Method for separating carbon dioxide from a gas mixture and device therefor

Abstract

The invention relates to a method for separating carbon dioxide (2) from a gas mixture (4), wherein the gas mixture (4) is passed through a solid sorbent (6), wherein the carbon dioxide (2) contained in the gas mixture (4) is sorbed by the sorbent (6) and a sorbate (8) is formed, and a flushing fluid (10) is heated to a temperature which is suitable for desorbing the carbon dioxide (2) from the sorbate (8) and flushing with the flushing fluid (10), after which the flushing fluid (10) is cooled to release the carbon dioxide (2), characterised in that a heat pump (12) is provided, having an evaporator (14) and a condenser (16), between which a coolant (18) circulates, and the evaporator (14) and the flushing fluid (10) to be cooled are made to thermally interact, and the condenser (16) is made to thermally interact with the flushing fluid (10) to be heated.

Description

[0001] Description

[0002] Method for separating carbon dioxide from a gas mixture and apparatus for this purpose

[0003] The invention relates to a method for separating carbon dioxide from a gas mixture according to the preamble of claim 1 and to a device for separating carbon dioxide from a gas mixture according to the preamble of claim 12.

[0004] To limit the global temperature increase to 1.5 K or less, negative carbon dioxide emissions must be achieved for approximately 20 years. This requires not only net-zero emissions from energy conversion processes but also the removal of carbon dioxide from ambient air using Direct Air Capture (DAC) technology. For thermodynamic reasons, energy is needed to capture carbon dioxide from air or exhaust gases, transport it, and either store it safely to prevent re-emission (CCS: Carbon Capture and Storage) or use it as a carbon feedstock for the chemical industry (CCU: Carbon Capture and Utilization). The energy required for DAC must emit less carbon dioxide than is captured.

[0005] Generally, two methods are available for separating carbon dioxide from air: temperature swing adsorption (TSA) and pressure swing adsorption (PSA). From our perspective, TSA is the more robust method. Regarding the sorbent and the reactor, the process can be operated, for example, on a fixed bed or in a fluidized bed reactor, or using a liquid sorbent. To date, DAC has been demonstrated on a pilot scale in Iceland using a fixed-bed reactor and TSA, operated by Climeworks. Here, adsorption takes place at a temperature of < 50°C with an air fan and a fixed-bed absorber. Geothermal energy, readily available in Iceland, is used for thermal regeneration, which occurs at temperatures above 100°C.However, continuous renewable energy in the form of geothermal energy is rarely available worldwide, which is why the separation process of carbon dioxide from a gas mixture, such as air, must be powered by available energy, some of which is itself generated by emitting carbon dioxide.

[0006] The object of the invention is to provide a method and a device by which carbon dioxide is separated from a gas mixture such as air, thereby reducing the consumption of carbon dioxide for the energy provided.

[0007] The solution to the problem consists of a method with the features of claim 1 and a device with the features of claim 12.

[0008] The solution to the problem consists of a method for separating carbon dioxide from a gas mixture, wherein the gas mixture is passed through a solid sorbent and the carbon dioxide (CO2) contained in the gas mixture is absorbed by the sorbent, forming a sorbate. A purge fluid is then heated to a temperature suitable for desorbing the carbon dioxide from the sorbate and flushing it out with the purge fluid, after which the purge fluid is cooled to release the carbon dioxide. The invention is characterized in that a heat pump is provided, comprising an evaporator and a condenser, between which a refrigerant circulates. The evaporator and the purge fluid to be cooled are brought into thermal contact, and the condenser is brought into thermal contact with the purge fluid to be heated.When the rinsing fluid is heated, it preferably evaporates, and conversely, when the rinsing fluid is cooled, it preferably condenses.

[0009] The advantage of the present invention over the prior art is that, firstly, a heat pump is used to heat the rinsing fluid for flushing the carbon dioxide out of the sorbate, which in itself already consumes significantly less energy than other heating methods and can be operated in a CO2-neutral manner, and secondly, that this heat pump is integrated into the circuit in such a way that the heat energy that the rinsing fluid has after flushing can be used to heat the rinsing fluid after the carbon dioxide has been removed.

