METHOD FOR THE PRODUCTION OF LIQUEFIED NATURAL GAS

Abstract

A method for the production of liquefied natural gas is provided. The method may include providing a high pressure natural gas stream, splitting the high pressure natural gas stream into a first portion and a second portion, and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream. The refrigeration needed for cooling and liquefaction of the natural gas can be provided by a closed nitrogen refrigeration cycle and letdown of the second portion of the high pressure natural gas stream.

Claims

1. A method for the production of liquefied natural gas (LNG), the method comprising the steps of: a) providing a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle is configured to provide refrigeration within a heat exchanger; b) purifying a first natural gas stream in a first purification unit to remove a first set of impurities to produce a purified first natural gas stream; c) cooling and liquefying the first natural gas stream in the heat exchanger using the refrigeration from the nitrogen refrigeration cycle to produce an LNG stream, wherein the first natural gas stream has an LNG refrigeration requirement, wherein the LNG stream is liquefied at a first pressure P.sub.H; d) purifying a second natural gas stream in a second purification unit to remove a second set of impurities to produce a purified second natural gas stream; e) partially cooling the second natural gas stream in the heat exchanger; f) withdrawing the partially cooled second natural gas stream from an intermediate section of the heat exchanger; g) expanding the partially cooled second natural gas stream to a medium pressure P.sub.M in a natural gas expansion turbine to form a cold natural gas stream, wherein the medium pressure P.sub.M is at a pressure lower than the first pressure P.sub.H; and h) warming the cold natural gas stream in the heat exchanger by heat exchange against the first natural gas stream to produce a warm natural gas stream at the warm end of the heat exchanger, wherein the natural gas expansion turbine drives a first booster, wherein the LNG refrigeration requirement is supplied by a combination of refrigeration from the nitrogen refrigeration cycle and step h).

2. The method as claimed in claim 1, wherein the first booster is configured to compress the second natural gas stream or a stream derived from the second natural gas stream.

3. The method as claimed in claim 1, wherein the first booster is configured to compress a stream selected from the group consisting of the first natural gas stream, the first purified natural gas stream, the second natural gas stream, the purified second natural gas stream, the partially cooled natural gas stream, the warm natural gas stream, and a nitrogen fluid within the nitrogen refrigeration cycle.

4. The method as claimed in claim 1, wherein the first set of impurities has a freezing point at or above the liquefaction temperature of methane at the first pressure P.sub.H.

5. The method as claimed in claim 1, wherein the second set of impurities comprises water.

6. The method as claimed in claim 1, wherein the nitrogen refrigeration cycle comprises a recycle compressor, a turbine, a booster and a plurality of coolers, wherein the turbine and booster are configured such that the turbine is configured to power the booster.

7. The method as claimed in claim 1, wherein the first natural gas stream and the second natural gas stream come from the same natural gas source.

8. The method as claimed in claim 7, wherein the natural gas source is a natural gas pipeline having a pressure between 15 and 100 bara.

9. The method as claimed in claim 1, wherein the first natural gas stream comes from a first natural gas source, and the second natural gas stream comes from a second natural gas source, wherein the first and second natural gas sources are different sources.

10. The method as claimed in claim 9, wherein the first natural gas source comprises a natural gas pipeline.

11. The method as claimed in claim 10, wherein the natural gas pipeline has a pressure between 15 and 100 bara.

12. The method as claimed in claim 1, wherein the first purification unit and the second purification unit are the same unit.

13. The method as claimed in claim 1, wherein the first purification unit and the second purification unit are separate units, wherein the first purification unit is configured to remove at least water and carbon dioxide, and wherein the second purification unit is configured to remove at least water.

14. A method for the production of liquefied natural gas (LNG), the method comprising the steps of: a) providing a nitrogen refrigeration cycle; b) cooling and liquefying a first natural gas stream in a heat exchanger by heat exchange with nitrogen from the nitrogen refrigeration cycle to produce an LNG stream, wherein the LNG stream is liquefied at a first pressure; c) expanding a second natural gas stream to a second pressure to produce an expanded natural gas stream; and d) warming the expanded natural gas stream in the heat exchanger to produce a warmed natural gas stream, wherein step d) provides a portion of the refrigeration used to cool and liquefy the first natural gas stream.

15. The method as claimed in claim 14, wherein the first natural gas stream comes from a first natural gas source, and the second natural gas stream comes from a second natural gas source, wherein the first and second natural gas sources are different sources.

16. The method as claimed in claim 14, wherein the first natural gas liquefied in step b) is derived from the expanded natural gas stream, wherein the first pressure and the second pressure are about the same.

