US20230375265A1

US20230375265A1 - Gas processing methodology utilizing reflux and additionally synthesized stream optimization

Info

Publication number: US20230375265A1
Application number: US17/974,768
Authority: US
Inventors: Kelly Mackenzie; Eugeniusz Grynia; Stuart Mackenzie
Original assignee: Gas Liquids Engineering Ltd
Current assignee: Gas Liquids Engineering Ltd
Priority date: 2022-05-17
Filing date: 2022-10-27
Publication date: 2023-11-23
Also published as: WO2023220806A1

Abstract

Gas processing methodology for high efficiency recovery of propane and/or ethane from a natural gas feed stream. The method is conducted without turboexpansion, and in some embodiments, without the use of a refrigeration system. A natural gas stream is processed to have gas and liquid portions. The gas portions are cooled and flow to a refluxed absorber column and the liquid portions flow to a lower pressure distillation column. Bottoms of the absorber column are depressurized into a separator, with the separator overhead vapor being used as a source of absorber column reflux. The separator liquids are fed into the lower pressure distillation column and the distillation column overhead vapor stream is used to cool the feed and/or reflux streams. The overhead vapour stream from the lower pressure distillation can be recycled to the absorber, either as a recycle or a source of reflux.

Description

FIELD OF THE INVENTION

The present invention relates to gas processing and more particularly the present invention relates to the recovery of selected fractions from a natural gas feed stream.

BACKGROUND OF THE INVENTION

A variety of gas processing plant configurations exist for the extraction of hydrocarbon fractions from natural gas streams. Most industrial processing facilities seeking high levels of propane and ethane extraction from natural gas utilize turboexpanders for cryogenic recovery of these components, and such processes have become a preferred method. Examples of such processes are documented in U.S. Pat. No. 4,157,904, Campbell et al. issued Jun. 12, 1979, U.S. Pat. No. 4,690,702, Paradowski et al. issued Sep. 1, 1987, and. U.S. Pat. No. 5,275,005, Campbell et al. issued Jan. 4, 1994.
In a typical natural gas separation process, a natural gas stream under pressure is cooled by heat exchange with a cold fluid stream, condensing liquids from the natural gas. The condensed liquids are then separated and fractionated in a distillation column to provide the desired separation of components. In some configurations, an additional lean liquids stream is condensed for use as reflux in the distillation, to absorb the components desired for extraction.
The majority of the commercial extraction processes employ a turboexpander in order to achieve high propane and ethane recovery. One major drawback to the use of a turboexpander is that turboexpanders have high operating efficiency at their design operating conditions, but lose efficiency at other operating conditions. The loss of turboexpander efficiency results in significant reductions of propane or ethane recovery at off-design conditions, such as at plant throughputs below the design rate. Existing designs that do not employ a turboexpander and instead use mechanical refrigeration such as propane refrigeration for cooling are not capable of achieving very high propane or ethane recoveries.
Mak describes in U.S. Pat. No. 6,837,070, issued Jan. 4, 2005, an extraction process which does not require a turboexpander, yet can achieve propane recoveries in excess of 95%. This process utilizes the depressurized absorber bottoms as a coolant for the absorber reflux, however the process does not cool the reflux to low enough temperatures to efficiently achieve high levels of propane recovery and lowers the attainable ethane recovery to levels that are not competitive with turboexpander based processes.
In the disclosure of the Mak document, there is a very good discussion of other relevant art. The Mak disclosure states:

- “Various improvements on the basic concept of cryogenic gas separation have been developed. For example, Rambo et al. describe in U.S. Pat. No. 5,890,378 a system in which (a) the absorber is refluxed, (b) in which the de-ethanizer condenser provides the reflux for both the absorber and the de-ethanizer while the cooling requirements are met using a turboexpander, and (c) in which the absorber and the de-ethanizer operate at substantially the same pressure. Although Rambo's configuration advantageously reduces capital cost for equipment associated with providing reflux for the absorption section and the de-ethanizer, propane recovery significantly decreases as the operating pressure in the absorber rises, especially at a pressure above 500 psig, where separation of ethane from propane in the de-ethanizer becomes increasingly difficult. Consequently, Rambo's system is generally limited by the upper operating limit of the de-ethanizer pressure. Increasing of the absorber pressure while maintaining desirable propane recovery becomes difficult, if not impossible in Rambo's process configuration. Moreover, operating the absorber and de-ethanizer at a pressure at or below 500 psig typically necessitates higher residue gas recompression, thereby incurring relatively high operating cost. To circumvent at least some of the problems associated with relatively high cost associated with residue gas recompression, Sorensen describes in U.S. Pat. No. 5,953,935 a plant configuration in which an additional fractionation column is included. The absorber reflux in Sorensen's plant configuration is produced by compressing, cooling, and Joule Thomson expansion of a slipstream of feed gas. Although Sorensen's configuration generally provides an improved propane recovery with substantially no increase in plant residue compression horsepower, propane recovery significantly decreases as the operating pressure in the absorber rises, especially at a pressure above about 500 psig. Furthermore, ethane recovery using such known systems designed for propane recovery is normally limited to about 20% recovery.”

Stothers, in U.S. Pat. No. 6,098,425, issued Aug. 8, 2000, discloses a thermodynamic separation process. The process continues to rearrange and introduce new operations to facilitate component separation. In the teachings, it is indicated that a gas fractionator tower, de-ethanizer, and recycle gas fractionator tower can produce high propane recoveries, in a configuration such that the fractionator vessel pressure is higher than the de-ethanizer pressure, and uses a reflux stream distilled from the de-ethanizer overheads in a third column.
In furtherance of improvements to this area of technology, Mak, in U.S. Pat. No. 7,051,553, issued May 30, 2006, teaches that in systems with a de-ethanizer and chilled overheads condenser, the de-ethanizer overheads stream can be used as a source of reflux in in the absorber column to achieve high propane and improved ethane recoveries in a turboexpanded process. The text states:

- “In especially preferred configuratons ranging from propane recovery to ethane recovery, the typical temperature ranges are illustrated as follows. The further cooled vapor stream 108 is split into a first portion that is expanded in a turbo-expander 150 to form expanded stream 109, typically at −100° F. to −115° F., which is introduced into the absorber 110, and a second portion stream 130 is still further cooled in heat exchanger 120 to typically −90° F. to −135” F., and reduced in pressure via a Joule-Thomson valve 132 before entering the absorber 110 as a reflux stream, typically at −125° F. to −140° F.
- Absorber 110 forms an overhead product 114, typically at −100° F. to −135°, which is employed as a refrigerant in heat exchangers 120, 122, and 124 before a residue gas re-compressor 160 recompresses the residue gas. Thus, it should be recognized that the overhead product cools the first and second absorber reflux, 146 and 130, respectively, and may further be employed as refrigerant to cool at least one of the vapor portions of the natural gas feed from the first and second separators. The absorber 110 further produces bottoms product 112, typically at −100° F. to −115° F., which also acts as a refrigerant in heat exchanger 120 to further cool the first and second reflux streams 146 and 130. The heated bottoms product 112, typically at −65° F. to −85° F., is then introduced into the distillation column 140, which separates the desired bottom product 142 (e.g., propane, or ethane/propane) from lean residue gas 144. The lean residue gas 144 may then be cooled with a cooler before entering separator 190 that produces a distillation column reflux 148 and the lean absorber reflux stream 146, typically at −85° F. to −115° F.”.

