CN103998882B - Method and apparatus for removing nitrogen from cryogenic hydrocarbon compositions - Google Patents

Abstract

Nitrogen is removed from the cryogenic hydrocarbon composition. The low temperature hydrocarbon composition is divided into a first portion and a second portion having the same composition and phase as the first portion. The first portion is fed to a nitrogen stripper column operating at a stripping pressure from which nitrogen-stripped liquid is withdrawn. Feeding the second portion into a nitrogen-stripped liquid or into a liquid hydrocarbon product stream or into a process vapour, the liquid hydrocarbon product stream or process vapour being produced from the nitrogen-stripped liquid as described above, and involving the step of depressurizing at least the nitrogen-stripped liquid to a flash pressure below the stripping pressure. The second portion bypasses the nitrogen stripper column between splitting the stream and feeding the second portion into the nitrogen stripped liquid or liquid hydrocarbon product stream or process vapor.

Description

Method and apparatus for removing nitrogen from cryogenic hydrocarbon compositions

technical field

The present invention relates to a method and apparatus for separating nitrogen from cryogenic hydrocarbon compositions.

Background technique

Liquefied natural gas (LNG) forms 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 hydrocarbons. It is often desirable to liquefy natural gas in an LNG plant at or near the source of the natural gas stream for a number of reasons. For example, natural gas is easier to store and transport over long distances as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored under high pressure.

WO2011/009832 describes a method for treating a multiphase hydrocarbon stream produced from natural gas, wherein lower boiling components such as nitrogen are separated from the multiphase hydrocarbon stream to produce content of this lower boiling component in the liquefied natural gas stream. It utilizes two consecutive gas/liquid separators operating at different pressures. The multiphase hydrocarbon stream is fed to a first gas/liquid separator at a first pressure. The bottoms stream of the first gas/liquid separator is passed to a second gas/liquid separator providing vapor at a second pressure lower than the first pressure. The vapor is compressed in the overhead compressor and returned to the first gas/liquid separator as a stripping vapor stream.

One disadvantage of the method and apparatus as described in WO2011/009832 is that two large gas/liquid separators are required.

Contents of the invention

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

- providing a low temperature hydrocarbon composition comprising a liquid phase comprising nitrogen and methane;

- feeding the first nitrogen stripper feed stream at stripping pressure to a nitrogen stripper, said nitrogen stripper comprising at least one internal stripping section disposed within the nitrogen stripper, said the first nitrogen stripper feed stream comprises a first portion of the cryogenic hydrocarbon composition;

- withdrawing nitrogen-stripped liquid from the liquid storage space of the nitrogen-stripping column below said stripping section;

- producing at least a liquid hydrocarbon product stream and a process vapor from said nitrogen-stripped liquid comprising at least the step of depressurizing said nitrogen-stripped liquid to a flash pressure below said stripping pressure;

- compressing the process vapor to at least the stripping pressure, whereby compressed vapor is obtained;

- passing a stripping vapor stream comprising at least a stripped portion of said compressed vapor into a nitrogen stripper at a level below gravity of said stripping section;

- discharging as off-gas a vapor fraction comprising a vented fraction of the top vapor obtained from the top portion of the nitrogen stripper;

- splitting said cryogenic hydrocarbon composition stream into said first portion and a second portion having the same composition and phase as said first portion;

- decompressing said second portion to said flash pressure;

- feeding said second portion into at least one of: nitrogen stripped liquid, liquid hydrocarbon product stream and process vapor;

Wherein said stream is split to said feed second portion, said second portion bypasses said nitrogen stripper.

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

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

- a nitrogen stripper in fluid communication with said cryogenic feed line, said nitrogen stripper comprising at least one internal stripper section disposed within the nitrogen stripper;

- an overhead vapor discharge line communicating with said nitrogen stripper via a headspace within said nitrogen stripper;

- a nitrogen-stripped liquid discharge line communicating with a liquid storage space within the nitrogen stripper column below the gravity of the stripping section;

- an intermediate pressure reducer in the discharge line of said nitrogen-stripped liquid, said intermediate pressure reducer being fluidly connected to said nitrogen stripper and arranged to receive storage from said nitrogen stripper nitrogen-stripped liquid in the liquid space and decompressing the nitrogen-stripped liquid, the intermediate pressure reducer being located at the interface between the stripping pressure side and the flash pressure side comprising the nitrogen stripping column superior;

- a liquid hydrocarbon product line disposed on said flash pressure side to discharge a liquid hydrocarbon product stream resulting from said nitrogen-stripped liquid;

- a process vapor line disposed on said flash pressure side to receive process vapor generated from said nitrogen-stripped liquid;

- a process compressor disposed in said process vapor line, configured to receive said process vapor and compress said process vapor to provide compressed vapor at a process compressor discharge of said process compressor, said a process compressor at said interface between the stripping pressure side and the flash pressure side;

- a stripping vapor line in fluid communication with the nitrogen stripper at a level below gravity of the stripping section and arranged to receive the compressed air from the process compressor at least the stripped portion of the vapor;

- an initial stream splitter in said cryogenic feed line, said initial stream splitter being arranged to split said cryogenic hydrocarbon composition into a first fraction and a second fraction having the same composition and phase as said first fraction part;

- a first feed line for transferring a first portion from said initial stream splitter to said nitrogen stripper;

- a second feed line for transferring the second portion from the initial stream splitter to at least one of: a nitrogen stripped liquid line, a liquid hydrocarbon product line and a process vapor line, the second feed line bypasses the nitrogen stripper.

Description of drawings

The invention is further described below using examples and with reference to the accompanying drawings, in which:

Figure 1 schematically represents a process flow diagram representing the method and apparatus incorporating one embodiment of the present invention; and

Figure 2 schematically represents a process flow diagram representing the method and apparatus incorporating another embodiment of the present invention.

In the figures, the same reference numerals will be used to designate the same or similar parts. Furthermore, a single reference number will be used to identify a pipe or line and the flow conveyed by that line.

detailed description

This specification relates to the removal of nitrogen from cryogenic hydrocarbon compositions comprising a liquid phase comprising nitrogen and methane. The cryogenic hydrocarbon composition is divided into a first portion and a second portion having the same composition and phase as said first portion. The first portion is fed to a nitrogen stripper column operating at stripping pressure, from which nitrogen stripped liquid is extracted. feeding said second portion into a nitrogen-stripped liquid or a liquid hydrocarbon product stream or process vapor produced from said nitrogen-stripped liquid, and A step involving at least depressurizing the nitrogen stripped liquid to a flash pressure below the stripping pressure. A stripping vapor stream comprising at least a stripped portion of the process vapor after compression is passed to a nitrogen stripper column.

Between stream splitting and feeding the second portion into the nitrogen-stripped liquid and/or liquid hydrocarbon product stream and/or process vapor, the second portion bypasses the nitrogen stripping column.

Thus, the liquid loading of the nitrogen stripper is reduced compared to where the entire feed of the cryogenic hydrocarbon composition is fed to the nitrogen stripper, and at the same time sufficient liquid can be maintained in the nitrogen stripper to Efficient stripping is facilitated using a stripping vapor stream. Thus, the nitrogen stripper can be smaller in size compared to the case of WO2011/009832, where the first gas/liquid separator receives the entire multiphase hydrocarbon stream to be treated.

Using the presently proposed solution, the amount of nitrogen maintained in the resulting liquid hydrocarbon product stream can be kept below a specified maximum nitrogen target while not all of the cryogenic hydrocarbon composition is passed through the nitrogen stripper. Liquid hydrocarbon product streams can be stored and transported at their cryogenic temperatures and at about atmospheric pressure.

The proposed solution also produces a waste gas consisting of a vapor fraction comprising a blow-off fraction of the top vapor obtained from the top part of the nitrogen stripping column. The vapor fraction may contain significant amounts of nitrogen, perhaps 50 mol% to 95 mol% nitrogen. However, preferably at a fuel gas pressure not higher than the stripping pressure, the vapor fraction can still be used as a fuel gas stream.

