Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2002-06-07
2003-05-13
Esquivel, Denise L. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S912000, C062S932000
Reexamination Certificate
active
06560989
ABSTRACT:
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 refrig
Harris Christopher Francis
Longhurst Daniel Lewis
McNeil Brian Alfred
Ott Christopher Michael
Roberts Mark Julian
Air Products and Chemicals Inc.
Drake Malik N.
Esquivel Denise L.
Fernbacher John M.
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