PROCESS OF HEAT INTEGRATING FEED AND COMPRESSOR DISCHARGE STREAMS WITH HEAVIES REMOVAL SYSTEM IN A LIQUEFIED NATURAL GAS FACILITY
An LNG facility employing an optimized heavies removal system. The heavies removal system can comprise at least one distillation column and at least two separate heat exchangers. Feed and/or compressor discharge streams can be used to provide heat duty to the heat exchangers in a thermally efficient manner to facilitate the removal of heavy components from an overall LNG facility.
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This application claims priority benefit under 35 U.S.C. Section 119(e) to U.S. Provisional Patent Ser. No. 61/349,297 filed on May 28, 2010 the entire disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTIONThis invention relates to systems and processes for liquefying natural gas. In another aspect, the invention concerns LNG processes and facilities employing a heavies removal system. In another aspect, the invention concerns heat integrating feed and compressor discharge streams with a heavies removal system in an LNG facility.
BACKGROUND OF THE INVENTIONCryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form.
Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers.
Storing natural gas in its liquefied form can help balance periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream through indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, mercury and mercury components, acid gases, and nitrogen, as well as a portion of ethane and heavier components) from the natural gas stream at different points during the liquefaction process.
In general, LNG facilities are designed and operated to provide LNG to a single market in a specific region of the world. Because specifications for the final characteristics of the natural gas product, such as, for example, higher heating value (HHV), Wobbe index, methane content, ethane content, C3+ content, and inerts content vary widely throughout the world, LNG facilities are typically optimized to meet a certain set of specifications for a single market. In large part, achieving the stringent final product specifications involves effectively removing certain components from the natural gas feed stream. LNG facilities may employ one or more distillation columns to remove these components from the incoming natural gas stream. Oftentimes, the heavies removal system is configured in a two column arrangement utilizing a high pressure demethanizer followed by a lower downstream column. In addition, at least one of the columns used to separate the heavier components from the natural gas stream can generally be operated at or near the critical pressure of the components being separated. These limitations, coupled with rigid product specifications, results in distillation columns that are typically designed to operate within a relatively narrow range of conditions. When situations arise that force the columns out of design range (e.g., start-up of the facility or fluctuations in feed composition), the resulting column operation may result in product loss and/or a LNG product that does not meet the desired product specifications.
SUMMARY OF THE INVENTIONIn one embodiment of the present invention, a process for liquefying a natural gas stream in a liquefied natural gas (LNG) facility, the process includes: (a) cooling at least a portion of the natural gas stream in an upstream refrigeration cycle to thereby produce a cooled natural gas stream; (b) introducing at least a portion of the cooled natural stream into an inlet of a first distillation column; (c) introducing at least a portion of the natural gas stream to provide heat duty to a first reboiler associated with the first distillation column to produce a first heated vapor fraction and a first heated liquid fraction from a reboiler inlet stream while cooling the portion of the natural gas stream and returning the cooled portion of the natural gas stream to the LNG facility; (d) using the first distillation column and the first reboiler and associated first heated vapor fraction from the reboiler inlet stream to separate all incoming streams into a first column vapor stream, a first column liquid bottoms stream, wherein the first column vapor stream exits through an overhead outlet, wherein the first column liquid bottoms stream exits through a bottoms outlet; (e) introducing at least a portion of the first column vapor stream exiting the first distillation column through the overhead outlet into the LNG facility; (f) introducing at least a portion of the first column liquid bottoms stream from the first distillation column into a second heat exchanger; (g) introducing at least a portion of the natural gas stream or at least a portion of a compressor discharge stream to provide heat duty to the second heat exchanger to thereby produce a second heated stream while cooling the portion of the natural gas stream or the portion of the compressor discharge stream; and (h) introducing at least a portion of the second heated stream into a second distillation column.
The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying figures by way of example and not by way of limitation, in which:
Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not as a limitation of the invention. It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations that come within the scope of the appended claims and their equivalents.
The present invention can be implemented in a facility used to cool natural gas to its liquefaction temperature to thereby produce liquefied natural gas (LNG). The LNG facility generally employs one or more refrigerants to extract heat from the natural gas and reject to the environment. Numerous configurations of LNG systems exist and the present invention may be implemented in many different types of LNG systems.
