EP2344821B1 - Method for producing liquid and gaseous nitrogen streams, a helium-rich gaseous stream, and a denitrogened hydrocarbon stream, and associated plant - Google Patents

Description

The present invention relates to a process for producing a liquid nitrogen stream, a nitrogen gas stream, a helium rich gas stream and a denitrogenized hydrocarbon stream, from a stream charge containing hydrocarbons, helium and nitrogen.

Such a method is particularly applicable to the treatment of charge streams consisting of liquefied natural gas (LNG) or also natural gas (NG) in gaseous form.

This process applies to new natural gas liquefaction units or new natural gas processing units. The invention also applies to improving the performance of existing units.

In these installations, natural gas must be de-nitrogenized before being sent to the consumer, or before being stored or transported. Indeed, natural gas extracted from underground deposits often contains a significant amount of nitrogen. It also frequently contains helium.

The known denitrogenization processes make it possible to obtain a denitrogenated hydrocarbon stream which can be sent to a storage unit in liquid form in the case of LNG, or to a gas distribution unit in the case of the NG.

These denitrogenation processes also produce nitrogen-rich streams which are used either to supply nitrogen necessary for the operation of the plant or to provide a nitrogen-rich fuel gas which serves as a fuel for the gas turbines of the compressors. used during the implementation of the method. Alternatively, these nitrogen-rich streams are released into the atmosphere in a torch after incineration of impurities, such as methane.

The aforementioned methods are not entirely satisfactory, particularly because of the new environmental constraints applying to the production of hydrocarbons. Indeed, so that the nitrogen produced by the process can be used in the production unit, or released into the atmosphere, it must be very pure.

The fuel streams produced by the process and intended for use in gas turbines must, on the contrary, contain less than 15 to 30% of nitrogen for burning in special burners designed to limit the production of nitrogen oxides. released into the atmosphere. These discharges occur in particular during the start-up phases of the installations used for the implementation of the process, in which the denitrogenation process is not yet very effective.

In addition, for economic reasons, the energy efficiency of such denitrogenation processes must be continuously improved. Processes of the above-mentioned type do not make it possible to valorize the helium contained in the natural gas extracted from the subsoil, this helium nevertheless being a rare gas of great economic value.

To at least partially overcome these problems, US 2007/0245771 discloses a process of the aforementioned type, which simultaneously produces a stream of liquid nitrogen, a helium-rich stream, and a gas stream containing about 30% nitrogen and about 70% hydrocarbons. This gas stream rich in nitrogen is intended in this installation to form a fuel stream.

However, this process is not entirely satisfactory since the quantity of pure nitrogen produced is relatively small. In addition, the fuel stream contains a large amount of nitrogen that is not compatible with all existing gas turbines, and is likely to generate many polluting emissions.

US 2004/231359 discloses a method of removing nitrogen from condensed natural gas to produce a subcooled liquefied natural gas. This process is not intended to produce a stream of liquid nitrogen, nor a gaseous stream rich in helium.

US 4,778,498 discloses a process for producing methane gas under pressure producing a gaseous phase containing helium.

US 5,339,641 describes a process for producing liquid nitrogen and a helium-rich gas.

An object of the invention is to obtain an economical process of denitrogenation of a hydrocarbon feed stream, which makes it possible to recover the nitrogen and helium contained in the feed stream, while limiting emissions to a minimum. harmful to the environment.

For this purpose, the subject of the invention is a method according to claim 1.

The method according to the invention may comprise one or more of the features of claims 2 to 10.

• selon la revendication 11.

The invention also relates to an installation

• according to claim 11.

The installation according to the invention may comprise one or more of the features
claims 12 and 13.

• la Figure 1 est un schéma synoptique fonctionnel d'une première installation de mise en oeuvre d'un premier procédé de production selon l'invention ;
• la Figure 2 est une vue analogue à la figure 1 d'une deuxième installation de mise en oeuvre d'un deuxième procédé de production selon l'invention ;
• la Figure 3 est une vue analogue à la figure 1 d'une troisième installation de mise en oeuvre d'un troisième procédé de production selon l'invention ;
• la Figure 4 est une vue analogue à la figure 1 d'une quatrième installation de mise en oeuvre d'un quatrième procédé de production selon l'invention ;
• la Figure 5 est une vue analogue à la figure 1 d'une cinquième installation de mise en oeuvre d'un cinquième procédé de production selon l'invention ; et
• la Figure 6 est une vue analogue à la figure 1 d'une sixième installation de mise en oeuvre d'un sixième procédé de production selon l'invention.