17. A method for the production of liquefied natural gas (LNG), the method comprising the steps of: a) providing a high pressure natural gas stream; b) splitting the high pressure natural gas stream into a first portion and a second portion; c) cooling and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream; d) providing a first portion of refrigeration via a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle comprises a recycle compressor, a turbine, a booster and a plurality of coolers, wherein the turbine and booster are configured such that the turbine is configured to power the booster; e) providing a second portion of refrigeration by expanding the second portion of the high pressure natural gas; and f) using the first portion of refrigeration and the second portion of refrigeration to achieve the cooling and liquefaction of the first portion of the high pressure natural gas stream in step c).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0096] 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.

[0097] FIG. 1 provides an embodiment of the prior art.

[0098] FIG. 2 provides an embodiment of the present invention.

[0099] FIG. 3 provides an embodiment of the present invention with both LIN and LNG production.

[0100] FIG. 4 provides another embodiment of the present invention with both LIN and LNG production.

[0101] FIG. 5 provides an embodiment of the present invention with LIN and medium pressure natural gas production.

DETAILED DESCRIPTION

[0102] 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.

[0103] In one embodiment, the method can include integrating a natural gas letdown system with a nitrogen refrigeration cycle. In one embodiment, the nitrogen refrigeration cycle is a closed loop refrigeration cycle. In this embodiment, the natural gas letdown essentially provides free refrigeration energy since the natural gas would have been alternatively letdown across a valve (i.e., the resulting drop in temperature of the natural gas would have been absorbed by the surroundings and would not have been recovered in any meaningful way). With the addition of a natural gas turbine booster, LNG can be co-produced with a significant power savings, while also potentially reducing the size of the nitrogen refrigeration cycle. In another embodiment, a purification unit, storage, loading and utility systems may also be included.

[0104] Referring to FIG. 2, a process flow diagram of an embodiment of the current invention is shown. In FIG. 2, high pressure natural gas 2 is preferably split into two portions, with one portion being liquefied and the other portion providing a portion of the refrigeration used to cool and liquefy the natural gas. First portion of the natural gas stream 102 is purified in first purification unit 130, wherein acid gases, water and mercury are preferably removed. Preferably, any impurity within the natural gas that would solidify prior to the natural gas liquefying or damage the downstream equipments is removed in first purification unit 130. The resulting purified first portion of the natural gas stream 104 is then withdrawn from first purification unit 130 and introduced to heat exchanger 40 for liquefaction therein. In embodiments in which the natural gas feed contains heavy hydrocarbons, it is preferable to withdraw purified first portion of the natural gas 104 from an intermediate section of heat exchanger 40 and separate the heavy hydrocarbons 8 using gas liquid separator 5. Alternatively, the gas-liquid separator may be replaced by a distillation column or other separation devices known in the art. Instead of collecting heavy hydrocarbons 8 separately as shown in FIG. 1, heavy hydrocarbons 8 may be expanded and then warmed in heat exchanger 40. The resulting warmed stream can be combined with other natural gas streams (e.g., cold natural gas stream 144 and first portion of the LNG 146) within heat exchanger 40. This advantageously captures some of the cold energy from heavy hydrocarbons 8, and if warm natural gas stream 108 is subsequently used for fuel, it also provides additional energy for that purpose.

[0105] Vaporized natural gas from gas liquid separator 5 is reintroduced to heat exchanger 40, wherein it subsequently liquefies to produce LNG 6. In one embodiment, first portion of the LNG 146 can be removed from LNG 6, expanded in second valve V2, and then warmed in heat exchanger 40, thereby providing additional refrigeration, to produce warm natural gas stream 108. The remaining portion can then be expanded across third valve V3, thereby producing low pressure LNG 148.

[0106] Refrigeration for the system is provided by two sources. The first refrigeration source can be via a conventional nitrogen refrigeration cycle 50. Nitrogen gas is compressed in nitrogen recycle compressor 10, cooled in cooler 11, compressed further in booster of first turbine booster 20, cooled in cooler 21, then further compressed in booster of second turbine booster 25 before being cooled again in cooler 26. The resulting compressed nitrogen is then cooled in heat exchanger 40, wherein a first portion is removed and expanded in turbine of second turbine booster 25 and the remaining portion is removed and expanded in turbine of first turbine booster 20. The resulting expanded nitrogen streams are then introduced to heat exchanger 40, where they are warmed via indirect heat exchange against the natural gas and other nitrogen streams.