Preferably, the off-gas is consumed at a fuel gas pressure not higher than the stripping pressure. However, the need for a dedicated fuel gas compressor can be avoided. Furthermore, by choosing a stripping pressure at a pressure in excess of the fuel gas pressure, any compression applied to the process vapor has additional associated benefits, such as increasing the enthalpy of the process vapor, which allows the process vapor to be used as stripping steam.

It is suggested that the method and apparatus presented herein are most beneficial when the cryogenic hydrocarbon composition contains 1.5 mol % (preferably 1.8 mol %) up to 5 mol % nitrogen. Existing alternatives also work adequately at nitrogen levels below about 1.8 mol% and/or below about 1.5 mol%.

The proposed method and apparatus are particularly suitable for application in conjunction with hydrocarbon liquefaction systems, such as natural gas liquefaction systems, to remove nitrogen from crude liquefaction products produced in hydrocarbon liquefaction systems. It has been found that even when the crude liquefied product or cryogenic hydrocarbon composition contains a relatively high amount of nitrogen from 1 mol % (or about 1 mol %) up to 5 mol % (or up to about 5 mol %), the resulting liquid hydrocarbon product can satisfy 0.5 The nitrogen content within the index of nitrogen between 1 mol%. The remainder of the nitrogen is vented as part of the vapor fraction in the off-gas along with a controlled amount of methane.

Figure 1 shows an apparatus comprising an embodiment of the present invention. Cryogenic feed line 8 is in fluid communication with nitrogen stripper 20 via first inlet system 21 .

The liquefaction system 100 may be arranged upstream of the cryogenic feed line 8 . The liquefaction system 100 serves as a source of cryogenic hydrocarbon compositions. The liquefaction system 100 is in fluid communication with the cryogenic feed line 8 via the main reduced pressure system 5 which is in communication with the liquefaction system 100 via the crude liquefaction product line 1 . In the embodiment shown, the main pressure reduction system 5 consists of a dynamic unit (eg expander turbine 6) and a static unit (eg Joule Thomson valve 5), but other variations are possible.

The first feed line 10 connects the low-temperature feed line 8 and the first inlet system 21 of the nitrogen stripper 20 via an initial flow splitter 9, the initial flow splitter 9 is arranged between the low-temperature feed line 8 and the first feed between line 10.

The second feed line 11 is connected on its upstream side to the initial flow splitter 9 . The second feed line 11 bypasses the nitrogen stripper 20, as described further below. The initial splitter 9 is configured to divide the cryogenic hydrocarbon composition flowing through the cryogenic feed line 8 into a first portion passed to the first feed line 10 and a second portion passed to the second feed line 11 . One benefit of the second feed line 11 and the initial splitter 9 is that the size of the nitrogen stripper 20 can be smaller than if the cryogenic feed line 8 and the first feed line 10 are directly connected without a splitter such that The entire low temperature hydrocarbon composition enters the nitrogen stripper 20 via the first inlet system 21 .

A bypass flow control valve 15 is provided in the second feed line 11 . The bypass flow flow control valve is functionally connected to a flow controller FC arranged in the first feed line 10 . The flow controller FC is configured to maintain the flow of the first portion through the first feed line 10 at under predetermined target traffic.

The nitrogen stripper 20 includes an internal stripping section 24 disposed within the nitrogen stripper 20 . Overhead vapor discharge line 30 communicates with nitrogen stripper 20 via top portion 26 within nitrogen stripper 20 . A discharge line 40 for the nitrogen-stripped liquid communicates with the nitrogen stripper 20 via a liquid storage space 28 within the nitrogen stripper 20 , which is below the gravity of the stripping section 24 .

Nitrogen stripper 20 may include vapor/liquid contact enhancements to enhance component separation and nitrogen removal. Depending on the tolerance of nitrogen in the nitrogen stripping liquid and the amount of nitrogen in the cryogenic feed line 8, a total of between 2 and 8 theoretical stages may generally be required. In one particular embodiment, 4 theoretical stages are required. Such contact enhancing means may be provided in the form of trays and/or packings, in the form of structured or unstructured packings. At least part of the vapor/liquid contact enhancing means suitably forms part of the internal stripping section 24 .

An intermediate pressure reducer 45 is disposed in the discharge line 40 of the nitrogen-stripped liquid and is thereby fluidly connected to the nitrogen stripper column 20 . The intermediate pressure reducer 45 is functionally coupled to a liquid level controller LC cooperating with the liquid storage space 28 of the nitrogen stripper 20 .

The intermediate pressure reducer 45 is provided on the interface between the stripping pressure side and the flash pressure side including the nitrogen stripper 20 . The flash pressure side includes a liquid hydrocarbon product line 90 configured to discharge the liquid hydrocarbon product stream resulting from the nitrogen stripped liquid 40 and a process vapor line 60, the process vapor line 60 is arranged to receive process vapor generated from nitrogen stripped liquid 40. Via the initial stream splitter 9 and the second feed line 11 discussed briefly above, the cryogenic feed line 8 is connected to at least one of: a discharge line 40 for nitrogen-stripped liquid, a liquid hydrocarbon product line 90 and a process Steam line 60. Preferably, the second feed line 11 does not pass through any indirect heat exchanger for indirect heat exchange with any process stream.

In the embodiment shown, the flash pressure side also includes a cryogenic storage tank 210 connected to liquid hydrocarbon product line 90, an optional volatiles supply line 230, and an optional terminal for storing the liquid hydrocarbon product stream. Internal flash separator 50.

If such an end flash separator 50 is provided (as is the case in the embodiment of FIG. 1 ), the second feed line 11 suitably feeds the end flash separator 50 . Additionally, such an end flash separator 50 may be configured in fluid communication with the nitrogen stripper column 20 via an intermediate pressure reducer 45 and a discharge line 40 for nitrogen-stripped liquid. End flash separator 50 may then be connected to cryogenic storage tank 210 via liquid hydrocarbon product line 90 . A cryopump 95 may be present in liquid hydrocarbon product line 90 to assist in delivering the liquid hydrocarbon product to cryogenic storage tank 210 .

As shown in the embodiment of FIG. 1 , process vapor line 60 may be connected to optional end flash separator 50 via flash vapor line 64 and flash vapor flow control valve 65, and via optional volatiles supply line 230. Connected to cryogenic storage tank 210. One advantage of the latter connection is that it allows processing at least part of the volatiles from cryogenic storage tank 210 as part of the process vapor.

A process compressor 260 is also disposed at the interface between the stripping pressure side and the flashing pressure side. Preferably, process compressor 260 is driven by an electric motor. Process compressor 260 is disposed in process vapor line 60 to receive process vapor and compress the process vapor. Compressed vapor discharge line 70 is fluidly connected to process compressor discharge outlet 261 of process compressor 260 . Suitably, the process compressor 260 is provided with anti-surge control and a recirculation cooler which is used when the process compressor is in recirculation and during start-up (not shown in the figure).

Stripping vapor line 71 is in fluid communication with nitrogen stripper column 20 via second inlet system 23 arranged at a level below the gravity of stripping section 24 and preferably above liquid storage space 28 . Stripping vapor line 71 is connected to compressed vapor discharge line 70 via optional bypass splitter 79 . A stripping steam valve 75 is provided in the stripping steam line 71 .

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

The combustion device 220 is arranged to receive at least a fuel portion of the vapor from the top discharge line 30 . The combustion apparatus may comprise a plurality of combustion units, and/or it may comprise one or more of, for example, a furnace, a boiler, an incinerator, a dual fuel diesel engine or combinations thereof. A boiler and a dual fuel diesel engine can be coupled to a generator.

Vapor recycle line 87 is optionally configured to receive at least a vapor recycle portion of the vapor from overhead discharge line 30 . Vapor recycle line 87 bypasses nitrogen stripper 20 and feeds back at least one of: liquid hydrocarbon product line 90 and process vapor line 60 . A vapor recycle flow control valve 88 is preferably provided in the vapor recycle line 87 . One benefit of the proposed vapor recycle line 87 is that it allows the nitrogen content of the liquid hydrocarbon product stream 90 to be selectively increased. Vapor recycle line 87 suitably feeds the end flash separator 50 if an optional end flash separator 50 is provided.

