WO2001088447A1 - Recuperation amelioree de liquides de gaz naturels (ngl) au moyen de refrigeration et de reflux provenant des installations de gaz naturel liquefie (lng) - Google Patents

Recuperation amelioree de liquides de gaz naturels (ngl) au moyen de refrigeration et de reflux provenant des installations de gaz naturel liquefie (lng) Download PDF

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Publication number
WO2001088447A1
WO2001088447A1 PCT/US2001/015721 US0115721W WO0188447A1 WO 2001088447 A1 WO2001088447 A1 WO 2001088447A1 US 0115721 W US0115721 W US 0115721W WO 0188447 A1 WO0188447 A1 WO 0188447A1
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WO
WIPO (PCT)
Prior art keywords
sfream
stream
gas
ngl
methane
Prior art date
Application number
PCT/US2001/015721
Other languages
English (en)
Inventor
Rong-Jwyn Lee
Jame Yao
Jong Juh Chen
Douglas G. Elliot
Original Assignee
Phillips Petroleum Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US09/733,533 external-priority patent/US6401486B1/en
Application filed by Phillips Petroleum Company filed Critical Phillips Petroleum Company
Priority to BRPI0110912-0A priority Critical patent/BR0110912B1/pt
Priority to DZ013341A priority patent/DZ3341A1/fr
Priority to AU6163301A priority patent/AU6163301A/xx
Priority to AU2001261633A priority patent/AU2001261633B2/en
Publication of WO2001088447A1 publication Critical patent/WO2001088447A1/fr

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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0242—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 3 carbon atoms or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0032—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/0035—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by gas expansion with extraction of work
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0032—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/004—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by flash gas recovery
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    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
    • F25J1/0052—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream
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    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
    • F25J1/0052—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream
    • F25J1/0055—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream originating from an incorporated cascade
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
    • F25J1/008—Hydrocarbons
    • F25J1/0085—Ethane; Ethylene
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
    • F25J1/008—Hydrocarbons
    • F25J1/0087—Propane; Propylene
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0203—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle
    • F25J1/0208—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop
    • F25J1/0209—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop as at least a three level refrigeration cascade
    • F25J1/021—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop as at least a three level refrigeration cascade using a deep flash recycle loop
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0211—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle
    • F25J1/0212—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a single flow MCR cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J1/0214—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle
    • F25J1/0215—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle with one SCR cycle
    • F25J1/0216—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle with one SCR cycle using a C3 pre-cooling cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
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    • F25J1/0219—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. using a deep flash recycle loop
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0228—Coupling of the liquefaction unit to other units or processes, so-called integrated processes
    • F25J1/0229—Integration with a unit for using hydrocarbons, e.g. consuming hydrocarbons as feed stock
    • F25J1/0231—Integration with a unit for using hydrocarbons, e.g. consuming hydrocarbons as feed stock for the working-up of the hydrocarbon feed, e.g. reinjection of heavier hydrocarbons into the liquefied gas
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    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J1/0235—Heat exchange integration
    • F25J1/0237—Heat exchange integration integrating refrigeration provided for liquefaction and purification/treatment of the gas to be liquefied, e.g. heavy hydrocarbon removal from natural gas
    • F25J1/0238—Purification or treatment step is integrated within one refrigeration cycle only, i.e. the same or single refrigeration cycle provides feed gas cooling (if present) and overhead gas cooling
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    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/32—Details on header or distribution passages of heat exchangers, e.g. of reboiler-condenser or plate heat exchangers

Definitions

  • the present invention relates generally to methods and apparatus for high recovery of hydrocarbon liquids from methane-rich natural gases and other gases, e.g., refinery gases. More particularly, the present invention provides methods and apparatus for more efficiently and economically achieving high recovery of ethane, propane, propylene and heavier hydrocarbon liquids (C 2+ hydrocarbons) in association with liquefied natural gas production.
  • natural gas includes some heavier hydrocarbons and impurities, e.g., carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas may be further processed to separate and recover heavier hydrocarbons as natural gas liquids (NGL's) thereby producing pipeline quality methane. The pipeline quality methane is then delivered to gas pipelines as the sales gas ultimately transmitted to end-users.
  • NNL's natural gas liquids
  • Natural gas comprising predominantly methane enters a LNG plant at elevated pressures and is pretreated to produce a purified feed stock suitable for liquefaction at cryogenic temperatures.
  • the pretreatment typically includes removal of acid gases, e.g., hydrogen sulfide and carbon dioxide, together with other contaminants, including moisture and mercury.
  • the purified gas is thereafter processed through a plurality of cooling stages using indirect heat exchange with one or more refrigerants to progressively reduce its temperature until total liquefaction is achieved.
  • the pressurized liquid natural gas is sub-cooled to reduce flashed vapor through one or more expansion stages to final atmospheric pressure suitable for storage and transportation.
  • the flashed vapor from each expansion stage, together with the boil off gas produced as a result of heat gain, are collected and used as a source of plant fuel gas with any excess recycled to the liquefaction process.
  • the cascade process presents the benefits of easy start-up and easy shutdown.
  • the cascade process consists of successive refrigeration cycles using propane, ethane or ethylene, and methane.
  • the thermal efficiency can be readily enhanced by the use of multi-stage compressors.
  • the propane pre-cooled mixed refrigerant process involves the use of a multi-component mixture of hydrocarbons, typically comprismg propane, ethane, methane, and optionally other light components in one cycle, and a separate propane refrigeration cycle to provide pre-cooling of natural gas and the mixed refrigerant to about -35°F.
  • the propane mixed refrigerant process advantageously provides improved thermal efficiency.
  • a significant disadvantage results from the use of extremely large spiral wound exchangers. Such exchangers are a long lead item requiring special facilities in the field to manufacture.
  • Examples of the propane mixed refrigerant process include those disclosed in United States Patent Nos. 4,404,008 and 4,445,916, incorporated herein by reference.
  • the single, mixed refrigerant process includes heavier hydrocarbons, e.g., butanes and pentanes, in the multi-component mixture and eliminates the pre-cooled propane refrigeration cycle. It presents the simplicity of single compression in the heat exchanger line and is particularly advantageous for small LNG plants.
  • turbo expander in combination with mechanical refrigeration cycles has also been adopted in many LNG processes.
  • Examples of the use of a turbo expander are disclosed in United States Patent Nos. 3,724,226; 4,065,278; 5,755,114;4,970,867, 5,537,827; and M'l Patent No. WO 95/27179.
  • natural gas typically contains various amounts of ethane, propane and heavier hydrocarbons.
  • the composition varies significantly depending on the source of the gas and gas reserve characteristics. Hydrocarbons heavier than methane need to be removed from LNG for various reasons. Hydrocarbons heavier than pentane, including aromatics, having high freezing points must be reduced to an extremely low level to prevent the freezing and plugging of process equipment in the course of the cooling and liquefaction steps. After separation of these heavy components from LNG, they provide excellent gasoline blending stock. Many patents have been directed to methods for removal of these heavy hydrocarbons. For instance, United States Patent No.
  • 5,325,673 discloses the use of a single scrub column in the pretreatment step operated substantially as an absorption column to remove freezable C 5+ components from a natural gas stream feeding to an LNG facility.
  • the recovered heavy liquid can subsequently be fractionated into various fractions for use as make-up refrigerants.
  • United States Patent No. 5,737,940 describes an exemplary system incorporated in a cascade process.
  • lighter natural gas liquid (NGL) components e.g., hydrocarbons having 2-4 carbon atoms
  • NGL natural gas liquid
  • hydrocarbons having 2-4 carbon atoms can also be a source of feedstock to refineries or petrochemical plants. Therefore, it is often desirable to maximize the recovery of NGL to enhance revenue.
  • NGL recovery plant it is common practice to design an NGL recovery plant so that the tail gas produced by the NGL recovery plant and comprising primarily methane is delivered to the LNG facility for liquefaction.
  • United States Patent Nos. 5,291,736 and 5,950,453 are typical examples of such combined facilities.
  • the cryogenic expansion process has become the preferred process for deep hydrocarbon liquid recovery.
