System for carbon dioxide capture

Abstract

A system for the capture of carbon dioxide (CO2) from a gas stream, the system comprises: an absorber configured to contact the gas stream with a sorbent, the sorbent operable to capture CO2 from the gas stream; a desorber configured to release the CO2 from the sorbent, wherein the desorber is arranged to receive a rich sorbent stream from the absorber via a rich line and to provide a lean sorbent stream to the absorber via a lean line; and a condenser system configured to receive an exhaust stream comprising CO2 and vapour from the desorber and to remove at least a part of the vapour from the exhaust stream as condensate; wherein the system is configured to determine a concentration of absorbent in the system, and, based on the determination, adjust the concentration to be within a predetermined range by providing top-up water to the system, wherein the top-up water comprises at least a part of the condensate from the condenser system.

Description

[0001] M&C PX221216W0

[0002] 1

[0003] System for Carbon Dioxide Capture

[0004] Technical field

[0005] The invention relates to a system for carbon dioxide capture.

[0006] Background

[0007] Carbon capture and storage is expected to be a significant way to reduce the effects of global warming from the combustion of fossil fuels.

[0008] Capture of carbon dioxide (CO2) may involve systems for extracting CO2 from a CO2 containing gas using an absorbent medium. Typically, this involves creating a gas flow over the absorbent medium under conditions where the medium will absorb CO2 from the gas, and then altering the conditions so that the medium releases the absorbed CO2 allowing it to be captured and stored. This process may be used to reduce atmospheric CO2 to mitigate the anthropogenic emissions that are associated with global warming, or climate change. Direct Air Capture (DAC) is the capture of CO2 from atmospheric air which, as the atmosphere contains less than 0.05% CO2, involves processing large volumes of air.

[0009] Some Direct Air Capture systems use a liquid medium to absorb CO2 in an absorber or liquid-air contactor. There is for example a sorbent containing solution, distributed within an absorber, subject to a high air flow necessary to process large quantities of air. The sorbent may be distributed by spray nozzles, which produces an aerosol of sorbent in solution with a high surface area. In liquid absorbent DAC, the lean liquid is used to absorb CO2 from the ambient air and then when it is loaded, a rich (loaded) liquid is bled off and taken to the regeneration system (desorption or stripper column). The desorber uses heat to release CO2 from the liquid, returning a lean liquid back to the absorber units. Alternatively, the air may be brought into contact with liquid films of sorbent solution propagated across solid surfaces, such as conventional structured packing / film fill.

[0010] Figure 1 shows a schematic diagram of a DAC system 1. The DAC system 1 contains an absorber 2 connected to a desorber 3 via a heat exchanger (HX) 4. A rich line 5 goes from the absorber 2 to the desorber 3 and a lean line 6 goes from the desorber 3 backM&C PX221216W0

[0011] 2

[0012] to the absorber 2. The desorber 3 is connected to a reboiler (RX) 7 for providing heat to release the CO2 and a condenser (CX) 8 at the top to condense water from the desorber 3. A condensate knock out tank 9 after the condenser 8 separates condensate and CO2. The condensate is then returned to the top of the desorber 3.

[0013] There is a continued need to provide improved, lower weight, compact and more energy efficient carbon dioxide capture systems.

[0014] Summary

[0015] According a first aspect there is provided a system for the capture of carbon dioxide (CO2 ) from a gas stream, the system comprises:

[0016] an absorber configured to contact the gas stream with a sorbent, the sorbent operable to capture CO2 from the gas stream;

[0017] a desorber configured to release the CO2 from the sorbent, wherein the desorber is arranged to receive a rich sorbent stream from the absorber via a rich line and to provide a lean sorbent stream to the absorber via a lean line; and

[0018] a condenser system configured to receive an exhaust stream comprising CO2 and vapour from the desorber and to remove at least a part of the vapour from the exhaust stream as condensate;

[0019] wherein the system is configured to determine a concentration of absorbent in the system, and, based on the determination, adjust the concentration to be within a predetermined range by providing top-up water to the system, wherein the top-up water comprises at least a part of the condensate from the condenser system.

[0020] The system may be a DAC system, wherein the gas stream comprises ambient air. The absorbent may be an amino acid salt.

[0021] The system may be configured to determine the concentration at least in one of the lean line, the rich line and a recirculation line of the absorber. For example, a concentration sensor may be fitted to one or more of these lines. The top-up water can be provided to at least one of the lean line, the rich line, and a recirculation line of the absorber. For example, the concentration of absorbent may be measured at a point or in a region of the lean line before the absorber, and then condensate provided as top-up water in the lean line downstream from that point or region.M&C PX221216W0

[0022] 3

[0023] The system typically comprises a main heat exchanger connected to the lean line and to the rich line. The main heat exchanger is configured to draw heat from the lean sorbent stream and provide heat to the rich sorbent stream, wherein the system may be configured to determine the concentration and to provide the top-up water in the lean line in a region between the main heat exchanger and the absorber. This region of the lean line may carry the lean sorbent stream when it is relatively cold and concentrated, increasing the risk of precipitation occurring. In an example, the system may be configured to measure the concentration of absorbent of the rich sorbent stream in the rich line before (upstream) of the main heat exchanger. Top-up water may be provided to the rich line at a point between the absorber and the main heat exchanger. In an example, the system may be configured to measure the concentration of both the rich sorbent stream and the lean sorbent stream.