[0010] The following definitions are introduced for the terms used here:

[0011] A gas mixture is, in particular, air, which naturally contains carbon dioxide, but it can also be an exhaust gas, e.g. from a cement plant.

[0012] Sorption is a collective term for processes that lead to the accumulation of a substance within a phase or at an interface between two phases. Accumulation within a phase is more precisely called absorption, while accumulation at the interface is called adsorption. The substance being absorbed is called the sorbent. The substance that has not yet been absorbed (in this case, carbon dioxide) is called the sorbent, while after sorption it is called the adsorbate or absorpt. The system consisting of the absorbed substance and the sorbent together is called the sorbate.

[0013] The rinsing fluid, which is preferably passed through the fixed-bed reactor in the gas or vapor phase, can also be considered a sorbent. It can dissolve the carbon dioxide from the solid sorbent, adsorbing, absorbing, or dissolving it in the process. The vapor phase refers to the simultaneous presence of a medium in both gaseous and liquid form, with the liquid phase existing in small droplets.

[0014] Thermal interaction encompasses all heat exchange processes based on conduction and radiation. This includes, for example, counterflow, parallel flow, and crossflow heat exchangers, as well as heat sinks around which a fluid (e.g., air, water) flows. The heat exchange media can be of different compositions, such as solid / liquid, gaseous / liquid, gaseous / solid, liquid / liquid, and gaseous / gaseous. Changes of state during thermal interaction are also possible. If fluids are used as heat transfer media in the heat exchange process, mixing of fluids within the heat transfer system should be avoided.

[0015] In a further embodiment of the invention, a process control system is provided for the described method, which is connected to an energy supply network and a threshold value for a state variable of the energy supply network is set, and the condenser of the heat pump is switched into thermal interaction with a first heat storage unit when the threshold value is reached.

[0016] Threshold and State Variable: The state variable of an energy supply network is a quantity that describes the energy content, the available energy quantities, and the network stability. It can be a direct state variable, such as the network frequency, or it can be an indirect state variable, such as a price on the electricity exchange, which indirectly reflects the technical state of the network. The threshold of the state variable can therefore be reached from above or below, depending on how the state variable is defined. The term "reached" thus also refers to being reached and exceeded, or fallen below, depending on which direction the threshold is reached. If the state variable is the price on the electricity market, then the heat treatment step for desorption takes place during the time when the electricity price is less than or equal to the threshold.If the state variable is, for example, an amount of energy fed into the energy grid by producers, then the threshold is reached from below and the heat treatment step for carbon dioxide desorption takes place when the amount of energy is greater than or equal to the threshold.

[0017] The advantage of this described configuration is that the heat pump's condenser only comes into thermal interaction with the heat storage unit when sufficient renewable energy, generated based on minimal carbon dioxide removal, is available. The state variable thus represents the amount of renewable energy generated in the energy supply network, and the threshold is reached when enough of this renewable energy is present. This means that the first heat storage unit is preferentially supplied with thermal energy via the heat pump and its condenser when this threshold is reached or exceeded, or, depending on the specific state variable, when it falls below it. This configuration also contributes to further reducing carbon dioxide emissions in the described process for separating carbon dioxide from the fluid mixture, particularly air.

[0018] The first heat storage unit can thus be supplied with thermal energy by the heat pump if the described conditions are met and continuously release energy to the process for heating the flushing fluid by being brought into thermal interaction with the flushing fluid to be heated. This means that even if the heat pump is not running and the first heat storage unit is not in thermal interaction with the first heat storage unit, the process can be operated continuously in this way. The thermal interaction between the flushing fluid and the condenser therefore occurs via the first heat storage unit, in an indirect manner, as it is not directly dependent on time. The first heat storage unit is therefore brought into thermal contact with the flushing fluid to be heated only when the aforementioned threshold value is not reached.

