AU2005243086B2

AU2005243086B2 - Method and device for liquefying a hydrocarbon-enriched flow

Info

Publication number: AU2005243086B2
Application number: AU2005243086A
Authority: AU
Inventors: Heinz Bauer; Hubert Franke; Rainer Sapper; Marc Schier
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2004-05-13
Filing date: 2005-05-10
Publication date: 2010-09-09
Anticipated expiration: 2025-05-10
Also published as: WO2005111522A1; RU2006143841A; NO20065721L; DE102004023814A1; AU2005243086A1; RU2374576C2

Description

WO 2005/111522 PCT/EP2005/0050 54 Method and device for liquefying a hydrocarbon-enriched flow The invention relates to a process for liquefying a 5 hydrocarbon-rich stream, in particular a natural gas stream, the liquefaction of the hydrocarbon-rich stream being carried out against a mixed refrigerant circuit cascade comprising three mixed refrigerant circuits, the first of the three mixed refrigerant circuits being 10 used for pre-cooling, the second mixed refrigerant circuit being used for the actual liquefaction and the third mixed refrigerant circuit being used for supercooling the liquefied hydrocarbon-rich stream, and the refrigerant mixtures being subjected to one-stage 15 or multi-stage compression. Furthermore, the invention relates to an apparatus for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, having a mixed refrigerant circuit 20 cascade comprising three mixed refrigerant circuits, against which the liquefaction of the hydrocarbon-rich stream is carried out, the first of the three mixed refrigerant circuits being used for pre-cooling, the second mixed refrigerant circuit being used for the 25 actual liquefaction and the third mixed refrigerant circuit being used for supercooling the liquefied hydrocarbon-rich stream, and having a plurality of one stage or multi-stage compressors, which are used for the compression of the refrigerants mixtures. 30 A process of the generic type and an apparatus of the generic type for liquefying a hydrocarbon-rich stream are disclosed by the German Laid-open specification 197 16 415. With the citation of the German Laid-open 35 specification 197 16 415, its disclosure content is incorporated in the disclosure content of the present patent application.