Optionally, nitrogen stripper 20 includes internal rectification section 22 in addition to internal stripping section 24 . Internal rectification section 22 is disposed within nitrogen stripper 20 at a higher gravity than stripping section 24 . The first inlet system 21 is gravity disposed between the internal rectification section 22 and the internal stripping section 24 . Top portion 26 is formed by the space within nitrogen stripper 20 above the gravity of rectification section 22 .

Optional internal rectification section 22 may include vapor/liquid contact enhancements similar to internal stripping section 24 to further enhance component separation and nitrogen removal.

Typically, nitrogen stripper 20 cooperates with a condenser to provide downward liquid flow through internal stripping section 24 and/or optional internal rectifying section 22 . For example, in Figure 1 the condenser is provided in the form of an overhead condenser 35 external to the nitrogen stripper 20, while in Figure 2 the condenser is provided in the form of an integral internal overhead condenser 235 which The condenser 235 is integrally arranged inside the top portion 26 of the nitrogen stripper 20 .

Such a condenser may advantageously be used to recondense at least a portion of the compressed process vapor from the compressed vapor discharge line 70 . For example, in the embodiment of FIG. 1 , an overhead condenser 35 is provided in the overhead vapor discharge line 30 . Inside the top condenser 35, the top vapor may pass in indirect heat exchange contact with the auxiliary refrigerant flow 132, whereby heat is transferred from the top vapor to the auxiliary refrigerant flow with a cooling duty. An auxiliary refrigerant flow control valve 135 is disposed in the auxiliary refrigerant line 132 .

A cooling duty controller 34 may be provided to control the cooling duty, which is the rate at which heat is transferred from the overhead vapor to the auxiliary refrigerant stream. Suitably, the cooling duty controller 34 is configured to control the cooling duty relative to the demand for heating power in response to the exhaust gas heating value indicator. In the embodiment shown, the cooling duty controller 34 takes the form of a pressure controller PC and an auxiliary refrigerant flow control valve 135, which are functionally coupled to each other .

Still referring to FIG. 1 , a top separator 33 is disposed on the downstream side of the top vapor discharge line 30 . The overhead vapor discharge line 30 discharges into an overhead separator 33 . An overhead separator 33 is provided to separate any non-condensable vapor fractions from any condensed fractions of overhead vapor. A vapor fraction discharge line 80 is provided to discharge the vapor fraction.

Suitably, a combustion device 220 is arranged on the downstream end of the vapor fraction discharge line 80 to receive at least the fuel portion of the vapor fraction in the vapor fraction discharge line 80 . Suitably, the configuration of the optional vapor recycle line 87 includes an optional vapor fraction splitter 89 which may be placed in the vapor fraction line 80, thereby allowing the vapor fraction line 80 to be separated from the vapor recycle line. Controlled fluid communication between 87.

A cold recovery heat exchanger 85 may be provided in the vapor fraction discharge line 80 to keep the vapor fraction 80 cool by exchanging heat with the cold recovery stream 86 before feeding the vapor fraction 80 to any combustion means.

In one embodiment, cold recovery stream 86 may comprise, or may consist of, a sidestream derived from the hydrocarbon feed stream in hydrocarbon feed line 110 of liquefaction system 100 . The resulting cooled side stream can be combined, for example, with the cryogenic hydrocarbon composition in cryogenic feed line 8 . Thus, cold recovery heat exchange in cold recovery heat exchanger 85 supplements the production rate of cryogenic hydrocarbon compositions. In another embodiment, cold recovery stream 86 may be contained in overhead vapor discharge line 30 (preferably in the portion of overhead vapor discharge line 30 through which the overhead vapor passes from nitrogen stripper 20 to overhead condenser 35) or may consist of the top vapor of said top vapor. The required power from the auxiliary refrigerant stream 132 in the overhead condenser 35 is thereby reduced.

The reflux system is arranged to allow at least a reflux portion 36 of the condensed fraction to enter the nitrogen stripper 20 at a level above the rectification section 22 . In the embodiment of FIG. 1 , the reflux system includes a condensed fraction discharge line 37 fluidly connected to the lower portion of the overhead separator 33, an optional reflux pump 38, and an optional condensed fraction splitter 39, The optional reflux pump 38 is provided in the condensed fraction discharge line 37 . Optional condensed fraction splitter 39 fluidly connects condensed fraction discharge line 37 to nitrogen stripper column 20 via reflux portion line 36 and reflux inlet system 25, and fluidly connects condensed fraction discharge line 37 to optional liquid recycle line 13 . Optional liquid recycle line 13 is in liquid communication with liquid hydrocarbon product line 90 . Liquid 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 flow to the liquid hydrocarbon product line 90 while remaining in the liquid phase. Thus, the liquid recycle line 13 may, for example, be directly connected to one or more of the following: nitrogen stripper 20, cryogenic feed line 8, first feed line 10, second feed line 11, nitrogen stripped Liquid discharge line 40, optional end flash separator 50 and liquid hydrocarbon product line 90. A recirculation valve 14 is provided in the optional liquid recirculation line 13 . An optional return flow valve 32 which is functionally controlled by a return flow controller (not shown) may preferably be provided in the return section line 36 .

Liquid recycle line 13 is preferably in liquid communication with liquid hydrocarbon product line 90 via a recycle path that does not pass through rectification section 22 (if provided). In this way, liquid recycle line 13 helps avoid feeding too much liquid to rectification section 22 and avoids passing recycled liquid through rectification section 22 . This advantageously avoids disturbing the equilibrium in the nitrogen stripper 20 .

An optional bypass splitter 79 is in fluid communication with the overhead vapor discharge line 30, which is preferably on the upstream side of the top condenser 35, if provided. An optional vapor bypass line 76 may be provided between an optional bypass splitter 79 and the overhead vapor discharge line 30 . A vapor bypass control valve 77 is preferably disposed in the vapor bypass line 76 . One benefit of this vapor bypass line 76 is that when excess process vapor is sometimes present, it can be disposed of along with the waste gas in the vapor fraction discharge line 80 without disturbing the material balance in the nitrogen stripper 20 . The vapor bypass line 76 suitably extends along the bypass path between the bypass splitter 79 and the top vapor discharge line 30 on the upstream side of the top condenser 35 . A bypass path extends between bypass splitter 79 and overhead vapor discharge line 30 and/or vapor fraction discharge line 80 . The bypass path does not pass through the internal stripping section 24 in the nitrogen stripper 20 . In this way, the non-stripping portion can be avoided from passing through the internal stripping section 24 , which helps avoid disturbing the equilibrium in the nitrogen stripper column 20 .

The liquefaction system 100 in this specification has so far been described very schematically. It may represent 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 liquefaction system. Examples of suitable liquefaction systems use a single refrigerant cycle process (usually a single mixed refrigerant-SMR process, such as PRICO described in the article "LNG Production on floating platforms" by KR Johnsen and P Christiansen, Gastech (Dubai) in 1998, but there are also Possibly a single component refrigerant, such as the BHP-cLNG process also described in the aforementioned article by Johnsen and Christiansen); a dual refrigerant cycle process (such as the commonly used propane-mixed-refrigerant process often abbreviated C3MR, such as e.g. described in U.S. Patent 4,404,008, or such as a dual-mixed refrigerant-DMR-process (an example of which is described in U.S. Patent 6,658,891), or such as a dual-cycle process in which each refrigerant cycle contains a single component refrigerant); and A process based on three or more compressor banks for three or more refrigeration cycles (an example of which is described in US Patent 7,114,351).

Other examples of suitable liquefaction systems are described in: US Patent 5,832,745 (ShellSMR), US Patent 6,295,833, US Patent 5,657,643 (both variants of Black and VeatchSMR), US Patent 6,370,910 (ShellDMR). Another suitable example of a DMR is the so-called Axens LIQUEFIN process, as described for example in the article entitled "LIQUEFIN: ANINNOVATIVE PROCESSTOREDUCELNGCOSTS" by P-Y Martin et al. presented at the 22nd World Gas Conference (2003) in Tokyo, Japan. Other suitable three-cycle processes include, for example, US Patent 6,962,060, WO2008/020044, US Patent 7,127,914, DE3521060A1, US Patent 5,669,234 (commercially known as an optimized cascade process), US Patent 6,253,574 (commercially known as a mixed fluid cascade process) , US Patent 6,308,531, US Application Publication 2008/0141711, Mark J. Roberts et al. "Largecapacity singletrain AP-X(TM) Hybrid LNG Process", Doha, Qatar, Gastech, 2002 (October 13-16, 2002). These suggestions are provided to illustrate the broad applicability of the present invention and are not intended to be an exclusive and/or exhaustive list of possibilities.