[0024] The system may further comprise a flow meter arranged to measure a flow rate of a sorbent stream in the system, wherein the system is configured to adjust a flow rate of the top-up water provided to the system based on the measured flow rate. The flow meter may be any known in-line or on-line measurement device or indicator to give a measure of the rate of flow. The flow meter may be arranged to provide a measure of the flow rate of absorbent through the section of pipe where the concentration measurement is made. The flow rate may be measured at a first point in the system and then calculated at a second different point based on the measurement. The second point may be the point in the system where the concentration is measured. For example, the one or more flow meters may be arranged on at least one of the rich line, the lean line and the recirculation line of the absorber.

[0025] The system can be configured to provide a part of the condensate back to the desorber. For example, the system may determine that a part of the condensate from the condenser system is required as top-up water to provide that part to the system (e.g. to the lean line), and may be configured to provide a remaining part to the desorber.

[0026] The condenser system may comprise a plurality of condensers. For example, the condenser system may comprise two or more condensers arranged in series. The condenser system may comprise a first condenser and a second condenser, wherein the first condenser is directly connected to the desorber to receive the exhaust streamM&C PX221216W0

[0027] 4

[0028] from the desorber and to condense a first part of the vapour comprised by the exhaust stream, and wherein the second condenser is directly connected to the first condenser to receive from the first condenser a remaining part of the exhaust stream comprising the CO2 and vapour and to condense at least a part of the vapour. A first condensate from the first condenser can be provided directly to the desorber, and a second condensate from the second condenser can be provided to the top-up water.

[0029] The system may comprise a heat pump system for providing heat to a reboiler of the desorber, wherein the heat pump system comprises a first heat pump for transferring heat from the lean line to an intermediate circuit, and a second heat pump for transferring heat from the intermediate circuit to the reboiler. The system may further comprise a heat exchanger connected to the lean line and to the first heat pump. The heat exchanger can allow further heat to be drawn from the lean line (after the main heat exchanger). The intermediate circuit may also be connected to and draw heat from the condenser system.

[0030] The system may further comprise one or more filters for filtering a sorbent stream of the system. The one or more filters may be configured to remove dirt, debris and / or low solubility by-products from the sorbent stream. The filter may be arranged in the lean line between the main heat exchanger and the absorber, where the lean sorbent stream is relatively cold and the concentration relatively high. The one or more filters can be arranged to filter at least one of the lean sorbent stream, the rich sorbent stream, and a recirculation stream of the absorber.

[0031] The system may further comprise a water reservoir (e.g. a tank) arranged to receive condensate from the condenser system and to receive additional water from an external source, and to provide the top-up water to the system. The system may comprise one or more valves for controlling the flow of liquid to and / or from the water reservoir. The one or more valves may comprise a first valve for controlling a flow of the additional water into the water reservoir, a second valve for controlling a flow of top-up water into the system, and a third valve for removing excess water from the water reservoir.

[0032] The system may further comprise a mixing device arranged to mix a sorbent stream downstream from a region where the top-up water is provided. The mixing device may comprise one or more of a tank or large vessel, a swirling section, a longer length of pipe, and a pipe bend.M&C PX221216W0

[0033] 5

[0034] The system may further comprise means for measuring the concentration, wherein the means are located downstream from the mixing device. For example, the system may comprise one or more concentration sensors arranged to measure the concentration of absorbent.

[0035] According to a second aspect there is provided a method of capturing carbon dioxide (CO2) from a gas stream. The method comprises:

[0036] providing an absorber for contacting the gas stream with a sorbent, the sorbent operable to capture CO2 from the gas stream;

[0037] providing a desorber for releasing the CO2 from the sorbent, wherein the desorber receives a rich sorbent stream from the absorber via a rich line and provides a lean sorbent stream to the absorber via a lean line; and

[0038] providing a condenser system for receiving an exhaust stream comprising CO2 and vapour from the desorber and for removing at least a part of the vapour from the exhaust stream as condensate;

[0039] determining a concentration of absorbent in the system, and, based on the determination, adjusting the concentration to be within a predetermined range by providing top-up water, wherein the top-up water comprises at least a part of the condensate from the condenser system.

[0040] The method may be carried out using the system of the first aspect. For example, the method may be carried out using a DAC system, wherein the gas stream comprises ambient air.

[0041] The method may further comprise determining a flow rate of the absorbent, and adjusting a flow rate of top-up water provided based on the determination. The method may further comprise filtering the absorbent. For example, the method may comprise filtering the lean sorbent stream downstream of the point of measuring the concentration and upstream of the point where top-up water is provided to the system.

[0042] Brief description of drawings

[0043] Figure 1 shows a schematic diagram of a DAC system;

[0044] Figure 2 shows a schematic diagram of an example of a DAC system;M&C PX221216W0

[0045] 6

[0046] Figure 3 shows a schematic diagram of another example of a DAC system;

[0047] Figure 4 shows a schematic diagram of another example of a DAC system;

[0048] Figure 5 shows a schematic diagram of another example of a DAC system;

[0049] Figure 6 shows a schematic diagram of another example of a DAC system;

[0050] Figure 7 shows a schematic diagram of another example of a DAC system;

[0051] Figure 8 shows a schematic diagram of another example of a DAC system;

[0052] Figure 9 shows a schematic diagram of another example of a DAC system;

[0053] Figure 10 shows a schematic diagram of another example of a DAC system;

[0054] Figure 11 shows a schematic diagram of another example of a DAC system;

[0055] Figure 12 shows a schematic diagram of another example of a DAC system; and Figure 13 shows a schematic diagram of a further example of a DAC system.