The invention will be better understood on reading the description which follows, given solely by way of example, and with reference to the appended drawings, in which:

• the Figure 1 is a functional block diagram of a first installation for implementing a first production method according to the invention;
• the Figure 2 is a view similar to the figure 1 a second installation for implementing a second production method according to the invention;
• the Figure 3 is a view similar to the figure 1 a third installation for implementing a third production method according to the invention;
• the Figure 4 is a view similar to the figure 1 a fourth installation for implementing a fourth production method according to the invention;
• the Figure 5 is a view similar to the figure 1 a fifth installation for implementing a fifth production method according to the invention; and
• the Figure 6 is a view similar to the figure 1 of a sixth installation for implementing a sixth production method according to the invention.

The Figure 1 illustrates a first installation 10 according to the invention for producing, from a liquid feed stream 12 obtained from a charge of liquefied natural gas (LNG), a hydrocarbon-rich denitrogenated LNG stream 14, a stream of nitrogen gas 16 for use in the plant 10, a stream of liquid nitrogen 18, and a stream 20 rich in helium.

As illustrated by Figure 1 , the installation 10 comprises an upstream portion 22 for cooling the load, and a downstream portion 24 for fractionation.

The upstream portion 22 comprises a liquid expansion turbine 26, an upstream heat exchanger 28, for cooling the charging current 12 by means of a cooling cycle 30.

In this example, the cooling cycle 30 is an inverted Brayton type closed cycle. It comprises a cycle heat exchanger 32, an upstream compressor 34 with stages, and a dynamic expansion turbine 36.

In the example of the Figure 1 , the upstream stage compression apparatus 34 comprises two stages, each stage comprising a compressor 38A, 38B and a refrigerant 40A, 40B cooled in air or in water. At least one of the compressors 38A of the upstream apparatus 34 is coupled to the dynamic expansion turbine 36 to increase the efficiency of the process.

The downstream fractionation portion 24 comprises a fractionation column 50 having a plurality of theoretical fractionation stages. The downstream portion 24 further comprises a first downstream downstream heat exchanger 52, a second downstream heat exchanger 54, and a third downstream heat exchanger 56.

The downstream portion 24 further comprises a downstream compressor apparatus 58 with stages and a first separation flask 60 at the top of the column.

The downstream compression apparatus 58 in this example comprises three compression stages connected in series, each stage comprising a compressor 62A, 62B, 62C placed in series with a refrigerant 64A, 64B, 64C cooled with water or with air .

A first production method according to the invention will now be described.

In what follows, we will designate by a single reference a fluid stream and the pipe that carries it. Similarly, the pressures considered are absolute pressures, and unless otherwise indicated, the percentages considered are molar percentages.

In this example, the liquid charging stream 12 is a stream of liquefied natural gas (LNG) comprising in moles 0.1009% helium, 8.9818% nitrogen, 86.7766% methane, 2.9215% d. ethane, 0.8317% propane, 0.2307% i-C4 hydrocarbons, 0.1299% n-C4 hydrocarbons, 0.0128% i-C5 hydrocarbons, 0.0084% n-C5 hydrocarbons, 0.0005% n-C6 hydrocarbons, 0.0001% benzene, 0.0050% carbon dioxide.

Thus, this stream 12 comprises a hydrocarbon molar content greater than 70%, a molar nitrogen content of between 5% and 30%, and a molar helium content of between 0.01% and 0.5%.

The charging current 12 has a temperature below -130 ° C, for example below -145 ° C. This current has a pressure greater than 25 bars, and in particular equal to 34 bars.

In this first embodiment, the charging current 12 is liquid, so that it constitutes a liquid charge stream 68 that can be used directly in the process.

The liquid charging stream 68 is introduced into the liquid expansion turbine 26, where it is expanded to a pressure of less than 15 bar, in particular equal to 6 bar up to a temperature below -130 ° C. and in particular equal to -150.7 ° C.

At the outlet of the liquid expansion turbine 26, a relaxed charge stream 70 is formed. This relaxed charge current 70 is divided into a first main feed stream 72, to be refrigerated by the refrigeration cycle 30, and a second secondary feed stream 74.

The first feed stream 72 has a mass flow rate greater than 10% of the expanded feed stream 70. It is introduced into the upstream heat exchanger 28, where it is cooled to a temperature below -150 ° C and in particular equal to -160 ° C to give a first cooled introduction stream 76.

In the upstream exchanger 28, the first introduction stream 72 is placed in heat exchange relation with the refrigerant stream flowing in the cycle 30, as will be described below.

The first cooled introduction stream 76 is expanded in a first expansion valve 78 to a pressure of less than 3 bars and is then introduced to an intermediate stage N1 of the fractionation column 50.

The second feed stream 74 is conveyed to the first downstream exchanger 52 at the bottom of the column, where it is cooled to a temperature below -150 ° C. and in particular equal to -160 ° C. to give a second stream. cooled introduction 80.

