LNG system with high pressure pre-cooling cycle

[0001] 1. Field of the Invention

[0002] This invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns an improved liquified natural gas (LNG) facility employing a high pressure pre-cooling cycle.

[0003] 2. Description of the Prior Art

[0004] The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.

[0005] Natural gas is frequently transported by pipeline from the supply source to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.

[0006] The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.

[0007] In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with a series of single-component refrigerants or one multi-component mixed refrigerant. A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.

[0008] Many LNG facilities employ a propane refrigeration cycle to pre-cool the natural gas stream prior to cooling and condensing the natural gas in the main downstream refrigeration cycle(s). Typically, propane pre-cooling cycles employ a propane compressor, propane cooler, one or more propane chillers, and associated piping for routing the propane refrigerant from the compressor, to the cooler, to the chiller(s), and back to the compressor. The propane compressor receives propane vapors and compresses the propane in one or more stages of compression. The compressed propane refrigerant discharged from the compressor is then cooled and condensed in the propane cooler via indirect heat exchange with ambient air or water. The pressure of the liquid propane refrigerant can then be let down to further cool the refrigerant. The resulting propane refrigerant can then be employed in the propane chiller(s) to cool the natural gas stream and, optionally, to cool other downstream refrigerants. As heat is transferred from the natural gas stream to the propane refrigerant in the propane chiller(s), at least a portion of the propane refrigerant vaporizes. The resulting propane vapor is returned to the propane compressor for compression.

[0009] There is a definite trend in the LNG industry towards increasing the capacity of existing LNG facilities and constructing new LNG facilities of higher capacity. As the demand for high capacity LNG facilities increases, conventional propane pre-cooling systems are becoming inadequate. High capacity LNG facilities require more refrigeration from the pre-cooling cycle than conventional LNG facilities. This increased refrigeration requirement necessitates an increase the flow rate of propane through the pre-cooling cycle. However, a number of disadvantages are associated with increasing the cooling capacity of a propane pre-cooling cycle by simply increasing the flow rate of propane through though the system. For example, increasing the flow rate of the propane refrigerant in the pre-cooling cycle requires larger compressors and larger piping. Obviously, the use of larger equipment and piping increases the cost ofthe facility. Further, larger equipment and piping takes up more space—an undesirable feature when plot space is limited, such as for offshore LNG facilities. In addition, the capacity of conventional propane refrigerant compressors is already at or near the limits of known technology. Thus, procuring a propane compressor of significantly higher capacity than currently available propane compressors would be very expensive, if not impossible.

[0010] It is, therefore, an object of the present invention to provide a novel natural gas liquefaction system employing a pre-cooling cycle that provides more refrigeration duty than conventional propane pre-cooling cycles.

[0011] A further object of the invention is to provide a high capacity pre-cooling cycle for LNG facilities that does not require significantly larger equipment and/or piping than conventional propane pre-cooling cycles.

[0012] Another object ofthe invention is to provide a high capacity pre-cooling cycle for LNG facilities that does not require a refrigerant compressor of significantly larger capacity than conventional compressors employed in propane pre-cooling cycles.

[0013] It should be understood that the above objects are exemplary and need not all be accomplished by the invention claimed herein. Other objects and advantages of the invention will be apparent from the written description and drawings.

[0014] Accordingly, one aspect of the present invention concerns a process for liquefying natural gas comprising: (a) cooling a natural gas stream in a high pressure pre-cooling cycle via indirect heat exchange with a pre-cooling refrigerant, where the pre-cooling cycle employs a pre-cooling compressor that discharges the pre-cooling refrigerant at a discharge pressure of at least 225 pounds per square inch atmospheric (psia) and the pre-cooling refrigerant has a boiling point temperature lower than −45° F. at one atmosphere; and (b) further cooling and at least partly condensing at least a portion of the natural gas stream in a subsequent cooling cycle via indirect heat exchange with a subsequent refrigerant having a lower boiling point temperature than the pre-cooling refrigerant.

[0015] Another aspect of the present invention concerns an apparatus for liquefying a natural gas stream. The apparatus comprises a pre-cooling refrigeration cycle and a subsequent refrigeration cycle located downstream of the pre-cooling refrigeration cycle. The pre-cooling refrigeration cycle includes a pre-cooling compressor, a pre-cooling chiller, and a pre-cooling refrigerant circulating through the pre-cooling compressor and pre-cooling chiller. The pre-cooling compressor is configured to discharge the pre-cooling refrigerant at a discharge pressure of at least 225 psia. The pre-cooling refrigerant has a boiling point temperature lower than −45° F. at one atmosphere. The subsequent refrigeration cycle includes a subsequent compressor, a subsequent chiller, and a subsequent refrigerant circulating through the subsequent compressor and subsequent chiller. The subsequent refrigerant has a lower boiling point temperature than the pre-cooling refrigerant.