[0056] Detailed

[0057]

[0058] Conventionally, condensate from the condenser after the desorber is returned to the top of the desorber tower, which is a requirement in an amine-based system. With existing technology, lean absorbent after the regeneration column (i.e. after the desorber) has heat removed and transferred to the rich absorbent stream.

[0059] It can be advantageous to extract more heat from the lean return. However, the absorbent is typically an amino acid salt, which can precipitate when the temperature is too low or the mixture too concentrated. In addition, some of the degradation products have low solubility and may precipitate out first. Without careful control, additional solvent evaporation in the absorber can lead to higher concentration and precipitates forming in the absorber which can be difficult to rectify. Changes in ambient air conditions can exacerbate the problem. For example, drier air (lower relative humidity) can promote additional solvent evaporation and thereby increase the risk of precipitation within the absorber. A lower temperature air can quench the temperature of the solvent and lead to lower levels of solubility and thereby an increased risk of precipitation with the absorber. Hence, there is a need to monitor and control the absorbent concentration orM&C PX221216W0

[0060] 7

[0061] water balance in the system and to reduce the risk or prevent the onset of precipitation in the system.

[0062] Embodiments described herein can allow condensate from the condenser at the top of the desorber to be returned to different locations in the system. The system can provide concentration management and control using condensate, which is returned to the process in a cool (e.g. the coolest) and more concentrated (e.g, the most concentrated) part of the system. The condensate may be returned after the heat recovery heat exchangers (HXs) in the lean return line or just prior to the delivery of absorbent into the absorber. Providing the condensate in the concentrated part of the system in the cold return line from the desorber, can prevent concentration build up and precipitation.

[0063] For example, described herein is a carbon capture system, such as a direct air capture (DAC) system. The system can be configured to measure the concentration of the cold return line (or in another part of the system having a low temperature and / or high concentration), then filter the line to remove by-products, dirt and debris (e.g. low solubility degradation species coming back from the desorber), and then provide cold condensate as required to maintain water balance in the system.

[0064] By also determining the flow rate of the cold return line, the flow rate of top-up water can also be controlled. This can be beneficial for catching thermal degradation products that may have been generated in the desorber column. For example, the system can catch salts when the liquid is concentrated and cool. The liquid becomes concentrated because condensate is not fully returned at the desorber column top, and instead condensate is provided (just) after the filter. It is important to ensure that there is no or minimal salt build up around the absorber spray(s) in order to maintain a functioning system.

[0065] Hence, embodiments described herein may provide a system for reducing or eliminating the risk of absorbent precipitation within the absorber units, which would otherwise be damaging to the operation of the system. Furthermore, embodiments may provide means for preventing or reducing build up of by-products and other dirt and debris within the desorber. For example, the embodiments may pre-filter the incoming absorbent (liquid) flows to remove dirt, debris and low solubility degradation products from the absorbent.M&C PX221216W0

[0066] 8

[0067] Figure 2 shows a schematic diagram of a DAC system 1. The same reference numerals have been used in different figures to denote equivalent or similar features for ease of understanding and are not intended to limit the illustrated embodiments.

[0068] The DAC system 1 comprises an absorber 2 connected to a desorber 3 via a heat exchanger (HX) 4. A rich line 5 goes from the absorber 2 to the desorber 3 and a lean (return) line 6 goes from the desorber 3 back to the absorber 2. The desorber 3 is connected to a reboiler (RX) 7 for providing heat to release the CO2 and a condenser (CX) 8 at the top to condense water from the desorber 3. A heat pump (HP) 12 transfers / upgrades heat from the condenser 8 to the reboiler 7. A vapour-liquid separator 9 (e.g. a so called knock-down tank) after the condenser 8 separates condensate and CO2.

[0069] In operation, the absorber 2 receives a CO2 containing gas stream (ambient air in the case of a DAC system). A sorbent stream flows through the absorber 2, a lean stream enters the absorber 2 via the lean line 6, and contacts the CO2 containing gas. The sorbent stream absorbs CO2 and becomes a rich stream that exits the absorber 2 via the rich line 5. Sorbent may be recirculated within the absorber 2 as a recirculation stream via a recirculation line 17 (also referred to as “bypass”, or “bypass line”), which increases the effective residence time of each portion of the lean stream of sorbent in the absorber 2.

[0070] The rich stream can be passed through heat exchanger 4 to recover some heat from the lean stream returning from the desorber 3. The desorber 3 receives the rich stream and heats it up to a temperature where the CO2 will be released form the sorbent. Reboiler 7 heats the rich stream, which may also generate steam to form vapour bubbles into which the desorbed CO2 can diffuse, leaving a lean stream of sorbent to return to the absorber 2 to repeat the process. The vapour and CO2 exit the desorber 3 as an exhaust stream, and a condenser 8 is used to cool the mixture causing the vapour to condense leaving a purer CO2 product stream.

[0071] The condensate is returned via the vapour-liquid separator 9 from the condenser 8 to the lean line 6 after HX 4. A measurement device 10 (e.g. a concentration sensor) is arranged to measure the concentration (Cl) of the liquid in the lean return line 6 before the condensate top-up point. By adjusting the amount of condensate provided to the leanM&C PX221216W0

[0072] 9

[0073] line 6 based on the measured concentration, the concentration of the lean return stream can be adjusted to within a predetermined (optimal) range. A filter 11 is arranged to filter the liquid in the lean line 6 after the HX 4. The filter 11 can be arranged to manage dirt / debris / fouling after the desorption step. For example, the filter may be configured to reduce or substantially remove any low solubility degradation products from the lean line 6.