The second cooled introduction stream 80 is expanded in a second expansion valve 82 to a pressure of less than 3 bars, and is then introduced to an intermediate stage N1 of the fractionation column 50.

In this example, the first cooled introduction stream 76 and the second cooled introduction stream 80 are introduced to the same stage N1 of the column 50.

A reboiling stream 84 is withdrawn from a lower stage N2 of the fractionation column 50 located under the intermediate stage N1. The reboiling current 84 passes into the first downstream exchanger 52, to be placed in heat exchange relation with the second introduction stream 74 and to cool the second stream 74. It is then reintroduced near the foot of the column. fractionation 50, below the lower stage N2.

The fractionation column 50 operates at low pressure, in particular less than 5 bar, advantageously less than 3 bar. In this example, the column 50 operates substantially at 1.3 bars.

The fractionation column 50 produces a bottom stream 86 for forming the nitrogen-rich liquefied stream 14. This denitrogenated LNG stream contains a controlled amount of nitrogen, for example less than 1 mol%.

The foot stream 86 is pumped at 5 bar in a pump 88 to form the hydrocarbon-rich denitrogen stream 14 and to be shipped to a storage operating at atmospheric pressure and form the denitrogenated LNG stream for exploitation. The stream 14 is a stream of LNG that can be transported in liquid form, for example in a LNG carrier.

The fractionation column 50 also produces a nitrogen-rich overhead stream 90 which is extracted from the top of this column 50. This overhead stream 90 has a molar content of hydrocarbons preferably less than 1%, and even more advantageously less than 1%. 0.1%. It has a molar helium content greater than 0.2% and advantageously greater than 0.5%.

In the example shown on the figure 1 the molar composition of the overhead stream 90 is as follows: 0.54% helium, 99.40% nitrogen and 0.06% methane.

The nitrogen-rich overhead stream 90 is then passed successively into the second downstream heat exchanger 54, into the first downstream heat exchanger 52, then into the third downstream heat exchanger 56 to be successively heated to -20 ° C.

At the outlet of the third downstream exchanger 56, a stream rich in heated nitrogen 92 is obtained. This stream 92 is then divided into a first minority portion 94 of nitrogen produced, and a second portion 96 of recycled nitrogen.

The minority portion 94 has a mass flow rate of between 10% and 50% of the mass flow rate of the stream 92. The minority portion 94 is expanded through a third expansion valve 98 to form the nitrogen gas stream 16.

This stream of nitrogen gas 16 has a pressure greater than atmospheric pressure and in particular greater than 1.1 bar. It has a molar nitrogen content greater than 99%.

The majority part 96 is then introduced into the downstream compression apparatus 58, where it passes successively into each compression stage through a compressor 62A, 62B, 62C and a refrigerant 64A, 64B, 64C.

The majority part 96 is thus compressed to a pressure greater than 20 bar and in particular substantially equal to 21 bar, to form a compressed recycled nitrogen stream 100.

The recycled compressed nitrogen stream 100 thus has a temperature above 10 ° C and in particular equal to 38 ° C.

The compressed recycled nitrogen stream 100 passes successively through the third downstream heat exchanger 56, then through the first bottom downstream heat exchanger 52, and then through the first downstream heat exchanger 54.

In the second downstream heat exchanger 54 and in the third downstream heat exchanger 56, the recycled nitrogen stream 100 circulates against the current and in heat exchange relation with the top nitrogen stream 90. Thus, the nitrogen stream of head 90 yields frigories to the recycled nitrogen stream 100.

In the first bottom heat exchanger 52, the recycled nitrogen stream 100 is further placed in heat exchange relationship with the reboilage stream 84 to be cooled by this stream 84.

After passing through the second downstream heat exchanger 54, the recycled nitrogen stream 100 forms a stream 102 of condensed, essentially liquid, recycled nitrogen. This liquid stream contains a liquid fraction greater than 90% and has a temperature below -160 ° C and advantageously equal to -170 ° C.

Then, the condensed stream 102 is expanded in a fourth expansion valve 104 to give a two-phase flow 106 which is introduced into the first separator tank 60.

The first separator balloon 60 produces a helium-rich gaseous head stream which, after passing through a fifth expansion valve 108, forms the helium rich gas stream 20.

The helium rich gas stream has a helium content of greater than 10 mol%. It is intended to be conveyed to a pure helium production unit for treatment. The method according to the invention makes it possible to recover at least 60 mol% of the helium present in the charging stream 12.

The first separator flask 60 produces a bottom stream of liquid nitrogen 110 at the bottom. This bottom stream 110 is separated into a minor portion of produced liquid nitrogen 112 and a major portion of reflux nitrogen 114.

