AU2007267116B2

AU2007267116B2 - Method for treating a hydrocarbon stream

Info

Publication number: AU2007267116B2
Application number: AU2007267116A
Authority: AU
Inventors: Eduard Coenraad Bras; Jill Hui Chiun Chieng; Akash Damodar Wani
Original assignee: SHELL INT RESEARCH; Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2006-05-30
Filing date: 2007-05-30
Publication date: 2010-08-12
Anticipated expiration: 2027-05-30
Also published as: US20090194461A1; WO2007138067A2; AU2007267116A1; WO2007138067A3

Description

WO 2007/138067 PCT/EP2007/055225 METHOD FOR TREATING A HYDROCARBON STREAM The present invention relates to a method of treating a hydrocarbon stream such as a natural gas stream, containing > 10 mole% CO 2 In particular the present invention relates to the 5 treatment of a natural gas stream, containing > 10 mole% C0 2 , involving recovery of at least some of the propane, butanes and higher hydrocarbons such as pentane from the natural gas. The recovery of hydrocarbons may be done for several purposes. One purpose may be the production of 10 hydrocarbon streams consisting primarily of hydrocarbons heavier than methane such as natural gas liquids (NGLs; usually composed of ethane, propane and butanes), liquefied petroleum gas (LPG; usually composed of propane and butane) or condensates (usually composed of butanes 15 and heavier hydrocarbon components). Another purpose may be the adjustment of e.g. the heating value of the hydrocarbon stream to correspond to desired specifications. Several processes and apparatuses for treating a 20 hydrocarbon stream are known. An example is given in US 2005/0268469 Al relating to a process for processing natural gas or other methane-rich gas streams to produce a liquefied natural gas (LNG) stream that has a high methane purity and a liquid stream containing 25 predominantly hydrocarbons heavier than methane. A problem of the known method is that it cannot properly treat hydrocarbon streams containing relatively high CO 2 concentrations. A further problem of the known method is that it is 30 rather complicated thereby resulting in high capital -2 expenses (CAPEX) , but at the same time it does not obtain a satisfactory recovery of ethane and heavier hydrocarbons, in particular propane. Other examples of such processes are described in US 5,960,644 and in "LPG recovery processes for baseload LNG plants examined", Chen-Hwa Chiu, Oil & Gas 5 Journal November 24, 1997. Again, a problem of these known methods is that they cannot properly treat hydrocarbon streams containing relatively high CO 2 concentrations. Object of the Invention It is the object of the present invention to substantially overcome or ameliorate one or more of the disadvantages of the prior art, or to at least provide a useful alternative. 10 Summary of the Invention The present invention provides a method for treating a hydrocarbon stream containing > 10 mole% C0 2 , the method at least comprising the steps of: (a) supplying a partly condensed feed stream containing > 10 mole% CO 2 to a first gas/liquid separator; 15 (b) separating the feed stream in the first gas/liquid separator into a gaseous stream and a liquid stream; (c) expanding the liquid stream obtained in step (b) and feeding it into a second gas/liquid separator at a first feeding point; (d) expanding the gaseous stream obtained in step (b), thereby obtaining an 20 at least partially condensed stream, and subsequently feeding it into the second gas/liquid separator at a second feeding point, the second feeding point being at a higher level than the first feeding point; (e) removing from the top of the second gas/liquid separator a gaseous stream, partially condensing it and feeding it into a third gas/liquid separator; 25 (f) separating the stream fed in the third gas/liquid separator in step (e) thereby obtaining a liquid stream and a gaseous stream; (g) feeding the liquid stream obtained in step (f), having a temperature above the freezing point of CO 2 at the prevailing conditions, into the second gas/liquid separator at a third feeding point, the third feeding point being at a higher level than the 30 second feeding point; and -3 (h) removing from the bottom of the second gas/liquid separator a liquid stream; wherein the gaseous stream removed from the second gas/liquid separator in step (e) is partially condensed by heat exchanging against a cold stream, which is selected 5 such that the partially condensed stream obtained in step (e) has a temperature greater than or equal to - 60"C, above the freezing point of CO 2 at the prevailing conditions, and wherein all streams being fed into the second gas/liquid separator fully originate from the feed stream. It has been found that using the surprisingly simple method according to the io present invention for treating a hydrocarbon stream such as a natural gas stream, containing > 10 mole% C02, the CAPEX can be significantly lowered. Further, also due to its simplicity, the method according to the present invention and apparatuses for performing the method have proven very robust when compared with known line-ups. Furthermore it has been found that according to at least a preferred embodiment is of the present invention a higher propane recovery can be obtained thereby resulting in a leaner methane-rich natural gas stream (that may be liquefied subsequently, if desired). The method according to the present invention has also been proven suitable for feed streams WO 2007/138067 PCT/EP2007/055225 -4 having a pressure well below 70 bar, at the same time keeping up a relatively high propane recovery. Whenever in the specification and claims reference is made to a pressure in bar, this is a pressure in bar 5 (absolute). The hydrocarbon stream may be any suitable gas 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 10 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. 15 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, besides CO 2 , also contain non-hydrocarbons such as H 2 0, 20 N 2 , mercury, H 2 S 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 H 2 0, mercury, H 2 S 25 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. The first, second and third gas/liquid separator may 30 be any suitable means for obtaining a gaseous stream and a liquid stream, such as a scrubber, distillation column, etc. If desired, four or more gas/liquid separators may be present.

