US20110239701A1

US20110239701A1 - Method of rejecting nitrogen from a hydrocarbon stream to provide a fuel gas stream and an apparatus therefor

Info

Publication number: US20110239701A1
Application number: US13/126,476
Authority: US
Inventors: Sander Kaart; Marco Dick Jager
Original assignee: Individual
Current assignee: Shell USA Inc
Priority date: 2008-11-03
Filing date: 2009-11-02
Publication date: 2011-10-06
Also published as: AU2009319191B2; EP2342517A2; AU2009319191A1; WO2010060735A3; JP2012514050A; WO2010060735A2; CA2741970A1; CN102713479A

Abstract

Method of, and apparatus for, rejecting nitrogen from a hydrocarbon stream to provide a fuel gas stream. A hydrocarbon stream is at least partially liquefied and subsequently expanded. The expanded hydrocarbon stream is fractionated in a fractionation column to provide an nitrogen-rich hydrocarbon stream and a nitrogen-lean hydrocarbon stream. The nitrogen-rich hydrocarbon stream is partially condensed in a condenser by cooling against a refrigerant circulated in a dedicated first refrigerant circuit, and phase-separated to provide a nitrogen-rejection stream and a nitrogen-lean reflux stream which is returned to the fractionation column. The nitrogen-lean hydrocarbon stream is partially vaporized and phase-separated to provide a vapour stream that is returned to the fractionation column and a liquefied nitrogen-lean hydrocarbon stream that is subjected to sub-cooling. The fuel gas stream is generated from the sub-cooled nitrogen-lean hydrocarbon stream.

Description

The present invention relates to a method for rejecting nitrogen from a hydrocarbon stream to provide a fuel gas stream, and an apparatus therefor. The present invention may also provide a liquefied hydrocarbon stream, such as a liquefied natural gas (LNG) stream.
A common hydrocarbon stream for a fuel gas stream comprises, or essentially consists of, natural gas (NG).
Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of 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 high pressure.
Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded to final atmospheric pressure suitable for storage and transportation.
However, notwithstanding pretreatment prior to liquefaction, hydrocarbon streams (including purified natural gas streams) may contain significant quantities of nitrogen. If special measures are not taken to remove at least a part of the nitrogen from the hydrocarbon stream, the fuel gas and any liquefied hydrocarbon stream produced may contain undesirably high nitrogen levels. Many LNG specifications require less than 1 mol % nitrogen in the final product.
EP 1 715 267 discloses a method of removing nitrogen from a liquefied natural gas feed comprising subjecting the liquefied natural gas to a first fractionation to provide a first nitrogen-enriched overhead vapour stream and a nitrogen-containing bottoms liquid stream. At least a portion of the nitrogen-containing bottoms liquid stream is then subjected to a second fractionation to provide a second nitrogen-enriched overhead vapour stream that is of lower purity than said first overhead vapour stream and a purified liquefied natural gas stream.
In the method illustrated in EP 1 715 267 for a propane pre-cooling, mixed refrigerant main cooling process (C3MR), the cooling duty in the first fractionation is provided by a condensed nitrogen reflux stream. The condensed nitrogen reflux stream is provided by heat exchange against an expanded cold liquefied natural gas stream from the main heat exchanger, providing an undesirable surplus of fuel gas. To prevent this, additional cooling duty for the nitrogen rejection is ultimately provided by the mixed refrigerant circuit used in the main cooling stage. This increases the load placed on the mixed refrigerant main cooling cycle, requiring larger refrigerant compressors of greater power, increased refrigerant cooling etc., resulting in a reduced power capacity limited by available compressor driver size.
It is an object of the present invention to address these problems, by providing an improved process for rejecting nitrogen from a hydrocarbon stream. In particular, the present invention seeks to provide a process which reduces the load on the main cooling cycle.
In a first aspect, the present invention provides a method of rejecting nitrogen from a hydrocarbon stream to provide a fuel gas stream, comprising at least the steps of:

- (a) at least partly liquefying a hydrocarbon stream in a heat exchanger to provide a cooled hydrocarbon stream;
- (b) expanding at least a part of the cooled hydrocarbon stream in a first expansion device to provide an expanded hydrocarbon stream;
- (c) fractionating the expanded hydrocarbon stream in a fractionation column to provide an upper nitrogen-rich hydrocarbon stream and a lower nitrogen-lean hydrocarbon stream;
- (d) condensing the upper nitrogen-rich hydrocarbon stream in a condenser by cooling against an expanded first refrigerant stream in a dedicated first refrigerant circuit to provide a partially condensed nitrogen-rich hydrocarbon stream and a heated first refrigerant stream;
- (e) separating the partially condensed nitrogen-rich hydrocarbon stream in a first separator to provide an upper nitrogen-rejection stream and a lower nitrogen-lean reflux stream which is returned to the fractionation column;
- (f) heating the lower nitrogen-lean hydrocarbon stream from the fractionation column in a reboiler against a first refrigerant feed stream in the first refrigerant circuit to provide a partially vapourised nitrogen-lean hydrocarbon stream and a cooled first refrigerant stream;
- (g) separating the partially vapourised nitrogen-lean hydrocarbon stream in a second separator to provide an upper second separator vapour stream which is returned to the fractionation column, and a lower liquefied nitrogen-lean hydrocarbon stream;
- (h) sub-cooling the lower liquefied nitrogen-lean hydrocarbon stream in a heat exchanger to provide a sub-cooled nitrogen-lean hydrocarbon stream; and
  generating a fuel gas stream from the sub-cooled nitrogen-lean hydrocarbon stream.

