AU2014201746B2 - Enhanced operation of lng facility equipped with refluxed heavies removal column - Google Patents

Abstract

improved methodology for starting up a INGi facility employing a refluxed heavies removal column. The improved metodology involves varying the temperature of the feed to the heavies removal colmn between start-up and normal operation. This allows a larger amount of the stream produced from the top of the heavis removal colmn during startup to be used to more rapidly start up the TANG facility.

Description

EMHANC® OPERATION OF LNG FAClOTY EQUIPPED WITH REFLUXED HEAVIES

REMOY AL COLL'MN 'Hie present application is atiiYisional application from Australian Patent Appliention No. 20 i 32013?H. rhe entire- disclosure ofwhLh is incorporated herein by reference.:

Th is invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns an improved methodology for staging.....up sand operating a liquefied natural gas (LNG) facility employing a refluxed heavies removal column.

The cryogenic liquefaction of natural gas is routinely practiced as a :-0630¾ of convening natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the y(flume of the natural gas by about 600-fdid and results in a product which eap be stored and transported at.near atmospheric pressure.

Natural ps is frequently transported by pipeline front foe supply source of supply to a distant market, Ifis desirable to operate the pipeline under a substantially constant and hip load factor but often the detiyerability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliveraMity 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 cars be delivered when demand exceeds supply, Such practice allows future demand peaks to be met with materia! from storage, ©m practical means lor dome this is to convert, the: gas to a liquefied state for storage andrtq then vaporize the liquid as demand requires.:

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. 1 his is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally pot practical because appreciable pressurization is required to significantly reduce; the Specific velunmof the gas. Such pressurization requires the use of more expensive storage containers.

In order to Store and transport natural gas in the liquid shite, the natural gas is preferably cooled to -151 cC to -162;V (-240°F to -260°F) where the liquefied natural gas (l.NG) possesses a near-atmospheric vapour pressure. Numerous systems exist in the prior art for the liquefaction: of natural gas in which the gas is liquefied by sequentially passing foe 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 vyith one or more refrigerants; such as propane, propylene, odsane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants pig. mixed reifigerant systems). A liqnefacfidn methodology which is particularly apptieableto the current invention employs an open methane cycle ha- die final refrigeration cycle wherein a pressurized LNG-bearing stream isflashed and the flash vapors (i.e., the flash gas: sfream|s]| :are subsequently employed as booling agents, recompressed, cooled, combined with the processed na|uraf gas feed stream and liquefied thereby producing the pressurized IMG-bearing stream.

In most LNG facilities it is necessary to remove heavy components (e.g, benzene, toluene;, xylene, and; or/cyclohexane) from the processed οοΡίπιΙ"gas stream in order to prevent:freezing of the heavy components in downstream beaLexcbangerS. It is known that refluxed heavies columns can provide significantly more effective and efficient heavies removal than, non-refluxed columns:. However, one drawback of using a refluxed heavies removal column in conventional LNG facilities has beep the significant delay in starting up thesLMG faedkies: caused by the refluxed heavies removal column. The main reason for this delay in starting up the LNG facility was that during start-up, the reflux stream to the heavies removal column originated from a lower outlet of the heavies removal column. During start-up., thw bulk of the feed stream entering the heavies removal column exited an upper bullet of tire heavies removal column, As a result, Only W shad! portion of the feed stream entering the heaves removal column during start-up exited the lower outlet and was available for routing back to the column as the reflux stream. As start-tip progressed, the Quantity of the teed stream available for use as reflux gradually increased to its optimum designed How rate ever a period of many hours or even days. However, the refluxed heavies removal column could not effectively remove heavies from5 the processed natural gas stream until the reflux stream was flowing at its designed rate, Thus,: conventional start-up of an LNG facility employing a refluxed heavies removal column took many: hours of even days. A fu til ter disadvantage: of conventional I ,NG plant start-up procedures was that tire processed natural gas stream exiting the Upper portion of the refluxed heavies removal column was simply fared because the elevated heavies concentration of this stream Would freeze in downstream heat exchangers. Thus, because the bulk of the processed natural gas stream; entering the refluxed heavies removal column during start-up exited the upper portion of the column and was subsequently flared, conventional start-up procedures for an LNG facility employing a refluxed heavies removal column wasted a sigm fiesnt portion of the pi octwsed natural gas .stream.

It is,: therefore, desirable to: provide a taster si-uri-up procedure for a LNG facility employing a refluxed heavies removal column.

Again it is desirable to provide a more efficient start-up procedure for a LNG facility employ mg a refluxed heavies removal eolunm, wherein the start-up procedure does not waste (e.g., Haw} ;·, significant portion of die processed natural gas suenm. ft should be understood that the above desires are exemplary and need not all be accomplished fry the invention claimed herein. Gther advantages of the invention will be apparent from the written description and drawings.

Throughout the description and claims of Mis specification the word “comprise” and variations Of that word, such as “comprises'' and “comprising", are not intended to exdude othec additives or component··; or integers. lie present invention relates to a method of starting up a cascade-type liquefied natural gas facility employing a refluxed jheavies removal column between two refrigeration cycles of the hicihiy, said method including the steps of: (a) operating the refluxed heavies removal column in an initiating mode., said initiating mode including initiating the flow of a natural gas stream through a feed inlet of the refluxed heavies removal column and into the refluxed heavies vohnnu, sakl refluxed heavies removal: column including a reflux inlet spaced from the feed inlet, said reflux inlet having substantially no hydro-eathen containing fluids flowing feerethrough and into the refluxed heavies removal column during operation in the initiating mode; (b) subsequent to step in), operating the refluxed heavies removal column in a start-up mode, said start-up mode including using the refluxed heavies iemoval column to separate the natural gas: stream into a first heavies stream and a first lights stream* said start-up mode ...including discharging the first lights stream from the refluxed heavies removal column, said start-up mode including routing at least a port-on ol the discharged first: lights streamjo the reflux inlet; add (c) subsequent to step (b), operating the refluxed heavies removal column in a normal mode, said normal mode including using the refluxed heavies removal column to separate the natural gas stream : into a second heavies stream and a second lights stream, said normal mode including discharging the second lights: stream from the refluxed heavies removal column, gap normal mode mefnding routing at least a portion of the discharged second lights stream to the reflux inlbfi in sonic embodiments, the start up method includes any one or combination of the following wherein said natural gas stream enters Me refluxed heavies removal column at a first inlet temperature during the start-up mode, said natural gas stream enters the refluxed heavies removal column at a second inlet temperature during fee normal mod<., said second inlet temperature is at least 2°f greater tha?· the first infer temperature: said second inlet temperature is in the range of from about 4 to about 12CF greater than the first inlet temperature; said sfaturai gas stream has a first vapor/liquid hydroearhbh separation point furing the start-up mode, said: natural gas stream has: a secon| vapor/hquid hydrocarbon separation point Cy(y.,q:: during the normai mode; wherein M and Y are integers representing the number of catipn atoms its the byfrocarien molecules of the respective natural gas stream, wherein X and Yotadh the range of from 2 to 1:0, wherein X isiatdeasf: 1 greater than Y; wherein X is in the range of from 3 to 5 and Y is in the range of from 5 to 7: Wherein Y is at least 2 greater than X; wherein X is 4 and Y is 6; and/or

