Separation of hydrogen-hydrocarbon gas mixtures using closed-loop gas expander refrigeration

Information

  • Patent Grant
  • 6560989
  • Patent Number
    6,560,989
  • Date Filed
    Friday, June 7, 2002
    22 years ago
  • Date Issued
    Tuesday, May 13, 2003
    21 years ago
Abstract
A method for the recovery of hydrogen and one or more hydrocarbons having one or more carbon atoms from a feed gas containing hydrogen and the one or more hydrocarbons, which process comprises cooling and partially condensing the feed gas to provide a partially condensed feed; separating the partially condensed feed to provide a first liquid stream enriched in the one or more hydrocarbons and a first vapor stream enriched in hydrogen; further cooling and partially condensing the first vapor stream to provide an intermediate two-phase stream; and separating the intermediate two-phase stream to yield a further-enriched hydrogen stream and a hydrogen-depleted residual hydrocarbon stream. Some or all of the cooling is provided by indirect heat exchange with cold gas refrigerant generated in a closed-loop gas expander refrigeration cycle.
Description




BACKGROUND OF THE INVENTION




The separation of gas mixtures containing hydrogen and light hydrocarbons is an important and widely-used operation in the refining and petrochemical industries. Many of these gas mixtures contain hydrogen, methane, major amounts of ethane and propane, and lower amounts of heavier saturated hydrocarbons. The recovery of hydrogen from such gas mixtures is an economically important operation in the refining industry. Other gas mixtures, for example gas mixtures produced by steam pyrolysis of saturated hydrocarbons, contain hydrogen, methane, and unsaturated hydrocarbons including ethylene and propylene. The recovery of ethylene and propylene from these mixtures is a large and economically important segment of the petrochemical industry. It is desirable in many cases to recover product quality hydrogen along with the main ethylene and propylene products. The recovery of methane-rich fuel gas also may be desirable.




The separation of these gas mixtures is usually accomplished by cryogenic condensation and fractionation methods, which require large amounts of refrigeration at low temperatures. Many methods have been proposed to provide this refrigeration for the recovery of C


2


or C


3


and heavier hydrocarbons in combination with an upgraded hydrogen product stream. These methods include work expansion of the upgraded hydrogen product gas, refrigeration systems using mixed refrigerants, conventional vapor compression refrigeration systems, Joule-Thomson expansion refrigeration, and various combinations of these refrigeration systems. Other processes utilize absorption for the recovery of C


2


or C


3


and heavier hydrocarbons and for the removal of light hydrocarbon impurities from the hydrogen product stream.




U.S. Pat. No. 5,979,177 describes a process utilizing a binary mixed refrigerant refrigeration system to recover ethylene and hydrogen from cracked gas in an ethylene plant. U.S. Pat. No. 5,626,034 describes a process utilizing two mixed refrigerant refrigeration systems to recover ethylene and hydrogen from cracked gas. However, most ethylene plants utilize cascaded vapor compression-type ethylene and propylene refrigeration systems supplemented with fuel gas expanders to recover ethylene and hydrogen as described in U.S. Pat. Nos. 5,452,581, 5,421,167 and 4,629,484.




A cold absorption process is described in U.S. Pat. No. 5,414,168 which utilizes an internally generated hydrocarbon stream as a solvent with work expansion of the upgraded hydrogen product gas to provide refrigeration for recovery of olefinic hydrocarbons and purified hydrogen from a catalytic dehydrogenation unit effluent gas stream. Another cold absorption process is disclosed in U.S. Pat. No. 5,333,462 which utilizes Joule-Thomson expansion of the separated hydrocarbon liquids to provide refrigeration for recovering heavy hydrocarbons and hydrogen from catalytic cracking off-gas and an auxiliary gas, which is partially condensed to provide the absorption solvent.




U.S. Pat. No. 4,256,476 discloses a process utilizing only Joule-Thomson expansion of the separated hydrocarbon liquids to recover ethane and hydrogen from thermal hydrocracking off-gases. U.S. Pat. No. 4,749,393 describes a cryogenic process utilizing work expansion of the upgraded hydrogen product gas and Joule-Thomson expansion of the separated hydrocarbon liquids to provide refrigeration for recovery of heavy hydrocarbons and hydrogen from hydrogen-lean feed gases. U.S. Pat. No. 4,559,069 describes a multistage fractional condensation process utilizing Joule-Thomson expansion of the separated hydrocarbon liquids and auxiliary vapor compression-type C


2


and C


3


refrigeration units to recover hydrogen and heavy hydrocarbons from multiple feed streams.




U.S. Pat. No. 6,266,977 describes a process to recover C


2


or C


3


and heavier hydrocarbons, including ethylene and/or propylene, utilizing a closed-loop gas expander refrigeration system but does not address the recovery of an upgraded hydrogen product stream or a methane-rich product stream.




Gas expander refrigeration systems of the open-loop and closed-loop type, including some which use nitrogen as the refrigerant, are described for use in hydrocarbon gas liquefaction processes in U.S. Pat. Nos. 6,041,620, 6,041,621, and 6,308,531; PCT Applications WO 95/27179 and WO 97/13109; and German Patent 24 40 215.




There is a need in the refining and petrochemical industries for improved refrigeration methods for the recovery of C


2


or C


3


hydrocarbons in combination with the recovery of hydrogen, particularly at warmer temperature levels of −50° F. to −300° F. The present invention, as described below and defined by the claims which follow, addresses this need with several closed-loop gas expander refrigeration systems for recovering C


2


or C


3


hydrocarbons, hydrogen, and optionally methane from hydrogen-hydrocarbon mixtures.




BRIEF SUMMARY OF THE INVENTION




The invention relates to a method for the recovery of hydrogen and one or more hydrocarbons having one or more carbon atoms from a feed gas containing hydrogen and the one or more hydrocarbons, which process comprises (a) cooling and partially condensing the feed gas to provide a partially condensed feed; (b) separating the partially condensed feed to provide a first liquid stream enriched in the one or more hydrocarbons and a first vapor stream enriched in hydrogen; (c) further cooling and partially condensing the first vapor stream to provide an intermediate two-phase stream; and (d) separating the intermediate two-phase stream to yield a further-enriched hydrogen stream and a hydrogen-depleted residual hydrocarbon stream. Some or all of the cooling in (a), or in (c), or in (a) and (c) is provided by indirect heat exchange with cold gas refrigerant generated in a closed-loop gas expander refrigeration cycle.




The cooling in (a) may be effected in a first heat exchange zone and the further cooling in (c) may be effected in a second heat exchange zone. The method may further comprise introducing the first liquid stream into a stripping column, and withdrawing therefrom a liquid stream further enriched in the one or more hydrocarbons and a residual vapor stream comprising hydrogen and portions of the one or more hydrocarbons.




The method may further comprise reducing the pressure of the hydrogen-depleted residual hydrocarbon stream of (d) to yield a reduced-pressure residual hydrocarbon stream and warming the reduced-pressure residual hydrocarbon stream in the second heat exchange zone by indirect heat exchange with the first vapor stream enriched in hydrogen to provide a portion of the cooling in (c), thereby providing a warmed residual hydrocarbon stream. The method may further comprise combining the residual vapor stream from the stripping column and the warmed residual hydrocarbon stream from the second heat exchange zone to provide a combined residual stream, and warming the combined residual stream by indirect heat exchange with the feed gas in the first heat exchange zone, thereby providing a portion of the cooling of the feed gas in (a).




The cold gas refrigerant generated in the closed-loop gas expander refrigeration cycle may provide cooling in the first and second heat exchange zones by the steps of




(1) compressing and cooling a refrigerant gas to provide a cooled compressed refrigerant gas and dividing the cooled compressed refrigerant gas into a first and a second cooled refrigerant gas stream;




(2) work expanding the first cooled refrigerant gas stream to provided a cooled work-expanded refrigerant gas stream;




(3) further cooling and reducing the pressure of the second cooled refrigerant gas stream to provide a cooled reduced-pressure refrigerant gas stream, wherein reducing the pressure is effected by either work expansion or Joule-Thomson expansion across a throttling valve;




(4) warming the cooled reduced-pressure refrigerant gas stream in the second heat exchange zone to provide at least a portion of the cooling of the first vapor stream in (c), thereby providing a warmed reduced-pressure refrigerant gas stream; and




(5) combining the cooled work-expanded refrigerant gas stream of (2) and the warmed reduced-pressure refrigerant gas stream of (4) to provide a combined reduced-pressure refrigerant gas stream and warming the combined reduced-pressure refrigerant gas stream in the first heat exchange zone to provide at least a portion of the cooling of the feed gas in (a), thereby warming the combined reduced-pressure refrigerant gas stream to provide the refrigerant gas of (1).




