Heavy hydrocarbon removal system for lean natural gas liquefaction

Abstract

A system and method for integrated heavy hydrocarbon removal in a liquefaction system having a lean natural gas source. An economizer located between a main cryogenic heat exchanger and a reflux drum is provided to cool an overhead vapor stream against a partially condensed stream. In addition, pressure of the natural gas feed stream is maintained into a scrub column. A pressure drop is provided by a valve located between the economizer and the reflux drum on a partially condensed stream withdrawn from the cold end of the warm section of the main cryogenic heat exchanger.

Claims

1. A method of steady state operation of a system for liquefying natural gas, the method comprising: (a) performing a closed-loop compression sequence on a warm first refrigerant stream withdrawn from a warm side of a main heat exchanger, the compression sequence comprising compressing and cooling the warm first refrigerant stream to produce at least one cooled, compressed first refrigerant stream; (b) withdrawing a natural gas feed stream from a natural gas feed source at a source pressure; (c) introducing the natural gas feed stream into a scrub column at a scrub column pressure, the scrub column having a top section and a bottom section; (d) separating the natural gas feed stream in the scrub column into a methane-rich vapor fraction, collected as a first overhead vapor stream at a top end of the scrub column, and a heavy hydrocarbon-enriched fraction, collected as a first bottoms liquid stream at a bottom end of the scrub column; (e) withdrawing the first bottoms liquid stream from the scrub column, the first bottoms liquid stream being a heavy hydrocarbon enriched natural gas stream; (f) withdrawing the first overhead vapor stream from the scrub column, the first overhead vapor stream being a methane-enriched natural gas stream; (g) introducing at a warm end of a warm section of a main heat exchanger, the first overhead vapor stream into a natural gas circuit, and each of the at least one cooled-compressed first refrigerant stream into a refrigerant circuit; (h) in at least one of the refrigerant circuits, withdrawing and reducing a pressure of an overhead refrigerant stream to produce a reduced pressure overhead refrigerant stream and introducing the reduced pressure overhead refrigerant stream into a cold side of the main heat exchanger; (i) providing indirect heat exchange between the warm side and the cold side of the main heat exchanger; (j) producing a product stream from the natural gas circuit at a cold end of the main heat exchanger, the product stream being at least partially liquefied; (k) withdrawing a two-phase natural gas stream from the natural gas circuit at a cold end of the warm section of the main heat exchanger; (l) reducing a pressure of the two-phase natural gas stream to form a reduced pressure two-phase natural gas stream; (m) introducing the reduced pressure two-phase natural gas stream into a reflux drum at an intermediate natural gas temperature; (n) separating the reduced pressure two-phase natural gas stream into a reflux drum liquid stream and a reflux drum vapor stream; (o) introducing the reflux drum vapor stream into the natural gas circuit at a location in the main heat exchanger that is closer to a cold end of the main heat exchanger than the cold end of the warm section; (p) increasing a pressure of the reflux drum liquid stream and introducing the reflux drum liquid stream into the top section of the scrub column; and (q) providing indirect heat exchange between the reflux drum vapor stream and the two-phase natural gas stream by which the two-phase natural gas stream is cooled against the reflux drum vapor stream.

2. The method of claim 1, further comprising: (r) operationally configuring any valves located between, and in flow communication with, the natural gas feed source and the scrub column to provide a total pressure drop of no more than one bar.

3. The method of claim 1, further comprising: (s) withdrawing a partially condensed refrigerant stream from one of the at least one refrigerant circuits at a cold end of the warm section of the main heat exchanger and at an intermediate refrigerant temperature; (t) separating the partially condensed refrigerant stream in a phase separator into an intermediate liquid refrigerant stream and an intermediate vapor refrigerant stream; (u) introducing each of the intermediate liquid refrigerant stream and the intermediate vapor refrigerant stream into a refrigerant circuit at a location in the main heat exchanger that is closer to the cold end of the main heat exchanger that the cold end of the warm section.

4. The method of claim 1, wherein step (i) further comprises: (i) providing indirect heat exchange between the warm side and the cold side of the main heat exchanger, the warm side of the main heat exchanger comprising at least one coil-wound bundle and the cold side of the main heat exchanger comprising a shell-side, each refrigerant circuit and the natural gas circuit comprising a portion of the at least one coil-wound bundle.

