AU2011314136B2

AU2011314136B2 - Two-stage membrane process

Info

Publication number: AU2011314136B2
Application number: AU2011314136A
Authority: AU
Inventors: Tom Cnop; Shain-Jer Doong; David Farr; George K. Xomeritakis
Original assignee: UOP LLC
Current assignee: Honeywell UOP LLC
Priority date: 2010-09-29
Filing date: 2011-09-27
Publication date: 2015-04-16
Anticipated expiration: 2031-09-27
Also published as: WO2012050816A3; CN103140571A; AU2011314136A1; BR112013006542A2; RU2013118555A; WO2012050816A2

Abstract

This invention comprises a two-stage process for purifying a gas, preferably natural gas. The process provides for a higher level of hydrocarbon recovery with lower compressor power consumption and a lower membrane area requirement than prior art processes.

Description

WO 2012/050816 PCT/US2011/053358 TWO-STAGE MEMBRANE PROCESS PRIORITY CLAIM OF EARLIER NATIONAL APPLICATION [0001] This application claims priority to U.S. Application No. 61/387,492 filed September 29, 2010. 5 BACKGROUND OF THE INVENTION [0002] This invention relates to a membrane process for purifying gases. More particularly, this invention relates to a more efficient two-stage process for purifying natural gas. [0003] Membrane processes have been widely used for bulk gas separation in the areas of 10 CO 2 and H 2 S removal from natural gas, H 2 recovery, air separation, etc. Gas separation membrane materials for current commercial applications are usually polymeric, which have somewhat limited selectivity for the gas species intended to be separated. As a result, complete separation of the components in the gas mixture is difficult to achieve. The product streams from a membrane process tend to be less than pure and product recovery is generally 15 well below 100%. For example, in the application of CO 2 removal from natural gas using a commercial cellulose acetate (CA) membrane, methane and other hydrocarbons can be lost with CO 2 in the permeate stream. Multistage membrane processes have been used commercially to further recover the hydrocarbons from the permeate stream, as shown in FIG. I for a two-stage membrane process and FIG. 3 for a two-step membrane process. Depending 20 on feed gas conditions, the hydrocarbon recovery for a two-stage membrane process will often exceed 95% and in some cases exceed 98% or even 99%. The major penalty is the cost of the added compressor and its associated power consumption. [0004] Solvent processes such as those based on amine solvents have also been widely used for CO 2 removal from natural gas, with generally above 99% hydrocarbon recovery, 25 although the (important) fuel consumption in the amine unit reboiler needs to be taken into account as well. However, compared to the solvent process, the membrane process provides various advantages such as ease of installation and operation in remote areas, reduced utility requirements, the elimination of a dehydration unit and for off-shore applications the absence of a sea motion effect which in the case of solvent processes can reduce efficiency. Since the 30 value of hydrocarbons is high and its loss incurs a significant penalty, there exists a need to - 1 develop an improved membrane process to increase the product recovery while reducing compressor power consumption. SUMMARY