PRIORITY CLAIM OF EARLIER NATIONAL APPLICATION

[0001] This application claims priority to U.S. Application No. 61/387,492 filed

September 29, 2010.

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 CO2 and H2S removal from natural gas, ¾ 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 well below 100%. For example, in the application of CO2 removal from natural gas using a commercial cellulose acetate (CA) membrane, methane and other hydrocarbons can be lost with CO2 in the permeate stream. Multistage membrane processes have been used

commercially to further recover the hydrocarbons from the permeate stream, as shown in FIG. 1 for a two-stage membrane process and FIG. 3 for a two-step membrane process. Depending 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 CO2 removal from natural gas, with generally above 99% hydrocarbon recovery, 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 value of hydrocarbons is high and its loss incurs a significant penalty, there exists a need to 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 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.

[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 membrane unit.

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

[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 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 CO2 concentration from the second stage membrane is typically kept close to the CC"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 with a residue CO2 concentration from the second stage membrane higher than the feed CO2 concentration. This, however, increases the size of the first stage membrane as well as the inter-stage compressor size and its power.

[0015] FIG. 1 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 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 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 CO2 concentration of the gas entering the second stage 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 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 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 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.

[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 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 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 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 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 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 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 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 CC"2 -rich stream for disposal or reinjection and a relatively hydrocarbon-rich stream for recycling back to the compressor.

[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 CO2, for example H2S, He or ¾(). The 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 CO2 composition of 12.5% (84.5% CH4, 1.5% C2Hg, 1 % C ₃ H ₈ and 0.5% N ₂ ), 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 CO2 content is down to 2%. A two-stage membrane process as shown in FIG. 1 is used to remove CO2 by keeping the CC"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 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 CO2 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 membrane process as shown in FIG. 2 with slightly higher compression power (Case lc). If the same feed stream is processed with the current invented process, the same hydrocarbon recovery, 98.4% can be achieved, but with 19%> less membrane area, while using equivalent compressor power (Case Id). 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 lb), while using 15% lower compressor power (Case le). 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

EXAMPLE 2

[0025] A natural gas stream with a C0 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. FC) is to be treated so that its C0 ₂ content is lowered to 1%. A two-stage membrane process as shown in FIG. 1 is used to remove C0 ₂ by keeping the C0 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 98.1%. 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 C0 ₂ composition from the residue of the second stage membrane at 21% (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).

Alternatively the current invented process allows increasing the hydrocarbon recovery to 99.2%) (22%o 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 2e), 15% more membrane area is required 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

Simulation results from Example 2

EXAMPLE 3

[0026] A natural gas stream with a C0 ₂ level of 35% (59% CH ₄ , 3% C ₂ H ₆ , 1 % C ₃ H ₈ and 2% N ), 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 C0 ₂ content is lowered to 8%. A two-stage membrane process as shown in FIG. 1 is used to remove C0 ₂ by keeping the C0 ₂ composition 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 CO2 composition from the residue of the second 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 3 c). A higher hydrocarbon recovery of 98.6% can be obtained using the alternative two-stage 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