METHOD AND APPARATUS FOR COOLING AND SEPARATING A HYDROCARBON STREAM

Abstract

A method and apparatus for cooling and separating a hydrocarbon stream such as natural gas. The method comprises providing a hydrocarbon stream, cooling the hydrocarbon stream (10) to provide a partially liquefied hydrocarbon stream, passing at least a fraction of the partially liquefied hydrocarbon stream into a distillation column through a first inlet, providing a lower feed stream, providing a refrigerant circuit comprising at least a refrigerant stream, heating the lower feed stream against the refrigerant stream to provide a heated lower feed stream and a cooled refrigerant stream, passing the heated lower feed stream into the distillation column through a second inlet lower than the first inlet, and separating the partially liquefied hydrocarbon stream in the distillation column to provide at least a first overhead gaseous stream and a first bottom liquid stream.

Description

The present invention relates to a method and apparatus for cooling and separating a hydrocarbon stream, particularly but not exclusively natural gas, and optionally subsequently liquefying the hydrocarbon stream. In another aspect, the present invention relates to a liquefaction plant comprising such an apparatus.

Several methods of cooling and 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, as it occupies a smaller volume and does not need to be stored at high pressure.

Liquefying a hydrocarbon stream such as natural gas generally involves two or more cooling stages, and one or more separation stages. At least one separation stage is to reduce the level of hydrocarbons in a natural gas stream heavier than methane, hereinafter termed “heavier hydrocarbons”. Conventionally, heavier hydrocarbons are separated from a natural gas stream prior to any significant cooling, and particularly liquefaction, for several reasons, including having different freezing or liquefaction temperatures than methane that may therefore cause them to block parts of a methane liquefaction plant.

Separation of heavier hydrocarbons from natural gas can be carried out by cooling the natural gas to partly condense it, and then using a fractionation column.

U.S. Pat. No. 5,960,644 describes a method for removing carbon dioxide, ethane and heavier components from a high pressure natural gas stream, wherein the high pressure natural gas stream is partly condensed and passed through first and second separators, before various streams are passed into a fractionation column. In order to supply heat to the lower part of the fractionation column, a fluid stream is removed from the fractionation column and is heated in a heat exchanger 46 to obtain a re-boiling stream, and the re-boiling stream is introduced into the fractionation column.

U.S. Pat. No. 5,960,644 uses an external refrigerant to partly condense the high pressure natural gas stream, and a heating medium to provide heat to the lower part of the fractionation column. The external refrigerant used to partly condense the high pressure natural gas stream is not described, and the heating medium is a compressed gaseous product stream requiring separate compression to pipeline pressure.

It is an object of the present invention to reduce the power that is needed to provide a given cooling duty of a refrigerant around a refrigerant circuit.

The present invention provides a method of cooling and separating a hydrocarbon stream such as natural gas comprising at least the steps of:

(a) providing a hydrocarbon stream;
(b) cooling the hydrocarbon stream to provide a partially liquefied hydrocarbon stream, wherein at least a fraction of the cooling is provided by partially or fully evaporating a cooled refrigerant stream;
(c) passing at least a fraction of the partially liquefied hydrocarbon stream into a distillation column through a first inlet;
(d) providing a lower feed stream;
(e) providing a refrigerant circuit comprising at least a compressed fully liquefied refrigerant stream;
(f) heating the lower feed stream against the refrigerant stream to provide a heated lower feed stream and the cooled refrigerant stream;
(g) passing the heated lower feed stream into the distillation column through a second inlet lower than the first inlet; and
(h) separating the partially liquefied hydrocarbon stream in the distillation column to provide at least a first overhead gaseous stream and a first bottom liquid stream.

The present invention also provides apparatus for cooling and separating a hydrocarbon stream such as natural gas comprising at least:

a first cooling stage to cool a hydrocarbon stream and provide a partially liquefied hydrocarbon stream;

a distillation column to receive and separate at least a fraction of the partially liquefied hydrocarbon stream provided through a first inlet;

a refrigerant circuit comprising at least a compressed fully liquefied refrigerant stream; and

a first heat exchanger to heat a lower feed stream against the compressed fully liquefied refrigerant stream to provide a cooled refrigerant stream and a heated lower feed stream to be passed into the distillation column through a second inlet lower than the first inlet; and

a heat exchange system in the first cooling stage arranged in the refrigerant circuit downstream of the first heat exchanger to cool the hydrocarbon stream against at least a fraction of the cooled refrigerant stream, thereby partly or fully evaporating the at least the fraction of the cooled refrigerant stream.

The present invention also provides a liquefaction plant comprising apparatus as herein defined.