This process presents another representation in the evolution of reflux streams, turbo expansion and sequenced operations which are rearranged or with some operations removed for attaining improved recoveries.
In a later publication, Mak et al., teach in United States Patent Publication No. 20210095921, published Apr. 1, 2021, integrated methods and configurations for propane recovery in both ethane recovery and ethane rejection. There is a discussion regarding the introduction of bottoms from the absorber being introduced into the stripper and further the use of multiple reflux streams. Although a meritorious procedure, the protocol still relies on turbo expansion. Mak at al., teach at paragraph [0040]:

- [0040]
- NGL recovery system 100 can comprise turbo expander 56. Turbo expander 56 Is configured to reduce the pressure of the vapor introduced thereto via vapor line 10 and second vapor line 14 and provide an expander discharge stream. Turbo expander 56 is any expander known in the art to be operable to provide the expansion described herein while producing work. An absorber inlet line 15 can fluidly connect turbo expander 56 and absorber 59, whereby the expander discharge stream produced in turbo expander 56 can be introduced into absorber 59. A stripper overhead line 21B can fluidly connect stripper 62 with absorber inlet line 15, via stripper overhead line 21, such that (during ethane recovery) substantially all of the stripper overhead in stripper overhead line 21 can be combined with the expanded second portion of the separator vapor in second vapor line 14 via absorber inlet line 15 and introduced into absorber 59.” [Emphasis mine]

Accordingly, the techniques disclosed in the Mak et al. would present disadvantageous economics relative to a recovery process which does not rely on turbo expansion. These are particularly noteworthy in respect of turndown in a protocol not using turbo expansion.
Other generally relevant documents in this area include Canadian Patent Nos. 2,388,266, 2,562,828, 2,593,886, European Patent No. EP 0721557, United States Patent Publication Nos. US20090107175, US20090293537, US20120096895, US20140007616, US20140130541, US20140260420, US20160187058 and U.S. Pat. Nos. 4,496,380, 4,507,133, 4,511,381, 5,152,148, 5,497,626, 5,685,170, 5,953,935, 6,098,425, 6,516,631, 6,837,070, 7,051,553, 7,191,617, 7,793,517, 9,103,585, 9,377,239 and WIPO documents WO2008005518, WO2009087307 and WO2014036322.
From a review of the rather extensive prior art in toto for this area of technology, it is evident that similarities exist in terms of reflux streams, towers, bottoms streams etc. Each one of the prior art references has merit, however the art has not unified select unit operations which have firstly been optimized and then rearranged in a novel manner to exploit substantial recovery percentages with commensurate economy and efficiency. Accordingly, there is still a need to provide improved methods and configurations for achieving very high propane and ethane recoveries, under variable flow conditions.
The present technologies to be discussed in detail herein after provides for such requisite improved methods.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method of recovering selected fractions of a natural gas feed stream with high efficiency in the absence of turbo expansion.
The technology has several aspects which provide for ethane recovery, propane recovery and methodology to switch between these.
By way of a preface to the details of the technologies, the present invention is directed to methods for gas processing, operational sequences for the processing and configurations of a gas plant.
Generally, the methods include cooling, partially condensing and separating a natural gas feed stream into gas and liquids streams. As a preliminary operation the gas may be treated to remove undesirable compounds by any of the known techniques. From the separation, the gas portion may be further cooled and passed into a refluxed absorber column called a fractionator. The liquid may be depressurized and passed into a distillation column.
The fractionator is utilized to contact a portion of the feed stream with cold and compositionally lean reflux liquids to absorb heavier hydrocarbons into the liquid and allow lean gas to leave the top of the absorber as overheads gas.
The fractionator overheads gas may be used to cool the reflux and feed gas streams before leaving the process as sales gas. The bottoms product of the fractionator may be depressurized and separated into gas and liquid streams using a vessel called the intermediate pressure vessel.
Accordingly, one object of one embodiment of the present invention is to provide a method of processing natural gas to recover selected hydrocarbons contained therein, comprising:

- cooling and partially condensing a natural gas feed stream;
- separating the cooled and partially condensed stream into a plurality of liquid streams and gas streams;
- passing the gas streams into a fractionator column and the liquid streams into a distillation column;
- recovering an overheads stream from the distillation column;
- treating the overheads stream to form a plurality of overhead streams;
- passing the overhead streams into the fractionator column as reflux;
- recovering a bottoms stream from said fractionator column;
- depressurizing and passing the bottoms stream into the intermediate pressure vessel;
- recovering an overheads stream from the intermediate pressure vessel;
- passing the overheads stream into the fractionator column as reflux;
- contacting the reflux streams with the natural gas feed stream in the fractionator column to liquefy hydrocarbon components present in the natural gas feed stream;
- recovering a bottoms stream from the intermediate pressure vessel;
- passing the bottoms stream into the distillation column; and
- recovering selected hydrocarbon components from the bottoms stream.

The intermediate pressure vessel, in one aspect of the invention, receives the depressurized fractionator bottoms and separates the stream into gas and liquids streams using either separation or distillation. The gas stream may then be utilized to cool the feed stream and/or fractionator reflux, prior to being compressed, cooled, and depressurized into the fractionator column, creating cold and lean liquids that serve as fractionator reflux. The liquids stream can then be used to cool the feed stream and/or fractionator reflux, and then be fed into said distillation column.
The distillation column, in the practicing an embodiment of the invention, receives the liquid portion of the feed stream as well as gas and liquids streams from the intermediate pressure vessel and distills them to produce a liquid product and an overheads stream.
The recovered liquids leave the process as the distillation column bottoms. The distillation column overhead stream may then be utilized to cool the feed stream and/or fractionator reflux, prior to being compressed, cooled, and depressurized into the top of the fractionator column, creating cold and lean liquids that serve as fractionator reflux. In one aspect of the invention, the distillation column comprises a deethanizer column. The distillation column produces an overhead stream that is used to cool the feed stream and/or absorber reflux, and is then compressed, cooled, and depressurized directly into the fractionator column at a position partway down the column. This configuration allows for propane recovery in excess of 99%, while rejecting over 99% of the ethane, with overall utility requirements that are similar to those used in turboexpander based processes.
As a particular benefit the method is conducted absent the use of turboexpansion as a unit operation.
The formation of the overheads stream forms a plurality of overhead streams utilized as reflux streams at different pressures.
It has been found that the unification of meritorious unit operations from the prior art with unique interplay and heat exchange, solves the issues currently in this area of technology. This has also been realized in turndown benefits; the methodology allows for varying fluid flow rates at least between the fractionator column and the distillation column.
Owing to the flexibility and elegance of the core features of the various embodiments, the methods allow for adaptation where the composition of the gas is variable. One can initially determine the composition of the natural gas and select a sequence of unit operations, such as cooling, heat exchange, depressurization and separation for energy optimization in effecting the method.
In respect of implementation into a plant, the technology is readily suited for plant implementation. As such, another object of one embodiment of the present invention is to provide a gas processing plant, comprising:

- a separator for receiving a cooled and partially condensed natural gas feed stream to separate the feed stream into a liquid stream and a gas stream;
- a fractionator column for receiving the gas stream and reflux streams produced from a distillation column;
- an intermediate pressure vessel in fluid communication with the fractionator column for producing an overheads stream, the overheads stream being utilized in the fractionator column as the reflux streams to liquefy hydrocarbons present in the feed stream, the fractionator column producing a bottoms stream which is depressurized into a distillation column to recover selected hydrocarbons; and
- a distillation column in fluid communication with the intermediate pressure vessel, the intermediate pressure vessel producing a bottoms stream for feeding into the distillation column.

