Post riser cracking

Reduction of the catalyst/hydrocarbon time in the riser, coupled with the elimination of post-riser cracking, reduces the saturation of the already produced olefins and allows the refiner to increase the reaction severity. The actions enhance the olefin yields and still operate within the wet gas compressor constraints. Elimination of post-riser residence time (direct connection of the reactor cyclones to the riser) or reducing the temperature in the dilute phase virtually eliminates undesired thermal and nonselective cracking. This reduces dry gas and diolefin yields. 

In most of today s FCC operations, the desired reactions take place in the riser. In recent years, a number of refiners have modified the FCC unit to eliminate, or severely reduce, post-riser cracking. Quick separation of catalyst from the hydrocarbon vapors at the end of the riser is extremely important in increasing the yield of the desired product. The post-riser reactions produce more gas and coke versus less gasoline and distillate. Presently, there are a number of commercially proven riser disengaging systems offered by the FCC licenser designed to minimize the post-riser cracking of the hydrocarbon vapors. 

Since the mid-1980s, FCC technology licensors and a number of oil companies have employed a number of RTD s to reduce non-selective post-riser cracking reactions. Two general approaches have been used to reduce post riser cracking. The most widely used approach is direct connection of the cyclones to the riser and on to the reactor vapor line. The second approach is quenching the reactor vapors downstream of the riser-cyclones (rough-cut cyclones). 

Mobil Oil developed and commercialized a "closed cyclone system" in which a rough cut cyclone was directly to the riser termination which dramatically reduced the residence time of hydrocarbon vapors in the dilute phase of the reactor vessel thereby dramatically reducing "post-riser cracking". 

Figure 15. Post Riser Cracking - Commercial Data from Ross (1990)...

Challenges, 6th. International Conference on Fluidization, Banff, Alberta, May 7-12 (1989). Avidan, A.A., F. J. Krambeck, H. Owen and P. H. Schipper, FCC Closed Cyclone System Eliminates Post-Riser Cracking, NPRA Annual Meeting, San Antonio, Texas, March 25-27 (1990). 

Avidan A.A., foambeck, F.J., Owen, H. and Schipper P.H., "FCC closed-cyclone system eliminates post-riser cracking", Oil and Gas J., Mar 26, 56-62 (1990b). 

Post-riser hydrocarbon residence time leads to thermal cracking and non-selective catalytic reactions. These reactions lead to degradation of valuable products, producing dry-gas and coke at the expense of... 

A conventional FCC unit can be an olefin machine with proper operating conditions and hardware. Catalysts with a low unit cell size and a high silica/alumina ratio favor olefins. Additionally, the addition of ZSM-5, with its lower acid site density and very high framework silica-alumina ratio, converts gasoline into olefins. A high reactor temperature and elimination of the post-riser residence time will also produce more olefins. Mechanical modification of the FCC riser for millisecond cracking has shown potential for maximizing olefin yield. 

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