Plants and processes for capturing and recovering CO2 from process fumes

Abstract

The present invention relates to a plant (10) and process for capturing and recovering CO2 from reheat furnace fumes. Carbon dioxide is captured in a CO2 capture and separation unit (C) by membrane separation, adsorptive separation using PSA technology, or chemical absorption and gas regeneration, and split into two CO2 streams (F1, F2). One of the two streams (F1) is fed to a scrubbing device (18), where the CO2 passes through a solution of alkali metal hydroxides (MOH), particularly NaOH or KOH, to form M2CO3, which reacts with a second CO2 stream (F2) in a downstream reactor (24) to form alkali metal bicarbonate (MHCO3). Heat recovered from the fumes can be utilized in various parts / stages of the plant / process of the present invention.

Description

[Technical Field]

[0001] The present invention relates to a process and production plant for capturing, separating and using carbon dioxide streams used in the production of bicarbonates, such as sodium bicarbonate, by a Carbon Capture and Usage (CCU) approach, in particular to capture and use CO contained in process fumes from furnaces, such as reheat furnaces. [Background technology]

[0002] In steel mills, the melting and reheating stages have a significant impact in terms of CO2 emissions as various process steps involve the combustion of hydrocarbons and carbonaceous materials.

[0003] In particular, melting furnaces are highly productive and produce small amounts of CO2 in their smoke, but the smoke contains a large amount of dust, while methane reheating furnaces produce a combustion smoke stream and are inferior in terms of dust.

[0004] This second category of smoke contains about 10% carbon dioxide, and the annual CO2 emissions per one of these furnaces could reach 200,000 tonnes.

[0005] Furthermore, these fumes are transported at high temperatures, which means that they must first be cooled and then introduced into the atmosphere in order to be treated.

[0006] These reheat furnaces therefore have a greater impact on climate change-causing emissions and are a burden to factories in terms of paying emissions taxes (known as carbon taxes), an impact that is expected to increase in the coming years.Other CO2-producing furnaces in the metal or steel sector are, for example, furnaces for the heat treatment of metal products. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Alex M. James et al. in A Pressure Swing Approach to Selective CO2 Sequestration Using Functionalized Hypercrosslined Polymers (Materials 2021, 14, 1605) Summary of the Invention

[0008] The object of the present invention is to overcome the aforementioned drawbacks and to propose a plant and associated process for capturing CO2 generated in furnaces such as reheating furnaces, heat treatment furnaces, melting furnaces, etc. A further object of the present invention is to find a solution for capturing and reusing CO2 in order to reduce the climate impact not only of steel production but also of the products that will be replaced by products produced by new technological processes.

[0009] Further objects and advantages of the present invention will become apparent from the following description. In a first aspect of the present invention, this object is achieved by a plant for capturing and recovering CO2 from process fumes, the plant comprising: (a) a furnace that produces heat and smoke containing CO2 during use; (b) a CO2 capture and separation unit downstream of the reheat furnace, the CO2 capture and separation unit including a first outlet and a corresponding first duct, and a second outlet and a corresponding second duct, each outlet and corresponding duct being supplied, in use, with a gas stream containing a portion of the captured and separated CO2, the gas stream having a different CO2 concentration than that supplied to the CO2 capture and separation unit; (c) a first gas scrubbing device, preferably an absorption column, adapted to scrub CO with aqueous MOH, where M is an alkali metal, preferably sodium (Na) or potassium (K), connected to a first duct for supplying CO; (d) a reactor adapted to carry out the reaction of an alkali metal carbonate, preferably Na or K, with CO, preferably in aqueous solution, connected to second ducts for feeding CO and the alkali metal carbonate, preferably Na or K, respectively, and to the first gas scrubbing device, and equipped with an outlet for extracting the alkali metal bicarbonate produced during use of the plant by the reaction of CO with the alkali metal carbonate.

[0010] The plant serves to recover CO2 in the form of alkali metal bicarbonate. This product has versatile applications in food, especially NaHCO3 (e.g., leavening agent), medicine (e.g., hemodialysis), and hygiene (bicarbonate cleaning). The proposed production of alkali metal bicarbonate can be carried out on an industrial scale. Here, the bicarbonate of an alkali metal M is produced by a reaction process between a carbonate of the alkali metal M and carbon dioxide in an aqueous solution according to the following formula (I):

[0011] [ka]

[0012] The required alkali metal carbonate is obtained according to the following formula (II):

[0013] [ka]

[0014] Here, an alkali metal hydroxide in aqueous solution reacts with carbon dioxide in suitable contact to form water and an alkali metal carbonate, both of which capture and recover carbon dioxide in the form of a salt.

[0015] Preferred alkali metals are sodium and potassium, with sodium being particularly preferred.

