EP4244558A1

EP4244558A1 - An integrated process for purifying and liquefying natural gas

Info

Publication number: EP4244558A1
Application number: EP21810735.7A
Authority: EP
Inventors: Fabrizio MELONI; Chiara LOZIO; Anton Marco FANTOLINI
Original assignee: Saipem SpA
Current assignee: Saipem SpA
Priority date: 2020-11-11
Filing date: 2021-11-03
Publication date: 2023-09-20
Anticipated expiration: 2041-11-03
Also published as: IT202000026978A1; EP4244558B1; WO2022101738A1; US20230400251A1

Abstract

The present invention describes a process for liquefying and simultaneously purifying natural gas from acidic compounds.

Description

AN INTEGRATED PROCESS FOR PURI FYING AND LIQUEFYING

NATURAL GAS

DESCRIPTION

Technical field of the invention

The present invention applies to the field of liquefying natural gas .

Background art

Liquefaction of natural gas is a particularly important industrial process , with a global production capacity of about 300 million tons per annum (MTPA) , mainly obtained from large-scale plants basel oad) with a capacity from 3 MTPA to 8 MTPA each .

The use of baseload plants has been j ustified so far by the economies of scale and by the increased energy ef ficiency, which can be obtained by increasing the production capacity; this was possible with signi ficant initial investments .

Before sending the natural gas for liquefaction, it is necessarily subj ected to a pre-treatment step for removing components and/or contaminants so as to allow an end-product to be obtained having determined features and also to allow the sending thereof to the cryogenic unit , in particular, to the liquefaction unit , in particular, avoiding the solidi fication of some components . Typical contaminants to be removed include CO2 , H2O, Hg, sul fur compounds and aromatic or heavy hydrocarbons .

The pre-treatment units are normally positioned upstream of the liquefaction unit and are independent thereof .

Typically, the following are used in plants of the basel oad type :

• physical and chemical solvents for removing CO2, sul furs and aromatics , mainly aqueous solutions , which saturate the water gas ;

• regenerable molecular sieves for removing the water ;

• non-regenerative sieves for removing the mercury;

• various technologies based on the principle of distillation for separating the heavy hydrocarbons .

The pre-treatment units significantly contribute to the energy consumption of the plant .

After the pre-treatment , the natural gas is sent to the liquefaction unit where it is gradually cooled .

Three main cooling steps can be identi fied : pre-cooling, in the mainly gaseous phase , liquefaction, with the passage of state to liquid phase , and sub-cooling, in liquid phase . For a baseload plant, the choice of the liquefaction technology, together with the choice of the cooling machines and plants, is fundamental, since it influences the total investment cost, production capacity and plant availability.

The world of processes for LNG baseload plants is dominated by technologies based on hydrocarbon refrigerants .

The most commonly used process is C3/MR, which was introduced by Air Products and Chemicals Inc. at the beginning of the Seventies and it is based on propane refrigeration cycles (C3) and a Mixed Refrigerant (MR) consisting of methane, ethane, propane, nitrogen and, sometimes, butane.

The following processes are also very common Single MR (with a single MR cycle) , Dual MR (with two MR cycles in series) and Conoco-Phillips Optimized Cascade, with three refrigeration cycles in cascade, with propane, ethane and methane, respectively.

The efficiency of the liquefaction technologies increases as the number of refrigeration cycles in series increases, at the expense of the complexity of the plant and the operating and installation cost

(CAPEX) thereof. It is known that, in general and in the field of application of this industry, the liquefaction efficiency is greater as the pressure of the natural gas increases; for example, for pressures comprised between 45 barg and 70 barg, the gain in terms of production amounts to about 0.5% for each additional bar .

Recent developments in the LNG industry and the drive to use natural gas as a "clean" fuel, replacing diesel fuel and diesel, have favored the development of the market of small-scale LNG plants, with an annual capacity from 0.05 MTPA to 1 MTPA.

This new market, driven by various factors from economies of scale, has introduced the use of new solutions for the purpose of reaching a wider range of clients and offering simpler products, with a smaller CAPEX, less equipment, easier manageability, and reduced delivery times, also accepting a lower energy efficiency.