In a further aspect, the present invention provides an apparatus for the rejection of nitrogen from a hydrocarbon stream the apparatus comprising at least:

- a first heat exchanger having a first inlet for a hydrocarbon stream and a first outlet for a cooled hydrocarbon stream, the first outlet of the heat exchanger;
- a first expansion device having an inlet connected to the first outlet of the first heat exchanger, and an outlet for an expanded hydrocarbon stream;
- a fractionation column having a first inlet connected to the outlet of the first expansion device, and a first outlet for an upper nitrogen-rich hydrocarbon stream, a second outlet for a lower nitrogen-lean hydrocarbon stream, a second inlet for a lower nitrogen-lean reflux stream and a third inlet for an upper second separator vapour stream;
- a condenser having a first inlet connected to the first outlet of the fractionation column, and a first outlet for a partially condensed nitrogen-rich hydrocarbon stream and a second inlet for an expanded first refrigerant stream and a second outlet for a heated first refrigerant stream;
- a first separator having a first inlet connected to the first outlet of the condenser and a first outlet for an upper nitrogen-rejection stream and a second outlet for a lower nitrogen-lean reflux stream, said second outlet connected to the second inlet of the fractionation column;
- a reboiler having a first inlet connected to the second outlet of the fractionation column, a first outlet for a partially vapourised nitrogen-lean hydrocarbon stream, a second inlet for a first refrigerant feed stream and a second outlet for a cooled first refrigerant stream;
- a second separator having a first inlet connected to the first outlet of the reboiler, a first outlet for an upper second separator vapour stream and a second outlet for a lower liquefied nitrogen-lean hydrocarbon stream, said second outlet connected to the third inlet of the fractionation column; and
- a second heat exchanger, which can be the first heat exchanger or a different heat exchanger, said second heat exchanger having a first inlet connected to the second outlet of the second separator and a first outlet for a sub-cooled nitrogen-lean hydrocarbon stream.

In a still further aspect, the invention provides a method of controlling the nitrogen concentration present in a fuel gas stream, the method comprising at least the steps of the method of rejecting nitrogen from the hydrocarbon stream to provide the fuel gas stream as defined above, and further comprising the steps of:

- heat exchanging the cooled first refrigerant stream against the heated first refrigerant stream in a fourth heat exchanger to provide an expander refrigerant feed stream and a compressor refrigerant feed stream; and expanding an expander refrigerant feed stream in a fourth expansion device to provide the expanded first refrigerant stream;
- compressing the compressor refrigerant feed stream in a first compressor to provide a compressed refrigerant steam;
- cooling the compressed refrigerant stream in a cooling device to provide a cooled compressed refrigerant stream; and heat exchanging the cooled compressed refrigerant stream in a fifth heat exchanger to provide the first refrigerant feed stream;
- providing a fifth heat exchanger bypass line from the cooled compressed refrigerant stream to the first refrigerant feed stream, the fifth heat exchanger bypass line containing a fifth heat exchanger bypass line valve
- bypassing the fifth heat exchanger; and
- controlling the fifth heat exchanger bypass valve to effect the relative proportion of the cooled compressed refrigerant stream being cooled in the fifth heat exchanger.

Embodiments 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 of a method of and apparatus for rejecting nitrogen for a hydrocarbon stream according to one embodiment; and

FIG. 2 is a diagrammatic scheme of a method of and apparatus for rejecting nitrogen for a hydrocarbon stream according to a second embodiment.

