EA035250B1 - Process for removing nitrogen from high-flow natural gas - Google Patents

Abstract

Process for separating the components of a gas mixture (1) to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, comprising the following steps: a) introduction of a stream of said gas mixture (1) to be treated at a flow rate greater than or equal to 2000000 Nm/h in a demethanization unit comprising N demethanization columns (7); b) partial condensation of a gas mixture (15), comprising less than 1 mol% of hydrocarbons having at least two carbon atoms, extracted from the demethanization unit (7) in order to obtain a liquid (29), at least one portion of which is treated in order to be extracted as denitrogenated natural gas product (30) and a second gas (26); c) introduction of said second gas (26) into a nitrogen removal unit (31) comprising N nitrogen removal columns, obtained from which are a gas (36) and a liquid (33), at least one portion of which is treated in order to be extracted as denitrogenated natural gas product (30); characterized in that said denitrogenated natural gas (30) thus produced comprises less than 5 mol% of nitrogen and in that N is greater than or equal to 3.

Description

The present invention relates to a method for separating components of a gas mixture containing methane, nitrogen and hydrocarbons heavier than methane.

In this regard, the present invention is applied to methods for removing nitrogen from natural gas with or without helium recovery.

Natural gas is in demand for use as a fuel intended for use in heating buildings, for providing heat for industrial methods of generating electricity, for use as a raw material for various synthetic methods for producing olefins, polymers and the like.

Natural gas is found in many fields remote from natural gas users. Natural gas usually consists of methane (C1), ethane (C2) and heavier compounds, such as hydrocarbons having at least three carbon atoms, such as propane, butane, etc. (C3 +).

It can often be beneficial to separate C2 and C3 + compounds from natural gas in order to sell them as separate by-products.

Specifically, their commercial use is generally wider than that of natural gas itself, since they can be used directly for chemical processes (for example, the production of ethylene from ethane), as motor fuels (C3 / C4 is a well-known motor fuel called LPG (GPL)) or for many other applications.

Another component often found in natural gas is nitrogen. The presence of nitrogen in natural gas can lead to difficulties in meeting the requirements for natural gas (usually the required minimum net calorific value).

Even more correct is the removal of hydrocarbons heavier than methane (C2 and C3 +), since they have a higher value of lower calorific value than methane, therefore, when they are removed, the value of lower calorific value is reduced, and then it should potentially increase by separation of nitrogen . Therefore, some efforts have been made to create means for removing nitrogen present in natural gas.

The exploited natural gas deposits contain increasing amounts of nitrogen. This is noticeable, since deposits that are rich enough so that processing for enrichment is not required before gas is put into commercial circulation are depleted and become increasingly rare.

These natural gas sources often also contain helium. The latter can be introduced into commercial consumption by pre-concentration before final processing and liquefaction.

Unconventional resources, such as shale gases, also share the same set of problems: in order to make them commercially viable, it may be necessary to increase their calorific value by treatment involving the removal of nitrogen from the gas.

The most widely used method for the separation of nitrogen and hydrocarbons heavier than methane is cryogenic separation. A cryogenic nitrogen separation method, more specifically a double column method, is described in US-A-4,778,498. Installations for the removal of nitrogen from natural gas usually process gases originating directly from wells at high pressure. After nitrogen has been removed, the treated gas must be returned to the network, often at a pressure close to the pressure under which it entered.

When exploiting natural gas deposits, many stages can be envisaged. One relatively common step after drying and removing impurities is the separation of liquids associated with natural gas (NGL).

This stage can have many advantages, but often the advantage is the commercialization of various heavy hydrocarbon products containing at least two carbon atoms (C2, C3, etc.), which are usually sold significantly more expensive than the natural gas product .

If natural gas contains nitrogen, there is a risk of producing natural gas with too low a calorific value due to the obtained low content of C2, C3, etc. Therefore, there is usually a need then to separate nitrogen from this gas to make it suitable for sale.

One common solution is to deal with two problems independently.

The first unit separates NGL (hereinafter referred to as the NGL unit), while the second unit separates nitrogen from natural gas (hereinafter referred to as the NRU unit).

The advantage of this solution is operational flexibility. For example, if the NRU installation includes a cooling cycle, the associated machines have limited reliability, and a failure in the compressor of the cycle will turn off the NRU, but will not turn off the NGL.

Unfortunately, this shutdown cannot be long, because then it would be necessary to burn products (due to its excessively low calorific value). In addition, this scheme is limited in terms of efficiency, since all gas is cooled and then heated again in the NGL unit, then cooled and heated again in the NRU.

