FIELD OF THE INVENTION

The invention concerns a process for sequestering CO2 from process gases through relative capture by sequestering agents and relative transformation into high value-added chemicals. BACKGROUND ART

Traditionally, CO2 capture and conversion requires the use of two different technologies: the one, called CCS (Carbon Capture and Sequestration) technology, to recover the molecule from process streams (flue gases) in which CO2 is diluted, at low pressure and in co-presence with other related substances (for example with other non-condensables in flue gases or with carbon monoxide in syngas streams from reformer or gasification). It usually consists of an absorption and a regeneration unit, if a solvent process is used; the other, called CCU (Carbon Capture and Utilization) to transform CO2 into compounds of greater added value, for circularity and decarbonization. Alternatively, the second technology involves CO2 sequestration without conversion (underground injection, deep sea injection, vitrifications...).

As far as CCS is concerned, the most widely used technology is the sweetening process, i.e. the coupling of an absorber and a regenerator as schematically shown in Figure 1. The liquid solution to break down CO2, usually a mixture of water and amine and more precisely water and methylethanolamine (MEA) (20-40%), is supplied to the upper part of the absorber. It meets in countercurrent the ascending flue gases (process gases), blown from below, and sequesters the fraction of CO2 contained therein. The flue gas exiting from the head of the absorber is constituted by N2 with saturation steam. The liquid stream that has trapped CO2 and any fractions of other acid components (e.g., SOx), passes through an energy recuperator and is then fed at the head of a regeneration tower. With different operating conditions (typically lower pressures and higher temperatures than absorption), the acid gases are released and conveyed to the top of the column, while the solution is being purified by descending and exits with low CO2 concentration. From the bottom of the regeneration column it is thus sent for recycling to the absorption unit after passing in the energy recovery element.

Such CO2 sequestration plants are particularly effective the more the CO2 is in high concentration. Although they are widely used in various industrial sectors such as refining, extraction and process chemistry, they also have some disadvantages that are progressively taking on value and impact:

Water/amine mixtures are toxic

Water/amine mixtures require a considerable energy effort for solvent regeneration Water/amine mixtures entail significant wear, corrosion, degradation problems Such plants solve the problem of capture, but not of CO2 re-utilization

SUMMARY OF THE INVENTION

The need to overcome the aforesaid problems is therefore felt.

The Applicant has found that it is possible to overcome these drawbacks with the process object of the present invention which provides for the complete review of the traditional scheme, in which the regeneration of the cycle of Figure 1 is carried out by means of the CO2 conversion unit, as schematically represented in Figure 2. The CO2-rich solution is fed to a system that converts the same CO2 into other products, separates it and makes the CO2- lean solution, but containing the CO2 sequestering agent available for a new capture cycle. Therefore, the object of the present invention is a process for the recovery of carbon dioxide from acid aqueous solutions comprising: a) Carbon dioxide sequestration in an absorption tower for the countercurrent passage of aqueous solutions capable of chemically sequestering CO2; b) regeneration of the aqueous solution coming from step a) enriched in CO2, in a CO2- lean solution, capable of chemically sequestering the CO2 to be recycled in step a); wherein said regeneration b) takes place by conversion into a high value-added chemical.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows in schematic form the CO2 capture process with absorber/ regenerator according to the prior art.

Figure 2 shows in schematic form the process object of the invention.

Figure 3 shows an implementation mode of the process according to the present invention in which in step b) CO2 is converted to calcium carbonate according to the metathesis reaction with calcium sulphate.

Figure 4 shows the solubility diagram of CaSO4 in water [°C vs solubility g / 100 g H2O], Figure 5 shows a solubility diagram of ammonium sulphate in water [°C vs solubility g / 100 g H2O] (dx). Data rearrangement from Handbook of Chemistry of Physiscs, D.R. Lide Ed., CRC Taylor Francis, 89 ^th Edition, 2008. Figure 6 represents in schematic form the crystallizer with draft tubes and baffles for the circulation and growth of the crystals and with the gas purge, in which step b-1 is carried out).

Figure 7 represents in schematic form a further apparatus, the operating unit (1) in which step b-1) of the process of the invention is carried out.

Figure 8 represents the flow lines of the ammonia- and CO2-rich aqueous solution coming from step a) possibly replenished with NH3 and CO2, in the operating unit 1 shown in Figure 7.