[0016] The furnaces can be of various nature, such as melting furnaces, reheat furnaces, heat treatment furnaces, etc. Particularly preferred types of furnaces are those that produce smoke with a relatively low dust content (e.g., a dust content of the order of 50 mg / Nmc), such as reheat furnaces or furnaces for heat treatment.

[0017] Reheat furnaces are commonly used in metal and steel processing. They are usually powered by natural gas and produce carbon dioxide during combustion at high temperatures, allowing the heat of the smoke to be utilized.

[0018] The reheat furnace can generate low-dust smoke that can be treated with alkali metal hydroxides and their derivatives to limit the pre-treatment required for sustainable production of alkali metal bicarbonates. When required, i.e., in the case of high-dust smoke, it is preferable to insert a dust removal system between the furnace and the CO2 capture unit, which is widely available and well known to those skilled in the art.

[0019] Taking into account the characteristics of the smoke generated in the furnace and the chemical reactions required to produce the alkali metal bicarbonate, the present invention describes a plant and process for producing alkali metal bicarbonate, which can use the carbon dioxide (or at least a portion thereof) produced by the reheat process in addition to using the heat of the smoke to produce the alkali metal bicarbonate.

[0020] This will reduce the environmental impact of these furnaces, allow the steel mill to expand its production facilities, allowing it to sell chemical products in addition to steel products, and also allow the zero-emission part of the mill that is the subject of this invention to offset emissions in other areas of the mill.

[0021] The CO2 capture unit captures the gas, separates it from the smoke (i.e., any other gases and powders it may contain), concentrates it, recovers it, and allows it to be reused in the form of alkali metal bicarbonate or its derivatives.

[0022] The gas stream or smoke fed to the capture unit has a specific CO concentration (e.g., expressed as vol%) that changes as it passes through the capture unit, creating two different gas streams whose concentrations vary from 40% to 95% by volume. Thus, a concentration difference expressed in a given concentration amount (e.g., m%, vol%, mol%, etc.) represents a difference in any other concentration amount, and these concentration amounts can be converted between them.

[0023] A gas stream is understood as a flow of a substance that is gaseous at room temperature but is not necessarily in the gaseous state in this case and may even be liquefied, especially in the case of fluids that have passed through a compression system.

[0024] As already mentioned, the thermal energy of the smoke produced by the reheat furnace can also be recovered and used in other parts of the plant or in related processes.

[0025] In this regard, in a preferred embodiment, the CO2 capture and recovery plant according to the invention comprises a first heat exchanger downstream of the furnace for recovering heat from the fumes inside the plant. The recovered heat is useful, for example, for heating a CO2 regeneration device or the solution being treated therein, or for evaporating water in a concentrator that is part of the extraction system from the alkali metal bicarbonate reactor and is adapted to process and recover the remaining solution (after crystallizing the salt), as described below.

[0026] To enable the isolation of the alkali metal bicarbonate, the CO2 capture and recovery plant according to the invention advantageously further comprises a crystallizer connected to the reactor outlet, a filter or centrifuge for separating the alkali metal bicarbonate produced during the reactor's use, and a concentrator downstream of the filter or centrifuge connected to the reactor for supplying the reactor with the concentrate produced during the plant's use. Any vapors produced in the concentrator can be used, after condensation, to supply a tank mixer which serves as a source of alkali metal hydroxide for the first gas washing device. Preferably, the reactor has a gas vent connected to the first duct, which allows the recirculation of CO2 not converted to alkali metal bicarbonate or CO2 formed by decomposition of carbonates and bicarbonates to handle any pressure peaks.

[0027] A reactor is understood as a reaction unit, i.e. a vessel, device, apparatus, plant for carrying out a chemical reaction. A particularly preferred reactor is a three-phase reactor, since there is a three-phase reaction between CO (gas), water (liquid) and sodium carbonate (at least partly solid, not in solution).

[0028] In a first embodiment, the capture unit utilizes chemical reaction or absorption to separate carbon dioxide from other components of smoke, and then performs a first purification of CO. For example, CO that is not captured by chemical reaction or absorption, such as with potassium carbonate or amines, is captured during the formation of carbonate salts (MCO) that combine CO and MOH, allowing both CO fractions to reach the reactor for the production of alkali metal bicarbonate, first by reaction with potassium carbonate or amine or other separation system, and second by reaction with MOH. The amine forms carbamates with CO, and the reaction equation is shown in Equation III below.

[0029] [ka] Potassium carbonate reacts with CO2 to form potassium bicarbonate, as shown in Equation IV below.

[0030] [ka]

[0031] During CO2 regeneration, the reaction occurs in the opposite direction, releasing gas and recovering the corresponding scavenger.

[0032] When producing potassium bicarbonate by capturing and recovering carbon dioxide, amine system capture is preferred.

[0033] Absorption with aqueous amine solutions is one of the most widely used processes for removing CO2 from gaseous mixtures. Amine solutions react with CO2 via two different mechanisms. Primary and secondary amines react with CO2 via two different mechanisms: first, a zwitterion is formed, which is then deprotonated by a second amine molecule, leading to the formation of carbamates (formula III). The carbamate ions then undergo partial hydrolysis to form bicarbonates. Thus, following the above reactions, the overall reaction V can be considered.