[0074] Figure 3 shows a schematic diagram of a DAC system 1. Similar to the DAC system described in relation to Figure 2 above, the DAC system 1 comprises an absorber 2 connected to a desorber 3 via a heat exchanger (HX) 4. A rich line 5 goes from the absorber 2 to the desorber 3 and a lean (return) line 6 goes from the desorber 3 back to the absorber 2. The desorber 3 is connected to a reboiler (RX) 7 for providing heat to release the CO2.

[0075] The DAC system 1 further comprises a condenser system comprising a split condenser (CX) 8A and 8B at the top of the desorber 3 to condense water from the desorber 3. The first condenser 8A may return condensate directly to the desorber 3, and the second condenser 8B provides condensate to the lean line 6 after HX 4. The condensate line from the first condenser 8A returns hot condensate, which is mostly water, back to the desorber 3. A measurement device 10 is arranged to measure the concentration (Cl) of the liquid in the lean line 6 before the condensate top-up point. By adjusting the amount of condensate added to the lean line 6 based on the measured concentration, the concentration of the lean sorbent stream can be adjusted to within a predetermined (optimal) range.

[0076] A filter 11 is arranged to filter the liquid in the lean line 6 after the HX 4. The filter 11 can be arranged to manage dirt / debris / fouling after the desorption step. For example, the filter may be configured to reduce or substantially remove any low solubility degradation products from the lean line 6.

[0077] Hence, the condensate (or a part thereof) from the second condenser 8B is returned to the lean line 6 after the lean return is measured to determine its concentration (Cl) and filtered, in order to adjust the lean return to correct the concentration and manage dirt / debris / fouling after desorption. If a relatively small amount of top-up condensate is required to correct the concentration of the lean return, as determined based onM&C PX221216W0

[0078] 10

[0079] measurements from the measurement device 10, then a part of the condensate from the second condenser 8B can also be returned to the desorber 3.

[0080] Heat from the second condenser 8B can also be used to increase the heat in the rich line 5 before the main heat exchanger 4. That is, the cool rich line 5 from the absorber 2 can be used to cool the second condenser 8B before being further heated by the main heat exchanger 4. This in turn leaves the lean sorbent stream after the HX 4 hotter, and can allow more useful heat to be extracted from the lean line 6 after HX 4.

[0081] The heat pump system of the DAC system 1 of Figure 3 comprises a first heat pump 12A arranged to recover heat from the first condenser 8A, and a second heat pump 12B arranged to recover heat from the lean line 6. Additional heat is extracted from the lean return via a second heat exchanger (HX) 14 connected to the second heat pump 12B. Heat pump 12B recovers heat from the lean return liquid at an intermediate temperature between that of the (main) heat exchanger 4 and the absorber 2, and thereby cools the lean return down before it is returned to the absorber. The heat is provided to an intermediate circuit 13, where it is combined with heat from the first condenser 8A. The first heat pump 12A can operate at a high coefficient of performance, because the temperature difference between the condenser 8A and the reboiler 7 is relatively small.

[0082] The reboiler 7 may require heat at 120-130 °C, while the condenser can be configured (e.g. by adjusting / setting the size of the condenser) to cool the exhaust stream to just below a dew point of the exhaust mixture, and thereby recovering most of the latent heat from the exhaust. By keeping the condensate temperature high, the heat required to reheat the condensate to the desorber temperature is small, while the COP of the heat pump is high, reducing the overall heat requirement of the system.

[0083] Additional moisture present in the exhaust stream after the first condenser 8A may be removed later at a lower temperature by the second condenser 8B. At least a part of this additional condensate from the second condenser 8B is injected back into the system to help maintain the water balance. In particular, condensate from the second condenser 8B is provided in the lean line 6 after the measurement device 10.

[0084] Because the second heat pump 12B only needs to raise the temperature of the heat from the lean return temperature to the temperature of the first condenser 8A, rather than allM&C PX221216W0

[0085] 11

[0086] the way to the temperature required for the reboiler 7, a greater choice of heat pumps are available, as heat pumps that can supply heat at above 100°C tend to be more complex and require specialist refrigerants. This means that we can run a high coefficient of performance (COP) dedicated heat pump between the condenser temperature and the reboiler for the full amount of heat required and feed the intermediate circuit 13 with heat from the first condenser 8A and from the lower temperature heat pump 12A such that the heat extracted from the intermediate circuit 13 matches the heat supplied to the intermediate circuit 13.

[0087] The temperature of the first condenser 8A can be set higher, to allow for the first heat pump 12A to operate at a high and consistent COP, so that it mainly recovers latent heat with little temperature reduction across the condenser 8A. The second condenser 8B can be configured (e.g. by setting an appropriate size of the condenser) so as to maximise both the heat content of the recovered condensate and the combined coefficient of performance of first and second heat pumps 12A, 12B so as to minimise the input energy requirements of the heat pump system.

