Natural Gas Liquids Recovery From Pressure Swing Adsorption and Vacuum Swing Adsorption

Abstract

Methods and systems for continuous pressure swing adsorption separation of a pressurized feed gas stream, the method including separating hydrocarbons heavier than methane from the pressurized feed gas stream to produce at least two product streams, a first product stream being substantially pure methane, and a second product stream being substantially comprised of components with a greater molecular weight than methane.

Claims

1. A method for continuous pressure swing adsorption separation of a pressurized feed gas stream, the method comprising the step of: separating hydrocarbons heavier than methane from the pressurized feed gas stream to produce at least two product streams, a first product stream being substantially pure methane suitable for transport by natural gas pipeline, and a second product stream being substantially comprised of components with a greater molecular weight than methane.

2. The method according to claim 1, wherein the step of separating includes a feed step carried out at a pressure between about 50 psia and about 500 psia to produce the stream being substantially pure methane.

3. The method according to claim 1, wherein the step of separating hydrocarbons can include recovering ethane in the first product stream being substantially pure methane or can include recovering ethane in the second product stream being substantially comprised of components with a greater molecular weight than methane.

4. The method according to claim 3, including the step of recovering ethane in the second product stream being substantially comprised of components with a greater molecular weight than methane, and further including the step of separating the ethane from propane and butane via pressure swing adsorption.

5. The method according to claim 1, wherein the method uses at least two fluidly coupled trains of PSA units, and further comprises the step of separating components of the second product stream being substantially comprised of components with a greater molecular weight than methane, including propane and butane.

6. The method according to claim 5, wherein each of the pressure swing adsorption trains of PSA units includes more than one individual adsorption bed.

7. The method according to claim 1, wherein the step of separating hydrocarbons comprises lengthening an amount of time of a product purge step in a PSA cycle to better separate two or more components which have similar desorption fronts within an adsorption bed.

8. The method according to claim 1, wherein the step of separating includes adjusting separation parameters such that CO.sub.2 in the pressurized feed gas stream can be separated to be in either the first product stream being substantially pure methane, or the second product stream being substantially comprised of components with a greater molecular weight than methane.

9. The method according to claim 1, wherein the step of separating can be carried out in a turndown mode, where the turndown mode is reduced by at least 50% relative to designed separation capacity, and still produce the first product stream being substantially pure methane, and the second product stream being substantially comprised of components with a greater molecular weight than methane.

10. The method according to claim 1, the method further comprising a bed-to-tank-to-bed equalization step, the bed-to-tank-to-bed equalization step reducing an amount of adsorbent beds required in the method, where a tank is a pressurizable vessel that does not contain any adsorbent and serves as an intermediate transit vessel for the gas moving from one bed to another bed.

11. The method according to claim 1, wherein the step of separating includes steps selected from the group consisting of: a feed step; an equalization down step; cocurrent depressurization occurring before, in between or after the equalization down step; countercurrent depressurization; light reflux; an equalization up step; and light product pressurization.

12. The method according to claim 1, where the first product stream being substantially pure methane is obtained from a single pass of the pressurized feed gas stream through a PSA bed system without recycle or additional PSA separation of the first product stream being substantially pure methane.

13. The method according to claim 1, where the step of separating is carried out at about ambient temperature without units for heating or cooling.

14. The method according to claim 1, where the substantially pure methane suitable for transport by natural gas pipeline is suitable for transport to and use by consumers.

15. The method according to claim 1, wherein the step of separating includes adjusting separation parameters such that CO.sub.2 in the pressurized feed gas stream is separated into the first product stream, such that the second product stream comprises less than about 500 ppm of CO.sub.2 by volume in liquid, less than about 0.5 mol. % methane, and such that the second product stream comprises substantially hydrocarbon components with a greater molecular weight than methane.

16. The method according to claim 1, wherein the step of separating includes the steps of: a feed step; a plurality of equalization down steps; a plurality of equalization up steps; cocurrent depressurization occurring before, in between, or after the equalization steps; countercurrent depressurization; light reflux; heavy reflux; and light product pressurization.

17. A pressure swing adsorption system, the system comprising: a plurality of adsorbent beds, the adsorbent beds comprising adsorbents comprising carbon, wherein the pressure swing adsorption system is operable to continuously and simultaneously separate components of a raw natural gas stream into a substantially pure methane stream and a product stream being substantially comprised of components with a greater molecular weight than methane.

18. The system according to claim 17, the system further comprising a second plurality of adsorbent beds, the adsorbent beds comprising adsorbents comprising carbon, and the second plurality of adsorbent beds operable to separate the components of the product stream being substantially comprised of components with a greater molecular weight than methane.

19. The system according to claim 17, wherein the substantially pure methane stream is obtained from a single pass of the raw natural gas stream passing through the pressure swing adsorption system without recycle or additional PSA separation of the substantially pure methane stream.

20. The system according to claim 17, where the system operates at about ambient temperature without units for heating or cooling.

21. The system according to claim 17, the system further comprising a tank operable for a bed-to-tank-to-bed equalization step, where a tank is a pressurizable vessel that does not contain any adsorbent and serves as an intermediate transit vessel for the gas moving from one bed to another bed.

