Method for separating nitrogen from LNG
Patent Information
- Authority / Receiving Office
- US · United States
- Patent Type
- Applications(United States)
- Current Assignee / Owner
- Filing Date
- 2023-12-13
- Publication Date
- 2026-08-13
Smart Images
Figure US20260235353A1-D00000_ABST
Abstract
Description
[0001] The invention relates to a method for separating a nitrogen-enriched fraction from a liquefied methane-rich, nitrogen-containing fraction, wherein
[0002] the liquefied methane-rich, nitrogen-containing fraction is subjected to a stripping process to which a methane-rich fraction is supplied as a stripping medium,
[0003] during the stripping process, a nitrogen-depleted, methane-rich liquid fraction and a nitrogen-enriched fraction are obtained,
[0004] the nitrogen-enriched fraction is condensed, cooled, and separated into a methane-rich liquid fraction and a nitrogen-enriched gas fraction in a rectificatory double-column process, and
[0005] the methane-rich liquid fraction and the nitrogen-enriched gas fraction are used to cool the nitrogen-enriched fraction.
[0006] During the liquefaction of a methane-rich fraction, in particular natural gas, a liquefied methane-rich fraction is obtained, especially LNG (liquefied natural gas), which is temporarily stored or immediately supplied for its further use. The required nitrogen specification of the liquefied methane-rich fraction is usually adjusted by depressurizing the liquefied methane-rich fraction to low pressure.
[0007] The subcooling conditions of the liquefied methane-rich fraction determine the nitrogen content of the liquefied methane-rich fraction after its expansion. If the expansion effect is not sufficient to achieve the given nitrogen specification, a stripping process can be provided to separate nitrogen from the liquefied methane-rich fraction. The stripping process or the stripping column used therefor is operated by means of a reboiler provided in the sump region and / or the supply of a sub-stream of the not yet liquefied methane-rich fraction as a stripping medium. The nitrogen-enriched gas fraction withdrawn at the head of the stripping column, whose nitrogen content is between 20 and 40 mol %, is generally used as fuel gas for the upstream liquefaction process of the methane-rich fraction. If use as fuel gas is not possible, a cryogenic separation of this nitrogen-enriched gas fraction into a nitrogen-enriched and a methane-rich gas or liquid fraction is often carried out.
[0008] This approach belonging to the prior art is described in further detail below with reference to the procedure shown in FIG. 1.
[0009] The liquefied methane-rich fraction (100) is first subcooled in the reboiler E4 against an evaporating bottom stream (109) of the stripping column (T) and, after expansion (a), supplied to the stripping column (T) at a pressure between 1.2 and 2.5 bar. A nitrogen-depleted methane-rich liquid fraction (101) is supplied from the sump of the stripping column (T), for further use, optionally with the aid of a pump (P1). As the stripping medium (102), a sub-stream of the methane-rich fraction to be liquefied is supplied to the stripping column (T), wherein it is cooled in the heat exchanger E1 and expanded to a pressure between 1.2 and 2.5 bar before being supplied into the stripping column (T) (b).
[0010] At the head of the stripping column (T), a nitrogen-enriched fraction (103) is withdrawn, the nitrogen content of which is between 20 and 40 mol %. This fraction is heated in a heat exchanger (E1) against the stripping medium (102) to be cooled, compressed to a pressure between 25 and 50 bar (C1) and then (104), after cooling in a further heat exchanger (E2) against process streams to be heated, supplied to the sump region of a high-pressure column (T1). A low-pressure column (T2) is arranged above the high-pressure column (T1), wherein both columns are connected to each other via a reflux condenser (K). While the high pressure column (T1) is usually operated at a pressure between 22 and 29 bar, the pressure of the low pressure column (T2) is between 1.6 and 2.2 bar.
[0011] A methane-rich liquid fraction (105) is withdrawn from the sump of the high-pressure column (T1), cooled in a further heat exchanger (E3) against process streams to be heated, and supplied to the middle region of the low-pressure column (T2) via a valve (c). In the head region of the high-pressure column (T1), a nitrogen-enriched liquid fraction (106) is withdrawn, cooled against process streams to be heated (E3), and supplied via a valve (d) to the low-pressure column (T2) above the feed point of the methane-rich liquid fraction (105). A sub-stream of the withdrawn nitrogen-enriched liquid fraction is added to the high-pressure column (T1) as reflux (106′).
[0012] At the head of the low-pressure column (T2), a nitrogen-enriched gas fraction (107) is withdrawn, warmed against process streams to be cooled (E2, E3), and then released to the atmosphere. A methane-rich liquid fraction (108) is withdrawn from the sump of the low-pressure column (T2) by means of a pump (P2), warmed against process streams to be cooled and evaporated (E2, E3), and then compressed to the pressure of the methane-rich fraction to be liquefied (C2). This fraction is then added, at a suitable location, to the methane-rich fraction to be liquefied. Alternatively, the methane-rich liquid fraction (108) can also be directly added to the methane-rich liquid fraction (101) obtained in the stripping column (T). However, this requires that an external cooling supply is available for cooling the process streams described above in the heat exchangers (E2, E3).