Preferably, but not necessarily, any compressors, in particular any refrigerant compressors, forming part of the hydrocarbon liquefaction process in the liquefaction system are driven by one or more electric motors and not mechanically driven by any steam and/or gas turbines. Such compressors may only be driven by one or more electric motors. Not all of the examples listed above use electric motors as refrigerant compressor drives. It is obvious that any drive other than an electric motor may be used in place of the electric motor to get the best benefit of the invention.

An example in which the liquefaction system 100 is based on eg C3MR or ShellDMR is schematically shown in FIG. 2 . It uses a low temperature heat exchanger 180, which in this case is in the form of a coiled heat exchanger comprising lower and upper hydrocarbon product tube bundles (181 and 182 respectively), lower and upper LMR tube bundles (183 and 184 respectively) and HMR tube bundles 185.

Lower and upper hydrocarbon product tube bundles 181 and 182 fluidly connect crude liquefied product line 1 with hydrocarbon feed line 110 . At least one refrigerated hydrocarbon pre-cooling heat exchanger 115 may be disposed in the hydrocarbon feed line 110 upstream of the cryogenic heat exchanger 180 .

The main refrigerant in the form of mixed refrigerant is provided in the main refrigerant circuit 101 . The main refrigerant circuit 101 includes a consumed refrigerant line 150 connected to a low temperature heat exchanger 180 (in this case the shell side 186 of the low temperature heat exchanger 180 ) and a compressed refrigerant line 120 . With the main suction end of the main refrigerant compressor 160 , the compressed refrigerant line 120 connects the main refrigerant compressor 160 discharge outlet with the MR separator 128 . One or more heat exchangers are disposed in the compressed refrigerant line 120 , including in this embodiment at least one ambient heat exchanger 124 and at least one chilled main refrigerant pre-cooling heat exchanger 125 . MR separator 128 is fluidly connected to lower LMR tube bundle 183 via light refrigerant fraction line 121 and to the HMR tube bundle via heavy refrigerant fraction line 122 .

The at least one refrigerated hydrocarbon pre-cooling heat exchanger 115 and the at least one refrigerated main refrigerant pre-cooling heat exchanger 125 are refrigerated by pre-cooling refrigerant (via lines 127 and 126, respectively). The same pre-cooling refrigerant can be shared by the same pre-cooling refrigerant cycle. Furthermore, the at least one refrigerated hydrocarbon pre-cooling heat exchanger 115 and the at least one refrigerated main refrigerant pre-cooling heat exchanger 125 may be combined into one pre-cooling heat exchanger unit (not shown). See US Patent 6,370,910 as a non-limiting example.

An optional external stripping vapor supply line 74, if provided, may suitably be at a point upstream of said at least one refrigerated hydrocarbon pre-cooling heat exchanger 115, said at least one refrigerated hydrocarbon pre-cooling heat exchanger 115 or (possibly, for example, if two or more refrigerated hydrocarbon pre-cooling heat exchangers are provided) at a point between two consecutive refrigerated hydrocarbon feed line 110 to obtain a portion of the hydrocarbon feed stream from hydrocarbon feed line 110.

At the transition point between the upper (182, 184) and lower (181, 183) tube bundles, the HMR tube bundle 185 is fluidly connected to the HMR line 141 in which the HMR control valve 144 is disposed. HMR line 141 is in fluid communication with shell side 186 of cryogenic heat exchanger 180, and via shell side 186 and in a heat exchange arrangement with each of lower hydrocarbon product tube bank 181 and lower LMR tube bank 183 and HMR tube bank 185. The consumed refrigerant line 150 is in fluid communication.

Above upper tube bundles 182 and 184 , near the top of cryogenic heat exchanger 180 , LMR tube bundle 184 is fluidly connected to LMR line 131 . A first LMR return line 133 establishes fluid communication between the LMR line 131 and the shell side 186 of the cryogenic heat exchanger 180 . An LMR control valve 134 is arranged in the first LMR return line 133 . The first LMR return line 133 passes through the shell side 186 and is arranged in heat exchange with each of the upper and lower hydrocarbon product bundles 182 and 181 and with each of the LMR bundles 183 and 184 and with the HMR bundle 185. The consumed refrigerant line 150 is in fluid communication.

Figure 2 shows one possible source of auxiliary refrigerant. The LMR line 131 splits into an auxiliary refrigerant line 132 and a first LMR return line 133 . The second LMR return line 138 is fluidly connected at its upstream end to the auxiliary coolant line 132 via a top condenser (which may be implemented in the form of an integral internal top condenser 235), and at its downstream end the second LMR return line 138 suitably It is finally connected to the consumed refrigerant line 150 via the first HMR line 141 .

The piping surrounding nitrogen stripper 20 in FIG. 2 is similar to that shown in FIG. 1 and will not be described in detail. Optional lines (including optional liquid recycle line 13, optional external stripping vapor supply line 74, optional vapor bypass line 76, and optional vapor recycle line 87) may be provided, but for clarity not reproduced in Figure 2.

Comparing the embodiment of Figure 2 with the embodiment of Figure 1, one difference should be noted in that the top condenser 35, top separator 33 and reflux system are implemented in the form of an integral internal top condenser 235 known in the art. Optional liquid recycle line 13 may also be provided in the case of FIG. 2 if desired, for example by gravitationally placing a partial liquid drain tray (not shown) between integral internal top condenser 235 and rectification section 22. An optional condensed fraction splitter 39 of the form shown).

Apparatus and methods for removing nitrogen from a cryogenic hydrocarbon composition comprising a liquid phase comprising nitrogen and methane may operate as follows:

Providing a cryogenic hydrocarbon composition 8 comprising a liquid phase comprising nitrogen and methane, preferably at an initial pressure of 2 to 15 bara absolute (bara) and preferably at a temperature below -130°C Down.

The low temperature hydrocarbon composition 8 may be obtained from natural gas or petroleum reservoirs or coal beds. Alternatively, the cryogenic hydrocarbon composition 8 may also be obtained from another source including, for example, synthetic sources such as the Fischer-Tropsch process. Preferably, the cryogenic hydrocarbon composition 8 comprises at least 50 mol% methane, more preferably at least 80 mol% methane.

In a typical embodiment, temperatures below -130° C. may 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 comprising a hydrocarbon-containing feed vapor may be heat exchanged with a main refrigerant stream, for example, in a low temperature heat exchanger 180, thereby liquefying the feed vapor of the feed stream To provide the crude liquefied stream in the crude liquefied product line 1. The desired cryogenic hydrocarbon composition 8 can then be obtained from crude liquefied stream 1 .

The main refrigerant flow may be produced by circulating the main refrigerant in the main refrigerant circuit 101 , whereby the consumed refrigerant 150 is compressed in the main refrigerant compressor 160 , forming a compressed refrigerant from the consumed refrigerant 150 . Refrigerant 120. Heat is removed from the compressed refrigerant discharged from the main refrigerant compressor 160 via one or more heat exchangers disposed in the compressed refrigerant line 120 . This produces partially condensed compressed refrigerant, which phase separates in MR separator 128 into a light refrigerant fraction 121 and a heavy refrigerant fraction 122, the light refrigerant fraction 121 consisting of the partially condensed compressed refrigerant. Composed of vapor components, the heavy refrigerant fraction 122 consists of partially condensed liquid components of the compressed refrigerant.

The light refrigerant fraction 121 passes continuously through the low temperature heat exchanger 180 via the lower LMR beam 183 and the upper LMR beam 184 , while the heavy refrigerant fraction 122 passes through the low temperature heat exchanger 180 via the HMR beam 185 until the transition point. While passing through these separate tube bundles, the respective light and heavy refrigerant fractions are cooled by the light and heavy refrigerant fractions re-evaporating in the shell side 186 (thus producing the consumed refrigerant 150), which completes the cycle. Simultaneously, hydrocarbon feed stream 110 passes through low temperature heat exchanger 180 successively via lower hydrocarbon stream 181 and upper hydrocarbon stream 182 and is liquefied and subcooled by the same evaporated light and heavy refrigerant fractions.