Embodiments and examples of the present invention will now be described by way of example only and with reference to the accompanying non-limited drawings in which:

FIG. 1 is a diagrammatic scheme for a first method of cooling and separating a hydrocarbon stream according to one embodiment of the present invention;

FIG. 2 is a diagrammatic scheme for a second method of cooling and separating a hydrocarbon stream as part of a liquefaction plant according to a second embodiment of the present invention;

FIG. 3 is a diagrammatic scheme for a third method of cooling and separating a hydrocarbon stream according to a third embodiment of the present invention;

FIG. 4 is a diagrammatic scheme for a fourth method of cooling and separating a hydrocarbon stream according to a fourth embodiment of the present invention; and

FIG. 5 is a diagrammatic scheme for a fifth method of cooling and separating a hydrocarbon stream according to a fifth embodiment of the present invention.

Embodiments of the present disclosure employ a refrigerant stream to heat a lower feed stream passing into a distillation column. The thus resulting cooled refrigerant stream is then advantageously employed to cool a hydrocarbon stream, thereby to partially condense or help to partially condense the hydrocarbon stream, before passing at least part of it to the distillation column.

As summarized in for instance FIG. 1, there is provided a method of cooling and separating a hydrocarbon stream such as natural gas comprising at least the steps of:

(a) providing a hydrocarbon stream 10;
(b) cooling the hydrocarbon stream 10 to provide a partially liquefied hydrocarbon stream 20;
(c) passing at least a fraction of the partially liquefied hydrocarbon stream 20 into a distillation column 12 through a first inlet 17;
(d) providing a lower feed stream 30;
(e) providing a refrigerant circuit 4 comprising at least a refrigerant stream 40;
(f) heating the lower feed stream 30 against the refrigerant stream 40 to provide a heated lower feed stream and a cooled refrigerant stream 60;
(g) passing the heated lower feed stream 50 into the distillation column 12 through a second inlet 18 that is located gravitationally lower than the first inlet 17; and
(h) separating the partially liquefied hydrocarbon stream 20 in the distillation column 12 to provide at least a first overhead gaseous stream 70 and a first bottom liquid stream 80.

In a preferred embodiment, the cooling in step (b) is carried out by partially or fully evaporating the cooled refrigerant stream 60, such as a sub-cooled liquefied refrigerant stream. In a further preferred embodiment, the refrigerant stream 40 in steps (e) and (f) is a compressed fully liquefied refrigerant stream, such that the cooled refrigerant stream 60 produced in step (f) is a sub-cooled liquefied refrigerant stream.

The use of a compressed fully liquefied refrigerant stream to heat the lower feed stream 30 sub-cools the liquefied refrigerant stream. Such a resulting sub-cooled liquefied refrigerant stream is advantageous because it produces less flash vapour when used for cooling, such as the cooling of the hydrocarbon stream.

Again with reference to FIG. 1, an apparatus for cooling and separating a hydrocarbon stream, such as natural gas, is also provided. The apparatus comprises at least:

a first cooling stage 14 to cool a hydrocarbon stream 10 and provide a partially liquefied hydrocarbon stream 20;

a distillation column 12 to receive and separate at least a fraction of the partially liquefied hydrocarbon stream 20 provided through a first inlet 17;

a refrigerant circuit 4 comprising at least a refrigerant stream 40; and

a first heat exchanger 16 to heat a lower feed stream 30 against the refrigerant stream 40 to provide a heated lower feed stream 50 to be passed into the distillation column 12 through a second inlet 18 located gravitationally lower than the first inlet 17.

In a preferred embodiment, the apparatus further comprises a heat exchange system 34 in the first cooling stage 14 arranged downstream of the first heat exchanger 16 to receive at least a fraction of the cooled refrigerant stream 60 and cool the hydrocarbon stream 10 against the at least the fraction of the cooled refrigerant stream 60, thereby partly or fully evaporating the refrigerant stream 60. The cooled refrigerant stream 60 is preferably a sub-cooled liquefied refrigerant stream. In a further preferred embodiment, the refrigerant stream 40 in the refrigerant circuit 4 is a compressed fully liquefied refrigerant stream.

One advantage of cooling the refrigerant stream through the heat exchange with the lower feed stream, is that the refrigerant stream then has more cooling duty to cool the hydrocarbon stream, and optionally other streams.

Another advantage of the present invention is that by cooling the refrigerant stream through the heat exchange with the lower feed stream, less compression power is required for the refrigerant circuit to provide the same cooling duty around its circuit, for example to cool the hydrocarbon stream.

Thus, the present disclosure can provide a method that requires less power to provide the duty for cooling and separating the same amount of hydrocarbon.

Described now in more detail, FIG. 1 shows a method of cooling and separating a hydrocarbon stream 10 according to one embodiment of the present invention.

The hydrocarbon stream 10 may be any suitable hydrocarbon stream such as, but not limited to, a hydrocarbon-containing gas stream able to be cooled. One example is a natural gas stream obtained from a natural gas or petroleum reservoir. 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 such a hydrocarbon stream is comprised substantially of methane. Preferably such a hydrocarbon stream comprises at least 50 mol % methane, more preferably at least 80 mol % methane.

Although the method according to the present invention is applicable to various hydrocarbon streams, it is particularly suitable for natural gas streams to be liquefied. As the skilled person readily understands how to liquefy a hydrocarbon stream, this is not discussed herein in detail.

