Patent Application
20190120550
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Natural gas liquids (NGL) may describe heavier gaseous hydrocarbons: ethane (C2H6), propane (C3H8), normal butane (n-C4H10), isobutane (i-C4H10), pentanes, and even higher molecular weight hydrocarbons, when processed and purified into finished by-products. Systems can be used to recover NGL from a feed gas using natural gas liquids plants.
In an embodiment, a natural gas liquid plant may be configured to operate in either ethane rejection or ethane recovery and may comprise an absorber configured to produce an ethane rich bottom stream and an ethane depleted vapor stream; a stripper fluidly coupled to the absorber configured to, during ethane rejection, fractionate the ethane rich bottom stream from the absorber into an ethane overhead product and a propane plus hydrocarbons product, and configured to, during ethane recovery, fractionate the ethane rich bottom stream into an ethane plus NGL stream and an overhead vapor stream; and an expander configured to, during ethane recovery, expand a vapor portion of a feed gas to the plant, and feed the expanded stream to the absorber.
In an embodiment, a method for operating a natural gas liquid plant in ethane recovery may comprise expanding a vapor portion of a feed gas to the plant to produce a chilled stream; feeding the chilled stream to an absorber; heating, by the exchanger, a vapor stream from the absorber; feeding the cooled ethane rich bottom stream to a stripper; and fractionating, by the stripper, the cooled ethane rich bottom stream into an ethane plus natural gas liquid stream and an overhead vapor stream.
In an embodiment, a method for operating an ethane rejection natural gas liquid plant in an ethane recovery mode may comprise fluidly coupling an expander to an absorber of the plant; expanding, by the expander, a vapor portion of a feed gas to the plant to produce a chilled stream; feeding the chilled stream to the absorber; fluidly coupling an exchanger to the absorber; cooling, by the exchanger, an ethane rich bottom stream from the absorber; heating, by the exchanger, a vapor stream from the absorber; feeding the cooled ethane rich bottom stream to a stripper; and producing, by the stripper, an ethane plus natural gas liquid stream.
For a more complete understanding of the present disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.
It should be understood at the outset that although illustrative implementations of one or more embodiments are illustrated below, the disclosed systems and methods may be implemented using any number of techniques, whether currently known or not yet in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, but may be modified within the scope of the appended claims along with their full scope of equivalents.
The following brief definition of terms shall apply throughout the application:
The term “comprising” means including but not limited to, and should be interpreted in the manner it is typically used in the patent context;
The phrases “in one embodiment,” “according to one embodiment,” and the like generally mean that the particular feature, structure, or characteristic following the phrase may be included in at least one embodiment of the present invention, and may be included in more than one embodiment of the present invention (importantly, such phrases do not necessarily refer to the same embodiment);
If the specification describes something as “exemplary” or an “example,” it should be understood that refers to a non-exclusive example;
The terms “about” or “approximately” or the like, when used with a number, may mean that specific number, or alternatively, a range in proximity to the specific number, as understood by persons of skill in the art field; and
If the specification states a component or feature “may,” “can,” “could,” “should,” “would,” “preferably,” “possibly,” “typically,” “optionally,” “for example,” “often,” or “might” (or other such language) be included or have a characteristic, that particular component or feature is not required to be included or to have the characteristic. Such component or feature may be optionally included in some embodiments, or it may be excluded.
All references to percentages of flow refer to volumetric percentages unless otherwise indicated.
Most natural gas plants are designed to condition the feed gas to meet the pipeline sales gas specification, for example including heating value specification, hydrocarbon dew point, and water content. Typically, natural gas plants can be used to extract the propane plus components. However, when the feed gas contains a higher amount of ethane, extraction of propane may not be sufficient due to the high heating value of the feed gas, which is mainly due to the presence of ethane.
Typically, the main revenue from the gas plant operation is generated from sales of the condensate components, including propane, butanes, pentanes, and heavier hydrocarbons. Therefore, typical gas plants may be configured to maximize propane recovery. In the past, the ethane content in the feed gas was valued only for its heating content, and there were no significant incentives for ethane recovery. However, with increasing demand from petrochemical facilities to use ethane as a feedstock, ethane may be more valuable if recovered. Considering this market potential, many natural gas liquids (NGL) recovery plants may be designed for propane recovery with the provision (or option) of converting the propane recovery plant to high ethane recovery in the future.