[0034] [ka]

[0035] Tertiary amines, which do not have a free proton, will not result in the formation of a carbamate, but will instead undergo a basic reaction, thus forming a bicarbonate ion, as shown in Equation VI below.

[0036] [ka]

[0037] To reuse the amine solution used for absorption, a regeneration step must be included, where stripping can be performed using steam at high temperatures (e.g., around 120-130°C). This increases the energy cost of the process, but this can be managed using the same furnace or heat recovered from other available sources. Typically, solutions of monoethanolamine, diethanolamine, and dimethylethanolamine, with added activation components, are used.

[0038] To perform chemical capture or absorption of CO2, in a first embodiment, the CO2 capture and separation unit comprises a second gas scrubbing device, preferably an absorption column, adapted to chemically bind or absorb CO2 with an aqueous solution preferably comprising an amine or potassium carbonate, and a regeneration device downstream of the second gas scrubbing device adapted to release CO2 absorbed or bound in the gas scrubbing device, wherein the second gas scrubbing device has a first outlet and is connected to the first gas scrubbing device by a first duct, and the regeneration device has a second outlet and is connected to the reactor by a second duct.

[0039] Those skilled in the art are aware of various types of absorption columns and will select from them the one that best suits their purpose based on their general knowledge. Chemical absorption can also be achieved by other chemical systems known to those skilled in the art. Physical types of capture can also be envisaged, such as various forms of adsorption known in the art.

[0040] Advantageously, a heat exchanger can be included in the capture / separation unit for heating / cooling the material streams. In one embodiment of the present invention, it is envisaged that the second gas scrubbing device and the regeneration device are connected via a second heat exchanger, which heats the stream of solution containing the captured CO2 discharged from the second gas scrubbing device and sent to the regeneration device, and accordingly cools the regeneration solution discharged from the regeneration device and heated by a heat source included in the plant, and supplies it to the second gas scrubbing device. Advantageously, the plant according to the invention comprises a compressor downstream of the regeneration device for compressing CO2. This system makes it possible to mutually adjust the material streams in the CO2 capture and treatment steps of the heat exchanger.

[0041] In a second embodiment, the CO capture and separation unit comprises at least one membrane separator, preferably a polymeric membrane, adapted to separate CO from a gas stream, thereby producing a first gas stream rich in CO and a second gas stream lean in CO, wherein the at least one membrane separator has a first outlet and a second outlet, and supplies the CO-lean gas stream to a first gas scrubbing device through a first duct connected to the first outlet, and supplies the CO-rich gas stream to a reactor through a second duct connected to the second outlet.

[0042] This second embodiment of the capture unit exploits the ability of membranes, particularly polymeric membranes, to separate gases from a gas stream. The separation effect is based on the differential diffusion mechanism within the membrane. In this case, the smoke produced by the furnace passes through a membrane separation unit, where separation occurs in two streams with different gas concentrations. The gas that permeates the membrane is enriched in the permeate, while it is depleted in the retentate. Separation occurs due to the difference in the diffusion rates of the individual components within the membrane material. The driving force for mass transport through the membrane is the partial pressure difference of the permeating components between the feed and permeate sides. From a process technology perspective, this difference is usually caused by a lower pressure on the permeate side.

[0043] In many cases, the separation efficiency of such membranes is so high that a single-stage plant is sufficient, but multi-stage plants are also conceivable. In this case, a variant of the membrane separation embodiment envisages that at least one additional membrane separator is inserted between the at least one membrane separator and the first gas scrubbing device, and that the additional membrane separator is fed with the CO2-poor fraction leaving the first membrane separator, the CO2-enriched fraction is fed to the second duct, and the CO2-poor fraction is fed to the first gas scrubbing device. Further membrane separators can also be inserted. All of these separators can be connected in series, with the retentate discharged from one separator being fed to the next separator, and each permeate from each separator being conducted countercurrently through the separator.

[0044] Typically, a (cross-flow) tangential feed is used. In tangential flow, the feed flows tangential to the membrane and is forced across it by a pressure gradient acting on both sides of the membrane itself. A permeate stream is formed perpendicular to the membrane and a retentate stream is formed tangential to the membrane. This type of flow is used, for example, to process fluids with a high suspended solids content.

[0045] Many different membrane variants exist. Spiral-wound membranes consist of a series of flat membrane pairs glued together on three sides, with a fourth membrane connected to a central permeate collection channel. The membranes are then wrapped around the channel. The two membrane sheets are separated by a spacer net for permeate drainage. Hollow-fiber membranes consist of several small tubes of gas-selective material inserted into a tube, known as flat or tubular modular membranes. A skilled artisan can evaluate parameters such as flux or flow rate, selectivity, fouling, and membrane cleaning to select the membrane best suited to their needs.