Depending on the source, the hydrocarbon stream 10 may contain one or more non-hydrocarbons such as H₂O, N₂, CO₂, Hg, H₂S and other sulphur compounds.

If desired, the hydrocarbon stream 10 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 and pre-pressurizing. As these steps are well known to the person skilled in the art, their mechanisms are not further discussed here.

Thus, the term “hydrocarbon stream” as used herein also includes a composition prior to any treatment, such treatment including cleaning, dehydration and/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, sulfur compounds, carbon dioxide and water.

Preferably, a hydrocarbon stream to be used in the present invention undergoes at least the minimum pre-treatment required to subsequently allow liquefaction of the hydrocarbon stream. Such a requirement for liquefying natural gas is known in the art.

The hydrocarbon stream 10 may also 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 hydrocarbon stream such as natural 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-C₄ hydrocarbons can be used as a source of natural gas liquids (NGLs) and/or refrigerant. Removal of hydrocarbons heavier than methane is often termed ‘NGL recovery’.

FIG. 1 shows a hydrocarbon stream 10 containing natural gas, which is cooled in a first cooling stage 14 to provide a cooled and partially liquefied hydrocarbon stream 20. The cooling in the first cooling stage 14 may cause partial condensation of the hydrocarbon stream 10.

The first cooling stage 14 may comprise a heat exchange system 34, e.g. in the form of one or more heat exchangers such as kettles, either in parallel, series or both, in a manner known in the art. Cooling in the first cooling stage 14 is provided by a first cooling stage refrigerant stream 15, which is warmed and partially or fully evaporated in the first cooling stage 14 to provide a warmed first cooling stage refrigerant stream 15a.

The cooling of the hydrocarbon stream 10 by the first cooling stage refrigerant stream 15 may be part of a liquefaction process, such as a pre-cooling stage, optionally involving a propane refrigerant circuit as described hereinafter with reference to FIG. 2, or the cooling may be a separate process. The first cooling stage refrigerant stream 15 may therefore be part of one or more refrigerant circuits hereinafter described, or may be provided separately.

Cooling of the hydrocarbon stream 10 may involve reducing the temperature of the hydrocarbon stream 10 to below −0° C., for example, in the range −10° C. to −70° C.

At least a fraction, optionally all, of the partially liquefied hydrocarbon stream 20 from the first cooling stage 14 is passed into a distillation column 12 through a first inlet 17.

The distillation column 12 may be of any suitable size and design known in the art. One common distillation column is a scrubber or scrub column, generally operating at an above ambient pressure in a manner known in the art. One purpose of the distillation column 12 is to provide a generally gaseous stream that is methane-enriched, and therefore more suitable for subsequent liquefaction.

Suitable operating parameters for a distillation column 12 are known in the art, and can be varied to vary the composition of at least one of the major product streams provided therefrom. Commonly, there are two major product streams provided from a distillation column 12, being a partially or fully gaseous overhead stream, and a bottom stream, generally being at least partially, usually fully, liquid. The bottom stream is intended to be enriched in the hydrocarbons heavier than methane in the hydrocarbon stream 10, especially butanes, pentanes and heavier hydrocarbons.

In order to reduce the amount of methane that may pass out of the distillation column 12 as part of the bottom stream, it is known in the art to provide a heated stream at or near the bottom of the distillation column 12 so as to encourage methane from the bottom of the distillation column 12 upwards to become or remain gaseous, and therefore pass out of the distillation column 12 as part of a gaseous overhead stream. Such a stream is conventionally heated by a separate boiler, and is therefore commonly termed a ‘re-boiling stream’.

In this regard, FIG. 1 shows a lower feed stream 30 intended to provide heat at or near the bottom of distillation column 12 in the manner of a re-boiling stream known in the art. As discussed herein above, conventionally, heat for the re-boiling stream is provided by a separate boiler, which requires a dedicated fuel source, and whose energy is not recoverable.

According to the method of the present disclosure, the lower feed stream 30 passes through a first heat exchanger 16 to heat the lower feed stream 30 against a compressed fully liquefied refrigerant stream 40 discussed hereinafter in more detail. The heat exchange in the first heat exchange 16 provides a heated lower feed stream 50, which passes into the distillation column 12 through a second inlet 18 lower than the first inlet 17, preferably at or near the bottom of the distillation column. This is to provide heat preferably at or near the bottom of the distillation column 12 to encourage the passage of methane in the partially liquefied hydrocarbon stream in the distillation column 20 to pass out of the distillation column 12 as part of the overhead gaseous stream 70.

The distillation column 12 preferably provides a first overhead gaseous stream 70 having a composition greater than 80 mol % methane, and having a temperature below 0° C., more preferably in the range −20° C. to −40° C. The distillation column 12 in FIG. 1 also provides a first liquid bottom stream 80, generally being enriched in hydrocarbons heavier than methane.

The compressed fully liquefied refrigerant stream 40 is part of a first refrigerant circuit 4, several arrangements for which are shown and described in FIGS. 2, 3 and 4 hereinafter by way of examples only.

