RU2607198C2 - Method and apparatus for removing nitrogen from cryogenic hydrocarbon composition - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to method and device for removing nitrogen from cryogenic hydrocarbon composition. At least a first portion of a cryogenic hydrocarbon composition is fed to a nitrogen stripper column as a first nitrogen stripper feed stream. Nitrogen-stripped liquid is drawn from nitrogen stripper column. Liquid hydrocarbon product stream and process vapour are produced comprising at least a step of depressurising nitrogen-stripped liquid to a flash pressure. Process vapour is compressed, and selectively split into a stripping portion and a non-stripping portion. Stripping vapour stream comprising at least stripping portion is passed into nitrogen stripper column. Vapour fraction is discharged as off gas, comprising a discharge fraction of overhead vapour from nitrogen stripper column and comprising at least bypass portion from non-stripping portion of compressed vapour, which bypasses a stripping section positioned in nitrogen stripper column.

EFFECT: technical result is prevention of imbalance in nitrogen stripper column and reduced loss of vapour.

17 cl, 2 dwg, 3 tbl

Description

The present invention relates to a method and apparatus for removing nitrogen from a cryogenic hydrocarbon composition.

Liquefied natural gas (LNG) is an economically important example of such a cryogenic hydrocarbon composition. Natural gas is a useful source of fuel, as well as a source of various hydrocarbon compounds. The liquefaction of natural gas in a natural gas liquefaction plant at or near a source of a natural gas stream is often desirable for a number of reasons. For example, natural gas can be more easily stored and transported over long distances in the form of a liquid, rather than in a gaseous form, since it will occupy a smaller volume and will not require storage under high pressure.

WO 2011/009832 describes a method for treating a multiphase hydrocarbon stream obtained from natural gas in which lower boiling components, such as nitrogen, are separated from a multiphase hydrocarbon stream to produce a liquefied natural gas stream with a lower content of such lower boiling components. Two sequential gas-liquid separators are used, operating at different pressures. A multiphase hydrocarbon stream is supplied to the first gas-liquid separator at a first pressure. The bottoms stream from the first gas-liquid separator enters the second gas-liquid separator, which provides steam at a second pressure that is lower than the first pressure. The vapor is compressed in the overhead compressor and returns to the first gas-liquid separator as a steam stream. The compressed vaporized gas from the cryogenic storage tank may be added to the steam stream. The first gas-liquid separator comprises a contact zone with contact enhancing means, such as trays or nozzles, located vertically between the inlet for the multiphase hydrocarbon stream in the first gas-liquid separator and the inlet for the steam stream. The low pressure fuel gas stream is obtained from the overhead steam stream discharged from the first gas-liquid separator, the low pressure fuel gas stream being supplied to the combustion device.

The disadvantage of the method and device described in WO 2011/009832 is that the equilibrium in the first gas-liquid separator can be disturbed if the amount of stripping steam changes significantly, which can be the case when the installation goes from operating in storage mode to loading mode .

The present invention provides a method for removing nitrogen from a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase, the method comprising:

- providing a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase;

- supplying a first feed stream for a nitrogen stripping column at a stripping pressure to a nitrogen stripping column containing at least one internal stripping section located inside the nitrogen stripping column, said first feed stream for a nitrogen stripping column containing a first portion of a cryogenic hydrocarbon composition;

- removal of nitrogen-depleted liquid from the region of the collector of the nitrogen stripping column located at the bottom of the stripping section;

- obtaining at least a stream of liquid hydrocarbon product and process steam from a nitrogen-depleted liquid, comprising at least a step of depressurizing the nitrogen-depleted liquid to a flash pressure;

- compressing said process steam to at least a stripping pressure, thereby obtaining compressed steam;

- selective division of the compressed steam into a stripped portion and an unboiled portion that does not contain a stripped portion, while the unboiled portion forms a bypassed (bypass) portion of the specified compressed steam;

- transmitting the stripping steam stream to the nitrogen stripping column at a level lower vertically of the indicated stripping section, wherein said stripping steam stream contains at least a stripping portion of said compressed steam;

- the passage of the intermediate steam through the condenser, by means of which indirect heat exchange of the intermediate steam with the auxiliary refrigerant stream and partial condensation of the intermediate steam take place, said intermediate steam containing at least an unpaired portion of said compressed steam, and wherein said heat exchange includes heat from the intermediate steam to the auxiliary refrigerant stream in cooling mode, resulting in the formation of excess liquid from the intermediate vapor, and into the cut whereby at least the indicated bypass (bypass) portion of the compressed steam remains in the vapor phase;

- abstraction of the vapor fraction in the form of an exhaust gas containing the exhaust fraction of the overhead vapor obtained from the overhead region of the nitrogen stripping column and containing at least a bypass (bypass) portion; and

- returning at least a portion of the portion of the liquid recycle to the liquid hydrocarbon product stream, wherein the portion of the liquid recycle contains at least a portion of the excess liquid;

in which from the specified selective division to the discharge of the bypassed (bypass) portion in the vapor fraction of the off-gas, the bypassed (bypass) portion bypasses at least one internal stripping section.

In another aspect, the present invention provides a device for removing nitrogen from a cryogenic hydrocarbon composition comprising a nitrogen and methane-containing liquid phase, the device comprising:

- a cryogenic feed line connected to a source of a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase;

- a nitrogen stripping column in fluid communication with the cryogenic feed line, said nitrogen stripping column comprising at least one internal stripping section located inside the nitrogen stripping column and a collection area vertically below the stripping section;

- a nitrogen-depleted liquid discharge line containing an intermediate pressure relief device in fluid communication with the region of the nitrogen stripping column collector, configured to receive nitrogen-depleted liquid from the collection region and depressurize the nitrogen-depleted liquid, said intermediate pressure relief device being located at the interface between the stripping pressure side including the nitrogen stripping column and the flash side ;

- a line of liquid hydrocarbon product located on the pressure side of flash flash in communication with the intermediate pressure relief device, to divert the flow of liquid hydrocarbon product obtained from a nitrogen-depleted liquid;

- a process steam line located on the pressure side of flash flash in communication with an intermediate pressure relief device for receiving process steam formed from a nitrogen-depleted liquid;

- a process compressor located in the line of process steam, configured to receive process steam and compress process steam to create compressed steam at the outlet of the process compressor, said process compressor being at a specified interface between the stripping pressure side and the flash side;

- a bypass divider, the inlet side of which is in fluid communication with the outlet of the process compressor to receive compressed steam, the first outlet side of the bypass divider is in fluid communication with a nitrogen stripping column through a stripping steam line and a second inlet system located at a level below the vertical stripping section and configured to receive at least stripping portion of the specified compressed steam from the process compressor, and the second the outlet side of the bypass divider is in fluid communication with the bypass line of steam containing an unpaired portion of compressed steam;

- a condenser located in fluid communication with a steam bypass line for supplying intermediate steam, which contains at least an unsuitable portion from the steam bypass line, the condenser comprising a heat exchange surface providing indirect heat exchange contact between the intermediate steam and the auxiliary refrigerant stream;

- a discharge line in communication with both the condenser and the overhead region of the nitrogen stripping column and configured to discharge the vapor fraction in the form of an evaporating gas containing the overhead vapor obtained from the overhead region of the nitrogen stripping column and a bypassed portion, containing non-condensed vapor from an intermediate vapor that has passed through a condenser; and

- a line of liquid recycle located on the inlet side in fluid communication with the condenser and on the outlet side in fluid communication with the liquid hydrocarbon product line;

in which a bypass path passes between the bypass divider and the lead line, the bypass path bypasses at least one inner stripping section.

Further in this document, the invention will be further illustrated by way of examples and with reference to the drawings, in which:

in FIG. 1 is a schematic flow diagram of a process representing a method and apparatus including an embodiment of the invention; and

in FIG. 2 is a schematic flow diagram of a process representing a method and apparatus including another embodiment of the invention.

In these figures, the same reference numerals will be used to mean the same or similar parts. In addition, one reference position will be used to designate a channel or line, as well as a stream transported along that line.

The present description relates to the removal of nitrogen from a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase. At least a first portion of the cryogenic hydrocarbon composition is supplied to the nitrogen stripping column as a first feed stream for the nitrogen stripping column. The nitrogen-depleted liquid is discharged from the nitrogen stripping column. A stream of liquid hydrocarbon product and process steam is produced, including at least the step of depressurizing the nitrogen-depleted liquid to flash pressure. Process steam is compressed and selectively divided into a stripping portion and an unboiled portion. A stripping steam stream containing at least a stripping portion enters the nitrogen stripping column down the vertical position of the stripping section therein. The intermediate vapor containing at least an unpaired portion of the compressed process steam passes through the condenser, resulting in the formation of excess liquid from the intermediate vapor, and as a result, at least the bypassed portion of the compressed steam remains in the vapor phase. The vapor fraction is discharged in the form of an exhaust gas containing an exhaust fraction of the overhead steam from the nitrogen stripping column and containing at least a bypass portion of compressed steam that bypasses the stripping section located in the nitrogen stripping column. A portion of the liquid recycle contains at least a portion of the excess liquid. At least a portion of the liquid recycle portion is returned to the liquid hydrocarbon product stream.