Additionally, typical gas fields may contain excessive amount of ethane (13% and higher) such that a propane recovery plant would fail to meet the heating value requirement (1200 Btu/scf) of the sales gas, which would require propane recovery plants to operate in ethane recovery, resulting in lower propane recovery.
Numerous separation processes and configurations are known in the art to fractionate the NGL fractions from natural gas. In a typical gas separation process, a high pressure feed gas stream may be cooled by heat exchangers, using propane refrigeration and turbo expansion, and the extent of cooling may depend on the hydrocarbon contents and desired levels of recoveries. As the feed gas is cooled under pressure, the hydrocarbon liquids may be condensed and separated from the cooled gas. The cooled vapor is expanded and fractionated in distillation columns (e.g. a deethanizer and/or a demethanizer) to produce (1) a residue gas containing mainly methane gas to a sales gas pipeline and (2) an ethane plus bottom that is to be transported by pipeline to a distant petrochemical facility.
Typically, current natural gas plants process relatively lean gases with ethane content less than 10%. While typical gas plants may be acceptable for a feed gas with a lower ethane content, they may not be suitable if the ethane content feed gas is high.
Typical natural gas liquid plants may be configured for either high ethane recovery or high propane recovery, and typically the ethane recovery process will decrease propane recovery to below 90% if operated on ethane rejection. For example, Rambo et al. describe in U.S. Pat. No. 5,890,378 a system in which the absorber is refluxed, in which the deethanizer condenser provides refluxes for both the absorber and the deethanizer while the cooling duties are supplied by turbo-expansion and propane refrigeration. Here, the absorber and the deethanizer operate at essentially the same pressure. Although Rambo's configuration can recover 98% of the C3+ hydrocarbons during propane recovery operation, high ethane recovery (e.g. over 80%) is difficult even with additional refluxes. The other problem is to maintain high propane recovery (e.g. over 95%) when the NGL plant is required to operate under an ethane rejection mode. The rejected ethane will contain a significant amount of propane, which typically lowers the overall propane recovery to below 90%.
To circumvent at least some of the problems associated with low ethane recoveries, Sorensen describes in U.S. Pat. No. 5,953,935 a plant configuration in which an additional fractionation column and reflux condenser are added to increase ethane recovery using cooling with turbo expansion and Joule Thompson expansion valves of portions of the feed gas. Although Sorensen's configuration may achieve high ethane recoveries, it fails to achieve high propane recovery when operated on ethane rejection. In addition, the ethane plus NGL product must be re-fractionated in a deethanizer to meet the liquefied petroleum gas (LPG) vapor pressure specification, subsequently increasing the overall energy consumption.
In yet other known configurations, high NGL recoveries were attempted with various improved fractionation and reflux configurations. Typical examples are shown in U.S. Pat. Nos. 4,278,457, and 4,854,955, to Campbell et al., in U.S. Pat. No. 6,244,070 to Lee et al., and in U.S. Pat. No. 5,890,377 to Foglietta. While such configurations may provide at least some advantages over prior processes, they are generally intended to operate on a definite recovery mode, either ethane recovery or propane recovery. Moreover, most of such known configurations require extensive modifications of turbo expanders and piping routing when the plants are retrofitted from propane recovery to ethane recovery or vice versa. In most cases, the capital and operating cost for the retrofit processes are relatively high and the revenue losses due to facility shutdown for installation are also high, making the operational change uneconomical.
To circumvent at least some of the problems associated with high ethane recovery while maintaining a high propane recovery, a twin reflux process (described in U.S. Pat. No. 7,051,553 to Mak et al.) employs configurations in which a first column receives two reflux streams: one reflux stream comprising a vapor portion of the NGL and the other reflux stream comprising a lean reflux provided by the overhead of the second distillation column. Similarly, U.S. Pat. App. No. 2010/0206003 to Mak et al. describes an improved natural gas liquid recovery method in which residue gas is integrated to the propane recovery design such that it can be used to reflux the demethanizer during high ethane recovery. However, even with these improvements, high ethane recovery (over 90%) is typically not feasible with additional reflux streams. All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Thus, although various configurations and methods are known to recover natural gas liquids, they typically suffer from one or more disadvantages. For example, while some known methods and configurations can be employed for ethane recovery and propane recovery, ethane rejection will typically result in a loss in propane recovery. Another drawback to the previously described systems is complexity of these systems, making them difficult to operate when changing ethane modes are required. Therefore, there is a need to provide methods and configurations for an NGL recovery plant that can recover high propane recovery of over 95% during ethane rejection, and can be modified to operate on ethane recovery of over 95% producing a pure ethane product for the petrochemical plants.