Another solution may be to fully combine NGL and NRU installations, in this case

- 1 035250 there is a problem that in the event of a malfunction in the cooling cycle of the NRU installation, it will be necessary to immediately turn off the entire unit.

Therefore, the authors of the present invention have developed a solution to eliminate the above problems.

An object of the present invention is a method for separating the components of a gas mixture to be processed containing methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of such hydrocarbons, comprising the following steps:

a) introducing a stream of said gas mixture to be treated, at a flow rate of 2,000,000 normal m ³ / h or more, into a methane removal unit containing N methane removal columns;

b) partial condensation of a gas mixture containing less than 1 mol.% hydrocarbons having at least two carbon atoms extracted from a methane removal unit to obtain a liquid, at least one part of which is treated to extract as a product of de-nitrated natural gas, and a second gas;

c) introducing said second gas into a nitrogen removal unit comprising N nitrogen removal columns from which gas and liquid are obtained, at least one part of which is treated to recover de-nitrated natural gas as a product;

characterized in that said de-nitrated natural gas thus obtained contains less than 5 mol% of nitrogen, and that N is 3 or more.

In other embodiments, the method of the present invention includes at least the following features.

The method defined above is characterized in that it comprises an additional step

d) treating said gas (36) from step c) in a second nitrogen treatment unit (B) to obtain a nitrogen gas stream containing not more than 2 mol% methane and a methane gas stream containing not more than 5 mol% nitrogen.

The method defined above is characterized in that the second installation (B) for nitrogen removal contains no more than N-1 columns for nitrogen removal.

The method defined above is characterized in that N is 6 or more.

The method defined above is characterized in that the second nitrogen removal unit (B) comprises from N-5 nitrogen removal columns to N-1 nitrogen removal columns, preferably from N-4 to N-2 nitrogen removal columns.

The method defined above is characterized in that steps b) and c) are carried out at a temperature below −50 ° C. and the fluid is not reheated above −50 ° C. between step b) and step c).

The invention will be described in more detail with reference to the drawing, illustrating the method of the present invention.

Stream 1 of natural gas, pre-treated (separation of water, CO ₂ , methanol, very heavy hydrocarbons, that is, having more than six or seven carbon atoms (such as, for example, C8 ⁺ hydrocarbons), containing at least 30 mol.% Methane, 0.1 mol% of hydrocarbons heavier than methane (that is, containing at least two carbon atoms) and 4 mol% of nitrogen are introduced into system 2, which provides at least partial condensation of said stream 1.

The pressure of this stream 1 is from 20 bar abs. (bar absolute pressure) up to 100 bar abs. (usually from 30 to 70 bar abs.), and the temperature is close to the ambient temperature, for example from 10 to 30 ° C.

System 2 is, for example, a heat exchanger. Mixture 3 emerging from this system 2 is in a two-phase (gas and liquid) state. This mixture 3 is introduced into a phase separation vessel 4.

The operating pressure is from 20 to 100 bar abs., Usually from 30 to 70 bar abs. The temperature of this vessel is from -100 to 0 ° C, as a rule, from -80 to -20 ° C.

The liquid phase 5 from the separation vessel 4 expands through the valve 6, then is pumped under pressure from 10 to 40 bar abs. and at a temperature, for example, from -110 to -30 ° C, to column 7 for methane removal.

By a methane removal column is meant a distillation unit designed to receive at least two streams of various compositions from feed streams originating from stream 1 of natural gas to be treated according to the method of the present invention.

At least two streams are the following: one gaseous, depleted in hydrocarbons having at least two carbon atoms, i.e. containing less than half of the heavy hydrocarbons contained in the feed gas (ethane, propane, butane, etc.), and the other in liquid form, containing less than 5 mol.% methane, originally present in stream 1 of natural gas to be processed.

A methane removal unit is understood to mean any system comprising at least one distillation column for methane enrichment of gas from the upper part and methane depletion of bottoms liquid.

At least one part of the gas phase 8 (usually only one part) from the separation vessel 4 is expanded by means of a turbine 9.

- 2 035250

The stream from the turbine 9 is introduced into the column 7 at a higher stage 10 compared to the stage at which the liquid 5 exiting the valve 6 is introduced.

A liquid stream 12 from heavier hydrocarbons than methane is recovered at the bottom 16 of column 7.

The reboiler 11 is placed at a level allowing re-boiling the bottoms liquid from the column 7 to reheat a portion of the liquid of the column to regulate the maximum limit of methane contained in the heavy hydrocarbon stream 12.

At least 50 mol% (typically at least 85 mol%) of the heavy hydrocarbons present in the gas mixture 1 to be treated are recovered in this stream 12. Preferably, at least 90% is recovered.

Preferably, the liquid hydrocarbon stream 12 does not contain more than 1 mol.% Methane.

To reheat the bottom of the column 7 (bottom = below the fluid inlet from the vessel 4), a heat exchanger 13 can be installed. This heat exchanger is fed with a gaseous feed stream 1. This reheating improves the balance between finding the maximum efficiency and purity of the stream leaving the column 7 to remove methane.

At the top of column 14 of column 7 (top = topmost column outlet) a methane-rich gas stream of 15 is recovered, typically containing less than 0.5 mol% of hydrocarbons having more than two carbon atoms (containing not more than half of the amount of heavy hydrocarbons having more than 2 carbon atoms present in the feed gas). The temperature of the gas stream 15 is below -80 ° C.

Therefore, cold can be returned by condensation of the methane-enriched gas under pressure. This condensation is carried out using a heat exchanger 17, fed as part of the gas stream 8 from the separation vessel 4, and a methane-rich gas stream 15 from the top 14 of the column 7 for methane removal.

This is just one example of the implementation of the method, which is the object of the present invention. But according to a particular embodiment of the present invention, a third stream to be condensed can be introduced into this heat exchanger.

According to yet another embodiment of the present invention, only one of the two described streams is subject to condensation.

Methane-enriched gas means a mixture of gases containing methane, nitrogen and, as a rule, less than 0.5% of hydrocarbons having more than two carbon atoms (containing no more than half of the amount of heavy hydrocarbons having more than 2 carbon atoms present in the feed gas) .

The stream (s) 18 (18a and 18b), cooled (cooled) in the heat exchanger 17, is expanded by, for example, at least one valve 19 (19a, 19b), then introduced into the upper part (upper part = above the feed 10 leaving from the turbine 9) columns 7.

Stream 20, reheated in heat exchanger 17, contains no more than half of the amount of heavy hydrocarbons having more than 2 carbon atoms present in the feed gas.

The gas stream 20, reheated in the heat exchanger 17 to a temperature of from -40 to -70 ° C, preferably of the order of -60 ° C, is then partially condensed by, for example, heat exchanger 21.

At the outlet of this heat exchanger 21, a two-phase (gas-liquid) stream 22 (containing from 20 to 80 mol.% Gas) exits.

Alternatively, the previous step can be dispensed with, that is, the passage of the stream 15, extracted from the top of the methane removal column 7, into the heat exchanger 17.

Therefore, it is possible to keep the temperature of stream 15 below -80 ° C (or even below -100 ° C) and introducing said stream 15 directly into heat exchanger 21 to produce stream 22.

Stream 22 is then directed to system A to remove nitrogen according to the invention described below.

In system A, to remove nitrogen, a two-phase stream 22, after possible expansion in a valve or turbine 23, is introduced into a phase separation vessel 25.

The liquid phase 29, obtained from the phase separation vessel 25, after possible expansion in the valve (not shown in the figure) is reheated using heat exchangers 27, then 21 and finally 2 to return methane-rich gas obtained at the end of the process to the effluent 30.

Exit stream 30 contains less than 5 mol.% Nitrogen.

The gas phase 26 obtained from the separation vessel 25 is partially condensed in the heat exchanger 27, then expanded after leaving the heat exchanger 27 by means of a turbine or valve before entering the distillation column 31.

The distillation column 31 is a nitrogen stripper, the purpose of which is to separate nitrogen from the methane-rich effluent, also called a nitrogen stripper.

The methane-enriched liquid contains less than 5 mol.% Nitrogen. Here the question arises that the distillation column is connected to the reboiler 32, but does not have an associated condensation system.

At the bottom of column 31, at a temperature below −100 ° C., preferably below −110 ° C., a very methane-rich stream 33 is recovered in liquid form. This stream 33 contains less than 5 mol% of nitrogen, preferably 3,035,250 but less than 4%. The liquid stream 33 is then mixed with the liquid phase 29 obtained from the phase separation vessel 25, and follows the same path to the effluent stream 30.

Part 32 of the mixed stream containing partially liquid phase 29 and liquid 33 and reheated with heat exchanger 27 is recycled to the lower part 34 of the nitrogen removal column 31.

At the top of column 35, a nitrogen-rich gas stream 36 is formed at temperatures below -110 ° C. Said nitrogen rich stream 36 contains at least 20 mol% of nitrogen.

The nitrogen-rich stream 36 is heated by sequentially arranged heat exchangers 27, 21, then 2. They can be the same heat exchanger according to one particular embodiment of the present invention. And according to another specific embodiment of the present invention, more than three heat exchangers can be used.

This results in a stream 37 at a temperature close to ambient temperature (typically above -10 ° C and below 50 ° C), sent to additional system B to remove nitrogen.

The purpose of system B for nitrogen removal is to produce a gas stream that is even richer in nitrogen than stream 37.

This system B may include, for example, at least one separation vessel and a nitrogen removal column. If the nitrogen requirements at the outlet of system B are strict (typically <100 ppm), it may be necessary to add a cycle compressor, such as a nitrogen compressor, to system B to ensure the reflux required to obtain nitrogen purity at the top of the nitrogen removal column B. systems

The method, which is the object of the present invention, makes it possible that there is no need to burn gas in the failure mode of the cooling cycle of the system for nitrogen removal (failure in the operation of the cycle compressor);

improving the efficiency of the method.

Specifically, if a malfunction occurs in system B for nitrogen removal, it will still be possible to continue the process and produce a significant part, usually at least 80% of the target products (de-nitrated methane), thanks to system A for nitrogen removal.

This is because the proposed solution should partially combine the nitrogen removal system with the system for extracting products originating from part of NGL. This partial combination consists in integrating at least the first separation vessel after the NGL methane removal column. At least one part of the natural gas product in liquid form will be recovered from this first separation vessel. This product will be, at least in part, de-nitrated, which in certain cases can satisfy the requirements for the calorific value of the product. In addition to this first vessel, a first nitrogen removal column can be integrated into the NGL portion, this allows an increase in the proportion of product that meets the requirements that is directly generated using the nitrogen removal system.

By the expression part of NGL is meant all the steps of the method of the present invention, the preceding steps c).

Then the last part remains, providing for very cold temperatures, in which the attainable temperature levels are below -140 ° C, preferably below -160 ° C, in which a cooling cycle can be applied if necessary.

Then a malfunction in the cooling cycle will lead to a shutdown of nitrogen removal, but it will be possible to maintain part of the production of de-nitrided natural gas, as well as the production of products originating from part of NGL.

In addition, the application of the methods of the present invention allows, in addition to improving the reliability of the installation, to optimize the total capital costs by optimizing the number of elements that make up the various installations for the implementation of this method in relation to the flow rate of the incoming stream in each installation.

More specifically, it will not be necessary to add as many systems B for nitrogen removal as systems A for nitrogen removal.

In the case of very large flow rates for the NGL part, one or several units are necessary, while the use of at least one unit in the NRU part than in the NGL part (for example, seven units for the NGL part and three units for the NRU part) allows In addition to improving the reliability of the installation, optimize the total capital cost by optimizing the number of units relative to the flow rate entering each installation.

The unit is a processing unit, containing, as a rule, a separate unit of equipment for each function in this method (one turbine 9, one separation vessel 4, etc.).

On the other hand, you can specifically double the number of one or more pieces of equipment within the same unit. If the flow rate is too large, it is necessary to use several identical units, that is, several processing units operating in parallel and with the same units of equipment.

Claims (4)

1. The method of separation of the components of a mixture (1) of gases to be treated, containing methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of such hydrocarbons, comprising the following stages: a) introducing a stream of said mixture (1) of gases to be treated, at a flow rate of 2,000,000 normal m ³ / h or more, into a methane removal unit containing N columns (7) for methane removal; b) partial condensation of a gas mixture (15) containing less than 1 mol% of hydrocarbons having at least two carbon atoms extracted from a methane removal unit (7) to obtain a liquid (29), at least one part of which is recovered in the form of a product (30) of de-nitrated natural gas, and a second gas (26); c) introducing said second gas (26) into a nitrogen removal unit (31) containing N nitrogen removal columns from which gas (36) and a liquid (33) are obtained, at least one part of which is mixed with said liquid (29) ) or at least one part thereof and is recovered as a product (30) of de-nitrated natural gas; wherein said de-nitrated natural gas (30) thus obtained contains less than 5 mol% of nitrogen, and wherein N is 3 or more; d) treating said gas (36) from step c) in a second nitrogen treatment unit (B) to obtain a nitrogen gas stream containing not more than 2 mol% methane and a methane gas stream containing not more than 5 mol% nitrogen, where the second installation (B) for nitrogen removal contains no more than N-1 columns for nitrogen removal. 2. The method according to claim 1, where N is 6 or more. 3. The method according to the preceding paragraph, where the second installation (B) for nitrogen removal contains from N-5 columns for nitrogen removal to N-1 columns for nitrogen removal, preferably from N-4 to N-2 columns for nitrogen removal. 4. The method according to any one of the preceding paragraphs, where stages b) and c) are carried out at a temperature below -50 ° C and the fluid is not reheated above -50 ° C between stage b) and stage c).