The advantage of separating a non-steaming portion that contains a bypass portion from the compressed process steam and at least a bypass portion entering the exhaust gas, while bypassing at least the stripping section located inside the nitrogen stripping column, is that the nitrogen stripping column may be protected against excess steam stripping through the steam section. Such excess intake can lead to disturbance of the equilibrium conditions. Due to the passage of the non-steamed portion through the condenser, it is possible to avoid that valuable parts of the process steam that are separated into the non-steamed portion, such as usually vaporous methane, are lost with the exhaust gas, but instead can be re-condensed and added to the excess liquid, which ultimately flows back into the liquid hydrocarbon product stream.

The vapor fraction in the exhaust gas usually has a calorific value. Preferably, the cooling mode in the condenser is adjusted to control the calorific value of the vented vapor fraction. The ability to adjust the calorific value preferably allows you to stabilize the calorific value of the vapor fraction in the exhaust gas against changes or fluctuations in the flow rate and / or composition of the bypassed portion of the compressed process steam compared to the flow rate and / or composition of the steam overhead from the nitrogen stripping column. Changes both in consumption and in compositions can be expected in the LNG installation during the transition from operation in storage mode to operation in loading mode. In loading mode, not only higher steam consumption, but also a poorer composition (in particular, containing more nitrogen). The ability to adjust the bypassed portion, as well as the cooling mode in the condenser, at the same time contribute to the ability to handle the additional steam load in loading mode.

Process steam may contain vapor methane, which was previously part of the crude liquefied product. Vapor methane, which was previously part of the crude liquefied product, may form in an LNG plant for various reasons. In the normal operation of a natural gas liquefaction plant, methane-containing steam is formed from a (crude) liquefied product in the form of:

- flash vapor resulting from the flash evaporation of the crude liquefied product during pressure relief; and

- evaporated gas resulting from thermal evaporation caused by heat supplied to the liquefied product, for example, in the form of heat leakage into storage tanks, LNG pipelines, and heat from the pumps of the LNG plant. In this mode of operation, known as the storage mode, the storage tanks are filled with a liquefied hydrocarbon product, as it leaves the unit, without any loading and transport operations being carried out at the same time. In storage mode, methane-containing vapors are formed on the installation side of the storage tanks.

The operating mode of the LNG facility while carrying out loading and transportation operations (usually ship loading operations) is known as loading mode operation. During operation in loading mode, evaporated gas is additionally generated in storage tanks on the side of the vessel, for example, due to the initial cooling of the vessel’s tanks; steam displacement from the vessel’s tanks; heat leakage through the pipeline and tanks connecting storage tanks and vessels, and heat from LNG loading pumps.

The proposed solution can facilitate the handling of these pairs during operations of both storage mode and boot mode. It combines the removal of nitrogen from a cryogenic hydrocarbon composition with the re-condensation of excess vaporous methane. This creates an elegant solution in situations where a little fuel is needed for the plant’s own needs, as can be the case with an electric drive system using electricity from an external power supply.

Although process steam may contain flash vapor and / or vaporized gas, it is particularly suitable for vaporized gas. The flow rate of vaporized gas is most susceptible to a change in a conventional LNG plant. Since the proposed solution allows selective division of the compressed steam into stripping and non-stripping portions, it makes it possible to selectively bypass the stripping section in the nitrogen stripping column for any quantity of process steam in addition to what is needed as a stripping steam. This makes the proposed solution particularly suitable for placing vaporized gas in process steam.

The figure 1 shows a device comprising an embodiment of the invention. The cryogenic feed line 8 is in fluid communication with the nitrogen stripping column 20 through the first inlet system 21. The first feed supply line 10 connects the cryogenic feed line 8 to the first inlet system 21 of the nitrogen stripping column 20, optionally through a feed stream divider 9 located between the cryogenic feed line 8 and the first feed line 10.

Upstream of the cryogenic feed line 8, a liquefaction system 100 may be provided. The liquefaction system 100 functions as a source of a cryogenic hydrocarbon composition. The liquefaction system 100 is in fluid communication with a cryogenic feed line 8 through a primary pressure relief system 5 that communicates with the liquefaction system 100 through a crude liquefied product line 1. In the embodiment shown, the primary pressure relief system 5 consists of a dynamic device, such as a turboexpander 6, and a static device, such as a Joule-Thomson valve 7, but other options are possible. Preferably, but not necessarily, any compressor that is part of the hydrocarbon liquefaction process in the liquefaction system, in particular any refrigerant compressor, is driven by one or more electric motors, without mechanical actuation of any steam and / or gas turbine. Such a compressor can be driven solely by one or more electric motors.

The nitrogen stripping column 20 comprises an internal stripping section 24 located inside the nitrogen stripping column 20. The steam removal line 30 of the overhead steam is connected to the nitrogen stripping column 20 through the overhead region 26 inside the nitrogen stripping column 20. The nitrogen depleted liquid discharge line 40 is connected to the nitrogen stripping column 20 through a collection region 28 inside the nitrogen stripping column 20 below the vertical of the stripping section 24.

The nitrogen stripping column 20 may comprise means for enhancing vapor / liquid contact to enhance separation of components and nitrogen removal. Depending on the permissible amount of nitrogen in the nitrogen-depleted liquid and the amount of nitrogen in the cryogenic feed line 8, a total of 2 to 8 theoretical stages may be necessary. In one particular embodiment, 4 theoretical steps were required. Such contact reinforcing means may be provided in the form of trays and / or nozzles, in the form of either a structured or unstructured nozzle. At least a portion of the vapor / liquid contact enhancing means respectively forms part of the inner stripping section 24.

An intermediate pressure relief device 45 is located in the discharge line 40 of the nitrogen-depleted liquid and is therefore in fluid communication with the nitrogen stripping column 20. An intermediate pressure relief device 45 is operatively connected to an LC level controller that interacts with a collection region 28 of a nitrogen stripping column 20.

An intermediate pressure relief device 45 is located at the interface between the stripping pressure side including the nitrogen stripping column 20 and the flash side. The flash side includes a liquid hydrocarbon product line 90 configured to divert a liquid hydrocarbon product stream obtained from a nitrogen-depleted liquid 40, and a process steam line 60 configured to receive a process vapor obtained from a nitrogen-depleted liquid 40. B the embodiment shown, the flash side further comprises a cryogenic storage tank 210 connected to liquid hydrocarbon line 90 of product for storing liquid hydrocarbon product stream, an optional line 230 supplying the vaporized gas and optional end flash separator 50.

If such a flash flash separator 50 is provided, such as, for example, in the embodiment of Figure 1, it may be in fluid communication with the nitrogen stripping column 20 through an intermediate pressure relief device 45 and a nitrogen depleted liquid discharge line 40. The flash flash separator 50 may furthermore be connected to a cryogenic storage tank 210 through a liquid hydrocarbon product line 90. A cryogenic pump 95 may be present in the liquid hydrocarbon product line 90 to facilitate transporting the liquid hydrocarbon product to the cryogenic storage tank 210.

If a feed stream divider 9 is provided, the cryogenic feed line 8 is also connected to at least one element from the group consisting of: a nitrogen depleted liquid discharge line 40, a liquid hydrocarbon product line 90, and a process steam line 60. For this, the second feed line 11 is connected from its inlet side to an optional source divider 9. This second feed line 11 bypasses the nitrogen stripping column 20. The bypass valve 15 flow control is located in the second line 11 of the feed. The flow control bypass valve is operatively connected to the flow controller FC provided in the first feed line 10. Accordingly, the second feed line 11 feeds an optional flash evaporator to the optional end separator 50.

An advantage of the optional second feed line 11 and the optional feed divider 9 is that the nitrogen stripping column 20 may be smaller than when the cryogenic feed line 8 and the first feed line 10 are directly connected without a divider, so that the cryogenic hydrocarbon composition enters the nitrogen stripping column 20 through the first inlet system 21.

Process steam line 60, as shown in the embodiment of FIG. 1 can be connected to an optional flash trap end separator 50 via a flash line 64 and a flash control valve 65 for flash steam, as well as to a cryogenic storage tank 210 through an optional vaporized gas supply line 230. An advantage of the latter compound is that it allows re-condensation of at least a portion of the vaporized gas from the cryogenic storage tank 210 using a condenser, which will be discussed later in this description.

In addition, process compressor 260 is provided at the interface between the steam pressure side and the flash side. Preferably, process compressor 260 is driven by an electric motor. Process compressor 260 is located in line 60 of process steam for receiving process steam and for compressing process steam. The compressed steam discharge line 70 is in fluid communication with the outlet 261 of the process compressor 260. Accordingly, the process compressor 260 is equipped with an anti-surge controller and a recirculate cooler, which are used when the process compressor is in recycle mode and during start-up (not shown in the drawing).

The stripping steam line 71 is in fluid communication with the nitrogen stripping column 20 through a second inlet system 23 located at a lower level vertically to the stripping section 24 and preferably above the region of the collector 28. The stripping steam line 71 is connected to the compressed steam discharge line 70 through the bypass a divider 79. A stripping steam valve 75 is provided in the stripping steam line 71.

Optionally, an external stripping steam supply line 74 is provided in fluid communication with the second inlet system 23 of the nitrogen stripping column 20. In one embodiment, as shown in FIG. 1, an optional external stripping steam supply line 74 is connected to a compressed steam discharge line 70. An external steam stripping valve 73 is provided in an optional external steam stripping line 74. In one embodiment, the optional external stripping steam line 74 is respectively connected to a hydrocarbon steam line in or upstream of the liquefaction system 100.