[0016] FIG. 1 is a simplified flow diagram of a cascaded refrigeration process for LNG production that employs a high pressure pre-cooling cycle.

[0017] A cascaded refrigeration process uses one or more refrigerants to transfer heat from a natural gas stream to a refrigerant, and ultimately transferring the heat from the refrigerant to the environment. In essence, the overall refrigeration system functions as a heat pump by removing heat energy from the natural gas stream as the stream is progressively cooled to lower and lower temperatures. The design of a cascaded refrigeration process involves a balancing of thermodynamic efficiencies and capital costs. In heat transfer processes, thermodynamic irreversibilities are reduced as the temperature gradients between heating and cooling fluids become smaller, but obtaining such small temperature gradients generally requires significant increases in the amount of heat transfer area, major modifications to various process equipment, and the proper selection of flow rates through such equipment so as to ensure that both flow rates and approach and outlet temperatures are compatible with the required heating/cooling duty.

[0018] As used herein, the term “open-cycle cascaded refrigeration process” refers to a cascaded refrigeration process comprising at least one closed refrigeration cycle and one open refrigeration cycle where the boiling point of the refrigerant/cooling agent employed in the open cycle is less than the boiling point of the refrigerating agent or agents employed in the closed cycle(s) and a portion of the cooling duty to condense the compressed open-cycle refrigerant/cooling agent is provided by one or more of the closed cycles. In the current invention, a predominately methane stream is employed as the refrigerant/cooling agent in the open cycle. This predominantly methane stream originates from the processed natural gas feed stream and can include the compressed open methane cycle gas streams. As used herein, the terms “predominantly,” “primarily,” “principally,” and “in major portion,” when used to describe the presence of a particular component of a fluid stream, shall mean that the fluid stream comprises at least 50 mole percent ofthe stated component. For example, a “predominantly” methane stream, a “primarily” methane stream, a stream “principally” comprised of methane, or a stream comprised “in major portion” of methane each denote a stream comprising at least 50 mole percent methane.

[0019] One of the most efficient and effective means of liquefying natural gas is via an optimized cascade-type operation in combination with expansion-type cooling. Such a liquefaction process involves the cascade-type cooling of a natural gas stream at an elevated pressure, (e.g., about 650 psia) by sequentially cooling the gas stream via passage through a multi-stage pre-cooling cycle, a multi-stage ethane or ethylene cycle, and an open-end methane cycle which utilizes a portion of the feed gas as a source of methane and which includes therein a multi-stage expansion cycle to further cool the same and reduce the pressure to near-atmospheric pressure. In the sequence of cooling cycles, the refrigerant having the highest boiling point is utilized first followed by a refrigerant having an intermediate boiling point and finally by a refrigerant having the lowest boiling point. As used herein, the terms “upstream” and “downstream” shall be used to describe the relative positions of various components of a natural gas liquefaction plant along the flow path of natural gas through the plant.

[0020] Various pretreatment steps provide a means for removing certain undesirable components, such as acid gases, mercaptan, mercury, and moisture from the natural gas feed stream delivered to the LNG facility. The composition of this gas stream may vary significantly. As used herein, a natural gas stream is any stream principally comprised of methane which originates in major portion from a natural gas feed stream, such feed stream for example containing at least 85 mole percent methane, with the balance being ethane, higher hydrocarbons, nitrogen, carbon dioxide, and a minor amount of other contaminants such as mercury, hydrogen sulfide, and mercaptan. The pretreatment steps maybe separate steps located either upstream of the cooling cycles or located downstream of one of the early stages of cooling in the initial cycle. The following is a non-inclusive listing of some of the available means which are readily known to one skilled in the art. Acid gases and to a lesser extent mercaptan are routinely removed via a chemical reaction process employing an aqueous amine-bearing solution. This treatment step is generally performed upstream of the pre-cooling stages. A major portion of the water is routinely removed as a liquid via two-phase gas-liquid separation following gas compression and cooling upstream of the pre-cooling cycle and also downstream of the first cooling stage in the pre-cooling cycle. Mercury is routinely removed via mercury sorbent beds. Residual amounts of water and acid gases are routinely removed via the use of properly selected sorbent beds such as regenerable molecular sieves.

[0021] The pretreated natural gas feed stream is generally delivered to the liquefaction process at an elevated pressure or is compressed to an elevated pressure generally greater than 500 pounds per square inch atmospheric (psia), preferably about 500 psia to about 3000 psia, still more preferably about 500 psia to about 1000 psia, still yet more preferably about 600 psia to about 800 psia. The feed stream temperature is typically near ambient to slightly above ambient. A representative temperature range being 60° F. to 150° F.

[0022] As previously noted, the natural gas feed stream is cooled in a plurality of multi-stage cycles or steps (preferably three) by indirect heat exchange with a plurality of different refrigerants (preferably three). The overall cooling efficiency for a given cycle improves as the number of stages increases but this increase in efficiency is accompanied by corresponding increases in net capital cost and process complexity.

