OA11014A - Aromatics and/or heavies removal from a methane-based feed by condensation and stripping - Google Patents

Abstract

A method and associated apparatus for removal of benzene, other aromatics and/or other heavier hydrocarbon components from a methane-based gas stream by condensation and stripping. It is desirable to remove benzene and other aromatics to prevent fouling and plugging of processing equipment and it is desirable to recover other heavier hydrocarbon components because of their value. Cooled feed stream (118) is fed to a column (60) and separated into methane-rich vapor stream (120) and benzene/aromatics/heavies liquid (114). The liquid (114) is sent to a heat exchanger (62) to recover refrigeration. Warm dry gas (108) is cooled in the heat exchanger (62) and delivered as stripping gas (109) to the column (60).

Description

011014

AROMATICS AND/OR HEAVIES REMOVAL FROM AMETHANE-BASED FEED BY CONDENSATION AND STRIPPING

This invention concems a method and associated apparatus for removing benzene,other aromatics and/or heavier hydrocarbon components from a methane-based gas stream by aunique condensation and stripping process.

BACKGROUND 5 Cryogénie liquéfaction of normally gaseous materials is utilized for the purposes of component séparation, purification, storage and for the transportation of said components in amore économie and convenient form. Most such liquéfaction Systems hâve many operations incommon, regardless of the gases involved, and consequently, hâve many of the same problems.One problem commonly encountered in liquéfaction processes, particularly when aromatics are 10 présent, is the précipitation and subséquent solidification of these species in the processequipment thereby resulting in reduced process efficiency and reliability. Another commonproblem is the removal of small quantifies of the higher valued, higher molecular weightChemical species from the gas stream immediately prior to liquéfaction of the gas stream in amajor portion. Accordingly, the présent invention will be described with spécifie reference to the 15 processing of naturel gas but is applicable to the processing of gas in other Systems whereinsimilar problems are encountered. 2 011014

It is common practice in the art of processing natural gas to subject the gas to cryogénie treatment to separate hydrocarbons having a molecular weight higher than methane (C2+) from the natural gas thereby producing a pipeline gas predominating in methane and a C2+ stream useful for other purposes. Frequently, the C2+ stream will be separated into individual 5 component streams, for example, C2, C3, C4 and C5+.

It is also common practice to ciyogenically treat natural gas to liquefy the samefor transport and storage. The primary reason for the liquéfaction of natural gas is thatliquéfaction results in a volume réduction of about 1/600, thereby making it possible to store andtransport the liquefied gas in containers of more economical and practical design. For example, 10 when gas is transported by pipeline from the source of supply to a distant market, it is désirableto operate the pipeline under a substantially constant and high load factor. Often thedeliverability or capacity of the pipeline will exceed demand while at other times the demandmay exceed the deliverability of the pipeline. In order to shave off the peaks where demandexceeds supply, it is désirable to store the excess gas in such a manner that it can be delivered 15 when the supply exceeds demand, thereby enabling future peaks in demand to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied Statefor storage and to then vaporize the liquid as demand requires.

Liquéfaction of natural gas is of even greater importance in making possible thetransport of gas from a supply source to market when the source and market are separated by 20 great distances and a pipeline is not available or is not practical. This is particularly true wheretransport must be made by ocean-going vessels. Ship transportation in the gaseous State isgenerally not practical because appréciable pressurization is required to significantly reduce thespécifie volume of the gas which in tum requires the use of more expensive storage containers. 3 011014

In order to store and transport natural gas in the liquid State, the natural gas ispreferably cooled to -240°F to -260°F where it possesses a near-atmospheric vapor pressure.Numerous Systems exist in the prior art for the liquéfaction of natural gas or the like in which thegas is liquefied by sequentially passing the gas at an elevated pressure through a plurality ofcooling stages whereupon the gas is cooled to successively lower températures until theliquéfaction température is reached. Cooling is generally accomplished by heat exchange withone or more réfrigérants such as propane, propylene, ethane, ethylene, and methane or acombination of one or more of the preceding. In the art, the réfrigérants are frequently arrangedin a cascaded manner and each réfrigérant is employed in a closed réfrigération cycle. Furthercooling of the liquid is possible by expanding the liquefied natural gas to atmospheric pressure inone or more expansion stages. In each stage, the liquefied gas is flashed to a lower pressurethereby producing a two-phase gas-liquid mixture at a significantly lower température. Theliquid is recovered and may again be flashed. In this manner, the liquefied gas is further cooledto a storage or transport température suitable for liquefied gas storage at near-atmosphericpressure. In this expansion to near-atmospheric pressure, some additional volumes of liquefiedgas are flashed. The flashed vapors from the expansion stages are generally collected andrecycled for liquéfaction or utilized as fuel gas for power génération.

As prevïously noted, a major operational problem in the liquéfaction of naturalgas is the removal of resîdual amounts ofbenzene and other aromatic compounds from thenatural gas stream immediately prior to the liquéfaction of a major portion of said stream and thetendency of such components to precipitate and solidify thereby causing the fouling and potentialplugging of pipes and key process equipment. As an example, such fouling can significantly 4 011014 reduce the heat transfer efficiency and throughput of heat exchangers, particularly plate-fin heatexchangers.

For technical and économie reasons it is not necessary to remove impurities suchas benzene completely. It is, however, désirable to reduce its concentration. Contaminantremoval from naturel gas may be accomplished by the same type of cooling used in theliquéfaction process wherein the contaminants condense in accordance with their respectivecondensation température. Except for the fact that the gas must be cooled to a lower températureto liquefy, as opposed to separating the benzene contaminant, the basic cooling techniques arethe same for liquéfaction and séparation. Accordingly, in respect of residual benzene, it is onlynecessary to cool the natural gas to a température at which a portion of the feed gas is condensed.This may be accomplished in a cryogénie séparation column included at an appropriate point inthe LNG recovery process to separate the condensed benzene from the main gas stream.

In the interest of efficient operation of the cryogénie séparation column, it isdésirable to utilize the condensed liquid at cryogénie températures, that must be withdrawn fromthe column, for heat exchange with a warm dry gas stream provided to the cryogénie séparationcolumn. This heat exchange scheme, however, présents a problem resulting from the excessivetempérature differential of the two streams supplied to the heat exchanger. Since the actualtempérature différence could exceed 100 “F, the thermal shock to the heat exchanger coulddamage or shorten useful life of the heat exchanger apparatus constructed of conventional materials.

Another considération related to efficient operation of a cryogénie séparationcolumn is providing heat exchanger Controls that allow automatîc start-up of the column. 5 011014

Still yet another problem in the processing of methane-rich gas streams is the lackof a cost-effective means for recovering the higher molecular weight hydrocarbons from the gasstream prior to liquéfaction of the stream in major portion or retuming the remainipg stream to apipeline or other processing step. The recovered higher molecular weight hydrocarbonsgenerally possess a greater value on a per unit mass basis than the remaining components in the gas stream.

SUMMARY OF THE INVENTION

It is an object of this invention to remove residual quantities of benzene and otheraromatics from a methane-based gas stream which is to be liquefied in major portion.

It is another object of this invention to remove the higher molecular weighthydrocarbons from a methane-based gas stream.

It is still yet another object of this invention to remove the higher molecularweight hydrocarbons from a methane-based gas stream which is to be liquefied in a majorportion.

It is yet still further an object of this invention to remove benzene, other aromaticsand/or the higher molecular weight hydrocarbons from methane-based gas stream in an energy- efficient manner.

It is still further an object of the présent invention that the process employed forthe removal of benzene, other aromatics and/or higher molecular weight hydrocarbons becompatible with and integrate into technology routinely employed in gas plants.

And further yet still, it is an object of this invention that the process and apparatusemployed for benzene, other aromatic and/or high molecular weight hydrocarbon removal froma methane-based gas stream be relatively simple, compact and cost-effective. 6 • 011014

It still further yet is an object of the présent invention that the process employedfor the removal of benzene, other aromatics and/or higher molecular hydrocarbons from amethane-based gas stream to be liquefied in major portion be compatible with and jntegrate intotechnology routinely employed in plants producing liquefied natural gas. 5 Yet still further an object of this is invention is to provide heat exchanger Controls which overcome the above-mentioned and other associated problems in handling lowtempérature fluids.

Another object of this invention is to provide an improved control method whichreduces initial equipment température requirements, and costs for heat exchange apparatus. 10 A more spécifie object is to control heat exchanger températures to allow cooling of a warm fluid stream against a low température fluid stream without introducing thermal shockto the heat exchange apparatus. A still further object of this invention is to control the heat exchanger to facilitateautomatic start-up of a cryogénie séparation column. 15 In one embodiment of this invention, benzene and/or other aromatics are removed from a methane-based gas stream by a process comprising (1) condensing a minor portion of themethane-based gas stream immediately prior to the step wherein a majority of said gas stream isliquefied thereby producing a two-phase stream, (2) feeding said two-phase stream into the uppersection of a stripping column, (3) removing from the upper section of said stripping column an 20 aromatic-depleted gas stream, (4) removing from the lower section of said stripping column anaromatic-rich liquid stream, (5) contacting via indirect heat exchange the aromatic-rich liquidstream with a methane-rich stripping gas stream thereby producing a warmed aromatic-bearingstream and a cooled methane-rich stripping gas stream, and (6) feeding said cooled methane-rich 7 011014 stripping gas stream to the lower section of the stripping column, and optionally (7) feeding saidaromatic-depleted gas stream to a liquéfaction step wherein the gas stream is liquefied in majorportion thereby producing liquefied naturel gas. .

In another embodiment of this invention, the higher molecular weight 5 hydrocarbons in a methane-based gas stream are removed and concentrated by a processcomprising (1) condensing a minor portion of the methane-based gas stream to produce a two-phase stream, (2) feeding said two-phase stream into the upper section of a stripping column, (3)removing from the upper section of said stripping column a heavies-depleted gas stream, (4)removing the lower section of said stripping column a heavies-rich liquid stream, (5) contacting 10 via indirect heat exchange the heavies-rich liquid stream with a methane-rich stripping gas stream thereby producing a warmed heavies-rich stream and a cooled methane-rich stripping gasstream, and (6) feeding said methane-rich stripping gas stream to the lower section of thestripping column.

In still yet another embodiment of this invention, the invention is an apparatus 15 comprising (1) a condenser wherein a minor portion of a methane-based gas stream is condensedthereby producing a two-phase stream, (2) a stripping column to which the two-phase stream isfed and from which is produced a vapor stream and a liquid stream, (3) a heat exchangercontaining an indirect heat exchange means which provides for indirect heat exchange between agas stream and the liquid stream thereby producing a cooled gas stream and a warmed liquid 20 stream, (4) a conduit between said condenser and the upper section of the stripping column forflow of said two-phase stream, (5) a conduit connected to the upper section of the strippingcolumn for removal of said vapor stream, (6) a conduit between said stripping column and saidheat exchanger for flow of said liquid stream, (7) a conduit between said heat exchanger and said 8 011014 stripping column for flow of said cooled gas stream, (8) a conduit connected to said heatexchanger for the flow of a said warmed liquid stream from the heat exchanger, and (9) aconduit connected to said heat exchanger for flow of said gas stream to the heat exchanger.

In yet another embodiment of this invention, the foregoing and other objectives5 and advantages are achieved in controlling a heat exchanger handling a low température fluid and a warm fluid by providing a by-pass conduit for the warm fluid, wherein a control valve inthe by-pass conduit is manipulated responsive to the température ratio of the heat exchangefluids. In accordance with another aspect of the invention automatic start-up Controls include ahigh selector for temporarily selecting a température to manipulate flow of the warm fluid that 10 facilitâtes start-up of the column, and then switches to manipulation of the warm gas flowresponsive to a desired température.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a simplified flow diagram of a cryogénie LNG production processwhich illustrâtes the methodology and apparatus of the présent invention for the removal of 15 benzene, other aromatics and/or higher molecular weight hydrocarbon species from a methane-based gas stream. « FIGURE 2 is a simplified flow diagram which illustrâtes in greater detail themethodology and apparatus illustrated in FIGURE 1. FIGURE 3 is a diagrammatic illustration of a cryogénie séparation column and20 the associated control System of the présent invention for maintaining a desired température ratio for the heat exchange fluids. FIGURE 4 is a diagrammatic illustration similar to FIGURE 3 for temporarilyselecting a température that will allow automatic start-up of the cryogénie séparation column. 9 011014

DESCRIPTION OF THE PREFERREPEMBQD1MENTS

While the présent invention in the preferred embodiments is applicable to (1) theremoval of benzene and/or other aromatics from a methane-based gas stream which is.to becondensed in major portion and (2) the removal of the more valuable, higher molecular weighthydrocarbon species from a methane-based gas stream which is to be condensed in majorportion, the technology is also applicable to the generic recovery of such species from methane-based streams (e.g., removal of natural gas liquids from naturel gas). Benzene and otheraromatics présent a unique problem because of their relatively high melting point températures.As an example, benzene which contains 6 carbon atoms possesses a melting point of 5.5 °C and aboiling point of 80.1 °C. Hexane, which also contains 6 carbon atoms, possesses a melting pointof-95°C and a boiling point of 68.95°C. Therefore when compared to other hydrocarbons ofsimilar molecular weight, benzene and other aromatic compounds pose a much greater problemwith regard to fouling and/or plugging of process equipment and conduit Aromatic compoundsas used herein are those compounds characterized by the presence of at least one benzene ring.

As used herein, higher molecular hydrocarbon species are those hydrocarbon species possessing,molecular weight greater than ethane, and this terrn will be used interchangeably with heavyhydrocarbons.

For the purposes of simplicity and clarity, the following description will beconfmed to the employaient of the inventive processes and associated apparatus in the cryogéniecooling of a natural gas stream to produce liquefied natural gas. More specifically, the followingdescription will focus on the removal of benzene and/or other aromatic species and/or highermolecular weight hydrocarbons (heavy hydrocarbons) in a liquéfaction scheme wherein cascadedréfrigération cycles are employed. However, the applicability of the inventive processes and 10 011014 associated apparatus herein described is not limited to liquéfaction Systems which employcascaded réfrigération cycles or which process naturel gas streams exclusively. The processesand associated apparatus are applicable to any réfrigération System wherein (a) benzene and/orheavier aromatics exist in a methane-based gas stream at concentrations which may foui or plug 5 process equipment, particularly the heat exchangers employed for condensing said stream, or(b) it is désirable for whatever reason to remove and recover higher molecular weighthydrocarbons from a methane-based gas stream.

