Patent Application
20180038639
The present invention relates to an apparatus and method for liquefaction of natural gas using available letdown energy of a high pressure natural gas pipeline. More specifically, embodiments of the present invention are related to liquefying a natural gas stream utilizing a combination of natural gas letdown and a nitrogen refrigeration cycle that provide for increased flexibility and efficiencies.
Many locations utilize a high pressure (transmission) network and a lower pressure (distribution) network to supply natural gas through an area. The transmission network acts as a freeway to send the gas to the general area, while the distribution network acts as the roads to send the gas to the individual users within the area. Pressures of these networks vary by location, but typical values are 30-60 bara for transmission and 5-20 bara for distribution.
Some small scale LNG plants such as peak shaving plants are often located close to these letdown stations and are able to utilize this “free energy” to produce liquefied natural gas (LNG). The process scheme used is an “open natural gas cycle” where part of the gas is letdown through a turbine and used to provide the refrigeration necessary to liquefy the rest of the natural gas that makes LNG. This letdown energy can supplement an additional refrigeration cycle such as nitrogen expansion cycle or mixed refrigerant cycle, or provide all of the system's refrigeration requirements. In the latter case, the plant does not require an external source of energy, but its major drawback is that the ratio of expanded natural gas flow over the LNG production is very high (usually 6 to 10) leading to a limited LNG production capacity due to low thermodynamic efficiency. Moreover, the LNG production is totally dependent on the availability of letdown gas. Fluctuations in the feed pressure, return pressure, or flowrate yields significant fluctuations in the operation of the liquefier.
Therefore, there is a need for an improved process for using the letdown energy of the high pressure natural gas to produce LNG that is more efficient and is less sensitive to fluctuations of the process conditions of the letdown stream
The present invention is directed to a process that satisfies at least one of these needs. In certain embodiments, a process is provided that optimally utilizes the natural gas letdown energy while accommodating the system fluctuations.
A typical small scale LNG scheme utilizes a nitrogen cycle (N2 recycle compressor and two turbine boosters) in a closed loop. However, certain embodiments of the present invention present flexible solutions that enable full utilization of the letdown energy by combining natural gas turbo-expansion for natural gas pre-cooling and a refrigeration cycle. Therefore, even when the letdown capacity varies, the production can be maintained while reducing costs. In certain embodiments of the invention, the refrigeration cycle used is either a nitrogen refrigeration cycle or a mixed refrigerant refrigeration cycle.
In one embodiment of the present invention, a method for the liquefaction of natural gas is provided. In one embodiment, the method can include the steps of: a) withdrawing a pressurized natural gas stream from a natural gas pipeline; b) boosting a first portion of the pressurized natural gas stream to a higher pressure using a first natural gas booster to produce a boosted pressurized natural gas stream; c) expanding a first portion of the boosted pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream; d) warming the first expanded natural gas stream in a heat exchanger against a second portion of the boosted pressurized natural gas stream to produce a first warmed natural gas stream; e) expanding a second portion of the pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream; f) warming the second expanded natural gas stream in the heat exchanger against the second portion of the boosted pressurized natural gas stream to produce a second warmed natural gas stream; and g) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product.
In optional embodiments of the invention:
In one embodiment of the present invention, a method for the liquefaction of natural gas is provided. In one embodiment, the method can include the steps of: a) withdrawing a pressurized natural gas stream from a natural gas pipeline operating at a first pressure; b) boosting a first portion of the pressurized natural gas stream in a first natural gas booster to a second pressure to produce a boosted pressurized natural gas stream; c) expanding a first portion of the boosted pressurized natural gas stream in a first natural gas expansion turbine to a third pressure to produce a first expanded natural gas stream; d) liquefying a second portion of the boosted pressurized natural gas stream in a natural gas liquefier using refrigeration provided by a refrigeration cycle; e) expanding a second portion of the pressurized natural gas stream in a second expansion turbine to a fourth pressure to produce a second expanded natural gas stream; f) warming the first expanded natural gas stream and the second expanded natural gas stream by in-direct heat exchange against the second portion of the boosted natural gas stream to produce a first and second warmed expanded natural gas stream; g) sending the first and second warmed expanded natural gas stream to a downstream facility, wherein the downstream facility has a natural gas demand, wherein the first natural gas expansion turbine is configured to provide compression power for the first natural gas booster.
In optional embodiments of the invention:
In another embodiment of the present invention, a method for the liquefaction of natural gas is provided. In one embodiment, the method can include the steps of: a) boosting a first pressurized natural gas stream to a higher pressure using a first natural gas booster to produce a boosted pressurized natural gas stream; b) expanding a second pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream; c) warming the first expanded natural gas stream in a heat exchanger against a second portion of the boosted pressurized natural gas stream to produce a first warmed natural gas stream; d) expanding a third pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream; e) warming the second expanded natural gas stream in the heat exchanger against the second portion of the boosted pressurized natural gas stream to produce a second warmed natural gas stream; and f) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product, wherein the first pressurized natural gas stream, the second pressurized natural gas stream, and the third pressurized natural gas stream all originate from a common high pressure natural gas pipeline.