5. The method of claim 4, wherein step (c) further comprises: (c) separating the natural gas feed stream into a first portion and a second portion, introducing the first portion of the natural gas feed stream into the scrub column at an intermediate location and introducing the second portion of the natural gas feed stream into the bottom end of the scrub column.

6. The method of claim 5, further comprising: (v) providing indirect heat exchange between the first overhead vapor stream and the first portion of the natural gas feed stream.

7. The method of claim 1, further comprising: (w) pre-cooling the natural gas feed stream by indirect heat exchange against a second refrigerant before performing step (c).

8. The method of claim 1, further comprising: (x) withdrawing a condensed natural gas stream from the natural gas circuit from a cold end of a middle section of the main heat exchanger, increasing the pressure of the condensed natural gas stream to form an increased pressure natural gas stream, and introducing the increased pressure natural gas stream into the reflux drum.

9. The method of claim 1, wherein step (p) comprises: (p) increasing a pressure of the reflux drum liquid stream, splitting the reflux drum liquid stream into a first portion and second portion, introducing the first portion of the reflux drum liquid stream into the top section of the scrub column, and mixing the second portion of the reflux drum liquid stream with the reflux drum vapor stream before performing step (o).

10. The method of claim 9, further comprising: (y) Performing an indirect heat exchange between the two-phase natural gas stream and a third refrigerant before performing step (1).

11. The method of claim 1, wherein step (h) further comprises splitting at least one of the reduced pressure overhead refrigerant streams into a first portion and a second portion, introducing the first portion into the cold side of the main heat exchanger, performing an indirect heat exchange between the second portion, the reflux drum vapor stream and the two-phase natural gas stream.

12. The method of claim 1, further comprising: (z) Increasing a pressure of the natural gas feed stream using a compressor before performing step (c).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic flow diagram depicting a HHC removal and an SMR natural gas liquefaction system and method in accordance with the prior art.

(2) FIG. 2 is a schematic flow diagram depicting a HHC removal and an SMR natural gas liquefaction system and method in accordance with a first exemplary embodiment of the present invention.

(3) FIG. 3 is a schematic flow diagram depicting HHC removal and a propane-mixed refrigerant (or “C3MR”) natural gas liquefaction system and method in accordance with a second exemplary embodiment of the present invention.

(4) FIG. 4 is a schematic flow diagram depicting HHC removal and an SMR natural gas liquefaction system and method in accordance with a third exemplary embodiment of the present invention.

(5) FIG. 5 is a schematic flow diagram depicting HHC removal and natural gas liquefaction system and method in accordance with a fourth exemplary embodiment of the present invention.

(6) FIG. 6 is a schematic flow diagram depicting HHC removal and natural gas liquefaction system and method in accordance with a fifth exemplary embodiment of the present invention.

DETAILED DESCRIPTION

(7) This present invention provides novel ways of achieving the temperature and pressure of the natural gas feed stream at the scrub column reflux drum for effectively providing reflux and condensing duty to the scrub column in integration with the natural gas liquefaction process.

(8) As described above, when the natural gas feed stream has a composition that is low (“lean”) in C2-C5 components and contains sufficient levels of heavy hydrocarbons, the conventional scrub column configuration is ineffective or energy inefficient. The inventors have found that the HHC removal effectiveness and the liquefaction efficiency can be improved by introducing an economizer heat exchanger between the MCHE and the reflux drum and changing the way in pressure of the feed gas is handled in the heavy hydrocarbon removal process.

(9) More specifically, the separation effectiveness and energy efficiency of the overall process can be improved by allowing the reflux drum to operate at a temperature significantly different than the feed gas temperature exiting the warm section of the MCHE. This decoupling of the reflux operating temperature from the rest of the refrigerant cycle provides an additional degree of freedom, which allows for better overall process optimization. The economizer warms the overhead vapor from the reflux drum to a temperature that is only few degrees colder than the MCHE warm section outlet temperature, which helps reduces the temperature differential at the warm end of the middle section of the MCHE and improves process thermal efficiency. The temperature difference depends upon the design temperature approach of the economizer, but is typically less than 5 degrees C. and is often less than 2 or 3 degrees C.