The first refrigerant circuit 4 comprises at least one refrigerant compressor and at least one refrigerant stream. One or more of the refrigerant streams may be divided and/or combined one or more times in the refrigerant circuit, to provide cooling to one or more other streams, units, fluids, etc. in a manner known in the art, generally by passing through one or more heat exchangers. The cooling by each refrigerant stream (being a full stream or a fraction or part thereof) is provided by the partial or full evaporation of the refrigerant stream at low pressure in a manner known in the art, the low pressure being provided by passage through one or more valves and/or other expanders.

Each refrigerant stream used in the present disclosure 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 a group comprising: nitrogen, methane, ethane, ethylene, propane, propylene, butanes, pentanes, etc.

Different stages, sections or steps of any part of a hydrocarbon cooling process may involve the same or different types of refrigerant in a manner known to the person skilled in the art, and the present invention is not therefore limited thereby.

The first heat exchanger 16 may comprise one or more heat exchangers in parallel, series or both in a manner known in the art.

The compressed fully liquefied refrigerant stream 40 generally has a temperature above 0° C., such as in the range 10-70° C. The temperature of the cooled refrigerant stream 60, which will be a sub-cooled liquefied refrigerant stream, is less than the refrigerant stream 40.

By providing compressed refrigerant stream 40 as a fully liquefied stream, a reduction in the cooling duty required of the lower feed stream 30 is achieved, compared to a corresponding compressed vapour, or only partly liquefied refrigerant stream. In this way, the heat input to the lower feed stream 30, supplied by the compressed fully liquefied refrigerant stream 40 can be controlled, and more particularly lowered, compared to the use of a partly or fully vaporised refrigerant stream, to provide a heated lower feed stream at a temperature suitable to optimise the separation in the distillation column 12. Use of a partly or fully vaporised refrigerant stream may require too much cooling duty of the lower feed stream 30 such that the heated lower feed stream 50 would be too hot, reducing the separation efficiency in the distillation column. Thus, a system is provided in which the cooling duty available from the lower feed stream 30 can be matched with the requirements of the compressed fully liquefied refrigerant stream 40.

The cooled, such as sub-cooled liquefied, refrigerant stream 60 is subsequently partially or fully evaporated to provide cooling as part of the first refrigerant circuit 4. At least a fraction of the cooling of the hydrocarbon stream 10 is carried out by partially or fully evaporating the cooled, such as sub-cooled liquefied, refrigerant stream 60. By providing the cooled refrigerant stream 60 as a sub-cooled liquefied refrigerant stream, by cooling compressed fully liquefied refrigerant stream 40, a reduction in the flash vapour produced by cooling the hydrocarbon stream 10 is achieved. The first refrigerant 4 circuit may be involved with treatment of the hydrocarbon stream 10 and/or with the first overhead gaseous stream 70, or be separate therefrom.

Examples of arrangements for the provision of the lower feed stream 30, the compressed fully liquefied refrigerant stream 40, the cooled, such as sub-cooled liquefied, refrigerant stream 60, and the first refrigerant circuit 4, are described hereinafter, but the present invention is not limited thereto.

FIG. 2 shows the hydrocarbon stream 10 passing through the first cooling stage 14 to provide a partially liquefied hydrocarbon stream 20, which fully passes into the distillation column 12 through the first inlet 17 to provide a first overhead gaseous stream 70 and a first bottom liquid stream 80 as described above.

FIG. 2 also shows the compressed fully liquefied refrigerant stream 40 passing through a first heat exchanger 16 to provide a cooled, such as sub-cooled liquefied, refrigerant stream 60 as described above.

As one possible embodiment of the present invention, FIG. 2 shows the first refrigerant circuit 4 comprising one or more first refrigerant compressors 42. The refrigerant passing around the first refrigerant circuit 4 may be any suitable refrigerant being a single component such as propane, or a mixed refrigerant. A refrigerant compressor includes any unit, device or apparatus able to increase the pressure of a refrigerant stream. This includes refrigerant compressors having a single compression process or step, or refrigerant compressors having multi-stage compressions or steps. Some stages or steps of a hydrocarbon cooling process may involve two or more refrigerant compressors in parallel, series or both. The present invention is not limited by the type or arrangement or layout of the refrigerant compressor or refrigerant compressors, particularly in any refrigerant circuit.

As part of the first refrigerant circuit 4, the first refrigerant compressor 42 provides a first compressed refrigerant stream 44 which passes through one or more coolers such as a water and/or air cooler 46, to provide a cooler compressed stream 48 which passes into an accumulator 52 in a manner known in the art.

From the accumulator 52, there is provided an accumulator stream 54, which is fully liquefied. Typically, at least a fraction of the accumulator stream 54 is provided as a first cooling refrigerant stream 56, which is expanded and/or evaporated through a suitable valve 58 to provide an expanded refrigerant stream 62 (which can act as the first cooling stage refrigerant stream 15 in FIG. 1) to pass into the first cooling stage 14 to at least partly cool the hydrocarbon stream 10, and to provide a warmed refrigerant stream 75 (like the warmed first cooling stage refrigerant stream 15a shown in FIG. 1), which passes back to the first refrigerant compressor 42.

