WO2022175886A1

WO2022175886A1 - Device and process for chemical sequestration and recovery of carbon dioxide

Info

Publication number: WO2022175886A1
Application number: PCT/IB2022/051467
Authority: WO
Inventors: Stefano CAVALLI; Marco Trevisan
Original assignee: Simlab Srl
Priority date: 2021-02-19
Filing date: 2022-02-18
Publication date: 2022-08-25
Also published as: IT202100003878A1

Abstract

A device and a process for the chemical sequestration and recovery of carbonic anhydride are described, wherein CO₂ is introduced in a basic solution contained in a reactor (9) to produce bicarbonates and/or carbonates. The pH, i.e., the concentration of OH-, is maintained essentially constant during the absorption of CO₂ and its transformation into HCO₃- or CO₃ ²-. The heat produced during the process can be recovered thanks to a pipe or jacket (1) inserted in/surrounding the reactor (9). The CO₂ not absorbed and built up in the upper part of the reactor (9) can be recirculated into the solution. Parallel to the production of bicarbonates, a portion of the solution is intended for a controlled decomposition of the bicarbonates in a dedicated regenerator (26) for producing high-purity carbonic anhydride. The heating of the bicarbonate solution can be accomplished by taking advantage of the above caloric recovery.

Description

DEVICE AND PROCESS FOR CHEMICAL SEQUESTRATION AND RECOVERY OF CARBON DIOXIDE

^ ^ H⁴

TECHNICAL FIELD

The invention relates to a device and a process for the chemical sequestration and recovery of carbon dioxide produced by combustion processes or biogas production plants.

BACKGROUND ART

In general, “Carbon Capture and Storage” (CCS) refers to any process or technology that allows capturing waste carbonic anhydride, transporting it to a storage site and storing it there, while preventing its subsequent release into the atmosphere.

The development of new carbon capture and storage technologies has been gaining more and more interest in recent decades as a potential means for mitigating the role in global warming and ocean acidification of carbon dioxide emissions from manufacturing and energy industry, transport, intensive farming and heating of buildings.

Despite the vast commitment of the scientific community to the effort of developing new carbon capture and storage techniques that are energy and economically efficient, to date, the only truly efficient technique turns out to be only the reforestation of previously deforested areas.

Various techniques currently under study including absorption, adsorption, gas separation by membrane or the use of gas hydrates are promising in terms of enrichment and purification of atmospheric CO2. However, these techniques do not guarantee a long-term carbon sequestration, but rather temporary storing with the aim of reusing carbon dioxide in subsequent industrial activities.

The studies relating to the chemical sequestration of carbon dioxide (also referred to carbonic anhydride) for the production of stable compounds that can be in turn either reused in industrial processes or stored for a long time in decommissioned mining sites turn out to be more promising. Finally, excellent results are also being achieved in the research segment involved in the development of new technologies for the synthesis of carbon dioxide-based fuels. Among these technologies, the most important are the development of methods for the methanation of CO2, the production of biodiesel through the cultivation of algae, and the synthesis of new aviation fuels by chemical synthesis. Although such technologies are very advanced, they can only reduce the environmental impact of future human activities by introducing low-impact energy cycles with zero or very low carbon dioxide balance. However, such technologies do not contribute to reducing the environmental impact already caused by human activities through the use of fossil fuels.

In addition to the industrial development of new technologies for carbon sequestration and capture and consequent reduction of the environmental impact of human activity related to the use of fossil fuels, there is a second, much broader research field dealing with the development of increasingly energy-efficient technologies capable of reducing the amount of fuels needed for equal work and/or recovering, as much as possible, low-enthalpy energy sources to be reused in secondary use cycles.

The study of new technologies allowing the thermal recovery from fumes and combustion gases to be reused for heating or the production of domestic hot water is of particular importance in this field. A remarkable step forward was introduced with the development of condensing boilers, which allow part of the calories to be recovered from the hot gases produced by combustion.

However, to date there are no technologies, neither in the industrial nor in the civil sector, that allow waste thermal energy to be recovered from the hot gases produced by combustion such as to cause a cooling of the same below 100 °C. Due to the chemical composition of combustion gases of fossil hydrocarbons, an excessive cooling of the same results in the production of very aggressive acid condensates capable of rapidly deteriorating any heat exchanger used for this purpose even if made with very resistant metals such as titanium.

DISCLOSURE OF THE INVENTION

The object of the invention is to overcome the aforesaid drawbacks, and precisely to propose a device and a process that allow simultaneously the sequestration and capture of carbon, i.e., of carbonic anhydride, from hot gases produced by the combustion of hydrocarbons or from biogas production plants, and the recovery of the thermal heat generated during the process.