The minority part 112 has a mass flow rate of less than 10%, and in particular between 0% and 10% of the mass flow rate of the bottom stream 110. The minority part 112 is expanded in a sixth expansion valve 116 to form the flow of liquid nitrogen product 18. The product nitrogen stream has a molar nitrogen content greater than 99%.

The major portion 114 is expanded to the column pressure through a seventh expansion valve 118, to form a first reflux stream, and then introduced to a top stage N3 of the fractionation column 50, located under the head of this column and above the intermediate stage N1. The molar fraction of nitrogen in the majority part 114 is greater than 99%.

In the example shown on the Figure 1 , the cooling cycle 30 is an inverted Brayton type closed cycle using an exclusively gaseous refrigerant stream.

In this example, the refrigerant stream is formed by substantially pure nitrogen whose nitrogen content is greater than 99%.

The refrigerant stream 130 delivered to the upstream exchanger 28 has a temperature below -150 ° C, and especially equal to -165 ° C and a pressure greater than 5 bar and in particular substantially equal to 9.7 bars. The coolant stream 130 flows through the cycle heat exchanger 32, where it is reheated by heat exchange with the first main feed stream 72.

Thus, the temperature of the heated refrigerant stream 132 at the outlet of the upstream exchanger 28 is less than -150 ° C and in particular equal to -153 ° C.

The heated stream 132 is reheated in the cycle heat exchanger 32 before being introduced into the succession of compressors 38A, 38B and refrigerants 40A, 40B of the upstream stage compression apparatus 34.

At the outlet of the upstream apparatus 34, it forms a compressed refrigerant stream 134 which is cooled by heat exchange with the cooling stream. heated refrigerant 132 from the upstream exchanger 28 in the cycle heat exchanger 32.

The cooled compressed current 136 thus has a pressure greater than 15 bar and in particular substantially equal to 20 bar and a temperature below -130 ° C and in particular substantially equal to -141 ° C.

The cooled compressed stream 136 is then introduced into the dynamic expansion turbine 36. It is dynamically expanded in the expansion turbine 36 to provide the refrigerant stream 130 at the temperature and pressure described above.

In an advantageous variant, the upstream and downstream compression devices 34 and 58 are integrated in the same multi-body machine, with a single motor for propelling the compressors 38A, 38B and the compressors 62A to 62C.

Examples of temperature, pressure, and mass flow rates of the different streams illustrated in the process of the Figure 1 are summarized in the tables below. CURRENT TEMPERATURE (° C) PRESSURE (Bars) FLOW (Kg / h) 12 - 149.5 34 177,365 70 - 150.7 6 177365 76 - 160 6 135,142 80 - 160 6 42,223 84 -163.6 1.4 168,931 86 - 159.7 1.4 154,923 14 -159.5 5 154,923 90 - 193.4 1.3 55,761 92 -20 1.3 55,761 16 -20.4 1.1 20219 100 38 21 35,541 106 - 173 9 35,541 20 -180.5 4 1,663 18 -182 4 560 114 -173 9 33,319 130 - 165 9.7 86,840 132 - 153 9.7 86,840 136 - 141.5 19.5 86,840

The energy consumption of the process is as follows: Compressor 62A: 1300 kW Compressor 62B: 1358 kW Compressor 62C: 1365 kW Compressor 38B 2023 kW Total: 6046 kW

A second installation 140 according to the invention is represented on the Figure 2 . This second installation 140 is intended for the implementation of a second production method according to the invention.

This installation 140 differs from the first installation 10 in that it comprises a second separator tank 142 interposed between the outlet of the fourth expansion valve 104 and the inlet of the first separator tank 60.

The second method according to the invention differs from the first method in that only part of the two-phase flow 106 resulting from the expansion of the cooled recycled nitrogen stream 102 in the fourth expansion valve 104 is received in the first separator tank 60.

Thus, the two-phase flow 106 formed at the outlet of the fourth expansion valve 104 is introduced into the second separator tank 142, and not directly into the first separator tank 60. In addition, the cooled nitrogen stream 102 does not pass through. through the second downstream exchanger 54.

The head stream 144 produced in the second separator tank 142 is passed through the second downstream exchanger 54 to be cooled, and is introduced as a cooled head stream 146 into the first separator tank 60.

Foot stream 148 pulled from the bottom of the second separator tank 142 is divided into a second nitrogen reflux stream 150 and a makeup supplement stream 152.

The second nitrogen reflux stream 150 is introduced, after expansion in an eighth expansion valve 154, to a top stage N4 of the fractionation column 50 located in the vicinity and below the N3 introduction stage of the first reflux stream 114 in the fractionation column 50.

In a variant shown in dotted lines on the Figure 2 the reflux streams 114, 150 are introduced at the same top stage N3 of the column 50.

