DESCRIPTION

Field of the Invention

The present invention relates to a process and an apparatus for recovering CO2 from a pressurised gaseous process flow comprising CO2. Preferably, the process and the apparatus of the present invention are used for recovering CO2 from pressurised process flows comprising hydrogen or syngas.

Background art

CO2 recovery is part of a vast area of significant environmental interest called CCS, i.e., Carbon Capture and Storage and, more recently, CCUS, Carbon Capture, Utilisation and Storage.

CCS and CCUS are aimed at mitigating the greenhouse impact and solving the current century's Grand Challenges such as global warming, climate change and melting glaciers, to name a few.

In the state of the art, CO2 is mainly sequestered by flue gas streams by means of so-called sweetening processes, i.e., processes that combine absorption units with regeneration units to selectively recover and release the acid gases, precisely CO2, by means of appropriate mixtures.

Such mixtures usually consist of water and amines and, preferably, of monoethanolamine in the case of sequestration of CO2 only. The percentages in the mixture vary depending on the case and the technologies up to a maximum of 35% by volume of amine.

Sweetening processes are known for their effectiveness, but also for some operational problems, such as energy expenditure, as well as the corrosiveness and toxicity of the absorption carrier. In particular, it is estimated that such processes are one of the main variable cost items of chemical plants, with approx. €100 per tonne of CO2 sequestered and are sometimes attributed to ammonia-based emissions generated by the progressive decomposition of the amines themselves subjected to considerable absorption and regeneration treatment cycles.

It is known to use water as a carrier since the absorbability of CO2 in water is favoured by a good Henry constant with respect to the other components that can constitute an exhaust stream and the same air. However, due to the considerable amount of water required and the increased efficiency of other fluids, such as amines, water- only abatement processes are not considered today.

Summary of the invention

In order to overcome the aforesaid problems, a process has been designed to allow the selective and efficient recovery of CO2 from a pressurised gaseous flow by exploiting the operating conditions of the flow itself and only water as a carrier.

An object of the present invention is therefore a process for recovering CO2 from a pressurised gaseous process flow containing CO2, comprising the following steps: a) the CO2 contained in said gaseous process flow is absorbed in water at a pressure between 5 and 100 atm obtaining CCh-enriched water; b) the CO2 is de-absorbed from the enriched water from step a) at a lower pressure than the pressure of step a) and the gaseous CO2 is separated from the water which is recycled and pressurised again for use in step a), said process being conducted: in a single apparatus comprising:

• an absorption section in which step a) is conducted; and

• a regeneration section in which step b) is conducted in which said absorption section and said regeneration section are in direct fluid communication or in two separate apparatuses consisting of:

• an absorber in which step a) is conducted; and

• a regenerator in which step b) is conducted in which said absorber or regenerator are in fluid communication with each other through a duct, possibly regulated by a thermal expansion valve.

Advantageously, the process object of the present invention allows to combine an economic, non-toxic and non-polluting liquid carrier for recovering CO2.

Advantageously, the process object of the present invention allows to recover considerable amounts of CO2 and to totally or partially clean the gaseous flow comprising the CO2 such as, for example, hydrogen and syngas streams from plant units or biogas from anaerobic digesters. Advantageously, the process object of the present invention allows to significantly reduce operating costs, environmental impact and process hazards. It further allows to use only electricity, not envisaging some heat sources for absorption and regeneration (complete electrification and, therefore, sustainability in the case of electricity supply from renewables).

List of figures

Figure 1: schematic depiction of an apparatus for carrying out a process for recovering CO2 from a process flow according to a first embodiment of the present invention;

Figure 2: schematic depiction of an apparatus for carrying out a process for recovering CO2 from a process flow according to a second embodiment of the present invention;

Figure 3: schematic depiction of an apparatus for carrying out a process for recovering CO2 from a process flow according to a third embodiment of the present invention;

Figure 4: block diagram depiction of a plant for the production of syngas in which the apparatus for carrying out a process for recovering CO2 is inserted in accordance with the embodiments of figure 1, 2 or 3;

Figure 5: block diagram depiction of a plant for the production of methane hydrogen in which the apparatus for carrying out a process for recovering CO2 is inserted in accordance with the embodiments of figure 1, 2 or 3,

Figure 6: schematic depiction of an apparatus for carrying out a process for recovering CO2 from a process flow according to a fourth embodiment of the present invention;

Figure 7 schematic representation of the apparatus of Figure 6 in which the energy developed in the expansion step of the water leaving the absorber is recovered to operate the pump to bring the water back to the required pressure;

Figure 8 schematic representation of the apparatus of Figure 6 in which the absorber and the de-absorber are in fluid communication, in which said fluid communication is regulated by a thermal expansion valve.

