METHOD FOR LIQUEFYING A HYDROCARBON-RICH STREAM

Abstract

A method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, is disclosed. A liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cycle cascade consisting of two or three refrigerant mixture cycles. No additional process steps are involved in the heat exchange between the hydrocarbon-rich stream which is to be precooled and the refrigerant mixture of the first refrigerant mixture cycle.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for liquefying a hydrocarbon-rich stream, in particular a stream of natural gas, where the liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cascade consisting of two refrigerant mixture cycles and where the first refrigerant mixture cycle is used for precooling and the second refrigerant mixture cycle is used for liquefying and supercooling the hydrocarbon-rich stream to be liquefied.

The invention further relates to a method for liquefying a hydrocarbon-rich stream, in particular a stream of natural gas, where the liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cycle cascade consisting of three refrigerant mixture cycles and where the first of the three refrigerant mixture cycles is used for precooling, the second refrigerant mixture cycle is used for the actual liquefaction and the third refrigerant mixture cycle for supercooling the liquefied hydrocarbon-rich stream.

In what follows, the term “precooling” should be understood to mean the cooling of the hydrocarbon-rich stream down to a temperature at which the separation of heavy, or higher-boiling, hydrocarbons takes place. The subsequent further cooling of the hydrocarbon-rich stream to be liquefied hereinafter comes under the term “liquefaction”.

Generic natural gas liquefaction methods in which the liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cycle cascade consisting of two refrigerant mixture cycles—generally designated as dual-flow LNG process—are sufficiently well known to the person skilled in the art; U.S. Pat. No. 6,105,389 can be named as an example.

The same applies to generic natural gas liquefaction methods in which the liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cycle cascade consisting of three refrigerant mixture cycles; the natural gas liquefaction method described in German disclosure 197 16 415 can be named as an example thereof.

With the citation of the two aforementioned patents, the content of their disclosure is hereby expressly incorporated by reference herein into the disclosure of this patent application.

Normally—as explained for example in the aforementioned U.S. Pat. No. 6,105,389—the exchange of heat in precooling, liquefaction and supercooling takes place in a combined multi-stream heat exchanger. All process streams are taken through a common type of heat exchanger. According to the required, or desired, capacity, several identical heat exchanger (units) located in parallel are provided.

As a matter of principle it holds true that natural gas liquefaction plants in which plate heat exchangers are used for precooling require several heat exchangers arranged in parallel above about 0.2 mtpa LNG capacity since the heating surface per heat exchanger block is limited. Consequently, for a liquefaction capacity of 5 mtpa 20 to 30 heat exchanger blocks arranged in parallel must be provided, as long as each contains all the process streams.

However, the following disadvantages result:

• • uniform distribution;
  • high cost of pipelines;
  • operating stability at part load operation, in particular the problem of entrainment; and
  • thermal imbalance when operating the plant only with precooling, i.e., without liquefaction and cooling because of missing process streams; this is of importance in particular during the start-up procedure for the liquefaction process.

The object of the present invention is to specify generic methods for liquefying a hydrocarbon-rich stream in which the aforementioned problems can be avoided.

To achieve this object, it is provided in the case of generic liquefaction processes in which liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cycle cascade consisting of two or three refrigerant mixture cycles that no additional process streams be involved in the heat exchange between the hydrocarbon-rich stream to be precooled and the refrigerant mixture of the first refrigerant mixture cycle.

By means of the method in accordance with the invention which represents a “specialization” of the heat exchangers, the number of heat exchangers, or heat exchanger blocks, for the same process task can be reduced substantially. As a concomitant result, the required expenditure for pipelines can be reduced.

The number of blocks per heat exchanger type can be kept below 16 at a liquefaction capacity up to 10 mtpa, preferably between 2 and 8 blocks. This allows symmetrical pipelines at an appropriate expenditure.

BRIEF DESCRIPTION OF THE DRAWINGS

The method in accordance with the invention and additional embodiments of the same are explained in more detail hereinafter using the embodiments shown in FIGS. 1 to 5.

FIG. 1 shows a natural gas liquefaction method in which the liquefaction takes place countercurrent to a refrigerant mixture cycle consisting of two refrigerant mixture cycles;

FIG. 2 shows a natural gas liquefaction method in which the liquefaction takes place countercurrent to a refrigerant mixture cycle consisting of three refrigerant mixture cycles;

FIG. 3 shows a natural gas liquefaction method as explained using FIG. 2 and in which at least one refrigerant mixture partial stream from the second refrigerant mixture cycle is used for the precooling of the natural gas; and

FIGS. 4/5 show natural gas liquefaction methods as explained using FIG. 2 in which the cooling of the refrigerant mixture of the second and ternary refrigerant mixture cycles takes place in dual-stream exchangers.