Naturel Gas Stream Liquéfaction

Cryogénie plants hâve a variety of forms; the most efficient and effective being a 10 cascade-type operation and this type in combination with expansion-type cooling. Also, sincemethods for the production of liquefied naturel gas (LNG) include the séparation ofhydrocarbons of molecular weight greater than methane as a first part thereof, a description of aplant for the cryogénie production of LNG effectively describes a similar plant for removing C2+ h hydrocarbons from a naturel gas stream. ,15 In the preferred embodiment which employs a cascaded réfrigérant System, the invention concems the sequential cooling of a naturel gas stream at an elevated pressure, forexample about 650 psia, by sequentially cooling the gas stream by passage through a multistagepropane cycle, a multistage ethane or ethylene cycle and either (a) a closed methane cyclefollowed by a single- or a multistage expansion cycle to further cool the same and reduce the 20 pressure to near-atmospheric or (b) an open-end methane cycle which utilizes a portion of thefeed gas as a source of methane and which includes therein a multistage expansion cycle tofurther cool the same and reduce the pressure to near-atmospheric pressure. In the sequence of 11 011014 cooîing cycles, the réfrigérant having the highest boiling point is utilized first followed by aréfrigérant having an intermediate boiling point and finally by a réfrigérant having the lowestboiling point.

Pretreatment steps provide a means for removing undesirable components such as 5 acid gases, mercaptans, mercury and moisture from the natural gas feed stream delivered to thefacility. The composition of this gas stream may vaiy significantly. As used herein, a naturalgas stream is any stream principally comprised of methane which originates in major portionfrom a natural gas feed stream, such feed stream for example containing at least 85% methane byvolume, with the balance being ethane, higher hydrocarbons, nitrogen, carbon dioxide and a 10 minor amounts of other contaminants such as mercury, hydrogen sulfide, mercaptans. The pretreatment steps may be separate steps located eïther upstream of the cooling cycles or locateddownstream 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 available to one skilled in theart Acid gases and to a lesser extent mercaptans are routinely removed via a sorption process 15 employing an aqueous amine-bearing solution. This treatment step is generally performedupstream of the cooling stages employed in the initial cycle. A major portion of the water isroutinely removed as a liquid via two-phase gas-Iiquid séparation following gas compression andcooling upstream of the initial cooling cycle and also downstream of the first cooling stage in theinitial cooling cycle. Mercury is routinely removed via mercury sorbent beds. Residual amounts 20 of water and acid gases are routinely removed via the use of properly selected sorbent beds suchas regenerable molecular sieves. Processes employing sorbent beds are generally locateddownstream of the first cooling stage in the initial cooling cycle. 12 011014

The resulting natural gas stream is generally delivered to the liquéfaction processat an elevated pressure or is compressed to an elevated pressure, that being a pressure greaterthan 500 psia, preferably about 500 to about 900 psia, still more preferably about 5J5O to about675 psia, still yet more preferably about 575 to about 650 psia, and most preferably about 600 5 psia. The stream température is typically near ambient to slightly above ambient Areprésentative température range being 60°F to 120°F.

As previously noted, the natural gas stream at this point is cooled in a plurality ofmultistage (for example, three) cycles or steps by indirect heat exchange with a plurality ofréfrigérants, preferably three. The overall cooling efficiency for a given cycle improves as the 10 number of stages increases but this increase in efficiency is accompanied by correspondingincreases in net capital cost and process complexity. The feed gas is preferably passed throughan effective number of réfrigération stages, nominally two, preferably two to four, and morepreferably three stages, in the first closed réfrigération cycle utilizmg a relatively high boilingréfrigérant. Such réfrigérant is preferably comprised in major portion of propane, propylene or 15 mixtures thereof, more preferably propane, and most preferably the réfrigérant 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 secondclosed réfrigération cycle in heat exchange with a réfrigérant having a lower boiling point. Suchréfrigérant is preferably comprised in major portion of ethane, ethylene or mixtures thereof, more 20 preferably ethylene, and most preferably the réfrigérant consists essentially of ethylene. Each ofthe above-cited cooling stages for each réfrigérant comprises a separate cooling zone.

Generally, the natural gas feed stream will contain such quantities of C2+components so as to resuit in the formation of a C2+ rich liquid in one or more of the cooling 13 011014 stages. This liquid is removed via gas-liquid séparation raeans, 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 as possible of the C2 and higher molecuiar weight hydrocarbons from the gas to produce a first gas stream predominating in methane and a second liquid stream containing significant amounts of ethane and heavier components. An effective number of gas/liquid séparation means are located at strategie locations downstream of the cooling zones for the removal of liquids streams rich in Ca+ components. The exact locations and number of gas/liquid separators will be dépendant on a number of operating parameters, such as the C2+ composition of the natural gas feed stream, the desired BTU content of the final 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 Cj+ hydrocarbon stream or streams may be demethanized via a single stage flash or a fractionation column. In the former case, the methane-rich stream can be repressurized and recycled or can be used as fuel gas. In the latter case, the methane-rich stream can be directly retumed at pressure to the liquéfaction process. The Cj+ 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 spécifie Chemicalconstituents (ex., Ç>, C3, C4 and C5+). In the last stage of the second cooling cycle, the gasstream which is predominantly methane is condensed (i.e., liquefied) in major portion, preferablyin its entirety. In one of the preferred embodiments to be discussed in greater detail in a latersection, it is at this location in the process that the inventive process and associated apparatus forbenzene, other aromatics and/or heavier hydrocarbon removal can be employed. The process 14 011014 pressure at this location is only slightly lower than the pressure of the feed gas to the first stageof the first cycle.

The liquefied natural gas stream is then further cooled in a third step or cycle byone of two embodiments. In one embodiment, the liquefied natural gas stream is further cooledby indirect heat exchange with a third closed réfrigération cycle wherein the condensed gasstream is subcooled via passage through an effective number of stages, nominally 2; preferably 2to 4; and most preferably 3 wherein cooling is provided via a third réfrigérant having a boilingpoint lower than the réfrigérant employed in the second cycle. This réfrigérant is preferablycomprised in major portion of methane and more preferably is predominantly methane. In thesecond and preferred embodiment which employs an open methane réfrigération cycle, theliquefied natural gas stream is subcooled via contact with flash gases in a main methane économiser in. a manner to be described later.

In the fourth cycle or step, the liquefied gas is further cooled by expansion andséparation of the flash gas from the cooled liquid. In a manner to be described, nitrogen removalfrom the System and the condensed product is accomplished either as part of this step or in aseparate succeeding step. A key factor distinguishing the closed cycle from the open cycle is the -initial température of the liquefied stream prior to flashing to near-atmospheric pressure, therelative amounts of flashed vapor generated upon said flashing, and the disposition of the flashedvapors. Whereas the majority of the flash vapor is recycled to the methane compressors in theopen-cycle system, the flashed vapor in a closed-cycle system is generally utilized as a fuel.

In the fourth cycle or step in either the open- or closed-cycle methane Systems, theliquefied product is cooled via at least one, preferably two to four, and more preferably threeexpansions where each expansion employs either Joule-Thomson expansion valves or hydraulic 15 0.1.1 01 4 expanders followed by a séparation of the gas-liquid product with a separator. When a hydraulicexpander is employed and properly operated, the greater efficiencies associated with the recoveryof power, a greater réduction in stream température, and the production of less vapor during theflash step will frequently be cost-effective even in light of increased capital and operating costs 5 associated with the expander. In one embodiment employed in the open-cycle System,additional cooling of the high pressure liquefied product prior to flashing is made possible byfirst flashing a portion of this stream via one or more hydraulic expanders and then via indirectheat exchange means employing said flashed stream to cool the high pressure liquefied streamprior to flashing. The flashed product is then recycled via retum to an appropriate location, 10 based on température and pressure considérations, in the open methane cycle.

When the liquid product entering the fourth cycle is at the preferred pressure of about 600 psia, représentative flash pressures for a three stage flash process are about 190,61and 14.7 psia. In the open-cycle System, vapor flashed or fractionated in the nitrogen séparationstep to be described and that flashed in the expansion flash steps are utilized as cooling agents in 15 the third step or cycle which was previously mentioned. In the closed-cycle System, the vaporfrom the flash stages may also be employed as a cooling agent prior to either recycle or use asfuel. In either the open- or closed-cycle System, flashing of the liquefied stream to nearatmospheric pressure will produce an LNG product possessing a température of -240°F to -260°F. 20 To maintain the BTU content of the liquefied product at an acceptable limit when appréciable nitrogen exists in the feed stream, nitrogen must be concentrated and removed atsome location in the process. Various techniques for this purpose are available to those skilled inthe art. The following are examples. When an open methane cycle is employed and nitrogen 16 01101 4 concentration in the feed is low, typically less than about 1.0 vol%, nitrogen removal is generallyachieved by removing a small side stream at the high pressure inlet or outlet port at the methanecompressor. For a closed cycle at nitrogen concentrations of up to 1.5 vol.% in the feed gas, theliquefted stream is generally flashed from process conditions to near-atmospheric pressure in asingle step, usually via a flash drum. The nitrogen-bearing flash vapors are then generallyemployed as fuel gas for the gas turbines which drive the compressors. The LNG product whichis now at near-atmospheric pressure is routed to storage. When the nitrogen concentration in theinlet feed gas is about 1.0 to about 1.5 vol% and an open-cycle is employed, nitrogen can beremoved by subjecting the liquefied gas stream from the third cooling cycle to a flash step priorto the foùrth cooling step. The flashed vapor will contain an appréciable concentration ofnitrogen and may be subsequently employed as a fuel gas. A typical flash pressure for nitrogenremoval at these concentrations is about 400 psia. When the feed stream contains a nitrogenconcentration of greater than about 1.5 vol% and an open or closed cycle is employed, the flashstep may not provide sufficient nitrogen removal. In such event, a nitrogen rejection column willbe employed from which is produced a nitrogen rich vapor stream and a liquid stream. In a preferred embodiment which employs a nitrogen rejection column, the high pressure liquefiedmethane stream to the methane economizer is split into a first and second portion. The firstportion is flashed to approximately 400 psia and the two-phase mixture is fed as a feed stream tothe nitrogen rejection column. The second portion of the high pressure liquefied methane streamis further cooled by flowing through a methane economizer to be described later, it is thenflashed to 400 psia, and the resulting two-phase mixture or the liquid portion thereof is fed to theupper section of the column where it fonctions as a reflux stream reflux. The nitrogen-rich vaporstream produced from the top of the nitrogen rejection column will generally be used as foel. 17 011014

The liquid stream produced from the bottom of the column is then fed to the first stage ofmethane expansion.

Refri gerative Cooling for Natural Gas Liquéfaction

Critical to the liquéfaction of natural gas in a cascaded process is the use of one or 5 more réfrigérants for transferring heat energy from the natural gas stream to the réfrigérant andultimately transferring said heat energy to the environment In essence, the réfrigération Systemfunctions as a heat pump by removing heat energy from the natural gas stream as the stream isprogressively cooled to lower and lower températures.

The liquéfaction process employs several types of cooling which include but are 10 not limited to (a) indirect heat exchange, (b) vaporization and (c) expansion or pressure réduction. Indirect heat exchange, as used herein, refers to a process wherein the réfrigérant orcooling agent cools the substance to be cooled without actual physical contact between therefrigerating agent and the substance to be cooled. Spécifie examples include heat exchangeundergone in a tube-and-shell heat exchanger, a core-in-kettle heat exchanger, and a brazed 15 aluminum plate-fm heat exchanger. The physical State of the réfrigérant and substance to becooled can vary depending on the demands of the System and the type of heat exchanger chosen.Thus, in the inventive process, a shell-and-tube heat exchange will typically be utilized where therefrigerating agent is in a liquid State and the substance to be cooled is in a liquid or gaseousState, whereas, a plate-fm heat exchanger will typically be utilized where the réfrigérant is in a 20 gaseous State and the substance to be cooled is in a liquid State. Finally, the core-in-kettle heatexchanger will typically be utilized where the substance to be cooled is liquid or gas and the 18 011014 réfrigérant undergoes a phase change from a liquid State to a gaseous State during the heatexchange.

Vaporization cooling refers to the cooling of a substance by the évaporation orvaporization of a portion of the substance with the System maintained at a constant pressure. 5 Thus, during the vaporization, the portion of the substance which evaporates absorbs heat fromthe portion of the substance which remains in a liquid State and hence, cools the liquid portion.

Finally, expansion or pressure réduction cooling refers to cooling which occurswhen the pressure of a gas-, liquid- or a two-phase System is decreased by passing through apressure réduction means. In one embodiment, this expansion means is a Joule-Thomson 10 expansion valve. In another embodiment, the expansion means is a hydraulic or gas expander.Because expanders recover work energy from the expansion process, lower process streamtempératures are possible upon expansion.

In the discussion and drawings to follow, the discussions or drawings may depictthe expansion of a réfrigérant by flowing through a throttle valve followed by a subséquent 15 séparation of gas and liquid portions in the réfrigérant chillers or condensers, as the case may be,wherein indirect heat-exchange also occurs. While this simplified scheme is workable andsometimes preferred because of cost and simplicity, it may be more effective to carry outexpansion and séparation and then partial évaporation as separate steps, for example acombination of throttle valves and flash drums prior to indirect heat exchange in the chillers or 20 condensers. In another workable embodiment, the throttle or expansion valve may not be aseparate item but an intégral part of the réfrigérant chiller or condenser (i.e., the flash occursupon entry of the liquefied réfrigérant into the chiller). In a like mariner, the cooling of multiple 19 011014 streams for a given réfrigération stage may occur within a single vessel (i.e., chiller) or withinmultiple vessels. The former is generally preferred from a capital equipment cost perspective.

