INTEGRATION METHODS OF GAS PROCESSING PLANT AND NITROGEN REJECTION UNIT FOR HIGH NITROGEN FEED GASES

Abstract

Gas processing plants and methods are contemplated in CO.sub.2 is effectively removed to very low levels from a feed gas to an NRU unit by adding a physical solvent unit that uses waste nitrogen produced by the NRU as stripping gas to produce an ultra-lean solvent, which is then used to treat the feed gas to the NRU unit. Most preferably, the physical solvent unit includes a flash unit and stripper column to produce the ultra-lean solvent.

Claims

1. A method of regenerating an ultra-lean solvent for deep CO.sub.2 removal of a treated feed gas, comprising: using an ultra-lean physical solvent in an absorber to remove CO.sub.2 from the treated feed gas to thereby form a CO.sub.2-loaded physical solvent and a CO.sub.2-depleted feed gas; removing in a nitrogen rejection unit N.sub.2 from the CO.sub.2-depleted feed gas to thereby produce a N.sub.2 waste stream and a pipeline gas; and regenerating the ultra-lean physical solvent from the CO.sub.2-loaded physical solvent in a process that includes flashing the CO.sub.2-loaded physical solvent and stripping the flashed CO.sub.2-loaded physical solvent using the N.sub.2 waste stream as a stripping gas.

2. The method of claim 1, wherein the ultra-lean physical solvent is a non-aqueous physical solvent.

3. The method of claim 2, wherein no dehydration of the CO.sub.2-depleted feed gas is required.

4. The method of claim 1, wherein the treated feed gas has a pressure of about 1,000 psig to 1,500 psig.

5. The method of claim 4, wherein the flashed CO.sub.2-loaded physical solvent has a pressure of equal to or less than 100 psig.

6. The method of claim 1, wherein the step of flashing comprises reducing a pressure of the CO.sub.2-loaded physical solvent in an amount of at least 1,000 psig.

7. The method of claim 1, wherein the CO.sub.2-depleted feed gas has equal or less than 3,000 ppmv CO.sub.2.

8. The method of claim 7, wherein the CO.sub.2-depleted feed gas has equal or less than 1,000 ppmv CO.sub.2.

9. The method of claim 1, wherein the treated feed gas has equal or less than 2 mol % CO.sub.2.

10. The method of claim 1, wherein the step of regenerating is performed without heating the CO.sub.2-loaded physical solvent.

11. The method of claim 1, wherein the step of regenerating is performed without heating the flashed CO.sub.2-loaded physical solvent.

12. The method of claim 1, further comprising: increasing a pressure of the N.sub.2 waste stream prior to using the N.sub.2 waste stream as a stripping gas.

13. The method of claim 12, wherein the N.sub.2 waste stream is increased to a pressure of about 5-50 psig.

14. The method of claim 1, further comprising: removing CO.sub.2 from a feed gas to produce the treated feed gas.

15. The method of claim 14, wherein the step of removing CO.sub.2 from a feed gas comprises contacting the feed gas with an amine solvent.

16. The method of claim 14, wherein the step of removing CO.sub.2 from a feed gas produces a CO.sub.2 loaded amine solvent, the method further comprising regenerating the CO.sub.2 loaded amine solvent.

17. The method of claim 14, wherein the feed gas is a hydrocarbonaceous feed gas.

18. The method of claim 14, wherein the feed gas has a N.sub.2 concentration of at least 10 mol %.

19. The method of claim 14, wherein the feed gas has a CO.sub.2 concentration of at least 2 mol %.

20. The method of claim 14, further comprising: increasing a pressure of the treated feed gas prior to removing CO.sub.2 from the treated feed gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Prior Art FIG. 1 is an exemplary configuration of a modified gas processing plant with a revamped amine unit for processing high nitrogen feed gas.

[0018] Prior Art FIG. 2 is another exemplary configuration of a modified gas processing plant with an additional amine unit for processing high nitrogen feed gas.

[0019] FIG. 3 is an exemplary configuration of a gas processing plant according to the inventive subject matter.

