US20090188279A1 - Method and apparatus for treating a hydrocarbon stream - Google Patents

Abstract

The present invention relates to a method of treating a hydrocarbon stream such as a natural gas stream, the method at least comprising the steps of: (a) supplying a partially condensed feed stream (10) to a first gas/liquid separator (2), the feed stream (10) having a pressure >50 bar; (b) separating the feed stream (10) in the first gas/liquid separator (2) into a first vaporous stream (20) and a first liquid stream (70); (c) expanding the first vaporous stream (20), thereby obtaining an at least partially condensed first vaporous stream (30); (d) supplying the at least partially condensed first vaporous stream (30) to a second gas/liquid separator (4); (e) separating the stream (30) as supplied in step (d) in the second gas/liquid separator (4) into a second vaporous stream (60) and a second liquid stream (40); (f) increasing the pressure of the second liquid stream (40) to a pressure of at least 50 bar, thereby obtaining a pressurized second liquid stream (50); and (g) returning the pressurized second liquid stream (50) to the first gas/liquid separator (2).

Description

• The present invention relates to a method of treating a hydrocarbon stream such as a natural gas stream, in particular in a process for the production of liquefied natural gas.
• Several methods of treating a natural gas stream are known, e.g. to remove undesired components from the natural gas and/or to meet the required specifications of a client.
• Also, several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at high pressures.
• Usually, the natural gas stream to be liquefied (mainly comprising methane) contains ethane, heavier hydrocarbons and possibly other components that are to be removed to a certain extent before the natural gas is liquefied. Also to this end, the natural gas stream is treated. One of the treatments may involve the removal of at least some of the ethane, propane and higher hydrocarbons such as butane and propane.
• A known method of treating a natural gas stream is disclosed in U.S. Pat. No. 5,291,736 relating to a method for the liquefaction of natural gas, at the same time separating hydrocarbons heavier than methane.
• As the treating process, whether or not forming part of a liquefaction process, is highly energy consuming there is a constant need to provide alternative processes of treating natural gas, wherein the energy consumption is reduced.
• It is an object of the invention to meet the above need and to provide a process in which the energy consumption is reduced.
• It is a further object of the present invention to provide an alternative method for treating a natural gas stream.
• One or more of the above or other objects are achieved according to the present invention by providing a method of treating a hydrocarbon stream such as a natural gas stream, the method at least comprising the steps of:
• • (a) supplying a partially condensed feed stream to a first gas/liquid separator, the feed stream having a pressure >(above) 50 bar;
  • (b) separating the feed stream in the first gas/liquid separator into a first vaporous stream and a first liquid stream;
  • (c) expanding the first vaporous stream obtained in step (b), thereby obtaining an at least partially condensed first vaporous stream;
  • (d) supplying the at least partially condensed first vaporous stream obtained in step (c) to a second gas/liquid separator;
  • (e) separating the stream as supplied in step (d) in the second gas/liquid separator into a second vaporous stream and a second liquid stream;
  • (f) increasing the pressure of the second liquid stream obtained in step (e) to a pressure of at least 50 bar, thereby obtaining a pressurized second liquid stream; and
  • (g) returning the pressurized second liquid stream (50) obtained in step (f) to the first gas/liquid separator.
• In an alternative embodiment, the invention relates to a method of treating a hydrocarbon stream such as a natural gas stream, the method at least comprising the steps of:
• • (a) supplying a partially condensed feed stream (10) to a first gas/liquid separator (2), the feed stream (10) preferably having a pressure >30 bar;
  • (b) separating the feed stream (10) in the first gas/liquid separator (2) into a first vaporous stream (20) and a first liquid stream (70);
  • (c) expanding the first vaporous stream (20) obtained in step (b), thereby obtaining an at least partially condensed first vaporous stream (30);
  • (d) supplying the at least partially condensed first vaporous stream (30) obtained in step (c) to a second gas/liquid separator (4);
