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
15/05/2020 A: Prince George, Bharat Petroleum Corporation Limited, prince.george@gmail.com
The slurry recycle to the riser contributes towards an increase in LCO make by 2-3wt% whereas slurry recycle to the stripper mainly results in ian ncrease in coke make. As you have mentioned that the regenerator dense bed temperature is low due to low coke make it is advised to inject slurry recycle into the stripper and not to the riser.
I suppose you are operating a Technip design DCC. Injecting torch oil i to maintain a regen dense temperature above 700 deg C is not needed. In such cases there will be severe afterburn with the dilute bed temperature going as high as 760 deg C.

We also have a Technip designed DCC. We maintain the Regen dense bed at 660 deg and do not inject torch oil. We do not experience afterburn either. The dilute bed temp is in the order of 670 deg C. Also try to increase the MF bottom temp at 350-355 deg C by drawing more LCO. This will make the bottoms heaviers and will help in more coke make with slurry recycle to the stripper.
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 Balaut, 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.