Patent Application
20100293997
The present invention relates to a method and apparatus for cooling a hydrocarbon stream. In another aspect, the present invention relates to a method of liquefying a hydrocarbon stream.
A common example of a hydrocarbon stream to be cooled and/or liquefied is 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.
Natural gas can be liquefied using a compressed refrigerant. U.S. Pat. No. 6,962,060 describes a compressor system for a multistage refrigeration of LNG comprising first and second refrigerant compressors, each with first and second stages, and piping means to combine the discharge from the second stages of the first and second compressors to provide a compressed gas. However, the need to combine the highest pressure discharges from the two second stages of the first and second compressors requires the two discharges to be at the exact same pressure, flow etc., to prevent any back pressure and possible surge. This has the disadvantage of limiting the variability allowed between the two discharges to avoid any combining problems. Refrigerant compressors commonly run at different discharge pressures, for example so as to temporarily avoid surge in one compressor, as is known in the art, but the high pressure of the two discharges makes it extremely difficult to accommodate different discharge pressures and maintain downstream flow, without any disturbance also leading to further complications upstream and/or downstream of the compressors.
Moreover, U.S. Pat. No. 6,962,060 only shows one arrangement of first and second compressors having first and second stages. It does not have any flexibility to provide a compression system for other refrigeration arrangements.
The present invention provides a method of cooling a hydrocarbon stream such as natural gas comprising at least the steps of:
(a) providing a refrigerant stream at a refrigerant pressure;
(b) passing the refrigerant stream through at least three heat exchange steps operating at different pressure levels;
(c) passing a hydrocarbon stream through at least two of the heat exchange steps of step (b) thereby progressively lowering the temperature of the hydrocarbon stream to provide a cooled hydrocarbon stream;
(d) expanding and evaporating a fraction of the refrigerant stream at each heat exchange step to a different pressure, to provide a first evaporated refrigerant stream at a first evaporation pressure, and at least two other evaporated refrigerant streams at evaporation pressures lower than the first evaporation pressure;
(e) compressing the first evaporated refrigerant stream through a highest-pressure compressor stage of a single compressor casing to the refrigerant pressure to provide at least a fraction of the refrigerant stream at the refrigerant pressure of step (a);
(f) compressing the other evaporated refrigerant streams through at least two parallel lower pressure compressor stages to provide two or more part-compressed refrigerant streams; and
(g) passing all of the part-compressed refrigerant streams through said highest pressure compressor stage of step (e).
The present invention further provides a method of liquefying a hydrocarbon stream, to provide a liquefied hydrocarbon stream, comprising cooling of the hydrocarbon stream in accordance with the method as defined above or using the apparatus as defined below.
The present invention further provides an apparatus for cooling a hydrocarbon stream such as natural gas at least comprising:
a refrigerant stream at a refrigerant pressure;
at least three heat exchangers comprising pressure reduction means for operating the heat exchange steps at different pressure levels
refrigerant passage means for passing the refrigerant stream passes through the at least three heat exchangers;
hydrocarbon passage means for passing a hydrocarbon stream through at least two of the heat exchangers steps to provide a cooled hydrocarbon stream;
a first evaporated refrigerant stream at a first evaporation pressure,
at least two other evaporated streams at evaporation pressures lower than the first evaporation pressure;
a highest-pressure compressor stage in a single compressor casing for compressing the first evaporated refrigerant stream to provide at least a fraction of the refrigerant stream at the refrigerant pressure;
at least two parallel lower pressure compressor stages for compressing the other evaporated refrigerant streams to provide one or more part-compressed refrigerant streams; and
a pathway to pass all of the part-compressed refrigerant streams through the highest pressure compressor stage in said single compressor casing.
Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:
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. Same reference numbers refer to similar components.
Embodiments of the present invention provide an improved method of cooling a hydrocarbon stream such as natural gas, which has greater flexibility in its refrigerant compressor arrangements.
It is presently proposed to provide a method of cooling a hydrocarbon stream using a single compressor discharge of refrigerant at the refrigerant pressure from the highest-pressure compressor stage of the refrigerant compressor(s). In other words, the highest-pressure compressor stage is located in a single compressor casing (possibly together with one or more lower compressor stages of the same compressor train).
