IT201600081328A1 - RECOVERY OF CARBON DIOXIDE FROM SYNTHESIS GAS IN PLANTS FOR THE PRODUCTION OF AMMONIA BY MEANS OF GRAVIMETRIC SEPARATION - Google Patents

Description

"RECOVERY OF CARBON DIOXIDE FROM SYNTHESIS GAS IN

INSTALLATIONS FOR THE PRODUCTION OF AMMONIA BY MEANS OF

GRAVIMETRIC SEPARATION "

DESCRIPTION

Field of the invention

The present invention finds application in the field of ammonia and urea synthesis.

State of the art

The complex synthesis gas production process generally comprises a reforming section from which a mixture of hydrogen, carbon dioxide, carbon monoxide, water and methane is obtained.

There may also be: nitrogen, argon, helium and other non-condensable inert elements or compounds.

The process then includes a shift section, in which the carbon monoxide is reacted with water giving carbon dioxide and hydrogen:

As shown in Figure 1, in known ammonia-urea processes, carbon dioxide is usually separated from the synthesis gas by washing with suitable solvents by means of a physical or chemical-physical absorption mechanism of carbon dioxide ("conventional CO2removai" section); the solvent is then suitably regenerated.

The residual carbon dioxide and carbon monoxide are subsequently eliminated by methanation ("methanation & compression") or by cryogenic washing with nitrogen ("purification and compression"), so as to obtain the mixture of hydrogen and nitrogen in the right ratio for the synthesis of ammonia ("ammonia synloop").

The recovered carbon dioxide is instead compressed in a subsequent section in order to reach the pressure suitable for the urea synthesis process ("urea plant").

The "conventional CO2removai" is not without drawbacks. Firstly, it has a high energy cost due to solvent regeneration.

Furthermore, the carbon dioxide is recovered at a low pressure, which therefore requires a compression phase to be able to reach the approximately 140-200 bar necessary for use in the synthesis of urea.

In addition, the use of a solvent for carbon dioxide separation has other disadvantages, including:

degradation in case of excessive operating temperature of the section;

precipitation in the event of low operating temperatures with the consequent risk of clogging of lines, valves, exchangers;

the formation of foams in the presence of solid particles and / or in case of excessive operating temperature, with the consequent risk of solvent entrainment downstream and plant shutdowns;

an increase in operating costs for the reintegration of any losses;

the difficulty in disposing of the solvent in case of leakage and / or replacement.

In addition to this, plants with conventional technology are not very adaptable to the possible need to increase operational capacities due to limitations of the carbon dioxide and syngas compression section.

Summary of the invention

The inventors of the present patent application have therefore developed a process that allows the gravimetric separation and recovery of carbon dioxide from synthesis gas (syngas) in plants for the production of ammonia by exploiting the properties of carbon dioxide in the supercritical phase. This separation is advantageously carried out in a suitable device for gravimetric (or gravity) separation. The process, the separator device and a plant comprising this device allow to overcome the drawbacks of the processes, devices and plants known in the art.

Object of the invention

In a first object, the present invention describes a process for the separation and recovery of carbon dioxide from synthesis gas (syngas) in plants for the production of ammonia.

In a second object, a device for the separation of carbon dioxide from synthesis gas (syngas) in plants for the production of ammonia is described.

A further object of the present invention is a plant for the production of ammonia and urea which comprises the carbon dioxide separator device.

Brief description of the figures

Figure 1 represents a diagram of the process according to the known art;

figure 2 represents a scheme of the process according to an embodiment of the invention;

Figure 3 represents the process of the invention in more detail;

figure 4 represents a scheme of the process according to an alternative embodiment of the invention (revamping);

figure 5 represents an alternative embodiment of the invention;

figure 6 is a plan view with some parts transparent for clarity, of a separator device, according to an embodiment of the invention;

Figures 7 and 8 are schematic sectional views made according to a cutting plane passing through the longitudinal direction X-X indicated in Figure 6, of a separator device according to an embodiment;

figure 9 is a schematic sectional view of a detail of a separator device, according to an embodiment, in which the behavior of the output flows is schematically shown;

Figure 10 is a sectional view of a separator device, made according to the cutting plane indicated by the arrows V-V in Figure 8;

Figure 11 is a diagram of a plant, according to an embodiment, comprising some separator devices.

Detailed description of the invention

In the present description, unless otherwise indicated, the pressure values are intended in units of measurement "bar g" (even where only "bar" is indicated).

According to a first object, the present invention describes a process for the separation and recovery of carbon dioxide from synthesis gas (syngas) in plants for the synthesis of ammonia.

As shown in Figure 2, the process of the present invention for the separation of carbon dioxide included in a flow of synthesis gas (syngas) coming from the shift and reforming sections in plants for the production of ammonia (synthesis gas flow ( syngas) initial), includes the phases of:

1) carry out a gravimetric separation of said gases; And

2) separately recover a prevalent flow of carbon dioxide (C02) and a prevalent flow of synthesis gas purified from C02 (purified syngas),

characterized in that in the initial synthesis gas (syngas) flow at least carbon dioxide is in the supercritical phase.

The term "prevailing" flow refers to the fact that the two separate flows of CO2e of purified syngas may contain a share of CO2e purified syngas respectively greater than the relative share present in the initial synthesis gas mixture (syngas) (in practice, because 100% separation was not achieved).

According to the present invention, the two flows are successively used:

- the flow of carbon dioxide (CO2) in a process for the synthesis of urea;

- the flow of purified syngas in a process for the production of ammonia.

For the purposes of the present invention, a flow of synthesis gas coming from the shift and reforming sections (syngas) is a flow which preferably comprises one or more of the elements or compounds selected from the group which includes: hydrogen, carbon monoxide, methane, water and carbon dioxide.

In one aspect of the invention, one or more of the elements or components selected from the group including: nitrogen, argon and helium, and other non-condensable aggregates may also be present.

According to a preferred aspect of the present invention, before the gravimetric separation step, the synthesis gas (syngas) is subjected to a pre-treatment step obtaining a dry syngas.

The term "dry" means that the synthesis gas (syngas) obtained is substantially free of water or may contain water in minimal traces, so as not to compromise the subsequent stages.

According to a preferred aspect, this pre-treatment step comprises a dehydration step.

In another preferred aspect, before being used in a process for the synthesis of ammonia, the purified syngas flow is subjected to a further purification step for the elimination of carbon dioxide, and possibly of the carbon monoxide, residues.

According to a particular aspect of the invention, this purification comprises a methanation step.

As an alternative to methanation, a purification step can be carried out by cryogenic washing with liquid nitrogen, according to the methods more detailed in the rest of the description.

Also, before being sent to the ammonia synthesis process, it may be necessary to perform an expansion step.

In a preferred aspect, the present invention describes a process as shown in Figure 3.

In particular, this process for the separation of the gases included in a flow of synthesis gas (syngas) 301 coming from the shift and reforming sections in plants for the production of ammonia, includes the steps of:

1) carry out a gravimetric separation of said gases; and 2) separately recovering a prevalent stream of carbon dioxide 320 and a stream of purified synthesis gas (from C02) 310.

The gas flow (syngas) 301 coming from the shift and reforming sections is a flow which preferably comprises one or more of the elements or compounds selected from the group which includes: hydrogen, carbon monoxide, methane, water and carbon dioxide.