In the first cooling cycle, cooling is provided by the compression of a higherboiling point gaseous réfrigérant, preferably propane, to a pressure where it can be liquefied byindirect heat transfer with a heat transfer medium which ultimately employs the environment as aheat sink, that heat sink generally being the atmosphère, a fresh water source, a sait water source,the earth or two or more of the preceding. The condensed réfrigérant then undergoes one or moresteps of expansion cooling via suitable expansion means thereby producing two-phase mixturespossessing significantly lower températures. In one embodiment, the main stream is split into atleast two separate streams, preferably two to four streams, and most preferably three streamswhere each stream is separately expanded to a designated pressure. Each stream then providesevaporative cooling via indirect heat transfer with one or more selected streams, one such streambeing the natural gas stream to be liquefied. The number of separate réfrigérant streams willcorrespond to the number of réfrigérant compressor stages. The vaporized réfrigérant from eachrespective stream is then retumed to the appropriate stage at the réfrigérant compressor (e.g., twoseparate streams will correspond to a two-stage compressor). In a more preferred embodiment,ail liquefied réfrigérant is expanded to a predesignated pressure and this stream then employed toprovide vaporative cooling via indirect heat transfer with one or more selected streams, one such stream being the natural gas stream to be liquefied. A portion of the liquefied réfrigérant is thenremoved from the indirect heat transfer means, expansion cooled by expanding to a lowerpressure and correspondingly lower température where it provides vaporative cooling via indirectheat transfer means with one or more designated streams, one such stream being the natural gasstream to be liquefied. Nominally, this embodiment will employ two such expansion 20 011014 cooling/vaporative cooling steps, preferably two to four, and most preferably three. Like the firstembodiment, the réfrigérant vapor from each step is retumed to the appropriate inlet port at thestaged compressor. . _ ...

In the preferred cascaded embodiment, the majority of the cooling for liquéfactionof the lower boiling point réfrigérants (i.e., the réfrigérants employed in the second and thirdcycles) is made possible by cooling these streams via indirect heat exchange with selectedhigher boiling réfrigérant streams. This manner of cooling is referred to as "cascaded cooling."

In effect, the higher boiling réfrigérants function as heat sinks for the lower boiling réfrigérantsor stated differently, heat energy is pumped from the natural gas stream to be liquefied to a lowerboiling réfrigérant and is then pumped (i.e., transferred) to one or more higher boilingréfrigérants prior to transfer to the environment via an environmental heat sink (ex., fresh water,sait water, atmosphère). As in the first cycle, réfrigérant employed in the second and thirdcycles are compressed via multi-staged compressors to preselected pressures. When possible andeconomically feasible, the compressed réfrigérant vapor is first cooled via indirect heat exchangewith one or more cooling agents (ex., air, sait water, fresh water) directly coupled toenvironmental heat sinks. This cooling may be via inter-stage cooling between compressionstages and/or cooling of the compressed product. The compressed stream is then further cooledvia indirect heat exchange with one or more of the previously discussed cooling stages for the higher boiling point réfrigérants.

The second cycle réfrigérant, preferably ethylene, is preferably first cooled viaindirect heat exchange with one or more cooling agents directly coupled to an environmental heatsink (i.e., inter-stage and/or post-cooling following compression) and then further cooled andfinally liquefied via sequential contact with the first and second or first, second and third cooling 21 011014 stages for the highest boiling point réfrigérant which is employed in the first cycle. Thepreferred second and first cycle réfrigérants are ethylene and propane, respectively.

When employing a three réfrigérant cascaded closed cycle System, the réfrigérantin the third cycle is compressed in a stagewise manner, preferably though optionally cooled viaindirect heat transfer to an environmental heat sink (i.e., inter-stage and/or post-coolingfollowing compression) and then cooled by indirect heat exchange with either ail or selectedcooling stages in the first and second cooling cycles which preferably employ propane andethylene as respective réfrigérants. Preferably, this stream is contacted in a sequential mannerwith each progressively colder stage of réfrigération in the first and second cooling cycles,

H respectively.

In an open-cycle cascaded réfrigération System such as that illustrated in FIGURE1, the first and second cycles are operated in a manner analogous to that set forth for the closedcycle. However, the open methane cycle System is readily distinguished from the conventionalclosed réfrigération cycles. As previously noted in the discussion of the fourth cycle or step, asignificant portion of the liquefied naturel gas stream originally présent at elevated pressure iscooled to approximately -260°F by expansion cooling in a stepwise manner to near-atmosphericpressure. In each step, significant quantifies of methane-rich vapor at a given pressure areproduced. Each vapor stream preferably undergoes significant heat transfer in methaneeconomizers and is preferably retumed to the inlet port of a compressor stage at near-ambienttempérature. In the course of flowing through the methane economizers, the flashed vapors arecontacted with warmer streams in a countercurrent manner and in a sequence designed tomaximize the cooling of the warmer streams. The pressure selected for each stage of expansioncooling is such that for each stage, the volume of gas generated plus the compressed volume of 22 011014 vapor from the adjacent lower stage results in efficient overall operation of the multi-stagedcompressor. Interstage cooling and cooling of the final compressed gas is preferred andpreferably accomplished via indirect heat exchange with one or more cooling agents directlycoupled to an environmental heat sink. The compressed methane-rich stream is then furthercooled via indirect heat exchange with réfrigérant in the first and second cycles, preferably ailstages associated with the réfrigérant employed in the first cycle, more preferably the first twostages and most preferably, only the first stage. Thé cooled methane-rich stream is furthercooled via indirect heat exchange with flash vapors in the main methane economizer and is thencombined with the natural gas feed stream at a location in the liquéfaction process where thenaturel gas feed stream and the cooled methane-rich stream are at similar conditions oftempérature and pressure, preferably prior to entry into one of the stages of ethylene cooling,more preferably immediately prior to the ethylene cooling stage wherein methane in majorportion is liquefied (i.e., ethylene condenser).

Optimization via Inter-stage and Inter-cycle Heat Tran.sfgL

In the more preferred embodiments, steps are taken to further optimize process efficiency by retuming the réfrigérant gas streams to the inlet port of their respectivecompressors at or near ambient température. Not only does this step improve overallefficiencies, but difficulties associated with the exposure of compressor components to cryogénieconditions are greatly reduced. This is accomplished via the use of economizers wherein streamscomprised in major portion of liquid and prior to flashing are first cooled by indirect heatexchange with one or more vapor streams generated in a downstream expansion step (i.e., stage)or steps in the same or a downstream cycle. In a closed System, economizers are preferably 23 011014 employed to obtain additional cooling from the flashed vapors in the second and third cycles.When an open methane cycle System is employed, flashed vapors from the fourth stage arepreferably retumed to one or more economizers where (1) these vapors cool via indirect heatexchange the liquefied product streams prior to each pressure réduction stage and (2) thesevapors cool via indirect heat exchange the compressed vapors from the open methane cycle priorto combination of this stream or streams with the main natural gas feed stream. These coolingsteps comprise the previously discussed third stage of cooling and will be discussed in greaterdetail in the discussion of FIGURE 1. In one embodiment wherein ethylene and methane areemployed in the second and third cycles, the contacting can be performed via a sériés of ethyleneand methane economizers. In a preferred embodiment which is illustrated in FIGURE 1 andwhich will be discussed in greater detail later, the process employs a main ethylene economizer,a main methane economizer and one or more additional methane economizers. These additional economizers are referred to herein as the second methane economizer, the third methaneeconomizer and so forth and each such additional methane economizer corresponds to a separatedownstream flash step.

Benzene. Other Aromatic and/or Heavier Hydrocarbon Removal

The inventive process for the removal of benzene, other aromatics and/or the higher molecular weight hydrocarbon species from a methane-based gas stream is an extremelyenergy efficient and operationally simple process. Because of the manner of operation, thecolumn referred to herein as a stripping column performs both stripping and fractionatingfonctions. The process comprises cooling the methane-based gas stream such that 0.1 to 20mol%, preferably 0.5 to about 10 mol%, and more preferably about 1.75 to about 6.0 mol% of 24 011014 the total gas stream is condensed thereby formïng a two-phase stream. The optimal molepercentage will be dépendant upon the composition of the gas undergoing liquéfaction and otherprocess-related parameters readily ascertained by one possessing ordinary skilled in the. art.

In one embodiment, the desired two-phase stream is obtained by cooling the entire 5 feed stream to such extent that the desired liquids percentage is obtained. In the preferredembodiment, the gas stream is first cooled to near the liquéfaction température and is then splitinto a first stream and a second stream. The first stream undergoes additional cooling and partialcondensation and is then combined with the second stream thereby producing a two-phase streamcontaining the desired percentage of liquids. This latter approach is preferred because of the 10 associated ease of operation and process control.

The two-phase stream is then fed to the upper section of a column wherein the stream contacts the rising vapor stream from the lower portion of the column thereby producinga heavies-rich liquid stream which functions as a reflux stream and a heavies-depleted vaporstream which is produced from the column. As used herein, “heavies” will refer to any 15 predominantly organic compound possessing a molecular weight greater than ethane. Thecolumn is unique in that it does ηοζ as previously noted, employ a condenser for refluxgénération and further, does not employ a reboiler for vapor génération.

As previously noted, a methane-rich stripping gas stream is fed to the column.This stream preferably originates from an upstream location where the methane-based gas stream 20 undergoing cooling has undergone some degree of cooling and liquids removal. Prior to introduction into the base of the column, this gas stream is cooled via indirect contact, preferablyin a countercurrent manner, with the liquid product produced from the bottom of the columnthereby producing a warmed heavies-rich stream and a cooled methane-rich stripping gas stream. 25 -011014

The methane-rich stripping gas may undergo partial condensation upon cooling and the resultingcooled methane-rich stripping gas containing two phases may be fed directly to the column.

The employment of the cooled methane-rich stripping gas which contains smallamounts of C3+ components in lieu of vapor generated from a reboiler which contains substantia] 5 amounts of C3+ components significantly reduces problems associated with fluids in the columnapproaching critical conditions whereupon poor component séparation results. This factorbecomes particularly significant when operating in the more preferred pressure range of about550 to about 675 psia. The critical température and pressure of methane is -116.4°F and 673.3psia. The critical température and pressure of propane is 206.2°F and 617.4 psia and the critical 10 température and pressure of n-butane is 305.7 °F and 551.25. The presence of appréciablequantities of C3+ components will (1) lower the critical pressure thereby approaching thepreferred operating pressures of the process and (2) raise the critical température. The resultingeffect is to make the séparation of the components via vapor/liquid contacting more difficult Asecond factor distinguishing the uses of the cooled methane-rich stripping gas over vapor from a 15 reboiler is the température différence between these respective streams and the liquid effluentfrom the last stage. Because it is preferred that the cooled methane-rich stripping gas be warmerthan the analogous vapor from a reboiler, this preferred stream possesses a greater ability to stripthe liquid phase of the lighter components. A température différence between the effluent liquidfrom the column and the effluent stripping gas to the column is more preferably 20°F to 110°F, 20 stiîl more preferably 40 °F to 90°F, most preferably about 60°F to about 80°F.

The number of theoretical trays in the column will be dépendant upon the composition, température and flowrate of the inlet vapor stream to the column and thecomposition, température, flowrate and liquid to vapor ratio of the two-phase stream fed to the 011014 upper section of the column. Such détermination is readily within the abilities of one possessingordinary skill in the art. The theoretical number of trays may be provided via various types ofcolumn packing (pall rings, saddles etc) or distinct contact stages (ex. trays) situated in .thecolumn or a combination thereof. Generally, two (2) to fifteen (15) theoretical stages are 5 required, more preferably three (3) to ten (10), still more preferably four (4) to eight (8), andmost preferably about five (5) theoretical stages. Trays are generally preferred when the columndiameter is greater than six (6) ft

Preferred Open-Cycle Embodiment of Cascaded Liquéfaction Process

The flow schematic and apparatus set forth in FIGURES 1 and 2 is a preferred 10 embodiment of the open-cycle cascaded liquéfaction process and is set forth for illustrativepurposes. Purposely missing from the preferred embodiment is a nitrogen removal System,because such System is dépendant on the nitrogen content of the feed gas. However as noted inthe previous discussion of nitrogen removal technologies, méthodologies applicable to thispreferred embodiment are readily available to those skilled in the art Presented in FIGURES 3 15 and 4 in greater detail for illustrative purposes is the inventive cryogénie column and in particular, the methodology for cooling and controlling the température of the stripping gas beingfed to the cryogénie column. Those skilled in the art will also recognized that FIGURES 1-4 areschematics only and therefore, many items of equipment that would be needed in a commercialplant for successful operation hâve been omitted for the sake of clarity. Such items might 20 include, for example, compressor Controls, flow and level measurements and correspondingcontrollers, additional température and pressure Controls, pumps, motors, filters, additional heat 27 011014 exchangers, valves, etc. These items would be provided in accordance with standard engineeringpractice.

To facilitate an understanding of FIGURES 1,2, 3 and 4, items numbered 1 thru99 generally correspond to process vessels and equipment directly associated with the 5 liquéfaction process. Items numbered 100 thru 199 correspond to flow Unes or conduits whichcontain methane in major portion. Items numbered 200 thru 299 correspond to flow lines orconduits which contain the réfrigérant ethylene or optionally, ethane. Items numbered 300 thru399 correspond to flow lines or conduits which contain the réfrigérant propane. To the extentpossible, the numbering System employed in FIGURE 1 has been employed in FIGURES 2,3, 10 and 4. In addition, the following numbering System has been added for additional éléments notillustrated in FIGURE 1. Items numbered 400 thru 499 correspond to additional flow lines orconduits. Items numbered 500 thru 599 correspond to additional process equipment such asvessels, columns, heat exchange means and valves, including process control valves. Itemsnumbered 600 thru 799 generally concem the process control System, exclusive of control 15 valves, and specifically includes sensors, transducers, controllers and setpoint inputs.

In almost ail control Systems, some combination of electrical, pneumatic or hydraulic signais are used. However, the use of any other type of signal transmission compatiblewith the process and equipment in use is withing the scope of this invention. With regard to theinvention depicted in FIGURES 1 through 4, lines designated as signal lines are depicted as dash 20 lines in the drawings. These lines are preferably electrical or pneumatic signal lines. Generallythe signais provided from any transducer are electric in form. However, the signais providedfrom flow sensors are generally pneumatic in form. The transducing of these signais is notalways illustrated for the sake of simplicity because it is well known in the art that if a flow is 28 0110.14 measured in pneumatic form it must be transduced to electric form if it is to be transmitted inelectrical form by a flow transducer.