[0046] In membrane separation, to improve separation efficiency, a sweep gas, a gas present on the permeate side of a membrane separator, can be used to lower the partial pressure of the permeating species and increase the driving force. This gas is different from the gas being separated. The driving force can be increased by manipulating the partial pressure either by increasing the feed pressure or by lowering the permeate pressure of a particular gas. In particular, the partial pressure of the permeate species can be reduced in two ways: by reducing the total pressure on the permeate side, for example by applying a vacuum, and / or by using a sweep gas on the permeate side.

[0047] This makes membrane separation advantageous when the pressure differential can be efficiently created and the separation efficiency limit is required, in which case the smoke must be separated into two CO2 streams for scrubbing with MOH and reaction with alkali metal carbonates.

[0048] Polymer membranes have high permeability to CO2 and good selectivity to other gases, depending on the polymer and the gas being separated. To achieve high separation rates and purity, a multi-stage solution is required. Membrane separation is particularly competitive in the processing of pressurized gases (>5 bar). The degree of separation can be controlled by selecting the polymer from a wide range of polymers known to those skilled in the art.

[0049] A third embodiment of the present invention, as previously mentioned, contemplates physical capture of CO2, i.e., through adsorption. Separation of materials by pressure swing adsorption (PSA) is particularly suitable.

[0050] PSA exploits the different adsorption behavior of gas molecules: under pressure, adsorbents bind CO2 better than other components of the gas, allowing it to permeate adsorbent materials such as hypercrosslinked functional polymers (e.g., those described by Alex M. James et al. in "A Pressure Swing Approach to Selective CO2 Sequestration Using Functionalized Hypercrosslined Polymers" (Materials 2021, 14, 1605)) or zeolites, carbon molecular configurations. When the adsorbent is finally saturated with the predominantly adsorbed CO2, the process can be reversed, allowing the adsorbent to be regenerated by releasing the pressure, or by flushing the adsorbent with a portion of the initially separated CO2 in the countercurrent direction. The CO2 that has passed through the adsorbent can be split into two different streams and fed from the capture unit through two different corresponding outlets and ducts to the first gas scrubbing device and reactor.

[0051] Capture units that combine different types of CO2 capture, selected from membrane separators, adsorptive separators (such as PSA), and chemical absorption systems, are also contemplated.

[0052] A second aspect of the present invention relates to a process for the capture and recovery of CO2 from process fumes, comprising the steps of:

[0053] (i) producing heat and smoke comprising CO2 using a furnace; (ii) capturing and separating the CO2 in a capture unit; (iii) splitting the captured and separated CO2 into a first CO2 stream and a second CO2 stream; (iv) washing the first stream with aqueous MOH to produce an alkali metal carbonate M2CO3, where M is preferably Na or K; (v) reacting the alkali metal carbonate with a second CO stream (F) to form the alkali metal bicarbonate MHCO; (vi) separating the alkali metal bicarbonate.

[0054] Preferably, the process at least partly utilizes the heat from the furnace, in particular the heat contained in the smoke.Possible utilizations of the heat have been described above in connection with the plant according to the invention.

[0055] Features described for one aspect of the invention are applicable mutatis mutandis to other aspects of the invention. Transfer is implicit, since one element of the plant (e.g., a membrane separator) corresponds to each step of the process (e.g., a membrane separator), and vice versa.

[0056] In an advantageous embodiment of the process for capturing and recovering CO2 according to the invention, the capture and separation of CO2 in step (ii) is carried out in the following way:

[0057] First, in (ii-a), the CO2 is washed with a solution containing amines or potassium carbonate, which absorb or chemically bind the CO2.

[0058] (ii-b) is then followed by the subsequent release or regeneration of the CO2 absorbed or bound in step (ii-a).

[0059] The portion of the CO2 that is not absorbed or bound in step (ii-a) forms a first CO2 stream, and the CO2 that is released or regenerated in step (ii-b) forms a second CO2 stream.

[0060] This carbon dioxide capture and separation fully mirrors the corresponding part of the plant shown above, and vice versa.

[0061] In a preferred variant of the invention, the process according to the invention, which uses the principle of chemical absorption, envisages that the solution liberated from CO2 is heated and then recycled in step (ii-a) by heating the solution containing the CO2 absorbed or bound between steps (ii-a) and (ii-b) with this heated solution in a heat exchanger before being recycled in step (ii-a). These additional steps make it possible to utilize inherent heat sources within the process without external energy load and to use two streams of substances at different temperatures in a "cross" fashion in heat exchange to heat / cool, depending on the needs of the process.

[0062] In an alternative embodiment of CO capture and separation, the process according to the invention envisages the advantageous use of membrane gas separation technology so as to be able to create two separate CO streams, whereby advantageously the capture and separation of CO in step (ii) is carried out by membrane separation to produce a CO concentrated stream which forms the second CO stream, and a CO depleted stream which forms the first CO stream.