- 2 Natural gas liquefaction plants are designed either as what are known as LNG baseload plants - that is to say plants for liquefying natural gas in order to supply natural gas as primary energy - or as what are known as 5 peak-shaving plants - that is to say plants for liquefying natural gas in order to cover the peak demand. LNG baseload plants are generally operated with 10 refrigerating circuits which comprise hydrocarbon mixtures. These mixed circuits are more efficient in terms of energy than expander circuits and permit correspondingly relatively low energy consumptions at the high liquefaction outputs of the baseload plants. 15 In generic liquefaction processes, the first mixed circuit is basically used for pre-cooling, the second mixed circuit for liquefaction and the third mixed circuit for supercooling the hydrocarbon-rich stream or 20 natural gas. Between the pre-cooling and liquefaction, the separation of higher-boiling-point hydrocarbons is carried out, if necessary. These are at least those 25 components of the hydrocarbon-rich stream or natural gas to be liquefied which would freeze out during the subsequent cooling, that is to say C 5 , hydrocarbons and aromatics. Often, in addition those hydrocarbons which would undesirably increase the heating value of the 30 liquefied natural gas, this means in particular propane and butane, are separated off before the liquefaction. This separation of higher-boiling-point hydrocarbons is normally done by what is known as an HHC (heavy 35 hydrocarbon) column (also known as a 'scrub column' in English usage) being provided, which is used to separate the heavy hydrocarbons and benzene from the hydrocarbon-rich stream to be liquefied. A process procedure of this type is likewise described in the -3 German Laid-open specification 197 16 415 already mentioned; see for example, Figure 2 therein and the associated figure description. 5 By predefining this cut between those components of the hydrocarbon-rich stream to be liquefied which ultimately represent the liquefied product, these are substantially methane and ethane, and those components which are (have to be) separated out for the 10 aforementioned reasons, with a given crude gas pressure, the temperature level of the separation of these components from the hydrocarbon-rich stream to be liquefied, designated C 3 1 separation in the following text, is defined within comparatively narrow limits. 15 If the first mixed circuit is now used exclusively for pre-cooling the hydrocarbon-rich stream to be liquefied before this C 3 + separation, a proportion of the total compressor output of about 40 to 50% is necessarily 20 allocated to this, while the remaining compressor output of 60 to 50% is distributed to the second and the third mixed circuit. However, in the sense of economic utilization of the 25 available compressors and drives, it is desirable that the (circuit) compressors of the three mixed circuits obtain approximately the same drive output, that is to say in each case about 33.33% of the total drive output. This applies in particular to large 30 liquefaction plants having a liquefaction output of more than 5 million tonnes of LNG per year, since the number of available compressors and drives for such orders of magnitude is highly restricted. By means of standardizing the drives and compressors of the three 35 refrigerating circuits, the liquefaction output from the liquefaction process that can be achieved with tried and tested drives and compressors can be maximized. An appropriate solution and procedure are described in German Patent Application 103 44 030, C-\NRPonbqD)CC\AX1A1199W54_1 DOC-5/0X/2010 -4 which is not a prior publication; with the citation of this Patent Application, its disclosure content is incorporated in the disclosure content of the present Patent Application. 5 German Patent Application 103 44 030 shows a generic liquefaction process in which the compressors of the mixed refrigerant circuits are driven by three substantially identical drives. However, drives, this applies in particular to gas turbines, are only available in discrete 10 output stages. Depending on the selected process or plant size, the use of three identical drives is therefore often not expedient. Instead, it would be sufficient if the required drive output could be furnished by only two identical or two approximately identical drives instead of 15 by three. It is desirable to provide a generic process and a generic apparatus which take account of the aforementioned problem. 20 According to the present invention, there is provided process for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, the liquefaction of the hydrocarbon-rich stream being carried out against a mixed refrigerant circuit cascade comprising three mixed 25 refrigerant circuits, the first of the three mixed refrigerant circuits being used for pre-cooling, the second mixed refrigerant circuit being used for the actual liquefaction and the third mixed refrigerant circuit being used for supercooling the liquefied hydrocarbon-rich stream, 30 and the refrigerant mixtures being subjected to one-stage or multi-stage compression, characterized in that the compressors are combined into two compressor streams and the C.\NRPorbl\DCCAXL\3(99'54_1 DOC-5AIn201 -5 drive of the compressors is carried out by means of two identical or two approximately identical drives. The present invention also provides apparatus for liquefying 5 a hydrocarbon-rich stream, in particular a natural gas stream, having a mixed refrigerant circuit cascade comprising three mixed refrigerant circuits, against which the liquefaction of the hydrocarbon-rich stream is carried out, the first of the three mixed refrigerant circuits being 10 used for pre-cooling, the second mixed refrigerant circuit being used for the actual liquefaction and the third mixed refrigerant circuit being used for supercooling the liquefied hydrocarbon-rich stream, and having a plurality of one-stage or multi-stage compressors, which are used for the 15 compression of the refrigerant mixtures, characterized in that the compressors are combined into two compressor streams and the compressors are assigned two identical or approximately identical drives. 20 In an embodiment of the invention, on the process side it is proposed that the compressors be combined into two compressor streams and the drive of the compressors be carried out by means of two identical or two approximately identical drives. 25 The apparatus according to one embodiment of the invention is characterized in that the compressors are combined into two compressor streams and the compressors are assigned two identical or approximately identical drives. 30 C.\NRPonb\DCCAXIA19054 I.DOC-51/I/2110 -6 The term "approximately identical drives" is to be understood to mean drives which differ by no more than 5% from one another in terms of their output. 5 The process and apparatus according to embodiments of the invention now make it possible to provide the necessary drive output of all the compressors by means of only two identical or approximately identical drives. 10 In this case, the drives are preferably constructed as gas turbines, electric motors and/or steam turbines. Developing embodiments of the invention, for the case in which there is a difference in output between the two 15 compressor streams, it is proposed that a generator be assigned to the more powerful compressor stream and an electric motor be assigned to the less powerful compressor stream and that the generator be coupled to the electric motor. 20 The excess power arising on the generator can be supplied to the electric motor, so that the latter can assist the drive of the less powerful compressor stream. 25 The process according to one embodiment of the invention, the apparatus according to the invention and further refinements of the same, which represent subjects of the dependent patent claims, are to be explained in more detail in the following text by using the exemplary embodiment 30 illustrated in the figure.