Said refluxed heavies removal column includes a stripping gas ink··, said refluxed inlet is spaced from and located above the feed and '.tripping gas inlets, said feed inlet is spaced from and located above the stripping gas inlet; said heavies removal column includes first and second sets of Internal packing, said firi set of internal packing its vertically disposed:: between the feed inlet and the snapping gas inia. said second set of internal packing is vertically disposed betvyeen the feed inlet and the reflux inlet.

In some embodiments, the start-up method of die present invention further includes any one or combination of the following: (d) undream of the refluxed heavies removal column, cooling the natural gas stream in a •finstrefrigeration cycle employing a first compressor to compress a first refrigerant,: (e) switching: torn die start-up mode to the normal mode by adfusfmg the difierendal pressure of the first refrigerant across the first eompressor; hi) downstream of the refluxed heavies removal column, cooling the natural gas stream in a second refrigeration cycle employing a second refrigerant including predominantly methane: (g) upstream of the first refrigeration cycle, cooling the natural gas stream in a third refrigeration cycle employing a third refrigerant comprising predominantiy propane or propylene, said first refrigerant comprising predominantly ethane or ethylene; (h) vaporizing liquefied natural gas produced by the liquefied natural.....gas facility during the normal mode.

Ip some embodiments, die start-up method includes inch-ding any one or combination of the following wherein step (e) includes decreasing: the differential pressure of the first refrigerant across the first compressor; or said first refrigerant includes predominantly propane, propylene* ethane, ethylene, or carbon dioxide,

The present invention pan also provide computer simulation process including the step of using a eompiter to simulate the method according to tire first aspect of the present invention.

The present Invention aKo relates to a liquefied natural gas product produced by die process of the present invention. Λ preferred embodiment of the present invention Is described in detail below with reference to fee attached drawing: figures, wherein: 1:10'. 1 is a: simplified flow diagram qf a eascaded-type LM0 fecility within which fee methodology of the present invention can be employed; and FIG;. 2 is a schematic sectional view of a refluxed heavies removal column that cart be controlled yia the inventive methodology. A cascaded refrigeration process Uses one or morerefrigerants for transferring heat energy from the natural gas stream to the reffigerant and Ultimately transferring said heat energy to fee environment. In essence, fee overall refrigeration system iimeibns as a heat pump by removing heat energy from the natural gas stream as the: stream is progressively cooled to slower: and lower temperatures. The design of a cascaded refrigeration process involves a balancing if thermodynamic efficiencies and capiM costs, In 'heat transfer processes, thermodynarme iijsmsibilities are «edacedas the temperature gradients: between beating and: cooling fluids become smaller, bid chaining such small temperature gradients generally requites slgmSdaid sncpases in die amount of heat transfer area, major modiheatiohs tc various process equipment; and the proper selection of How 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; :As used herein., the fepr opsn-eydio cascaded refrigeration process fsfears to a cascaded refrigeration process coTUprising at least; one dosed rcfHgeration cycle and one Open refrigeration cycle where tbs boiling: point of the refidgerant/cootiug agent employed in the open cycle is less than the boding point of the refrigerating agent or agents employed in the closed cyclsfe) and a portion of the cooling duty to condense the compressed open-cycle· reftigerant/cooling agent, is provided by one or more of the closed cycles. In the current invention, a ja-edoiadnately methane stream is employed as the refrigerant/co oling agent in. the open cycle. This predommantly methane stream originates from the processed natural gas feed stream and can iaclude the compressed open methane cycle gas streams. .As used herein, the; terms "predominantly1', "primarily", "principally1', and "in major portion", when used to describe· the presence of a particular cornpcnsni of a fluid stream, shall mean that the fuid stream comprises at least 50 mole percent of the stated component, For example, a "predominantly" methane stream, a “primarily" methane siresm, a stream "'principally'' comprised of methane, or a stream comprised "in major portion" of methane each denote a stream comprising at least 50 mob percent methane.

One of the; most eiScisnt and effective meads of liquefying natural gas is via m optimised cassads-typs operation in combination with, expaGsion-type cooling. Such a liquefaction process involves the cascade-type cooling of a natural gas stream at an elevated pressure, fe.g.. about 4.4S ΜΡ&amp;;§65() psia));by seqaendalfy cooling the gas stream via passage through a multistage propane cycle,: a multistage 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 multistage expansion cycle to further cool the same and reduce the pressure tc near-atmospheric pressure. In the sequence of cooling cycles, the refrigerant haying: the highest boiKng point is utilized first; followed by a retripraht 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 us ed to deseribs the relate positions of various components of a natural gas Jiquefaoticn plant along the flow path, of natural gas through the plant.

Various pretrcaiment steps provide a means for removing undesirable components, such as add gases, mercaptan, mercury, and moisture from the Batura! gas feed stream deliveredto the LNG facility The composition of this gas stream may vary significantly, As used herein, a natural gas stream is aay stream pmdipaily comprised of methane which originates in major portion from a natural gas feed stream, such feed stream for example contrdning at least 85 mole percent methane, with the balance being sthasCi higher hydrocarhous, nitrogen, carbon dioxide, and a minor amount of other contaminaxits such as mercury, hydrogen sulfide, and mercaptan. The preireatmsut steps may be separate steps located either upstream of the cooling cycles or located downstream. of one of the early stages of cooling in the initial eycle. The following in a non-Indusive listing of some of the available means which are readily known to pus skilled in the art . Acid gases and to a lesser extent, mercaptan are routinely removed, via a sorption process employing an aqueous- axniae-bsaring solution. This treatment step is generally performed upstream of the cooling stages in the initial cycle, A major portion of the water i routinely removed as a liquid via two-phase gas-liquid separation, following gas compression, and eoolfeg upstreaia df the initial cooling cycle and also downstream pf the first cooling stage in the initial 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 xegsuerable molecular sieves.