[0063] In a further alternative embodiment of CO capture and separation, the process according to the invention envisages the use of adsorptive separation techniques, in particular using PSA technology, in which case the capture and separation of CO in step (ii) is advantageously carried out by adsorptive separation and generation of two CO streams.

[0064] The plant and process according to the invention are applicable to the production of bicarbonates of different alkali metals, in particular sodium and potassium. [Brief explanation of the drawings]

[0065] [Figure 1] FIG. 1 is a basic diagram showing the principle of capture and recovery of CO2 generated from a reheat furnace in a metallurgical plant. [Figure 2] 1 shows a detail of the portion of FIG. 1 relevant to the carbonate to bicarbonate morphology. [Figure 3]1 shows a detailed diagram of the capture unit of FIG. 2, illustrating a first capture variant, in this case capture by chemical absorption. [Figure 4] FIG. 3 shows a diagrammatic detail of the capture unit of FIG. 2, illustrating a second capture variant, in this case capture by single-stage membrane separation. [Figure 5] FIG. 2 shows a diagrammatic detail of the capture unit of FIG. 2, illustrating a third capture variant, in this case capture by two-stage membrane separation. [Figure 6] 1 shows a cross section of a gas separation membrane. [Figure 7] FIG. 1 is a perspective view of a hollow fiber separation membrane. [Figure 8] 1 shows a cross section of a spiral membrane. [Figure 9] The configuration of the permeable membrane (left) and the trends in partial pressure of the separated gases versus the surface area through which they pass are shown. [Figure 10] The configuration of the permeable membrane in combination with the sweep gas and the trend of the partial pressure of the separated gas relative to the surface through which it passes are shown. DETAILED DESCRIPTION OF THE INVENTION

[0066] The following examples are directed to the production of sodium bicarbonate, but can be adapted to the production of other alkali metal bicarbonates, such as potassium bicarbonate or lithium bicarbonate, by replacing sodium with another alkali metal, such as potassium or lithium, as appropriate.

[0067] Figure 1 shows the basic diagram underlying the present invention. The reheat furnace produces combustion gases containing CO2, which are captured by a capture unit. The captured carbon dioxide is split into two streams. First, it is washed with an NaOH solution to produce sodium carbonate (Na2CO3). The second stream reacts with the carbonate produced by the first stream in a reactor to form sodium bicarbonate (NaHCO3), which is then separated. Heat recovered from the reheat furnace can be used in the bicarbonate production process.

[0068] With respect to the CO2 capture process, the present invention includes various embodiments, but the recovery and use portion (at least the majority of it) of the captured CO2 remains the same or similar in all variations.

[0069] FIG. 2 therefore shows in detail the parts common to various plants according to the invention, with box C indicating the part of the plant involved in the initial capture and separation of the gas. Reference numeral 10 is used to designate a CO2 capture and recovery plant, which discharges from a reheat furnace (not shown) and enters the plant 10 (arrow 12). Hot (i.e., high-temperature) gas from the reheat furnace (RHF), with a carbon dioxide concentration of approximately 10%, is cooled in a heat exchanger 14 and pumped to the CO2 capture system C through a compression system 16. The heat generated in the reheat furnace (Δ) is preferably used at least in part within the plant 10, as described below, although it can also be used for other purposes. After capture, the gas is split into two streams F1 and F2. The first stream F1 is directed to an absorption column 18, where it is combined with a countercurrent of caustic soda injected from a tank 20. The tank 20 receives a concentrated solution containing NaOH, which is diluted in a mixer 20 before being introduced into the plant 10. The concentration of the solution containing sodium hydroxide (NaOH) corresponds, for example, to 10-25 m%. In the absorption column, further CO2 sequestration occurs, for example, for about 45%, which corresponds to the first capture step, whereas the capture unit may sequestrate about 50% of the CO2. The gas, purified in this way from about 95% of the CO2, passes through a condenser 22 or other similar device and is disposed of via a chimney according to known procedures. Sodium carbonate (Na2CO3) is discharged from the absorption column 18 in the form of an aqueous solution produced by the reaction of NaOH with CO2 and sent to the reactor 24, which is simultaneously fed with the second carbon dioxide stream F2. CO2 is recovered in the reactor 24 in the form of sodium bicarbonate (NaHCO3) produced by the reaction of Na2CO3 with CO2. An air vent 26 located above the reactor 24 optionally feeds "unconsumed" carbon dioxide into the line of the first stream F1, thereby allowing any pressure peaks to be disposed of. From the reactor 24, the aqueous solution containing bicarbonate reaches a crystallizer 28 for precipitation of the salt.Downstream of the crystallizer 28 is a separation system 30, such as a filter or centrifuge, which separates the solid salt (NaHCO3) for different uses (arrow 32), while the separated water (arrow 34) containing bicarbonate residues is sent to a concentrator 36, which then heats a portion of the water (arrow 38) containing bicarbonate residues using heat recovered in various ways (e.g., from the reheat furnace itself) to evaporate the excess water, before sending it back to the reactor 24. This vapor then passes to a condenser 40, where it is cooled by condensing into water and can be extracted (arrow 42) for other uses, or recycled to the tank 20 to dissolve solid NaOH or dilute caustic soda already in any aqueous solution.

