FR3081047A1 - PROCESS FOR EXTRACTING NITROGEN FROM A NATURAL GAS CURRENT - Google Patents

Abstract

A method of extracting nitrogen from a natural gas feed stream comprising at least nitrogen, methane, CO2 and / or H2S, comprising the following steps: Step a): introducing the stream feed gas in a refrigeration unit comprising at least one main exchanger, wherein said gas stream is condensed at least partially; Step b): introducing the gas stream from step a) in a phase separator means at a pressure P to produce a gas stream and a liquid stream; Step c): the gas stream from step b) is introduced at a pressure P1 into a cryogenic separation means comprising at least one distillation column for separating the nitrogen from the said gas stream; Step d): a liquid stream enriched with CH4 obtained from the cryogenic separation is recovered, characterized in that P is greater than P1 which is greater than 20 bar absolute and in that in step d), said stream is recovered. CH4-enriched liquid resulting from the cryogenic separation by pumping the bottom product of said column and / or pumping said liquid stream resulting from step b) to a pressure P2 greater than 25 bar absolute and preferably greater than the critical pressure of said product.

Description

© PROCESS FOR EXTRACTING NITROGEN FROM A NATURAL GAS STREAM

FR 3 081 047 - A1 (57) Process for extracting nitrogen from a natural gas feed stream comprising at least nitrogen, methane, CO _{> 2} and / or H _{> 2} S, comprising the following steps:

Step a): introduction of the supply gas stream into a refrigeration unit comprising at least one main exchanger, in which said gas stream is at least partially condensed;

Step b): the gas stream from step a) is introduced into a phase separator means at a pressure P to produce a gas stream and a liquid stream;

Step c): the gas stream from step b) is introduced at a pressure P1 into a cryogenic separation means comprising at least one distillation column for separating the nitrogen from said gas stream;

Step d): a liquid stream enriched in CH _{> 4} from the cryogenic separation is recovered, characterized in that P is greater than P1 which is greater than 20 bar absolute and in that in step d), it is recovered said liquid stream enriched in CH _{> 4} coming from the cryogenic separation by pumping the tank product from said column and / or pumping said liquid stream coming from step b) at a pressure P2 greater than 25 bar absolute and preferably greater than the critical pressure of said product.

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DESCRIPTION

Title: Process for extracting nitrogen from a natural gas stream The subject of the invention is a process for extracting nitrogen from a natural gas feed stream comprising at least nitrogen, methane , CO2 and HLS.

Raw natural gas can contain a large number of annoying impurities to be removed. Nitrogen is an example. From a certain concentration of nitrogen in natural gas, it is typically not salable because of its low calorific value or simply because of a limitation of the quantity of inert gases, of which is a part nitrogen in natural gas. To remove nitrogen, we most often use a cryogenic process implemented in a unit called the nitrogen rejection unit [in French: Nitrogen Rejection Unit (NRU)].

In certain situations, natural gas contains acid gases such as CO2 and / or IHLS. These acid gases are generally extracted from natural gas during a pretreatment stage upstream of the nitrogen rejection unit.

The document US Pat. CO2 in particular, then finally a cryogenic separation to remove nitrogen and retain only methane.

The documents US3989478 and FR2917489 describe cryogenic systems for the purification of a stream rich in methane. Both of these systems use an adsorption system to knock down the CO2 before the liquefaction stage.

Usually the nitrogen discharge units contain several distillation columns in order to optimize the energy consumption.

Nitrogen discharge units are often based, for example, on so-called double column systems. In this type of system, part of the distillation is carried out at low pressure and low temperatures. The problem with these low temperatures is that acid gases such as CO2 and / or H2S can freeze in the equipment if they are not removed. This is the reason why these acid gases are removed during an upstream pretreatment step. Some nitrogen release units are tolerant of a certain CO2 content but are then limited to a content not exceeding a few hundred ppm.

One of the problems which the invention therefore intends to solve is that of providing a process for extracting nitrogen from a stream of natural gas containing high contents of acid gases by exempting itself from a pre-treatment step by upstream.

The inventors of the present invention then developed a solution which makes it possible to solve the problems raised above.