The mass flow rate of the second reflux stream 150 is greater than 90% of the flow of the mass flow of the foot stream 148.

The second additional cooling stream 152 is reintroduced into the overhead stream 90, upstream of the second downstream heat exchanger 54, in order to provide frigories for partially cooling and condensing the overhead flow 144 passing through the second downstream heat exchanger 54.

The mixing stream 156 resulting from the mixing of the overhead stream 90 and the cooling makeup stream 152 is introduced successively into the second downstream heat exchanger 54 and then into the first downstream heat exchanger 52 where it enters into a heat exchange relationship with the recycled nitrogen stream 100 and the second introduction stream 74 to cool these streams.

The second method according to the invention is also operated in a similar manner to the first method according to the invention.

In this process, the feed stream 12 is a stream of liquefied natural gas (LNG) comprising a composition identical to that described above.

In the example shown on the figure 2 the molar composition of the overhead stream 90 is as follows: helium 0.54%, nitrogen 99.35% and methane 0.11%.

Examples of temperature, pressure, and mass flow rates of the different streams illustrated in the process of the Figure 2 are summarized in the tables below. CURRENT TEMPERATURE (° C) PRESSURE (Bars) FLOW (Kg / h) 12 - 149.5 34 177,365 70 - 150.7 6 177,365 76 - 160 6 134,400 80 - 160 6 43,150 84 -163.6 1.4 169,069 86 - 159.7 1.4 155,100 14 -159.5 5 155,100 90 - 193.4 1.3 52,390 92 -32 1.3 52,678 16 -32.1 1.1 22 140 100 38 19.7 30,550 106 -180 5 30,550 146 - 186 4.7 3,940 150 -179.8 5 26,320 152 -179.8 5 288 20 -186.3 4.7 271 18 -186.3 4.7 28 114 -186.3 4.7 3,640 130 - 163 9.7 112,100 132 - 154 9.7 112,100 136 -140 19.2 112,100

The energy consumption of the process is as follows: Compressor 62A: 1482 kW Compressor 62B: 912 kW Compressor 62C: 708 kW Compressor 38B 2584 kW Total: 5686 kW

A third installation 160 according to the invention, for the implementation of a third method according to the invention is illustrated by the Figure 3 .

The third installation 160 differs from the first installation 10 by the presence of a fractionation section 162 and an upstream liquefaction exchanger 164 placed upstream of the liquid expansion turbine 26.

In this example, the charging current 12 is natural gas (NG) in gaseous form. It is introduced firstly into the liquefaction exchanger 164 to be cooled to a temperature below -20 ° C and substantially equal to -30 ° C.

The feed stream 12 is then fed to the fractionation section 162 which produces a treated gas 166 having a low C ₅ ⁺ hydrocarbon content and a section 168 of a C ₅ ⁺ hydrocarbon rich liquefied gas. The molar content of C ₅ ⁺ hydrocarbons in the treated gas 166 is less than 300 ppm.

The treated gas 166 is reintroduced into the liquefying exchanger 164 to be liquefied and to give a liquid charge stream 68 at the outlet of the liquefying exchanger 164.

Since the treated gas 166 is devoid of heavy constituents, such as benzene, the crystallization temperature of which is high, it can be liquefied easily and without risk of clogging in the liquefaction exchanger 164.

To provide the frigories necessary for cooling the charging current 12 and the treated gas 166, the third method according to the invention comprises passing the nitrogen-rich hydrocarbon stream 14 through the exchanger 164 after passing through the pump 88.

For this purpose, the liquid foot stream 86 of the fractionation column 50 is pumped at a pressure greater than 20 bar, advantageously at 28 bar, to be vaporized in the liquefaction exchanger 164 and to allow the cooling of the charging stream 12 and liquefaction of the treated gas 166.

The refrigeration provided by the vaporization of the denitrogenized hydrocarbon stream 14 represents more than 90%, advantageously more than 98%, of the refrigeration necessary for the liquefaction of the feed stream 12.

Similarly, a withdrawal stream 170 is taken from the stream of nitrogen 102 after it has passed through the bottom downstream exchanger 52 and before its introduction into the third downstream heat exchanger 56. The withdrawing stream 170 is then introduced into the liquefaction exchanger 164 before being delivered in the form of a stream of auxiliary nitrogen gas 172 at the outlet of the exchanger 164.

The mass flow rate of the withdrawal fraction 170 with respect to the mass flow rate of the nitrogen-rich top stream 90 is, for example, between 0% and 50%.

The third method according to the invention also operates in a similar manner to the first method according to the invention.