DETAILED DESCRIPTION OF THE DRAWINGS

In the embodiment of the method in accordance with the invention shown in FIG. 1, the natural gas stream to be cooled and liquefied is taken over line 1 to a first heat exchanger E1. Here the natural gas stream is cooled countercurrent to a partial stream P3 of the refrigerant mixture of the first refrigerant mixture cycle. Subsequently the natural gas stream is taken over line 2 to a second heat exchanger E2 in which it is cooled in succession countercurrent to two partial streams P5 and P7 of the refrigerant mixture of the first refrigerant mixture cycle.

The natural gas stream cooled in this way is subsequently taken over line 3 to a further heat exchanger E5 and liquefied in the exchanger countercurrent to the refrigerant mixture L2 of the second refrigerant mixture cycle and supercooled as necessary. Subsequently to this the liquefied natural gas stream (LNG) is taken over line 4 to its further use and/or to storage.

The last described liquefaction and supercooling of the precooled natural gas stream takes place in the case of the embodiment shown in FIG. 1 countercurrent to the second refrigerant mixture cycle L1 to L4 of the refrigerant mixture circuit cascade, where the refrigerant mixture compressed by means of single- or multi-stage compression LV is first taken to an aftercooler LK and subsequently over line 4 to a heat exchanger E3. A cooling and liquefaction of the refrigerant mixture of the second refrigerant mixture cycle takes place in the exchanger countercurrent to partial streams P9, P11 and P13 of the refrigerant mixture of the first refrigerant mixture cycle, which are available at suitable temperature levels.

The thus cooled and liquefied refrigerant mixture of the second refrigerant mixture cycle is subsequently taken over line L1 to heat exchanger E5 already mentioned, supercooled countercurrent to itself, drawn off over line L2 from heat exchanger E5, expanded and again taken through heat exchanger E5 countercurrent to the natural gas stream to be liquefied and if necessary supercooled. Subsequently the refrigerant mixture is drawn off over line L3 and taken to the single- or multi-stage circuit compressor LV already mentioned.

The partial streams P3, P5 and P7 already mentioned of the refrigerant mixture of the first refrigeration cycle which serve to precool the natural gas stream 1 or 2 in heat exchangers E1 and E2 are reunited in the multi-stage compressor unit PV of the first refrigerant mixture cycle.

The refrigerant mixture stream compressed in compression PV is taken over line P1 to a condenser PK and subsequently over line P2 to the first of three heat exchangers E4A, E4B and E4C. After each of the three aforementioned heat exchangers, partial streams of the refrigerant mixture at suitable temperature levels are drawn off through the lines P3, P5 or P7, expanded and subsequently—as already described—taken through heat exchangers E1 and E2 for the purpose of precooling the natural gas stream 1 or 2 which is to be liquefied.

To supercool the aforementioned refrigerant mixture partial streams P3, P5 and P7, partial streams P15, P17 and P19 are drawn off from them in turn, expanded and taken countercurrent through the three aforementioned heat exchangers E4A, E4B or E4C. These partial streams are subsequently in turn admixed to the particular streams from which they were drawn off over lines P16, P18 and P20 before compression PV.

Further developing the methods in accordance with the invention, it is provided that no further process streams be involved in the heat exchange E4A, E4B and E4C of the refrigerant mixture P2 of the first refrigerant mixture cycle to be cooled countercurrent to itself.

For the purpose of cooling, or supercooling, the refrigerant mixture L4 of the second refrigerant mixture cycle in heat exchanger E3, partial streams are similarly drawn off from the three refrigerant mixture partial streams P3, P5 and P7 over lines P9, P11 and P13, expanded and taken through the heat exchanger E3 countercurrent to the refrigerant mixture L4 of the second refrigerant mixture cycle. These refrigerant mixture partial streams are also subsequently admixed over lines P10, P12 and P14 to the refrigerant mixture partial streams in the lines P4, P6 and P8 before compression PV.

The embodiment of the method in accordance with the invention shown in FIG. 2 differs from the one shown in FIG. 1 in that an additional refrigerant mixture stream is now provided for the supercooling of the liquefied natural gas stream. Consequently, in what follows only the differences between the embodiments shown in FIGS. 1 and 2 will be discussed.

In the case of the example of the method shown in FIG. 2, the liquefaction of the natural gas stream precooled in heat exchangers E1 and E2 takes place in heat exchanger E5 countercurrent to the refrigerant mixture stream of the second refrigerant mixture cycle. Subsequently the liquefied natural gas stream is taken over line 4 to a further heat exchanger E6, supercooled in the heat exchanger countercurrent to the refrigerant mixture stream S3 of the third refrigerant mixture cycle and subsequently taken over line 5 to its further use and/or storage.