WO 2007/138067 PCT/EP2007/055225 -5 Also, the person skilled in the art will understand that the steps of expanding may be performed in various ways using any expansion device (e.g. using a throttling valve, a flash valve or a common expander). 5 The person skilled in the art will readily understand that treated hydrocarbon streams may be further processed, if desired. Also, further intermediate processing steps between the first and third gas/liquid separator may be performed. 10 According to a preferred embodiment of the present invention the second gas/liquid separator is a distillation column. Accordingly, according to a preferred embodiment, the invention relates to a method of treating a hydrocarbon stream such as a natural gas 15 stream, containing > 10 mole% CO 2 , the method at least comprising the steps of: (a) supplying a partly condensed feed stream (10) containing > 10 mole% CO 2 to a first gas/liquid separator (2); 20 (b) separating the feed stream (10) in the first gas/liquid separator (2) into a gaseous stream (20) and a liquid stream (30); (c) expanding the liquid stream (30) obtained in step (b) and feeding it (40) into a distillation column 25 (3) at a first feeding point (31); (d) expanding the gaseous stream (20) obtained in step (b), thereby obtaining an at least partially condensed stream (50), and subsequently feeding it into the distillation column (3) at a second feeding point 30 (32), the second feeding point (32) being at a higher level than the first feeding point (31); (e) removing from the top of the distillation column (3) a gaseous stream (60), partially condensing it WO 2007/138067 PCT/EP2007/055225 -6 and feeding it (70) into a third gas/liquid separator (4); (f) separating the stream (70) fed in the third gas/liquid separator (4) in step (e) thereby obtaining a 5 liquid stream (80) and a gaseous stream (90); (g) feeding the liquid stream (80) obtained in step (f), having a temperature above the freezing point of CO 2 at the prevailing conditions, into the distillation column (3) at a third feeding point (33), 10 the third feeding point (33) being at a higher level than the second feeding point (32); and (h) removing from the bottom of the distillation column (3) a liquid stream (100, 100a); wherein the gaseous stream (60) removed from the 15 distillation column (3) in step (e) is partially condensed by heat exchanging against a cold stream (120). When the distillation column is provided with distillation trays, a distillation stage corresponds to one distillation tray, and when the column is provided 20 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 distillation column is defined relative to introducing another stream, there is 25 at least one distillation stage between the two levels, the same applies to defining the level of removing a stream from the distillation column. The top of the distillation column is that part of the distillation column that is located above the uppermost distillation 30 stage, and the bottom of the column is that part of the distillation column that is located below the lowermost distillation stage. According to another preferred embodiment of the present invention the cold stream is selected such that 35 the partially condensed stream obtained in step (e) has a WO 2007/138067 PCT/EP2007/055225 -7 temperature above the freezing point of CO 2 at the prevailing conditions, preferably greater than or equal to -60 0 C, more preferably a temperature from -60 0 C to -40 0 C, even more preferably a temperature from -60 0 C to 5 -45 0 C, even more preferably a temperature from -60 0 C to -50 0 C, in particular from -55 0 C to -50 0 C. Further it is preferred that the temperature of the liquid stream obtained in step (f) is preferably greater than or equal to -60 0 C, more preferably a temperature 10 from -60 0 C to -40 0 C, even more preferably a temperature from -60 0 C to -45 0 C, even more preferably a temperature from -60 0 C to -50 0 C, in particular from -55 0 C to -50 0 C near the third feeding point. The person skilled in the art will readily understand 15 that the freezing point of CO 2 at the prevailing conditions may vary dependent on the prevailing pressure. According to a particularly preferred embodiment the cold stream is obtained from a separate source of liquefied natural gas (LNG), preferably from an LNG 20 storage tank, more preferably located at an LNG import terminal. With 'separate source' for the cold stream is meant that preferably no cold stream is used that is generated during the treating itself or downstream of the treating. 25 Further it is preferred that > 75% by weight of the propane present in the partially condensed feed stream is recovered in the liquid stream obtained in step (h), preferably > 80, more preferably > 85, even more preferably > 90, most preferably > 95% by weight. 30 Also it is preferred that the partially condensed feed stream contains > 15 mole% C0 2 ; and preferably < 70 mole% C0 2 , more preferably < 50 mole% C0 2 , even more preferably < 25 mole% CO 2