[0023] It is preferred for the feed gas to be initially cooled in a closed, high pressure, pre-cooling refrigeration cycle via an effective number of refrigeration stages (nominally two, preferably two to four, and more preferably three stages), with each cooling stage defining a separate cooling zone. The pre-cooling refrigeration cycle employs a pre-cooling refrigerant having a relatively low boiling point temperature. In the high pressure pre-cooling refrigeration cycle of the present invention, it is preferred for the pre-cooling refrigerant to have a boiling point temperature that is at least about 10 percent lower than the boiling point temperature of propane, on a Fahrenheit temperature scale. Preferably, the pre-cooling refrigerant has a boiling point temperature at one atmosphere of pressure in the range of from about −75° to about −45° F., more preferably in the range of from about −65° to about −50° F., and most preferably in the range of from −60° to −52° F. The pre-cooling refrigerant preferably has a latent heat of vaporization at one atmosphere of pressure and boiling point temperature in the range of from about 175 to about 205 British thermal units per pound (btu/lb), more preferably in the range of from about 180 to about 195 btu/lb, and most preferably in the range of from 185 to 192 btu/lb. The pre-cooling refrigerant preferably has a vapor pressure at 68° F. in the range of from about 130 to about 180 psia, more preferably in the range of from about 140 to about 170 psia, and most preferably in the range of from 145 to 160 psia. The pre-cooling refrigerant preferably has a liquid density at one atmosphere of pressure and boiling point temperature in the range of from about 25 to about 50 pounds per cubic foot (lb/ft³), more preferably in the range of from about 30 to about 45 lb/ft³, and most preferably in the range of 37 to 43 lb/ft³. The pre-cooling refrigerant preferably has a gas density at one atmosphere of pressure and boiling point temperature in the range of from about 0.1 to about 0.2 lb/ft³, more preferably in the range of from about 0.125 to about 0.175 lb/ft³, and most preferably in the range of from 0.140 to 0.155 lb/ft³. In one embodiment of the present invention, the pre-cooling refrigerant is comprised predominately of propylene. In another embodiment of the present invention, the pre-cooling refrigerant consists essentially of propylene.

[0024] After pre-cooling, the processed feed gas is further cooled in a closed subsequent refrigeration cycle via an effective number of refrigeration stages (nominally two, preferably two to four, and more preferably two or three). The subsequent refrigeration cycle preferably employs a subsequent refrigerant having a lower boiling point than the pre-cooling refrigerant. However, it is preferred for the boiling point temperatures at one atmosphere of the pre-cooling refrigerant and the subsequent refrigerant to be within about 150° F. of one another, more preferably within about 110° F. of one another, and most preferably within 90 to 105° F. of one another. The subsequent refrigerant is preferably comprised in major portion of ethane, ethylene, or mixtures thereof. More preferably, the refrigerant comprises at least about 75 mole percent ethylene, even more preferably at least 90 mole percent ethylene, and most preferably the refrigerant consists essentially of ethylene.

[0025] The processed natural gas feed stream is preferably combined with one or more recycle streams (i.e., compressed open methane cycle gas streams) at various locations in the subsequent refrigeration cycle thereby producing a liquefaction stream. In the last stage of the subsequent cooling cycle, the liquefaction stream is condensed (i.e., liquefied) in major portion, preferably in its entirety, thereby producing a pressurized LNG-bearing stream. Generally, the process pressure at this location is only slightly lower than the pressure of the pretreated feed gas to the first stage of the pre-cooling refrigeration cycle.

[0026] The pressurized LNG-bearing stream resulting from the final stage of the subsequent refrigeration cycle is then further cooled in a final refrigeration cycle or step referred to as the open methane cycle via contact in a main methane economizer with flash gases (i.e., flash gas streams) generated in this final cycle in a manner to be described later and via sequential expansion of the pressurized LNG-bearing stream to near atmospheric pressure. The flash gasses used as a refrigerant in the final refrigeration cycle are preferably comprised in major portion of methane. More preferably the flash gas refrigerant comprises at least 75 mole percent methane, still more preferably at least 90 mole percent methane, and most preferably the refrigerant consists essentially of methane. During expansion of the pressurized LNG-bearing stream to near atmospheric pressure, the pressurized LNG-bearing stream is cooled via at least one, preferably two to four, and more preferably three expansions, where each expansion employs an expander as a pressure reduction means. Suitable expanders include, for example, either Joule-Thomson expansion valves or hydraulic expanders. The expansion is followed by a separation of the gas-liquid product with a separator. When a hydraulic expander is employed and properly operated, the greater efficiencies associated with the recovery of power, a greater reduction in stream temperature, and the production of less vapor during the flash expansion step will frequently more than off-set the higher capital and operating costs associated with the expander. In one embodiment, additional cooling of the pressurized LNG-bearing stream prior to flashing is made possible by first flashing a portion of this stream via one or more hydraulic expanders and then via indirect heat exchange means employing said flash gas stream to cool the remaining portion of the pressurized LNG-bearing stream prior to flashing. The warmed flash gas stream is then recycled via return to an appropriate location, based on temperature and pressure considerations, in the open methane cycle and will be recompressed.