The bypass divider 79 is also in fluid communication with the condenser through at least a steam bypass line 76. A steam bypass control valve 77 is preferably provided in the steam bypass line 76. The steam bypass line 76 contains a non-desorbing portion of the compressed steam from the compressed steam discharge line 70. The condenser may be any type of indirect heat exchanger in fluid communication with the bypass divider 79 through the steam bypass line 76. Such a condenser is preferably used to re-condensate at least a portion of the compressed process steam from the compressed steam discharge line 70.

In FIG. 1 shows a suitable embodiment in which the condenser is provided in the form of a overhead condenser 35 external to the nitrogen stripping column 20. The overhead condenser 35 is in fluid communication with both the overhead steam withdrawal line 30 and the steam bypass line 76 to partially condense the intermediate steam stream that contains an unheated portion from the steam bypass line 76, in addition to any amount of steam overhead taken from the nitrogen stripping column 20. The condenser comprises a heat exchange surface that provides indirect heat exchange contact between the intermediate steam and the auxiliary refrigerant stream 132, whereby heat can be transferred from the intermediate steam to the auxiliary refrigerant stream 132 in the cooling mode. The auxiliary refrigerant flow control valve 135 is provided in the auxiliary refrigerant line 132.

In the embodiment of FIG. 1, the steam bypass line 76 respectively runs along the bypass between the bypass divider 79 and the overhead steam discharge line 30 on the inlet side of the overhead condenser 35. A bypass route passes between the bypass divider 79 and the steam removal line 30 of the overhead and / or the steam fraction discharge line 80. The bypass does not pass through the inner stripping section 24 in the nitrogen stripping column 20. In this way, the passage of the non-steamed portion through the inner stripping section 24 can be avoided, which helps to avoid imbalance in the nitrogen stripping column 20.

As can be seen again from FIG. 1, the overhead separator 33 is located on the discharge side of the overhead steam line 30. The steam overhead line 30 is unloaded into the head overhead separator 33. The overhead separator 33 is configured to separate any non-condensed vapor fraction from any condensed overhead vapor fraction.

The steam fraction discharge line 80 is configured to discharge the vapor fraction mentioned above. The vapor fraction discharge line 80 is in fluid communication with both the condenser and the overhead region 26 of the nitrogen stripping column 20. In embodiments, such as the embodiment of FIG. 1, in which the intermediate steam contains both overhead steam and unboiled steam, the steam removal line 80 is initially in communication with both the condenser and the head region 26 of the nitrogen overhead column 20 . The bypass in this embodiment continues to the steam fraction lead line 80.

The advantage of the steam bypass line 76 is that when there is an excess of process steam, it can be treated together with the vaporizing gas in the steam fraction discharge line 80 without disturbing the material balance in the nitrogen stripping column 20.

The condenser is also in fluid communication with the liquid recycle line 13. The liquid recirculate line 13 is in fluid communication with the liquid hydrocarbon product line 90. A fluid communication means that the liquid recycle line 13 is connected to any suitable location from which at least a portion of the liquid recycle portion can enter the liquid hydrocarbon product line 90, while remaining in the liquid phase. Thus, the liquid recycle line 13 can, for example, be directly connected to one or more elements selected from the group consisting of: a nitrogen stripping column 20, a cryogenic feed line 8, a first feed line 10, an optional second feed line 11 , a nitrogen depleted liquid withdrawal line 40, an optional flash trap end separator 50, and a liquid hydrocarbon product line 90. A recirculation valve 14 is provided in the line 13 of the liquid recirculate.

Optionally, the nitrogen stripping column 20 comprises an internal distillation section 22 in addition to the internal stripping section 24. The internal distillation section 22 is located inside the nitrogen stripping column 20 vertically higher than the stripping section 24. The overhead region 26 is preferably defined vertically above the distillation section 22. The first inlet system 21 is provided vertically between the inner distillation section 22 and the inner stripping section 24. Region 26 of the overhead is located vertically above the distillation section 22.

The optional internal distillation section 22 may include means for enhancing vapor / liquid contact, similar to that found in the internal stripping section 24, to enhance separation of components and nitrogen removal.

The irrigation system may be configured to allow at least a portion 36 of the irrigation from the condensed fraction to the nitrogen stripping column 20 at a level above the distillation section 22. In the embodiment of FIG. 1, the irrigation system includes a condensed fraction discharge line 37 fluidly coupled to the lower part of the overhead separator 33, an optional irrigation pump 38 provided in the condensed fraction discharge line 37, and a condensed fraction divider 39. The condensed fraction divider 39 fluidly couples the condensed fraction withdrawal line 37 to the nitrogen stripping column 20 through the irrigation portion line 36 and the irrigation inlet system 25 and to the liquid recycle line 13. An optional irrigation flow control valve 32 operably controlled by an irrigation flow regulator (not shown) may preferably be provided in the irrigation portion line 36.

In embodiments where the nitrogen stripping column 20 contains an optional internal distillation section 22, the liquid recycle line 13 is preferably in fluid communication with the liquid hydrocarbon product line 90 through a recycle path that does not pass through the distillation section 22, if provided . Thus, the liquid recycle line 13 helps to avoid supplying too much liquid to the distillation section 22 and to avoid the passage of liquid recycle through the distillation section 22. This is useful in order to avoid imbalance in the nitrogen stripping column 20.

A cooling mode controller 34 may be provided to control the cooling mode by determining the intensity with which heat is transferred from the intermediate steam to the auxiliary refrigerant stream. Accordingly, the cooling mode controller 34 is configured to control the cooling mode in accordance with the calorific value of the exhaust gas as applied to the heat demand. In the shown embodiment, the cooling mode controller 34 is in the form of a pressure controller PC and an auxiliary refrigerant flow control valve 135, which are functionally connected to each other.

A combustion device 220 is accordingly provided at the outlet end of the steam fraction discharge line 80 for receiving at least a fuel portion from the vapor fraction in the vapor fraction discharge line 80. The combustion device may comprise a plurality of combustion units and / or it may include, for example, one or more of the following devices: a furnace, a boiler, an incinerator, a dual-fuel diesel engine, or combinations thereof. A boiler and a dual-fuel diesel engine can be connected to an electric generator.

The amount of methane in the exhaust gas can be adjusted to meet the specific need for methane. This makes the exhaust gas suitable for use as a fuel gas stream, preferably at a fuel gas pressure not exceeding the stripping pressure, even in circumstances where the need for calorific value is variable.

The steam recirculate line 87 is optionally configured to receive at least a portion of the steam recirculate from the overhead line 30. The steam recirculate line 87 bypasses the nitrogen stripping column 20 and provides a reverse feed to at least one of the elements of the group consisting of: a liquid hydrocarbon product line 90 and a process steam line 60. The steam recirculate flow control valve 88 is preferably provided in the steam recirculate line 87. The advantage of the proposed line 87 of steam recirculate is that it allows you to selectively increase the nitrogen content in the stream 90 of the liquid hydrocarbon product. If an optional flash-off end separator 50 is provided, the steam recirculate line 87 respectively feeds into the flash-off end separator 50.

Accordingly, the configuration of the optional steam recycle line 87 includes an optional steam fraction divider 89, which may be provided in the steam fraction line 80, providing a controlled fluid communication between the steam fraction line 80 and the steam recycle line 87.

A heat recovery heat exchanger 85 may be provided in the steam fraction discharge line 80 to maintain the cold present in the steam fraction 80 by heat exchange with the cold recovery stream 86 before supplying the steam fraction 80 to any combustion device.

In one embodiment, the cold recovery stream 86 may comprise or consist of a side stream originating from a hydrocarbon feed stream in a hydrocarbon feed line 110 of the liquefaction system 100. The resulting cooled side stream may, for example, be combined with a cryogenic hydrocarbon composition in a cryogenic feed line 8. Thus, heat transfer for recovering cold in a heat exchanger 85 for recovering cold complements the performance of the cryogenic hydrocarbon composition. In another embodiment, the cold recovery stream 86 may comprise or consist of overhead steam in the overhead steam removal line 30, preferably a portion of the overhead steam removal line 30 through which the overhead steam flows from the nitrogen stripping column 20 to the overhead condenser 35. In this case, the performance required from the auxiliary refrigerant stream 132 in the overhead condenser 35 can be reduced.

The liquefaction system 100 in the present description has so far been presented very schematically. It may be any suitable hydrocarbon liquefaction system and / or process, in particular any natural gas liquefaction process producing liquefied natural gas, and the invention is not limited to the particular choice of a liquefaction system. Examples of suitable liquefaction systems utilize single refrigerant cycle processes (typically a single mixed refrigerant cycle — SMR processes such as PRICO described by KR Johnsen and R. Christiansen at “LNG Production on floating platforms” presented at the Gastech 1998 conference ( Dubai), but it is also possible to use a single-component refrigerant process, such as, for example, the BHP-cLNG process, also described in the aforementioned work by KR Johnsen and P. Christiansen); processes with two refrigerant cycles (for example, the often used process with mixed refrigerant and propane with the frequent abbreviation C3MR, described, for example, in US Pat. No. 4,440,408, or, for example, processes with two mixed refrigerants - DMR, an example of which is described in US 6658891 , or, for example, processes with two cycles in which each refrigerant cycle contains a single component refrigerant); and processes based on three or more compressor sequences for three or more refrigeration cycles, an example of which is described in US Pat. No. 7,114,351.