DETAILED DESCRIPTION

[0020] The inventor has discovered that CO.sub.2 can be effectively removed to very low levels from a feed gas to an NRU unit by adding a physical solvent unit that uses waste nitrogen produced by the NRU as stripping gas to produce an ultra-lean solvent, which is then used to treat the feed gas to the NRU unit. So treated gas will typically have equal or less than 0.001 mol % CO.sub.2 and can be fed to the NRU to produce a pipeline quality gas. Most typically, the feed gas has a relatively low CO.sub.2 concentration (e.g., 1-2 mol %) and has been subjected to a prior CO2 removal step as discussed in more detail below. While contemplated systems and methods can be employed in a grass roots installation, it should be appreciated that the inventive subject matter is particularly advantageous where an existing CO.sub.2 removal facility does not provide sufficient CO.sub.2 removal capability for a new or existing downstream NRU. Integration of the physical solvent unit will provide numerous advantages, including deep CO.sub.2 removal at capital requirement and low cost operation, simplified process flow, and elimination of an otherwise typically required drying step.

[0021] In contrast, a typical known gas processing configuration for nitrogen rejection is shown in Prior Art FIG. 1. Feed gas 1, typically at 1000 psig to 1500 psig, contains 15-20 mol % N.sub.2 and is treated in amine unit 51, producing a treated gas stream 2 with 1-2 mol % CO.sub.2, which is acceptable to avoid CO.sub.2 freezing in the NGL unit. However, if nitrogen rejection is required, the CO.sub.2 content must be further reduced to meet the CO.sub.2 specification by the downstream NRU 60, typically to 2000 ppmv or lower. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value with a range is incorporated into the specification as if it were individually recited herein. The treated gas 2 is dried in molecular sieve drier 52 forming dried stream 3, which is then processed in NGL recovery unit 53. The NGL recovery 53 unit can be designed for either propane recovery or ethane recovery. The NGL unit 53 produces an NGL stream 5 and a lean gas stream 4 that is compressed by a residue gas compressor 54 to a pressure of about 1000 psig to 1500 psig forming stream 6, which is then fed to the NRU 60 that produces N.sub.2 waste stream 14 that is vented to the atmosphere, and a N.sub.2 depleted stream 17 that is sent to the sales gas pipeline (most pipeline specifications require the gas to contain no more than 3 mol % nitrogen). As used herein, the term about in conjunction with a numeral refers to a range of that numeral starting from 20% below the absolute of the numeral to 20% above the absolute of the numeral, inclusive. For example, the term about 100 F. refers to a range of 80 F. to 120 F., inclusive, and the term about 100 psig refers to a range of 80 psig to 120 psig, inclusive.

[0022] The cold box and fractionation columns (not shown) in the NRU typically operate at very low temperatures, in most cases at 250 F. or lower, which means the residual CO.sub.2 content in the gas stream from a conventional amine unit that is ultimately delivered to the NRU exceeds levels at which CO.sub.2 freezing in the NRU becomes problematic (and renders the NRU inoperable). For this reason, amine unit 51 must be revamped for deeper CO.sub.2 removal. However, this may not be feasible if the amine unit is already operating at it maximum capacity, or will be expensive and thus an economically unattractive solution.

[0023] Alternatively, as shown in Prior Art FIG. 2 where like numerals depict like components as shown in Prior Art FIG. 1, a new amine unit 50 is used to treat the lean gas stream 4 from the NGL recovery unit 53 to reduce its CO.sub.2 content from, typically 1-2 mol % to 2000 ppm or lower, using a more aggressive amine such as DGA or activated MDEA. The treated gas from the new amine unit 50 is saturated with water and generally requires drying in a dehydration unit 55 (e.g., molecular sieve dehydration unit) producing a dry gas with CO.sub.2 content that is acceptable to the downstream NRU unit 60. Gas stream 6 is typically re-compressed by residue gas compressor 54 prior to entry into the NRU. Unfortunately, the addition of a new amine and dehydration renders such option often very costly. With respect to the remaining numerals, the same considerations for like components with like numerals as provided for Prior Art FIG. 1 apply.

[0024] In contrast, the configurations and methods according to the inventive subject matter will overcome the difficulties of known configurations in a conceptually simple and elegant manner that allows for implementation in a grass roots facility as well as in a retrofit. More particularly, a physical solvent unit is fluidly coupled between the amine unit and the NRU that reduces the residual CO2 concentration in the gas stream to a level acceptable for use in a NRU. In further preferred aspects, the physical solvent unit receives the gas stream that was previously subjected to CO2 removal after compression to a suitable pressure (e.g., pressure of the NRU or pipeline pressure).