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 present invention provides a method of rejecting nitrogen from a hydrocarbon stream to produce a fuel gas stream, in which the cooling duty for the nitrogen rejection is provided by a dedicated first refrigerant circuit.
The first refrigerant circuit is a dedicated circuit in the sense that neither the refrigerant being circulated in the first refrigerant circuit, nor cooling duty therefrom, is used for the cooling and at least partly liquefying of the hydrocarbon stream in step (a) and for the subcooling in step (h). In other words, the first refrigerant circuit is separate from other refrigerant circuits used in the cooling, liquefying and subcooling of the hydrocarbon stream.
By utilising a dedicated first refrigerant circuit, the cooling duty placed upon the heat exchanger in step (a), which produces the at least partly liquefied hydrocarbon stream and can be a main cooling step, is reduced. Consequently, the capacity of the main cooling refrigerant circuit which supplies the heat exchanger may be reduced compared to that of EP 1 715 267, for a constant LNG capacity. Viewed in another way, compared to EP 1 715 267 the method of the present invention provides an increased production of liquefied hydrocarbon for an equivalent main cooling refrigerant circuit.
The present invention is particularly suitable for retrofitting already existing liquefaction plants because it does not alter the demand upon the main cooling refrigerant circuit.
The present invention is applicable to many methods, such as those for the manufacture of a cooled hydrocarbon stream, for instance LNG from natural gas. The method may be applied to, for example AP-X liquefaction processes such as those described in U.S. Pat. No. 6,308,531, C3MR processes such as those described in U.S. Pat. No. 4,404,008 and Dual Mixed Refrigerant (DMR) processes, such as those described in U.S. Pat. No. 6,370,910.
Referring to the drawings, FIG. 1 shows a method of and apparatus 1 for rejecting nitrogen for a hydrocarbon stream 10 according to a first embodiment.
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 10 is comprised substantially of methane. Preferably such a hydrocarbon stream 10 comprises at least 50 mol % methane, more preferably at least 80 mol % methane. Although the method disclosed herein is applicable to various hydrocarbon streams, it is particularly suitable for natural gas streams to be liquefied.
Depending on the source, a hydrocarbon stream may contain one or more non-hydrocarbons such as H₂O, N₂, CO₂, Hg, H₂S and other sulfur compounds.
The method and apparatus disclosed herein can be used with hydrocarbon streams comprising significant quantities of nitrogen, for instance, in excess of 4 mol %.
As an example, if the hydrocarbon stream 10 were to comprise 4-5 mol % nitrogen, conventional methods which do not take steps to reject the nitrogen may produce fuel gas streams with nitrogen contents in excess of 40 mol %. This can result in significant problems if the fuel gas is used to power a gas turbine. Many aeroderivative gas turbines cannot tolerate nitrogen contents above 15 mol % in their fuel gas. Furthermore, even the more nitrogen-tolerant gas turbines, such as conventional heavy duty industrial gas turbines, cannot operate on fuel gas with a nitrogen content of above 40-45 mol %. Thus, there is a need to remove excessive nitrogen from the hydrocarbon stream 10 in order to reduce the nitrogen content in the fuel gas, and any liquefied hydrocarbon.
If desired, the hydrocarbon stream 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 acid gases such as CO₂and H₂S or other steps such as early cooling and pre-pressurizing.
The hydrocarbon stream 10 may have been pre-compressed and/or pre-cooled. Any pre-cooling stage will generally cool the stream to a temperature below 0° C., and preferably between −20 to −50° C., preferably using a pre-cooling refrigerant circuit. As these steps are well known to the person skilled in the art, their mechanisms are not further discussed here.
A further advantage of the method and apparatus disclosed herein is that nitrogen is rejected from the hydrocarbon stream without requiring additional compression power. For instance, the fuel gas can be sent for nitrogen rejection after it is produced. However, the method and apparatus disclosed herein treat the hydrocarbon stream to reject nitrogen during the liquefaction process, when the hydrocarbon stream is already compressed. For example the hydrocarbon stream 10 can be provided to the first heat exchanger 50 at a pressure in the region of 60 bar. The person skilled in the art will understand that pressure values in the present application are considered to be given in absolute pressure values, as opposed to gauge pressure values.
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 10 to be used herein 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.
A hydrocarbon stream commonly also contains 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 butanes generally need to be removed from natural gas to be liquefied because at LNG temperatures they may freeze out and cause blockage of parts of a methane liquefaction plant. In addition, the desired specification of the LNG may require the removal of or a reduction in the proportion of certain components. C2-4 hydrocarbons can be extracted and used as a source of natural gas liquids (NGLs) and/or refrigerant. To this end, the natural gas liquids to be recovered may be separated from the methane in for instance a high-pressure scrub column and subsequently be fractionated in a number of dedicated distillation columns to yield valuable hydrocarbon components. These may be valuable either as product streams per se or for use in the liquefaction plant and process, for example as a component of a refrigerant.
Returning to FIG. 1, the hydrocarbon stream 10 is passed to the inlet 52 of a heat exchanger 50, which may be a first heat exchanger, and is preferably the main cooling stage of a liquefaction plant. The hydrocarbon stream 10 is at least partly, preferably fully, liquefied in the heat exchanger 50 to provide a cooled hydrocarbon stream 60 at the outlet 54.
The heat exchanger 50 is shown symbolically as a single unit in FIG. 1, although it may comprise one or more first heat exchangers, either in series, parallel or both. The cooling duty of the one or more heat exchangers 50 can be met by one or more main refrigerant circuits, in a manner known in the art. The cooled hydrocarbon stream 60 may have a temperature below −100° C., more preferably below −120° C.
The cooled hydrocarbon stream 60 is then passed to the inlet 102 of a first expansion device 100, such as a valve and/or expander, more preferably a turboexpander or Joule-Thomson valve, where it is expanded to provide an expanded hydrocarbon stream 110 at the outlet 104. Preferably the pressure reduction in the expansion is more than a de minimis pressure reduction that results from flow through conventional pipes and conduits. The pressure reduction may be by at least 10 bar, preferably by at least 35 bar, to in order to further lower the temperature of the stream. The expansion of the cooled hydrocarbon stream 60, for instance to a pressure of less than or equal to 25 bar, preferably in a range of from about 15 to about 25 bar, more preferably of about 20 bar, can further cool the stream. Also, a pressure of less than or equal to 25 bar, particularly in the range of from about 15 to about 25 bar, is considered to be beneficial for the separation of nitrogen from the stream 110.
