Multi-stage membrane filtration system and method, a carbon capture plant and method using multi-stage membrane filtration for water management

Abstract

The system comprises a plurality of membrane filtration units in series. Each membrane filtration unit comprises a filtration membrane separating a concentrate side from a permeate side of the respective membrane filtration unit. Each membrane filtration unit further comprises a concentrate discharge duct, fluidly coupled to the respective concentrate side, to remove a concentrate therefrom, and a permeate discharge duct, fluidly coupled to the respective permeate side, to remove permeate therefrom. A first fluid inlet for each membrane filtration unit is fluidly coupled to the concentrate side of the membrane filtration unit. Moreover, each membrane filtration unit, except the most downstream one, further comprises a second fluid inlet, fluidly coupled to the permeate side of the membrane filtration unit.

Description

Multi-stage membrane filtration system and method, a carbon capture plant and method using multi-stage membrane filtration for water managementDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure relates generally to technologies for removing water from high-concentration salt solutions, such as solvent solutions used in carbon capture systems. The disclosure further relates to carbon capture plants, adapted to remove carbon dioxide from a flue gas containing moisture. Embodiments disclosed herein concern ammonia-based carbon capture plants and methods.BACKGROUND ART

[0002] Carbon dioxide (CO2) is a greenhouse gas which is believed to be one of the major contributors to global warming and climate changes. Carbon dioxide is generated by several industrial processes, where thermal power is generated by combustion of fossil fuels, such as natural gas and oil.

[0003] Carbon capture and storage (in short CCS) is a technology effective to make deep cuts in greenhouse gas emissions. Post combustion capture (in short PCC) is a process which uses an aqueous absorption solution (herein referred to also as solvent) containing compounds such as ammonia, potassium salts and the like, to capture carbon dioxide from flue gas generated by combustion of fossil fuel

[0004] The solvent contacts CCh-rich flue gas in an absorber and causes carbon dioxide to remain trapped in the solvent. CCL-rich solvent is then transferred to a regenerator, where carbon dioxide is removed and delivered to a carbon dioxide recovery line for further processing, while the CCL-lean solvent obtained by removing carbon dioxide therefrom is recirculated towards the absorber.

[0005] Flue gas usually contains water in the form of moisture. Part of the moisture can condense in a direct contact cooler, arranged upstream of the absorber. However, a certain amount of water remains in the flue gas which flows through the absorber and condenses in the solvent. The percentage amount of water in the solvent tendstherefore to increase due to condensation of the flue gas moisture content in the solvent.

[0006] In order to maintain the correct solvent concentration, water needs to be removed from the solvent cycle. Typically, this is achieved by vaporizing water in the regenerator using heat from a reboiler. However, this process consumes a significant amount of energy, thereby negatively impacting the efficiency of the carbon capture process.

[0007] It is therefore an aim of the subject matter disclosed herein to provide a system which is adapted to remove water from a high-concentration salt solution, such as a solvent in a carbon capture system, and which can be used, for instance, to remove water from the solvent cycle in a carbon capture plant in a more efficient, and less energy-consuming manner.SUMMARY

[0008] According to one aspect, disclosed herein is a system for removing water by membrane filtration from a concentrated salt solution, the system comprising a plurality of membrane filtration units in series. Each membrane filtration unit comprises a filtration membrane separating a concentrate side from a permeate side of the respective membrane filtration unit. Each membrane filtration unit further comprises a concentrate discharge duct, fluidly coupled to the respective concentrate side, to remove a concentrate therefrom, and a permeate discharge duct, fluidly coupled to the respective permeate side, to remove permeate therefrom. A first fluid inlet for each membrane filtration unit is fluidly coupled to the concentrate side of the membrane filtration unit. Moreover, each membrane filtration unit, except the most downstream one, further comprises a second fluid inlet, fluidly coupled to the permeate side of the membrane filtration unit. Each second fluid inlet unit and the first fluid inlet of the most upstream membrane filtration unit are fluidly coupled to a common concentrated salt solution infeed.

[0009] As will become apparent from the following detailed description, the above outlined structure enables removal of water from a highly concentrated salt solution using a membrane filtration unit arrangement and feasible pressure differentials acrossthe filtration membranes of the sequentially arranged filtration units. The second fluid inlet fluidly coupled to the permeate side of each membrane filtration unit, except the most downstream one, allows delivering of salt solution which maintains the correct salt concentration in the permeate side of each membrane filtration unit.

[0010] The system can be used in a carbon capture plant, where flue gas having a high moisture content is processed for removing carbon dioxide therefrom. In such plant, a high-concentration salt solution can be treated in the membrane filtration units to remove water therefrom. The removed water compensates for the added water in the system caused by the moisture content of the treated flue gas.

[0011] According to a further aspect, therefore, disclosed herein is a carbon capture plant, for instance an ammine-based carbon capture plant, which comprises an absorber, including: a flue gas inlet; a CCh-lean solvent inlet; and a CCh-rich solvent outlet. The plant further comprises a regenerator, which in turn includes: a CCh-rich solvent inlet, fluidly coupled with the CCh-rich solvent outlet of the absorber; a CO2- lean solvent outlet, fluidly coupled with the CCh-lean solvent inlet of the absorber; and a CO2 outlet. The carbon capture plant also includes a membrane filtration unit as outlined above, adapted to remove water from the solvent, between the absorber and the regenerator.