(10) In addition, a pressure let-down valve is placed between the MCHE and the reflux drum. This has two benefits over the conventional scrub column configurations. First, with the majority pressure drop taken at this let-down valve, very little (or no) pressure drop needs to be provided at the inlet of the scrub column itself, thereby maintaining higher feed gas density and lower feed volumetric flow in the warm section of the MCHE. This reduces the required size of the MCHE and associated capital cost. Secondly, taking the pressure drop at this location achieves cooling to the feed gas itself, off-loading a portion of the condensing duty required from the warm section of the MCHE and benefiting the HHC removal effectiveness and the overall liquefaction efficiency. Providing the pressure let-down valve in this location also helps maintains proper approach temperature in the economizer between the MCHE and the reflux drum.

(11) Moreover, additional reflux can be provided using fully condensed LNG streams taken anywhere from the system, including but not limited to LNG stream from the middle section outlet, subcooled LNG stream from cold section outlet, and LNG production pumped from the LNG storage tank.

(12) Optionally, supplemental refrigeration and condensing duty can be provided by using an additional cooler or adding an additional cooling circuit in the economizer. Cooling medium can be taken from any stream in the system that is colder than the feed gas temperature at the MCHE warm section outlet.

(13) Finally, and as noted above, a portion of the feed gas stream is directly used as a stripping gas to the scrub column. This avoids the use of extra heating source and more importantly helps maintain a proper liquid to vapor flow ratio in the column. It helps achieve better overall liquefaction efficiency and maintain column operability and improves HHC removal effectiveness.

(14) The articles “a” and “an”, as used herein and unless otherwise indicated, mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used.

(15) The terms “fluid communication” and “fluid flow communication” as used in the specification and claims, both refer to the nature of connectivity between two or more components that enables liquids, vapors, and/or two-phase mixtures to be transported between the components in a controlled fashion (i.e., without leakage) either directly or indirectly. Coupling two or more components such that they are in fluid flow communication with each other can involve any suitable method known in the art, such as with the use of welds, flanged conduits, gaskets, and bolts. Two or more components may also be coupled together via other components of the system that may separate them, for example, valves, gates, or other devices that may selectively restrict or direct fluid flow.

(16) The term “conduit,” as used in the specification and claims, refers to one or more structures through which fluids can be transported between two or more components of a system. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and/or gases.

(17) The term “natural gas”, as used in the specification and claims, means a hydrocarbon gas mixture consisting primarily of methane.

(18) The term “mixed refrigerant” (also abbreviated as “MR”), as used in the specification and claims, means a fluid comprising at least two hydrocarbons and for which hydrocarbons comprise at least 80% of the overall composition of the refrigerant.

(19) The terms “heavy component” or “heavy hydrocarbon”, as used in the specification and claims, means a hydrocarbon that has a boiling point higher than methane at standard pressure.

(20) As used herein, the term “indirect heat exchange” refers to heat exchange between two fluids where the two fluids are kept separate from each other by some form of physical barrier.

(21) As used herein, the term “warm stream” is intended to mean a fluid stream that is cooled by indirect heat exchange under normal operating conditions of the system being described. Similarly, the term “cold stream” is intended to mean a fluid stream that is warmed by indirect heat exchange under normal operating conditions of the system being described.

(22) As used therein, the term “warm side” is intended to mean a portion of a heat exchanger through with one or more warm streams flow. Similarly, the term “cold side” is intended to mean a portion of the heat exchanger through which one or more cold streams flow.

(23) The term “scrub column” refers to a type of distillation column, which is a column containing one or more separation stages, composed of devices such as packing or trays, that increase contact and thus enhance mass transfer between upward rising vapor and downward flowing liquid flowing inside the column. In this way, the concentration of lighter (i.e. higher volatility and lower boiling point) components increases in the rising vapor that collects as overhead vapor at the top of the column, and the concentration of heavier (i.e. lower volatility and higher boiling point) components increases in the descending liquid that collects as bottoms liquid at the bottom of the column. The “top” of the distillation column refers to the part of the column at or above the top-most separation stage. The “bottom” of the column refers to the part of the column at or below the bottom-most separation stage. An “intermediate location” of the column refers to a location between the top and bottom of the column, between two separation stages.

(24) In the case of a scrub column, the natural gas feed stream is introduced (as a gaseous stream or as a partially condensed, two-phase stream) into the scrub column at an intermediate location of the column or, more typically, at the bottom of the column. The upward rising vapor from the feed stream is then brought into contact, as it passes through one or more separation stages inside the scrub column, with a downward flowing liquid reflux stream, thereby “scrubbing” components heavier than methane from said vapor (i.e. removing at least some of said less volatile components from the vapor). This results, as noted above, in the natural gas feed stream being separated into a methane-rich vapor fraction collected as an overhead vapor (referred to herein as a “first overhead vapor”) at the top of the scrub column, and a liquid fraction, enriched in hydrocarbons heavier than methane, collected as a bottoms liquid (referred to herein as a “first bottoms liquid”) at the bottom of the scrub column.

