CN114008396A

CN114008396A - Method and plant for producing liquefied natural gas

Info

Publication number: CN114008396A
Application number: CN202080044349.9A
Authority: CN
Inventors: 海因茨·鲍尔; M·瓦尔特
Original assignee: Linde LLC
Current assignee: Messer LLC
Priority date: 2019-08-02
Filing date: 2020-07-10
Publication date: 2022-02-01
Also published as: WO2021023393A1; US20220307765A1; EP4007881A1; AU2020324268A1

Abstract

The invention relates to a process for the preparation of Liquefied Natural Gas (LNG), wherein a feed Natural Gas (NG) comprising methane and higher hydrocarbons including benzene is cooled in a first cooling step to a first temperature level using a first mixed refrigerant (WMR) and then subjected to counter-current absorption using an absorption liquid to form a methane-enriched and benzene-depleted gas fraction, wherein in a second cooling step a part of the gas fraction is cooled to a second temperature level using a second mixed refrigerant (CMR) and liquefied into said Liquefied Natural Gas (LNG). In the proposed plant, the first and second mixed refrigerants (WMR, CMR) are propane-lean or propane-free and the absorption liquid is formed from another part of the gas fraction, which is condensed above the countercurrent absorption and recycled into the countercurrent absorption without pump. The invention also relates to a corresponding device (100, 200).

Description

Method and plant for producing liquefied natural gas

Description

The present invention relates to a method and an apparatus for producing liquefied natural gas according to the preambles of the independent claims.

Background

For liquefaction and pressureless storage, the natural gas must be cooled to a low temperature of about-160 ℃. In this state, lng can be economically transported by cargo ships or trucks because it has only 1/600 volumes of gaseous substances at atmospheric pressure.

Natural gas typically contains a mixture of methane and higher hydrocarbons, as well as nitrogen, carbon dioxide, and other undesirable components. These components must be partially removed prior to liquefaction to avoid freezing during liquefaction or to meet customer requirements. Methods used for this purpose, such as adsorption, absorption and cryogenic rectification, are generally known.

For details on the processes used in the liquefaction of Natural Gas, reference is made to the specialist literature, for example the article "Natural Gas" in Ullmann's Encyclopedia of Industrial Chemistry, published on-line on 7, 15.2006, DOI: 10.1002/14356007.a17_073.pub2, especially section 3, "Liquefaction".

In particular, mixed refrigerants composed of different hydrocarbon components and nitrogen are used in natural gas liquefaction. For example, methods are known which use two Mixed Refrigerant circuits (DMR). In this way, for example, natural gas which contains higher hydrocarbons such as ethane, propane, butane, etc. in addition to methane, but which has been previously freed of acid gases in a suitable manner and dried, can be subjected to separation and subsequent liquefaction of the higher hydrocarbons. The separation of higher hydrocarbons is accompanied by the separation of benzene, which is undesirable in the remaining liquefied natural gas. Benzene is used in the corresponding process as a key or marker component and can also be used as an indicator component for the separation.

The methods known from the prior art for liquefying natural gas using corresponding mixed refrigerant circuits have generally proven to be in practice in need of improvement for the reasons explained below.

It is therefore an object of the present invention to improve the liquefaction of natural gas using two mixed refrigerant circuits.

Disclosure of Invention

On this background, the invention proposes a method for producing liquefied natural gas and a corresponding plant according to the preambles of the respective independent claims. Embodiments are the subject matter of the dependent claims and the following description, respectively.

Before explaining the features and advantages of the present invention, some basic principles of the present invention are further explained and the terms used below are defined.

The present application uses the terms "pressure level" and "temperature level" to characterize pressure and temperature, thereby indicating that the corresponding pressure and temperature need not be in the form of precise pressure or temperature values used in the corresponding device. However, such pressures and temperatures are typically shifted within a specific range, for example ± 10% of the mean. The respective pressure and temperature levels can be in non-intersecting ranges or in overlapping ranges. In particular, for example, the pressure level includes an unavoidable or expected pressure loss. The corresponding applies to the temperature level. The pressure level in bar here is absolute pressure.