  • (e) separating the stream (30) as supplied in step (d) in the second gas/liquid separator (4) into a second vaporous stream (60) and a second liquid stream (40);
  • (f) increasing the pressure of the second liquid stream (40) obtained in step (e) to a pressure of at least 30 bar, thereby obtaining a pressurized second liquid stream (50); and
  • (g) returning the pressurized second liquid stream (50) obtained in step (f) to the first gas/liquid separator (2).
• It has surprisingly been found that using the method according to the present invention, a significant reduction of energy consumption may be obtained. The method according to the invention is especially advantageous as the feed stream is available at a relatively high pressure, typically >(above) 50 bar, preferably above 55 bar, more preferably above 60 bar.
• Whenever in the specification and claims reference is made to a pressure in bar, this is a pressure in bar (absolute).
• According to the present invention no expensive refrigerant scheme has to be used to cool the first vaporous stream.
• The hydrocarbon stream may be any suitable stream to be treated, but is usually a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the natural gas stream may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.
• Usually the natural gas stream is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol % methane, more preferably at least 75 mol %, such as at least 80 mol % methane.
• Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The natural gas stream may also contain non-hydrocarbons such as H₂O, mercury, N₂, CO₂, H₂S and other sulphur compounds.
• If desired, the feed stream containing the natural gas may be pre-treated before feeding it to the first gas/liquid separator. This pre-treatment may comprise removal of undesired components such as H₂O, mercury, N₂, CO₂, H₂S and other sulphur compounds, or other steps such as pre-cooling or pre-pressurizing. As these steps are well known to the person skilled in the art, they are not further discussed here.
• Usually the feed stream has a temperature in the range from ambient to 90° C., preferably from 20° C. to 80° C. Preferably the pressure of the feedstream is in the range from more than 50 bar to 100 bar, more preferably from more than 55 bar to 90 bar, even more preferably from more than 60 bar to 80 bar.
• The first and second gas/liquid separators may be any suitable means for obtaining a vaporous stream and a liquid stream, such as a vessel, a scrubber, a distillation column, etc. Usually the first gas/liquid separator comprises a column having 1-30 trays, preferably 1-15 trays. In the embodiment of the invention described with reference toFIG. 1, the second gas/liquid separator usually comprises a simple vessel with only one tray. In the embodiment of the invention described with reference toFIG. 2, the second gas/liquid separator preferably comprises a column having 1-30 trays, more preferably 1-15 trays.
• Alternatively the first and second gas/liquid separators may each be provided with packing (random or structured). When the gas/liquid separator is provided with trays, a distillation stage corresponds to one tray, and when the gas/liquid separator is provided with packing (random or structured) a distillation stage corresponds to a theoretical stage.
• Where in the specification and in the claims a level of introducing a stream into the gas/liquid separator is defined relative to introducing another stream, there is at least one distillation stage between the two levels, the same applies to defining the level of removing a stream from the gas/liquid separator. The top of the gas/liquid separator is that part of the gas/liquid separator that is located above the uppermost distillation stage, and the bottom of the gas/liquid separator is that part of the gas/liquid separator that is located below the lowermost distillation stage.
• The first liquid stream and the second vaporous stream may be used as product streams or may be further processed, if desired.
• In step (f) of the method of the present invention, the pressure of the second liquid stream obtained in step (e) is increased to a pressure of at least 50 bar, thereby obtaining a pressurized second liquid stream. Preferably, the pressure of the second liquid stream is increased to a pressure in the range from more than 50 bar to 100 bar, more preferably from more than 55 bar to 90 bar, even more preferably from more than 60 bar to 80 bar.
• Typically, the pressure of the second liquid stream is in the range from 0 to 5 bar higher than the pressure in the first gas/liquid separator, preferably from 0 to 2 bar higher, even more preferably from 0 to 1 bar higher, in particular substantially the same pressure.
• It is preferred according to the present invention that in step (a) the feed stream is supplied as at least two different streams to the first gas/liquid separator, the feed stream comprising a higher feed stream and a lower feed stream. In this embodiment, the higher feed stream is fed at a warmer (i.e. higher) point of the first gas/liquid separator than the lower feed stream (that is fed at a lower, i.e. colder, point of the first gas/liquid separator).
• Further it is preferred that the higher feed stream is cooled, preferably against the second vaporous stream obtained in step (e). To this end a heat exchanger may be used.
• Also it is preferred that the first liquid stream obtained in step (b) is supplied to a third gas/liquid separator thereby obtaining a third vaporous stream and a third liquid stream. Preferably the third vaporous stream is combined with the second vaporous stream.
• In a further aspect the present invention relates to an apparatus for treating a hydrocarbon stream such as a natural gas stream, the apparatus at least comprising:
• • a first gas/liquid separator for separating a partially condensed feed stream into a first vaporous stream and a first liquid stream;
  • an expander for expanding the first vaporous stream;
  • a second gas/liquid separator for separating the expanded first vaporous stream into a second vaporous stream and a second liquid stream; and
  • a pressurizing unit for increasing the pressure of the second liquid stream to at least 50 bar before being returned to the first gas/liquid separator.
• Preferably the first gas/liquid separator comprises at least two inlets for the feed stream, including an inlet for a higher feed stream and an inlet for a lower feed stream.
• It is especially preferred that the apparatus further comprises a heat exchanger for cooling the higher feed stream against the second vaporous stream.
• Further it is preferred that the apparatus further comprises a third gas/liquid separator for separating the first liquid stream into a third vaporous stream and a third liquid stream. Preferably the third vaporous stream can be combined with the second vaporous stream.
• Hereinafter the invention will be further illustrated by the following non-limiting drawing. Herein shows:
• FIG. 1 schematically a process scheme in accordance with an embodiment of the present invention; and
• FIG. 2 schematically a process scheme in accordance with another embodiment of the present invention.
• For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components.
• FIG. 1 schematically shows a process scheme enabling selective low temperature separation of heavy hydrocarbons (C₅⁺) in a gas plant with flexibility to recover/reject LPGs.
• The process scheme (or apparatus) is generally indicated withreference number1.
• A partially condensedhydrocarbon feed stream10 such as natural gas is supplied to a first gas/liquid separator2 at a certain inlet pressure and inlet temperature. In the embodiment ofFIG. 1 thefeed stream10 is fed as two different streams, viz. ahigher feed stream10aand alower feed stream10b.If desired thefeed stream10 may be split in more than two sub-streams. Thehigher feed stream10ais pre-cooled inheat exchanger6 and fed to theseparator2 atfirst inlet11; thelower feed stream10bis fed to theseparator2 atsecond inlet12. In the shown embodiment, stream10ais cooled against another stream in the process (i.e. stream60). However, any other cooling may be used, if desired.
• Typically, thefeed stream10 has a temperature in the range from ambient to 90° C., preferably from 20° C. to 80° C. Preferably the pressure of the feedstream is in the range from more than 50 bar to 100 bar, more preferably from more than 55 bar to 90 bar, even more preferably from more than 60 bar to 80 bar. The temperature and pressure of thestreams10aand10bis chosen to optimise a gas/liquid separation step inseparator2. If desired, the pressure of thestreams10aand10bmay have been adjusted invalves13 and14, respectively.
• As mentioned above,stream10 is fed to the gas/liquid separator2 asstreams10aand10b.There, thefeed stream10 is separated into a first vaporous (i.c. overhead)stream20 and a first liquid (i.c. bottom)stream70. Theoverhead stream20 leaves theseparator2 atfirst outlet15 and is enriched in methane (and usually also ethane) relative to thefeed stream10.
• Thebottom stream70 leaves theseparator2 atsecond outlet16 and is generally liquid;stream70 may contain hydrocarbons that can be separately processed to form liquefied petroleum gas (LPG) products. Usually, thebottom stream70 is subjected to one or more fractionation steps to collect various natural gas liquid products.