Depending on the source, the hydrocarbon feed stream 110 may contain varying amounts of components other than methane and nitrogen, including one or more non-hydrocarbon components other than water (e.g., _CO2 , Hg, _H2 S and other sulfur compounds); and one or more hydrocarbons heavier than methane (such as notably ethane, propane and butane, and possibly lesser amounts of pentane and aromatic hydrocarbons). Hydrocarbons having a molecular weight of at least the molecular weight of propane may be referred to herein as C3+ hydrocarbons, and hydrocarbons having a molecular weight of at least the molecular weight of ethane may be referred to herein as C2+ hydrocarbons.

If desired, the hydrocarbon feed stream 110 may have been _pretreated to reduce and/or remove one or more undesired components, such as _CO2 and H2S, or have been subjected to other steps, such as prepressurization, etc. Such steps are well known to those skilled in the art and their mechanisms are not discussed further herein. Thus, the composition of the hydrocarbon feed stream 110 varies depending on the type and location of the gas and the pretreatment(s) applied.

The crude liquefaction stream 1 may comprise between 1 mol% and 5 mol% nitrogen at an original temperature between -165°C and -120°C and a liquefaction pressure typically between 15 bara and 120 bara. In many cases, the original temperature may be between -155°C and -140°C. In this narrower range, the required cooling load in the liquefaction system 100 is lower than that required at lower temperatures, while the subcooling at pressures above 15 bara is high enough to avoid depressurization to Overproduction of flash gas between 1 and 2 bara.

The low-temperature hydrocarbon composition 8 can be obtained from the raw liquefied stream 1 by mainly decompressing the raw liquefied stream 1 from the liquefaction pressure to the initial pressure. When the low-temperature hydrocarbon composition 8 reaches the initial stream splitter 9, the low-temperature hydrocarbon composition 8 is divided into a first part and a second part in the initial stream splitter 9, and the first part is the first part in the first feed line 10. A nitrogen stripper feed stream, said second portion is in the form of a bypass feed stream in the second feed line 11. The second part has the same composition and phase as the first part.

A first nitrogen stripper feed stream 10 derived from a cryogenic hydrocarbon composition 8 is fed via a first inlet system 21 to a nitrogen stripper column 20 at a stripping pressure.

The stripping pressure is usually equal to or lower than the initial pressure. In a preferred embodiment, the stripping pressure is chosen to be between 2 and 15 bar abs. Preferably, the stripping pressure is at least 4 bara, because with slightly higher stripping pressures, the stripping vapor in stripping vapor line 71 can benefit from some additional Enthalpy (form of heat of compression). Preferably, the stripping pressure is at most 8 bara to facilitate separation efficiency in the nitrogen stripper column 20 . Furthermore, if the stripping pressure is in the range between 4 and 8 bara, the waste gas in vapor fraction line 80 can readily be used as a so-called low pressure gas stream without further compression.

In one embodiment, the raw temperature of crude liquefaction stream 1 is -161°C, and the liquefaction pressure is 55 bara. The main decompression can be done in two stages: first a dynamic stage using an expansion turbine 6 to reduce the pressure from 55 bara to about 10 bara, followed by a further depressurization in a static stage to a pressure of 7 bara using a Joule Thomson valve 7. In this case, the stripping pressure was assumed to be 6 bara.

An overhead vapor stream 30 is obtained from the top portion 26 of the nitrogen stripper 20 . Vapor fraction 80 obtained from overhead vapor stream 30 and comprising the vented fraction of overhead vapor 30 is vented as off-gas. Suitably, at least the fuel portion of the vapor fraction 80 is passed to the combustion device 220 at a fuel gas pressure not higher than the stripping pressure.

Nitrogen-stripped liquid 40 is withdrawn from the liquid storage space 26 of the nitrogen stripper column 20 . The temperature of the nitrogen stripped liquid 40 is generally higher than the temperature of the first nitrogen stripper feed stream 10 . Typically, the temperature of the nitrogen-stripped liquid 40 is expected to be higher than the temperature of the first nitrogen stripper feed stream 10 and be between -140°C and -80°C, preferably between -140°C and -120°C .

The nitrogen-stripped liquid 40 is then depressurized, preferably using an intermediate pressure reducer 45, to a flash pressure which is lower than the stripping pressure and suitably in the range between 1 and 2 bar absolute. Preferably, the flash pressure is in the range between 1.0 and 1.4 bara. At a slightly higher difference between the flash pressure and the stripping pressure, the stripping vapor in stripping vapor line 71 may benefit from some additional heat of compression added to process stream 60 in process compressor 260 .

The intermediate pressure reducer 45 may be controlled by a liquid level controller LC set such that if the accumulated liquid level in the liquid storage space 26 of the nitrogen stripper 20 increases above a target level, the liquid level controller LC will The face controller LC increases the flow through the intermediate reducer. As a result of the depressurization, the temperature typically drops below -160°C. The liquid hydrocarbon product stream 90 thus produced can be maintained at atmospheric pressure, typically in an open insulated cryogenic storage tank.

Process vapor 60 is also produced. Process vapor 60 may comprise flash vapor 64 resulting from depressurization of nitrogen-stripped liquid 40 and/or depressurization of bypass feed stream 11 .

A second portion of the low temperature hydrocarbon composition 8 in the form of bypass feed stream 11 may be passed to and enter the optional end flash separator 50 , for example. The cryogenic hydrocarbon composition 8 stream is split into first and second portions such that the second portion 11 has the same composition and phase as the first portion 10 .

The bypass flow control valve 15 may be used to control the split ratio, which is defined as the flow of the second portion relative to the flow of the cryogenic hydrocarbon composition in the cryogenic hydrocarbon composition line 8 . The bypass flow control valve 15 is controllable by a flow controller FC to maintain a predetermined target flow of the first nitrogen stripper feed stream 10 into the nitrogen stripper column 20 . The flow controller FC will increase the opening of the bypass flow flow control valve 15 if there is a surplus of flow over the target flow and will decrease the opening if there is a flow deficit compared to the target flow.

As a general rule, the split ratio can advantageously be selected between 50% and 95%. Lower values are generally recommended for higher nitrogen contents in cryogenic hydrocarbon compositions, and higher values are preferred for lower nitrogen contents. In one embodiment, the nitrogen content in the low-temperature hydrocarbon composition 8 is 3.0 mol%, so the selected split ratio is 75%.

The second portion from the initial stream splitter 9 is then fed to at least one of the nitrogen-stripped liquid discharge line 40, the liquid hydrocarbon product line 90, and the process vapor line 60, while bypassing Prior to nitrogen stripper 20, the second portion is also depressurized to the flash pressure. Suitably, the optional second portion is passed to an optional end flash separator 50 . However, in any single pass from the initial stream splitter 9 to said subsequent feed, the second portion originating from the initial stream splitter 9 preferably does not undergo any functional indirect heat exchange. In this context, the term "functional indirect heat exchange" is intended to exclude inherently "non-functional" heat exchange and /or trivial heat exchange.

Volatile gases 230 typically result from the addition of heat to liquid hydrocarbon product stream 90 whereby a portion of liquid hydrocarbon product stream 90 evaporates to form volatile gases. In a typical LNG plant, the production of volatile gases can exceed the flash gas flow rate by several times, especially during operation of the project in the so-called loading mode, so if there is insufficient on-site demand for heating power to use contained in All the methane in the volatiles, then it is important to recondense not only the flash vapor but also the volatiles.

To facilitate the transfer of volatiles to process vapor stream 60 , optional volatiles supply line 230 preferably connects the vapor space in cryogenic storage tube 210 with process vapor line 60 . To facilitate transfer of flash gas 64 to process vapor stream 60 and further denitrogenate liquid hydrocarbon product stream 90, preferably after depressurizing the nitrogen-stripped liquid, the nitrogen-stripped liquid is fed to an optional In the optional end flash separator, the nitrogen stripped liquid is phase separated at flash separation pressure into a liquid hydrocarbon product stream 90 and a flash vapor 64. The flash separation pressure is at or below the flash pressure and suitably ranges between 1 and 2 bar absolute. In one embodiment, the flash separation pressure is expected to be 1.05 bara.