In one aspect of the invention, one or more of the elements or components selected from the group including: nitrogen, argon and helium, and other non-condensable aggregates may also be present.

In a preferred aspect, before undergoing gravimetric separation step 1), the flow 301 is subjected to a pre-treatment step so as to obtain a dry gas flow 302.

In a preferred aspect of the invention, this step is a dehydration step (DEHYD).

Optionally in the pre-treatment step, before the dehydration step a), the flow 301 can also be subjected to a compression step a ') (in Figure 4 indicated with LPC1).

Preferably, the compression is carried out up to a pressure of about 50-60 bar.

After compression (LPC1), flux 302 is treated in a heat exchanger (HEX1) and cooled to a temperature of about 20-50 ° C.

In a preferred aspect, the flow 302 obtained from the dehydration step is subjected to a step a '') of further compression (LPC2).

Before the gravimetric separation step 1), the flow 302 can optionally be cooled (HEX2); preferably, this flow 302 is brought to a temperature of 20-50 ° C.

For the purposes of the present invention, the pre-treatment steps, ie preceding the gravimetric separation step, have the purpose of bringing at least the carbon dioxide contained in the initial synthesis gas mixture (syngas) to the supercritical state.

With reference to Figure 3, in a preferred aspect of the invention, the phase 1) of gravimetric separation of the synthesis gas (syngas) can comprise a phase la) of gravimetric separation (LPS) carried out at intermediate pressure.

Preferably, this step la) is carried out at a pressure of about 90-400 bar and, preferably, of 100-200 bar.

From the first phase of gravimetric separation at intermediate pressure (LPS) two flows are obtained: a head flow 310 rich in synthesis gas purified from CO2 (purified syngas) and a bottom flow 320 rich in carbon dioxide (CO2).

According to an embodiment of the invention, the head stream 310 is subjected to a gravimetric separation step 1b), conducted at high pressure (HPS2).

This phase is preferably carried out at a pressure of about 90-400 bar and, preferably, of about 200-300 bar.

According to the present invention, two streams are obtained from step 1b): a top stream 330 rich in synthesis gas purified from C02 (purified syngas) and a bottom stream 340 rich in carbon dioxide (CO2).

In a preferred aspect of the present invention, between the two gravimetric separation steps LPS and HPS2, an HPC1 compression step of the head stream 310 can be carried out.

Preferably, this compression is operated up to a pressure of about 90-400 bar and, more preferably, of about 200-300 bar.

Before the gravimetric separation phase at high pressure (HPS2) and after the compression phase (HPC1), a cooling phase (HEX3) of the head stream 310 can be carried out, to bring it to the temperature between about -50 ° C and 50 ° ° C.

The overhead stream 330 obtained from the high pressure gravimetric separation step (HPS2), can therefore be used in a step 3) of preparing ammonia.

In a preferred aspect of the invention, before being used in a process for the synthesis of ammonia, the overhead stream 330 can be subjected to one or more, and preferably all, the steps of:

3a) heating (in the HEX5 exchanger),

3b) purification from carbon dioxide and carbon monoxide by HPM methanation,

3c) cooling (in the HEX5 exchanger).

In a particular aspect of the invention, the above steps 3a), 3b) and 3c) are preceded by an expansion step EX1, and necessary in view of the operating pressure of the separator HPS1.

In particular, the overhead stream 330 used in the ammonia synthesis process is preferably expanded so as to reach the working pressure of the synthesis reactor.

In a preferred aspect, this pressure is about 120-240 bar. As regards step 3a), on the other hand, the overhead stream 330 is preferably heated to a temperature of about 250-350 ° C.

Phase 3b), on the other hand, is preferably carried out at high pressure and, preferably, at the working pressure of the synthesis reactor, which, in a preferred aspect, is about 120-240 bar.

Preferably, from the methanation step 3b) a synthesis gas flow 390 with a carbon dioxide and carbon monoxide content lower than 10 ppm is obtained.

As described above, this flow 390 can then be subjected to a cooling phase (in the HEX5 exchanger).

As described above, a bottom flow 320 rich in carbon dioxide (C02) is also obtained from step la) of intermediate pressure gravimetric separation (LPS).

For the purposes of the process of the present invention, this stream 320 is subjected to a further gravimetric separation step HPS1.

In particular, this phase is carried out at high pressure and, preferably, at a pressure of about 90-400 bar and, more preferably, of about 200-300 bar.

From this HPS1 phase, a top flow 350 rich in synthesis gas purified from C02 (purified syngas) and a bottom flow 360 rich in carbon dioxide (CO2) are obtained.

In a preferred aspect of the present invention, between the two gravimetric separation steps LPS and HPS1, an HPC2 compression step of the bottom flow 320 can be carried out.

Preferably, this compression is operated up to a pressure of about 90-400 bar and, more preferably, of about 200-300 bar.

Before the HPS1 gravimetric separation phase and after the HPC2 compression phase, a cooling phase (HEX4) of the bottom flow 320 can be carried out to bring it to a temperature of about -50 ° C - 50 ° C.

In a particularly preferred aspect of the invention, the overhead stream 350 can be employed in an ammonia preparation step 3).

In an even more preferred aspect, the overhead stream 350 is employed for the preparation of ammonia together with the overhead stream 330 obtained from the high pressure gravimetric separation step 1b) (HPS2).

In particular, for this purpose, the head flow 350, together with the head flow 330, gives rise to a flow of synthesis gas purified from C02 (purified syngas) 370, which is subjected to steps 3a), 3b), 3c), possibly preceded by the expansion phase EX1, as described above.

As regards the C02 (C02) rich bottom flow 360 obtained from the high pressure gravimetric separation step (HPS1), in a preferred aspect of the present invention, this is employed in a urea preparation step 4).

As described above, a bottom flow 340 rich in carbon dioxide (CO2) is also obtained from the gravimetric separation at high pressure (HPS2).

In a particularly preferred aspect of the invention, the bottom flux 340 can be employed in step 4) of preparing the urea.

In this regard, the bottom flow 340 obtained from the high pressure gravimetric separation step HPS2, together with the bottom flow 360 obtained from the high pressure gravimetric separation step HPS1, give rise to a bottom flow 380 of carbon dioxide (C02) which it is used for the preparation of urea.

In a preferred embodiment, before being used for the preparation of urea, the bottom flow 360, possibly comprising the bottom flow 340 (thus originating the flow 380), is subjected to a step of:

4a) HEX6 heating.

More specifically, in step 4a) the carbon dioxide is heated to a temperature of about 90-150 ° C.

In a particular aspect of the invention, before step 4a) the carbon dioxide can be subjected to an expansion step (EX2).

Preferably, this expansion leads to a pressure of about 140-200 bar.

According to a particular embodiment of the process of the present invention, not shown in the figures, the recycling of one or both of the following flows can be carried out:

- the bottom flow 340 obtained from the gravimetric separation at high pressure HPS2 comprising mainly C02;

- the overhead flow 350 obtained from the gravimetric separation at high pressure HPS1 mainly comprising synthesis gas purified from C02 (purified syngas).

In particular, one or both of the streams 340,350 can be recycled to the intermediate pressure gravimetric separation step (LPS), possibly after having undergone one or more of the pre-treatment steps indicated above.

Advantageously, the process of the present invention can also be used for the so-called revamping of already existing "traditional" plants for the synthesis of ammonia, in which the CO2 separation is carried out by washing with a solvent.