In one embodiment, the present invention can be implemented in a mixed refrigerant LNG system. Examples of mixed refrigerant processes can include, but are not limited to, a single refrigeration system using a mixed refrigerant, a propane pre-cooled mixed refrigerant system, and a dual mixed refrigerant system.
In another embodiment, the present invention is implemented in a cascade LNG system employing a cascade-type refrigeration process using one or more predominately pure component refrigerants. The refrigerants utilized in cascade-type refrigeration processes can have successively lower boiling points in order to facilitate heat removal from the natural gas stream being liquefied. Additionally, cascade-type refrigeration processes can include some level of heat integration. For example, a cascade-type refrigeration process can cool one or more refrigerants having a higher volatility through indirect heat exchange with one or more refrigerants having a lower volatility. In addition to cooling the natural gas stream through indirect heat exchange with one or more refrigerants, cascade and mixed-refrigerant LNG systems can employ one or more expansion cooling stages to simultaneously cool the LNG while reducing its pressure.
Referring now to
The operation of the LNG facility illustrated in
The cooled natural gas stream from high-stage propane chiller 33a flows through conduit 114 to a separation vessel, wherein water and in some cases propane and heavier components are removed, typically followed by a treatment system 40, in cases where not already completed in upstream processing, wherein moisture, mercury and mercury compounds, particulates, and other contaminants are removed to create a treated stream. The stream exits the treatment system 40 through conduit 116. Thereafter, a portion of the stream in conduit 116 can be routed through conduit A to a heavies removal zone illustrated in
The combined vaporized propane refrigerant stream exiting high-stage propane chillers 33 and 33A is returned to the high-stage inlet port of propane compressor 31 through conduit 306. The liquid propane refrigerant in high-stage propane chiller 33A provides refrigeration duty for the natural gas stream 110. Two-phase refrigerant stream can enter the intermediate-stage propane chiller 34 through conduit 310, thereby providing coolant for the natural gas stream (in conduit 116) and to yet-to-be-discussed streams entering intermediate-stage propane chiller 34 through conduits 204 and 310. The vaporized portion of the propane refrigerant exits intermediate-stage propane chiller 34 through conduit 312 and can then enter the intermediate-stage inlet port of propane compressor 31. The liquefied portion of the propane refrigerant exits intermediate-stage propane chiller 34 through conduit 314 and is passed through a pressure-reduction means, illustrated here as expansion valve 44, whereupon the pressure of the liquefied propane refrigerant is reduced to thereby flash or vaporize a portion thereof. The resulting vapor-liquid refrigerant stream can then be routed to low-stage propane chiller 35 through conduit 316 and where the refrigerant stream can cool the methane-rich stream and a yet-to-be-discussed ethylene refrigerant stream entering low-stage propane chiller 35 through conduits 118 and 206, respectively. The vaporized propane refrigerant stream then exits low-stage propane chiller 35 and is routed to the low-stage inlet port of propane compressor 31 through conduit 318 wherein it is compressed and recycled as previously described.
As shown in
Turning now to ethylene refrigeration cycle 50 in
The cooled stream in conduit 120 exiting low-stage propane chiller 35 can thereafter be split into two portions, as shown in
The remaining liquefied ethylene refrigerant exiting high-stage ethylene chiller 53 in conduit 220 can re-enter ethylene economizer 56, to be further sub-cooled by an indirect heat exchange means 61 in ethylene economizer 56. The resulting sub-cooled refrigerant stream exits ethylene economizer 56 through conduit 222 and can subsequently be routed to a pressure reduction means, illustrated here as expansion valve 62, whereupon the pressure of the refrigerant stream is reduced to thereby vaporize or flash a portion thereof. The resulting, cooled two-phase stream in conduit 224 enters low-stage ethylene chiller/condenser 55.
