C3+ recovery with membranes

Abstract

A method of separating hydrocarbons containing three or more carbon atoms from an off-gas stream is provided. This method includes separating a light ends stream from a fractionator, thereby producing a stream rich in hydrocarbons containing three or more carbon atoms, and a stream lean in hydrocarbons containing three or more carbon atoms, separating the stream lean in hydrocarbons containing three or more carbon atoms in a membrane unit, thereby producing a permeate stream enriched in hydrocarbons containing three or more carbon atoms and a retentate stream, and separating the stream rich in hydrocarbons containing three or more carbon atoms in one or more separation columns, thereby producing one or more streams selected from the group consisting of a propylene stream, a propane stream, a butane stream, a light cat naptha stream, and a heavy cat naptha stream.

What is claimed is: 
     
         1 . A method of separating hydrocarbons containing three or more carbon atoms from an off-gas stream, comprising:
 a) separating a light ends stream ( 204 ) from a fractionator ( 203 ) comprising hydrocarbons containing three or more carbon atoms in main absorption unit ( 210 ), thereby producing a stream rich in hydrocarbons containing three or more carbon atoms ( 211 ), and a stream lean in hydrocarbons containing three or more carbon atoms ( 212 ),   b) separating the stream lean in hydrocarbons containing three or more carbon atoms ( 212 ) in a membrane unit ( 501 ), thereby producing a permeate stream enriched in hydrocarbons containing three or more carbon atoms ( 503 ) and a retentate stream ( 502 ), and   c) separating the stream rich in hydrocarbons containing three or more carbon atoms ( 211 ) in one or more separation columns ( 217 ), thereby producing one or more streams selected from the group consisting of a propylene stream, a propane stream, a butane stream, a light cat naptha stream, and a heavy cat naptha stream.   
     
     
         2 . The method of  claim 1 , wherein the permeate stream enriched in hydrocarbons containing three or more carbon atoms ( 503 ) is recycled to the fractionator ( 203 ). 
     
     
         3 . The method of  claim 1 , wherein the permeate stream enriched in hydrocarbons containing three or more carbon atoms ( 503 ) is sent to the one or more separation columns ( 217 ). 
     
     
         4 . The method of  claim 1 , further comprising a secondary absorption unit ( 214 ),
 b1) wherein the stream lean in hydrocarbons containing three or more carbon atoms ( 212 ) is introduced into the secondary absorption unit ( 214 ), thereby producing a secondary absorber off-gas stream ( 215 ) which is sent to membrane unit ( 501 ), thereby producing the permeate stream enriched in hydrocarbons containing three or more carbon atoms ( 503 ) and the retentate stream ( 502 ), and   b2) wherein the permeate stream enriched in hydrocarbons containing three or more carbon atoms ( 503 ) is recycled to the fractionator ( 203 ).   
     
     
         5 . The method of  claim 1 , further comprising a carbon dioxide removal unit ( 701 ),
 b1) wherein the stream lean in hydrocarbons containing three or more carbon atoms ( 212 ) is introduced into the carbon dioxide removal unit ( 701 ), thereby producing a carbon dioxide lean stream ( 703 ) which is sent to membrane unit ( 501 ), thereby producing the permeate stream enriched in hydrocarbons containing three or more carbon atoms ( 503 ) and the retentate stream ( 502 ), and   b2) wherein the permeate stream enriched in hydrocarbons containing three or more carbon atoms ( 503 ) is recycled to the fractionator ( 203 ).   
     
     
         6 . The method of  claim 1 , wherein the one or more separation columns ( 217 ) comprise one or more units selected from the group consisting of a debutanizer column, a gasoline splitting column, a depropanizer column, and propane splitting column. 
     
     
         7 . A method of separating hydrocarbons containing three or more carbon atoms from an off-gas stream, comprising:
 a) combining a light ends stream ( 204 ) from a fractionator ( 203 ) comprising hydrocarbons containing three or more carbon atoms and a permeate stream ( 911 ) to form a combined stream ( 912 ),   b) separating the combined stream ( 912 ) in main absorption unit ( 210 ), thereby producing a stream rich in hydrocarbons containing three or more carbon atoms ( 211 ), and a stream lean in hydrocarbons containing three or more carbon atoms ( 212 ),   c) separating an at least partially condensed stream lean in hydrocarbons containing three or more carbon atoms ( 212 ) in a cold separator ( 902 ), thereby producing a liquid stream ( 903 ), and a vapor stream ( 904 ),   d) warming the vapor stream ( 904 ) to ambient temperature, then separating the ambient temperature stream in a membrane unit ( 909 ), thereby producing the permeate stream ( 911 ) and a retentate stream ( 910 ), and   e) separating the stream rich in hydrocarbons containing three or more carbon atoms ( 211 ) in one or more separation columns ( 217 ), thereby producing one or more streams selected from the group consisting of a propylene stream, a propane stream, a butane stream, a light cat naptha stream, and a heavy cat naptha stream.   
     
     
         8 . The method of  claim 7 , wherein the one or more separation columns ( 217 ) comprise one or more units selected from the group consisting of a separator, a debutanizer column, a gasoline splitting column, a depropanizer column, and propane splitting column.