[0012] Additional details, embodiments and features of the membrane filtration system and of the carbon capture plant are described below and set forth in the appended claims.

[0013] According to a further aspect, disclosed herein is also a method for removing water by membrane filtration from a concentrated salt solution. According to an embodiment, the method comprises the following steps: feeding a concentrated salt solution from a concentrated salt solution infeed to a concentrate side of a first membrane filtration unit comprising a first filtration membrane which separates a concentrate side from a permeate side of the first membrane filtration unit; removing a concentrate from the concentrate side of the first membrane filtration unit; removing a permeate from the permeate side of the first membrane filtrationand delivering the permeate removed from the first membrane filtration unit to a concentrate side of a second membrane filtration unit, the second membrane filtration unit comprising a second filtration membrane which separates the concentrate side from a permeate side of the second membrane filtration unit; removing a concentrate from the concentrate side of the second membrane filtration unit; removing a permeate from the permeate side of the second membrane filtration unit and delivering the permeate removed from the second membrane filtration unit to a concentrate side of a third membrane filtration unit, the third membrane filtration unit comprising a third filtration membrane which separates the concentrate side from a permeate side of the third membrane filtration unit; and feeding a concentrated salt solution from the concentrated salt solution infeed to the permeate side of all membrane filtration units except the most downstream membrane filtration unit.

[0014] According to another aspect, a method for removing water by membrane filtration from a concentrated salt solution is disclosed herein, wherein the method uses a system comprising a plurality of membrane filtration units in series; wherein each membrane filtration unit comprises: a filtration membrane separating a concentrate side from a permeate side of the respective membrane filtration unit; a concentrate discharge duct, fluidly coupled to the respective concentrate side, to remove a concentrate therefrom; a permeate discharge duct, fluidly coupled to the respective permeate side, to remove permeate therefrom; a first fluid inlet, fluidly coupled to the concentrate side of the membrane filtration unit; wherein each membrane filtration unit, except the most downstream one, further comprises a second fluid inlet, fluidly coupled to the permeate side of the membrane filtration unit. The method comprises the following steps: generating a pressure difference across each filtration membrane; flowing a salt solution through each filtration membrane from the respective concentrate side to the permeate side of each filtration unit; removing concentrate from the concentrate side of each membrane filtration unit; transferring permeate from the permeate side of each membrane filtration unitto the concentrate side of the next membrane filtration unit; maintaining a correct salt concentration in the permeate side of each membrane filtration unit, except the most downstream membrane filtration unit, by feeding a salt solution through the second fluid inlet in the permeate side.

[0015] Disclosed herein is also a method for removing carbon dioxide from a flue gas containing carbon dioxide and moisture; the method comprising the step of condensing moisture contained in the flue gas to water; removing water from the flue gas through an absorber; removing water from the absorber by a membrane filtration process.

[0016] Further features and embodiments of the methods of the present disclosure are described below, reference being made to the enclosed drawings, and are outlined in the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Reference is now made briefly to the accompanying drawings, in which:Fig. l illustrates a schematic of a multi-stage reverse osmosis system according to the present disclosure;Fig.2 illustrates a multi-stage reverse osmosis system according to the present disclosure in more detail;Figs.3 and 4 illustrate two embodiments of a carbon capture plant based on mixed salt process and including a reverse osmosis system for managing water in the carbon capture system; andFig.5 illustrates a carbon capture plant based on the chilled ammonia process and including a reverse osmosis system for managing water in the carbon capture system.DETAILED DESCRIPTION

[0018] In the following description, reference is specifically made to a membrane filtration system and method using reverse osmosis technology. It shall however be understood, that other embodiments can use different kinds of filtration membranes, specifically nanofiltration membranes.

[0019] Fig. l illustrates a simplified schematic of a multi-stage reverse osmosis system 1 according to the present disclosure. In the embodiment depicted in Fig.l, the reverse osmosis system 1 comprises four reverse osmosis units, labeled 3A, 3B, 3C, 3D. However, it should be noted that the number of reverse osmosis units shown in Fig. 1 is provided as an example only, and a greater or lesser number of reverse osmosis units can be utilized. Reference number 3 is employed to denote a generic reverse osmosis unit.

[0020] Each reverse osmosis unit 3 comprises a semi-permeable membrane, also referred herein as reverse osmosis membrane. The semi-permeable membranes, or reverse osmosis membranes of reverse osmosis units 3A, 3B, 3C, 3D are labeled 5A, 5B, 5C, 5D respectively. Reference number 5 designates a generic semi-permeable membrane. Each semi-permeable membrane 5 separates the inner volume of the respective reverse osmosis unit 3 into a concentrate side 7 and a permeate side 9. The concentrate sides of the reverse osmosis units 3A, 3B, 3C, 3D are labeled 7A, 7B, 7C, 7D respectively. The permeate sides of the reverse osmosis units 3A, 3B, 3C, 3D are labeled 9A, 9B, 9C, 9D, respectively. Reference numbers 7 and 9 indicate a generic concentrate side and a generic permeate side, respectively.