In another embodiment of the present invention, a method for the liquefaction of natural gas is provided. In one embodiment, the method can include the steps of: a) expanding a first pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream; b) warming the first expanded natural gas stream in a heat exchanger against a second pressurized natural gas stream to produce a first warmed natural gas stream; c) expanding a third pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream; d) warming the second expanded natural gas stream in the heat exchanger against the second pressurized natural gas stream to produce a second warmed natural gas stream; and e) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product, wherein the first natural gas turbine is configured to drive a first booster, wherein the first booster is configured to compress a stream selected from the group consisting of the first pressurized natural gas stream, the first warmed natural gas stream, the second pressurized natural gas stream
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.
While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.
In one embodiment of the present invention, the system presented in
In certain embodiments, the natural gas operating pressure at cold box inlet can be limited by the critical pressure of the gas. This is because the HHC condensation requires the operating pressure to be less than the critical pressure for the separation of liquid and vapor to occur. Therefore, in certain embodiments, the limit to the natural gas critical pressure will set the maximum discharge pressure of the first natural gas booster 17 and thus the flow going to the first natural gas expander 15. In certain embodiments, the letdown flow rate available is higher than the flow rate required to reach the booster maximum suction pressure. When this occurs, second natural gas turbine 25 can be utilized.
In one embodiment, second natural gas turbine 25 can be configured to drive a generator, thereby producing additional electricity. This turbine 25 is completely independent from the first turbine 15, and uses the extra letdown flow available to produce electricity. In this way, the natural gas liquefaction stream can be maintained at its optimum pressure through a range of letdown flows and pressures.
Additionally, in certain embodiments, the nitrogen cycle flow may be adjusted such that the LNG production can be maintained independently from the letdown flow variation.
In another embodiment not shown, the energy of the second natural gas turbine 25 may drive a booster which is compressing expanded LNG flash after the letdown valve to the LNG tank. The advantage of such system is to provide free cold energy at both the warm end (thanks to the natural gas expansion) and the cold end (thanks to LNG flash) of the liquefier with no natural gas losses, as it is recompressed to the low pressure network. Therefore, this embodiment is particularly efficient and is especially suited when using a bullet tank type storage, which has sufficient pressure to send the flash gas back at the warm end of the heat exchanger.
This by-pass 60 advantageously (1) enables an increase of the heat exchange efficiency in the heat exchanger and (2) reduces the power consumption of the nitrogen cycle compressor by cooling down its suction temperature.
In summary, embodiments of the invention provide for many improvements over conventional liquefaction techniques. For example, by increasing the feed pressure of the natural gas using a combination turbine booster (15, 17), the heat exchange efficiency is greatly improved, which allows for either an increase in LNG production capacity by keeping the same equipment size or reducing the size of the equipment, and therefore the overall footprint of the plant while maintaining current production capacity.
Additionally, expansion of natural gas enables to pre-cool the warm end of the heat exchanger reducing the specific power of the nitrogen cycle. The embodiments of the invention are very robust as they can adapt to a wide range of natural gas flow rates. This is due to the decoupling of the natural gas turbines 15, 25 with the ability to maintain the natural gas liquefaction pressure 17 with the first natural gas turbine 15.
In certain embodiments, the significant refrigeration brought to the warm end of the main exchanger by the natural gas letdown can allow for the removal of the warm nitrogen turbine and booster to reduce capital cost.
Moreover, the design of the main heat exchanger can optionally stay very similar to a standard nitrogen cycle plant, which means that no major changes in design are required.
In certain embodiments, all the expansion of the natural gas is carried out at ambient or warm temperatures, which results in limited risk of heavy hydrocarbon freezing at turbine outlets.
Additionally, for an incremental additional capital cost (natural gas turbine booster 15, 17 and turbine-generator 25), there can be a significant power savings as there is a corresponding reduction in nitrogen cycle size and power. This is shown in Table I below.
Table II below, provides data for an embodiment in which the flowrates and pressures of various streams could be adjusted based on the pressure of the natural gas coming from the pipeline in order to keep the LNG production at a constant pressure and flowrate.
The flows, pressure variations and impact on the machinery between Case 01 and Case 04 presented in Table II are merely one example, and are included herein for illustrative purposes.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device.
The singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. The terms about/approximately a particular value include that particular value plus or minus 10%, unless the context clearly dictates otherwise.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.
This application is a non-provisional application of U.S. Provisional Applicant No. 62/371,497, filed Aug. 5, 2016, which is herein incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62371497 | Aug 2016 | US |