  • the feed gas at elevated pressure is pretreated for the removal of acid gases, moisture and other contaminants to produce a purified feed stock suitable for further processing at cryogenic temperatures.
  • the purified feed gas is then cooled to partial condensation by heat exchange with other process streams and/or external propane refrigeration, depending upon the richness of the gas.
  • the condensed liquid after removal of the less volatile components is then separated and fed to a fractionation column, operated at medium or low pressure, to recover the heavy hydrocarbon constituents desired.
  • the remaining non-condensed vapor portion is turbo-expanded to a lower pressure, resulting in further cooling and additional liquid condensation.
  • the resultant two-phase stream is fed to the top section of the fractionation column where the cold liquids act as the top reflux to enhance recovery of heavier hydrocarbon components.
  • the remaining vapor combines with the column overhead as a residue gas, which is then recompressed to a higher pressure suitable for pipeline delivery or for liquefaction in an LNG facility after being heated to recover available refiigeration.
  • a column operated as described above acts mainly as a stripping column, the expander discharge vapor leaving the column overhead that is not subject to rectification still contains many heavy components. These components could be further recovered through an additional rectification step.
  • the NGL recovery column in these systems is often required to operate at a relatively high pressure, typically above 550 psig, in order to maintain an efficient and economical utilization of mechanical refrigeration employed in the LNG process. While benefitting from lower refrigeration energy by maintaining a high liquefaction pressure, the separation efficiency within the recovery column may be significantly reduced due to less favorable separating conditions, i.e., lower relative volatility inside the column, hi addition, prior art processes fail to effectively provide reflux to the recovery column. As a result, none of these processes are capable of efficiently maintaining a high NGL recovery, i.e., the NGL recovery does not typically exceed 80% with these processes.
  • the present invention provides an integrated process for recovery of the components of a feed gas containing methane and heavier hydrocarbons while maximizing NGL recovery and minimizing capital expenditures and operating costs incurred with the LNG facility.
  • the present invention is also intended to improve separation efficiency within an NGL recovery column while maintaining column pressure as high as practically possible to achieve an efficient and economical utilization of mechanical refrigeration in the liquefaction process. This is achieved by the introduction of an enhanced reflux specifically suitable for the purpose of the recovery column.
  • the price of liquid ethane has been cyclical, rising and falling in response to the demand for use as petrochemical feed stock. When the price of liquid ethane is high, gas processors can generate additional revenues by increasing the recovery of ethane.
  • the present invention is designed to permit flexible transition between operation for ethane recovery or ethane rejection.
  • the present invention in the broadest sense, provides an integrated process and apparatus for cryogenically recovering ethane, propane and heavier components during natural gas liquefaction processes via a distillation column, in which the reflux derived from various sources in the liquefaction process is essentially free of the components to be recovered.
  • the provision of an enhanced reflux stream, which is lean on the NGL components, to the distillation column permits a high recovery of NGL components even when the column is operated at a relatively high pressure.
  • the process involves introducing a cooled gas or condensate (i.e., two phase) feed stream into a first distillation column, e.g., an NGL recovery column, at one or more feed trays.
  • the gas or condensate feed stream is separated into a first liquid stream primarily comprising NGL components to be recovered and a methane-rich overhead stream essentially free of NGL components.
  • the methane-rich overhead stream is further cooled to total liquefaction and is preferably sub-cooled.
  • This liquefied, and preferably sub-cooled, methane-rich stream under pressure is subsequently flashed to near atmospheric pressure in one or more steps with the LNG stream collected in the final flash step being delivered to the LNG tank for storage.
  • the flashed vapor stream or streams are heated and compressed to a higher pressure for delivery as a fuel gas stream.
  • the first liquid stream (i.e., the liquid stream produced by the NGL recovery column) is introduced into a second distillation column, e.g., an NGL purifying column, at one or more feed trays.
  • the first liquid stream is separated into an NGL product stream produced from the bottom and a first vapor portion stream primarily comprismg all of the remaining lighter components from the overhead.
  • the first vapor portion stream and optionally, a portion of the excess flashed vapor stream are combined.
  • the combined stream is compressed and cooled to substantial condensation and thereafter introduced to the top of the NGL recovery column as a reflux stream.
  • This reflux stream will contain an extremely low concentration of the heavy components to be recovered in the NGL product stream.
  • This stream enhances the recovery efficiency within the column and reduces the loss of NGL components in the methane-rich overhead stream to a minimum.
  • a high NGL recovery is therefore achieved even with a relatively high operating pressure, i.e., a pressure of about 600 psig, for the NGL recovery column.
  • the economic advantages of the present invention can be further improved by thermally linking a side reboiler for the first distillation column with the overhead condenser for the second distillation column. More specifically, the first vapor portion stream is cooled in countercurrent heat exchange with a liquid stream withdrawn from a tray located below the feed trays of the first distillation column. The cooled first vapor portion stream is separated into a liquid fraction for introduction into the second distillation column as a top reflux stream and a lighter, vapor fraction with further reduced NGL components which is liquefied alone or upon combination with a portion of the excess flashed stream and introduced into the first distillation column as a top reflux. Thus the NGL recovery efficiency in the second column is enhanced.
  • Heat is transferred from the NGL purification column to the NGL recovery column by the above-cited liquid stream which is withdrawn and returned to the NGL recovery column where it provides a stripping action in the bottom portion of the column, thereby reducing volatile components, e.g., methane, in the first liquid stream from the bottom.
  • the recovery efficiency can be improved in another embodiment of the present invention by the introduction of a second reflux stream to the upper, rectification section of the first distillation column.
  • the second reflux stream enters the distillation column preferably in the middle of the rectification section, as a middle reflux stream which provides a bulk rectification effect and reduces the NGL components to be recovered in the up-flow vapor stream. Residual NGL components in the upward vapor stream are efficiently recovered via the first reflux stream (i.e., top reflux stream).
  • a slipstream from the feed gas stream can be taken and cooled to substantial condensation or even sub-cooled to form the second reflux stream.
  • the feed gas stream contains much heavier components, e.g., hexane and aromatics, which tend to freeze at cryogenic temperatures.
  • the feed gas stream can be first cooled to partial condensation where most of these components will be condensed in the liquid phase and separated out in a separator. Such resulting liquid stream can be fed to the lower section of the NGL recovery column or combined with a stream being routed to the lower section of the NGL recovery column. A slipstream can then be taken from the non-condensed vapor portion and further cooled to substantial condensation to form the second reflux stream.
  • this reflux stream can be fed to the top of the NGL recovery column either as a separate stream or in combination with the previously described top reflux or first reflux stream.
  • the second reflux stream as described herein can function as the sole reflux stream to the column and can be fed to the top of the NGL recovery column.
  • Another feature providing a significant economic advantage in the present invention is the cooling of a portion of the feed gas stream by countercurrent heat exchange with a refrigerant stream comprismg a portion of the first liquid stream (i.e. liquid withdrawn from the lower portion of the first distillation column).
  • the refrigerant stream is partially vaporized and may be separated into a second liquid stream for introduction into the second distillation column and a second vapor stream for introduction into the first distillation column as a stripping gas stream for compression and cooling.
  • the introduction of stripping gas stream supplements the heat requirements in the first distillation column for stripping volatile components from the in-situ liquid stream.
  • the top reflux stream to the first distillation column is generated by recycling a small portion of the pressurized LNG stream prior to flashing.
  • This reflux stream has an extremely low content of the NGL components to be recovered and, accordingly, enhances separation efficiency.
  • This reflux scheme can be advantageous for the liquefaction process where the LNG can be deeply sub-cooled using very cold mechanical refrigeration to reduce the vapor produced in the flashing steps to a minimum.
  • Typical examples of this embodiment include liquefaction processes using mixed refrigerant with or without propane pre-cooling and cascaded refrigeration in a closed circuit.
  • the reflux stream to the NGL recovery column is provided in accordance with any of the methodologies set forth herein but the stripping gas stream employed in the NGL recovery column is provided via a portion of the feed gas stream rather than the vaporization of a portion of the liquid existing in or removed from the NGL recovery column either by one or more internal or external reboilers.
