Steam assisted vacuum desorption process for carbon dioxide capture

Abstract

A method for separating gaseous carbon dioxide from a mixture by cyclic adsorption/desorption using a unit containing an adsorber structure with sorbent material, wherein the method comprises the following steps: (a) contacting said mixture with the sorbent material to allow said gaseous carbon dioxide to adsorb under ambient conditions; (b) evacuating said unit to a pressure in the range of 20-400 mbar.sub.abs and heating said sorbent material to a temperature in the range of 80-130 C.; and (c) re-pressurization of the unit to ambient atmospheric pressure conditions and actively cooling the sorbent material to a temperature larger or equal to ambient temperature; wherein in step (b) steam is injected into the unit to flow-through and contact the sorbent material under saturated steam conditions, and wherein the molar ratio of steam that is injected to the gaseous carbon dioxide released is less than 20:1.

Claims

1. A method for separating gaseous carbon dioxide from a gas mixture containing said gaseous carbon dioxide as well as further gases different from gaseous carbon dioxide by cyclic adsorption/desorption using a sorbent material adsorbing said gaseous carbon dioxide, using a unit containing an adsorber structure with said sorbent material, the unit being evacuable to a vacuum pressure of 400 mbar.sub.abs or less, and the adsorber structure being heatable to a temperature of at least 80 C. for the desorption of at least said gaseous carbon dioxide and the unit being openable to flow-through of the gas mixture and for contacting it with the sorbent material for the adsorption step, wherein the method comprises the following sequential and in this sequence repeating steps: (a) contacting said gas mixture with the sorbent material to allow at least said gaseous carbon dioxide to adsorb on the sorbent material under ambient atmospheric pressure conditions and ambient atmospheric temperature conditions in an adsorption step; (b) evacuating said unit to a pressure in the range of 20-400 mbar.sub.abs and heating said sorbent material in said unit to a temperature in the range of 80-130 C. in a desorption step and extracting at least the desorbed gaseous carbon dioxide from the unit and separating gaseous carbon dioxide from water by condensation in or downstream of the unit; (c) actively cooling the sorbent material under a pressure of the one in step (b) to a temperature larger or equal to ambient atmospheric temperature and re-pressurisation of the unit to ambient atmospheric pressure conditions; wherein in step (b) steam is injected into the unit to flow-through and contact the sorbent material under saturated steam or superheated steam conditions with a superheated steam temperature of up to 130 C. at the pressure level in said unit, and wherein the molar ratio of steam that is injected during the entire step (b) to the gaseous carbon dioxide released during the entire step (b) is less than 40:1.

2. The method according to claim 1, wherein step (b) comprises the following sequential steps: (b1) evacuating said unit to an initial pressure in the range of 20-200 mbar.sub.abs; (b2) preheating the sorbent material; (b3) optionally injecting an initial portion of steam in a pre-purge step; (b4) further heating the sorbent material to a temperature in the range of 80-130 C. and extracting a first portion of said gaseous carbon dioxide from the unit and separating gaseous carbon dioxide from water by condensation in or downstream of the unit; (b5) injecting steam into the unit to flow-through and contact the sorbent material under saturated steam or superheated steam conditions with a superheated steam temperature of up to 130 C. at a pressure level in the range of 50-400 mbar.sub.abs, while continuing to heat the sorbent material to a temperature in the range of 80-130 C. and extracting a second portion of said gaseous carbon dioxide from the unit and separating gaseous carbon dioxide from water by condensation in or downstream of the unit.

3. The method according to claim 2, wherein in step (b2) the sorbent material is heated to a temperature in the range of 45-75 C. using a heat exchanger integrated into the sorbent material, and/or in step (b4) the sorbent material is heated to a temperature in the range of 90-120 C. using a heat exchanger integrated into the sorbent material, and/or wherein step (c) includes active cooling of the adsorber structure to a temperature of less than 60 C.

4. The method according to claim 2, wherein in step (b3) an initial portion of steam is injected in a pre-purge step.

5. The method according to claim 2, wherein a low-temperature heat source providing heat in the range of 40-80 C. supplies the heat for steam generation in a steam heat exchange or generation unit for use during step (b5) and/or during step (b3), and wherein a high-temperature heat source providing heat in the range of 80-130 C. supplies the heat used for heating said sorbent material during step (b5).

6. The method according to claim 2, wherein the molar ratio of steam that is injected during the entire step (b) to the gaseous carbon dioxide released during the entire step (b) is less than 10:1, and/or wherein the steam used in step (b5) has a saturation pressure in the range of 50-400 mbar.sub.abs, and a corresponding saturation temperature in the range of 33-76 C.