If reference is made here to an "expander", this is generally understood to be known turbo-expanders with radial impellers arranged on a shaft. The corresponding expander can be mechanically braked or coupled to a device such as a compressor or a generator, for example. The expansion of the mixed refrigerant in the context of the present invention is generally not performed using an expander, but rather using an expansion valve.

The "heat exchanger" for use in the context of the present invention may be constructed in any manner conventional in the art. The heat exchanger serves for indirectly transferring heat between at least two fluid streams, which are conducted, for example, counter-currently to one another, in this case in particular a relatively hot feed natural gas stream or a gaseous fraction formed therefrom, and one or more cold mixed refrigerant streams. The corresponding heat exchanger can be formed by a single or a plurality of heat exchanger sections connected in parallel and/or in series, for example by one or more coiled heat exchangers or corresponding sections. In addition to the types of coiled heat exchangers already mentioned, other types of heat exchangers can also be used in the context of the present invention.

The relative spatial concepts of "upper", "lower", "above", "below", "above …", "below …", "beside …", "side-by-side", "vertical", "horizontal", and the like, refer herein to the arrangement of components relative to one another in normal operation. The arrangement of two components "one above the other" is to be understood here to mean that the upper end of the lower of the two components is at a lower or the same geodetic height than the lower end of the upper of the two components, and that the perpendicular projections of the two components coincide. In particular, the two components are arranged exactly one above the other, i.e. the central axes of the two components run on the same vertical line. However, the axes of the two components need not be exactly perpendicular to one another, but may also be offset relative to one another.

In the context of the present invention, a countercurrent absorber is used. Reference is made to the relevant textbooks concerning the design and implementation of the corresponding devices (see, for example, K.Sattler: "Thermitche Trennverfahren.Grundling, Auslegeng, apparatus." Weinheim: Wiley-VCH, 3 rd edition 2001). From the countercurrent absorber, a liquid fraction ("bottoms") and a gaseous fraction ("overhead gas") can generally be withdrawn from a lower region ("bottoms") and an upper region ("overhead"). Countercurrent absorbers are generally known in the separation art. It is used for phase countercurrent absorption and is therefore also referred to as a countercurrent column. In the case of countercurrent absorption, the discharged gas phase flows upwards through the absorption column. The absorbed solution phase is fed from above and discharged from below, flowing towards the gas phase. The gas phase is "washed" with the solution phase. In the corresponding absorption column, an internal member is generally provided to ensure contact in a stepwise manner (tray, spray zone, rotating disk, etc.) or in a continuous manner (random dumping of packing, fillers, etc.). A liquid stream, also referred to as "absorption liquid", is fed into the upper region of the counter-current absorber, whereby components are washed out of the deeper-fed gaseous stream.

If in the following "feed natural gas" is mentioned, it is to be understood as already mentioned that the natural gas is in particular subjected to acid gas removal and optionally further treatment. In particular, heavy hydrocarbons, such as butanes and/or pentanes and hydrocarbons having six or more carbon atoms, may also have been separated from the corresponding feed natural gas. The feed natural gas is in particular anhydrous and has a methane content of, for example, more than 85% and comprises in particular ethane and propane in the remaining remainder. And may also contain nitrogen, helium and other light components.

If "liquefied natural gas" is mentioned in the following, it is to be understood that a cryogenic liquid which is present at atmospheric boiling point of methane or below, in particular at-160 to-164 ℃, has more than 85%, in particular more than 90% methane and has a methane content which in any case is higher than that of the feed natural gas used. In particular, liquefied natural gas has a much lower benzene content than the feed natural gas and has only the maximum content of benzene as shown below.

Features and advantages of the invention

A method for producing liquefied natural gas using two mixed refrigerants is disclosed in, for example, US 6,119,479 a. In this process, the higher hydrocarbons contained in the feed natural gas can be separated from the feed natural gas in a counter-current absorber as desired.