The pretreatsd natural gas feed stream is generally delivered to the Mquefaction process at an elevated pressure or is compressed to ah elevated pressure generally greater than 3.44 MPa (500 psia), preferably about 3.44 MPa to about 20,67 MPa (about 500 psia to about 3000 psia), still tnoreprlfeably about 3.44 MPa to; about 6.894 Ml'a (about 500 psia to about 1000 psia), still, yet more preferably about 4.14 MPa to about 5.51 MPa (about 600 psia to about 800 psia), The feed stream temperature is typically near ambient to slightly above ambient, A representative temperature range being 15°C to 65.5°C (60°P to 150°F). 1

As previously aete^ file fiataral gas feed stream is cooled in. a plurality of multistage cycles or steps (preferably three) by indirect heat exchange with a plurality of different refrigerants (preferably three), The overall cooling efficiency tor a given, cycle nuproveb as the number of stages increases but this increase is. efficiency is accompanied by corresponding increases its net capital cost and process complexity.

The feed gas is preferably passed through an effective number of refrigeration stages, nominally two, preferably two to four, and mors preferably three stages, in the first closed refrigeration cycle utfiizing a relatively high boiling refrigerant. Such relatively high boiling point refrigerant Is preferably cornprisedmnisior portion sfprQpqne, propylene, or mixtures thereof more pfeferahly the:refi%erant; comprises at least about 75 mole percent propane, even more, preferably at least 90 mole percent propane, and most preferably the refrigerant consists essentially of propane. Thereafter, the processed feed gas flows through an effective number of stages, nominally two, preferably two to four, and more preferably two or three, in a second closed refrigeration cycle in heat exchange with a retigeranfihatpg a lower boiling paint Such lower boiling point; refrigerant is pretbrabiy comprised in major portion of ethane, cthy.le.ue, or ndxlures thereof, more preferably the refrigeraut 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. Bach cooling stage comprises a separate cooling zone. As preyiorrsly noted, 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 loeations m the second, eyele thereby producing n liquefaction stream. In the last stags of the second cooling cycle, file Hquefactibn stream is condensed (ie , liquefied) in;rniyor portion, preferably in its entirety, thereby producing a pressurized LNG-bsmng stream Generally, the process pressure at tins location is only slightly lotvar than the pressure of the preheated feed gas to the first stage of the first cycle.

Generally, fee natural gas; feed stream will contain such quantities of Cy*· components so as to result in the formation of a Cyr 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 0¾ and higher molecular weight hydrocarbons as possible from fee gas to produce a g&amp;s streana predominating: in methane and a liquid stream containing significant amounts of ethane and heavier components. An effective number of gas/iiqaid 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 gaa/liquid separation means, preferably oonvcrdsonal^gas/hquid sepaistors, will be dependant on a hmfoer of QperatiQg parameters, such as the C2~ composition of the natural, gas feed ah earn, the dashed BTU content of the LNG product, the value of the C|h components for other applications, and other factors routinely considered by those skilled ία the art of LNG plant and gas plant operation. The CL Eydrocarhon stream or streams may be demethanizer! via a single stage flash or a fractionation column. In the latter case, Jbe resulting methane-rich stream can be directly returned at pressure to the liquefaction prefesm is. the: former case, this rmifraoe-rMi sirearn can ha repressurihsd arid reoycle or can be used as fed gas, The CL hydrocarbon stream or streams or the demethanized, GL hydrocarbon. stream may be used as fuel or tnav ba further processed, such as by fractionation in one or mote fractionation zones to produce individual streams rich in specific chemical constituents (e,g„ Q, C%, C4, arid CL).

The pressurized LNiLbearixig sfreamis then further cooled in a third cycle or step referred to as the open methane cycle tea contact in a main methane economizer with flash gases (is., flash gas streams) generated in this third cycle in a manner to be described later and via sequential expansion of the pressurised LNG-beamg stream to near atmosphmc pressure, The flash gasses used as a refrigerant in the third refrigeration cycle are preferably comprised in major portion of methane, more preferably the flash gas refrigerant comprises at least ?5 mole percent methane, still more preferably at least 90 mole percent methane, and most preferably the refrigerant consists essenfialy of methane. During expansion of the pressurized LNG*bearing stream, to sear atmospheric pressure, the pressurized LMGLteariag stream is cooled via at least one, preferably two to four, and pore preferably three expansions where each expansion employs an expander as a pressure reduction means. Suitable expanders Include, for example, either loule-Thcmson 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 v-'itli the recover/ of power, a greater reduction in stream temperature, had the production of less vapor during the flash expansion. step will S'equestly more than off-set the higher capital and operating costs associated with the expander. In one embodiments additional, cooling of the pressurised LNG-bsarmg stream, prior tosflashing is made possible by first flashing a portion. of this stream via one or more hydraulic expanders arid then via in direct heat exchange means employing said flash gas stream to cool the romaimag paftton. of the pressurihed IhJflMjeaang stream prior to flashing. The warmed flash gas stream is then recycled via* return to an appropriate location, based ontemperature and pressure considerations, in the open methane cycle aodwill he repornpressed.

The In pe faction process described herein may use one of several types of ocohng which include but fire not lirritedto (a) indirect heat exchange, (b) vaporization; and (c) expansion ox pressure reduction, Indirect heat exchanges as used herein, refers to a process %B«rein tte refiigerast cools the substance to be cooled without actual physical contact between the FetKgt^^ing agent and the substance to be cooled.

Specific examples of indirect: heat exchange means include heaf exchange undergone in a sheii-and“tufcs heat exchanger, a core-in-ketiie heat exchanger, and a brazed, aluminum plate-fin heal, exchanger, The physical state of the· refrigerant and substance to he cooled can vary depending cm the demands of the system. and the type of heat exchanger chosen. Tk% a shelhand-tuhe heat exchanger will typically be utilized where the refligs.ratir.sg agent is iu 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 eore-in-ketilo heat exchange.?, -As ah. example, aluminum and aluminum alloys are preferred materials of construction for the core but such nraterxals 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 stats and the substance to bs cooled is in a liquid or gaseous state. Finally, the coreuh-kettie heat exchanger will typically be utilized where the substance to be cooled is liquid or gas and the refrigerant undergoes a phase change· horn a liquid state to a gaseous state duriag the heat exchange.