In one embodiment, one or more separators may be provided for receiving cooled and partially condensed natural gas feed streams to separate the feed streams into liquid streams and gas streams.
The fractionator column receives the gas stream and reflux streams and the intermediate pressure vessel separates the depressurized fractionator bottoms into a gas stream and a liquid stream. The gas stream is utilized in the absorber column as reflux to liquefy hydrocarbons present in the feed stream.
The distillation column in fluid communication with the intermediate pressure vessel produces an overheads stream employed in the absorber column as reflux streams to liquefy hydrocarbons present in the feed stream. The distillation column produces a bottoms stream which is depressurized in the distillation column to recover selected hydrocarbons in the absence of further unit operations.
Contributory to the effectiveness of the technology herein is the heat exchange between the streams for exemplary heat recovery and utilization. With the stream heat exchange, synthesis commingling and redirection, high process energy efficiency is attainable. This is a further benefit of the novel operation unification discussed herein, supra.
The unification of the operations is yet another object of the present invention. This object is directed to a method of processing natural gas with a fractionator column and distillation column in fluid communication to recover selected hydrocarbons contained therein, comprising a plurality of operations, comprising:

- a separation operation where a natural gas stream is processed to form a liquid stream and a gas stream which are separated;
- an intermediate pressurization operation fluid communication with the fractionator column for producing an overheads stream;
- an overheads processing and distribution operation where an overheads stream is recovered from the distillation column and processed into a plurality of overhead streams distributed into predetermined locations in the fractionator column;
- a distillation operation in the fractionator column where the natural gas feed stream contacts the overhead streams to liquefy hydrocarbon components present in the natural gas stream; and
- a bottoms recovery and processing operation where a bottoms stream is recovered from the fractionator column and processed by depressurization into an intermediate pressure vessel in the intermediate pressurization operation.

The fractionator bottoms may be used to cool the feed and reflux streams, and fed into the distillation column.
Advantageously, exchange operations between each of the separation, overheads processing and distribution, refrigeration and bottoms recovery operations recover and distribute heat between all streams associated with the operations.
Where the distillation tower is operated as a deethanizer, propane recovery is in excess of 99%, without the required use of turboexpanders. Further, the method accepts a range of natural gas feed pressures, including low pressures such as approximately 5000 kPag and can operate at an increased natural gas feed pressure and a higher chiller temperature with reduced chilling duty. For some feed gas compositions and feed pressures, the requirement for chilling can be eliminated altogether. For chilling, the method uses refrigerants such as propane, or ammonia at temperatures of approximately −40° C.
Heat for the deethanizer reboiler may be provided by heat exchange with a heat medium fluid or the hot discharge gas of a compressor. Other scenarios are within the purview of one skilled.
As an option, the deethanizer may use a side reboiler heated by the process fluid to further improve the recovery and energy efficiency of the process.
Regarding the reflux streams, the same may be compressed to pressures in the range of approximately 6000 kPag to 10000 kPag and depressurized into the fractionator.
Owing to the flexibility of the methods, some heat exchangers or chillers may be removed from the process and the stream flows altered to reduce the equipment costs, such that the propane recovery is reduced.
Where the distillation column is operated as a demethanizer. The ethane recovery is in excess of 99% similarly as above in the case of the deethanizer, without the required use of turboexpanders or cryogenic pumps within the process.
In this variation of the process, a range of natural gas feed pressures may be utilized, including pressures above approximately 7000 kPag.
For switching between the propane or ethane recovery the method can selectively recover either ethane and lower boiling point components, or propane and lower boiling point components, from a natural gas stream.
It has been found that propane recovery is in excess of 99% in a propane recovery mode with ethane rejection and ethane recovery is in excess of 99% in ethane recovery mode, without the required use of turboexpanders or cryogenic pumps within the process.
For feed gas compositions and pressures that require chilling to reach the required recovery, the method uses refrigerants such as propane, or ammonia, at temperatures of approximately −40° C. for ethane recovery, and −40° C. or lower temperatures for propane recovery.
Having thus generally described the invention, reference will now be made to the accompanying drawings, illustrating preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram in accordance with a first embodiment of the present invention;

FIG. 2 is a process flow diagram in accordance with a second embodiment of the present invention;

FIG. 3 is a is a process flow diagram in accordance with a third embodiment of the present invention;

FIG. 4 is a is a process flow diagram in accordance with a fourth embodiment of the present invention;

FIG. 5 is a process flow diagram in accordance with a fifth embodiment of the present invention;

FIG. 6 is a process flow diagram in accordance with a further embodiment of the present invention;

FIG. 7 is a process flow diagram in accordance with another embodiment of the present invention;

FIG. 8 is a process flow diagram in accordance with yet another embodiment of the present invention;

FIG. 9 is a process flow diagram in accordance with yet another embodiment of the present invention;

FIG. 10 a process flow diagram in accordance with yet a still further embodiment of the present invention; and

FIG. 11 a process flow diagram in accordance with yet another embodiment of the present invention.