A classic plant which includes the recovery of carbon dioxide from ammonia synthesis gases (syngas) can therefore be converted to exploit the technology made available by the present invention.

In this regard, as shown in figure 4, a traditional process for the synthesis of ammonia comprising (existing units):

a reforming and shift section, from which a flow of 300 synthesis gas (syngas) is obtained;

- a step of removing carbon dioxide by washing with suitable solvents obtaining a stream of carbon dioxide 600 and a stream of purified synthesis gas 400; - a phase of use of the carbon dioxide thus separated 600 in a process for the synthesis of urea, possibly after suitable compression;

- a further purification step of the synthesis gas 400, obtaining a further purified synthesis gas stream 500;

- a step of use of said further purified synthesis gas 500 in a process for the synthesis of ammonia, can be modified in such a way that at least a portion 300a of the synthesis gas (syngas) obtained from the reforming and shift section is subjected to a phase of removal of carbon dioxide by gravimetric separation, according to the process described above, ie exploiting the properties of carbon dioxide in the supercritical phase.

According to a preferred aspect, the synthesis gas (syngas) stream 300 subjected to separation comprises one or more of the elements or compounds selected from the group which includes: hydrogen, carbon monoxide, methane, water and carbon dioxide.

In an even more preferred aspect of the invention, one or more of the elements or components selected from the group comprising: nitrogen, argon and helium, and other non-condensable aggregates may also be present.

As regards, on the other hand, the step of further purification of the flow 400, this is preferably carried out by means of methanation, with which the carbon dioxide C02 and possibly also the carbon monoxide are removed.

Alternatively, a purification can be carried out by cryogenic washing with liquid nitrogen, according to the methods described below.

From the gravimetric separation step, a purified stream of carbon dioxide 900 and a stream of purified synthesis gas (syngas) 1000 are thus obtained.

As far as flow 900 is concerned, this is sent to the urea preparation process.

In a preferred aspect of the invention, this flow 900 can be sent to the urea synthesis process together with the flow 600 obtained from the recovery of carbon dioxide by washing with a solvent and subsequent compression.

As regards, instead, the purified synthesis gas flow 1000, this is sent to a process for the preparation of ammonia.

In a preferred aspect of the invention, before being sent to the ammonia preparation, the stream 1000 can be subjected to a high pressure purification step, giving a further purified synthesis gas stream 700.

According to preferred aspects of the present invention, one or more of the process steps according to the embodiment described above as revamping, can be carried out according to the methods reported below.

For example:

- the gravimetric removal step of the carbon dioxide can comprise one or more of the intermediate pressure gravimetric separation steps LPS or high pressure HPS1 and / or HPS2; - the dehydration step of the portion 300a of synthesis gas coming from the reforming and shift steps can be preceded by a LPC1 compression and possibly HEX1 cooling step;

- the dehydration step of the portion 300a of the synthesis gas coming from the reforming and shift steps can be followed by a LPC2 compression and possibly HEX2 cooling step;

- before this high pressure gravimetric separation step HPS1, an HPC2 compression step can be carried out, possibly followed by a HEX4 cooling step;

- before this high pressure gravimetric separation step HPS2 a compression step HPC1 can be carried out, possibly followed by a cooling step HEX3;

- before this purification step, an expansion step EX1 and possibly a heating step HEX5 can be carried out;

- after this purification step, a HEX5 cooling step can optionally be carried out;

- after this high pressure gravimetric separation step HPS1, an expansion step EX2 and a cooling step HEX6 can be carried out;

- operate one or more of the recycles described above.

According to an alternative embodiment of the present invention shown in Figure 5, in the process described above the purification step of the gas stream purified from C02 (purified syngas) can be carried out by cryogenic washing with liquid nitrogen.

In this case, the overhead stream 330 purified from C02 (puri fi f and syngas) passes through one or more molecular sieves (HMPS) for the removal of carbon dioxide.

In a subsequent phase the gas is subjected to washing with liquid nitrogen in the HP LIN section in order to remove carbon monoxide, methane and any other non-condensable aggregates.

In this way a mixture of hydrogen and nitrogen is obtained in the correct stoichiometric ratio and / or suitable for the preparation of ammonia in the synthesis reactor.

In accordance with a further object of the invention and in particular with reference to Figure 6, a separator device 10 of a mixture of fluids which can be used for the purposes of the process described above is described.

Indeed, in an even more preferred aspect of the invention, the process, and more in detail, the intermediate pressure (LPS) and high pressure (HPS1 and HPS2) gravimetric separation steps are carried out in a separator such as the one described below.

As indicated above, in the operating conditions of pressure and / or temperature of the gravimetric separation process, at least carbon dioxide and possibly one or more of the other fluids included in the synthesis gas mixture (syngas) is in the supercritical state.

Furthermore, it cannot be ruled out that under certain operating conditions of pressure and / or temperature, inside the separator one or more of the fluids of the mixture is in the liquid state.

In particular, the mixture object of the gravimetric separation inside the separator device 10 is the synthesis gas mixture (syngas) described above.

Therefore, such a mixture preferably comprises hydrogen, carbon monoxide, methane, water and carbon dioxide.

In a preferred aspect, nitrogen, argon and other non-condensable aggregates may also be present.

The separation of the components of the synthesis gas mixture (syngas) is a separation by gravity or gravimetric separation which therefore exploits the different density of the components of the mixture.

In particular, the difference in density between supercritical carbon dioxide and the other components of the mixture is exploited.

In particular, in the operating conditions the carbon dioxide density di is greater than the density dn of each of the other components of the synthesis gas mixture (syngas) (dl> dn).

According to the present invention, the separator device 10 comprises at least one casing 11.

A longitudinal direction X-X coinciding or parallel to the longitudinal development axis of said casing 11 is defined. Preferably, said longitudinal direction X-X is substantially parallel to the horizontal direction. In accordance with a preferred embodiment, said longitudinal direction X-X is substantially perpendicular to the direction of acceleration due to gravity "G". According to an embodiment, said longitudinal direction is sub-horizontal. According to an embodiment, said casing 11 is oriented substantially horizontally or sub-horizontally.

According to an embodiment, said casing 11 can be made in a single piece or in separate pieces and, for example, can comprise flanged heads.

In accordance with a preferred embodiment, said casing 11 is a pressure vessel, for example it is suitable for containing fluids at a pressure higher than the pressure external to the casing 11 itself. In accordance with a preferred embodiment, said casing is impermeable to fluids.

Said casing 11 comprises at least one inlet edge 12 which delimits at least one inlet opening 13, suitable for receiving an inlet flow 100 in said separator device 10.

For the purposes of the present invention, this inlet flow 100 is represented by the synthesis gas mixture (syngas) described above to be subjected to separation.

The casing 11 of the device at least partially delimits at least one separation chamber 18, suitable for being traversed by a flow in transit 105 having a speed of advancement Vx. According to an embodiment, said transit flow 105 passes through said separation chamber 18 from the inlet opening to the outlet opening.

In the present description, the path of the transit flow 105 therefore defines an "upstream" portion, ie closer to the inlet opening, and a "downstream" portion, ie closer to the outlet openings of the separator device 10.

Said casing 11 comprises at least a first surface 19 and at least a second surface 20, facing said separation chamber 18, in which said second surface 20 is located below with respect to said first surface 19.