Referring to FIGURE 1, gaseous propane is compressed in multistage compressor18 driven by a gas turbine driver which is not illustrated. The three stages of compression 5 preferably exist in a single unit although each stage of compression may be a separate unit andthe units mechanically coupled to be driven by a single driver. Upon compression, thecompressed propane is passed through conduit 300 to cooler 20 where it is liquefied. Areprésentative pressure and température of the liquefied propane réfrigérant prior to flashing isabout 100°F and about 190 psia. Although not illustrated in FIGURE 1, it is préférable that a 10 séparation vessel be located downstream of cooler 20 and upstream of a pressure réductionmeans, illustrated as expansion valve 12, for the removal of residual light components from theliquefied propane. Such vessels may be comprised of a single-stage gas-liquid separator or maybe more sophisticated and comprised of an accumulator section, a condenser section and anabsorber section, the latter two of which may be continuously operated or periodically brought 15 on-line for removing residual light components from the propane. The stream from this vesselor the stream from cooler 20, as the case may be, is passed through conduit 302 to a pressureréduction means, illustrated as expansion valve 12, wherein the pressure of the liquefied propaneis reduced thereby evaporating or flashing a portion thereof. The resulting two-phase productthen flows through conduit 304 into high-stage propane chiller 2 wherein gaseous methane 20 réfrigérant introduced via conduit 152, naturel gas feed introduced via conduit 100 and gaseousethylene réfrigérant introduced via conduit 202 are respectively cooled via indirect heat exchangemeans 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 main methane economizer 74 which will be .29 011014 discussed in greater detail in a subséquent section and wherein the stream is cooled via indirectheat exchange means 98. The resulting cooled compressed methane recycle stream produced viaconduit 158 is then combined with the heavies depleted vapor stream in conduit 12Q from the heavies removal column 60 and fed to the methane condenser 68. 5 The propane gas from chiller 2 is retumed to compressor 18 through conduit 306.

This gas is fed to the high stage inlet port of compressor 18. The remaining liquid propane ispassed through conduit 308, the pressure further reduced by passage through a pressure réductionmeans, illustrated as expansion valve 14, whéreupon an additional portion of the liquefiedpropane is flashed. The resulting two-phase stream is then fed to chiller 22 through conduit 310 10 thereby providing a codant for chiller 22. The cooled feed gas stream from chiller 2 flows viaconduit 102 to a knock-out vessel 10 wherein gas and liquid phases are separated. The liquidphase which is rich in C3> components is removed via conduit 103. The gaseous phase isremoved via conduit 104 and then split into two separate streams which are conveyed viaconduits 106 and 108. The stream in conduit 106 is fed to propane chiller 22. The stream in b 15 conduit 108 becomes the feed to heat exchanger 62 and is ultimately the stripping gas to theheavies removal column 60. Ethylene réfrigérant from chiller 2 is introduced to chiller 22 viaconduit 204. In chiller 22, the feed gas stream, also referred to herein as a methane-rich stream,and the ethylene réfrigérant streams are respectively cooled via indirect heat transfer means 24and 26 thereby producing cooled methane-rich and ethylene réfrigérant streams via conduits 110 20 and 206. The thus evaporated portion of the propane réfrigérant is separated and passed throughconduit 311 to the intermediate-stage inlet of compressor 18. Liquid propane réfrigérant fromchiller 22 is removed via conduit 314, flashed across a pressure réduction means, illustrated asexpansion valve 16, and then fed to third stage chiller 28 via conduit 316. 30 011014

As illustrated in FIGURE 1, the methane-rich stream flows from the intermediate-stage propane chiller 22 to the low-stage propane chiller/condenser 28 via conduit 110. In thischiller, the stream is cooled via indirect heat exchange means 30. In a like manner, the ethyleneréfrigérant stream flows from the intermediate-stage propane chiller 22 to the low-stage propanechiller/condenser 28 via conduit 206. In the latter, the ethylene réfrigérant is totally condensedor condensed in nearly its entirety via indirect heat exchange means 32. The vaporized propaneis removed from the low-stage propane chiller/condenser 28 and retumed to the low-stage inlet atthe compressor 18 via conduit 320. Although FIGURE 1 illustrâtes cooling of streams providedby conduits 110 and 206 to occur in the same vessel, the chilling of stream 110 and the coolingand condensing of stream 206 may respectively take place in separate process vessels (ex., aseparate chiller and a separate condenser, respectively). In a similar manner, the precedingcooling steps wherein multiple streams were cooled in a common vessel (ex., chiller) may beconducted in separate vessels. The former arrangement is a preferred embodiment because of thecost of multiple vessels and the requirement of less plant space.

As illustrated in FIGURE 1, the methane-rich stream exiting the low-stagepropane chiller is introduced to the high-stage ethylene chiller 42 via conduit 112. Ethyleneréfrigérant exits the low-stage propane chiller 28 via conduit 208 and is preferably fed to aséparation vessel 37 wherein light components are removed via conduit 209 and condensedethylene is removed via conduit 210. The séparation vessel is analogous to the vessel earlierdiscussed for the removal of light components from liquefied propane réfrigérant and may be asingle-stage gas-liquid separator or may be a multiple stage operation which provides greaterselectivity in the removal of light components from the system. The ethylene réfrigérant at thislocation in the process is generally at a température of about -24 °F and a pressure of about 285 31 011014 psia. The ethylene réfrigérant via conduit 210 then flows to the ethylene economizer 34wherein ît is cooled via indirect heat exchange means 38 and removed via conduit 211 andpassed to a pressure réduction means illustrated as an expansion valve 40 whereupon theréfrigérant is flashed to a preselected température and pressure and fed to the high-stage ethylene 5 chiller 42 via conduit 212. Vapor is removed from this chiller via conduit 214 and routed to theethylene economizer 34 wherein the vapor functions as a coolant via indirect heat exchangemeans 46. The ethylene vapor is then removed from the ethylene economizer via conduit 216and feed to the high-stage inlet on the ethylene compressor 48. The ethylene réfrigérant which isnot vaporized in the high-stage ethylene chiller 42 is removed via conduit 218 and retumed to 10 the ethylene economizer 34 for further cooling via indirect heat exchange means 50, removedfrom the ethylene economizer via conduit 220 and flashed in a pressure réduction meansillustrated as expansion valve 52 whereupon the resulting two-phase product is introduced intothe low-stage ethylene chiller 54 via conduit 222.

Removed from high-stage ethylene chiller 42 via conduit 116 is a methane-rich 15 stream. This stream is then condensed in part via cooling provided by indirect heat exchangemeans 56 in low-stage ethylene chiller 54 thereby producing a two-phase stream which flows viaconduit 118 to the benzene/aromatics/heavies removal column 60. As previously noted, themethane-rich stream in conduit 104 was split so as to flow via conduits 106 and 108. Thecontents of conduit 108 which is referred to herein as the methane-rich stripping gas is fîrst fed to 20 heat exchanger 62 wherein this stream is cooled via indirect heat exchange means 66 therebybecoming a cooled methane-rich stripping gas stream which then flows by conduit 109 to thebenzene/heavies removal column 60. Liquid contaïning a significant concentration of benzene,other aromatics and/or heavier hydrocarbon components is removed from the benzene/heavies 32 011014 removal column 60 via conduit 114, preferably flashed via a flow control means which can alsofunction as a pressure réduction means 97, preferably a control valve, and transported to heatexchanger 62 by conduit 117. Preferably, the stream flashed via flow control.meaps 97 is flashedto a pressure about or greater than the pressure at the high stage inlet port to the methane 5 compressor. Flashing also imparts greater cooling capacity to said stream. In the heat exchanger62, the stream delivered by conduit 117 provides cooling capabilities via indirect heat exchangemeans 64 and exits said heat exchanger via conduit 119. In the benzene/aromatics/heaviesremoval column 60, the two-phase stream introduced via conduit 118 is contacted with thecooled methane-rich stripping gas stream introduced via conduit 109 in a countercurrent manner 10 thereby producing a benzene/heavies-depleted, methane-rich vapor stream via conduit 120 and abenzene/heavies-enriched liquid stream via conduit 117.

The stream in conduit 119 is rich in benzene, other aromatics and/or other heavierhydrocarbon components. This stream is subsequently separated into liquid and vapor portionsor preferably is flashed or fractionated in vessel 67. In each case a liquid stream rich in benzene, 15 other aromatics and/or heavier hydrocarbon components and is produced via conduit 123 and asecond methane-rich vapor stream is produced via conduit 121. In the preferred embodimentwhich is illustrated in FIGURE 1, the stream in conduit 121 is subsequently combined with asecond stream delivered via conduit 128 and the combined stream fed via conduit 140 to the highpressure inlet port on the methane compressor 83. 20 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 cooledcompressed methane recycle or réfrigérant stream in conduit 158 is combined in the preferredembodiment with the heavies depleted vapor stream from the heavies removal column 60 33 011014 delivered via conduit 120 and fed to the low-stage ethylene condenser 68. In the low-stageethylene condenser, this stream is cooled and condensed via indirect heat exchange means 70with the liquid effluent from the low-stage ethylene chiller 54 which is routed to the low-stageethylene condenser 68 via conduit 226. The condensed methane-rich product from the low- 5 stage condenser is produced via conduit 122. The vapor from the low-stage ethylene chiller 54withdrawn via conduit 224 and low-stage ethylene condenser 68 withdrawn via conduit 228 arecombined and routed via conduit 230 to the ethylene economizer 34 wherein the vapors functionas coolant via indirect heat exchange means 58. The stream is then routed via conduit 232 fromthe ethylene economizer 34 to the low-stage side of the ethylene compressor 48. 10 As noted in FIGURE 1, the compressor effluent from vapor introduced via the low-stage side is removed via conduit 234, cooled via inter-stage cooler 71 and retumed tocompressor 48 via conduit 236 for injection with the high-stage stream présent in conduit'216.Preferably, the two-stages are a single module although they may each be a separate module andthe modules mechanically coupled to a common driver. The compressed ethylene product from 15 the compressor is routed to a downstream cooler 72 via conduit 200. The product from thecooler flows via conduit 202 and is introduced, as previously discussed, to the high-stagepropane chiller 2

The liquefied stream in conduit 122 is generally at a température of about -125°Fand a pressure of about 600 psi. This stream passes via conduit 122 through the main methane 20 economizer 74, wherein the stream is further cooled by indirect heat exchange means 76 ashereinafter explained. From the main methane economizer 74 the liquefied gas passes throughconduit 124 and its pressure is reduced by a pressure réduction means which is illustrated asexpansion valve 78, which of course evaporates or flashes a portion of the gas stream. The 34 011014 flashed stream is then passed to methane high-stage flash drum 80 where it is separated into a gasphase discharged through conduit 126 and a liquid phase discharged through conduit 130. The < gas-phase is then transferred to the main methane economizer via conduit 126.whecein.the vapor functions as a coolant via indirect heat transfer means 82. The vapor exits the main methane 5 economizer via conduit 128 where it is combined with the gas stream delivered by conduit 121.These streams are then fed to the high pressure inlet port of compresser 83.

The liquid phase in 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 the second methane economizer 87 via conduit 132 and is·» 10 expanded or flashed via pressure réduction means illustrated as expansion valve 91 to furtherreduce the pressure and at the same time, vaporize a second portion thereof. This flash stream isthen passed to intermediate-stage methane flash drum 92 where the stream is separated into a gasphase passing through conduit 136 and a liquid phase passing through conduit 134. The gasphase flows through conduit 136 to the second methane economizer 87 wherein the vapor cools 15 the liquid introduced to 87 via conduit 130 via indirect heat exchanger means 89. Conduit 138 serves as a flow conduit between indirect heat exchange means 89 in the second methane « economizer 87 and the indirect heat transfer means 95 in the main methane economizer 74. Thisvapor leaves the main methane economizer 74 via conduit 140 which is connected to theintermediate stage inlet on the methane compressor 83. 20 The liquid phase exiting the intermediate stage flash drum 92 via conduit 134 is further reduced in pressure by passage through a pressure réduction means illustrated as aexpansion valve 93. Again, a third portion of the liquefied gas is evaporated or flashed. Thefluids from the expansion valve 93 are passed to final or low stage flash drum 94. In flash drum 35 011014 94, a vapor phase is separated and passed through conduit 144 to the second methane economizer87 wherein the vapor functions as a codant via indirect heat exchange means 90, exits thesecond methane economizer via conduit 146 which is connected to the first methane economizer74 wherein the vapor functions as a coolant via indirect heat exchange means 96 and ultimately 5 leaves the first methane economizer via conduit 148 which is connected to the low pressure porton compressor 83.

The liquefied naturel gas product frora flash drum 94 which is at approximatelyatmospheric pressure is passed through conduit 142 to the storage unit The low pressure, lowtempérature LNG boil-off vapor stream from the storage unit and optionally, the vapor retumed 10 from the cooling of the rundown Unes associated with the LNG loading System, is preferablyrecovered by combining such stream or streams with the low pressure flash vapors présent ineither conduits 144,146, or 148; the selected conduit being based on a desire to match vaporstream températures as closely as possible.

As shown in FIGURE 1, the high, intermediate and low stages of compressor 83 15 are preferably combined as single unit. However, each stage may exist as a separate unit wherethe units are mechanically coupled together to be driven by a single driver. The compressed gasfrom the low-stage section passes through an inter-stage cooler 85 and is combined with theintermediate pressure gas in conduit 140 prior to the second-stage of compression. Thecompressed gas from the intermediate stage of compressor 83 is passed through an inter-stage 20 cooler 84 and is combined with the high pressure gas in conduit 140 prior to the third-stage ofcompression. The compressed gas is discharged from the high-stage methane compressorthrough conduit 150, is cooled in cooler 86 and is routed to the high pressure propane chiller viaconduit 152 as previously discussed. 36 0110.14 FIGURE 1 depicts the expansion of the liquefied phase using expansion valveswith subséquent séparation of gas and liquid portions in the chiller or condenser. While thissimplified scheme is workable and utilized in somé cases, it is often more efficient and-effectiveto carry out partial évaporation and séparation steps in separate equipment, for example, anexpansion valve and separate flash drum might be employed prior to the flow of either theseparated vapor or liquid to a propane chiller. In a like manner, certain process streamsundergoing expansion are idéal candidates for employment of a hydraulic expander as part of thepressure réduction means thereby enabling the extraction of work energy and also lowertwo-phase températures.

•I

With regard to the compressor/driver units employed in the process, FIGURE 1depicts individual compressor/driver units (i.e., a single compression train) for the propane,ethylene and open-cycle methane compression stages. However in a prefened embodiment forany cascaded process, process reliability can be improved significantly by employing a multiplecompression train comprising two or more compressor/driver combinations in parallel in lieu ofthe depicted single compressor/driver units. In the event that a compressor/driver unit becomesunavailable, the process can still be operated at a reduced capacity.