Embodiments of the disclosure relate to natural gas liquids plants as well as phase implementation of natural gas liquids plants from ethane rejection or high propane recovery to high ethane recovery. Systems and methods disclosed herein relate to processing natural gas, especially as it relates to the methods of configuring a natural gas liquid (NGL) plant for fully rejecting ethane and changing the configuration (e.g. retrofitting) of the NGL plant for over 95% ethane recovery, while maintaining high propane recovery.
The present invention is directed to methods and configurations of a phase implementation of a propane recovery plant (ethane rejection) to ethane recovery without (substantial) losses in propane recovery, where the plant may comprise an absorber and a stripper that are closely coupled with a feed gas/residue gas/refrigeration reflux system.
When the system is operating in ethane rejection, the contemplated methods and systems may produce an ethane rich sales gas and a propane plus NGL product stream, and during ethane recovery, the methods and systems may produce a lean gas to sales and a Y-grade NGL product stream to a downstream facility.
In some embodiments, a dried feed gas may be split into two portions at the inlet of the NGL plant battery limit, with a first portion at about 30% to 60% of the feed gas, where the first portion may be chilled and partially condensed and separated, forming a first liquid, while a vapor is further chilled to a lower temperature and separated, forming a second liquid, with the combined liquids let down in pressure and fed to the feed exchanger.
When the system is operating in ethane rejection, the stripper overhead may be partially condensed in the feed exchanger, forming a reflux that may be fed as reflux to the absorber. The feed exchanger may comprise at least six cores, which may include one or more of refrigerant liquid, separator liquids, absorber overhead, absorber bottom, fractionator overhead, and/or feed gas.
When the system is operating in ethane rejection, the stripper may fractionate the ethane rich NGL from the absorber into an ethane overhead product and a propane plus hydrocarbons product. The methods and systems described here may be configured to achieve over 95% propane recovery, while rejecting 98% of the ethane content from the NGL.
Also, when the system is operating in ethane recovery, a turbo expander and/or an absorber (bottom) exchanger may be added to the system to provide more chilling to the system, such that the NGL plants provide ethane recovery of at least 95% and propane plus recovery of at least 98%.
Disclosed embodiments of an NGL recovery plant may comprise an absorber and a stripper (which may function as a deethanizer/demethanizer) fluidly coupled, and the plant may be changed from ethane rejection to ethane recovery or vice versa with minor process adjustment. The same equipment and piping can be used for both operations and no retrofit may be required to meet the minimum 95% ethane and high propane recovery (for example, if the plant is built to this embodiment configuration, where pre-existing plants may also be retrofit towards this embodiment configuration).
It should be recognized that the disclosed plant may be used to condition the feed gas to meet the sales gas heating value specification and ethane recovery targets in ethane recovery operation.
The feed gas to the system can be a variable feed gas with variable hydrocarbons content and ethane content and is supplied at a temperature of about 100° F. and a pressure of about 900 psig. As used herein, the term “about” in conjunction with a numeral refers to that numeral +/−10, inclusive. For example, where a temperature is “about 100° F.”, a temperature range of 90-110° F., inclusive, is contemplated.
Referring now to
In one exemplary configuration as depicted in
Stream 36 may be separated in a separator 52 into a vapor stream 7 and a liquid stream 8. Vapor stream 7 may be further chilled in the feed exchanger 54, forming stream 9, which may then be separated in a separator 53 into vapor stream 13 and liquid stream 10. Liquid stream 10 may be letdown in pressure and combined with the letdown liquid stream 8, forming a further chilled stream 11, where stream 11 may be fed to the feed exchanger 54 to be heated, forming stream 12. Stream 12 may be fed to the mid-section of the deethanizer 156. The recovery of the refrigeration from the letdown stream enhances the operating efficiency of the process.
