KR20070091323A - Method and apparatus for producing a liquefied natural gas stream - Google Patents

Abstract

Method and apparatus producing a LNG stream, wherein before liquefaction heavier hydrocarbon components are removed from a NG stream to be liquefied, the method at least comprising the steps of: providing a vaporous feed stream (1) of natural gas; feeding the feed stream (1) into a distillation column (10); drawing a bottom stream (17) and an overhead stream (16) from the distillation column (10); and liquefying at least a part of the overhead stream (16) thereby obtaining a LNG stream; wherein the step of feeding the feed stream (1) into the distillation column (10) comprises the sub steps of: splitting the feed stream (1) into first (3a) and second (3b) substreams; feeding the first substream (3a) into the distillation column (10) via a first feed point (7a), at a pressure not lower than the feed stream pressure minus a pressure drop brought about by the said splitting of the feed stream (1); cooling the second substream (3b); and feeding the cooled second substream (7) into the distillation column (10) at a second feed point (7b) overhead of the first feed point (7a).

Description

METHOD AND APPARATUS FOR PRODUCING A LIQUEFIED NATURAL GAS STREAM}

The present invention relates to a method and apparatus for producing a liquefied natural gas (LNG) stream, wherein the LNG stream mainly comprises methane (preferably greater than 90 mol%).

In the absence or other attractive means of transport, it is common to liquefy natural gas and transport it to carriers. Natural gas is significantly reduced in volume through liquefaction, allowing for more efficient transport. In order to produce liquefied natural gas (LNG), a liquefaction process is used. The liquefaction process generally includes a cryogenic region that includes one or more refrigeration cycles, so that the natural gas is cooled through one or more stages to ambient temperatures near or slightly below the boiling point of the natural gas at ambient temperature. This boiling point is generally about -160 ° C.

Refrigeration fluids are generally used in refrigeration cycles, which may be formed of a mixture or a single component. The refrigerant is typically vaporized in one or more cryogenic heat exchangers where the natural gas is cooled. The vaporized refrigerant is then compressed to high temperature and high pressure. In an ambient cooler, heat from the refrigerant is released into a cooling medium such as water or air and then cooled by expansion. It is very common in liquefaction processes involving multiple cycles that the continuous refrigeration cycle is cooled by the first refrigeration cycle.

In today's liquefaction process, it is also common to remove certain components from natural gas before it is cooled in cryogenic heat exchangers. Among the components generally removed are carbon dioxide, sulfur containing compounds, water and hydrocarbons having a higher molecular weight than butane. In this specification, the last of these is called "heavy hydrocarbon". These components must be removed from the natural gas, otherwise they become solids at the cryogenic temperatures at which liquefaction takes place.

Water and acid gases are preferentially removed from the raw natural gas stream, and there are many physical and / or chemical processes for this. The resulting stream of sweetened and dried natural gas is subjected to the removal of heavy hydrocarbons. Heavy hydrocarbons are generally removed by partial condensation of the natural gas mixture, followed by separation of the gaseous phase in which the heavy hydrocarbons are lean and the liquid phase rich in heavy hydrocarbons. The most common method is to use a scrub column for this separation. The scrub column is a distillation column, which includes a series of separation stages between the bottom and top, the heavy hydrocarbon-rich mixture exits the bottom in the form of a bottom stream, and the light natural gas mixture is an overhead stream ( discharge from the top in the form of an overhead stream).

U.S. Patent 5 685 170 discloses a system and process for recovering propane, butane and heavy hydrocarbon components from natural gas to produce a gas stream consisting primarily of methane and ethane.

Various aspects of a method for pretreatment of a natural gas stream using a single scrub column to remove freezable C _{5 +} components to provide LNG products, and a line-up of scrub columns, are described in US Pat. 673.

In one aspect, the natural gas feed stream is divided into several feed substreams, cooled, and introduced into the scrub column at different feed points near the top and middle of the scrub column. The first portion of natural gas is cooled by expansion in the Joule-Thompson valve and introduced into the scrub column under reduced pressure. The second and third portions are first cooled in a cooler prior to depressurization and separated into a condensation liquid stream and a gas stream. This condensed liquid stream is fed into the scrub column at a lower feed point than the vapor stream.