7. The method according to claim 1, wherein a plurality of serial individual units is supplied with steam generated by a plurality of steam generation or steam heat exchange units, wherein apart from a most upstream steam generation unit these steam supplies are provided between adjacent individual units, and wherein steam is provided to a first unit by said most upstream heat exchanger at a first pressure, and the steam downstream of the first unit is, along with extraction of gaseous carbon dioxide originating from the first unit, at least partly condensed downstream of the first unit in a heat exchange and/or steam generation unit thereby providing at least part of the heat for the generation of steam to a subsequent unit at a lower second pressure than said first pressure, wherein the steam downstream of the subsequent unit is, along with extraction of gaseous carbon dioxide originating from the subsequent unit, at least partly condensed downstream of the subsequent unit in a heat exchange and/or steam generation unit thereby providing at least part of the heat for the generation of steam to a further unit at a lower third pressure than that in the previous unit, followed by at least partial condensation of the steam downstream of the further unit, along with extraction of gaseous carbon dioxide originating from the further unit, and/or followed by a serial sequence of further units, each at sequentially lower pressure, and the steam downstream of each unit is, along with extraction of gaseous carbon dioxide originating from the respective unit at least partly condensed downstream of the respective unit in a heat exchange and/or steam generation unit thereby providing at least part of the heat for the generation of steam to a next unit at lower pressure than in the previous unit.

8. The method according to claim 1, wherein one single heat source is used for providing the heat for the steam generation and the heat for the heating of the sorbent material in step (b) using a heat exchange fluid passing through a heat exchanger element provided in the adsorber structure and a steam heat exchange unit in a parallel flow arrangement or in a serial flow arrangement where in the case of a serial flow arrangement it first passes through the heat exchanger element provided in the adsorber structure and subsequently through the steam heat exchange unit.

9. The method according to claim 1, wherein steam originating from injected steam and desorbed water from the gas mixture, in a mixture with gaseous carbon dioxide extracted from the unit in step (b), is condensed in a condensation heat exchanger separating the carbon dioxide at least partly from the condensed water.

10. The method according to claim 1, wherein steam and gaseous carbon dioxide extracted from the unit in step (b) is first compressed in a re-compressor and then passes a kettle re-boiler condensing the steam and separating it from the carbon dioxide and using the released heat for generating steam for use in step (b).

11. The method according to claim 1, wherein the sorbent material is a weakly basic ion exchange resin, including one with adsorbing amine groups, or is an amine functionalised cellulose.

12. The method according to claim 1, wherein a heat exchange or steam generation unit is located in the interior space of the unit generating the steam for step (b) within the wall boundaries of the unit to be evacuated and upstream of the sorbent material, and/or wherein the steam condensation heat exchanger for the condensation of steam and/or separation of gaseous carbon dioxide is located in the interior space of the unit within the wall boundaries of the unit to be evacuated and downstream of the sorbent material.

13. The method according to claim 1, wherein steam downstream of the unit, condensed or not, is used, if need be after further supply with heat, as steam for step (b) of the same or another unit or for the generation of steam for step (b) of the same or another unit.

14. Use of a method according to claim 1 for the separation of carbon dioxide from an ambient air stream.

15. The method according to claim 1, wherein the gas mixture is air or flue gas.

16. The method according to claim 1, wherein step (b) comprises the following sequential steps: (b1) evacuating said unit to an initial pressure in the range of 50-150 mbar.sub.abs, without actively heating or cooling the sorbent material; (b2) preheating the sorbent material, to a temperature in the range of 45-75 C.; (b3) optionally injecting an initial portion of steam in a pre-purge step; (b4) further heating the sorbent material to a temperature in the range of 90-120 C. and extracting a first portion of said gaseous carbon dioxide from the unit and separating gaseous carbon dioxide from water by condensation in or downstream of the unit; (b5) injecting steam into the unit to flow-through and contact the sorbent material under saturated steam or superheated steam conditions with a superheated steam temperature of up to 130 C. at a pressure level in the range of 100-300 mbar.sub.abs while continuing to heat the sorbent material to a temperature in the range of 90-120 C. and extracting a second portion of said gaseous carbon dioxide from the unit and separating gaseous carbon dioxide from water by condensation in or downstream of the unit.

17. The method according to claim 2, wherein within steps (b2) to (b5) the pressure in the unit is allowed to increase with respect to step (b1) and/or is maintained in the range 100-300 mbar.sub.abs.

18. The method according to claim 2, wherein the heat transfer in steps (b2) and (b4) is effected by a heat exchanger element provided in the adsorber structure, involving a heat exchange fluid circulated through the heat exchanger element.

19. The method according to claim 2, wherein step (c) includes active cooling of the adsorber structure to a temperature of less than 50 C., effected by a heat exchanger element provided in the adsorber structure, involving a heat exchange fluid circulated through the heat exchanger element.

20. The method according to claim 2, wherein in step (b3) an initial portion of steam is injected in a pre-purge step, wherein in this pre-purge step (b3) the steam injected is characterized by a saturation temperature in the range of 33-58 C., corresponding to a saturation pressure range of 50-150 mbar.sub.abs.

21. The method according to claim 2, wherein in step (b3) an initial portion of steam is injected in a pre-purge step, wherein the total injected steam volume in the pre-purge step (b3), at the pressure prevailing in the unit, corresponds to less than 10 times the volume of the desorption chamber of the unit and/or wherein the molar ratio of steam that is injected during the pre-purge step (b3) to the gaseous carbon dioxide released during the entire step (b) is less than 0.5:1.

22. The method according to claim 2, wherein a low-temperature heat source providing heat in the range of 45-75 C. supplies the heat for steam generation in a steam heat exchange or generation unit for use during step (b5) and/or during step (b3), and wherein said low-temperature heat source also supplies the heat for pre-heating of said sorbent material during step (b2), and wherein a high-temperature heat source providing heat in the range of 90-120 C. supplies the heat used for heating said sorbent material during step (b5).