Small-scale plants also differ from the baseload ones in the liquefaction technology: to favor the simplicity of the process and use in offshore applications, the commonest technologies are based on the use of non-hydrocarbon refrigerants, such as nitrogen (including the design based on Moss' patent RS NO305525B1) , hydrocarbon refrigerants, such as the

Single MR and, more recently, cycles have also been proposed using natural gas.

Nitrogen refrigeration cycles

Nitrogen cycles are based on the Brayton thermodynamic cycle (inverted) .

In an ideal process, the global efficiency is independent of the type of refrigerant and, in fact, the efficiency of the machines is equal to 100%, the release of energy occurs reversibly without load or working losses and the expansion/compression of the gas is reversible and isentropic.

Whereas, in a real cycle, different effects, represented by imperfect equipment, load losses, non- isentropic expansions and compressions, which are neither reversible nor isotherm, must be taken into account .

Small scale natural gas pre-treatment

Small-scale plants differ from the baseload ones in their small dimensions and capacity; these features enable the applicability of alternative technologies for pre-treating the gas, which operate at a lower temperature than traditional ones.

In particular, the use of regenerative molecular sieves is known for removing CO2, aromatic and sulfur compounds, with the option of carrying out the regeneration of the pre-treatment units.

Further technologies allow the removal of undesired components from the gas (CO2 and sulfur compounds) , by means of sublimation or deposition by freezing on cold surfaces of the same.

Some of these technologies are described, for example, in patent documents US 4.265.088, WO 2009/047341, US 7.073.348.

Prior art documents CN 105737515A (figure 1 of the present patent application) and CN 105890281A (figure 2 of the present patent application) describe a natural gas liquefaction process, comprising a step of removing the CO2 after the liquefaction of the natural gas, exploiting the natural characteristics between LNG and the step with solid CO2.

In the process described by CN 105375151, the solidified carbon dioxide is separated in the solidliquid separator 14.

In the process described by CN 105890281 the raw natural gas is cooled at -40°C through the heat exchanger for pre-cooling the natural gas 1, then it enters the separator for heavy hydrocarbons 4 to remove the heavy hydrocarbons; after removing the heavy hydrocarbons, the natural gas enters the heat exchanger for a further cooling of the natural gas 8 and is cooled at -160°C, then it enters the separator at a low temperature of carbon dioxide 9, to remove the solid carbon dioxide. After removing the carbon dioxide, the natural gas passes through the valve 11 and continues through the exit 17 at the LNG storage pressure.

The removal of carbon dioxide directly from LNG, i.e. downstream of the natural gas liquefaction, raises serious problems regarding the operability of the main exchanger by the accumulation of solid CO2 therein, with the probable stratification on the exchange surface with the refrigerant.

In general, the affordability of a small-scale liquefaction plant, which exploits nitrogen cycles (or nitrogen-natural gas) , is largely affected by the inefficiency of the cycle itself with respect to the hydrocarbon cycles of the baseload plants.

Furthermore, the energy efficiency is further compromised by the failed integration between the pretreatment units and the cooling and liquefaction units. Summary of the invention

The inventors of the present patent application have developed a small-scale process for treating and liquefying the natural gas (NG) with high energy efficiency in the removal of acidic compounds. Obj ect of the invention

In a first obj ect , the present invention describes a process for puri fying and liquefying the natural gas (NG) .

According to a particular aspect , such process is a small -scal e process .

Particular embodiments of the described process represent further obj ects of the invention .

In a second obj ect , a plant is described for puri fying and liquefying the natural gas (NG) .

Brief description of the drawings

Figure 1 shows the diagram of an NG liquefaction plant according to what is described by the prior art document GN 105737515 .

Figure 2 shows the diagram of an NG liquefaction plant according to what is described by the prior art document GN 105890281 .

Figure 3 shows the general diagram of the process of the invention .

Figure 4 shows the diagram of a process for liquefying NG according to a first embodiment of the invention .

Figure 5 shows the diagram of a process for liquefying NG according to a second embodiment of the invention . Detailed description of the invention

In a first object, the present invention describes a process for purifying and liquefying the natural gas (NG) .

