Configurations and methods for NGL recovery for high nitrogen content feed gases

Abstract

A low cost and efficient design is used to convert a propane recovery process based on low nitrogen content feed gas to an ethane recovery process based on a high nitrogen feed gas while achieving over 95 mole % ethane recovery while maintaining a 99% propane recovery, and achieved without additional equipment.

Claims

1. A NGL plant configured to convert from a propane recovery configuration to an ethane recovery configuration, the NGL plant comprising: an absorber; a fractionator; a first heat exchanger; a separator; an expander; and a chiller, wherein the absorber is configured to produce an absorber overhead product and an absorber bottom product, wherein the fractionator is configured to produce a fractionator overhead product and a fractionator bottom product, wherein in the propane recovery configuration: the first heat exchanger is configured to cool a feed gas to form a cooled feed gas, the separator is configured to separate the cooled feed gas into a first vapor stream and a first liquid stream, the expander is configured to expand the first vapor stream to form an expanded first vapor stream, and wherein the expanded first vapor stream is directed to a mid-section of the absorber, and the first liquid stream is directed to a bottom of the absorber; wherein in the ethane recovery configuration: the feed gas is split into a first portion and a second portion, the first heat exchanger is configured to cool the first portion to form a cooled first portion, wherein the cooled first portion is fed to the absorber as a top reflux, the chiller is configured to chill the second portion of the feed gas utilizing propane refrigeration to form a chilled second portion, the first heat exchanger is configured to cool the chilled second portion to form a cooled second portion, the separator is configured to separate the cooled second portion into a second vapor stream and a second liquid stream, the expander is configured to expand the second vapor stream to form an expanded second vapor stream, and wherein the expanded second vapor stream is fed to the mid-section of the absorber, and the second liquid stream is fed to the bottom of the absorber.

2. The NGL plant of claim 1, wherein, in the propane recovery configuration, the NGL plant is configured to let the first liquid stream down in pressure and then heat the first liquid stream prior to flowing to the bottom of the absorber, and wherein, in the ethane recovery configuration, the NGL plant is configured to let the second liquid stream down in pressure and combine the second liquid stream with the fractionator overhead product prior to flowing to the bottom of the absorber in the ethane recovery configuration.

3. The NGL plant of claim 1, wherein the absorber is configured to receive a vapor portion of the fractionator overhead product as the top reflux in the propane recovery configuration and to receive a first portion of the absorber overhead product as part of the top reflux in the ethane recovery configuration; wherein the fractionator overhead product is configured to flow to the chiller in the propane recovery configuration and to combine with the second liquid stream before flowing to the bottom of the absorber in the ethane recovery configuration; and wherein the absorber bottom product is configured to flow to the mid-section of the fractionator in the propane recovery configuration and to a top tray of the fractionator in the ethane recovery configuration.

4. The NGL plant of claim 3, wherein the fractionator overhead product is further configured to flow to a second separator after flowing to the chiller in the propane recovery configuration, wherein the second separator is configured to separate the fractionator overhead into the vapor portion and a liquid portion in the propane recovery configuration.

5. The NGL plant of claim 4, wherein the fractionator is configured to receive a first portion of the liquid portion of the fractionator overhead product at the top tray of the fractionator in the propane recovery configuration, wherein the absorber is configured to receive a second portion of the liquid portion of the fractionator overhead product in the propane recovery configuration.

6. The NGL plant of claim 3, wherein the absorber overhead product is heated in the first heat exchanger before being split into a first portion of the absorber overhead product and a second portion of the absorber overhead product in the ethane recovery configuration, wherein the second portion of the absorber overhead product comprises a sales gas in the ethane recovery configuration.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a schematic of one exemplary process and configuration for C.sub.3 recovery with a low nitrogen content feed gas according to the inventive subject matter. The solid lines pertain to the C.sub.3 recovery operation while the fathom lines pertain to the C.sub.2 recovery operation.

(2) FIG. 2 is a heat and mass table for the process shown in FIG. 1.

(3) FIG. 3 is a schematic of one exemplary process and configuration for C.sub.2 recovery with a high nitrogen content feed gas according to the inventive subject matter. The solid lines pertain to the C.sub.2 recovery operation while the fathom lines pertain to the C.sub.3 recovery operation.

(4) FIG. 4 is a heat and mass table for the process shown in FIG. 3.

(5) FIG. 5 is heat composite curve for core exchanger 51 operating in the C.sub.3 recovery mode with a low nitrogen content feed gas according to the inventive subject matter.

(6) FIG. 6 is heat composite curve for core exchanger 51 operating in the C.sub.2 recovery mode with a high nitrogen content feed gas according to the inventive subject matter.

DETAILED DESCRIPTION

(7) The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

(8) FIG. 1 shows an exemplary C.sub.3 recovery process, in which feed gas 1 enters an NGL plant at 100 F. and about 900 psig, with a feed gas composition as shown in the overall heat and mass table of FIG. 2. Feed gas stream 2 is cooled to about 40 F. by heat exchange with residual gas stream 12 from the absorber, forming stream 4 which is separated in separator 52, producing liquid stream 70, and vapor stream 8. Vapor stream 8 is expanded in expander 53 to about 430 psig, forming stream 11 at about 95 F., which is fed to the lower section of absorber 54. The power produced from expander 53 is used to drive re-compressor 65. Liquid stream 70 is let down in pressure in JT valve 71 to about 430 psig, forming stream 10 at about 75 F., and is then heated to 20 F. by the feed gas in exchanger 51, forming stream 32, prior to feeding to the bottom of absorber 54.

(9) Absorber 54 is refluxed with two streams; liquid stream 74 and the vapor stream 80, producing an ethane depleted overhead stream 12 at 60 F., and an ethane rich bottom stream 13 at 25 F. The refrigerant content in the overhead stream 12 is recovered by chilling the feed gas 1, and the bottom stream 13 is pumped by pump 55 and heated by feed gas 1 to about 90 F., forming stream 7, prior to entering the mid-section of fractionator 58. The fractionator produces an ethane rich overhead stream 15 at 16 F., and a propane rich bottom stream 16 at 210 F. Side reboilers 59 and 60 are used to reduce the reboiler duty for energy conservation while the fractionator 58 bottom temperature is controlled by reboiler 61, maintaining the ethane content in stream 16 (NGL) to below 0.01 mole %.

(10) The fractionator overhead stream 15 is cooled by propane refrigeration in chiller 62 to about 20 F., forming stream 30, which is separated in separator 63 into vapor stream 14 and liquid stream 31, supplying refluxes for absorber 54 and fractionator 58.

(11) Overall heat and material balance for the C.sub.3 recovery process is shown in the table of FIG. 2.

(12) FIG. 3 shows an exemplary C.sub.2 recovery process, in which feed gas 1 enters a NGL plant at 100 F. and about 900 psig, with a feed gas composition as shown in the overall heat and mass table of FIG. 4. Feed gas stream 1 is split into two portions, stream 2 and stream 3, where stream 2 constitutes about 20% to 30% of the total feed gas rate, and is cooled by the residue gas in exchanger 51.

(13) The other portion, stream 3, is cooled by propane chiller 62 to about 22 F., forming stream 30, which is further cooled in exchanger 51 to about 40 F., forming stream 80, which is separated in separator 52, producing liquid stream 70 and vapor stream 8. Vapor stream 8 is expanded in expander 53 to about 430 psig, forming stream 11 at about 105 F., which is fed to the lower section of absorber 54.

(14) The power produced from expander 53 is used to drive re-compressor 65. Liquid stream 70 is let down in pressure to about 450 psig in JT valve 71 and combined with the fractionator overhead vapor stream 15 and fed to the bottom section of absorber 54.

(15) Absorber 54 is refluxed with two reflux streams, feed gas stream 5 and the residue gas recycle stream 27, producing an ethane depleted overhead stream 12 at 150 F., and an ethane rich bottom stream 13 at 66 F. The absorber overhead stream 12 is used in chilling the feed gas stream 2 and residue gas recycle stream 25 in exchanger 51, and the absorber bottom stream 13 is pumped by pump 55 and is sent to fractionator 58 as reflux stream 77. Fractionator 58 produces an ethane depleted overhead stream 15 and ethane rich bottom stream 16. Side reboilers 59 and 60 are used to reduce reboiler duty for energy conservation, and the temperature of the bottom liquid in fractionator 58 is maintained at 82 F. by reboiler 61, maintaining the methane content in stream 16 (NGL) to below 0.01 mole %.

(16) Overall heat and material balance for the high nitrogen feed gas operation on C.sub.2 recovery is shown in the table of FIG. 4.

(17) It should be particularly appreciated that the contemplated configurations shown in FIGS. 1 and 3 may be used for high nitrogen feed gases for either ethane or propane recovery by repositioning valves and piping. Alternatively, a NGL plant can be designed so that it is transitionable between a C.sub.2 recovery mode and C.sub.3 recovery mode with minimum impact on the process. For example, an NGL plant can be configured with piping and components represented by both the solid lines and the dotted lines in FIGS. 1 and 3, with valves at the intersections of solid and fathom lines. The valves can be operated manually or automatically to transition between recovery modes. In this manner, NGL plants can process a feed gas that has an increase in nitrogen content over time, such as the feed gas from a crude oil reservoirs that is processed using nitrogen-enhanced methods.

(18) With the contemplated plant designs, C.sub.3 recovery can be maintained at over 99% during the C.sub.3 recovery mode, while C.sub.2 recovery can be maintained at 95% while maintaining a 99% C.sub.3 recovery. When C.sub.2 recovery is required, the propane chiller is used for cooling a portion of the feed gas, and when C.sub.3 recovery is desirable, the propane chiller is used as a reflux condenser for the absorber and fractionator.

(19) When operating on C.sub.2 recovery mode, the absorber bottom liquid stream is fed directly to the top tray of the fractionator column by valve switching, and when C.sub.3 recovery is required, the absorber bottom stream is heated and routed to the mid-section of the fractionator. Thus, it should be noted that during C.sub.3 recovery, the fractionator overhead vapor is chilled and partially condensed with propane refrigeration and the absorber bottoms, producing a vapor and liquid stream. The ethane rich vapor stream is further chilled by the absorber column overhead forming a reflux stream. During C.sub.2 recovery, the fractionator overhead is routed to the bottom of the absorber for rectification and recovery of the ethane and heavier components.

(20) With respect to suitable feed gas streams, it is contemplated that various feed gas streams are appropriate, and especially suitable fed gas streams may include various hydrocarbons of different molecular weight. With respect to the molecular weight of contemplated hydrocarbons, it is generally preferred that the feed gas stream predominantly includes C.sub.1-C.sub.6 hydrocarbons. However, suitable feed gas streams may additionally comprise acid gases (e.g., carbon dioxide, hydrogen sulfide) and other gaseous components (e.g., hydrogen). Consequently, particularly preferred feed gas streams are natural gas and natural gas liquids.

(21) Thus, it should be especially recognized that in contemplated configurations, the cooling requirements for the first column are at least partially provided by product streams and recycle gas, and that the C.sub.2/C.sub.3 recovery can be varied by employing a different reflux stream. With respect to the C.sub.2 recovery, it is contemplated that such configurations provide at least 85%, more preferably at least 90%, and most preferably at least 95% recovery, while it is contemplated that C.sub.3 recovery will be at least 98%, more preferably at least 98%, and most preferably at least 99%. Further related configurations, contemplations, and methods are described in co-owned International Patent Applications with the publication numbers WO 2005/045338 and WO 2007/014069, both of which are incorporated by reference herein.

(22) Thus, specific embodiments and applications of C.sub.2 recovery and C.sub.2 rejection configurations and methods therefore 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.

(23) As used herein, and unless the context dictates otherwise, the term coupled to is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms coupled to and coupled with are used synonymously.

(24) Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.

(25) 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. Unless the context dictates the contrary, all ranges set forth herein should be interpreted as being inclusive of their endpoints and open-ended ranges should be interpreted to include only commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. such as) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

(26) As used in the description herein and throughout the claims that follow, the meaning of a, an, and the includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of in includes in and on unless the context clearly dictates otherwise.