Figure 9 represents the flow lines for CaCO3 crystals.

Figure 10 represents the absorption tower in which step a) is carried out is provided at the top with a washing tower to eliminate traces of ammonia from the purified gases.

Figure 11 on the left shows in schematic form an absorption and washing tower integrated with traditional pump-around systems whereas the one on the right shows an integrated absorption and washing tower provided with ancillary cooling units.

Figure 12 on the left shows in schematic form an integrated absorption and washing unit, provided with a cooling coil inside a chimney plate; whereas the one on the right represents an integrated absorption and washing unit, in which the cooling is carried out by injecting a liquid ammonia solution used for ammonia make-up.

Figure 13 shows the diagram of the reabsorption and washing section, developed according to the process simulator Aspen Hysys 10, as shown in example 1.

Figure 14 shows in Table 1 the results obtained with the aforesaid simulator of the absorption and washing section shown in Figure 13 of step a) of the process of the invention carried out at 1.3 bar.

Figure 15 shows the temperature profiles vs. steps for the absorption tower on the left and for the washing tower (scrubber) on the right of step a) process of the invention carried out at 1.3 bar.

Figure 16 shows in Table 2 the results obtained with the aforesaid simulator of the absorption and washing section shown in Figure 13 of step a) of the process of the invention carried out at 10 bar.

Figure 17 shows the temperature profiles vs. steps for the absorption tower on the left and for the washing tower (scrubber) on the right of step a) process of the invention carried out at 10 bar. Figure 18 shows in Table 3 the results obtained with the aforesaid simulator of the absorption and washing section shown in Figure 13 of step a) of the process of the invention carried out at 30 bar.

Figure 19 shows the temperature profiles vs. steps for the absorption tower on the left and for the washing tower (scrubber) on the right of step a) process of the invention carried out at 30 bar.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention the definition "comprising" does not exclude the presence of further components/steps not explicitly listed after such definition, while the definition "constituted by.. / formed by., consisting of...excludes the presence of further steps/ components in addition to those expressly listed after this definition.

Process gas means any flow (stream) of gases coming from processes for the preparation of syngas, reforming, gasification, Fischer Tropsch etc.

Flue gas means any flow (stream) of gases outflowing from combustion systems with main content of CO2, steam, nitrogen.

A particular application of the process of the invention is the one illustrated in Figure 3.

In this particular embodiment of the process of the invention:

- in step a) an aqueous solution of ammonia is used as an aqueous solution capable of chemically sequestering CO2;

- in step b) calcium sulphate is added to the aqueous solution of ammonia enriched in carbon dioxide coming from step a) and calcium carbonate and ammonium sulphate are formed by the following metathesis reaction (1)

CaSC>4 + (NHfECCE ⁼ CaCCE + (NHfESCh

(1) and the aforesaid salts are recovered.

In step a) the temperature of the aqueous solution of ammonia depends directly on the pressures used in absorption. The absorption column used in step a) of the process of the invention can exert any type of pressure. For example the pressures in step a) are comprised between 1 and 50 atm, preferably between 20 and 50 atmospheres, even more preferably between 40 and 50 atmospheres. The temperature of the solution capable of capturing CO2 in step a) must not exceed the boiling temperatures of ammonia, preferably it must be lower than 50°C more preferably it is comprised between 25°C and 40°C, even more preferably it is 30°C.

In the process of the invention according to the implementation mode of Figure 3, a calcium- based salt is used for it to immediately crystallize calcium carbonate by precipitating within the solution. Calcium salts used as reagents in step b) must have an anionic part that can bind with ammonium in solution, forming a fertilizer salt. The invention also provides that the two salts must form at different times and/or in different zones of the conversion unit, as is the case with the process schematically shown in Figure 3.

Calcium sulphate has been chosen because it is a by-product of many industrial processes; therefore, the process fully falls within the circular economy processes in which, in addition to CO2, waste material of other industrial processes is used, which are reused for the production of high value-added chemicals.

Specifically step b) preferably comprises the following steps: b. l) addition of calcium sulphate and formation of the aforesaid two salts, precipitation and recovery of the precipitated calcium carbonate from the aqueous solution by filtration; b.2) recovery of ammonium sulphate from the filtered aqueous solution coming from step b. l).

Calcium sulphate may be supplied in step bl) in powder form.