OF THE INVENTION [0005] The invention provides a process for purifying a hydrocarbon gas comprising 5 sending a gas stream through a first stage membrane unit to produce a hydrocarbon-enriched residue stream and a hydrocarbon-reduced permeate stream; compressing this permeate stream and sending it through a second stage membrane unit divided into a first section and a second section wherein each section produces a residue stream and a permeate stream and where the residue stream of the first section feeds the second section. 10 [0005a] The invention also provides a process for purifying a hydrocarbon gas comprising sending a gas stream through a first stage membrane unit to produce a first stage residue stream and a first stage permeate stream, compressing part or whole of said first stage permeate stream in a compressor to produce a compressed permeate stream, sending said compressed permeate stream through a second stage membrane unit having a first section and 15 a second section wherein said first section of said second stage membrane unit produces a first permeate stream and a first residue stream when said compressed permeate stream passes through said first section of said second stage membrane unit wherein said first residue stream is sent through said second section of said second stage membrane unit and said first permeate stream of said first section is sent to flare or used as a waste fuel and wherein a 20 second residue stream and a second permeate stream are produced when said compressed permeate stream passes through said second section of said second stage membrane wherein the second permeate stream is recycled to an inlet of said compressor and said second residue stream of said second section of said second stage membrane unit is recycled to an inlet of said first stage membrane unit. 25 [0006] The hydrocarbon gas may be natural gas or another hydrocarbon containing gas. The impurities that are removed with this invention include carbon dioxide, hydrogen sulfide, helium and water. [0007] In a preferred embodiment of the invention, the permeate from the second section of the second stage membrane unit is recombined with the permeate from the first stage 30 membrane unit. -2- BRIEF DESCRIPTION OF THE DRAWINGS [0008] FIG. 1 shows a two-stage membrane process based upon prior art systems. [0009] FIG. 2 shows an alternative two-stage membrane process based on prior art systems. 5 [0010] FIG. 3 shows a two-step membrane process based on prior art systems. [0011] FIG. 4 shows a three-stage membrane process based upon prior art systems. [0012] FIG. 5 shows a two-stage membrane process based upon the present invention. DETAILED DESCRIPTION OF THE INVENTION [0013] In a traditional two-stage membrane process, the permeate gas from the first stage 10 membrane unit is recompressed and sent to a second stage membrane unit. The residue stream from the second stage membrane unit is recycled and combined with the feed gas to the first stage membrane unit, whereas the second stage permeate gas is typically vented, flared, reinjected or used as fuel gas. In a traditional two-stage membrane process, the residue CO 2 concentration from the second stage membrane is typically kept close to the - 2a - WO 20121050816 PCT/US2011/053358