Detailed description

For the purposes of the present invention, the definition "comprising” does not exclude the presence of additional components not expressly mentioned in the list after such a definition.

The expression "consisting of’ excludes the presence of additional components after such a definition.

For the purposes of the present invention, the definition of "water" means industrial water that is commonly used in industrial plants, or partially or totally demineralised water, the partial or total demineralisation of which is carried out with the sole purpose of avoiding incrustations of the plants in which it is used.

The process according to the present invention allows to recover CO2 from a pressurised gaseous process flow containing CO2 by the use of a carrier, in particular water. Preferably, the process for recovering CO2 is associated with industrial processes, for example hydrogen or syngas production, as shown in the graph of figures 4 and 5, operating pressurised. It is thereby possible to efficiently exploit the pressures of the gaseous process flow leaving the plant and recover CO2. Otherwise, the recovery process is also possible together with the related apparatus following a compression of the process flow until adequate pressures are reached to carry out the process, as will be clarified below.

A further effect of the use of the process according to the present invention is also to totally or partially clean the process flow from CO2 for further uses.

The process according to the present invention comprises step a) of absorption, in which the CO2, contained in the gaseous process flow, is absorbed in water. Such absorption is favoured by the pressure at which the gaseous process flow operates. Specifically, it should be noted that step a) of the process for recovering CO2 is carried out at a pressure between 5 and 100 atm, preferably between 20 and 100 atm and more preferably between 40 and 100 atm. Specifically, the pressure of step a) is substantially equal to the pressure of the process flow comprising CO2.

It should be noted that the absorption of CO2 in water is favoured by a good Henry constant increased by the pressures at which step a) of the process operates.

In accordance with an embodiment of the present invention, the water is provided to step a) at a pressure between 5 and 100 atm, preferably between 20 and 100 atm and more preferably between 40 and 100 atm, in the most preferred condition the water pressure is substantially equal to the pressure of the process flow provided to step a).

In step a) the water is provided so as to come into contact with the countercurrent or equi -current process flow as a function of the insertion of the process flow in step a) with respect to where the water is diffused.

The contact between water and process flow therefore allows the selective elimination of CO2 from the process flow. Thereby, following absorption, step a) allows to obtain water enriched in CO2 and a process flow cleaned from the CO2.

The process for recovering CO2 comprises a subsequent de-absorption step of the CO2 from the CC -enriched water from step a) and is obviously conducted at a lower pressure than that of step a). Thereby, step b) not only allows to recover the CO2 for any subsequent steps or treatment, but also to recycle, at least in part, the water in step a). In fact, following the elimination of CO2 from the water, it can be reused following a pressure increase at step a). Although the pressure at step b) is always lower than the pressure at step a), in any case the pressure at step b) is preferably between 1 and 80 atm, more preferably between 1 and 10, and in the most preferred condition the pressure at step b) is between 1 and 5 atm. Thus, at step b) the gaseous CO2 is separated from the water which is then recycled and pressurised to be used in step a) with any additions where a part of the gaseous CO2 is wet.

Preferably, step b) is conducted by atomising the CCh-enriched water. Subsequently, step b) includes, by means of a phase separator, condensing and recovering the water and the CO2 for subsequent treatments.

In accordance with the present invention, the process for recovering CO2 is carried out by means of an apparatus indicated overall with 1, G, 1” in the figures. Specifically, the C02 recovery process can be carried out in an apparatus 1 schematically shown in figure 1 and 3 or in two separate apparatuses ,I” figures 2 and 6.

In accordance with an embodiment, the single apparatus 1 comprises an absorption section la in which step a) is conducted and a regeneration section lb in which step b) is conducted. Specifically, the absorption section la and the regeneration section lb are in direct fluid communication

In accordance with the alternative embodiment, the separate apparatuses , 1” consist respectively of an absorber 3 in which step a) is conducted and a regenerator 4 in which step b) is conducted. Specifically, the absorber 3 and the regenerator 4 are in fluid communication with each other through a duct 5, possibly regulated by a thermal expansion valve 6.