23. The method according to claim 2, wherein the molar ratio of steam that is injected during the entire step (b) to the gaseous carbon dioxide released during the entire step (b) is less than 10:1, and wherein the pressure in steps (b3)-(b5), is in the range of 100-300 mbar.sub.abs and wherein the temperature in steps (b2) and (b3) is increased to a value in the range 45-75 C. and wherein the temperature in steps (b4) and (b5) is increased to a value in the range 90-120 C., and/or wherein the steam used in step (b5) has a saturation pressure in the range of 100-300 mbar.sub.abs and a corresponding saturation temperature in the range of 45-69 C.

24. The method according to claim 7, wherein the sequence is followed by a serial sequence of further units, each at sequentially lower pressure, and the steam downstream of each unit is, along with extraction of gaseous carbon dioxide originating from the respective unit at least partly condensed downstream of the respective unit in a heat exchange and/or steam generation unit thereby providing at least part of the heat for the generation of steam to a next unit at lower pressure than in the previous unit.

25. The method according to claim 2, wherein steam originating from injected steam and desorbed water from the gas mixture, in a mixture with gaseous carbon dioxide extracted from the unit in step (b), (b2), (b3), (b4) and (b5) is condensed in a condensation heat exchanger separating the carbon dioxide at least partly from the condensed water, and wherein the condensation heat generated in the condensation heat exchanger is used, if need be after further increase of the temperature by means of a heat pump, for generation of steam for use in step (b) for the same unit or for other units and/or for heating of other units through heat exchanger elements provided in their adsorber structures, during step (b2).

26. The method according to claim 3, wherein in step (b3) an initial portion of steam is injected in a pre-purge step.

27. The method according to claim 3, wherein a low-temperature heat source providing heat in the range of 40-80 C. supplies the heat for steam generation in a steam heat exchange or generation unit for use during step (b5) and/or during step (b3), and wherein a high-temperature heat source providing heat in the range of 80-130 C. supplies the heat used for heating said sorbent material during step (b5).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

(2) FIG. 1 shows the basic concept of integrating a steam purge with a temperature vacuum swing desorption;

(3) FIG. 2 shows an embodiment coupling multiple units with one steam generation heat exchanger and one steam condensation heat exchanger;

(4) FIG. 3 shows an embodiment in which the steam generation heat exchanger and the heat exchanger structure of the adsorption structure are supplied by the same heat source in parallel;

(5) FIG. 4 shows an embodiment in which the steam generation heat exchanger and the heat exchanger structure of the adsorption structure are supplied by the same heat source in series;

(6) FIG. 5 shows an embodiment in which the steam generation heat exchanger and the heat exchanger structure of the adsorption structure are supplied by two different heat sources at different temperatures;

(7) FIG. 6 shows an embodiment coupling the steam generation and condensation heat exchanger with a heat pump to recover heat of condensation to steam generation;

(8) FIG. 7 shows an embodiment coupling multiple units at different pressure levels in a cascade formation;

(9) FIG. 8 shows an embodiment integrating the steam generation and condensation heat exchanger inside the unit and coupling them with a heat pump to recover heat of condensation to steam generation;

(10) FIG. 9 shows an embodiment re-compressing the gas leaving the unit before condensing the vapor in a kettle re-boiler;

(11) FIG. 10 shows a possible process sequence;

(12) FIG. 11 shows the measured flow of CO2 being desorbed in a delayed steam delivery process;

(13) FIG. 12 shows the measured effective CO2 partial pressure in a delayed steam delivery process;

(14) FIG. 13 shows the measured cumulative desorption capacity for a delayed steam delivery process;

(15) FIG. 14 shows the measured temperature of the heat source and the sorbent in a delayed steam delivery process;

(16) FIG. 15 shows the effective CO2 partial pressure achieved with a purge flow of steam at two flow rates 2.5 g/h and 5 g/h under a vacuum of 200 mbar.sub.abs;

(17) FIG. 16 shows the measured O2 concentration during the desorption of CO2 captured from ambient atmospheric air in a conventional temperature vacuum swing process; and

(18) FIG. 17 shows details of the measured O2 concentration during the desorption of CO2 captured from ambient atmospheric air in a conventional temperature vacuum swing process.

DESCRIPTION OF PREFERRED EMBODIMENTS

(19) FIG. 1 shows the basic concept and the basic elements of integrating a steam purge with a temperature vacuum swing desorption. The sorbent material, possibly contained in an adsorber structure 6 is contained in a unit 1 with a closed wall structure which is capable of withstanding vacuum pressures of normally 400 mbar.sub.abs and lower. The sorbent material is first loaded with the carbon dioxide by allowing contact between a gas mixture containing the carbon dioxide and the sorbent material. Once the desired level of loading has been reached, the unit is sealed with the appropriate valves 3 and evacuated to the desired vacuum pressure of typically in the range of 20-200 mbar.sub.abs preferably 50-150 mbar.sub.abs to achieve the pressure swing. The sorbent material within the adsorber structure 6 is also heated to a temperature between 80-130 C. preferably 90-120 C. with an adsorber structure heat exchanger 2 to achieve the temperature swing. During this phase the pressure can change and is maintained in the range 50-400 mbar.sub.abs, preferably 100-300 mbar.sub.abs. Steam 4 is produced from liquid water 17 in a steam generation heat exchanger 5 which is driven by heat input Q and injected into the unit at a non-condensing condition such that the effective partial pressure of carbon dioxide is reduced below that of the applied vacuum pressure. The steam passes through the adsorber structure 6 and the contained sorbent material along arrow 9. Desorbed gas 7 in combination with desorbed water (if present) and purge steam is withdrawn by a vacuum pump from the unit. The steam is condensed at the condensation heat exchanger 8 leaving behind the desired high purity carbon dioxide 16, liquid water 17 and releasing the condensation heat Q.