[0019] The heat pump used is preferably designed such that a temperature between 100°C and 160°C is reached at the condenser. This is a so-called high-temperature heat pump, which basically operates on the same principle as a conventional heat pump. However, through the use of specific refrigerants or appropriate compression, a temperature above 100°C is achieved, enabling the conversion of aqueous-based flushing fluids into the gaseous phase. This means that the flushing fluid is heated to a temperature above its boiling point. In this way, the flushing fluid, in gaseous or vaporous form, can be passed through the solid sorbate, thereby more effectively flushing or dissolving the carbon dioxide from the saturated sorbent, i.e., the sorbate. In one embodiment, the flushing fluid is water or is aqueous-based and comprises at least 90% water.

[0020] In a further embodiment of the invention, the method is designed such that a second heat storage device is provided which is in thermal contact with the rinsing fluid during its cooling.

[0021] In another embodiment, the flushing fluid serves as the refrigerant for the heat pump. This has the advantage that fewer heat exchange processes need to take place between different media, which means further thermal optimization of the process and less energy loss.

[0022] A further component of the invention is a device for separating carbon dioxide from a fluid mixture, comprising the features of claim 12. This device includes a fixed-bed reactor in which a sorbent can be arranged and a flow-through device for passing a fluid mixture through the fixed-bed reactor. The device further comprises a valve for alternately opening and closing the flow-through device and a flushing fluid supply, by means of which flushing fluid can be passed through the fixed-bed reactor. A flushing fluid cooling device and a carbon dioxide separation device for separating the carbon dioxide from the flushing fluid are also provided. Additionally, a flushing fluid heating device is included for heating the flushing fluid and for further introducing the flushing fluid into the fixed-bed reactor via the valve device.The invention is characterized in that a heat pump is provided which has an evaporator and a condenser, wherein the evaporator is in thermal interaction with the flushing fluid cooling device and the condenser is in thermal interaction with the flushing fluid heating device.

[0023] The described device offers the same advantages over the prior art as already explained in the section on the method. These advantages include, in particular, the energy-efficient operation of the device for separating carbon dioxide from a fluid mixture with the lowest possible emission of carbon dioxide itself.

[0024] In another embodiment, the purge fluid heating device is part of the heat pump's evaporator itself. Furthermore, it is advantageous for the purge fluid cooling device to be part of the heat pump's condenser, allowing both the evaporator and condenser to be operated with the purge fluid, thus reducing or eliminating heat exchange losses. This design of the device further increases the efficiency of the heat pump and, consequently, the process. In this configuration, the purge fluid serves as the heat pump's refrigerant.

[0025] Further embodiments and features are explained in more detail with reference to the following figures. These are purely schematic representations of the method and the device, which do not constitute a limitation of the scope of protection. Features with the same designation but in different embodiments are indicated by the same reference numerals, possibly with an asterisk.

[0026] This shows:

[0027] Figure 1 shows a method and a device for separating carbon dioxide from a gas mixture in a fixed-bed reactor using a heat pump.

[0028] Figure 2 shows the device and method according to Figure 1, wherein the device is connected to a power supply network via a process control system.

[0029] The process and the associated device for separating carbon dioxide 2 from a gas mixture 4, as described in Figure 1, are referred to as Direct Air Capture (DAC) process 1. It will be described starting with a fixed-bed reactor 32 on the left side of the figure. A solid sorbent 6 is provided in the fixed-bed reactor 32, which is applied, for example, to a supporting honeycomb structure to achieve a large surface area. The design is technically similar to a catalytic converter used in automotive engineering. Furthermore, a flow-through device 34 is provided, which is designed in the form of a blower, by means of which a fluid mixture 4, usually air or exhaust gas from a thermal process, is blown into the fixed-bed reactor 32. This is accomplished by means of a valve device 36, which is located upstream of the fixed-bed reactor 32.