It should be noted that in both embodiments of the apparatus, in order to achieve an effective absorption of the CO2 in the water at step a) the pressure in the absorption section la and in the absorber 3 is between 10 and 100 atm, more preferably between 20 and 100 atm and even more preferably between 40 and 100 atm, in the most preferred condition the pressure is substantially equal to the pressure of the incoming process flow.

With regard to the regeneration section lb and the regenerator 4, where step b) is conducted even if the pressure is lower than the pressure of step a), in any case is preferably between 1 and 80 atm, more preferably between 1 and 10 atm and in the most preferred condition the pressure is between 1 and 5 atm and more preferably. Preferably, the pressure in the regeneration section lb and in the regenerator 3, where the CO2 is degassed, depends on the CO2 storage pressure downstream of the apparatus, specifically:

- 1 atm if the CO2 is released into the atmosphere or if there are dedicated compression systems;

- 8 - 10 atm if the CO2 is stored in cylinders;

- other pressures depending on the CO2 storage/use technology.

In accordance with an embodiment, both the absorption section la and the absorber 3 comprise a water diffuser 7 near the head of the absorption section la and the absorber 3 respectively in order to atomise the water. Preferably, the atomised water is at a pressure substantially equal to the pressure of the absorption section la or the absorber 3. Such a pressure is achieved by pumping means 14, for example a volumetric pump.

As previously anticipated, the recovery process includes a contact between atomised water and countercurrent or equi-current process flow.

According to a preferred embodiment, the absorption section la and the absorber comprise an inlet 8 positioned between the diffuser 7 and the end of the absorption section la or the absorber 3. Thereby, in step a) the process flow enters the absorption section la or the absorber through the inlet 8 so that the process flow contacts the atomised water in countercurrent. Specifically, the gaseous process flow will tend to go from the end to the head of the absorption section la or of the absorber 3, while the atomised water will tend to go from the head to the end of the absorption section la or of the absorber 3.

Preferably, in the present countercurrent configuration, a packing zone 13 is included between the diffuser 7 and the inlet 8 to increase the contact surface between the process flow containing CO2 and the water.

In accordance with an alternative embodiment, the absorption section la and the absorber comprise an inlet 8’, figure 3, positioned near the diffuser 7 or between the diffuser 7 and the head of the absorption section la or the absorber 3. Thereby, in step a) the process flow enters the absorption section la or the absorber through the inlet 8' so that the process flow contacts the atomised water in equi-current. Specifically, the gaseous process flow due to the inlet pressure is pushed from the head to the end of the absorption section la or the absorber 3 as the atomised water and will subsequently tend to turn going from the end to the head of the absorption section la or the absorber 3.

In accordance with an alternative embodiment to the previous ones, not illustrated in the figures, the absorption section la and the absorber 3 include bubbling the process flow in a liquid bed placed at the end of the absorption zone la or the absorber 3 respectively. It should be noted that the liquid bed is an accumulation of CC -enriched water following bubbling at the end of the absorption zone la or the absorber 3. Preferably, the absorption section la and the absorber 3 comprise conveying means, for example in fluid communication with the inlet 8, configured to convey the process flow into the liquid bed to bubble the process flow within the liquid bed. Thereby, working pressurised, step a) is performed. That is, the present embodiment has the functionality and technical features of a water column known to the person skilled in the art and used herein with a pressurised process flow to facilitate CO2 separation.

It should be noted that the embodiments described above specifically of the water column and the countercurrent flow are combinable so as to accumulate the liquid bed on the bottom by means of a diffuser 7 and optimise the CO2 absorption process

Thereby, step a) makes it possible to clean the process flow from the CO2 which will tend to continue towards the head of the absorption section la or of the absorber 3 from where it leaves the apparatus 1, G. Preferably, the absorption section la and the absorber 3 comprise a head outlet channel 15 that allows the extraction of the process flow cleaned of the CO2. In detail, step a) envisages, after the absorption of CO2, also the extraction of the cleaned process flow by means of the outlet channel. More preferably, a demister 16 with optional droplet separator is also included for further cleaning the cleaned process flow before exiting the absorption section la or the absorber 3. Therefore, following the first step a) the process makes it possible to obtain a precipitate of CC -enriched water 18 at the bottom of the absorption section la or of the absorber 3 and a process flow cleaned of CO2 exiting from the head of the absorption section la or of the absorber 3.

Subsequently, the process includes the transition of the CC -enriched water from the absorption section la or from the absorber 3 to the regeneration section lb or to the regenerator 4 respectively in fluid communication where step b) occurs.

According to a preferred embodiment of the single apparatus 1, schematically depicted in figure 1 or 3, it comprises a plurality of holes 9 positioned in the end of the absorption section la and configured to put the absorption section la in fluid communication with the regeneration section lb. Such a plurality of holes 9 is made at the bottom of the absorption section la e.g., drilled or punched and preferably in a winding configuration (as reported in the inventor's publication: Bozzano, Dente, Manenti, Masserdotti, Corna, “Fluid Distribution in Packed Beds. Part 2. Experimental and Phenomenological Assessment of Distributor and Packing Interactions", Industrial & Engineering Chemistry Research, 53, 3165-3183, 2014 relating to air/water contact). Specifically, step b) includes the atomisation of the enriched water by means of the plurality of holes 9. Such an atomisation is carried out thanks to the pressure difference between the absorption section la and the regeneration section lb. In fact, the high pressure in the absorption section la strongly pushes the CCh-enriched water through the plurality of holes 9, atomising it and favouring the separation of the CO2 dissolved therein. In other words, the CCh-enriched water is atomised and due to the pressure reduction in the regeneration section lb, it de-absorbs the gaseous CO2.

In accordance with an alternative embodiment to the previous one of the apparatus 1’, 1”, schematically depicted in figures 2 and 6, it comprises a duct 5 connecting the absorber 3 and the regenerator 4, possibly regulated by the thermal expansion valve 6. Such a duct includes on the one hand a connection with the bottom of the absorber 3 on the other a connection with the regenerator 4 by means of a plurality of holes, for example sprays, nebuliser, rotating disks or atomisation systems 9’. Also in this case as in the previous one, the passage holes are positioned near the head of the regenerator 4. Such holes 9’ allow the passage of CCE-enriched water and its atomisation by virtue of the pressure difference between the regenerator 4 and the absorber 3. Furthermore, the optional valve 6 allows to regulate the level of enriched water on the bottom of the absorber 3. Specifically, step b) includes the atomisation of the enriched water by means of the plurality of holes 9’ . Such an atomisation is achieved by virtue of the pressure difference between the absorber 3 and the regenerator 4. In fact, the high pressure in the absorber 3 strongly pushes the CC -enriched water through the plurality of holes 9’, atomising it and favouring the separation of the CO2 dissolved therein. In other words, the CC -enriched water is atomised and due to the pressure reduction in the regenerator 4 de-absorbs the gaseous CO2.

In accordance with an alternative embodiment to the previous one, the duct 5 with the possible valve 6 can also be applied to the single apparatus 1 in place of the perforated bottom of the absorption section la.

In accordance with further embodiments not illustrated in the figures, the single apparatus 1 or the separate apparatuses , 1” comprise CC -enriched water heating systems to promote the release of the CO2. Such heating systems can comprise elements inside the absorption section la, the regeneration section lb, the absorber 3 or the regenerator 4 so as to heat the enriched water prior to the atomisation of step b). Such heating systems can comprise, for example, a coil, plate or tubes which, when in direct contact with the enriched water, allow heating. Alternatively, such heating systems are positioned outside the apparatuses such as, for example, a heat exchanger or electrical resistance acting on the duct 5 when there are separate apparatuses. In accordance with these embodiments, the process includes a heating step b) prior to atomisation in the regenerator 4 or in the regeneration section lb. It should be noted that in accordance with embodiments in which there is a heating prior to atomisation, a cooling of the water in evaporative towers is preferable, for example, once step b) has been performed and before the water is recycled to step a). This is especially true in cases of large water flows such as, for example, in large plants known to the person skilled in the art.

According to a preferred embodiment, the regeneration section lb and the regenerator 4 comprise a phase separator 10 configured to separate the de-absorbed CO2 and the degassed water. Preferably, the phase separator 10 is preceded by a accumulation convergent 11 placed downstream of the atomisation in the regeneration section lb and the regenerator 4 configured to direct the degassed water and the released CO2 to the phase separator 10.

Preferably, the phase separator 10 is configured to separate the degassed water by condensing it at the bottom of the regeneration section lb or of the regenerator 3 by means of a condensation trap. Instead, with regard to the released CO2 stream, from the water, the phase separator 10 is configured to direct it away from the bottom of the end of the regeneration section lb or of the regenerator 4 to be subsequently expelled from the regeneration section lb or from the regenerator 4 by an outlet channel 12. Subsequently, the CO2 flow can pass through a system of baffles and demisters to eliminate any aerosols and droplets dragged to exit with a minimal moisture content. Finally, such moisture can be recycled together with the degassed water in step a) after compression.

Specifically, step b) envisages that in the regeneration section lb or in the regenerator 4, the C0 ₂ -free water is collected at the end of the regeneration section lb or at the regenerator 4 where it is pressurised and recycled to the absorption section la or to the absorber 3. Specifically, the regeneration section lb and the regenerator 4 comprise a recycling channel 17 that puts the bottom of the regeneration section lb and the regenerator 4 in fluid communication where the degassed water and the diffuser 7 has precipitated, passing through the pumping means 14. The process thereby allows recycling the water as a carrier. It should be noted that in some cases the process includes a supply of water to the recycled water to compensate for any leaks, before this is pumped and atomised inside the absorption section or in the absorber.

Advantageously, the process allows to exploit the pressures of the process flow by reducing the single operating cost of the pumping systems to insert the water into the absorption section or into the absorber.

A particularly preferred form of the process of the invention is that schematically shown in figure 7. In this form, as also shown in the following example 3, the unit P100 arranged downstream of the absorber T-100 allows the energy recovery of the water on which the C02 is absorbed, during decompression and consequently expansion of the liquid mass before being sent to the regenerator VI 00. The recovered energy allows the pump P101 to operate to pressurise the water that is recycled to the absorber T100. This apparatus can also include a second pump PI 02 located downstream of the pump P101 to compress the water entering the absorber at the required pressure.

Following are four application examples of the process for recovering CO2 and the related apparatus 1, G, 1” in the case of:

- correction of the CO2/CO/H2 ratio for methanol synthesis plants;

- purification of hydrogen from CO2 downstream of reforming and water-gas shift reactors. APPLICATION EXAMPLE 1 (figure 4)

An application case is represented by the synthesis of methanol from plastics, according to Italian patent application No. 102019000013239 of the type schematically depicted in Figure 4, in which the process flow comprising CO2 comes from successive steps carried out in different sections.

Specifically, the process has been suitably simulated in Aspen Hysys (AspenTech suite). In particular, in the process object of the first application example, downstream of some heat treatments of the plastics, a syngas process flow is obtained to be then converted into methanol. Specifically, Table 1 shows the composition of the syngas process flow. It should be noted that the CO2 contained therein must be partially removed in order to provide the optimal methanol synthesis conditions (ratio S = 2.001). To this end, 47% of the CO2 present must be selectively removed with the use of a single electrical energy consumption (duty) specific to this case of 87,768 kW due to the pump 14.

Where the operating conditions are shown below:

- the process flow pressure at the inlet 8 is 8000 kPa (78.95 atm) while the temperature is 25°C;

- the pressure of the process fluid in the passage between the absorption section la and the regeneration section lb is 8000 kPa (78.95 atm) while the temperature is 20.88 °C;

- the pressure of the process fluid cleaned of CO2 in the outlet channel 15 is 7900 kPa (77.97 atm) while the temperature is 20.76 °C;

- the pressure of the CO2 released in the outlet channel 12 is 100 kPa (0.99 atm) while the temperature is 20 °C;

- the pressure of the degassed water in the recycling channel 17 upstream of the pumping means 14 is 100 kPa (0.99 atm) while the temperature is 20°C;

- the pressure of the degassed water in the recycling channel 17 downstream of the pumping means 14 is 8000 kPa (78.95 atm) while the temperature is 20.67°C, such conditions are also the same at the outlet from the diffuser 7 in the absorption section l a.

Table 1.

At the outlet from the process there is the sweetened syngas shown in table 2 and the CO2 flow shown in table 3.

