Patent Application
20090282862
The present invention relates to a method and apparatus for producing a cooled liquefied hydrocarbon stream, such as natural gas. The method and/or apparatus may be used in and/or for a process for liquefying a hydrocarbon stream e.g. for the production of liquefied natural gas.
Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at a high pressure.
U.S. Pat. No. 6,658,892 B2 relates to processes and systems for liquefying natural gas wherein a common separator (e.g. flash tank) and vapour compressor are used by multiple trains within the system. A problem of the arrangement in U.S. Pat. No. 6,658,892 is that each train still requires its own dedicated equipment and running costs until the common separator. All equipment associated with liquefying natural gas is expensive, both in terms of capital and running costs.
It is an object of the present invention to reduce the capital and/or running costs of a liquefaction plant involving liquefying apparatus.
It is a further object to provide an alternative method and apparatus for liquefying natural gas.
One or more of the above or other objects can be achieved by the present invention. The invention provides a method of producing a cooled liquefied hydrocarbon stream, the method at least comprising the steps of:
providing a first liquefied hydrocarbon stream by passing a hydrocarbon first feed stream through a first liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit;
providing a second liquefied hydrocarbon stream by passing a hydrocarbon second feed stream through a second liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit;
combining the first liquefied stream with the second liquefied stream to provide a combined liquefied stream; and
further cooling the combined liquefied stream against a refrigerant to provide a cooled liquefied hydrocarbon stream, such as liquefied natural gas (LNG).
The present invention also provides an apparatus for the production of a cooled liquefied hydrocarbon stream such as natural gas from two or more liquefied hydrocarbon streams, the apparatus at least comprising:
a first liquefying system to provide a first liquefied hydrocarbon stream comprising at least two cooling stages, at least one of which has a closed refrigerant circuit;
a second liquefying system to provide a second liquefied hydrocarbon stream comprising at least two cooling stages, at least one of which has a closed refrigerant circuit;
a combiner to combine the first liquefied stream and the second liquefied stream to provide a combined liquefied stream; and
a further cooling stage arranged to cool the combined liquefied stream against a refrigerant to provide a liquefied hydrocarbon product stream.
Embodiments and examples of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings, in which:
Although the method and apparatus described herein are applicable to various hydrocarbon-containing streams, it is particularly suitable for natural gas streams to be further cooled after liquefaction.
In particular, the methods described herein comprise
(a) providing a first liquefied hydrocarbon stream;
(b) providing a second hydrocarbon stream;
(c) combining the first liquefied hydrocarbon stream with the second liquefied hydrocarbon stream thereby providing a combined liquefied hydrocarbon stream; and
(d) further cooling the combined liquefied stream against a refrigerant thereby providing a further cooled liquefied hydrocarbon stream.
It has surprisingly been found that by combining the two or more liquefied streams and further cooling in a single final cooling stage, capital and running costs can be reduced.
The methods described herein may be part of a method for the production of a cooled liquefied hydrocarbon stream such as liquefied natural gas from two or more hydrocarbon streams, such streams being from one feed stream or source, or from a plurality of feed streams or sources.
The person skilled in the art readily understands how to cool and liquefy a hydrocarbon stream. Generally a feed stream is provided, and passed through a liquefying system.
In various embodiments described herein, the first liquefied hydrocarbon stream may be provided by passing a first hydrocarbon feed stream through a first liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit. Likewise, the second liquefied hydrocarbon stream may be provided by passing a second hydrocarbon feed stream through a second liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit.
In these or other embodiments described herein, the first liquefied hydrocarbon stream may be generated in a first liquefying system, and the second liquefying hydrocarbon stream in a second system liquefying system. Each liquefying system may have at least two cooling stages. The first and second liquefying systems may have a common first cooling stage and at least one separate second cooling stage each.
In these or other embodiments, the refrigerant used in the further cooling of step (d) may be a single component refrigerant.
Apparatuses described herein comprise:
a first liquefying system to provide a first liquefied hydrocarbon stream;
a second liquefying system to provide a second liquefied hydrocarbon stream;
a combiner to combine the first liquefied stream and the second liquefied stream to provide a combined liquefied stream; and
a further cooling stage a refrigerant for the combined liquefied stream to provide a cooled liquefied product stream.