[0088] If the first condenser 8A is operated so that about 88% to 71% of the available water vapour is recovered, then the second condenser 8B can be operated at a much colder temperature to recover the remaining water. The rich line 5, which may be at ambient temperature (or below), can be used for cooling the second condenser 8B. A part of the condensate from the second condenser 8B may be returned together with the hot condensate from the first condenser 8b to the desorber 3. Because the mass flow rate from the second condenser 8B may be only between 12% and 29% of the available moisture, and only a relatively small fraction of this might be mixed with the hot condensate from the first condenser, the mixed temperature of any combined flows back to the desorber 3 remains relatively high. In an example, the condensate temperature of a mixed flow from the first and second condensers back to the desorber 3 can be above 78°C when the condenser outlet temperature of the first condenser 8A is between 85 and 100°C and the second condenser 8B is cooled to 25°C or below. The temperature of any mixed flow can be greater if a greater part of condensate from the second condenser 8B is provided to the lean line 6. For example, if substantially 100% of the condensate recovered by the second condenser 8B is provided to the lean line 6, then the temperature of the condensate returned to the desorber 3 is the same as the outletM&C PX221216W0

[0089] 12

[0090] temperature of the first condenser 8A. This can reduce the heat requirement for reheating the returned condensate in the desorber 3.

[0091] Figure 4 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 3 above with a split condenser 8A, 8B, but with only the one heat pump 12 illustrated.

[0092] Condensate is collected from the second condenser 8B and top-up water is added to the lean line 6 as required. The DAC system 1 comprises a flow meter 15 for measuring the flow rate of the lean sorbent stream in the lean line 6. The flow meter 15 may be arranged on the lean line between the heat exchanger 4 and the measurement device 10 as shown in Figure 4. The system is configured to measure the flow rate (Fl), concentration (Cl) and then filter the lean line 6 in order to re-adjust the lean return to correct concentration and manage dirt / debris / fouling after the desorption. In this embodiment, a part of the adjustment water to the lean line 6 is provided by condensate from the second condenser 8B, with further water added from a second source (not shown).

[0093] Figure 5 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 3 above. The DAC system 1 comprises a flow meter 15 for measuring the flow rate of the lean return in the lean line 6. The flow meter 15 may be arranged on the lean line between the heat exchanger 4 and the measurement device 10 as shown in Figure 5. The system is configured to measure the flow rate (Fl), concentration (Cl) and then filter the lean line 6 in order to re-adjust the lean return to correct concentration and manage dirt / debris / fouling after the desorption. In this embodiment, a part of the top-up water to the lean line 6 is provided by condensate from the second condenser 8B, with further water added from a second source (not shown).

[0094] Figure 6 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 2 above. In addition, the DAC system 1 comprises a water reservoir 16 (e.g. a tank) for receiving condensate from the condenser 8 and for receiving further water (if necessary) from a second source (not shown) via a first control valve 17A, a second control valve 17B is used to control the flow of top-up water provided to the lean line 6. The amount of top-up water that is added via the second valve 17B can be determined based on the concentration Cl and the flow rate Fl as measured by the measurement device 10 and the flow meter 15 respectively. The amount of top-up waterM&C PX221216W0

[0095] 13

[0096] can be set as a flow rate, or an average flow rate. For example, the second valve 17B can be controlled to provide top-up water at a constant or continuous rate. Alternatively, a simpler type of valve, such as an on-off valve, may be used to provide bursts of top up water to provide a determined average amount of top-up water over a given time period. A third valve 17C allows the water reservoir 16 to provide water draw off to prevent the water reservoir from overflowing.

[0097] The embodiment allows water to be added to the water reservoir in a controlled manner to maintain the water balance of the system. The system may comprise further flow meters (not shown) to measure flow into the tank (e.g. the condensate flow rate and / or the metered flow of water from the external source via controllable flow valve 17A) and flow out of the tank (metered flow of top-up water via controllable flow valve 17B into lean line 6 and metered flow of draw-off water out of system via 17C). Water draw-off via the controlled valve 17C can ensure a fully functioning control system. Several high humidity days in a row can lead to excess water build up and therefore the provision of a water draw off control can be advantageous.

[0098] Figure 7 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 5 above, and comprising a water reservoir 16 with connected control valves 17A, 17B and 17C as described in relation to Figure 6 above.

[0099] The DAC system 1 comprises additional heat removal in the lean line. Condensate from the condenser 8B is collected in a water reservoir 16 (e.g. a tank), and additional top up water can be added to that reservoir 16 in a controlled manner to ensure there is sufficient water for water balancing. Water draw off via valve 17C can be used to ensure that the reservoir 16 does not overflow.

[0100] Water from the reservoir is provided to the lean line 6 in a controlled manner after the lean sorbent stream has its flow rate measured (Fl), has been filtered to remove dirt / debris / fouling (including low solubility degradation products) and has been measured for concentration (Cl). Hence, the concentration can be adjusted by controlled addition of top-water to the lean line 6 to the correct concentration and to thereby avoid or reduce precipitation in the absorber system.M&C PX221216W0

[0101] 14

[0102] Figure 7 also shows a pre-heat arrangement for the rich line 5, which can extract heat from the condensate so that the condensate return is not too hot. The first stage hot condensate (from the first condenser 8A) is heat exchanged with the rich line 5 first, then the (slightly) heated rich line is used to cool the second condenser 8B. Alternatively, the cold rich line 5 can cool the second condenser 8B first, before first stage hot condensate is heat exchanged with it.

[0103] Figure 8 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 6 above. In this embodiment, the DAC system 1 comprises means 10B for measuring the concentration (Cl) of absorbent in the recirculation line 17 (carrying a recirculation stream) of the absorber 2, as well as a second flow meter 15B arranged to measure the flow rate of the recirculation stream in the recirculation line 17.