CO

2 concentration in the feed gas as this provides a good trade-off between compression power and membrane area. A two-step membrane process (FIG. 3) is an alternative set-up that is used in certain applications. [0014] If higher hydrocarbon recovery is desirable, one can operate the membrane system 5 with a residue CO 2 concentration from the second stage membrane higher than the feed CO 2 concentration. This, however, increases the size of the first stage membrane as well as the inter-stage compressor size and its power. [0015] FIG. I shows such a two-stage membrane process based upon the prior art. A feed 2 is shown going to a first stage membrane 4. The gas that does not pass through the first 10 stage membrane is the residue, shown as product 6. A permeate 8 that passes through the first stage membrane is compressed in compressor 10 to produce a compressed permeate 12 that then contacts a second stage membrane 14. Residue 16 from second stage membrane 14 is then shown being combined with feed 2. Permeate 18 that contains carbon dioxide and other impurities removed from feed 2 is the secondary product. It's not uncommon that a slip 15 stream of permeate 8 or permeate 18 is used as a source of fuel gas. [0016] Another option to increase the hydrocarbon recovery for the conventional two stage membrane process is to recycle a portion of the second stage permeate gas back to the compressor, as shown in FIG. 2. The recycled permeate gas, in addition to increasing the gas flow to the compressor, also raises the CO 2 concentration of the gas entering the second stage 20 membrane, which increases the size of the second stage membrane. In FIG. 2, as in FIG. 1, feed 2 is shown going to a first stage membrane 4 with a residue that does not pass through the first stage membrane shown as product 6. A permeate 8 that passes through the first stage membrane is compressed in compressor 10 to produce a compressed permeate 12 that then contacts a second stage membrane 14. Residue 16 from second stage membrane 14 is then 25 shown being combined with feed 2. Permeate 18 that contains carbon dioxide and other impurities removed from feed 2 is the secondary product. However, a portion of the permeate 18 is split off as a recycled permeate 20 that is combined with permeate 8 and then passes through the second stage membrane 14 again. [0017] FIG. 3 shows an alternate open art process which is used to increase hydrocarbon 30 recovery by splitting the first membrane stage into two sections and recycling permeate gas from the second section. A feed 2 is shown going to the first section 40 of the first stage membrane, producing a hydrocarbon enriched residue 41 and hydrocarbon reduced permeate -3- WO 20121050816 PCT/US2011/053358 18. The residue 41 feeds a second section 42, which produces a further hydrocarbon enriched product 6 and a hydrocarbon reduced permeate 43, which has a greater hydrocarbon content than the permeate from the first section 18. This second section permeate 43 is compressed in a compressor 10 to create a compressed gas 44 that is combined with the feed 2. 5 [0018] Another option to increase the hydrocarbon recovery is to recover some of the hydrocarbons from the second stage permeate in a third membrane unit as shown in FIG. 4 which illustrates a three stage membrane process that is based on the prior art schemes and differs from the present invention as shown in FIG. 5. In FIG. 4, a feed 2 is shown going to a first stage membrane 4 with a residue that does not pass through the first stage membrane 10 shown as product 6. A permeate 8 that passes through the first stage membrane is compressed in compressor 10 to produce a compressed permeate 12 that then contacts a second stage membrane 14. Residue 16 from second stage membrane 14 is then shown being combined with feed 2. Permeate 18 that contains carbon dioxide and other impurities removed from feed 2 is shown being sent from second stage membrane 14 to a second 15 compressor 30. A second compressed permeate 32 is then sent to third stage membrane 34 with residue 36 being sent from third stage membrane 34 to compressed permeate 12 and third stage membrane permeate 38 being the secondary product from the system. [0019] In the present invention, the second stage membrane unit of a two-stage membrane process is divided into two sections. The residue of the first section of the second 20 stage membrane is feeding the second section of the second stage membrane. The permeate from the first section of the second stage membrane contains less hydrocarbons than the permeate from the second section of the second stage membrane and so is more suitable for disposal or reinjection than the full second stage permeate from a standard two stage membrane process. The permeate from the second section of the second stage membrane 25 unit, which contains more hydrocarbons than the permeate from the first section is recycled back to the inlet of the compressor. The residue of the second section is recycled to the inlet of the first stage membrane, as in a traditional two-stage membrane configuration. FIG. 5 is a schematic showing this invention. A feed 2 is shown going to a first stage membrane 4 with a residue that does not pass through the first stage membrane shown as product 6. A 30 permeate 8 that passes through the first stage membrane is compressed in compressor 10 to produce a compressed permeate 12 that then contacts a first section 15 of a second stage membrane. Residue 22 from first section 15 of the second stage membrane is sent to a -4- WO 20121050816 PCT/US2011/053358 second section 25 of the second stage membrane. Residue 26 is shown combining with feed 2. Permeate 28 from second stage 25 of the second membrane is shown being combined with permeate 8 to be recycled through the two sections of the second stage membrane. Permeate 18 that has a lower hydrocarbon content than permeate 28 and contains carbon dioxide and 5 other impurities removed from feed 2 is the secondary product from the system. [0020] In comparison with the two-stage membrane process based on a prior art (FIGS. 1 and 2), this invention provides a way to split the second stage permeate gas into a relatively