C:NRPonblCC\AXL\1399054 I.DOC-AM/2010 - 6a In the procedure described by using the figure, the cooling and liquefaction of the hydrocarbon-rich stream, which is supplied to the heat exchanger El via line 1, is carried out against a mixed refrigerant circuit cascade, comprising 5 three mixed refrigerant circuits. These generally have different compositions as are described, for example, in the German Laid-open specification 197 16 415 mentioned previously. 10 The hydrocarbon-rich stream to be liquefied is cooled in the heat exchanger El against the two evaporating mixed refrigerant partial streams 4b and 4d from the first mixed circuit 4a to 4e and is then supplied via line la to a separating unit S merely illustrated as a black box. 15 The previously described C 3 + separation is carried out in this separating unit S, the components separated out from the hydrocarbon-rich stream to be liquefied being drawn off from the separating unit S via the line lb. 20 According to an advantageous refinement of the process according to an embodiment of the invention, not illustrated in the figure, at least a partial stream of one of the two partial streams 3b and 3d of the second mixed refrigerant 25 circuit 3a to 3e, which will be discussed in more detail in the following text, can is used to provide the refrigeration in the separating unit S. In this case, the choice as to which of the two partial streams 3b and/or 3d is in turn used as the at least one partial stream to provide this 30 refrigeration will be determined by the temperature level(s) required in the separating unit S.

U NRI'onMD)C(w oI'NI IDOC-5/"/210 - 6b The hydrocarbon-rich stream to be liquefied is then supplied via line lc to a second heat exchanger E2 and is liquefied in the latter against the evaporating mixed refrigerant partial stream 3b of the second refrigerating circuit 3a to 5 3b. After the liquefaction has been carried out, the hydrocarbon-rich stream is supplied via line ld to a third heat exchanger E3 and is supercooled in the latter against 10 the mixed refrigerant stream 2b of the third refrigerating circuit 2a to 2c. The supercooled liquid product is then supplied to its further use via line le. As can be seen from the figure, the compressors V2, V3, V3' 15 and V4 of the refrigerating circuits 2a to 2c, 3a to 3f and 4a to 4e are now combined, according to the invention, to form two compressor streams. In this -7 case, the first compressor stream is formed by the compressor V4 of the pre-cooling circuit and the high pressure compressor V3 of the liquefaction circuit, while the second compressor stream is formed by the 5 compressor V2 of the supercooling circuit and the low pressure compressor V3' of the liquefaction circuit. An embodiment in which the compressors V4 and V3' form the first compressor stream and the compressors V2 and V3 form the second compressor stream is to be seen as an 10 equivalent alternative. The circuit compressor of the liquefaction circuit is "divided up" according to the invention into two compressors or compressor streams. The consequence of 15 this, as will be explained in the following text, is that a partial stream 3c of the refrigerant mixture of the liquefaction circuit is supplied to a low-pressure compressor V3', and a partial stream 3e of the refrigerant mixture of the liquefaction circuit is 20 supplied to a high-pressure compressor V3. In this case, the two aforementioned compressors compress the mixed refrigerant streams, preferably to the same end pressures. 25 In each case a drive GT1 and GT2 is assigned to the two compressor streams, according to the invention these being two identical or two approximately identical drives. 30 Suitable drives are, in particular, gas turbines, electric motors and/or steam turbines. Not illustrated in the figure are the coolers or heat exchangers connected downstream of the compressors V2, 35 V3, V3' and V4, in which the refrigerant mixture is cooled down against a suitable cooling medium, for example water or air.

-8 The refrigerant mixture of the first mixed circuit compressed in the compressor V4 is supplied via the line 4a to the heat exchanger El and divided up into two partial streams 4b and 4d after cooling has been 5 carried out in the said heat exchanger. The refrigerant mixture in these partial streams 4b and 4d, following expansion in the valves d and e or expansion devices, is evaporated to different pressure levels in the heat exchanger El and subsequently supplied via the line 4c 10 or 4e to the compressor V4 upstream of the first stage (partial stream 4c) or to an intermediate pressure level (partial stream 4e). The refrigerant mixture of the second refrigerating 15 circuit 3a to 3f compressed in the compressor" V3 is supplied via the lines 3f and 3a to the heat exchangers El and E2 and cooled in the latter. That partial stream 3b of this mixed refrigerant stream which is led through the heat exchanger E2, after expansion has 20 taken place in the valve b, is evaporated in the heat exchanger E2 against process streams to be cooled down and is subsequently supplied via line 3c to the input stage of the compressor V3'. 25 That partial stream 3d of the refrigerant mixture of the second mixed refrigerant circuit 3a to 3f which is already drawn off downstream of the heat exchanger El is expanded in the valve c and subsequently evaporated in the heat exchanger El against process streams to be 30 cooled down, before it is supplied via line 3e to the compressor V3. With this process procedure, the aforementioned mixed refrigerant partial stream 3d contributes to the pre-cooling of the hydrocarbon-rich stream in the heat exchanger El. 35 In order that this can be achieved, the partial stream 3d of the refrigerant mixture of the second mixed refrigerant circuit 3a to 3f which is used for the pre cooling of the hydrocarbon-rich stream must be C\NRPonbl\DCCAXL\311))54_I.DOC.5/UN/20 10 -9 evaporated to a pressure which is higher than the evaporation pressure of the mixed refrigerant partial stream 3b of the second mixed refrigerant circuit 3a to 3f. 5 By means of the selection of the intermediate pressure at which the mixed refrigerant partial steam 3e is evaporated and supplied to the compressor V3, and by means of the control of the volume distribution of the two mixed refrigerant partial streams 3b and 3d, the division of the 10 refrigerating output of the second mixed circuit to the heat exchangers El and E2 and therefore to the pre-cooling and liquefaction of the hydrocarbon-rich stream to be liquefied can be set virtually as desired. 15 If there is a difference in output between the two compressor streams then, according to an advantageous refinement of the invention, as is also illustrated in the figure, a generator G can be assigned to the more powerful compressor stream and an electric motor M can be assigned to 20 the less powerful compressor stream. The current generated by means of the generator G drives the electric motor M which, as a result, assists the less powerful compressor stream or its drive GT2. 25 The process according to the invention for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, therefore provides a liquefaction process in which the compressor streams are coupled to only two identical or approximately identical drives. As a result, in a large 30 number of applications, optimum adaptation to obtainable drives can be implemented, which results in a reduction in the necessary investment and operating costs.