Stream 13 may be letdown in pressure in JT valve 60 forming stream 14, where stream 14 may be fed to the absorber 55. Absorber 55 may produce an ethane rich bottom liquid stream 17 and a propane depleted vapor stream 23. The propane depleted vapor stream 23 may be heated in the feed exchanger 54 to produce residue gas stream 16. Bottom liquid stream 17 may be pumped by pump 57, forming stream 18, which may be about 100 psi higher than the absorber pressure. Stream 18 may be chilled in feed exchanger 54, forming stream 19 which may be fed as reflux to the deethanizer 156.
During the ethane rejection operation (as shown in
As an example of suitable conditions of the process shown in
The heat recovery efficiency of the ethane rejection process (described above in
In another exemplary embodiment, as depicted in
The additional equipment required for the ethane recovery operation (shown in
The front section of the ethane recovery process may be the same as the ethane rejection case (as described in
During operation of the plant 200 for ethane recovery, the demethanizer 256 is configured to fractionate the absorber bottom stream 19 into an ethane plus NGL stream 25 and an overhead vapor stream 20. The overhead vapor stream 20 may be fed to the bottom of the absorber 55 for reabsorption of the ethane content (as opposed to being heated and returned to the absorber 55 as reflux, as in
As described above, the absorber 55 may produce an ethane rich bottom liquid stream 17 and an ethane depleted vapor stream 23. The bottom liquid stream 17 may be pumped by pump 57, forming stream 18, which may be about 10 to 20 psi higher than the absorber pressure, as needed to feed the demethanizer 256 downstream. To further improve ethane recovery, stream 18 may be fed to the exchanger 259 and chilled to form stream 19, which is then fed to the demethanizer 256. The vapor stream 23 from the absorber 55 may also be fed to the exchanger 259 and heated to form stream 30, which is then further heated in the feed exchanger 54, producing the residue gas stream 16. Alternatively, the absorber bottom stream 18 can be fed directly to the demethanizer 256 (however ethane recovery may not be as effective with this configuration, i.e. ethane recovery may be reduced by about 1 to 2%).
The heat recovery efficiency of the ethane recovery process is shown in heat composite curve in
With respect to suitable feed gas streams, it is contemplated that different feed gas streams are acceptable, and especially feed gas streams may contain a high level of ethane and heavier hydrocarbon content. With respect to the gas compositions, it is generally preferred that the feed gas stream predominantly includes C1-C6 hydrocarbons and nitrogen and other inert compounds (but may exclude CO2 due to potential freeze issues). The contemplated preferred feed gas streams are associated and non-associated gas from oil and gas production units.
Thus, specific embodiments and applications for improved natural gas liquids recovery 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. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Having described various devices and methods herein, exemplary embodiments or aspects can include, but are not limited to:
In a first embodiment, a natural gas liquid plant configured to operate in either ethane rejection or ethane recovery may comprise an absorber configured to produce an ethane rich bottom stream and a propane depleted vapor stream; a stripper fluidly coupled to the absorber configured to, during ethane rejection, fractionate the ethane rich bottom stream from the absorber into an ethane overhead product and a propane plus hydrocarbons product, and configured to, during ethane recovery, fractionate the ethane rich bottom stream into an ethane plus NGL stream and an overhead vapor stream; and an expander configured to, during ethane recovery, expand a vapor portion of a feed gas to the plant, and feed the expanded stream to the absorber.
A second embodiment can include the plant of the first embodiment, further comprising an exchanger configured to, during ethane recovery, counter-currently contact the ethane rich bottom stream from the absorber with the ethane depleted vapor stream from the absorber, thereby heating the vapor stream and chilling the ethane rich bottom stream before the ethane rich bottom stream is fed to the stripper.
A third embodiment can include the plant of the first or second embodiments, wherein the expanded vapor stream from the expander to the absorber provide increased chilling to the absorber when compared with the plant during ethane rejection.
A fourth embodiment can include the plant of any of the first to third embodiments, wherein the chilled ethane rich bottom stream that is fed to the stripper provides increased chilling to the stripper when compared with the plant during ethane rejection.
A fifth embodiment can include the plant of any of the first to fourth embodiments, wherein, during ethane recovery, the overhead vapor stream from the stripper is fed to the bottom of the absorber for reabsorption of the ethane content.
A sixth embodiment can include the plant of any of the first to fifth embodiments, wherein, during ethane recovery, the ethane plus natural gas liquids stream (from the stripper) contains about 1 mole % methane content.