As already explained using the first and the second refrigerant mixture cycle, the refrigerant mixture of the third refrigerant mixture cycle is also initially compressed in a single- or multi-stage compression SV and taken to an aftercooler SK and subsequently to heat exchanger E3 over line S1. The refrigerant mixture—jointly with the refrigerant mixture from the second refrigerant mixture cycle—is cooled in the heat exchanger countercurrent to several refrigerant mixture partial streams of the first refrigerant mixture cycle and at least partially condensed.

The cooled refrigerant mixture of the third refrigerant mixture cycle is taken to heat exchanger E5 over line S2, cooled further here, completely condensed and subsequently supercooled in heat exchanger E6. The supercooled refrigerant mixture is drawn off from the latter over line S3, expanded and again taken through heater exchanger E6 countercurrent to the natural gas stream to be supercooled. Subsequently the heated refrigerant mixture of the third refrigerant mixture cycle is again taken over line S4 to compression SV, which has already been described.

FIG. 3 shows an embodiment of the method in accordance with the invention in which a partial stream of the refrigerant mixture of the second refrigerant mixture cycle—in addition to the refrigerant mixture from the first refrigerant mixture cycle—is used for precooling the natural gas stream to be liquefied.

For this purpose, a refrigerant mixture partial stream from the refrigerant mixture stream cooled in heat exchanger E3 is drawn off over line L5, expanded and taken at a suitable temperature level through heat exchanger E2 in counterflow to the natural gas stream 2 which is to be cooled. The heated refrigerant mixture partial stream is subsequently taken to compression LV over line L6.

A further partial stream of refrigerant mixture L1 of the second refrigerant mixture cycle cooled in heat exchanger E3 is drawn off over line L7, expanded and taken to heat exchanger E3 for the purpose of preparing refrigerant. This refrigerant mixture partial stream is taken over line L8 to the compressor unit LV after passing through heat exchanger E3.

FIGS. 4 and 5 show embodiments of the method in accordance with the invention in which the cooling of refrigerant mixture L4 of the second refrigerant mixture cycle takes place countercurrent to refrigerant mixture partial streams from the first refrigerant mixture cycle Pa, Pa′, Pb or Pb′ and the cooling of refrigerant mixture S1 from the third refrigerant mixture cycle takes place countercurrent to refrigerant mixture partial streams from the first refrigerant mixture cycle Pc, Pc′, Pd or Pd′ in dual-stream exchangers E3A, E3B, E3C or E3D. The dual-stream exchangers E3A, E3B, E3C or E3D are preferably configured as plate exchangers.

This method of proceeding requires a redesign of heat exchangers E2 and E3; the remaining heat exchangers can in principle remain unchanged.

This embodiment of the method in accordance with the invention for liquefying a hydrocarbon-rich stream has the advantage that all refrigerant mixture partial streams of the first refrigerant mixture circuit Pa, Pb, Pc and Pd are carried in separate flow passages of dual-stream exchangers E3A, E3B, E3C and E3D which are optimized for the particular task and thereby substantially improve performance, in particular at startup and at part load. Against this, there is the disadvantage that the greater number of heat exchanger models causes increased engineering expense.

The embodiment shown in accordance with the invention of the method in FIG. 5 for liquefying a hydrocarbon-rich stream differs from the one shown in FIG. 4 only in that the refrigerant mixture of the second refrigerant mixture cycle is evaporated at two different temperature levels. As a result, the heat exchanger E5 shown in FIG. 4 is divided into two heat exchangers E5A and E5B.

The inventive methods for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, in contrast to the known collective pipelines which in the event of different pressure drops cause faulty distribution because of the non-symmetrical pipelines, allow the realization of sufficiently symmetrical pipelines and thus appropriate equal distribution by avoiding different pressure drops.

With the monobloc concept only one refrigerant mixture stream is responsible for several streams to be cooled per heat exchanger section. As a result, the heat output of the individual refrigerant mixture streams is higher than is compatible with stable entrainment. Entrainment is economical only in a load range of 1:3. Faulty entrainment in refrigerant circuits leads to demixing of gas and liquid and can compromise the stability of the operation and even the mechanical strength of a heat exchanger.

In addition, the inventive methods reduce complexity with respect to the necessary heat exchangers since predominantly only two-stream exchangers are used; as a result a thermal imbalance can largely be avoided in the event of the failure of individual circuits.

