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Cryogenic separation of gaseous mixtures

  • US 4,900,347 A
  • Filed: 04/05/1989
  • Issued: 02/13/1990
  • Est. Priority Date: 04/05/1989
  • Status: Expired due to Term
First Claim
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1. A cryogenic separation method for recovering C1+ hydrocarbons from cracked hydrocarbon feed gas comprising methane, ethene and ethane, wherein cold pressurized gaseous streams are separated in a plurality of dephlegmator units, each of said dephlegmator units being operatively connected to accumulate condensed liquid in a lower dephlegmator drum vessel by gravity flow from an upper dephlegmator heat exchanger comprising a plurality of vertically disposed indirect heat exchange passages through which gas from the lower drum vessel passes in an upward direction for cooling with refrigerant fluid by indirect heat exchange within said heat exchange passages, whereby gas flowing upwardly is partially condensed on vertical surfaces of said passages to form a reflux liquid in direct contact with the upward flowing gas stream to provide a condensed stream of cooler liquid flowing downwardly and thereby enriching condensed dephlegmator liquid gradually with C2+ hydrocarbon components;

  • comprising the steps of;

    introducing dry feed gas into a primary dephlegmation zone having a plurality of serially connected, sequentially colder dephlegmator units for separation of feed gas into a primary methane-rich gas stream recovered at low temperature and at least one primary liquid condensate stream rich in C2+ hydrocarbon components and containing a minor amount of methane;

    passing at least one primary liquid condensate stream from the primary dephlegmation zone to serially connected demethanizer fractionators, wherein a moderately low cryogenic temperature is employed in a first demethanizer fractionator unit to recover substantially all of the methane from the primary liquid condensate stream in a first demethanizer overhead vapor stream and to recover a first C2+ liquid demethanizer bottoms stream substantially free of methane, wherein said demethanizer overhead vapor stream is cooled with moderately low temperature coolant to provide liquid reflux for recycled to a top portion of the first demethanizer fractionator;

    further separating at least a portion of the first demethanizer overhead vapor stream in an ultra-low temperature final demethanizer fractionator unit to recover a liquid ethene-rich predominantly C2 hydrocarbon crude product stream and a final demethanizer ultra-low temperature overhead vapor stream substantially free of C2+ hydrocarbons, wherein a major amount of total demethanization heat exchange duty is provided by moderately low temperature refrigerant and overall energy requirements for refrigeration utilized in separating C2+ hydrocarbons from methane and lighter components are decreased; and

    fractionating said second crude ethene stream and said first ethene-rich C2 hydrocarbon crude product stream to obtain a pure ethene product.

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