In this example, the charging current 12 is a stream of natural gas in gaseous form comprising in moles 0.1000% helium, 8.9000% nitrogen, 85.9950% methane, 3.0000% ethane. , 1.0000% of propane, 0.4000% of i-C4 hydrocarbons, 0.3000% of n-C4 hydrocarbons, 0.1000% of i-C5 hydrocarbons, 0.1000% of hydrocarbons. n-C5 hydrocarbons, 0.0800% n-C6 hydrocarbons, 0.0200% benzene, 0.0050% carbon dioxide.

The liquid charging stream 68 then comprises the same composition as the LNG stream 12 described for the first and second processes according to the invention.

In the example shown on the figure 3 the molar composition of the overhead stream 90 is as follows: helium 1.19%, nitrogen 98.64% and methane 0.16%.

Examples of temperature, pressure, and mass flow rates of the different streams illustrated in the process of the Figure 3 are summarized in the tables below. CURRENT TEMPERATURE (° C) PRESSURE (Bars) FLOW (Kg / h) 12 38 40 182,700 166 - 38 35 177,470 68 - 152 34 177,470 70 - 152.8 6 177,470 76 - 159.5 6 139,733 80 - 160 6 37,779 84 -161.5 2.7 174,559 86 - 158.3 2.7 165,811 14 -157.2 28 165,811 90 - 186.7 2.6 24,896 92 -20 2.6 24,896 16 -20.7 2.5 11,083 100 38 39.7 13,813 106 - 177 9 13,813 20 -180.41 5 370 18 -179.8 5 248 114 -176.9 9 13,195 130 - 165.8 9.7 61,629 132 - 155 9.7 61,629 136 -143 19.2 61,629

The energy consumption of the process is as follows: Compressor 62A: 632 kW Compressor 62B: 388 kW Compressor 62C: 325 kW Compressor 38B 1440 kW Total: 2785 kW

A fourth installation 180 according to the invention, intended for the implementation of a fourth method according to the invention is represented on the figure 4 . This fourth installation 180 differs from the third installation 170 by the presence of two separator balloons 60, 142 as in the second installation.

Its operation is moreover analogous to that of the third installation 160.

A fifth installation 190 according to the invention is represented on the Figure 5 for the implementation of a fifth method according to the invention.

The fifth installation 190 differs from the fourth installation 180 in that the cooling cycle 30 is a semi-open cycle. For this purpose, the refrigerating fluid of the refrigeration cycle 30 is formed by a bypass stream 192 of the compressed recycled nitrogen stream 100 taken at the outlet of the upstream compression apparatus 58, at a first pressure P1 substantially equal to 40 bars.

The mass flow rate of the bypass stream 192 is less than 99% of the mass flow rate of the majority portion 96.

The bypass current 192 is introduced into the cycle heat exchanger 32 to form, at the outlet of the exchanger 32, the cooled compressed stream 136, and then after expansion in the turbine 36, the refrigeration stream 130 introduced into the upstream exchanger 28.

The refrigerating stream 130 thus has a molar nitrogen content greater than 99% and a hydrocarbon content of less than 0.1%.

After passing through the exchanger 32, the heated refrigerating stream 132 is introduced into the compressor 38A coupled to the turbine 36, then to the refrigerant 40A, before being reintroduced into the compressed recycled nitrogen stream 100, between the the penultimate stage and the last stage of the compression apparatus 58, at a second pressure P2 less than the first pressure P1.

A sixth installation 200 according to the invention is represented on the figure 6 .

The sixth installation 200 according to the invention differs from the fourth installation 180 in that the cycle exchanger 32 is constituted by the same heat exchanger as the third downstream exchanger 56.

The heated refrigerant stream 132 coming from the upstream exchanger 28 is introduced into the third downstream heat exchanger 56 where it is placed in heat exchange relation with the mixing stream 156 coming from the second downstream heat exchanger 52 and with the nitrogen stream. recycled tablet 100 from the downstream compression apparatus 58.

Similarly, the compressed refrigerant stream 134 passes into the third downstream heat exchanger 56 to be cooled before it is introduced into the dynamic expansion turbine 36.

The operation of the sixth method according to the invention is moreover analogous to that of the fourth method according to the invention.

Thanks to the processes according to the invention, it is possible to produce, in a flexible and economical manner, substantially pure nitrogen gas 16, substantially pure liquid nitrogen 18, and a helium-rich stream which can be recovered later. in a helium production plant.

The process further produces a denitrogenated hydrocarbon rich stream 14 which may be used in liquid or gaseous form.

All the fluids produced by the process are therefore usable and recoverable as such.

This process can be used indifferently with a charging stream 12 consisting of liquefied natural gas or natural gas in gaseous form.

The amount of liquid nitrogen produced by the process can be controlled in a simple manner by adjusting the thermal power taken by the second feed stream 72 into the refrigerant stream 130 of the refrigeration cycle 30.