Therefore, the carbon dioxide having said density dn goes towards said first surface 20, while the other components of the synthesis gas mixture (syngas) each having said density dn go towards said second surface 19.

According to an embodiment, said first surface 19 comprises a first line 119 and said second surface 20 comprises a second line 120, in which the second line 120 is placed below the first line 119. In this way, when the device separator 10 is thus oriented, the fluid with density dn lower than di will tend to accumulate towards line 119 while carbon dioxide will tend to accumulate towards line 120.

In accordance with an embodiment, a useful distance "h" is defined equal to the distance between said first line 119 and said second line 120. When the shape of the casing 11 of the separator device 10 is cylindrical, and said separator device 10 is oriented substantially horizontal, said distance h is equal to the internal diameter of the casing 11 of the separator device 10.

Said first surface 19 comprises at least a first trailing edge 14 which defines at least a first exit opening 15, suitable for supplying a first outflow 103 from the separator device 10. Said second surface 20 comprises at least a second trailing edge 16 which delimits at least a second outlet opening 17, suitable for supplying a second outlet flow 104 from the separator device 10.

For the purposes of the present invention, said first outlet stream 103 is represented by the synthesis gas purified from CO2 (purified syngas), while said second outlet stream 104 is represented by carbon dioxide (CO2).

Said separation chamber 18 comprises at least a first outlet portion 23 that opens into said first outlet opening 15 and at least a second outlet portion 24 that opens into said second outlet opening 17.

Advantageously, said separator device 10 comprises fluidic separation elements 30, 31, 32, suitable for influencing the transit flow 105. Said fluidic separation elements 30, 31, 32 at least partially delimit channels 22 for the passage of the fluid in transit 105 in said separation chamber 18. Said fluidic separation elements 30, 31, 32 are impermeable to fluids. Preferably, said fluidic separation elements 30, 31, 32 are made of metallic material.

With a further advantage, said fluidic separation elements 30, 31, 32 are suitable for influencing the flow in transit 105 so that at least a portion of the flow in transit 105 in said channels 22 is a substantially laminar flow. With the terminology "substantially laminar" is meant to indicate the reduced presence, and preferably the absence, of turbulent phenomena in the flow in transit 105 inside said separation chamber 18. Preferably, the flow in transit 105 inside said channels 22 is substantially laminar and more preferably it is laminar.

In this way, carbon dioxide having a density of is directed towards said second outlet opening 17 determining said second outlet flow 104 which comprises a portion of the rest of the mixture which is less than the portion included in said inlet flow 100. Al at the same time, the rest of the mixture of synthesis gas purified from C02 (purified syngas) goes towards said first outlet opening 15 determining said first outlet flow 103 which comprises a portion of carbon dioxide less than the portion included in said flow inbound 100.

Advantageously, the separator device 10 is suitable to be used for any portion of carbon dioxide included in the synthesis gas mixture (syngas) of the inlet stream 100.

According to an embodiment, said fluidic separation elements 30, 31, 32 have a vertical or sub-vertical orientation. Preferably, said fluidic separation elements 30, 31, 32 are vertically oriented. Preferably, said fluidic separation elements 30, 31, 32 are substantially parallel to each other.

In accordance with a preferred embodiment, said fluidic separation elements 30, 31, 32 are unsuitable to perform a structural function. In other words, said fluidic separation elements 30, 31, 32 are unsuitable to act as structural reinforcement for said casing 11. Therefore, in one aspect of the invention, said elements 30, 31, 32 have no structural function. According to an alternative aspect of the invention, however, said one or more fluidic separation elements 30, 31, 32 have a structural function.

Said fluidic separation elements 30, 31, 32 at least partially delimit channels 22, whereby a channel opening y-y is defined as the minimum distance between two fluidic separation elements 30, 31, 32. A substantially vertical dimension of channel z-z, oriented transversely to said channel opening y-y. According to an embodiment, said substantially vertical dimension of channel z-z is directed vertically and is parallel to the direction of the acceleration of gravity "G". Preferably, said substantially vertical dimension z-z of channel is greater than said opening of channel y-y.

By reducing the opening of channel y-y of said channels 22, it is possible to maximize the speed of the fluid, while remaining in the laminar field. One skilled in the art will appreciate that in a substantially laminar regime turbulent phenomena of the fluid in transit 105 are avoided, increasing the separation efficiency. This improves the uniformity of the distribution of the forward speed Vx of the passing flow 105. The laminar flow regime along the X-X axis of the separator device 10 allows to minimize the interference of the speed Vx of the flow 105 with the separation speed. A person skilled in the art will appreciate that, otherwise, the onset of turbulent phenomena would cause mixing caused by the presence of components of the flow velocity 105 parallel to the separation speed (i.e. orthogonal to the longitudinal direction X-X) with consequent reduction of the separation efficiency for the same of residence times in the separator device 10.

In accordance with a preferred embodiment, said fluidic separation elements 30, 31, 32 are parallel to each other and equally spaced. In this way, said y-y channel opening is substantially constant.

According to an embodiment, said channels 22 are in fluid communication with said first outlet portion 23 and said second outlet portion 24.

According to an embodiment, said fluidic separation elements 30, 31, 32 extend in the longitudinal direction X-X along the entire separation chamber 18. In this way, said channels 22 extend in the longitudinal direction X-X along the entire separation chamber 18. According to an embodiment, said fluidic separation elements 30, 31, 32 extend between said first surface 19 and said second surface 20.

In accordance with a preferred embodiment, said separation chamber 18 comprises a proximal portion of separation chamber 39 and a distal portion of separation chamber 38, in which said distal portion of separation chamber 38 is located downstream of said portion separation chamber 39, and wherein said distal portion of separation chamber 38 comprises a greater number of fluidic separation elements 30, 31, 32 than said proximal portion of separation chamber 39. In accordance with an embodiment , said distal portion of separation chamber 38 comprises approximately twice the number of fluidic separation elements 30, 31, 32 with respect to said proximal portion of separation chamber 39. According to one embodiment, said distal portion of separation chamber separation 38 comprises double or double number minus one of fluidic separation elements 30, 31, 32 with respect to said proximate portion the separation chamber le 39. The increase in the number of fluidic separation elements 30, 31, 32 near the outlets of the separator device 10 allows to minimize the propagation of any turbulence induced by the outlet openings 15 and 17, where the first outlet stream 103 and second outlet stream 104 must substantially accelerate and change direction.

In accordance with a preferred embodiment, said channels 22 thicken near the outlet portions 23 and 24. In other words, said channels 22 increase in number, thus reducing the distance between two fluidic separation elements 30, 31, 32 adjacent. In this way, said channels 22 increase in number resulting in a decreased opening of channel y-y where a greater source of turbulence is expected, induced by the acceleration of the fluid leaving the separator device 10, a fluid which must change its speed both in the direction both in intensity. The turbulence must be reduced to avoid mixing between the first outlet stream 103 and the second outlet stream 104.

According to an embodiment, said channels 22 are delimited by said fluidic separation elements 30, 31, 32, by at least a portion of said first surface 19 and by at least a portion of said second surface 20. In other words, the walls of each channel 22 comprise portions of said fluidic separation elements 30, 31, 32, at least a portion of said first surface 19 and at least a portion of said second surface 20. In this way the separation efficiency is increased.

According to an embodiment, said fluidic separation elements 30, 31, 32 have a prevalent extension along the longitudinal direction X-X and along the direction of acceleration due to gravity "G". In this way the separation efficiency is increased.