The expanded hydrocarbon stream 110 is then passed to a first inlet 152 of a fractionation column 150 at a first feeding level, to provide an upper nitrogen-rich hydrocarbon stream 160 at a first outlet 154, which is preferably at or near the top of the fractionation column 150, and a lower nitrogen-lean hydrocarbon stream 170 at a second outlet 156, which is preferably at or neat the bottom of the fractionation column 150. The upper nitrogen-rich hydrocarbon stream 160 removes at least a part of the nitrogen from the hydrocarbon stream. The upper nitrogen-rich hydrocarbon stream 160 also provides the reflux stream to the fractionation column 150 as discussed below.
The upper nitrogen-rich hydrocarbon stream 160 is passed to the first inlet 202 of a condenser 200, where it is partially condensed to provide a partially condensed nitrogen-rich hydrocarbon stream 210 at the first outlet 204 of condenser 200. The condensing is carried out against an expanded first refrigerant stream 860, present in a first dedicated refrigerant circuit. The first dedicated refrigerant circuit is separate from the one or more main refrigerant circuits that provide the cooling duty of the one or more heat exchangers 50.
The expanded first refrigerant stream 860 is passed to a second inlet 206 of the condenser 200 where it cools the upper nitrogen-rich hydrocarbon stream 160 and exits the condenser 200 at second outlet 208 as a heated first refrigerant stream 870.
The expanded first refrigerant stream 860 and heated first refrigerant stream 870 are provided in a dedicated first refrigerant circuit 800. By “dedicated” is meant that this circuit is separate from the main refrigerant circuit and any pre-cooling refrigerant circuit and subcooling refrigerant circuit if present, such that the first refrigerant circuit does not share a compressor and/or compressor driver with any other refrigerant circuit. The first refrigerant circuit will be discussed in greater detail in relation to FIG. 2.
The partially condensed nitrogen-rich hydrocarbon stream 210 is then passed to the inlet of a first separator 250, which may be a gas/liquid separator, which provides an upper nitrogen-rejection stream 260 at a first outlet 254 and a lower nitrogen reflux stream 270 at a second outlet 256. The composition of the upper nitrogen-rejection stream 260 can be varied. When the upper nitrogen-rejection stream 260 comprises high purity nitrogen, this can be vented to the atmosphere.
Lowering the temperature of the condenser 200 increases the proportion of nitrogen in the upper nitrogen-rejection stream 260. For instance, a temperature of about −150° C. at about 25 bar in the partially condensed nitrogen-rich hydrocarbon stream 210 can provide an upper nitrogen-rejection stream 260 comprising >99 mol % nitrogen and a liquefied hydrocarbon stream 520 having <1 mol % nitrogen. The pressure in the fractionation column 150 can affect the temperature required of first separator 250. For example, by increasing the pressure in the fractionation column 150, the same nitrogen content of the upper nitrogen-rejection stream 260 can be obtained at a higher temperature up to the critical point of the stream.
Alternatively, if the temperature of the condenser 200 is raised sufficiently, the hydrocarbon content of the upper nitrogen-rejection stream 260 can be raised e.g. up to 80-90 mol % nitrogen, such that the upper nitrogen-rejection stream 260 can be used as fuel gas for equipment which can tolerate higher nitrogen-content hydrocarbon streams, such as a gas boiler.
In a further embodiment which is not shown in FIG. 1, a portion of the cold present in the upper nitrogen-rejection stream 260 can be recovered. For instance, the cold of the upper nitrogen-rejection stream 260 can be used to cool one or more of: the hydrocarbon stream 10, or a refrigerant stream, such as a refrigerant stream in the first refrigerant circuit 800 or any pre-cooling or main-cooling stage refrigerant, as well as providing optional direct pre-cooling of the hydrocarbon stream 10. In addition, the cold from the upper nitrogen-rejection stream 260 could be used to further cool the sub-cooled nitrogen-lean hydrocarbon stream 410, as will be found in the discussion for FIG. 2.
The lower nitrogen-lean reflux stream 270 which exits the first separator 250 at the second outlet 256, is returned to the fractionating column 150 at a second feeding level above the first feeding level, preferably near the top of the column.
A lower nitrogen-lean hydrocarbon stream 170 exits the fractionating column 150 via second outlet 156, preferably near the bottom of the column. The lower nitrogen-lean hydrocarbon stream 170 is passed to the first inlet 302 of a reboiler 300, where it is heated against a first refrigerant feed stream 810, also present in the first refrigerant circuit 800, which enters the reboiler 300 at a second inlet 306. The reboiler 300 provides a partially vapourised nitrogen-lean hydrocarbon stream 310 at a first outlet 304 and cooled first refrigerant stream 820 at second outlet 308.
The function of the reboiler 300 disclosed herein can be contrasted with that of the reboiler 47 disclosed in FIG. 2 of prior art document EP 1 715 267 discussed above. In order to change the duty in the reboiler 47 disclosed in this prior art document, it is necessary to change the temperature of cooled high pressure feed gas 17 withdrawn from the heat exchanger 16 as discussed at paragraph [0035] of this document. This prior art arrangement passes the duty to the second section of the heat exchanger, such as a subcooling stage.
The cited European patent application states at paragraph [0014] that one object is to provide for reject of part of the nitrogen from any LNG process with minimal additional equipment and minimal impact on plant performance. This objective cannot be met by the prior art system because the cooling duty for nitrogen-rejection column is provided by the vapourisation of LNG, resulting in a lowered plant productivity. This vapourisation produces additional fuel gas which must be disposed of or reliquified. In order to replace the LNG which has been vapourised, an increased load must be placed on the heat exchanger, producing a significant impact on plant performance.
In the method and apparatus disclosed herein, the duty of reboiler 300 is controlled entirely independently of the duty placed upon the heat exchanger 50, 400. This is particularly advantageous because the apparatus disclosed herein can be retrofitted to an existing liquefaction unit, without altering the cooling duty placed upon the heat exchanger impacting on the unit performance.
The partially vapourised nitrogen-lean hydrocarbon stream 310 is passed to an inlet 352 of a second separator 350, such as a gas/liquid separator, where it is separated into an upper second separator vapour stream 360 at a first outlet 354 and a lower liquefied nitrogen-lean hydrocarbon stream 370 at a second outlet 356.
The upper second separator vapour stream 360 is passed to a third inlet 158 of the fractionating column 150 at a third feeding level below the first feeding level, preferably near the bottom of the column.
The lower liquefied nitrogen-lean hydrocarbon stream 370 is passed to the first inlet of a heat exchanger 400, which can be a second heat exchanger. The heat exchanger 400 is preferably a sub-cooling stage shown symbolically as a single unit in FIG. 1, although it may comprise one or more second heat exchangers, either in series, parallel or both.