In a first aspect of the invention, the object is achieved by a device for the chemical sequestration and recovery of carbonic anhydride comprising a reactor, in particular of the column type, adapted to allow an acid-base neutralization reaction between a base, in particular NaOH, and acids formed by diffusing carbonic anhydride-containing gases in an aqueous basic solution contained in the reactor, which is provided with

(a) a first injector, in particular for diffusion, configured to maintain the pH within the reactor at a level above or equal to a given value, e.g., 13.5, by the introduction of a base, preferably an anhydrous base;

(b) a loading system for feeding said first injector configured to provide certain amounts of base, preferably anhydrous base, to the reactor;

(c) a system for continuous monitoring the pH of the solution contained within the reactor configured to control said loading system of the first injector;

(d) a first gaseous diffuser, adapted to diffuse a gas having a pressure greater than or equal to 0.4 bar and at a temperature lower than or equal to 200 °C inside a liquid;

(e) a supply system of said first gaseous diffuser that allows the continuous supply of carbonic anhydride-containing gas to the device;

(f) preferably a system for monitoring the temperature of a solution contained in said reactor;

(g) optionally, a system for monitoring the level of the solution within the reactor;

(h) an inlet for introducing a solution into the reactor;

(i) an outlet for extracting a solution, preferably saturated with bicarbonates and/or carbonates, from said reactor.

The preferred base is NaOH, but other bases, such as KOH, may be envisaged based on the desired bicarbonates/carbonates. The base is advantageously added in anhydrous form, it is also conceivable to add a base in the form of a concentrated solution, in this case the injector is advantageously a metering pump.

The inlet for introducing a solution or water is preferably located in the upper part of the reactor, while the outlet is preferably located at the bottom of the reactor, allowing the solution to escape by gravity. The first gaseous diffuser may be made of various materials, such as silicone material or EPDM (Ethylene Propylene Diene Monomer). The injection of CO2, coming from combustion gas or biogas plants or even from other sources, through this diffuser is intended as a primary inj ection, which can be integrated as seen below by the internal recirculation of unabsorbed CO2 after the first injection.

The mechanism of absorption of carbon dioxide in basic solutions, in particular in aqueous sodium hydroxide solutions can be illustrated as follows:

Sodium hydroxide can be considered to be completely dissociated in aqueous solution, thanks to its high solubility (1000 g/1 at 25 °C)

When carbon dioxide is introduced into the NaOH solution, CO2 is physically absorbed into the aqueous solution:

^C02(g) ^{® C0}2(aq) (0)

Next, the aqueous CO2 reacts with OH to generate HCO3^' and CO3^2':

Equation (1) is a second-order reaction that can be considered a pseudo-first order reaction assuming the constancy of the CO2 concentration. Equations (1) and (2) are reversible reactions with a very high rate in high pH ranges, reaction (2) occurs immediately after reaction (1). Aqueous CO2 does not exist in the solution after absorption, because it is immediately consumed by reactions (1) and (2). Equation (2) is dominant in the first part of the process because the absorbent is present with a high alkalinity, which further increases the concentration of CO3² compared to that of HCO3 . The net reaction occurring in the first range of the CO2 absorption reaction is:

Subsequently, just as CO2 continuously feeds the NaOH solution during the reaction, CO2 is continuously absorbed, which results in the consumption of OH and the accumulation of CO3^2' according to equation (1) and promotes the reaction of equation (2) to the right. However, the increase in CO3^2' concentration favours the reaction of equation (2) to the left. The net reaction during the second process stage is:

^Na2^C03(aq) + ^C02(g) + H₂0₍₁₎ ® 2NaHC0_3(-aq·₎ After complete consumption of OH via reactions (1) and (2), some CO2 is absorbed via the solution in water according to equation (0).

To solve the problem highlighted above, the inventors have verified and confirmed with empirical experiments the mechanism of carbonation of CO2 in aqueous NaOH solution, as will be illustrated later with reference to Figures 1 to 3. It was possible to determine the pH levels at which the three phases of CO2 absorption can be distinguished. This point proved to be crucial to define the management, e.g., by means of a PLC (Programmable Logic Controller, often used as acronym) algorithm of the device according to the invention with the purpose of maximizing the absorption effect (step 1), of controlling the purity of the produced chemical substance (step 3) and of stopping the process when all the base is consumed without proceeding with the absorption of CO2 through its dissolution in water since such an amount of CO2 is not chemically bound and can be easily re-dispersed in the atmosphere.

Thanks to the in-depth study of the above mechanisms and the idea of controlling them through pH management (periodic addition of a base to keep it constant) it has become possible to simultaneously (a) chemically sequester and capture carbonic anhydride with the consequent production of stable carbonates useful for industrial reuse or storage in decommissioned mining sites and (b) thermally recover the heat produced both by the plant itself and any hot gases produced by the combustion of hydrocarbons or similar if used as a source of carbonic anhydride.

Precisely the in-depth knowledge of the mechanisms led the inventors to a new approach, instead of increasing the starting concentration of NaOH (with an obvious problem of the safe management of the plant), the idea of the invention is to work keeping the starting concentration of NaOH almost stable over time through a periodic or constant addition of reagent.