Claims (13)

1. Process for producing a liquid nitrogen stream (18), a gaseous nitrogen stream (16), a gaseous stream (20) which is rich in helium and a denitrided stream (14) of hydrocarbons from a feed stream which contains hydrocarbons, nitrogen and helium, the process comprising the following steps:

   - expanding the feed stream (12) in order to form an expanded feed stream (70);

   - dividing the expanded feed stream (70) into a first introduction stream (72) and a second introduction stream (74);

   - cooling the first introduction stream (72) within an upstream heat exchanger (28) by heat exchange with a gaseous refrigerant stream (130) which is obtained by dynamic expansion in a cooling cycle (30) in order to obtain a first cooled introduction stream (76);

   - cooling the second introduction stream (74) by means of a first downstream heat exchanger (52) in order to form a second cooled introduction stream (80);

   - introducing the first cooled introduction stream (76) and the second cooled introduction stream (80) into a fractionating column (50) which comprises a plurality of theoretical fractionating stages;

   - tapping at least one reboiling stream (84) and circulating the reboiling stream (84) in the first downstream heat exchanger (52) in order to cool the second introduction stream (74);

   - tapping, at the bottom of the fractionating column (50), a bottom stream (86) which is intended to form the denitrided stream (14) of hydrocarbons;

   - tapping, at the head of the fractionating column (50), a head stream (90) which is rich in nitrogen;

   - reheating the head stream (90) which is rich in nitrogen by means of at least one second downstream heat exchanger (54, 56) in order to form a reheated stream (92) which is rich in nitrogen;

   - tapping and expanding a first portion (94) of the reheated stream (92) which is rich in nitrogen in order to form the gaseous nitrogen stream (16);

   - compressing a second portion (96) of the reheated stream (92) which is rich in nitrogen in order to form a compressed, recycled nitrogen stream (100), and cooling the compressed, recycled nitrogen stream (100) by means of circulation through the first downstream heat exchanger (52) and the or each second downstream heat exchanger (54, 56);

   - liquefying and partially expanding the recycled nitrogen stream (100) in order to form an expanded nitrogen rich stream (106);

   - introducing at least a portion (106; 146) obtained from the expanded nitrogen rich stream (106) into a first separation container (60);

   - recovering the gaseous head stream from the first separation container (60) in order to form the helium rich stream (20);

   - recovering the liquid stream (110) from the bottom of the first separation container (60) and separating that liquid stream (110) into a liquid nitrogen stream (18) and a first reflux stream (114);

   - introducing the first reflux stream (114) as reflux into the head of the fractionating column (50).
2. Process according to claim 1, characterised in that the whole of the expanded nitrogen rich stream (106) is introduced into the first separation container (60) directly after the expansion thereof.
3. Process according to claim 1, characterised in that the nitrogen rich expanded stream (106) is introduced into a second separation container (142) which is positioned upstream of the first separation container (60), the head stream (144) from the second separation container (142) being introduced into the first separation container (60), at least a portion of the bottom stream (148) of the second separation container (142) being introduced as reflux into the head of the fractionating column (50).
4. Process according to claim 3, characterised in that the bottom stream (148) of the second separation container is separated into a second reflux stream (150) which is introduced into the fractionating column (50) and a supplementary cooling stream (152), the supplementary cooling stream (152) being mixed with the nitrogen rich head stream (90) before it is introduced into the second downstream heat exchanger (54).
5. Process according to claim 4, characterised in that the operating pressure of the fractionating column (50) is less than 5 bar, advantageously less than 3 bar.
6. Process according to any one of the preceding claims, characterised in that the cooling cycle (30) is a closed cycle of the inverted Brayton type, the process comprising the following steps:

   • reheating the refrigerant stream (130) in a cycle heat exchanger (32) up to substantially ambient temperature;

   • compressing the reheated refrigerant stream (132) in order to form a compressed refrigerant stream (134), and refrigerant in the cycle heat exchanger (32) by means of heat exchange with the reheated refrigerant stream (132) from the first upstream heat exchanger (28) in order to form a cooled, compressed refrigerant stream (136);

   • dynamically expanding of the cooled, compressed refrigerant stream (136) in order to form the refrigerant stream (130), and introducing the refrigerant stream (130) into the first upstream heat exchanger (28).
7. Process according to claim 6, characterised in that the cycle heat exchanger (32) is formed by one (56) of the downstream heat exchangers (52, 54, 56), the compressed refrigerant stream (134) being cooled at least partially by heat exchange in the downstream heat exchanger (56) with the nitrogen rich head stream (90) from the head of the fractionating column (50).
8. Process according to any one of claims 1 to 5, characterised in that the cooling cycle (30) is a semi-open cycle, the process comprising the following steps:

   • tapping at least one fraction of the nitrogen rich recycled stream (100) which is compressed at a first pressure (P1) in order to form a tapped stream (192) which is rich in nitrogen;

   • cooling the nitrogen rich tapped stream (192) which in a cycle heat exchanger (32) in order to form a cooled, tapped stream;

   • dynamically expanding of the cooled, tapped stream from the cycle heat exchanger (32) in order to form the refrigerating stream (130), and introducing the refrigerant stream (130) into the upstream heat exchanger (28);

   • compressing the refrigerant stream (132) from the upstream heat exchanger in a compressor and re-introducing that stream into the recycled nitrogen stream (100) which is compressed at a second pressure (P2) less than the first pressure (P1).
9. Process according to any one of the preceding claims, characterised in that the feed stream (12) is a gaseous stream, the process comprising the following steps:

   • liquefying the feed stream (12) in order to form a liquid feed stream (68) by means of introduction through a liquefying heat exchanger (164);

   • vaporising the denitrided stream (14) of hydrocarbons from the bottom of the fractionating column (50) by means of heat exchange with a gaseous stream (166) which is from the feed stream (12) in the liquefying heat exchanger (164).
10. Process according to claim 9, characterised in that the cooling provided by the vaporisation of the denitrided stream (14) of hydrocarbons constitutes more than 90%, advantageously more than 98%, of the cooling necessary for liquefying the feed stream (121).
11. Installation (10; 140; 160; 180; 190; 200) for producing a liquid nitrogen stream (18), a gaseous nitrogen stream (16), a gaseous stream (20) which is rich in helium and a denitrided stream (14) of hydrocarbons from a feed stream (12) which contains hydrocarbons, nitrogen and helium, the installation comprising:

    - means (26) for expanding of the feed stream (12) in order to form an expanded feed stream (70);

    - means for dividing the expanded feed stream (70) into a first introduction stream (72) and a second introduction stream (74);

    - means (28; 30) for cooling the first introduction stream (72) comprising an upstream heat exchanger (28) and a cooling cycle (30), in order to obtain a first cooled introduction stream (76) by means of heat exchange with a gaseous refrigerant stream (130) which is obtained by dynamic expansion in the cooling cycle (30);

    - means for cooling the second introduction stream (74) comprising a first downstream heat exchanger (52) in order to form a second cooled introduction stream (80);

    - a fractionating column (50) comprising a plurality of theoretical fractionating stages;

    - means for introducing the first cooled introduction stream (76) and the second cooled introduction stream (80) into the fractionating column (50);

    - means for tapping at least one reboiling stream (84) and means for circulating the reboiling stream (84) in the first downstream heat exchanger (52) in order to cool the second introduction stream (74);

    - means for tapping, at the bottom of the fractionating column (50), a bottom stream (86) which is intended to form the denitrided stream (14) of hydrocarbons;

    - means for tapping, at the head of the fractionating column (50), an head stream (90) which is rich in nitrogen;

    - means for reheating the nitrogen rich head stream (90) comprising at least a second downstream heat exchanger (54, 56) in order to form a reheated stream (92) which is rich in nitrogen;

    - means for tapping and expanding a first portion (94) of the nitrogen rich reheated stream (92) in order to form the gaseous nitrogen stream (16);

    - means (58) for compressing a second portion (96) of the nitrogen rich reheated stream (92) in order to form a recycled nitrogen stream (100) and means for cooling the compressed, recycled nitrogen stream (100) by means of circulation through the first downstream heat exchanger (52) and the or each second downstream heat exchanger (54, 56);

    - means (104) for partially liquefying and expanding the recycled nitrogen stream (100) in order to form an expanded nitrogen rich stream (106);

    - a first separation container (60);

    - means for introducing at least a portion obtained from the expanded nitrogen rich stream (106) into the first separation container (60);

    - means for recovering the gaseous head stream from the first separation container (60) in order to form the helium rich stream (20);

    - means for recovering the liquid stream (110) from the bottom of the first separation container (60) and for separating that stream into a liquid nitrogen stream (112) and a first reflux stream (114);

    - means for introducing the first reflux stream (114) as reflux into the head of the fractionating column (50).
12. Installation (10; 160) according to claim 11, characterised in that it comprises means for introducing the whole of the expanded nitrogen rich stream (106) into the first separation container (60).
13. Installation (140; 180; 190; 200) according to claim 11, characterised in that it comprises a second separation container (142) which is positioned upstream of the first separation container (60) and means for introducing the expanded nitrogen rich stream (106) into the second separation container (142), the installation comprising means for introducing the head stream (144) from the second separation container (142) into the first separation container (60) and means for introducing at least a portion of the bottom stream (148) of the second separation container (142) as reflux into the head of the fractionating column (50).

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