In this case in step b.l) CaCCh precipitates immediately due to the low solubility product in water (P.S.= 6.6* 10' ⁴ g per 100 g of water) and is separated from the bottom of the crystallizer.

In step b2) the liquid solution, coming from the filtration of calcium carbonate, instead, is sent to a second crystallizer/evaporator where an oversaturation of the solution is generated both by evaporation (decrease in pressure) and by consequent decrease in temperature, thus allowing the precipitation of ammonium sulphate, which is in turn recovered at the bottom of the second crystallization unit.

Achieving the over saturation conditions of the solution requires the vaporization of a part of the solvent mixture. The higher the vaporization, the lower the volume of liquid inside the equipment and, therefore, a smaller unit is supposed to be installed. However, a greater vaporization of the solvent mixture results in good energy consumption which can be compensated for, if there is no thermal waste to be recovered, by low enthalpic currents since the temperatures involved are lower than the boiling temperature of ammonia at the preferred pressure conditions. The over saturation conditions therefore depend on the plant context and are prior art that can be calculated by the expert. The vaporized solvent mixture is constituted only by water and ammonia since the solutes remain in solution until their crystallization. The solvent stream is condensed and recycled at the absorption unit after make-up of water and ammonia.

According to another embodiment of the process shown in schematic form in Figure 3, it provided that calcium sulphate is added in the form of aqueous solution.

In this case, it is avoided that CaCO3 covers the CaSO4 particles before they dissolve, thus preventing their total dissolution. The solution must be supplied possibly at a temperature close to the dissolution peak (T = 40°C) to minimize the quantities of water involved (Figure 4). In such a case, the crystallization unit in which step bl) is carried out may assume the configuration of Figure 6.

This type of conventional crystallizer is defined as a crystallizer with draft tube baffle crystallizer.

This crystallizer filled with a liquid for a volume comprised between 50 and 90%, preferably 80% the total volume of said crystallizer is provided:

• with a deflector indicated in the figure with Baffle, with a mechanical agitator indicated in the figure with Stirrer placed inside a tube (Draft Tube), preferably of truncated conical shape, completely immersed in the liquid, and ending with an elutriation chamber where the precipitated calcium carbonate is collected;

• with at least one inlet for the aqueous solution coming from step a)

• with an inlet for feeding the aqueous solution of calcium sulphate,

• with an outlet for the aqueous solution containing ammonium sulphate and

• with an outlet located at the level of the zone occupied by the gases respectively for the humid gases (NH 3/CO2);

• with an outlet of the aqueous solution that is reintroduced into the upper part of the crystallizer in the part not occupied by the liquid and in direct contact with the latter.

Preferably this crystallizer comprises two inlets for the aqueous solution coming from step a) of which the first is located in the zone below the deflector or Baffle and the second in the bottom of the elutriation chamber or Elutriation leg.

In particular, step b.1. of the process carried out in the crystallizer of Figure 3, comprises the following steps.

The aqueous solution of calcium sulphate is supplied to the crystallizer Draft tube baffle crystallizer at a temperature close to the dissolution peak (T = 40°C), The calcium carbonate crystallizes and precipitates in the elutriation chamber, before being expelled from the bottom by means of a solids valve;

A part of the ammonia- and carbon dioxide-rich solution is supplied from the bottom of the same neck to keep the crystal bed fluidized to prevent its packing. Part of the solution evaporates (water and ammonia) and is recycled upstream at the absorber after condensation. The solution discharged of calcium, CO2 and CaCO3 particles is taken from the blind torus outside the Baffle, preferably after filtration for example with membrane, and is sent to a second crystallization unit similar to the previous one where step b-2) of crystallization of ammonium sulphate takes place.

Also this crystallization that occurs in step b.2) takes place thanks to two phenomena of equal importance:

- the evaporation of the solvent

- decrease in temperature which generate the conditions of oversaturation for the precipitation of the salt. The evaporated stream (water and ammonia) is then recycled at the absorption column after condensation, pumping and make-up with ammonia.

Alternatively, step b. l) can be carried out in an operating unit, such as the one represented in Figures 6, 7 and 8, which from now on we will define as a descaling unit that in the aforesaid figures is indicated with 1.

This unit is constituted by a substantially cylindrical column, whose head comprises the crystallizer indicated in the figure with Draft tube Baffle Crystallizer .