To optimize the absorption of CO2 contained in the gas introduced into the reactor, the device according to the invention further comprises:

(j) an extraction system for extracting a portion of gas not absorbed in the solution contained in the reactor; and

(k) a second gaseous diffuser adapted to diffuse a gas having a pressure greater than or equal to 0.4 bar and at a temperature lower than or equal to 200 °C inside a liquid, wherein said extraction system feeds said second gaseous diffuser. The gas, i.e., the carbonic anhydride not absorbed after the first injection, is not lost, but reintroduced into the basic solution by increasing the yield of the CCh actually recovered. Instead, in a variant of the invention, the device according to the invention comprises in the upper part of the reactor a CCh detector and a vent controllable by said CCh detector or by a relative control unit in such a way that, with CCh concentrations in the gas above a solution contained in said reactor being below a predetermined level, the vent opens to expel the gas with a too low carbonic anhydride concentration. Gases stripped of CCh and no longer usable for the purpose of the invention are thus removed from the system.

In a preferred embodiment of the invention, the reactor is provided with a pipe or jacket for recirculating a carrier fluid which is preferably connected with a heat exchanger to a device for producing hot water. Such a system allows the thermal recovery of excess heat generated by the acid-base neutralization reaction in the reactor, of heat generated by the dissolution of the base added to the reaction solution and/or of heat transferred to the solution from any hot gases produced by combustion or biogas plants introduced into the reactor. The use of a carrier fluid advantageously allows through a recirculation system the recovery of such heat for various uses, but in particular for producing hot water up to a temperature of 60 °C, while contemporarily keeping the reactor temperature below 80 °C. The pipe may pass into the reactor and thus into a solution contained therein as it may be wrapped around the reactor. A jacket involving practically the entire reactor is very efficient. The gas entering through the relative diffusers into the solution helps to stir the solution and distribute the heat evenly. Using stirrers may be also contemplated.

In a particularly preferred alternative embodiment of the invention, the device for the chemical sequestration and recovery of carbonic anhydride comprises a regenerator for the regeneration of carbonic anhydride from bicarbonates which is connected to the outlet of the reactor. The connection allows it to be supplied with the solution or mixture contained in the reactor during its use.

Advantageously, the regenerator is a column-shaped reactor. Preferably, the regenerator comprises an inlet, which is connected to the reactor outlet, which advantageously comprises a nozzle distributor, e.g., one with an annular arrangement. A nozzle distributor makes it possible to create turbulences and therefore kinetic energy inside the regenerator and facilitates the release of carbon dioxide from the bicarbonates produced in the reactor. Advantageously, the nozzles are arranged circularly and their outlets directed towards the walls of the regenerator so that they direct the flow directly onto the internal surface of the reactor itself.

The nozzle distributor is preferably located in the lower part of the regenerator, in particular in the lower third of the reactor.

In a preferred embodiment of the regenerator, it comprises, preferably at its head, a gas outlet, in particular CO2, leaving the solution or mixture present in the regenerator during its use.

In addition, the regenerator is advantageously provided, preferably in its upper part, in particular at the apical area at least in the upper quarter of the regenerator, with a condenser, such as a pipe or jacket for recirculating a carrier fluid, for cooling rising water vapours and for condensing them. Preferably, the condenser is placed before and adjacent to the gas outlet in order to allow the maximum condensation of any residual water vapour, thus guaranteeing purity for the outflow of carbon dioxide greater than 97.8%.

In a preferred embodiment of the invention, the connection from the reactor to the regenerator comprises a heat exchanger which is connected to the reactor jacket or pipe to be fed therefrom. The heat that develops in the reactor, thanks to the neutralization reaction, the dissolution of the base and possibly the exhaust gas that feeds the reactor in a hot form, is transmitted to the jacket or pipe, is exchanged in the heat exchanger and transmitted to the fluid that passes through it, then to the solution or mixture which then enters the regenerator and is heated to allow the release of CO2.

In another embodiment of the invention, the reactor pipe or jacket is connected, preferably in a circuit, to a second reactor tubing or jacket, at its lower part so that the heat developed inside the reactor can be used directly to heat the solution or mixture inside the regenerator itself.

Of course, it may also be envisioned to provide the reactor and the regenerator without a jacket or pipe for the heat exchange and to heat the fluid passing from the reactor to the regenerator or the regenerator with other heating devices.

Preferably, the reactor comprises at least one opening for inspection and/or maintenance thereof.

In a highly advantageous embodiment of the invention, the device according to the invention is provided with a control unit configured to perform the following algorithm:

(i) activating said first and/or second gaseous diffuser so as to introduce CC>2-containing gas into a basic solution, in particular a NaOH solution, contained in the reactor; (ii) monitoring the pH value of said basic solution simultaneously with step (i);

(iii) deactivating said first and/or second gaseous diffusers when the pH value measured in step (ii) falls below a threshold value, e.g., 13.5;

(iv) activating said loading system so as to add a base into said solution, preferably an anhydrous base, until a certain pH value, e.g., 14, above said threshold value is reached and, with the achievement of said certain pH value, resuming the introduction of carbonic anhydride into said solution;

(v) repeating step (iv) until saturation of the solution with bicarbonate and/or carbonate is achieved; and

(vi) preferably extracting the saturated solution at a stability pH that results in only bicarbonate being present in solution, e.g., 8.5; or, alternatively to steps (iii) to (v), activating said loading system so as to add continuously a base into said solution, preferably a hydrated base in liquid form, so as to keep the pH value above a threshold value, e.g., 13.5, until saturation of the solution with bicarbonate and/or carbonate is achieved.

