PROCESS AND PLANT FOR PRODUCING LIQUEFIED NATURAL GAS

Abstract

A process for producing liquefied natural gas, in which natural gas feed having methane and higher hydrocarbons including benzene is cooled down to a first temperature level in a first cooling step using a first mixed coolant and then subjected to a countercurrent absorption using an absorption liquid to form a methane-enriched and benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed coolant and liquefied to give the liquefied natural gas. In the plant proposed, the first and second mixed coolants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping. The present invention likewise provides a corresponding plant.

Claims

1-14. (canceled)

15. A process for producing liquefied natural gas, in which natural gas feed containing methane and higher hydrocarbons, including benzene, is cooled down to a first temperature level in a first cooling step using a first mixed refrigerant, and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed refrigerant and liquefied to give the liquefied natural gas, wherein the first and second mixed refrigerants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping.

16. The process according to claim 15, wherein a countercurrent absorber is used in the countercurrent absorption, which is operated with a head condenser arranged above an absorption region of the countercurrent absorber, wherein the head condenser is used for condensing the further portion of the gas fraction.

17. The process according to claim 16, wherein the head condenser is integrated into the countercurrent absorber or is at least partially arranged within the countercurrent absorber.

18. The process according to claim 15, wherein the first mixed refrigerant comprises in total more than 90 mole percent, preferably more than 95 mole percent ethane, isobutane and n-butane and in total less than 10 mole percent, preferably less than 5 mole percent nitrogen, methane, propane and hydrocarbons having five or more carbon atoms.

19. The process according to claim 15, wherein the second mixed refrigerant comprises in total more than 98 mole percent nitrogen, methane and ethane and in total less than 2 mole percent propane and heavier hydrocarbons.

20. The process according to claim 15, wherein a first heat exchanger is used in the first cooling step, wherein the first mixed refrigerant in gaseous form is subjected to, in particular, single-stage compression in a first mixed refrigerant circuit, condensed by cooling, subcooled, expanded, heated in the first heat exchanger and, in particular, completely evaporated thereby, and subsequently subjected to the compression again.

21. The process according to claim 20, wherein a second heat exchanger is used in the second cooling step, wherein the second mixed refrigerant in gaseous from is subjected to an in particular multi-stage compression in a second mixed refrigerant circuit, condensed by cooling, subcooled, expanded, heated in the second heat exchanger and, in particular, completely evaporated thereby, and subsequently subjected to compression again.

22. The process according to claim 21, wherein the second mixed refrigerant is used after heating in the second heat exchanger and before compression during the condensation of the further portion of the gas fraction from the countercurrent absorption and further heated thereby.

23. The process according to claim 21, wherein the first heat exchanger (E1) is used to cool down the first mixed refrigerant (WMR) and/or the first (E1) and the second (E3) heat exchangers are used to cool down the second mixed refrigerant (CMR).

24. The process according to claim 15, wherein in countercurrent absorption, a rising gas phase is provided at least in part by feeding in further natural gas feed which was not subjected to the first cooling step and/or at least in part by evaporating a portion of a sump liquid formed in the countercurrent absorption.

25. The process according to claim 15, wherein the natural gas feed contains at least 80% methane and, in the methane-free remainder, at least 50% ethane and propane.

26. A method according to claim 15, wherein the liquefied natural gas contains at least 90% methane, wherein the methane content in the liquefied natural gas is higher than in the natural gas feed.

27. A plant configured to produce liquefied natural gas, having a first heat exchanger configured to cool natural gas feed containing methane and higher hydrocarbons, including benzene, to a first temperature level in a first cooling step using a first mixed refrigerant, a countercurrent absorber configured to subject the natural gas feed to countercurrent absorption using an absorption liquid after the first cooling step by forming a benzene-depleted gas fraction, having a second heat exchanger configured to cool down a portion of the gas fraction in a second cooling step to a second temperature level using a second mixed refrigerant and is liquefied to give the liquefied natural gas, wherein the plant is configured to use low-propane or propane-free first and second mixed refrigerants, and means are provided which are configured to form the absorption liquid from a further portion of the gas fraction, wherein they condense this above the countercurrent absorption and return it to the countercurrent absorption without pumping.

28. The plant according to claim 27, wherein the plant is configured to carry out a process for producing liquefied natural gas, in which natural gas feed containing methane and higher hydrocarbons, including benzene, is cooled down to a first temperature level in a first cooling step using a first mixed refrigerant, and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed refrigerant and liquefied to give the liquefied natural gas, wherein the first and second mixed refrigerants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 illustrates a plant according to an embodiment of the present invention in the form of a simplified process flow diagram.

