Fluid catalytic cracking (FCC) is a process that is typically driven toward LPG and naphtha production, but it also produces light cycle oil (LCO), a relevant middle-cut stream that can be incorporated into the diesel pool (provided that the severity of the diesel hydrotreating unit is sufficient to absorb the low-cetane-index LCO stream); thus, for market considerations, the FCC unit may be pushed to increase the LCO/naphtha production ratio. Operating the FCC in the maximum LCO mode requires careful optimization of process conditions to overcome the drawback of increased bottoms (slurry oil). The present work complements a previous one (in which an empirical model for the FCC yield profile exploring the low-conversion region was devised) by discussing the option of slurry oil recracking and an optimization study of the most profitable operating mode. A simplified empirical model for slurry oil cracking was developed from experimental data from a fixed fluidized-bed pilot plant using two samples of slurry oil originally produced at a large-scale FCC pilot plant: (a) from a high-conversion run (21.4 wt % yield of slurry oil) and (b) from a low-conversion run (35.5 wt % yield of slurry oil). The empirical models were coupled with a simplified process simulator and a constrained nonlinear optimization tool, and the evaluation performed indicated that operating the FCC unit at high conversion (thus in maximum naphtha mode) remains more profitable, particularly due to a relatively small increase in LCO yield compared to a large increase in slurry oil yield in maximum middle distillation mode. If any specific circumstance demands for a low-conversion operation in the FCC, the empirical model obtained showed that an ideal path to achieve a lower conversion would be to allow for slurry oil recycling, to keep a high reaction temperature and a high contact time between catalyst and cracked gaseous products in the riser, and to lessen severity through reducing the catalyst-to-oil ratio, e.g., by increasing feed temperature to the riser.
Silva et al. (Wed,) studied this question.