More specifically, the process comprises the steps of :

1) pre-cooling a natural gas flow 0 in a cryogenic exchanger (CE) obtaining a pre-cooled natural gas flow 1,

2) pre-treating the pre-cooled natural gas flow 1 obtained from step 1 inside a pre- treatment unit (PK1) obtaining a purified flow 3,

3) heat recovery using the flow purified 3 inside the cryogenic exchanger (CE) obtaining a flow 5 at a higher temperature,

4) compression of said flow 5 at a higher temperature by means of a first compressor GK1 obtaining a first compressed recirculation flow 7,

5) cooling said first compressed recirculation flow 7 in a first cooler of the natural gas GC1 obtaining a compressed and cooled flow 8,

6) separation from said compressed and cooled flow 8 of a first portion 17, which is subjected to the steps of: 6a) further cooling, liquefaction and possible sub-cooling, inside said cryogenic exchanger (CE) obtaining a flow of liquefied natural gas 18,

6b) expansion of said flow of liquefied natural gas 18 by means of a first valve VI obtaining a flow of liquefied natural gas 19 at a lower pressure,

7) separation from said compressed and cooled flow 8 of a recirculation flow portion of the natural gas 13, which is subjected to the steps of:

7a) cooling, inside said cryogenic exchanger (CE) , obtaining a first cooled recirculation flow of the natural gas 14,

7b) expansion of said first cooled recirculation flow of the natural gas 14 in an expander GE obtaining a second recirculation flow of the natural gas 15,

7c) subjecting said second recirculation flow of the natural gas 15 to a step of heat recovery inside said cryogenic exchanger (CE) obtaining a third recirculation flow 16 at a higher temperature, which is reunited with said higher temperature flow 5, to form an overall recirculation flow 6 to be subjected to step 4 ) .

According to a particular aspect of the present invention, in the pre-treatment step 2) , a first high- pressure flow 22 can be used, which is separated from the first compressed recirculation flow 7 obtained from step 4 ) .

Such first high-pressure flow 22 is used to carry out the pre-treatment of the pre-cooled flow 1, e.g. by means of a heat exchange.

Furthermore, for the purposes of the present invention, one or more of the cooling steps according to steps 1) , 6a) , 7a) reported above, can be carried out by means of a flow of nitrogen 46, which circulates inside a closed nitrogen refrigeration cycle 100.

Such closed nitrogen refrigeration cycle 100 is supplied by a flow of nitrogen 60 and is described below in greater detail.

In an embodiment of the invention, after step 1) , said pre-cooled flow 1 can be expanded by means of a second valve V2 obtaining a further pre-cooled and expanded flow 2.

The expansion results in a further cooling of the flow 1.

Therefore, the successive pre-treatment step 2) can be carried out on the pre-cooled flow 1 or on the further pre-cooled and expanded flow 2.

In particular, said pre-treatment step 2) comprises one or more purification processes, which are known in the art . Such purification processes aim at separating the carbon dioxide, hydrogen sulfide, water, other sulfur compounds and aromatic or heavy hydrocarbons (>C5) .

For example, molecular sieves can be used or removal systems by means of freezing.

According to a preferred aspect of the present invention, the purified flow 3 obtained after the pretreatment step 2) has a CO2 content of less than 250 ppmv and preferably less than 50 ppmv.

According to another preferred aspect of the present invention, the purified flow 3 obtained after the pre-treatment step 2) has an H2S content of less than 10 mg/Nm³ and preferably less than 5 mg/Nm³.

According to a further preferred aspect of the present invention, the purified flow 3 obtained after the pre-treatment step 2) has an H2O content of less than 5 ppmv and preferably less than 1 ppmv.

In an embodiment of the invention, the purified flow 3 obtained after the pre-treatment step 2) can be further expanded by means of a third valve V3 obtaining a purified and further expanded flow 4.

Therefore, the heat recover step 3) can be carried out on the purified flow 3 or on the purified and further expanded flow 4. As described above, as for step 4) this is carried out on the higher temperature flow 5, with which the third natural gas recirculation flow 16 at a higher temperature is reunited, forming the overall recirculation flow 6, which is then compressed in the first compressor GK1, obtaining a first compressed recirculation flow 7.

As reported above, after the compression, the first compressed recirculation flow 7 is cooled in a first cooler for the natural gas (GC1) obtaining a cooled flow 8.