Similar numerals employed in the Figures denote similar elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 , shown is a process flow diagram for a first embodiment of the present invention for propane recovery and ethane rejection.
A natural gas feed stream 10 at a pressure above, for example, approximately 5000 kPag flows through heat exchanger 12 cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 leaving exchanger 12 is then passed through a chiller 18, which partially condenses the natural gas stream. The chiller 18 may operate at, for example, approximately −40° C. and may use common refrigerants such as propane, or ammonia. Other suitable refrigerant examples are well within the purview of one skilled in the art.
The partially condensed natural gas stream 20 is then separated in separator 22 and the gas portion 24 leaving separator 22 is further cooled by heat exchange at 26 with a sales gas stream 28 and a reflux stream 30, discussed in greater detail herein after. The gas stream 32 is then depressurized across a valve 34 entering, as stream 36, to the bottom of a refluxed fractionator tower 38 at, for example, approximately 3500 kPag. The gas flows upwardly through tower 38 where it is contacted with the downwardly flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of tower 38 as illustrated and are depressurized across a valve 42 directly into the top of a second distillation column 44, operating in the example as a deethanizer at approximately 1000 kPag. The deethanizer (second distillation column 44) is also fed the condensed liquids 46 from the separator 22. Liquids 46 are depressurized via valve 48 into the deethanizer 44.
Distillation of these liquids occurs in the deethanizer 44, with the bottoms product reboiled in circuit 50 to produce a low ethane content C3+ liquid product 52. The deethanizer reboiler circuit can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.
The deethanizer overheads 54 flow to a heat exchanger 56, which is used to further cool the stream after compression in compressor 58 to a pressure above the fractionator operating pressure followed by cooling in an air exchanger 60. The overheads stream 62 is then chilled and partially condensed in a chiller 64, and further condensed in an exchanger 66 with the sales gas, prior to being separated in separator 68.
The liquid stream 70 from the separator 68 is used as a medium quality source of reflux, with a composition that is approximately half methane and half ethane. The medium quality reflux stream 70 is fed into a mid-point of the fractionator tower 38 as illustrated in the Figure.
The gas stream 72 from the separator 68 can optionally be split into two streams 30 (referenced herein previously) and 73, with the ratio of the split determining the degree of propane recovery versus the refrigeration and reflux compression energy requirement. For the highest propane recovery, all of the gas stream from separator 68 can be used to generate high quality reflux liquids. The stream 30 is used to cool the feed to the fractionator 38 in a heat exchanger 76. The stream 74 then flows to another heat exchanger 78 which is used to further cool the stream after compression in compressor 80 and cooling in an air exchanger 82.
The stream 84 is compressed to approximately 7000 kPag, and then chilled in chiller 86 and cooled with heat exchanger 88 with the sales gas stream 90 from fractionator tower 38. Depressurization of this stream 92 is achieved by valve 94 to the fractionator tower 38 and this depressurization condenses a portion of the stream into high quality liquefied natural gas reflux, composed mainly of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwardly flowing natural gas into the liquid phase. The fractionator overheads 90 have propane and heavier components effectively absorbed by the reflux and this exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux streams, fractionator feed, and inlet natural gas in heat exchangers 88, 66, 26, and 12 and then the sales gas exits the process for further compression, if required.
Turning now to FIG. 2 , shown is a process flow diagram in another embodiment of the invention for ethane recovery.
In this embodiment, natural gas stream 10 at a pressure above approximately 7000 kPag flows through a heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 is then passed through a series of heat exchangers and a chiller, 18, 100 and 102, respectively, which partially condenses the natural gas stream. The chiller 100 may operate at approximately −40° C., and may use common refrigerants such as propane, or ammonia. The partially condensed natural gas stream 104 is then separated in separator 22 and the gas portion 24 is then depressurized across a valve 34, entering the bottom of fractionator tower 38 at approximately 3500 kPag.
The gas flows upwards through the refluxed fractionator tower 38 where it is contacted with the downwards flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of the tower 38 and are depressurized across a valve 42 directly into the top of second distillation column 44 operating as a demethanizer at approximately 1000 kPag. The demethanizer 44 is also fed the condensed liquids 46 from the separator 22 which are depressurized across a valve 48 into the demethanizer.
Distillation of these liquids occurs in the demethanizer 44 with the bottoms product reboiled in a reboiler circuit 50 to produce a low methane content C2+ liquid product 52. The demethanizer reboiler circuit 50 can be heated by heat exchange with the feed gas stream, another process stream of appropriate temperature, or a heat medium stream. The demethanizer overheads 54 flows to a series of heat exchangers 102, 18 which are used to cool the feed gas stream.
The demethanizer overheads then flow to a compressor 58 which compresses the gas stream 106 to a pressure above the fractionator operating pressure, and then the gas is cooled in an air exchanger 60. This gas is then split into two streams 108,110 with the ratio of the split determining the degree of ethane recovery versus the refrigeration and reflux compression energy requirement.
One of these two streams 108 can be chilled and depressurized into a mid-point of the fractionator tower 38. The other of these two split streams, can be further compressed to approximately 9000 kPag in compressor 111, cooled with an air cooler 112, chilled with chiller 114, cooled by heat exchangers 116, 118 with the fractionator tower overheads 120 and depressurized across a valve 122 into the fractionator tower 38 to generate high quality reflux liquids.
Depressurization of this stream 124 to the fractionator tower 38 pressure condenses a portion of the stream into high quality liquefied natural gas reflux, consisting primarily of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the ethane and heavier components from the upwards flowing natural gas, into the liquid phase. The fractionator overheads 120 have had ethane and heavier components effectively absorbed by the reflux, and exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux stream, and inlet natural gas in heat exchangers (118, 116, 12), and then the sales gas exits the process for further compression, if required.
FIG. 3 illustrates yet another variation of the present invention for switchable ethane recovery.
The process may be arranged so that the ethane recovery can be switched on and off. The figure illustrates the ethane rejection mode. Generally speaking, the process is very similar to that discussed regarding FIG. 1 , with the exception of the following:
The feed gas pressure for high ethane recovery is desirably above approximately 7000 kPag. The distillation column 44 operates as a demethanizer as opposed to a deethanizer. Overheads stream 54 from the demethanizer 44 is redirected from the exchanger 56 to the exchanger 76.
The gaseous stream 72 from the separator 68 is redirected from exchanger 76 to exchanger 78.
Stream 74 from the exchanger 76 is redirected from the exchanger 76 to the exchanger 56. The demethanizer reboiler 44 can instead be heated using a heat medium, or by heat exchange with process streams such as the feed gas stream 16.
Turning now to FIG. 4 , shown is a propane recovery, ethane rejection process flow diagram.
Natural gas stream 10 at a pressure above approximately 5000 kPag flows through heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process at 14.
The natural gas stream 16 is then passed through a heat exchanger 126 which partially condenses the natural gas stream. The partially condensed natural gas stream 128 is then separated in separator 22 and the gas portion 24 is further cooled in a chiller 27. The chiller 27 may operate at approximately −40° C., and may use common refrigerants such as propane, or ammonia.
The partially condensed gas stream is then depressurized across valve 34, entering the bottom of a fractionator tower 38 at approximately 3500 kPag. The gas flows upwardly through the refluxed fractionator tower 38 where it is contacted with the downwardly flowing reflux liquids.
The fractionator tower 38 bottoms liquid 40 exit the bottom of the tower 38 and are depressurized across valve 42 directly into the top of a second distillation column 44 operating as a deethanizer at approximately 1000 kPag.
The deethanizer 44 is also fed the condensed liquids 46 from separator 22, which are depressurized across valve 48 into the deethanizer 44.
Distillation of these liquids occurs in the deethanizer 44 with the bottoms product reboiled in circuit 50 to produce a liquid product 52, in this example, a low ethane content C3+.
The deethanizer reboiler circuit 50 can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.
The deethanizer overheads 54 flow to heat exchanger 126 which uses the overheads to cool the feed gas. The overheads then flow to another heat exchanger 56 which is used to further cool the stream after compression with compressor 58 to a pressure above the fractionator operating pressure, and cooling in air exchanger 60.
The overheads stream is then split into two streams 108 and 110 each to be used as a source of reflux in the absorber.
Reflux stream 108 is cooled by heat exchange 56 with the deethanizer overheads stream, then chilled with chiller 132, partially condensed and depressurized across valve 134 into a midpoint of the fractionator tower 38 as a source of medium quality reflux.
The other split reflux stream 110 is further compressed at 111, cooled with air exchanger 112, heat exchange at 116, chilled at 114, and further cooled and condensed by heat exchange at 118 with the sales gas. Depressurization of this stream across valve 122 to the fractionator tower 38 pressure condenses a portion of the stream into high quality reflux. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwardly flowing natural gas, into the liquid phase.