The volume occupied by the steam of this operating unit is comprised between 5 and 15%, preferably it is 10% of the total volume.

This operating unit (1) is formed by: o a first region defined Supply Skirt Region as indicated in the aforesaid Figures delimited below by the bottom of said column and above by a sprayer or Gas sparger like in the figures of ammonia and carbon dioxide; o a second region or bubble column region delimited below by the aforesaid sprayer Gas sparger and above by the V-shaped manifold, in which second region liquid-gas contact takes place and for this reason it is equipped with at least one perforated single or multiple chimney plate, and said second region also being equipped with at least one inlet for CO2/NH3 gases that are dispensed into said region by means of at least one sprayer; o a third region or Crystallization region delimited below by the V-shaped collector V connection) and above by the aforesaid crystallizer Draft-tube baffle crystallizer .

This third region is also provided: o with the aforesaid V-shaped manifold (V connection) which is welded to the operating unit through a ring or Welding ring and which ends below with an elutriation chamber ox Elutriation leg as indicated in the figures as substantially inverted cone-shaped, where calcium carbonate is collected; o With said crystallizer (Draft-tube baffle crystallizer) in turn provided with: a deflector (Baffle), with a mechanical agitator (Stirrer) placed inside a tube (Draft Tube), preferably of truncated conical shape, completely immersed in the liquid, said crystallizer being further provided with an inlet for feeding the aqueous solution of calcium sulphate, with an outlet for the aqueous solution containing ammonium sulphate and with an outlet located at the level of the zone occupied by the gases respectively for humid gases (NH3/CO2).

The third region is preferably provided with at least one outlet preferably two outlets from which the not yet dissolved gas-rich aqueous solution diverted by the V-shaped manifold (V connection) and/or by the deflector (Baffle) exits and is re-introduced into said region through at least one inlet, preferably two inlets, arranged in the zone occupied by the gases. In particular, step bl) of the process of Figure 3 is carried out in said descaling unit shown in Figures 7, 8, 9 according to operating modes shown below.

Water, ammonia and carbon dioxide are supplied from below in the first region or Supply skirt region as indicated in the aforesaid figures, the water fills the body of the unit in all its regions, for a volume comprised between 85 and 95% preferably for 90% the total volume of said operating unit (1).

This water, supplied in the first region, comes from the absorption tower already containing ammonia and CO2, but it can possibly be further enriched in ammonia and CO2 through the sprayer or sparger, reaches the second region, where liquid gas contact takes place.

Once the ascent of the second region is completed, it reaches the third crystallization region at a temperature of about 40°C, i.e. at the temperature of maximum solubility of the gypsum in water.

The water/ammonia/Carbon Dioxide (WACD) solution enters the “V” connector. The V connector is welded to the wall of the unit (1) by means of a perforated ring, which acts as a flow homogenizer, surmounted in turn by a slight widening of the mantle. This zone favours the separation of ammonia and carbon dioxide bubbles that are not yet dissolved in the WACD solution.

The gas is removed from the top in humid form and recycled to the feed;

The WACD solution, on the other hand, turns towards the centre of the unit, following the V profile and near the centre enters the third crystallization region, where the aqueous solution of calcium sulphate (ASCS) at 40°C is also supplied from the top.

The agitator or Stirrer placed at the centre of the truncated conical tube or Draft Tube recalls the rich solution upwards, favouring the mixing with ASCS and activating metathesis (1).

CaCO3 is formed by heterogeneous nucleation induced from the outside and due to the poor solubility it continuously grows freeing the mixture from calcium and carbonate ions, leaving therein on the contrary the ammonium and sulphate ions in water (AAS mixture), said CaCO3 particles remaining in suspension until they reach a critical mass for which they lose speed, decant at the bottom of the V connector and roll inside the elutriation cone Elutriation leg.

The AAS mixture is tapped through an outlet located at the top and laterally to the unit (1), precisely beyond the deflector or Baffle submerged in the liquid. The deflector has the task of keeping the external zone as clean as possible from CaCO3 particles, which can hardly ascend a fluid-dynamically nil zone (blind arm). This outlet is preferably provided with a membrane to recover the solution without entrainments of small suspended CaCO3 particles. Also in this case step b2) is carried out in a further crystallizer where the evaporation of the solvent takes place by reducing the pressure until reaching the oversaturation conditions, in which the ammonium sulphate precipitates thanks also to the reduction in the temperature and the evaporated stream constituted by water and ammonia, after condensation and makeup with ammonia is recycled at the absorption column.