For this purpose, the feed natural gas can be cooled in a first cooling step to a temperature in the range from-20 ℃ to-70 ℃ depending on the composition and then fed into a countercurrent absorber. The countercurrent absorber can have a bottom heater. The bottoms liquid separated in the countercurrent absorber contains at least a portion of the higher hydrocarbons from the feed natural gas. A part of the bottom liquid may be recycled as absorption liquid onto the countercurrent absorber and, if desired, may also be partly fed into the overhead gas of the countercurrent absorber after it has been withdrawn therefrom. In this way, the overhead gas of the countercurrent absorber consumes at least a portion of the higher hydrocarbons and is then subjected to a second cooling step that results in liquefaction. Benzene is also used here as a key component, which is allowed to be contained in the overhead gas of the countercurrent absorber and thus in the natural gas to be liquefied, in particular in a molar amount of less than 1 ppm. The content of other higher hydrocarbons is obtained from the above; however, these are generally less critical. Thus, benzene in particular is considered critical in natural gas liquefaction because it can solidify at the low temperatures used.

In the first cooling step and the second cooling step of the method just described, a mixed refrigerant is used in the respective refrigerant circuits. In particular, the first Mixed Refrigerant (WMR) can be subjected here in the gaseous state to compression, condensation by cooling, supercooling, expansion, heating in a first heat exchanger, in particular complete evaporation here, and then again to compression in the sequence given below. The supercooling of the first mixed refrigerant can take place in particular in a first heat exchanger, and the preceding cooling can take place in a further heat exchanger. Furthermore, a second Mixed Refrigerant (CMR) can be subjected in the gaseous state to compression, condensation by cooling, supercooling, expansion, heating in a second heat exchanger, in particular completely evaporating here, and then again to compression. The supercooling of the second mixed refrigerant can take place in particular in the second heat exchanger, and the preceding cooling can take place in the first and second heat exchangers.

The first Heat Exchanger and the second Heat Exchanger are in particular embodied as Coil Heat exchangers (CWHE) of the type known per se, wherein the heating of the mixed refrigerant takes place after its expansion, in particular on the shell side, i.e. in the shell space surrounding the Heat Exchanger tubes, in which the mixed refrigerant expands. The medium to be cooled is conducted on the tube side, i.e. through correspondingly provided heat exchanger tubes. The heat exchanger tubes are provided in bundles in the respective heat exchanger, so that the terms "tube side" or "bundle side" are used herein for the respective current guidance.

Similar types of methods and apparatus are also disclosed, for example, in US 6,370,910A and AU 2005224308B 2.

Natural gas liquefaction processes must be able to flexibly accommodate different plant capacities and operating conditions. The method described using two mixed refrigerant circuits is preferably used when large ambient temperature fluctuations lead to distinctly different refrigerant condensation conditions. These issues can be more effectively considered if a mixture of refrigerant components is used rather than a single pure component such as propane.

Furthermore, the corresponding process does not contain large amounts of liquid hydrocarbons with molecular weights greater than air, which would constitute a significant safety risk. The corresponding hydrocarbons may accumulate in deeper areas and may cause explosions. In this sense, propane is considered the most hazardous refrigerant due to its combination of high volatility and high molecular weight. The method using two mixed refrigerant circuits and a correspondingly reduced propane content therein is therefore a preferred solution for floor-limited plant layouts with limited installation space, such as modular plants and/or floating plants.

A compact plant layout (e.g., as is necessary for offshore facilities) can be achieved by minimizing the number of plant components and reducing the space between plants, which may be dictated by safety considerations. Known hazardous equipment components include liquid hydrocarbon pumps (which risk leakage and liquid outflow) and all types of equipment that contain large quantities of liquid propane.

The present invention solves the stated problem by eliminating the hydrocarbon pump and largely propane as the refrigerant component in the corresponding mixed refrigerant. These advantages are achieved by the measures according to the invention and the corresponding advantageous embodiments set forth below.

In the process proposed according to the invention for the production of liquefied natural gas, feed natural gas of the above-mentioned type comprising methane and higher hydrocarbons including benzene is cooled overall in a first cooling step using a first ("hot") mixed refrigerant to a first temperature level, in particular-20 ℃ to-70 ℃, and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction. The benzene-depleted gas fraction here in particular has a benzene molar content of less than 1ppm, the benzene content in the feed natural gas being significantly higher than this, for example from 5 to 500 ppm. The gas fraction formed is enriched in methane and depleted in higher hydrocarbons, in particular with respect to the feed natural gas.