(25) As used herein, the term “separator” or “phase separator” refers to a device, such as drum or other form of vessel, in which a two phase stream can be introduced in order to separate the stream into its constituent vapor and liquid phases. A reflux drum is a type of phase separator that is operationally configured to provide liquid reflux for a distillation column.

(26) Solely by way of example, certain exemplary embodiments of the invention will now be described with reference to FIGS. 2 to 6. In the figures, elements that are similar to those of a previous embodiment are represented by reference numerals increased by a multiple of 100. For example, the main cryogenic heat exchanger 110 of FIG. 1 has the same structure and function as the main cryogenic heat exchanger 210 of FIG. 1. Such elements should be regarded as having the same function and structure unless otherwise stated or depicted herein, and the discussion of such elements may therefore not be repeated for multiple embodiments.

(27) In the embodiments depicted in FIGS. 2 to 6, the main cryogenic heat exchanger, used to liquefy the natural gas, is shown as being a coil-wound heat exchanger. Although use of a coil wound heat exchanger is currently the preferred technology, the main exchanger could alternatively be a plate and fin heat exchanger, or another type of heat exchanger known in the art or developed in the future. Similarly, although in the embodiments depicted herein depict the coil bundles of the main heat exchanger as being housed in a single shell, thereby forming a single unit, the main heat exchanger could comprise a series of two or more units, with having its own casing/shell, or with one or more of the bundles being housed in one casing/shell, and with one or more other bundles being housed in one or more different casings/shells. The refrigerant cycle used to supply cold refrigerant to the main heat exchanger may likewise be of any type suitable for carrying out the liquefaction of natural gas. Exemplary cycles known and used in the art, and that could be employed in the present invention, include single mixed refrigerant cycle (SMR), the propane pre-cooled mixed refrigeration cycle (C3MR), nitrogen expander cycle, methane expander cycle, dual mixed refrigerant cycle (DMR), and cascade cycle.

(28) Referring now to FIG. 2, in this embodiment, the natural gas feed stream 202 is separated in a first portion 202a and a second portion 202b before being introduced into the scrub column 236. The first portion 202a is pre-cooled in an economizer 232 to a suitable temperature, preferably below 0 degrees C., and more preferably between −10 degrees C. and −40 degrees C. The cooled first portion is then introduced into the scrub column 236 through the feed stream inlet 235, where it is separated into a methane-rich overhead vapor stream 239 and a bottom liquid stream 240, which is enriched in hydrocarbons heavier than methane. Preferably there is zero or very low pressure drop (e.g. less than one bar) across the inlet valve 234 such that the feed gas entering the scrub column 236 at the inlet 235 is slightly below the original pressure of the feed gas stream 202. For example, if the feed gas stream 202 enters the inlet valve 234 at 65 bara, the outlet pressure from the inlet valve 234 is nominally 64 bara (not including any pressure drops due to connecting conduits and the economizer 232 passages). The second portion 202b is used as stripping gas to the bottom section 238 of the scrub column 236. The flow rate of the second portion 202b is regulated by an inlet valve 207 that is preferably configured and operated to provide a pressure drop of less than one bar.

(29) The overhead vapor stream 239, is withdrawn from the top section 237 of the scrub column 236 and the bottom liquid stream 240 is withdrawn from the bottom section 238 of the scrub column 236. The top section 237 is also known in the art as the rectification section of a distillation column while the bottom section 238 is also known in the art as the stripping section of a distillation column. The boundary of the two sections is dependent on the location of the feed stream inlet 235. The two sections can be filled with structured packing or contrasted with trays for counter-current contact of liquid and vapor flows inside the scrub column 236.