FIG. 2 also shows the option of dividing the accumulator stream 54 by a stream splitter 64 to provide the compressed fully liquefied refrigerant stream 40. The proportion of the divided streams 56, 40 from the accumulator stream 54 can be calculated in order to provide the desired subsequent heat exchanges.

It can be seen from FIG. 2 that the compressed fully liquefied refrigerant stream 40 provided by the stream splitter 64 is a relatively warm stream following the compression in the first refrigerant compressor 42. At least part of this heat can be used to heat the lower feed stream 30 prior to use of the cooled, such as a sub-cooled liquefied, lower feed stream refrigerant stream 60 as a normal refrigerant stream.

The cooled lower feed stream refrigerant stream 60 may be used as a normal refrigerant stream at any suitable location or locations, and through any suitable heat exchange system including any suitable heat exchangers and/or cooling stages, either as part of the present method cooling and separating a hydrocarbon stream, and/or elsewhere in the liquefaction plant 1, and/or in an independent process or plant. Thus, the refrigerant circuit 4 may comprise a heat exchange system arranged downstream of the first heat exchanger 16, wherein at least a fraction of the cooled, such as a sub-cooled liquefied, refrigerant stream 60 may be used to provide cooling to another stream.

At least a fraction of the cooled, such as a sub-cooled liquefied, refrigerant stream 60 at least partially cools the hydrocarbon stream 10. This may be achieved by passage of the cooled, such as a sub-cooled liquefied, refrigerant stream 60 through a valve 19 to provide an expanded refrigerant stream 60a, and passage thereof into the first cooling stage 14. All of the heated/warmed refrigerant from the first cooling stage 14 can return to the first refrigerant compressor 42 through line 75.

In this way, at least a fraction, optionally all, of the cooled, such as a sub-cooled liquefied, refrigerant stream 60 provides at least a part of the cooling of at least part of or all of the hydrocarbon stream 10 in the first cooling stage 14.

FIG. 3 shows a number of other and/or alternative embodiments of the present invention using the first refrigerant circuit 4 shown in FIG. 2.

In a first alternative of the present invention shown in FIG. 3, at least a fraction, optionally all, of the first liquid bottom stream 80 provides the lower feed stream 30, which passes through the first heat exchanger 16 to provide the heated lower feed stream 50. Optionally, the heated lower feed stream 50 passes into a lower feed gas/liquid separator 51, which may be external or internal to the distillation column 12, to provide a second overhead gaseous stream 50a for use in the distillation column 12, and a second bottom stream 50b for use as an NGL stream.

FIG. 3 also shows the cooled, such as a sub-cooled liquefied, refrigerant stream 60 providing, as stream 60a after valve 19, at least a part of the cooling of the hydrocarbon stream 10 in the first cooling stage 14. This could provide a separate warmed refrigerant stream 60b from the first cooling stage 14, which could be returned to the first refrigerant compressor 42 as part of the return stream 75 as described above (along with the expanded refrigerant stream 62 also passing into the first cooling stage 14), or could be separately returned to the first refrigerant compressor 42 at any other suitable location, one example of which is as a return stream 60c upstream of the first refrigerant compressor 42 (shown in dashed line in FIG. 2 for clarity).

FIG. 3 also shows the option of dividing the accumulator stream 54 by the stream splitter 64 to provide a usually majority second refrigerant stream 66 which can be expanded by a valve 68 to provide cooling in a second heat exchanger 72 as described hereinbelow, and to provide a second return stream 74 to the refrigerant compressor 42. The second heat exchanger 72 may be associated, aligned or integral with the first cooling stage 14 in a manner known in the art.

FIG. 3 also shows non-limiting examples of alternative sources of the compressed fully liquefied refrigerant stream 40 from the first refrigerant circuit 4.

As an example, the compressed stream 44 from the refrigerant compressor 42 could be divided by a stream splitter to provide a first split stream 76 as the source prior to the cooler 46.

Alternatively, the source of the refrigerant stream 40 may be from a separate supply, such as a separate refrigerant circuit.

For example, FIG. 3 also shows a second refrigerant circuit 6 comprising one or more second refrigerant compressors 82 able to provide a second compressed refrigerant stream 84, which then passes through one or more second coolers 86 and then the second heat exchanger 72 to provide a cooler second compressed stream 92. The cooler second compressed stream 92 is shown in FIG. 3 passing through a third separator 94, generally to provide a lighter overhead stream 96 and a heavier bottom stream 98. In a manner known in the art, the lighter overhead stream 96 and the heavier bottom stream 98 pass though a third heat exchanger 102 to be cooled, are separately expanded outside the third heat exchanger 102, and then returned therein to provide cooling to the streams passing through the third heat exchanger 102. This provides a second return refrigerant stream 104 to the second refrigerant compressor 82.