The heat exchanger 400 may be the same as, or different to the heat exchanger 50. For instance heat exchangers 50 an 400 can be the same heat exchanger, different heat exchangers present within the same cold box or shell, or may be different heat exchangers located apart from one another. The heat exchangers 50, 400 can be plate and fin or shell and tube heat exchangers and are more preferably coil wound heat exchangers.
Cooling duty may be provided to the one or more heat exchangers 400 by one or more sub-cooling refrigerant circuits, in a manner known in the art. Such one or more sub-cooling refrigerant circuits may be shared with the one or more main refrigerant circuits that provide cooling duty to the one or more heat exchangers 50, or separate therefrom. But the one or more sub-cooling refrigerant circuits are separate from the dedicated first refrigerant circuit.
The heat exchanger 400 can sub-cool the lower liquefied nitrogen-lean hydrocarbon stream 370 to provide a sub-cooled nitrogen-lean hydrocarbon stream 410 at a first outlet 404. Preferably the lower liquefied nitrogen-lean hydrocarbon stream 370 is cooled to a temperature below −140° C.
The sub-cooled nitrogen-lean hydrocarbon stream 410 is used to generate a fuel gas stream 510 by methods known in the art. For instance, as shown in FIG. 1, the sub-cooled nitrogen-lean hydrocarbon stream 410 can be passed to an inlet 462 of a second expansion device 450, such as a valve and/or expander, preferably a turbo-expander or a Joule-Thomson valve. Second expansion device 450 provides an expanded nitrogen-lean hydrocarbon stream 460 at an outlet 454.
The expanded nitrogen-lean hydrocarbon stream 460 can then be passed to an inlet of a third separator 500, such as a gas/liquid separator, to provide a fuel gas stream 520 at a first outlet 464, preferably at or near the top of the separator and a liquefied hydrocarbon stream 520, such as a LNG stream, at a second outlet 466, preferably at or near the bottom of the separator.
The fuel gas stream 510 can be passed to one or more end-compressors (not shown) if pressurisation, for instance to export gas pressure of about 30-50 bar, is required.
FIG. 2 shows a second embodiment in which the method and apparatus for rejecting nitrogen has been integrated with a main cryogenic heat exchanger 700. Such a heat exchanger is used in the AP-X, C3MR and DMR methods of liquefying natural gas streams which are known in the art. The first refrigerant circuit 800 is also described in more detail in this embodiment.
Hydrocarbon stream 10 is passed to main cryogenic heat exchanger (MCHE) 700 via a first inlet 52. In a similar manner to the embodiment of FIG. 1, the hydrocarbon stream may have been pre-cooled, for instance in a pre-cooling stage, in a known manner.
The MCHE 700 comprises a main cooling stage 50 a, which corresponds to the heat exchanger 50 of FIG. 1, and a sub-cooling stage 400 a, which corresponds to the heat exchanger 400 of FIG. 1. The MCHE 700 may comprise one or more associated refrigerant circuits, which are not shown in FIG. 2, such as main cooling refrigerant circuits and sub-cooling refrigerant circuits, or a single linked circuit, for instance when light and heavy mixed refrigerant fractions are utilised.
Preferably the MCHE 700 is cooled by a mixed refrigerant comprising two or more components of the group comprising: nitrogen, methane, ethane, ethylene, propane, propylene, butane and pentanes. The refrigerant circuit may involve any number of refrigerant compressors, coolers and separator to provide one or more refrigerant streams to the MCHE 700 in a manner known in the art.
For instance, light and heavy mixed refrigerant streams 710 and 720 respectively may be provided by a mixed refrigerant circuit and passed through the MCHE 700 for further cooling. The light and heavy mixed refrigerant streams 710, 720 can then be withdrawn from the MCHE 700, expanded by one or more valves and/or expanders (not shown) before re-entering the MCHE 700 to provide cooling therein.
The MCHE 700 can be a spool wound heat exchanger, able to cool and at least partly, preferably fully, liquefy the hydrocarbon stream 10 to provide a cooled hydrocarbon stream 60 between the liquefying bundle of main cooling stage 50 a and the sub-cooling bundle of sub-cooling stage 400 a. The cooled hydrocarbon stream 60 is expanded and passed to the fractionating column 150 as described for FIG. 1.
A bypass line 62, with a bypass first pressure reducing device 64 can be provided to transfer at least a part of the cooled hydrocarbon stream 60 to the lower liquefied nitrogen-lean hydrocarbon stream 370, via a combiner 372. The bypass line 62 allows the proportion of the cooled hydrocarbon stream 60 passed for nitrogen rejection to be varied.
First refrigerant circuit 800 shown in FIG. 2 provides the cooling duty for the nitrogen rejection. The refrigerant in the first refrigerant circuit 800 preferably comprises nitrogen, more preferably >90 mol % nitrogen, with the balance provided by light hydrocarbons, such as one or more of methane, ethane and propane.
In first refrigerant circuit 800, a compressor refrigerant feed stream 880 can be passed to the inlet 892 of a first compressor 890. The first compressor 890 compresses the compressor refrigerant feed stream 880 to provide a compressed refrigerant stream 900 at an outlet 892. The compressed refrigerant stream 900 can be passed to an inlet 912 of a cooling device 910, such as an air or water cooler, to provide a cooled compressed refrigerant stream 920 at an outlet 914.
The cooled compressed refrigerant stream 920 can then be passed to a first inlet 932 of a fifth heat exchanger 930. The fifth heat exchanger 930 further cools the cooled compressed refrigerant stream 920 to provide first refrigerant feed stream 810 at an outlet 934. The first refrigerant feed stream 810 is passed to the second inlet 306 of the reboiler 300.
In the fifth heat exchanger 930, the cooled compressed refrigerant stream 920 can be heat exchanged against a second refrigerant in a second refrigerant circuit, such as a mixed refrigerant in the main cooling circuit or a pre-cooling refrigerant. Alternatively, the cooled compressed refrigerant stream 920 can be heat exchanged against at least a part of the upper nitrogen-rejection stream 260.
In an alternative embodiment not shown in FIG. 2, a partially open first refrigerant circuit 800 can be provided in which the cooled compressed refrigerant stream 920 can be combined with at least a part of the upper nitrogen-rejection stream 260 to provide the first refrigerant stream 810.
The fifth heat exchanger 930 can be provided with a fifth heat exchanger bypass line 940 from the cooled compressed refrigerant stream 920 to the first refrigerant feed stream 810. The fifth heat exchanger bypass line 940 is provided with a fifth heat exchanger bypass line pressure reduction device 950, such as a valve. The reboiler duty of the fractionation column 150 is controlled by the fifth heat exchanger bypass line 940, thereby setting the nitrogen ejected in the upper nitrogen-rejection stream 260, and the nitrogen content of the fuel gas stream 510 via the amount of nitrogen remaining in lower liquefied nitrogen-lean hydrocarbon stream 370.