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 vaporisation, 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 portico. Finally, expansion or pressure redaction 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 essbodiment, this expansion means is a jouls-Thomson e-xpansion valve. In another embodiment, the expansion means is either a hydraulic or gas expander. Because expanders recover work energy foonithe expansion process, tower process stream temperatures are possible upon expansion

The flo w schematic; forth in .FIG. 1 represents a preferred embodiment of an LNG facility in which the methodology of the present invention can be employed. FIG, 2 represents aprsferred embodiment of a refluxed heavies removal column fcr use with the meShodplogy of the present invention. As used herein, the term fheaviesrernovai colurem*' shall denote; a vessel operable to separate a heavy componeni(s) of a hydroearbon-ccdtaising stream fiom a lighter component^ of the hydiOcarborvcGntainiug stream, As used herein, the term AeiTuxsd heavies removal sqtonnr shall denote a heavies removal column that- employs a reflux stream to aid in separating heavy and light hydrocarbon cemponenis, loose skilled in the ad will recognized that FIGS, 1 and 2 are schematics only and, therefore, many items of equipment that would he needed in a commercial plant for successful operation have beea 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·, T©¥&amp;cihtam:mmd<^tod&amp;g ofHGS. 1 and 2, the following numbering nomenclature was employed, hems numbered 1 through 99 are process vessels and equipment which are directly associated with the liquefaction process, Items numbered 100 through 193 correspond to flow lines or conduits which contain predomiinaatiy methane streams. Items nhrnberediSOO through 299 correspond to dory lines or conduits which contain predominantly ethylene streams, items numbered 300 thrcaigh 393 correspond to flow lines or conduits whieh contain predoniinantly propane stream

Referring to FIG. 1, during normal operation of the LNG taciity, gaseous propane is compressed hi a smifiistage (preferably three-stage) compressor 18 driven by a gas turbine driver (not illustrated). The three stages of concession preferably exist in a single 'js.it although each stage of compression may be a separate upit and the units mechanically coupled to be· driven by a single driver. Upon compression, the compressed propane is passed through conduit 300 to a cooler 20 where it is cooled and liquefied. A representative pressuM and temperature of thedigaeiied propane refrigerant prior to flashing is about 37,7G°C (100aP) and about 1.30 MPa (190 psia). The stream front cooler 20 is passed through conduit 302 to a pressure reduction means, illustrated as expansion valve 12, -wherein the pressure of the liquefied propane is reduced, thereby evaporating or flashing a portion thereof The resultsig through conduit 304 into a highrgtage propane eMler 2 wherein gaseous methane retHgerant 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, respectively produced via conduits 154......102, end 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 97, A portion of the stream cooled in heat exchange means $7 is removed from methane economizer 74 via conduit 155 end subsequently used, alter further cooling, as a reflux stream in a heavies removal column 60, as discussed la greater detail below with reference to FIG. 2.. The portion of the cooled stream from heat exchange means 97 that is not removed for use as: a reflux stream is farther cooled in indirect heat exchange means 98. The resulting cooled methane recycle -streamproduced via conduit 158 is then combined in conduit 120 with the heavies depleted (is,, %htdiydrocsrboxi dob.) vapor stream from heavies removal column 60 and fed to an ethylene condenser 68.

Th© propane gas from chiller 2 is returned: to compressor 18 through conduit. 306. This gas is fed to the high-stage Met port of compressor 18, The remaining liquid propane is passed through conduit 308. the pressure further reduced by passage through a pressure reduction means, illustrated as expansion valve 14, whereupon an additional portion of the liquefied propane is flashed. The resulting two-phase strearn is then fed to ^-tatenoe^le^ge.^c^ane -chiller 22 through conduit 310, thereby pro viding a coolant for chiller 22. The cooled feed gas stream from chiller 2 flows via conduit 102 to a knock-out vessel 10 wherein gas and liquid phases are separated. The liquid phase, which, is ridi 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 ted to propane· chiller 22. Tire stream in conduit 108 is employed as a stripping gas in refluxed heavies remo val column 60 to aid in the removal of heavy hydrocarbon components from the processed natural gas stream, a# discussed. in more detail below with reference to FIG. 2. Bthylene refrigerant from chiller 2 is introduced to chiller 22 via conduit :2-04, la chiller 22, the feed gas stream, also referred to herein as a mstbane-aeh stream, and the ethylene refrigerant streams m> respectively cooled via indirect heat transfer means 24 and 26, thereby producing cooled rneihaGs-vich and: ethylene refrigerant streams via conduits 110 and 206. The thus evaporated portion of ; the propane refrigerant is separated and passed through conduit 313 to the intermediate* stage mist of compressor 18. Liquid propane- refrigerant from chiller 22 is removed via conduit 314, flashed across a pressure-reduction means,: illustrated as expansion valve 16, and then fed to a low-stage propane chiilsr/condenssr 2S via conduit 316,

As illustrated in HO. 1, the methane-tic^ stei&amp;ft IdWftsm knermadiate-si age propane chiller 22 to the low-stage propane cMHer/condenser 28 via conduit 110. In chiller 28, the stream is cooled via indirect heat exchange means "0. In alike manner, the ethylene refrigsrant stream flows from the intermediate-stage propane caiHer 22 to low-stage propane chiller/oondeussr 28 via conduit 206. In the latter, the ethylene refrigerant is totally condensed or condensed in nearly its entirety via hxdirect heat exchange means 32-, The vaporised propane is removed from low-stage propane chifle-r/condenser 28 and returned to the iov,'-stage inlet of compressor 18 via conduit 320.

As illustrated in PIG. 1, the methane-rich stream exiting low-stage propane duller 28 is introduced to highrStage ethylene driller42 via conduit 112, Bthylene refrigerant exits low-stage propane 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 -31°C (about *2-4“F] and a pressure of about 1 -,,:9.6 MPa (about 281 psia). The ethylene refrigerant then flows tons ethylene economizer 34 viieaieai j|;js mesas 3:8, remold via conduir2 U, and passed to a pressure reduction means, illustrated as as expansion valve 4Θ, whereupon the reft igeraat is flashed to a preselected temperature and pressure and fed: tg: hlgh-stage ethylene chiller 42 via conduit 212, Vapor is removed from cMlier 42 via conduit 214 find routed to ethylene economizer 3 4 wherein fee vapor functions as a coolant via iadfreetbsai exchange means 46. The ethylene vapor is then removed from ethylene economizer 34 via conduit 2'. 6 and feed to fee high-stage inlet of ethylene compressor 48. The ethylene refrigerant which is not vaporized in high-stage ethylene chiller 42 is removed via conduit 218 ar id 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-^hase product is introduced into a low-stage ethylene chiller 54 via conduit 222.