[0027] Generally, the natural gas feed stream will contain such quantities of C₂+ components so as to result in the formation of a C₂+ rich liquid in one or more of the cooling stages. This liquid is removed via gas-liquid separation means, preferably one or more conventional gas-liquid separators. Generally, the sequential cooling of the natural gas in each stage is controlled so as to remove as much of the C₂ and higher molecular weight hydrocarbons as possible from the gas to produce a gas stream predominating in methane and a liquid stream containing significant amounts of ethane and heavier components. An effective number of gas/liquid separation means are located at strategic locations downstream of the cooling zones for the removal of liquids streams rich in C₂+ components. The exact locations and number of gas/liquid separation means, preferably conventional gas/liquid separators, will be dependant on a number of operating parameters, such as the C₂+ composition of the natural gas feed stream, the desired BTU content of the LNG product, the value of the C₂+ components for other applications, and other factors routinely considered by those skilled in the art of LNG plant and gas plant operation. The C₂+ hydrocarbon stream or streams may be demethanized via a single stage flash or a fractionation column. In the latter case, the resulting methane-rich stream can be directly returned at pressure to the liquefaction process. In the former case, this methane-rich stream can be repressurized and recycled or can be used as fuel gas. The C₂+ hydrocarbon stream or streams or the demethanized C₂+ hydrocarbon stream may be used as fuel or may be further processed, such as by fractionation in one or more fractionation zones to produce individual streams rich in specific chemical constituents (e.g., C₂, C₃, C₄, and C₅+).

[0028] The liquefaction process described herein may use one of several types of cooling which include but are not limited to (a) indirect heat exchange, (b) vaporization, and (c) expansion or pressure reduction. Indirect heat exchange, as used herein, refers to a process wherein the refrigerant cools the substance to be cooled without actual physical contact between the refrigerating agent and the substance to be cooled. Specific examples of indirect heat exchange means include heat exchange undergone in a shell-and-tube heat exchanger, a core-in-kettle heat exchanger, and a brazed aluminum plate-fin heat exchanger. The physical state of the refrigerant and substance to be cooled can vary depending on the demands of the system and the type of heat exchanger chosen. Thus, a shell-and-tube heat exchanger will typically be utilized where the refrigerating agent is in a liquid state and the substance to be cooled is in a liquid or gaseous state or when one of the substances undergoes a phase change and process conditions do not favor the use of a core-in-kettle heat exchanger. As an example, aluminum and aluminum alloys are preferred materials of construction for the core but such materials may not be suitable for use at the designated process conditions. A plate-fin heat exchanger will typically be utilized where the refrigerant is in a gaseous state and the substance to be cooled is in a liquid or gaseous state. Finally, the core-in-kettle heat exchanger will typically be utilized where the substance to be cooled is liquid or gas and the refrigerant undergoes a phase change from a liquid state to a gaseous state during the heat exchange.

[0029] Vaporization cooling refers to the cooling of a substance by the evaporation or vaporization of a portion of the substance with the system maintained at a constant pressure. Thus, during the vaporization, the portion of the substance which evaporates absorbs heat from the portion of the substance which remains in a liquid state and hence, cools the liquid portion. Finally, expansion or pressure reduction cooling refers to cooling which occurs when the pressure of a gas, liquid or a two-phase system is decreased by passing through a pressure reduction means. In one embodiment, this expansion means is a Joule-Thomson expansion valve. In another embodiment, the expansion means is either a hydraulic or gas expander. Because expanders recover work energy from the expansion process, lower process stream temperatures are possible upon expansion.

[0030] The flow schematic and apparatus set forth in FIG. 1 represents a preferred embodiment of the inventive LNG facility employing a high pressure pre-cooling refrigeration cycle. Those skilled in the art will recognize that FIG. 1 is a schematic only and, therefore, many items of equipment that would be needed in a commercial plant for successful operation have been omitted for the sake of clarity. Such items might include, for example, compressor controls, flow and level measurements and corresponding controllers, temperature and pressure controls, pumps, motors, filters, additional heat exchangers, and valves, etc. These items would be provided in accordance with standard engineering practice.

[0031] To facilitate an understanding of FIG. 1, the following numbering nomenclature was employed. Items numbered 1 through 99l are process vessels and equipment which are directly associated with the liquefaction process. Items numbered 100 through 199 correspond to flow lines or conduits which contain predominantly methane streams. Items numbered 200 through 299 correspond to flow lines or conduits which contain predominantly ethylene streams. Items numbered 300 through 399 correspond to flow lines or conduits which contain the pre-cooling refrigerant.