Other examples of suitable liquefaction systems are described in patents: US 5832745 (Shell SMR); US 6295833 and US 5657643 (both are Black & Veatch SMR variants); US 6,370,910 (Shell DMR). Another suitable example of a DMR process is the so-called Axens LIQUEFIN process, described, for example, in an article by PY Martin et al entitled “LIQUEFIN: AN INNOVATIVE PROCESS TO REDUCE LNG COSTS” presented at the 22nd World Gas Conference in Tokyo, Japan ( 2003). Other suitable three-loop processes are described, for example, in US Pat. Nos. 6,962,060; WO 2008/020044; US 7127914; DE 3521060A1; US 5669234 (commercially known as optimized cascade process); US 6253574 (commercially known as cascaded mixed refrigerant process); US 6308531; in the publication of application US 2008/0141711; Mark J. Roberts et al, "Large Capacity Single Train AP-X (TM) Hybrid LNG Process", Gastech 2002, Doha, Qatar (October 13-16, 2002). These references are provided to demonstrate the broad applicability of the invention, and are not an exclusive and / or exhaustive list of possibilities.

Preferably, but not necessarily, any compressor that is part of the hydrocarbon liquefaction process in the liquefaction system, in particular any refrigerant compressor, is driven by one or more electric motors, without mechanical actuation of any steam and / or gas turbine. Such a compressor can be driven solely by one or more electric motors. Not all examples above use electric motors as drives for refrigerant compressors. It should be understood that any drives other than electric motors can be replaced with an electric motor to benefit most from the present invention.

An example in which the liquefaction system 100 is based, for example, on C3MR or Shell DMR, is briefly illustrated in Figure 2. It uses a cryogenic heat exchanger 180, in this case in the form of a spiral heat exchanger containing lower and upper bundles (181 and 182, respectively) of pipes for a hydrocarbon product, the lower and upper bundles (183 and 184 respectively) of pipes for light refrigerant fraction (LMR) and bundle 185 of pipes for heavy refrigerant fraction (HMR).

The lower and upper bundles 181 and 182 of pipes for the hydrocarbon product fluidly couple the crude liquefied product line 1 to the hydrocarbon feed line 110. At least one pre-cooling heat exchanger 115 for the chilled hydrocarbon may be provided in the hydrocarbon feed line 110 upstream of the cryogenic heat exchanger 180.

The primary refrigerant in the form of mixed refrigerant is supplied to the primary refrigerant circuit 101. The main refrigerant circuit 101 contains a waste refrigerant line 150 connecting the cryogenic heat exchanger 180 (in this case, the annular zone 186 of the cryogenic heat exchanger 180) to the main suction side of the main refrigerant compressor 160, and a compressed refrigerant line 120 connecting the outlet of the main refrigerant compressor 160 with MR a separator 128. One or more heat exchangers is provided in a compressed refrigerant line 120 including, in the present example, at least one heat exchanger 124 using an ambient heat transfer medium and at least one pre-cooling heat exchanger 125 with a cooled main refrigerant. The MR separator 128 is in fluid communication with the lower tube bundle 183 for the LMR through line 121 of the light refrigerant fraction and with the tube bundle for HMR through line 122 of the heavy refrigerant fraction.

At least one pre-cooling heat exchanger 115 of the cooled hydrocarbons and at least one pre-cooling heat exchanger 125 of the cooled main refrigerant are cooled by the pre-cooling refrigerant (along lines 127 and 126, respectively). The same pre-refrigerant can be transferred from the same cycle to the pre-refrigerant. In addition, at least one pre-cooling heat exchanger 115 for a chilled hydrocarbon and at least one pre-cooling heat exchanger 125 with a cooled main refrigerant can be combined into one pre-cooling heat exchanger (not shown). Reference is made to US Pat. No. 6,370,910 as a non-limiting example.

The optional external steam stripping line 74 (if provided) may suitably be connected to the hydrocarbon feed line 110, or at a point upstream from at least one pre-cooling of cooled hydrocarbons 115, downstream from at least one pre-heat exchanger 115 cooling cooled hydrocarbons, or (for example, if it is possible to provide two or more heat exchangers for pre-cooling cooled hydrocarbons) between two -inflammatory precooling heat exchangers cooled hydrocarbons to produce part of the hydrocarbon feed stream from line 110 of the hydrocarbon feedstock.

At the transition point between the upper (182, 184) and lower (181, 183) tube bundles, the HMR tube bundle 185 is in fluid communication with the HMR line 141 in which the HMR control valve 144 is provided. The HMR line 141 is in fluid communication with the annulus 186 of the cryogenic heat exchanger 180 and, through said annulus 186 and in the heat exchange configuration, with each one of the lower pipe bundle 181 for the hydrocarbon product, the lower pipe bundle 183 for the LMR and the pipe bundle 185 for HMR, with a line of 150 spent refrigerant.

Above the upper tube bundles 182 and 184, near the upper part of the cryogenic heat exchanger 180, the tube bundle 184 for the LMR is in fluid communication with the LMR line 131. The first LMR return line 133 establishes a fluid communication between the LMR line 131 and the annulus 186 of the cryogenic heat exchanger 180. An LMR control valve 134 is provided in the first LMR return line 133. The first LMR return line 133 is in fluid communication with the spent refrigerant line 150 through said annulus 186 and in a heat exchange configuration with each one of the upper and lower bundles 182 and 181 of pipes for the hydrocarbon product and each one of the bundles 183 and 184 of pipes for LMR and 185 tube bundle for HMR.

In FIG. 2 shows one possible source of auxiliary refrigerant. The LMR line 131 is divided into the auxiliary refrigerant line 132 and the first LMR return line 133. The second LMR return line 138 at its inlet end is fluidly connected to the auxiliary refrigerant line 132 through a head overhead condenser (for example, head overhead condenser 35 of Figure 1 or an integrated internal overhead condenser 235 shown in Figure 2), and at the outlet end, a second the LMR return line 138 is ultimately connected to the spent refrigerant line 150, respectively, through the first HMR line 141.

The line at the top around the nitrogen stripping column 20 in Figure 2 is similar to the line shown in Figure 1 and will not be described in detail again. Optional lines including an optional second feed line 11, an optional external steam stripping line 74 and an optional steam recycle line 87 may be provided, but have not been shown in FIG. 2 for purposes of clarity.

However, one of the differences that should be noted between the embodiment of FIG. 2 and the embodiment of FIG. 1 is that the overhead condenser 35, the overhead separator 33 and the irrigation system were made in the form of an internal overhead overhead condenser 235, which is made inside an overhead region 26 in a nitrogen stripping column 20. Capacitors, such as overhead condenser 235, are known in the art. The liquid recycle line 13 is in fluid communication with a partial liquid collection plate 27 provided inside the nitrogen stripping column 20 above the vertical distillation section 22 and below the internal overhead condenser 235. The partial fluid withdrawal plate 27 functions equivalently to the divider 39 of the condensed fraction of figure 1.

Regardless of the type of (overhead) overhead condenser 35 or overhead overhead condenser 235, the condenser is preferably in fluid communication with both the steam bypass line 76 and the overhead region 26 of the nitrogen stripping column 20, resulting in an intermediate pair, passing through the condenser, preferably contains both a single portion from the steam bypass line 76 and head steam obtained from the head region 26 of the nitrogen stripping column 20.

A device and method for removing nitrogen from a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase can work as follows.

The cryogenic hydrocarbon composition 8 containing a nitrogen and methane-containing liquid phase is preferably provided at an initial pressure of 2 to 15 bar abs. (0.2-1.5 MPa) and preferably at a temperature below -130 ° C.

Cryogenic hydrocarbon composition 8 can be obtained from reservoirs of natural gas, or oil, or coal seams. Alternatively, the cryogenic hydrocarbon composition 8 can also be obtained from another source, including, for example, an artificial source, such as the Fischer-Tropsch process. Preferably, the cryogenic hydrocarbon composition 8 contains at least 50 mol. % methane, more preferably at least 80 mol. % methane.

In typical embodiments, a temperature of less than -130 ° C. can be achieved by passing the hydrocarbon feed stream 110 through the liquefaction system 100. In such a liquefaction system 100, a hydrocarbon feed stream 110 containing a vaporous hydrocarbon-containing feed may be heat exchanged, for example, in a cryogenic heat exchanger 180, with a main refrigerant stream, thereby causing the vaporous feed to be liquefied from the feed stream to form a crude liquefied stream in line 1 of the crude liquefied product. The desired cryogenic hydrocarbon composition 8 can then be obtained from the crude liquefied stream 1.

The main refrigerant stream can be formed by circulating the main refrigerant in the main refrigerant circuit 101, in which the spent refrigerant 150 is compressed in the main refrigerant compressor 160 to form compressed refrigerant 120 from the used refrigerant 150. Heat is removed from the compressed refrigerant taken out from the main refrigerant compressor 160, through one or more heat exchangers provided in the compressed refrigerant line 120. This results in the formation of a partially condensed compressed refrigerant which is phase separated in an MR separator 128 into a light refrigerant fraction 121 consisting of vaporous components of a partially condensed compressed refrigerant and a heavy refrigerant fraction 122 composed of liquid components of a partially condensed compressed refrigerant.