[0030] Since the fluid mixture 4, whether conventional air or exhaust gas, always contains carbon dioxide 2, carbon dioxide 2 is introduced into the fixed-bed reactor 32 via this mixture and also via the sorbent 6. The sorbent 6 is, for example, a zeolite whose selectivity for carbon dioxide 2 is appropriately adjusted by its composition (e.g., zeolite 5A or zeolite 13X), or silica gel or activated aluminum oxide, whose selectivity for carbon dioxide 2 can be adjusted by impregnation with imines or amines, or by treatment with salt or hydroxide solutions. This sorbent 6 is applied to a support, for example in a honeycomb structure, with the largest possible surface area, so that as much carbon dioxide as possible can be deposited on the surface of the support structure coated with the sorbent 6.When the absorption rate decreases due to the saturation of the sorbent 6 with carbon dioxide 2, the sorbent 6, now saturated, is referred to as sorbate 8. In this state, the flow of fluid mixture 4 is interrupted by the valve device 36, and a vaporous or gaseous flushing fluid 10 is introduced into the fixed-bed reactor 32 via the now formed sorbate 8. This flushing fluid 10 can, for example, be steam at a sufficiently high temperature, which supplies the sorbate 8 with the necessary heat to release the carbon dioxide 2 from the sorbate 8 by desorption through an increase in temperature, thereby restoring the sorbent 6.The amount of carbon dioxide 2 that the vaporous or gaseous purge fluid 10 can absorb from the sorbate 8 is calculated as the product of the CO2 partial pressure and the total gas volume. This amount increases with the achievable temperature according to a Langmuir isotherm, whereby the heat supplied via the purge fluid 10 must not only raise the temperature of the sorbate 8 but also provide the enthalpy of desorption of the carbon dioxide 2. The sorbent 6 remains spatially unchanged on the support material in the fixed-bed reactor. Once the sorbent 6 has been sufficiently regenerated by desorption of the carbon dioxide 2 from the sorbate 8, the valve device 36 is switched again, and gas mixture 4 is passed through the fixed-bed reactor once more until the sorbent 6 is saturated.

[0031] The CO2-laden purge fluid 10 is drawn out of the fixed-bed reactor 32 and cooled via a cooling device in the form of a condenser, thereby transitioning from the vapor phase 10' to the liquid phase 10". In the liquid phase 10" the saturation concentration of carbon dioxide 2 in the purge fluid 10 is determined according to Henry's Law by the temperature-dependent CO2 solubility constant and the CO2 partial pressure above the liquid phase 10" and is therefore significantly lower than in the vapor phase 10'. Water at 25°C, for example, has a CO2 solubility constant of 1.5 g / (liter bar), which would correspond to a CO2 volume concentration of 0.06% in the vapor phase. Subsequently, the carbon dioxide 2 is extracted from the liquid purge fluid 10" by pressure release and fed to further processing, for example, a cleaning and compression process, which is not described in detail here.The purge fluid 10, freed from carbon dioxide 2, is then fed back into the process and converted back into steam or gaseous form in the purge fluid heating device 44 in order to introduce it again into the fixed bed reactor 32 through the valve device 36.

[0032] The process and device described above are further advantageously enhanced by providing a heat pump 12, which heats the cleaning fluid 10 in the cleaning fluid heating device 44. This heat pump 12 has an evaporator 14 and a condenser 16, which are connected to each other via a refrigerant 18. The refrigerant 18 is evaporated in the evaporator 14, compressed into gaseous form by a compressor (not explicitly shown here), and condensed back into a liquid in the condenser. During condensation, the refrigerant releases heat via a heat exchange process to a heat storage device 28 or directly to the cleaning fluid heating device 44.The refrigerant 18, thus condensed, remains under the high pressure introduced by the compressor and is reduced in pressure in an expansion valve (not shown here) and returned to the evaporator in liquid form at reduced pressure (which is usually higher than atmospheric pressure).

[0033] The first advantage of the described heat pump 12 in the DAC process 1 according to Figure 1 is that the heat for heating and evaporating the flushing fluid 10 can be provided by the heat pump 12 in an energy-efficient manner. Furthermore, the heat pump 12 enables the heated flushing fluid 10, which is discharged from the fixed-bed reactor 32, to be cooled, thereby transferring the heat energy to the evaporator 14. The heating and cooling of the flushing fluid 10 are each introduced into the heat pump 12 via a heat exchange process, i.e., through thermal interaction. It is thus introduced via the evaporator 14 and discharged again via the condenser 16, where it is used for heating.The thermal loss in the process due to the respective thermal interaction of the evaporator 14 and the condenser 16 in the heating and cooling process of the rinsing fluid 10 is low and minimizes the thermal loss and thus the energy that has to be introduced into the process.