For instance, if the reboiler 300 provides a higher heating duty, more nitrogen will be rejected in the second separator 350 to the upper second separator vapour stream 360 because proportionally more of the lighter components such as nitrogen will be vapourised. More nitrogen vapour will therefore rise through fractionation column 150, increasing the nitrogen content of upper nitrogen-rich hydrocarbon stream 160. Any methane present in the upper nitrogen-rich hydrocarbon stream 160 can be recondensed in condenser 200 and returned to the fractionation column 150 after in lower nitrogen-lean reflux stream 270 from first separator 250. Correspondingly, if the reboiler 300 provides a lower heating duty this will result in less nitrogen rejection in the upper nitrogen rich hydrocarbon stream 160, and therefore a lower nitrogen content in upper nitrogen-rejection stream 260.
The fifth heat exchanger bypass line pressure reduction device 950 can be operated by a controller Q, which is placed on the fuel gas stream 510 and monitors the nitrogen content of the stream. The controller Q can signal the fifth heat exchanger bypass line pressure reduction device 950 to change the flow in the fifth heat exchanger bypass line 940 thus changing the duty of reboiler 300 and the quantity of nitrogen-lean hydrocarbon stream 370.
The reboiler 300 provides a cooled first refrigerant stream 820 at second outlet 308. The cooled first refrigerant stream 820 can be passed to the first inlet 832 of a fourth heat exchanger 830. The fourth heat exchanger 830 further cools the cooled first refrigerant stream 820 against heated first refrigerant stream 870 which is passed to a second inlet of the fourth heat exchanger 830 to provide an expander refrigerant feed stream 840 at a first outlet 834 and the compressor refrigerant feed stream 880 at a second outlet 838.
The expander refrigerant feed stream 840 is passed to the inlet of a fourth expansion device 850, such as a valve and/or an expander, preferably a turboexpander or Joule-Thomson valve, where it is expanded to provide an expanded first refrigerant stream 860 at outlet 854.
The expanded first refrigerant stream 860 is passed to the second inlet 206 of the condenser 200 where it can be partially vapourised to provide a mixed liquid and vapour stream as it cools the upper nitrogen-rich hydrocarbon stream 160 from the fractionation column 150. The heated first refrigerant stream 860 exits the condenser 200 at second outlet 208 as heated first refrigerant stream 870 which is passed to the second inlet 836 of the fourth heat exchanger 830 to provide compressor refrigerant feed stream 880 after heat exchange, thus completing the first refrigerant circuit 800.
With regard to the remainder of the nitrogen-rejection method and apparatus, the reference numerals in FIG. 2 have identical designations and purposes to those numerals of the same number discussed for FIG. 1.
Returning to FIG. 2, the lower liquefied nitrogen-lean hydrocarbon stream 370 provided by second separator 350 is passed to a second inlet 402 of the MCHE 700, where it is sub-cooled in sub-cooling stage 400 a, corresponding to the heat exchanger 400 of FIG. 1. Sub-cooling stage 400 a provides a sub-cooled nitrogen-lean hydrocarbon stream 410 at second outlet 404 of the MCHE 700.
The sub-cooled nitrogen-lean hydrocarbon stream 410 is passed to the first inlet 552 of a third heat exchanger 550, where it is cooled against an intermediate fraction of the nitrogen-lean hydrocarbon stream 660 withdrawn from an endflash unit 650, to provide a pre-cooled nitrogen-lean hydrocarbon stream 560 at a first outlet 554 of the third heat exchanger 550.
In a further embodiment not shown in FIG. 2, the third heat exchanger 550 may alternatively be fed with at least a part of the upper nitrogen-rejection stream 260, which can be used to further cool the sub-cooled nitrogen-lean hydrocarbon stream 410.
The pre-cooled nitrogen-lean hydrocarbon stream 560 is then passed to an inlet 602 of a third expansion device 600, such as a valve and/or expander, more preferably a turboexpander or Joule-Thomson valve, where it is expanded to provide an expanded nitrogen-lean hydrocarbon stream 610 at an outlet 604. The expanded nitrogen-lean hydrocarbon stream 610 can then optionally be passed through a further second pressure reducing device 620 before being passed to a first inlet 652 of an endflash unit 650 at a fourth feeding level.
Fuel gas stream 510 exits the endflash unit 650 from first outlet 654, preferably at or near the top of the unit and liquefied hydrocarbon stream 520, such as an LNG stream, exits the endflash unit 650 at second outlet 656, preferably at or near the bottom of the unit. The fuel gas stream 510 can be passed to a fuel gas network (not shown) for distribution to fuel gas users such as gas turbines and gas boilers. Alternatively and/or additionally the fuel gas stream 510 can be optionally compressed in one or more fuel gas compressors and sent for export. The liquefied hydrocarbon stream 520 can be sent for storage or transport.
An intermediate fraction of the nitrogen-lean hydrocarbon stream 660 can be withdrawn from the endflash unit 650 at third outlet 657, and passed to a second inlet 556 of the third heat exchanger 550 where it can be used to cool the sub-cooled nitrogen-lean hydrocarbon stream 410. The intermediate fraction of the nitrogen-lean hydrocarbon stream 660 leaves the third heat exchanger 550 at second outlet 558 as heated intermediate fraction nitrogen-lean hydrocarbon stream 670, which is then returned to a second inlet 658 of the endflash 650 at a fifth feeding level below that of the fourth feeding level, preferably near the bottom of the unit. The heated intermediate fraction nitrogen-lean hydrocarbon stream 670 can be fitted with a flow controller FIC, attached to a third pressure reduction device 680, which can be a valve, present in the heated intermediate fraction nitrogen-lean hydrocarbon stream 670. The flow controller FIC is in communication with a nitrogen sensor Q1, which monitors the nitrogen content of the liquefied hydrocarbon stream 520. The flow controller can adjust the ratio of heated intermediate fraction nitrogen-lean hydrocarbon stream 670 fed to the endflash unit 650 compared to the reflux stream in the endflash unit generated from the expanded nitrogen-lean hydrocarbon stream 610 after passing through second pressure reducing device 620 and being fed to the endflash unit 650.
When the flow rate of heated intermediate fraction nitrogen-lean hydrocarbon stream 670 is reduced by third pressure reduction device 680, the residence time of intermediate fraction of the nitrogen lean hydrocarbon stream 660 in the third heat exchanger 550 will be increased. The third heat exchanger 550 will therefore heat the intermediate fraction of the nitrogen lean hydrocarbon stream 660 to a higher temperature, providing a hotter heated intermediate fraction nitrogen-lean hydrocarbon stream 670. When a hotter heated intermediate fraction nitrogen-lean hydrocarbon stream 670 is passed to the endflash unit 650, it will provide more stripping vapour, boil off more nitrogen and thus strip more nitrogen from the liquid falling through the endflash unit. Thus, by changing the ratio of reflux to stripping vapour the quality of the liquefied hydrocarbon stream such as LNG can be varied.
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. For instance, the second expansion device 450 and third separator 500 of the embodiment of FIG. 1 could be employed to process the sub-cooled nitrogen-lean hydrocarbon stream 410 of the embodiment of FIG. 2, or the third heat exchange 550, third expansion device 600 and endflash unit 650 of FIG. 2 could be employed to process the sub-cooled nitrogen-lean hydrocarbon stream 410 of the embodiment of FIG. 1.