Preferred Embodiment of the Inventive Removal Process and Apparatus

Presented in FIGURE 2 is a preferred embodiment of the benzene, other aromatic and/or heavier hydrocarbon component removal process and associated apparatus. As previously noted, the two-phase stream fed to the benzene/aromatics/heavies removal column 60 via conduit 118 results from the cooling and partial condensing of the stream in conduit 116 via cooling provided by heat exchange means 56 in ethylene chiller 54. In one embodiment, the entire 37 011014 stream in conduit 116 is cooled. In a preferred embodiment illustrated in FIGURE 2, the two-phase stream is obtained by cooling and partially condensing a portion of the stream in conduit116 and this portion is then combined with the remaining portion of the stream originating via conduit 116.

Referring to FIGURE 2, the stream delivered via conduit 116 is split into a fîrststream flowing in conduit 450 and a second stream flowing in conduit 452. The stream inconduit 532 flows through an optional valve 532, preferably a hand control valve, to conduit 454which delivers the fîrst stream to ethylene chiller 54 wherein the stream undergoes at least partialcondensation via indirect heat exchange means 56 and exits said means via conduit 458. Thesecond stream in conduit 452 flows through a valve 530, preferably a control valve, into conduit 456 which is then combined with the fîrst stream delivered via conduit 458. The combined streams, now a two-phase stream, is delivered to column 60 via conduit 118. From anoperational perspective, the length of conduit 118 should be sufficient to insure adéquate mixingof the two streams such that equilibrium conditions are approached. The amount of liquids in thetwo-phase stream in conduit 118 is preferably controlled via maintaining the streams at a desiredtempérature. This is accomplished in the following manner. A température transducing device688 in combination with a sensing device such as a thermocouple situated in conduit 118provides an input signal 686 to a température controller 682. Also provided to the controller byoperator or computer algorithm is a setpoint température signal 684. The controller 682 respondsto the différences in the two inputs and transmits a signal 680 to the flow control valve 530which is situated in a conduit wherein flows the portion of the stream delivered via conduit 116which does not undergo cooling via heat exchanger means 56 in chiller 54. The transmitted 38 . 011014 signal 680 is scaled to be représentative of the position of the control valve 530 required tomaintain the flowrate necessary to obtain the desired température in conduit 118.

These feedstreams to the process step wherein benzene, other aromaticand/orheavy hydrocarbon components are removed are the two-phase process stream from ethylenechiller 54 delivered via conduit 118 to the upper section of column 60 and the methane-richstopper gas delivered via conduit 108. Although depicted in FIGURE 1 as originating from thefeed gas stream from the first stage of propane cooling, this stream can originate from anylocation within the process or may be an outside methane-rich stream. As illustrated in FIGURE2, at least a portion of the methane-rich stripper gas undergoes cooling in heat exchanger 62 viaindirect heat exchange means 62 prior to entering the base of column 60. Effluent streams fromthis inventive process step are the heavies-depleted gas stream from column 60 produced viaconduit 120 and the warmed heavies-rich stream produced via conduit 119. As illustrated inFIGURE 2, a heavy-rich stream is produced from column 60 and undergoes warming in heatexchanger 62 via indirect heat exchange means 66. It is in this manner that the column effluentproduced via conduit 114 cools the stripping gas fed to the column via conduit 109.

The number of theoretical stages in column 60 will be dépendent on thecomposition of the feedstreams to the column. Generally, two (2) to fifteen (15) theoreticalstages will be required. The preferred number of stages is three (3) to ten (10), still morepreferably is four (4) to eight (8) and from an operational and cost perspective, the mostpreferred number is about five (5). The theoretical stages may be made available via packing,plates/trays or a combination thereof. Generally, packing is preferred in columns of less thanabout six (6) ft?diameter and plates/trays on columns of greater than about six (6) ft diameter.As illustrated in FIGURE 2, the upper section of column wherein the two-phase stream in 39 011014 conduit 118 is fed is designed to facilitate gas/liquid séparation. The top of the columnpreferably contains a means for demisting or removing entrained liquids from the vapor stream.This means is to be located between the point of entry of conduit 118 and the pointuf exit of conduit 120.

As illustrated in FIGURE 2, the heavies-rich liquid stream produced via conduit114 flows through control valve 97 and conduit 117 to beat exchanger 62 wherein said streamprovides cooling via indirect heat transfer means 64 and is produced from heat exchanger 62 viaconduit 119 as a warmed heavies-rich stream. Depending on the operational pressure ofdownstream processes, the cooling ability of this stream can be enhanced by flashing to a lowerpressure upon flow through control valve 97. This process stream produced via conduit 119 maybe utilized directly or undergo subséquent treatment for the removal of lighter components. Inthe preferred embodiment illustrated in FIGURE 2, the stream is fed to a demethanizer 67.

The flowrate of heavies-rich liquid from column 60 may be controlled via variousméthodologies readily available to one skilled in the art. The control apparatus illustrated inFIGURE 2 is a preferred apparatus and is comprised of a level controller device 600, also asensing device, and a signal transducer connected to said level controller device, operably locatedin the lower section of column 60. The controller 600 establishes an output signal 602 that eithertypifies the flowrate in conduit 114 required to maintain a desired level in column 60 or indicatesthat the actual level has exceeded a predetermined level. A flow measurement device andtransducer 604 operably located in conduit 114 establishes an output signal 606 that typifies theactual flowrate of the fluid in conduit 114. The flow measurement device is preferably locatedupstream of the control valve so as to avoid sensing a two-phase stream. Signal 602 is providedas a set point signal to flow controller 608. Signais 602 and 608 are respectively compared in 40 011014 flow controller 608 and controller 608 establishes an output signal 614 responsive to the différence between signais 602 and 606. Signal 614 is provided to control valve 97 and valve 97 is manipulated responsive to signal 614. A setpoint signal (not illustrated) représentative of a desired level in column 60 may be manually inputted to level controller 600 by an operator or in 5 the alternative, be under computer control via a control algorithm. Depending on the operatingconditions, operator or computing machine logic is employed to détermine whether control willbe based on liquid level or flowrate. In response to the variable flowrate input of signal 606 andthe selected setpoint signal, the controller 608 provides an output signal 614 which is responsiveto the différence between the respective input and setpoint signais. This signal is scaled so as to 10 be représentative, as the case may be, of the position of the control valve 97 required to maintainthe flowrate of fluid substantially equal to the desired flowrate or the liquid level substantiallyequal to the desired liquid level, as the case may be.

In the heat exchanger 62, the heavies-rich stream, which cools the methane-richstripping gas stream, is routed to the heat exchanger via conduit 117. The heavies-rich stream 15 flows thru indirect heat exchange means 66 and is produced from the heat exchanger via conduit119. The degree to which the methane-rich stripping gas is cooled by the heavies-bearing streamprior to entry into the column may be controlled via various méthodologies readily available toone skilled in the art. In one embodiment, the entire methane-rich stripping gas stream is fed tothe heat exchanger and the degree of cooling controlled by such parameters as the amount of 20 heavies-rich liquid stream made available for heat transfer, the heat transfer surface areas available for heat transfer and/or the résidence times of the fluids undergoing heating or coolingas the case may be. In a preferred embodiment, the methane-rich stripping gas stream deliveredvia conduit 108 flows through control valve 500 into conduit 400 whereupon the stream is split 41 011014 and transferred via conduits 402 and 403. The stream flowing through conduit 403 ultimatelyflows through indirect heat transfer means 64 in heat exchanger 62. A means for manipulatingthe relative flowrates of fluid in conduits 402 and 403 is provided in either conduits 402 or 403or both. The means illustrated in FIGURE 2 are simple hand control valves, designated 502 and 5 504, which are respectively attached to conduits 404 and 407. However, a control valve whose position is manipulated by a controller and for which input to the controller is comprised of asetpoint and signal représentative of flow in the conduit, such as that discussed above for theheavies-bearing stream, may be substituted for one or both of the hand control valves. In anyevent, the valves are operated such that the température approach différence of the streams in 10 conduits 117 and 404 to heat exchanger 62 does not exceed 50 °F whereupon damage to the heatexchanger might resuit The cooled fluid leaves the indirect heat transfer means 64 via conduit405 and is combined at a junction point with uncooled methane-rich stripping gas delivered viaconduit 407 thereby forming the cooled methane-rich stripping gas stream which is delivered to the column via conduit 109. 15 Operably located in conduit 109 is a flow transducing device 616 which in combination with a flow sensing device such as an orifice plate (not illustrated) establishes anoutput signal 618 that typifies the actual flowrate of the fluid in the conduit Signal 618 isprovided as a process variable input to a flow controller 620. Also provided either manually orvia computer output is a set point value for the flowrate represented by signal 622. The flow 20 controller then provides an output signal 624 which is responsive to the différence between therespective input and setpoint signais and which is scaled to be représentative of the position ofthe control valve required to maintain the desired flowrate in conduit 109. 42 011014

In another embodiment, the relative flowrate of fluid through conduits 402 and 403 can be controlled via locating a température sensing device and a transducer connected to saîd device, if so required, in conduit 109 and using the resulting output and a setpoint _ température as input to a flow controller which would generate an output signal responsive to the 5 différence in the two signais and scaled to be représentative of a control valve position requiredto maintain the desired flowrate in conduit 109. Such control valves could be substituted for hand valves 502 and/or 504.

In still yet another embodiment depicted in FIGURE 3, the température of thestripping gas to column 60 is controlled in the following manner. Température transducer 704 in 10 combination with a measuring device such as a thermocouple operably located in conduit 117provides an output signal 708 which is représentative of the actual température of liquid flowingin conduit 117. Signal 708 is provided as a first input to the ratio calculator 700. Ratiocalculator 700 is also provided with a second température signal 706 représentative of thetempérature of fluid flowing into conduit 109. Signal 706 originates in température transducer 15 702 whose output signal 706 is responsive to a sensing element such as a thermocouple operably located in conduit 109. In response to signais 706 and 708 ratio calculator 700 provides anoutput signal 710 which is représentative of the ratio of signais 706 and 708. Signal 710 isprovided as an input to ratio controller 712. Ratio controller 712 is also provided with a set pointsignal 714 which is représentative of the desired température ratio for the fluids flowing in 20 conduits 109 and 114, Responsive to signais 710 and 714, ratio controller 712 provides an output signal 716 which is responsive to the différence between signais 710 and 714. Signal 716is scaled to be représentative of the position of control valve 534, which is operably located in 43 011014 by-pass conduit 718, required to maintain the desired ratio represented by set point signal 714.Control valve 534 is manipuîated responsive to signal 716.

In accordance with the most preferred control methodology depicted in FIGURE4 where like référencé numerals are used for éléments shown in the previous Figures, an 5 automatic start-up of column 60 is facilitated by high selector 728. It is noted that the set point724 of température controller 722 is desirably set at a température compatible with the liquid inthe column 60. On start-up however, the température in conduit 109 will be at or near ambienttempérature. Accordingly connecting signal 726 directly to manipulate valve 536 would causevalve 536 to close and not allow flow of the warm dry gas to a cryogénie séparation column 60 10 during startup. This problem is overcome by temporarily selecting signal 742 to manipulate valve 536 as described below.

Responsive to signais 706 and 724 température controller 722 provides an outputsignal 726 responsive to the différence between signais 706 and 724. Signal 726 is scaled to bereprésentative of the position of control valve 536 which is operably located in conduit 108 15 required to maintain the actual température of the fluid in conduit 109 substantially equal to thedesired température représentative by signal 724. As previously stated, however, the desiredvalue for set point signal 724 will not allow start-up of the column. Accordingly signal 726 isprovided to a signal selector 728. Signal selector 728 is also provided with a control signal 742which is responsive to the différence between signais 736 and 740 and is scaled to be 20 représentative of the position of control valve 536 required to maintain the température of fluidin conduit 119 substantially equal to the desired température represented by signal 740. On startup of the column, the actual température of fluid in conduit 119 will be less than the desiredtempérature represented by signal 740. Accordingly, connecting signal 742 to valve 536 would 44 011014 cause valve 536 to open so as to lower the température represented by signal 706. High selector728 décidés which of the control signais 726 and 742 manipulate the valve 536.

Start-up proceeds like this. Feed gas is introduced into the top of the cryogénieséparation column 60 in the upper section. When the température of the feed gas cools to the 5 condensing température of the impurity to be removed, liquid begins to build a level in thecolumn 60. Level controller 600 senses the level and its output opens valve 97 responsive tosignal 614. Low température liquid is then passed to heat exchanger 62 and exchanges heat witha warm dry gas stream through conduit 108 and valve 536. Valve 536 is initially opened bysignal 742 on set point température. After dry gas flow is initiated température transducer 702 10 senses a sharply colder température resulting in signal 726 being selected by the high selector728. The start-up Controls assist the’operator in providing a smooth safe start-up and reduce thelevel of human attention required.