The above pH values are optimal for a Na0H-C02-NaHC03 Na2C03 system or other systems using strong bases such as KOH. The preferred pH range comprised between 13.5 and 14 refers to the step of maximum absorption of CO2. Once the solution is saturated with carbonates, then the product can be advantageously refined in the form of bicarbonate by lowering the pH to the threshold of 8.5.

The time of saturation of the solution with bicarbonate and/or carbonate may be easily calculated, e.g., by a control unit operating the system and all its components, from the volume of the solution, the amounts of base introduced, the temperature of the solution and the type of carbonate/bicarbonate produced.

The above algorithm can be integrated to manage the individual steps described, in particular the regeneration of CO2 by deviating a portion of the bicarbonate solution and heating it, and preferably introducing the bicarbonate solution under pressure by means of respective nozzles in a regenerator and in particular by taking advantage of the heat available in the reactor. In this regard, the aforementioned control unit is adapted to manage all the process steps described herein and to control for this purpose all the elements described for the device according to the invention so as to cause the device to carry out the steps of the relative process. A second aspect of the invention relates to a process for the chemical sequestration and recovery of carbonic anhydride comprising the following steps:

(I) preparing a basic solution having a pH greater than or equal to 14 in a reactor,

(II) introducing a carbonic anhydride-containing gas into said basic solution and simultaneously monitoring pH,

(III) with pH values below a threshold value, e.g., 13.5, discontinuing the introduction of carbonic anhydride and addition of a base into said solution, preferably an anhydrous one, until a certain pH value, e.g., 14, above said threshold value is reached and, with the achievement of said certain pH value, resuming the introduction of carbonic anhydride into said solution,

(IV) repeating step (III) until saturation of the bicarbonate and/or carbonate solution is achieved, and

(V) preferably extracting from the reactor the solution saturated with bicarbonates/carbonates formed by the neutralization reaction between carbonic acid and the base upon reaching the stability pH, indicating that only bicarbonates are presents, e.g., 8.5, and sending to a crystallizer to recover solid bicarbonate, or, alternatively to steps (III) to (IV), continuously adding a base into said solution, preferably a hydrated base in liquid form, so as to keep the pH value above a threshold value, e.g., 13.5, until saturation of the solution with bicarbonate and/or carbonate is achieved.

Preferably, the introduction of carbon dioxide is accomplished by introducing gas from combustion or a biogas plant and/or by taking carbonic anhydride not absorbed by the solution from the volume in the reactor above said solution. This allows to recover most of the carbonic anhydride from the gas, only when the gas that builds up above the solution is practically free of CO2, determined with a regular control of the concentration of carbonic anhydride in the volume above the solution in the reactor, the process comprises a step wherein the gas is expelled from said volume if the concentration of CO2 falls below a threshold value. Advantageously, the process according to the invention comprises the step of using the heat resulting from the neutralization reaction of step (II) and/or the heat created by the dissolution of the base added in step (III) and/or the heat contained in the introduced gas to heat other systems, in particular to produce hot water. Various prior art documents, such as, for example, CA 2695 006 Al, US 2011/091955 Al, US 2012/189529 Al, KR 101351464 Bl, US 2013/089482 Al e JP 2012206872 A describe the capture of CO2 in carbonates or bicarbonates, but these do not teach the use of the heat available in the reactor to heat other systems or provide the energy to other reactions, e.g., to produce hot water or to release the carbonic anhydride from the bicarbonates produced in the reactor.

In a highly preferred embodiment of the process according to the invention, in parallel with at least one of the steps (III) to (V), in a step (VI), the extraction of a portion of solution from the reactor is accomplished, once a pH thereof lower than or equal to 10.5 is reached, and the heating of said portion, preferably at a temperature between 95 and 107 °C to favour the release of CO2 from the bicarbonates.

At pH values below 10.5, bicarbonates prevail over carbonates, from an energy point of view it is preferable to release carbonic anhydride from bicarbonates and not from carbonates.

In this temperature range, bicarbonates present in the solution will easily tend to degenerate releasing a carbon dioxide molecule.

This CO2 release reaction is particularly facilitated if in addition to the heat provided during heating, an appropriate amount of kinetic energy is also transferred to the solution. For this reason, the portion of solution or mixture taken from the reactor, preferably already heated, is injected under high pressure into a regeneration reactor, preferably through a nozzle distributor. Advantageously, the nozzles are arranged circularly and their outlets directed towards the walls of the regenerator so that they direct the flow directly onto the internal surface of the reactor itself.