According to an embodiment of the present invention, the obtainment of a flow of natural gas cooled at the liquefaction pressure of steps 4) and 5) can be obtained with a plurality of successive partial steps .

To this end, for example, as shown in the diagram in figure 4, the cooled flow 8 is sent for further compression steps in one or more further compressors (GK2, GK3) obtaining further compressed recirculation flows (9,11) , respectively.

Since a respective cooling step is carried out in further respective natural gas coolers (GC2, GC3) , after each compression step, further compressed and cooled recirculation flows are obtained (10,12) , respectively .

As for step 5) , the cooling can be obtained according to techniques known in the field, e.g. air or water heat exchange, or by means of other fluids.

Therefore, for the purposes of the present invention, the successive steps 6) and 7) are carried out on portions of the compressed and cooled flow 8 or on a further compressed and cooled recirculation flow 10,12 or, in any case, on the last compressed and cooled flow obtained.

For the purposes of the present invention, the capacity of said flow 17 subjected to step 6a) is comprised between 10-40% and preferably between 15-30% of the capacity of the flow 8, 10, 12 or, in any case, on the last compressed and cooled flow obtained.

According to an embodiment of the present invention, a first compressed flow portion 23, a second compressed flow portion 24, or further compressed flow portions can be separated from one or more of the further compressed flows 9,11 or, in any case, from each compressed flow obtained, , respectively, which are not subjected to cooling, but which are reunited with the first compressed flow 22 forming a treatment flow 2.2.' , which, as described above, can be used in the pre-treatment step 2) .

In particular, such treatment flow 22' is used in the pre-treatment flow of the pre-cooled flow 1 or 2, e.g. by means of heat exchange.

According to a preferred aspect of the present invention, the flow of liquefied natural gas 18 obtained from step 6a) has a temperature from -161°C to -141°C and on average of -147°C and a pressure of 0-10 barg and on average of 5 barg.

For the purposes of the present invention, the closed nitrogen refrigeration cycle 100 mentioned above is a cycle, in which one or more low-pressure nitrogen flows and one or more high-pressure nitrogen flows circulate, independently of each other.

According to an embodiment of the present invention (shown in figure 3) , said closed nitrogen refrigeration cycle 100 comprises the steps of:

A) subjecting a low-pressure nitrogen recirculation flow 40 to a step of compression in a first compressor of the nitrogen cycle NK1, obtaining a first high-pressure nitrogen recirculation flow 41, and to a successive step of cooling in a first cooler of the nitrogen cycle NCI, obtaining a first cooled high-pressure nitrogen flow 42,44, B ) subj ecting said first cooled high-pressure nitrogen flow 42 , 44 to a step of heat exchange in the cryogenic exchanger CE obtaining a further cooled high- pressure nitrogen flow 45 ,

C ) subj ecting said further cooled high-pressure nitrogen flow 45 to a step of expansion in an expander of the nitrogen cycle NE , obtaining a low-pressure nitrogen flow 46 ,

D) subj ecting said low-pressure nitrogen recirculation flow 46 to a step of heat recovery in the cryogenic exchanger CE , obtaining the low-pressure nitrogen flow 40 to be subj ected to step A) .

According to an embodiment of the present invention, the compression step A) of the low-pressure nitrogen flow 40 can be carried out in a plurality of successive partial steps .

To this end, as shown, for example , in the diagram in figure 4 , the first cooled high-pressure nitrogen flow 42 is sent to a further compressor NK2 obtaining a further high-pressure nitrogen recirculation flow 43 .

Since , after each compression step, a respective cooling step is carried out , the further high-pressure nitrogen recirculation flow 43 is sent to a further cooler of the nitrogen cycle NC2 obtaining a further cooled high-pressure nitrogen recirculation flow 44 . Therefore , for the purposes of the present invention, step B ) is carried out on a first cooled high-pressure nitrogen recirculation flow 42 or on a further cooled high-pressure nitrogen recirculation flow 44 or, in any case , on the last compressed and cooled nitrogen flow obtained .