[0034] Furthermore, a first heat storage unit 28 and a second heat storage unit 30 are provided, the use of which will be discussed in more detail below. This will be illustrated in particular with reference to the extended representation of Figure 1 in Figure 2. Figure 2 shows the same device and the same method already described in Figure 1, but it also has an additional feature: a process control system 20 is provided, which is connected to an energy supply network 22. The energy supply network 22 is generally a public electrical energy supply network 22, but it can also be a heat network, for example a district heating network, or it can be a company-internal or municipal electrical or heat network. A state variable 26 is defined for this energy supply network 22, which describes the network state with respect to the amount of energy supplied.This state variable 26 can, for example, be the grid frequency or the amount of energy supplied to the grid, for instance, related to an area, time, and / or line length. This state variable 26 reflects the energy content of the energy supply network 22 and, in particular, indicates whether sufficient energy is available for current consumption, whether there is an energy surplus, or an energy demand. The state variable 26 can also be measured indirectly, since, for example, the electricity price decreases on a power exchange when there is a high feed-in of energy into the energy supply network 22. Thus, the state variable 26 for the energy supply network 22 can also be the electricity price. Furthermore, a threshold value 24 is provided for the state variable 26, which empirically indicates that sufficient energy is available in the energy supply network 22.

[0035] If there is an energy surplus in the energy supply network 22, it is usually because a large amount of regeneratively generated energy is fed into the energy supply network 22.

[0036] If the state variable 26 is, for example, the energy price on a power exchange, then the process control system 20 is connected to this power exchange and a signal is transmitted which, if it falls below the threshold value 24, causes the heat pump 12 to be activated. It is also possible that the process control system 20 is connected to a power exchange via a conductor 48 of the energy supply network 22. By selectively switching on the heat pump 12 when – in this case – the threshold value 24 is reached or fallen below, a grid-supporting behavior is generated that stabilizes the energy supply network 22 and is therefore advantageous for the operator of the energy supply network 22.

[0037] Since renewable energy sources are irregular and unpredictable, the state variable 26 may remain above the threshold value 24 for an extended period, preventing the heat pump 12 from operating in a grid-friendly manner and thus from achieving optimized CO2 emission reduction. In this case, it is advantageous to increase the heat pump 12's operating rate when the threshold value 24 remains below the threshold for a prolonged period, allowing the first heat storage tank 28 to be preheated and, if necessary, superheated. This means that the condenser 16 can be in thermal interaction with the flushing fluid 10 in the flushing fluid heating device 44 and with the first heat storage tank 28. This interaction can occur independently or simultaneously. The first heat storage tank 28 is then used to heat the flushing fluid 10 when the heat pump 12 is not operating due to a lack of renewable energy sources.The same applies to a second energy storage device, which is integrated into the cooling process of the flushing fluid 10 and thermally interacts with the flushing fluid cooling device 40. This second heat storage device 30 can absorb heat from the cooling process when the heat pump 12 is not in operation. It can then also provide this heat to the evaporator 14, or it can feed the heat into another system, for example into a district heating network.

[0038] Reference symbol list

[0039] 1 DAC procedure (device)

[0040] 2 Carbon dioxide (CO2)