[0013] The process described above requires process-related interaction with other plant components of the entire liquefaction plant—which is typically a natural gas liquefaction plant—such as the fuel gas system, the cooling supply, etc. This does not pose a fundamental problem when designing a new plant. However, the situation is different if such a process control used to separate nitrogen from a methane-rich liquid fraction, in particular from LNG, is to be integrated into a plant already planned or in operation. This is desired or necessary when the composition, in particular the nitrogen content, of the methane-rich fraction to be liquefied changes or when a standardized liquefaction process, in particular a liquefaction process consisting of a plurality of parallel streams, is used.
[0014] The object of the present invention is to provide a generic method for separating a nitrogen-enriched fraction from a liquefied methane-rich, nitrogen-containing fraction, which can be retrofitted to existing plants without requiring any significant process-related connection with other plant components.
[0015] To achieve this object, a method is proposed which is characterized in that at least a sub-stream of the re-evaporated methane-rich liquid fraction is supplied to the stripping process as a stripping medium.
[0016] According to the invention, it is possible to dispense with the need to supply a methane-rich liquid fraction as a stripping medium to the stripping process from the outside. This task is now taken over by the methane-rich liquid fraction or a sub-stream of this fraction withdrawn from the sump of the low-pressure columns.
[0017] The process according to the invention makes it possible to separate a nitrogen-enriched fraction from a liquefied, nitrogen-containing methane-rich fraction as well as to separate the separated nitrogen-enriched fraction into a nitrogen-enriched gas fraction and a methane-rich liquid fraction in an independent plant section, which can essentially dispense with process-related interaction with other plant sections of the entire liquefaction plant. This means that retrofitting existing systems is also easily feasible.
[0018] More advantageous embodiments of the method according to the invention are the subject matter of the dependent claims.
[0019] The method according to the invention is explained in more detail below with reference to the exemplary embodiments shown in FIGS. 2 to 4.
[0020] In these, the liquefied methane-rich fraction (1) is first subcooled in a reboiler (E4) against an evaporating sump stream (25) of the stripping column (T) and, after its expansion (a), supplied to the stripping column (T) at a pressure between 1.2 and 2.5 bar. This expansion (a) is advantageously carried out in an expander. It is also advantageous if the liquefied methane-rich fraction (1) is in supercooled form.
[0021] A nitrogen-depleted methane-rich liquid fraction (2) is withdrawn from the sump of the stripping column (T), optionally with the aid of a pump (P1), and supplied for its further use. At the head of the stripping column (T), a nitrogen-enriched fraction (3) is withdrawn, the nitrogen content of which is between 20 and 40 mol %. This fraction is heated in a heat exchanger (E1) against the stripping medium to be cooled, which will be discussed below, compressed to a pressure between 25 and 50 bar (C1) and then (4), after cooling in a further heat exchanger (E2) against process streams to be heated, supplied to the sump region of a high-pressure column (T1). A low-pressure column (T2) is arranged above the high-pressure column (T1), wherein both columns are connected to each other via a reflux condenser (K).
[0022] A methane-rich liquid fraction (5) is withdrawn from the sump of the high-pressure column (T1), cooled in a further heat exchanger (E3) against process streams to be heated, and supplied to the middle region of the low-pressure column (T2) via a valve (c). In the head region of the high-pressure column (T1), a nitrogen-enriched liquid fraction (6) is withdrawn, cooled against process streams to be heated (E3), and supplied via a valve (d) to the low-pressure column (T2) above the feed point of the methane-rich liquid fraction (5). A sub-stream of the withdrawn nitrogen-enriched liquid fraction is added to the high-pressure column (T1) as reflux (6′).
[0023] At the head of the low-pressure column (T2), a nitrogen-enriched gas fraction (10) is withdrawn, warmed against process streams to be cooled (E2, E3), and then at least partially released to the atmosphere (11). In the embodiment shown in FIG. 2, a sub-stream (12) of this nitrogen-enriched gas fraction is added to the nitrogen-enriched fraction (3), withdrawn from the stripping column (T), before its compression. This process control allows the adjustment of an optimal and constant composition of the nitrogen-enriched fraction to be compressed (4).
[0024] A methane-rich liquid fraction (7) is withdrawn from the sump of the low-pressure column (T2) by means of a pump (P2). In the embodiment shown in FIG. 2, this fraction is heated and evaporated (E2, E3) against process streams to be cooled, and after its cooling (E1) against the nitrogen-enriched fraction (3) withdrawn from the stripping column (T), expanded to a pressure between 1.2 and 2.5 bar (e) and supplied to the stripping column (T) as a stripping medium (8). In this case, a sub-stream (9) of the heated methane-rich fraction can be added to the nitrogen-enriched fraction (3) withdrawn from the stripping column (T) before its compression after a decompression (f).