[0107] The second refrigeration source is provided by using the excess pressure differential of the high pressure natural gas. In this embodiment, second portion of the natural gas stream 106 is split from high pressure natural gas 2, and then purified in second purification unit 131 of at least water and potentially mercury to produce purified second portion of the natural gas 132. While the embodiment shown in FIG. 2 includes two separate purification units, it is possible to use a single purification unit to fully purify the entire natural gas stream prior to splitting the natural gas into two streams. However, it is preferable to split the streams prior to purification since the natural gas used to provide refrigeration (i.e., the portion not liquefied), does not need to have carbon dioxide removed, since the natural gas turbine outlet stream 144 is at a sufficiently warm temperature such that carbon dioxide will not freeze within this stream. In another embodiment, units 130 and 131 may be combined into a single unit, and the moisture free stream (e.g., 132) is removed at an intermediate location of the vessel and the moisture and CO.sub.2 free stream (e.g., 104) is removed from the end of the vessel opposite the feed location.

[0108] Purified second portion of the natural gas 132 is then compressed in booster of natural gas turbine booster 120, cooled in cooler 140 to produce compressed natural gas stream 142. Compressed natural gas stream 142 can then be partially cooled in heat exchanger 40, before being expanded in turbine of natural gas turbine booster 120 to form cold natural gas stream 144. Alternatively, in an embodiment not shown, natural gas stream 142 can be sent, prior to cooling, directly to natural gas turbine 120 for expansion. This can help limit the temperature of 144 to avoid heavy hydrocarbon condensation and potential solidification. Cold natural gas stream 144 is then reintroduced to heat exchanger 40, wherein it is warmed via indirect heat exchange and collected as warm natural gas stream 108 from the warm end of the heat exchanger. In one embodiment, cold natural gas stream 144 can be combined with heavy hydrocarbons 8 and optionally first portion of the LNG 146 within the heat exchanger, or the different streams can warm individually within the heat exchanger and be combined following their warming.

[0109] The booster of natural gas turbine booster 120 can be located at many different locations depending on the natural gas source and return pressures. For example, it may be located at 1) the NG stream to be expanded (FIG. 2) if the feed pressure and/or return pressure are low, 2) the total natural gas feed flow before splitting the flow to be expanded and flow to be liquefied (FIG. 3), or 3) on the discharge of the turbine at the warm end of the exchanger (e.g., stream 108) in the case of high natural gas feed pressure and high natural return pressure (not shown), or 4) on the natural gas stream to be liquefied (e.g., stream 104) if the feed pressure is low (not shown). Alternatively the turbine may be used to drive an electrical generator or dissipated by oil brake (not shown).

[0110] A comparison of the embodiment shown in FIGS. 1 and 2 can be found in Table I below:

TABLE-US-00001 TABLE I Comparison of Energy Requirements for FIG. 1 and FIG. 2 Base (Typical LNG LNG production by production by N2 NG letdown and N2 cycle) FIG. 1 cycle. FIG. 2 NG supply 32 bara 342 1019 (mtd) NG to letdown 5.6 0 677 bara (mtd) LNG Production 342 342 (mtd) N2 cycle power input 7155 4158 (kW) LNG Specific power 502 292 (kWh/mt) Power Reduction (%) 42% LIN production (mtd) LIN Specific Power (kWh/mt)

[0111] In the setup shown of FIG. 2, the power required to produce 342 mtd of LNG is reduced to approximately 4158 kW, meaning the specific power of this setup is approximately 292 kWh/mt. As such, this represents a decrease of approximately 42% in power requirements.

[0112] Regarding FIG. 3, a process flow diagram of an embodiment for the co-production of liquid nitrogen and LNG using a nitrogen refrigeration cycle in combination with natural gas letdown. In FIG. 3, natural gas can be acquired from a natural gas source, compressed in natural gas booster 101 to produce high pressure natural gas 2. High pressure natural gas 2 is preferably split into two portions, with one portion being liquefied and the other portion providing a portion of the refrigeration used to cool and liquefy the natural gas. First portion of the natural gas stream 102 is purified in first purification unit 130, wherein acid gases, water and mercury are preferably removed. Preferably, any impurity within the natural gas that would damage or solidify prior to the natural gas liquefying is removed in first purification unit 130. The resulting purified first portion of the natural gas stream 104 is then withdrawn from first purification unit 130 and introduced to heat exchanger 40 for liquefaction therein. In embodiments in which the natural gas feed contains heavy hydrocarbons, it is preferable to withdraw purified first portion of the natural gas 104 from an intermediate section of heat exchanger 40 and separate the heavy hydrocarbons 8 using gas liquid separator 5. Alternatively, the gas-liquid separator may be replaced by a distillation column or other separation devices known in the art. Instead of collecting heavy hydrocarbons 8 separately as shown in FIG. 1, heavy hydrocarbons 8 may be expanded and then warmed in heat exchanger 40. The resulting warmed stream can be combined with cold natural gas stream 144 within heat exchanger 40. This advantageously captures some of the cold energy from heavy hydrocarbons 8, and if warm natural gas stream 108 is subsequently used for fuel, it also provides additional energy for that purpose.