Claims

1. A method of rejecting nitrogen from a hydrocarbon stream to provide a fuel gas stream comprising at least the steps of:

(a) at least partly liquefying a hydrocarbon stream in a heat exchanger to provide a cooled hydrocarbon stream;

(b) expanding at least a part of the cooled hydrocarbon stream in a first expansion device to provide an expanded hydrocarbon stream;

(c) fractionating the expanded hydrocarbon stream in a fractionation column to provide an upper nitrogen-rich hydrocarbon stream and a lower nitrogen-lean hydrocarbon stream;

(d) condensing the upper nitrogen-rich hydrocarbon stream in a condenser by cooling against an expanded first refrigerant stream in a dedicated first refrigerant circuit to provide a partially condensed nitrogen-rich hydrocarbon stream and a heated first refrigerant stream;

(e) separating the partially condensed nitrogen-rich hydrocarbon stream in a first separator to provide an upper nitrogen-rejection stream and a lower nitrogen-lean reflux stream which is returned to the fractionation column;

(f) heating the lower nitrogen-lean hydrocarbon stream from the fractionation column in a reboiler against a first refrigerant feed stream in the first refrigerant circuit to provide a partially vapourised nitrogen-lean hydrocarbon stream and a cooled first refrigerant stream;

(g) separating the partially vapourised nitrogen-lean hydrocarbon stream in a second separator to provide an upper second separator vapour stream which is returned to the fractionation column, and a lower liquefied nitrogen-lean hydrocarbon stream;

(h) sub-cooling the lower liquefied nitrogen-lean hydrocarbon stream in a heat exchanger to provide a sub-cooled nitrogen-lean hydrocarbon stream; and

(i) generating a fuel gas stream from the sub-cooled nitrogen-lean hydrocarbon stream.

2. The method of claim 1 wherein step (a) comprises fully liquefying the hydrocarbon stream in a first heat exchanger to provide a cooled hydrocarbon stream.

3. The method of claim 1 wherein in step (i) comprises:

(1) expanding the sub-cooled nitrogen-lean hydrocarbon steam in a second expansion device to provide an expanded nitrogen-lean hydrocarbon stream; and

(2) separating the expanded nitrogen-lean hydrocarbon stream in a third separator to provide the fuel gas stream and a liquefied hydrocarbon steam.