CO

2 -rich stream for disposal or reinjection and a relatively hydrocarbon-rich stream for recycling back to the compressor. 10 [0021] Variations to the present invention are those where part of the first stage permeate, part of the permeate from the first section membrane or permeate from the second section membrane is used as a fuel gas, for example to drive the compressors. [0022] Other variations of the present invention are those where the present invention is used in the separation of other components than C0 2 , for example H 2 S, He or H 2 0. The 15 valuable stream can either be the product 6 or permeate 18. [0023] The following examples demonstrate the advantages of the current invention versus prior art processes. EXAMPLE 1 [0024] A natural gas stream with a CO 2 composition of 12.5% (84.5% CH 4 , 1.5% C 2

H

6 , 20 1% C 3

H

8 and 0.5% N2), a flow rate of 34 MMSCFD (911,000 Normal cubic meter/day) at 950 psig (6,550 kPag) and 129'F (53.9'C) is to be treated so that its CO 2 content is down to 2%. A two-stage membrane process as shown in FIG. 1 is used to remove CO 2 by keeping the CO 2 composition from the residue of the second stage membrane at 12.5% or the same as the feed (condition A). The obtained hydrocarbon recovery from simulation is 97.4% as 25 shown in Case la in the table below. In order to increase the hydrocarbon recovery, the process conditions are adjusted (condition B) to have a CO 2 composition from the residue of the second stage membrane at 34% (Case lb below). The obtained hydrocarbon recovery from simulation is 98.4%, which is a 38% reduction in hydrocarbon losses versus Case la. The same hydrocarbon recovery can also be obtained using the alternative two-stage 30 membrane process as shown in FIG. 2 with slightly higher compression power (Case Ic). If the same feed stream is processed with the current invented process, the same hydrocarbon -5- WO 20121050816 PCT/US2011/053358 recovery, 98.4% can be achieved, but with 19% less membrane area, while using equivalent compressor power (Case 1 d). Alternatively, using equal membrane area to the two-stage membrane process (condition B, Case lb), the current invention process can provide a recovery of 98.9% (30% reduction in hydrocarbon losses versus Case 1b), while using 15% 5 lower compressor power (Case 1 e). Concluding, the current invention process provides equivalent hydrocarbon recovery to open art processes using 19% less membrane area or reduces hydrocarbon losses by 30% while also using 15% lower compressor power consumption. TABLE 1: Simulation results from Example 1 Hydrocarbon Low heating Relative Relative Relative Case recovery, value of Hydrocarbon power membrane mol% disposal gas Losses consumption area (Btu/scf) la Prior art two-stage 97.4 160 100 100 100 (FIG. 1 condition A) lb Prior art two-stage 98.4 105 62 134 113 (FIG. 1 condition B) 1c Prior art two-stage 98.4 105 60 143 112 (FIG.2 alternative) 1d Current invention 98.4 105 60 136 91 (FIG. 5) Current invention le (FIG. 5) matching 98.9 75 43 114 115 140% relative compression power 10 EXAMPLE 2 [0025] A natural gas stream with a CO 2 level of 7% (90% CH 4 , 1.5% C 2

H

6 , 1% C 3

H

8 and 0.5% N 2 ), a flow rate of 500 MMSCFD (13,397,000 Normal cubic meter/day) at 950 psig (6,550 kPag) and 70'F (21 .1 C) is to be treated so that its CO 2 content is lowered to 1 %. A two-stage membrane process as shown in FIG. I is used to remove CO 2 by keeping the 15 CO 2 composition from the residue of the second stage membrane at 7% or the same as the feed (condition A). The obtained hydrocarbon recovery from simulation is 9 8 .1%. Results are shown as Case 2a in the table below. In order to increase the hydrocarbon recovery, the process conditions are adjusted to have a CO 2 composition from the residue of the second -6- WO 20121050816 PCT/US2011/053358 stage membrane at 210% (condition B). The obtained hydrocarbon recovery from simulation is 98.9% as shown for Case 2b in the table below. If the same feed stream is processed with the current invented process, the same hydrocarbon recovery, 98.9% can be achieved, but with a 12% lower compressor power consumption and 16% lower membrane area (Case 2c). 5 Alternatively the current invented process allows increasing the hydrocarbon recovery to 99.2% (22% lower hydrocarbon losses) for the same compression power while also using 12% less membrane area. Results for this case are shown below as Case 2d. A three-stage membrane process as shown in FIG. 4 can also be used to further increase the hydrocarbon recovery of 99.2%, but as shown in Table 2 (Case 2c), 15% more membrane area is required 10 and power consumption for the two compressors is equivalent to the current invention process. Furthermore, the three-stage process has the disadvantage of requiring two compressors as well as a separate third stage membrane unit which adds to the complexity of the process. TABLE 2 15 Simulation results from Example 2 Hydrocarbon Low heating Relative Relative Relative Case recovery, value of Hydrocarbon power membrane mol% disposal gas Losses consumption area (Btu/scf) 2a Prior art two-stage 98.1 205 100 100 100 (FIG. 1 condition A) 2b Prior art two-stage 98.9 130 58 122 108 (FIG. 2 condition B) 2c Current invention 98.9 130 58 107 90 (FIG. 5) 2d Current invention 99.2 105 45 121 95 (FIG. 5) 2e Prior art three- 99.2 105 45 121 109 stage (FIG. 4) EXAMPLE 3 [0026] A natural gas stream with a CO 2 level of 35% (59% CH 4 , 3% C 2