The light refrigerant fraction 121 passes sequentially through the lower LMR bundle 183 and the upper LMR bundle 184 of the cryogenic heat exchanger 180, while the heavy refrigerant fraction 122 passes through the HMR bundle 185 of the cryogenic heat exchanger 180 to the transition point. When passing through these respective tube bundles, the corresponding light and heavy fractions of the refrigerant are cooled with the help of light and heavy fractions of the refrigerant, which evaporate in the annulus 186, again forming the spent refrigerant 150, which completes the cycle. Simultaneously, the hydrocarbon feed stream 110 passes through a cryogenic heat exchanger 180, sequentially through the lower hydrocarbon bundle 181 and the upper hydrocarbon bundle 182, and is liquefied and supercooled by the same evaporation of light and heavy refrigerant fractions.

Depending on the source, the hydrocarbon feed stream 110 may contain various amounts of components other than methane and nitrogen, including one or more non-hydrocarbon components other than water, such as CO ₂ , Hg, H ₂ S and other sulfur compounds; and one or more hydrocarbons heavier than methane, such as, in particular, ethane, propane and butanes, and possibly smaller amounts of pentanes and aromatic hydrocarbons. Hydrocarbons with a molecular weight corresponding to at least the mass of propane may be referred to herein as C ₃ + hydrocarbons, and hydrocarbons with a molecular mass corresponding to at least the mass of ethane may here be referred to as C ₂ + hydrocarbons.

Optionally, the hydrocarbon feed stream 110 may be pretreated to reduce and / or remove one or more undesirable components, such as CO ₂ and H ₂ S, or sent to other stages, such as pre-compression or the like. Such steps are well known to those skilled in the art, and their mechanisms are not discussed further here. The composition of the hydrocarbon feed stream 110 thus varies depending on the type and location of the gas source and the applied pre-treatment (s).

The crude liquefied stream 1 may contain from 1 mol. % to 5 mol. % nitrogen, have an initial temperature of -165 ° C to -120 ° C and usually a liquefaction pressure of 15 to 120 bar abs. (1.5-12.0 MPa). In many cases, the initial temperature can be from -155 ° C to -140 ° C. Within this narrower range, lower cooling capacity in the liquefaction system 100 is required than when lower temperatures are desired, while the amount of subcooling at a pressure above 15 bar abs. (1.5 MPa) is high enough to avoid excessive formation of flash vapor when depressurizing up to 1-2 bar abs. (0.1-0.2 MPa).

The cryogenic hydrocarbon composition 8 can be obtained from the crude liquefied stream 1 using a main pressure relief of the crude liquefied stream 1 from the liquefaction pressure to the initial pressure. The first stream 10 of feedstock for the stripping column of nitrogen is obtained from the cryogenic hydrocarbon composition 8 and fed into the column 20 stripping nitrogen at a pressure of stripping through the first inlet system 21.

The stripping pressure is usually less than or equal to the initial pressure. The stripping pressure in preferred embodiments is selected in the range of 2 to 15 bar abs. (0.2-1.5 MPa). Preferably, the stripping pressure is at least 4 bar abs. (0.4 MPa), since at a slightly higher stripping pressure the stripping steam in the stripping steam line 71 can benefit from some additional enthalpy (in the form of heat of compression), which is added to the process stream 60 in the process compressor 260. Preferably, the stripping pressure is not more than 8 bar abs. (0.8 MPa) to facilitate separation efficiency in the nitrogen stripping column 20. In addition, if the stripping pressure is in the range of 4 to 8 bar abs. (0.4-0.8 MPa), the exhaust gas in the steam fraction line 80 can be easily used as the so-called low-pressure fuel stream without the need for additional compression.

In one example, the initial temperature of the crude liquefied stream 1 was −161 ° C., while the liquefaction pressure was 55 bar abs. (5.5 MPa). The main pressure relief can be carried out in two stages: the first dynamic stage is carried out using a turbo expander 6 to reduce the pressure from 55 bar abs. (5.5 MPa) to about 10 bar abs. (1.0 MPa), followed by a further pressure relief of up to 7 bar abs. (0.7 MPa) in the static stage using valve 7 Joule-Thomson. It was assumed that the stripping pressure in this case was 6 bar abs. (0.6 MPa).

A stream of 30 overhead steam is obtained from the overhead region 26 of the nitrogen stripping column 20. The vapor fraction 80 obtained from the overhead steam stream 30 and containing the withdrawn steam fraction 30 of the overhead is discharged in the form of exhaust gas. Accordingly, at least a fuel portion of the vapor fraction 80 is supplied to the combustion device 220 at a fuel gas pressure that does not exceed the stripping pressure.

The nitrogen-depleted liquid 40 is withdrawn from the collection area 26 of the nitrogen stripping column 20. The temperature of the nitrogen-depleted liquid 40 is usually higher than the temperature of the first feed stream 10 of the stripping nitrogen column. As a rule, it is assumed that the temperature of the nitrogen-depleted liquid 40 is higher than the temperature of the first stream 10 of raw materials for the stripping column of nitrogen, and ranges from -140 ° C to -80 ° C, preferably from -140 ° C to -120 ° C.

The nitrogen-depleted liquid 40 is then subjected to a pressure relief, preferably by means of an intermediate pressure relief device 45, to a flash pressure that is lower than the stripping pressure, corresponding to a range of 1 to 2 bar abs. (0.1-0.2 MPa). Preferably, the flash pressure is in the range of 1.0 to 1.4 bar abs. (0.10-0.14 MPa). With a slightly higher difference between the flash pressure and the stripping pressure in the stripping line 71, the stripping steam can benefit from some additional heat of compression that is added to the process stream 60 in the process compressor 260.

The intermediate pressure relief device 45 may be controlled by an LC level controller configured to increase the flow rate through the intermediate pressure relief device if the level of liquid accumulated in the region of the collector 26 of the nitrogen stripping column 20 exceeds a target level. As a result of depressurization, the temperature usually drops below -160 ° C. The stream 90 of the liquid hydrocarbon product, which is formed in this case, can usually be stored at atmospheric pressure in an open isothermal cryogenic storage tank.

Process steam 60 also forms. Process steam 60 may comprise flash vapor 64, which is often generated by depressurizing a nitrogen-depleted liquid 40 and / or depressurizing a feed bypass stream 11 (discussed further below).

The first feed stream 10 for the nitrogen stripping column contains the first portion of the cryogenic hydrocarbon composition 8. It may contain the entire cryogenic hydrocarbon composition, but in practice it is preferable to divide the cryogenic hydrocarbon composition 8 into the first portion 10 and the second portion 11 having the same composition and phase as and the first portion 10. The second portion is preferably diverted, in the form of a bypass flow of the feed, from the side of the steam pressure to a suitable location on the side of the flash pressure.

The division ratio, defined as the flow rate of the second portion relative to the flow rate of the cryogenic hydrocarbon composition in line 8 of the cryogenic hydrocarbon composition, can be adjusted using the bypass valve 15 flow control. This flow control bypass valve 15 can be adjusted using the FC flow controller to maintain a predetermined target flow rate of the first feed stream 10 for the nitrogen stripping column to the nitrogen stripping column 20. The FC flow controller will increase the open portion of the flow control bypass valve 15 if there is an excess flow in excess of the target flow, and decrease the open portion if there is insufficient flow compared to the target flow.

As a general recommendation, the division ratio can preferably be selected from 50% to 95%. Lower values are generally recommended for a higher nitrogen content in the cryogenic hydrocarbon composition, while higher values are preferred for a lower nitrogen content. In one example, the nitrogen content in the cryogenic hydrocarbon composition 8 was 3.0 mol. %, whereby the selected division ratio was 75%.

The second portion leaving the splitter 9 of the feed stream may also be depressurized to the indicated flash pressure before it is subsequently fed to at least one line from the group consisting of: a nitrogen depleted liquid discharge line 40, a liquid hydrocarbon product line 90, and a line 60 process steam, while bypassing the nitrogen stripping column 20. Accordingly, an optional second portion is supplied to an optional flash flash separator 50.

Process steam 60 may contain vaporized gas. Evaporated gas 230 is typically formed by adding heat to the liquid hydrocarbon product stream 90, whereby a portion of the liquid hydrocarbon product stream 90 evaporates to form the vaporized gas. In a typical LNG plant, the formation of evaporated gas can exceed the flow rate of flash vapor several times, especially during operation of the plant in the so-called loading mode, and therefore, an important advantage lies not only in the re-condensation of flash vapor, but also in the re-condensation of vaporized gas, and also if there is not enough local heat demand for the use of all methane contained in the evaporated gas.

In order to facilitate the transfer of the vaporized gas to the process steam stream 60, preferably an optional vaporized gas supply line 230 connects the steam region in the cryogenic storage tank 210 to the process steam line 60. In order to facilitate the transfer of flash flash vapor 64 to process steam stream 60 and to further decontaminate liquid hydrocarbon product stream 90, preferably nitrogen-depleted liquid after depressurization is fed to an optional flash flash separator where it undergoes phase separation at flash pressure 90 liquid hydrocarbon product and vapor 64 flash evaporation. The flash pressure is equal to or lower than the flash pressure, and accordingly is in the range of 1 to 2 bar abs. (0.1-0.2 MPa) in a stream of 90 liquid hydrocarbon product and a pair of 64 flash evaporation. It is envisaged that in one embodiment, the instantaneous separation pressure is 1.05 bar abs. (0.105 MPa).