[0032] Referring to FIG. 1, natural gas enters the LNG facility via conduit 100 and is pre-cooled in high-stage, intermediate-stage, and low-stage pre-cooling chillers 2, 22, 28. In pre-cooling chillers 2, 22, 28, the natural gas stream is cooled via indirect heat exchange with the pre-cooling refrigerant. The pre-cooling refrigeration cycle employs a pre-cooling compressor 18 to compress and circulate the pre-cooling refrigerant through pre-cooling chillers 2, 22, 28. Pre-cooling compressor 18 is preferably a multi-stage (preferably three-stage) compressor which is driven by a gas turbine (not illustrated). The three stages of compression associated with pre-cooling compressor 18 preferably exist in a single unit, although each stage of compression may be a separate unit and the units mechanically coupled to be driven by a single driver.

[0033] The compressed pre-cooling refrigerant is discharged from compressor 18 via conduit 300. The discharged pre-cooling refrigerant in conduit 300 preferably has a pressure of at least about 225 psia, more preferably at least about 250 psia, and most preferably in the range from 275 to 350 psia. The discharged pre-cooling refrigerant in conduit 300 preferably has a temperature in the range of from about −300° to about −50° F., more preferably in the range of from about −250° to about −100° F., and most preferably in the range of from −200° to −150° F. It is preferred for pre-cooling compressor 18 to be capable of providing a maximum inlet-to-discharge pressure increase in the range of from about 200 to about 350 psi, more preferably in the range of from 240 to 280 psi.

[0034] The compressed pre-cooling refrigerant in conduit 300 is passed to a cooler 20 where it is cooled and liquefied. In a preferred embodiment of the present invention, cooler 20 employs ambient air and/or water as the cooling medium for removing heat from the compressed pre-cooling refrigerant. The pre-cooling refrigerant stream discharged from cooler 20 is passed through conduit 302 to a pressure reduction means, illustrated as expansion valve 12, wherein the pressure of the liquefied pre-cooling refrigerant is reduced, thereby evaporating or flashing a portion thereof. The resulting two-phase product then flows through conduit 304 into high-stage pre-cooling chiller 2 wherein gaseous methane refrigerant introduced via conduit 152, natural gas feed introduced via conduit 100, and gaseous ethylene refrigerant introduced via conduit 202 are respectively cooled via indirect heat exchange means 4, 6, and 8, thereby producing cooled gas streams respectively produced via conduits 154, 102, and 204. The gas in conduit 154 is fed to a main methane economizer 74 which will be discussed in greater detail in a subsequent section and wherein the stream is cooled via indirect heat exchange means 98.

[0035] When heat is transferred from the natural gas stream to the pre-cooling refrigerant in high-stage pre-cooling chiller 2, at least a portion of the pre-cooling refrigerant vaporizes. The vaporized portion of the pre-cooling refrigerant from chiller 2 is returned to a high-stage inlet port of compressor 18 via conduit 306. Preferably, the gaseous pre-cooling refrigerant entering the high-stage inlet port of compressor 18 via conduit 306 has a pressure of at least about 15 psia, more preferably at least about 20 psia, and most preferably in the range of from 25 to 35 psia. The gaseous pre-cooling refrigerant in conduit 306 preferably has temperature in the range of from about −250° to about −75° F., more preferably in the range of from about −225° to about −100° F., and most preferably in the range of from −200° to −150° F. The pre-cooling refrigerant in conduit 306 preferably has a density of at least about 0.18 lb/ft³, more preferably at least about 0.2 lb/ft³, and most preferably in the range of from 0.225 to 0.3 lb/ft³.