The refrigerant gas may be selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.




The method may further comprise warming the further-enriched hydrogen stream of (d) in the first and second heat exchange zones to provide by indirect heat exchange a portion of the cooling of the feed gas in (a) and a portion of the cooling of the first vapor stream in (c).




The cooling in (a) and (c) may be effected in a first heat exchange zone and the method may further comprise introducing the first liquid stream of (b) into a distillation column and withdrawing therefrom a liquid stream enriched in hydrocarbons containing two or more carbon atoms and a residual vapor stream enriched in methane. The intermediate two-phase stream of (c) may be introduced into the distillation column.




The method may further comprise warming the residual vapor stream in the first heat exchange zone to provide by indirect heat exchange at least a portion of the cooling of the feed gas in (a). The method also may further comprise cooling and partially condensing the further-enriched hydrogen stream of (d) in a second heat exchange zone to provide an additional intermediate two-phase stream, and separating the additional intermediate two-phase stream to yield a hydrogen product stream and an additional hydrogen-depleted residual hydrocarbon stream. In addition, the hydrogen product stream may be warmed in the first and second heat exchange zones to provide by indirect heat exchange a portion of the cooling of the feed gas in (a) and a portion of the cooling of the further-enriched hydrogen stream.




The method may further comprise reducing the pressure of the additional hydrogen-depleted residual hydrocarbon liquid stream to yield a reduced-pressure residual hydrocarbon liquid stream, warming the reduced-pressure residual hydrocarbon liquid stream in the second heat exchange zone to yield a two-phase residual hydrocarbon liquid stream, separating the two-phase residual hydrocarbon stream to yield a residual hydrocarbon vapor stream and an enriched hydrocarbon liquid stream, and introducing the enriched hydrocarbon liquid stream into the distillation column as reflux. In addition, the residual hydrocarbon vapor stream may be warmed in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a).




A portion of the feed gas stream may be cooled by indirect heat exchange with one or more hydrocarbon-rich liquid streams withdrawn from a lower part of the distillation column to provide a cooled feed stream and one or more vaporized hydrocarbon-rich streams, the one or more vaporized hydrocarbon-rich streams may be returned to the distillation column to provide boil-up therein, and the cooled feed stream may be combined with the partially condensed feed of (a).




The cold gas refrigerant generated in the closed-loop work expander refrigeration cycle may provide cooling in the first and second heat exchange zones by the steps of




(1) providing a compressed refrigerant gas, cooling the compressed refrigerant gas to provide a cooled compressed refrigerant gas, and dividing the cooled compressed refrigerant gas into a first and a second cooled refrigerant gas stream;




(2) work expanding the first cooled refrigerant gas stream to a first pressure to provided a cooled work-expanded refrigerant gas stream;




(3) further cooling and reducing the pressure of the second cooled refrigerant gas stream to a second pressure to provide a cooled reduced-pressure refrigerant gas stream, wherein reducing the pressure is effected by either work expansion or Joule-Thomson expansion across a throttling valve, and the second pressure is lower than the first pressure;




(4) warming the cooled reduced-pressure refrigerant gas stream in the second heat exchange zone to provide at least a portion of the cooling of the further-enriched hydrogen stream of (d), thereby providing a warmed reduced-pressure refrigerant gas stream;




(5) further warming the warmed reduced-pressure refrigerant gas stream in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a), thereby providing a further-warmed reduced-pressure refrigerant gas;




(6) warming the cooled work-expanded refrigerant gas stream of (2) in the first heat exchange zone to provide at least a portion of the cooling of the feed gas in (a), thereby providing a warmed work-expanded refrigerant gas; and




(7) compressing the further-warmed reduced-pressure refrigerant gas of (5) and the warmed work-expanded refrigerant gas of (6) to provide the compressed refrigerant gas in (1).




The refrigerant gas may be selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.




In another embodiment, the cooling in (a) and (c) may be effected in a first heat exchange zone, the first liquid stream of (b) may be introduced into a stripping column, and a liquid stream enriched in hydrocarbons containing two or more carbon atoms and a residual vapor stream enriched in methane may be withdrawn from the column. In addition, the further-enriched hydrogen stream of (d) may be cooled and partially condensed in a second heat exchange zone to provide a two-phase stream, and the two-phase stream may be separated to yield a hydrogen vapor product stream and an additional hydrocarbon-enriched liquid stream.




The method may further comprise reducing the pressure of the additional hydrocarbon-enriched liquid stream to yield a reduced-pressure hydrocarbon-enriched liquid stream, warming the reduced-pressure hydrocarbon-enriched liquid stream in the second heat exchange zone to provide an additional two-phase stream, separating the additional two-phase stream to provide a vapor stream containing hydrocarbons and residual hydrogen and a liquid stream further enriched in hydrocarbons, and introducing the liquid stream further enriched in hydrocarbons into the top of the stripping column. The vapor stream containing hydrocarbons and residual hydrogen may be warmed in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a).




The method may further comprise warming the hydrogen vapor product stream in the second heat exchange zone to provide by indirect heat exchange a portion of the cooling of the further-enriched hydrogen stream and further warming the hydrogen product stream in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a). The residual vapor stream may be warmed in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a). A portion of the feed gas stream may be cooled by indirect heat exchange with one or more hydrocarbon-rich liquid streams withdrawn from a lower part of the stripping column to provide a cooled feed stream and one or more vaporized hydrocarbon-rich streams, the one or more vaporized hydrocarbon-rich streams may be returned to the stripping column to provide boil-up therein, and the cooled feed stream may be combined with the partially condensed feed of (a).




The cold gas refrigerant generated in the closed-loop gas expander refrigeration cycle may provide cooling in the first and second heat exchange zones by the steps of




(1) providing a compressed refrigerant gas, dividing the compressed refrigerant gas into a first compressed refrigerant gas stream and a second compressed refrigerant gas stream, and work expanding the first compressed refrigerant gas stream to a first pressure to provided a first cooled work-expanded refrigerant gas stream;




(2) cooling the second compressed refrigerant gas stream in the first heat exchange zone to provide a cooled second compressed refrigerant gas stream;




(3) dividing the cooled second compressed refrigerant gas stream into a first portion and a second portion, work expanding the first portion to the first pressure to provide a second cooled work-expanded refrigerant gas stream, and further cooling the second portion in the first heat exchange zone to provide an intermediate cooled compressed refrigerant gas stream;




(4) warming the second cooled work-expanded refrigerant gas stream in the first heat exchange zone to provide a partially-warmed second work-expanded refrigerant gas stream and provide by indirect heat exchange a portion of the cooling of the feed stream in (a), and combining the partially-warmed second work-expanded refrigerant gas stream with the first cooled work-expanded refrigerant gas stream of (1) to provide a combined cooled work-expanded refrigerant gas stream;




(5) warming the combined cooled work-expanded refrigerant gas stream in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a), thereby providing a first warmed refrigerant gas stream;




(6) further cooling the intermediate cooled compressed refrigerant gas stream of (3) to provide a cold compressed refrigerant gas stream, reducing the pressure of the cold compressed refrigerant gas stream to a second pressure by either work expansion or Joule-Thomson expansion across a throttling valve, wherein the second pressure is lower than the first pressure, to provide a cold reduced-pressure refrigerant gas stream;




(7) warming the cold reduced-pressure refrigerant gas stream to provide by indirect heat exchange a portion of the cooling of the further-enriched hydrogen stream of (d) in the second heat exchange zone and a portion of the cooling of the feed gas of (a) in the first heat exchange zone, thereby providing a second warmed refrigerant gas stream; and




(8) compressing the first and second warmed refrigerant gas streams to provide the compressed refrigerant gas in (1).




The refrigerant gas may be selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.











BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS





FIG. 1

is a schematic flow diagram of a first exemplary embodiment of the present invention.





FIG. 2

is a schematic flow diagram of a second exemplary embodiment of the present invention.