[0113] Vaporized natural gas from gas liquid separator 5 is reintroduced to heat exchanger 40, wherein it subsequently liquefies to produce LNG 6. While not shown specifically in FIG. 3, as in FIG. 2, in one embodiment, first portion of the LNG 146 can be removed from LNG 6, expanded in second valve V2, and then warmed in heat exchanger 40, thereby providing additional refrigeration, to produce warm natural gas stream 108. The remaining portion can then be expanded across third valve V3, thereby producing second portion of the LNG 148. In the embodiment shown in FIG. 3, all of LNG 6 is expanded in valve V3 and used as product.

[0114] Refrigeration for the system is provided by two sources. The first refrigeration source can be via a conventional nitrogen refrigeration cycle 50. Nitrogen gas is compressed in nitrogen recycle compressor 10, cooled in cooler 11, compressed further in booster of first turbine booster 20, cooled in cooler 21, then further compressed in booster of second turbine booster 25 before being cooled again in cooler 26. The resulting compressed nitrogen is then cooled in heat exchanger 40, wherein a first portion is removed and expanded in turbine of second turbine booster 25, a second portion is removed and expanded in turbine of first turbine booster 20. The resulting expanded nitrogen streams are then introduced to heat exchanger 40, where they are warmed via indirect heat exchange against the natural gas and other nitrogen streams.

[0115] The second refrigeration source is provided by using the excess pressure differential of the high pressure natural gas. In this embodiment, second portion of the natural gas stream 106 is split from high pressure natural gas 2, and then purified in second purification unit 131 of at least water and preferably mercury to produce purified second portion of the natural gas 132. While the embodiment shown in FIG. 3 includes two separate purification units, it is possible to use a single purification unit to fully purify the entire natural gas stream prior to splitting the natural gas into two streams. However, it is preferable to split the streams prior to purification since the natural gas used to provide refrigeration (i.e., the portion not liquefied), does not need to have carbon dioxide removed, since the natural gas turbine outlet stream 144 is at a sufficiently warm temperature such that carbon dioxide will not freeze within this stream. In another embodiment, units 130 and 131 may be combined into a single unit, and the moisture free stream (e.g., 132) is removed at an intermediate location of the vessel and the moisture and CO.sub.2 free stream (e.g., 104) is removed from the end of the vessel opposite the feed location.

[0116] Purified second portion of the natural gas 132 can then be partially cooled in heat exchanger 40, before being expanded in turbine 121 of natural gas turbine booster 120 to form cold natural gas stream 144. Alternatively, in an embodiment not shown, purified second portion of the natural gas stream 132 can be sent, prior to cooling, directly to natural gas turbine 121 for expansion. This can help limit the temperature of 144 to avoid heavy hydrocarbon condensation and potential solidification. Cold natural gas stream 144 is then reintroduced to heat exchanger 40, wherein it is warmed via indirect heat exchange and collected as warm natural gas stream 108 from the warm end of the heat exchanger. In one embodiment, cold natural gas stream 144 can be combined with heavy hydrocarbons 8 within the heat exchanger, or the different streams can warm individually within the heat exchanger and be combined following their warming.

[0117] The booster 101 of natural gas turbine booster 120 can be located at many different locations depending on the natural gas source and return pressures. For example, it may be located at 1) the NG stream to be expanded (FIG. 2) if the feed pressure and/or return pressure are low, 2) the total natural gas feed flow before splitting the flow to be expanded and flow to be liquefied (FIG. 3), or 3) on the discharge of the turbine at the warm end of the exchanger (e.g., stream 108) in the case of high natural gas feed pressure and high natural return pressure (not shown), or 4) on stream to be liquefied (e.g., stream 104) if the feed pressure is low (not shown). Alternatively the turbine may be used to drive an electrical generator or dissipated by oil brake (not shown).