(30) The overhead vapor stream 239 is warmed by the economizer 232, which provides indirect heat exchange against the feed gas stream 202. The warmed overhead vapor stream 244 then flows into the warm section (warm bundle) 214 of a MCHE 210, in which it is cooled to a temperature typically between −40 degrees C. and −60 degrees C., and typically also partially condensed. The partially condensed natural gas stream 245 is then withdrawn from the warm section 214 of the MCHE 210 and is further cooled in an economizer 252 against the overhead vapor stream 251 from the reflux drum 250. The cooled feed gas stream 246 exiting the economizer 252 is expanded across a pressure let-down JT valve 253 to a lower pressure such that sufficient liquid is formed in the reflux drum. Depending on the feed gas composition, the reflux drum is often operated at 2-10 bar below the critical pressure of the feed. The sub-critical pressure feed stream is then introduced into the reflux drum 250 at inlet 247, where it is phase separated to form the bottoms liquid stream 254 and the overhead vapor stream 251.

(31) The operating pressure and temperature of the reflux drum 250 (which is the same as the outlet pressure and temperature of the JT valve 253) is such that the density ratio of the liquid phase to the vapor phase in the reflux drum 250 is higher than 1 and, preferably, higher than 4. In addition, the surface tension of the liquid phase in the reflux drum 250 is high enough to have a clear phase boundary, preferably higher than 2 dyne/cm. The bottoms liquid stream 254 from the reflux drum 250 is then pumped, using a liquid pump 255, and returned to the top end of the scrub column 236 as a reflux stream 256 in order to provide the necessary reflux for operation of the scrub column and washing down heavy hydrocarbons from the feed gas. As noted above, the overhead vapor stream 251 is warm in the economizer 252 against the partially condensed natural gas stream 245 exiting the warm section 214 of the MCHE 210 and before being sent to the middle section 215 of the MCHE 210.

(32) The components and operation of the refrigerant compression system 260 is essentially the same as the refrigerant compression system 160 described in connection with FIG. 1. Accordingly, reference numerals are not provided in FIG. 2 for the elements of the refrigerant compression system 260.

(33) In comparison to the conventional arrangement shown in FIG. 1, the method and system of the embodiment of the present invention depicted in FIG. 2 therefore differs in the manner in which the majority of the feed pressure let-down is taken at the inlet 247 of the reflux drum 250 and the reflux drum 250 operating temperature is significantly lower (e.g. 5-30 degrees C. lower) than the temperature of the streams 245, 278, 221a, 221b exiting the warm end of the warm section 214 of the MCHE 210. As a result, the feed gas stream is maintained at higher pressure in the natural gas circuit 217a through the warm section 214 of the MCHE 210 than in the natural gas circuit 117a of FIG. 1. Moreover, in the embodiment of FIG. 2, the operating temperature of the cold MR separator 279 is much warmer (5-30 degrees C., preferably at least 5 degrees C. and, more preferably, at least 10 degrees C.) than the temperature in the reflux drum 250. By decoupling the operating temperatures of the cold MR separate 279 and the reflux drum 250 allows for more freedom to independently optimize the refrigeration loop and the heavy hydrocarbon removal system 230. In addition, the economizer 252 also helps maintain a tighter temperature differential at the warm end the middle section (bundle) 215, meaning that streams 257, 280, 281 have a smaller temperature differential as they enter the warm end of the middle section 215 than streams 157, 180, 181 of FIG. 1. Finally, replacing or supplementing the dedicated reboiler 142 in FIG. 1 with a stripping gas (second portion 202b of the feed gas stream 202) reduces or avoids the need for an external heat input to the system. All of the above allows the overall liquefaction efficiency to improve significantly, as demonstrated in the Example provided herein.

(34) Similar improvement to the process can be achieved with other refrigerant cycles, such as propane precooled mixed refrigerant process (C3-MR). Referring now to FIG. 3, another exemplary embodiment of the invention is depicted, in which refrigerant duty is provided by a propane refrigerant cycle and a mixed refrigerant cycle. The propane refrigerant cycle pre-cools both the feed gas and the mixed refrigerant.