A reboiler is provided to vaporize the liquid accumulated in the bottoms of the scrub column and fractions of the heavy stream rich in heavy hydrocarbons. The reboiler also performs the function of controlling the temperature of the scrub column bottom so that the temperature of the scrub column bottom is not so low that unwanted components such as carbon dioxide do not accumulate in the bottom stream.

Known embodiments have many disadvantages. First, because liquefaction of natural gas at low pressure requires more energy, depressurizing the feed stream before feeding it into the scrub column reduces the efficiency of the subsequent liquefaction step.

The disadvantage of the reboiler is that, despite the essence of the liquefaction process being the cooling of the natural gas, it heats the natural gas. The use of reboilers adversely affects the overall efficiency of the liquefaction process.

It is an object of the present invention to minimize the one or more disadvantages.

It is a further object of the present invention to provide another method for producing a liquefied natural gas stream comprising mainly liquefied methane.

According to the present invention, one or more of these or other objects are achieved by providing a process for producing a liquefied natural gas stream, wherein prior to liquefaction, the heavy hydrocarbon component having a higher molecular weight than butane is removed from the natural gas stream to be liquefied, the process comprising

Providing a feed stream of natural gas substantially gaseous at the feed stream pressure and temperature;

Feeding the feed stream into a distillation column having at least two separation stages;

Extracting a bottoms stream from the bottom of the distillation column and extracting an overheads stream from the top of the distillation column with a relatively less amount of heavy hydrocarbon components than the bottoms stream; And

Liquefying at least a portion of said overhead stream to obtain a liquefied natural gas stream,

Feeding the feed stream into the distillation column,

Separating the feed stream into first and second substreams at a selected split ratio;

-Feeding said first substream into said distillation column through a first feed point below said distillation column at a pressure above said feed stream pressure minus the pressure drop caused by said separation of said feed stream; ;

Cooling the second substream in a heat exchanger to a temperature lower than the supply temperature;

A substep consisting of feeding the cooled second substream into the distillation column at the second feed point higher than the first feed point.

For the purposes of this specification, the separation ratio is defined as the mass flow rate of the first substream divided by the mass flow rate of the second substream.

An advantage of the present invention is that in dedicated decompressors such as (turbo) expanders or Joule-Thomson valves, they do not intentionally lower the pressure of the feed stream as well as the first and second substreams.

Since the first substream is fed into the distillation column essentially at a pressure above the feed stream pressure minus the pressure drop caused by the separation of the feed stream, there is no need to lower the pressure at the second feed point. Thus, since distillation is performed without a significant pressure drop of natural gas, it is energy-efficient if the overhead stream is liquefied.

Another consequence of not intentionally depressurizing the pressure of the first substream is that the temperature is kept close to the feed stream temperature, and it is preferred that the first substream is not heated. The advantage of this is that less heating is usually required at the bottom of the distillation column, for example, in order to prevent it from cooling too much, for example through a reboiler.

By selecting the separation ratio sufficiently high, there is no need to add additional heating power at all, thus eliminating the need to provide a reboiler for control of the bottom temperature.

It has been found that the separation ratio can be selected such that the bottom temperature of the distillation column is maintained at -10 ° C or higher.

The temperature of the bottom of the distillation column can be controlled by providing a selectable or controllable variable separation ratio to select or control the separation ratio.

In addition, the present invention is embodied in an apparatus for producing a liquefied natural gas stream, in which heavy hydrocarbon components having a molecular weight greater than butane can be removed from the natural gas stream to be liquefied prior to liquefaction, wherein

A feed stream line for conveying a natural gas feed stream that is substantially gaseous at the feed pressure and temperature;

At least two separation stages for separating heavy hydrocarbon components from the natural gas, a bottom stream outlet opening disposed at the bottom of the distillation column for the discharge of a bottom stream, and a quantity of heavier hydrocarbon components relative to the bottom stream is relatively A distillation column having an overhead stream discharge opening disposed on top of the scrub column for discharge of less overhead stream; And

A cryogenic region from which at least a portion of the overhead stream can be liquefied to obtain a liquefied natural gas stream,

The feed stream line includes a feed stream junction fluidly connecting the main branch with the first and second branches to separate the feed stream into first and second substreams according to a selected separation ratio. Is connected to the feed stream junction at a pressure above the pressure drop of the feed stream resulting from the separation of the feed stream at a first feed point below the distillation column, the second branch being a second feed of the distillation column higher than the first feed point. Connected to the feed stream junction at the point, the second branch is provided with a heat exchanger.

For the purpose of this specification, it is to be understood that the heat exchanger comprises at least a so-called spool-wound type heat exchanger.

Most broadly, the present invention is applicable to any distillation column capable of removing heavy hydrocarbon components of higher molecular weight than butane in hydrocarbon gas mixtures. However, one or more preferred embodiments of the present invention require a scrub stream to be fed into the column. In that case, the distillation column becomes a so-called scrub column.

Hereinafter, with reference to the accompanying drawings, these or other features of the present invention will be described by way of example.

For this purpose, a single reference numeral is assigned to a line and a stream traveling through that line. Like reference numerals refer to like elements.

1 is a schematic diagram illustrating a process flow according to a first aspect of the present invention,

2 is a schematic diagram illustrating a process flow according to a second aspect of the present invention,

3 is a schematic diagram illustrating a process flow according to a third aspect of the present invention,

4 is a schematic diagram showing an alternative process flow according to a third aspect of the invention,

5 is a schematic diagram illustrating an alternative process flow according to a second aspect of the present invention;

6 is a schematic diagram showing a process flow according to a fourth aspect of the present invention,

7 is a schematic diagram illustrating a process flow according to a fifth aspect of the present invention,

8 is a schematic diagram showing a process flow in which the feed stream is not separated.

1 is a schematic diagram illustrating a process flow including a system for separating heavy hydrocarbon components having a higher molecular weight than butane from a hydrocarbon gas mixture as part of an apparatus for producing an LNG stream comprising primarily methane. For the purposes of this specification, hydrocarbon gas mixtures are considered to be in the form of natural gas mixtures, which are previously treated to remove water, CO ₂ , and sulfur using known methods. In general, the pretreated natural gas mixture comprises light components comprising gaseous methane and ethane and C ₃ and C ₄ , and heavy components (C _{5 +} ) that can potentially solidify upon liquefaction of methane. .

The apparatus of FIG. 1 comprises a natural gas feed line arranged to receive and transport a feed stream of a hydrocarbon gas mixture from which heavy hydrocarbons are removed. The feed stream line is separated from the main branch 1 into a first substream branch 3a and a second substream branch 3b, respectively. In order to separate the feed stream in the main branch 1 into first and second substreams that can flow through the first and second substream branches 3a, 3b, respectively, a feed stream junction 2 is provided. . A feed stream junction 2 is arranged to separate the feed stream according to a specific separation ratio defined as the mass flow rate of the first substream divided by the mass flow rate of the second substream. Those skilled in the art will understand that feed stream junction (2) is not a phase separator but separates the main feed stream into two or more substreams.

Both the first and second substream branches 3a, 3b are in fluid communication with the scrub column 10. The scrub column 10 of the present embodiment is a distillation column provided with a plurality of trays 11 which enable the separation of light hydrocarbon components from heavy hydrocarbon components in a plurality of separation stages. The temperature of the scrub column typically varies and the lower the stage, the lower. The scrub column 10 is provided at the bottom of the scrub column 10, for example, at the bottom of the scrubbing column 10, at the bottom of the scrubbing column 10 and at the bottom of the scrubbing column 10. It is further arranged to include an overhead stream outlet opening 12 for discharging an overhead stream enriched in light hydrocarbon components, for example via an outlet line 16. The overhead stream 16 is connected with a cryogenic region (not shown) for LNG production.

The first branch 3a connects the feed stream junction 2 and the first feed point 7a of the scrub column 10. In order to feed the first substream 3a to one of the lower trays 11, the first feed point 7a is relatively close to the bottom of the scrub column 10. The substream 3a is preferably supplied below the lowest phase separation tray 11. Since the first branch 3a is essentially free of pressure reducing devices, the feed stream junction 2 is in fluid connection to the first feed point 7a essentially without pressure loss. The connection is preferably dimensioned such that under normal operating conditions the pressure loss does not exceed 5 bar, and more preferably does not exceed 2 bar. Furthermore, the first substream 3a is not heated.