The third heat exchanger 102 may be fully or part of a second cooling stage of a hydrocarbon cooling process. A second cooling stage is preferably separate from the first cooling stage. That is, the second cooling stage comprises one or more separate heat exchangers using a second refrigerant circulating in a second refrigerant circuit, although the refrigerant of the second refrigerant circuit may also pass through one or more heat exchangers of the first cooling stage. Such a second cooling stage is sometimes also termed a ‘main cooling’ stage.

In a manner similar to that described above in relation to the first refrigerant circuit 4, the second refrigerant circuit 6 may provide one or more part or split streams as the source of the compressed fully liquefied refrigerant stream 40. For example, the second compressed refrigerant stream 84 could be divided to provide a third split stream 106, and the second refrigerant stream after the cooler 86 could provide a fourth split stream 108.

Similarly, the warmed refrigerant stream 60b from the first cooling stage 14 could be returned into the second refrigerant circuit 6 at any suitable location, such as a third return stream 60d after the second heat exchanger 72, or as a fourth return stream 60e into the third separator 94.

FIG. 3 also shows at least a fraction, preferably all, of the first overhead gaseous stream 70 from the distillation column 12 being subsequently liquefied in the third heat exchanger 102 to provide a liquefied hydrocarbon stream 120 such as LNG.

FIG. 4 shows a hydrocarbon stream 10 being cooled in heat exchanger system 34a of first cooling stage 14 similar to FIGS. 1 to 3, to provide a partially liquefied hydrocarbon stream 20.

In another alternative embodiment of the present invention, FIG. 4 shows that the hydrocarbon stream 10 can be divided by a stream splitter 21 into at least a first part stream 10a and a second part stream 10b in a manner known in the art. The ratio of the division of the hydrocarbon stream 10 into the first and second part streams 10a, 10b may be any ratio or proportion. Preferably, any part stream to be provided through the first inlet 17 into the distillation column 12 is the majority of the hydrocarbon stream 10, such as described in WO 2006/061400 A1, incorporated herein by way of reference.

The first part feed stream 10a passes through the heat exchange system 34a in first cooling stage 14 as described above to provide the partially liquefied hydrocarbon stream 20 which passes into the distillation column 12.

The second part stream 10b provides the lower feed stream 30 for passage into first heat exchanger 16 to provide a heated lower feed stream 50, which also passes into the distillation column 12.

As shown in FIGS. 2 and 3, the refrigerant stream 40 can be provided from the first refrigerant circuit 4 after the accumulator 52, optionally as a fraction of the second refrigerant stream 66 being provided to the second heat exchanger 72 (not shown in FIG. 4), represented as fraction stream 78.

In another alternative embodiment of the present invention, the hydrocarbon stream 10 is provided from an initial stream 8, which passes through a fourth heat exchanger 112, which may comprise one or more heat exchangers in series, parallel or both. The fourth heat exchanger 112 may form part of the first cooling stage 14. Cooling for the fourth heat exchanger 112 can be provided either by use of the cooled, such as a sub-cooled liquefied, refrigerant stream 60 after passing through the valve 19 to provide an expanded refrigerant stream 60a, and/or by provision of at least a fraction of the expanded refrigerant stream 62 in the first refrigerant circuit 4 prior to its use as the first cooling stage refrigerant stream 15 in the first cooling stage 14. Dashed line 63 show passage of an optional refrigerant stream into the fourth heat exchanger 112, and return line 63a could also be the path of return of the expanded refrigerant stream 60a and any optional refrigerant stream 63 back into the first refrigerant circuit 4 after use in the fourth heat exchanger 112. An optional second fraction of the expanded refrigerant stream 60g may be passed to heat exchanger system 34a of the first cooling stage 14. FIG. 4 shows second fraction expanded refrigerant stream 60g being passed to first cooling stage refrigerant stream 15 which is then passed to heat exchange system 34a of the first cooling stage 14.

Optionally, the fourth heat exchanger 112 is a part of the first cooling stage 14.

In this way, FIG. 4 shows embodiments of the present invention wherein a fraction of the hydrocarbon stream 10 provides at least a fraction of the lower feed stream 30, preferably by division of the hydrocarbon stream 10 into at least first and second part streams 10a, 10b, and wherein the first part stream 10a is cooled to provide the partially liquefied hydrocarbon stream 20, and the second part stream 10b provides at least a fraction of the lower feed stream 30.

FIGS. 3 and 4 also show various alternatives for the provision of cooling to the first cooling stage 14, and various embodiments for the use of the cooled, such as a sub-cooled liquefied, refrigerant stream 60 to provide cooling, preferably as part of the refrigerant circuit 4 through one or more of the same or different heat exchangers.

FIG. 5 shows further alternative embodiments of the present invention.

In FIG. 5, the hydrocarbon stream 10 passes through the first cooling stage 14 as described above to provide a partially liquefied hydrocarbon stream 20 which passes into a feed stream gas/liquid separator 32 to provide a feed stream overhead gaseous stream 100a, and a feed stream liquid bottom stream 100b.