(35) In this embodiment, the feed gas stream 302 cooled in one or more propane kettles (collectively represented by block 382 and also referred to as a precooler) to a temperature preferably below zero degrees C. and, more preferably, to between −20 degrees C. and −35 degrees C. before being sent to the scrub column 336. Low pressure propane refrigerant streams 384, 331c, 331b, 331a (collected from the a series of evaporator kettles operated at different pressures and temperatures) are compressed in the propane compressor 385 to form a high pressure discharge propane stream 386. The high pressure discharge propane stream 386 is then cooled and fully condensed in one or more aftercooler 387 to form and high pressure liquid propane refrigerant stream 388. The high pressure liquid propane refrigerant stream 388 is then evaporated at multiple pressure to provide sequential cooling to the feed gas stream 302 and the high pressure mixed refrigerant stream 374. The warm low pressure mixed refrigerant 361 from the MCHE 310 is compressed by a series of compressors 364, 371, and cooled by a series of after coolers 366, 373, to form a high pressure mixed refrigerant stream 374. After being cooled and partially condensed through the series of propane kettles 382, the cooled high pressure mixed refrigerant stream 383 is phase separated in a phase separator 375 into a mixed refrigerant liquid (MRL) stream 376 and a mixed refrigerant vapor (MRV) stream 377. The MRL stream 376 is further subcooled in the warm 314 and middle sections 315 of the MCHE 310 before being expanded through a JT valve 325 to form a low pressure cold refrigerant stream 326. The low pressure cold refrigerant stream 326 is then sent to the shell side of the middle section 315 of the MCHE 310 to provide refrigeration to the system. The MRV stream 377 is further cooled, condensed and subcooled sequentially in the warm, middle and cold sections of the MCHE 310 before being expanded through a JT valve 328 to form another low pressure cold refrigerant stream 329. The low pressure cold refrigerant stream 329 is then sent to the shell side of the cold section 316 of the MCHE 310 to provided refrigeration to the system.

(36) The system 300 shown in FIG. 3 differs from system 200 in that the first economizer (economizer 232 in system 200) is not needed because the feed gas stream 202 has already been precooled in the propane kettles 382. It also differs in that there is no cold MR separator between the middle 315 and the warm sections 314 of the MCHE 310 in system 300. However, as in system 200, the feed gas stream 345 exiting the warm section 314 of the MCHE 310 is further cooled in an economizer 352, located between the MCHE 310 and the reflux drum 350. The feed gas stream 346 exiting the economizer 352 is expanded across a pressure let-down JT valve 353 to a pressure that is blow its critical pressure. It is then phase separated in the reflux drum 350 into its liquid and vapor phases to produce a liquid stream 354 and an overhead vapor stream 351. The operating pressure and temperature of the reflux drum 350 (same as the outlet pressure and temperature of the JT valve 353) is such that the density ratio of the liquid phase to the vapor phase in the drum is higher than 1 and, preferably, higher than 4. The surface tension of the liquid phase in the reflux drum 250 is high enough to have a clear phase boundary—preferably 2 dyne/cm.

(37) Comparing system 300 to the system 100 of the prior art from the perspective of operation of the heavy hydrocarbon removal systems 330, 130, the majority of the pressure drop in the feed gas occurs just before the inlet 347 of the reflux drum 350. This allows the reflux drum 350 operating temperature to be much colder than the temperature of the feed gas stream 345 exiting the warm section 314 of the MCHE 310 and the pressure of the feed gas can be maintained relatively high (e.g. 1-10 bara higher than the same stream in FIG. 1.) in the warm 314 and middle sections 315 of the MCHE 310 as compared to system 100 (the prior art). All of the above helps achieve better overall liquefaction.

(38) Such arrangement for C3-MR process also allows more flexible operation as composition of the feed gas stream 302 changes. For example, as the composition of the feed gas stream 302 becomes leaner, system 300 allows the removal of HHC to be achieved efficiently by taking more pressure drop at the JT valve 353, while keeping operational parameters of the refrigerant compression system 360 and the scrub column 336 relatively constant.

(39) Referring now to FIG. 4, in system 400 an additional reflux stream 489 is provided using a portion of the fully liquefied LNG stream exiting the feed gas circuit 117b at the cold end of the middle section 415 of the MCHE 410. The pressure of the additional reflux stream 489 is increased by a pump 490 and the increased pressure reflux stream 491 flows into the reflux drum 450, where it is mixed with the overhead vapor stream 451 coming from the cold end of the warm section 414 of the MCHE 410. This additional reflux helps supplement the reflux flow and duty. It also helps maintain the reflux drum at a temperature much colder (e.g. 5-30 degrees C.) than the overhead vapor stream 451 coming from the cold end of the warm section 414 of the MCHE 410, especially when the feed gas source 401 is at a lower pressure (e.g. 30-45 45 bara, or a pressure that is already below the feed gas critical pressure) and self-cooling through the pressure let-down valve 453 is not sufficient to achieve the desired temperature.