The fractionator overheads 120 at this point in the process have had propane and heavier components effectively absorbed by the reflux and the exit the fractionator 38 as sales gas.
The sales gas is used to cool the reflux streams, fractionator feed, and inlet natural gas in heat exchangers and then the sales gas exits the process for further compression, if required.
FIG. 5 is a process flow diagram depicting propane recovery with ethane rejection).
Similar to the previous embodiments, natural gas stream 10 at a pressure above approximately 5000 kPag flows through heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process at 14.
The natural gas stream 16 is then passed through heat exchanger 126, which partially condenses the natural gas stream. The partially condensed natural gas stream 128 is then separated with separator 22 with gas portion 24 further cooled in chiller 27.
Chiller 27 may operate at approximately −40° C.
The partially condensed gas stream (9) is further cooled by heat exchange with the sales gas stream as shown and depressurized across valve 34, entering the bottom of a fractionator tower 38 at a pressure of approximately 3500 kPag.
The gas flows upwardly through the refluxed fractionator tower 38 where it is contacted with the downwardly flowing reflux liquids.
The fractionator tower 38 bottoms liquids 40 exit the bottom of the tower 38. and are depressurized across valve 42 directly into the top of distillation column 44, operating as a deethanizer at approximately 1000 kPag.
The deethanizer 44 is also fed the condensed liquids 46 from the separator 22, which are depressurized across valve 48 into the deethanizer 44. Distillation of these liquids occurs in the deethanizer 44, with the bottoms product reboiled in circuit 50 to produce liquid product 52 which in this example is a low ethane content C3+ product.
The deethanizer reboiler circuit 50 can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.
The deethanizer overheads 54 flow to heat exchanger 126 which uses the overheads to cool the feed gas. The overheads then flow to another heat exchanger 56, which is used to further cool the stream after compression at 58 to a pressure above the fractionator operating pressure, and cooling in air exchanger 60.
The overheads stream is then split into two streams 108 and 110 each to be used as a source of reflux in the absorber.
Reflux stream 108 is cooled by heat exchanger 56 with the deethanizer overheads stream, then chilled at 132, further cooled and condensed by heat exchange at 138 with the sales gas, and depressurized across valve 134 into a mid-point of the fractionator tower 38 as a source of medium quality reflux.
The other split reflux stream 110 is further compressed at 111, cooled with air exchanger 112, heat exchange at 116, chilled at 114 and further cooled and condensed by heat exchange 118 with the sales gas.
Depressurization of this stream across a valve 122 to the fractionator tower pressure condenses a portion of the stream into high quality reflux. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwards flowing natural gas, into the liquid phase.
The fractionator overheads 120 at this point have had propane and heavier components effectively absorbed by the reflux, and the exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux streams, fractionator feed, and inlet natural gas in heat exchangers and then the sales gas exits the process for further compression, if required.
These processes can economically achieve higher ethane and/or propane recoveries than the traditional turbo-expander processes with lower plant inlet pressures. Generally, depending upon gas composition, it is desired to have a plant inlet pressure of approximately 7000 kPa to achieve 99% ethane recovery. When inlet pressures are higher, energy, capital cost, and operating cost can be saved by having lower refrigerant temperatures than −40° C. Depending upon markets conditions for ethane sales and energy purchase, some scenarios may favor lower ethane recovery for reasons of economics. Lower ethane recovery can be easily achieved with warmer refrigerant temperatures, but raising the demethanizer pressure and/or the gas fractionator pressure can also be considered.
Similarly, an inlet pressure of approximately 5000 kPa is generally desired for 99% propane recovery, but with higher inlet pressures, lower refrigerant temperatures can save capital cost, energy cost, and operating cost and still achieve an economical 99% propane recovery.
Referring now to FIG. 6 , shown is a process flow diagram for a first embodiment of the present invention for propane recovery and ethane rejection, without the use of a refrigeration system for chilling.
A natural gas feed stream 10 at a pressure above, for example, approximately 6500 kPag flows through heat exchanger 12 cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 leaving exchanger 12 is then passed through heat exchanger 18′, which further cools and may partially condense the natural gas stream, by exchanging heat with the deethanizer (distillation column 44) overheads stream 54. The natural gas stream is then further cooled by exchanging heat with the cold liquids leaving process separators 22 and 140, in heat exchangers 100 and 102, further partially condensing gas. The partially condensed natural gas stream 20 is then separated in separator 22 and the gas portion 24 leaving separator 22 is depressurized across a valve 34 entering, as stream 36, to the bottom of a refluxed fractionator tower 38 at, for example, approximately 3500 kPag. The gas flows upwardly through tower 38 where it is contacted with the downwardly flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of tower 38 as illustrated and are depressurized across a valve 42 into the intermediate pressure vessel 140 operating at approximately 1000 kPag. The liquids stream 142 leaving the intermediate pressure vessel 140 are then used to cool the reflux stream entering the top of the fractionator 38, and the natural gas feed stream, with heat exchangers 88 and 102. The fractionator bottoms liquids are then fed into the deethanizer 44. The condensed liquids 46 from the separator 22 are depressurized across valve 48 through heat exchanger 100 in order to cool the natural gas feed stream, and then the stream is also fed into the deethanizer 44.
Distillation of these liquids occurs in the deethanizer 44, with the bottoms product reboiled in circuit 50 to produce a low ethane content C3+ liquid product 52. The deethanizer reboiler circuit can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.
The deethanizer overheads 54 flow to heat exchanger 18′ which is used to cool natural gas feed stream 16. The deethanizer overheads 106 then flow to heat exchanger 56 which is used to cool the stream after compression in compressor 58 to a pressure above the fractionator operating pressure followed by cooling in air exchanger 60.
The overheads stream 336 is then further cooled with fractionator overheads gas in exchanger 66, and fed into the fractionator 38 at a mid-point of the fractionator tower as a source of medium quality reflux.
The gas stream 154 leaving the intermediate pressure vessel 140, can be split into two streams 156 and 158, in order to provide an appropriate volume of reflux liquids to the fractionator 38. The stream 158 that is not required for fractionator reflux, is fed to the deethanizer 44. The stream 156 intended for fractionator 38 reflux then flows to another heat exchanger 178 which is used to further cool the stream after compression in compressor 111 and cooling in an air exchanger 112.
The stream has been compressed to approximately 7000 kPag, and is then cooled in exchanger 88 by heat transfer with the depressurized liquids stream 142 leaving the intermediate pressure vessel. Depressurization of this stream 124 is achieved by valve 122 to the fractionator tower 38 and this depressurization condenses a portion of the stream into high quality liquefied natural gas reflux, composed mainly of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwardly flowing natural gas into the liquid phase.
The fractionator overheads 120 have propane and heavier components effectively absorbed by the reflux and this exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux, and inlet natural gas in heat exchangers 66 and 12, and then the sales gas exits the process for further compression, if required.
Turning now to FIG. 7 , shown is a process flow diagram in another embodiment of the invention for propane recovery, requiring a refrigeration system for chilling.
Natural gas feed stream 10 at a pressure above, for example, approximately 5000 kPag flows through heat exchanger 12 cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 leaving exchanger 12 is then passed through heat exchanger 18′, which further cools and may partially condense the natural gas stream, by exchanging heat with the deethanizer (distillation column 44) overheads stream 54. The natural gas stream is then further cooled with chiller 100′. The chiller 100′ may operate at approximately −40° C., and may use common refrigerants such as propane, or ammonia. The partially condensed natural gas stream 20 is then separated in separator 22 and the gas portion 24 leaving separator 22 is depressurized across a valve 34 entering, as stream 36, to the bottom of a refluxed fractionator tower 38 at, for example, approximately 3500 kPag. The gas flows upwardly through tower 38 where it is contacted with the downwardly flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of tower 38 as illustrated and are depressurized across a valve 42 into intermediate pressure vessel 140 operating at approximately 1000 kPag. The liquids stream 142 leaving the intermediate pressure vessel 140 are then used to cool the reflux stream entering the top of the fractionator 38. The fractionator bottoms liquid stream is then fed into the deethanizer 44. The condensed liquids 46 from the separator 22 are depressurized across valve 48, and then the stream is also fed into the deethanizer 44.
Distillation of these liquids occurs in the deethanizer 44, with the bottoms product reboiled in circuit 50 to produce a low ethane content C3+ liquid product 52. The deethanizer reboiler circuit 50 can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.
The deethanizer overheads 54 flow to a heat exchanger 18, which is used to cool natural gas feed stream 16. The deethanizer overheads 106 then flow to heat exchanger 56 which is used to cool the stream after compression in compressor 58 to a pressure above the fractionator operating pressure followed by cooling in air exchanger 60.
The overheads stream 62 is then further cooled by heat exchange with the sales gas in heat exchangers 64′ and 66, and is then fed into the fractionator 38 at a mid-point of the fractionator tower as a source of medium quality reflux.