22. A method for continuous pressure swing adsorption separation of a pressurized feed gas stream, the method comprising the step of: separating hydrocarbons heavier than a target hydrocarbon from the pressurized feed gas stream to produce at least two product streams, a first product stream being substantially pure target hydrocarbon, and a second product stream being substantially comprised of components with a greater molecular weight than the target hydrocarbon.

23. The method according to claim 22, wherein inlet pressure for the pressurized feed gas stream can range from about 30 psia to about 250 psia.

24. The method according to claim 22, wherein inlet temperature is between about 278 K to about 348 K.

25. The method according to claim 22, wherein the second product stream comprises at least about 90 mol. % hydrocarbons heavier than ethane, substantially no CO.sub.2, and no more than about 0.5 mol. % of methane and about 0.5 mol. % of ethane.

26. The method according to claim 22, wherein the step of separating includes the following PSA steps: a feed step; a heavy reflux step; at least one equalization down step; a countercurrent depressurization step; a light reflux step; at least one equalization up step; and a light product pressurization step.

27. A pressure swing adsorption system, the system comprising: a plurality of adsorbent beds, wherein the pressure swing adsorption system is operable to continuously and simultaneously separate components of a pressurized feed gas stream into a substantially pure target hydrocarbon stream and a product stream being substantially comprised of components with a greater molecular weight than the target hydrocarbon stream.

28. The system according to claim 27, wherein the adsorbent beds comprise at least one material selected from the group consisting of: carbon-based adsorbents; silica gels; activated aluminas; zeolite imidazole frameworks (ZIFs); metal organic frameworks (MOFs); molecular sieves; other zeolites; and combinations thereof.

29. The system according to claim 27, wherein inlet temperature is between about 278 K to about 348 K.

30. The system according to claim 27, wherein the system includes 6 adsorption beds operating with 9 cycle steps and includes: a feed step; a heavy reflux step; two equalization down steps; a countercurrent depressurization step; a light reflux step; two equalization up steps; and a light product pressurization step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following descriptions, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the disclosure and are therefore not to be considered limiting of the disclosure's scope as it can admit to other equally effective embodiments.

[0034] FIG. 1A shows a schematic of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0035] FIG. 1B shows a graphic representation of the steps occurring in separate beds during a PSA cycle for certain unit steps shown in FIG. 1A.

[0036] FIG. 2A shows a schematic of an example PSA cycle step schedule using 7 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0037] FIG. 2B shows a graphic representation of the steps occurring in separate beds during a PSA cycle for certain unit steps shown in FIG. 2A.

[0038] FIG. 3A shows a schematic of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0039] FIG. 3B shows a graphic representation of the steps occurring in separate beds during a PSA cycle for certain unit steps shown in FIG. 3A.

[0040] FIG. 4A shows a schematic of an example PSA cycle step schedule using 7 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0041] FIG. 4B shows a graphic representation of the steps occurring in separate beds during a PSA cycle for certain unit steps shown in FIG. 4A.

[0042] FIG. 5A shows a schematic of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0043] FIG. 5B shows a graphic representation of the steps occurring in separate beds during a PSA cycle for certain unit steps shown in FIG. 5A.

[0044] FIG. 6A shows a schematic of an example PSA cycle step schedule using 7 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0045] FIG. 6B shows a graphic representation of the steps occurring in separate beds during a PSA cycle for certain unit steps shown in FIG. 6A.

[0046] FIG. 7A shows a schematic of an example PSA cycle step schedule using 5 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0047] FIG. 7B shows a graphic representation of the steps occurring in separate beds during a PSA cycle for certain unit steps shown in FIG. 7A.

[0048] FIG. 8 shows a schematic of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0049] FIG. 9 shows a schematic of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

[0050] FIG. 10 shows a schematic of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons.

DETAILED DESCRIPTION

[0051] So that the manner in which the features and advantages of the embodiments of systems and methods of natural gas liquids recovery from pressure swing adsorption and vacuum swing adsorption as well as others, which will become apparent, may be understood in more detail, a more particular description of the embodiments of the present disclosure briefly summarized previously may be had by reference to the embodiments thereof, which are illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only various embodiments of the disclosure and are therefore not to be considered limiting of the present disclosure's scope, as it may include other effective embodiments as well.

[0052] Referring first to FIG. 1A, a schematic is provided of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons. In FIG. 1A proceeding from left to right, the individually labeled blocks, such as Feed for example, represent cycle steps, where the time for a cycle step (some amount of unit step(s) as shown in the Figures) can range from about 5 seconds or about 15 seconds to many minutes in duration. As well, the duration of each cycle step can also vary depending on the separation to be carried out. In the present disclosure, the quantity of unit steps can vary and the time period for each unit step and cycle step can vary. Depending on the unit steps and cycle steps, idle steps may or may not be part of a PSA method or system.