The above portion is preferably heated by using the heat available in the reactor for producing bicarbonates and carbonates, in an embodiment of the invention by means of a heat exchanger found in the connection between the reactor and the regenerator which is fed by a jacket or pipe that surrounds the reactor or by connecting a jacket or pipe that surrounds the reactor to a respective jacket or pipe that surrounds the regenerator in its lower part. The heat exchange in the various alternatives is accomplished advantageously through a diathermic oil.

The bicarbonate decomposition reaction is accomplished, in particular under the temperature and turbulence conditions described above inside the regenerator, by releasing a high purity CO2 flow that can be reused for industrial uses. At the outlet of the regenerator, the absorbent solution will be discharged of bicarbonates with a pH higher than the input pH, and also richer in carbonates. This solution may be cooled by means of a heat exchanger, in particular at a temperature lower than 70 °C, and returned to the main reactor. This scheme allows a cyclic reuse of the absorption solution and a total reuse of the recovered thermal energy, ensuring an efficient decarbonisation of the fumes or exhaust gases and, at the same time, the production of a flow of purified CO2, useful for the industrial or commercial reuse thereof.

Preferably, the process according to the invention is performed in a device according to the invention.

The features and advantages described for one aspect of the invention may be transferred mutatis mutandis to the other aspect of the invention. In this context, it is obvious that in the process steps, the relative components of the device are usable to perform the corresponding process steps, e ., the loading system to additionally introduce a base, whereas a control unit of the device according to the invention can be configured to perform an algorithm which comprises as individual stages the process steps by activating or deactivating the relative component of the device adapted to perform the corresponding process step.

The industrial applicability is obvious from the moment when it becomes possible to sequester and capture CO2 from various sources and transform it into bicarbonates and/or carbonates and at the same time optimise the yield of absorbed CO2 and recover the heat generated during the process.

Said objects and advantages will be further highlighted in the disclosure of a preferred embodiment example of the invention provided by way of illustration and not limitation Variants and further features of the invention are the subject matter of the dependent claims. The description of the preferred exemplary embodiment of the device and process for the chemical sequestration and recovery of the carbonic anhydride is given by way of example and not limitation, with reference to the appended drawing. In particular, unless specified otherwise, the number, shape, size and materials of the system and of the individual components may vary, and equivalent elements may be applied without deviating from the invention concept.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

Fig. 1 illustrates in a diagram the function pH = f(t) where t is the CO2 absorption time for a batch of 2501. Fig. 2 illustrates the trend of the concentration of the OH ions as a function of the absorption time for the set-up of Figure 1.

Fig. 3 illustrates the trend of the consumption rate of the OH ions as a function of the absorption time for the set-up of Figure 1.

Fig. 4 illustrates the trend of pH as a function of the absorption time with the indication of four additions of NaOH for another set-up.

Fig. 5 illustrates the trend of the concentration of the OH ions as a function of the absorption time for the set-up of Figure 4.

Fig. 6 illustrates the consumption rate of the OH ions as a function of the absorption time for the set-up of Figure 4.

Fig. 7 illustrates an exemplary embodiment of a device according to the invention, which can be fed, for example, by waste combusted gas.

Fig. 8 illustrates in various views a CO2 regenerator connected to the reactor of the device according to the invention, namely a top view (Fig. 8a), two sectional side views along the lines A-A (Fig. 8b) and B-B (Fig. 8c) of Figure 8a, a side view of the part of the regenerator presenting the inlet for the basic solution (Fig. 8d), the section along the line D-D (Fig. 8e) of Figure 8d, a further top view (Fig. 8f) turned with respect to Fig. 8a, the section along the line C-C (Fig. 8g) of Fig. 8f showing only the upper part of the reactor; and in Fig. 8h a section along a line orthogonal to the line D-D of Fig. 8d.

In a set-up of the invention, with a device as described later with reference to Figure 7 and with a reaction solution of 2501 with a pH = 12.9 at the point t = 0 s, CO2 compressed from a relative cylinder started being diffused through a diffusion disc at a flow rate of 25 1/min (at 20 °C at sea level pressure) into the solution. The pH of the solution was continuously monitored. The solution was continuously mixed via an air blower that recirculated gas from the top of the reactor to the bottom of the solution to recover any CO2 not absorbed during a previous injection.