[0104] The DAC system 1 is configured to provide condensate to the lean line 6 after the lean sorbent stream (from the desorber 3) has been combined with the recirculation stream (from the absorber 2). A filter 11 is arranged on the lean line 6 at a point after the lean sorbent stream has been combined with the recirculation stream to filter out any dirt and by products from both streams before they are provided to the absorber 2.

[0105] This arrangement may be advantageous with larger levels of absorbent recirculation around the absorber 2 and when the lean return is hotter than the air temperature. The recirculated absorbent may be close to the cold air temperature, and when the hotter lean return from the desorber 3 mixes with the cold recirculated absorbent from the absorber 2, then the cooling provided by the recirculated absorbent can force low solubility species in the lean sorbent stream to precipitate out before the filter 11. One or more additional filters (not shown) may be arranged before the measurements of flow rate and concentration on the lean line 6 to reduce the risk of damage to the measurement means, for example due to salt build up in the measurement sections. All the lines typically comprise means for measuring the temperature in them.

[0106] Figure 9 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 8 above. Instead of adding condensate to the lean line 6, condensate from the condenser 8 is provided to the recirculation line 17 of the absorber. Means for measuring the concentration 10B and a flow meter 15B are arranged on the recirculation line 17. Separate filters 11A and 11B are arranged on the lean line 6 and on theM&C PX221216W0

[0107] 15

[0108] recirculation line 17 respectively. The arrangement of flow meter 15B, concentration measurement means 10B and filter 10B in the recirculation line 17 can be in any order. In an alternative embodiment, the filter 11B may be arranged first, to protect the cleanliness of the flow and concentration measurement devices 15B, 10B.

[0109] Figure 10 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 4 above, wherein the DAC system 1 comprises a first condenser 8A and a second condenser 8B. In addition, the DAC system 1 comprises a water reservoir 16 and associated valves 17A, 17B and 17C as described in relation to Figure 6 above.

[0110] Instead of providing condensate to the lean line 6 or to the recirculation line 17, the system is arranged to provide condensate to the (cold) rich line 5. The DAC system 1 comprises additional means 10C for measuring the concentration in the rich line 5 and a flow meter 15C for measuring the flow rate in the rich line 5, as well as a filter 11C for filtering the rich sorbent stream before the measurement devices 10B, 10C, 15B and 15C.

[0111] In operation, water from the reservoir 16 can be added in a controlled manner to the absorbent stream after it has gone through the absorber 2 and before it is recirculated (or rich absorbent is bled off). This arrangement may be beneficial for colder climates, where a problematic point is after the absorbent has been cooled down by the air flow in the absorber 2. Whilst the filtering and water balancing is happening after the spray heads (in the absorber 2), the spray heads may still be protected because the line is filtered at a point of low(est) temperature and high(est) loading. It may be beneficial to filter and re-balance with water after the absorber 2 on a recirculation system because this can protect the recirculation system. The absorbent may be susceptible to additional precipitation after loading and cooling, especially on cold days. The absorbent flow rate is measured on both the recirculation line 17 and the rich line 5. By determining the flow rate of condensate provided to the rich line 5, the absorbent flow rate prior to condensate top up can be determined by summing the flow rates of the rich line 5 and recirculation line 17 and subtracting the condensate top up flow rate. The concentration of absorbent is measured both before and after top up.

[0112] Figure 11 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 6 above. The DAC system 1 comprises a mixing device 18 configuredM&C PX221216W0

[0113] 16

[0114] to mix the lean sorbent stream in the lean line 6 after the point of water top-up from the reservoir 16. The mixing device may comprise one or more of a tank or large vessel, a swirling section, a longer length of pipe, or a pipe bend for example. Further means 11 D for measuring the concentration of the lean sorbent are arranged after the mixing device 18.

[0115] In operation, the concentration in the lean line 6 is re-measured after the mixing device 18 to provide a means for direct feedback control. The mixing device 18 can ensure that the second concentration measurement is from a fully mixed flow. Hence, the DAC system 1 may be able to more accurately determine the concentration of absorbent and thereby provide improved feedback control for providing top-up of water from the reservoir 16.

[0116] Figure 12 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 11 above. In this embodiment, means 10C for measuring the concentration of absorbent and a flow meter 15C for measuring the flow rate are arranged on the rich line 5 before the heat exchanger 4.

[0117] In operation, the concentration and flow rate can be re-measured from the rich line 5 (the rich draw) to provide a means for direct feedback control. A potential benefit of the concentration measurement at the absorber outflow is that evaporation losses in the absorber 2, which can fluctuate due to weather conditions, is already taken into account. The feedback control target can therefore compensate for these evaporative losses automatically.

[0118] Figure 13 shows a schematic diagram of a DAC system 1 similar to that described in relation to Figure 12 above. The DAC system 1 comprises further means 10D for measuring the concentration of the lean sorbent stream arranged on the lean line 6 after the mixing device 18, and a further flow meter 15D arranged on the lean line 6 after the mixing device 18.

[0119] In operation, the concentration and flow rate can be re-measured twice: first after the top up water is mixed into the lean sorbent stream, and from the rich draw (or equivalently from the recirculated absorbent draw) to allow for direct feedback control. A potential benefit of performing the concentration measurement at both the inlet and outlet of theM&C PX221216W0

[0120] 17

[0121] absorber 2 is that there is direct feedback for the top up controller while the effect of evaporation losses in the absorber can also be taken into account. The outflow feedback control concentration target may be delayed by the absorbent hold-up time in the absorber 2. A double measurement can provide improved controller performance.