[0036] The liquid portion of the pre-cooling refrigerant that is not vaporized in high-stage pre-cooling chiller 2 exits pre-cooling chiller 2 via conduit 308 and its pressure further reduced by passage through a pressure reduction means, illustrated as expansion valve 14, whereupon an additional portion of the liquid pre-cooling refrigerant is flashed. The resulting two-phase stream is then fed to intermediate-stage pre-cooling chiller 22 through conduit 310, thereby providing a coolant for chiller 22. The cooled feed gas stream from chiller 2 flows via conduit 102 to separation equipment 10 wherein gas and liquid phases are separated. The liquid phase, which can be rich in C₃+ components, is removed via conduit 103. The gaseous phase is removed via conduit 104 and then split into two separate streams which are conveyed via conduits 106 and 108. The stream in conduit 106 is fed to intermediate-stage pre-cooling chiller 22. The stream in conduit 108 becomes the feed to heat exchanger 62 and ultimately becomes the stripping gas to heavies removal column 60, discussed in more detail below. Ethylene refrigerant from chiller 2 is introduced to chiller 22 via conduit 204. In chiller 22, the feed gas stream, also referred to herein as a methane-rich stream, and the ethylene refrigerant streams are respectively cooled via indirect heat transfer means 24 and 26, thereby producing cooled methane-rich and ethylene refrigerant streams via conduits 110 and 206. The evaporated portion of the pre-cooling refrigerant is separated and passed through conduit 311 to the intermediate-stage inlet port of compressor 18. The gaseous pre-cooling refrigerant introduced into the intermediate-stage inlet port of compressor 18 via conduit 311 preferably has a pressure of at least about 40 psia, more preferably at least about 60 psia, and most preferably in the range of from 70 to 100 psia. The vaporized pre-cooling refrigerant in conduit 311 preferably has a temperature in the range of from about −50° to about 100° F., more preferably in the range of from about 0° F. to about 75° F., and most preferably in the range of from 10° to 50° F. The pre-cooling refrigerant in conduit 311 preferably has a density greater than about 0.5 lb/ft³, more preferably in the range of from 0.55 to 0.75 lb/ft³. The unvaporized liquid pre-cooling refrigerant in chiller 22 is removed via conduit 314, flashed across a pressure reduction means, illustrated as expansion valve 16, and then fed to low-stage pre-cooling chiller 28 via conduit 316.

[0037] As illustrated in FIG. 1, the methane-rich stream flows from intermediate-stage pre-cooling chiller 22 to the low-stage pre-cooling chiller 28 via conduit 110. In chiller 28, the stream is cooled via indirect heat exchange means 30. In a like manner, the ethylene refrigerant stream flows from the intermediate-stage pre-cooling chiller 22 to low-stage pre-cooling chiller 28 via conduit 206. In the latter, the ethylene refrigerant is totally condensed or condensed in nearly its entirety via indirect heat exchange means 32. The vaporized pre-cooling refrigerant is removed from low-stage pre-cooling chiller 28 and returned to the low-stage inlet port of compressor 18 via conduit 320. The gaseous pre-cooling refrigerant introduced into the low-stage inlet port of compressor 18 via conduit 320 preferably has a pressure of at least about 80 psia, more preferably at least about 110 psia, and most preferably in the range of from 120 to 175 psia. The temperature of the pre-cooling refrigerant in conduit 320 is preferably in the range of from about 0° to about 200° F., more preferably in the range of from about 25° to about 150° F., and most preferably in the range of from 50° to 100° F. The gas density of the vaporized pre-cooling refrigerant in conduit 320 is preferably at least about 0.9 lb/ft³, more preferably in the range of from 0.95 to 1.25 lb/ft³.

[0038] After cooling the natural gas stream in chillers 2, 22, 28 of the pre-cooling refrigeration cycle, the pre-cooled methane-rich stream is then further cooled in a subsequent refrigeration cycle employing a predominately ethylene refrigerant. The subsequent refrigeration cycle employs high-stage, intermediate-stage, and low-stage ethylene chillers 42, 54, and 68 to sequentially cool the methane-rich stream. The methane-rich stream exiting low-stage pre-cooling chiller 28 is introduced to high-stage ethylene chiller 42 via conduit 112. Ethylene refrigerant exits low-stage pre-cooling chiller 28 via conduit 208 and is preferably fed to a separation vessel 37 wherein light components are removed via conduit 209 and condensed ethylene is removed via conduit 210. The ethylene refrigerant at this location in the process is generally at a temperature of about −24° F. and a pressure of about 285 psia. The ethylene refrigerant then flows to an ethylene economizer 34 wherein it is cooled via indirect heat exchange means 38, removed via conduit 211, and passed to a pressure reduction means, illustrated as an expansion valve 40, whereupon the refrigerant is flashed to a preselected temperature and pressure and fed to high-stage ethylene chiller 42 via conduit 212. Vapor is removed from chiller 42 via conduit 214 and routed to ethylene economizer 34 wherein the vapor functions as a coolant via indirect heat exchange means 46. The ethylene vapor is then removed from ethylene economizer 34 via conduit 216 and fed to the high-stage inlet port of ethylene compressor 48. The ethylene refrigerant which is not vaporized in high-stage ethylene chiller 42 is removed via conduit 218 and returned to ethylene economizer 34 for further cooling via indirect heat exchange means 50, removed from ethylene economizer via conduit 220, and flashed in a pressure reduction means, illustrated as expansion valve 52, whereupon the resulting two-phase product is introduced into a low-stage ethylene chiller 54 via conduit 222.