As can be seen from the diagram in Figure 1, the two steps of carbonic anhydride absorption and its transformation into carbonate/bicarbonate shown above are well defined, the first at a pH of 11.9 to 11.0 and the second at a pH of 8.4 to 7.7. Assuming complete dissociation of NaOH in water, the hydroxide ion concentration may be easily calculated: [OH ](ti) — lO^^ri^)-14) to determine the OH ion concentration from pH. Figures 2 and 3 illustrate respectively the concentration of the OH ions as a function of the absorption time and the consumption rate of the OH ions as a function of the absorption time. As evident from the diagram in Figure 2, the greatest amount of CO2 is during the first step; and, as shown by Figure 3, the consumption rate of OH during this step is almost constant with an average value of 11.14 mmol/s (= 40.1 mol/h = 1.76 kg/h). During the second step, only a small, smaller by two orders of magnitude, amount (0.3 mol/h = 0.013 kg/h) of CO2 is absorbed. This is due to the fact that during this step only the amount of CO2 involved in reaction (2) following the left-directed reaction arrow is involved in the absorption mechanism. Such behaviour has also been described by Miran Yoo et.al. ( Carbon dioxide capture capacity of sodium hydroxide aqueous solution, Journal of Environmental Management 114 (2013) 512- 519) under the same initial conditions. The amount of CO2 absorption in the third step is negligible for the purpose of this work. Mirian Yoo et al. studied the absorption capacity as a function of the starting concentration of NaOH and showed an increase in the evident absorption capacity of solutions with a higher starting concentration of NaOH, but within the same order of magnitude. However, precisely these results led the inventors to a different approach, instead of increasing the starting concentration of NaOH (with an obvious problem of the safe management of the plant), the idea of the invention is to work keeping the starting concentration of NaOH almost stable over time through a constant addition of reagent, a solution not mentioned by Yoo.

How to manage the periodic addition of NaOH? To test the above hypothesis, the working pH range is set between 12.7 and 12.9. When pH reaches the lower limit of 12.7, the CO2 flow is stopped and new NaOH is added to the solution until a pH of 12.9 is again reached. Then, the CO2 flow is resumed again.

Figures 4 and 5 respectively illustrate the trend of pH and concentration of OH as a function of the absorption time indicating in Figure 4 with circles the time points of NaOH addition. The approach according to the invention allows to maintain semi-fixed conditions during the absorption step that allow the constancy of the absorption capacity and efficiency to be controlled without increasing too much the pH in the reactor. Figure 6 reflects the consumption rate of hydroxide ions. With regard to absorption efficiency, Table 1 shows that, except for the first cycle, the absorption efficiency of carbonic anhydride ranges from 83,9% to 96,5%. The high absorption efficiency is probably also due to a reflux flow of gas through the solution via a second diffuser which reintroduces CO2 not immediately absorbed but built up at the top of the reactor.

Table 1

The total absorption capacity measured in the above set-up with the above conditions is 65.5 kg/day with an average efficiency of 92%. Fig. 7 illustrates in three different views an exemplary embodiment of a device according to the invention, which can be fed, for example, by waste combusted gas. The top middle view shows the device seen from above, whereas the left and right views show the device in sections along line B-B and line A-A of the middle view, respectively. The device can be made of AISI 304 steel, and is equipped with an automatic loading system of the solid base, such as a screw or bigbag emptying loading system.

In a column reactor 9 with a lid 17 and a bottom 22 which can be opened and closed with relative screws also for inspection and maintenance of the reactor 9. The bottom 22 ends into a kind of funnel. The reactor 9 is supported by a frame 26. Inside the reactor 9, a basic solution is provided, a carbon-rich gas (CO2) is introduced through a primary injection system 16, 18, 20

When, for example, the solution contained within the reactor 9 reaches a pH of 11.2 or less, the automatic loading system is operated by discharging a defined amount of the base into the injector supply mouth 15. The usable base can be of different nature depending on the type of carbonate desired, a preferred embodiment of the invention involves the use of NaOH for the production of sodium bicarbonate. The amount of base loaded reaches by gravity the diffusion injector 19 where it begins to hydrate and diffuse into the main reactor chamber 9. The high carbon gas (CCh) is injected at a pressure between 0.55 and 0.9 bar through a mouth 16 and then along a primary injection channel 18 and allowed to bubble through the gaseous diffuser 20 within the solution contained in the reactor 9. Upon contact with the solution, the carbon dioxide contained in the gas is absorbed by dissolution of the aqueous solution, producing "carbonic acid", which in turn is instantly neutralized by the base contained in the aforementioned solution, first producing a mixture of carbonates and bicarbonates. The high carbon gases usable in reactor 9 can be of a different nature, in a preferred embodiment of the invention gases from biogas production systems, exhausted gases from combustion plants, or enriched gases from atmospheric CCh sequestration systems can be employed.

A second gas diffusion system with a mouth 14 and a related secondary injection channel 12 lets the carbonic anhydride collected at the top of the reactor bubble through the gaseous diffuser 11 inside the main reactor chamber 9. In this regard, for the recirculation of CCh not immediately absorbed during its first injection, the secondary diffusion system is fed through a blower pump that through the intake 6 withdraws from the upper part of the reaction chamber 9 the gas which could possibly not be absorbed by the solution during the first injection. Such system ensures the desired emission standards through a continuous monitoring system of the amount of residual CCh placed right after a vent 25.