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WO 2007/138067 PCT/EP2007/055225 -8 Furthermore the partially condensed feed stream preferably has a pressure > 20 bar, more preferably from 30 to 100 bar, even more preferably from 38 to 70 bar. Further it is preferred that the pressure in the 5 second gas/liquid separator is from 10 to 50 bar, preferably from 30 to 40 bar, more preferably from 30 to 37 bar, even more preferably from 33 to 36 bar, in particular about 35 bar. Typically, the pressure of any feed stream to the 10 second gas/liquid separator is in the range from 0 to 5 bar higher then the pressure in the second 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. 15 Although the gaseous stream obtained in step (f) may be used for various purposes it is preferably sent to a gas network. Alternatively, it may e.g. be liquefied thereby obtaining a liquefied hydrocarbon stream such as liquefied natural gas (LNG). Typically, the CO 2 20 concentration in the gaseous stream obtained in step (f) is so low (e.g. less than 50 ppmv) that the CO 2 will not freeze at the lower temperature encountered in liquefaction of the gaseous stream obtained in step (f). Further, the liquid stream removed from the bottom of 25 the second gas/liquid separator is preferably subjected to distillation thereby obtaining two or more distilled streams. In a special embodiment the partially condensed feed stream has been previously cooled against a cold stream, 30 preferably against a cold stream that has been obtained from a separate source of a liquefied hydrocarbon stream, in particular LNG, preferably obtained from an LNG storage tank, more preferably located at an LNG import terminal.