H

6 , 1% C 3

H

8 and 2% N 2 ), a flow rate of 200 MMSCFD (5,360,000 Normal cubic meter/day) at 600 psig (4,240 kPa) and 130'F (54.4'C) is to be treated so that its CO 2 content is lowered to 8%. A 20 two-stage membrane process as shown in FIG. 1 is used to remove CO 2 by keeping the CO 2 -7composition from the residue of the second stage membrane at 35% or the same as the feed (condition A). The obtained hydrocarbon recovery from simulation is 97.7%. Results are shown as Case 3a in the table below. In order to increase the hydrocarbon recovery, the process conditions are adjusted to have a CO 2 composition from the residue of the second 5 stage membrane at 60% (condition B). The obtained hydrocarbon recovery from simulation is 98.4% as shown for Case 3b in the table below. If the same feed stream is processed with the current invented process, the same hydrocarbon recovery, 98.4% can be achieved, but with a 13% lower compressor power consumption while using equivalent membrane area (Case 3c). A higher hydrocarbon recovery of 98.6% can be obtained using the alternative two-stage 10 membrane process as shown in FIG. 2 using higher compression power (Case 3d). The current invented process can achieve the same higher hydrocarbon recovery of 98.6% using equivalent compression power, but with 21% less membrane area (Case 3e). TABLE 3 Simulation results from Example 3 Hydrocarbon Low heating Relative Relative Relative Case recovery, value of Hydrocarbon power membrane mol% disposal gas Losses consumption area (Btu/scf) 3a Prior art two-stage 97.7 45 100 100 100 (FIG. 1 condition A) 3b Prior art two-stage 98.4 31 71 124 105 (FIG. 2 condition B) 3c Current invention 98.4 31 71 108 106 (FIG. 5) 3d Prior art two-stage 98.6 26 59 143 112 (FIG.2 alternative) 3e Current invention 98.6 26 59 143 88 (FIG. 5) 15 [0027] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification, they are to be interpreted as specifying the presence of the stated features, integers, steps or components referred to, but not to preclude the presence or addition of one or more other feature, integer, step, component or group thereof. 20 [0028] Further, any prior art citation or statement provided in the specification is not to be taken as an admission that such art constitutes, or is to be understood as constituting, part of the common general knowledge in Australia. -8-

Claims

1. A process for purifying a hydrocarbon gas comprising sending a gas stream through a first stage membrane unit to produce a first stage residue stream and a first stage 5 permeate stream, compressing part or whole of said first stage permeate stream in a compressor to produce a compressed permeate stream, sending said compressed permeate stream through a second stage membrane unit having a first section and a second section wherein said first section of said second stage membrane unit produces a first permeate stream and a first residue stream when said compressed permeate stream passes through said 10 first section of said second stage membrane unit wherein said first residue stream is sent through said second section of said second stage membrane unit and said first permeate stream of said first section is sent to flare or used as a waste fuel and wherein a second residue stream and a second permeate stream are produced when said compressed permeate stream passes through said second section of said second stage membrane wherein the second 15 permeate stream is recycled to an inlet of said compressor and said second residue stream of said second section of said second stage membrane unit is recycled to an inlet of said first stage membrane unit.

2. The process of claim 1, wherein said second residue stream is combined with said gas stream. 20

3. The process of claim 1 or claim 2, wherein said second permeate stream is combined with said first stage permeate stream.

4. The process of any one of claims 1 to 3, wherein the molar amount of hydrocarbons not recovered from said gas stream in said first stage residue stream is at least 5% lower compared to previous art processes with equivalent compression power 25 consumption wherein said second residue stream has a higher concentration of carbon dioxide than said gas stream.

5. The process of any one of claims 1 to 3, wherein said first stage residue stream comprises equivalent hydrocarbon gas recovered from said gas stream compared to previous art processes with said compressor consuming at least 5% less power than the comparison 30 previous art processes wherein said second residue stream has a higher concentration of carbon dioxide than said gas stream. -9-

6. The process of any one of claims 1 to 5, wherein the combined membrane area to achieve a targeted hydrocarbon recovery is at least 5% lower compared to previous art processes wherein said second residue stream has a higher concentration of carbon dioxide than said gas stream. 5

7. The process of any one of claims 1 to 6, wherein said gas stream comprises a natural gas or another hydrocarbon containing gas.

8. The process of any one of claims 1 to 7, wherein said permeate streams comprise at least one impurity selected from the group consisting of carbon dioxide, hydrogen sulfide, helium and water. 10

9. The process of any one of claims 1 to 3, wherein said first stage residue stream comprises at least 95% of said hydrocarbon gas from said gas stream on a molar basis.

10. The process of claim 8, wherein said at least one impurity is carbon dioxide.

11. A hydrocarbon gas purified by the process of any one of claims 1 to 10.

12. The process of any one of claims 1 to 10, substantially as hereinbefore described 15 with reference to any of the Examples and/or Figure 5. - 10 -