Through a monitoring system of the amount of base introduced inside the reactor the achievement of the limit amount of reagent reached can be accurately determined. Such quantity depends on the reactor volume and the chemical species to be produced and is determined by the saturation value of this chemical species at the desired temperature, e.g., at 20 °C. Once this amount of reagent in solution has been reached, the automatic loading system is stopped and the high carbon gas is allowed to bubble until the stability pH is reached. In the case of the production of sodium bicarbonate, the stability pH is set at 8.5. This value ensures that the chemical species in solution inside the reactor 9 is formed exclusively by bicarbonate.

When the solution in the main reactor reaches the stability pH, the automatic emptying system activates a solenoid valve 24 by withdrawing through a relative drain 23 a known volume of solution from the main reactor 9 and sending it to a crystallization tank (not shown). In the crystallization tank, through the use of a heat pump system, the temperature of the solution is raised to 20 °C causing the crystallization and consequent solid separation of the produced bicarbonate which can then be filtered and dried. The discharged solution of the excess bicarbonate is reintroduced into the main reactor via the inlet 8 through a pump controlled by the level control system 3, 4, 5. The well 2 located on the side of the reactor serves for housing a temperature probe. By contrast, the well 7 serves for housing a pH probe.

During all activity steps, a carrier fluid is made to flow into the jacket 1 of the reactor 9 from the inlet 21 to the outlet 13 thereof. Such carrier fluid is used in order to extract all the excess heat produced by the base hydration reaction, the acid-base neutralization reaction and the transfer of heat of any high carbon gases produced by combustion plants to the solution contained in the main reactor chamber 9. Such solution with a jacket ensures the dual benefit of maintaining the temperature of the reaction chamber within a desired range (in an exemplary embodiment it is envisaged to maintain the temperature in the reaction chamber in the range of 50 to 80 °C) and to recover the generated heat, e.g., for the production of hot water at 55-60 °C reusable in secondary thermal circuits. The flow of hot water produced by the heat pump used by the crystallization tank can be added to such flow of hot water.

Figure 8 (8a to 8h) illustrates in various views a CO2 regenerator 26 connected to the reactor of the device according to the invention. In the upper part, an outlet 28 for the produced CO2 can be seen. The upper third of the column 27 of the regenerator is equipped with a cooling coil 36 in order to condense any water vapours. The coil 36 has an inlet 40 and an outlet 38 for countercurrent cooling. The bottom sees an outlet 30 of the regenerator 26 in order to output the solution or mixture contained in the regenerator 26. The solution or mixture of carbonates and/or bicarbonates from the reactor (not shown) is introduced through the inlet 32 which leads to two nozzle rings 34 allowing the solution or mixture to be sprayed into the regenerator 26 in order to create significant turbulences. The entrance area corresponds to sector X.

Claims

1) A device for the chemical sequestration and recovery of carbonic anhydride comprising a reactor (9), in particular of the column type, adapted to allow an acid-base neutralization reaction between a base, preferably NaOH, and acids formed by diffusing carbonic anhydride- containing gases in an aqueous basic solution contained in the reactor (9), which is provided with

(a) a first injector (15, 19), in particular for diffusion, configured to maintain the pH within the reactor (9) at a level above or equal to a given value, e.g., 13.5, by the introduction of a base, preferably an anhydrous base;

(b) a loading system for feeding said first injector (15, 19) configured to provide certain amounts of base, preferably anhydrous base, to the reactor (9);

(c) a system for continuous monitoring the pH (7) of the solution contained within the reactor configured to control said loading system of the first injector (15, 19);

(d) a first gaseous diffuser (16, 18, 20), adapted to diffuse a gas having a pressure greater than or equal to 0.4 bar and at a temperature lower than or equal to 200 °C inside a liquid;

(e) a supply system of said first gaseous diffuser (11, 12, 14) that allows the continuous supply of carbonic anhydride-containing gas to the reactor (9);

(f) preferably a temperature monitoring system (2) of a solution contained in said reactor (9);

(g) optionally, a system for monitoring the level (3, 4, 5) of the solution within the reactor;

(h) an inlet (8) for introducing a solution into the reactor (9);

(i) an outlet (23, 24) for extracting a solution from said reactor (9).

2) The device according to claim 1, characterized in that it further comprises:

(j) an extraction system (6) for extracting a portion of gas not absorbed in a solution contained in the reactor (9); and

(k) a second gaseous diffuser (11, 12, 14) adapted to diffuse a gas having a pressure greater than or equal to 0.4 bar and at a temperature lower than or equal to 200 °C inside a liquid, wherein said extraction system (6) feeds said second gaseous diffuser (11, 12, 14). 3) The device according to claim 1 or 2, characterized in that it comprises in the upper part of the reactor (9)

(l) a CO2 detector; and

(m) a vent (25) controllable by said CO2 detector or a relative control unit such that, with CO2 concentrations in the gas above a solution contained in said reactor (9) being below a predetermined level, it opens to expel the gas with a too low carbonic anhydride concentration.

4) The device according to any one of the preceding claims, characterized in that said reactor (9) is provided with a pipe or jacket (1) for recirculating a carrier fluid which is preferably connected with a heat exchanger to a hot water production device.