[0021] Each reverse osmosis unit 3 comprises a concentrate discharge duct 11, labeled 11 A, 11B, 11C, 11D for the four reverse osmosis units 3A-3D shown in Fig. l. The concentrate discharge ducts 11A-11D are fluidly coupled to the respective concentrate sides 7A-7D of the respective reverse osmosis units 3A-3D. The concentrate discharge ducts 11A-11D are fluidly coupled through a collector or header 12 to a concentrate removal line 13.

[0022] Each reverse osmosis unit 3 further includes a respective permeate discharge duct 15. The permeate discharge ducts are labeled 15A, 15B, 15C, 15D, respectively.

[0023] For each pair of sequentially arranged upstream reverse osmosis unit and respective downstream reverse osmosis unit 3, the permeate discharge duct 15 of the upstream reverse osmosis unit is fluidly coupled to the concentrate side 7 of the respective downstream osmosis unit 3. Thus, for instance, the permeate discharge duct 15A is fluidly coupled to the concentrate side 7B of the reverse osmosis unit 3B, the permeate discharge duct 15B is fluidly coupled to the concentrate side 7C of thereverse osmosis unit 3C, the permeate discharge duct 15C is fluidly coupled to the concentrate side 7D of the reverse osmosis unit 3D. The permeate discharge duct 15D of the most downstream reverse osmosis unit 3D is fluidly coupled to a water discharge line 17, which removes almost pure water (i.e. water with a low or negligible concentration of salts) from the reverse osmosis system 1.

[0024] A pump, or another pressure boosting device, labeled 16 is positioned along each permeate discharge duct 15 fluidly coupling the permeate side 9 of an upstream reverse osmosis unit 3 with the concentrate side 7 of the respective downstream reverse osmosis unit 3. The pumps arranged along the permeate discharge ducts 15 A, 15B, 15C are labeled 16A, 16B, 16C respectively.

[0025] Each reverse osmosis unit 3 comprises a first fluid inlet 19, fluidly coupled to the concentrate side 7 of the respective reverse osmosis unit 3. The first fluid inlets are labeled 19A, 19B, 19C, 19D for the reverse osmosis units 3A, 3B, 3C,3D.

[0026] Additionally, each reverse osmosis unit 3A, 3B, 3C, except the most downstream one 3D, further comprises a second fluid inlet 21A, 21B, 21C, fluidly coupled to the permeate side 9A, 9B, 9C of the reverse osmosis unit 3A, 3B, 3C.

[0027] In the embodiment of Fig.1, the first fluid inlet 19B, 19C, 19D of each reverse osmosis unit 3B, 3C, 3D, except the most upstream one (reverse osmosis unit 3A), is fluidly coupled to the permeate discharge duct 15 A, 15B, 15C of the directly upstream reverse osmosis unit 3A, 3B, 3C. Moreover, the second fluid inlet 21A, 21B, 21C of each reverse osmosis unit 3A, 3B, 3C (i.e all reverse osmosis units except the most downstream one 3D) and the first fluid inlet 19A of the most upstream reverse osmosis unit 3 A are fluidly coupled to a common concentrated salt solution infeed 25. A pump, or other pressure boosting device 18 is positioned between the common concentrated salt solution infeed 25 and the first fluid inlet 19A of the most upstream reverse osmosis unit 3 A.

[0028] In some embodiments, between each pair of sequentially arranged reverse osmosis units, except between the first and second most upstream reverse osmosis units 3 A, 3B, a recirculation line 27 fluidly couples the concentrate discharge duct 11 of the downstream reverse osmosis unit 3 with the second fluid inlet 21 of the reverseosmosis unit 3 directly upstream thereof. Thus, in the embodiment of Fig.1, for instance, a first recirculation line 27.1 is arranged between the concentrate discharge duct 11C of the third reverse osmosis unit 3C and the second fluid inlet 21B of the second reverse osmosis unit 3B. In the same way, a second recirculation line 27.2 is arranged between the concentrate discharge duct 1 ID of the most downstream reverse osmosis unit 3D and the second fluid inlet 21C of the third (second last) reverse osmosis unit 3C.

[0029] The reverse osmosis system 1 of Fig.1 disclosed so far operates as follows. A water solution containing a high concentration of salts, such as a solvent of a carbon capture plant, is fed through the common concentrated salt solution infeed 25 to the reverse osmosis system 1. The primary objective of this system is to extract water from the solution while preserving the majority of chemical compounds present, effectively yielding a substantially pure water output. This would not be possible with a reverse osmosis system of the prior art since, due to the high concentration of salts in the concentrated salt solution, the osmotic pressure that would need to be overcome would be beyond feasible limits.

[0030] Typical salt concentrations in the concentrated salt solution can range for instance between 50 and 100 g / 1.

[0031] To overcome this limitation, the multi-stage reverse osmosis system 1 described here ensures that in each stage, i.e. in each reverse osmosis unit 3A, 3B, 3C, 3D, the difference in salt concentration between the concentrate side and the permeate side is sufficiently low to allow water to pass through the semi-permeable membrane 5 A, 5B, 5D, 5E from the concentrate side 7A, 7B, 7C, 7D to the permeate side 9A, 9B, 9C, 9D with operating pressures (i.e., pressure differences between the two faces of the semi-permeable membrane 5A, 5B, 5C, 5D) of an acceptable magnitude. Exemplary but not limiting values of the pressure difference between the concentrate side 7 A, 7B, 7C, 7D and the permeate side 9A, 9B, 9C, 9D may range, for instance, between 30 bar and 90 bar, preferably between 40 bar and 80 bar.

[0032] By employing an adequate number of stages, the permeate side (9D) of the furthest downstream reverse osmosis unit (3D) will accumulate nearly pure water, originating from the highly concentrated salt solution supplied through the commonconcentrated salt solution inlet (25).

[0033] More specifically, the first reverse osmosis unit 3A receives a first flow of concentrated salt solution through the first fluid inlet 19A in the concentrate side 7A thereof. A second flow of concentrated salt solution is delivered to the permeate side 9 A through the second fluid inlet 21 A. Since both the first fluid inlet 19A and the second fluid inlet 21 A receive concentrated salt solution from the common concentrated salt solution infeed 25, the salt concentration is the same in the two flows. Therefore, the osmotic pressure difference between permeate and concentrate side is 0 bar. In some embodiments most of the concentrated salt solution delivered through the common concentrated salt solution infeed 25 is delivered to the concentrate side 7A of the first reverse osmosis unit 3 A, for instance 70% of the incoming concentrated salt solution flow is delivered through the first fluid inlet 19A of the first, i.e. most upstream reverse osmosis unit 3 A. A substantially smaller flowrate, for instance 25% of the total flowrate of concentrated salt solution is fed to the permeate side 9A of the first reverse osmosis unit 3 A.

[0034] The pump 18 ensures a differential pressure between the concentrate side 7 A and the permeate side 9A of the first reverse osmosis unit 3A, such that water flows through the reverse osmosis membrane (i.e. semi-permeable membrane) 5A of the first reverse osmosis unit 3A. The remaining solution, i.e. the total incoming concentrated salt solution minus the water permeating through the semi-permeable membrane 5A, leaves the first reverse osmosis unit 3A through the first concentrate discharge duct 11 A.

[0035] Due to the pressure difference across the first reverse osmosis membrane 5A, the salt concentration of the solution contained in the permeate side 9A of the first reverse osmosis unit 3 A is lower than the salt concentration of the solution leaving the first reverse osmosis unit 3A, and lower than the salt concentration of the incoming concentrated salt solution delivered through the common concentrated salt solution infeed 25, while the salt concentration in the solution leaving the concentrate side 7A of the first reverse osmosis unit 3 A through the concentrate discharge duct 11A is higher. For instance, if the incoming concentrated salt solution delivered through the common concentrated salt solution infeed 25 has a salt freight SF equal to 1 (SF = 1),the salt freight of the solution leaving the concentrate side 7A through the first concentrate discharge duct 11A can be SF=1.01.

[0036] The liquid with lower salt concentration collected on the permeate side 9A of the first reverse osmosis unit 3 A is gradually transferred through permeate discharge duct 15A to the concentrate side 7B of the second reverse osmosis unit 3B. A small flowrate, for instance around 4%, of the total incoming concentrated salt solution entering the system 1 through the common concentrated salt solution infeed 25, is fed to the permeate side 9B of the second reverse osmosis unit 3B. The pump 16A increases the pressure of the liquid flowing from the permeate side 9A of the first reverse osmosis unit 3A to the concentrate side 7A of the second reverse osmosis unit 3B. Thus, a pressure difference is established across the semi-permeable membrane 5B of the second reverse osmosis unit 3B. The recirculation line 27.1 is foreseen to returns concentrate from the downstream reverse osmotic unit 3C back to the permeate inlet 9B. To ensure proper operation of the reverse osmosis unit 5B, the salt concentration on the permeate side 9B thereof will be maintained lower than the salt concentration in the concentrate side 7B thereof by increasing the salt concentration in the fluid recirculated through the first recirculation line 27.1. This is achieved by using solvent from the salt solution infeed 25 via the respective second fluid inlet 2 IB and discharging fluid too low in salt concentration via the concentrate discharge duct 11C to header 12.

[0037] The pressure difference causes water to flow through the semi-permeable membrane 5B, thus reducing the salt concentration in the permeate side 9B of the second reverse osmosis unit 3B. The flowrate of liquid pumped along the permeate discharge duct 15 A, minus the water which flows through the semi-permeable membrane 9B, is removed from the second reverse osmosis unit 3B through the respective concentrate discharge duct 1 IB and is collected, along with the concentrated salt solution exiting the concentrate side 7A of the first reverse osmosis unit 3 A, in the concentrate removal line 13, wherefrom the concentrated salt solution is removed from the reverse osmosis system 1.

[0038] Since the liquid collecting in the concentrate side 7B of the second reverse osmosis unit 3B has a lower salt concentration as it comes from the permeate side 9 A of the first reverse osmosis unit 3A, the salt solution leaving the second reverseosmosis unit 3B has a salt freight lower than 1, for instance around 0.75 (SF=0.75).