[0118] The primary difference between the embodiment of FIG. 2 and the embodiment of FIG. 3 is that in FIG. 3, low pressure gaseous nitrogen is introduced as feed into the nitrogen refrigeration cycle and LIN is coproduced with LNG. In one particular embodiment, gaseous nitrogen (GAN) is introduced into, and compressed by, nitrogen compressor 15 before being cooled in cooler 16 and then added to the refrigeration cycle. Those of ordinary skill in the art will recognize that the nitrogen compressor 15 can be optional, since its use can be dependent on the pressure of the GAN feed stream. In another embodiment, a third portion of the cooled nitrogen is removed from the heat exchanger 40, subcooled in nitrogen subcooler 45, and expanded across valve V4 before being introduced to nitrogen gas liquid separator 55. Nitrogen vapor 57 is withdrawn from the top of nitrogen gas liquid separator 55 and then warmed in heat exchanger 40, wherein it is then recompressed by nitrogen compressor 15 before again rejoining the refrigeration cycle. Liquid nitrogen is withdrawn from the bottom of nitrogen gas liquid separator 55 and preferably one portion 51 is sent to be vaporized in subcooler 45, while the other portion 52 is sent to a liquid nitrogen storage tank (not shown).

[0119] As such, FIG. 3 provides for an embodiment in combining LIN+LNG+natural gas letdown. As before, the nitrogen refrigeration cycle includes a recycle compressor, and at least one turbine booster. However, because it produces LIN (e.g., removes nitrogen molecules from the loop), it also includes a step of adding gaseous nitrogen feed to the system. In the embodiment shown in FIG. 3, the gaseous nitrogen makeup is at low pressure, and therefore it also includes a nitrogen feed compressor, as well as a subcooler to provide liquid nitrogen product. As in other embodiments, the natural gas supply is split between a flow to be liquefied and a flow to be expanded back to low pressure. As noted previously, the natural gas booster 101 may be located at various locations depending on the flow ratio and pressure of the natural gas feed and letdown pressures used.

[0120] Regarding FIG. 4, a process flow diagram of an embodiment having a partial integration of a nitrogen liquefier with a natural gas liquefier is shown. In FIG. 4, natural gas can be acquired from a natural gas source, compressed in natural gas booster 101 to produce high pressure natural gas 2. High pressure natural gas 2 is preferably split into two portions, with one portion being liquefied and the other portion providing a portion of the refrigeration used to liquefy the natural gas. First portion of the natural gas stream 102 is purified in first purification unit 130, wherein acid gases, water and mercury are preferably removed. Preferably, any impurity within the natural gas that would damage equipment or solidify prior to the natural gas liquefying is removed in first purification unit 130. The resulting purified first portion of the natural gas stream 104 is then withdrawn from first purification unit 130 and introduced to heat exchanger 440 for liquefaction therein. In embodiments in which the natural gas feed contains heavy hydrocarbons, it is preferable to withdraw purified first portion of the natural gas 104 from an intermediate section of heat exchanger 440 or before entering exchanger 440 and separate the heavy hydrocarbons 8 using gas liquid separator 5 or distillation column. In one embodiment, heavy hydrocarbons 8 may be expanded and then warmed in heat exchanger 440. The resulting warmed stream can be combined with other natural gas streams (e.g., cold natural gas stream 144) within heat exchanger 440. This advantageously captures some of the cold energy from heavy hydrocarbons 8, and if warm natural gas stream 108 is subsequently used for fuel, it also provides additional energy for that purpose. Vaporized natural gas from gas liquid separator 5 is reintroduced to heat exchanger 440, wherein it subsequently liquefies to produce LNG 6.

[0121] Refrigeration for the system can be provided by three sources, a first nitrogen refrigeration cycle 50, a second nitrogen refrigeration cycle 450, and by expansion of high pressure natural gas. In first nitrogen refrigeration cycle 50, nitrogen gas coming from first nitrogen refrigeration cycle 50 and second nitrogen refrigeration cycle 450 is compressed in shared nitrogen recycle compressor 410, and cooled in cooler 411. The resulting compressed nitrogen is then split into two streams, with a first portion going to first nitrogen refrigeration cycle 50 and the second portion going to second nitrogen refrigeration cycle 450.

[0122] With respect to first nitrogen refrigeration cycle 50, the nitrogen can be compressed further in booster of first turbine booster 20, cooled in cooler 21, further compressed in booster of second turbine booster 25 before being cooled again in cooler 26. The resulting compressed nitrogen is then cooled in heat exchanger 40, wherein a first portion is removed and expanded in turbine of second turbine booster 25, a second portion is removed and expanded in turbine of first turbine booster 20. The resulting expanded nitrogen streams are then introduced to heat exchanger 40, where they are warmed via indirect heat exchange against the natural gas and other nitrogen streams, and then sent back to shared nitrogen recycle compressor 410.