C \NRPonhIt\DCCAXI'l'9'IU5 I DO)C-5'NK211tI - 10 Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be 5 understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The reference in this specification to any prior publication 10 (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of 15 endeavour to which this specification relates.

Claims

1. Process for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, the liquefaction of the 5 hydrocarbon-rich stream being carried out against a mixed refrigerant circuit cascade comprising three mixed refrigerant circuits, the first of the three mixed refrigerant circuits being used for pre-cooling, the second mixed refrigerant circuit being used for the actual 10 liquefaction and the third mixed refrigerant circuit being used for supercooling the liquefied hydrocarbon-rich stream, and the refrigerant mixtures being subjected to one-stage or multi-stage compression, characterized in that the compressors are combined into two compressor streams and the 15 drive of the compressors is carried out by means of two identical or two approximately identical drives.

2. Process according to Claim 1, characterized in that the drives are constructed as gas turbines, electric motors 20 and/or steam turbines.

3. Process according to Claim 1 or 2, there being a difference in output between the two compressor streams, characterized in that a generator is assigned to the more 25 powerful compressor stream and an electric motor is assigned to the less powerful compressor stream and the generator is coupled to the electric motor.

4. Process according to one of Claims 1 to 3, 30 characterized in that at least a partial stream of the refrigerant mixture of the second mixed refrigerant circuit is used for the pre-cooling of the hydrocarbon-rich stream. C.\NRPrbl\DCC\AXIXI990I54_1 DOC-5/AIK21.I10 - 12

5. Process according to one of Claims 1 to 4, characterized in that the partial stream of the refrigerant mixture of the second mixed refrigerant circuit that is used 5 for the pre-cooling of the hydrocarbon-rich stream is evaporated to a pressure which is higher than the evaporation pressure of the remaining partial stream of the refrigerant mixture of the second mixed refrigerant circuit and is supplied to the compressor of the second mixed 10 refrigerant circuit at an intermediate pressure level.

6. Process according to one of Claims 1 to 5, characterized in that the volumes and/or evaporation pressures of the two partial streams of the second mixed 15 refrigerant circuit can be varied.

7. Apparatus for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, having a mixed refrigerant circuit cascade comprising three mixed refrigerant circuits, 20 against which the liquefaction of the hydrocarbon-rich stream is carried out, the first of the three mixed refrigerant circuits being used for pre-cooling, the second mixed refrigerant circuit being used for the actual liquefaction and the third mixed refrigerant circuit being 25 used for supercooling the liquefied hydrocarbon-rich stream, and having a plurality of one-stage or multi-stage compressors, which are used for the compression of the refrigerant mixtures, characterized in that the compressors are combined into two compressor streams and the compressors 30 are assigned two identical or approximately identical drives. C :NRPorbl\DCC\AXL\1b 054 1.DOC-5/IKI21 I - 13

8. Apparatus according to Claim 7, characterized in that the drives are gas turbines, electric motors and/or steam turbines. 5

9. Apparatus according to Claim 7 or 8, there being a difference in output between the two compressor streams, characterized in that a generator is assigned to the more powerful compressor stream and an electric motor is assigned to the less powerful compressor stream and the generator is 10 coupled to the electric motor.

10. A process substantially as hereinbefore described with reference to the accompanying drawings. 15

11. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.