[0039] The permeate side 9B of the second reverse osmosis unit 3B receives water passing through the semi-permeable membrane 5B, which combines with the recirculation flow (first recirculation line 27.1) from the concentrate side 7C of the downstream reverse osmosis unit 3C. Under ideal conditions the water flow through the membranes of the different reverse osmosis modules 3A, 3B, 3C, 3D is the same. If that is not the case the salt concentrations of the different stages need to be adjusted to adjust the osmotic pressure. This is achieved by using the small flow of high concentration salt solution from the common concentrated salt solution infeed 25 (in this example, as mentioned, around 4% of the total flow rate fed through common concentrated salt solution infeed 25). In case the water passage in the downstream reverse osmosis unit 3C is smaller than the flow in the upstream reverse osmosis unit 3B, diluted recycle is disposed off using the concentrate discharge ducts (draw-off connections) 11C or 1 ID. Commercial designs will target to feature a reduction in water permeation capacity from one stage to the next to achieve easy control of the salt concentration by adding some of the concentrated salt solution infeed 25 and disposing the balance to the concentrate removal line (effluent side) 13.

[0040] Liquid flowing in the permeate side 9B of the second reverse osmosis unit 3B flows through the respective permeate discharge duct 15B towards the concentrate side 7C of the third reverse osmosis unit 3C. The pump 16B boosts the pressure of the solution flowing through said permeate discharge duct 15B, such that the pressure in the concentrate side 7C of the third reverse osmosis unit 3C is higher than the pressure in the respective permeate side 9C, which is fluidly coupled to the common concentrated salt solution infeed 25 through the second fluid inlet 21C.

[0041] Similarly to the second reverse osmosis unit 3B, also the permeate side 9C of the third reverse osmosis unit 3C is fluidly coupled at 21C to the common concentrated salt solution infeed 25 wherefrom high concentration solution can be added to the permeate, while concentrate from the fourth reverse osmosis unit 3D recirculates to the permeate side of the third reverse osmosis unit 3C through recirculation line 27.1. The small flowrate (in the example 1% of the total incoming concentrated salt solution entering the system 1) of concentrated salt solution from the commonconcentrated salt solution infeed 25 increases the concentration in the recirculated fluid from recirculation line 27.2, while fluid too low in salt concentration is discharged via fourth concentrate discharge duct 1 ID. The correct salt concentration is thus maintained in the permeate side 9C of the third reverse osmosis unit 3C.

[0042] The pressure difference across the permeable, reverse osmosis membrane 5C causes water to permeate therethrough, such that the salt concentration in the permeate side 9C of the third reverse osmosis unit 3C drops. The salt freight of the liquid leaving the concentrate side 7C of the third reverse osmosis unit 3C through concentrate discharge duct can be around 0.5 (SF=0.5), which is lower than the salt freight (SF=1) in the incoming salt solution delivered to the permeate sides 9A, 9B, 9C, 9D of all the reverse osmosis units 3A, 3B, 3C, 3D through the common concentrated salt solution infeed 25.

[0043] By recycling, through recycle liner 27.2, a fraction of low-concentration solution from the fourth concentrate discharge duct 1 ID to the second fluid inlet 21C of the third reverse osmosis unit 3C, the salt concentration in the permeate side 9C of the third reverse osmosis unit 3C is maintained constantly lower than the salt concentration in the concentrate side 7C thereof, such that water can continue to permeate through the semi-permeable membrane 5C under the effect of the pressure differential across said permeable membrane.

[0044] Liquid collecting in the permeate side 9C of the third reverse osmosis unit 3C is transferred through the respective permeate discharge duct 15C towards the concentrate side 7D of the fourth reverse osmosis unit 3D. The pump 16C increases the pressure of the fluid flowing through the permeate discharge duct 15C, such that a pressure difference is maintained across the semi-permeable membrane 5D of the fourth reverse osmosis unit 3D. The pressure difference can be, similarly as in the previous reverse osmosis units 3A, 3B, 3C , between 30 bar and 90 bar, preferably between 40 bar and 80 bar, values provided as exemplary and non-limiting.

[0045] The last RO unit 3D is preferably of a conventional type, i.e. including only a first fluid inlet 19D at the concentrate side, a concentrate discharge duct 1 ID and a fluid outlet (permeate discharge duct 15D) at the permeate side thereof.

[0046] Driven by the pressure difference across the semi-permeable membrane 5D, water flows through the semi-permeable membrane 5D from the concentrate side 7D to the permeate side 9D of the fourth reverse osmosis unit 3D. Concentrate solution leaves the concentrate side 7D through the concentrate discharge duct 1 ID. The salt freight of this salt solution can be around 0.1 (SF=0.1), while substantially pure water collects in the permeate side 9D of the fourth reverse osmosis unit 3D and is removed from the reverse osmosis system 1 through the water discharge line 17.

[0047] As mentioned, a fraction of the low-concentration solution flowing through the concentrate discharge duct 1 ID is recycled towards the permeate side 9C of the third reverse osmosis unit 3C to maintain a concentration difference between the concentrate side 7C and the permeate side 9C, such that water continues to flow through the semi-permeable membrane 5C, driven by the pressure difference therethrough.