For countercurrent absorption, known means can in principle be used. Here, the gas fraction can also be (substantially) free of hydrocarbons having five and, where appropriate, more carbon atoms, so that a depletion to zero can be (substantially) achieved. However, higher hydrocarbons may also be contained, and the bottoms liquid formed in the countercurrent absorption may also have a proportion of methane. The degree of separation or enrichment and depletion achieved in countercurrent absorption depends on the respective tolerable content of the component and the corresponding fraction subsequently used.

In the context of the present invention, in a second cooling step, a portion of the gas fraction from the counter-current absorption, correspondingly lean (or substantially free) of benzene (and other higher hydrocarbons), is cooled to a second temperature level, in particular from-145 ℃ to-165 ℃, using a second ("cold") mixed refrigerant and liquefied into liquefied natural gas. The liquefied natural gas formed in this way may be subjected to any further treatment or conditioning (expansion, subcooling, etc.).

In the context of the present invention, the first and second mixed refrigerants are propane-lean (having a content of less than 5 mole percent propane) or (substantially) propane-free, and the absorption liquid for countercurrent absorption is formed by a further portion of the gas fraction from countercurrent absorption, which (geodetically) condenses above the countercurrent absorption and is recycled into the countercurrent absorption without a pump. With respect to the term "above …", please see the above definition.

The present invention reduces or eliminates the use of significant amounts of propane-containing media by the proposed measures. As previously mentioned, propane is considered a hazardous refrigerant due to a combination of high volatility and high molecular weight. The corresponding refrigerant must inevitably be conveyed by means of a machine in which there is an increased possibility of propane escaping. This is no longer the case in the context of the present invention, so that it is also advantageously particularly suitable for device arrangements with limited installation space, for example modular devices and/or floating devices, in which the floor area is limited and safety technology devices requiring additional installation space are difficult to install.

Since the absorption liquid used for the countercurrent absorption is formed by a further portion of the gas fraction from the countercurrent absorption, condensed on top of the countercurrent absorption and recirculated to the countercurrent absorption without a pump, there is also no need for the medium (possibly containing propane) to be disadvantageously used with a pump which would bring about the stated problems.

The present invention thus provides a solution in which the use of a significant amount of propane-containing medium is essentially dispensed with, either by using a previously propane-containing mixed refrigerant with a lean or no propane or by conveying propane-containing overhead gas from countercurrent absorption without a pump. Surprisingly, it was found here that the methods proposed in the context of the present invention have the same or higher thermodynamic efficiency than the known methods. In the context of the present invention, the investment costs can be reduced without increasing the operating costs.

In the process proposed according to the invention, a countercurrent absorber is advantageously used in countercurrent absorption, which countercurrent absorber is operated with an overhead condenser arranged above the absorption zone of the countercurrent absorber, wherein the overhead condenser is used for condensing a further portion of the gas fraction. Here, "absorbent region" is to be understood as a region having an inner member as described above.

The overhead condenser can be integrated into the countercurrent absorber or arranged at least partially within the countercurrent absorber. The integrated overhead condenser comprises a heat exchange structure in a common column shell, in which a mass transfer structure of the aforementioned type is also arranged, wherein the heat exchange structure, such as cooling coils or the like, is separated from the area containing the mass transfer structure, in particular by a liquid-blocking tray or a liquid-tight tray. The latter allows a controlled reflux of condensate to the region with the mass transfer structure. In contrast, the externally arranged overhead condensers are not arranged in a common column shell with mass transfer structures.

In the process proposed according to the invention, the first mixed refrigerant advantageously has a total of more than 90 mole percent of ethane, isobutane and n-butane and a total of less than 10 mole percent, preferably less than 5 mole percent, of nitrogen, methane, propane and hydrocarbons having five or more carbon atoms. Small amounts of propane have proved to be unproblematic in comparison with the known processes. In contrast, the second mixed refrigerant advantageously has greater than 98 mole percent total of nitrogen, methane, and ethane and less than 2 mole percent total of propane and higher hydrocarbons.