The process vapor 60 is compressed to at least the stripping pressure, thereby obtaining a compressed vapor stream 70 . Stripping vapor stream 71 is obtained from compressed vapor stream 70 and passed to nitrogen stripper column 20 via second inlet system 23 . The stripping vapor may percolate upwardly through the stripping section 23 in countercurrent contact with the liquid percolating downwardly through the stripping section 23 .

If external stripping vapor supply line 74 is provided in fluid communication with second inlet system 23 , external stripping vapor may optionally be fed to nitrogen stripper column 20 via second inlet system 23 . Major damage to the nitrogen stripper 20 can thus be avoided, for example in the event that the process compressor 260 does not function to provide a sufficient amount of the compressed vapor stream 70 .

Obtaining the stripping vapor stream 71 from the compressed vapor stream 70 may involve dividing the compressed vapor stream 70 into a stripping vapor stream 71 and a vapor bypass section which does not contain the stripping portion and which may optionally be injected into the overhead vapor Line 30 thereby bypasses nitrogen stripper 20 . Selective injection may be controlled using vapor bypass control valve 77 . Suitably, vapor bypass control valve 77 is controlled by a pressure controller on compressed vapor line 70 arranged to increase the opening portion of vapor bypass control valve 77 in response to increasing pressure in compressed vapor line 70 . The flow rate of the vapor bypass portion allowed to flow through vapor bypass line 76 into top vapor stream 30 is expected to be particularly high during the so-called loading mode, where the amount of volatiles is generally higher than during the so-called hold-off mode. Mode process is much higher. Preferably, vapor bypass control valve 77 is fully closed during normal operation in hold mode.

In a preferred embodiment, a partially condensed intermediate stream is formed from overhead vapor 30 . This involves indirect heat exchange of the overhead vapor 30 with the auxiliary refrigerant stream 132 whereby heat is transferred from the overhead vapor 30 to the auxiliary refrigerant stream 132 at a selected cooling duty. The resulting partially condensed intermediate stream comprises a condensed fraction and a vapor fraction.

In the context of this specification, cooling duty reflects the rate at which heat is exchanged in a condenser, which may be expressed in units of power such as watts or megawatts. The cooling duty is related to the flow of auxiliary refrigerant undergoing heat exchange with the top vapor.

The auxiliary refrigerant 132 stream preferably has a bubble point under standard conditions (ISO 13443 standard: 15° C. at 1.0 atmospheres) at a temperature below the bubble point of the overhead vapor stream 30 . This facilitates recondensing the relatively higher amount of methane present in overhead vapor stream 30 , which in turn facilitates controllability of the methane content in vapor fraction 80 . For example, the auxiliary refrigerant may contain between 5 mol% and 75 mol% nitrogen. In a preferred embodiment, the auxiliary refrigerant stream is formed by the slip stream of the main refrigerant stream, more preferably by the slip stream of the light refrigerant fraction. The latter case is shown in FIG. 2 but is also applicable to the embodiment of FIG. 1 . This slip flow can be conveniently passed back into the main refrigerant circuit via the shell side 186 of the low temperature heat exchanger 180, where it can still assist in the extraction of heat from the flow in the upper and/or lower bundles.

In one embodiment, one contemplated composition of the secondary refrigerant contains between 25 mol% and 40 mol% nitrogen, between 30 mol% and 60 mol% methane, and up to 30 mol% _C2 (ethane and/or ethylene), Thus, the auxiliary refrigerant contains at least 95% of these components and/or nitrogen and methane together at least 65 mol%. Compositions within these ranges can be readily obtained from the main refrigerant circuit if a mixed refrigerant is used for the subcooling of the liquefied hydrocarbon stream.

It is also possible to use a separate refrigeration cycle for the purpose of partially condensing the overhead vapor stream 30 . However, using the slip stream from the main refrigerant flow has the advantage that the amount of additional equipment installed is minimal. For example, no additional auxiliary refrigerant compressors and auxiliary refrigerant condensers are required.

The condensed fraction is separated from the vapor fraction in an overhead separator 33 at a separation pressure which may be lower than the stripping pressure and preferably in the range between 2 and 15 bar abs. The vapor fraction is discharged via vapor fraction discharge line 80 . The condensed fraction is discharged from the overhead separator 33 into the reflux system, for example via the condensed fraction discharge line 37 .

Thus, overhead condenser 35 allows recondensation of the vaporous methane that previously formed part of the crude liquefied product 1 up to the excess of methane in vented vapor fraction 80 by adding (compressing) any such vaporous methane-containing stream to the process vapor stream the extent of the target amount. Once forming process vapor 60 or a portion of compressed process vapor 70, the vaporous methane may seek to exchange heat with auxiliary refrigerant 132 whereby vaporous methane is selectively condensed from the overhead vapor from nitrogen stripper 20 while allowing Most of the nitrogen is emitted with the exhaust. Thus, it is possible to remove enough nitrogen from the cryogenic hydrocarbon composition 8 to produce a liquid hydrocarbon product stream 90 within the desired maximum specification for nitrogen content, while not creating more heat capacity in the exhaust than is required.

The vaporous methane that previously formed part of the crude liquefied product 1 may be formed for a number of reasons. During normal operation of a natural gas liquefaction facility, methane-containing vapors are formed from (crude) liquefaction products as follows:

- flash gas obtained from the flashing of the crude liquefied product during depressurization; and

- Volatile gases resulting from thermal evaporation due to heat added to the liquefied product, for example in the form of heat infiltration into storage tanks, LNG pipelines, and heat input from plant LNG pumps. During said mode of operation, referred to as holding mode operation, the storage tanks are filled with liquefied hydrocarbon product as it leaves the plant without any conveyor loading operations taking place at the same time. When in hold mode, methane-containing vapors are generated on the equipment side of the storage tank.

The operating mode of the LNG plant when there is a continuous conveyor loading operation (typically a ship loading operation) is called loading mode operation. During loading mode operation, volatile gases are additionally generated on the ship's side of the tank, for example due to initial cooling of the ship's tank, heat infiltration through pipes and vessels connecting the tank and ship, and heat input from the LNG loading pump.

The proposed solution can handle these vapors during hold mode and load mode operation. It combines the removal of nitrogen from the cryogenic hydrocarbon composition 8 and the recondensation of excess vaporous methane. This forms an excellent solution in situations where small amounts of plant fuel are required, as in the case of electrically driven plants using electricity from an external grid.

The heating value of the discharged vapor fraction 80 is appropriately adjusted by adjusting the cooling duty in the top condenser 35 . This can be done by cooling duty controller 34 . By adjusting the cooling duty of heat transfer from the overhead vapor to the auxiliary refrigerant stream, the relative amount of methane in the exhaust gas can be adjusted. As a result, the heating value of the vented vapor fraction can be adjusted to match specific needs for heating power. This makes the exhaust gas suitable for use as a fuel gas stream, even in situations where the heating value needs are variable.

When the vapor fraction 80 is delivered to the combustion device 220 as fuel and consumed by the combustion device 220 , the calorific value can be adjusted to match the actual demand of the heating power of the combustion device 220 .

The adjusted calorific value can be selected according to the suitability of the intended use of the exhaust gas as fuel gas. Calorific value can be determined according to DIN51857 standard. For many applications, the adjusted heating value can be proportional to the lower heating value (LHV; sometimes called net heating value), which can be defined as the temperature at which the combustion products The heat released when returning to 150°C. This assumes that the latent heat of vaporization of water in the reaction product is not recovered.

However, for the purpose of adjusting the heating value in the context of the present disclosure, it is not necessary to determine the actual heating value of the vapor fraction being vented on an absolute basis. Usually, it is sufficient to adjust the calorific value relative to the actual need for heating power in order to minimize any deficit or excess in the heating power delivered.