[0122] In general, the embodiments provide means for measuring the concentration, means for measuring the flow rate, means for topping up water (to maintain the water balance of the system and to prevent or hinder precipitation) and means for filtering the sorbent stream. These four different means of the embodiments may be referred to collectively herein as a “4-block unit”.

[0123] Such a 4-block unit may be located on the lean line from the desorber and / or may be located on the bypass route (the recirculation line) when liquid is being recirculated around the absorber and / or may be located on the absorber inlet pipe or prior to the spray heads or the packing wetting distribution nozzles of the absorber.

[0124] Other embodiments may provide a 5-block unit comprising a concentration sensor, a flow meter, a filter, a regulated top-up tee section and a final concentration sensor. Optionally, the system may comprise a swirl mixing section (or a longer length of pipe, or a bend or elbow or a tank or vessel) prior to the final concentration sensor to ensure the top up water is fully mixed prior to the second concentration measurement. This final concentration measurement can provide a (feedback) target for the controller to regulate to (the control objective).

[0125] The 4-block unit or 5-block unit may be placed upstream of the lean return feed, and / or on the recirculated feed, and / or a section where lean return and recirculated feeds are already mixed, and / or on the outflow to the recirculation, and / or on the outflow to the desorber (rich line draw).

[0126] Alternatively, the target concentration measurement may be measured at the outlet of the absorber in the bypass loop (if present) or in the rich line draw. In this case, the regulation of concentration can also take the evaporation losses within the absorber into account. The system can be configured to adj ust / reg ulate the concentration of absorbent to reduce or eliminate precipitation within the absorber. This embodiment can beM&C PX221216W0

[0127] 18

[0128] described as a 6-block unit comprising a concentration sensor, a flow meter, a filter, a regulated top-up tee section, a rich line concentration sensor, and a rich line flow meter.

[0129] The concentration measurement may be measured both immediately after top-up water mixing and in the rich line draw, giving an immediate concentration measurement postdilution and also a final concentration on the rich line draw.

[0130] The concentration measurement device itself can be any sensor used to measure salt concentration in a liquid preferably with an in-line technique (where measurements occur without the need to remove samples) but also possible to use an on-line technique (where removed samples are returned to the process automatically). Concentration measurement techniques include pH measurement, capacitive measurement, conductivity, optical, refractometry, spectroscopic (including near infrared spectroscopy such as Fourier Transform Infrared spectroscopy, Non-Dispersive Infrared spectroscopy, or other spectroscopic approaches such as Raman spectroscopy, attenuated total reflectance (ATR) spectrographic techniques, Laser induced fluorescence spectroscopy), tomographic techniques (nuclear magnetic resonance imaging, x-ray imaging, electrical resistance, or electrical capacitance), or calorimetric techniques and other types of concentration measurement techniques.

[0131] The flow meter may be any known in-line or on-line measurement device or indicator to give a measure of the rate of flow through the section of pipe where the concentration measurement is made. Measurement of the flow could be achieved before and / or after the filter. When the flow rate is measured after the metered water top up section, then a flow rate can be calculated / derived for the flow rate at the point of concentration measurement.

[0132] The top-up valve may be any type of controllable flow device such as a controllable valve or regulator and the water top-up inflow branch may be a tee or any other equivalent connected pipework system (multiple smaller tees).

[0133] The lean (unloaded or lowest CO2 loaded) absorbent coming from the desorber is typically the most concentrated liquid in the system because of the condensate not being returned in its entirety to the top of the desorber column. Embodiments described herein can allow the concentration of the lean return to be measured, then the line is filtered toM&C PX221216W0

[0134] 19

[0135] remove low solubility species and dirt or solids in the flow (this is to catch thermal degradation products and catch salt where the system is most concentrated). Top-up water is added to re-adjust the concentration to the target value, with the top up water flow rate controlled based on measured flow rate of the lean return line and its concentration. The top-up water can comprise condensate water generated during cooling of the CO2-rich gas flowing out of the top of the desorber. This may be substantially pure water, which is suitable for re-injecting into the system. Due to the low volatility of amino acid sorbents, they may be particularly suitable for the described embodiments that provide condensate to the absorber, rather than returning the condensate to the desorber column.

[0136] Useful heat may have already been extracted from the condensate stream so that the condensate stream may be cool or cold when being re-injected. Depending on weather conditions and on the concentration and flow rate measured using the concentration measurement device and flow meter, the target water flow rate is controlled to a target value so as to ensure that precipitation does not form in the absorber. This may require additional water over and above the condensate from the condenser system. Additional top-up water can be provided to supplement the condensate, in order to reach a target flow rate value. At other times not all the condensate water may be needed and only a fraction of it is required to regulate the concentration to the desired value. Any excess condensate can be drawn off and provided as a byproduct of the process.

[0137] Atmospheric conditions affect the operation of the absorber and on a given day there may be changes in relative humidity and temperature which lead to either a net water loss in the system or a net water gain. When there is a net water loss in the system, additional top-up water may be added to the condensate return. When there is a net water gain in the system, a smaller fraction of condensate may be returned and some condensate may be accumulated in a separate tank for later use.