(20) FIG. 2 shows an embodiment coupling multiple adsorption units 1 with one steam generation heat exchanger 5 for energy consumption minimization purposes. This involves coupling more than one unit 1 to a single steam generation heat exchanger 5 by corresponding tubing. Such a setup can be combined with coupling the multiple units 1 to a single condensation heat exchanger 8. In this manner, multiple units could be treated with steam from one set of process devices, which simplifies control, reduces the number of components, and allow us to optimize energy consumption and maintenance.

(21) FIG. 3 shows an embodiment in which the steam generation heat exchanger and the heat exchanger structure of the adsorption structure are supplied by the same heat source in parallel. The steam is produced at a steam generation heat exchanger 5 using the same high temperature heat source 11 as that supplying the adsorber structure heat exchanger 2. Heat can be transferred, for example with a heat transfer fluid in a parallel fashion through the steam generation heat exchanger 5 and the adsorber structure heat exchanger 2.

(22) FIG. 4 shows an embodiment in which the steam generation heat exchanger and the heat exchanger structure of the adsorption structure are supplied by the same heat source in series allowing for better adaptation of the temperature levels for further minimizing energy consumption. In this case the steam generation heat exchanger 5 is in series and downstream of the adsorber structure heat exchanger 2 as shown in FIG. 4, the latter being supplied by heat by the high temperature heat source 11. As the steam saturation temperature, for example 60 C. for a desorption pressure of 200 mbar.sub.abs, is typically lower than the goal desorption temperature to be reached in the adsorber structure 6, for example 95 C., this embodiment maydepending on the design of the steam generation heat exchangerproduce superheated steam. In the embodiment involving a series connection of heat transfer fluid, steam production can only begin after the exit temperature of the adsorber structure heat exchanger has exceeded the saturation temperature. Therefore different process sequences may need to be applied.

(23) FIG. 5 shows an embodiment in which the steam generation heat exchanger and the heat exchanger structure of the adsorption structure are supplied by two different heat sources at different temperatures. The steam generation heat exchanger 5 in this case is supplied from a different, preferably lower temperature heat source 12 than the adsorber structure heat exchanger 2, which is supplied by the high temperature heat source 11. It should be noted that the lower temperature heat source 12 can also in parallel or in series be used for preheating the adsorber structure in a first phase and only once the adsorber structure is at a sufficient temperature to use the high temperature heat source 11. This embodiment illustrates at least one significant advantage of the combination of steam purge with vacuum desorption of the present invention over prior art: While the heat that is needed to bring the sorbent material to desorption temperature through the adsorber structure heat exchanger 2 is at least partly required to be at a temperature level of around 85-130 C., preferably 90-120 C., the combination of steam purge and vacuum desorption allows for steam generation (i.e. water evaporation) in the steam generation heat exchanger 5 at a significantly lower temperature level in the range of 35-80 C. preferably 45-75 C.corresponding to the saturation temperatures within the range of possible desorption pressures 50-400 mbar.sub.abs preferably 100-300 mbar.sub.abs. At this temperature level, heat can be available at potentially much lower cost than at the higher temperature level. This important aspect of this invention is possible due to the applied vacuum within the unit and is by no means obvious from the prior art.

(24) FIG. 6 shows an embodiment coupling the steam generation and condensation heat exchanger with a heat pump to recover heat of condensation to steam generation. In this case, the steam generation heat exchanger 5 and condensation heat exchanger 8 are connected with a heat pump 10. The heat of vaporization Q can thusly be recovered in the heat of condensation Q. To achieve this, only a small portion of electricity, relative to the amount of heat recovered Q, will be required to power the heat pump which needs to upgrade the temperature level of the heat only by a small temperature difference, as further laid out in the examples below. A further expansion of this embodiment could see multiple units connected to a single heat pump or multiple units connected to a combination of steam generation and condensation heat exchangers with a heat pump.

(25) FIG. 7 shows an embodiment coupling multiple units at different pressure levels in a cascade formation. In this case multiple units are connected in sequence and desorbed at decreasing pressures. FIG. 7 shows a possible example of three units 1-1, 1-2, 1-3 desorbing at pressures P1, P2, P3 where P1>P2>P3. High temperature level steam 4-1 is produced at one steam generation heat exchanger 5 with a heat input Q and fed into the first unit 1-1 at pressure P1. The desorbed gaseous carbon dioxide and steam 7-1 exit the first unit and flow into a combined steam generation-condensation heat exchanger 13 where the mixed steam and gaseous carbon dioxide flow 7-1 leaving the first unit is condensed at saturation conditions P1 and T1 producing a high purity gaseous carbon dioxide 16 and liquid water flow 17. The heat of the condensed steam at P1 and T1 is transferred through the combined steam generation-condensation heat exchanger 13 and used to produce the medium temperature steam flow 4-2 at pressure P2 with a saturation temperature of T2 which flows further into the second unit 1-2 at desorption pressure P2 where it again supports desorption. Finally at the last unit 1-3, steam at P3 with desorbed gaseous carbon dioxide 7-3 is again condensed at P3 and T3 by a condensation heat exchanger 8 leaving behind high purity gaseous carbon dioxide 16 and liquid water 17. The special feature of this embodiment is the fact that the decreasing desorption pressures and corresponding decreasing steam saturation temperatures of the subsequent units are chosen in a way such that in each combined steam generation-condensation heat exchanger a sufficient temperature gradient exists that enables efficient heat transfer. This in turn enables a very efficient recovery of the heat of condensation of the steam leaving the units. It is to be understood that this embodiment is not limited to three units and that the pressures of desorption in the multiple units for steps (b4) and (b5) fall within the ranges which are deemed attractive for desorption (50-400 mbar.sub.abs preferably 100-300 mbar.sub.abs).

(26) FIG. 8 shows an embodiment integrating the steam generation and condensation heat exchanger inside the unit and coupling them with a heat pump to recover heat of condensation to steam generation. In this case, the steam generation 5 and condensation 8 heat exchangers are integrated into the unit and connected with a heat pump 10. In this manner steam can be produced and condensed directly in situ, i.e. at the site of use in very close proximity to the adsorber structure 2. Further liquid water 17 arising from steam condensation can be internally transferred to the steam generation heat exchanger 5 to be once again used for steam generation. Because all process steam is generated and condensed within the unit 1 in this embodiment, the dimensions of piping for gas transport do not need to take steam into account. This is particularly crucial for higher molar ratios of steam to desorbed gas where the steam volume flow can for example represent 95% of the volume throughput of the piping and imply potentially uneconomical conduit sizes. Particularly for prior art processes requiring high dilution, i.e., high molar flows of steam to desorbed gas (e.g. US2011/0226872 A1), costs of piping, connection, valves and process equipment may be a very large and potentially prohibitive cost element. A further option of this embodiment sees the internally integrated steam generation heat exchanger 5 being supplied from an external heat source.

(27) FIG. 9 shows an embodiment re-compressing the gas leaving the unit before condensing the vapor in a kettle re-boiler. The steam and desorbed gaseous carbon dioxide 7 leave the unit 1 and pass through a re-compressor 14 which increases the pressure and correspondingly the saturation temperature. The re-compressor is well insulated to prevent heat losses and the mixture of steam and desorbed gas leave the re-compressor with a saturation temperature higher than the saturation temperature of the gas mixture leaving the unit. Reducing heat losses from the re-compressor is known to those skilled in the art to reduce the efficiency however it assures a good quality of re-compressed steam. The re-compressed gas flow passes through a kettle re-boiler 15 where the steam is condensed leaving behind a high purity gaseous carbon dioxide flow 16 and a liquid water 17. The herein released condensation energy Q generates steam at the saturation temperature and desorption pressure of the unit and is fed directly into the unit 4. In this manner, the steam generation heat exchanger is avoided. This procedure represents a special kind of heat pump in which the vapor is at the same time a process gas and the heat pump working fluid. In this embodiment, one re-compressor 14 and/or kettle re-boiler 15 can be used for multiple units 1.

(28) In all the above mentioned embodiments, liquid water arising from the condensed steam can be reused for steam generation. Ideally this water can be available at the saturation temperature of the condensed steam. Reusing this water flow can reduce the input heat Q by the sensible portion. For a possible steam generation temperature of 60 C. (at a saturation pressure of 200 mbar.sub.abs), the heat required for producing steam from 10 C. liquid water represents roughly 10% of the total energy input which may represent an attractive energy saving for certain processes. Further reusing this water for steam generation emits significantly less waste water than a through flow process which reduces the load on water demand, water purification and steam preparation, ultimately reducing process costs.

(29) In the above mentioned embodiments it can be advantageous to achieve a homogenous steam distribution. This can be achieved by injecting the steam on the opposite side of the adsorber structure from the vacuum extraction port as shown in for example FIG. 1. In this manner, the good gas distribution properties of certain adsorber structures can be utilized to provide good contact of steam with sorbent material.

(30) As a certain quantity of condensed water will likely be present in the unit due to the condensation on the unit walls of injected steam and desorbed water, the unit can be tilted in such a manner that this water flows towards the vacuum extraction port and is thusly removed from the unit.

(31) A steam barrier can be integrated into the unit to prevent excessive steam losses to the walls of the unit. The advantage of this aspect is a lower steam demand, a more homogenous steam distribution within the unit and a more effective steam generation and condensation on the heat exchangers. Such a steam barrier can for example be a thin metal sheet or high temperature resistant plastic sheet which is set at a defined spacing from the unit walls. As the thermal mass of such a steam barrier is small, steam condensation will quickly raise the temperature to the saturation temperature of steam limiting further steam condensation. Steam condensation of the unit walls can thus be significantly reduced.