At least a fraction, preferably all, of the feed stream liquid bottom stream 100b is provided as the lower feed stream 30 which passes through the first heat exchanger 16 to provide the heated lower feed stream 50, and then passes into the distillation column 12 as described hereinabove.

Meanwhile, at least a fraction, preferably all, of the feed stream overhead gaseous stream 100a passes through a fifth heat exchanger 116 to provide a cooled partially liquefied hydrocarbon stream 20a which also passes into the distillation column 12. Cooling for the fifth heat exchanger 116 can be provided by a suitable cooling and/or refrigerant stream 114 from one or more of the sources of a refrigerant stream described herein, or from a separate source, in a manner known in the art, at least a fraction of which is derived from cooled, such as a sub-cooled liquefied, refrigerant stream 60, to provide a warmed further refrigerant stream 114a.

Cooling of the feed stream overhead gaseous stream 100a increases the temperature difference between the subsequent cooled partially liquefied hydrocarbon stream 20a and the heated lower feed stream 50 as they enter the distillation column 12, thus increasing the separation, or the efficiency of the separation, of the content of the distillation column 12 into its first overhead gaseous stream 70 and first bottom liquid stream 80, preferably increasing the amount of methane, which passes out of the distillation column 12 as part of the first overhead gaseous stream 70.

From the distillation column 12, the first gaseous overhead stream 70 passes through a second cooling stage 22, comprising one or more heat exchangers in parallel, series or both in a manner known in the art. Optionally, the second cooling stage 22 comprises the third heat exchanger 102 and the cooler second compressed stream 92 shown in FIG. 3.

From the second cooling stage 22, a partially, preferably fully, liquefied hydrocarbon stream 120 passes to a final separator such as an end-flash vessel 24, to provide a final overhead stream such as end flash gas 24a, and a final bottom stream such as an enriched liquefied hydrocarbon stream 90.

In another embodiment of the present invention, optionally, at least a fraction 90b of the final bottom stream 90 is split from the main product stream 90a (such as LNG). This fraction 90b is able to provide, via a pump 94, a secondary lower feed 30a which passes through a secondary heat exchanger 16a to provide a secondary heated lower feed stream 50a, which itself passes into the distillation column 12. Heating for the secondary lower feed stream 30a can be provided by a secondary refrigerant stream 40a, being the same as, a fraction of, or separately derived from, the compressed fully liquefied refrigerant stream 40. The secondary heat exchanger 16a provides a cooled secondary refrigerant stream 60f, which may be combined with or associated with the cooled refrigerant stream 60, or otherwise separately used. Thus, the distillation column 12 may receive more than one heated lower feed stream, either separately or combined into a single stream.

The present disclosure provides an advantageous method of using a least a fraction of a refrigerant stream to heat rather than cool another stream, prior to its subsequent use as a refrigerant stream in a refrigerant circuit as medium against which to cool another stream. Such a refrigerant stream can be provided at, by or from a number of locations around a refrigerant circuit, and the heated refrigerant stream cam be returned to the refrigerant circuit at a number of suitable locations. Thus, the present disclosure provides flexibility in the provision and return of the refrigerant stream to and from a refrigerant circuit.

Table 1 below gives an overview of phases, mass rates, pressures, temperatures and certain compositions for some of the streams and various parts of an example process of the arrangement shown in FIG. 2.

	TABLE 1
	Total Mass
	Rate	Temp	Pressure	C1	C2+
Stream	Phase	Kg/Sec	° C.	Bar	Mol %
10	V/L	209.3	20.0	60.0	0.825	0.17
10a	V/L	146.5	20.0	60.0	0.825	0.17
20	V/L	146.5	−37.0	59.0	0.825	0.17
30	V/L	62.8	20.0	60.0	0.825	0.17
40	L	66.1	52.0	18.5	0.000	1.00
44	V	142.0	83.7	19.0	0.000	1.00
48	L	142.0	52.0	18.5	0.000	1.00
50	V/L	62.8	50.0	59.5	0.825	0.17
54	L	142.0	52.0	18.5	0.000	1.00
56	L	75.8	52.0	18.5	0.00	1.00
60	L	66.1	22.4	18.2	0.000	1.00
60a	V/L	66.1	−40.0	1.1	0.000	1.00
62	V/L	75.8	−40.0	1.1	0.000	1.00
70	V	165.5	−25.2	58.0	0.884	0.10
75	V	142.0	−39.9	1.1	0.000	1.00
80	L	43.8	−39.9	1.1	0.000	1.0
V = Vapour, L = Liquid

The present disclosure also requires less compression power and less cooling in the first refrigerant circuit because of the cooling provided to the refrigerant stream by its use for heating the lower feed stream.

Table 2 below provides a comparison for the power duties required by or in the first cooling stage 14, the first heat exchanger 16, the first refrigerant compressor 42 and its subsequent cooler(s) 46, between;

(i) an example of the present disclosure as shown in FIG. 2; and
(ii) the same arrangement for the first refrigerant circuit, but with separate or external heating being provided to the first heat exchanger to heat the lower feed stream (provided as a fraction of the hydrocarbon stream), and whose heating stream is not subsequently used in the first refrigerant circuit. The first refrigerant circuit of the comparison example is only used to provide same amount of cooling to the first cooling stage.

