ROBUST RECOVERY OF NATURAL GAS LETDOWN ENERGY FOR SMALL SCALE LIQUEFIED NATURAL GAS PRODUCTION

Abstract

A method for liquefaction of natural gas using refrigeration from a combination of sources including a refrigeration cycle and letdown energy of natural gas is provided. The natural gas to be liquefied (LNG) is boosted in pressure using a booster that is powered by expansion of a portion of the natural gas flow from the booster through a first natural gas turbine. A second flow of natural gas is expanded in a second natural gas turbine, and the resulting expanded stream, along with the natural gas expanded in the first natural gas turbine, are warmed against the natural gas to be liquefied. The flow rate of the natural gas in the second natural gas turbine is decoupled from the booster, thereby allowing for variation in flow rates and pressures while maintaining a constant production of LNG.

Claims

1. A method for the liquefaction of natural gas, the method comprising 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.

2. The method as claimed in claim 1, wherein the first natural gas turbine is configured to power the first natural gas booster.

3. The method as claimed in claim 1, wherein the second natural gas turbine is configured to power a generator such that electricity is produced during step f).

4. The method as claimed in claim 1, wherein the second natural gas turbine is configured to power a second natural gas booster.

5. The method as claimed in claim 1, wherein the refrigeration cycle comprises a refrigerant by-pass configured to remove a portion of refrigerant coolant from an intermediate section of the heat exchanger thereby reducing the amount of cooling provided from the refrigeration cycle to the second portion of the boosted pressurized natural gas stream.

6. The method as claimed in claim 1, wherein the first warmed natural gas stream and the second warmed natural gas stream are combined within the heat exchanger or combined before entering the heat exchanger.

7. The method as claimed in claim 1, further comprising the steps of: removing an impurity from the pressurized natural gas stream using an adsorption bed, wherein the impurity is selected from the group consisting of carbon dioxide, water, and combinations thereof; and regenerating the adsorption bed using a low pressure natural gas, wherein the low pressure natural gas is selected from the group consisting of the first warmed natural gas stream, the second warmed natural gas stream, and combinations thereof.

8. The method as claimed in claim 1, wherein the first expanded natural gas stream and the second expanded natural gas stream are expanded to about the same pressure.

9. The method as claimed in claim 1, further comprising the steps of: sending a flow of a low pressure natural gas stream to a user; adjusting the amount of the second portion of the pressurized natural gas stream expanded in the second natural gas turbine based on the flow of the low pressure natural gas sent to the user, wherein the low pressure natural gas is selected from the group consisting of the first warmed natural gas stream, the second warmed natural gas stream, and combinations thereof.

10. The method as claimed in claim 1, wherein the amount of LNG product produced in step h) is unchanged by the amount of the second portion of the pressurized natural gas stream expanded in a second natural gas turbine in step f).

11. The method as claimed in claim 1, wherein the pressure of the second portion of the boosted pressurized natural gas stream during liquefaction is unchanged by the combined flow rate of the first expanded natural gas stream and the second expanded natural gas stream.

12. The method as claimed in claim 1, further comprising the steps of: modifying the amount of the second portion of the pressurized natural gas stream expanded in the second natural gas turbine in step f); and adjusting the refrigeration provided from the refrigeration cycle to the second portion of the boosted pressurized natural gas stream in step h) in order to keep the amount of the LNG product produced within a targeted range.

13. The method as claimed in claim 1, wherein the refrigeration cycle is selected from the group consisting of a nitrogen refrigeration cycle and a mixed refrigerant refrigeration cycle.

14. The method as claimed in claim 1, wherein the refrigeration cycle has only one turbine-booster.

15. The method as claimed in claim 1, wherein the nitrogen cycle comprises two turbine-boosters.

16. A method for the liquefaction of natural gas, the method comprising 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.

17. The method as claimed in claim 16, wherein the flow rate of the second portion of the pressurized natural gas stream expanded in the second expansion turbine is adjusted based on changes in the natural gas demand of the downstream facility.

18. The method as claimed in claim 17, wherein the flow rate of the second portion of the boosted pressurized natural gas stream is independent of the natural gas demand of the downstream facility.