(32) According to one embodiment of the invention, the sorbent material used for the process is a granular weak basic ion exchange resin suitable for the capture of CO.sub.2 from ambient atmospheric air. Another sorbent material suitable for use with this invention can be amine functionalized cellulose as described in WO2012/168346.

(33) Further embodiments of the present invention include various combinations of the above disclosed embodiments. For example, heat sources at two different temperature levels can be combined with a heat pump recovering the heat of condensation from multiple units and re-using condensed water for renewed steam generation.

Example 1: Steam Assisted Desorption Process

(34) The process is shown in FIG. 10 and comprises, after a step of loading the adsorber with gaseous carbon dioxide under ambient atmospheric pressure and temperature, the following stages: S1Evacuation: To the desorption pressure of 200 mbar.sub.abs, the oxygen partial pressure drops correspondingly, S2Pre-Heat: A heat source supplies heat to the adsorber structure heat exchanger to raise the temperature of the sorbent material to 60 C., S3Pre-Purge: Steam flow is started after the adsorber structure has reached the Pre-Heat temperature to flush out oxygen. The oxygen concentration drops progressively to near zero, S4Temperature Swing under Vacuum: From the Pre-Heat temperature of 60 C. to the final desorption temperature of 110 C. Desorption of gaseous carbon dioxide achieves a capacity of 0.5 mmol/g once the final temperature is reached, S5Steam Purge under Vacuum: Steam is flown at a constant flow rate at a constant sorbent material temperature. Gaseous carbon dioxide is released rapidly and brings the total desorbed capacity to 1 mmol/g, S6Cooling: The sorbent material is cooled under vacuum with the adsorber structure heat exchanger to below 50 C., S7Re-pressurization. The unit is re-pressurized to atmospheric pressure before the adsorption is restarted.

Example 2: Experimental Performance of One Process Embodiment

(35) One embodiment of a steam assisted desorption process was investigated for its application to direct air capture of CO.sub.2. The process comprised evacuation, temperature swing under vacuum up to the desorption temperature, steam purge under vacuum, cooling and re-pressurization. During the previous adsorption step, 40 g of amine functionalized adsorbent material was loaded with CO.sub.2 by flowing ambient atmospheric air with a relative humidity of 60% and a temperature of 30 C. through a vacuum chamber in which the sorbent is held. During the desorption step, the chamber was evacuated to 200 mbar.sub.abs and subsequently heated with an external heat source at 110 C. to about 100 C. During this stage (Temperature Swing under Vacuum) a quantity of CO.sub.2 is released corresponding to 0.54 mmol/g. Subsequent injection of the steam at a constant flow rate of 2.5 g/h produced a very rapid further desorption of CO.sub.2 (FIG. 11) due to the rapid and deep reduction in partial CO.sub.2 pressure (FIG. 12) down to 25 mbar.sub.abs. During the 20 minutes of this Steam Purge under Vacuum stage, an additional 0.45 mmol/g of CO.sub.2 were released bringing the total desorption capacity to 1 mmol/g (FIG. 13). The overall dilution of the CO.sub.2 with steam at the end capacity measured by the cumulative gas volumes was 3.3:1 moles H.sub.2O to moles CO.sub.2. The temperatures of the heat source and the sorbent material are shown in FIG. 14. The sorbent material temperature climbs steadily during stage Temperature Swing under Vacuum. Upon the application of steam in stage Steam Purge under Vacuum, the temperature initially drops due to the rapid desorption and the associated energy demand for the reaction.

Example 3: Effect of Various Steam Flow Rates on One Embodiment

(36) The process of Example 2 was repeated with various steam flow rates, heat source temperatures, desorption pressures and various sorbent materials. The results are summarized in the following Table 1. All experiments were conducted with 40 g of amine functionalized adsorbent.

(37) TABLE-US-00001 TABLE 1 Selected steam assisted desorption experiments Stage Stage Steam Heat source Temperature Purge temperature/ Steam Swing under under Dilution at Desorption Sorbent flow Vacuum Vacuum Total max Pressure temperature rate Capacity Capacity Capacity capacity (mbar.sub.abs) ( C.) (g/h) (mmol/g) (mmol/g (mmol/g) (nH.sub.2O:nCO.sub.2) 400 95/88 20 0.118 0.536 0.653 26:1 200 110/100 2.5 0.542 0.458 1.000 3.3:1 200 110/100 5 0.53 0.475 1.005 6.3:1 200 102/96 10 0.445 0.597 1.043 20:1 100 102/96 10 0.556 0.450 1.006 6:1 200 95/88 5 0.359 0.490 0.815 10:1 200 95/88 60 0.318 0.809 1.148 45:1 50 95/88 20 0.612 0.528 1.145 25:1

(38) A reduction of the desorption pressure at a constant desorption temperature favors the release of CO.sub.2 in Temperature Swing under Vacuum as does an increase in the desorption temperature at a constant pressure. An increased release of CO.sub.2 in stage Temperature Swing under Vacuum reduces the necessary steam demand of stage Steam Purge under Vacuum to achieve an attractive cyclic yield. Increasing the steam flow rate increases the cyclic yield by increasing the CO.sub.2 released in stage Steam Purge under Vacuum.