  • the portion of the feed gas stream can be fed either directly into the column or first cooled via indirect heat exchange with the first liquid stream which is produced from the bottom of said column. In the later mode, additional cooling via the first liquid stream is possible by first flashing the first liquid stream to a pressure slightly above the operating pressure of the NGL purification column.
  • the methods and apparatus of the present invention efficiently integrate NGL recovery into the natural gas liquefaction process and permit high recoveries of propane and heavier components, e.g., recoveries exceeding 95% of those components originally present in the feed gas. Certain of the embodiments when properly optimized permit the recovery of at least 99% of the propane and heavier hydrocarbons originally found in the feed gas.
  • the high recovery of heavier hydrocarbons achieved with the methods of the present invention may be advantageously used to clean gas feeds contaminated by cyclo- hexane, benzene and other heavy hydrocarbons which have been determined to create potential freezing problems and, accordingly, must be thoroughly removed. This high NGL recovery is achieved while eliminating the NGL plant, as typically employed in the prior art, in the front-end of the LNG facility.
  • the flexible design of the present invention permits an easy transition between operations designed to either recover or reject ethane in order to accommodate rapidly changing values of liquid ethane.
  • the integration methods proposed in the present invention can also be easily adapted for use with any liquefaction process regardless the refrigeration system used.
  • Fig. 1 illustrates a schematic representation of an enhanced NGL recovery process utilizing refrigeration and reflux from a LNG plant wherein a top reflux stream to the NGL recovery column is employed;
  • Fig. 2 illustrates a schematic representation of a LNG plant employing a typical open cycle cascaded refrigeration process with enhanced NGL recovery
  • Fig. 3 illustrates a schematic representation of a LNG plant employing a typical propane pre-cooled, mixed refrigeration process with enhanced NGL recovery
  • Fig. 4 illustrates a schematic representation of a LNG plant employing a typical single, mixed refiigeration process with enhanced NGL recovery;
  • Fig. 5 illustrates an alternative embodiment of an enhanced NGL recovery process utilizing refiigeration and reflux from a LNG plant and a second reflux stream to the NGL recovery column;
  • Fig. 6 illustrates an alternative embodiment of an enhanced NGL recovery process utilizing refiigeration and reflux from a LNG plant wherein the reflux to the NGL recovery column is a portion of the liquefied natural gas recycled under pressure;
  • Fig. 7 illustrates an alternative embodiment of an enhanced NGL recovery process utilizing refrigeration and reflux from a LNG plant wherein the feed gas is employed as a stripping gas in the NGL recovery column;
  • Fig. 8 illustrates an alternative embodiment of an enhanced NGL recovery process utilizing refrigeration and reflux from a LNG plant wherein the generation of a stripping gas from column liquid in the NGL recovery column is enhanced; and Fig. 9 illustrates another alternative embodiment of an enhanced NGL recovery process utilizing refrigeration and reflux from a LNG wherein a simplified NGL purifying system is employed.
  • the present invention permits the separation and recovery of substantially all of the NGL components, i.e., ethane, propane and heavier hydrocarbons, from a compressed natural gas in a LNG process.
  • the present invention achieves these high recoveries while eliminating the need for a separate NGL plant in the front-end of the LNG facilities by introducing to the distillation column an enhanced reflux having an extremely low content of the NGL components to be recovered.
  • the introduction of lean reflux permits the column to be operated at higher pressures while still maintaining high recovery of NGL and, accordingly, the refrigeration system can be utilized more efficiently in the liquefaction process.
  • the capital requirements, as well as operating costs, for recovering substantially all of the NGL components present in the feed gas in a LNG process may be greatly reduced.
  • lean reflux when used with respect to a distillation column, it refers to the components to be recovered in the bottom liquid stream.
  • a lean reflux for recovery of propane and heavy hydrocarbons means that the reflux stream has a low content of the cited components.
  • upper and lower as used with respect to a distillation column, these terms are to be understood as relative to one another, i.e., that withdrawal of a stream from an upper region of a column is at a higher position than a stream withdrawn from a lower region thereof, h an exemplary, but non-limiting embodiment, upper may refer to the upper half of a column, whereas lower may refer to the lower half of a column.
  • middle it is to be understood that a middle region is intermediate to an upper region and a lower region.
  • upper, middle, and lower are used to refer to a cryogenic distillation column, it should not be understood that the column is strictly divided into thirds by these terms.
  • Fig. 1 illustrates a schematic configuration of one exemplary embodiment of the invention where at least about 95%, preferably above 98%, of the propane, propylene and heavier hydrocarbons, i.e., the C 3+ hydrocarbons, from a feed gas which will be ultimately liquefied as LNG product may be recovered.
  • a dry feed gas at a flow rate of about 480 MMSCFD is introduced into the illustrated process through inlet stream 10 at a pressure of about 1015 psia and a temperature typically close to ambient, for example about 100°F in this illustration.
  • stream 10 has been pre-freated as necessary to remove undesirable components, including acid gases, mercaptans, mercury and moisture, from the natural gas delivered to the facility.
  • Stream 10 is split into two streams 12 and 14. The smaller portion, stream 14, is directed through gas/liquid exchanger 18 in NGL recovery block 100 where it is in countercurrent heat exchange with liquid withdrawn from the bottom of NGL recovery column 50 and liquid from separator 22.
  • This inlet gas provides heat for NGL recovery column 50, while chilling the inlet gas to a temperature of about -61°F.
  • the larger portion of inlet gas, stream 12 flows to exchanger block 300 where it is cooled to about -40°F by utilizing refrigeration in the liquefaction process.
  • the cooling steps in the refrigeration system used in the liquefaction process may differ significantly, depending on the process used, and are collectively illustrated as simplified exchanger block 300, which will be described in more detail later. Cooled feed gas stream 16 from exchanger block 300 is combined with the cooled feed gas stream 14a from gas/liquid exchanger 18.
  • the combined sfream 20 at about -42°F and 990 psia is directed and separated into liquid stream 24 comprising any condensed heavier hydrocarbons and into cooled vapor stream 30 comprising lighter and more volatile components in separator 22.
  • Liquid sfream 24 is expanded through expansion device 26 and preheated in gas/liquid exchanger 18 prior to introduction into a distillation column, e.g., NGL recovery column 50, as stream 28 for further fractionation!
  • a distillation column e.g., NGL recovery column 50
  • the preheating of expanded liquid stream 24 in exchanger 18 can be bypassed in some cases.
  • Cooled vapor stream 30 flows to expander 34 where it is expanded to a pressure slightly above the operating pressure of NGL recovery column 50.
  • the vapor in stream 30 may by-pass expander 34 through control valve 34a.
  • Stream 32 from the expander discharge possesses a temperature and pressure of at about -84°F and about 610 psia and is fed to NGL recovery column 50 right below the upper rectifying section.
  • cooled stream 16 leaving exchanger block 300 can be directly fed to NGL recovery column.
  • expansion device 26 is not required.
  • cooled feed gas stream 14a can be delivered directly to NGL recovery column 50 either alone or after being combined with the cooled gas in line 38.
  • cooled feed gas stream 14a can be fed to separator 22 and the resulting gas stream fed to the NGL recovery column or combined with cooled stream 16 and fed to the NGL recovery column.
  • the NGL recovery column operated at about 600 psia is a conventional distillation column containing a plurality of mass contacting devices, trays or packings, or some combinations of the above. It is typically equipped with one or more liquid draw trays in the lower section of the column to permit heat inputs to the column for stripping volatile components off from the bottom liquid product .
  • Liquid collected in draw tray 50a is withdrawn via stream 46a and heated by countercurrent heat exchanger in side reboiler 48 prior to re-introduction to the NGL recovery column.
  • liquid condensed in the lower draw fray 50b is withdrawn via sfream 46b, partially vaporized in gas/liquid exchanger 18, and re-introduced to the NGL recovery column.
  • Bottom liquid stream 44 also referred to herein as the first liquid stream, containing substantially all of the heavier hydrocarbons is withdrawn from NGL recovery column 50 and directly introduced into the middle portion of a second distillation column, i.e., NGL purifying column 70.