Claims

1-10. (canceled)

11. A method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, wherein a liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cycle cascade consisting of two refrigerant mixture cycles and wherein a first refrigerant mixture cycle is used for precooling and a second refrigerant mixture cycle is used for liquefaction and supercooling of the hydrocarbon-rich stream to be liquefied, wherein no additional process steps are involved in a heat exchange between the hydrocarbon-rich stream which is to be precooled and the refrigerant mixture of the first refrigerant mixture cycle.

12. A method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, wherein a liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cycle cascade consisting of three refrigerant mixture cycles and wherein a first of the three refrigerant mixture cycles is used for precooling, a second refrigerant mixture cycle for an actual liquefaction and a third refrigerant mixture cycle for supercooling the liquefied hydrocarbon-rich stream, wherein no additional process steps are involved in a heat exchange between the hydrocarbon-rich stream which is to be precooled and the refrigerant mixture of the first refrigerant mixture cycle.

13. The method according to claim 11, wherein at least one partial stream of the refrigerant mixture of the second refrigerant mixture cycle is used for the precooling of the hydrocarbon-rich stream, wherein the partial stream from the refrigerant mixture of the second refrigerant mixture cycle is involved in the heat exchange between the hydrocarbon-rich stream and the refrigerant mixture of the first refrigerant mixture cycle.

14. The method according to claim 11, wherein the heat exchange between the hydrocarbon-rich stream to be precooled and the refrigerant mixture of the first refrigerant mixture cycle is implemented in at least one straight-tube heat exchanger.

15. The method according to claim 14, wherein the heat exchanger is a plate heat exchanger or a coil-type heat exchanger.

16. The method according to claim 11, wherein no additional process streams are involved in a heat exchange countercurrent to itself of the refrigerant mixture of the first refrigerant mixture cycle.

17. The method according to claim 16, wherein the exchange of heat of the refrigerant mixture of the first refrigerant mixture cycle countercurrent to itself is implemented in at least one straight tube heat exchanger.

18. The method according to claim 17, wherein the heat exchanger is a plate heat exchanger or a coil-type heat exchanger.

19. The method according to claim 11, wherein a cooling of the refrigerant mixture of the second refrigerant mixture cycle takes place countercurrent to refrigerant mixture partial streams of the first refrigerant mixture cycle in a separate heat exchanger.

20. The method according to claim 19, wherein the separate heat exchanger is a plate heat exchanger or a coil-type heat exchanger.

21. The method according to claim 12, wherein a cooling of the refrigerant mixture of the third refrigerant mixture cycle takes place countercurrent to refrigerant mixture partial streams of the first refrigerant mixture cycle in a separate heat exchanger.

22. The method according to claim 21, wherein the separate heat exchanger is a plate heat exchanger or a coil-type heat exchanger.

23. The method according to claim 12, wherein a cooling of the refrigerant mixture of the second refrigerant mixture cycle takes place countercurrent to refrigerant mixture partial streams of the first refrigerant mixture cycle and a cooling of the refrigerant mixture of the third refrigerant mixture cycle takes place countercurrent to refrigerant mixture partial streams of the first refrigerant mixture cycle in dual-stream heat exchangers.

24. The method according to claim 23, wherein the dual-stream heat exchangers are plate heat exchangers.

25. A method for liquefying a hydrocarbon-rich stream, comprising the steps of: liquefaction of the hydrocarbon-rich stream countercurrent to a refrigerant mixture cycle cascade consisting of two refrigerant mixture cycles, wherein a first refrigerant mixture cycle is used for precooling and a second refrigerant mixture cycle is used for liquefaction and supercooling of the hydrocarbon-rich stream to be liquefied, wherein no additional process steps are involved in a heat exchange between the hydrocarbon-rich stream which is to be precooled and the refrigerant mixture of the first refrigerant mixture cycle.

26. A method for liquefying a hydrocarbon-rich stream, comprising the steps of: liquefaction of the hydrocarbon-rich stream countercurrent to a refrigerant mixture cycle cascade consisting of at least two refrigerant mixture cycles;wherein a first refrigerant mixture cycle precools the hydrocarbon-rich stream and wherein a second refrigerant mixture cycle liquefies and supercools the hydrocarbon-rich stream;and wherein no additional process steps are involved in a heat exchange between the hydrocarbon-rich stream and the first refrigerant mixture cycle in the precooling step.

27. The method according to claim 26, wherein the liquefaction of the hydrocarbon-rich stream countercurrent to the refrigerant mixture cycle cascade includes a third refrigerant mixture cycle; wherein the second refrigerant mixture cycle liquefies the hydrocarbon-rich stream and wherein the third refrigerant mixture cycle supercools the liquefied hydrocarbon-rich stream.