In various embodiments described herein, the first and second liquefying systems each comprise at least two cooling stages. At least one of the at least two cooling stages in each of the first and second liquefying systems may have a closed refrigerant circuit. Each liquefying system may comprise a first cooling stage and a second cooling stage arranged downstream of the first cooling stage. The first cooling stage may be a pre-cooling stage and the second cooling stage may be a main cryogenic cooling stage. The first and second liquefying systems may have a common first cooling stage, which may be a common pre-cooling stage, which may have 1, 2, 3, 4, or 5, preferably 4, heat exchangers. The refrigerant for the second cooling stage may be a mixed refrigerant. The refrigerant in the further cooling stage may be a single component refrigerant, e.g. nitrogen, a mixed refrigerant, or a natural gas. The refrigerant of the further cooling stage may be in a closed refrigerant circuit.
The hydrocarbon feed stream for the method and/or apparatus, or the hydrocarbon feed stream streams for the liquefying systems, may be any suitable hydrocarbon-containing stream or streams, generally termed ‘feed streams’, to be treated, but they are usually natural gas streams obtained from natural gas or petroleum reservoirs. As an alternative the natural gas streams may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.
Usually the natural gas stream(s) are comprised substantially of methane. Preferably a feed stream for the method and apparatus described herein comprises at least 60 mol % methane, more preferably at least 80 mol % methane.
Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The natural gas streams may also contain non-hydrocarbons such as H2O, N2, CO2, H2S and other sulphur compounds, and the like.
If desired, a feed stream containing the natural gas may be pre-treated before passing it to a liquefying system. This pre-treatment may comprise removal of any undesired components present, such as CO2 and H2S, or other steps such as pre-cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, they are not further discussed here.
An advantage of this arrangement is using only one further cooling for two liquefied streams, which streams or liquefying systems may or may not be the same.
Optionally, the first cooling stage of each liquefying system may be combined or be “common”, providing the further advantage of further reduction of capital and running costs.
The term “natural gas” as used herein relates to any hydrocarbon-containing composition that is at least substantially methane. This includes a composition prior to any treatment, such treatment including cleaning or scrubbing, as well as any composition having been partly, substantially or wholly treated for the reduction and/or removal of one or more compounds or substances, including but not limited to sulfur, carbon dioxide, water, and C2+ hydrocarbons.
Two or more feed streams used in the method and apparatus described herein could be the same or different. Each feed stream could be derived from the same feed stream source, such as the same natural gas well. Each feed stream could be provided by division from the same source.
Any pre-treatment of a feed stream may be the same or different. Preferably, each feed stream is wholly or substantially, i.e. >90%, more preferably >95%, and even more preferably >99%, the same, in terms of its parameters and constituents.
A liquefying system may be embodied in various ways, and generally involves one or more heat exchangers and refrigerant circuits.
A liquefying system useable with the method and apparatus described herein may involve one or more cooling stages, and each cooling stage may involve one or more heat exchangers, steps, levels or sections. One arrangement involves the first stage being a pre-cooling stage, and the second cooling stage being a main cryogenic stage.
A pre-cooling stage may involve reducing the temperature of a feed stream to below −0° C., for example in the range −10° C. to −30° C.
A main cryogenic cooling stage may involve cooling a feed stream to below −90° C. or below −100° C., for example between −100° C. to −130° C., which usually creates a hydrocarbon stream which is now liquefied, such as liquefied natural gas.
Each cooling stage generally involves one or more refrigerant circuits, usually one refrigerant circuit per dedicated heat exchanger or sets of heat exchangers, which has at least one compressor for compressing the refrigerant after passing it against the stream to be cooled or liquefied. Each refrigerant circuit may also involve one or more heat exchangers, such as air and/or water coolers or other condensers, to help cool the refrigerant by heat exchange with a coolant such as water.
Refrigerant circuits are known in the art. Whilst each refrigerant circuit can be separate, one or more parts of a refrigerant circuit can be connected or interconnected with another refrigerant circuit(s), or at least involve an interconnection of actions or combination of materials and/or flow with other circuit(s).