[0123] As in FIG. 3, the embodiment of FIG. 4 also includes low pressure gaseous nitrogen introduced as feed and LIN is coproduced. Gaseous nitrogen (GAN) is introduced into, and compressed by, nitrogen compressor 15 before being cooled in cooler 16 and then added to the refrigeration cycle. Those of ordinary skill in the art will recognize that the nitrogen compressor 15 can be optional, since its use can be dependent on the pressure of the GAN feed stream. Additionally, the remaining portion of the compressed nitrogen is removed from the heat exchanger 40, subcooled in nitrogen subcooler 45, and expanded across valve V4 before being introduced to nitrogen gas liquid separator 55. Nitrogen vapor 57 is withdrawn from the top of nitrogen gas liquid separator 55 and then warmed in heat exchanger 40, wherein it is then recompressed by nitrogen compressor 15 before again rejoining the refrigeration cycle. Liquid nitrogen is withdrawn from the bottom of nitrogen gas liquid separator 55 then split into first portion 51 which is vaporized in subcooler 45 to provide heat exchange for the LIN subcooling and second portion 52 as LIN production preferably sent to a storage tank (not shown).

[0124] The second refrigeration source can be second nitrogen refrigeration cycle 450, which is comprised of shared nitrogen recycle compressor 410, shared cooler 411, and non-shared equipment such as third turbine booster 420, cooler 421, fourth turbine booster 425, and cooler 426.

[0125] The third source of refrigeration is provided by using available excess pressure differential of high pressure natural gas. In this embodiment, second portion of the natural gas stream 106 is split from high pressure natural gas 2, and then purified in second purification unit 131 of at least water and preferably mercury to produce purified second portion of the natural gas 132. While the embodiment shown in FIG. 4 includes two separate purification units, it is possible to use a single purification unit to fully purify the entire natural gas stream prior to splitting the natural gas into two streams. However, it is preferable to split the streams prior to purification since the natural gas used to provide refrigeration (i.e., the portion not liquefied), does not need to have carbon dioxide removed, since the natural gas turbine outlet stream 144 is at a sufficiently warm temperature such that carbon dioxide will not freeze within this stream. Alternatively, units 130 and 131 may be combined into a single unit such that the moisture free stream 132 is removed at an intermediate location of the vessel and the moisture and CO.sub.2 free stream 104 is removed from the end of the vessel opposite the feed 2 location.

[0126] Purified second portion of the natural gas 132 may be partially cooled in heat exchanger 440, before being expanded in natural gas turbine 121 to form cold natural gas stream 144. Alternatively, stream 132 can be sent, prior to cooling in the heat exchanger, to turbine 121 for expansion to limit the temperature of 144 due to CO.sub.2 freezing or heavy hydrocarbon condensation. Cold natural gas stream 144 is then reintroduced to heat exchanger 440, wherein it is warmed via indirect heat exchange and collected as warm natural gas stream 108 from the warm end of the heat exchanger. In one embodiment, cold natural gas stream 144 can be combined with heavy hydrocarbons 8 within the heat exchanger, or the two streams can warm individually within the heat exchanger and be combined following their warming.

[0127] The booster 101 of natural gas turbine booster 120 can be located at many different locations depending on the natural gas source and return pressures. For example, it may be located at 1) the NG stream to be expanded (FIG. 2) if the feed pressure and/or return pressure are low, 2) the total natural gas feed flow before splitting the flow to be expanded and flow to be liquefied (FIG. 3), or 3) on the discharge of the turbine at the warm end of the exchanger (e.g., 108) in the case of high natural gas feed pressure and high natural return pressure (not shown), or 4) on stream to be liquefied (e.g., 104) if the feed pressure is low (not shown). Alternatively the turbine may be used to drive an electrical generator or dissipated by oil brake (not shown).

[0128] As noted above, the embodiment of FIG. 4 preferably includes a stand-alone nitrogen liquefier 350, that shares a common nitrogen recycle compressor (e.g., 410), with the second nitrogen refrigeration cycle 450. As such, such an embodiment can advantageously produce LIN and LNG at locations that have both a nitrogen liquefaction unit and access to natural gas.

[0129] The embodiment of FIG. 4 has a 12% efficiency improvement compared to the embodiment shown in FIG. 3, primarily due to the additional turbine boosters which can be positioned at temperatures in the cycle to independently optimize the LNG and LIN trains.

[0130] Additionally, the shared recycle compressor 410 provides a lower capital cost compared to an independent nitrogen liquefier plus independent LNG plant, since the embodiment effectively eliminates one recycle compressor, which typically is the largest capital cost equipment of the system. In addition, there is a small efficiency improvement due to a single, large machine compared to two, small machines. Similarly as indicated before, the location of the booster for the natural gas letdown can vary with natural gas source and letdown pressure.