[0048] A flow rate of nearly pure water, representing approximately 5% of the total flow rate of concentrated salt solution, fed to the reverse osmosis system 1 through the common concentrated salt solution infeed 25, is extracted therefrom. The remaining 95% of the total flowrate is collected in collector or header 12 and fed to the concentrate removal line 13.

[0049] As can be understood from the above description of the system schematically shown in Fig.l, the cascade of sequentially arranged reverse osmosis units is able to remove a controlled amount of almost pure water from a highly concentrated salt solution, with a salt content which may be as high as 50-100 g / 1, with a feasible pressure difference across the sequentially arranged reverse-osmoses membranes 5A, 5B, 5C, 5D. As will be explained later with reference to Figs. 3, 4 and 5, the multi-step or cascade reverse osmosis system can be used in a carbon capture system, to manage the water content therein.

[0050] With continuing reference to Fig.l, a more detail embodiment of the multi- step reverse osmosis system 1 according to the present disclosure is shown in Fig.2. In Fig.2 the same reference numbers indicate parts that are identical or corresponding to those illustrated in Fig.1 and described above. These parts will not be described again.

[0051] In the embodiment of Fig.2 the reverse osmosis system 1 includes five reverseosmosis units, instead of four as in Fig. l. The most downstream reverse osmosis unit is labeled 3E. The most downstream osmosis unit 3E has the same structure and fluid connections as the reverse osmosis unit 3D of Fig. l, while the reverse osmosis units 3B, 3C and 3D of Fig.2 have the same structure as the reverse osmosis units 3B or 3C of Fig.l.

[0052] In Fig.2 reference numbers 41A, 41B, 41C, 41D and 41E refer to flow rate control valves which control the concentrated salt flow from the common concentrated salt solution infeed 25 to the permeate side 9A, 9B, 9C, 9D of each reverse osmosis unit 3, except the most downstream one 9E.

[0053] Reference numbers 51 A, 5 IB, 51C, 5 ID indicate respective liquid / gas separators arranged along the permeate discharge ducts 15 A, 15B, 15C, 15D which connect to one another each pair of sequentially arranged reverse osmosis units 3A, 3B, 3C, 3D. The liquid / gas separators 51 A, 5 IB, 51C, 5 ID, if present, remove gaseous species from the permeate flow upstream of the respective pumps 16 A, 16B, 16C, 16D, to prevent possible pump damages. Gaseous species removed from the liquid-gas separators 51A-51D are collected in a gas removing collector 52 and can be recycled. For instance.

[0054] Additionally, in the diagram of Fig.2, reference number 61.1 indicates a flow control valve arranged on the first recirculation line 27.1 which connects the concentrate discharge duct 11C of the reverse osmosis unit 3C with the second fluid inlet 21B of the reverse osmosis unit 3B. Reference number 61.2 indicates a flow control valve arranged on the second recirculation line 27.2 which connects the concentrate discharge duct 1 ID of the reverse osmosis unit 3D with the second fluid inlet 21C of the reverse osmosis unit 3C. Reference number 61.3 indicates a flow control valve arranged on a recirculation line 27.3 which connects the concentrate discharge duct 11D of the reverse osmosis unit 3D with the second fluid inlet 21C of the reverse osmosis unit 3C.

[0055] Flow control valves 71 A, 71B, 71C, 71D. 71E are arranged along the respective concentrate discharge ducts 11 A, 1 IB, 11C, 1 ID, 1 IE of the five reverse osmosis units 3A, 3B, 3C, 3D, 3E.

[0056] A multi-stage reverse osmosis system 1 as described above can be used, for instance and in particular, in a carbon capture plant to control the amount of water in the salt solution processed through the plant. Fig.3 illustrates a carbon capture plant 100 based on a mixed salt process, known in the art, which will not be described in detail. Only the main components of the carbon capture plant 100 will be mentioned here.

[0057] In short, the carbon capture system 100 comprises a CO2-rich flue gas inlet 101, a direct contact cooler 103, a direct contact heater 105, an absorber section 107, including a first absorber 107A and a second absorber 107B, as well as a water wash column 107C. The carbon capture plant 100 further comprises a regenerator 109, which includes a carbon dioxide removing line 111 and a reboiler 113

[0058] A stream of chilled CCh-rich flue gas flows through line 115 in the absorber section 107, wherein carbon dioxide is removed from the CCh-rich flue gas and is absorbed in the salt solution which circulates in the first absorber 107A and in the second absorber 107B. The resulting chilled CCh-lean flue gas exits the absorber section 107 along a flue gas discharger line 117 and is heated in the direct contact heater 105 prior to be discharged in the environment through a stack 119.

[0059] Streams of CCh-rich salt solution, which is loaded with carbon dioxide removed from the flue gas, are fed through lines 121 A and 12 IB, departing from the bottom of the first absorber 107A and second absorber 107B, towards the regenerator 109.

[0060] In the regenerator 109 carbon dioxide is separated from the salt solution and collected in the carbon dioxide removing line 111 and CO2-lean salt solution is returned from the regenerator 109 to the absorber section 107. More specifically, two streams of CCh-lean salt solution are recycled from the regenerator 109 through lines 123A and 123B to the first absorber 107A and to the second absorber 107B, respectively, to absorb carbon dioxide from the infeed chilled CCh-rich flue gas fed by the direct contact cooler 103.