As shown in
In low-stage ethylene chiller/condenser 55, the cooled stream (which can comprise the stream in conduit 122 optionally combined with streams in conduits D and 168) can be at least partially condensed through indirect heat exchange with the ethylene refrigerant entering low-stage ethylene chiller/condenser 55 through conduit 224. The vaporized ethylene refrigerant exits low-stage ethylene chiller/condenser 55 through conduit 226 and can then enters ethylene economizer 56. In ethylene economizer 56, the vaporized ethylene refrigerant stream can be warmed through an indirect heat exchange means 64 prior to being fed into the low-stage inlet port of ethylene compressor 51 through conduit 230. As shown in
The cooled natural gas stream exiting low-stage ethylene chiller/condenser 55 in conduit 124 can also be referred to as the “pressurized LNG-bearing stream” As shown in
The liquid portion of the reduced-pressure stream exits high-stage methane flash drum 82 through conduit 142 to then re-enter main methane economizer 73, wherein the liquid stream can be cooled through indirect heat exchange means 74 of main methane economizer 73. The resulting cooled stream exits main methane economizer 73 through conduit 144 and can then be routed to a second expansion stage, illustrated here as intermediate-stage expansion valve 83 but could include an expander. Intermediate-stage expansion valve 83 further reduces the pressure of the cooled methane stream which reduces the stream's temperature by vaporizing or flashing a portion thereof. The resulting two-phase methane-rich stream in conduit 146 can then enter intermediate-stage methane flash drum 84, wherein the liquid and vapor portions of this stream can be separated and can exit the intermediate-stage flash drum 84 through respective conduits 148 and 150. The vapor portion (also called the intermediate-stage flash gas) in conduit 150 can re-enter methane economizer 73, wherein the vapor portion can be heated through an indirect heat exchange means 77 of main methane economizer 73. The resulting warmed stream can then be routed through conduit 154 to the intermediate-stage inlet port of methane compressor 71, as shown in
The liquid stream exiting intermediate-stage methane flash drum 84 through conduit 148 can then pass through a low-stage expansion valve 85 and/or expander, whereupon the pressure of the liquefied methane-rich stream can be further reduced to thereby vaporize or flash a portion thereof. The resulting cooled, two-phase stream in conduit 156 can then enter low-stage methane flash drum 86, wherein the vapor and liquid phases can be separated. The liquid stream exiting low-stage methane flash drum 86 through conduit 158 can comprise the liquefied natural gas (LNG) product. The LNG product, which is at about atmospheric pressure, can be routed through conduit 158 downstream for subsequent storage, transportation, and/or use.
The vapor stream exiting low-stage methane flash drum (also called the low-stage methane flash gas) in conduit 160 can be routed to methane economizer 73, wherein the low-stage methane flash gas can be warmed through an indirect heat exchange means 78 of main methane economizer 73. The resulting stream can exit methane economizer 73 through conduit 164, whereafter the stream can be routed to the low-stage inlet port of methane compressor 71.
Methane compressor 71 can comprise one or more compression stages. In one embodiment, methane compressor 71 comprises three compression stages in a single module. In another embodiment, one or more of the compression modules can be separate, but can be mechanically coupled to a common driver. Generally, one or more intercoolers (not shown) can be provided between subsequent compression stages.
As shown in
Upon being cooled in propane refrigeration cycle 30 through heat exchanger means 37, the methane refrigerant stream can be discharged into conduit 130 and subsequently routed to main methane economizer 73, wherein the stream can be further cooled through indirect heat exchange means 79. The resulting sub-cooled stream exits main methane economizer 73 through conduit 168 and can then combined with stream in conduit 122 exiting high-stage ethylene chiller 53 and/or with stream in conduit D exiting the heavies removal zone (e.g. first predominately vapor stream from first distillation column 650 in
Turning now to
In an embodiment as shown in
Referring to
Referring now to
As shown in
The second distillation column 660 separates the incoming streams. A second column overhead vapor stream (also called “second overhead stream”) is withdrawn through conduit 622 from second distillation column 660. A portion of the second column overhead vapor stream exiting second distillation column 660 can enter cooling pass 684 of third heat exchanger 652, wherein the stream can be cooled and at least partially condensed using air, water, or other suitable coolant. The resulting condensed or two-phase stream can then be routed through conduit 624 to a reflux accumulator, wherein the stream can be separated into a vapor and liquid phase.