However, at least one of the cooling stages may have a closed refrigerant circuit, such that the refrigerant is not mixed with refrigerant from a different stage, refrigerant circuit or liquefying system. Each closed refrigerant circuit has a dedicated compressor, which is not shared with other refrigerant circuits and/or cooling stages. Pre-coolers and coolers can however be shared between a closed refrigerant circuit and another refrigerant circuit. In addition, the refrigerant in the closed refrigerant circuit is typically not, or at least not during normal operation, commingled with another refrigerant.
In one embodiment of the method and apparatus described herein, the first and second liquefied hydrocarbon streams are provided by first and second, preferably parallel, liquefaction systems respectively, each system using a mixed refrigerant as herein defined.
The method and apparatus described herein may involve more than two liquefied hydrocarbon streams, and/or more than two feed streams, and/or more than two liquefying systems. Such multiple streams may also involve a combined further cooling stage as described hereinafter, optionally for some or all of such multiple streams. The use of a common or combined further cooling stage provides the advantage of reduced capital and running costs, especially where the cooling requirement of a further cooling stage is less than, possibly relatively small in comparison with, the cooling requirement of other cooling stages, such that previously separate further cooling stages can be combined without any significant extra energy requirement.
Each liquefying system may use the same or different liquefying parameters. Each stage and/or any similar stages of each liquefying system may use the same or different parameters, such as flowrate, temperature, pressure, etc. Each liquefying system, and/or each stage of each liquefying system, may involve recycle of one or more streams or products, as is well known in the art.
Preferably, each liquefying system comprises at least two cooling stages, preferably a first cooling stage and a second cooling stage, more preferably a first cooling stage being a pre-cooling stage and a second cooling stage being a main cryogenic cooling stage.
The first and second liquefying systems could have a common first cooling stage, preferably a common pre-cooling stage having 1, 2, 3, 4 or 5 heat exchangers, more preferably having 4 heat exchangers.
The second cooling stage may have a closed refrigerant circuit. Preferably, the second cooling stages of the first and second liquefying systems are separate closed refrigerant circuits.
The refrigerant of the further cooling stage is preferably a dedicated refrigerant, and is in a closed refrigerant circuit.
The present invention includes a combination of any and all of the methods and apparatuses herein described.
For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. The same reference numbers refer to similar components.
Referring to the drawings,
To provide two liquefied hydrocarbon streams,
The first feed stream 10 passes through a first liquefying system 100 comprising two cooling stages, in this example being a first cooling stage 12 to provide a cooled stream 20, and a second cooling stage 14 to provide a first liquefied stream 30.
The second feed stream 10a passes through a second liquefying system 200, being in this example a first cooling stage 12a to provide a first cooled stream 20a, and a second cooling stage 14a to provide a second liquefied stream 30a.
The first and second liquefying systems 100, 200 can be different or the same, i.e. have the same or different volumes, flowrates, process conditions etc. The first and second cooling stages 12a, 14a of the second liquefying system 200 may be the same or different to the first and second cooling stages 12, 14 of the first liquefying system 100. Each of the first and second cooling stages 12, 12a, 14, 14a of each liquefying system 100, 200 may also be the same or different to each other.
Preferably, a first cooling stage of each liquefying system provides different cooling, i.e. different temperature reduction, to a stream passing therethrough, compared with a second cooling stage.
Preferably, the cooling for the first cooling stage of a liquefying system is provided by a first refrigerant circuit or circuits (not shown in
Preferably, the cooling for the second cooling stage of a liquefying system is provided by a second refrigerant circuit or circuits (also not shown in
At least one of the first and second cooling stages of the first liquefaction system and at least one of the first and second cooling stages of the second liquefaction system has a closed refrigerant circuit.
In
Alternatively, the first liquefied stream 30 and the second liquefied stream 30a are combined at or in the further cooling, including any apparatus, device, unit or part thereof or therefor, which provides or helps to provide further cooling. The combining of the streams 30, 30a may not require full integration or mixing for their passage through the further cooling.
According to the method described herein, the combined liquefied stream 40 undergoes further cooling to provide a cooled liquefied hydrocarbon stream. The further cooling may be similar or different in concept, design, arrangement or equipment to the first and second cooling stages 12, 12a, 14, 14a of the first and second liquefying systems 100, 200, and may involve the same, similar or different process conditions as the first and second cooling stages 12, 12a, 14, 14a of the first and second liquefying systems 100, 200.