One advantage of this is that there is no requirement for any balancing of discharge pressure streams from different compressors at the refrigerant pressure. This has the further advantage of increasing the variability allowed in discharges from lower pressure compression stages or steps. There can therefore be wider compressor variability, such as may be required to avoid surge, at the lower pressures of such discharges.
Another advantage of the present invention is that there is little or no additional CAPEX or OPEX required to create any new stream flow arrangement required by the present invention, such as the embodiment shown in the accompanying
With simple re-arrangements of the interconnections between the evaporated and part-compressed streams and the compressor stages, much greater flexibility can be achieved in matching the overall cooling duty curve provided by the refrigerant stream through the heat exchange steps against the compressor duty requirement.
The hydrocarbon stream may be any suitable gas stream to be cooled, but is usually a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the natural gas stream may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.
Usually the natural gas stream is comprised substantially of methane. Preferably the hydrocarbon feed stream comprises at least 50 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 composition varies depending upon the type and location of the gas. Hydrocarbons heavier than methane generally need to be removed from natural gas for several reasons, such as having different freezing or liquefaction temperatures that may cause them to block parts of a methane liquefaction plant. C2-4 hydrocarbons can be used as a source of natural gas liquids.
The natural gas stream may also contain non-hydrocarbons such as H2O, N2, CO2, Hg, H2S and other sulphur compounds.
If desired, the hydrocarbon stream containing the natural gas may be pre-treated before use either as part of a hydrocarbon cooling process, or separately. This pre-treatment may comprise reduction and/or removal of non-hydrocarbons such as CO2 and H2S or other steps such as early cooling, pre-pressurizing. As these steps are well known to the person skilled in the art, their mechanisms are not further discussed here.
Preferably, a hydrocarbon stream to be used in the present invention undergoes at least the minimum pre-treatment required to subsequently liquefy the hydrocarbon stream. Such a requirement for liquefying natural gas is known in the art.
A refrigerant stream used in the present invention may be formed from a single component such as propane or nitrogen, or may be a mixed refrigerant formed from a mixture of two or more components selected from the group comprising: nitrogen, methane, ethane, ethylene, propane, propylene, butanes, pentanes, etc.
The present invention may comprise, or may be part of, a multistage cooling process involving two or more cooling stages, each stage having one or more steps, parts etc. For example, each cooling stage may comprise one to five heat exchange steps. A first cooling stage could reduce the temperature of a hydrocarbon stream to below 0° C., usually in the range −20° C. to −160° C., more usually −20° C. and −70° C. Such a first cooling stage is sometimes also termed a ‘pre-cooling’ stage.
Any second cooling stage is typically separate from a first cooling stage. That is, the second cooling stage comprises one or more separate heat exchange steps using a second refrigerant circulating in a second refrigerant circuit, although the refrigerant of the second refrigerant stream may also pass through one or more heat exchange steps of the first cooling stage. Such a second cooling stage is sometimes also termed a ‘main cooling’ stage.
Referring to the drawings,
Typically, each heat exchange step 12, 14, 16, 18 comprises a single heat exchanger such as a kettle or plate-and-fin heat exchanger known in the art. Arrangements of series of kettles or plate-and-fin heat exchangers are known in the art.
Typically, each heat exchange step 12, 14, 16, 18 progressively lowers the temperature of the hydrocarbon stream 20, to provide therefrom a cooled hydrocarbon stream 30. For example, the heat exchange steps, 12, 14, 16, 18 shown in
One example of the above arrangement is a first or pre-cooling stage in a hydrocarbon cooling process, which may be one stage in a hydrocarbon liquefaction plant such as an LNG plant.
The four heat exchange steps 12, 14, 16, 18 operate at different pressure levels, and in each of the heat exchange steps 12, 14, 16, 18, a fraction of the refrigerant stream 10 is expanded and evaporated. The expanding usually is accomplished by passing the refrigerant stream though pressure reduction means (not shown) in the form of an expander, a valve, or the like. Usually, the remaining fraction of the refrigerant stream passes to the next heat exchange step, until the final remaining fraction passing into the fourth heat exchange step 18 is fully expanded and evaporated. As each heat exchange step 12, 14, 16, 18 is operating at a different pressure, each heat exchange step 12, 14, 16, 18 will provide an evaporated refrigerant stream therefrom at a different evaporation pressure.