FIG. 3

is a schematic flow diagram of a third exemplary embodiment of the present invention.











DETAILED DESCRIPTION OF THE INVENTION




The invention relates to processes for the recovery of hydrogen and one or more hydrocarbons having one or more carbon atoms from a feed gas containing hydrogen and the one or more hydrocarbons. The process utilizes a cryogenic separation method which includes cooling and partially condensing the feed gas to provide a partially condensed feed, separating the partially condensed feed to provide a first liquid stream enriched in the one or more hydrocarbons and a first vapor stream enriched in hydrogen, further cooling and partially condensing the first vapor stream to provide an intermediate two-phase stream, and separating the intermediate two-phase stream to yield a further-enriched hydrogen stream and a hydrogen-depleted residual hydrocarbon stream. Refrigeration to provide some or all of the cooling is obtained by indirect heat exchange with cold refrigerant gas streams generated in any of several closed-loop gas expander refrigeration cycles.




Three embodiments of the invention are described below with reference to the schematic flowsheets of

FIGS. 1

,


2


, and


3


. These flowsheets and the following descriptions are exemplary and do not necessarily limit the invention to any of the specific details shown in the flowsheets and described below.




A first embodiment of the invention is illustrated in the schematic flowsheet of FIG.


1


. Pretreated feed gas is provided in line


101


, typically at a pressure of 100 to 1000 psia and at ambient temperature, and contains hydrogen, one or more light hydrocarbons selected from methane, ethane, ethylene, propane, propylene, and optionally carbon monoxide, nitrogen, and/or C


4




+


hydrocarbons. The feed gas is pretreated in an upstream pretreatment step (not shown) to remove water and other components which can freeze out in the downstream processing. The feed gas is cooled and partially condensed in feed cooler or first heat exchange zone


103


by indirect heat exchange with several cold process streams (described later) to yield partially condensed feed in line


105


. The partially condensed feed is separated in stripping column feed drum


107


to provide a first liquid stream enriched in hydrocarbons in line


109


and a first vapor stream enriched in hydrogen in line


111


.




The first liquid stream may be reduced in pressure across valve


113


and introduced into optional stripping column


115


, in which lighter hydrocarbons, residual hydrogen, and other light gases are stripped and withdrawn in overhead line


117


. The heavier hydrocarbon fraction is withdrawn as liquid from the bottom of the stripping column via line


119


and contains C


2




+


product components such as ethylene and/or propylene. A portion of the stripping column bottoms stream is vaporized in heat exchanger


121


and returned to the column as boilup or stripping vapor. The stripping column typically operates at 50 to 400 psia in the temperature range of −200° F. to +50° F.




The first vapor stream enriched in hydrogen in line


111


is cooled and partially condensed in hydrogen recovery heat exchanger or second heat exchange zone


123


by indirect heat exchange with cold process streams (defined later) to yield a partially condensed stream typically at −200° F. to −300° F. in line


125


. The partially condensed stream is separated in hydrogen recovery drum


127


to yield a hydrogen product stream in line


129


and a hydrogen-depleted residual hydrocarbon stream in line


131


. The hydrogen product stream in line


129


is warmed in second heat exchange zone


123


by indirect heat exchange to provide a portion of the cooling for the stream entering in line


111


, and then is further warmed in first heat exchange zone


103


by indirect heat exchange to provide a portion of the cooling for the feed gas entering in line


101


. The final hydrogen product in line


130


is typically at ambient temperature and 15 to 30 psi below the feed gas pressure in line


101


. This hydrogen product stream, which typically contains 80 to 97 mole % hydrogen, may be further purified by a pressure swing adsorption or membrane process if desired.




The hydrogen-depleted residual hydrocarbon stream in line


131


may be reduced in pressure across valve


133


and is warmed in second heat exchange zone


123


by indirect heat exchange to provide an additional portion of the cooling for the stream entering in line


111


. The warmed stream in line


135


may be combined with the stripped gas stream in line


117


, in which case the combined stream in line


137


is further warmed in first heat exchange zone


103


by indirect heat exchange to provide a portion of the cooling for the feed gas entering in line


101


. The warmed residual hydrogen-hydrocarbon stream is withdrawn via line


138


and can be used as fuel.




A major portion of the refrigeration for this embodiment is provided by a closed-loop gas expander refrigeration system. A refrigerant gas, for example nitrogen, is withdrawn in line


139


from first heat exchange zone


103


and compressed to 600 to 1500 psia in refrigerant compressor


141


. Other refrigerants may be used such as, for example, methane, a mixture of nitrogen and methane, or air. The compressed refrigerant gas is cooled in passage


144


of first heat exchange zone


103


to provide a cooled compressed refrigerant gas, which is divided into a first refrigerant gas stream withdrawn in line


145


and a second refrigerant gas stream in heat exchanger passage


147


. The second refrigerant gas stream is further cooled in heat exchanger passage


147


to provide cooled refrigerant gas in line


149


.




The first refrigerant gas stream in line


145


is work expanded in warm expander


150


to provided a cooled work-expanded refrigerant gas stream in line


151


. The further cooled refrigerant gas in line


149


is work expanded in cold expander


153


to provide a cooled reduced-pressure refrigerant gas stream in line


155


. Alternatively, instead of work expansion, the gas in line


149


can be reduced in pressure and cooled by Joule-Thomson expansion across a throttling valve (not shown). The cooled reduced-pressure refrigerant gas stream in line


155


is warmed in second heat exchange zone


123


to provide at least a portion of the cooling of the stream entering in line


111


, thereby providing a warmed reduced-pressure refrigerant gas stream in line


157


. The warmed reduced-pressure refrigerant gas stream in line


157


and the warmed work-expanded refrigerant gas stream in line


151


may be combined, in which case the combined stream in line


159


is warmed in first heat exchange zone


103


by indirect heat exchange to provide a portion of the cooling for the feed gas entering via line


101


and for the refrigerant flowing through passages


144


and


147


. This provides a warmed reduced-pressure refrigerant gas stream in line


139


which is the refrigerant gas described above.




In stripping column


115


, the first liquid stream in line


109


is separated to produce a light overhead gas stream in line


117


and a C


2




+


- or C


3




+


-enriched hydrocarbon product stream in line


119


that can be further separated and purified in additional columns if desired. The light overhead gas stream in line


117


from the stripping column can be recovered separately or combined with the hydrogen-depleted residual hydrocarbon stream in line


135


from the hydrogen recovery heat exchanger


123


and rewarmed in feed cooler


103


to be recovered as a fuel stream in line


138


. Optionally, a refluxed de-methanizer or de-ethanizer column can be utilized in place of stripping column


115


to increase recovery of the desired hydrocarbon products. Alternatively, the first liquid feed stream in line


109


can be recovered directly from feed drum


107


without stripping or distillation, either as a liquid or vapor product that can be rewarmed in feed cooler


103


if desired to recover refrigeration.




Multiple partial condensation stages can be utilized to provide multiple feed streams to the column or to produce separate hydrocarbon products. For example, a C


3


-rich hydrocarbon product could be produced from a warmer partial condensation stage and a C


2


-rich hydrocarbon produced from a colder partial condensation stage. Stripping columns or refluxed distillation columns could be added to remove lighter impurities from one or both hydrocarbon products.




Alternatively, if the feed gas is lean in C


2


and heavier hydrocarbons, or if no C


2




+


hydrocarbon product is desired, only methane and upgraded methane-rich fuel gas would be recovered. Referring to

FIG. 1

, the upgraded methane-rich fuel gas product could be the hydrogen-depleted hydrocarbon stream in line


131


, or the stripped gas stream in line


117


if stripping column


115


is utilized, or a combination of both, as in line


137


. If the stripping column is utilized, the bottom liquid stream in line


119


could be vaporized in feed cooler


103


to provide refrigeration therein.




A second embodiment of the invention is illustrated in the schematic flowsheet of FIG.


2


. Pretreated feed gas is provided in line


201


, typically at a pressure of 100 to 1000 psia and ambient temperature, and contains hydrogen, one or more light hydrocarbons selected from methane, ethylene, propane, propylene, and optionally carbon monoxide, nitrogen, and/or C


4




+


hydrocarbons. The feed gas is pretreated in an upstream pretreatment step (not shown) to remove water and other components which can freeze out in the downstream processing.