According to an embodiment, said fluidic separation elements 30, 31, 32 comprise at least one bundle of septa 31, 32. According to an embodiment, said septa 31, 32 are in the form of lamellae having a prevalent long extension the longitudinal direction X-X and along the direction of acceleration due to gravity "G". According to an embodiment, said septa 31, 32 are substantially parallel to each other. According to an embodiment, said at least one bundle of baffles 31, 32 comprises first baffles 31 and second baffles 32, in which said first baffles 31 extend along the direction of the acceleration of gravity "G" greater than said second ones septa 32. In accordance with an embodiment, said at least one bundle of septa 31, 32 comprises first septa 31 and second septa 32, in which said first septa 31 have a greater extension along the longitudinal direction X-X than said second septa 32. In accordance with an embodiment, said at least one bundle of baffles 31, 32 comprises said first baffles 31 arranged alternately with respect to said second baffles 32.

According to an embodiment, said fluidic separation elements 30, 31, 32 are made in a single piece with said casing 11. According to an embodiment, said fluidic separation elements 30, 31, 32 are made in the form of at least one insert which is inserted into said casing 11. According to an embodiment, said casing 11 is fitted onto said fluidic separation elements 30, 31, 32.

According to an embodiment, at least one of said first outlet portion 23 and said second outlet portion 24 are devoid of fluidic separation elements 30, 31, 32. In this way, fluidic communication between said channels is favored. 22 and said first and second outlet openings 15, 17. In a preferred embodiment, the diameter of said outlet openings 15, 17 can be smaller than the diameter of the separator device 10. In accordance with this embodiment, at least some of said fluidic separation elements 30, 31, 32 in correspondence with the outlet portions 23, 24 and externally to these portions 23 and 24 are modeled and possibly partially eliminated in order to allow the flow of outgoing flows 103 and 104 from the periphery of the separator towards the outlet openings 15, 17. According to an embodiment, between said first outlet portion 23 and said second outlet portion 24 there is at least a portion of said el fluidic separation elements 30, 31, 32.

In this way, it is possible to guide the flow towards the outlet openings 15, 17 avoiding turbulence.

According to an embodiment, at least one of said first outlet flow 103 and said second outlet flow 104 has an outlet pressure substantially equal to the pressure of the inlet flow 100. The pressure drops are in fact to be considered lower than 3 % and more preferably less than 1%. According to an embodiment, at least one of said first outlet flow 103 and said second outlet flow 104 has an outlet pressure substantially comprised between about 90% and about 99.9% of the pressure of the inlet flow 100 , preferably between about 95% and about 99.9%, more preferably between about 97% and about 99.9%, and even more preferably between about 99% and about 99.9%.

The separation efficiency is influenced by various factors. For example, other conditions being equal (for example temperature, pressure and residence time of the fluid in transit 105 inside the separation chamber 18) the higher the feed speed Vx and the lower the separation efficiency, as the onset of turbulent phenomena is favored. For example, all other conditions being equal, the efficiency increases asymptotically with the increase in the residence time of the transit flow 105 in the separator 10. For example, all other conditions being equal, the separation efficiency increases with increasing of the difference between the densities of and dn. For example, all other conditions being equal, the separation efficiency increases with increasing pressure as an increase in pressure causes an increase in the density difference between carbon dioxide and the other components of the gas mixture purified by C02. (purified syngas) a decrease in the diffusivity coefficients. For example, the separation efficiency increases with decreasing temperature, as this causes an increase in the density difference between carbon dioxide and the other components of the gas mixture purified by C02 (purified syngas), as well as a decrease in the diffusivity coefficients. In particular, for temperatures below the critical temperature of carbon dioxide, for example 31.1 ° C for pure CO2, a liquid phase is likely to arise near the outlet opening 17, i.e. in correspondence with the section where a composition is obtained such as to allow the liquefaction of the C02. This liquefaction can be favored by a suitable thermal device 36 which cools the second outlet stream 104.

For example, the smaller the opening of the y-y channels in a direction transverse to the direction of acceleration due to gravity "G", the greater the separation efficiency, as the flow in laminar regime is favored at the same speed of advancement Vx , pressure, temperature and composition of the inlet mixture 100.

According to an embodiment, said first trailing edge 14 defines a first exit direction Ul-Ul substantially orthogonal to the section of said first exit opening 15 and said second trailing edge 16 defines a second direction d outlet U2-U2 substantially orthogonal to the section of said second outlet opening 17. At least a portion of said first outlet flow 103 is substantially directed according to said first outlet direction Ul-Ul and at least a portion of said second flow in output 104 is directed substantially parallel to said second output direction U2-U2.

According to an embodiment, said first exit direction U1-U1 is substantially parallel to said second exit direction U2-U2. According to an embodiment, it is possible to realize outlets with angles lower than 90 ° with respect to the longitudinal direction X-X, i.e. the angle between the longitudinal direction X-X and each of the exit directions Ul-Ul and U2 -U2 can be less than 90 °.

Such angles can be chosen in such a way as to minimize the variation in momentum of the output flows 103 and 104.

In accordance with a preferred embodiment, said first exit direction Ul-Ul is substantially parallel and not coincident with said second exit direction U2-U2, so that an exit distance "D" is defined between said first exit direction Ul-Ul or an extension thereof and said second exit direction U2-U2. The provision of said outlet distance "D" allows to minimize the fluidic interference between said first outlet flow 103 and said second outlet flow 104, which could favor the onset of turbulent phenomena (decreasing the efficiency of separation).

In order to limit the onset of turbulence in the outlet portions 23, 24, it is envisaged to gradually reduce the walls of each outlet duct 37 so that they assume a frusto-conical shape, by means of filling elements 35 such as filling material. In this way the outgoing fluid will gradually accelerate.

In accordance with an embodiment, said casing 11 comprises at least one leading edge 21, which extends downstream of said leading edge 12, and delimits an entry 28 opening wider than said opening inlet 13. In other words, said casing 11 comprises an inlet portion 29 located downstream of said inlet edge 12. In this way, the inlet flow 100 is subjected to a section variation which slows it down.

According to an embodiment, said separator device 10 comprises at least one static device 33, suitable for uniformly distributing the forward speed Vx of the passing flow 105. Preferably, said static device 33 is suitable for distributing uniformly on at least a portion of a fluidic section orthogonal to the longitudinal direction X-X the speed of advancement Vx of the flow in transit 105. In this way, it is possible to uniformly distribute the speed of advancement of the flow in transit in the face of a minimum loss of pressure or cargo. This favors the formation of a substantially laminar flow in the channels 22 and allows to increase the separation efficiency. In fact, the pressure drops of the separator device 10 are almost totally concentrated in the static device 33 whose operation requires very low pressure drops.

According to an embodiment, said static device 33 comprises at least one porous septum. According to an embodiment, said static device 33 comprises at least one perforated or grilled plate.

According to an embodiment, said casing comprises filling elements 35 which influence the shape of the separation chamber 18. In this way, it is possible to obtain an optimized transit flow 105. According to an embodiment, said filling elements 35 comprise at least one between said first surface 19 and said second surface 20.

According to an embodiment, said separation device 10 comprises at least one trap member (not shown), suitable for recovering components in the liquid phase.

Said casing 11 can be variously shaped. In accordance with an embodiment, said casing 11 has a substantially rectilinear longitudinal axis. According to an embodiment, said casing 11 has a curved longitudinal development axis. In this way it is possible to maintain compact dimensions, in the face of an increase in the longitudinal extension of the separation chamber 18. Or, with the same longitudinal extension of the separation chamber 18, the overall dimensions of the separator device 10 are reduced. According to an embodiment, said casing 11 has a substantially cylindrical shape. According to an embodiment, said casing 11 has a prismatic shape.

In accordance with an advantageous embodiment, said longitudinal direction X-X is substantially horizontal and said fluidic separation elements 30, 31, 32 are oriented vertically.

According to an embodiment, said casing 11 comprises at least one inlet duct 25 which extends upstream of said inlet edge 12.

According to an embodiment, said casing 11 comprises at least a first outlet duct 27 which extends downstream of said second outlet edge 16. Preferably, said first outlet duct 27 is crossed by said second flow 104 output represented by carbon dioxide.

In accordance with an embodiment, said casing 11 comprises at least a second outlet duct 26 which extends downstream of said first outlet edge 14. Preferably, said second outlet duct 26 is crossed by said first flow in output 103 represented by the mixture of synthesis gas purified from carbon dioxide.

In accordance with an embodiment, at least one of said second outlet duct 26 and said first outlet duct 27 comprises outlet duct walls 37 having a substantially funnel-like shape. In this way, they narrow the lumen of the outlet duct 26, 27 gradually increasing the respective speeds and thus limiting the onset of turbulence. According to an embodiment, at least one of said first inlet duct 25, said second outlet duct 26 and said second outlet duct 27 comprises at least one flanged element 34, suitable for connecting with a fluidic element, such as for example a pipe 41, 42, 43, 44.

In accordance with an embodiment, said separator device 10 comprises a thermal device 36, suitable for influencing the temperature of at least one of said passing flow 105, said inlet flow 100, said first outflow 103 and said second inlet flow outlet 104. In accordance with a preferred embodiment, said thermal device 36 is suitable for cooling said second outlet flow 104. In accordance with one embodiment, said thermal device 36 comprises a jacket that embraces at least a portion of said casing 11. Said thermal device 36 can be located both inside the separation chamber 18 and outside the separation chamber 18.

Thanks to the provision of such a separator device 10 it is possible to obtain a second outlet flow 104 comprising carbon dioxide (C02) in the supercritical phase or in the liquid phase when said separator device 10 is fed at the inlet with a mixture comprising, precisely, carbon dioxide . As described above, for the purposes of the present invention, one or more of the gravimetric separation steps LPS, HPS1 and HPS2 described above are carried out in the separator device 10 of the invention.

In a preferred aspect, all three phases LPS, HPS1 and HPS2 are conducted in the separator device 10.

More in detail, each of said gravimetric separation steps comprises the steps of:

A) injecting the synthesis gas mixture (syngas) into the separator device 10;

B) creating a flow of said mixture of synthesis gas (syngas) 100 inside said device 10;

C) collecting two separate flows, of which a first flow 103 mainly comprising a mixture of synthesis gas purified from C02 (purified syngas) and a second flow 104 comprising C02.

According to a preferred aspect, the flow of the synthesis gas mixture (syngas) injected in phase A) comprises carbon dioxide in the supercritical phase.

Optionally, one or more other components of said synthesis gas mixture (syngas) can also be in the supercritical phase.

According to another preferred aspect, the flow of said mixture of synthesis gas (syngas) carried out in phase B) is a laminar flow and, even more preferably, free from turbulence.

In accordance with another object of the present invention, a plant 40 is described (Figure 11) comprising at least one separator device 10, 10 ', 10' ', 10' '' according to any of the previously described embodiments.

According to an embodiment, said plant 40 is a plant for purification and separation of CO2 from synthesis gas (syngas).

In accordance with a preferred embodiment, said plant 40 is a synthesis gas (syngas) purification and supercritical carbon dioxide separation plant. In other words, said plant 40 purifies synthesis gas (syngas) and at the same time separates supercritical or liquid carbon dioxide.

According to an embodiment, said plant comprises a plurality of separator devices 10 ', 10' ', 10' '' with each other in fluid connection thanks to at least one connecting pipe 41. According to an embodiment, said plant 40 comprises at least one first stage separator 10 ', or first separator device 10', and at least one second stage separator 10 '', 10 '' '. According to an embodiment, said at least one second stage separator 10 '', 10 '' 'comprises at least a first second stage separator 10' ', or second separator device 10' ', and at least a second second stage separator 10 '' 'stage, or 10' 'third separator device.

In accordance with an embodiment, said plant 40 comprises at least one inlet pipe 42, suitable for receiving a flow 200 entering the plant, in fluid connection with said first inlet opening 13 of at least one separator device 10, at least one first outlet pipe 43, suitable for supplying a first outgoing flow 201 from the plant and in fluid connection with at least a first outlet opening 15 of said separator device 10, and a second outlet pipe 44, suitable for supplying a second outgoing flow 202 from the plant and in fluid connection with at least a second outlet opening 17 of said separator device 10.

According to an embodiment, at least two separator devices 10 ', 10 ", 10'" are connected together in series. The provision of two or more separator devices 10 ', 10 ", 10'" in series allows to realize two or more separation stages. In other words, it allows to realize a multistage separation.

Advantageously, in such a plant the pressure drops between the inlet pipe 42 and each of the outlet pipes 43, 44 are much lower, if not negligible, with respect to known solutions.

By "connection in series" it is meant that the first outflow 103 from a first outlet 15 of a first separator device 10 ', preferably represented by a flow of synthesis gas (syngas) purified from CO2, is sent to a second separator device 10 '', in principle the same as the first separator device 10 'but not necessarily of the same size. Preferably, this outlet flow 103 from a first outlet opening 15 of a first separator device 10 'represents the inlet flow 100' 'of said second separator device 10' '. In the case of three or more separator devices in series, the flow of synthesis gas purified from C02 (purified syngas) leaving a separator device therefore represents the inlet flow of the subsequent separator device. In this way, the degree of purification of the synthesis gas (syngas) from CO2 is improved. In fact, the purified synthesis gas flow (syngas) leaving the first separator device 10 'is injected into the second separator device 10' 'which supplies a further purified C02 purified syngas synthesis gas flow.

Alternatively, by "connection in series" it is meant that the second outlet flow 104 'from a second outlet opening 17 of a first separator device 10', preferably represented by a flow of CO2 separated from the synthesis gas, is sent to a second stage separator 10 '' ', in principle the same as the first separator device 10' but not necessarily of the same size. Preferably, this second outflow 104 'from a second outlet 17 of a separator device 10' represents the inlet flow into the subsequent separator device. In the case of three or more 10 ', 10' ', 10' '' separator devices in series, the separated C02 flow leaving a first stage 10 'separator therefore represents the inlet flow 100' ', 100' '' of the at least one separator of the second stage 10 ", 10" '.

So:

- the degree of purification of the purified syngas is improved; in fact, the purified synthesis gas flow (syngas) leaving the first separator device 10 ', or separator of the first stage 10', is injected into the first separator of the second stage 10 "which provides a flow of purified syngas ) further purified, to be used in the ammonia preparation process;

- the degree of CO2 purification to be used in the urea preparation process is improved.