According to an aspect of the present invention, in the closed nitrogen refrigeration cycle 100 the nitrogen flows vary the temperature and pressure conditions according to the indicative values reported in the following table :

According to an embodiment o f the present invention, one portion 47 and a further portion 48 are separated, respectively, from the first high-pressure nitrogen recirculation flow 41 and/or from the further high-pressure nitrogen recirculation flow 43 or, in any case , from each high-pressure nitrogen recirculation flow obtained, which can be reunited, forming a treatment nitrogen flow 49 , which can be used in the pre-treatment step 2 ) , e . g . for regenerating the pretreatment units . Therefore, the pre-treatment step can be carried out by using one or more of the flows selected from: the first high-pressure flow 22, the treatment flow 22' , the first portion 47 of the high-pressure nitrogen recirculation flow or the portion 48 of the further portion of the first high-pressure nitrogen recirculation flow or the nitrogen treatment flow 49 or, according to an alternative not shown in the figures, an external current.

According to an embodiment, for example, shown in the diagram in figure 4, as a function of the pressure thereof, at a higher temperature the flow 5 obtained from step 3) can be sent, not to the first natural gas compressor GK1, but directly to the second compressor GK2 (flow 5' ) or to the third compressor GK3 (flow 5' ’ ) or to any successive compressor, after being reunited with the further cooled compressed recirculation flows (8,10) , respectively, or in general, with each cooled compressed recirculation flow obtained.

According to an embodiment of the present invention, for example, shown in the diagram in figure 5, a portion 27 of the second recirculation flow of the natural gas 15 obtained from the expansion step

7b) is used in the pre-treatment step 2) . According to another embodiment of the present invention, a portion 26 of said liquefied natural gas flow 19 obtained from step 6b) is used in the pretreatment step 2 ) , from which a flow 28 is recovered, which is subj ected to an expansion step by means of a fourth valve V4 obtaining a natural expanded gas flow 29 , which is reunited with the natural gas flow 5 at a higher temperature obtained from step 3 and sent to step 4 ) .

Advantageously, in this way, the l iquefied natural gas 19 refrigeration units are exploited .

For the purposes of the present invention, a flow of removed compounds 25 , 50 is obtained from the pretreatment step 2 ) , comprising acidic or other compounds removed by means of the pre-treatment step, which can be sent to other plant units , e . g . for treating the fuel gas, or it can be recovered as a by-product or freed into the atmosphere .

According to a second obj ect of the invention, a plant is described for puri fying and liquefying the natural gas (NG) comprising ( the numbers and references coincide for the steps of the process and the plant elements ) :

( I ) a natural gas circuit comprising : a section for pre-treating said natural gas

(PK1) ,

- a section for compressing (GK) , cooling (GC) and expanding said natural gas (GE) ,

- a point for recovering the liquefied natural gas for the storage or introduction thereof into a convenient distribution network (not shown in the figures ) ;

(II) a nitrogen circuit comprising:

- a tank of nitrogen (60) ,

- a section for compressing (NK) , cooling (NG) and expanding (NE) the nitrogen NE) ; and

(III) a cryogenic exchanger (GE) comprising:

- a section for cooling, liquefying and sub-cooling the natural gas .

For the purposes of the present invention, inside said natural gas circuit, flows of natural gas circulate, which can be: low-pressure, high-pressure, compressed or expanded, cooled or heated.

For the purposes of the present invention, inside said nitrogen circuit, several flows of nitrogen circulate, which can be: low-pressure, high-pressure, compressed or expanded, cooled or heated.

According to a preferred aspect of the present invention, said cryogenic exchanger (GE) is built so as to allow thermal exchanges between one or more of said flows of natural gas with one or more of said flows of nitrogen and/or natural gas .

According to a preferred aspect of the present invention, the plant for puri fying and liquefying the natural gas (NG) of the present invention is the plant in which the steps of the process described above are carried out .

From the above description of the present invention, the advantages of fered by the present invention will be immediately apparent to those ski lled in the art .

In particular, the process provided allows optimi zing the liquefaction of the natural gas and the puri fication thereof with an optimum compromise between plant ef ficiency and complexity; by virtue of these features , the process lends itsel f well to offshoretype applications .

Furthermore , the combination with a nitrogen cycle allows operating more safely and more compactly, inter alia, avoiding importing, storing and managing hydrocarbon refrigerants .

These further characteristics make the process of the invention also ideal for applications of the floating type (on boats) .