[0025] For example, FIG. 3 exemplarily depicts a gas treatment plant for treatment of a feed gas that comprises CO.sub.2 and N.sub.2. Here, the hydrocarbonaceous feed gas 1 (e.g., from a oil and/or gas production well), typically at 1000 psig to 1500 psig, contains 15-20 mol % N.sub.2 and is treated in amine unit 51, producing a treated gas stream 2 with 1-2 mol % CO.sub.2, which is acceptable to avoid CO.sub.2 freezing in the NGL unit 53. However, with the feed gas composition of the example, nitrogen rejection is required to meet the CO.sub.2 specification by the downstream NRU 60 (e.g., in most cases 2000 ppmv or lower). The so treated feed gas 2 is dried in molecular sieve drier 52 (or other drying unit, e.g., glycol dryer) forming dried stream 3, which is then processed in NGL recovery unit 53. The NGL recovery 53 unit can be designed for propane recovery and/or ethane recovery as desired and produces an NGL stream 5 and a lean gas stream 4. The lean stream 4 is then compressed by residue gas compressor 54 to a pressure of about 1000 psig to 1500 psig forming stream 6, which is then fed to the physical solvent unit that receives a N.sub.2 waste stream from the NRU and that ultimately produces a CO.sub.2-depleted feed gas, a CO.sub.2-rich flash stream and N.sub.2-rich stripper overhead stream as further described in more detail below.

[0026] In the physical solvent unit, gas stream 6 having relatively high nitrogen content is fed to absorber 55 that is configured to receive an ultra-lean solvent 7. In especially preferred aspects, the ultra-lean solvent is a physical solvent (preferably propylene carbonate) having a very low residual CO.sub.2 content (typically no more than 0.1 mol %, and even more typically no more than 0.01 mol %). It should be appreciated that among other advantages, a particularly desirable technical effect of using an ultra-lean physical solvent on a previously decarbonized solvent (i.e., solvent from which CO.sub.2 was previously removed in a separate and distinct device) is that the residual CO2 concentration can be reduced more effectively at increased pressure due to Henry's law. Moreover, use of an ultra-lean solvent allows even further deep CO.sub.2 removal, which would not be achievable with a chemical solvent under elevated pressure. Still further, use of an ultra-lean physical solvent advantageously allows regeneration that does not require heating (particularly in combination with a N.sub.2 stripping step) and so eliminates or reduces greenhouse gas emissions associated with solvent recovery.

[0027] Absorber 55 produces a CO.sub.2-depleted feed gas as overhead stream 8 containing in some embodiments equal or less than 3,000 ppmv CO.sub.2, in other embodiments equal or less than 2,000 ppmv CO.sub.2, and in yet other embodiments equal or less than 1,000 ppmv CO.sub.2, as well as a CO.sub.2-loaded solvent 9 at relatively high pressure that is predominantly determined by the residue gas compressor 54. The CO.sub.2-loaded solvent 9 is then letdown in pressure in a hydraulic turbine 56 (or other suitable pressure reduction device) to about 50 psig, and is flashed via stream 10 to the separator 57, which produces a CO.sub.2 rich flash stream 11 that can be used as a low pressure fuel gas. Flashed solvent stream 12 is further letdown in pressure in JT valve 61 forming stream 13, which is fed to solvent stripper 58. The N.sub.2 waste stream 14 from the NRU 60 is used in stripping the flashed solvent stream to produce an ultra-lean solvent 16, which is pumped by pump 59 to about 1000 to 1500 psig forming stream 7 that is re-circulated to the absorber 55.

[0028] The CO.sub.2-depleted feed gas 8 from the physical solvent unit is further processed in the NRU 60, producing a pipeline gas 17 that now meets the pipeline specification (e.g., having equal or less than 3 mol %, and more preferably equal or less than 2 mol % nitrogen). At least a portion of the rejected nitrogen leaves the NRU 60 as N.sub.2 waste stream 18 that is compressed by nitrogen blower 62 (e.g., to about 5-50 psig) and to form stream 14 which is fed to the physical solvent unit as stripping gas in stripper 58. It should be appreciated that the N.sub.2 waste stream can be produced in the NRU at 5 to 10 psig such that nitrogen blower 62 may not be required. N.sub.2 stripper 58 uses the N.sub.2 waste stream as stripping gas and produces a N.sub.2 rich striper overhead stream 15 stream that can now be vented to the atmosphere or routed to sequestration.