Table 2

Table 3

The above results were obtained with an energy consumption expressed in kWh per kg of CO2 removed of 0.159 kWh/kgCC with a cost of 15.94 €/tonC0 ₂ assuming the cost of electricity as 0.1000 €/kWh.

APPLICATION EXAMPLE 2 (figure 5)

A classical methane hydrogen production unit consists of a primary reformer and a shift reactor. The primary reformer transforms methane into syngas by means of steam, supplied abundantly above the stoichiometric by the following reaction:

CH ₄ + H ₂ O = 3 H ₂ + CO

Subsequently, the shift reactor transforms the carbon monoxide into CO2, obtaining further Eh by means of part of the residual steam:

12 CO + H2O = CO2 + H2

The block diagram of figure 5 schematically shows the process in accordance with the second exemplary example that also in this case has been simulated in AspenHysys. Contrary to the above example, 1 in the production of hydrogen it is necessary to remove almost all CO2 in order to achieve market standards. Table 4 shows the composition of the process flow entering the apparatus.

Table 4

The composition of the cleaned process flow and the C02 flow are reported in tables 5 and 6, respectively. Table 5

Table 6

The data reported therein were calculated assuming an absorbent liquid temperature of 20.83°C and an operating gas pressure entering to the absorption unit of 80bara. In these operating conditions the electricity consumption in kWh per kg of CO2 removed is 0.098 kWh/kgCC with a cost of 9.8 C/tonCC assuming the cost of electricity as 0.1000 €/kWh.

EXAMPLE 3 - C02 capture system for gaseous stream intended for the synthesis of methanol with energy recovery in the expansion phase of the CO 2 -containing liquid. The process diagram shown in Figure 7 representative of the CO2 regulation module in question is composed as follows: the unit T-100 represents a liquid gas contact section, stream number 2 after being contacted with the CCE-rich syngas exits from the bottom of the unit in the CCh-enriched stream number 4. The stream number 4 is then expanded at low pressure, releasing the captured CO2. The P-100 unit represents an energy recuperator that exploits the pressure jump to generate electric current for use in the unit P-101 to compress the degassed fluid. The choice of this unit in terms of mechanical features and efficiency must be submitted to expert personnel capable of assessing the specific situation. The unit V-100 is provided with a volume that allows gas-liquid separation, at the top of the unit a stream mainly consisting of CO2 is collected while the regenerated liquid is collected on the bottom. The unit P-101 allows the liquid to be compressed, exploiting the energy recovered from the unit P-100, while the unit P-102 covers the remaining pressure jump necessary to bring the absorbent liquid flow rate back to the head of the contact section. Although not always necessary, it is possible to introduce fresh water into the system (WATER MAKE UP) to compensate for any leaks.

A process simulation in Aspen HYSYS VI 1 was used to validate the performance of the CO2 capture system intended for the synthesis of methanol at a pressure of 65 bara. The composition change along the absorption column is shown in Table 1. Finally, table 2 shows a summary of the operating conditions of the whole process with the main intensive and extensive process variables.

Table 1: Molar compositions of the input and output streams from the module The optimum stoichiometric number value SN or the molar ratio (H2-CO) / (CO2+CO) for the synthesis of methanol is obtained with a total electric consumption of 41.76 kW at pump P-102 used for the high pressure pumping of the water used for the absorption of CO2. The energy demand on a mass basis of CO2 can therefore be estimated at a value of 0.151 kWh/kgco2.

Table 2 the mass flow rates in kg/h of the main streams

Table 2: Operating conditions of all process streams involved in the simulation shown in Figure 7

EXAMPLE 4 C02 capture system for gaseous stream intended for the synthesis of methanol without energy recovery in the expansion phase of the C02-containing liquid. If the technician responsible for choosing the unit P-100 does not consider the implementation of an energy integration as cost-effective, it is possible to apply the system with the same operating conditions, flow rates and compositions, but with higher electric consumption in the configuration shown in figure 8 where the energy recovery unit and the associated compression unit are replaced by a simple thermal expansion valve. This results in a significant increase in consumption to the compression unit P- 102 which in this configuration with the same operating conditions has an electric consumption of 96.1 kW, bringing the energy demand on a mass basis of CO2 to a value of 0.348 kWh/kgcoi.