Again, the process of the present invention allows optimizing the pre-treatment and liquefaction steps, which are favored by mutually opposite operating conditions .

Furthermore, by virtue of the increased efficiency, the process of the invention can be carried out with a lower energy consumption, thereby reducing the operating costs of the plant (OPEX) and, ultimately, the environmental impact; this is even more apparent when part of the natural gas entering the plant is consumed to produce the energy to be used for pre-treatment and liquefaction.

Claims

1. A process for purifying and liquefying natural gas (NG) comprising the steps of:

1) pre-cooling a natural gas flow (0) in a cryogenic exchanger (CE) obtaining a pre-cooled natural gas flow ( 1 ) ,

2) pre-treating the pre-cooled natural gas flow (1) obtained from step 1) inside a pre-treatment unit (PK1) obtaining a purified flow (3) ,

3) heat recovery inside the cryogenic exchanger (CE) obtaining a flow (5) at a higher temperature,

4) compression of said flow (5) at a higher temperature by means of a first compressor (GK1) obtaining a first compressed recirculation flow (7) ,

5) cooling said first compressed recirculation flow (7) in a first cooler of the natural gas (GC1) obtaining a compressed and cooled flow (8) ,

6) separation from said compressed and cooled flow (8) of a first portion (17) , which is subjected to the steps of:

6a) further cooling, liquefaction and possible sub-cooling, inside said cryogenic exchanger (CE) obtaining a flow of liquefied natural gas (18) ,

6b) expansion of said flow of liquefied natural gas (18) by means of a first valve VI obtaining a flow of liquefied natural gas (19) at a lower pressure,

23 7) separation from said compressed and cooled flow

(8) of a recirculation flow portion of the natural gas (13) , which is subjected to the steps of:

7a) cooling, inside said cryogenic exchanger (CE) , obtaining a first cooled recirculation flow of the natural gas (14) ,

7b) expansion of said first cooled recirculation flow of the natural gas (14) in an expander (GE) obtaining a second recirculation flow of the natural gas ( 15) ,

7c) subjecting said second recirculation flow of the natural gas (15) to a step of heat recovery inside said cryogenic exchanger (CE) obtaining a third recirculation flow (16) at a higher temperature, which is reunited with said higher temperature flow (5) , to form an overall recirculation flow (6) to be subjected to step 4) , wherein one or more of the cooling steps according to steps 1) , 6a) , 7a) reported above, can be carried out by means of a flow of nitrogen (46) , which circulates inside a closed nitrogen refrigeration cycle 100.

2. A process according to the preceding claim, wherein, after step 1) and/or after said step 2) the pre-cooled flow (1) and/or said purified flow (3) of natural gas are expanded, obtaining an expanded pre- cooled flow (2) and/or a purified and further expanded flow, respectively (4) .

3. A process according to any one of the preceding claims, wherein said step 4) is carried out on the higher temperature flow (5) , with which a third natural gas recirculation flow (16) is reunited, forming a second overall recirculation flow (6) , which is then compressed in the first compressor (GK1) , obtaining a first compressed recirculation flow (7) .

4. A process according to any one of the preceding claims, wherein step 4) and step 5) are carried out in a plurality of successive partial compression steps in further compressors (GK2,GK3) obtaining further compressed recirculation flows (9,11) and respective cooling in further coolers (GC2,GC3) obtaining further compressed and cooled recirculation flows (10,12) .

5. A process according to any one of the preceding claims, wherein, in the pre-treatment step 2) , a first high-pressure flow (22) is used, which is separated from the first compressed recirculation flow (7) obtained from step 4) .

6. A process according to any one of the preceding claims, wherein portions of said further compressed recirculation flows (23, 24) are separated from said one or more of the further compressed flows (9,11) , which are reunited together with the first high- pressure flow (22) , forming a high-pressure treatment flow (22' ) , which is used in the pre- treatment step 2) .

7. A process according to any one of the preceding claims, wherein, in step 6) , a first portion (17) is separated having a capacity of about 10-40% of the capacity of said compressed and cooled flow (8) or of said further compressed and cooled recirculation flows (10,12) or, in any case, from each compressed and cooled flow.