  • Stream 44 is separated in NGL purifying column 70 operated at a pressure of about 440 psia into an NGL product stream 64 comprising mainly propane, propylene and heavier hydrocarbons, i.e., the C 3+ hydrocarbons, and a vapor comprising mainly ethane and lighter hydrocarbons.
  • the purity of the NGL product stream is controlled by external heat input via bottom reboiler 62.
  • the NGL product stream exits column 70 at about 235°F and is cooled to about 120°F via exchanger 66 for delivery as product stream 68.
  • Vapor stream 52 is withdrawn from the top of NGL purifying column 70 through an overhead line. Vapor phase stream 52 is cooled to partial condensation in side reboiler 48 prior to return to reflux drum 54 at a temperature of about -9°F.
  • the heat carried by vapor stream 52 is effectively transferred to the NGL recovery column as external heat input. This is accomplished by a unique thermal integration between the overhead condenser and the side reboiler for NGL purifying column 70.
  • the partially condensed stream is separated in reflux drum 54 into vapor and liquid phases. The liquid accumulated in reflux drum 54 is withdrawn via line 58 where it is pumped via reflux pump 60 for re-introduction to the NGL purifying column as top reflux.
  • Vapor sfream 72 withdrawn from reflux drum 54 comprises mainly methane and ethane which were present in feed stream 44.
  • the concentration of propane and higher components in vapor sfream 72 is very low.
  • Vapor stream 72 is directed into exchanger block 300 for recovering available refrigeration. In cases where the available refrigeration is limited or not needed, sfream 72 can bypass exchanger block 300 and simplify the exchanger block design.
  • a combined stream formed by warmed stream 72a and optionally, excess flashed vapor stream 102, if any, is compressed to a higher pressure at about 635 psia in compressor 96 prior to being cooled in after-cooler 98.
  • the cooled, combined vapor stream 104 returns to exchanger block 300 where it is further cooled to substantial condensation using refrigeration employed in the liquefaction process thereby producing stream 42.
  • the substantially condensed sfream 42 is introduced to NGL recovery column 50 as top reflux.
  • Stream 42 characterized by a very low content of C 3+ hydrocarbons, reduces the equilibrium loss of C 3+ hydrocarbons in the overhead vapor to a minimum.
  • the introduction of a lean reflux sfream in the present invention permits the column to be operated at a relatively high pressure, e.g., about 600 psia in this example, while maintaining high recovery of C 3+ hydrocarbon liquids.
  • LEO Lighter and more volatile gases primarily rich in methane are withdrawn from the top of NGL recovery column 50 via overhead sfream 40.
  • This stream may be compressed in compressor 36 which preferably utilizes work extracted from expander 34 before delivery to exchanger block 300.
  • the overhead stream 40 can be directly sent to exchanger block 300 without further compression as shown with dashed line 40a in cases where the expander 34 is not available or the work extracted from the expander is used for other services.
  • the overhead sfream 40 is directly introduced to exchanger block 300.
  • lean reflux stream 42 may also be vapor stream 72 from NGL purifying column 70 or a portion of flashed vapor stream 80 alone, or any combination of these two streams.
  • the methane-rich overhead stream from NGL recovery column 50 at about -112°F and about 600 psia is totally liquefied and in most cases substantially sub-cooled in exchanger block 300 utilizing appropriate refrigeration from refrigeration block 200.
  • Sub-cooled LNG at an elevated pressure is delivered via stream 74 from exchanger block 300 to expansion block 400 where it is expanded to near atmospheric pressure through one or more expansion steps.
  • Expansion block 400 illustrates a typical arrangement with one expansion step.
  • Sub-cooled LNG is expanded through expansion means 76 to about 25 psia causing partial vaporization in discharge line 78.
  • a hydraulic turbine optionally can be employed as an expansion means to reduce flashing as a result of pressure reduction. Any flashed vapor in expanded LNG stream 78 is separated from the liquid portion in separator 82.
  • the liquid portion withdrawn from separator 82 comprises LNG product sfream 84 for delivery to storage.
  • the expansion provided in expansion block 400 can also be carried out in multiple stages. For instance, a liquefaction process utilizing cascaded refrigeration, either closed or open cycle, typically involves 3 or 4 expansion stages.
  • a liquefaction process utilizing cascaded refrigeration typically involves 3 or 4 expansion stages.
  • additional flashed vapor will be generated when the final LNG product 84 is delivered to the LNG tank for storage due mainly to further pressure let-down and heat gain in the cryogenic storage tank at the ambient pressure.
  • This flashed vapor stream is typically compressed and combined with the flashed vapor stream 80 via the use of a boil-off gas compressor.
  • Flashed vapor stream 80 from separator 82 enters exchanger block 300 for recovery of available cold refrigeration.
  • the warmed, flashed vapor sfream 86 leaves exchanger block 300 at about 66°F and is compressed to a pressure of about 425 psia via methane compressor 88.
  • the compressed vapor stream is then cooled to about 100°F through after-cooler 90 prior to being used as fuel gas via sfream 92. It should be noted that, depending upon the pressures of the expansion steps and the final fuel gas supply pressure, more than one compression and cooling step may be required. Any portion of excess flashed vapor stream 102 may be combined with the warm vapor sfream 72a for recycle to the top of NGL recovery column 50 as reflux after being further compressed and cooled to substantial condensation.
  • FIG. 2 illustrates in a non-limiting manner an open cycle cascaded refrigeration process in conjunction with the embodiment of the present invention illustrated in Fig. 1.
  • three refrigerant loops or cycles are employed (e.g. propane, ethylene and methane).
  • Each refrigeration cycle employs an effective number of refrigeration stages, such effective number being nominally two and preferably 2 to 4.
  • the preferred number of stages in the first, second and third cycles that being 3, 2 and 3, respectively are employed.
  • the preferred refrigerant for the first refiigeration cycle is propane.
  • the preferred refrigerant for the second refrigeration cycle is ethylene or ethane, most preferably ethylene.
  • the preferred refrigerant for the third refrigeration cycle is preferably methane, but may contain small concentrations of nitrogen and other light hydrocarbons.
  • Propane refrigerant sfream 202a withdrawn from the propane surge drum 220 at about 100°F and approximately 190 psia is directed to a pressure reduction device, e.g., expansion valve 204a, and expanded to a lower pressure, thereby flashing a portion of the propane refrigerant sfream and lowering its temperature.
  • a pressure reduction device e.g., expansion valve 204a
  • the resulting two-phase sfream is directed into the high-stage propane chiller 310a as a coolant in indirect heat exchange with the following: a) feed gas, such as stream 12, b) the second refrigerant stream 240, such as ethylene, and the c) Combined vapor stream 104. via conduits 302a, 208a, and 206a respectively in chiller 310a.
  • the flashed propane vapor sfream 210a from the chiller 310a is fed to the high-stage inlet port of the propane compressor 212 through the high-stage suction line.
  • Liquid propane sfream 202b from chiller 310a is directed to a pressure reduction valve 204b to further reduce its pressure, thereby flashing an additional portion of propane refrigerant sfream and further lowering its temperature.
  • the resulting two-phase sfream is directed into the inter-stage propane chiller 310b as a coolant in indirect heat exchange with the cooled feed gas split from conduit 302a and the second refrigerant vapor stream from conduit 206a via conduits 302b and 206b, respectively.
  • the flashed propane vapor sfream 210b from the chiller 310b is fed to the inter-stage inlet port of propane compressor 212 through the inter-stage suction line.
  • Liquid propane stream 202c from chiller 310b is further directed to a pressure reduction valve 204c to reduce its pressure, thereby flashing another portion of propane refrigerant stream and lowering its temperature still further.
  • the resultant two-phase sfream is directed into low-stage propane chiller 310c as a coolant in indirect heat exchange with the cooled feed gas split from conduit 302b and the second refrigerant stream from conduit 206b via conduits 302c and 206c, respectively.
  • the flashed propane vapor stream 210c from chiller 310c is fed through the low-stage suction line to the low-stage inlet port of the propane compressor 212.
  • Propane vapor is compressed in a three-stage propane compressor 212 typically driven by a gas turbine. Although they may be separate units tandem driven by a single driver, the three stages preferably form a single unit.