[0061] Moisture contained in the CCh-rich flue gas is partly drawn into the salt solution removed through lines 121A, 121B from the absorber section 107. Thiscontinuous addition of water in the salt solution circulating in the absorber section 107 and in the regenerator 109 shall be continuously removed, to maintain the correct balance of water and other species in the salt solution.

[0062] This is achieved through the reverse osmosis system 1 positioned along one of the lines which fluidly couple the absorber section 107 to the regenerator 109. In the exemplary embodiment of Fig.3, the reverse osmosis system 1 is positioned along the line 123B. Part or the entire flow of CCh-lean salt solution flowing through line 123B can be processed through the reverse osmosis system 1 to remove a controlled amount of water therefrom, which corresponds to the amount of water which is transferred from the flue gas to the salt solution in the absorber section 107.

[0063] With continuing reference to Fig.3, Fig.4 illustrates alternative embodiments of the carbon capture plant 100 comprising a reverse osmosis system 1 according to the present disclosure. The same reference numbers used in Figs. 3 and 4 designate the same parts, which will not be described again. Fig.4 illustrates two exemplary alternative positions of the reverse osmosis system 1, which can be positioned along the line 121 A (position IX) or along the line 121B (position 1 Y).

[0064] The reverse osmosis system 1 can be used in any kind of carbon capture plant, specifically in ammonia-based CO2 capture plants. With continuing reference to Figs. 3 and 4, Fig.5 illustrates a schematic of a CO2 capture plant 200 based no a chilled ammonia process (CAP), which is known in the art and which will not be described in detail.

[0065] In short, the carbon capture system 200 comprises a CCh-rich flue gas inlet 201, a direct contact cooler 203, a direct contact heater 205, an absorber section 207, including an absorber 107A and a water wash column 207C. The carbon capture plant 200 further comprises a regenerator 209, which includes a carbon dioxide removing line 211 and a reboiler 213

[0066] A stream of chilled CCh-rich flue gas flows through line 215 in the absorber section 207, wherein carbon dioxide is removed from the CCh-rich flue gas and is absorbed in the salt solution which circulates in the absorber 207A. The resulting chilled CCh-lean flue gas exits the absorber section 207 along a flue gas dischargerline 217 and is heated in the direct contact heater 205 prior to be discharged in the environment through a stack 219.

[0067] A stream of CCh-rich salt solution, which is loaded with carbon dioxide removed from the flue gas, is fed through line 221, departing from the bottom of the absorber 207A, towards the regenerator 209.

[0068] In the regenerator 209 carbon dioxide is separated from the salt solution and collected in the carbon dioxide removing line 211 and CCh-lean salt solution is returned from the regenerator 209 to the absorber 207A. More specifically, a streams of CCh-lean salt solution is recycled from the regenerator 209 through line 223 to the top of absorber 207, to absorb carbon dioxide from the chilled CCh-rich flue gas fed by the direct contact cooler 203.

[0069] Moisture contained in the CCh-rich flue gas is partly drawn into the salt solution removed through line 221 from the absorber section 207. To maintain the correct balance of water and other species in the salt solution a reverse osmosis system 1 is located in one of the lines 212, 223 which connect the absorber 207A and the regenerator 209. In the embodiment of Fig.5 the reverse osmosis system 1 is arranged along line 221.

[0070] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A system for removing water by membrane filtration from a concentrated salt solution, the system comprising a plurality of membrane filtration units in series; wherein each membrane filtration unit comprises: a filtration membrane separating a concentrate side from a permeate side of the respective membrane filtration unit; a concentrate discharge duct, fluidly coupled to the respective concentrate side, to remove a concentrate therefrom; a permeate discharge duct, fluidly coupled to the respective permeate side, to remove permeate therefrom; a first fluid inlet, fluidly coupled to the concentrate side of the membrane filtration unit; wherein each membrane filtration unit, except the most downstream one, further comprises a second fluid inlet, fluidly coupled to the permeate side of the membrane filtration unit; and wherein each second fluid inlet unit and the first fluid inlet of the most upstream membrane filtration unit are fluidly coupled to a common concentrated salt solution infeed.

2. The system of claim 1 , wherein for each pair of sequentially arranged membrane filtration units, comprising an upstream membrane filtration unit and downstream membrane filtration unit of said plurality of membrane filtration units, the permeate discharge duct of the upstream membrane filtration unit is fluidly coupled to the first fluid inlet of the respective downstream membrane filtration unit.

3. The system of claim 2, further comprising a pressure boosting device along each permeate discharge duct fluidly connecting the permeate side of the upstream membrane filtration unit and the concentrate side of the downstream membrane filtration unit.

4. The system of any one of the preceding claims, wherein the permeate side of the most downstream membrane filtration unit is fluidly coupled to a water discharge line.

5. The system any one of the preceding claims, comprising a flowratecontrol valve adapted to deliver a controlled flowrate of concentrated salt solution from the common concentrated salt solution infeed to the respective membrane filtration unit, the controlled flowrate decreasing from the most upstream to the most downstream membrane filtration unit.