[0131] A comparison of the embodiments shown in FIGS. 1-4 can be found in Table II below.

TABLE-US-00002 TABLE II Comparison Data for FIGS. 1-4 LNG + LIN Base (Typical LNG LNG production by production by NG LNG + LIN production production by N2 NG letdown and N2 letdown and N2 by NG letdown and N2 cycle) FIG. 1 cycle. FIG. 2 cycle (FIG. 3) cycle (FIG. 4) NG supply 32 bara 342 1019 1019 1019 (mtd) NG to letdown 5.6 0 677 677 677 bara (mtd) LNG Production 342 342 342 342 (mtd) N2 cycle power input 7155 4158 10555 9974 (kW) LNG Specific power 502 292 353 313 (kWh/mt) Power Reduction (%) 42% 30% 38% LIN production 301 301 (mtd) LIN Specific Power 440 440 (kWh/mt)

[0132] In an optional embodiment, fourth gas stream 351 can be cooled and/or liquefied within heat exchanger 40 to produce cooled/liquefied fourth gas stream 352. In one embodiment, fourth gas stream 351 is selected from the group consisting of natural gas; ethane; ethylene; acetylene; C.sub.3-C.sub.6 alkanes, alkenes and alkynes; nitrogen; hydrogen; and helium. In embodiments in which gas stream 351 is hydrogen or helium, gas stream 352 is preferably not liquefied. Otherwise, cooled stream 352 is preferably liquefied. Advantageously, this optional embodiment allows for three separate gases to be liquefied (e.g., streams 52, 352 and 6).

[0133] The embodiments shown in FIG. 3 and FIG. 4 are preferably located near, on, or have access to an industrial site with a large constant letdown flow of natural gas (e.g., a cogen unit, or steam methane reformer facility), as well as a source of nitrogen (e.g., near an air separation unit ASU or nitrogen pipeline). Nitrogen is often available near an ASU as they are commonly designed for O.sub.2 production. Nitrogen may be extracted with a small cost to the ASUs precooling system.

[0134] The embodiment shown in FIG. 4 includes a specific embodiment of producing LNG and LIN, however, the invention is not to be so limited. Instead, an embodiment of the invention can include liquefaction of a first gas and a second gas, through the use of two refrigeration cycles, in which the two refrigeration cycles share a common recycle compressor. In a preferred embodiment, the refrigeration cycles are nitrogen refrigeration cycles. In one embodiment, the two liquefiers could each produce either LIN or LNG or liquid hydrogen or liquid helium or any type of other industrial gases. In another embodiment, either or both of the liquefiers may have an expansion device configured to expand a higher pressure gas source.

[0135] Embodiments of the invention can have wide applications in the industry. For example, an embodiment of the invention may include identifying an underutilized liquefaction system, and then adding a second liquefier nearby (e.g., an LNG liquefier). The original liquefier can be slightly modified in order to allow for its previously underutilized recycle compressor to provide compression for both refrigeration cycles. This allows for the new liquefier to produce its liquid in a much more efficient manner. In another embodiment, the second liquefaction unit is preferably located nearby a high and low pressure pipeline network (e.g., natural gas pipeline) such that the system is able to use the refrigeration from expansion of the natural gas.

[0136] In another embodiment, two new liquefiers can be built to satisfy a market demand. For example, the first liquefier can be a nitrogen liquefaction unit and the second liquefier can be a natural gas liquefaction unit, both using nitrogen refrigeration cycles. It can be economically advantageous that at least one of the liquefiers is a standardized plant (e.g., a modular type design that can be designed and produced in bulk). In many cases, the capacity which the standardized plant has been designed for is greater than the capacity needed for this specific application. A similar concept could apply to the relocation of an existing liquefier. Therefore, the second liquefier can be built such that its refrigeration cycle uses the same recycle compressor as the one from the first liquefier. It is also common that such liquefaction plants are located near an industrial area, therefore benefiting from a wide natural gas pipeline network. One or both liquefiers would benefit from adding a natural gas expansion refrigeration to each nitrogen refrigeration cycle, as described herein.

[0137] Similarly, if the standardized plant were undersized for a particular application (e.g., produce liquid nitrogen), the second liquefaction unit could be designed to make up the difference. In this embodiment, the second liquefaction unit could be configured to create both a liquid nitrogen product, as well as an LNG product.