[0070] FIG. 3 shows an embodiment of the capture unit C from the previous figure. The part of the plant where CO2 is further captured for recovery as carbonate / bicarbonate is the same as in FIG. 2 and is not shown here. Capture unit C operates according to the chemical absorption principle. After being cooled by exchanger 14, the smoke passes through a compression system 16 and then enters the first stage of an absorption column 44. The cooled smoke is introduced at the bottom of column 44 and rises upward. At the same time, an absorption liquid (such as potassium carbonate or an amine in aqueous solution) flows countercurrently across the column and combines with the CO2 contained in the smoke to form potassium bicarbonate or carbamate, removing approximately 50% or more of the carbon dioxide. High pressure (e.g., above 3 bar) and low temperature (preferably below 70°C) are advantageous in the carbon dioxide removal process. The smoke thus purified from the first portion of CO2 in column 44 and not captured by K2CO3 or the amine forms the aforementioned first carbon dioxide stream F1 and is fed to absorption column 18. The liquid potassium carbonate or carbamate solution rich in the captured CO2 is sent to the regeneration column 48 via a heat exchanger 46. The captured CO2-rich solution is heated as it passes through the exchanger 46, so that it can have a temperature sufficient to release the CO2. In practice, in the regeneration column 48, the high-temperature, low-pressure (atmospheric) liquid solution is inserted from above and crosses countercurrently with a stream of steam 56 produced by a reboiler 54 equipped with a heat source, further increasing the temperature of the steam (above 100°C), removing the CO2 component therein, and then mixing it with water vapor. The water vapor is produced from the regenerated amine or potassium carbonate solution by the heat source 54 and cooled in the regenerator 48. The mixture of water vapor and carbon dioxide passes through a condenser 50, from which condensed water (arrow 52) is discharged and fed to the regeneration column 48 and purified carbon dioxide to form the second compressed CO2 stream F2. The solution containing the "absorbent" elements accumulates at the bottom of the column 48 as the CO2 is released (captured by the steam) and is mixed with the cooled and reintroduced condensate water (arrow 52).The total is then sent to a reboiler equipped with a heat source 54, as previously described, to evaporate some of the water and circulate in vapor form (arrow 56) so that fresh CO2 can be drawn into the regeneration column 48, while the heated "absorption" liquid returns (arrow 58) through the heat exchanger 46 where it is cooled and its heat is transferred to the stream entering the regeneration column 48 of the absorption column 44. The heat source 54 can, for example, utilize recovered heat from the heat exchanger 14 or other system heat recovery.

[0071] Figure 4 shows an alternative to an amine capture system, or via K2CO3. Capture unit C includes a membrane separation system. The fumes cooled by heat exchanger 14 preferably pass through blower or compressor 16 and enter cross-flow membrane separator 60, where membrane M separates the gas into a CO2-lean (-CO2) and a CO2-rich (+CO2) fraction. The lean carbon dioxide fraction is sent to absorption column 18 as stream F1, and the enriched CO2 fraction is sent through compressor 62 or a vacuum pump to reactor 24 as stream F2.

[0072] In contrast to Figure 4, Figure 5 consists of a two-stage membrane separation system (obviously, multiple separation stages are possible to increase the recovery efficiency). A further membrane separator 64 is inserted between the membrane separator 60 and the absorption column 18, which is fed with the CO2-lean fraction discharged from the first membrane separator 60. This further membrane separator 64 separates the gas into a CO2-depleted fraction (-CO2) and a CO2-enriched fraction (+CO2). The CO2-depleted fraction (-CO2) is fed to the absorption column 18 as stream F1, while the CO2-enriched fraction (+CO2) is fed as stream F2 (through 68).

[0073] 3-5, solid lines represent room temperature flows, dashed lines represent cold flows, dotted lines represent hot flows, and dashed and dotted mixed lines represent compressed gas flows. The capture unit may be any other CO2 capture system known to those skilled in the art.

[0074] FIG. 6 shows a cross section of a porous membrane M of a membrane separator. A porous support 84, with an exemplary thickness of 50-100 μm, is shown, followed by channels 86, followed by a selective layer 88 (typically 1,000 Å thick) that represents the actual separation element. All of this is protected by a coating layer 90. Such a composite membrane thus has a very thin selective layer bonded to a microporous support layer that provides the mechanical strength to support the pressure differential between the feed and permeate sides.