[0029] While all physical solvents (and various non-physical solvents following Henry's law) are generally contemplated suitable for use herein, especially preferred physical solvents include FLUOR SOLVENT (propylene carbonate), NMP (normal-methyl pyrrolidone), SELEXOL (dimethyl ether of polyethylene glycol), and TBP (tributyl phosphate). As already noted above, physical solvents provide numerous advantages over chemical solvents and other CO.sub.2 removing processes (e.g., membrane separation, PSA, etc.) and especially allow increased solubilization of CO.sub.2 at increased pressure, the capability to remove dissolved CO.sub.2 by flashing without the need for steam regeneration. Moreover, where the physical solvent is a non-aqueous solvent, the CO.sub.2-depleted feed gas will not require a downstream dehydration unit.

[0030] Consequently, it is generally preferred that the absorber in the physical solvent unit will operate at elevated pressure, and contemplated elevated pressures include 500-700 psig, 700-1,000 psig, 1,000-1,500 psig, and even higher. Viewed from another perspective, it is generally preferred that the absorber operates at a pressure that is suitable for feeding the CO.sub.2-depleted feed gas directly into the NRU and/or pipeline without further need of re-compression. Thus, suitable pressures will be at least 700 psig, at least 1,000 psig, at least 1,200 psig, at least 1,500 psig, or even higher, but generally less than 3,000 psig. In some aspects, the absorber may even operate at supercritical pressures. Absorbers for physical solvents to capture CO.sub.2 are well known in the art, and all such absorbers are deemed suitable for use herein.

[0031] The so produced CO.sub.2-loaded solvent is preferably reduced in pressure to a pressure that allows flashing of the CO.sub.2 to so remove at least 50%, more preferably at least 70%, even more preferably at least 80%, and most preferably substantially all (i.e., greater of equal than 90%) of the previously dissolved CO.sub.2. Consequently, and depending on the absorber pressure and pressure reduction, the CO.sub.2-loaded solvent is reduced in pressure in an amount of at least 500 psig, more typically at least 700 psig, and most typically at least 1,000 psig. Viewed from a different perspective, the residual pressure in the flashed solvent will typically be between 0-50 psig, or between 50 and 100 psig, or between 20 and 20 psig.

[0032] There are numerous pressure reduction devices known in the art, and all of them are deemed suitable for use herein. However it is especially preferred (but not necessary) that the pressure reduction device is suitable to recover at least some energy. For example, suitable pressure reduction devices could be hydraulic turbines to reduce energy consumption (e.g., via generation of electrical energy, or mechanical energy for pumping flashed solvent, etc.). Therefore, it should be noted that one of the advantages of flashing the physical solvent is that significant quantities of CO.sub.2 can be removed without expenditure of heat energy while at the same time recovering or generating energy from the expansion step. Moreover, so flashed solvent is easily separated in a separator (flash vessel) and further processed while the CO.sub.2-rich flash stream can be routed to an incinerator or sequestration unit. Most typically, the CO.sub.2-rich flash stream will comprise at least 50 mol % CO.sub.2, in further embodiments at least 70 mol % CO.sub.2, and in still further embodiments at least 90 mol % CO.sub.2, with the remaining components mainly being N.sub.2, CH.sub.4, and inert compounds.

[0033] In further contemplated aspects of the inventive subject matter, the flashed solvent is then reduced in pressure, most typically to a residual pressure of between atmospheric pressure (zero psig) and 50 psig to allow for stripping with a low-pressure N.sub.2 waste stream from the NRU. In most instances, pressure reduction is performed across a JT-valve, but other pressure reduction devices are also deemed suitable for use herein. However, it should be noted that the flashed solvent may also be subjected to stripping without further pressure reduction (especially where the flashing step already produces a low-pressure flashed solvent). N.sub.2 stripping is in most cases performed in a conventional stripping column that receives the flashed and pressure-reduced solvent, and the nitrogen used for stripping is at least in part derived from the NRU. Depending on the particular N.sub.2 content of the hydrocarbonaceous feed gas, it is contemplated that at least 10%, at least 30%, at least 50%, at least 70%, or at least 90% of the N.sub.2 waste stream from the NRU is used to strip the flashed and pressure reduced solvent. The overhead product of the stripping column is typically vented to the atmosphere as N.sub.2-rich stripper overhead stream, but may also be further processed (e.g., via PSA, membrane process, etc.) or routed to sequestration.