19. The method as claimed in claim 16, further comprising the step of monitoring the first pressure; and adjusting the flow rates of the first portion of the boosted pressurized natural gas stream and the second portion of the pressurized natural gas stream based on the first pressure while maintaining the flow rate of the LNG stream produced in step d).

20. A method for the liquefaction of natural gas, the method comprising 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.

21. A method for the liquefaction of natural gas, the method comprising 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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] 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.

[0032] FIG. 1 shows an embodiment of the prior art.

[0033] FIG. 2 shows an embodiment in accordance with the present invention.

[0034] FIG. 3 shows a second embodiment in accordance with the present invention.

[0035] FIG. 4 shows a third embodiment in accordance with the present invention.

DETAILED DESCRIPTION

[0036] 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.

[0037] FIG. 1 is an example of a typical small LNG scheme that utilizes a nitrogen cycle (N.sub.2 compressor and two turbine boosters) in a closed loop 10. Natural gas NG is cooled and condensed into LNG in passages separate and adjacent to the N.sub.2 in the heat exchanger 40. In most small scale LNG plants, heavy hydrocarbons (HHC), which freeze at LNG temperatures, condense and are removed from the natural gas via a knock out drum 50. The specific power of such plant is highly dependent on the natural gas feed pressure and usually varies between 450 and 550 kWh/ton of LNG produced.

[0038] In one embodiment of the present invention, the system presented in FIG. 2 combines two natural gas turbines 15, 25 and a standard nitrogen liquefier 10. In the embodiment shown, the first natural gas turbine 15 is driving a first natural gas booster 17 that is used to set the pressure of the natural gas to a specified optimum value at the inlet of the cold box 20. This additional pressure boost to the natural gas stream is advantageous, since a high natural gas pressure (1) improves the heat exchange efficiency, (2) shrinks the size of the equipment, and (3) reduces overall costs. However, this pressure must also be maintained below the maximum allowable working pressure of the equipment design.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] FIG. 3 presents an alternative embodiment in which a second booster 27 replaces the generator. The temperature of the booster aftercooler 30 may be adjusted such that no condensation appears at the discharge of second natural gas turbine 25. Alternatively (in an embodiment not shown), components which condense may be removed prior to expansion. Depending on the pressure ratio and flows, the natural gas can also be expanded prior to boosting in second booster 27. As this embodiment does not require a transformation of the mechanical energy into electrical energy, the embodiment presented in FIG. 3 is generally more efficient and cost competitive compared to the embodiment presented in FIG. 2.

[0043] 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.

[0044] FIG. 4 presents another embodiment that is particularly useful for varying demands for the low pressure natural gas. As the low pressure natural gas flow and pressures vary, the relative amount of natural gas letdown energy varies compared to the N.sub.2 cycle energy. As a result, the warm end of the heat exchanger may receive more cold than is needed. In addition to the loss of thermal efficiency , the warm end of the heat exchanger could get to a temperature that is colder than it was designed for, which could lead to structural issues, as well as premature freezing of components within other parts of the heat exchanger, particularly the heavy hydrocarbons. To alleviate this issue, certain embodiments of the invention can include a by-pass 60 of cold nitrogen at the warm end of the heat exchanger.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

TABLE-US-00001 TABLE II Response of the system to a change of NG Feed pressure Case 01 Case 04 (FIG. 2) (FIG. 2) Note NG Feed Pressure bar abs 31 40 +9 bar change in the feed gas pressure Liquefaction bar abs 48 48 Kept Constant Pressure Letdown Pressure bar abs 6.5 6.5 Kept Constant Letdown Flow MMSCFD 29 29 Constant Demand LNG Production MMSCFD 21 21 Constant Demand Flow to NG MMSCFD 14 5 Adjusted to keep the discharge Turbine (15) pressure constant Flow to NG MMSCFD 15 24 Adjusted to deliver the rest of the Turbine (25) letdown gas N.sub.2 Cycle Power kW 6,600 6,260 5% (10) Reduced power mainly due to the higher expansion power (higher pressure ratio and flow) of NG Turbine 25 NG Turbine (15) kW 750 230 69% Power Reduced Power due to the reduced pressure ratio, and therefore flowrate of Turbine 15 NG Turbine (25) kW 550 1,008 +83% Power Higher power generation of Turbine 25

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.