The feed gas may be divided into a first feed gas stream in line


202


and a second feed gas stream in line


203


. The first feed gas stream in line


202


is cooled and partially condensed in feed cooler or first heat exchange zone


204


by indirect heat exchange with several cold process streams (described later) to yield partially condensed feed in line


205


. The second feed gas stream in line


203


is cooled and partially condensed in reboiler heat exchangers


206


and


207


(described later) to yield partially a condensed feed stream in line


208


. The two partially condensed feed streams are combined in line


209


and the combined stream is separated in first feed drum


210


to provide a first liquid stream enriched in hydrocarbons in line


211


and a first vapor stream enriched in hydrogen in line


212


. The first liquid stream in line


211


may be reduced in pressure across valve


213


and is introduced into distillation column


214


.




The first vapor stream in line


212


is further cooled and partially condensed in feed cooler or first heat exchange zone


204


to yield a partially condensed intermediate stream in line


215


, which is separated in second feed drum


216


to yield a further-enriched hydrogen vapor stream in line


217


and a residual hydrocarbon liquid stream in line


218


. The residual hydrocarbon stream in line


218


may be reduced in pressure across valve


219


and introduced into distillation column


214


. The further-enriched hydrogen vapor stream in line


217


is further cooled and partially condensed in hydrogen recovery heat exchanger or second heat exchanger zone


220


to provide a partially condensed stream in line


221


, which is separated in hydrogen recovery drum


222


to provide a hydrocarbon-enriched liquid stream in line


223


and a hydrogen vapor product stream in line


224


.




The hydrogen vapor product stream in line


224


is warmed in second heat exchange zone


220


to provide by indirect heat transfer a portion of the cooling for the stream entering in line


217


, and then is further warmed in first heat exchange zone


204


to provide by indirect heat transfer a portion of the cooling for the feed stream entering in line


202


and the stream entering in line


212


. Final hydrogen product gas is withdrawn via line


225


and typically contains 80 to 97 mole % hydrogen.




The hydrocarbon-enriched liquid stream in line


223


may be reduced in pressure across valve


226


and the reduced-pressure stream in line


227


warmed in second heat exchange zone


220


to provide by indirect heat exchange a portion of the cooling for the stream entering in line


217


. The warmed and partially vaporized stream in line


229


is separated in reflux drum


230


into a liquid hydrocarbon stream in line


231


and a waste hydrocarbon vapor in line


232


. The waste hydrocarbon vapor in line


232


, which comprises chiefly hydrogen, methane, and other light gases such as nitrogen and carbon monoxide, is warmed in first heat exchange zone


204


to provide by indirect heat transfer a portion of the cooling for the feed stream entering in line


202


. The resulting warmed gas stream is withdrawn as a low BTU fuel gas in line


233


.




The liquid hydrocarbon stream in line


231


, which may contain chiefly methane with some ethane and ethylene, may be reduced in pressure across valve


234


and is introduced as reflux into the top of distillation column


214


. A methane-rich overhead stream is withdrawn from the top of the column via line


235


and is warmed in first heat exchange zone


204


to provide by indirect heat transfer a portion of the cooling for the feed stream entering in line


202


. The warmed stream is withdrawn as a high BTU fuel gas in line


236


. Recovered C


2




+


hydrocarbon liquid product is withdrawn via line


237


and may be transferred via pump


238


and line


239


into reboiler heat exchanger


206


, where it can be warmed to provide cooling for feed gas entering in line


203


as earlier described. Final C


2




+


hydrocarbon product then is withdrawn via line


240


. One or more additional liquid sidestreams are withdrawn from distillation column


214


via lines


241


and


242


, vaporized in reboilers


207


and


206


respectively, and returned via lines


243


and


244


to provide boilup vapor to the distillation column.




A major portion of the refrigeration for this embodiment is provided by a closed-loop gas expander refrigeration system. Refrigerant gas streams are withdrawn via lines


241


and


242


from feed cooler or first heat exchange zone


204


. These streams, which may comprise nitrogen as the refrigerant, are typically at two different pressures as explained later. Other refrigerants may be used such as, for example, methane, a mixture of nitrogen and methane, or air. The two refrigerant streams in lines


241


and


242


enter refrigerant compressor


243


at the suction inlets of the appropriate compressor stages. The refrigerant gas is compressed to provide compressed refrigerant in line


244


, and optionally may be further compressed in compressors or companders


245


and


246


which are driven by refrigerant work expansion as described later.




Compressed refrigerant gas, typically at to 600 to 1500 psia, flows via line


247


into first heat exchange zone


204


and is cooled in passage


248


therein. The cooled gas is divided into a first refrigerant stream which is withdrawn via line


249


and a second refrigerant stream which is further cooled in passage


250


. A further cooled refrigerant stream is withdrawn via line


251


. The first refrigerant stream in line


249


is work expanded to a first pressure in warm expander


252


to provide a cooled, work-expanded stream in line


253


, and this stream is warmed in first heat exchange zone


204


to provide by indirect heat exchange a portion of the cooling for the feed gas entering in line


202


and for cooling the refrigerant flowing through passages


248


and


250


. The warmed refrigerant is withdrawn via line


242


as earlier described.




The further cooled refrigerant stream in line


251


is work expanded in cold expander


254


to a second pressure, which is typically lower than the first pressure, to provide a cooled, work-expanded stream in line


255


. This stream is warmed in second heat exchange zone


220


to provide by indirect heat exchange a portion of the cooling for the stream entering in line


217


. The warmed refrigerant in line


256


is further warmed in first heat exchange zone


204


to provide by indirect heat exchange a portion of the cooling for the feed gas entering in line


202


and for cooling the refrigerant flowing through passages


248


and


250


. The warmed refrigerant is withdrawn via line


241


as earlier described.




A third embodiment of the invention is illustrated in the schematic flowsheet of FIG.


3


. Pretreated feed gas is provided in line


301


, typically at a pressure of 100 to 1000 psia and ambient temperature, and contains hydrogen, one or more light hydrocarbons selected from methane, ethylene, propane, propylene, and optionally carbon monoxide, nitrogen, and/or C


4




+


hydrocarbons. The feed gas is pretreated in an upstream pretreatment step (not shown) to remove water and other components which may freeze out in the downstream processing.




The feed gas may be divided into a first feed gas stream in line


302


and a second feed gas stream in line


303


. The first feed gas stream in line


302


is cooled and partially condensed in feed cooler or first heat exchange zone


304


by indirect heat exchange with several cold process streams (described later) to yield partially condensed feed in line


305


. The second feed gas stream in line


303


is cooled and partially condensed in reboiler heat exchangers


306


and


307


(described later) to yield partially a condensed feed stream in line


308


. The two partially-condensed feed streams are combined in line


309


and the combined stream is separated in first feed drum


310


to provide a first liquid stream enriched in hydrocarbons in line


311


and a first vapor stream enriched in hydrogen in line


312


. The first liquid stream in line


311


is introduced into stripping column


313


.




The first vapor stream in line


312


is cooled and partially condensed in feed cooler or first heat exchange zone


304


to yield a partially condensed intermediate stream in line


315


, which is further cooled in hydrogen recovery heat exchanger or second heat exchanger zone


316


to provide a partially condensed stream in line


317


, which is separated in hydrogen recovery drum


318


to provide a hydrocarbon-enriched liquid stream in line


319


and a hydrogen vapor product stream in line


320


.




The hydrogen vapor product stream in line


320


is warmed in second heat exchange zone


316


to provide by indirect heat transfer a portion of the cooling for the stream entering in line


315


, and then is further warmed in first heat exchange zone


304


to provide by indirect heat transfer a portion of the cooling for the stream entering in line


312


and for the feed stream entering in line


302


. Final hydrogen product gas is withdrawn via line


321


and typically contains 80 to 97 mole % hydrogen. The hydrocarbon-enriched liquid stream in line


319


may be reduced in pressure across valve


322


and the resulting stream in line


323


is warmed in second heat exchange zone


316


to provide by indirect heat exchange a portion of the cooling for the stream entering in line


315


. The warmed and partially vaporized stream in line


324


is separated in second feed drum


325


into a liquid hydrocarbon stream in line


326


and a waste hydrocarbon vapor in line


327


. The waste hydrocarbon vapor in line


327


, which comprises chiefly hydrogen, methane, and other light gases such as nitrogen and carbon monoxide, is warmed in first heat exchange zone


304


to provide by indirect heat transfer a portion of the cooling for the feed stream in line


302


. The warmed stream is withdrawn as a low BTU fuel gas in line


328


.




The liquid hydrocarbon stream in line


326


, which may contain chiefly methane with some ethane and ethylene, is pumped by feed pump


329


if necessary and introduced as feed into the top of stripping column


313


. A methane-rich overhead stream is withdrawn from the top of the stripping column via line


330


and is warmed in first heat exchange zone


304


to provide by indirect heat transfer a portion of the cooling for the feed stream entering in line


302


. The warmed stream is withdrawn as a methane-rich synthetic natural gas product in line


331


. Recovered C


2




+


hydrocarbon product is withdrawn from the bottom of stripping column


313


via line


337


. One or more liquid sidestreams are withdrawn from stripping column


313


via lines


332


and


333


, vaporized in reboilers


307


and


306


respectively, and returned via lines


334


and


335


to provide boilup vapor to stripping column


313


.




A major portion of the refrigeration for this embodiment is provided by a closed-loop gas expander refrigeration system. Warmed refrigerant gas streams are withdrawn via lines


336


and


337


from feed cooler or first heat exchange zone


304


. These streams, which may comprise nitrogen as the refrigerant, are typically at two different pressures as explained later. Other refrigerants may be used such as, for example, methane, a mixture of nitrogen and methane, or air. These two streams in lines


336


and


337


enter refrigerant compressor


338


at the suction inlets of the appropriate compressor stages. The refrigerant gas is compressed to provide compressed refrigerant in line


339


, and optionally may be further compressed in compressors or companders


340


and


341


which are driven by refrigerant work expansion as described later.




Compressed refrigerant gas typically at 600 to 1500 psia in line


342


is divided into first and second compressed refrigerant gas streams in lines


343


and


344


respectively. The first compressed refrigerant stream in line


343


is work expanded in warm expander


345


to provide a cooled work-expanded refrigerant stream in line


346


. The second compressed refrigerant gas streams in line


344


is cooled in passage


347


of first heat exchange zone


304


, a portion of the cooled refrigerant stream is withdrawn via line


348


, and the remainder is further cooled in passage


349


to provide cold refrigerant in line


350


.




The cooled refrigerant stream in line


348


is work expanded in cold expander


351


to provide cooled work-expanded refrigerant in line


352


, which is warmed in passage


353


to provide by indirect heat exchange a portion of the cooling for feed gas entering in line


302


, the stream entering in line


312


, and refrigerant flowing in passage


349


. The refrigerant in passage


353


is combined with the cooled work-expanded refrigerant stream in line


346


and the combined stream is further warmed in passage


354


to provide a portion of the cooling for feed gas entering in line


302


and refrigerant flowing in passage


347


. The warmed refrigerant is withdrawn via line


337


as earlier described.




Cold refrigerant in line


350


is further cooled in second heat exchange zone


316


and is reduced in pressure by Joule-Thomson expansion across valve


355


to provide cold refrigerant in line


356


. Alternatively, work expansion by a turboexpander (not shown) may be used to provide the cold refrigerant in line


356


. The cold refrigerant in line


356


is warmed in second heat exchange zone


316


to provide a portion of the cooling for the stream entering in line


327


, and then is further warmed in first heat exchange zone


304


to provide by indirect heat transfer a portion of the cooling for the feed gas entering in line


302


and refrigerant flowing in passages


347


and


349


. The final warmed refrigerant gas is withdrawn via line


336


as earlier described.




The following Examples illustrate the present invention but do not limit the invention to any of the specific details described therein.




EXAMPLE 1




The embodiment of

FIG. 2

is operated using a nitrogen refrigerated cryogenic separation process with two expanders for the recovery of an ethylene/propylene-rich liquid product, a hydrogen product, and a methane-rich high BTU fuel gas stream from the off-gas of a fluid catalytic cracking (FCC) unit. The dried feed gas in line


201


has a flow rate of 16,145 lb moles per hour with a composition of 13.3 mole % hydrogen, 6.8% nitrogen and carbon monoxide (CO), 35.4% methane, 11.3% ethylene, 15.8% ethane, 11.2% propylene and 6.2% propane and heavier hydrocarbons, at 96° F. and 455 psia. Most of the feed is cooled and partially condensed in feed cooler


204


and the remainder is cooled in reboilers


206


and


207


of demethanizer column


214


.




Condensed liquid having a flow rate of 9563 lb moles per hour at −109° F. and containing 28.9 mole % methane and lighter components, 16.8% ethylene, 25.1 % ethane, 18.7% propylene and 10.5% propane and heavier hydrocarbons, is separated in first feed drum


210


and flows via line


211


into demethanizer column


214


. The uncondensed vapor in line


212


is further cooled and partially condensed in feed cooler


204


. The partially condensed stream in line


215


is separated in second feed drum


216


to provide a condensed liquid stream in line


218


with a flow rate of 1430 lb moles per hour at −175° F. containing 75.7 mole % methane and lighter components, 12.4% ethylene, 10.2% ethane, 1.4% propylene and 0.3% propane and heavier hydrocarbons. This stream also is introduced into demethanizer column


214


.




Uncondensed vapor in line


217


is then further cooled and partially condensed in hydrogen recovery heat exchanger


220


and separated in hydrogen recovery drum


222


. Condensed liquid is withdrawn from this drum in line


223


with a flow rate of 2975 lb moles per hour at −291° F. containing 2.9 mole % hydrogen, 23.7% nitrogen and CO, 72.0% methane and 1.4% ethylene and ethane. This condensed liquid is flashed across valve


226


to −290° F. and 145 psia and the flashed stream in line


227


is warmed to a temperature −203° F. and partially vaporized in hydrogen recovery heat exchanger


220


. The two-phase stream in line


229


is separated in reflux drum


230


and unvaporized liquid is withdrawn therefrom via line


231


at 480 lb moles per hour containing 0.1 mole % hydrogen, 4.4% nitrogen and CO, 88.1% methane and 7.4% ethylene and ethane. This liquid is introduced as reflux into the top of demethanizer column


214


.




The vapor stream in line


232


is withdrawn from reflux drum


230


at 2495 lb moles per hour and contains 3.4 mole % hydrogen, 27.4% nitrogen and CO, 68.9% methane and 0.3% ethylene and ethane. The stream is further warmed in feed cooler


204


and is recovered at 93° F. and 140 psia as a low BTU fuel stream in line


233


. The hydrogen-enriched vapor product in line


224


from hydrogen recovery drum


222


, at a flow rate of 2177 lb moles per hour containing 90.7 mole % hydrogen, 8.4% nitrogen and CO, and 0.9% methane, is warmed in hydrogen recovery heat exchanger


220


and feed cooler


204


, and is recovered at 93° F. and 435 psia as a hydrogen product stream in line


225


.




In demethanizer column


214


, the liquid feed and reflux streams are separated to produce a light overhead gas stream in line


235


and an ethylene/propylene-enriched bottom liquid stream in line


237


. The ethylene/propylene-rich liquid product, which is withdrawn at a flow rate of 7152 lb moles per hour and contains 24.9 mole % ethylene, 35.7% ethane, 25.3% propylene and 14.2% propane and heavier hydrocarbons, is recovered from the bottom of demethanizer column


214


via line


237


at −16° F. and 146 psia and is pumped to 550 psia and warmed to 93° F. in the demethanizer column reboiler


206


. This ethylene/propylene-enriched liquid product stream is withdrawn in line


240


and can be further separated in additional distillation columns to produce, for example, purified ethylene and propylene products if desired. The light overhead vapor stream in line


235


, having a flow rate of 4321 lb moles per hour at −190° F. and containing 1.9 mole % hydrogen, 5.3% nitrogen and CO, 92.0% methane and 0.8% ethylene and ethane, is warmed separately in feed cooler


204


and recovered at 93° F. and 135 psia as a high BTU fuel stream in line


236


.




Most of the refrigeration required for this cryogenic separation process is supplied by a closed-loop nitrogen recycle refrigeration system. Two low pressure nitrogen streams, one in line


241


at 2200 lb moles per hour at 93° F. and 47 psia, and the other in line


242


at 22,091 lb moles per hour at 93° F. and 282 psia, are compressed to 1050 psia in refrigerant compressor


243


and in expander-driven compressors or companders


245


and


246


. The compressed nitrogen in line


247


is cooled to 100° F. in an aftercooler (not shown) and then is further cooled in passage


248


of feed cooler


204


. Most of the cooled nitrogen, 22,091 lb moles per hour at −21° F. is withdrawn at an intermediate point of the feed cooler via line


249


, expanded to −150° F. and 289 psia in warm expander


252


, and returned via line


253


to feed cooler


204


to provide low level refrigeration for the feed gas entering in line


202


.




The remainder of the nitrogen, having a flow rate of 2200 lb moles per hour, is cooled in passage


250


to −145° F., expanded to −299° F. and 50 psia in cold expander


254


, and sent to hydrogen recovery heat exchanger


220


to provide low temperature refrigeration for hydrogen recovery. This expanded nitrogen stream is warmed to −203° F. in the hydrogen recovery heat exchanger. Both expanded nitrogen streams in lines


253


and


256


are then warmed separately to 93° F. in the feed cooler to be recycled to the nitrogen compressor at two pressure levels via lines


241


and


242


. A conventional vapor compression refrigeration system utilizing propylene, propane, or freon, for example, is utilized to provide the large amount of high level refrigeration at 43° F. via line


257


, since this type of refrigeration system is generally more efficient than expander refrigeration at temperature levels above about −25° F.




This process recovers 98.0% of the ethylene, 99.85% of the ethane and essentially 100% of the propylene and heavier components from the feed gas in line


201


as a hydrocarbon liquid product in line


240


containing less than 0.0025 mole % methane and lighter impurities. The process also recovers 92.1% of the hydrogen in the feed gas at a purity of 90.7 mole % hydrogen in line


225


. Separate low and high BTU fuel streams in lines


233


and


236


also are recovered with methane purities of 68.9 mole % and 92.0 mole % respectively.




For ethylene and ethane recovery described in this example, it may be desirable to utilize additional expanders to meet the refrigeration requirements of the separation process in a more energy-efficient manner, such as when the feed gas is at a lower pressure or when a higher purity hydrogen product is desired. The closed-loop refrigerant could be expanded to three or more temperature levels from one or more pressure levels and also could be returned to the refrigerant compressor at several pressure levels. If the hydrogen product is required at a significantly lower pressure than the feed, it is also possible to supplement the closed-loop recycle refrigeration by work expansion of the hydrogen. Alternately, if some or all of the hydrocarbon product is recovered as a vapor, a significant amount of refrigeration can be recovered from the vaporization of the recovered liquid and it may be possible to eliminate one or more of the expanders. A similar process can be used to recover propylene and/or propane and hydrogen without ethylene/ethane recovery.




EXAMPLE 2




The embodiment of

FIG. 3

is operated using a nitrogen refrigerated cryogenic separation process with two expanders and a separate low temperature Joule-Thomson expansion refrigeration loop for the recovery of ethylene and heavier hydrocarbons in combination with a hydrogen product stream and a methane-rich gas (synthetic natural gas or SNG) product stream from a mixture of off-gases from various petrochemical units.




Pretreated, dried feed gas in line


301


is provided at a flow rate of 7407 lb moles per hour with a composition of 21.9 mole % hydrogen, 4.3% nitrogen, oxygen (O


2


) and carbon monoxide (CO), 43.6% methane, 5.8% ethylene, 16.2% ethane, 1.7% propylene and 6.5% propane and heavier hydrocarbons, at 99° F. and 335 psia. A major portion of the feed via line


302


is cooled and partially condensed in feed cooler


304


and a small portion of the feed via line


303


is cooled in stripping column reboilers


306


and


307


. Partially condensed feed in line


309


is separated in first feed drum


310


to provide condensed liquid in line


311


at 3835 lb moles per hour and −160° F. containing 43.1 mole % methane and lighter components, 10.6% ethylene, 30.6% ethane, 3.2% propylene and 12.5% propane and heavier hydrocarbons. This liquid is introduced into stripping column


313


.




The uncondensed vapor in line


312


is further cooled and partially condensed in feed cooler


304


and in hydrogen recovery heat exchanger


316


. The partially-condensed stream in line


317


is separated in hydrogen recovery drum


318


and a condensed liquid is withdrawn via line


319


at 1742 lb moles per hour and −258° F. containing 96.8 mole % methane and lighter components, 1.5% ethylene, and 1.7% ethane and heavier hydrocarbons. This liquid is flashed across valve


322


to −259° F. and 70 psia, and then is warmed to −234° F. and partially vaporized in hydrogen recovery heat exchanger


316


to reduce the amount of nitrogen and other light components in the unvaporized liquid. The two-phase stream in line


324


is separated in second feed drum


325


, and a liquid is withdrawn therefrom via line


326


at 1505 lb moles per hour containing 0.1 mole % hydrogen, 3.9% nitrogen, O


2


and CO, 92.4% methane, and 3.6% ethylene and ethane. This liquid optionally may be pumped via pump


329


before introduction into stripping column


313


.




The vapor stream from second feed drum


325


is withdrawn via line


327


at 237 lb moles per hour and contains 13.7 mole % hydrogen, 56.9% methane, and 29.4% nitrogen, O


2


and CO. This vapor stream is further warmed in feed cooler


304


and recovered at 97° F. and 65 psia as a low BTU fuel stream in line


328


. The hydrogen-enriched vapor is withdrawn from hydrogen recovery drum


318


via line


320


at 1830 lb moles per hour and contains 85.6 mole % hydrogen, 5.9% methane, and 8.5% nitrogen, O


2


and CO. This stream is warmed in hydrogen recovery heat exchanger


316


and feed cooler


304


, and is recovered at 97° F. and 320 psia as a hydrogen product stream in line


321


. The hydrogen product stream can be further purified in a pressure swing adsorption or membrane system to provide a higher purity hydrogen product if desired.




In stripping column


313


, the liquid feed streams are separated to produce a light overhead gas stream in line


330


and an ethane/propane-enriched bottom liquid stream in line


337


. The ethane/propane-rich liquid product in line


337


, which is withdrawn at a flow rate of 2140 lb moles per hour and contains 0.5 mole % methane, 17.9% ethylene, 53.5% ethane, 5.7% propylene, and 22.4% propane and heavier hydrocarbons, is recovered at −5° F. and 183 psia. This ethane/propane-enriched liquid stream can be further separated in additional distillation columns to produce, for example, separate ethane/ethylene and propane/propylene products if desired. A light overhead vapor stream is withdrawn from the stripping column via line


330


at a flow rate of 3201 lb moles per hour and −150° F. and contains 0.7 mole % hydrogen, 92.9% methane, 3.4% ethylene and ethane, and 3.0% nitrogen, O


2


and CO. This vapor stream is warmed in feed cooler


304


and recovered at 97° F. and 176 psia as a high BTU, low nitrogen synthetic natural gas stream via line


331


.




Most of the refrigeration required for this cryogenic separation process is supplied by a closed-loop nitrogen recycle refrigeration system. Two low pressure nitrogen streams, one in line


336


at 255 lb moles per hour, 97° F., and 58 psia and the other in line


337


at 17,057 lb moles per hour, 97° F., and 204 psia, are compressed to 1000 psia in refrigerant compressor


338


and expander-driven compressors or companders


340


and


341


. These are driven by the expanders described below. The compressed nitrogen in line


342


is cooled to 100° F. in an aftercooler (not shown), and a first portion of the nitrogen in line


343


at 5199 lb moles per hour is work expanded to 86° F. and 209 psia in warm expander


345


and flows via line


346


to feed cooler


304


to provide high level refrigeration therein.




The second portion of the compressed nitrogen stream in line


344


is cooled in flow passage


347


of feed cooler


304


to −110° F. This cooled nitrogen stream is divided and a first portion is withdrawn from an intermediate point of feed cooler


304


via line


348


at a flow rate of 11,858 lb moles per hour, expanded to −233° F. and 209 psia in the cold expander


351


, and returned via line


352


to feed cooler


304


to provide low level refrigeration in passageway


353


therein. The second portion of the cooled nitrogen at 255 lb moles per hour is further cooled in passage


349


to −258° F., withdrawn via line


350


, further cooled in hydrogen recovery heat exchanger


316


, and is reduced in pressure by Joule-Thomson expansion across valve


355


to −294° F. and 63 psia. The cooled, expanded nitrogen in line


356


is warmed in hydrogen recovery heat exchanger


316


to provide low temperature refrigeration therein, is further warmed to 97° F. in feed cooler


304


to provide refrigeration therein, and is withdrawn via line


336


and recycled to refrigerant compressor


338


as earlier described. The two work-expanded nitrogen streams in line


346


and passageway


353


are combined and warmed in passageway


354


of feed cooler


304


to provide refrigeration therein, and the warmed nitrogen is withdrawn at 97° F. via line


337


to nitrogen compressor


338


at an intermediate pressure level.




The process of this Example recovers 88.6% of the ethylene, 95.2% of the ethane, 99.5% of the propylene and propane, and essentially 100% of the C


4


and heavier components in the feed gas to yield the liquid hydrocarbon product in line


355


which contains 0.5 mole % methane impurity. The process also recovers 96.55% of the hydrogen in the feed gas at a purity of 85.6 mole % hydrogen in line


321


. More than 92% of the methane in the feed gas is recovered as a synthetic natural gas product stream in line


331


containing 92.9 mole % methane with 0.7% hydrogen and 3.0% nitrogen and other light impurities.




A higher purity hydrogen product can be achieved by cooling the nitrogen Joule-Thomson expansion stream in line


356


to a colder temperature by expansion to a lower pressure and temperature. This would allow hydrogen recovery drum


318


to be operated at a lower temperature. For example, if the flow of the nitrogen stream in line


344


were to be increased by about 35% and the cooled nitrogen stream in line


350


and the vapor in line


324


from first feed drum


310


were to be cooled to −296° F. in hydrogen recovery heat exchanger


316


by expanding the nitrogen across valve


355


to −299° F. and 48 psia, a hydrogen purity of 95.5 mole % could be achieved in hydrogen recovery drum


318


.




Many alternative flow schemes are possible for the closed-loop refrigeration system of the present invention. These alternatives may result in lower power requirements and/or lower capital cost, depending on the particular requirements for refrigeration at various temperature levels. These refrigeration requirements are determined primarily by the feed gas pressure and composition and the level of hydrocarbon product recovery and hydrogen product purity required. A portion of the closed-loop refrigerant can be expanded to a higher pressure level in one of the expanders and returned to the refrigerant compressor at an intermediate pressure level as in

FIG. 2

, or all of the expanders can operate at the same pressure levels, as in FIG.


3


. Alternatively, a portion of the closed-loop refrigerant could be removed from the refrigerant compressor at an intermediate stage, cooled separately and expanded in one of the expanders to the lowest pressure level or to another intermediate pressure level. The separate Joule-Thomson expansion loop utilized to provide refrigeration for the hydrogen recovery section of the process in

FIG. 3

would typically operate at the lowest pressure level but may be operated at any intermediate pressure level. The Joule-Thomson expansion could be replaced by work expansion if the amount of low temperature refrigeration required is large enough to justify the cost of an additional expander, as in FIG.


2


.




In this invention, the closed-loop gas expander refrigeration system can supply refrigeration at any required temperature level, but operates most efficiently and economically in the range of about −50° F. to −300° F. At this low temperature level, very high C


2


or C


3


hydrocarbon and hydrogen recovery is possible even with relatively low pressure feed gases, and thus minimal feed compression would be required. The closed-loop recycle refrigerated process can achieve much higher recoveries than processes that utilize work expansion of feed gas or light residue gas, where recovery is limited by the refrigeration available between the feed gas inlet pressure and the residue gas delivery pressure.




In utilizing nitrogen as the refrigerant, this process will have a lower capital cost than similar processes that utilize conventional cascade refrigeration systems or mixed refrigerant refrigeration systems due to the relatively low cost and high efficiency of nitrogen compressors and expanders as compared to hydrocarbon compression equipment. No complex refrigerant make-up systems are required because nitrogen is normally available in most refinery and petrochemical facilities for use as inert gas or for purging of equipment. Alternatively, air, methane, or mixtures of nitrogen and methane could be utilized as the refrigerant gas if desired.




Since most of the closed-loop refrigerant typically is maintained above 100 psia and preferably above 200 psia throughout the process, pressure drop losses are small compared to hydrocarbon or freon refrigerants that are generally vaporized at much lower pressures for refrigeration. Typically the gaseous refrigerant of the present invention is compressed to at least 600 psia, and preferably at least 1000 psia, to provide the most energy-efficient process. Higher pressures are usually even more energy-efficient, but the power savings must be evaluated against the additional cost of higher-pressure equipment.




A plant using any of the processes described above also will have a lower capital cost than plants which utilize absorption for hydrocarbon and hydrogen recovery, since those processes require multiple distillation columns to absorb and strip the hydrocarbon product and light gases from the absorption solvents, in addition to any columns required to remove light or heavy impurities from the hydrocarbon product(s). Also, a significant amount of external refrigeration usually is required to refrigerate the solvents in order to achieve high C


2


and hydrogen recovery.



Claims
  • 1. A method for the recovery of hydrogen and one or more hydrocarbons having one or more carbon atoms from a feed gas containing hydrogen and the one or more hydrocarbons, which process comprises:(a) cooling and partially condensing the feed gas to provide a partially condensed feed; (b) separating the partially condensed feed to provide a first liquid stream enriched in the one or more hydrocarbons and a first vapor stream enriched in hydrogen; (c) further cooling and partially condensing the first vapor stream to provide an intermediate two-phase stream; and (d) separating the intermediate two-phase stream to yield a further-enriched hydrogen stream and a hydrogen-depleted residual hydrocarbon stream; wherein some or all of the cooling in (a), or in (c), or in (a) and (c) is provided by indirect heat exchange with cold gas refrigerant generated in a closed-loop gas expander refrigeration cycle.
  • 2. The method of claim 1 wherein the cooling in (a) is effected in a first heat exchange zone and the further cooling in (c) is effected in a second heat exchange zone.
  • 3. The method of claim 2 which further comprises introducing the first liquid stream into a stripping column, and withdrawing therefrom a liquid stream further enriched in the one or more hydrocarbons and a residual vapor stream comprising hydrogen and portions of the one or more hydrocarbons.
  • 4. The method of claim 3 which further comprises reducing the pressure of the hydrogen-depleted residual hydrocarbon stream of (d) to yield a reduced-pressure residual hydrocarbon stream and warming the reduced-pressure residual hydrocarbon stream in the second heat exchange zone by indirect heat exchange with the first vapor stream enriched in hydrogen to provide a portion of the cooling in (c), thereby providing a warmed residual hydrocarbon stream.
  • 5. The method of claim 4 which further comprises combining the residual vapor stream from the stripping column and the warmed residual hydrocarbon stream from the second heat exchange zone to provide a combined residual stream, and warming the combined residual stream by indirect heat exchange with the feed gas in the first heat exchange zone, thereby providing a portion of the cooling of the feed gas in (a).
  • 6. The method of claim 2 wherein the cold gas refrigerant generated in the closed-loop gas expander refrigeration cycle provides cooling in the first and second heat exchange zones by the steps of(1) compressing and cooling a refrigerant gas to provide a cooled compressed refrigerant gas and dividing the cooled compressed refrigerant gas into a first and a second cooled refrigerant gas stream; (2) work expanding the first cooled refrigerant gas stream to provided a cooled work-expanded refrigerant gas stream; (3) further cooling and reducing the pressure of the second cooled refrigerant gas stream to provide a cooled reduced-pressure refrigerant gas stream, wherein reducing the pressure is effected by either work expansion or Joule-Thomson expansion across a throttling valve; (4) warming the cooled reduced-pressure refrigerant gas stream in the second heat exchange zone to provide at least a portion of the cooling of the first vapor stream in (c), thereby providing a warmed reduced-pressure refrigerant gas stream; and (5) combining the cooled work-expanded refrigerant gas stream of (2) and the warmed reduced-pressure refrigerant gas stream of (4) to provide a combined reduced-pressure refrigerant gas stream and warming the combined reduced-pressure refrigerant gas stream in the first heat exchange zone to provide at least a portion of the cooling of the feed gas in (a), thereby warming the combined reduced-pressure refrigerant gas stream to provide the refrigerant gas of (1).
  • 7. The method of claim 6 wherein the refrigerant gas is selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.
  • 8. The method of claim 2 which further comprises warming the further-enriched hydrogen stream of (d) in the first and second heat exchange zones to provide by indirect heat exchange a portion of the cooling of the feed gas in (a) and a portion of the cooling of the first vapor stream in (c).
  • 9. The method of claim 1 wherein the cooling in (a) and (c) is effected in a first heat exchange zone and wherein the method further comprises introducing the first liquid stream of (b) into a distillation column, and withdrawing therefrom a liquid stream enriched in hydrocarbons containing two or more carbon atoms and a residual vapor stream enriched in methane.
  • 10. The method of claim 9 which further comprises introducing the intermediate two-phase stream of (c) into the distillation column.
  • 11. The method of claim 9 which further comprises warming the residual vapor stream in the first heat exchange zone to provide by indirect heat exchange at least a portion of the cooling of the feed gas in (a).
  • 12. The method of claim 9 which further comprises cooling and partially condensing the further-enriched hydrogen stream of (d) in a second heat exchange zone to provide an additional intermediate two-phase stream, and separating the additional intermediate two-phase stream to yield a hydrogen product stream and an additional hydrogen-depleted residual hydrocarbon stream.
  • 13. The method of claim 12 which further comprises warming the hydrogen product stream in the first and second heat exchange zones to provide by indirect heat exchange a portion of the cooling of the feed gas in (a) and a portion of the cooling of the further-enriched hydrogen stream.
  • 14. The method of claim 12 which further comprises reducing the pressure of the additional hydrogen-depleted residual hydrocarbon liquid stream to yield a reduced-pressure residual hydrocarbon liquid stream, warming the reduced-pressure residual hydrocarbon liquid stream in the second heat exchange zone to yield a two-phase residual hydrocarbon liquid stream, separating the two-phase residual hydrocarbon stream to yield a residual hydrocarbon vapor stream and an enriched hydrocarbon liquid stream, and introducing the enriched hydrocarbon liquid stream into the distillation column as reflux.
  • 15. The method of claim 14 which further comprises warming the residual hydrocarbon vapor stream in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a).
  • 16. The method of claim 9 wherein a portion of the feed gas stream is cooled by indirect heat exchange with one or more hydrocarbon-rich liquid streams withdrawn from a lower part of the distillation column to provide a cooled feed stream and one or more vaporized hydrocarbon-rich streams, the one or more vaporized hydrocarbon-rich streams are returned to the distillation column to provide boil-up therein, and the cooled feed stream is combined with the partially condensed feed of (a).
  • 17. The method of claim 12 wherein the cold gas refrigerant generated in the closed-loop work expander refrigeration cycle provides cooling in the first and second heat exchange zones by the steps of(1) providing a compressed refrigerant gas, cooling the compressed refrigerant gas to provide a cooled compressed refrigerant gas, and dividing the cooled compressed refrigerant gas into a first and a second cooled refrigerant gas stream; (2) work expanding the first cooled refrigerant gas stream to a first pressure to provided a cooled work-expanded refrigerant gas stream; (3) further cooling and reducing the pressure of the second cooled refrigerant gas stream to a second pressure to provide a cooled reduced-pressure refrigerant gas stream, wherein reducing the pressure is effected by either work expansion or Joule-Thomson expansion across a throttling valve, and the second pressure is lower than the first pressure; (4) warming the cooled reduced-pressure refrigerant gas stream in the second heat exchange zone to provide at least a portion of the cooling of the further-enriched hydrogen stream of (d), thereby providing a warmed reduced-pressure refrigerant gas stream; (5) further warming the warmed reduced-pressure refrigerant gas stream in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a), thereby providing a further-warmed reduced-pressure refrigerant gas; (6) warming the cooled work-expanded refrigerant gas stream of (2) in the first heat exchange zone to provide at least a portion of the cooling of the feed gas in (a), thereby providing a warmed work-expanded refrigerant gas; and (7) compressing the further-warmed reduced-pressure refrigerant gas of (5) and the warmed work-expanded refrigerant gas of (6) to provide the compressed refrigerant gas in (1).
  • 18. The method of claim 17 wherein the refrigerant gas is selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.
  • 19. The method of claim 1 wherein the cooling in (a) and (c) is effected in a first heat exchange zone and wherein the method further comprises introducing the first liquid stream of (b) into a stripping column and withdrawing therefrom a liquid stream enriched in hydrocarbons containing two or more carbon atoms and a residual vapor stream enriched in methane.
  • 20. The method of claim 19 which further comprises cooling and partially condensing the further-enriched hydrogen stream of (d) in a second heat exchange zone to provide a two-phase stream and separating the two-phase stream to yield a hydrogen vapor product stream and an additional hydrocarbon-enriched liquid stream.
  • 21. The method of claim 20 which further comprises reducing the pressure of the additional hydrocarbon-enriched liquid stream to yield a reduced-pressure hydrocarbon-enriched liquid stream, warming the reduced-pressure hydrocarbon-enriched liquid stream in the second heat exchange zone to provide an additional two-phase stream, separating the additional two-phase stream to provide a vapor stream containing hydrocarbons and residual hydrogen and a liquid stream further enriched in hydrocarbons, and introducing the liquid stream further enriched in hydrocarbons into the top of the stripping column.
  • 22. The method of claim 21 which further comprises warming the vapor stream containing hydrocarbons and residual hydrogen in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a).
  • 23. The method of claim 20 which further comprises warming the hydrogen vapor product stream in the second heat exchange zone to provide by indirect heat exchange a portion of the cooling of the further-enriched hydrogen stream and further warming the hydrogen product stream in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a).
  • 24. The method of claim 20 which further comprises warming the residual vapor stream in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a).
  • 25. The method of claim 20 wherein a portion of the feed gas stream is cooled by indirect heat exchange with one or more hydrocarbon-rich liquid streams withdrawn from a lower part of the stripping column to provide a cooled feed stream and one or more vaporized hydrocarbon-rich streams, the one or more vaporized hydrocarbon-rich streams are returned to the stripping column to provide boil-up therein, and the cooled feed stream is combined with the partially condensed feed of (a).
  • 26. The method of claim 20 wherein the cold gas refrigerant generated in the closed-loop gas expander refrigeration cycle provides cooling in the first and second heat exchange zones by the steps of(1) providing a compressed refrigerant gas, dividing the compressed refrigerant gas into a first compressed refrigerant gas stream and a second compressed refrigerant gas stream, and work expanding the first compressed refrigerant gas stream to a first pressure to provided a first cooled work-expanded refrigerant gas stream; (2) cooling the second compressed refrigerant gas stream in the first heat exchange zone to provide a cooled second compressed refrigerant gas stream; (3) dividing the cooled second compressed refrigerant gas stream into a first portion and a second portion, work expanding the first portion to the first pressure to provide a second cooled work-expanded refrigerant gas stream, and further cooling the second portion in the first heat exchange zone to provide an intermediate cooled compressed refrigerant gas stream; (4) warming the second cooled work-expanded refrigerant gas stream in the first heat exchange zone to provide a partially-warmed second work-expanded refrigerant gas stream and provide by indirect heat exchange a portion of the cooling of the feed stream in (a), and combining the partially-warmed second work-expanded refrigerant gas stream with the first cooled work-expanded refrigerant gas stream of (1) to provide a combined cooled work-expanded refrigerant gas stream; (5) warming the combined cooled work-expanded refrigerant gas stream in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a), thereby providing a first warmed refrigerant gas stream; (6) further cooling the intermediate cooled compressed refrigerant gas stream of (3) to provide a cold compressed refrigerant gas stream, reducing the pressure of the cold compressed refrigerant gas stream to a second pressure by either work expansion or Joule-Thomson expansion across a throttling valve, wherein the second pressure is lower than the first pressure, to provide a cold reduced-pressure refrigerant gas stream; (7) warming the cold reduced-pressure refrigerant gas stream to provide by indirect heat exchange a portion of the cooling of the further-enriched hydrogen stream of (d) in the second heat exchange zone and a portion of the cooling of the feed gas of (a) in the first heat exchange zone, thereby providing a second warmed refrigerant gas stream; and (8) compressing the first and second warmed refrigerant gas streams to provide the compressed refrigerant gas in (1).
  • 27. The method of claim 26 wherein the refrigerant gas is selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.
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