The gas stream 154 leaving the intermediate pressure vessel 140, can be split into two streams 156 and 158, in order to provide an appropriate volume of reflux liquids to the fractionator 38. The stream 158 that is not required for fractionator reflux, is fed to the deethanizer. The stream 156 intended for fractionator 38 reflux then flows to heat exchanger 178 which is used to further cool the stream after compression in compressor 111 and cooling in an air exchanger 112.
The stream 84′ has been compressed to approximately 7000 kPag, and is then cooled by heat exchange with the sales gas in exchangers 116 and 118, and is also chilled in chiller 114. The stream 86 is then cooled in exchanger 88 by heat transfer with the depressurized liquids stream 142 leaving the intermediate pressure vessel. Depressurization of this stream 124 is achieved by valve 122 into the fractionator tower 38 and this depressurization condenses a portion of the stream into high quality liquefied natural gas reflux, composed mainly of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwardly flowing natural gas into the liquid phase. The fractionator overheads 120 have propane and heavier components effectively absorbed by the reflux and this exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux streams, and inlet natural gas in heat exchangers 118, 66, 116, 64′ and 12, and then the sales gas exits the process for further compression, if required.
FIG. 8 illustrates yet another variation of the present invention for ethane recovery.
In this embodiment, natural gas stream 10 at a pressure above approximately 7000 kPag flows through a heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 is then passed through a chiller 100′, which partially condenses the natural gas stream. The chiller 100′ may operate at approximately −40° C., and may use common refrigerants such as propane, or ammonia. The natural gas stream is then split, such that appropriate reflux stream cooling and tower temperature profiles can be achieved. One of the split streams 170 is depressurized across valve 172, into separator 176 at approximately 3500 kPag. The gas stream leaving the separator 176 is fed into fractionator 38, and the liquid stream 180 leaving the separator 176 is depressurized across valve 184 into the demethanizer 44. The other portion of the partially condensed natural gas feed stream 101 is further cooled and condensed by heat exchange with sales gas and the demethanizer overheads in exchangers 102 and 18′. The stream is then depressurized and separated into gas and liquid streams in separator 22, with the gas portion 24 being fed into the fractionator tower 38 at approximately 3500 kPag.
The gas flows upwards through the refluxed fractionator tower 38 where it is contacted with the downwards flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of the tower 38 and are depressurized across valve 42 directly into the top of second distillation column 44 operating as a demethanizer at approximately 1000 kPag. The demethanizer 44 is also fed the condensed liquids 46 from the separator 22 which are depressurized across a valve 48 into the demethanizer. The demethanizer 44 is also fed the condensed liquids 180 from the separator 176 which are depressurized across a valve 184 into the demethanizer 44.
Distillation of these liquids occurs in the demethanizer 44 with the bottoms product reboiled in a reboiler circuit 50 to produce a low methane content C2+ liquid product 106. The demethanizer reboiler circuit 50 can be heated by heat exchange with the feed gas stream, another process stream of appropriate temperature, or a heat medium stream. The demethanizer overheads 54 is used to cool and further condense the feed gas stream 104 using heat exchanger 18′.
The demethanizer overheads then flow to heat exchanger 116, which further cools the overheads after compression and air cooling (58, 60, 111, 112). The overheads stream is then chilled at 114, and further cooled by heat exchange with the fractionator overheads stream 120, in heat exchanger 118. The stream is then depressurized across valve 122 into the fractionator tower 38, which condenses a portion of the stream into high quality liquefied natural gas reflux, consisting primarily of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the ethane and heavier components from the upwards flowing natural gas, into the liquid phase. The fractionator overheads 120 have had ethane and heavier components effectively absorbed by the reflux, and exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux stream, and inlet natural gas in heat exchangers 18′, 102, 12), and then the sales gas exits the process for further compression, if required.
FIG. 9 illustrates yet another variation of the present invention, configured such that ethane recovery can be switched on and off using a single apparatus.
In this embodiment, in ethane recovery mode, natural gas stream 10 at a pressure above approximately 7000 kPag flows through a heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 is then passed through a chiller 100′, which partially condenses the natural gas stream. The chiller 100′ may operate at approximately −40° C., and may use common refrigerants such as propane, or ammonia. The natural gas stream is then split, such that appropriate reflux stream cooling and tower temperature profiles can be achieved. One of the split streams 170 is depressurized across valve 172, into separator 176 at approximately 3500 kPag. The gas stream 188 leaving the separator 176 is fed into fractionator 38, and the liquid 180 stream leaving the separator 176 is depressurized across valve 184 into the demethanizer 44. The other portion of the partially condensed natural gas feed stream 101 is further cooled and condensed by heat exchange with sales gas and the demethanizer overheads in exchangers 102 and 18. The stream is then depressurized and separated into gas and liquid streams in separator 22, with the gas portion 24 being fed into the fractionator tower 38 at approximately 3500 kPag.
The gas stream 24 flows upwards through the refluxed fractionator tower 38 where it is contacted with the downwards flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of the tower 38 and are depressurized across a valve 42 directly into the top of second distillation column 44 operating as a demethanizer at approximately 1000 kPag. The demethanizer 44 is also fed the condensed liquids 46 from the separator 22 which are depressurized across a valve 48 into the demethanizer. The demethanizer 44 is also fed the condensed liquids 180 from the separator 176 which are depressurized across a valve 184 into the demethanizer.
Distillation of these liquids occurs in the demethanizer 44 with the bottoms product reboiled in a reboiler circuit 50 to produce a low methane content C2+ liquid product 106. The demethanizer reboiler circuit 50 can be heated by heat exchange with the feed gas stream, another process stream of appropriate temperature, or a heat medium stream. The demethanizer overheads 54 is used to cool and further condense the feed gas stream 104 using heat exchanger 18.
The demethanizer overheads 76 then flow to heat exchanger 116, which further cools the overheads after compression and air cooling (58, 60, 111, 112). The overheads stream is then chilled, and further cooled by heat exchange with the fractionator overheads stream 120, in heat exchanger 118. The stream 124 is then depressurized across valve 122 into the fractionator tower 38, which condenses a portion of the stream into high quality liquefied natural gas reflux, consisting primarily of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the ethane and heavier components from the upwards flowing natural gas, into the liquid phase. The fractionator overheads 120 have had ethane and heavier components effectively absorbed by the reflux, and exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux stream, and inlet natural gas in heat exchangers (118, 102, 12), and then the sales gas exits the process for further compression, if required.
In order to switch the system from ethane recovery mode, to ethane rejection mode, a number of valves must be changed in position to appropriately redirect stream flows. Changing the positions of valves 300, 304, 308, 312, 316, 320, 324, 328, 332, 336, 340, and 344, and operating the distillation tower 44 as a deethanizer, instead of a demethanizer, allows the process to operate in ethane rejection mode. In this mode, a natural gas feed stream 10 at a pressure above, for example, approximately 5000 kPag flows through heat exchanger 12 cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 leaving exchanger 12 is then passed through a chiller 100′, which partially condenses the natural gas stream. The chiller 100′ may operate at, for example, approximately −40° C. and may use common refrigerants such as propane, or ammonia. Other suitable refrigerant examples are well within the purview of one skilled in the art.
The partially condensed natural gas stream 20 is then further cooled by heat exchange at 102 and 18 with a sales gas stream 28 and a reflux stream 30, then depressurized across valve 172 into separator 176. The order of this chilling, heat exchange, depressurization, and separation can occur in a different order, as most advantageous from an energy standpoint for the particular gas composition. The gas portion 188 leaving separator 176 is discussed in greater detail herein after. The gas stream 188 then enters the bottom of a refluxed fractionator tower 38 at, for example, approximately 3500 kPag. The gas flows upwardly through tower 38 where it is contacted with the downwardly flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of tower 38 as illustrated and are depressurized across a valve 42 directly into the top of a second distillation column 44, operating in the example as a deethanizer at approximately 1000 kPag. The deethanizer (second distillation column 44) is also fed the condensed liquids 180 from the separator 176. Liquids 180 are depressurized via valve 184 into the deethanizer 44.
Distillation of these liquids occurs in the deethanizer 44, with the bottoms product reboiled in circuit 50 to produce a low ethane content C3+ liquid product 52. The deethanizer reboiler circuit can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.
The deethanizer overheads 54 flow to a heat exchanger 200, which is used to further cool the stream after compression in compressor 58 to a pressure above the fractionator operating pressure followed by cooling in an air exchanger 60. The overheads stream 62 is then chilled and partially condensed in a chiller 206, and further condensed in an exchanger 210 with the sales gas, prior to being separated in separator 22.
