FCC process combining molecular separation with staged conversion

Abstract

This invention relates to fluid catalytic cracking (FCC) process for improving the quality of distillates suitable as blend stocks for diesel fuels. The FCC process combines staging the FCC conversion process with interstage molecular separation of multi-ring aromatic species. Lower and higher severity reaction zones in the riser of the FCC reactor are combined with selective molecular separations to improve the production of diesel quality distillates.

1 . A catalytic cracking process for enhancing the production of distillate suitable as a diesel fuel or blend stock for diesel fuel which comprises: 
 (a) contacting hydrocarbon feed with catalyst in a two zone FCC reactor, said reactor comprising a first low severity zone and a second higher severity zone and said feed comprises a fresh feed and a recycled feed wherein the fresh feed is contacted with catalyst in the first zone and recycled feed is contacted with catalyst in the second zone to form combined cracked product;    (b) separating combined cracked product from catalyst;    (c) fractionating separated cracked product from step (b) into a naphtha fraction, a fraction containing diesel fuel or blend stock for diesel and a bottoms fraction;    (d) passing the fraction from step (c) containing diesel fuel or blend stock for diesel fuel through a selective low energy molecular separation to produce a diesel fuel or blend stock for diesel fuel and a light cycle oil;    (e) passing the bottoms fraction from step (c) through a second selective low energy molecular separation to produce a second bottoms fraction and a feed fraction; and    (f) passing the feed fraction from step (e) as recycled feed to the second zone of step (a).    
   
   
       2 . A catalytic cracking process for enhancing the production of distillate suitable as a diesel fuel or blend stock for diesel fuel which comprises: 
 (a) contacting hydrocarbon feed with catalyst in a FCC process containing a first and second riser, said first riser comprising a low severity zone and said second riser comprising a higher severity zone, and said feed comprises a fresh feed and a recycled feed wherein the fresh feed is contacted with catalyst in the first riser to form a first cracked product and recycled feed is contacted with catalyst in the second riser to form a second cracked product;    (b) combining the first and second cracked product to from a combined cracked product and separating combined cracked product from catalyst;    (c) fractionating separated combined cracked product from step (b) into a naphtha fraction, a fraction containing diesel fuel or blend stock for diesel and a bottoms fraction;    (d) passing the fraction from step (c) containing diesel fuel or blend stock for diesel fuel through a selective low energy molecular separation to produce a diesel fuel or blend stock for diesel fuel and a light cycle oil;    (e) passing the bottoms fraction from step (c) through a second selective low energy molecular separation to produce a second bottoms fraction and a feed fraction; and    (f) passing the feed fraction as recycled feed to the second riser of step (a).    
   
   
       3 . A catalytic cracking process for enhancing the production of distillate suitable as a diesel fuel or blend stock for diesel fuel which comprises: 
 (a) contacting hydrocarbon feed with catalyst in a riser of a FCC reactor, said riser comprises a low severity zone and a higher severity zone and said feed comprises a fresh feed and a recycled feed wherein the fresh feed and recycled feed are first contacted with catalyst in the higher severity zone to form combined cracked product;    (b) separating cracked product from catalyst;    (c) fractionating separated cracked product from step (b) into a naphtha fraction, a fraction containing diesel fuel or blend stock for diesel and a bottoms fraction;    (d) passing the fraction from step (c) containing diesel fuel or blend stock for diesel fuel through a selective low energy molecular separation to produce a diesel fuel or blend stock for diesel fuel and a light cycle oil;    (e) passing the bottoms fraction from step (c) through a second selective low energy molecular separation to produce a second bottoms fraction and a recycled feed fraction; and    (f) combining the fresh feed and recycled feed fraction; and    (g) contacting combined fresh feed and recycled feed with catalyst in step (a).    
   
   
       4 . The process of claims  1 ,  2  or  3  wherein the selective low energy molecular separation comprises a membrane.  
   
   
       5 . The process of  claim 1  or  3  wherein the higher severity zone comprises vapor residence time of 1.0 to 5.0 seconds, catalyst/oil ratio of 4 to 10 wt/wt, and outlet temperature of 500 to 571° C.  
   
   
       6 . The process of claims  1  or  3  wherein the lower severity zone comprises a catalyst/oil ratio in the range of 3 to 8 wt/wt, vapor residence times of 0.5 to 5.0 seconds and outlet temperature of 500 to 571° C.  
   
   
       7 . The process of  claim 2  wherein the low severity zone comprises a catalyst/oil ratio=2 to 5 wt/wt, vapor residence time 0.5 to 2 seconds, and outlet temperature 482 to 538° C.  
   
   
       8 . The process of  claim 2  wherein the higher severity zone comprises a catalyst/oil ratio 5 to 10 wt/wt, vapor residence time 1.5 to 5 seconds, and riser top (outlet) temperature 527 to 571° C.  
   
   
       9 . The process of  claim 4  wherein the membrane separates feed into an aromatics rich permeate and a saturates rich retentate.  
   
   
       10 . The process of  claim 4  wherein the membrane is a polyurea/urethane, polyurethane imide or polyester imide copolymer membrane.  
   
   
       11 . The process of  claim 4  wherein membrane separation is practiced under perstraction or pervaporation conditions.  
   
   
       12 . The process of claims  1 ,  2  or  3  wherein the molecular separation is by solvent extraction.  
   
   
       13 . The process of  claim 5  wherein the higher severity zone has a catalyst/oil ratio that is greater than catalyst/oil ratio in the lower severity zone.  
   
   
       14 . The process of  claim 13  wherein catalyst/oil ratio is adjusted by feed flow rates.  
   
   
       15 . The processes of claims  1 ,  2  or  3  wherein conversion of feed in the higher severity zone to 221° C.− (430° F.−) is at least 10% greater than is conversion of feed in the lower severity zone.