  • Compressed propane vapor stream 214 flows through condenser 216 where it is liquefied at about 100°F and about 190 psia in the illustrated system, prior to being returned via line 218 to propane surge drum 220.
  • Exemplary temperatures for the three propane refrigeration levels, respectively, in the illustrated example are about 62°F, about 12°F, and about -25°F.
  • the second refiigeration, cycle that being illustrated as the ethylene refrigeration cycle in Fig. 2 is a closed two-stage system.
  • an economizer 246 is incorporated in the ethylene refrigeration.
  • the ethylene refrigerant sfream 240a exits the low-stage propane chiller 310c, in most cases, totally condensed and is directed to ethylene surge drum 244 at about -21°F and approximately 300 psia.
  • Ethylene refrigerant sfream 222a withdrawn from ethylene surge drum 244 is directed to the ethylene economizer 246 where it is sub-cooled prior to a pressure reduction device such as an expansion valve 224a.
  • the sub-cooled ethylene is expanded to a lower pressure across expansion valve 224a, thereby flashing a portion of ethylene refrigerant sfream and lowering its temperature.
  • the amount of ethylene flashed in this pressure reduction step is reduced as a result of sub-cooling prior to expansion via the use of ethylene economizer 246.
  • the resulting two-phase stream is directed into the high-stage ethylene chiller 330a as a coolant in indirect heat exchange with the main feed gas split stream 12a from the low-stage propane chiller 310c and cooled vapor sfream 104c from the methane economizer 360, via conduit 324a and 326a respectively.
  • a portion of cooled feed gas stream 12a might bypass the ethylene refrigeration loop as stream 12b.
  • the flashed ethylene vapor stream 230a from the chiller 330a is fed to the ethylene economizer 246 to provide refrigeration for sub-cooling ethylene refrigerant as previously described thereby producing a warmed high stage ethylene gas stream 232a which is fed through high-stage suction line to the high-stage inlet port of ethylene compressor 234.
  • Liquid ethylene sfream 222b from the chiller 330a is directed to the ethylene economizer 246. It is then directed to a pressure reduction valve 224b to further reduce its pressure and lowering its temperature further.
  • the resulting two-phase sfream from expansion device 224b is introduced the low-stage ethylene chiller 330b as a coolant in indirect heat exchange with methane-rich overhead vapor stream 40a from NGL recovery column 50 as depicted in Fig. 1 and cooled vapor sfream from conduit 326b via conduits 324c and 326c respectively.
  • both streams leave the low-stage ethylene chiller 330b totally liquefied as liquid streams 40b and 42 respectively.
  • the flashed ethylene vapor stream 230b from low-stage chiller 330b returns to the ethylene economizer to provide refrigeration for ethylene sub-cooling thereby producing a warmed low stage ethylene gas stream 232b which is fed to the low-stage inlet port of ethylene compressor 234 through low-stage suction line.
  • Ethylene vapor is compressed in the illustrative two-stage compressor 234 typically driven by a gas turbine.
  • the compressed ethylene vapor sfream is cooled to partially reject the heat of compression to the atmosphere via after-cooler 238b.
  • the compressed ethylene sfream 236 produced by compressor 234 is cooled in after- cooler 238a.
  • the refiigerant vapor stream 240 After the preceding steps of compression and cooling, the refiigerant vapor stream 240 returns to the propane chillers for further cooling and condensation as previously described, thereby completing the ethylene cycle.
  • Exemplary temperatures for two ethylene refrigeration levels, respectively, in the illustrated example are about -70°F, and about -126°F.
  • the combined vapor sfream 104 leaves the high-stage propane chiller 310a via conduit 208a as sfream 104a and is directed to the methane economizer 360 for further cooling.
  • a portion of sfream 104a is withdrawn from the middle of the methane economizer 360 at about -35°F via sfream 104c.
  • Sfream 104c is subsequently condensed 42 via ethylene chillers 330a and 330b and used as the top lean reflux sfream.
  • the remaining portion leaves the economizer 360 at a colder temperature of about -110°F via sfream 104b.
  • Stream 104b is preferably combined with stream 40a from the NGL recovery column 50 and then condensed in the chiller 330b as liquefied LNG stream 40b, thereby simplifying the chiller design, although it can be condensed in a separate conduit of chiller 330b and thereafter combined with liquefied sfream 40a to form stream 40b.
  • the liquefied LNG sfream 40b from the low-stage ethylene chiller 330b at an elevated pressure is further cooled in the methane economizer 360.
  • the sub-cooled LNG stream 74 exits the methane economizer 360 and is directed to the expansion block 400 as shown in Fig. 2.
  • LNG stream 74 undergoes a series of pressure reduction stages, preferably corresponding in number to the number of open methane cycle refrigeration stages, to near atmospheric pressure.
  • LNG sfream 74 is first directed to an expansion valve 76a and expanded to a lower pressure, resulting in flashing a portion of the pressurized LNG stream and lowering its temperature.
  • the resultant two-phase stream is fed to the high stage flash drum 82a wherein any flashed vapor is removed from the top as high stage flashed vapor stream 80a.
  • liquid expanders i.e. hydraulic turbines
  • Flashed vapor sfream 80a from flash drum 82a is fed to the methane economizer
  • Compressed high stage flash vapor sfream 86a is thereafter introduced through high-stage suction line to the high- stage inlet port of methane compressor 88.
  • LNG stream 74b is removed from flash drum 82a and is directed to sub-cooler 410. It is then expanded through a pressure reduction valve 76b to further reduce its pressure and lower its temperature.
  • the resulting two- phase sfream from expansion device 76b is introduced into the inter-stage flash drum 82b wherein any flashed vapor is removed from the top as inter-stage flashed vapor sfream 80b.
  • the inter-stage flashed vapor sfream 80b from flash drum 82b is fed to sub-cooler 410 and then methane economizer 360 to recover available cold refrigeration.
  • the flashed vapor sfream is warmed to near ambient temperature thereby producing a warmed inter-stage flashed vapor sfream 86b which is sent through the inter-stage suction line to the inter-stage inlet port of methane compressor 88.
  • LNG stream 74c is removed from flash drum 82b and preferably undergoes the third pressure expansion through expansion valve 76c directly thereby lowering its temperature further, although it can be re-introduced to sub-cooler 410 prior to such expansion for further cooling.
  • the resulting two-phase sfream from expansion valve 76c is introduced to the low-stage flash drum 82c wherein any flashed vapor is removed from the top as the low-stage flashed vapor sfream 80c.
  • Final LNG product sfream 84 is removed from the bottom of flash drum 82c and delivered to storage.
  • the low-stage flashed vapor stream 80c from flash drum 82c is fed to sub-cooler 410 and subsequently to the methane economizer 360 to recover its 'available cold refrigeration thereby producing a warmed low stage flashed vapor stream 86c which is subsequently infroduced through inter-stage suction line to the low-stage inlet port of methane compressor 88.
  • the warmed flashed vapor sfreams are compressed via a two or three-stage compressor 88 depending on the requirement of fuel gas supply pressure.
  • After-coolers such as 90a, 90b, or 90c are often provided after each compression stage to partially reject the heat of compression to the atmosphere.
  • Stream 102 comprised of any excess flashed vapor is further compressed and cooled via compressor 96 and after-cooler 98 respectively.
  • the cooled vapor sfream 104 returns to exchanger block 300 where it is further cooled to substantial condensation using propane and ethylene refrigeration as described earlier.
  • compressor 96 is typically considered as the final compression stage, and is part of compressor 88 which preferably consists of three stages.
  • the vapor sfream 72 withdrawn from the reflux drum 54 shown in Fig. 1 can be combined either with excess flashed vapor sfream 102 or with high stage flash vapor sfream 86a as shown via the dashed line.
  • vapor sfream 72 although not shown in Fig. 2, may be infroduced to the methane economizer 360 by combining with stream 80a for recovering its cold refiigeration in cases where it temperature is relatively cold.
  • Such steam may also provide refrigeration via a separate conduit methane economizer 360.
  • Table 1 summarizes the results of a numerical simulation of the embodiment of the invention illusfrated above for a target recovery of C 3+ hydrocarbons exceeding 98%. As indicated in Table 1, recovery of 99.2% of propane and 100% of all C 4 + hydrocarbons can be achieved via this embodiment.
  • FIG. 3 Another example representing a typical arrangement of exchanger block 300 and refrigeration block 200 utilizing the propane pre-cooled mixed refrigeration cycle in conjunction with the embodiment of the present invention of Fig. 1 is described below with a schematic shown in Fig. 3.
  • Recovery of C 3+ hydrocarbons in excess of 95% from a dry gas feed of about 400 MMSCFD is desired.
  • the dry feed gas has been pre- treated as necessary to remove undesirable components, and contains about 90 mol% methane, 4.9 mol% ethane, 2.3 mol% propane, 2.0 mol% butanes, 0.7 mol% C5 and heavier components, and the remaining non-hydrocarbon components.
  • the application and operation of the present invention shown in Fig. 1 in this example is essentially the same as the previous example and is briefly described here.
  • the dry feed gas enters the facility through inlet stream 10 and is first cooled to about -43 °F via two paths similar to the previous example prior to entering separator 22 for separation of condensed liquid, if any, as stream 24.
  • Liquid stream 24 is expanded through expansion device 26 and preheated in gas/liquid exchanger 18 prior to introduction into NGL recovery column 50 for further fractionation.
  • Cooled vapor stream 30 is expanded at about 610 psia via expander 34 and then fed as a cooled gas or condensate (i.e., two phase) stream 32 to NGL recovery column 50 right below the upper rectifying section.
  • the NGL recovery column a conventional distillation column containing a plurality of mass transfer devices, is operated at about 600 psia.
  • stream 32 is fractionated into a bottom liquid stream 44 containing substantially all of the heavier hydrocarbons and an overhead stream 40 comprising lighter and more volatile gases primarily rich in methane.
  • Liquid sfream 44 is further introduced into the middle portion of the NGL purifying column 70 where ethane and lighter components are stripping off the bottom NGL product 64.
  • the NGL product stream exits column 70 at about 230°F and is cooled to about 120°F via exchanger 66 for delivery to product sfream 68.
  • the vapor stream 72 from the NGL purifying system comprises mainly methane and ethane, with essentially free of propane and higher components utilizing a thermal integration of side reboiler 48 as previously described.
  • the warm vapor sfream 72a is combined with a portion of excess flashed vapor sfream 102, if any and compressed via compressor 96 to a pressure sufficient to return the compressed stream when liquefied to the top of NGL recovery column 50 as the top reflux.
  • the stream is cooled in after-cooler 98.
  • the resulting cooled vapor sfream 104 returns to exchanger block 300 where it is further cooled to substantial condensation and produced as sfream 42.
  • the substantially condensed sfream 42 characterized by a very low content of C 3+ hydrocarbons, is introduced to the NGL recovery column 50 as top reflux to achieve high recovery of C 3+ hydrocarbons.
  • the overhead stream 40 from the top of NGL recovery column 50 at about -101°F and about 600 psia is first compressed in compressor 36 utilizing work extracted from expander 34 prior to entering the exchanger block 300. It is totally liquefied and in most cases deeply sub-cooled in exchanger block 300 utilizing appropriate refrigeration from refrigeration block 200.
  • Sub-cooled LNG at an elevated pressure is delivered via sfream 74 from exchanger block 300 to expansion block 400 where it is expanded to near atmospheric pressure through typically one expansion step to about 20 psia causing partial vaporization and production of a two phase LNG-bearing stream 78.
  • a hydraulic turbine optionally can be employed as an expansion means to reduce flashing as a result of pressure reduction. Any flashed vapor in expanded LNG stream 78 is separated from the liquid portion in separator 82.
  • the LNG product stream 84 withdrawn from the separator 82 is then delivered to storage tank.
  • Flashed vapor stream 80 from separator 82 enters exchanger block 300 for the recovery of available cold refrigeration.
  • Warmed, flashed vapor stream 86 leaves exchanger block 300 at about 65°F and is compressed in one or more stages to a fuel gas at a pressure of about 420 psia via fuel gas compressor 88.
  • the compressed vapor sfream is then cooled to about 100°F through after-cooler 90 prior to being used as fuel gas stream 92. Any portion of excess flashed vapor sfream 102 may be combined with the warm vapor sfream 72a for recycle to the top of NGL recovery column 50 as reflux sfream 42.
  • Fig. 3 illustrates in more detail a typical arrangement of exchanger block 300 and refiigeration block 200 utilizing the propane pre-cooled mixed refiigeration cycle in conjunction with the embodiment of the present invention illusfrated in Fig. 1.
  • an illustrative three-stage propane refiigeration cycle in closed circuit is configured essentially the same as that in Fig. 2 and operates in a similar manner accordingly.
  • the three-stage propane refrigeration cycle provides high level cooling for a) feed gas, such as stream 12 from Fig. 1, and b) the mixed refrigerant vapor stream 502 from refiigerant compressor after-cooler 524b.
  • Exemplary temperatures for the three propane refrigeration levels, respectively, in the illusfrated example are about 60°F, about 10°F, and about ⁇ 30°F.
  • Partially condensed, mixed refrigerant leaving conduit 206c via sfream 502a is infroduced into separator 504.
  • the condensed portion is removed from the bottom of separator 504 as stream 506 at about -26 °F and about 640 psia.
  • Condensed refrigerant sfream 506 is further cooled in exchanger 320 via conduit 506a to about -188°F.
  • Sub-cooled refiigerant stream 514 is directed to a pressure reduction means, e.g., expansion valve 516, to lower the pressure thereby producing expanded refrigerant stream 518 which returns to exchanger 320 as a coolant.
  • Non-condensed vapor refrigerant sfream 508 from separator 504 is divided into two portions, those portions being sfreams 510 and 512.
  • Main portion sfream 510 flows through exchanger 320 where it is liquefied and, optionally, sub-cooled to about -235°F via conduit 510a.
  • Remaining vapor portion stream 512 passes through exchanger 340 where it is liquefied and sub-cooled thereby producing stream 524 via indirect heat exchange with flashed vapor sfream 80 from expansion block 400 in Fig. 1.
  • Other streams entering exchanger 340 include combined vapor stream 104 from after-cooler 98 and overhead vapor stream 72 from reflux drum 54 as depicted in Fig. 1.
  • streams 72 and 80 are warmed before exiting exchanger 340 at about 65°F as sfreams 72a and 86, respectively.
  • stream 104 is cooled and exits exchanger 340 as sfream 104a at about -26°F.
  • Sub-cooled refrigerant stream 524 exiting from exchanger 340 at about -245°F is combined with the other sub-cooled refrigerant stream from conduit 510a and thereafter directed to a pressure reduction means, e.g., expansion valve 526, to a lower pressure before being returned as stream 528 to exchanger 320 as a coolant.
  • expanded refiigerant sfream 528 is combined with the other expanded refiigerant sfream 518 in exchanger 320 where it flows through said exchanger via conduit 528a.
  • the combined refiigerant sfream provides the refrigeration necessary for cooling the following sfreams in exchanger 320: • feed gas sfream 12a from low-stage propane chiller 310c;
  • cooled vapor sfream 104a from exchanger 340, via conduits 322a, 322b, and 322c, respectively, thereby producing respective streams 16, 74 and 42.
  • a hydraulic turbine may be used as a pressure reduction means for the sub-cooled refrigerant in place of expansion valves 516 or 526 illusfrated therein.
  • work can also be extracted by a hydraulic turbine, thereby lowering the refrigerant temperature further and enhancing liquefaction efficiency and overall plant throughput.
  • the combined mixed refrigerant exits exchanger 320 as warmed and vaporized stream 520 at about -30°F and about 50 psia.
  • Warmed refrigerant stream 520 is then compressed and cooled.
  • An exemplary compression and cooling configuration is illustrated in Fig. 3 with two stages.
  • Stream 520 is first compressed to about 250 psia via low stage refrigerant compressor 522a and then cooled to about 100°F via low stage refrigerant after-cooler 524a.
  • the cooled and compressed sfream is further compressed and cooled to form stream 502 at about 655 psia and about 100°F via high stage refiigerant compressor 522b and after-cooler 524b, thus completing the closed circuit.
  • Table 3 summarizes the results of a numerical simulation of the embodiment of the invention illustrated above for a target recovery of C 3+ hydrocarbons exceeding 98%. As indicated in Table 3, recovery of 98.4% of propane and 100% of all C 4 + hydrocarbons can be achieved via this embodiment utilizing another refrigeration process, namely the propane pre-cooled mixed refrigerant process. Table 3
  • Figs. 1 and 5 In addition to the open cycle cascaded refrigeration process represented in Fig. 2 and the propane pre-cooled mixed refiigerant process represented in Fig. 3, other mechanical refrigeration cycles for liquefying natural gas known to the art can also be integrated with the present invention.
  • One such process being the single, mixed refiigerant process.
  • the single, mixed refiigerant process includes heavier hydrocarbons, e.g., butanes and pentanes, in the multi-component, mixed, refrigeration stream and in so doing, eliminates the need for a propane pre-cooled refrigeration cycle.
  • Fig. 4 illustrates the embodiment of the present invention as depicted in Fig. 1 further including the single, mixed refrigerate process via exchanger block 300 and refrigeration block 200.
  • mixed refrigerant stream 502 exits the final compression and cooling stage from high stage after-cooler 524b partially condensed as it contains some heavier components in the mixture.
  • the partially condensed refrigerant stream 502 is introduced into separator 504 from which the condensed portion is removed from the bottom of the separator as stream 506.
  • the non-condensed vapor refrigerant sfream 508 from separator 504 is divided into two portions corresponding to sfreams 510 and 512, respectively.
  • the condensed refrigerant sfream 506 is pumped via high stage refrigerant pump 538 as sfream 536 for combination with the main vapor portion stream 510.
  • the combined sfream flows through exchanger 320 where it is liquefied and in most cases sub-cooled in conduit 510a.
  • the remaining vapor portion stream 512 passes through exchanger 340 where it is also liquefied and sub-cooled in indirect heat exchange with the flashed vapor stream 80 from expansion block 400 and the overhead vapor stream 72 from reflux drum 54 as illusfrated in Fig. 1 thereby producing a liquefied and subcooled stream 524.
  • Streams 72 and 80 are warmed inside exchanger 340 before exiting as sfreams 72a and 86, respectively.
  • Sub-cooled refrigerant sfream 524 from exchanger 340 is combined with the other sub-cooled refrigerant sfream exiting from conduit 510a in exchanger 320.
  • the combined stream is then directed to a pressure reduction means, e.g., expansion valve 526, and expanded to a lower pressure for return to exchanger 320 as coolant sfream 528.
  • the combined refrigerant sfream provides via conduit 528a the refiigeration necessary for cooling the following: • feed gas sfream 12; • methane-rich vapor stream 40a from NGL recovery column 50 in Fig. 1; and
  • Fig. 4 illustrates an exemplary two-stage system for performing this compression and cooling.
  • Sfream 520 is first compressed via low stage refiigerant compressor 522a and then cooled via low stage refiigerant after-cooler 524a thereby producing cooled refiigerant sfream 526 which is directed to the high stage suction scrubber 528 for removal of any condensed refrigerant.
  • the non-condensed refrigerant withdrawn from scrubber 528 is subsequently compressed to final pressure via high stage refiigerant compressor 522b thereby producing a compressed refrigerant sfream 532.
  • the condensed refiigerant separated in scrubber 528 is pumped via refrigerant pump 530 and the resulting sfream 534 combined with compressed refrigerant stream 532.
  • the combine stream then flows through after-cooler 524b, thereby producing stream 502, thus completing the closed circuit.
  • Recovery efficiency is further improved in another embodiment of the present invention wherein a second reflux is infroduced to the NGL recovery column.
  • Fig. 5 represents a schematic embodiment illustrating this improvement to further enhance recovery efficiency.
  • a small slipstream 106 about 12.5%o in the illustrative example, from the pre-cooled feed gas stream 12a in exchanger block 300 is taken for further cooling to substantial condensation by utilizing appropriate refiigeration.
  • slipstream 106 may be sub-cooled depending upon the refrigeration level available for the liquefaction process.
  • Sub-cooled stream 108 exits exchanger block 300 at about -170°F and about 975 psia. Stream 108 is thereafter infroduced into the middle of the rectification section of NGL recovery column 50 as a middle reflux after pressure reduction to the column pressure via expansion valve 110.
  • the feed gas is pre-cooled to a temperature where most of the components having high freezing points are condensed and separated in the liquid phase in separator 22.
  • the vapor sfream withdrawn from separator 22 comprises very few of these high freezing point components, thus eliminating the concerns of freezing.
  • the feed gas contains much heavier components, e.g., hexane, C 6+ alkanes and aromatics, which tend to freeze when cooled to cryogenic temperatures, in particular temperatures below
  • slipstream 30a taken from the vapor portion withdrawn from the top of separator 22 as illusfrated with a dashed line in Fig. 5 can be used as sfream 106 or as an alternative sfream 12 can be cooled to such extent that two phases exist, the resulting stream separated via a separator into gas and liquid streams and the gas stream employed as stream 106 and the liquid sfream optionally fed directly or in combination with another sfream to the distillation column 50. In the cascade process, such additional cooling is readily available in one of the refiigeration stages in the second refiigeration cycle.
  • Table 4 summarizes the overall performance of a LNG process incorporating a second reflux stream as described with reference to Fig. 5. As indicated in Table 4, propane recovery is improved to 99.1%>.
  • the second reflux may be fed to the top of the NGL recovery column alone or in combination with the other top reflux sfream 42. While this will simplify the design of the upper rectification section, the recovery efficiency may be reduced slightly.
  • illusfrated in Fig. 6 high recovery of NGL components can also be achieved by recycling a portion of the sub-cooled LNG at elevated pressure as the top reflux to NGL recovery column 50.
  • the LNG sfream again containing a very low content of NGL components, serves as an enhanced lean reflux to achieve high recovery efficiency in this embodiment.
  • the system illusfrated in Fig. 6 is essentially the same as that illusfrated in Fig. 1 and operates in a similar manner. The difference resides in the source of the top reflux to the NGL recovery column 50.
  • stream 40 withdrawn from the overhead of NGL recovery column 50 is totally liquefied and, in most cases, sub-cooled in exchanger block 300 via conduit 112. Appropriate refrigeration from refrigeration block 200 is used for this liquefaction and sub-cooling.
  • methane-rich overhead sfream 40 Prior to introduction into exchanger block 300, methane-rich overhead sfream 40 maybe raised in pressure via expander/compressor 36 utilizing work extracted from expander 34 when available as previously described. At least a portion of the sub-cooled LNG sfream is re-introduced to the top of NGL recovery column 50 as reflux via line 42.
  • a cryogenic pump 116 may be used to return this reflux to the top of the recovery column as illusfrated via the dashed line.
  • the main portion of the sub-cooled LNG sfream is further cooled before exiting the exchanger block 300 as stream 74 at a much colder temperature about -242 °F.
  • the flow rate of flashed vapor sfream 80 from expansion block 400 is greatly reduced before being directed to the fuel gas system after recovering refiigeration and compression. Additional heat input is provided to the lower stripping section of recovery column 50 to further strip lighter components off bottom liquid sfream 44. This also leads to a reduction in overhead vapor sfream 72 from reflux drum 54 associated with
  • NGL purifying column 70 This overhead vapor stream 72 is also directed to the fuel gas system. Further, a second reflux such as that disclosed in Fig. 5 may be incorporated to further improve recovery efficiency as illustrated previously.
  • the aforementioned method can also be effectively applied for the recovery of C 5+ components alone as those heavy components tend to freeze out and need to be removed prior to final liquefaction.
  • the temperature profile inside the NGL recovery block 100 will typically be warmer and the reflux sfream should be reduced.
  • the NGL recovery block 100 can be further simplified by eliminating the side and bottom reboilers, and others depicted in Fig. 7 as an alternate embodiment of the present invention.
  • the cooled feed gas sfream 16 from the exchanger block 300 is infroduced to the NGL recovery column 50 right below the upper rectifying section.
  • the separator 22, expander 34 and compressor 36 are not required in this example because the feed gas pressure is close to the operating pressure of the NGL recovery column 50.
  • the NGL recovery column 50 contains two sections, trays or packings, preferably packings in both sections. To simplify its design, the typical one or more liquid draw trays for facilitating heat inputs to the lower section of the column are eliminated.
  • the smaller portion, stream 14, instead of being cooled such as via exchanger 18 in Fig. 1, is directly sent to the bottom of the NGL recovery column 50 as a stripping gas to reduce the content of lighter NGL components in the bottom liquid sfream to the maximum extent.
  • stream 14 can be cooled prior to introduction into the bottom of the column via indirect heat exchange with at least a portion of the first liquid sfream (i.e. the bottom liquid sfream) which is produced from the bottom of said column. In the later mode, additional cooling via the first liquid sfream is possible by first flashing the first liquid sfream to a pressure slightly above the operating pressure of the NGL purification column.
  • the bottom liquid sfream 44 containing substantially all of the C5 and heavier hydrocarbons is withdrawn from NGL recovery column 50. It is expanded to about 235 psia and used as a cooling media for reflux exchanger 48a prior to being introduced into the middle portion of NGL purifying column 70 operated at about 220 psia. Within column 70, all C4 and lighter components are removed from the top reflux system as a vapor stream 72, and a stabilized condensate with a Reid vapor pressure (RVP) of less than 12 psia is produced from the bottom.
  • RVP of stabilized condensate is confrolled by external heat input via bottom reboiler 62.
  • column 70 The operation of column 70 is adjusted such that the concentration of C5 and higher components in the vapor sfream 72 is maintained very low.
  • the NGL product stream exits column 70 at about 325°F and is cooled to about 120°F via exchanger 66 for delivery to product stream 68.
  • the vapor phase stream 72 withdrawn from reflux drum 54 at about 113°F will bypass the exchanger block 300 and directly combined with the warm flashed vapor stream 86a from flash drum 82a via the dashed line shown in Fig. 2.
  • the combined sfream is compressed to a higher pressure at about 645 psia and cooled in after-cooler 98. In a typical arrangement demonstrated in Fig.
  • the LNG-bearing gases primarily rich in methane are withdrawn from the top of NGL recovery column 50 via overhead sfream 40a.
  • This stream is directly sent to the exchanger block 300 where it is combined with sfream 104b for liquefaction at an elevated pressure via the ethylene chiller 330b and thereafter undergoes a typical three- stage expansion as detailed in Fig. 2 to produce near ambient LNG for storage.
  • Table 5 summarizes the results of a numerical simulation for the above-cited embodiment for the removal of C 5 and heavier.
  • the vapor sfream 40a from the column 50 contains less than 0.1 ppmv of aromatics, including benzene, cyclo- hexane, and toluene and none for C7 and heavier components.
  • any entrained liquid will have a composition represented by reflux sfream 42.
  • such sfream will contain less than 0.3 ppmv of benzene, thereby significantly reducing the freezing concern incurred by the phenomena of liquid enfrainment.
  • FIG. 8 Embodiment Employing Enhanced Stripping Gas Generation Another aspect of the present invention which offers a significant economic advantage is the cooling of the feed gas by countercurrent heat exchange with a refiigerant sfream comprising a portion of bottom liquid sfream 44 or liquid withdrawn from the lower portion of NGL recovery column 50.
  • Illusfrated in Fig.8 is an alternative arrangement of a cryogenic NGL recovery process incorporating this modification.
  • a side liquid stream 120 is withdrawn from the lower portion of NGL recovery column 50. This liquid is directed to pressure reduction valve 122 to reduce its pressure and thereby flash a portion of the liquid refrigerant.
  • Sfream 126 carries the partially vaporized liquid exiting exchanger 18 to suction knockout drum 128 where it is separated into vapor and liquid portions.
  • the vapor portion withdrawn from the top of knockout drum 128 through line 130 is directed to recycle compressor 132 where it is compressed to a pressure slightly higher than that of the NGL recovery column.
  • the compressed gas from compressor 132 is cooled in cooler 134 prior to re-introduction to NGL recovery column 50 as a stripping gas.
  • the liquid portion accumulated at the bottom of knockout drum 128 is withdrawn via line 136.
  • This liquid portion comprismg primarily propane and heavier hydrocarbons, is pumped by recycle pump 138 to NGL purifying column 70 for further fractionation.
  • stripping gas (sometimes referred to as enrichment gas) supplements the heat requirements in NGL recovery column 50 for stripping volatile components from the bottom liquid stream 44. It also enhances the relative volatility of the key components and, accordingly, improves the separation efficiency of the column, particularly when the column is operated at a relatively high pressure as in the NGL recovery column illusfrated here.
  • FIG. 9 Embodiment Employing Simplified NGL Purification System
  • FIG. 9 where only NGL recovery block 100 is illusfrated, bottom liquid stream 44 from NGL recovery column 50 is split into two portions. One portion 44b is directly infroduced into the middle portion of the NGL purifying column 70, as illusfrated in Figs. 1, 4, 5 and 6. The other portion 44a is directed to reflux exchanger 48 where it is substantially sub-cooled.
  • the sub-cooled liquid 44c from reflux exchanger 48 is introduced to the top of NGL purifying column 70 as reflux to reduce the equilibrium loss of heavy hydrocarbons in vapor sfream 72.
  • An exemplary source for the cold stream for reflux exchanger 48 is a liquid side-draw from NGL recovery column 50 as illusfrated in Fig. 9. Consequently, the reflux drum and pumps can be eliminated.

Abstract

La présente invention concerne des procédés et des appareils destinés à améliorer la récupération des composants relativement moins volatiles (68) à partir d'un courant gazeux riche en méthane (10) sous pression, en vue de produire un produit de NGL (68) et, en même temps, à récupérer de manière séparée les composants relativement plus volatiles qui sont liquéfiés de manière à produire un produit de LNG (84). Les procédés selon la présente invention permettent d'améliorer la séparation et l'efficacité dans la colonne de récupération de NGL, tout en conservant la pression de la colonne, de manière à utiliser de façon efficace et économique la réfrigération mécanique disponible (200). Les procédés selon la présente invention sont particulièrement utiles dans l'élimination d'un courant gazeux de cyclohexanes, de benzène et d'autres hydrocarbones lourds présentant un danger. On obtient les avantages de la présente invention en introduisant un reflux amélioré (42) pauvre en composants de NGL dans la colonne de récupération de NGL (50). De plus, on peut obtenir d'autres avantages en liant de manière thermique un rebouilleur latéral (48) destiné à la colonne de récupération de NGL (50) au condensateur suspendu (52) destiné à la colonne de purification de NGL (70). La mise en oeuvre des procédés selon la présente invention permet de récupérer facilement du propane et des composants plus lourds en excès de 95 %.
PCT/US2001/015721 2000-05-18 2001-05-16 Recuperation amelioree de liquides de gaz naturels (ngl) au moyen de refrigeration et de reflux provenant des installations de gaz naturel liquefie (lng) WO2001088447A1 (fr)

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BRPI0110912-0A BR0110912B1 (pt) 2000-05-18 2001-05-16 processo e aparelho para recuperar componentes relativamente menos voláteis e processo para liquefazer uma corrente de gás pressurizado rico em metano
DZ013341A DZ3341A1 (fr) 2000-05-18 2001-05-16 Récuperation ameliorée de liquides de gaz naturels (ngl) au moyen de réfrigération et de reflux provenant des installations de gaz naturel liquéfié (lng)
AU6163301A AU6163301A (en) 2000-05-18 2001-05-16 Enhanced ngl recovery utilizing refrigeration and reflux from lng plants
AU2001261633A AU2001261633B2 (en) 2000-05-18 2001-05-16 Enhanced NGL recovery utilizing refrigeration and reflux from LNG plants

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US09/733,533 US6401486B1 (en) 2000-05-18 2000-12-08 Enhanced NGL recovery utilizing refrigeration and reflux from LNG plants

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