In one example, the further cooling stage 18 is a sub-cooling stage, adapted to reduce the temperature of the combined liquefied stream 40 to a temperature between −150° C. to −160° C., to provide a cooled liquefied hydrocarbon stream 50.
The further cooling stage 18 may also involve one or more steps, levels or sections. The cooling for the further cooling stage 18 can be provided by at least one (third) refrigerant, which refrigerant(s) is preferably circulating in a refrigerant circuit (not shown in
Any refrigerant circuit for the further cooling stage 18 may be a ‘stand alone’ circuit, or may partly or wholly pass through one or more parts or units of the first and/or second liquefying systems 100, 200. Alternatively or additionally, at least some cooling of the refrigerant for the further cooling stage may be indirectly provided by a part or unit of the first and/or second liquefying systems 100, 200. Many such systems or arrangements for cooling a refrigerant are known in the art.
Optionally, the cooled liquefied hydrocarbon stream 50 can be passed into a final separator wherein vapour can be removed for use as a fuel in the plant, for example for the gas turbines running compressors used in the refrigeration circuits, and a liquefied hydrocarbon product, such as a liquefied natural gas, which can be transferred to a storage vessel or other storage or transportation apparatus.
As an example, the final separator can be an end flash separator 22 as shown in
Thus,
In
The arrangement shown in
The two liquefied hydrocarbon streams 30b, 30c in
The arrangement of first and second cooling stages in
In the first cooling stage 12b, the four heat exchangers 32a,b,c,d can operate at different pressures, achieved by expansion valves 31a,b,c,d, especially when using a single component refrigerant such as propane. Propane can be used at different pressure levels, and after vapourisation in each heat exchanger, it can pass into two compressors 34a, 34b, powered by driver D, which help recompress the refrigerant vapour as part of a first refrigerant circuit 101 prior to its condensation and reuse through the four heat exchangers 32a,b,c and d. The recompressed refrigerant vapour may be passed through cooler 44 to provide prior cooling before being passed to heat exchangers 32a,b,c,d. The use of the four heat exchangers, and the operation of the first refrigerant circuit 101 for the first cooling stage 12b, is known to the person skilled in the art.
Similarly, heat exchangers 32b, 32c and 32d could each be split into three separate heat exchangers respectively, to provide twelve heat exchangers in total for the first combined cooling stage of this alternative embodiment. Each set of three heat exchangers corresponding to heat exchangers 32a, 32b, 32c and 32d could operate at a different refrigerant pressure, in a similar manner to the scheme shown in
In each second cooling stage 14b, 14c in
The second refrigerant circuits 201, 202 can be different, but preferably are the same or similar, and generally involve the passage of a second refrigerant, which second refrigerant may be the same or different for each second refrigerant circuit 201, 202. Preferably each second refrigerant is the same, and is a mixed refrigerant, preferably a mixed refrigerant of two or more components, more preferably two or more components selected from the group comprising nitrogen, methane, ethane, ethylene, propane, propylene, butane and pentane.
For the example shown in
In the embodiment shown in
The first and second liquefied hydrocarbon streams 30b, 30c are then combined by a combiner 16 as described above to provide a combined liquefied stream 40, which can then undergo further cooling by a further cooling or sub-cooling stage 18, shown in
In one example, the third refrigerant can be nitrogen, whose use in a sub-cooling stage 18 is known in the art. Generally, the nitrogen refrigerant can further cool the combined liquefied hydrocarbon stream 40 to provide a further cooled liquefied hydrocarbon stream 50, having a temperature below at least −140° C., preferably below −150° C.
Thus,
In
The first common pre-cooling stage 12c shown in the scheme of
Each second cooling stage 14d, 14e has its own separate second refrigerant circuit which may be the same or different, but which circuits are preferably similar in arrangement and use of refrigerant. By way of example, the second refrigerant circuit 201 for the upper second cooling stage 14d shown in
Suitable components, streams, flows and temperatures for the second refrigerant circuit 201 are all well known in the art, especially where the second refrigerant for the second refrigerant circuit 201 is a mixed refrigerant as described above.