The warmed heavies-rich liquid stream from heat exchanger 62 is fed via conduit119 to the demethanizer column 67 which contains both rectifying and stripping sections. The 15 rectifying and stripping sections may contain distinct stages (e.g., trays, plates) or may providefor continuous mass transfer via column packing (eg., saddles, racking rings, woven wire) or acombination of the preceding. Generally, packing is preferred for columns possessing a diameterof less than about six (6) ft and distinct stages preferred for columns possessing a diameter ofgreater than about six (6) ft. The number of theoretical stages in both the rectifying and stripping 20 sections is dépendant on the desired composition of the final products and the composition of thefeed stream. Preferably the stripping or lower section contains 4 to 20 theoretical stages, morepreferably 8 to 12 theoretical stages, and most preferably about 10 theoretical stages. In a similarmanner, the upper or rectifying section of the column preferably contains 4 to 20 theoretical 45 011014 stages, more preferably 8 to 13 theoretical stages, and most preferably about 10 theoretical stages. A conventional reboiler 524 is provided at the bottom to provide stripping vapor.In the preferred embodiment presented in FIGURE 2, liquid from the lower-most stage in thedemethanizer is provided to the reboiler via conduit 428 wherein said fluid is heated via anindirect heat transfer means 525 with a heating medium delivered via conduit 440 and retumed via conduit 442 which is connected to flow control valve 526 which is in tum connected toconduit 444. Vapor from the reboiler is retumed to the demethanizer column via conduit 430and liquids are removed from the reboiler via conduit 432. Said stream in conduit 432 mayoptionally be combined in conduit 436 with a second liquids stream produced from the bottom ofthe demethanizer via optional conduit 434. The total liquids stream produced from thedemethanizer via conduits 436 and/or 432, as the case may be, may optionally flow thru cooler520 and produced via conduit 438. A means for controlling liquid flow is inserted into one orboth of the preceding conduits. In one embodiment as illustrated in FIGURE 2, the flow controlmeans is comprised of control valve 522 which is inserted between conduits 438 and 123. Theposition of the control valve 522 is manipulated by a flow controller 632 which is responsive tothe différences between a setpoint input signal 628 from a level control device 626 and the actualflowrate of fluid in conduit 438 represented by signal 631. A set point flowrate 630 for levelcontroller 626 may be provided via operator or computer algorithm input. Output from thecontroller 632 is signal 634 which is scaled to be représentative of the position of the controlvalve 522 required to maintain the desired flowrate in conduit 438 to maintain the desired level in 67. 46 011014

Although various control techniques are readily available for regulating the flowrate of stripping vapor to the column 67 via conduit 430, a preferred technique is based on the température of the retum vapor. A température transducing de vice 636 in combination with a sensing device such as a thermocouple situated in conduit 430 provides an input signal 638 to a 5 température controller 642. Also provided to the controller by operator or computer algorithm isa setpoint température signal 640. The controller 642 responds to the différences in the twoinputs and transmits a signal 644 to the flow control valve 526 which is situated in a conduitcontaining the heating medium, preferably conduits 440 or 444, most preferably conduit 444 asillustrated. The transmitted signal 644 is scaled to be représentative of the position of the 10 control valve 526 required to maintain the flowrate necessary to obtain the desired température in conduit 440. A novel aspect of the demethanizer column is the manner in which reflux liquideare generated. As illustrated in FIGURE 2, the overhead product exits the demethanizer column67 via conduit 410 whereupon at least a portion of said stream is partially condensed upon 15 flowing through indirect heat exchange means 510 in heat exchanger 62 which is cooled via the heavies-rich liquid product from the heavies removal column 60. In a preferred embodiment, the« heavies-rich liquid product is first employed for cooling of at least a portion of the overheadvapor stream and then employed for cooling of the methane-rich stripping gas stream. Thecondensed liquids resulting from cooling via the heavies-rich liquid stream become the source of 20 the reflux for demethanizer column 67. Preferably, the heat exchange between the two designated streams occurs in a countercurrent manner. In one embodiment, the entire streammay flow to heat exchanger 62 in the manner previously discussed for the cooling of the entiremethane stripping gas. In a preferred embodiment which is illustrated in FIGURE 2, the 47 011014 overhead vapor product in conduit 410 is split into streams flowing in conduits 412 and 414.

The stream in conduit 414 is cooled in heat exchanger 62 by flowing said stream through indirectheat exchange means 510 in exchanger 62 and the resulting cooled stream is produced viaconduit 418. The relative flowrates of the vapor streams in conduits 412 and 414 or 418 are 5 controlled by a flow control means, preferably a flow control valve through which overheadvapor may flow without flowing through the heat exchanger thereby avoiding the control of atwo-phase fluid. Vapor flowing in conduit 412 flows through flow control means 512 and isproduced therefrom via conduit 416. Conduits 416 and 418 are then joined thereby resulting in acombined cooled two-phase stream which flows through conduit 420. Situated in conduit 420 is 10 a température transducing device 646, in combination with a température sensing device, preferably a thermocouple, pro vides a signal 648 représentative of the actual température of thefluid flowing in conduit 420 to température controller 652. A desired température 650 is alsoinputted to the controller 652 either manually or via a computational algorithm. Based on acomparison of the input via the transducing device 646 and the setpoint 650, the controller 652 15 then provides an output signal 654 to the valve 512 which is scaled to manipulate the valve 512in an appropriate manner such that the setpoint température is approached or maintained. Theresulting two-phase fluid in conduit 420 is then fed to separator 514 from which is produced amethane-rich vapor stream via conduit 422 and a reflux Iiquid stream via conduit 424. In anotherpreferred embodiment, the preceding methodology is employed but the heavies-rich stream in 20 conduit 117 is first employed for cooling of the stream delivered via conduit 414 prior to coolingthe stream delivered via conduit 414. As illustrated in FIGURE 1, the methane rich vapor streamin conduit 121 can be retumed to the open methane cycle for subséquent liquéfaction. Thepressure of the demethanizer and associated equipment is controlled by automatically 48 011014 manipulating control valve 518 responsive to a pressure transducer device 656 operably locatedin conduit 422. The control valve is connected on the inlet side to conduit 422 and on the outletside to conduit 121 which preferably is directly or indirectly connected to the low pressure inletport on the methane compressor, the pressure transducing device 656 in combination with a 5 sensing device, provides a signal 658 to a pressure controller 660 which is représentative of theactual pressure in conduit 422. A set point pressure signal 662 is also provided as input to thepressure controller 660. The controller then generates a response signal 664 représentative of thedifférence between the pressure sensing device signal 658 and the setpoint signal 662. Signal664 is scaled in such a manner as to activate the valve 518 according for approach and 10 maintenance of the setpoint pressure. In one embodiment, the controller and control valve andoptionally, the pressure sensing transducer 656 are embodied in a single device commonly calleda back pressure regulator.

The reflux from the separator ultimately flows to the demethanizer. In thepreferred embodiment illustrated in FIGURE 2, the reflux leaves the separator 514 via conduit 15 424, flows thru pump 516, and then flows thru conduit 425, control valve 519, and conduit 426 whereupon the stream is introduced into the upper section of the demethanizer column. In thisembodiment, the flowrate of reflux is controlled via input from a level control device 666 whichis responsive to a sensing device located in the lower section of the separator 514. Controller666 generates a signal 668 représentative of the flowrate in conduit 426 required to maintain the 20 desired level in separator 514, signal 668 is provided as a setpoint input to flow controller 670 towhich is also fed a signal 671 which typifies the actual flowrate in conduit 425. The controller670 then generates a signal 674 to control valve 519 which is représentative of the différence in 49 011014 signais and scaled to provide for appropriate liquids flow through the flow control valve 519such that liquid level in separator 514 is controlled.

The controllers previously discussed may use the various well-known modes nfcontrol such as proportional, proportional-integral, or proportional-integral-derivative (PID). A 5 digital computer having backup accommodations is preferred in the preferred embodimentdepicted in FIGURE 4 for calculating the required control signais based on measured processvariables as well as set points supplied to the computer. Any digital computer having softwarethat allows operation of a real time environment for reading values of extemal variables andtransmitting signais to extemal devices is suitable for use. The PID controllers shown in 10 FIGURES 2,3, and 4 can utilize the various modes of control such as proportional, proportionalintégral or proportional-integral-derivative. In the preferred embodiment a proportional-integralmode is utilized. However, any controller having capacity to accept two or more input signaisand produce a scaled output signal représentative of the comparison of the two input signais iswithin the scope of the invention. 15 The scaling of an output signal by a controller is well known in the control

Systems art Essentially, the output of a controller can be scaled to represent any desired factoror variable. An example of this is where a desired température and an actual température arecompared by controller. The controller output might be a signal représentative of a flow rate of"control" gas necessary to make the desired and actual températures equal. On the other hand, 20 the same output signal could be scaled to represent a pressure required to make the desired andactual températures equal. If the controller output can range from 0-10 units, then the controlleroutput signal could be scaled so that an output having a level of 5 units corresponds to 50%percent or a specified flow rate or a specified température. The transducing means used to 011014 50 measure parameters which characterize a process in the various signais generated thereby maytake a variety of forms or formats. For example the control éléments of this System can beimplemented using electrical analog, digital electronic, pneumatic, hydraulic, mechanical, orother similar types of equipment or combination of such types of equipment. Sélective control loops are used in a variety of process situations for selecting anappropriate control action. Typically a normal control signal is overridden by a secondarycontrol signal that has a higher priority in the event of certain process conditions. For example,hazardous conditions can be avoided, or désirable features such as automatic start-up can beimplemented by temporarily selecting a secondary control signal.

The spécifie hardware and/or software utilized in such feedback control Systems iswell known in the field of process plant control. See for example Chemical Engineering’sHandbook, 5th Ed., McGraw-Hill, pgs. 22-1 to 22-147.

While spécifie cryogénie methods, materials, items of equipment and controlinstruments are referred to herein, it is to be understood that such spécifie récitals are not to beconsidered limiting but are included by way of illustration and to set forth the best mode inaccordance with the présent invention.

EXAMPLE I

This Example shows via computer simulation the efficiency of the processdescribed in the spécification for the removal of benzene and heavier components from amethane-based stream immediately prior to liquéfaction of the methane-based stream in majorportion. The flowrates are représentative to those existing in a 2.5 million metric tonne/yearLNG plant employing the liquéfaction technology set forth in FIGURES 1 and 2. The benzeneconcentrations in the methane-based gas streams employed in this Example are considered to be 51 011014 représentative of those possessed by many candidate natural gas streams at this location in the process. However, the methane-based gas streams are considered to be relatively lean in the heavier hydrocarbon components (i.e., C3+). Simulation results were obtained using Hyprotech's

Process Simulation HYSIM, version 386/C2.10, Prop. Pkg PR/LK. 5 Presented in Table 1 are the compositions, températures, pressures and phase conditions of the influent and effluent streams to the heavies removal column. The simulation isbased upon the column containing 5 theoretical stages. The partially condensed stream, alsoreferred to as the two-phase stream, which will latter undergo liquéfaction in major proportion isflrst fed to the uppermost stage in the column (Stage 1). The température of this stream is - 10 112.5°F and the pressure is 587.0 psia. As previous noted, this stream has undergone partial condensation such that the stream is 98.24 mol% vapor.

The cooled methane-rich stripping gas fed into the lowermost stage (Stage 5) istaken from the upstream location depicted in FIGURE 1. This stream is cooled fromapproximately 63 °F to -10°F via countercurrent heat exchange with the heavies-rich liquid 15 stream produced from Stage 5. During such heat exchange as depicted in FIGURE 2, this streamis heated from approximately -78 °F to approximately 62 °F. This stream may also be employedto cool the overhead vapors from the demethanizer column. Presented in Table 2 are thesimulated températures, pressures, and relative flowrates of each phase on a stagewise basiswithin the column. Presented in Table 3 for each stage are the respective liquid and vapor 20 equilibrium compositions.

The warmed heavies-rich stream is then fed to the demethanizer column whichcontains rectifying and stripping sections wherefrom is produced a methane/ethane rich stream 52 011014 which preferably is recycled back as feed to the high stage inlet port on the methane compresserand a stream rich in natural gas liquids.

The efficiency of the process for aromatics/heavy removal is iilustrated by acomparison of the combined nitrogen, methane and ethane mole percentages in the feed streams 5 to Stages 1 and 5 and the product from Stage 1. These percentages for each stream arerespectively 99.88,99.89 and 99.94 mol percent The process therefore produces a productstream richer in these light components than either of the two gaseous feed streams.

The efficiency of the process for benzene and heavier aromatics removal isiilustrated by a comparison of the enrichment ratios which is defined to be the mole percent of 10 said component in the liquid product from Stage 5 divided by the mole percent of said component in the vapor product from Stage 1. Using benzene as an example, the respective molefractions are 0.1616E-04 and 0.00352. This results in an enrichment ratio of approximaiely 220.

An additional basis for illustrating the efficiency of the process are the enrichmentratios for the C3+ components in the feed streams to Stages 1 and 5 and the liquid product stream 15 produced from Stage 1. This ratio varies from about 45 for propane to about 200 for n-octane.The respective ratios between the product streams varies from about 50 for propane to about20,000 for n-octane.

EXAMPLE II

This Example, like that previously presented, shows via computer simulation the 20 efficiency of the process described in the spécification for the removal of benzene and heaviercomponents from a methane-based gas stream immediately prior to liquéfaction of the stream inmajor portion. The flowrates are représentative of those existing in a 2.5 million metrictonne/year LNG plant employing the liquéfaction technology set forth in FIGURES 1 and 2. The 53 011014 benzene concentrations in the methane-rich feed streams employed in this Example areconsidered to be représentative of the concentrations existing for many candidate gas streams atthis location in the process. However, the concentrations of ethane and heavier components inthe gas stream hâve been increased significantly thereby representing a richer gas stream and 5 placing a greater burden on the process for the removal of both these components and benzene.This example illustrâtes in greater detail the ability of the process to simultaneously removebenzene and heavier hydrocarbon components. In addition, this Example illustrâtes the ability ofthe benzene removal process to tolerate significant process upsets in the form of significantincreases in ethane and heavier hydrocarbon concentrations without significantly affecting the 10 efficiency and operability of the benzene removal process. Furthermore, this example illustrâtesthe ability of the process to recover heavies hydrocarbons as a separate liquefied stream.Simulation results were obtained using Hyprotech’s Process Simulation HYSIM, version386/C2.10, Prop. Pkg PR/LK.

Presented in Table 4 are the compositions, températures, pressures and phase 15 conditions of the influent and effluent streams to the heavies removal column. The simulation isbased upon the column containing 5 theoretical stages. The partially condensed stream, alsoreferred to as the two-phase stream, which will undergo liquéfaction in major proportion is firstfed to the uppermost stage in the column (Stage 1). The température of this stream is -91.2°Fand the pressure is 596.0 psia. As noted in the Spécification, this stream has undergone partial 20 condensation such that the stream is 94.04 mol% vapor.

The methane-rich stripping stream fed into the lowermost stage (Stage 5) is taken frora the upstream location depicted in FIGURE 1. This stream is cooled from approximately - 54 011014 10 F via countercurrent heat exchange with the liquid product stream produced from Stage 5. Asnoted in Table 4, this stream has undergone partial condensation in the course of cooling.

Presented in Table 5 are the simulated températures, pressures, and relativeflowrates of each phase on a stagewise basis within the column. Presented in Table 6 for each 5 stage are the respective liquid and vapor equilibrium compositions.

The efficiency of the process for heavies removal is illustrated by a comparison of the combined nitrogen, methane and ethane mole percentages in the feed streams respectively toStages 1 and 5 and the product stage from Stage 1. These percentages are respectively 97.85,97.30, and 99.37 mol percent. The process produces a product stream significantly richer in 10 these components than either of the two gaseous feed streams.

The efficiency of the process for benzene and heavier aromatics removal is illustrated by a comparison of the enrichment ratios which for benzene is as defined in Example1. The respective mole fractions are 0.003E-04 and 0.00923 thus resulting in an enrichmentratio of approximately 30. 15 An additional basis for illustrating the efficiency of the process are the enrichment ratios for the C3+ components in the feed streams to Stages 1 and 5 and the liquid product streamproduced from Stage 1. This ratio varies from about 19 for propane to about 30 for n-octane.