[0034] Among other technical advantages it should be recognized that stripping of the flashed solvent with the N.sub.2 waste stream from the NRU not only produces an ultra-lean physical solvent having residual CO2 content of (typically no more than 0.1 mol %, and even more typically no more than 0.01 mol %), but also makes effective use of the N.sub.2 waste stream that would otherwise be vented to the atmosphere.

[0035] Such stripping is in significant contrast to U.S. Pat. No. 6,174,348 that teaches use of a chemical solvent, which must be regenerated with a steam reboiler as the solvent is a chemical solvent and as the flashing does not effectively removes the CO.sub.2 from the solvent. Moreover, the nitrogen is not provided from within the process (feed gas via the NRU) but obtained from an external air separation unit that must be collocated with the stripper, thereby further adding to the energy inefficiency of this system. Lastly, the '348 system also fails to recover energy from the flashing that could be at least in part used to drive the ultra-lean solvent as described in FIG. 3.

[0036] It should also be appreciated that contemplated plants and methods can be implemented in a grass roots installation or as a retrofit to an already existing acid gas/nitrogen removal unit. With respect to the primary CO.sub.2 removal unit in contemplated plants and methods it is generally contemplated that any type of acid gas removal unit is suitable for use herein (e.g., solvent based, membrane-based, sorbent based, etc.), however, chemical solvent-based units are particularly preferred. Therefore, in most aspects of the inventive subject matter, an amine absorber and amine solvent regenerator will be used to reduce CO.sub.2 concentration in the feed gas from >10 mol % to 1-5 mol % CO.sub.2, and more typically 1-2 mol % CO.sub.2. Secondary CO.sub.2 removal units will preferably use a physical solvent unit that reduces CO.sub.2 concentration from 1-2 mol % CO.sub.2 to equal or less than 3,000 ppm, equal or less than 2,000 ppm, equal or less than 1,000 ppm, and even lower. As noted before, it should therefore be appreciated that where the physical solvent unit reduces CO.sub.2 by flash and N.sub.2-stripping, an ultra-lean solvent can be produced in a simple and energy efficient manner, particularly where the nitrogen stream is obtained from the same NRU that is used to reduce the nitrogen concentration of the feed gas.

[0037] Consequently, the inventor also contemplates a method of processing a feed gas that includes CO.sub.2 and N.sub.2 in which CO.sub.2 is removed from the feed gas in a primary CO.sub.2 removal unit to a first CO.sub.2 concentration (e.g., 1-5 mol %, or 1-3 mol %, or 1-2 mol %, or 0.5-2 mol %) to so form a treated feed gas, and in which an ultra-lean solvent is used in a secondary CO.sub.2 removal unit to further remove CO.sub.2 from the treated feed gas to so form a CO.sub.2-loaded solvent and a CO.sub.2-depleted feed gas having a second CO.sub.2 concentration (equal to or less than 3,000 ppmv CO.sub.2, equal to or less than 2,000 ppmv CO.sub.2, equal to or less than 1,000 ppmv CO.sub.2, or between 2,000 and 200 ppmv CO.sub.2, or between 2,000 and 500 ppmv CO.sub.2). Nitrogen is then removed from the CO.sub.2-depleted feed gas in a NRU to thereby produce a N.sub.2 waste stream and a pipeline gas, and at least a portion of the N.sub.2 waste stream is used as a stripping gas in the secondary CO.sub.2 removal unit to thereby produce the ultra-lean solvent from the CO.sub.2-loaded solvent.

[0038] Therefore, in view of the above and attached figures, it should also be appreciated that the inventors contemplate a method of regenerating an ultra-lean solvent for deep CO.sub.2 removal of a treated feed gas that has equal or less than 2 mol % CO.sub.2 in which an ultra-lean physical solvent is used in an absorber to remove CO.sub.2 from the treated feed gas to thereby form a CO.sub.2-loaded solvent and a CO.sub.2-depleted feed gas. Nitrogen is then removed from the CO.sub.2-depleted feed gas in a NRU to thereby produce a N.sub.2 waste stream and a pipeline gas, and the ultra-lean physical solvent is then regenerated from the CO.sub.2-loaded solvent via flashing the CO.sub.2-loaded solvent and stripping the flashed CO.sub.2-loaded solvent using the N.sub.2 waste stream as a stripping gas.

[0039] Thus, specific embodiments and applications for the configuration of acid gas removal unit and nitrogen rejection unit have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the present disclosure. Moreover, in interpreting the specification and contemplated claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms comprises and comprising should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.