Process steam 60 is compressed to at least a stripping pressure, thereby producing a compressed steam stream 70. Stripping steam stream 71 is obtained from the compressed steam stream 70 and sent to the nitrogen stripping column 20 through the second inlet system 23. This stripping steam can seep up through the stripping section 23, in countercurrent contact with liquids leaking down through the stripping section 23.

If an external stripping steam supply line 74 is provided in fluid communication with the second inlet system 23, the external stripping steam can selectively be supplied to the nitrogen stripping column 20 through the second inlet system 23. In this way, basic damage to the nitrogen stripping column 20 can be avoided, for example, when the process compressor 260 does not provide a sufficient flow of compressed steam stream 70.

The receipt of the stream 71 stripping steam from the stream 70 of compressed steam includes the selective division of the stream 70 of compressed steam into a stripping portion and an unboiled portion. An unboiled portion contains a bypassed portion of the compressed steam, wherein the bypassed portion may also be referred to herein as the bypassed portion of steam. It does not contain a steaming portion. The steam stream 71 contains at least a stripping portion.

Selective entry can be adjusted using the bypass control valve 77 steam. Accordingly, the steam bypass control valve 77 is controlled by a pressure regulator in the compressed steam line 70, which is configured to increase the open portion of the steam bypass control valve 77 in response to an increase in pressure in the compressed steam line 70. It is assumed that the flow rate of the bypassed portion of the steam, which can pass through the steam bypass line 76 to the overhead steam stream 30, is particularly high in the so-called loading mode, during which the amount of vaporized gas is usually much higher than usually during the so-called storage mode. Preferably, the steam bypass control valve 77 is completely closed during normal storage operation.

A partially condensed intermediate stream is formed from the intermediate vapor as the intermediate vapor passes through the condenser. The intermediate steam in this case contains at least an unpaired portion of compressed steam. In preferred embodiments, such as illustrated in FIG. 1, the intermediate steam also comprises steam overhead 30. This can be achieved by selectively introducing an unpaired portion of compressed steam into the overhead steam stream 30, thereby forming an intermediate pair. The formation of a partially condensed intermediate stream, respectively, includes indirect heat exchange of the intermediate steam with the auxiliary refrigerant stream 132 and partial condensation of the intermediate steam, as a result of which heat is transferred from the intermediate steam to the auxiliary refrigerant stream 132 in the selected cooling mode. The resulting partially condensed intermediate stream includes a condensed fraction containing excess liquid and a vapor fraction. The vapor fraction contains a bypassed portion of compressed steam that remains in the vapor phase during partial condensation.

The condensed fraction is separated from the vapor fraction in the overhead separator 33 at a separation pressure that may be lower than the stripping pressure, and is preferably in the range of 2 to 15 bar abs. (0.2-1.5 MPa). The vapor fraction is discharged along line 80 of the discharge of the vapor fraction in the form of exhaust gas. It contains a vented overhead vapor fraction obtained from the overhead region 26 of the nitrogen stripping column 20, as well as at least a bypassed portion of steam. The condensed fraction is discharged from the overhead separator 33 into the irrigation system, for example, via the condensed fraction discharge line 37.

Thus, all the way from the selective division of the compressed steam into the stripping and non-stripping portions to the discharge of the bypassed portion to the vapor fraction of the offgas, the bypassed portion bypasses at least one inner stripping section 24. In other words, on the way from the bypass divider 79 to the overhead steam extraction line 30 and / or the steam fraction removal line 80, the bypassed portion does not pass through at least one inner stripping section 24. It is hereby achieved that any compressed steam in the compressed steam line 70 exceeding the amount of stripping steam consumed during the normal operation of the nitrogen stripping column 20 under equilibrium conditions deviates around the stripping 24 so that the equilibrium of stripping inside the nitrogen stripping column 20 is not disturbed. In preferred embodiments, the transfer portion bypasses not only the stripping section 24, but the entire nitrogen stripping column 20, as shown in the embodiment of FIG. 1.

At least a portion of the condensed fraction withdrawn from the overhead separator 33 is fed to the liquid recirculate line 13 to form a portion of the liquid recirculate. The recirculation valve 14 can accordingly be adjusted using a flow regulator provided in the condensed fraction discharge line 37 and / or a level regulator provided on the overhead separator 33. A portion of the liquid recycle contains at least a portion of the excess liquid. At least a portion of the portion of the liquid recycle is returned to the liquid hydrocarbon product stream, while at least a portion of this is retained in the liquid phase. This can be accomplished by feeding a portion of liquid recycle to at least one of the elements of the group consisting of: a nitrogen stripping column 20, a cryogenic hydrocarbon composition 8, a first feed stream 10 for a nitrogen stripping column, an optional bypass stream 11 of the feed, a nitrogen-depleted liquid 40 optional flash vaporizer and separator 50 and liquid hydrocarbon product stream 90.

The condenser, which in the embodiment of FIG. 1 is configured as a overhead condenser 35 in this way, allows re-condensation of the methane vapor, which was previously part of the crude liquefied product 1 (or cryogenic hydrocarbon composition 8), by the addition of any such vapor-containing stream methane to a stream of (compressed) process steam. Preferably, the methane is condensed provided that it exceeds the target methane content in the vented vapor fraction 80. Previously forming part of the process steam 60 or the compressed process steam 70, methane vapor can find its way to heat exchange with auxiliary refrigerant 132 by which it selectively condenses from the steam 30 overhead of the nitrogen stripping column 20, while simultaneously allowing most of the nitrogen to be vented off-gas. In this case, it becomes possible to remove a sufficient amount of nitrogen from the cryogenic hydrocarbon composition 8 to obtain a stream 90 of liquid hydrocarbon product within the desired maximum standard for nitrogen content, while not creating more thermal power in the exhaust gas than necessary.

The vapor fraction 80 in the exhaust gas usually has a calorific value. The calorific value of the extracted steam fraction 80 is accordingly regulated by adjusting the cooling mode in the overhead condenser 35. This can be done using the controller 34 cooling mode. By adjusting the cooling mode, in which heat is transferred from the overhead steam to the auxiliary refrigerant stream, the relative methane content in the exhaust gas can be controlled. As a result, the calorific value of the extracted steam fraction can be adjusted to match a specific heat demand. This makes the exhaust gas suitable for use as a fuel gas stream, even in circumstances in which the demand for calorific value is variable.

When the vapor fraction 80 is supplied to and consumed by the combustion device 220 as fuel, the calorific value can be controlled by the combustion device 220 in accordance with the actual heat demand.

Adjustable heating value may be selected in accordance with the possible circumstances of the intended use of the off-gas as fuel gas. The calorific value can be determined in accordance with DIN 51857. For many applications, the regulated calorific value can be proportional to the lower calorific value (LHV; sometimes called calorific value), which can be defined as the amount of heat released when a certain amount of fuel is burned (initially at 25 ° C) and bringing the temperature of the combustion products to 150 ° C. This suggests that the latent heat of water vaporization in the reaction products is not taken into account.

However, in order to control the calorific value in the context of the present invention, the actual calorific value of the vented vapor fraction does not need to be determined on an absolute basis. As a rule, it turns out to be sufficient to regulate the calorific value relative to the actual need for thermal power in order to minimize any drawback and excess of the provided thermal power.

In the context of the present description, the cooling mode reflects the intensity with which heat transfer occurs in the condenser, which can be expressed in units of power (for example, in watts or megawatts). The cooling mode is associated with the consumption of auxiliary refrigerant sent for heat exchange with steam overhead.

Preferably, the cooling mode is automatically adjusted in response to a signal that is linked by a causal relationship to the controlled calorific value. In embodiments in which the vapor fraction is supplied to one or more selective methane consumers, such as, for example, the combustion device 220 shown in FIG. 1, the regulation can be carried out in accordance with the required heat output, whereby the partial consumption of methane is adjusted to achieve calorific value that meets the need. Accordingly, the auxiliary refrigerant flow control valve 135 can be controlled by the pressure regulator PC to maintain the desired target flow rate of the auxiliary refrigerant stream 132 through the overhead condenser 35. The actual pressure in the steam fraction lead line 80 is due to a causal relationship with adjustable calorific value.

The pressure regulator PC will be configured to reduce the open part of the auxiliary refrigerant flow control valve 135 when the pressure drops below a predetermined target level, which indicates a higher methane consumption rate than the steam supply rate 80. On the other hand, the pressure regulator PC will configured to increase the open portion of the auxiliary refrigerant flow control valve 135 if the pressure exceeds a predetermined target level.

It is assumed that the vapor fraction 80 contains from 50 mol. % to 95 mol. % nitrogen, preferably from 70 mol. % to 95 mol. % nitrogen or from 50 mol. % to 90 mol. % nitrogen, more preferably from 70 mol. % to 90 mol. % nitrogen, even more preferably from 75 mol. % to 95 mol.% nitrogen, most preferably from 75 mol. % to 90 mol. % nitrogen. It is assumed that the condensed fraction 37 contains less than 35 mol. % nitrogen.