The subject of the present invention is a process for extracting nitrogen from a natural gas feed stream comprising at least nitrogen, methane, CO2 and / or HLS, comprising the following steps:

Step a): introduction of the feed gas stream into a refrigeration unit comprising at least one main exchanger, in which said gas stream is at least partially condensed;

Step b): the gas stream from step a) is introduced into a phase separator means at a pressure P to produce a gas stream and a liquid stream;

Step c): the gas stream from step b) is introduced at a pressure P1 into a cryogenic separation means comprising at least one distillation column for separating the nitrogen from said gas stream;

Step d): a liquid stream enriched in CHU from the cryogenic separation is recovered, characterized in that P is greater than P1 which is greater than 20 bar absolute and in that in step d), said stream is recovered liquid enriched in CHU from cryogenic separation by pumping the bottom product from said column and / or pumping said liquid stream from step b) at a pressure P2 greater than 25 bar absolute and preferably greater than the critical pressure of said product.

The solution object of the present invention is therefore not to further reduce the content of CO2 and / or H2S in the gas to be treated while ensuring sufficient solubility of CO2 and / or H2S in the gas to be treated (mainly methane ) to avoid crystallization at any point in the process.

The upstream pretreatment step (for example TSA) to mainly cut down the CO2 is therefore withdrawn.

According to other embodiments, the invention also relates to:

A process as defined above, characterized in that the liquid stream from step c) is pumped and then mixed with the liquid stream from step b), said mixture thus obtained is then pumped at a pressure P2 greater than 25 bars absolute and preferably higher than the critical pressure of said mixture.

A process as defined above, characterized in that the liquid stream from step b) and the liquid stream from step c) are pumped independently of each other at a pressure P2 greater than 25 bar absolute and preferably greater than the critical pressure of each liquid product, then these streams are introduced into the main exchanger and mixed downstream of said main heat exchanger.

A process as defined above, characterized in that the implementation temperature of step b) is higher than the freezing temperature of CO2 and / or H2S of the composition of the gas stream, at pressure P

A method as defined above, characterized in that said feed stream comprises at least 0.1 mol% of CO2 and / or at least 0.1 mol% of H2S.

A process as defined above, characterized in that the feed stream does not undergo a pre-treatment step intended to reduce the molar concentration of CO2 and H2S below 0.1 mol%.

A process as defined above, characterized in that the pressure P2 is greater than 40 bar absolute.

A method as defined above, characterized in that said refrigeration unit is supplied by an external refrigeration cycle in which a refrigerant circulates in a closed loop.

A process as defined above, characterized in that said refrigeration cycle comprises the following steps:

• compression of the feed gas; then • cooling of said compressed gas in the main heat exchanger;

• expansion of at least part of said cooled gas;

• spraying of the previously relaxed product.

A process as defined above, characterized in that said cooling fluid is chosen from methane, nitrogen, a mixture of the two or hydrocarbons having more than two carbon atoms.

The heat exchanger can be any heat exchanger, any unit or other arrangement adapted to allow the passage of a certain number of flows, and thus allow a direct or indirect heat exchange between one or more lines of refrigerant, and a or more feed streams.

Advantageously, the cryogenic separation unit only contains a high pressure distillation column. Typically the pressure is greater than 20 bar absolute. Consequently, the frigories necessary for the condensation of the gas stream to be treated must be provided by an external refrigeration cycle integrated into the installation allowing the implementation of the process which is the subject of the present invention. The pressure and temperature conditions of this refrigeration cycle are determined in order to optimize the operating conditions of the main heat exchanger and according to the specifications of the products used.

More specifically, the refrigeration cycle is an external refrigeration cycle consisting of the following stages:

• Compression of the gas to be treated;

• Cooling in a heat exchanger;

• At least part of the cooled gas is at least partially condensed in indirect exchange with the reboiling of the distillation column (the output pressure of the cycle compressor is chosen in order to be able to carry out this condensation while minimizing the temperature difference in this exchanger);

• At least part of the cooled gas that has been condensed is expanded in a valve and then vaporized in indirect exchange with the condenser of the distillation column (the inlet pressure of the cycle compressor is chosen in order to be able to carry out this vaporization while minimizing the temperature difference in this exchanger).

The fluid may for example be nitrogen at a supercritical pressure at the outlet of the compressor. In such a case, the indirect exchange cooling with the reboiling of the column is not really a condensation because there is no longer any phase change under these supercritical conditions. In such a case, it is necessary to understand a simple cooling implying a significant change in density (at least 5%).

Alternatively, the refrigeration cycle could as well be opened (i.e. one of the products is used as the circulating fluid in the refrigeration cycle).

For example, a refrigeration cycle with residual nitrogen from the head of the column could be envisaged.

The invention will be described in more detail with reference to [Fig. 1], [Fig. 1] illustrates a particular embodiment of a method according to the invention implemented by an installation as shown schematically.

The same reference designates a liquid flow and the pipe which conveys it, the pressures considered are absolute pressures and the percentages considered are molar percentages.