Turning now to
In
Referring to
Referring to
Referring now to
As shown in
Referring to
In one embodiment of the present invention, the LNG production systems can be simulated on a computer using process simulation software in order to generate process simulation data in a human-readable form. In one embodiment, the process simulation data can be in the form of a computer print out. In another embodiment, the process simulation data can be displayed on a screen, a monitor, or other viewing device. The simulation data can then be used to manipulate the operation of the LNG system and/or design the physical layout of an LNG facility. In one embodiment, the simulation results can be used to design a new LNG facility and/or revamp or expand an existing facility. In another embodiment, the simulation results can be used to optimize the LNG facility according to one or more operating parameters. Examples of suitable software for producing the simulation results include HYSYS™ or Aspen Plus® from Aspen Technology, Inc., and PRO/II® from Simulation Sciences Inc.
The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.
Claims
1. A process for liquefying a natural gas stream in a liquefied natural gas (LNG) facility, the process comprising:
- (a) cooling at least a portion of the natural gas stream in an upstream refrigeration cycle to thereby produce a cooled natural gas stream;
- (b) introducing at least a portion of the cooled natural gas stream into an inlet of a first distillation column;
- (c) introducing at least a portion of the natural gas stream to provide heat duty to a first reboiler associated with the first distillation column to produce a first heated vapor fraction and a first heated liquid fraction from a first reboiler inlet stream while cooling the portion of the natural gas stream and returning the cooled portion of the natural gas stream to the LNG facility;
- (d) using the first distillation column and the first reboiler and associated first heated vapor fraction from the reboiler inlet stream to separate all incoming streams into a first column vapor stream, a first column liquid bottoms stream, wherein the first column vapor stream exits through an overhead outlet, wherein the first column liquid bottoms stream exits through a bottoms outlet;
- (e) introducing at least a portion of the first column vapor stream exiting the first distillation column through the overhead outlet into the LNG facility;
- (f) introducing at least a portion of the first column liquid bottoms stream from the first distillation column into a second heat exchanger;
- (g) introducing at least a portion of the natural gas stream or at least a portion of a compressor discharge stream to provide heat duty to the second heat exchanger to thereby produce a second heated stream while cooling the portion of the natural gas stream or the portion of the compressor discharge stream; and
- (h) introducing at least a portion of the second heated stream into a second distillation column.
2. The process according to claim 1, wherein the cooled natural gas stream or at least a portion of the cooled natural gas stream from step (a) is expanded across a valve or expander to thereby produce a lower pressure stream before step (b).
3. The process according to claim 1, wherein the cooled natural gas or at least a portion of the natural gas stream is separated into a liquid fraction and a vapor fraction prior to step (b).
4. The process according to claim 3, wherein the liquid and the vapor fraction are introduced into the first distillation column in separate locations.
5. The process according to claim 4, wherein the vapor fraction is combined with the first column vapor stream of step (d).
6. The process according to claim 2, wherein the lower pressure stream is separated into a liquid fraction and a vapor fraction prior to step (b).
7. The process according to claim 6, wherein the liquid and the vapor fraction are introduced into the first distillation column in separate locations.
8. The process according to claim 1, wherein the first distillation column includes one or more side reboilers.
9. The process according to claim 1, wherein the first distillation column includes one or more pumparounds.
10. The process according to claim 1, wherein the first distillation column includes one or more reboilers and one or more pumparounds.
11. The process according to claim 1, wherein the first distillation column includes an internal condenser.
12. The process according to claim 1, wherein one or more side product streams are taken from the first distillation column;
13. The process according to claim 1, wherein a gas stream introduced directly into the first distillation column provides heat duty to replace the heat duty supplied by the first reboiler of step (c).
14. The process according to claim 1, wherein a gas stream introduced directly into the first distillation column provides heat duty to the first distillation column to supplement the heat duty supplied by the first reboiler according to step (c).
15. The process according to claim 1, wherein at least a portion of the natural gas stream of step (g) includes any recycled stream or compressor discharge stream.
16. The process according to claim 1, wherein the heat duty supplied to the second heat exchanger of step (g) is supplied by any compressor discharge stream.
17. The process according to claim 1, wherein reflux provided to the first distillation column is liquefied natural gas or a portion of the overhead stream from the second distillation column of step (h).
Type: Application
Filed: May 12, 2011
Publication Date: May 17, 2012
Applicant: CONOCOPHILLIPS COMPANY (Houston, TX)
Inventor: Wesley R. Qualls (Katy, TX)
Application Number: 13/106,446