The liquid stream 46 from the separator 22 is used as a medium quality source of reflux, with a composition that is approximately half methane and half ethane. The medium quality reflux stream is fed into a mid-point of the fractionator tower 38 as illustrated in the Figure.
The gas stream from the separator 22 can optionally be split into two streams 24 (referenced herein previously) and 74, with the ratio of the split determining the degree of propane recovery versus the refrigeration and reflux compression energy requirement. For the highest propane recovery, all of the gas stream from separator 22 can be used to generate high quality reflux liquids. The stream 74 is used to cool the feed to the fractionator 38 in a heat exchanger 18. The stream 76 then flows to another heat exchanger 116 which is used to further cool the stream after compression in compressor 111 and cooling in an air exchanger 112.
The stream 115 is compressed to approximately 7000 kPag, and then chilled in chiller 114 and cooled with heat exchanger 118 with the sales gas stream 120 from fractionator tower 38. Depressurization of this stream 124 is achieved by valve 122 to the fractionator tower 38 and this depressurization condenses a portion of the stream into high quality liquefied natural gas reflux, composed mainly of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwardly flowing natural gas into the liquid phase.
The fractionator overheads 120 have propane and heavier components effectively absorbed by the reflux and this exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux streams, fractionator feed, and inlet natural gas in heat exchangers 118, 102, 210 and 12 and then the sales gas exits the process for further compression, if required.
Turning now to FIG. 10 , shown is a process flow diagram in another embodiment of the invention for ethane recovery, that does not require a refrigeration system or turboexpander for cooling.
A natural gas feed stream 10 at a pressure above, for example, approximately 6500 kPag flows through heat exchanger 12 cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 15 leaving exchanger 12 is then passed through heat exchangers 400, 402, 404, and 406, which further cools the natural gas stream by heat exchange with the sales liquids, demethanizer 44 overheads, condensed feed liquids, and intermediate liquids streams 408, 410, 412, and 414 which further cools partially condenses the natural gas stream for separation in separator 416. The order of these heat exchangers, depressurization, and separation can occur in a different order, as most advantageous from an energy standpoint for the particular gas composition. The liquids outlet from separator 416 is depressurized across valve 418, then is used to cool the gas outlet of separator 404, and also cool the feed gas stream, and is fed into the demethanizer tower 44 operating at approximately 1000 kPag. The gas outlet from separator 416 is then cooled by the depressurized liquids from separator 416, and the gas stream is then depressurized to approximately 3500 kPag, forming more liquids which are separated in separator 420. The liquids stream leaving separator 420 are depressurized through valve 422 into an intermediate pressure distillation tower 424. The gas stream 426 leaving separator is fed to a refluxed fractionator tower 428. The gas flows upwardly through fractionator tower 428 where it is contacted with the downwardly flowing reflux liquids. The fractionator tower 428 bottoms liquids 430 exit the bottom of tower 428 as illustrated and are depressurized across a valve 432 into intermediate pressure distillation tower 424. The intermediate pressure distillation tower 424 accepts several sources of heavier, primarily liquid streams, and distills them, with the overheads of this column being the lean stream used for reflux in the fractionator column.
The liquids stream 432 leaving the intermediate pressure distillation column 424 are then used to cool the reflux stream in exchanger 434, and the partially condensed natural gas feed stream in exchanger 406, before being fed to the demethanizer 44
Distillation of the liquid streams occurs in the demethanizer 44, with the bottoms product reboiled in circuit 436 to produce a low methane content C2+ liquid product 438. The demethanizer reboiler circuit 436 can be heated using an appropriate temperature process stream within the process, such as the natural gas feed stream, or an appropriate heat medium.
The demethanizer overheads 410 flow to heat exchanger 402, which is used to cool natural gas feed stream 18. The demethanizer overheads 440 then flow to a heat exchanger 442 which is used to cool the stream after compression in compressor 446 to a pressure above the fractionator operating pressure followed by cooling in an air exchanger 448.
The overheads stream 450 is then further cooled by heat exchange with the sales gas in heat exchanger 452, and is then depressurized across valve 454 into separator 456. The gas portion of this stream is fed into the fractionator 428, and the liquid portion is further fed into the intermediate pressure distillation tower 424.
The gas stream 37 leaving the intermediate pressure fractionation tower 424 can be split into two streams 458 and 460, in order to provide an appropriate volume of reflux liquids to the fractionator 428. The stream 460 that is not required for fractionator reflux, is fed to the demethanizer. The stream 462 intended for fractionator 428 reflux then flows to another heat exchanger 464 which is used to further cool the stream after compression in compressor 466 and cooling in an air exchanger 468.
The stream 470 has been compressed to approximately 7000 kPag, and is then cooled by heat exchange with the sales gas in exchanger 434. Depressurization of this stream is achieved by valve 472 into the fractionator tower 428 and this depressurization condenses a portion of the stream into high quality liquefied natural gas reflux, composed mainly of liquid methane. This reflux stream flows downwardly through the fractionator tower 428, effectively absorbing the propane and heavier components from the upwardly flowing natural gas into the liquid phase.
The fractionator overheads 474 have ethane and heavier components effectively absorbed by the reflux and this exits the fractionator 428 as sales gas. The sales gas is used to cool the feed and recycle streams in heat exchangers 452 and 12, and then the sales gas exits the process for further compression, if required.
Turning now to FIG. 11 , shown is a process flow diagram in another embodiment of the invention for ethane recovery, using refrigeration for cooling.
In this embodiment, natural gas stream 10 at a pressure above approximately 7000 kPag flows through a heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 is then passed through a chiller 100′, which partially condenses the natural gas stream. The chiller 100′ may operate at approximately −40° C., and may use common refrigerants such as propane, or ammonia. The natural gas stream is then split, such that appropriate reflux stream cooling and tower temperature profiles can be achieved. One of the split streams 500 is depressurized across valve 502, into separator 504 at approximately 3500 kPag. The gas stream 504 leaving the separator 504 is fed into fractionator 508, and the liquid stream 510 leaving the separator 504 is depressurized into intermediate pressure distillation tower 512. The other portion of the partially condensed natural gas feed stream 514 is further cooled and condensed by heat exchange with sales gas and the intermediate pressure distillation tower overheads in exchangers 516 and 518. The stream is then depressurized and separated into gas and liquid streams in separator 520, with the gas portion 522 being fed into the fractionator tower 506 at approximately 3500 kPag.
The gas flows upwards through the refluxed fractionator tower 506 where it is contacted with the downwards flowing reflux liquids. The fractionator tower 506 bottoms liquids 524 exit the bottom of the tower 506 and are depressurized across a valve 526 directly into the top of intermediate pressure distillation tower 512. The intermediate pressure distillation tower 512 is also fed the condensed liquids 528 from the separator 520 which are depressurized across a valve 530 into the intermediate pressure distillation tower 512. The intermediate pressure distillation tower 512 is also fed the condensed liquids 510 from the separator 504 which are depressurized across a valve 532 into the intermediate pressure distillation tower 512.
Distillation of these liquids occurs in the intermediate pressure distillation tower 512 with the bottoms product reboiled in a reboiler circuit 534. The reboiler circuit 534 can be heated by heat exchange with another process stream of appropriate temperature, or a heat medium stream. The intermediate pressure overheads then flow to heat exchanger 536, which further cools the overheads after compression at 538, air cooling at 540, and cooling by heat exchange at 542 with the demethanizer overheads. The overheads stream further cooled by heat exchange with the fractionator overheads stream 544, in heat exchanger 546. The stream is then depressurized across valve 548 into the fractionator tower 506, which condenses a portion of the stream into high quality liquefied natural gas reflux, consisting primarily of liquid methane. This reflux stream flows downwardly through the fractionator tower 506, effectively absorbing the ethane and heavier components from the upwards flowing natural gas, into the liquid phase. At this point, the fractionator overheads 544 have had ethane and heavier components effectively absorbed by the reflux, and exit the fractionator 506 as sales gas. The sales gas is used to cool the reflux stream, and inlet natural gas in heat exchangers (550, 516, 12), and then the sales gas exits the process for further compression, if required.
The intermediate distillation column liquids are depressurized into a demethanizer column operating at approximately 1000 kPag. Distillation of these liquids occurs in the demethanizer 552 with the bottoms product reboiled in a reboiler circuit 554 to produce a low methane content C2+ liquid product 556. The demethanizer reboiler circuit 554 can be heated by heat exchange with the feed gas stream, another process stream of appropriate temperature, or a heat medium stream. The demethanizer overheads 558 are used to cool and further condense the feed gas stream 104 using heat exchanger 518, and to cool the high quality reflux stream using exchanger 542. The demethanizer overheads are then compressed, cooled, chilled, and depressurized (560,562,564,566) into the intermediate pressure distillation tower