After cooling in indirect heat exchange means 44, the methane-rich : stream is removed from high-stage ethylene efefkft 42 via conduit 116. The strsamm conduit 116 is then carried to a feed inlet of heavies removal column GO wherein heavy hydrocarbon components are removed from the methans-fieh stream, as described in further detail below with reference to FIG. 2. A hsaviea-rich liquid stream containing a significant concentration of C4-*· hydrocarbons, such as benzene, toluene, xylene, cyclohexane, other aromatics, and/or heavier hydrocarbon components, is removed from the bottom of heaves removal column 60 via conduit 114. The heavies-rieli stream in conduit 1|4 is subsequently separated into liquid and vapor portions or preferably is Hashed or fractionated in vessel 67. In either cane, a second heavies -rich liquid ftreamis produced via conduit 123 and a second mefeane-rfeh vapor stream is produced via conduit 12.1. In the preferred embodiment, which is illustrated in FIG.: i, 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 msfeans compressor 83. High-stage ethylene chiller 42 alsabnebdes an indisc| heat extbsnger means 43: 'which receives ami cools the stream wifedraro from methane economiser 24 via conduit 155,: as dismissed above. The resulting cooled stream from indirect heat exchanger means 43 is conducted via conduit. 157 to low-stage sthylsae chiller 54. In low-stage ethylene chiller 54 the stream from conduit 157 is cooled via kdireet heat exchange means 5C

Alter cooling ϊβ indirect beat exchange means 56, tie stream exits low-stage ethylene chiller 54 and is carried. via conduit 159 to a reflux inlet of hsavlea removal column 60. where it is employed as &amp; reflux stream.:

As previously noted, the gas in conduit 154 is fed to main methane econonhrkr 74 Whefeia the stream is cooled m indirect heat exchange means 97, A portion of the cooled stream from hsat exchange means 97 is then further cooled in indirect heat exchange means 98, The resulting cooled stream is removed from methane economiser 74 via, conduit 158 and is thereafter combined with the heaviss-depieted vapor stream, exiting the top of heavies removal column 60, dskvered vda conduit :5,1 !Sr and 120, and fed to a low-stage eihyiens condenser 68, hi low-stage ethylene condenser 68, this stream. is enoledaud CDadensed! via rndkect heat exchange means 70 With die liquid efrluedt 54 which. Is; routed to low-stags ethylene condenser 68 via ccnduii. 226. The condensed, mcth&amp;ns-rich product from low-stage condenser 68 is produced via conduit 122. The vapor from low-stage sthyfene chiller 1% withdrawn via conduit 224, and low-stage efhybse condenser 61, withdrawn via conduit 228, are combined mdronted, via conduit 230, to ethylene economiser 34 wherein the vapors iimefiol as a coolant via indirect best exchange means 58. The stream is then routed via conduit 232hum ethylene ectnomker 34 to the low-stage inlet of ethylene compressor 48,

As noted in FIG·, f, the compressor effeMsfram:vapor introduced wa the low-stag© side of ethylene compressor 48 is removed via conduit 234? copied via inter-stage cooler 71, and returned to compressor 48 via conduit 236 for injection with tie high-stage strewn present in conduit 214 Preferably, the two-stages are a single module although they may each be a separate!inodule and the modules mechanically coupled to a eamraoa driver. The compressed eth^eneproiast hem compressor48 is routedto a dawrisifekB Cpekr 72 vdft conduit 2Θ0. Tto produst from cooler 72 flows via conduit 202 and is introduced^ as pwvidnsly discassed, to Mgh-stags propane chiller 2.

The pressurised LNG-beathig stream, preferably a liquid stream in its entirety, in conduit 122 is preferably at a temperature in the range of from about -'!28':C to about -45°G (about -20© to about -50TF), more preferably in the range of from about -! 15°C to about ~73.3°C (about -175 to about -l00eF)',..mo^.^^f^^M:the'iai^e:of from -10i°C 10 -87.2e€ (-1 SO to -125*F). The pressure of fee stream in conduit 122 is preferably in the rang© of from about 3.44 MPa to about 4,82 MPa (about 500 to about, 700 psia), most qfre&amp;abiy in the range of from 3,79 MPa to 4,99 MPa (550 to 725 psia). The stream in conduit 122 is directed to main methane economizer 74 wherein tbe stream is ftriher cooled by indirect heat exchange means-lieat exchanger pass 76 as kefeinafter explained. It is preferred for mala methane economiser 74 to inciude a pMsiity of heat exchanger passes which, pro vide for the indirect exchange of heat between various predominantly methane streams in the economizer 74. Preferably, methane economiser 74 comprises one or more, plate-fin heat exchangers. The cooled stream from beat exchanger pass 75 exits methane economizer 74 via conduit 124. It is preferred fox the temperature of the stream in conduit 124 to be at least about 104F 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 it· the range of from about -129°C to about -H)70C (about •200 to about. ~150Τ). The pressure of the stream to. 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 th ereby generating a two-phase stream. The two-phase stream from expansion vah^e 78 is then passed to high-stag© methane flash drum 80 where ids separated, into a flash gas stream discharged through eohdui 126 and a liquid phase stream (ig,, pressmised Lhl'Q-bearing stream) discharged through, conduit 130, The flash gas stream. is then transferred to raammsthane economiser 74 via conduit 126 wherein the stream Suctions sis a coolant in heat exchanger pass 82. The j^edcMnaitly methane stream is warmed in heat exchanger pass 82, at least ir. part, by indirect heat exchange with the predominantly methane stream in heat exchanger pass 76 The warmed sfrepa exits heat exchanger pass 82 and methane economizer 74 via conduit 128.

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 vis htdiiect heat exchange means 88. The pooled 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 ilaah drain 92 where the stream is separated into a gas phase passing through conduit 136 sad a liquid phase passing through conduit 134. Toe gas phase flows through conduit 135 to second methane economiser §7 wherein the vapor cools the liquid introduced to economizer 87 via, conduit 130 via indirect heat, exchanger means 89. Conduit 158 serves as a Sow conduit between indirect heat exchange means δ9 in second methane economksr 87 and heat exchanger pass 95 in main methane economizer 74. The warmed vapor stream horn heat exchanger pass 95 exxs mam. methane economizer 74 via. conduit 140, is- combined with the iirst nitrogen-reduced stream in conduit 406, and the combined stream is coadueted to tte Msririediatevstage inlet of methane compressor 83 . 'Phc liquid phase exiting intermediate-stage flash dram 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 Anal or low-stage flash dram 94, In flash drum 94, a vapor phase is separated and passed through conduit 144 to second methane econontizer 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 cconorafeer 74 wherein the vapor functions as a coolant vis heat exchanger pass 96. The Warmed vapor stream from heat exchanger pass 96 exits main methaac economizer 74 via conduit 148, is eofcabiaed; wfflL -the· second jritrogen^reduced:SteeTO:hi conduit 408, and the combined stream is conducted to the low-s tage inlet of compressor 83.