WO 2007/138067 PCT/EP2007/055225 -9 Hereinafter the invention will be further illustrated by the following non-limiting drawing. Fig. 1 schematically depicts a process scheme in accordance with the present invention. 5 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. Figure 1 schematically shows a process scheme 10 (generally indicated with reference no. 1) for the treating of a hydrocarbon stream such as natural gas whereby propane and heavier hydrocarbons are recovered to a certain extent. The process scheme of Figure 1 comprises a first 15 gas/liquid separator 2, a second gas/liquid separator 3 (in the form of a distillation column), a third gas/liquid separator 4, a first expander 6, a second expander 7 (in the from of a throttling valve), a first heat exchanger 8, a separate source 13 of a cold stream 20 (in the embodiment of Figure 1 an LNG storage tank at an LNG import terminal), a gas network 14 and a distillation unit 15. The person skilled in the art will readily understand that further elements may be present if desired. 25 During use, a partly condensed feed stream 10 containing natural gas is supplied to the inlet 21 of the first gas/liquid separator 2 at a certain inlet pressure and inlet temperature. Typically, the inlet pressure to the first gas/liquid separator 2 will be between 10 and 30 100 bar, preferably at or above 30 bar, more preferably at or above 38 bar and preferably below 90 bar, more preferably at or below 70 bar. The temperature will usually between 0 and -60 0 C. To obtain the partly condensed feed stream 10, it may have been pre-cooled in 35 several ways. In the embodiment of Figure 1, the feed WO 2007/138067 PCT/EP2007/055225 - 10 steam has been heat exchanged in heat exchanger 11 against stream 90 (to be discussed hereafter) and subsequently in heat exchanger 5 against stream 130 originating from the LNG storage tank 13. It goes without 5 saying that instead of stream 130 a common external refrigerant such as propane or another cooler such as an air or water cooler may be used. If desired the feed stream 10 may have been further pre-treated before it is fed to the first gas/liquid 10 separator 2. As an example, C0 2 , H 2 S and hydrocarbon components having the molecular weight of pentane or higher may also at least partially have been removed from the feed stream 10 before entering the first separator 2. In the first gas/liquid separator 2, the feed 15 stream 10 (fed at inlet 21) is separated into a gaseous overhead stream 20 (removed at first outlet 22) and a liquid bottom stream 30 (removed at second outlet 23). The overhead stream 20 is enriched in methane (and usually also ethane) relative to the feed stream 10. 20 The bottom stream 30 is generally liquid and usually contains some components that are freezable when they would be brought to a temperature at which methane is liquefied. The bottom stream 30 may also contain hydrocarbons that can be separately processed to form 25 liquefied petroleum gas (LPG) products. The stream 30 is expanded in the second expander 7 to the operating pressure of the distillation column 3 (usually about 35 bar) and fed into the same at the first feeding point 31 as stream 40. If desired a further heat exchanger (not 30 shown) may be present on line 40 to heat the stream 40. The second expander 7 may be any expansion device such as a common expander as well as a flash valve. The gaseous overhead stream 20 removed at the first outlet 22 of the first separator 2 is at least partially 35 condensed in the first expander 6 and subsequently fed as WO 2007/138067 PCT/EP2007/055225 - 11 stream 50 into the distillation column 3 at a second feeding point 32, the second feeding point 32 being at a higher level than the first feeding point 31. If desired a further heat-exchanging step may take place between the 5 first expander 6 and the second feeding point 32. If desired (and as indicated with dashed lines in Figure 1) the gaseous overhead stream 20 may be split into two streams; the 