The auxiliary refrigerant stream 132 preferably has a boiling point under standard conditions at a lower temperature than the starting point of a 30-steam overhead stream under standard conditions (standard conditions according to ISO 13443: 15 ° C at 1.0 atmosphere (0.1 MPa) This facilitates the re-condensation of the relatively high amount of methane present in the overhead steam stream 30, which in turn facilitates the adjustability of the methane content in the vapor fraction 80. For example, the auxiliary refrigerant may contain from 5 mol% up to 75 mol% nitrogen. In a preferred embodiment, the auxiliary refrigerant stream is formed by means of a discharge stream from the main refrigerant stream, more preferably by means of a light stream of a light fraction of refrigerant. This latter case is illustrated in figure 2, but can also be used in an embodiment of FIG. 1. Such a diverted flow can conveniently be directed back to the main refrigerant circuit through the annulus 186 of the cryogenic heat exchanger 180, where it can still be promoted heat from the flow in the upper and / or lower bundles of pipes.

For example, the intended composition of the auxiliary refrigerant contains from 25 mol. % to 40 mol. % nitrogen; from 30 mol. % to 60 mol. % methane and up to 30 mol. % C ₂ (ethane and / or ethylene), as a result of which the auxiliary refrigerant contains at least 95% of these components, and / or the total content of nitrogen and methane is at least 65 mol. % Composition within these ranges can be easily accessed from the main refrigerant circuit if mixed refrigerant is used to supercool the liquefied hydrocarbon stream.

You can also use a separate cooling circuit to partially condense a stream of 30 steam overhead. However, the use of a flow diverted from the main refrigerant stream has the advantage that the amount of additional equipment that needs to be installed is minimal. For example, an additional auxiliary refrigerant compressor and auxiliary refrigerant condenser are not required.

If the nitrogen stripping column 20 is equipped with an optional internal distillation section 22, as described above, the overhead steam stream 30 is preferably obtained from the head of the nitrogen stripping column 20 above the distillation section 22.

At least a portion of the irrigation 36 from the condensed fraction enters the distillation section 22 of the nitrogen stripping column 20, starting from the level above the distillation section 22. From here, the portion of the irrigation can seep down through the distillation section 22, in contact with vapors rising upward through the distillation section 22. In the case of the embodiment of FIG. 1, the condensed fraction may enter the nitrogen stripping column 20 through the irrigation inlet system 25. A portion of the irrigation is respectively obtained from the condensed fraction and loaded into the nitrogen stripping column 20 using an optional irrigation pump 38 (and / or it can come under the action of gravity) and line 36 of the portion of the irrigation. In the case of the embodiment of FIG. 2, the condensed fraction is separated inside the head region of the nitrogen stripping column 20 and thus is already available over the distillation section 22.

The irrigation portion may contain the entire condensed fraction, but, optionally, the condensed fraction is separated in an optionally provided divider of the condensed fraction 39 into a portion of liquid recycle, which is introduced via line 13 of liquid recycle, for example, into the first feed stream 10, and a portion of irrigation, which is introduced into a nitrogen stripping column 20 through an irrigation inlet system 25 and an irrigation portion line 36.

The ability to divide the condensed fraction into the irrigation portion 36 and the liquid recycle portion 13 is preferable to deflect any excess liquid of the condensed fraction around the distillation section 22 in the form of liquid recirculate so as not to interfere with the distillation section 22. Embodiments in which the portion of the liquid recycle is recycled to the stripping column nitrogen 20, bypassing the internal distillation section 22, can be carried out by feeding a portion of the liquid recycle to the stripping column 20 nitrogen at a point vertically below the rectifying section 22.

Partial condensation may also include direct and / or indirect heat exchange with other streams in other successive overhead heat exchangers. For example, a heat recovery heat exchanger 85 may be such an overhead heat exchanger in which partial condensation of the overhead stream further includes indirect heat exchange with the vapor fraction 80.

The optional steam recycle line 87 can be used selectively, respectively by selectively opening the steam recycle control valve 88 to increase the amount of nitrogen that remains in the liquid hydrocarbon product stream 90. This can be done by diverting a portion of the vapor recycle from the vapor fraction, relieving the pressure of a portion of the vapor recycle to flash pressure and subsequently pumping a portion of the vapor recycle into nitrogen-depleted liquid 40. The remaining portion of the vapor fraction 80, which does not enter the steam recycle line 87, can form a fuel portion that can be transferred to the incinerator 220.

In some embodiments, the target amount of nitrogen dissolved in stream 90 of the liquid hydrocarbon product is from 0.5 to 1 mol. %, preferably as close to 1.0 mol. %, but not more than 1.1 mol. % The steam recirculate flow control valve 88 controls the amount of vapor fraction stream 80 that is fed back, for example, to the flash flash separator 50, while bypassing the nitrogen stripping column 20. By this, the amount of nitrogen in the stream 90 of the liquid hydrocarbon product can be affected. To further contribute to achieving the target nitrogen content, the steam recirculate flow control valve 88 may be controlled in accordance with a signal from a QMI quality meter, which is optionally provided in the liquid hydrocarbon product line 90.

The proposed method and device is particularly suitable for use in combination with a hydrocarbon liquefaction system, such as a natural gas liquefaction system, in order to remove nitrogen from a crude liquefied product. It was found that even when the crude liquefied product (or cryogenic hydrocarbon composition) contains a sufficiently high amount of nitrogen, from 1 mol. % (or from about 1 mol.%) to 5 mol. % (or up to about 5 mol.%), the resulting liquid hydrocarbon product may correspond to a nitrogen content that is within the range of 0.5 to 1 mol. % nitrogen. The rest of the nitrogen is discharged as part of the vapor fraction in the exhaust gas, together with a controlled amount of methane.

It is assumed that the method and device proposed in the present invention are most useful when the crude liquefied product or cryogenic hydrocarbon composition contains from 1.5 mol. %, preferably from 1.8 mol. % to 5 mol. % nitrogen. Existing alternative approaches may also work adequately when the nitrogen content is below about 1.8 mol. % and / or below about 1.5 mol. %

Static simulation was carried out on the embodiment shown in figure 1, both for the storage mode (table 1) and for the loading mode (table 2). It was assumed that the cryogenic hydrocarbon composition 8 consists of more than 90 mol. % mixture of nitrogen and methane (98.204 mol.%). In the example, the amount of nitrogen (1.654 mol.%) And methane (98.204 mol.%) Is more than 99.8 mol. %, while the remaining part (0.142 mol.%) consists of carbon dioxide (0.005 mol.%). Carbon dioxide leaves the process with nitrogen-depleted liquid 40 and a stream 90 of liquid hydrocarbon product. The division ratio in the divider 9 of the original stream was approximately 75% in both cases.

It can be seen that both in the storage mode and in the loading mode, despite the large difference in the amount of process steam, the amount of methane in the vented vapor fraction 80 can remain at about 80 mol. % and, to a large extent, in the range of 10 mol. % to 25 mol. %, while the nitrogen content in the stream 90 of the liquid hydrocarbon product is maintained within the target values, close to 1.0 mol. %, but not exceeding 1.1 mol. %

In storage mode, about 2.0 kg / s of vaporized gas, consisting of about 17 mol. % nitrogen and 83 mol. % methane was added to the process through the supply line of evaporated gas 230, while in the loading mode this amount was about 4.4 kg / s.

In the storage mode, steam was not directed through the steam bypass line 76, while in the loading mode 30% of the compressed steam 70 was directed through the steam bypass line 76 to accommodate additional steam caused by the additional influx of vaporized gas. The amount of liquid recycle 13 in the loading mode also increased from about 8% to about 41% of the condensed fraction in the condensed fraction discharge line 37. The additional addition of the condensed fraction is the result of additional re-condensed methane.

When calculating the liquefaction system 100, the circuit shown in FIG. 2 was used with mixed refrigerant in the compressed refrigerant line 120 with the composition shown in table 3 in column “120”.

In storage mode, the pressure in line 120 of the compressed refrigerant was 58 bar abs. (5.80 MPa), in loading mode it was higher - 61 bar abs. (6.10 MPa). The total pressure drop in the lower and upper beams (183 and 184, respectively) of the LMR cryogenic heat exchanger tubes was 13 bar (1.30 MPa) in both cases. The pressure drop caused by the auxiliary refrigerant flow control valve 135 was 39 bar (3.90 MPa) in the case of storage mode and 42 bar (4.20 MPa) in the boot mode, so that the pressure in the annulus 186 of the cryogenic heat exchanger 180 was the same in both storage mode and boot mode.

The relative flow rate of auxiliary refrigerant stream 132 was 11% of the total LMR flow in line 131 of the LMR. In boot mode, it was 18%. In addition, the actual flow rate was 1.6 times higher than in the case of the storage mode, but the separation of the HMR and LMR in the MR separator 128 was slightly more in favor of the HMR when operating in boot mode than in storage mode.

In the above example, it was assumed that the cryogenic hydrocarbon composition does not contain hydrocarbons heavier than methane (C ₂ + hydrocarbons), as would be the case if the cryogenic hydrocarbon composition is made from unconventional gas sources such as coalbed methane, shale gas, or, perhaps some artificial sources. However, the proposed methods and device can also be used when the cryogenic hydrocarbon composition contains up to about 15 mol% of C ₂ + hydrocarbons, including one or more hydrocarbons selected from the group consisting of ethane, propane, i-butane, n-butane and pentane. In fact, it is not expected that these additional C ₂ + hydrocarbons will change the functioning of the proposed methods and devices, since it is assumed that none of these C ₂ + hydrocarbons will be detected in the steam 30 of the overhead or in the exhaust gas in the steam fraction discharge line 80, like carbon dioxide from the example.