Advantageously, in the context of the present invention, a first heat exchanger is used in the first cooling step, wherein the first mixed refrigerant is subjected in the first mixed refrigerant circuit in the gaseous state to, in particular, single-stage compression, condensation by cooling, supercooling, expansion, heating in the first heat exchanger, in this case in particular complete evaporation, and is then subjected to compression again. The supercooling of the first mixed refrigerant can take place in particular in a first heat exchanger, and the preceding cooling can take place in a further heat exchanger. In contrast to the method according to the invention, the compression of the first mixed refrigerant is therefore carried out in particular in a single stage and without intermediate cooling, which involves the risk of partial condensation and requires the condensate to be conveyed to the high-pressure side of the compressor. This disadvantage is addressed here.

In the method according to the invention, furthermore, it is advantageous to use a second heat exchanger in the second cooling step, wherein the second mixed refrigerant is subjected in the second mixed refrigerant circuit in the gaseous state to, in particular, multistage compression, condensation by cooling, supercooling, expansion, heating in the second heat exchanger, in this case in particular complete evaporation, and is then subjected to compression again. The supercooling of the second mixed refrigerant can take place in particular in the second heat exchanger, and the preceding cooling can take place in the first and second heat exchangers.

The first heat exchanger and the second heat exchanger can be embodied as coil heat exchangers, as described above, and in particular each have one or two (series) bundles in a common shell.

In the context of the present invention, the accumulator for the second mixed refrigerant, which receives the second mixed refrigerant after its condensation, may be designed in particular for a pressure which is 2 to 10bar higher than the suction pressure of the first compressor of the compressor or compressors used in compressing the second mixed refrigerant.

In particular, for compressing the first and second mixed refrigerants, a series of three compressors may be used, wherein the first compressor compresses the first mixed refrigerant and the other two compressors compress the second mixed refrigerant. These compressors can be designed for (almost) the same shaft power, i.e. 331/3 ± 3% of the total power consumption.

The second mixed refrigerant is advantageously used after heating and evaporation in the second heat exchanger and before compression in condensing a further portion of the gaseous fraction from the countercurrent absorption and is further heated there. A particularly advantageous utilization of the second mixed refrigerant is obtained in this way.

For cooling the first mixed refrigerant, a first (but not second) heat exchanger is advantageously used in the context of the present invention and/or for cooling the second mixed refrigerant, a second (and additionally the first) heat exchanger is used. Further cooling after the compression or after the compression step can be carried out in a known manner, for example using an air cooler or a water cooler.

In the context of the present invention, in countercurrent absorption, in an alternative, the ascending gas phase is formed at least in part by feeding additional feed natural gas that has not been subjected to the first cooling step. In this way, a reboiler is saved, but higher separation efficiency is required in countercurrent absorption. However, the ascending gas phase may also be provided at least in part by vaporizing a portion of the bottoms liquid formed in the countercurrent absorption.

In the context of the present invention, a liquid expander that does work may be used at any location instead of an expansion valve. Thereby reducing power consumption.

The invention is applicable to typical natural gas, whereby the feed natural gas may in particular comprise at least 80% methane and at least 50% ethane and propane in the remaining methane-free fraction. The liquefied natural gas advantageously comprises at least 90% methane, wherein the methane content in the liquefied natural gas is higher than the methane content in the feed natural gas.

The invention also relates to a plant for the production of liquefied natural gas, the specific features of which are set forth in the corresponding independent claim. With regard to further features and embodiments and preferred embodiments of such a device, reference is expressly made to the above explanations with regard to the method according to the invention and its corresponding advantageous embodiments. Advantageously, such a device is adapted to perform a method as set forth previously in the different embodiments.

The invention is explained in more detail below with reference to the accompanying drawings, which show a natural gas liquefaction plant according to one embodiment of the invention.

Drawings

Figure 1 shows an apparatus according to one embodiment of the invention in the form of a simplified process flow diagram.

Fig. 2 shows, in the form of a simplified process flow diagram, an apparatus according to a further embodiment of the invention.

Detailed Description

An apparatus according to a particularly preferred embodiment of the present invention is shown in highly simplified schematic process flow diagram form in fig. 1 and is generally designated 100.

The feed natural gas NG is supplied to the plant 100 shown in fig. 1, which is first divided into two substreams. The first substream is cooled in a first cooling step in a first heat exchanger E1, which may in particular be configured as a coiled heat exchanger, to a first temperature level of, for example, -20 ℃ to-70 ℃, and is subsequently fed approximately centrally into the countercurrent absorber T1.