'additional' stream 20a may be expanded in expander 6a and fed into the distillation 10 column at a further feeding point 37. Preferably, the pressure in the distillation column 3 is from 10 to 50 bar, preferably from 30 to 40 bar, more preferably from 30 to 37 bar, even more preferably from 33 to 36 bar, in particular about 35 bar. 15 From the top of the distillation column 3, at first outlet 34, a gaseous overhead stream 60 is removed that is partially condensed in first heat exchanger 8 while heat exchanging it against cold stream 120 (originating from LNG storage tank 13), and is fed into third 20 gas/liquid separator 4 (at inlet 41) as stream 70. Preferably the amount, flow rate and temperature of the cold stream 120 are selected such that the partially condensed stream 70 has a temperature greater than or equal to -60 0, more preferably from -60 0 C to -40 0 C, 25 even more preferably a temperature from -60 0 C to -45 0 C, even more preferably a temperature from -60 0 C to -50 0 C, in particular from -55 0 C to -50 0 C. The stream 70 being fed into the third gas/liquid separator 4 at inlet 41 is separated thereby obtaining a 30 gaseous stream 90 (at outlet 42) and a liquid stream 80 (at outlet 43). The liquid stream 80 removed at outlet 43 is pumped via pump 9 and fed into the distillation column 3 at a third feeding point 33, the third feeding point 33 being WO 2007/138067 PCT/EP2007/055225 - 12 at a higher level than the second feeding point 32. Preferably the third feeding point 33 is at the top of the distillation column 3. Preferably, the temperature of the liquid stream 80 5 is at or above -60 0 C near the third feeding point 33, more preferably a temperature from -60 0 C to -40 0 C, even more preferably a temperature from -60 0 C to -45 0 C, even more preferably a temperature from -60 0 C to -50 0 C, in particular from -55 0 C to -50 0 C near the third feeding 10 point 33. Herewith it is prevented that the CO 2 freezes and blocks the system, while at the same time a high recovery of propane may be obtained in the distillation column 3. The gaseous stream 90 obtained at the outlet 42 of 15 the third gas/liquid separator 4 is forwarded to the gas network 14 after heat exchanging against the feed stream 10 in heat exchanger 11 and optionally compressing in compressor 12 (which is functionally coupled to first expander 6). 20 Usually, a liquid bottom stream 100 is removed from the second outlet 35 of the distillation column 3 and is subjected to one or more distillation steps in a distillation unit 15 to collect various natural gas liquid products. As the person skilled in the art knows 25 how to perform distillation steps, this is not further discussed here. If desired, and as shown in Figure 1, a part of the liquid bottom stream 100 may be returned to the bottom of the distillation column 3 as stream 110, the remainder of 30 stream 100 being indicated with stream 100a. Table I gives an overview of the pressures and temperatures of a stream at various parts in an example process of Fig. 1. Also the mole% of propane is indicated. The feed stream in line 10 of Fig. 1 comprised WO 2007/138067 PCT/EP2007/055225 - 13 approximately the following composition: 64% methane, 8% ethane, 4% propane, 2% butanes and pentane, 20% Co 2 and 2% N 2 . Other components such as H 2 S and H 2 0 were previously removed. TABLE I Line Pressure (bar) Temperature Mole% (OC) propane 10 39.3 -10 4.5 20 39.2 -10 3.8 30 39.2 -10 20.5 40 35.0 -12 20.5 50 35.0 -16 3.8 60 35.0 -39 0.2 70 35.0 -50 0.2 80 35.0 -50 0.6 90 35.0 -50 0.06 100 34.8 50 60.7 5 The same line-up as Fig. 1 was used, but instead a higher temperature for stream 70 was used, viz. -30 OC instead of -50 OC. It was found that according to the preferred embodiment of the present invention a significantly higher propane recovery (98%) was obtained 10 in stream 100a, whilst the same line-up with a higher temperature for stream 70 (viz. -30 OC) resulted in a propane recovery of only 28%. This is shown in Table II.