[0138] Operational problems can occur when the natural gas turbine drives an electric generator without extracting the refrigeration energy of expansion. Furthermore, in some instances, the flow rate and pressures of the natural gas can often fluctuate. This can cause issues with respect to fluctuations in produced energy, since electrical systems are not always able to accept the resulting fluctuations of electricity sent to the grid from the generator. Similarly, the resulting fluctuations in cold created by the natural gas expansion can yield fluctuations in other utilities.

[0139] In certain embodiments of the invention, the above referenced problems can be mitigated through the use of an LNG and/or LIN storage tank, as the storage tank provides a buffer for the fluctuations of the refrigeration balance. For example, minor fluctuations in natural gas conditions can be accounted for by adjusting the load of the nitrogen refrigeration cycle and the quantity of LNG and/or LIN being liquefied. Large or long term fluctuations can be accounted for by stopping the liquefier and compensating by the tank level. In addition, significant short term fluctuations can be accounted for by adjusting a bypass valve to allow high pressure natural gas to bypass the liquefier and going straight to the MP GAN stream (not shown). In another embodiment, the method can include monitoring various process conditions (e.g., pressure, flow rate, gas composition, etc. . . . ) of the natural gas source, and/or streams downstream of the natural gas source. Based on these monitored process conditions, various set points can be adjusted in order to further optimize the system. For example, a set point that can be adjusted can include expansion ratio for the various turbines, along with flow rates of various streams throughout. In one embodiment, the set points for the flow rate and inlet pressure to the natural gas turbine can be controlled within an acceptable operating range of the liquefaction equipment by adjustment of the natural gas bypass valve and/or a turbine inlet control valve. In one embodiment, the method can include a central process controller that is configured to receive the various monitored process conditions and then determine whether a selected set point should be adjusted based on the monitored process conditions. The monitoring devices can communicate with the controller via all known methods, for example, both wirelessly and via wired electrical communication.

[0140] FIG. 5 provides for a process flow diagram with liquid nitrogen production being supplemented with refrigeration from letdown of natural gas. The additional energy provided by the natural gas letdown reduces the power and size of the nitrogen refrigeration cycle for a fixed LIN production depending on the amount of energy which can be removed from the natural gas letdown (i.e., flow and pressure ratio of the NG letdown).

[0141] In this type of embodiment, it is preferable that the system be proximate to a nitrogen source (e.g., ASU with available nitrogen production, or other small dedicated nitrogen generator, or nitrogen pipeline) as well as a source of pressurized natural gas suitable for letdown. While it is understood that there will be variations in the natural gas flow and pressure, the liquefier can accommodate some of these variations by a corresponding adjustment in LIN production and or power from the nitrogen refrigeration cycle.

[0142] The method shown in FIG. 5 has one natural gas turbine booster for the warm section of the exchanger and one nitrogen turbine booster for the cold section. However, for improved efficiency and flexibility, an additional warm turbine booster (as shown in FIG. 2) can be included in certain embodiments of the invention.

[0143] With respect to purification, water should be removed and depending on natural gas composition, pressure and temperature prior to natural gas expansion, acid gases such as CO.sub.2, and other impurities which freeze at colder temperatures may be removed from the natural gas as well. The natural gas may be cooled before being expanded and can reach a temperature of approximately 60 C. to 100 C. before entering the heat exchanger, is re-warmed and returned to the low pressure header. Since CO.sub.2 will only freeze at lower temperatures, it is not required to remove CO.sub.2 from the stream being expanded.

[0144] Since the liquefier is intended to be in industrial facilities with constant natural gas letdown, nitrogen source, etc, these facilities often have much less impurities in the feed natural gas. For example odorization (addition of sulfur containing mercaptans) is not used in these areas. Therefore, the purification system maybe simplified compared to a similar unit installed at a non-industrial site.

[0145] Those of ordinary skill in the art will recognize that other types of refrigeration cycles may be used. Therefore, embodiments of the invention are not intended to be limited to the particular refrigeration cycles shown and described within the detailed specification and in the accompanying figures. Additionally, while the embodiments shown in the figures and discussed herein, typically show that the natural gas expansion turbine can be connected to a natural gas booster, certain embodiments of the invention are not intended to be so limited. Rather, in certain embodiments of the invention, the natural gas expansion turbine 121 can drive a booster that is located within one of the refrigeration cycles, for example the nitrogen refrigeration cycle. In this embodiment, the booster can be configured to compress a refrigeration fluid (for example, nitrogen) within the refrigeration cycle.

[0146] 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 as 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, if there is language referring to order, such as first and second, it 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 can be combined into a single step.

[0147] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0148] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

[0149] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0150] 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.

[0151] 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.

[0152] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.