The respective ratios between the product streams varies from about 67 for propane to about 19,000 for n-octane. 55 011014 TABLE 1 FEEDSTREAM AND SIMULATED PRODUCT STREAMCOMPOSITIONS AND PROPERTIES Feed Streams* Product Streams* - Stage 1 Stage 5 Stage 1 Stage 5 Nitrogen 0.0022 0.0007 0.002169 0.000107 CO2 0.7587E-04 0.8806E-04 0.000075 0.000279 Methane 0.9726 0.9686 0.974167 0.559178 Ethane 0.0242 0.0296 0.023043 0.357346 Ethylene 0.0000 0.0000 0.000000 0.000000 Propane 0.0005 0.0006 0.000404 0.026993 i-Butane 0.8998E-04 0.0001 0.000055 0.009050 n-Bùtane 0.0001 0.0001 0.000059 0.013291 i-Pentane 0.3442E-04 0.4031E-04 0.000011 0.006026 n-Pentane 0.3340E-04 0.4031E-04 0.881E-05 0.006391 n-Hexane 0.2424E-04 0.3023E-04 0257E-05 0.005627 n-Heptane Û.3230E-04 0.4031E-04 0.125E-05 0.008054 n-Octane 0.1615ΕΌ4 0.2015E-04 0.221E-06 0.004132 Benzene 0.1616E-04 0.2015E-04 0.258E-05 0.003526 n-Nonane 0.0000 0.0000 0.000000 0.000000 Température -112.45°F -10.00°F -112.32’F -78.09’F Pressure 587.01 psia 601.00 psia 587.00 psia 589.00 psia Vapor % 98.24% 100% 100% 0.00% Flowrate (lb mole/hr) 60347.00 1203.0 61311.53 238.46 'Compositions are on mole fraction basis. 56 011014 TABLE2 SIMULATION RESULTS OF FLOW CHARACTERISTICS AND FLUED PROPERTIES WITHÏN THE COLUMN Stage No. Pressure psia Température op FIow Rates (1b mole/hr) Liquid Vapor Feed Product Streams 1 587.0 -112.3 1060.3 60347.0' 61311.52 2 587.5 -108.2 917.8 2024.9 3 588.0 -101.1 761.5 1882.4 4 588.5 -90.8 619.0 1726.1 5 589.0 -78.1 1583.5 1203.03 238.54 'Feed to Stage 1 is 98.24 mol % vapor. ’Prodùçt removed from Stage 1,100 mol % vapor. 3Feed to Stage 5,100 mol % vapor. 4Product removed from Stage 5,0 molVo vapor. 57 011014 TABLE 3 10 SIMULATED LIQŒD/VAPOR STREAM COMPOSITIONSLEAVING EACH THEORETICAL STAGE (Mole Fraction)

Nitrogen COj Methane Etbane Propane i-Butane n-Butane Stage 1 Vapor 0.002169 0.00075 0.974167 0.023043 0.000404 0.000055 0.000055 Liquid 0.000772 0.000173 0.874962 0.105444 0.006229 0.002030 0.002965 Stage 2 Nsçqt 0.000811 0.000110 0.967766 0.030734 0.000436 0.000057 0.000059 Liquid 0.000263 0.000252 0.832784 0.145068 0.007288 0.002348 0.003425 Stage 3 Vapor 0.000565 0.000144 0.954226 0.044398 0.000514 0.000063 0.000064 Liquid 0.000159 0.000317 0.761049 0211924 0.009202 0.002861 0.004152 Stage 4 Vapor 0.000547 0.000163 0.933571 0.064781 0.000745 0.000082 0.000080 Liquid 0.000131 0.000329 0.669188 0.295174 0.013204 0.003786 0.005372 Stage 5 Vapor 0.000571 0.000154 0.913194 0.084077 0.001548 0.000194 0.000191 Liquid 0.000107 0.000279 0.559178 0.357346 0.026933 0.009050 0.013291 15 58 TABLE 3 011014 SIMULATED LIQUEDZVAPOR STREAM COMPOSITIONSLEAVING EACH THEORETICAL STAGE (Mole Fraction)(CONTINUED) î-Pentane n-Pentane n-Hexane n-Heptane n-Octane Bcnzene Stage 1 Vapor 0.000011 8.81E-06 2.57E-06 1.25E-06 2.21E-07 2.58E-06 Liquid 0.001331 0.001408 0.001236 0.001768 0.000907 0.000775 Stage 2 Vapor 0.000011 8.54E-06 2.39E-06 1.12E-06 1.90E-07 2.35E-06 Liquid 0.001536 0.001625 0.001427 0.002042 0.001047 0.000894 Stage 3 Vapor 0.000011 8.64E-06 2.30E-06 1.03E-06 1.68E-07 2.17E-06 Liquid 0.001854 0.001961 0.001720 0.002461 0.01262 0.001078 Stage 4 Vapor 0.000014 0.000010 2.60E-06 1.14E-06 1.80E-07 231E-06 Liquid 0.002328 0.002446 0.002125 0.003031 0.001554 0.001332 Stage 5 Vapor 0.000033 0.000024 6.08E-06 2.57E-06 3.93E-07 “ 4.83E-O6 Liquid 0.006026 0.006391 0.005627 0.008054 0.004132 0.003526 59 TABLE 4 011014 FEEDSTREAM AND SIMULATED PRODUCT STEAMCOMPOSITIONS AND PROPERTIES (Mole Fraction)

Feed Streams' Product Streams’ Stage 1 Stage 5 Stage 1 Stage 5 Nitrogen 0.0024 0.0006 0.002301 0.000060 CO2 0.7074E-04 0.8851E-04 0.000072 0.000106 Methane 0.9478 0.9361 0.966005 0.346889 Ethane 0.0283 0.0363 0.025421 0.145714 Ethylene 0.0000 0.0000 0.000000 0.000000 Propane 0.0120 0.0145 0.005277 0.227598 i-Butane 0.0024 0.0030 0.000467 0.062744 n-Butane 0.0028 0.0036 0.000367 0.078635 i-Pentane 0.0010 0.0013 0.000049 0.030295 n-Pentane 0.0008 0.0011 0.000026 0.024383 n-Hexane 0.0013 0.0018 0.000012 0.043792 n-Heptane 0.0007 0.0010 0.170E-05 0.024376 n-Octane 0.0002 0.0003 0.111E-06 0.006019 Benzene 0.0003 0.0004 0.283E-05 0.009229 n-Nonane 0.4853E-05 0.6724E-05 0.851E-09 0.000160 Température -9120’F -10.00’F -88.19’F -31.98°F Pressure 596.01 psia 610 psia 596.00 psia 598.00 psia Vapor % 94.04% 98.94% 100% 0.00% Flowrate (Ib mole/hr) 57109.78 7668.00 62724.19 2053.60 'Compositions are on mole fraction basis 60 011014 TABLE 5 SIMULATION RESULTS OF FLOW CHARACTERISTICS AND FLUID PROPERTIES WITHIN THE COLUMN Stage No. Pressure psia Température °F Flow Rates (lb mole/hr) Liquid Vapor Feed Product Streams 1 596.0 -88.2 3345.9 57109.8* 62724.22 2 596.5 -67.6 2905.8 8960.3 3 597.0 -52.5 2680.0 8520.2 4 597.5 -42.3 2439.5 8294.4 5 598.0 -32.0 8053.9 7668.03 2053.64 'Feed to Stage 1 is 94.04 mol % vapor. 2Product removed from Stage 1,100 mol % vapor. 3Feed to Stage 5, 98.94 mol % vapor. «Product removed from Stage 5, 0 mol % vapor. 61 01 T 01 4 TABLE 6 SIMULA TED LIQUID/VAPOR STREAM COMPOSITIONSLEAVING EACH THEORETICAL STAGE (Mole Fraction) I Nitrogen | co2 Méthane | Ethane 1 Propane j i-Butane j n-Bntane Stage 1 Vapor 0.00231 0.000072 0.966005 0.025421 0.005277 0.000467 0.000367 Liquid 0.000359 0.000153 0.589261 0.132705 0.130329 0.033700 0.041711 Stage 2 Vapor 0.000640 0.000108 0.941610 0.047192 0.008898 0.000776 0.000615 Liquid 0.000085 0.000178 0.476845 0.190340 0.161161 0.039734 0.048783 Stage 3 Vapor 0.000561 0.000115 0.921470 0.062431 0.013142 0.001134 0.000905 Liquid 0.000069 0.000157 0.415375 0.208673 0.187549 0.044244 0.053820 Stage 4 Vapor 0.000569 0.000106 0.913713 0.064872 0.017638 0.001540 0.001229 Liquid 0.000065 0.000130 0.380377 0.191896 0216335 0.050645 0.061013 Stage 5 Vapor 0.000583 0.000097 0.917993 0.055497 0.021253 0.002204 0.001837 Liquid 0.000060 0.000106 0.346889 0.145714 0227598 0.062744 0.078635 62 011014 TABLE 6 SIMULATED LIQUID/VAPOR STREAM COMPOSITIONS LEAVING EACH THEORETICAL STAGE (Mole Fraction) (CONTINUED) - - î-Pentane n-Pentane n-Hexane n-Heptane n-Octane Benzene n-Nonane S tage 1 Vapor 0.000049 0.000026 0.000012 1.70E-06 1.11E-07 2.83E-06 8.51E-10 Liquid 0.015796 0.012679 0.022699 0.012625 0.003116 0.004784 0.000083 Stage 2 Vapor 0.000084 0.000046 0,000021 3.26E-06 2.23E-07 4.90E-06 I.78E-O9 Liquid 0.018298 0.014662 0.026170 0.014543 0.003588 0.005516 0.000095 Stage 3 Vapor 0.000126 0.000069 0.000034 5.40E-06 3.87E-07 7.60ΕΌ6 3.21E-09 Liquid 0.019970 0.015971 0.028414 0.015775 0.003891 0.005988 0.000103 Stage 4 Vapor 0.000171 0.000095 0.000047 7.71E-06 5.67E-07 0.000010 4.82E-09 Liquid 0.022257 0.017730 0.031314 0.017348 0.004276 0.006598 0.000114 Stage 5 Vapor 0.000273 0.000154 0.000079 0.000013 9.77E-07 0.000017· 8.41E-09 Liquid 0.030295 0.024383 0.043792 0.024376 0.006019 0.009229 0.000160

Claims (47)