The liquefied natural gas product flora low-stage flash drum 94, which is at approximately atmospheric^pleasure, 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 cm then he vaporized at an onshore LNG terminal ibr transport in the gaseous state via conventional natural gas pipelines.

As shown in FIG. 1, the high, intermediate, and low stages of compressor 83 are preferaMy combined as single unit. However, each stage may exist a? a. separate amt 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 gag in conduit 140 prior to the second-stage of compression. The compressed gas from the intermediate stage of compressor S3 is passed through an hter-stsge eoo5er 84 and is connbked with the high pressure gas provided via conduits 121 and 128 pnor to the third-stage of compression, The compressed gas (i.s., compressed open methane cycle gas stream) is discharged fiO.ro high stage methane compressor through, conduit 150, is cooled in cooler 86, nod is routed to: the high pressure propane chiller 2 vis. conduit 152 as previously discussed,

Tbs stream is cooled in ebilter 2 via indirect heat exchange means 4 and flows to main methane economiser 74 via conduit 1 54. The compressed open methane· cycle gas strsaamfirem chilisr 2 Which eaters ths mainmetbaae economizer 74 undergoes cooling in its entirety via flow throughindirect heat exchange means 98. This cooled stream is then removed via conduit 158 and combined with the processed natural gss feed stream upstream of the first stage of ethylene cooling,

Referring now to FIS. 2, refluxed heavies column 60 generally in chicles an upper zone 61, a middle zone 62, and a lower zone 65, Upper zone- 61 receives Hie reflux stream in conduit 159 via a reflux inlet 66, piddle zone 62 receives the processed natural gas scream, irr conduit 118 via a feed inlet 69. Lower zone 65 receives the stripping gas stream in conduit 108 via a stripping gas Met 73. Upper zone 61 and middle zone 62 are separated by upper internal packing 75, while· middle· zone 62 and lower zone 65 are separated by lower internal packing 77. Internal packing 75,77 can be any conventional structure Mown in the art for eshmeing contact between two ccantercurreut streams ir. a vessel,: Refluxed heavies removal column 60 also includes an upper outlet 79 and a lower outlet 81.

In accordance with the preseat invention, heavies removal eoteran 60 can be operated in three distinct modes: an initiating mods, a start-up mode, and a normal mode. The taiflaung mode involves initiating the flow of a hydrocarbon-containing stream into heavfef removal edlumn 60 wa feed inlet 69, itrpiedistely prior to the initiating mode, substantially no hydreoaxboa-oontasung streams flow into or through heavies removal1 column 60. During the initiating mode, substaniallv no hydrocarbon-CGMainmg stream® ari:ih^p#eed:;ih^^ea^te^vd eohiaaa 60 through reflux inlet 66 and stripping gas inlet 73.

The start-up mode of operation involves continuing the flow of the hydrOcarbonm-ont arning stream (e. g.t pmWbsssi astral: gas streair| into: heavies removal cclumo 60 via feed islet 69, 'During ike start-up mode, the stream entering column 60 via feed inlet 69 is separated into a light vapor stream, which exits eokrnn 60 via upper outlet 79, and &amp; heavy liquid stream, which exits column 60 via lower cutlet 81. During the start-up mode, at least a portion of tbs light vapor stream exiting upper outlet 79 via conduit 119 is routed back to heavies removal column 6Q sad introdueed into upper done 61 of heavies removal column 60 via reflux inlet 66, Referring now to FIG. 2, during start-up, the routing pf the light vapor stream in. conduit 119 back to reflux inlet 66 of heavies removal column 60 takes place by initially routing the sir. earn to the open-methane rel%eraion cycle via conduit 120, heat exchange means 70, and conduit 122. The stream exits tbs opcoi-inethaue cycle aid is fed to methane- compressor 83. From methane compressor 83 the stream fe thenroutedhack to heavies removal column. 60 via the following conduits and components: conduit 150, cooler 86, conduit 152, heat exchange means 4, conduit 154, hoar, exchange means 92, conduit 155, boat exchange means 43. conduit 157, heat exchange means 56, and conduit 159. Referring to FIGS. 1 and 2, during the start-up mode, at least a portion of the heavy hqnid stosam exiting lower outlet 81 of heavies removal column 60 via conduit 114 is routed back to reflux inlet 66 of heavies removal, column via the folio wing conduits and components: vessel 67, conduit 121, conduit 128, methane compressor 8|, conduit 150, cooler 86, conduit 152. heat exchange means 4, conduit 154, heat exchange means 97, conduit 155, heat exchange means 43, conduit 157, heat exchange means 56, and conduit 159.

Referring again to FIG, 2, during the normal mode of operation, die feed stream enters middle zone 62 of heavies removal column 60 via feed inlet 69, the reflux stream enters upper acne 61 of heavies removal columu 60 via reflux inlet 66, and the stripping gas stream, enters lower zone 65 of heavies removal column 60 via stripping gas Met 73. During the normal, mods, the downwardly Sowing liquid reflux stream is contacted in upper internal packing 75 with the upwardly flowing vapor portion of the feed stream, while the downwardly flowing liquid portion of the feed stream is i contacted in lower interr·at packing 27 with the upward flowing stripping gas. In this manner, heavies removal column 6S is operable to produce a heavfeg-deplsied (i.e,, lights-rieli) streamvfeupper outlet 79 add abssries-rich Stream via lower outlet 81 during the normal mode. During tire normal mods, the feed introduced into heavies removal column 60 ;via feed inlet 69 typically has a Cs+ concentration of at least 0.1 mole percent; a C* epicestttratioa of at least 2 mole percent, a benzene concentration of at least 4 ppmw (parts per million by weight), a cyclohexane concentration of at least 4 ppmw, and/or a combined ePnceatiMon ofxpend and toluene of at feast; ID ppmw.