• Theoverhead stream20 is led to anexpander3, thereby at least partially condensing thestream20, thereby obtainingstream30. Subsequently,stream30 is fed to a second gas/liquid separator4 atinlet21. In thesecond separator4, the partially condensedstream30 is separated into a second vaporous (i.c. overhead)stream60 and a second liquid (i.c. bottom)stream40. Theoverhead stream60 leaves theseparator4 atoutlet22 and is generally vaporous; thebottom stream40 leaves theseparator4 atoutlet23 and is generally liquid.
• Then thestream40 is pressurized in pressurizingunit5 to a pressure of at least 50 bar. The pressurizingunit5 may be any suitable means for increasing the pressure such as a pump. Thepressurized stream50 leaving the pressurizingunit5 is subsequently returned to the first gas/liquid separator2, preferably at the warm (i.c. high) part thereof, atthird inlet17 of thefirst separator2.
• The firstliquid stream70 and the secondvaporous stream60 may be used as product streams or may be further processed, if desired.
• In the embodiment as shown inFIG. 1, the secondvaporous stream60 is used to cool thehigher feed stream10ainheat exchanger6.
• Furthermore, the firstliquid stream70 is (after being optionally depressurized in valve33) fed (asstream70a) to a third gas/liquid separator7 (at inlet34) thereby obtaining (at outlet31) a thirdvaporous stream80 and (at outlet32) a thirdliquid stream90.
• The thirdvaporous stream80 is combined with the second vaporous stream65 (i.e.stream60 after being heat exchanged in heat exchanger6) atjunction point18 and is subsequently compressed incompressor8 thereby obtainingproduct gas100 which will usually be subjected to a liquefaction step in one or more heat exchangers (not shown) thereby obtaining liquefied natural gas (LNG). In case that stream100 is to be liquefied, some further treatment steps may take place to remove any contaminants that may solidify during the liquefaction process. As an example a (n optionally additional) CO₂removal step may take place.
• Stream80 may be compressed to about the same pressure of the secondvaporous stream65 beforestream80 is combined with the secondvaporous stream65 at thejunction point18.
• FIG. 2 schematically shows an alternative embodiment of the present invention to provide an integrated gas dew pointing and condensate stabilizing process, wherein thethird column7 is in the form of a debutanizer/stabilizer, thereby obtaining a thirdvaporous stream80 being enriched in butane and lower hydrocarbons (such as methane, ethane and or propane) relative to the thirdliquid stream90.
• Furthermore,FIG. 2 shows that the thirdvaporous stream80, before being combined withstream65 injunction point18, has previously been cooled (asstream80a) against (an air cooler or water cooler or, as shown) an external refrigerant inheat exchanger55, fed (asstream80b) to a fourth gas/liquid separator19 atinlet41, and removed atoutlet42 from the fourth gas/liquid separator19 (as stream80). The fourth gas/liquid separator19 functions as an overhead condenser drum. The liquidbottom stream110 removed atoutlet43 from the fourth gas/liquid separator19 is pressurized inpump51 and returned asstream120 to the top (at inlet33) of thedebutanizer7.
• A part of the bottom stream90 (or ‘condensate’) of the debutanizer/stabilizer7 is split off atsplitter56, heat exchanged asstream130 against an external stream in heat exchanger52 (functioning as a reboiler) and returned asstream140 to the bottom (at inlet35) of the debutanizer/stabilizer7. The major part of thecondensate stream90 is (after splitter56) heat exchanged against the firstliquid stream70 inheat exchanger53 and subsequently againststream10binheat exchanger54 and used as a product stream.
• In addition to or instead of heat exchanging stream70 (or70a) against stream90 (in heat exchanger53), stream70 (or70a) may be heat exchanged againststream80a, for example inheat exchanger55.
• If desired, one or more further gaseous and/or liquid streams (not shown) may be introduced into the debutanizer/stabilizer7.
• The line-up as used inFIG. 2 allows to produce aproduct gas stream80 with a surprisingly high content of LPGs (i.e. propane and/or butane) and acondensate stream90 with a surprisingly high content of C₅⁺ (i.e. pentane and higher components). As indicated above,stream80 may be used as a separate product stream, but will usually combined withstream65 to enrich the latter stream.
• Table I gives an overview of the estimated pressures and temperatures of a stream at various parts in an example process ofFIG. 2. Also the mole fraction of methane is indicated. The feed stream inline10 ofFIG. 2 comprised approximately the following composition: 75.2 mole % methane, 9.2 mole % ethane, 4.3 mole % propane, 2.1 mole % butanes, 5.2 mole % C₅⁺, 1.2 mole % N₂and 2.7 mole % CO₂. H₂S and H₂O were previously removed.

