Methods for recovering nitrogen from process gas streams

Abstract

Methods and systems for recovering nitrogen and alkenes (e.g. ethylene, propylene) from process gas streams, including multi-step condensing of the process gas stream, are provided herein.

What is claimed is: 
     
         1 . A method for recovering nitrogen from a process gas stream comprising:
 cooling the process gas stream comprising nitrogen and C 2 -C 4  alkene in a first condenser vessel with a first cooling medium under suitable conditions to produce a first condensate comprising impurities, a first cooled process gas stream comprising nitrogen and C 2 -C 4  alkene, and a first gaseous nitrogen stream;   cooling the cooled process gas stream in a second condenser vessel with a second cooling medium under suitable conditions to produce a second condensate comprising C 2 -C 4  alkene, a nitrogen-containing vent gas stream, and second gaseous nitrogen stream; and   combining at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream.   
     
     
         2 . The method of  claim 1 , wherein the process gas stream comprises one or more petrochemical vent gas streams. 
     
     
         3 . The method of  claim 1 , wherein the process gas stream comprises at least about 1.0 wt % nitrogen and less than about 80 wt % C 2 -C 4  alkene. 
     
     
         4 . The method of  claim 1 , wherein the impurities are hydrogen, water, C 1 -C 20  hydrocarbons, and/or oxygenates. 
     
     
         5 . The method of  claim 1 , wherein the nitrogen-containing vent gas stream comprises at least about 90 wt % nitrogen based on the total weight of the nitrogen-containing vent gas stream. 
     
     
         6 . The method of  claim 1 , wherein the first condenser vessel and/or the second condenser vessel each independently comprises a coil heat exchanger, a shell and tube heat exchanger or a plate heat exchanger. 
     
     
         7 . The method of  claim 1 , wherein the first cooling medium and the second cooling medium is liquid nitrogen and/or gaseous nitrogen. 
     
     
         8 . The method of  claim 1 , wherein the first cooling medium and/or the second cooling medium is provided at a temperature at least about −196° C. and a pressure of less than or equal to about 1500 kPa. 
     
     
         9 . The method of  claim 1 , wherein the combination of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream comprises greater than 90 wt % nitrogen, based on the total weight of the combination. 
     
     
         10 . The method of  claim 1 , wherein the combination of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream comprises nitrogen, based on the total weight of the combination, in an amount greater than the nitrogen-containing vent gas stream, based on the total weight of the nitrogen-containing vent gas stream. 
     
     
         11 . The method of  claim 1 , wherein the C 2 -C 4  alkene is ethylene and/or propylene. 
     
     
         12 . A system for recovering nitrogen gas from a process gas stream comprising:
 a process gas stream comprising nitrogen and C 2 -C 4  alkene;   a first cooling medium stream;   a first condensate stream comprising impurities;   a cooled process gas stream comprising nitrogen and C 2 -C 4  alkene;   a first gaseous nitrogen stream;   a second cooling medium stream;   a second condensate stream comprising C 2 -C 4  alkene;   a nitrogen-containing vent gas stream;   a second gaseous nitrogen stream;   a combined nitrogen stream;   a first condenser vessel operated under suitable conditions to produce the first condensate stream comprising impurities, the cooled process gas stream comprising nitrogen and C 2 -C 4  alkene, and the first gaseous nitrogen stream, wherein the first condenser vessel comprises:
 a first heat exchanger; 
 a first inlet for providing the process gas stream; 
 a second inlet for providing the first cooling medium stream; 
 a first outlet for removal of the first gaseous nitrogen stream; 
 a second outlet for removal of the first condensate stream; and 
 a third outlet for removal of the cooled process gas stream; 
   a second condenser vessel operated under suitable conditions to produce the second condensate stream comprising C 2 -C 4  alkene, the nitrogen-containing vent gas stream, and second gaseous nitrogen stream, wherein the second condenser vessel comprises:
 a second heat exchanger; 
 a fourth inlet for providing the cooled process gas stream; 
 a fifth inlet for providing the second cooling medium stream; 
 a fourth outlet for removal of the nitrogen-containing vent gas stream; 
 a fifth outlet for removal of the second gaseous nitrogen stream; and 
 a sixth outlet for removal of the second condensate stream, and 
   a nitrogen vessel comprising at least two of:
 a seventh inlet for providing the first gaseous nitrogen stream; 
 an eighth inlet for providing the nitrogen-containing vent gas stream; 
 a ninth inlet for providing the second gaseous nitrogen stream; and 
 a seventh outlet for removal of the combined nitrogen stream. 
   
     
     
         13 . The system of  claim 12  further comprising one or more petrochemical vent gas streams and means for combining the one or more petrochemical vent gas streams to form the process gas stream. 
     
     
         14 . The system of  claim 12 , wherein the first cooling medium is liquid nitrogen or gaseous nitrogen and the second cooling medium is liquid nitrogen. 
     
     
         15 . The system of  claim 12 , wherein the first heat exchanger and the second heat exchanger are each independently a coil heat exchanger, a shell and tube heat exchanger or a plate heat exchanger. 
     
     
         16 . The system of  claim 12 , wherein the process gas stream comprises at least about 1.0 wt % nitrogen and less than about 80 wt % C 2 -C 4  alkene. 
     
     
         17 . The system of  claim 12 , wherein the nitrogen-containing vent gas stream comprises at least about 90 wt % nitrogen, based on the total weight of the nitrogen-containing vent gas stream, and/or the nitrogen vessel comprises greater than 90 wt % nitrogen, based on the total weight of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream. 
     
     
         18 . The system of  claim 12 , wherein nitrogen vessel comprises nitrogen, based on the total weight of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream, in an amount greater than the nitrogen-containing vent gas stream, based on the total weight of the nitrogen-containing vent gas stream. 
     
     
         19 . The system of  claim 12 , wherein the C 2 -C 4  alkene is ethylene and/or propylene