8. A process according to any one of the preceding claims, wherein said closed nitrogen refrigeration cycle (100) comprises the steps of:

A) subjecting a low-pressure nitrogen recirculation flow (40) to a step of compression in a first compressor of the nitrogen cycle (NK1) , obtaining a first high-pressure nitrogen flow (41) , and to a successive step of cooling in a first cooler of the nitrogen cycle (NCI) , obtaining a first cooled high- pressure nitrogen flow (42,44) ,

B) subjecting said first cooled high-pressure nitrogen flow (42,44) to a step of heat exchange in the cryogenic exchanger (CE) obtaining a further cooled high-pressure nitrogen flow (45) ,

26 C) subjecting said further cooled high-pressure nitrogen flow (45) to a step of expansion in an expander of the nitrogen cycle (NE) , obtaining a low-pressure nitrogen flow (46) ,

D) subjecting said low-pressure nitrogen recirculation flow (46) to a step of heat recovery in the cryogenic exchanger (CE) , obtaining the low- pressure nitrogen flow (40) to be subjected to step A) .

9. A process according to claim 8, wherein the compression in step A) of the low-pressure nitrogen recirculation flow (40) is carried out in a plurality of successive partial steps, obtaining a further high- pressure nitrogen recirculation flow (43) , which is then subjected to a corresponding cooling step, obtaining a further cooled high-pressure nitrogen recirculation flow (44) .

10. A process according to claim 8 or 9, wherein a portion (47) of the first high-pressure nitrogen recirculation flow and a portion (48) of the further high-pressure nitrogen recirculation of the first high- pressure nitrogen recirculation flow are separated from said high-pressure nitrogen recirculation flow (41) and/or from said further high-pressure nitrogen recirculation flow (43) , which are reunited together

27 forming a treatment nitrogen flow (49) , which is used in the pre-treatment step 2) .

11. A process according to any one of the preceding claims from 4 to 10, wherein a higher temperature flow (5' , 5' ' ) obtained from step 3) is sent directly to one of the further compressors (GK2,GK3) of said partial steps 4) .

12. A process according to any one of the preceding claims, wherein a portion (27) of the second recirculation flow of the natural gas obtained from step 7b) is used in the pre-treatment step 2) .

13. A process according to any one of the preceding claims, wherein a portion (26) of said liquefied natural gas flow obtained from step 6b) is used in the pre-treatment step 2) .

14. A process according to any one of the preceding claims, wherein the pre-treatment step 2) can be carried out using, as an alternative to, or in addition to one or more of said: first high-pressure flow (22) , said high-pressure treatment flow (22' ) , said portion (27) of the second recirculation flow of the natural gas, said portion (47) of the first high- pressure nitrogen recirculation flow and/or said portion (48) of the further high-pressure nitrogen

28 recirculation flow or said nitrogen treatment flow

(49) , one or more external currents.

15. A process according to claim 13, wherein a recovery flow (28) is obtained from step 2) , which is subjected to a step of expansion by means of a fourth valve v4, obtaining a flow of expanded natural gas (29) , which is reunited with the higher temperature natural gas flow (5) to be used in step 4) .

16. A process according to any one of the preceding claims, wherein a flow (25, 50) of removed compounds is obtained from the pre-treatment step 2) .

17. A plant for purifying and liquefying natural gas (NG) in which the process is carried out according to any one of the preceding claims comprising:

(I) a natural gas circuit inside which flows of natural gas circulate, which can be: low-pressure, high- pressure, compressed or expanded, cooled or heated, comprising :

- a section for pre-treating said natural gas (PK1) ,

- a point for recovering the liquefied natural gas for the storage or introduction thereof into a convenient distribution network;

29 (II) a nitrogen circuit inside which several flows of nitrogen circulate, which can be: low-pressure, high- pressure, compressed or expanded, cooled or heated, comprising :

- a tank of nitrogen (60) ,

- a section for compressing (NK) , cooling (NC) and expanding (NE) the nitrogen; and

(III) a cryogenic exchanger (CE) comprising:

- a section for cooling, liquefying and sub-cooling the natural gas .

18. A plant according to the preceding claim, wherein said cryogenic exchanger (CE) is built so as to allow thermal exchanges between one or more of said flows of natural gas with one or more of said flows of nitrogen and/or natural gas.

30