Typically, the first heat exchange step 12 provides a first evaporated refrigerant stream 40 at a first evaporation pressure, typically at the ‘highest’ pressure closest to the refrigerant pressure of the refrigerant stream 10. The highest evaporation pressure may also be termed ‘a high-high-pressure’, and the first evaporated refrigerant stream 40 may also be termed a ‘high-high-pressure evaporated refrigerant stream’ 40.
The second heat exchange step 14 receives a first remaining portion 10a of the original refrigerant stream 10, and expands and evaporates a fraction of the first remaining portion 10a to provide a second or other refrigerant stream 50 at a pressure lower than the first evaporation pressure of the first evaporated refrigerant stream 40. The second evaporated stream 50 may also be termed a ‘high-pressure evaporated refrigerant stream’.
A second remaining portion 10b of the first remaining portion 10a not expanded or evaporated in the second heat exchange step 14 passes to the third heat exchange step 16, wherein another fraction of the refrigerant stream 10 is expanded and evaporated to provide a third or other evaporated refrigerant stream 60 at an evaporation pressure lower than the first evaporation pressure. The third or other evaporated refrigerant stream 60 may also be termed an ‘intermediate-pressure evaporated refrigerant stream’.
A third remaining portion 10c of the refrigerant stream 10 passes to the fourth heat exchange step 18, where it is expanded and evaporated to provide a fourth or other evaporated refrigerant stream 70 at an evaporation pressure lower than the first evaporation pressure. The fourth evaporated stream 70 may also be termed a ‘low-pressure evaporated refrigerant stream’ 70.
The process, set-up and arrangement of heat exchange steps 12, 14, 16 and 18, and the multi-stage expansion and evaporation of the refrigerant stream 10, are known in the art, examples of which are shown in U.S. Pat. Nos. 6,962,060 and 6,637,238.
The expansions and evaporations of part or all of the remaining fractions 10a, 10b and 10c of the refrigerant stream 10 in each of the second, third and fourth heat exchange steps 14, 16, 18 allow further or lower-temperature cooling of the hydrocarbon stream 20 as it passes therethrough. However, each of the evaporated refrigerant streams 40, 50, 60 and 70 must then be re-compressed back to the refrigerant pressure of the original refrigerant stream 10.
As shown in
As shown in
The arrangement of two or more compressors, compression stages or a combination of same, to form a train is well known in the art. Typically, a compression train comprises one compression stage in a single casing, or two, three or four conjoined compression stages in a single casing. In the latter cases, the compressed refrigerant from the lowest compressor stage passes through each subsequently higher compression stage, usually in combination with one or more higher pressure feeds from one or more intermediate inlets.
The part-compressed refrigerant stream 60a from the third compressor stage 26 then passes directly into the highest-pressure compressor stage 22. The highest-pressure compressor stage 22 is comprised in a single casing, either alone or, for instance, together with the other, lower pressure, stage of train A.
The second or high evaporated refrigerant stream 50 passes into a second or high pressure compressor stage 24. The fourth or low evaporated refrigerant stream 70 passes into a fourth or low-pressure compression stage 28, the part-compressed refrigerant stream 70a from which passes directly into the second or high-pressure compressor stage 24 along with the second evaporated refrigerant stream 60.
The second and fourth compressor stages 24, 28 comprise a second compression train B, separate from the first compression train A. Thus, at least two of the lower pressure compressor stages 24, 26, 28 are in at least two compressor trains A and B.
The second and fourth compressor stages 24, 28 are also parallel with the third compressor stage 26, such that there are at least two parallel lower pressure compressor stages shown in the general scheme 1 of
The discharge from the second compressor stage 24 is still a part-compressed refrigerant stream 50a, which passes through an outlet 24a of the second compressor train B. As shown in
In this way, all of the part-compressed refrigerant streams 50a, 60a and 70a commonly pass through the highest pressure compressor stage 22 of a single casing.