4. The method of claim 1 wherein step (i) comprises:

(1) cooling the sub-cooled nitrogen-lean hydrocarbon stream in a third heat exchanger to provide a pre-cooled nitrogen-lean hydrocarbon stream;

(2) expanding the pre-cooled nitrogen-lean hydrocarbon stream in a third expansion device to provide an expanded nitrogen-lean hydrocarbon stream; and

(3) separating the expanded nitrogen-lean hydrocarbon stream in an endflash unit to provide the fuel gas stream and a liquefied hydrocarbon stream, preferably a LNG stream.

5. The method of claim 4 wherein step (i) further comprises:

(4) withdrawing an intermediate fraction of the nitrogen-lean hydrocarbon stream from the endflash unit and passing this to the third heat exchanger;

(5) heating the intermediate fraction of the nitrogen-lean hydrocarbon stream against the sub-cooled nitrogen-lean hydrocarbon stream to provide the pre-cooled nitrogen-lean hydrocarbon stream and a heated intermediate fraction nitrogen-lean hydrocarbon stream; and

(6) passing the heated intermediate fraction nitrogen-lean hydrocarbon stream to the end flash unit.

6. The method of claim 1 comprising the further step of heat exchanging the nitrogen-rejection stream from the first separator against one or more of the group consisting of: the sub-cooled nitrogen-lean hydrocarbon stream, the hydrocarbon stream and a refrigerant stream from the first or second heat exchanger.

7. The method of claim 1 wherein the first refrigerant stream in the first refrigerant circuit comprises a nitrogen.

8. The method of claim 1 wherein the first refrigerant circuit is a partially open circuit comprising a first refrigerant which is drawn from the upper nitrogen-rejection stream.

9. The method of claim 1 further comprising the steps of: heat exchanging the cooled first refrigerant stream against the heated first refrigerant stream in a fourth heat exchanger to provide an expander refrigerant feed stream and a compressor refrigerant feed stream; and expanding an expander refrigerant feed stream in a fourth expansion device to provide the expanded first refrigerant stream.

10. The method of claim 9 further comprising the steps of: compressing the compressor refrigerant feed stream in a first compressor to provide a compressed refrigerant steam.

11. The method of claim 10 further comprising cooling the compressed refrigerant stream in a cooling device to provide the first refrigerant feed stream.

12. The method of claim 10 further comprising cooling the compressed refrigerant stream in a cooling device to provide a cooled compressed refrigerant stream; and heat exchanging the cooled compressed refrigerant stream in a fifth heat exchanger to provide the first refrigerant feed stream.

13. The method of claim 12 wherein the cooled compressed refrigerant stream is heat exchanged against a second refrigerant in a second refrigerant circuit.

14. The method of claim 12 further comprising providing a fifth heat exchanger bypass line from the cooled compressed refrigerant stream to the first refrigerant feed stream, the fifth heat exchanger bypass line containing a fifth heat exchanger bypass line valve.

15. A method of controlling the nitrogen concentration present in a fuel gas stream, the method comprising at least the steps of:

bypassing the fifth heat exchanger in the method of claim 14; and

controlling the fifth heat exchanger bypass valve to effect the relative proportion of the cooled compressed refrigerant stream being cooled in the fifth heat exchanger.

16. An apparatus for the rejection of nitrogen from a hydrocarbon stream to provide a fuel gas stream, the apparatus comprising at least:

a first heat exchanger having a first inlet for a hydrocarbon stream and a first outlet for a cooled hydrocarbon stream, the first outlet of the heat exchanger;

a first expansion device having an inlet connected to the first outlet of the first heat exchanger, and an outlet for an expanded hydrocarbon stream;

a fractionation column having a first inlet connected to the outlet of the first expansion device, and a first outlet for an upper nitrogen-rich hydrocarbon stream, a second outlet for a lower nitrogen-lean hydrocarbon stream, a second inlet for a lower nitrogen-lean reflux stream and a third inlet for an upper second separator vapour stream;

a condenser having a first inlet connected to the first outlet of the fractionation column, and a first outlet for a partially condensed nitrogen-rich hydrocarbon stream and a second inlet for an expanded first refrigerant stream and a second outlet for a heated first refrigerant stream;

a first separator having a first inlet connected to the first outlet of the condenser and a first outlet for an upper nitrogen-rejection stream and a second outlet for a lower nitrogen-lean reflux stream, said second outlet connected to the second inlet of the fractionation column;

a reboiler having a first inlet connected to the second outlet of the fractionation column, a first outlet for a partially vapourised nitrogen-lean hydrocarbon stream, a second inlet for a first refrigerant feed stream and a second outlet for a cooled first refrigerant stream;

a second separator having a first inlet connected to the first outlet of the reboiler, a first outlet for an upper second separator vapour stream and a second outlet for a lower liquefied nitrogen-lean hydrocarbon stream, said second outlet connected to the third inlet of the fractionation column; and

a second heat exchanger, which can be the first heat exchanger or a different heat exchanger, said second heat exchanger having a first inlet connected to the second outlet of the second separator and a first outlet for a sub-cooled nitrogen-lean hydrocarbon stream.

17. The apparatus of claim 16 further comprising:

a fourth heat exchanger having a first inlet connected to the second outlet of the reboiler, a first outlet for an expander refrigerant feed stream, a second inlet connected to the second outlet of the condenser, and a second outlet for a compressor refrigerant feed stream;

a first compressor having an inlet connected to the second outlet of the fourth heat exchanger and an outlet for a compressed refrigerant stream;

a cooling device having an inlet connected to the outlet of the first compressor and outlet for a cooled compressed refrigerant stream;

a fifth heat exchanger having an inlet connected to the outlet of the cooling device and an outlet connected to the second inlet of the reboiler; and

a fourth expansion device having an inlet connected to the first outlet of the fourth heat exchanger and an outlet connected to the second inlet of the condenser.