[0041] 4 Gas mixture

[0042] 6 sorbents

[0043] 8 Sorbat

[0044] 10 rinsing fluid

[0045] 10' vaporous flushing fluid

[0046] 10" liquid rinsing fluid

[0047] 12 Heat pump

[0048] 14 evaporators

[0049] 16 Condenser

[0050] 18 Refrigerants

[0051] 20 Process control system

[0052] 22 Energy supply network

[0053] 24 Threshold

[0054] 26 State variable

[0055] 28 first heat storage

[0056] 30 second heat storage

[0057] 32 Fixed-bed reactor

[0058] 34 Flow-through device

[0059] 36 Valve device

[0060] 38 Rinsing fluid supply

[0061] 40 Rinse fluid cooling device / condenser

[0062] 42 CO2 separation device

[0063] 44 Rinse fluid heating device / evaporator

[0064] 46 cold storage units

[0065] 48 Control room

Claims

Patent claims 1. A process for separating carbon dioxide (2) from a gas mixture (4), wherein the gas mixture (4) is passed through a solid sorbent (6), wherein the carbon dioxide (2) contained in the gas mixture (4) is sorbed by the sorbent (6) and a sorbate (8) is formed, and a rinsing fluid (10) is heated to a temperature suitable for desorbing the carbon dioxide (2) from the sorbate (8) and rinsing it out with the rinsing fluid (10), after which the rinsing fluid (10) is cooled to release the carbon dioxide (2), characterized in that - a heat pump (12) is provided which has an evaporator (14) and a condenser (16) between which a refrigerant (18) circulates and - the evaporator (14) and the rinsing fluid (10) to be cooled are brought into thermal interaction and - the condenser (16) is brought into thermal interaction with the rinsing fluid (10) to be heated.

2. Method according to claim 1, characterized in that - a process control system (20) is provided, - which is connected to an energy supply network (22) and - a threshold value (24) is set for a state variable (26) of the energy supply network (22) and - the condenser (16) of the heat pump (12) is switched into thermal interaction with a first heat storage unit (28) when the threshold (24) is reached.

3. Method according to claim 1 or 2, characterized in that the first heat storage device (28) is brought into thermal interaction with the flushing fluid (10) to be heated.

4. Method according to claims 2 and 3, characterized in that the first heat storage device (28) is brought into thermal contact with the flushing fluid (10) to be heated when the threshold value (24) is not reached.

5. Method according to one of the preceding claims, characterized in that the heat pump (12) is designed in such a way that a temperature between 100 °C and 160 °C is reached at the condenser (16).

6. Method according to one of the preceding claims, characterized in that the rinsing fluid (10) is heated to a temperature above its boiling point.

7. Method according to one of the preceding claims, characterized in that the rinsing fluid (10) is water or consists of at least 90% water.

8. Method according to one of the preceding claims, characterized in that a second heat storage device (30) is provided which is in thermal contact with the flushing fluid (10) during its cooling.

9. Method according to claim 8, characterized in that the first and the second heat storage device are brought into thermal interaction.

10. Method according to one of the preceding claims, characterized in that the rinsing fluid is passed through the solid sorbate in gaseous or vaporous form.

11. Method according to one of the preceding claims, characterized in that the flushing fluid is used as a refrigerant for the heat pump.

12. Device for separating carbon dioxide (2) from a gas mixture (4) with - a fixed-bed reactor (32) in which a sorbent (6) can be arranged and - a flow-through device (34) for the flow of a fluid mixture (10) through the fixed-bed reactor (32), - a valve device (36) for alternately opening the flow device (34) and a flushing fluid supply (38) by means of which flushing fluid (10) can be conveyed through the fixed bed reactor (32), - a rinsing fluid cooling device (40), - a carbon dioxide separation device (42) for separating the carbon dioxide (2) from the rinsing fluid (10) and - a flushing fluid heating device (44) for heating the flushing fluid (10) for further introduction of the flushing fluid (10) via the valve device (36) into the fixed bed reactor (32), characterized in that - a heat pump (12) is provided which has an evaporator (14) and a condenser (16), wherein - the evaporator (14) is in thermal interaction with the rinse fluid cooling device (40). and - the condenser (16) is in thermal interaction with the flushing fluid heating device (44).

13. Device according to claim 11, characterized in that the flushing fluid heating device (44) is part of the evaporator of the heat pump (12').

14. Device according to claim 11 or 12, characterized in that the flushing fluid cooling device (38) is part of the condenser of the heat pump (12').

15. Device according to any one of claims 12 to 14, characterized in that the The flushing fluid is the refrigerant of the heat pump (12').

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