[0039] After cooling in indirect heat exchange means 44, the methane-rich stream is removed from high-stage ethylene chiller 42 via conduit 116. This stream is then condensed in part via cooling provided by indirect heat exchange means 56 of intermediate-stage ethylene chiller 54, thereby producing a two-phase stream which flows via conduit 118 to heavies removal column 60. As previously noted, the methane-rich stream in line 104 was split so as to flow via conduits 106 and 108. The contents of conduit 108, which is referred to herein as the stripping gas, is first fed to heat exchanger 62 wherein this stream is cooled via indirect heat exchange means 66 thereby becoming a cooled stripping gas stream which then flows via conduit 109 to heavies removal column 60. A heavies-rich liquid stream containing a significant concentration of C₄+ hydrocarbons, such as benzene, cyclohexane, other aromatics, and/or heavier hydrocarbon components, is removed from heavies removal column 60 via conduit 114, preferably flashed via a flow control means 97, preferably a control valve which can also function as a pressure reduction, and transported to heat exchanger 62 via conduit 117. Preferably, the stream flashed via flow control means 97 is flashed to a pressure about or greater than the pressure at the high-stage inlet port to methane compressor 83. Flashing also imparts greater cooling capacity to the stream. In heat exchanger 62, the stream delivered by conduit 117 provides cooling capabilities via indirect heat exchange means 64 and exits heat exchanger 62 via conduit 119. In heavies removal column 60, the two-phase stream introduced via conduit 118 is contacted with the cooled stripping gas stream introduced via conduit 109 in a countercurrent manner thereby producing a heavies-depleted vapor stream via conduit 120 and a heavies-rich liquid stream via conduit 114.

[0040] The heavies-rich stream in conduit 119 is subsequently separated into liquid and vapor portions or preferably is flashed or fractionated in vessel 67. In either case, a heavies-rich liquid stream is produced via conduit 123 and a second methane-rich vapor stream is produced via conduit 121. In the preferred embodiment, which is illustrated in FIG. 1, the stream in conduit 121 is subsequently combined with a second stream delivered via conduit 128, and the combined stream fed to the high-stage inlet port of the methane compressor 83.

[0041] As previously noted, the gas in conduit 154 is fed to main methane economizer 74 wherein the stream is cooled via indirect heat exchange means 98. The resulting cooled compressed methane recycle or refrigerant stream in conduit 158 is combined in the preferred embodiment with the heavies-depleted vapor stream from heavies removal column 60, delivered via conduit 120, and fed to low-stage ethylene chiller 68. In low-stage ethylene chiller 68, this stream is cooled and condensed via indirect heat exchange means 70 with the liquid effluent from conduit 222 which is routed to low-stage ethylene chiller 68 via conduit 226. The condensed methane-rich product from low-stage condenser 68 is produced via conduit 122. The vapor from low-stage ethylene chiller 54, withdrawn via conduit 224, and low-stage ethylene chiller 68, withdrawn via conduit 228, are combined and routed, via conduit 230, to ethylene economizer 34 wherein the vapors function as a coolant via indirect heat exchange means 58. The stream is then routed via conduit 232 from ethylene economizer 34 to the low-stage inlet port of ethylene compressor 48.

[0042] As noted in FIG. 1, the compressor effluent from vapor introduced via the low-stage side of ethylene compressor 48 is removed via conduit 234, cooled via inter-stage cooler 71, and returned to compressor 48 via conduit 236 for injection with the high-stage stream present in conduit 216. Preferably, the two-stages are a single module although they may each be a separate module and the modules mechanically coupled to a common driver. The compressed ethylene product from compressor 48 is routed to a downstream cooler 72 via conduit 200. The product from cooler 72 flows via conduit 202 and is introduced, as previously discussed, to high-stage pre-cooling chiller 2.

[0043] The pressurized LNG-bearing stream, preferably a liquid stream in its entirety, in conduit 122 is preferably at a temperature in the range of from about −200° to about −50° F., more preferably in the range of from about −175° to about −100° F., most preferably in the range of from −150° to −125° F. The pressure of the stream in conduit 122 is preferably in the range of from about 500 to about 700 psia, most preferably in the range of from 550 to 725 psia.

[0044] The stream in conduit 122 is directed to a main methane economizer 74 wherein the stream is further cooled by indirect heat exchange means/heat exchanger pass 76 as hereinafter explained. It is preferred for main methane economizer 74 to include a plurality of heat exchanger passes which provide for the indirect exchange of heat between various predominantly methane streams in the economizer 74. Preferably, methane economizer 74 comprises one or more plate-fin heat exchangers. The cooled stream from heat exchanger pass 76 exits methane economizer 74 via conduit 124. It is preferred for the temperature of the stream in conduit 124 to be at least about 10° F. less than the temperature of the stream in conduit 122, more preferably at least about 25° F. less than the temperature of the stream in conduit 122. Most preferably, the temperature of the stream in conduit 124 is in the range of from about −200 to about −160° F. The pressure of the stream in conduit 124 is then reduced by a pressure reduction means, illustrated as expansion valve 78, which evaporates or flashes a portion of the gas stream thereby generating a two-phase stream. The two-phase stream from expansion valve 78 is then passed to high-stage methane flash drum 80 where it is separated into a flash gas stream discharged through conduit 126 and a liquid phase stream (i.e., pressurized LNG-bearing stream) discharged through conduit 130. The flash gas stream is then transferred to main methane economizer 74 via conduit 126 wherein the stream functions as a coolant in heat exchanger pass 82 and aids in the cooling of the stream in heat exchanger pass 76. Thus, the predominantly methane stream in heat exchanger pass 82 is warmed, at least in part, by indirect heat exchange with the predominantly methane stream in heat exchanger pass 76. The warmed stream exits heat exchanger pass 82 and methane economizer 74 via conduit 128, and into the high-stage inlet of methane compressor 83. Predominantly methane stream exiting heat exchanger pass 82 via conduit 128 to be at least about 10° F. greater than the temperature of the stream in conduit 124, more preferably at least about 25° F. greater than the temperature of the stream in conduit 124. The temperature of the stream exiting heat exchanger pass 82 via conduit 128 is preferably warmer than about −50° F., more preferably warmer than about 0° F., still more preferably warmer than about 25° F., and most preferably in the range of from 40° to 100° F.