[0075] In another implementation (not shown), the capture unit C can include a system for capturing and separating CO2 by adsorption, such as a system operating according to the PSA principle. In this regard, two-reactor systems are known in the state of the art, in which each reactor is equipped with an adsorbent permeable to CO2, while other gases are retained (adsorbed) on the adsorbent. The first reactor is initially supplied with a high-pressure (5-10 bar) gas mixture, while the supply to the second reactor is closed. The CO2 discharged from the reactor can be used in the second stage of the process according to the invention. As the load on the adsorbent from the adsorbed gas increases, the pressure in the first reactor decreases, the supply to the first reactor is closed, and the supply to the second reactor is opened, starting the CO2 separation. At the same time, under reduced pressure, the gas adsorbed in the first reactor is desorbed from the adsorbent and discharged from the system. When the pressure in the second reactor decreases and reaches a certain value, its supply is interrupted and directed back to the first reactor. Regeneration begins at a lower pressure in the second reactor, so that there is a continuous exchange between adsorption separation and adsorbent regeneration between the first and second reactors due to corresponding pressure changes in the reactors.

[0076] 7 shows a membrane separator with multiple hollow fibers M1 in tubes 61 that, during use, are crossed by a feed 92. A portion of the gas passes through the fiber walls as permeate 93 (i.e., in this case, the carbon dioxide-rich fraction), and the remaining stream is discharged as retentate 91 (i.e., the carbon dioxide-lean fraction).

[0077] FIG. 8 shows an alternative membrane configuration in which multiple membrane sheets are spirally wound together, creating spaces between each sheet for the feed FS and the permeate PS. Numeral 98 indicates the outer cover, and opening 100 indicates the permeate outlet. A sweep gas can be introduced at location 96. The feed and retentate flows are perpendicular to the section, while the permeate flows perpendicular to the other flows in a spiral pattern. In these membrane separators, a single module has a cross-flow configuration, but by connecting them appropriately in series and always passing the permeate through the previous spiral, a counter-flow configuration can be achieved for the feed and retentate flows.

[0078] Figure 9 shows on the left a membrane separator 64 divided schematically into two sectors by a membrane M. A feed 92 enters the separator 64. A retentate stream RF exits one sector (in the same direction as the feed 92), and a permeate stream PF exits the other sector, perpendicular to the other flow. The horizontal graph (right) shows that the partial pressure of the permeate remains constant with respect to the membrane surface, while the pressure on the feed side decreases.

[0079] Figure 10 shows a membrane separator 65 on the left, divided schematically into two sectors by a membrane M. A feed 92 enters the separator 65. A retentate stream RF exits one sector (in the same direction as the feed), while a permeate stream PF exits the other sector in the opposite direction. In this case, a sweep gas SG is introduced countercurrent to the feed and retentate RF flows, resulting in a change of direction relative to the stream PF in Figure 9. The horizontal graph (right) shows that the partial pressure of the permeate increases with respect to the membrane surface toward the beginning of the surface and decreases on the feed side.

Claims

1. CO from process fumes ２ A plant (10) for capturing and recovering (a) Heat and CO during use ２ a furnace that generates smoke containing (b) a CO 2 reheating furnace downstream of the reheating furnace, the CO 2 reheating furnace having a first outlet and a corresponding first duct, and a second outlet and a corresponding second duct; ２ a capture and separation unit (C), wherein each outlet and its corresponding duct contains, in use, said captured and separated CO ２ and a gas stream (F1, F2) containing a portion of the CO ２ CO fed to the capture and separation unit (C) ２ CO concentration different from (12) ２ CO having a concentration ２ a capture and separation unit; (c) a first gas washing device (18), preferably an absorption column, which is used to remove CO with aqueous MOH; ２ wherein M is an alkali metal, preferably sodium (Na) or potassium (K), and ２ a first gas washing device (18) connected to said first duct (F1) for supplying (d) an alkali metal carbonate, preferably Na or K, and CO ２ a reactor (24) adapted to carry out a reaction with CO ２ and an alkali metal carbonate (preferably Na or K) connected to said second duct (F2) and said gas scrubbing device (18), which in use ２ and a reactor (24) provided with an outlet for extracting the alkali metal bicarbonate produced by the reaction of the alkali metal carbonate with the nitrate.

2. 2. A CO2 treatment system according to claim 1, characterized in that it comprises, downstream of the furnace, (e) a first heat exchanger (14) for recovering the heat of the fumes in the plant (10). ２ Capture and recovery plant (10).

3. CO according to claim 1 or 2 ２ A capture and recovery plant (10) comprising: (f) a crystallizer (28) connected to the outlet; (g) a filter or centrifuge (30) for separating the alkali metal bicarbonate produced during use in said reactor (24); (h) a concentrator (36) connected downstream of the filter or centrifuge (30) to the reactor (24) and to the first gas washing device (18) for feeding the concentrate produced during use to the reactor (24) and for feeding the vapors produced in the concentrator (36) to the first gas washing device (18) after condensation in a tank that serves as a source of alkali metal hydroxide, Preferably, the plant (10) is characterized in that the reactor (24) further comprises a condenser having a gas vent (26) connected to the first duct (F1).