5) The device according to any one of the preceding claims, characterized in that it comprises a regenerator (26), preferably in the form of a column reactor, for regenerating carbonic anhydride which is connected via its inlet (32) to the outlet (23) of the reactor (9).

6) The device according to claim 5, characterized in that the inlet (32) comprises a nozzle distributor (34), wherein the nozzles are advantageously arranged circularly and their outlets are directed towards the walls of the regenerator (26) so that they direct the flow directly onto the inner surface of the reactor (26) itself.

7) The device according to any one of claims 5 or 6, characterized in that it comprises in the upper part an outlet (28) for gas and a condenser (36).

8) The device according to any one of claims 5 to 7, characterized in that said reactor (9) is provided with a pipe or jacket (1) for recirculating a carrier fluid which feeds a heat exchanger inserted in the connection between the reactor (9) and the regenerator (26) or which feeds a jacket or pipe that wraps the regenerator in its lower part.

9) The device according to any one of the preceding claims, characterized in that it is provided with a control unit configured to perform the following algorithm: (i) activating said first and/or second gaseous diffusers (16, 18, 20; 11, 12, 14) so as to introduce CC -containing gas into a basic solution contained in the reactor (9);

(ii) monitoring the pH value of said basic solution simultaneously with step (i);

(iii) deactivating said first and/or second gaseous diffusers (16, 18, 20_,11, 12, 14) when the pH value measured in step (ii) falls below a threshold value, e g., 13.5;

(iv) activating said loading system (15, 19) so as to add a base into said solution, preferably an anhydrous base, until a certain pH value, e.g., 14, above said threshold value is reached and, with the achievement of said certain pH value, resuming the introduction of carbonic anhydride into said solution;

(vi) preferably extracting the saturated solution at a stability pH that results in only bicarbonate being present in solution, e.g., a pH of 8.5, or, alternatively to steps (iii) to (v), activating said loading system so as to add continuously a base into said solution, preferably a hydrated base in liquid form, so as to keep the pH value above a threshold value, e.g., 13.5, until saturation of the solution with bicarbonate and/or carbonate is achieved; and optionally at least one of the steps according claims 10 to 18.

10) A process for the chemical sequestration and recovery of carbonic anhydride comprising the following steps:

(I) preparing a basic solution having a pH greater than or equal to a predetermined pH value, e.g., 13.5, in a reactor,

(III) with pH values below a threshold value, e.g., 13.5, discontinuing the introduction of carbon dioxide and addition of a base into said solution, preferably an anhydrous one, until a certain pH value, e.g., 14, above said threshold value is reached and, with the achievement of said certain pH value, resuming the introduction of carbonic anhydride into said solution, (IV) repeating step (III) until saturation of the solution with bicarbonate and/or carbonate is achieved, and

(V) preferably extracting the solution saturated with bicarbonates/carbonates formed by the neutralization reaction between carbonic acid and the base from the reactor, in particular upon reaching a stability pH, e.g., 8.5, indicating that only bicarbonates are present, and sending to a crystallizer to recover bicarbonate/carbonate solid, or, alternatively to steps (III) to (IV), continuously adding a base into said solution, preferably a hydrated base in liquid form, so as to keep the pH value above a threshold value, e.g., 13.5, until saturation of the solution with bicarbonate and/or carbonate is achieved.

11) The process according to claim 10, characterized in that the introduction of carbonic anhydride is accomplished by introducing combustion gas and/or by taking carbonic anhydride not absorbed by the solution from the volume in the reactor above said solution. 12) The process according to claim 10 or 11, characterized in that it comprises a step (VI) wherein the concentration of carbonic anhydride in the volume above the solution in the reactor (9) is regularly determined, and the gas in said volume is expelled if the concentration falls below a threshold value. 13) The process according to any of claims 10 to 12, characterized in that it comprises the step of using the heat resulting from the neutralization reaction of step (II) and/or from the heat created by the dissolution of the base added in step (III) and/or the heat contained in the introduced gases to heat other systems, in particular to produce hot water. 14) The process according to any of claims 10 to 13, characterized in that it is performed with a device according to any of claims 1 to 9.

15) The process according to any one of claims 10 to 14 characterized in that, parallel to at least one of steps (III) to (V), in a step (VI), the extraction of a solution portion from the reactor (9) is accomplished, once a pH thereof lower than or equal to 10.5 is reached, and the heating of said portion, preferably at a temperature comprised between 95 and 107 °C. 16) The process according to claim 15, characterized in that said portion is injected into a regenerator (26), in particular in the form of a column reactor, through a nozzle distributor (34), wherein preferably the nozzles are arranged circularly and their outlets are directed towards the walls of the regenerator (26) so as to direct the flow directly onto the inner surface of the reactor (26) itself.

17) The process according to one of claims 15 or 16, characterized in that the heating of the portion takes place by using the heat available in the reactor (26).

18) The process according to one of claims 15 to 17 characterized in that any water vapour which has formed during heating in step (VI) is condensed and separated from the CO2 gas that is formed.