• 	TABLE I
  				Methane
  		Pressure	Temperature	[mole
  	Line	[bar]	[° C.]	fraction]

  	10	67.7	61.1	0.752
  	20	66.8	−1.0	0.807
  	30	42.2	−21.3	0.807
  	40	42.2	−21.3	0.291
  	50	69.7	−19.2	0.291
  	60	42.2	−21.3	0.831
  	65	41.7	85.6	0.831
  	70	67.0	4.9	0.287
  	80	42.7	10.0	0.456
  	90	9.5	173.3	0.0
  	100	49.6	78.8	0.795
  	110	8.9	10.0	0.027

• The person skilled in the art will readily understand that many modifications may be made, without departing from the scope of the appended claims.
• As an example, theexpander3 andcompressor8 may be functionally coupled.

Claims (20)

1. A method of treating a hydrocarbon stream, the method at least comprising the steps of:

(a) supplying a partially condensed feed stream to a first gas/liquid separator, the feed stream having a pressure >50 bar;

(b) separating the feed stream in the first gas/liquid separator into a first vaporous stream and a first liquid stream;

(c) expanding the first vaporous stream obtained in step (b), thereby obtaining an at least partially condensed first vaporous stream;

(d) supplying the at least partially condensed first vaporous stream obtained in step (c) to a second gas/liquid separator;

(e) separating the stream as supplied in step (d) in the second gas/liquid separator into a second vaporous stream and a second liquid stream;

(f) increasing the pressure of the second liquid stream obtained in step (e) to a pressure of at least 50 bar, thereby obtaining a pressurized second liquid stream; and

(g) returning the pressurized second liquid stream obtained in step (f) to the first gas/liquid separator.

2. The method according toclaim 1, wherein in step (a) the feed stream is supplied as at least two different streams to the first gas/liquid separator, the feed stream comprising a higher feed stream and a lower feed stream.

3. The method according toclaim 2, wherein the higher feed stream is cooled before it is supplied to the first gas/liquid separator.

4. The method according toclaim 3, wherein the higher feed stream is cooled against the second vaporous stream obtained in step (e).

5. The method according toclaim 1, wherein the first liquid stream obtained in step (b) is supplied to a third gas/liquid separator thereby obtaining a third vaporous stream and a third liquid stream.

6. The method according toclaim 5, wherein the third vaporous stream is combined with the second vaporous stream.

7. The method according toclaim 5, wherein the third liquid stream is heat exchanged against the first liquid stream, before it is supplied to the third gas/liquid separator.

8. The method according toclaim 5, wherein the third liquid stream is heat exchanged against the lower feed stream.

9. The method according toclaim 5, wherein the third column is a debutanizer, thereby obtaining a third vaporous stream being enriched in butane and lower hydrocarbons relative to the third liquid stream.

10. The method according toclaim 1, wherein the second vaporous stream, is liquefied, thereby obtaining a liquefied hydrocarbon stream.

11. An apparatus for treating a hydrocarbon stream, the apparatus at least comprising:

a first gas/liquid separator for separating a partially condensed feed stream O into a first vaporous stream and a first liquid stream;

an expander for expanding the first vaporous stream;

a second gas/liquid separator for separating the expanded first vaporous stream into a second vaporous stream and a second liquid stream; and

a pressurizing unit for increasing the pressure of the second liquid stream to a pressure of at least 50 bar before being returned to the first gas/liquid separator.

12. The apparatus according toclaim 11, wherein the first gas/liquid separator comprises at least two inlets for the feed stream, including a first inlet for a higher feed stream and a second inlet for a lower feed stream.

13. The apparatus according toclaim 12, wherein the apparatus further comprises a heat exchanger for cooling the higher feed stream against the second vaporous stream.

14. The apparatus according toclaim 11, wherein the apparatus further comprises a third gas/liquid separator for separating the first liquid stream into a third vaporous stream and a third liquid stream.

15. The apparatus according toclaim 14, wherein the third vaporous stream can be combined with the second vaporous stream.

16. The method according toclaim 2, wherein the first liquid stream obtained in step (b) is supplied to a third gas/liquid separator thereby obtaining a third vaporous stream and a third liquid stream.

17. The method according toclaim 3, wherein the first liquid stream obtained in step (b) is supplied to a third gas/liquid separator thereby obtaining a third vaporous stream and a third liquid stream.

18. The method according toclaim 4, wherein the first liquid stream obtained in step (b) is supplied to a third gas/liquid separator thereby obtaining a third vaporous stream and a third liquid stream.

19. The method according toclaim 6, wherein the third liquid stream is heat exchanged against the first liquid stream, before it is supplied to the third gas/liquid separator.

20. The method according toclaim 6, wherein the third liquid stream is heat exchanged against the lower feed stream.

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