(39) Practically there are certain limitations on these parameters which define the operation of the preferred embodiment. Pumping out the desorbed gas at a vacuum pressure of significantly less than 100 mbar.sub.abs can represent increased process cost for DAC applications where not only pump work but also the capital investment for larger volume throughput pumps must be considered. Therefore desorption pressures of significantly less than 100 mbar.sub.abs may become economically critical. Conversely, desorption pressures higher than 200 mbar may result in low yields and high steam demands as demonstrated by the results of a 400 mbar desorption pressure experiment. Desorption temperatures as here shown should be as high as possible to achieve the largest release of CO.sub.2 in stage temperature swing under vacuum. In the temperature range 95 C.-110 C., the price of heat changes little, making it attractive to work with the highest possible temperature which avoids damage to the amines groups of the adsorbent.

(40) Currently, typical amine sorbents experience damage above roughly 120 C. Steam flow rates define the size of process equipment for the heat recovery system and the gas and water conduits. In consideration of capital costs for infrastructure such as steam generators, condensers, water handling equipment and piping a molar flow rate of greater than 40:1 steam to CO.sub.2 may be economically unfeasible when applied to DAC. Further, reduction in the steam demand further reduces the energy demand and the capital cost of process equipment. FIG. 15 shows the partial CO.sub.2 pressure for two desorption processes at 200 mbar.sub.abs and 110 C. heat source temperature using 2.5 g/h and 5 g/h steam flows. Although the CO.sub.2 partial pressure is roughly halved by increasing the steam flow, it is clear from Table 1 that the capacity in stage Steam Purge under Vacuum and the total capacity show a nearly negligible increase. This is due to the flat and rectangular form the adsorption isotherms of the selected amine functionalized sorbent materials and shows that further increases in capacity can only be brought about with very significant increases in steam demanda conclusion which is by no means obvious from the prior art.

(41) With such considerations, one possible attractive and practical operation parameter set for the investigated process was found to consist of 100-115 C. heat source temperature, 150-250 mbar.sub.abs desorption pressure and a steam to CO.sub.2 molar ratio of less than 8:1 corresponding to less than approx. 0.1-0.2 kg steam/h/kg sorbent. This combination as shown above yields the desired approx. 1 mmol/g in cyclic operation. This parameter set is specific to the selected process parameters and investigated sorbent and is not obvious from the prior art.

(42) The behavior of this process of this invention is substantially superior to a conventional pure steam purge at atmospheric pressure as it is known in the prior art. In particular, the dilution necessary to reach the same cyclic capacity for a pure steam purge is in direct proportion to the partial pressure of CO.sub.2. To reach 25 mbar.sub.abs CO.sub.2 partial pressure as achieved with this process, a pure steam purge process at atmospheric pressure would require 40 moles of steam for each mole of CO.sub.2 released. Such a steam demand might not be economically feasible from the viewpoint of energy for steam generation and capital costs for infrastructure such as steam generators, water handling equipment and piping. The process of the preferred embodiment achieves the same CO.sub.2 partial pressure with significantly lower steam demand. Secondly, the process of this invention represents a significant improvement over the conventional temperature vacuum swing process due to a doubling in the cyclic capacity. Further, because the low steam demand of the preferred process represents only a small increase in the additional energy which must be supplied for stage Steam Purge under Vacuum, an overall decrease in specific energy demand per ton of CO.sub.2 is achieved, compared to a conventional temperature vacuum swing process. A comparison between processes with and without the recovery of the heat of vaporization of steam is given in Example 4.

Example 4: Energy Analysis of Sorbent Regeneration Processes

(43) The possible process of Example 1 was analyzed for energy demand with one possible heat recovery embodiment consisting of a heat pump between steam generation and condensation heat exchangers. For comparison the energy demand is also reported without heat recovery. The energy analysis omits electricity demand of the vacuum system which is typically on the order of 100 kWh per ton CO2. The operation parameters and resulting cyclic capacities determined in Example 2 are used for the analysis along with the physical properties of an amine functionalized adsorbent shown in the following parameter table. The sorbent mass was derived from one possible configuration for the unit.

(44) TABLE-US-00002 Sorbent Specific heat 1.4 kJ/kg/K Water cyclic capacity 3 . . . 8 mmol/g (40% . . . 80% relative humidity) Heat of H.sub.2O desorption 47 kJ/mol H.sub.2O Heat of CO.sub.2 desorption 70 kJ/mol CO.sub.2 Process Steam dilution ratio in Stage 3.3:1 mol H.sub.2O:mol CO.sub.2 Steam Purge under Vacuum Sorbent mass within unit 700 kg COP Heat Pump 8 Heat of vaporization of water 2260 kJ/kg Volume purge ratio for Stage 10 Pre-Purge

(45) The total thermal energy, steam and electrical energy demand is shown in the following table for the case of one preferred embodiment with the heat recovery system.