When operating during the normal mode, the lieavies-dspieted stream exiting heavies remo val column 60 via upper outlet 79 preferably has a lower concentration of €,.+ hydrocarbon components than the feed entering inlet 69, more preferably the heavies-depleted stream exiting upper outlet 79 has a C.;.+ concentratiori of loss than 0.1 mole percent, a C4 conoentration of less than 2 mole percent, &amp; bonze©» concentration ofless than 4 gprnvv, a cyclohexane concentration ofless than 4 ppmw; and a cpnibinsd concentration of xylene anddofeene of fess than 10 ppm®·. When operating during the normal mode, the keaviec-rich stream exiting heavies removal eolunm 60 via lower outlet 8 i preferably has a higher concentration of €+· hydrocarbons than the feed ehtefihg feed inlet 69. During fee normal mode, it Is preferred for the stripping gas entering; heavies removal column 60 via stepping gas inlet 56 to comprise a higher proportion of light hydrocarbons than the feed to feed miet 69 of heavies reinpvar column 60. More preferably the reflux stream entering reflux inlet 66 of heavies removal column 60 du#g the nonaM mode comprises at least about 90 mole percent methane, sdll more preferably at least: about ,95 ruble percent methane, and most· preferably at least 97 mole percent methane. When operating during the normal mode, it is preferred for the stripping gas entering heavies removal column. 60 tda stripping gas inlet; 73 to have substantially the same composition as the feed stream entering heavies removal column 60 via feed inlet 69. deferring to PIGS. 1 and 2, when the LNG facility illustrated in. FIG. 1 is staried upj the flow of the natural gas stream is initiated in conduit 100. The natural gas stream is then sequentially copied via indirect heat transfer in heat exchange means 6,24,30, and 44. In accordance with one embodiment cf the present invention, the propane and ethylene ituxigeratioa cycles are controlled during stari-up in a manner so that the. cooled natural gas stream exiting heat exchange means 44 of high-stags ethylene chiller 42 and entering feed inlet 69 of heayies removal column 60 is a two-phase stream, Pfeferablpj the bvo-phase stream entering feed inlet 69 of heavies removal column 60 during start-up includes a vapor phase that contains predominantly light hydroc;rrbct5. components and a liquid phase that contains predominantly heavy hydrocarbon, components.

As used herein, Ike tea "vapor/iiopidhydracarbon separation point" or simply "hydrocarbon separation point'' shall be used to identity a point of separation bgweeathe vapor and liquid phases of a hyfeOcarbon-cnntamijig stteambssedonthe number of carbon atoms In the hydrocarbon molecules of the phases. When the hydrocarbon separation poat is representsdby tas fcrnniteT^M)» then a predonanant molar portion of Cx- hydrocarbon molecules are present in the vapor phase while a preefonmnnt molar portion of Ci<x+1)4· hydrocarbon molecules are present in the Hqnki phase. For example, if the hydrocarbon separation point of a certain two-phase hydrocarbon-containing stream is Cin, then a predominant portion (ie,, more than SO mole- percent) of the Cy- hydrocarbons are present in the liquid phase while, a predominant meter portion of tbs C^- hydrocarbons are present hi the vapor phase. In other words, if the hydrocarbon separation point is Cm the vapor phase would contain more than SO mole percent of the C* hydrocarbons present in the two-phase stream, more than 50 mole percent of the C5 hydrocarbons present in the two-phase stream, mors than 50 mole percent ofiths Cs hydrocarbons': present in the two-phase stream, and more than 50 mole percent of the Ct hydrocarbons present in the two-phase stream, while the liquid phase would contain more than 50 mole percent of the CJ3 05, C7.. C? etc. hydrocarbons present in the two-phase stream,

The stream entering feed inlet 69 of heavies removal column 60 daring the start-up mode preFerabiy bas a hydincarhon scpmtion point which cab be represented as follows: CX:.;XM7, wherein X is an integer in the range of horn 2 to 10. More preferably, X is in the range of horn 2 to 6. still more preferably in the range of Shorn 3 to 5, and most preferably X is 4. When the feed to inlet 69 of heavies removal column 6© has the above-deseribed hydfboarhon separation pohit, it is ensured that a significant portion of the light hydrocarbomeontainihg vapor phase exits upper outlet W and a significant portion of the heavy hydrocarbon-containing liquid phase exits lower outlet 81 during start-up, The hydrocarbon separation point of the two-phase stream entering feed inlet 69 of heavies removal column 60 is controlled by controlling its temperature. As the temperature of the feed stream increases, the value ofX increases. Conversely, as the temperature of the feed stream decreases, the value of X decreases.

Preferably, the temperature of the stream catering feed inlet 65 of heavy removal column 60 during start-up is in the range of from about -73,3X to about ~623C (about -100: to about. -80°F), -?3.3°€·*ο about -.68°© (dbout -100 to -90"?), mostprefsrabiy in the range of from -71.S°C to -69,1°G (-97.5 to -92.5’F) ©bring the normal mode of operatibii, the stream entering feed islet 69 of heavies removal column 60 preferably has a hydrocarbon separation. poise winch, das be represented as fellows; whereinY is as integer in the range off from 2 to 10.

Mors prefer ably, Y1¾ in the range of from 4 to 8, still more preferably in the range of from 5 to 7, and most preferably Y is 6. Preferably. Y is at least 1 .greater than. X. Most preferably, Y is 2 greater than X. When the feed to islet 69 of heavies remora! column 60 has the above-described hydrocarbon separation point, optimal heavies removal can be achieved during the normal mode.

In order to switch from the start-up operational mode to the normal operational mods, the hydrocarbon separation point of the· teed to heavies removal column 6(J is increased. As memioned aboveiitheihydtoiparbos separation point of the. stream entering feed inlet 6.9 of heavies removal column 60 is controlled by controlling its temperature. Thus, is order to switch from the start-up mode to the normal mode, the temperature of tire feed catering heavies removal column 60 via feed Met 69 is increased. A preferred way of controlling the temperature of the feed ssterihg heavies removal column 60 via feed inlet 69 is to control the speed of ethylene compressor 48 Ethylene compressor 48 is preferably a multi-stage axial or centrifugal compressor, wherein the pressure differential between the islet and outlet of the compressor can be increased by iacroaaing the speed of the compressor and decreased by decreasing the speed of the compressor. It ia preferred for the speed (and pressure differential) of ethylene compressor 48 to be greater during;the start-up mode than during the normal mode. Thus· provides for mors chilling of the processed natural gas stream in indirect heat exchange means 44 of high-stage ethylene chiller 42 during start-up than during normal operation. Thus, the temperature of the feed entering heavies removed column 60 via conduit 116 is lower during start-up than during norms! operation. In order to shift from the si art-up racds to fee normal mode, it is preferred tor the speed of ethylene compressor 48 to be reduced, thereby changing the temperature and hydrocarbon aep ar a lion point of the· fesd to heavies retrieval column 60 as described herein. '

Preferably, the temperature of the iced entering heavies removal column 60 via feed inlet 69 during the normal mode is at feast about 2°F warmer than the feed entering heavies removal column. <50 yia feed inlet: 69 during the start-up mode, more preferably at least 4'!F warmer and most preferably id the range of ixom 4 to 12SE wamisr. lreferal%,:::tSe oftlie stream entering fesd inlet 69 ofheavies removal: column oG during the normal mode is in the mage of from about -73.38C to about -59°C (about -100 to about -75¾. ipore preferably in. the fangs of from shout -70*C to about -62,2°C (about -95 to about -803F). moat preferably In the range of from -69,2 to -65¾ (-92.5 to -85°F).