Preferably, the cooling duty is automatically adjusted in response to a signal that is thus related to the adjusted heating value. In embodiments where the vapor fraction is passed to one or more selective consumers of methane (such as the combustion device 220 shown in FIG. 1 ), control can be accomplished in response to the required heating power, thereby controlling the fraction of flow to obtain the calorific value that matches the need. Suitably, the auxiliary refrigerant stream flow control valve 135 is controllable by the pressure controller PC to maintain a predetermined target flow of the auxiliary refrigerant stream 132 through the top condenser 35 . The actual pressure in the vapor fraction discharge line 80 is thus related to the adjusted heating value. When the pressure drops below a predetermined target level (which indicates that the rate of consumption of methane is higher than the rate of supply of vapor fraction 80 ), pressure controller PC is set to decrease the opening portion of auxiliary refrigerant stream flow control valve 135 . Conversely, when the pressure exceeds a predetermined target level, the pressure controller PC is set to increase the opening portion of the auxiliary refrigerant flow flow control valve 135 .

The vapor fraction 80 is expected to contain between 50 mol% and 95 mol% nitrogen, preferably between 70 mol% and 95 mol% nitrogen or between 50 mol% and 90 mol% nitrogen, more preferably between 70 mol% and 90 mol% nitrogen, still more Preferably between 75 mol% and 95 mol% nitrogen, most preferably between 75 mol% and 90 mol% nitrogen. Condensed fraction 37 is expected to contain less than 35 mol% nitrogen.

If nitrogen stripper 20 is equipped with optional internal rectification section 22 as described above, overhead vapor stream 30 is preferably obtained from the top portion of nitrogen stripper 20 above rectification section 22 . Starting at a level above rectification section 22 , at least a reflux portion 36 of the condensed fraction enters nitrogen stripper 20 . In the case of the embodiment of FIG. 1 , the condensed fraction may pass through an optional reflux pump 38 (and/or it may flow under the influence of gravity). The reflux portion is then obtained from the condensed fraction and charged to nitrogen stripper column 20 via reflux inlet system 25 and reflux portion line 36 . In the case of the embodiment of FIG. 2 , the condensed fraction is separated inside the top part of the nitrogen stripper 20 and thus has been obtained above the rectification section to be contacted with the vapor rising upwards through the rectification section 22 Instead, it percolates downward through the rectification section 22 .

The reflux portion may contain the entire condensed fraction, but optionally the condensed fraction is split in an optionally provided condensed fraction splitter 39 into a liquid recycle portion and a reflux portion, the liquid recycle portion being discharged via liquid recycle line 13 to e.g. The reflux portion of the first feed stream 10 is discharged into the nitrogen stripper 20 via the reflux inlet system 25 and the reflux portion line 36 . The ability to split the condensed fraction into reflux portion 36 and liquid recycle portion 13 facilitates diverting any excess condensed fraction around rectification section 22 , eg, so as not to interfere with rectification section 22 operation. The recycle valve 14 may be suitably controlled using a flow controller provided in the condensed fraction discharge line 37 and/or a liquid level controller provided on the top separator 33 .

Partial condensation may also involve direct and/or indirect heat exchange with other streams in other successively arranged overhead heat exchangers. For example, cold recovery heat exchanger 85 may be an overhead heat exchanger by which partial condensation of the overhead stream also includes indirect heat exchange with vapor fraction 80 .

Optional vapor recycle line 87 may be selectively used by selectively opening vapor recycle control valve 88 to increase the amount of nitrogen held in liquid hydrocarbon product stream 90 as appropriate. This can be accomplished by extracting a vapor recycle portion from the vapor fraction, depressurizing the vapor recycle portion to flash pressure, and then injecting the vapor recycle portion into the nitrogen stripped liquid 40 . The remainder of the vapor fraction 80 that is not passed into the vapor recycle line 87 may form a fuel portion that may be passed to the combustion device 220 .

In some embodiments, the target amount of nitrogen dissolved in the liquid hydrocarbon product stream 90 is between 0.5 and 1 mol%, preferably close to 1.0 mol%, however not exceeding 1.1 mol% if possible. Vapor recycle flow control valve 88 regulates the amount of vapor fraction stream 80 that is fed back into, for example, end flash separator 50 while bypassing nitrogen stripper 20 . Thus, the amount of nitrogen in the liquid hydrocarbon product stream 90 may be affected. To further assist in meeting the target nitrogen content, vapor recycle flow control valve 88 may be controlled in response to a signal from a mass measurement instrument QMI optionally disposed in liquid hydrocarbon product line 90 .

Static simulations were performed at the embodiment shown in Figure 1 for both hold mode (Table 1) and load mode (Table 2). It is assumed that the cryogenic hydrocarbon composition 8 comprises more than 90 mol% of a mixture of nitrogen and methane (98.204 mol%). In the example, the amount of nitrogen (1.654 mol%) and methane (98.204 mol%) exceeds 99.8 mol%, the balance of 0.142 mol% consists of carbon dioxide (0.005 mol%). Carbon dioxide exits the process via nitrogen stripped liquid 40 and liquid hydrocarbon product stream 90 . In both cases, the split ratio of the initial vapor splitter 9 was about 75%.

It can be seen that the amount of methane in the vent vapor fraction 80 can be maintained at about 80 mol% and suitably between 10 mol% and 25 mol% in the hold mode and the load mode, despite the large difference in the amount of process vapor , while maintaining the nitrogen content in the liquid hydrocarbon product stream 90 within a target of approximately 1.0 mol% and no more than 1.1 mol%.

In hold mode, about 2.0 kg/s of volatile gas consisting of about 17 mol% nitrogen and 83 mol% methane is added to the process via volatile gas supply line 230, while in loading mode the volatile gas is about 4.4 kg /s.

In hold mode, no vapor is directed through vapor bypass line 76, while in charge mode, 30% of the compressed vapor 70 is directed through vapor bypass line 76 to accommodate the additional volatile gas inflow. additional vapor. In loading mode, the liquid recycle 13 is also increased from about 8% of the condensed fraction in the condensed fraction discharge line 37 to about 41% of said condensed fraction. The flow of additional condensed fractions is caused by additional recondensed methane.

Using the setup shown in FIG. 2 for the liquefaction system 100 under calculation, the mixed refrigerant in compression refrigerant line 120 has the composition as listed in Table 3 in the column labeled "120".

Table 3: Mixed refrigerant composition (in mol%)

In hold mode the pressure in the compressed refrigerant line 120 is 58 bara and in charge mode it is higher at 61 bara. In both cases the combined pressure drop in the lower and upper LMR tube bundles (183 and 184 respectively) of the cryogenic heat exchanger was 13 bar. The pressure drop applied by the auxiliary refrigerant stream flow control valve 135 is 39 bar in the hold mode case and 42 bar in the charge mode, so that the shell pressure in the shell side 186 of the low temperature heat exchanger 180 is same.

The relative flow of auxiliary refrigerant stream 132 is 11% of the total LMR flow in LMR line 131 . In charging mode, the relative flow of auxiliary refrigerant stream 132 is 18% of the total LMR flow in LMR line 131 . Also, the actual flow rate is 1.6 times that in the hold mode case, but the separation between HMR and LMR in MR splitter 128 is made slightly more HMR favored in load mode operation than in hold mode operation.

In the above examples, it is assumed that the low temperature hydrocarbon composition does not contain hydrocarbons heavier than methane (C2 ₊ hydrocarbons), such as the following situation: the low temperature hydrocarbon composition is derived from an unconventional gas source, such as coal bed methane, Shale gas or possibly some synthetic source. However, the proposed method and apparatus are also applicable where the cryogenic hydrocarbon composition contains up to about 15 mol% C2 ₊ hydrocarbons (including one selected from the group consisting of ethane, propane, isobutane, n-butane and pentane). one or more). In practice, these additional C2 ₊ hydrocarbons are not expected to alter the operation of the proposed process and apparatus, since none of these C2 ₊ hydrocarbons are expected to be present in the overhead vapor 30 or vapor fraction discharge like the carbon dioxide of the examples In the exhaust gas of line 80.

A person skilled in the art will appreciate that the invention can be carried out in many different ways without departing from the scope of the appended claims.