4. The CO according to any one of claims 1 to 3 ２ A capture and recovery plant (10) comprising: ２ A capture and separation unit (C) (b-1) A second gas scrubbing device (44), preferably an absorption column, which is preferably an aqueous solution containing an amine or potassium carbonate to remove CO ２ a second gas scrubbing device (44) adapted to chemically bind or absorb (b-2) downstream of the second gas washing device (44), CO absorbed or bound in the second gas washing device (44); ２ and a regeneration device (48) adapted to release the second gas washing device (44) is provided with the first outlet and is connected to the first gas washing device (18) by the first duct (F1); The plant (10), wherein the regeneration device (48) is provided with the second outlet and is connected to the reactor (24) by the second duct (F2).

5. The CO according to claim 4 ２ A capture and recovery plant (10), wherein the second gas scrubbing device (44) and the regeneration device (48) are connected via a second heat exchanger (46), and the captured CO2 discharged from the second gas scrubbing device (44) and sent to the regeneration device (48) is ２ and at the same time cooling the regenerated solution discharged from the regeneration device (48), which is heated by a heat source (54) contained in the plant (10) and sent to the second gas washing device (44), the plant (10 preferably comprising a CO ２ A plant (10) comprising a compressor (50) for compressing

6. The CO according to any one of claims 1 to 3 ２ A capture and recovery plant (10) comprising: ２ The capture separation unit (C) comprises a first membrane separator (60), preferably comprising a polymeric membrane, for separating CO from the gas stream. ２ and CO ２ a first gas stream rich in CO ２ and adapted to produce a second gas stream having a reduced CO content, the first membrane separator (60) comprising the first outlet and the second outlet, and adapted to deliver the CO content to the first gas washing device (18) via the first duct (F1) connected to the first outlet. ２ a gas stream lean to the CO 2 content is fed to the reactor (24) via the second duct (F2) connected to the second outlet. ２ 1. A plant (10) characterized in that it supplies a gas stream enriched with

7. The CO according to claim 6 ２ A capture and recovery plant (10) comprising: ２ The CO ２ The concentrated fraction is fed to the second duct (F2), and the CO ２ 1. A plant (10) characterized in that at least one further membrane separator (64) is inserted between the first membrane separator (60) and the first gas washing device (18), which feeds a lean fraction to the second gas washing device (18).

8. The CO ２ The capture and separation unit (C) comprises an adsorption capture and separation system, in particular a PSA system, for extracting CO from the smoke. ２ The CO2 according to any one of claims 1 to 3, characterized in that it is adapted to separate ２ Capture and recovery plant (10).

9. CO2 according to any one of claims 1 to 8, characterized in that the reactor (24) is a three-phase reactor. ２ Capture and recovery plant (10).

10. CO2 according to any one of claims 1 to 9, characterized in that the furnace is a reheat furnace. ２ Capture and recovery plant (10).

11. CO from process fumes ２ 1. A process for capturing and recovering a molten metal, the process comprising: (i) Using a furnace to heat and CO ２ producing smoke comprising: (ii) CO in the capture unit (C) ２ capturing and separating the (iii) Captured and separated CO ２ The first CO ２ Stream (F1) and second CO ２ a stream (F2) and (iv) washing the first stream with aqueous MOH to remove alkali metal carbonate M ２ CO ３ wherein M is preferably Na or K; (v) mixing the alkali metal carbonate and the second CO ２ By reaction with the stream (F2), alkali metal bicarbonate MHCO ３ and (vi) separating the alkali metal bicarbonate; Preferably, the process uses, at least in part, heat generated from the furnace.

12. CO in step (ii) ２ and separation is carried out using at least one of the following options: (A) (ii-a) Preferably, it contains an amine or potassium carbonate, and CO ２ CO in aqueous solution that absorbs or chemically binds ２ and (ii-b) The CO subsequently absorbed or bound in step (ii-a) ２ and wherein the unabsorbed or bound CO in step (ii-a) ２ A part of the first CO ２ forming a stream, and the CO released in step (ii-b) ２ But the second CO ２ A stream is formed, preferably CO ２ The solution from which CO has been removed is heated and then recycled to step (ii-a) by heating together with the heated solution, which is then recycled to step (ii-a) by removing the CO absorbed or bound between steps (ii-a) and (ii-b) in heat exchanger (46) before being recycled to step (ii-a). ２ Including, (B) separating the second CO ２ CO forming stream (F2) ２ a concentrate stream and the first CO ２ CO forming stream (F1) ２ Generate a lean stream and (C) by adsorptive separation, preferably by PSA technology; 12. The CO according to claim 11, ２ Process for capturing and recovering

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