and has been based on three values for the reaction enthalpy for the chemical absorption, 50, 75, 100 KJ/mol CO2, which cover the range of values for chemical absorption liquids. For water evaporation an average value of 37.5 KJ/mol H2O was used for the temperature range 100 to 120° C. The reboiler duty increases with an increase in the reaction enthalpy for the chemical absorbent chosen.
In Table 1 it can be seen that for steam fractions equal to 0.8 and lower the compressor is not able to provide sufficient energy for the regeneration of the absorption liquid. At a steam fraction of 0.9 and higher there is sufficient heat available for regeneration of absorption liquid. At higher steam fraction the overall compression energy requirement will increase, and the optimal conditions are those at which the water vapour content is high enough to provide the heat to the reboiler via the vapour compression process but not higher than that, as this would unnecessarily increase the energy requirement. The water vapour content is determined by the characteristics of the absorption liquid and will be dependent on CO2-loading of the absorption liquid and temperature. For the example of compression to 10 bar it is shown that the “break-even” steam fraction would be between 0.8 and 0.9. This “break-even” point is also dependent on the compression ratio of the vapour compression process. In another example the pressure ratio for the vapour compression is varied between 1.25 and 10 assuming a single-stage compression process and the vapour exiting the desorber at 100° C./1 bar. To understand the trends and the potential for heat recovery, in this calculation a 100% compressor efficiency was used and it was assumed that all latent heat could be recovered.
Table 2 presents results the specific heat recovery (H) and the specific compressor power consumption (P) for four compression ratios (1.25, 2.5, 5, 10) and a range of steam fractions of the exit vapour from the CO2 desorber (0.5 to 0.999). Also presented are the estimated minimum specific reboiler duties for the CO2-desorption process for three values of the reaction enthalpy. These reboiler duties can be compared with the heat recovery from the compression process (H). The values where the heat from the compression process exceeds the reboiler duty for all values of the reboiler duty are shown in bold for clarity. At low compression ratio the steam fraction needs to be high for to achieve high enough heat recovery from the compression process, e.g. at 1.25 bar pressure after compression, the steam fraction needs to be at least 0.99 to 0.995 to achieve. At 10 bar pressure after compression the steam fraction can be somewhat lower, i.e. more than 0.8 to 0.9 to achieve break-even. Higher steam fractions lead to steep increases in the specific power consumption as shown in Table 2. This indicates that careful optimisation is needed for the specific absorption liquid, CO2-loading and regeneration process design to determine the optimum compression conditions that will minimise compression power consumption and achieve sufficient heat recovery from the compression process.
The simplified analysis here indicates that for the relevant pressure conditions of 1.25 to 10 bar the steam fraction will need to be in the range 0.8 to 0.995 to achieve the situation where the heat of the compression process covers is sufficient to supply the reboiler duty. At lower vapour fraction the heat generated by the compression will be insufficient to fully cover the reboiler duty. At higher vapour fractions the equipment might be become quite large and expensive as large amounts of water vapour need to be treated relative to the amount of CO2. Also, the compressor power consumption increases quite rapidly which is not desired.
The process configuration shown in
The absorption liquid entering the regeneration system had a liquid loading equivalent to 0.3 mol/mol that can be achieved through CO2-capture from the air. The desorber was operated at a pressure of 150 kPa (1.5 bar) under isothermal conditions (˜112° C.) for all process simulations. The steam compressor was assumed to have an 80% efficiency. The rich solution 140 was preheat in heat exchangers 312 using the hot lean liquid exiting the bottom of the desorber column 310. The heated rich liquid is subsequently fed into desorber column 310.
The resulting process flow sheets are given in
The process configuration shown in
The rich absorption liquid 340 entering the regeneration system at a temperature of 104.5° C. (α=0.3, 14420 kg/hr) had a liquid loading equivalent to 0.3 mol/mol that can be achieved through CO2-capture from the air. The desorber 310 was operated at a pressure of 150 kPa (1.5 bar) and a bottom temperature of 115° C. with the regenerated, lean absorption liquid 320 leaving the regeneration at a temperature of 112.2° C., (α=0.036, 14120 kg/hr) having a liquid loading equivalent to 0.036 mol/mol.
The vapour mixture leaving the desorber 350 exchanged heat with the incoming water stream 442 (@40° C., 8150 kg/h) that resulted in the production of steam 450 at a temperature of 110° C. and a pressure of 150 kPa (8150 kg/hr). CO2 and condensate exit in stream 475 at 45° C., a pressure of 150 kPa (CO2: 290 kg/h). This steam is compressed using the compressor 460 to 290 kPa (8150 kg/hr) with the adiabatic compression resulting in an outlet temperature of 195° C. in line 462 and condenses in the reboiler 315 at a temperature of 132.5° C. in the condensate 470 transferring heat to the absorption liquid which exits the reboiler in stream 318. The steam compressor was assumed to have an 80% efficiency and the electricity consumption of the process equals 1.2 MWh/tonne CO2.
Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.
Where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022900421 | Feb 2022 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2023/050110 | 2/20/2023 | WO |