In [Fig. 1], The installation includes a source of natural gas 1, comprising at least methane, nitrogen, CO2 and / or IHLS. Typically the stream of natural gas from this source 1 comprises at least 50 mol% of nitrogen and at least 20 mol% of methane.

The stream 3 of natural gas 1 is introduced into a compressor 2. The stream 3 thus compressed is then introduced into a heat exchanger 4. Typically the stream 3 is compressed in the compressor 2 at a pressure greater than 20 bar absolute. The stream 3 is then cooled 5 in the heat exchanger 4 to a temperature between - 50 ° C and -100 ° C.

The stream 5 thus cooled is introduced into a means 6 for separating the liquid / gas phases. Beforehand, the current 5 may have undergone expansion in an expansion means 7, typically a valve. The phase separator means generates two streams, one gaseous 8 and the other liquid 9. The gaseous stream 8 is enriched in nitrogen and methane while the liquid stream 9 is depleted in nitrogen and methane but enriched in more products. heavy, such as hydrocarbons having at least two carbon atoms and CO2 and IHLS.

The gas stream 8 is introduced at an intermediate level 13 into a distillation column 10 composed of trays (or structured or unstructured packing) located between one end located in the tank 11 and another end located at the head 12. Said column 10 comprises a condenser 16 and a reboiling means 17. The current 8 is introduced into the column 10 at a pressure P1 greater than 20 bar absolute.

At the head 12 of column 10, a stream 14 comprising at least 95 mol% of nitrogen and less than 5 mol% of methane, preferably at least 99 mol% of nitrogen and less than 1%, is extracted at a temperature T1 methane molar. This stream 14 does not include CO2, HHL, or heavier hydrocarbons than methane. Typically T1 is between -120 ° C and -160 ° C.

Part 15 of stream 14 is introduced into condenser 16 then is recycled 18 to the head 12 of column 10. The other part 19 of stream 14 which is not introduced into condenser 16 is introduced into heat exchanger 4 in order to be heated 20. This current 20 is introduced into an expansion means 21. The current 20 then undergoes an expansion of at least 10 bar absolute. The stream 22 thus expanded is introduced into the heat exchanger 4 in order to be reheated. The stream 23 obtained contains at least 95 mol% of nitrogen, preferably at least 99 mol% of nitrogen

In tank 11 of column 10, a stream 24 comprising at least 80 mol% of methane, at most 10 mol% of N2, and enriched in CO2 and H2S is extracted at a temperature T2. Typically T2 is between -120 ° C and - 85 ° C. Part 25 of this stream 24 is pumped by means of a pump 26 at a pressure P2 greater than 25 bar absolute and preferably greater than the critical pressure of said product. Typically P2 is greater than 40 bars absolute, even 50 bars absolute. The current 25 is introduced into the heat exchanger 4 in order to be reheated 27.

The liquid stream 9 is pumped using a pump 28 at a pressure P3 greater than 25 bar absolute and preferably greater than the critical pressure of said product. Typically P3 is greater than 40 bar absolute, or even 50 bar absolute. Advantageously P3 is identical to P2. The stream 9 thus pumped is introduced into the heat exchanger 4 in order to be reheated 29. The streams 27 and 29 are mixed, the mixture 30 comprises at least 50 mol% of methane and is enriched in CO2 and H2S.

Alternatively, the streams extracted both from the bottom of the column 10 and from the liquid phase from the phase separator means 6 can be first mixed and then pumped once mixed before being introduced into the heat exchanger. heat 4. This alternative has the advantage of minimizing the number of rotating machines (one pump instead of two).

The process which is the subject of the present invention makes it possible to treat natural gas streams of different qualities having more or less nitrogen as well as several thousand molar ppm of acid gas, or even more than 1% mol of acid gas.

Furthermore, the process which is the subject of the present invention makes it possible to treat streams having different compositions, that is to say the composition of which can vary.

The methane produced by the process which is the subject of the present invention can be pumped at very high pressure in order to respect the pressure of the gas pipeline into which it will be introduced, while avoiding the addition of a compressor.

By way of example, the following [Table 1] illustrates an implementation of the method according to the invention.

[Table 1]

Current NOT 3 5 8 9 25 30 14 18 20 22 23 Temperature Deg C 55.00 -95.00 -95.00 -95.00 -107.32 28.91 -150.19 -160.00 45.00 -84.62 50.09 Pressure barabs 30.00 29.40 29.40 29.40 28.54 75.40 28.35 28.20 27.75 2.78 2.18 Molar flow Nm3 / h 132304 132304 116154 16150 33897 50047 253257 171000 82257 82257 82257 n ₂ mol% 64.8596 64.8596 73.0542 5.9236% 10.0000 8.6845 99.0379% 99.0379% 99037 99.0379% 99.0379 CO ₂ mol% 0.6926 0.6926 0.4496 2.4398% 1.5407 1.8309 0.0000% 0.0000% 0.0000 0.0000% 0.0000 H ₂ S mol% 0.4728 0.4728 0.1004 3.1514% 0.3441 1.2500 0.0000% 0.0000% 0.0000 0.0000% 0.0000 Methane mol% 24.0561 24.0561 25.1004 16.5452% 83.6765 62.0131 0.9621% 0.9621% 0.9621 0.9621% 0.9621 Ethane mol% 4.0725 4.0725 1.1832 24.8524% 4.0543 10.7659 0.0000% 0.0000% 0.0000 0.0000% 0.0000 Propane mol% 2.8665 2.8665 0.1045 22.7311% 0.3581 7.5779 0.0000% 0.0000% 0.0000 0.0000% 0.0000 i-Butane mol% 0.4708 0.4708 0.0034 3.8329% 0.0115 1.2447 0.0000% 0.0000% 0.0000 0.0000% 0.0000 n-Butane mol% 1.1047 1.1047 0.0039 9.0219% 0.0132 2.9203 0.0000% 0.0000% 0.0000 0.0000% 0.0000 i-Pentane mol% 0.3179 0.3179 0.0002 2.6029% 0.0008 0.8405 0.0000% 0.0000% 0.0000 0.0000% 0.0000 n-Pentane mol% 0.3544 0.3544 0.0002 2.9019% 0.0006 0.9369 0.0000% 0.0000% 0.0000 0.0000% 0.0000 n-Hexane mol% 0.7321 0.7321 0.0000 5.9970% 0.0001 1.9353 0.0000% 0.0000% 0.0000 0.0000% 0.0000

Claims (10)

1. Method for extracting nitrogen from a natural gas feed stream comprising at least nitrogen, methane, CO2 and / or IHLS, comprising the following steps: Step a): introduction of the feed gas stream into a refrigeration unit comprising at least one main exchanger, in which said gas stream is at least partially condensed; Step b): the gas stream from step a) is introduced into a phase separator means at a pressure P to produce a gas stream and a liquid stream; Step c): the gas stream from step b) is introduced at a pressure P1 into a cryogenic separation means comprising at least one distillation column for separating the nitrogen from said gas stream; Step d): a liquid stream enriched in CHU from the cryogenic separation is recovered, characterized in that P is greater than P1 which is greater than 20 bar absolute and in that in step d), said stream is recovered liquid enriched in CHU from cryogenic separation by pumping the bottom product from said column and / or pumping said liquid stream from step b) at a pressure P2 greater than 25 bar absolute and preferably greater than the critical pressure of said product. 2. Method according to the preceding claim, characterized in that the liquid stream from step c) is pumped and then mixed with the liquid stream from step b), said mixture thus obtained is then pumped at a higher pressure P2 at 25 bar absolute and preferably higher than the critical pressure of said mixture. 3. Method according to claim 1, characterized in that the liquid stream from step b) and the liquid stream from step c) are pumped independently of each other at a pressure P2 greater than 25 bars absolute and preferably greater than the critical pressure of each liquid product, then these streams are introduced into the main exchanger and mixed downstream of said main heat exchanger. 4. Method according to one of the preceding claims, characterized in that the implementation temperature of step b) is higher than the freezing temperature of CO2 and / or H2S of the composition of the gas stream, at the pressure P. 5. Method according to one of the preceding claims, characterized in that said feed stream comprises at least 0.1 mol% of CO2 and / or at least 0.1 mol% of H2S. 6. Method according to one of the preceding claims, characterized in that the feed stream does not undergo a pre-treatment step intended to reduce the molar concentration of CO2 and H2S below 0.1 mol%. 7. Method according to one of the preceding claims, characterized in that the pressure P2 is greater than 40 bar absolute. 8. Method according to one of the preceding claims, characterized in that said refrigeration unit is supplied by an external refrigeration cycle in which a refrigerant circulates in a closed loop. 9. Method according to the preceding claim, characterized in that said refrigeration cycle comprises the following steps: - compression of the feed gas; then - cooling of said compressed gas in the main heat exchanger; - expansion of at least part of said cooled gas; - spraying of the previously relaxed product. 10. Method according to one of claims 8 and 9, characterized in that said refrigerant is chosen from methane, nitrogen, a mixture of the two or hydrocarbons having more than two carbon atoms.