[0025] In the embodiment shown in FIG. 3, the methane-rich liquid fraction (7) withdrawn from the sump of the low-pressure column (T2) is also initially heated against process streams to be cooled (E3). A part of this fraction (19) is then subcooled against a sump stream (25) in the reboiler (E4) of the stripping column (T) and accordingly serves as a heating medium. In addition, the liquefied methane-rich fraction (1) can also be incorporated as a heating stream in the reboiler (E4) before its expansion (a). This methane-rich fraction (19) is then separated into two sub-streams (20, 21). One of the sub-streams (20) is expanded to a pressure between 1.2 and 2.5 bar (h) and supplied directly to the stripping column (T). The other sub-stream (21) is returned to the non-subcooled sub-stream of the methane-rich fraction (18). This methane-rich fraction is then heated and evaporated (E2) against the nitrogen-enriched fraction (4) to be cooled, and after cooling in the heat exchanger E1, expanded to a pressure between 1.2 and 2.5 bar (j) and also supplied to the stripping column (T) as a stripping medium (8).
[0026] The process control described above improves the heat integration between the double column (T1, T2) and stripping column (T) and increases their efficiency.
[0027] Moreover, in the two embodiments described above, supplying according to the invention at least one sub-stream (8, 20) of the methane-rich fraction (7), withdrawn from the sump of the low-pressure column (T2), as a stripping medium to the stripping column (T) does not require any compression of this fraction.
[0028] The method according to the invention can react flexibly to a changing composition of the methane-rich fraction (1) supplied to the stripping column (T) as well as to partial load operation. Due to the integration of cold and the lack of necessity to compress the methane-rich fraction (7) withdrawn from the low-pressure column (T2), the method according to the invention is highly efficient in terms of energy consumption and investment costs.
[0029] In the embodiment shown in FIG. 4, the methane-rich fraction (30) is compressed to a pressure between 35 and 60 bar (C2) after evaporation and superheating (E3, E2). The compressed methane-rich fraction is then liquefied (E1) against the nitrogen-enriched fraction (3) withdrawn from the stripping column (T). It is then expanded to a pressure of 1.2 to 2.5 bar (e) and supplied to the stripping column (T) as a two-phase stripping medium (31).
[0030] In so doing, a sub-stream of the partially or completely compressed methane-rich fraction (32) can be discharged. Due to the compression (C2), this stream can be used both as high-pressure fuel gas as well as a pretreated feed stream for re-liquefaction.
[0031] Furthermore, a sub-stream of the methane-rich fraction (30) can be directly expanded (g) to a pressure between 1.2 and 2.5 bar upstream of the compressor (C2) and supplied to the stripping column (T) as an additional stripping medium (33).
[0032] Due to the additional compressor (C2), the stripping column (T) can be operated at a lower pressure, and the nitrogen-depleted methane-rich liquid fraction (2) can therefore be discharged at a lower temperature. This means a higher yield of LNG in the tank, as less expansion gas is produced upon dropping to tank pressure, which is between 1.05 and 1.1 bar.
Claims
1. A method for separating a nitrogen-enriched fraction from a liquefied methane-rich, nitrogen-containing fraction, comprising:the liquefied methane-rich, nitrogen-containing fraction is subjected to a stripping process to which a methane-rich fraction is supplied as a stripping medium,during the stripping process, a nitrogen-depleted, methane-rich liquid fraction (2) and a nitrogen-enriched fraction are obtained,the nitrogen-enriched fraction is compressed, cooled, and separated into a methane-rich liquid fraction and a nitrogen-enriched gas fraction in a rectificatory double-column process, andthe methane-rich liquid fraction and the nitrogen-enriched gas fraction serve to cool the nitrogen-enriched fraction,wherein at least one sub-stream of the re-evaporated methane-rich liquid fraction is supplied to the stripping process as a stripping medium.
2. The method according to claim 1, wherein at least a sub-stream of the re-evaporated methane-rich liquid fraction is cooled or at least partially condensed as a stripping medium before being supplied into the stripping process.
3. The method according to claim 1, wherein the heating of the reboiler stream of the stripping process takes place by subcooling at least a sub-stream of the methane-rich liquid fraction.
4. The method according to claim 3, wherein a sub-stream of the methane-rich supercooled liquid fraction is supplied to the stripping process.
5. The method according to claim 1, wherein at least a sub-stream of the re-evaporated methane-rich fraction is compressed, preferably to a pressure between 35 and 60 bar, liquefied, expanded, and supplied to the stripping process as a stripping medium.
6. The method according to claim 2, wherein the heating of the reboiler stream of the stripping process takes place by subcooling at least a sub-stream of the methane-rich liquid fraction.
7. The method according to claim 2, wherein at least a sub-stream of the re-evaporated methane-rich fraction is compressed, preferably to a pressure between 35 and 60 bar, liquefied, expanded, and supplied to the stripping process as a stripping medium.