WO 2007/138067 PCT/EP2007/055225 - 14 TABLE II Component Molar Molar Molar fraction of fraction of fraction of stream 100a in stream 10 stream 100a Fig. 1 with a in Fig. 1 in Fig. 1 temperature of -30 OC for stream 70 Flow rate 9 0.66 0.33 [kmol/s] Methane 0.639 0.000 0.000 Ethane 0.077 0.012 0.007 Propane 0.045 0.607 0.346 i-Butane 0.011 0.148 0.213 Butane 0.009 0.125 0.221 i-Pentane 0.003 0.041 0.081 Pentane 0.002 0.026 0.051 % Propane 98% 28% recovery The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention. As an example, the compressors may comprise two or more compression stages. 5 Further, each heat exchanger may comprise a train of heat exchangers.

Claims

1. Method for treating a hydrocarbon stream containing > 10 mole% CO 2 , the method at least comprising the steps of: (a) supplying a partly condensed feed stream containing > 10 mole% CO 2 5 to a first gas/liquid separator; (b) separating the feed stream in the first gas/liquid separator into a gaseous stream and a liquid stream; (c) expanding the liquid stream obtained in step (b) and feeding it into a second gas/liquid separator at a first feeding point; 10 (d) expanding the gaseous stream obtained in step (b), thereby obtaining an at least partially condensed stream, and subsequently feeding it into the second gas/liquid separator at a second feeding point, the second feeding point being at a higher level than the first feeding point; (e) removing from the top of the second gas/liquid separator a gaseous is stream, partially condensing it and feeding it into a third gas/liquid separator; (f) separating the stream fed in the third gas/liquid separator in step (e) thereby obtaining a liquid stream and a gaseous stream; (g) feeding the liquid stream obtained in step (f), having a temperature above the freezing point of CO 2 at the prevailing conditions, into the second gas/liquid 20 separator at a third feeding point, the third feeding point being at a higher level than the second feeding point; and (h) removing from the bottom of the second gas/liquid separator a liquid stream; wherein the gaseous stream removed from the second gas/liquid separator in step 25 (e) is partially condensed by heat exchanging against a cold stream, which is selected such that the partially condensed stream obtained in step (e) has a temperature greater than or equal to - 60"C, above the freezing point of CO 2 at the prevailing conditions, and wherein all streams being fed into the second gas/liquid separator fully originate from the feed stream. 30

2. Method according to claim 1, wherein the hydrocarbon stream containing > 10 mole% C02 is a natural gas stream containing > 10 mole% CO 2 . - 16

3. Method according to claim I or 2, wherein the temperature of the liquid stream obtained in step (f) is at or above -60*C near the third feeding point.

4. Method according to any one of the preceding claims, wherein the cold stream is obtained from a separate source of liquefied natural gas (LNG).

5 5. Method according to any one of the preceding claims, wherein > 75% by weight of the propane present in the partially condensed feed stream is recovered in the liquid stream obtained in step (h).

6. Method according to any one of the preceding claims, wherein the partially condensed feed stream contains > 15 mole% CO 2 . 10

7. Method according to any one of the preceding claims, wherein the partially condensed feed stream has a pressure > 20 bar.

8. Method according to any one of the preceding claims, wherein the pressure in the second gas/liquid separator is in the range from 10 to 50 bar.

9. Method according to any one of the preceding claims, wherein the is gaseous stream obtained in step (f) is sent to a gas network.

10. Method according to any one of claims 1-8, wherein the gaseous stream obtained in step (f) is liquefied thereby obtaining a liquefied hydrocarbon stream.

11. Method according to any one of the preceding claims, wherein the liquid stream removed from the bottom of the second gas/liquid separator is subjected to 20 distillation thereby obtaining two or more distilled streams.

12. Method according to any one of the preceding claims, wherein the partially condensed feed stream has been previously cooled against a cold stream.

13. Method according to claim 12, wherein the cold stream has been obtained from a separate source of a liquefied hydrocarbon stream. 25

14. Method according to any one of the preceding claims, wherein the temperature of the liquid stream obtained in step (f) is at or above -55*C near the third feeding point.

15. Method according to any one of claims 1-13, wherein the temperature of the liquid stream obtained in step (f) is from -55*C to -50*C near the third feeding point. 30

16. Method according to any one of the preceding claims, wherein the cold stream is selected such that the partially condensed stream obtained in step (e) has a temperature from -60*C to -40*C. - 17

17. Method according to any one of the preceding claims, wherein the cold stream is selected such that the partially condensed stream obtained in step (e) has a temperature greater than or equal to -55"C.

18. Method according to claim 17, wherein the cold stream is selected such 5 that the partially condensed stream obtained in step (e) has a temperature of from -55"C to -50"C.

19. Method for treating a hydrocarbon stream containing >10 mole% CO 2 , substantially as hereinbefore described with reference to any one of the embodiments as that embodiment is shown in the accompanying drawings. 10 Dated 9 July, 2010 Shell Internationale Research Maatschappij B.V. Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON