Ethane recovery and ethane rejection methods and configurations

Abstract

Contemplated plants for flexible ethane recovery and rejection by allowing to switch the top reflux to the demethanizer from residue gas to the deethanizer overhead product and by controlling the flow ratio of feed gas to two different feed gas exchangers. Moreover, the pressure of the demethanizer is adjusted relative to the deethanizer pressure for control of the ethane recovery and rejection.

Claims

1. A method of changing ethane recovery to ethane rejection operation in an NGL plant, comprising: changing a top reflux of a demethanizer from a demethanizer overhead product to a deethanizer overhead product for ethane rejection; reducing a demethanizer pressure to a pressure that is lower than a deethanizer pressure for ethane rejection; and increasing flow of a first portion of a feed gas relative to flow of a second portion of the feed gas for ethane rejection; wherein the demethanizer is configured to receive a second reflux below the top reflux, wherein the second reflux is the first portion of the feed gas, wherein the second portion of the feed gas is cooled in a demethanizer side reboiler heat exchanger and then combined with a third portion of the feed gas to form a partially condensed feed stream, wherein the partially condensed feed stream is separated into a liquid stream and a vapor stream, wherein the demethanizer is configured to receive the liquid stream and the vapor stream.

2. The method of claim 1, wherein the demethanizer is configured to produce a demethanizer bottom product that is fed to a deethanizer that is configured to produce the deethanizer overhead product.

3. The method of claim 1, wherein the feed gas is cooled before the step of sub-cooling by expanding the feed gas in a turbo expander.

4. The method of claim 1, wherein the first portion of the feed gas is cooled in a feed gas heat exchanger.

5. The method of claim 4, wherein the demethanizer overhead product is heated in the feed gas heat exchanger.

6. The method of claim 1, wherein the demethanizer pressure is between 445 psig and 475 psig or at least 475 psig, and wherein the deethanizer pressure is between 319 psig and 450 psig.

7. The method of claim 1, wherein the demethanizer produces a demethanizer bottom product that is fed to a deethanizer.

8. The method of claim 1, wherein an operating pressure in the demethanizer is between 445 psig and 475 psig or at least 475 psig, and wherein an operating pressure in a deethanizer is between 319 psig and 450 psig.

9. The method of claim 1, wherein a deethanizer produces a deethanizer bottom product, the method further comprising feeding the deethanizer bottom product into a depropanizer.

10. The method of claim 1, wherein the feed gas has a pressure of at least 1000 psig.

11. The method of claim 1, further comprising expanding the feed gas in a turbo expander prior to subcooling the first portion of the feed gas.

12. The method of claim 11, wherein the feed gas is expanded in the turbo expander prior to cooling the second portion of the feed gas.

13. The method of claim 1, wherein a propane recovery of at least 99% is achieved during ethane rejection.

14. The method of claim 1, where the first portion and the third portion of the feed gas are cooled by the demethanizer overhead product in a feed gas heat exchanger.

15. The method of claim 14, wherein the deethanizer overhead product is cooled by the demethanizer overhead product in the feed gas heat exchanger.

16. The method of claim 1, wherein each of the vapor stream and the liquid stream are expanded before feeding to the demethanizer.

17. The method of claim 16, wherein the vapor stream is expanded using a turbo expander.

18. The method of claim 16, wherein the liquid stream is expanded using a JT valve.

19. The method of claim 1, wherein the demethanizer is configured to receive the liquid stream below the vapor stream.

20. The method of claim 1, wherein the demethanizer is configured to produce a demethanizer bottom product, the method further comprising lowering a temperature of the demethanizer bottom product for ethane rejection.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic of an exemplary plant configuration according to the inventive subject matter.

(2) FIG. 2 is a composite heat curve during ethane recovery according to the inventive subject matter.

(3) FIG. 3 is a composite heat curve during ethane rejection according to the inventive subject matter.

DETAILED DESCRIPTION

(4) The inventors have discovered that high propane recovery of 99.9% can be achieved for the ethane recovery and ethane rejection operation by changing the origin of the reflux from residue gas to deethanizer overhead, and by varying the feed gas split ratios to two feed exchangers. In contemplated methods and configurations, the demethanizer is operated at a higher pressure than the deethanizer pressure during ethane recovery, and at a lower pressure than the deethanizer pressure during ethane rejection or propane recovery.

(5) Thus, it should be recognized that during ethane recovery, residue gas compression horsepower is reduced as the demethanizer operates at a higher pressure than the deethanizer. On the other hand, during ethane rejection, it should be noted that the deethanizer overhead can be directed to the demethanizer for refluxing without further compression as the demethanizer pressure is lowered to below that of the deethanizer. Consequently, using contemplated configurations and methods, ethane recovery of at least 95%, more typically at least 98% during ethane recovery is achieved.

(6) In one preferred aspect of the inventive subject matter, contemplated plants include a demethanizer and a deethanizer, wherein the demethanizer is configured to receive a top reflux (relative to other streams) that is provided by a residue gas recycle stream during ethane recovery. When ethane rejection is desired, the top reflux is provided by deethanizer overhead gas. Moreover, it is generally preferred that the demethanizer is refluxed with a second reflux stream (preferably at least two trays below the top reflux) that is provided by a portion of subcooled feed gas. Feed gas cooling is preferably achieved by use of one or more turboexpanders and/or one or more demethanizer side reboilers.

(7) Using the above inventive configurations and methods, the volume ratio of methane to ethane content in the demethanizer bottom is controlled at about 2%, as necessary to meet the ethane product specification during ethane recovery. During ethane rejection, the methane to ethane volume ratio is increased to 10% such that more deethanizer overhead vapor is generated for refluxing the demethanizer, which thus eliminates the need for residue gas recycle.

(8) Consequently, methods and configurations are now available to achieve ethane recovery of at least 95%, preferably at least 98%, and propane recovery of at least 95%, preferably at least 98%, more preferably at least 99%, and most preferably at least 99.9% during ethane recovery. Moreover, contemplated methods and configurations also achieve propane recovery of at least 99.9% during ethane rejection. 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 commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.

(9) It should still further be appreciated that the configurations and methods presented herein can process high pressure hydrocarbon feed gases (e.g. at least 1.400 psig, and more preferably at least 1600 psig, and even higher). At such pressures, two stages of turbo-expansion are preferably included to so eliminate propane refrigeration typically required inconventional designs. In especially preferred configurations, the demethanizer side reboilers are also used for stripping the methane component in the feed gas by using the heat content of the feed gas, and turbo-expansion of the feed gas subsequently provides the cooling duty in the demethanizer.

(10) FIG. 1 depicts an exemplary gas processing plant for ethane recovery and ethane rejection using a feed gas with a composition as shown in Table 1:

(11) TABLE-US-00001 TABLE 1 Mole % CO2 0.4 Nitrogen 0.4 Methane 88.9 Ethane 5.2 Propane 2.7 i-Butane 0.5 n-Butane 1.1 n-Pentane 0.3 i-Pentane 0.3 n-Hexane 0.1

(12) More particularly, dried feed gas stream 1, at a temperature of about 95 F. and a pressure of about 1600 psig, is letdown in pressure to about 1100 psig via first turboexpander 51, forming stream 2 at about 55 F. The expander power is used to drive one of the residue gas compressors 52. The expanded gas is then split into two portions 3/4 and 5, with portion 3/4 being fed to the upper feed exchanger 56 and the other portion 5 being fed to the lower exchanger 64.

(13) In the upper exchanger 56, the demethanizer overhead gas stream 26 at about 108 F. is used to chill and subcool the residue gas (or deethanizer overhead) stream 20 from about 110 F. to about 130 F. and a portion of the feed gas stream 3 from about 54 F. to about 130 F. The residue gas stream 14 from the demethanizer is warmed up to about 58 F. prior to compression in the residue gas compressor 52. During ethane recovery, these two subcooled streams (21 and 11) are used to form the first and second reflux streams (22 and 12 via JT valves 75 and 76, respectively) to the demethanizer 58. The first reflux 22 is fed to the top of the demethanizer, and the second reflux 12 is fed to a position at the demethanizer that is at least two trays below the top tray. The residual refrigerant content in the demethanizer overhead gas is recovered by chilling a portion of the feed gas stream 4 from about 54 F. to about 20 F. forming stream 7. During ethane rejection, residue gas recycle flow is stopped by closing valve 80, and valve 79 is opened such that the top reflux is provided by deethanizer overhead vapor stream 32 via streams 49 and 20. The deethanizer overhead vapor is chilled from about 23 F. to about 108 F. forming an ethane rich reflux stream which is used during the ethane rejection operation.

(14) In lower exchanger 64, the refrigerant content of the upper and lower side reboilers in the demethanizer are recovered via streams 23 and 24 by chilling the feed gas to about 21 F. forming stream 6. The chilled feed gas streams from the upper and lower exchangers are combined and separated in feed gas separator 57. The separator liquid stream 9 is letdown in pressure via JT valve 77 and fed as stream 10 to the lower section of the demethanizer 58, and separator vapor stream 8 is expanded in the second turboexpander 53 forming stream 19 at about 90 F., which is fed to the mid section of demethanizer 58.

(15) During ethane recovery, the temperature of demethanizer bottom product 25 is heated to about 104 F. by the heat medium flow in reboiler 65 for controlling the methane component to the ethane component in the bottom liquid at a ratio of 2 volume %. A gas analysis is typically used to fine, tune the reboiler temperature. During ethane rejection, the demethanizer bottom temperature stream 25 is lowered to about 64 F. in reboiler 65 such that the ratio of the methane component to the ethane component in the liquid is increased to about 10 volume %. The higher methane content is used in refluxing the demethanizer during the ethane rejection operation, which significantly reduces the power consumption of the residue gas compressor.

(16) During ethane recovery, the demethanizer overhead vapor 26, at a pressure of about 472 psig, is heated from about 93 F. to about 110 F. by the residue gas recycle stream 20 and the feed gas streams 3 and 4, and then compressed by the first and second compressors 52 (via stream 15) and 54 to about 620 psig driven by turbo expanders 51 and 53. The gas stream 16 is further compressed to about 1185 psig by residual gas compressor 55. The compressor discharge 17 is cooled by air cooler 81 forming stream 18, and during ethane recovery, a portion 48 (about 20% of the total flow) of the residue gas stream 18 is recycled as stream 20 to the upper exchanger 56 as top demethanizer reflux 22. The remaining portion is sales gas stream 99.

(17) During ethane recovery, the demethanizer 58 operates at a pressure of about 475 psig, and the deethanizer 59 operates at a pressure of about 319 psig, and the demethanizer bottoms stream 25 is fed directly to the deethanizer by pressure differential without the use of bottoms pump 72 via stream 27. During ethane rejection, the demethanizer pressure is lowered to a pressure of about 445 psig, and the deethanizer pressure is increased to a pressure of about 450 psig, thus requiring operation of bottoms pump 72. The deethanizer pressure is increased such that during ethane rejection, the deethanizer overhead stream 32 can be recycled back to the demethanizer as a top reflux (which replaces the residual gas recycle stream 48). The deethanizer overhead stream 29 is partially condensed using propane refrigeration in chiller 70, and the two phase stream 30 is separated in reflux drum 60. The separator liquid stream 31 is pumped by reflux pump 73 forming stream 33 for refluxing the deethanizer. The separator vapor stream 32 is the ethane product stream during ethane recovery. During ethane recovery, the deethanizer 59 (reboiled by reboiler 66) produces an overhead vapor stream 32 which can be exported as an ethane product and a bottoms liquid stream 28 which is further fractionated in depropanizer 61 into a propane product stream 41 and a butane plus product stream 35. Depropanizer 61 produces overhead stream 34 that is chilled in chiller 68 to produce stream 36 which is fed through drum 62 and separated from stream 37 into product stream 41 and depropanizer reflux via reflux pump 74. Reboiler 67 provides necessary heat for separation in column 61. During ethane rejection, the deethanizer overhead is recycled back to the demethanizer, and the bottoms is fractionated in the depropanizer 61 into a propane product stream 41 and a butane plus product stream 35.

(18) It should be appreciated that contemplated methods and configurations are also suitable where a relatively high-pressure supercritical feed gas (e.g., 1500 psig or higher) with relatively low propane and heavier content (about 3 mole %) is processed. Most preferably, the supercritical pressure feed gas is expanded to below its critical pressure (e.g., 1200 psig or lower) using a turboexpander, and the expanded vapor is split into three portions: The first portion is then chilled and subcooled, providing reflux to the demethanizer while the second portion is chilled, separated, and its vapor portion is fed to the stripping section of the demethanizer, and the third portion is used to recover the refrigerant content in the demethanizer side reboilers. Thus, suitable gas processing plants will include a first turboexpander that is configured to expand a feed gas to sub-critical pressure (e.g., between 1100 psig and 1200 psig), a first heat exchanger that subcools the feed gas to form a mid reflux to the demethanizer, and a second turboexpander that expands a vapor phase of the cooled feed gas to produce a feed stream to the demethanizer. It is especially preferred that first and second turbo-expanders are mechanically coupled to drive residue gas compressors. Most preferably, a second heat exchanger is thermally coupled to the demethanizer to at least recover the refrigeration content of the side reboilers in the demethanizer.

(19) Moreover, it should also be recognized that contemplated configurations and methods are suitable to process rich gas streams (e.g., content of C3+ at least 10 mol % with at least 75 mol % of hydrocarbons being C2+). In such scenario, all of the feed gas is expanded across the turbo expander and the operating pressure of the demethanizer is lowered to provide the front end chilling duty. An exemplary rich feed gas composition is provided in Table 2 below.

(20) TABLE-US-00002 TABLE 2 Mole % CO2 0.4 Nitrogen 1.1 Methane 0.0 Ethane 74.8 Propane 11.2 i-Butane 6.9 n-Butane 1.4 n-Pentane 2.7 i-Pentane 0.7 n-Hexane 0.7

(21) To provide the front end cooling requirement, operating pressure of the demethanizer is lowered, and the feed gas stream 3 for production of the second reflux stream 12 is stopped. Thus, the flow to the turboexpander 53 is increased. This reduction in demethanizer pressure, the increase in turboexpander cooling, and the use of residue gas recycle provides sufficient cooling duty for the rich gas process.

(22) It is contemplated that at least a portion of the feed gas can be cooled to supply the reboiler duties of the demethanizer. With respect to the heat exchanger configurations, it should be recognized that the use of side reboilers to supply feed gas and residue gas cooling and reflux duty will minimize total power requirement for ethane recovery and ethane rejection. Therefore, propane refrigeration can be minimized or even eliminated, which affords significant cost savings compared to known processes. Consequently, it should be noted that in the use of two turboexpanders coupled to the demethanizer and deethanizer operation allows stripping, and eliminating or minimizing propane refrigeration in the ethane recovery process, which in turn lowers power consumption and improves the ethane recovery. Further aspects and contemplations suitable for the present inventive subject matter are described in our International patent application WO 2005/045338 and U.S. Pat. No. 7,051,553, and U.S. Pat. App. No. 2010/0011809, all of which are incorporated by reference herein.

(23) Thus, specific embodiments and applications of ethane recovery and ethane rejection configurations and methods therefor 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.