6. The system of any one of the preceding claims, wherein a pressure boosting device is positioned between the common concentrated salt solution infeed and the first fluid inlet of the most upstream membrane filtration unit.

7. The system of any one of the preceding claims, wherein for each pair of sequentially arranged membrane filtration units, comprising an upstream membrane filtration unit and downstream membrane filtration unit of said plurality of membrane filtration units, except between the first most upstream membrane filtration unit and the second most upstream membrane filtration unit, a recirculation line is arranged, which fluidly couples the concentrate discharge duct of the downstream membrane filtration unit with the second fluid inlet of the upstream filtration unit.

8. The system of any one of the preceding claims, wherein each membrane filtration unit is a reverse osmosis unit, and wherein each filtration membrane is a reverse osmosis membrane.

9. A carbon capture plant comprising: an absorber, comprising: a flue gas inlet; a CCh-lean solvent inlet; and a CCh-rich solvent outlet; a regenerator, comprising: a CCh-rich solvent inlet, fluidly coupled with the CO2- rich solvent outlet of the absorber; a CCh-lean solvent outlet, fluidly coupled with the CCh-lean solvent inlet of the absorber; and a CO2 outlet; a system according to one or more of claims 1 to 8, to remove water from the solvent, between the absorber and the regenerator.

10. The carbon capture plant of claim 9, wherein the membrane filtration system is positioned along a line which fluidly couples the CCh-lean solvent outlet of the regenerator with the CCh-lean solvent inlet of the absorber.

11. The carbon capture plant of claim 9 or 10, wherein the carbon capture plant is selected from the group comprising: an ammonia-based process plant; amixed-salt process plant or a chilled-ammonia process plant12. The carbon capture plant of any one of claims 9 to 11, wherein the membrane filtration system is positioned along a line which fluidly couples the CO2- rich solvent outlet of the absorber with the C Ch-rich solvent inlet of the regenerator.

13. A method for removing water by membrane filtration from a concentrated salt solution, comprising the following steps: feeding a concentrated salt solution from a concentrated salt solution infeed to a concentrate side of a first membrane filtration unit comprising a first filtration membrane which separates a concentrate side from a permeate side of the first membrane filtration unit; removing a concentrate from the concentrate side of the first membrane filtration unit; removing a permeate from the permeate side of the first membrane filtration and delivering the permeate removed from the first membrane filtration unit to a concentrate side of a second membrane filtration unit, the second membrane filtration unit comprising a second filtration membrane which separates the concentrate side from a permeate side of the second membrane filtration unit; removing a concentrate from the concentrate side of the second membrane filtration unit; removing a permeate from the permeate side of the second membrane filtration unit and delivering the permeate removed from the second membrane filtration unit to a concentrate side of a third membrane filtration unit, the third membrane filtration unit comprising a third filtration membrane which separates the concentrate side from a permeate side of the third membrane filtration unit; and feeding a concentrated salt solution from the concentrated salt solution infeed to the permeate side of all membrane filtration units except the most downstream membrane filtration unit.

14. The method of claim 13, wherein the filtration units are reverse osmosis units and the filtration membranes are reverse osmosis membranes.

15. The method of any one of claims 13 or 14, further comprising the step of recycling concentrate from the concentrate side of the third membrane filtrationunit to the permeate side of the second membrane filtration unit.

16. The method of any one of claims 13 to 15, further comprising the following steps: increasing the pressure of the concentration salt solution fed to the concentrate side of the first membrane filtration unit; and increasing the pressure of the permeate removed from the permeate side of each first and second membrane filtration unit before delivering the permeate to the second and third membrane filtration unit, respectively.

17. A method for removing water by membrane filtration from a concentrated salt solution, with a system comprising a plurality of membrane filtration units in series; wherein each membrane filtration unit comprises: a filtration membrane separating a concentrate side from a permeate side of the respective membrane filtration unit; a concentrate discharge duct, fluidly coupled to the respective concentrate side, to remove a concentrate therefrom; a permeate discharge duct, fluidly coupled to the respective permeate side, to remove permeate therefrom; a first fluid inlet, fluidly coupled to the concentrate side of the membrane filtration unit; wherein each membrane filtration unit, except the most downstream one, further comprises a second fluid inlet, fluidly coupled to the permeate side of the membrane filtration unit; the method comprising the following steps: generating a pressure difference across each filtration membrane; flowing a salt solution through each filtration membrane from the respective concentrate side to the permeate side of each filtration unit; removing concentrate from the concentrate side of each membrane filtration unit; transferring permeate from the permeate side of each membrane filtration unit to the concentrate side of the next membrane filtration unit; maintaining a correct salt concentration in the permeate side of each membrane filtration unit, except the most downstream membrane filtration unit, by feeding a concentrated salt solution from a common concentrated salt solution infeed through the second fluid inlet in the permeate side.

18. The method of claim 17, wherein the salt solution fed through the second fluid inlet comprises concentrate discharged from the concentrate side of thenext membrane filtration unit.

19. A method for removing carbon dioxide from a flue gas containing carbon dioxide and moisture; the method comprising the step of condensing moisture contained in the flue gas to water; removing water from the flue gas through an ab- sorber; removing water from the absorber by a membrane filtration method according to any one of claims 13 to 18.

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