A seventh embodiment can include the plant of the sixth embodiment, wherein during ethane rejection, the stripper functions as a deethanizer.
An eighth embodiment can include the plant of any of the first to seventh embodiments, wherein during ethane recovery, the stripper functions as a demethanizer.
A ninth embodiment can include the plant of any of the first to eighth embodiments, wherein the plant produces at least 95% (or at least about 95%) propane recovery during ethane rejection.
A tenth embodiment can include the plant of any of the first to ninth embodiments, wherein the plant produces at least 95% (or 99%, at least 99%, or about 99%) propane recovery during ethane recovery.
In an eleventh embodiment, a method for operating a natural gas liquid plant in ethane recovery may comprise expanding a vapor portion of a feed gas to the plant to produce a chilled stream; feeding the chilled stream to the absorber; heating, by the exchanger, a vapor stream from the absorber; feeding the cooled ethane rich bottom stream to a stripper; and fractionating, by the stripper, the cooled ethane rich bottom stream into an ethane plus natural gas liquid stream and an overhead vapor stream.
A twelfth embodiment can include the method of the eleventh embodiment, further comprising cooling, by an exchanger, a bottom stream from an absorber, wherein the bottom stream comprises an ethane rich bottom stream.
A thirteenth embodiment can include the method of the eleventh or twelfth embodiments, wherein, during ethane recovery, the absorber operates at a lower temperature than when the plant is operated in ethane rejection.
A fourteenth embodiment can include the method of any of the eleventh to thirteenth embodiments, wherein, during ethane recovery, the ethane plus natural gas liquids stream (from the stripper) contains about 1 mole % methane content.
A fifteenth embodiment can include the method of any of the eleventh to fourteenth embodiments, further comprising feeding the overhead vapor stream from the stripper to the bottom of the absorber for reabsorption of the ethane content.
In a sixteenth embodiment, a method for operating an ethane rejection natural gas liquid plant in an ethane recovery mode may comprise fluidly coupling an expander to an absorber of the plant; expanding, by the expander, a vapor portion of a feed gas to the plant to produce a chilled stream; feeding the chilled stream to the absorber; fluidly coupling an exchanger to the absorber; cooling, by the exchanger, an ethane rich bottom stream from the absorber; heating, by the exchanger, a vapor stream from the absorber; feeding the cooled ethane rich bottom stream to a stripper; and producing, by the stripper, an ethane plus natural gas liquid stream.
A seventeenth embodiment can include the method of the sixteenth embodiment, wherein, during ethane recovery, the absorber operates at a lower temperature than during ethane rejection.
An eighteenth embodiment can include the method of the sixteenth or seventeenth embodiments, further comprising producing, by the stripper, an overhead vapor stream, and feeding the overhead vapor stream from the stripper to the bottom of the absorber for reabsorption of the ethane content.
A nineteenth embodiment can include the method of any of the sixteenth to eighteenth embodiments, wherein, during ethane recovery, the ethane plus natural gas liquids stream (from the stripper) contains about 1 mole % methane content.
A twentieth embodiment can include the method of any of the sixteenth to nineteenth embodiments, wherein the plant produces at least 95% propane recovery during ethane recovery.
While various embodiments in accordance with the principles disclosed herein have been shown and described above, modifications thereof may be made by one skilled in the art without departing from the spirit and the teachings of the disclosure. The embodiments described herein are representative only and are not intended to be limiting. Many variations, combinations, and modifications are possible and are within the scope of the disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated as further disclosure into the specification and the claims are embodiment(s) of the present invention(s). Furthermore, any advantages and features described above may relate to specific embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages or having any or all of the above features.
Additionally, the section headings used herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or to otherwise provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings might refer to a “Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a limiting characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.
Use of broader terms such as “comprises,” “includes,” and “having” should be understood to provide support for narrower terms such as “consisting of,” “consisting essentially of,” and “comprised substantially of.” Use of the terms “optionally,” “may,” “might,” “possibly,” and the like with respect to any element of an embodiment means that the element is not required, or alternatively, the element is required, both alternatives being within the scope of the embodiment(s). Also, references to examples are merely provided for illustrative purposes, and are not intended to be exclusive.
While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted or not implemented.
Also, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.