As a first alternative, the hydrocarbon stream may not pass through all the same heat exchange steps 12, 14, 16, 18 as the refrigerant stream 10. Stream 20a in
The first evaporated stream 40 passes into a highest-pressure compressor stage 22 to be compressed and provide a fraction of the original refrigerant stream 10 at the refrigerant pressure.
In the second general scheme 2 shown in
The fourth evaporated stream 70 passes into the fourth compressor stage 28, and the part-compressed refrigerant stream 70a therefrom passes directly into the second compressor stage 24 along with the second evaporated stream 50. The part-compressed refrigerant stream 50a from the second compressor stage 24 then passes directly into the highest-pressure stage 22.
Meanwhile, the third evaporated refrigerant stream 60 passes into a parallel third compressor stage 26, which comprises a fourth compressor train D. The fourth compressor train D thus comprises only one compressor stage 26, which is separate from the compressor stages 22, 24, 28 in the third compressor train C.
The discharge of a part-compressed refrigerant stream 60a from the third compressor stage 26 passes through an outlet 26a of the second compressor train D to combine with the first evaporation stream 40 by a combiner 36, and so also pass through the highest-pressure stage 22.
Table 1 lists a number of examples of arrangements for four compressor stages to recompress evaporated refrigerant streams from four different heat exchange steps according to the present invention, using the nomenclature of the compressor stages used in
Table 1 confirms how the discharge for each train not compressing a refrigerant stream up to the highest compressor, is transferred either straight to the highest pressure stage 22, or into an earlier compressor stage in the same train as the highest pressure stage 22, through which all such refrigerant will then pass.
Examples 1 and 2 in Table 1 are shown in accompanying
Examples 4 and 5 in Table 1 illustrate that a part-compressed refrigerant stream discharged from either compressor stage 26 as compressor train H, or compressor stage 28 as compressor stage J, can be first passed into the second compressor stage 24 rather than directly into the highest pressure compressor stage 22. As the second compressor stage 24 is part of the same compressor train G, I as the highest-pressure compressor stage 22, the part-compressed refrigerant stream will still commonly pass through the highest-pressure compressor stage 22.
Table 1 also provides Example 8 comprising three compressor trains O, P and Q, of which compressor trains P and Q comprise only one compressor stage 24, 26 respectively. However, the part-compressed discharge from each of the lower pressure compressor stages 24 and 26 can be commonly passed through the highest-compressor stage 22.
Table 1 relates to examples involving four compressor stages. The present invention is not limited thereto, and arrangements involving 3 or 5+ compressor stages are also within the scope of the present invention. The skilled man can see different combinations of compressor stages in different compressor trains, wherein the discharge from one or more lower pressure compressor stages can pass into the inlet or inlets of one or more higher-pressure compressor stages, as long as all the part-compressed streams commonly pass through the highest pressure compressor stage.
In summary, disclosed are methods and compressor line-ups for compressing three or more evaporated refrigerant streams through three or more refrigerant compressor stages, wherein:
(i) a first evaporated refrigerant stream (40) is compressed through a common highest-pressure compressor stage (22) to provide at least a fraction of a fully-compressed refrigerant stream (10) at a refrigerant cooling pressure;
(ii) the other evaporated refrigerant streams (50, 60, 70) are compressed through at least two parallel lower pressure compressor stages (24, 26, 28) to provide one or more part-compressed refrigerant streams (50a, 60a, 70a); and
(iii) all of the part-compressed refrigerant streams (50a, 60a, 70a) are commonly passed through the common highest pressure compressor stage (22).
Different arrangements or line-ups of the compressor stages provide different profiles for the re-compression of the refrigerant stream. In this way, the present invention provides flexibility for better matching the compression required of the evaporated refrigerant streams either with the attendant compressor power requirement, or with the cooling duty requirement, or a combination of same, so as to make the arrangement or set-up more efficient.
Persons skilled in the art will readily understand that the present invention may be modified in many ways without departing from the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07122288.9 | Dec 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP08/66623 | 12/2/2008 | WO | 00 | 7/27/2010 |