(40) It should be noted that such additional reflux could be provided using one or more fully condensed LNG streams taken anywhere from the system 400, including but not limited to an LNG stream from the cold end of the middle section 415, the subcooled LNG stream 403, the LNG product stream 406, or even final LNG product pumped from the LNG storage tank 404.

(41) In yet another embodiment, as depicted in FIG. 5, system 500 includes supplemental refrigeration and condensing duty provided by using an additional cooler 592 located between the economizer 552 and the pressure let-down valve 553. Cooling medium for the cooler 592 can be sourced from any stream in the system 500 that is colder than the temperature of the partially condensed stream 545. For example (not shown), a portion of the CMRL stream 524 could be expanded and directed to the cooler 592 to help cool the partially condensed stream 545. A spent CMRL slip stream from the cooler 592 is sent back to the shell side of the MCHE 510, preferably at an intermediate location between the warm 514 and the middle sections 515 of the MCHE 510. This arrangement helps maintaining the reflux drum 550 at a temperature much colder (e.g. 5-30 degrees C. colder) than the overhead vapor stream 545, especially when the feed gas source 501 is at lower pressure and self-cooling through the JT valve 553 is not sufficient to achieve the desired temperature.

(42) System 500 also includes a reflux pump-forward option. With this option, a portion of the pumped reflux liquid stream 556 is directed to and mixed with the overhead vapor stream 551 instead of being sent to the top section 537 of the scrub column 536. The mixing point can either be before the economizer 552 (as indicated by stream 593a) or after the economizer 552 (as indicated by stream 593b). This option provides additional operational flexibility. For example, as the feed gas stream 502 become richer, more liquid will be formed in the reflux drum 550. If no other operational change is desired, the amount of pump-forward liquid can be increased, and vice versa.

(43) Referring to FIG. 6, another exemplary embodiment is shown as system 600. In system 600, an additional cooling circuit is added to the economizer 652. A portion of the CMRL stream 624 is expanded and directed to the economizer 652 to help cool the overhead vapor stream 645. A spent CMRL slip stream 697 from the economizer 652 is sent back to the shell side of the MCHE 610, preferably an intermediate location 698 between the warm 614 and the middle sections 615 of the MCHE 610. Similar to system 500, this arrangement also helps maintaining the reflux drum 650 at a temperature much colder than the overhead vapor stream 645 as it exits the warm section 614 of the MCHE 610. Optionally, a feed booster compressor 694 could be added to increase the pressure of the feed gas stream 602, allowing higher self-cooling capability at the pressure let-down valve 653 at the inlet 647 of the reflux drum 650.

EXAMPLE

(44) Table 1 below shows a comparison between a set of simulated operating conditions of various streams of system 100 (FIG. 1) and system 200 (FIG. 2). The data in this table illustrates that using economizer between the MCHE 210 and the reflux drum 250 and introduce pressure drop at the inlet 247 of the reflux drum 250 can significantly improve the overall liquefaction efficiency. The liquefaction efficiency is typically measured by specific power, which is calculated by dividing the total refrigeration power by the production rate. However specific power means higher liquefaction efficiency. The feed pressure is maintained higher than that in the prior art in both the warm and middle sections of the MCHE. Specifically, as can be seen from the table, the feed gas through warm section of the system 200 is about 10 bara higher than that in system 100; while the feed gas through middle section of the system 200 is about 3 bara higher than that in system 100. Maintaining higher feed gas pressure helps achieve higher liquefaction efficiency.

(45) TABLE-US-00001 TABLE 1 System 100 (prior art) System 200 Feed gas stream Ref # 102 202 P 59.00 59.00 T 33.13 33.65 Column Pressure Ref # 136 236 P 48.42 58.57 Feed at Warm Ref # 145 245 section outlet P 42.98 53.14 T −72.54 −52.78 Feed at Ref # 246 Economizer P — 52.79 outlet T — −71.53 Reflux Drum Ref # 150 250 P 42.98 46.26 T −72.54 −77.00 Feed at Middle Ref # 151 257 section inlet P 42.98 45.92 T −72.54 −53.87 Specific Power, 428.1 402.0 kWh/tonne Relative Spec 100.0% 93.9% Power P: Pressure in bara T: Temperature in degrees C.

(46) It will be appreciated that the invention is not restricted to the details described above with reference to the preferred embodiments but that numerous modifications and variations can be made without departing from the spirit or scope of the invention as defined in the following claims.