(46) TABLE-US-00003 With waste Without waste heat recovery heat recovery Cyclic capacity 1 mmol/g Steam Mass 1.0 ton H2O/ton CO2 Steam flow rate - stage Steam 64 kg/h Purge under Vacuum Sensible heat 619 kWh/ton CO2 Heat of H2O desorption 0 890 kWh/ton CO2 (@3 mmol H2O/g) 2374 kWh/ton CO2 (@8 mmol H2O/g) Heat of CO2 desorption 441 kWh/ton CO2 Heat steam generation 0 628 kWh/ton CO2 Electricity heat pump 190 kWh/ton CO2 0 (COP = 8) (@3 mmol H2O/g) 375 kWh/ton CO2 (@8 mmol H2O/g) Total thermal energy demand 1060 kWh/ton CO2 2578 kWh/ton CO2 (@3 mmol H2O/g) (@3 mmol H2O/g) Total electrical energy 190 kWh/ton CO2 0 demand

(47) As a comparison, the same analysis was repeated for a pure steam purge desorption process conducted at atmospheric pressure as known in the prior art without recovery of the heat of vaporization with a steam quantity necessary to reach the same effective CO2 partial pressure as the preferred embodiment. The results are shown below:

(48) TABLE-US-00004 Cyclic capacity 1 mmol/g Steam Mass 19.6 ton H.sub.2O/ton CO.sub.2 Steam flow rate 1077 kg/h Electricity for heat pump 0 kWh/ton CO.sub.2 Heat for steam production 12270 kWh/ton CO.sub.2 External heating of sorbent 0 kWh/ton CO.sub.2 Total thermal energy demand 12270 kWh/ton CO.sub.2

(49) It is seen that a pure steam process of the prior art has 5 times higher energy and 17 times higher steam demand than the process of the disclosed invention, inducing high costs. Considering facility demand, with a maximum gas flow speed of 20 m/s in steam piping (which is a typical value used for plant design), the flow rate for the prior art process requires piping with a 170 mm bore (i.e. DN 200) with the correspondingly sized valves and connections. The process of the disclosed invention can use much more economical 45 mm (i.e. DN 50) piping with more readily available and significantly less expensive process equipment.

(50) As a further comparison a conventional temperature vacuum swing process without stage Pre-Heat and stage Pre-Purge is analyzed with a desorption pressure of 200 mbar.sub.abs and 110 C. heat source temperature.

(51) TABLE-US-00005 Cyclic capacity 0.54 mmol/g Steam Mass 0 ton H.sub.2O/ton CO.sub.2 Steam flow rate 0 kg/h Electricity see above. 0 kWh/ton CO.sub.2 Heat for steam production 0 kWh/ton CO.sub.2 External heating of sorbent 3543 kWh/ton CO.sub.2 Total thermal energy demand 3543 kWh/ton CO.sub.2

(52) Clearly the main drawback of this process compared to the preferred embodiment of the process disclosed in this invention is that the cyclic capacity is half that of the preferred embodiment. Correspondingly, the specific energy demand is significantly higher than that of the process of the preferred embodiment.

Example 5: Improvement of Sorbent Lifetime

(53) The process of those embodiments comprising the process stage Pre-Purge strongly reduces exposure of the sorbent to high temperature oxygen which is known to significantly reduce cyclic capacity. Stage Pre-Purge was evaluated experimentally with an amine functionalized adsorbent in a conventional temperature-vacuum swing process. It was found that by pre-purging the sorbent chamber with an inert gas and removing oxygen before beginning the temperature swing, the reduction in cyclic capacity can be reduced by 50%. Fresh sorbent was tested before and after 200 temperature-vacuum swing (TVS) cycles achieving sorbent temperatures of more than 90 C. with and without Pre-Purge. The results are shown in the table below:

(54) TABLE-US-00006 Fresh sorbent Sorbent purge cyclic Sorbent purge cyclic purge cyclic capacity after 200 TVS capacity after 200 TVS capacity cycles with Pre-Purge. cycles without Pre-Purge 1.27 mmol/g 1.11 mmol/g 1.02 mmol/g

(55) FIG. 16 show the oxygen concentration in the desorption gas during a typical temperature vacuum swing desorption of CO.sub.2 captured from ambient atmospheric air. The concentration of oxygen in the desorption gas sinks rapidly from a near atmospheric concentration as CO.sub.2 gas is released and effectively purges the vacuum chamber of the majority of oxygen achieving a sub 1% concentration roughly 26 minutes after the start of desorption. FIG. 17 shows a zoomed portion of the FIG. 16 focusing on oxygen concentrations below 100 ppm. The oxygen concentration sinks below 1 ppm roughly at 28.7 min after the start of desorption.

LIST OF REFERENCE SIGNS

(56) TABLE-US-00007 1 Unit 1-1, -2, -3 Units in a sequential arrangement 2 Adsorber structure heat exchanger 3 Adsorption valves 4 Inlet steam 4-1, -2, -3 Steam flow into various units 5 Steam generation heat exchanger 6 Adsorber structure 7 Desorption gas (gaseous carbon dioxide) 7-1, -2, -3 Desorption gas (gaseous carbon dioxide) and steam flow of various units 8 Condensation heat exchanger 9 Steam flow through adsorber structure 10 Heat pump 11 Heat source high temperature 12 Heat source low temperature 13 Combined condensation/steam generation heat exchanger 14 Re-compressor 15 Kettle re-boiler 16 High purity carbon dioxide 17 Liquid water 18 Vacuum pressure (mbar.sub.abs) 19 Oxygen partial pressure (mbar.sub.abs) 20 Steam flow (kg/h) 21 Sorbent temperature ( C.) 22 Cumulative desorption capacity (mmol/g) Q General heat flow S1-S7 Process steps P1, P2, P3 Various vacuum pressure levels T1, T2, T3 Various temperature levels