In addition, a second substream of the feed natural gas NG expanded by valve V6 is fed into the lower region of the countercurrent absorber T1 where it rises substantially in the gaseous state. From the upper region of the countercurrent absorber T1, gas is discharged, which is cooled in an overhead condenser E2, which may be configured, for example, as a plate heat exchanger, and fed into the overhead space of the countercurrent absorber T1. The liquid separated here is recycled as reflux to the countercurrent absorber T1 and the heavier components are washed out of the feed natural gas and converted into the bottoms liquid of the countercurrent absorber T1.

The bottoms liquid from countercurrent absorber T1 can be expanded through valve V5 and withdrawn from plant 100 as heavy ends HHC (English: heavy hydrocarbons). Instead, the overhead gas (i.e. the methane-rich gas fraction) of the countercurrent absorber T1 is cooled to liquefaction temperature in a second heat exchanger E3, which may also be configured as a coiled heat exchanger, and after expansion is discharged from the plant 100 as liquefied natural gas LNG through valve V4.

The apparatus 100 includes two mixed refrigerant circuits. In the first mixed refrigerant circuit, the first ("hot") mixed refrigerant WMR is subjected to single-stage compression in the gas state in the compressor C1 and is subcooled and thereby condensed in the air cooler and/or water cooler E4. The condensate may be withdrawn in a separation vessel D1. This condensate is first cooled further on the bundle side in the first heat exchanger E1, then expanded through the valve V1 and fed into the shell space of the first heat exchanger E1, where it is heated, completely evaporated and then subjected to compression again.

In contrast to the method according to the invention, the compression of the first mixed refrigerant is carried out here, in particular in the single-stage compressor C1, without intermediate cooling, which involves the risk of partial condensation and requires the condensate to be conveyed to the high-pressure side of the compressor. This disadvantage is addressed here.

Furthermore, in the plant 100, the second mixed refrigerant CMR is subjected to a staged compression in the gaseous state in the compressors LP C2 and HP C2 and is sub-cooled, for example in air and/or water coolers E5 and E6, respectively. Further cooling is performed in the first heat exchanger E1 on the bundle side and then in the second heat exchanger E3. After subsequent expansion in valve V2, is fed into buffer vessel D2. The condensate withdrawn therefrom is expanded by means of a valve V3 and fed on the shell side into a second heat exchanger E2 and heated there and completely evaporated. The gaseous second mixed refrigerant CMR is used as refrigerant in the aforementioned overhead condenser E2 before it is subjected to compression again.

By installing the overhead condenser E2 above the counter-current absorber T1, the reflux pump can be eliminated, the overhead condenser operating with the sensible heat of the second mixed refrigerant leaving the second heat exchanger E3 as a vapor. Conversely, the reflux formed by the gas from the countercurrent absorber T1 is recycled to the countercurrent absorber T1 purely by gravity.

An apparatus according to another embodiment of the present invention is shown in highly simplified schematic process flow diagram form in fig. 2 and is generally designated 200.

A first difference with the embodiment of the plant 100 according to fig. 1 is that the countercurrent absorber T1 is not fed with a substream of the feed natural gas, but instead a reboiler E7 is provided which vaporizes a part of the bottom liquid of the countercurrent absorber T1 and thus forms a part of the ascending gas phase in the countercurrent absorber T1.

A further difference with the embodiment of the plant 100 according to fig. 1 is also that the overhead condenser E3 is moved in the form of a corresponding heat exchanger structure into the overhead space of the countercurrent absorber T1, whereby corresponding structural space is saved if necessary.

Finally, as shown herein, expansion of the liquefied natural gas LNG leaving the second heat exchanger E3 is provided by an expander X1 and corresponding expansion of the cooled second mixed refrigerant CMR is provided in an expander X2. Similarly, valve V1 could also be replaced by expander X3 (not shown).

Claims

1. Process for the preparation of Liquefied Natural Gas (LNG), wherein a feed Natural Gas (NG) comprising methane and higher hydrocarbons including benzene is cooled to a first temperature level in a first cooling step using a first mixed refrigerant (WMR) and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction, wherein a part of the gas fraction is cooled to a second temperature level and liquefied into the Liquefied Natural Gas (LNG) in a second cooling step using a second mixed refrigerant (CMR), characterized in that the first and second mixed refrigerants (WMR, CMR) are propane-depleted or propane-free and the absorption liquid is formed from another part of the gas fraction which is condensed above the countercurrent absorption and recycled into the countercurrent absorption without a pump.

2. The process according to claim 1, wherein a countercurrent absorber (T1) is used in the countercurrent absorption, which countercurrent absorber is operated with an overhead condenser (E2) arranged above the absorption zone of the countercurrent absorber (T1), wherein the overhead condenser (E2) is used for condensing a further part of the gas fraction.

3. The process of claim 2, wherein the overhead condenser is integrated in the countercurrent absorber (T1) or is at least partially arranged within the countercurrent absorber (T1).

4. The process according to one of the preceding claims, wherein the first mixed refrigerant (WMR) has more than 90 mole percent, preferably more than 95 mole percent, of ethane, isobutane and n-butane in total, and less than 10 mole percent, preferably less than 5 mole percent, of nitrogen, methane, propane and hydrocarbons having five or more carbon atoms in total.

5. The process of one of the preceding claims, where the second mixed refrigerant has greater than 98 mole percent total of nitrogen, methane, and ethane and less than 2 mole percent total of propane and heavier hydrocarbons.

6. The process according to one of the preceding claims, wherein a first heat exchanger (E1) is used in the first cooling step, wherein the first mixed refrigerant (WMR) is subjected in a first mixed refrigerant circuit in gaseous state, in particular to a single-stage compression, to condensation by cooling (E4), to supercooling, to expansion, to heating in the first heat exchanger (E1), in this case in particular to complete evaporation, and then to compression again.

7. The process according to claim 6, wherein in the second cooling step a second heat exchanger (E3) is used, wherein the second mixed refrigerant (CMR) is subjected in a second mixed refrigerant circuit to, in particular, multistage compression in a gaseous state, condensed by cooling, subcooled, expanded, heated in the second heat exchanger (E3), in this case in particular completely evaporated, and then subjected to compression again.

8. A process according to claim 7, wherein the second mixed refrigerant (CMR) is used in condensing a further part of the gas fraction from countercurrent absorption after heating in the second heat exchanger (E3) and before compression and is heated further there.

9. Method according to claim 7 or 8, wherein for cooling the first mixed refrigerant (WMR) the first heat exchanger (E1) is used and/or for cooling the second mixed refrigerant (CMR) the first heat exchanger (E1) and the second heat exchanger (E3) are used.

10. The process according to one of the preceding claims, wherein in counter-current absorption the rising gas phase is provided at least partly by feeding additional feed Natural Gas (NG) not subjected to the first cooling step and/or at least partly by evaporating a part of the bottom liquid formed in counter-current absorption.

11. The process according to one of the preceding claims, wherein the feed Natural Gas (NG) comprises at least 80% methane and at least 50% ethane and propane in the remaining methane-free fraction.

12. The method according to one of the preceding claims, wherein the Liquefied Natural Gas (LNG) comprises at least 90% methane, wherein the methane content in the Liquefied Natural Gas (LNG) is higher than the methane content in the feed Natural Gas (NG).

13. An apparatus (100,200) adapted to produce Liquefied Natural Gas (LNG), the apparatus having: a first heat exchanger (E1) adapted to cool a feed Natural Gas (NG) comprising methane and higher hydrocarbons including benzene to a first temperature level in a first cooling step using a first mixed refrigerant (WMR); a counter-current absorber (T1) adapted to subject the feed Natural Gas (NG) to counter-current absorption using an absorption liquid after the first cooling step to form a benzene-depleted gas fraction; -a second heat exchanger adapted to cool a part of the gas fraction to a second temperature level using a second mixed refrigerant (CMR) and liquefy it to the Liquefied Natural Gas (LNG) in a second cooling step, characterized in that the plant is adapted to use a first and a second mixed refrigerant (WMR, CMR) lean or free of propane, and means are provided adapted to form an absorption liquid from another part of the gas fraction, wherein these means condense the absorption liquid above counter-current absorption and recycle it without a pump into counter-current absorption.

14. The device (100,200) according to claim 13, the device being adapted to perform the method according to one of claims 1 to 12.