[0053] Referring to Beds 1-6, which include at least one adsorbent material that is selective for hydrocarbons, for example an adsorption bed comprising a heterogeneous high surface area carbon-containing adsorbent, a first step labelled Feed is carried out at a constant, high pressure, optionally the highest-available pressure of the PSA cycle. A light product stream containing lighter species, such as for example methane and/or ethane, is produced also at a high pressure, optionally about the highest-available pressure of a PSA cycle. Heavier hydrocarbon components and other components with a molecular weight greater than methane are adsorbed to the adsorbent at high pressure. For example, the Feed step in the present disclosure can be carried out at between about 689 kPa (50 psia) and about 3,447 kPa (500 psia). The temperature of a gas composition at the Feed step in embodiments of the present disclosure can be between about 278 K to about 318 K, about 278 K to about 348 K, or between about 278 K to about 323 K.

[0054] In certain embodiments, the adsorbent is selected from a group including, but not limited to, zeolites, activated carbon, silica gel, and alumina. In some embodiments, activated, porous carbon particles derived from low-cost carbon sources are used as an adsorbent. Highly-microporous carbon particles advantageously have a much higher surface area than typical activated carbon. In another embodiment, the adsorbent can be carbon-based molecular sieves. In other embodiments, the adsorbent can include, or not include, metal-oxide based molecular sieves or metal organic frameworks. In certain embodiments, the adsorbent can include nanoparticles. The adsorbent material can be presented in a variety of physical forms, including but not limited to powders, beads, pellets, granules, rods, and coatings on sheets or encapsulated between metal fibers. The adsorbent material should have a large working capacity observed for hydrocarbons, such as methane and ethane, especially in a system operating between about 100 kPa to about 3500 kPa (about 14.7 psia to about 500 psia). Separate beds may use the same adsorbent materials or different adsorbent materials. Separate trains of PSA beds may use the same adsorbent materials in one or more layers within each bed or different adsorbent materials in one or more layers within each bed.

[0055] Still referring to Bed 1 in FIG. 1A, after the Feed step, next two consecutive light end equalization down steps, denoted by Eqd1 and Eqd2, are carried out from the light end of the bed to reduce the pressure of the bed and enrich the bed with heavier species as they desorb from the adsorbent material. Next, a countercurrent depressurization step, denoted by Cnd, is carried out, in which gas is withdrawn from the feed end of the bed to constitute a heavy product while the pressure of the bed reaches the lowest pressure, or close to the lowest pressure, of the PSA cycle. The lowest pressure in the PSA process cycle in embodiments of the disclosure here can be about 1 psia or about 1.5 psia. Vacuum may or not be applied to increase heavy product recovery. Afterwards, a light reflux step, denoted by LR, is carried out at a constant low pressure, optionally, not necessarily, the lowest-available pressure of the PSA cycle, during which a small fraction of the light product stream containing the lighter species is fed into the light end of a bed to produce additional heavy product enriched in the heavier species.

[0056] Next, two consecutive light end equalization up steps, denoted by Equ2, Equ1, are carried out through the light end that individually take all the gas coming from light end equalizations down steps, (Eqd1, Eqd2), taking first the gas coming from the last down equalization step Eqd2 (for example at Bed 4) and taking last the gas coming from the first down equalization step Eqd1 (for example at Bed 3), resulting in each case with a partial re-pressurization of Bed 1. Afterward, a light product pressurization step, denoted by LPP, is carried out, wherein a small fraction of the light product stream containing the lighter species is fed into the light end of the bed to finalize the re-pressurization of the bed to the highest pressure prior to starting the Feed step corresponding to the next cycle.

[0057] FIG. 1B shows a graphic representation of the steps in a PSA cycle for certain unit steps in FIG. 1A. In a bed undergoing a Feed step at high pressure, feed stream 100 enters a bed, thereby producing a light end stream 102 at the light end of the bed that is enriched with the lighter species and at essentially the pressure of the feed stream. A light product stream 104 is withdrawn and a portion of light end stream 102 is withdrawn for light reflux stream 106, and a portion of light end stream 102 is withdrawn for light product pressurization stream 108. During a first equalization down in a bed (Eqd1) a first equalization up occurs in another bed (Equ1) shown by stream 110, and during a second equalization down in a bed (Eqd2) a second equalization up occurs in another bed (Equ2) shown by stream 112.

[0058] During light reflux, light reflux stream 106 drives heavy product at low pressure via stream 114, and this is combined with heavy product from countercurrent depressurization in stream 116. A heavy product stream at the heavy (feed) end of a bed that is enriched with the heavier species leaves a bed at pressures ranging between the feed pressure and the lowest pressure of the cycle, which may be less than atmospheric pressure with the aid of a vacuum pump.

[0059] The process may utilize any arbitrary number of equalization down steps with the same number of corresponding equalization up steps. In some embodiments, equalization tanks without adsorbent material are used to reduce the required number of adsorbent beds, and the number of equalization tanks mediating an equalization step is either equal to the number of down equalization steps or equal to that number minus one. An increase in the number of adsorbent beds used and/or equalization tanks used can lead to an increase in the number of equalization steps used.

[0060] Referring now to FIG. 2A, a schematic is provided of an example PSA cycle step schedule using 7 beds to achieve production of a substantially pure methane product, for example suitable for pipeline transport and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons. FIG. 2A is similar to the configuration shown in FIG. 1A, with similarly labelled cycle steps meaning the same as that described for FIG. 1A, except that a 7.sup.th bed is shown, and an additional equalization up step Equ3 and an additional equalization down step Eqd3 are shown as part of the process. In other configurations, more or fewer than 6 or 7 beds can be used with or without any number of equalization tanks, where the equalization tanks do not contain adsorbent material, but help reduce the number of required adsorbent beds for a given separation. In addition to countercurrent depressurization, cocurrent depressurization steps also can be utilized.

[0061] FIG. 2B shows a graphic representation of the steps in a PSA cycle for certain unit steps in FIG. 2A. FIG. 2B is similar to the configuration shown in FIG. 1B, with similarly labelled units being the same as that described for FIG. 1A, except that with a 7.sup.th bed as shown in FIG. 2A an additional equalization up step Equ3 and an additional equalization down step Eqd3 are shown as part of the process, with stream 118 in FIG. 2B.

[0062] Referring now to FIG. 3A, a schematic is provided of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons. FIG. 3A is similar to the configuration shown in FIGS. 1A and 2A, with similarly labelled cycle steps meaning the same as that described for FIGS. 1A and 2A, except that an additional heavy reflux step HR is shown as part of the process. In other configurations, more or fewer than 6 or 7 beds can be used with or without any number of equalization tanks, where the equalization tanks do not contain adsorbent material, but help reduce the number of required adsorbent beds for a given separation. In addition to countercurrent depressurization, cocurrent depressurization steps also can be utilized.

[0063] FIG. 3B shows a graphic representation of the steps in a PSA cycle for certain unit steps in FIG. 3A. FIG. 3B is similar to the configurations shown in FIGS. 1B and 2B, with similarly labelled units being the same as that described for FIG. 1A, except that with a heavy reflux step as shown in FIG. 3A, an additional heavy reflux step HR is shown as part of the process, with stream 120 in FIG. 3B showing a portion of gas from the light reflux step in one bed proceeding for use in the HR step in another bed. After heavy reflux, product stream 122 returns light product to light end stream 102. In general, reflux steps such as light reflux and heavy reflux are used in pressure swing adsorption processes to help produce products at greater recovery rates and at greater purity. In FIG. 3B a compressor pump 124 is shown to indicate that stream 120 is pressurized to ultimately produce product stream 122 which comprises light product at high pressure. Also shown is optional vacuum pump 126 which can apply a vacuum to stream 116 and a bed in which countercurrent depressurization is taking place to produce heavy product at low pressure.

[0064] One of ordinary skill in the art will understand other compressor and vacuum pumps can be applied as necessary between beds to create desired pressure swings within a pressure swing system during operation. In certain embodiments of systems and methods of the present disclosure, vacuum pumps and applied vacuum is optional.

[0065] Referring now to FIG. 4A, a schematic is provided of an example PSA cycle step schedule using 7 beds to achieve production of a substantially pure methane product, for example suitable for pipeline transport and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons. FIG. 4A is similar to the configuration shown in previous figures, for example FIG. 3A, with similarly labelled cycle steps meaning the same as that described for previous figures, except that an additional idle step I is shown as part of the process. In other configurations, more or fewer than 6 or 7 beds can be used with or without any number of equalization tanks, where the equalization tanks do not contain adsorbent material, but help reduce the number of required adsorbent beds for a given separation. In addition to countercurrent depressurization, cocurrent depressurization steps also can be utilized.

[0066] FIG. 4B shows a graphic representation of the steps in a PSA cycle for certain unit steps in FIG. 4A. FIG. 4B is similar to the configuration shown in FIGS. 1B, 2B, and 3B, with similarly labelled units being the same as that described for previous figures. As noted, FIG. 4B represents an idle step I also shown in FIG. 4A. In some embodiments, an optional idle step is used to allow other beds in a PSA system to match up for sequencing purposes. An idle step is a period of time in a PSA cycle where a bed is not producing gas, regenerating, or adsorbing gas.

[0067] Referring now to FIG. 5A, a schematic is provided of an example PSA cycle step schedule using 6 beds to achieve production of a substantially pure methane product, for example suitable for pipeline transport and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons. FIG. 5A is similar to the configurations shown in previous figures, with similarly labelled cycle steps meaning the same as that described for previous figures. In other configurations, more or fewer than 6 or 7 beds can be used with or without any number of equalization tanks, where the equalization tanks do not contain adsorbent material, but help reduce the number of required adsorbent beds for a given separation. In addition to countercurrent depressurization, cocurrent depressurization steps also can be utilized.

[0068] FIG. 5B shows a graphic representation of the steps in a PSA cycle for certain unit steps in FIG. 5A. FIG. 5B is similar to the configuration of previously labeled figures, with similarly labelled units being the same as that described for previous figures.

[0069] Referring now to FIG. 6A, a schematic is provided of an example PSA cycle step schedule using 7 beds to achieve production of a substantially pure methane product, for example suitable for pipeline transport or consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons. FIG. 6A is similar to the configuration shown in previous figures, with similarly labelled cycle steps meaning the same as that described for previous figures. In other configurations, more or fewer than 6 or 7 beds can be used with or without any number of equalization tanks, where the equalization tanks do not contain adsorbent material, but help reduce the number of required adsorbent beds for a given separation. In addition to countercurrent depressurization, cocurrent depressurization steps also can be utilized.

[0070] FIG. 6B shows a graphic representation of the steps in a PSA cycle for certain unit steps in FIG. 6A. FIG. 6B is similar to the configuration of previously labeled figures, with similarly labelled units being the same as that described for previous figures. FIG. 6B includes stream 113 which shows a transfer of gas from one bed to another during Eqd3 and Equ3.

[0071] Referring now to FIG. 7A, a schematic is provided of an example PSA cycle step schedule using 5 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline or consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons. FIG. 7A is similar to the configuration shown in previous figures, with similarly labelled cycle steps meaning the same as that described for previous figures. In other configurations, more or fewer than 5 beds can be used with or without any number of equalization tanks, where the equalization tanks do not contain adsorbent material, but help reduce the number of required adsorbent beds for a given separation. In addition to countercurrent depressurization, cocurrent depressurization steps also can be utilized.

[0072] FIG. 7B shows a graphic representation of the steps in a PSA cycle for certain unit steps in FIG. 7A. FIG. 7B is similar to the configuration of previously labeled figures, with similarly labelled units being the same as that described for previous figures.

[0073] FIGS. 8-10 show schematics of example PSA cycle step schedules using 6 beds to achieve production of a substantially pure methane product, for example suitable for transport in a pipeline and consumer use, or to achieve production of a substantially pure target molecular weight hydrocarbon separated from other higher molecular weight hydrocarbons. In FIG. 8, a countercurrent depressurization step is followed by an idle step, and the idle step precedes a light reflux step. In FIG. 9, an idle step falls in between LR and Equ2. In FIG. 10, an idle step precedes a CnD step and follows Eqd2. FIGS. 8-10 show the flexibility in design for PSA schedules in embodiments of the present disclosure. While the time of unit steps corresponding to individual cycle steps may be increased or decreased to impact cycle times, idle steps may in some embodiments be necessary to keep gas flows internally consistent between adsorbent beds or tanks.

[0074] For example, comparing FIG. 8 to FIG. 1A, the countercurrent depressurization step of FIG. 8 has been decreased to 2 unit steps of time, rather than 3 as shown in FIG. 1A. This may be desired if less heavy product needs to be withdrawn at low pressure during countercurrent depressurization during a separation. Unit steps can be the same amount of time or different amounts of time within a PSA system and between PSA systems, optionally resulting in idle steps, to achieve a desired separation between hydrocarbon components of varying molecular weight.

Examples

[0075] In the examples that follow, one objective is to have a continuous feed PSA cycle, regardless of how that is achieved by dividing up the number of unit steps within a unit block, where the number of unit blocks is equal to the number of beds. In the first example, with the aid of FIGS. 1A and 1B, there are 6 unit blocks because it is a 6-bed PSA cycle. In the first example, there are 2 unit steps in the fraction of the unit block corresponding to one of the 6 beds. This means there are 12 total unit steps in the first example, 2 for each bed. In some PSA systems, every other unit step should be about the same duration within the cycle, for example odd numbered unit steps being substantially the same length of time and even numbered unit steps being substantially the same length of time. Such a schedule can help keep the flow of gases within a multi-bed system internally consistent and balanced. To be a continuous feed PSA cycle, the feed step of each bed should occupy two unit steps, as shown in the first example in FIG. 1A.

[0076] One of ordinary skill in the art would understand that the unit blocks could very well include 18 unit steps, i.e., 3 unit steps for each bed, and that the feed step of each bed would then occupy 3 unit steps. The durations of the other cycle steps could occupy just 1 unit step or several unit steps, as shown by the example in FIG. 1A, where, e.g., the feed step occupies 2 unit steps, an EqD step occupies 1 unit step and the CnD step occupies 3 unit steps. The durations of all the other cycle steps relative to the feed step could vary depending on the number of unit steps in a unit block, with the duration of the unit step time having no limitations or restrictions and with the number of unit steps in a unit block having no limitations or restrictions, unless they are imposed by the PSA process design. With these objectives in mind, non-limiting examples are provided below.

[0077] Example 1 provides an example 6-bed, 8-cycle step (12 unit step) adsorption bed separation of the components of a raw natural gas stream with an initial feed pressure of 100 psia and 298 K. In other situations, more or fewer adsorption beds could be used, at different temperatures and pressures, and with optional equalization tanks. Example 1 follows the layout shown in FIGS. 1A and 1B. The feed gas composition is shown in Table 1. In Example 1, the unit step time of 60 seconds was used (with 2 unit steps per unit block as described previously), while the cycle step durations in this schedule ranged between 60 seconds and 180 seconds.

TABLE-US-00001 TABLE 1 Feed gas composition for Example 1. Feed Gas Composition Component Component Mol. fraction C1 Methane 80.0% C2 Ethane 11.0% C3 Propane 3.8% C4 Butane 1.7% C5+ Pentane and Heavier 0.8% CO.sub.2 Carbon Dioxide 1.8% N.sub.2 Nitrogen 0.9%

[0078] The example multi-bed PSA process produces a substantially pure methane product stream (sales gas) and also achieves high ethane, propane, and butane recovery in the heavy product stream, as shown in Table 2.

TABLE-US-00002 TABLE 2 Light and heavy product streams for Example 1. Heavy Product Light Product Component Recovery % Mol. fraction Recovery % Mol. fraction C1 4.4% 15.8% 95.6% 98.4% C2 98.3% 48.6% 1.5% 0.2% C3 99.7% 17.1% 0.0% 0.0% C4 100.0% 7.8% 0.0% 0.0% C5+ 100.0% 3.5% 0.0% 0.0% CO.sub.2 87.4% 7.1% 13.3% 0.3% N.sub.2 1.9% 0.1% 97.0% 1.1%

[0079] There is flexibility in the PSA process to enable CO.sub.2 to be separated in the light product stream alternative to the heavy product stream. For example, Table 3 shows that the CO.sub.2 has been mostly separated into the light product, while still achieving high ethane, propane, and butane recovery in the heavy product stream.

TABLE-US-00003 TABLE 3 Light and heavy product streams for alternative embodiment of Example 1. Heavy Product Light Product Component Recovery % Mol. fraction Recovery % Mol. fraction C1 2.2% 9.4% 97.8% 96.1% C2 93.5% 55.6% 6.0% 0.8% C3 98.9% 20.3% 0.0% 0.0% C4 100.0% 9.4% 0.0% 0.0% C5+ 100.0% 4.2% 0.0% 0.0% CO.sub.2 10.9% 1.1% 89.7% 2.0% N.sub.2 2.0% 0.1% 97.9% 1.1%

[0080] Example 2 provides an example 7-bed, 10-cycle step (14 unit step) adsorption bed separation of the components of a raw natural gas stream with an initial feed pressure of 500 psia and 298 K. In other situations, more or fewer adsorption beds could be used, at different temperatures and pressures, and with optional equalization tanks. Example 2 follows the layout shown in FIGS. 2A and 2B. The feed gas composition is shown in Table 4. In this example, the unit step time of 60 seconds was used, while the cycle step durations in this schedule ranged between 60 seconds and 180 seconds.

TABLE-US-00004 TABLE 4 Feed gas composition for Example 2. Feed Gas Composition Component Component Mol. fraction C1 Methane 80.0% C2 Ethane 11.0% C3 Propane 3.8% C4 Butane 1.7% C5+ Pentane and Heavier 0.8% CO.sub.2 Carbon Dioxide 1.8% N.sub.2 Nitrogen 0.9%

[0081] The example multi-bed PSA process produces a substantially pure methane product stream (sales gas) and also achieves high ethane, propane, and butane recovery in the heavy product stream, as shown in Table 5.

TABLE-US-00005 TABLE 5 Light and heavy product streams for Example 2. Heavy Product Light Product Component Recovery % Mol fraction Recovery % Mol fraction C1 2.2% 8.6% 97.0% 98.3% C2 98.5% 52.9% 1.5% 0.2% C3 99.3% 18.4% 0.7% 0.0% C4 100.0% 8.5% 0.0% 0.0% C5+ 100.0% 3.8% 0.0% 0.0% CO.sub.2 88.5% 7.8% 13.3% 0.3% N.sub.2 1.9% 0.1% 98.0% 1.1%

[0082] There is flexibility in the PSA process to enable CO.sub.2 to be separated in the light product stream alterative to the heavy product stream, as shown in Table 6.

TABLE-US-00006 TABLE 6 Light and heavy product streams for alternative embodiment of Example 2. Heavy Product Light Product Component Recovery % Mol. fraction Recovery % Mol. fraction C1 2.0% 8.8% 97.8% 95.9% C2 94.3% 56.8% 6.0% 0.8% C3 95.0% 19.8% 5.0% 0.2% C4 100.0% 9.5% 0.0% 0.0% C5+ 100.0% 4.2% 0.0% 0.0% CO.sub.2 7.5% 0.7% 91.7% 2.0% N.sub.2 2.0% 0.1% 97.9% 1.1%

[0083] Example 3 provides an example 7-bed, 10-cycle step (14 unit step) adsorption bed separation of the components of a raw natural gas stream with an initial feed pressure of 500 psia and temperatures of 278 K, 298 K, and 318 K. In other situations, more or fewer adsorption beds could be used, at different temperatures and pressures, and with optional equalization tanks. Example 3 follows the layout shown in FIGS. 2A and 2B. The feed gas composition is shown in Table 7.

TABLE-US-00007 TABLE 7 Feed gas composition for Example 3. Feed Gas Composition Component Component Mol. fraction C1 Methane 80.0% C2 Ethane 11.0% C3 Propane 3.8% C4 Butane 1.7% C5+ Pentane and Heavier 0.8% CO.sub.2 Carbon Dioxide 1.8% N.sub.2 Nitrogen 0.9%

[0084] A multi-bed PSA process can achieve high ethane, propane, and butane recovery under a wide range of feed gas temperatures (from about 278 K to about 318 K), as shown in Table 8.

TABLE-US-00008 TABLE 8 Heavy product streams for alternative embodiments of Example 3. 278 K 298 K 318 K Heavy Product Heavy Product Heavy Product Recov- Mol. Recov- Mol. Recov- Mol. Component ery % fraction ery % fraction ery % fraction C1 2.0% 7.9% 2.2% 8.6% 2.3% 9.2% C2 99.3% 54.6% 98.5% 52.9% 98.8% 53.1% C3 96.5% 18.3% 99.3% 18.4% 99.8% 18.5% C4 99.8% 8.7% 100.0% 8.5% 100.0% 8.5% C5+ 100.0% 3.8% 100.0% 3.8% 100.0% 3.8% CO.sub.2 73.6% 6.6% 88.5% 7.8% 77.6% 6.8% N.sub.2 1.0% 0.0% 1.9% 0.1% 1.6% 0.1%

[0085] In further separations of the heavy product carried out after the separation of methane from raw natural gas, C2 (ethane) can be separated from C3, C4, C5+, CO.sub.2, and N.sub.2. Using multiple PSA units or trains fluidly coupled together, each having one or more adsorbent beds, each component of raw natural gas can be separated.

[0086] Example 4 provides an example 6-bed, 9-cycle step (12 unit step) adsorption bed separation of the components of a raw natural gas stream with an initial feed pressure of no more than 100 psia and no less than 60 psia, but preferably between about 70 psia and about 80 psia with the feed temperatures between about 278 K to 363 K. The lowest pressure in the process is between about 2.8 psia and about 7 psia. Example 4 follows the layout shown in FIGS. 3A and 3B. The general gas composition range in which this example is applicable is shown in Table 9.

TABLE-US-00009 TABLE 9 Feed gas composition range for Example 4. Feed Gas Composition Lower Limit of Upper Limit of Component Component Mol. % Range Mol. % Range C1 Methane 70.0% 88.0% C2 Ethane 5.0% 14.0% C3 Propane 3.0% 7.0% C4 Butane 0.4% 3.0% C5+ Pentane and Heavier 0.3% 3.0% CO.sub.2 Carbon Dioxide 0.0% 3.0% N.sub.2 Nitrogen 0.0% 2.0%

[0087] Example 4 provides a multi-bed PSA process where at least about 95% of the C3+ is recovered in the heavy product, and all nitrogen is rejected into the light product with the heavy product gas having no more than 0.5 mol. % of methane. The light product, containing mostly methane, will meet specifications generally accepted to allow for pipeline transportation and/or consumer use.

[0088] Subsequently, if further separation of ethane from other non-methane hydrocarbons in the heavy product is desired, then an additional PSA unit comprising the same 6-bed, 9 cycle step process can be coupled to the first PSA unit to enact this additional separation. In other words, the 6 bed PSA system shown in FIGS. 3A and 3B can be repeated in series for subsequent separation of hydrocarbon species heavier than methane. The inlet pressure for the subsequent separation can range from about 30 psia to about 250 psia with the inlet temperature between about 278 K to about 323 K, and the lowest pressure in the system being between about 2.8 and about 7.0 psia. At least 90 mol. % of the C3+ is recovered with the product gas having substantially no CO.sub.2, no more than about 0.5 mol. % of methane, and having most of the ethane removed. Table 10 provides a range of gas compositions in which the separation of ethane from other non-methane hydrocarbons is applicable.

TABLE-US-00010 TABLE 10 Inlet range of heavy gas composition for ethane separation. Heavy Feed Gas Composition Following Initial Methane Separation Lower Limit of Upper Limit of Component Component Mol. % Range Mol. % Range C1 Methane 0.0% 3.0% C2 Ethane 40.0% 70.0% C3+ Propane 15.0% 60.0% CO.sub.2 Carbon Dioxide 0.0% 10.0%

[0089] Table 11 shows the recovery percentage of C3+ after ethane separation.

TABLE-US-00011 TABLE 11 C3+ product range after ethane separation. C3+ Product Composition Lower Limit of Upper Limit of Component Component Mol. % Range Mol. % Range C1 Methane 0.0% 2.0% C2 Ethane 0.0% 0.3% C3+ Propane and heavier 90.0% 99.0% CO.sub.2 Carbon Dioxide 0.0% 0.0%

[0090] In Example 4, where 2 series-linked 6-bed separations take place, in both adsorption bed separations, the first for methane separation and the second for ethane separation, the following PSA steps occur: a feed step; a heavy reflux (HR) step; two equalization down steps (Eqd1, Eqd2); a countercurrent depressurization step (CnD); a light reflux step (LR); two equalization up steps (Equ2, Equ1); and a light product pressurization step (LPP). The LRR, shown in FIG. 3B, is the light reflux ratio that represents the fraction of the gas leaving the feed step to be used as feed in the LR step.

[0091] Example 5 provides an example of a 7-bed, 10-cycle step (14 unit step) adsorption bed separation and follows the layout shown in FIGS. 4A and 4B. One purpose of this cycle is similar to that of Example 4 (FIGS. 3A and 3B), except that the countercurrent depressurization step is made longer to ensure better regeneration. Similar to Example 4, the purified methane product will meet specifications generally accepted to allow for pipeline transportation. The sequence involves the following PSA steps: a feed step; a heavy reflux step (HR); two equalization down steps (Eqd1, Eqd2); a countercurrent depressurization step (CnD); an idle step (I); a light reflux step (LR); two equalization up steps (Equ2, Equ1); and a light product pressurization step (LPP). The LRR is the light reflux ratio that represents the fraction of the gas leaving the feed step to be used as feed in the LR step.

[0092] Example 6 provides an example of a second 6-bed, 9-step PSA cycle and follows the layout shown in FIGS. 5A and 5B. One purpose of this cycle is the same as that of Example 4, for the removal of both methane and N.sub.2 and partial removal of both CO.sub.2 and ethane from a raw natural gas stream. The range of acceptable gas compositions for separation is the same as Example 4 (Table 9). One difference in inlet conditions, however, between Examples 4 and 6 is that the feed pressure is between about 80 psia and about 200 psia here for Example 6 versus for Example 4 with an initial feed pressure of no more than 100 psia and no less than 60 psia, but preferably between about 70 psia and about 80 psia.

[0093] The separation outcome of Example 6 is similar to Examples 4 and 5, and purified methane product that meets pipeline specifications is produced. The sequence involves the following PSA steps: a feed step, a first equalization down step (Eqd1), a heavy reflux step (HR), a second equalization down step (Eqd2), a countercurrent depressurization step (CnD), a light reflux step (LR), two equalization up steps (Equ2, Equ1), and a light product pressurization step (LPP). The LRR is the light reflux ratio that represents the fraction of the gas leaving the feed step to be used as feed in the LR step. The LRR and the light product pressurization stream in a given PSA system or method can vary from about substantially 0% to about substantially 100%, for example about 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95% of a light product stream produced at high pressure, depending on the desired separation requirements and inlet conditions of a natural gas stream. In certain embodiments exemplified here, the LRR is between about 4% and about 20% of a light product stream produced at high pressure and LPP is about between 4% and about 20% of the of a light product stream produced at high pressure.

[0094] Example 7 is an example of a 7-bed, 11-step PSA cycle similar in purpose to Example 6, but is applicable when feed pressure is equal to or greater than 150 psia. Example 7 is represented via FIGS. 6A and 6B. The sequence involves the following PSA steps: feed step; two equalization down steps (Eqd1, Eqd2); a heavy reflux step (HR); a third equalization down step (Eqd3); a countercurrent depressurization step (CnD); a light reflux step (LR); three equalization up steps (Equ3, Equ2, Equ1); and a light product pressurization step (LPP). The LRR is the light reflux ratio that represents the fraction of the gas leaving the feed step to be used as feed in the LR step.

[0095] As discussed in Example 4, if the heavy products produced in Examples 5, 6 and 7 require subsequent separation of ethane from the other non-methane hydrocarbons purified, then the cycle and sequence presented in Example 4 can be used for further separation purposes. A 6-bed 9-step cycle, from Example 4, will effectively separate ethane from all other hydrocarbons present, so a substantially pure ethane product is produced and a second NGL product meeting commercial specifications that is substantially free from ethane is also produced. The need for this additional separation step may be due to commercial or market considerations or they can be due to vapor pressure considerations. For example, ethane has a much higher vapor pressure than propane and other heavy hydrocarbons, so storage vessels and transportation pipelines for NGLs need to be maintained at much higher pressures if ethane is present in an NGL product. Therefore, there is a distinct advantage in being able to separate hydrocarbons by example systems and methods of the present disclosure, for example to isolate methane and to isolate ethane.

[0096] Example 8 is a 5-bed, 7-step PSA cycle represented by FIGS. 7A and 7B. One purpose of this cycle is the same as that of the 6-bed, 9-step PSA cycle shown in Example 4 and generally for the removal of ethane from a stream containing predominantly hydrocarbons greater than methane and is applicable when the feed pressure for the separation is no more than about 30 psia. The sequence involves the following PSA steps: a feed step, a heavy reflux step (HR), an equalization down step (Eqd1), a counter depressurization step (CnD), a light reflux step (LR), an equalization up step (Equ1), and a light product pressurization step (LPP). The LRR is the light reflux ratio that represents the fraction of the gas leaving the feed step to be used as feed in the LR step.

[0097] The singular forms a, an, and the include plural referents, unless the context clearly dictates otherwise.

[0098] In the drawings and specification, there have been disclosed embodiments of systems and methods for natural gas liquids recovery from pressure swing adsorption and vacuum swing adsorption of the present disclosure, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The embodiments of the present disclosure have been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the disclosure as described in the foregoing specification, and such modifications and changes are to be considered equivalents and part of this disclosure.