One skilled in the art will understand that the present invention may be practiced in various ways without departing from the scope of the appended claims.

Claims (49)

1. The method of removing nitrogen from a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase, and this method includes: - providing a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase; - supplying a first feed stream for a nitrogen stripping column at a stripping pressure to a nitrogen stripping column containing at least one internal stripping section located inside the nitrogen stripping column, said first feed stream for a nitrogen stripping column containing a first portion of a cryogenic hydrocarbon composition; - discharge of nitrogen-depleted liquid from the region of the collector of the nitrogen stripping column located below the stripping section; - obtaining at least a stream of liquid hydrocarbon product and process steam from a nitrogen-depleted liquid, comprising at least a step of depressurizing the nitrogen-depleted liquid to a flash pressure; - compressing said process steam to at least a stripping pressure, thereby obtaining compressed steam; - selective division of the compressed steam into a stripping portion and an unboiled portion that does not contain a stripping portion, while the unboiled portion contains a bypassed portion of the specified compressed steam; - transmitting the stripping steam stream to the nitrogen stripping column at a level lower vertically of the indicated stripping section, wherein said stripping steam stream contains at least a stripping portion of said compressed steam; - the passage of the intermediate steam through the condenser, by means of which indirect heat exchange of the intermediate steam with the auxiliary refrigerant stream and partial condensation of the intermediate steam take place, said intermediate steam containing at least an unpaired portion of said compressed steam, and wherein said heat exchange includes heat from the intermediate steam to the auxiliary refrigerant stream in cooling mode, resulting in the formation of excess liquid from the intermediate vapor and into the cut tate which at least said bypass portion of the compressed vapor remains in the vapor phase; - removal of the vapor fraction in the form of an exhaust gas containing a vented overhead vapor fraction obtained from the overhead region of the nitrogen stripping column and containing at least said bypass portion; and - returning at least a portion of the portion of the liquid recycle to the liquid hydrocarbon product stream, wherein the portion of the liquid recycle contains at least a portion of the excess liquid; in which from the specified selective division to the discharge of the specified bypass portion in the vapor fraction of the vaporized gas, the specified bypass portion bypasses at least one inner stripping section. 2. The method of claim 1, wherein said partial condensation of the intermediate vapor results in a partially condensed intermediate stream comprising a condensed fraction and said vapor fraction, wherein said vapor fraction contains said bypass portion and wherein said condensed fraction contains said excess liquid, moreover, the method further includes: - selectively introducing an unpaired portion of said compressed steam into the overhead vapor obtained from the overhead region of the nitrogen stripping column, thereby forming said intermediate vapor; and - separation of the condensed fraction from the vapor fraction at a separation pressure to the specified discharge of the vapor fraction in the form of exhaust gas. 3. The method according to claim 2, wherein the nitrogen stripping column further comprises at least one internal distillation section located inside said nitrogen stripping column higher vertically than said steam stripping section in said nitrogen stripping column; moreover, the specified method further includes: - passing at least a portion of the irrigation from the condensed fraction to the inlet of the distillation section in a nitrogen stripping column from a level above the distillation section. 4. The method according to p. 3, further comprising: - dividing the condensed fraction into a specified portion of the irrigation and a portion of the liquid recycle, as a result of which the portion of the liquid recycle does not contain the specified portion of the irrigation; and wherein said return of at least a portion of the portion of the liquid recycle to the liquid hydrocarbon product stream includes: - deviation of a portion of the liquid recirculate around the distillation section. 5. The method according to any one of paragraphs. 1-4, in which the vapor fraction in the exhaust gas has a calorific value, the method further comprising adjusting the cooling mode to control the calorific value of the exhausted vapor fraction. 6. The method according to any one of paragraphs. 1-4, in which after said selective division the stripping portion has the same composition and phase as the unboiling portion. 7. The method according to any one of paragraphs. 1-4, in which the stripping pressure is in the range from 2 to 15 bar abs. (0.2-1.5 MPa) and / or in which the flash pressure is from 1 to 2 bar abs. (0.1-0.2 MPa). 8. The method according to any one of paragraphs. 1-4, further comprising supplying at least a fuel portion of the vapor fraction to the combustion device at a fuel gas pressure not exceeding the stripping pressure. 9. The method according to any one of paragraphs. 1-4, in which the process steam contains vaporized gas obtained by adding heat to the liquid hydrocarbon product stream, whereby a portion of the liquid hydrocarbon product stream is vaporized to form said evaporated gas. 10. The method according to any one of paragraphs. 1-4, in which flash vapor is generated during said depressurization of said nitrogen-depleted liquid to said flash pressure and in which process steam comprises said flash vapor. 11. The method according to any one of paragraphs. 1-4, in which the specified provision of the specified cryogenic hydrocarbon composition includes: - heat transfer of a feed stream containing a hydrocarbon-containing vaporous feedstock in a cryogenic heat exchanger with a main refrigerant stream, thereby leading to liquefaction of the vaporous feedstock from the feedstock to obtain a crude liquefied stream; and - obtaining a cryogenic hydrocarbon composition from a crude liquefied stream. 12. The method according to any one of paragraphs. 1-4, further including: - abstraction of a portion of vapor recycle from the vapor fraction; - depressurization of the indicated portion of the vaporous recycle to the pressure of flash evaporation; - the introduction of a portion of the vaporous recycle in at least one selected from the group consisting of: a nitrogen-depleted liquid, a stream of liquid hydrocarbon product and process steam. 13. The method according to any one of paragraphs. 1-4, in which the auxiliary refrigerant contains from 5 mol. % to 75 mol. % nitrogen. 14. The method according to any one of paragraphs. 1-4, in which the vapor fraction contains from 50 mol. % to 95 mol. % nitrogen. 15. A device for removing nitrogen from a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase, and this device contains: - a cryogenic feed line connected to a source of a cryogenic hydrocarbon composition containing a nitrogen and methane-containing liquid phase; - a nitrogen stripping column in fluid communication with the cryogenic feed line, said nitrogen stripping column comprising at least one internal stripping section located inside the nitrogen stripping column and a collection area located vertically below the stripping section; - a nitrogen-depleted liquid discharge line containing an intermediate pressure relief device in fluid communication with the region of the nitrogen stripping column collector, configured to receive nitrogen-depleted liquid from the collection region and depressurize the nitrogen-depleted liquid, said intermediate pressure relief device being located at the interface between the stripping pressure side including the nitrogen stripping column and the flash side ; - a line of liquid hydrocarbon product located on the pressure side of the flash, in communication with the intermediate pressure relief device, to divert the flow of liquid hydrocarbon product obtained from a nitrogen-depleted liquid; - a process steam line located on the pressure side of flash flash in communication with an intermediate pressure relief device for receiving process steam formed from a nitrogen-depleted liquid; - a process compressor located in the line of process steam, configured to receive process steam and compress process steam to create compressed steam at the outlet of the process compressor, said process compressor being at a specified interface between the stripping pressure side and the flash side; a bypass divider whose inlet is in fluid communication with the outlet of the process compressor to receive compressed steam, and the first inlet of the bypass divider is in fluid communication with a nitrogen stripping column through a stripping steam line and a second inlet system located at a lower vertical level of the stripping section and configured to receive at least a stripping portion of the specified compressed steam from the process compressor, and the second side Product bypass divider is in fluid communication with the bypass line pair comprising neotparnuyu portion of the compressed vapor; - a condenser located in fluid communication with a steam bypass line for supplying intermediate steam, which contains at least an unsuitable portion from the steam bypass line, the condenser comprising a heat exchange surface providing indirect heat exchange contact between the intermediate steam and the auxiliary refrigerant stream; - a discharge line in communication with both the condenser and the overhead region of the nitrogen stripping column and configured to discharge the vapor fraction in the form of exhaust gas containing the overhead vapor obtained from the overhead region of the nitrogen stripping column and a bypassed portion, containing non-condensed vapor from an intermediate vapor that has passed through a condenser; and - a line of liquid recycle located on the inlet side in fluid communication with the condenser and on the outlet side in fluid communication with the liquid hydrocarbon product line; in which the bypass path passes between the bypass divider and the lead line, the bypass path bypasses at least one inner stripping section, and in which the bypass steam line runs along the bypass path. 16. The device according to p. 15, in which the condenser is also in fluid communication with the region of the overhead of the nitrogen stripping column, as a result of which said intermediate pair contains both an unpaired portion from the steam bypass line and the overhead pair obtained from the region head cut of a nitrogen stripping column. 17. The device according to claim 16, wherein the nitrogen stripping column further comprises at least one internal distillation section located inside said nitrogen stripping column, vertically higher than said steam stripping section in said nitrogen stripping column, said device further comprising a system irrigation, configured to pass at least a portion of the condensed fraction from the condenser to enter the distillation section in a nitrogen stripping column, from a level above the distillation section, and a condensed fraction divider configured to receive the condensed fraction from the condenser, the liquid recirculate line being in fluid communication with the condenser through the condensed fraction divider and in fluid communication with the liquid hydrocarbon product line through the path for recycle that bypasses the distillation section.

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