[0045] The liquid-phase stream exiting high-stage flash drum 80 via conduit 130 is passed through a second methane economizer 87 wherein the liquid is further cooled by downstream flash vapors via indirect heat exchange means 88. The cooled liquid exits second methane economizer 87 via conduit 132 and is expanded or flashed via pressure reduction means, illustrated as expansion valve 91, to further reduce the pressure and, at the same time, vaporize a second portion thereof. This two-phase stream is then passed to an intermediate-stage methane flash drum 92 where the stream is separated into a gas phase passing through conduit 136 and a liquid phase passing through conduit 134. The gas phase flows through conduit 136 to second methane economizer 87 wherein the vapor cools the liquid introduced to economizer 87 via conduit 130 via indirect heat exchanger means 89. Conduit 138 serves as a flow conduit between indirect heat exchange means 89 in second methane economizer 87 and heat exchanger pass 95 in main methane economizer 74. The warmed vapor stream from heat exchanger pass 95 exits main methane economizer 74 via conduit 140 and is introduced into the intermediate-stage inlet port of methane compressor 83.

[0046] The liquid phase exiting intermediate-stage flash drum 92 via conduit 134 is further reduced in pressure by passage through a pressure reduction means, illustrated as a expansion valve 93. Again, a third portion of the liquefied gas is evaporated or flashed. The two-phase stream from expansion valve 93 are passed to a final or low-stage flash drum 94. In flash drum 94, a vapor phase is separated and passed through conduit 144 to second methane economizer 87 wherein the vapor functions as a coolant via indirect heat exchange means 90, exits second methane economizer 87 via conduit 146, which is connected to the first methane economizer 74 wherein the vapor functions as a coolant via heat exchanger pass 96. The warmed vapor stream from heat exchanger pass 96 exits main methane economizer 74 via conduit 148 and is introduced into the low-stage inlet port of compressor 83.

[0047] The liquefied natural gas product from low-stage flash drum 94, which is at approximately atmospheric pressure, is passed through conduit 142 to a LNG storage tank 99. In accordance with conventional practice, the liquefied natural gas in storage tank 99 can be transported to a desired location (typically via an ocean-going LNG tanker). The LNG can then be vaporized at an onshore LNG terminal for transport in the gaseous state via conventional natural gas pipelines.

[0048] As shown in FIG. 1, the high, intermediate, and low stages of compressor 83 are preferably combined as single unit. However, each stage may exist as a separate unit where the units are mechanically coupled together to be driven by a single driver. The compressed gas from the low-stage section passes through an inter-stage cooler 85 and is combined with the intermediate pressure gas in conduit 140 prior to the second-stage of compression. The compressed gas from the intermediate stage of compressor 83 is passed through an inter-stage cooler 84 and is combined with the high pressure gas provided via conduits 121 and 128 prior to the third-stage of compression. The compressed gas (i.e., compressed open methane cycle gas stream) is discharged from high stage methane compressor through conduit 150, is cooled in cooler 86, and is routed to the high-stage pre-cooled chiller 2 via conduit 152 as previously discussed. The stream is cooled in pre-cooling chiller 2 via indirect heat exchange means 4 and flows to main methane economizer 74 via conduit 154. The compressed open methane cycle gas stream from chiller 2 which enters the main methane economizer 74 undergoes cooling in its entirety via flow through indirect heat exchange means 98. This cooled stream is then removed via conduit 158 and combined with the processed natural gas feed stream upstream of the first stage of ethylene cooling.

[0049] In one embodiment of the present invention, the LNG production system illustrated in FIG. 1 is simulated on a computer using conventional process simulation software. Examples of suitable simulation software include HYSYS™ from Hyprotech, Aspen Plus® from Aspen Technology, Inc., and PRO/II® from Simulation Sciences Inc.

[0050] The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.

[0051] The inventor hereby states his intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.