In accordance with an embodiment, when at least two separator devices are connected in series, it may be advantageous to recycle the output flow recovered from the second separator device 10 '', placed in series with the first, into the input of the first separator device 10 '. This configuration is illustrated, for example, in figure 11, in which the combination of non-return valves 46 with the control valves 45 allows to manage three separator devices 10 ', 10' ', 10' '', passing from a configuration in parallel to a series configuration with the possibility of recycling the flows exiting the separators of the second stage 10 '', 10 '' ', which are re-injected at the inlet of the separator of the first stage 10'. Preferably, the outgoing flows from the second stage separators that are recycled are the outgoing flows which still require further purification, and in particular the second outgoing flow 104 '' from the first separator of the second stage 10 '' and the first flow 103 '' 'from the second separator of the second stage 10' ''.

Any flows recycled will have a pressure slightly lower than the mixture inlet 100 of the separator 10 'of the first stage and therefore a compressor and / or booster (not shown in the figures) can be provided to recover the pressure drops and a system for regulating the temperature to cool the booster delivery.

The possibility of switching from series to parallel configuration increases the flexibility of management of the synthesis gas (syngas) purification and C02 recovery plant.

According to an embodiment, at least two separator devices 10 are connected to each other in parallel.

By "parallel connection" it is meant that the same inlet pipe is in connection with the inlet openings 13 of at least two separator devices 10. The provision of at least two separator devices 10 connected to each other in parallel allows the volume of fluid to be increased separated for the same time, with respect to the provision of a single separator device 10.

According to an embodiment, said plant 40 comprises at least one feedback pipe 47, suitable for returning at least one outflow 103, 104 to the inlet of at least one separator device 10 of the previous separation stage.

In this way, the overall separation efficiency increases. In accordance with a preferred embodiment, said plant 40 comprises a feedback pipe 47 which carries at least a portion of the second outflow 104 '' from the first separator of the second stage 10 '' to the inlet of the first stage 10 ', and in which said plant 40 further comprises a feedback pipe 47 which carries at least a portion of the first outlet flow 103 '' 'from the second separator of the second stage 10' '' to the inlet of the first stage separator 10 '.

In this way, the overall separation efficiency increases. According to an embodiment, at least one of said connecting pipe 41, said inlet pipe 42, said first outlet pipe 43, said second outlet pipe 44 and said feedback pipe 47 comprises a non-return valve 46 .

According to an embodiment, at least one of said connecting pipe 41, said inlet pipe 42, said first outlet pipe 43, said second outlet pipe 44 and said feedback pipe 47 comprises a control valve 45, suitable to regulate the fluid flow that flows in the pipe 41, 42, 43, 44, 47.

Note the high flexibility of the separator 10 when the composition of the initial mixture varies, especially with reference to the concentration of carbon dioxide, as well as the possibility of easily changing the configuration of the separators from parallel to series without system modifications but with simple connections of pipes provided of control valves 45 and non-return valves 46 (see figure 11). Other known technologies do not allow this flexibility.

Furthermore, the technologies already known require additional compressors and a lot of energy to recompress the CO2 so that it can be reused since the CO2 recovered by these technologies is always at a much lower pressure than the pressure of the inlet flow.

From the above description, the advantages offered by the present invention will be evident for the person skilled in the art.

In this regard, the process described for the purification of the synthesis gas (syngas) from carbon dioxide allows a considerable saving in terms of energy, thanks to the fact that the high energy cost for the regeneration of the solvent is avoided.

Not to be underestimated is also the fact that the described gravimetric separation leads to the recovery of carbon dioxide in conditions that allow its ready exploitation for the synthesis of urea; in fact it is not necessary to subject carbon dioxide at low pressure, for example recovered from a separation with suitable solvents, to a compression phase up to about 140-200 bar.

On the other hand, the purified synthesis gas can also be used directly for the production of ammonia by exploiting the pressure at the outlet of the purification step, without the need for a subsequent compression step.

The set of aspects described above, in particular, leads to a decrease in operating costs (OPEX).

The important advantage linked to the great flexibility of the process according to the present invention will also be recognized, which allows to operate without the limitations imposed by the sizing of some apparatuses and machinery such as, for example, turbo-compressors for carbon dioxide and synthesis gas, when it becomes necessary to increase the operating capacity of the plant.

Finally, the overcoming of the problems related to the use of solvent (precipitation, degradation, formation of foams) will be recognized, as well as the costs associated with the possible reintegration and disposal of the solvent itself.

As regards more specifically, however, the separator device described, this allows to:

- decrease the energy consumption of the process without reducing the separation efficiency;

- to obtain satisfactory separation efficiencies with a compact footprint;

- to reduce management costs and the frequency of maintenance interventions;

- it is possible to obtain satisfactory separation efficiencies in the face of reduced residence times of the gaseous mixture and / or fluid;

- a versatile and modular separator device is supplied, which can be connected by piping in various configurations of a separation plant.

To the embodiments described above, a person skilled in the art, in order to satisfy contingent and specific needs, can make numerous modifications, adaptations and replace elements with other functionally equivalent ones, without however departing from the scope of the following claims.

LIST OF REFERENCES

10 Separator device, or separator

11 Casing

12 Entrance edge

13 Entrance opening

14 First trailing edge

15 First exit opening

16 Second trailing edge

17 Second exit opening

18 Separation chamber

19 First surface

20 Second surface

21 Edge of entrance portion

22 Channels

23 First portion of exit

24 Second portion of exit

25 Inlet duct

26 First exit duct

27 Second exit duct

28 Opening of the entrance portion

29 Portion of entrance

30 Elements of fluidic separation

31 First septa

32 Seconds Septa

33 Static device

34 Flanged element

35 Filling elements

36 Thermal device

37 Outlet duct walls

38 Distal portion of separation chamber

39 Proximal portion of the separation chamber

40 Implant

41 Connecting piping

42 Inlet piping

43 First outlet pipe

44 Second outlet pipe

45 Control valve

46 Non-return valve

47 Feedback piping

100 Flow in

100 '' Inlet flow in the first separator of the second stage 100 '' 'Inlet flow in the second separator of the second stage 101 First component

102 Second component

103 First stream out

103 '' First flow out of the first separator of the second stage 103 '' 'First flow out of the second separator of the second stage 104 Second flow out

104 '' Second flow out of the first separator of the second stage 104 '' 'Second flow out of the second separator of the second stage 105 Flow in transit

119 First line

120 Second line

200 Flow entering the plant

201 First flow leaving the plant

202 Second flow outgoing from the plant

10 'First stage separator

10 '' First separator of the second stage

10 '' 'Second separator of the second stage

x-x Longitudinal direction

Vx Speed of advancement

D Exit distance

h Useful distance

G Direction of acceleration due to gravity Ul-Ul First exit direction

U2-U2 Second exit direction

Claims (40)

