Q & A > Question Details
1. What is the contribution of lower/upper LCN recycles and lower/upper C-4 recycles towards dry gas and c3/c4 yields, assuming that riser outlet and regenerator dense bed temperature are constant.
2. What is the contribution of lower and upper slurry recycles towards coke and slurry make? We have a low CCR feed and coke/slurry yield is low. So, continuous torch oil injection is there to maintain regenerator dense bed temperature.
 
Answers
11/05/2016 A: Eric Hennings, Technip Stone & Webster, EHennings@technip.com
Recycles to the “lower” elevation will see the most severe conditions (mix temperature and cat-to-oil ratio). This will result in high conversion and additional dry gas.
Reslurry to reactor bed will increase coke more than reslurry to riser.
The best method of increasing coke is to add coke precursors to feed. Precursors include (in descending order) higher concarbon, higher nitrogen, non-hydrotreated VGO, less severe-hydrotreated VGO.
28/03/2016 A: Satyendra Balot, MRPL, sbalaut@yahoo.com
1. When LCN is recycled in acceleration zone at higher elevation, it eventually increases WHSV and hence over cracking of recycle stream can be minimized leading to improvement of propylene selectivity substantially.
light cracked naphtha (LCN) injected right at the bottom produces very high undesirable dry gas consisting of hydrogen, methane, ethane and ethylene, since ethylene is not recovered in most FCC units. The undesirable dry gas make can be dropped considerably with improvement in propylene selectivity to about 25 to 30% if light cracked naphtha is introduced at relatively higher elevation within the riser bottom zone.
It was found that the conversion of C4 olefin stream to propylene is in the range of 15 to 30% depending on the riser bottom condition. It was also found that this olefins also oligomarize to gasoline range compound. As the coke from C4 olefin cracking is very less, the coke on catalyst before meeting the hydrocarbon stock feed is in the range of 0.05 to 0.1 wt %. As a result, catalyst doses not deactivate while meeting with fresh VGO (vacuum gas oil) or heavy feedstock. Moreover, residual coke on catalyst in the range of 0.05 to 0.1 wt % does reduce dry gas make from cracking of VGO or heavy feedstock, as this coke preferentially sits on very high active acid sites which otherwise make more coke and dry gas.