[0050] FIG. 2 illustrates a plant according to a further embodiment of the invention in the form of a simplified process flow diagram.

DETAILED DESCRIPTION OF THE DRAWINGS

[0051] In FIG. 1, a plant according to a particularly preferred embodiment of the present invention is shown in the form of a greatly simplified schematic process flow diagram and is designated as a whole by 100.

[0052] The plant 100 illustrated in FIG. 1 is supplied with natural gas feed NG, which is first divided into two partial streams. A first partial stream is cooled down in a first heat exchanger E1, which can in particular be designed as a wound heat exchanger, in a first cooling step to a first temperature level of, for example, −20° C. to −70° C. and then fed approximately centrally into a countercurrent absorber T1.

[0053] Furthermore, the second partial flow of the natural gas feed NG, which is expanded via a valve V6, is fed into a lower region of the countercurrent absorber T1, where it rises essentially in gaseous form. Gas is withdrawn from an upper region of the countercurrent absorber T1 and is cooled down in a head condenser E2, which can be designed, for example, as a plate heat exchanger, and fed into a head space of the countercurrent absorber T1. Liquid precipitating here is returned as a return flow to the countercurrent absorber T1 and washes out heavier components from the natural gas feed, which pass into a sump liquid of the countercurrent absorber T1.

[0054] The sump liquid of the countercurrent absorber T1 can be expanded via a valve V5 and discharged from the plant 100 as a heavy fraction HHC (heavy hydrocarbon). Head gas of the countercurrent absorber T1, i.e., a methane-rich gas fraction, is, in contrast, cooled down to a liquefaction temperature in a second heat exchanger E3, which can also be designed as a wound heat exchanger, and, after expansion, discharged via a valve V4 as liquefied natural gas LNG from the plant 100.

[0055] The plant 100 comprises two mixed refrigerant circuits. In a first mixed refrigerant circuit, a first (“warm”) mixed refrigerant WMR is subjected to single-stage compression in gaseous form in a compressor C1 and subsequently cooled down in an air cooler and/or water cooler E4 and thereby condensed. Condensate can be obtained in a separation vessel D1. This is first further cooled down in the first heat exchanger E1 on the bundle side, then expanded via a valve V1 and fed into the jacket space of the first heat exchanger E1, where it is heated, completely evaporated and subsequently subjected to compression again.

[0056] In contrast to processes not according to the invention, the compression of the first mixed refrigerant takes place here, in particular, in the single-stage compressor C1 without intermediate cooling, which would constitute a risk of partial condensation and a need to convey the condensate to the high-pressure side of the compressor. This disadvantage is remedied here.

[0057] Furthermore, in the plant 100, a second mixed refrigerant CMR is subjected to a gradual compression in compressors LP C2 and HP C2 in gaseous form and subsequently cooled down in each case, for example in air coolers and/or water coolers E5 and E6. Further cooling takes place on the bundle side in the first heat exchanger E1 and then in the second heat exchanger E3. After subsequent expansion in a valve V2, feeding into a buffer vessel D2 takes place. Condensate withdrawn therefrom is expanded via a valve V3 and fed jacket-side into the second heat exchanger E2, where it is heated and completely evaporated. The gaseous second mixed refrigerant CMR is used as refrigerant in the aforementioned head condenser E2 before it is again subjected to compression.

[0058] A return pump can be dispensed with by installing the head condenser E2, which is operated using tactile heat of the second mixed refrigerant, which leaves the second heat exchanger E3 as a vapor above the countercurrent absorber T1. In contrast, the return flow formed from the gas from the countercurrent absorber T1 is returned to the countercurrent absorber T1 purely by the effect of gravity.

[0059] In FIG. 2, a plant according to a further embodiment of the present invention is shown in the form of a greatly simplified schematic process flow diagram and is designated as a whole by 200.

[0060] A first difference from the embodiment of the plant 100 according to FIG. 1 here is that the countercurrent absorber T1 is not supplied with a partial flow of the natural gas feed, but instead a reboiler E7 is provided which evaporates a portion of the sump liquid of the countercurrent absorber T1 and thus forms a portion of the rising gas phase in the countercurrent absorber T1.

[0061] A further difference from the embodiment of the plant 100 according to FIG. 1 is, furthermore, that the head condenser E3 in the form of corresponding heat exchanger structures is displaced into the head space of the countercurrent absorber T1, whereby corresponding installation space is potentially saved.

[0062] Finally, as illustrated here, an expansion of the liquid natural gas LNG leaving the second heat exchanger E3 is provided via an expansion machine

[0063] X1 and a corresponding expansion of the cooled second mixed refrigerant CMR in an expansion machine X2. Analogously, the valve V1 can also be replaced by a expansion machine X3 (not shown).