The process as schematically described in Figure 3 may preferably contemplate an electrolysis step prior to step b) to eliminate the metal cations from gypsum.

Preferably the absorption column where the capture of CO2 takes place in the process object of the invention is provided with a washing tower arranged at the head of said absorption column, as represented in Figure 10.

This washing tower is supplied with water only and step a) comprises the following steps. The purified gas exiting the absorption column and containing traces of ammonia is sent to said washing column where washing water passes countercurrently. At the head of said washing column the purified gases free of ammonia exit while the water at the tail of said absorption tower containing ammonia is cooled and recycled at the head of the absorption column near the make-up of the ammonia solution.

The washing tower may be integrated into a single assembly with the absorption column, like for example shown in Fig. 11.

In the figure on the left the cooling can be carried out by means of pump around, or by means of an external ancillary unit (EXTERNAL ANCILLARY UNIT) arranged near the make-up (NH3 make-up) of the ammonia solution, as represented in the figure on the right.

In the case of the pump around, generally used in distillation columns, the ammonia solution is extracted from a colder lower stage and introduced into an upper stage near which also the make-up of the ammonia solution takes place.

In the case of the external ancillary unit, preferably a heat exchanger, the solution is extracted from a step and cooled by means of the external ancillary unit and reintroduced into the same step, near which the make-up of the ammonia solution is arranged.

Other embodiments of a single absorption and washing column are shown in Figure 12.

In the figure on the left, the cooling system is obtained by means of a single chimney plate equipped with a tube bundle that crosses it throughout the circular evolution.

This tube bundle can be connected to the make-up of the ammonia solution and is able to release cold ammonia solution through appropriate lateral or frontal openings in order to achieve direct cooling of the liquid descending the unit.

In the figure on the right, cooling is achieved by supplying cold ammonia solution from the outside in the quantities necessary for make-up and at a temperature such as to achieve the cooling of the plate.

Below are the following exemplary embodiments of the process of the invention for illustrative purposes.

Example 1

A simulation of step a) of the process of the invention is carried out, which envisages in addition to the capture of CO2 through absorption column (T101) on which the ammonia solution is passed, also the elimination of traces of ammonia through the washing tower (T102) as shown in Figure 13.

The simulation of step a) of the process of the invention is carried out using AspenHysis v.10 carried out at 1.3 bar, 10 bar and 30 bar (absolutes), respectively.

In Table 1 of Figure 14, in Table 2 of Figure 16 and in Table 3 of Figure 18, the results obtained by simulating step a) at the aforesaid pressures are shown. Figures 15, 17 and 19 show instead the temperature profiles as a function of the number of steps of the absorption column on the left and the washing tower.

As can be seen from a comparison of the data reported in the above tables note that the CO2 exiting the absorption column (see stream liquid waste solution) goes from 14% when the process is carried out at 1.3 bar until reaching about 48% at 30 bar. At the same time, as the pressure increases, the volumes of liquid to be recycled decrease considerably (see stream liquid recycle) (3.99 vs. 0.41 m3/h)

Example 2. Complete plant for flue gas recovery, mass balances

A generic stream of 1000 kg/h of flue gas at 130°C, 1 atm and molar composition is considered as follows: N2 = 72%, CO2= 12%, H2O = 14% and 02 = 2%. The massive flow rate is qualitative to simplify the efficiency estimates of the entire plant. To capture this diluted amount of CO2, the above stream is brought to 30 atm by compression and dehydration. This stream is fed to the absorber, which receives a liquid mixture equal to 516 kg/h of water and 344 kg/h of ammonia. The feed of the solution can also take place in several points according to the diagrams illustrated in the embodiments of the invention. To convert a mole of CO2 according to reaction (1), it is needed: 1 mole of water, 2 moles of ammonia and a mole of gypsum. To recover and transform 1 ton of CO2, therefore, 3.09 tons of gypsum (anhydrous), 0.77 tons of ammonia and 0.41 tons of reaction water (does not include water of dissolution, in any case recycled within the process) are needed to produce 2.27 tons of CaCO3 to be supplied to cement plants and 3 tons of fertilizer.