	TABLE 2
		Duty (kW)
		Comparison	Disclosure
	Apparatus	(ii)	(i)	Difference
	14	31530	31530	0.0%
	16	6130	6130	0.0%
	46	61450	49500	−19.4%
	42	29920	24110	−19.4%

Table 2 confirms that the embodiment of the present disclosure shown in FIG. 2 provides a near 20% reduction in the power required for the first refrigerant compressor 42, and a similar near 20% reduction in the cooling required in the first cooler(s) 46. These are significant power and cooling duty reductions in industrial scale of providing a liquefied hydrocarbon stream.

Table 2 confirms that the embodiment of the present disclosure shown in FIG. 2 requires less compression power for the refrigerant circuit to provide the same cooling duty around its circuit, which reduces the overall power required to cool and separate the same amount of a hydrocarbon stream as the comparison example, which could equate to that shown in U.S. Pat. No. 5,960,644.

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.

Claims

1. A method of cooling and separating a hydrocarbon stream such as natural gas comprising at least the steps of:

(a) providing a hydrocarbon stream;

(b) cooling the hydrocarbon stream to provide a partially liquefied hydrocarbon stream, wherein at least a fraction of the cooling is provided by partially or fully evaporating a cooled refrigerant stream;

(c) passing at least a fraction of the partially liquefied hydrocarbon stream into a distillation column through a first inlet;

(d) providing a lower feed stream;

(e) providing a refrigerant circuit comprising at least a compressed fully liquefied refrigerant stream;

(f) heating the lower feed stream against the compressed fully liquefied refrigerant stream to provide a heated lower feed stream and the cooled refrigerant stream;

(g) passing the heated lower feed stream into the distillation column through a second inlet that is located gravitationally lower than the first inlet; and

(h) separating the partially liquefied hydrocarbon stream in the distillation column to provide at least a first overhead gaseous stream and a first bottom liquid stream.

2. The method according to claim 1 wherein at least a fraction of the first liquid bottom stream provides at least a fraction of the lower feed stream.

3. The method according to claim 1 wherein a fraction of the hydrocarbon stream provides at least a fraction of the lower feed stream.

4. The method according to claim 3 wherein the hydrocarbon stream is divided into at least first and second part-streams, and wherein the first part-stream is cooled to provide the partially liquefied hydrocarbon stream, and the second part-stream provides at least a fraction of the lower feed stream.

5. The method according to claim 1 wherein a fraction of the partially liquefied hydrocarbon stream provides at least a fraction of the lower feed stream.

6. The method according to claim 5 wherein the partially liquefied hydrocarbon stream is passed into a first gas/liquid separator to provide a feed stream overhead gaseous stream and a feed stream liquid bottom stream, and wherein at least a fraction of the feed stream overhead gaseous stream is provided to the first inlet of the distillation column, and at least a fraction of the feed stream liquid bottom stream is provided as at least a fraction of the lower feed stream.

7. The method according to claim 1 wherein at least a fraction of the first overhead gaseous stream is at least partly condensed and passed into a final gas/liquid separator to provide at least a final bottom stream, and wherein at least a fraction of the final bottom stream is a lower feed stream.

8. The method according to claim 1 wherein at least a fraction of the cooling in step (b) is provided by the refrigerant circuit.

9. The method according to claim 1 wherein the refrigerant circuit comprises one or more refrigerant compressors, and wherein the compressed fully liquefied refrigerant stream is provided from the refrigerant circuit downstream of a refrigerant compressor(s).

10. The method according to claim 1 wherein a refrigerant circuit comprises one or more refrigerant accumulators, and wherein the compressed fully liquefied refrigerant stream is provided from a refrigerant accumulator.

11. The method according to claim 1 wherein the compressed fully liquefied refrigerant stream is propane.

12. The method according to claim 1 wherein at least a fraction of the first overhead gaseous stream is subsequently liquefied to provide a liquefied hydrocarbon stream such as LNG.

13. An apparatus for cooling and separating a hydrocarbon stream such as natural gas, comprising at least:

a first cooling stage to cool a hydrocarbon stream and provide a partially liquefied hydrocarbon stream;

a distillation column to receive and separate at least a fraction of the partially liquefied hydrocarbon stream provided through a first inlet;

a refrigerant circuit comprising at least a compressed fully liquefied refrigerant stream;

a first heat exchanger to heat a lower feed stream against the compressed fully liquefied refrigerant stream to provide a cooled refrigerant stream and a heated lower feed stream to be passed into the distillation column through a second inlet that is located gravitationally lower than the first inlet; and

a heat exchange system in the first cooling stage arranged in the refrigerant circuit downstream of the first heat exchanger to cool the hydrocarbon stream against at least a fraction of the cooled refrigerant stream, thereby partly or fully evaporating the at least the fraction of the cooled refrigerant stream.

14. A liquefaction plant comprising apparatus as claimed in claim 13.