[0138] The described technology has several potential advantages over conventional technology:

[0139] • It can accurately regulate the concentration of sorbent in the solvent prior to the absorbent being provided to the absorber units;

[0140] • It can use the condensate return from the system to provide a fraction of the top-up water;M&C PX221216W0

[0141] 20

[0142] • The control system can be responsive to weather condition changes to regulate to a target concentration in the lines. This may be particularly beneficial when the concentration measurement is performed in the rich draw-off line too as with the 6-block arrangement.

[0143] • It can largely eliminate dirt, dust, debris and low solubility degradation products from entering the absorber via the liquid lines;

[0144] • The concentration control can be arranged to provide condensate at the coldest and most concentrated part of the system to make the filtering of low solubility degradation products as effective as possible;

[0145] • The water balance can be actively controlled to a pre-determined (optimum) range.

[0146] • Any excess water can be drawn-off as a byproduct.

[0147] While specific embodiments of the invention have been described above, it will be appreciated that further embodiments are possible. The descriptions above are intended to be illustrative, not limiting. It will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

[0148] Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

M&C PX221216W021CLAIMS:

1. A system for the capture of carbon dioxide (CO2) from a gas stream, the system comprising:an absorber configured to contact the gas stream with a sorbent, the sorbent operable to capture CO2 from the gas stream;a desorber configured to release the CO2 from the sorbent, wherein the desorber is arranged to receive a rich sorbent stream from the absorber via a rich line and to provide a lean sorbent stream to the absorber via a lean line; anda condenser system configured to receive an exhaust stream comprising CO2 and vapour from the desorber and to remove at least a part of the vapour from the exhaust stream as condensate;wherein the system is configured to determine a concentration of absorbent in the system, and based on the determination, adjust the concentration to be within a predetermined range by providing top-up water to the system, wherein the top-up water comprises at least a part of the condensate from the condenser system.

2. A system according to claim 1 , wherein the system is configured to determine the concentration in at least one of the lean line, the rich line and a recirculation line of the absorber.

3. A system according to claim 1 or 2, wherein the top-up water is provided to at least one of the lean line, the rich line, and a recirculation line of the absorber.

4. A system according to any one of the preceding claims, further comprising a main heat exchanger connected to the lean line and to the rich line and configured to draw heat from the lean sorbent stream and provide heat to the rich sorbent stream, wherein the system is configured to determine the concentration and to provide the top-up water in the lean line in a region between the main heat exchanger and the absorber.

5. A system according to any one of the preceding claims, further comprising a flow meter arranged to measure a flow rate of a sorbent stream in the system, wherein the system is configured to adjust a flow rate of the top-up water provided to the system based on the measured flow rate.M&C PX221216W0226. A system according to any one of the preceding claims, wherein the system is configured to provide a part of the condensate back to the desorber.

7. A system according to any one of the preceding claims, wherein the condenser system comprises a first condenser and a second condenser, wherein the first condenser is directly connected to the desorber to receive the exhaust stream from the desorber and to condense a first part of the vapour comprised by the exhaust stream, and wherein the second condenser is directly connected to the first condenser to receive from the first condenser a remaining part of the exhaust stream comprising the CO2 and vapour and to condense at least a part of the vapour.

8. A system according to claim 7, wherein a first condensate from the first condenser is provided directly to the desorber, and wherein a second condensate from the second condenser is provided to the top-up water.

9. A system according to any one of the preceding claims, and comprising a heat pump system for providing heat to a reboiler of the desorber, wherein the heat pump system comprises a first heat pump for transferring heat from the lean line to an intermediate circuit, and a second heat pump for transferring heat from the intermediate circuit to the reboiler.

10. A system according to claim 9, further comprising a heat exchanger connected to the lean line and to the first heat pump.

11. A system according to any one of the preceding claims, further comprising one or more filters for filtering a sorbent stream of the system.

12. A system according to claim 11, wherein the one or more filters are arranged to filter at least one of the lean sorbent stream, the rich sorbent stream, and a recirculation stream of the absorber.

13. A system according to any one of the preceding claims, further comprising a water reservoir arranged to receive condensate from the condenser system and to receive additional water from an external source, and to provide the top-up water to the system.M&C PX221216W02314. A system according to claim 13, further comprising one or more valves for controlling the flow of liquid to and / or from the water reservoir.

15. A system according to claim 14, wherein the one or more valves comprises a first valve for controlling a flow of the additional water into the water reservoir, a second valve for controlling a flow of top-up water into the system, and a third valve for removing excess water from the water reservoir.

16. A system according to claim 15, further comprising a mixing device arranged to mix a sorbent stream downstream from a region where the top-up water is provided.

17. A system according to claim 16, further comprising means for measuring the concentration, wherein the means are located downstream from the mixing device.

18. A method of capturing carbon dioxide (CO2) from a gas stream, the method comprising:providing an absorber for contacting the gas stream with a sorbent, the sorbent operable to capture CO2 from the gas stream;providing a desorber for releasing the CO2 from the sorbent, wherein the desorber receives a rich sorbent stream from the absorber via a rich line and provides a lean sorbent stream to the absorber via a lean line; andproviding a condenser system for receiving an exhaust stream comprising CO2 and vapour from the desorber and for removing at least a part of the vapour from the exhaust stream as condensate;determining a concentration of absorbent in the system, and based on the determination, adjusting the concentration to be within a predetermined range by providing top-up water, wherein the top-up water comprises at least a part of the condensate from the condenser system.

19. A method according to claim 18, further comprising determining a flow rate of the absorbent, and adjusting a flow rate of top-up water provided based on the determination.M&C PX221216W02420. A method according to claim 18 or 19, further comprising filtering the absorbent.

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