1. 011014 The daims defining the invention are as follows: 1 · A process for removing and concentrating the higher molecular weight hydrocarbon species from a methane-based gas stream including the steps of: (a) condensing a minor portion of the methane-based gas stream thereby producing a two-phase stream; - - (b) feeding said two-phase stream into the upper section of a column; (c) removing from the upper section of said column a heavies-depleted gas stream; (d) removing from the lower section of said column a heavies-rich liquid stream; (e) contacting via indirect heat exchange the heavies-rich liquid streamwith a methane-rich stripping gas stream thereby producing a warmed heavies-richstream and a cooled methane-rich stripping gas stream; (f) feeding said cooled methane-rich stripping gas stream to the lowersection of the column; and (g) contacting the two-phase stream and the cooled methane-richstripping gas stream in said column thereby producing the heavies-depleted gas streamand the heavies-rich liquid stream.
2. 2. A process according to claim 1, wherein step (a) is comprised of splittingthe methane-based gas stream into a first stream and a second stream, cooling said firststream thereby producing a partially condensed first stream, and combining saidpartially condensed first stream with the second stream thereby producing said two-phase stream.
3. 3. A process according to claim 2, wherein the amount of liquids in saidtwo-phase stream is controlled by determining for the methane-based gas stream a two-phase stream température corresponding to the desired liquids content at equilibriumconditions, measuring the température of the two-phase stream, maintaining constantthe flowrate of the first stream and the amount of cooling imparted to said stream, andadjusting the flowrate of said second stream responsive to the two-phase streamtempérature such that the two-phase stream température approximates the calculatedtwo-phase stream température.
4. 4. A process according to any of the preceding daims, additionallyincluding the step of 64 011014 (h) sequentially cooling the methane-based gas stream prior to step (a)by flowing said stream through at least one indirect heat exchange means in contactwith a first réfrigérant stream thereby producing a cooled methane-based gas streamthrough at least one indirect heat exchânge means in contact with a second réfrigérantstream wherein the boiling point of the second réfrigérant stream is less than the 'boiling point of the first refrigerating stream thereby producing the feedstream to step(a).
5. 5. A process according to claim 4, wherein said first réfrigérant stream iscomprised in major portion of propane and said second réfrigérant stream is comprisedin major portion of ethane, ethylene or a mixture thereof.
6. 6. A process according to claim 4 or 5, further comprising: (i) withdrawing a side stream ffom the methane-based gas stream at alocation downstream of one of the indirect heat exchange means and employing saidside stream as the methane-rich stripping gas in step (e).
7. 7. A process according to any of daims 4, 5 or 6, wherein said cooling byat least one indirect heat exchange means in contact with a first réfrigérant stream iscomprised of flowing said gas stream to be cooled through two or more indirect heatexchange means in a sequential manner and wherein the first réfrigérant to each suchindirect heat exchange means has been flashed to a progressively lower température andpressure in a sequentially consistent manner and wherein said cooling by at least oneindirect heat exchange means in contact with a second réfrigérant stream is comprisedof flowing said gas stream to be cooled through two or more indirect heat exchangemeans in a sequential manner and wherein the second réfrigérant to each indirect heatexchange means has been flashed to a progressively lower température and pressure ina sequentially consistent manner.
8. 8. A process according to claim 7, wherein three indirect heat exchangemeans are employed for cooling by the first réfrigérant stream and two or three indirectheat exchange means are employed for cooling by the second réfrigérant stream.
9. 9. A process according to claim 7 or 8, wherein the pressure of themethane-based feed gas is 3450 to 6210 kPa (500 to 900 psia).
10. 10. A process according to claim 7, 8 or 9, wherein the pressure of themethane-based feed gas is 3970-4485 kPa (about 575 to about 650 psia).
11. 11. A process according to any of the preceding daims, additionally 65 011014 including (j) feeding the warmed heavies-rich stream of step (e) to a demethanizercomprised of a fractionator, a reboiler and a condenser thereby producing a heavies-richliquid stream and a methane-rich vapor stream.
12. 12. A process according to claim 1Γ, wherein a major portion of the coolingduty for the condenser is provided by the heavies-rich liquid stream produced by step(d) or step (e).
13. 13. A process according to claim 11, wherein a major portion of the coolingduty for the condenser is provided by flowing through an indirect heat exchange meansin contact with the heavies-rich liquid stream of step (d) and the resulting treatedheavies-rich liquid stream becomes the heavies-bearing feedstream to step (e).
14. 14. A process according to claim 12, wherein the cooling duty is providedby splitting the overhead vapor stream into a first vapor stream and a second vaporstream, cooling and partially condensing said first stream via indirect heat exchangewith the heavies rich liquid stream of step (d) thereby producing a cooled, partiallycondensed first stream, combining said first stream and said second stream, feeding saidcombined stream to a gas-liquid separator from which is produced the reflux stream tothe fractionating column and the methane-rich vapor stream.
15. 15. A process according to claim 14, wherein the flowrate of the refluxstream is controlled by calculating for the overhead vapor stream a two-phase streamtempérature corresponding to the desired liquids content at equilibrium conditions,measuring the température of the two-phase stream, maintaining constant the flowrateof the first stream and the amount of cooling imparted to said stream, and adjusting theflowrate of said second stream responsive to the two-phase stream température suchthat the calculated two-phase stream température is approached.
16. 16. A process according to claim 12, additionally including between steps (d)and (e) the additional step of: (k) flashing the heavies-rich liquid stream to a lower pressure therebyfurther decreasing the température of said stream.
17. 17. A process according to claim 16, additionally including the step of (l) condensing the heavies depleted gas stream thereby producing aliquefied natural gas stream.
18. 18. A process according to claim 17, wherein said condensing is comprised 66 011014 of flowing the heavies depleted gas stream through an indirect heat exchange meanscooled by said second réfrigérant stream.
19. 19. A process according to claim 18, wherein the pressure of the methane-based gas stream is 3450 to 6210 kPa (500 to 900 psia).
20. 20. A process according to claim 19, additionally including the steps of (m) flashing in one or more steps the liquefied product of step (1) toapproximately atmospheric pressure thereby producing an LNG product stream and oneor more methane vapor streams; (n) compressing a majority of the vapor streams of step (m) to apressure of 3450 to 6210 kPa (500 to 900 psia); (o) cooling said compressed vapor stream of step (n); and (p) combining the resulting cooled stream with the methane-based gasstream fed to step (a) or the resulting product from one of the indirect heat exchangemeans of step (j).
21. 21. A process according to claim 20, wherein the methane-fich vapor streamof step (h) is combined with one of the vapor streams of step (m) prior to step (n).
22. 22. A process according to claim 20, wherein the pressure of the methane-based feed gas and the gas stream from step (n) is about 575 to about 650 psia.
23. 23. A process according to any of the preceding daims, wherein the columnprovides two to fifteen theoretical stages of gas-liquid contacting.
24. 24. A process according to any of the preceding daims, wherein the columnprovides three to ten theoretical stages of gas-liquid contacting.
25. 25. A process according to claim 22, wherein the column provides two tofifteen theoretical stages of gas-liquid contacting.
26. 26. A process according to claim 22, wherein the column provides three toten theoretical stages of gas-liquid contacting.
27. 27. A process according to any one of the preceding daims, wherein saidhigher molecular weight hydrocarbon species that are removed and concentrated includebenzene and other aromatics and there are produced a benzene/aromatic depleted gasstream and a benzene/aromatic rich liquid stream.
28. 28. . An apparatus including: (a) a condenser; (b) a column; 67 011014 (c) a heat exchanger providing for indirect heat exchange between two fluids; (d) a conduit between said condenser and the upper section of thecolumn for flow of a two-phase stream to the column; (e) a second conduit connected to the upper section of the column-forthe removal of a vapor stream from the column; (f) a conduit between said column and heat exchanger for flow of acooled gas stream from the heat exchanger; (g) a conduit between said column and said heat exchanger for flow ofa liquid stream from the column; (h) a conduit connected to the heat exchanger for flow of a warmedliquid stream from the heat exchanger; and (i) a conduit connect to the heat exchanger for flow of a gas stream tothe heat exchange.
29. 29. An apparatus according to claim 28, additionally including (j) a first conduit; (k) a splitting means connected to the first conduit; (l) a second conduit and a third conduit connected to said splittingmeans where said second conduit is connected to the condenser; (m) a control valve connected at the inlet side to the second conduit; (n) a conduit connected to the outlet side of said control valve; (o) a junction or combining means connected to said conduit of element(n) and the conduit of element (d) prior to connection with the column; (p) a température sensing means with sensing element situated inconduit of element (d) between said junction means and connection with the column;and (q) a control means operably attached to control valve of element (m)and operably responsive to input received from the température sensing device ofelement (p) and a température setpoint.
30. 30. An apparatus according to claim 28, additionally including (r) a pressure réduction means situated in conduit (g).
31. 31. An apparatus according to claim 28, 29 or 30, wherein said columncontains 2 to 12 theoretical stages. 68 011014
32. 32. An apparatus according to claim 28, 29, 30 or 31, additionally includingone or more indirect heat exchange means situated in a sequential manner, conduitsbetween each heat exchange means for the sequential flow of a common fluid throughthe heat exchangers whereupon the last conduit is connected to the condenser ofelement (a), conduits to and from each heat exchanger providing for the flow of à'refrigerating agent to each heat exchanger and wherein the conduit of element (i) is inflow communication with one of the above conduits for flow of a common fluidbetween heat exchangers.
33. 33. An apparatus according to claim 32, wherein propane is employed as therefrigerating agent in at least two of the heat exchange means; and ethane, ethylene or amixture thereof is employed as the refrigerating agent in at least two heat exchangemeans.
34. 34. An apparatus according to any one of daims 30-33, additionallyincluding: (s) a fractionation column; (t) a reboiler; (u) a condenser; (v) an overhead conduit connecting the upper section of the column tothe condenser for removal of the overhead vapor, a reflux conduit connected thecondenser to the column for the return of the reflux fluid, a vapor product conduitconnected to the condenser for removal of uncondensed vapors; (w) a bottoms conduit connecting the lower section of the column to thereboiler, a vapor conduit for retuming stripping vapor to the column, and a bottomsproduct line connected to the reboiler for removal of unvaporized product from thereboiler; and wherein the conduit of element (h) is connected to the fractionation column at a pointbetween the top and the bottom theoretical stages.
35. 35. An apparatus according to claim 34, wherein the condenser of element(u) is comprised of an indirect heat exchange means and coolant to such means isprovided by a junction connecting the cooling side of the indirect heat exchange meansto the conduit of element (g).
36. 36. An apparatus according to claim 34 or 35, wherein the condenser ofelement (u) is comprised of an indirect heat exchange means and said coolant to such 69 011014 means is provided by a junction connecting the cooling side of the indirect heatexchange means to the conduit of element (q) downstream of pressure réduction means(r).
37. 37. An apparatus according to claim 34, 35 or 36, additionally including (x) a conduit connected to condenser of element (a); - - (y) a compressor connected at the inlet port to the vapor conduit line ofelement (v); and (z) a conduit connecting the outlet port of said compressor element (z)to the conduit of element (y).
38. 38. An apparatus according to claim 32, additionally including: (ad) a fractionation column; (ab) a reboiler; (ac) a condenser; (ad) an overhead conduit connecting thé upper section of the column tothe condenser for removal of the overhead vapor, a reflux conduit connected thecondenser to the column for the return of the reflux fluid, a vapor product conduitconnected to the condenser for removal of uncondensed vapors; (ae) a bottoms conduit connecting the lower section of the column tothe reboiler, a vapor conduit for returning stripping vapor to the column, and a bottomsproduct line connected to the reboiler for removal of unvaporized product from thereboiler; and wherein the conduit of element (h) is connected to the fractionationcolumn at a midpoint location.
39. 39. An apparatus according to claim 38, additionally comprising (af) a compressor connected at the inlet port to the vapor conduit line ofelement (ad) and (ag) conduit connecting the outlet port of said compressor to one of thecommon flow conduits of claim 32.

    39. Apparatus comprising: . (a) a cryogénie séparation column for partially condensing a feed gasstream in an LNG recovery process; (b) means for withdrawing a liquid condensate stream from saidcryogénie séparation column; 70 011014 (c) a heat exchanger associated with said cryogénie séparation coluxnn; (d) means for passing said liquid condensate stream through said heat exchanger; (e) means for passing a warm dry gas stream through said heatexchanger and thereafter to said cryogénie séparation column, wherein said warmdrygas stream is cooled by indirect heat exchange with said liquid condensate stream insaid heat exchanger; (f) a bypass conduit having a first control valve operably located thereinfor bypassing said warm dry gas stream around said heat exchanger; (g) means for establishing a first signal représentative of the actualtempérature of said warm dry gas stream exiting said heat exchanger; (h) means for establishing a second signal représentative of the actualtempérature of said liquid condensate stream entering said heat exchanger; (i) means for dividing said first signal by said second signal to establisha third signal représentative of the ratio of said first signal and said second signal; (j) means for establishing a fourth signal représentative of a desiredvalue for the ratio represented by said third signal; (k) means for comparing said third signal and said fourth signal andestablishing a fifth signal which is responsive to the différence of said third signal andsaid fourth signal, wherein said fifth signal is scaled to be représentative of the positionof said first control valve required to maintain the actual ratio represented by said thirdsignal substantially equal to the desired ratio represented by said fourth signal; and (m) means for manipulating said first control valve in said bypassconduit in response to said fifth signal.
40. 40. Apparatus in accordance with claim 39, additionally comprising:means for establishing a sixth signal scaled to be représentative of the flow rate of said liquid condensate stream required to maintain a desired liquid level insaid cryogénie séparation column; and means for controlling the flow rate of said liquid condensate streamresponsive to said sixth signal.
41. 41. Apparatus in accordance with claim 40, additionally comprising: a second control valve operably located so as to control flow of said warm dry gas stream; and 71 011014 means for manipulating said second control valve responsive to atempérature selected from the pair of températures consisting of: i. the actual température of said warm dry gas stream exiting said heatexchanger; and ii. the actual température of said liquid condensate stream exiting saidheat exchanger.
42. 42. Apparatus in accordance with claim 41, wherein said means for manipulating said second control valve comprises: means for establishing a seventh signal représentative of the actualtempérature of said liquid condensate stream exiting said heat exchanger; means for establishing an eighth signal représentative of the desiredtempérature of said liquid condensate stream exiting said heat exchanger; means for comparing said seventh signal and said eighth signal toestablish a ninth signal responsive to the différence of Said seventh signal and saideighth signal, wherein said ninth signal is scaled to be représentative of the position ofsaid second control valve required to maintain the actual température of said liquidcondensate stream exiting said heat exchanger represented by said seventh signalsubstantially equal to the desired température represented by said eighth signal; means for establishing a tenth signal représentative of the desiredtempérature of said warm dry gas stream exiting said heat exchanger represented bysaid second signal; means for comparing said second signal and said tenth signal to establishan eleventh signal responsive to the différence between said second signal and saidtenth signal, wherein said eleventh signal is scaled to be représentative of the positionof said second control valve required to maintain the actual température of said warmdry gas stream exiting said heat exchanger substantially equal to the desired valuerepresented by said tenth signal; means for establishing a twelfth signal selected as the one of said ninthsignal and said eleventh signal having the higher value; and means for manipulating said second control valve responsive to said twelfth signal.
43. 43. A method for controlling température in a heat exchanger equipped with a bypass conduit having a first control valve operatively connected therein, said heat 72 011014 exchanger being associated with a cryogénie séparation column that removes a benzenecontaminant from a feed stream in and LNG recovery process, said method comprising: withdrawing a liquid condensate stream at a cryogénie température fromsaid cryogénie séparation column; passing said liquid condensate stream through said heat exchanger; -passing a warm dry gas stream through said heat exchanger and thereafter introducing said warm dry gas stream into said cryogénie séparation column,wherein said warm dry gas stream is cooled by indirect heat exchange with said liquidcondensate stream in said heat exchanger; establishing a first signal représentative of the actual température of saidwarm dry gas stream exiting said heat exchanger; establishing a second signal représentative of the actual température ofsaid liquid condensate stream entering said heat exchanger; dividing said first signal by said second signal to establish a third signalreprésentative of the ratio of said first signal and said second signal; establishing a fourth signal représentative of a desired value for said third signal; comparing said third signal and said fourth signal and establishing a fifthsignal which is responsive to the différence between said third signal and said fourthsignal, wherein said fifth signal is scaled to be représentative of the position of saidfirst control valve required to maintain the actual ratio represented by said third signalsubstantially equal to the desired ratio represented by said fourth signal; and manipulating said first control valve in said bypass conduit in responseto said fifth signal.
44. 44. A method in accordance with claim 43, additionally comprising thefoliowing steps: establishing a sixth signal scaled to be représentative of the flow rate ofsaid liquid condensate steam required to maintain a desired liquid level in saidcryogénie séparation column; and controlling the flow rate of said liquid condensate stream responsive tosaid sixth signal.
45. 45. A method in accordance with claim 43 or 44, wherein a second controlvalve is operably located so as to control flow rate of said warm dry gas stream, said 75 011014 method additionally comprising the following steps: manîpulating said second control valve responsive to a température selected from the pair of températures consisting of: i) the actual température of said warm dry gas stream exiting said heat exchanger; and - - ii) the actual température of said liquid condensate stream exiting saidheat exchanger.
46. 46. A method in accordance with claim 45, wherein said step of manîpulating said second control valve comprises: establishing a seventh signal représentative of the actual température of said liquid condensate stream exiting said heat exchanger; establishing an eighth signal représentative of the desired température of said liquid condensate stream exiting said heat exchanger; comparing said seventh signal and said eighth signal to establish a ninth signal responsive to the différence between said seventh signal and said eighth signal,wherein said ninth signal is scaled to be représentative of the position of said secondcontrol valve required to maintain the actual température of said liquid condensatestream exiting said heat exchanger represented by said seventh signal substantiallyequal to the desired température represented by said eighth signal; establishing a tenth signal représentative of the desired température ofsaid warm dry gas stream exiting said heat exchanger represented by said secondsignal; comparing said second signal and said tenth signal to establish aneleventh signal responsive to the différence between said second signal and said tenthsignal, wherein said eleventh signal is scaled to be représentative of the position of saidsecond control valve required to maintain the actual température of said warm dry gasstream exiting said heat exchanger substantially equal to the desired value representedby said tenth signal; establishing a twelfth signal selected as the one of said ninth signal andsaid eleventh signal having the higher value; and manîpulating said second control valve responsive to said twelfth signal.
47. 47. A method in accordance with any of daims 43-46, wherein said LNG recovery process is a cascade réfrigération process employing three different 74 01101 4 réfrigérants. •t