During the norma! operational mode, if. is preferred for the temperature of the reflux, stream entering heavies removal column 60 via reflux inlet 65 to be cooler than the temperature of the feed stream entering heavies removal column 60 via feed inlet 69, more preferably at least about 5°F cooler, still more preferably at least about Lvf cooler, and most preferably at least 35’F cooler. Preferably, the temperature of the reflux stream entering reflux inlet 66 of heavies removal column 60 during the normal mode is in. the range of fi'crn about-197 to about -73.3¾ (about -160 to about 400¾ more preferably &amp; the range of from about -98,3¾ to about -84.41¾ (about -143 to about -1200F), most preferably in the range of froth-94.4eC to -87.2¾ (-138 to ~125°F). During the normal operational mode, it is preferred.for thetempefature of tide gripping gas stream entering heavies removal column 60 via stripping gas inlet 73 to be warmer than thetempgrature of the feed stream entering heavies removal column 60 via feed inlet 69, more:preferably at |sa|t about 5aF warmer, still more preferably at least about 20"F v/armer, and most preferably at least 40°F warmer. Preferably, the temperature of the stripping gas stream entering stripping gas inlet 66 of heavies removal column 60 during the normal mode is in the range of from· about -59¾ to abotrt-IB5# (about;-7f to about -0¾ more preferably in the range of from about -51°C to about -26®C (about -60 to about -151¾ m3st;pmferablyia:t|s:rg®ge of feomL-40 to:~16.6°d (-40 to -30SP),

The above-described methodology allows a LNG facility employing a refluxed, heavies removal column, to be started up faster than conventional methods because, during startup, a sigsifeeanfly greater amount of the separated natural gas stream exiting the heavies removal: can be used to help start-up dovmTfream equipment (β||.> tbeOpenmefosnis cooling cycle), Eadditisa, the present invention a!s$ allows the LNG facility to be started up more rapidly because an adequate reflux stream to the heavies removal cohuna is established much more rapidly than. under conventional methods. la ode embodiment of the present invention, the LNG production systems illustrated in. FIGS. 1 and 2 are simulated on a computer rising conventional process simulation software; Examples ofisuftablSsSinmladOn softwmieiinelnde HYSYS™ from Syprefreeh, Aspen: Plus® Aspen Tedinols|y, toe.;, and ί?Κ£)/Ιί® from Simulation Sciences Inc.

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

The inventors hereby state their intent to rely on the Doctrine of Equivalents to d etermine and assess: the reasonably fair scope of the pr<^snt:i|ivemion ascertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.

Claims (7)

1. THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

   1. A method of starting up a cascade-type liquefied natural gas facility employing a refluxed heavies removal column between two refrigeration cycles of the facility, said method including the steps of: (a) operating the refluxed heavies removal column in an initiating mode, said initiating mode including initiating the flow of a natural gas stream through a feed inlet of the refluxed heavies removal column and into the refluxed heavies column, said refluxed heavies removal column including a reflux inlet spaced from the food inlet, said reflux inlet having substantially no hydrocarbon-containing fluids flowing therethrough and into the refluxed heavies removal column during operation in the initiating mode; (b) subsequent to step (a), operating the refluxed heavies removal column in a start-up mode, said start-up mode including using the refluxed heavies removal column to separate the natural gas stream into a first heavies stream and a first lights stream, said start-up mode including discharging the first lights stream from the refluxed heavies removal column, said start-up mode including routing at least a portion of the discharged first lights stream to the reflux inlet; and (c) subsequent to step (b), operating the refluxed heavies removal column in a normal mode said normal mode including using the refluxed heavies removal column to separate the natural gas stream into a second heavies stream and a second lights stream, said normal mode including discharging the second lights stream from the refluxed heavies removal column, said normal mode including routing at least a portion of the discharged second lights stream to the reflux inlet.
2. 2. The start-up method of claim 1, including any one or combination of the following wherein said natural gas stream enters the refluxed heavies removal column at a first inlet temperature during the start-up mode, said natural gas stream enters the refluxed heavies removal column at a second inlet temperature during the normal mode, said second inlet temperature is at least 2°F greater than the first inlet temperature; said second inlet temperature is in the range of from about 4 to about 12°F greater than the first inlet temperature; said natural gas stream has a first vapor/liquid hydrocarbon separation point Οφ+η during the start-up mode, said natural gas stream has a second vapor/liquid hydrocarbon separation point Cy(y+i) during the normal mode, wherein X and Y are integers representing the number of carbon atoms in the hydrocarbon molecules of the respective natural gas stream, wherein X and Y are in the range of from 2 to 10, wherein X is at least 1 greater than Y; wherein X is in the range of from 3 to 5 and Y is in the range of from 5 to 7; wherein Y is at least 2 greater than X; wherein X is 4 and Y is 6; and/or said refluxed heavies removal column includes a stripping gas inlet, said refluxed inlet is spaced from and located above the feed and stripping gas inlets, said feed inlet is spaced from and located above the stripping gas inlet; said heavies removal column includes first and second sets of internal packing, said first set of internal packing is vertically disposed between the feed inlet and the stripping gas inlet, said second set of internal packing is vertically disposed between the feed inlet and the reflux inlet,
3. 3. The start-up method of any one of claims 1 or 2, further including any one or combination of the following: (d) upstream of the refluxed heavies removal column, cooling the natural gas stream in a first refrigeration cycle employing a first compressor to compress a first refrigerant; (e) switching from the start-up mode to the normal mode by adjusting the differential pressure of the first refrigerant across the first compressor; (f) downstream of the refluxed heavies removal column, cooling the natural gas stream in a second refrigeration cycle employing a second refrigerant including predominantly methane; (g) upstream of the first refrigeration cycle, cooling the natural gas stream in a third refrigeration cycle employing a third refrigerant comprising predominantly propane or propylene, said first refrigerant comprising predominantly ethane or ethylene; (h) vaporizing liquefied natural gas produced by the liquefied natural gas facility during the normal mode.
4. 4. The start-up method of claim 3, wherein step (e) includes decreasing the differential pressure of the first refrigerant across the first compressor.
5. 5. The start-up method of claim 3 or 4, wherein said first refrigerant Includes predominantly propane, propylene, ethane, ethylene, or carbon dioxide.
6. 6, A computer simulation process including the step of using a computer to simulate the method of any one of claims 1 to 5.
7. 7. A liquefied natural gas product produced by the process of any one of claims 1 to 5.