Claims

We claim:

1. A method of processing natural gas to recover selected hydrocarbons contained therein, comprising:

cooling and partially condensing a natural gas feed stream;

separating the cooled and partially condensed stream into a plurality of liquid streams and gas streams;

passing said gas streams into a fractionator column and said liquid streams into a distillation column;

recovering an overheads stream from said distillation column;

treating said overheads stream to form at least one overhead stream;

passing said at least one overhead stream into said fractionator column as a recycle stream or source of reflux liquids;

contacting said source of reflux liquids as a reflux stream with said natural gas feed stream in said fractionator column to liquefy hydrocarbon components present in said natural gas feed stream or recycling said recycle stream; and

recovering a bottoms stream from said fractionator column.

2. The method as set forth in claim 1, wherein said overheads stream is treated to form a plurality of overhead streams.

3. The method as set forth in claim 1, wherein said bottoms stream from said fractionator are depressurized in a separator.

4. The method as set forth in claim 3, further including the step of utilizing an intermediate pressure vessel as said separator.

5. The method as set forth in claim 4, further including treating an intermediate pressure vessel overheads stream to form at least one reflux stream.

6. The method as set forth in claim 5, further including the step of passing at least one reflux stream into said fractionator column as a source of reflux.

7. The method as set forth in claim 3, further including the step of depressurizing the separator bottoms into said distillation column and recovering selected hydrocarbon components from said bottoms stream.

8. The method as set forth in claim 1, wherein said method is conducted absent the use of turboexpansion as a unit operation.

9. The method as set forth in claim 1, wherein said treating said at least one overheads stream to form a plurality of overhead streams includes forming a plurality of reflux streams at different pressures.

10. The method as set forth in claim 1, further including the step of varying fluid flow rates at least between said fractionator column and said distillation column.

11. The method as set forth in claim 1, wherein said method has a turndown range between 30% and 100% of flowrate in said method where 99% recovery is maintained.

12. The method as set forth in claim 1, wherein said method utilizes at least cooling, heat exchange, depressurization and separation unit operations, said method further including initially determining the composition of the natural gas; and selecting a sequence of said unit operations based on said composition of said natural gas for energy optimization in effecting said method.

13. A gas processing plant, comprising:

a separator for receiving a cooled and partially condensed natural gas feed stream to separate said feed stream into a liquid stream and a gas stream;

a fractionator column for receiving said gas stream and reflux streams produced from a distillation column;

an intermediate pressure vessel in fluid communication with said fractionator column for producing an overheads stream, said overheads stream being utilized in said fractionator column as said reflux streams to liquefy hydrocarbons present in said feed stream, said fractionator column producing a bottoms stream which is depressurized into a distillation column to recover selected hydrocarbons; and

a distillation column in fluid communication with said intermediate pressure vessel, said intermediate pressure vessel producing a bottoms stream for feeding into said distillation column.

14. The gas processing plant as set forth in claim 13, further including a network of heat exchangers positioned to recover and distribute heat from all streams of said streams during processing.

15. The gas processing plant as set forth in claim 13, wherein said overheads stream is compressed, cooled and divided into a plurality of overhead streams.

16. The gas processing plant as set forth in claim 13, wherein at least one of said overhead streams is compressed, cooled and depressurized for use in said fractionator column as a reflux stream.

17. The gas processing plant as set forth in claim 13, wherein said overhead streams are pressurized at a different pressure relative to one another.

18. The gas processing plant as set forth in claim 13, wherein said overhead streams may exist in a gas, partially condensed, or liquid state.

19. A method of processing natural gas with a fractionator column and distillation column in fluid communication to recover selected hydrocarbons contained therein, comprising a plurality of operations, comprising:

a separation operation where a natural gas stream is processed to form a liquid stream and a gas stream which are separated;

an intermediate pressurization operation fluid communication with said fractionator column for producing an overheads stream;

an overheads processing and distribution operation where an overheads stream is recovered from said distillation column and processed into a plurality of overhead streams distributed into predetermined locations in said fractionator column;

a distillation operation in said fractionator column where said natural gas feed stream contacts said overhead streams to liquefy hydrocarbon components present in said natural gas stream; and

a bottoms recovery and processing operation where a bottoms stream is recovered from said fractionator column and processed by depressurization directly in said distillation column in the absence of turboexpansion to recover selected hydrocarbons.

20. The method as set forth in claim 19, wherein said method further includes heat exchange operations between each of the separation, overheads processing and distribution, refrigeration and bottoms recovery operations to recover and distribute heat between all streams associated with said operations.

21. The method as set forth in claim 19, wherein said distillation column is operated as a deethanizer.

22. The method as set forth in claim 19, wherein said distillation column is operated as a demethanizer.

23. The method as set forth in claim 19, wherein said operations are modifiable to recover ethane, propane, hydrocarbons, or a combination thereof.

24. The method as set forth in claim 19, wherein said separation operation includes cooling and partially condensing said natural gas feed stream.

25. The method as set forth in claim 19, wherein said bottoms recovery and processing operation includes introducing said bottoms stream at a pressure of between 500 kPag and 2000 kPag into said distillation column relative to a pressure of between 2000 and 7000 kPag prior to introduction.