The second branch 3b connects the feed stream junction 2 and the second feed point 7b of the scrub column 10. In order to feed the first substream to one of the trays higher than the lower tray, the second feed point 7b is located higher than the first feed point 7a.

The second branch 3b is provided with a heat exchanger 6 which divides the second branch 3b into a warm portion 3b and a cold portion 7. The heat exchanger 6 is arranged essentially for cooling the second substream 3b without intentionally depressurizing it. The heat exchanger 6 can be any suitable heat exchanger, such as a so-called spool-wound heat exchanger.

Under normal operating conditions, the pressure drop of the second substream is less than 6 bar, preferably less than 3 bar. The heat exchanger 6 has at least one refrigerant supply 4 and one exhausted or vaporized refrigerant removal 5. The heat exchanger 6 may be an integral heat exchanger or a dedicated heat exchanger, which may provide cooling for other purposes. It is preferable that the heat exchanger 6 becomes an exclusive heat exchanger using an external refrigerant.

Although not required in the present invention further related to separating the feed stream into first and second substreams 3a, 3b, in the aspect of FIG. 1, a third feed point 7c is provided in the scrub column 10. It is advantageous to be. The third feed point 7c is located near the top of the scrub column 10 higher than the second feed point 7b. An optional scrub stream line 18 connects the third feed point 7c with the scrub stream source. The scrub stream source serves to feed a multi-phase stream or other liquid capable of scrubbing heavy hydrocarbons to facilitate the downstream transport of the heavy hydrocarbons in the scrub column 10. The scrub stream is a stream of cooler natural gas, condensate from an overhead condenser, LNG, chilled LNG, chilled condensate, mixtures thereof, or any other stream having properties suitable for removing heavy hydrocarbons from natural gas. It may include one or more groups (group) consisting of.

In operation, the apparatus of FIG. 1 acts as follows. The feed stream of the pretreated hydrocarbon gas mixture is provided via line 1 at feed stream pressure and temperature. The pressure of the feed stream is generally 20 to 80 bar, more typically 40 to 65 bar. The temperature of the feed stream is generally from 0 to 50 ° C, typically 15 to 25 ° C, more typically 15 to 20 ° C.

The feed stream is separated into first and second substreams 3a, 3b in feed stream junction 2, with minor and major substream forms being preferred. The minor substream 3a is fed into the scrub column 10 through the first feed point 7a, the pressure of which is caused by the separation of the feed stream 1 in the feed stream junction 2 at the feed stream pressure. More than minus the pressure drop. In practice, this means that the pressure in the minor substream 3a is not intentionally lowered.

The second substream 3b, which is usually a major substream, is cooled in the heat exchanger 6 to a temperature lower than the supply temperature. In general, the major substream 3b is not cooled below -50 ° C, preferably not below -20 ° C. Preferably, the major substream 3b is cooled to -10 deg.

The cooled major substream is at a position higher than the position at which the minor substream 3a is fed into the scrub column 10 through the cooling section 7 of the second feed point 7b and the second subbranch 3b. In, the scrub column 10 is fed into.

The temperature of the optional scrub stream of the scrub line 18 is below the temperature of the second substream entering through the second feed point 7b.

The relatively cold major substream 7 together with the relatively warm minor substream 3a contributes to maintaining the desired temperature gradient inside the scrub column 10.

It is possible to control the separation ratio between the major and minor substreams. It is also possible to control the temperature and / or temperature gradient below the scrub column 10. It is known that in order to lower the temperature of the scrub column sufficiently to efficiently separate heavy hydrocarbons from the mixture, it is preferable to select a separation ratio of less than 1/5. It is more preferable to select a separation ratio smaller than 1/10.

In order to achieve the beneficial effect of reducing the need for external heat required to keep the temperature of the scrub column lower than -10 [deg.] C., it is known to select a separation ratio of greater than 1/100. In order to be able to completely eliminate the need for a reboiler, it is desirable to choose a separation ratio greater than 1/50. Thus, in the preferred embodiment, there is no reboiler and as a result no reboiling occurs between the overhead stream outlet opening 12 and the third feed point 7c.

Preferably, the scrub stream 18 is fed to the scrub column 10 through a third feed point 7c higher than the second feed point 7b. The temperature of the scrub stream 18 is typically lower than the temperature of the cooled minor substream, which is typically -70 to -10 ° C. This further aids in maintaining the desired temperature gradient inside the scrub column 10.

The overhead product 16 exits through the overhead stream outlet opening 12, which is natural gas from which heavy hydrocarbons have been sufficiently removed. Stream 17 is the bottom product rich in heavy hydrocarbons exiting through outlet opening 8. In particular, the top product 16 is a natural gas lean in heavy hydrocarbons, while the natural gas gas stream is further cooled down to the final liquefaction in the cryogenic region (not shown), while the requirements for preventing solid formation are met. Since those skilled in the art will readily understand how to liquefy the top product 16 (eg, using a heat exchanger) in the cryogenic region, no further explanation is given. The bottom product 17 can be found in any application, one of which is to further process it to form liquefied petroleum gas (LPG).

2-5 are schematic diagrams illustrating alternative process flows that result in alternative apparatus. In the case of these drawings, portions already described with reference to FIG. 1 are denoted by the same reference numerals and will not be described again. In addition, their functions and actions are as described above.

2 to 5 show an aspect in which the scrub stream 18 exits at least partially from the feed stream 1.

In the case of FIG. 2, the main difference from the embodiment of FIG. 1 is that the second feed stream junction 20 provided in the second branch 7 upstream of the second feed point 7b and downstream of the heat exchanger 6. ) In the presence of The second branch 7 leads downstream of the feed stream junction 20, and the third branch 22 is formed for conveying the third substream of the feed stream 1. The third branch 22 is provided with a second heat exchanger 26, the downstream side of which is connected to the scrub stream line 18.

The second heat exchanger 26 is arranged to further cool the third substream 22 to a temperature lower than the temperature of the second substream, without essentially intentionally depressurizing it. Under normal operating conditions, the pressure drop of the third substream 22 is less than 6 bar, preferably less than 3 bar. As shown in FIG. 2, the second heat exchanger 26 is provided with at least one refrigerant supply 24, which removes the consumed or vaporized refrigerant 25 and supplies the supply to the first heat exchanger 6. It is possible to form the refrigerant supply unit 4 for.

Alternatively, the first and second heat exchangers are each provided with at least one refrigerant supply and removal section independently. The second heat exchanger 26 may be an integral heat exchanger or a dedicated heat exchanger that also provides cooling for other duties.

Referring to FIG. 3, an alternative to FIG. 2 is schematically illustrated, in which a second feed stream junction 20 is provided in the second branch 3b upstream of the first heat exchanger 6. The second heat exchanger 26 is provided in parallel with the first heat exchanger 6 instead of the series arrangement of FIG. 2. The second branch 3b leads downstream of the second feed stream junction 20 and the third branch 22 is formed to convey the third stream of the feed stream 1. As before in FIG. 2, the downstream side of the second heat exchanger is connected to a scrub stream line 18.

The first and second heat exchangers 2, 26 respectively have one or more refrigerant supplies 4, 24 and removal portions 5, 25 of exhausted refrigerant.

The first and second heat exchangers 6, 26 can be combined in one housing, whereby the refrigerant can be operated at one pressure level.

Referring to FIG. 4, an example based on parallel cooling of the second and third substreams is shown schematically, wherein the first and second heat exchangers are integrated in one housing, each indicated by a flow path. It is. FIG. 5 shows an example of an integrated heat exchanger embodying series cooling of the aspect of FIG. 2. In this example, the second feed stream junction 20 is located outside of the heat exchanger housing and the second and third branches can be directed into and out of the heat exchanger housing. Alternatively, although currently considered less practical, feed stream junction 20 may be located inside the heat exchanger housing.

Thus, for the embodiment of FIGS. 2-5, the scrub stream source connected to the scrub column 10 via the third feed point 7c higher than the second feed point 7b may be the second feed stream junction 20. And a second heat exchanger 26.

In operation, the device of FIGS. 2-5 acts similarly to the device of FIG. 1. However, to form the third substream, the scrub stream of line 18 is obtained by extracting the fraction from the second substream 3b. The residue is conveyed as the second substream 3b. The third substream is cooled in a second heat exchanger 26 downstream of the second feed stream junction 20 to a temperature lower than the temperature of the second substream cooled by the first heat exchanger 6.

Another embodiment will be described below with reference to FIG. 6. Once again, the parts already described with reference to FIG. 1 are denoted by the same reference numerals and will not be described again. In addition, their functions and actions are as described above.

In the case of the aspect of FIG. 6, the overhead condenser is provided to the discharge line 16 in the form of an overhead heat exchanger 14. The heat exchanger 14 has at least one refrigerant supply 30 and one removal 31 of exhausted or vaporized refrigerant. The heat exchanger 14 may be an integral heat exchanger or a dedicated heat exchanger that also provides cooling for other duties. The discharge line 16 fluidly connects the downstream outlet of the heat exchanger 14 to the separator 27. The separator 27 has a condensate outlet 35 discharged to the line 15 and a gas outlet 33 discharged to the line 13. Line 15 may be directly connected to scrub column 10 via third feed point 7c and line 18. In FIG. 6, an optional reflux pump 19 is provided between line 15 and line 18.

The overhead condenser 14 and separator 27 may also be a single device which is integrated in one housing or whose functions are combined.

During operation, the aspect of FIG. 6 acts as follows. The overhead product overhead stream exiting the scrub column 10 via line 16 is directed to the overhead condenser 14, where it is partially condensed with refrigerant. This partial condensate forms a mixed phase stream of gas and condensate and is directed to separator 27. The gas discharged from the separator 27 via the line 13 is natural gas from which heavy hydrocarbons are sufficiently removed, and liquefied to obtain LNG. Condensate in the form of condensed liquid is extracted from the mixed phase stream to obtain scrub stream 18 or added to another scrum stream provided to scrub column 10. The reflux pump 19 can be used to bring the liquid to the desired pressure level.

An advantage of the embodiment of FIG. 6 is that since the number of trays (corresponding to the height of the scrub column 10) to which the second substream 3b is fed into the distillation column 10 can be selected, the second substream ( The temperature of 3b) can be selected freely. Thus, without constraints, the temperature of the second substream of line 7 can be selected to optimize the cooling cycle. The temperature profile of the bottom of the scrub column 10 and the temperature of the bottom product discharged through the outlet 8 and the line 17 can be optimally controlled by selecting or controlling the separation ratio. An advantage of the embodiment of FIGS. 2-5 is that it avoids the use of a reboiler in the form of an overhead separator 27 and / or a reflux pump 19.

It can be appreciated that the aspect of FIG. 6 may be combined with one of FIGS. 2-5.

In all of the foregoing embodiments, the third substream forms at least half of the major fraction of the second substream, ie the original second substream separated in feed stream junction (2). The third substream is typically cooled to a temperature of -100 ° C to -10 ° C. The third substream is preferably cooled to a temperature below -30 ° C. Preferably, the third substream is cooled to a temperature of at least -60 ° C. Next, the third substream is introduced into the scrub column 10 at the third feed point 7c.

7 schematically illustrates another aspect of the present invention. In comparison with the embodiment of FIG. 1, since the function of the third feed point 7c has been transferred to the second feed point 7b, fewer devices are required. For this purpose, the second feed point 7b is provided near the top of the scrub column 10 which is usually the scrum stream inlet. In this way, no special reflux device is needed. The heat exchanger of the second branch is shown as a plurality of heat exchangers 6, 6 ′ acting in series with each other. It will be appreciated that the heat exchanger may be provided in the form of a single device.

The second substream of the second branch 3b is cooled to a sufficiently low temperature to form the gas / liquid mixture before being fed into the line 7. The temperature is typically from -60 ° C to -10 ° C. Preferably, the second substream is cooled to below -30 ° C. Preferably, the third substream is cooled to above -60 ° C.

Advantages of the embodiments of FIGS. 2-5 and 7 over the embodiment of FIG. 6 are that some of the natural gas does not pass through the second heat exchanger 26 or the second member 6 ′ of the heat exchanger. The flow rate passing through the second heat exchanger 26 or the second member 6 'of the heat exchanger is less than the flow rate passing through the overhead condenser 14.

Comparative example

8 shows a comparative example, in which the feed stream of feed stream line 1 is not separated into substreams, but optionally in heat exchanger 6 before being fed into scrub column 10 via feed point 7d. Is cooled. The feed point 7d may be at or near the bottom of the scrub column, or somewhat higher than the feed point 7a.

For a typical feed gas and typical ambient conditions, the mass and energy balances were calculated in relation to the process flow diagrams shown in FIGS. 6, 7 and 8.

In the process of FIG. 8, the relative power (including end-flash power for production) calculated to bring the C _{5 +} content in the stream of line 13 to 0.03 mol% is 13.1 kW / was tpd.

In the process of FIG. 7, the separation ratio was set at 8%, allowing the main part of the feed stream to proceed through heat exchangers 6, 6 ′. The temperature of the second substream of line 7 has dropped to about -20 ° C. The calculated relative power (including end-flash power for production) is 13.1 kW / tpd and the C _{5 +} content in the stream of line 16 is 0.06 mol%.

Thus, separation of the feed stream provides a choice to remove elements that produce a reflux stream, such as overhead separator 27 and / or reflux pump 19, although the separation becomes slightly poor. At the same time, the control over the temperature gradient of the scrub column 10 is improved, so that the mass flow in the bottom of the scrub column 10 is significantly reduced, which makes it slimmer.

In the process of FIG. 6, the separation ratio was selected to 6%. Separation was performed in the same manner as in FIG. 7, but 12.9 kW / tpd was used in which the C _{5 +} content of line 13 was 0.06 mol% and the relative power was decreased by 1.5%. In view of the large amount of product to be treated, a 1.5% reduction in power consumption is a significant improvement. Due to this reduction in power consumption, the additional cost of having a reflux device is offset. Compared to the process of FIG. 8, the control over the temperature gradient of the scrub column 10 is improved, which can be made slimmer, since the material flow in the scrub column 10 bottom is considerably reduced.

Claims (21)

A process for producing a liquefied natural gas stream, wherein, prior to liquefaction, heavy hydrocarbon components having a molecular weight greater than butane are removed from the natural gas stream to be liquefied Providing a natural gas feed stream 1 which is substantially gas at feed stream pressure and temperature; Feeding said feed stream (1) into a distillation column (10) having at least two separation stages (11); Extracting the bottom stream 17 from the bottom of the distillation column 10 and from the top of the distillation column 10 an overhead stream 16 having a relatively smaller amount of heavy hydrocarbon components than the bottom stream 17. Extracting; And Liquefying at least part of the overhead stream 16 to obtain a liquefied natural gas stream, Feeding the feed stream 1 into the distillation column 10, Separating said feed stream (1) into a first substream (3a) and a second substream (3b) at a selected split ratio; The first sub through the first feed point 7a below the distillation column 10, at a pressure above the feed stream pressure minus the pressure drop caused by the separation step of the feed stream 1. Feeding stream (3a) into the distillation column (10); Cooling the second substream (3b) in a heat exchanger (6) to a temperature lower than the supply temperature; Liquefied natural, comprising the step of feeding the cooled second substream 7 into the distillation column 10 at the second feed point 7b higher than the first feed point 7a Process for producing a gas stream. The method of claim 1, Process for producing a liquefied natural gas stream, characterized in that the substantially gaseous feed stream (1) comprises more than 90 vol%, preferably more than 95 vol% gas. The method according to claim 1 or 2, The first substream (3a) is fed into the distillation column (11) below the lowest separation stage (11). The method according to any one of claims 1 to 3, Liquefied natural gas, characterized in that the pressure drop of the first substream 3a between the separation point of the feed stream 1 and the first feed point 7a is less than 6 bar, preferably less than 3 bar. Process for producing a stream. The method according to any one of claims 1 to 4, The selected separation ratio is kept below 1, preferably below 1/5, more preferably below 1/10, wherein the separation ratio is divided by the mass flow rate of the second substream 3b ( A process for producing a liquefied natural gas stream, characterized in that it is defined as the mass flow rate of 3a). The method of claim 5, The selected separation ratio is maintained above 1/100, preferably above 1/50. The method according to any one of claims 1 to 6, The second substream (3b) is cooled in the heat exchanger (6) using an external refrigerant. The method according to any one of claims 1 to 7, The distillation column 10 is provided in the form of a scrub column and the scrub stream 18 is at a third feed point higher than the second feed point 7b at a temperature lower than the temperature of the cooled second substream 7. 7c) through a scrub column. The method of claim 8, Process for producing a liquefied natural gas stream, characterized in that the scrub stream (18) is substantially liquid. The method according to claim 8 or 9, The scrub stream 18 is Drawing a fraction from said second substream (3b) to form a third substream (22), the residue of which is conveyed as a second substream (3b), A process for producing a liquefied natural gas stream, which is obtained by cooling the third substream in a second heat exchanger. The method according to claim 8 or 9, The scrub stream 18 is A process for producing a liquefied natural gas stream, which is obtained by partially condensing the overhead stream (16) to form a mixed phase stream of gas and condensate and extracting the condensate from the mixed phase stream. The method of claim 11, A portion of the overhead stream (16) used as the scrub stream (18) is characterized in that it does not expand between the scrub column and the third feed point (7c). The method according to any one of claims 8 to 12, The temperature of the scrub stream (18) is low enough to form a condensate of heavy hydrocarbons. The method according to any one of claims 1 to 13, The first substream 3a is separated between the separation of the feed stream 1 and the feeding of the first substream 3a into the distillation column 10 at the first feed point 7a. Process for producing a liquefied natural gas stream, characterized in that not heated. Apparatus for producing a liquefied natural gas stream, prior to liquefaction, heavy hydrocarbon components having a molecular weight greater than butane can be removed from the natural gas stream to be liquefied, the apparatus being at least A feed stream line 1 for conveying a substantially gaseous natural gas feed stream 1 at a feed pressure and a temperature; At least two separation stages 11 for separating heavy hydrocarbon components from the natural gas, bottom stream outlet openings 8 arranged below the distillation column 10 for the discharge of bottom stream 17, And a distillation column having an overhead stream discharge opening 12 disposed on top of the scrub column 10 for the discharge of the overhead stream 16 with a relatively smaller amount of heavy hydrocarbon components than the bottom stream 17. 10; And A cryogenic region from which at least a portion of the overhead stream 16 can be liquefied to obtain a liquefied natural gas stream, The feed stream line 1 is a feed stream which fluidly connects the main branch with the first and second branches in order to separate the feed stream into first and second substreams 3a and 3b according to the selected separation ratio. A junction (2), wherein the first branch (3a) is at least a pressure drop of the feed stream resulting from the separation of the feed stream (1) at a first feed point (7a) below the distillation column (10). Is connected to a feed stream junction (2), the second branch (3b) being fed to the feed stream junction (2) at a second feed point (7b) of the distillation column (10) higher than the first feed point (7a). And a second heat exchanger (6) is provided in the second branch (3b). The method of claim 15, Apparatus for producing a liquefied natural gas stream, characterized in that the first feed point (7a) is located below the lowest separation stage (11) of the distillation column (10). The method according to claim 15 or 16, The distillation column 10 is in the form of a scrub column, the apparatus further comprising a scrub stream source connected to the scrub column via a third feed point 7c higher than the second feed point 7b of the scrub column. Apparatus for producing a liquefied natural gas stream, characterized in that. The method of claim 17, The scrub stream source comprises a second junction 20 provided in the second branch 3b upstream of the second feed point for feeding the third branch 22 coming out of the second branch 3b; , Wherein the third branch (22) is provided with a second heat exchanger (26). The method of claim 17 or 18, The scrub stream source comprises a condenser 14 provided to receive the overhead stream 16 downstream of the scrub column in cooperation with a separator 27 having a condensate outlet 33 and a gas outlet 35; And the condensate outlet (33) is connected to the scrub column via the third feed point (7c). The method according to any one of claims 17 to 19, Apparatus for producing a liquefied natural gas stream, characterized in that no expander is present between the overhead stream outlet opening (12) and the third feed point (7c). The method according to any one of claims 15 to 20, Apparatus for producing a liquefied natural gas stream, characterized in that the first branch (3a) is not provided with a heat exchanger for heating the first branch (3a).

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