In
The end flash system shown in U.S. Pat. No. 5,611,216, which is incorporated herein by way of reference, is another example. As shown in U.S. Pat. No. 5,611,216, a liquefied hydrocarbon stream can be passed through a methane economizer and then further cooled by a series of expansions, where each expansion uses either Joule-Thomson expansion valves, or hydraulic expanders, followed by separation of the gas-liquid product with a separator. Additional cooling may be effected by flashing at least a portion of the liquefied hydrocarbon stream via one or more expanders, and/or a heat exchanger employing the flashed vapours from each flash or separation involved.
In the
The tertiary heat exchangers may comprise one or more heat exchangers in series or parallel, and different arrangements are known and are possible for the heat exchange of the further cooling stage in the examples and embodiments described herein.
From the second tertiary heat exchanger 76, the further cooled liquid stream passes via an expansion valve 77 into a second separator, being a second flash drum 78, where the stream is separated into a natural gas vapour phase passing through line 52, and a liquid phase which passes through a further expansion valve 82 and into a third separator 79, where the stream is again separated into a natural gas vapour phase 53, and a final product hydrocarbon stream 50, which can pass through a further expansion valve 83 prior to storage (in tank 81) and/or transportation.
Any vapour, such as boil-off gas, from the tank 81 can be compressed in compressor 56 to provide a compressed stream 54, which can be combined with the vapour stream 53 from the third separator 79, to provide a combined vapour stream 55. The combined stream 55 and the vapour stream 52 from the second flash drum 78 pass through the second tertiary heat exchanger 76 to provide the cooling to the liquefied hydrocarbon stream 75 passing countercurrently therethrough. Both the vapour streams from the second tertiary heat exchanger 76, and the vapour stream 51 from the first flash drum 74, then provide the cooling in the first tertiary heat exchanger 71.
The arrangement for the further cooling shown in
The three exit streams of refrigerating vapour 61, 62, 63 from the first tertiary heat exchanger 71 are then sent to separate inlets of one or more compressors (three compressors 92, 94, 96 are shown in
A fraction of the combined refrigerant stream 64 may be removed or divided as stream 64a for use as a fuel, usually a high pressure fuel, for example in the liquefying plant or elsewhere.
The combined refrigerant stream 64 or remainder fraction is further compressed in compressor 98 and cooled in cooler 99 to provide a third refrigerant stream 65. The third refrigerant stream 65 is additionally cooled in the first heat exchanger 32a of the pre-cooling stage 12c, to provide a cooled refrigerant stream 66, which passes into the first tertiary heat exchanger 71. A part of the cooled third refrigerant stream 66, after part-passage through the first tertiary heat exchanger 71, can outflow from the first tertiary heat exchanger 71 as a supply stream 70, for combination with the cooled feed stream 10c (provided by the pre-cooling stage 12c), to provide the combined cooled feed stream 10d.
The arrangement shown in
Thus,
Table 1 gives a representative working example of temperatures, pressures and flows of streams at various parts in an example process described herein referring to
Table 2 gives an overview of the separate and overall power requirements of an example process described herein referring to
The prior art example refers to the process shown in U.S. Pat. No. 5,611,216, which uses propane (C3), ethylene (C2) and methane (C1), as the first, second and third refrigerants therein. The example of
The results show that by increasing the loading of the second cooling stage described herein, the reductions in power for the pre-cooling and further cooling stages are so significant as to outweigh the increase in second cooling stage loading. The overall specific power requirements for the example of
Further, there is an increase in LNG production by the process described herein.
In each of the examples described above or shown herein, the liquefying system to provide the liquefied hydrocarbon stream is shown as having first and second cooling stages. Other liquefying systems are known in the art which may involve more or less cooling stages, or a liquefying stage wherein the feed stream therefor is pre-treated, e.g. pre-cooled, elsewhere, for example by one or more heat exchangers in a separate part of a liquefaction plant using a cooling line or stream such as reject gas or fuel gas. Thus, the term ‘liquefying system’ as used herein is not limited to a system having two distinct cooling stages.
The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06121089.4 | Sep 2006 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2007/059959 | 9/20/2007 | WO | 00 | 3/20/2009 |