Claims (14)

1. from low temperature hydrocarbon composition, remove the method for nitrogen, described low temperature hydrocarbon composition comprises the liquid phase containing nitrogen and methane, and described method comprises:

-providing package is containing the low temperature hydrocarbon composition of the liquid phase containing nitrogen and methane;

-the first nitrogen stripping device incoming flow under stripping pressure is fed to nitrogen stripping tower, described nitrogen stripping tower comprises at least one the inner stripping portion section be arranged in nitrogen stripping tower, and described first nitrogen stripping device incoming flow comprises the Part I of low temperature hydrocarbon composition;

-from liquid through nitrogen stripping of the liquid storage spatial extraction of the nitrogen stripping tower below described stripping portion section;

-prepare at least liquid hydrocarbon product stream and process steam by the described liquid through nitrogen stripping, at least comprise the described liquid pressure-reducing through nitrogen stripping to the step lower than the flashing pressure of described stripping pressure;

-described process steam is compressed at least stripping pressure, obtain compressed vapour thus;

-stripping steam is spread in the nitrogen stripping tower at the level place be handed to below described stripping portion section gravity, described stripping steam stream comprises at least Stripping section of described compressed vapour;

-discharge vapor fraction as waste gas, described vapor fraction comprises the discharge cut of the overhead vapor of the top section available from described nitrogen stripping tower;

-described low temperature hydrocarbon combined stream is split into described Part I and there is the composition identical with described Part I and the Part II of phase;

-described Part II is decompressed to described flashing pressure;

-described Part II is fed at least one in the liquid of nitrogen stripping, liquid hydrocarbon product stream and process steam;

Wherein from described diverting flow to described charging Part II, described Part II walks around described nitrogen stripping tower,

Described method is further comprising the steps of:

-control the split ratio that described low temperature hydrocarbon composition is split into described Part I and described Part II, under thus the flow of described Part I being remained on predeterminated target flow, described split ratio is defined as the flow of the described Part I relative to described first and second parts total flow together.

2. method according to claim 1, wherein said stripping pressure 2 to 15 absolute pressures bar between scope in and/or wherein said flashing pressure be 1 to 2 absolute pressure bar between.

3. method according to claim 1 and 2, it also comprises and to be passed to by least fuel meat of described vapor fraction not higher than the burner under the fuel gas pressure of stripping pressure.

4. method according to claim 1 and 2, wherein said process steam comprises the Volatile Gas obtained by heat being added into described liquid hydrocarbon product stream, and a part for liquid hydrocarbon product stream is evaporated and forms described Volatile Gas thus.

5. method according to claim 1 and 2, wherein described, the described liquid pressure-reducing through nitrogen stripping is produced flashed vapour to the process of described flashing pressure, and wherein said process steam comprises described flashed vapour.

6. method according to claim 5, wherein described in prepared by the described liquid through nitrogen stripping, at least liquid hydrocarbon product stream and process steam also comprise the steps: under the flash separation pressure being equal to or less than flashing pressure, in the flash separator of end, the described liquid through nitrogen stripping are separated into liquid hydrocarbon product stream and flashed vapour.

7. method according to claim 6, the wherein said at least one be fed to by described Part II in the liquid of nitrogen stripping, liquid hydrocarbon product stream and process steam comprises: after described Part II is decompressed to described flashing pressure, described Part II is fed to described end flash separator.

8. method according to claim 1 and 2, wherein said nitrogen stripping tower also comprises gravity at least one inner rectifying portion section higher than the described stripping portion section in described nitrogen stripping tower, and described method also comprises:

-by the intermediate flow of the overhead vapor forming section condensation of the top section available from the nitrogen stripping tower above described rectifying portion section, the intermediate flow of described partial condensation comprises condensate fraction and vapor fraction, described formation comprises by making described overhead vapor and the heat exchange of auxiliary refrigerant stream and thus heat being passed to auxiliary refrigerant stream with certain cooling load from described overhead vapor, thus overhead vapor described in partial condensation;

-under separating pressure, make described condensate fraction be separated with described vapor fraction;

-make the level of at least refluxing portion of described condensate fraction above described rectifying portion section enter rectifying portion section in described nitrogen stripping tower.

9. method according to claim 1 and 2, the wherein said described low temperature hydrocarbon composition that provides comprises:

-make the incoming flow of the feed stream comprising hydrocarbon-containifirst in low temperature heat exchanger with the heat exchange of main refrigerant stream, the feed stream of described incoming flow of liquefying thus is to provide fluidized flow; And

-obtain described low temperature hydrocarbon composition by described fluidized flow.

10. method according to claim 1 and 2, it also comprises the by-passing part of described compressed vapour is optionally injected described overhead vapor, and walk around at least stripping portion section of described nitrogen stripping tower thus, described by-passing part does not comprise Stripping section.

11. methods according to claim 1 and 2, it also comprises:

-extract vapor recirculation part from described vapor fraction;

-by described vapor recirculation part reduced pressure to flashing pressure;

-described vapor recirculation part is injected following at least one: through the liquid of nitrogen stripping, liquid hydrocarbon product stream and process steam.

12. methods according to claim 1 and 2, wherein said vapor fraction comprises the nitrogen between 50mol% to 95mol%.

13. methods according to claim 8, wherein said condensate fraction comprises the nitrogen being less than 35mol%.

14. 1 kinds for removing the method for nitrogen from low temperature hydrocarbon composition, described low temperature hydrocarbon composition comprises the liquid phase containing nitrogen and methane, and described device comprises:

-being connected to the low temperature feeding line in the source of low temperature hydrocarbon composition, described low temperature hydrocarbon composition comprises the liquid phase containing nitrogen and methane;

-nitrogen stripping the tower that is communicated with described low temperature feeding line fluid, described nitrogen stripping tower comprises at least one the inner stripping portion section be arranged in nitrogen stripping tower;

-overhead vapor discharge pipe line, described overhead vapor discharge pipe line is communicated with described nitrogen stripping tower via the headroom in described nitrogen stripping tower;

-through the discharge pipe line of the liquid of nitrogen stripping, the discharge pipe line of the described liquid through nitrogen stripping is communicated with the liquid storage space in the nitrogen stripping tower below the gravity of described stripping portion section;

-intermediate relief device in the discharge pipe line of the described liquid through nitrogen stripping, described intermediate relief device fluid is connected to described nitrogen stripping tower, and arrange in order to reception from the liquid through nitrogen stripping in the liquid storage space of described nitrogen stripping tower and by the described liquid pressure-reducing through nitrogen stripping, on the interface of described intermediate relief device between the stripping pressure side comprising described nitrogen stripping tower and flashing pressure side;

-be arranged at liquid hydrocarbon product pipeline on described flashing pressure side, to discharge the liquid hydrocarbon product stream produced by the described liquid through nitrogen stripping;

-be arranged at process steam pipeline on described flashing pressure side, to receive the process steam produced by the described liquid through nitrogen stripping;

-be arranged at process compresses machine in described process steam pipeline, it is arranged in order to receive described process steam and to compress described process steam, compressed vapour is provided, on the described interface of described process compresses machine between stripping pressure side and flashing pressure side with the process compresses machine floss hole place at described process compresses machine;

-stripping steam pipeline, the level place of described stripping steam pipeline below the gravity of described stripping portion section is communicated with described nitrogen stripping tower fluid, and arranges at least Stripping section in order to receive from the described compressed vapour of described process compresses machine;

-initial flow current divider in described low temperature feeding line, described initial flow current divider is arranged in order to described low temperature hydrocarbon composition is divided into Part I and has the composition identical with described Part I and the Part II of phase;

-the first feeding line, described first feeding line is used for the Part I from described initial flow current divider to be sent to described nitrogen stripping tower;

-the second feeding line, described second feeding line be used for the Part II from described initial flow current divider to be sent to following at least one: through the liquid line of nitrogen stripping, liquid hydrocarbon product pipeline and process steam pipeline, described second feeding line walks around described nitrogen stripping tower;

-flow governor, described flow governor is constructed by the split ratio controlling low temperature hydrocarbon composition and be divided into the first and second parts, thus under making the flow of described Part I remain on predeterminated target flow.