CLAIMS 1. Process for the separation of carbon dioxide from a synthesis gas mixture coming from the shift and reforming sections in plants for the production of ammonia, comprising the steps of: 1) carry out a gravimetric separation of the gases of said mixture; 2) separately recovering a stream of carbon dioxide and a stream of synthesis gas purified from carbon dioxide, characterized in that at least the carbon dioxide contained in said mixture of synthesis gas subjected to gravimetric separation is in the supercritical phase. Process according to the preceding claim, wherein said synthesis gas mixture comprises one or more of the elements and compounds selected from the group which includes: hydrogen, carbon monoxide, methane, water and carbon dioxide. Process according to any one of the preceding claims, wherein said synthesis gas mixture further comprises one or more of the elements and compounds selected from the group which includes: nitrogen, argon, helium and other incondensable aggregates. 4. Process according to any one of the preceding claims, in which a pre-treatment step of said synthesis gas mixture is carried out before step 1), obtaining a dry flow. 5. Process according to claim 4, wherein said pre-treatment step is a dehydration step. Process according to claim 4 or 5, in which the pre-treatment step is preceded by a compression step, possibly followed by a cooling step. 7. Process according to the preceding claim, in which said compression step is carried out up to a pressure of about 50-60 bar. Process according to any one of claims 4 to 7, in which the pre-treatment step is followed by a further compression step, possibly followed by a further cooling step. 9. Process according to claim 6 or 8, in which said cooling and further cooling steps are carried out, independently of each other, up to a temperature of about 20 ° C-50 ° C. 10. Process according to any one of the preceding claims, wherein said gravimetric separation step 1) comprises an intermediate pressure gravimetric separation step la) carried out at a pressure of about 90-400 bar and preferably of about 100-200 bar, obtaining a overhead stream predominantly of synthesis gas purified from carbon dioxide (310) and a bottom stream (320) predominantly of carbon dioxide. 11. Process according to the preceding claim, wherein said head flow (310) is subjected to a gravimetric separation step 1b) HPS2 carried out at a pressure of about 90-400 bar and preferably of about 200-300 bar, obtaining a flow of head (330) mainly of synthesis gas purified from carbon dioxide and a bottom flow (340) mainly of carbon dioxide. Process according to any one of claims 10 or 11, in which after step la) and before step 1b) a compression step (HPC1) of the head flow (310) is carried out at a pressure of about 90-400 bar and preferably of about 200-300 bar. 13. Process according to any one of claims 10 to 12, wherein after step la) and before step 1b), and preferably after the compression step (HPC1) of the overhead stream (310), a step of cooling (HEX3) at a temperature of about -50 ° C - 50 ° C. Process according to any one of the preceding claims 10 to 13, wherein the overhead stream (330) obtained from the step (HPS2), is used in a step 3) of preparing ammonia. Process according to the preceding claim, wherein said overhead stream (330), before being used in an ammonia preparation step 3) is subjected to one or more of the steps of: 3a) heating, 3b) purification from carbon dioxide and carbon monoxide, 3c) cooling. 16. Process according to claim 15 above in which the head stream 330 before being subjected to one or more of the phases 3a), 3b), 3c) is subjected to an expansion phase (EX1) at a pressure of about 120-240 bar. Process according to claim 15 or 16, wherein in step 3a) the overhead stream (330) employed is heated to a temperature of about 250-350 ° C. 18. Process according to any one of claims 15 to 17, in which step 3b) is carried out at a pressure of about 120-240 bar. 19. Process according to any one of claims 15 to 18, wherein from step 3b) a stream of purified synthesis gas with a carbon dioxide and carbon monoxide content lower than about 10 ppm is obtained. Process according to any one of the preceding claims 10 to 19, wherein the bottom flow (320) is subjected to a high pressure gravimetric separation step conducted at a pressure of about 90-400 bar and preferably about 200-300 bar, obtaining a top flow (350) mainly of synthesis gas purified from carbon dioxide and a bottom flow (360) mainly of carbon dioxide. 21. Process according to the preceding claim, wherein said overhead flow (350), possibly together with the overhead flow (330) thus originating a flow (370), is subjected to step 3) of preparing ammonia. Process according to the preceding claim, wherein also said head flow 350, together with the head flow 330 thus originating a flow 370, is subjected to the process according to any one of claims 15 to 19. 23. Process according to any one of the preceding claims 10 to 22, wherein the bottom flow (360) is used in a step 4) of preparing the urea. Process according to any one of claims 10 to 23, wherein prior to the high pressure gravimetric separation step (HPS1), the bottom stream (320) is subjected to a compression step (HPC2) at a pressure of about 90 -400 bar and preferably about 200-300 bar. 25. Process according to any one of claims 10 to 24, in which after the compression step (HPC2) and before the separation step (HPS1), a cooling step (HEX4) is carried out at a temperature of about -50 ° C - 50 ° C. Process according to any one of claims 10 to 25, wherein said bottom flow (360) before being used in urea preparation step 4) is subjected to a heating step 4a) (HEX6). 27. Process according to the preceding claim, wherein said flow (360) is heated to a temperature of about 90-150 ° C. 28. Process according to claim 26 or 27, wherein before said step 4a) the flow (360) is subjected to an expansion step (EX2), preferably, at a pressure of about 140-200 bar. 29. Process according to any one of the preceding claims 11 to 28, wherein said bottom flow (340), possibly together with said bottom flow (360) originating a flow (380), is used in step 4) of preparing the urea. Process according to the preceding claim, wherein said bottom flow (360), together with the bottom flow (340) originating the flow (380) is subjected to the process according to any one of claims 26 to 28. 31. Process according to any one of claims 15 to 30, wherein said purification step 3b) is carried out by high pressure methanation. 32. Process according to any one of claims 15 to 31, in which said purification step 3b) is carried out by cryogenic washing with liquid nitrogen. 33. Process according to the preceding claim, in which said purification step by cryogenic washing is preceded by a step of passage through one or more molecular sieves (HMPS) for the removal of carbon dioxide. 34. Process according to any one of the preceding claims, in which the recycling of one or both of the following flows is carried out: - the bottom flow (340) obtained from the gravimetric separation at high pressure (HPS2) mainly comprising CO2; - the overhead flow (350) obtained by gravimetric separation at high pressure (HPS1) mainly comprising synthesis gas purified from C02 (purified syngas), wherein said recycling of one or both flows (340,350) is carried out at the intermediate pressure gravimetric separation step (LPS). Process according to the preceding claim, wherein said one or more recycling is preceded by one or more of the pre-treatment steps according to any one of claims 4 to 9. 36. A process for preparing ammonia comprising the step of carrying out the process according to any one of the preceding claims. 37. A process for preparing urea comprising the step of carrying out the process according to any one of the preceding claims 1 to 36. 38. Method for the conversion / modification of a traditional ammonia synthesis plant comprising: - a reforming and shift section, from which a flow (300) of synthesis gas (syngas) is obtained; - a carbon dioxide removal section by washing with suitable solvents obtaining a stream of carbon dioxide (600) and a stream of purified synthesis gas (400); - a section for the use of carbon dioxide thus separated (600) in a process for the synthesis of urea, possibly after suitable compression; - a further purification section of the synthesis gas (400), obtaining a further purified synthesis gas stream (500); - a section for the use of said further purified synthesis gas (500) in a process for the synthesis of ammonia, comprising the step of modifying this plant in such a way that: - at least a portion (300a) of the synthesis gas (syngas) obtained from the reforming and shift section is subjected to a carbon dioxide removal step in the supercritical phase by means of a gravimetric separation process. 39. A plant for the production of ammonia and / or urea comprising one or more devices for the gravimetric separation in the supercritical phase of carbon dioxide from a stream of synthesis gas. The plant according to the preceding claim, wherein said one or more devices for the gravimetric separation in the supercritical phase of carbon dioxide from a flow of synthesis gas are in fluid connection with each other in series or in parallel.